sol-gel chemistry of transition metal...
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Prog. Solid St. Chem. Vol, 18, pp. 250341, 1988 007%6786/88 $0.00 + .50 Printed in Great Britain. All rights reserved Copyright © 1989 Pergamon Press plc
SOL-GEL CHEMISTRY OF TRANSITION METAL OXIDES
J. Livage, M. Henry and C. Sanchez
Laboratoire de Chimie de la Mati6re Condensde, CNRS (UA 302), Universit6 Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France
i. INTRODUCTION
The sol-gel process provides a new approach to the preparation of glasses and
ceramics. Starting from molecular precursors, an oxide network is obtained via inorganic
polynterization reactions. These reactions occur in solutions and the term "sol-gel
processing" is often broadly used to describe the synthesis of inorganic oxides by "wet
chemistry" methods. These processes offer many advantages as compared to the conventional
"powder" route, such as :
Homogeneous multi-component systems can be easily obtained by mixing the molecular
precursor solutions 1,2
Temperatures required for material processing can be noticeably lowered leading to unusual
glasses or ceramics 3
The rheological properties of sols or gels allow the formation of fibers, films or
composites by such techniques as spinning 4, dip-coating 5 or impregnation 6
This explains why the sol-gel process has received so much scientific and
technological attention during the last decade. Several international meetings are now
devoted mainly to this topic, namely the "International Workshop on Glasses and Glass-
Ceramics from Gels" 7-10, "Ultrastructure Processing of Ceramics, Glasses and Composites"
11-13 and "Better Ceramics through Chemistry" 14,15
One unique property of the sol-gel process is the ability to go all the way from
the molecular precursor to the product, allowing a better control of the whole process and
the synthesis of "tailor-made" materials. Therefore, a real mastery of the sol-gel process
would require an emphasis which relates chemical reactivity to gel formation and powder
morphology. The present paper reviews successively the chemistry of the molecular
precursors, the aggregation phenomena involved in the sol-gel to material transformation,
the physical properties and applications of transition metal oxide gels.
The chemistry of the sol-gel process is based on hydroxylation and condensation of
molecular precursors. These reactions have been extensively studied in the case of silica
16. Unfortunately, much less data is available for transition metal oxide precursors. Two
different routes are usually described in the literature depending on whether the precursor
is an aqueous solution of an inorganic salt or a metal organic compound. The aqueous
chemistry of transition metal ions is described in the first section of this paper. This
topic can be quite complicated because of the numerous molecular species which can exist
depending on the oxidation state, the pH or the concentration. Moreover, in the case of non
tetravalent cations, oxides, hydroxides and even oxo-hydroxides can be obtained 17,18
The most versatile precursors for the sol-gel synthesis of oxides are undoubtely
JPSSC 18:4-A 259
260 J. Livage et al.
19 metal alkoxides which are very reactive toward nucleophilic reagents such as water
Hydrolysis and condensation of transition metal alkoxides are described in the second
section. These alkoxides appear to be much more reactive than silicon alkoxides. They must
be handled with great care, in a dry environment and are often stabilized via chemical
modification 20 This high chemical reactivity is due to the lower electronegativity of the
metal as compared with silicon, and the metal atom's ability to exhibit several
coordination states. As a result of the latter property, coordination expansion
spontaneously occurs when the metal alkoxide reacts with water.
Structural evolution during the sol to gel and gel to solid transitions need to be
fully understood before a real mastery of the sol-gel process can be reached. The
properties of a gel and its response to heat treatment are very sensitive to the structure
already created during the sol stage. Therefore the formation of colloidal aggregates often
determines the main properties of the resulting powder and its ability for the extent to
which the powder can be sintered. By varying the chemical conditions under which silica is
polymerized, structures can be formed which range from randomly branched polymers to
colloidal particles 21 The aggregation of colloidal SiO 2 particles and the growth of
silica polymers have been extensively studied during the last few years. They usually give
rise to very tenuous objects which have very low densities even for large radii of gyration
and can be described as fraetal aggregates 22
Monodispersed transition metal oxide colloids are currently synthesized which can
exhibit anisotropic shapes 23. Particle-particle interactions then lead to the formation of
anisotropic aggregates in which all individual particles are mutually oriented. These
ordered aggregates, called "tactoids" 24 will be described in the third section. They can
lead to anisotropic coatings that behave as host structures for intercalation 25
Sols and gels are usually considered as intermediates in the processing of glasses
and ceramics. Therefore, drying and densification are very important processes that need to
be fully understood 26-28. The present paper does not intend to describe the properties and
applications of transition metal oxide-based materials obtained via the gel route. These
will be reviewed briefly at the end of the paper. The fourth section shows that transition
metal oxide gels are actually diphasic materials made of solvent molecules trapped in a
solid network. Specific electronic and ionic properties arising from the two phases
together with their interface have been observed. They lead to new applications such as
antistatic coatings or electrochromic devices 29
A survey of the literature shows that most studies are concerned with the sol-gel
processing of silicates 30. Fewer papers have been published about A1203, TiO 2 or ZrO 2 and
very few papers deal with other transition metal oxides 29. Therefore, the present article
is mainly concerned with transition elements of the "d" group. However, most of the ideas
described here can be extended to other elements belonging to the "p" (B, AI, P .... ) or
"f" (rare-earths) groups.
2. AQUEOUS CHEMISTRY OF INORGANIC PRECURSORS
The aqueous chemistry of inorganic salts is quite complicated owing to the occur-
rence of hydrolysis reactions which convert the ions to new ionic species or to
precipitates. The hydrolysis of salts can involve the cation, the anion or even both. We
will start by considering the hydrolysis of metal cations, which was first studied by N.
Bjerrum at the beginning of the 20th century 31 At the same time, A. Werner 32 and P.
Pfeiffer 33 proposed the concept of "aquo-acidity" which describes cation hydrolysis as
Sol-Gel Chemistry of Transition Metal Oxides 261
the stepwise removal of protons from hydrating water molecules. However, until the work of
L.G. Sillen 34 , the formation of polynuclear hydrolysis products was almost ignored. This
author proposed a mechanism of hydrolysis in which hydroxyl groups are added to the cation
which leads to the formation of condensed species. Iso o and heteropoly oxometalates are now
well known 3S , and detailed experimental data on the hydrolysis of cations can be found in
the literature 18,36 Unfortunately, it is difficult to account for these data
quantitatively on a theoretical basis. However, a model was recently proposed which allows
the calculation of the partial charge distribution of any complex in order to predict their
chemical reactivity. When two atoms combine, a partial electron transfer occurs so that
each atom acquires a positive or negative partial charge 6 i . It is usually assumed that the
electronegativity Xi of an atom changes linearly with its charge 38 :
xi = x~ + ~i~i (1)
where X~ ° is the electronegativity of the neutral atom and N~ is the "hardness" which may
be defined as 37.
.~ - k/f T (2)
that depends on the electronegativity scale (k = 1.36 for Pauling's
to the principle of electronegativity equalization stated by R.T.
charge transfer should stop when the electronegativities of all
k is a constant
scale). According
Sanderson 39 , the
constituent atoms become equal to the mean electronegativity X given by 37 :
= Zi Pi/~ + kz
Zi (pi/~) (3)
where Pi corresponds to the stoichiometry of the i th atom in the compound and z is the
total charge of the ionic species. Electronegativity actually corresponds to the electronic
chemical potential and electronegativity equalization is nothing else than the well-known
thermodynamic principle of chemical potential equalization in the equilibrium state. The
partial charge 6 i can be deduced from eq. (1),(2) and (3) leading to:
6~ = (~ - x[)Ik~ (4)
6 i can be easily calculated knowing the electronegativity X~ of all neutral atoms, the
stoichiometric composition for the ionic species and its charge z. The Partial Charge Model
can be applied to both inorganic and metal-organic precursors. It is based on simple ideas
and is easy to handle. It corresponds to a thermodynamic approach and leads to a relatively
good quantification of inductive effects. However, several limitations do arise, namely :
In its present form, the Partial Charge Model does not take into account the real
structure of the chemical species.
- Resonance effects and ~ overlapping are not included.
It is difficult to account for coordination variations which occur during the chemical
process.
Nevertheless, this model can be applied successfully to describe the chemical
reactions involved in the sol-gel process and provides a useful guide for inorganic
polymerization reactions.
2.1. Hydrolysis of metal cations
2. i.I. Formation of inorganic precursors. When dissolved in pure water, a cation M z+
becomes solvated by the surrounding water molecules according to :
M z ÷ + :0 ~ M +-- 0
262 J. Livage et al.
In the case of transition metal ions, this solvation leads to the formation of a
partially covalent bond. A partial charge transfer occurs from the filled 3a I bonding
orbital of the water molecule to the empty d orbitals of the transition metal ion. The
positive partial charge on the hydrogen atoms then increases and the water molecule, as a
whole, becomes more acidic. Depending on the magnitude of the electron transfer, the
following reactions occur :
[M - OH2 ]z+ = [M-OH] (z'l)+ + H + = [M=O] (z'2)+ + 2H +
Three kinds of ligands must then be considered in a non complexing aqueous medium : aquo
ligands (OH2) , hydroxo ligands (-OH), and oxo ligands (=0).
Let N be the number of water molecules covalently bound to the cation M z+
(coordination number). The rough formula for any inorganic precursor can then be written as
[MONH2,.h ](z'h)+, where h is defined as the molar ratio of hydrolysis. When h=0, the pre-
cursor is an "aquo-ion" [M(OH2)N] z+ while for h=2N , it is an "oxo-ion" [MON](2N'Z)"
If o<h<2N, the precursor can be either an oxo-hydroxo complex [MOx(OH)N.x ](N+x'z)"
(h>N), an hydroxo-aquo complex [M(OH)h(OH2)N.h ](z'h)+ (h<N) or an hydroxo complex :
[M(OH),] ("'z)" (h=N).
2.1.2. The "charge-pH" diagram. Let us consider a typical transition metal such as
chromium, which exhibits two stable oxidation states, namely Cr(Vl) and Cr(lll). Only three
Cr(Vl) precursors have been experimentally characterized in aqueous solutions 18 .
[CrO (OH) z]° h=6
[CrO] (OH) ] h=7
[CrO~'] 2" h=8
Cr(VI) gives rise to only oxo-hydroxo or oxo complexes but never to aquo
complexes. For Cr(III) however, five precursors have been reported 18 o
As a
aqueous solutions but never oxo-complexes,
[Or(OH2)6 ]3* [Or(OH) (OH 2)s ]2+ [ Cr (OH)2 (OH2)4 ]+
[Cr(OH)3 (0H2)3 ]0
[Cr(OH)4 ]"
consequence, Cr(III)
+8
0 2-
OH-
+3
+1
0 7 14 pH
Fig. i : The "charge-pH" diagram.
h=0
h=l
h=2
h=3
h=4
forms only aquo, aquo-hydroxo or hydroxo complexes in
These observations can be summed up in a
qualitative way using a "charge-pH" diagram
17,40 as shown in figure i. This diagram gives
the nature of the precursors as a function of
the formal charge z of the cation M z+ and the
pH of the aqueous solution. Three domains can
be defined namely : "aquo" [M(OH2)N] z+ ,
[MONH2N.h ](z'h)+, and "oxo" "hydroxo"
[MON](2N'z)'. Such a diagram shows that low-
valent cations (z<+4) give rise to aquo-
hydroxo and/or hydroxo complexes over the
whole range of pH, while high-valent cations
(z>+5) form oxo-hydroxo and/or oxo complexes
over the same range of pH. Tetravalent cations
(z=+4) are on the border line, and therefore
lead to a large number of possible precursors.
Sol-Gel Chemistry of Transition Metal Oxides 263
2.1.3. Quantitative analysis. The Partial Charge Model can be used in order to calculate
the magnitude of charge transfer between ligands (oxo, hydroxo, aquo) and cations M z÷ . A
"charge-pH" diagram can thus be established in close agreement with experimental data.
Using the model, acidic or basic forms of a given cation in an aqueous solution can also be
predicted 37 Under acidic conditions the reaction to be considered is the cleavage of the
O-H bond arising from the metal atom's large polarization :
~+ S" 6 ÷ M - O - H + H20 = M - 0" + H30 +
This occurs as long as 8(OH)>O in the [MONH2N.p] (z'p)+ precursor, leading to the
reaction :
[MONH2N ]z+ + PH20 = [MONH2N.p] (z'p)+ + pH30 +
The limiting condition 6(OH)=0 leads to the following relations :
- mean electronegativity X = ~(OH) = 2.71
z - n&(H)-6(M) charge conservation p =
1-8(H)
Partial charges ~(H) and ~(M) can thus be calculated leading to :
p = 1.45z - 0.45 N - 1.07(2.71-X~)/~ (5)
Relation (5) shows that the number of protons p removed through spontaneous
hydrolysis directly depends on the formal charge z, the coordination number N and the
electronegativity X~ of the metal. These last two parameters are a direct function of the
size of the cation M z+ which can thus be taken into account. When applying relation (5)
three possible cases have to be considered (cf Table i) :
i) p<0, (2N-p>2N) : the [M(OH2)H] z+ precursor does not exhibit any acidic behavior.
A base such as OH" must be added in order to initiate hydrolysis. This situation occurs for
Ag + and Mn 2+ cations for example.
M Z N X~ 2N-p Couple
Ru +8 4 1,78 -i,I [Ru04]°
Mn +7 4 1,63 0,5 [MnO4]'/[MnO3(OH)]°
Cr +6 4 1,59 2,1 [CrO2(OH)2]°/[CrO(OH)3 ]+
V +5 6 1,56 8,4 [VO2(OH2)4]+/[VO(OH)(OH2)4] 2+
Ti +4 6 1,32 10,2 [TiO(OH2)5]2+/[Ti(OH)(OH2)5] 3÷
Zr +4 8 1,29 15,1 [Zr(OH)(OH2)7]3+/[Zr(OH2)8] 4÷
Fe +3 6 1,72 11,2 [Fe(OH)(OH2)5]2+/[Fe(OH2)6] 3+
Mn +2 6 1,63 12,7 [Mn(OH2)6] 2+
Ag +i 2 1,68 4,3 [Ag(OH2)e] ÷
Table 1 - Some inorganic precursors in their most acid forms.
ii) p>2N, (2N-p<0) : the [MON](2N'z)" precursor does not exhibit any basic behavior
and cannot be protonated by H3 O+ in aqueous solutions. A typical example is RuO 4 .
iii) 0<p<2N, (0<2N-p<2N) : under acidic conditions, two species corresponding to
h=E(p) and h=E(p+l) are in equilibrium where E(p) indicates the whole part of p. Typical
examples are Mn(VII), Cr(VI), V(V), Ti(IV) and Fe(III).
Under basic conditions, the limiting reaction is the cleavage of the M-O bond
arising from the low polarization of the metal atom :
___+ M + (M-OH)aq aq + OHaq
This reaction occurs as soon as a hydroxyl ion can be formed through solvation. The
(z-q)+ precursor, leading to 37 : limiting case corresponds to S(OHaq)=-i in the [MONH2N.q]a q
q = i + 1.25z - 0.92(2.49-X~)/~ (6)
264 J. Livage et al.
(2N-q) corresponds to the number of protons that cannot be removed from the precursor even
at very high pH. Two cases can be encountered when applying relation (6) (cf.Table 2).
M Z N X~ 2N-q Couple
Ru +8 5 1.78 -0.5 [RuO~] 2"
Mn +7 4 1.63 -i.I [Mn04]"
Cr +6 4 1.59 0.2 [CrO~]/[CrO3(OH)]"
V +5 4 1.56 1.4 [VO3(OH)]2"/[VO2(OH2)]
Ti +4 5 1.32 5.0 [MO(OH)4]2"/[M(OH)5]" Zr 1.29
Fe +3 4 1.72 3.8 [FeO(OH)3]2"/[Fe(OH)4]"
Mn +2 3 1.63 3.1 [Mn(OH)B]'/[Mn(OH)2(OHz)] °
Ag +I 2 1.68 2.3 [Ag(OH)2]'/[Ag(OH)(OH2)] °
Table 2 - Some inorganic precursors in their most basic forms.
i) q>2N (2N-q<0) : The most basic form of M is an oxo-ion [MON](2N'z)'. Typical
examples are Ru(VlII), Mn(VII).
ii) O<q<2N (0<2N-q<2N) : twe species corresponding to h=E(q) and h-E(q+l) are in
equilibrium at very high pH. These may be exo-hydroxe complexes (V(V), Ti(IV), Zr(IV) and
Fe(lll)) or hydroxe-aquo complexes (Mn(ll), Ag(1)).
2.1.4. Initiation of condensation reactions. Condensation in aqueous solutions can occur
through two simple mechanisms that can be related to the coordination unsaturation :
i) If the preferred coordination is already fulfilled in the molecular precursor,
condensation occurs via a substitution reaction. In this case an entering group OX and a
leaving group OY must be present around M :
X M - OX + M - OY ~ M - 0 - M + OY
in order to keep the coordination number of the metal unchanged.
ii) If the preferred coordination is not fulfilled in the molecular precursor,
addition reactions become possible :
X M - OX + M - OY ~ M - 0 - M - OY
An increase of the coordination number occurs so that no OY group need to be eliminated.
In aqueous solutions, three kinds of precursors have to be considered according to
the "charge-pH" diagram (cf. Fig.l).
- Oxo-ions [MON](2N'z)" • the partial charge on M is usually slightly positive while the
partial charge on 0 is strongly negative (6(0)<<0). As a consequence oxo ligands are very
good nucleophiles but very poor leaving groups. Condensation therefore occurs only via
addition when the precursor is unsaturated.
- aquo-ions [M(OH2)N] z+ : the partial charge en M is usually strongly positive (6(M)>>0),
while the charge on the H20 molecule is slightly positive (6(H20)>0). Aquo ligands thus
show no nucleophilic property and act only as leaving groups. Condensation cannot occur
with such precursors because no entering group is available.
precursors [MONH2N.h] Cz'h)+ : both nucleophilic ligands (oxo or hydroxo) Other and
leaving ligands (hydroxo or aquo) are present around the metal. Condensation through
substitution reactions can thus begin as soon as one hydroxo ligand appears in the
coordination sphere. Following the "charge-pH" diagram this means that we must move into
the hydroxo domain in order to get condensed species (oligomers, sols, gels or
precipitates). This can be done by :
Sol-Gel Chemistry of Transition Metal Oxides 265
- adding a base or an oxidizer to an aquo precursor :
[Fe(OH2)6] 3+ + 3 OH" ~ [Fe(OH)3(OHz)3] ° + 3 HzO
[Mn(OH2)6] 2+ + H202 , [Mn(OH)4(OH2)2 ]° + 2 H + + 2 H20
- adding an acid or a reducing agent to an oxo precursor :
[WO412" + 2 H3 O+ , [WO2(OH)2(OH2)2] °
2[Mn04]" + 3 H202 + 6 H20 , 2[Mn(OH)4(OH2)2 ]° + 30 Z + 2 OH"
- or even via thermohydrolysis of an aquo precursor :
[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2+ + H30 +
In this case, the temperature has to be increased because the enthalpy change All
hydrolysis reaction is positive 18
of the
2 .2 . Condensation via olation
2.2.1. Mechanism. According to the literature 41, "olation" leads to the formation of a
hydroxo or "oi" bridge M-OH-M. Such a condensation process occurs with hydroxo-aquo
precursors [M(OH)x(OH2)N.x ](z'x)+ where x < N. Basically it corresponds to a nucleophilic
substitution (SN) in which M-OH is the nueleophile and H20 the leaving group. Several kinds
of bridges can occur as shown in figure 2 . Following Baran 42 bridges will be symbolized
~- ~+ ~+ H M - - O H ~ +..~.~M~jOH 2 ~ M--O--M + H20 2(OH)1
M _ M \
M / ~ M ~ . 9H2 ~ M / O H - M + H20 3(0H)I
~- H ,~+ (~ + / 0 H-- - - - - -~ ~+ ~+ cO~
H20-- M - - * ~ M - - OH 2 . M M ~o~_ o
+ 2 H20 2(0H)2
H H /o
H20 OH ,~+ ,~_ H
+ H20 2 ( OH )3
AI 3+
I G a 3*
I I
o
z+ [M(OH2)N ] z * - - - [M(OH2)N_I ] * H20 S N 1
Be 2. Mg2 ÷
I I ,r ,r in 3* TI 3* Zn 2. Cd 2*
J I I I V2" Fe 3. C J * Ti3*Ni 2+ d + F,Z*Mn=*A, g* C~*C,u =*
I I I I I I I I I, I i , ~ i J , r , 1 2 3 4 5 6 7 8
Li* N a*K*RIoCs*
Hg 2+
I Mn 3÷
I log ~(s") 9 10
Fig.2. Olation mechanisms and lability of some aquo-ions.
266 J. Livage et al.
as × (OH)y where x is the number of metal atoms linked by one "oi" bridge and y the number
of bridges between these x metal atoms. As oxygen cannot form more than four covalent
bonds, the limiting value for x is 3.
In all cases an aquo ligand must be removed from the coordination sphere. The
kinetics of olation therefore strongly depends on the lability of the M-OH 2 bond. This
lability depends mainly on the charge, size, electronegativity and electronic configuration
of the M atom as shown in figure 2 43,44 " the smaller the charge and the larger the ionic
radius, the faster the M-OH 2 bond will be broken. In addition, it is well known that
transition elements whose electronic configuration is d 3 (Cr B+ ,V 2+) ,d 6 low spin (Co 3+) or
d 8 (Ni 2+) are kinetically inert owing to their high crystal field stabilization energy in
octahedral coordinations 45. For these elements the rate constant for solvent exchange
ranges typically between 10 .4 and 10 .6 s "I 46
In other cases, olation can be extremely fast especially for low valent precursors
(O~_z-h<2) and is limited only by diffusion (k>10 ? M" I s" I). Rates are much slower for highly
charged precursors (z-h~2), particularly when the size of the cation is small. The
dimerization rate constant k of the Fe 3+ precursors is rather low 47 :
H 2(0H)I: [Fe(OH)(OH2)5] 2+ + [Fe(OH2)6] 3+ = [(H20)sFe-O-Fe(OH2)5] 5+ + H20 (k=2.5.10 "2 M'Is "I) o}
]4+ 2(0H)2: 2[Fe(OH)(OH2)5] 2÷ = [(H20)4Fe e(OH2) 4 + 2 H20 (k = 10"1-103 M'Is "I)
0
while it is much faster for VO 2÷ or Cu 2+ 4?,48 .
H 2(0H)I: [VO(OH)(OH2)4 ]+ + [VO(OH2)5] 2÷ = [(H20)40V-O-VO(OH2)4] 3+ + H20 (k = i M'Is "I )
2(0H)2:2[VO(OH)(OH2)41+ = [(H20)30 0(0H2)3 ]2+ + 2 H20 (k = 104 MIs "I)
-OH
2(0H)2 : 2[Cu(OH)(OH2)5 ]+ = [(H20)4C /0~ - o,CU(OH2)412÷~ ~ + 2 H20 (k = 108 M'Is "I)
2.2.2. Polycations. Charged precursors (z-h ~I) cannot condense indefinitely to form a
solid phase. This is mainly due to the fact that the nucleophilic strength of the hydroxo
group 6(OH) varies during the condensation process. In the typical dimerization reaction of
Cr(lll) :
° I 2[Cr(OH)(OH2)5] 2+ = [(H20)4Cr \ ~ Cr(OH2)4] 4+ + 2 H20
O
OH groups are negatively charged in the monomer (6(OH)=-0.02) while they become positively
charged in the dimer (6(OH)~+0.OI). The partial charge of hydroxo groups can change in sign
during the condensation process, owing to the departure of donor water molecules. From a
chemical stand point, this means that OH loses its nucleophilic power in this polycationic
compound. Condensation is then limited to dimers mainly for entropic reasons. More
condensed polycations can however be formed if the nucleophilic strength of the starting
monomer is higher. As an example, let us consider the dimerization of Ni(ll) species :
2[Ni(OH)(OH2)31÷ = [(H20)2Ni\ { i(OH2)212+ + 2 H20
O
6(OH)=-0.07 in the monomer and °0.03 in the dimer. The hydroxo group remains negatively
charged and keeps some nucleophilic character. Condensation can proceed further towards a
Sol-Gel Chemistry of Transition Metal Oxides 267
tetramer whose presumed structure is shown in figure 3 (structure E) :
2[Ni2(OH)2(OH2)4 ]2+ = [Ni4(OH)4(OH2)4] 4+ + 4 H20
The partial charge becomes 6(0H)=+0.06 in this tetramer and condensation stops
stage in agreement with experiments 49
at this
A (A) [M2 (OH) (OH2)× ] 3+
M = Mn 2+, Co 2+ Ni2+ 18
C
D
(B) [M 2(OH) 2(OH2) x](2z'2)+ M = VO 2+, Cr3+, Fe3+, Ti3+, Cu2+ 18
(C) [Cr2(OH)(OH2)I0 ]5+ 50
(D) [M 4 (OH)6 (OH 2)12 ]6+
M = Cr 3+ 51
(E) [M 4 (OH)4 (OH 2 )4 ]4+ M = Co 2+ , Ni 2+ 49
(F) [M 4 (OH)8 (OH 2 )I 6 ]8+
M = Zr 4+ , Hf 4+ 52,53
oM ®oH OHio Fig.3. Transition metal polycations,
Figure 3 gives other examples of transition metal polycations 18'49"53.It is easy to
show that in each case the partial charge on the hydroxo ligand is close to zero or weakly
positive. The Partial Charge Model is thus able to explain why condensation stops before an
infinite network is formed. These polycations must then be considered as end points in
hydrolysis and condensation reactions of monomeric precursors in a given range of pH.
Precursor 6(OH) 6(M)
[Ti(OH)2(OH2)4] 2+ - 0.01 + 0.88
[V(OH)2(OH2)4 ]2+ + 0.01 + 0.68
[Zr(OH)2(OH2)6 ]2+ - 0.07 + 0.87
[Hf(OH)2(OH2)6 ]2+ - 0.06 + 0.81
Table 3 - Nucleophilic strength of h = 2 precursors of tetravalent elements.
The formation of oxo-aquo precursors for tetravalent metals can also be easily
explained by the Partial Charge Model. Table 3 compares the nucleophilic strength of OH
groups for several h=2 precursors. The nucleophilic strength is quite low for Ti(IV) and
V(IV) which means that condensation is difficult. As condensation is inhibited, a
prototropic transfer between the two geminal hydroxo groups can occur :
6 + OH O
OH OH 2
The oxo l i g a n d t h u s fo rmed c a n make a s t r o n g d o u b l e bond w i t h t h e h i g h l y e l e c t r o p h i l i c
m e t a l and c a n n o t be e a s i l y p r e t e n a t e d a g a i n . As a c o n s e q u e n c e t h e s t a b l e fo rm o f t h e h~2
268 J. Livage et al.
precursor is an oxo-aquo precursor [MO(OH2)5] 2+ rather than a geminal dihydroxo-aquo
precursor [M(OH)2(OH2)4] 2÷ in good agreement with experiments 54,55,56. Such a mechanism
does not occur with zirconium and hafnium. Hydroxo groups in the h-2 precursors are
nucleophilic enough to initiate further condensation. Therefore cyclic tetramers with
2(OH) 2 bridges are formed rather than monomeric oxo-aquo ions 52,53
2.2.3. Precipitation and gelation. Zero charged precursors (h-z) are able to nucleate a
solid phase through infinite condensation of "oi" groups. The final term of this process
must then be a hydroxide M(OH)z provided oxolation does not occur. In order to know whether
oxolation has to be taken into account when considering aquo-hydroxo precursors
[M(OH)h(OH2)N.h] (z'h)+ or hydroxides M(OH)z, let us consider the following equilibrium :
6 ÷ 6" 6 ÷ 6
--M-- -- = --M--~. .-
This reaction is basically a 1,3 electrophilic rearrangement where a proton jumps between
two adjacent hydroxo ligands, with at least one of them being in a bridging position. The
partial charge of the water molecule created by this prototropic transfer can be either
positive or negative :
i) 6(H20 ) < 0 : There is a net attractive force between the cation M(6 +) and the
aquo llgand (6"). Water elimination is thus prevented and the reverse prototropic transfer
occurs reforming the "oi" bridge which was originally broken. In such a situation the "oi"
bridge remains stable and oxolatlon does not occur.
ii) 6(H20 ) > 0 : There is a net repulsive force between the cation M(6 +) and the
aquo ligand (6+). Water can be removed and the reverse transfer becomes impossible leading
to the irreversible formation of an oxo bridge. In such a situation the "oi" bridge is
unstable and oxolation can compete with olation.
Table 4 gives the calculated values of 6(H20) for some transition metal aquo-
hydroxo precursors and hydroxides. It is seen that as soon as 6(H20)<0 an hydroxide M(OH) z
can be isolated 57 This is no more the case when 6(H20)>0 for oxolation can now occur. In
such conditions an oxy-hydroxide can be obtained with trivalent
Solid hydroxide Soluble precursor 6(H20 ) formed by pure 6(H20)
olation
[Mn(OH)2(OH2)4] ° - 0,02 Mn(OH) 2 - 0,06
[Fe(OH)2(OH2)4] ° - 0,01 Fe(OH) 2 - 0,02
[M(OH)2(OH2)4] °(*) - 0,003 M(OH) 2 - 0,01
[ Sc (OH)3 (OH 2 )3 ] ° Sc(OH)3 - 0,05 - 0,i0
[Y(OH)3 (OH2)3 ] ° Y(OH)3
[V(OH)3(OH2)3 ]° + 0,01 V(OH) 3 + 0,02
[Cr(OH)3(OH2)3 ]° + 0,01 Cr(OH) 3 + 0,03
[Mn(OH)3(OH2)3 ]° + 0,02 Mn(OH) 3 + 0,04
[Fe(OH)3(OH2)3 ]° + 0,03 Fe(OH) 3 + 0,07
[Co(OH)3(OH2)3 ]° + 0,03 Co(OH) 3 + 0,08
[TiO(OH)2(OH2)3 ]° + 0,01 TiO(OH)2 + 0,02
[VO(OH)2(O~)3 ]° + 0,05 VO(OH) 2 + 0,12
[Zr(OH)4(OH2)4] ° + 0,002 Zr(OH) 4 + 0,005
[Hf(OH)4(OH2)4] ° + 0,01 Hf(OH) 4 + 0,03
(*)M = Co, Ni, Cu.
Table 4 - Stability of hydroxides M(OH) z
elements while hydrous
Crystalline phases known
Mn(OH)2,MnO
Fe(OH)2,FeO
M(OH)2, MO
Y(OH)~ YOOH Sc(OH)~,ScO.OH
Y203 , Sc203
VO.OH, V203
CrO.OH,Cr203
MnO.OH, Mn203
FeOOH, Fe203
CoOOH
T i O 2
VO 2
ZrO 2
HfO 2
deduced from the Partial Charge Model.
Sol-Gel Chemistry of Transition Metal Oxides 269
oxides are obtained only with tetravalent elements. However, it must be pointed out that
these oxy hydroxides are formed under very specific conditions. They should not be
considered as the final term of nucleation and growth processes which would lead to the
oxide MOz/2 if 6(HzO)>0.
The formation of a gel rather than a precipitate from aquo-hydroxo inorganic pre- 58.
cursors is a rather complicated process which depends critically upon many parameters
- A pH-gradient is induced by the gelifying agent which may be NaOH, NH 3 , NaHCO 3 , Na2CO 3 ,
(NH2)2CO, or any hydroxyl exchanger.
The concentration of both reagents may be quite different.
- The addition mode and the speed of agitation of the solution must be controlled.
- The order of mixing of the reactants and the geometry of the vessel play a role.
- The temperature can either favor or inhibit gel formation.
The chemical composition of the aqueous solution can induce modification of the
precursors at a molecular level.
All these parameters must be taken into account because nucleation and growth
involve mainly olation reactions which are diffusion-controlled processes. As a
consequence, colloidal gels are obtained which are not very stable when prepared in a pure
form. Metals that lead to stable "oi" bridges give rise to well defined hydroxides M(OH) z
59 Other metals that do not form stable hydroxo bridges lead to hydrated amorphous
gelatinous precipitates MOx/2(OH)z.x.YH20 when a base is added to the aquo precursors.
These precipitates are not well defined. They lose water continuously through oxolation
finally leading to the oxide MOz/2 60,61,62 Other complications can arise with
multivalents elements such as Mn, Fe and Co because electron transfers may occur in the
solution, the solid phase, or even at the oxide-water interface. The following examples
will briefly show how these different reactions may be analyzed.
2.2.4. Sols and gels of divalent metal oxides. We will consider mainly Co 2+, Ni 2+ and Cu 2+
cations because other divalent metals (V 2÷ , Cr 2+ , Mn 2+ and Fe 2+) are easily oxidized in
aqueous solution.
Green transparent nickel hydroxide gels can be obtained by dissolving the freshly
precipitated hydroxide in tartric acid and adding sodium or potassium hydroxide in molar
proportions (>0.5 M) 63. Similar results are obtained when nickel acetate is dissolved in
glycerol and treated by an alcohol solution of potassium hydroxide 64 After dialysis and
dessication, the solid phase is Ni(OH)2 and not NiO showing the stability of the ol bridges
in this system. No structural characterization has been undertaken for these gels.
Owing to the easy oxidization of Co 2+ in strongly alkaline solutions, different
results are obtained with cobalt. In this case gelation is slower and the color changes
from pink to purple to green and after many days to brown 63 Oxidation of Co 2+ towards
Co 5÷ obviously occurs under such conditions :
3 Co 2+ + 3 H20 + 1/2 02 , Co304 + 6H +
This reaction was used by Sugimoto and Matijevie to produce monodispersed Co304 sols 65 In
this case it is interesting to point out that sols can be obtained only in the presence of
acetate ions. No precipitation is observed under the same conditions when other Co (II)
salts (nitrate, chloride and sulfate) are used.
Copper hydroxide gels are more difficult to produce and the following conditions
must be fulfilled in order to make them 66,67 .
i) The starting precursor must be copper (II) acetate. Nitrates, chlorides or sulfates
always give rise to gelatinous precipitates.
ii) The added base must be diluted ammonia without any excess.
270 J. Livage et al.
iii) A small amount of sulfate ions must be added in order to get a stable gel.
These gels are highly anisotropic and show interesting aggregation phenomena which
have been studied in our laboratory.
Copper (II) hydrous-oxide sols can also be made by heating a solution of nitrate or
sulfate or a mixture of copper (II) nitrate and potassium phosphate 68. By heating copper
tartrate complexes (Fehling's solution) with glucose, uniform copper (I) hydrous oxide sols
can be obtained with various particle shapes and sizes 69
2.2.5. Sols and gels of trivalent metal oxides. Hydrous chromic oxide gels can be made
by treating Cr(III) sulfate, nitrate, chloride or acetate precursors with ammonia or
potassium hydroxide 70,71 Highly vibrant monolithic gels can be produced only when acetate
ions are present in excess 70-72. The color of these gels is blue-grey when NH 3 is used and
bottle-green with KOH . This difference may well be due to complexation between Cr 3+ and
NH 3 . These gels are amorphous to X-rays, but small fractions of crystalline CrOOH and
microcrystalline Cr(OH) 3 can sometimes be detected 73,74,?5. EXAFS measurements have shown
that the gels have the stoichiometry [Cr(OH)3(OH2)3].nH20 and that hydroxyl groups condense
to form Cr-O-Cr bonds without decreasing the coordination number of Cr 3+ 76 The final term
of this oxolation is ~-Cr203 with no intermediate phase such as CrOOH, which is in
agreement with the predicted instability of ol bridges in the h ~ 3 precursor (table 4). By
ageing chromium salts such as KCr(SO4)2.16H20, Cr2(SO4) 3 and Cr(NO3) 3 at high
temperature hydrous chromic oxide sols can be prepared 77,78 Some sulfate and phosphate
ions are necessary in order to obtain monodispersed sols 78
The behaviour of Fe 3+ is quite different despite similar eleetronegativity and
coordination number. Gelatinous precipitates are obtained instead of gels when a base
such as NH 3 or NaOH is added to precursors such as chlorides, sulphates, nitrates,
perchlorates, acetates or oxalates. This may be correlated with the rate of olation of the
aquo-hydroxo precursors :Fe 3+ (3d 5) exhibits no crystal field stabilization in an
octahedral symmetry. Consequently, olation is fast as shown by the rate of dimerization of
the [Fe(OH)(OH2)5] 2+ ion : k- 450 M'Is "I at 25oc 79. In contrast, Cr 3+ (3d 3) shows a high
crystal field stabilization in the same symmetry. This implies a low reactivity of Cr 3+
ions towards nucleophilie substitution and thus olation rates must slow down in a rather
drastic way. In agreement, the rate of dimerization of the [Cr(OH)(OH2)(C204)2]2"ion is k =
10 .5 MIs "I at 25°C 80. As monolithic gels are preferentially formed when the rate of
condensation is slow, gels are easily formed with Cr 3+ while only gelatinous precipitates
are obtained with Fe 3+ .
These gelatinous precipitates are amorphous and seem to have a composition
intermediate between ~-FeOOH (goethite) and o-Fe203 (haematite) 81'82. A crystal structure
has been proposed for a compound whose composition is close to 2Fe203.FeOOH.4H2 O83 . The gel
is supposed to be an amorphous form of this material 84,85. Upon aging, ~-FeOOH is formed
at pH>I0 while ~-Fe203 is obtained at pH<4 86,87. In agreement with the high partial charge
6(H20 ) in the h=3 [Fe(OH)3(OH2)3 ]° precursor, no microerystalline Fe(OH) 3 similar to
Cr(OH) 3 can be detected. Another difference between
hydrolysis kinetics of the aquo-ion 88,89 .
[Or(OH2)6] 3+ + H20 = [Cr(OH)(OH2)5] 2+ + H3 O+
[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2 + H3 O÷
[Fe(OH)(OH2)5 ]2+ + H20 = [Fe(OH)2(OH2)4] + + }{30 ÷
As a result, acidic ferric
Fe 3+ and Cr 3+ lies in the
kl : 1.4 105s'I
kl - 3. i0 zs" I
% ~ 6.1 104s "I
solutions are highly unstable and precipitate through
spontaneous hydrolysis. The mechanism of this precipitation was extensively studied 90-97
and appears to proceed as follows 98,99,100 :
Sol-Gel Chemistry of Transition Metal Oxides 271
- The h=l precursor [Fe(OH)(OH2)5 ]2+ can undergo a dimerization reaction and nucleate the
~-FeOOH phase through mixed olation/oxolation reactions.
- At room temperature the h=2 precursor [Fe(OH)2(OH2)5] + can form a polycation whose mean
composition is [Fe403(OH)4 ]2n+n with a molecular weight around 104 g/mole (n=25)~ This
polycation gives rise to spheres about 2-4 nm in diameter which are responsible for the
brown-red color of the colloidal solutions. Mixed oxo-hydroxo bridges 2(O)i, 2(0)2, 2(0H)I
and 2(0H)2 seem to be present in this polycation. A structure was proposed 101 , in which
the iron atoms are in a tetrahedral coordination in the core and in an octahedral
coordination near the surface. However, other results 102 suggest that all iron atoms are
octahedrally coordinated. Upon ageing, or adding a base, aggregation occurs leading to ~-
FeOOH needles with the same diameter as the original polycation. These needles then undergo
an oriented aggregation process giving rod-like particles which can form fibrous tactoids
responsible for the gelatinous aspect of the precipitate. In the presence of chloride ions
fl-FeOOH precipitates are formed rather than ~-FeOOH 103,104 while in the presence of
sulfate ions a basic salt precipitates 105,106 The synthesis of this Fe-polycation has
been reviewed 107
- At high temperature, the h=2 precursor does not form a polycation. It nucleates directly
into ~-Fe203 particles that may exhibit various morphologies 108,109
Iron oxide sols or gels can also be made through the oxidation of Fe(II) precursors
or the reduction of Fe(III) salts. Depending on the experimental conditions, the solid
phases thus formed can be Fe304 , ?-Fe203 or 6-FeOOH.
i) Magnetite Fe304 can be made through reduction of ~-Fe203 with hydrazine and is
formed following a dissolution recrystallization mechanism 110. The situation appears much
more complex when it is made by slow-oxidation of Fe(OH) 2 .
- Under basic conditions, nucleation takes place near the surface of Fe(OH) 2 particles and
the growth involves a contact-recrystallization mechanism inside the gel phase 111,112
Under neutral or weakly acidic conditions, some Fe(II) precursors are oxidized snd
copreeipitation of the ferric hydroxo complexes thus formed, with ferrous precursors leads
to a green product called green-rust 111 Surface oxidation of the green-rust particles
leads to colloidal magnetite Fe304 . Such a mechanism is probably involved when mixed
Fe(II)/Fe(III) precursors are used in order to obtain ferrofluids 113. Typically, an
aqueous mixture of ferric chloride and ferrous chloride is added, under strong agitation,
to an ammonia solution. A black gelatinous precipitate is instantaneously formed and can be
isolated from the solution by centrifugation or magnetic decantation without washing with
water. An alkaline ferrofluid is then made by peptization with tetramethylammoni~1
hydroxide. An acidic sol is obtained when the precipitate is stirred with aqueous
perchloric acid, centrifuged and peptized by adding distilled water. In all cases
peptization is possible only when the Fe(II)/Fe(III) ratio is lower than 0.15114'115
ii) The final term for the oxidation of Fe304 is ~-Fe203116 This transformation
is induced by air, H3 O÷, Fe 3+, Fe(OH)3 , Ag ÷ or other oxidizing agents 116-118 All these
reactions are characterized by an electron transfer at the water-solid interface, coupled
with an other electron transfer between Fe(ll) and Fe(lll) ions inside the particle.
Chemisorption at the interface induces a reduction of surface Fe 3+ cations in oetahedral
positions by trapping electrons from the solid phase which are normally deloealized.
Desorption or in-situ oxidation of this reactive Fe(ll) occurs, while charge compensation
leads to Fe 3+ migration from the core towards the surface with the creation of oxygen
vacancies (cf 5.4.2). The final product of these processes is aggregated 7-Fe203 particles.
iii) Finally, fast oxidation of Fe(II) by H202 leads to either crystalline 6-
FeOOH 111,119 or amorphous phases 120
272 J. Livage et al.
2.2.6. Sols and gels of tetravalent metal oxides. Hydroxo-aquo precursors of tetravalent
Ti, Zr or Hf can be easily hydrolyzed 121'122'123. These cations can therefore form stable
sols. However, their growth mechanism mainly involves olation so that clear gels are
rather difficult to obtain. TiO 2 gels have been made by adding sodium carbonate to an
aqueous solution 124,125,126 or by acid peptization 127 . Similarly, ZrO 2 gels can be made by
neutralization of chloride or nitrate precursors with urea or by peptization. The structure
of these colloidal gels remains unknown, but mixed oxo/hydroxo bridges seem to be
present 61 , 62,128 The only structural study concerns amorphous ZrO 2 for which a sheet-like
structure with zirconium atoms linked through 3(O)1 and/or 3(OH)1 bridges was proposed 129
With other dioxide sols and gels such as VO 2 ,CrO 2 or MnO2, redox reactions cannot
be neglected. Sols of MnO 2 are readily obtained by reduction of KMnO 4 with As(OH)3130 ,
Na2S204131 , Mn2+132 , NH 4 + 133 , glucose, fructose or galactose 134.Gels have also been
formed 59.
2.3. Condensation via oxolation
2.3.1. Mechanism. Oxolation leads to the formation of oxo bridges M-O-M between two metal
cations M. Such a condensation process is observed when no aquo ligand is available in the
coordination sphere of the metal. Typically, this occurs for oxo-hydroxo precursors
[MOx(OH)N.x ](N+x'z)" where x<N. Two basic mechanisms have to be considered for oxolation
reactions.
i) When the metal coordination is not fully saturated, nucleophilic
addition (AN) with M-OH and/or M-O as nucleophiles can occur, as shown in figure 4. Ligands
need not be removed and chains or cycles are formed very rapidly 135,136,13Z. Typical
examples are given by [MO3(OH)]" species (M - W,Mo) which form cyclic tetramers
[M4OIz(OH)4] 4" . The kinetic constants of such reactions are larger than lOSM-~s "I in
agreement with a pure addition mechanism 138
2(0)2 or face bridges 2(0)3 are easily formed.
/
< A
According to this mechanism, edge bridges
s
O / \
+ O -- M - ' - ~O-~ - 2(O)3
-%
Fig. A. Formation of small polymers according to a
nucleophilic addition mechanism.
(A)(B)(C) chains ; (D) cycles.
ii) When the metal coordination is
already fully saturated, nucleo-
philic substitution must occur
with M-OH as a nucleophile and OH"
or HzO as leaving groups. This
reaction can be decomposed into
two basic steps :
- a nucleophilic addition leading
to an unstable 2(0H)I bridge :
6" 6 + H M-OH + M-OH , M-O-M-OH
followed by a fl-elimination
leading to the departure of one
water molecule :
Sol-Gel Chemistry of Transition Metal Oxides 273
6 + ~" H OH
- M - O - M - , M - O - M + H20
This basic mechanism will be called ANflE i in order to indicate the two step process and the
prototropie transfer within the transition state.
The first step can be catalyzed by bases which strongly favour the nucleophilic
attack :
M - OH + OH" , M - O" + H20
M - O" + M - OH , M - O - M + OH"
This mechanism will be called AN~E 2 in order to indicate a concerted elimination.
The second step can be catalyzed by acids which strongly favor the elimination of
the leaving group :
H 6" H M - 0 - M - OH + H 30 + , [ M - O - M - OH2] + + H20
1 H20 H ]+
M - O - M + H 30 + c [ M - O - M + H 20
The positive charge of the "oi" bridge greatly increases its acidity, favoring proton
removal. As a water molecule is eliminated from the transition state this mechanism will be
called AN~E I .
These mechanisms explain why, in contrast to olation, oxolation occurs ever a
wide range of pH. Moreover, as a proton has to be transferred before elimination occurs,
the rate limiting step can be either the proton transfer (AN~Ei) or the elimination of the
leaving group (ANflE I and AN~E2). Oxolation kinetics thus strongly depends on both the metal
M and the pH. The reaction rate usually goes through a minimum around the isoleetric point
of the solution (precursor [MOz. N(OH)2N.Z] ° predominent in solution). Considering the h = 7
precursor, the dimerization reaction of Cr(VI) can be written as follows 138,139,140 .
[HCrO4] + [HerO4]" = [Cr207 ]2" + H20 k = i M'Is "I and k ~ 5.10 .4 M'Is "I
while the polymerization of vanadates leads to 141.
[VO3 (OH) ]2" + [VOa(OH)2 ]" = [V206 (OH) ]3" + H20 ~ = 3.1 104 M'Is "I
[VO3(OH)] 2" + [V204(OH)3]" = [V309] 3" + 2 H20 k = 5 102 M'Is "I
Oxolation following an ANflE mechanism is a slow process and cannot proceed as fast
as olation as it is never under diffusion control. The different types of bridges that can
be formed via oxolation are shown below :
M---OH + HO---M M--0--M +
/~OH + HO--M /~O--M +
Isolated 3(0)I and 4(0)I bridges are not
H~O 2 (°)1
H20 3 (0)1
+ H20 4 (O)I
[M30(OAc)6 (0H2)3 ]÷ (M = Cr,Fe,Ru) and in [Cu40Cl 6 (Ph3PO) 4 ] complexes
+ H2° 4 ( ° ) I
very common. They can be found in
142
2.3.2. Polyanions. One of the main differences between aquo-hydroxo and oxo-hydroxo
precursors, is the fact that even when the charge is zero (x=z-N), condensation through
oxolation may not go beyond a limited degree of polymerization. This is again due to the
loss of nucleophilie strength of hydroxo groups after condensation has occurred :
274 J. Livage et al.
2[CrO2(OH)2] ° ) [(HO)O2Cr-O-CrO2(OH)] ° + H20
6(OH) = -0.01 8(OH) = +0.04
This dimer behaves as an acid and can lose protons to form polyanions :
{ [Cr205(0H) 2]° = [Cr206(OH)]" + H +
[Cr206 (OH) ]" = [Cr207] 2" + H +
However, as condensation must occur before ionization takes place, such species are often
called "polyacids". Depending on M, more or less condensed species can be obtained. Let us
consider as an example the decavanadic acid that can be formed by the polycondensation of
h=5 precursors [VO(OH)31% :
I0 [VO(OH)3]° ) [H6V10028 ]° + 12 H20
6(OH)= -0,09 6(OH) = +0.003
Ten vanadium atoms are required to make the hydroxo group positive. Spontaneous
deprotonation leads to a polyoxy-ion [HzVI0028 ]4" whose structure is shown in figure 5G. At
higher pH further deprotonation occurs leading to :
[H2V10028 ]4" = [V10028 ]6" + 2 H* Figure 5 shows also the structures of some well known transition metal polyanions
35,143-161. These polyanions are probably formed through a mixed AN/AN~E mechanism as shown
in figure 6 137when the rate of the AN~E reaction is fast. Geometric constraints lead to
more open structures particularly when the reaction rate is slow (figure 7) 35,162-171
Fig.5. Structures of compact isopolyanions
(A) [W4012(OH)4 ]4" , (B) [W4016] 8" 35,143
(C) [M6019] 8" M = Nb, Ta 144,145
[M6019 ]2" M = W 146, Mo14?,148
(D) [MzO24] 6" M = W 35,143, Mo 149,150
(E) ~-[Mo8026] 4" 151-154
(F) [MOsO26(OH)2 ]6" 155,156
(G) [MI0028] 6" M = V157"159; Nb 16°
(H) [Au206] 6" 161
A
B
o o
~\ Oo AN
Fig.6. Mechanism of formation of isopoly-
anions according to Tytko and Glemser 137
Formation of the [M4011(OH)5] 3" (A) and
[M4012 (OH)4 ]4- (B) tetramers through
successive addition of [MO3(OH)]'tetrahedra
and protonation. (C) Structure of the
[M4OI2(OH)4] 4" tetramer and growth (D) of
the isopolyanion through an AN~E mechanism.
Sol-Gel Chemistry of Transition Metal Oxides 275
¢
V
Fig.7. Structures of non compact
isopolyanions.
(A) [M207] 2 M = Cr 162 Mo 35
[M207] 4 M = V 16S
(B) [Cr3010 ]2" 164
(C) [VsO9 Is 16s
(D) [Cr4013 ]2" 165
(E) [V4012] 4 ss,16s
(F) ~-[Mo8026 ]4" 153,154,166
(G) [H2W12042 ]I0" 167
(H) [H2W12040 ]6" 168
(I) [W10032 ]4" 169,170
(j) [Mo360112(OH2)1618- 171
It should be noted that the formation of most isopolyanions involves a change in the
coordination of the metal from 4 to 6. This change occurs because protonation increases the
electrophilic strength of the metal M as shown in table 5. As 6(M) becomes larger than
+0.50 octahedral coordination is preferred because it allows a larger charge transfer
towards the metal. This explains why pyrovanadates (precursor h=7 [VO3(OH)]2" ) and
metavanadates (precursor h=6 [VO2(OH)2]" ) have a tetrahedral structure 35'143 while vanadium 172
oxide gels and decavanadates (precursor h=5 [VO(OH)3] °) have an octahedral structure
With niobium in the h=6 precursor, the higher positive charge explains why niobium must
keep octahedral coordination even at very high pH (hexaniobate ion) :
6[NbO2(OH)2 ] " , [H2Nb6019 ]6" + 5 H20
With Mo(VI) and W(VI), the h=7 precursors [M03(OH)]" are on the border line between
both coordinations while h=6 precursors [MO2(OH)2] ° appear unstable in tetrahedral
coordination. Thus for Mo(VI) a great variety of polyanions can be formed in which this
element can have two different coordinations as in ~-[Mo8026 ]4"
Precursor X 6(0) 6(OH) 6(M) pK
[V04] 3 1.583 -0.74 +0.01
[HV0412" 2.056 -0.57 -0.59 +0.29 14.4
[H2V04] 2.378 -0.44 -0.30 +0.48 8.95
[H3V04] ° 2.611 -0.35 -0.09 +0.62 3.74
[Mo0412 2.046 -0°57 +0.29
[I{Mo04 ]" 2.431 -0.42 -0.25 +0.51 3.89
[H2MoO4 ] ° 2.693 -0.32 -0.02 +0.67 3.61
[WO4 ]2 2.055 -0.57 +0.27
[HWO 4 ] 2.439 -0.42 -0.25 +0.50 3.50
[H2WO 4]° 2.701 -0.31 -0.01 +0.64 4.60
[Nb04 ]3 1.550 -0.77 +0.07
[H2Nbo4]- 2.027 -0.58 -0.61 +0.38
Table 5 - Variation of partial charges with protonation for some tetrahedral inorganic
precursors.
JPSSC 18:4-B
276 J. Livage et al.
At very low pH, positively charged oxo-aquo precursors are formed owing to the
low nucleophilic strength of hydroxo groups (cf.2.2.2). Condensation through oxo bridges
can occur leading to acidic polycations such as [Mo20(OH2)x] 2+ or [Mo20(OH)(OH2)×] ÷ 173
Tetrahedral species such as [CrO2(OH)2] ° cannot condense beyond a certain point,
leading to the formation of polyacids. This will no longer be the case if hydration occurs.
Hydrated phases can nucleate which have very different structures and can be transformed
into the anhydrous oxide MOz/2 upon heating. Moreover as coordination becomes saturated in
the h=z precursor, only slow AN~E mechanisms are involved, leading to the formation of
clear gels when an acid is added. Some of the probable growth mechanisms for these gels
will now be described.
2.3.3. Sols and gels of pentavalent metal oxides. Vanadium pentoxide gels can be made by
adding nitric acid to a vanadate salt or by hydration of the amorphous oxide V205 174 The
best method however, is to use a proton exchange resin which yields a relatively pure
product quite rapidly, without dialysis or washing 175. Polyvanadic acid solutions can
thus be prepared by ion exchange in a resin from sodium or ammonium metavanadate solutions
176,177. The freshly prepared decavanadic acid is yellow and turns dark-red within a few
hours. Decavanadic acid (M.W.-lOO0g/mole) predominates below 10"3M and transforms into
polymeric species (M.W.=2.106g/mole) above 10"3M 176. Aggregation occurs above 2.10"2M and
finally gelation is observed if the vanadium concentration is larger than 0.i M. Some
vanadium reduction occurs during the polycondensation process and about 1% of the vanadium
ions are in the V(IV) oxidation state as shown by ESR 178
The fibrous nature of the
............. gel is well established
(figure 8). Electron and X-
ray diffraction studies 172
have shown that these
fibers actually look like
flat ribbons about i00 A
wide and IOA thick. Accor-
ding to the 2D structure
observed along the ribbons,
V205 layers are formed by
fibrils 27A wide linked to-
gether side by side. Water
molecules can be inter-
calated leading to a gel or
Fig.8. Fibrous texture of V205 gels. a colloidal solution. The
xerogel obtained by drying
these gels at room temperature has a water content about 1.6 H20 per V205 which correspond
to one interfoliar water layer 179. Swelling of this xerogel can be followed by SAXS and
SANS 180
Acidification of vanadate solutions around pH-2 leads to the formation of the h=5
precursor which can be formulated [VO(OH)3]°. In this monomeric precursor with tetrahedral
structure, vanadium is highly electrophilic (6(V) - + 0.62). Addition of any nucleophilic
ligand is thus expected and transition towards an octahedral coordination must occur.
This can be achieved in two ways :
i) tetrahedral'h=5 precursors are acidic species : [VO(OH)3 ]° = [VO2(OH)2 ]" + H +
6(0)=-0.35 6(0)=-0.44
Sol-Gel Chemistry of Transition Metal Oxides 277
Addition and condensation of several such tetrahedral nucleophilic precursors lead to the
formation of decavanadic acid as shown in 1.3.2. (x~4) :
(10-x)[VO(OH)3]° + x[VO2(OH)2 ]" = [H6.×V10028] x" + 12 H20
ii) when x = 0 in the previous equilibrium, water molecules (6(0)=-0.40) appear to be
better nucleophiles than h=5 precursors (6(0)=-0.35) and figure 9 shows a possible pathway
towards fiber formation. The first step corresponds to an increase in the coordination of
the V(V) atom from four to six through the addition of two water molecules. An octahedral
complex is formed with a long V-OH 2 bond along the z axis, opposite to the short V=0 double
bond. The other water molecule has an hydroxo ligand in a trans position. Olation can occur
readily leading to a chain compound whose stoichiometry corresponds to [VO(OH)3(OH2)] ~. In
this case olation occurs before oxolation because the same complex contains both a good
leaving group (6(H20)=+0.I0) and a good nucleophile (6(OH)=-0.14). Once the chains are
formed, condensation through oxolation can occur between two chains in order to transform
unstable 2(0H)I bridges into stable 3(0)I bridges. Further condensation between these
double chains leads to a fibre-like structure as evidenced by electron diffraction 172
The coexistence of decavanadic acid o o
/}'k"~',OH OH 2
ODH -
5 . 7 5 ~ i b
3 . 6 0
Fig.9. Mechanism of formation of V205
fibers through olation (SN) and
oxolation (AN~E) from the monomeric
h=5 precursor.
of NbCI 5 or TaCI 5 with ammonia or
Peptization of this precipitate by
and fibrous polymeric species in V205 gels
can thus be understood in a very simple
manner. Both species are in an equilibrium
which can be shifted in either direction
by varying the V(V) concentration.
Niobium and tantalum behave quite
differently from vanadium : VCI 5 is un-
known while NbCI 5 is stable as dimeric
molecules Nb2CIIo with an octahedral
structure 57 Also, Nb=O and Ta=O double
bonds are not stable, which explains why
VOCI 3 is a monomerie tetrahedral complex
while NboCI 3 is an infinite octahedrally
coordinated polymer in which condensation
has occurred through 2(0)I and 2(CI)2
bridges. Therefore, formation of mixed
aquo-hydroxo-oxo complexes is not possible
with Nb(V)and Ta(V) inorganic precursors
mainly because they remain octahedrally
coordinated even at very high pH.
Consequently, amorphous gelatinous
precipitates are formed through hydrolysis
acidification of an alkali-niobate or tantalate.
washing or dialysis leads to sols and gels 181
2.3.4. Sols and gels of hexavalent metal oxides. Colloidal tungstic acid is usually
obtained by adding hydrochloric acid to a sodium tungstate solution 182 As with vanadium,
acidification can be performed with a proton exchange resin in order to obtain colloidal
solutions free of foreign ions 183,184 After exchange, a clear yellow-colored solution is
obtained which becomes progressively turbid and turns to a gel and then to a precipitate
within a few hours. Light-yellow precipitates are obtained when the tungsten concentration
is low (< 0.5M) while the precipitates are dark-yellow at higher concentrations (>0.7M). X-
ray diffraction has shown that the light-yellow xerogel corresponds to W03.2 H20 hydrate
278 J. Livage et al.
Fig.10. Lamellar structure of WO3.H20 xerogels obtained
through polycondensation of [WO(OH)4(OH2)]°.
while the dark-yellow one is
WO3.H20 hydrate. The colloidal
particles thus obtained have a
plate-like shape and are able to
form long range ordered tactoids
184 (figure i0).
As in the case of vanadium,
the h=6 precursor formed by
acidification around pH=2
[MO2(OH)2 ]° is able to change
its coordination number from 4
(tetrahedral) to 6 (oetahedral)
owing to the high partial charge
on tungsten atom (6(W)=+0.64).
Addition of nucleophilic ligands
can occur again in two ways :
i)Tetrahedral h=6 species are acidic: [MO 2(OH) 2] ° = [MO 3(OH)]" + H +
6(O)=-0.31 6(0)=-0.42
Addition and condensation of these tetrahedral precursors lead to isopolyanions •
(10-x)[WO2(OH)2] ° + x [WO3(OH)]" ' [H4.xW10032 ]x" + 8 H20
- , ] x - + 5 H20 M = Mo,W (6-x)[MO 2(0H)2] ° + x [MO 3(OH)] [Hz-xM6019
ii) If x ~ O, water molecules ean enter into the coordination sphere. As the h=6
precursor has two oxo ligands, two water molecules can be added in a trans position
relative to the short M=O double bonds. In this case condensation can occur only through
oxolation leading to linear or cyclic species because the functionality of the precursor is
f= 2 :
I o
H ~ * W ",,o % //? %"oH
? JVo~ H206-
n[MO2(OH)2(OH2)z] ° , [MO3(OH2)2]" + n H20
Ho/i\o. 0"2
/
Fig.ll. Mechanism of formation of
WO3.H20 layers
No precipitation occurs because low
molecular weight cycles are easily formed.
This explains why Mo(Vl) does not give
rise to precipitates nor gels when ion-
exchange techniques are used 185,186,167
Mo(VI) thus behaves in a similar way as
Cr(VI), but seems to have an octahedral
coordination 188 with two water molecules
preventing precipitation. Such behavior
is not observed with W(VI) because
another possibility (figure ii) is the
disso-ciation of one water molecule by the
reverse mechanism proposed in 2.2.2. The
other oxo ligand remains stable and the
water molecule in the trans position plays
the same role as before. An oetahedral
[WO(OH)4(OH2)] ° h=6 precursor is formed
which can grow in a bidimensional way
through oxolation because olation is
prevented. The sheets thus formed can make
hydrogen bridges leading to the layered
structure of tungstic hydrates WO3.2H~O
Sol-Gel Chemistry of Transition Metal Oxides 279
and WO3.H20 189 shown in figure I0. The water dissociation process is very slow with Mo(VI)
but occurs upon ageing or heating leading to isostructural hydrates MoO3.2H20190'191 and
MoO3.H20 192,193,194 or to ~-MoO3.H20 a white-colored hydrate 195,196
2.4. Role of the anions
In our previous discussion on the hydrolysis of cations, the role of the counter
anion was completely neglected. The metal atom was assumed to be surrounded by aquo,
hydroxo or oxo species only. This situation occurs when pH modifications are obtained with
an ion exchange resin. However, in most cases a counter anion is present when an inorganic
2 F m , A C e . . . . . . B
Q l e " t' ' t " i ' , . , % ,
,IV r~ ~ A _ . . . ~ • , . ~ . ~
• . . ~ ' . . . ~ / ~ ' ~ f M ~
i I I ' i t l I 11 l l l l ~ i • g
, I , . i . , ' - • • ~ V ~ ' ~ A : k ~ ; : : ~td._. / ; ?
t ' : ~I i t Y / " ~ " i k " ! D
. f
t - i , . -
e- Y
~ ~ - ~ F
Fig. 12. Various morphologies of particles as a function of the type of counter-ions present in solution according to E. Matijevic.
(A) Cl" 23 (~.Fe203) (E) H2PO ~ 23 (~_Fe203)
(B) CIO4 108 (~_Fe203) (F) Cl" 23 (~-FeOOH)
(C) NO3 108 (~_Fe203) (G) HSO 4 105 (Fe3(OH)s(SO4)2.2H20)
(D) CI'/EtOH 109(~_Fe203)
280 J. Livage et al.
salt is dissolved into water. In some cases organic or inorganic anionic species are added
to the solution in order to control the precipitation process. It is well known that other
anions besides hydroxide ions play a decisive role in homogeneous precipitation of metal
oxides 23,197-202. Some anions are strongly coordinated to metal cations and thus end up in
the precipitate while others can be removed by leaching. In most cases anions strongly
affect the particle morphology and colloid stability (figure 12) 23'105'108'109'201
Many techniques are now available to produce a large number of well-defined
monodispersed colloidal particles. However it is still difficult, if not impossible, to
predict the morphology of these particles. Anions seem to play both a chemical and a
physical role. At the beginning of the process, they are able to coordinate the metal ion
giving rise to a new molecular precursor whose chemical reactivity toward hydrolysis and
condensation is expected to be different. Once colloidal species are formed, the anions
change the double layer composition and the ionic strength of the solution therefore
modifying aggregation processes.
This section will attempt to describe the chemical role of anions in the aqueous
chemistry of inorganic precursors in order to show how they can orient the chemical
composition and the structure of colloidal particles. The following discussion shows that
the morphology of the particles cannot be a unique function of the chemistry involved
during nucleation and therefore physico-chemical factors must also play a decisive role. At
the present time, it is difficult to make a clear difference between growth by monomeric
units and growth through aggregation. As a result, no attempt will be made to correlate the
morphology observed by TEM and the chemical role of anions.
2.4.1 Complexation of metal cations. Associated species [M(OH)h(X)(OH2)~.h.I ](z'h'1)+ can
be formed when both positively charged hydrolyzed cations [M(OH)h(OH2)N.h] (z'h)+ and
negatively charged anions X" are simultaneously present in an aqueous solution. Such M-X
associations have been clearly shown by optical spectroscopy 203,204, N.M.R 205,206 or X-
ray scattering 207,208. The full coordination N of a metal cation in an aquo or hydroxo-
aquo precursor is already satisfied. Therefore the coordination of the anionic species X"
with such a precursor occurs via a nucleophilic substitution. However, the question arises
whether one can predict if such species remain stable in an aqueous medium or whether they
readily dissociate. Water actually plays a double role. It behaves as a solvent with a high
static dielectric constant (e=80) which favors the dissociation of ionic species. It is
also ~ a-donor molecule which reacts as a nucleophilic ligand. Therefore we have to check
whethe~ the M-X bond is stable against both ionic dissociation and hydrolysis.
i) Let us consider an associated species in which a monovalent anion X'is
coordinated to the hydrolyzed cation. Ionic dissociation corresponds to the following
reaction : 1(z.h.1) + H20 [M(OH)h(X)(OH2)N . . . . h lJaq. + H20 - [M(OH)h(OH2)N h 1(z'h)+aq. + X'aq. (7)
A partial charge transfer occurs between M and X within the M-X chemical bond leading to a
modification of the negative charge of the anion. Two possible cases arise :
- X" is more electronegative than H20 ligands (x(H20) - 2.49). Electrons are attracted
by X and the overall transfer goes from the precursor to X, increasing the negative charge
of the anion (6(X)<-!) The M-X bond become~ more ionic and the high dielectric constant of
the aqueous solvent favors ion-pair formation. Equilibrium (7) is displaced toward the
right and the associated species are not stable against ionic dissociation. X'does not
exhibit any ability to complex with the metal cation.
- X is less eleotronegative than H20 ligands. Electrons can be transferred from X to
the precursor. The negative charge of the anion decreases (6(X)>-I) giving rise to a more
Sol-Gel Chemistry of Transition Metal Oxides 281
covalent M-X bond which is not dissociated by the solvent. Equilibrium (7) is displaced
toward the left and the anion X" remains coordinated to the metal atom.
The ability of an anionic species X x" to form complexes with a cation M z+ will
therefore depend mainly on the magnitude of electron transfer from X to M within the M-X
bond. This electron transfer leads to a charge variation Ax of the anion, before (x) and
after (6(X)) eomplexation, which can be easily calculated with the Partial Charge Model:
Ax=x+6(X). A rough estimate of how much equilibrium (7) is displaced toward the left can be
made by looking at the relative charge variation of the anion :
ax 6(X) - - = 1 + - - (8) x x i00
In the case of monovalent anions this leads to : Ax = 1 + 6(X). Anion X" does not complex
when Ax<O. Its ability to form complexes increases with Ax when Ax>0, i.e. when the
electronegativity of the anion decreases. The above considerations are based on
electrostatic interactions only. Entropic and resonance effects observed with chelating
anions (E.D.T.A., ~-hydroxy acids) can increase their ability to form complexes and
209-213 therefore such species are often used to control precipitation processes
ii)The associated species with Ax>O [M(OH)h(X)(OH2)N.h.I ](z'h'1)+ are in the
presence of a large excess of water molecules. Therefore, they must also be stable against
hydrolysis :
] ( z - h - I ) + + H2 0 ](z-h-l)+ + (9 ) [ M ( O H ) h ( X ) ( O H 2 ) N . h . 1 ,aq " = [ M ( O H ) h + l ( O H 2 ) N . h . l , a q " HXaq "
This equilibrium goes through a transition state in which a proton can be
transferred from a water molecule towards the X group : [M(OH)h+ I(HX)(OH2)N.h.I ](z'h'1)+
Again, charge considerations lead to two possibilities :
6(HX)<O : from purely electrostatic considerations, the negatively charged HX species
remains attracted by the positively charged M z÷ cation. Equilibrium (9) is displaced
towards the left and the anion X" remains coordinated to the metal.
6(HX)>0 : nucleophilic substitution by water molecules becomes possible and the
associated species is not stable towards hydrolysis. It could be stable however in the
presence of aprotic solvents such as DMSO.
2.4.2. Complexation of Fe 3+ aqueous precursors. The hydrolysis of [Fe(OH2)613+ species has
been described previously. As an illustration, let us consider now whether this aqueous
precursor can be complexed by a monovalent anion X" such as CIO4, NO3, HSO4, H2PO ~ or
CH3COO'. According to the literature, such anions behave as bidendate ligands and should be
able to replace two water molecules giving rise to [Fe(X)(OH2)4] 2+ species. According to
the previous discussion, this complexed species has to be stable against : 3÷
Ionic dissociation : [Fe(X)(OH2)4] . = [Fe(OH2)6]aq + Xaq.
,y rolysis : = +
Depending on the strength of the acid HX in aqueous solution, the hydrolyzed species can be
reprotonated leading to the non hydrolyzed [Fe(OH2)6] 3+ precursor. Table 6 reports charge
calculations performed on both coordinated species using the Partial Charge Model.
x- ClO~ .% .so i He% CH 3 COl
2.86 2.76 2.64 2.49 2.24
6(X) -0.92 -0.84 -0.50 -0.34 +0.40
Ax +0.08 +0.18 +0.50 +0.66 +1.40 i
6(HX) I -0.52 -0.42 -0.15 +0.02 +0.70
Table 6 . Partial Charges $(X) and 6(HX) in [Fe(X)(OH2)4]2+ and [Fe(OH)(HX)(OH2)3 ]2+
species respectively, as a function of the mean electronegativity X of the anion Xaq.
282 J. Livage et al.
According to table 6, the M-X bond becomes less and less ionic when the
electronegativity of X" decreases. The complexed species then become more stable towards
ion pair formation. On the other hand, 6(HX) increases so that hydrolytic dissociation
becomes possible as soon as 6(HX)>0.
'~,x
-+1.0 \ -1 .0 -
-+0 .5 - 0 . 5 -
A©O CI-
- - 0 . 5 + 0.5
- - 1 , 0 + 1 . 0 -
Fig.13. Variation of Ax - I+6(X) and 6(HX) versus X for some
[Fe(X)(OH2)4] 2÷ precursors.
~(HX)~
X" monovalent anions in
An electronegativity range can be estimated graphically if we plot Ax=l+6(x) and
6(HX) versus X as shown in figure 13. Anion X" remains coordinated to the metal (Ax>0,
~(HX)<0) for intermediate electronegativities only, roughly speaking, between 2.55<~<2.90
for the example shown in figure 13. Ionic dissociation prevails for higher electro-
negativities (Ax<0) while hydrolytic dissociation occurs for lower electronegativities
(6(HX)>0). Therefore, HCO3, CI" and CH3CO0" cannot give stable complexes in these
experimental conditions. The ability to form complexes for a given anion X" also depends on
the hydrolysis ratio h of the precursor, i.e. on the pH of the solution. Table 7 reports
partial charge calculations performed on hydrolyzed neutral species corresponding to h=2.
One can see that highly electronegative anions such as perchlorates are not able
to coordinate Fe 3+ ions because of ionic dissociation. They behave as counter ions.
Sol-Gel Chemistry of Transition Metal Oxides 283
However, the hydrolysis ratio h can be decreased by lowering the pH and complexation
occurs under highly acidic conditions as shown in table 6 and in agreement with
experimental observations 207,208 A similar behavior has been reported for Ti(IV) in
highly concentrated HCIO 4 214 On the other hand, less eleetronegative anions such as HC03
are able to coordinate the metal cation when the hydrolysis ratio is high, i.e. at high
pH. As a consequence, they behave mostly as counter ions except under basic conditions.
X ~ 6(x) Ax 6(HX)
CIO4 2.86 -1.26 -0.26 -0.94
HSO4 2.64 -0.92 +0.08 -0.65
HC03 2.49 -0.72 +0.28 -0.45
Table 7. Partial charges 6(X) and 6(HX) in the neutral species [Fe(OH)2X(OH2)2 ]°,
I,=3
I':211 I I h:, .=0 2.0 2.1 2.2 I 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
Aoo- c,- HCO; HSO; NO~ C,O;
Fig.14. Electronegativity range as a function of the hydrolysis ratio h for which X"
monovalent anions form stable complexes [Fe(OH)h(X)(OH2)4.h ](z'h)+
As a rough guide, the electronegativity range for which anions form stable
complexes with metal cations shifts towards low electronegativities when the pH of the
aqueous solution increases as shown in figure 14. As a consequence, highly electronegative
anions usually behave as counter ions except at very low pH. Anions having a low
electronegativity also behave as counter ions or molecules (because of hydrolytic
dissociation), except at high pH. Some anions however, having a mean electronegativity
close to that of H20 (X=2.49) are able to form stable complexes over the whole range of pH.
Such anions (sulfates) will therefore have a strong effect on both hydrolysis and
condensation processes. They will induce deep modifications of the structure and morphology
of colloids and precipitates. They can even remain coordinated to the metal cation up to
the end of the precipitation giving rise to basic salts such as those observed when Fe 3+
ions are precipitated in the presence of SO~'. On the other hand, pure ~-Fe203 is obtained
with other anions which, depending on the pH, can be complexing or not (cf. figure 12).
This discussion concerning Fe 3÷ inorganic precursors can be easily extended to any other
aquo-hydroxo complex. For each element, electronegativity ranges may be computed as
previously described allowing a quantitative description of complexation phenomena in
aqueous solutions.
2.4.3. Hydrolysis and condensation of Fe 3+ . Anion complexation of metal cations leads to
new precursors whose chemical reactivity can be noticeably different. The modification of
Fe 3+ aqueous species by a strong chelating ligand such as EDTA,[(OOCCH2)2N-CH2-CH2-
N(CH2COO)2 ]4" , has been studied carefully. Both hydrolysis and condensation constants were
measured before 89,99,203,215 and after 216 complexation leading to the following results :
Hydrolysis :
[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2" + H30 + ~ = i0 "5
[Fe(OH2) 2 EDTA]" + H20 = [Fe(OH)(OH2)EDTA] 2" + H3 O+ ~ = 10 .25
284 J. Livage et al.
Condensation :
2[Fe(OH)(OH2)5] 2+ = [Fe2(OH)2(OH2)8] 4+ + 2 H20
2[Fe(OH)(OH2)2EDTA]2" = [Fe2(OH)2(EDTA)2] 4" + 2 H20
It can be seen that hydrolysis is prevented by
condensation is favored. This is mainly due to charge
complexation. A partial charge calculation shows that (table 8):
Precursor 6(Fe) 6(H) 6(OH)
[Fe(OH2)6] 3+ + 0.59 + 0.34
[Fe(OH2)2(EDTA)]" + 0.43 + 0.20
[Fe(OH)(OH2)5] 2÷ + 0.55 + 0.30 - 0.01
[Fe(OH)(OH2)(EDTA)] 2" + 0.40 + 0.17 - 0.25
Table 8 : Complexation of h-0 and h-i aquo precursors of Fe 3+ by EDTA 4" (EDTA=C10HI208N2).
K d = 6.10 .4
K~ - 102.95
EDTA complexation while
modifications induced by
The positive partial charge on the protons of the water molecules in the non hydrolyzed
precursors decreases upon complexation. The EDTA modified precursor is therefore a weaker
acid and the deprotonation of coordinated water molecules is more difficult.
The condensation process begins with a nucleophilic attack by the negatively charged OH
group onto the positively charged metal atom. This process therefore is easier as 6(OH)
becomes more negative and 6(M) more positive. Table 8 shows that EDTA complexation leads to
a decrease of 6(Fe) and an increase of 6(OH). Therefore, it is not obvious to determine
which factor will prevail. However, a rough estimate of the condensation ability could be
given by the product 6(M).6(OH) that varies from -5.10 .3 up to -10 "I upon complexation. The
larger variation comes from 6(OH) and dimerization of the modified precursor should be
easier as confirmed by the equilibrium constants.
2.4.4. Formation of basic salts. Complexing anions coordinated to the dissolved metal ion
do not only change the charge distribution within the aqueous precursor. They can also play
a role as network formers in the structure of condensed phases. Some of them end up in the
solid giving rise to the precipitation of basic salts.
Figure 14 shows the structures which can nucleate from the h-2 aqueous zirconium
precursor [Zr(OH)2(OH2)6] 2+. All these structures have been experimentally determined by X-
ray diffraction 217-221 Non-eomplexing anions (~-0%) such as CI" or CIO4 are not able to
displace water molecules. They are not involved in the formation of condensed species and
hydrous zirconia ZrOz.nH20 can precipitate at high pH. A cyclic tetramer [Zr(OH)2(OH2)4]~ +
is formed via 2 (OH)2 bridges in which zirconium is surrounded by four terminal water
molecules and four bridging OH groups (square antiprism). Complexation occurs with nitrate
which exhibits a weak complexing ability (~-4%). Two terminal water molecules are replaced
by one NO3 group and a chain polymer [Zr(OH)2(NO3)(OH2)z] ~ is formed in which zirconium
remains in eight-fold coordination (dodecahedron). It should be mentioned that nitrates
remain as terminal groups : they do not link chains together and should not be considered
as network formers. Sulfates have a higher complexing ability (~-32%). Thus, they are able
to replace all coordinated water molecules leading to [Zr(OH)2SO4] n species in which the
zirconium is eightfold coordinated (square antiprism). Moreover, SO~'anions behave as
network formers, bridging three different [Zr(OH)2 ]2n+,n chains together. Stronger
complexation is expected with HPO~" (a-50%) or CrO~" (~-53%) ions. Chromate compounds
exhibit a layered structure in which [Zr3(OH)6CrO4]~ n+ sheets are linked together by CrO~"
tetrahedra. Zirconium exhibit both eightfold (dodecahedron) and sevenfold (pentagonal
bipyramid) coordinations. Another structure was suggested for the phosphate derivative 219
Sol-Gel Chemistry of Transition Metal Oxides 285
b
C O H 20
d
o>
Fig. 15. X-ray structures of some basic salts of zirconium.
(A) {[Zr4(OH)s(OH2)1618+,8CI04 " ) 52,53 : 6(CI04)=.I.I 7
(B) ([Zr(OH)2(NO3)(OH2)2]n+,nNO" ) 217
(C) [Zr(OH)2(SO4)(OH2) ] 218 :
(D) [Zr(OH)2(S04) ] 217,219 .
(E) [Zr(OH)2(Cr04) ] 220 .
[Zr(OH)2(H2P04)2 ] 221 :
: 6 (NO 3 )=-0.96
~ (HS04)=-0.37
6 (HS04)=-0.68
6 (HerO 4)=-0.47
6 (H2 P04 ) =- 0.50
Stronger complexation of chro-
mate ions (~=64%) can be obser-
ved if h is reduced to 1.5.
[Zr4(OH)6(CrO4~lSn+-.n chains with
zirconium in sevenfold coordi-
nation (pentagonal bipyramid)
linked together by Cr04"
tetrahedra are found in the
resulting compound.
2.4.5. Monodispersed chromium
hydrous oxide sols. The produc-
tion of monodispersed powders is
of the utmost importance for the
ceramic industry. Therefore
great efforts have been made in
order to control nucleation and
growth processes that lead to
the formation of a precipitate.
It appears that the fundamental
requirement for the preparation
of monodispersed particles in
aqueous solutions is to control
of the rate of generation of the
solutes species that are precur-
sors to precipitates 222,223
The goal is to reach a critical
supersaturation of the particle
forming species so that only one
burst nucleation occurs. Care
must be taken to avoid secondary
nucleation 201 This effect has
been well illustrated by E.
Matijevic et al. who showed that
spherical amorphous particles of
chromium hydrous oxide can be
generated by hydrothermal ageing of solutions containing sulfate 224,225 or phosphate
ions 78 but not in the presence of CI', NO3 or CH3CO0" ions. It appears that complexing
anions have a specific role in the nucleation process. Under identical experimental
conditions, but in the absence of these anions, only solute hydrolysis products are
obtained, and no solid particles precipitate. Electron microscopy shows that strands of
polymeric materials are obtained prior to the formation of spherical particles 226
Moreover, chemical analysis indicates that sulfates are bound in both solute chromium
complexes and polymeric species but not in the spherical chromium hydroxide particles
225,226,227 Therefore, the role of sulfates seems to be restricted to the nucleation step,
or to the cross-linking of polymeric chromium hydroxide chains. As nucleation involves
mainly neutral species, the following monomeric precursors have to be taken into account
22Z : [Cr(OH)3(OH2)3]o, [Cr(OH)2(HSO4)(OH2)2 ]° . Polymeric species giving rise to embryos
are formed from the condensation of these monomers. At high sulfate concentrations,
286 J. Livage et al.
condensation involves mostly modified precursors. This gives rise to chain polymers formed
through olation and linked together by sulfate bridges. Partial charge calculations show
that in such polymers, HSO~ remains coordinated to the chromium atom. At lower sulfate
concentrations, condensation between modified and non modified precursors becomes more
important :
[Cr(OH)2(HSO4)(OH2)2] ° + [Cr(OH)3(OH2)3] ° ~ [Cr2(OH)5(HSO4 )(0H2)4] ° + 2H20
However, in this case the Cr-HSO 4 bond becomes more ionic (6(HSO4)<-I) and sulfate ions
lose their complex-forming ability giving rise to ion pair formation as follows :
[Cr2(OH)5(HSO4)(OH2)2 ]° = [Cr2(OH)6(OH2)4] ° + HSO4 + H3 O+
Hydrous chromium oxide should then precipitate, free from sulfate ions, in agreement with
Matijevic's results 225 At intermediate sulfate concentrations, both condensation
processes occur simultaneously. However, chromium oxide precipitation displaces the
previous equilibrium. As a consequence, the polymeric basic salt should be progressively
transformed into hydrous Cr203 .
3. METAL ORGANIC MOLECULAR PRECURSORS
Metal alkoxides M(OR) n are versatile molecular precursors for the sol-gel
synthesis of metal oxides. They are known for almost all transition metal elements,
including the lanthanides 19. The number and stability of transition metal alkoxides
decreases from left to right across periodic table. The alkoxy group OR (R = saturated or
unsaturated organic group) is a hard ~-donor and stabilizes the highest oxidation state of
the metal. Therefore, alkoxides of main group elements and d O transition metals (Ti, Zr)
are rather well-known, while those corresponding to the soft d n late transition metals have
been much less studied 228. Moreover, the chemistry of electron-rich metal alkoxides has
long been restricted by oligomerization reactions which lead to the formation of insoluble
polymeric species (Fe, Co, Ni, Cu...) 229. Some alkoxides which are already being widely
used in industry are commercially available at relatively low cost (Si, Ti, AI, Zr). Many
others can be found for small-scale applications but at much higher prices (V, Mn, Fe, Co,
Ni, Cu, Y, Nb, Ta) 230 Otherwise, transition metal alkoxides have to be prepared in the
laboratory following the usual methods for the synthesis of metal alkoxides 19 The sol-gel
processing of silicates from silicon alkoxides has been extensively studied 28,30,231
Unfortunatly, there is a lack of data concerning the hydrolysis and condensation of
transition metal alkoxides. Therefore, the chemical reactivity of these alkoxides, mostly
Ti(OR)4, will be compared to the chemical reactivity of the corresponding silicon alkoxides
Si(OR) 4 . The main differences arise from the following two points :
- The lower electronegativity of transition elements leads to a much higher electrophilic
character of the metal.
- The possibility exists for most transition metals to exhibit several coordinations so
that full coordination is usually not satisfied in the molecular precursor, which allows
coordination expansion.
As a result, transition metal alkoxides are much more reactive. They must be handled with
care, in the absence of moisture. They readily form precipitates rather than gels when
water is added.
3.1. Hydrolysis and condensation of metal alkoxides
Electronegative alkoxo groups (OR) make the metal atom highly prone to
Sol-Gel Chemistry of Transition Metal Oxides 287
nucleophilic attack. Metal alkoxides are therefore extremely reactive with water leading te
the formation of hydroxides or hydrous oxides. The overall reaction can be written as
follows :
M(OR)n + nH20 ----+ M(OH)n + nROH
This reaction is actually much more complex than it might seen. Two chemical processes,
namely hydrolysis and condensation, are involved in the formation of an oxide network from
metal alkoxides. Hydrolysis of the alkoxide occurs upon adding water or a water/alcohol
solution, and a reactive M-OH hydroxo
usually proposed in the literature 20,37
H-I + M-OR(a) , H~ : H/O --+ M-OR
(b)
group is generated. A three steps mechanism is
/ H0-M +-- O~ M-0H + R0H
(c) kH (d)
The first step (a) is a nucleophilic addition of a water molecule to the positively charged
metal atom M. This leads to a transition state (b) where the coordination number of M has
increased by one. The second step involves a proton transfer within (b) leading to the
intermediate (c). A proton from the entering water molecule is transfered to the nega-
tively charged oxygen of an adjacent OR group. The third step is the departure of the
better leaving group which should be the most positively charged species within the
transition state (c).
The whole process, (a) to (d), follows a nucleophilic substitution mechanism. Charge
distribution governs the thermodynamics of this reaction which will be highly favored
when:
The nucleophilic character of the entering molecule and the electrophilic character of
the metal atom are strong : 6(0)<<0 and 6(M)>>O.
The nucleofugal character of the leaving molecule is high : 6(ROH)>>O.
On the other hand, the rate of the nucleophilic substitution depends on :
The coordination unsaturation of the metal atom in the alkoxide given by the difference
between the maximum coordination number N of the metal atom in the oxide and its oxidation
state z. The larger (N-z), the lower the activation energy associated to the nucleophilic
addition of step (a) should be.
The ability of the proton to be transferred within the intermediate (b). The more acidic
the proton, the lower the activation energy associated with this transfer will be.
Condensation is also a complex process and can occur as soon as hydroxo groups are
generated. Depending on experimental conditions, three competitive mechanisms have to be
considered namely : alcoxolation, oxolation and elation.
i) Alcoxolation is a reaction by which a bridging oxo group is formed through the
elimination of an alcohol molecule. The mechanism is basically the same as for hydrolysis
with M replacing H in the entering group :
M-~ + M-OR --+ M-Ok:--+ M-OR ~ M-O-M +-- O~/*" ~ M-O-M+ ROH
(a) ~ (b) (c) ~ (d)
Consequently, the thermodynamics and kinetics of this reaction are governed by the
same parameters as for hydrolysis.
ii) Oxolation follows the same mechanism as alcoxolation, but the R group of the
leaving species is a proton
M-O + M-OH M-0:--+ M-OH
(a) ~ (b) The leaving group is thus a water molecule.
I M-O-M '~ - - :0~ :' M-O-M + H20
(c) ~ (d)
288 J. Livage et al.
iii) Olation can occur when the full coordination of the metal atom is not
satisfied in the alkoxide (N-z~O). In this case bridging hydroxo groups can be formed
through the elimination of a solvent molecule. This latter can be either H20 or ROH
depending on the water concentration in the medium :
M - OH + M +-- O , M - O - M + ROH
M - OH + M +-- O , M - O - M + H20
The thermodynamics of this nucleophilic substitution are governed by the charge
distribution. The reaction is strongly favored when the nucleophilic character of the
entering group and the electrophilic strength of the metal are high : 6(0)<<0 and 6(M)>>0.
Moreover, since no proton transfer is involved within the transition state and since the
metal coordination is not saturated, the reaction rate is usually quite fast.
These four reactions (hydrolysis, alcoxolation, oxolation and olation) may be
involved in the transformation of a molecular precursor into an oxide network. The
structure and morphology of the resulting oxide strongly depend on the relative
contribution of each reaction. These contributions can be optimized by carefully adjusting
the experimental conditions which are related to both internal (nature of the metal atom
and alkyl groups, structure of the molecular precursor) and external (water/alkoxide ratio,
catalyst, concentration, solvent, temperature) parameters.
3.1.1. Nature of the metal atom. Since transition elements are more electropositive than
silicon, hydrolysis of transition metal alkoxides is much easier. Actually it is well known
that silicon alkoxides are not very reactive with water. On the other hand, transition
metal alkoxides react vigorously and a strongly exothermic reaction is observed as soon as
the alkoxide is brought into contact with water. A rough estimate of the partial charge
distribution (table 9) in metal alkoxides shows that the partial positive charge is much
higher for transition metals than for silicon. This explains why transition metal
alkoxides are very unstable towards hydrolysis 19,20,181,232. They must be handled very
carefully, in a dry environment and stabilizing agents are often added in the sol-gel
processing of transition metal oxides 20
Alkoxide Zr(OEt)4 Ti(OEt)4 Nb(OEt)s Ta(OEt)5 VO(OEt)3 W(OEt)6 Si(OEt)4 I ~(M) + 0.65 + 0.63 + 0.53 + 0.49 + 0.46 + 0.43 + 0.32 1
Table 9 : Positive partial charge on M for some metal ethoxides.
Another peculiarity of transition metal alkoxides is that coordination expansion
of the metal readily occurs upon hydrolysis. Hydrolysis rates are thus expected to be much
higher than for Si(OR)4 where the fourfold coordination of silicon is already satisfied. A
survey of the literature concerning hydrolysis rates of Tetraethoxysilane (~) gives values
ranging between 10 .4 and 10 .6 M'Is "I at pH-3 233-238 Extrapolation of this constant at
pH=7 gives ~=5 10 .9 M'Is "1 234. Although very little data is available for most transition
metal alkoxides, a minimum value of ~=lO'3M'Is "I at pH-7 can be roughly estimated for
Ti(OR) 4 239-243 which is at least five orders of magnitude larger than for Si(OR)4.
Hydrolyzed species such as M(OR)3(OH ) (M - Si,Ti) can undergo two condensation processes :
a l c o x o l a t i o n M(OR)3OH + RO-M(OR)2OH , (RO)3M-O-M(OR)2OH + ROH
oxolation M(OR)30H + HOoM(OR)3 , (RO)3M-O-M(OR)3 + H20
Sol-Gel Chemistry of Transition Metal Oxides 289
The charge distribution calculated within the transition states M2(OR)6(OH) 2 for M = Si, Ti
and R = Et are given in table i0 :
M 6(M) 6(OH) 6(H20) 6(EtOH)
Ti +0.64 -0.36 -0.25 +0.02
Si +0.33 -0.34 -0.21 +0.13
Table i0 : Charge distribution for two transition states during condensation.
In both cases the hydroxo groups are highly negatively charged allowing a nucleophilic
attack of the positively charged metal atom. After proton transfer, a positively chsrged
species must be removed. Table i0 shows that in both cases, water retains a negative
partial charge while ethanol carries a positive one. Therefore condensation of hydrolyzed
alkoxides should proceed via alcoxolation rather than oxolation. This conclusion has been
checked for Tetramethoxysilane by IR absorption 244 and for Tetraethoxysilane by 29Si NMR
233,245,2:46 In the case of transition metal alkoxides, alcoxolation leads to well defined
oxoalkoxides which can be isolated as single crystals. Their structures has been resolved
by X-ray diffraction for M = Ti 247 Nb 248 and Zr 249 and are shown in figure 16:
3 Ti(OEt)4 + 4 Ti(OEt)3(OH ) ~ TiFO4(OEt)20 + 4 EtOH
2 Nb(OEt)3(OH)2 + 6 Nb(OEt)4(OH ) , NbsO6(OEt)10 + i0 EtOH
5Zr(OMe) 4 + 8 Zr(OMe)3(OH ) , Zr1308(OMe)36
B
C
Fig.16. X-ray structures of some transition
metal oxo-alkoxides : (A) TizO 4 (OEt)20
(]3) NbsO10(OEt)20 ; (C) Zr1308(OMe)36
+ 8 MeOH
The main feature of these molecular
compounds is that the usual metal atom
coordination number is always satisfied.
The low coordination of the metal atom
in non-hydrolyzed transition metal alke-
xides must be correlated with their high
rates of condensation. Condensation is a
rather slow process for silicon alkoxides,
a global rate kc=10 "4 M-ls-1 where has
been measured for TEOS 250,251 . Such rate
constants are difficult to measure for
Ti(OR)4, owing to the rapid precipitation
of the oxide. However, a global rate cons-
tant of about 30 s "I was found for TiO 2
precipitated from Ti(OEt)4 243 Conden-
sation of Ti(OPri)4 is also extremely fast
as evidenced by the following linear
growth-rate deduced from turbidity
measurements 242 : r(nm.min'1)=0.9 [Ti] 4"I
This means that for [Ti]=0.1 M the growth-
rate is about 1.8 nms "I which is rather p
high. Similar behavior has been found for
the hydrolysis rates of Zr(oPrn)4 in
ethanol where the time t elapsed between
mixing and precipitation is given by 252 .
t-1(s'1)=0.9[H20] 3 [Zr(OPrn)4] I (mol.l "I)
3.1.2. Nature of the organic ligand. The hydrolysis rate constants for a series of silicon
alkoxides were measured by several authors 234-237 who pointed out that the rate of
290 J. Livage et al.
hydrolysis decreases with increasing size of the alkyl groups (table Ii). It seems that
these results can be extended to transition metal alkoxides : hydrolysis of titanium n-
alkoxides also becomes slower when the size of the alkyl group increases 239,253. Table ii
shows that the partial charge distribution in the alkoxide depends on the alkyl group,
giving rise to more or less polar M-OR bonds 37
R 6(Ti) -6 (OR) 6(H) 6(Si) 6(OR) 6(H) ~I02M" Is I [H+] I
CH 3 +0.66 -0.16 +0.12 +0.36 -0.09 +0.14 571
CzH 5 +0.63 -0.16 +0.i0 +0.32 -0.08 +0.ii 1.9
n-C4~ +0.61 -0.15 +0.09 +0.30 -0.08 +0.09 0.83
n-C6HI 3 +0.60 -0.15 +0.08 +0.29 -0.07 +0.08 0.3
n-CgHI 9 +0.59 -0.15 +0.07 +0.28 -0.07 +0.08
Ti(OR) 4 Si(OR) 4
Table Ii : Charge distribution in Ti(OR) 4 and Si(OR) 4 n-alkoxides.
The positive partial charge of the metal atom (M ~ Si,Ti) decreases with the
length of the alkyl chain. The sensitivity of the alkoxide towards hydrolysis should then
decrease, in agreement with experiments 235,239,253. Moreover, the positive partial charge
of the hydrogen atom decreases in the same way. Proton transfer should then become more
difficult, which is an effect that could be related to the decrease of the kinetic constant
235
Experimental results are often explained in terms of steric hindrance as well. It
has been shown that, for isomeric titanium butoxides, the hydrolysis rate are in the order
tertiary > secondary > normal 239. A reverse behavior was observed for silicon butoxides
where the measured gelation time is 32 h for Si(OBun)4, 236 h for Si(OBun)(oBut)3 and 500 h
for Si(OBuS)4 236,23?. The hydrolysis of titanium tetra-tert-amyloxide behaves the same way
as required on the basis of effective
slow but becomes much faster for
distribution alone cannot explain the
effects of the alkyl chain should also
Mesomeric effects may also
Bistan and Gomory 253 studied the
shielding, i.e. hydrolysis of the first OR group is
the following ones 254. Steric hindrance and charge
hydrolysis rate of metal alkoxides. Inductive + I
probably be taken into account.
affect the hydrolysis of metal-organic precursors.
hydrolysis reaction of a number of alkoxides and
aryloxides Ti(OR)4 (R = C6H5, C6HsCH2, m-CH3C6H4). They concluded that aryloxides are more
resistant to hydrolysis than aliphatie alkoxides. The same behavior is observed with
W(OC6H5) 6 and W(OEt)6 255 On the other hand, silicon phenoxide Si(OC6H5) 4 appears to be
more reactive with water than Si(OEt)4 255,256 This difference in reactivity between
transition metals and silicon may he explained by two competing effects :
- The -I inductive effect of the aryloxo group increases the positive charge on the
metal atom.
- The mesomeric +E effect of the aromatic ring increases the ~-donor ability of aryloxo
groups and reduces the positive charge of the metal atom. The magnitude of this +E effect
strongly depends on the availability of the metal d-orbitals.
For silicon phenoxide the -I inductive effect is probably the strongest rendering
the silicon atom highly prone to nucleophilic attack. Meanwhile, for transition metal
aryloxides mesomerie effects should be predominent. This is also supported by the fact
that the hydrolytic stability also depends upon the nature of the aryloxo groups, i.e.
ortho-and para-substituted groups are more stable than classical phenoxides.
Condensation is also strongly affected by the nature of the alkyl chain. K.C. Chen
and J.D. Mackenzie showed that the gelation time of silicon alkoxides increases with the
length of the alkyl chain 257 For transition metal alkoxides, under neutral or basic
Sol-Gel Chemistry of Transition Metal Oxides 291
conditions, and without any chemical modification, gelation is never possible. Depending on
the chain length precipitates or polymer colloids are formed. Precipitation of TiO 2 from
Ti(OR) 4 is observed when R=Et 240 ,241 pr i 240,242 while linear polymers seem to be formed
when R=Bu n 239,258,259 or R=Am n 260 Experiments performed in our laboratory confirm that
precipitation cannot be avoided, even under mild hydrolysis conditions, when R = Et, Pr n or
Pr i . However, stable sols can be obtained when Ti(OBun)4 or Ti(OAmt) 4 are hydrolyzed under
the same conditions. Analytical ultracentrifuging of these sols leads to the following
mean molecular weights : M.W.=5600 g/mole for R=Bu n and M.W.=3800 g/mole for R=~n t .
This corresponds to molecular species containing at most several tens of titanium atoms.
This supports the formation of small polymeric species whose degree of condensation depends
on the R group. The larger the R group is, the smaller the resulting polymer.
The main characteristics of oxide powders (particle size, surface area,
morphology and crystalline phases), obtained via hydrolysis and condensation of metal
alkoxides strongly depend on the identity of the alkyl group. For example, both anatase and
rutile phases can be present in a TiO 2 powder obtained after calcination of a gel. The
ratio rutile/anatase can be varied by changing the molecular weight of the metal-organic
precursor 261~262 The same phenomenon has been found recently in the hydrolytic
condensation of zirconium alkoxides 263. The alkyl group affects the morphology and the
particle size of the resulting materials. These in turn affect the sintering and
monoelinic-tetragonal transformation of ZrO 2.
3.1.3. Molecular structure of the alkoxide. - Oligomerization : The above discussion does
not take into account one of the main features of transition metal alkoxides. In such
compounds the oxidation state z of the metal is generally smaller than its normal
coordination number N. The full coordination of the metal is therefore not satisfied in
monomerie alkoxides M(OR)z. Consequently, the metal atom tends to increase its coordination
number by using its vacant d orbitals to accept oxygen or nitrogen lone pairs from
nucleophilic ligands. In non polar solvents one finds that coordination expansion of the
metal occurs via alkoxy-bridging which leads to the formation of more or less condensed
oligomers in which the metal attains a higher coordination number. This oligomerization is
basically a nucleophilic addition of a negatively charged OR group to a positively charged
metal atom M. It corresponds to an alcolation reaction which could proceeds as follows:
The degree of association depends on the nature of the metal atom. Within a given
group, the molecular complexity increases with the atomic size of the metal (table 12).
According to Bradley 264 alkoxides should adopt the smallest possible structural unit
consistent with all atoms attaining their higher coordination number. The insolubility of
divalent transition metal alkoxides (Cu, Fe, Ni,Co,Mn) may thus be attributed to their
highly polymeric nature 265
Compound Ti(OEt)4 Zr(OEt)4 Hf(OEt) 4 Th(OEt) 4
Covalent radii (•) 1.32 1.45 1.44 1.55
Molecular complexity 2.9 3.6 3.6 6.0
Table 12 : Degree of oligomerization for some transition metal ethoxides as a function of
metal size.
The molecular complexity also depends on the nature of the alkoxy group. It
decreases with increasing branching and bulkiness of the OR group because of steric
JPSSC 18:4-C
292 J. Livage et al.
hindrance effects 19. The molecular complexity of metal alkoxides is usually estimated from
molecular weight measurements in solution or by mass spectrometry 19,228 Direct evidence
for the oligomerization of titanium alkoxides was recently provided by X-ray absorption
experiments 266,26z. The shape and intensity of the prepeak observed before the absorption
edge show that titanium is tetracoordinated in Ti(OPri)4 and Ti(OAmt)4 while it is
pentacoordinated in Ti(OEt)4 , Ti(OBu")4 and Ti(OPrn)4. Moreover Ti...Ti distances of about
3.09A were clearly found in the EXAFS spectra of these last alkoxides showing that
oligomers are formed, while bulky (OPr i ) and (OAm t ) groups lead to monomers showing no
Ti...Ti correlations. In the case of silicon alkoxides the oxidation state z = 4 of Si and
its usual coordination number N - 4 are identical. Therefore Si(OR)4 precursors are always
monomeric and exhibit a tetrahedral structure 19
Solvate formation : Metal alkoxides are often dissolved in organic solvents before
hydrolysis is performed. These solvents usually correspond to the parent alcohol and are
far from being chemically inert with respect to the alkoxide. As a general rule, dilution
should lead to lower association. However, the nature of the solvent has to be taken into
account. Bradley observed that while Ti(OEt)4 remains trimeric in an inert solvent such as
benzene, the same was no longer true in a polar solvent such as EtOH 19. This was due to
the nucleophilic properties of the alcohol which causes dissociation and solvation of the
oligomer as follows:
2[Ti3(OEt)12 ] + 6 EtOH = 3[Ti2(OEt)6, 2 EtOH]
These experiments point out a very important property of metal alkoxides. Alkoxy
bridging is not the only method for coordination expansion. The alternative process of the
addition of a solvent donor molecule is also found. Metal alkoxides then behave as Lewis
acids and react with Lewis bases leading to solvate formation. Because of solvation, the
molecular structure of alkoxide precursors generally depends on the nature of the solvent
19 Zirconium alkoxides, for instance, exhibit a reduced molecular complexity when
dissolved in the parent alcohol rather than in an inert solvent. This is due to the
tendency of zirconium to expand its coordination with alcohol molecules instead of alkoxo
groups. The stability of such solvates increases with the positive charge of the metal atom
and its tendency to acquire a higher coordination number. Therefore, stable Zr(OPri)4.iPrOH
(6(Zr)=+0.64 ; N-z=3) 268,269 and Ce(OPi)4.iPrOH (6(Ce)-+0.75 ; N-z=4) 270 solvates can be
isolated as single crystals, while Ti(OEt)4.EtOH (6(Ti)-+0.63 ; N-z=2) can only be observed
in the solution at low temperature 2ZI No solvate has ever been characterized for Si(OEt)4
in ethanol (6(Si)=+0.32; N-z=0) 19
Hydrolysis/Condensation reactions : Alkoxy bridges appear to be more stable towards
hydrolysis than solvate bonds. Starting from a given alkoxide, different molecular
precursors can be obtained depending on the solvent used. Therefore, different hydrolysis
rates are expected which leads to completely different oxide materials. Precipitation
occurs when Zr(OPrn)4 is dissolved in n-propanol. This can however be avoided when
Zr(OPrn)4 is dissolved in a non-polar aprotic solvent such as cyclohexane, and leads to the
formation of polymeric gels and ZrO 2 monoliths 2Z2 As a result, hydrolysis and
condensation rates of Zr(OPrn)4 are much faster when the alkoxide is dissolved into
propanol than cyclohexane owing to the presence of solvate bonds in the former and alkoxy
bridges in the latter.
The same phenomenon was observed for titanium alkoxides. Monodispersed TiO 2 powders
have been synthesized by controlled hydrolysis of Ti(OEt) 4 in EtOH 240,241. Precipitation
also occurs when Ti(OPri)4 is dissolved in iPrOH but the monodispersity is lost 240-242
Ti(OEt)4 exhibits an oligomeric structure through ethoxy bridges while Ti(OPrl)4 remains
monomeric. Therefore, hydrolysis is much faster for this latter precursor than for the
Sol-Gel Chemistry of Transition Metal Oxides 293
former. Condensation, being a fast process in both cases, means that monodispersed TiO 2
powders can be obtained with Ti(OEt) 4 where hydrolysis rates are lower than condensation
rates. This is not possible with Ti(OPri)4 where hydrolysis and condensation rates are of
the same order of magnitude.
All these experiments show that the molecular structure of the precursor has to be
taken into account in order to describe its chemical reactivity. This was first
demonstrated by Bradley et al. 254 who carefully studied the hydrolysis of a large number
of alkoxides. They proposed different structural models to account for the hydrolysis
behavior of transition metal alkoxides (Ti, Zr, Nb, Ta, Ce). For each model the molecular
complexi1~y deduced from ebulliometric experiments can be related to the hydrolysis ratio h
assuming a sixfold coordinated metal atom. The structure of the molecular precursor is not
supposed to be modified upon hydrolysis, and condensation occurs between oligomerized
species. Four structural models names paqb have been proposed to account for experimental
results :
- The p3q4 model based on non solvated trimeric units Ti3(OR)12 (R=Et, Pr n, Bun).
- The p2q3 model based on solvated dimeric units M2(OR)8(ROH)2 (M=Zr, Ce, Ti).
- The plq3 model based on solvated monomers M(OR)4(ROH) 2 (M=Ti, Zr, Ce).
- And the plq2 model based on solvated monomers M(OR)5(ROH) of pentaalkoxides (M =
Ta, Nb).
The pioneer work of Bradley emphasizes the fact that both alcolation and solvate
formation can play a decisive role in hydrolysis/condensation reactions.
3.1.4. Hydrolysis ratio. The main external parameter is the hydrolysis ratio h which can be
defined as :
h [H20] (i0)
[M(OR) z ]
Bradley showed that for a given model paqb a mathematical relation can be established
between the average condensation degree n and the hydrolysis ratio 271 .
I/n = i/a - I/h (ii)
This relation shows that condensation could be adjusted by a careful control of the
hydrolysis ratio. However, quantitative predictions show some discrepencies with
experiments. For instance, the trimeric structure proposed for Ti(OEt) 4 in which titanium
atoms have a sixfold coordination 254 does not agree with XANES-EXAFS experiments 266,267
which suggest a fivefold coordination in Ti(OR) 4 (R = Et, Bun). This may explain why the
first hydrolysis product predicted from Bradley's model, Ti604(OEt)16, does not correspond
to the X..ray data on single crystals giving TiTO4(OEt)20 (figure 15) 247,274 Three main
domains could be considered in a rough qualitative analysis :
- h<l : In this domain condensation is mainly governed by alcolation and alcoxolation
reactions. The functionality of precursor towards alcoxolation is always smaller than one,
while for alcolation it could go up to z-i (i.e. three for a tetravalent metal). Under such
conditions, an infinite network is seldom obtained. Gelation or precipitation cannot occur
as long as hydrolysis remains carefully controlled (no local excess of water). Both
processes, alcolation or alcoxolation lead to molecular transition metal oxo-alkoxides
which can be isolated as single crystals from the solution (figure 16). Alcolation cannot
occur with silicon alkoxides owing to the fact that N-z=0. However, molecular compounds can
be for~ed through alcoxolation (dimers, trimers, tetramers .... ) which have been
characterized by 29Si NMR in solution 246
Oxo-alkoxides are the organic counterparts of polyanions and polycations which can
be obtained in aqueous solutions under careful control of the pH. Moreover, the structure
294
of these molecular clusters is
isostructural with Mo706 ~__ and
paratungstate Z [ H 2 W 1 2 0 4 2 1 1 0 "
J. Livageetal.
close to their inorganic analogs. Ti704(OEt)20 is
Nb8010(OEt)20 has the same structural units as
- ishs z : Charge calculations are reported in table 13, in order to describe the role of
the hydrolysis ratio. Ti(OPri)4 was chosen as an example because of its monomeric
structure. Table 13 gives the results obtained for h~0,1,2,3 or 4, in the transition states
Ti(OH)h(OR)4.h(OH2) and hydrolyzed species Ti(OH)h(OR)4. h :
Precursor h 6(OPr i ) 6(OH) 6(priOH) 6(H20) 6(Ti)
Ti (OPri)4 0 -0.15
Ti(OPri )4 (OH2) I -0.08 -0.38 +0.01 -0.28 +0.62
Ti (OPr i )30H 1 -0.08 -0.38 +0.02 -0.28 +0.62
Ti(OPri)3(OH)(OH 2 ) 2 -0.00 -0.36 +0.i0 -0.26 +0.64
Ti(OPri)2(OH)2 2 +0.04 -0.36 +0.15 -0.25 +0.64
Ti(OPri)2(OH)2(OH 2) 3 +0.13 -0.34 +0.25 -0.22 +0.65
Ti(OPri)(OH) 3 3 +0.28 -0.32 +0.41 -0.18 +0.67
Ti(OPri)(OH)3(OH2) 4 +0.38 -0.30 +0.52 -0.16 +0.68
Ti(OH) 4 4 -0.19 +0.01 +0.76
Table 13 : Influence of the hydrolysis ratio h upon the charge distribution in monomeric
precursors.
This table shows that the first steps of hydrolysis (h<2) can readily occur when
6(OR)<0 and 6(Ti)>0. As previously discussed, competition between oxolation and
alcoxolation may occur in this domain. Owing to the positive partial charge on ~PrOH,
alcoxolation should be favored thermodynamically . Under such conditions, chain polymers
can be obtained according to the following simplified scheme :
I I I IZ Ii n M ( O H ) ( O R ) 3 ~ . . . - 0 - R - O - R - 0 - R - 0 - O - 0 - . . . + n R O H
Such polymers were first obtained by Boyd and Winter with Ti(OBun)4 239,258. Under
similar conditions, spinnable sols were synthetized by Kamiya et al. 275,276, from which
SiO 2 or Tie 2 fibers could be drawn.
Upon further hydrolysis, the partial charge of the OR group becomes more and more
positive. This means that the prototropic transfer could become the rate limiting step. As
a consequence, hydrolysis may not go to completion even when h=4. This prediction is in
agreement with experimental data showing that the fourth alkoxy group is very difficult to
remove via hydrolysis or alcoxolation 239,241,258-260 Therefore, condensation via
oxolation becomes highly competitive when the full coordination is already satisfied (TMOS,
TEES). However, in the case of transition metal alkoxides, the alternative pathway,
elation, can occur preferentially because the required charge conditions (6(OH)<<0,
6(M)>>0 and N-z>>0) are fulfilled. The formation of elated polymers in this domain is
strongly supported by the fact that upon ageing, solvent is released via syneresis.
- h>z : Cross-linked polymers, particulate gels or precipitates can be obtained when an
excess of water is added to the alkoxide. It has been observed 263,277,278 that the
hydrolytic ratio strongly affects the mean size and weight of macromolecules which can be
formed. This observation seems to be general for Si, Ti, Zr alkoxides. Using an excess of
water, monodispersed powders based on Ti02, ZrO 2 and Ta205 have been obtained via
controlled precipitation of Ti(OEt)4 240, Zr(OPrn)4 279 and Ta(OEt) 5 280. As precipitation
is an extremely fast process in these experiments, it is highly probable that elation and
not oxolation is the predominent pathway for condensation.
Sol-Gel Chemistry of Transition Metal Oxides 295
3.1.5. Role of the catalyst. Another way to control hydrolysis and condensation processes
is to adjust the pH of the water used to perform hydrolysis. This can be done with an acid
such HCI or HNO3, or a base such as NH 3 or NaOH.
- Acid catalysis : Negatively charged OR groups can be easily protonated by H3 O+ ions :
M - OR + H30 + , M +--:O~ + H20
Under such conditions, the prototropic transfer and the departure of the leaving
group can no longer be the rate limiting steps. As a consequence, all OR groups can be
hydrolyzed as long as enough water is added. Hydrolysis rates can thus be greatly improved
by using an acid catalyst. This seems to be a general conclusion for all alkoxides
234,235,281,282. In the presence of H30 + , condensation occurs between these rapidly formed
hydrolyzed species M(OH)x(OR)z.x. Let us consider a typical polymer such as :
H O - - 0 - . . . - 0 - - 0 - . . . - 0 - - 0 - . . . - 0 - - O R A C
(charge calculations performed on different moieties of this polymer (A,B,C,D) are gathered
in table 14 :
SITE 6(OR) 6(Ti)
A -0.01 +0.70
B +0.22 +0.76
C +0.04 +0.71
D -0.08 +0.68
Table 14 : Charge distribution along a titanium oxo polymer.
It is easily seen that reactivity towards protonation decreases in the order :
D>>A>C>>B. OH groups are thus preferentially generated at the end of chains which leads to
rather linear polymers 283,284. The control of gelation rates is thus possible by using
acid catalysts together with substoichiometric hydrolysis ratio. Under such experimental
conditions spinnable sols 275,276 or monolithic gels 244,277 can be reproducibly obtained.
It must be pointed out that more acidic conditions (close to [H÷]=[Ti]), strongly inhibit
the condensation process. Protonation of the hydroxo group becomes possible, leading to
mixed aquo-hydroxo species such as those encountered with inorganic precursors.
The use of hyperacid catalysts such as trifluoromethanesulfonic acid (CF3SO3H) or
trifluoroacetic acid (CF3COOH) is also possible. However in this case,the reaction pathway
may be completely different and involves extremely reactive intermediates such as
sililenium ion (>Si ~) 285
- Base catalysis : Under acidic conditions hydrolysis and condensation can be uncorrelated
233. This is no longer the case with basic catalysts. Using NH 3 as a catalyst, it was shown
that hydrolysis of silicon alkoxides was activated 234,235 This could be due to a
nucleophilic activation of silicon through the coordination of the nitrogen lone pair.
Conversely, using NaOH as a catalyst, Bradley 281 showed that hydrolysis of Ti(OBuS)4 was
more difficult than under neutral or acidic conditions. In this case, nucleophilic addition
of OH" can occur which decreases the positive charge of the titanium atom.
Using NH 3 or NaOH, condensation is always activated through the formation of
highly nucleophilic species such as M - O" :
M - OH + :B - M - O" + BH + (B = OH', NH3)
This reactive condensation precursor will attack the more positively charged metal
296 J. Livage et al.
atom. According to table 14, the order of reactivity will be B>>C A>D. Strongly cross-
linked polymers are expected to be formed in agreement with literature 283,284. Under such
conditions, depending upon the hydrolitic ratio, non-spinnable sols or particulate gels are
obtained. This is also the case if olation is a competitive pathway for condensation.
3.1.6. Other physical parameters. The hydrolysis ratio and nature of the catalysts are the
most important external parameters in sol-gel processing. However, other parameters such as
concentration, nature of the solvent, and temperature can also play a decisive role in
reactions pathways.
Dilution, for instance, could help to separate hydrolysis and condensation
processes when acid catalysts and high hydrolytic ratios are used. This has been shown for
TEOS by several authors using 29Si NMR 233,238,286,287
Another effect of dilution is to prevent growth through aggregation. According to
Yoldas, 263'277'2T8 the mean polymer size decreases as the precursor concentration increases
for Ti(OR)4 and Zr(OR)4 systems. This is intimately linked to the occurence of sol-gel
transition which is strongly affected by aggregation processes.
Solvent effects are much more subtle. Solvents having a high dielectric constant
(formamide, propylene carbonate and water in large excess) can induce different pathways
for hydrolysis and condensation reactions through the cleavage of the polar M - O - C
bonds. It is usually assumed that cleavage occurs at the M - 0 bond 288,289. However, this
may not be the case when tertiary alkoxides are hydrolyzed. In such conditions-highly
reactive intermediates such as carbocations may not be neglected.
Increasing the temperature generally activates both hydrolysis and condensation
processes. As a consequence, with poorly reactive precursors such as Si(OR)4 , the
temperature may be increased to activate the sol-gel transition. On the other hand, for
strongly reactive precursors such as transition metal alkoxides, the temperature must be
lowered in order to slow down hydrolysis and condensation processes as shown by Rehspringer
et al. 290 in BaTiO 3 processing.
3.2. Chemical modification of metal alkoxides
One of the main drawback or advantages of transition metal alkoxides is their high
reactivity with water. They must be handled with great care, in a dry box and precipitation
is usually observed rather than gelation. A survey of literature shows that chemical
additives are almost always used in order to improve the sol-gel process and obtain better
materials. Such additives can be solvents 257, acidic or basic catalysts 282 stabilizing
agents, 291,292 or drying control chemical additives 293,294. In most cases they are
nucleophilic XOH molecules that react with the alkoxide giving rise to a new molecular
20 precursor
M(OR) n + x XOH = M(OR)n.x(OX)x + xROH
The chemical reactivity of the alkoxide with nucleophilic species mainly depends
on the following :
- The electrophilic power of the metal atom increases when its electronegativity decreases.
- The ability of the metal atom to increase its coordination that can be estimated as the
difference (N-z) between its usual coordination number N in the oxide and its oxidation
state z. For a given group, (N-z) increases when going down the periodic table.
- The nucleophilic strength of the chemical modifiers.
Addition or substitution reactions lead to new molecular precursors which react
differently with respect to hydrolysis and condensation. The charge distribution among the
Sol-Gel Chemistry of Transition Metal Oxides 297
metal atom and its ligands is modified leading to enthalpy changes while entropy changes
occur when the coordination number increases. Both effects lead to a modification of the
nucleophilic reactions together with a differentiation of the ligand reactivity for
hydrolysis and condensation. It should be noted that the chemical reactivity and the
funetiennality of a M(OR)z.x(OX)x mixed alkoxide is not often simply deduced from the
behavior of the parent compounds M(OR)z and M(OX)z. Substitution reactions decrease the
functionnality while addition reactions leave it unchanged. Thus, substitution promotes a
decoupling between hydrolysis and condensation. Less electronegative ligands are first and
rather quickly removed upon hydrolysis while more electronegative ones (the modifiers)
should be mainly removed during condensation reactions. As a consequence, the growth of the
particles becomes more anisotropic which promotes the formation of polymeric gels.
Molecular modifications have a strong effect on parameters such as gelation time,
particle morphology, porosity, etc... The sol-gel transition in polymer chemistry is
usually given by equation (12) 295,296
t = [C0k(f2-2f)]'1 (12)
Three parameters can then be varied in order to optimize the sol-gel process, namely C O
(monomer concentration), k (bimoleeular condensation rate) and f (functionality which
depends on the hydrolysis rate). A good rule of thumb for the sol-gel chemist is reported
in table 15. It suggests that, depending on the relative hydrolysis and condensation rates,
different products can be obtained.
Hydrolysis Condensation Result rate rate
SLOW SLOW COLLOIDS/SOLS
FAST SLOW POLYMERIC GELS
FAST FAST COLLOIDAL GEL OR GELATINOUS PRECIPITATE
SLOW FAST CONTROLLED PRECIPITATION
Table 15 : Products obtained according to the relative rates of hydrolysis
condensation.
and
3.2.1. Alcohol interchange. Metal alkoxides react with a variety of alcohols to set up the
following equilibrium :
M(OR) z + x R'OH = M(OR)z.x(OR')x + x ROH
In general, the facility for interchange increases when the steric hindrance of the alkoxy
R group decreases ; OMe>OEt>OPri>OBu t " The facility for the interchange reaction also
depends strongly on the nature of the metal atom 19 More particularly, transition metal
alkoxides exhibit faster exchange rates than silicon alkoxides. This point can be
illustrated by NMR experiments performed on the metal probe. Recent 29Si NMR measurements
have clearly shown that exchange between silicon alkoxides and solvent molecules can take
place at room temperature 233,245 . The following reaction has been studied 233 :
Si(OEt) 4 + x HOPr i ) Si(OEt)4.x(HOPri)x + x EtOH
It has been shown, using 29Si NMR (figure 17a), that exchange between ethoxy group and
isopropanol molecule takes place under acidic catalysis on a time scale of about twenty
hours. Conversely, 51V NMR experiments performed in our laboratory have shown that the
following exchange reaction :
VO(OPri)3 + x (HOAm t) ~ VO(OPr~)3.× (OAmt) x + x HOPr ~
takes place instantaneously at room temperature (figure 17b) without a catalyst. Such
alcohelysis reactions are widely used for the synthesis of metal alkoxides. It is well
298 J. Livage et al.
¢o
A " B ~
iJl . . . . . . ' l l i l _ _ . ,
PPml i i J J i J I , I , I , i 8 ( )
- 8 0 - 1 0 0 - 1 2 0 - 4 0 0 - 5 0 0 - 6 0 0 -700 - O'
Fig.17. (a) 29Si NMR spectrum of a solution of Si(OEt) 4
in propanol-2 with an acid catalyst after
20 hours.
(b) 51V NMR spectrum of a solution of VO(OAmt)3
in iPrOH with no acid catalyst after I0
minutes.
known that hydrolysis and conden-
sation rates depend on the nature
of the alkyl group. Therefore, it
should be possible to adjust the
rate of gelation of a given
alkoxide by using different
solvents 257
Similar experiments have been
performed in our laboratory with
titanium alkoxides. TiO 2 preci-
pitates are readily formed when a
stoichiometrie amount of water
(H20/Ti~2) is added to Ti(OPri)4
while stable colloidal solutions
are obtained with Ti(OAmt)4. On
the other hand, gelation occurs
within a few minutes when
Ti(OPri)4 is dissolved into AmtOH
prior to hydrolysis. Formation of
a mixed Ti(OPri)2(OAmt) 2 alkoxide
occur in which (OAm t ) groups
should be first hydrolyzed
according to partial charge
calculations 20
3.2.2. Metal chloride alkoxldes. Metal alkoxides are known to react with halogen or
hydrogen halides giving rise to halide alkoxides. Chloride alkoxides can also be very
easily obtained through the reaction of metal chlorides with alcohols 19
TiCI 4 + 3 EtOH , TiCI2(OEt)z.EtOH + 2 HCI
The reactivity of metal chlorides decreases with increasing electropositive character of
the metal, i.e. when going down the periodic table. The reaction of SiCI 4 with EtOH can be
pushed to completion with the formation of Si(OEt) 4 . Under the same conditions TiCI 4 and
ZrCI 4 undergo only partial substitution : TiCI2(OEt)2.EtOH 297 and ZrCI3(OEt).EtOH 298
while ThCI 4 forms only addition compounds i.e. alcoholates : ThCI4.4EtOH 19 Such chloride
alkoxides can be considered to be chemical modifications of the alkoxides. They are very
easy to synthesize and can be used as molecular precursors for the sol-gel processing of
transition metal oxides.
Niobium pentoxide gels are quite difficult to obtain from inorganic, NbCI 5 , or
metal organic, Nb(OEt)5, precursors. Both are highly reactive with water and precipitation
occurs rather than gelation. Niobium chloride alkoxides are readily formed when NbCI 5 is
dissolved into an alcohol 181 :
NbcI 5 + 3 ROH , NbCI2(OR)3 + 3 HCI
Solutions of these chloride alkoxides are quite stable. They can be stored in a
dry environment without any special care. Gels can be easily obtained through hydrolysis of
these solutions with an excess of water. The rate of gelation depends on the alcohol used.
It is much faster the longer the alkyl chain. Gelation occurs within a few seconds with
PriOH, a few hours with EtOH and several days with MeOH 181
Electrochromie WO 3 layers have been made from tungsten chloride alkoxides 20
Tungsten hexaehloride is dissolved in ethanol where upon a violent reaction the solution
Sol-Gel Chemistry of Transition Metal Oxides 299
turns blue. The chemical reaction can be written as follows 19 .
WCI 6 + 2 EtOH , WCI3(OEt)2 + 1/2 CI 2 + 2 HCI
Reduction of W(VI) to W(V) can be avoided by using WOCI 4 , instead of WCI 6 . Stable
solutions of oxychloride alkoxides are thus obtained. They can be kept for months and used
for making electrochromic layers by dip-coating 20
3.2.3. Acetic acid. Stable metal alkoxo-acylates can be formed when acetic acid is added to
an alkoxide 299 Acetic acid is often used as an acid catalyst in the sol-gel processing of
metal alkoxides M(OR)n : M = Si 275, AI 300, Ti 301,302 or Zr 20,263. Acid catalysis is
known to increase hydrolysis rates and acetic acid is currently used to decrease the
gelation time of Si(OR)4282. A reverse effect was actually observed with transition metal
alkoxides such as Ti(0R) 4 or Zr(OR) 4 . Precipitation readily occurs when pure water is added
to the alkoxide while homogeneous and transparent TiO 2 or ZrO 2 gels are obtained in the
presence of acetic acid 20,301 Gelation times then increase up to a few minutes or even
days. This can be attributed to the complexing ability of the acetate ligand.
An exothermic reaction takes place when acetic acid is added to Ti(OBun)4 , which
leads to a clear solution. X-ray absorption experiments on the Ti(OBun)4 precursor shows
that the coordination number of Ti increases from 5 to 6 upon acetic acid addition 266 13 C
and IH NMR of the modified precursor show that acetate groups are bonded to titanium while
infra-red spectra indicate that CH3CO0" behaves as a bidentate ligand (chelating and
bridging). A stoichiometric chemical reaction takes place for a one to one ratio which can
be written as follows :
Ti(OBun)4 + AcOH , Ti(OBun)3OAc + BuOH
Infra-red and NMR experiments show that (BunOH) groups are first removed upon hydrolysis
while chelating acetates remain bonded much longer to titanium and thereby slow down the
gelation process 301 Calculations based on the Partial Charge Model are in agreement with
these experiments. Titanium has a high positive charge (6=+0.61) in Ti(OBun)4 and its full
coordination is not satisfied. Therefore, nucleophilic addition of AcOH is possible giving
rise to the intermediate : Ti(OBun)4(AcOH). The charge distribution in this intermediate
shows that AcOH is negatively charged (6--0.7) while BuOH is positively charged (6=+0.1).
An alcohol molecule is then removed which leads to the substituted alkoxide
Ti(OBun)3(OAc). Hydrolysis of this new precursor begins via a nucleophilie addition of H20:
Ti(OBun)3(OAc) + H20 ........ ~ Ti(OR)3(OAc)(OH 2)
A charge distribution calculation shows that AcO remains negatively charged (6=-0.6) while
(Buno) is positively charged (6=+0.2). (OBu n) groups are then removed first upon hydrolysis
in agreement with NMR experiments 266 As acetate groups are not immediately removed
through hydrolysis or condensation, the functionality of Ti(OBun)3(OAc) is smaller than
that of Ti(OBun)4. The more (OAc) groups located around titanium, the smaller the
functionality will be and therefore the slower gelation occurs. In agreement with this
analysis, the gelation time strongly increases as the molar ratio HOAc/Ti approaches 2 301
3.2.4. Chelating ligands. - Acetylacetone is known to be a rather strong chelating ligand
and many metal ~-diketonates have already been reported in the literature 303 The enolic
form of ~-diketones contains a reactive hydroxyl group which reacts readily with metal
alkoxides 19,303 Therefore, acetylacetone has often been reported in the sol-gel
literature as a stabilizing agent for metal alkoxide precursors : W(OEt) 6 304, Zr(OPri)4
291,305, Ti(OPri)4 266, Ti(OBun)4 306 or Al(OBuS)3 292. Patents have even been obtained in
which acetylacetone is used to improve the process 307,308. Recently, TiO 2 colloids have
been stabilized up to high pH with acetylacetone 309 X-ray absorption experiments show
300 J. Livage et al.
that Ti(OPri)4 is a four fold coordinated monomer. An exothermic reaction takes place when
acetylacetone is mixed to Ti(OPri)4 in a one to one ratio. IH and 13C NMR spectra, together
with infra-red experiments show that acac ligands are bonded to titanium. XANES suggests
that the coordination number increases up to 5 20 A stoichiometric reaction takes place
which can be written as follows :
Ti(OPri)4 + acaeH , Ti(OPri)3(acac) + priOH
Titanium coordination turns to 6 as soon as water is added to the new precursor. Ti...Ti
correlations become visible in the EXAFS spectrum showing that hydrolysis leads to
condensed species. NMR and I.R. spectra show that (OPr i ) groups are hydrolyzed first. All
acac ligands cannot be completely removed, even when a large excess of water is added.
Precipitation or gelation was not observed. Small colloidal particles about 5 nm in
diameter are obtained. These colloids are much smaller than those obtained without acac
modification (15 nm) which shows that this new ligand prevents condensation 20
- Hydrogen peroxide : Some papers 181,292,305 report the formation of gels in the presence
of H202. According to R. Roy et al. 310 the reaction of H202 with alkoxides results in
aerohydrogels that appear to have a fibrillar microstructure. Monolithic Nb202 gels can
also be easily obtained when H202 is added to NbCI 5 rather than H20 181. In both cases, the
resulting gels exhibit a yellow-orange color arising from the formation of peroxy species
311,312 Complex polymerization processes involving peroxy compounds are thus involved
during gelation. Peroxy ions 022" are known to be strong chelating ligands that are able to
react with the metal atom and increase its coordination 311 . Coordination numbers up to 7
have been found for peroxotitanium (IV) compounds 312. Let us suppose that peroxy species
can be formed as follows :
Ti(OEt)4 + H202 , Ti(OEt)202 + 2 EtOH
A charge distribution calculation shows that the peroxy group is negatively charged
(6(O2)~-0.89). It should therefore be strongly bound to titanium. A transition state
Ti(OEt)202.H20 could be formed when one water molecule is added to the new precursor. The
negatively charged peroxy group remains bound to Ti while the positively charged EtOH
molecule can be removed, giving rise to hydrolyzed species such as :
Ti(OEt)202 + H20 , Ti(OEt)202.H20 , Ti(OEt).O2(OH) + EtOH
O~" ligands increase the positive partial charge of the metal atom and the alkoxy
groups, making both the nueleophilic attack of water molecules and the departure of alkoxy
groups more facile. Moreover, these peroxo groups are not removed upon hydrolysis which
decreases the functionality of the alkoxide to a value close to 2. This could explain the
fibrillar microstructure observed by TEM 310
3.2.5. Organically modified gels. Organically modified silicates (ORMOSILS) have been
recently developed 314-316 In these compounds, non-hydrolyzable Si-C bonds are formed
which behave as network modifiers or network formers depending on the chemical reactivity
of the organic group 317 Such a modification cannot be extended to transition metal oxides
because the more ionic M-C bond would be destroyed upon hydrolysis. Organic modification
could however be performed using polyhydroxylated compounds such as polyols (glycerol,
polyethyleneglycol or polyethanolamine) or o-hydroxyacids (glycolic, salicylic or mandelic
acids). These species can react with metal alkoxides giving rise to mixed alkoxide
derivatives 306,318-320. These derivatives appear to be very stable because of chelate and
steric hindrance effects. Therefore, they are not removed during hydrolysis and
condensation leading to new, mixed organic-lnorganic materials. These compounds can be
calcinated in order to obtain a ceramic powder 290. They can also be used as such, like
ORMOSILS, and offer a wide range of new possibilities.
Sol-Gel Chemistry of Transition Metal Oxides 301
Electrolyte gels have been made via the reaction of a polyol (glycerol) and a
carboxylic acid (acetic acid) with a titanium alkoxide Ti(OBun)4 321. More or less viscous
gels are obtained upon hydrolysis in which both organic (Ti-OH2C-CHOH-CH20-Ti) and
inorganic (Ti-O-Ti) bridges are formed. They remain stable even upon heating at 80°C.
Layers deposited from these gels exhibit high proton conductivities at room temperature
(o=5.10 .4 Scm'1). Such gels have been used as electrolytes for making electrochromic
display devices 321
Reactions with maeromolecules such as cellulosic 322 or polysaccharides 323 lead
to other organically modified TiO 2 gels. In such compounds a good control of the cross
linking of hydroxy group is achieved by merely mixing the non-hydrolyzed alkoxide with the
polymeric material in different amounts. Application of these materials are manyfold :
production of high viscosity fluids for hydraulic fracturing 323
gelation of cellulosics or textile materials to make water repelant or flame retardant
fabrics 324,325
Very few papers concerning organic-inorganic copolymers involving transition
metals have been described 326,327. A good example is provided with TiO 2 gels organically
modified with vinyl acetylacetone. A Ti(OBun)4 alkoxide first reacts with the organic
modifier in a one to one ratio. A double polymerization process is then initiated via
partial hydrolysis of the alkoxy groups and radical polymerization of the vinyl functions
using azobisisobutyronitrile as a catalyst. A viscous product is obtained that can easily
be deposited onto a substrate giving photochromic coatings which turn blue upon U.V.
irradiation 20
4. ORDERED AGGREGATION AND INTERCALATION
All colloidal systems have an in-built tendency to become unstable and aggregate
spontaneously. This is a random process governed by Brownian motion, and as a result,
colloidal aggregates usually exhibit disordered open structures. The concept of fractal
geometry is often used to describe such aggregates 328. The fractal dimension can be
measured by Small Angle X-ray (SAXS) or Neutron Scattering (SANS). It is deduced from the
slope of the scattering curve in the Porod region 329,330. Models such as "cluster
aggregation" or "diffusion limited aggregation" are then computed in order to account for
the observed fractal dimensions. Most of the studies published in the literature deal with
silica gels. Transition metal oxide colloids however may exhibit a large variety of shapes
201 and two possibilities have to be considered for aggregation 24
i) if for any mutual orientation of the colloidal particles, the potential energy maximum
is less than kT, all collisions will be non-elastic and the multi-particle aggregate
ultimately formed will be completely disordered and isotropic.
ii) if for a particular orientation of the two colliding particles, the potential energy
maximum is less than kT, while it is in excess of kT for other orientations, an ordered
and, therefore anisotropic aggregate is bound to result. Such aggregates usually occur when
colloidal particles are strongly anisotropic (platelets, rods). They lead to sols or gels
that exhibit specific properties such as streaming birefringence, rheopexy or chemical
intercalation.
4.1. Anlsotropic Aggregates
4.1.1. Tactoid formation. Electrostatic repulsion between charged colloidal particles
302 J. Livage et al.
usually prevents aggregation and flocculation. In some cases, these interactions can lead
to a long-range ordering in which charged colloids are placed along a periodic array as in
a crystal. Such systems, known as "colloidal crystals" 331, are observed with monodispersed
spherical colloidal particles such as latex or SiO 2 . The distance between particles is of
the same order as the optical wavelength giving rise to visible light scattering. These
colloidal crystals then appear iridescent. The natural opals are a well-known example of
such a long-range ordering.
More interesting is the case of colloidal particles that exhibit a strongly
anisotropic shape such as rods or platelets. Colloidal solutions of these non-spherical
particles may separate under suitable conditions into a concentrated, anisotropic phase and
a dilute, isotropic phase. Interactions between solid particles are quite strong in the
concentrated phase where colloidal particles are mutually oriented giving rise to the so-
called "tactoids". Colloidal particles are randomly dispersed into the isotropic dilute
phase called "atactosol" 24,332 Two main types of orientations have been observed :
Platelike colloids lead to sediments which have a periodicity along the axis
perpendicular to the layers. The tactoids which are formed by such oriented aggregates may
be called "smectic" tactoids. They are characterized by a brilliant luster giving rise to
the so-called "schiller layers". Tungstic acid or ~-FeOOH are typical examples of such
systems 333,334
Rodlike particles are arranged with their main axis parallel to each other. They may be
called "nematic" tactoids. The best known examples are V205 sols that give rise to
typically ellipsoidal tactoids 335 The special shape of these tactoids results from the
competition between two energy terms ; the interaction between rodlike particles that lead
to a cylinder and the interfacial energy between a viscous taetoid and the surrounding
dilute sol that lead to a spherical droplet.
The size of tactoids increases with the concentration of the sol. In the case of
V205, they can reach a length of about 250 ~m. Such tactoids are made of approximately 1013
colloidal particles. Below a critical concentration, tactoids are not formed and the
colloidal solution remains isotropic unless a shear stress is applied. Vanadium pentoxide
sols exhibit unusual properties such as thixotropy or streaming birefringence 336 arising
from the anisotropy of colloidal V205 particles. This can also explain the amazing
phenomenon of "rheopexy" ; on rolling a test tube, containing a liquified thixotropic V205
sol, between the palms of the hands, it is observed that gelation of the sol is accelerated
significantly 337. The mean distance between colloidal particles can be progressively
reduced by suitable changes of the dispersion medium. On careful addition of electrolytes,
V205 tactoids gradually shrink to a fraction of their original size, maintaining and even
enhancing their internal anisotropy. Ordered, reversible aggregates can then be changed
into ordered, irreversible aggregates giving rise to crystalloids in which all particles
are mutually oriented 332 Smectic tactoids of ~-FeOOH can,by drying slowly, be readily
changed into solid sheets in which individual layers are maintained, reminiscent of
structures like mica 338
4.1.2. Anisotropie layers deposited from ordered colloids. The spontaneous orientation of
anisotropic colloidal particles can be preserved and even enhanced, upon slow removal of
the solvent. Anisotropic coatings can therefore be obtained that exhibit specific
properties. One of the best known examples is undoubtely the V205 layers deposited from
gels that have been extensively studied during the last decade 339
V205 gels are made of entangled fibers (figure 8). Electron microscopy shows that
these fibers actually look like flat ribbons approximately I x i0 x 102 nm in size. X-ray
Sol-Gel Chemistry of Transition Metal Oxides 303
and electron diffraction experiments 172 show that these ribbons exhibit a two-dimensional
structure defined by a unit cell : a = 27.0 A and b = 3.6 A . This 2D structure is not
modified upon swelling and seems to be closely related to the layered structure of
orthorhombic V205 . Fibers are built of basic blocks containing I0 vanadium atoms along the
a direction. Some strongly bound water molecules, or OH groups, link these blocks together
giving rise to the corrugated structure of the ribbons 340 X-ray absorption experiments
show that vanadium is surrounded by five oxygen ate,ms with a short V=0 distance (1.58 A) as
in crystalline V205 . There is, however, no evidence for a long V-O bond between adjacent
layers 341
Under ambient conditions, the water content of V2Os.nH20 xerogels corresponds to
n=l.8. Thermal analysis 179,342 shows that water can be removed reversibly , upon heating
or under vacuum, down to a composition V205,0.5H20. Below this value further condensation
occurs and the thermal dehydration process becomes no longer reversible, leading to
crystalline V205 . According to infra-red and Raman studies the nature of water molecules
depends on the water stoichiometry 343,344. For high water content (n>l.8) water molecules
exchange hydrogen bonds with the oxide network while for low water contents (n=0.5) they
are directly bonded to vanadium atoms 345, in agreement with ESR and ENDOR experiments 178
001 j 003
, ,; S 10 15 20 25 30 ~ o
U
5
002 003
II II t,0ob I ]
10 115 20 25 30 ~ O
Fig.18. X-ray diffraction pattern of a V205.nH20
layer :
a) n = 1.8, basal spacing d = 11.5
b) n = 0.5, basal spacing d = 8.7 A
V205 layers deposited from gels
exhibit an anisotropic structure that
can be easily detected by X-ray
diffraction. Reflection geometry X-
ray diffraction patterns are typical
of a one-dimensional order corres-
ponding to the turbostratic stacking
of the ribbons one upon another along
a direction perpendicular to the
substrate 179 All diffraction peaks
can be indexed as 001 (figure 18).
The anisotropy of these coatings was
also clearly demonstrated by E.S.R.
178,infra_red 345 and polarized EXAFS
346 spectroscopies. The basal spacing
d, deduced from the position of the
001 peaks, increases with the amount
of water in the sample : d=8.7 A for
a xerogel dried under vacuum
(V205,0.5H20) and d=ll.5 A for a
xerogel dried under ambient condi-
tions (V205,1.8 H20 ). By comparison
with similar layered clay systems,
the 2.8 A increase of the d-spacing
was attributed to the reversible intercalation of one water molecule layer between the
V205 ribbons.
The swelling process of V2Os.nH20 xerogels at low water contents (n<20) was
followed by X-ray diffraction and Wide Angle Neutron Scattering 180. A stepwise swelling
process was first observed up to about n=6. The basal distance d increases by steps of 2.8
corresponding to the thickness of a single water layer. In this domain, interactions
between the oxide ribbons remain quite strong and the swelling process can be described as
the intercalation of water molecules into a layered host lattice. Beyond n=6, the basal
304 J. Livage et al.
spacing d increases progressively and a continuous swelling seems more appropriate to
describe the water uptake process (figure 19). Interactions between particles become weaker
and their mean distance increases continuously with the amount of water added, as in usual
colloidal solutions. The composition V205.6H20 corresponds to a turning point between solid
state chemistry (n<6) and colloidal chemistry (n>6).
4 0
3 0
2 0
10
~d(A)
__1]_
_.~_1]._. WATER LAYERS
5 10 15 n i l20
Ln(I)
% k I'"' ++ % 7 x ' / i .HI.# n=192
+~. .i ~#tll, p, " ~. f [~,.,.~+~+ +%+, ,j~/~ I1~4 'd'~'~+ltllll#l n. 15 5 IE ~,. '+++ ~+l+,~++.+~°,. _
.% %1- n= Sl -1~- ~,~. ~.~
0.0 2.25 4.5 6.75 9.0 10"2Q[~-1)
Fig. 19. Variation of the basal distance d as
a function of the water content of a
V20 s.nH20 layer.
n f 1 2 0 / V 2 0 5 800 300 100 50 20 10 5 1
Lna(A) _ ' . . . . . . I ~(,~)
: ~ r S e n C ~ . T ". ~ 2510::
First "'- ~ 50 F ~ j _ T:ainr:~t IOn ~ ' " ~ " 3 0
2 I Regl/e I I I I lo -6 -5 -4 -3 -2 -1 0
Ln~
Fig,21. Swelling of V205.nH20 gels
as a function of the oxide
volume fraction.
Fig.20. Scattering curve of V205.nD20
gels as a function of the
amount of water.
Small Angle Neutron Scattering experiments
were also performed to follow the swelling 347 process to higher water content
Scattering curves for V 205.nD20 samples in
the concentration range 80<n<200 clearly
exhibit maxima in the angular dependence
(figure 20). The d-values corresponding to
these maxima are plotted in figure 21 as a
function of the volume fraction ¢ of V205 .
They can be described by : in(d) = kln(¢).
Assuming additivity of the volume
fractions of V20 s and water, the slope of
about -I observed in the concentrated
regime should correspond to a ID swelling
procedure of plate-like particles. In
fact, this concentration range can be
divided into two parts. The first part (regime I) has a slope of -0.9 for n<6 (or d<25 A).
The gel looks like an hydrated powder and the swelling procedure is characterized by an
increase of the basal spacing by steps of about 2.8 A. The second part has a slope of about
-I.i for 10<n<80 (or 50<d<250 ~). The gel is in a thixopic elastic state. A first
transition occurs in the range 25-50 ~, where the gel becomes an inelastic, pasty material.
The slope -0.60 observed in the more diluted regime II, where the gel turns from a
thixotropic liquid to a slightly viscous one, suggests a 2D swelling. The range between I00
Sol-Gel Chemistry of Transition Metal Oxides 305
and 200A corresponds to a second transition range, where the swelling process progressively
turns from ID to 2D.
Such behavior can be described using the following model. The thickness of the
V205-ribbons was estimated around 8.8 A by X-ray and neutron powder diffraction. The value
obtained from the extrapolation of the one dimensional swelling regime to the dry state is
approximately the same (7.4 A). In regime I, swelling is governed by the increase of the
interparticular distance perpendicular to the largest surface of the particles. When the
mean distance between ribbons reaches values comparable to the width of the particles, the
swelling becomes two dimensional• As a result, d-spacing that reflects the interparticular
distance increases more slowly with water content.
4.1.3. Magnetic ordering in 7-Fe203 colloids. Ferrimagnetic spinel iron oxide particles
about i0 nm in diameter can be prepared by increasing the pH of an aqueous mixture of Fe 2÷
and Fe 3+ salts. Stable sols are then obtained by peptizing the flocculate in an acidic or a
basic medium 113 Aggregation of the colloidal particles depends mainly on the sign and
the magnitude of surface charges 114 in relation to the acidity of the medium. Magnetic
dipolar interactions (=kT) are much smaller than electrostatic interactions (=lOkT) and are
not likely to contribute to the primary aggregation process. However, magnetic ordering has
been observed in colloidal aggregates that seem to behave as superferromagnets rather than
348 superparamagnets
Colloidal aggregates can be frozen in place by adding a water soluble polymer 349
or by surfaction and dispersion in a toluene-polymer mixture, in order to perform
correlative electron microscopy and M6ssbauer spectroscopy experiments. Figure 22 clearly
shows the effect of surface charges on the aggregation state : small clusters (n=5) are
formed in an acidic medium (figure 22A), while small chains (n=15) are observed at slightly
higher pH (figure 22B). Much longer strings (n=50) or large compact aggregates are found
around the point of zero charge (figure 22C).
o
A /'. .~; : '
• , - . % .
% • .. ,.%.
l-
• r . . . , < z.: ?~ .~, :
ij :j ;i ::: !: .: - ~. -
VELOCITY (ram/s)
Fig.22. Electron micrographs and M6ssbauer spectra (300K) of 7-Fe203 colloids in frozen-in
sols.
306 J. Livage et al.
Small clusters give M6ssbauer spectra A or B depending on the aggregate concentration.
These spectra are typical of magnetically uniaxial particles which undergo
superparamagnetic relaxation. The change A~B corresponds to a weak evolution of the
particle magnetic coupling. Strings exhibit the same features (A~B) depending on their
size. Because of neighboring effects in the small clusters case and of branching and
coiling in the strings case, the change A~B is related to the average number of first
neighbors per particle. It occurs when one particle gets around 3 neighbors. Spectrum C is
typical of large, compact aggregates. It is nearly identical to the M6ssbauer spectrum of
bulk 7-Fe203 and was interpreted as enhanced interparticle interactions. Exchange coupling
between facing spins at the surface of adjacent particles likely becomes operative, leading
to superferromagnetic ordering 349. The magnetic moments of all elementary particles then
tend to be parallel to each other.
Large aggregates can also be obtained by adding an electrolyte to the aqueous sol.
Anionic charges can compensate positive surface charges in cationic sols. Anions and
hydration water molecules separate the iron oxide colloids. Magnetic interactions between
particles are then expected to decrease, especially those due to exchange coupling : super-
ferromagnetic ordering must vanish. In agreement, superferromagnetic ordering decreases
when the complexing ability of the anions increases : NO3"<CIO4"<SO42<HPO42 350
4.2. Intercalation properties of V205 ~els
Intercalation of guest species into host lattices with layered structures has
received ever increasing attention during the last decade. Intercalation is a reversible
process. The host matrix retains its basic structural integrity during the course of
forward and backward reactions while expansion of the lattice perpendicular to the layered
planes is observed. Host lattices must exhibit a strong 2D anisotropy, therefore very few
oxides are able to give rise to reversible intercalation. In the case of orthorhombic V205
for instance, intercalation seems to be restricted to small cations such as Li + 351 V205
actually behaves as a three-dimensional framework rather than a van der Waals host. Li+ions
are inserted into the channels of the orthorhombic network and the weak V-O bonds between
layers persist in LixV205 352
Vanadium pentoxide gels exhibit a layered structure in which the internal 2D
structure of the ribbons is closely related to that of the (a,c) planes of orthorhombic
V205 . Vanadium-oxygen bonds between the layers are however much weaker in the gel than in
the crystalline oxide. Therefore, reversible intercalation of guest species becomes
possible. This was already observed in the case of water intercalation leading to a
stepwise swelling process during the first hydration stages. V205 gels actually offer a
very versatile host structure for intercalation. Intercalation reactions involving
crystalline compounds are quite slow. They usually require heating under reflux for several
days. Gels are much more reactive species, full intercalation can be performed, at room
temperature, within a few hours or even minutes 25
4.2.1. Intercalation of metal cations. Ion exchange between protons of the hydrous
V205.nH20 xerogel and metal cations occurs as soon as the gel is dipped into an aqueous
solution of metal chlorides 353 The rate of ionic H+/M + exchange is controlled by
diffusion in the gel phase. It can be monitored by measuring the pH of the solution which
decreases when protons are released 354. Intercalation does not affect the monodimensional
order arising from the stacking of V205 ribbons. The basal spacing d increases, showing
that guest species are intercalated. The d increase however depends on the nature of the
Sol-Gel Chemistry of Transition Metal Oxides 307
intercalated cation. It varies with the charge/ionic radius ratio, related to the hydration
enthalpy U h of the cation. All data gathered are centered around two values : d=ll A and d
=13.6 A (figure 23) suggesting that the M + cation is intercalated with one or two water
layers 353 Intercalated species containing one water layer are obtained with cations
having a low U h value (mainly monovalent cations) whereas intercalation with two water
layers occurs with cations having a high U h value (mainly divalent cations). These
observations can be explained as resulting from the competition between two energy terms :
- The energy required to separate V205 layers increases with the basal spacing variation.
The energy required to remove water molecules from the solvation sphere of the cation
increases with the charge/radius ratio.
o d ( A )
Li + Ca2+C~2+Fe2+ Mg2+
~-27X
N ÷ Cs y R I ~ . H 4 = = I e / r 1 2 3 1=
Fig.23. Variation of the basal distance
d as a function of the charge/radius
ratio of cations intercalated into the
layered structure of V205 gels.
It is interesting to point out that V205
xerogels can be dissolved when dipped into pure
water. A non-limited swelling process is
observed, leading to a gel or a colloidal
solution. Such a phenomenon does not occur in
the aqueous solutions of metal cations. A
limited swelling process is observed which
stops after the intercalation of one or two
water layers. This is probably due to the
positive charge of the intercalated cations
that attracts the negatively charged V205
ribbons thus preventing further swelling by
additional water molecules. These results have
been recently extended to the intercalation of
alkali ions in the presence of non aqueous organic solvents 355 An increase of the basal
spacing d is observed which depends not only on the charge/ionic radius ratio, but also on
the nature of the solvent. Two solvation stages have been deduced from interlayer
distances. For water, they correspond to the intercalation of either one or two solvent
layers together with the metal cation.
Sodium intercalation has been recently used to synthesize vanadium bronzes at low
temperature 356 A V205,1.8 H20 xerogel is deposited onto a glass substrate, then dipped
into an aqueous solution of NaCI(IM). Intercalation occurs, leading to a Na0.33V205,1.8 H20
compound characterized by a basal spacing d=10.9 A. Water is then removed upon heating, and
crystallization of the monoclinic Na0.33V205 bronze occurs at 320°C instead of 700°C by
usual solid state reactions. The anisotropy of the layer is conserved even after
crystallization of the bronze so that the tunnels present in the structure remain parallel.
Ionic diffusion along these tunnels therefore becomes easier and such bronzes exhibit
remarkable properties as reversible cathodes in lithium batteries 356
4.2.2. Intercalation of molecular ions. Alkylammonium ions, CnH2n+1 N+ (CH3)3, with n
ranging from 1 to 18, have been intercalated into the layered structure of V205 gels. X-ray
diffraction patterns exhibit a serie of 001 peaks typical of the turbostratic stacking of
the V205 ribbons 357
Figure 24 reports the variation of the basal spacing d as a function of the number n of
carbon atoms in the alkyl chain. The orientation of the alkyl chain within the interlayer
volume can be easily deduced from this variation. The angle ~ between the chain and the
layer plane is given by 1.27sin ~=Ad/An. The 1.27 factor corresponds to the projection of a
C-C bond onto the main axis of the alkyl chain. Three domains are clearly seen:
Domain I (e=0) corresponds to short alkyl chains (n<6). Alkylammonium ions are
JPSSC 18:4-D
308 J. Livage et al.
4(]
3C
20
10
dool (.~)
I ~90 °
.~- IT o I - " : / ~ ~ 5 3 •
7 - - j
, - /~- i:1 o . . . . .
I
i
I I I I I I i i I I "v 2 4 6 8 10 12 14 16 n a b
ooi
C
Fig.24. Variation of the basal distance d
of V205 .xH 20 gels as a function of n number
of carbon atoms of the alkyl chain of the
intercalated alkylammonium ions.
Fig.25. Position of alkylammonium ions
between the V205 layers as deduced from
figure 24 :(a) n<6, a=O °
(b) 6<n<12, 0°<=<90 ° ; (c) n>12, a=90 °
intercalated parallel to the layer planes in order to minimize the energy required to
separate the V205 layers (figure 25a).
- Domain III (==90 ° ) corresponds to long alkyl chains (n>12). Van der Waals interactions
between alkyl chains become predominant. Therefore, alkylammonium ions are aligned parallel
to one another in a direction perpendicular to the layer planes (figure 25c).
- Domain II (a=42 °) corresponds to an intermediate situation where both energy terms, layer
separation and Van der Waals interactions, are of the same order of magnitude. Alkyl chains
are still aligned parallel to one another, but they cannot stand perpendicular to the
layers (figure 25b).
Cobalticinium and ferricinium molecular ions have also been intercalated into V205
gels. In both cases, the basal distance increases up to about 13.2 ~ 358. The basic V205
ribbon structure is not modified upon intercalation. A noticeable improvement of the ID
stacking is even observed, especially with Co(C5H5); . The ~d=4.4 A increase of the basal
spacing suggests that cyclopentadienyl rings are perpendicular to the layers and somehow
inserted into the corrugated sheet structure of the ribbons.
4.2.3. Swelling in organic solvents. The first stages of the swelling process of V205 gels
in an aqueous solution can be described as the intercalation of one to several water
layers. The same process occurs when water is replaced by an organic solvent. The basal
spacing d between V205 ribbons increases by steps when the gel is dipped into a polar
organic solvent, while the internal 2D structure of the ribbons remains unchanged 179. Some
solvents (propylene carbonate) form a double-layer intercalate (d=21.5 A) while others
(DMSO) lead to a single layer compound (d=16.5 A). Some solvents however (DMF) do not
intercalate at all. The problem of reliable intercalation criteria is therefore opened.
This appears to be a rather difficult problem as many different interactions have to be
taken into account, namely solid-solid, solid-solvent and solvent-solvent interactions.
Neither dipole moments nor relative permittivities or even Gutman's donnor numbers allow
one to predict possible intercalation. The best parameter appears to be the Hildebrand
parameter 6 (square root of the cohesive energy density) which is related to the
Sol-Gel Chemistry of Transition Metal Oxides 309
vaporization energy and the molar volume of the solvent 359. No intercalation is usually
observed in V205 gels when 6 is smaller than 13 call/2cm "3/2.
In some cases however, a chemical reaction (proton or electron exchange) occurs
between the gel and the organic compound. A molecular ion is formed and intercalation
proceeds via an ion exchange process. Pyridine, benzidine and alkylamines for instance have
been intercalated into V205 .n H20 gels 360,361 Although some vanadium reduction occurs,
infra-red studies show that intercalation is mainly governed by a proton transfer, reaction
involving interlayer water molecules. This denotes the Bronsted acid character of these
gels. Protonation of organic bases leads to the formation of pyridinium, benzidinium or
alkylammonium ions. Intercalation of tetrathiofulvalene (TTF) has also been reported. A
black flocculate is obtained and the resulting material (TTFxV205 with x<l.8) is an ill-
organized, insoluble solid, that contains a large amount of water 362. The process appears
to be non reversible and can hardly be described as intercalation. A severe reduction of
vanadium ions by the organic molecules or solvent (ethanol) presumably occurs. The layered
structure of the gel is destroyed and the amount of water increases with the amount of
363 V(IV) ions as it was previously shown with other reducing reagents
5. PHYSICAL PROPERTIES AND APPLICATIONS OF TRANSITION METAL OXIDE GELS.
The sol-gel process is mainly used for making glasses, ceramics, films or fibers.
The gel state is then nothing more than an intermediate stage in the processing of these
materials. Drying and densification quickly follow the chemical synthesis of gels. Many
examples can be found in the literature in which transition metal oxide gels are used to
make ceramic powders 2,364 optical coatings 5 or fibers 4. Such applications will not be
described here. The discussion will rather be focused on the physical properties of gels or
xerogels before calcination in order to show that they can lead to new applications in the
field of materials science. Gels or xerogels are diphasic systems in which solvent
molecules (usually water) are trapped inside an oxide network. Such materials can be
considered to be water-oxide composites. Therefore they exhibit specific properties arising
from the intimate mixing of both phases. Transition metal ions often exhibit several
valence states giving rise to mixed valence compounds. Electronic properties due to a
hopping process within the solid phase can be observed. Water molecules are adsorbed at the
surface of the oxide particles. They can be more or less ionized, depending on the acidity
of the oxide, giving rise to H3 O÷ or OH" species. Ionic properties arising from ion
diffusion within the liquid phase can thus be expected. Both phases are involved in the
electrochemical properties of transition metal oxide gels. Electron diffusion occurs
through the solid phase and ion diffusion through the liquid phase. Because of the very
large interface between both phases, electron transfer at the oxide-water interface can be
greatly enhanced leading to specific photochemical properties.
5.1. Electronic properties
5.1.l. Small polaron hopping. A general condition for semieondueting behavior of transition
metal oxides is that the metal ions should be capable of existing in several valence
states, so that conduction can take place by electron transfer from low to high valence
states. A strong electron-phonon coupling is usually observed in transition metal oxides
leading to the formation of a so-called "small polaron" 365. The strong interaction between
the unpaired electron and the polar oxide network leads to a polarization of the lattice
310 J. Livage et al.
and a displacement of the oxygen ions around the low valence transition metal ion. When
these distortions are limited to the nearest neighbors, the unpaired electron becomes
trapped in its own potential well 366. A "small-polaron" is formed, characterized by its
binding energy Wp which is usually about 0.5 eV for most transition metal oxides 367. Small
polaron hopping between two neighboring sites occurs when both sites have the same
potential energy. This is achieved by lattice distortions and phonons must be involved in
the hopping process. Conduction then has the character of a thermally activated process in
368 The hopping rate however which the activation energy W h should be given by Wh=I/2 Wp .
depends on two factors :
- A phonon term corresponding to the probability for both sites to have the same potential
energy.
- An electronic term corresponding to the probability for the electron to tunnel from one
site to the other during this coincidence.
A detailed analysis of small polaron diffusion is rather difficult and can be found in many
review papers 368. A general formula for electrical conductivity in transition metal oxides
was proposed by Austin and Mott 366.
e 2 W a = w -- c(l-c) exp(-2oR)exp(-~) (13)
RkT
where :
- u is a phonon frequency related to the Debye temperature 8 by hv-k0.
- R is the distance between transition metal ions.
c is the ratio of ion concentration in the low valence state reported to the total
concentration of transition metal ions.
a is the rate of the electronic wave function decay, exp(-2~R) corresponds to the
tunnelling transfer.
W is the thermal activation energy of the hopping process.
One of the most striking features of the small polaron conductivity is that the
thermal activation energy W decreases with the temperature.
At high temperature (T>8/2), the small polaron hopping is activated by an optical
multiphonon process. The activation energy is given by W =Wh+i/2Wd, where W d corresponds
to a disorder term in the case of non-crystalline oxides 369
- As the temperature is lowered the phonon spectrum freezes out and the polaron term W h
drops continuously to zero, leading to a decrease in the observed activation energy W
below 8/2. A detailed analysis of the electrical conductivity variation in this temperature
range was proposed by Schnakenberg 370
- At very low temperature (T<0/4) an acoustical phonon assisted hopping takes place and the
activation energy becomes W - 1/2 W d .
5.1.2. Semiconducting V205 xerogels. The semiconducting properties of V205 layers deposited
from gels have been extensively studied because of their potential application as
antistatic coatings in the photographic industry 371,372,373. Such xerogels, when dried
near room temperature, still contain some water and care must be taken in order to separate
electronic and ionic contributions to the electrical conductivity 3?4 Purely electronic
conductivity can be observed when the xerogel is under vacuum or in the presence of a dry
atmosphere. The water content then corresponds to V205,0.5 H20. The electrical behavior
appears to be purely ohmic, both a.c. and d.c. conductivities are identical and no
transient regime is observed when a d.c. voltage is applied across the sample 375. The
room-temperature conductivity depends on the amount of reduced vanadium ions. It increases
quite fast with the V 4+ concentration 376 Some discrepancies are observed in the
Sol-Gel Chemistry of Transition Metal Oxides 311
literature concerning the conductivity of V205 layers deposited from gels. Conductivities
as high as a=0.1 Scm "I at 300K have been reported 371. It seems that this value somehow
depends on the way electrodes are deposited onto the sample. All results however do agree
with the very low mobility of the charge carriers, values ranging between 10 .5 and 10 .6
em2V'Is "I are currently reported 373 The temperature dependence of the d.c. conductivity,
plotted as log(aT) versus (T "I) is shown in figure 26. The non-linear variation, together
with the low mobility of the charge carriers, is typical of a small polaron hopping
process. Theoretical models suggested by Mott
the experimental results 371,373
hn ~T.101
- . 4
'7 E . .~
- . 7
O . , a U
366 or Schnakenberg 370 fit quite well with
i i i i. , , L i. 0~ T(K "1) 3 . 2 3.6 4.0 4 4 4.8 5.2 5,6 6 0 10 /
,HI .... ..... , . . . . .
0
ii N
[ \\
\\
I \ \ \
V (volts) 10 V H 20 VTH
Fig.26. Temperature dependence of the
electrical conductivity of V205
layers deposited from gels.
Fig.27. Intensity-Potential characteristic
showing the switching effect in a
V205,1.8 H20 xerogel.
The small-polaron hopping process can be either thermally or optically activated.
According to the theory, the optical activation energy Wop t should roughly correspond to
Wopt=4 Wt h 369 Intervalence transfer then usually corresponds to a broad absorption around
leV, i.e. in the red part of the optical spectrum. Therefore, most mixed valence compounds
exhibit a typical blue color. An optical absorption study of V205 gels was performed by J.
Bullot et al. 377 They found an optical gap of 2.2eV close to the gap of crystalline V205.
An Urbach tail was observed on the low energy side, whose slope increases with the amount
of V 4+ . Moreover, the absorption due to the optically induced polaron hopping was detected
in the near infra-red region. The absorption band maximum (0.geV) is close to the energy
predicted from conductivity data. It depends on the V 4+ concentration and suffers a
redshift when the amount of V 4+ increases.
A threshold switching process was observed in V205 layers deposited from gels
378 Two gold electrodes 0.i mm apart were evaporated in a coplanar geometry at the surface
of the layer. The device is formed by applying a high voltage of about IOOV between the
electrodes. After a few cycles, the device starts switching. A typical I-V characteristic
is shown in figure 27. The threshold voltage is around 25 V, the minimum holding current
close to 500 #A and the on/off ratio 400. These values depend on the way the device is
made. On/off ratio as high as 800 were obtained but these results are hardly reproducible.
Optical microscopy shows that some filaments grow between the electrodes when the forming
voltage is applied. These filaments correspond presumably to the formation of VO 2 and the
switching effect should be due to the metal-insulator transition of VO 2 around 60°C. This
could explain why switching is no longer observed above this temperature .
312 J. Livage et al.
5.2. Ionic properties
5.2.1. Particle hydrates. From a chemical stand point, transition metal oxides gels or
xerogels are hydrous oxides. They correspond to the general formula MOx.nH20 and can be
defined as "particle hydrates" according to the classification suggested by W.A. England et
al. 379. Following these authors, particle hydrates consist of charged particles separated
by an aqueous solution "S". The structure within a particle is that of the anhydrous oxide
and the particles are linked together to form agglomerates. The full coordination at the
surface of the particles is preserved by water molecules. Additional water molecules occupy
the inter-particle region to produce a connected, viscous liquid region through the
composite solid. A protonation equilibrium exists at the oxide-water interface. This makes
the liquid-like region acidic or basic depending on the nature of the oxide network. Acidic
dissociation is promoted by small metal atoms of high positive charge and basic
dissociation by large metal atoms of low charge. Particle hydrates exhibit some common
features such as good ion exchange properties or fast proton conduction 380. The liquid
content favours densifieation and particle hydrates can often be compressed into
transparent pellets by cold pressing.
5.2.2. Ion exchange. Inorganic ion-exchangers have been widely studied during the last
few decades. The rapid development in nuclear energy, hydrometallurgy, high-purity
materials, water purification, etc. has reinforced attempts to find new, highly selective
ion-exchanging materials, resistant to chemicals, temperature and radiation. Hydrous oxides
appear to be good candidates as ion-exchangers. They can compete with commercial organic
or natural inorganic (clays) products. Hydrous oxides of polyvalent metals behave as cation
or anion exchangers. Their dissociation can be represented schematically as follows 379 .
M-OH = M + + OH" (14)
M-OH = M-O" + H + (15)
Scheme (14) takes place in acid solutions where the hydrous oxide acts as an anionic ion
exchanger. Scheme (15) corresponds to a cationic ion exchanger in a basic medium.
Dissociation, near the isoelectric point of amphoteric oxides, such as ZrO2, TiO 2 or ThO 2
occurs in both ways which enables simultaneous development of both ion exchange processes.
The resultant charge on the particles may be switched reversibly from positive to negative
by changing pH. There is a characteristic pH value for any particular oxide at which the
overall charge on the surface is zero. This pH value is called the zero point of charge
(ZPC) and is readily determined by potentiometric titration. The shape of the pH titration
curve, however, depends on the preparation of the hydrous oxide.
The general formula for a tetravalent M(IV) hydrous oxide having a mean
oxygen/metal ratio R per particle can be written as follows 379 .
MOz.nH20 - [MOa(OH)b(H20)R. (a+b) ] 4 . ( 2 a + b ) + S
where the brackets correspond to the molar formula of the solid particle and S is the
aqueous region of the material. This region is basic, S = Sb, if (2a+b)<4 and acidic; S =
SA, if (2a+b)>4, where :
S A - (2a+b-4)H3 O+ + [(2+n-R)-(2a+b-4)]H20
S B - (4-2a-b)OH" + [(2+n-R)-(4-2a-b)]H20
The two equilibria may be represented by acidic and basic dissociation constants, K A and
~. Cation exchange corresponds to the replacement of the H3 O÷ ions in S by M ÷ ions coming
from an external solution S'. This equilibrium is characterized by a relative formation
constant ~. Cation exchange is then favored by a high pH, a large concentration of M + ions
or a large value of KAK N . For hydrous oxides, K M apparently increases with decreasing the
Sol-Gel Chemistry of Transition Metal Oxides 313
size of M ÷. Therefore, cation exchange of alkali metals decreases in the order
Cs+>Rb+>K+>Na+>Li+ "
The exchange properties of silica gels are already well known. Transition metal
oxide gels find applications in isolation, removal and treatment of radioactive materials
and purification of water. Among the main characteristics of these compounds are their
stability in strong radiation fields and retention of ion exchange properties above 100°C.
5.2.3 Fast proton conduction. The development of solid state proton conductors has received
stimulus from the practical side due to possible applications in low-temperature fuel-
cells, storage batteries and electrochromic devices. Hydrous oxides have been shown to be
rather good proton conductors and therefore proton diffusion has been extensively studied
during the last decade, mainly by a.c. conductivity measurements and 1H NMR.
The temperature dependence of the conductivity of hydrous oxides usually exhibits
a kink below 0°C, corresponding to the freezing of included water. Conductivity then
decreases faster and higher activation energies are observed. Above 60°C, some water
molecules leave the network and the conductivity drops. An analysis of the literature shows
that proton conductivity usually ranges between i0 "6 Scm "I and 10 .4 Scm "I with activation
energies around 0.30 eV. These values do not depend strongly on the nature of the oxide and
can be accounted for by proton diffusion through water molecules adsorbed at the surface of
small colloidal particles. Therefore, conductivity increases quite fast with water content
of the hydrous oxide MOx.nH20 , i.e. as a function of the water pressure above the sample.
Quite different values are found for hydroxides such as AI(OH)3.H20 (~=6.10 "8 Scm "~) or
framework hydrates such as antimonic acid Sb205,nH20 (a=7.5.10 "3 Scm "I) 381
A high proton conductivity necessitates a large concentration of mobile protons as
well as a high proton mobility. The first factor is optimised in highly acidic oxides which
contain high-valent cations. Proton motion in aqueous solutions usually occurs via the
classical drift of H3 O+ or via the tunnelling of a proton through an hydrogen bond. This
last process has a much higher probability in the case of solid hydrous oxides. However,
two conduction pathways are available. One is entirely within the interconnected liquid
phase between the particles, while the other is via the surface of the isolated particles.
It is not clear at this point whether proton diffusion occurs through the liquid or at the
surface. Nevertheless, good proton mobility should be expected in those hydrous oxides
which have a large water content and a high oxidation state.
Proton NMR relaxation times have been measured for several hydrous oxides 382,383
According to the authors, protons are found in three different environments :
i) at the surface as hydroxyl groups.
ii) in acid solution in micropores (diameter < i00 A).
iii) in acid solution in macropores (diameter > 103 A).
Pores result from the agglomeration of oxide particles. The solution in the
macropores should be almost liquid-like while that in the micropores will be more
constrained and viscous. Proton conduction involves chemical exchange between environments
of various viscosity. It must be pointed out however, that there is no simple link between
conductivity and NM_R data.
5.2.4 Mixed conduction in V205 gels. Hydrous oxides appear to be good proton conductors
379. When prepared from gels, they are easy to compress into pellets or to deposit as thin
layers. Therefore they would be very good candidates for solid-state ionic devices.
However, it is not clear whether the measured conductivity arises from the bulk or from
water adsorbed at the surface of the sample. Proton conductivity must then be studied as a
314 J. Livage et al.
function of the water stoichiometry and related to the water adsorption isotherms 384
Figure 28 shows the dependence of the water content of a V205 layer deposited from gels as
a function of the water pressure above the sample. As shown by X-ray diffraction 180, the
water content increases by steps corresponding to the intercalation of one to several water
layers between the ribbon-like colloidal particles. For a relative humidity larger than 80%
a continuous swelling is observed that can lead, if enough water is added, to a colloidal
solution.
Electronic conductivity predominates at low water pressures. The water content of the
xerogel corresponds to VZ05, 0.5 H20 and the basal distance to d-8.7 A. This means that no
water remains intercalated between the ribbon-like vanadium oxide particles. The thermal
activation energy for conductivity decreases with the temperature. Such a behavior is
typical of small polaron hopping between V 4÷ and V 5+ ions in the oxide network 373
c / /
E
½ %
z
I I I J. 0 . 2 5 0.5 0 .75
relat ive I~umidity P/P$ (H20)
Fig.28. Water adsorption isotherm
of a V205.nil20 xerogel.
10
i_
0 . 2 5 0.5 0 . 7 5 I r
re lat ive h u m i d i t y P/Ps (H20)
Fig.29. Variation of the a.c. conductivity of a
V205,nH20 gel as a function of the relative
humidity of the surrounding atmosphere.
- Conductivity increases quite quickly with the water pressure above the sample (figure 29)
177,385. A high a.c. conductivity is observed in ambient conditions (o=10 .2 Scm "I at 300K).
The water content of the xerogel is V205,1.6 H20 and the basal distance d=ll.5 A
corresponds to the intercalation of one water layer. The log(aT) vs f(T "I ) curve shows two
Arrhenius behaviors with a kink around -10°C, typical of proton conductivity in particle
hydrates 177. A dielectric study of this xerogel in a broad frequency range (105-1010Hz)
suggests three different behaviors for the intercalated water 386 .
- A low frequency effect due to proton diffusion.
- Two dielectric relaxations due to water molecules which are strongly or weakly bound to
the ribbons.
- A dielectric relaxation which should be due to a fast rotation of H30 + ions.
It has to be pointed out that both curves in figure 28 and figure 29 are quite similar. A
plateau is observed around ambient conditions that corresponds to the intercalation of the
first water layer. This gives rise to the sigmoidal shape of the conductivity isotherm that
looks like a type II Brunauer isotherm. Such an isotherm is typical of a multilayer
adsorption process in which the first layer is much more strongly bound than the following
ones.
This study was extended to framework hydrates (HUO 2PO 4 ,nH20) and particle hydrates
(Ce(HPO4)2.nH20) . All these compounds have a bidimensional character as a common feature.
Conductivity occurs either in a layer or on a surface. The main difference then has to be
drawn between the bonding of water molecules responsible for proton conduction. The most
strongly bound water molecules give a well ordered lattice through which proton diffusion
Sol-Gel Chemistry of Transition Metal Oxides 315
will be solid-like. As soon as the water molecules are less tightly bound to the solid
network, they become disordered and give rise to a liquid like behavior 384
The example of V205 layers shows that the electrical conductivity of a gel, or
xerogel, cannot be fully described unless both the solid and the liquid phases are taken
into account. Mixed conduction occurs in V205 gels. Electron hopping is observed at low
water content while proton diffusion predominates as soon as the swelling process begins.
This could account for some discrepancies in the literature and explains one of the main
advantages of V205 antistatic coatings that keep their electrical properties under both dry
or humid atmospheres 387
5.3 Electrochemical properties
5.3.1 Electrochromic display devices. Electrochromie layers based on amorphous WO 3 thin
films have been extensively studied during the last decade 388 . Such films can exhibit two
stable states, one is transparent while the other one is blue. Reversible coloration and
bleaching can be easily obtained in an electrochemical cell. A double injection process is
observed that can be described as follows :
WO 3 + xe" + xM + = MxWO 3 (M + = H + , Li + )
Electrochromic WO 3 layers have been used to make display devices 389, rear-view mirrors 390
or smart windows 391
Amorphous WO 3 thin films are usually deposited by vacuum evaporation or
sputtering, however sol-gel derived eleetrochromic layers have also been made recently 392
Several techniques for the preparation of WO 3 films from solutions have been published
during the last few years. Amorphous WO 3 .nH20 can be formed upon hydrolysis of metal-
organic precursors such as tungsten hexaphenoxide 393 or tungsten ethoxide 302,394
Tungstic acid colloidal solutions have been obtained by ion exchange from an aqueous
solution of sodium tungstate 183 Peroxotungstic acid coated films were also investigated
for electrochromic applications 395 They are obtained by dissolution of a freshly
precipitated tungstic acid in an hydrogen peroxide solution. More recently, tungsten alkoxo
chlorides were obtained upon dissolution of WOCI 4 into an alcohol 20 Stable solutions are
obtained which can be easily deposited and hydrolyzed by dip-coating 20. Crystalline
WO3.nH20 (n=l,2) layers have recently been deposited from gels and colloidal solutions.
They appear to be strongly anisotropic as shown by X-ray diffraction and infrared dichroism
184 This lamellar structure of the film favors the intercalation of guest species.
W03.1~20 films chemically intercalate long-chain alkylammonium and electrochemically
intercalate Li + ions. They can therefore be used for making display devices.
Other transition metal oxides also exhibit electrochromic properties and can be
deposited via the sol-gel process. TiO 2 films have been made from Ti(OBun)4. They turn
from white to blue reversibly. V205 films deposited from a polyvanadic acid sol turn from
yellow to green upon an applied voltage of ± 1.5V. They have a memory effect of more than
20 hours 396
The sol-gel technique offers many advantages for making electrochromic devices :
Thin layers can be easily deposited under ambient conditions by dip-coating, spin-coating
or spraying. Large surfaces can be coated at low cost 5
According to the literature, eleetrochromic characteristics of WO 3 films are very
sensitive to the method of preparation It has been reported that sputtered films are
easier to color and bleach than evaporated ones. This is probably due to the smaller water
content of the latter. Water has to be incorporated into WO 3 films in order to obtain
faster coloration 397 Sol-gel deposited films always contain some water making
316 J. Livage et al.
electrochemical ion diffusion easier. Moreover, evaporated amorphous WO 3 films always
adsorb water when placed in an ambient atmosphere. It has been shown that such hydrated
layers could be described as xerogels of hydrated tungsten hydroxy-oxides. The high
electrochromic reversibility and the short response time of these layers was attributed to
398 their porous, spongy structure
Multi-layer all-gel devices have recently been made in which all active layers
(electrochromic and electrolyte) are deposited from gels. Such cells exhibit a rather long
response time, a good cycling behavior, and a very long memory 321. They open the way for
new micro-ionic devices.
5.3.2 Reversible cathodes for lithium batteries. Lithium batteries, based on the
reversible insertion of Li + ions into a host lattice have been extensively studied 399
Most work was focused on transition metal chalcogenides that exhibit layered structures.
Vanadium oxides (V205, V6012... ) also appear to be good candidates for such applications.
They offer high stoichiometric energy densities : values up to 600 Wh/kg have been reported
for V205 352 However, as already mentioned, this oxide behaves as a 3D framework rather
than a Van der Waals host 352. It is hoped that better reversibility can be achieved with
amorphous oxides for which structural changes should be limited. Therefore vanadate glasses
have been suggested as reversible cathodes for lithium batteries 400,401. Amorphous V205
made by splat-cooling also exhibits interesting properties as a reversible cathode 402
Electrochemical experiments performed with LiAsF 6 in a cyclic ether as an electrolyte show
that up to 1.8 Li + ions per V205 can be inserted reversibly between 3.5 V and 2 V (vs.
Li+/Li). Contrary to crystalline V205, the open circuit voltage continuously decreases with
the amount of inserted Li + , suggesting that no phase transition occurs.
Reversible electrochemical intercalation of Li + ions into V2Os.I.6H20 xerogels was
reported a few years ago 403 Electrochemical experiments were made using a triple
electrode device and a LiCiO4-propylene carbonate solution as an electrolyte. Li + ions are
intercalated when a negative voltage is applied to the V205 electrode. Two LixV2Oscompounds
are formed upon reduction corresponding to x-l.l and x-l.6. The process appears to be
reversible and Li + ions are removed upon electrochemical oxidation. An X-ray study of the
layer shows that the well-ordered stacking of the V205 ribbons is destroyed upon insertion,
giving rise to a disordered material. However, the I-D order is restored during the
oxidation cycle. Another electrochemical study was published recently, using a V205 xerogel
as the cathode and metallic lithium as the negative electrode 404. This xerogel was
partially dehydrated at 230°C. The remaining water appears to be strongly bound, as no
adverse effect on the lithium counter electrode was detected. A polymeric electrolyte was
used in order to avoid swelling by the solvent. The discharge curve appears to be nearly
linear between 3.5-2.2V (vs Li), with none of the inflections and plateaux characteristic
of crystalline V205 . The average cycling efficiency during the first 46 cycles is 99.7%.
However this good performance is accompanied by a modest energy density : 420 Wh/kg for
insertion of i.i Li/V205 . Gels are quite suitable for making layers and should be quite
useful in the processing of thin film micro-batteries.
5.4. Interfaclal properties
5.4.l.Photochemistry of colloidal semiconductors. When a semiconductor is brought into
contact with an aqueous solution, an electron transfer occurs at the oxide-water interface
until the electrochemical potentials of both phases (Fermi level and mean redox potential)
become equal. As a result of this transfer, the oxide surface becomes charged with respect
Sol-Gel Chemistry of Transition Metal Oxides 317
to the solution. This charge is actually distributed over a region the thickness of which
depends on the doping level of the semiconductor. For a weakly doped oxide, the space
charge layer is typically more than i000 A. The electric field created by the electron
transfer can be described by the typical band bending model of the semiconductor-
electrolyte junction. Electron-hole pairs can be created when photons of energy larger than
the band gap (h~>Eg) are adsorbed at the surface of the oxide. The electrical field of the
junction provides electron-hole separation within the space charge region. For n-type
semiconductors, holes move toward the surface while electrons move toward the bulk. The
opposite is observed for p-type semiconductors. Redox reactions are thus expected at the
oxide-water interface 405,406
The photochemical properties of oxide suspensions have already been widely
studied. However, such studies are best carried out using colloidal particles with a
diameter smaller than i0 rim. Such solutions are optically transparent and offer a larger
oxide-water interface 406 Moreover, the size of colloidal particles may be smaller than
the space charge thickness so that the semiconductor-electrolyte junction model cannot be
applied. Charge separation at the junction is no longer effective and charge carrier
mobility should be described by usual diffusion theory 407. When the particles are small,
both charge carriers can reach the surface. Light-induced charge separation and redox
reactions can be coupled without intervention of bulk diffusion. Thus a single colloidal
semiconductor particle can be treated with appropriate catalysts so that different regions
of the same particle function either as anodes or cathodes 405
The photochemical properties of transition metal oxide colloids Fe203 408 WO 3
409,410 or MnO 2 411 have been widely studied, although most of the work has been performed
on TiO 2 in an aqueous or organic medium. This oxide can lead to a large variety of
photochemical reactions such as water splitting 412, photocatalysis 413 or photodegradation
of pollutants 414. Transparent TiO 2 sols are usually produced via hydrolysis of TiCI 4 in
water or hydrolysis of Ti(OPri)4 in acid aqueous solutions 415 Particles a few i00 A in
diameter are obtained which exhibit good photochemical characteristics. However these sols
only absorb U.V. light and are not stable above pH 3. Chemical modification of the alkoxide
with a strong chelating ligand such as acetylacetone was reported to give transparent sols
that remain stable up to pH i0. Moreover, charge transfer from the (acac) ligand to the Ti
atom gives rise to a strong absorption of visible light that improves the photochemical
efficiency of TiO 2 sols. These modified colloids are stronger reducing agents than other
TiO 2 colloids 416
A photoelectrochemical cell consists of two electrodes immersed into an aqueous
electrolyte and connected electrically by a wire. The main electrode is a semiconductor
with one face in contact with the electrolyte and the other face connected to the shorting
wire by an ohmic contact. The majority carriers move toward the bulk of the electrode where
they are collected by the wire and transferred to the counter electrode, usually a metal
that does not react chemically with the electrolyte 41T. In such a cell, the potential
corresponding to zero excess charge in the semiconductor (i.e. the point of zero charge) is
called the flat band potential Vfb. This is a very important parameter that gives an
estimate of the reducing power of the electrons generated upon illumination of the n-type
semiconductor 407 that behaves as a photoanode. Very few papers actually report the sol-gel
processing of photoelectrodes despite the fact that this process can offer many advantages:
- Oxide layers of large area can be easily deposited onto a metallic substrate and sintered
at relatively low temperature.
- Metastable crystalline phases can be obtained such as anatase in the case of TiO 2 instead
of rutile 418
318 J. Livage et al.
- Oxides such as TiO 2 actually have a wide band gap (Eg=3.2eV). Therefore they are of
little interest for photoelectrolysis unless means are found to enhance both their
electrical conductivity and light response into the visible region. This can be obtained by
doping with impurities such as Cr 3+ or AI 3+ . Doping can be made at a molecular level by
mixing the appropriate solutions of molecular precursors which yields highly homogeneously
doped photoanodes 419,420
- Photoelectrochemical and photochemical 421,422 experiments permit such measurements as
flat band potential values that can give useful information to characterize the water oxide
(colloid or gel) interface. Moreover, the d.C. photocurrent variation as a function of the
incident wave length gives an access to the band profile of gels or xerogels.
5.4.2. Electron transfer at the Fe304 colloids interface. Interfacial properties of metal
oxides do not usually take into account the bulk of the solid network. Acid peptization of
metal oxides is usually described as arising from the protonation of surface M-OH groups
leading to positively charged particles. Such a description may become no longer valid when
metal ions exhibit several valence states. Electron hopping occurs in mixed valence oxides
and the whole oxide network may be involved in surface charge modifications. Such a process
can become especially important for colloidal particles having a large surface/volume
ratio. As an example, redox reactions of Fe304 colloids have been shown to involve the
whole spinel lattice and not only the surface of the particle. Such behavior results from
electron delocalization in mixed valence Fe304 together with close structural similarity
between the reduced (Fe304) and oxidized (7-Fe203) forms.
At room temperature Fe304 exhibits a cubic inverse spinel structure. Octahedral
sites are equally occupied by Fe 2+ and Fe 3+ ions while tetrahedral sites are occupied by
Fe 3+ ions only. It is a mixed valence compound where electron hopping leads to
delocalization over the octahedral sites. As a consequence, octahedrally coordinated iron
ions exhibit an average charge of +2.5. The molecular formula may thus be written as
Fe3+[Fe~'5+]O4 where brackets label octahedral sites. 7-Fe203 is also a spinel oxide with
almost the same lattice parameter, but it contains no Fe 2+ ions and octahedral sites are
iron deficient. It can be described as Fe3+[Fe~3DI/3]04 where D corresponds to iron
vacancies.
It has been shown that oxidation of Fe304 into 7-Fe203 can occur under anaerobic
conditions. One of the possible pathways for the process is outlined in figure 30 for Fe304
colloids in a weakly acidic medium (pH=2).In this case all Fe 2÷ ions are released from the
spinel framework while protons are consumed 118. Fe2+ desorption proceeds without
significant structural changes and the overall stoichiometry is found to be 2 H + consumed
per Fe 2+ ion released in solution. Surface protonation of Fe-OH hydroxyl groups appears to
be the driving force of the process. The potential energy at protonated octahedral sites is
lowered leading to electron localization and formation of -O-Fe-OH~ species (figure 30b).
As O-Fe 2+ bonds are weakened, hydrolysis occurs, leading to soluble [Fe(OH2)6] ++ with
formation of iron vacancies and bare oxygen at the surface (figure 30c). Simultaneously,
electron localization at the surface leaves unpaired Fe 3÷ ions in the core of the particle
which are acceptor states for mobile electrons. As Fe 2+ ions are released in solution, an
electron flux occurs towards the surface leaving a positively charged core. Charge
neutrality is locally maintained by outward migration of iron ions towards the surface
leaving vacancies within the lattice (figure 30d). The surface is thus progressively
renewed: new superficial iron ions are coordinated by water molecules while bared oxygen
adsorb protons from the solution (figure 30c). Peptization occurs as soon as
Fe(II)/Fe(III)s0.15, leading to a stable cationic sol while at the end of the process the
Sol-Gel Chemistry of Transition Metal Oxides 319
colloid is converted into ?-Fe203 . It must be pointed out that this transformation is
partially reversible. Once Fe 2÷ have been removed from Fe304 colloids, they can be
readsorbed just by raising the pH up to 5 or 6, but the particle does not transform into
Fe304 anymore. After adsorption of Fe 2÷ at the surface electron transfers towards
octahedral Fe 3+ and delocalization over the v-Fez03 lattice occurs but no migration of iron
ions inside the particle is observed. This process leads to an epitaxial growth of a Fe304
layer and adsorption stops as soon as all octahedral sites within the core have an average
charge of +2.5. As no significant iron diffusion occurs, it is supposed that simultaneous
proton diffusion towards the core is involved in order to maintain charge balance within
the lattice 116
( ~ ( ~ 3 ~ F e 2 5 *
H H H H H H IH H H Y H oFe 3÷ ...... ; 6 d d 6 6 ....... E ...... 6 H ~ H 6 6H"E)'H 6 ' ' ' ~ "Fe2*
0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 O ~ O u O ~ O ¢ O P O ~ O uFe vacancy 0 ¢ 0 ~ 0 ¢ 0 ~ 0 e 0 0 / ( 0 ¢ 0 ~ 0 a~"O @ 0 (~[Fe (OH2)612÷
OeOeO~OeO®OeO OoOeOeOo/OeOeO
= H ;H H I H H IH H: H H H H H H H ' - ' , i , , D ..... 6 6 6 6 ' = ........ 6 6,6 6 6,6--
o® o /O .6 . , 6 .o .6 .6 .oTo} oAo o o I ooo, ,o, ,ooo, ,o •
Od 0 ~ 0 cO-'6 O ~0 • 0 ODO®O ° O n O ~ O ~ O
hopping I electrons
Fig.30. Schematic process of the transformation Fe304 ~ 7-Fe2O 3 in weakly acidic medium.
(A) Configuration of the octahedral sublattice in Fe304.
(B) Protonation and electron localization at surface sites.
(C) Desorption of Fe 2÷ and migration of iron towards the surface.
(D) Fe 2÷ content has decreased and vacancies have appeared.
This transformation Fe304 ----+ 7-Fe203 is also observed under various conditions
(aerobic oxidation, Fe 3+ adsorption, etc...) 116"118. Analysis of this reaction reveals the
same electronic process: electron transfer through the interface relayed by electron
transfer within the particle. The intrinsic structural transformation is likely to be the
same but adsorption phenomena that induce electron transfer at the interface and outward
conditions that rule the behavior of the superficial Fe 2÷ may be quite different. The
fundamental role of electron delocalization in surface phenomena is nicely corroborated by
the fact that replacing Fe 2+ by another divalent cation such as Co 2÷ prevents electron
delocalization. In agreement with this result, surface hydrolysis of Fe2CoO 4 is strongly
inhibited in weak acid medium, and Co 2÷ adsorption onto 7-Fe203 is also very limited. This
explains the outstanding behavior of spinel iron oxide colloids which may be used as
colloidal electron exchanger in aqueous solutions.
6. MONOGRAPH
The present monograph provides a brief review of the published literature on
transition metal oxide sols and gels. The main elements are classified according to their
atomic number. Multicomponent systems are discussed separately at the end of the monograph.
320 J. Livage et al.
This review mainly points out the nature of the precursors, the experimental procedure and
the main applications of the resulting materials.
6.1. Transition metal oxide gels
6.1.1. Titanium oxide. TiO 2 gels have been known for a long time. They can be made by
dissolving sodium titanate in concentrated hydrochloric acid, then adding a weak base such
as ~C03, (NH4)2CO 3 or Na2CO 3 in order to avoid high pH gradients 124,125,126 TiO 2 sols
can be easily obtained through thermohydrolysis of TiCI 4 or TiO(N03) 2 under acidic
conditions 60,423 The colloidal particles are crystalline and have anatase or rutile
structure depending on the pH and the nature of the counter-ions 60,423 Some authors have
studied the parameters which influence gel formation while others have focused their
attention on processing in order to obtain fibers, coatings or monodispersed powders.
Sol and gel formation : Most recent studies are devoted to metal-organic routes using
Ti(OR) 4 alkoxides precursors. Monolithic TiO 2 gels can be synthesized from Ti(OR) 4 (R= Et,
Bu n , Pr I , Pr n , Bu s ) using substoichiometric hydrolysis ratios (l<h<4) and inorganic acid
catalysts (HCI, HNO3) 244,277. TiO2.based gels or colloids can also be obtained after a
chemical modification of titanium alkoxides 20 . This modification is performed mainly with
acetic acid 266,309,420, acetylacetone 266,309 or hydrogen peroxide 310. A good review of
gel synthesis using inorganic precursors was published by Woodhead 424
- TiO 2 fibers : TiO 2 sols or gels allow fibers to be drawn when viscosity is carefuly
controlled. Spinnable sols can be made through acid hydrolysis (HCI) of Ti(OPri)4 in
ethanol using substoichiometric hydrolysis ratio 276 Chemical modification with
acetylacetone offers an alternative route. The modified precursor Ti(OPri)2(acac)2 is
hydrolyzed in ethanol in the presence of acidic (HCI) or basic (NH4OH) catalysts leading to
sols or transparent monoliths 425. Fibers can also be obtained by unidirectional freezing
of a gel made through partial neutralization of TiCI 4 with KOH followed by dialysis 426
- TiO 2 coatings : Coatings on various metals have been made by dip coating in various
inorganic sols. Dispersable sols can be obtained using phase-transfer or extraction
techniques 427. Membranes with controlled porosity for ultrafiltration have been obtained
through hydrolysis of Ti(OPri)4 or Ti(OBun)4 and peptization by HCI or HNO 3 in the presence
of cellulose 428,429. Porosity appears to depend mainly on the firing temperature.
- Powders for ceramics : Monodispersed submicronic TiO 2 powders can be obtained by using
either inorganic or organic precursors. The inorganic route involves thermohydrolysis of
TiOS04 430,431 or TiCI 4 in the presence of Na2SO 4 121 In both cases monodispersed spheres
about 0.4 ~m in diameter are obtained. With alkoxides precursors, two main routes are
available :
i) Controlled precipitation of Ti(OEt) 4 in EtOH with an excess of water 240,241,432,433
These powders can easily be doped with Nb(OEt) 5 and Ta(OEt) 5 434 Hydroxypropyl cellulose
can be used in order to improve the monodispersion 435
ii) Hydrolysis of Ti(OEt) 4 or Ti(OPri)4 aerosols leads to monodispersed spheres whose
diameter can be varied from 0.06 to 0.6 ~m 122,243,436
Other TiO2-based powders for ceramics can be obtained either by precipitation of
Ti(OPri)4 in iprOH 242,437,438 and Ti(OBui)4 in iPrOH 436 or by spray techniques using
Ti(OR)4/ROH mixtures (R - Et, Pr i , Bu n ) 261. This leads to dense TiO 2 ceramics when heated
around 900°C. TiO2-based anionic exchangers have been synthesized through acid hydrolysis
of Ti(OR) 4 439 Spherical TiO 2 powders with diameters in the range of 1-2000 ~m can also be
obtained from gels made from inorganic 440,441 or organic precursors 441
Sol-Gel Chemistry of Transition Metal Oxides 321
6.1.2. Vanadium pentoxide. V205 gels have been known for a long time and can be
synthesised by different routes.
- Acidification of sodium or ammonium metavanadate solutions by hydrochloric or nitric acid
followed by washing or dialysis 174
- Acidification of sodium or ammonium metavanadate solutions with a proton exchange resin 174-176
Dissolution of amorphous V205 prepared by splat cooling into water 442-444
Pouring the molten V205 oxide directly into water 445
- Hydrolysis of vanadium oxoalkoxides VO(OR)3 (R = Et, Pr i , Pr n , Bu n , Am t) in the presence
of excess water 446,447
The structure and properties of V205 gels have been reviewed recently 339,448
6.1.3. Chromium oxide. Monodispersed sols of hydrous chromic oxide have been synthetised
by thermohydrolysis of various Cr(III) salts (CrCI3, Cr(NO3)3, KCr(S04)2) in the presence
of sulphate or phosphate ions 78
Monolithic green or blue-grey hydrous chromic oxide gels are easily formed when
Cr(IIl) salts (CrCI 3, Cr(N03) 3, Cr2(SO4)3, Cr(OOCCH3)3) are treated by an aqueous basic
solution of NH40H or KOH with an excess of acetate ions 70,71,74,76 The structure of these
gels was studied by EXAFS, Infra-Red spectroscopy, TEM and magnetic measurements 74,76
Catalysis is one of the main applications of these gels 449
Hydrosols can also be obtained via hydrolysis and condensation of CrCI3.4EtOH ,
CrCI3.3EtOH, Cr(OEt)3.EtOH and Cr(OEt)3 in ethanol 450
6.1.4. Manganese oxides. Pure hydrosols of Mn(OH)2 can be obtained via hydrolysis and
condensation of Mn(OEt)2 in ethanol 450
Hydrous MnO 2 sols and colloids are readily obtained through the reduction of KMnO 4
with reducing agents such as As(OH)3130 , Na2S204131, Mn2+ 132,411,451, NH~ 133 or glucose
134. Transparent sols of manganese (IV) oxides and manganese III oxides can also be
prepared by 7-irradiation of KMnO 4 solutions 411 No structural studies have been performed
on these sols.
6.1.5. Hydrous ferric oxide. Monodispersed hydrous ferric oxide sols can be obtained
through controlled thermolysis of Fe(III) salts (chloride, nitrate, sulphate and
perchlorate) 108 Chlorides first lead to monodispersed acicular ~-FeOOH which, upon
further aging, give rise to monodispersed ~-Fe203 spheres. Monodispersed ellipsoidal ~-
Fe203 particles are directly formed with nitrates or perchlorates 108 If FeCI 3 is aged in
a water/ethanol mixture, fl-FeOOH is more rapidly formed and leads to monodispersed cubic ~-
Fe203 particles, 109 while with triethanolamine monodispersed ~-Fe203 discs are obtained
110. In the presence of a reducing agent such as hydrogen peroxide or hydrazine, the
Fe2+/Fe 3+ ratio can be adjusted until the formation of monodispersed Fe304 sols 110 occurs.
Finally, if Fe2(S04) 3 is used as a Fe 3+ precursor, monodispersed basic salts are obtained :
Fe3(SO4)2(OH)5.2H20 and Fe4(SO4)(OH)I 0 105. The formation of these hydrous iron oxide sols
and the aggregation process that occurs upon aging have been extensively studied 90-97 7-
Fe20 B sols can be obtained through peptization and oxidation of precipitates of hydrous
Fe304 using weakly polarizing bases such as tetramethyl ammonium hydroxide or a strong acid
such as perchloric acid 113,114 Pure hydrosols of hydrous ferric oxide can also be
obtained through hydrolysis and condensation of Fe(OEt)3 in ethanol 450
Gelatinous precipitates of hydrous ferric oxide can be obtained using a large
variety of techniques 107: dialysis, hydrolysis of inorganic precursors through dilution,
322 J. Livage et al.
ionic exchange, phase transfer extraction with long chain organic amines, neutralization
with a weak base such as NaHCO 3 100, peptization of a precipitate 452 or decomposition of
ferrous oxalate by hydrogen peroxide 120
6.i.6. Cobalt nickel and copper oxides. Monodispersed cubic Co304 particles have been
synthesized by aging CoOOH precipitated from cobalt acetate 65. Pure hydrous cobalt oxide
hydrosols are obtained through hydrolysis and condensation of Co(OEt) 3 in ethanol 450
Ni(OH)2 and Co(OH) 2 gels can be synthetized upon dialysis of nickel or cobalt
tartrate precipates 63 Green Ni(OH)2 gels can also be obtained through neutralization of
nickel(II) acetate dissolved in glycerol with alcoholic KOH 64
Monodispersed Cu20 sols are formed upon ageing copper (If) tartrate in the presence
of glucose 69 Ellipsoidal CuO or Cu(OH)2 particles can be obtained upon ageing copper(II)
nitrates or sulphates 68
Sky-blue copper(II) hydroxide gels can be obtained through neutralization of
copper acetate with ammonia in the presence of a small amount of sulfate ions 66,6z or
through neutralization of CuCI 2 with NaOH 58
6.1.7. Hydrous yttrium oxide. Sols and gels can easily be obtained from yttrium nitrate by
ion exchange techniques 453 Structural characterization of such sols has been done by
EXAFS, SAXS, light scattering and TEM 454. Peptization of yttrium hydroxide precipitates
also leads to colloidal solutions 455
6.1.8. Zirconium oxide. Monolithic ZrO 2 gels can be synthesized from Zr(OR) 4 (R = Et, Pr I ,
Pr n , Bu n) using substoichiometric hydrolysis ratios (l<h<4) and inorganic acid catalysts
(HCl, HN03) 263. Stabilization of Zr(oPrn)4 via chemical modification 20 can be performed
with acetic acid 263, acetylacetone 291,305 or hydrogen peroxide 305. Using different
solvents also leads to monolithic gels upon mild hydrolysis of Zr(oPrn)4 272 or Zr(OBun)4
456. ZrO2 gels obtained from inorganic precursors were reviewed by Woodhead 424. Structural
studies have been performed on amorphous ZrO 2 gels 129,441
ZrO 2 fibers : Two main methods have been used to get ZrO 2 fibers :
- Extrusion and calcination of zirconium acetate 457
- Unidirectional freezing of aqueous solutions made from ZrOCI 2 426,458
ZrO 2 coatings: They have been made mainly by dip-coating from colloidal solutions using
inorganic precursors 42? or alkoxides 429,459,460,461. Chemical modification of Zr(oPrn)4
by acetic acid 429,459,460, acac or etac 461 and ethylene glycol 461 allows a better
control of the viscosity. The dip-coating process can thus be easily optimized.
Powders for ceramics : Thermohydrolysis is the cheapest way to obtain monodispersed ZrO 2
powders from inorganic precursors such as ZrOCI2, ZrO(NO3)2, ZrCI 4 or ZrO(S04)
455,462,463,464 The formation of monodispersed ZrO 2 sols was followed by TEM 465
Controlled precipitation of zirconium alkoxides, Zr(OPri)4 or Zr(oPrn)4 in EtOH
279,364,433,466, allows the synthesis of submieronie monodispersed ZrO 2 based powders.
6.1.9. Niobium and tantalum pentoxldes. Monodispersed Ta205 powders can be synthesized
through controlled precipitation of Ta(OEt)5 in an ethanol/butanol-i (1:4) mixture with an
excess of water (h=3-10) 280
Ta205 sols can be used to make storage capacitor dielectrics for microelectronies
by hydrolysis-condensation of Ta(OEt)5 in ethanol or toluene with an acid catalyst such as
HCI or CH3COOH 232,467. Thin films 1750 A thick were obtained from such sols by a spin
coating technique 467
Sol-Gel Chemistry of Transition Metal Oxides 323
Various methods can be used in order to synthesize Nb205 gels 181 :
- Hydrolysis of NbcI 5 followed by a careful washing and the addition of hydrogen
in order to remove chloride ions.
- Hydrolysis of chloride-alkoxides such as Nb(0R)3CI 2.
- Hydrolysis of niobium ethoxide in various alcohols.
peroxide
6.1.10. Tungsten oxide WO 3. Colloidal tungstic acid can be obtained when acidification of a
sodium tungstate solution with HCI is followed by washing or dialysis 468,469. A pure sol
can be easily obtained by acidification of a sodium tungstate solution through a proton-
exchange column 183,470 Electrochromic thin films have been deposited from WO 3 sols in
order to make display devices 183 These sols can also be made from tungsten chloride-
alkoxides 321 or alkoxides. In this case, a chemical stabilization of W(OEt) 6 by
acetylacetone in butanol must be made 304
6.1.11. Noble metal oxides. Gelification can also be achieved with noble transition
elements such as Au(lll). Hydrous Au203 gels have been synthesized through acidification of
Na Au(OH)4 with an inorganic acid 471 Colloids IrO2.xH20 can be prepared by hydrolysis of
hexachloroiridate (III) or (IV) at pH=7 472. Finally, colloidal RuO2.2H20 can be
synthesized by dissolving KRuO4and poly(styrene/maleic anhydride) (i:i) in water and adding
aqueous H202 at pH=7 473. The main application of these noble transition metal colloids is
in the field of photocatalytic materials 474
6.2. Materials
The sol-gel process is especially suitable for making multicomponent ceramics or
glasses. Only materials derived from sol-gels are rewiewed here. Other wet techniques such
as coprecipitation, freeze drying, spray drying and liquid drying will not be considered.
6.2.1. Ferroelectric ceramics. Barium titanate BaTiO 3 is the most often studied material
for high dielectric constant ceramic capacitors. The alkoxide route using Ti(OEt) 4 290,475
or Ti(OPri)4 476-478 and barium alkoxides such as Ba(OEt) 2 290, Ba(oPrn)2 476 or Ba(OH)2 in
CH30 H 478 have been mainly used to obtain thin films 478 or monolithic gels 290 Barium may
be substituted by strontium in these ceramics. Strontium titanate powders can be obtained
through controlled precipitation of a double alkoxide SrTi(OPri)6 479,480 W doped
strontium titanate can be synthesized giving c values as high as 40,000 through chemical
modification of Ti(OBun)4 with ethylene glycol and citric acid 481 Strontium is introduced
as Sr(N03)2, tungsten as tungstic acid H2WO 4 and hydrolysis is performed under acidic
(HN03) conditions. Finally complex formulations can be obtained by mixing Zr(OBun)4,
Ti(OBun)4 , Nb(OEt) 5, Sb(OEt) 3, La(N03)3, Ba(OH) 2 and Sr(OH) 2 in butanol-2. They lead to
fine perovskite powders which can be used for piezoelectric and electrooptic applications 482
Lead titanate PbTiO 3 can also be used in high dielectric constant ceramic
capacitors. It can be made as a monolithic gel from Pb(OAc)2.nH20 and Ti(OPri) 4 precursors
in methoxyethanol using acid catalyzed (HN03) hydrolysis 483,484,485. Thin films have been
made by spin-coating 486 The substitution of some titanium by zirconium lead to the so
called PZT compositions. Films of various compositions are obtained by dip-coating 487 or
spin coating 488,489 Titanium and zirconium are usually introduced as alkoxides :
Ti(OPri)4 487, Ti(OBun)4 488,489, Zr(oPrn)4 488,489, Zr(OEt)4 487 ; while lead is
introduced as lead acetate or lead ethyl-2 hexanoate 487,488
JPSSC 18:4-E
324 J. Livage et al.
Alkali niobates and tantalates are also very important ferroelectric materials
which can be obtained by the sol-gel process. LiNbO 3 490,491 ; Nal. x Li x NbO3 492 ; KNbO 3 ;
KTaO 3 and K(Ta,Nb)O 3 476 have been made mainly from alkoxides.
6.2.2. Magnetic ceramics. Spherical mixed cobalt and nickel ferrite particles have been
synthesized by ageing Fe(II), Co(II) and Ni(II) hydroxides in the presence of nitrate or
sulfate ions 493. Ferromagnetic NiFe204 films are deposited by dip-coating from solutions
containing Nickel(II) ethyl-2-hexanoate and Fe(III) ethyl-2-hexanoate 487
Barium ferrite powders BaFe12 O19 can be obtained from a goethite gel and Ba(OR) 2
in ethanol 494. Through hydrolysis-condensation of Ba(OEt) 2 and Fe(OEt) 3 magnetic Ba2Fe204
can also be synthesized 495
6.2.3. Other ceramics. Many binary systems have been made by the sol-gel process :
. TiO2_AI203 (Ti(OPri)4/iproH) 432 and AI2TiO 5 from organometallic precursors 496
_ ZrO2.AI203 (Zr(OPrn)4/EtOH) 432, and AI203-ZrO 2 composites made by dispersing ZrO 2 fibers
in AI203 gel (Al(OBuS)3/qINO3 or HCI) 497
Y3AI5012 made from yttrium and aluminium alkoxides 480 or Y(NO3) 3 and Al(OPri)3 in
ethanol with base catalyst which leads to a translucent gel 498
- Y203-AI205 transparent gels from Al(OBuS)3 and yttrium acetate hydrolyzed at pH 5.5 499
- LaYO 3 thermomechanic ceramics made by basic (NH 3) hydrolysis of La(OEt) 3 and Y(OEt) 3 500
- TiO2-CeO 2 films made by dip-coating from a solution containing Ti(OPri)4, CeCI 4 and
acetic acid 501
- ZrO2-Ce203 thermomechanic ceramics from Ce(acac) 3 and Zr(OBun)4 in ethanol 502
ZrO2-Cr203 thin films by solvent extraction 503
TiO2-V205 fibrous gels from VO(OEt)3 or decavanadic acid sols 447
6.2.4. Glasses and vitroceramics. Low thermal expansion coefficient glasses are mainly
based on TiO2-SiO 2 systems. TEOS and titanium alkoxides Ti(OEt) 4 315,504, Ti(OPri)4 275
Ti(OBun)4 504,505,506 and Ti(OPri)2(acac)2 425 are mixed and hydrolyzed under acidic
conditions (HCI, CH3COOH , PTSA).
Alkali resistant glasses are obtained in the SiO2-ZrO 2 system. Gels can be made by
mixing TEOS and zirconium n-propoxide 275,315 Zr(acac) 4 507 or ZrO(NO3) 2 508. Coatings 50
nm thick can be obtained from Si(OEt)4/Zr(oPrn)4 mixtures hydrolyzed under an atmosphere of
95% relative humidity 509. Monolithic films or fibers can be obtained when hydrolysis is
performed under acidic conditions 275
Other vitreous compositions studied included :
SiO2-ZrO 2 photoresponsive polymers made by polymerization of Zr(OBun)4 in
tetrahydrofuran and other solvents such as : benzene, ethanol, CS 2 and acetone with
freshly crushed silica gel 456
Si02-Fe203 made from TEOS and Fe(OEt)3 2?5
Si02-Y203 high temperature glasses made from Y(NO3) 3 and TEOS/EtOH (1:3) 510
SiO2-TiO2-ZrO 2 films made by dip-coating from ethanolic solution containing TEOS,
Ti(OBun)4 and Zr(OPrn)4 with HCI and/or formamide 511
Si02-TiO2-ZrO 2 glasses obtained through hydrolysis of a mixture of TEOS, Ti(OBun)4 and
Zr(oPrn)4 in ethanol 512
SiO2-ZrO2-AI203-Na20 alkali resistant glasses obtained through hydrolysis of TEOS,
Zr(oPrn)4, Al(OBuS) 3 and NaOEt mixtures at h<l.8 with chemical modifiers (acetic acid,
ethylene glycol and pentanol) followed by a peptization process with nitric or perch~oric
acid 513
Sol-Gel Chemistry of Transition Metal Oxides 325
SiO2-TiO2-AI203-Li20 low thermal expansion coefficient glasses made through the
hydrolysis of a Si(OMe)4, Ti(OPri)4 , Al(OBuS)3 and LiOMe alkoxide mixture in methanol in
the presence of a DCCA such as formamide 514
Na20-B203-V205-SiO 2 gels made by mixing Si(OEt) 4, VO(OEt) 3, B(OBun)3 and NaOMe in
methanol using NH 3 as a catalyst and a wet atmosphere for hydrolysis 515 Similar colored
gels are obtained when Co(OAc) 2 or ~i(OAc) 2 is used instead of VO(OEt) 3 .
6.2.5. Catalysts. The sol-gel process offers many advantages for making catalysts. Powders
with high surface area and optimized pore size distributions can be obtained. Since
homogeneous mixing can be made at the molecular scale, the chemical reactivity of the oxide
surface can be greatly enhanced.
- Hydrogen adsorption is achieved at the surface of chromium oxide gels which provide good
catalysts for the dehydro-cyclization of paraffins 516. These gels have been prepared by
slow precipitation with dilute ammonia from dilute chromium nitrate solutions, and by
gelation from chromic acetate and reduction of chromic acid by alcohol or other reducing
agents such as dugar or oxalic acid 449 . The highest recorded rate for the dehydro-
cyclization of n-heptane was obtained from chromium oxide gels obtained through reduction
with oxalic acid 449 Chromium oxide microspheres for catalyzing fluorination processes can
also be obtained using sol-gel techniques 517 Hydrolysis is achieved by mixing chromium
chloride with ammonia and hexamethylenetetramine. Gelation occurs by injecting the solution
into a glass column using ethyl-2 hexanol as the extraction solvent. Highly dispersed
particles with narrow size distributions have been obtained from Ti(OPr~)4 and cobalt
nitrate dissolved in ethyleneglycol 518. The average size of the particles can be varied in
the range 30-120 A by diluting the alkoxide precursor during the synthesis. This leads to
modified catalytic activity for the hydrogenation of propionaldehyde.
If drying is performed in hypercritical conditions, a highly porous material called
"aerogel" is obtained. Aerogels exhibit better catalytic properties (activity, selectivity,
resistance to desactivation) than usual xerogel catalysts 519,520. Anatase TiO 2 aerogels
made from Ti(OPri)4 or Ti(OBun)4 allow partial oxidation, at room temperature under U.V.
irradiation, of paraffins, olefins and alcohols into ketones and aldehydes 521 NiO/AI203
or NiO/SiO2/AI203 aerogels made from nickel acetate in methanol, Al(OBuS)3 and Si(OMe) 4 are
almost 100% selective towards partial oxidation of paraffins or olefins. Isobutene can be
converted into methacroleine and acetone 519,522 Similar aerogels and Cr203/AI203 aerogels
also allow the conversion of olefins into nitriles 520, while Fe203/SiO 2 and Fe203/AI203
aerogels exhibit Fisher-Tropsch reaction rates two or three orders of magnitude higher than
those of the conventional reduced iron catalysts 523. Reduced oxides such as MoO 2 in
aerogel form can be made from Mo(acac)3 in methanol/ammonia solution. They have been used
for electrochemical generator catalysts 520 Finally, mixed oxide aerogels (TiO2-ZrO 2 ,
TiO2-SiO2, ZrO2-Si02, MgO-TiO2, MgO-ZrO2) made from Ti(OBun)4 , Zr(OPri)4 , Si(OMe) 4 and
Mg(OMe) 2 precursors, can replace SiO 2 or AI203 as substrates for catalysts 519,520
- The transition metal oxide catalytic phase can also be used as a coating on SiO 2 or
various glassy substrates. Amorphous TiO 2 coatings made from an ethanolic solution of
Ti(OBun)4 can be deposited onto glass spheres of the Si02/AI203/CaO/K20/MgO/Na20 system and
treated by a solution of Pd(CBHs) 2 in pentane 524. The catalytic activity of such catalysts
towards olefin hydrogenation is comparable with that of the best conventional systems. A
mono-atomic layer of amorphous niobium oxide can be deposited onto the surface of SiO 2 by
reacting surface silanol groups with Nb(OEt5) in dry hexane followed by chemical treatment
with H20 and 02 525 Such a catalyst is active and selective for ethene formation from
ethanol.
326 J. Livage et al.
6.2.6. High Te Superconducting Ceramics. A tremendous effort has been applied to synthesize
high temperature superconducting ceramics by the sol-gel process. The versatility of this
process will allow one to obtain dense ceramics, fibers or films from sols or gels
intermediates. Bulk ceramics and thick films have already been made by solution techniques.
In the case of the 90K superconducting phase YBa2Cu307 such materials have been achieved
through coprecipitation 526-528 , by controlled precipitation with colloidal mixtures of
hydroxides and acetates 453,529,530, solutions of neodecanoates 531, ethylhexanoates 532 Some other solution processes have included the control of the rheology by use of
ethyleneglycol and citrates 535,534 or metacrylates 535. Because of the difficulty to
dissolve copper alkoxides very few processes have been described using alkoxides 536
However, some soluble alkoxides like Cu(OCH2CH2NEt2) 2 o'r Cu(OCH2CH2OBu)2 have been recently
successfully used 537
Unfortunately, these precursors decompose through oxides and barium carbonates around
500°C. The reaction leading to the pure material occurs then only around 850°C upon a long
time, and the sintering effect is weak. Although better homogeneity is achieved, the trans-
port properties of the superconducting ceramics obtained up to date by sol-gel processes
are still dominated by the grain boundaries and they do not show better critical currents
than conventional ones. The best results have actually been obtained with carbon-free
precursors like nitrates 538-541 or hyponitrites 537 that decompose easily to oxides upon
heating. The superconducting phase can then be obtained around 650=C, yielding submicronic
grains with, unfortunately, a poor diamagnetic signal intrinsic to the small grain size 537 .
7 . CONCLUSION
Interest in the sol-gel process began about 20 years ago 542. Many significant
results have been obtained since then and products such as optical coatings or fibers have
already appeared on the market. However, the future of the sol-gel technology still depends
on whether it will be able to make better and cheaper materials or even completely new
materials 543. Therefore a real mastery of the process is required from both scientific
knowledge and technological expertise point of view. One of the main advantage of the sol-
gel process is the ability to go all the way from the molecular precursor to the product,
making possible to synthesize tailor-made materials. However, many parameters are involved
along the process : chemistry during hydrolysis and condensation of the precursors,
physical chemistry of aggregation, gelation, drying and finally physics to account for the
properties of the material. Each step has to be optimized depending on the required
application.
The sol-gel process is based on inorganic polymerization reactions. Thus,
chemistry is one of the main points for further development of the process. The chemical
reactivity of silica precursors is beginning to be rather well understood but this is not
yet the case for transition metal oxides. Six chemical reactions are mainly involved in the
sol-gel process, namely ; hydrolysis, modification, olation, alcolation, oxolation and
alcoxolation. More reliable experimental data and accurate characterization of all the
chemical species involved in these reactions have to be obtained before a real science of
inorganic polymerization can be established. Many efforts are being made in order to adapt
the existing techniques to this problem. The ideal method should be able to give "in-situ"
and dynamical information all the way, from small molecular species to large colloidal
particles and gels. Spectroscopies such as X-ray absorption, IR-Raman, X-ray or Neutron
Scattering, high resolution liquid and solid state N.M.R. have been used during the last
Sol-Gel Chemistry of Transition Metal Oxides 327
few years. They appear to give significant results. From a theoretical point of view, the
chemical reactivity of a molecular MX n precursor mainly depends on the polarity of the M-X
bond and the nature of the solvent ROH. As shown in this paper, the thermodynamics of these
reactions can be described on the basis of electronic chemical potential # or
electronegativity X considerations (X=-#) 37. A molecular precursor ME n chemically reacts
with ROH when its mean electronegativity is smaller (x(MXn)<x(ROH)), while solvation occurs
in the reverse case (x(MXn)>X(ROH)). Water has a high electronegativity (X(H20) = 2.49) so
that alkoxides cannot be easily handled and are readily hydrolyzed in the presence of
moisture. Inorganic precursors must then be used in aqueous solutions. They are quite
electronegative and therefore will not react as readily with most ROH reagents. Their
chemical reactivity is quite low and they are not easily modified by chemical additives. On
the other hand, metal alkoxides have a low electronegativity so that they react with many
chemical reagents besides water. They offer a large variety of chemical reactions 20 and
are therefore versatile precursors for the sol-gel process.
Once the colloidal particles are formed, chemistry does not play such an important
role in the sol-gel process. Aggregation occurs which dictate the ultrastructure of the
oxide. However, it mainly depends on physico-chemical parameters such as particle-particle
or particle-solvent interactions. Thus, either colloidal or polymeric gels are obtained
278. According to the literature, polymeric gels are made almost exclusively from metal
alkoxides while colloidal gels are typically formed from metallic salt aqueous solutions 2
However, such a simplified classification should be considered with care. Polymeric V205
gels have been obtained from inorganic precursors 176 and colloidal SiO 2 from silicon
alkoxides 544 Small-angle scattering experiments are currently done in order to study
aggregation and gelification 330 Computer models have been proposed to account for the
observed scattering curves, and the fractal geometry of aggregates is still a matter of
debate 22
The functionality f of the molecular precursors is sometimes taken into account
329. As for organic polymers this will be a very important parameter. A three-dimensionnal
network is usually obtained when f is larger than 2, while chain polymers are expected when
f is close to 2. Gelation processes should therefore be strongly dependent on the
functionality of the precursors. As mentioned in this paper, the shape of the primary
colloidal particles is often governed by the chemical conditions. Strongly anisotropic
colloids can sometimes be obtained which lead to ordered aggregation and anisotropic
coatings. However, it must be pointed out that, up to now, it remains impossible to relate
chemistry and morphology.
Despite the present lack of knowledge, it may be assumed that the sol-gel
processing of transition metal oxides will continue to grow in the near future. It offers
unique advantages for making monodispersed powders 23, multicomponent ceramics 2, coatings
5 , fibers 4 or even completely new mixed organic-inorganic materials 317. However, one of
the main drawbacks of the sol-gel process remains the long time required for drying and
densification. Rapid drying causes cracking and monolithic materials are difficult to make.
Two general approaches have been proposed to circumvent this problem, namely hypercritical
drying 545 and the so-called DCCA (Drying Control Chemical Additives) 546. They have been
almost exclusively used for silica rather than for transition metal oxides. Theoretical
analysis of the drying process has been also recently proposed 28 This should provide the
necessary basis for the development of the sol-gel process. Anyway, as shown in this paper,
gels or xerogels, can actually be considered as liquid-solid composites. They exhibit some
interesting physical properties and can be used for making antistatic coatings, micro-
batteries or electrochromic display devices.
328 J. Livage et al.
Acknowledgments : We are greatly indebted to Prof. E. MATIJEVIC for authorizing and
providing the reproduction of electron micrographs of figure 12 and to D r . K. CHEMSEDDINE
for providing electron micrograph of figure i0. Special thanks are also due to D r J.P.
JOLIVET and P. BARBOUX for helpful discussions and preparation of the final manuscript.
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