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CHAPTER 4

EFFECT OF ALKALINE EARTH METAL AND TRANSITION

METAL DOPANTS ON THE STRUCTURAL, OPTICAL AND

ELECTRONIC PROPERTIES OF YTTRIUM STABILIZED

ZIRCONIA NANOPARTICLES

4.1 INTRODUCTION

The tetragonal and cubic phases could exist when doped with

alkaline earth metal oxides (MgO, CaO, etc.) or by transition metal oxides

(Y2O3, etc.) or by rare earth metal oxides (CeO2, etc.) at room temperature

(Bechepeche et al. 1999). According to Iwasaki et al. (1992), the mechanical

strength of stabilized zirconia is improved without lowering the ion

conductivity by providing a solid electrolyte comprising stabilized zirconia

and a metal oxide dispersed within grains or grain boundaries of stabilized

zirconia. Stabilization of zirconia in particular phase is recognized as the

stabilized zirconia. It can be obtained by addition of about 5-10 mol%,

particularly about 8 mol% of a stabilizer such as yttrium, cerium, calcium or

magnesium. With the stabilized zirconia, the first group metal oxide can be

added which do not generally form solid solution with the host and the second

group metal oxide that forms the solid solution can also be added as well

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without changing their physical properties and chemical stability. For

example, alumina, chromia and mullite can be easily added to stabilized

zirconia which forms as a composite oxide and a second group metal oxide

such as magnesia, barium oxide and calcia could form solid solution with

stabilized zirconia. When the second group oxide is used as the dopant, the

metal oxide may be in the form of separate grains which are dispersed within

the stabilized zirconia grains depending upon the preparation conditions.

Nevertheless, such a metal oxide may be partially solid dissolved, particularly

around the stabilized zirconia grains. The limitation of the metal oxide in

mol% in the stabilized zirconia is preferably up to about 30%, more preferably

0.01 to 20% and specifically 0.1 to 5% (Iwasaki et al. 1992). Addition of a

lower concentration of metal oxide improves the strength of the stabilized

zirconia whereas a greater concentration of metal oxide in stabilized zirconia

lowers the ionic conductivity significantly.

Manganese ion stabilizes zirconia in the cubic phase and moreover

it delays the cubic to tetragonal (c�t) phase transformation responsible for

slow conductivity decay (>1000 h) even at high temperature of about 850–

1000° C (Herle and Vasquez 2004). Theoretical (Ostanin et al. 2007, Jia et al.

2009) and experimental research (Clavel et al. 2008, Yu et al. 2008, Zippel et

al. 2010, Srivastava et al. 2011) in Mn doped YSZ is however limited and is

mostly oriented in exploiting the magnetic properties especially room

temperature ferromagnetism for spintronics application. For Mn-stabilized

zirconia special magnetic properties were recently predicted depending on the

number of oxygen vacancies (Ostanin et al. 2007). Cu/ZrO2 materials were

proposed by Velu et al. (2000), Liu et al. (2002), Matter et al. (2004) and

Fisher et al. (1999) as catalysts in the process of oxidative steam reforming of

methanol. However, to the best of the author knowledge there are no reports

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available analyzing the effect of these transition metal ions with yttria

stabilized zirconia. Here, in the present study it is believed that the well

stabilized cubic YSZ can be formed by adding these dopants.

Ni doped YSZ is widely used as anode material in SOFC (Park et

al., 2011). A number of researchers have addressed the interaction of YSZ

with small additions of NiO, most of the research work has been focused on

phase stabilization (Kondo et al., 2003), aging (Mori et al. 2003) and electrical

properties (Kondo et al. 2003, Linderoth et al. 2001, Herle and Vasquez 2004).

It has been reported by Kondo et al. (2003) that addition of nickel enhances

the ionic conductivity of zirconia. Moreover, Herle and Vasquez (2004) have

proved that lower concentration of NiO is advantageous as it can significantly

reduce the sintering temperature.

ZnO is well known not only as a semiconductor but also as a

probable oxygen-ion conductor due to enrichment of oxygen vacancies at

higher temperature. It has been reported that ZnO could be used as an effective

sintering aid and an optimal scavenger for grain boundary in yttria-stabilized

zirconia. Small addition of ZnO is found to be effective in reducing the

sintering temperature and promoting the densification rate of the ceramics.

The 5.0 wt% ZnO-doped YSZ has ~96% relative density, as compared to

~89% relative density for the undoped sample. The total conductivity of 8YSZ

was evidently increased by doping small amount of ZnO. At intermediate

temperature (~300° C), the maximum enhancement of grain boundary

conductivity was observed with 5.0 wt% ZnO dopant (Liu and Lao 2006). In

this case also there are no more related reports available. Here, in the present

study the 5 mol% Zn has been used to stabilize the cubic phase of YSZ.

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For SOFC application it is important to stabilize the cubic structure,

in particularly, at nanoscale. In recent years, research has been directed

towards stabilized nano-zirconia at room temperature by various dopants

inorder to obtain phase stability over time as well as to improve the ionic

conductivity by increasing the oxygen vacancies. This demands a clear

understanding of the influence of various dopants on the structural and

electronic band structure of the stabilized zirconia specifically at nanoscale as

their properties are entirely different from the bulk counter-part. However

there is lack of research directed on such concept of material science which

will pay way for better understanding of the factors that impulse the phase

stabilization and ionic conductivity. This chapter clearly depicts the influence

of various dopants on the structural, optical and electronic properties of yttria

stabilized zirconia and effective stabilization of cubic phase at room

temperature by alkali earth metal oxide and transition metal oxide dopants.

4.2 EXPERIMENTAL SYNTHESIS

AR grade Zirconium oxychloride (Himedia) and Yttrium nitrate

(Himedia) were used as the precursor. The precursors used for the doping

were AR grade Barium nitrate (Merck), Magnesium nitrate (Merck),

Manganese nitrate (Merck), Nickel nitrate (Merck), Copper nitrate (Merck)

and Zinc nitrate (Merck). Appropriate amount of the precursor salts in the

molar ratio Zr:Y:M::0.87:0.08:0.05 (where 5 mol% of M refers to the cationic

dopant added) were dissolved in required amount of double distilled water by

stirring. Oxalic acid was added drop-wise to the above mixed solution under

vigorous stirring until the solution turns pale white gel in the case of Ba, Mg

and Zn as dopant whereas a light brownish gel, light greenish gel and bluish

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gel in the case of Mn, Ni and Cu respectively as dopant. Stirring was

continued till the gel gets dissolved and forms a solution. Precipitation of the

precursor solution by drop-wise addition of liquid ammonia was done to

maintain the pH of the solution at 8. Ultrasonication was further continued for

45 min and the temperature of the solution was maintained at room

temperature by immersing it in a cold water bath. The precipitate was aged for

12 h at room temperature and then the supernatant water was decanted. The

obtained precipitate was again subjected to ultrasonication for another 15 min

by maintaining the temperature at room temperature by using a cold water

bath. The precipitate was then washed with water and ethanol several times

and dried in hot air oven at 100˚C. The dried precipitate was ground to fine

powder using agate mortar and pestle. All the synthesized powders were

calcined at 700˚C for 2 h. The powders doped with alkali metal were subjected

to prolonged heat treatment for 8 h at 700˚C. For convenience the samples are

named as Mg-YSZ, Ba-YSZ, Mn-YSZ, Ni-YSZ, Cu-YSZ and Zn-YSZ.

4.3 RESULTS AND DISCUSSION

4.3.1 Structural Characterization

The identification of phases in the synthesized zirconia powder

sample was carried out by X-ray powder diffraction studies using Cu K�

radiation (1.54 Å), and comparing the interplanar distances and intensity

values with those of the corresponding standard peaks using JCPDS files. The

crystallite size of the nano-sized zirconia was evaluated from X-ray powder

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diffraction data using Scherrer formula (Cullity and Stock 2001) for the most

intense peak (1 1 1) plane of zirconia crystal using the equation 2.1.

It is evident from the XRD patterns shown in Figure 4.1 that YSZ

shows poor crystallinity when doped with alkali earth metal ions such as Mg

and Ba. In order to obtain well crystalline features the calcination time was

further extended to 8 h and there the well defined high intensity diffraction

peaks can be seen as in Figure 4.2. The observed peak position and their

relative intensity are indexed to cubic phase of zirconia and shows increase in

crystallite size as well.

It can be concluded that when doping with Ba in YSZ, there is

larger crystallinity delay that requires prolonged calcination time when

compared to pure YSZ and transition metal doped YSZ as well. This may be

due to the large ionic radius of Ba2+ (142 pm) which suppress the grain growth

rate in shorter calcination time. Therefore, it requires more time to get

complete crystallization. As a result of the metal ion incorporation, strain is

induced in the lattice due to the difference in their ionic radii which reduces

the particle size as observed by the peak broadening and poor crystallinity

indicating quantum confinement. At further continuation of heat treatment, the

lattice strain gets relieved to some extent thereby improving the crystallinity of

the material.

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Figure 4.1 XRD patterns of (a) Mg-YSZ and (b) Ba-YSZ calcined at

700˚C for 2 h

Figure 4.2 XRD patterns of (a) Mg-YSZ and (b) Ba-YSZ calcined at

700˚C for 8 h

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Figure 4.3 XRD patterns of (a) YSZ (b) Mn-YSZ (c) Ni-YSZ (d) Cu-

YSZ and (e) Zn-YSZ calcined at 700˚C for 2 h

Figure 4.4 Extended view of high intensity (111) plane in the XRD patterns of (a) YSZ (b) Mn-YSZ (c) Ni-YSZ (d) Cu-YSZ and (e) Zn-YSZ calcined at 700˚C for 2 h

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For the 5 mol% transition metal doped YSZ all the diffraction

peaks could be indexed to cubic symmetry with a peak shift to higher 2�

values. The XRD patterns of YSZ doped with 5 mol% of transition metal

oxide shows better crystallinity compared to alkali earth metal doped YSZ

when calcined for shorter time of 2 h. The Ni-YSZ shows better crystallinity

compared to other samples. The crystallite size and crystallinity of the material

depends on the dopant added.

The peak broadening observed is due to the non uniform strain

which confirms the presence of nanometric YSZ. It can be noted that the phase

formed is predominantly cubic due to incorporation of dopants. Moreover, the

diffraction planes of the cubic and tetragonal phase are almost nearer to each

other and hence clear determination of diffraction planes in the case of broader

peaks are difficult. Therefore, it cannot be suggested that the material is purely

cubic. It may be a combination of cubic and tetragonal phases (pseudocubic

phase). This is clearly shown in Figure 4.3 where the standard JCPDS data 89-

9069 and 79-1769 for cubic (red lines) and tetragonal (black lines) phase

respectively are given for comparison with the experimental data.

The intensity of the diffraction peak decreases with increasing

vacancy concentrations which are consistent with the PL results. The (111)

peak broadens with increasing stacking fault and the location of the (111) peak

is shifted to higher angles. In the extremely small crystals, the greatest shift

occurs when the stacking fault is located in the center of the cube (Makinson

et al. 2000). The lattice constant of stabilized ZrO2 varies depending on the

amount and the nature of the stabilizing element (Schubert et al. 2009). As the

particle size is very small the fraction of atoms in the surface layer is large and

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these atoms are less strongly bonded to their neighbours than atoms in the

bulk. As a result, the unit-cell dimensions at the surface are larger than in the

core. The unit-cell parameters are an average measurement, based on the

average interatomic distances. Thus, smaller particle size leads to larger

average interatomic distances and hence the unit-cell parameter would be

larger compared to the bulk.

According to Esposito et al. (2011), the structural properties depend

markedly on the precursor and on the synthesis procedure. Esposito et al.

(2010) has proposed the concept of crystallization delay in the Cu doped

Zirconia. This is caused by the incorporation of dopant metal ions into the

ZrO2 lattice i.e. they occupy the position of Zr4+ ions. Based on their

assumption, the same hypothesis was followed in the present case. With the

ionic radius of the dopant the variation in lattice parameter was observed as

summarized in the Table 4.1.

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Table 4.1 Variation of lattice parameters of zirconia with respect to various metal ion dopants

Sample name Mean Crystallite

size D (nm)

Strain x 10-4

Dislocation density

x 1015 lines/m2

Lattice constant

a (Å)

Volume V

(Å3)

c-ZrO2 JCPDS 89-9069

- - - 5.135 135.40

c-ZrO2 Tsunekawa et al., (2003)

bulk - - 5.07 130.32

c-ZrO2 Manna et al., (2010)

~11 - - 5.1215 134.34

8YSZ 18.29 32.21 2.99 5.1043 132.99

Mg-YSZ (700˚C, 2 h)

8.22 71.87 14.77 5.1174 134.01

Mg-YSZ (700˚C, 8 h)

18.56 31.76 2.90 5.1075 133.23

Ba-YSZ

(700˚C, 2 h)

8.20 75.26 14.86 5.1229 134.45

Ba-YSZ

(700˚C, 8 h)

18.53 31.82 2.91 5.1109 133.51

Mn-YSZ (700˚C, 2 h)

10.29 57.10 9.44 5.0912 131.96

Ni-YSZ

(700˚C, 2 h)

17.89 32.86 3.12 5.0925 132.06

Cu-YSZ

(700˚C, 2 h)

8.23 71.62 14.86 5.1045 133.00

Zn-YSZ

(700˚C, 2 h)

7.84 75.20 16.28 5.1050 133.05

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The lowering of the lattice parameters values of the samples

compared to those of pure zirconia were an unquestionable proof of the dopant

ion–zirconium replacement. The decrease in unit cell parameters of cubic

phase is supported by the shift in 2� to higher values. The ionic radii of Mn2+,

Ni2+, Cu2+ and Zn2+ are 67, 44, 57 and 60 pm which are smaller than that of

Zr4+ with ionic radii 84 pm (Shannon 1974). Therefore substitution of

transition metal dopant ion into zirconia lattice leads to lattice volume

shrinkage. The metal ions used as dopants are generally of smaller size and

lower valence (than Zr4+ ions) that may result in a decrease in the unit cell

volume and generation of positive holes with lattice defects (oxygen

vacancies).

The typical FTIR spectra of alkali earth metal and transition metal

doped Yttria stabilized Zirconia are shown in Figure 4.5 and Figure 4.6

respectively which are considered as the finger print of the material. The broad

peak exhibited in the FTIR Spectra in the range 3000 – 3800 cm-1 corresponds

to the stretching vibration of physically adsorbed –OH with the metal ion on

the surface (Truffault et al. 2010). The band located around 1400 cm-1 and

1625 cm-1 represents bending vibration of water molecules (Phoka et al. 2009,

Truffault et al. 2010, Wang et al. 2012). The peak centered around 2340 –

2350 cm-1 can be attributed to the coupling effect of stretching and bending

vibration of –OH groups (Sarkar et al. 2007). The stretching frequency of

metal-oxygen (M-O) band is found below 600 cm-1. It has been predicted by

Srinivasan et al. (2010) that the M-O absorption bands become broader in the

FTIR spectra as the particle size decreases due the enhanced surface effects.

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Figure 4.5 FTIR spectra of (a) Mg-YSZ and (b) Ba-YSZ calcined at

700˚C for 8 h

Figure 4.6 FTIR spectra of (a) Mn-YSZ (b) Ni-YSZ (c) Cu-YSZ and (d)

Zn-YSZ calcined at 700˚C for 2 h

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4.3.2 Optical and Electronic Properties

Figure 4.7 and 4.8 shows the diffuse reflectance spectra of alkali

earth metal ion doped yttrium stabilized zirconia and transition metal ion

doped yttrium stabilized zirconia respectively. The energy band gap is

determined from UV Diffuse reflectance data by transforming it into a

function of reflectance as proposed by Kubelka-Munk. The Kubelka-Munk

plot for determining band gap energy is shown is Figure 4.9 and 4.10 for the

alkali earth metal doped yttrium stabilized zirconia and transition metal doped

yttrium stabilized zirconia respectively. KM plot is plotted with the nth power

of product of function of reflectance F(R) and photonic energy (Eg = h)

against the photonic energy. Since Zirconia is considered as a direct band gap

semiconductor the value of n is taken as 2 for allowed transitions (Joy et al.,

2012). The energy band gap is found out by extrapolating the linear portion of

the graph to the X-axis.

The formation of defects, such as oxygen vacancies, lead to

reflectance at lower energies due to the presence of donor levels located inside

the forbidden band (Manna et al. 2010). The oxygen vacancies in ZrO2

crystals can induce the formation of new energy levels in the band gap region.

The observed red-shift of the cut-off wavelength could be due to oxygen

vacancy that is in fact responsible for lowering the band gap energy. Among

all the transition metal doped YSZ, Mn-YSZ shows poor reflectance which is

due to the lower optical features of Mn by nature. Comparing the band gap

energy of the metal ion doped YSZ tabulated in Table 4.2, Ba-YSZ and Zn-

YSZ show comparatively high energy band gap which is due to quantum

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confinement effect as supported by the PL results and smaller nanosize of the

crystallites as deduced from the XRD results.

Figure 4.7 Diffuse Reflectance Spectra of (a) Mg-YSZ and (b) Ba-YSZ

calcined at 700˚C for 2 h

Figure 4.8 Diffuse Reflectance Spectra of (a) Mn-YSZ (b) Ni-YSZ (c) Cu-

YSZ (d) Zn-YSZ and (e) YSZ calcined at 700˚C for 2 h

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Figure 4.9 KM plot of (a) Mg-YSZ and (b) Ba-YSZ calcined at 700˚C

for 2h

Table 4.2 Variation of energy band gap of zirconia with respect to alkali

metal and transition metal ion dopants

Sample Name Energy band gap Eg (eV)

YSZ 5.1

Mg-YSZ 4.85

Ba-YSZ 5.08

Mn-YSZ 4.85

Ni-YSZ 4.93

Cu-YSZ 5.00

Zn-YSZ 5.06

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Figure 4.10 KM plot of (a) Mn-YSZ (b) Ni-YSZ (c) Cu-YSZ (d) Zn-YSZ

and (e) YSZ calcined at 700˚ C for 2 h

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Figure 4.11 Photoluminescence Spectra of (a) YSZ (b) Mg-YSZ and (c)

Ba-YSZ

Figure 4.12 Photoluminescence Spectra of (a) Mn-YSZ, (b) Ni-YSZ, (c)

Cu-YSZ and (d) Zn-YSZ

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The origin of photoluminescence in zirconia is discussed in detail

in Section 3.3. All the undoped and doped YSZ show almost similar PL

emission. They show three prominent peaks in the UV, violet and green region

of the electro-magnetic spectrum with emission wavelength centered around

361 nm, 400 nm and 493 nm corresponding to excitation at 293 nm as shown

in Figure 4.11 and 4.12.

The intense zirconia emission peak at 361 nm in the ZrO2 calcined

in air can be due to the ionized oxygen vacancies (F+ and F− centers) from the

conduction band (Joy et al. 2011). Here UV emission can arise as a result of

the radiative recombination of a photogenerated hole with an electron

occupying the oxygen vacancy. It is reported by Lai et al. (2000) that there is

formation of surface traps in the ZrO2 leading to electron transitions, even

with a small amount of excitation energy. The PL emission band observed at

400 nm might be due to the transitions from the surface trap states in the

conduction band to lower energy levels close to the valance band. This is in

agreement with the results reported in the literature (Neppolian et al. 2007, Lai

et al. 2000). A very broad emission peak in the green region is seen in the case

of BaYSZ and ZnYSZ which is due to quantum confinement effect of reduced

particle size and the presence of more oxygen vacancy in the material. The

broad band may be due to the inhomogeneous broadening from a distribution

of surface or defect states because of the narrow particle size distribution (Joo

et al. 2003). The presence of a certain type of dopant may inhibit the

luminescence of other centers; here Zn-YSZ acts as a "killer".

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4.3.3 Morphological Characterization

Figure 4.13 FESEM image and corresponding EDAX spectrum of (a) Mg-

YSZ and (b) Ba-YSZ

The FESEM images of the transition metal doped YSZ (Figure

4.14) reveal agglomerated uniform sized spherical shaped nanoparticles

whereas alkaline earth metal doped YSZ (Figure 4.13) exhibit irregular shape

and non-uniform particle size distribution which is due to the low crystallinity.

Further, the severe agglomerations of the particles are due to the change in

surface energy considerably by the addition of dopants. The corresponding

EDAX spectrum shows the composition of the elements present in the sample.

The atomic ratio of Zr:Y:M (where M is the dopant metal ion) obtained from

the EDAX analysis agrees well with the initial composition taken for synthesis

of the nanomaterial.

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Figure 4.14 FESEM images of (a) Mn-YSZ, (b) Cu-YSZ (c) Ni-YSZ with corresponding EDAX spectrum and (d) Zn-YSZ with corresponding EDAX spectrum

4.4 CONCLUSION

The dopants influence the crystal growth and phase of the material.

Moreover, it can be inferred that crystallite size reduction can be made by

addition of such dopants as it suppresses grain growth and crystallization.

Usually a very smaller size of the particle (<10 nm) reduces crystallinity of the

sample and the material exhibits amorphous nature. All the synthesized

samples are nanosized <20 nm as determined from XRD analysis by using

Scherrer formula. All the metal ion doped YSZ are predominantly cubic

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phased. Crystallization delay was observed in Ba-YSZ compared to YSZ and

other metal ion doped YSZ. For transition metal doped YSZ peak shifts to

higher 2� value marked by decrease in lattice constant as a result of

incorporation of metal ions into the YSZ lattice. Since diffraction peaks of

tetragonal and cubic phase are closer to each other determination and

confirmation of the phase in the case of broader peaks is impossible and still

higher sophisticated analysis is required to refine the peak to get a improved

report and absolute phase determination. Variation of intensity of the

diffraction peaks was observed associated with the vacancy concentrations

which are consistent with the PL results. FTIR confirms the formation of the

material.

Red shift in reflectance spectra was observed due to formation of

new energy levels in the forbidden gap. Band gap energy of various metal ion

doped YSZ have been determined by using KM plot. Ba-YSZ and Zn-YSZ

show high values of energy band gap which is due to quantum confinement

effect. From the photoluminescence spectra, the observed UV emission

evidences the presence of of F-centres in the sample. Ba-YSZ and Zn-YSZ

shows a very broad peak in the green region due to quantum confinement of

reduced particle size and the presence of oxygen vacancy. This result was in

good agreement with the determined higher band gap values. The surface

morphology of the transition metal doped YSZ was observed to compose

agglomerated almost uniform sized spherical nanoparticles whereas alkaline

earth metal doped YSZ shows agglomerated irregular shaped non-uniform

sized particles in the nano regime.