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Nano Res
1
Covalent functionalization/polycarboxylation of
tungsten disulfide inorganic nanotubes (INTs-WS2)
Daniel Raichman, David A. Strawser, and Jean-Paul Lellouche ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0630-9
http://www.thenanoresearch.com on November 7 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0630-9
Covalent
functionalization/
polycarboxylation
of tungsten
disulfide
inorganic
nanotubes
(INT-WS2)
Daniel Raichman,
David A. Strawser,
and Jean-Paul
Lellouche*
Department of
Chemistry,
Nanomaterials
Research Center,
Institute of
Nanotechnology &
Advanced Materials,
Bar-Ilan University,
Ramat-Gan
5290002, Israel
Efficient polycarboxylation of surface shell S atoms of WS2 inorganic nanotubes (INT-WS2) by a
modified, acidic, highly electrophilic Vilsmeier-Haack (VH) reagent.
Covalent functionalization/polycarboxylation of
tungsten disulfide inorganic nanotubes (INTs-WS2)
Daniel Raichman, David A. Strawser, and Jean-Paul Lellouche ()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Tungsten disulfide
nanotubes, INT-WS2,
Vilsmeier-Haack reagents,
polycarboxylation,
inorganic nanotube
functionalization
ABSTRACT
Inorganic nanotubes of tungsten disulfide (INT-WS2) are insoluble in common
solvents and practically inert, hindering their usefulness in both research and
commercial applications. The covalent attachment of functional species onto the
surface of INT-WS2 is a critical first step in realizing the potential that INT-WS2
offer for high-performance materials and products. Although a few attempts
have been reported regarding preparing modified nanotubes, only a limited
range of surface functionalities is possible with these methods. We have
developed a versatile method, based on a modified, highly electrophilic acidic
Vilsmeier-Haack reagent, to produce covalently bonded, polycarboxylated
functional WS2 nanotubes that are dispersible in polar liquids, including water.
The surface polycarboxylated shell provides a means for additional
derivatization, enabling matching compatibility of derivatized nanotubes to
both hydrophobic and hydrophilic materials. Nanocomposites incorporating
derivatized INT-WS2 are expected to show improved properties as a result of
enhanced interfacial compatibility, made possible by the large number of classes
of functionalization available through the initial polycarboxylation step.
1. Introduction
Nanomaterials are of great interest to the scientific
community due to the unique properties that result
from their small size (< 100 nm in at least one
dimension). For example, small nanoparticles
(NPs) typically exhibit very large surface areas,
with 1 mg of NPs of 1 nm3 containing the same
surface area as 1 kg of particles of 1 mm3. Although
this large surface area is valuable in applications,
such as nanocomposites that rely on optimal
interfacial interactions between all of the
components, it is a challenge to prepare
well-dispersed NP-based suspensions that are
required to obtain optimum performance of the
final product.
In order to study the chemical and physical
properties of NPs or to produce useful products,
especially nanocomposites that incorporate
nanoscale fillers, the NPs must be compatible with
a variety of media. For example, carbon nanotubes
(CNTs) are insoluble, nearly inert, and difficult to
disperse into most common liquids. However,
today, various commercial products are available
that incorporate CNTs including bicycle
components, anti-fouling paints for ships, flexible
transistors, and electrostatic discharge shields used
in spacecraft technology [1]. These products were
made possible only through research that led to
methods of dispersing the CNTs in a variety of
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Address correspondence to Jean-Paul Lellouche, [email protected]
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2 Nano Res.
liquids as well as polymers. CNTs can be made
dispersible through physisorption of surfactants
[2-7] or via surface functionalization [8-14]. From
Iijima’s report of the discovery of CNTs in 1991 [15],
extensive research was conducted on surface
modifications of CNTs, particularly regarding the
best process to oxidize CNTs in a manner that
would produce desired surface functional groups,
such as hydroxyls (-OH), acids (-COOH), and
carbonyls (R(R’)C=O), while at the same time
minimizing damage, such as etching of the CNT
surface or even complete cutting of the tubes into
smaller pieces [16-18]. In addition to modifying the
interactions with other materials, these surface
functionalizations enable surface properties to be
tailored even further by attachment of additional
chemical groups through reaction with the above
“anchor” chemical groups.
Inorganic transition metal dichalcogenide
materials, such as tungsten and molybdenum
disulfides (WS2 and MoS2, respectively), are of
interest to the scientific community because of their
unique layered structure and functional properties,
with nano-sized particles (NPs) tending to exhibit a
different set of properties compared to the bulk
forms. These metal dichalcogenide nanomaterials
have emerged as one of the most promising classes
of nanomaterials since the discovery of CNTs. As
with early research in the field of CNTs, a number
of potential applications have been proposed [19]
including areas such as energy storage [20], field
effect transistors [21], nanocomposite coatings [22,
23], battery anodes [24], light-emitting diodes [25],
self-lubricating medical devices [26], and
high-performance lubricants [27-34]. In addition,
the outstanding shock absorbing ability of WS2 NPs
[35-37] holds potential for new impact and shock
resistant materials. Nanocomposite materials,
formed by adding small amounts (less than 5% by
weight) of nano-sized fillers into a polymer matrix,
are of particular interest, showing improved
mechanical properties, higher thermal properties,
and improved performance as barriers to heat,
moisture, and solvents [23, 38, 39] than composites
prepared with conventional fillers [21, 22].
Considerable research has been conducted on
polymer-nanocomposites that incorporate WS2 NPs
into matrices of epoxy [40],
polystyrene/poly(methylmethacrylate) [41],
poly(propylene fumarate) [42], nylon 12 [43], and
poly(phenylene) sulphide [44]. Due to the superior
mechanical properties of WS2 NPs, such as high
stiffness and strength [45], ultrahigh-performance
polymer nanocomposites have been produced [46].
In addition, commercial lubricants are presently
available that include WS2 NPs that impart unique
tribological properties [47] to the final products.
Although there are many potential applications
in a variety of fields for these metal dichalcogenide
NPs and INTs, research has been hampered
analogously to early CNT research because these
dichalcogenide materials are insoluble in common
solvents, difficult to disperse into most liquids and
resins, and tend to have limited compatibility when
admixed with common polymers. This directly
mirrors the difficulties that were encountered in
the early years of research with CNTs. And
although the CNT literature contains a number of
methods to functionalize the CNTs, as presented
above, few of these methods are readily adaptable
to metal dichalcogenides because of the difference
in composition between the two.
To overcome these limitations due to phase
incompatibility, considerable effort has been made
to functionalize metal dichalcogenide NPs, as first
reported by Tahir et al. in 2006 [48]. Chemically
modified MoS2 NPs were produced that were
dispersible both in water and common organic
solvents. This was accomplished by attaching a
cationic Ni(II) complex with chelating ligands to
the outer sulfur layer/atoms of the MoS2 NPs. This
approach of using a cationic metal with high sulfur
affinity to coordinate with the sulfur was chosen
because they believed that “… a direct anchoring of
organic ligands to the sulfur surface is not
possible…” Although successful, this method lacks
versatility due to the limited selection of chelating
ligands as well as rather weak bonding between
the Ni(II) and S reactive centers. Another method
to introduce covalently bound functional groups
onto WS2 NPs was developed by Shahar et al. [49].
This method relies on inherent hydroxylated
surface defects that are few in number, thereby
severely limiting the quantity of functional groups
that serve as anchors for further modification in
subsequent functionalization steps.
In contrast to these studies, we have
successfully developed a functionalization method
for INT-WS2 that uses readily available materials
and equipment to produce an acidic, covalently
bound shell of polyCOOH functional groups. This
breakthrough in efficient, robust functionalization
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3 Nano Res.
of INT-WS2 provides researchers with a highly
versatile tool to tailor the compatibility of
functionalized INT-WS2 (f-INT-WS2) with nearly
any other material. This functionalization method
produces polycarboxylated INT-WS2 (polyCOOH
f-INT-WS2) that are readily dispersible in polar
liquids, including water. Thus, these
polycarboxylation sites can serve as anchor points
for secondary-step derivatizations, providing a
versatile route to additional surface modifications
through the well-known reactivity of carboxylic
acid groups. These surface modifications enable
matching interfacial compatibility to both
hydrophilic and hydrophobic materials. In
addition, subsequent functionalizations can include
bis- or multi-reactive species that enable covalent
bonding with other materials/polymeric phases in
specific compositions. This is an especially
important feature for the controlled fabrication of
hybrid nanocomposites that may contain both
co-fillers and polymerizable monomers, for
example.
Realizing the important ramifications of this
new derivatization method, we undertook in
parallel with the present study an optimization
study [50], via design of experiments (DoE)
methodology of several reaction parameters to
maximize the yield of polyCOOH f-INT-WS2. In
addition to finding optimum values of reaction
factors, an interaction analysis indicated that
reaction parameters might be adjusted to tune the
level of poly-carboxylation obtained.
While screening various solvents and reagents
to try to produce surface functionalized INTs, we
found that DMF in combination with halogenated
acetic acid produced readily dispersible INTs.
Finding this to be reproducible, a more extensive
study using a variety of solvents revealed that
DMF was an essential component in this
polycarboxylation reaction. More specifically, this
innovative polycarboxylation functionalization
method exploits the high electrophilic reactivity of
a modified Vilsmeier-Haack (VH) reagent [51].
Classical VH reactions generally use in situ
generated VH iminium salts/reagents arising from
cold (-50–0 °C) mixtures of dimethyl formamide
(DMF, secondary N-formyl amine) and POCl3 or
SOCl2, that are effective in formylating a large
variety of electron rich substrates via iminium salts
of type A (Scheme 1a).
These types of VH reactions have been
extensively studied and found to be extremely
versatile, producing not only formylated products
but also various oxygen and nitrogen heterocycles
[52-57] as well.
In this context, we discovered that a mixture
containing DMF (secondary N-formyl amine),
O-alkylating 2-bromoacetic acid (2-BrCH2COOH),
and silver acetate (Ag(I)OAc) as a bromophilic
agent to enhance abstracting the halogen from
bromoacetic acid, very effectively polycarboxylates
INT-WS2 according to the mechanism presented in
Scheme 1(c). The essential role of anhydrous DMF
in this reaction was demonstrated by either its
removal from the reaction mixture or replacement
with other polar, non-protic solvents (e.g., DME,
1,4-dioxane, THF, etc.), resulting in unsuccessful
polycarboxylation. This strict requirement of DMF
(secondary N-formyl amine) as an essential
component in the reaction mixture led us to
propose the VH-like reaction mechanism, detailed
in Scheme 1(c), with emphasis on the key
ionization assistance induced by bromophilic Ag(I)
cations.
The presence and chemical reactivity of the
grafted carboxylic acid groups covalently bound
onto the outer S layer of INT-WS2 were quantified
via the quite sensitive UV-spectrophotometric
Kaiser test for reactive primary amines [56] after
derivatization of the polycarboxylated shell with a
diamine (1,3-diaminopropane), thereby producing
polyNH2 f-INT-WS2 comprising a
poly(amido-primary NH2) shell for fluorescence
coupling. In addition and quite similarly,
polyCOOH f-INT-WS2 have been reacted with
cysteamine, resulting in polySH f-INT-WS2
containing a terminal polySH shell that was
quantified using Ellman’ s fluorescence based test
[59].
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4 Nano Res.
(a)
(b)
(c)
Scheme 1 (a) Classical highly electrophilic Vilsmeier-Haack (VH) iminium salt intermediate A; (b) generation of the electrophilic
VH iminium complex B with 2-bromoacetic acid (POCl3 replacement) and halide (Br) abstraction mediated by the Ag(I)OAc salt;
(c) sulfur mediated nucleophilic addition of the electrophilic VH iminium complex B onto the outermost layer of S atoms of
INT-WS2 to produce polyCOOH f-INT-WS2.
2. Experimental
2.1. Materials
Tungsten disulfide inorganic nanotubes
(INT-WS2) have been provided by NanoMaterials
Ltd (Yavne, Israel). All reagents and solvents
were purchased from commercial sources and
used without further purification.
Thermogravimetric analysis was performed on a
TA Q600-0348, model SDT Q600
(Thermofinnigan) using a temperature profile of
25–800 °C at 10 °C/min under nitrogen flow (180
mL/min) with sample masses of 5-15 mg. IR
spectra were recorded on an FT-IR Tensor 27
spectrometer (Bruker) using ATR. Nanomaterial
surface charges were evaluated by ξ potential
measurements with a Zetasizer Nano-ZS
(Malvern Instruments Ltd., Worcestershire, UK)
in water (pH unadjusted) at 25 °C and 150 V.
Untreated and VH-treated INTs were
characterized with a G2, FEI High Resolution
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Nano Res.
TEM (Tecnai). Dispersions of INT-WS2 and
f-INT-WS2 were prepared with a low-power
ElmaSonic S30 bath sonicator (Elma GmbH & Co.,
Singen, DE). The chemically accessible
polyCOOH shell present on the surface of the
polyCOOH f-INT-WS2 was quantified by (i) the
Kaiser test after shell derivatization using
1,3-diaminopropane and by (ii) Ellman’ s test
after a subsequent similar shell derivatization
with cysteamine.
2.2. Polycarboxylation of INT-WS2 (polyCOOH
f-INT-WS2)
To a solution of 2-bromoacetic acid
(2-BrCH2COOH, 1.0 g, 7.19 mmol) in anhydrous
DMF (3 mL) was added AgOAc (10 mg, 0.059
mmol) and dry INT-WS2 (200 mg). The mixture
was heated in an oil bath to 80 °C and stirred
over 2 days at the same temperature. After
cooling to room temperature, the mixture was
centrifuged (11,000 RPM, 5 min) and the
supernatant discarded. The solids were cleaned
by 5 cycles of washing with EtOH followed by
centrifugation (11,000 rpm, 5 minutes). The
cleaned solids were dried under vacuum to
obtain 190 mg of polyCOOH f-INT-WS2.
2.3. Diamine coupling onto polyCOOH f-INT-WS2
(polyNH2 f-INT-WS2)
To a solution of
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDC, 20 mg, 4 mmol) in dichloromethane (DCM,
12 mL) was added polyCOOH f-INT-WS2 (200
mg) and 4-dimethylaminopyridine (DMAP, 10
mg, 0.08 mmol). The mixture was stirred for 2
hours at room temperature followed by addition
of 1, 3-diaminopropane (NH2-(CH2)3-NH2, 800 µl,
9.58 mmol) and stirring continued at room
temperature overnight. The mixture was
centrifuged (11,000 RPM, 5 min) and the
supernatant discarded. The solids were worked
up as described for polyCOOH f-INT-WS2. The
product contained 0.77 mmol NH2 groups/g of
polyNH2 f-INT-WS2, as determined by the Kaiser
Test [45].
2.4. Cysteamine Coupling onto polyCOOH
f-INT-WS2 (polySH f-INT-WS2)
To a solution of EDC (3.0 g, 19.32 mmol) in DCM
(40 mL) was added polyCOOH f-INT-WS2 (1.8 g).
The suspension was stirred for 2 hours at room
temperature followed by addition of cysteamine
(NH2-(CH2)2-SH, 4 g, 51.85 mmol) and DMAP (20
mg, 0.16 mmol) and stirring continued for 2 days
at room temperature. The mixture was
centrifuged (11,000 RPM, 5 min) and the
supernatant discarded. The solids were worked
up as described for polyCOOH f-INT-WS2 to
obtain 1.6 g of product. The product contained
0.80 mmol SH groups/g of polySH f-INT-WS2, as
determined by Ellman’s Test [46].
3. Results and discussion
INT-WS2 were functionalized with a shell of
carboxylic acid groups followed by further shell
modifications with 1,3-diaminopropane or
cysteamine species, resulting in f-INT-WS2 with
surfaces decorated with terminal polyCOOH,
polyNH2, or polySH functional shells. A large
excess of 1,3-diaminopropane was used to
suppress cross-linking between INTs and to
ensure all accessible carboxyl sites were
derivatized. Likewise, a large excess of
cysteamine was used to ensure all accessible
carboxyl sites were derivatized
The polycarboxylation was accomplished by
reaction of INT-WS2 with 2-bromoacetic acid in
dry DMF (as both solvent and reagent) using
Ag(I)OAc to assist in bromide abstraction/DMF
ionization. The presence of carboxylic acid
groups (polyCOOH shell) on polyCOOH
f-INT-WS2 was confirmed by characterizations
including thermogravimetric analysis (TGA),
Fourier-transform infrared spectroscopy (FTIR),
Kaiser and Ellman’ s tests, as well as Zeta
potential. Zeta potential was measured in water
with no adjustment to pH.
Stability of dispersions in water of
polyCOOH f-INT-WS2 was superior to
dispersions of unmodified INT-WS2. The
dispersions were prepared in water by low
power sonication (37 kHz, 280 W, 1 minute) in a
bath sonicator.
Thermogravimetric analyses were performed
in order to determine the amount of organic
material bound to the functionalized INTs.
Figure 1(a) displays the TGA graphs of
unfunctionalized and functionalized INT-WS2
(polyCOOH f-INT-WS2, polyNH2 f-INT-WS2, and
polySH f-INT-WS2), and Figure 1(b) displays the
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Nano Res.
first-derivative plots. The unfunctionalized
material shows a total weight loss of less than 3%
(25-800 °C under nitrogen). As expected, the
three functionalized samples all display
significant weight losses, ranging between 11%
and 19% (25-800 °C under nitrogen). In addition,
the weight loss profiles for polyNH2 f-INT-WS2
and polySH f-INT-WS2 are significantly different,
which supports the fact that different species
have been grafted.
(a)
(b)
Figure 1 TGA graphs of unfunctionalized and
functionalized INT-WS2. (a) Weight (%) vs. temperature
(°C). (b) First-derivative curves of TGA graphs displayed in
Figure 1(a).
FTIR spectroscopy was used to confirm the
presence of characteristic absorptions of the
chemically modified f-INT-WS2. Figure 2 displays
stacked spectra of the untreated INT-WS2 and the
three functionalized samples. The spectrum of
unmodified INT-WS2 is nearly featureless,
whereas the other three spectra show
characteristic peaks that indicate the presence of
carboxyl, amine, and thiol groups, respectively.
Figure 2 FTIR spectra of unfunctionalized and
functionalized INT-WS2. (1) INT-WS2; (2) polyCOOH
f-INT-WS2; (3) poly NH2 f-INT-WS2; (4) polySH
f-INT-WS2.
The significant peaks can be assigned as
follows: 1730 cm-1 to 1640 cm-1, C=O of carboxylic
acid and amide functions, respectively; 2972 cm-1
and 1454 cm-1, C-H bend and C-H stretch
respectively; 3500 cm-1 to 3600 cm-1, O-H, N-H,
and S-H stretch; 867 cm-1, C-S stretch; 1039 and
1267 cm-1, C=S stretch; 2052 cm-1 results from
protonated primary amines (-NH3+).
potential analyses were performed to assess
changes in surface charges of the chemically
modified f-INT-WS2. The pH of the dispersions
was not adjusted. As expected, the incorporation
of the polyCOOH shell onto the surface of
modified f-INT-WS2 increased the negative
charge ( potential) on this material as well as
lowered the pH. The incorporation of the
polyNH2 shell resulted in a decrease of the
potential as well as an increase in pH. The
potential was -25 mV @pH 6.1, -34.7 mV @pH 5.9,
and -18.9 mV @pH 6.5 for unmodified INT-WS2,
polyCOOH f-INT-WS2, and polyNH2 f-INT-WS2,
respectively. These significant changes in the
potential values for the functionalized INTs
clearly indicate that subsequent surface
modifications have occurred.
XRD analyses were performed to determine
if a change in the crystallographic properties of
the polyCOOH f-INT-WS2 occurs due to the
VH-based chemical treatment. The resulting XRD
spectra (Figure 3, black: untreated control, red:
polyCOOH f-INT-WS2) agree well with the
literature reference spectrum (red vertical bars)
for unmodified INT-WS2, indicating that the
chemical treatment causes no significant change
in crystal structure.
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Nano Res.
Figure 3 XRD plot of unfunctionalized INT-WS2 (black) and polyCOOH f-INT-WS2 (red), showing good agreement with the
literature reference (red vertical bars JCPDS 08-0237).
Figure 4 displays HRTEM images, at low-
and high-magnifications, of polyCOOH
f-INT-WS2 and unmodified INT-WS2. No
discernible difference in the morphologies of
either of the two materials is revealed, indicating
that the polycarboxylation procedure does not
result in damage to the INTs.
(a) (b)
(c) (d)
Figure 4 HRTEM images at low- and high-magnification
respectively of (a, c) unmodified and (b, d) polyCOOH
f-INT-WS2. No change in the morphology of the INTs is
evident due to the chemical modification.
The stability of suspensions of unmodified
INT-WS2 and polyCOOH f-INT-WS2 in water was
compared by dispersing ultrasonically (low
power bath sonicator) for one minute 5 mg of
INTs in 1.5 mL of distilled water in capped vials
while leaving the suspensions undisturbed at
room ambient. Figure 5 displays a photograph of
the two suspensions after 18 h. A significant
portion of the unmodified INT-WS2 has settled
out, while the polyCOOH f-INT-WS2 remain
suspended with no visual evidence of
sedimentation.
Figure 5. Suspensions of (A) unmodified INT-WS2 and (B)
polyCOOH f-INT-WS2 in H2O after low-power sonication
and standing undisturbed 18 h at room temperature.
4. Conclusions
A novel method for the polycarboxyl surface
functionalization (polyCOOH shell) of INT-WS2
via an electrophilic reactive modified
Vilsmeier-Haack reagent is described. This
polycarboxylated shell can serve as an anchoring
shell for subsequent second-step attachment of a
wide variety of organic molecules/polymers,
including other components such as NPs, for
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Nano Res.
example, onto the nanotube surface using quite
versatile, simple organic chemistry (EDC
activation of polyCOOH), enabling surface
property tuning to match those of any contacting
material (polymeric phases, solvents, etc.).
Moreover, by employing reactive functional
groups such as those described in this study
(polyNH2/polySH shells), the chemically
modified f-INT-WS2 can be covalently bound to a
variety of reactivity-complementing materials.
These surface modifications should prove quite
useful in a wide range of applications in which
interfacial phase/component compatibility is
presently problematic. For example,
nanocomposite materials that incorporate such
functional f-INT-WS2 in any given polymer
matrix are expected to display significantly
improved properties such as mechanical strength,
impact resistance, and fracture performance
compared to those obtained when using
unmodified INT-WS2. A wide range of research
areas and applications can now benefit from (i)
the inclusion of these functionalized nanotubes
and (ii) the very versatile chemistry-tailored
platform.
Acknowledgements
We thank NanoMaterials Ltd. for their generous
gift of WS2 nanotubes and the Israel National
Nanotechnology Initiative Focal Technology Area
proposal “Inorganic Nanotubes: From
Nanomechanics to Improved Nanocomposites”
for partial funding of this research. References
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