Solid state synthesis of tungsten carbide nanorods and nanoplatelets by a single-step pyrolysis

Article abstract of DOI:10.1021/jp0540003

We report a simple and efficient single-step synthesis of tungsten carbide nanorods and nanoplatelets by direct pyrolysis of a hybrid composite material of 12-tungstophosphoric acid and hexadecyltrimethylammonium bromide in a closed Swagelok cell at 1000?°C. The product was characterized by XRD, TGA, SEM, TEM, XPS, and CV. The diameter of the nanorods is 30-50 nm, and the length varies from 200 to 500 nm. The size of the platelets is around 55 nm. The WC exhibits an interesting structural surface with kinks, steps, and terraces which is evidenced by HRTEM studies. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0540003

19056
2005, 109, 19056-19059
Published on Web 09/23/2005
Solid State Synthesis of Tungsten Carbide Nanorods and Nanoplatelets by a Single-Step
Pyrolysis
Sangaraju Shanmugam, David S. Jacob, and Aharon Gedanken*
Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for
AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel
ReceiVed: July 20, 2005; In Final Form: September 12, 2005
We report a simple and efficient single-step synthesis of tungsten carbide nanorods and nanoplatelets by
direct pyrolysis of a hybrid composite material of 12-tungstophosphoric acid and hexadecyltrimethylammonium
bromide in a closed Swagelok cell at 1000 °C. The product was characterized by XRD, TGA, SEM, TEM,
XPS, and CV. The diameter of the nanorods is 30-50 nm, and the length varies from 200 to 500 nm. The
size of the platelets is around 55 nm. The WC exhibits an interesting structural surface with kinks, steps, and
terraces which is evidenced by HRTEM studies.
Introduction
Experimental Section
The composite hybrid was prepared by the reaction of 12-
phosphotungstic acid and hexadecyltrimethylammonium bro-
mide. Elemental and thermogravimetric analysis revealed that
the molar ratio of PW to CTAB in the product was 1:3. The
hybrid composite was synthesized by dissolving CTAB in
distilled water, and this solution was added slowly to the PW
solution by vigorous stirring. The molar ratio of PW and CTAB
was kept at 1:3. The white precipitate was filtered and dried at
room temperature. From the C, H, N analysis, the formula of
as-synthesized product was found to be [C21.95H41.19N1.33]3-
PW12O40.
Transition metal carbides in general, tungsten carbide in
particular, find potential applications due to their special
properties, such as high melting point, superior hardness, low
friction coefficient, high oxidation resistance, and good electrical
conductivity.1,2 Arie et al. prepared a WC nanoneedle on a
tungsten tip by catalytic deposition. The product tested as a
scanning tunneling microscope tip.3 WC was shown to have
platinum-like behavior for the chemisorption of hydrogen and
oxygen, its applicability as an alternative electrocatalyst of Pt.
The early studies showed that this combination of an early
transition metal with carbon yielded materials with attractive
catalytic activity, stability, selectivity, and resistance to
poisoning.4-9 Recently, there have been several reports on WC
as a non-noble metal electrocatalyst in the polymer electrolyte
fuel cell (PEFC).10-12 The most important tungsten carbides are
WC and W2C. W2C is thermodynamically unstable at low
temperatures, while WC is a stable compound. Zellner et al.
recently examined the electrochemical stability of WC and W2C
in acidic solutions and found that WC is stable.13
The synthesis of WC was carried out by using a Swagelok
cell, which was assembled from stainless steel Swagelok parts.
3
A /4 in. union part was plugged from both sides by standard
caps. For this synthesis, 0.5 g of a composite hybrid of 12-
phosphotungstic acid (H3PW12O40, PW) and hexadecyltrim-
ethylammonium bromide (C16H33N(CH3)3Br, CTAB) was in-
troduced into the cell at room temperature under atmospheric
conditions. The filled cell was closed tightly with the other plug
and then placed inside an iron pipe at the center of the furnace.
The temperature was raised at a heating rate of 40 °C/min. The
closed vessel cell was heated at 1000 °C for 10 h. The reaction
took place at an autogenic pressure of the precursor. The closed
vessel cell (Swagelok) heated at 1000 °C was gradually cooled
to room temperature and opened with the release of a little
pressure, and 0.325 g of black powder was obtained. The total
yield of the obtained material was 65% (relative to the starting
material). The final black product was treated with HCl 35%
and analyzed by elemental (C, H, N) analysis. The content of
the carbon was found to be 6.14 wt %. Energy dispersive X-ray
analysis of the sample showed a W to C ratio of 1:1.
There are several routes to prepare tungsten carbide, including
direct carburization of tungsten powder, solid state metathesis,
reduction-carburization, mechanical milling, and polymeric
precursor routes using metal alkoxides.14-18 WC production
generally proceeds as a two-step process. First, the oxide is
reduced to a high purity tungsten in a hydrogen atmosphere.
The tungsten metal is then mixed with the required amount of
carbon and reacts at a temperature of 1400-1600 °C to produce
tungsten carbide.19-21 Herein, we describe a novel and efficient
single-step direct pyrolysis of a composite hybrid of 12-
phosphotungstic acid (H3PW12O40, PW) and hexadecyltrim-
ethylammonium bromide (C16H33N(CH3)3Br, CTAB) in a
specially designed Swagelok union, which resulted in WC
nanorods and nanoplates with quantitative yields.
Results and Discussion
The phase purity of the product was examined by X-ray
diffraction (XRD) using a Bruker AXSD* advanced powder
* Corresponding author. E-mail: gedanken@mail.biu.ac.il.
10.1021/jp0540003 CCC: $30.25 © 2005 American Chemical Society

Letters
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19057
the presence of nanorods and platelets (Figure 2a). The low
magnification TEM image of the product consists mainly of
rods and platelets (Figure 2b,c). The diameters of the WC rods
are 30-50 nm with varying lengths of 600-1000 nm, and the
sizes of the platelets are around 55 nm. The high resolution
TEM (JEOL 3010 operated at 200 kV) micrographs clearly
indicate that the rod is highly crystalline in nature, which is
shown in Figure 2d. From the high resolution TEM (HRTEM)
studies, it is evident that the growth of the nanorod is mainly
along the (001) direction, while the platelets are grown in the
direction of the (100) plane
The high resolution TEM image of Figure 2c is presented in
Figure 3. It illustrates that the platelet consists of surface features
such as steps, kinks, and terraces. Figure 3a shows the lattice
fringes with a d spacing of 0.252 nm, which closely matches
the (100) plane of hexagonal WC. The fast Fourier transform
(FFT) pattern of the flat surface of the platelet is consistent with
the d values observed experimentally. The platelet also contains
a grain boundary (001)/(101) which is shown in Figure 3b.
Figure 1. XRD pattern of WC product obtained at 1000 °C for 10 h.
X-ray diffraction using Cu KR radiation at 40 kV and 45 mV.
Figure 1 shows the XRD pattern of the product, and it can
readily be indexed to a hexagonal WC with [space group P6m2
(187)] lattice constants of a ) 2.90 Å and c ) 2.88 Å, which
match with the literature values (JCPDS 00-051-0939). The
morphology of the product was studied with transmission
electron microscopy (TEM) on a JEOL-TEM 100 SX micro-
scope. The scanning electron microscopy (SEM) image shows
Figure 4a shows the 4f electron photoelectron emission
spectra of WC. It exhibits the W 4f5/2 and W 4f7/2 peaks at
binding energies of 34.40 and 32.28 eV, respectively, which
correspond to the carbidic-modified W. It also demonstrates the
oxygen-modified W on the particle’s surface at 36.2 and 38.17
Figure 2. (a) SEM image of WC nanostructures. (b) TEM image showing the coexistence of WC nanorods and nanoplatelets. (c) TEM image of
an individual nanoplatelet. (d) HRTEM image of an individual nanorod, with the arrow showing lattice fringes of the WC(001) plane.

19058 J. Phys. Chem. B, Vol. 109, No. 41, 2005
Letters
Figure 3. High resolution electron micrographs of the WC platelet shown in Figure 2c. (a) TEM image showing steps, kinks, and terraces. The
lattice fringes correspond to the (100) lattice plane of WC. The inset shows a fast Fourier transform (FFT) pattern of the flat surface. (b) HRTEM
image showing grain boundary (001)/(100) planes marked with dotted lines. The lattice fringes correspond to the (001) and (100) planes of WC.
of 283.3 eV (carbidic) and 284.8 eV (graphitic). The value of
the carbidic binding energy is in good agreement with previous
data.22 The energy dispersive X-ray (EDX) and elemental
analysis also showed an excess of free carbon (0.14%) on the
WC surface. The electrochemical stability of WC was measured
in 0.5 M H2SO4 using a WC-coated glassy carbon (GC)
electrode employed as a working electrode. Figure 4b shows
the cyclic voltammogram of WC after the steady state condition.
The voltammogram showed a large current increase above 0.6
V, indicating the oxidation of WC. In the reverse scan, there
was a cathode current due to the reduction of the oxidized
species during the anodic scan.
To determine the effect of temperature on the formation of
WC, we performed several experiments at different temperatures
in the 800-1000 °C range. It is evident from the XRD analysis
that WO2 is the predominant product at 800 °C, and a mixture
of W, W2C, and WC was observed at 900 °C. When the same
reaction was carried out at 1000 °C, but at shorter times, 3 and
6 h, a mixture of WC and W2C was obtained (SI). Thus, the
formation of WC as the sole product in a 10 h reaction at 1000
°C can be explained as follows: First, the precursor is converted
to WO2 and free carbon at low temperatures (800 °C); the free
carbon reduces the WO2 to metallic W at 900 °C. The produced
W can further react with carbon, giving rise to tungsten carbide
(1000 °C). We have also carried out similar experiments in order
to understand the formation of WC by direct pyrolysis of the
precursor by varying the anion and cation. We obtained entirely
different results. When the anion is changed from heteropolya-
nion (PW12O40)3- to isopolyainon (W10O32)4- with the same
Figure 4. (a) X-ray photoemission spectra of the core electron binding
cation, CTAB, we find open shallow tubes. Early results point
out to the formation of carbon nanotubes and W nanoparticles.
The formation of WC by the direct pyrolysis of a single
precursor is a potential route for the production of WC in large
scale.
energy of W 4f of WC and (b) cyclic voltammogram of WC in 0.5 M
sulfuric acid. Scan rate: 25 mV S-1
.
eV. The stoichiometric WC reacted strongly with atmospheric
oxygen to passivate the surface.21 X-ray photoelectron spec-
troscopy (XPS) showed that the proposition of W0 to W6+ is
very small, indicating that the surface is not oxidized. XPS also
determined the free carbon contamination of the carbide surface.
Two types of carbon peaks were detected at binding energies
Conclusions
In conclusion, we have demonstrated the synthesis of single
phase WC nanorods and nanoplatelets by a simple and efficient

Letters
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19059
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single-step direct pyrolysis of a hybrid composite material of
12-tungstophosphoric acid and hexadecyltrimethylammonium
bromide at relatively low temperatures. The WC nanoplatelets
exhibited interesting surface features, which can be exploited
in catalysis. The adopted method is not only applicable for
tungsten-based polyoxometalates, but it can also be extended
to other metals. The method presented here can be easily
upscaled to produce WC nanorods in large quantities.
Acknowledgment. The authors thank Dr. Yudith Grinblat
and Dr. Y. Gofer for the TEM and XPS analysis. S.S. is thankful
to the European community for his postdoctoral fellowship.
Supporting Information Available: XRD patterns of
products obtained at 1000 °C for 3 and 6 h and EDX analysis
of WC. This material is available free of charge via the Internet
at http://pubs.acs.org.
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