Preparation of tungsten carbide nanosheets with large surface area using an in situ DWCNT template

Abstract

Tungsten carbide (WC) nanosheets were prepared using an in situ double-walled carbon nanotube (DWCNT) template with high dispersion. The growth of tungstenic acid (H₂WO₄) was restrained because of homogenous DWCNTs that were dispersed in ethylene glycol (EG). H₂WO₄ sheets approximately 50 nm long and 20 nm wide were obtained. WC nanosheets with large surface areas were prepared when the tungstenic precursor was carbonized at a low temperature using a liquid carbon source. The WC was used as a precursor for depositing Pt nanoparticles, and the obtained WC–Pt displayed a high electrochemical activity (Pt, 10 wt%). The results indicated that WC with a peculiar morphology and size could be used in electrochemistry, horniness alloys, and in other related fields.

---

1. Introduction

Tungsten carbide (WC) is an interstitial compound of C atoms that are filled into a W crystal, which possesses high strength and rigidity as a covalent compound, high melting point as an ionic crystal, and electromagnetism as a transition tool and surface coating.^1–3 However, it is difficult for horniness alloys to form through sintering because of the high melting point of WC. Once the carbide size is reduced to the order of nanometers, the compaction temperature will be low and the sintering time will be reduced. Nano- and microsized WC can be compacted at 500 and 1200 °C, respectively.⁴ In addition, nanosized WC exhibits a markedly improved strength, hardness, and tough sintered alloy.^5–8

WC is gaining attention for commercial fuel cell applications because of its high melting point and noble metal-like activity.^9–15 Since 1960, WC has been used as an anodic material for hydrogen¹⁶ or methanol fuel cells.¹⁷ Studies have focused on WC in chemical and electrochemical catalysis. The carbide exhibits high stability in acidic solutions^18–20 and has high toleration to carbon monoxide and bisulfide.^21–23 However, WC-based anodic catalysts function poorly as electrocatalysts, despite their resistance carbide to carbon monoxide poisoning.^24,25 The traditional metallurgical procedure is usually performed at high temperatures and yields materials with large particle sizes and low specific surface areas.

Nanosized WC can be used in a wide range of applications. Several methods have been proposed to synthesize the carbide, such as the conventional furnace method based on direct interactions between elements, carbon-induced reduction of tungsten oxide,²⁶ reduction of carbothermal hydrogen,²⁷ and the reaction of tungsten with a methane–hydrogen mixture.²⁸ However, the traditional methods exhibit difficulties during the preparation of nanosized WC with large specific surface areas because of high temperature, large size precursor, and insufficient interaction between the tungsten precursor and carbon sources. In addition, the prepared WC exhibits nanosized particle morphology. To the best of our knowledge, few studies focusing on WC with the morphology of nanosheets have been reported.^29,30 In this study, highly dispersive double-walled carbon nanotubes (DWCNTs) were used as the carrier and template in an in situ approach to prepare well dispersed, small sized WC precursors and deoxidize the oxides at low temperatures with a liquid carbon source. WC nanosheets with large surface areas were prepared with a template that allowed sufficient interaction between the tungsten precursor and carbon sources. The WC was used as a precursor for depositing Pt nanoparticles, and the obtained WC–Pt displayed a high electrochemical activity (Pt, 10 wt%).

2. Experimental

2.1 Preparation of tungstenic acid (H₂WO₄)

H₂WO₄ was deposited on DWCNT bundles by direct precipitation with DWCNTs as an in situ template. A homogenous DWCNT/ethylene glycol (EG) suspension was nondestructively prepared as previously reported.^28,31–33 The diameters of the dispersed DWCNTs and DWCNT bundles were approximately 6 nm and 10–200 nm, respectively, and the length of the nanotubes ranged from 1 μm to 2 μm. Following the preparation of the DWCNT/EG suspension, sodium tungstate (Na₂WO₄) was added to the suspension. The complex suspension was transferred into a round flask to obtain the tungstenic precursor. HCl was added to the suspension when the complex suspension was heated to 100 °C in a reflux device. The reaction temperature was sustained for 2 h to enable sufficient H₂WO₄ deposition onto DWCNT bundles. The as-received sample was cooled to room temperature, filtered, and washed with excess deionized water. The sample was then dried at room temperature in a low-vacuum system.

2.2 Carbonization of tungstenic precursor

The as-prepared precursor was carbonized by ethanol with a single-step method. The prepared precursor was placed into a reaction furnace, and two flanges were applied on both ends of the reaction tube to seal the reaction chamber. The reaction system was drawn into a vacuum state prior to heating. After the reaction system was heated to 600 °C, air was pumped into the tube at a flow rate of 300 ml min⁻¹ for 1 h. High-purity nitrogen was then introduced into the chamber at a flow rate of 300 ml min⁻¹ to function as a protective gas. When the air was completely eliminated from the reaction chamber, the system was further heated to 950 °C. Ethanol was propelled into the reactor by an electronic squirming pump and nitrogen flow with a supply rate of 3 ml h⁻¹. The furnace was turned off and cooled to room temperature after 3 h. The received products were then collected from the furnace.

2.3 Pt deposition on tungsten carbide

Pt particles were deposited on WC in EG with a good dispersion state. EG functioned as a reduction and dispersion medium. Accurately measured chloroplatinic acid (H₂PtCl₆) was added to the EG solution and then heated to 140 °C in a reflux device for 5 h. After cooling to room temperature, the as-received sample was filtered, washed with excess deionized water, and dried at room temperature in a low-vacuum system. Pt loading was controlled at 10 wt%.

2.4 Characterization of sample

The sample microstructure was studied by high-resolution transmission electron microscopy (HRTEM, JEOL 2100F, accelerating voltage, 200 kV), scanning electron microscopy (SEM, XL30), X-ray diffraction (XRD, D8 Advance, Bruker) by Cu Kα radiation, and thermogravimetric analysis (TGA, Diamond TG/DTA6300, heating rate, 10 °C min⁻¹ from room temperature to 1000 °C at a N₂ flow rate of 100 ml min⁻¹). Energy dispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition of the selected area. N₂ adsorption and desorption isotherms of tungsten materials were measured at 77 K using a BELSORP instrument (BEL, Japan, Inc.). Details regarding the analytical methods can be found in ref. 34.

2.5 Electrochemical measurements

The electrochemical activities of WC–Pt materials were characterized by cyclic voltammetry (CV). Experiments were performed in a three-electrode cell using an EG & G potentiostat (CHI 618C) at room temperature. Working electrodes were prepared by spreading a catalyst coating and 5% Nafion mixture in ethanol on a glass carbon cylinder with a diameter of 3 mm. Pt loading of the working electrode was controlled at 0.2 mg cm⁻². Pt foil and a saturated calomel electrode (SCE) were used as the counter electrode and electrolyte, respectively. CV profiles were recorded at a scan rate of 100 mV s⁻¹ for potentials against SCE that ranged from −0.241 V to 0.999 V.

3. Results and discussion

The microstructures of the semifinished sample that were prepared from a DWCNT/EG suspension, Na₂WO₄, and HCl were examined by SEM, HRTEM, XRD, and TGA (Fig. 1). From Fig. 1a, it can be observed that the sample has undergone self-assembly to form floccules. Small particle deposition on the nanotube bundles is evident at higher magnifications of the floccules (Fig. 1b). Fig. S1a of the electronic supplementary information (ESI)† depicts the macro-morphologic features of the material, which exhibits a powder-like appearance. The sample was further characterized with XRD, and characteristic peaks (Fig. 2a) with 2θ at 13°, 23°, 28.4°, 33.8°, 35.2°, 36.5°, 50°, 55.6° and 63.5° were observed, which are indexed as planes of (010), (200), (011), (−201), (220), (221), (410), (420), and (510) for H₂WO₄·H₂O, respectively (JCPDS card no. 18-1420). The diffraction peaks (2θ at 16.6° and 26°) may be attributed to (110) and (002) of C. Therefore, the sample yielded diffraction peaks of H₂WO₄·H₂O and C phases (curve a, Fig. 2), which indicated the occurrence of the following chemical reaction: Na₂WO₄ + 2HCl + H₂O → H₂WO₄·H₂O↓ + 2NaCl. Crystal seeds of H₂WO₄ can be formed homogenously in the reaction suspension due to the uniform dispersion of DWCNTs, Na₂WO₄, and HCl in EG (Fig. 3 left). In the absence of DWCNTs, the seeds will continue to expand along their own surface and will eventually coalesce to form larger particles that settle down and thereby affect the homogeneity of the mixture. The presence of dispersed nanotube bundles will prevent the union of the crystal seeds of H₂WO₄, thereby restricting the formation of secondary seed growth. This leads to the direct deposition and growth of H₂WO₄ seeds using DWCNT bundles as a support template (Fig. 3 right). Fig. 1a and the diffraction pattern of the C phase suggest that the DWCNTs are homogenously dispersed during the deposition of H₂WO₄·H₂O. Thus, H₂WO₄ sheets can be formed due to the bundle surface that has not been completely covered by H₂WO₄ seeds.

Fig. 1 Typical SEM image (a), HRTEM images (b and c) and TGA curves (d) of the semifinished sample prepared from DWCNTs/EG suspension, Na₂WO₄ and HCl.

Fig. 2 XRD patterns of the semifinished sample before (a) and after (b) heated to 600 °C, and carbonized at (c) 850 °C and (d) 950 °C.

Fig. 3 Schematic illustration of crystal seeds of H₂WO₄ deposited on DWCNT and formed nanosheet.

The morphology of the H₂WO₄ was also investigated by HRTEM. Several nanoparticles and nanotube bundles were observed (Fig. 1b). Sheet morphologies with length and width of 50 and 20 nm, respectively, were observed when the morphology of the sample was magnified (Fig. 1c). In addition, CNTs were randomly found in the sample. These results indicated that the morphology of the deposited samples correlated to that of DWCNTs. Sheet morphologies were formed when DWCNT bundles were partly surrounded by the deposited sample from the chemical reaction of Na₂WO₄ and HCl. Furthermore, nanosized samples were maintained if the resultant sheets were not in contact with each other. The visualizations were realized for dispersed DWCNT bundles that were homogenously distributed in the EG solvent. Therefore, DWCNT templates significantly influenced the size and morphology of the deposited sample from the reaction of Na₂WO₄ and HCl.

Thermoanalysis was performed to clarify the carbonization of the loaded as-prepared sample on DWCNTs. The TGA curve (curve 1, Fig. 1d) exhibited a reduced weight with increasing temperature. A sharp weight loss was observed in the sample when the temperature was increased from room temperature to 600 °C. This can be attributed to the decomposition of H₂WO₄ (H₂WO₄ → WO₃ + H₂O) from the reaction of Na₂WO₄ and HCl. The sample weights were slightly reduced for temperatures close to 1000 °C (Fig. 1d), which corresponded to the DTA curve (curve 2) with no obviously absorbable or exothermal peaks, indicating that H₂WO₄ was completely decomposed at approximately 600 °C. Furthermore, reaction between the WO₃ and DWCNTs carbon source did not occur even with temperatures as high as 1000 °C.

After heating to 600 °C and carbonization at 950 °C, the semifinished sample was further characterized using XRD based on thermoanalysis results. Clear characteristic peaks (Fig. 2b) with 2θ at 23.6°, 33.6°, 41.5°, 48.4°, and 54.5° were found, which are indexed as (200), (220), (222), (400), and (420) planes of WO₃ (JCPDS card no. 46-1096). The results may be attributed to the reaction of H₂WO₄·H₂O → WO₃ + 2H₂O. The XRD patterns for the WO₃ phase were observed and are shown in curve b. In addition, DWCNTs can be burned completely with pumped air at 600 °C. The macro-morphologic feature of the sample is shown in ESI Fig. S1b.† ESI Fig. S2† reveals the morphology of the sample, SEM images, and EDS results corresponding to the XRD data.

If the temperature was further increased to 950 °C, characteristic peaks (Fig. 2d) with 2θ at 31.6°, 35.7°, 48.5°, 58.5°, 64.3°, 73.5°, and 77.2° are found, which are indexed as (001), (100), (101), (110), (111), (002), and (102) planes of WC (JCPDS card no. 65-4539). Therefore, the WC phase was clearly observed in the XRD pattern, and no W₂C phase existed in the sample. Another strong peak at 40.3° appeared if the temperature was increased only to 850 °C (Fig. 2c), which may be attributed to the W₂C phase. The result indicates that liquid ethanol has undergone decomposition to form gaseous carbon. The WO₃ phase was completely deoxidized to sufficiently make contact with the gaseous carbon source from ethanol. Therefore, the WC phase appeared after heating WO₃ at 950 °C in an N₂ environment. ESI Fig. S1c† shows the macro-morphologic feature of WC. The temperature used in the current study for carbonization was lower than the temperature used in traditional methods.¹⁴ The complete WC phase in the XRD pattern demonstrated that the microstructure of the tungstenic precursor of H₂WO₄·H₂O and liquid carbon sources significantly affected the formation of the WC phase. The EDS results from the selected area of the SEM image (ESI Fig. S3†) also indicate that the atomic ratio of W and C was near 1 : 1. A gaseous carbon source can be easily obtained when liquid ethanol is heated. In addition, the gaseous carbon source can easily move to W atoms and deoxidize the tungstenic precursor. If DWCNTs were used as the carbon source (air was not pumped into the reaction tube at 600 °C), WC cannot form due to the high thermal stability of the DWCNTs, which prevents the carbon atoms from reaching the W atoms at such low temperatures (approximately 950 °C).

Fig. 4a and b illustrate the N₂ adsorption–desorption isotherms and the corresponding pore size distribution for H₂WO₄/DWCNTs before and after carbonization, respectively. The isotherms were obtained at 77 K, and the pore size distributions were calculated from the desorption branch of N₂ isotherms via the BJH method.³⁵ The specific surface areas of H₂WO₄/DWCNTs and WC from the BET method were found to be 39.8 and 16.2 m² g⁻¹, respectively. The corresponding total pore volumes were 0.1023 and 0.0566 m³ g⁻¹ (P/P₀ = 0.966), respectively, suggesting that the sample exhibited higher specific surface areas and pore volumes compared to traditional products.¹⁴ The results were attributed to the prepared nanosized H₂WO₄·H₂O and WC with sheet morphologies. The reduced specific surface area and pore volume indicated a different morphology of WC when H₂WO₄·H₂O was heated and carbonized. The result was caused by porous structures that diffused the C atom into W, and thus, highly crystalline WC occupied certain pores.

Fig. 4 N₂ adsorption–desorption isotherms (a) and pore radius distributions (b) of the semifinished sample before and after heated to 600 °C, and carbonized at 950 °C.

Fig. 5 shows the TEM, EDS, and HRTEM images of WC–Pt materials that are prepared in homogenous EG systems. Pt particles were uniformly precipitated on the supports (Fig. 5a to d). Several nanosized materials conglomerated on each other, and nanosheets were also observed (Fig. 5b). Lengths and widths of the nanosheets varied from 60 nm to 500 nm, which were larger than those of nanoparticles. Most of the nanoparticles were scattered on or around the nanosheets. Fig. 5c and d reveal the homogeneous distribution of the nanoparticles with sizes that range from 3 nm to 5 nm. The inter-planar distances (Fig. 5d) indicate that the nanoparticles are Pt particles. The EDS result (Fig. 5e, selected area in Fig. 5b) indicates that the Pt loading is approximately 10 wt%. In addition, Fig. 2d demonstrates that the atomic ratio of W and C is approximately 1 : 1. The Cu peak observed in the EDS spectrum arises from the Cu grid used for the preparation of the TEM sample. This observation suggests that the nanosheet is most likely WC, and the composite is composed of WC and Pt. The inter-planar distances measured in the HRTEM (Fig. 5e) image were approximately 0.23 nm, 0.2 nm, and 1.2 nm, which were in accordance with the (111), (200), and (311) facets of Pt (JCPDS card no. 04-0802, XRD data Fig. 6). It is implied that the small-size nanoparticles were Pt particles that were decomposition products from the H₂PtCl₆ precursors. Fig. 5f and g depict the typical morphology of the nanosheets, with a length and width of approximately 80 nm and 60 nm. Lattice fringes indicate the highly crystalline structure of the prepared WC with sheet morphologies. The inter-planar distance measured in the HRTEM (Fig. 5g) image was approximately 0.29 nm, which was in accordance with the (001) facet of WC, and the result was consistent with the XRD data (Fig. 2).

Fig. 5 Typical TEM, EDS and HRTEM images of WC–Pt material prepared in homogenous EG systems.

Fig. 6 XRD pattern of WC–Pt material prepared in homogenous EG systems.

WC–Pt exhibited a current density and electrochemically active surface area of 0.011 A cm⁻² and 66.67 m² g⁻¹ (Fig. 7), respectively, which indicated a high electrochemical activity given that 10 wt% Pt was used in the study. Small-valued peaks were observed in CV curves (ESI Fig. S4†) if the prepared WC nanosheet was used to measure electrochemical activity. Therefore, synergistic and catalytic properties of the WC nanosheet in the catalyst could be speculated.

Fig. 7 CV curve of WC–Pt material at a scanning rate of 100 mV s⁻¹. Note: WC–Pt material loading on the working electrode was controlled to be 0.2 mg cm⁻².

The high electrochemical activity of the nanosheets was attributed to the morphology and nanosize sheet characteristic of the prepared WC phase. Sheet morphology and nanosize features of the materials were affected by the DWCNT template, and WO₃ and WC could be prepared by carbonization. In addition, the small sizes of WC that were prepared at low carbonization temperatures were maintained. Furthermore, the liquid carbon source was evaporated at carbonization temperatures, and sufficient contact between the gaseous carbon source and WO₃ was ensured. Therefore, WC nanosheets could be prepared by in situ DWCNT templates and liquid carbon sources at low carbonization temperatures.

4. Conclusions

WC nanosheets were prepared with lengths and widths that varied from 60 nm to 500 nm by using highly dispersive DWCNTs as a template. The addition of HCl into a DWCNT and tungstenic precursor suspension formed H₂WO₄ seeds, in which the crystal seed was deposited on DWCNT bundles as supports. The second growth of the crystal grain of H₂WO₄ could be restrained because of homogenous DWCNTs that were dispersed in EG solvent. Therefore, H₂WO₄ sheets could be prepared with length and width of approximately 50 and 20 nm, respectively. WC with a homogenous phase and good crystallization could be prepared after heating the prepared H₂WO₄ with a pumped liquid carbon source. WC sheets could be potentially applied in electrochemistry, horniness alloys, and other related fields because of their interesting morphologies and sizes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51202095 and 51264010), the Department of Science & Technology, and Human Resources & Social Security of Jiangxi Province (20122BAB216013, 20121BBE50027, [2012]195), and the China Scholarship Council Science Foundation (201208360129).

Notes and references

1. A. Kumar, K. Singh and O. P. Pandey, Ceram. Int., 2011, 37, 1415–1422.
2. K. Feng, J. Xiong, L. Sun, H. Fan and X. Zhou, J. Alloys Compd., 2010, 504, 277–283.
3. B. Huang, L. D. Chen and Q. Bais, Scr. Mater., 2006, 54, 441–445.
4. Z. Yan, M. Cai and P. K. Shen, Sci. Rep., 2013, 3, 1646–1651.
5. G. R. Goren-Muginstein, S. Berger and A. Rosen, Nanostruct. Mater., 1993, 3, 19–30.
6. J. P. C. Urbina, C. Daniel and C. Emmelmann, Phys. Procedia, 2013, 41, 752–758.
7. L. Bennett, J. R. Cuthill, A. Mcalister, N. Erickson and R. Watson, Science, 1975, 187, 858–859.
8. K. M. Tsai, Int. J. Refract. Met. Hard Mater., 2011, 29, 188–201.
9. V. Keller, P. Wehrer, F. Garin, R. Ducros and G. Maire, J. Catal., 1995, 153, 8–16.
10. C. D. Patrick, L. L. Jean and P. Claude, Catal. Today, 2001, 65, 195–200.
11. P. Hasin, J. Phys. Chem. C, 2014, 118, 4726–4732.
12. M. V. Iyer, L. P. Norcio, E. I. Kugler and D. B. Dadyburjor, Ind. Eng. Chem. Res., 2003, 42, 2712–2721.
13. N. Ji, T. Zhang, M. Y. Zheng, A. Q. Wang, H. Wang, X. D. Wang and J. G. Chen, Angew. Chem., Int. Ed., 2008, 47, 8510–8513.
14. E. Antolini and E. R. Gonzalez, Appl. Catal., B, 2010, 196, 245–266.
15. C. A. Ma, C. B. Xu, M. Q. Shi, G. H. Song and X. L. Lang, J. Power Sources, 2013, 242, 273–279.
16. H. Binder, A. Kohling, W. Kuhn, W. Lindner and G. Sandstede, Nature, 1969, 224, 1299–1300.
17. D. Baresel, W. Gellert, J. Heidemeyer and P. Scharner, Angew. Chem., Int. Ed., 1971, 10, 194–195.
18. Z. X. Yan, G. Q. He, M. Cai, H. Meng and P. K. Shen, J. Power Sources, 2013, 242, 817–823.
19. H. Chhina, S. Campbell and O. Kesler, J. Power Sources, 2007, 164, 431–440.
20. M. Rahsepar, M. Pakshir, P. Nikolaev, A. Safavi, K. Palanisamy and H. Kim, Appl. Catal., B, 2012, 127, 265–2729.
21. D. R. Mclntyre, G. T. Burstein and A. Vossen, J. Power Sources, 2002, 107, 67–73.
22. J. B. Christian, S. P. E. Smith, M. S. Whittingham and H. D. Abruua, Electrochem. Commun., 2007, 9, 2128–2132.
23. A. R. Ko, Y. W. Lee, J. S. Moon, S. B. Han, G. Z. Cao and K. W. Park, Appl. Catal., A, 2014, 477, 102–108.
24. V. S. Palanker, R. A. Gajyev and D. Sokolsky, Electrochim. Acta, 1977, 22, 133–136.
25. H. Okamoto, G. Kawamura, A. Ishikawa and T. Kudo, J. Electrochem. Soc., 1987, 134, 1645–1658.
26. G. D. Rieck, Tungsten and its compounds, Pergamon Press, England, 1967.
27. C. Liang, F. Tian, Z. Wei, Q. Xin and C. Li, Nanotechnology, 2003, 14, 955–958.
28. F. F. P. Medeiros, S. A. D. Oliveira, C. P. D. Souza, A. G. P. D. Silva, U. U. Gomes and J. F. D. Souza, Mater. Sci. Eng., A, 2001, 315, 58–62.
29. C. H. Liang, L. Ding, C. Li, M. Pang, D. S. Su, W. Z. Li and Y. M. Wang, Energy Environ. Sci., 2010, 3, 1121–1127.
30. C. L. Guo, Y. T. Qian and P. J. Han, J. Nanomater., 2011, 987530–987531.
31. Z. P. Wu, J. N. Wang and J. Ma, Carbon, 2009, 47, 324–327.
32. L. F. Su, J. N. Wang, F. Yu, Z. M. Sheng, H. Chang and C. Pak, Chem. Phys. Lett., 2006, 420, 421–425.
33. Z. P. Wu, M. M. Li, Y. Y. Hu, Y. S. Li, Z. X. Wang, Y. H. Yin, Y. S. Chen and X. Zhou, Scr. Mater., 2011, 64, 809–812.
34. B. C. Lippens and J. H. De Boer, J. Catal., 1965, 4, 319–413.
35. Z. Slanina and J. F. Crifo, Int. J. Thermophys., 1992, 13, 465–476.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08029g

---