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Facile Synthesis of WSe2 Nanoparticles and Carbon Nanotubes Swati V. Pol,† Vilas G. Pol,*,† Jose M. Calderon-Moreno,‡,| and Aharon Gedanken† Center for AdVanced Materials and Nanotechnology, Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel, Applied Physics Department, UniVersitat Politecnica de Catalunya, AV. Canal Olimpic, Castelldefels, 08860 Barcelona, Spain, and Institute of Physical Chemistry Ilie Murgulescu, Romanian Academy, Splaiul Independentei 202, Bucharest 060021, Romania ReceiVed: December 20, 2007; In Final Form: January 11, 2008
A single-step, nonaqueous, solventless, facile chemical reaction is carried out to synthesize a WSe2 nanoparticles/carbon nanotubes [WSe2/C] composite at moderate temperature. The use of highly toxic H2Se gas is avoided, reacting elemental selenium powder with W(CO)6 at 750 °C under their autogenic pressure in a closed reactor. Without further processing the as-prepared WSe2/C nanocomposite, detailed characterizations are carried out, and a proposed reaction mechanism is discussed. X-ray diffraction measurements are consistent with the hexagonal phase of WSe2, and the highly crystalline nature of WSe2 nanoparticles is also confirmed by high-resolution transmission electron microscopy pictures. Scanning electron microscopy measurements reveal that WSe2 nanoparticles and carbon nanotubes are formed, with the composition being tracked by C, H, N, S and energy-dispersive X-ray analysis. The Brunauer, Emmett, and Teller surface area analysis technique is implemented for the determination of nitrogen gas adsorption on the surface of the WSe2/C nanocomposite.
Introduction Tungsten diselenide (WSe2) has a hexagonal structure consisting of single sandwiched Se and W layers with a thickness of about 3.3 Å. There is a strong covalent bonding within the Se-W-Se layers but only fairly weak van der Waals interactions between neighboring sandwich layers.1 WSe2 is a semiconductor with a band gap in the range of 1.2-2 eV, depending on how the nonstoichiometry differs from the ideal W:Se ratio of 1:2. WSe2 is a black or gray odorless material, has good thermal stability and a high melting point.2 Because of its high optical absorption, the layered arrangement between the cations, the high resistance against photocorrosion, and the magnitude of its band gap, WSe2 is an important material in photoelectrochemical conversion and photovoltaic solar energy conversion. WSe2 also plays an important role in a number of technologies such as high temperature solid lubrication and rechargeable batteries.3,4 Very recently, ultralow thermal conductivity5 has been reported in disordered, layered WSe2 crystals. Nanoparticles of WSe2 can be synthesized by a chemical reaction between W(CO)6 and selenium6 dissolved in a paraxylene solution. WSe2 thin films can be obtained by many processes, such as the reaction of WO3 thin films in a H2Se atmosphere, a solid-state reaction between the constituents sequentially deposited in a thin film form, electrodeposition, rf sputtering, and van der Waals rheotaxy.7-9 Single crystals of WSe2 were grown via a vapor-transport technique, employing SeCl4, chlorine, or iodine as a transport agent. Although a few papers have already reported on the chemical vapor deposition (CVD) of WSe2, it is difficult to handle WF6 and H2Se as precursors because they produce HF as a byproduct of the reaction.10-12 Films of WS2 were grown on different substrates between 300-700 °C and were reported to be stable and * To whom correspondence should be addressed. E-mail: vilaspol@ gmail.com. † Bar-Ilan University. ‡ Universitat Politecnica de Catalunya. | Romanian Academy.
crystalline with a preferential orientation. Recently, Boscher et al. prepared WSe2 films on glass substrates at atmospheric pressure via CVD13 at 500-650 °C. Tenne et al. probed the nested polyhedra14 of MX2 (M ) W, Mo; X ) S, Se) by highresolution transmission electron microscopy (HR-TEM) and scanning tunneling microscopy (STM). Rao CNR et al. reported on the fabrication of WSe2 nanotubes15 by the reduction of the corresponding triselenides in hydrogen or by the decomposition of the ammonium selenometalates in a hydrogen atmosphere. The comparative tribological performance of single-layer WSex and bilayer WSex/diamondlike carbon [DLC] coatings on the steel was demonstrated by Fominski et al. Because of the addition of DLC, the friction coefficient was reduced by 15%, compared to the single-layer WSe2 film, while the wear resistance16 was increased by a factor of 4. This article presents solventless, single-step, uncomplicated efficient chemical reaction for the fabrication of a crystalline WSe2 nanoparticle/carbon nanotube composite. The application of the highly poisonous H2Se gas is avoided by using Se powder and handling it under inert atmosphere inside a glove box. The mechanistic elucidation of the formation of WSe2 particles and carbon nanotubes is provided, based on the obtained analytical data and previously published reports. Synthesis of WSe2 Nanoparticles and Carbon Nanotubes The precursors, selenium powder and tungsten hexacarbonyl [W(CO)6], were purchased from Aldrich and used as received. For the synthesis of WSe2/C, a stoichiometric amount of W(CO)6 [1408 mg] and pure Se powder [632 mg] is introduced into a 5-mL stainless steel [SS] reactor at room temperature in a nitrogen-filled glove box. The filled reactor is tightly closed with the threaded plug and kept inside the 2.5 in. diameter iron pipe inside the tube’s furnace. The temperature of the tube’s furnace is raised to 750 °C at a rate of 20 °C/min and maintained at 750 °C for 1 h. The SS reactor heated at 750 °C is gradually cooled (∼5 h) to room temperature and opened, and a black
10.1021/jp7119685 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008
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Figure 1. Wide-angle XRD pattern of (a) WSe2/C fabricated from a 1 h chemical reaction between W(CO)6 and Se powder at 750 °C; (b) bulk EDAX spectrum of a WSe2/C nanocomposite.
powder is obtained. 1488 mg of the product is obtained [72.94% yield] for WSe2 nanoparticles and C nanotubes, termed as a WSe2/C nanocomposite. This crystalline product was directly characterized without further processing by various morphological, compositional, structural, and surface area analysis techniques. Results and Discussion Figure 1a shows the X-ray diffraction pattern (XRD, Bruker AXS D* Advance Powder X-ray diffractometer) of the asprepared WSe2/C nanocomposite fabricated during a 1 h chemical reaction between W(CO)6 and Se at 750 °C. The peaks can be readily indexed to those of the hexagonal phase (p63/ mmc space group) of WSe2 with lattice constants a ) 3.286 and c ) 12.980 Å (PDF No. 38-1388). The XRD pattern confirms the purity of formed WSe2 particles. However, no reflection lines are obtained from the carbon nanotubes. The nature of the formed carbon in the present system is further understood by Raman spectroscopy and discussed in the
following section. The evidence that the as-prepared WSe2 nanoparticles are indeed composed of WSe2 without any impurities comes from the EDAX analysis presented in Figure 1b. The EDAX measurements give a Se/W molar concentration ratio of ∼2. Here the additional signal for the presence of carbon is verified. The SEM image (Figure 2a) presents the mixture of agglomerated particles (∼200 nm) and long carbon fibers. The diameters of the carbon fibers are in the range of 50-100 nm with lengths of several micrometers with a globular tip (Figure 2b). The carbonaceous fibers are indeed hollow and not solid, as confirmed by TEM measurements. The formed carbon nanotubes (Figure 2c) are polydispersed with diameters from 20 to 120 nm. The HR-TEM of carbon nanotubes (Figure 2d) is semi-graphitic in nature. The measured interlayer spacing is 0.34 nm, very close to the graphitic layers. The selected area electron diffraction X-ray (SAEDX) measurements also confirmed that the formed nanotubes are of carbon (Figure 2e). The additional Cu signal is from the TEM copper grid, while
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Figure 2. Scanning electron micrographs of (a) a WSe2/C nanocomposite fabricated from a 1 h chemical reaction between W(CO)6 and Se elements at 700 °C, (b) carbon fibers shown at high resolution, (c) TEM of carbon nanotubes, (d) HR-TEM of carbon nanotube, (e) SAEDS on the carbon nanotubes, (f) Raman spectrum of carbon nanotubes, and (g) nitrogen adsorption-desorption isotherms for the WSe2/C sample.
the W is depicted from the nearby WSe2 particle. A Raman spectrum (Figure 2f) demonstrates the nature (graphitic/ amorphous) of carbon nanotubes. The first-order Raman spectrum of polycrystalline graphite consists of two peaks. The first, located at 1605 cm-1 (the G peak), originates from the in-plane lattice vibrations. The second peak is located at about 1375 cm-1 (the D peak) and only occurs in graphite with small crystal sizes. This disorder-induced mode corresponds to a peak in the vibrational density of states of graphite and is observed when the crystallite size is sufficiently small.17 The G peak is weaker than the D peak, verifying that the provided temperature [750 °C] is insufficient for the formation of highly graphitic walls in the tubes. The N2 adsorption-desorption isotherm (Micromeritics, Gemini 2375) of a WSe2/C sample is presented in Figure 2g. The measured Brunauer, Emmett, and Teller (BET) surface area is 66.2 m2/g, and a total pore volume of 0.0033 cm3/g is
recorded. The BET surface area analysis technique is also employed for the determination of nitrogen gas adsorption on the surface of the WSe2/C nanocomposite. The maximum (78.50 cm3/g) nitrogen gas adsorption was recorded at 0.9954 relative pressures (757.54 mmHg) at a liquid nitrogen temperature (77 K). Therefore, at room temperature (298 K), the amount of adsorbed nitrogen on the WSe2/C nanoparticles embedded with carbon is 303.8 cm3/g. The crystalline nature of WSe2 nanoparticles is further studied by HR-TEM measurements. The SEM picture (Figure 2a) shows some agglomerates of WSe2 nanoparticles. These agglomerates are comprised of 5-10 nm WSe2 nanoparticles, evidenced by the TEM measurements (JEOL-2010 HR-TEM) presented in Figure 3a. The WSe2 particles are polydispersed with a mixture of irregular shapes. One of the T-shaped particles is shown under high resolution (Figure 3b). The edge of one of the WSe2 particles is presented in a HR-TEM image (Figure 3c). The measured distance
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Figure 3. Transmission electron micrographs of (a) WSe2 particles [bar ) 20 nm], (b) T-shaped WSe2 particle, and (c) the HR-TEM of a single WSe2 rod and its SAED.
between the (002) lattice planes is 0.62 nm, which is very close to the literature value (0.649 nm) for the hexagonal lattice of the WSe2 phase (PDF No. 38-1388). The SAED pattern of the WSe2 nanorod is shown in the inset of Figure 3c. The dotted diffraction rings can be indexed to the reflections of the WSe2 (002) and (004) planes and prove the highly crystalline nature of the WSe2 nanoparticles. This section presents the results related to the amount of the various elements in the reactants and compares their weight in the products. For WSe2/C, the calculated element percentages of W, C, O, and Se in W(CO)6 + Se precursors are 36.03, 14.15, 18.85, and 30.97%, respectively. The measured percentage of carbon (Eager 200 C, H, N, S analyzer) in the WSe2/C product is 10 wt %, while a negligible amount of oxygen is measured. In 1.488 g of W(CO)6 + Se precursors, the calculated weights of W, C, and Se are 735, 289, and 632 mg, while the measured weights of these elements in the WSe2/C product are 720, 150, and 618 mg, respectively. This shows that the amounts of W and Se are significantly reduced in this autogenic pressurized chemical reaction, while the amount of carbon is reduced by about 28%, due to the formation of CO gases at 750 °C. The SEM image already confirmed that the formed product is a mixture of carbon nanotubes and WSe2 nanoparticles. This means that only a partial amount of WSe2 acts as the catalyst to grow the carbon nanotubes, while due to the insufficient carbon, the rest of the WSe2 remains as particles. It is known that the thermal decomposition of W(CO)6 can produce crystalline tungsten nanoparticles18 (eq 1). Selenium is a nonmetallic powder and has low melting (221 °C) and boiling (685 °C) points. The W and the Se vapor reacted at 750 °C to form WSe2 (eq 2) particles. The layered hexagonal19 structure of WSe2 is known to be an excellent solid lubricant due to its low shear strength and high melting point. The CO undergoes a Boudard20-23 disproportionation reaction to form elemental C and CO2 (eq 3) (750 °C)
W(CO)6 98 W + 6CO
(1)
W(s) + 2Se(g) f WSe2(s)
(2)
CO f C + CO2 (Boudard reaction)
(3)
During the reduction of CO, the layered WSe2 particles act as catalysts to orient the fabrication of in situ generated carbon layers on the surface of the WSe2 catalyst, leading to the formation of carbon nanotubes. We have also considered the Yarmulke mechanism24 for the growth of carbon on the catalyst,
originally reported by Smalley et al. With the formation in the early stage of a graphene cap (the so-called yarmulke) with its edges strongly chemisorbed onto the metal (in the present case WSe2), the newly arriving carbon will continue to rise by forming a tubular morphology. There are three places to which the additional carbon can go: (1) The original surface shell can continue to grow around the particle, which ultimately results in the overcoating25 and deactivation of the catalyst. (2) A second cap can form underneath22 the first, spaced by roughly the interspacing of graphite. As additional caps form, older caps are forced to rise by forming a cylindrical tube whose open end remains chemisorbed onto the catalytic particle. (3) Carbon can be added to the cylindrical section of a growing layer. In our case, the second scheme seems to be applicable (can be seen in Figure 2b). In conclusion, the efficient, solvent-free process for the synthesis of a WSe2/C nanocomposite is demonstrated. The detailed compositional, morphological, structural, and BET surface area analyses are carried out and a probable reaction mechanism is discussed. References and Notes (1) Voss, D.; Kruger, P.; Mazur, A. Pollmann, J. Phys. ReV. B 1999, 60, 14311. (2) Haraldsen, H. Angew. Chem., Int. Ed. 1966, 5, 51. (3) Tsirlina, T.; Cohen, S.; Cohen, H.; Sapir, L.; Peisach, M.; Tenne, R.; Matthaeus, A.; Tiefenbacher, S.; Jaegermann, W.; Ponomarev, E. A.; Levy-Clement, C. Sol. Energy Mater. Sol. Cells 1996, 44, 457. (4) Reshak, A. H.; Auluck, S. Phys. ReV. B 2003, 68, 195107. (5) Chiritescu, C.; Cahill, D. G.; Nguyen, N.; Johnson, D.; Bodapati, A.; Keblinski, P.; Zschack, P. P. Science 2007, 315, 351. (6) Delphine, S. M.; Jayachandran, M.; Sanjeeviraja, C. Mater. Chem. Phys. 2003, 81, 78. (7) Jebaraj Devadasan, J.; Sanjeeviraja, C.; Jayachandran, M. Mater. Chem. Phys. 2002, 77, 397. (8) Tenne, R.; Galun, E.; Ennaoui, A.; Fiechter, S.; Ellmer, K.; Kunst, M.; Koelzow, C.; Pettenkofer, C.; Tiefenbacher, S.; Scheer, R.; Jungblut, H.; Jaegermann, W. Thin Solid Films 1996, 272, 38. (9) Guettari, N.; Ouerfelli, J.; Bernede, J. C.; Khelil, A. Pouzet, J.; Conan, A. Mater. Chem. Phys. 1998, 52, 83. (10) Miller, D. E. Energy Res. Abstr. 1986, 11. (11) Bourezg, R.; Couturier, G.; Salardenne, J. J. Chim. Phys. Phys.Chim. Biol. 1991, 88, 2021. (12) Sienicki, W. Pol. J. Chem. 1992, 66, 1139. (13) Boscher, N. D.; Carmalt, C. J.; Parkin, I. P. J. Mater. Chem. 2006, 16, 122. (14) Hershfinke, M.; Gheber, L. A.; Volterra, V.; Hutchison, J. L.; Margulis, L.; Tenne, R. J. Am. Chem. Soc. 1994, 116, 1914. (15) Nath, M.; Rao, C. N. R. Chem. Commun. 2001, 2236. (16) Fominski, V. Yu.; Nevolin, V. N.; Romanov, R. I.; Titov, V. I.; Scharff, W. Tribology Letters 2004, 17, 2. (17) Lifshitz, Y. Diamond Relat. Mater. 1999, 8, 1659.
5360 J. Phys. Chem. C, Vol. 112, No. 14, 2008 (18) Magnusson, M. H.; Deppert, K. J. Mater. Res. 2000, 15, 1564. (19) Reshak, A. H.; Auluck, S. Phys. ReV. B 2003, 68, 195107. (20) Sivarajan, R.; Brinson, B.; Johnson, M. P.; Gu, Z.; Saini, R. K.; Willis, P.; Marriott, T.; Billups, W. E.; Margrave, J. L.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem. B 2003, 107, 1360. (21) Pol, S. V.; Pol, V. G.; Gedanken, A.; Perkas, N. J. Phys. Chem. C 2007, 111, 134. (22) Pol, S. V.; Pol, V. G.; Gedanken, A. AdV. Mater. 2006, 18, 2023.
Pol et al. (23) Odoni, A.; Pol, V. G.; Pol, S. V.; Aurbach, D.; Gedanken, A. AdV. Mater. 2006, 18, 1431. (24) Dai, H.; Rinzler, A.; Nikolaev, P.; Thess, A.; Colbert, D.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471. (25) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 484.
