What is Tungsten Disulfide? The WS2 and WS2–carbon composite powders had pure crystal structures of a hexagonal WS2 phase without impurity peaks of WO3, even though the two samples had different crystal. The (002) reflection indicating stacking of the WS2 layers was only observed in the WS2–carbon composite powders. The XRD patterns had broad peaks with low intensities. The amorphous WS2 and WS2–carbon composite powders were prepared because of a low sulfidation temperature of 400°C. The thermogravimetry (TG) curve of the WS2–carbon composite powders. The TG curve had two distinct weight losses below 700°C. The weight loss by complete oxidation of WS2 into WO3 under air atmosphere was as low as 6.5 wt%. Therefore, the two-step weight losses observed near 200 and 400°C were mainly due to the decomposition of carbon components. The composition of the WS2–carbon composite powders with a dense structure occurred in two steps. The carbon content of the WS2–carbon composite powders calculated from the TG results was about 30 wt%. The composite powders had morphology similar to that of the WO3–carbon composite powders, indicating that the sulfidation process did not significantly change the morphology of the powders. The high-resolution TEM images shown in Figures 3c and 3d reveal slightly crystalline WS2 that consists of a few graphitic layers near the surface of the powder. The interlayer distance of the (002) plane measured from the TEM image is 0.63 nm14. However, the stacked graphitic layers were not found in the inner part of the composite powders. The uniform distributions of the W, S, and C components all over the WS2–carbon composite powder. Complete sulfidation of the powders occurred. The uniform distribution of carbon all over the powders disturbed the growth of WS2 crystals. The morphologies of the bare WS2 powders. The bare WS2 powders were also spherical like the WS2–carbon composite powders. The high-resolution TEM images in Figures 4c and 4d show the non-stacked WS2 layers and amorphous structure, respectively. The dense structures of the WS2 and WS2–carbon composite powders without pores resulted in low BET surface areas.
The electrochemical properties of the bare WS2 The electrochemical properties of the bare WS2 and WS2–carbon composite powders. The cyclic voltammograms (CVs) of the WS2–carbon composite powders were measured at a scan rate of 0.4 mV s−1 in the 0.001–3 V voltage range. The mesoporous WS2 powders with high crystallinity prepared by a vacuum-assisted impregnation route had sharp reduction and oxidation peaks in their CV curves14. However, the amorphous WS2–carbon composite powders had broad reduction and oxidation peaks in all cycles. The clear reduction peaks for the lithium insertion to WS2 to form LixWS2 (WS2 + xLi+ + xe− = > LixWS2) and the conversion reaction (WS2 + 4Li+ + 4e− = > W + 2Li2S) were not observed15. The steady CV profiles after the initial cycle indicated high stability and reversibility of the WS2–carbon composite powders for Li+ insertion and extraction. He cycle curves of the bare WS2 powders with amorphous structure also had no distinct plateaus. The rate performance of the WS2-carbon composite powders, wherein the current density increased stepwise from 50 to 500 mA g−1 and then returned to 50 mA g−1. The WS2-carbon composite powders exhibited the final discharge capacities of 761, 630, 561, 524, and 502 mA h g−1 at current densities of 50, 150, 300, 400, and 500 mA g−1, respectively. When the current density returned to 50 mA g−1, the discharge capacity recovered to 678 mA h g−1.
Phase-pure WS2–carbon composite powders with uniform distribution Phase-pure WS2–carbon composite powders with uniform distribution of W, S, and C components were prepared by applying ultrasonic spray pyrolysis. The WS2–carbon composite powders had a spherical shape, dense and amorphous crystal structure. Amorphous WO3–carbon composite powders with high carbon content were transformed into amorphous WS2–carbon composite powders. However, a sulfidation process transformed highly crystalline bare WO3 powders prepared by spray pyrolysis into amorphous bare WS2 powders. The cyclic voltammograms and initial discharge and charge curves revealed the amorphous structure of the WS2–carbon composite powders. The WS2–carbon composite powders had better electrochemical properties than the bare WS2 powders. Spherical WO3–carbon and WO3 powders were prepared directly by spray pyrolysis from spray solutions of ammonium metatungstate with and without a sucrose additive, respectively. The spray pyrolysis system consisted of a droplet generator, a quartz reactor, and a powder collector. A 1.7 MHz ultrasonic spray generator with six vibrators was used to generate many droplets, which were carried into the high-temperature tubular reactor by the carrier gas at a flow rate of 10 L min−1. The ammonium metatungstate and sucrose concentrations were 0.2 and 0.1 M, respectively. The as-prepared WO3–carbon and WO3 powders were converted to WS2–carbon and WS2 by adopting a sulfidation process under a reducing atmosphere with thiourea as the sulfur source. The as-prepared powders were placed in a small alumina boat located inside a larger alumina boat. Thiourea was loaded outside the small alumina boat to form the WS2. The sulfidation process was performed at 400°C for six h in the presence of a 10% H2/Ar mixture gas.
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