Synthesis and Characterization of WO x Nanowires and their Conversion to WS 2 Nanotubes
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Synthesis and Characterization of WOx Nanowires and their Conversion to WS2 Nanotubes Lifeng Dong, Aitor Maiz1, Jun Jiao Department of Physics, Portland State University, Portland, OR 97201 1
Catlin Gabel School, Portland, OR 97225
ABSTRACT In this paper, we report for the first time the conversion of tungsten oxide (WOx) nanowires to tungsten sulfide (WS2) nanotubes using hydrogen (H2) gas and sulfur (S) powder as precursors instead of hydrogen sulfide (H2S) gas. Both the morphology and diameter of WS2 nanotubes were affected by the use of WOx nanowires. The diameter and length of the WOx nanowires were controlled by the growth temperature. A series of experiments confirmed that the higher the temperature was, the larger and longer the diameter and length of the WOx nanowires. When the temperature increased from 600 °C to 800 °C, the nanowire diameter increased from ~ 15 nm to ~50 nm. Furthermore, electron microscopy characterization reveals that the conversion of WS2 nanotubes from WOx nanowires started from outside the WOx nanowires and the conversion was not symmetrically uniform along the radial directions of the WOx nanowires. INTRODUCTION Since the first report on the synthesis of WS2 nanotubes in 1992 [1], their promising electronic and mechanical properties [2,3] have led to the exploration of a number of methods for their synthesis. Most of these methods have focused on the conversion of WOx nanowires or nanoparticles to WS2 nanotubes using H2S and H2 [4,5] or only H2S as the precursor gases [6]. To avoid the direct use of the corrosive and toxic H2S gas, we have investigated the synthesis of WS2 nanotubes by converting WOx nanowires using H2 gas and S powder as the reaction precursors. In this paper, different growth temperatures (600-1000 °C) and O2 partial pressures were used to optimize the synthesis of WOx nanowires. Also, to understand the formation of WOx nanowires and their conversion to WS2 nanotubes, various electron microscopy and microanalysis techniques were utilized to characterize their internal structures, morphologies and composition distributions. Furthermore, based on the electron microscopy results, the conversion mechanism of WOx nanowires to WS2 nanotubes is discussed. EXPERIMENTAL DETAILS Synthesis of WOx nanowires and conversion to WS2 nanotubes Synthesis of WOx nanowires and conversion to WS2 nanotubes were carried out in a horizontal tube furnace. For the WOx nanowire synthesis, tungsten (W) foils were used as growth substrates. Prior to use, the W foils (Alfa Aesar, 99.95%, 0.1mm in thickness) were polished, ultrasonicated in acetone for 5 min, rinsed with deionized water and dried with flowing nitrogen (N2) gas. Then, these foils were placed in the center of a quartz tube, which was inserted into a
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tube furnace. After this, the furnace was heated up at a rate of 20 ˚C/min. When the system reached the desired growth temperature, such as 600˚C, 700˚C, 800˚C or 1000 ˚C, the growth process was maintained for 30 minutes at atmospheric pressure. After the growth
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