On the thermal annealing conditions for self-synthesis of tungsten carbide nanowires from WC_x films

Abstract

The thermal annealing conditions in nitrogen ambient for the self-synthesis of tungsten carbide nanowires from sputter-deposited WC(x) films were investigated. Experimental results show that the temperature window for the growth of nanowires lies in the range of 500-750 °C with the corresponding annealing time interval ranging from 2.5 to 0.25 h. The diameter, length, and density of the grown nanowires are in the range of 10-15 nm, 0.1-0.3 µm, and 210-410 µm(-2), respectively. The degree of carbon depletion in the annealed WC(x) films plays a crucial role in determining both the shape and density of the self-synthesized nanowires. Nanowires synthesized at lower temperatures were seen to be smaller in dimension but higher in density. Material analysis reveals that the phase transition from WC to W(2)C arising from decarburization of the WC(x) film during thermal annealing should be responsible for the self-synthesis of nanowires.

Structural, electronic properties and stability of tungsten mono- and semi-carbides: A first principles investigation

“…Alongside bulk W/C materials, an increasing interest has been devoted recently to new nano-sized systems such as W/C nanopowders [9][10][11], nanowires [12,13], nanotubes [14] and nanocomposites [15,16]; see also review [8].…”

Development of Gas Sensors Based on Tungsten Oxide Nanowires in Metal/SiO2/Metal Structure and Their Sensing Responses to NO2

“…Note that TONWs had good length controllability (wire length % 0.15 -0.4 mm) within a wide range of thermal budgets used for the thermal annealing. 17,19) According to the currentvoltage (I-V) measurement, the proposed three-layer structure with an area of 1 Â 1 mm 2 conducts currents of 2 Â 10 À10 and 4 Â 10 À7 A at 2 V before and after thermal annealing, respectively, indicating the firm linking of TONWs between the top and bottom electrodes. Essentially, a single TONW with typical lengths of 0.15 -0.4 mm and diameters of 17 -25 nm has SVRs of around ð1:6 { 2:35Þ Â 10 6 cm À1 , which are equivalent to specific surface areas (SSAs) of around 20.8 -30.8 m 2 /g, when a density of 7.71 g/ cm 3 was used for the TONW.…”

“…The present results are different from those of our previous work. 16,17) in annealing temperature. It is suspected that, because the present sample was subjected to PECVD and BOE, it might contain extra oxygen-related particles inside the device structure.…”

“…In general, the self-catalytic growth of TONWs should be attributed mainly to the formation of -W 2 C structures initially caused by carbon depletion in the WC x films and the subsequently accompanying oxidation during thermal annealing. 16,17) The nanocrystalline structure of TONWs was also examined by TEM and SAED analysis. Figure 5 presents typical TEM images of a single nanowire obtained from the WC x film after thermal annealing at 750 C for 30 min.…”

“…[16][17][18][19][20][21] TONWs with typical diameters of 17 -23 nm, typical lengths of 0.2 -0.3 mm, and typical densities of 150 -720 mm À2 have been successfully grown. Essentially, the use of TONWs as gas sensing materials is very attractive because they exhibit physical and chemical properties that are very different from those of their bulk or film counterparts.…”