Synthesis and gas-sensing properties of the silicon nanowires/vanadium oxide nanorods composite

Abstract

As air pollution is becoming more and more serious in recent years, gas-sensing devices have attracted intensive attention. In particular, NO2 is one of the most toxic gases in the atmosphere, which tends to produce acid rain and photochemical smog. Thus, there is a strong demand of cheap, reliable and sensitive gas sensors targeting NO2. Gas sensors fabricated on silicon substrates with room-temperature operation are very promising in power saving, integrated circuit processing and portable detectors. More important, the silicon nanowires (SiNWs)-based devices are compatible with very large scale integration processes and complementary metal oxide semiconductor technologies. In the present work, the novel nanocomposite structure of (SiNWs)/vanadium oxide (V2O5) nanorods for NO2 detection is successfully synthesized. The SiNWs are fabricated by a combination of nanosphere lithography and metal-assisted chemical etching. Vanadium films are deposited on SiNWs by DC magnetron sputtering, and then V2O5 nanorods are synthesized with subsequent thermal annealing process for full oxidation in air. The morphology and crystal structure of product obtained are characterized by field-emission scanning electron microscopy and X-ray diffraction. The characterization results indicate that V2O5 nanorods are uniformly distributed on the surfaces of SiNWs. The increased specific surface area of SiNWs/V2O5 nanocomposite provides more adsorption sites and diffusion conduits for gas molecules. Therefore, the novel structure of the nanocomposite is conducive to gas-sensing. In addition, the sputtering time has an obvious influence on the morphology of vanadium oxide. With the increase of the sputtering time, the specific surface area and the number of p-n heterojunctions formed in the nanocomposite are both less than those of nanocomposite with appropriate sputtering time. The gas-sensing properties are examined by measuring the resistance change towards 0.5-4 ppm NO2 gas at room temperature by the static volumetric method. Results show that the nanocomposite with shorter deposition time has better gas-sensing properties to low-concentration NO2 gas than those of bare SiNWs and nanocomposite with longer deposition time. On the contrary, the responses of the nanocomposite to other high-concentration reducing gases are very low, indicating good selectivity. The enhancement in gas sensing properties may be attributed to the change in width of the space charge region, which is similar to the behavior of p-n junction under forward bias, in the high-density p-n heterojunction structure formed between SiNWs and V2O5 nanorods. In conclusion, these results demonstrate that the SiNWs/V2O5 nanocomposite has great potential for future NO2 gas detection applications with low consumption and good performance.