Study And Preparation Of Porous Tungsten Oxide Gas-sensing Thin Film Based On Anodization

Since the 20th century, with the rapid development of industrial technology, more and more gaseous pollutants were emitted to the atmosphere. As a kind of strong toxic gas, nitrogen dioxide (NO2) in the atmosphere is gradually threatening the safety and health of the people. So far, a variety of gas sensitive materials have been developed, when tungsten trioxide (WO3) is considered as one of the most promising materials in the field of gas sensors materials. Based on the above background, the preparation of a novel porous WO3 film with high performance was studied in this article.Metallic tungsten (W) films were first deposited on alumina (Al2O3) substrates by means of DC magnetron sputtering process. Porous WO3 films were then prepared by anodic oxidation of the W films. A preliminary optimization of anodizing process conditions was carried out. For comparison, a compact WO3 film was deposited on an alumina substrate via DC reactive magnetron sputtering process. The anodization time was increased by adding rapid thermal process (RTP) before the anodization of W films, which improved the performance of porous WO3 films. The morphology, crystal structures and gas sensing properties of the samples were analyzed and compared by using a field emission scanning electron microscope (FESEM), an X-ray diffractometer (XRD) and gas sensing characteristics measurement.The result indicates that both the porous WO3 film and sputtered WO3 film were monoclinic phase, but had different preferential orientation of crystal faces. The porous WO3 films have smaller average grain size and more porous morphology, which means larger specific surface area.Compared with sputtered WO3 film gas sensor, the porous WO3 exhibited markedly higher sensitivity, faster response recovery rate and lower optimal operating temperature to different concentration of NO2 gas due to the porous morphology and small grain size. The porous WO3 film has higher sensitivity than the sputtered WO3 film even operated at lower temperature. According to the result of gas selectivity test, porous WO3 film exhibited highly selective to NO2 gas. The improved porous WO3 samples prepared by adding rapid thermal process (RTP) before the anodization process showed similar crystal structure, average grain size and morphology, when the corrosion level of films increased with the extension of anodization time. The improved porous WO3 samples exhibited high sensitivity, excellent reversibility, and also good stability.