There are many factors that could affect the hydrogen sensing performance of the Al- and V-doped TiO2 nanofilms. Nanotubular geometry, polymorph, element doping, and testing temperature affected the hydrogen sensing properties of the nanofilm sensors.
Varghese et al. found that undoped TiO2 nanotubes with a smaller diameter (22 nm) could have a higher sensitivity for 1,000 ppm H2 at 290°C . Anatase, the polymorph of TiO2, has been reported to be highly sensitive IWR 1 to reducing gases like hydrogen and carbon monoxide . The hydrogen atom could diffuse to the interstitial sites of TiO2. As the c/a ratio of anatase phase is almost four times that of the rutile phase, the anatase TiO2 phase thus has a greater contribution to hydrogen sensitivity . In the present oxide system, the nanofilms consisted of anatase phase favorable for hydrogen sensing at different temperatures. There are more defects and dislocations in the anatase structures than other crystalline structures [38, 39].
Al and V atoms had an atomic radius different from Ti atom. Thus, Al and V doping could produce more lattice vacancy to capture electrons and accelerate the electron change which is beneficial for the chemical adsorption of hydrogen at the surface and therefore enhance the hydrogen sensitivity. Furthermore, an increased operating temperature of the nanofilm sensor could accelerate the diffusivity of the hydrogen atoms to the surface of the nanofilms and thus lead to a higher sensitivity. As a ceramic oxide fabricated on robust metal substrate, the doped nanofilm provides a robust sensor unit working at either room temperature HSP90 BGB324 cost or elevated temperatures. The hydrogen sensing capability shown by the Al- and V-doped nanofilms makes it possible to further explore the semiconducting characteristics and hydrogen sensing behaviors of various kinds of TiO2 nanofilms with different dopant levels (i.e., Al/V ratio). Conclusions In summary, Ti-Al-V-O oxide nanofilms
with anatase structures were prepared by anodization and annealing. Annealing at different temperatures was found to result in different hydrogen sensing performances. Al and V doping was found to reduce the bandgap of TiO2 oxide. The Al- and V-doped anatase nanofilms demonstrated a p-type hydrogen sensing characteristics, which was quite different from the undoped TiO2 nanotubes. The Ti-Al-V-O nanofilms annealed at 450°C demonstrated sensitivity for 1,000 ppm H2 at elevated operating temperatures, while Ti-Al-V-O nanofilms annealed at 550°C had good sensing response at both room temperature and elevated temperatures. Acknowledgments This work was supported by Shanghai Pujiang Program (no. 07pj14047) and 863 Plan of China (no. 2006AA02A1). We thank the contribution from SEM lab at Instrumental Analysis Center of SJTU. References 1. Dresselhaus MS, Thomas IL: Energy and power.