Comparative study of ultraviolet light and visible light on the photo-assisted conductivity and gas sensing property of TiO2

The photoconductivity and the gas sensing property of TiO 2 at room temperature under ultraviolet (UV) irradiation were compared with those under visible light irradiation (≥420   nm), respectively. It was found that UV light could cause the photoconductivity of TiO 2 and its sensing response to both ethylene and oxygen, while the visible light at 420   nm could cause the photoconductivity of TiO 2 and its sensing response to oxygen but not to ethylene. This indicates that the photo-assisted gas sensing performance of TiO 2 was dependent on its light-excited behavior: The intrinsic excitation of TiO 2 induced by UV light could produce both the holes and electrons, which exhibited a respective response to ethylene and oxygen, while the extrinsic excitation of TiO 2 induced by visible light maybe only produced the free electrons but not free holes, resulting in only a response to oxygen. Furthermore, it was found that the visible-light enhanced conductivity maybe depend on the Ti–bonded hydroxyls (Ti–OH) at TiO 2 surface. With the removal of Ti–OH by increasing temperature, the visible light could not produce the photoconductivity of TiO 2 . It was proposed that Ti–OH groups act as the shallow trapped level to cause the extrinsic excitation of TiO 2 .