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

- Photoconductivity and gas sensing property of TiO2 were somewhat dependent on the light excitation behavior.
- Intrinsic excitation of TiO2 induced by UV light could cause the sensing response to both ethylene and oxygen.
- Extrinsic excitation of TiO2 induced by visible light (420 nm) could cause the sensing response to oxygen but not ethylene.
- Extrinsic excitation of TiO2 induced by visible light maybe depend on the Ti-OH species at TiO2 surface.
- Extrinsic excitation of TiO2 could be regarded as the electron transfer from O2p to Ti3d in the Ti-OH structure.

The photoconductivity and the gas sensing property of TiO2 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 TiO2 and its sensing response to both ethylene and oxygen, while the visible light at 420 nm could cause the photoconductivity of TiO2 and its sensing response to oxygen but not to ethylene. This indicates that the photo-assisted gas sensing performance of TiO2 was dependent on its light-excited behavior: The intrinsic excitation of TiO2 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 TiO2 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 TiO2 surface. With the removal of Ti-OH by increasing temperature, the visible light could not produce the photoconductivity of TiO2. It was proposed that Ti-OH groups act as the shallow trapped level to cause the extrinsic excitation of TiO2.

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