Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of 〈0 0 1〉 preferential orientation, surface temperature and humidity

• Preferential orientation enhances the reactivity by an order of magnitude.
• The reaction rate increases with catalyst temperature up to 40% relative humidity.
• The reaction rate increases with relative humidity for randomly oriented TiO2 films 〈0 0 1〉.
• Orientated films show high reactivity in a broad range of reaction conditions.

We present a systematic in situ study of the environmental reaction conditions on the photocatalytic activity of sputter deposited anatase TiO2 films with controlled amounts of preferential 〈0 0 1〉 orientation. In particular, the effects of relative humidity (RH) and substrate temperature (TsurfTsurf) are investigated. It is found that {0 0 1} facets, which are present in higher abundance on highly oriented samples, exhibit an order of magnitude higher reactivity for gas-phase photocatalytic oxidation of the indoor air pollutant acetaldehyde (CH3CHO) than {1 0 1} facets do, and a functional dependence of the reaction rate on facet orientation is determined. It is proposed that water adsorbed on the film contributes to two counteracting effects on the photocatalytic activity: (i) It provides hole acceptors to complete the photo-induced redox cycle and subsequent OH radical formation for pollutant degradation, and (ii  ) it creates a diffusion barrier between the catalyst interface and pollutant molecules adsorbed in the water layer. As a consequence, increasing TsurfTsurf at high RH has the beneficial effect of removing excess water and reducing the diffusion barrier, thereby improving the photocatalytic activity. A comparison is also made with a commercial anatase TiO2 film, with less developed surface crystallinity and random facet distribution, where the improvement is even more pronounced. Films with a higher degree of orientation exhibit much more stable performance over a range of operating conditions, which suggests that it is possible to tune the effects of water and exposed facet orientation to achieve optimum activity and make TiO2 films amenable to a larger (RH, TsurfTsurf) parameter space for practical applications.

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