Factors affecting photocatalytic activity of TiO₂

There are many factors affecting TiO2 photocatalysis, such as specific surface area, crystal phase, crystallite size, and crystallinity. However, there is controversy regarding the most important factor among them. In this chapter, we prepared TiO2 by conducting dialysis in a sol–gel synthesis and investigated the photocatalytic activity. Purification of TiO2 sol by dialysis minimizes the effect of impurities originating from raw materials on the crystallization of TiO2 during the thermal treatment. As a result, we reveal that fine anatase powder with high crystallinity is the most appropriate for the degradation of 4-chlorophenol in water, whereas the crystallite size is the key factor determining the oxygen evolution from water. In the case of visible light–responsive TiO2 doped with metal ions, the photocatalytic activity is controlled by the metal ion dopants having different valence states. These factors are related to the efficiency to suppress the recombination of the photogenerated charge carriers.