The oxygen photoevolution (or water-photooxidation) reaction at the surfaces of TiO₂ and related metal oxides has been attracting strong attention from the point of view of solar energy conversion (solar water splitting). We have recently reported that the oxygen photoevolution reaction is initiated by a nucleophilic attack of an H₂O molecule to a surface-trapped hole (STH), accompanied by bond breaking. On this process, part of the trapped holes was extinguished by the recombination with electrons, which generates the PL at 840 nm. Thus, the PL observation gives the detail mechanisms of the oxygen photoevolution reaction.¹⁾ In this work, we investigated about the properties of the photogenerated holes at the sub-surface layer of single crystal (rutile) TiO₂(110) and (100) surfaces by observing the PL with a focus being placed on how it changes with increasing of pH value of the solution. Figure 1 shows the PL intensity plotted against the pH of the electrolyte solution. We can see that the intensity of PL was drastically increased at the pH less than 5, which is close to the isoelectric point. This result was explained as follows; Under the alkaline condition, photogenerated holes are trapped not only STH but also -O- species on the surface, which reduced the intensity of PL. On the other hand, in the acidic solution, most holes were trapped at STH, resulting in the strong intensity of PL.
1) A. Imanishi et al., J. Am. Chem. Soc., 129(2007)11569.