Surface residual stress dependence on photoinduced highly hydrophilic conversion and back-reaction in the dark of rutile single crystals

The wettability changes of TiO2 surface, photo-induced hydrophilic conversion and back-reaction in the dark, were evaluated using rutile (100) and (001) single crystals with diamond polishing (DP) and chemical mechanical polishing (CMP) treatments. Dynamic hardness measurements indicated that the DP surface had a residual compressive stress; however, the CMP surface did not. The rate of hydrophilic conversion was greatly suppressed (approximately one fourteenth) on the DP surface compared to the CMP surface, showing that the photoinduced hydrophilicity was greatly suppressed on the surface with compressive stress although the number of photogenerated carriers at the DP surface was estimated to decrease to only ca. half that at the CMP surface. In addition, when a heat treatment relaxed the compressive stress, the hydrophilicity was greatly increased. The back-reaction in the dark, i.e., the degradation of the photo-produced hydrophilic state, was ca. two times faster on the DP surface with compressive stress. The results strongly suggest that the pressure effect caused by the compressive stress could be the main reason for the degradation of the hydrophilicity and acceleration of the back reaction.