Crystal-face and illumination intensity dependences of the quantum efficiency of photoelectrochemical etching, in relation to those of water photooxidation, at n-TiO2 (rutile) semiconductor electrodes

The quantum efficiency of photoelectrochemical etching (ηPE) in aqueous H2SO4, photocurrent–potential curves, and the photoluminescence intensity (IPL) were measured for photoetched (mostly the (1 0 0)-face exposed) and non-photoetched (various faces exposed) n-TiO2 (rutile) electrodes as a function of the illumination intensity (I). The photocurrent onset for the photoetched electrode under low I showed a negative shift relative to the non-photoetched electrode, indicating that the (1 0 0) face has high activity for the water photooxidation under low I. This conclusion was supported by low ηPE for the photoetched electrode under low I compared with the non-photoetched electrode. Such crystal-face dependences became negligible under middle and high I. The IPL showed a strong saturation with increasing I. A possible mechanism for surface reactions is proposed to explain the above results as well as the formation of rectangular nano-holes or grooves with the (1 0 0) face by photoetching. The mechanism has led to a detailed assignment of the PL to a transition from conduction-band electrons to surface-trapped holes at the atomic grooves in the (1 0 0) face. The results indicate the importance of atomic-scale regulation of the TiO2 surface for improvement of the photoelectrochemical and photocatalytic activity.