Some physical properties of the TiO2 semiconductor electrode

Some physical properties of n-TiO₂ single crystals have been studied by electrochemical and photoelectrochemical techniques. X-ray diffractometry confirmed the TiO₂ basic rutile type structure. For the TiO₂/0.5 N NaOH system in the dark a Tafel slope of 0.11 V/decade was observed between 0.50 and 0.61 V versus SCE, suggesting a one-electron process. The potential, at which the gaseous oxygen evolution was confirmed, was well over 2.0 V versus SCE. On the other hand the potential for oxygen evolution was markedly decreased to ?0.75 V versus SCE, when the TiO₂ was illuminated. From these facts and the observation of passivation, transpassivation and current saturation an oxygen-pumping mechanism is proposed for the polarization behaviour of the TiO₂ electrode in the dark. The spectral responses of photocurrent generated from TiO₂ at 0.0 V bias versus SCE showed that the peak illumination current at 360 nm is larger than the dark current by three orders of magnitude. The band gap of TiO₂ was determined as 3.1 eV by assuming that the photoelectrolysis of water involves an indirect energy transition. An anomalous photoresponse was observed at 1.8 eV, which is due to the presence of surface states at the TiO₂/electrolyte interface.