The effect of potential on the electron-trapping process of surface states in nanocrystalline TiO₂ film electrode

The effect of electrode potential (U) on the electron-trapping process of surface states in nanocrystalline TiO₂ film electrode was investigated by current-potential (I-U) curves and transient photocurrent spectra. A simple model, the transfer property of photogenerated carriers in alkaline electrolyte varied with U, was built up. As a result, two kinds of surface states, deep surface state O_L^- (S_c) formed by nonprotonic lattice oxygen ion and shallow surface state Ti–O–^̈O–^̈O^- (S_o) formed by chemisorbed O₂, were formed on the hydroxylated surface of TiO₂ particles, which could efficiently trap the photogenerated electrons. The variation of U could remarkably change the relative position between Fermi level of TiO₂ (E_Fn) and surface state level (E_S), consequently affecting the electron-trapping probability of surface states. As U≫0.4 V, the more negative E_Fn could significantly suppress the electron-trapping process of S_c and S_o, improving the intensity and stability of anodic photocurrent. When U was decreased, the E_Fn shifted to positive direction, which resulted in the facilitative trapping process of S_{c<- /inf>} and the decrease in anodic photocurrent. As U≤-0.4 V, the E_Fn was more positive than the energy level of S_o (E_So). Therefore, S_o could easily trap the photogenerated electrons, and the produced cathodic current resulted in the further decrease in anodic photocurrent.