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

Abstract

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 OL− (Sc) formed by nonprotonic lattice oxygen ion and shallow surface state Ti–O–̈O–̈O− (So) 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 (EFn) and surface state level (ES), consequently affecting the electron-trapping probability of surface states. As U>0.4V, the more negative EFn could significantly suppress the electron-trapping process of Sc and So, improving the intensity and stability of anodic photocurrent. When U was decreased, the EFn shifted to positive direction, which resulted in the facilitative trapping process of Sc and the decrease in anodic photocurrent. As U⩽−0.4V, the EFn was more positive than the energy level of So (ESo). Therefore, So could easily trap the photogenerated electrons, and the produced cathodic current resulted in the further decrease in anodic photocurrent.