ELECTROSTATIC-FIELD MEASUREMENTS AND BAND FLATTENING DURING ELECTRON-TRANSFER PROCESSES AT SINGLE-CRYSTAL TIO2 ELECTRODES BY ELECTRIC-FIELD-INDUCED OPTICAL 2ND-HARMONIC GENERATION

Optical second harmonic generation (SHG) is employed as a local, time-resolved measurement of the electrostatic fields at the surface of single-crystal TiO2 electrodes in contact with aqueous electrolyte solutions. The interfacial SHG at a fundamental wavelength of 584 nm is dominated by the electric field-induced second harmonic (EFISH) response from the first 20 nm of the space charge layer at the surface of the semiconductor. A substantial decrease in the amount of SHG from the electrode while under potentiostatic control is observed upon illumination of the surface with UV light (320 nm) whose energy is above the bandgap for TiO2 (3.0 eV or 410 nm). Comparisons of the drop in SHG upon UV illumination with photovoltage measurements for TiO2 electrodes at open circuit verify that this decrease is due to a reduction in the strength of the electrostatic fields (''band flattening'') within the space charge region upon UV illumination. The EFISH response from the surface decreases with increasing power of the UV illumination, corresponding to a band flattening of up to 70% for the highest power densities. Upon termination of the UV illumination, a slow (10-12 s) rise time is observed for the return of the surface SHG to its normal levels. The unexpected length of this rise time is attributed to the chemical discharge of surface charge trap sites on the semiconductor electrode surface and can be shortened considerably by the addition of a hole scavenger such as sodium sulfite to the electrolyte solution.