OPTICAL 2ND-HARMONIC GENERATION AS A PROBE OF ELECTROSTATIC FIELDS AND FLAT-BAND POTENTIAL AT SINGLE-CRYSTAL TIO2 ELECTRODES

The surface-sensitive technique of optical second harmonic generation (SHG) is applied to the measurement of dc electrostatic fields and the flatband potential at the TiO2 semiconductor electrode-electrolyte in terface. A large SHG response is observed from n-type single-crystal T iO2(001) electrodes at potentials for which a depletion layer exists a t the surface. This response is attributed to the electric field-induc ed SHG from the polarized TiO2 lattice in the space charge layer at th e semiconductor surface. The SHG from the interface varies linearly wi th the applied potential and reaches a minimum at the flatband potenti al of the electrode. The potential at which this minimum occurs shifts with pH at the expected rate of 60 mV/pH unit. A simple model of the electrostatic fields as a function of the band bending at the interfac e can be used to explain the observed dependence of the SHG on applied potential. These experiments suggest that SHG can be used as a local, time-resolved spectroscopic measurement of the electrostatic fields a nd surface charge density at single-crystal TiO2 electrodes.