Electrostatic Field Measurements and Band Bending at Single Crystal Titanium Dioxide Semiconductor Electrodes by Electric Field-Induced Optical Second Harmonic Generation.

The surface-sensitive technique of optical second harmonic generation (SHG) is applied to the measurement of the electrostatic fields, band bending and band flattening at the n-TiO_2(001) semiconductor electrode-electrolyte interface. When the TiO _2 electrode is held at depletion region potentials, the strong electric fields present in the space charge region polarize the TiO_2 lattice, which gives rise to a large SHG response via an electric field-induced second harmonic (EFISH) mechanism. Thus, SHG is a direct probe of the electrostatic fields and band bending present in the first 20 nm of the TiO_2 /electrolyte interface. The SHG intensity from the interface varies linearly with applied potential and reaches a minimum at the flatband potential of the electrode. A substantial decrease in the SHG intensity from the TiO_2 electrode is observed upon illumination of the surface with supra-bandgap ultraviolet (UV) light, even under potentiostatic control. Comparisons of the drop in SHG upon UV illumination with photovoltage measurements for TiO_2 electrodes verify that this decrease is due to a reduction in the surface electrostatic fields ("band flattening"). Upon termination of the UV illumination, a slow (~ 10 second) rise time is observed for the return of the surface SHG to its original level due to charge trapping at the electrode surface. Time-resolved SHG is used to probe the transient changes in the surface electrostatic fields that occur when the pulsed UV pump and SHG probe beams are overlapped temporally on the TiO_2 surface. On top of the steady-state band flattening, an additional transient band flattening is observed which is attributed to the finite time required for photogenerated holes to migrate to the interface under the influence of the surface electrostatic fields. The average transit time of 25 ps is independent of applied potential or solution composition, and yields a hole drift velocity for the surface region of the TiO_2 electrode. The SHG intensity returns to its steady-state level in 3-4 ns due to the removal of the excess holes at the surface by electrochemical charge transfer and surface recombination processes.