Surface States for Photoelectrodes of Gallium Phosphide (GaP) with Surface-Specific Electronic Spectra and Phase Measurements

Abstract

Gallium phosphide (GaP) photoelectrodes have received tremendous attention owing to their applications in photocatalysis and photoelectrocatalytic reduction of CO2. Surface electronic states of GaP are important in such catalysis applications. However, knowledge of surface states of GaP under ambient conditions is lacking. Here, we combined azimuth-dependent electronic sum-frequency generation (ESFG) spectroscopy with phase measurements to investigate the surface states for n-type and p-type GaP(100) semiconductors. ESFG spectroscopic studies enabled us to identify three surface states of the GaP crystals under ambient conditions. These experiments have also shown that all of the spectral features come from surface contributions for both the n-type and p-type GaP(100) crystals and that both surface dipoles and surface charges were responsible for the electronic transitions of isotropic and anisotropic components. Combined with azimuth-dependent phase measurements, surface charges were found to account for the isotropic surface ESFG components: negative for n-type and positive for p-type GaP(100). Finally, we conducted a thorough theoretical analysis of surface and bulk contributions for azimuth-dependent ESFG responses. With these spectral and phase signatures, we have further quantified surface and bulk contributions along different orientations for the n-type and p-type GaP(100) crystals.