Photoinduced Surface Electric Fields and Surface Population Dynamics of GaP(100) Photoelectrodes

Abstract

Gallium phosphide (GaP) photoelectrodes have received remarkable focus due to their applications in photocatalysis and photoelectrocatalysis of CO2 reduction reactions. Understanding the dynamical mechanisms of surfaces of photoelectrodes is essential in improving their working efficiencies in any application. However, knowledge of photoinduced surface dynamics of these materials is lacking. Here, we investigate surface dynamics of n-type and p-type GaP(100) semiconductors by utilizing time-resolved electronic sum frequency generation (TR-ESFG). Transient ESFG spectra showed that four surface states in both n- and p-type GaP(100) were involved in subsequent kinetics. Transient spectral signatures of the surface states showed that photoexcited electrons move toward the surface regions for p-type GaP, while photoexcited holes move to the surface regions for n-type GaP. These carriers first build up surface electric fields, resulting in fluence-dependent band flattening. The buildup rates of the surface electric fields were found to be on the order of 2.86 ± 0.30 ps–1 for n-type and 2.50 ± 0.25 ps–1 for p-type. Subsequently, a relatively slow process occurs, being attributed to population dynamics of surface states dependent upon applied fluences. We found that surface population behaves as a bimolecular process with rates of 0.020 ± 0.002 cm2 s–1 for n-type and 0.035 ± 0.002 cm2 s–1 for p-type GaP. The four surface states, shallow and deep for both n- and p-type GaP(100), were found to be involved in both surface electric fields and surface carrier populations, contrary to previous hypotheses. Our time-resolved surface-specific approach provides unique information on surface dynamical behaviors of photoelectrodes under ambient conditions.