Time-Resolved Spectroscopy of ZnTe Photocathodes for Solar Fuel Production

The negative conduction band potential and small bandgap of ZnTe make the material a promising photoelectrode for solar fuels production, photocatalyst, and solar cell component. However, the factors controlling the underlying efficiencies of the light-driven processes on ZnTe are not well understood. Here we report a combined spectroelectrochemical and transient absorption (TA) spectroscopic investigation of ZnTe photoelectrodes for CO2 reduction. In the visible region TA spectra are dominated by a broad positive photoinduced absorption at 540 nm following initial charge carrier relaxation (< 540 nm). The 540 nm spectral feature is shown to be related to deeply trapped photoelectrons with charge carrier recombination occurring via a trapping detrapping model on the microsecond time scale. Significantly these deeply trapped electrons are insensitive to the presence of electron acceptors and to the applied potential of the ZnTe electrode. Trapping at such states is proposed to be a significant factor limiting the photoelectrochemical activity of ZnTe. Near-IR spectral features associated with shallow trapped/conduction band electrons exist at > 1150 nm. Shallow trapped electrons are generated and accumulate at potentials where photoelectrochemical H-2 evolution and CO2 reduction occur, and we show these charges are able to undergo interfacial electron transfer to an acceptor molecule. The passivation of sites related to deep traps is proposed to be the key to optimize the photocatalytic and photoelectrochemical performance of ZnTe.