ZnGa2Te4 Thin-Film Absorbers for Photoelectrochemical CO2 Reduction

Photoelectrochemical (PEC) carbon dioxide reduction reaction (CO2RR) has been considered as a promising route to convert and store solar energy into chemical fuels. It is crucial to find suitable photoelectrode materials that are photo-catalytically active and exhibit excellent photochemical stability. One of the promising contenders is ZnTe with the ~2.26 eV band gap and prolonged stability under CO2RR PEC conditions. Herein, a new telluride based thin-film ZnGa2Te4 photocathode with lower band gap and stronger visible light absorption compared to ZnTe is synthesized and characterized using a combinatorial sputtering technique. A two-step annealing method with excess Te supply is implemented to synthesize nearly stoichiometric ZnGa2Te4 absorber material with a zincblende-derived tetragonal crystal structure confirmed by synchrotron X-ray and electron diffraction. Theoretical calculations show that ZnGa2Te4 has suitable direct bandgap (~1.86 eV) and high absorption coefficient ~105 cm-1, in agreement with experimentally prepared films. Transient absorption spectroscopy reveals the biexponential decay dynamics, with time constants, ..tau..1 ~ 0.04, and ..tau..2 ~ 0.65 ..mu..s in microsecond time scales and provides the optical transition pathways for this semiconductor thin film. PEC measurements show that the ZnGa2Te4 photocurrent densities are comparable to the widely investigated ZnTe photocathodes or even surpass it under simulated sunlight condition. ZnGa2Te4 samples demonstrate promising photoelectrochemical stability, maintaining consistent performance under illumination. The inclusion of diaryliodonium additive substantially increases its CO2RR selectivity to ~60%. These findings open a new avenue for the synthesis of telluride-based thin-film photocathodes for further exploration and will motivate future research to integrate this potential photocathode material into PEC devices.