Exploring ZnGa₂Te₄ as a Next-Generation Photocathode for Efficient 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.  In this work, we report the synthesis and characterization of a novel telluride-based thin-film ZnGa₂Te₄ photocathode featuring a low bandgap and strong visible-light absorption, fabricated via a combinatorial sputtering approach. A two-step annealing process with an excess Te supply was employed to obtain nearly stoichiometric ZnGa₂Te₄ absorbers exhibiting a zincblende-derived tetragonal crystal structure, as confirmed by synchrotron X-ray and electron diffraction analyses. Theoretical calculations show that ZnGa₂Te₄ has suitable direct bandgap (∼1.86 eV) and high absorption coefficient ∼10⁵ cm⁻¹, consistent with experimentally prepared films. Transient absorption spectroscopy reveals the biexponential decay dynamics, with time constants, τ₁ ∼ 0.04, and τ₂ ∼ 0.65 μs in microsecond time scales and provides the optical transition pathways for this semiconductor thin film. PEC measurements demonstrate that the ZnGa₂Te₄ photocathodes deliver photocurrent densities comparable to, or exceeding, those of widely studied ZnTe systems under simulated sunlight. Notably, the addition of a diaryliodonium co-catalyst significantly enhances CO₂RR selectivity to approximately 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.