CO2 Photoreduction via Quantum Tunneling: Thin TiO2-Coated GaP with Coherent Interface To Achieve Electron Tunneling

摘要

Photocatalysts for CO2 photoreduction such as GaP often suffer from poor stability due to a high reduction environment as required for CO2 reduction, e.g., -1.9 V vs SCE to reach the current density of 10 mA/cm(2) at pH = 5.8 on p-GaP (Hallman, M. Nature 1978, 275, 115). The coating with stable oxides appears to be a promising solution, but many fundamental aspects such as the epitaxial relation between materials, the coating layer thickness, and the consequent photoactivity remain unknown. Here, using a thin TiO2-coated GaP as a model system, we have systematically investigated the dependence of photoactivity upon the deposition of TiO2 on GaP surfaces. By combining the modified phenomenological theory of Martensitic crystallography and a stochastic surface walking global search, we resolve the atomic-level structures of the interface between GaP and TiO2, a set of coherent interfaces between TiO2 and GaP ((001)(TiO2)//(100)(GaP), (101)(TiO2)//(110)(GaP), and (112)(TiO2)//(100)(GaP)). Due to the negligible interface strain, the epitaxial TiO2 coating layer is found to be as stable as bulk TiO2 even with a thickness below 1 nm, suggesting the presence of the well-ordered interface is a critical condition for promoting electron-hole separation in photocatalysis. We demonstrate that a photoelectron generated from GaP bulk can readily tunnel through the thin TiO2 layer (0.5-3 nm) without significant decay, which can decrease the CO2 reduction barrier significantly by 1.02 eV as compared to that without electron tunneling; alternatively, the photoelectron may also undergo lattice hopping to the TiO2 surface to promote hydrogen evolution as required in water splitting. Our results rationalize the recent finding that the high CO2 reduction ability of TiO2 coated GaP at a critical thickness (<10 nm). Using the theoretical framework developed here, we predict that the TiO2 coating strategy could be extended naturally to a range of photoabsorbing materials, including zincblende semiconductors and organic-inorganic hybrid halide perovskites, which can achieve a coherent interface and exhibit photoactivity toward water splitting and CO2 reduction.