Operando Ambient-Pressure X-Ray Photoelectron Spectroscopy of Adsorbate-Induced Band Bending and Photocatalytic Intermediates in CO₂ Reduction Reaction

Photocatalytic CO₂ reduction reaction (PC-CO₂RR) has attracted significant attention in the renewable energy field due to the urgent need to address climate change. However, the current efficiency of PC-CO₂RR is limited by its low conversion rates. To overcome this bottleneck, it is crucial to gain a deeper understanding of the material variations and charge transfer processes that occur during the photocatalytic reaction. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful surface-sensitive technique for measuring the electronic and chemical properties of catalysts and detecting adsorbents during chemical reaction. In this work, MoS₂ was selected as the model catalyst due to its simple surface and unified crystal orientation, which helps minimize complicating effects during the catalytic reaction. The APXPS results revealed that adsorbed H₂O molecules lead to downward band bending, facilitating the transfer of photo-generated electrons. Conversely, adsorbed CO₂ molecules caused dissipation of surface electrons, resulting in upward band bending. Density Functional Theory (DFT) calculations further corroborate these findings, indicating shifts in the band structure upon introducing CO₂ or H₂O into the model. Furthermore, the APXPS results also revealed that the sequence of introducing CO₂ and H₂O can alter the formation of bent CO₂^δ–, which is regarded as the key intermediate in determining the conversion efficiency. Experimental results also showed that the production of CO was enhanced when pre-adsorption of H₂O was applied, proving that H2O can the promote PC-CO₂RR. These findings provide new insights into designing more efficient PC-CO₂RR systems.