Two-dimensional (2D) materials have been extensively studied in recent years due to their unique properties and great potential in energy-related applications, such as hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). To gain in-depth atomic-level understanding on the correlation between the catalytic surface and reaction mechanism, we employed ambient pressure X-ray photoelectron spectroscopy (APXPS) to investigate the electronic and chemical properties of MoS2 interface at various environments. Our APXPS data showed a pristine MoS2 ultrathin film surface exposed to CO2/H2O ambient formed several intermediates, including physisorbed (linear form, l-CO2) or chemisorbed (bent form, b-CO2) CO2 states and oxygenate species, such as hydroxyl, formate or carbonate groups. The adsorption configuration of CO2 (l-CO2 vs. b-CO2) depends on the experimental conditions. If we first introduced CO2 then H2O, more physisorbed CO2 were detected; in contrast, if H2O were introduced first then CO2, more chemisorbed CO2 were obtained. Identifying the first CO2 adsorption step is to promote initiation of reaction steps. Furthermore, the valence band offset was also observed due to surface band bending during CO2 reaction. While CO2 can induce upward surface band bending due to an acceptor molecule CO2 has accepted electron from MoS2 semiconductor surface, co-dosing H2O into system facilitates the adsorbing molecules to take holes from MoS2 semiconductor surface, consequently, induces downward band bending on semiconductor surface.
Our APXPS results suggest a strong correlation between the electronic and chemical properties of the material surface during catalytic reaction. The results in this study have added our understanding of the underlying mechanism involved in catalytic reaction, and helped us gain insights in designing and improving CO2 reduction catalysts.