Yi-Rung Lin1 2 Wen-Hui Cheng1 Matthias Richter1 Joseph DuChene1 Cora Went1 Zakaria Al Balushi1 Deep Jariwala3 Li-Chyong Chen2 Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States
2, Center for Condensed Matter Sciences, National Taiwan University, Taipei, , Taiwan
3, Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States

Molybdenum disulfide (MoS2) and its related layered transition-metal dichalcogenides (TMDs) have attracted much attention as potential electrocatalysts for converting carbon dioxide to fuels due to their lower price compared with precious metals and their prominent catalytic features. MoS2 and MoSe2 have recently been shown to perform as excellent electrocatalysts in ionic-liquid-based systems for the CO2 reduction reaction (CO2RR)1,2. However, achieving selectivity in the CO2RR is challenging due to the numerous possible chemical reaction pathways and their very similar reduction potentials, often leading to a multitude of CO2RR products. Theoretical calculations3 indicate that the conduction band (CB) edge position of TMD materials can be tuned by adjusting the layer thickness (i.e. monolayer vs. bilayer), as well as by chemical alloying (e.g. MoSSe). The TMDs therefore offer a suitable material system for adjusting the CB edge of the catalyst relative to a given reduction potential for CO2RR. Herein, we report the thickness-controllable, large-area (1 cm2) synthesis of MoS2-xSex thin-films for the CO2RR via metal–organic chemical vapor deposition (MOCVD) followed by a post-selenization process. The post-treatment enables the Se incorporation into the MoS2 films as well as tunability of the S/Se ratio in MoS2-xSex alloys. As a first step, we evaluated bulk crystals of MoS2, MoSSe, and MoSe2 for electrochemical CO2RR in aqueous K2CO3 solution (pH = 6.8) at -0.4 V vs. RHE. The results showed that both MoSSe and MoSe2 produce 4 times more CO and CH4 than MoS2. To further explore the effect of the CB edge position relative to the CO2RR, we examined the performance of thickness-controlled, MoS2-xSex electrodes incorporated on glassy carbon substrates for electrocatalysis. We found that the MoS2-xSex (x=0.46) films own the highest faradaic efficiencies about 16.6% , which is much higher than MoS2 film. The CO2RR product analysis as a function of film composition will be discussed and compared to the relevant CO2RR reduction potentials.