Yi-Rung Lin1 2 Joseph DuChene1 Giulia Tagliabue1 Wen-Hui Cheng1 Matthias Richter1 Deep Jariwala1 Wei-Hsiang Lin1 Cora Went1 Zakaria Al Balushi1 Li-Chyong Chen2 Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States
2, National Taiwan University, Taipei, , Taiwan

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 chemically 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) growth of Mo(S,Se)2 thin-films synthesized via metal–organic chemical vapor deposition (MOCVD) as the catalysts for CO2RR. As a first step, we evaluated bulk crystals of MoS2, MoSSe, and MoSe2 for electrochemical CO2RR in aqueous K2CO3 solution (pH = 6.8) at -0.6 V vs. RHE. The results show 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 synthesized thickness-controllable, large-area Mo(S,Se)2 electrodes integrated with degenerately-doped Si substrates via MOCVD. This process enables the incorporation of organic selenium vapor source during the MOCVD growth process to change the S/Se ratio in Mo(S,Se) alloys. The CO2RR product analysis as a function of film composition and thickness will be discussed and compared to the relevant CO2RR reduction potentials.

1. Asadi, M. et al. Robust carbon dioxide reduction on molybdenum disulphide edges. Nat. Commun. 5, 4470 (2014).
2. Asadi, M. et al. Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid. Science 353, 467-469 (2016)
3. Kang, J. et al. Band offsets and heterostructures of two-dimensional semiconductors, Appl. Phys. Lett. 21, 012111 (2013)