Structural Aspects of MoSx Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction

Molybdenum sulfides (MoSx) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoSx with adjustable S/Mo ratio (2.8–4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoSx films transform into highly HER-active amorphous MoS2 (overpotential 210–250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately −0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoSx, no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoSx HER electrocatalysts as well as new insights into their activity and structure.