Abstract
Molybdenum sulfides (MoS (x) ) 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 MoS (x) with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS(2) with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS (x) films transform into highly HER-active amorphous MoS(2) (overpotential 210-250 mV at 10 mA/cm(2) in 0.5 M H(2)SO(4)) 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 MoS (x) , no changes in the structure of crystalline MoS(2) catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm(2), being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS (x) HER electrocatalysts as well as new insights into their activity and structure.