Structural Transformation Identification of Sputtered Amorphous MoS_<i>x</i> as an Efficient Hydrogen-Evolving Catalyst during Electrochemical Activation

Molybdenum sulfide, MoSx, is considered as attractive hydrogen evolution catalyst since it is free of noble metals and shows a low overpotential. Especially, amorphous molybdenum sulfide has attracted attention because of its high catalytic activity. However, the catalytic mechanism of the hydrogen evolution reaction is not yet fully understood. Therefore, in our study, layers of MoSx were deposited by reactive magnetron sputtering, varying the substrate temperature in the range from room temperature (RT) to 500 °C. The morphology and structure of the films change significantly as a function of temperature, from an amorphous to a highly textured 2H-MoS2 phase. The highest catalytic activity was found for amorphous layers deposited at RT, showing an overvoltage of 180 mV at a current density of −10 mA cm–2 in a 0.5 M sulfuric acid electrolyte (pH 0.3) after electrochemical activation. As detected by Raman spectroscopy, the RT deposited catalyst consists of [Mo3S13]2– and [Mo3S12]2– entities which are interconnected via [S2]2– and S2– ligands. When the potential was swept from +0.2 to −0.3 V vs RHE, a massive release of sulfur in the form of gaseous H2S was observed in the first minutes as detected by differential electrochemical mass spectroscopy (DEMS). After electrochemical cycling for 10 min, the chains of these clusters transformed into a layer-type MoS2–x phase observed by in situ Raman spectroscopy. In this transformation process, H2S formation gradually vanishes and H2 evolution becomes dominant. The transformed phase is considered as a sulfur-deficient molybdenum sulfide characterized by a high number of molybdenum atoms located at the edges of nanosized MoSx islands, which act as catalytically active centers