Abstract
Systematic and in-depth studies of the structure, composition, and efficiency of hydrogen evolution reactions (HERs) in MoSx films, obtained by means of on- and off-axis pulsed laser deposition (PLD) from a MoS2 target, have been performed. The use of on-axis PLD (a standard configuration of PLD) in a buffer of Ar gas, with an optimal pressure, has allowed for the formation of porous hybrid films that consist of Mo particles which support a thin MoSx~2+δ (δ of ~0.7) film. The HER performance of MoSx~2+δ/Mo films increases with increased loading and reaches the highest value at a loading of ~240 μg/cm2. For off-axis PLD, the substrate was located along the axis of expansion of the laser plume and the film was formed via the deposition of the atomic component of the plume, which was scattered in Ar molecules. This made it possible to obtain homogeneous MoSx~3+δ (δ~0.8–1.1) films. The HER performances of these films reached saturation at a loading value of ~163 μg/cm2. The MoSx~3+δ films possessed higher catalytic activities in terms of the turnover frequency of their HERs. However, to achieve the current density of 10 mA/cm2, the lowest over voltages were −162 mV and −150 mV for the films obtained by off- and on-axis PLD, respectively. Measurements of electrochemical characteristics indicated that the differences in the achievable HER performances of these films could be caused by their unique morphological properties.
Highlights
Chalcogenides of transition metals, molybdenum sulfides (MoSx), form compounds with different local packings of metal (Mo) and chalcogen (S) atoms, which are organized into several crystalline and amorphous phases
For the on- and off-axis pulsed laser deposition (PLD) catalysts, nearly identical linear voltammetry (LV) curves were detected after the first and 2000th cycles of Cyclic voltammograms (CV) testing. These results indicate no loss of catalytic performance and the remarkable stabilities of the electrocatalysts prepared on the glassy carbon (GC) substrate by the two PLD modes
Submicron and nanosized Mo-based particles were formed during the pulsed laser ablation of the target, and they were incorporated into the bulk of the catalytic coatings deposited on the GC substrate
Summary
Chalcogenides of transition metals, molybdenum sulfides (MoSx), form compounds with different local packings of metal (Mo) and chalcogen (S) atoms, which are organized into several crystalline and amorphous phases. If structures are crystalline, such as the layered 2H-MoS2 and 1T-MoS2 phases, which are nanometer sized, they can initiate a strong catalytic effect that activates a hydrogen evolution reaction (HER) during water electrolysis [1,2,3,4,5]. For this reason, thin-film molybdenum sulfide coatings are among the most used nonprecious nanoelectrocatalysts for hydrogen evolution. In the thermodynamically stable crystalline 2H-MoS2 phase, basal plane activation and edge site exposure/orientation on the cathode surface are the most frequently employed strategies [6,7]. The edge sites of the crystalline 2H-MoS2 phase exhibit the highest catalytic activity; in order to activate the basal planes, they should be modified by defect formation [8,9,10,11]
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