Doping WSe2 with various metals promotes the HER. Ni doping was found as the most efficient dopant, forming grafted layers on the structures. The mechanisms of the enhanced activity are unveiled and discussed.

Transition-metal dichalcogenides (TMDs) are being widely pursued as inexpensive, earth-abundant substitutes for precious-metal catalysts in technologically important reactions such as electrochemical hydrogen evolution reaction (HER). However, the relatively high onset potentials of TMDs relative to Pt remain a persistent challenge in widespread adoption of these materials. Here, we demonstrate a one-pot synthesis approach for substitutional Mn-doping of MoSe 2 nanoflowers to achieve appreciable reduction in the overpotential for HER along with a substantial improvement in the charge-transfer kinetics. Electron microscopy and elemental characterization of our samples show that the MoSe 2 nanoflowers retain their structural integrity without any evidence for dopant clustering, thus confirming true substitutional doping of the catalyst. Complementary density functional theory calculations reveal that the substitutional Mn-dopants act as promoters, rather than enhanced active sites, for the formation of Se-vacancies in MoSe 2 that are known to be catalytically active for HER. Our work advances possible strategies for activating MoSe 2 and similar TMDs by the use of substitutional dopants, not for their inherent activity, but as promoters of active chalcogen vacancies.

Layered transition metal dichalcogenides are noble-metal free electrocatalysts for the hydrogen evolution reaction (HER). Instead of using the common hydrothermal synthesis, which requires high pressure and temperature, herein a relatively simple and controlled colloidal synthesis was used to produce an alloy of MoWSe with nanoflower morphology as a model system for the electrocatalysis of hydrogen evolution in both acidic and alkaline environments. The results show that MoWSe alloys exhibit better catalytic activity in both acidic and alkaline solutions with low overpotentials compared to pure MoSe and WSe. Moreover, the electrode kinetics was studied using electrochemical impedance spectroscopy (EIS) and the results indicate that the alloys exhibit improved catalytic activity with low Tafel slopes, making them appealing for HER in either environment. Additionally, when MoSe nanoflowers (NFs) are prepared by using different metal salts and chalcogenide precursors, changes in the HER catalytic activity were observed, despite the morphology and crystal structure similarities. This finding suggests that different results reported in the literature could originate from different synthetic methods of the TMD, emphasizing that a better understanding of the relationship between the synthetic route and the catalytic performance is still lacking.

Core-shell nanoparticles provide a unique morphology to exploit electronic interactions between dissimilar materials conferring them new or improved functionalities. MoS2 is a layered transitionmetal disulfide that has been studied extensively for the hydrogen evolution reaction (HER) but still suffers from low electrocatalytic activity due to its poor electronic conductivity. To understand the fundamental aspects of the MoS2-Au hybrids with regard to their electrocatalytic activity, a single to a few layers of MoS2 were deposited over Au nanoparticles via a versatile procedure that allows for complete encapsulation of Au nanoparticles of arbitrary geometries. High-resolution transmission electron microscopy of the Au@MoS2 nanoparticles provides direct evidence of the core-shell morphology and also reveals the presence of morphological defects and irregularities in the MoS2 shell that are known to be more active for HER than the pristine MoS2 basal plane.Electrochemical measurements show a significant improvement in the HER activity of Au@MoS2 nanoparticles relative to free-standing MoS2 or Au-decorated MoS2. The best electrochemical performance was demonstrated by the Au nanostars -the largest Au core employed hereencapsulated in an MoS2 shell. Density-functional theory calculations show that charge transfer occurs from the Au to the MoS2 layers, producing a more conductive catalyst layer and a better electrode for electrochemical HER. The strategies to further improve the catalytic properties of such hybrid nanoparticles are discussed.

MoSe2 is a 2D layered transition metal dichalcogenide that has attracted much attention because its properties may be easily altered by both morphology control and doping by substitutional transition metals. Here, the study of Ru-doped MoSe2 nanoflowers is presented, and the effect of Ru doping on their optical, electronic, and catalytic properties is presented. A significant enhancement in their catalytic properties toward the hydrogen evolution reaction (HER) is evident, showing an overpotential as low as 143 mV (at 10 mA cm–2) for samples by substituting 11.4% of the Mo with Ru. In order to gain understanding of the dopants’ interaction with the host and the nature of the atomic-scale substrate for the catalytic reaction, density functional theory (DFT) calculations are employed to trace the modulation of the density of states (DOS) near the Fermi level and to model possible dopant sites. The Ru dopants have two additional d electrons and a high DOS near the Fermi level. The optical absorption spectra were significantly affected by Ru doping, and the optical band gap of MoSe2 increased due to the Burstein–Moss effect. The increased charge carrier density enhances the conductance of the samples, but the most significant change is the reduction in the charge transfer resistance during the HER upon doping.

The alkaline hydrogen evolution reaction (HER) plays a key role in photo(electro)catalytic water splitting technologies, particularly in water-alkali electrolyzers. Unfortunately, although transition metal dichalcogenide (TMD) materials, especially MoS2 and MoSe2, are considered efficient, Earth-abundant catalysts for the HER in an acidic electrolyte, they are much less effective under high pH conditions due to a sluggish water dissociation process (Volmer-step) and strong adsorption of the OH- intermediate on their surfaces. Herein we show a novel synergetic effect obtained by tailoring the S/Se ratio in Mo(SxSe1-x)2 alloys. We were able to influence the metal oxidation state and d-band to optimize the catalytic sites for HOH dissociation and OH- desorption while maintaining favourable M-H energetics. The Mo(SxSe1-x)2 (x = 0.58) catalyst exhibited high performance with an onset potential of -43 mV in 0.5 M KOH, i.e., ∼3 and ∼4-fold less than that for MoSe2 and MoS2, respectively. The results obtained in the current study have implications for the rational design of cost-effective electro-catalysts for water reduction based on TMD alloys.

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Pt°-NPs, prepared by the reduction of Pt(IV) salts with borohydride, do not catalyse the reduction of water in the presence of the strongly-reducing ˙C(CH3)2OH radicals. However, supporting the same metal nanoparticles (M°-NPs) with SiO2 alters the catalytic properties enabling the reaction. This effect depends both on the nature of M° and concentration of the composite nanoparticles. At low nanocomposite concentration: for M = Au nearly no effect is observed; for M = Ag the support decreases the catalytic reduction of water and for M = Pt the support initiates the catalytic process. At high nanocomposite concentration: for M = Au the reactivity is considerably lower and for M = Ag or Pt no catalysis is observed. Furthermore, for M = Ag or Pt H2 reduces the ˙C(CH3)2OH radicals.