Using Exciton/Trion Dynamics to Spatially Monitor the Catalytic Activities of MoS2 during the Hydrogen Evolution Reaction

Selective activation of MoS2 grain boundaries for enhanced electrochemical activity

Molybdenum disulfide (MoS2) has emerged as a promising material for catalysis and sustainable energy conversion. However, the inertness of its basal plane to electrochemical reactions poses challenges to the utilization of wafer-scale MoS2 in electrocatalysis. To overcome this limitation, we present a technique that enhances the catalytic activity of continuous MoS2 by preferentially activating its buried grain boundaries (GBs). Through mild UV irradiation, a significant enhancement in GB activity was observed that approaches the values for MoS2 edges, as confirmed by a site-selective photo-deposition technique and micro-electrochemical hydrogen evolution reaction (HER) measurements. Combined spectroscopic characterization and ab-initio simulation demonstrates substitutional oxygen functionalization at the grain boundaries to be the origin of this selective catalytic enhancement by an order of magnitude. Our approach not only improves the density of active sites in MoS2 catalytic processes but yields a new photocatalytic conversion process. By exploiting the difference in electronic structure between activated GBs and the basal plane, homo-compositional junctions were realized that improve the photocatalytic synthesis of hydrogen by 47% and achieve performances beyond the capabilities of other catalytic sites.

The practice of reaction window in an electrocatalytic on-chip microcell

To enhance the efficiency of catalysis, it is crucial to comprehend the behavior of individual nanowires/nanosheets. A developed on-chip microcell facilitates this study by creating a reaction window that exposes the catalyst region of interest. However, this technology's potential application is limited due to frequently-observed variations in data between different cells. In this study, we identify a conductance problem in the reaction windows of non-metallic catalysts as the cause of this issue. We investigate this problem using in-situ electronic/electrochemical measurements and atom-thin nanosheets as model catalysts. Our findings show that a full-open window, which exposes the entire catalyst channel, allows for efficient modulation of conductance, which is ten times higher than a half-open window. This often-overlooked factor has the potential to significantly improve the conductivity of non-metallic catalysts during the reaction process. After examining tens of cells, we develop a vertical microcell strategy to eliminate the conductance issue and enhance measurement reproducibility. Our study offers guidelines for conducting reliable microcell measurements on non-metallic single nanowire/nanosheet catalysts.

Monitoring Transformations of Catalytic Active States in Photocathodes Based on MoSx Layers on CuInS2 Using In Operando Raman Spectroscopy

The electrochemical activation of CuInS₂ /MoS_x for photoelectrochemical (PEC) H₂ production was revealed for the first time through in operando Raman spectroscopy. During the activation process, the initial metallic MoS_x phase was transformed to semiconducting MoS_x , which facilitates charge carrier transfer between CuInS₂ and MoS_x . Ex situ X-ray photoelectron spectroscopy and Raman spectroscopy suggest the existence of MoO₃ after the activation process. However, apart from contradicting these results, in operando Raman spectroscopy revealed some of the intermediate steps of the activation process.

On-Chip Microdevice Unveils Reactant Enrichment Effect Dominated Electrocatalysis Activity in Molecular-Linked Catalysts

Molecular functionalization has been intensely studied and artificially constructed to advance various electrocatalytic processes. While there is a widely approved charge-doping effect, the underlying action for reactant distribution/transport remains long neglected. Here an on-chip microdevice unravels that the proton enrichment effect at prototypical methylene blue (MB)/MoS₂ interfaces rather than charge doping contributes to the hydrogen evolution reaction (HER) activity. Back-gated electrical/electrochemical tests detect quantitatively a strong charge injection from MB to MoS₂ realized over diploid carrier density, but these excess carriers are unqualified for the actual enhanced HER activity (from 32 to 125 mA cm^-2 at -0.29 V). On-chip electrochemical impedance further certifies that the proton enrichment in the vicinity of MoS₂, which is generated by the nucleophilic group of MB, actually dominates the HER activity. This finding uncovers the leading function of molecular-linked catalysts.