Layered metal dichalcogenide materials (MX 2 ) have great potential for solar energy conversion. However, as-grown MX 2 materials often contain edge and terrace defects that degrade semiconducting properties and hinder their solar performance. Herein, we demonstrate a simple approach to removing surface defects and improving the solar performance by using UVgenerated ozone to oxidize the surface of WSe 2 nanoplates and single crystals, followed by a simple soak in aqueous solutions to remove the oxide. Structural characterizations reveal that defective edges and basal plane defect sites are selectively oxidized and subsequently etched, and the ratio of the nonstoichiometric WSe x species is reduced. After this treatment, p-type WSe 2 single crystals show increased electron accumulation on the surface and significantly enhanced photoelectrochemical solar conversion efficiency. These results and insights will be useful in the improvement and utilization of layered MX 2 materials based on both Se and S for solar energy conversion and other device applications.

Polymer electrolyte membrane water electrolyzers (PEMWEs) are devices of paramount importance, enabling the large-scale storage of hydrogen. A transition towards lower catalyst loadings and intermittent operation is needed for widespread utilization, but the extent of degradation of catalyst layer constituents and further structural changes have not been widely explored. The multitude and complexity of degradation mechanisms requires characterization that can explore surfaces and interfaces at a range of length-scales to probe all changes of constituents within the catalyst later. This paper presents such an approach, featuring scanning electron microscopy, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, X-ray photoelectron and absorption spectroscopies, and transmission X-ray microscopy with X-ray absorption near-edge structure chemical mapping, to study degradation of the catalyst layer with a focus on comparing intermittent and steady-state operation. Catalyst changes including dissolution, oxidation, and agglomeration were observed, as well as redistribution and dissociation of the ionomer, and the smaller scale changes were found to cause the formation of voids and segregation of constituents at the larger scale. These findings highlight the importance of detailed analysis of catalyst layer degradation to propose mitigation strategies and improve long-term performance at various operating conditions.

Functionalization of carbon supports with heteroatom dopants is now widely regarded as a promising route for stabilizing and strengthening the interactions between the support and metal catalysts. Tuning the type and density of heteroatom dopants allows for the tailoring of nanoscale catalyst−support interactions; however, an understanding of these phenomena has not yet been fully realized because of the complexity of the system. In this work, computational modeling, materials synthesis, and advanced nanomaterial characterization are used to systematically investigate the intriguing effect of the two most common nitrogen functionalities in the carbon-based supports on the interactions with selected transition metals toward realizing catalytic applications. Specifically, this study utilized density functional theory to evaluate adsorption energies and modes of adsorption for 12 metals located in groups 8−11 and periods 4−6 with pyridinic and graphitic N defects. Based on these results, further electronic structure investigation of the period 4 metals was conducted to elucidate periodic group trends. Experimental work included synthesis and nanomaterial characterization of a subset of materials featuring three metals each supported on two types of N-doped carbon supports and undoped graphene. Characterization of nanomaterials with scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed that N functionalities enhanced the interactions with the selected transition metals when compared to the undoped support and demonstrated that the nature of the defect influences these interactions. Both computations and experiments agreed that Fe and Co are biased toward the graphitic sites over pyridinic sites, while Ni has an affinity to both defects without a statistically significant preference. This work established a correlation between computational and experimental work and a framework that can be expanded to other metals and alternative dopants beyond nitrogen in tailoring nanoscale catalyst−support interactions for a breadth of catalytic applications.

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