Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution

Low-cost, layered transition-metal dichalcogenides (MX_2) based on molybdenum and tungsten have attracted substantial interest as alternative catalysts for the hydrogen evolution reaction (HER). These materials have high intrinsic per-site HER activity; however, a significant challenge is the limited density of active sites, which are concentrated at the layer edges. Here we unravel electronic factors underlying catalytic activity on MX_2 surfaces, and leverage the understanding to report group-5 MX_2 (H-TaS_2 and H-NbS_2) electrocatalysts whose performance instead mainly derives from highly active basal-plane sites, as suggested by our first-principles calculations and performance comparisons with edge-active counterparts. Beyond high catalytic activity, they are found to exhibit an unusual ability to optimize their morphology for enhanced charge transfer and accessibility of active sites as the HER proceeds, offering a practical advantage for scalable processing. The catalysts reach 10 mA cm^(−2) current density at an overpotential of ∼50–60 mV with a loading of 10–55 μg cm^(−2), surpassing other reported MX2 candidates without any performance-enhancing additives

Highly selective chemical vapor deposition of tin diselenide thin films onto patterned substrates via single source diselenoether precursors

The distorted octahedral complexes [SnCl4{nBuSe(CH2)nSenBu}]  (n = 2 or 3), (1) and (2), obtained from reaction of SnCl4 with the neutral bidentate ligands and characterized by IR/Raman and multinuclear (1H, 77Se{1H} and 119Sn) NMR spectroscopy and X-ray crystallography, serve as very effective single source precursors for low pressure chemical vapor deposition (LPCVD) of microcrystalline, single phase tin diselenide films onto SiO2, Si and TiN substrates. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) imaging show hexagonal plate crystallites which grow perpendicular to the substrate surface in the thicker films, but align mostly parallel to the surface when the quantity of reagent is reduced to limit the film thickness. X-ray diffraction (XRD) and Raman spectroscopy on the deposited films are consistent with hexagonal SnSe2 (P3m1; a = b = 3.81 Å; c = 6.13 Å), with strong evidence for preferred orientation of the crystallites in thinner (0.5–2 µm) samples, consistent with crystal plate growth parallel to the substrate surface. Hall measurements show the deposited SnSe2 is a n-type semiconductor. The resistivity of the crystalline films is 210 (±10) mΩ cm and carrier density is 5.0 × 1018 cm–3. Very highly selective film growth from these reagents onto photolithographically patterned substrates is observed, with deposition strongly preferred onto the (conducting) TiN surfaces of SiO2/TiN patterned substrates, and onto the SiO2 surfaces of Si/SiO2 patterned substrates. A correlation between the high selectivity and high contact angle of a water droplet on the substrate surfaces is observed