Preferential horizontal growth of tungsten sulfide on carbon and insight into active sulfur sites for the hydrogen evolution reaction.

Transition metal dichalcogenides (TMDs) have attracted considerable attention as active electrocatalysts for the hydrogen evolution reaction (HER). Since TMD catalysts are commonly supported on carbon to endow electrical conductivity, understanding the growth behaviour of TMDs on carbon surfaces is crucial, and yet remains to be explored. In this work, we investigated the growth behaviour of tungsten sulfide (WS) on carbon surfaces inside the confined nanopores. Experimental and computational studies revealed the preferential bonding between the basal planes of WS and carbon surfaces, as well as the subsequent horizontal growth of WS. As a result, subnanometer WS clusters were formed at a low WS loading, and grew into monolayer WS nanoplates with increased WS loadings. In contrast, a TMD analogue, MoS, favors edge plane bonding with carbon surfaces and subsequent stacking of nanoplate layers, leading to multilayer MoS nanoplates with increased MoS loadings. A time-dependent growth of WS further corroborated the formation of WS nanoplates at the expense of ultrasmall WS nanoclusters. Interestingly, the sample prepared with a short sulfidation time, which was mainly comprised of WS nanoclusters, showed higher HER activity compared to the sample prepared with a prolonged sulfidation time, which mostly contained WS nanoplates. The higher HER activity of WS nanoclusters is attributed to the larger density of active bridging S sites, compared to the WS nanoplates. These findings may provide important insights into the growth behaviour of layered TMD materials at the nanoscale, as well as potential active species in WS for the HER.