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 (WSx) on carbon surfaces inside the confined nanopores. Experimental and computational studies revealed the preferential bonding between the basal planes of WSx and carbon surfaces, as well as the subsequent horizontal growth of WSx. As a result, subnanometer WSx clusters were formed at a low WSx loading, and grew into monolayer WS2 nanoplates with increased WSx loadings. In contrast, a TMD analogue, MoS2, favors edge plane bonding with carbon surfaces and subsequent stacking of nanoplate layers, leading to multilayer MoS2 nanoplates with increased MoS2 loadings. A time-dependent growth of WSx further corroborated the formation of WS2 nanoplates at the expense of ultrasmall WSx nanoclusters. Interestingly, the sample prepared with a short sulfidation time, which was mainly comprised of WSx nanoclusters, showed higher HER activity compared to the sample prepared with a prolonged sulfidation time, which mostly contained WS2 nanoplates. The higher HER activity of WSx nanoclusters is attributed to the larger density of active bridging S-2(2-) sites, compared to the WS2 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 WSx for the HER.