Post O2 Plasma-Induced Fabrication of 1T-WS2/a-WO3 Heterostructure for Superior Hydrogen Evolution Reaction Electrocatalysts Via Hydrogen Spillover

Abstract

2D transition metal dichalcogenide (TMDC) group has been widely spotlighted as hydrogen evolution reaction (HER) electrocatalysts to replace the conventional Pt electrocatalyst due to their distinctive features of robustness, low cost, and accessibility to structural modification. However, the electrochemical activity of TMDC materials is mostly originated from its exposed edge sites, whereas its basal plane is relatively inert. There have been attempts to increase the relative fraction of active edge sites in the basal plane or employ the phase transition from semiconductive 2H phase to metallic 1T phase. The practical uses have been restricted compared to nobel metal Pt.Plasma engineering is a facile method to functionalize their basal plane and generate vacancies into the lattice as active sites to improve electrocatalytic activity. In our previous works, MoS2 and WS2 has been synthesized to have abundant defects with maximized active edge sites to impart superior HER performance by employing Ar+H2S plasma treatment on tungsten (W) thin metal film with a thickness of 1 nm. However, to surpass the intrinsic electrochemical properites, we introduce 1T-WS2/a-WO3 (WSO) heterostructure via post O2 plasma treatment by exploiting a plasma enhanced-chemical vapor deposition (PE-CVD). There has been a limitation in obtaining the best electrocatalytic performances from the plasma assisted 1T-WS2 films because of its much lower hydrogen affinity of 1T-WS2. On the contrary, tungsten trioxide (WO3) shows strong hydrophilic property in that W atomic site of WO3 can easily absorb H2O. In this regard, 1T-WS2 with hydrophilic amorphous WO3 (a-WO3) can have the outstanding HER activity by inducing hydrogen spillover effect that is a hydrogen transport phenomenon from a-WO3 to 1T-WS2. a-WO3, which has relatively stronger hydrogen affinity, plays an important role in transferring the absorbed hydrogen to 1T-WS2 as active sites. As a result, the HER performance of WSO thin film can be optimized by means of controlling the amount of a-WO3 depending on the post O2 plasma treatment time. It was also confirmed from density functional theory (DFT) calculation that the hydrogen movement on the WSO surface related to hydrogen absorption energy difference between 1T-WS2 and WO3.