TiS₂ Monolayer as an Emerging Ultrathin Bifunctional Catalyst: Influence of Defects and Functionalization

We have envisaged the hydrogen evolution and oxygen evolution reactions (HER and OER) on a two-dimensional (2D) noble-metal-free titanium disulfide (TiS₂ ) monolayer, which belongs to the exciting family of transition metal dichalcogenides (TMDCs). Our theoretical investigation to probe the HER and OER on both the H and T phases of 2D TiS₂ is based on electronic-structure calculations witihin the framework of density functional theory (DFT). Since TiS₂ is the lightest compound among the group-IV TMDCs, it is worth exploring the catalytic activity of a TiS₂ monolayer through the functionalization at the anion (S) site, substituting with P, N, and C dopants as well as by incorporating single sulfur vacancy defects. We have investigated the effect of functionalization and vacancy defects on the structural, electronic, and optical response of a TiS₂ monolayer by determining the density of states, work-function, and optical absorption spectra. We have determined the HER and OER activities for the functionalized and defective TiS₂ monolayers based on the reaction coordinate, which can be constructed from the adsorption free energies of the intermediates (H*, O*, OH* and OOH*, where * denotes the adosrbed state) in the HER and OER mechanisms. Finally, we have shown that TiS₂ monolayers are emerging as a promising material for the HER and OER mechanisms under the influence of functionalization and defects.