Janus Pt XnY2−n ( X , Y = S , Se , Te ; 0≤n≤2 ) Monolayers for Enhanced Photocatalytic Water Splitting

Abstract

We investigate Janus and alloy structures of $\mathrm{Pt}$${X}_{n}{Y}_{2\ensuremath{-}n}$ ($X$, $Y$ = $\mathrm{S}$, $\mathrm{Se}$, $\mathrm{Te}$; $0\ensuremath{\le}n\ensuremath{\le}2$) materials on the basis of first-principles plane-wave simulations. Using cluster-expansion theory to study alloys of $\mathrm{Pt}$${X}_{n}{Y}_{2\ensuremath{-}n}$ monolayers at various concentrations, for half coverage ($n=1$), our results indicate that Janus-type structures are not energetically the most favorable for $\mathrm{Pt}$$XY$ monolayers; however, they are dynamically and thermally stable. To distinguish Janus $\mathrm{Pt}$$XY$ structures, we report the Raman-active modes and compared them with those of bare $\mathrm{Pt}$${X}_{2}$ monolayers. The electronic band gaps calculated with use of hybrid functionals are on par with available experimental data. Spin-orbit coupling significantly modifies the electronic band structure of $\mathrm{Pt}$$XY$ monolayers. Because of the electronegativity differences of different chalcogen atoms on each surface of Janus $\mathrm{Pt}$$XY$ structures, the arising dipole moment significantly modifies the band alignments on both surfaces. We find that hydrogen-evolution and oxygen-evolution reactions occur on different surfaces and that applied strain enhances the catalytic activity. We also investigate the monovacancy and stacking effects on the electronic properties of $\mathrm{Pt}$${X}_{2}$ and $\mathrm{Pt}$$XY$ structures. Our results indicate that due to their intrinsic dipole moments and band gaps, Janus $\mathrm{Pt}$$XY$ monolayers are perfect candidates for water-splitting reactions.