Janus PtXnY2-n (X, Y= S, Se, Te; 0 ≤ n ≤ 2) Monolayers for Enhanced Photocatalytic Water Splitting

We investigate Janus and alloy structures of PtXnY2-n (X, Y = S, Se, Te; 0 <= n <= 2) materials on the basis of first-principles plane-wave simulations. Using cluster-expansion theory to study alloys of PtXnY2-n monolayers at various concentrations, for half coverage (n = 1), our results indicate that Janus-type structures are not energetically the most favorable for PtXY monolayers; however, they are dynamically and thermally stable. To distinguish Janus PtXY structures, we report the Raman-active modes and compared them with those of bare PtX2 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 PtXY monolayers. Because of the electronegativity differences of different chalcogen atoms on each surface of Janus PtXY 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 PtX2 and PtXY structures. Our results indicate that due to their intrinsic dipole moments and band gaps, Janus PtXY monolayers are perfect candidates for water-splitting reactions.