Structural transformations of Hydrogen- and Sulfur-annealed Pt-based chalcogenides electrocatalysis

Phase transformations in transition metal dichalcogenides have been widely studied in recent years. However, the transformation in platinum-based chalcogenides from a tetragonal (3D) to hexagonal (2D) structure has not been thoroughly researched. In this study, X-ray photoelectron spectroscopy, a powerful measurement technique, was utilized to analyze the Pt oxidation state change originating from the structural transformation in tetragonal PtS crystals grown via chemical vapor deposition to hexagonal PtS2 crystals. The Pt(IV) content of the as synthesized PtS increased by approximately-six times upon annealing under H-2 and S environments; i.e., the Pt(IV)/Pt(II) ratio increased from 0.09 to 0.53. Transmission electron microscopy images of the annealed sample reveal the formation of PtS2 on the surface of the PtS crystals, clearly demonstrating the structural transformation caused by annealing under H-2 and S environments. Interestingly, the efficiency of the material for the hydrogen evolution reaction (HER) was found to be proportional to the PtS2/PtS ratio. These findings can facilitate the design of enhanced optoelectronics and catalysis through the phase and structural transformations of Pt-based chalcogenides.