Acid-induced surface reconstruction of NiSeTe revealed by NAP-XPS

Surface activation of transition metal chalcogenides (TMCs) is commonly attributed to electrochemical reduction and defect formation under applied bias. Herein, we demonstrate that for single-crystalline NiSeTe, a significant fraction of the surface transformation typically associated with electrochemical activation instead arises from purely chemical, sulfuric acid-driven (H2SO4) reconstruction occurring prior to any applied potential. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is employed to monitor the evolution of pristine, acid-exposed, and washed surfaces. Acid treatment selectively removes native TeOx species and Ni-O(H) environments, while heterogeneous sulfate-derived species collapse into a transient S(VI)-rich adlayer. Subsequent washing eliminates weakly bound sulfates and yields a chemically simplified, oxide-free NiSeTe surface with a thin, stable sulfate termination. Notably, operando, bias-controlled NAP-XPS measurements with simultaneous chronoamperometric current monitoring reveal that cathodic polarization under HER-relevant conditions does not further reduce the acid-reconstructed surface, indicating that sulfuric acid exposure already generates an oxide-free NiSeTe termination prior to electrochemical biasing. In contrast, anodic polarization under OER-relevant conditions promotes tellurium re-oxidation. These findings establish acid-driven chemical restructuring as a dominant contributor to surface activation in NiSeTe and underscore the importance of disentangling chemical and electrochemical effects in chalcogenide electrocatalysts.