Surface Structure to Tailor the Electrochemical Behavior of Mixed-Valence Copper Sulfides during Water Electrolysis.

The semiconducting behavior of mixed-valence copper sulfides arises from the pronounced covalency of Cu-S bonds and the exchange coupling between Cu and Cu centers. Although electrocatalytic study with digenite CuS and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured CuS and CuS are prepared and used as electrode materials to study their electrochemistry. Powder X-ray diffraction (PXRD) and microscopic studies have found the exposed surface of CuS to be d(0015) and d(002) for CuS. Tetrahedral () Cu, distorted octahedral (O) Cu, and trigonal planar (T) Cu sites form the d(0015) surface of CuS, while the (002) surface of CuS consists of only Cu. The distribution of Cu and Cu sites in the lattice, predicted by PXRD, can further be validated through core-level Cu 2p X-ray photoelectron spectroscopy (XPS). The difference in the electrochemical response of CuS and CuS arises predominantly from the different copper sites present in the exposed surfaces and their redox states. In situ Raman spectra recorded during cyclic voltammetric study indicates that CuS is more electrochemically labile compared to CuS and transforms rapidly to CuO/CuO. Contact-angle and BET analyses imply that a high-surface-energy and macroporous CuS surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of CuS and CuS to CuO/CuO/Cu(OH) electroactive species. The performance of the in situ formed copper-oxides towards electrocatalytic water-splitting is superior compared to the pristine copper sulfides. In this study, the redox chemistry of the CuS/CuS has been correlated to the atomic arrangements and coordination geometry of the surface exposed sites. The structure-activity correlation provides in-depth knowledge of how to interpret the electrochemistry of metal sulfides and their in situ potential-driven surface/bulk transformation pathway to evolve the active phase.