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 I and Cu II centers. Although electrocatalytic study with digenite Cu 9 S 5 and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured Cu 9 S 5 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 Cu 9 S 5 to be d(0015) and d(002) for CuS. Tetrahedral ( T d ) Cu II , distorted octahedral (O h ) Cu II , and trigonal planar (T p ) Cu I sites form the d(0015) surface of Cu 9 S 5 , while the (002) surface of CuS consists of only T d Cu II . The distribution of Cu I and Cu II 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 Cu 9 S 5 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 Cu 9 S 5 is more electrochemically labile compared to CuS and transforms rapidly to CuO/Cu 2 O. Contact-angle and BET analyses imply that a high-surface-energy and macroporous Cu 9 S 5 surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of Cu 9 S 5 and CuS to CuO/Cu 2 O/Cu(OH) 2 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 Cu 9 S 5 /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.