Electrochemical control over stoichiometry via cation intercalation into Chevrel-phase sulphides (Cu_xMo₆S₈, x=1-3)

This work presents a systematic investigation of the electrochemical intercalation of aqueous copper cations into the Chevrel phase (CP) Mo6S8 and its effect on the host's electronic and structural characteristics as a function of stoichiometry. The electrochemical potentials at which copper intercalates into the CP Mo6S8 host in 1 M CuSO4 were identified for CuxMo6S8 (x = 1-3) as 0.23 V, 0.11 V, and 0.055 V (vs. Cu/Cu2+) for Cu1Mo6S8, Cu2Mo6S8, and Cu3Mo6S8, respectively, and correlated via ex situ PXRD to stoichiometric content. Potentials were also identified that yield non-stoichiometric phases with traceable Cu content-dependent structural changes. Ab initio DFT calculations were performed to probe thermodynamic stability of CuxMo6S8 CP compositions and the metal-to-semiconductor transitions as a function of copper content. Lastly, we demonstrate that the SK-pre-edge intensity is a successful probe for intercalant content: as copper equivalents increase, so does charge transfer to the partially occupied S 3p orbitals, resulting in a decrease in the pre-edge intensity corresponding to the excitation of a S 1s-to-3p/Mo 4d hybridized orbital.