Enhanced Photoelectrochemical Hydrogen Evolution over Crystalline Cu₂Sn_0.38Ge_0.62S₃ Particles Grown from LiCl-RbCl Flux

Cu2Sn0.38Ge0.62S3 (CTGS) crystalline particles grown from a LiCl–RbCl eutectic flux exhibited efficient photoelectrochemical (PEC) hydrogen evolution from water, providing a cathodic photocurrent of −9.6 mA cm–2 at 0 V vs a reversible hydrogen electrode (RHE) under simulated sunlight, and a 45% incident-photon-to-current conversion efficiency (IPCE) at 0 VRHE under 600 nm monochromatic light. The bulk and surface properties of the flux-grown photocatalyst crystals included a low concentration of electronic defects, a relatively narrow particle size distribution with a mean of approximately 10 μm, the formation of crystal facets and the suppressed formation of oxide-like surface impurities. These factors all contributed to the enhanced PEC performance. CTGS crystals produced in a LiCl-RbCl flux were found to have a greater amount of Cu-deficient secondary phase such as Cu2(Sn,Ge)3S7, resulting in an increased carrier concentration in the bulk material as well as the formation of a preferred interface with an n-type CdS surface modifier. The present synthesis concept based on crystal growth in a flux provides insights into methods of tuning both the bulk and surfaces of photocatalyst crystals, including interfaces formed at heterojunctions.