Synthesis of Stoichiometric Cu ₃ BiS ₃ Thin Films through Sulfurization of Oxide Precursors

The wittichenite-type compound (Cu3BiS3, CBS) shows great potential for photovoltaic and solar-to-fuel applications due to its favorable optoelectronic properties. However, precise control over composition and phase stability in thin films remains challenging to achieve due to phase complexity and a limited kinetic stability window. This research focuses on obtaining homogeneous and stable CBS phases with ideal composition through a simple sulfurization process of an oxide Cu–Bi–O (CBO) precursor film in the thermodynamically permissible temperature range (350–425 °C). The findings confirm successful conversion of oxide film to the wittichenite phase, free of secondary phases, at 350 °C with a direct bandgap energy of 1.61 eV. The energy band positions from photoelectron spectroscopy show favorable energetics toward water splitting and CO2 reduction. Photoelectrochemical performance of a prototype CBS/TiO2 heterojunction device demonstrates a photocurrent density of 0.12 mA/cm2 and 0.96 mA/cm2 for water splitting and CO2 reduction, respectively. The presented method can be further explored to fabricate CBS films in various configurations for PEC and solar cell devices.