Controlled Growth of delafossite CuFeO₂ Photoelectrodes for Superior Water Splitting Performance

Fossil-fuel dependence has undermined both energy security and environmental sustainability, driving intensive research into renewable green-hydrogen production. Solar-driven photoelectrochemical (PEC) water splitting is a compelling route, and delafossite-based semiconductors are especially attractive owing to their favorable physicochemical properties. However, high-performance electrodes are often fabricated via conventional “wet” chemical synthesis, which typically relies on toxic solvents and complex precursors. Moreover, producing phase-pure delafossite CuFeO₂ (CFO) thin films with controlled morphology and high electronic quality remains challenging. Here, we introduce plasma-assisted magnetron sputtering as a green, solvent-free strategy to fabricate efficient, phase-pure 3R delafossite CFO photoelectrodes. Optimized magnetron co-sputtering conditions were developed to achieve high-quality films, which were evaluated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy with energy-dispersive X-ray analysis (SEM–EDAX), UV–Vis spectroscopy, and electrochemical impedance spectroscopy (EIS). PEC performance was found to be strongly governed by film architecture. Increasing the thickness from 50 to 300 nm and the number of stacked layers from 1 to 6 progressively enhanced photocatalytic activity: the photocurrent increased from 0.3 mA cm^-2 at 0.4 V_RHE for a 50 nm single-layer film to 0.5 mA cm^-2 for a two-layer configuration, reaching 0.72 mA cm^-2 for a six-layer electrode. Multilayer CFO also exhibited improved operational stability relative to single-layer counterparts. Importantly, the best-performing six-layer film achieved strong activity without the addition of a co-catalyst or electron scavenger, which we attribute to synergistic contributions from improved light harvesting, a reduced effective band gap, and optimized surface roughness. Overall, the magnetron co-sputtering–based materials design offers a scalable and practical pathway to efficient and stable CFO photoelectrodes for PEC tandem-cell applications.