Photoelectrochemical performance and charge dynamics of supersonic cluster beam deposited CuFe2O4 thin films

• A stable CuFe 2 O 4 thin film electrode was fabricated by supersonic cluster beam deposition. • Cu 2+ reduction occurs below 0.4 V vs. RHE, suppressing charge injection kinetic constant. • A major contribution to the photocurrent efficiency is due to the direct transition at 2.8 eV. Developing efficient and stable photoelectrodes is a major challenge in photoelectrochemical energy conversion technologies. Although CuFe 2 O 4 is a promising p-type semiconductor, its implementation as a photocathode has been limited by the difficulty of synthesizing uniform and stable films and by the scarce investigation of its charge-transfer kinetics. In this work, a 70 nm thick CuFe 2 O 4 layer was fabricated for the first time by supersonic cluster beam deposition, enabling the direct growth of a homogeneous and nanogranular film on a FTO substrate. After air annealing at 400 °C, compositional and structural characterizations revealed the formation of a polycrystalline tetragonal spinel-type CuFe 2 O 4 film, with a Cu x O segregation at the surface. Despite its low thickness, the electrode displayed strong visible light absorption and a p-type photoelectrochemical activity in neutral electrolyte (0.1 M Na 2 SO 4 ), with a good electrochemical stability. Interestingly, below 0.4 V vs . RHE applied bias, the charge injection kinetic constant drop corresponds to the reduction of Cu 2+ surface species to Cu + . The absorbed photon to current conversion efficiency spectra revealed a peak efficiency at 2.8 eV and confirmed a bias-dependent feature associated with the Cu x O interfacial layer.