p–n junction formation between CoPi and α-Fe2O3 layers enhanced photo-charge separation and catalytic efficiencies for efficient visible-light-driven water oxidation

Abstract

α-Fe₂O₃ has attracted considerable attention as an n-type semiconductor (SC) photoanode with a narrow band gap for visible-light-driven water oxidation. The loading of CoPi as an active catalyst for electrochemical water oxidation (Science, 2008, 321, 1072–1075) onto a variety of the α-Fe₂O₃ electrodes surely can improve photoelectrochemical (PEC) water oxidation. However, the mechanism of the improved PEC water oxidation by CoPi on the α-Fe₂O₃ electrodes and the role of CoPi are incredibly diverse. Herein, CoPi was loaded onto an α-Fe₂O₃ electrode prepared by a mixed metal-imidazole casting (MiMIC) method. The XPS and Mott–Schottky plots suggested the formation of a p–n junction at the interface between CoPi (p-type) and α-Fe₂O₃ (n-type), which is observed for the first time among the hitherto-reported α-Fe₂O₃/CoPi-based electrodes. The p–n junction formation significantly improved the PEC water oxidation performance for the present α-Fe₂O₃/CoPi electrode to attain an onset potential (E_on) of 0.65 V for photocurrent generation, charge separation (η_sep = 33%), and catalytic efficiency (η_cat = 70%) at 420 nm and 1.23 V vs. RHE, which are superior or comparable to those of the state-of-the-art α-Fe₂O₃/CoPi-based electrodes. Photoelectrochemical impedance spectroscopy (PEIS) analysis indicated that the rate constant (k_O2 = 2.1 s⁻¹) for water oxidation at the surface was higher compared to that (k_O2 = 1.4 × 10⁻⁹ s⁻¹) for the α-Fe₂O₃ electrode at 0.68 V by nine orders of magnitude. The rate constants (k_rec/s⁻¹) for the surface recombination of photogenerated carriers for the α-Fe₂O₃/CoPi electrode were lower than those for α-Fe₂O₃ over the whole potential range employed by a factor of 1.4–5.3. The lower E_on, higher η_sep and η_cat values for the α-Fe₂O₃/CoPi electrode compared with the α-Fe₂O₃ electrode did not result from the catalytic ability of the CoPi layer, but from the efficient charge transfer from the α-Fe₂O₃ surface to the active site on the CoPi layer through the formed p–n junction.