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

alpha-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 alpha-Fe2O3 electrodes surely can improve photoelectrochemical (PEC) water oxidation. However, the mechanism of the improved PEC water oxidation by CoPi on the alpha-Fe2O3 electrodes and the role of CoPi are incredibly diverse. Herein, CoPi was loaded onto an alpha-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 alpha-Fe2O3 (n-type), which is observed for the first time among the hitherto-reported alpha-Fe2O3/CoPi-based electrodes. The p-n junction formation significantly improved the PEC water oxidation performance for the present alpha-Fe2O3/CoPi electrode to attain an onset potential (Eon) of 0.65 V for photocurrent generation, charge separation (etasep = 33%), and catalytic efficiency (etacat = 70%) at 420 nm and 1.23 V vs. RHE, which are superior or comparable to those of the state-of-the-art alpha-Fe2O3/CoPi-based electrodes. Photoelectrochemical impedance spectroscopy (PEIS) analysis indicated that the rate constant (kO2 = 2.1 s^-1) for water oxidation at the surface was higher compared to that (kO2 = 1.4 x 10^-9 s^-1) for the alpha-Fe2O3 electrode at 0.68 V by nine orders of magnitude. The rate constants (krec/s^-1) for the surface recombination of photogenerated carriers for the alpha-Fe2O3/CoPi electrode were lower than those for alpha-Fe2O3 over the whole potential range employed by a factor of 1.4-5.3. The lower Eon, higher etasep and etacat values for the alpha-Fe2O3/CoPi electrode compared with the alpha-Fe2O3 electrode did not result from the catalytic ability of the CoPi layer, but from the efficient charge transfer from the alpha-Fe2O3 surface to the active site on the CoPi layer through the formed p-n junction.