Unveiling the Synchronized Effect of Bulk and Surface Dual Modification of In Situ Nb-Doping and Microwave-Assisted Co(OH)_x Cocatalyst for Boosting Photoelectrochemical Water Splitting of Fe₂O₃ Photoanodes

Herein, in order to improve the conductivity and slack water oxidation kinetics of the hematite (α-Fe₂O₃) photoanode, we propose the Nb-doped and Co(OH)_x cocatalyst-deposited α-Fe₂O₃ thin film photoanode (Nb-HT/Co(OH)_x) via in situ diluted hydrothermal and microwave-assisted methods. The as-prepared Nb-HT/Co(OH)_x thin-film photoanode exhibited a photocurrent density of 1.78 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode (RHE), which is 1.7-fold higher than that of the Bare-Fe₂O₃ photoanode. The dual effect of Nb-doping and Co(OH)_x deposition markedly improved the PEC performance through enhancing the charge carrier mobility and donor density in hematite, as well as accelerating the interfacial charge transfer kinetics at the electrode/electrolyte interface of the α-Fe₂O₃ thin film photoanode. The Nb-HT/Co(OH)_x photoanode displayed a high charge separation efficiency of 90% (at 1.23 V_RHE) and excellent stability over a 10 h period without any decrease. Detailed electrochemical analyses using electrochemical impedance spectroscopy (EIS), open-circuit potential (OCP), and accumulated charge density techniques disclosed the charge separation and transfer processes. This strategy of bulk and surface modification highlights a new approach to constructing a stable photoanode for sustainable solar energy conversion.