Inducing hollow and porous hematite nanorod photoanodes by rare earth and transition metal doping for enhanced solar water splitting

Abstract

Hematite serves as a promising photoanode material in photoelectrochemical (PEC) water splitting systems. However, its inherent limitations of short hole diffusion length and insufficient carrier lifetime pose a significant challenge for practical application. Here, we report a hollow and porous hematite nanorod photoanode using hybrid microwave annealing induced rare earth europium (Eu) and transition metal niobium (Nb)-doped core/shell FeOOH nanorods synthesized by a two-step hydrothermal method, which enhances photocurrent density (J_ph) and reduces turn-on voltage (V_on) simultaneously by three synergistic effects: (i) hollow and porous nanorod formation to shorten hole diffusion distance; (ii) surface asymmetric oxygen vacancies and Eu³⁺ ↔ Eu²⁺ states to generate highly active sites; and (iii) suppression of surface segregation of Nb and Sn to reduce surface states. As a result, the hollow and porous Eu, Nb co-doped Fe₂O₃ photoanode loaded with a RuFe₂(OH)_x cocatalyst achieves a J_ph of 3.49 mA cm⁻² and a V_on of 0.67 V_RHE under simulated 1 sun irradiation (100 mW cm⁻²), which is 2 times higher J_ph and a more negative V_on of ∼250 mV than that of Nb : Fe₂O₃. This work demonstrates the successful combination of the two-step hydrothermal method, rare earth and transition metal co-doping, and hybrid microwave annealing to design and construct efficient nanorod-based photoanodes.