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

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 Eu 3+ ↔ Eu 2+ 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 Fe 2 O 3 photoanode loaded with a RuFe 2 (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 : Fe 2 O 3 . 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. A hollow and porous Eu,Nb : Fe 2 O 3 nanorod photoanode was fabricated by hybrid microwave annealing (HMA) induced conversion of shell Eu- and core Nb-doped FeOOH nanorods, simultaneously enhancing photocurrent density and reducing turn-on voltage.