Regulating Sn self-doping and boosting solar water splitting performance of hematite nanorod arrays grown on fluorine-doped tin oxide via low-level Hf doping

Hematite (α-Fe2O3) nanorod arrays grown on fluorine-doped tin oxide (FTO) substrate exhibit outstanding solar water splitting efficiency, benefiting from Sn self-doping induced by high-temperature annealing. However, this Sn self-doping couldn't be freely controlled without changing the optimized annealing conditions, which limits the further improvement of their photoelectrochemical (PEC) properties. Here, we report a facile hydrothermal synthesis with subsequent annealing approach to regulate the Sn diffusion via hafnium (Hf) doping as well as enhance the PEC performance of hematite photoanode. Upon increasing the Hf doping concentration, the Sn self-doping content was continuously suppressed. The very low doping-level of Hf (i.e., atomic Hf/Fe = 0.13 ∼ 1.54%) was sufficient for enhancing the electrical conductivity. The Hf-doped α-Fe2O3 with the optimized dopant concentration (Hf/Fe = 1.34%, denoted as 0.25-Hf-Fe2O3) showed a photocurrent density of 1.79 mA/cm2 at 1.23 V vs. RHE, 70% higher than that of the Sn self-doped one (Pristine-Fe2O3). The donor density of 0.25-Hf-Fe2O3 increased 2.5 times compared to Pristine-Fe2O3 while its space-charge resistance and charge transfer resistance declined by 40% and 22%, respectively, verifying Hf doping improves the charge carrier density and accelerates the charge transfer for solar water oxidation. We offered here a prospective dopant alternative for preparing superior hematite-based photoanode.