Selective placement of modifiers on hematite thin films for solar water splitting

Abstract

The design of nanostructured materials for photoelectrochemical water splitting relies on a detailed understanding of the reactional bottlenecks. For hematite, a model system for photoanodes, the challenges concern poor charge transfer and separation, carrier recombination rate, and sluggish water oxidation kinetics. Several methods have been proposed to address each individually, with complex multi-step processes offered as solutions to improve overall performance. Here, we introduce a single polymeric precursor solution that enables the design of hematite (α-Fe₂O₃) with synergistic bulk and interfacial engineering using Ga³⁺, Hf⁴⁺ and NiFeO_x. The solution causes Ga³⁺ to dope hematite lattice to reduce polaronic effects, while simultaneously induces Hf⁴⁺ enrichment at both surface and grain boundaries, improving charge separation and reducing recombination. Hf⁴⁺ also led to a refined microstructure derived from interface stabilization, which associated with Ga³⁺ bulk doping and NiFeO_x electrodeposition resulted in a thin film with 65% of overall photoelectrode efficiency. As a consequence, the modified hematite photoanode (176 nm-thick) delivered a water oxidation photocurrent of 2.30 mA cm⁻² in contrast to 0.37 mA cm⁻² for the pristine system measured at 1.23 V against hydrogen reversible electrode (RHE). The results suggest the simplicity of this new polymeric solution may offer a cost-effective, scalable and versatile alternative for multiple chemical modifications in oxides beyond hematite.