Heterogeneous doping of metastable β-Fe₂O₃ thin film photoanodes for enhanced solar water splitting

β-Fe₂O₃, a metastable polymorph of iron oxide with a cubic bixbyite structure, has been introduced as a promising photoanode material for solar water splitting. However, its metastable nature makes it difficult to grow as a single-phase material and poses challenges for effective doping. Here, we report the heteroepitaxial growth of β-Fe₂O₃ thin film photoanodes on indium tin oxide (ITO) films deposited on YSZ single-crystal substrates by pulsed laser deposition. Epitaxial film growth enables the study of dopant behaviour without interference from microstructural factors often encountered in polycrystalline materials. The incorporation of Ti⁴⁺ and Zn²⁺ was examined to assess the effects of donor (n-type) and acceptor (p-type) doping on β-Fe₂O₃ compared to undoped, nominally intrinsic (i) films. Photoanodes with heterogeneous p-i-n doping profiles were subsequently fabricated, resulting in substantial improvements in the charge separation and charge transfer efficiency as compared to uniformly doped films.