Band alignments of complex heterojunctions from first principles: the case of the α -Fe ₂ O ₃ /BaTiO ₃ interface

Abstract Photoactive-ferroelectric nanostructured composites often exhibit superior performance compared to their individual components, and the interfacial band profile has been regarded as the major contributor to the reported enhancements. In this context, band alignment could serve as a guiding parameter for the screening of novel architectures; therefore, reliable theoretical approaches are desirable to facilitate this formidable task. A practical framework for band-alignment calculations within density functional theory is that developed by Van de Walle et al. , which combines superlattice calculations performed in the generalised gradient approximation (GGA) with independent bulk calculations using the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). Here, we evaluate the applicability of this 'GGA-HSE’ method for band alignments in complex oxide heterojunctions, using the α -Fe ₂ O ₃ /BaTiO ₃ interface as a representative and technologically relevant case. First, we apply this method to calculate the valence- and conduction-band offsets of model ferroelectric- and halide-based interfaces, establishing a reference line against full HSE band-alignment calculations. Our results demonstrate that the GGA-HSE method yields band offsets consistent with HSE values and is therefore not limited to prototypical oxides. With this validation in place, we then apply the method to the α -Fe ₂ O ₃ /BaTiO ₃ interface. We find that this system exhibits a straddling type-I profile, in which the charge carriers are confined within the photoactive component of the heterostructure. To rationalise the observed enhancement in photocatalytic performance of α -Fe ₂ O ₃ /BaTiO ₃ nanocomposites, we propose a band-structure-related mechanism involving thermally assisted charge transfer and ferroelectric polarisation effects.