Band Edge Engineering of BiOX/CuFe₂O₄ Heterostructures for Efficient Water Splitting

Layered bismuth oxyhalides (BiOX, X = Cl, Br, and I) are promising visible light-responsive photocatalysts but suffer from inadequate electron transportation from the bulk to the surface. Construction of heterostructures has been considered as a convenient approach to improve the spatial charge carrier separation and enhance the efficiencies of the surface-reactive charges for catalysis. Here, a series of heterostructures has been successfully designed for n-type bismuth oxyhalides and p-type spinel ferrites CuFe2O4 (CFO) by a facile and generalized protocol via the hydrothermal method followed by the co-precipitation method. The heterostructure introduces built-in electric field at the interface that facilitates vectorial charge transfer, which demonstrated significantly improved visible light-driven photocatalytic activity toward H2 generation without using any noble metal co-catalyst. A conventional type-I and type-II charge transfer mechanism has been followed for BiOBr/CFO and BiOI/CFO heterostructures, respectively, which may effectively lower charge transfer resistance compared to that for bare BiOBr and BiOI, suggesting facile charge transfer. Remarkably, a direct Z-scheme BiOCl/CFO heterostructure has been formed between BiOCl and CFO with an intimate interfacial contact, which demonstrated 5.7 times higher H2 generation activity than pure BiOCl and two fold improved catalytic efficiency compared to type-II BiOI/CFO heterostructures under visible light. Very low resistance in electrochemical impedance spectra confirmed the superiority of the direct Z-scheme in promoting the charge separation and transfer and increase in carrier density. Moreover, the optimal space charge layer width and the redox potentials have been achieved for BiOCl/CFO through the engineering of band edge potentials, which reduces the fast recombination rate. This work offers a paradigm for the design of highly engineered BiOX-based heterostructures with tuned band structures for efficient photocatalytic water splitting.