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 CuFe 2 O 4 (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 H 2 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 H 2 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.