Synergistic photocatalytic effect of BiOBr-BiOI heterojunctions due to appropriate layer stacking.

The increasing interest in acquiring efficient visible-light active photocatalytic materials has led to the formation of heterojunctions with different combinations of semiconductors. Despite the fact that increasingly more complex structures are proposed, there are still many unclear factors affecting their performance and limiting their prompt implementation. In this work, we used the spray pyrolysis technique to deposit individual visible light-active BiOBr and BiOI films and formed the heterojunctions BiOBr-BiOI and BiOI-BiOBr to determine the effect of the stacking order of semiconductors. These materials were widely characterized; their structural, optical, (photo)electrochemical, and photocatalytic properties were evaluated, revealing that the configuration BiOI-BiOBr boosted the photocatalytic indigo carmine dye removal under simulated sunlight irradiation, but the opposite layout quenched it. The high efficiency is attributed to a better use of the incident radiation and the effective migration of the photogenerated carriers. BiOBr - with a wider band gap and a less negative conduction band with respect to BiOI - provides its good attributes to the heterostructure, such as high stability and low recombination rates, when it is at the surface. We demonstrated that in thin-film heterostructures, the order in which the layers are stacked becomes decisive for the photocatalytic performance and that the energy band gap and the relative band positions of both semiconductors are the principal features that govern the photocatalytic mechanism. These findings provide a key to designing more efficient photocatalysts without several unsuccessful trials.