Improving the photocatalytic performance remains a key challenge in perovskite nanocrystal-based systems.[1] One effective strategy is to introduce a semiconductor-semiconductor electronic junction, designed to promote charge separation at the interface and increase the accessibility of photogenerated carriers.[2] Colloidal nanocrystal heterostructures, that are nanoparticles composed of at least two distinct materials sharing an interface, emerge in this context as highly promising solution. Indeed, proper band alignment at the heterojunction can suppress charge recombination, thus improving the overall efficiency of the photocatalytic process.[3]

To this end, we optimized the synthesis of a family of perovskite-chalcohalide heterostructures based on CsPbX₃/Pb₄S₃X₂ (X= Cl, Br, I).[4] These semiconductor-semiconductor junctions have recently attracted considerable attention as they can effectively convert sunlight into electron-hole pairs, which are then separated through the electronic structure of the material. By combining absorption and ultraviolet photoelectron spectroscopies, we characterized the band alignments of these materials, which can be tuned through the halide composition. This compositional control allows for a variety of different band configurations, suitable for many photocatalytic reactions. 

As a proof-of-concept, we evaluated the heterostructures performances in the photooxidative coupling of p-substituted thiophenols [5] under visible-light irradiation, at room temperature, in air and in the absence of a sacrificial electron donor. Our CsPbBr₃/Pb₄S₃Br₂ heterostructures achieved up to 94 % selectivity toward disulfide formation in 90 minutes when using p-OCH₃ thiophenol. We also propose a plausible mechanism for this reaction, based on experiments with several scavenger species such as 1,4 benzoquinone (superoxide anion scavenger), N,N-diisopropylethylamine (hole scavenger) and the radical trapping agent 2,2′,6,6′-tetramethylpiperidine-1-oxyl (TEMPO). These results highlight the crucial role of the type-II heterojunction in promoting charge separation and the efficient electron delocalization across semiconductor domains. This synergistic behavior enhances the overall reduction capability of the heterostructures nanoparticles, thus improving their photocatalytic performances. These promising results pave the way to further photocatalytic investigations and are a significant first step towards the real-world application of these complex yet fascinating heterostructures.