Heavy-metal chalcohalides are inorganic semiconductors [1], with potential as next-generation optoelectronic materials based on earth-abundant elements. Compared to other semiconductors, usually obtained through colloidal synthesis, chalcohalides combine the simple chemistry of metal halides like CsPbBr₃ with the improved stability of metal chalcogenides like CdS, making them ideal candidates to be explored in the form of nanocrystals. Among them, bismuth-based compounds [2],[3] are particularly promising because of their low toxicity and the high absorption cross-sections, which makes them optimal choices for light harvesting applications. Despite these potential advantages, however, these materials have been explored relatively sparsely so far and primarily in the form of bulk or microcrystals [4].

Herein, a new synthetic route to obtain metallic Bi/Bi₁₃S₁₈Br₂ colloidal nano-heterostructures with dumbbell morphology is presented. This one-pot procedure was developed by modifying the synthesis of Bi-S-X nanocrystals [5] with the controlled introduction of a tertiary amine, which acts as a mild reducing agent. This induces an in-situ nucleation of the metallic bismuth domains on the surface of the semiconductor. By combining electron microscopy and X-ray diffraction we elucidated the  non-trivial growth mechanism of these heterostructures, which proceeds via dual nucleation event (first Bi₁₃S₁₈Br₂ and then metallic-Bi on its surface) followed by a controlled deposition of material at the interface between the two domains.

The combination of a semiconducting and a metallic domain makes our Bi/Bi₁₃S₁₈Br₂ heterostructures interesting materials for photocatalytic applications, as the presence of a heterojunction is expected to enhance the separation of photogenerated carriers and increase their availability for promoting chemical reactions [6]. To assess the photocatalytic activity of these heterostructures, we tested them for the photodegradation of organic dyes like Rhodamine B and Methylene Blue as a proof of concept. Our tests revealed that the Bi/Bi₁₃S₁₈Br₂ heterostructures can efficiently photodegrade the dyes without relevant structural, morphological, and compositional modification for several reuse cycles, demonstrating excellent robustness and stability in polar solvents, like ethanol. The mechanism of photocatalysis will be also discussed. These promising results motivated us to pursue a more in-depth exploration of these and similar heterostructures for photocatalytic applications, which is currently ongoing.