Mixed Valence of Bismuth in Hexagonal Chalcohalide Nanocrystals

We report the colloidal synthesis of bismuth chalcohalide nanocrystals, which adopt a hexagonal phase that we describe with the Bi13S18X2 (in which X = Br and I) structure. Such a model structure displays columns of Bi atoms that form dimers of subvalent Bi (formally Bi24+), which we here ascribe to Peierls-type distortions. We suggest that the Bi24+ dimers are at the origin of the anomalously low band gap of this material, with the lowest energy electronic transition showing analogies with an intervalence charge transfer. Our synthetic approach and insights into the structural and electronic features of the hexagonal bismuth chalcohalides are fundamental to sustain the rapidly increasing use of this class of (nano)materials for diverse applications, such as photocatalysis and thermoelectrics.

Automated summary

We have developed a colloidal synthesis method for bismuth chalcohalide nanocrystals with a hexagonal phase. The structure of these nanocrystals can be described by the Bi13S18X2 (where X = Br and I) model. It was found that the presence of Bi24+ dimers, formed by columns of Bi atoms, and Peierls-type distortions are responsible for the anomalously low band gap and intervalence charge transfer in this material. Our findings and synthetic approach provide fundamental understanding for the use of hexagonal bismuth chalcohalides in diverse applications such as photocatalysis and thermoelectrics.