Colloidal Metal Chalcohalide Nanocrystals

We present a colloidal approach to explore the Bi-S-Br phase diagram that we also extended to other chalcogens and halogens. Our approach yielded phase pure bismuth chalcohalide nanocrystals (BimEnXp NCs) by co-injecting both the chalcogen (E = S, Se) and the halogen (X = Cl, Br, I) precursors in hot solutions of Bi-carboxylate complexes in a non-coordinating solvent. We synthesized orthorhombic BiEX (BiSBr, BiSI, and BiSeBr) NCs and trigonal Bi13E18X2 (Bi13S18Br2 and Bi13S18I2) NCs; in addition, we could synthesize NCs of a previously unknown BiSCl polymorph. The bismuth chalcohalide NCs were stable at standard laboratory conditions and did not degrade upon heating up to 250 °C. The bismuth chalcohalide NCs formed colloidally stable dispersions in solvents of different polarities, which were obtained upon post-synthesis surface chemistry modification. This enabled the processability of the NC dispersions into robust solid films with reduced rugosity (variations in height amplitude close to 5 %), which displayed a high absorption coefficient (up to above 105 cm-1) in the Vis-NIR spectral range (with an indirect band gap ranging between about 0.8 and 2.5 eV, depending on NC composition) and n-type character. We tested the light harvesting properties of the bismuth chalcohalide NC solids in photoelectrochemical cells, which proved able to extract current densities of the order of the mA/cm2 under prolonged solar-simulated irradiation with a quantum efficiency of the photon-to-current conversion process above 10 % across the entire visible spectral range.