Colloidal Bismuth Chalcohalide Nanocrystals

Abstract

Here we present a colloidal approach to synthesize bismuth chalcohalide nanocrystals (BiEX NCs, in which E=S, Se and X=Cl, Br, I). Our method yields orthorhombic elongated BiEX NCs, with BiSCl crystallizing in a previously unknown polymorph. The BiEX NCs display a composition-dependent band gap spanning the visible spectral range and absorption coefficients exceeding 10⁵ cm^-1 . The BiEX NCs show chemical stability at standard laboratory conditions and form colloidal inks in different solvents. These features enable the solution processing of the NCs into robust solid films yielding stable photoelectrochemical current densities under solar-simulated irradiation. Overall, our versatile synthetic protocol may prove valuable in accessing colloidal metal chalcohalide nanomaterials at large and contributes to establish metal chalcohalides as a promising complement to metal chalcogenides and halides for applied nanotechnology.

Keywords: Bismuth Chalcohalides; Colloidal Synthesis; Light-Harvesting; Nanocrystals; Photoelectrochemistry.

Figure 1

a) TEM image of the as‐synthesized BiSBr NCs. b) Rietveld fitting of the synchrotron XPD diffractogram of the BiSBr NCs (to facilitate the comparison with data collected on lab‐grade setups, the 2θ values of the XPD horizontal axis were converted to emulate the CuKα₁ radiation). c) Crystal structure of the BiSBr NCs highlighting the 1D ribbons along the c‐axis and the square pyramidal coordination of Bi atoms (according to the CPK coloring: pink is for Bi, yellow is for S, and dark red is for Br).

Figure 2

a) Rietveld fit of the XPD data based on the calculated BiSCl model obtained ab‐initio and comparison with the reflection list of the known BiSCl structure (to facilitate the comparison with data collected on lab‐grade setups, the 2θ values of the XPD horizontal axis were converted to emulate the CuKα₁ radiation). b) Superimposed crystal structures of both the known (grey) and the disclosed (colored) BiSCl polymorphs (according to the CPK coloring: pink is for Bi, yellow is for S, and green is for Cl).

Figure 3

a) Extinction spectra and daylight picture of toluene colloidal dispersions of BiSCl, BiSBr, and BiSI NCs. b) Absorption coefficients and daylight picture of thin films of BiSCl, BiSBr, and BiSI NCs. c) Computed band structure and density of states (with atomic orbital projections) of BiSBr (band structures for BiSCl and BiSI are shown in Figure S39).

Figure 4

a) XRD patterns of the BiSBr NCs upon one month storage at ambient conditions then heated up to 350 °C. b) 1H NMR spectra of the as‐synthesized BiSBr NCs upon ligand exchange with either 1‐dodecanethiol or dimethyldidodecylammonium bromide in toluene‐d8. c) FTIR spectra of the BiSBr NCs capped with either oleic acid or dimethyldidodecylammonium bromide upon solution phase ligand exchange with methylammonium bromide.

Figure 5

a) Daylight picture of a BiSI NC film cast on a ITO substrate used as a working electrode for photoelectrochemical measurements. b) Photocurrent density extracted at a 0.25 V bias under solar‐simulated illumination. c) Incident photon to current conversion efficiency at a 0.25 V bias under monochromatic illumination. d) Tentative energy level diagram of the photoelectrochemical cell based on BiSI NC photoelectrodes.