Composition-Dependent Photoelectrochemical Properties of Low-Toxic Bismuth Chalcohalide Nanorods Synthesized via a Solution-Phase Method (Supporting Information)

Bismuth chalcohalides (Bi–S–X, where X = Cl, Br, or I) are promising candidates for photovoltaic and optoelectronic applications due to their low toxicity, narrow bandgaps, and solution processability. Here, we report a colloidal synthesis of Bi13S18I2 nanorods (NRs) with tunable widths ranging from 7 nm to 44 nm. Surface modification with 3-mercaptopropionic acid (MPA) enabled the resulting NRs to disperse uniformly in aqueous solution, free from by-products. By varying synthesis temperature, the bandgap can be systematically engineered from 1.67 eV to 1.31 eV due to the quantum size effect. The NRs exhibit broad visible-to-NIR absorption, and retain their optical and structural integrity after ligand exchange with MPA for aqueous dispersion. This approach was extended to synthesize Bi13S18(Br,I)2 alloy NRs, whose composition was tuned by varying the I/(I + Br) precursor ratio. The resulting alloyed NRs maintained a similar morphology, the width of 6–8 nm and the length of 250–300 nm, regardless of their composition, but their absorption spectra tended to be blue-shifted with an increase in the I content. Photoelectrochemical measurements revealed n-type semiconductor behavior, with photocurrent generation under visible and NIR illumination. This study establishes a versatile synthetic framework for engineering composition, morphology, and electronic structure in bismuth chalcohalide NRs for environmentally friendly optoelectronic device applications.