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

<div>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.</div><p></p>