Tuning Crystal Growth of Colloidal Cs ₃ Bi ₂ I ₉ Perovskite‐Like Nanocrystals via a Solvent‐Assisted Reprecipitation Approach

Lead-free bismuth-based perovskite-like materials have emerged as promising alternatives to lead halide perovskites due to their enhanced stability and lower toxicity. Colloidal approaches for the synthesis of such materials offer the advantage of morphology engineering and precise control over size and shape. However, conventional approaches require long-chain organic ligands, which hinder charge transport and limit their applicability in optoelectronic devices. Herein, a ligand-free Solvent-Assisted Reprecipitation (SARP) approach is introduced, exploiting solvent-mediated synthesis to control the nucleation and growth of Cs₃Bi₂I₉ nanocrystals. The use of solvent descriptors, such as Kamlet-Taft parameters, enables preliminary screening of suitable solvents, suggesting five key candidates, including the green solvents triethyl phosphate and Cyrene, best suited to control crystal size, aspect ratio, and morphology. Electrostatic stabilization, induced by the solvent, promoted the formation of small Bi⁰ islands at the surface, which is key to ensuring colloidal stability and processability. Computational and experimental analyses agree on highlighting the role of solvents in controlling nucleation and growth of the nanocrystals, paving the way for the rational design of ligand-free nanomaterials for a platform of different applications.