Synthesis of AgBiS ₂ and AgInS ₂ Based on Formation Energies in Sulfur‐Rich Reducing Flame

Emerging reactive spray technologies, adapted from well‐established gas‐phase metal oxide synthesis, provide a powerful and scalable route for producing non‐oxide nanoparticles. The high surface area ternary metal sulfides are particularly attractive due to their relevance in photocatalysis, energy storage, and gas sensing. Enclosed flame spray pyrolysis (EFSP) enabled the synthesis of ternary metal sulfides AgInS 2 and AgBiS 2 by precisely tuning O 2 , fuel, and N 2 co‐flow and metal/sulfur (M/S) ratio to form reducing, sulfur‐rich flames. Such flames suppressed oxide formation and promoted rapid nucleation of phase‐pure uniform nanoparticles via combustion of high‐enthalpy precursor‐solvent combinations. Structural and compositional analyses using confirmed high crystallinity, consistent lattice distances, crystal structure, and homogeneous elemental distributions, with average particle sizes of ∼11 nm for AgInS 2 and nearly double for AgBiS 2 . A methodology was developed for dispersing the particles in aqueous solution using CTAC surfactant. Dynamic light scattering data demonstrated colloidal dispersions (hydrodynamic size 150 (±4) nm) with positively charged colloids (zeta‐potentials (ZP) +15 to +40 mV, hydrodynamic size 150 (±4) nm) according to electrophoretic measurements. UV–vis spectroscopy revealed direct band gaps of 1.45 eV for AgInS 2 and 1.50 eV for AgBiS 2 . These results pave the way for applications of mixed‐metal sulfide nanoparticles in colloidal solutions.