Ternary Silver Halide Nanocrystals Sasitha C. Abeyweera, † Kowsalya D. Rasamani, † and Yugang Sun* Department of Chemistry, Temple University, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States CONSPECTUS: Nanocrystalline silver halides (AgX) such as AgCl, AgBr, and AgI, a class of semiconductor materials with characteristics of both direct and indirect band gaps, represent the most crucial components in traditional photographic processing. The nanocrystal surfaces provide sensitivity specks that can turn into metallic silver, forming an invisible latent image, upon exposure to light. The photographic processing implies that the AgX nanoparticles possess unique properties. First, pristine AgX nanoparticles absorb light only at low efficiency to convert surface AgX into tiny clusters of silver atoms. Second, AgX nanoparticles represent an excellent class of materials to capture electrons efficiently. Third, small metallic silver clusters can catalyze the reduction of AgX nanoparticles to Ag nanoparticles in the presence of mild reducing reagents, known as self-catalytic reduction. These properties indicate that AgX nanoparticles can be partially converted to metallic silver with high precision, leading to the formation of hybrid AgX/Ag nanoparticles. The nanosized metallic Ag usually exhibit intense absorption bands in the visible spectral region due to their strong surface plasmon resonances, which make the AgX/Ag nanoparticles a class of promising visible-light-driven photocatalysts for environmental remediation and CO 2 reduction. Despite the less attention paid to their ability of capturing electrons, AgX nanoparticles might be a class of ideal electron shuttle materials to bridge light absorbers and catalysts on which electrons can drive chemical transformations. In this Account, we focus on ternary silver halide alloy (TSHA) nanoparticles, containing two types of halide ions, which increase the composition complexity of the silver halide nanoparticles. Interdiffusion of halide ions between two types of AgX at elevated temperatures has been developed for fabricating ternary silver halide alloy crystals, such as silver chlorobromide optical fibers for infrared communications. This solid state process is not feasible for synthesizing TSHA nanoparticles since it is hard to form two different types of AgX nanoparticles in direct contact. In contrast, coprecipitation of silver ions with different halide ions via colloidal chemistry represents the most promising strategy to synthesize TSHA nanoparticles. Forming uniform and phase-pure ternary silver halide nanocrystals requires that the rate ratio for precipitating both halide ions remains constant throughout the synthesis. However, the significant difference in solubility among different AgX usually leads to a nonuniform compositional distribution in the resulting nanoparticles because the halide ions corresponding to the less soluble AgX precipitate faster at the early reaction stage. This Account summarizes the methods recently developed for the successful synthesis of phase-pure TSHA nanoparticles with uniform sizes and morphologies, which involve precise control over the balanced diffusion of different halide ions to react with silver ions. Typical methods include the use of microemulsion capsules and high-viscosity solvents to lower and even the diffusion coefficients of various halide ions, thus maintaining the precipitation rates of both AgX in single nanoparticles at a constant ratio. The availability of high-quality TSHA nanoparticles provides promising opportunities to explore their new properties and applications. 1. INTRODUCTION Silver halide crystals (e.g., AgCl, AgBr, and AgI) have been widely used in photography for centuries, ever since the discovery of the conversion of cerargyrite (silver ore composed of AgCl) to dark metallic silver, a chemical reaction that was identified by the great Swedish chemist Carl Wilhelm Scheele in 1777. 1 Although the revolution of digitization significantly reduced the use of silver halides in photography, emergence of nanoscience enables the nanometer-sized silver halide crystals to be useful as antimicrobial agents, 2,3 water oxidation catalysts, 4,5 and photocatalysts for environment remediation. 6-8 Alloying two types of silver halide crystals into ternary silver halide nanocrystals renders additional controllability over their optical properties. For example, silver halides are transparent in the 0.6-20 μm range and widely used as the critical components of optical fibers in the mid-infrared (IR) region. Varying the Cl/Br ratio in silver chlorobromoide (AgCl x Br 1-x ) crystals tunes their refractive index, making AgCl x Br 1-x an ideal class of materials in the fields of IR optical signal processing, fiber-optical communication, optical waveguiding, and targeted photothermal therapy. 9-11 The excellent and tunable transparency of AgCl x Br 1-x in the mid-IR region enables the AgCl x Br 1-x fibers with tapered tips, which are coupled with scanning near-field optical microscopy (SNOM), to image chemical and biological samples that are either placed in air or immersed in water in the spectral range of 5-15 μm. 12,13 The corresponding spatial resolution is determined by the apex size of the AgCl x Br 1-x fibers at the tapered end, which can be improved by attaching AgCl x Br 1-x Received: April 21, 2017 Article pubs.acs.org/accounts © XXXX American Chemical Society A DOI: 10.1021/acs.accounts.7b00194 Acc. Chem. Res. XXXX, XXX, XXX-XXX