Probing the Stability and Band Gaps of Cs ₂ AgInCl ₆ and Cs ₂ AgSbCl ₆ Lead-Free Double Perovskite Nanocrystals

Lead toxicity has sparked interest into alternative halide nanomaterials with properties similar to CsPbX ₃ perovskites. A promising alternative suggested from bulk studies is the family of double perovskites of the form Cs ₂ AgMX ₆ . Here, we report the synthesis of colloidal Cs ₂ AgInCl ₆ and Cs ₂ AgSbCl ₆ nanocrystals via injection of acyl halides into a metal acetate solution under atmospheric conditions and relatively mild temperatures. We demonstrate the synthesis of single-crystalline cubic nanocrystals of ca. 10 nm side length and their morphological similarities to other double perovskite nanostructures in terms of their [200] facet termination and decoration with Ag ⁽⁰⁾ smaller nanocrystallites. To compare the stabilities of the synthesized materials, we develop a titration assay based on the degradation of nanocrystals with amines as a proxy for degradation by humidity, which provides a quantifiable stability metric. This measurement shows that Cs ₂ AgSbCl ₆ releases more than twice the decomposition energy compared to Cs ₂ AgInCl ₆ or CsPbCl ₃ and degrades in the presence of approximately one molar equivalent of amine, whereas the other two materials require more than a 100-fold excess. Using facile chemical titration to quantitatively determine chemical stability provides an additional tool to aid in the basic understanding of what makes some of these materials more environmentally stable than others.