Compositionally Induced Twin Defects Control the Shape of Ternary Silver Halide Nanocrystals

We demonstrate that the anion composition of ternary silver bromoiodide, AgBr_1-xI_x, nanocrystals determines their shape through the introduction of twin defects as the nanocrystals are made more iodide-rich. AgBr_1-xI_x nanocrystals grow as single-phase, solid solutions with the rock salt crystal structure for anions compositions ranging from 0 ≤ x < 0.38. With increasing iodide content, the morphology of the nanocrystals evolves from cubic to truncated cubic to hexagonal prismatic. Structural characterization indicates the cubic nanocrystals are bound by {100} facets, whereas the hexagonal platelet nanocrystals possess {111} facets as their top and bottom surfaces. Calculations based on first-principles density functional theory show that iodide substitution in AgBr stabilizes {111} surfaces and that twin defects parallel to these surfaces possess a low formation energy. Our experimental observations and calculations are consistent with a growth model in which the presence of multiple twin defects parallel to a {111} surface enhances lateral growth of the side facets and changes the nanocrystal shape. This new method to control nanoscale morphology based on anion composition provides a route to study structure-activity relationships in silver halide photocatalysts.