Mechanism of morphology variations in colloidal CuGaS₂ nanorods

Cu_2- S nanocrystals can serve as templates and intermediates in the synthesis of a wide range of nanocrystals through seeded growth, cation exchange, and/or catalytic growth. This versatility can facilitate and accelerate the search for environmentally benign nanocrystals of high performance with variable shapes, sizes, and composition. However, expanding the compositional space via Cu_2- S nanocrystals while achieving necessary uniformity requires an improved understanding of the growth mechanisms. Herein we address several unusual and previously unexplained aspects of the growth of CuGaS₂ nanorods from Cu_2- S seeds as an example. In particular, we address the origin of the diverse morphologies which manifest from a relatively homogeneous starting mixture. We find that CuGaS₂ nanorods start as Cu_2- S/CuGaS₂ Janus particles, the majority of which have a {101̄2}/{101̄2} interface that helps to minimize lattice strain. We propose a mechanism that involves concurrent seed growth and cation exchange (CSC), where epitaxial growth of the Cu_2- S seed, rather than the anticipated catalytic or seeded growth of CuGaS₂, occurs along with cation exchange that converts growing Cu_2- S to CuGaS₂. This mechanism can explain the incorporation of the large number of anions needed to account for the order-of-magnitude volume increase upon CuGaS₂ rod growth (which cannot be accounted for by the commonly assumed catalytic growth mechanism) and variations in morphology, including the pervasive tapering and growth direction change. Insights from the CSC growth mechanism also help to explain a previously puzzling phenomenon of regioselective nucleation of CuInSe₂ on kinked CuGaS₂ nanorods.