Kinetics of Concurrent Seed Growth and Cation Exchange in Transforming Cu_2-xS Nanocrystals to CuGaS₂ Nanorods

Cation exchange can convert nanocrystals that have already been achieved with a well-controlled size, size distribution, and shape to a broad range of compositions. However, cation exchange can often be accompanied by changes in the nanocrystal morphology/shape as exemplified by the synthesis of I-III-VI₂ nanocrystals. We examine the temperature-dependent kinetics of concurrently occurring seed epitaxial growth and cation exchange that convert nearly spherical Cu_2-xS seeds into CuGaS₂ nanorods with varying lengths and degrees of tapering. A simple model is developed to quantify and explain experimentally observed reaction kinetics. Direct epitaxial growth of Cu_2-xS seeds occurs with an activation energy of 96 kJ/mol, while cation exchange to convert the growing seed to CuGaS₂ requires overcoming a 202 kJ/mol energy barrier. Understanding how each reaction rate evolves over time provides insights into the tapering mechanism and a means of predicting when the onset of tapering occurs. The predicted onset is then exploited to synthesize nanorods with a minimized tapering. Our findings provide the basis for developing precise control over the composition and morphology of nanocrystals synthesized through a combination of cation exchange and solution epitaxy.