Real-Time Monitoring of Competing Nanoparticle Formation Pathways during Cation Exchange Using Benchtop Light Scattering

Cation exchange is an increasingly common pathway for nanoparticle synthesis, as it modifies the composition in a controllable way while maintaining other key features, including crystal structure and morphology. However, cation exchange pathways can compete with other nanoparticle formation pathways, depending on the system and reaction conditions. Simple strategies for monitoring such reactions can therefore be informative. Here, we use benchtop light scattering with a laser pointer as a simple tool to monitor putative cation exchange reactions and to help differentiate, in real time, between pathways that involve cation exchange versus pathways that involve dissolution and reprecipitation. We use the transformation of digenite copper sulfide into manganese sulfide as a model system. When a laser pointer shines through the reaction flask as digenite copper sulfide nanoparticles react with Mn2+ at 100 °C, light scattering is observed continuously, indicating that nanoparticles are present during the entire reaction as would be required for a cation exchange pathway. At higher temperatures, light scattering disappears and then reappears, indicating that nanoparticles are not always present and that a different pathway involving dissolution and reprecipitation is operable. Using this approach, along with additional control experiments, we were able to identify the threshold temperature below which zincblende MnS, a metastable polymorph, forms through a cation exchange pathway. We were also able to establish that at higher temperatures, the thermodynamically favored product, rocksalt MnS, forms through a dissolution/reprecipitation pathway. These results provide useful insights into the temperature dependence of a model cation exchange reaction and suggest that light scattering could provide high-level insights, in real time on the benchtop, into nanoparticle reaction pathways that involve postsynthetic modifications where multiple competing pathways could be possible.