Single-Particle Imaging of Anion Exchange Reactions in Cuprous Oxide

Ion exchange is a predominant and flexible route to tailor the composition and crystal structure of various materials. In situ monitoring of the ion exchange process at the single-particle level is critical to better understand the reaction mechanism and engineer high-performance materials. We report herein a dark-field imaging approach to in situ investigate the anion exchange reactions between individual Cu₂O microparticles and S^2- or Cl^- assisted by the hydrolysis of Sn⁴⁺, which are visualized by directly observing the color change of single Cu₂O microparticles. The variation of the scattering intensity is applied for quantitative analysis of anion exchange kinetics, revealing that this reaction process is dependent on the morphology, size, environmental pH, and reactant concentration. We directly observe that the corners of Cu₂O are the preferential exchange sites, and the reaction activity is surface dependent. Moreover, the reaction rate constant and diffusion coefficient are estimated to be 1.1 × 10^-2 s^-1 and 9.4 × 10^-11 cm²/s. Furthermore, a single-particle colorimetric assay is also fabricated for visual detection of S^2-.