Oscillatory Reaction Activity of Single Cuprous Oxide Microparticles with NO2

Here, we report on using dark-field microscopy (DFM) as a simple and low-cost imaging platform to visually resolve the kinetics of single cuprous oxide (Cu2O) microparticles for NO2 removal in a real-time manner. Unexpectedly, we find that the redox reaction between Cu2O microparticles and NO2 is oscillating with the reaction time. Specifically, the oscillatory behavior of single Cu2O microparticles for NO2 reduction shows a large particle-to-particle variability, which is also dependent upon the NO2 pressure and Cu2O facets. A combined DFM imaging, spectroscopic, scanning electron microscopy, and density functional theory study uncovers that Cu2O is gradually transformed to copper nitrate hydroxide [Cu2(NO3)(OH)3], and this oscillatory reaction is attributed to the cyclic formation and structural collapse of Cu2(NO3)(OH)3. The present findings open an alternative avenue for probing structure–performance relationships, which are anticipated to benefit the creation of functional materials for air purification.