Oscillatory Reaction Activity of Single Cuprous Oxide Microparticles with NO 2 .

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 (Cu 2 O) microparticles for NO 2 removal in a real-time manner. Unexpectedly, we find that the redox reaction between Cu 2 O microparticles and NO 2 is oscillating with the reaction time. Specifically, the oscillatory behavior of single Cu 2 O microparticles for NO 2 reduction shows a large particle-to-particle variability, which is also dependent upon the NO 2 pressure and Cu 2 O facets. A combined DFM imaging, spectroscopic, scanning electron microscopy, and density functional theory study uncovers that Cu 2 O is gradually transformed to copper nitrate hydroxide [Cu 2 (NO 3 )(OH) 3 ], and this oscillatory reaction is attributed to the cyclic formation and structural collapse of Cu 2 (NO 3 )(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.