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.

Imaging adsorption of iodide on single Cu2O microparticles reveals the acid activation mechanism.

Imaging an adsorption reaction taking place at the single-particle level is a promising avenue for fundamentally understanding the adsorption mechanism. Here, we employ a dark-field microscopy (DFM) method for in situ imaging the adsorption process of I- on single Cu2O microparticles to reveal the acid activation mechanism. Using the time-lapsed DMF imaging, we find that a relatively strong acid is indispensable to trigger the adsorption reaction of I- on single Cu2O microparticle. A hollow microparticle with the increase in size is obtained after the adsorption reaction, causing the enhancement of the scattering intensity. Correlating the change of the scattering light intensity or particle size with adsorption capacity of I-, we quantitatively analyze the selective uptake, slightly heterogeneous adsorption behavior, pH/temperature-dependent adsorption capacity, and adsorption kinetics as well as isotherms of individual Cu2O microparticles for I-. Our observations demonstrate that the acid-initiated Kirkendall effect is responsible for the high-reaction activity of single Cu2O microparticles for adsorption of I- in the acidic environment, through breaking the unfavorable lattice energy between Cu2O and CuI as well as generating high-active hollow intermediate microparticle.