Visualizing the Cu/Cu₂O Interface Transition in Nanoparticles with Environmental Scanning Transmission Electron Microscopy

Abstract

Understanding the oxidation and reduction mechanisms of catalytically active transition metal nanoparticles is important to improve their application in a variety of chemical processes. In nanocatalysis the nanoparticles can undergo oxidation or reduction in situ, and thus the redox species are not what are observed before and after reactions. We have used the novel environmental scanning transmission electron microscope (ESTEM) with 0.1 nm resolution in systematic studies of complex dynamic oxidation and reduction mechanisms of copper nanoparticles. The oxidation of copper has previously been reported to be dependent on its crystallography and its interaction with the substrate. By following the dynamic oxidation process in situ in real time with high-angle annular dark-field imaging in the ESTEM, we use conditions ideal to track the oxidation front as it progresses across a copper nanoparticle by following the changes in the atomic number (Z) contrast with time. The oxidation occurs via the nucleation of the oxide phase (CuO) from one area of the nanoparticle which then progresses unidirectionally across the particle, with the Cu-to-CuO interface having a relationship of Cu{111}//CuO{111}. The oxidation kinetics are related to the temperature and oxygen pressure. When the process is reversed in hydrogen, the reduction process is observed to be similar to the oxidation, with the same crystallographic relationship between the two phases. The dynamic observations provide unique insights into redox mechanisms which are important to understanding and controlling the oxidation and reduction of copper-based nanoparticles.

Rydberg excitons in Cu2O microcrystals grown on a silicon platform

“…1b) show pronounced texturing in {111} direction in both cases, indicating that the oxidation proceeds via an epitaxial relationship of Cu 2 O {111} ∥ Cu {111}. This relationship is in accordance with literature reports on the thermal oxidation of copper thin films 22 and nanoparticles with sizes down to few nanometers 25 . Similar to a previous report on epitaxial Cu 2 O growth on MgO 15 , the observed terrace-like structures on the Cu 2 O surfaces are suggesting a two-dimensional growth mode for individual microcrystals.…”

Mixed Copper/Copper‐Oxide Anchored Mesoporous Fullerene Nanohybrids as Superior Electrocatalysts toward Oxygen Reduction Reaction

“…The HR‐TEM images shown in Figure c display hexagonally organized highly ordered mesopores with well‐defined lattice fringes ( d = 0.246) and pore walls along with the selected area electron diffraction (SAED) pattern (Figure d, inset). The SAED pattern in Figure d, inset displays spacing between the lattice fringes of the crystalline particle corresponds to the (111) plane of fcc Cu (0.207 nm) (JCPDS no: 85–1326) and octahedron, lattice planes that are aligned parallel to the (111) facets of Cu 2 O (0.244 nm) . The average diameter of the Cu NPs was found to be ≈34 nm (averaged over three batches) and the population distribution is given in Figure e.…”

Promotive Effect of Sn2+ on Cu0/Cu+ Ratio and Stability Evolution of Cu/SiO2 Catalyst in the Hydrogenation of Dimethyl Oxalate

“…Cu 0 can be oxidised to Cu + by ester, while H2 can also reduce Cu + into Cu 0 in the reaction [21]. LaGrow et al also observed the interface transition of Cu and Cu2O using ESTEM via switching hydrogen and oxygen environments [22]. It is reasonable to explain why different catalytic systems have different Cu + /Cu 0 ratios that are favorable for the reaction; that may be why the Cu-0.6%Sn/SiO2 catalyst with the Cu 0 /Cu + ratio of 1.06 has the best catalytic activity and stability in our catalysts.…”