Double oxide shell layer formed on a metal nanoparticle as revealed by aberration corrected (scanning) transmission electron microscopy

Determination of the extent of oxidation in batches of metal nanoparticle is essential to predict the behaviour of the material. Using aberration corrected transmission electron microscopy (TEM) it was possible to detect the formation of an oxide shell, of thickness 3nm, on the surface of copper nanoparticles. Further analysis shows that this shell actually consists of two layers, both of which were polycrystalline in nature with domains in the size range of 12 nm, and having a thickness of 1.5 nm each. Energy dispersive X-ray spectroscopy confirms that the layers arise due to the formation of oxides, but it was not possible to determine their exact nature. Analysis of the intensity variation within images obtained via probe corrected scanning TEM combined with a high angle annular dark field detector indicates that the shell consists of an inner layer of cuprous oxide (Cu2O) and an outer layer of cupric oxide (CuO). This work was complemented by conventional TEM which provided size distribution and revealed that the majority of particles have a core consisting of a single crystal of copper. This demonstrates the ability of TEM to determine the extent of oxidation of nanoparticles and its potential to be applied to a wide range of homogenous and heterogeneous nanoparticles.