In-situ electron-beam-induced reduction of CuO: A study of phase transformations in cupric oxide

The reduction of CuO to Cu has been studied using high resolution electron microscopy and electron energy loss spectroscopy. The reduction proceeds via at least two intermediate phases - Cu4O3 and Cu2O. This is the first experimental observation and the first successful synthesis of the rare Cu4O3 phase. The CuO particles were observed to reduce via these two phases with a continuous refinement of particle size and a redistribution of material over the support film (carbon). In the case of large CuO crystals, the entire particle transformed to Cu4O3 before further reduction or decrease in particle size occurred. This is the first direct observation of the mechanism involved in the formation of the very small metallic Cu particles normally observed in reduced supported CuO. The reduction was effected by a high current density electron beam in the residual gas atmosphere of an electron microscope. The similarity between this technique and the normal hydrogen reduction at high temperatures is discussed. The presence of the Cu2O and Cu4O3 was determined by application of the optical diffraction method to small regions of a high resolution electron micrograph. This enabled the measurement of both interplanar spacings and interplanar angles, which combine together to allow an unambiguous identification of the phases present.