Reduction of CuO nanowires confined by a nano test tube

Using in situ transmission electron microscopy observations of the thermally induced reduction of CuO nanowires sheathed by a carbon shell, we show that a confined nanoscale geometry leads to changes in the oxide reduction mechanism from a surface dominated process to the bulk dominated process. It is shown that the reduction of carbon-confined CuO nanowires occurs via oxygen vacancy clustering in the bulk that results in the nanowire fragmentation into Cu2O segments encapsulated by the carbon shell while the reduction of un-confined CuO nanowires proceeds via the nucleation and growth of Cu2O islands on the nanowire surface. The comparative in situ TEM observations demonstrate that the surface coating layer reduces the thermal stability of the oxide nanowires, which is in contrast to the commonly anticipated effect of enhancing the nanostructure stability by developing a surface protective coating layer. Our density functional theory analyses reveal that the effects of oxygen vacancy ordering at the surface and in the bulk of CuO are comparable in energy, which support the alternative reduction process observed in the bulk of the sheathed CuO nanowires.