Shrinking of hollow Cu2O and NiO nanoparticles at high temperatures

The structural stability of hollow Cu2O and NiO nanoparticles associated with reduction and oxidation reactions at high temperatures was studied by transmission electron microscopy (TEM). Hollow Cu2O and NiO in annealing under 5.0 × 10−5 Pa was observed to have shrunk at 473 and 623 K, respectively, where the reduction reactions from oxides to metals started. As a result of shrinking associated with reduction, hollow oxides turned into solid metal nanoparticles after annealing at higher temperatures for a long time. In addition, hollow oxides shrunk and collapsed through high-temperature oxidation. It was found that shrinking of hollow oxides during oxidation occurs at temperature where the diffusion coefficients of slower diffusing species reach around 10−22 m2 s−1. Annealing at high temperatures both in a vacuum and in air leads to atomic movement that results in the annihilation of nano-holes inside hollow nanoparticles, and a consequent reduction in the extra inner-surface energy.