Novel Route from a Wurtzite to a Rock-Salt Structure in CoO Nanocrystals: In Situ Transmission Electron Microscopy Study

The phase transformation from a metastable wurtzite (WZ) CoO to stable rock-salt (RS) CoO nanocrystals via an intermediate spinel (SP) Co3O4 was examined based on in situ transmission electron microscopy (TEM). Specifically, the atomic evolution during the phase transformation was monitored using high-resolution TEM images of individual nanocrystals. A two-step phase transformation was identified during the heating experiment: rapid oxidation of WZ CoO to SP Co3O4 and reduction of SP Co3O4 to RS CoO. In a low temperature range from 200 to 280 °C, the oxidation reaction, which was the first phase transformation from WZ CoO to SP Co3O4, occurred spontaneously via remnant oxygen in the TEM column. On the other hand, the reduction reaction, which was the second phase transformation from SP Co3O4 to RS CoO at high temperatures of over 280 °C, occurred under a low oxygen partial pressure. The phase transformation phenomena were analyzed from thermodynamical and microstructural viewpoints.