Onset coarsening/coalescence of cobalt oxides in the form of nanoplates versus equi-axed micron particles

The change of specific surface area and pore size distribution coupled with N2 adsorption–desorption hysteresis isotherm, in particular that typical to cylindrical pores, were used to determine the onset coarsening/coalescence in the temperature range of 500–800 °C for Co(OH)2 derived Co3O4 nanoplates and 700–1000 °C for CoO-derived Co3O4 powders (backtransformed to CoO above 900 °C) which are equi-axed in shape and microns in size. The vigorous onset coarsening/coalescence of the nanoplates and equi-axed micron particles was found to occur within minutes having apparent activation energy of 37 ± 7 kJ/mol (based on t0.7, i.e. time for 70% surface area reduction) and 113 ± 8 kJ/mol (based on t0.3), respectively. The surface area reduction process of the nanoplates was found to be controlled by (1 1 1)-specific coalescence besides a coarsening–repacking process more common to the equi-axed particles. The present static experimental results of coarsening–coalescence of the Co3O4 (below 900 °C) or CoO particles (above 900 °C) supports our previous supposition that CoO and Co3O4 nanocondensates could readily assemble as nanochain aggregates and further coalesce into a close packed manner below 1000 °C by the radiant heating effect in a dynamic laser ablation process.