Coarsening, coalescence and sintering of hexagonal ZnO elongated nanoparticles

The pore size distribution coupled with N2 adsorption–desorption hysteresis isotherm and the change of specific surface area were studied in the temperature range of 600–900 °C for hexagonal ZnO nanoparticles. Cylindrical pores are characteristic of the dry pressed ZnO nanoparticles with elongated shape (aspect ratio 2–3) and well-developed {0 1 1¯ 0} and (0 0 0 1) surfaces, and hence not a good indicator of subsequent coarsening/coalescence and/or sintering events. On the other hand, the vigorous onset coarsening, coalescence and then sintering of the elongated nanoparticles was satisfactorily differentiated to occur within minutes having apparent activation energy of 39 ± 7, 47 ± 4, and 55 ± 5 kJ/mol based on t0.3, t0.5 and t0.7, i.e. time for 30, 50 and 70% surface area reduction, respectively. The proposition of coarsening/coalescence processes being differentiable from a later sintering process by specific activation energies is in accord with the increase and then decrease of the average pore size. The minimum temperature for {0 1 1¯ 0} and (0 0 0 1)-specific coarsening/coalescence of the nanorods is 516 °C based on the extrapolation of steady specific surface area reduction rates to null.