A template-free, sonochemical route to porous ZnO nano-disks

A template-free, sonochemical aqueous route was used to synthesize hexagonal-shaped ZnO nanocrystals with a combined micro- and mesoporous structure. The products are much more porous when the sonohydrolysis is carried out under argon than their sonication under air. This has been attributed to the higher average specific heat ratio γ (=C_p/C_v) of argon gas, leading to higher bubble collapse temperatures. Small-angle XRD (SAXRD) studies show that the microporosity is lost at 250 °C, while the mesoporous structure persists till a very high temperature (550 °C). The BET surface area of the products synthesized under argon and air are 35 and 13 m²/g, respectively. The pore size is distributed from 1 nm (micropore) to 3.1-3.4 nm (mesopore), while the ZnO nanoparticles are 6.3 ± 1.2 nm. The possible mechanisms of the self-assembled pore formation are attributed to the organic porous framework of basic zinc acetate. The excitonic absorption of the ZnO occurs at 349 nm. The photoluminescence (PL) spectra of the ZnO nano-disks show the red-shifted band edge exciton transitions and the presence of deep levels due to oxygen vacancies or surface-deep traps, because of the porous structure.