Fabrication of ZnO nanoparticles by pulsed laser ablation in aqueous media and pH-dependent particle size: An approach to study the mechanism of enhanced green photoluminescence

ZnO nanoparticles have been fabricated by pulsed laser ablation (PLA) of a zinc target in surfactant-free aqueous solutions, and surface chemistry effects on the green photoluminescence of ZnO nanoparticles were also investigated. The conspicuously stable, well-defined ZnO nanoparticles with very narrow size distribution were obtained by ablation of the Zn target under surfactant-free acidic or basic conditions due to the increased surface charge of nanoparticles compared to those prepared in deionized water. However, the ZnO nanoparticles produced in NaCl solution strongly coalesced due to the decreased surface charge of nanoparticles. Moreover, the green-to-exciton emission intensity ratio progressively increased with the particle average size decrease. This suggests that larger surface area for smaller nanoparticles produced more oxygen defects in the ZnO nanoparticles. Further experimental results on passivation of the nanoparticles with a surfactant, lauryl dimethylaminoacetic acid betaine (LDA), and post-reduction of the surface charge of the ZnO suspension, indicated that the green emission from ZnO nanoparticles obtained under the acidic condition originated partially from surface oxygen defects, while the green emission from those under the basic condition is primarily due to the defects below the crystalline ZnO nanoparticle surface, not from the surface itself.