Evolution of ZnO Nanorods from Faceted Crystals to Near-Spherical Nanoparticles Under Controlled Laser Irradiation

Owing to its favorable physical and chemical characteristics, zinc oxide (ZnO) has become one of the most extensively investigated metal oxide semiconductors. Pulsed laser irradiation in liquids provides a post-synthesis route for modifying ZnO nanostructures. However, correlating laser parameters with the systematic evolution of particle morphology, size distribution, and surface chemistry remains challenging. In this work, a facile and additive-free pulsed laser irradiation approach was employed to engineer the morphology and surface chemical composition of ZnO nanoparticles. As-prepared hydrothermally synthesized ZnO nanorods dispersed in deionized water were irradiated with a Q-switched Nd:YAG laser for 30 and 90 min and compared against the untreated powder. TEM and SEM analyses revealed a progressive morphological transformation in which the strongly agglomerated, faceted nanorods partially reshape into a mixed rod and particle morphology after 30 min and evolve into well-dispersed, nearly spherical nanoparticles with diameters of approximately 12-48 nm after 90 min. XRD confirmed that the hexagonal wurtzite structure is retained throughout the treatment with no secondary or impurity phases, while the mean crystallite size increases from 15.14 nm for the powder to 18.80 nm after 90 min of irradiation as a result of photothermal fusion. XPS demonstrated that zinc preserves its +2 oxidation state during laser processing, whereas the surface O/Zn ratio and the relative fractions of lattice oxygen and hydroxyl species evolve systematically with irradiation time, revealing a concurrent modification of the surface chemistry. These results establish the laser post-irradiation duration as a simple and effective control parameter for tailoring both the morphology and surface chemical state of ZnO nanostructures.