Controlling zinc oxide particle growth using various solutions conditions

Solution phase synthesis of metal oxide nanoparticles is an important area of research because it provides an economical and potentially green route to synthesize nanoparticles of controlled size, shape, and phase in high yield. Optical, electrical, and magnetic properties are often size dependent for particles with sizes at and below a few tens of nanometers. For ZnO, the size dependence of the band gap can be exploited in applications such as solar cells, sensors, and light emitting diodes. ZnO particle growth from initially homogenous solution is sensitive to solution conditions (e.g., concentration of precursors) and further elucidating the link between solution conditions and nanoparticle growth mechanisms is expected to lead to improved control over ZnO nanoparticles properties (e.g., size and shape). Zinc oxide nanoparticle growth from homogenous solution was monitored using high-resolution transmission electron microscopy (HRTEM) and in situ UV-Vis spectroscopy. Particle sizes determined from in situ UV-Vis spectroscopy were calculated using both the effective mass model and the tight binding model. Particle growth was fit using the classic coarsening model and the simultaneous coarsening and oriented aggregation model in order to evaluate the contribution of oriented aggregation to particle growth. Solution variables included varying precursor molar ratio, concentration of precursors, identity and concentration of surfactant, and temperature. Overall, two variables yielded the greatest control over ZnO particle growth as measured by the growth rate constants for coarsening and oriented aggregation and HRTEM characterization. First, increasing the number concentration of ZnO nanoparticles by increasing precursor concentration strongly enhanced growth by oriented aggregation, and this was shown to result from an increase in particle-particle interactions. Second, surfactant additives modified growth both by coarsening and oriented aggregation, with some inhibiting growth by both mechanisms. Results presented demonstrate that ZnO nanoparticle growth by coarsening and oriented aggregation can be selectively inhibited or promoted by judicious selection of solution conditions and surfactant additive.