Influence of different additives on the morphology, defect state and luminescence of ZnO nanoparticles

Zinc oxide (ZnO) is an important multifunctional material with applications in various environmental, drug delivery, sensing, and optoelectronics domains. Binding of different molecules on ZnO enhances the properties and characteristics of nanoparticles through surface modification. In this study, varieties of ZnO nanoparticles (ZnO-NP) are prepared using different additives via simple, low-cost precipitation technique at room temperature. The surface state of ZnO-NP was controlled by three surfactants and two silanes, namely: cetyltrimethyl-ammonium bromide, tween 20, heptaethylene glycol monododecyl ether, 3-aminopropyl triethoxysilane, and tetraethoxysilane. The modified ZnO samples were characterized by various analytical techniques, X-ray diffraction, Raman, FT-IR spectroscopy, surface area analysis and electron paramagnetic resonance. The surface area of the samples significantly increases when silanes are used during the synthesis. Scanning electron microscopy shows the formation of either rounded nanoparticles or layered structures. Depending on the type of additives the zeta potential of colloids increase from 25.92 mV up to 40.36 mV. In this study we show that the chemical nature of additives and the stability of colloids play an important role in the generation of luminescent radiative processes. The green luminescence of ZnO colloids is either enhanced or quenched due to the interaction of additives with the particles surface through covalent, electrostatic, and hydrogen bonds. The ZnO surface becomes more or less permissive toward the movement of surface defects, with formation of oxygen vacancies (VO+ and VO++) responsible for the luminescent response.