Effect of Crystal Size on the Structural and Functional Properties of Water-Stable Monodisperse ZnO Nanoparticles Synthe
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Effect of Crystal Size on the Structural and Functional Properties of WaterStable Monodisperse ZnO Nanoparticles Synthesized Via a Polyol-Route Yesusa Collantes1 and Oscar Perales-Perez2 1
Department of Physics, University of Puerto Rico, Mayaguez 00980, PR, 00680-9044 USA
2
Department of Engineering Science and Materials, University of Puerto Rico, Mayaguez, PR, 00680-9044 USA
ABSTRACT: Highly monodispersed ZnO nanoparticles (NPs) have been synthesized in polyol medium. The control on crystal size was attempted at 180ºC by monitoring the heating rate of reacting solutions and the cooling rate (quenching) at the end of the reaction time. The possibility to promote crystal growth by heterogeneous nucleation was also evaluated; in this approach, presynthesized 5-nm pure ZnO nanocrystals were used as seeds in fresh Zn-polyol solutions at suitable seeds/ZnO w/w ratios. As-synthesized samples were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Fourier Transform Infrared spectroscopy (FT-IR), Absorbance (UV-vis) and Photoluminescence spectroscopy (PL). XRD measurements confirmed the formation of well crystallized ZnO-wurtzite with absence of secondary phases in both seeds and grown crystals. FT-IR analyses evidenced the presence of organic moieties on the surface of the nanoparticles that are associated to the functional groups of polyol by-products; these adsorbed species would have prevented particles from aggregation. PL measurements (excitation wavelength 345 nm) reveled that a tuning in the emission bands of ZnO NPs can be achieved through synthesis conditions and crystal size. HRTEM measurements evidenced the formation of bare ZnO NPs of 2 nm, 6 nm, 20 nm and clusters of small nanocrystals. INTRODUCTION Zinc Oxide (ZnO) is a high quality semiconductor material with a wide band gap of 3.37 eV. ZnO, with a hexagonal wurtzite structure (a = 3.29Å, and c = 5.24 Å), is transparent in the UV region and exhibits a large excitation binding energy at room temperature (60 meV) [1]. ZnO has been important for several decades due to its applications in optics and optoelectronics [2,3]. Currently, ZnO nanoparticles (NPs) are being widely studied because of their unique physicochemical properties at the nanoscale and consequent potential applications in biology and medicine, including tissue imaging, biological fluorescence labeling, and diagnosis and cancer therapy. ZnO is an excellent candidate for biological applications due to its non-toxicity and ability to biodegrade [4,5] coupled with high thermal and chemical stabilities [6]. However, some of the fundamental properties of ZnO continue unclear [1,7] and detailed study of the dependence of the optical properties of ZnO as a function of the crystal size in NPs of similar shape and synthesis process has not been properly addressed yet. On this basis, the present work is focused on the size-controlled synthesis and characterization of ZnO NPs through a modified polyol approach where heterogeneous nucleation conditions were promoted by usi
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