Facile and reproducible synthesis of nanostructured colloidal ZnO nanoparticles from zinc acetylacetonate: Effect of experimental parameters and mechanistic investigations

A facile and reproducible route to nanostructured colloidal ZnO nanoparticles was developed by controlled hydrolysis and condensation of zinc acetylacetonate in alkaline conditions. By reaction of an ethanolic solution of Zn(acac)2 with NaOH in a 1:2 molar ratio, after reflux, ZnO spherical nanoparticles were obtained that displayed a homogeneous size distribution; particle diameters ranged from 6 to 10 nm, as evidenced by transmission electron microscopy (TEM) analysis. The same reaction was carried out also in water, glycerol and 1,2-propanediol, to investigate the effect of the solvent viscosity and dielectric constant on the final features of the obtained material. Irrespective of the nature of the solvent, X-ray diffraction (XRD) analysis shows the formation of hexagonal ZnO, whereas the presence of residual unreacted Zn(acac)2 could be ruled out. Indeed, different particle sizes and very different morphologies were obtained. Also the reflux step was shown to be a key factor in avoiding the fast precipitation of a floc and achieving a pure compound, which Introduction Research in the field of ZnO nanostructures of different shapes and morphologies has witnessed an amazing development in the last 10 years.[1] This interest can be mainly traced back to the electrical and optical properties of ZnO[2] as well as to its outstanding electronic properties and functional performances,[3] which are in many cases related to defectivity.[4] Also its photocatalytic properties have been recently extensively reviewed.[5] Among the very different morphologies that ZnO can display (nanowires, nanotubes, [a] Istituto di Scienze e Tecnologie Molecolari, ISTM-CNR and Dipartimento di Scienze Chimiche, Università degli Studi di Padova, and INSTM, UdR Padova, Via Marzolo 1, 35131 Padova, Italy E-mail: silvia.gross@unipd.it [b] Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany [c] Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Università di Trento, Via Mesiano 77, 38100 Trento, Italy Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.200900506. Eur. J. Inorg. Chem. 2009, 5017-5028 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5017 was isolated and thoroughly characterised. The composition of the obtained ZnO was determined by elemental analysis, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA), showing the formation of pure ZnO. IR spectroscopy evidenced the presence of adsorbed organic ligands on the colloid surfaces. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) revealed the presence of medium- to high-strength acidic sites on the ZnO surface. To gain a deeper insight into the formation mechanisms of these nanostructures, time-resolved UV/Vis and XAS studies were performed on the ethanol solution used for the synthesis of the oxide and also on the solid specimen, obtained after the refluxing step. No remarkable changes could be evidenced in the solution after the addition of an understoichiometric amount of NaOH, but the growth of the ZnO nanoparticles could be followed by UV/Vis spectra