The nucleation kinetics of ZnO nanoparticles from ZnCl 2 in ethanol solutions† A. G. Vega-Poot, a G. Rodr  ıguez-Gattorno, * ab O. E. Soberanis-Dom  ınguez, a R. T. Pati~ no-D  ıaz, a M. Espinosa-Pesqueira c and G. Oskam * a Received 25th June 2010, Accepted 12th August 2010 DOI: 10.1039/c0nr00439a The first stages of the synthesis of ZnO nanoparticles by forced hydrolysis of ZnCl 2 with NaOH and water in ethanol have been investigated using UV-Vis spectrophotometry. At sufficiently low water concentrations, focusing of the nanoparticle size distribution was observed during the nucleation and growth phase, followed by a defocusing phase when coarsening becomes significant. During nucleation and growth, only the smaller particles grow while the larger particles have an essentially zero growth rate, indicating that the growth rate decreases rapidly with particle size. As the average particle size remains nearly constant in this regime, the absorbance increase with time can be used to determine the nucleation rate. The nucleation rate was found to depend on both the water concentration and the reactant concentrations. The results are discussed in terms of a mechanism where water determines the precursor formation kinetics thus controlling the nucleation rate. 1. Introduction Several solution-phase routes to the synthesis of zinc oxide nanoparticles and nano-architectures have been reported, based on either hydrolysis of a zinc salt in an organic solvent 1–13 or non- hydrolytic methods. 14–17 The main steps during a precipitation synthesis can be summarized as follows: (i) precursor formation (e.g. neutralization reactions), (ii) nucleation, (iii) growth, and (iv) aging processes such as aggregation and coarsening. 9,18,19 The degree of overlap of these processes is an essential reaction parameter that determines the ability to control the particle shape, morphology, and size distribution. In order to tailor the properties of the nanomaterial to specific applications, it is essential to develop a fundamental understanding of the different processes that lead to particle formation and their kinetics. UV-Vis spec- trophotometry has provided a fast and reliable tool to determine the ZnO nanoparticle growth kinetics, due to the dependence of the optical band gap on the nanoparticle size up to relatively large dimensions. However, because of the nature of reactions involved (e.g. acid–base reactions), the kinetics of the processes during the first three stages of colloidal synthesis are usually fast, and it has been difficult to observe the nucleation and growth processes. The aging processes have been studied in detail, and in previous work the effects of the anion of the zinc salt, the solvent, and the hydroxide-providing reagent on the coarsening kinetics have been reported. 2–8 In addition, it was shown that oriented attachment processes can affect the aging kinetics. 16,20,21 The mechanism of nanoparticle formation and the associated kinetics can be envisaged to involve the formation of zero-charge precursor molecules Zn n (OH) m (H 2 O) l L k , where L is a ligand, and subsequent homogeneous nucleation. For the system of zinc acetate in ethanol using LiOH, the complex nature of the precursor chemistry has been convincingly shown, and the simultaneous presence of ZnO nanoparticles, a zinc double hydroxide salt (Zn-HDS), as well as precursor molecules as mentioned above was illustrated. 22–24 In classical nucleation theory, the nucleation rate, J, is deter- mined by an Arrhenius-type rate constant containing the Gibbs free energy needed to form a nucleus of critical size, DG*: J ¼ J 0 J N ¼ J 0 A exp  DG* kT  ¼ J 0 A exp  16pg 3 V 2 m 3ðkT Þ 3 ðln SÞ 2 ! (1) where J 0 represents a rate constant that depends on the type of reactions that take place, J N is the nucleation rate for a certain reaction, given by a constant A, which contains a frequency factor, and the exponential Arrhenius factor. The exponential term contains the supersaturation, S, given by the precursor concentration divided by the solubility of the solid material formed. The surface energy is represented by g, with V m the molar volume, k the Boltzmann constant, and T the temperature. According to classical nucleation kinetics, the time dependence of the nucleation reaction is governed by the time dependence of the supersaturation, assuming a constant frequency factor and J 0 . Recently, the applicability of the classical nucleation theory to the formation of nanoparticle colloids has been called into question, related to intrinsic inconsistencies as well as disagree- ment with experimental results. 25–30 For example, for clusters with an extremely low bulk material solubility, the critical nucleus size would be smaller than the size of precursor mole- cules. In this case, the nucleation kinetics are related to the chemical reaction kinetics and the molecular mechanisms involved. 25 a Departamento de F ısica Aplicada, CINVESTAV-IPN, Merida, Yucat  an, 97310, Mexico b Centro de Investigaci  on en Ciencia Aplicada y Tecnolog ıa Avanzada, Instituto Polit ecnico Nacional, Irrigaci  on, Mexico, D.F., 11500, Mexico c Departamento de Tecnolog ıa de Materiales, Instituto Nacional de Investigaciones Nucleares (ININ), Ocoyoacac, Mexico, D.F., 52750, Mexico † Electronic supplementary information (ESI) available: Figures illustrating the time dependence of (a) the absorbance maximum and (b) l 1/2 for different [NaOH]/[ZnCl 2 ] ratios at a fixed water concentration of 100 mM and 1 mM ZnCl. See DOI: 10.1039/c0nr00439a 2710 | Nanoscale, 2010, 2, 2710–2717 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/nanoscale | Nanoscale