Room-temperature organometallic synthesis of soluble and crystalline ZnO nanoparticles of controlled size and shape.

ZnO is a wide-band-gap semiconductor (3.37 eV) that displays interesting luminescent properties, which include the recent demonstration of ultraviolet lasing from nanowires. These properties have stimulated the search for new synthetic methodologies for well-controlled ZnO nanostructures. Several reports on high-temperature physical or chemical ZnO syntheses have recently been published. The chemical methods appear to be of particular interest since they offer the potential of facile scale-up, and occur at moderate temperatures (100–200 8C). The synthesis is generally carried out in water or alcohols using zinc salts as starting materials in the presence of a base. These methods are convenient and may lead to nanomaterials of controlled morphology. However, the synthesis makes use of ionic species, which may react with the growing oxide and modify the properties of the final material, whether chemical or physical. An alternative method using the organometallic complex [Zn(C2H5)2] (1) as a precursor was reported by Klabunde et al. However, it involves the transformation of 1 into an alkoxide prior to hydrolysis and heat treatment. Complex 1 was also recently used for the synthesis of ZnO following a “high-temperature organometallic method” derived from that originally reported by Bawendi et al. in the presence of ligands, such as tri-n-octyl phosphane oxide (TOPO) and amines. In separate studies, we have evidenced the interest of organometallic complexes for the preparation of metal nanoparticles of controlled size and shape. This methodology was extended to the synthesis of metal oxide nanoparticles through a two-step approach : 1) the formation of metal nanoparticles through decomposition of the organometallic precursor, and 2) the oxidation of the as-formed nanoparticles. This approach has allowed the preparation of SnO2 and In2O3 particles for gas sensing [16,17] and was also used for the preparation of ZnO particles from the bisalkyl zinc precursor [Zn(c-C6H11)2] (2). [18] However, we reasoned that most organometallic complexes are air-sensitive and decompose exothermically in air. A controlled oxidation of the precursor in solution could therefore lead in one step to oxidized nanomaterials (oxides or hydroxides), the shape and size of which could in principle be controlled by the ligands or surfactants present. Herein we report the room-temperature synthesis of ZnO nanoparticles of controlled size and shape in solution. This route provides crystalline nanoparticles of regular disk or rod shapes depending on the characteristics of the solution, in terms of solvent, ligand, and concentration. The oxide particles are fully soluble in organic solvents giving rise to clear and luminescent solutions from which regular monolayers can be deposited. When a solution of the dicyclohexylzinc(ii) compound 2 in THF is left standing at room temperature in open air, the solvent slowly evaporates and leaves a white luminescent residue, which was characterized by XRD and TEM as being agglomerated nanoparticles of ZnO with a zincite structure that display no defined size or shape. If, however, ligands such as long chain amines are added under argon to a solution of 2 in THF, and the resulting mixture is treated as before, welldefined nano-objects are formed, the size and shape of which depend upon the reaction conditions. For example, the reaction can be carried out in THF using hexadecylamine (HDA) as a ligand. After a reaction time of 17 h and evaporation of the solvent (concentration of reagents: 0.042 molL ; standard procedure), homogeneous nanorods of approximately 8.1 D 2.6 nm are obtained, which can be redissolved in THF (Figure 1a). Several parameters were identified as important for controlling the size, shape, and homogeneity of the nanomaterials, namely the nature of the ligand, the relative concentration of the reagents, the solvent, the overall concentration of the reagents, the reaction time, the evaporation time, and the reaction/evaporation temperature. These parameters and their influence on the ZnO nanoobjects produced are listed in Table 1 and, in some cases, illustrated in Figures 1 and 2. All new materials were characterized by X-ray diffraction (XRD) and/or selectedarea electron diffraction (SAED) and consistently display the same pattern. The XRD pattern corresponds to the hexagonal zincite phase, space group P63mc (see Supporting Information; S1). It is evident from Table 1 that the control of the size and morphology of the ZnO particles results from the nature of the solution (solvent, ligands, concentrations, and so on). We have previously shown, in particular by NMR spectroscopy, that the shape control of metal nanoparticles was related to ligand coordination. A similar effect seemed however very surprising in the case of oxides and led us to undertake an NMR study. The addition of HDA to 2 in [D8]THF leads to H and C{H} NMR spectra that suggest the coordination of the amine to 2 through nitrogen atoms. After oxidation, the C{H} NMR spectra of the reaction mixture do not reveal the presence of any trace of 2 but show peaks attributed to HDA. The signals corresponding to the carbon atoms in the a, b, and g positions relative to nitrogen at d= 42.69, 34.58, and 27.31 ppm, respectively, are very broad (see Figure 3). This phenomenon has been observed previously on ruthenium nanoparticles and can be attributed to a fast exchange between free ligand molecules in solution and coordinated ligand molecules linked to ZnO through nitrogen atoms. This therefore demonstrates that throughout the process of [*] Dr. B. Chaudret, Dr. M. Monge, Dr. M. L. Kahn, Dr. A. Maisonnat Laboratoire de Chimie de Coordination du CNRS 205, route de Narbonne 31077 Toulouse C&dex 04 (France) Fax: (+33)5-6155-3003 E-mail: chaudret@lcc-toulouse.fr