Structure-Property Correlations in ZnO Tetrapods and Spheres prepared by Chemical Vapour Synthesis Revathi Bacsa * , Yolande Kihn ** , Marc Verelst ** , Jeannette Dexpert ** , Wolfgang Bacsa ** Philippe Serp * * Laboratoire de Chimie de Coordination, Composante ENSIACET, 118 Route de Narbonne Toulouse 31077, France, Revathi.Bacsa@ensiacet.fr ** CEMES CNRS UPR, 29 rue Jeanne Marvig, Univ. Toulouse, 31055 Toulouse France ABSTRACT We present experimental results on the synthesis and characterization of ZnO nanorods, tetrapods and nanospheres prepared by the gas phase oxidation of zinc vapor followed by the collection of the oxide particles in an aerosol. Spherical nanoparticles or fine nanorods of ZnO consisting principally of tetra pod like structures are obtained selectively depending on the process parameters. Intermediate structures can also be synthesized. Trans- mission electron microscopy of the particles and rods shows that these are single crystals with diameters in the 5- 50nm range. We find that the tetrapods have high absorption in the UV and nearly 100% reflectivity in the visible. UV absorption coefficients for suspensions of tetrapods are found to be substantially higher (x10) than for commercial nanocrystalline powders. Keywords: ZnO, nanoparticlea, nanorods, tetrapos, nanospheres, TEM, UV absorption, Raman spectroscopy 1 INTRODUCTION Due to its tunable electrical properties and stable chemical structure, zinc oxide has been used in varistor and sensor applications. High purity nano-crystalline ZnO with controlled particle size is used in medicine and cosmetic industry for transparent UV screens since it absorbs both UV A and UV B and has a low photo catalytic activity when compared to TiO 2 [1-3]. To ensure high absorption coefficients and transparency, the development of scaleable synthetic procedures that allow the large scale preparation of size and shape controlled ZnO is necessary. While solution methods suffer from the disadvantages of being laborious and susceptible to contamination by anions or organic additives, the gas phase chemistry has the advantage of producing high purity crystalline products in a single step reaction and is often preferred due to its lower cost [4,5]. Aerosol processes are increasingly being used for the large scale production of metal and oxide nanoparticles [6]. Various precursors have been used for the gas phase synthesis of ZnO that include Zn metal, diethyl zinc and more recently, organometallic precursors such as heterocubane [7-9]. In this com-munication we report the gas phase synthesis of high purity, nanocrystalline and non agglomerated spheres and rods of ZnO produced via a one step gas to particle conversion in an aerosol using zinc metal as the source. We find that spherical or rod shaped particles can be produced selectively in the aerosol by controlling the temperature of zinc vapor and that of oxidation. 2 EXPERIMENTAL The reactor used was a 60 cm long tubular quartz gas flow reactor (Figure 1). All experiments were performed under atmospheric pressure. Zinc metal powder (325 mesh size) contained in an alumina boat was introduced in the central zone of the three zone furnace maintained under flowing Ar/N 2 . Figure 1. Schematic diagram of the reactor used for the synthesis. Figure 2. X-ray powder diffractogram of the nano- particles produced at Zn temperatures of 650, 800 and 900ºC. A mixture of air/oxygen +Ar (in the ratio 1:1) in parallel or cross flow is introduced so as to react with Zn vapour at the point B. Zn temperature was varies from 650-900°C and 666 NSTI-Nanotech 2008, www.nsti.org, ISBN 978-1-4200-8503-7 Vol. 1