JOURNAL OF CHEMISTRY Materials Synthesis of zinc oxide nanoparticles with controlled morphology Lingna Wang and Mamoun Muhammed* Materials Chemistry Division, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. E-mail: mamoun@matchem.kth.se Received 12th May 1999, Accepted 6th September 1999 A chemical precipitation method has been used for the synthesis of ZnO nanoparticles with controlled morphology. The precursor powders were prepared using several precipitation reagents and using ammonium carbamate as a precipitating reagent led to unusual rod-shape morphology. The precursor was decomposed by heating in air resulting in the formation of spherical or rod-like shapes of zinc oxide. A ¯ow injection synthesis technique has been developed to synthesize nanophase particles of zinc oxide. The precursor and decomposed products were analyzed using IR, SEM, XRD and TGA techniques. The average size of the particles of ZnO obtained using the ¯ow injection technique was approximately 20 nm while the crystallite size as measured from the X-ray pattern was 10±15 nm. Zinc oxide has a broad range of applications, e.g., in pigments, rubber additives, gas sensors, varistors and transducers. 1 Several studies on the fabrication of mixed metal oxides containing ZnO have been reported. The studies were carried out in order to ®ne-tune ZnO properties for special applica- tions. It has recently been demonstrated that nanophase zinc oxide can be used in photocells of the Gra È tzel type, 2 which results in improved current generation ef®ciency. Zinc oxide with a particle size in the range 100±200 nm has proved to be an excellent UV absorbing material, which can be used in sunscreen lotions to enhance the sun protection factor. 3 Pure zinc oxide is an insulator and improving its conductivity extends its use to many new applications. Zinc oxide with increased conductivity is suited for applications where static charge build-up must be prevented. The electrical conductivity can be signi®cantly increased by doping or the introduction of defects into the ZnO crystal lattice, which can improve the electrical conductivity to the high end for semiconductors. In conventional powder metallurgy, the use of ultra®ne zinc oxide powder has signi®cant advantages; e.g., it lowers the sintering temperature. Moreover, a smaller grain size leads to an increased density of the sintered materials. Nanophase materials have been prepared using physical methods, e.g., gas evaporation. 4 Chemical methods have shown several distinct advantages for the synthesis of nanophase particles. 5 Several chemical methods for the manufacture of zinc oxide and mixed metal oxides have been reported, e.g., preparation of ®ne zinc oxide by means of spray pyrolysis; 6 sol±gel technique 7±9 and thermal decomposition. 10 The synthesis of zinc oxide from organic solutions has also been reported, e.g. precipitation from alcohols and amines. 11±13 In some of these studies, the control of particle morphology and the rate of particle growth have been considered in order to avoid the formation of large particles. However, for nanophase powders, morphology manipulation and the preparation of unaggregated particles have received much less attention. Different synthesis techniques and appropriate reagents for the chemical control of nanophase particles with distinct shapes still present major challenges in this ®eld. Chemical methods, including precipitation from inorganic or organic solutions and sol±gel techniques can conveniently provide control of nucleation, growth and ageing of particles in the solution. The methods rely on advanced solution and coordination chemistry theories enabling the synthesis of the required precursor particles utilizing a variety of parameters to enable control of the solid formation process. A major contribution to the growth of particles is through Ostwald ripening, where small particles with lower solubility product dissolve and re-precipitate on the surface of larger particles in solution. Agglomeration takes place in solution as the particles clog together to minimize surface energy. These processes are especially important when the precipita- tion takes place in solution where particles have free access to each other. In order to minimize or eliminate this access, the precipitation reaction should take place in con®ned zones separated from each other rather than in the bulk solution. A microemulsion can be made to contain aqueous droplets as the dispersed phase separated from each other by the continuous organic phase. In such a system, it is possible to carry out the precipitation in the con®ned water droplets. 14,15 In the microemulsion system, the extent of particle growth is reduced because precipitation occurs in the isotropic solution of the microdroplets (10±100 nm in diameter) which are surrounded by the continuous phase. In this way, particle growth, due to the interaction between different aqueous solutions, can be inhibited and very small sized particles are formed. 14 Another way to carry out the reaction within a con®ned volume is through the use of a set-up similar to that of ¯ow injection analysis. In this paper, several approaches describing the synthesis of zinc oxide powder in solution and in con®ned zones are investigated. Experimental Reagents Analytical grade reagents; ZnCl 2 and NH 2 CO 2 NH 4 (MERCK) of at least 99% purity were used as received without further puri®cation. Stock solutions of ZnCl 2 and NH 2 CO 2 NH 4 were prepared by the dissolution of the appropriate amount of each salt in distilled water. Bulk synthesis The synthesis of ZnO precursors was carried out using chemical precipitation. Solutions of ZnCl 2 and NH 2 CO 2 NH 4 were simultaneously mixed under stirring until the precipitation was complete; gas evolved from the solution during the reaction. The solutions were ®ltered and the resulting solids were washed with distilled water several times until no chloride was detected in the ®ltered water (checking with Ag z solution). The precipitates were then dried at 105 ³C in an oven overnight. J. Mater. Chem., 1999, 9, 2871±2878 2871 This Journal is # The Royal Society of Chemistry 1999