Solution-based methods are widely used for the synthesis of semiconductor nanoparticles where the small size and high surface to volume ratio can lead to many unique properties. The assembly of mesoscopic structures using nanoparticle building blocks have become increasingly important in many technologies. For example, wide band-gap metal oxide nanoparticles are critical components in high surface area electrodes for dye sensitized solar cells. In these systems, the control of particle size and morphology is important in producing materials with tailored optical and electrical properties.
Particle growth usually occurs by atom attachment but in some cases, longer length scale effects such as epitaxial attachment of particles can also be important. Growth rates for metal oxide nanoparticles span many orders of magnitude depending on the temperature, solubility, and surface energy of the crystal. In order to control particle size, growth can be terminated by thermal quenching or by injection of a suitable adsorbate. In this paper we describe the growth kinetics of ZnO and TiO2 nanoparticles and show how an understanding of the growth kinetics can be used to synthesize particles with tailored size and morphology.
The growth of nanometer sized ZnO particles from solution follows Ostwald ripening kinetics according to the Lifshitz-Slyozov-Wagner (LSW) theory, where the average particle size cubed is equal to the rate constant times time. Since the rate constant k is proportional to temperature, the average particle size, and hence the resultant optical properties, can be tailored by selection of the appropriate growth time and temperature. Due to the low solubility, the growth rate of TiO2 nanoparticles is slow and relatively high temperatures are needed to produce particles several nanometers in size. We also show how adsorption of alkanethiols and alkanephosphonic acids influence the growth kinetics.
Peter C. Searson
Department of Materials Science and Engineering, The Johns Hopkins University
3400 N. Charles St.
Baltimore, MD 21218 USA