Metal oxide particles in the nanometer size range (e.g., 1 to 100 nanometers or 1 to 10 nanometers) are of increasing interest in a variety of applications such as catalysis, electronic devices, and sensors. Many of the methods used to prepare metal oxide particles in this size range, however, are problematic.
Metal oxide particles in the nanometer size range have been prepared by reacting metal-containing precursors under various conditions. For example, tin oxide nanoparticles have been produced using a variety of techniques, such as, thermolysis of organometallic precursors such as described, for example, in Nakamoto et al., Kagaku to Kogyo, 78, 503 (2004); sol-gel processes as described, for example, in Briois et al., Chem. Mater., 16, 3885 (2004); oxidation of SnCl2 as described, for example, in Jiang et al., J. Phys. Chem. B, 109, 8774 (2005); sonochemistry as described, for example, in Zhu et al., Chem. Mater., 12, 2557 (2000); and hydrothermal processes as described, for example, in Shen et al., Mater. Lett., 58, 3761 (2004).
Other metal oxides, such as zinc oxide, have been prepared by solution phase methods where a metal salt is reacted with hydroxide ions as described, for example, by Spanhel and Anderson, J. Am. Chem. Soc., 113, 2826 (1991). With these methods, control over particle size requires control over the rate of nucleation and growth from a supersaturated solution as well as the use of processes such as coarsening, oriented attachment, and aggregation.
While these methods are effective for producing particles in the nanometer size range (typically, greater than 10 nanometers), the particle surfaces are usually passivated (i.e., surface functionalized). A passivated metal oxide tends to be less effective than a non-passivated metal oxide in applications such as catalysis or sensing.
Metal oxide particles in the nanometer size range have been prepared within dendritic structures. The dendritic structures have been loaded with metal oxoanions from a soluble metal oxoanion salt and then reacted to produce metal oxide particles. These metal oxide particles are typically coordinated on the outer surface by functional groups within the dendritic structure. Such coordination can disadvantageously affect the utility of the metal oxide particles for various applications such as catalysis or sensing.