Control of Nanoparticle Morphology In Supercritical Carbon Dioxide Synthesis

            Supercritical carbon dioxide-assisted deposition of nanoparticles is a viable, sustainable alternative to solution-based methods for nanoparticle synthesis^1,2. Carbon dioxide is abundant, nontoxic, and nonflammable, and the sc-CO2 synthesis process produces no net carbon dioxide. The byproduct organic ligands from organometallic precursors dissolve readily in supercritical carbon dioxide (sc-CO2), eliminating the need for high temperature vacuum drying often necessary in processes using aqueous solution chemistry. Supercritical CO2 possesses increased diffusivity compared to liquids, allowing for more efficient transport of precursor in a deposition reaction³. Moreover, sc-CO2 has lower viscosity than liquids and no surface tension. Nanoparticles have been deposited using sc-CO2 for several applications. For example, platinum and palladium nanoparticles have been synthesized on carbon nanotubes for use as electrocatalysts in fuel cells^4,5.

While the sc-CO2 route does possess several advantages over other methods for nanoparticle synthesis, there are important challenges that must be overcome in order to realize the widespread adoption of sc-CO2 as a medium for nanoparticle synthesis. Paramount among these challenges is the lack of knowledge regarding the relationship between process inputs and outputs. Examples of inputs include reaction temperature, reaction pressure, and substrate surface chemistry, while examples of process outputs include nanoparticle size distribution and morphology. The relationship between such inputs and outputs has not been well studied or characterized.

The work presented here describes preliminary efforts to understand the effect of surface chemistry on nanoparticle morphology during the supercritical carbon dioxide deposition process. Specifically, silicon wafers are modified to yield hydrophilic and hydrophobic surfaces. Hydrophilic surfaces are achieved by functionalizing the wafers with either hydroxyl groups or 3-aminopropyltriethoxysilane (APTES). Hydrophobic surfaces are achieved by coating the wafers with either a fluorocarbon thin film or octadecyltrichlorosilane (OTS). Metallic nanoparticles are deposited on these surfaces and the morphology is characterized via scanning electron microscopy. Results indicate a strong dependence of nanoparticle morphology on surface chemistry, especially in regard to the hydrophobicity of the surface. Furthermore, the morphology of the nanoparticles appears insensitive to reaction temperature and pressure.

           

           

            (1)        Leitner, W. Accounts of Chemical Research 2002, 35, 746.

            (2)        Beckman, E. J. Industrial & Engineering Chemistry Research 2003, 42, 1598.

            (3)        Cabañas, A.; Blackburn, J. M.; Watkins, J. J. Microelectronic Engineering 2002, 64, 53.

            (4)        Ye, X.-R.; Lin, Y.; Wang, C.; Engelhard, M. H.; Wang, Y.; Wai, C. M. Journal of Materials Chemistry 2004, 14, 908.

            (5)        Ye, X. R.; Lin, Y.; Wai, C. M. Chemical Communications 2003, 642.