(252c) 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.