Chemistry Reference
In-Depth Information
High pressure
Solution of precursor
and SCF
Line to fill vessel with SCF
and solute
Nozzle with valve to
control
depressurisation
Low pressure
Precursor precipitates
out of solution
Fig. 3 Schematic to explain basic principles and operation of the rapid expansion of
supercritical solutions (RESS) technique.
rapid depressurisation of the solution, often through a nozzle (Fig. 3). As
the pressure decreases the solvating power of the SCF diminishes rapidly,
resulting in rapid supersaturation and nucleation. Particle morphology
can be influenced by the reaction parameters, such as the pressure
gradient, temperature and nozzle geometry and dimensions. A number of
issues limit the practical application of RESS and these are the tendency
of particles to agglomerate and the relatively limited solubility of
appropriate solid substrates in suitable supercritical media. Variations of
the RESS technique include rapid expansion of supercritical solvents into
liquids (RESOLV), which limits agglomeration and allows a reactive step
to be introduced, such as the formation of CdS from Cd(NO
3
)
2
.
30
Another
variant of RESS is the use in chemical vapour deposition (SCT-CVD)
techniques used to prepare metal and metal oxide thin films.
31
This
technique involves the expansion of an SCF containing the relevant metal
precursor salt into a chamber in the presence of a solid substrate. As the
solution expands the metal precursor precipitates onto the solid and then
decomposes on the surface of the substrate.
5
Alternatively SCF solutions can be used to transport a desired solute
into a porous material, such as a metal oxide or a porous polymer; it can
then be deposited prior to depressurisation.
5,32-36
Deposition can occur
on defect sites within the porous material or it can be induced by
changing the components of the SCF mixture. This technique can be
considered as an impregnation or deposition from a supercritical fluid,
and has the advantage of utilising the high diffusivity and low viscosity of
SCFs to deposit active components deep into the pore structures of
support materials.
A significant body of work has been performed with thermal reactions
in various SCFs, where the SCF is used as a reactant.
5
The use of SC-H
2
O
and subcritical H
2
O for the hydrothermal preparation of metal oxides is
well known and has been performed both in batch and flow systems.
Flow systems have been shown to provide greater control over prepar-
ation parameters, such as pressure and temperature, which allows for a
