Environmental Engineering Reference
In-Depth Information
were also synthesized in sc-hexane. Kameo et al. produced Pd and Ag nanoparticles by reduction of palladium acetate and
silver acetylacetonate in supercritical co
2
[84].
18.7.3
scf for hydrolysis
In supercritical water (scW) Ziegler et al. synthesized copper oxide (cu
2
o) nanoparticles from the copper nitrate precursor by
hydrolysis [85]. The reaction was performed with and without ligands. In the absence of alkanethiol ligands, polydisperse
nanoparticles of cu
2
o with size range of 10-35 nm were obtained by hydrolysis; however, addition of 1-hexanethiol results in
the formation of cu nanocrystals of about 7 nm. The alkanethiol ligand helps to stabilize the synthesis of nanocrystals and con-
trol their oxidation by reduction to cu nanoparticles. ligands that bind on the nanoparticle surface can block the growth of
nanoparticles, with a stabilization process [86].
han and coworkers synthesized Ag nanoparticles in water-in-isooctane continuous phase using w/o reverse micelles and
sodium bis(2-ethylhexyl)succinate as surfactant [87]. Water-in-co
2
tetraethylene glycol dodecyl ether was added as the cosur-
factant. The reactants AgNo
3
and KBh
4
were separately loaded in two micellar solutions and then mixed. supercritical co
2
was used to eliminate the organic phase. The obtained Ag nanoparticles were in the size range between 2 and 5 nm (minimum
size) and 6 and 20 nm (maximum size) and obtained by varying the pressure and the w/o ratio.
hong et al. synthesized Tio
2
nanoparticles by controlled hydrolysis in water-in-co
2
reverse micelles [88]. The average particle
size increases from 8 to 18 nm by increasing the water to surfactant ratio. cason and Roberts reported cu nanoparticle preparation
in w/c reverse micelles using sc-ethane [89]. The authors compared the rate of particle growth in liquid solvents, and the process
is faster in scF due to improved transport properties. The same research group also used reverse micelles (w/c) in compressed
propane or supercritical ethane to produce Ag and cu nanoparticles by reduction of the corresponding precursors [90].
18.7.4
scf for thermal deposition
cansell et al. reported the thermal degradation of metallic precursors dissolved in a supercritical fluid (ammonia) [91]. The
precursors of cu and Fe acetylacetonates that show good solubility in ammonia were used for nanoparticle synthesis; the tem-
perature of the reactor was then increased, thus inducing decomposition of the precursors and the precipitation of the
corresponding oxides and nitrites. Aggregates of about 50 nm were obtained. The same group subsequently proposed the pro-
tocol for nanoparticle synthesis of several other compounds including cr, co, cu, Ni, Al, Ti, and Ga using supercritical
ammonia-methanol mixtures.
18.7.5 supercritical h
2
o
over the past 10 years, synthesis of micro- and nanosized metal oxides has been reported by using the supercritical water
method (FT-scW) by continuous hydrothermal synthesis [92]. In this protocol, an aqueous solution of starting materials is
pressurized and fed into a mixing tee that combines reactants with preheated water. Rapid heating results from the mixing and
allows the hydrothermal reactions. The hydrothermal reaction rate and metal oxide solubility can be greatly varied when water
in its near-critical or supercritical state is used, since the reaction solvent properties are strongly dependent on thermodynamic
conditions in these regions. Therefore, the technique can be used to change size, morphology, and crystal structure of many
types of particles [93].
scW is an excellent reaction medium for hydrothermal synthesis since it can vary the rate of reaction and equilibrium by a
shift of the dielectric constant and solvent density with temperature and pressure. In this regard, cote et al. produced nanocrys-
tals of α-Fe
2
o
3
and co
3
o
4
using two disparities of the continuous hydrothermal technique: cold mixing and hot mixing of the
reactants [94]. In the experiments, compressed water (subcritical) was used. In a subsequent work they also obtained nanocrys-
talline coFe
2
o
4
using the same process variations [95]. In case of metal oxide nanoparticles, Viswanathan and Gupta reported
Zno nanoparticle synthesis [96]. The reaction was carried out in a continuous tubular reactor starting from zinc acetate and
using various flow rates and feed concentrations to obtain spherical nanoparticles with the average particle size of 39 nm.
18.8
solar enerGy
due to global warming, special attention has been paid to the development of green techniques with available renewable energy
sources as a replacement for conventional energy sources. Till date various conventional and nonconventional methods for
nanoparticle synthesis have been reported, but these reported methods have some limitations such as harsh reaction conditions,
