Environmental Engineering Reference
In-Depth Information
In the case of aluminum nanoparticles a solution of liAlh
4
and Alcl
3
in ThF was used for the synthesis, and the resulting
nanoparticles were found to be in the size range of 10-20 nm [76]. The technique has been also used for the synthesis of binary
and ternary alloyed nanopowders containing iron, cobalt, and nickel. These alloys were synthesized using electrolyte bases on
Aotani's formulation. Binary and ternary alloys were deposited galvanostatically at 8000 A m
-2
, and particles with a mean diam-
eter of 100 nm were produced [77].
18.6.2
semiconductor nanopowders
semiconducting materials are the foundation of modern electronics and have applications in photochemistry, in dye-sensitized
solar cells, and in the photocatalytic treatment of chemical waste. Nanocrystalline cuo has been prepared in the potentiostatic
mode [78]. The work was based on a previous voltametric study that revealed that at an applied potential range between −0.65
and −1V/sse it was possible to avoid the formation of a mixture of cu
2
o and cu. TeM micrographs showed agglomerates of
nanoparticles of various sizes; in addition, isolated particles were also found with diameters ranging between 7 and 20 nm.
In literature, lei et al. reported this technique for the synthesis of tungsten nanoparticles. The electrolyte contained sodium
tungstate, trisodium citrate, ferrous sulfate, and citric acid. A titanium alloy horn was used as the cathode and a platinum slice
as the anode. A 20-khz ultrasound generated by an ultrasound generator was used for the synthesis. Because of the combined
effect of the electrochemical reaction and the cavitation effect of the ultrasound, iron-tungsten aggregates at the cathode were
dispersed into the electrolyte and iron atoms were dissolved in the acidic environment. In this manner tungsten nanoparticles
having body-centered-cubic (bcc) structure were obtained by controlling the density of the electric current, the ultrasound pulse
period, and the amplitude [73].
18.7
supercritical solvents for nanoparticle synthesis
A supercritical fluid (scF) is a substance at a temperature and pressure above its critical point, where distinct liquid and gas
phases do not exist. It can flow out through solids like a gas and dissolve materials like a liquid. scFs are found to be a suitable
substitute for organic solvents in various laboratory and industrial processes. carbon dioxide and water are the most commonly
used scFs, being used for decaffeination and power generation, respectively. on the basis of the characteristic properties and
applications discussed, various research groups have applied scFs for nanoparticle synthesis.
A general classification of scF-based nanoparticle synthesis techniques can be proposed based on the role of scF in the
process. certainly, scFs have been proposed as solutes, solvents, antisolvents, and reaction media. e. Reverchon has put up a
detailed review on scFs for nanoparticle synthesis [79].
18.7.1
rapid expansion of supercritical solutions
The rapid expansion of supercritical solutions (Ress) is associated with the saturation of the scF with a solid substrate; then
depressurization into a low-pressure chamber through a heated nozzle produces a rapid nucleation of the substrate in the form
of very small particles that are collected from the gaseous stream. The resulting morphology depends on the chemical structure
of the material and on the Ress parameters such as temperature, pressure drop, impact distance of the jet against a surface,
nozzle geometry [80]. The fast release of the solute in the gaseous medium results in the production of very small particles. This
process is particularly attractive since it eludes organic solvents. In this regard, sun et al. reported Ag nanoparticle synthesis
wherein water was used in supercritical co
2
(w/c) as a microemulsion, and the modified supercritical solvent was used to dis-
solve AgNo
3
[81]. A w/c microemulsion containing AgNo
3
was rapidly expanded into a room-temperature solution of sodium
borohydride for the synthesis of Ag nanoparticles with an average particle size of 7.8 nm. In a subsequent work Meziani et al.
reported nanocrystalline Ag particles with a bimodal distribution, with the smaller ones centered around 3.1 nm and larger ones
around 10 nm [82].
18.7.2
scf as reductant
A variety of reports have documented the synthesis of nanoparticles using scF as a reductant; for example, Korgel et al.
reported Ag nanoparticle synthesis by reduction of silver acetylacetonates in sc-co
2
in the presence of organic ligands that
were used as stabilizers for the nanoparticles [83]. Using the same technique, silicon nanoparticles in the range of 2 and 20 nm
