Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 20

Nanoparticles can also be synthesized by vapor phase reactions. In general, this
synthesis procedure is conducted at elevated temperatures and under a vacuum; a
vacuum is needed to maintain a low concentration of growth species to promote
subsequent diffusion-controlled growth. Owing to the vapor pressure characteristics of
compounds such as TiO 2 , a much higher concentration of droplets was obtained using
the same equipment (Visca and Matijevic, 1979).
In the vapor phase synthesis of nanoparticles, conditions are maintained such that
the vapor phase mixture is kept thermodynamically unstable; conditions that include the
presence of a supersaturated vapor (Swihart, 2003). If the conditions include high
supersaturation, and the reaction kinetics permit, vapors rapidly nucleate and form a
large number of extremely small particles. A decrease in temperature leads to a more
rapid decrease in the equilibrium vapor pressure and a relatively higher level of
supersaturation (Flagan and Lunden, 1995). After the formation of nucleation, the
remaining supersaturation is reduced by either condensation or reaction between the
vapor phase molecules and the resulting particles. The particles then grow by Brownian
coagulation (Granqvist and Buhrman, 1976), and product particles are generally
collected by thermophoretic deposition.
There are several factors that determine the characterization of the synthesized
nanoparticles. These factors include temperature, flow rate of the carrier gas, precursor
concentration, operation pressure, and the growth time; the size of the particles can be
altered by changing the temperature, the flow rate of the carrier gas, and the precursor
nuclei concentration (Maisels et al., 2000; Ohno, 2002; Wegner et al., 2002; Kim et al.,
2004; Simchi et al., 2007). In addition, the rate of production of nanoparticles can be
dramatically increased when the synthesis reactor is operated at a higher pressure, with
correspondingly shorter growth times. Short growth times are also achieved by rapid
cooling; neck formation in the agglomerate particles that do form is diminished by
starting the growth process at a high initial temperature (Flagan and Lunden, 1995).
Despite the aforementioned challenges, various nanoparticles can be synthesized
by vapor phase reactions. Nepijko et al. (2000) synthesized silver nanoaprticles of 2-3
nm in diameter by the gas aggregation technique. Another example is the Au
nanoparticles. Au nanoparticles have been grown on various oxide substrates such as
iron oxide (Haruta, 1997) and -alumina (Grisel and Nieuwenhuys, 2001).
MnO nanoparticles (Chang et al., 2005) can be synthesized by using a vapor
phase reaction method. MnCl 2 powders and several silicon substrates were placed on the
quartz boat. Temperature and pressure were maintained at 778 °C and 0.05 MPa,
respectably, at a constantly-mixed gas flow (Ar 2 :H 2 ) in a horizontal furnace. The
particles formed are found to be composed of nearly round shape with diameters ranging