Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 27

The critical point for water lies at 374 °C and 218 atm; above these conditions,
water is said to be supercritical. Supercritical fluids have been shown to exhibit
characteristics of both a liquid and a gas (Cushing et al., 2004). Water acts as reactant at
supercritical conditions, accelerating the kinetics of hydrolysis reactions. Due to an
increase in the temperature condition, there is a corresponding increase in the solubility
and reactivity of ionic species and the viscosity of water is decreased, thus leading to
greater mobility. This increased mobility allows Oswald ripening to continue at a faster
rate, further increasing product uniformity.

When compared with other methods, the hydrothermal method has several
benefits:
1) The hydrothermal method can synthesize many kinds of nanomaterials at

relatively lower temperatures, compared to temperatures required by traditional
solid-state reactions. Unlike the coprecipitation or sol-gel methods, products
obtained by the hydrothermal method are usually crystalline and do not require a
post-annealing treatment (Willard et al., 2004). Therefore, the sintering process
can be avoided, leading to a decrease in agglomeration between the particles.

2) Characteristics of products obtained by the hydrothermal method can change

depending on the controlling time, temperature, pH, and water content (Cheng et
al., 1994; Yanqing et al., 2001). Cheng et al. (1994) developed a method for
preparing nanoparticles, using phase-pure rutile and anatase from aqueous TiCl 4
and based on a hydrothermal process. They found that acidic conditions favored
rutile while basic conditions favored anatase. Interestingly, they discovered that
higher temperatures favor more highly dispersed products and mineralizers such
as SnCl 4 and NaCl tended to reduce the average grain size, whereas NH 4 Cl led to
increased agglomeration. Research on the synthesis of nanophase TiO 2
continues. By expanding the work of Cheng et al. (1994), others have determined
that the phase purity of the products obtained from hydrothermally processed
aqueous TiCl 4 primarily depends on concentration, with higher concentrations of
TiCl 4 favoring the rutile phase, and particle size depending primarily on the
reaction time (Yanqing et al., 2001). To this end, Rozman et al. (1995)
synthesized (Mn, Zn)Fe 2 O 4 ferrite nanoparticles by the neutralization of mixed
metal hydroxides (Rozman and Drofenik, 1995). As the control temperature and
time, about 11 nm nanoparticles were synthesized at 95 °C for 50 h and 140 °C
for 0.5 h. The shorter reaction time resulted in a nanoparticle surface with a
greater amount of hydroxide groups, whereas the reduced temperatures allowed
additional water to be incorporated in the crystal structure, causing an increase in
lattice distortions.

3) The purity of the product prepared in appropriate conditions could be high owing

to recrystallization in the hydrothermal solution.