Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 21

from 20 to 40 nm. Here, hydrogen played an important role for selecting a particular
product during the growing process. The final product was not Mn 3 O 4 but MnO due to
the decrease in the oxygen partial pressure by the reaction with hydrogen. However,
with the absence of hydrogen, only Mn 3 O 4 was formed.

2.2.5 (Flame) Aerosol Synthesis

Aerosol processes are commonly used in the large-scale commercial production
of ultrafine particles (d p < 100 nm) and materials such as titania and silica (Ulrich and
Rieh, 1982; Hartmann et al., 1989; Ahonen et al., 2001; Huisman et al. 2003; Backman
et al., 2004). This use is due to the ease of formation of metal oxides from inexpensive
water-soluble precursors (Pruss et al., 2000). A variety of chemical precursors have been
used, including metal salts such as TiCl 4 to synthesize TiO 2 , and SiCl 4 to synthesize
SiO 2 , metal alkoxides (Visca and Matijevic, 1979; Huisman et al., 2003).
Material properties, the aerosol volume concentration (volume of particles per
unit volume of gas), the resident time, and process temperature all play a key role in the
system (Ulrich and Rieh, 1982; Backman et al., 2004; Jiang et al., 2007). These are
significant factors that have the potential to determine coalescence or sintering, as well
as product morphology and crystallinity.

Precursor materials are injected into the burner as a gas, droplets, or solid
particles. Usually, liquid or solid precursors rapidly evaporate as they are exposed to
high flame temperatures. Then, condensable molecules produced by either physical or
chemical processes self-nucleate to form particles. After a high temperature step, the
aerosol stream slowly cools to a lower temperature, allowing the particles to collect.
Subsequent collision and coalescence leads to the formation of larger particles.
Sometimes, aggregates are physically held together by bonds of varying strength, and
these can then combine to form aggregates held together by necks formed as a result of
sintering. These agglomerates can be relatively easily separated into their aggregate
components.

The collision/coalescence mechanism for particle formation is based on a series
of steps assumed to proceed as follows (Bandyopadhyaya et al., 2004):
1) A chemical (or physical) process converts the aerosol precursor to condensable

molecules.

2) The condensable molecules self-nucleate to form a cloud of stable nuclei.

3) The stable nuclei initially coalesce to form larger particles.

4) Coalescence ceases or slows significantly, leading to the formation of

agglomerate structures.

5) Coalescence and neck formation may continue for particles within the

agglomerate structures.