products do not neutralize the electrical charges of the particles. A typical example of
such an unfavorable reaction (e.g., for oxide synthesis) occurs after the addition of
water. As shown in Eq. (4.17), where a chloride is assumed as the precursor, in
conventional terms the addition of water shifts the equilibrium more towards the
oxide. However, in plasma environment water molecules dissociates and form H
þ
and OH
À ions, so that two additional reaction routes are possible:
ðiÞ MeCl n þ
m
2
O 2 þ
n
2
H 2 ) MeO m þ nHCl
ð4:21Þ
ðiiÞ MeCl n þ ðm þ xÞOH
À
þ ðm þ xÞH
þ
)
MeO m þ xHClO þ ðn À xÞHCl þ m þ x À
n
2
H 2
ð4:22Þ
where x is assumed to be much greater than n.
The addition of water has two effects:
The consequent formation of hydrochloric acid (and possibly hypochlorous acid
(HClO)), increases the reaction enthalpy, such that the temperature of the gas,
determined after passing the reaction zone, is generally higher.
The electrical charges of the particles are quenched, so that particle growth is not
inhibited.
In detail, the positively charged particles are neutralized by collision with OH
À
ions, thereby blocking the mechanism that limits particle growth by the following
process:
H 2 O ) H
þ
þ OH
À
particle
nþ
þ nOH
À
) ðparticle
nþ
þ nOH
À
Þ
neutral
ð4:23Þ
The neutralized particles carry a hydroxide layer at the surface, so the production
of larger particles with a broad particle size distribution might also be expected. The
near-perfect narrow size distribution of ZrO 2 powder produced without water
addition, compared to the drastic effect of adding water to the reaction gas, under
otherwise similar condition, on the synthesis product are shown in Figure 4.24a
and b, respectively. The zirconia specimen produced without water addition showed
a grain size of approximately 8 nm and most of the grains were of equal size. In
contrast, the material produced with added water was characterized by a broad
distribution of particle sizes, ranging from 10 to 50 nm. Such a dramatic difference
between these two batches of the same material clearly demonstrated the validity of
this simple model.
The narrow size distribution of particles shown in Figure 4.24a is characteristic of
the microwave plasma process working in range I and also occurs with smaller
particles. An example of the particle size distribution of a ZrO 2 powder (mean
particle size about 3 nm; distribution determined using particle mass spectrometry)
is shown in Figure 4.25.
Interestingly, the particle size distribution in Figure 4.25 was extremely narrow
(maximum 3.1 nm) when compared to the values shown in Figures 4.8 and 4.19.
Despite the narrow size distribution, the distribution function remained
68j 4 Gas-Phase Synthesis of Nanoparticles
such an unfavorable reaction (e.g., for oxide synthesis) occurs after the addition of
water. As shown in Eq. (4.17), where a chloride is assumed as the precursor, in
conventional terms the addition of water shifts the equilibrium more towards the
oxide. However, in plasma environment water molecules dissociates and form H
þ
and OH
À ions, so that two additional reaction routes are possible:
ðiÞ MeCl n þ
m
2
O 2 þ
n
2
H 2 ) MeO m þ nHCl
ð4:21Þ
ðiiÞ MeCl n þ ðm þ xÞOH
À
þ ðm þ xÞH
þ
)
MeO m þ xHClO þ ðn À xÞHCl þ m þ x À
n
2
H 2
ð4:22Þ
where x is assumed to be much greater than n.
The addition of water has two effects:
The consequent formation of hydrochloric acid (and possibly hypochlorous acid
(HClO)), increases the reaction enthalpy, such that the temperature of the gas,
determined after passing the reaction zone, is generally higher.
The electrical charges of the particles are quenched, so that particle growth is not
inhibited.
In detail, the positively charged particles are neutralized by collision with OH
À
ions, thereby blocking the mechanism that limits particle growth by the following
process:
H 2 O ) H
þ
þ OH
À
particle
nþ
þ nOH
À
) ðparticle
nþ
þ nOH
À
Þ
neutral
ð4:23Þ
The neutralized particles carry a hydroxide layer at the surface, so the production
of larger particles with a broad particle size distribution might also be expected. The
near-perfect narrow size distribution of ZrO 2 powder produced without water
addition, compared to the drastic effect of adding water to the reaction gas, under
otherwise similar condition, on the synthesis product are shown in Figure 4.24a
and b, respectively. The zirconia specimen produced without water addition showed
a grain size of approximately 8 nm and most of the grains were of equal size. In
contrast, the material produced with added water was characterized by a broad
distribution of particle sizes, ranging from 10 to 50 nm. Such a dramatic difference
between these two batches of the same material clearly demonstrated the validity of
this simple model.
The narrow size distribution of particles shown in Figure 4.24a is characteristic of
the microwave plasma process working in range I and also occurs with smaller
particles. An example of the particle size distribution of a ZrO 2 powder (mean
particle size about 3 nm; distribution determined using particle mass spectrometry)
is shown in Figure 4.25.
Interestingly, the particle size distribution in Figure 4.25 was extremely narrow
(maximum 3.1 nm) when compared to the values shown in Figures 4.8 and 4.19.
Despite the narrow size distribution, the distribution function remained
68j 4 Gas-Phase Synthesis of Nanoparticles