50 4 Gas-Phase Synthesis of Nanoparticles
relatively broad particle-size distribution, which is typical for unbiased random
processes.
Besides pure metals, as rods or powders, volatile compounds are used as precursor, too. In many instances, this is of economical advantage. Using these
modifications, this process for synthesis is called “chemical vapor synthesis”.
Generally, it applies a tubular furnace with temperatures up to 1500 K as a source
of heat. A carrier gas, argon or nitrogen, transports the evaporated precursor
through the hot reaction zone. As precursor, one uses chlorides, carbonyls, or
metalorganic compounds. The final selection depends on the properties, availability, and price of these compounds. Additionally, it is important to note that these
precursors lead to typical reaction products in the off-gas and may leave some
traces behind that could be dissolved in the matrix of the particles or adsorbed at
the particle’s surface. This may be severely disturbing as, for example, chlorine or
hydrochloric acid reacts with high probability with organic materials applied for
functionalizing the surface of the particles.
Box 4.6 Chemical Reactions to Synthesize Oxides
To obtain the metal oxide MeO y , starting from a chloride, a typical reaction is:
MeCl
O
MeO
Cl 2
x
y
y
x
+
⇒
+
2
2
2
.
(4.12)
Generally, this reaction is carried out at a temperature in the range from 1200
to 1500 K. To reduce the reaction temperature water is added:
MeCl
H O
O
MeO
HCl
2
2
x
y
x
y x
x
+
+
−
⇒
+
2
2
2
.
(4.13)
In many cases, the advantage of reduced reaction temperature is compensated
by the huge disadvantage of getting highly corrosive hydrochloric acid, HCl as
a byproduct in the system. If available, the use of carbonyls is recommended;
for example, the synthesis of Fe 2 O 3 :
2
13
2
10
Fe CO
O
Fe O
CO
5
2
2 3
2
( ) +
⇒
+
.
(4.14)
For most of the carbonyls, carbonyl chlorides, or nitrosyl carbonyls, this process
works in a temperatures window between 600 and 800 K.
To replace chlorides, carbonyls, or metalorganic compounds are often used as
precursors. In most cases, this allows reduction of the operating temperature of
the tubular furnace for the reaction from ca. 1500 K down to approximately 650 K,
with the consequence of significant energy savings. Avoiding chlorides avoids the
relatively broad particle-size distribution, which is typical for unbiased random
processes.
Besides pure metals, as rods or powders, volatile compounds are used as precursor, too. In many instances, this is of economical advantage. Using these
modifications, this process for synthesis is called “chemical vapor synthesis”.
Generally, it applies a tubular furnace with temperatures up to 1500 K as a source
of heat. A carrier gas, argon or nitrogen, transports the evaporated precursor
through the hot reaction zone. As precursor, one uses chlorides, carbonyls, or
metalorganic compounds. The final selection depends on the properties, availability, and price of these compounds. Additionally, it is important to note that these
precursors lead to typical reaction products in the off-gas and may leave some
traces behind that could be dissolved in the matrix of the particles or adsorbed at
the particle’s surface. This may be severely disturbing as, for example, chlorine or
hydrochloric acid reacts with high probability with organic materials applied for
functionalizing the surface of the particles.
Box 4.6 Chemical Reactions to Synthesize Oxides
To obtain the metal oxide MeO y , starting from a chloride, a typical reaction is:
MeCl
O
MeO
Cl 2
x
y
y
x
+
⇒
+
2
2
2
.
(4.12)
Generally, this reaction is carried out at a temperature in the range from 1200
to 1500 K. To reduce the reaction temperature water is added:
MeCl
H O
O
MeO
HCl
2
2
x
y
x
y x
x
+
+
−
⇒
+
2
2
2
.
(4.13)
In many cases, the advantage of reduced reaction temperature is compensated
by the huge disadvantage of getting highly corrosive hydrochloric acid, HCl as
a byproduct in the system. If available, the use of carbonyls is recommended;
for example, the synthesis of Fe 2 O 3 :
2
13
2
10
Fe CO
O
Fe O
CO
5
2
2 3
2
( ) +
⇒
+
.
(4.14)
For most of the carbonyls, carbonyl chlorides, or nitrosyl carbonyls, this process
works in a temperatures window between 600 and 800 K.
To replace chlorides, carbonyls, or metalorganic compounds are often used as
precursors. In most cases, this allows reduction of the operating temperature of
the tubular furnace for the reaction from ca. 1500 K down to approximately 650 K,
with the consequence of significant energy savings. Avoiding chlorides avoids the