oxidizing elements is considered as negative, and the reducing elements as positive,
similar to the oxidation number concept familiar to chemists. Accordingly, the
elemental valency of C, Sn, In, and H is +4, +2, +3, and +1, respectively, and
oxidizing valency of oxygen is taken as -2. The valency of nitrogen is considered to
be zero. Based on this concept, the oxidizing valency of aluminum nitrate and the
reducing valency of urea are
In(NO3)3 = -15; [In = +3, 3N = 0, 9O= (9 x -2) = -18],
CH4N2O = +6; [C = +4, 4H = (4 x +1) = +4, 2N = 0, O = (1 x -2) = -2].
Sn(NO3)2 = -8 [Sn = +4, 3N = 0, 6O = (6 x -2) = -12]
9 (In(NO3)3) + 1 (Sn(NO3)2) + 6n = 0
[ n = 23.833 g] for the ratio In2O3 : SnO2 =
(90:10)
8.5 (In(NO3)3) + 1.5 (Sn(NO3)2) + 6n = 0
[ n = 23.25 g] for the ratio In2O3 : SnO2 =
(85:15)
8 (In(NO3)3) + 2 (Sn(NO3)2) + 6n = 0
[ n = 22.67 g] for the ratio In2O3 : SnO2 =
(80:20)
5 (In(NO3)3) + 5 (Sn(NO3)2) + 6n = 0
[ n = 19.17 g] for the ratio In2O3 : SnO2 =
(50:50)
10 (In(NO3)3) + 6n = 0 [ n = 25 g] for In2O3
10 (Sn(NO3)2) + 6n = 0 [ n = 13.333 g] for SnO2
Accordingly, for the complete combustion of indium nitrate: urea mixture, the molar
ratio becomes 15/6 = 2.5. Like that for the complete combustion of tin nitrate: urea
mixture, the molar ratio becomes 8/6 = 1.333.
2.6.1 Preparation of In2O3
In a typical synthesis, an aqueous solution containing ions of In2O3 was
prepared by dissolving typical amount of high purity analytical grade chemicals i.e.,
27
indium metal ingots (99%, Aldrich, India supplied from Merck Chemical Company),
distilled water (~ 100 ml) and nitric acid (99% Merck, India) in a glass beaker. Urea
(99%, Merck, India) and 25% NH3 (0.91 g cm-3) was then added to the solution
containing In ions. Amount of urea was calculated based on total valence of the
oxidizing and reducing agents for maximum release of energy during combustion.
Oxidant/fuel ratio of the system was adjusted by adding nitric acid and ammonium
hydroxide; and the ratio was kept at unity. The resulting translucent solution was
heated on a hot plate (at about 100 ºC) until it turned into a viscous solution. The
solution boils upon heating and undergoes dehydration accompanied by foam. The
foam then ignites by itself due to persistent heating giving a voluminous and fluffy
product of combustion. The combustion product was subsequently characterized as
single phase nanopowders of In2O3. Yellow ashes obtained after combustion were
then collected for structural characterization and other morphological studies. The
system was homogeneous during the whole process and no precipitation was
observed. The entire processing steps are illustrated in Fig. (2.3)
14In(NO3)3.3H2O + 3NH2CONH2 ⎯
⎯→7In2O3 + 48NO2 + 3CO2 + 48H2O
(2.3)
2.6.2 Preparation of SnO2
In a typical synthesis, an aqueous solution containing ions of SnO2 was
prepared by dissolving typical amount of high purity the analytical grade chemicals
i.e., tin metal ingots (99%, Aldrich, India supplied from Merck Chemical Company),
distilled water (~ 100 ml) and nitric acid (99% Merck, India) in a glass beaker. Urea
(99%, Merck, India) and 25% NH3 (0.91 g cm-3) was then added to the solution
containing Sn ions. Amount of urea was calculated based on total valence of the
oxidizing and reducing agents for maximum release of energy during combustion.
28
Oxidant/fuel ratio of the system was adjusted by adding nitric acid and ammonium
hydroxide; and the ratio was kept at unity. The resulting translucent solution was
heated on a hot plate (at about 100 ºC) until it turned into a viscous solution. The
solution boils upon heating and undergoes dehydration accompanied by foam. The
foam then ignites by itself due to persistent heating giving a voluminous and fluffy
product of combustion. The combustion product was subsequently characterized as
single phase nanopowders of SnO2. White ashes obtained after combustion were then
collected for structural characterization and other morphological studies. The system
was homogeneous during the whole process and no precipitation was observed.
According to the principle of propellant chemistry [22], for a stoichiometric
combustion reaction between a fuel and an oxidizer, the ratio of the net oxidizing
valency of the metal nitrate to the net reducing valency of the fuel should be unity.
The total oxidizing valency of Sn(NO3)2.3H2O works out to be -15. Urea (N2H4CO)
was taken as a fuel, which contains one carboxylic and four hydroxyl group for
coordinating the metal ions, which facilitate the formation of a viscous gel [23]. The
total reducing valency of urea works out to be +6. Hence, in order to have the
stoichiometric combustion reaction, one mole of tin nitrate needs 15/6 (or 2.5) mol of
urea. This ratio of oxidant: fuel (1:2.5) gives a satisfactory viscous gel to initiate
combustion. Since oxidant-fuel composition was optimum, sluggish decomposition
was not possible and therefore the reaction not leftover carbonaceous material in the
as-prepared powder. These observations are in good agreement with those reported in
the literature [24]. The further fuel-rich ratios were not suitable due to a sluggish and
incomplete combustion.
3Sn(NO3)2.2H2O + 4NH2CONH2 ⎯
⎯→3SnO2 + 7N2 + 4CO2 +14H2O
(2.4)
29
2.6.3 Preparation of ITO with different proportions
An aqueous solution containing ions of indium nitrate and tin nitrate in the
ratio of 95:05, 90:10, 85:15, 80:20 and 50:50 and amount of doping was prepared by
dissolving stoichiometric amount of high purity In(NO3)3.3H2O (99%, Merck, India)
and Sn(NO3)2.2H2O (99%, Merck, India) in 100ml distilled water in a glass beaker.
Urea (99%, Merck, India) was then added to the solution containing In(NO3)3.3H2O
and Sn(NO3)2.2H2O ions. Amount of urea was calculated based on total valence of
the oxidizing and reducing agents for maximum release of energy during combustion
[25]. In the preparation of nanoparticles of other ceramic oxides using combustion
process polyvinyl alcohol and urea were used as the complexing agent and fuel,
respectively.
In these cases, usual calcinations of the combustion product were
essential to get a single phase nano material. In the present combustion method, urea
was used as the complexing agent instead of polyvinyl alcohol and urea was replaced
with ammonia. Using this complexing agent and oxidant fuel system, it was possible
to get a single phase ITO nanoparticles in single-step combustion without the usual
calcinations for prolonged duration at high temperature.
Amount of Urea =
Valency of metal ions × m.w. of urea × req. wt.
3 × m.w. of Compound
(2.5)
Oxidant/fuel ratio of the system was adjusted by adding nitric acid and
ammonium hydroxide; and the ratio was kept at unity. At such a condition, maximum
energy is released. The solution containing the precursor mixture was heated using a
hot plate at ~250°C in a ventilated fume hood. The solution boils upon heating and
undergoes dehydration accompanied by foam. The foam then ignites by itself due to
persistent heating giving a voluminous and fluffy product from combustion. The
system was homogeneous during the whole process and no precipitation was
observed. The combustion product was subsequently characterized as single phase
30
nanocrystals of ITO in the ratio of 95:05, 90:10, 85:15, 80:20 and 50:50. The
schematic diagram of experimental technique is shown in Fig. 2.3.
2.7 Conclusion
This chapter presents the various powder preparation techniques. The
combustion synthesis method has been elaborately discussed with the principles of
combustion process and the steps involved in the preparation of monophase oxide
powders. Preparation of In2O3, SnO2 and ITO powders under the optimized conditions
is presented.
31
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32
Fig. 2.1 Two basic approaches to nanomaterials fabrication: top-down (from left to
right) and bottom-up (from right to left)
Fig. 2.2 Comparison of the “top-down” and “bottom-up” approach to nanomaterials synthesis
Clear Solution of Indium
Clear Solution of
Tin
95:05, 90:10, 85:15,
80:20 and 50:50
Indium & Tin Nitrate
Solution
Dehydration stage
Combustion
(Ignition)
above 300ºC
As prepared powder
Calcination at
different high
temperatures
Compounds
Fig. 2.3 Flow chart for the preparation of ITO