5.3.5
Solvothermal Methods
To prepare crystalline oxide materials, high temperatures, associated with the refluxing of reactants in high-boiling solvents, is often called for. There are many
problems associated with such solvents, including possible toxicity, expense and
their inability to dissolve simple salts. A simple method to circumvent this problem is to use solvothermal methods, employing solvents well above their boiling
points in enclosed vessels that support high autogenous pressures. Solvothermal
(called hydrothermal when the solvent is water) methods are widespread in their
use, particularly in the preparation of crystalline solids, including silicate materials
such as zeolites [64].
Recently, Rajamathi and Seshadri [65] have reviewed the uses of solvothermal
methods for the preparation of oxide and chalcogenide nanoparticles. For oxide
nanoparticles, these methods can involve hydrolysis, oxidation and thermolysis, all
performed under hydrothermal or solvothermal conditions. Some of the more
striking examples are provided here.
5.3.5.1 Oxidation
An unusual reaction reported by Inoue et al. [66] is the direct oxidation of Ce metal
in 2-methoxyethanol at temperatures between 200
C and 250
C. Most of the
product obtained was bulk CeO 2 as a yellow solid, but in addition, they obtained a
brown solution of 2 nm CeO 2 nanoparticles. The CeO 2 nanoparticles could be
salted out by the addition of NaCl, and redispersed into solution at will. The solutions obeyed the Beer–Lambert law for the concentration dependence of the optical
extinction, suggesting that the nanoparticulate dispersion was a genuine solution.
Starting with Zn powder and GaCl 3 in water, Qian and coworkers [67] have used
the fact that the oxidation of Zn to Zn
2þ in water is associated with the production
of hydroxy ions according to:
Zn þ 2H 2 O ! Zn
2þ þ 2OH
À þ H 2 "
The OH
À ions help in the hydrolysis of Ga
3þ , to eventually give, in the presence of
Zn
2þ , the spinel ZnGa 2 O 4 . The authors used a 10–20% over-stoichiometry of Zn
and the reaction was carried out at 150
C for 10 h. 10 nm ZnGa 2 O 4 nanoparticles
were obtained. Excess ZnCl 2 formed in the reaction could be washed away with
water.
5.3.5.2 Hydrolysis
Chemseddine and Moritz [68] have used the polycondensation of titanium alkoxide
Ti(OR) 4 in the presence of tetramethylammonium hydroxide, to obtain highly
crystalline anatase nanoparticles with different sizes and shapes [Figure 5.4]. A
general, stepwise reaction for such polycondensation could be written:
5.3 Routes for the Preparation of Isolated Oxide Nanoparticles 105
Solvothermal Methods
To prepare crystalline oxide materials, high temperatures, associated with the refluxing of reactants in high-boiling solvents, is often called for. There are many
problems associated with such solvents, including possible toxicity, expense and
their inability to dissolve simple salts. A simple method to circumvent this problem is to use solvothermal methods, employing solvents well above their boiling
points in enclosed vessels that support high autogenous pressures. Solvothermal
(called hydrothermal when the solvent is water) methods are widespread in their
use, particularly in the preparation of crystalline solids, including silicate materials
such as zeolites [64].
Recently, Rajamathi and Seshadri [65] have reviewed the uses of solvothermal
methods for the preparation of oxide and chalcogenide nanoparticles. For oxide
nanoparticles, these methods can involve hydrolysis, oxidation and thermolysis, all
performed under hydrothermal or solvothermal conditions. Some of the more
striking examples are provided here.
5.3.5.1 Oxidation
An unusual reaction reported by Inoue et al. [66] is the direct oxidation of Ce metal
in 2-methoxyethanol at temperatures between 200
C and 250
C. Most of the
product obtained was bulk CeO 2 as a yellow solid, but in addition, they obtained a
brown solution of 2 nm CeO 2 nanoparticles. The CeO 2 nanoparticles could be
salted out by the addition of NaCl, and redispersed into solution at will. The solutions obeyed the Beer–Lambert law for the concentration dependence of the optical
extinction, suggesting that the nanoparticulate dispersion was a genuine solution.
Starting with Zn powder and GaCl 3 in water, Qian and coworkers [67] have used
the fact that the oxidation of Zn to Zn
2þ in water is associated with the production
of hydroxy ions according to:
Zn þ 2H 2 O ! Zn
2þ þ 2OH
À þ H 2 "
The OH
À ions help in the hydrolysis of Ga
3þ , to eventually give, in the presence of
Zn
2þ , the spinel ZnGa 2 O 4 . The authors used a 10–20% over-stoichiometry of Zn
and the reaction was carried out at 150
C for 10 h. 10 nm ZnGa 2 O 4 nanoparticles
were obtained. Excess ZnCl 2 formed in the reaction could be washed away with
water.
5.3.5.2 Hydrolysis
Chemseddine and Moritz [68] have used the polycondensation of titanium alkoxide
Ti(OR) 4 in the presence of tetramethylammonium hydroxide, to obtain highly
crystalline anatase nanoparticles with different sizes and shapes [Figure 5.4]. A
general, stepwise reaction for such polycondensation could be written:
5.3 Routes for the Preparation of Isolated Oxide Nanoparticles 105