Geoscience Reference
In-Depth Information
Room temperature
Supercritical condition
Room temperature
Organic phase
Organic phase
Homogeneous phase
Hydrolysis
M
cr+
M(OH)
n
Supercritical H
2
O
10 mi
n
M
cr+
M
cr+
Water phase
Water phase
CH
CH
M
cr+
M
cr+
M
cr+
HO
CH
−
OH
OH
OH
Figure 10.23 The strategy for the synthesis of metal oxide nanocrystals in the organic
ligand-assisted SCF technique
[44]
.
(a)
(b)
(c)
(d)
3nm
3nm
60 nm
60 nm
60 nm
60 nm
Figure 10.24 TEM photographs of ceria nanocrystals obtained through organic ligand-
assisted supercritical hydrothermal synthesis. The mole ratios of decanoic amine to decanoic
acid were (a) 0, (b) 1:5, (c) 1:1, and (d) 2:1.
Source: Photographs courtesy: Prof. T. Adschiri, Tohoku University, Japan.
several new applications. Researchers like Adschiri, Arai, Reverchon, Byrappa,
Lester, and Poliakoff have contributed extensively on the synthesis of metal oxides
and proposed a systematic mechanism
[8,41,42,58,105,108,109]
. Metal oxides like:
Al
2
O
3
,Ga
2
O
3
,In
2
O
3
, SiO
2
, GeO
2
, ZnO, V
2
O
5
,VO
2
,V
2
O
3
, TiO
2
,CeO
2
, ZrO
2
,
CoO,
-MnO
2
,Cu
2
O, CoFe
2
O
4
, ZnFe
2
O
4
,
ZnAl
2
O
4
,Fe
2
CoO
4
, BaZrO
3
, BaTiO
3
,BaFe
12
O
19
, LiMn
2
O
4
, LiCoO
2
, and La
2
O
3
have been prepared by the above method. Usually, the particles obtained in the sub-
critical water conditions are larger than those in the scH
2
O, because there is a parti-
cle growth with an increase in the residence time, whereas under supercritical
conditions such a phenomenon has not been observed. The hydrothermal reaction
rate in scH
2
O is higher, and the solubility of the metal oxides is much lower than
that in subcritical water. This leads to the generation of higher degree of super-
saturation. The nucleation rate is expected, by the function of degree of supersatu-
ration, and the surface energy according to the nucleation theory. Thus, extremely
high nucleation rate can be expected at supercritical conditions, which leads to the
formation of nanosize particles.
Figure 10.26
shows the mechanism for the fine
α
-Fe
2
O
3
,
γ
-Fe
2
O
3
, NiO, Co
3
O
4
,Mn
3
O
4
,
γ
