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156 Multifunctional Photocatalytic Materials for Energy

(A) (B)

20nm 20 nm

(C) (D)

20nm

Fig. 8.2 (A) TEM images of TiO 2 NPs with a diameter of ~7 nm prepared from 0.10 M
titanium isopropoxide in 4:1 ethanol/water; (B) ~15 nm TiO 2 NPs prepared from 0.04 M titanium
isopropoxide in 1:2 ethanol/water; (C) ~25 nm TiO 2 NPs prepared from 0.02 M titanium
isopropoxide in 1:8 ethanol/water; and (D) a corresponding high-resolution TEM image for
~7 nm TiO 2 NPs.
Reprinted with permission from S.Y. Chae et al., Preparation of size-controlled TiO 2
nanoparticles and derivation of optically transparent photocatalytic films. Chem. Mater.
15 (2003) 3326–3331.

Fig. 8.2 shows the TEM images of different sizes that were obtained by controlling
the ratio of the solvents [14].
The sol-gel method is another way to obtain TiO 2 NPs. It is a low-temperature tech-
nique that does not require a high-temperature annealing process, which provides a
new technique for fabricating TiO 2 NPs in the flexible substrate (e.g., PET and PEN).
Through catalysis by an acid or a base sol, a three-dimensional (3D) gel network can
be obtained after hydrolysis of a titanium precursor in excess water; the corresponding
mechanisms are shown here [15]:

Ti(OR) + 4 HO ® Ti (OH) + 4 ROH ( Hydrolysis)
4 2 4