Page 261 - Multifunctional Photocatalytic Materials for Energy

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lengths ranging from 170 to 210 nm and widths ranging from 25 to 30 nm. Fig. 11.4F
shows the TiO 2 nanocrystallines, and the entire inner surface of the anodic aluminum
oxide (AAO) channels was covered [88]. As a result, because of the absorption and
reaction of the surface-related precursor molecules, the anisotropic TiO 2 nanostruc-
tures were obtained. Maili reported an ionic liquid-assisted hydrothermal method in
which 3-carboxymethyl-1-methylimidazoliurn bisulfate, titanium isopropoxide, HCl,
and H 2 O were used to prepare rutile TiO 2 NRs. Fig. 11.4C shows the SEM image of
the TiO 2 NRs [83]. By using TTIP and Ti(OC 3 H 7 ) 4 as precursors, Chen et al. produced
well-aligned rutile TiO 2 NRs on (100) oriented sapphire substrates. The densely packed
rutile TiO 2 NRs were grown with similar heights and all in one direction (Fig. 11.4D
and E). A combination of chemical vapor deposition and microwave hydrothermal
methods were also developed for the preparation TiO 2 NRs [89,90].

11.2.2.4 TiO 2 nanofibers
In comparison with 1D TiO 2 nanostructures, TiO 2 NFs possess additional advantages
attributed to their special nanosize and enlarged surface area for catalytic activity
[91–95]. The most utilized technology to fabricate TiO 2 NFs is the electrospinning
method, which includes the following steps: (i) preparing the sol; (ii) adding sol to
the polymer template; (iii) electrospinning the solution for NFs; and (iv) calcination
for single-phase TiO 2 NFs [96–103]. Xia et al. obtained ultra-long TiO 2 NFs using
Ti(OC 3 H 7 ) 4 , poly (vinyl pyrrolidone) (PVP), acetic acid, and ethanol as precursor via
the electrospinning method [104]. In their work, the diameter of the TiO 2 /PVP NFs
was about 78 nm, which could be varied according to the electric field, the concen-
tration of PVP and Ti(OC 3 H 7 ) 4 , and the feeding rate. Then the samples were heated at
500°C to remove the additional PVP in order to obtain pure TiO 2 NFs (Fig. 11.5A and
B). Another typical structure is hollow TiO 2 NFs. Yuan and coworkers successfully
obtained hollow TiO 2 NFs by using activated carbon fibers (ACF) as templates via
precursor impregnation. The ACFs were immersed in a mixed solution of titanium iso-
propoxide and anhydrous ethanol according to a designated volume ratio (1:6). Then
the filled ACFs and solution were transferred to a stainless steel autoclave at 150°C for
24 h. Finally, the as-prepared TiO 2 NFs were calcined at 600°C for 5 h to remove the
template (Fig. 11.5C and D) [105].

11.2.3 Formation of 2D TiO 2 nanostructures
In the preparation of 1D TiO 2 nanostructures, 2D TiO 2 NSs are the intermediate part of
the hydrothermal process, and they also play an important role in the development of
TiO 2 materials. Therefore the key to obtaining 2D TiO 2 NSs is to precisely control the
reaction conditions of the formation mechanism. Sun's group obtained typical TiO 2
NSs via a traditional hydrothermal method [106,107]. By combining continuous hy-
drothermal routes [108], room temperature synthesis [109], and ionic liquid synthesis
approaches [110], a new organic-stabilizer-free synthesis was produced recently for
the formation of TiO 2 NSs. Yang's group synthesized anatase TiO 2 NSs with (001)
facets using a modified solvothermal method. A 14.5 mL mixed solution of TiF 4 and