Page 259 - Multifunctional Photocatalytic Materials for Energy

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the suspension was centrifuged with deionized water several times to reach pH = 7,
thus obtaining the elongated TiO 2 nanotubes [56,57].

11.2.2.2 TiO 2 nanowires
Recently, TiO 2 NWs have been extensively prepared by various methods [53–55,64–
66]. The three main steps in the formation of TiO 2 NWs from particles are as fol-
lows: (i) exfoliation of the layered Na 2 Ti 3 O 7 , (ii) formation of nanosheets (NSs), and
(iii) formation of NWs. Highly ordered and controlled TiO 2 NWs were reported to
be obtained from electrophoretic deposition of TiO 2 colloidal suspensions with an
anodic alumina membrane (AAM) template [67]. Solvothermal and physical vapor
deposition [68,69] in which TiO 2 NWs were prepared also have been reported [70,71].
Based on the crystal orientations, TiO 2 NPs can be aligned to form TiO 2 NWs through
the sol-gel method via oriented attachment [72,73]. Rodríguez-Reyes and Dorantes-
Rosales prepared TiO 2 NWs by utilizing titanium tetraisopropoxide, isopropanol, and
acetic acid as the sol [74]. Without any substrates or seeding layer, TiO 2 NWs can also
be synthesized by treating TiO 2 powders in an alkaline solution via a hydrothermal
process [75,76]. Typically, the NWs are randomly oriented and exist in powder form.
In order to obtain well-aligned and highly controlled TiO 2 NW arrays, Zheng's group
used TiO 2 NP-based substrates combined with a hydrothermal process [60–62]. The
reaction was carried out by spinning the TiO 2 polymeric sol, followed by annealing at
450°C for 1 h. The as-prepared rutile TiO 2 NWs held excellent nanosized uniformity
(Fig. 11.3A and B). Very experienced with the vapor-liquid-sol method, Sung et al.
first kept a quartz substrate covered with Ti (500 nm) and Au NPs (50 nm) at 1050°C

Fig. 11.3 Cross-sectional view (B) and top view (A) of TiO 2 NWs on FTO substrates [60–62].
SEM images of TiO 2 NWs grown on sapphire (C) and quartz substrates (D), respectively [63].