Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 26

Fig. 8 a Schematic diagram of anodization experiments. b Anodization leads to the formation of
a compact oxide on Ti (in most neutral and acidic electrolytes). If dilute fluoride electrolytes are
used, highly ordered nanoporous or nanotubular anodic oxide layers can be formed. In the case of
nanoporous oxide, the layer consists of vertically aligned nanosized channels in an oxide matrix
while in the case of nanotubular morphology the layer is composed of an ordered array of oxide
nanotubes. (Reprinted with permission from Ref. [ 150 ]. Copyright Wiley-VCH)
and tetramethylammonium hydroxide (Me 4 NOH) as synergistic morphology-con-
trolling reagents [ 148 ]. In contrast to anatase TiO 2 with low-index facets like (101),
(001), and (100), recently Li et al. reported the fabrication of an anatase TiO 2
nanosheet array assembled by (116) facet-oriented nanocrystallites. The (116)
facets were made parallel to the surface of a FTO substrate via a two-step process,
which exhibited 50 % higher photocatalytic activity than (001) facet-oriented
nanosheet arrays in the degradation of methyl blue (MB) under UV light irradiation
[ 149 ].
2.1.3 Electrochemical Anodization Method
Anodic oxidation represents a facile and well-established method to form nano-
structures in a self-organiting way [ 104 ]. The controlled oxidation of titanium metal
under electrochemical anodization provides another method of TiO 2 nanomaterial
production [ 52 ]. In general, the anodization process is conducted in a two-electrode
electrochemical cell at a constant potential in aqueous or organic electrolyte with Ti
foil as an anode and platinum foil as a cathode (Fig. 8 a) [ 150 ]. Under optimized
anodized conditions, highly ordered nanoporous or nanotubular architectures with
high aspect ratios could be successfully achieved [ 151 ]. Anodization is particularly
useful in the synthesis of TiO 2 nanotubes from titanium foil. Anodization has been
extensively studied after pioneering work in 2001 by Gong et al. in which they
reported the formation of nanotubes up to 0.5 mm in length by electrochemical
anodization of titanium foil in HF aqueous electrolyte [ 152 ].
Among the various forms of semiconductor nanostructures, one-dimensional
(1D) highly ordered architectures, such as nanowires, nanorods, nanofibers,
nanotubes with high surface area-to-volume ratios possess useful and unique
properties compared to their bulk counterparts [ 39 ]. The highly ordered nature of