Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 17

2.1 Preparation Methods
2.1.1 Sol-Gel Method
The sol-gel method is a powerful technique in the development of nanostructured
semiconductor films with diverse morphologies. Sol-gel method is typically a two-
step process of synthesis and deposition, and can easily tailor the material prop-
erties during synthesis under optimized conditions [ 65 , 66 ]. In a typical sol-gel
process, a colloidal suspension or sol is first obtained from the hydrolysis and
polymerization reactions of the precursors, which are usually inorganic metal salts
or metal organic compounds such as metal alkoxides. The phase conversion from
the liquid sol into the solid gel phase is realized by complete polymerization and
evaporation of the solvent. Finally, thin films of the resulting solid gel can be
achieved via a variety of deposition techniques such as spin-coating, dip-coating,
spray pyrolysis, doctor blade, electrophoretic, and template assisted deposition
onto a conducting glass substrate (e.g., fluorine-doped tin oxide, FTO or indium-
tin oxide, ITO glasses) or other preferred surface [ 67 - 69 ]. The film is then
annealed at 200-450 C in air for 1-2 h, which is essential to reduce grain
boundaries and enhance crystallinity, thus improving electrical conductivity.
Nanostructured TiO 2 materials are usually prepared from the hydrolysis of a
titanium precursor, which is normally performed via an acid-catalyzed hydrolysis
step of titanium (IV) alkoxide (e.g., titanium isopropoxide, TTIP) or halide pre-
cursor (e.g., TiCl 4 ) followed by condensation. Properties of the resulting TiO 2
nanomaterials, including the crystallinity, morphology, structure size, surface area,
porosity, degree of dispersion, and crystal phase, heavily depend on the reaction
conditions, including the temperature, reaction time, solvent, solution pH, type of
precursor, drying conditions, and post-treatment [ 20 , 35 , 70 ]. Among various
methods, the sol-gel method is one of the most widely used methods for TiO 2
nanostructure preparation, due to its relatively low cost, flexibility of substrate, and
diversity of nanostructures such as nanoparticles, nanorods, nanoporous films, and
nanowires [ 1 ].
Template-assisted sol-gel processes are a well-established strategy for the
design of various functional TiO 2 nanostructures. This method utilizes the mor-
phological properties of known and characterized templates in order to assemble
materials with a particular morphology by reactive deposition or dissolution
methods. By controlling the morphology of the template material, it is possible to
prepare numerous new materials with a regular and controllable morphology (e.g.,
mesoporous structure) on the nano and micro scale [ 71 , 72 ]. The template-assisted
method can be generally divided into three sequential steps as shown in Fig. 3 a:
(1) assembly of a template (e.g., composed of polymer latex spheres), (2) infil-
tration and deposition of TiO 2 nanoparticles or titanium precursor, and (3)
selective removal of the used template to yield an inverse porous structure [ 73 ].
The used templates can be soft templates (surfactants or block polymers) or hard
templates (porous silica, polystyrene spheres, or porous carbon) [ 74 ].