Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 20

Interestingly, some designs are created using two kinds of templates. For
example, a double layer couples a high surface area mesoporous anatase underlayer
prepared using PI-b-PEO block polymer as template, with an optically and elec-
trically active 3D periodic TiO 2 PC overlayer reversed from a PS template. Such a
design is expected to allow effective dye sensitization, electrolyte infiltration, and
charge collection from both the mesoporous and the PC layers, which acts as the
photoanode in DSSCs [ 68 ]. Additionally, mesoscale colloidal PS particles and
lithographically patterned SU8 macropores can be used as the dual templates, with
the colloidal particles assembled within the macropores. The as-prepared template
produces hierarchical TiO 2 electrodes for DSSCs with sufficient surface area from
mesoscale pores and effective light scattering from micropores [ 99 ].
Alongside the effective organic templates, several inorganic materials including
porous anodic alumina (PAA) [ 100 ], ZnO [ 101 ], Cd(OH) 2 [ 102 ] and SiO 2 [ 103 ]
have been successfully employed as templates to construct TiO 2 architectures,
such as ordered nanowires/nanotubes, mesoporous hollow spheres, and hierar-
chical nanoplates. One of the most commonly used templates, a PAA membrane
with high density and high aspect ratio pores, is prepared by anodic oxidation of an
aluminum sheet in a solution of sulfuric, oxalic, or phosphoric acids. It is fre-
quently used as a template in nanowire or nanotube array synthesis through the
sol-gel process described in Sect. 2.2.2 [ 104 ]. The PAA allows nanowires or
nanotubes to grow on transparent conducting indium tin oxide (ITO) substrates
without bending or breaking. This technique is capable of achieving a surface
roughness factor equal to a typical nanoparticle layer [ 100 ]. In particular, a number
of hybrid templated processes have been developed to construct attractive TiO 2
nanostructures. For example, 3D interconnected nanoporous TiO 2 nanotube arrays
on fluorine-doped tin oxide (FTO) glass were prepared using a sol-gel process
assisted by PVC-g-P4VP block copolymer and a ZnO nanorod template [ 105 ]. An
organic-inorganic hybrid template (Fig. 4 ), Al 2 O 3 -POEM, is specially designed to
fabricate a crack-free, organized mesoporous TiO 2 photoanode with high surface
area, good interconnectivity, and uniform pores, yielding a high energy conversion
efficiency of 7.3 % in DSSCs [ 78 ]. Other synthesis methods have also been
assisted by templates, including hydrothermal [ 106 , 107 ], atomic layer deposition
(ALD) [ 108 - 110 ], microwave, and sonochemical methods [ 77 ].
2.1.2 Hydrothermal/Solvothermal Method
The hydrothermal/solvothermal methods have also been used to prepare a variety
of TiO 2 nanomaterials [ 111 - 113 ]. In such processes (Fig. 5 ), the synthesis reaction
generally occurs in aqueous/organic solutions containing required material pre-
cursors within temperature controlled steel pressure vessels called autoclaves. The
reaction temperature can be raised above the boiling point of the water/organic
solvent, reaching the pressure of vapor saturation. The temperature and the amount
of solution added to the autoclave largely affect the internal pressure produced.
The solvothermal method is almost identical to hydrothermal method except that