Page 267 - Multifunctional Photocatalytic Materials for Energy

250 Multifunctional Photocatalytic Materials for Energy

agglomerates, porous spheres with periodic porosity, and hierarchical hollow spheres,
that possess different sizes, crystallinities, and porosities and that can be prepared
in spontaneous processes without templates. Several methods have been reported for
producing porous or hierarchical hollow TiO 2 spheres with defined shapes and con-
trollable porosity, such as hydrothermal and solvothermal reactions, chemical and
thermal transformations of porous spheres, spray- or aerosol-assisted processes, and
emulsion syntheses.
Lyonnard et al. recently prepared a hollow spherical TiO 2 nanostructure by spray
drying concentrated sols. The uniformity of the spheres were well controlled by the
ionic strength of the TiO 2 sol and the amount of salt added (Fig. 11.7D) [129]. As shown
in Fig. 11.7E, Lee et al. obtained hierarchical nanoporous F-doped TiO 2 spheres via
a low-temperature hydrothermal treatment by using a TiF 4 predecessor in HCl (1 M)
kept at 180°C. The modified hydrothermal treatment mentioned above led to the for-
mation of spherical TiO 2 particles with a diameter of nearly 1 μm from agglomerated
anatase crystals with sizes ranging from 11 to 15 nm along with mesopores around
3 nm [130]. Another hierarchical sphere of titanate morphology was obtained by Xie
et al. via a variety of solvothermal and hydrothermal reactions. The size and morphol-
ogy of the as-prepared samples was easily tunable by adjusting the reaction conditions
(Fig. 11.7F) [131]. Using a simple spray-drying process, Iskandar's group recently
produced submicron spherical brookite NPs under controlled sizes ranging from about
200 to 400 nm (Fig. 11.7G) [132,133]. In practice, macroporous brookite TiO 2 was
produced by spray drying a suspension of brookite NPs and polystyrene latex (PSL)
NPs for templating. Li's group reported another acid-induced solvothermal reaction to
form hollow TiO 2 spheres from precursors, including titanium oxosulfate and TiOSO 4 .
The formation of TiO 2 spheres with a diameter ranging from 1.6 to 3.1 μm and tunable
morphology was adjusted by changing the alcohol and the reaction time (Fig. 11.7H)
[134,136]. Moreover, by utilizing polystyrene beads as the templates, Li et al. suc-
cessfully fabricated hollow TiO 2 spheres with mesoporous shells. The key point of the
technology was using a plasma treatment to modify the surface of the hydrophobic
polystyrene beads to achieve available hydroxyl groups. Immersed in a mixed solution
of ethanol, ammonia, and titanium isopropoxide, the surface-modified polystyrene
spheres formed amorphous titania shells via a sol-gel process. The polystyrene tem-
plate could be totally removed by dissolution in tetrahydrofuran to obtain the hollow
TiO 2 spheres with a shell thickness of 25 nm (Fig. 11.7I) [130,135].

11.3 Solar WS by nanostructured TiO materials
2

Depletion of fossil fuels and the damaging environmental pollution arising from com-
bustion are recognized as two of the main challenges today and for the near future.
Furthermore, combustion of fossil fuels can cause global warming and the dire climatic
consequences known as the greenhouse effect [137,138]. Hydrogen is a carbon- neutral
energy carrier alternative to the diminishing fossil fuels, but a hydrogen economy re-
quires efficient hydrogen production. Solar energy is a renewable energy source and
can produce a chemically clean fuel product; therefore the conversion of solar energy