Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 11

Fig. 1 a The physical image and b the simple operating principle of DSSCs based on TiO 2
nanomaterials. (Reprinted with permission from Ref. a [ 17 ], b [ 9 ]. Copyright Wiley-VCH and
Nature Publishing Group)

circuit in the cell [ 17 ]. The voltage generated under illumination depends on the
difference between the Fermi level of the electron in the semiconductor materials
and the redox potential of the electrolyte (Fig. 1 b) [ 18 , 19 ].
Notably, the core of the system is the nanoporous semiconductor, composed
primarily of TiO 2 materials, which not only supplies numerous adsorption sites for
dye sensitizer but also functions as an electron acceptor and electronic conductor
[ 16 ]. TiO 2 possesses several unique chemical and physical properties which make it
the most popular candidate for semiconductors in DSSCs. First, the conduction
band edge of TiO 2 lies slightly below the excited state energy level of many
sensitized dyes, which is a necessary condition for efficient electron injection.
Second, TiO 2 also has a large dielectric constant (e = 80 for anatase) for effective
electrostatic shielding of the injected electrons from the oxidized dye molecules
adsorbed on the TiO 2 surface, thus avoiding their recombination before regenera-
tion of the dyes by the redox electrolyte. The relatively high refractive index of
TiO 2 (n = 2.5 for anatase) also provides efficient diffusive light scattering inside
the nanoporous film, thus significantly increasing the light harvesting potential. In
addition, TiO 2 is stable over a wide range of environments, such as high temper-
ature and high acidity. Lastly, TiO 2 is inexpensive, abundant, and nontoxic [ 2 , 20 ].
In order to improve upon the aforementioned useful properties, over the past several
decades extensive research interests and efforts have focused on the design, fab-
rication, and modification of versatile TiO 2 photoanodes.

1.2 Applications in Photoelectrochemical Water Splitting

In addition, another ideal way for direct conversion of solar energy into practical
energy sources is through the generating of hydrogen from solar photoelectro-
chemical

splitting of water using semiconductors as photoelectrodes [ 21 ].