Page 276 - Multifunctional Photocatalytic Materials for Energy

258 Multifunctional Photocatalytic Materials for Energy

the calcination temperature as the crystalline phase evolves from the transformation
of H 2 Ti 3 O 7 to TiO 2 , to anatase, and finally to rutile [29]. Different band gaps (2.85,
2.8, 2.9, 3.05, 3.1, and 2.9 eV) of TiO 2 nanobelts can be obtained at different reaction
temperatures (100°C, 400°C, 600°C, 800°C, 900°C, and 1000°C). This result suggests
that the band gap values are associated with the phase transition from TiO 2 with a
band gap value of 2.8 eV and to anatase with a band gap of 3.1 eV [22,23]. However,
multiphase materials can usually be obtained, whereas rutile nanomaterials are always
obtained after a higher temperature process and with higher degrees of crystallinity
than others [111,201]. It is reported that the reactivity of the TiO 2 (110) facet is pri-
marily governed by surface species such as oxygen vacancies and bridging hydroxyls.
So varying these surface moieties may adjust the surface electronic structure of these
materials. Despite having a generally lower photoactivity than anatase, TiO 2 is receiv-
ing increasing attention as an alternative photocatalyst because of its relatively open
structure [202]. Many works have reported impressive photocatalytic activity of dif-
ferent phases of TiO 2 for hydrogen production [203,204]. More importantly, different
crystalline planes have different reduction or oxidation potentials, so photogenerated
electrons and holes may migrate to different planes separating from each other upon
photoexcitation.

11.3.3 Development of visible light-sensitized photocatalysts
Because it is environmentally friendly, has excellent physicochemical stability, and
is not high in cost, TiO 2 is seen as an essential wide band gap semiconductor, and its
photocatalytic applications have been widely explored [205]. However, it has an in-
trinsic drawback; that is, its wide band gap allows only 3%–5% of the solar spectrum
to be effectively utilized. Thus many efforts have aimed at finding various strategies
to enhance the light absorption of TiO 2 and achieve efficient utilization of the solar
spectrum by sensitizing TiO 2 with metals, nonmetals, and other semiconductors.

11.3.3.1 Bulk doping with metal and nonmetal elements

The photoelectrical activities of TiO 2 materials are based on their structure and chem-
ical constituents. Titanium (cation) and oxygen (anion) can be replaced by metal
and nonmetal doping, particularly on the surface of the nanostructures [206–208].
Therefore importing a secondary active cation or anion species into the lattice of TiO 2
is suitable for sensitizing them to visible light as well as suppressing the recombination
of electron-hole pairs. Numerous physical and chemical methods have been used to
incorporate nitrogen into the TiO 2 lattices. Pioneering work focused on N-doped TiO 2
showed that nitrogen doping in TiO 2 leads to a significant decrease in the band gap of
TiO 2 , making it active in the visible light region. Sathish et al. synthesized spherical-
shaped, N-doped TiO 2 through a simple chemical method, which proved to be an
effective way of nitrogen doping because of the mixing of N 2p states with O 2p states
on top of the VB or the formation of an N-induced midgap level. This N-doped TiO 2
greatly absorbed visible light because of its narrow band gap, resulting in the electrons
and holes generating under visible light irradiation [209]. In another work, different