Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 37

Among these methods, ion doping has been widely adopted to adjust the position
of either the conduction band (CB) or valence band (VB) of TiO 2 . This could have
the desired effect of making the electrons excitable under visible light irradiation to
produce photoelectron-hole pairs. The mechanism of the visible absorption is
produced from interaction between the 2p orbitals of the dopant and the 2p orbitals
of the oxygen in the newly formed valence band or the creation of dopant isolated
states above the valence band maximum [ 319 - 321 ]. Nonmetal ion doping is usually
considered more promising than metal ion doping because metal ion doping can
introduce undesirable defects which can serve as the recombination centers for
photoelectron-hole pairs, and thus reduce the pollutant degradation efficiency
[ 322 ]. Nitrogen is the most promising dopant and can be easily introduced into the
TiO 2 structures, due to its comparable atomic size with oxygen, small ionization
energy and high stability [ 323 ]. Since Sato et al. found that the addition of NH 4 OH
to TiO 2 sol, followed by calcination of the precipitated powder, could generate a
material which showed a visible light response, many strategies have been devel-
oped to produce N-doped TiO 2 materials [ 324 , 325 ]. In the past decades, doping of
nitrogen into TiO 2 structures has been realized via both wet and dry preparation
methods. Physical techniques (i.e., sputtering and ion implantation) based on the
direct treatment of TiO 2 with energetic nitrogen ions have also been developed
[ 321 , 326 ]. Meanwhile, gas phase reaction methods (i.e., atomic layer deposition
and pulsed laser deposition) have been successfully used to prepare N-doped TiO 2
materials [ 269 , 327 ]. The sol-gel method has proven to be the most versatile
technique for the synthesis of N-TiO 2 nanoparticles because of its low cost, rela-
tively simple equipment, and easy control of the resulting nanostructures [ 328 ]. In
brief, the simultaneous growth of TiO 2 and N doping can be realized by hydrolysis
of titanium precursors (i.e., titanium tetrachloride, titanium tetra-isopropoxide,
tetrabutylorthotitanate) in the presence of nitrogen sources (i.e., aliphatic amines,
nitrates, ammonium salts, ammonia, and urea) [ 328 - 330 ].
Recent studies have also revealed that doping TiO 2 with other elements, such as S,
F, C, and B shifts the optical absorption edge to longer wavelengths [ 267 , 331 - 333 ].
For example, F-doped flower-like TiO 2 nanostructures (Fig. 15 a) have been syn-
thesized in the presence of HF by a mild hydrothermal process and exhibited high
photoelectrochemical activity for water-splitting and the photodegradation of
organic pollutants (Fig. 15 b) [ 334 ]. Mesoporous C-doped TiO 2 materials were
prepared by a hydrothermal synthetic approach using sucrose as a carbon-doping
source, followed by a post-thermal treatment. The resulting C-doped TiO 2 photo-
catalyst showed reduced recombination of electron-hole pairs due to the reduction of
surface defects and promoted visible-light photocatalytic activity (Fig. 15 c, d)
[ 335 ]. In order to further improve the photocatalytic activity, co-doping TiO 2 with
double non-metal elements (i.e., N-S [ 336 ], N-B [ 329 , 337 ], F-N [ 338 ], C-N [ 339 ])
has attracted more attention. For example, F-N co-doped TiO 2 nanoparticles with
dominant (001) facets were prepared by calcination a TiOF 2 precursor in NH 3 gas
flow. The resulting nanoparticles showed drastically enhanced absorption and
excellent water oxidation performance under visible light irradiation [ 338 ].