Agriculture Reference
In-Depth Information
12.6 Nano-titanium Dioxide Photocatalytic Technology
Photocatalytic technology, especially that using nano-titanium dioxide (TiO
2
), has
been reported to show good performance in removing recalcitrant organic pollu-
tants, has advantages such as low cost, high stability, suitable band gap, etc., and
has become the most widely used technique. In 1972, electromotive force was
observed when placing TiO
2
under xenon lamp irradiation with electrolysis of
water to generate hydrogen, indicating that TiO
2
converts light energy into chem-
ical energy stored in hydrogen. TiO
2
is characterized by chemical stability,
nontoxic nature, rich source, high quantum efficiency, and catalytic efficacy.
Currently, the major applications of TiO
2
include solar cell materials, photo-
sensitive materials, and sensors. In addition, TiO
2
is also used in water purification,
sterilization, deodorization, decontamination, defogging, and other environmental
fields.
Photocatalytic performance of TiO
2
depends on its semiconductor structure.
According to the energy band theory, the low energy band structure of the semi-
conductor material consists of a valence band with low energy and a conduction
band with high energy, with a band gap existing between the valence and conduc-
tion bands. TiO
2
is a semiconductor with a band gap of 3.2 eV, and only under UV
irradiation (
<
385 nm), the electrons of the valence band absorb the energy of the
photon and jump into the conduction band to form electron-hole pairs. The
photogenerated electrons and holes adsorbed onto the surface of the semiconductor
transfer the charged species, i.e., the hole-captured substances adsorbed onto the
surface of the electronic semiconductor or electrons of the solvents, so that pollut-
ants are activated and oxidized. An electron acceptor is reduced by receiving the
electrons, and a heterogeneous catalysis occurs through such continuous charge
transfer. However, UV radiation only occupies 4 % of sunlight, whereas visible
light occupies about 43 % of sunlight, thus making this technology less efficient. In
recent years, numerous studies have been carried out on photogenerated carriers to
reduce the recombination rate, expand the excitation wavelength range, etc.,
including semiconductor doping, surface sensitization, noble metal deposition,
nonmetal doping, and so on.
The main methods of TiO
2
doping include metal doping, nonmetal doping, and
co-doping, and among them, metal doping is classified into transition metal doping
and rare-earth metal doping. Transition metal doping decreases the energy gap of
the photocatalyst, increases the electron-hole pair recombination, and subsequently
enhances the photocatalytic performance. In contrast, some of the more common
nonmetal doping methods use doping elements such as N, S, C, H, halogens, etc.
Asah doped small amounts of N (0.75 %) into the lattice, instead of oxygen, and
achieved the activity under visible light, and was the first to prepare TiNx-doped
TiO
2
with visible excitation property. Chen reported that black TiO
2
nanocrystals,
which introduce chaos on the TiO
2
crystal surface through hydrogenation, can
effectively utilize energy in the infrared region. In recent years, there has been a
gradual development of halogen doping. Yu carried out hydrolysis of titanium tetra-
