Environmental Engineering Reference
In-Depth Information
toxic and nonbiodegradable organics to carbon dioxide (cO
2
), water, and inorganic constituents [6, 44, 45, 49-52]. However,
there are some drawbacks in using TiO
2
nanoparticles, including the following: the use of a UV light-limited irradiation source
because only light below 388 nm can produce electron and hole pairs; recombination of electron and holes; and separation and
agglomeration problems [44, 49, 53]. These limitations lead to a decrease in the photocatalytic efficiency of TiO
2
nanoparticles
and eventually reduce the photodegradation of contaminants in wastewater. Hence, numerous studies have been undertaken to
utilize TiO
2
nanoparticles more efficiently, such as doping noble metals (Ag, platinum (pt), gold (Au)), metals, metal ions, or
metal oxides, on TiO
2
[5, 42, 44, 45, 48, 50, 54-59], and/or the immobilization of TiO
2
on a rigid support, such as Ac and silicon
carbide (Sic) [6, 7, 49, 51, 60-62]. The deposited metals and doped ions onto TiO
2
can extend the light absorption capacity into
the visible range and suppress the recombination of electrons and holes by acting as electron traps in photocatalysts, leading to
better availability of the holes and consequently to better degradation efficiency [45, 55].
Sobana et al. [45] demonstrated that the photocatalytic degradation of dyes was improved by doping noble metal Ag nanopar-
ticles on TiO
2
as electron traps. results showed that Ag nanoparticles could enhance electron-hole separation by transferring
the trapped electrons to the adsorbed O
2
, acting as an electron acceptor to produce reactive O
2
−
⋅ OH⋅ was also generated during
the process. In this study, the observed optimum loading ratio to achieve the highest photocatalytic degradation efficiency of
dyes was approximately 1.5% Ag concentration in relation to TiO
2
concentration. However, due to the high prices of noble
metals, most of the researchers focused on the use of other metals as dopants, such as transition metals and alkaline metals.
park et al. [5] demonstrated the enhancement in photocatalytic degradation of 4-chlorophenol (4-cp) by doping cu into TiO
2
nanoparticles. The increase in photocatalytic activity was due to the successful entry of cu atoms into the TiO
2
lattice, which
created charge compensating anion vacancies in the lattice points of TiO
2
. Hydrochloric acid (Hcl), cO
2
, and water were the
products formed after the photocatalysis of 4-cp. In the photocatalysis of 2-propanol, bimetal-incorporated TiO
2
photocatalysts
(FeZn-TiO
2
) were prepared, and the photocatalytic activity was investigated by park et al. [48]. The highest activity was
achieved at the element ratio of 1.5 between Fe and Zn. In this Fe/Zn ratio, the charge recombination of photoexcited electrons
and holes was circumvented, thereby enhancing the degradation efficiency. penpolcharoen et al. [44] showed the positive effect
of nanohematite, embedded onto TiO
2
, in the photocatalysis of sucrose and nO
3
-
, wherein the photogenerated electrons were
trapped by iron (III) ion (Fe
3+
), which led to an improvement in the electron-hole separation and increased photocatalytic
activity. Jeon et al. [42] investigated the photonic efficiency of nanosized molybdenum-doped TiO
2
-mixed (Mo/Ti) oxide pho-
tocatalysts in the degradation of dichloroacetate (DcA). The optimum Mo content was 0.5 mol% to obtain the highest degra-
dation efficiency of 28%.
The degradation efficiency of TiO
2
could be improved by doping with alkaline metals, such as lithium (li), sodium (na), and
potassium (K). According to bessekhouadet et al. [56], these alkaline metals lead to the creation of negative charges on the
surface of the photocatalyst, facilitating adsorption of cationic molecules of malachite green oxalate (MG) contaminants
through electrostatic interactions. The photoformed electrons then move from the lowest unoccupied molecular orbital (lUMO)
band of MG to the conduction band of TiO
2
. This process enhances electron production by the photocatalyst under illumination,
thereby increasing the generation of radicals for the degradation of organic contaminants. An optimum content of dopant on
TiO
2
can increase the ability of the doped photocatalyst in absorbing the light spectrum near the visible region. This phenomenon
was revealed by Fan et al. [54], where TiO
2
was doped with 0.4% cerium and an enhancement in the photocatalytic degradation
of phenol was observed when compared with that of pure TiO
2
. The absorption spectrum of TiO
2
increased from 390 to 450 nm
after doping, with cerium causing more light to be absorbed for photocatalytic activities.
Aside from noble, transition, and alkaline metals as dopants on TiO
2
, metal oxide-modified heterogeneous nanoparticles are
also important for maximizing the efficiency of photocatalytic degradation of wastewater contaminants. Dong et al. [59] indi-
cated that cationic and anionic dyes in mesoporous TiO
2
-SiO
2
nanocomposites exhibit high decolorization and degradation
efficiency because of the synergistic effect of coupled adsorption and photocatalytic oxidation provided by the nanocomposites.
The cationic and anionic dyes were absorbed by surface Si-OH and Ti-OH groups, respectively. Meanwhile, Han et al. [55]
confirmed that the photocatalytic degradation of phenol begins with the breakup of carbon-carbon bonds by TiO
2
-SrO photo-
catalysts, during which butane (c
4
H
10
) and propanal (c
3
H
6
O) ions are detected in the aqueous solution. The fastest photocata-
lytic degradation rate of phenol (14h) has been achieved using 60wt% of SrO in TiO
2
photocatalysts under visible light,
compared with the 20 and 40 wt% of SrO. Meanwhile, doping ZnFe
2
O
4
onto TiO
2
successfully enhanced the photocatalytic
degradation of rhodamine b, as reported by liu et al. [58] Moreover, O
2
is an important oxidant in photooxidation, acting as an
electron acceptor to decrease the recombination rate between photogenerated electrons and holes. O
2
also promotes the gener-
ation of OH⋅ radicals for the degradation of organic contaminants.
The photocatalytic activity of TiO
2
significantly affects the degradation of wastewater contaminants. Thus, aside from metals
or metal oxides, ions can be doped onto TiO
2
to enhance its photocatalytic activity. Venkatachalam et al. [50] indicated that
doping zirconium ions (Zr
4+
) onto TiO
2
nanoparticles enhanced the photocatalytic degradation of 4-cp. Zr
4+
successfully enters
the TiO
2
lattice and increases photocatalytic activity. charge-compensating anion vacancy was created in the TiO
2
lattice points,
