**2.3. Co-doping and tri-doping**

the doped non-metal with the O2p states shifts the valence band edge upward and narrows

nanoparticles has been reported to exhibit greater photocatalytic activity under visible light

alyst for non-metal doping. Zeng et al. [69] reported the preparation of a highly active modi-

performance of the photocatalyst was ascribed to excellent crystallinity, strong light harvesting and fast separation of photogenerated carriers. Moreover, the enhancement of charge separation was attributed to the formation of paramagnetic [O-Ti4+-N2−-Ti4+-VO] cluster. The surface oxygen vacancy induced by vacuum treatment trapped electron and promoted to generate super oxygen anion radical which was a necessary active species in photocatalytic process.

photocatalyst slightly decreases with increasing N content. In addition, the sub-band energies

the interstitial N species and the sub-band gap energies were found to have decreased from

because of the radius of N (17.1 nm) being higher compared to O (14 nm) and the electroneutrality can be maintained by oxygen vacancies, that are provided by replacement of three

vacancies from 4.2 to 0.6 eV, suggesting that N favors the formation of oxygen vacancies [72]. In contrast, O atoms (14 nm) could be substituted easily by F atoms (13.3 nm) because of their

visible light due to the high-density states that were evaluated to be below the maxima valence

absorption in UV, visible and infrared light illumination with enhanced electrons and holes separation. Surface vacancies and Ti3+ centres of the hydrogenated F-doped catalyst coupled with enhanced surface hydrophilicity facilitated the production of surface-bound and free hydroxyl radicals. Species present on the surface of the catalyst triggered the formation of new

 has also been ascribed to efficient separation of electron-hole pairs as well as an increased creation of surface radicals such as hydroxyl The band gap can also be narrowed by doping

lytic reduction of Cr(IV) showed that the co-doping and calcination played an important role in the microstructure and photocatalytic activity of the catalysts. The co-doped samples calcined at 500°C showed the highest activities ascribed to the synergistic effect in enhancing crystal-

Phongamwong et al. [70] investigated the photocatalytic activity of CO<sup>2</sup>

related to the impurity energy level were observed in the N-TiO<sup>2</sup>

similar ionic radius [73]. Yu et al. [64] reported that the F-doped TiO2

tic effect between fluorine and hydrogen in hydrogenated F-doped TiO<sup>2</sup>

S atoms (18 nm) compared to O atoms (14 nm). S incorporation in TiO2

allowing for higher photocatalytic activity [74]. N, S and C co-doped TiO<sup>2</sup>

lization of anatase and (N, S and C) co-doping. The carbon doped TiO<sup>2</sup>

Ti3+ occupied states under the conduction band of the hydrogenated F-doped TiO2

ing the band gap energy [73]. Enhanced photocatalytic performance of N-doped TiO<sup>2</sup>

band, although there was no shift in the band edge of TiO<sup>2</sup>

with S, since replacement of S into TiO<sup>2</sup>

change the lattice spacing of the TiO<sup>2</sup>

oxygen vacancies by two nitrogen atom [71]. N-TiO<sup>2</sup>

photocatalyst. The nitrogen and carbon doped TiO2

reduction under vis-

) is able to absorb

which enabled light

has been reported to

samples photocata-

) is reported to

, thus narrow-

over pure

photocatalyst because of

photocatalyst reduces the oxygen energy

(F-TiO2

can be performed easily due to larger radius of

(C-TiO<sup>2</sup>

with a reduction in the band gap width from 3.2 to 1.7 eV

. Samsudin et al. found a synergis-

) appears to be the most efficient and extensively investigated photocat-

photocatalyst and they have found that the band gap of N-TiO<sup>2</sup>

photocatalyst. In contrast, the replacement of O by N is difficult

nanoparticle via a novel modular calcination method. The excellent photocatalytic

the band-gap energy of the doped TiO2

44 Photocatalysts - Applications and Attributes

(N-TiO<sup>2</sup>

ible light over modified N-TiO<sup>2</sup>

2.18 eV with 10 wt% N-TiO<sup>2</sup>

N-doped TiO<sup>2</sup>

fied N-TiO<sup>2</sup>

TiO2

TiO2

irradiation compared to other non-metal dopants.

Although single metal doped and non-metal doped TiO2 have exhibited excellent performance in decreasing the electrons and holes recombination, but they suffer from thermal stability and losing a number of dopants during catalyst preparation process [77]. Therefore, co-doping of two kinds of atoms into TiO<sup>2</sup> has recently attracted much interest [78]. The electronic structure of TiO2 can be altered by co-doping on TiO2 by formation of new doping levels inside its band gap. Abdullah et al. [77] reported that the doping levels situated within the band gap of TiO2 can either accept photogenerated electrons from TiO2 valence band or absorb photons with longer wavelengths. Therefore, suggesting that the TiO<sup>2</sup> absorption range can be expanded.

Zang et al. [79] evaluated the photocatalytic degradation of atrazine under UV and visible light irradiation by N,F-codoped TiO<sup>2</sup> nanowires and nanoparticles in aqueous phase. It was found that photocatalytic degradation of atrazine was higher in the presence of N,F-codoped TiO2 nanowires than that of N,F-codoped TiO<sup>2</sup> nanoparticles. The higher photocatalytic performance in the presence of N,F-codoped TiO<sup>2</sup> nanowires was attributed to the higher charge carrier mobility and lower carrier recombination rate. Moreover, the speed of electron diffusion across nanoparticle intersections is several orders of magnitude smaller compared to that of nanowire because of frequent electron trapping at the intersections of nanoparticles and increasing the recombination of separated charges before they reach the TiO2 nanoparticles surface. Park et al. [80] showed the best performance for novel Cu/N-doped TiO<sup>2</sup> photoelectrodes for dye-sensitized solar cells. It was found that the Cu/N-doped TiO<sup>2</sup> nanoparticles provided higher surface area, active charge transfer and decreased charge recombination. Moreover, the addition of suitable content of Cu- to N-doped TiO<sup>2</sup> electrode effectively inhibited the growth of TiO2 nanoparticles and improved the optical response of the photoelectrode under visible light irradiation. Chatzitakis et al. [81] studied the photoelectrochemical properties of C, N, F codoped TiO<sup>2</sup> nanotubes. It was found that increasing surface area is not followed by increase in the photoconversion efficiency, but rather that an optimal balance between electroactive surface area and charge carrier concentration occurs.

Zhao et al. [82] investigated the photocatalytic H2 evolution performance of Ir-C-N tridoped TiO2 under UV-visible light irradiation. The photocatalytic activity of TiO<sup>2</sup> nanoparticles was reported to be improved by Ir-C-N tridoped TiO<sup>2</sup> under UV-visible light, due the synergistic effect between Ir, C and N on the electron structure of TiO<sup>2</sup> . It was found that Ir existed as Ir4+ by substituting Ti in the lattice of TiO<sup>2</sup> nanoparticles, whereas the C and N were also incorporated into the surface of TiO2 nanoparticles in interstitial mode. The absorption of TiO2 nanoparticles was expanded into the visible light region and the band gap was narrowed to ~3.0 eV, resulting in improved photocatalytic H<sup>2</sup> evolution under UV-visible light irradiation. Tan et al. [83] investigated the photocatalytic degradation of methylene blue by W–Bi–S-tridoped TiO2 nanoparticles. It was found that the absorption edge of TiO2 was expanded into visible-light region after doping with W, Bi and S and the catalytst showed the best photocatalytic activity, than that of TiO<sup>2</sup> , S-TiO<sup>2</sup> , W–S–TiO<sup>2</sup> and Bi–S–TiO2 . This might be attributed to the synergistic effect of W, Bi and S.

for energy and environmental related applications. Lin et al. [90] also investigated photore-

influenced the photocatalytic efficiency, and the highest catalytic activity was observed for

/NrGO nanocomposites with the highest N doping content. Moreover, modified TiO<sup>2</sup>

/NrGO. Qu et al. [91] prepared the graphene quantum dots (GQDs) with high quantum

nanoparticles) nanocomposites and the photocatalytic activity was tested towards the

photocatalytic degradation of methyl orange under UV-vis light irradiation (ʎ = 380–780 nm).

of N, S co-doped graphene quantum dots (N, S-GQDs)-reduced graphene oxide- (rGO)-TiO<sup>2</sup>

showed an extended photoresponse range, improved charge separation and transportation

NT and pure TiO<sup>2</sup>

can improve the utilization of solar light for energy conversion and environmental therapy.

pension in water which makes its recovery difficult, therefore hindering the application of photocatalytic in an industrial scale. As a result, many studies have attempted the modifica-

ticles because of their layered morphology, chemical as well as mechanical stability, cation

posites have attracted much attention for application in both water and air purification and have been prepared by numerous researchers. Belver et al. [97] investigated the removal of

nanostructure. The substitution of Ti ions with W resulted in the increase of its crystal size and the distortion of its lattice and moderately narrower band gap of photocatalysts. Mishra et al.

is highly dependent on the clay texture (as 2:1 clays show highest activity than 1:1) apart from

steel [96], etc. Clays have been reported to be a significant support material for TiO<sup>2</sup>

exchange capacity, non-toxic nature, low cost and availability. Therefore, TiO<sup>2</sup>

VOC and dye. They found that the photocatalytic performance of TiO<sup>2</sup>

their surface area and porosity. Moreover, the reactions involving TiO<sup>2</sup>

nanoparticles on support materials such as clays [93, 94] quartz [95], stainless

NT) nanocomposites for photodegradation of methyl orange under visible

nanoparticles mentioned above is the formation of uniform sus-

O vapor in the gas-phase under the irradiation of a Xe lamp using

/NrGO) nanocomposites. They found

http://dx.doi.org/10.5772/intechopen.79374

Modified Titanium Dioxide for Photocatalytic Applications

adsorption on the catalyst surface and

nanoparticles and enhance the activity on

nanoparticles. Tian et al. [92] reported the preparation

nanoparticles was approximately 2.7

nanocomposites simultaneously

NT, respectively. Suggesting that GQDs

/clay photocatalysts. It was found that the

/clay catalyst exhibited higher photocatalytic performance

/clay nanocomposites for photocatalytic degradation of

/clay, which was explained by the presence of W ions in the TiO<sup>2</sup>

/NrGO nanocomposites strongly

photoreduction rate

nanotubes (GQDs/

nanoparticles

NT is 1.8 and 16.3

nanopar-

/clay nanocom-

/clay nanocomposites

/Clay photocatalyst

/

47

duction of CO<sup>2</sup>

TiO2

TiO2

of TiO2

TiO2

nanotubes (TiO2

**2.6. Immobilized TiO<sup>2</sup>**

tion of TiO2

Another drawback of TiO<sup>2</sup>

with H2

/nitrogen (N) doped reduced graphene oxide (TiO<sup>2</sup>

that the quantity and configuration of N dopant in the TiO<sup>2</sup>

promoting photogenerated electron transfer that resulted in a higher CO<sup>2</sup>

yield (about 23.6% at an excitation wavelength of 320 nm) and GQDs/TiO<sup>2</sup>

degradation of methyl orange. It was found that the GQDs deposited on TiO<sup>2</sup>

properties. Moreover, the apparent rate constant of N, S-GQDs+rGO + TiO<sup>2</sup>

rGO demonstrated a synergistic effect, enhancing CO<sup>2</sup>

can expand the visible light absorption of TiO2

atrazine under solar light using a novel W-TiO2

photocatalytic activity of W-TiO2

[98] reported the preparation of TiO2

than that of an un-doped TiO2

times as higher than that of bare TiO2

times higher compared to rGO + TiO<sup>2</sup>

Furthermore, the photocatalytic activity of GQDs/TiO<sup>2</sup>

light irradiation. It was found that the S-GQDs+rGO + TiO<sup>2</sup>
