**2.1. Metal doping**

source. An efficient photocatalyst converts solar energy into chemical energy which can be used for environmental and energy applications such as water treatment, air purification,

Research in the development of efficient photocatalytic materials has seen significant progress in the last 2 decades with a large number of research papers published every year. Improvements in the performance of photocatalytic materials have been largely correlated with advances in nanotechnology. Of many materials that have been studied for

possesses may merits such as high photocatalytic activity, excellent physical and chemical stability, low cost, non-corrosive, nontoxicity and high availability [1–4]. The photocatalytic activity of titania depends on its phase. It exists in three crystalline phases; the anatase, rutile and brookite. The anatase phase is metastable and has a higher photocatalytic activity, while the rutile phase is more chemically stable but less active. Some titania with a mixture of both anatase and rutile phases exhibit higher activities compared to pure anatase and rutile phases [5–7]. When titania is irradiated with light of sufficient energy, electrons from the valence band are promoted to the conduction band, leaving an electron

tion band. The free electrons in the conduction band are good reducing agents while the resultant holes in the valence band are strong oxidizing agents and can both participate in

Titania however suffers from a number of drawbacks that limit its practical applications in photocatalysis. Firstly, the photogenerated electrons and holes coexist in the titania particle and the probability of their recombination is high. This leads to low rates of the desired chemical transformations with respect to the absorbed light energy [8, 9]. The relatively large band gap energy (~ 3.2 eV) requires ultraviolet light for photoactivation, resulting in a very low efficiency in utilizing solar light. UV light accounts for only about 5% of the solar spectrum compared to visible light (45%) [1, 10]. In addition to these, because titania is non-porous and has a polar surface, it exhibits low absorption ability for non-polar organic pollutants [10–13]. There is also the challenge to recover nano-sized titania particles from treated water

aggregation and agglomeration which affect the photoactivity as well as light absorption [15–18]. Several strategies have been employed in the open literature to overcome these draw-

visible region of the spectrum thereby increasing the utilization of solar energy; preventing the electron/hole pair recombination and thus allowing more charge carriers to successfully

well as preventing the aggregation and agglomeration of the nano-titania particles while easing their recovery from treated water. Several reviews have been published in recent years on the development of strategies to eliminate the limitations of titania photocatalysis [1, 19–25]. Most of these however focus on pollutant removal from wastewater, water splitting for hydro-

review some of the latest publications mainly covering the last 5 years, on strategies that have

conversion and reaction mechanisms [1, 21, 25–31]. In this chapter, we

backs. These strategies aim at extending the wavelength of photoactivation of TiO<sup>2</sup>

in regards to both economic and safety concern [14]. The TiO2

diffuse to the surface; increasing the absorption affinity of TiO<sup>2</sup>

conversion to hydro-

nanoparticles also suffer from

towards organic pollutants as

into the

; titania) has been extensively researched because it

, in the valence band and an excess of negative charge in the conduc-

self-cleaning surfaces, hydrogen production by water cleavage and CO<sup>2</sup>

carbon fuels.

deficiency or hole, h<sup>+</sup>

redox reactions.

gen production, CO<sup>2</sup>

photocatalysis, titanium dioxide (TiO<sup>2</sup>

38 Photocatalysts - Applications and Attributes

Metal doping has been extensively used to advance efforts at developing modified TiO<sup>2</sup> photocatalysts to operate efficiently under visible light. The photoactivity of metal-doped TiO<sup>2</sup> photocatalysts depends to a large extent on the nature of the dopant ion and its nature, its level, the method used in the doping, the type of TiO<sup>2</sup> used as well as the reaction for which the catalyst is used and the reaction conditions [32]. The mechanism of the lowering of the band gap energy of TiO2 with metal doping is shown in **Figure 1**. It is believed that doping TiO2 with metals results in an overlap of the Ti 3d orbitals with the d levels of the metals causing a shift in the absorption spectrum to longer wavelengths which in turn favours the use of visible light to photoactivate the TiO2 .

Doping of TiO2 nanoparticles with Li, Na, Mg, Fe and Co by high energy ball milling with the metal nitrates was found to widen the TiO2 visible light response range. In the Na-doped sample, Ti existed as both Ti4+ and Ti3+ and the conversion between Ti4+ and Ti3+ was found to prevent the recombination of electrons and (e<sup>−</sup> ) and holes (h+ ). The metal ion doping promoted crystal phase transformations that generated electrons (e<sup>−</sup> ) and holes (h+ ) [33]. Mesoporous TiO2 prepared by sol gel technique and doped with different levels of Pt (1–5 wt% nominal loading) resulted in a high surface area TiO2 with an enhanced catalytic performance in photocatalytic water splitting for the Pt-doped samples. The 2.5 wt%Pt-TiO<sup>2</sup> had showed the optimum catalytic performance and a reduction in the TiO2 band gap energy from 3.00 to 2.34 eV with an enhanced electron storage capacity, leading to a minimization of the electron-hole recombination rate [34]. Noble metal nanoparticles such as Ag [35], Pt [34], Pd [36], Rh [37] and Au [38] have also been used to modify TiO2 for photocatalysis and have been reported to efficiently hinder electron-hole recombination due to the resulting Schottky barrier at the metal-TiO2 interface. The noble metal nanoparticles act as a mediator in storing and transporting photogenerated electrons from the surface of TiO2 to an acceptor. The photocatalytic activity increases as the charge carriers recombination rate is decreased.

Haung et al. [44] prepared Pt/TiO2

anatase phase of Pt/TiO2

found that the TiO2

band gap of TiO2

effect with P25 TiO<sup>2</sup>

Indium-doped TiO2

The doping of TiO2

during the TiO2

lytic CO<sup>2</sup>

TiO2

photosensitizing the Rh/TiO2

values and found that the phase of TiO2

The decrease in the anatase composition of TiO2

Liu et al. [45] prepared the palladium doped TiO2

pollutant degradation was 7.3 times higher compared to TiO<sup>2</sup>

Repouse et al. [46] prepared a series of noble metal promoted TiO2

humic acid to considerably improve the reaction rate of Rh/TiO2

catalyst.

reduction activity of the In-TiO2

Numerous studies reported that doping of TiO<sup>2</sup>

the reduction peaks in Cr-doped TiO<sup>2</sup>

of the high surface area and extended light absorption range.

nanoparticles. Inturi et al. [48] compared the doping of TiO2

doping resulted in an increase in surface area because of suppression of TiO2

introduction of the impurity level below the conduction band level of the TiO2

synthesis. The light absorption ability of the In-TiO2

and found that the dispersion of the small metal crystallites on TiO2

tase phase showed better degradation efficiency than the Pt/TiO<sup>2</sup>

anatase/rutile intersection of a Pt/TiO2

nanoparticles from TiO2

resulted in the degradation rate decrease, suggesting that anatase composition in the Pt/TiO<sup>2</sup> system played a crucial role of increasing the photocatalytic degradation of Acid Red 1 dye.

reduction method and tested it the photocatalytic degradation of organic pollutant. It was

absorption of ultraviolet light also enhanced after using chemical reduction method, however, all these changes had no effect on degradation of organic pollutant. But the degradation

in the degradation of bisphenol A under solar irradiation. They also found the presence of

[48, 52], V [53], Cu [54], Ni [51] and Zn [55], has been studied by different research groups.

catalytic activity, attributable to a change in the electronic structure resulting in the absorption region being shifted from UV to visible light. The shift results from charge-transfer transition between the d electrons of the transition metals and the conduct or valence band of TiO2

Mo, Ce, Co, Cu, Ni, Y and Zr and it was found that Cr, Fe and V showed improved conversions in the visible region while, the incorporation of the other transition metals (Mn, Mo, Ce, Co, Cu, Ni, Y and Zr) exhibited an inhibition effect on the photocatalytic activity. The Cr-doped

in the reduction potential of titania and chromium. Therefore, the higher photocatalytic

 demonstrated a superior catalytic performance and the rate constant was found to be approximately 8–19 times higher than the rest of the metal doped catalysts. It was reported that

have recently been used for photocatalytic reduction of CO<sup>2</sup>

was significantly improved due to the deposition of Pd nanoparticles; the Pd/TiO<sup>2</sup>

prepared at various hydrolysis pH

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

anatase/rutile intersection.

) photocatalyst using chemical

(P25) by wet impregnation

but had a clearly adverse

was enhanced due to the

with transition metals improve the photo-

shifted to much lower temperatures, due to the increase

nanoparticles with Cr, Fe, V, Mn,

did not affect the optical

ana-

41

organic

[47]. Indium

particle growth

. The photocata-

as a consequence

obtained depended largely on the hydrolysis pH. The

, and the decrease in the composition of TiO<sup>2</sup>

Modified Titanium Dioxide for Photocatalytic Applications

sample had a lower recombination rate compared to the

(P25).

due to the longer recombination pathway. Though, the Pt/TiO<sup>2</sup>

(Pd-TiO2

grain size was reduced while the specific surface area increased and the

. The Pt-promoted catalyst exhibited the highest photocatalytic efficiency

photocatalyst. Fluorescence data revealed that humic acid is capable of

was about 8 time that of pure TiO2

with transition metals such as Cr [48], Co [48], Fe [48–50], Ni [48, 51], Mn

**Figure 1.** Band-gap lowering mechanism of metal-doped TiO2 .

In a recent review by Low et al. [21] the deposition of Au onto TiO2 surface is reported to result in electron transfer from photo-excited Au particles (> 420 nm) to the conduction band of TiO2 , which showed a decrease in their absorption band (∼550 nm) and the band was recovered by the addition of electron donors such as Fe2+ and alcohols. Zhang et al. [39] reported that the visible light activity of coupled Au/TiO2 can be ascribed to the electric field enhancement near the metal nanoparticles. Moreover, numerous researchers coupled Au and Ag nanoparticles onto TiO2 surface to use their properties of localized surface plasmonic resonance (LSPR) in photocatalysis [40]. Wang et al. [41] and Hu et al. [42] reported an improved photocatalytic performance due to the Pt nanoparticle which increased the electron transfer rate to the oxidant. It was observed that photocatalytic sacrificial hydrogen generation was influenced by several parameters such as platinum loading (wt%) on TiO2 , solution pH, and light (UV, visible and solar) intensities [43]. Moreover, complete discoloration and dye mineralization were achieved using Pt/TiO2 as catalyst; the results were attributed to the higher Pt content of the photocatalyst prepared with the highest deposition time. For Pt-TiO2 catalysts the best discoloration and dye mineralization were obtained over the catalyst prepared by photochemical deposition method and using 120 min of deposition time in the synthesis. These results may be due to the higher Pt content of the photocatalyst prepared with the highest deposition time.

Haung et al. [44] prepared Pt/TiO2 nanoparticles from TiO2 prepared at various hydrolysis pH values and found that the phase of TiO2 obtained depended largely on the hydrolysis pH. The anatase/rutile intersection of a Pt/TiO2 sample had a lower recombination rate compared to the anatase phase of Pt/TiO2 due to the longer recombination pathway. Though, the Pt/TiO<sup>2</sup> anatase phase showed better degradation efficiency than the Pt/TiO<sup>2</sup> anatase/rutile intersection. The decrease in the anatase composition of TiO2 , and the decrease in the composition of TiO<sup>2</sup> resulted in the degradation rate decrease, suggesting that anatase composition in the Pt/TiO<sup>2</sup> system played a crucial role of increasing the photocatalytic degradation of Acid Red 1 dye.

Liu et al. [45] prepared the palladium doped TiO2 (Pd-TiO2 ) photocatalyst using chemical reduction method and tested it the photocatalytic degradation of organic pollutant. It was found that the TiO2 grain size was reduced while the specific surface area increased and the absorption of ultraviolet light also enhanced after using chemical reduction method, however, all these changes had no effect on degradation of organic pollutant. But the degradation was significantly improved due to the deposition of Pd nanoparticles; the Pd/TiO<sup>2</sup> organic pollutant degradation was 7.3 times higher compared to TiO<sup>2</sup> (P25).

Repouse et al. [46] prepared a series of noble metal promoted TiO2 (P25) by wet impregnation and found that the dispersion of the small metal crystallites on TiO2 did not affect the optical band gap of TiO2 . The Pt-promoted catalyst exhibited the highest photocatalytic efficiency in the degradation of bisphenol A under solar irradiation. They also found the presence of humic acid to considerably improve the reaction rate of Rh/TiO2 but had a clearly adverse effect with P25 TiO<sup>2</sup> photocatalyst. Fluorescence data revealed that humic acid is capable of photosensitizing the Rh/TiO2 catalyst.

Indium-doped TiO2 have recently been used for photocatalytic reduction of CO<sup>2</sup> [47]. Indium doping resulted in an increase in surface area because of suppression of TiO2 particle growth during the TiO2 synthesis. The light absorption ability of the In-TiO2 was enhanced due to the introduction of the impurity level below the conduction band level of the TiO2 . The photocatalytic CO<sup>2</sup> reduction activity of the In-TiO2 was about 8 time that of pure TiO2 as a consequence of the high surface area and extended light absorption range.

In a recent review by Low et al. [21] the deposition of Au onto TiO2

several parameters such as platinum loading (wt%) on TiO2

photocatalyst prepared with the highest deposition time. For Pt-TiO2

visible light activity of coupled Au/TiO2

40 Photocatalysts - Applications and Attributes

**Figure 1.** Band-gap lowering mechanism of metal-doped TiO2

onto TiO2

achieved using Pt/TiO2

in electron transfer from photo-excited Au particles (> 420 nm) to the conduction band of TiO2

which showed a decrease in their absorption band (∼550 nm) and the band was recovered by the addition of electron donors such as Fe2+ and alcohols. Zhang et al. [39] reported that the

.

the metal nanoparticles. Moreover, numerous researchers coupled Au and Ag nanoparticles

photocatalysis [40]. Wang et al. [41] and Hu et al. [42] reported an improved photocatalytic performance due to the Pt nanoparticle which increased the electron transfer rate to the oxidant. It was observed that photocatalytic sacrificial hydrogen generation was influenced by

ible and solar) intensities [43]. Moreover, complete discoloration and dye mineralization were

oration and dye mineralization were obtained over the catalyst prepared by photochemical deposition method and using 120 min of deposition time in the synthesis. These results may be due to the higher Pt content of the photocatalyst prepared with the highest deposition time.

surface to use their properties of localized surface plasmonic resonance (LSPR) in

as catalyst; the results were attributed to the higher Pt content of the

surface is reported to result

, solution pH, and light (UV, vis-

catalysts the best discol-

can be ascribed to the electric field enhancement near

,

The doping of TiO2 with transition metals such as Cr [48], Co [48], Fe [48–50], Ni [48, 51], Mn [48, 52], V [53], Cu [54], Ni [51] and Zn [55], has been studied by different research groups. Numerous studies reported that doping of TiO<sup>2</sup> with transition metals improve the photocatalytic activity, attributable to a change in the electronic structure resulting in the absorption region being shifted from UV to visible light. The shift results from charge-transfer transition between the d electrons of the transition metals and the conduct or valence band of TiO2 nanoparticles. Inturi et al. [48] compared the doping of TiO2 nanoparticles with Cr, Fe, V, Mn, Mo, Ce, Co, Cu, Ni, Y and Zr and it was found that Cr, Fe and V showed improved conversions in the visible region while, the incorporation of the other transition metals (Mn, Mo, Ce, Co, Cu, Ni, Y and Zr) exhibited an inhibition effect on the photocatalytic activity. The Cr-doped TiO2 demonstrated a superior catalytic performance and the rate constant was found to be approximately 8–19 times higher than the rest of the metal doped catalysts. It was reported that the reduction peaks in Cr-doped TiO<sup>2</sup> shifted to much lower temperatures, due to the increase in the reduction potential of titania and chromium. Therefore, the higher photocatalytic efficiency of Cr/TiO<sup>2</sup> in the visible light can be attributed to strong interaction (formation of Cr-O-Ti bonds). Fe-doped TiO<sup>2</sup> nanoparticles were used in the visible light degradation of para-nitrophenol and it was found that the Fe-dopant concentration was crucially important in determining the activity of the catalyst. The maximum degradation rate of para-nitrophenol observed was 92% in 5 h when the Fe(3+) molar concentration was 0.05 mol%, without addition of any oxidizing reagents. The excellent photocatalytic activity was as a result of an increase in the threshold wavelength response as well as maximum separation of photogenerated charge carriers [49]. On the other hand, Fe-doped TiO<sup>2</sup> evaluated for solar photocatalytic activity for the degradation of humic acid showed a retardation effect for the doped catalysts compared to the bare TiO2 specimens, which could be attributed to surface complexation reactions rather than the reactions taking place in aqueous medium. The faster removal rates attained by using bare TiO2 could be regarded as substrate specific rather than being related to the inefficient visible light activated catalytic performance [50]. Ola et al. [56] reported that the properties of V doped TiO<sup>2</sup> were tuned towards visible light because of the substitution of the Ti4+ by V4+ or V5+ ions since the V4+ is centred at 770 nm while the absorption band of V5+ is lower than 570 nm. Moradi et al. [57] obtained high photocatalytic activity of Fe doped TiO2 and studied the effects of Fe3+ doping content on the band gap and size of the nanoparticles. It was found that the increase in the doping content decreased the band gap energy and particle size from 3.3 eV and 13 nm for bare TiO<sup>2</sup> to 2.9 eV and 5 nm for Fe10-TiO2 , respectively.

of TiO2

TiO2

lite size and accordingly, the doped TiO<sup>2</sup>

· − , HO<sup>2</sup> , H<sup>2</sup> O2 ).

appropriate for the extension of the photocatalytic activity of TiO2

**Figure 2.** Band-gap energy narrowing mechanism for non-metal-doped TiO2

.

reactive oxygen species (O2

**2.2. Non-metal doping**

nanoparticles generally increases by La3+ particle doping by diminishing the crystal-

Based on theoretical calculations, it was proposed that during the electrochemical process, new Ho-f states and surface vacancies were formed and may reduce the photon excitation energy from the valence to the conduction band under visible light irradiation. The photocatalytic activity under visible light irradiation was attributed not to ·OH but to other forms of

 nanoparticles have been comprehensively doped at the O sites with non-metals such as C [61], B [62], I [63], F [64], S [65], and N [66]. Non-metal dopants are reported to be more

pared to metal dopant [67, 68]. This can be ascribed to the impurity states which are near the valence band edge, however, they do not act as charge carriers, and their role as recombination centres might be minimized [53]. As shown in **Figure 2**, the mixing of the p states of

nanoparticle displayed higher adsorption capacity.

Modified Titanium Dioxide for Photocatalytic Applications

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

43

into visible region com-

The rare earth metals doped TiO2 catalyst also have good electron trapping properties which can result in a stronger absorption edge shift towards longer wavelength, obtaining narrow band gap. Bethanabotla et al. [58] carried out a comprehensive study on the rare earth doping into a TiO2 and found that the rare earth dopants improved the aqueous-phase photodegradation of phenol at low loadings under simulated solar irradiation, with improvements varying by catalyst composition. Differences in defect chemistry on key kinetic steps were given as the explanation for the enhanced performance of the rare earth doped samples compared to pure titania. Reszczyńska et al. [59] prepared a series of Y3+, Pr3+, Er3+ and Eu3+ modified TiO<sup>2</sup> nanoparticles photocatalysts and results demonstrate that the incorporation of RE3+ ions into TiO2 nanoparticles resulted in blue shift of absorption edges of TiO2 nanoparticles and could be ascribed to movement of conduction band edge above the first excited state of RE3+. Moreover, incorporated RE3+ ions at the first excited state interact with the electrons of the conduction band of TiO2 , resulting in a higher energy transfer from the TiO<sup>2</sup> to RE3+ ions. But observed blue shift could be also attributed to decrease in crystallite size of RE3+–TiO2 in comparison to TiO2 . The Y3+, Pr3+, Er3+ and Eu3+ modified TiO<sup>2</sup> nanoparticles exhibited higher activity under visible light irradiation compared to pure P25 TiO2 and can be excited under visible light in the range from 420 to 450 nm. In a similar work on rare earths (Er, Yb, Ho, Tb, Gd and Pr) titania nanotubes (RE-NTs), [60] the RE3+ species were found to be located at the crystal boundaries rather than inside the TiO2 unit cell and an observed excitation into the TiO2 absorption band with resulting RE3+ emission confirmed energy migration between the TiO<sup>2</sup> matrix and RE3+. The presence of the rare earth component was found to reduce recombination of the electrons and holes successfully by catching them and also by promoting their rapid development along the surface of TiO2 nanoparticles. Lanthanide ions doping did not impact the energy gap of TiO2 nanoparticles, however this enhanced the light absorption of catalyst. The surface range of TiO2 nanoparticles generally increases by La3+ particle doping by diminishing the crystallite size and accordingly, the doped TiO<sup>2</sup> nanoparticle displayed higher adsorption capacity. Based on theoretical calculations, it was proposed that during the electrochemical process, new Ho-f states and surface vacancies were formed and may reduce the photon excitation energy from the valence to the conduction band under visible light irradiation. The photocatalytic activity under visible light irradiation was attributed not to ·OH but to other forms of reactive oxygen species (O2 · − , HO<sup>2</sup> , H<sup>2</sup> O2 ).

### **2.2. Non-metal doping**

efficiency of Cr/TiO<sup>2</sup>

to the bare TiO2

V doped TiO<sup>2</sup>

bare TiO2

into a TiO2

TiO2

TiO2

TiO2

band of TiO2

rather than inside the TiO2

the surface of TiO2

Cr-O-Ti bonds). Fe-doped TiO<sup>2</sup>

42 Photocatalysts - Applications and Attributes

3.3 eV and 13 nm for bare TiO<sup>2</sup>

The rare earth metals doped TiO2

carriers [49]. On the other hand, Fe-doped TiO<sup>2</sup>

in the visible light can be attributed to strong interaction (formation of

specimens, which could be attributed to surface complexation reactions rather

could be regarded as substrate specific rather than being related to the inefficient

were tuned towards visible light because of the substitution of the Ti4+ by V4+

para-nitrophenol and it was found that the Fe-dopant concentration was crucially important in determining the activity of the catalyst. The maximum degradation rate of para-nitrophenol observed was 92% in 5 h when the Fe(3+) molar concentration was 0.05 mol%, without addition of any oxidizing reagents. The excellent photocatalytic activity was as a result of an increase in the threshold wavelength response as well as maximum separation of photogenerated charge

the degradation of humic acid showed a retardation effect for the doped catalysts compared

than the reactions taking place in aqueous medium. The faster removal rates attained by using

visible light activated catalytic performance [50]. Ola et al. [56] reported that the properties of

or V5+ ions since the V4+ is centred at 770 nm while the absorption band of V5+ is lower than

the effects of Fe3+ doping content on the band gap and size of the nanoparticles. It was found that the increase in the doping content decreased the band gap energy and particle size from

to 2.9 eV and 5 nm for Fe10-TiO2

can result in a stronger absorption edge shift towards longer wavelength, obtaining narrow band gap. Bethanabotla et al. [58] carried out a comprehensive study on the rare earth doping

tion of phenol at low loadings under simulated solar irradiation, with improvements varying by catalyst composition. Differences in defect chemistry on key kinetic steps were given as the explanation for the enhanced performance of the rare earth doped samples compared to pure titania. Reszczyńska et al. [59] prepared a series of Y3+, Pr3+, Er3+ and Eu3+ modified TiO<sup>2</sup> nanoparticles photocatalysts and results demonstrate that the incorporation of RE3+ ions into

ascribed to movement of conduction band edge above the first excited state of RE3+. Moreover, incorporated RE3+ ions at the first excited state interact with the electrons of the conduction

range from 420 to 450 nm. In a similar work on rare earths (Er, Yb, Ho, Tb, Gd and Pr) titania nanotubes (RE-NTs), [60] the RE3+ species were found to be located at the crystal boundaries

The presence of the rare earth component was found to reduce recombination of the electrons and holes successfully by catching them and also by promoting their rapid development along

nanoparticles, however this enhanced the light absorption of catalyst. The surface range

unit cell and an observed excitation into the TiO2

nanoparticles. Lanthanide ions doping did not impact the energy gap of

and found that the rare earth dopants improved the aqueous-phase photodegrada-

570 nm. Moradi et al. [57] obtained high photocatalytic activity of Fe doped TiO2

nanoparticles resulted in blue shift of absorption edges of TiO2

. The Y3+, Pr3+, Er3+ and Eu3+ modified TiO<sup>2</sup>

visible light irradiation compared to pure P25 TiO2

, resulting in a higher energy transfer from the TiO<sup>2</sup>

blue shift could be also attributed to decrease in crystallite size of RE3+–TiO2

with resulting RE3+ emission confirmed energy migration between the TiO<sup>2</sup>

nanoparticles were used in the visible light degradation of

evaluated for solar photocatalytic activity for

, respectively.

nanoparticles exhibited higher activity under

and can be excited under visible light in the

catalyst also have good electron trapping properties which

and studied

nanoparticles and could be

to RE3+ ions. But observed

in comparison to

absorption band

matrix and RE3+.

TiO2 nanoparticles have been comprehensively doped at the O sites with non-metals such as C [61], B [62], I [63], F [64], S [65], and N [66]. Non-metal dopants are reported to be more appropriate for the extension of the photocatalytic activity of TiO2 into visible region compared to metal dopant [67, 68]. This can be ascribed to the impurity states which are near the valence band edge, however, they do not act as charge carriers, and their role as recombination centres might be minimized [53]. As shown in **Figure 2**, the mixing of the p states of

**Figure 2.** Band-gap energy narrowing mechanism for non-metal-doped TiO2 .

the doped non-metal with the O2p states shifts the valence band edge upward and narrows the band-gap energy of the doped TiO2 photocatalyst. The nitrogen and carbon doped TiO2 nanoparticles has been reported to exhibit greater photocatalytic activity under visible light irradiation compared to other non-metal dopants.

be more active than N-TiO<sup>2</sup>

light active C-TiO<sup>2</sup>

, therefore, C-TiO<sup>2</sup>

tively. Ji et al. [61] reported the preparation of C-TiO<sup>2</sup>

Although single metal doped and non-metal doped TiO2

can be altered by co-doping on TiO2

can either accept photogenerated electrons from TiO2

nanowires than that of N,F-codoped TiO<sup>2</sup>

formance in the presence of N,F-codoped TiO<sup>2</sup>

longer wavelengths. Therefore, suggesting that the TiO<sup>2</sup>

tube wall was composed of anatase TiO2

in photocatalytic activity than bare TiO2

ment doping into the lattice of TiO<sup>2</sup>

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

two kinds of atoms into TiO<sup>2</sup>

ited the growth of TiO2

properties of C, N, F codoped TiO<sup>2</sup>

Zhao et al. [82] investigated the photocatalytic H2

light irradiation by N,F-codoped TiO<sup>2</sup>

of TiO2

TiO2

TiO2

et al. [76] investigated the photocatalytic removal of nonylphenol (NP) compound using visible

enhance the visible light utilization and affect the structural properties of the as-synthesized photocatalysts. Moreover, it was reported that after C doping and changing the calcination temperature, the band gap was narrowed from 3.17 to 2.72 eV and from 2.72 to 2.66 eV, respec-

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

gap. Abdullah et al. [77] reported that the doping levels situated within the band gap of TiO2

Zang et al. [79] evaluated the photocatalytic degradation of atrazine under UV and visible

found that photocatalytic degradation of atrazine was higher in the presence of N,F-codoped

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

provided higher surface area, active charge transfer and decreased charge recombination.

trode under visible light irradiation. Chatzitakis et al. [81] studied the photoelectrochemical

followed by increase in the photoconversion efficiency, but rather that an optimal balance

nanoparticles and improved the optical response of the photoelec-

nanotubes. It was found that increasing surface area is not

evolution performance of Ir-C-N tridoped

increasing the recombination of separated charges before they reach the TiO2

trodes for dye-sensitized solar cells. It was found that the Cu/N-doped TiO<sup>2</sup>

between electroactive surface area and charge carrier concentration occurs.

under UV-visible light irradiation. The photocatalytic activity of TiO<sup>2</sup>

Moreover, the addition of suitable content of Cu- to N-doped TiO<sup>2</sup>

surface. Park et al. [80] showed the best performance for novel Cu/N-doped TiO<sup>2</sup>

. The C-TiO<sup>2</sup>

light adsorption toward longer wavelength and hindered charge recombination.

ascribed to the C doping, which narrowed the band gap energy of TiO<sup>2</sup>

with anatase/rutile. It was found that the doping of C into TiO<sup>2</sup>

has received special attention [75]. Noorimotlagh

Modified Titanium Dioxide for Photocatalytic Applications

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

with a diameter of around 200 nm and the

have exhibited excellent performance

valence band or absorb photons with

absorption range can be expanded.

nanotubes exhibited much better performance

, amorphous carbon, crystalline carbon and carbon ele-

under UV and visible light. The obtained results were

by formation of new doping levels inside its band

nanowires and nanoparticles in aqueous phase. It was

nanoparticles. The higher photocatalytic per-

nanowires was attributed to the higher charge

has recently attracted much interest [78]. The electronic structure

lattice may

45

, extended the visible

nanoparticles

nanoparticles

nanoparticles was

electrode effectively inhib-

photoelec-

N-doped TiO<sup>2</sup> (N-TiO<sup>2</sup> ) appears to be the most efficient and extensively investigated photocatalyst for non-metal doping. Zeng et al. [69] reported the preparation of a highly active modified N-TiO<sup>2</sup> nanoparticle via a novel modular calcination method. The excellent photocatalytic 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. Phongamwong et al. [70] investigated the photocatalytic activity of CO<sup>2</sup> reduction under visible light over modified N-TiO<sup>2</sup> photocatalyst and they have found that the band gap of N-TiO<sup>2</sup> photocatalyst slightly decreases with increasing N content. In addition, the sub-band energies related to the impurity energy level were observed in the N-TiO<sup>2</sup> photocatalyst because of the interstitial N species and the sub-band gap energies were found to have decreased from 2.18 eV with 10 wt% N-TiO<sup>2</sup> photocatalyst. In contrast, the replacement of O by N is difficult 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 oxygen vacancies by two nitrogen atom [71]. N-TiO<sup>2</sup> photocatalyst reduces the oxygen energy 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 similar ionic radius [73]. Yu et al. [64] reported that the F-doped TiO2 (F-TiO2 ) is able to absorb visible light due to the high-density states that were evaluated to be below the maxima valence band, although there was no shift in the band edge of TiO<sup>2</sup> . Samsudin et al. found a synergistic effect between fluorine and hydrogen in hydrogenated F-doped TiO<sup>2</sup> which enabled light 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 Ti3+ occupied states under the conduction band of the hydrogenated F-doped TiO2 , thus narrowing the band gap energy [73]. Enhanced photocatalytic performance of N-doped TiO<sup>2</sup> over pure TiO2 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 TiO2 with S, since replacement of S into TiO<sup>2</sup> can be performed easily due to larger radius of S atoms (18 nm) compared to O atoms (14 nm). S incorporation in TiO2 has been reported to change the lattice spacing of the TiO<sup>2</sup> with a reduction in the band gap width from 3.2 to 1.7 eV allowing for higher photocatalytic activity [74]. N, S and C co-doped TiO<sup>2</sup> samples photocatalytic 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 crystallization of anatase and (N, S and C) co-doping. The carbon doped TiO<sup>2</sup> (C-TiO<sup>2</sup> ) is reported to be more active than N-TiO<sup>2</sup> , therefore, C-TiO<sup>2</sup> has received special attention [75]. Noorimotlagh et al. [76] investigated the photocatalytic removal of nonylphenol (NP) compound using visible light active C-TiO<sup>2</sup> with anatase/rutile. It was found that the doping of C into TiO<sup>2</sup> lattice may enhance the visible light utilization and affect the structural properties of the as-synthesized photocatalysts. Moreover, it was reported that after C doping and changing the calcination temperature, the band gap was narrowed from 3.17 to 2.72 eV and from 2.72 to 2.66 eV, respectively. Ji et al. [61] reported the preparation of C-TiO<sup>2</sup> with a diameter of around 200 nm and the tube wall was composed of anatase TiO2 , amorphous carbon, crystalline carbon and carbon element doping into the lattice of TiO<sup>2</sup> . The C-TiO<sup>2</sup> nanotubes exhibited much better performance in photocatalytic activity than bare TiO2 under UV and visible light. The obtained results were ascribed to the C doping, which narrowed the band gap energy of TiO<sup>2</sup> , extended the visible light adsorption toward longer wavelength and hindered charge recombination.
