**3. Titanium dioxide–based hybrid materials as active photocalatysts**

Photocatalysis is a phenomenon in which chemical reactions are accelerated by the action of light. The most important stage of the process is the light-induced excitation of electrons from the valence band to the conduction band. This takes place provided that the energy of the incident radiation is equal to or greater than the band gap of the photocatalyst and leads to the creation of electrons (e<sup>−</sup> ) and holes (h+ ). The electrons combine with atmospheric oxygen to produce active O<sup>2</sup> , while the holes combine with water or atmospheric water vapour to form OH• radicals. These hydroxide radicals are strong oxidising agents and can thus easily oxidise and decompose various organic pollutants (such as oils and fats). The active oxygen, on the other hand, triggers reduction reactions. In the photocatalysis process, oxidation and reduction reactions occur simultaneously. During photocatalysis, the created electrons and holes are subject to surface or voluminous recombination as well as taking part in redox reactions. The process of photocatalysis is affected by a number of factors: rate of reaction, mass of catalyst, wavelength, initial reagent concentration and luminous flux [75–80].

A key factor in the process is the photocatalyst used. Many semiconductor materials are available on the market for use in photocatalysis processes, but efforts are constantly being made to develop new materials that are highly active in the visible and near ultraviolet ranges, while also being biologically inert and photostable [81]. Among the wide range of photocatalysts in use, the most promising material is TiO<sup>2</sup> , in view of its high photochemical activity. It is also regarded as a cheap, nontoxic material that is photostable and chemically and biologically inert [82].

Research has been carried out to investigate photocatalytic activity using two forms of TiO<sup>2</sup> : anatase and rutile. The amorphous form of TiO<sup>2</sup> is considered to exhibit practically no such activity [83]. Photocatalytic activity is affected not only by the type of photocatalyst used, but above all by its physicochemical properties: specific surface area and pore type, degree of hydroxylation of the surface, particle size and degree of agglomeration and the degree of crystallinity and number of defects in the crystalline structure [84]. Differences in the performances of photocatalysts are attributed largely to physicochemical properties such as the width of the band gap, the rate of recombination of e<sup>−</sup> –h+ pairs and the number of hydroxyl groups on the TiO<sup>2</sup> surface.

The phase composition of the studied material is a very important factor in determining the photocatalytic activity of TiO<sup>2</sup> . It has been frequently reported that anatase exhibits much greater photoactivity than rutile [85–87]. Tanaka et al. [88] and Kumar et al. [89] have suggested that the higher activity of anatase results from its lower capacity to adsorb oxygen, the higher degree of hydroxylation of the TiO<sup>2</sup> surface and the high specific surface area, which provides more active sites for the adsorption of pollutants. Too high a surface area entails the presence of a large number of structural defects, which means that the recombination of charge carrier pairs proceeds much more rapidly. The recombination rate is slowed by a larger number of OH− groups on the photocatalyst surface, which trap the holes generated [83, 90–92].

Titanium dioxide is the most widely used catalyst for photocatalytic degradation of organic compounds, but there are some limitations in using TiO<sup>2</sup> for practical applications, which include:

• its large band gap;

products on semi-industrial or full industrial scale. The transfer of optimum conditions of synthesis from the laboratory to larger-scale processes is often problematic and should continue to

Photocatalysis is a phenomenon in which chemical reactions are accelerated by the action of light. The most important stage of the process is the light-induced excitation of electrons from the valence band to the conduction band. This takes place provided that the energy of the incident radiation is equal to or greater than the band gap of the photocatalyst and leads to

form OH• radicals. These hydroxide radicals are strong oxidising agents and can thus easily oxidise and decompose various organic pollutants (such as oils and fats). The active oxygen, on the other hand, triggers reduction reactions. In the photocatalysis process, oxidation and reduction reactions occur simultaneously. During photocatalysis, the created electrons and holes are subject to surface or voluminous recombination as well as taking part in redox reactions. The process of photocatalysis is affected by a number of factors: rate of reaction, mass of

A key factor in the process is the photocatalyst used. Many semiconductor materials are available on the market for use in photocatalysis processes, but efforts are constantly being made to develop new materials that are highly active in the visible and near ultraviolet ranges, while also being biologically inert and photostable [81]. Among the wide range of photocata-

is also regarded as a cheap, nontoxic material that is photostable and chemically and biologi-

Research has been carried out to investigate photocatalytic activity using two forms of TiO<sup>2</sup>

activity [83]. Photocatalytic activity is affected not only by the type of photocatalyst used, but above all by its physicochemical properties: specific surface area and pore type, degree of hydroxylation of the surface, particle size and degree of agglomeration and the degree of crystallinity and number of defects in the crystalline structure [84]. Differences in the performances of photocatalysts are attributed largely to physicochemical properties such as the

The phase composition of the studied material is a very important factor in determining the

greater photoactivity than rutile [85–87]. Tanaka et al. [88] and Kumar et al. [89] have suggested that the higher activity of anatase results from its lower capacity to adsorb oxygen, the

provides more active sites for the adsorption of pollutants. Too high a surface area entails

–h+

. It has been frequently reported that anatase exhibits much

surface and the high specific surface area, which

). The electrons combine with atmospheric oxygen

, in view of its high photochemical activity. It

is considered to exhibit practically no such

pairs and the number of hydroxyl

:

, while the holes combine with water or atmospheric water vapour to

**3. Titanium dioxide–based hybrid materials as active photocalatysts**

) and holes (h+

catalyst, wavelength, initial reagent concentration and luminous flux [75–80].

lysts in use, the most promising material is TiO<sup>2</sup>

anatase and rutile. The amorphous form of TiO<sup>2</sup>

width of the band gap, the rate of recombination of e<sup>−</sup>

surface.

higher degree of hydroxylation of the TiO<sup>2</sup>

be the subject of intensive research.

the creation of electrons (e<sup>−</sup>

to produce active O<sup>2</sup>

160 Titanium Dioxide

cally inert [82].

groups on the TiO<sup>2</sup>

photocatalytic activity of TiO<sup>2</sup>


Various strategies have been adopted for improving or enhancing the photocatalytic efficiency of TiO<sup>2</sup> (see **Figure 3**). These methods can be summarised as either morphological modifications, such as increasing surface area and porosity, or chemical modifications, by the incorporation of additional components into the TiO<sup>2</sup> structure. Modifications include:


**Table 3** contains information on selected methods of modifying titanium dioxide.

The absorption spectrum for titanium dioxide can be shifted towards the visible range by the incorporation of additional particles which cause significant changes in the material's semiconductor properties. Doping was the first technique used by researchers to modify the

**Figure 3.** Various modification methods for titania-based photocatalysts.



**Dopant**

 **Precursor** **Doping with non-metal**

Iron

 FeCl3∙6H

NH3∙H

Tungsten

 Ti(OC W(OC

**Doping with non-metal**

Nitrogen

 NH3 gas

TiO2 nanorods

Thermal treatment of TiO2 nanorods at

Morphology: sample NTR-60—TiO2 rods ∼50 nm in

Photooxidation of

[108]

thiol molecules to the

sulfonic acid species

length, sample NTR-150—

TiO2 rods ∼900 nm in length

and ∼95 nm in width, rutile

structure

500°C for 1 h in the presence of NH3 gas

prepared by

hydrothermal

route on a

fluorine-doped

tin oxide (FTO)-

coated glass

substrate

Trimethylamine

TiO2 nanorods

Hydrothermal treatment of TiO2 at 200°C for

Anatase structure,

Photodegradation

[109]

of methyl orange

(MO)—90% for

sample NTO-4

(doping with 3%w/w

crystallite size: is decreasing

from 22 to 10 nm with

increasing of N content,

band gap: is decreasing

from 3.22 to 2.85 eV with

N)

increasing of N content,

BET surface area: from 81 to

101 m2/g

120 min with different amount (from 1 to 5%

w/w compared to TiO2) of trimethylamine

synthesised

by the

hydrothermal

treatment

H2 5

)<sup>5</sup>

H2 5

)4, toluene,

–

The resulting solution was stirred for

30 min and then atomised; the substrate

temperature was kept at 500°C; deposition

time was 45 min

2

O, HCl

O, TiCl

2

3,

–

Solution containing TiCl3, ammonia and

Anatase structure, the

Photodegradation of

[106]

particle diameter decreases

phenol

(from 25 to 15 nm) while

the specific surface area

increases (from 50 to

76 m2/g) with the increasing

iron content

Anatase structure

Destruction of

[107]

resazurin redox dye

water was mixed at room temperature for

16 h, final pH = 8.5; hydrothermal treatment:

24 h at 110°C; drying: 80°C for ≥10 h,

calcination: 500°C for 4 h

**Type of matrix**

**Conditions of preparation**

**Properties of obtained** 

**Potential application**

 **Ref.**

162 Titanium Dioxide

**material**

Advanced Hybrid Materials Based on Titanium Dioxide for Environmental and Electrochemical... http://dx.doi.org/10.5772/intechopen.69357 163


**Table 3.** Modification of titanium dioxide.

electron structure of titanium dioxide [116]. Foreign ions or atoms are introduced into the titanium dioxide crystalline lattice with the aim of modifying and improving its physicochemical properties. The success of the process is largely dependent on the type and the quantity of dopant, which usually does not exceed a few percent. Both metals and non-metals may be used as dopants in titanium dioxide [83, 117–120].

The incorporation of metals (iron, chromium, tungsten, platinum, etc.) into titanium dioxide leads to a reduction in the band gap and thus an increase in absorption of radiation in the UV-Vis range. The activity of TiO<sup>2</sup> doped with metal ions depends largely on the valence of the dopant. If those ions have the same charge as the Ti ion in the crystal, the effect will be a change in the interactions between the metal atoms. The incorporation of metal ions of lower valence than Ti4+ (Y3+, La3+, Nd3+, Pd2+) favours a change in the size of the band gap and a reduction in the density of point defects [121–124].

Reports on the doping of titanium dioxide with iron confirm that the quantity of iron used, which may range from 0.05 to 50% Fe, has a significant effect on the increase in photocatalytic activity. In this process, a titanium atom in the anatase phase may be subject to substitution or else some of the iron atoms are incorporated into the titanium dioxide crystal lattice in anatase form to produce a composite, while some aggregates form the oxides Fe<sup>2</sup> O3 and Fe3 O4 . The doping process leads to the generation of shallow charge traps in the crystal structure, which decreases the recombination rate of electron-hole pairs. Introducing iron ions into the TiO<sup>2</sup> lattice not only leads to a lower electron-hole recombination rate but also increases excitability by visible light. Titanium dioxide doped with iron exhibits better photocatalytic activity under UV and also under visible light irradiation [106, 125–129].

The doping of TiO<sup>2</sup> with chromium not only increases its photocatalytic activity but also causes the photocatalyst to acquire ferromagnetic properties without losing its conductive properties. The enhancement of photocatalytic activity results from the formation of vast oxygen vacancies. The oxygen vacancies in TiO<sup>2</sup> act as electron traps which can bind the photoinduced electrons and play a significant role in inhibiting the recombination rate of photoinduced electron-hole pairs [130]. In turn, tungsten can be incorporated into the TiO<sup>2</sup> structure in oxide form (WO<sup>x</sup> ). This enhances the material's photocatalytic activity by reducing charge carrier recombination and by increasing light absorption in the visible portion of the spectrum [131].

Doping with non-metals is usually carried out to extend the photocatalytic activity of TiO<sup>2</sup> in the UV-Vis range. The introduction of non-metals into the oxygen sub-lattice may cause a change in the position of the valence band and thus reduce the band gap. Promising results have been obtained by doping titanium dioxide with non-metals such as nitrogen, carbon, iodine, sulphur and fluoride. Such doping narrows the band gap or leads to the appearance of new internal levels between the valence band and the conduction band [108–111]. When TiO<sup>2</sup> is doped with nitrogen, the dopant may replace an oxygen ion (in the case of a material with anatase structure) or a titanium ion (in the case of a rutile structure) [108, 109]. In case of doping with carbon, the dopant may replace oxygen or titanium or else occupy an inter-nodal position, depending on the energy of formation of the product and the presence of oxygen in the reaction environment. If doping is carried out in oxygen-rich conditions,

**Dopant**

 **Precursor** **Hybrids with nano-materials**

Graphene

glycol (EG)

Graphite powder,

–

Simple mixing at room temperature for 24 h

Anatase and rutile structure

rhodamine B

Photodegradation of

[115]

and sonication, drying: dried in oven at 60°C

titanium dioxide, HCl,

methanol, H2SO4,

KMnO4, H

methanol

**Table 3.** Modification of titanium dioxide.

O2

2, NaNO3,

Graphene oxide, poly(Llysine) (PLL), ethylene

TiO2 hollow

Solvothermal method

–

Decomposition of

[114]

MB dye

microspheres

**Type of matrix**

**Conditions of preparation**

**Properties of obtained** 

**Potential application**

 **Ref.**

164 Titanium Dioxide

**material**

substitution of carbon for titanium takes place or else it is incorporated in the inter-nodal position due to its small atomic size. If the environment contains little oxygen, the carbon atom takes the oxygen position, forming the structure C-Ti-O-C [117, 132, 133]. The doping of titanium dioxide with iodine, on the other hand, leads to increased visible light absorption and increased photocatalytic activity below the visible range. This phenomenon results from retardation of the recombination of electron-hole pairs due to the capture of electrons by the iodine. Substitution of iodine atoms for oxygen or titanium results in a narrowing of the band gap [134]. Doping titanium dioxide with sulphur is more difficult, due to the fact that the dopant replaces oxygen in the oxide crystal lattice, and the differences in the radii of the two atoms are significant [110, 111].

Reports on the doping of titanium dioxide indicate that a small quantity of dopant will not lead to major changes in the porous structure but may cause significant changes in the phase composition of TiO<sup>2</sup> and in the size of crystallites. In the case of doping with nitrogen, an increase in the quantity of the dopant has been found to increase the thermal stability of anatase and to alter the temperature of transformation of anatase to rutile [135–137].

Another method for modifying the electron structure of titanium dioxide is the formation of hybrid oxide systems. Among synthetic hybrid oxide systems, TiO<sup>2</sup> /ZrO<sup>2</sup> materials, which thanks to the addition of zirconium dioxide have much greater surface area and mechanical strength than pure TiO<sup>2</sup> , deserve special attention. An admixture of ZrO<sup>2</sup> prevents the phase change of anatase to rutile and causes a reduction in the particle diameter of the resulting hybrid. These factors contribute to the improved photocatalytic activity of TiO<sup>2</sup> /ZrO<sup>2</sup> oxide systems. There are many publications with information on the application of TiO<sup>2</sup> /ZrO<sup>2</sup> hybrids in photocatalysis. The use of TiO<sup>2</sup> /ZrO<sup>2</sup> hybrid materials in the photo-oxidation of organic compounds or degradation of dyes originating from various industrial processes is well known. It also has applications in the photo-reduction of atmospherically harmful oxides, like CO<sup>2</sup> and NO<sup>x</sup> , resulting for example from the combustion of fossil fuels. The advantages of the TiO<sup>2</sup> /ZrO<sup>2</sup> hybrid are its mechanical strength, nontoxicity and corrosion resistance and the ability to conduct photocatalytic processes using sunlight. These advantages may stimulate increasing demand for this material in the near future [59, 113, 138].

Zhou et al. [113] determined the photocatalytic properties of a TiO<sup>2</sup> /ZrO<sup>2</sup> system obtained by the sol-gel method. Physicochemical analysis showed the products to have an anatase crystalline structure. An increase in the molar content of zirconium dioxide leads to a decrease in the crystallinity of the resulting materials, while an increase in the temperature of calcination increases their crystallinity. The specific surface areas of the materials (for all variant methods of synthesis) lay in the range 187.0–219.2 m<sup>2</sup> /g. Photocatalytic analysis indicated a fall in the effectiveness of photocatalysis as the temperature of calcination of the materials was increased.

The sol-gel method was also used by Fan et al. [59] to obtain a TiO<sup>2</sup> /ZrO<sup>2</sup> system. It was found that the proposed method led to mesoporous materials with a well-crystallised anatase structure. The systems were found to have high specific surface areas, in the range 136.9–148.9 m<sup>2</sup> /g.

Combining titania with zinc oxide can also lead to a hybrid oxide system with good photocatalytic properties. The resulting material can be used, for instance, in the degradation of organic impurities such as detergents, dyes and pesticides present in various types of wastewater. The TiO<sup>2</sup> /ZnO hybrid material can be synthesised by both physical and chemical processes, which enables enhancement of its properties, for example by widening its light absorption spectrum. Additionally, the photocatalytic activity of oxides may help reduce the susceptibility of pollutants to form aggregate structures [139].

substitution of carbon for titanium takes place or else it is incorporated in the inter-nodal position due to its small atomic size. If the environment contains little oxygen, the carbon atom takes the oxygen position, forming the structure C-Ti-O-C [117, 132, 133]. The doping of titanium dioxide with iodine, on the other hand, leads to increased visible light absorption and increased photocatalytic activity below the visible range. This phenomenon results from retardation of the recombination of electron-hole pairs due to the capture of electrons by the iodine. Substitution of iodine atoms for oxygen or titanium results in a narrowing of the band gap [134]. Doping titanium dioxide with sulphur is more difficult, due to the fact that the dopant replaces oxygen in the oxide crystal lattice, and the differences in the radii of the two

Reports on the doping of titanium dioxide indicate that a small quantity of dopant will not lead to major changes in the porous structure but may cause significant changes in the phase

increase in the quantity of the dopant has been found to increase the thermal stability of ana-

Another method for modifying the electron structure of titanium dioxide is the formation of

thanks to the addition of zirconium dioxide have much greater surface area and mechanical

change of anatase to rutile and causes a reduction in the particle diameter of the resulting

/ZrO<sup>2</sup>

organic compounds or degradation of dyes originating from various industrial processes is well known. It also has applications in the photo-reduction of atmospherically harmful oxides,

the ability to conduct photocatalytic processes using sunlight. These advantages may stimu-

the sol-gel method. Physicochemical analysis showed the products to have an anatase crystalline structure. An increase in the molar content of zirconium dioxide leads to a decrease in the crystallinity of the resulting materials, while an increase in the temperature of calcination increases their crystallinity. The specific surface areas of the materials (for all variant methods

effectiveness of photocatalysis as the temperature of calcination of the materials was increased.

that the proposed method led to mesoporous materials with a well-crystallised anatase structure. The systems were found to have high specific surface areas, in the range 136.9–148.9 m<sup>2</sup>

Combining titania with zinc oxide can also lead to a hybrid oxide system with good photocatalytic properties. The resulting material can be used, for instance, in the degradation

, deserve special attention. An admixture of ZrO<sup>2</sup>

tase and to alter the temperature of transformation of anatase to rutile [135–137].

hybrid. These factors contribute to the improved photocatalytic activity of TiO<sup>2</sup>

systems. There are many publications with information on the application of TiO<sup>2</sup>

hybrid oxide systems. Among synthetic hybrid oxide systems, TiO<sup>2</sup>

late increasing demand for this material in the near future [59, 113, 138].

Zhou et al. [113] determined the photocatalytic properties of a TiO<sup>2</sup>

The sol-gel method was also used by Fan et al. [59] to obtain a TiO<sup>2</sup>

and in the size of crystallites. In the case of doping with nitrogen, an

, resulting for example from the combustion of fossil fuels. The advantages

hybrid are its mechanical strength, nontoxicity and corrosion resistance and

/ZrO<sup>2</sup>

hybrid materials in the photo-oxidation of

/ZrO<sup>2</sup>

/g. Photocatalytic analysis indicated a fall in the

/ZrO<sup>2</sup>

materials, which

prevents the phase

/ZrO<sup>2</sup>

system obtained by

system. It was found

/g.

oxide

/ZrO<sup>2</sup>

atoms are significant [110, 111].

composition of TiO<sup>2</sup>

166 Titanium Dioxide

strength than pure TiO<sup>2</sup>

and NO<sup>x</sup>

/ZrO<sup>2</sup>

like CO<sup>2</sup>

of the TiO<sup>2</sup>

hybrids in photocatalysis. The use of TiO<sup>2</sup>

of synthesis) lay in the range 187.0–219.2 m<sup>2</sup>

Cheng et al. [140] determined the photocatalytic properties of a hybrid material (UTZ) consisting of 3D nanospherical TiO<sup>2</sup> with a "hedgehog" shape and one-dimensional ZnO in the form of "nanospindles". The resulting system was highly homogeneous and contained the crystalline structure of anatase (TiO<sup>2</sup> ) and the hexagonal wurtzite structure (ZnO). The TiO<sup>2</sup> / ZnO system was found to offer significantly better photocatalytic performance than pure ZnO or TiO<sup>2</sup> in the decomposition of methyl orange (MO) and nitrophenol. This high photocatalytic activity was probably due to the existence of a closely bound heterostructural surface between the ZnO and TiO<sup>2</sup> , enabling charge separation and reducing the rate of recombination of electron-hole pairs.

The marked improvement in photocatalytic activity in the case of titania/graphene hybrids is linked to the fact that the graphene component enables the transfer and/or trapping of electrons photogenerated in the oxide semiconductor structure, allowing the holes to form reactive sites. Therefore, charge recombination is suppressed, leading to improvement of the photocatalytic performance [141].

Yan et al. [114] obtained a novel three-dimensional (3D) reduced graphene oxide/TiO<sup>2</sup> (rGO/TiO<sup>2</sup> ) hybrid composite by wrapping TiO<sup>2</sup> hollow microspheres with rGO sheets via a facile solvothermal route using poly(L-lysine) (PLL) and ethylene glycol (EG) as coupling agents. It was confirmed that the hybrid materials, containing mixed phases of TiO<sup>2</sup> (with content of rutile - 20.8%), demonstrate higher photocatalytic activity in the decomposition of MB dye. Ni et al. [142] synthesised high-photoactive GP strongly wrapped three-dimensional anatase TiO<sup>2</sup> . The prepared material demonstrated excellent photocatalytic activity under UV irradiation for the degradation of MB, much higher than that of commercial P25 titania. Similar results were presented by Thomas et al. [115], who synthesised high-performance functionalised FLG (FFLG) decorated with TiO<sup>2</sup> photocatalyst, by simple mixing without any calcination or high-pressure treatment. The FFLG/TiO<sup>2</sup> system produced a higher rate of degradation of Rhodamine B (Rhd B) as compared with pure TiO<sup>2</sup> nanoparticles and FLG-TiO<sup>2</sup> (non-functionalised FLG).

Although titanium dioxide is an excellent candidate for photocatalytic applications, due to its band gap size, nontoxicity, chemical stability, inert nature and relatively low cost, it is subject to certain limitations, chiefly resulting from its relatively low activity in the visible light range and its high exciton recombination rate. For this reason, much research is carried out with the aim of improving and reinforcing the photocatalytic activity of TiO<sup>2</sup> and increasing its spectral sensitivity. This may be achieved by modifying TiO<sup>2</sup> during or after its synthesis, with the choice of a suitable method of activation (doping with metals or nonmetals, coupling with other semiconductor materials, increasing its crystallinity by calcination, or synthesis of hybrid materials). These solutions can lead to a material with enhanced photocatalytic properties, including increased sensitivity in the UV and visible light ranges, and with reduced recombination rate due to the provision of charge traps. New research trends also relate to the combination of TiO<sup>2</sup> with polymers or various forms of carbon nanotubes, fullerenes, graphene oxide (GO) or reduced GO (R-GO), with the aim of obtaining multifunctional materials with a wide range of applications. With this in mind, it should be emphasised how many opportunities and technological solutions are available for implementation with the goal of obtaining unique titania-based materials.
