TiO 2 : A Semiconductor Photocatalyst

Titanium dioxide (TiO 2 ) is considered as an inert and safe material and has been used in many applications for decades. TiO 2 have been widely studied, due to its interesting general properties in a wide range of fields including catalysis, antibacterial agents, in civil as nano-paint (self-cleaning) and especially photocatalysis, and that affect the quality of life. Thus, the development of nanotechnologies TiO 2 nanoparticles, with numerous novel and useful properties, are increasingly manufactured and used. TiO 2 doped with noble metal are good candidates in the performance these applications. The fascinating physical and chemical features of TiO 2 depend on the crystal phase, size and shape of particles. For example, varying phases of crystalline TiO 2 have different band gaps that rutile TiO 2 of 3.0 eV and anatase TiO 2 of 3.2 eV, determine the photocatalytic performance of TiO 2 . This chapter explains basic information on TiO 2 and theoretical concepts of nanostruc-ture of TiO 2 nanoparticles as a semiconductor photocatalyst. the of the of in solid for


Introduction
An ideal photocatalyst should posseses following nature: biologically and chemically inert, inexpensive, nontoxic, stable towards photocorrosion and certainly highly active and suitable towards the visible/UV light photoreaction. An additional criterion for such ideal photocatalyst is better redox potential of the H 2 O/ • OH couple (EOP = 2.80 V) [1]. Among the available semiconductors, TiO 2 , ZnO, ZnS and WO 3 have gap energies sufficient for catalyzing a wide range of chemical reactions [2].
Binary metal sulphide semiconductors such as CdS, CdSe or PbS are regarded as unstable photocatalyst due to easy photoanodic corrosion property and their toxic nature [2,3]. Furthermore, the iron oxide semiconductors are not suitable as they readily perform the photocathodic reaction and finally produced corrosive materials [1]. ZnO on the other hand is known to have the similar quality of band gap energy like TiO 2 (3.0 eV) but it is highly unstable towards pH condition, where precipitate of Zn (OH) 2 is formed on the particle surface and resulted in photocatalyst deactivation [2].
Of these, titanium dioxide (TiO 2 ) is the most ideal semiconductor for photocatalytic destruction of organic compounds [2,4]. It also provides an excellent conciliation between catalytic performance and stabilization in aqueous media [5].

Titanium dioxide (TiO 2 )
The strong oxidation and reduction power of photoexcited TiO 2 was successfully discovered by Honda-Fujishima [6,7]. Since its innovation, extensive efforts have been made in the development of TiO 2 photocatalyst owing to its potential application in the removal of various types of organics and in both aqueous and air phase [8,9]. It resulted in flexible applicability either in the form of a suspension, or immobilization [10][11][12][13].
Regardless the high superior photocatalytic efficiency of TiO 2 , it is popular for its unique wider applications as electroceramic, antifogging agent and as therapy for cancer [14,15]. Moreover, it also acts as an antibacterial agent due to its strong oxidation activity and superhydrophilicity and was employed for electrolysis of water (H 2 O) to produce H 2 and for harvesting solar energy as dye-sensitized solar cells [16]. Additionally, it possesses superior pigmentary property, UV light absorption capability and durability [17]. The high refractive indices of rutile and anatase phase of TiO 2 laid a pathway for its versatility nature.
In general, the crystallinity, impurities, specific surface area and density of the surface hydroxy groups are the physical parameters that have influence over the photocatalytic activity of TiO 2 . Apart from that the band gap energy, charge-carrier and recombination as well play a crucial role for photoactivation [18,19]. Among the available phases the anatase is the most stable form with 8-12 kJ/mol. This describes the quantity of energy transferred during phase transformation. The rest phases such as brookite is very uncommon [14,[20][21].
In the terms of density, rutile is greater than anatase, i.e., 4.26 and 3.90 g/mL respectively. However, in terms of utilization and ability, anatase phase is more efficient due to its open structure [14]. The anatase phase remains as most active photocatalyst with greater stability after repeated catalytic cycles illuminated under a photon energy between 300 nm < λ < 390 nm [22]. In addition, it is also chemically stable in aqueous media and in broad range of pH (0 < pH < 14) [23]. Hence TiO 2 with its major anatase phase had more applicability as photocatalyst [24]. Furthermore, it also exhibits relatively high reactivity and chemical stability only under UV light (λ < 387 nm) [7]. Thus, its limited utilization of the activation energy becomes a major drawback. Therefore, to overcome this limitation of TiO 2 , an extensive research needs to be emphasized to develop a titania based photocatalyst that can exhibit high reactivity under visible light spectrum (λ > 400 nm) and can persuades the utilization of solar spectrum, even under poor luminance of interior lighting [7].
The reduction in energy gap between the CB and VB further leads to the easy recombination of the formed electrons and holes. This recombination further decreases the interfacial charge-carrier transfer. The size of the TiO 2 further causes difficulties in separating them from the waste's stream. All these issues stress the reseachers to find a suitable composition/dopant for its modification [25].

Modification of TiO 2 photocatalyst
The photoactivity of TiO 2 is hindered by the narrow UV wavelength spectrum for photoactivation under ambient conditions. The generation of this UV requires additional power source, which shoots the activation cost of the photocatalyst. Therefore, the utilization of renewable energy source could be a better sustainable choice for the photoactivation [26] This is a study on the effect of the photoreduction of dinitrogen in a gassolid regime using Cr as a dopant for phenol degradation. The prepared Cr-TiO 2 showed an enhanced activity and displacement of the Fermi level at the TiO 2 interface. But the presence of Cr as donor ion did not favor the charge separation with respect to pure TiO 2 . [34] Iron (Fe) Fe 3+ ion has a unique half-filled electronic structure which resulted in enhanced activity compared to Fe 4+ and Fe 2+ . The stable Fe 3+ ion detrapped the electron and hole to adsorbed oxygen and surface hydroxyl group, thereby surpressed the electron-hole recombinations. [35] This study is about the effect of Fe doping into TiO 2 for the degradation of chloroform (CHCl 3 ). It observed that Fe provided shallow trapping sites for the charge-carriers and increase the photocatalytic efficiency. It also found that Fe can act as trap for both electron and hole, at high dopant concentration.
[ 36] This study is about the Fe-doped with TiO 2 for the degradation of 1, 2-dichloroethane. The study proved that Fe 3+ alleviates the surface phenomenon and act as both electron-hole traps. [37] The effect of Fe doping for the inactivation of E. coli was studied. The Fe-doped TiO 2 proved the enhancement in the inactivation of E. coli. Fe played as a source for the inhibition of the anatase crystal growth. [38]

Molybdenum (Mo)
The Mo as a cationic dopant was studied in 2009. The dopant increased the surface acidity of TiO 2 and accelerated the interfacial charge transfer process. Thus, prepared doped photocatalyst degraded the cationic dyes, rhodamine B (RhB) and methylene blue (MB) and anionic dyes, methyl orange (MO) and congo red (CR) at both alkaline and acidic pH. [39] Zinc (Zn) Similarly, the same group studied the effect of Zn as a dopant in 2010 and applied the prepared photocatalyst for the degradation of CR. They produced a small crystallite size and stable filled electronic configuration of Zn 2+ -TiO 2 . Moreover, this photocatalyst provided a shallow trap for the charge-carriers contributing to the overall activity.  Researchers prepared a narrow band gap N-TiO 2 with oxygen vacancies. Thus, prepared N-TiO 2 failed to oxidize formate and NH 3 -OH + under visible light illumination. Their preparation method resulted the N as a weak anion donor. [44] This is the study about the preparation of N-TiO 2 by thermal decomposition of the Ti 4+ -bipyridine complex and had moderately better removal of NO x under artifical visible light. [45] The researchers prepared TiO 2x N x by solvothermal route, produced violet color of particles and further calcination between 200 and 800°C produced weak violet, bright yellow, weak yellow and gray; no white color of particles. Their findings proved the thermal stability of Ti-N bonding with TiO 2x N x . [46] N-TiO 2 was prepared using Ti-melamine and Ti-salen complex as precursors and reported a higher photoactivity was obtained for the Ti-melamine by degrading MB degradation. The reasoned for the low photoactivity of the Ti-salen complex was due to the low amount of N doping and smaller Ti-N bonds. [47] N-doped layered titanates (Cs 0. 68  Multitype N-TiO 2 containing both subtitutional (N-Ti-O and Ti-N-O) and interstitial NO characteristics was prepared. The subtitutional and interstitial technique resulted in intraband states at 0.14 and 0.73 eV above the VB resulting in faster visible light driven photodegradation of gas phase toluene. [49] Carbon (C) A carbonaceous coke-like species embedded in the TiO 2 matrix was obtained by pyrolysis of alcohol in the sol-gel titania processing and calcined at 250 and 400°C. The resulted C-TiO 2 calcined at 250°C exhibited a maximum photoactivity for degradation of 4-chlorophenol (4-CP) under visible light than that the later. Further calcination to the higher tempearature caused the elimination of carbon from the titania matrix. Thus, showed significance of the calcination temperature. [50] A rutile TiO 2 doped with carbon was prepared by a pyrolysis method using combustion products namely CO 2 and H 2 O and used for water splitting reaction. Their doping method resulted in a band gap of 2.32 eV against 3.0 eV and showed the supremacy of the preparation method. [51] TiO 2 -G (titania-glycine) was successfully prepared through solution combustion route by glycine (G), hexemethylene (H) and oxalyldihydrazide (O) as fuel. The precursors employed contributed for the carbide ion and thus reduced band gap energy and resulted in a superior photoactivity, crystallinity and surface area. The photoactivity was successfully experimented by degrading aqueous phase MB under solar irradiation. [52] TiO 2 -C was prepared through a simple carbon sources, which is tetrabutylammonium hydroxide (C 16  Attempt to shift the optical response of TiO 2 from the UV to the visible spectrum will have a profound positive effect on the photocatalytic efficiency [27]. The visible light reception potential of these photocatalysts is either highly unstable under their illumination (e.g. CdS and CdSe) or have limited photoactivity (e.g. WO 3 and Fe 2 O 3 ) [26]. Therefore, doping/hybridizing TiO 2 with transition metals like Fe, Co, Ni, etc. [28], non-metals like N, O, C, B, etc. [29], noble metals like Ag, Pt, Au or its metal ions incorporation [30], incorporating carbon nanotubes [31] and dyed sensitizers [32] have been proposed to improve its photocatalytic efficiency under solar light.
These modifications either by doping or hybridization are based on the concept of balancing both the half-reaction rates of the photocatalytic reaction by adding electron acceptor, or modifying the photocatalyst structure and composition [26]. The enhancement shown by doped ions is by providing charge trapping sites which leads to the reduction of electron-hole recombination [33]. In addition, the excited electron is expelled by the electron acceptors and hence hinders the recombination of electron-hole pairs [26].
In general, parameters such as types of doping/hybrid, its concentration, synthesizing technique and physico-chemical characteristics of the photocatalyst will have control over the photocatalytic activity. It was observed that the formation of both free • OH and active O 2 species is improved with the increase in charge separation efficiency [26]. The detailed literature reviews of these metals and non-metals dopant/hybrid are tabulated in Tables 1 and 2 respectively.

Modification of TiO 2 with graphene oxide
The incorporation of highly conductive carbon materials can also enhance the electron-hole pair separation of the photocatalyst. Graphene oxide commonly known as GO, is one such high conductive carbon materials that can be employed as a dopant/hybrid. Graphene can be regarded as the origin of all graphitic forms and can be curled, rolled or stacked to shape buckyball fullerenes, carbon nanotubes or graphite [58][59][60]. With free-standing 2-dimensional (2D) crystal and one-atom thickness properties, it has emerged with wide applications in several fields but employed mostly in nanotechnology for the improvisation of materials chemistry [61]. The unique single atom-thick planar sheet of sp 2 hybridized carbon atoms contribute to efficient storing and shuttling of electrons [61]. Moreover, it attracted the scientific community tremendously because of its distinctive electronic properties, superior chemical stability and soaring specific surface area [61].

Type Remark Reference
Fluorine (F) The researchers chose F as an anionic dopant and obtained F-TiO 2 via spray hydrolysis from fluorotitanic acid (H 2 TiF 6 ) precursor. Their work showed that F can excited under visible light but does not contributed for the intrinsic absorption properties rather that the extrinsic properties of TiO 2 absorption. [55] Boron (B) Boron was selected as a potential anion dopant and obtained B-TiO 2 which resulted in wider band gap energy, larger thermodynamic and faster charge-carrier transfer. [56] Phosphorus (P) Phosphorus as dopant was experimented via phosphoric acid (H 3 PO 4 ) and had higher photooxidation of n-pentane in air. It was due to the formation of Ti ions in tetrahedral coordination of TiO 2 .
[57] The exfoliated graphene sheets employ a theoretical surface area of around 2600 m 2 /g, and as a result graphene appears as an attractive high-surface area 2D photocatalyst support [62]. Besides it has potential ideal electron sinks or electron transfer bridges [63]. This was attributed to its exceptional structure that allow ballistic transport, in which electrons can travel without scattering at mobilities exceedingly approximately 15,000 m 2 /V/s at room temperature [63]. They are also foremost responsive for chemical doping, adsorbed or bound species and structure distortion [64]. Further incorporation of inorganic materials with modified graphene enormously improves their electronic, electrocatalytic and photocatalytic characteristics [32]. Thus, it proves the potential to enhance the fast electron transfer that highly benefits photocatalysis [32].
Recently modifying TiO 2 surface with carbonaceous materials propounded to induce visible-light responsive property. Few types of carbonaceous materials such as graphitic or coke-like carbon [50,65], or carbonate structural fragments bonding with titanium were employed for this purpose. Graphene oxide supported TiO 2 is expected to create synergistic effect that enhances the solar photocatalytic activity. The synergistic effect attributes to its unique separation efficiency of electrons and holes between TiO 2 and graphene oxide [32,66]. The photo-reductions initiated in the transformation of graphene oxide to graphene lays a platform for continuous electron conducting network through cross-surface charge percolation and permitted graphene to act as an efficient exciton sink [66].
Nguyen-Phan et al., [32] adopted a simple one-step colloidal blending method as an environmentally friendly that preserves the TiO 2 properties and combines the advantages of graphene oxide. The prepared composites showed superior adsorptivity and photocatalytic activity under both UV and visible light [32]. This was proved through a photocatalytic degradation study by adopting MB as model pollutant excited under artificial solar energy. The study indicated that graphene oxide acted as an adsorbent, electron acceptor and photosensitizer in the process of accelerating photodecomposition [32].
Recently Hu et al., 2012 also reported that graphene oxide/TiO 2 hybrid (GOT) demonstrated an excellent adsorption and photocatalysis performance under visible radiation by degrading MB dye under solar irradiation. The phenomenon was due to the electron sink in GOT that contributed for the photoactivity [67].

Noble metal deposited TiO 2 photocatalyst
Apart from these metals, non-metals and graphene oxide as dopant/hybrid, few reseachers have also utilized the noble metals like platinum (Pt), gold (Au) and silver (Ag) as a potential source of dopant. The expensive Pt deposition on both rutile and anatase All their study showed that Pt had beneficial effects for the photocatalytic oxidation. The Pt as a dopant/hybrid has increased the electron mobility rate of TiO 2 . This mobility has contributed for the electron transfer to the adsorbed oxygen (O − ads ) especially in rutile TiO 2 . A negligible such effect was observed in the anatase phase [68][69][70][71][72].
Owing to the versatile potential of the Au, it was also chosen as a dopant by the reseachers [73,74]. The study by reseachers showed that Au can behaves like a semiconductor rather than metallic. The size of Au plays a major role in the interfacial electron transfer to adsorbed oxygen (O − ads ). The Fermi level of the Au particle will be lowered than that of adsorbed oxygen (O − ads ) in TiO 2 and this depends on the particle size of Au. The Au also contributed for the hindering effect of the surface recombination. Thus, they exhibited an improvised visible light photoactivity. Finally, the influence of Ag as a dopant/hybrid was studied by Szabo et al., Christopher et al., 2010. Their study showed that Ag as a dopant contributed for the uniform morphology of the TiO 2 and increased the probability function of excited oxygen atoms via electron transfer from adsorbed oxygen (O − ads ) produced from O 2 2− to the hole. The Ag also further exhibited a unique Surface Plasmon Resonance (SPR) property. Thus, exhibited SPR lowered the electron-hole recombination rate. All these potentially improvised the photoactivity of TiO 2 with a better degradation of the pollutants either under natural or artificial visible light illumination [30,75].

Magnetic particles deposited TiO 2 photocatalyst
Though these noble metals dopant/hybrid had a significant contribution on the photocatalytic mechanism of TiO 2 , they are highly expensive and further increasing the cost of the prepared photocatalyst. Most of the modification techniques solve the issues of photocatalytic efficiency however, leaving behind the separation difficulty. Such separation could make them reuse and contribute for economy of the treatment cost. Most commonly these photocatalysts are nanoparticles and requires high speed centrifugation or membrane filtration. However, adopting these techniques further burdens the economy of the treatment. Hence doping/hybridizing the photocatalyst with a ferromagnetic material could be a best alternative.
Hence coating the photocatalyst with magnetic particles emerges to be a promising method to separate the photocatalyst from treated stream [76]. Nanoparticles combine with magnetic core and photoactive shell using magnetic granules and semiconductor photocatalyst is reported to possess magnetic property and separation could be achieved easily by applying simple magnetic field [77]. For these purpose magnetic cores such as magnetite (Fe 3 O 4 ), maghemite (Fe 2 O 3 ), nickel ferrite (NiFe 2 O 4 ), etc. were chosen. Though separation of photocatalysts was achieved their photocatalytic activity was found to decrease than that of pure TiO 2 [8,78,79].
Beydoun et al. prepared the magnetic photocatalyst by coating TiO 2 particles onto Fe 3 O 4 particles. They observed that the magnetic core was easily oxidized and suppressed the photoactivity of the TiO 2 [76,80,81]. Alternatively, Chen et al., 2001 prepared the magnetically separable photocatalyst by coating TiO 2 particles onto γ-Fe 2 O 3 particles. Their preparation method transformed ferromagnetic γ-Fe 2 O 3 to α-Fe 2 O 3 paramagnetic material and resulted in poor separation efficiency [79]. Such phase transformation from γ-Fe 2 O 3 to α-Fe 2 O 3 was triggered by annealing temperature. Therefore, difficulties arise to synthesize TiO 2 coated particles with high photoactivity without loss of magnetic property by an iron oxide phase transition, as well as of high crystallinity without agglomeration, or formation of impurities by solid diffusion [9]. Chung et al., 2004 reported a TiO 2 -coated NiFe 2 O 4 photocatalyst by multi-step ultrasonic spray pyrolysis method. Their complicated synthesize method resulted the photocatalyst in micron size. Owing to the micron size of the photocatalyst the activity and the separation efficiency declined drastically [4]. Similarly, Xu et al., 2007 prepared a magnetically separable nitrogen-doped photocatalyst, TiO 2x N x / SiO 2 /NiFe 2 O 4 (TSN) by a simple method, which consists of a NiFe 2 O 4 as magnetic core, a SiO 2 as magnetic barrier and nitrogen as visible-light-active dopant. Their prepared TSN was found to possess a great photocatalytic activity by removing MO in the presence of artificial UV and visible light illumination [9].
In recent years the M type hexaferrites, MFe 12 O 19 (M = Ba, Sr., Pb, etc) gained interest over the spinel ferrite (NiFe 2 O 4 ), since the magnetic properties of M type hexaferrites allow them to serve as highly stable permanent magnet. One such M type hexaferrites, strontium ferrite (SrFe 12 O 19 ), is regarded as an excellent magnetic material [82]. There was so far no report discussed about nanoscale hexaferrites as carriers for magnetic photocatalyst before Fu et al., 2006 synthesized TiO 2 / SrFe 12 O 19 composite nanoparticles with core-shell structure. Despite the fact that the photocatalytic activity of the composite is slightly lower than that of Degussa P25, the separation of composite particles was well achieved with an external magnetic field, thus proved the separation incapability of commercial photocatalyst Degussa P25 [82].
Researches on protective layer-coated permanent magnets nanoparticles have been studied for both fundamental magnetic investigations and practical engineering applications. In such investigations, coated nanoparticles attracted the attention as the coating hinders the nanocomposites from coarsening and agglomeration. In practical engineering applications, coating works well in magnetic applications as an insulate phase to achieve high electric resistivity and behaves as a binder to ease the consolidation of the nanoparticles [18,19].
Coating magnetic nanoparticles with silica (SiO 2 ) is becoming a significant topic in the research of magnetic nanocomposites. The formation of SiO 2 interlayer on the surface of magnetic nanoparticles helps to screen the magnetic dipolar attraction between magnetic nanoparticles. It also protects from leaching of the core magnetic materials during the dispersion in the aqueous phase. Moreover, SiO 2 coating could be easily activated to provide its surface with various functional groups due to the presence of abundant silanol groups in it. Finally, SiO 2 interlayer plays a very significant role in providing a chemically inert surface for magnetic layer. Hence inclusion of a protective SiO 2 coating will suppress the electron-hole recombination rate that occurs in the photocatalyst and benefits both the photo and magnetic activity.

Doped/hybrid TiO 2 photocatalytic mechanism
The basic mechanism of TiO 2 photocatalyst is described based on Figure 1, which inititated by the absorption of the photon hv 1 with energy equal to the band gap of anatase TiO 2 (3.20 eV). Electron-hole pair is produced on the surface of titania as schematized. As known, the electron from the CB is promoted and produced positive hole in VB. The excited state electrons and holes get trap in the metastable states as well as dissipate energy as heat. Besides, they also can react with the electron donor or acceptor adsorbed on its surface. Therefore, the • OH is produced with high oxidation potential which plays an important role in photocatalysis [1,17].
The interstitial metal doped TiO 2 on the other hand beneficially produced the new energy level in the band gap of TiO 2 by the dispersion of metal nanoparticles in titania matrix. As shown in Figure 1, electron from the CB can be excited from the defect state by photon energy equals to hv 2 . The improvement of the electrons trapping to inhibit the electron-hole recombination during its photoactivation as well described the additional advantageous of the metal ion doping. Thus, decreasing in the charge-carrier recombination resulted in enhanced photoactivity of the photocatalyst.
The modification of TiO 2 with the non-metal ion doping provides four main opinions regarding the changes in the nature mechanism of photocatalyst as described below [7]:  [84].
All these obtained modification in the mechanism will contribute for the better and sustainable treatment of the organic pollutants.

Conclusions
Though various studies have been carried out to find an ideal semiconductor photocatalyst, TiO 2 however remains as a benchmark and active photocatalyst among them and was proved in both laboratory and pilot studies. Other oxides such as ZrO 2 , SnO 2 , WO 2 and MoO 3 do not have the similar application prospects as TiO 2 due to the fact that these oxides are much less active, chemically and biologically instable Several commercial TiO 2 photocatalysts are produced worldwide. Among them Degussa P25®, an amorphous TiO 2 , emerged as the best photocatalyst due to its better utilization of the UV light. It has a phase ratio of 25:75 (rutile:anatase). It is also considered as standard photocatalyst for environmental applications. The wider band gap, greater recombination of electron-hole pair and low interfacial charger carrier transfer, limit the visible light or sustainable solar energy utilization of TiO 2 photocatalyst. The limitations were successfully achieved by synthesizing a new and modified TiO 2 -based composite nanophotocatalysts through a series of simple preparation processes. The nano-size morphology of the composite photocatalysts well created the quantum effect that improved the photocatalytic properties.