Why tio2 for photocatalysis

An efficient photocatalyst converts solar energy into chemical energy which can be used for environmental and energy applications such as water treatment, air purification, self-cleaning surfaces, hydrogen production by water cleavage and CO 2 conversion to hydrocarbon fuels. 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 photocatalysis, titanium dioxide TiO 2 ; titania has been extensively researched because it possesses may merits such as high photocatalytic activity, excellent physical and chemical stability, low cost, non-corrosive, nontoxicity and high availability [ 1 , 2 , 3 , 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 , 6 , 7 ]. 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 redox reactions.

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 ]. 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 , 11 , 12 , 13 ].

There is also the challenge to recover nano-sized titania particles from treated water in regards to both economic and safety concern [ 14 ].

The TiO 2 nanoparticles also suffer from aggregation and agglomeration which affect the photoactivity as well as light absorption [ 15 , 16 , 17 , 18 ]. Several strategies have been employed in the open literature to overcome these drawbacks. Several reviews have been published in recent years on the development of strategies to eliminate the limitations of titania photocatalysis [ 1 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ].

Most of these however focus on pollutant removal from wastewater, water splitting for hydrogen production, CO 2 conversion and reaction mechanisms [ 1 , 21 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. In this chapter, we review some of the latest publications mainly covering the last 5 years, on strategies that have been researched to overcome the limitations of TiO 2 for general photocatalytic applications and the level of success that these strategies have been able to achieve.

Based on the current level of research in this field, we also present some perspectives on the future of modified TiO 2 photocatalysis. A large number of research works have been published on TiO 2 modification to enhance its photocatalytic properties. These modifications have been done in many different ways which include metal and non-metal doping, dye sensitization, surface modification, fabrication of composites with other materials and immobilization and stabilization on support structures.

The properties of modified TiO 2 are always intrinsically different from the pure TiO 2 with regards to light absorption, charge separation, adsorption of organic pollutants, stabilization of the TiO 2 particles and ease of separation of TiO 2 particles. Metal doping has been extensively used to advance efforts at developing modified TiO 2 photocatalysts to operate efficiently under visible light.

The photoactivity of metal-doped TiO 2 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 2 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 TiO 2 with metal doping is shown in Figure 1. It is believed that doping TiO 2 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 TiO 2.

Band-gap lowering mechanism of metal-doped TiO 2. The 2. Noble metal nanoparticles such as Ag [ 35 ], Pt [ 34 ], Pd [ 36 ], Rh [ 37 ] and Au [ 38 ] have also been used to modify TiO 2 for photocatalysis and have been reported to efficiently hinder electron-hole recombination due to the resulting Schottky barrier at the metal-TiO 2 interface. The noble metal nanoparticles act as a mediator in storing and transporting photogenerated electrons from the surface of TiO 2 to an acceptor.

The photocatalytic activity increases as the charge carriers recombination rate is decreased. In a recent review by Low et al. Zhang et al. Moreover, numerous researchers coupled Au and Ag nanoparticles onto TiO 2 surface to use their properties of localized surface plasmonic resonance LSPR in photocatalysis [ 40 ].

Wang et al. For Pt-TiO 2 catalysts the best discoloration and dye mineralization were obtained over the catalyst prepared by photochemical deposition method and using 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. Liu et al. It was found that the TiO 2 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.

Repouse et al. The Pt-promoted catalyst exhibited the highest photocatalytic efficiency in the degradation of bisphenol A under solar irradiation. Drawing on Membrane Photocatalysis for Fouling Mitigation. Chemistry of Materials , 33 6 , Prezhdo, Jin Zhao.

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The band gap to less than 3. Photocatalytic activity of TiO 2 Nanomaterial The synthesis of TiO 2 nanoparticles by a combined sol—gel ball milling method has been done by Farbod and Khademalrasool [ 81 ]. Photocatalytic activity of doped - TiO 2 The performance of photocatalyst can be improved by depositing or incorporating metal ion or non-metal dopants into the TiO 2 [ 90 , 91 ].

Doping concentration. Au MB, Dye Acid-7 , 3. Pd Organic Waste, 99 6. Sn Pencillin, MB , 7. Sb MO 8. Co MB 9. WO 3 Organophosphorous pesticides La Formic acid Cd MO V RhB, 3,4-chloroaniline , Ce 3,4-chloroaniline, Nitrogen MB, MO , , , Sulphure MO, MB , Floride Phenol, Chlorophenol , Photocatalytic activity of Coupled TiO 2 6.

Nanocomposite Organic Pollutant Ref. ISBN: [ Links ] It is considered that the photocatalytic activity can be increased with increasing the concentration of the catalyst [ 14 ]. However, the level of catalyst concentration has to be determined because the excess catalyst prevents the diffusion of the light into the solution which might result in unfavorable light scattering [ 15 ].

As a photocatalyst, titanium dioxide TiO 2 has attracted substantial attention for a long time and is considered as one of the most promising materials for commercial use due to its outstanding optical and electronic properties, photoactivity, high chemical stability, low cost, nontoxicity, reusability, and eco-friendliness [ 8 , 16 , 4 ]. Much more attention has been focused on the different preparation methods of TiO 2 in order to improve its performance compared to commercial TiO 2 powders for example, Degussa P The crystallographic structure of TiO 2 is shown in Figure 5 [ 17 ].

Well-known phases of TiO 2 are anatase, rutile, and brookite. Among all these phases, anatase is the most active allotropic phase regarding photocatalytic activity when compared to rutile, brookite, and TiO 2 -B artificial phase.

Rutile is thermodynamically stable at ambient conditions, whereas anatase is kinetically stable, and it transforms to rutile at higher temperatures [ 18 ] depending on the particle size, ambient pressure, and other parameters [ 19 ]. Brookite phase is also metastable but difficult to synthesize, hence it is seldom studied [ 20 ].

Table 2 represents the oxide polymorphs of TiO 2 with experimental condition and method for synthesis as well as particle size.

At room temperature, the rutile phase is thought to be a more stable phase as compared to the anatase in the case of bulk TiO 2. The properties of brookite are poorly known because of the difficulties associated with synthesis of pure brookite phase.

Tay et al. Two primary factors must be taken into account while designing the systems for conducting photocatalytic reactions. The photocatalytic efficiency can be enhanced significantly by addressing the following primary issues: i increase the time of charge separation and recombination, ii broaden the solar spectrum range for photocatalyst, and iii changing the yield of a particular product [ 1 , 8 ].

Semiconductor photocatalysis system can be designed by considering the stability of the material under the light, product selectivity, and activation wavelength range [ 8 ]. TiO 2 has been the most widely studied photocatalyst for many years. Another major drawback of TiO 2 is photogenerated electron-hole recombination, which deteriorates the photocatalytic activity [ 27 ].

Therefore, an apparent goal is the reducing bandgap of TiO 2 in order to shift the absorption band to the visible region and to enhance the electron-hole separation process. The modifications of TiO 2 have been accomplished by different strategies such as doping, codoping, sensitization, and coupling [ 28 — 32 ].

The coupling of two semiconductors provides different energy levels, which give a chance to improve a more efficient charge separation to enhance the lifetime of charge carriers and to increase interfacial charge transfer [ 33 ]. Rare-earth elements are ideal dopants because of their 4f electronic configuration and spectroscopic properties [ 34 ]. Modification of electronic structure as well as crystal structure of TiO 2 can be performed by doping rare-earth elements. Lanthanide ions could act as effective electron scavengers to trap the CB electrons from TiO2.

Xu et al. Wang et al. Cerium is one of the most reactive rare-earth metals and can be utilized in a broad range of applications in the field of catalysis and photocatalysis. It is also one of the most generous rare-earth elements, making up about 0. However, CeO 2 does not show good thermostable behavior because of its rapid sintering under influence of elevated temperatures [ 43 , 44 ]. Cerium oxide ceria , CeO 2 has the face-centered cubic fcc fluorite structure with eight coordinate cations oxygen atoms and four coordinate anions Figure 6.

The complete unit cell, Ce 4 O 8 , measures 0. Ceria being an n-type semiconductor has a bandgap of 2. The favorable bandgap of ceria makes it active under UV-Vis range and its utilization as a photocatalyst in advanced oxidation processes. Ceria is used in numerous catalytic reactions such as photocatalytic reactions and water splitting reactions, to name a few [ 45 ].

Recently, ceria has been explored as a dopant in aqueous phase photocatalytic reactions [ 36 , 46 ]. The photocatalytic activity of Ce-TiO 2 for degradation of 2-mercaptobenzothiazole was reported by Li et al. The activity of Ce-TiO 2 was higher in comparison with pure TiO 2 , and this was attributed to the higher surface area and better charge separation of photocatalyst [ 39 ]. Formation of 4f level and defect level were considered as a reason for delayed electron-hole recombination and activation of photocatalyst under UV visible light range [ 39 ].

The phase transformation of anatase to rutile was hindered by Ce ions for low concentrations [ 47 , 48 ]. Liu et al. For higher amount of Ce, phase segregation of ceria on the surface of TiO 2 and rutile was formed [ 49 ]. Experimental results showed that better thermal stability for TiO 2 can be achieved by doping Ce ions to prevent particle sintering and subsequent pore collapsing [ 50 ]. Positive impact on crystallographic characteristic as well as sintering of photocatalyst was achieved by Ce doping.

Cao et al. The photodegradation test of methylene blue MB was carried out to assess the photocatalytic activity of Ce-TiO 2. Interesting behavior of first rising and then falling the degradation efficiency with an increase of Ce content was found.

However, when the Ce content exceeds a certain amount, the TiO 2 crystal lattice reduces the crystallinity because of high impurities and defects. This causes shortening of the trapping center distance, increases the chance of recombination of the photogenerated charge carriers in the trapping center, and thus reduces the photocatalytic efficiency in return.

Reli et al. Eskandarloo et al. It was found that mixing TiO 2 with CeO 2 could facilitate efficient charge transfer, which can ease the separation of the photogenerated charge and in return increase photocatalytic efficiency [ 54 ]. Apart from here mentioned synthesis methods for Ce-TiO 2 photocatalyst, their morphology and particle size are summarized in Table 3.

Various photocatalytic reaction mechanisms have been proposed so far depending upon the morphology and phases present, different mechanisms apply for photocatalytic reactions. In this reaction mechanism, an excited electron from dye due to visible light transfers to Ce ion. Photocatalytic degradation of dyes through semiconductor materials follows a complex path.

Many researchers presented dye sensitization and subsequent charge transfer as a major reaction mechanism. Verma et al. The unique architecture of Ce-TiO 2 system was synthesized via simple sol-gel method with varying Ce content. Doping of Ce inhibited transformation of anatase to stable rutile phase. As it was mentioned earlier that anatase has better photocatalytic activity, Ce doping showed a positive effect on photocatalytic activity which was measured by degradation of MB.

Novel architecture material showed enhanced activity under a combination of ultraviolet and visible light. Reason for higher photocatalytic activity was sought to be the presence of CeO 2 at the interface of anatase and rutile, which thus facilitates excellent interfacial charge transfer as shown in Figure 8. However, CB electron of CeO 2 is not capable of reduction of O 2 due to its unfavorable redox potential position. This electron can transfer to CB of anatase and subsequently to rutile via interfacial charge transfer.

An electron in CB of rutile can thermodynamically have accepted by O 2 adsorbed on the surface and create highly reactive superoxide anions by reduction reaction. Some studies suggest that the hydroxyl radicals play a key role as reactants for oxidation of organic substrates in aqueous photocatalysis. Therefore, hydroxyl radicals yield is essential to determine the photocatalytic efficiency and for comparison of different photocatalytic materials.

In order to assess the photocatalytic efficiency of a TiO 2 -based catalyst, an organic and inorganic pollutant can be used as the environment. They can be divided into three categories: azo dyes [ 58 ], organic compounds [ 59 ], and inorganic gases [ 60 ].

Dye degradation is a popular method for assessing the efficiency because of its being fast and easy. Dyes have relatively high extinction coefficients to activate the photocatalyst, and they consequently will absorb a significant amount of the incoming light, which makes it difficult to assess the full photocatalytic activity [ 61 ]. Methylene blue MB is most commonly used as a model dye for the adsorption of organic dyes from aqueous solution [ 62 ].

It is a heterocyclic aromatic hydrocarbon with a molecular formula, C 16 H 18 N 3 SCl [ 63 ] molecular weight Methylene blue dye is commonly used in the textile, food, and chemical industries [ 64 , 65 ]. As it is a commonly used dye in numerous industrial sectors, it can mix up with domestic water and pollute it.

Therefore, it is crucial to remove this toxic dye from contaminated water by converting it to inorganic benign compounds by adding photocatalyst materials to wastewater [ 66 , 67 ]. Methylene blue is found to give several environmental and health damages through contamination of water. Toxicity rate of MB is four and acute exposure by intravenous injection causes sweating, dizziness, confusion, vomiting, chest pain, and nausea [ 70 ].

The structures of the dye molecules directly decide the absorption characteristic of dyes for light. In the electron absorption spectra of dyes, there are several absorption bands, which reflect the state of motion of the electrons. The absorption wavelength, absorption intensity, and the shape of the absorption band are related directly to the structure of dye molecules. Therefore, it is possible to evaluate the structural variation of dyes by investigating the variation of the electron absorption spectra during the process of degradation of the dyes.

It has been reported that the photodecomposition of RhB aqueous solution in the presence of TiO 2 particles as a photocatalyst has two pathways: 1 the photocatalytic pathway which would occur under UV irradiation. The bandgap of anatase TiO 2 is 3. In the photosensitization pathway, where TiO 2 cannot be activated by visible light, dyes will absorb visible light irradiation and can be excited, which will drive the process of photodegradation.

However, the existence of TiO 2 photocatalyst is a prerequisite and a crucial requirement to ensure electron carriers to contact with electron acceptors adsorbed on the TiO 2 surface, which will help in the process of photodecomposition. Xie et al. Priyanka et al.

TiO 2 being extensively used as a photocatalyst for degradation of organic dyes, modification of it by Ce doping for visible light activation has been presented in this review. Various synthesis processes along with different morphologies of Ce-doped TiO 2 are also summarized. Complex reaction mechanisms involved with Ce-doped TiO 2 with important aspects were discussed.

Even though the above-mentioned reaction mechanisms occurred simultaneously, it is very difficult to conclude which mechanism is more dominating during photocatalytic degradation of dye. It would be a mere oversimplification of complex reaction mechanism to present with any one of above-mentioned reaction mechanism alone.

Photocatalytic reaction mechanisms for brookite and complex systems require more elaboration. When contributions from surface area, bandgap, presence of phases, concentration of Ce, and morphology of nanomaterial are included, the complex yet complete reaction mechanism of Ce-doped TiO 2 system can be modeled. Such reaction mechanism provides the ground basis for designing better material for utilization of visible light for novel applications of visible light active photocatalysts.