Photochemical applications of solar energy:photocatalysis and photodecomposition ofwaterK. KalyanasundaramDOI: 10.1039/9781849737722-00182

Two main aspects of solar energy applications are reviewed, viz. photocatalysis

and the photodecomposition of water into hydrogen and oxygen. In most of the

cases solid phase photocatalysts are used and this topic is often referred to as

heterogeneous photocatalysis.

1 Introduction and scope

This review covers the research publications that appeared during the years2011 and 2012 in two major research areas: photocatalysis and the photo-decomposition of water to hydrogen and oxygen using semiconductingmaterials such as TiO2 and related oxides. Majority of the catalysts used aresolid phase catalysts dispersed as particulates or as electrodes and hencethese studies are referred to as heterogeneous photocatalysis. Intenseresearch activity is underway in both the areas discussed here as evidencedby over 500 publications appearing during the two-year review period.Several review publications have appeared capturing the highlights onphotocatalysis120 and photochemical water splitting.2129 On the photo-catalysis front, several new photocatalysts have been identified, with whichmany of the photoreactions catalyzed by titania (TiO2) in the UV can nowbe sensitized with visible light with high efficiency.

The field of artificial photosynthesis has benefited with the crystalstructure reports at 1.9 A resolution of the photosynthetic water oxidationcomplex of Photosystem II.30 The oxygen evolving complex (OEC) involvedin the water splitting consists of 4 Mn atoms and 1 Ca atom and is locatednear the lumenal surface of photosystem II. At 1.9 A resolution, it ispossible to place the coordinated water molecules and this has facilitateddesign of suitable model complexes. There have been several reportsidentifying a growing number of homogeneous and heterogeneous wateroxidation catalysts whose mode of operation resemble more closer to thecurrently accepted models on how water oxidation to molecular oxygentakes places in natural photosynthesis.

Studies of photodecomposition of water using semiconductor electrodesand particle photocatalysts now use water oxidation or reduction asadditional co-catalysts. The rates and the quantum efficiency of wateroxidation and reduction processes are enhanced in the presence of theseredox catalysts. Few photocatalytic systems have been identified, which forthe first time effect complete decomposition of water to H2 and O2 in a

Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, SwissFederal Inst. of Technology (EPFL), Station 6, EPF-Lausanne, CH-1015, Lausanne,Switzerland. E-mail: k.kalyanasundaram@epfl.ch

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c The Royal Society of Chemistry 2013

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sustained manner with reasonable efficiency. These studies are reviewed inthe second part of this chapter.

2 Photocatalysis

Titania still remains as the most extensively studied photocatalyst material invarious morphological forms and photocatalysis studies using TiO2 are firstreviewed. This is followed by studies on other oxide and non-oxide semi-conductors including many that are active under visible light irradiation.

2.1 Titania (TiO2 ) based photocatalysisPhotocatalysis refers to the process of using solar energy to effect chemicalconversion of organic and inorganic molecules and in the removal of toxicpollutants and wastes present in water and in air. Photoinduced generationof strong oxidants and/or reductants is achieved via light absorption by anorganic molecule, inorganic metal complex or a semiconducting material. Ina seminal paper published in 1972 Fujishima and Honda31 reported onpossible photodecomposition of water through bandgap excitation of oxidesemiconductors such as TiO2.

Fig. 1 shows conceptually various processes that take place followingoptical excitation with photons of energy EZEbg. Photo excitation ofsemiconducting oxides such as TiO2 leads to generation of holes in thevalence band and electrons in the conduction band. While a good part of thephotogenerated charge carriers recombine, they also diffuse apart to reachthe surface rapidly where they can trapped or transformed to other reactiveintermediates (equations 18):

TiO2 ! hvb ecb 1

hvb ! htr; ecb ! e-tr 2

Fig. 1 Schematic representation of various processes that take place following optical exci-tation of a wide bandgap oxide semiconductor with photons of energy EZEbg. (Adapted withpermission from: T. Ochiai and A. Fujishima, J. Photochem. Photobiol., 2012, 13, 247.)

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htr H2O! OH H 3

htr RH! R H 4

htr O2 ! 1O2 5

etr H2O2 ! OH OH 6

etr O2 ! O2 7

etr R H ! RH 8

In a sequence of steps, the electrons and holes can oxidize and reducesurface-adsorbed molecules through interfacial charge transfer. Hydroxylradicals have been identified as one of the most active and nonselectiveinitiators of photocatalytic oxidation of organic substrates, particularlyin relation to weakly adsorbing species such as alcohols and aromatics.Secondary radical formation from the oxidation of organic substrates oftenincreases the complexity in working out mechanistic details. Thermo-dynamic limits of photooxidation or photoreduction are set by the relativedisposition of the energy levels of valence and conduction band of thesemiconductor. Most active photocatalysts used are TiO2, ZnO, WO3 andrecently other oxides such as BiVO4.

TiO2 as a white pigment has been used in the fabrics and paint industryfor a long time. Earlier studies have shown that, in addition to water oxi-dation, other industrially and environmentally useful photooxidations canbe carried out using this photocatalyst. A few publications have appearedtracing the history of photocatalysis as practiced today.32 Keidel in hispaper33 invoked the photosensitivity of TiO2 as a possible cause of thedegradation of paints and degradation of fabrics as early as 1929. Howeverthere are reports that the term photocatalysis was first used as early as1910 in a Russian textbook on photochemistry by J. Plotnikov34 and a fewyears later by Landau in 1913.35 Eibner reported results of his studies on thedegradation of inorganic and organic paints via photolysis in the presence ofZnO as a catalyst.36 It was reported that UV absorption produces activeoxygen species on the TiO2 surface, causing the photobleaching of dyes. Inaddition to the early work of Landau and Plotnikov, Terenin and cow-orkers studied in 1930s several photochemical reactions in solution and inheterogeneous media There was a report on the photobleaching of dyes byTiO2 both in vacuum and in oxygen in 1939.

37 Though the term photo-cataysis was known, the process was referred to as photosensitization.Markham discussed some of these early studies on the photocatalyticproperties of oxides such as TiO2 , ZnO and Sb2O3 in a review paper.

37

Studies on photocatalysis using titania and related oxide semiconductorscan be broadly grouped in three main categories: i) preparation of alternateversions of TiO2 photocatalysts in different electronic content, morpholo-gical forms with additional facet control; ii) efficacy of standard titaniabased photocatalysis for photodegradation of various industrial andgeneral toxic wastes and iii) development of novel/new applications of

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semiconductor-based photocatalysis. TiO2 occurs in three stable poly-morphic forms: anatase (tetragonal), rutile (tetragonal) and brookite(orthorhombic). Anatase (band gap energy Ebg=3.2 eV) is considered as themost photoactive forms, (in particular certain crystallogrpahic planes) thanthe rutile form (Ebg=3.0 eV). The most widely used form of TiO2 is asaqueous suspensions of P-25 particulates (prepared originally by Degussathrough aerosol process, now sold under the name Aeroxide P25). P25 is amixture composed of approximatively 75% anatase and 25% rutile. P-25has fairly high surface area (BET), 54m2/g with an average particle dia-meter of ca. 20 nm. Due to its smaller bandgap, rutile can be photoactivatedusing near UV photons of the solar radiation (ca 4% of solar radiation).

Nearly all photochemical studies report some sort of comparison of thecatalyst efficiency. Quantitative comparison of catalyst efficiency amongstvarious photocatalysts is difficult and complex for several reasons. Thedifferences in morphology, surface structure and surface chemistry (whichdepends often on the pre-treatment history) and preferential adsorption ofsubstrates on one form of photocatalyst over the other are some of thefactors that are difficult to quantify. Since P25 is a readily available com-mercial powder, it has been used as a standard photocatalyst. Synergisticeffect in catalysis refers to the situation where the photocatalytic activity ofa mixture exceeds the sum of activities of the components. It is generallyaccepted that this P25 exhibits better catalytic efficiency and there have beena lot of speculations on this synergistic enhanced.

2.1.1 Titania photocatalyst in different morphologies - nanostructuring.Advances in nanoscience and nanotechnology permit today synthesis ofphotocataysts tailored to any specification (size and architecture) defined atnanometric scale. Efficiency of catalytic reactions can then be optimized.Synthesis of catalyst materials can be approached in two different ways:start with macro-sized larger materials and process them (cleaving andpolishing) to get particles of lower dimensions (top-to-bottom approach).Alternatively, one can start with simple atoms and molecules and use var-ious assembling procedures to make larger nanoparticles (bottom-upapproach). Nanostructured materials that are zero-, one- and two-dimen-sional have been prepared and found to have their own unique properties.Spherical nanoparticles, nanocrystals and quantum dots are examples ofzero-D materials. Nanorods/nanowires/nanofibres and nanotubes are 1-Dmaterials. Nanosheets and nanoribbons have 2-D morphology. Sol-gel andhydrothermal followed by high temperature sintering (calcination) are mostcommon methods.

Hierarchical structures are systems of relatively large dimensions com-posed of smaller building blocks connected together in a repetitive, oftenhighly periodic, manner. 1D nanostructure can be used as template to growone- or two-sided comb-like structures or even brush-like hierarchicalarchitectures. Hierarchical systems can be fabricated in principle bycombing 0D, 1D, and/or 2D nanoparticles to form a wide range ofarchitectures. Electrochemical anodization and electrospinning are alsobeing used extensively to prepare organized nanostructures for photo-catalysis applications.3841

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Photocatalysis studies using particulates (slurry systems) is the simplest andcommon approach. Studies using larger electrodes in photoelectrochemicalcells have added advantage that oxidation and reduction processes can bespatially separated and quantitative studies on factors that affect the per-formance (role of added co-catalyst layers for example) are feasible. Recentlythere have been a number of studies of thin film electrodes fabricated usingnanoparticles and they try to integrate the advantages of nanoparticles inincreasing the collection efficiency of photogenerated charge carriers.

Two key factors that control the catalytic performance of any photo-catalyst are the intrinsic electronic and surface properties. For particulatesystems and mesoporous films, the particle size, surface area, porosity/porevolume, pore size, crystallinity and types of facets exposed at the surface aresome of the key parameters that are to be controlled. Nanostructuring ofthe photocatalyst in different morphologies such as nanorods, fibers, tubes,sheets and interconnected architectures is one viable approach. In the lastdecade there has been a phenomenal growth in the experimental methodfor the preparation and characterization of nanostructures in zero, one,two and three dimensions. Catalysts in the form of spheres are of zero-dimension offering very high surface area. One dimensional tubes and fibersoffer short distances for photogenerated charge carriers to escape and reachthe surface rapidly. Two dimensional sheets and films provide additionalcontrol on substrate distribution/adsorption. One and two-dimensionalnanostructures can be used as templates to grow larger hierarchical 3-Dstructures which allow the architecture control even further.

2.1.1.1 Photocatalysis using P25 and sol-gel derived Titania nanoparticles.Controlled hydrolysis of suitable precursors and solvent composition is amethod well known as sol-gel process for the preparation of finelydivided monodispersed form of colloidal metals and metal oxides. Anumber of studies have used this method to prepare photocatalysts.4245

TiO2 sols can be prepared via hydrolysis and polycondensation reactions ofTi-alkoxides Ti(OR)n in aqueous solutions. The solvent from the asprepared colloidal solutions can be removed to prepare dry powders. Alter-natively the sol solution, after suitable concentration can be coated ontoconductive glass and other substrates to form thin films. Doctor blading andscreen printing are the common methods for planar/flat surfaces.For conformal film deposition on non-planar structures (steps, stacks ortrenches), spray coating is preferable. Sol-gel route is a powerful technique toprepare thin films or large homogeneous (monodisperse) concentration andstoichiometric control. The method also permits facile co-doping of othermaterials, non-metals such as boron or nitrogen in particular.

One other fundamental problem to face in the quantitative analysis ofphotocatalysis studies is the manner in which various modifications ofthe photocatalyst are performed. Other materials are introduced in/onto thepristine catalyst using different procedures. Metal ions and anions can bereadily adsorbed on to the photocatalyst by stirring the TiO2 particles in anaqueous solution containing these salts. When the additives are introducedduring the sol-gel hydrolysis process itself, the additives can form an inneror outer shell over that of the pristine titania particles. The term doping is

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often used in a loose manner, though usage of the terms such as core-shellsystems may be more appropriate. Akpan and Hameed44 have reviewedrecent studies of sol-gel method for doping TiO2 photocatalysts.

In principle, doping refers to the introduction of foreign elements in thehost matrix of the parent/pristine semiconductor, at sufficiently low con-centration (o1%) so as not to disturb the crystallographic phase. Fortitania, there is the creation of additional conduction electrons by theaddition of heterovalent cations having a valency higher than that of Ti4

(V5, Nb5, Ta5, Mo6, W5, etc) (n-type doping) or the creation of holesby heterovalent cations having a valency smaller than 4 (Al3, Ga3, Cr3)(p-type doping). Doping is resorted to for two main reasons: to improve theseparation of photogenerated charge carriers and to improve the visiblelight (solar radiation) response of the photocatalyst. TiO2 by itself is a widebandgap semiconductor and harvest hardly 3% of the solar radiationreaching the earths surface. There have been several cases of anion- andcation- doping of titania resulting in materials that show enhanced lightabsorption in the visible light region.

The spectroscopic origin of the new absorption in the visible region hasnot been unambiguously established in many cases, due to lack of anyphotocatalytic activity for photoexcitation in the visible light region. Insome case studies of photodegradation of organic dyes, dye oxidationproceeds only for a limited period of time. On the contrary even depositionof noble metal ions and inorganic anions (such as phosphate or fluoride) onthe surface of the photocatalyst has been found to affect charge recombi-nation by rapid trapping of one of the charge carriers and thereby enhancethe photocatalytic activity.

The TiO2 and the P-TiO2 transparent films that strongly adhere to thesurface of fused-silica substrate were prepared by the sol-gel method andspin-coating process. The effects of calcination temperature and phosphorusdopants on the properties of the photocatalyst have been studied. In thepristine TiO2 case, anatase-to-rutile phase transformation was observedabove 900 1C with pure rutile form obtained at 1000 1C. But after introducingphosphorus atoms into the Ti-O framework, the anatase-TiO2 can withstandtemperature up to 900 1C, without the formation of rutile-TiO2. As expected,the average size of the titania crystallites increased with calcinationtemperature. After exposing to 365-nm UV light for 12 h, the P-TiO2 filmscalcined between 600 1C and 900 1C can photocatalytically decompose at ahigher rate.44b Thin films of TiO2 have been prepared

42 on borosilicate glassvia a sol-gel dip coating method, with and without polyethylene glycol. Thephotoactivity of textile dyes such as congo red has been studied.

Malengreaux et al.43 prepared TiO2 thin films by a dip-coating processusing a non-aqueous sol-gel method. This study investigated the influence ofthe operating variables such as nature of the substrate, sol concentration,withdrawing speed of the dip-coater and number of layers on the physico-chemical properties of the films. Photocatalytic activity of the films wasevaluated by following the degradation of methylene blue under artificialUV light at 25 1C. Plantard et al.45 have compared the photocatalyticproperties of sol-gel method derived TiO2 films with that of a commercialP-25 powders.

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2.1.1.2 Spherical nanoparticles of titania, hollow spheres. Historicallynanosized particulates have been used in photocatalysis for many decades.Commercially available Degussa P25 particle is one common catalystsample used worldwide. In addition to aerosol-derived anatase-rutile mix-ture P25, sol-gel hydrolysis route has been used to prepare monodisperseparticles of different sizes. Sol-gel derived particles and mesoporous filmsare to be annealed at higher temperature to improve the crystallinity of thematerial (though such treatments reduce the surface area available).Crystallinity is important for efficient generation of charge carriers.

An important extension of the spherical particles is hollow spheres,prepared via a templating procedure.4654 In addition to providing highersurface area, hollow structures permit multiple refractions and reflections oflight, assisting the light harvesting process. Hollow spheres are prepared bysol-gel hydrolysis in the presence of well defined polystyrene beads and thetemplate removed subsequently through calcination. There have been anumber of reports on the synthesis of hollow spheres of titania and assayson their performance as photocatalysts. Flower-like anatase TiO2hierarchical spheres assembled from nanosheets as substrates in a hydro-thermal growth process in the presence of glycine. These flower-like spheresare about 2 um in diameter with sheet thickness about 20 nm.

Jiao et al.46 have reported the use of phosphate as a morphologycontrolling agent to prepare hollow anatase TiO2 single crystals and mesocrystals. Hollow and mesocrystals show enhanced catalytic activity forwater and CO2 reduction. Hollow mesoporous titania microsphere (HTS)was fabricated49 in a sol-gel process by alternatively coating of titaniaprecursor Titanium tetrabutoxide (TBOT) and polydimethyl-diallylammo-nium chloride (PDDA) polyelectrolyte onto cationic polystyrene sphere(PS) template, followed by calcination. The resultant uniform mesoporousHTS (diameter 1.22 mm) features a low shell thickness/diameter ratio(B2%) and a high specific surface area (B77.5 m2/g), and thus leads to ahigh photocatalysis. The porous TiO2 hollow spheres were obtained byusing TiOSO4 as a titanium source and carbon nanospheres as a sacrificialtemplate. Their particle size, diameter and morphology can be readilycontrolled by varying growth parameters, including reaction temperature,time and reagent concentration.51

Leshuk et al.52 have reported on the synthesis of robust, monodisperse,mesoporous titanium dioxide (TiO2 ) submicrometer hollow spheres througha single step hydrothermal silica etching reaction under mild conditions. Theresulting processing conditions yielded TiO2 hollow spheres with a surfacearea ofB300 m2/g and anatase phase crystallization, which exhibit enhancedphotodegradation of methylene blue. A template-free solvolthermal route forthe preparation of anatase TiO2 hollow spheres using TiCl4 and a mixture ofacetone-alcohol mixtures has also been reported.50 Control of the hollowspheres size was achieved by adjusting the ratio of alcohols to acetone. Theas-prepared TiO2 hollow nanostructures exhibited good photocatalyticactivity for the degradation of phenol compared to Degussa P25.

2.1.1.3 Titania as thin films. Thin films of photocatalysts can be depos-ited from nanoparticles or deposited by other chemical methods.5559

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Procedures have been developed using Atomic Layer Deposition method(ALD) for the preparation of highly photoactive organic-inorganic hybridtitanicone films using TiCl4 and ethylene glycol.

56 Lyandres et al.58 haveused substrate RF bias and reactive gas partial pressure to control preferredorientation and photoreactivity of TiO2 films by reactive direct current(DC) magnetron sputtering and studied the efficiency of the films for thephotooxidation of acetaldehyde. They observe an optimum RF bias of50V at which the films exhibit biaxial texture with the c-axis parallel tothe surface with maximum crystallinity and degree of orientation, corre-sponding to a maximum in the reactivity as well.

2.1.1.4 Facet control of the photocatalysts. In light activated semi-conductor oxides, the electrons and holes are intimately linked to the sur-face species. Different surface sites are exposed depending on the exposedcrystal facet in solution. Some of the facets can be strongly oxidizing whilesome others can exhibit reducing tendency. Association of facet controlwith catalytic selectivity has been studied extensively in conventionalcatalysis and the methods have since been exported to photocatalysis.Photocatalysts in the form of 3D structures (cubes, pyramids, spinel andother anisotropic structures) have several unique crystal facets exposed.There have been a number of studies with targeted synthesis of titaniaphotocatalysts with one or more facets.6065

Ohno et al.60 demonstrated the preferential reductive and oxidative facetson TiO2 crystals using the photocatalytic reduction of H2PtCl6 and oxida-tion of Pb(NO3)2 probes, respectively. They identified rutile {110} andanatase {011} as the reductive sites, whereas rutile {011} and anatase {001}acted as the oxidative sites. The reductive anatase {011} was further con-firmed visually by single particle fluorescence spectroscopy. Whereas thephotocatalytic activity of faceted catalysts can (and should) be substrate-specific in nature, it is thought that a balance between redox reactions, forexample, facets targeting oxidation of organic compounds and those tar-geting reduction of molecular oxygen, needs to be achieved for optimalphotocatalytic activity.

The inherent mechanisms of facet nucleation and morphology evolutionof anatase in the presence of fluoride ions has been studied recently.63

Simply tuning the Ti/F ratio in the synthetic mixture leads to single anatasecrystal exposed with different facets like {001}, {010}, or {110}. And com-plex sphere structure exposed with {001} facets can be formed by secondarynucleation and growth. Prolonging the hydrothermal treatment time causesselective etching on {001} facets, whereas defluorination via thermal calci-nation produces many pores on the surface. The photodegradation ofpositively and negatively charged, and zwitterionic dyes indicates that thetype of reactant, adsorption mode and surface area play significant roles inphotocatalysis. In a related study, Wang et al.64 have successfully preparedanatase TiO2 hollow microspheres with (001) facets in a one-step process inthe presence of H2O2 and HF solution. The product exhibited enhancedperformance in the photodegradation of methylene blue. A mechanism forformation of the TiO2 hollow spheres with exposed {001} facets bydissolution-recrystallization and Ostwald ripening is proposed.

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Hollow anatase TiO2 single crystals and mesocrystals with dominant{101} facets have been prepared65 by a new route with PO4

3/F asmorphology controlling agent and they have been used to study the extentof bulk recombination of charge carriers in faceted photocatalysts. It wasfound that with respect to solid crystals, being hollow crystals andmesocrystals can substantially improve photocatalytic activity (O2/H2evolution from water splitting, CH4 generation from photoreduction ofCO2) as a result of the synergistic effects of shortened bulk diffusion lengthof carriers for the decreased bulk recombination and increased surface area.

2.1.1.5 Nanotubes and nanofiber form of titania photocatalysts. Nano-tubes and nanofibers represent the two-dimensional morphological forms.They are readily prepared principally via electrochemical anodic oxidation,sol-gel template methods and hydrothermal methods. A number of reviewshave appeared on the synthesis and catalytic properties of titania nanotubearrays.6673 Anodic oxidation is a versatile method permitting the pre-paration of either compact or porous nanotubes, depending on the nature ofthe electrolyte medium. Electrochemical oxidation route permits synthesisof nanotubes of tunable length, The formation of nanotubes takes place viaelectric field assisted dissolution of Ti metal ions, electric field assistedoxidation of Ti metal to form TiO2 and dissolution of Ti and TiO2 due toetching by fluoride ions. The relative rates of these control the length andthe aspect ratio of the nanotubes. The aspect ratio can also be controlledwith change in the composition of the electrolytes present and the solvent(DMSO, ethylene glycol, N-methylformamide, etc.)

Ti-containing precursor reagents undergo sol-gel hydrolysis. The TiO2formed is allowed to polymerize or deposit onto the template. Subsequentremoval of the template material and calcination yields crystalline TiNT.The literature reports a wide variety of templates. Advantages of thisapproach include tight control over the size and morphology of the TiNTbased on the geometry of well-understood template materials. Hydro-thermal process employs high temperature and pressure under alkalineconditions to generate single layer nanosheets of titania which self-assemble(roll) into tubes due to the high surface energy of sheets.

Fang et al.74 prepared highly ordered titanium oxide (TiO2 ) nanotubes byelectrolytic anodization of titanium electrodes. Morphological evolution andphase transformations of TiO2 nanotubes on a Ti substrate and that of free-standing TiO2 membranes during the calcinations process were studied. Sunet al.75 prepared TNT via hydrothermal route and used them to construct anovel photocatalytic membrane reactor for the photodegradation of brilliantblue dye. The activity was 45 times higher in the membrane reactor for allTNT samples tested. Hydrogen treatment of titania particles under highpressure (200 barH2) at 200 1C for 5 days in a stainless steel container leads tothe formation of a Black hydrogenated form of titania. These black particlesshow enhanced visible light response, possibly due to the introduction ofdisorders in the surface layers. Danon et al.76 prepared black TiO2 nanotubesand examined their activity towards acetaldehyde oxidation under visible light.

2.1.1.6 Photonic crystals - improving the light harvesting capacity ofphotocatalysts. Photonic crystals are a class of opto-electronic material

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designed to confine, control and manipulate photons so as to increase thelight harvesting properties. They are three-dimensional periodic dielectriccomposites with lattice parameters on the order of the wavelength of light.There are three basic requirements for the design of photonic crystals:lattice parameters in the range of several tenths of mm to several mmrefractive index ratio between the air and the material should be high whilethe optical absorption is low and ability to form well-ordered lattice. titaniawith a high refractive index (2.6 at 500 nm for anatase) and a wide bandgapof 3 eV is well suited as a substrate. Since the efficiency of any photocatalystsdepends on the extent of light absorption, there have been several attemptsto improve the light absorption properties of titania-based catalysts. Being awide bandgap material (with a bandgap in the range of 3.03.2 eV), TiO2absorbs only 3% of the solar radiation. Use of photonic crystals and surfaceplasmonic effects using noble metal deposits are some of the methodsexamined.

Procedures have been developed for the preparation of photonic crystalsof titania using polystyrene latex beads as templates during sol-gel hydro-lysis of titanium alkoxides followed by calcinations to remove the carbontemplate. There have been few studies of photonic crystals in photocatalysisand dye-sensitized solar cells.7784 Wu et al.79 for example used colloidaltemplating to prepare three-dimensionally ordered macroporous titaniawith a range of pore diameters. Calcination at different temperatures toremove the templates resulted in different crystalline phases. The structuraland photonic properties were characterized and their effects on photo-catalytic activity are presented as well. A strong effect of the pore diameteron the photocatalytic activity was observed and correlated with the photonenergy involved in the photodegradation process of organics.

2.1.2 Bulk doped titania catalysts. Dopants are external componentsadded to a host lattice and they can be present as interstitially dissolvedforeign atoms or by substitutional dissolution. Difference in sizes of theforeign atom with respect to the native atom can introduce strain in thelattice. Aliovalent doping refers to the situation where the dopant ions havedifferent valency as compared to those in the host lattice. In these case, theireffective charge will affect the electroneutrality condition and thereby affectthe defect equilibria. Nonstoichiometric oxides with excess metal in theform of predominating interstitial metal ions exhibit the same qualitativeeffects of aliovalent foreign cations.

2.1.2.1 Reduced titania (blue) via hydrogenation or F, Nb doping.Heating of titania samples in hydrogen atmosphere at elevated temperatureleads to the formation of reduced form of titania which is black incolor.85 Excess of metal compared to oxygen stoichiometrically results inthe formation of formally Ti3 ions. But this can be pictured either as anO-vacancy as in TiO2x or as interstitial Ti as in Ti1 xO2. F and Nb dopingof stoichiometric TiO2 also leads to lattice Ti

3 ions in a pseudo-octahedralenvironment. Experimental evidence for the formation of reduced titaniacomes from various data: absorption bands in the visible light regionresponsible for the dark color; occurrence of new state in the bandgap atabout 0.8 eV below the CB of TiO2 attributed to reduced Ti

3 ions; shift in

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the core level binding energies of reduced form of Ti-atoms in the XPS andiv) the presence of more than one EPR signal associated with different typesof paramagnetic Ti3 ions in the lattice.

2.1.2.2 Metal ions doping of titania. There have been several reports onmetal ion-doped titania photocatalysts with enhanced visible light absorp-tion properties.86103 In many cases catalytic activity upon irradiation in thevisible light absorption bands is poor or non-catalytic (turnover with respectto the catalysto1). Depending on the redox state and the energy level of thedopants, they may function as electron- or hole-acceptors. The insertedenergy levels provide photons of sub-bandgap energy to create chargecarriers through additional channels (e.g., dopant d-band to CB or VB tothe dopant d-band). Application of high energy routes such as high energyRF and magnetron sputtering can introduce novel tetravalent dopants(Fe4, Cr4, V4) that match the valency of Ti4 in TiO2 . Co-doping with aconjugate metal ion such as Rh3 or Sb5 can preserve the charge equalityof the doped photocatalysts, resulting in improved photocatalytic activity.Doping in general can create structural and aliovalency defects that aredifficult to circumvent and they can become potential sources for chargerecombination. In such cases, introduction of dopants can lead to a decreasein the photocatalytic efficiency.

Herrmann86 has elaborated on why the Cr(III) doping of TiO2 is ingeneral catastrophic for photocatalysis. Although Cr-doped (0.85 at%)TiO2 absorbs in the visible region, it only becomes a photoconductorthrough band-gap illumination and the doping causes a considerabledecrease in photoconductivity by three orders of magnitude. Correspond-ingly, its photocatalytic activity is diminished, as can be noted in fivedifferent reactions representative of various media in which titania is active.

There have been few reports of enhanced photocatalytic activity of titaniain the presence of Cr(VI) ions as surface adsorbed species. Cr(VI) ions candesorb and get reduced to Cr(III). Since fluoride ions are known toenhance surface adsorption, immobilization has been explored using flur-ochromate(VI) K[CrO3F] instead of K2Cr2O7.

91 Wang and coworkers87

have compared the effect of chromium on photo-oxidation of propan-2-ol,for which hydroxyl radicals are the catalytically active intermediates, andstearic acid oxidation which proceeds mainly via direct hole transfer. TheCr/TiO2 samples were prepared by co-precipitation from aqueous TiCl4solutions. The photo-oxidation of liquid propan-2-ol oxidation to acetonewas followed by gas chromatography. The oxidation of stearic acid tocarbon dioxide on irradiation of a pressed disc of stearic acid/TiO2 wasfollowed by using FTIR spectrometry of the product CO2.

Modification of the pristine TiO2 photocatalyst with metal ions, parti-cularly d metal ions such as Fe(III), Cr(III), Ru(IV), Mo(VI) results in theinsertion of impurity energy levels between the parent valence and con-duction bands. Rauf et al.101 have reviewed studies of photodegradation ofazo dyes in the presence of TiO2 doped with selective transition metal ions(Cr3, Fe3 and Cu2). At chromium concentrations below 1% (typical forion doping without altering the crystal lattice), both the lifetime and pho-tocatalytical activity were reported to decrease drastically to a nearly

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constant low level. At much higher concentrations employed (1015%),Cr-ions are possibly present as surface adsorbed ions. SurprisinglyRhodamine B underwent 25% more degradation in the presence of such ahigh concentration of Cr(III) ions.

Single molecule fluorescence microscopy has been used to study photo-degradation of fluorescent dye resourufin on Sb-doped TiO2 nanorods.

103

Two potential oxidants (OHd, and O2d radicals) can oxidize the dye

causing a decrease in the fluorescence intensity. Using two distinctquenchers (Superoxide dismutase for OHd and DMSO for O2

d radicals) toremove selectively one of the two oxidants, authors can follow the specificsites where each of the two oxidants operates. Enhanced photodegradationof organic dyes in the presence of Cu(II) ions arise from scavenging of theelectrons to form Cu . In the case of low levels of Fe3, Fe3 traps photogenerated holes and forms Fe4 which reacts with the surface adsorbedhydroxyl ions to produce hydroxyl radicals and O2

on the surface lattice ofdoped TiO2.

Visible light induced photocatalytic activity of synthesized materials wasstudied for 4-chlorophenol (4-CP) oxidation.97 A considerable photo-catalytic activity was observed in the case of F-Cr(VI)@TiO2 and TiO2@MCrO4 type materials, M= Mn2, Ba2, CO2

, Cr3). Besenbacher andcoworkers102 prepared high-quality polycrystalline Fe-doped TiO2 (Fe-TiO2) porous films via one-step electrochemical oxidation. Properties suchas the impurity concentration and the microstructure influence the perfor-mance of the material for photovoltaic and photocatalysis applications andthey have been controlled by adjusting the electrolyte composition.

The efficiency of ethidium bromide photodegradation has been studiedusing a series of five Fe-doped TiO2.

95 The photocatalysts were preparedusing a combined sol-gel/reverse microemulsion method and the ironcontent varied from 0.5 to 5%. The initial amorphous oxide was overnightdried at 100 1C and then calcined at 450 1C for 2 hours. The photo-degradation was studied in the presence of oxygen or H2O2.

The mechanism of enhanced photocatalysis of TiO2 with Fe3 was

studied by Zhang et al.96 using Sulfadiazine (SD) as the model compoundunder different photolysis conditions. The results are interpreted with amechanism of enhanced photocatalysis of TiO2 by Fe

3 in the presentstudy. Both oxygen and Fe3 act as the electron acceptors of TiO2.

A study has been reported investigating the photocatalytic activity ofTiO2 samples enriched with several percentages of Mn for the degradationof gaseous acetaldehyde in the presence of both O2 and UV-visible light atatmospheric pressures and room temperature.98 Mn doped TiO2 sampleswere prepared by a modified sol gel method. The photocatalyst wasobtained by precipitating titanium dioxide on a sol of manganese dioxide.Photolysis was carried out using different light sources in the UV and visiblelight region. At low loading, as in the case of 0.1% Mn doped TiO2 , thesub-band-gap response was present in the whole range from 3.15 eV toalmost 2 eV demonstrating that the introduction of Mn in the TiO2 latticedoes increase significantly the material sub-band-gap absorption. Dopingof TiO2 samples with higher levels than 1% of Mn, although it resultsin higher absorbance in the visible, it leads to substantial lower

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photocatalytic decomposition yields of CH3CHO under both UV andvisible irradiation.

An effective co-doping approach is described to modify the photoelec-trochemical properties of anatase TiO2 by doping with nonmetal (N or C)and transition metal (Nb or Ta) impurities.104 Here, compensated andnon-compensated co-doped TiO2 systems are constructed with differentproportions and dopant species, and then their dopant formation energiesand electronic properties are calculated to study the stability and visible-light photoactivity by first-principles density functional theory incorporat-ing the LDAU formalism, respectively.

2.1.2.3 Plasmonic effects of noble metal ions loaded titania photocatalysts.Metal nanoparticles in the size regime 110nm exhibit characteristic size andshape dependent properties due to better charge transfer that cannot occur inbulk materials because the exposed surface atoms are 50 times less thanquantum size particles . Noble-metal nanoparticles (NPs) such as Au and Agcan respond to visible light due to the localized surface plasmon resonance(LSPR), which is produced by the collective oscillations of the surface elec-trons, exhibiting great potential for extending the light absorption range ofwide band gap semiconductors. This surface plasmon resonance effect hasbeen studied in various forms of titania photocatalysts.105117

Lu et al.105 prepared photonic crystals of TiO2 using 240nm polystyrenebeads and then infiltrated 15nmAu nanoparticles (NP) into photonic crystalsof TiO2. The light absorption by Au NP was amplified. Rate constant for thephotodegradation of 2,4-dichlorophenol was 2.3 times faster as compared tothe reference catalyst made using mesoporous film (TiO2 MP/Au NP).Kochaveedu et al.106 decorated Silica-TiO2 core shell particles (SiO2@TiO2 )with Au nanoparticles to examine plasmonic effects. An optimized systememploying Au NP arrays with 15nm size and 700/mm2 density showed bestcatalytic efficiency due to a synergistic effect of the firm contact between AuNPs and TiO2 . Kaur and Pal

108 reported results on the effect of supportedAu nanoparticles of various sizes and shapes on its cocatalytic activityimparted to TiO2 during photocatalytic oxidation of salicylic acid.

Vertically oriented TiOxNy nanopillar arrays with embedded Ag nano-particles have also been examined for photocatalysis in the visible lightregion.113 N-doped titania nanopillars were prepared by reactive DCsputtering to produce TiN followed by high temperature oxidation(450800 1C) in the presence of oxygen. By controlling the oxidationconditions, the band gap of the prepared TiOxNy can be tuned to differentwavelength within the range of visible light. Higher temperature oxidation(>800 1C) leads to rutile formation. Surface plasmon effects of Au nano-particles deposited on N-doped titania in the photocatalytic reduction ofCr(VI) has also been reported.110

Shiraishi et al.107 found that Pt nanoparticles supported on AnataseTiO2 also exhibit absorption in the visible light region assigned to surfaceplasmon resonance SPR. The intensity of these SPR band however wasweak as compared to that obtained with Au nanoparticles. Ag nanoparticlesdecorated SiO2 nanospheres were also found to exhibit remarkablesurface plasmon mediated photocatalytic activities.109 The as-prepared

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Ag-decorated SiO2 NSs showed a significantly red shifted and relativelybroad SPR absorption when compared with the individually dispersed Agnanoparticles. Owing to the considerably broad SPR absorption thatspanned from the visible to the near-infrared region, Ag-decorated SiO2 NSssurpassed N-doped P-25 TiO2 powder and individually dispersed Ag nano-particles in photocatalytic activity. Facile synthesis of a variety of metal-decorated TiO2 nanoparticles using a novel photoirradiation approach hasbeen reported,111 wherein polyoxometalate (POM) molecules sandwichedbetween TiO2 and metal nanoparticles were employed as UV-switchableagents to control the metal loading onto TiO2 surface. A TiO2 /POM/metalnanocomposite essentially act as a cocatalytic system, wherein not only thephotocatalytic efficiency of this system is significantly improved, this com-posite also showed good activity for visible light photocatalysis.

Cao et al.,116 found that, for the methyl orange photodegradation, acomposite photocatalyst AgI/AgCl/TiO2 showed much higher photo-catalytic activity than AgCl/TiO2 and AgI/TiO2 under visible-light irra-diation (lW400 nm). When the molar percentage of AgI to initial AgCl is20% (sample SE-20%), the maximal degradation efficiency of MO reached85.8% after irradiation for 120 min. The enhancement of photocatalyticactivity of the composite photocatalyst AgI/AgCl/TiO2 will be attributed toits good absorption in the visible-light region. Photoluminescence studiesusing terephthalic acid as a probe has been used to probe the mechanisticdetails. The detection of reactive species by radical scavengers displays thatO2

d and H2O2 are the main reactive species for the degradation of MOunder visible-light irradiation.

The role of ionic silver, Ag, and metallic silver, Ag0, on the potentialphotocatalytic activity of a titanosilicate ETS-10 semiconductor hasbeen studied.117 Unlike as-synthesized ETS-10 and Ag-ETS-10 samples,Ag0-ETS-10 samples showed absorbance in the visible region. Theabsorption bands atB350 nm andB460 nm observed for Ag0-ETS-10 wereattributed to the surface plasmon resonance of Ag0 nanoparticles on thesurface of ETS-10 crystals. In contrast to as-synthesized silica sample, whichwas essentially inactive in this illumination range, all Ag and Ag0 samplessupported on titanosilicate showed apparent enhanced activity for thedegradation of MB under visible light (420630 nm) irradiation.

2.1.2.4 Anion doping of titania. Anionic doping has been a recent newinnovative concept associated with the narrowing of the band gap energyobtained by substituting foreign anions to O2

anions. In a pioneeringpaper, Asahi et al.,118 reported on the preparation of visible light activephotocatalysts, N-doped TiO2 prepared via sputtering of TiO2 in N2/Ar gasmixture and subsequent annealing in N2 atmosphere at 550 1C. Visible lightactive N-doped titania catalysts have also been prepared via ammonialysis(high temperature treatment with NH3 gas). Nitrogen-doped TiO2 nano-catalysts with a homogeneous anatase structure were successfully synthe-sized through a microemulsion-hydrothermal method by using someorganic compounds such as triethylamine, urea, thiourea, and hydrazinehydrate. During the last two years, several papers appeared dealing withanion-doped titania photocatalysts.119130

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For nitrogen doping (N-doping different from n-type doping), accordingto the valence induction law, it should be proved (i) that nitrogen isreally present in the nitride state N3, (ii) that N3 anions are in O2

latticebulk positions and (iii) that, under oxidizing working conditions, titaniahas no tendency to perform self-cleaning by expulsion of N3 anionsfrom the anionic sub-lattice via their oxidation with a favorableprogressive decrease of the ionic radius of element N from 1.71 A to 0.55,0.25, 0.16 and eventually to 0.13 A, corresponding to the progressivelyincreasing oxidation states of N equal to 3, 0, 1, 3 and 5,respectively.)

The visible light response and enhanced catalytic activity can arise fromthe substitutional doping (Ti-N-Ti) or via interstitial N impurities (Ti-O-Nor Ti-N-O). The N3 anions can go at the lattice bulk positions of O2

causing a bandgap narrowing driving by mixing of N 2p states with the O 2pstates.129 Umezawa and coworkers130 have made Density Functional The-ory (DFT) Calculations to determine the deep impurity states in N-dopedanatase. Hybridization of N p with Ti d states of Ti is believed to give rise toa new band at the valence band edge. A plausible mechanism for the visiblelight absorption of the catalyst has been proposed.

Rice grain-shaped Nitrogen-doped titanium dioxide (N-TiO2 ) nano/mesostructures were fabricated through a combination of sol-gel and elec-trospinning methods.127 The fibers formed initially are continuous butbreak up into rice grain-shaped nanostrucutures upon thermal treatment at500 1C for 1 hour. The average size of the particles is in the range of5080 nm. The presence of N in the TiO2 lattice was confirmed by X-rayphotoelectron spectroscopy (XPS). The band-gap of TiO2 reduced from3.19 eV to 2.83 eV upon increasing doping level of N from 0% to 5%(w/w),respectively. The N-TiO2 rice grains showed an enhanced UV light-assistedphotocatalysis compared to pure TiO2 in the photodegradation of AlizarinRed S dye, an industrially important anthraquinone dye.

Wang et al.131 have reported on the effects of annealing temperature onthe photocatalytic activity of nitrogen-doped (N-doped) titanium oxide(TiO2) thin films deposited on soda-lime-silica slide glass by radio frequency(RF) magnetron sputtering. As-deposited N-doped TiO2 thin films havebeen found to be nearly amorphous. The rutile and anatase phases coexistedwhen the N-doped TiO2 thin films were annealed at 623 and 823K for 1 h,respectively. Chi et al.123 have used micro-arc oxidation method to prepareN-doped and N-Eu co-doped titania catalysts and studied their photo-catalytic performance.

Photodegradation of the antibiotic Cefazolin has been found to occur atenhanced rate when N-doped TiO2 catalysts are employed as compared toDegussa P25.122 The decomposition of 3-chlorophenol on N-modified TiO2photocatalyst has also been reported.121 Selvam and Swaminathan125 havedeveloped the visible light photolysis of N-doped TiO2 in the presence ofnitrobenzene as a synthetic method for quinaldines.

2.1.3 Surface modifications of titania photocatalysts. Titania dispersedas particulates in aqueous solutions expose a large surface area. Being anamphoteric oxide, the surface is filled with hydroxyl groups and its partially

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ionized forms. It has been found that simple adsorption of ions to thesurface can influence significantly the photocatalytic properties.

2.1.3.1 Superhydrophilic and superhydrophobic titania photocatalysts forair cleaning. Depending on the method of preparation and thermal treat-ments, titania can have isoelectric point anywhere between 4 to 6. In thepresence of moist air/humidity, acids or bases, the surface hydroxyl groupscan undergo reversible protonation, deprotonation. Wetting of the surfacedepends critically on the surface properties. Freshly calcined TiO2 samplesurface is very hydrophilic, as indicated by low contact angle measured forwetting by water droplets. Upon free standing in air, the surface becomespartially hydrophobic. The hydrophobicity can be increased significantly byadsorption of long chain fatty acids. Thus, with suitable acid/base treatmentand additives adsorption, it is possible to tune the hydrophobic/ hydrophilicproperties of titania. Such super hydrophilic materials find applicationsin air-cleaning and as window coatings.132139 Fujishima and coworkershave exploited this tenability to make Self-cleaning windows. Adsorbedorganic products (dirts) are photodegraded by the titania pigments of thewindow glasses, leaving the surface superhydrophilic. Simple washing ofthe window glass removes the degraded organics leaving a clean surface.Self cleaning windows are available commercially from Pilkington, Saint-Goban and other glass companies. Fujishima and others have recentlyreviewed new photocatalysis applications of this kind.132134

Using long chain fatty acids, it is possible to prepare TiO2 surfaces thatare hydrophobic or superhydrophobic. Fujishima and coworkers have usedhydrophobic compounds such as octadodecylphosphonic acid (ODP),octadecyl-trimethoxysilane (ODS), or fluoroalkylsilane.

Photodecomposition of the adsorbed layer using photocatalysts convertsthese surfaces to be hydrophilic and even superhydrophilic. Using pat-terning photomasks, it is possible to make wettability patterns on a self-assembled monolayer of hydrophobic molecules such as ODS. Wettabilitypatterns are used in many fields such as offset printing and in fluidmicrochips.

Ramakrishna and coworkers135 have used electrospinning method toprepare rice-shaped nano/mesostructures deposited on glass substrates. Thewater contact angle decreased with increase in the film thickness resulting insuperhydrophilic transparent coatings for possible use in air cleaning and inwindow coatings. The self-cleaning property of the TiO2 film consisting ofrice-shaped nano/mesostructures is compared with that of commerciallyavailable Degussa P-25 in the photodegradation of Alizarin red dye andfound to be more effective. Xi et al.136 have used a adsorptive self-assemblymethod to prepare TiO2 thin films for self-cleaning applications. By tuningthe number of process cycles (i.e., adsorption-then-heating) of TiO2 col-loidal suspension, we could facilely prepare large-area TiO2 films at adesired thickness and with uniform crystallite morphology. As-preparedanatase TiO2 thin films also display superhydrophilicity and excellentphotocatalytic activity for self-cleaning application.

Verma et al.136 have reported on a solution-based adsorptive self-assembly approach to fabricate anatase TiO2 thin films on different glass

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substrates such as simple plane glass and patterned glass at variablecompositions (normal soda lime glass or solar-grade borofloat glass).As-prepared nanostructured TiO2 thin films on glass substrates do not causedeterioration in optical transmission of glass. The films display super-hydrophilicity and excellent photocatalytic activity for self-cleaning appli-cation. Photocatalytic degradation of methyl orange indicates that thesethin films are indeed highly effective, in comparison to other commercialTiO2 thin films under identical testing conditions. Mesoporous titania-silicacomposites that have photocatalytic activity have been synthesized bymixing ethoxysilane oligomers and titanium dioxide nanoparticles in thepresence of a nonionic surfactant (n-octylamine).139,140 The resultingnanomaterials create effective adhesive and crack-free coatings for stone.These coatings give self-cleaning properties to stone and improve itsmechanical resistance.

2.1.3.2 Surface modification with adsorption of anions. Phosphate anionsare known to adsorb strongly on titania surfaces. A simple strategy togreatly increase the thermal stability of nanocrystalline anatase has beendescribed through this adsorption by Jing et al.141 Efficient TiO2-basedphotocatalysts prepared under ultraviolet irradiation, via the surfacemodification with phosphate anions. Compared to un-modified TiO2, themodified TiO2 calcined at high temperature (over 700 1C) exhibits muchhigh photocatalytic activity for degrading Rhodamine B (or phenol) solu-tion, even superior to the commercial P25 TiO2. In a related study theseauthors142 prepared nanocrystalline TiO2 films by a doctor blade methodfollowed by post-treatment with sodium orthophosphate solution.Phosphate-modified TiO2 was effective for the photodecomposition ofwater . Zhu et al.143 prepared a multilayer photoactive coating containingsurface fluorinated TiO2 nanoparticles and hybrid matrices by sol gelapproach based on renewable chitosan was applied on poly(lactic acid)(PLA) film by a step wise spin-coating method. The photocatalytic activityof resulting highly transparent films has been studied for the degradation ofMethyl orange as a pollutant.

2.1.3.3 Titania photocatalysts on various supports (zeolites, clays, etc).To improve the surface area, titania photocatalysts have been deposited ona number of high surface area support materials such as Zeolites and clayminerals.144152 TiO2 hybridized with HZSM-5 zeolites has been preparedby a simple solid state dispersion method and studied the photocatalyticoxidation of NOx over TiO2 /HZSM-5 having different Si/Al radios and atdifferent humidity levels.146 Co impregnation has been examined in Zeolitesupported TiO2 photocatalysts for photoinduced water splitting reaction.Different sources of Co (as nitrate or chloride) and zeolites (Na-Y andNa-Mordenite) have been studied. Although nitrate based catalyst showedincreased visible light absorption, they produced 30% lower H2 rate incomparison to chloride salts.144 Verbruggen et al. report145 on a fast, lowcost immobilization method involving a one step suspension coating ofpre-synthesized colloidal TiO2 particles and titania nanotubes on glassbeads and studied the efficiency of the supported catalyst in the gas phasephotodegradation of ethylene.

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Vermiculite is a layered magnesium aluminium silicate with 2:1 crystallinestructures. It consists of two silica tetrahedral sheets and one magnesiumoctahedral sheet. Heat treatment at high temperatures enables it to becomea highly porous exfoliated material with high adsorption and selectiveion-exchange capabilities. TiO2 photocatalyst was incorporated into acid-treated vermiculite and the photocatalytic activity of TiO2/vermiculitecomposites was evaluated by removal of methylene blue (MB).148 Flyash cenospheres, a low cost alumino-silicate rich by-products generatedin coal-fired power plants has been used to support hollow sphere TiO2.

149

The polypyrrole-sensitized TiO2 layer on fly ash cenosphere (PPy-TiO2/FAC) was successfully used as a photocatalyst for methylene bluephotodegradation.

TiO2/fly-ash cenosphere substrate has also been used150 to prepare

photocatalysts modified with heteropolyacid by sol-gel followed solvo-thermal method at 40 1C. The modified catalyst was found to be veryeffective in causing photodegradation of ciprofloxacin antibiotic. Themolecular/ions surface imprinted TiO2/fly-ash cenospheres photocatalysthas been prepared by molecular imprinted technology with coordinationcompounds of La3 and oxytetracycline as molecular template. Thisphotocatalyst showed preferential selectivity in the photodegradation ofantibiotics. For example, the relatively coefficient of selectivity degradationon oxytetracycline could reach 4.5146 compared with the ciprofloxacin.

Aluminum silicate fiber is a fire-resistant and thermal insulation materialand has been widely used in industry and building. A novel TiO2 -aluminumsilicate fiber (TAS) nanocomposite, synthesized by a sol-gel method, isproposed to use as a photocatalyst for the removal of multiple pollutants.The TAS nanocomposite has excellent mechanical and thermal propertiesand high corrosion resistance. The TAS calcined at 500 1C exhibited thebiggest BET surface area and highest photocatalytic activity for the removalof) in a simulated coal combustion flue gas SO2, NO and also elementalmercury.151

There are few studies on nanocomposite titania photocatalysts withconducting polymers.153,154 Liao et al.153 have described the synthesis ofpolyaniline (PANI) modified core-shell mesoporous TiO2 photocatalyst(PANI/m-TiO2) by hydrothermal synthesis followed by chemisorption.PANI/M-TiO2 possessed a unique core-shell structure allowing multiplereflection or scattering of light in the photocatalyst and led to the increase ofoptical path length. Hence increasing loading of PANI and associated pathlength increase leads to significant enhancement in the visible light absorptionof the photocatalyst. For the photodegradation of Rhodamine B dye, thecatalytic activity increased with increasing amounts of PANI, with optimalperformance obtained at 6%. In addition to Rhodamine, the photo-degradation of 4-chlorophenol has also been examined. In a related workconducting polymer of polyaniline PANI -TiO2 nanocomposites were syn-thesized via a hydrothermal method154 and followed by a low-temperaturecalcination treatment process. It was found that such a PANI/TiO2 nano-composite exhibits higher photocatalytic activity and stability than bare TiO2and TiO2 xN(x) toward the liquid-phase degradation of methyl orange (MO)under both UV and visible light (420nmolo800 nm) irradiation.

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2.1.3.4 Nanocomposites of photocatalysts and Carbon allotrophs. Inrecent years procedures have been developed for the synthesis andcharacterization of various allotrophic forms of carbon, particularly asnanostructures. There have been several studies of dispersing titania onvarious allotrophic forms of carbon, on graphene in particular.155168

Graphene can be viewed as one atom thick sheet of graphite. Few- andsingle-layer transferable graphene nanosheets are usually obtained bymechanical exfoliation (Scotch-tape method) of bulk graphite and byepitaxial chemical vapor deposition. Graphene oxide graphite (also referredto as reduced graphene oxide rGO) is one that has been oxidized to inter-sperse the carbon layers with oxygen molecules, and then reduced, toseparate the carbon layers completely into individual or few layer graphene.

Carbon nanotubes, graphene and graphene oxide, C60 and relatedmaterials have been found to be good support materials and even as tem-plates for titania-based photocatalysts. They all exhibit excellent chargecarrier mobility, a large surface area and good electrical conductivity. Usedas a template, graphene can help control the morphology of the TiO2nanoparticle by controlling the nucleation and growth processes. In nearlyall cases, photocatalytic performance improves significantly when the TiO2photocatalyst is dispersed as nanocomposites with different carbon allo-trophs. A one-step colloidal blending method has been used to prepare aTiO2-graphene oxide composites for the photocatalytic degradation ofmethylene blue dye.165 Remarkable improvement in the dye degradationwas found with the GO nanocomposite as compared to the parent DegussaP25 catalyst.

Several studies have indicated TiO2-CNT (carbon nanotube) nano-composites to exhibit enhanced photocatalysis compared to pure TiO2 for theoxidation of organic compounds. Eder has provided a recent review on thesynthesis, properties of CNT-inorganic oxide photocatalysts.155 The CNTshave superior electronic properties such as large electron storage and canaccept photons and excited electrons in mixtures with TiO2 and henceretarding recombination of the charge carriers. Two possible mechanismshave been discussed in the literature: a photoinduced transfer of electronsfrom TiO2 to the CNT thereby enhancing the charge separation or CNT itselfacting as a sensitizer pumping electrons into the conduction band of TiO2.Electrospinning technique has been used to prepare Anatase-multiwallcarbon nanotube (MWCNT) composites with negligibly small percentage ofrutile and brookite polymorphs. The nanocomposites are obtained by hightemperature sintering of as-spun composite fibers. Enhanced photocatalysiswas confirmed for the nanocomposite as compared to pristine TiO2 ricegrains and P25 particulates for Alizarin Red oxidation.158,159

The role of dimensionality of the substrate and hosts as nanotubesor nanosheets on the photocatalytic activity has been examined recently.Carbon nanotube-titania nanosheet and graphene-titania nanosheetcomposites with low carbon defect densities are compared.156 The resulting2D-2D graphene-titania nanosheet composites yield superior electroniccoupling compared to 1D-2D carbon nanotube-titania nanosheet compo-sites, leading to greater enhancement factors for CO2 photoreduction

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under ultraviolet irradiation. Kim et al.157 prepared Graphene/Carboncomposite nanofibers (CCNF) with attached TiO2 nanoparticles andstudied their performance in the photodegradation of methylene blue dye.A significant increase in the reaction rate was observed with TiO2-CCNFmaterials under visible light. This increase is due to the high migrationefficiency of photoinduced electrons and the inhibition of charge-carrier recombination due to the electronic interaction between TiO2 andgraphene.

The effect of activated carbon on the structural and photocatalyticproperties of titania has been studied using a series of catalysts prepared byhydrothermal and reflux methods. Hydrothermal method gave catalystswith mesoporous structures while reflex method yielded micropores withdecreased mesoporosity. The photocatalyst prepared via reflex methodshowed a better performance for the light induced removal of Rhodamine Bdye.160 Hierarchical porous TiO2-carbon hybrid composites (TiO2@C)with a hollow structure have been successfully fabricated by a one-potlow-temperature solvothermal approach in the presence of dodecylamine.The hybrid hollow sphere catalysts exhibited enhanced activity for thephotodegradation of Rhodamine B dye.167 Ravindrananathan Thampiet al.162 prepared a series of C-doped, W- doped and C, W-codoped TiO2samples using modified sol-gel techniques in the presence of MelamineBorate. Visible light-induced photocatalytic activity of C-doped materialwas superior to that of an undoped catalyst.

A series of TiO2-reduced graphene oxide RGO nanocomposites havebeen prepared by one step hydrothermal reaction using TiCl4 and grapheneoxide without any reducing agents. The synthesized nanocomposites ofbiphasic TiO2 with RGO showed enhanced catalytic activity compared toconventional TiO2 photocatalyst, P25. The photocatalytic activity isstrongly affected by the concentration of RGO in the nanocomposites,164

TiO2 nanoparticles supported on two different forms of activated carbon(Darco G60 and Norit SA UF) have been prepared and tested for thedegradation of Acid Red 88 dye.168

2.1.4 Applications. Degradation of toxic and industrial waste organiccompounds to small (C1) chemicals has been the major application ofheterogeneous photocatalysis. While Degussa P25 particulates are used asstandard or reference photocatalyst, organic dyes Methylene Blue,Methyl Orange, Eosin and Rhodamine B are often used as bench markreference dyes. Herein we review some recent studies on the photo-degradation of simple organic molecules as well larger organic dyes.169180

2.1.4.1 Photodegradation of organic molecules. Li et al.169 have used acombination of experimental techniques to study the mechanism ofphotodegradation of methyl orange on TiO2, whether the primary oxidationstep involve direct electron transfer between the substrate and positive holesor secondary active species. Active species such as holes, electrons, hydroxylradicals (dOH), and superoxide radicals (O2

d) involved in the photo-degradation process of methyl orange (MO) over TiO2 photocatalyst weredetected by several techniques. Using different types of active species

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scavengers, the results showed that the MO oxidation was driven mainly bythe participation of O2

d, holes and dOH radicals.The degradation pathways for citric acid degradation by

TiO2-heterogeneous photo-catalysis were studied through the analysis of itsdegradation intermediate products.178 The most important product formedwas 3-oxoglutaric acid, but several other compounds, like acetoacetic,lactic, pyruvic, malic, glyoxylic, acetic and formic acids plus acetone werefound. While citric acid is present, the intermediates cannot be photo-catalytically degraded. Degradation of nitrophenol has been studied usingTiO2 photocatalysts (commercial P25) and sulphuric acid-treated activatedcarbon catalysts.180

2.1.4.2 Photodegradation of toxic pollutants (drugs, bacterial, industrialwastes, etc). Removal of bacterial and pathogenic microorganism con-taminants in drinking water and waste water is one of the major objectivesof photocatalysis. A number of publications appeared in this area.181192

Photocatalysis and other toxic waste removal schemes attempt toconvert toxic pollutants to smaller components which are biodegradable.Biodegradation is a natural process carried out by microorganisms usingoxygen and Sturm test is often used to determine if a given chemical isbiodegradable or not. In a recent report, the use of photocatalysis to effectbiodegradability of the antibiotic Tetracycline was investigated using Sturmtest to assess the biodegradability. Tests showed that the byproducts are notbiodegradable.190

UV-irradiation is generally used to destroy the pollutants. Photolysis isusually carried out using entire UV-light (200400 nm, labeled as UV-Cradiation) or limited to near UV region (above 340 nm, labeled as UV-A).Increasingly UV-A light is used for the removal of bacteria, viruses andprotozoan cysts. Two reports have appeared recently examining therelative efficiency of these two types of irradiation on the bacterial modelEscherichia coli (E. coli) and Degussa P25 as the photocatalyst.184 Totaldestruction is not feasible and often the bacteria can repair some of theirDNA damages, becoming more resistant to such treatments. Cyanobacteriaare known to produce a group of potent toxins (neurotoxins and hepato-toxins). Vielela et al., have tested a flat plate glass reactor coated withDegussa P25 TiO2 to follow the degradation of the [D-Leu]-Microcystin-LRtoxin. 150 min. photolysis was necessary to reduce the toxin level to WHOsguidelines for the drinking water.185

Lanao et al.186 studied disinfection of another bacteria, Enterooccus sp.,using a combination of TiO2 and H2O2 and UV-B range photolysis. Onlyradiation in the UV-B range was effective in the inactivation.

The presence of pharmaceutical waste is another major health hazardconcern in the environmental protection. Drugs are designed to have aphysiological effect on humans and animals at very low/trace levels. Oftenthey are often non-biodegradable and hence remain in the environment fora long time. Photocatalytic degradation of drugs has been shown to beeffective in several cases.187192

The UV-A/TiO2 photocatalysis has been used to study the degradation ofthe antibiotic Amoxicillin in aqueous suspensions.187 Eight different

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commercial titania samples were tested and Degussa P25 was found to bemost active, giving 93% mineralization. Similarly Ioannou et al.191 foundP25 was effective to photodegrade (to 80% level) Atenolol and Propranolol(b-blockers used for high blood pressure control) in aqueous solutions. Theefficacy of photolysis in the visible light region using doped titania catalystssuch as carbon-doped titania has been tested for the photodegradation ofthe antibiotic Norfloxacin.189 The influences of catalyst dosage, initial probecompound concentration and solution pH levels on the decay performanceand reaction kinetics were investigated and optimized.

2.1.4.3 Photocatalysis and atmospheric pollution. TiO2 for air purification.There is increasing awareness and concern that the atmosphere above theearth is getting polluted increasingly with increasing number of gaseous andparticulate wastes coming from combustion and other human activities. UNagencies have identified five greenhouse gases (CO2, CH4, N2O, Chloro-flurocarbons (CFCs) and SF6) as chemical agents that cause significant cli-mate changes, including a decrease in the ozone layer of upper atmosphere.Few review publications have appeared on this topic.193195 Photo- andelectrocatalysis studies are being employed and to fix CO2 in the form ofuseful C1 chemicals such as methanol and reforming gas (COH2),196210 todecompose nitrous oxide (N2O) and to oxidize methane gas.

CO2 reduction has an important role. Depending on the photocatalystand experimental conditions, photoreduction of CO2 in aqueous solutionscan give rise to one or more of the following products: CO, CH4, CH3OH,HCHO, and HCOOH (equations 915):

2CO2 2e ! CO CO32 9

CO2 2H 2e ! COH2O Eo 0:53V 10

CO2 2H 2e ! HCOOH Eo 0:61V 11

CO2 4H 4e ! HCHOH2O Eo 0:48V 12

CO2 6H 6e ! CH3OHH2O Eo 0:38V 13

CO2 8H 8e ! CH4 2H2O Eo 0:24V 14

2CO2 12H 12e ! C2H4 4H2O 15

One of the products, CO, can be combined with H2 gas produced fromwater reduction to form the syn-gas and further reductions to methanol ormethane. Sustained photoreduction with high turnover numbers (TON) andturnover frequency rates (TOF) are important for practical applications.Titania photocatalyst carrying various metal deposits such as Pd, Rh, Pt,Au, Cu, or Ru.as co-catalysts have been tested for the reduction of CO2.Various aspects of the photocatalytic reduction of CO2 with water onvarious types of active TiO2 photocatalysts have been reviewed.

196199

Photoreduction under heterogeneous gas-solid conditions produce methane

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as the main product while methanol also forms as a predominant producton highly dispersed TiO2 catalysts anchored on zeolites and mesoporoussilica.200210

In 1987, Thampi, Kiwi and Graetzel203 reported the photo-methanationof CO2 at room temperature and atmospheric pressure using dispersedruthenium and ruthenium oxides loaded onto TiO2. This work was followedup by Anpo and coworkers,204,205 who reported on the formation ofmethanol and CO. Ogura et al.206 found H2 and CH4 as reduction productswhen CeO2-TiO2 photocatalyst was irradiated by visible light. Rajalakshmiet al.207 have reported results of their recent studies on the photoreductionof CO2 on three different TiO2 catalysts (including P25) in aqueous alkalinesolutions. Methane, methanol and ethanol are observed as the majorproducts. Activity in terms of total hydrocarbons production and (methane methanol) yields during six hours run follows the order: P-25>RM-TiO2WUV-100. Ethanol is formed in significant amounts in the case of UV-100and RM-TiO2 but not with P-25. Probable reaction pathways that lead tothe formation of these products have been proposed.

The mechanism of photocatalytic reduction of CO2 with H2O in the gasphase over PtTiO2 films produced by the solgel deposition over glassbeads was investigated.208 Accumulation of Hydrogen gas had an inhibitiveeffect on the photoreduction process, as seen in the slower rate of methaneformation in the presence of added H2. A binary co-catalyst of Pt and Cu2Owith a core-shell structure has been found to be effective for thephotoreduction of CO2, water mixture to methane and CO.

209 It has beenproposed that Cu2O shell provides sites for the preferential activation ofCO2 while Pt core extracts the photogenerated electrons from TiO2. Thesame binary catalyst is also active for the photoreduction of water to H2.

There have been a number of reports on the photoreduction of CO2using various transition metal complexes, particularly metal-carbonyl-polypyridines. A few review publications have appeared summarizing theresearch progress in this area.200,201 Ishitani and coworkers have reported211

on the photoreduction of CO2 using a number of polypyridyl carbonylcomplexes of Ru and in particular on the selective formation of formic acidas the reduction product. HCOOH is an important industrial chemical andalso used as a preservative and an insecticide. a trinuclear complex with twophotosensitizer units and one catalyst unit photocatalyzed CO2 reduction(F(HCOOH)=0.061, TON(HCOOH)=671) with the fastest reaction rate(TOF(HCOOH)=11.6 min1).

Methane is an important green house gas, whose concentration in theatmosphere has to be reduced. It is preferable that the conversion ofmethane yields other industrially useful chemicals along with H2 gas.

212 Insteam reforming methane is converted to H2 and CO2. Taylor

213 hasreported on the oxidation of methane in the presence of water to H2 andmethanol using WO3 and TiO2 as photocatalysts. With WO3 visible lightirradiation is possible. The photocatalyst with the highest level of conver-sion of methane was found to be the lanthanum-doped tungsten oxide.Photocatalytic steam reforming of methane was examined recently byShimura et al.,214 using metal-loaded K2Ti6O13 photocatalysts. Althoughthe production rate was improved by loading Pt cocatalyst on the K2Ti6O13

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photocatalyst, unfavorable formation of CO and gradual deactivation ofphotocatalyst were observed. On the other hand, a Rh-loaded K2Ti6O13sample showed two times higher activity than the Pt-loaded one did,and promoted the selectively without deactivation for many hours. TheRh-loaded titanate catalyst was also tested for the photoredcution of waterto H2.

A novel air-cleaner based on photocatalysis coupled to plasma assistedcatalysis with a TiO2 impregnated Ti-mesh filter has been developed byFujishima and coworkers.215 The air-cleaners were tested by the tobaccosmoke deodorization tests according to both the standard test methodfor air cleaner and the proposed test method. In the standard test method,80100% decrease of the concentrations of gaseous compounds in tobaccosmoke in 1m3 box has been achieved for 30min by the air-cleaner. In arelated work, N-doped titania has been tested as a photocatalyst for airpurification.216 Since N-doped TiO2 absorbs in the visible region, ambientfluorescent lamp light can be used as the light source. A N-TiO2 photo-catalyst prepared via sol-gel process was loaded with Pt catalyst and thePt/N-TiO2 successfully tested in a closed reactor containing a mixtureof volatile organic compounds (VOC) Acetone, ethanol, 2-propanol, TCE,n-hexane and toluene.

Likewise important is nitrous oxide N2O pollutant removal. This is a verystable greenhouse gas. With a lifetime of over 100 years, it is 200 times moreharmful than CO2. Because of its stability it migrates to the stratospherewere N2O is involved in reactions leading to stratospheric ozone reduction.The main source of Nitrous oxide is soil, coming from synthetic nitrogenfertilizers used in agriculture. Other sources are fossil fuel combustion,ammonia from livestock manure and from industrial production of nitricacid and in adipic acid synthesis for use in nylon industry. There has been asustained interest to find suitable thermal and photocatalysts to decomposeN2O. Various metals supported on pure and mixed oxides and zeolitesystems have been studied.217220

A review from Matsuoka and Anpo193 reported that transition metaloxides (Ti, V, Mo, Cr) incorporated within the framework of zeolites as wellas transition metal ions (Cu, Ag, Pr3) exchanged within the zeolitecavities exhibited high and unique photocatalytic activities for variousreactions such as the decomposition of N2O into N2 and O2 or the reductionof CO2 with H2O to produce CH4 and CH3OH. According to Anpo andMatsuoka, the use of zeolites is one of the most promising approaches indesigning efficient photocatalytic systems to eliminate global air and waterpollution. Firstly because their physicochemical properties such as pore sizediameter or channel structure allow to control the photocatalytic activity, aswell as the selectivity of the reaction product. Then because transition metalions can be introduced in the zeolites frameworks by simple methods(ion exchange or hydrothermal synthesis) and the obtained mono-atomicdispersion of metal oxides or ions, results in remarkable enhancement of thephotocatalytic performance as compared with corresponding bulk metaloxides such as TiO2.

Toma et al.210 studied photocatalytic removal of nitrogen oxides onDegussa P25 as a function of catalysts concentration, catalyst morphology

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