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Encyclopedia ofNanoscience andNanotechnology

Nanocrystalline TiO2 for Photocatalysis

Hubert Gnaser, Bernd Huber, Christiane Ziegler

Universität Kaiserslautern, Kaiserslautern, Germany

CONTENTS

1. Introduction2. Electronic and Charge-Transfer Processes

in Photocatalysis3. Preparation of Nanostructured Materials

and Thin Films4. Structural Properties of Nanocrystalline

TiO2 Films5. Electrical Properties of Nanocrystalline

TiO2 Films6. Photocatalytic Properties

of Nanocrystalline TiO2

7. Photocatalytic Applicationsof Nanocrystalline TiO2

GlossaryReferences

1. INTRODUCTIONThe development of novel materials and the assessment oftheir potential application constitutes a major fraction oftoday’s scientific reasearch efforts. In fact, there exist var-ious major governmental research and development pro-grams related to nanostructured materials. Furthermore, itis estimated that nanotechnology has grown into a multi-billion dollar industry and may become the most domi-nant single technology of the twenty-first century. To allowfor this fact, this encyclopedia [1] encompasses a seriesof contributions devoted to a very prominent field of cur-rent materials research activities, namely, nanoscience andnanotechnology. The importance of these developments isreflected also in a number of recent books and articlesreviewing this rapidly evolving field [2–10].

This article focuses on a specific class of such novelnano-scaled materials, nanocrystalline TiO2, and its photo-catalytic properties. The title of this article encompassesthree main terms (“(photo)catalysis,” “nanocrystalline,” and“TiO2”) which, individually, stand for very important areasof scientific research and of, perhaps even more important,technological applications. Their synergistic combination, as

indicated by the present theme, has stimulated great hopesin accomplishing thereby achievements with paramount ben-efits for human beings and the global environment. To out-line the present state of that quest is the major goal of thisarticle.

“Catalysis” is probably the most familiar of the threeterms mentioned. A catalyst is incorporated in essentiallyeverybody’s automobile, with the goal of reducing or eveneliminating the engine’s toxic gaseous components by con-verting them into less harmful (albeit not necessarily benign)substances. As is the case in all catalytic reactions, the cat-alyst itself is not part of the reaction, but is expected toenhance its rate, that is, the velocity of the transformationfrom the original components (the “educts” in the chemist’sterminology) into the final ones (the “products”). Hence, acatalyst is an entity that accelerates a chemical reaction with-out being consumed itself in the process. Without catalysts,various chemical reactions of great importance would pro-ceed too slowly [11]. The economic significance of catalysisis enormous. In the U.S. alone, the annual value of productsmanufactured with the use of catalysts is roughly in the vicin-ity of one trillion dollars [12]. Indeed, more than 80% of theindustrial chemical processes in use nowadays rely on one ormore catalytic reactions [13]. A number of those, includingoil refining, petrochemical processing, and the manufactur-ing of commodity chemicals (olefins, methanol, ethylene gly-col, etc.), are already well established. But many others, aswill be seen in this contribution, represent challenges requir-ing the development of entirely new approaches. But apartfrom their industrial importance, catalytic phenomena effectvirtually all aspects of our lives. They are crucial in manyprocesses occurring in living things, where enzymes are thecatalysts. They are important in the processing of foods andthe production of medicines. The reader may have noticedthat we have as yet refrained from specifying the meaningof photocatalysis; which will be one of the major topics ofthis article. This term refers to a catalytic process that istriggered by illuminating the system by visible light or ultra-violet irradiation. Ideally, that light flux would be the sun’sradiance.

Next we shall consider the meaning of “nanocrystalline.”First, it is noted that in today’s science world ratherinflationary used, the prefix “nano” refers to a fraction of

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Encyclopedia of Nanoscience and NanotechnologyEdited by H. S. Nalwa

Volume 6: Pages (505–535)

506 Nanocrystalline TiO2 for Photocatalysis

one part in one billion (109� and, hence, its correct usagewould require it being connected to some kind of unit (e.g.,of length, time, energy, mass, etc.). In the present context(and in that of “nanotechnology”), “nano” most often relatesto the dimension, that is, the size of an object. Therefore,nanocrystalline in the ensuing discussions will designate par-ticles (of crystalline structure and, primarily, with the chem-ical composition of titanium dioxide) whose typical sizes arein the range of a few to several nanometers (nm), that is, ofthe order of the one billionth part of one meter. Obviously,these are extremely tiny objects and can be “seen” and stud-ied only with the help of sophisticated analytical instrumentslike an electron microscope.

At first glance, it may appear that such tiny particles area rather modern contrivance, but this is probably a prema-ture conclusion. In fact, it is quite firmly established thatnm-sized particles (mostly very refractory ones like corun-dum, diamond, or silicon carbide) are ubiquitous in the uni-verse [14] and that they were already present at the timeand the location of the formation of the solar system. This“stardust” originated from stellar outflows and supernovaejecta, which may have occurred eons before the gas anddust condensed into what is now the sun, the earth, and theplanets. In fact, this dust has intrigued astronomers sincethe days of William Herschel who noted, in the 1780’s, theexistence of small regions in the sky where there appearedto be a complete absence of stars [15]. These regions aremost easily seen against the rich star-fields of the Milky Way.Evidence of the presolar origin of these nanocrystalline par-ticles comes primarily from their isotopic abundance pat-tern [16], which deviates typically to such an extent fromany other known matter that a terrestrial or solar originis virtually impossible. (Most of these particles that havebeen investigated were extracted from primitive meteoritesin which they were incorporated during the formation stageof the solar system; these did not experience any later mod-ification and, hence, preserved the presolar dust particlesunaltered [17].)

Only now, some billions of years later, mankind has ini-tiated the manufacture and application of such nanocrys-talline materials. Nanostructured materials with crystal sizesin the range of 5–50 nm of a variety of materials, includ-ing metals and ceramics, have been artificially synthesized bymany different techniques in the past couple of years [2, 3,5–7]. Such new ultrafine-grained materials have propertiesthat are often significantly different and greatly enhancedas compared to coarser-grained or bulk substances. Thesefavorable changes in properties result generally from theirsmall grain sizes, the large percentage of atoms in grainboundaries and at surfaces, the large surface-to-bulk ratio,and the interaction between individual crystallites. Sincethese features can be tailored to a considerable extent, dur-ing synthesis and processing, such nanophase materials arethought to have great technological potential even beyondtheir current applications.

Let us finally turn to a brief discussion of the third term,“TiO2” ( i.e., titanium dioxide). TiO2 has three different crys-tal structures [18]: rutile, anatase, and brookite; only the for-mer two of them are commonly used in photocatalysis. Likefor many other metal oxides (also for titanium oxide) havethe respective structural, optical, and electronic properties

been elucidated through several decades of intense scientificresearch (for a review see, e.g., [19]); some of them will bereferred to in the course of the present overview. The fea-ture probably most important in the present context is thefact that TiO2 is a semiconductor with a bandgap of ∼ 3.2 eV.On the other hand, TiO2, in its nanocrystalline form, consti-tutes an enormously important commercial product. In fact,the world production of titanium dioxide white pigmentsamounts to some 4.5 million tons per annum and the globalconsumption may be considered a distinct economic indi-cator. White pigments of TiO2 have average particle sizesof around 200–300 nm, optimized for the scatter of whitelight, resulting, thereby, in a hiding power. Reducing thecrystallite size (to ≤ 100 nm), the reflectance of visible light(vis) decreases and the material becomes more transparent;it is widely employed, for example, in paints, plastics, paper,or pharmaceuticals. Nanocrystalline TiO2 exhibits, in addi-tion, a pronounced absorption of ultraviolet (UV) radiation.Because of this high UV absorption and the concurrent hightransparency for visible light, TiO2 particles with a size of<100 nm have found widespread use in such diverse areasas sun cosmetics, packaging materials, or wood protectioncoatings. Hence, although perhaps not generally realized,TiO2 is ubiquitous in our everyday life. Apart from this well-established range of usage, an increasing number of catalyticapplications of nanocrystalline TiO2 have emerged in recentyears.

At this point, it appears appropriate to return to a moregeneral view of the present topic. Nanostructured materialshave generally the potential [3] for incorporating and takingadvantage of a number of size-related effects in condensedmatter ranging from electronic effects (so-called “quantumsize effects”) caused by spatial confinement of delocalizedvalence electrons and altered cooperative (“many-body”)atom phenomena, like lattice vibrations or melting, to thesuppression of such lattice-defect mechanisms as dislocationgeneration and migration in confined grain sizes. The pos-sibilities to assemble size-selected atom clusters into newmaterials with unique or improved properties will likely cre-ate a revolution in our ability to engineer materials withcontrolled optical, electronic, magnetic, mechanical, andchemical properties for many pending future technologicalapplications.

Among those nanocrystalline materials, semiconductorsappear to play a pivotal role in such distinct fields as [20]:

(i) heterogeneous photocatalysis;(ii) photoelectrochemistry, including electrochemical

photovoltaic cells;(iii) photochemistry in zeolites, intercalated materials,

thin films, and membranes (like self-assembled struc-tures);

(iv) supramolecular photochemistry.

This diversity is thought to be largely due to the fact thatheterogeneously dispersed semiconductor surfaces provideboth a fixed environment to influence the chemical reactivityof a wide range of adsorbates and, in addition, a means toinitiate light-induced redox reactivity in these weakly associ-ated molecules [21]. Upon photoexcitation of semiconductornanoparticles, either in solutions or fixed to a suitable sub-strate, simultaneous oxidation and reduction reactions may

Nanocrystalline TiO2 for Photocatalysis 507

occur; molecular oxygen is often assumed to serve as the oxi-dizing agent. Such semiconductor-mediated redox reactionsare commonly grouped under the rubric of heterogeneousphotocatalysis [21].

In a heterogeneous photocatalytic system, photo-inducedmolecular reactions take place at the surface of the cat-alyst. Depending on where the initial excitation occurs,photocatalysis can be generally divided into two classes ofprocesses [22]: In the case that the initial photoexcitationoccurs in the adsorbate molecule, which then interacts withthe ground state catalyst substrate, the process is referredto as a catalyzed photoreaction. On the other hand, whenthe initial excitation takes place in the catalyst substrate andthe excited catalyst transfers an electron or energy into aground state molecule, this process is referred to as a sensi-tized photoreaction [22]. Apparently, a considerable degreeof synergism may be crucial when, for example, a transitionmetal oxide photocatalysts is combined with supramolecularspectral sensitizing ligand complexes used to harvest lightand vectorially transfer photo-generated electrons and holesalong selected energetic pathways.

In 1972, Fujishima and Honda reported [23] the photocat-alytic splitting (i.e., the simultaneous oxidizing and reducing)of water upon illumination of a TiO2 single-crystal electrode;in analogy to natural photosynthesis, they demonstrated thephotoelectrolysis of water making use of a photoexcitedsemiconductor in what was essentially a photochemical bat-tery. In that system, an n-type TiO2 semiconductor elec-trode, which was connected through an electrical load to aplatinum counter electrode, was exposed to near-UV light(cf. Fig. 1). When the surface of the TiO2 electrode wasilluminated with light of wavelength shorter than ∼415 nm,photocurrent was observed to flow [23, 24]. Furthermore,oxygen evolution (i.e., an oxidation reaction) on the TiO2and hydrogen evolution (a reduction) on the Pt electrodewas observed. The photoexcitation of TiO2 injects electronsfrom its valence band into its conduction band; the electronsflow through the external circuit to the Pt cathode wherewater molecules are reduced to hydrogen gas and the holesremain in the TiO2 anode where water molecules are oxi-dized to oxygen. These observations indicate that water canbe decomposed by means of UV-VIS light according to the

hν

+

-

VB

CB

TiO2

load

H2O

O2

PtH2O

H2

e-e-

e-e-

e-

Figure 1. Schematic arrangement for the photosplitting of water in anelectrochemical cell (in the actual setup, both electrodes are immersedin an aqueous solution and the chambers are separated by an ionicallyconducting porous material). When the TiO2 photoanode is irradiatedwith light, O2 evolves from it, whereas H2 evolves from the Pt counter-electrode, while electrons will flow through an external load.

following scheme

TiO2 + 2h� → 2e− + 2h+ electron-hole pairformation in TiO2

H2O+ 2h+ → �1/2�O2 + 2H+ reaction at the TiO2electrode

2H+ + 2e− → H2 reaction at thePt electrode

H2O+ 2h� → �1/2�O2 +H2 overall reaction

It appears to be generally accepted that this discoveryboosted extensive research efforts in the era of heteroge-neous photocatalysis [21, 25–27]. These studies, often car-ried out in an interdisciplinary fashion with the participationof physicists, chemists, and chemical engineers, focused onissues that are of great relevance both economically as wellas ecologically like energy renewal and storage [28–30], thedecomposition of organic compounds in polluted air andwastewaters [31–33], chemical energy generation [34, 35],and photovoltaic devices [36, 37]. Most of these eitheralready proven or envisaged applications are intimatelylinked to the extraordinary properties of nanocrystallineTiO2. In fact, nanocrystalline metal-oxide semiconductorssuch as TiO2 have been applied successfully in moderntechnologies including solar energy conversion, gas sensors,catalysis, and photocatalysis [38–42].

Following the first examination in 1977, using TiO2 todecompose cyanide in water [43, 44], a great deal of efforthas been devoted in recent years to developing heteroge-neous photocatalysts with high photocatalytic activities fortheir applications in solving environmental cleanup and pol-lution remediation problems [31, 32, 45, 46]. Photocatalyticreactions on TiO2 surfaces are very important in such envi-ronmental processes, as the oxidation of organic materialsand the reduction of heavy metal ions in industrial wastewaters. Apart from the utilization for water and air purifi-cation, TiO2 photocatalysis has been shown useful for thedestruction of microorganisms such as bacteria [47] andviruses [48], for the inactivation of cancer cells [46, 49], theclean-up of oil spills [50, 51], and other applications [45]; amore detailed account will be given later in this article.

As mentioned, TiO2 is a semiconductor with a bandgapof 3.2 eV; when excited by light of energy equal to orexceeding that value, electrons are promoted from thevalence band to the conduction band leaving positive holesin the valence band. These electrons and holes are capa-ble of, respectively, reducing and oxidizing compounds atthe TiO2 surface. If the electrons and holes do not recom-bine (and produce heat), they can follow various reactionpathways; it is commonly accepted that the hole is quicklyconverted to the hydroxyl radical (•OH) upon oxidation ofsurface-adsorbed water and that the hydroxyl radical is themajor reactant responsible for the oxidation of organic com-pounds. Typical reductive and oxidative reactions could be


the following [25, 52–54]

TiO2 + h� → TiO2�e− + h+� electron-hole

pair formation

e− +Mn+ → M�n−1�+ reduction reaction

h+ +H2O�ads� → •OH+H+ oxidation ofadsorbed water

•OH+R�ads� → •R�ads� +H2O oxidation oforganic species

where Mn+ is the oxidized compound and R�ads� theadsorbed organic species. Because the production of thehydroxyl radical is considered the decisive step, the determi-nation and optimization of the corresponding quantum yieldin illuminated TiO2 is an important task [55, 56].

According to those concepts, TiO2 nanoparticles areexpected to show a unique surface chemistry due to theirlarger surface area [57]. The origin of the distinct photo-catalytic activities exhibited by nanoparticles of TiO2 is cru-cial in understanding the reaction mechanisms, for example,if they are purely due to the increased surface area or ifthey have to be addressed to the presence of distorted siteson the surface or to the whole lattice of the particles. Inorder to commercialize these treatment techniques, it is ofgreat importance to improve the preparative methods oftitania, because the photocatalytic activity and phase tran-sition behavior of TiO2 are significantly influenced by thepreparative conditions [58–63]. As previously mentioned,these catalytic processes constitute a major issue of this workand will be outlined in the following sections.

The photocatalytic activity of TiO2 is very useful notonly in environmental purification by decomposition oforganic substances, but also in the materials industry such asmirrors and glasses due to its self-cleaning [64] and antifog-ging effects. The latter has been attributed to the photo-induced hydrophilic nature of the surface [65, 66]. A furtherenhancement of the photocatalytic activity could be effectedby means of composite TiO2 materials; examples would bemetal doping, mixing with insulating substances like SiO2 orAl2O3 [67], and monolayer coverage by SiO2 [68].

Another prominent future application of nanocrystallinesemiconductors is thought [69–71] to lie in photovoltaics,that is, the conversion of sunlight into electrical power.The limited reserves of fossil fuels and the increasing con-cern of global warming due to the greenhouse effect causedby the combustion of those fuels has triggered, in the lastdecades, widespread efforts into the development of photo-voltaic devices. With an energy supply from the sun to theearth of 3 × 1024 Joule per year (about 10,000 times theglobal annual energy consumption), this enterprise appearsall but unreasonable. In such solar cells, photon incident ona semiconductor can create electron-hole pairs, basically aresult of the photoelectric effect, discovered by Becquerelalready in 1839 [72]. At a junction between two differentmaterials, this may establish an electrical potential differ-ence across this interface. Until now, the material of choicefor manufacturing such junctions has been (doped) silicon(crystalline or amorphous), with compound semiconductorsalso being considered more recently. While this traditional

approach clearly has room for further improvements, it mayultimately face limitations in terms of cost efficiency (manu-facturing costs per unit of energy produced). Novel materialsand fabrication schemes are therefore explored. A promis-ing approach consists of electrochemical photovoltaic cellsutilizing nanoporous semiconducting electrodes fabricatedby lightly sintering nanosized TiO2 particulates, followed byspectral sensitization with an appropriate dye.

Metal oxide particles with diameters of some 10 nmcan be prepared as paste and spread out over a surfaceof fluorine-doped SnO2 conducting glass to form a three-dimensional network of interconnected nanoparticles. Thesenanostructured metal oxide layers, and in particular thoseconstructed from anatase titanium dioxide (TiO2�, havearoused great interest because of their unprecedented prop-erties as electrodes. They find application in dye-sensitizedsolar cells, which nowadays show light-to-electricity conver-sion efficiencies of 10% [69, 73–77]. The nature of electronmigration in these electrodes has been debated in past yearsas the experimental results and their interpretation does notconverge to a generally accepted model. Instead, the exactrole of electron trapping and the concomitant screening ofthe electric field remain unclear. It is reported that soonafter electrons are injected into the conduction band of TiO2a large fraction of them get trapped in surface states. Migra-tion of these electrons must then proceed with a hopping-type process in which the electrons remain most of the timein localized states [78–83]. On the other hand, it is alsoreported that the injection of electrons into the conductionband shows the so-called “free-electron” absorption, whichextends over a wide spectral range from the visible to theinfrared [84–88]. This suggests that not all injected elec-trons become trapped but that a substantial fraction of themremain in the conduction band.

This article is organized in the following way: Section 2outlines the electronic and charge-transfer processes as rel-evant for photocatalysis, with a special emphasis towardsnanocrystalline TiO2. Since in photocatalysis and relatedapplications the respective nanostructured materials areemployed either in colloidal solutions or attached to a suit-able support (e.g., as electrodes or thin films), both of thesepreparation techniques are discussed (Section 3). Further-more, various approaches for surface and thin-film modifi-cation are described, as well as novel deposition methodsand structures. The structural and electronic properties ofnanocrystalline TiO2 films are examined in Sections 4 and5, respectively. The photocatalytic properties of nanocrys-talline TiO2 constitute the central theme of Section 6, high-lighting the dependence of the photocatalytic activity ondifferent parameters like film structure and phase, surfacemorphology, electronic properties, and the effects inducedby various surface modifications. Representative examplesof photocatalytic applications utilizing nanocrystalline TiO2materials are presented in Section 7. Finally, an extensiveset of references is provided that should be useful for fur-ther study: although the number is substantial, no attemptwas made, however, to be comprehensive; any such attemptmight be bound to fail due to the rapidity with which thisfield is evolving.


2. ELECTRONICAND CHARGE-TRANSFERPROCESSES IN PHOTOCATALYSIS

2.1. Electronic Excitationsand Charge-Carrier Trapping

A photocatalytic process is initiated by the absorption ofphotons by a molecule or the substrate to produce highlyreactive electronically excited states. The efficiency is con-trolled by the system’s light absorption properties. Threefundamental steps are of relevance: (1) the electronic excita-tion of a molecule upon photon absorption, (2) the band-gapexcitation of the semiconductor substrate, and (3) the inter-facial electron transfer. Since a detailed account of theseprocesses has been given in a lucid treatise by Linsebigleret al. [22], we shall briefly summarize here only the moreimportant points, referring thereby partly to that work. Theprobability of an electronic transition can be calculated fromquantum mechanical perturbation theory and is propor-tional to the square of the amplitude of the radiation fieldand the square of the transition dipole moment [89, 90].The latter may be computed via the molecular wave func-tion which, in turn, depends on the product of the electronicspatial wave function, the electronic spin wave function, andthe nuclear wave function. Further arguments [89] lead tosome general selection rules in terms of which electronictransitions are allowed and which might be forbidden. Typ-ically, the excitation of a weak transition will not effectivelyinduce a photochemical reaction, because few of the inci-dent photons will be absorbed (low cross-section); however,an overall high reaction rate may still be possible in the caseof a high quantum yield, that is, if the probability of prod-uct molecule formation per absorbed photon is high. Thephotochemical efficiency will also be determined by whichdeexcitation channels are dominant. In particular, the perti-nent lifetimes of the involved processes are to be considered.Whereas the absorption of a photon occurs very rapidly onthe order of 10−15 s, deexcitation events are much slower,favoring the decay channel which minimizes the lifetime ofthe excited state.

The initial process for heterogeneous photocatalysis oforganic and inorganic compounds by semiconductors is thegeneration of electron-hole pairs in the semiconductor par-ticles. Once excitation across the bandgap has occurred,the lifetime is sufficient (in the nanosecond regime [91])for the created electron-hole pair to undergo charge trans-fer to adsorbed species on the semiconductor surface fromsolution or gas phase. Figure 2 exemplifies these processes.The enlarged section shows the excitation of an electronfrom the valence band to the conduction band initiatedby light absorption with energy equal or greater than thebandgap of the semiconductor. The figure also illustratesseveral deexcitation pathways for the electrons and holes.The electron transfer to adsorbed species or to the solventresults from migration of electrons or holes to the sur-face. At the surface, the semiconductor can donate elec-trons to reduce an electron acceptor (often oxygen in anaerated solution), corresponding to pathway c in Figure 2;conversely, a hole can migrate to the surface where an elec-tron from a donor species can combine with the surface hole

hν

+

hν

+

-

+

-

--+

+

-

++

D

D+

A

A-

a b

c

d

VB

CB

Figure 2. Schematic illustration of the photoexcitation in a semicon-ductor particle followed by deexcitation events. CB and VB designatethe conduction and valence band, respectively. For further details seetext.

oxidizing the donor (pathway d). Competing with chargetransfer to adsorbed species is electron and hole recombi-nation, occurring either in the volume (pathway b) or onthe surface of the semiconductor (pathway a�; in both casesheat will be released. Naturally, electron and hole recom-bination is detrimental to the efficiency of a semiconductorphotocatalyst. Modifications to semiconductor surfaces, suchas metal addition, dopants, or combination with materials,can be beneficial in decreasing the recombination and con-currently increasing the quantum yield of the process.

An efficient means to retard the recombination of pho-toexcited electron-hole pairs may proceed via the trappingof charge carriers. The occurrence of surface and bulk irreg-ularities resulting from the preparation process is associatedwith surface electron states; these may serve as charge car-rier traps and can suppress the recombination of electronsand holes. The charge carriers trapped in such states arelocalized to a specific site on the surface or in the bulk; theirpopulation is dependent on the energy difference betweenthe trap and the bottom of the conduction band. Experi-mental observations of such trapped states in TiO2 will bereported later.

On the basis of laser flash photolysis measurements[45, 92], the characteristic times for the individual stepsoccurring during heterogeneous photocatalysis on TiO2 havebeen derived. Whereas the primary process of charge-carriergeneration is extremely fast (∼fs), charge-carrier recombina-tion may occur on time scales of 10–100 ns. Charge-carriertrapping can be very fast (100 ps) for the (reversible) trap-ping of a conduction-band electron in a shallow trap, butmoderately fast (∼10 ns) for a deep trap or for the surfacetrapping of a valence-band hole, resulting in a surface-boundhydroxyl radical. Finally, interfacial charge transfer can beslow (∼100 ns) for the oxidation of an electron donor orvery slow (∼ms) for the reduction of an electron acceptor.In general, the valence-band holes are powerful oxidants(+1�0 to +3�5 V versus NHE depending on the type of semi-conductor and pH), while the conduction-band electrons aregood reductants (+0.5 to −1.5 V vs NHE) [93]; most organicphotodegradation reactions utilize the oxidizing power ofthese photo-generated holes.


2.2. Band-Edge Positionand Band Bending

The ability of a semiconductor to undergo photo-inducedelectron transfer to adsorbed species on its surface is gov-erned by the band energy position of the semiconductor andthe redox potentials of the adsorbate. The relevant potentialof the acceptor must be below (more positive than) the con-duction band potential of the semiconductor. By contrast,the potential of the donor needs to be above (more nega-tive than) the valence-band position of the semiconductor inorder to donate an electron to the vacant hole. The band-edge positions of several semiconductors are depicted inFigure 3; the internal energy scale is given both with respectto the vacuum level (left scale) and for comparison to nor-mal hydrogen electrode (NHE) in a solution of an aqueouselectrolyte at pH = 1.

When a semiconductor is brought into contact withanother phase (e.g., liquid, gas, or metal), the transferof mobile charges across this junction occurs until elec-tronic equilibrium is reached. This redistribution of chargesinvolves the formation of a space-charge layer, that is, thecharge distribution on each side of the interface differsfrom the bulk material (cf. Fig. 4). Depending on whetherthe electrons accumulate or deplete at the semiconductorside, an accumulation or depletion layer is formed, causingconcurrently a shift in the electrostatic potential and a down-ward (or upward) bending of the bands in the semiconduc-tor. The depletion of electrons may reach such an extent thattheir concentration at the surface decreases below the intrin-sic level. As a consequence, the Fermi level is closer to thevalence than to the conduction band; this situation is calledan inversion layer, as the semiconductor is p-type at thesurface and n-type in the bulk. These features are well docu-mented [94] and will not be further discussed here. Of someinterest in the present context is the situation encountered

0.0

-0.5

-1.0

-2.0

-1.5

0.5

1.0

2.0

3.0

4.0

1.5

2.5

3.5

-4.5

-4.0

-3.5

-2.5

-3.0

-5.0

-5.5

-6.5

-7.5

-8.5

-6.0

-7.0

-8.0

0.0

E [eV]

vacuum

ENHE [eV]

GaAs(n,p)

GaAsP(n,p)

GaP(n,p)

CdSe(n)

CdS(n) ZnO

(n)WO3

(n)SnO2

(n)

TiO2

(n)

SiC(n,p)

∆E=1.4eV

2.25eV

2.25eV 1.7

eV2.5eV

3.2eV

3.2eV

3.8eV

3.2eV

3.0eV

Eu2+/3+

H2/H+

[Fe(CN)6]3-/4-

Fe2+/Fe3+

Ru(bipy)2+/3+

Ce4+/3+

Figure 3. Bandgap energies of various semiconductors in an aqueouselectrolyte at pH = 1.

CB

+ -++

+ -

--

--+

++

+

+

E

Eref

VB

+ -+ +

+

-

---

-++

E

Eref

CB

VB

semiconductor electrolyte

(a) (b)

Ec Ec

EVEV

EFEF

CB

VB

++ +

+-

-+

+

E

Eref

--

-+-

++

+--

-

+

VB

E

ErefCB

-

-+

++

(c) (d)

Ec Ec

EV EV

EF EF

-

--

+

Figure 4. Space-charge layer of an n-type semiconductor in contactwith another phase (e.g., an electrolyte or gas): (a) flat band situation,(b) accumulation layer, (c) depletion layer, and (d) inversion layer.

when an n-type semiconductor like TiO2 is in contact withan electrolyte as in a photoelectrochemical cell [71]; suchdevices are thought to have great potential both in pho-tovoltaics for producing electric current and as fuel cellsfor the generation of hydrogen via photo-cleavage of water.Because of these potential applications, the characteristicsof the semiconductor-electrolyte interface have been inves-tigated in great details [93, 95, 96].

In particular, the potential distribution within a sphericalsemiconductor particle could be derived [97] using a lin-earized Poisson–Boltzmann equation. As discussed in [69],two limiting cases are of special importance for light-inducedelectron transfer in semiconductor dispersions. For largeparticles, the total potential drop within the particle is

�� = kT

2e

(w

LD

)2

(1)

where w is the width of the space charge layer and LD =��0�kT /e2Nd�

0�5 is the Debye screening length [94], whichdepends on the dielctric constant, �, of the material and onthe number density of ionized dopants, Nd. This potentialvariation is identical with that of a planar Schottky deple-tion layer [98]. For very small semiconductor particles (withradius R) the potential drop within the particle becomes

�� = kT

6e

(R

LD

)2

(2)


From the latter equation, it is found that the electrical fieldin nano-sized semiconductors will usually be small and thathigh dopant levels are required to produce a significantpotential difference between the center and the surface. Forexample [69], in order to obtain a 50 meV potential drop in ananocrystalline TiO2 particle with R= 6 nm, a concentrationof 5 × 1019 cm−3 of ionized donor impurities is necessary.Undoped TiO2 nanocrystallites have a much lower carrierconcentration and the band bending within the particles istherefore negligibly small.

2.3. Photo-Induced Charge-TransferProcesses on the Catalyst Surface

The principle of electron and hole transfer at the photoex-cited semiconductor particle has already been alluded topreviously in Fig. 2. Both free and trapped charge carri-ers participate in these interfacial redox reactions. Due tothe quantization effects, by decreasing the particle’s size, itis possible to shift the conduction band to more negativepotentials and the valence band to more positive values. Itwas concluded [99] that the shift of the bandgap is propor-tional to 1/R2, R being the particle size. Therefore, redoxprocesses that cannot occur in bulk materials can be facili-tated in quantized semiconductor particles. Figure 5 showsschematically such possible transfer reactions for an adsor-bate at the surface of a semiconductor. When there areaccessible energy levels in the substrate, an electron maybe transferred from the donor (D) into a substrate leveland then into the acceptor orbital as shown in Figure 5(a).This scheme operates in the photosensitization of semicon-ductor particles by dye molecules. An electron is injectedfrom the excited state dye molecule into the semiconduc-tor, which then reduces another adsorbate particle. Earlyexperimental confirmation [100, 101] of these processes usedthe reduction of methyl viologen in colloidal semiconductorsystems and the water splitting process. Later, such reduc-tive processes have been investigated for many different sys-tems (see, e.g., [102]); some illustrative examples will bepresented in Section 6.

For an initial excitation on the semiconductor substrate(Fig. 5(b)), a positively charged hole is created at the band-edge of the valence band. An electron is transferred into

-

hν

CB

VB

D

A

CB

VB

CB

VB

D

A

D+

A-

+

-

+ e-

e-

-

hνCB

VB

D

A

CB

VB

D*

A

CB

VB

D+

A-

(a)

(b)

Figure 5. Sensitized photoreaction with (a) an initial excitation of theadsorbate, or (b) an initial excitation of the solid.

the empty acceptor orbital and, simultaneously, an electronis donated from the filled donor level to recombine with theoriginal hole. This is a very general process for wide bandgapoxide semiconductors like TiO2 and others. For example,the oxidation of many organic substances in colloidal sus-pensions has been investigated [102]. The energetics of thesemiconductor valence band and the oxidation potential ofthe redox couple influence this photocatalytic oxidation. Forexample, the enhancement in the efficiency of halide oxida-tion at TiO2 follows the sequence Cl− < Br− < I−, corre-lating with the decrease in the oxidation potential. Again,some recent examples will be presented in Section 6.

The kinetic analysis [103] of electron transfer in colloidalsemiconductor systems is often complex. Apart from theenergetics of the conduction band of the semiconductorand the redox potential of the acceptor, factors such asthe surface charges of the colloids, adsorption of the sub-strates, participation of surface states, and competition withcharge recombination influence the rate of charge transfer atthe semiconductor interface [102]. This fact is evident fromthe widely differing rates of experimentally observed chargetransfer rates, with time scales ranging from picoseconds tomilliseconds for different experimental conditions and vari-ous semiconductor systems.

2.4. Quantum-Size Effects

Size quantization effects in metals or semiconductorshave attracted considerable attention in the past decade[104–107]. Semiconductor nanoparticles may experience atransition in terms of electronic properties from those typ-ical for a solid to that of a molecule, in which a com-plete electron delocalization has not yet occurred. Thesequantum-size effects arise when the Bohr radius of the firstexciton (an interacting electron-hole pair) and the semicon-ductor becomes comparable with or larger than that of theparticle; the Bohr radius [94]

rB = 4��0��2/�e2m∗� (3)

depends on the dielectric constant � and the effective massm∗ of the charge carriers (electrons and holes). The lat-ter is frequently radically different for holes and electronsand, in some cases, m∗ is more than an order of magni-tude smaller than the free-electron mass me. Hence, suchquantum-size effects play a role for crystallites of approx-imately 1–10 nm in diameter. Under these conditions, theenergy levels available for the electrons and holes in the con-duction and valence bands become discrete and the effec-tive bandgap of the semiconductor is increased. To a firstapproximation, the energy spacing between quantized lev-els is inversely proportional to the effective mass and thesquare of the particle diameter. A schematic energy diagramresulting from such confinement effects is shown in Figure 6.

Several attempts have been carried out to compute theelectronic energy levels in such quantum dots [99, 108–110].According to these concepts, the energy of the lowest excitedstate of a semiconductor particle with radius R is givenapproximately by

E�R� = Eg +�2�2

2R2

[1m∗

e

+ 1m∗

h

]− 1�8e2

�R(4)


CB

VB

shallowtrap deep trapEg

deep trap

HOMO

LUMO

surfacestate

bulk semiconductor cluster

Figure 6. Schematic correlation diagram relating bulk-semiconductorelectronic states to quantum crystalline states. Adapted from [110],M. G. Bawendi et al., Annu. Rev. Phys. Chem. 41, 477 (1990). © 1990,Annual Reviews.

Here Eg is the bandgap of the bulk semiconductor, the sec-ond term is the quantum energy of localization, increasingas R−2 for both electron and hole, and the third term is theCoulomb attraction [99]; whereas the Coulomb term shiftsE�R� to smaller energy as R, the quantum confinementcontribution increases E�R� as R2. As a result, the appar-ent bandgap will always increase for small enough R. Butwhile the shift can be appreciable (∼1 eV) for small bandgap materials like InSb, it might be considerably smaller(∼0.2 eV) for semiconductors with a large bandgap likeTiO2 or ZnO [99]. Apart from a large effect on the opti-cal properties (e.g., a change in color of the material dueto the blue shift of the absorption), size quantization mayalso lead to major changes in the photocatalytic properties.While these effects have been studied in great detail for sev-eral compound semiconductor materials like CdS, ZnO, orPbS, related data for TiO2 appear to be still rather limited.

2.5. Optical Properties

Semiconductors absorb light below a threshold wavelengthg which is related to the bandgap energy Eg by [93]

g �nm� = 1240/Eg �eV� (5)

Within the solid, the extinction of light follows an exponen-tial law

I = I0 exp�−z� (6)

where z is the penetration depth and is the reciprocalabsorption length. For TiO2, for example, = 2�6×104 cm−1

at a wavelength of 320 nm; this implies that such light isextinguished to 90% after passing a distance of 390 nm inthe semiconductor. Near threshold, the value of increaseswith increasing photon energy. Frequently, a proportionalityof the type

h� = C�h� − Eg�n (7)

provides a satisfactory description of the absorption whereh� is the photon energy. C is a constant scaling with theeffective masses of the charge carriers, but is independentof the photon frequency. The exponent n has a value of0.5 for a direct semiconductor and 2 for an indirect one[94]. Experimental data [111, 112] obtained on both anataseand rutile TiO2 thin films indicated, however, that the actualsituation might be more complex.

In colloidal solution, semiconductor particles reduce thelight intensity of the incident beam both by scattering andabsorption. In the absence of quantum-size effects the extinc-tion spectrum is described by the Mie theory [113, 114].This theoretical approach can be applied to an assembly ofspherical particles if the interparticle distance is larger thanthe wavelength of the incident light (i.e., the particles scat-ter independently); if, furthermore, the particle size is muchsmaller than the wavelength, the energy-dependent absorp-tion cross-section �� for the irradiation of a solution con-taining the particles can be derived [115–117]:

�� = 9Vp

�

c

�3/2s �2

��1 + 2�s�2 + �2

2

(8)

where �� = �1 + i�2 is the complex, frequency-dependentdielectric constant of the semiconductor particle, �s is thedielectric constant of the embedding medium (the solvent),Vp is the volume of a particle, � is the frequency of theincident light, and c is the velocity of light. For the caseof a dilute system of particles with the number density n,�� can be related to the reciprocal absorption length�� = n��. It follows that the imaginary partof the dielectric constant is a direct measure of the lightabsorption by the particles; it increases steeply near theabsorption edge, that is, for � close to the threshold fre-quency. As noted in [69], Mie’s theory has been widelyemployed to interpret the extinction spectra of colloidal sys-tems [118]. For example, the brilliant ruby or yellow colorscaused, respectively, by colloidal gold or silver particles aredistinctly explained by this theory.

3. PREPARATION OFNANOSTRUCTURED MATERIALSAND THIN FILMS

Several distinct techniques have been utilized in recent yearsto synthesize nanocrystalline TiO2 thin films by chemical,electrochemical, and organized assembly methods [69, 119–121]. A simple approach involves casting of the thin filmdirectly from colloidal suspensions [122]. This method ofpreparation is relatively simple and inexpensive comparedwith other existing methods such as chemical vapor deposi-tion or molecular beam epitaxy. Preparation of nanoclustersin polymer films and Langmuir–Blodgett films has also beenconsidered. The sol–gel technique has been found to be use-ful in developing nanostructured semiconductor membraneswith either a two-dimensional or three-dimensional con-figuration. Organic-template-mediated synthesis has beenemployed to develop nanoporous materials. The nanostruc-tured materials are highly porous and can easily be surfacemodified with sensitizers, redox couples, and/or other nano-structured films.

3.1. Preparation fromColloidal Suspensions

Nanostructured semiconductor films of TiO2 have been pre-pared frequently from colloidal suspensions [123–133]. Bycontrolling the preparative conditions of the precursor col-loids, it is possible to tailor the properties of these films.


Typically, these thin TiO2 films exhibit interesting pho-tochromic, electrochromic, photocatalytic, and photoelec-trochemical properties that are inherited from the nativecolloids. The synthetic procedure involves the preparationof ultrasmall particles (diameter 2–10 nm) in aqueous orethanolic solutions by controlled hydrolysis. The colloidalsuspension is coated onto a conducting glass plate (an opti-cally transparent electrode (OTE)) and dried; finally, thefilm is annealed at 200–400 �C in air for some 1–2 h.The conducting surface facilitates direct electrical contact ofthe nanostructured thin film. This simple approach of coat-ing preformed colloids onto a surface and annealing gener-ally produces an oxide film, which is robust and exhibits anexcellent stability in both acidic and alkaline media, a fea-ture very important in several potential applications. Gener-ally, some optimization is required for thicker films and forhigher colloidal concentrations in order to avoid crackingof the films upon drying. Further details of the methodol-ogy of preparation can be found in [134–136]. Transmis-sion electron micrographs of nanostructured films preparedfrom colloidal suspensions show a three-dimensional net-work of oxide nanocrystals of particle diameter as smallas a few nanometers. No significant aggregation or sinter-ing effects are observed during the annealing process. X-raydiffraction analysis also confirms the crystallinity of thesenanostructured films. Composite films, which in some casesmay exhibit improved properties as compared to single-component films, can be manufactured by mixing two ormore components prior to casting of the film.

Titania sol and gel prepared through the hydrolysis oftetrabutyl titanate in acid water solution are sensitive toultraviolet (UV) irradiation and turn into blue color [137].Electron spin-resonance measurement showed that the pho-tochromism was ascribed to the presence of titanium (III)ions in the Ti-O-Ti network. The titanium (III) ions couldbe oxidized by the oxygen in the atmosphere, and then thesol and gel faded slowly. The absorption peaks in the opticalabsorption spectra of the titania gel were attributed to thetransition of 3-dimensional electrons of a trivalent titaniumin certain environments.

The morphology of TiO2 particles affects their catalyticactivity and electrical properties. In recent years, manymethods for preparing TiO2 nanoparticles and thin filmshave been studied [138]. TiO2 nanocrystals prepared by thesol–gel method often have fully hydroxylated surfaces andthese hydroxyl groups have a strong influence on the cat-alytic and photocatalytic properties such as electron-transferrates and reducing properties [139]. In order to developphotocatalysts with high activities, it is very important toprepare porous anatase nanoparticles with a high specificsurface area. Furthermore, the preparation method shouldbe simple and should be a low temperature process. Mix-tures of rutile and anatase precipitates could be obtainedby the hydrothermal treatment of an alcohol solution of Tialkoxide at 573 K [140], while anatase nanoparticles wereprepared by heat treatment of a H2O–EtOH solution ofTiOSO4 at 373 K [141]. Thus, the anatase and rutile particleswere usually formed in a solution by conventional soft chem-ical synthesis methods. The preparation of monodispersedoxide particles by the “sol–gel method” enables the manu-facture of oxide particles through a gel state with a regulated

particle growth rate [142–144]. In a recent study [145],anatase nanoparticles were prepared in a Ti-peroxy gel with-out the collapse of the gel during the particle formationprocess. The gel was made from titanium tetraisopropoxide(Ti(O iPr)4� and H2O2.

The crystallization rate of titania gels was found to bemuch higher in water than in methanol or n-hexane [146];also, the crystallite size of anatase prepared in water waslarger than those in organic solvents. Processing parametersvery often control the crystallite size and phase. Nonag-glomerated, ultrafine anatase particles have been gener-ated by hydrothermally treating sol–gel-derived hydrousoxides [147]. The degree of crystallinity and purity of thesynthesized materials may affect their structural evolutionduring any heat treatment.

TiO2 thin films with different surface structures were pre-pared from alkoxide solutions containing polyethylene glycol(PEG) via the sol–gel method [148]. The larger the amountof PEG added to the precursor solution, the larger the sizeand number of pores produced in the resultant films. WhenPEG was added to the gel, the films decomposed completelyduring heat treatment. The adsorbed hydroxyl content ofsuch porous thin films is found to increase with increasingamount of PEG. However, the transmittance of the filmsdecreases due to the scattering of light by pores of largersize and a higher number in the films. Photocatalytic degra-dation experiments show that methyl orange is efficientlydecolorized in the presence of the TiO2 thin films by expos-ing its aqueous solution to ultraviolet light. However, in filmsdeposited on soda-lime glass [149], diffusion of sodium andcalcium ions from the glass into the nascent TiO2 films wasfound to be detrimental to the photocatalytic activity of theresulting films. Sodium and calcium diffusion into nascentTiO2 films was effectively retarded by a 0�3 �m SiO2 inter-facial layer formed on the soda-lime glass [149, 150].

TiO2 thin film photocatalysts coated onto glass plates wereprepared [151] by thermal decomposition of tetraisopropylorthotitanate with a dip-coating process using alpha-terpineol as a highly viscous solvent. Two types of ligands—polyethylene glycol 600 and (ethoxyethoxy)ethanol—wereadded to the dip-coating solution as the stabilizer of tita-nium alkoxide and thin films were obtained after calcinationat 450 �C for 1 h. The film thickness obtained with a singledipping was proportional to the viscosity of the dip-coatingsolutions. The thin films obtained were transparent with athickness of 1 �m. The crystal form of the films was anatasealone. The thin films were formed with aggregated nano-sized TiO2 single crystals (7–15 nm), and the size of the TiO2crystals became smaller for the polymer-added systems.

Transparent anatase TiO2-based multilayered photocat-alytic films synthesized via a sol–gel process on porousalumina and glass substrates showed a sponge-like micro-structure and a mean crystallite dimension of ca. 8 nm [152].Doping such films with iron(III) impeded the photocatalyticactivity.

The effects of calcination on the microstructures of nano-sized titanium dioxide powders prepared by vapor hydroly-sis was investigated in detail [153–155]: Among the factorsexamined [153], large surface area and good dispersionof the powders in the reaction mixture are favorable tophotoactivity. Conversely, prolonged calcination at high


temperatures is detrimental to photoactivity. Powders pro-duced at higher temperatures are predominantly anataseand are generally more photoactive.

The formation, structure, and photophysical propertiesof functional mixed films of 5,10,15,20-tetra-4-(2-decanoicacid)phenyl porphyrin (TDPP) with TiO2 nanoparticlesformed from the two-dimensional sol–gel process of tetra-butoxyltitanium (TBT) at the air/water interface was reported[156]. The composite multilayer films were assembled bytransferring the mixed monolayer onto quartz plates. Thesensitization of TDPP upon TiO2 nanoparticles was con-firmed by the spectral changes of UV-visible absorption andfluorescence of TDPP in the composite films. Furthermorethe photosensitization greatly affected the photocatalyticactivity of TiO2 particles with respect to the degradation ofmethylene blue.

Crystalline titania thin films were obtained [157] on glassand various kinds of organic substrates at 40–70 �C by deposi-tion from aqueous solutions of titanium tetrafluoride. Trans-parent films consisting of small anatase particles (∼20 nm)exhibited excellent adhesion to relatively hydrophilic sur-faces. Uniform coatings were successfully prepared on sub-strates with complex shapes such as cotton and felt fiber.Growth rate and particle size were controlled by both thedeposition conditions and the addition of an organic surfac-tant. Organic dyes were incorporated into the anatase filmsusing organic-dye dissolving solutions and a surfactant.

3.2. Chemical Precipitation

Rutile-phase nano-sized TiO2 powders having a high spe-cific surface area of 180 m2/g were prepared by homoge-neous precipitation at ambient or very low temperatures(<100 �C) [158]. Ultrafine SnO2 TiO2-coupled particlescould be synthesized [159] by homogeneous precipitation;they were employed for photocatalytic degradation of azodye active red X-3B in aerated solution. The results showthat a very rapid and complete decolorization of the azodye can be achieved, and the photoactivity of the coupledparticles is higher than that of pure ultrafine TiO2, and theoptimum loading of SnO2 on TiO2 is 18.4%. The enhanceddegradation rate of X-3B using coupled photocatalysts isattributed to increased charge separation in these systems.

3.3. Gas Condensation and Consolidation

Another method to synthesize nanostructured materials isby way of gas condensation followed by the in-situ consol-idation under high-vacuum conditions [2]. This approachcan produce ultrafine-grained materials which may exhibitsize-related effects. The basic aspects of the generation ofnanometer-sized materials via gas condensation are concep-tually rather simple [2]: A precursor material, either an ele-ment or a compound, is evaporated in a gas maintained at alow pressure, usually well below one atmosphere. The evap-orated atoms or molecules lose energy via collisions withthe gas atoms and undergo a homogeneous condensationto form atomic or molecular clusters in the highly supersat-urated vicinity of the precursor source. In order to main-tain small cluster sizes, by minimizing further atom/moleculeaccretion and cluster-cluster coalescence, the clusters once

nucleated must be removed from the region of high super-saturation. Since the aggregates are already entrained in thecondensing gas, this is readily accomplished by setting upconditions for moving this gas.

Typically, there are three fundamental rates that essen-tially control the formation of the clusters in the gas-condensation process [160]. These are

(i) the rate of supply of atoms to the region of supersat-uration where condensation occurs,

(ii) the rate of energy removal from the hot atoms viathe condensing medium, the gas,

(iii) the rate of removal of the cluster upon nucleationfrom the supersaturated region.

The clusters that are collected via thermophoresis on thesurface of a cold finger usually form very open, fractal struc-tures. The clusters are held on the collector surface ratherweakly, via Van-der-Waals forces, and are easily removed bymeans of a scraper. The material removed is consolidatedin a compaction unit at typical pressures of 1–2 GPa; thescraping and consolidation are carried out under ultra-high-vacuum conditions in-situ after the removal of the gas fromthe chamber, in order to maximize the cleanliness of theparticle surfaces and the interfaces that are formed.

3.4. Film Deposition by Sputteringand Vacuum-Based Techniques

Crystalline titanium dioxide films are often deposited byvarious techniques employing vacuum conditions, using,for example, RF magnetron [161–166], DC sputtering[167–171], chemical vapor deposition [172, 173], plasmaspraying [174, 175], or related techniques [176, 177]. Rapidelectroplating of photocatalytically highly active TiO2-Znnanocomposite films on steel was achieved [178] and thegas-phase oxidation of CH3CHO was employed as an indi-cator of the photocatalytic activity. Not surprisingly, the filmand surface morphologies and the crystallinity are stronglydependent on the total and the oxygen partial pressure, thedeposition rate, and the phase composition; the resultingphotocatalytic properties can be modified over a wide rangeby those parameters. Such films may show good unifor-mity of thickness over large areas, high optical transmittance(∼80%) in the visible region, and considerable mechanicaldurability [169].

Transmission electron microscopy was used to study [179]the structure, morphology, and orientation of thin TiO2 filmsprepared by reactive magnetron sputtering on glass slidesat different substrate temperatures (100 to 400 �C). Themicrostructure and photocatalytic reactivity of TiO2 filmshave been shown to be functions of deposition tempera-ture. In the temperature range examined, all film sampleshave a porous nanostructure and the dimension of parti-cles grew with increasing deposition temperature. Films areamorphous at temperatures of 100 �C and only the anatasephase forms at 200 �C and above. Films deposited between200 to 300 �C show a preferred orientation, while films at400 �C change into complete random orientation. Deposi-tion at 250 �C yields high efficiency in photocatalytic degra-dation owing to the high degree of preferred orientation andnanocrystalline/nanoporous anatase phase.


Another frequently employed and convenient way for thepreparation of TiO2 thin films is pulsed laser deposition[180], although this technique does not produce nanocrys-talline structures.

3.5. Surface and Film Modifications

The surface of as-deposited films has frequently been mod-ified in the quest to enhance the catalytic activity. Forexample, Fe [181] or Sn [182] ions have been implantedinto transparent and colorless TiO2 thin films fabricated onmicroscope glass slides by DC magnetron reactive sputter-ing using Ar and O2 as working gases. The efficiency ofthis procedure could not as yet be proven unambiguously.Apart from grain size, the presence of reactive species onthe surface [183] may influence the photocatalytic activity.Implanted metal ions (V or Cr) were observed [184] locatedat the lattice positions of Ti4+ in TiO2 and were stabilizedafter calcination of the samples in an O2 ambient at around775 K. Spectroscopic studies showed that the presence ofthese substitutional metal ion species are, in fact, responsiblefor a large shift in the absorption spectra of these catalyststoward visible light regions. Porous anatase TiO2 films weredensified by Zn+ ion implantation up to the ion penetra-tion depth [185]. After the subsequent annealing at 800 �C,the phase transformation from anatase to rutile accompa-nied with grain growth up to the film thickness was observed.In addition, the phase transformation was not induced bythe annealing up to 800 �C with or without preceding Ar+

ion implantation. Thus, the implanted impurity Zn assistedthe phase transformation. Annealing in O2 tends to reducethe rate of phase transformation and create ZnTiO3. Opticalabsorption above the photon energy of 2.9 eV was increasedremarkably by the Zn+ or Zn+ and O+ ion implantation andsubsequent annealing and is due to the phase transformation.

The presence of active species such as Ti3+ and hydoxylon the surface of ultrafine TiO2 particles, prepared by a col-loidal chemical approach and subjected to different heatingtreatments, was inferred [186] from optical absorption andphotoelectron spectra; these species may enhance the pho-tocatalytic activity of particles. Treating TiO2 powder by ahydrogen plasma resulted in a reduction of the oxide par-ticles and electrons trapped at the oxygen-defect sites werefound [187]. Also laser ablation of TiO2 photocatalysts aim-ing at the enhancement of the activity was reported [188].

Noble metal particles of Au, Pt, and Ir were depositedon nanostructured TiO2 films using an electrophoreticapproach [189]. The improved photoelectrochemical per-formance of the semiconductor-metal composite film wasattributed to the shift in the quasi-Fermi level of the com-posite to more negative potentials. Continuous irradiationof the composite films over a long period causes the pho-tocurrent to decrease as the semiconductor-metal interfaceundergoes chemical changes.

Doping of nanostructured TiO2 both as particles and inthin films with a variety of metals has been reported, somecommon examples being Pt [190, 191], Pb [192], Au [193],or others [194]. A shift of the UV-vis absorption towardslonger wavelengths was observed upon Pb doping, whichindicates a decrease in the bandgap of TiO2. In TiO2 filmsembedding Au nanoparticles [193], the specific resistance of

the films experienced a rising phase, followed by a dramaticdrop with an increasing number of Au particles. UltrafinePt particles [191] were embedded into rutile TiO2 particlesby decomposing a colloidal organic-Pt complex, resulting invery narrow size distribution with a mean diameter of 3 nm.These nm-sized Pt particles were found to grow epitaxiallyon the TiO2 crystallites with a well-defined crystallographicrelationship.

Doping a nano-structured TiO2 electrode sensitized withtetrasulfonated gallium phthalocyanine with tetrasulfonatedzinc porphyrin (ZnTsPP) greatly enhances the photoelec-tric conversion at long wavelengths, with 20- and 60-fold improvement of the quantum efficiency at 680 and700 nm [195].

Semiconductor/metal composite nanoparticles have beensynthesized [196] by chemically reducing HAuCl4 on the sur-face of preformed TiO2 nanoparticles. These gold-cappedTiO2 particles (diameter 10–40 nm) were stable in acidic(pH 2–4) aqueous solutions. The role of the gold layerin promoting the photocatalytic charge transfer has beenprobed using thiocyanate oxidation at the semiconductorinterface. More than 40% enhancement in the oxidation effi-ciency was found with TiO2/Au nanoparticles capped withlow concentration of the noble metal.

Magnetic photocatalysts were synthesized by coating tita-nium dioxide particles onto colloidal magnetite and nano-magnetite particles [197]. The photoactivity of the preparedcoated particles was lower than that of single-phase TiO2and was found to decrease with an increase in the heattreatment. These observations were explained in terms ofan unfavorable heterojunction between the titanium dioxideand the iron oxide core, leading to an increase in electron-hole recombination.

TiO2-based powders, doped with a small amount ofSiO2, were prepared by a sol–gel method and were sub-sequently heated to precipitate fine anatase crystals [198].The obtained powders have large specific surface areas(∼200 m2/g) and upon treating them chemically with aque-ous NaOH, the photocatalytic property of the powders wasextremely improved and the powders showed higher activitythan the undoped TiO2 powders.

In composite TiO2-SiO2 thin films prepared by a sol–gelprocess, the refractive index and the photocatalytic activitydecrease with increasing SiO2 content in these films [199].Alkoxide sol–gel processing has been investigated for thesynthesis of stable SiO2-TiO2 high-permeability catalyticmembranes to be used in alkene isomerization [200].Nanocrystalline TiO2 was prepared on mesoporous sil-ica both by sol–gel processes [201] and by an impregna-tion method with titanium complexes featuring differentligands [202].

Binary mixed oxide of Fe/Ti (1:1 composition) with homo-geneous distribution of iron into the TiO2 has been preparedby sol–gel impregnation using metal alkoxide precursors andfiring at different temperatures (500, 700, and 900 �C) [203].The mixed oxide exhibits excellent absorption in the visiblespectral region (570–600 nm). The photocatalytic activity ofthe Fe/Ti oxide reduces to a large extent at a high sinteringtemperature of the sample due to the presence of a increas-ing amount of the inactive (Fe2/TiO5� pseudobrookite phase.Nanostructured TiO2/SnO2 binary oxides were prepared by


combustion of stearic acid precursors [204], with metal pre-cursors being dispersed in the stearic acid at the molecularlevel. It was found that preparative methods affected thecrystalline structure of the powders and the anatase phaseof TiO2 was stabilized by the addition of SnO2.

Spray “painted” (spray deposited) titanium dioxidecoatings were sensitized [205] with chemically depositedcadmium selenide thin films; the structural, optical, and pho-toelectrochemical characterization of these composite filmsindicate the importance of thermal treatments in improvingthe photocurrent quantum yields. Up to 400 �C, the effectof air annealing is to shift the onset of absorption to longerwavelengths and improve the photocurrent substantially.

Organic compounds may play a crucial role in chemicalprocesses for ceramic coatings [206]. Organic compoundsremained in a fixed position in the coating, which was pre-pared from the chemically modified titanium tetraisopropox-ide solution and heated at temperatures as high as 673 K. Itwas not until the organic compounds decomposed to carbondioxide and the gas phase was left from the coating that thenanostructure, consisting of nano-sized pores and anatasecrystallites with preferred orientation, developed at 723 K.

Cobalt(II) 4,4′,4′′,4′′′-tetrasulfophthalocyanine, covalentlylinked to the surface of titanium dioxide particles, TiO2-CoTSP, was shown [207] to be an effective photocatalyst forthe oxidation of sulfur (IV) to sulfur (VI) in aqueous sus-pensions. Upon bandgap illumination of the semiconductor,conduction-band electrons and valence-band holes are sep-arated; the electrons are channeled to the bound CoTSPcomplex resulting in the reduction of dioxygen, while theholes react with adsorbed S(IV) to produce S(VI) in theform of sulfate.

The photoactivity of the Pt/TiO2 system in the visibleregion was improved [208] by the addition of the sensitizer([Ru(dcbpy)2(dpq)]2+� [where dcbpy = 4,4′-dicarboxy 2,2′-bipyridine and dpq = 2,3-bis-(2′-pyridyl)-quinoxaline] lead-ing to an efficient water reduction.

Photocatalytic properties for hydrogen production wereinvestigated [209] on layered titanium compounds interca-lating CdS in the interlayer, which were prepared by directcation exchange reactions and sulfurization processes. Thephotocatalytic activity of the compounds intercalating CdSwas superior to those of simple CdS and the physical mix-ture of CdS and metal oxides. The improvement might beattributed to the formation of microheterojunctions betweenthe CdS nanoparticles and the layers of oxides.

3.6. Novel Deposition Methodsand Structures

In recent years, there has been increased interest in studyingand manufacturing nanoscaled TiO2 materials as nanoparti-cles [210], nanowires [211], nanorods [212], whiskers [213],and nanotubes [214–217].

There are many synthetic routes for the creation ofnanocrystals of oxides and the controlled hydrolysis of metalalkoxides is the most generalized solution-phase syntheticstrategy [218]. Increased photocatalytic activity was reportedrecently for TiO2 prepared by ultrasonic irradiation and gly-cothermal methods. This novel method [219] for prepar-ing highly photoactive nanometer-sized TiO2 photocatalysts

with anatase and brookite phases has been developed byhydrolysis of titanium tetraisopropoxide in pure water ora 1 + 1 EtOH–H2O solution under ultrasonic irradiation;the photocatalytic activity of TiO2 particles prepared by thismethod exceeded that of Degussa P25 and was the firstreport that showed high photocatalytic activity of a photo-catalyst containing an 80% anatase and 20% brookite phase.

A novel and convenient nonhydrolytic approach to thepreparation of uniform, quantum confined TiO2 nanocrys-tals, using an intramolecular adduct stabilized alkoxideprecursor, was described recently [220]. In contrast to estab-lished aqueous sol–gel-techniques, the processing in hydro-carbon solvents at high temperatures allows access to verysmall free-standing crystallites, and opens up new possibili-ties for control over size distribution, surface chemistry, andparticle agglomeration.

It has been reported that the columnar morphologiesin sputtered TiO2 films enhances the photocatalytic [221]and photovoltaic [222] efficiency. Following these studies,enhanced surface-reaction efficiency has been demonstratedin the photocatalysis of sculptured thin films of TiO2 [223].In obliquely deposited films with variously shaped columnssuch as zigzag, cylinder, and helix, the columnar thicknessand spacing play an important role in the enhancement ofthe effective surface area, while the columnar shape is lessimportant. The optimum morphology for a surface reactionhas been obtained at the deposition angle of 70�, where thephotocatalytic activity is 2.5 times larger than that at 0�. Themorphology of these obliquely deposited thin films appearswell suited for application as solar cells, electro- and pho-tochromic devices, and photocatalysts.

The template method for synthesizing nanostructuresinvolves the synthesis of the desired material within thepores of a nanoporous membrane or other solid. Thisapproach has been used in several experiments [224–229] forthe preparation of TiO2 nanotubes and nanorods; typically,porous aluminum oxide (PAO) nano-templates were used.

Compact, continuous, and uniform anatase nanotubuleswith diameters in the range 50–70 nm were producedinside PAO nano-templates by pressure impregnating thePAO pores with titanium isopropoxide and then oxidativelydecomposing the reagent at 500 �C [230]. Cleaning the sur-face of the template and repeating the process several timesproduced titania nanotubules with a wall thickness of ∼3 nmper impregnation. The tube exteriors appeared to be faithfulreplicas of the pores in which they were formed.

Nano-TiO2 whiskers were prepared by various techniques[213, 231]; using, for example, controlled hydrolysis oftitanium butoxide [231] it was found that the nano-TiO2whiskers obtained were anatase and grew selectively in the[001] direction with a diameter of about 4 nm and a lengthof about 40 nm. Acetic acid played an important role in theoriented growth of nano-TiO2 whiskers.

Highly ordered TiO2 nanowire (TN) arrays were prepared[211] in anodic alumina membranes by a sol–gel method.The TNs are single crystalline anatase phase with uniformdiameters around 60 nm. At room temperature, photolu-minescence measurements of the TN arrays show a visiblebroadband with three peaks, which are located at about 425,465, and 525 nm that are attributed to self-trapped excitons,F , and F + centers, respectively.


4. STRUCTURAL PROPERTIESOF NANOCRYSTALLINE TiO2 FILMS

4.1. The Lattice Structure of Rutileand Anatase

Three different crystal structures of TiO2 exist [18]: rutile,anatase, and brookite; only the former two of them are com-monly used in photocatalysis, with anatase typically exhibit-ing the higher photocatalytic activity [232]. The structureof rutile and anatase can be described in terms of chainsof TiO6 octahedra. The two structures differ by the dis-tortion of each octahedron and the actual pattern of thechains. Figure 7 depicts the unit cell structures of rutile andanatase crystals [233–235]. Each Ti4+ ion is surrounded by anoctahedron of six O2− ions. The octahedron in rutile showsa slightly orthorhombic distortion, whereas the respectiveoctahedra in anatase are significantly distorted, resultingin a symmetry that is lower than orthorhombic. The Ti-Tidistances in anatase are greater (0.379 and 0.304 nm as com-pared to 0.357 and 0.296 nm in rutile) while the Ti-O dis-tances are shorter than in rutile (0.1934 and 0.1980 nm inanatase versus 0.1949 and 0.1980 nm in rutile). In the rutilestructure, each octahedron is in contact with 10 neighboringones (two sharing edge oxygen pairs and eight sharing cor-ner oxygen atoms), whereas in the anatase crystal each octa-hedron is in contact with eight neighbors (four sharing anedge and four sharing a corner). These differences in latticestructure result in different mass densities (� = 4�250 g/cm3

for rutile and � = 3�894 g/cm3 for anatase) and electronicband structures for the two forms of TiO2.

Synthetic titanium oxide crystallizes in two polymorphs:anatase and rutile. Anatase is metastable and transformsexothermally and irreversibly to rutile. Some properties ofTiO2 may strongly depend on its polymorphic phase. Theanatase-rutile transformation is strongly influenced by thesynthesis method, atmosphere, grain growth, and impuri-ties. Some additives, such as ZrO2 and Al2O3, retard theanatase-rutile transformation, whereas others, such as CoOand ZnO, accelerate such a process [236]. The anatase-to-rutile phase transformation of doped nanostructured titaniawas studied [237] using differential thermal analysis (DTA)and X-ray diffraction (XRD). The presence of Cu and Niwas found to enhance transformation as well as sintering.

Titanium Oxygen

81.21˚

90˚ 78.12˚

92.43˚

Figure 7. Crystal structures of rutile (left) and anatase (right) TiO2.

On the other hand, La retarded both transformation anddensification.

Transmission electron microscopy (TEM) is typically usedto investigate the crystal size distribution, grain-boundarydisorder, and defect structure in nanocrystalline TiO2 mate-rials prepared by the various techniques outlined in the pre-vious section. In a recent study of films with an average grainsize of 15 nm prepared by reactive sputtering [238], evidenceof both ordered and disordered grain-boundary regions wasfound and planar defects were observed in grain interiorsidentified as (011) deformation twins. Also, crystallographicshear defects can occur as a result of aggregation of oxygenvacancies in understoichiometric titanium oxide. Figure 8shows results of TEM investigations [239] of nanocrystallineTiO2 films prepared from colloidal suspensions; the TiO2crystallites were nominally pure anatase phase with a size of16 nm.

4.2. Structure of Crystalline TiO2 Surfaces

The geometric structure of crystalline surfaces of TiO2 hasbeen studied predominantly on (macroscopically) large sin-gle crystals; in particular, the (110) surface of rutile, beingthermodynamically the most stable one [240], has beeninvestigated extensively by a broad variety of surface sciencetools [241–243]. Among the different questions addressedtherein, a prominent one was related to the possible typesof oxygen defects at the surface; in fact, three distinct oxy-gen vacancy sites were tentatively identified: lattice, single-bridging, and double-bridging vacancies [244]. With thewidespread use of scanning-tunneling microscopy (STM),atomically resolved investigations of TiO2 surfaces becamefeasible and a more detailed picture of the rutile (110) sur-face structure emerged [245–248]. Consistently, these studiescorroborate the longstanding notion about the prominentimportance of surface defects as the active sites for vari-ous types of chemical reactions [249–251], for example, for

Figure 8. Cross-section transmission electron microscopy image of ananocrystalline TiO2 anatase film. The nominal crystallite size is 16 nm.


the dissociation of water [252, 253]. The number of stud-ies on the structural properties of anatase single crystalsis considerably smaller, which appears to be largely due tothe difficulties encountered in preparing such surfaces in adefect-free state. Nevertheless, a rather distinct picture ofthe anatase TiO2 surface structure [254–257] and its prop-erties in terms of adsorption/desorption reactivity [258–260]has been achieved.

The structure and composition of a nanocrystalline sur-face may have a particular importance in terms of chemi-cal and physical properties because of their small size. Forinstance, nanocrystal growth and manipulation relies heav-ily on surface chemistry [261]. The thermodynamic phasediagrams of nanocrystals are strongly modified from thoseof the bulk materials by the surface energies [262]. More-over, the electronic structure of semiconductor nanocrystalsis influenced by the surface states that lie within the bandgapbut are thought to be affected by the surface reconstruc-tion process [263]. Thus, a picture of the physical propertiesof nanocrystals is complete only when the structure of thesurface is determined.

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform adetailed investigation of the surface and the interior struc-tural properties of nanocrystalline materials, such as rutileand anatase with diameter of a few nanometers. Detailedexperiments using X-ray absorption near-edge structurespectroscopy (XANES) demonstrated [264] that the pres-ence of both defects and surface states strongly influencethe X-ray absorption spectra, even though the first nearest-neighbor geometrical arrangement around the central Tiatom in both rutile and anatase is quite similar: the dif-ferences in the XANES spectra arise from the outer-lyingatomic shells, indicating that “medium” to “long-range”effects play an important role to the near-edge features.In another study of this kind [265], a shorter Ti-O dis-tance for surface TiO2, resulting from Ti-OH bonding wasobserved together with a minor disorder of the latticein smaller nanoparticles. Nevertheless, the Ti sites largelyremain octrahedral even in particles with diameters of 3 nm.Because the interfacial electron/hole transfer occurs via sur-face Ti or O atoms, the observed structural changes aroundthe surface Ti atoms in small TiO2 particles could be respon-sible [265] for the unique photocatalytic properties.

A qualitative analysis of opaque nanostructured glass-supported TiO2 films was carried out [266] using scanningforce microscopy (SFM), and surface parameters such asaverage grain diameter, roughness exponent, and fractaldimension [267] were determined. The TiO2 surfaces exhibitdistinct roughness due to the large aggregates formed by theinterconnected TiO2 particles. Fractal dimension was foundto range between 2.10 and 2.45, depending on the scannedrange and the preparation method. The surface morphol-ogy of nanocrystalline materials prepared by compactingnanometer-sized clusters was investigated by SFM [268];these materials had a grain size of 5–15 nm and containedabout 1019 interfaces per cubic centimeter. Upon heat treat-ment, grains were found to fuse together forming bamboo-like structures and then lined up as tubular structures.

The influence of the iron concentration in mixed-oxide (TiO2 and Fe2O3� thin films prepared by reactive

radio-frequency sputtering on the structural properties ofthe layers has been studied [269]. This characterizationallowed the correlation of the inhibition of the grain growthof titania to the presence of iron oxide and its segregationat grain boundaries. This behavior could be ascribed to asuperficial-tension phenomenon. As a possible applicationof these thin films, it was observed that they were able tosense CO down to the level requested for environmentalmonitoring.

A study [270] of the structure and morphology of a tita-nium dioxide photocatalyst (Degussa P25) reveals multipha-sic material consisting of an amorphous state, together withthe crystalline phases anatase and rutile in the approximateproportions 80/20. Transmission electron microscopy pro-vides evidence that some individual particles are a mixtureof the amorphous state with either the anatase phase orthe rutile phase, and that some particles, which are mostlyanatase, are covered by a thin overlayer of rutile that man-ifests its presence by the appearance of Moiré fringes. Thephotocatalytic activity of this form of titanium dioxide isreported as being greater than the activities of either of thepure crystalline phases, and an interpretation of this obser-vation has been given in terms of the enhancement in themagnitude of the space-charge potential, which is createdby contact between the different phases present and by thepresence of localized electronic states from the amorphousphase.

5. ELECTRICAL PROPERTIESOF NANOCRYSTALLINE TiO2 FILMS

Most studies into the carrier transport in nanocrystallineTiO2 were carried out with the films in contact with elec-trolytes, mostly due to their use in highly efficient electro-chemical solar cells, often called “Grätzel cells” [38, 71]. Inthis application, the pore surface is covered with an ultrathinorganic dye layer and contacted with an electrolytic solutionthat penetrates the pore structure. The experimental workindicates [271–274] that in this configuration the electrolytescreens any electric field within the porous structure andestablishes diffusion conditions for the carrier propagation.On the other hand, investigations in which the nanoporesare in contact with an insulating medium (a gas or vacuum)may allow one to obtain quantitative insight into the elec-tronic properties of the material and the basic feature of car-rier transport. In a series of measurements [275–277] on theelectron transport in nanoporous TiO2 films with gas-filled,insulating pores employing Pt/TiO2, Schottky barrier struc-tures indicate a barrier height of 1.7 eV, compatible with anelectron affinity of 3.9 eV for the TiO2 films. Below ∼300 K,tunneling transport through the barrier occurs, resulting inbarrier lowering effects. Carrier drift mobilities, recombi-nation lifetimes, and their dependence on injection levelin TiO2 are reported. It is found that the mobility-lifetimeproduct is independent of injection level, while drift mobilityand recombination lifetime change strongly with injection.A trap-filling model appears as the most plausible modelcompatible with the experimental findings [277]. Compari-son with recent experiments on nanoporous films in contactwith electrolytes indicate that the transport and recombina-tion mechanism is qualitatively similar for the two cases.


Various observations indicate that electron transport inthe nanocrystalline TiO2 dominates the transient responseof the system. Transient photocurrent measurements reveala very slow (∼millisecond), multiphasic response to bothcontinuous wave [278, 279] and pulsed [280–282] illumina-tion. The characteristic rise or decay time of the responseis dependent upon the intensity of the light source [278,281–283]. Comparison with the transient response withoutelectrolyte indicates that it is the TiO2, and not the elec-trolyte, which is responsible for the very long tail [284, 275].A slow and multiphasic time dependence has also beenobserved in the rereduction of oxidized dye molecules in aredox inactive environment [285]; the same work indicatesthat the rate of dye reduction is controlled by the concen-tration of electrons introduced into the TiO2 by externallyapplied bias. The wide range of time scales is consistent withthe assumption that electron transport within the TiO2 is therate limiting step.

The slow processes are attributed to the trapping of elec-trons by a high density of localized states in the TiO2. Sincethe TiO2 grains are normally crystalline [286], the localizedstates are believed to be concentrated at the grain bound-aries and on the very large surface. There is evidence forintraband-gap states in bias-dependent optical absorbancespectra [287] and surface photovoltage spectra. It would,therefore, be extremely useful to correlate the density andnature of those states with the electronic transport proper-ties of the material.

Investigating electron migration in nanostructuredanatase TiO2 films with intensity-modulated photocurrentspectroscopy [288], it was found that, upon illumination, afraction of the electrons accumulated in the nanostructuredfilm is stored in deep surface states, whereas anotherfraction resides in the conduction band and is free to move.These data indicate that the average concentration of theexcess conduction band electrons equals about one electronper nanoparticle, irrespective of the type of electrode, thefilm thickness, or the irradiation intensity.

The photocurrent in thin film TiO2 electrodes preparedby sol–gel methods was studied in [289] as a function offilm thickness. Films with thickness smaller than the spacecharge layer were found to show a larger photocurrent thanfilms with thickness larger than the space charge layer. Itwas concluded that the increase in photocurrent is due tothe effective electron-hole separation throughout the wholefilm thickness and the reduction of bulk recombination. Theuse of TiO2 thin-film electrodes for photocatalytic devicesmight therefore be useful to gain high device performance.

In a series of papers, Dittrich et al. carried out extensiveinvestigations of the electrical conductivity in nanoporousTiO2 films [290–296]. Studying the temperature- and oxy-gen partial pressure-dependent conductivity � of rutile andanatase, they noted [292] that � is thermally activated withEA = 0�85 eV, independent of the absolute value of � anddepends on p(O2� by a power law for p(O2� < 1–10 mbar.The electrical properties of reduced nanoporous TiO2 aredetermined by surface chemical reactions which lead to theformation of shallow donor and deep trap states. Further-more, this group examined in detail the photovoltage innanocrystalline TiO2 [293, 295] and the injection currents inthese porous specimens [294, 296].

Such a power dependence of � on the oxygen partial pres-sure was noted also in recent work [297] investigating electri-cal and defect thermodynamic properties of nanocrystallinetitanium oxide. At high O2 pressures, p(O2� > 1 mbar,the conductivity is constant, whereas at values p(O2� <10−14 mbar a steep increase of � with decreasing pressurewas found, following a power dependence � ∝ �p(O2��

n withn = −1/2 [297]. The plateau of conductivity at high oxy-gen pressures can be interpreted as being a domain of ionicconductivity, an unexpected behavior for titanium dioxide.In a coarse-grained material, dominant hole conductivity isobserved in this partial pressure range. This difference maybe due to the high density of grain boundaries in nanocrys-talline ceramics, which can be preferred paths for diffu-sion at reduced temperatures. Furthermore, an increase inionic conductivity is also expected due to enhanced defectformation in the space charge regions adjacent to grainboundaries [298]. At low oxygen partial pressure, nanocrys-talline TiO2 exhibits enhanced electronic conductivity ascompared to coarse-grained TiO2. The power exponent n =−1/2 can be explained under the assumption that doublycharged titanium interstitials are formed. The intrinsic disor-der of titanium dioxide is reputedly of the cationic Frenkel-type [299–302], although alternative defect models basedon Schottky disorder are also described in the literature[303, 304]. In the domain of ionic conductivity, the activa-tion energy of conduction is ∼1�0 ± 0�1 eV [297], a valuetypical of migration enthalpies for ionic defects. By contrast,the activation energy in the reduction-controlled regime wasfound to be ∼3�9± 0�2 eV.

In titanium oxide thin films prepared by a d.c. sputter-ing technique onto glass substrates with average grain sizesof 100–200 nm, the surface structure and phase morphologyof the films was found [305] to depend on the depositionconditions. The current-voltage characteristics of these filmsare ohmic for values of applied voltage lower than 0.5 V.For higher values, the mechanism of electrical conductionis determined by space-charge-limited currents [306]; then,a power-law dependence was observed with n ∼ 2.3–2.9. Inmuch thicker Ti oxide films (1.9–8 �m) deposited by sput-tering [307], both the surface roughness and the internalsurface area increased with film thickness; this resulted in anenhancement of the incident photon-to-current efficiency.

Electrical and optical spectroscopic studies of TiO2anatase thin films deposited by sputtering showed [308, 309]that the metastable phase anatase differs in electronic prop-erties from the well-known, stable phase rutile. (From thebroadening of the X-ray diffraction peaks, the average grainsize of the films is estimated to be in the range of 30–40 nm[308].) Resistivity and Hall-effect measurements revealed aninsulator-metal transition in a donor band in anatase thinfilms with high donor concentrations. Such a transition isnot observed in rutile thin films with similar donor con-centrations. This indicates a larger effective Bohr radiusof donor electrons in anatase than in rutile, which in turnsuggests a smaller electron effective mass in anatase. Thesmaller effective mass in anatase is consistent with thehigh-mobility, bandlike conduction observed in anatase crys-tals. It is also responsible for the very shallow donor ener-gies in anatase. Luminescence of self-trapped excitons was


observed in anatase thin films, which implies a strong lat-tice relaxation and a small exciton bandwidth in anatase.Optical absorption and photoconductivity spectra show thatanatase thin films have a wider optical absorption gap thanrutile thin films. The extrapolated optical absorption gaps ofanatase and rutile films were found to be 3.2 and 3.0 eV,respectively, at room temperature.

The observation of space charge limited currents (SCLC)in nanoscaled pure and chromium-doped titania wasreported [310] and both the free-charge carrier density andthe trapped-charge carrier density were given.

Photoconductivity was also studied in compound systems;for example, in a TiO2-C60 bilayer system the conductivityincreases significantly in the fullerene upon irradiation atwavelengths <300 nm [311].

Although being an efficient photocatalyst for the detox-ification of organically charged waste water, titanium diox-ide suffers from the drawback of poor absorption propertiesbecause of a bandgap of 3.2 eV. Thus, wavelengths shorterthan 400 nm are needed for light-induced generation ofelectron-hole pairs. That is the reason why doping with tran-sition metal ions is interesting for inducing a reduction of thebandgap. However, this doping changes other physical prop-erties such as lifetime of electron-hole pairs and adsorptioncharacteristics [312].

6. PHOTOCATALYTIC PROPERTIESOF NANOCRYSTALLINE TiO2

Most experimental investigations reported a higher photo-catalytic efficiency in the anatase TiO2 phase; as a possi-ble reason it was suggested that the recombination of theelectron-hole pairs produced by UV irradiation occurs morerapidly on the surface of the rutile phase, and the amount ofreactants and hydroxides attached to this surface is smallerthan on the surface of the anatase phase.

The study of the photocatalytic activity of nanocrystallineTiO2 materials is a longstanding research effort [313–315];in most lab-scale experiments it was evaluated by meansof the degradation observed for typical substances (e.g.,aqueous methyl orange, methylene blue, etc.) upon expo-sure of the specimen to UV irradiation. In such a way,the possible influence of light intensity, structural proper-ties, surface morphology, phase and chemical composition,resulting from various deposition or preparation methods,could conveniently be explored [316–320]. Furthermore,any correlation with the optical or electrical properties ofthe nanocrystalline films could thereby be investigated. Inaddition, alternative approaches have come under scrutiny.A new simple method for characterizing photocatalyticactivity by measuring photo-generated transient charge sep-aration at the surface of semiconductor photocatalysts wasproposed [321]. In this technique, the charge separation gen-erated by a pulse dye laser is obtained as a function of theincident laser energy; thereby, the photocatalytic activity andthe type of surface reaction (reduction or oxidation) in tita-nium dioxide films were rapidly determined. In the followingsections some examples of such studies will be given.

6.1. Dependence of Photocatalytic Activityon Film Structure and Phase

The photoactivities of ultrafine TiO2 nanoparticles inanatase, rutile, or mixed phases were tested in the photocat-alytic degradation of phenol [322]. For TiO2 nanoparticles,mainly in the anatase phase and mixed-phases, the photo-catalytic activities increased significantly with the content ofthe amorphous part decreasing. The completely crystallizedrutile nanoparticles exhibited size effects in this photocat-alytic reaction and the photocatalytic activity of rutile-typeTiO2 nanoparticles with a size of 7.2 nm was much higherthan that with 18.5 nm or 40.8 nm and was comparable tothat of anatase nanoparticles.

A modified sol–gel process was used [323] to preparenano-structured TiO2 catalysts of controlled particle size(i.e., 6, 11, 16, and 20 nm). The effect of TiO2 parti-cle size on gas-phase photocatalytic oxidation of toluenewas examined under dry and humid conditions. Main reac-tion products were CO2 and water, although small amountsof benzaldehyde were also detected. The smaller particlesize (i.e., 6 nm) led to higher conversion and completemineralization of toluene into CO2 and H2O. Electronicand structural effects (i.e., size and ensemble effects) wereresponsible for the excellent performance of a 6 nm TiO2catalyst for toluene photodegradation. The dependency ofthe photoactivity on the crystallite size of anatase titania forthe decomposition of trichloroethylene (TCE) was investi-gated [324]. It was found that the photoactivity of all tita-nia samples was linearly increased as the crystallite size ofthe anatase phase increased, regardless of the preparationmethod, as long as there was no significant rutile phase.

The enhancement of the photocatalytic activity of TiO2was investigated as a function of added amount of otheroxides to promote desired oxidation or reduction reac-tions [325]. Mixed oxides of Nb2O5 or Li2O with TiO2 wereprepared by the sol–gel process. The target material ofdichloroacetic acid (DCA) was chosen for oxidation reac-tions and K2Cr2O7 for reduction. While the Nb-oxide hada deleterious effect on the decomposition rate of DCA, theexcess electrons due to the doping of Nb2O5 into TiO2 pro-moted the reduction process for Cr6+. Li2O (1 wt%) withTiO2 was found to be the most efficient photocatalyst forDCA oxidation, resulting in photocatalytic activity of 50%.

A highly sensitive biochemical oxygen demand (BOD)sensor using a commercial TiO2 semiconductor and photo-catalytic pretreatment was developed to evaluate low BODlevels in river waters [326]. The photocatalytic oxidation wasinvestigated as a function of irradiation times, TiO2 concen-trations, and pH. The optimal irradiation time was 4 min.The sensor response was increased with increasing pH andthe responses obtained by photocatalysis to river sampleswere higher than those obtained without photocatalysis.

6.2. Influence of Surface Morphologyand Defects

Photocatalytic reduction of CO2 to organic compounds wascarried out [327] in a semiconductor suspension systemunder simulated solar power using a TiO2 catalyst. Exper-imental results show that the photocatalytic activity can be


improved by depositing Pd or Ru on the TiO2 surface. Filmsof TiO2 dispersed or coated with platinum were depositedon glass and Pt-buffered polyamide substrates, respectively,by magnetron sputtering [328]. The photocatalytic activity ofthe films was evaluated through the decomposition of aceticacid under UV irradiation. The Pt-dispersed TiO2 film withapproximately 1.5 wt% platinum shows a maximum activitydue to the formation of anatase phase with a fine grain size.Platinum particles ∼2 nm in thickness coated on anatasefilm greatly improves activity. The activity shows a steplikedependence of film thickness, where the critical thicknessvaries between 150 and 200 nm depending on the depositiontemperatures. The correlation between defects and activitywas verified by measuring either the temperature depen-dence of electric resistance or the shift of binding energyfrom X-ray photoelectron spectroscopy (XPS).

In crystalline TiO2 films deposited by reactive RF mag-netron sputtering on glass substrates without additionalexternal heating, the photocatalytic activity was evaluated bythe measurement of the decomposition of methylene blueunder UV irradiation [162]. The results showed that crys-talline anatase, anatase/rutile, or rutile films can be suc-cessfully deposited on unheated substrates. Anatase TiO2films with a more open surface, a higher surface roughness,and a larger surface area, formed at higher total pressures,exhibit the best photocatalytic activity. The photocatalyticactivity of polycrystalline anatase TiO2 films was found [329]to be affected by the crystalline orientation that dependson the deposition temperature; it was greater on the (112)-oriented than on the (001)-oriented film. The former filmexhibited a columnar structure resulting in a larger sur-face area for photocatalytic reaction than the films withthe (001)-preferred orientation. Furthermore, the introduc-tion of structural defects associated with oxygen vacancieswas found [169] to create some energy levels around themid-gap, indicating that they could work as recombinationcenters of photo-induced holes and electrons, causing thedecrease in photocatalytic activity. Therefore, the decreasein the structural defects associated with oxygen vacanciesappears to be important for improving the photocatalyticactivity of the films.

A marked difference of the photocatalytic activitybetween the TiO2 films coated on quartz and glass substrateswas confirmed [330], which would be interpreted in terms ofthe difference in the photocarrier’s diffusion length inducedby impurity Na+ ions. These results lead to a conclusionthat the crystallinity and defects of TiO2 as well as the filmthickness and surface area have a great influence on thephotocatalytic activity. An enhanced photocatalytic activityof TiO2 could be achieved also by deposition of the films onsulfonated glass substrates [331] or by using special supportmaterials [332, 333].

Photo-oxidative self-cleaning and antifogging effects oftransparent titanium dioxide films has attracted considerableattention for the past decade [334, 335]. In order to under-stand the photo-induced hydrophilic conversion on titaniumdioxide coatings in details, it is inevitably necessary to under-stand the relationship between the photo reaction and thesurface crystal structure; this can be done, for example, byan evaluation of the photo-induced hydrophilic conversionon the different crystal faces of rutile single crystals and also

polycrystalline anatase titanium dioxide [336]. Self-cleaningand antifogging effects of TiO2 films prepared by magnetronsputtering were investigated [161, 163] in terms of the photo-catalytic behavior by measuring the decomposition of methy-lene blue and the reduction of the contact angle betweenwater and TiO2 under ultraviolet irradiation. The phase con-version from the rutile to the anatase TiO2 film leads to anenhancement of the activity; the anatase films with the bestphotocatalytic behavior are prepared at higher total pres-sures (>1.50 Pa) and characterized by a high decompositionefficiency, a contact angle about 10� after irradiation, and agood stability in darkness.

Titanium dioxide thin films prepared with various sur-face morphologies by metalorganic chemical vapor deposi-tion were found to exhibit reversible wettability control bylight irradiation [337]. These TiO2 surfaces became highlyhydrophilic by UV irradiation, and returned to the initialrelatively hydrophobic state by visible-light (VIS) irradia-tion. The hydrophobic-hydrophilic conversion induced byUV light was ascribed to the increase in dissociated wateradsorption on the film surface. By contrast, the conver-sion from hydrophilic to hydrophobic by VIS irradiation wascaused by the elimination of water adsorbed on the sur-face due to the heat generated. Changes of the water con-tact angle between hydrophilic states and hydrophobic onesstrongly depended on the roughness of the film surface.

The self-cleaning property of thin transparent TiO2 coat-ings on glass under solar UV light was studied [338] for twocompounds: palmitic (hexadecanoic) acid and fluoranthene,which are both present in the atmospheric solid particles.The removal rates of layers of these compounds sprayed onthe self-cleaning glass were found to be sufficient for theexpected application. The identified intermediates (about40 for each compound) show the gradual splitting of thepalmitic acid chain and the oxidative openings of the aro-matic rings of fluoranthene. In the case of palmitic acid,the products give some indications about the photocatalyticmechanism. About 20% of the organic carbon contained inthe initial compounds was transformed into volatile carbonylproducts.

An extreme photo-induced hydrophilicity was achieved[339, 340] when TiO2 films were covered by SiO2 overlay-ers (with 10–20 nm in thickness). These multilayer filmsexhibited much more extreme hydrophilicity than a TiO2film without overlayer. The surface analyses revealed thatthe enhanced photo-induced hydrophilic surface of the mul-tilayer films exhibited an improved photocatalytic activitytowards decomposition of organic substances on their sur-faces. An extreme light-induced superhydrophilicity was alsoreported [341] for mesoporous TiO2 thin films (crystallitesize ∼15 nm, surface area ∼50 m2/g, pore size ∼3.6 nm).For such films, the water contact angle was found to bereduced essentially to zero upon UV-irradation for a dura-tion of about 60 min. In addition, the photocatalytic activityof those films could be enhanced by treating the substratesurfaces with an H2SO4 solution [342].

In order to investigate the cathodic photoprotection ofthe steel from corrosion, stainless steel was coated withTiO2 thin films, applied by a spray pyrolysis [343]. Itwas concluded that these coatings exhibit both a cathodic


photo-protection effect against corrosion and the frequentlyreported photocatalytic self-cleaning effect.

6.3. Influence of Electronic Properties

Detailed spectroscopic investigations of the processes occur-ring upon bandgap irradiation in colloidal aqueous TiO2suspensions in the absence of any hole scavengers showed[344] that while electrons are trapped instantaneously, thatis, within the duration of the laser flash (20 ns), at leasttwo different types of traps have to be considered for theremaining holes. Deeply trapped holes are rather long-livedand unreactive, that is, they are not transferred to theions of model compounds for photocatalytic oxidation likedichloroacetate or thiocyanate. Shallowly trapped holes, onthe other hand, are in a thermally activated equilibrium withfree holes which exhibit a very high oxidation potential. Theoverall yield of trapped holes can be considerably increasedwhen small platinum islands are present on the TiO2 sur-face, which act as efficient electron scavengers competingwith the undesired e−/h+ recombination. While molecularoxygen, O2, reacts in a relatively slow process with trappedelectrons, the adsorption of the model compounds on theTiO2 surface prior to the bandgap excitation appears to bea prerequisite for an efficient hole scavenging.

6.4. Enhanced Photocatalytic ActivityVia Surface Modifications

A driving force for research in heterogeneous photocataly-sis using TiO2 (and semiconductors in general) is the cre-ation and application of systems capable of using naturalsunlight to degrade a variety of organic and inorganic con-taminants in wastewater or polluted air. As mentioned, thephotocatalytic activity depends strongly, among other fac-tors, on the wavelength range response. Since the bandgapof TiO2 is ∼3.2 eV, it is active only in the ultraviolet regionwhich amounts to <10% of the overall solar intensity. Prin-cipally, there are several remedies to circumvent (at leastpartially) this limitation: (i) Deposition of metals on thesemiconductor; (ii) using multicomponent semiconductors;(iii) surface modification with sensitizing dyes. The merits ofthese options will be outlined briefly in the following.

6.4.1. Deposition of Metals on the SurfaceThe selectivity and efficiency of a photochemical reactioncan be improved by modifying the surface with a noblemetal. The deposition of metal particles on oxide surfaceshas been the subject of several recent reviews [345–348] andtherefore, there is, no need to duplicate it here. In termsof photocatalytic activity, a drastic enhancement has, for thefirst time, been observed for the photocatalytic conversionof water into hydrogen and oxygen upon a fractional cover-age of the TiO2 surface with platinum [349]. After excitationthe electron migrates to the metal where it becomes trappedand electron-hole recombination is suppressed. The hole isthen free to diffuse to the semiconductor surface where oxi-dation of organic species can occur. These processes areschematically depicted in Figure 9. The presence of themetal can be beneficial also because of its own catalytic

hν

+

hν

+

-

+

-

-

metal

SchottkyBarrier

semiconductor

VB

CB

Figure 9. Metal-modified semiconductor photocatalyst particle.

activity. The Pt/TiO2 system is probably the most frequentlystudied metal-semiconductor pair (see, e.g., [350, 351]);Figure 10 exemplifies the enhancement of the photo-catalytic activity of nanocrystalline TiO2 by platinization[350]: three commercially available TiO2-catalysts, namely,Degussa P25, Sachtleben Hombikat UV100, and MillenniumTiONA PC50, were platinized by a photochemical impreg-nation method with two ratios of platinum deposits (0.5 and1 wt%). The photocatalytic activities of these samples weredetermined using three different model compounds, EDTA,4-chlorophenol (4-CP), and dichloroacetic acid (DCA). Pla-tinization resulted in all cases in an enhancement of theactivity; Figure 10 shows the degradation of DCA as a func-tion of illumination time (light intensity 23 W/m2 at a wave-length of 320–400 nm) for pure TiO2 and the two platinizedspecimens. After 2 h of illumination, the initial concentra-tion of 120 ppm total organic carbon (TOC) was reducedto 2.3 ppm at pH 3 employing the best photocatalyst, inthis case, Hombikat UV100 with 0.5 wt% Pt. For this sys-tem, an initial photonic efficiency (i.e., number of degradedmolecules per number of incident photons) of ∼43% wasobtained [350]. Apart from platinum, an enhanced pho-tocatalytic activity has been also noted for other metalsand semiconductors. Their influence on the photocatalyticactivities has been studied in detail, for example, utilizing

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

TO

C/T

OC

0

illumination time (min)

without Pt 0.5 wt% Pt 1 wt% Pt

Figure 10. Degradation of dichloroacetic acid (expressed as the relativechange of TOC) with platinized TiO2 in comparison to pure TiO2 as afunction of illumination at pH 3. Data from [350], D. Hufschmidt et al.,J. Photochem. Photobiol. A: Chem. 148, 223 (2002).


the following systems: Au-modified TiO2 [352, 353], silver-modified titanium particles [354], transition-metal dopedTiO2 photocatalysts [355–357], or rare-earth-doped TiO2nanoparticles [358].

It should be noted in this context that various analyticaltechniques like transmission electron microscopy or scan-ning force microscopy are often very useful in determiningthe size of the particles and their distribution in the bulkand at the surface of these nanocrystalline materials.

6.4.2. Composite SemiconductorsThe advantage of composite semiconductors is usuallytwofold: first, to extend the photo-response by couplinga large bandgap semiconductor with another featuring asmaller one and, second, to retard the recombination ofphoto-generated charge carriers by injecting electrons intothe lower lying conduction band of the large bandgap mate-rial. Two types of geometries have been employed: cap-ping the nanocrystallites of one semiconductor with thoseof the second or bringing the nanocrystalline particles ofthe two materials into intimate contact. The principle ofcharge exchange and separation for both arrangements isillustrated in Figure 11. Let us consider the case of cou-pling CdS with TiO2; the energy of the excitation light istoo small to directly excite the TiO2 particle, but it is largeenough to excite an electron from the valence band acrossthe bandgap of CdS (Eg = 2�5 eV) to the conduction band.According to the energetics, the hole produced in the CdSvalence band remains in the CdS particle, whereas the elec-tron transfers to the conduction band of the TiO2 particle;this increases the charge separation and efficiency of thephotocatalytic process. The separated electrons and holesare then free to undergo electron transfer with adsorbates atthe surface. While the mechanisms of charge separation in acapped system are similar, in a capped semiconductor onlyone of the charge carriers is accessible at the surface for cat-alytic reactions. Several semiconductors have been studied

B

B-

A

A+

(a)

(b)

hν'

+

CB

VB

-TiO2

A

A+

+

-CB

VB

hν SnO2

hν'

+

CB

VB

-

TiO2

CdS

CdS

+

-CB

VB

hν

Figure 11. Photo-induced charge separation in composite semiconduc-tor particles: (a) capped and (b) coupled semiconductor nanocrystal-lites. Photo-generated charge carriers move in opposite directions.

thoroughly in combination with TiO2: TiO2 CdS [359–361],TiO2 CdSe [362], TiO2 coupled SnO2 [363], TiO2 cappedSiO2 [364], and some mixed-oxide systems like Fe2O3 TiO2[365] or ZrO2 TiO2 [366].

6.4.3. Surface Modificationwith Sensitizing Dyes

Surface sensitization of a wide bandgap semiconductor pho-tocatalyst like TiO2 via adsorbed dyes can increase the effi-ciency of the excitation step and, as a consequence, theactivity. This process can also expand the wavelength rangeof excitation. Some common dyes that are used as sensi-tizers include erythrosin B, eosin, rhodamines, cresyl vio-let, thionine, porphyrins, [Ru(bpy)3]2+ and its analogues,and many others (see, e.g., [102] for a more comprehen-sive list). The individual charge-transfer and excitation stepsinvolved in the dye sensitizer surface process are exemplifiedin Figure 12. The primary excitation of an electron in thedye molecule occurs in either the singlet or triplet excitedstate of the molecule; if the oxidative energy level of theexcited state of the dye molecule, with respect to the con-duction band energy level of the semiconductor, is morenegative, then the dye molecule can transfer the electronto the conduction band of the semiconductor. The surfaceaccepts electrons from the dye molecules which, in turn,can be transferred to reduce an organic acceptor moleculeadsorbed on the surface. The dye-sensitized semiconduc-tor can also be used in oxidative degradation of the dyemolecule itself after charge transfer; this appears to be animportant process in view of the large quantities of dye sub-stances in wastewater from the textile industries and others.In passing, it can be noted that the process of utilizing sub-bandgap excitation with dyes that absorb strongly in the vis-ible for photosensitization is frequently employed in colorphotography and other imaging science applications. Thisapproach of light-energy conversion is also similar to plantphotosynthesis, in which chlorophyll molecules act as lightharvesting antenna molecules.

Generally, the high porosity and strong bonding char-acter of nanostructured TiO2 (and other) semiconductorfilms facilitate the surface modification with organic dyesor organometallic complexes. For example, photoconversionefficiencies in the range 10–15% in diffused daylight havebeen reported [367] for nanostructured TiO2 films modifiedwith a ruthenium complex. The charge injection from a sin-glet excited sensitizer into the conduction band of a largebandgap semiconductor is thought to be an ultrafast process

-

CB

VB

hν

S*

S

CB

VB

CB

VB

A A A-S+S+

-

Figure 12. Sequence of excitation and charge-transfer steps using a dye-molecule sensitizer. In the first step, the sensitizer (S) is excited by anincident photon of energy h� and an electron is transferred into theconduction band of the semiconductor particle; the electron then canbe transferred to reduce an organic acceptor molecule (A) adsorbed onthe surface.


occurring at a picosecond time scale; in the case of differentorganic dyes, it has been shown [368, 369] that charge injec-tion takes place within 20 ps. A similar fast electron transferhas also been noted for [Ru(H2O)2]2− on a TiO2 surface atvery low coverage [370]. Progress in femtosecond laser spec-troscopy opened a venue for investigations on even muchshorter time scales. In fact, in recent studies charge car-rier injection times in the range of 20–200 fs were reported[371–375] for various dyes on nanocrystalline TiO2 particles.

Significant enhancement effects of electron acceptors(additives) such as hydrogen peroxide, ammonium per-sulfate, potassium bromate, and potassium peroxymono-sulfate (oxone) on the TiO2 photocatalytic degradationof various organic pollutants were observed already inearly investigations [376]. The results showed that theseadditives markedly improved the degradation rate of 2,4-dichlorophenol. The enhanced photocatalytic oxidation ofsulfide ions on phthalocyanine modified titania was ascribed[377] to the additional formation of superoxide radicals.

Sensitization of wide bandgap semiconductor electrodesby dyes absorbing visible light are routinely used also in dye-sensitized photoelectrochemical cells, in order to achievehigh photon-to-current conversion efficiencies [378, 379].The preparation and dynamics of interfacial photosensi-tized charge separation in metal oxides such as TiO2 filmshas been reviewed [70, 71, 380]. Principally, the photo-physical reactions occurring in those dye-sensitized injec-tion solar cells, which are based on a dye adsorbed ontoa porous TiO2 layer, are very similar to those relevant tophotocatalysis. Because of their importance as an energysource, Figure 13 presents a schematic drawing [69] of such

por-TiO2 electrolyte I- / I3-

TCO glass with PtTCO glass

Ehν

I- / I3-

S+/ Se-

e-

e-

e- e-

e-

S+/ S*

dye

load

∆V

e-

1

23

4

5

Figure 13. Schematic outline of a dye-sensitized photovoltaic cell,showing the electron energy levels in the different phases. The systemconsists of a semiconducting nanocrystalline TiO2 film onto which aRu-complex is adsorbed as a dye and a conductive counterelectrode,while the electrolyte contains an I−/I−3 redox couple. The cell volt-age observed under illumination corresponds to the difference, �V ,between the quasi-Fermi level of TiO2 and the electrochemical potentialof the electrolyte. S, S∗, and S+ designate, respectively, the sensitizer,the electronically excited sensitizer, and the oxidized sensitizer. See textfor details. Adapted from [69], A. Hagfeldt and M. Grätzel, Chem Rev.95, 49 (1995). © 1995, American Chemical Society.

a dye-sensitized nanocrystalline TiO2 solar cell, depictingthe relevant energy levels and the pathway for the photo-excited electrons. In this specific example, a ruthenium com-plex [367] was adsorbed as a dye onto the TiO2 and anI−/I−3 redox couple was used in the electrolyte. Contrary toconventional semiconductor devices, in the dye-sensitizedcells the function of light absorption is separated from thecharge-carrier transport. The Ru-complex has to absorb theincident sunlight and to effect, via this energy, the electron-transfer reaction (numbers ©1 and ©2 in Fig. 13). Apart fromsupporting the dye, the TiO2 film acts as an electron accep-tor and electronic conductor: the electrons injected into theTiO2 conduction band drift across the nanocrystalline filmto the conducting glass support which functions as currentcollector (©3 in Fig. 13). At the counterelectrode, the elec-tron is transferred to the redox couple in the electrolyte (©4 )which, in turn, serves to regenerate the dye (©5 ). The cellvoltage observed under illumination, �V , is determined bythe difference between the Fermi-level of TiO2 and the elec-trochemical potential of the electrolyte (cf. Fig. 13 [69]).

7. PHOTOCATALYTIC APPLICATIONSOF NANOCRYSTALLINE TiO2

While many examples of the photocatalytic activity ofnanocrystalline TiO2 have been presented already in theforegoing sections, the following discussions will focus onnovel and important (and sometimes large-scale) applica-tions. A very prominent area appears to be environmentalcatalysis [31, 32, 45] which, in recent years, has expandedconsiderably beyond the traditional fields like NOx removalfrom stationary and mobile sources, or the conversion ofvolatile organic compounds (VOC). According to [381],these potential new areas include:

(i) catalytic technologies for liquid or solid wastereduction or purification;

(ii) use of catalysts in energy-efficient catalytic tech-nologies and processes;

(iii) reduction of the environmental impact in the use ordisposal of catalysts;

(iv) new ecocompatible refinery, chemical or nonchem-ical catalytic processes;

(v) catalysis for greenhouse gas control;(vi) use of catalysts for user-friendly technologies and

reduction of indoor pollution;(vii) catalytic processes for sustainable chemistry;(viii) reduction of the environmental impact of transport.

Hence, (photo)catalysis in environmental applications canbe instrumental in promoting the quality of life and environ-ment, in promoting a more efficient use of resources, and inpromoting sustainable processes and products.

Because of the tremendous importance of those environ-mentally related areas, the use of nanocrystalline TiO2 forsuch photocatalytic applications is illustrated in this sectionby means of selected examples. Before doing so, it is stressedthat there exists at least one other rather important issuein this context. In fact, hydrogen production from aqueoussolutions using semiconductor particles such as, CdS, TiO2,WO3 as photocatalysts is envisaged to become a potential


major application of these materials, and new concepts andapproaches are developed continuously. For example, a newphotocatalytic reaction that splits water into H2 and O2was designed [382] by a two-step photo-excitation systemcomposed of a IO−

3 /I− shuttle redox mediator and two dif-

ferent TiO2 photocatalysts, Pt-loaded TiO2-anatase for H2evolution and TiO2-rutile for O2 evolution. Simultaneousgas evolution of H2 (180 �mol/h) and O2 (90 �mol/h) wasobserved from a basic (pH = 11) NaI aqueous suspensionof two different TiO2 photocatalysts under UV irradiation( > 300 nm, 400 W high-pressure Hg lamp). An exten-sive review [383] assesses photocatalytic efficiencies with ref-erence to hydrogen production by means of light energyin the presence and absence of loaded metals, electron-donors/acceptors, and hole scavengers.

7.1. Reduction/Removal of Toxic Gases

The conversion of nitrogen oxides to less toxic compounds isimportant both because of their toxicity and the global atmo-spheric pollution. NOx can be converted to N2 and othernitrogen compounds by reduction. TiO2-loaded zeolites andthe vanadium silicate-1 were found [384] to decompose NOunder irradiation, in particular, TiO2 included in zeolite cav-ities results in complete decomposition into N2 and O2. Tita-nium oxide catalysts prepared within the Y-zeolite cavitiesvia an ion-exchange method exhibit [385] high and uniquephotocatalytic reactivities for the decomposition of NO intoN2 and O2, as well as the reduction of CO2 with H2O show-ing a high selectivity for the formation of CH3OH. It wasalso found that the charge transfer excited state of the tita-nium oxide species, (T3+-O−)∗, plays a vital role in theseunique photocatalytic reactions. In yet another approach, anefficient catalytic reduction of NO at low temperature bymeans of NH3 could be achieved using Mn-, Cr-, or Cu-oxides on a nanocrystalline TiO2 support [386].

The NOx removal process was studied experimentally ina pulsed corona discharge combined with the TiO2 photo-catalytic reaction [387]. NO2 was found to adsorb easily onthe photocatalyst surface, whereas NO was hardly adsorbed.Addition of water vapor enhanced the NO2 adsorption. Itwas concluded that the main role of the plasma-chemicalreaction in this system is the oxidation of NO into NO2.A considerable part of NO2 is adsorbed on the photocatalystsurface, and is transformed to HNO3 through photocatalyticreaction with OH.

The photocatalytic degradation of VOCs in the gas phaseconstitutes another very important example in this range ofapplications. Utilizing variously prepared TiO2 photocata-lysts (e.g., deposited on glass fiber cloth, as pellets or asthin films), the photo-induced reactions of trichloroethylene,acetone, methanol, and toluene were investigated [388–390].The photocatalytic degradation rate was observed [388] toincrease with increasing initial concentration of the VOCs,but remained almost constant beyond a certain concen-tration. It matched well with the Langmuir–Hinshelwood(L-H) kinetic model [11]. For the influence of water vaporin a gas-phase photocatalytic degradation rate, there was anoptimum concentration of water vapor in the degradation oftrichloroethylene and methanol. Furthermore, water vaporenhanced the photocatalytic degradation rate of toluene,

whereas it inhibited that of acetone. As for the effect of pho-ton flux, it was found that photocatalytic degradation occursin two regimes with respect to photon flux: for illumina-tion levels distinctly blow 1000–2000 �W/cm2, the photocat-alytic degradation rate increased linearly with photon flux,whereas for power densities above that value, the rate wasfound to scale with the square root of the flux. Figure 14shows some of those data [388], depicting in panel (a), thedegradation of methanol as a function of UV illuminationtime for five different initial concentrations. (Using TiO2anatase nanocrystallites with 7 nm diameter in a solution,in this work photocatalytic TiO2 films were deposited ontoglass substrates by dip-coating.) The reaction kinetics werefound to follow the L-H model, in which the reaction rate rvaries proportionally with the surface coverage � accordingto

r = k� = kKc

1+Kc(9)

where c is the concentration of the VOC and k and K are,respectively, the reaction rate constant and the adsorptionequilibrium constant. Figure 14(b) exemplifies this finding,showing the initial reaction rates r0 derived from data likethose in Figure 14(a) as a function of the respective initialmethanol concentrations c0. The solid line in Figure 14(b)is a fit to the data according to Eq. (9).

0 2 4 6 8 100

5

10

15

20

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

(a)methanol

conc

entr

atio

n (1

0–3 m

ol/m

3 )


(b)

initi

al r

eact

ion

rate

r 0 (1

0–3 m

ol/m

3 min

)

initial concentration c0 (10–3 mol/m3)

Figure 14. (a) Photocatalytic degradation of methanol with differentinitial concentrations as a function of UV illumination time (light inten-sity 2095 �W/cm2 at a wavelength of 254 nm) at a H2O concentrationof 0.38 mol/m3 and a reaction temperature of 45 �C. (b) Initial reactionrates r0 as derived from the data in (a) versus the initial methanol con-centrations; the solid line is a fit according Eq. (9). Data from [388],S. B. Kim and S. C. Hong, Appl. Catal. B: Environ. 35, 305 (2002).


7.2. Degradation of Organic Compounds

The degradation of organic compounds is probably themost widely used photocatalytic application of nanocrys-talline TiO2 and other semiconductor materials. In an aque-ous environment, the holes created under UV irradiationare scavenged by surface hydroxyl groups to generate •OHradicals that then promote the oxidation of organics. Thisradical-mediated oxidation has been successfully employedin the mineralization of several hazardous chemical contam-inants such as hydrocarbons, haloaromatics, phenols, halo-genated biphenyls, surfactants, and textile and other dyes[102].

The possible photocatalytic decomposition of a broadrange of organic compounds has been investigated usingnanocrystalline TiO2 particles. Detailed studies reportedthe oxidation of dissolved cyanide [391], the degradationof various kinds of acids [392–398], and of several herbi-cides [399–402], for the photocatalytic oxidation of toluene,benzene, cyclohexene, and benzhydrol [403–406] or forthe 1,1′-dimethyl-4,4′-bipyridium dichloride decomposition[407]. In another application, a titanium oxide photocata-lyst of ultra-high activity has been employed for the selectiveN-cyclization of an amino acid in aqueous suspensions [408].Anatase crystallites of average diameter of ∼15 nm wereplatinized by impregnation from aqueous chloroplatinic acidsolution followed by hydrogen reduction. The catalyst wassuspended in an aqueous L-lysine (Lys) solution and photo-irradiated under argon at ambient temperature to obtainL-pipecolinic acid.

The photocatalytic degradation and oxidation of phenoland phenol-based compounds has been examined quite fre-quently [409–414]. The decomposition of aqueous phenolsolutions to carbon dioxide have been studied using naturalsunlight in geometries simulating shallow ponds [415]. Thephotocatalyst was titanium dioxide freely suspended in thesolution or immobilized on sand or silica gel. Photodegra-dation rates were approximately three times faster with thefree suspension than with the immobilized catalyst underthe same conditions, and were dependent on the time ofyear and the time of day. The seasonal variation correlatedroughly with seasonal solar irradiance tabulations for theUV component of the spectrum. For 10 ppm of phenol, themaximum rate of solar degradation resulted in a decreasein concentration to 10 ppb in less than 80 min with totalmineralization in 110 min.

An efficient degradation of aqueous phenol was achieved[416] by a new rotating-drum reactor coated with a TiO2photocatalyst, in which TiO2 powders loaded with Pt areimmobilized on the outer surface of a glass drum. The reac-tor can receive solar light and oxygen from the atmosphereeffectively. It was shown experimentally that phenol can bedecomposed rapidly by this reactor under solar light: withthe used experimental conditions, phenol with an initial con-centration of 22.0 mg/dm3 was decomposed within 60 minand was completely mineralized through intermediate prod-ucts within 100 min.

The photocatalytic degradation of various types of dyesappears to be another prominent and extensively exploredapplication of nanocrystalline TiO2 in environmental cataly-sis [417–423].

In a recent study [424], the photocatalytic degradation offive dyes in TiO2 aqueous suspensions under UV irradiationhas been investigated; it was attempted to determine theindividual steps of such a degradation process by varying thearomatic structures, using either anthraquinonic (AlizarinS (AS)), or azoic (Crocein Orange G (OG), Methyl Red(MR), Congo Red (CR)) or heteropolyaromatic (Methy-lene Blue (MB)) dyes. Figure 15 exemplifies the photocat-alytic degradation of three of these dyes (CR, OG, and MR)as a function of UV irradiation. The initial reaction rateswere found to fall in the range from 1.9 �mol/l min (forCR) to 3.6 �mol/l min (for MR). In addition to a promptremoval of the colors, TiO2/UV-based photocatalysis wassimultaneously able to fully oxidize the dyes, with a com-plete mineralization of carbon into CO2. Sulfur heteroatomswere converted into innocuous SO2−

4 ions. The mineraliza-tion of nitrogen was more complex. Nitrogen atoms in the3-oxidation state, such as in amino groups, remain at thisreduction degree and produced NH+

4 cations, subsequentlyand very slowly converted into NO−

3 ions. For azo-dye (OG,MR, CR) degradation, the complete mass balance in nitro-gen indicated that the central N N azo group wasconverted into gaseous dinitrogen, which is the ideal issuefor the elimination of nitrogen-containing pollutants. Thearomatic rings were submitted to successive attacks by pho-togenerated •OH radicals leading to hydroxylated metabo-lites before the ring opening and the final evolution of CO2induced by repeated reactions with carboxylic intermediates.

The photocatalytic degradation of acid derived azo dyesin aqueous TiO2 suspensions follows apparently first-orderkinetics [425, 426]. The site near the azo bond (C N N-bond) is the attacked area in the photocatalytic degrada-tion process, while the TiO2 photocatalytic destruction ofthe C N( ) bond and N N bonds leads to fadingof the dyes. The pH effect on the TiO2 photocatalytic degra-dation of the acid-derived azo dyes varies with dye struc-ture. Hydroxyl radicals play an essential role in the fissionof the C N N conjugated system in azo dyes in TiO2photocatalytic degradation. Metalized azo dyes were studied

0 50 100 150 2000

20

40

60

80

conc

entr

atio

n (µ

mol

/l)


CR OG MR

Figure 15. Photocatalytic degradation of three different dyes, CongoRed (CR), Crocein Orange (OG), and Methyl Red (MR), given interms of the concentration versus the time of illumination. Data from[424], H. Lachheb et al., Appl. Catal. B: Environ. 39, 75 (2002).


[427] under TiO2 photocatalytic and photosensitized condi-tions in aqueous buffering solutions. The size and strength ofintramolecular conjugation determines apparently the light-fastness of the dyes; the more powerful OH radicals in TiO2photocatalytic process are highly reactive towards the azodyes.

7.3. Wastewater and Soil Remediation

The major causes [428] of surface water and groundwa-ter contamination are industrial discharges, excess use ofpesticides, fertilizers (agrochemicals), and landfilling domes-tic wastes. Typically, the wastewater treatment is basedupon various mechanical, biological, physical, and chemicalprocesses. After filtration and elimination of particles in sus-pension, the biological treatment is the ideal process (natu-ral decontamination). Unfortunately, organic pollutants arenot always biodegradable; a promising approach then relieson the formation of highly reactive chemical species, whichdegraded the more recalcitrant molecules into biodegrad-able compounds. These are called the advanced oxidationprocesses (AOPs). Although there exist differences in theirdetailed reaction schemes, their common feature is the pro-duction of OH radicals (•OH); these radicals are extraordi-narily reactive species (oxidation potential 2.8 V). They arealso characterized by a low selectivity of attack, which is auseful attribute for an oxidant used in wastewater treatmentand for solving pollution problems. These photocatalyticdegradation of wastewater employing nanocrystalline TiO2has been examined in various set-ups [429] and pilot-plantscale solar photocatalytic experiments have been realized[428].

Several recent studies reported on the removal or reduc-tion of metals or metal-containing contaminants in waste-water, based on the principles outlined in the foregoingparagraph. Those investigations examined, for example, theremoval of cadmium and mercury from water using mod-ified TiO2 nanoparticles [430, 431], the radical, mediatedphoto-reduction of manganese ions in UV-irradiated titaniasuspensions [432], the simultaneous photocatalytic Cr(VI)reduction and dye oxidation in a TiO2 slurry reactor [433],or the removal of iron(III) cyanocomplexes [434].

While the efficient use of a photocatalytic process in thepresence of TiO2 to degrade many different types of pol-lutants in wastewater has been confirmed repeatedly, thequestion of how to efficiently separate and reuse TiO2 fromtreated wastewater became a notable problem in the appli-cation of a TiO2 photo-oxidation process. A recent study[435] aimed to develop an advanced process for dyeingwastewater treatment, in which dyeing wastewater was ini-tially treated by an intermittently decanted extended aer-ation (IDEA) reactor to initially remove biodegradablematters and further treated in a TiO2 photocatalytic reac-tor for complete decolorization and high chemical oxygendemand (COD) removal. Suspended TiO2 powder used inthe photo-oxidation was separated from slurry by a mem-brane filter and recycled to the photo reactor continuously.

Photocatalytic destruction of chlorinated solvents in waterwith solar energy was investigated [436] using a near-commercial scale, single-axis tracking parabolic trough sys-tem with a glass pipe reactor mounted at its focus. In

the photocatalytic degradation of industrial residual waters,the use of peroxydisulfate (S2O2−

8 ) as an additional oxidant(electron scavenger) was observed to have an outstandingeffect, producing an important increase in the degradationrate [437]. The impact of pH and the presence of inorganicions and organic acids commonly found in natural waters onrates of TiO2 photocatalyzed trinitrotoluene (TNT) trans-formation and mineralization was examined [438]. Raisingthe pH slightly increased the rate of TNT transformation,primarily as a result of an increased rate of TNT photoly-sis, but significantly reduced rates of mineralization due toincreased electrostatic repulsion between the catalyst surfaceand anionic TNT intermediates. The presence of inorganicanions did not substantially hinder TNT transformation atalkaline pH, but mineralization rates were diminished whenthe anion either adsorbed strongly to the photocatalyst orwas an effective hydroxyl radical scavenger.

Immobilized TiO2 photocatalysts were used to sterilizeand reclaim the wastewater of bean sprout cultivation froma continuous hydrocirculation system [439]. The photocata-lysts effectively killed bacteria and degraded organic pollu-tants in the wastewater. Stimulation of bean sprout growthand suppression of decaying pathogens were also induced bythe TiO2 photocatalytic activity.

Photocatalytic decomposition of seawater-soluble crudeoil fractions using high surface area colloid nanoparticlesof TiO2 under UV irradiation was explored [440]; althoughno mineralization occurred due to photolysis, importantchemical changes were observed in the presence of TiO2,with the degradation reaching 90% (measured as dissolvedorganic carbon, (DOC)) in waters containing 9–45 mg C/l ofseawater-soluble crude oil compounds after 7 days of artifi-cial light exposure. During light exposure, transient interme-diates that showed higher toxicity than the initial compoundswere observed, but were subsequently destroyed. Hetero-geneous photocatalysis using TiO2 was considered to be apromising process to minimize the impact of crude oil com-pounds on contaminated waters.

TiO2-photocatalytic degradation of a cellulose effluentwas evaluated [441] using multivariate experimental design.The effluent was characterized by general parameters suchas adsorbable organic halogens (AOX), TOC, COD, color,total phenols, acute toxicity, and by the analysis of chlori-nated low molecular weight compounds using GC/MS. Theoptimal concentration of TiO2 was found to be around 1 g/l.After 30 min of reaction more than 60% of the toxicity wasremoved and after 420 min of reaction, none of the initialchlorinated low molecular weight compounds were detected,suggesting an extensive mineralization.

Photocatalysts, based on titanium dioxide, were used forthe purification of contaminated soil polluted by oil [442].Commercially produced slurry of titanium dioxide was mod-ified with barium, potassium, and calcium. The experimentswere performed under natural conditions in summer months(July and August) applying direct solar-light irradiation. Themost active photocatalyst for soil purification was titaniumdioxide modified with calcium.

Two different photocatalysts, namely, Hombikat UV100(Sachtleben Chemie) and P25 (Degussa) have been used inbatch experiments [443] to compare their ability to degradethe toxic components of a biologically pretreated landfill


leachate. A strong adsorption of the pollutant molecules wasobserved for both TiO2-powders, with a maximum of almost70% TOC reduction for Hombikat UV100.

7.4. Purification of Drinking Water

Pathogens in drinking water supplies can be removed bysand filtration followed by chlorine or ozone disinfection.These processes reduce the possibility of any pathogensentering the drinking water distribution network. How-ever, there is doubt about the ability of these methodsto remove chlorine-resistant microorganisms including pro-tozoan oocysts. Titanium dioxide (TiO2) photocatalysis isa possible alternative/complementary drinking water treat-ment method and several studies [444, 445] reported astrong and swift photocatalytic inactivation of bacteria andbacteriophages in aqueous solutions. For example, the rateof disinfection was explored using TiO2 electrodes preparedby the electrophoretic immobilization of TiO2 powders withdifferent crystallinity. These electrodes were tested for theirphotocatalytic bactericidal efficiency with E. coli K12 as amodel test organism [446]. Similar studies were reported fornatural water from a river [447].

Cyanobacterial toxins produced and released bycyanobacteria in freshwater around the world pose aconsiderable threat to human health if present in drinkingwater sources. Therefore, various treatments have beenapplied to remove these toxins. The effectiveness of TiO2photocatalysis for the removal of microcystin-LR fromwater has been established [448]. Not only does the processrapidly remove the toxin but also the by-products appear tobe nontoxic. The photocatalytic process has also significantlyreduced the protein phosphatase 1 (PP1) inhibition. Proteinphosphatase 1 inhibition is potentially one of the mostserious harmful effects to humans who may consume watercontaminated by microcystins. Figure 16 shows some ofthese data, namely, the reduction of the microcystin-LRconcentration and the PP1 inhibition as a function of theillumination time. The results indicate that about 86% of

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Figure 16. Destruction and protein phosphatase (PP1) inhibition ofmicrocystine-LR via TiO2 photocatalysis as a function of the durationof UV illumination (xenon lamp with 480 W at a wavelength of 330–450 nm). Data from [448], I. Liu et al., J. Photochem. Photobiol. A:Chem. 148, 349 (2002).

microcystin-LR was destroyed within the first 5 min ofphotocatalysis, with 97% of the toxin removed in 20 min.The addition of 0.1% H2O2 to the photocatalytic systemwas found [448] to further enhance the degradation rate:99.6% of microcystin-LR was destroyed within 5 min andno toxin was left after 10 min of photocatalysis.

Photocatalytic inactivation of different bacteria and bacte-riophages in drinking water at different TiO2 concentrationswith or without concurrent exposure to O2 was studied in[449] using UV irradiation (5.5 mW/cm2 at 365 nm). Forexample, for this light intensity, the most effective inacti-vation of Escherichia coli CN13 was obtained at 1 g/l sus-pension of TiO2, resulting in a reduction by five orders ofmagnitude in 5 min. Under the same experimental condi-tions, MS2 bacteriophage was reduced by four orders ofmagnitude, also in 5 min. The addition of O2 into the exper-imental environment increased the inactivation of Deinococ-cus radiophilus by four orders of magnitude in 60 min.

7.5. Miscellaneous PhotocatalyticApplications

It may have become apparent from the foregoing dis-cussions and examples that the solution of environmentalproblems constitutes one of the (if not the) major drivingforces in research and development in photocatalysis usingnanometer-sized TiO2 (and other semiconductor) particles.Another one, of course, is the production of hydrogen fromwater splitting. Apart from these main applications, thereexist, on the other hand, many attempts to explore novelareas for the photocatalytic use of nanocrystalline TiO2materials. To give a flavor of the diversity of these efforts,some selected (and mostly recent) examples follow.

Nano-sized titanium oxide (TiO2� thin films have beenexplored for alcohol-sensing applications. TiO2 thin filmswith different doping concentrations were prepared on alu-mina substrates [450] using the sol–gel process using thespin-coating technique for ethanol and methanol alcohol.Experimental results indicated that the sensor is able to mon-itor alcohols selectively at ppm levels; the films are stoichio-metric with carbon as the dominant impurity on the surface.The morphologies and crystalline structures of the films werestudied by scanning electron microscopy (SEM) and XRD.X-ray diffraction patterns showed that the films are pureanatase phase up to an annealing temperature of 600 �C.As the annealing temperature increased to 800 �C, a smallamount of rutile phase formed along with the anatase phase.

Optical waveguides were prepared by depositing a sol–gel-derived titania film onto a silica substrate [451]. The titaniafilm is mesoporous, with pore sizes ranging from 3 to 8 nm.Deposition of the titania does not change the critical angleof total internal reflection. Thus, the titania-coated wave-guides propagate light in an attenuated total reflection mode,despite the relatively high refractive index (n = 1.8 in air) ofthe titania film relative to the silica substrate (n = 0�5).

The light output of electric lighting gradually decreasesdue to stain buildup on lamps and covers during operation.Roadway, and especially tunnel lighting, experiences a largeamount of contamination due to dust, carbon particles foundin vehicle engine exhausts and other airborne contaminants,


which results in the rapid deterioration of the light output.Photocatalytic reactions caused by TiO2 are known to decom-pose such stains. This reaction is caused by the absorptionof UV light ( ≤ 400 nm, corresponding to the bandgapof ∼3 eV) irradiated from lamps or the sun, and followedby oxidation. Extensive field tests revealed [452] that a finefilm coating of TiO2 on lamps and luminaires can effec-tively decompose various organic compounds such as vehicleexhaust gases, oil, nicotine, etc. This leads to an improvementof the luminous performance of installed lighting systems andreduces the cost of maintenance by approximately one-half.

It has been recently found [453] that photocatalytic TiO2coated with polycarbonate (PC) releases a huge amount ofexothermic energy in the temperature range between 200and 400 �C (ca. 1.85 kJ/g). The strong interaction betweenoxygen-deficient sites in TiO2 and carbonyl groups of PCmediated by a good PC solvent is found to be a prerequi-site for a release of the enormous amount of exothermicenergy. This finding suggests that PC-coated TiO2 powdersor related oxides work as a combustion-assisting agent in arelatively lower temperature range and can be utilized forincineration applications in order to suppress the formationof extremely toxic dioxins.

A somewhat unusual application reported [454] the pho-tocatalytic deposition of a gold particle onto the top of aSiN cantilever tip, employing the photocatalytic effect oftitanium dioxide. When the titanium dioxide immersed ina solution including gold ions is subject to optical expo-sure, the excited electrons in the conduction band reducegold ions into gold metal. Illumination by an evanescentwave generated with a total reflection configuration limitsthe deposition region to the very tip. In the experiments,100–300 nm gold particles on SiN cantilever tips for atomicforce microscopes were obtained. In a related vein, photo-induced deposition of copper on nanocrystalline TiO2 filmswas proposed [455].

Solar photocatalytic oxidation processes (PCO) for degra-dation of water and air pollutants have received increasingattention. In fact, some field-scale experiments have demon-strated the feasibility of using a semiconductor (TiO2� in solarcollectors and concentrators to completely mineralize organiccontaminants inwaterandair [456].Althoughsuccessfulprein-dustrial solar tests have been carried out, there are still dis-crepancies and doubt concerning process fundamentals suchas the roles of active components, appropriate modelling ofreaction kinetics, or quantification of photo-efficiency. Chal-lenges to development are catalyst deactivation, slow kinet-ics, low photo-efficiency and unpredictable mechanisms. Thedevelopmentof specificnonconcentrating collectors fordetox-ification and the use of additives such as peroxydisulfate havemade competitive use of solar PCO possible.

GLOSSARYCharge transfer The transfer of a charge carrier (electron orhole) fromanexcited semiconductor toanadsorbed species onits surface. This transfer may initiate a reaction (oxidation orreduction) in the adsorbedmolecule.Dye-sensitized semiconductor Adsorbing a suitable dye onthe surface of a wide band gap semiconductor (like TiO2) can

enhance the efficiency of the excitation step and, hence, thecatalytic activity.Electron-hole pair The absorption of a photon of sufficientenergy may excite in a semiconductor an electron from thevalence band to the conduction band, thereby creating a holein the valence band.Nanocrystalline A material composed of individual crys-tallites which have a size in the range of nanometer (nm);1 nm = 10−9 m.Photocatalysis Acatalytic reaction triggeredor enhancedbyilluminating the system with visible or ultraviolet irradiation.This reaction involves normally the electronic excitation of thecatalyst via the absorption of photons and an interfacial chargetransfer to an adsorbed species. Typically, the photocatalyst isnot consumed in the reaction.Photocatalytic degradation The removal or reduction of(usually unwanted) substances via a photocatalytic reaction.Quantum yield The probability of product formation peradsorbed photon in a photocatalytic reaction.Titanium oxide Titanium dioxide with the nominal compo-sition TiO2 is a semiconductor with a band gap of ∼3.2 eV; itexists in three different crystalline modifications, two of which(anataseandrutile) are commonlyemployed inphotocatalysis.

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