Desinfeccion Fotocatalitica de Agua en Semiconductores de Oxido

7/30/2019 Desinfeccion Fotocatalitica de Agua en Semiconductores de Oxido

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Photocatalytic water disinfection on oxidesemiconductors: Part 1 basic concepts of

TiO2 photocatalysisT. Bak1, J. Nowotny*1, N. J. Sucher2 and E. D. Wachsman3

Water disinfection (removal of microbial agents) using sunlight is an emerging technology, which

has the capacity to address the global shortage of drinking water. Therefore, intensive

investigations in many laboratories aim to develop photocatalyst for water disinfection. The

research is focused on titanium dioxide (TiO2), which is the most promising candidate for high

performance photocatalyst able to address the commercial requirements. The present work (Part

1) considers the effect of defect disorder on semiconducting and photocatalytic properties of

TiO2 (rutile) in water disinfection using solar energy. It is shown that photocatalytic properties of

TiO2 in water are closely related to the light induced reactivity of TiO2 with water leading to the

formation of active species, such as OH*, H2O2 and O{

2 , which have the capacity to oxidise

microorganisms. It is also shown that the ability of TiO2 to form the active radicals is closely

associated with the presence of point defects in the TiO2 lattice and the related semiconducting

properties. Therefore, photocatalytic properties of TiO2 may be modified in a controlled manner

by changes in its defect disorder. Consequently, defect chemistry may be used as the framework

in the development of TiO2 with controlled properties that are desired for solar water disinfection.

The following work (Part 2) considers the structure of bacteria and their reactivity/photoreactivity

with TiO2 in aqueous environments. Both Part 1 and 2 bring together the concepts of TiO2

photocatalysis and the concepts of microbiology in order to derive the theoretical models that are

needed for the development of high performance photocatalysts for solar water disinfection.

Keywords: Water purification, Titanium dioxide, Solar energy, Photocatalysis

This paper is part of a special issue on Energy Conversion Systems

Introduction

The United Nations has estimated that worldwide, 1?1

billion people lack access to clean water supply and 2?4

billion to proper sanitation.1 The lack of either or both

contributes to a vicious cycle of ill health and poverty.

Drinking of contaminated water is a major cause ofgastro-intestinal disease including (infectious) diarrhea.

Therefore, there is an increasingly urgent need, in the

global scale, to address this problem.

Pollution of water resources is affecting mostly the

worlds poor. The development and implementation of

efficient and cost effective means for the purification,

disinfection and provision of water is crucial to remedy

this situation. Closely linked to this water crisis is the

impending energy crisis. It is in this context that the

use of semiconductor based photocatalysts for water

purification using solar energy has attracted intense

interest.2,3

Photocatalysis is the science of catalytic processes

activated by the light energy (photons) rather than heat

(thermal energy). Thus, most photocatalytic reactions

do not require the expenditure of heat energy and cantake place at room temperature.4

The best studied photocatalyst is titanium dioxide

(TiO2).58 A large body of work has demonstrated that

TiO2 is the most promising photocatalyst for water

purification. The process consists in the decomposition

of organic toxic compounds (detoxification) and micro-

organisms, including viruses, bacteria, fungi and algae

(disinfection). Therefore, the main sources of the em-

pirical data, which can be used to verify the theoretical

models on photocatalysis, are the reports on photo-

catalytic properties of TiO2.

The aim of the present study, involving the present

work (Part 1) and the following paper (Part 2),9

is tobring together the concepts and basic terms of solid state

chemistry and TiO2 photocatalysis on the one hand and

1Solar Energy Technologies, University of Western Sydney, Australia2Centre for Complementary Medicine Research, University of Western

Sydney, Australia3Energy Research Center, University of Maryland, USA

*Corresponding author, email [email protected]


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the concepts of microbiology on the other hand. The

objective is to consider the effect of the light inducedelectroactivity of TiO2 based photosensitive oxide semi-

conductors on their photocatalytic properties in waterdisinfection by using solar energy as the only driving

force of the process.

The present work (Part 1) considers the basic conceptsof TiO2 semiconductors and general models of photo-catalysis on semiconductors. It is shown that photo-

catalytic activity of TiO2 is determined by several keyproperties, including charge transport, electronic struc-

ture, as well as surface and near surface properties. It isargued that progress in TiO2 photocatalysis requires

better understanding of photoreactivity of TiO2 withwater and its contaminants, such as microorganisms andtoxic organic compounds. The research aims to derive

theoretical models and collect well defined empirical data

that are needed to verify these models. The latter may beused to develop photocatalysts with enhanced perfor-mance. The present work also considers the concepts of

TiO2 based photosensitive semiconductors in terms of thekey functional properties. It is shown that these key

properties are closely related to the presence of pointdefects in the TiO2 lattice. Therefore, these propertiesmay be modified using defect engineering. Finally, the

present work considers the basic concepts of defectchemistry and the application of defect engineering in the

formation of high performance TiO2 based photocatalysts.

The following work (Part 2)9 considers the structureof basic types of microorganisms (prokaryotes andeukaryotes) in terms of their functional components.The focus is on bacteria, both Gram positive and Gram

negative. The mechanism of penetration of the reactiveOH* radicals towards the cell membrane and thecytoplasm, through the open structure of bacteria orporins is discussed and the critical stages of oxidation

resulting in killing of bacteria are considered. Theobjective of Part 2 is to consider the mechanism of lightinduced reactivity of bacteria with TiO2 oxide semi-

conductors (acting as photocatalysts) in water. Thefocus is on the photoreactivity of TiO2 with water,resulting in the formation of reactive radicals, whichexhibit high oxidation power leading to killing thebacteria. This work considers also the main hurdles thatmust be overcome in order to bring the technology ofwater disinfection to commercialisation.

Basic properties of TiO2Titanium dioxide is very reactive with both light andwater. At the same time, TiO

2exhibits an exceptional

resistance to corrosion and photocorrosion in aqueousenvironments. Therefore, TiO2 in water exhibits stableproperties over a prolonged period of time.

Titanium dioxide (titania) exists in several crystallineforms. The most common forms are: rutile, anatase andbrookite. These structures are formed of TiO6 octahedrain different arrangements.10 The most common formis rutile, which is the only thermodynamically stablestructure of TiO2. The rutile structure is shown in Fig. 1.

Owing to its high refractive index (2?9) and brightness,TiO2 is used mainly for pigments providing whitenessand opacity to paints, plastics, papers, inks, cosmetics,

fibers, as well as medicines and toothpaste. Because of

its strong UV light absorbance, TiO2 is applied as asunscreen blocker. The applications of titania alsoinclude self-cleaning coatings of building materials,

hydrophyllic coatings of glass, antifogging coatings formirrors and antiseptic coatings in sanitary areas.8

Several applications of titania are outlined in Table 1.

TiO2 is a non-stoichiometric compound, which has beencommonly recognised as an oxygen deficient compound of

1 Rutile structure

Table 1 Applications of titanium dioxide and the performance-related properties

Application Performance associated properties

Photocatalytic water disinfection & detoxification Selective photoreactivity with water resulting in the formation of active radicalsPhotoelectrochemical water splitting Selective photoreactivity with water leading to water splitting into oxygen and

protons at anodePigments While colourSkin protection from UV light High reflective indexChemical gas sensors Selective reactivity with specific gases, such as oxygen,

hydrocarbons, hydrogen, carbon oxide and alcoholDielectrics High dielectric constantAntireflection coatings Optical transmittanceConducting coatings High charge transport of TiO2 solid solutions with penta-valent ionsDye-sensitized solar cells Specific properties of the TiO2/dye

interface and the related charge transferAntipollution coatings Photoreactivi ty with gases causing air pollution, such

as nitrogen oxides, leading to their removal from the gas phaseSelf-cleaning coatings Photoreactivity of adsorbed species resulting in the oxidation and removalSuperhydrophyll ic coatings Photoreactivi ty with waterAntifogging coatings Photoreactivity with water

Antiseptic coatings Photoreactivi ty with microbial agents in the presence of waterLasers Optical transmittanceTemperature sensors Well defined electrical properties

Bak et al. Photocatalytic water disinfection on oxide semiconductors: Par


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the formula TiO22x, where x is the effective deviation fromstoichiometry.10 Awareness is growing, however, that:

N The non-stoichiometry of TiO2 is complex andinvolves defects in both oxygen and titanium sub-

lattices: oxygen vacancies, titanium vacancies andtitanium interstitials.

N The performance related properties of TiO2 are

closely related to its defect disorder.2

Among several titanium oxides, TiO2 (rutile) is stableover a wide range of temperatures and oxygen activities.There is an excellent agreement between different

literature reports on the effect of oxygen activity on

the extent of oxygen deficit determined by thermogra-vimetry at elevated temperatures.1016

TiO2 may be reduced or oxidised within a single phase

regime leading to the formation, or removal, of pointdefects in the crystal lattice. These include oxygen

vacancies and titanium interstitials, which are donor-type defects. Their ionisation results in the formation of

quasi-free electrons, which are responsible for n-type

charge transport. Their concentration may be deter-mined from defect diagrams.17 Recent studies show that

prolonged oxidation of undoped TiO2 may lead to p-type semiconductivity (conducting via electron holes),which are associated with the presence of titanium

vacancies.17,18 Ionisation of these acceptor-type defectsresults in the formation of electron holes. Consequently,

the real chemical formula of titanium dioxide, reflecting

the presence of defects in both oxygen and cationsublattices, is better represented by Ti1xO22y.

Most of the non-stoichiometry data reported in the

literature for TiO2 are related to bulk properties. How-ever, the nanosize TiO2 exhibit entirely different proper-

ties, which are mainly influenced by the surface tovolume ratio. The non-stoichiometry of the surface layer

and the related defect disorder are very different fromthose of the bulk phase.17,18 The details on non-stoichiometry of TiO2 and the related defect disorder

are considered in the present work.

The microstructure of commercial specimens depends

on the applied processing procedure and grain size. Forexample, TiO2 of Degusa (P25) contains 20% of the

rutile phase and 80% of the anatase phase, while the

Millenium specimens (PC-10, PC-50 and PC-500) havethe anatase structure.19 Heating of the anatase form of

TiO2 in air results in its transition into the rutile form at600 K. The effect of the crystallite size on the

temperature of this transition has been reported byWu et al.,20 who studied a nanocrystalline form of TiO2.

The literature reports represent a common perceptionthat photocatalytic properties of oxides are determined

by the crystalline structure and phase composition. Thisperception, however, fails to recognize that the oxide

lattice includes a range of structural defects, such as point

defects, linear defects and planar defects. TiO2 is not anexception. The recent studies have shown that the key

properties of TiO2 are closely related to the concentration

of point defects.2 Therefore, defect chemistry, may beused as the framework for the processing of TiO2 with

desired properties.2,17,18

Annealing of TiO2 in extremely reduced conditionsresults in the formation of lower oxide phases of

titanium, which can be considered as a homologousseries of oxides and their chemical composition may be

expressed by the general formula TinO2n21.2127 The

formation of crystallographic shears was initially pos-

tulated by Magneli,24,25 Wadsley,27 and Hyde.23 Bursill

and Hyde21,22

reported that maximum concentration ofoxygen vacancies in TiO22x corresponds to x50?001 and

above this concentration the vacancies are eliminated by

the formation of crystallographic shear planes. Therelated new phases are termed Magneli phases.

The valence band of TiO2 is formed of filled 2porbitals of double valent oxygen ions, while the

conduction band is formed of empty 3d states of four-

valent titanium ions (Fig. 2). The difference between the

energy at the top of the valence band EV and the bottom

of the conduction band EC, forms the forbidden energy

gap, which for rutile is 3?05 eV.2,2836 Reduction of TiO2to Ti2O3 and then to TiO results in a reduction of the

band gap to 0?13 eV and zero respectively (TiO is ametallic conductor).37

The electronic structure of nanostructured materials

with ultrafine grain/crystalline size is entirely differ-

ent than that of single crystals. Therefore, electronic

structure may be modified through the modification of

particle size. There has been an accumulation of effortsto reduce the TiO2 band gap by reduction of its grain

size. Hoffmann et al.38 reported that below a certaincritical grain size (10 nm) the band gap increases. This

effect has been confirmed by studies of Wang et al.39

who observed that the band gap of the 2?7 nm grain size

TiO2 (rutile) is 3?32 eV, while the band gap for TiO2single crystal is 3?05 eV. However, there are experi-

mental and theoretical evidences that TiO2 nanotubeshave reduced band gap.3941 This indicates that nano-

particles with dominating concave curvatures at surfacesexhibit a decrease of the band gap, while convex

curvatures lead to band gap increase. The effective band

gap may also be reduced by the imposition of midgap

bands, which may be formed by the incorporation of

aliovalent ions.7,8,4244

TiO2 photocatalysis

ReactivityWhile the properties of oxides, including TiO2, are

closely related to their defect disorder, the reactivity andthe related charge transfer are determined by the

chemical potential of electrons mn

2 Electronic structure of TiO2 (EC, conduction band; EV,

valence band; Eg, band gap , EF, Fermi level

Bak et al. Photocatalytic water disinfection on oxide semiconductors: Part 1


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mn~monzkTln an (1)

where mon is the chemical potential of electrons in the

standard state and an is the activity of electrons (in the

first approximation the an term is equal to the con-

centration of electrons). Therefore, knowledge of the

concentration of the electronic charge carriers is essen-

tial in correct assesment of the reactivity of TiO2,

including its reactivity with water. The term an is theproduct of the concentration of quasi-free electrons n,

and the activity coefficient f

an~fn (2)

Quasi-free electrons are formed by ionisation of ionic

defects. Consequently, the chemical potential of elec-

trons is determined by the point defects disorder. The

basic concepts of defect chemistry for TiO2 are

considered in the following section.

Photocatalytic effect of TiO2TiO2 exhibits promising photocatalytic properties for

the oxidation of water contaminants, including micro-organisms and toxic compounds. The photocatalytic

effect consists in the light induced reactivity of TiO2 with

organic compounds, and bacteria, leading to their

oxidation into harmless products, such as H2O and

CO2. The photocatalytic reactions take place at the

surface of the TiO2 photocatalyst, which is exposed to

light.

Absorption of light by TiO2 results in ionisation over

the band gap. However, the light induced electrons and

electron holes are available to chemical reactions only if

separated in an electric field. Alternatively, the light

induced electronic charge carriers recombine. The

charge separation, leading to enhanced concentration

of electron holes at the surface, results in a substantial

increase of the oxidation power of TiO2, when immersed

in water. This consequently results in mineralisation of

organic compounds.

The bactericidal effect is schematically represented in

Fig. 3 showing the concentration of bacteria in water, in

the presence and absence of light and TiO2, including the

following combinations:

N Absence of light and TiO2 (Fig. 3a). In this case

bacteria have the tendency to grow (their concentra-

tion increases as a function of time).

N Presence of TiO2 and absence of light (Fig. 3b). As

seen, the presence of TiO2 in water has no effect on

the process of bacterial growth.N The presence of light and absence of TiO2. The effect

of light on the concentration of the colony forming

units of bacteria depends on the light spectrum:

(i) the light energy, in the absence of the UV light

component, is mainly converted to heat leading

to enhanced grow of bacteria (Fig. 3c)

(ii) the UV light component results in killing of

bacteria. Consequently, their concentration

decreases in time (Fig. 3d).

N The presence of both light and TiO2 (Fig. 3e). In this

case the light energy is converted into the chemical

energy resulting, in the decomposition of bacteria.

The strength of photocatalytic effect of TiO2 dependson its specific properties, such as surface vs. bulk

composition.

Up to now, the effect of properties of oxide semicon-

ductors on their photocatalytic performance has been

considered mainly in terms of the critical importance of

electronic structure.2,48,42,43

3 Schematic representation of effect of light and TiO2 on

change in population of bacteria (number of colony

forming units, CFU): a absence of light and TiO2; b pre-sence of TiO2 in dark; c illumination without UV com-

ponent, light is converted into heat; d presence of UV

component in light; e presence of both, TiO2 and full

spectrum light

4 Solar spectrum in terms of number of photons as func-

tion of their energy



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Light induced semiconducting propertiesThe solar to chemical energy conversion in semiconduc-

tors is closely related to their electronic structure,

specifically the band gap Eg. When the photon energy

is equal to, or greater than, the band gap energy, its

absorption can lead to ionisation over the band gap and

formation of an electron-hole pair. For commonly

available TiO2 the band gap is >3?05 eV for rutile

and 3?2 eV for anatase.

19

As seen in Fig. 4, thecommonly available TiO2 allows only a very small part

of the entire solar spectrum to be converted into

chemical energy that can be used for the oxidation

reactions at the surface of TiO2. Therefore, the key

question is: How can we form TiO2 with enhanced

photocatalytic activity? In order to address this ques-

tion, we need first to define some terms that are related

to the photocatalytic performance.

The photon flux available for conversion J, is

J~

?

Eg

N(E)dE (3)

As seen in Fig. 4, representing the solar spectrum interms of the number of photons N(E), versus photon

energy E, the photon flux (area under the curve) for the

standard (large grain) TiO2 (for which Eg53 eV) is very

small. Consequently, the most critical issue in the

development of photocatalysts is to enhance the light

absorption through reduction of the band gap to a lower

value in order to increase the amount of absorbed light

energy. This may be achieved, for example, through the

imposition of midgap bands. According to Asahi et al.43

the band gap reduction may also be achieved by lifting

the energy level of the valence band through mixing 2p

states of oxygen and s states of dopant, such as nitrogen.

Similar effect was reported by Kudo et al.44

Exposition of semiconducting photocatalyst to light

leads to intrinsic ionisation over the band gap and

results in the formation of an electron-hole pair

hn?ezh. (4)

where, according to the notation proposed by Kroger

and Vink,45 e9 and hN denote electron and electron holein the lattice (this notation allows to represent the charge

transfer during reactions between the lattice defects10).

However, the life time of the light-induced charge

carriers is limited to nanoseconds as these have tendency

to recombine, if not separated. The recombination is not

desired as this leads to energy losses. These losses may

be reduced when the light-induced charge carriers areseparated in an electric field formed near the surface.

Such field is usually formed across the space charge

layer, compensating the surface charge, when the

photocatalyst is immersed in water. The recent studies

show that the electric field may also be engineered by the

imposition of chemical concentration gradients within

the light penetration distance from the TiO2/H2O

interface. Such gradients may be imposed by segregation

or diffusion.2

The basic quantity of semiconductors, describing their

ability to donate or accept electrons, is the chemical

potential of electrons that is expressed by equation (1).

This quantity is equivalent to the Fermi level EF.10

Theeffect of light on semiconducting properties may be

considered in terms of the split of the Fermi level, into

two quasi-Fermi levels related to electrons (EF)n andholes (EF)p, as it is schematically represented in Fig. 5.As seen, in this case the semiconductor exhibits positivesurface charge, which is characteristic for photoanode. It

is essential to note that these quasi-Fermi levels do notcorrespond to equilibrium, as the split takes place onlywhen the semiconductor is exposed to light and excesscharge carriers are generated. This is in contrast to theFermi level quantity in the FermiDirac statistics, whichcorresponds to thermodynamic equilibrium.10

The electric field, forming the surface potential barrieris represented in either upward or downward bandbending. In the first case, the charge separation leads tothe transport of electron holes towards the surface andthe transport of electrons toward the bulk. This kind ofcharge separation causes a substantial increase of theoxidation potential at the surface. Such an effect is

typical for the anodic site. The picture is different whenthe downward band bending occurs resulting in thetransport of electrons to the surface. This, consequently,results in an increase of the reduction power (cathodic

site).

Photocatalytic water disinfection

Basic reactionsThe pioneering experiment of Fujishima and Honda,46

showing that TiO2 may be used as a photocatalysts tosplit water into hydrogen and oxygen, paved the way for

the potential application of TiO2 in water disinfection.The use of TiO2 as a catalyst in the light induced

photoelectrochemical oxidation of microorganisms wasintroduced in 1985 by Matsunaga et al.47 These reportsresulted in an enormous interest in TiO2 as a potentialphotocatalyst for water disinfection (removal of bac-teria) and detoxification (removal of toxic organiccompounds) using solar energy.

A wide range of light induced primary and secondaryreactions take place at the interface between TiO2 andwater. The most common oxidation reaction, takingplace at anodic sites between the light induced electronholes (minority charge carriers) and water, leads to theformation of OH* radicals and hydrogen ions (protons)

H2Ozh.?HzzOH (5)

The reactive hydroxyl radicals OH*, play an importantrole in water purification. These radicals react with

5 Schematic representation of light induced split of Fermi

level EF, into components related to electrons (EF)n and

electron holes (EF)p, for n-type semiconductor



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microorganisms leading ultimately to the formation ofharmless products, such as CO2 and H2O.

The reduction potential of cathodic sites is related tothe excess concentration of electrons. The most impor-

tant reaction at these sites is reduction of oxygen leadingto the formation of singly ionised molecular (super-oxide) species

O2ze?O{

2 (6)

These species may then react with protons leading to theformation of hydrogen peroxide

O{2 z2Hzze?H2O2 (7)

The species formed as a result of the reactions (5)-(7)

then react with organic molecules resulting in theiroxidation and the formation of stable molecules, whichare not toxic.48

The hydroxyl radical is considered as the secondstrongest oxidising agent after fluorine.4 Its reactivity

normally results in complete mineralisation of mostorganic compounds leading to the formation of carbondioxide and water as the final products.49 The oxidationprocesses based on the generation of OH* are often

referred to as advanced oxidation processes.50

Under irradiation with short wavelength UV (200280 nm), H2O2 may dissociate into two OH* radicals,

some of which might also be activated directly by shortwavelength UV irradiation.50 Thus, irradiation of TiO2with short wavelength UV will result in a combinationof photocatalytic and photochemical reactions.

Another well known reaction, leading to the genera-tion of OH* radicals, is between iron ions and hydrogenperoxide

Fe2zzH2O2zHz?Fe3zzOHzH2O (8)

Reaction (8), which is often referred to as the Fenton

reaction, indicates that the presence of the Fe ions hasenhanced effect on the formation of hydroxyl radicals.The Fenton reaction may take place between the

bacterial protein, involving the iron-sulphur cluster,and hydrogen peroxide that can penetrate into cellinterior. The locally produced hydroxyl radicals can

oxidize the cell directly as a result on intra-cellchemistry.2,9

A general model of a photocatalytic system, involving

a semiconducting photocatalyst and microbial cell inwater as well as the light induced reactions at the TiO2/H2O interface, leading to the formation of active

radicals, and the reactivity of these radicals with

microbial cell is represented schematically in Fig. 6.

Experimental evidenceThere has been an accumulation of data providing the

evidence that TiO2 is the best candidate for highperformance photocatalysts. In their seminal experi-

ments, Matsunaga et al.47 demonstrated that yeast cells(Saccharomyces cerevisiae), Gram positive (Lactoba-

cillus acidophilus) and Gram negative bacteria (Esche-richia coli) were completely sterilised when they wereincubated with TiO2 (1 g L

21 Aerosil P-25, 99?99%

anatase and platinum black powder) and irradiatedby a halide lamp (400 W) or Xenon lamp (300 W) for

12 h. Green algae (Chlorella vulgaris) were partiallyresistant and could not be completely killed by this

treatment within 2 h. Neither TiO2/Pt nor light irra-diation alone was effective. The sterilisation efficiencyincreased with increasing light intensity and white

fluorescent light was ineffective. These workersobserved a reduction in the respiratory activity of yeastcells that was accompanied by a decrease in the

intracellular content of Coenzyme A. However, the cell

wall appeared to be unaffected by TiO2 and light.Interestingly, irradiated TiO2 appeared to be effectiveonly when the particles were in close contact with the

cells. Separation of particles from the microorganisms,using a dialysis membrane, rendered the treatment

ineffective. Similarly, neither addition of catalase,

albumin or cysteine had any affect on the respirationand viability of the cells. Together, these data suggest

that the bactericidal effect of toxic substances such asH2O2 and free radicals are not responsible but rather

the contact of irradiated TiO2 with microorganisms ledto the direct oxidation of the microbial cell: thephotochemical oxidation of Coenzyme A and a con-

comitant reduction in their respiratory and metabolicactivity resulting in cell death.

Since the publication of this groundbreaking work of

Matsunaga et al.,47 a large number of studies have beenpublished confirming the bactericidal effect of TiO2powders.8,5159 The focus has been on the kinetics of thebactericidal effects and the influence of the quality and

quantity of light and TiO2 as well as the chemicalcomposition of water and, especially, its pH. So far,however, little is known on the precise molecular

mechanisms and chemical reactions underlying the bac-tericidal effects of UV irradiated TiO2. Consequently,our understanding of this issue remains largely incom-

plete to date.

A substantial step forward in this direction has beenmade by Rincon and Pulgarin,51,52,57 who reported

photocatalytic inactivation of E. coli bacteria by TiO2.

They have determined simultaneously the reactionprogress and its effect on pH, as a function of time indark and also during exposure to light.

Increased dispersion of particles results in an increasein the catalytically active surface area. At the same time,however, the increase of dispersion results in a decrease

6 Model representing photocatalytic performance of TiO2,

related anodic and cathodic reactions leading to forma-

tion of active radicals, as well as reactivity of these

radicals with microbial cell



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of light penetration. Kim et al.55 observed that anoptimum of disinfection for some food pathogenicbacteria is at 1 g L21 for TiO2 of the surface area2?95 m2 g21. Saito et al.58 reported similar findings.

In general, the increased light intensity leads to anincreased rate of killing of bacteria. According to Wei

et al.59 the cell killing rate increases proportionately tothe increase in incident light intensity, in the range 180

1660 mE s21 m22.So far, the reports on TiO2 photocatalysis did not take

into account the reactivity between water and TiO2,

which results in the incorporation of protons into theTiO2 lattice. However, these species seem to have asubstantial effect on defect chemistry and the relatedproperties of TiO2. Their incorporation may be asso-ciated with the removal of oxygen vacancies

H2OzV..

O ?2H.zO|O (9)

where protons in the TiO2 lattice are associated with thelattice oxygen.60,61 Alternatively, hydrogen incorporationmay lead to the formation of negatively charged titaniumvacancies

2H2O?4H.zVTiz2O

|

O (10)

Unresolved problems

Semiconducting propertiesEssentially, all photocatalytic reactions are accompaniedby a charge transfer between the semiconductor andreacting substances. Therefore, the progress in photo-catalytic water purification requires better understand-ing of the electronic mechanism of photocatalyticreactions. The reactivity is determined by the ability of

the photocatalyst to donate or accept electrons, which isdetermined by the light induced chemical potential ofelectrons at the surface of the photocatalyst. So far,however, little is known in this matter as the majority ofstudies aimed to establish the correlation between

photocatalytic activity and the physical/chemical prop-erties that are not directly related to photoreactivity,such as structure, microstructure, dispersion, pH, light

intensity and surface area. Therefore, the progress inphotocatalysis requires collecting the empirical data onthe relationship between semiconducting properties,such as the chemical potential of electrons, and the

photocatalytic performance. Awareness is growing thatthese properties are directly related to defect disorder.

Therefore, there is a need to increase the present state ofunderstanding on the effect of defect disorder, anddefect related properties, on photocatalytic oxidation ofbacteria.

ReproducibilityMost of the studies reported in the literature are basedon the trial and error approach. Moreover, most of thereported experimental data are not well defined and arenot reproducible.2 For example, the reported data on theeffect of doping by aliovalent ions on photocatalyticproperties are conflicting. The conflicting reports resultfrom the applied doping procedures, which are not well

defined. The effect of doping on properties of TiO2depends on the incorporation mechanism of the ionsapplied as dopants. It was shown that dopants can be

incorporated into a polycrystalline specimen either intothe grain boundary area and/or into the lattice of thebulk phase.2 In the latter case, the incorporation maylead to the formation of concentration gradients, which

are not well defined. Consequently, there is a need todevelop processing procedures, which lead to theformation of photocatalytic systems with reproducibleproperties.

Effect of hydrogenThe photocatalytic reactions take place in water, whichis the reactive environment. In other words, thephotocatalytic activity of TiO2 in water disinfection is

closely related to the reactivity and photoreactivitybetween TiO2 and water. So far, little is known in thismatter. Specifically, there is a need to understand thereactivity of hydrogen ions (protons) with TiO2 and to

establish the effect of this reactivity on surface and bulkproperties of TiO2.

61 It appears that this reactivityresults in the formation of a solid solution of TiO 2 with

hydrogen. Its properties depend on the concentration ofhydrogen. So far, little is known on the effect of

hydrogen on properties of TiO2.

Effect of platinumThe main stream of research on TiO2 photocatalysis is

concentrated on the effect of platinum (and other noblemetals), deposited in the form of small surface islets,on the performance of TiO2 based photocatalysts.

6,7,58

The research aims to assess the effect of Pt dispersion,and the related ratio of the Pt/TiO2 surface area, onperformance. While the effect of platinum is relativelywell established, it is unlikely that Pt can be applied incommercial photocatalysts due to its high costs.Therefore, there is a need to search for alternativesolutions that are less expensive.

Structure and reactivity of bacteriaThe progress in photocatalysis requires knowledge of thereactivity/photoreactivity of both reacting partners: theTiO2 photocatalyst and the bacteria. The research areaof TiO2 photocatalysis is interdisciplinary itself as itinvolves the concepts of photoelectrochemistry, solidstate chemistry, surface science, catalysis and materialsengineering. Therefore, the research teams involved instudies on TiO2 photocatalysis must represent theexpertise in these areas as well as the science ofmicrobiology. The latter is essential in understandingthe reactivity of bacteria with solids that is controlled by

the structure of the cell wall and the functional genes.

62

The photoreactivity of microorganisms with TiO2 isconsidered in Part 2 of this paper series of two papers.9

Point defects and photocatalysisThe most recent studies indicate that properties of TiO2and its reactivity are closely related to defect disorderand the associated semiconducting properties.2 It hasbeen shown that the critical performance related proper-ties for photoreactivity of TiO2, including electronicstructure, charge transport and surface properties, arerelated to the concentration of point defects. Therefore,the photocatalytic performance may be enhanced by

defect engineering.The photocatalytic properties of TiO2 for water

purification, leading to removal of microorganisms from



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water, are closely related to the reactivity of TiO2 withwater and its decomposition products, oxygen andhydrogen. Therefore, there is a need to increase thepresent level of understanding on the following effects:

(i) the effect of defect disorder on semiconducting

properties

(ii) the effect of light on semiconducting properties(iii) the effect of semiconducting properties on the

reactivity and photoreactivity of TiO2 withwater

(iv) the effect of light on the electrochemical

properties of the TiO2/H2O interface.So far, little is known in this matter. Therefore, the aimof the present work is to consider the following:

(i) defect disorder of TiO2(ii) effect of defect disorder on semiconducting

properties

(iii) effect of light on the reactivity and photoreactiv-ity of TiO2 with water.

The following section shows that desired functionalproperties, such as electronic structure and charge trans-port, may be modified by the imposition of controlled

defect disorder. The considerations on the effect of lighton the reactivity of TiO2 with water are preceded by ashort overview on defect chemistry of TiO2 and therelated semiconducting properties. The effect of semi-conducting properties on reactivity and photoreactivityis then considered in terms of the performance relatedproperties, including electronic structure, charge trans-port, surface properties and near to surface concentra-tion gradients.

Defect disorder of TiO2Definition of terms

Titanium dioxide is a non-stoichiometric compound. Itscrystal lattice includes point defects, which are atomicsize imperfections, such as oxygen vacancies, cationvacancies and cation interstitials as well as electrons andelectron holes. The incorporation of foreign aliovalentions, including cations and anions, leads to the forma-tion of electron donors and acceptors. The point defectshave a crucial effect on properties, including electricalproperties, the mass transport kinetics, reactivity,catalytic properties and light induced properties.

The TiO2 lattice includes the following point defects:

N Cation vacancies represent empty sites in the cationsublattice. The missing four valent metal ion, Ti4z,may be considered as negatively charged (relative to

the lattice) species.

N Oxygen vacancies are empty sites in the oxygen

sublattice, which are formed when oxygen is removedfrom the lattice. Lack of the negative charge related

to the missing doubly ionised oxygen ion, O22, maybe considered as positively charged (relative to the

lattice) species.

N Interstitial cations represent titanium ions located ininterstitial positions (oxygen ions are unlikely to form

interstitial ions due to their large radius).

N Aliovalent ions may be incorporated either substitu-

tionally or interstitially. These includes both impu-

rities and the dopants introduced intentionally.

N Electronic defects include electrons and electronholes, which take part in conduction or remain

trapped on certain defects.

The formation of intrinsic defects may be represented by

the reactions between the TiO2 lattice and oxygen. Thesereactions, and the related equilibrium constants, are

shown in Table 2, using the KrogerVink notation.45

Defect disorder must satisfy the charge neutralitycondition, which requires that the crystal is electrically

neutral. Consequently, assuming full ionisation of

defects, the concentration of all charged defects underconsideration must satisfy the following condition

2V..O z3Ti...

i z4Ti....

i zD.zp~

nz4VTi zA0 (11)

where n and p denote the concentrations of electrons andelectron holes respectively, and [DN] and [A9] denote the

concentrations of singly ionised donor- and acceptor-

type foreign ions respectively. The condition expressedby equation (11) involves both thermodynamically re-

versible defects (oxygen vacancies, titanium vacancies

and interstitials and electronic defects), and also thedefects, which are thermodynamically irreversible (for-

eign ions).

The effect of titanium vacancies on defect disorder ofTiO2 must be considered in both, thermodynamic and

also kinetics terms. While these defects are thermodyna-

mically reversible, they are relatively immobile.2,17,6366 Incommonly applied experimental conditions their concen-

tration may be considered as constant. Then these defects

may be treated as acceptor-type impurities.17

The equilibrium constants K1, K4 and Ki (see Table 2)have been derived by Bak et al.17 using three indepen-

dently measured defect related properties (electrical

conductivity, thermoelectric power and thermogravi-metry). The constants K2 and K3 were reported by

Kofstad.13

Table 2 Defect reactions and related equilibrium constants for TiO2 (reproduced with permission from T. Bak, Oxidesemiconductors, unpublished collection; copyright 2011 T. Bak)

Defect reaction Constant DHu/kJ mol21 DSu/J mol

21 K21

1O|O'V

..

O z2ez1

2O2

K1~V..

O n2p(O2)

1=2 493.1 106.5

2 Ti|Tiz2O|

O'Ti...

i z3ezO2 K2~Ti...

i n3p(O2) 879

.2 190.8

3 Ti|Tiz2O|

O'Ti....

i z4ezO2 K3~Ti....

i n4p(O2) 1025

.8 238.3

4 O2'VTiz4h.z2O|O K4~VTi p

4p(O2){1 354.5 2202.1

5 nil=e9zhN Ki5np 222.1 44.6

lnK~DS0

R

{DH0

RT*n and p denote the concentrations of electrons and electron holes respectively; square brackets denote concentrations of ionicspecies.



9/14

Combination of expressions 15 from Table 2 andcondition (11) results in the equation representing theeffect of oxygen activity p(O2), on the concentration of

electronic charge carriers

4K4K{4i p(O2)n

8zn5{(D.{A0)n4{Kin

3{

2K1p(O2){1=2n2{3K2p(O2)

{1n{4K3p(O2){1~0 (12)

As seen from equation (12), the effect of p(O2) on theconcentration of electrons depends on the combinationof all defects. This equation may be used for the

derivation of a full defect disorder diagram, includingtitanium vacancies, in the form of the plot of theconcentrations of the reversible defects as a function of

p(O2). The diagram, showing the concentrations ofdefects as a function of oxygen activity at 1123 K, is inFig. 7. Such diagrams may be used for the prediction ofthe experimental conditions required for the impositionof both n- and p-type properties as well as mixedconduction.

In equilibrium, the data represented by the defectdisorder diagram in Fig. 7 is well defined by the

conditions of the equilibrium (temperature and oxygenactivity). While the defect disorder diagram in Fig. 7was determined at full ionisation of point defects (except

titanium interstitals), similar procedure may lead toderivation of defect diagrams at alternative (partial)ionisation degrees.

Effect of donors and acceptors

The penta valent ions, such as niobium6769

and trivalentions, such as chromium70,71 have been commonly usedas dopants to modify semiconducting properties of

TiO2. The incorporation of niobium ions leading to theformation of donor sites, results in a shift of the Fermilevel upwards. On the other hand, incorporation of

chromium, into the titanium sites of the TiO2 latticeleads to the formation of acceptors. These result inreducing the concentration of electrons and, ultimately,in converting n-type TiO2 into a p-type TiO2.

The concept of using defect engineering in themodification of reactivity of oxide semiconductors,including TiO2, is schematically represented in Fig. 8.

As seen, this concept is based on controlled impositionof the chemical potential of electrons mn, either bychanges of oxygen activity or the incorporation ofaliovalent ions leading to the formation of donors (D N)

or acceptors (A9). Changing mn up and down results inthe increase of the reduction and oxidation powerrespectively.

Defect engineering in formation of standardsAs outlined in the section above, data reported on TiO2based photocatalysts are frequently not well defined interms of their processing conditions and the relatedproperties. Therefore, the reported effects, such as theeffect of specific dopants on photocatalytic activity, arefrequently not compatible. Consequently, the progress inphotocatalysis requires introducing reference materials asstandards. It is essential that standards are well defined interms of photocatalytic and other properties. Suchstandards may be formed by application of well definedprocessing procedure, including the imposition of con-

trolled defect disorder and subsequent controlled cooling.The concept of defect engineering in the formation ofstandards allows conversion of a raw material: TiO2 ofunknown defect disorder, into TiO2 with controlleddefect disorder and well defined properties.2

Commercial materials are frequently used as stan-dards. However, most of the commercially availableTiO2 specimens are not well defined in terms of theirprocessing conditions, internal oxygen activity, defectdisorder and the related semiconducting properties.

Therefore, there is an urgent need to establish thestandard, reference specimens, which are well defined.

Performance related propertiesThe performance of photocatalysts is determined by theability of the photocatalyst to absorb sunlight and its

8 Schematic representation of effect of oxygen activity

p(O2), and concentration of donors [DN] and acceptors

[A9] on chemical potential of electrons mn

7 Defect disorder diagram of undoped TiO2 at 1123 K in

terms of concentrations of defects as function of oxy-

gen activity



10/14

subsequent conversion into the chemical energy required

for oxidation of organic contaminants, including micro-organisms and toxic compounds. Consequently, the

performance is closely related to electronic structure

and, specifically, the width of the band gap.

Semiconductors effectively absorb photons when their

energy is equal or higher than the forbidden gap. Then

light absorption leads to light induced ionisation over

band gap. The band gap of commonly available TiO2,

rutile, is 3?05 eV. Reduction of band gap may be

achieved by imposition of defects leading either to the

imposition of midgap levels or the shift of the valence

band.43,44

Besides the band gap, the ability of the photocatalyst

to convert the solar energy into the chemical energy

depends on the number of factors, including:

N Charge transport. The performance of photocatalystsrequires efficient charge transport. The energy losses

related to charge transport should be minimised in

order to maximise the conversion. This may be

achieved by minimisation of the internal resistance

through increase of the concentration of electronic

charge carriers and their mobility.

N Defect disorder of the outermost surface layer. Animportant-issue in photocatalysis is efficient charge

transfer between the adsorbed species and the

photocatalyst. This may be achieved by optimisation

of the population of the active surface sites, which can

form an active complex with the adsorbed reacting

molecule. It was shown that the surface active sitesfor water oxidation are titanium vacancies.72

N Concentration gradients in near surface layer. Theelectron-hole pairs formed as a result of light induced

ionisation have a tendency to recombine resulting in

energy losses (formation of heat). The recombination

related energy losses may be reduced when the lightinduced charge carriers are separated in an electric

field formed by the surface charge resulting in the

formation of a subsurface space charge, or chemical

concentration gradients within the surface layer. Such

field is formed spontaneously at the solid/liquid

interface between TiO2 and water. Recent studiesindicate that the electric field may be modified by the

imposition of chemically induced electric field by

using the phenomena of segregation and diffusion.2

It has been shown that the above performance related

properties, including electronic structure, charge trans-

port, charge transfer and charge separation, are closelyrelated to defect disorder of TiO2.

2,63 Therefore, defectchemistry may be used as the framework for the

modification of these properties in a controlled manner

in order to maximise the photocatalytic performance.

Collective and local reactivity factors

The prerequisite of the reactions between TiO2 and

water or its solutes is the adsorption of the reactingspecies on the TiO2 surface and the subsequent charge

transfer. The reactivity is determined by the ability of

TiO2 to donate or accept electrons and the chemical

affinity or ionisation potential of the adsorbed species.There has been a general perception that the reactivity of

TiO2 (with water and organic solutes in water) is closelyrelated to collective properties of TiO2. So far, little is

known about the effect of local surface properties, which

are closely related to the presence of point defects, on

reactivity.The collective factor is related to collective properties

in a macro scale, which are representative of the entirebulk phase or its surface layer as a continuum. The keycollective factor is the Fermi level EF.

The defect engineering may be used for shifting up ordown the chemical potential of electrons, compared tothe energy of the electrochemical couples Hz/H2 andO2/H2O, in order to allow spontaneous charge transfer.Therefore, the collective properties are also expected tocontrol the reactivity of TiO2 with organic moleculesdissolved in water.

While the collective factor is the driving force of the

charge transfer within the photoelectrochemical cell,

2

ithas been recently shown that the mechanism ofphotoreactivity at the TiO2 surface, and the relatedcharge transfer, must be considered in terms of both thecollective factor and the local factor.

The local factor is related to local interactions at anatomic scale between the adsorbed molecules andsurface active sites formed by individual surface defects.These defects, which are directly involved in the

9 Schematic representation of effect of titanium vacan-

cies on partial water oxidation leading to formation of

hydroxyl radicals

10 Schematic representation of effect of oxygen vacan-cies on reduction of dissolved oxygen leading to for-

mation of superoxide species



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reactivity between H2O and TiO2, play an essential rolein the charge transfer, for example, the transfer of anelectron hole from the TiO2 surface to the H2Omolecule. Recent studies have shown that titanium

vacancies at the outermost surface layer (with theassociated electron holes) are the favourable activesurface sites that allow effective charge transfer betweenthe H2O molecule and the TiO2 surface. The proposed

reactivity model, involving the reaction between thewater molecule and the TiO2 surface site, leading to theformation of an active complex, that is the precursor of

the formation of hydroxyl radicals, is shown in Fig. 9. Inanalogy, oxygen vacancies and the associated trivalenttitanium ions may be considered as the local active sitefor the formation of superoxide species (Fig. 10).

The development of high performance photocatalystsfor water splitting and water purification requires thatboth collective and local factors are taken into account.The latter factor is determined by defect disorder of theoutermost surface layer. Therefore, the reactivity ofTiO2 with water should be considered in terms ofthe donor- and acceptor-type defects in this layer.

Consequently, knowledge of the surface versus bulkdefect disorder is essential for processing of TiO2 withenhanced photocatalytic performance.

Reactivity of TiO2 with bacteria

The reactivity of TiO2 with water has been generallyconsidered in terms of water splitting into its elements orradicals. The predominant ionic species in water, at the

absence of photocatalyst, are protons (hydronium ions)and hydroxyl ions, OH2, which are formed by waterdissociation

H2O?HzzOH{ (13)

In the presence of a semiconducting photocatalystswater may react with its surface and form hydroxylradicals, OH*. This reaction is accompanied by singleelectron transfer (per one water molecule)

H2O?OHzHzze (14)

The OH* radicals play an important role in photo-catalytic oxidation of organic compounds and bacteria,leading to the formation of harmless products. Moredetailed discussion of the reactivity between TiO2 andbacteria is the subject of Part 2 of this paper.9

Future research directionsThe key requirement in the development of highperformance TiO2 based photocatalysts for waterdisinfection is to enhance the performance relatedproperties, which are essential for effective and fastkilling of microorganisms in water. The problems tosolve include:

N Production of hydroxyl radicals. The production ofOH* radicals depends on the chemical potential ofelectrons. Therefore, there is a need to establish therelationship between the chemical potential of elec-trons and photocatalytic activity. Knowledge of suchrelationship is essential to optimise the chemicalpotential of electrons in order to maximise the

efficiency of their production.N Minimisation of distance between reacting species.

The OH* radicals are extremely reactive. These

species, which are generated at the surface ofphotocatalyst, have the tendency to oxidise micro-organisms, if they are encountered during theirlifetime. Alternatively, these species will react with

each other leading to system disactivation. Therefore,there is a need to minimise the distance between thesurface of the photocatalyst and the microorganisms.This may be achieved through the modification of

these surface properties, which are favourable foradsorption of microorganisms.

N Removal of oxidation products. Efficient progress of

the oxidation process, taking place at or near thesurface of photocatalyst, requires to remove theoxidation products, including the end products, suchas CO2 and H2O, as well as the remaining parts of themicroorganisms after their oxidation.

N Reactivity enhancement. This effect is related to theproduction of radicals during the decomposition ofmicroorganisms. The related concept is outlined inthe following paper.9

ConclusionPhotocatalytic water disinfection (removal of microor-ganisms) using solar energy is the realistic option toaddress the water shortage in the global scale. The highperformance photocatalyst required for water disinfec-tion may be formed by the conversion of the commonlyavailable TiO2 (a raw material) into the TiO2 basedoxide semiconductors with controlled properties, which

are desired for photocatalytic performance. It is shownthat the key functional properties, which are related tothe performance of the photocatalysts are closely related

to the presence of point defects in the TiO2 lattice.Therefore, defect chemistry may be used as the frame-work in the formation of high performance photocata-

lyst using defect engineering.73

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Desinfeccion Fotocatalitica de Agua en Semiconductores de Oxido

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