Review Article Photocatalysis of Titanium Dioxide for ...downloads.hindawi.com/archive/2014/973484.pdf · photocatalysis-mediated water disinfection. 2. Water Oxidation e concept

Review ArticlePhotocatalysis of Titanium Dioxide for Water DisinfectionChallenges and Future Perspectives

M J Wu12 T Bak2 P J OrsquoDoherty1 M C Moffitt12 J Nowotny2

T D Bailey1 and C Kersaitis1

1 School of Science and Health University of Western Sydney Penrith NSW 2751 Australia2 University of Western Sydney Solar Energy Technologies Penrith NSW 2751 Australia

Correspondence should be addressed to M J Wu mwuuwseduau

Received 9 May 2014 Revised 18 August 2014 Accepted 25 August 2014 Published 7 September 2014

Academic Editor Rong-Jun Xie

Copyright copy 2014 M J Wu et alThis is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The performance of metal oxides such as titanium dioxide (TiO2) in the conversion of solar energy into chemical energy is

determined by semiconducting properties The conversion process is closely related to the light-induced reactivity between oxidesemiconductors andwater whichmay lead to partial water oxidation and consequentlywater disinfection Key performance-relatedproperties are considered here including light absorption light-induced ionisation over the band gap charge separation chargetransport charge transfer and the chemical reactions taking place at anodic and cathodic sitesOptimisation of these interconnectedperformance-related properties is discussed along with the photocatalytic application in water disinfection

1 Introduction

Over the last decade research activities which aimed atthe development of photocatalysts for water purification byphotosensitive oxide materials such as TiO

2 have intensified

[1ndash5] The underlying concept of water purification involvesthe utilisation of solar energy to oxidise water moleculesfor the production of reactive oxygen species and otheroxidising radicals which are toxic to microorganisms inwater TiO

2has been the focus of intensive research activities

due to its high photocatalytic activity under the photonenergy of ultraviolet and possibly visible light chemicaland thermal stability resistance to chemical breakdownand strong mechanical properties Its application in waterdisinfection is enhanced by the ability of TiO

2to completely

destroy organic pollutants and microorganisms [6ndash11] Itappears however thatmost of the reported experimental dataon photocatalytic water purification are not comparable evenfor the same chemical systems due to lack of reproducibilityTherefore there is an urgent need to assess the reasons forthis incompatibility Generation of meaningful antimicrobialdata may lead to derivation of theoretical models Suchmodels could then be used to compare photocatalytic systems

and predict the effect of basic properties such as chemicalcomposition structure and semiconducting properties onperformance

As is known to the present oxide materials are welldefined in terms of reproducibility when they are in thermo-dynamic equilibriumwith the gas phase of controlled oxygenactivity In the case of TiO

2 its properties are controlled by

the conditions of the equilibrium [12] At lower tempera-tures (below equilibrium) they are profoundly influenced bycooling procedures such as cooling rate and the associatedgas phase composition In many instances however TiO

2

specimens are processed at room temperature using softchemistry This results in properties being determined by theapplied experimental procedures which are frequently notreproducible

In this review we consider the light-induced reactivity ofTiO2-based semiconductors againstmicroorganisms inwater

through the formation of active radicals that exhibit a highoxidation power The related effects are considered in termsof a range of photo-induced reactions of TiO

2with water

The reactivity between TiO2and water is determined by the

semiconducting properties and the associated charge transferat the H

2OTiO

2interfaceThe quantitative evaluation of free

Hindawi Publishing CorporationInternational Journal of PhotochemistryVolume 2014 Article ID 973484 9 pageshttpdxdoiorg1011552014973484

2 International Journal of Photochemistry

radicals which can be employed in the development pro-cess of photocatalytic materials is discussed The survey ofpossible microorganisms in water contamination highlightsmicrobial diversity which may have varying sensitivity to thephotocatalysis-mediated water disinfection

2 Water Oxidation

The concept of the conversion of solar energy into chemicalenergy required for water oxidation by n-type semiconduc-tors such as TiO

2 is based on light-induced generation of

electron-electron hole pairs [12 13] The primary reaction ofTiO2withwater is the anodic reaction which results in a total

or partial oxidation of water depending on the number ofelectrons removed from water molecules

Total water oxidation is associated with the transfer offour electrons from two water molecules leading to theformation of one molecule of oxygen (O

2) and four protons

The latter species can be reduced to hydrogen at the cathodicsite Thus total oxidation ultimately leads to the generationof oxygen and hydrogen gas in a stoichiometric ratio

An alternative reactivity of TiO2with water results in

partial water oxidation which is associated with the removalof two electrons from two water molecules [13]

The partial water oxidation may be represented by thefollowing anodic reaction

2H2Olarrrarr 2HOlowast + 2H+ + 2e1015840 (1)

where e1015840 denotes quasi-free electron in the TiO2lattice Alter-

native anodic reaction results in the formation of hydrogenperoxide and protons

2H2O + 2h∙ larrrarr H

2O2+ 2H+ (2)

In the reaction h∙ is an electron hole The charge neutralityrequires that the negative charge of TiO

2 associated with

the reactions (1) and (2) is removed at the cathodic siteThe predominant cathodic reaction is based on reduction ofoxygen dissolved in water for the generation of superoxideanion

O2+ e1015840 larrrarr O

2

minus (3)

The reaction (1) results in the formation of hydroxylradicals while hydrogen peroxide and superoxide are formedin reactions (2) and (3) respectively These radicals exhibita substantial oxidation power They attack multitudes oforganic and biological compounds in their vicinity includingmicrobial cell wall and all cellular constituents proteinslipids and the DNA [13] Generation of hydroxyl radicalsthrough the partial oxidation of water is sufficient to killmicroorganisms such as bacteria destroy viruses and removealternative organic contaminants from water The associatedcathodic reaction (3) leads to the formation of superoxidespecies which also exhibit a strong oxidation power Thisresults in the decomposition of organic molecules in waterand also contributes to microbe elimination [13]

The superoxide species formed as a result of reaction (3)may be reduced at cathodic sites to hydrogen peroxide

O2

minus+ 2H+ + e1015840 larrrarr H

2O2

(4)

The hydroxyl radical species have a very short half-life They can react with themselves also resulting in theformation of hydrogen peroxide

HOlowast +HOlowast larrrarr H2O2 (5)

Like hydroxyl radicals superoxide and hydrogen per-oxide can damage carbohydrates lipids and proteins Theproperty differentiating H

2O2from these other oxidants is

its longer half-life and the ability to readily diffuse into livingcells through the cell wall In fact it has an approximate half-life in water of 5 h [14] however it is not a free radical perse for it lacks unpaired electrons As a result its reactionrate with many compounds is slow compared to the othertrue free radicals generated by partial water oxidation [13]Its conjugate base (HOOminus) is also capable of acting as areductantThe toxicity ofH

2O2is exacerbated in the presence

of ferrous ions or to a lesser degree other transitional metalions such as cuprous ion (Cu+)

The chemistry associated with the reactivity of oxidesemiconductors and water is complex Theoretically thereaction mechanism and the associated charge transfer maybe modified by a change in the chemical affinity of electronsor ionisation potential These quantities are closely related tothe Fermi level EF (a measure of the chemical potential ofelectrons) for the outermost surface layer of the semicon-ductor Although the surface has different properties fromthe interior (bulk) of the solid it is not isolated from thebulk Therefore the determination of both surface and bulksemiconducting properties is crucial in better understandingof the reactivity of TiO

2with water and the associated

photocatalytic properties [12]

3 Performance-Related Properties

Conversion of solar energy into chemical energy by oxidesemiconductors is a complicated process and involves severalsteps including light-induced ionisation charge separationcharge transport charge transfer and both anodic andcathodic reactionsThen the total efficiency of the conversiondepends on the efficiencies of all these individual stepsIdentification of specific material properties that impactdirectly on each step is a crucial task leading to improvementof photocatalytic performance This section considers theseperformance-related properties as well as the research strat-egy in the development of high-performance photocatalysts

Titanium dioxide is able to absorb only a small partof the solar spectrum which is represented in Figure 1Specifically the related energy threshold is associated withthe band gap that is equal to 3 eV and 32 eV for rutile andanatase respectively Consequently decreasing the band gapis expected to result in better utilisation of solar energy andin an increased number of photo-generated electrons andelectron holes One of the common techniques in band gapmodification is doping with aliovalent ions

The electronic charge carriers which are formed as aresult of light-induced ionisation have a tendency to recom-bine The recombination rate may be reduced by impo-sition of an electric field within the photon penetration

International Journal of Photochemistry 3

Photon energy (eV)1 2 3 4

Solar energy spectrum

110

210

410

310

Num

ber o

f pho

tons

(sminus

1 mminus

2 eVminus

1 )

Anatase (sim32 eV)Rutile (sim30 eV)

times1021

Figure 1The solar spectrum showing parts that can be absorbed byrutile and anatase

layer resulting in charge separation As the photocatalystoperates in aqueous solution such an electric field is formedspontaneously in the vicinity to the solidliquid interfaceAdditionally any concentration gradient of charged speciesalso generates an internal electric field As a result of segrega-tion the chemical composition of the surface layer is differentthan that of bulk phase The resulting concentration gradientdepends on the conditions of processing temperature andoxygen activity in the gas phase and the rate of coolingUnderstanding of these effects will provide the knowledgethat may help in engineering of an optimised electric fieldwhich is associated with maximised performance

The electronic charge carriers (electrons and electronholes) which are formed as a result of light induced ioni-sation must diffuse from the site of their generation to thereaction sites at the surface This process is affected by ohmicresistance Therefore an improved electronic transport canbe achieved by an increase in the concentration of chargecarriers andor their mobilities A large concentration ofelectrons or electron holes in wide-gap semiconductors suchas TiO

2is usually associated with the presence of the shallow

states of intrinsic and extrinsic defects within the bandgap Then thermal ionisation of these states is sufficientto generate a great number of electronic carriers in theconduction or valence bands even at room temperatureHowever the modification of mobility is more complex [15]

The performance of anodic and cathodic sites is deter-mined by the local affinity to electrons and the ability todonate electrons respectively These can be considered interms of both collective and local factors [12]

The collective factor is related to the position of the Fermilevel at the surface This quantity is reflective of the abilityof electrons to be accepted or donated The position of theFermi level is related to work function which is defined asthe work required for transferring an electron from the Fermilevel to infinity Usually work function exhibits fluctuationsalong the surface as it is shown in Figure 2 The local workfunction values which are higher or lower than a certaincritical value (120601cr) correspond to anodic and cathodic sites

Anodic site

Cathodic site

Position

Wor

k fu

nctio

n

120601cr

120601 gt 120601cr

120601 lt 120601cr

Figure 2 Work function versus position at the surface of aphotocatalyst

respectively However the local affinity to electrons and theability to donate electrons are strongly influenced by the localactive sites associated with the charged lattice species and therelated local electric field Typical anodic sites are titaniumvacancies which are strong acceptors of electrons as wellas acceptor-type foreign ions Typical donor-type sites areoxygen vacancies and titanium interstitials as well as extrinsicadditions such as niobium and tantalum ions The effectiveoxidation power for anodic sites is the combination of bothcollective properties and properties of local active sites Inanalogy the reduction power of cathodic sites should beconsidered in terms of both collective and local factors

The anodic and cathodic reactions are representedschematically in the upper part of Figure 3 The local electricfields represented by band bending are responsible for themigration of photogenerated electrons (black circle) andelectron holes (open circle) to the cathodic and anodic sitesrespectively The progress of the reactions at these sitesrequires that the electronic charge carriers are transferredacross the solidliquid interface and are available to theadsorbed molecules of water and oxygen resulting in thegeneration of hydroxyl radicals and superoxide speciesrespectively (Figure 3) Hence the charge transfer is the mostimportant reaction step of photocatalytic water purification

The radical species which are formed at the anodic andcathodic sites have a strong oxidation power The majoroxidants produced are the hydroxyl radical the superoxideanion and hydrogen peroxide These species have the capac-ity to damage the cell wall It is important to note howeverthat the ability of exogenously generated hydroxyl radicalsto damage microorganisms or other organic contaminantsdepends on the distance ofmicroorganisms from the reactionsites at the photocatalyst surface When the distance is toolarge the radicals have a greater chance to react betweenthemselves Superoxide and hydrogen peroxide which are oflower reactivity and longer half-life relative to the hydroxylradical can diffuse further from the site of formation andhave the ability to penetrate through the cell wall and cellmembrane Once inside the cell these oxidants can initiateFenton-like reactions involving transition metal catalysissuch as ferrous ions (Fe2+) (Figure 3) [12] The resulting


Microbial cell

h

Active radicals

Cathodicreaction Anodic

reaction

Cell wall

Cellmembrane

O2 + eminus rarr O minus2

O minus2 + 2H+ + eminus rarr H2O2

h∙ + H2O rarr OHlowast + H+

O minus2 + Fe3+ rarr Fe2+ + O2

H2O2 + Fe2+ rarr Fe3+ + OHminus + OHlowast

x x

Figure 3 The upper part represents anodic and cathodic reactions at the surface of semiconducting photocatalysts leading to the formationof active radicals in water The lower part represents the interaction between these radicals and the microbial cell

hydroxyl species formed in vivo have greater capacity todamage the cell and are toxic to all waterborne pathogens

Each of the performance-related properties such as bandgap electrical conductivity flat band potential or concentra-tion of surface active sites can be modified by an appropriateprocessing procedure The change of each of these propertiesaffects one or more elementary steps of the overall water oxi-dation reactionHowever the properties themselves are inter-related Processes aimed at the improvement of one propertymay lead to deterioration of others Past studies on theprocessing of TiO

2with enhanced performance commonly

aimed at reducing the band gap In many instances thisapproach may result in the formation of an insulating systemin which the charge transport is blocked Other approachesalso include imposition of chemically induced electric fieldsusing the phenomena of diffusion and segregation

Reduction of the width of band gap to the level that allowsan enhanced light absorption could lead to the developmentof high-performance photocatalysts A common method isthe incorporation of aliovalent ions which can result in theformation of mid-gap levels and a reduction of the effectiveenergy gap required for light-induced ionisation [16] Thisapproach leads to an effective increase in photocatalytic activ-ity only when the additional performance-related propertiesdo not play a dominant role

An essential issue is related to the effect of recombinationIt can be reduced by enhancing the charge separation byimposition of chemically induced electric fields at the inter-face of TiO

2and water [17] The phenomenon of segregation

can be used as a technology for imposition of these electricfields The light-induced electrons and holes which areformed within the light penetration distance must travelto the surface in order to be involved in photoreactions asexpressed by the reactions (1) and (3) High performance

requires that these charge carriers are transported rapidlyThis can be achieved by imposition of electronic chargecompensation regime [17]

Efficient water oxidation leading to the formation of theactive radicals is possible when the reactive surface exhibitsoptimal concentration of surface active sites for water adsorp-tion and the formation of active complexes that subsequentlydecompose into active radicals [14] Obviously the quantityof free radicals is a critical determinant for the sanitisingefficiency of TiO

2materials Establishment of assays for

measuring free radicals will facilitate the development ofphotocatalytic apparatus

4 Measurement of Free Radicals

The quantification of free radicals presents a significantchallenge because of their instability and short half-lifeFor example the reaction rate constant of the hydroxylradical with DNA is 108Molminus1Sminus1 and its half-life is innanoseconds [18] Therefore indirect analysis using selectiveprobemolecules which fluoresce or display othermeasurablecharacteristics upon being oxidised must be employed [1920]

From the views of both engineering and biology it isimportant to measure the concentrations of the key radicalsgenerated by TiO

2materials Hydroxyl radical level can

be monitored by using dyes such as 30-(p-hydroxyphenyl)fluorescein (HPF) or 31015840-(p-aminophenyl)fluorescein (APF)which are oxidized by OH∙ with high specificity (Figure 4)[21 22]

Hydrogen peroxide is a thermodynamically powerfuloxidant Interestingly it is formed in living organisms duringthe course of normal metabolism such as ATP generation in


Highly fluorescentAlmost nonfluorescent

OO

O O

O

O

XH

X

HO∙

COOminus

COOminus

Ominus

Figure 4 Illustration of chemistry for HPFAPF in hydroxyl quantification X = O for HPF and X = NH for APF Excitation wavelength is488 nm and emission is measured with 505ndash550 nm filter

mitochondria and bacterial cell membranes lipid catabolismand superoxide dismutation [23ndash25] and therefore bacterialcells have the innate ability to detoxify small quantitiesof H2O2 As mentioned earlier its toxicity can be greatly

enhanced by Fenton-like reactions through generation of thehydroxyl radical [26 27] Understandably the concentrationof H2O2is proportional to disinfection efficiency in the field

of photocatalysis and water treatmentMultiple assay formatsare available for the detection of H

2O2 including chemilu-

minescence spectrophotometric electrochemistry and flu-orometric methods [28ndash31] In addition the fluorescent dyesScopoletinHPF andAPF are useful in its quantificationwiththe assistance of a catalystmdashhorseradish peroxidise Suchassays are suited for high through-put analysis

Commercial kits for the measurement of H2O2are

also available for example Amplex Red reagent (N-acetyl-3 7-dihydroxyphenoxazine) from Invitrogen [32] AmplexRed is a colourless substrate that reacts with H

2O2with a

1 1 stoichiometry to produce highly fluorescent resorufin(excitationemission maxima = 570585 nm) Its chemicalreaction is shown in Figure 5 The assay is highly specificand sensitive with aH

2O2detection limit of sim5 pM although

this has also been reported as being up to 50 nM [31]Because the stoichiometry of Amplex Red and H

2O2is 1 1

the assay results are linear Care should be taken in dealingwith the Amplex Red dye which is somewhat unstable Athigh concentrations (50120583molL) it can be autooxidized andproduce O

2

∙minus and H2O2 Low concentrations of Amplex Red

(10 120583molL) minimize this problemAs no single assay is perfect concomitant use of multiple

assays for the reliable measurement of the free radicals ofinterest would seem expedient Many methods have beendiscussed in the literature however the fluorophore-based

assays described above namely those utilising HPFAPF andAmplex Red would be best for this purpose given theirhigh sensitivity and specificity Specificity here encompassesselectivity for radicals and accuracy Because of the short half-life the ability of a method to measure a reactive oxygenspecies (ROS) quickly is critical The portability of an assayis also desirable The proposed assays meet these criteria andthus are considered suitable for free radical measurement Itis also essential to confirm the portability of photocatalysis-treated water by comparing its free radical level to normalclean water

5 Analysis of Photocatalysis-MediatedAntimicrobial Activity

In addition to detecting the presence of the free radicalsthe antimicrobial effects of photocatalysis-mediated activityon microbes commonly associated with water contamina-tion should be analysed As shown in Table 1 there aremanymicroorganismswhich could potentially contaminate awater source including bacteria (Legionella Coliform Enter-obacteriaceae Vibrio Shigella Helicobacter Clostridium andSalmonella) protozoans (Cryptosporidium and Giardia) andviruses Faecal matter particularly human faecal matter isthe key source of contamination by pathogenic organisms[33ndash35] History tells us that one of the great scourges ofcities in Europe and North America in the 19th century wasoutbreaks ofwaterborne diseases such as cholera and typhoidEven nowadays in many parts of the developing world theseserious diseases remain a leading cause of death Indeed theWHO reports that mortality due to water associated diseasesexceeds 5 million people per year Of these over 50 are


N

OHHO

O CH3

N

HO OO O

HRP

H2O2 H2O

Nonfluorescent Highly fluorescent

Figure 5 Scheme of Amplex Red assay for H2O2measurement

Table 1 Microorganisms found in contaminated waters

Microorganisms Orgnismal features Potential health effects Sources of contamination

Cryptosporidium A single-celledprotozoan parasite

Gastrointestinal illness (eg diarrhoeagastroenteritis vomiting cramps) Human and animal faecal waste

Giardia lamblia A single-celledprotozoan parasite


Legionella Gram-negativebacterium

Legionnairersquos disease a type ofpneumonia

Found naturally in water and multiplies inheating systems

Coliforms (includingfaecal coliform and Ecoli)

Gram-negativerod-shaped bacteria

Not a health threat in itself it is used toindicate whether other potentiallyharmful bacteria may be present

Coliforms are naturally present in theenvironment as well as faeces faecalcoliforms and E coli only come from humanand animal faecal waste

Vibrio Gram-negativerod-shaped bacteria Gastroenteritis Human and animal faecal waste

Shigella Gram-negativerod-shaped bacteria Bacillary dysentery or shigellosis Human and animal faecal waste

Helicobacter pylori Gram-negativespiral-shaped bacteria stomach ulcers An emerging water-borne bacterium

possibly due to human faecal wasteClostridiumperfringens

Gram-positiverod-shaped bacteria

Gastrointestinal illness (eg diarrhoeacramps) Human and animal faecal waste

Enterococci Gram-positive ovoidor round bacteria Gastroenteritis Human and animal faecal waste

Salmonella Gram-negativerod-shaped bacteria Gastroenteritis Human and animal faecal waste

Viruses (enteric) Virus Gastrointestinal illness (eg diarrhoeavomiting cramps) Human and animal faecal waste

microbial intestinal infections with cholera being the topcause The question is which microorganism should be usedto assess the efficacy of photocatalysts

Escherichia coli (E coli) is a Gram-negative rod-shapedbacterium and a very common contaminating bacterialspecies in water It is routinely used as an indicator for faecalcontamination in water and a model organism for entericbacteria Hence E coli is a logical choice of microorganismfor evaluation of photocatalysis-mediated water disinfec-tion Due to the stark variation in cell wall structure ofGram-positive and Gram-negative bacteria and becausecell wallplasma membrane damage forms a major part ofmolecular mechanism for photocatalysis-mediated bacteri-cidal activity [9] it is sensible to also include a Gram-positive bacterium for analysis Indeed Rincon and Pulgarindemonstrated the difference of TiO

2-based photocatalytic

efficacy between E coli and the Gram-positive Bacillus spwith Bacillus being more resistant to the treatment [36]The Gram-positive Staphylococcus aureus (S aureus) andGram-negative E coli are currently used in our evaluationprotocols Comparison should also be carried out withLegionella Cryptosporidium and Giardia lamblia since pre-vious studies found that these organisms are highly resis-tant to traditional disinfection practices [37] Experimentalwork in this regard was in fact conducted in 2009 byNavalon et al and demonstrated that waters containingCryptosporidium parvum and Giardia lamblia at low con-centrations can be efficiently disinfected in continuous flowby using a commercial fibrous ceramic TiO

2photocatalyst

[38]Confocal fluorescence microscopy can also be employed

in the evaluation process The fluorophore propidium iodide


(PI) for example could be used to indicate biocidal effec-tiveness as the membranes of viable cells are impermeableto PI Membrane-compromised or dead bacteria howeverwould allow entry of the dye which strongly binds to DNAIn contrast SYTO green-fluorescent dyes such as SYTO 9(Invitrogen) can enter both living and dead cells Thereforesimultaneous usage of the two dyes is highly informativein quantifying dead and healthy bacterial cells Note thata flow cytometer could be used for sample analysis in theplace of a confocal microscope if only cell counting is soughtThe advantage of the microscopy is to obtain additionalstructural cellular details In addition an antibiotic suchas Ampicillin should be used as a positive control whendetermining photocatalysis-mediated biocidal activity

According to the photocatalytic disinfection mechanismproposed by Sunada et al [9] using E coli the free radicalsgenerated upon irradiation of the semiconductor particlescaused bacterial inactivation via partial decomposition of theexternal cell wall and membrane This is widely acceptedHowever the exact kinetics are determined by the surfacearea of semiconductor the level of photon absorption thedegree of microbial contamination and ultimately the quan-tity of free radicals Differentmicrobes have different externalstructures and therefore should exhibit diverse inactivationkinetics under a given photocatalytic condition

Bacterial cells readily develop resistance to antibioticsparticularly following repeated and inappropriate use Suchresistance presents a major health problem In light ofthe potential biocidal activity of photocatalysis it becomesimportant to ask do bacteria develop resistance to photo-catalysis-mediated antimicrobial activity If so is the level ofresistance the same as that for conventional antibioticsThesequestions have not yet been asked in this area because thistechnique is not widely applied in practice We speculate thatresistance to photocatalysis-generated free radicals will beless than that for antibiotic drugs since free radical-inducedcell death is the primary mechanism of photocatalysis waterdisinfection rather than being secondary after the effectof antibiotics However we need to engage the topic andinvestigate it An experiment can be designed by workingon a population of E coli for a number of continuous pho-tocatalytic treatment-survival analysis cycles If the survivalrates are increasing along the exposure treatments then theresistance will be a problem Any such resistance mechanismobserved should be keenly studied

6 Difference between Photocatalysisand Antibiotics

Kohanski et al [39] recently demonstrated that the threemajor classes of bactericidal antibiotics all stimulate theproduction of highly deleterious hydroxyl radicals in Gram-negative and Gram-positive bacteria which ultimately leadsto cell death by inducing oxidative DNA and protein damageThis is regardless of their known modes of action be it tar-geting protein synthesis DNA replication and repair and cellwall homeostasis Both Escherichia coli and Staphylococcusaureus under the treatment of bactericidal antibiotics wereshown to produce hydroxyl free radicals Given the radical

formation of photocatalysis these findings give promise tothe application of photocatalysis-mediated water disinfec-tion

A UV source and a TiO2appliance are the basic require-

ments for photocatalysis-mediated water disinfection appli-cable to remote areas that lack potable water This also com-plements the current drinking water treatment consisting ofa series of systems for coagulation and flocculation filtrationand oxidation with chlorine [40] Drinking water is notsterile however Bacteria can be found in the distributionsystem and at the tap Most of these organisms are harmlessbut some opportunist pathogens such as Pseudomonas aerug-inosa and Aeromonas spp may multiply during distributiongiven suitable conditions Currently there is some debate as towhether these organisms are responsible for any waterbornegastrointestinal disease in the community but P aeruginosa isknown to cause infections in immunocompromised patientsand weakened patients in hospitals The contamination ofdrinking water by pathogens causing diarrhoeal disease is themost important aspect of drinking water quality

7 Future Prospects

The engineering and understanding of semiconductor pho-tocatalyst TiO

2will continue to advance Its large scale

application in water disinfection is a matter of when not ifDespite the intensive research of the past decades desirablephotocatalytic efficacy is yet to be achieved to a level suitablefor practical applications The reality is that access to cleanwater is still a major problem in many parts of the worldThe seventh cholera pandemic since it was started in 1961arrived in South America in 1991 and caused 4700 deathsin one year [41] Photocatalysis-based water sanitisation willplay a major role in peoplersquos daily lives Such a reality is nottoo far away as the usage of TiO

2has been widely applied to

the other industries including examples such as self-cleaningautomobiles with a layer of TiO

2paint Undoubtedly it is

the future breakthrough in technology and engineering thatwill turn photocatalysis into a driving force for maintainingcritical water resources

8 Conclusions

Photocatalysis is undoubtedly a desirable tool in dealing withmicrobial contamination of drinking water sources Muchremains to be done in terms of maximising its efficiency byenhancing the performance-related properties of oxidemate-rials for photocatalytical oxidation of organic contaminantsin water Future work should be focused on optimisation ofthese properties through material design and engineeringThere is an urgent need to have a concerted approach fromfree radical measurement to antimicrobial assessment

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper


References

[1] Y Liu J Li X Qiu and C Burda ldquoNovel TiO2nanocatalysts for

wastewater purification tapping energy from the sunrdquo WaterScience and Technology vol 54 no 8 pp 47ndash54 2006

[2] T Tachikawa S Tojo K Kawai et al ldquoPhotocatalytic oxidationreactivity of holes in the sulfur- and carbon-doped TiO

2pow-

ders studied by time-resolved diffuse reflectance spectroscopyrdquoThe Journal of Physical Chemistry B vol 108 no 50 pp 19299ndash19306 2004

[3] R Asahi T Morikawa T Ohwaki K Aoki and Y TagaldquoVisible-light photocatalysis in nitrogen-doped titaniumoxidesrdquo Science vol 293 no 5528 pp 269ndash271 2001

[4] S-H Lee S Pumprueg BMoudgil andW Sigmund ldquoInactiva-tion of bacterial endospores by photocatalytic nanocompositesrdquoColloids and Surfaces B Biointerfaces vol 40 no 2 pp 93ndash982005

[5] R Nakamura and Y Nakato ldquoMolecular mechanism of wateroxidation reaction at photo-irradiated TiO2 and related metaloxide surfacesrdquo Diffusion and Defect Data B Solid State Phe-nomena vol 162 pp 1ndash27 2010

[6] A Fujishima X T Zhang and D A Tryk ldquoTiO2photocatalysis

and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008

[7] M Cho H Chung W Choi and J Yoon ldquoLinear correlationbetween inactivation of E coli and OH radical concentration inTiO2photocatalytic disinfectionrdquoWater Research vol 38 no 4

pp 1069ndash1077 2004[8] D M A Alrousan M I Polo-Lopez P S M Dunlop P

Fernandez-Ibanez and J A Byrne ldquoSolar photocatalytic dis-infection of water with immobilised titanium dioxide in re-circulating flow CPC reactorsrdquo Applied Catalysis B Environ-mental vol 128 pp 126ndash134 2012

[9] K Sunada T Watanabe and K Hashimoto ldquoStudies on pho-tokilling of bacteria on TiO

2thin filmrdquo Journal of Photochem-

istry and Photobiology A Chemistry vol 156 no 1ndash3 pp 227ndash233 2003

[10] C Navntoft P Araujo M I Litter et al ldquoField tests of thesolar water detoxification SOLWATER reactor in Los PereyraTucuman Argentinardquo Journal of Solar Energy Engineering vol129 no 1 pp 127ndash134 2007

[11] J Gamage and Z Zhang ldquoApplications of photocatalytic disin-fectionrdquo International Journal of Photoenergy vol 2010 ArticleID 764870 11 pages 2010

[12] J Nowotny Oxide Semiconductors for Solar Energy ConversionTitanium Dioxide CRC Press Boca Raton Fla USA 2012

[13] N J Sucher M C Carles J Nowotny and T Bak ldquoPho-tocatalytic water disinfection on oxide semiconductors part2mdashstructure functional properties and reactivity of microbialagentsrdquo Advances in Applied Ceramics vol 111 no 1-2 pp 16ndash33 2012

[14] J Nowotny T Bak M K Nowotny and L R SheppardldquoTiO2surface active sites for water splittingrdquo Journal of Physical

Chemistry B vol 110 no 37 pp 18492ndash18495 2006[15] F M Hossain A V Evteev I V Belova J Nowotny and G E

Murch ldquoStructural electronic and optical properties of titaniananotubesrdquo Advances in Applied Ceramics vol 111 no 1-2 pp72ndash93 2012

[16] K Wilke and H D Breuer ldquoThe influence of transition metaldoping on the physical and photocatalytic properties of titaniardquoJournal of Photochemistry and Photobiology A Chemistry vol121 no 1 pp 49ndash53 1999

[17] T Bak J Nowotny N J Sucher and E Wachsman ldquoEffectof crystal imperfections on reactivity and photoreactivity ofTiO2(Rutile) with oxygen water and bacteriardquo The Journal of

Physical Chemistry C vol 115 no 32 pp 15711ndash15738 2011[18] W H Glaze and J W Kang ldquoAdvanced oxidation processes

for treating groundwater contaminated with TCE and PCElaboratory studiesrdquo Journal-AmericanWaterWorks Associationvol 88 no 5 pp 57ndash63 1988

[19] J M BurnsW J Cooper J L Ferry et al ldquoMethods for reactiveoxygen species (ROS) detection in aqueous environmentsrdquoAquatic Sciences vol 74 no 4 pp 683ndash734 2012

[20] O C Zafiriou N V Blough E Micinski B Dister D Kieberand J Moffett ldquoMolecular probe systems for reactive transientsin natural watersrdquoMarine Chemistry vol 30 no 1ndash3 pp 45ndash701990

[21] K Setsukinai Y Urano K Kakinuma H J Majima and TNagano ldquoDevelopment of novel fluorescence probes that canreliably detect reactive oxygen species and distinguish specificspeciesrdquoThe Journal of Biological Chemistry vol 278 no 5 pp3170ndash3175 2003

[22] C A Cohn S R Simon and M A A Schoonen ldquoCompari-son of fluorescence-based techniques for the quantification ofparticle-induced hydroxyl radicalsrdquo Particle and Fibre Toxicol-ogy vol 5 no 1 article 2 2008

[23] I Fridovich ldquoSuperoxide radical and superoxide dismutasesrdquoAnnual Review of Biochemistry vol 64 pp 97ndash112 1995

[24] J M McCord and I Fridovich ldquoSuperoxide dismutase the firsttwenty years (1968ndash1988)rdquo Free Radical Biology and Medicinevol 5 no 5-6 pp 363ndash369 1988

[25] G Gille and K Sigler ldquoOxidative stress and living cellsrdquo FoliaMicrobiologica vol 40 no 2 pp 131ndash152 1995

[26] H J H Fenton ldquoLXXIIImdashOxidation of tartaric acid in pres-ence of ironrdquo Journal of the Chemical Society Transactions vol65 pp 899ndash910 1894

[27] S Goldstein D Meyerstein and G Czapski ldquoThe Fentonreagentsrdquo Free Radical Biology amp Medicine vol 15 no 4 pp435ndash445 1993

[28] R J Kieber and G R Helz ldquoTwo-method verification ofhydrogen peroxide determinations in natural watersrdquo Analyt-ical Chemistry vol 58 no 11 pp 2312ndash2315 1986

[29] J T Corbett ldquoThe scopoletin assay for hydrogen peroxideA review and a better methodrdquo Journal of Biochemical andBiophysical Methods vol 18 no 4 pp 297ndash307 1989

[30] L A Marquez and H B Dunford ldquoTransient and steady-state kinetics of the oxidation of scopoletin by horseradishperoxidase compounds I II and III in the presence of NADHrdquoEuropean Journal of Biochemistry vol 233 no 1 pp 364ndash3711995

[31] S Watabe Y Sakamoto M Morikawa R Okada T Miura andE Ito ldquoHighly sensitive determination of hydrogen peroxideand glucose by fluorescence correlation spectroscopyrdquo PLoSONE vol 6 no 8 Article ID e22955 2011

[32] M Zhou Z Diwu N Panchuk-Voloshina and R P HauglandldquoA stable nonfluorescent derivative of resorufin for the fluoro-metric determination of trace hydrogen peroxide applicationsin detecting the activity of phagocyte NADPH oxidase andother oxidasesrdquoAnalytical Biochemistry vol 253 no 2 pp 162ndash168 1997

[33] World Health Organization Guidelines for Drinking-waterQuality Incorporating 1st and 2nd Addenda Volume 1 Recom-mendations WHO Geneva Switzerland 3rd edition 2008


[34] A Fenwick ldquoWaterborne infectious diseasesmdashcould they beconsigned to historyrdquo Science vol 313 no 5790 pp 1077ndash10812006

[35] J P S Cabral ldquoWater microbiology Bacterial pathogens andwaterrdquo International Journal of Environmental Research andPublic Health vol 7 no 10 pp 3657ndash3703 2010

[36] A-G Rincon and C Pulgarin ldquoUse of coaxial photocatalyticreactor (CAPHORE) in the TiO

2photo-assisted treatment of

mixed E coli and Bacillus sp and bacterial community presentin wastewaterrdquo Catalysis Today vol 101 no 3-4 pp 331ndash3442005

[37] S Regli ldquoDisinfection requirements to control for microbialcontaminationrdquo in Regulating Drinking Water Quality C EGilbert and E J Calabrese Eds Lewis Chelsea Mich USA1992

[38] S NavalonM Alvaro H Garcia D Escrig and V Costa ldquoPho-tocatalytic water disinfection of Cryptosporidium parvum andGiardia lamblia using a fibrous ceramic TiO

2photocatalystrdquo

Water Science and Technology vol 59 no 4 pp 639ndash645 2009[39] M A Kohanski D J Dwyer B Hayete C A Lawrence and J

J Collins ldquoA common mechanism of cellular death induced bybactericidal antibioticsrdquo Cell vol 130 no 5 pp 797ndash810 2007

[40] J Fawell and M J Nieuwenhuijsen ldquoContaminants in drinkingwater environmental pollution and healthrdquoThe British MedicalBulletin vol 68 no 1 pp 199ndash208 2003

[41] P R Reeves and R Lan ldquoCholera in the 1990srdquo British MedicalBulletin vol 54 no 3 pp 611ndash623 1998

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


radicals which can be employed in the development pro-cess of photocatalytic materials is discussed The survey ofpossible microorganisms in water contamination highlightsmicrobial diversity which may have varying sensitivity to thephotocatalysis-mediated water disinfection

2 Water Oxidation

The concept of the conversion of solar energy into chemicalenergy required for water oxidation by n-type semiconduc-tors such as TiO

2 is based on light-induced generation of

electron-electron hole pairs [12 13] The primary reaction ofTiO2withwater is the anodic reaction which results in a total

or partial oxidation of water depending on the number ofelectrons removed from water molecules

Total water oxidation is associated with the transfer offour electrons from two water molecules leading to theformation of one molecule of oxygen (O

2) and four protons

The latter species can be reduced to hydrogen at the cathodicsite Thus total oxidation ultimately leads to the generationof oxygen and hydrogen gas in a stoichiometric ratio

An alternative reactivity of TiO2with water results in

partial water oxidation which is associated with the removalof two electrons from two water molecules [13]

The partial water oxidation may be represented by thefollowing anodic reaction

2H2Olarrrarr 2HOlowast + 2H+ + 2e1015840 (1)

where e1015840 denotes quasi-free electron in the TiO2lattice Alter-

native anodic reaction results in the formation of hydrogenperoxide and protons

2H2O + 2h∙ larrrarr H

2O2+ 2H+ (2)

In the reaction h∙ is an electron hole The charge neutralityrequires that the negative charge of TiO

2 associated with

the reactions (1) and (2) is removed at the cathodic siteThe predominant cathodic reaction is based on reduction ofoxygen dissolved in water for the generation of superoxideanion

O2+ e1015840 larrrarr O

2

minus (3)

The reaction (1) results in the formation of hydroxylradicals while hydrogen peroxide and superoxide are formedin reactions (2) and (3) respectively These radicals exhibita substantial oxidation power They attack multitudes oforganic and biological compounds in their vicinity includingmicrobial cell wall and all cellular constituents proteinslipids and the DNA [13] Generation of hydroxyl radicalsthrough the partial oxidation of water is sufficient to killmicroorganisms such as bacteria destroy viruses and removealternative organic contaminants from water The associatedcathodic reaction (3) leads to the formation of superoxidespecies which also exhibit a strong oxidation power Thisresults in the decomposition of organic molecules in waterand also contributes to microbe elimination [13]

The superoxide species formed as a result of reaction (3)may be reduced at cathodic sites to hydrogen peroxide

O2

minus+ 2H+ + e1015840 larrrarr H

2O2

(4)

The hydroxyl radical species have a very short half-life They can react with themselves also resulting in theformation of hydrogen peroxide

HOlowast +HOlowast larrrarr H2O2 (5)

Like hydroxyl radicals superoxide and hydrogen per-oxide can damage carbohydrates lipids and proteins Theproperty differentiating H

2O2from these other oxidants is

its longer half-life and the ability to readily diffuse into livingcells through the cell wall In fact it has an approximate half-life in water of 5 h [14] however it is not a free radical perse for it lacks unpaired electrons As a result its reactionrate with many compounds is slow compared to the othertrue free radicals generated by partial water oxidation [13]Its conjugate base (HOOminus) is also capable of acting as areductantThe toxicity ofH

2O2is exacerbated in the presence

of ferrous ions or to a lesser degree other transitional metalions such as cuprous ion (Cu+)

The chemistry associated with the reactivity of oxidesemiconductors and water is complex Theoretically thereaction mechanism and the associated charge transfer maybe modified by a change in the chemical affinity of electronsor ionisation potential These quantities are closely related tothe Fermi level EF (a measure of the chemical potential ofelectrons) for the outermost surface layer of the semicon-ductor Although the surface has different properties fromthe interior (bulk) of the solid it is not isolated from thebulk Therefore the determination of both surface and bulksemiconducting properties is crucial in better understandingof the reactivity of TiO

2with water and the associated

photocatalytic properties [12]

3 Performance-Related Properties

Conversion of solar energy into chemical energy by oxidesemiconductors is a complicated process and involves severalsteps including light-induced ionisation charge separationcharge transport charge transfer and both anodic andcathodic reactionsThen the total efficiency of the conversiondepends on the efficiencies of all these individual stepsIdentification of specific material properties that impactdirectly on each step is a crucial task leading to improvementof photocatalytic performance This section considers theseperformance-related properties as well as the research strat-egy in the development of high-performance photocatalysts

Titanium dioxide is able to absorb only a small partof the solar spectrum which is represented in Figure 1Specifically the related energy threshold is associated withthe band gap that is equal to 3 eV and 32 eV for rutile andanatase respectively Consequently decreasing the band gapis expected to result in better utilisation of solar energy andin an increased number of photo-generated electrons andelectron holes One of the common techniques in band gapmodification is doping with aliovalent ions

The electronic charge carriers which are formed as aresult of light-induced ionisation have a tendency to recom-bine The recombination rate may be reduced by impo-sition of an electric field within the photon penetration




110

210

410

310

Num

ber o

f pho

tons

(sminus

1 mminus

2 eVminus

1 )


times1021








Anodic site

Cathodic site

Position

Wor

k fu

nctio

n

120601cr

120601 gt 120601cr

120601 lt 120601cr






Microbial cell

h

Active radicals


reaction

Cell wall

Cellmembrane






x x






















OO

O O

O

O

XH

X

HO∙

COOminus

COOminus

Ominus









2O2with a




2O2is 1 1


2








N

OHHO

O CH3

N

HO OO O

HRP

H2O2 H2O





























2photocatalyst












7 Future Prospects








8 Conclusions





References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




110

210

410

310

Num

ber o

f pho

tons

(sminus

1 mminus

2 eVminus

1 )


times1021








Anodic site

Cathodic site

Position

Wor

k fu

nctio

n

120601cr

120601 gt 120601cr

120601 lt 120601cr






Microbial cell

h

Active radicals


reaction

Cell wall

Cellmembrane






x x






















OO

O O

O

O

XH

X

HO∙

COOminus

COOminus

Ominus









2O2with a




2O2is 1 1


2








N

OHHO

O CH3

N

HO OO O

HRP

H2O2 H2O





























2photocatalyst












7 Future Prospects








8 Conclusions





References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Microbial cell

h

Active radicals


reaction

Cell wall

Cellmembrane






x x






















OO

O O

O

O

XH

X

HO∙

COOminus

COOminus

Ominus









2O2with a




2O2is 1 1


2








N

OHHO

O CH3

N

HO OO O

HRP

H2O2 H2O





























2photocatalyst












7 Future Prospects








8 Conclusions





References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



OO

O O

O

O

XH

X

HO∙

COOminus

COOminus

Ominus









2O2with a




2O2is 1 1


2








N

OHHO

O CH3

N

HO OO O

HRP

H2O2 H2O





























2photocatalyst












7 Future Prospects








8 Conclusions





References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


N

OHHO

O CH3

N

HO OO O

HRP

H2O2 H2O





























2photocatalyst












7 Future Prospects








8 Conclusions





References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










7 Future Prospects








8 Conclusions





References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


References




2pow-















































2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of









2photocatalystrdquo














Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

Documents

Review Article Photocatalysis of Titanium Dioxide for ...downloads.hindawi.com/archive/2014/973484.pdf · photocatalysis-mediated water disinfection. 2. Water Oxidation e concept