Pure and modied TiO2 photocatalysts and their environmental

applications

Wonyong Choi*

School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea

Semiconductor photocatalysis is a process that harnesses light energy in chemical conversions. In particular, its applications to

environmental remediation have been intensively investigated. The characteristics of TiO2, the most popular photocatalyst, is

briey described and selected studies on the degradation/conversion of various recalcitrant pollutants using pure and modied TiO2photocatalysts, which were carried out in this group, are reviewed. Photocatalytic reactions are multi-phasic and take place at

interfaces of not only water/TiO2 and air/TiO2 but also solid/TiO2. Examples of photocatalytic reactions of various organic and

inorganic substrates that are converted through the photocatalytic oxidation or reduction are introduced. TiO2 has been modied

in various ways to improve its photocatalytic activity. Surface modications of TiO2 that include surface platinization, surface

uorination, and surface charge alteration are discussed and their applications to pollutants degradation are also described in

detail.

KEY WORDS: photocatalyst; titanium dioxide; pollutant degradation; TiO2 surface modication; advanced oxidation process.

1. Introduction

Semiconductor photocatalysis has been extensivelystudied as a vital device that utilizes solar photoenergy[1]. The intrinsic bandgap in semiconductors serves as alight harvesting center. Photocatalytic reactions occur-ring on semiconductor surface have many applicationsincluding chemical fuel synthesis (e.g., H2 productionthrough water splitting) [2], selective oxidation [3,4],degradation of organic compounds [5,6], disinfection[7,8], metal corrosion prevention [911], lithography[12,13], etc. In particular, their application to theremediation of polluted water and air has been dem-onstrated to be a technically viable process and TiO2 hasbeen the most popular and successful photocatalyst forthis purpose [5, 6,1416]. This technology has a uniqueversatility in that it can be applied to various environ-mental media of water, air, and even solid phases.Photocatalytic conversion or degradation with illumi-nated TiO2 has been demonstrated for a huge number ofsubstances, which is largely ascribed to the strong oxi-dation potential of its valence band (VB) holes andsurface OH radicals [5,17]. TiO2 is ideally suited as apractical environmental photocatalyst because it isextremely stable, non-toxic, safe to handle, inexpensive,and photoactive under solar light. Although the TiO2photocatalysis seems to be a narrowly dened subjectof research, to our surprise, it has been continuously

producing a large number of research papers, newndings, patents, and commercial products for morethan 30 years.

Research activities on TiO2 photocatalysis can beclassied into many dierent subjects such as reac-tion kinetics and mechanisms, material synthesis andmodication, thin lm and coating fabrication, surfaceand colloid chemistry, photoelectrochemistry, chargerecombination and transfer dynamics, and reactor de-sign and engineering. The level of current TiO2 researchactivities widely ranges from the very fundamental suchas the ash laser spectroscopic studies of chargerecombination dynamics [1821] to the development ofcommercial products such as air puriers and self-cleaning glasses [2224]. As for the application, TiO2 isprobably second to none in its diversity, which is sum-marized in Table 1. Thanks to the interdisciplinarynature of TiO2-related research and the diversity of itsapplications, TiO2 is certainly one of the most frequentlyand thoroughly studied materials in the world. On theother hand, a tremendous number of works on TiO2modication to improve its eciency have been carriedout in various ways, which include impurity doping[2527], sensitization [2831], surface modication[3237], and fabrication of composites with othermaterials [38,39]. Modied TiO2 can be very dierentfrom pure TiO2 in many aspects such as light absorp-tion, charge recombination dynamics, interfacial chargetransfer kinetics, surface structure and charge, andadsorption of substrates. As a result, the photocatalyticreactions of modied TiO2 markedly dier from those of

*To whom correspondence should be addressed.

E-mail: wchoi@postech.ac.kr

Catalysis Surveys from Asia, Vol. 10, No. 1, March 2006 ( 2006) 16DOI: 10.1007/s10563-006-9000-2

1571-1013/06/03000016/0 2006 Springer Science+Business Media, Inc.

pure TiO2. This article is intended to review selectedtopics related with TiO2 as an environmental photo-catalyst and most works discussed in this paper arelimited to those performed in the authors laboratory inthe last 5 years.

2. TiO2 photocatalysis as an advancedoxidation process

2.1. General characteristics of TiO2 photoactivation

Titanium dioxide generates a pair of a conductionband (CB) electron and a VB hole in the solid oxidelattice upon absorbing a photon with energy greaterthan 3.2 eV (or k

environmental cleanup processes. TiO2 photocatalystshave shown excellent performances in oxidizing andsubsequently mineralizing various organic pollutants [5].As a result of the photocatalytic oxidation, all elementspresent in a molecule are mineralized to inorganic spe-cies: carbon to CO2, hydrogen to H2O, halogens tohalide ions, sulfurs to sulfates, and phosphorus tophosphates, respectively. Here we describe severalexamples of TiO2 photocatalytic oxidation/reductionreactions of organic and inorganic pollutants.Tetramethylammonium hydroxide (TMAH) is being

consumed in quantity in semiconductor industry whereit is used as a silicon etchant [41]. Since this chemical isvery stable and recalcitrant against degradation, theconventional wastewater treatment process is not e-cient at all in removing it. Figure 2 compares the time-dependent decays of (CH3)4N

+ and the formation ofintermediates and products in UV-illuminated TiO2suspension [15]. The photocatalytic degradation of(CH3)4N

+ proceeded with sequential demethylation togenerate (CH3)3NH

+, (CH3)2NH2+, CH3NH3

+, andNH4

+. Each demethylation step consists of a series ofinterfacial reactions and is initiated by an H-atomabstraction from the methyl group (reaction scheme ingure 2). This provides an example how the photocat-alytic oxidation can be successfully applied to degradinga recalcitrant water pollutant. However, the photocat-alytic oxidation process applied to the wastewatertreatment is often very costly and not easily scaled up,which hinders its commercial exploitation. The signi-cant gap between the scientic feasibility and the prac-tical engineering needs to be bridged by the developmentof more ecient photocatalytic materials and ingeniousengineering.Polychlorinated dibenzo-p-dioxins (PCDDs) are one

of the most problematic and toxic pollutants and clas-sied into persistent organic pollutants (POPs) becauseof their refractory character. PCDDs can be also

degraded by TiO2. Figure 3 compares the direct phot-olytic and photocatalytic degradation of four PCDDs(loaded on a glass plate or a TiO2-coated glass plate)under UV irradiation and ambient air [14]. The directphotolysis with k>300 nm did not induce any notice-able degradation of PCDDs but their photocatalyticdegradation on TiO2 was greatly enhanced, and resultedin 85% conversion within 15-h irradiation for OCDD(octachlorodibenzo-p-dioxin). Neither stable intermedi-ates nor less chlorinated dioxin congeners from OCDDdegradation were detected under the present analyticalconditions where authentic less chlorinated dioxin waseasily detected. Since PCDDs and their degradationproducts reside on TiO2 surface in the air throughoutthe photoirradiation, stable degradation intermediates,if any, seemed to be volatilized into the air or not to beextracted by toluene from the TiO2 surface. Similarphotolytic and photocatalytic behaviors were observedfor HpCDD (hepta-), TCDD (tetra-), and MCDD(mono-): insignicant photolytic degradation andgreatly enhanced photocatalytic degradation on TiO2for all of them. In situ diuse reectance FTIR spectraof OCDD adsorbed on TiO2 showed that the aromaticvibrational peaks of OCDD was gradually reduced withirradiation, which indicates that the benzene ring of thedioxin molecule is destroyed as a result of photocatalyticreaction [14].

C6Cl4O2C6Cl4 OH! C6Cl4O2C6Cl4OH 3The photocatalytic oxidation begins with the additionof an OH radical to the dioxin benzene ring, which leadsto the formation of a hydroxycyclohexadienyl radical(reaction 3). The preferred position of OH radical attackon dioxin seems to be dependent on the number andposition of chlorines. The hydroxycyclohexadienylradical immediately reacts with O2 in the air, thenthe cleavage of the aromatic ring follows, whichwas suggested from the FTIR analysis of OCDD

Vacuum0

TiO2 TiO2Rutile

3.0

eV

Anatase

3.2

eVSrTiO3

3.2

eV

ZrO25.

0 eV

ZnOFe2O3

WO3

2.8

eV

2.2

eV

3.2

eV

SnO2

3.8

eV

CdS

2.5

eV

Si

SiC

1.1

eV

3.0

eV

E vs. NHE

-3.0

-4.0

-5.0

-6.0

-7.0

-8.0

-4.5

-3.5

-5.5

-6.5

-7.5

-1.5

-0.5

+0.5

+1.5

+2.5

+3.5

-0

-1.0

+1.0

+2.0

+3.0

O2/O2- (-0.33 V )O2/HO2 (-0.05 V )H+/H2 (0.0 V )Fe3+/Fe2+ (0.77 V )

OH/OH- (1.9 V )

OH/H2O (2.73 V )

@ pH = 0

O2/H2O (1.23 V )

Figure 1. Energy-level diagram showing the CB and VB edge positions of various semiconductors at pH 0 along with selected redox potentials.

The energy scales are referenced against both the vacuum level and the normal hydrogen electrode (NHE).

W. Choi/Pure and modied TiO2 photocatalysts18

degradation. In general, the photocatalytic oxidationprocess is well suited for the destruction of POPs andother recalcitrant organic pollutants that are present atvery low concentrations in environmental media.

The photocatalytic reactions initiating on the TiO2surface have the multi-phasic character. The photocat-alytic degradation reactions of organic substances takeplace not only at the TiO2/water and TiO2/air interfacesbut also at the TiO2/solid interfaces. Figures 4 and 5show the evidences (SEM images) that the photocata-lytic reactions are able to proceed at the TiO2/(organicpolymer) and TiO2/(carbon soot) interfaces, respec-tively. We prepared the TiO2 particle-embedded PVClms and irradiated them with UV light. The SEMimages (gure 4) show that the degradation of the PVCmatrix started from the PVCTiO2 interface and led tothe formation of cavities around TiO2 particle aggre-gates [42]. The PVCTiO2 lm that was irradiated undernitrogen atmosphere showed little sign of degradation.The cross-sectional SEM images of soot layer on TiO2lm (gure 5) exhibit the progressive degradation ofsoot as the irradiation time increases [43]. The imageshows that the soot layer of 2 lm thickness completelydisappeared after 32 h irradiation, which corresponds toa soot oxidation rate of 65 nm/h. The production ofCO2 from the photocatalytic degradation of both PVCand soot was conrmed by gas chromatographic mea-surements. Therefore, this is essentially a combustion

Irradiation Time (hr)0 2 4 6

Conc

. (M

)

0

20

40

60

80

100NH4+CH3NH3+(CH3)2NH2+(CH3)3NH+(CH3)4N+NO2-NO3-Total N

H+

(CH3)3N+H

(CH3)3N+ C H2O2(CH3)3N+ C H2C H3

-ecb

- HCHO

HO2

OH

-O2

O2-H2OOH

( successive demethylation )

(CH3)3N (CH3)3N+

(CH3)3N+

C H2O(CH3)3N+

Figure 2. Photocatalytic degradation of (CH3)4N+ and the formation of intermediates and products as a function of the irradiation time in

aqueous TiO2 suspension at pH 3.4. The bottom reaction scheme proposes a photocatalytic demethylation mechanism.

Irradiation Time (hour)0 2 4 6 8 10 12

Conc

entra

tion

(g c

m2 )

0

1

2

3

4

5

6

7

OCDDHpCDD TCDDMCDD

direct photolysis

Figure 3. Comparison of the photocatalytic degradation of four

PCDD congeners as a function of irradiation time under UV

irradiation (k>300 nm). Direct photolysis was negligible for allcongeners.

W. Choi/Pure and modied TiO2 photocatalysts 19

process of organic solid in which oxygen moleculesphotoactivated in the ambient air condition combinewith organic carbon to generate CO2.Although the oxidants should be produced on the

surface of TiO2 lm, a strict two-dimensional surfacereaction at the PVC/TiO2 or soot/TiO2 interface cannotaccount for the degradation of the organic solid bulk.This implies that the active oxidants generated on TiO2surface desorbed and migrated into the bulk of the or-ganic solid. The phenomenon that TiO2 photocatalyst isable to oxidize a substrate that is remote from the activesurface site has been repeatedly observed in other studies

[44,45]. The mobility of photooxidants should play acritical role in the photocatalytic degradation of solidsubstrates because the substrates are immobile in thiscase. The SEM images in gure 6 clearly verify that thephotooxidants generated on TiO2 are migratory. Thesoot lm deposited alongside the TiO2 lm was de-graded with developing a gap between the edges of sootand TiO2 domains: the gap distance continuously in-creased with UV illumination up to 80 lm [43]. Theactive oxidants formed on irradiated TiO2 surface des-orb and migrate across the glass surface to reach thesoot domain.

Figure 4. SEM images of the PVCTiO2 (1.5 wt%) composite lm surface. (a) before irradiation; (b) 25 h irradiated; (c) 50 h irradiated;

(d) 100 h irradiated.

Soot

TiO2Glass

Soot

Glass Glass

a b c

1 mTiO2

TiO2

Figure 5. Cross-sectional SEM images of soot-coated TiO2 lms on a glass plate. The UV light was illuminated from the TiO2 side.

(a) before illumination, (b) 18 h illuminated, (c) 32 h illuminated.

W. Choi/Pure and modied TiO2 photocatalysts20

Photooxidants other than OH radicals may contributeto TiO2 photocatalytic oxidation. We investigated thephotocatalytic oxidation of arsenite (As(III)) to arsenate(As(V)) in aqueous TiO2 suspension [16,46]. The preox-idation of As(III) to As(V) is recommended in thetreatment of arsenic-contaminated waters since As(V) isless toxic and more easily removed by adsorbents.As(III) could be rapidly converted into As(V) in illumi-nated TiO2 suspension but the main photooxidants donot seem to be OH radicals since the addition of excessOH radical scavenger, tert-butylalcohol, did not reducethe oxidation rate at all. It has been proposed that su-peroxides play an important role as oxidants (reaction 4).

AsIII O2 2H ! AsIV H2O2 4

AsIV O2 ! AsV O2 5The photocatalytic oxidation rate was signicantlyreduced in the presence of superoxide dismutase. Su-peroxides are generally considered as a weak oxidant.However, in this specic case of arsenite photooxida-tion, they seem to be ecient oxidants.Most photocatalytic degradation reactions occurring

on TiO2 are initiated by an oxidation step such as an OHradical attack or VB hole transfer. However, not allsubstances can be degraded in this way. Perchlorinatedcompounds such as CCl4 and CCl3CO2

) are goodexamples. CCl4 and CCl3CO2

) without any CH bondreact with neither OH radicals nor VB holes. However,

their photocatalytic degradation in TiO2 suspensionshas been successfully demonstrated, which was ascribedto the role of CB electrons (reactions 6, 7) [47,48].

CCl4 ecb ! CCl3 Cl 6

CCl3CO2 ecb ! CCl2CO2 Cl 7

The reductive dechlorination step is followed by a seriesof thermal radical reactions, which lead to full degra-dation. Such reductive degradation is usually enhancedin the presence of electron donors (e.g., alcohols andorganic acids). The reducing power of CB electrons inTiO2 is generally not strong enough to dechlorinatechlorohydrocarbons but the reduction potentials ofperchlorocompounds are positive enough to initiatereactions 6 and 7 upon reacting with CB electrons. CBelectrons in TiO2 may initiate the degradation of organiccompounds in a reductive way for limited cases only.

The reactivity of CB electrons in TiO2 can be alsoutilized in the reductive conversion of heavy metal ions.Photocatalytic reduction of metal ions leads to conver-sion to lower oxidation states (e.g., Cr(VI) Cr(III))[49,50] or deposition onto TiO2 surface as a zero-valentmetal (e.g., Ag+ Ag0) [50]. For example, the platinummetal deposition on the surface of TiO2 particles, which isfrequently performed to enhance the photocatalyticactivity of TiO2, is typically done by the photocatalyticreduction of platinum ions. The UV-illumination ofaqueous TiO2 suspension in the presence of PtCl6

2) andelectron donors results in the deposition of Pt0 on theTiO2 surface (reaction 8) [32].

PtCl26 4ecb ! Pt0 6Cl 8Figure 7 shows a TEM image of Pt nanoparticles thatwere photocatalytically deposited on TiO2 via reaction8. Such reductive conversion of heavy metal ions isenabled not only under UV irradiation but also undervisible light when dyes that serve as a sensitizer are co-present. Figure 8 illustrates a case in which excited dyestransfer electrons to metal ions through TiO2 CB [51].As a result, dyes are oxidized and metal ions are reducedsimultaneously under visible light. The ternary system(TiO2/dye/metal ion) exhibits highly enhanced conver-sion eciencies for both dye (Acid Orange 7: AO7) andheavy metal ion (Cr(VI)) under visible light, comparedwith the binary systems (TiO2/dyes or TiO2/metal ions).

As discussed in the above, the photocatalytic reac-tions are very versatile owing to their multi-phasic nat-ure and the kind of chemical substances that can bedestructed or transformed photocatalytically is almostunlimited. The target of photocatalytic conversion is notlimited to chemical pollutants. Microorganisms, patho-gens, and algae can be killed photocatalytically usingTiO2 on the basis of the similar mechanisms involvingphotooxidants, which is being intensively studied fordisinfection applications [7,8].

(a)

Glass

dUV

TiO2

TiO2 SootUV

Soot

OH

(b)

d

TiO2

Soot

Figure 6. Remote photocatalytic degradation of soot near the edge of

TiO2 domain. (a) the schematic illustration of the photocatalytic

degradation of soot layer near the borderline of soot and TiO2domains where a gap distance, d develops between edges of TiO2 and

soot domain with illumination time. (b) The SEM image shows the

developing gap after 6 h irradiation.

W. Choi/Pure and modied TiO2 photocatalysts 21

On the other hand, the development of photocatalyticreactors is vitally important from the practical aspect.However, compared to a large number of researchworks devoted to the mechanistic studies of photocata-lytic reactions and the synthesis and development ofhigh eciency photocatalytic materials, much less eorthas been made in the area of photocatalytic engineeringand reactor development for commercial exploitation[52]. Most studies of photocatalytic purication of waterhave been carried out using TiO2 slurry that is not veryuseful for practical applications. TiO2 particles in watershould be removed after treatment or immobilized onsupports. Immobilized TiO2 photocatalytic reactors aregenerally less ecient than the slurry-type reactors be-cause of the reduced catalyst surface area, the reducedexposure of the catalyst surface area to light, and themass-transfer limitation and suer from the surfacedeactivation and the lack of long-term durability ofimmobilized TiO2 coatings. In this respect, the slurry-type reactor coupled with membrane ltration isattractive. We recently constructed a pilot-scale photo-catalyst-membrane hybrid reactor (500 L volume) andcharacterized its performance in terms of the degrada-tion eciency of organic pollutants and the degree ofmembrane fouling under various operational conditions[53]. Figure 9 shows the schematic diagram of thereactor system that has a submerged membrane moduleand an air blower that plays the multiple roles of mixingthe suspension, supplying oxygen, and inhibiting themembrane fouling. 4-Chlorophenol of 100 ppb could becompletely removed within 2-h batch operation. Incontinuous runs, no fouling of the membrane (or nosuction pressure build-up) took place with an intermit-tent operation with the 9-min suction and 3-min pauseperiod.

3. Modication of TiO2 photocatalysts

3.1. Properties and reactivities of surface platinized TiO2

The surface platinization of TiO2 has been a popularphotocatalyst modication technique, since Kraeutlerand Bard [32] rst introduced it, because the platinizedTiO2 (Pt/TiO2) exhibits enhanced activity for manyphotocatalytic reactions [5456]. The presence of Ptdeposits on TiO2 is believed to retard fast charge-pairrecombination by serving as an electron sink

Figure 7. Pt nanoparticles deposited on the surface of TiO2 particles

(Degussa P25) through a photoreductive conversion of PtCl62).

Visible Light Illumination Time (h)0 1 2 3 4

[Cr(V

I)] (

M)

0

50

100

150

200

TiO2/Cr(VI)TiO2/Cr(VI)/AO7TiO2/Cr(VI)/AO7 (dark)Cr(VI)/AO7Cr(VI) only

0 1 2 3 4

[AO7

] (M

)

0

20

40

60

80

100

TiO2/AO7TiO2/Cr(VI)/AO7TiO2/Cr(VI)/AO7 (dark)Cr(VI)/AO7AO7 only

a

b

VB

CB

Dye

Mn+

TiO2

O2

Dye*

M(n-1)+

M(n-2)+

M0

e-

hDegradation1

2

3

4 5

Figure 8. Visible light-induced simultaneous oxidation of dyes

and reduction of metal ions on TiO2 particles. Conversion of

(a) AO7 (C0 = 100 lM) and (b) Cr(VI) (C0 = 100 lM as Cr2O72))

in the binary or ternary systems under visible light illumination

(k>420 nm). The experimental conditions were air-equilibrated;pH = 3.0; [TiO2] = 0.5 g/L.

W. Choi/Pure and modied TiO2 photocatalysts22

(Schottky-barrier electron trapping) and to facilitatethe interfacial electron transfer to dioxygen or otherelectron acceptors, which has been supported by elec-trochemical and time-resolved spectroscopic investiga-tions [21,57].To demonstrate the role of Pt deposits on TiO2 as an

electron sink, the Fe3+-mediated photocurrents that arecollected onto an inert electrode are compared betweenpure TiO2 and Pt/TiO2 suspensions (gure 10) [58].A higher photocurrent is obtained in the suspension ofPt/TiO2. This should be ascribed to the fact that CBelectrons are trapped in the surface Pt phase andconsequently more electrons are transferred onto thecollector electrode via the electron shuttle (Fe3+/Fe2+).To take another example, the electron trapping abilityof the Pt phase was successfully utilized in the devel-opment of dye-sensitized TiO2 photocatalysts. Figure 11describes the working principle of ruthenium bipyridyl-complex (RuIIL3) sensitized TiO2 photocatalyst undervisible light [30]. Figure 12 clearly shows that loadingPt nanoparticles on TiO2 surface dramatically enhancesthe visible light-sensitized dechlorination of CCl4 [30].In the absence of Pt on TiO2, most electrons injectedfrom the excited sensitizer into TiO2 CB recombinewith the oxidized dye (path 2 in gure 11), which limitsthe overall photonic eciency smaller than 10)3. WithPt loaded on TiO2, both the electron trapping into thePt phase (path 3) and the back electron transfer(path 2) typically occur within 1 ls [21,29], and there-fore the electron trapping on Pt can compete with theback electron transfer. As a result, the interfacialCB electron transfer to the electron acceptor (CCl4) canbe highly enhanced when Pt is deposited on thesensitized TiO2.

Pt deposits on TiO2 not only enhance the photocat-alytic activity by serving as an electron sink and conse-quently retarding fast charge-pair recombination butalso change reaction pathways by providing catalytic

Overflow P

1

Flowmeter

Controlpannel

Inlet

Drain

BLB lamp(5X2 EA)

Air BlowerFeeding

pumpReactor (500 L)

Suctionpump

Storage tank

Membrane(hollow fiber module)

Effluent

Figure 9. Schematic of a pilot-scale photocatalyst-membrane hybrid reactor. During the reactor operation, the concentration of suspended TiO2particles and the ow rate of euent were kept at a constant level of 0.50.1 g/L and 2 L/min, respectively.

Time (h)0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

I (A

cm

2 )

0

100

200

300

400

500

600

Pt / TiO2 + Fe3+

TiO2 + Fe3+

Ligh

t ON

e-

h+hn

Fe3+

Fe2+ D

D +

e-

Pt

Figure 10. Comparison of photocurrent generation in UV-illuminated

deoxygenated suspensions of TiO2 + Fe3+ and Pt/TiO2 + Fe

3+ with

acetate used as an electron donor (D). Experimental conditions were

[TiO2] = [Pt/TiO2] = 0.5 g/L; [Acetate]0 = 0.2 M; pHi = 1.950.05;

platinum collector electrode held at + 0.6 V (vs. SCE); illuminated

with 30 W-black lamp; N2 purged continuously during the test.

W. Choi/Pure and modied TiO2 photocatalysts 23

sites. The catalytic activity of Pt is well known but itscatalytic role in photocatalytic reactions using Pt/TiO2has been little investigated. Here we describe a fewexamples.Photocatalytic degradation of ammonia in aqueous

TiO2 suspension was slow and resulted in the quantita-tive conversion to NO2

) and NO3). On the other hand,

the photocatalytic degradation of ammonia with Pt/TiO2 was much faster and accompanied with a signi-cant reduction in the total N-mass, which implies thepresence of missing products [34]. In order to accountfor the decit in the N-mass balance, GC/MS analysiswas performed to detect the peak at m/e = 30 (15N2)from the photocatalytic oxidation of isotope-labeled15NH3 on Pt/TiO2. A similar experiment using nakedTiO2 did not produce even a trace of m/e = 30 signal.This conrmed that NH3 on Pt/TiO2 photocatalyticallytransformed into N2. Pt deposits on TiO2 stabilize the

intermediate atomic nitrogen species and thus facilitatetheir recombination into dinitrogen. Figure 13 com-pares the anoxic photocatalytic conversion of dimeth-ylamine ((CH3)2NH) between the pure and platinizedTiO2 suspensions as another example [59]. The photo-degradation of dimethylamine on Pt/TiO2 was muchfaster and yielded products dierent from thoseobtained with pure TiO2. In particular, not only deme-thylated amines but also N-methylated amines wereproduced as a byproduct in the deaerated Pt/TiO2 sus-pension. About 30% of dimethylamine was convertedinto trimethylamine in the deaerated Pt/TiO2 suspensionwithin 1 h of irradiation, whereas no conversion intotrimethylamine was observed in the presence ofdissolved O2. This case also demonstrates the catalytic

VB

CB

RuIIL3

RuII*L3Pt

Cl3CX

Cl2CX + Cl- TiO2

e-

h D/D+

1

2

34

5

Figure 11. Visible light-induced reductive degradation of perchlori-

nated compounds (Cl3CX: X = Cl for CCl4, X = CO2) for

CCl3CO2)) on Pt/TiO2/Ru

IIL3 particles. The number represents the

major electron pathways: 1, electron injection from the excited

sensitizer to CB; 2, back electron transfer to the oxidized sensitizer

(RuIIIL3); 3, electron migration and trapping in Pt deposits; 4,

interfacial electron transfer to a perchlorinated molecule on Pt; 5,

sensitizer regeneration by electron donors.

Irradiation time (min)0 10 20 30 40 50 60

[ C

l- ] (

M)

0

300

600

900

1200

1500Pt/TiO2/RuIIL3 with 0.1 M IPAPt/TiO2/RuIIL3TiO2/RuIIL3 with 0.1 M IPATiO2/RuIIL3

Figure 12. Time-dependent chloride production from CCl4 degrada-

tion on TiO2/RuIIL3 and Pt/TiO2/Ru

IIL3 under visible light. The

eects of adding 0.1 M isopropyl alcohol (IPA) on the chloride

production are compared as well. The experimental conditions were:

[TiO2] = 0.5 g/L, pHi = 3, [RuIIL3]i = 10 lM, [CCl4] = 1 mM,

k>420 nm, and initially N2-saturated.

Irradiation Time (min)0 50 100 150 200 250

Conc

. (M

)

0

100

200

300

400

500

DimethylamineMethylamineAmmoniaTotal N

TiO2/N2

Pt/TiO2/N2

30

Irradiation Time (min)0 50 100 150 200 250

Conc

. (M

)

0

100

200

300

400

500

DimethylamineTrimethylamineMethylamineAmmoniaTotal N

30

(a)

(b)

Figure 13. Photocatalytic conversion of dimethylamine [(CH3)2NH]

on (a) naked and (b) Pt/TiO2 in the anoxic suspension. (C0 = 500 lM;[TiO2] = 0.5 g/L; pHi = 10.4) .

W. Choi/Pure and modied TiO2 photocatalysts24

role of Pt in the photocatalytic reactions. The degrada-tion of TCA on Pt/TiO2 is discussed as the thirdexample. The photocatalytic degradation of TCAshould be initiated by CB electron transfer as mentionedearlier (see reaction 7) [48]. The resulting dichloroacetateradical rapidly reacts with dissolved O2, leading tocomplete destruction with no stable intermediates pro-duced.

CCl2CO2 O2 ! OOCCl2CO2 !! 2Cl 9

Therefore, the photocatalytic degradation of TCA inpure TiO2 suspension needs O2 as a reagent and ismarkedly retarded in deaerated suspension [33]. How-ever, when Pt/TiO2 was used as a photocatalyst, thedegradation rate was enhanced in the absence of O2,which implies that the Pt catalyst provides an alternativedegradation path. It is proposed that the reaction ofdichloroacetate radical with VB holes (reaction 10) isenabled on Pt/TiO2 and is responsible for this anoxicpathway [33].

CCl2CO2 hvb ! CCl2 CO2 10The resulting dichlorocarbene (CCl2) could be hydro-lyzed to yield chloride ions in an anoxic solution [60].When the anoxic path (reaction 7 + reaction 10) isdominant, the presence of O2 reduces the reactivity byscavenging CB electrons. This anoxic mechanism seemsto be eective only in the presence of Pt deposits onTiO2. As the last example of the Pt catalytic eect,gure 14 shows the photocatalytic oxidation of gaseousCO on Pt/TiO2 photocatalysts prepared with threecommercial TiO2 samples (Degussa P25, ISK STS-01,Hombikat UV-100). The photocatalytic conversion ofCO to CO2 was quantitative in the presence of O2 andproceeded on Pt/TiO2 at much faster rate than on bareTiO2 [54].On the other hand, the reported Pt eects in the

photocatalytic degradation of substrates have not beenalways positive and even contradictory for the samesubstrate. For example, Chen et al. [61] reported that theplatinization of TiO2 drastically reduced the photocat-alytic degradation rate of trichloroethylene (TCE) andDriessen et al. [62] observed a similar phenomenon. Onthe contrary, Crittenden et al. [63] reported that such asignicant retardation in TCE degradation was notobserved with Pt/TiO2. Our recent study explains whythere are discrepancies in the reported observations[64]. The photocatalytic degradation rates of TCE,PCE (perchloroethylene), and DCA (dichloroacetate) inair-equilibrated aqueous suspensions are comparedin gure 15 when a series of Pt/TiO2 samples preparedwith varying Pt photo-deposition time (1, 2, 5, 30, and60 min) were used as a photocatalyst. As for DCAdegradation, a volcano-shape activity curve, which hasbeen frequently observed in other studies [30,56] wasobtained. However, the platinization eect on the

degradation of TCE and PCE is drastically dierentfrom the general case. TiO2 that was photo-platinizedfor 1, 2, and 5 min had detrimental eects on the deg-radation of TCE and PCE while the 30-min photode-position of Pt recovered the photoactivity to a levelsimilar to that of bare TiO2. This result clearly demon-strates that the Pt eects in TiO2 photocatalytic reac-tions depend on not only the Pt loading but also thekind of substrates, hence cannot be generalized. Our

Illumination Time (min)0 2 4 6 8

[CO]

/[CO]

0

0.0

0.2

0.4

0.6

0.8

1.0

TiO2(D)TiO2(I)TiO2(H)Pt/TiO2(D)Pt/TiO2(I)Pt/TiO2(H)Dark Pt/TiO2(H)

[CO]0 = 30 ppmv

Figure 14. Time-dependent proles of photocatalytic removal of CO

on bare TiO2 and Pt/TiO2. Three commercial TiO2 samples (TiO2(D)

[Degussa P25], TiO2(H) [Hombikat UV-100] and TiO2(I) [ISK STS-01])

are compared for their photoactivities. The experimental conditions

were [O2] = 20 vol.%, Pt loading of 3 wt%, UV intensity of 3 mW/

cm2, and no water vapor added.

Pt Photodeposition Time (min)0

5

10

15

20

25

30

d[Cl

-

]/d

t (M

/min

)

d[Cl

-

]/d

t (M

/min

)

0

1

2

3

4

5

6TCEDCAPCE

0 1 2 5 30 60

Figure 15. Eects of Pt photodeposition time on the photocatalytic

degradation rates of TCE, PCE, and DCA. Experimental conditions

were [Pt/TiO2] = 0.5 g/L; [TCE]0 = 0.4 mM; [PCE]0 = 0.25 mM;

[DCA]0 = 1 mM; pH = 4.0; air-equilibrated [Note that the abscissa

scale (photodeposition time) is not linear].

W. Choi/Pure and modied TiO2 photocatalysts 25

study revealed that the oxidation state of Pt deposits isvery important in determining the initial degradationrates of chlorinated organic compounds. TiO2 withoxidized Pt species (Ptox/TiO2) was less reactive thanTiO2 with metallic Pt (Pt

0/TiO2) for all substrates tested.In particular, Ptox/TiO2 strongly inhibited the degrada-tion of TCE and PCE whereas it was more reactive thanbare TiO2 for other compounds. The main eect ofphotodeposition time (in gure 15) was related to thechange in the oxidation state of deposited Pt species. X-ray photoelectron spectroscopic analysis showed thatPtII and PtIV were the major species in the earlyphotodeposition period (12 min) but Pt0 was dominantafter longer photodeposition (3060 min). It should berealized that the Pt eects in photocatalysis are sub-strate-specic and highly dependent on how the plati-nization is done.

3.2. Properties and reactivities of surface uorinatedTiO2

The surface uorination of TiO2 (FTiO2) is a simpleligand exchange between uoride anions (F)) and sur-face hydroxyl groups on TiO2 in water (reaction 11)[35,36].

TiOH F $ TiFOH pKF 6:211

It was recently reported that the surface uorination ofTiO2 improves the photocatalytic oxidation rate ofphenol [35] and tetramethylammonium (TMA) [36] in aspecic pH range. Since the surface uorides themselvesshould not be reactive with VB holes [E0(F/F-) = 3.6 V vs. NHE] [65] the higher photocatalyticoxidation rate in the FTiO2 suspension has been as-cribed to the enhanced generation of mobile free OHradicals (reaction 12) whereas most OH radicals gener-ated on naked TiO2 surface prefer to remain adsorbed(reaction 13) [35].

TiFH2OorOHhvb! TiFOHfreeH12

TiOH hvb ! TiOH 13Eects of surface uorination on the photocatalyticreactivities are very dierent depending on the kind ofsubstrates to be degraded. FTiO2 is more eective thanpure TiO2 for the photocatalytic oxidation of AcidOrange 7 and phenol, but less eective for the degra-dation of dichloroacetate [37]. It is proposed that theOH radical-mediated oxidation pathways are enhancedon FTiO2, whereas the hole transfer-mediated oxida-tions are largely inhibited due to the hindered adsorp-tion (or complexation) of substrates on FTiO2. As forthe photocatalytic reduction, the dechlorination of TCAis much retarded on FTiO2. The photocurrents

collected in TiO2 suspensions, which are mediated byelectron shuttles (methyl viologen or ferric ions) as ingure 10, are also markedly reduced in the presence ofF. The surface TiF group seems to act as an elec-tron-trapping site and to reduce interfacial electrontransfer rates by tightly holding trapped electronsbecause of the strong electronegativity of the uo-rine.The surface uorination eect is observed not onlyat TiO2/water interface but also at TiO2/air interface.The enhanced desorption of mobile OH radicals fromFTiO2 surface into the air could be clearly veriedthrough a study of remote photocatalytic oxidation [66].A stearic acid-coated glass plate and a TiO2-coated platewere faced to each other and held together, but sepa-rated by a small intervening gap (30 lm) as illustrated ingure 16a. A black-light UV lamp (10 W: SankyoDenki) irradiated the sample from the TiO2 side.A control experiment was carried out with reversingthe TiO2-coated glass plate upside down. As shown ingure 16b, the remote photocatalytic degradationof stearic acids over FTiO2 was much faster thanover bare TiO2. It indicates that the generation of freeOH radical as air-borne oxidant is enhanced overFTiO2.

TiFH2Oad hvb! TiF OHair borne H 14

(b)

(a)

2TiOF-

2TiO

control1.2

1.0

0.8

0.6

0.4

0.2

0.0

SA C

onc.

(arb

. unit

)

2018161412100 2 4 6 8Irradiation Time (h)

ghtUV li

sasGl mfil2OTiF-erSpac

oating cdicacStearisasGl

ic

Figure 16. (a) Illustration of the experimental setup for the remote

photocatalytic oxidation of stearic acid (SA). (b) Remote photocata-

lytic degradation of SA over TiO2 vs. FTiO2 lm as a function of UV

irradiation time. The degradation of SA was monitored by the IR

absorption of CH stretching band.

W. Choi/Pure and modied TiO2 photocatalysts26

3.3. Surface charge modications

In TiO2 photocatalytic systems where the substrateadsorption is essential as a prerequisite step, the surfacecharge modication of TiO2 inuences the photocata-lytic reactivity for ionic substrates by altering the elec-trostatic interaction between the catalyst surface andthe substrate. The surface charge of bare TiO2 ispositive at acidic condition (pH 5) due to the presenceof TiOH2+ groups, near neutral at pH 57, andnegative at basic condition (pH 7) owing to TiO

groups [5,67]. This pH-dependent surface charge ofTiO2 can be controlled by loading other metal oxides oradsorbing charged substrates. We introduced anionicsurface charge by loading silica [68] or naon polymer(cation-exchange resin) [69] onto TiO2 and demon-

strated that the surface charge-modied TiO2 highlyaccelerated the photocatalytic degradation of cationicsubstrates. Figure 17a shows that the surface charges ofboth silica-loaded TiO2 (SiO2/TiO2) and naon-coatedTiO2 (Nf/TiO2) are signicantly shifted to the negativevalues compared with that of bare TiO2. As a result,SiO2/TiO2 exhibited a highly enhanced activity for thedegradation of TMA (cationic substrate) (gure 17b)[68]. Nf/TiO2 showed an enhanced reactivity for thedegradation of TMA as well [69]. The visible light-sen-sitized degradation of cationic dyes (e.g., methylene blue)was also enhanced with Nf/TiO2 [69]. In particular, thesensitized degradation of rhodamine B (RhB) followed adierent path when the surface of TiO2 was coated withnaon. The N-de-ethylation of RhB that leads to thegeneration of rhodamine-110 was a prevailing path withNf/TiO2 whereas the cleavage of the chromophoric ringstructure was dominant with bare TiO2 [69]. The pho-tocatalytic degradation of N-nitrosodimethylamine(NDMA: an emerging water pollutant) was also enhancedwith Nf/TiO2 [70]. Although NDMA is a neutralmolecule, the highly concentrated protons within thenaon layer facilitate the formation of Lewis acidcomplex with NDMA and enhance its photocatalyticdegradation.

4. Conclusions

TiO2 photocatalysis is being investigated from di-verse points of view that are related with materialssynthesis and modication, reaction kinetics andmechanisms, reactor engineering, and surface chemistryto take a few examples. The application areas of TiO2photocatalysis are very diverse, which seems to be lar-gely responsible for the longevity of this eld. Photo-catalytic reactions taking place on the surface of TiO2can be applied to the degradation of pollutants in wa-ter, air, and even solid phases, which makes this tech-nology very versatile. Mechanistic understanding aboutheterogeneous photocatalytic reactions is far fromcomplete due to the complex nature despite intensiveresearch eorts. The current status of knowledge sug-gests that the mechanisms in photocatalytic reactionsare hard to generalize and should be understood on acase-by-case basis. Some examples discussed in thisarticle also reveal such aspects. Although pure TiO2 is areasonably good photocatalyst, a variety of methodshave been employed to improve the eciency andovercome the inactivity in the visible light region. Onlya few examples are introduced in this article. Veryecient photocatalysts that are modied from TiO2or new materials are yet to be developed for success-ful commercialization. In addition, more eorts inphotocatalytic engineering and reactor developmentare required to commercialize the photocatalyticdetoxication technology.

Irradiation Time (hr)0 1 2 3 4 5 6

[ TMA

] (M

)

0

20

40

60

80

100

120

SiO2/TiO2TiO2

pH2 4 6 8

Zeta

Pot

entia

l (mV)

30

20

10

0

10

20

30

TiO2

SiO2/TiO2

Nf/TiO2

(a)

(b)

Figure 17. (a) Zeta potentials of bare TiO2, SiO2/TiO2, and Nf/TiO2particles suspended in water ([catalyst]0 = 2 mg/L) as a function of

pH. (b) Photocatalytic degradation of TMA in aqueous suspensions of

bare TiO2 vs. SiO2/TiO2.

W. Choi/Pure and modied TiO2 photocatalysts 27

Acknowledgments

This work was supported by the Hydrogen Energy R&DCenter (21st Century Frontier R&D Program, MOST)and Brain Korea 21 program.

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