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Catalysis Today 224 (2014) 21–28

Contents lists available at ScienceDirect

Catalysis Today

j our na l ho me page: www.elsev ier .com/ locate /ca t tod

ye decolorization test for the activity assessment of visible lighthotocatalysts: Realities and limitations

ugyeong Baea, Sujeong Kima, Seockheon Leeb, Wonyong Choia,∗

School of Environmental Science and Engineering/Dept. of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang90-784, Republic of KoreaCenter for Water Resource Cycle Research, Korea Institute of Science and Technology (KIST) , Seoul 136-791, Republic of Korea

r t i c l e i n f o

rticle history:eceived 24 September 2013eceived in revised form9 November 2013ccepted 9 December 2013vailable online 10 January 2014

eywords:hotocatalytic activity testisible light photocatalystye degradationnvironmental photocatalystye sensitization

a b s t r a c t

The development of photocatalysts with visible light activity has been extensively investigated. Theiractivities are usually tested by measuring the degradation rate of different organic compounds. Amongthese organic substrates, dyes are the most widely employed due to their rapid decolorization and simplekinetic analysis using a spectrophotometric method. However, the dye test has much uncertainty in theevaluation of photocatalytic activity. To assess the validity of the dye test, six visible-light photocatalysts(N–TiO2, C–TiO2, C60(OH)x/TiO2, Pt/WO3, BaBiO3, and Bi2WO3) were tested and compared for the degra-dation of five organic dyes (anionic: acid orange 7, indigo carmine, and new coccine; cationic: methyleneblue and rhodamine B) in this study. This study aimed to assess how the measured activities dependon the kind of test dyes and how reliable the dye test is as an activity evaluation method. The activi-ties determined by the dye test were highly specific to the kind of dye and photocatalyst. For example,N–TiO2 is the most active photocatalyst for the degradation of acid orange 7 at pH 3 but is one of theleast active at pH 9; Pt/WO3 is the best photocatalyst for the degradation of methylene blue but not muchactive for the degradation of acid orange 7. This is ascribed to the fact that the dye test is significantlyinfluenced by various factors such as the dye sensitization of catalyst particles, the absorption spectral

overlap between dyes and photocatalysts in the visible region, the electrostatic interaction (attractive orrepulsive), and the properties of dye degradation intermediates. In general, the dye decolorization effi-ciency was poorly correlated with the dye mineralization efficiency, which limits the practical value ofthe dye test. Therefore, the practice of dye test for the activity assessment of visible light photocatalystsshould not be recommended and the activity results obtained for a specific combination of a dye and aphotocatalyst should not be generalized.

. Introduction

Photocatalysts have been widely investigated for various energynd environmental applications including hydrogen production1], degradation of organic pollutants [2], bacterial disinfection3,4], and CO2 reduction [5]. In particular, the photocatalyticemediation of contaminated water and air has been extensivelynvestigated to demonstrate its viability as a useful cleanup process.arious contaminants such as chlorinated aromatics [6], chlori-ated hydrocarbons [7], heavy metal ions [8,9], and volatile organicompounds [10,11] can be degraded or transformed by photo-

atalysis. Among numerous kinds of organic compounds, dyes arehe most tested substrates in photocatalytic studies because notnly they are common industrial pollutants [12], but also their

∗ Corresponding author. Tel.: +82 54 279 2283; fax: +82 54 279 8299.E-mail address: wchoi@postech.edu (W. Choi).

920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2013.12.019

© 2013 Elsevier B.V. All rights reserved.

degradation can be simply monitored by a colorimetric method.Dye discoloration test is now widely used as a de facto standardmethod of photocatalytic activity assessment although the con-cerns about the dye test problems have been repeatedly raised[13,14]. The research publications on photocatalysis that used thedye decolorization as a test method have rapidly increased for thelast decade (see Fig. 1). In particular, the dye test is being widelyused as an activity test method for visible-light active photocata-lysts. Table 1 shows some recent examples that used the dye test inthe studies of visible light photocatalysis: the dye test studies areclassified according to the kind of photocatalytic materials such asdoped TiO2[15–27], composite semiconductor [28–31], and binarymetal oxide [32–39].

The dye decolorization as an activity test method for visible-

light active photocatalysts suffers from many problems such as: (1)Dye itself absorbs visible light and its degradation can be initiatedfrom the excited dye (e.g., direct photolysis [40], dye sensitization[41,42]), not the excited photocatalyst. The decolorization result

22 S. Bae et al. / Catalysis Today 224 (2014) 21–28

Table 1Literature study examples of photocatalyst activity test using dye decolorization under visible light.

Photocatalyst type Type of dye Ref.

Doped semiconductor N–TiO2 Acid orange 7 (Azo) 15Methylene blue (xanthene) 16–18

C–TiO2 Acid orange 7 (Azo) 19–21Methylene blue (xanthene) 22–24Rhodamine B (phenothiazine) 25

S–TiO2 Methylene blue (xanthene) 26Indigo carmine (indigo) 27

Composite semiconductor CdS/TiO2 Methylene blue (xanthene) 28, 29WO3/TiO2 Acid orange 7 (Azo) 30

Methylene blue (xanthene) 31Binary metal oxide semiconductor Bi2WO6 Acid orange 7 (Azo) 32

Methylene blue (xanthene) 33

otllcvooIb[gtmbulbapt

ewdm

Fd(r

Bi2VO4

btained via the excited dye does not represent the intrinsic pho-ocatalytic activity of the tested materials. In addition, the visibleight absorption by the dyes themselves attenuates the incidentight flux available to the photocatalyst (i.e., dyes shield the photo-atalyst from irradiation), which may underestimate the intrinsicisible light activity of the catalyst material. (2) The decolorizationf dye reflects the selective transformation of chromophoric groupsnly, and not necessarily the full degradation (or mineralization).t has been frequently demonstrated that there is poor correlationetween the color removal and TOC (total organic carbon) removal43,44]. Therefore, the selective degradation of a specific functionalroup in a dye molecule should lead to the rapid decolorization, buthe overall degradation can be inefficient. (3) The decolorization

onitored by absorbance measurement at a single wavelength cane inaccurate because the generation of intermediate products issually accompanied by the spectral change. Therefore, the Beer’s

aw that relates the absorbance to the dye concentration cannote applied to such system. (4) Most commercially available dyesre impure (typically 70–90%). They contain many unknown com-onents that may interfere with the photocatalytic degradation ofhe parent dye, which makes dyes unsuitable as a test substrate.

Keeping the above general problems in mind, this study aims tovaluate the validity and limitation of the dye test in a systematic

ay. We measured the activities for various combinations of fiveyes and six visible-light photocatalysts and assessed how theeasured activities depend on the kind of test dyes and how

Year2000 2002 2004 2006 2008 2010 2012

Num

ber o

f pub

licat

ions

0

200

400

600

800

1000

ig. 1. Annual number of papers published in the subject area of “photocatalyticye degradation”. The literature search was carried out at the Scoupus websitewww.scoupus.com) using the key words “photocatal*” and “dye degradation,emoval, decolorization, or decoloration”.

Rhodamine B (phenothiazine) 34–36Methylene blue (xanthene) 37Rhodamine B (phenothiazine) 38, 39

reliable the dye test is as an objective evaluation method. Despitethe wide popularity of dye tests in the field of environmental pho-tocatalysis, this is the first comprehensive study that comparedand assessed the validity of various dye tests.

2. Experimental

2.1. Reagents and materials

Six visible light active photocatalysts were selected for thisstudy and pure TiO2 (P25) was compared as a control sample(listed in Table 2). The following visible-light photocatalysts weresynthesized according to the literature method: nitrogen-dopedTiO2 (N–TiO2) [45], carbon-doped TiO2 (C–TiO2) [46], fullerol-anchored TiO2 (C60(OH)x/TiO2) [47] and Pt/WO3[43], BaBiO3[48],and Bi2WO6[49]. Five dyes that were selected as the substrate forthe activity test are acid orange (AO7, Aldrich), new coccine (NC,Aldrich), indigo carmine (IC, Aldrich), methylene blue (MB, Aldrich),and rhodamine B (RhB, Aldrich). Their structures and properties arelisted in Table 3. Methyl orange (MO, Aldrich) was also used as asubstrate in a separate activity test.

2.2. Photocatalysts characterization

All photocatalysts were characterized, and their properties arelisted in Table 2. X-ray powder diffraction analysis using Cu K�radiation (Marc Science Co. M18XHF) was carried out to measurethe crystalline phase. The BET surface area of powder samples wasmeasured using nitrogen as an adsorptive gas. The zeta potentialsof the photocatalyst particles suspended in water were measuredusing an electrophoretic light scattering spectrophotometer (ELS8000, Otsuka) equipped with a He–Ne laser and a thermostatedflat board cell. Diffuse reflectance UV/visible absorption spectra(DRUVS) were recorded using a spectrophotometer (Shimadzu UV-2600) with an integrating sphere attachment and BaSO4 was usedas the reference.

2.3. Photocatalytic activity measurement

Photocatalyst powder was dispersed in distilled water at theconcentration of 0.5 g/L. An aliquot of the dye substrate stock solu-tion was subsequently added to the suspension to make a desiredsubstrate concentration (100 �M). The pH of the suspension wasadjusted to 3 or 9 with HClO4 or NaOH solution, and then the sus-

pension was stirred for 30 min in the dark to reach the adsorptionequilibrium of dyes on the photocatalyst surface. A 300 W Xe arclamp (Oriel) was used as a light source. The light beam was passedthrough a 10 cm IR water filter and a cutoff filter (� > 420 nm

S. Bae et al. / Catalysis Today 224 (2014) 21–28 23

Table 2Properties of visible light photocatalysts.

Color N–TiO2 C60(OH)x/TiO2 C–TiO2 Pt/WO3 BaBiO3 Bi2WO6 TiO2 (P25)

Yellowish Brownish Brown Grey Blackish brown Yellowish WhiteBET surface (m2/g) 119 57 247 4.7 <0.5 <0.5 56Crystallite Sizea (nm) 13 22 5.7 45 28 74 21pHpzc

b 5.3 3.2 5.0 2.0 –c –c 6.5Ref. 45 47 46 43 48 49 –

n.

fcd(taaei(mi

2

tfca

3

3

dt4itNet(Tithatdrtef

l(d39cd

with other photocatalysts. The visible light activities of BaBiO3 andBi2WO6 are even lower than those of pure TiO2 (P25) in most cases.The overall trend in the activity variation in Table 4 is hard to begeneralized. This indicates that the visible light activity of a specific

Wavelength (nm)400 500 600 700 800

Abs

(K.M

.)

0.0

0.2

0.4

0.6

0.8

1.0

A. N-TiO2

B. C60(OH) X/TiO2

C. C-TiO2

D. Pt/WO3

E. BaBiO3

F. Bi2WO6

G. TiO2 (P25)

A

B C

D

E

F

G

(a)

400 500 600 700 800

Abs

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(b)E

D

CB

A

A. AO7B. NCC. ICD. MBE. RhB

a Calculated from Debye–Scherrer equation.b Point of zero charge (measured at point of zero zeta potential).c Not determined because BaBiO3 and Bi2WO6 were not well dispersed in solutio

or visible and � > 300 nm for UV irradiation) and focused onto aylindrical pyrex reactor (30 mL) with a quartz window. The inci-ent photon flux was measured by using an optical power meter1830-C, Newport) and determined to be about 155 mW/cm2 inhe wavelength range of 420–550 nm. The reactor was open to thembient air to prevent the depletion of dissolved dioxygen, andgitated magnetically during irradiation. Sample aliquots werextracted from the reactor at a periodic time interval during thellumination and filtered through a 0.45 �m PTFE syringe filterMillipore) to remove photocatalyst particles. Multiple measure-

ents of the photocatalytic activity were carried out under thedentical reaction condition to confirm the reproducibility.

.4. Analysis

The removal of dye color was monitored using a UV/visible spec-rophotometer (Shimadzu UV-2401PC). The monitored wavelengthor each dye is listed in Table 3. The change of the organic carbonontent in the irradiated dye solution was monitored using a TOCnalyzer (Shimadzu TOC-VSH).

. Results and discussion

.1. Dye-specific activity test

Six visible light active photocatalysts tested in this study are veryifferent among one another in their working mechanisms. All pho-ocatalysts have a significant absorption in visible region between00–500 nm (see Fig. 2a). The elevated background (460–800 nm)

n Pt/WO3 spectrum is due to the presence of Pt, which is similaro the previously measured spectra of Pt/TiO2

6 and Pt/WO3[43].–TiO2 and C–TiO2 have non-metal dopants that have mid-gapnergy levels in the band gap region. The photo-induced elec-ronic transition between the mid-gap levels and the conductionor valence) band is responsible for the visible light activity [45,46].he visible light activity of fullerol-TiO2 is ascribed to the photo-nduced charge transfer between the ground state of fullerol andhe conduction band of TiO2[47]. Pt/WO3, BaBiO3, and Bi2WO6ave smaller band gaps that can be directly excited by visible lightbsorption. Table 3 shows that dyes can be classified by their struc-ure, charge, and functional group. Five tested dyes are clearlyifferent in their spectral absorption and intensity in the visibleegion: the maximal absorption band of AO7, NC, and IC are weakerhan that of MB and RhB (see Fig. 2b). Various types of dyes weremployed to obtain the comprehensive assessment of the dye testor different kinds of visible active photocatalysts.

Table 4 summaries the results of the dye tests under visibleight irradiation in a matrix form which has seven photocatalystsincluding pure TiO2 as a control sample) in the column and fiveyes in the row. Most activity tests were carried out at the initial pH

. As for AO7, MB, and RhB, the tests were done for both pH 3 and pH

to check out their pH dependence. There were no significant pHhanges during the photodegradation of dyes in all cases. Anionicyes (AO7, NC, and IC) are generally more degraded than cationic

dyes (MB and RhB). The activity variation across the table showsthat the dye test method is highly specific to the kind of dyes andphotocatalysts. The most active photocatalyst is different for eachdye. For example, the most active photocatalyst is C–TiO2 for AO7,but C–TiO2 and fullerol-TiO2 for NC and IC, respectively. Similarly,the most reactive (degradable) dye is different for each photocat-alyst. Therefore, each photocatalyst is the most active at least forone dye except for BaBiO3 and Bi2WO6 which are generally muchlower in the activity because their surface area is too low compared

Wavelength (nm)

Fig. 2. (a) Diffuse reflectance UV/visible spectra of six visible-light active photocat-alysts. (b) UV/visible absorption spectra of five dyes employed as the test substrate([Dye] = 10 �M; pH = 3).

24

S. Bae

et al.

/ Catalysis

Today 224

(2014) 21–28

Table 3Properties of tested dyes.

Name of dye Molecular formula/weight Classification Purity (dye content %) Structure �max (nm) (εmax, M-1 cm-1) Ref.

Acid orange 7(AO7)

C16H11N2NaO4S 350.3 Azo-anionic 87 485 (2500) 15, 19–21, 30, 32

New coccine(NC)

C20H14N2O10S3 604.5 Azo-anionic 75 507 (3000) 12

Indigo carmine(IC)

C16H8N2Na2O8S2 466.3 Indigo-anionic 93 610 (2100) 27

Methylene blue(MB)

C16H18ClN3S 373.9 Phenothiazine-cationic 89 663 (9100) 16–18, 22–24, 26, 28, 29, 31, 33, 37

Rhodamine B(RhB)

C28H11N2NaO4S 479.0 Xanthene-zwitter ionic 80 554 (13,500) 25, 34–36, 38, 39

S. Bae et al. / Catalysis Today 224 (2014) 21–28 25

Table 4Visible light activities of seven photocatalysts measured with various dyesa .

Dye No catalyst (photolysis control) N–TiO2 C60(OH)x/TiO2 C–TiO2 Pt/WO3 BaBiO3 Bi2WO6 TiO2 (P25)

AO7 3(±2) 84(±2) 80(±2) 97(±6) 49(±3) 21(±1) 21(±3) 60(±1)AO7 (pH 9) 4(±6) 18(±4) 47(±4) 94(±4) 26(±2) 4(±0) 5(±4) 20(±4)NC 2(±3) 29(±3) 32(±4) 54(±6) 18(±3) 4(±1) 1(±1) 10(±1)IC 2(±1) 46(±2) 99(±1) 89(±3) 87(±3) 36(±5) 22(±2) 64(±6)MB 3(±3) 4(±1) 13(±2) 21(±0) 29(±1) 6(±2) 2(±0) 4(±1)MB (pH 9) 6(±3) 5(±2) 23(±5) 32(±1) 32(±4) 27(±4) 25(±7) 11(±1)RhB 6(±1) 19(±0) 47(±1) 39(±1) 33(±5) 5(±3) 6(±3) 25(±2)RhB (pH 9) 2(±2) 19(±1) 31(±5) 36(±2) 35(±1) 4(±1) 4(±1) 8(±0)

after

r ye are

patcfs[

atmTzswmbdp(tndfippto

3d

eppcanoTascaptdbtl

tization pathway on the surface of semiconductor as long as theexcited dye has the energy level higher than the conduction bandedge. This pathway does not require the excitation of the bandgap and enables the visible light-induced degradation of dyes on

BET s urf ace area (m2/g)

0 50 100 150 200 250 300

[ D]/[

D] 0 x

100

(%)

20

40

60

80

100

N-TiO2C60 (OH)X/TiO2

Pt/WO3

C-TiO2

(a) AO7

TiO2 (P25)

BaBiO3

Bi2WO 6

2

0 50 100 150 200 250 300

[ D]/[

D] 0 x

100

(%)

0

10

20

30

40

C-TiO2

N-TiO2

C60 (OH)X/TiO2

TiO2 (P25)

Pt/WO3

(b) MB

BaBiO3

Bi2WO 6

a The listed numbers represent percentage (%) of dye removal (100х�[D]/[D]0

epresents the concentration of dye). The most active photocatalysts for a specific d

hotocatalyst that is assessed with a specific dye cannot be gener-lized to other dyes. This also clearly confirms that the single dyeest cannot represent the overall activity of a visible active photo-atalyst. This general behavior is very similar to what we reportedor the substrate-specific photocatalytic activities that were mea-ured for various commercial TiO2 samples under UV irradiation50].

The effect of pH is important in controlling the photocatalyticctivity because pH controls the surface charge and the adsorp-ion of charged substrates. Since most commercial dyes are charged

olecules, the pH effect should be important in the dye test as well.he catalyst surface takes positive charge below pHpzc (point ofero charge) and negative charge above pHpzc. Therefore, anionicubstrates are more adsorbed and degraded faster at pH < pHpzc

hereas the adsorption and degradation of cationic substrates areuch favored at pH > pHpzc because of the electrostatic interaction

etween the substrate and catalyst surface. The pH effects on theegradation of charged dye substrates were investigated by com-aring the degradation of AO7 (anionic), MB (cationic), and RhBzwitter ionic) between pH 3 and pH 9. For most photocatalystsested in this study, the surface charge is positive at pH 3 andegative at pH 9. As a result of the electrostatic interaction, theegradation of anionic AO7 is clearly favored at pH 3 than at pH 9or all tested catalysts (see Table 4). On the other hand, cationic MBs degraded faster at pH 9. The zwitter ionic RhB does not show clearreference to acidic or basic condition. Therefore, the most activehotocatalyst for a given dye is also dependent on pH. For example,he best photocatalyst for AO7 is N–TiO2 at pH 3 but N–TiO2 is onef the least active photocatalysts at pH 9.

.2. Correlation between surface area and visible light activity forye degradation

The specific surface area (SSA) is one of the most critical param-ters in determining the catalytic activity. SSA also influences thehotocatalytic activities but the relationship between SSA and thehotocatalytic activity is weak in many case [50]. Fig. 3 shows theorrelation between SSA and the dye degradation activity for AO7nd MB (at pH 3). Although AO7 shows some correlation, MB doeso correlation between SSA and the activity. As for the degradationf MB, Pt/WO3 shows the highest activity despite its lowest SSA.his should be ascribed to the negative surface charge of Pt/WO3t pH 3 (pHpzc ∼2) whereas other photocatalysts have the positiveurface charge at pH 3. The cationic MB is attracted electrostati-ally onto the negatively charged surface of Pt/WO3, which shouldccelerate the degradation of MB. As a result, Pt/WO3 is the besthotocatalyst for the degradation of MB at both pH 3 and 9. Onhe other hand, Pt/WO3 is the least active photocatalyst for the

egradation of AO7 (at pH 3) because of the electrostatic repulsionetween the anionic dye and the negative surface charge. Inciden-ally, BaBiO3 and Bi2WO6 that have extremely low SSA show muchower activity than other photocatalysts: in most cases they are

2 h irradiation of visible light (� > 420 nm) at pH 3 (or pH 9 when indicated); [D] indicated in bold numbers.

even less active than pure TiO2 (P25) that does not absorb anyvisible light.

3.3. Dye sensitization effect in photocatalysis

It is well known that dyes can be degraded through the sensi-

BET s urf ace area (m /g)

Fig. 3. The correlation between the dye degradation activity for (a) AO7 and (b)MB and the BET surface area of photocatalysts under visible light illumination.([Catalyst] = 0.5 g/L; [Dye]0 = 100 �M; pHi = 3; � > 420 nm).

2 s Today 224 (2014) 21–28

wbcdiTedtaTescicd

irmsdlauMompasg

3

cecdvttwFvaTlaTpooe(tauohbdrb

[D]/[D]0 x 100 (%)

0 20 40 60 80 100 120

[ TO

C]/[

TOC

] 0 x 1

00 (%

)

0

20

40

60

80

100

120

UV ligh t (2 h)

Visible light (8 h)

N-TiO2

w/o cat.

N-TiO2

C60 (OH)x/TiO2

Bi2WO 6

TiO2 (P25)Bi2WO 6

TiO2 (P25)

w/o cat.

(a) for AO7

Pt/WO3

BaBiO3

BaBiO3

C60 (OH)x/TiO2

[D]/[D]0 x 100 (%)

20 40 60 80 100 120

[ TO

C]/[

TOC

] 0 x 1

00 (%

)

0

10

20

30

40

50

60

for N- TiO2

for Pt/ WO3

AO7

NCMB

RhB

IC

(b)

AO7NC

MB

IC

RhB

Fig. 4. (a) The correlation between the decolorization of AO7 (color removal) andthe mineralization (TOC removal) with various visible light photocatalysts underUV (� > 300 nm) irradiation for 2 h and visible light (� > 420 nm) irradiation for 8 h.(b) The correlation between the decolorization of test dyes and the mineraliza-tion with N–TiO2 and Pt/WO3 under visible light (� > 420 nm) irradiation for 8 h([Catalyst] = 0.5 g/L; [Dye] = 100 �M; pH = 3).

6 S. Bae et al. / Catalysi

ide bandgap semiconductors like TiO2 of which bandgap cannote excited under visible light. On the visible light-irradiated photo-atalysts, the discoloration of dyes can be contributed by both theye sensitization and the bandgap excited photocatalysis. Assess-

ng the contribution from each path is not an easy task. As shown inable 4, the pure TiO2 (P25) was also tested as a control catalyst tostimate the sensitization effect under visible light irradiation. Allyes could be degraded on pure TiO2 under visible light althoughhe degradation efficiency highly varies from dye to dye. AO7, IC,nd RhB were significantly degraded while MB was little degraded.his is ascribed to the weak thermodynamic driving force for thelectron injection from excited MB to TiO2 CB as well as the electro-tatic repulsion between the cationic dye (MB) and the positivelyharged TiO2 surface under acidic condition [51]. Since MB exhib-ted particularly low sensitization effect at both acidic and basiconditions, MB should be the most appropriate among the testedyes in assessing the intrinsic visible light activity of photocatalysts.

The presence of the dye sensitization effect should generate anntrinsic error in evaluating the photocatalytic activity. The coloremoval kinetics, which is related with the destruction of the chro-ophore group, can be further complicated by the concurrent

ide processes such as N-deethylation (in RhB degradation) and N-emethylation (in MB degradation) [52], which is discussed in the

ater part. A previous study claimed that the degradation of MB bybsorption spectrum analysis may cause misunderstanding of gen-ine visible light activity because of the dye sensitization effect.oreover, dyes cannot absorb visible light after the destruction

f the chromophore structure and the resulting degradation inter-ediates may not be further degraded due to the limited oxidative

ower of visible active photocatalyst [53]. On the basis of the aboverguments, it is apparent that the activity assessed through mea-uring the dye decolorization efficiency may not represent theenuine photocatalytic activity.

.4. Dye decolorization versus mineralization

The decolorization of a dye indicates the destruction of thehromophore group, but not necessarily the full degradation (min-ralization). To examine how the degree of decolorization isorrelated with that of mineralization, the photocatalytic degra-ation of AO7 was tested with various photocatalysts under bothisible light and UV light irradiation and compared for not onlyhe color removal but also the TOC removal. Since the mineraliza-ion was much less under visible light, the visible light irradiationas extended to 8 h whereas UV light was irradiated for 2 h.

ig. 4 summarizes the results. The extent of mineralization underisible light is generally much lower than that under UV irradi-tion despite the longer irradiation time of visible light (Fig. 4a).his is mainly because the visible light photocatalysis generatesess energetic electrons and holes and consequently the photocat-lytic reaction is thermodynamically and kinetically limited [14].he band edge positions in semiconductors determine the redoxotentials of electrons in CB and holes in VB. Although the decol-rization of dyes can be achieved by the selective transformationf chromophoric groups by less energetic electrons/holes, the min-ralization of dyes requires the holes with highly positive potentialor OH radicals) which are generated preferably under UV irradia-ion. As for TiO2-based visible light photocatalysts (N–TiO2, C–TiO2,nd fullerol-TiO2), the intrinsic bandgap (∼3 eV) cannot be excitednder visible light while the visible light activation proceeds with-ut generating the VB holes (or OH radicals). The less energeticoles generated in the sub-bandgap states under visible light can

e effective in destructing chromophoric groups but further degra-ation to CO2 (i.e., mineralization) cannot be achieved. The coloremoval efficiencies of the titania-based catalysts are all similaretween the visible and UV irradiation conditions but the TOC

0 i

removal efficiencies are much higher under UV irradiation. On theother hand, the TOC removal activities of narrow-bandgap semi-conductors (BaBiO3, and Bi2WO6) are negligibly small under notonly visible light but also UV irradiation although, the decoloriza-tion efficiencies vary depending on the kind of semiconductors. OnUV-irradiated semiconductors with narrow bandgaps, the hot elec-trons and holes are generated but rapidly thermalized to the bandedge energy, which wastes the excess photon energy. Therefore, theactivity difference between visible and UV irradiation conditions isnot significant for narrow-bandgap semiconductors.

In a similar way to Fig. 4a, the photocatalytic degradation ofdifferent dyes was tested with N–TiO2 under visible light irradi-ation to compare the color removal with the TOC removal. Thenear complete decolorization of RhB and IC under visible light wasaccompanied by less degree of TOC removal. NC and MB were alsohardly mineralized. N–TiO2 exhibited the highest mineralization

efficiency for AO7 but it did not exceed 50% even at the point of nearcomplete decolorization. Since N–TiO2 is unable to generate OHradicals under visible light illumination, it has restricted oxidationpower [54]. In general, the level of photocatalytic mineralization

S. Bae et al. / Catalysis Today 224 (2014) 21–28 27

[Dye]0 (uM)0 50 100 150 200 250 300

Initi

al R

ate

(um

ol /

L m

in)

0.0

0.2

0.4

0.6

0.8

AO7IC

Fig. 5. Effect of the initial concentration of dye on its decolorization rate in the visiblel

oaoTcs

3

atcpeacmttwtcattadrOhleest

3

oab

ight-irradiated suspension of N–TiO2. ([Catalyst] = 0.5 g/L; pHi = 3; � > 420 nm).

f dye is affected by not only the oxidizing power of the photocat-lyst, but also the dye properties (e.g., molecular charge, the kindf functional groups, the kind of dye degradation intermediates).herefore, the photocatalytic activity measured with a specific dyeannot be generalized to different dyes as the data in Table 4 clearlyhow.

.5. Dye concentration effect

The degradation rates of dyes depend on their concentrationnd this dye concentration effect can be also complex. The ini-ial degradation rate of dye may increase with increasing the dyeoncentration because the concentration of dyes adsorbed on thehotocatalyst surface is enhanced at higher concentration. How-ver, when both the substrate (dye) and the photocatalyst canbsorb the visible light in the same wavelength region, the photo-atalytic degradation kinetics may be retarded because excess dyeolecules in the solution absorb visible light photons and attenuate

he light flux incident onto the catalyst surface [55]. To investigatehis effect, AO7 and N–TiO2, both of which absorb mainly in theavelength region of 400–500 nm (see Fig. 2), were selected and

he dye degradation rate was compared with varying the dye con-entration (10–300 �M). The degradation of IC with N–TiO2 waslso compared as a control case since IC has much lower absorp-ion spectral overlap with N–TiO2 (see Fig. 2). Fig. 5 shows thathe dye degradation rates as a function of the dye concentrationre very different between AO7 and IC. Although AO7 has a highegree of absorption spectral overlap with N–TiO2, its degradationate monotonously increases with increasing [dye] up to 300 �M.n the other hand, the degradation rate of IC was retarded atigher concentration although IC has much lower spectral over-

ap with N–TiO2 than AO7. The results imply that the light shieldingffect by dye is not straightforward. The dye-induced light shieldingffect might be counterbalanced by the dye sensitization effect thathould be higher at higher dye concentrations, which complicateshe practice of the dye test.

.6. Formation of intermediates and absorption peak shift

Most tests of photocatalytic decolorization of dyes are carriedut by monitoring the absorbance at a specific wavelength. Thebsorbance is directly converted to the dye concentration on theasis of Beer’s law (A = εbC), which is valid only if dye degradation

Fig. 6. UV/visible absorption spectral change of MO in the presence of (a) C–TiO2

(b) Pt/WO3 ([Catalyst] = 0.5 g/L; [Dye]0 = 100 �M; pHi = 3; � > 420 nm).

intermediates do not have any interfering absorbance in themeasured wavelength. When a degraded intermediate has avisible absorption band which is different from that of the parentdye, the spectral shift is accompanied in the course of the dyedegradation. For example, the hypsochromic shift (blue shift) by N-demethylation of MB and N-deethylation of RhB has been observed,which indicates that N-demethyl/deethylation concurs with thecleavage of RhB chromophore [56,57]. As an example, Fig. 6 com-pares the absorption spectral shift during the visible light-induceddegradation of methyl orange (MO) with C-TiO2 and Pt/WO3. MOhas both amine (-N(CH3)2) and sulfonate (-SO3

-) groups and itsphotocatalytic degradation can be accompanied by the absorptionpeak shift, which is induced by either the demethylation in thedimethylamino group or the hydroxylation of benzene rings inMO [58]. In particular, the generation of demethylated productsinduces a marked blue-shift [58]. The photocatalytic degradationof MO on C–TiO2 exhibited a significant spectral shift (Fig. 6a)whereas the spectral shift with Pt/WO3 is insignificant (Fig. 6b). Itis interesting to note that the degradation of MO on C–TiO2 slightlyincreased the absorbance in the initial stage of the degradation,which implies that colored intermediates are momentarily gen-erated from the photocatalytic degradation of MO [56,58]. Suchintermediates do not seem to be generated with Pt/WO3. Therefore,in the case like MO/C–TiO2 system, the absorbance monitoringat a single wavelength cannot provide accurate data about thedye degradation kinetics. This example clearly demonstrates that

the dye degradation mechanism can be different depending onthe kind of photocatalysts. In such case, the time-profiles of theabsorbance decay monitored at a specific wavelength may notproperly represent the complex photocatalytic activity behaviors.

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. Conclusions

The development of visible light active photocatalysts is beingntensively investigated to achieve higher solar conversion efficien-ies and their activities can be assessed by various methods. Onef the most common methods is the dye decolorization test, whichas been systematically evaluated in this study. The dye test has areat merit for its simplicity and rapidity, which is usually done byonitoring the dye absorbance at a specific wavelength as a func-

ion of irradiation time. However, despite the strong merit of theacile measurement, it suffers from many problems which makehe dye test unsuitable as a standard method to assess the visi-le light photocatalytic activity. The activities determined by theye test depend on not only the intrinsic photocatalytic activityut also other parameters/phenomena that are related with vari-us interactions between the dye and the photocatalyst such as thebsorption spectral overlap in the visible region, the electrostaticnteraction (attractive or repulsive), the dye sensitization of cata-yst particles, and the properties of dye degradation intermediates.herefore, the practice of the dye test for the activity assessment ofew photocatalytic materials should not be recommended (when-ver possible although it is convenient and useful to some extent)nd the activity results obtained for a specific combination of a dyend a photocatalyst should not be generalized.

The visible light photocatalytic activities should be measuredith test substrates that do not absorb visible light. As demon-

trated with the TiO2/UV system [50], the photocatalytic activitiesre highly substrate-specific and the test results can be veryifferent depending on the choice of the test substrates. It isecommended that several substrates are selected from differentlasses of compounds. In our previous study [50], we proposedhat the photocatalytic activity assessment be carried out usinghe following four substrates: phenol, dichloroacetate, tetramethy-ammonium, and trichloroethylene to take the substrate-specificitynto account. They are the aromatic, anionic, cationic, and chlorohy-rocarbon compounds, respectively, which are markedly different

n their molecular properties and structure. They are also proposeds test substrates for visible light activity assessment. Nevertheless,hen the dye test is the only option available for the activity assess-ent, a test dye that has a minimal absorption spectral overlap with

he visible light photocatalyst and/or a minimal dye sensitizationffect (e.g., MB) should be selected and the TOC removal should beeasured as well. In addition, it is always better to use multiple

yes instead of a single dye.

cknowledgements

This work was supported by the Green City Technologylagship Program funded by KIST (KIST-2012-2E23322), thelobal Frontier R&D Program on Center for Multiscale Energyystem (2011-0031571), and the KOSEF EPB center (No. 2008-061892).

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