ChineseJournalofCatalysis35(2014)1781–1792
催化学报2014年第35卷第11期|www.chxb.cn available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc Review Recentdevelopmentsinphotocatalyticdyedegradationupon
irradiationwithenergy‐efficientlightemittingdiodes
Wan‐KuenJoa,RajeshJ.Tayadea,b,*
DepartmentofEnvironmentalEngineering,KyungpookNationalUniversity,Daegu702‐701,Korea
DisciplineofInorganicMaterialsandCatalysis,CSIR‐CentralSaltandMarineChemicalsResearchInstitute,CouncilofScientificandIndustrialResearch,
GBMarg,Bhavnagar‐364002,Gujarat,India
a
b
A R T I C L E I N F O
A B S T R A C T
Articlehistory:
Received15June2014
Accepted24July2014
Published20November2014
Keywords:
Lightemittingdiodes
Photocatalysis
Degradation
Dyes
Wastewater
Light emitting diodes (LEDs) are gaining recognition as a convenient and energy‐efficient light
source for photocatalytic application. This review focuses on recent progress in the research and
developmentofthedegradationofdyesinwaterunderLEDlightirradiationandprovidesabrief
overviewofphotocatalysis,detailsoftheLEDscommonlyemployed,adiscussionoftheadvantages
ofLEDsovertraditionalultravioletsourcesandtheirapplicationtophotocatalyticdyedegradation.
Wealsodiscusstheexperimentalconditionsused,thereportedmechanismsofdyedegradationand
thevariousphotocatalyticreactordesignsandpayattentiontothedifferenttypesofLEDsused,and
their power consumption. Based on a literature survey, the feasibility, benefits, limitations, and
futureprospectsofLEDsforuseinphotocatalyticdyedegradationarediscussedindetail.
©2014,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
Anurgentneedexists for solutionstocurrent water pollu‐
tionproblems.Therecentrapidgrowthoftheindustrialsector
hasledtoenvironmentalproblemsandtohighlevelsofpollu‐
tionworldwide.Additionally,thereisanincreaseindemandfor
water in the industrial, agricultural, and domestic sectors,
which generate large amounts of polluted wastewater. The
generalclassesofcompoundsthatoccurincontaminatedwater
aresolvents,dyes,dioxins,dibenzofurans,pesticides,polychlo‐
rinatedbiphenyls(PCBs),chlorophenols,asbestos,arsenic,and
heavymetals[1,2].Amongthese,dyesareaseriouscontributor
topollution.Dyesareoftendifficulttodecomposeinwateras
they have composite molecular structures that cause them to
be more stable toward light and resistant to biodegradation
[3,4]. A considerable amount of dye‐containing wastewater is
generated in industries such as fabrics, leather, paper, food,
cosmetics, agricultural research, pharmaceuticals, electroplat‐
ing,anddistillation.Thiscausesdamagetotheenvironmentas
dyes are toxic to aquatic life [5,6]. Additionally, wastewater
fromthedyeindustrygenerallycontainsresidualdyestuff,in‐
termediarydyes,andnon‐reactedrawmaterialssuchasodor‐
ousamines,inorganicandorganometallicsalts,andwastesol‐
vents. These are found in different quantities and concentra‐
tionsandarefromdifferentstagesofthedyeproductionpro‐
cess. Dye wastewater is generally considered to have an ele‐
vatedchemicaloxygendemand(COD)becauseofthepresence
oforganiccompounds, ahigh inorganic andorganicdissolved
substance content, inconsistent pH, and low degradability by
biologicalreagents. Duringthe reductionof dyes andtheirin‐
termediates the creation of strong carcinogenic or mutagenic
compoundscanoccur,whichhas a detrimentalimpactonmi‐
croorganismsandaquaticlife[6].Humanconsumptionofwa‐
tercontaminatedwiththesecompoundscancauseavarietyof
*Correspondingauthor.Tel:+91‐278‐2567760.Ext.7180;Fax:+91‐278‐2567562;E‐mail:[email protected]
DOI:10.1016/S1872‐2067(14)60205‐9|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.35,No.11,November2014 1782
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
adverse health effects such as wide‐ranging immune suppres‐
sion,breathingproblems,centralnervoussystem(CNS)disor‐
ders, behavioral problems, allergic reactions, tissue necrosis,
andinfectionsoftheskinandeyes[7]. Dye molecules usually consist of two main components:
chromophoresandauxochromes.Thechromophoreabsorbsa
certain wavelength of light to produce the color. The auxo‐
chromesupplementsthechromophoreandhelpsthemolecule
dissolveinwater,thusenhancingitscolor.Dyesshowconsid‐
erable structural variety and are classified by their chemical
compositionandthefabrictypetheyareappliedto.Dyesmay
alsobeclassifiedonthebasisoftheirsolubilityinvarioussol‐
vents.Theseincludeacidic,basic,direct,mordant,reactiveand
metalcomplexbaseddyes.Insolubledyesincludevarioustypes
ofazoic,sulfur,vatanddispersedyes.Furthermore,dyesmay
also be characterized based on the presence and type of azo
and/or anthraquinone unit. More than 100000 commercial
dyesarecurrentlyavailableonthemarketandthroughoutthe
worldmorethan7×105tonsofdyestuffareproducedannually
[8].Itisestimatedthat10%–15%islostinwastewaterduring
manufacturing and application processes. This constitutes a
huge environmental problem as these dyes are resistant to
removal by irradiation with light or washing with water (or
otherchemicals)becauseoftheirrobustchemicalcomposition
[9]. The primary methods of water treatment such as coagula‐
tion, flocculation, filtration, electro‐flocculation, reverse osmo‐
sis, and adsorption do not degrade pollutants but instead de‐
creasetheirlevelsbyconvertingthepollutantsfromoneform
toanother,therebycreatingsecondarypollution[10].Because
ofthenon‐biodegradabilityandhighsolubilityofdyesinwater,
activatedsludgeprocesseshavebeenfoundtobeineffectivefor
dyeremoval,anddyesareresistanttoaerobictreatment.Ithas
also been reported that the production of carcinogenic com‐
poundssuchasaromaticaminescanoccurduringtheanaero‐
bic treatment of dyes [11]. An alternative method used to de‐
gradedyesinwastewaterisoxidation[12].Theoxidationpro‐
cess uses oxidants such as molecular oxygen, ozone, or H2O2.
However,alimitationofthisprocessisthepooroxidationpo‐
tentials of the oxidants and thus long treatment time is re‐
quired. Therefore, there is a need to discover new materials
withhigheroxidationpotentialstotreatdyewastewater[13].
The “advanced oxidation” process can also be used in which
hydroxyl radical species are generated to degrade the dyes in
wastewater [14]. This technique requires a high energy light
irradiationsource(usuallyanultraviolet(UV)lightsource)and
anoxidanttogeneratehydroxylradicals.Typicalsystemsem‐
ployed to date are UV/hydrogen peroxide, UV/ozone,
UV/Fenton reagent and UV/TiO2 [15]. These photocatalytic
degradationshavebeencarriedoutinthepresenceofnatural
sunlight or a mercury vapor lamp [16,17]. However, ener‐
gy‐efficient light emitting diodes (LEDs) have recently been
used as an alternative light source for the photocatalytic deg‐
radationofvariouspollutantspresentinwater[18,19]andin
air[20,21]. LEDs are emerging as a new irradiation source and many
researchersarestudyingthe photocatalyticactivityofsynthe‐
sizedphotocatalystsunderLEDirradiationandexploringpho‐
tocatalytic reactor designs. To date, only two review articles
havebeenpublishedthatfocusontheuseofLEDsinphotoca‐
talysis and in plant tissue culture [22,23]. This review mainly
focusesonrecentdevelopmentsinphotocatalyticdyedegrada‐
tion and the mechanisms by which it occurs. We also briefly
discuss the basic principles of photocatalysis, the details of
some of the LEDs used, the advantages of LEDs over conven‐
tionalUVlightsources,thedevelopmentofphotocatalyticdeg‐
radationreactorsusingLEDs,andthefutureprospectsofLEDs
inthisfield.
2. Basicprinciplesofphotocatalysis Heterogeneous semiconductor photocatalysis has been
widely explored over the last few decades for various envi‐
ronmentalapplication.Thesestudiestypicallyinvestigatedthe
useofdifferentsourcesoflightirradiationandthenatureofthe
solidsemiconductoronthedegradationofliquidandgas‐phase
pollutants[24].Photocatalysiscanbedefinedasachangeinthe
rateofchemicalreactionsortheirinitiationunderlightinthe
presenceofaphotocatalyst.Photocatalystsareaclassofcom‐
poundthatproduceelectron‐holepairsupontheabsorptionof
lightquantaandtheyinducechemicaltransformationsinreac‐
tion substrates that come into contact with them. They then
undergoregenerationtotheir original electronic composition.
Many semiconductors have been synthesized and studied as
photocatalysts including zinc oxide (ZnO, 3.2 eV), titanium di‐
oxide (TiO2, 3.2 eV), strontium titanate (SrTiO3, 3.4 eV), iron
oxide (Fe2O3,2.2 eV),cadmiumsulfide (CdS, 2.5 eV),tungsten
trioxide (WO3, 2.8 eV), zinc sulfide (ZnS, 3.6 eV), ilmenite
(FeTiO3, 2.8 eV), zirconium dioxide (ZrO2, 5.0 eV), vanadium
oxide(V2O5,2.8eV),niobiumpentoxide(Nb2O5,3.4eV),andtin
oxide(SnO2,3.5eV).Ofthese,TiO2hasbeenfoundmostsuita‐
bleforgeneralenvironmentalremediation[25–27].
TiO2iswidelyusedasaphotocatalystbecauseitisinexpen‐
sive, stable in biological and chemical environments, and is
stable to photocorrosion. TiO2 has a unique property in that
natural(solar)UVlightgenerateselectron‐holepairsforredox
reactions. This is because TiO2 has a suitably sized bandgap
between its valence band (VB, +3.0 eV) and conduction band
(CB,–0.2eV)resultinginabandgapof3.2eVallowingenergy
of near‐UV light with a wavelength greater than 387 nm to
generateelectron‐holepairs.AlthoughZnOhascharacteristics
similartoTiO2andappearstopresentasuitablealternative,it
dissolves in solutions at low pH and cannot be used for the
photocatalyticdegradationofpollutants[28]. Whensufficientlyenergeticphotonsstrikeasemiconductor,
anelectronmaybeexcitedoutofitsenergylevelfromtheval‐
ance band and thus leave a hole. This phenomenon is termed
electron‐hole pair generation. These electron‐hole pairs are
continuously generated in the presence of a constant energy
source. Unselective degradation occurs at the surface of the
photocatalyst via similar and successive redox reactions in
whichtheoxidizedorganiccompoundsaretheendproducts.A
schematicpresentationofthemechanismofthegenerationof
oxidativespeciesfromaphotocatalyticstudyisshowninFig.1. Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
1783
O2
Conduction band (CB)
e-
Band gap
Active radical
Species
(O2● - , ●OH)
hv
Valance band (VB)
O2●
h+
●OH
+
Dyes
Intermediate
products
H2O or OH
CO2 + H2O
Fig.1.Themechanismofphotocatalysis.
The following series of reactions shows the generation of
radicalspeciesupontheactivationofaTiO2semiconductor:
TiO2+hυ→TiO2(e−(CB)+TiO2h+(VB)) (1)
h+(VB)+H2OorOH→H++•OH (2)
e−(CB)+O2→O2•− (3)
O2•−+H+→HO2• (4)
2HO2•+2H+→H2O2+2•OH (5)
2HO•2→O2+H2O2 (6)
H2O2+e−(CB)→OH−+•OH (7)
Theradicalspeciesthataremostactiveduringoxidationare
O2• −, HO2• and •OH. Employing photocatalysis for complete
oxidation(asopposedtopartialoxidation)toprovideanalter‐
native to the conventional removal of pollutants from
wastewaterhasattractedgrowinginterestamongresearchers
[29,30]. 3. HistoryandfundamentalsofLEDs ThefirstinfraredLEDwasinventedandpatentedbyRobert
BiardandGaryPittmanforTexasinstrumentsin1961.In1962,
NickHilonyack invented theredvisibleLEDusing gallium ar‐
senidephosphideasasubstrateforthediode.Thefirstyellow
LED was invented by M. George in 1972. These LEDs were
bright and were mainly used as indicators in equipment. The
commercial production of LEDs began in the 1970s. Further
advances in LED materials technology resulted in the manu‐
factureofunitscapableofahigheramountoflightoutput. Overthepastfewdecades,continuousandrapidtechnolog‐
ical advances have occurred in the development of solid‐state
technology.Theseadvancesincludethedevelopmentoflower
cost and environmentally friendly LEDs. The first LEDs were
only capable of dim red output, and green LEDs were devel‐
opedshortlyafter.However,overthepast15yearsthedevel‐
opmentofsolid‐statetechnologyhasprovidedmorepowerful
LEDsinawidespectrumofcolors.Intheearly1990s,thede‐
velopment of the first blue LED provided an opportunity to
createpracticallyanycoloroflight,makingittechnicallypossi‐
ble to generate white light from semiconductor devices. The
economicmass‐productionofwhiteLEDsisthecurrenttarget
ofresearchersandmanufacturersinthisfield,andthisislikely
to end our current dependence on inefficient incandescent
lamps.
LEDscanemitlightatdifferentwavelengths(infrared,visi‐
ble,orUV)basedonthecompositionandconditionofthesem‐
iconducting materials. LEDs comprise a solid‐state technology
basedentirelyonp‐njunctiondevicesthataredevelopedusing
semiconductorssuchasgalliumarsenide(GaAs),galliumarse‐
nide phosphide (GaAsP), gallium phosphide (GaP), or indium
gallium nitride (InGaN). LEDs are far more efficient than in‐
candescent lamps at converting electricity into visible light
while being robust and small. Furthermore, they can be used
forupto100000h,whichisaround100timeslongerthanin‐
candescentlamps.LEDsprovidemonochromaticlight,andcan
beusedinapplicationwherethereisaneedforhighbrightness
andsingle‐colorirradiation.Recently,LEDshavebeenfoundto
be an alternative for traditional UV irradiation, and they have
beenusedinvariousapplicationincludingUVcuring,disinfec‐
tion,sensorsandphotocatalyticapplication[31–41].
LEDs are considered capable of generating cold light be‐
causeoftheirlowoperatingtemperature.Indeed,5mmLEDs
areusuallyonly10–25°Cwarmerthantheambienttempera‐
tureduringoperationwhereasincandescentbulbscanbesev‐
eral hundred degrees warmer under similar conditions. The
materialusedtomakeaLEDdictatestheenergyofthephotons
that leave the diode. Each wavelength of light has a certain
amount of energy associated with the photons being carried.
ThecloserthewavelengthsaretoUVandshorterwavelengths,
themoreenergytheycontain.Thehighestamountoflightgen‐
eratedbymostLEDsisarounditspeakwavelength.LEDsmay
produce light of different wavelengths including infrared, UV,
andvisiblelight. AbasicLEDconsistsofa“pandn”typesemiconductorjunc‐
tion.Upontheapplicationofpotentialacrossthejunction,cur‐
rent flow injects charge carriers across the junction and the
emission of light takes place. The p‐n junctions in LEDs nor‐
mally consist of a mixture of Group III and Group V elements
likegalliumarsenide,galliumarsenidephosphide,andgallium
phosphide.ThebasicelementofaLEDisasemiconductorin‐
tegrated circuit, which is joined to two electrical wires. This
unit is mounted in a reflector cup supported by a lead frame
anditisfixedinasolidepoxylens. Inthisp‐njunction,thep‐typeregionisdominatedbyposi‐
tive charges and the n‐type region is dominated by negative
charges. After the application of sufficient voltage across the
LED’s electrical contacts, current flows and electrons move
acrossthejunctionfromthen‐typesemiconductorregioninto
1784
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
thep‐typesemiconductorregion.Thenegatively‐chargedelec‐
tronsthencombinewithpositivecharges.Eachcombinationof
chargesatthejunctionisrelatedtoadropinenergylevelanda
quantumofelectromagneticenergyisliberatedintheformof
light. The frequency and, therefore, the apparent color of the
emittedphotonsischaracteristicofthesemiconductormateri‐
al. Consequently, different color LEDs can be obtained by
changingthesemiconductorcompositionofthechipused. 4. LEDsvs.conventionalUVirradiationsources The disadvantages associated with conventional mercury
UVlampsarethattheyaredelicate,containmercury(whichis
hazardous and creates disposal problems after use), have a
shortworkinglife‐spanofonly100–1000h,andtheyareprone
to gas leakage. Additionally, lamp‐blast is possible in medium
andhigh pressurelamps.Furthermore,theoperatingtemper‐
atureofmedium/highpressureUVlampsisintherangeof600
to900°Cand,therefore,theyneedtobecooledduringthere‐
action, which results in increased energy consumption. How‐
ever,LEDs(bothUVandvisible)arerobust,safe,compact,cool,
non‐toxic, inexpensive, environmentally friendly and have a
long life‐span of around 100000 h. Moreover, LEDs can emit
light at different wavelengths based on the composition and
conditionofthesemiconductingmaterials[31]. 5. Application of LEDs to the photocatalytic degradation
ofdyes The first article on the use of UV‐LEDs was published in
2005 by Chen et al. [32], who explored the use of UV‐LEDs
(peakwavelength:375nm;spectrumhalfwidth:12nm;optical
poweroutput: 1mW) as an alternative sourceof light forthe
oxidation of perchloroethylene (PCE). Since then, there have
been a number of application of UV and visible LEDs to the
photocatalytic degradation of aqueous dye pollutants such as
methylene blue (MB), rhodamine B (RhB), malachite green
(MG),reactivered22(RR‐22),methylorange(MO),congored
(CR),andreactiveblack5 (RB‐5).Table1listsdyesthathave
been degraded using different LEDs with their wavelength,
type, power, and the photocatalysts used for the degradation
[31–57]. Fig. 2 shows the chemical structure of the synthetic
Table1
ListofLEDsusedfortheremovalofdyes.
Dye
MB
LEDwavelength
(nm)
255,310,365
LEDtype
LEDsnumberanditspower
Chip
7,14,21(0.031,0.065,0.099
m/Wcm2)
Bulb
One,24W
Bulb
One,17.5W
Chip
6×21(fourside),powernotmen‐
tioned
Bulb
600mW/Cm2
Bulb,d5mm
5,10–12mWforeachLED
LEDstrip
30,6W
Bulb,d5mm
15,10–12mWforeachLED
Bulb
5×20LEDarrays,400μW/cm2
Bulb
250mW
Bulb
5×20LEDarray,6W
Bulb
6×2,notmentioned
MB,PR,MR MB,PR,MR
MB
405
450
376
MB
MB
MB,MO
MB,MG,RhB
MB
MB,MO
MB
MB
365
390–410
400–800
390–410
540
370
450
475
MB,PR,MR
MB
MB
475
410
375
Bulb
Bulb
Bulb
1,3W
5×2,7W
10×10
BR
240
Bulb
6,1W
390–410
385
405
Bulb,d5mm
Blub
Bulb
4,10–12mWforeachLED
6,3W
4×20,notgiven
375
Bulb
15,2mWforeachLED
MO
AO‐7
360
390–395
Bulb,d5mm
Bulb,d5mm
6×6strip,435µW
2×11,3–35W
MB RB‐5
RhB,MO
RhB
RhB
RhB
463
240–355
450–475
390–410
465
443
Bulb
Bulb
BULB
Bulb,d5mm
Bulb
Bulb
1,10W
6,1Weach
3,1W
5,10–12mWforeachLED
1,3W
1,4W
CR
DR‐23
RR‐22
MO
TiO2(P25Degussa)
Dyedegradation
(%)
90
[33]
CdSmicrosphere
CdSmicrosphere
TiO2(P25Degussa)
100
100
91
[34]
[34]
[35]
Catalyst
ZnOandTiO2
TiO2(P25Degussa)and
N‐dopedTiO2
TiO2
TiO2(P25Degussa)
TiO2(P25Degussa)
Ag/AgBrheterostructure
TiO2(P25Degussa)andNickel
modifiedTiO2
CdSnanostructures
AgBr/ZnOcomposite
Grapheneoxideoxidemodified
ZnOnanorods
Peroxydisulfate
(S2O82–)
Titaniumnanotubearray
Nano‐TiO2/S2O82–
DegussaP‐25TiO2coatedquartz
plate
P‐25TiO2andP‐25TiO2coated
glasswafer
DegussaP‐25TiO2
Glucoseoxidase/
TiO2/polyurethane
NitrogenmodifiedTiO2
Peroxydisulfate(S2O82–)
PolymermodifiedZnO
TiO2(P25Degussa)
Bi2MoO6
Sb2O3/WO3
Ref.
60
[36]
100
[31,37]
100
[38]
61,99,62
[39]
91
[35]
100
[40]
95
[41]
Notmentioned
[42]
100
92
98
[43]
[44]
[45]
90
[46]
100
90
95
[55]
[48]
[49]
100
[50]
89
99
[51]
[54]
67
99
100,40
100
99
45
[53]
[54]
[58]
[37,55]
[56]
[57]
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
1785
SO3
HN
HO
N
H 3C
O3S
CH3
N
S
N
CH3
Cl
CH3
OH
N
SO3
N
H
SO3
N
H
Methylene blue
H3 C
Na+O O
S
O
HO
OCH3
N
N
N
SO3Na
N
N
NH2
NaO3SOH2CH2CO2S
H 2N
S
O
Methyl orange
O- Na+
NaO
O
H3C
H3 C
O
CH3 Cl
N
CH3
N
CH3
O
S
N
N=N
O
O
S
NaO
Rhodamine B
O
S
O
O
ONa
S
O
HO
CH3
COOH
Malachite green
O
S
O
O
CH3
Cl
N
N
N
O
Congo red
O O
S
O- Na+
N
N
Reactive red
H 3C
COOH
Methyl red
Phenol red
Methyl blue
N
N
CH3
H 2N
N=N
O
O
S
ONa
O
Reactive black 5
O
Cl
CH3
N
O H
Acid orange 7
SO3H
OH
N N
N
N
N
N
OH
N
N
N
CH3
CH3
O
S
NaO
O
Basic Red 4
N N
O
N
H
N
H
Direct red 23
O
S
ONa
O
N
H
CH3
Fig.2.ChemicalstructureofthesyntheticdyesdegradedbyLEDirradiation.
dyes studied in photocatalytic degradation experiments using
LEDirradiation.
Wang et al. [49] carried out a reactive red 22 dye photo‐
catalyticdegradation studyunderirradiationbyUV‐LEDsand
thisisoneofthedyeslistedinFig.2.Alargenumberofstudies
intothedegradationofdyesusingLEDshavebeenconducted.
Here,wesummarizetheworkconductedonthedegradationof
various dyes under irradiation by UV and visible LEDs. The
propertiesofthedyescoveredinthisreviewaregiveninTable
2.Theproposedgeneralmechanismofphotocatalyticdyedeg‐
radationusingvariousphotocatalystsuponirradiationbyLEDs
isgiveninFig.3.
Wangetal.[49]demonstratedthatUV‐LEDscanbeusedas
aUVlightirradiationsourceforthephotocatalyticmineraliza‐
tionofreactivered22.Specifically,theyinvestigatedthepho‐
tocatalytic degradation of RR‐22 in aqueous solution by
UV‐LEDs with TiO2 as a photocatalyst. They studied this pho‐
tocatalytic degradation using a rectangular planar fixed‐film
reactor in recirculation mode and investigated various condi‐
tions including the initial dye concentration (ranging from 10
to100mg/L),periodicillumination(0.36–0.90mW/cm2),light
intensityvariationintherangeof1.0–7.3mW/cm2,anddiffer‐
ent TiO2 coating arrangements. They concluded that the de‐
compositionofreactivered22inaqueoussolutionsviaaTiO2
photocatalyzed process using UV‐LED irradiation was techni‐
callyfeasibleunderadequateexperimentalconditions. Methylenebluehasoftenbeenusedasamodelpollutantin
water,andmanyresearchershavestudiedthedegradationand
decolorization of methyleneblue dyetodetermine the photo‐
catalytic activity of standard and synthesized materials under
Table2
Propertiesofselecteddyes.
Sr.No.
1
2
3
4
5
6
7
8 9
10
Dyename
Molecularmass(amu) Chemicalformula
Methyleneblue
319.86
C16H18ClN3S
Malachitegreen
927.02
C52H54N4O12
Phenolred
354.38
C19H14O5S
Methylorange
327.32
C14H14N3NaO3S
Reactivered22
590.51
C19H16N2Na2O11S3
Congored
696.665
C32H22N6Na2O6S2
Reactiveblack5
991.82
C26H21N5Na4O19S6
Basicred46
357.5
C18N6H21N6
Acidorgange7
350.3
C16H11N2NaSO4
DirectRed23
813.72
C35H25N7Na2O10S2
Class
thiazin
triarylmethane
triphenylmetha‐ne
azo
azo
azo
azo
monoAzo
monoAzo
doubleAzo
Absorption,λmax(nm)
644
628
558
462
511
500
597
531
307–313
507
Solubilityinwater
soluble
verysoluble
soluble
solubleinhotwater
soluble
soluble
soluble
soluble
soluble
soluble
1786
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
damineB.Inthisset‐up,thephotocatalyst(P25Degussa)was
suspendedinaparticulardyesolutionandthenirradiatedwith
UV‐LEDs. They also investigated the effects of operational pa‐
rameters such as the mass of the photocatalyst, the dye con‐
centration, the pH, and the addition of hydrogen peroxide on
the decolorization and mineralization of methylene blue to
identify the most favorable conditions for remediation. Fur‐
thermore, they demonstrated the possibility of the complete
degradation of methylene blue dye (3.12×10–5 mol/L) and
RhodamineBdye(2.08×10−5mol/L)basedonachemicaloxy‐
gendemand(COD)analysisandreportedthattheUV‐LEDand
TiO2 process could successfully degrade the methylene blue
dyeunderoptimalconditions.Additionally,theyreportedthat
the optimum conditions for Rhodamine B dye photocatalytic
degradationintermsofcatalystcontent,dyeconcentrationand
pHwere1.6g/L,6.26×10–5mol/L,and3.05,respectively,and
in the case of methylene blue they were 1.2 g/L, 3.12×10–5
mol/L,and8.84,respectively.Theyalsoproposedamechanism
forthephotocatalyticdecompositionofRhodamineBthatwas
in agreement with that of the photocatalytic degradation of
Rhodamine B dye using conventional UV sources (Scheme 1).
O2
O2 ‐
e‐ e‐ e‐
Photocatalysts
h + h+ h+
HO‐
H2O or OH‐
LED light
Dye + Photocatalysts
Degraded products
Fig.3.GeneralmechanismofdyedegradationuponLEDirradiation.
UV‐LEDirradiation[31,33–45].
In 2009, Tayade et al. [31,37,47] demonstrated a simple
photocatalyticsetupusingfiveUV‐LEDsforthephotocatalytic
decompositionandmineralizationofmethyleneblueandRho‐
(C2H5)2N
O
N(C2H5)2
COOH
m/z = 443
O
C2H5HN
O
(C2H5)2N
N(C2H5)2
COOH
COOH
m/z = 415
C2H5HN
NHC2H5 H2N
O
m/z = 415
O
N(C2H5)2
m/z = 387
O
(C2H5)2N
O
NH2
m/z = 387
NH2
H2N
O
m/z = 387
NHC2H5
COOH
m/z = 359
C2H5HN
O
H
N
H m/z = 282
O
O
N
N
COOH
H
m/z = 282
COOH
m/z = 359
m/z = 359
O
H
NH2
COOH
H2 N
N
COOH
COOH
COOH
COOH
C2H5HN
NHC2H5
HN
O
NH
COOH
NH2
m/z = 268
COOH
m/z = 331
H
H2N
O
O
N
COOH
NH2
O
m/z = 254
COOH
m/z = 258
O
NH2
m/z = 316
O
m/z = 244
m/z = 230
Cleavage of Chromophore
O
O
OH
m/z = 122
O
OH
OH
O
m/z = 166
OH
OH
O
HO
OH
m/z = 155
OH
O
HO
m/z = 146
O
O
m/z = 132
O
OH
OH
HO
OH
OH
OH
OH
OH
m/z = 92
m/z = 90
m/z = 90
Mineralization
CO2 + H2O + NO3- + NH4+
Scheme1.MechanismofthephotocatalyticmineralizationofRhodamineB.
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
Specifically, they reported that the mineralization of Rhoda‐
mineBoccursviaaN‐de‐ethylationprocesswiththeN‐de‐eth‐
ylated product being oxidized to adipic acid, 2‐hydroxypropa‐
noic acid, propane‐1,2,3‐triol and butane‐1,3‐diol. The formed
acidsarethenmineralizedtoCO2,H2O,NO3–,andNH4+[31,47].
Similarly,Joonwichienetal.[36]studiedthephotocatalytic
degradation of methylene blue upon UV‐LED exposure using
ZnO and TiO2 particles with a magnetic field surrounding the
reactor.ThebandgapsfortheZnOandTiO2particleswere3.37
and3.2eV,respectively.TheyirradiatedamixtureofMBsolu‐
tionandaphotocatalystthatwascontainedinasampleholder
composed of quartz using a UV‐LED lamp (OMRON, ZUV‐L8V,
Kyoto, Japan) at a wavelength of 365 nm and an intensity of
600mW/cm2.Inthissystem,lightwasprovidedfromthebot‐
tom of the sample holder at a distance of 10 mm, and a mag‐
netic field was applied around the sample holder. They con‐
cludedthatthephotocatalyticdegradationofthedyedepended
onoperationalparameterssuchastheinitialconcentrationof
thedye,thesettlingduration,thetypeofphotocatalyst,andthe
intensityoftheappliedmagneticfield[36]. Saccoetal.[38]usedwhiteandblueLEDsasvisiblelightir‐
radiation sources for the photocatalytic degradation of meth‐
ylene blue and methyl orange in aqueous solution. To accom‐
plishthis,theysynthesizedN‐dopedTiO2viathedirecthydrol‐
ysisoftitaniumalkoxidewithammonia.Becauseofvariations
inammoniadoping,thebandgapofthetitaniumdioxide(TiO2)
ranged from 2.5 to 3.3 eV. Their photoreactor was equipped
with a strip of 30 white light LEDs (wavelength of emission:
400–800nm)withanominalpowerofca.6W,orbyasimilar
number of blue light LEDs (wavelength of emission: 400–550
nm)withthesamepowerasthewhitelights.TheLEDstripwas
covered on the outside of the reactor to ensure uniform light
exposureofthereactionmixture.Theirresultsshowedthatthe
highest photocatalytic degradation of methylene blue (7.5
ppm)wasobtainedforphotocatalystswithabandgapof2.6eV.
They also used the same set‐up to mineralize methyl orange
dyetoconfirmthecatalyst’sphotocatalyticefficiency.
Natarajan et al. [39] developed a UV‐LED and TiO2‐coated
quartz tube photocatalytic reactor. This reactor was success‐
fullyusedforthemineralizationofthreedyes:methyleneblue,
malachitegreen,andRhodamineB.Specifically,theycoatedthe
interiorsurfaceofthequartztubewiththeP‐25Degussapho‐
tocatalyst after which the dye solution was passed through it,
asshowninFig.4.ThetotalamountofTiO2depositedonthe
insideofthequartztubewas2mg.Thesetubeswerethenirra‐
diatedby15UV‐LEDs(diameter:5mm,wavelength:390–410
nm, optical rising time: 30 ns). The radiant intensity of each
LEDwas350mcdandtheradiantfluxwasaround12mWat20
mA.
Their study confirmed the photocatalytic mineralization of
thedyesbyUV‐Visspectroscopy,highpressureliquidchroma‐
tography(HPLC)andchemicaloxygendemand(COD)analysis.
Furthermore,theyfoundthatthesuccessofthephotocatalytic
degradationofthedyesdecreasedasfollows:malachitegreen
> Rhodamine B > methylene blue. They also investigated
whetherexperimentalparameterssuchastheflowrateofthe
solutionthroughthecoatedtubereactor,theconcentrationof
1787
Fig. 4. Photocatalytic reactor using a TiO2 coated quartz tube and
UV‐LEDs.
thedye,andthepHinfluencedphotocatalyticdecompositionof
themalachitegreendye.
Based on a LC‐MS analysis of the samples collected at dif‐
ferentintervalsduringthephotocatalyticdegradationofmala‐
chite green (MG) they found that the decomposition of mala‐
chitegreenbytheN‐demethylationprocessleadstothe crea‐
tionofintermediateswithamassvalueofm/z=315,andthat
these N‐demethylated intermediates were further demethyl‐
ated, giving mass values of 301, 287, 273, 259, 229, and 244.
Conversely,theadditionofahydroxylgrouptomalachitegreen
upon reaction with hydroxyl radicals led to the formation of
hydroxylatedintermediateswithm/z=345and361thatwere
demethylatedgivingmassvaluesof329,315,and298.Subse‐
quently, the N‐demethylated intermediates were cleaved and
oxidized by the formed hydroxyl radicals leading to the crea‐
tion of intermediates with m/z values of 229, 214, and 211.
These intermediates underwent further demethylation and
oxidation yielding 2‐(3,4‐dihydroxyphenyl)‐2‐hydroxyacetic
acid, benzaldehyde, benzenamine, nitrobenzene, phenol, and
benzene. The generated oxidized intermediary products were
thenfurtherconvertedintoCO2,NO3−,NH4+,andH2O.Overall,
in this study they found that the photocatalytic surface of the
TiO2 coated quartz tube was reusable and that the UV‐LED
photocatalytic mineralization of malachite green dye (and, by
extension, other dyes) using this TiO2 coated tube system is
economicallyviableandpractical[37,39]. Daietal.[35,41,44]alsousedmethyleneblueasatestcom‐
poundwithaLEDirradiation(wavelength:450nm)setupand
a plasmonic Ag/AgBr heterostructure as the photocatalytic
material. Specifically, they used a 5 × 20 LED array (Wuxi
Chengtian Co., Ltd.) printed on a copper clad laminate as the
light source, and a reaction vessel containing 100 mL of 10
mg/Lmethyleneblueand0.05gofthephotocatalystatroom
temperature. The distance between the LED light source and
thereactorwas1cm,thelightintensitythatreachedthereac‐
tor was 400 μW/cm2, and the reaction mixture was stirred
continuously during the photocatalytic degradation. They
1788
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
foundthatusingthissystem,95%methylenebluedyedecom‐
positionwasobservedover240min[35].
Repo et al.[34]synthesized CdSmicrospheresby ahydro‐
thermal method for the photocatalytic decomposition of dyes
uponexposuretonear‐UVandblueLEDlightirradiation.They
investigatedthedecompositionofmethyleneblue,phenolred,
and methyl red under irradiation by different LEDs (wave‐
lengths: 405 and 450 nm). The power consumption of the
near‐UV LED was 24 W and it was 17.5 W for the blue LED.
Additionally, the radiation efficiency of the catalyst and
dye‐watermixtureswere1.2and3.2Wbasedontheintensity
oftheradiation(39and73mW/cm2)foragapof5cmbetween
the lamp and the solution, and a dye concentration of 3–5
mg/L.Undertheseconditions,acompletedecolorizationofthe
dyeoccurredwithin3h.
Additionally,thephotocatalyticactivityofCdSmicrospheres
with different size was investigated. The size of the micro‐
spheres was varied by applying different amounts of mercap‐
toethylamine hydrochloride (MEA) as a capping agent during
their preparation. The photocatalytic decomposition of meth‐
ylene blue dye in the presence of blue LEDs (3 W) and solar
light[43]wasusedasthemodelsystem.Specifically,thepho‐
tocatalytic activity of the synthesized CdS photocatalysts with
bandgaps of 2.34–3.10 eV was investigated. The smallest
nanostructurehadabandgapof2.74eVandthelargesthada
bandgapof2.34eV.Basedontheseresults,theyconcludedthat
smaller nanostructures have enhanced photocatalytic efficien‐
cybecauseofanincreaseinthebandgapofthenanostructures
and the resulting optimization of the redox potential of the
nanostructures. Under exposure to blue LED light, the CdS
photocatalyst exhibited higher photocatalytic decomposition
activitythanthecommercialTiO2photocatalyst(P25Degussa).
However, upon reusing the catalyst they found that after the
first three cycles a gradual decrease in photocatalytic activity
occurred,andafter5cyclesitwasreducedto50%.
Daietal.[35]developedaUV‐LED/TiO2deviceforthedeg‐
radationofmethyleneblueanditwascomposedofaUV‐LED
with an output wavelength of 376 nm as the UV light source.
Thedeviceusedinthisstudywascomposedoftwoopenboxes
madeofpolymethylmethacrylate(PMMA)withathicknessof1
mm.FourUV‐LEDlightsourceswereputinthesmallboxwith
lightsoneachsideofthebox.Thesmallboxwasinsertedintoa
bigger box, and the reaction medium was introduced into the
space between them. The light source consisted of 6 × 21
UV‐LEDchiparraysprintedonanAlsubstrateanditwaspow‐
ered by an external source. A total of 100 mL wastewater
mixedwithaTiO2suspensionwasplacedintheUV‐LED/TiO2
deviceafterwhichtheUV‐LEDlightpowerwasadjustedusing
a direct current power supply. The properties of this
UV‐LED/TiO2 device were investigated by varying the initial
dye concentrations, the mass of the catalyst, the light power,
andthepH.Theyfoundthatatinitialdyeconcentrationsof5,
10, 20, and 30 mg/L the rate constants were 1.002, 0.406,
0.269, and 0.129 h–1, respectively. They also investigated the
effect of light intensity and found that operating the
UV‐LED/TiO2devicewithalightpowerof193.8µW/cm2was
optimalforcatalyticactivityandenergyefficiency.
Itisalwaysdifficulttorecovernanosizedphotocatalystsaf‐
terphotocatalyticexperiments.Therefore,photocatalyticreac‐
tors have been developed using immobilized photocatalysts
suchasTiO2depositedontosubstrates,ortitaniumnanotubes
grownontitaniummetalplates.Forexample,aphotocatalytic
reactorbasedonaquartzcellfittedwithaTiO2nanotubearray
andcontainingUV‐LEDswasusedforthedegradationofcongo
red [55]. In this reactor, a titanium nanotube array grown on
the surface of a titanium metal plate (30 mm × 10 mm) was
placedatthecenterofthequartzcelland5mlcongoreddye
solutionwasintroducedtothecell.Thequartzcellwasirradi‐
ated using 4 UV‐LEDs (2 LEDs on each of the two sides). The
decompositionofcongoredoccurredonthetitaniumnanotube
arraysurface.Theexperimentsrevealedthattheinitialrateof
degradation of congo red was 0.77×10–7 mol/L. The electrical
energy per order (EE0), for low contaminant concentrations is
definedasthenumberofkWhofelectricalenergyrequiredto
reducetheconcentrationofapollutantbyoneorderofmagni‐
tude(90%)in1m–3pollutedwater.Theconsumptionofelec‐
trical energy (EE0) for the degradation of congo red dye was
228kWhm–3order–1usingaTiO2nanotubearraycalcinedat
450 °C, while the initial rate of degradation under irradiation
byUVlightalonewas0.03×10–7mol/Landtheelectricalener‐
gywas14285kWhm–3order–1.Thedevelopedphotocatalytic
surfacehadgoodreusability,andthedegradationofcongored
wasfoundtodecreasebyonly5%afterfiveuses.Adegradation
pathwaywasproposedbyNatarajanetal.[55]andisgivenin
Scheme2.
AbatchreactorcontainingUV‐LEDsandpotassiumperoxy‐
disulfate(K2S2O8)withaLEDcoolingarraywasinvestigatedfor
thedegradationofreactiveblack5.Theworkingvolumeofthe
reactorwas100mLandtheperoxydisulfateconcentrationwas
varied(10–200mmol/L),aswasthedyeconcentration(5–40
ppm).TheeffectofpHwasalsostudiedfrompH=2topH=10.
Six UV‐LEDs (1 W) were used to carry out the photocatalytic
experiments. In a beaker, the dye solution and K2S2O8 were
stirredundertheLEDsataconstanttemperatureof25°C.The
effects of different UV‐LED exposures and applied currents
werealsoinvestigated.Basedonthepowerconsumptionofthe
system, the electrical energy per order for the degradation of
reactive black 5 was calculated for both traditional UV lamps
andLEDs[54]. A closed water circuit with an integrated real‐time,
in‐streamsensorwasdesignedbyNickelsetal.[50].Thereac‐
torcomprisesareactorvesselwithinletsandoutletsaswellas
a glass chamber (84 mm inner diameter and 70 mm height)
and a base coated with photocatalytic material (Evonik TiO2
AeroxideP25).15UV‐LEDsweremountedonthereactorcover
andthephotocatalystcoatedsurfacewasattachedtothereac‐
torbaseatadistanceof65mmfromtheUV‐LEDs.UltraBright
DeepVioletLED370EUV‐LEDswereusedastheUVlightirra‐
diation source to promote the photocatalytic degradation of
methylorange[50].Theinletsandoutletswere4mmdiameter
glass tubes located 10 mm from the bottom of the reactor,
which was filled with the test liquid in volumes ranging from
100to250mL.Acentrifugalpumpwasusedtomixthesolu‐
tionand circulate waterinthe reactor.The photocatalyticde‐
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
1789
NH2
N
N
O
S
O
S OO
N
O
-O
H2 N
N
m/z = 650
NH2
H2N
N
N
N
N
O
S OO
N
N
m/z = 491
m/z = 387
NH2
NH2
H2 N
N
N
N
N
N
m/z = 443
N
H2 N
N
N
H2 N
N
m/z = 392
H2 N
N
N
H2 N
m/z = 288
NH2
H2 N
N
m/z = 197
m/z = 185
NH2
NH2
OH
NH2
NaO S O
O
m/z = 245
O S ONa
O
m/z = 143
m/z = 211
Cleavage of chromophore
OH
O
O
O
O
NH2
-O
m/z = 139
O+
OH
OH
m/z = 167
O
OH
O
O
m/z = 158
m/z = 93
O
m/z = 108
m/z = 78
m/z = 77
O
Other low molecular
m/z = 102 mass intermediates
Mineralization
CO2 + H2 O + NO3 - + NH4 -
Scheme2.ProposedphotocatalyticdegradationpathwayforcongoreduponUV‐LEDexposure.
composition of methyl orange was investigated by a series of
experiments in which the initial concentration of the dye, the
irradiance,andtheliquidvolumewerevaried.
An annular‐type photocatalytic reactor was developed by
Tokodeetal.[51]toinvestigatethephotocatalyticdecomposi‐
tionofmethylorange(2.5×10–5mol/L)undercontrolledpe‐
riodicilluminationwithUV‐LEDstoenhancethephotoniceffi‐
ciency of photocatalytic decomposition in water. An array of
LEDs (LED sources: FoxUV™, 5 mm diameter with a peak
wavelengthof360nm,aviewingangleof15°,andanaverage
radiantpowerof435µWinstrips)wasusedforirradiationand
theouterchamberwascoatedwithareflectivealuminumfilm
tominimizephotoniclossfromthereactor.Thisstudyshowed
anincreaseinphotonicefficiencywithareductionintheduty
cyclethatdependedonthedutycycletimeratherthanlightor
darktimeperiodssuggestingthatthereactionisnotlimitedby
mass diffusion or by adsorption/desorption to and from the
TiO2 powder surface. Moreover, the photonic efficiency of the
CPI increased with a decrease in power levels under lower
UV‐power densities. For the photoreactor used in this study
(with lower power densities), the photonic efficiency from
controlled periodic illumination followed the same trend as
that under continuous illumination indicating a non‐limited
carrierrecombinationprocess.
Loetscheretal.[40]developedatitania‐acryliccoilreactor
for the degradation of methyl orange and methylene blue. A
lightweight,economic,portablereactorwasfabricatedusinga
coatingofTiO2onaUV‐transparentacrylicsheet.Theydemon‐
1790
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
strated that pollutants including dyes like methyl orange,
methyleneblue,aswellasleadandbacteriacouldberemoved
using this photocatalytic reactor under illumination by LEDs
and compact fluorescent backlights (CFL). Furthermore, they
reported that the photocatalytic efficiency of the reactor was
unchangedfor1100handthattheremovaland/ordegradation
of these pollutants proceeded without change over the time
scaleoftheexperiment.
Thephotocatalyticdecompositionofdyeswasalsoinvesti‐
gatedusingcatalystsotherthantitaniumoxideunderUV‐LED
irradiation. Sun et al. [56] synthesized laminar structured
Bi2MoO6 with a 2.52 eV bandgap energy by the hydrothermal
method.Theythenconductedexperimentsinadispersionme‐
diumusing0.05gofthephotocatalystand50mLRhodamineB
solution(0.1μmol/L)underblueLEDirradiationwithcontin‐
uous magnetic stirring. The results of their experiments sug‐
gestthatelectronsandholesplayanimportantroleinthedeg‐
radation of Rhodamine B with holes playing the most im‐
portantrole.
Rhodamine B dye degradation under UV‐LED irradiation
was also reported in a system employing a polymer modified
ZnO photocatalyst using a column glass reactor [58]. Specifi‐
cally,theyusedthree1WLEDs(radiantwavelength:450–475
nm) located firmly against the reactor spaced 120° from each
other as a visible light source. The photocatalytic reactor was
kept open to air to allow sufficient oxygen to access the reac‐
tion solution. Around 40% phenol and methyl orange decom‐
positionwasachievedusingthisset‐upafter2hofirradiation.
RhBwascompletelydegradedwithinthesameirradiationtime
[58].
6. FutureprospectsofLEDs Recentdevelopmentsinsolid‐statesemiconductortechnol‐
ogyhaveprovidedavarietyofLEDsusefulforgenerallighting
aswellasmorespecificapplicationresultinginincreasedpro‐
ductionand,therefore,decreasedcost.TheadvantagesofLEDs
arethatnocoolingisrequired,theyaresmall,andareusefulin
differenttypes ofphotocatalyticreactorset‐ups.Furthermore,
LED irradiation is unidirectional and thus, an appropriately
designed reactor will not leak UV light and can achieve high
photocatalyticefficiency.Becauseofthisunidirectionalirradia‐
tion the loss of light is lower than that of traditional UV light
lamps. Reactors employing traditional UV irradiation sources
canwasteupto80%ofthelightthattheyproduce.Conversely,
LEDs emit specific wavelengths of light that are completely
used. As there are many advantages over conventional tradi‐
tional UV sources, many researchers have focused on the de‐
velopment of photocatalytic reactors based on LEDs and the
synthesisofphotocatalyststhatcanprovidehighphotocatalytic
activity in these systems. To develop a photocatalytic reactor
with the optimum photocatalytic efficiency, it is necessary to
select appropriate LEDs and design appropriate reactors.
However, this is a challenging task. Additionally, LEDs can be
operated at low voltage, enabling them to be powered using
solar photovoltaicpanels. Studieshave shownthat forcertain
application,replacingmercurylampswithUV‐LEDscanleadto
areductioninCO2productionof25tons.
Overall,theavailableliteratureindicatesthatLEDscanplay
a major role in the future development of photocatalytic pro‐
cessesforenvironmentalpollutionabatement,andcanenable
thedevelopmentofphotocatalyticreactorswithhighthrough‐
put that are environmentally friendly, energy‐efficient, and
compact. UV‐LEDs are also useful for the development of mi‐
croreactors, which can exhibit improved properties such as
higher lighting homogeneity and penetration compared with
large‐scale reactors. However, further progress in the devel‐
opmentofhigh‐powerUV‐LEDsisneededtoproducereliable,
energy‐efficient, and small photocatalytic reactors. Such ad‐
vances will help meet the increasing demand for the imple‐
mentationofcleanenergytechnology.
7. Conclusions Over the last two to three decades, extensive research has
beenconductedintotheuseofphotocatalysisfortheremoval
of dyes from wastewater. These studies have dealt with the
synthesisofnewphotocatalystswithhighersurfaceareas,dif‐
ferent morphology, different phase structure, and varying
composition of metals and non‐metals. These tests have pri‐
marily been conducted using UV lamps or other methods to
promotethephotocatalyticdecompositionofdyesuponexpo‐
sure to solar light. However, the use of traditional UV light
sources is problematic because they require cooling strategy,
havehigh‐energycosts,andcanbeharmful.Systemsemploying
sunlighthavehighinitialinstallationcostsandrequirethereg‐
ular tracking of sunlight. Moreover, the rate of degradation
usingsunlightislowerthanthatforirradiationbyconventional
UVsources.Consequently,researchershavebeeninvestigating
alternative light sources such as LEDs for the photocatalytic
decomposition of various organic compounds and dyes. How‐
ever, to date, few dyes have been photocatalytically degraded
under irradiation by LEDs at low concentrations. Accordingly,
thereisaneedtoinvestigatedifferentdyestoconfirmthepho‐
tocatalytic decomposition of each dye upon exposure to LED
irradiation.BothUVandvisibleLEDsareavailable,andappro‐
priatephotocatalystsmayhelpachievehigherlevelsofphoto‐
catalyticdegradation.Currently,investigationsintoUV‐LEDsas
analternativesourceofUVlightaregenerallyconductedusing
standard P25 Degussa photocatalysts. However, some effort
has been made to modify P25 Degussa, and the synthesis of
other photocatalysts such as CdS and Bi2MoO6 has been con‐
ducted.
Basedontheavailableliterature,itisclearthatanincrease
in the number and power of available LEDs will allow re‐
searcherstoattainbetterphotocatalyticdegradationofdyesin
shorter reaction time. Moreover, the studies conducted have
shownthatvisibleLEDscanalsobeusedforthephotocatalytic
decomposition of various dyes by employing photocatalysts
withabandgapintherangeof2.0–3.1eV. Overall,theavailableliteratureindicatesthefeasibilityofUV
andvisibleLEDsforthephotocatalyticdecolorizationandmin‐
eralization of dyes in water. Moreover, they can be useful for
theremovalofotherpollutantspresentinwaterandair.Inves‐
Wan‐KuenJoetal./ChineseJournalofCatalysis35(2014)1781–1792
tigationsintoLEDilluminationforthedegradationofdyeshave
indicated that they are effective when applied to low concen‐
trationsofdyes,buttheirintensitywillneedtobeincreasedto
enable their application to industrial effluent. Additionally,
theseemergingtechniquesmustbeevaluatedusingavarietyof
pollutants to validate the use of LEDs in the photocatalytic
treatmentofdyecontaminatedwastewater. 2008,47:5847
[17] LazarMA,TayadeRJ,BajajHC,JasraRV.NanoHybids,2012,1:
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
Acknowledgments This study was conducted with the support of the MSIP
(Ministry of Science, ICT & Future Planning, Project No.
132S‐5‐3‐0610) and the National Research Foundation of Ko‐
rea (NRF) funded by the Korean Government (MEST, No.
2011‐0027916).Oneoftheauthors(RJT)alsothanksMr.Siva‐
kumarThillai,Dr.S.ShinandMr.JoonY.Leefortheirhelpwith
theliteraturesearch.
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GraphicalAbstract
Chin.J.Catal.,2014,35:1781–1792 doi:10.1016/S1872‐2067(14)60205‐9
Recentdevelopmentsinphotocatalyticdyedegradationunder irradiationwithenergy‐efficientlightemittingdiodes
Dye solution + Photocatalyst
LED
Irradiation
Wan‐KuenJo,RajeshJ.Tayade*
KyungpookNationalUniversity,Korea;
CSIR‐CentralSaltandMarineChemicalsResearchInstitute,India
This review focuses on photocatalytic dye degradation by energy‐efficient
lightemittingdiodesandsummarizesrecentdevelopmentsinphotocatalyt‐
ic reactors, photocatalysts, and the degradation mechanism of irradiation
uponusingvariouslightemittingdiodes.
Coloured
water
Colourless
water
1792
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