ContentslistsavailableatScienceDirect
Applied
Catalysis
B:
Environmental
jo u r n al ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b
N-doped
carbon
quantum
dots/TiO
2
composite
with
improved
photocatalytic
activity
Natércia
C.T.
Martins
a,
Joana
Ângelo
a,
Ana
Violeta
Girão
b,
Tito
Trindade
b,
Luísa
Andrade
a,
Adélio
Mendes
a,∗aLEPABE,DepartamentodeEngenhariaQuímica,FaculdadedeEngenharia,UniversidadedoPorto,RuaDr.RobertoFrias,4200-465Porto,Portugal bDepartamentodeQuímica-CICECO,UniversidadedeAveiro,CampusdeSantiago,Aveiro,Portugal
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received29January2016
Receivedinrevisedform5April2016 Accepted8April2016
Availableonline11April2016 Keywords:
N-dopedcarbonquantumdots P25
Photocatalyst NOphoto-oxidation Methylenebluedegradation
a
b
s
t
r
a
c
t
AnewcompositeofTiO2 (P25)andN-dopedcarbonquantumdots(P25/NCQD)waspreparedbya hydrothermalmethodandwasusedforthefirsttimeascatalystofthephoto-oxidationofNOunder UVandvisiblelightirradiation.P25/NCQDcompositeexhibitedaNOconversion(27.0%)morethantwo timeshigherofthatobservedforP25(10%)undervisiblelightandtheselectivityoftheprocesswas increasedfrom37.4%to49.3%.ThecompositealsoshowedbetterphotocatalyticperformancethanP25 intheUVregionwithincreasesof36.3%onNOconversionand16.8%onselectivity.Moreover, com-paredwithP25,thephotodegradationratioofmethylenebluewasenhancedfrom68%to91%afterUV irradiationfor1h.NCQDplayedacrucialroleonthephotocatalyticactivityimprovementofP25/NCQD, increasingvisiblelightabsorption,slowingtherecombinationandimprovingthechargetransfer.
©2016PublishedbyElsevierB.V.
1. Introduction
Nitrogenoxides(NOx)arewellknownairpollutantsthatcause
severeenvironmentalandhealthproblems.AlthoughNOx(NOand
NO2)canbeformedduringnaturalprocessessuchaslightning,the
majorityoftheemissionsareproducedbyanthropogenic activi-tiesmainlybycoalpowerplantsandroadtraffic[1,2].Alongwith SO2,NOxisresponsiblefortheformationofacidrainthatcauses
corrosionof buildingsand harmsforests, cropsand aquaticlife [3].HighconcentrationsofNOx intheatmospherecanalso
con-tributetotheformationoftroposphericozoneandphotochemical smogthatcanleadtoseriousrespiratoryproblemssuchas emphy-semasandbronchitis[4,5].Althoughstricterlegislationhasbeen adoptedbytheEuropeanUnionmembers,NOxlevelsinthe
atmo-spherearestillabovethelimitvaluerecommendedbythelatest EUdirective[6],particularlyinurbanareas[7].Ontheotherhand, watercontaminationwithorganicpollutants(e.g.dyes,herbicides, phenoliccompounds)fromindustry,agricultureandotherhuman activitiesarealsoasubjectofseriousconcern[8].Thereleaseof pollutedwastewatersintotheaquaticecosystemrepresentsboth environmentaland publichealthrisksbecauseoftheirnegative ecotoxicologicaleffectsandbioaccumulationinaquaticlife[9].For
∗ Correspondingauthor.
E-mailaddress:[email protected](A.Mendes).
allthereasonsdiscussedabove,thedevelopmentofnewand effi-cientmethodsforNOx andorganicpollutantsremovalfromthe
environmentisofgreatimportance.
Inthelastdecade,photocatalyticoxidationusing semiconduc-torsascatalystshasbecomeoneofthemostpromisingapproaches forremovinginorganic(e.g.NOx)[10,11]andorganic(e.g.dyes)
[12]pollutantsfromgasorliquidphases.Amongsemiconductor photocatalysts,titaniumdioxide(TiO2)hasreceivedconsiderable
attentionduetoitshighphotocatalyticactivity,highchemical sta-bility,low toxicity andlow cost [13].WhenTiO2 is exposed to
sunlight,photonswithenergyequal orhigherthanitsbandgap energyareabsorbedleadingtothepromotionofelectrons(e−)from thevalencetotheconductionbandandtheconsequentformation ofpositivelychargedvacancies(holes,h+)inthevalenceband.Both
speciesmigratetothesurfaceofthesemiconductorwherethey par-ticipateinoxidation(h+)andreduction(e−)reactionsforminghigh
reactiveradicalspecies.Holeshaveapotentialsufficientlypositive tooxidizewatermoleculesandOH−adsorbedontothe semicon-ductorsurfaceproducingOH•radicals,whichwillthenpromotethe oxidationofharmfulpollutantslikeNOxororganicdyes[14].Onthe
otherhand,photoexcitedelectronsreactwithoxygenmoleculesto formsuperoxideanionsO2•−thatparticipateintheredox
reac-tionsinvolvedinthedegradationofthepollutants[3,11].Recently, someauthorsreportedthatholescanalsobetrappedbyTiO2
sur-facelatticeoxygenions,eitherprotonated(>OHs−)ordeprotonated
(>Os2−), togenerate surface latticeradicals(−OHs•/−Os•−)that
http://dx.doi.org/10.1016/j.apcatb.2016.04.016 0926-3373/©2016PublishedbyElsevierB.V.
canthanparticipateinseveralredoxreactionsthatoriginatethe degradationofpollutants[15].
Several materials incorporating photocatalytic TiO2 have
alreadybeenpreparedforairandwaterpurificationapplications, however,onlyveryfewweremadecommerciallyavailable[16–18]. Moreover,theresultsofNOxphotoabatementtests,using
materi-alscontainingTiO2,underrealoutdoorconditionsarestillbehind
theexpectations.ThisismainlyduetothefactthatTiO2hasone
importantdrawback:itswidebandgap(3.2eVforanatase)requires photocatalyticactivation by ultravioletirradiation (about 4% of thesolarspectrum),giving risetovery low solar photoconver-sionefficiencies.Therefore,forpracticaluse,thedevelopmentof photocatalystsactivatedbyvisiblelightisextremelyimportant.
ManystrategieshavebeenadoptedtoextendTiO2
photocat-alyticactivityintothevisibleregion,includingdyesensitization [19,20], noblemetal deposition[21,22], dopingwithmetal and non-metalelements[23,24]and couplingwithnarrowbandgap semiconductors [20,25]. In particular, the modification of TiO2
with carbon nanomaterials is being increasingly investigated. TiO2/carboncompositeshaveshownimprovedopticalabsorbance
inthevisiblespectralrangeandenhancedchargeseparation effi-ciency,whencomparedwithpristineTiO2 [26,27].Severaltypes
of carbon nanomaterials, suchas, carbon nanotubes, fullerenes andgraphene,havebeenusedfortheproductionofTiO2/carbon
compositeswithpromisingphotocatalyticactivitiesinthevisible region[28–30].Recently, a newmemberof carbon family, car-bonquantumdots(CQD), haveattractedconsiderableattention duetotheiruniqueproperties. Liketraditional semiconductors, CQDshowphotoluminescencepropertiesandinadditionpossess theadvantagesoflowtoxicity,watersolubilityandresistanceto photobleaching[31].Asaresult,theyhavebeenwidelyappliedin manyfieldssuchaschemicalsensing,bioimaging,electrocatalysis andphotocatalysis[32].Inlastyears,N-dopingofCQDhasshown greatpotentialforenhancethephotocatalyticactivityofTiO2/CQD
composites.Zhangetal.[33]synthetizedN-dopedcarbonquantum dots(NCQD)andcombineditwithrutileTiO2toformTiO2/NCQD
compositeswhichexhibitedenhancedvisible-lightphotocatalytic activitytowardsthedegradationofrhodamineBwhencompared withN-freeTiO2/CQDcomposites.These authorshavereported
thatN-dopinglowerstheworkfunctionofCQD,whichis proba-blythemainreasonfortheenhancedphotocatalyticactivityofthe TiO2/NCQD.
To the best of our knowledge there are no reports in the literatureconcerningtheuseofTiO2/NCQDcompositesinthe
pho-tocatalyticremovalofNOx.Moreover,studiesoftheoxidationof
NOxusingphotocatalystswithvisiblelightactivityarestillscarce.
InthisworkNCQDwerepreparedbymicrowave-assistedmethod andthenhydrothermallycombinedwithP25TiO2toobtainanew
composite,P25/NCQD.Thecompositewasusedascatalystsofthe photo-oxidationofNO underUVand visiblelight andwasalso testedinthephotodegradationofmethyleneblue(MB)underUV light.Inbothcasesthecompositeshowedhigherphotocatalytic activitythancommercialP25TiO2.Thereasonsfortheenhanced
photocatalyticactivityofP25/NCQDhavebeendiscussedbasedon theexperimentalresultsandonrecentliterature.
2. Materialsandmethods
2.1. Materials
Glycerol (ASC reagent≥99.5%), ammonium phosphate diba-sic(ASC reagent≥98%)and 4,7,10-trioxa-1,13-tridecanediamine (TTDDA,97%)werepurchasedfromAldrichandusedasreceived. Aeroxide®TiO
2P25wasobtainedfromEvonikIndustries.Dialysis
wasdoneusingSpecta/PorFloat-A-LyzerG2dialysistubes(MWCO 500–1000au).
2.2. SynthesisofN-dopedcarbonquantumdots(NCQD)
NCQD were synthetized using an adaptation of themethod reportedbyLuietal.[34].5gofglycerolweremixedwith2mL ofammoniumphosphatesolution(20mM).Then1mLofTTDDA wasaddedundervigorous stirring.Thesolutionwasputinthe ultrasoundsduring3minandthenitwastransferredtoadomestic microwaveovenandheatedduring7minat700W.Aftercooling downtoroomtemperaturetheresultantdarkredsolutionwas dilutedwith20mLofwateranddialyzedagainstpurewaterduring 3days.Uponcompletionofthepurificationprocessbydialysis,the yellowsolutionobtainedwaslyophilizedtocollectNCQD. 2.3. PreparationofP25/NCQDcomposite
P25/NCQDcompositewasproducedbyahydrothermalmethod. 1gofP25wasdispersedinasolutionofdistilledwater(40mL)and ethanol(20mL).Thesuspensionwasstirredforfiveminutesand then2.5mLofanaqueousNCQDsolution(4mg/L)wasadded.The mixturewaskeptstirringfor5handthenwastransferredintoa 125mLTeflon-sealedautoclaveandmaintainedat130◦Cfor4h. Finally,theresultingcompositewasrecoveredbycentrifugation (15minat5000rpm),washedwithdistilledwatertwotimesand driedat60◦Cduring24h.
2.4. Materialscharacterization
Morphological,structuralandanalyticalcharacterizationofthe obtainedNCQDandP25/NCQDcompositewasperformedby Elec-tronMicroscopy. Bright-FieldTransmissionElectronMicroscopy (BF-TEM)and High-Resolution(HR) TEMimageswere obtained usingaJEOL2200FSelectronmicroscopeequippedwithOxford EnergyDispersiveX-rays(EDX)detectorandin-columnOmega fil-ter,operatedat200kV.Adropoftheobtainedsampleswashighly dilutedinultra-purewaterandthenplacedontothesurfaceofan AgarScientificcoppergridwithaholeyamorphouscarbonfilmand lefttodryinair.Afterbeingcompletelydried,allthesampleswere immediatelykeptinavacuumboxtillbeingusedforexamination undertheTEMmicroscope.Observation,acquisitionandimaging analysiswerecarriedoutusingGatanDigitalMicrographSoftware 1.84.1282(GatanInc.©1996–2010).Scanning-TEMEDXmapping
wasperformedusingINCAEnergySoftware(OxfordInstruments Analytical©2006–2011).Fouriertransforminfrared(FTIR)spectra
wererecordedonaBOMEM(modelMB154)spectrophotometer scanning from4000to 500cm−1. Zetapotentialmeasurements wereperformedusingaZetaSizerNanoSeries(Malvern) equip-ment.X-rayphotoelectronspectroscopy(XPS)wasobtainedusing aKRATOSaxisultraHAS-VisionX-rayphotoelectronspectrometer withamonochromaticX-raysource(Al).Thermogravimetric anal-ysis(TGA)curvesofthesampleswereobtainedbyaTG209F1Iris Thermal-microbalance(Netzsch)innitrogenatmosphereusingan inputflowrateof30mLmin−1andaheatingrampof3◦Cmin−1up to600◦C.Diffusereflectancespectraofthepowdersampleswere recordedonaShimatzuUV-3600UV–vis-NIRspectrophotometer, equippedwitha150mmintegratingsphereandusingBaSO4 as
100%reflectancestandard.Thespecificsurfaceareaofthe cata-lystswasmeasuredat77KonaQuantaChromeNova-1200(USA) equipment using Brunauer-Emmett-Teller nitrogen adsorption-desorptionmethod(BET).Beforebeinganalysed,thesampleswere treated at150◦C for2hunder vacuumtoremove any surface-adsorbedspecies.PowderX-raysDiffraction(XRD)patternsofthe preparedsampleswererecordedonaPhillipsEmpyrean diffrac-tometersystemoperatedat45kV/40mAincontinuousmode,with
astepsizeof0.0130andscansteptimeof2698.92s,controlled by thePANalytical X’Pert HighScore software. The basic lattice parameterswereobtainedusingCheckcell,amodifiedversionof Celreffree software(http://www.inpg.fr/LMGP).Electrochemical impedancespectroscopy(EIS)measurementswereperformedwith aZENNIUMworkstation,withafrequencyrangebetween100mHz and 100kHz and the magnitude of the modulation signalwas 10mV.Allmeasurementswereperformedatroomtemperature and open-circuitvoltage.Preparedthin films(activeareaof ca. 0.238cm2)appliedontheFTOcoatedsubstrateswereusedas
work-ingelectrodes,99.9%platinumwirewasusedascounter-electrode (AlfaAesar,Germany)andAg/AgCl/sat.KClwasusedasreference electrode(Metrohm,Switzerland).Themeasurementswere con-ductedusing anaqueoussolutionof H2SO4 0.5Maselectrolyte
(pH=0.8).
2.5. Photocatalyticactivitytests 2.5.1. NOphoto-oxidation
The experimental setup used in NO photo-oxidation tests consistsoffourmainsections:(i)feed,(ii)reactor,(iii)NOx quan-tificationand(iv)computermonitoring/control.Inthefirstsection, thegasstreamwiththedesiredNOconcentration,relative humid-ityand flow rate,is prepared andfed tothephotoreactor. The reactionsectioniscomposedbyaphotoreactor(withaPyrex®
win-dow)designedtoholdthesamples,minimizingdeadandstagnant volumes.Alampwiththedesiredwavelengthisplacedabovethe reactor.NOandNO2concentrationsarequantifiedusinga
chemi-luminescenceanalyser(ThermoElectron42C).Acomputercontrols theexperimentalsetupandacquirestherelevantdata.Thereactor isplacedinsideathermostaticcabintoensurecontrolledand con-stanttemperature.Detailedinformationaboutthesetupusedcan befoundelsewhere[11].
Samplesweretestedaspowderfilms(5×5cm2)obtained
pour-ingthepowderevenlyoveranaluminumslabandthenpressingat 5barfor5min.Thisprocedureyieldedhomogeneouspowderfilms ca.0.5mmthick.Photocatalytictestswereperformedat25◦Cwith afeedrateof0.7Lmin−1ofNOat1ppmvinairand50%ofrelative
humidity.AssourceofUVlightanultravioletlamp(UV-A,highest emissionat365nm)withtwo6Wblack-light-bluebulbs (VL-206-BLB,VilbertLourmat,France)wasemployed.Thevisiblelighttests wereconductedwitha regularfluorescentlamp(PhilipsMaster TL-MiniSuper806W/840)equippedwithafiltertoobtainlightof >400nm.Theirradianceatthecatalystsurface,estimatedwith aradiometer,was10Wm−2and50Wm−2fortheUVandvisible lighttests,respectively.
In theoperating conditionsused in this work,the products resultingfromthephoto-oxidationofNOarenitrite(NO2−),nitrate
(NO3−)andNO2[11].SinceNO2isevenmoreharmfulthanNOto
humanhealthandtotheenvironment,thedesiredreaction prod-uctsaretheionicspecies.Therefore,two parameterswereused toevaluatetheperformanceof thepreparedphotocatalysts,NO conversion(Eq.(1))andselectivitytowardstheformationofionic species(Eq.(2)): Conversion=
Cin NO−CNOout Cin NO ×100 (1) Selectivity= 1− C out NO2 Cin NO−CoutNO ×100 (2)whereCNOandCNO2standfortheconcentrationofNOandNO2,
respectively,andthesuperscripts(inandout)refertothereactor’s inletandoutletstreams.Thephoto-oxidationofNOwasfollowed during85hunderUVradiationand120hundervisibleradiation.
2.5.2. Methylenebluedegradation
Photocatalyticdegradationofmethyleneblue(MB)dyewas per-formedina250mLcylindricalglassvesselcontaining100mLof MBsolution(0.01g/L)and 50mgofthephotocatalyst.The mix-turewasfirstlystirredduring30minin thedarktoensurethe establishmentofanadsorption-desorptionequilibrium.Afterthat, thephotoreactorvesselwasirradiatedwithUVlightproducedby anultravioletlamp(UV-A,highestemissionat365nm)withtwo 6Wblack-light-bluebulbs(VL-206-BLB,VilbertLourmat,France) positioned10cmabovethesurfaceofreactionsolution(irradiance atthesurfaceofthemixtureof10W/m2).Aliquotsof3mLwere
collectedevery30mininaperiodof4h,thenwerecentrifuged andthesupernatantwasanalysedinaUV–vis spectrophotome-ter(ShimatzuUV-3600UV–vis-NIRspectrophotometer)torecord theabsorptionbandmaximum(665nm)ofMBasafunctionofthe time;theconcentrationofMB(C)isproportionaltothemaximum absorbanceat665nm(A).Thereby,theMBchangeofconcentration (C/C0)wasobtainedfromtheabsorbancehistory(A/A0),whereC0
istheconcentrationafteradsorption–desorptionequilibriumand A0isthecorrespondentabsorbance.Allexperimentswererepeated
threetimesandtheaveragecomputed.
3. Resultsanddiscussion
3.1. CharacterizationofN-dopedcarbonquantumdots
AlthoughNCQDwerepreparedusinganadaptationofamethod reportedontheliterature[34],intheconditionsemployedinthis research,particleswithdifferentcharacteristicswereobtained(e.g. sizeandcrystallinity)andnewfeatureswerefound.BF-TEM anal-ysisrevealedthattheaspreparedNCQDpresentaquasi-spherical morphologywithdiameters rangingfrom6to13nmandmean of9.4nm(Fig.1A).HR-TEMimagingofoursamplesdemonstrated thatinthiscasemostoftheNCQDarenanocrystallinewith well-resolvedlatticefringes,whichdiffersfromthefindingsreported byLiuetal.[34].Fig.1BshowsarepresentativeHR-TEMimageof NQCDinwhichtheinterplanardistances2.096and2.029Å cor-respondrespectivelytothe(010)and(011)latticeplanesofthe hexagonalgraphitic carbon(ICDDreferencecode01-075-1621). The powderXRD patternsfor theNCQD samples (Fig. S1, Sup-porting Information) are in good agreement with the HR-TEM resultsshownabove,thusshowingthecrystallinegraphitic car-bonstructure.Althoughthebroadpeakcentredabout20◦(2)may suggestthepresenceofamorphousNCQD,therewasnoevidence forthis typeofcarbonbyHR-TEM.Therefore thebroadeningof suchdiffractionpeakmightberelatedtothenanometersizeofthe particles.Thefewrelativelysharppeaksaround2.9–2.3Åmight beindicativealsoofmicrocrystallitespresentinthecarbon sam-ples.AlthoughnotdetectedbyHR-TEM,thesediffractionpeakscan beattributedtoanothercrystallinehexagonalcarbonphase(ICDD referencecode00-026-1081)thatcouldbebeenformedduringthe NCQDsynthesis[35,36].
The FTIR spectrum of NCQD (Fig. S2, Supporting Informa-tion)displayscharacteristicvibrationalbandsat3380cm−1(O H and N H stretching), 2922and 2868cm−1 (CH2, C H
antisym-metric and symmetric stretching), 1645cm−1 (C O stretch in amideand carboxylic groups),1568cm−1 (N Hdeformation in amides),1455cm−1(OHinplanebendingincarboxylicgroups)and 1118cm−1 (C Ostretch).Mostofthenonpassivated CQD sam-plesdescribedintheliteraturepresentanegativezetapotential duetothepresenceofabundantcarboxylicgroupsontheir sur-face[37,38].TheNCQDpreparedinthisworkshowinopposition apositivezetapotential(13.6±3.37mV,pH=7.6).Thismeansthat conversionofthecarboxylicgroups,thathavebeenformedduring thepyrolysisofglycerol,intoamidegroups,occurredinlarge
exten-Fig.1. (A)BF-TEMimageofNCQDandhistogramfortheparticlesizedistribution;(B)HR-TEMimageofNCQD.
sionafterpassivationwithTTDDA.Thestrongbandat1568cm−1 intheFTIRspectrumconfirmsthishypothesis.Thepositivezeta potentialofNCQDcolloidalsosuggeststhepresenceofprotonated aminegroups onthesurfaceoftheparticleswhendispersed in water(pH<7)[39,40].
ThechemicalcompositionoftheNCQDwasfurther character-izedbyX-rayphotoelectronspectroscopy(XPS).TheXPSsurvey spectrum(Fig.S3,SupportingInformation)showsthattheNCQD aremainlycomposedofcarbon(75.3%),oxygen(17.2%)and nitro-gen(7.61%). Lui et al.[34] have synthesized surfacepassivated carbonquantumdotsbymicrowavepyrolysisofglycerolinthe presenceofTTDDA.Wehaveusedasimilarroutetosynthesize thecarbonquantumdotshoweverwefoundbyXPSanalysisthat theseparticleswerenotonlysurfacepassivatedwithamide/amine groups,butalsothecarboncoresweredopedwithnitrogen.In fact,the deconvolutedhigh resolution N1s spectrum ofNCQD (Fig. 2B) reveals the presence of three peaks at 398.9, 400.1 and 401.4eV, which are attributed to different types of nitro-gen:amide/amine,pyrrolicandgraphitic,respectively[41,42].This meansthat TTDDA actsnot only aspassivation agent but also as precursor for N-doping. Zhai et al. [42] have also reported theproductionofN-dopedsurfacepassivatedCQDbymicrowave assistedpyrolysisofcitricacidinthepresenceofaprimaryamine (1,2-ethylenediamine). The deconvoluted high resolution C 1s spectrumoftheNCQD(Fig.2A)showsfourmainpeaksat284.7, 285.7,286.0,and287.3eVattributedtoC C/C C,C N,C Oand C Orespectively[43].Fromallthecharacterizationdoneitwas possibletoconcludethatthesurfaceoftheNCQDisfunctionalized withdifferentorganicgroupssuchasamide,amine,hydroxyland carboxyl.InadditionthecarboncoresaredopedwithN.
3.2. CharacterizationofP25/NCQDcomposite
Anewcomposite(P25/NCQD)ofP25decoratedwithN-doped carbonquantumdotswaspreparedusingahydrothermalmethod. TEMimagesclearlyillustratethesuccessfulcouplingofNCQDonto P25and,inthefinalcomposite,smallersphericalparticlesofNCQD intermixedwithlargercrystallinenanoparticlesofP25(TiO2
mix-tureof80%anataseand20%rutile)areeasilyspotted,asshown inFig.3A.TheHR-TEMimageinFig.3Bshowsthelatticespacing of2.016Åforthe(101)diffractionplaneofthecrystallineNCQD. Otherwellresolvedinterlayerdistanceswerealsoidentifiedforthe (200)crystallineplaneofanatase(1.872Å)andforthe(111)plane
Fig.2. HighresolutionXPSspectraofNCQD:(A)C1sand(B)N1s.
of rutile (2.184Å).STEM-EDX elementalmapping of P25/NCQD wasalsocarriedout(Fig.S4,SupportingInformation).Theimages obtainedclearlyshowareaswhereTiandOareabsentsuggesting thepresenceofNCQDintheseareas.XRDpatternoftheP25/NCQD showssharpandintensereflectionsattributedtoanataseandrutile TiO2polymorphsconfirmingthepresenceofP25(Fig.S1,
Support-ingInformation).However,thetypicalXRDreflectionsforcarbon werenotobservedintheP25/NCQDsampleduetotheirverylow contentinthefinalmaterialandalsolowcrystallinitywhen com-paredtothatofP25.
Fig.4showsTGAcurvesofP25andofP25/NCQDcomposite. Theweightlossbefore120◦CofbothP25 andthecompositeis attributedtotheeliminationofadsorbedwaterandisnotshown inFig.4.The0.36%weightlossobservedbetween120◦Cand250◦C forP25andP25/NCQDshouldresultfromdehydroxylationof
sur-Fig.3. (A)BF-TEMimageofP25/NCQD;(B)HR-TEMimageofP25/NCQDshowingthelatticefringesofbothP25(AnataseandRutilepolymorphs)andNCQD.
Fig.4. TGAcurvesofP25andofP25/NCQDcomposite.
Fig.5.FTIRspectraofP25andofP25/NCQDsamples.Thearrowshighlightthemain differencesbetweenthetwospectra.
faceattachedwaterandOHgroups[44,45].Forthecomposite,a thirdweightlossof0.69%isobservedbetween250◦Cand520◦C correspondingtotheslowcombustionofNCQD.Thismeansthat thecontentoftheNCQDonthecompositeisabout0.69%.
FTIRanalyseswereperformedinordertoinvestigatethe inter-actionbetweenP25andNCQD(Fig.5).P25spectrumdisplaysthree characteristicpeaksat3370,1630and650cm−1associatedwith stretching vibrationsof hydrogen-bonded water molecules and hydroxylgroups,bendingvibrationsofO HgroupandTi O Ti bridgingstretchingmoderespectively[46,47].IntheFTIR spec-trumofP25/NCQDthreenewfeaturesappeared.Asmallshoulder at2900cm−1isassignedtoC HstretchinginNCQDandapeak
Fig.6. XPShighresolutionspectraofP25andofP25/NCQDsamples.(A)N1sand (B)Ti2p.
at1170cm−1isassociatedwithC Ostretchingvibration.Itwas alsoobservedthatthebroadabsorptionbelow1000cm−1became widerand shiftedtowardhighwavenumberwhencompared to that of P25. This behaviour was associated by several authors [28,48]toacombinationofTi O TiandTi O Cvibrationsand suggeststhatthecouplingbetweenP25andNCQDisachievedby theformationoftheTi O Cbond.
Thepresence ofnitrogenwasdetectedbyXPS inP25/NCQD andnotinP25sample(Fig.6A).ThisconfirmsthattheNCQDare presentintheP25/NCQDcomposite.TheTi2phighresolution spec-traforP25/NCQDandP25areshowninFig.6B.ForP25/NCQDthe bindingenergiesofTi2p3/2andTi2p1/2arepositionedat458.8 and 464.7eV,respectively.Compared toP25, theTi2p peaksof P25/NCQDareshiftedtowardsahigherbindingenergy.Theshiftof 0.3-0.5eVsuggeststhatthechemicalenvironmentofTiinthe com-positeschangedduetostronginteractionbetweenP25andNCQD withformationofTi O Cbonds[45,48].
Fig.7.DiffusereflectancespectraofP25andP25/NCQD.
ThediffusereflectancespectraofP25andP25/NCQD compos-iteareshowninFig.7.P25exhibitsareflectanceof100%above 425nm,whilethecompositeshowsaconsiderabledecreaseofthe reflectanceintherangeof425–800nm.At425nmthereflectance is69.8%forP25/NCQD.Thismeansthatthecompositespresent significantabsorptioninthevisibleregion.Thebandgapsofthe twocompoundswereestimatedfromdiffusereflectancespectra followingamethodbasedonTaucequationanddescribed else-where[49].ForP25twobandgapswerefoundat3.15and3.05eV correspondingtothetwo crystallineforms ofTiO2,anataseand
rutile,respectively.P25/NCQDshowedabandgapof2.95eV.The slightdecreaseofthebandgapofthecompositewhencompared withP25maybeattributedtochemicalbondingbetweenP25and NCQDwiththeformationofTi O Cbond[28,50].Therefore,NCQD haveanimportantroleintheenhancementofvisiblelight absorp-tionintheP25/NCQDcomposite,whichisexpectedtoimproveits photocatalyticactivityundervisiblelightirradiation.
3.3. PhotocatalyticactivityofP25/NCQDcomposite 3.3.1. NOphoto-oxidation
P25/NCQDwas tested ascatalysts ofthe photo-oxidation of NOunderUVandvisiblelight.After85hofUVlightirradiation, thecompositeP25/NCQDshowedaNOconversionof79.6%anda selectivityof47.9%(Fig.8AandB).Thesevaluesweremuchhigher thanthoseobtainedforP25(58.4%ofNOconversionand41.0%of selectivity).Therefore,comparedwithP25,thecompositeshowsa 36.3%increaseonNOconversionanda16.8%increaseonselectivity. Moreover,afterreachingsteady-statethephotocatalystsshowed goodstabilityforatleast48h.
Whensubmittedtovisiblelight,P25/NCQDalsoshowed supe-riorphotocatalyticactivitycomparedwithP25duringatleast120h (Fig.9AandB).WhileinthepresenceofP25aconversionof10%at steadystatewasobtained,withP25/NCQDtheachievedconversion was27%.Thismeansthattheconversionincreases2.7timeswhen usingP25/NCQDasphotocatalyst.Theselectivityofthe photocat-alyticprocessisalsoconsiderablyhigherwhenusingthecomposite (49.3%)comparedwithP25(37.4%).
TothebestofourknowledgeP25/NCQDsynthesizedinthiswork isthefirstexampleofacompositeofTiO2 andcarbonquantum
dotswithphotocatalyticactivitytowardsNOdegradationunder visiblelight(>400nm).P25/NCQDcompositeshowedabetter photocatalyticperformancethanN-dopedTiO2particlesprepared
byseveralresearchers[46,51,52].
Tocheckiftheenhancementofthephotocatalyticactivityofthe P25/NCQDcompositeisonlyrelatedtothepresenceoftheNCQD orifitisalsoconnectedwiththehydrothermaltreatmentsuffered bythepowders,thephotoactivityofP25hydrothermallytreated at130◦Cduring4h(P25-HT)wasalsotested.Asshown inFig. S5(seeSupportingInformation),P25-HTpresentahigherNO
con-Fig.8.NOconversion(A)andselectivity(B)forP25,P25/NCQDcompositeduring 85hunderUVradiation.
Fig.9.NOconversion(A)andselectivity(B)forP25,P25/NCQDcompositeduring 120hundervisiblelightirradiation.
versionthanuntreatedP25bothunderUVandvisibleirradiation suggestingthatthehydrothermaltreatmentcontributespartially totheenhancementofthecatalystsphotoactivity.Thisbehaviour couldbeattributedtotheformationofmorehydroxylgroupsin theTiO2surface.AccordingtoYuetal.[53],morehydroxylgroups
Fig.10.Plotsof−ln(C/C0)versustforthephotodegradationofMBbyP25,P25/NCQD
andmixP25+NCQD.
cangeneratemorehydroxylradicals,enhancetheadsorptionofO2
moleculesandreducerecombinationofphotogeneratedelectrons andholes.Itshouldbehighlightedthataphysicalmixture (with-outhydrothermaltreatment)ofP25andNCQD(mixP25+NCQD) showedsignificantlylowerphotocatalyticactivitythanP25/NCQD (Fig.S5,SupportingInformation).Thisconfirmsthatthe hydrother-maltreatmenthasaroleontheenhancementofthephotocatalytic activitynotonlyfosteringtheformationofhydroxylgroupsbut alsopromotingamoreefficientattachmentoftheNCQDtoTiO2
surface.
3.3.2. Photocatalyticactivityinaqueousmedium
Thephotocatalytic activity of P25 and P25/NCQD composite in aqueous medium, under UV irradiation, was assessed using methyleneblue(MB).AsshowninFig.S6(seeSupporting Infor-mation), P25/NCQD composite presents a considerably higher photocatalyticactivitytowardsMBdegradationascomparedwith commercialP25.After1hofUVirradiation91%ofMBwas pho-todegradatedby P25/NCQDwhereas only68%of theinitialdye wasdecomposedbyP25.Moreover,thephotocatalyticactivityof P25/NCQDcompositeishigherthanthatofaphysicalmixtureof P25andNCQD(mixP25+NCQD)showingthathydrothermal treat-mentpromotesamoreefficientattachmentoftheNCQDtoTiO2
surface.ThedecayofMBduringtheinitialperiod(120min)ofUV excitationwasfittedtoapseudo-first-orderkinetics:−ln(C/C0)=kt,
wherekistherateconstantandttheirradiationtime.Fig.10plots −ln(C/C0)historyforallphotocatalysts.Fromthisplotitwas
pos-sibletoestimatetherateconstantsfortheMBdegradationinthe presenceofthesecatalysts.Valuesof0.019,0.026and0.044min−1 wereobtainedforP25,mixP25+NCQDandP25/NCQDrespectively. ThismeansthatthephotocatalyticdegradationratesofMBover P25/NCQDare2.32higherthanthatoverP25.SimilarMB photo-catalyticdegradationrateswerefoundbyotherresearcherswhen usingTiO2/carboncompositesascatalystsunder UVlight
how-ever,in themajorityof thecaseswithlargerconcentrations of carbon(>1wt.%)thanthatusedinthiswork(0.69wt.%)[27].Tests ofthephotocatalyticperformanceofP25/NCQDcompositeswere notimplementedundervisiblelightsincetherearereportsthat statethatMBcansufferself-photosensitizeddegradationoverTiO2
whenirradiatedwithvisiblelightandthuswouldrepresenta non-photocatalyticcontributiontotheoveralldegradationofthedye [54].
3.4. Discussionontheenhancedphotocatalyticactivityof P25/NCQDcomposite
The performance of a photocatalyst can be determined by several factors such as specific surface area, crystallinity, light absorptionrangeand efficiencyofcharge separationand
trans-port[28,55].Generally,photocatalystswithlargerspecificsurface area exhibit higher photocatalytic activity because a larger surfaceareaprovidesmoreactivesitesfortheadsorptionof reac-tantmolecules.ThespecificsurfaceareaoftheP25andP25/NCQD composite was measured using Brunauer-Emmett-Teller nitro-genadsorption-desorptionmethod(BET)andvaluesof54.9and 53.8m2g−1,respectively,wereobtained.Thismeansthatthehigher
photocatalytic activity of P25/NCQD is not related to a surface areaenhancement. Thesimilaritybetweenthe XRDpatternsof P25andP25/NCQD(Fig.S1,SupportingInformation)revealsthat crystallinity,crystalsizeandmodificationsonthecrystalstructure ofTiO2 arealsonotresponsibleforthedifferencesbetweenthe
photocatalyticactivityofthetwocompounds.
Theexperimentalresultsprovedthathydrothermaltreatment canimparthigherphotoactivitytoTiO2.However,thepresenceof
NCQDisnecessarytoachievethehighestNOdegradationrates(Fig. S5,SupportingInformation).ThereforeNCQDhaveakeyroleonthe enhancementofphotocatalyticactivity.
P25/NCQDdisplaysenhanced visiblelightabsorption(Fig.7) and narrowing of the bandgap (from 3.05eV to2.95eV) when comparedwithP25.Carbondopingisnotexpectedtooccursince usuallyC-dopedTiO2 issynthesizedattemperatureshigherthan
600◦C[50].Thisleadsustoconcludethattheslightdecreaseof thebandgapcouldberelatedwithchemicalbondingbetweenP25 andNCQDwithformationofTi O Cbond.Infact,FTIRandXPS resultsseemtosupportthishypothesis.Moreover,severalauthors reportedthatthepresenceofTi O CbondinTiO2/carbon
com-posites favours charge transfer andcan effectively extendlight absorptiontolongerwavelengths[28,50].Ontheotherhand,NCQD canactaselectronacceptortotraptheelectronsfromP25 con-ductionbandandthushinderelectron-holerecombinationleaving morechargecarrierstoformreactivespecies(e.g.OH•andO2•−)
thatcanpromotethedegradationofpollutants[48,56].
Inaphotocatalyticprocessthefasttransportofcharge carri-ersisimportanttopreventrecombination.Inordertostudythe chargetransferprocessonP25andP25/NCQDtheelectrochemical impedancespectraofthetwophotocatalystsweremeasured. Com-paringtheNyquistplotsforP25andP25/NCQD(Fig.S7,Supporting Information),it wasobservedthat thediameterofthe semicir-cleathighfrequenciesissmallerforP25/NCQD,whichindicates adecreasein thechargetransferresistanceonthesurface[28]. ThissuggeststhatonP25/NCQDcompositethetransportofcharge carriersisfaster,contributingtoabetterseparationofthe photo-generatedelectron-holepairsandconsequentlytoadecreaseofthe recombination.
Insummary,NCQDcontributetotheenhancedphotocatalytic activityofP25/NCQDcompositeinthreeways:extendingthelight absorptionintothevisiblerange,slowingtherecombinationand improvingthechargetransfer.
4. Conclusions
A new photocatalyst (P25/NCQD) of P25 decorated with N-dopedcarbonquantumdotswaspreparedusingahydrothermal method.The P25/NCQDcomposite showed enhanced photocat-alyticperformanceforNOphoto-oxidationwhencomparedwith P25,bothunderUVandvisiblelight.NOconversionincreasedfrom 10%to27%andfrom58.4%to79.6%undervisibleandUV irradia-tion,respectively.Moreover,theselectivityoftheprocessincreased from37.4%to49.3%andfrom41.0%to47.9%undervisibleandUV irradiation,respectively.ThephotocatalyticactivityofP25/NCQD inaqueousmediumwasalsoassessedusingmethyleneblue(MB). UnderUVradiationtheMBdegradationratewas2.32higherthan theobtainedforP25.
Although the hydrothermal treatment contributed to the enhancedphotocatalyticactivityofP25/NCQD,themainroleonthe photocatalyticperformanceofthesenanocompositeswasassigned to NCQD. These nanoparticles can strongly interact with TiO2
extendingtheresponseofthecompositetovisiblelight. NCQD, intheP25/NCQDphotocatalyst,canalsoactasefficientelectron acceptorsand transporters,slowingtherecombinationrateand consequentlyincreasingphotocatalyticactivity.
Acknowledgments
Luísa Andradeand Natércia Martinsacknowledge the Euro-peanResearchCouncilforfundingwithinprojectBI-DSC—Building IntegratedDyesensitizedSolarCells(ContractNumber:321315). This work was also supported by research project SolarCon-cept(PTDC/EQU-EQU/120064/2010),co-financedbytheEuropean Unionandthe PortugueseGovernmentthrough thePortuguese FoundationforScienceandTechnology(FCT),intheframework oftheQRENInitiativeandtheEuropeanRegional Development FundthroughtheOperationalProgramforCompetitiveness Fac-tors.AnaGirãoandTitoTrindadethanktheprojectCICECO-Aveiro Instituteof Materials (Ref.FCT UID/CTM/50011/2013), financed by national fundsthrough the FCT/MEC and when appropriate co-financedbyFEDERunderthePT2020PartnershipAgreement. JoanaÂngeloisgratefultothePortugueseFoundationforScience andTechnology(FCT)forherPh.Dgrant(SFRH/BD/79974/2011). AnaGirãothanksFCTforfundingherPostdoctoralResearchGrant (SFRH/BPD/66407/2009)andacknowledgesDr.NunoJoãoforXRD acquisition.TheauthorsarethankfultoCEMUPfortheXPSanalysis.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion, athttp://dx.doi.org/10.1016/j.apcatb.2016. 04.016.
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