composites dioxide nanoﬁbers Visible light reduction photocatalytic of Cr(VI) by surface modiﬁedCNT/titanium Journal of Molecular Catalysis A: Chemical

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ContentslistsavailableatScienceDirect

Journal of Molecular Catalysis A: Chemical

j o ur na l h o 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 / m o l c a t a

Visible light photocatalytic reduction of Cr(VI) by surface modiﬁed CNT/titanium dioxide composites nanoﬁbers

Alaa Mohamed

^a,c,∗

, T.A. Osman

^b

, M.S. Toprak

^a

, M. Muhammed

^a

, Eda Yilmaz

^d

, A. Uheida

^a,∗

aDepartmentofMaterialsandNanoPhysics,KTH−RoyalInstituteofTechnology,SE16440,Stockholm,Sweden

bMechanicalDesignandProductionEngineeringDepartment,CairoUniversity,12613Giza,Egypt

cProductionEngineeringandPrintingTechnologyDepartment,AkhbarElYomAcademy,12655Giza,Egypt

dNationalNanotechnologyResearchCenter,BilkentUniversity,06800Ankara,Turkey

a r t i c l e i n f o

Articlehistory:

Received26June2016

Receivedinrevisedform22July2016 Accepted11August2016

Availableonline12August2016

Keywords:

Photocatalyticreduction Chromium(VI) Compositenanoﬁbers Visiblelight

a b s t r a c t

InthisworkwereportahighlyefﬁcientphotocatalyticreductionofCr(VI)basedonPAN-CNT/TiO2-NH2

compositenanofibersfabricatedbyusingelectrospinningtechniquefollowedbychemicalcrosslinking ofsurfacemodifiedTiO2NPsfunctionalizedwithaminogroup.Thestructureandmorphologyofthefab- ricatedcompositenanofiberswerecharacterizedbyFTIR,SEM,TEM,TGA,andXPS.Theresultsindicate thatthecompositenanofiberspossessexcellentphotoreductionperformanceforCr(VI)undervisible light(125W)after30min,whichismuchfasterthanpreviousreports.Theeffectsofvariousexperi- mentalparameterssuchascatalystdose,irradiationtime,initialconcentrationofCr(VI),andpHonthe photoreductionefficiencyofCr(VI)wereinvestigated.ThehighestphotoreductionefficiencyofCr(VI) wasobtainedatlowacidityandlowamountofTiO2/CNTphotocatalyst.Thekineticexperimentaldata wasattainedandfittedwellwithapseudo-first-ordermodel.TheUV–visspectrophotometerandXPS analysesprovedthatchromateCr(VI)wasreducedtoCr(III).Inaddition,itcanbeconcludedthatthe additionofthephenolenhancesthephotocatalyticreductionofCr(VI).Furthermore,thephotoreduction mechanismhasalsobeendiscussed.Finally,thefabricatedcompositenanofiberswerefoundtobestable afteratleastfiveregenerationcycles.

1. Introduction

Chromium plays an essential role in plant and animal metabolism,andiswidelyusedinmanyindustrialprocessessuch aselectroplating,textiledyeing,paint,leathertanneries,andpig- mentindustriesascriticalindustrymaterials[1].Crexistsmainly inhexavalentCr(VI)andtrivalentCr(III)formsinthenaturalenvi- ronment[2].ThehexavalentchromiumCr(VI)ishighlytoxicand carcinogenictohumans,animals,and plants.The WorldHealth Organization(WHO)recommendsthemaximumallowablelimit forthedischargeofCr(VI)intoinland surfacewateris 0.1ppm, andintothedrinkingwateris0.05ppm.Therefore,thepreferred treatmentisa reduction ofCr(VI)totheless harmfulCr(III), in ordertoavoidthedeleteriousimpactoftheCr(VI)onthehuman health. Toxicity of Cr(III) is relatively low and it is one of the essentialmicronutrientforhumanhealth[3].Inreality,industrial

∗ Correspondingauthor.

E-mailaddresses:alakha@kth.se(A.Mohamed),salam@kth.se(A.Uheida).

wastewaterconsistsofamixtureoforganicandinorganicpollu- tants.Therefore,phenoliccompoundsareusedasamodelpollutant becausetheyarewidelyusedinthepreparationofresins,herbi- cides,andfungicideswhicharehighlytoxictomostaquaticlife [4,5].Therefore,thereisanurgentneedtoremovephenolfromthe wastewater.

Hence, the reduction of Cr(VI) into Cr(III) received great attentionintheenvironmentalremediationprocesses.Different techniqueshavebeenreportedforthetreatmentofCr(VI)pollution includingchemical reduction,ion exchange,sorption, photocatalytic, and bacterial reduction [6–10]. However, most of these methodsrequireeitherhighenergyorlargequantitiesofchem- icalsand arenotwidelyused[8,9].Recently, thephotocatalytic processeshavereceivedconsiderableattentionbecausebeingeco- nomicallyviable,facile,andeffectivemethodforarapidefﬁcient destructionofenvironmentalpollutants[11,12].Manysemicon- ductorcatalysts,suchasTiO2,ZnO,ZnS,ZrO2,CdSandWO3,have beenstudiedtoinvestigatethephotocatalyticreductionofCr(VI)to Cr(III)[13–19].Amongvarioussemiconductorcatalysts,TiO₂was consideredasoneofthemostpromisingcandidatesduetoitsopti-

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solutionafterreactionisessential,becauseitisdifficulttoseparate andrecoverafterprocessingwithwastewater,causingsecondary pollutionand thisprocess istime consumingand costly,which limitsitsapplicationforwaterpurification.Therefore,researchers are focusing on the development of polymer based compos- itematerials,mainlybyincorporationordepositionofmetalor semiconductor and metal semiconductor NPs in/on polymeric nanofibersduetotheirenhancedpropertiesandpotentialappli- cationincatalysisandenvironmentalremediation[23–25].Inthis regard,electrospinningtechniqueisaneconomicandeffectiveway ofsynthesizingpolymer nanofibers[26,27],which displaylarge specificsurfacearea,finefabricstructure,highaspectratio,flex- iblesurfacefunctionality,tunablesurfacemorphologiesandbetter adsorptionaswellasfiltrationproperties[28,29].Polyacrylonitrile (PAN)isthemostwidelyusedpolymerformanufacturinghighper- formancefibersduetoitsexcellentcharacteristicsandcommercial availability,aswellasitsnon-toxicnature[30].Accordingtoour knowledge,fewstudiesworkingonthereductionofCr(VI)under visiblelightirradiationusingnanocompositesmaterials[31–35].

Thesestudieshavealot ofproblemlikelongerirradiationtime (2–4h)andhighpowerintensity(>125W)toobtainthemaximum reduction.

Inthis work,wedevelop anewsystembased oncomposite nanofibersconsistingofPAN, andCNTfabricatedusinganelec- trospinningtechniquefollowedbyfurthercrosslinkingofsurface amino-modifiedTiO2NPstothesePAN/CNTnanofibrousmatrices inordertoincreasetheadsorptionofheavymetalsduetothelarge numberofbindingactivesitesincorporatedonthesurfaceofTiO₂ NPs.Ourpreviousworkverifiesthesuccessofthissystemunder UVandvisiblelightirradiationcomparedtoearlierreports[36,37].

Theobjectivesofthisstudyweretofabricatecompositenanofibers containingPAN polymer, MWCNT, and surface amino-modified TiO₂NPsandtodevelopanefficientandeconomicphotocatalytic compositenanofibersforthephotoreductionofCr(VI)inaqueous solutionsunder visible light irradiation.The effectof operating parametersincludesinitialsolutionpH,theamountofphotocat- alyst,andCr(VI)concentrationonthephotocatalyticreductionof Cr(VI)wereinvestigated.Furthermore,thesynergisticphotocat- alyticmechanismhasalsobeendiscussed.

2. Experimental

2.1. Materials

Polyacrylonitrile, PAN (MW=150,000); N,N- dimethylformamide (DMF), sodium hydroxide (NaOH) and hydrochloricacid(HCI),titaniumdioxidepowder(TiO2 Degussa P-25),and3-aminopropyltriethoxysilane(APTES),werepurchased from Sigma Aldrich. Multi-walled carbon nanotubes, MWCNTs (purity 95wt%; diameter: 10–40nm; length: 20␮m; speciﬁc surface area 460m²/g) were synthesized and the procedure is describedelsewhere[38].Potassiumdichromate(K₂Cr₂O₇), and caffeic acid(3,4-dihydroxycinnamic acid, 99%)were purchased

theexcesssolvent.Inaddition,thesurfacefunctionalizationofTiO₂ nanoparticleswiththeaminogroupwascarriedoutaccordingtoa well-establishedproceduredescribedinRef.[39].Thecrosslinking ofthePAN/CNTcompositenanoﬁberstoTiO₂-NH₂NPswascarried outasdescribedelsewhere[36].

Themorphology of thecomposite nanoﬁberswas examined usingScanningElectronMicroscopy(SEM,GeminiZeiss-Ultra55) and Transmission Electron Microscopy (TEM, JEM-2100F, Joel).

Fouriertransforminfraredspectroscopy(FTIR,NicoletiS10)was usedtoindicatethespectraofPANandPAN-CNT/TiO₂-NH₂com- positenanofibersbeforeand afterCr(VI)reduction.Thethermal stabilitiesofthecomposite nanofibersamplesweredetermined byusingthermogravimetricanalysis(TGA),TGAQ500,TAinstru- ment.Thiswasdonebyheatingthesamplefromroomtemperature until 800^◦C witha heating rateof 20^◦Cmin⁻¹ undersynthetic air. The concentrationof Cr(VI) in the solutionwas measured usingUV–vis/NIRspectrophotometer(modelLAMBDA750,Perkin Elmer).SurfacechemicalcompositionsofPAN-CNT/TiO2-NH2com- positenanofiberswereanalyzedusingThermoScientificK-Alpha x-ray photoelectron spectrometer with monochromated Al K␣ radiation.Samplesurfacewasneutralizedagainstchargingwith flood gunemissionduring themeasurementsand allthe spec- trawerecorrectedaccordingtotheC1speakagainstadditional chargingeffects.

2.3. Photocatalyticreductionexperiments

Photocatalyticexperimentswereconductedinacolumn(2cm diameter, 30cm height) in which composite nanoﬁbers matof 5cm×5cmwasplacedinthemiddleofthecolumn.A30mLaliquot ofCr(VI)withinitialCr(VI)concentrationof20mg/Lwasused.The columnwasshakingatroomtemperaturefor30minandcovered fromanysourceoflighttoassurethattheadsorptionequilibrium ofCr(VI)wasreached. Thesolutionwaspumpedata ﬂowrate of7mL/min.Thelight intensityobtainedfromtheXenonlamp (125W,420nm)wasdeterminedtobe100mW/cm².Duringillu- mination,3mLofthesuspensionwastakenfromthecolumnat scheduledintervals.TheCrconcentrationpriortoandafterphoto- catalyticreductionwasmeasuredthreetimesusingaUV–vis/NIR spectrophotometer.

3. Resultsanddiscussions

3.1. Catalystcharacteristics

TheSEMandTEMimagesofphotocatalyticmaterialcomposed ofPAN-CNT/TiO₂-NH₂areshowninFig.1.TiO₂NPsaredistributed onthesurfaceofnanofibers,whichconfirmsthatTiO2NPsattached tothesurfaceofnanofibersduetothecrosslinkingprocedure.The compositenanofibersappearsmoothanduniformwithanaverage diameterof126±4nm.

In orderto confirmthesurface functionalizationof thefabricated composite nanofibers, FTIR spectra of PAN nanofibers, and PAN-CNT/TiO2-NH2 composite nanofibers before and after

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Fig.1. (a)SEMand(b)TEMmicrographofPAN-CNT/TiO2-NH2compositenanoﬁbers.

3500 3000 2500 2000 1500 1000 500

Cr-Cr=OO

(c) (b)

% Tr an sm itt an c e

Wavenumber (cm

^-1

)

(a)

C-H C=N

NH₂

O-H C=O N-H

C-C N-O C-H C-H

Fig.2.FTIRspectraof(a)PAN(b)PAN-CNT/TiO2-NH2and(c)PAN-CNT/TiO2-NH2

loadedwithCr(VI).

thephotoreductionof Cr(VI)wereobtainedas shownin Fig.2.

Thespectrumfor PAN nanofibersexhibited characteristicpeaks ofnitrile(2342cm⁻¹),carbonyl(1700cm⁻¹)andC Hstretching (3159cm⁻¹)[40].FromFig.2b,thepeakcorrespondingtonitrile is markedly decreased due to theconversionof nitrile toami- doxime,afterthe crosslinkingof (PAN-CNT)to(TiO2-NH2).The absorptionintherange3100–3700cm⁻¹ isassignedtoN Hand O Hvibrations.ThebendingvibrationsoftheaminegroupNHor NH2 observedat1680cm⁻¹confirmtheconversionofthenitrile grouptoamidoxime[41].Thebandobservedat3159,1520,and 1152cm⁻¹assignedtothealiphaticC Hbendingvibrationofthe CH₂ofpolymericchain,whilethebandobservedat900cm⁻¹ is assignedtoN O.AfterthephotoreductionofCr(VI)atpH2,Fig.2c two new peaksat 620 and 570cm⁻¹ appear in the FTIR spec- trumof compositenanofibers,which areattributedtotheCr–O andCr=ObondsfromtheCr(VI)species.Inaddition,thebandat 1200–1470cm⁻¹correspondingtoN HandO Hbendingiscon- siderablyincreasedduetothepresenceofCr(VI)suggestingthatthe amineandoximegroupofamidoximeareinvolvedinthebinding ofchromiumduringCr(VI)uptake[42].

Fig. 3 shows TGA thermograms of PAN nanofibers, PAN- CNTcompositesnanofibers, andPAN-CNT/TiO2-NH2 composites nanofibersinthetemperaturerangefromroomtemperatureto 800^◦C.ThethermogramofPANnanofibersshowsthree decom- positionsteps.Inthefirststageupto290^◦C,therewasnoweight

0 100 200 300 400 500 600 700 800 0

20 40 60 80 100

Wei g h t ( % )

Temperature (

^o

C)

40.6 %

9.6 %

(a) (b) (c)

Fig.3. TGAthermogramsof(a)PANnanofibers,(b)PAN-CNTnanofibers,and(c) PAN-CNT/TiO2-NH2compositesnanofibers.

loss.About,∼45%ofweightlosswasobservedinthesecondstage from290^◦Cto300^◦C,indicatingthatasignificantchemicalreac- tiontookplace,andvolatilegassesevolved.Inthelaststageupto 720^◦C,completedecompositionwasobserved.Furthermore,the weightofthePAN-CNTcompositenanofibersdecreasedrapidlyin thetemperaturerangeof338–650^◦C,duetothecombustionand decompositionofcompositenanofiberstakingplaceatthistem- peraturerange.Afterthetemperaturewasincreasedto650^◦C,the CNTremainedandnomoreweightlossoccurred,whichmeantthat nanofiberswereremovedcompletely.ForthePAN-CNT/TiO₂-NH₂ compositenanofibers,theweightdecreasedrapidlyinthetemper- aturerange338–705^◦C,afterthattheTiO2remainedandnomore weightlossoccurred,whichmeantthatnanofiberswereremoved completely.Moreover,theTiO₂contentinthecompositescouldbe easilycalculatedfromtheweightremainderafterthesampleswere heatedover705^◦C.SincethesamplePANnearlycompletelydisap- pearedover720^◦C,theCNT/TiO₂andCNTcontentsincompositions weredeterminedtobe40.6wt%and9.6wt%forthesamplePAN- CNT/TiO2-NH2andPAN-CNTcompositenanofibers,respectively.

3.2. PhotocatalyticperformanceofPAN-CNT/TiO2-NH2

compositesnanoﬁbers

3.2.1. Effectofcatalystcontent

Theamountofcatalystisanimportantparameterinoptimiz- ingtheoperationalconditions.Inthisstudy,theeffectofcatalyst loadingonthephotoreduction ofCr(VI) wasinvestigatedusing

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0 10 20 30 40 50 60 0.0

Irradiation Time (min)

250 300 350 400 450

Wavelength (nm) Fig.4.EffectofcatalystdosageonphotoreductionefﬁciencyofCr(VI)(Cr(VI)=20ppm,andpH2).

acatalystdosage(TiO₂/CNT)rangingfrom10to35mgtoavoid anineffectiveexcessamountofthephotocatalystandtheresults obtainedareshownin Fig.4.Theresultsindicatethatthepho- toreductionefﬁciencyofCr(VI)graduallyincreasesasthecatalyst dosageincreasesfrom10to35mg.Thismaybeattributedinterms ofavailabilityoftheactivesitesonthephotocatalystsurfaceand thepenetrationofvisiblelightthroughthewater,leadingtoan increaseinthephotoreductionofCr(VI)[43,44].

3.2.2. Effectofirradiationtime

In this study, the photoreduction activity of Cr(VI) using PAN-CNT/TiO2-NH2andPAN/TiO2-NH2compositenanoﬁberswas investigatedunderthevisible lightasa functionof thecontact time.ThephotoreductionexperimentsofCr(VI)wereconducted atinitialCr(VI)concentrationof20mg/L,pH2andtheamountof photocatalystis20mg.TheresultsobtainedareshowninFig.5.It canbeseenthatabout60%ofphotoreductionwasachievedinless than15minirradiationtimeusingPAN-CNT/TiO₂-NH₂composites nanoﬁbers,andacompletereductionwasobservedafter30min.

Thismaybeduetotheavailabilityofmoreactivebindingsiteson thesurfaceofTiO₂NPsthatcrosslinkedtothecompositenanofibers inordertoincrease theadsorptionof Cr(VI),thereforeenhance thephotoreduction efficiency. Furthermore,the PAN-CNT/TiO₂- NH₂compositenanofibersleadtohighloadingofCr(VI)inashort time,duetoincreasedactivesurfacesiteswhichwillfacilitatehigh exposureoflightandthenhighphotoreductionefficiency.Accord- ingtopreviousstudiesundervisiblelightathighpowerof500W [31–35],ittakesabout2–4htogetacompletephotoreduction.On theotherhandforthePAN/TiO2-NH2compositenanofibersabout 50%ofphotoreductionwasachievedinlessthan20minirradia- tiontime andnochangesinthephotoreductionefficiencywere observedafter30minirradiationtime.Theseresultsindicatedthat thephotocatalyticreductionefficiencyincreasedwiththeincorpo- rationofCNTsandTiO₂.Thistrendresultedfromthehighsurface areaofCNTs/TiO2photocatalytic.Inaddition,CNTscaneffectively generateagreaternumberofelectronsandholes,andaccelerate theprocess of thephotocatalytic reactionto enhancethe photocatalyticactivity.The photoreduction efficiencyof Cr(VI)was determinedfromtheresultsobtainedfromUV–visspectroscopy.

Theresultsobtainedareshown inFig.5b inwhich thepeak at 350nmcorrespondingtoCr(VI)wasshiftedto302nm,correspond- ingtoCr(III).Thekineticexperimentsdataofthephotocatalytic reductionofCr(VI)areshowninFig.5cwassuccessfullyﬁttedwell usingcommonlyappliedpseudo-ﬁrst-orderequation[45],which canbeexpressedasfollows:

ln

_C

0

C

=kat (1)

WhereC₀istheinitialconcentrationofCr(VI)andCistheconcen- trationofCr(VI)ataspeciﬁctime,andkaistherateconstantof pseudo-ﬁrstordermodel(min⁻¹).

3.2.3. EffectofpHonthephotoreductionofCr(VI)

pH of the solution plays a major role in the photocatalytic processasitisknowntoinfluencethesurfacechargeofthesemi- conductor thereby affectingthe adsorption, interfacial electron transfer,and the photoreductionprocess [46]. Theeffect ofpH onthephotocatalyticreduction efficiencyofCr(VI)ispresented inFig.6.Asobserved,thephotoreductionefficiencyofCr(VI)was highlydependentonthe pHwithmaximum photoreductionat pH=2. TheamountofCr(VI)decreased intheaqueoussolution withincreasingpH.AsseenfromFig.6that100%photoreduction efficiencyofCr(VI)wasobtainedatpH2.Then,thephotoreduc- tionofCr(VI)decreasedsharplyto72%aspHvalueincreasedto 9.Thevariation inreductionefficiencyof Cr(VI)atdifferentpH valuesmaybeattributedtotheaffinitiesofPAN-CNT/TiO₂-NH₂ compositesnanofibersforthedifferentspeciesofCr(VI)existing atacidicpHvaluesnamelyH2CrO40,HCrO4−,CrO42−,andCr2O72−

[40].TheNH₂groupsonthesurfaceoftheTiO₂nanoparticlescan eitherbeprotonatedtoformNH₃⁺atlowpHorbedeprotonated toformNH2···OH athighpH.Itisclearthatnegativelycharged HCrO4−and Cr2O72− areeasilytobeadsorbedtothepositively chargedPAN-CNT/TiO₂-NH₂compositesnanofibersatlowpHval- uesduetotheelectrostaticattraction,thereforeahigheryieldof photoreduction[47,48].Theelectrostaticrepulsionbetweenneg- ative Cr(VI)species and negatively chargedPAN-CNT/TiO₂-NH₂ compositesnanofibersincreasedwithincreasingpHvalues,and therebyresultedinthedecreaseofthereductionofCr(VI)[49].To investigatethekineticsofCr(VI)photoreductionunderdifferent pHvalues,theexperimentaldataweresuccessfullyfittedusingthe pseudo-firstorderasshownin(Fig.S1,Supplementarydata).

3.2.4. EffectofCr(VI)initialconcentration

TheeffectofinitialCr(VI)concentrationonthephotoreduction efficiencyofCr(VI)ontoPAN-CNT/TiO₂-NH₂compositenanofibers wasstudied at initialCr(VI) concentration of 10–100mg/L and pH=2. The obtainedresult areshown in Fig. 7 shows thatthe photoreductionefficiencyofCr(VI)graduallydecreaseswiththe increaseoftheinitialCr(VI)concentrationfrom10to100mg/L.

Thecompletereductioncanbeachievedafter30minat10–20mg/L initialCr(VI)concentration,whiletherespectivevaluedecreasesto 79%at100mg/L.Apossibleexplanationhastodowiththefactthat increasedCr(VI)concentrationincreasesthesolutionabsorbance and,therefore,agreaterfractionofthelightisinterceptedbefore reachingthecatalystsurface,thusdecreasingthedegreeofreduc-

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Fig.5.(a)PhotoreductionofCr(VI)undervisiblelightirradiation(b)UV–viscurvesofCr(VI)beforeandafterphotoreduction(c)Fittingofpseudo-ﬁrstordermodel,(Cr (VI)=20ppm,pH=2,catalystamount=20mg).

250 300 350 400 450

pH 9 pH 7

pH 5

pH 4 pH 3

Absorption (a.u.)

Wavelength (nm)

pH 2

(a)

0 10 20 30 40 50 60

0.0 0.2 0.4 0.6 0.8 1.0

C/C0

pH 2 pH 3 pH 4 pH 5 pH 7 pH 9

(b)

Fig.6.(a)UV–visspectraofphotoreductionofCr(VI)atdifferentpH(b)PhotoreductionofCr(VI)atdifferentpHontoPAN-CNT/TiO2-NH2compositenanoﬁbers.

tion[33].Thekineticexperimentsforthephotoreductionofvarious concentrationsofCr(VI)areshownin(Fig.S2,Supplementarydata) wassuccessfullyﬁttedusingpseudo-ﬁrst-order.Accordingto(Fig.

S2,Supplementarydata)withtheincreaseoftheinitialconcentra- tionofCr(VI),therateconstantkadecreased.Thiscanbeattributed totheincreaseinCr(VI)concentration,whichdecreasesthepath lengthofphotonsenteringintothereactionmixture,andfewer

photonsreachthecatalystsurface.Inaddition,theunchangeable value of light intensity,the amount of catalyst and irradiation timeleadtothedecreaseoftheavailabilityofactivesites.Con- sequently,thephotoreductionefﬁciencyofCr(VI)decreasesasthe concentrationincreases[3,50,51].Moreover,anincreaseinCr(VI) concentrationcanleadtothesaturationofthelimitednumberof

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250 300 350 400 450 Wavelength (nm)

0 10 20 30 40 50 60

0.0

Fig.7. (a)UV–visspectraofphotoreductionofCr(VI)atdifferentconcentration(b)PhotoreductionofCr(VI)atdifferentinitialconcentrationontoPAN-CNT/TiO2-NH2

compositenanoﬁbers.

accessibleactivesitesonthephotocatalystsurface,resultingina reductioninthephotoreductionefﬁciency.

3.2.5. XPSdataanalysis

Inordertoinvestigatetheinfluenceactivityofthephotocat- alyticcompositenanofibersonthephotoreductionofCr(VI),the compositenanofibers afteradsorptionstep inthedarkand the photoreductionstep under visiblelight irradiation wasdirectly examinedbyXPS.Fig.8showntheXPSpatternsofthecompos- itenanofibers,whichshowedtheCr2pspectrarecordedforCr(VI) andCr(III).AstheCr2ppeakisadoublet,thepeakcomponentat lowerbindingenergycorrespondstoCr2p_3/2orbital,whilethose athigherbindingenergycorrespondtoCr2p_1/2orbital.Beforethe photoreductionprocesstookplace,bandscorrespondingtoCr(VI) appearedatabindingenergyof579.2and588.3eV,whichcon- firmstheadsorptionofCr(VI)onthesurfaceofPAN-CNT/TiO2-NH2

compositenanofibers.Aftertheirradiationofthenanocomposites withvisiblelight,newsignificantbandsappearcorrespondingto Cr(III)bindingenergyof577.1and586.5eV,whichconfirmsthe reductionofCr(VI)toCr(III)ontoPAN-CNT/TiO2-NH2composite nanofibers[52].Thewide-scanXPSspectrumshowsalsofourpeaks at458.2eV,531.3eV,284.6eV,and399.3eVcorrespondingtoTi2p, O1s,C1s,andN1s,respectively,indicatingtheexistenceofTi,O, C,andNelements.ThepeaksintheTi2pspectrumat458.2eVand 464.1eVrepresentedtoTi2p_3/2andTi2p_1/2,respectively,indicat- ingthattitaniumboundedtooxygenremainsinoxidationstateIV forthetitanium-oxocluster.TheO1sspectrumhasabroadpeakat 531.3eVthatisindicativeofoxygeninmetaloxidessuchasTiO₂. Inaddition,thepeaksintheN1sregionat399.3eVcanbeassigned totheNoftheaminefunctionality.

3.2.6. PhotoreductionofCr(VI)inthepresenceofphenol

ThePAN-CNT/TiO2-NH2compositenanofiberswastestedsimi- larlytotheindustrialwastewaterconsistsofamixtureoforganic andinorganicpollutants.Inthisexperiment,wetestedthePAN- CNT/TiO2-NH2compositenanofibersat20ppmofCr(VI)aqueous solution and 20ppm of phenol as a combination of pollutants incontinuousmode[53,54].Fig.9shows thekineticfirst-order reaction for thedegradation efficiency of phenol and the pho- toreductionofCr(VI)intheabsenceorpresenceofphenol.This synergismisbasedonthephotogeneratedelectronsandholeson thesurfaceofthecompositenanofibers[22–26].Theresultsindi- catedthattherateofCr(VI)photoreductionwasabout1.4times higherinthepresenceofphenolthaninitsabsence,whichcan beexplainedbasedonthemechanismasdescribedinthefollow-

ingsections.Therefore,simultaneousredoxreactionsincreasethe efﬁciencyofthereaction,withaconcomitantdecreaseofwater treatmentcost.

3.2.7. Proposedreductionmechanism

ThemechanismofphotoreductionofCr(VI)canbesimplified schematicallyasshowninFig.10.TheNH₂groupsonthesurface oftheTiO₂/CNTcomposite nanofiberscaneitherbeprotonated toformNH3 atlowpH.ItisclearthatnegativelychargedCr(VI) speciesareeasytobeadsorbedtothepositivelychargedTiO2/CNT composite nanofibersat lowpH values dueto theelectrostatic interaction.AftervisiblelightirradiationCr(VI)reducedtoCr(III)on thesurfacesofTiO2/CNTcompositenanofibersandreleaseintothe solutionbyelectrostaticrepulsionbetweentheprotonatedsurfaces ofTiO₂/CNTandthecationCr(III). ThephotoreductionofCr(VI) achievedundervisiblelight,whereTiO2/CNTNPsleadstothegen- erationofelectron-holepairsatthesurfaceofthephotocatalyst (Eq2).Afterthemigrationofelectron-holepairstothesurfaceof theparticles,thephotogeneratedelectronsreduceCr(VI)toCr(III) (Eq.(3)),andtheholesmayleadtogenerationofO₂(Eq.(4))and produce^•OHradicalsintheabsenceofanyorganics(Eq.(5))[55].

Phenolcanscavengethevalencebandholeinthephotocatalytic reactionsystemleadingtoaninhibitionofrecombinationofelec- tronand holepairsonthecatalystsurface andacceleratingthe reductionofCr(VI)byphotogeneratedelectrons[56].Inthepres- enceofphenol,theholescanproduce^•OHradicalsmorethanin thepresenceofCr(VI),whichcanfurtherdegradethephenolto CO₂ andH₂O(Eq.(6))[57].Therefore,theholescanalsodirectly oxidizethephenol(Eq.(7)).

TiO₂/CNT+h→h⁺+e⁻ (2)

Cr₂O72−+14H⁺+6e⁻→2Cr³⁺+7H₂O (3)

2H₂O+4h⁺→ O₂+4H⁺ (4)

H₂O+h⁺→^•OH+H⁺ (5)

•OH+Phenol →CO2+H2O (6)

H⁺+Phenol →CO₂+H₂O (7)

3.2.8. Catalystreuse

Thereuseofthecatalystisconsideredasanimportantaspect and an economic necessity. In these experiments, the PAN- CNT/TiO₂-NH₂ composite nanoﬁbers were used in consecutive photocatalytic conditions in order toevaluatethe durability of thecompositenanoﬁbers.Attheendofeachcycle,thecomposite

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595 590 585 580 575 570 Cr 2p_1/2

Cr 2p_3/2

Dark

Intensity (a.u.)

Binding Energy (eV) Cr 2p

Visible Light

472 468 464 460 456

458.2 eV Ti 2p

Intensity (a.u.)

Binding Energy (eV)

543 540 537 534 531 528 525

531.3 eV O 1s

Intensity (a.u.)

Binding Energy (eV)

294 291 288 285 282

284.6 eV C 1s

Intensity (a.u.)

408 405 402 399 396 393

399.3 eV

Intensity (a.u.)

N 1s

Fig.8. XPSspectraofthePAN-CNT/TiO2-NH2compositenanoﬁbersforCr(VI)beforeandafterphotocatalyticreductionprocess.

nanoﬁberswaswashedwithdeionizedwaterandthendriedinair.

ThephotoreductionefficiencyofPAN-CNT/TiO₂-NH₂ composites nanofibersslightlydecreasedwiththecyclenumberrepeatedas showninFig.11.Afterfiveconsecutiveadsorption-photoreduction cycles,thephotoreductionefficiencyofthecompositesnanofibers decreasedbyabout3%,whichimpliesthatthecatalystretainedits photoreductionactivityforCr(VI).Theslightdecreaseofthepho- toreduction performance of thePAN-CNT/TiO₂-NH₂ composites nanofibersmight bedue totheadsorptionof theCr(III) gener- atedafterphotocatalyticreactions,whichresultsinthedecrease ofadsorptionandactivesitesonthesurfaceofPAN-CNT/TiO2-NH2

compositesnanoﬁbers.Theseresultshavedemonstratedthatthe goodstabilityandreusabilitypropertywouldgreatlypromotethe

practicalapplicationsofcompositenanoﬁbersinthereductionof heavymetalpollutantsfromwastewater.

4. Conclusions

Accordingtotheresultsobtainedinthiswork,thefabricated compositenanofibersdisplaypromisingphotocatalyticreduction efficiencyforCr(VI)inaqueoussolutionundervisiblelightirra- diation.Thekineticsofthephotoreductionprocessshowedthat completephotoreductionwasachievedafterapproximately30min andtheexperimentaldatafolloweda pseudo-first-ordermodel.

ThephotoreductionefﬁciencyofCr(VI)washigherinacidicsolu- tionsthanthatinalkalinesolutionsduetotheCr(VI)speciesand

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0 10 20 30 40 50 60 0.0

0 5 10 15 20 25

0

Fig.9. PhotoreductionofCr(VI)intheabsenceorpresenceofphenol.(Cr(VI)=20ppm,phenol=20ppm,catalystamount=20mg,andpH2).

Fig.10. ProposedmechanismofphotocatalyticreductionofCr(VI)undervisible-lightirradiation.

0 20 40 60 80 100

Photoreduction efficiency (%)

Regeneration cycle

1st 2nd 3rd 4th 5th

Fig.11.ReusabilityofthecompositenanoﬁbersforthephotoreductionofCr(VI).

theprotonationdegreeofthephotocatalyticsurface.Theaddition ofphenolenhancesthephotoreductionofCr(VI),duetoitsability

toadsorbonthecatalystsurface,whichcanalsoactasaholescav- enger.TheUV–visspectrophotometerandXPSanalysesprovedthat chromateCr(VI)wasreducedtoCr(III).Furthermore,theﬂexibil- ityandthereuseofthePAN-CNT/TiO2-NH2compositenanoﬁbers, revealtheirpromising potentialforadvanced wastewatertreat- ment.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.molcata.2016.08.

010.

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