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

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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,∗

a_Department_of_Materials_and_NanoPhysics,_KTH_−Royal_Institute_of_Technology,_SE_16440,_Stockholm,_Sweden b_Mechanical_Design_and_Production_Engineering_Department,_Cairo_University,₁₂₆₁₃_Giza,_Egypt

c_Production_Engineering_and_Printing_Technology_Department,_Akhbar_El_Yom_Academy,₁₂₆₅₅_Giza,_Egypt d_National_{Nanotechnology}_Research_Center,_Bilkent_University,₀₆₈₀₀_Ankara,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received26June2016

Receivedinrevisedform22July2016 Accepted11August2016

Availableonline12August2016

Keywords: Photocatalyticreduction Chromium(VI) Compositenanoﬁbers Visiblelight

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InthisworkwereportahighlyefﬁcientphotocatalyticreductionofCr(VI)basedonPAN-CNT/TiO2-NH2

compositenanoﬁbersfabricatedbyusingelectrospinningtechniquefollowedbychemicalcrosslinking ofsurfacemodiﬁedTiO2NPsfunctionalizedwithaminogroup.Thestructureandmorphologyofthe

fab-ricatedcompositenanofiberswerecharacterizedbyFTIR,SEM,TEM,TGA,andXPS.Theresultsindicate thatthecompositenanofiberspossessexcellentphotoreductionperformanceforCr(VI)undervisible light(125W)after30min,whichismuchfasterthanpreviousreports.Theeffectsofvarious experi-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,and pig-mentindustriesascriticalindustrymaterials[1].Crexistsmainly inhexavalentCr(VI)andtrivalentCr(III)formsinthenatural envi-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).

wastewaterconsistsofamixtureoforganicandinorganic pollu-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, photocat-alytic, and bacterial reduction [6–10]. However, most of these methodsrequireeitherhighenergyorlargequantitiesof chem-icalsand arenotwidelyused[8,9].Recently, thephotocatalytic processeshavereceivedconsiderableattentionbecausebeing eco-nomicallyviable,facile,andeffectivemethodforarapidefﬁcient destructionofenvironmentalpollutants[11,12].Many semicon-ductorcatalysts,suchasTiO2,ZnO,ZnS,ZrO2,CdSandWO3,have beenstudiedtoinvestigatethephotocatalyticreductionofCr(VI)to Cr(III)[13–19].Amongvarioussemiconductorcatalysts,TiO2was consideredasoneofthemostpromisingcandidatesduetoits

opti-http://dx.doi.org/10.1016/j.molcata.2016.08.010

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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 nanofibersduetotheirenhancedpropertiesandpotential appli-cationincatalysisandenvironmentalremediation[23–25].Inthis regard,electrospinningtechniqueisaneconomicandeffectiveway ofsynthesizingpolymer nanofibers[26,27],which displaylarge specificsurfacearea,finefabricstructure,highaspectratio, flex-iblesurfacefunctionality,tunablesurfacemorphologiesandbetter adsorptionaswellasfiltrationproperties[28,29].Polyacrylonitrile (PAN)isthemostwidelyusedpolymerformanufacturinghigh per-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, andCNTfabricatedusingan elec-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,theamountof photocat-alyst,andCr(VI)concentrationonthephotocatalyticreductionof Cr(VI)wereinvestigated.Furthermore,thesynergistic photocat-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-NH2 com-positenanofibersbeforeand afterCr(VI)reduction.Thethermal stabilitiesofthecomposite nanofibersamplesweredetermined byusingthermogravimetricanalysis(TGA),TGAQ500,TA instru-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-NH2 com-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_._During illu-mination,3mLofthesuspensionwastakenfromthecolumnat scheduledintervals.TheCrconcentrationpriortoandafter photo-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 the fab-ricated 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

30

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25

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20

00 1500

1000

500

Cr=O Cr-O

(c)

(b)

%

Tr

an

sm

itt

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e

Wavenumber (cm

-1

)

(a)

C=N C-H 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 to ami-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 Hbendingis con-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

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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,indicatingthatasignificantchemical reac-tiontookplace,andvolatilegassesevolved.Inthelaststageupto 720◦C,completedecompositionwasobserved.Furthermore,the weightofthePAN-CNTcompositenanofibersdecreasedrapidlyin thetemperaturerangeof338–650◦C,duetothecombustionand decompositionofcompositenanofiberstakingplaceatthis tem-peraturerange.Afterthetemperaturewasincreasedto650◦C,the CNTremainedandnomoreweightlossoccurred,whichmeantthat nanofiberswereremovedcompletely.ForthePAN-CNT/TiO2-NH2 compositenanofibers,theweightdecreasedrapidlyinthe temper-aturerange338–705◦C,afterthattheTiO2remainedandnomore weightlossoccurred,whichmeantthatnanofiberswereremoved completely.Moreover,theTiO2contentinthecompositescouldbe easilycalculatedfromtheweightremainderafterthesampleswere heatedover705◦C.SincethesamplePANnearlycompletely disap-pearedover720◦C,theCNT/TiO2andCNTcontentsincompositions weredeterminedtobe40.6wt%and9.6wt%forthesample PAN-CNT/TiO2-NH2andPAN-CNTcompositenanofibers,respectively. 3.2. PhotocatalyticperformanceofPAN-CNT/TiO2-NH2

compositesnanoﬁbers 3.2.1. Effectofcatalystcontent

Theamountofcatalystisanimportantparameterin optimiz-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.Theresultsindicatethatthe pho-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%ofphotoreductionwasachievedinlessthan20min irradia-tiontime andnochangesinthephotoreductionefficiencywere observedafter30minirradiationtime.Theseresultsindicatedthat thephotocatalyticreductionefficiencyincreasedwiththe incorpo-rationofCNTsandTiO2.Thistrendresultedfromthehighsurface areaofCNTs/TiO2photocatalytic.Inaddition,CNTscaneffectively generateagreaternumberofelectronsandholes,andaccelerate theprocess of thephotocatalytic reactionto enhancethe pho-tocatalyticactivity.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)andCisthe concen-trationofCr(VI)ataspeciﬁctime,andkaistherateconstantof pseudo-ﬁrstordermodel(min−1).

3.2.3. EffectofpHonthephotoreductionofCr(VI)

pH of the solution plays a major role in the photocatalytic processasitisknowntoinfluencethesurfacechargeofthe semi-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,the photoreduc-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-NH2compositesnanofibersatlowpH val-uesduetotheelectrostaticattraction,thereforeahigheryieldof photoreduction[47,48].Theelectrostaticrepulsionbetween neg-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,thusdecreasingthedegreeof

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reduc-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 Ab so rp ti on (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)withtheincreaseoftheinitial concentra-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

Inordertoinvestigatetheinfluenceactivityofthe photocat-alyticcompositenanofibersonthephotoreductionofCr(VI),the compositenanofibers afteradsorptionstep inthedarkand the photoreductionstep under visiblelight irradiation wasdirectly examinedbyXPS.Fig.8showntheXPSpatternsofthe compos-itenanofibers,whichshowedtheCr2pspectrarecordedforCr(VI) andCr(III).AstheCr2ppeakisadoublet,thepeakcomponentat lowerbindingenergycorrespondstoCr2p3/2orbital,whilethose athigherbindingenergycorrespondtoCr2p1/2orbital.Beforethe photoreductionprocesstookplace,bandscorrespondingtoCr(VI) appearedatabindingenergyof579.2and588.3eV,which con-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 464.1eVrepresentedtoTi2p3/2andTi2p1/2,respectively, indicat-ingthattitaniumboundedtooxygenremainsinoxidationstateIV forthetitanium-oxocluster.TheO1sspectrumhasabroadpeakat 531.3eVthatisindicativeofoxygeninmetaloxidessuchasTiO2. Inaddition,thepeaksintheN1sregionat399.3eVcanbeassigned totheNoftheaminefunctionality.

3.2.6. PhotoreductionofCr(VI)inthepresenceofphenol

ThePAN-CNT/TiO2-NH2compositenanofiberswastested simi-larlytotheindustrialwastewaterconsistsofamixtureoforganic andinorganicpollutants.Inthisexperiment,wetestedthe PAN-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].Theresults indi-catedthattherateofCr(VI)photoreductionwasabout1.4times higherinthepresenceofphenolthaninitsabsence,whichcan beexplainedbasedonthemechanismasdescribedinthe

follow-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/CNTNPsleadstothe gen-erationofelectron-holepairsatthesurfaceofthephotocatalyst (Eq2).Afterthemigrationofelectron-holepairstothesurfaceof theparticles,thephotogeneratedelectronsreduceCr(VI)toCr(III) (Eq.(3)),andtheholesmayleadtogenerationofO2(Eq.(4))and produce•OHradicalsintheabsenceofanyorganics(Eq.(5))[55]. Phenolcanscavengethevalencebandholeinthephotocatalytic reactionsystemleadingtoaninhibitionofrecombinationof elec-tronand holepairsonthecatalystsurface andacceleratingthe reductionofCr(VI)byphotogeneratedelectrons[56].Inthe pres-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₊_H₂_O ₍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 Int e ns it y ( a .u .)

Binding Energy (eV) Cr 2p Visible Light 472 468 464 460 456 458.2 eV Ti 2p In te ns it y ( a .u.)

Binding Energy (eV)

543 540 537 534 531 528 525 531.3 eV O 1s In te n s it y ( a .u .)

294 291 288 285 282 284.6 eV C 1s Int e ns it y ( a .u .)

408 405 402 399 396 393 399.3 eV In te n s it y ( 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).Theslightdecreaseofthe pho-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)inaqueoussolutionundervisiblelight irra-diation.Thekineticsofthephotoreductionprocessshowedthat completephotoreductionwasachievedafterapproximately30min andtheexperimentaldatafolloweda pseudo-first-ordermodel. ThephotoreductionefficiencyofCr(VI)washigherinacidic solu-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 Ph o to re d u c ti on e ff ic ie n cy (% ) Regeneration cycle 1st 2nd 3rd 4th 5th

Fig.11.ReusabilityofthecompositenanoﬁbersforthephotoreductionofCr(VI).

theprotonationdegreeofthephotocatalyticsurface.Theaddition ofphenolenhancesthephotoreductionofCr(VI),duetoitsability

toadsorbonthecatalystsurface,whichcanalsoactasahole scav-enger.TheUV–visspectrophotometerandXPSanalysesprovedthat chromateCr(VI)wasreducedtoCr(III).Furthermore,the ﬂexibil-ityandthereuseofthePAN-CNT/TiO2-NH2compositenanoﬁbers, revealtheirpromising potentialforadvanced wastewater treat-ment.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.molcata.2016.08. 010.

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