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Catalysis Today
jou rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d
Carbon sphere templates for TiO 2 hollow structures: Preparation, characterization and photocatalytic activity
Balázs Réti
a, Gabriella Ilona Kiss
a, Tamás Gyulavári
a, Kornelia Baan
b, Klara Magyari
c, Klara Hernadi
a,d,∗aDepartmentofAppliedandEnvironmentalChemistry,FacultyofSciencesandInformatics,UniversityofSzeged,Rerrichtér1,H-6720Szeged,Hungary
bDepartmentofPhysicalChemistryandMaterialScience,UniversityofSzeged,TiszaLajoskrt.84,H-6720Szeged,Hungary
cInterdisciplinaryResearchInstituteonBio-Nano-Sciences,Babes-BolyaiUniversity,M.Kogalniceanu1,400084Cluj-Napoca,Romania
dResearchGroupofEnvironmentalChemistry,InstituteofChemistry,FacultyofSciencesandInformatics,UniversityofSzeged,Dómtér7,H-6720Szeged, Hungary
a r t i c l e i n f o
Articlehistory:
Received21June2016
Receivedinrevisedform22October2016 Accepted21November2016
Availableonline28November2016
Keywords:
Carbonspheres Titaniumdioxide Hollowstructure Photocatalysis Phenol
a b s t r a c t
TiO2 hollowstructures(HS)weresynthesizedbycarbonspheretemplateremovalmethod.Nanome- tersizedcarbonspheres(CS)werepreparedbymildhydrothermaltreatmentofordinarytablesugar (sucrose).Thesizeofthesespherescanbecontrolledbytheparametersofthehydrothermaltreatment (e.g.timeandpH).TheobtainedCSswerecharacterizedbyscanningelectronmicroscopy(SEM),Raman spectroscopy,infraredspectroscopy(IR),X-raydiffraction(XRD)andthermogravimetry(TG).CSswere successfullycoatedwithTiO2viasol–gelmethod.ThephasecompositionoftheTiO2hollowsphereswere controlledbytheannealingtemperatureduringcrystallizationandCSstemplateremoval.TiO2hollow structures(HSs)werecharacterizedbySEM,XRD,Ramanspectroscopy,TGandenergy-dispersiveX-ray spectroscopy(EDX).PhotocatalyticperformanceoftheTiO2HSswasevaluatedbyphenoldegradationin abatch-typefoamreactorunderlowpoweredUV-Airradiation.Thedegradationreactionwasfollowed byhigh-performanceliquidchromatography(HPLC)andtotalorganiccarbon(TOC)measurementtech- niques.Photocatalyticactivitytestresultspointedoutthatincreasedrutilecontentuptoacertainextent (resultingmixedphaseanatase-rutileTiO2)effectsadvantageouslythephotocatalyticperformanceof TiO2HSsandtheuniquemorphologyprovedtoenhancethephotocatalyticactivity(sixtimes)aswellas TOCremovalefficiency(twelvetimes)comparedtothesamplewhichwaspreparedbythesamemethod withouttheCSs.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Sincetheindustrial revolutionthewaterpollution gradually becamemoreandmoresignificant,andnowadaysitisaburning environmentalissue[1,2].Amongthemanyhazardouswaterpol- lutants[3,4],phenolisoneofthemoststudiedbecauseitcanbe originatedfrombothanthropogenicandnaturalsources[5–7].The purificationofthisessentialmediaisimperative.Fortunately,there arealready manyeffectivetechnologicalandtechnicalsolutions toachievetheeliminationofvariouswatercontaminants.How- ever,there arechemicals which arenot removablefrom water by conventional methods due to their stability and/or toxicity
∗Correspondingauthorat:DepartmentofAppliedandEnvironmentalChemistry, FacultyofSciencesandInformatics,UniversityofSzeged,Rerrichtér1,SzegedH- 6720,Hungary.
E-mailaddress:hernadi@chem.u-szeged.hu(K.Hernadi).
towardsmicroorganisms(e.g.pesticides,antibiotics,pharmaceu- ticalmetabolites,etc.).Advancedoxidationprocesses(AOPs)are effectivemethodsfortheneutralizationofthesepersistentcon- taminants[8].Heterogeneousphotocatalysisisapromisingbranch ofAOPtechnologies.
Titaniumdioxide(TiO2)isann-typesemiconductortransition metaloxidepossessingmanyadvantageouspropertiestobecon- sideredoneofthemostpromisingphotocatalystmaterials[9,10].It ischeap,photostable,non-toxicandbiocompatible.Heterogeneous photocatalysisis a very complex photoinduced process onthe surfaceofsemiconductorparticles[11–14].Duringthephotoexci- tationofasemiconductorparticlewithenergyequalorgreaterthan itsbandgap(Eg)anelectronisexcitedtotheconductionband(CB) fromthevalenceband(VB)leavingavacancy(hole;h+)behind.
TiO2 polymorphspossessinherentlydifferentbandgapenergies (Eg∼3.2eV=388nm;Eg∼3.0eV=413nmforanataseandrutile, respectively)[15].Thephotogeneratedchargecarriers(e−andh+)
http://dx.doi.org/10.1016/j.cattod.2016.11.038 0920-5861/©2016ElsevierB.V.Allrightsreserved.
arecapabletomigratetothesurfaceoftheparticleorbeingtrapped atvarioussites[13].Ifthechargecarriersareabletoreachthe surfacetheymaytakepartinredoxreactionswithappropriate donorandacceptorspecies.Inaqueousmedia,importantdonorand acceptormoleculesareOH−(H2O)anddissolvedO2,respectively, andtheyformhighlyreactiveandoxidativeOH•andO2•−radicals [16].Theoverallphotocatalyticactivityofamaterialisdependent ofmanyfactors[17].Inmostcases,crystalphasecompositionis determinativeregardingthephotocatalyticperformance.However, mixedphase(particularlyanatase-rutile)TiO2photocatalystsmay possesselevatedactivitiesmostpresumablyduetotheirinterfacial interactions[18–24].
There is a tremendous effort in the investigation of carbon (nano)materialssincethediscoveryoffullerenes[25].Researchers preparedcarbonmaterialswithvarioussizesandshapes(e.g.fibers, onions, horns, (nano)tubes, etc.) [26]. Carbon spheres (CS) are recentlyregainedscientificinterestduetotheirpromisingappli- cationinbatterycathodes[27],fuelcells[28]andcatalystsupports [29,30].CSscanbepreparedbynumeroussynthesistechniques:
arc-discharge,CVD,hydrothermalmethodetc.[31].Aconvenient routeto produce micro- or nanosized CSsis the hydrothermal dehydrationand carbonizationofdifferentcarbohydrates(most commonlyglucose)[32,33].
Titaniumdioxidehollowspherescanbepreparedbyusingthe abovementionedCSsasremovabletemplates.Thus,thediameterof theTiO2hollowstructuresisfinelytunable.Theseobjectsareinter- estingnotjustbecausetherelowapparentdensity[34]buttheir uniqueopticalproperties[35–37].Theseattributescanbeexploited in either photocatalyticapplications orin DSSCs [38]. Asmen- tioned,themostcommonlyusedroutetoprepareTiO2(andother metaloxide)hollowspheresistemplateremovalmethod[39].Lv etal.studiedtheefficiencyofsurfacefluorinatedTiO2HSprepared viahydrolysis–precipitatemethod(usingsulfonatedpolystyrene beadsastemplates)inbrilliantredX3Bphotocatalyticdegradation reaction[40].Aoetal.preparedTiO2HSsbyprecipataionofTiO2 ontothesurfaceofhydrothermallypreparedCSsanddemonstrated enhancedphotocatalyticactivityinphotocatalyticdecomposition ofmethyleneblue[41].
Herein,wedescribethepreparationofcarbonspheretemplates fromordinarytablesugarviafacilehydrothermalmethodandtheir useastemplatesforTiO2hollowstructuresynthesis.Weintended toinvestigatetheeffectofvarioussynthesisparameters(time,pH) ontheyieldandsizeoftheCSs.TheCSsamplewithappropriatesize distributionwasselectedasremovabletemplatefortheprepara- tionofTiO2hollowstructures.Wehavestudiedthecrystalphase compositionofthehollowspheresformedduringheattreatment atdifferenttemperaturesandtheeffectofthisparameterontheir photocatalyticactivityinphenoldecompositionreaction.
2. Experimental
2.1. Materials
Allchemicalswereusedasreceivedwithoutfurtherpurification.
Duringtheexperiments,Milli-Qwater(Millipore,18.2Mcm)and absoluteethanolwasused(VWRProlabo).Carbonspheresynthe- sis:Ordinarytablesugar(sucrose,MagyarCukorZrt.,KoronásTM) wasused as carbon source.The desired pHwas adjusted with hydrochloricacid(37wt%,a.r.,Molar)orwithsodiumhydroxide (50wt%, a.r., Molar). TiO2 hollow structure synthesis: The tita- niumprecursor wastitanium(VI)butoxide (Fluka,≥97%purum, [Ti(O-CH2CH2CH2CH3)4]).Photocatalytictestreactions:Themodel pollutantwasphenol(VWR, extrapure) and duringthe exper- iments oxygen gas (Messer, 2.5) was used. Methanol (VWR
HiperSolvChromanorm)wasusedduringtheeluentpreparation fortheHPLCanalysis.
2.2. Samplepreparation
First, CSs were synthesized via hydrothermal treatment. All hydrothermalexperimentswerecarriedoutinapreheateddrying ovenat180◦CusingaTeflon-linedstainlesssteelautoclavewith totalvolumeof275cm3.DuringeachCSpreparations,80cm3of 50g/dm3sucrosesolutionwastreated(Vfill/Vtot=29%).Theeffect ofhydrothermal treatmenttime (3,6, 12,18h)wasstudiedon theresultedproduct.ToinvestigatetheeffectofpH,thesucrose solutionwasadjustedby HClor NaOHsolutionstoachieve the desiredvalues(3,7,12;forthesakeofnotation,theunmodified sucrose solutionwillbeconsideredas pH7 sincesucrose does notinherentlychangethepHwhendissolvedinwater,thusthe pHofthissolutionis∼7).Afterthehydrothermaltreatment,the autoclave wasleftto cooltoroomtemperature naturally,then thebrownish-blackproductwascollected.Thesampleswerecen- trifuged(4000rpm,20min)andredispersedinwaterthreetimes.
Then,theywerefilteredwithamembranefilterapparatus(Milli- pore,DuraporePVDFmembrane,47mm,0.1m)andwashedwith hotwater,thanwiththreealiquot5,15,45V/V%ethanol/water mixturestoremoveresidualorganiccontaminants.Thesolidprod- uctwasdriedinairat70◦Cfor18h.SamplelabelingfortheCSs willbethefollowing:CS-t-pH,where‘t’isthetimeofhydrothermal treatment,‘pH’isthepHofthestartingsucrosesolution.
TheCSsamplewiththepreferredpropertieswasusedasatem- plateforthesynthesisofTiO2hollowstructures.2goftheCSswas suspended in130cm3 ethanol viaultrasonicationand0.64cm3 waterwasaddedtothesuspension(namedasmixture“A”).4cm3 titanium(IV)butoxidewasaddedto70cm3absoluteethanolunder vigorousstirring(namedasmixture“B”).Mixture“B”wasadded dropbydrop(∼2cm3/min)tomixture“A”undervigorousstirring.
Themolarratiowasn(Ti):n(H2O)=1:3.Afterthefulladditionof thetwomixtures,itwaslefttobestirredfor1handthenitwas filteredandwashedwith10cm3ethanolthreetimes.Theproduct wasdriedinairat70◦Cfor18h.Theabovedescribedprocesswas repeatedthreetimesandtheportionswereunited.Thewholecoat- ingprocesswasrepeatedunderthesameconditionsexpectthatfor startingmaterialwasthepreviouslycoatedcarbonspheres(instead ofpristineCSs)andallprecursor,ethanolandwaterquantitieswere tripled.Aliquotamountofsampleswereannealedinastaticfur- naceinairwithaheatingrateof5◦C/minfor4hat400,500,600, 700◦CtoremovetheCScoreandsimultaneouslyconverttheamor- phoustitaniaphasetocrystallineTiO2.TiO2referencesamplewas preparedviaexactlythesamemethodasTiO2HSswere,except thatduringthesynthesisprocedureCSswerenotadded.Refer- encesamplewascalcinatedwithheatingrateof5◦C/minfor4hat 500◦Cinair.(CharacterizationofthereferenceTiO2canbefound inSupplementarymaterial.)
2.3. Characterizationtechniques
Thestructure and morphologyof theprepared sampleswas investigatedwithscanningelectronmicroscopy(HitachiS-4700 TypeIIFE-SEM).Crystalstructureandphasecompositionwasmea- suredwithX-raydiffractometry(RigakuMiniFlexIIDiffractometer) usingCuK␣radiation.Thermogravimetricanalysis(NetzschSTA 409PCconnectedtoaPfeifferQMS200massspectrometersystem) wasperformedinoxygenflow(40cm3/min)with5◦C/minheating rateusing∼100mgsample.Ramanspectrumwastaken(Thermo ScientificDXRRamanMicroscope)utilizing532nmlaserirradia- tion.FT-IRspectrumwastakenusingaBioradFTS-60AFT-IRdevice usinganATRmoduleonanair-drysample.Theconcentrationof phenolwasmeasuredwithaHPLCtechnique(MerchHitachisys-
Fig.1.Theeffectofsynthesistimeontheyieldofthecarbonspheres.
tem).LiChrospherRP-18ecolumn andV(methanol):V(H2O)=1:1 eluentwasusedduringtheHPLCanalysis.Totalorganiccarboncon- tentofphenolsolutionwasdeterminedbeforeandafter(samples werecentrifugedfirst)thephotocatalytictestreactions(Analytik JenaMultiN/C3100).
2.4. Evaluationofphotocatalyticefficiencies
Photocatalyticdegradationofthephenolmodelpollutantwas carried out in a cylindrical shape batch-type suspension foam reactorequippedwiththermostatingjacketmadefromglass.The diameter of the inner tube was 45mm (V=250cm3). To pro- vide sufficient mixing and dissolved oxygen level, oxygen gas was bubbled though a fritted glass to the solution with flow rate of 430–450cm3/min. The UV-A lamp (Sylvania Blacklight F6W/T5/BL350) was immersed in the center in a quartz tube (25mmin diameter). All photocatalytic experimentswere per- formedat 25±0.5◦C. The initialphenol concentration and the photocatalyst load was 1×10−4mol/dm3 and 0.5g/L, respec- tively. The photocatalyst sample was added to the 200cm3 1×10−4mol/dm3 phenolsolutionand sonicated for5min then transferredtothephotoreactor.Thephotocatalystsuspensionwas lefttobestirredindarkfor30mintoreachadsorption/desorption equilibrium.Atthatpoint,theUVlampwasswitchedonandsam- plesweretakeninregulartimeintervals.Totalirradiationtimewas 90min.Eachphotocatalyticexperimentsweredoneinduplicateto checkreproducibility.
Aftertheexperiment,theacquiredsampleswerecentrifugedat 16000RCF.Eachsamplewasfilteredbyasyringefilter(Whatman, Anotop25+,0.02m)toremoveeventhefinesparticlespriorthe HPLCanalysis(0.9cm3/minflowrate,UV–visdetectionat210nm).
TOCcontentwasdeterminedtoevaluatetheoverallmineralization potentialofthephotocatalysts.
3. Resultsanddiscussion
3.1. Carbonspheres
3.1.1. Theeffectofsynthesistime
First,asanimportantaspectofanysyntheticprocedure,the effectofthedurationofhydrothermaltreatmentwasstudiedon theyieldandthesizedistributionoftheCSs.Afterthepreparation ofCSs,theyield(definedasmCS/msucrose)wascalculatedandits dependenceonthesynthesistimeisshowninFig.1.Bytheendof thethreehourreactiontheyieldwasverylow(1.4%),whichcan beexplainedbythelownucleationrateoftheCSseeds.Increased reactiontimeleadstoincreasedyield,butaftertwelvehoursthe changeinyieldislesseningandmostprobablyleadingtosaturation
duetotheconsumptionofthesucrose(accordingtoourdatathe estimatedmaximumyieldisaround30%).Basedonthesefinding, sufficientyieldsareachievablebyapplyingatleast12hreaction time.Themorphologyandsize(diameter)oftheCSswereinvesti- gatedbySEMtechnique(Fig.2).Afterjustthreehoursofreaction time(CS-3-7),well-definedspherestructureswereobserved.The particlesizedistributionwasrathernarrow;mostofthespheres hadadiameteraround350nm.Sixhoursofhydrothermaltreat- mentresultedsimilarproduct(CS-6-7)comparedtothesample CS-3-7.Therathernarrowsizedistributionisshiftedtowardsbig- gerparticlesizes;theaverageparticlesizewasaround590nm.
ThesizedistributionoftheCS-12-7samplewaswidercompared tothepreviouslymentionedone;aswellastheaveragesizeof thesphereswasincreasedto∼660nm.Inthissamplejointspheres wereformedbuttheycouldbeobservedinarelativelylowamount.
OntheSEMpictureofCS-18-7,CSswithgreatersize werealso detectable,andjointstructuresweremoreprominent.Theaver- agesizeofthespheres isincreasedto∼700nmand widersize distributioncouldbeobserved.Overall,atshortersynthesistimes thenucleationofCSswasmoresignificant,andlateronthenucle- ationsloweddownand thegrowthofCSbecamethedominant process.Combiningtheexperienceswiththeonesregardingthe yields,furtherexperimentsweredonewith12hofsynthesistime.
3.1.2. TheeffectofpH
Afterfinding themostconvenient but satisfactory synthesis time,theeffectofpHwasinvestigatedonthemorphologyofthe CSs.ThepHvaluesofthesucrosesolutionsweresetto3and12 beforethe hydrothermal treatment (samplewith pH 7, CS-12- 7,hasalready beendemonstrated intheprevioussection).The yieldofthesesynthesizeswere23%,26%and28%fortheCS-12-3, CS-12-7,CS-12-12,respectively.Onlyaslightincreasewasnotice- ablewithincreasingpH.Fig.3showsconspicuouslythedifference betweentheCS-12-3synthesizedunderacidic,andtheCS-12-12 underbasicconditions.CS-12-3samplepreparedinacidicmedia showedquitewideparticlesizedistributionandhaveanaverage particlesizearound810nm.Larger,jointparticlesarealsopresent inthesample.CS-12-7sampleownsimilarparticlesizedistribu- tiontoCS-12-3,however,theaverageparticlesizewasdecreased toaround660nm.CS-12-12sampleshowednarrowersizedistri- butioncomparedtothepreviouslydiscussedsamples.Theaverage particlesizewasaround400nmandthepresenceofjointparticles wasnotprominent.Accordingtotheseresultsitcanbeconcluded thatthepHofthesucrosesolutionhassignificantimpactonthesize ofthepreparedCSs.Aplausibleexplanationofthephenomenon canbethefollowing:Sucrosefirstundergoesdehydrationreaction resulting HMF (5-(hydroxymethyl)-2-furaldehyde) which then take part in polymerization and aldol condensation reactions [32,33,42].Itisreportedthataldolreactionofcyclicandaromatic ketones/aldehydesaremorefavorableunderalkalineconditions [43].Thus, thismayleadtofasteraccumulationofpolymerized specieswhicheventuallyresultsrapidnucleation.Subsequently, particlegrowthismoredecisiveinthelaterperiodofthehydrother- malprocess resultingincreased numberof smallerparticles. In acidicmedia,thenucleationissomewhatelongated,followedby particlegrowthresultinglargerparticles and wideparticlesize distribution.SinceCS-12-12sampleshowedgoodyieldandquite uniformCSsithasbeenstudiedfurtherandusedastemplatefor thepreparationofTiO2hollowstructures.
3.1.3. CharacterizationofCSs
CS-12-12samplewasstudiedwithvarioustechniques(Fig.4).
XRDwasusedtoinvestigatethepossiblecrystallinityoftheCSs (Fig.4,A).Onlyonebroaddiffractionat∼22◦dominatesthediffrac- togram which belongs toamorphous carbon [44].Reflection at
Fig.2.Morphologyofcarbonspherespreparedviahydrothermaltreatmentatdifferentsynthesistimes(indicatedintheupperleftcorneroftheSEMpictures).Particlesize distributionsofCSsareshownonthehistogramsrighttothecorrespondingSEMpictures.PleasenotethattheSEMmicrographsandthehistogramsaredirectlycomparable duetotheirsamescaleandformatting.
∼26.5◦ wasnot detectedwhich would indicate well-structured graphiticsegments.Nocrystallineimpuritiesweredetected.
RamanspectroscopywasusedtoanalyzetheCSs(Fig.4B).Inthe Ramanspectra,typicalRamanfeaturesofcarbonmaterialswere present,namelytheD(∼1350cm−1)andG(∼1580cm−1)bands.
WhiletheDbandcanbeattributedtothepresenceofsp3hybridized carbonatoms,theGbandisanindicatorofconjugatedsp2carbon atoms.Inhighlygraphiticcarbonmaterials2Dband(∼2700cm−1) isalsopresent,howeveritismissingfromtheRamanspectraof theCSs.BothDandGbandsarebroad,meaningaratheramor- phousstructure.ThehighintensityGbandcomparedtotheDband indicatesthatmostofthematerialconsistsofconjugatedcarbon framework.
FT-IRtechniquewasusedtofurtherinvestigatethechemical characteristicsoftheCSs(Fig.4C).Thespectrum isfeature-rich and quitecomplex. Broad bandappear inthe 1800–3800cm−1 whichindicatesthestretchingvibrationsofstructuralOHgroups
andphysisorbedwater.Lowintensitybandsataround2922and 2970cm−1canbeassignedtoC Hstretchingvibrations.Thevery intensebandsituated at1701cm−1 belongstheC Ostretching vibrations.Anotherintensebandat1611cm−1 ismostprobably thesignof the C C vibration in aromaticring; however, some oxygencontaininggroups(e.g.cyclicethers)couldappearinthe sameregion[45].Intenseandbroadbandispresentbetween1100 and1400cm−1whichindicatesC Ovibrationinhighlyconjugated, aromaticorinvariousotherchemicalenvironments[46].
Thethermal behavior ofCSswasalso investigated(Fig.4D).
HeatingtheCSsinoxygenatmospherestwointenseweightloss can be seen. The first is between 220 and 250◦C, the second is 410–440◦C and thetwo regions arelinked witha moderate mass-reducingperiod. The former is most probablydue to the decomposition of various oxygen containing functional groups whilethelatterisduetothecombustionofthecarbonitself[47,48].
Fig.3. TheeffectofpHonthemorphologyofthepreparedcarbonspheres.PleasenotethattheSEMmicrographsandthehistogramsaredirectlycomparableduetotheir samescaleandformatting.
Thelowdecompositionandcombustiontemperatureisconvenient incaseoftheremovalofthesetemplatematerials.
Accordingtoourresults,CSswereconsistofamorphouscarbon withnographiticdomains.However,RamanandFT-IRresultsindi- catedsignificantconjugated/aromaticsegmentsaswellasvarious oxygencontainingfunctionalgroups.
3.2. Titaniumdioxidehollowstructures
CS-12-12carbonspheresamplewerecoatedwithTiO2layervia sol-gelmethod.Thismaterialwasthencalcinatedinairtoremove theCStemplateandtoconverttheamorphoustitaniaintocrys- tallinetitaniumdioxide.ElaborationofTiO2HSsfromas-prepared, non-heat-treated(nht)CS-titaniawasstudiedbyTG-MStechnique (Fig.5).The TG-DTG showedsimilarfeatures totheTG-DTG of uncoatedCSs(Fig.4D).Thefirstmajor,welldefinedweightlos- ingperiodwasat around∼220–250◦C whilethesecond wasa moreelongated,broadregionwhichwasstartedrightafterthe firstoneandwasbetween∼250–480◦C.Theelongatedcombus- tionofCStemplatescomparedtouncoatedCSscanbeattributed tothepresenceofTiO2 shell.Above∼480◦C, noweight-change canbedetected.Theremaining27.4%massiscontributedtothe incombustiblecrystallineTiO2.Theweightlosswasapproximately 73%independentlyfromthecalcinationtemperatureincaseofall TiO2HSs.Consequently,ifat700◦CCSsandallcombustiblecom- poundsareburnedaway;andthereisnovariationintheweight lossbetweenthecalcinationtemperaturesthennoorganicresidue fromtheprecursor/solventorCSsisleftinTiO2HS,notevenatthe lowesttemperature.ThiswasfurtherconfirmedbytheMSresults (Fig.5,rightpanel).COandCO2wereidentifiedasmaingaseous products.Thetemperaturerangeoftheevolutionofthetwogases isidenticaltothemainmassreductionregionsmentionedinTG- DTGresults.Thus,consideringtheweightlosingrateandtheMS resultsitcanbededucedthatevenat400◦C4hofcalcinationtime isenoughtocompletelyeliminateallcarbon,nottomentionhigher temperatures.
Fig. 6 shows the morphology of the TiO2 hollow structures annealedatdifferenttemperatures.Roundforms,wholeanddam- agedstructurescanbeseeninthepictures.Smaller,fragmented shellparticlesarescatteredbetweenlargerhollowspheres.Shell thicknessisbetween20and40nmregardlessoftheannealingtem- perature.Thisthicknessisthickenoughtopreservetheshapeof thetemplateandthinenoughtobepermeableforthephotonsto exploitthespecialshapeoftheobject.
Crystallinity of the TiO2 samples was investigated by XRD technique(Fig.7).Diffractionintensitiesoftheindividualdiffrac- tograms were normalized to the intensity of anatase (101) reflection.Allsamplesshowcharacteristic,rathersharpreflections markedwiththeappropriateMillerindices.Incaseofthesamples annealedat400◦Cand500◦ConlyanatasephaseTiO2ispresent.As expected,athighertemperaturesrutilephasewasformed.Rutile contentofsamplescalcinatedat600◦Cand700◦Cwerecalculated accordingtothewell-knownequationdeterminedbySpurretal.
[49]andtheycontained37w/w%and65w/w%rutile,respectively.
Fullwidthathalfmaximum(FWHM)ofanatase(101)reflectionis decreasingproportionallywiththeincreasingheattreatmenttem- peraturefrom0.47◦ to0.26◦,from400◦Cto700◦Crespectively.
Itcanbededucedthatamoreordered,well-crystallizedstructure provideslessrecombinationcentersforthephotogeneratede−/h+ pairs.At600◦C,rutilephaseTiO2appears.FWHMofrutile(110) reflectionwasdecreasedfrom0.28◦to0.22◦,from600◦Cto700◦C respectively.BasedontheX-raydiffractograms,thecrystallitesizes weredeterminedusingtheScherrerequation(seeSupplementary materialTableS01).Asanticipated,thecrystallitesizesincreased graduallywithincreasingannealingtemperature.
SpecificsurfaceareaofTiO2 HSsampleswasdeterminedby BETmethod(seeSupplementarymaterialTableS01).Also,detailed measurementwasexecutedona representativesampleofTiO2 HS(annealedat600◦C)inordertoinvestigatenotjustthespe- cificsurfaceareabutalsotheporestructure(Fig.8).Thespecific surfaceareaandthetotalporevolumewerefoundtobe26m2/g and0.14cm3/g,respectively.Theobtainedspecificsurfaceareaval- uesofTiO2HSsarerathersmallcomparedtoporousmetaloxide
Fig.4.X-raydiffractionpattern,RamanandFT-IRspectraandTGcurveofCS-12-12 areshownfromtoptobottom,labeledA–D,respectively.
materials[50].Thespecificsurfaceareacanberoughlyapproxi- matedtogaininformationonthemagnitudeofthespecificsurface areaofsphericalhollowobjects.Assumingnon-porous,perfectly sphericalshellofanatasewhichcanbeelaboratedonthesurface ofD=400nmCSswiththicknessof30nm,thespecificsurfacearea iscalculated17m2/gwhichisinreasonableagreementwiththe obtainedspecificsurfaceareavaluesofTiO2HSs(seeTableS01).
TheN2 adsorptionmeasurementrevealedthepresenceofmeso- andmacropores;however,theorderofmagnitudeofthesepore volumesisrathersmall(10−4cm3nm−1g−1).Thus,theyhaveonly smallcontributiontospecificsurfacearea.Thereisadecreasing trendinthespecificsurfaceareaoftheTiO2HSswithincreasing annealingtemperature.Thisisduetotheconsolidation,densifica- tionandsinteringoftheTiO2particles/crystallitesandTiO2HSsas well.
Briefly, crystallineTiO2 hollowstructures werepreparedvia completeremovalofCStemplatesatdifferenttemperatures.Inde- pendentlyfromtheannealingtemperature,thehollowspheroid shapewaspreserved;however,insomeextentthesphereswere fragmented.TheTiO2HSsdoesnotshowwell-definedporestruc- tureanditcanbeconsideredasrathersolid/denseshellofTiO2
withahollowinside.
3.3. Photocatalyticactivity
PhotocatalyticperformanceofthepreparedTiO2HSswasinves- tigatedwiththephotocatalyticdecompositionreactionofphenol underlowpoweredUV-Airradiation.Thephotocatalyticactivity wasrepresentedbytheapparentrateconstant (kapp).Thiswas calculatedfromthefirst40minirradiationperiodbylinearfitting ofthe−ln(c/c0)vstrelationassumingpseudofirstorderreaction kinetic.Phenolphotocatalyticdegradationcurvesaswellthepho- tocatalyticactivitiesaresummarizedinFig.9.AllpreparedTiO2 HS sampleshave shown considerable photocatalytic activityin phenoldegradationreaction.Sincenocarboncontaminationorig- inatedfromtheusedtitaniumprecursor,solventsandCSswere detectedtheeffectofsuchimpurityonthephotocatalyticactivity isimprobable.N2 adsorptionmeasurementsshowedthespecific surfaceareaoftheTiO2 HSsisdecreasingwithincreasingcalci- nationtemperaturewhichindicatesthatTiO2HS700◦Chashigh photocatalyticactivitydespiteofitsratherlowspecificsurfacearea.
Photocatalyticactivity wasincreasedparallel tothe calcination temperaturewhichcanbeexplainedbythecrystallizationofthe samples.Withtheappearanceofrutilephasethephotocatalytic activityhasincreasedfurthertoacertainlimit.Lightabsorption propertiesoftheTiO2 HSsampleswereinvestigatedbydiffused reflectancespectroscopy(seeSupplementarymaterialFig.S08).In caseoftitaniumdioxidehollowstructures,theresultsindicated increasedlightharvestingfeatureinthemostoftheUVregioncom- paredtothedensereferencesample.Thishigherlightabsorption (appearaslowerreflectanceinFig.S08)cancontributetotheele- vatedphotocatalyticefficiencyofTiO2HScontrarytodenseTiO2 referencesample.
Itiswell-knownthatthecoexistenceofanataseandrutilephase canenhancethephotocatalyticactivity.Theincreasedphotocat- alytic activityof mixed anatase/rutile phase TiO2 is due to the differencein thebandposition ofanataseand rutileaswellas tothesolid-solidinterfacecangreatlyinfluencethephotogener- atedchargecarrierseparation.Thisinterfaceisresponsibleforfast chargecarrierdiffusiontothesurfaceoftheparticlethusinflu- encingthechargetransferandrecombinationprocesses[51,52].
Also,itisbelievedthatrutilephaseisabletoacceptelectronsfrom anatasephase,thusactingasan“electronsink”sincethisprocess isthermodynamicallyfavorabletotheirbandpositions[53].
Totalorganiccarbonremovalcapabilitywasalsostudied.The changeofTOCremovalefficiencyandthephotocatalyticactivity
Fig.5.TG-DTGcurve(left)andMSresults(right)ofnon-heat-treatedtitaniacoatedCSs.
Fig.6.MorphologyoftheTiO2hollowstructuresannealedatdifferenttemperatures.
Fig.7.X-raydiffractionpatternofTiO2hollowstructuresannealedatdifferent temperatures.
wasingoodagreement.HighpercentageofTOCremovalindicates notjustefficientphenoldegradationbutalsopotentmineralization properties.AsmalldropofTOCremovalefficiencywasnoticedin caseoftheTiO2HSannealedat700◦Cwhichcanbeexplainedbythe differentphotodegradationpathwayofmineralizationofphenol
onanataseandrutilephaseTiO2[54].Thus,forapplicationswhere efficientmineralizationoforganiccontaminantsisdesiredmixed phaseTiO2canbeusedadvantageously.
Asa reference, a TiO2 sample wasprepared with thesame methodastheTiO2 HSsbutwithoutCStemplate.Thephotocat- alyticactivityofthissample couldbecomparedtotheTiO2 HS calcinatedatthesametemperature(500◦C).Asaresult,TiO2HS showed6timeshigherphotocatalyticactivityand12timeshigher TOCremovalefficiencyduring90minUV-Airradiationcompared toitscounterpartpreparedwithoutCStemplate.Thus,addition ofCStemplatetoanexistingTiO2preparationmethodmakeshuge differenceinphotocatalyticactivitythatisdefinitelyworthexploit- ingastheresultsindicated.
4. Conclusion
Inthiswork,wereportthesuccessfulpreparationoftitanium dioxidehollowstructuresbyremovalofcarbonspheretemplates synthetizedviahydrothermaltreatmentofsucrosesolutionusing ordinarytablesugar.Firstly,hydrothermalsynthesisparameters ofCSpreparationwereinvestigatedemphasizingonthereaction timeandthepHofthesucrosesolution.Bothfactorshavedetermi- nativeeffectonthesizedistributionoftheCSproduct.Increasing reactiontimeresultedincreasedyieldandparticlesizeaswell.The
Fig.8. N2adsorptionmeasurement(left)andporesizedistribution(right)ofrepresentativeTiO2HSsample(calcinatedat600◦C).
Fig.9.PhenolphotocatalyticdegradationcurvesofthedifferentTiO2hollowstruc- turesamples(top).Photocatalyticactivity[striatecolumn]andtotalorganiccarbon contentremovalefficiency[densecolumn]oftheTiO2hollowstructuresamples (bottom).
studyontheeffectofinitialpHofthesucrosesolutionrevealedthat underacidicconditionslargerCSsareformed;however,inalkaline mediasmaller,morehomogeneousparticlesizewasachievedand theaverageparticlesizewasaround400nmwithacceptableyield.
CharacterizationsoftheCSsrevealedthatthesespheresconsist ofamorphouscarbon.However,theycontainsignificantamount ofconjugated/aromaticsegmentsandvariousoxygencontaining functiongroupswhichisadvantageousforthepreparationofTiO2 shellsontheirsurface.CSswerecoatedwithTiO2viasol-gelmethod andcalcinatedatdifferenttemperaturestoremovetheinnercar- bonspherecoreandtoconverttheamorphoustitanialayerinto crystallineTiO2.TiO2 hollowstructureswereobtainedandtheir morphologywasinvestigated.Afterannealing,thesphericalchar- acteristicwaspreservedandTiO2shellswith20–40nmthickness wereformed.Asexpected,someimperfectandfragmentedshell structureswerealsopresent.PhotocatalyticactivityoftheTiO2HS wasstudiedbyphenoldegradationreactionunderlow-powered UV-Airradiation.AllTiO2HSsamplesshowedsignificantphotocat-
alyticactivityaswellasgoodmineralizationefficiencymeasured byTOCtechnique.TiO2 HSscontainingrutilephase (alongwith anatase)showedevenhigherphotocatalyticactivitycomparedto onlyanatasecontainingonesmostprobablyduetosynergiceffect betweenthetwopolymorphs. TiO2 hollowstructurephotocata- lystprovedtohavesixtimeshigherphotocatalyticactivity(and twelvetimeshigherTOCremovalefficiency)thanitscounterpart preparedbythesamemethodwithouttheapplicationofCStem- plate.Detailedinvestigationoftheopticalproperties(measured andsimulated)oftheTiO2HSwillbethetopicofournextreport.
Hopefully,ourworkcontributestotheapplicationofrenew- able,biomassbasedmaterialsaswellastheirutilizationineffective photocatalyticorenergyharvestingprocesses.
Acknowledgments
Theauthorswishtoexpresstheirdeepestandsincerestrecogni- tionofProf.AndrásDombi,akeyfigureinthetopicofphotocatalytic materialsforthedegradationofcontaminantsofenvironmental concern.TheresearchandBRweresupportedbyNKFIunderthe NN114463grantandtheSwissContribution(SH/7/2/20).
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod.2016.11.
038.
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