photocatalytic properties of hierarchical zno flowers synthesized by a

Photocatalytic activity was evaluated using the degradationof organic dyemethylorange.The sucroseaddedZnO flowers showed improved activity, whichwas mainlyattributedtothebetter crystallin

jo u rn a l h om epa g e :w w w e l s e v i e r c o m / l o ca t e / a p s u s c

Photocatalytic properties of hierarchical ZnO flowers synthesized by a

a Key Laboratory of Photonic and Electric Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, PR China

b Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150080, PR China

c Department of Physics, Northeast Forestry University, Harbin 150040, PR China

a r t i c l e i n f o

Article history:

Received 6 November 2011

Received in revised form 4 April 2012

Accepted 5 April 2012

Available online 24 July 2012

Keywords:

ZnO flowers

Photocatalytic properties

Hydrothermal method

Sucrose

a b s t r a c t

In this work, hierarchical ZnO flowers were synthesized via a sucrose-assisted urea hydrother-mal method.The thermogravimetric analysis/differential thermal analysis (TGA–DTA) and Fourier transform infraredspectra (FTIR)showedthatsucroseactedasacomplexing agentinthe synthe-sis processand assistedcombustion duringannealing Photocatalytic activity was evaluated using the degradationof organic dyemethylorange.The sucroseaddedZnO flowers showed improved activity, whichwas mainlyattributedtothebetter crystallinityasconfirmedbyX-ray photoelec-tronspectroscopy(XPS)analysis.Theeffectofsucroseamount onphotocatalyticactivity wasalso studied

© 2012 Elsevier B.V All rights reserved

Inthelastdecade,zincoxide(ZnO)nanostructureshavearoused

tremendous attention due to its distinguished performance in

piezoelectric systems,optoelectronics, photovoltaicenergy

con-version,photocatalyticdecompositionoforganicpollutantsandas

chemicalsensingelements.Also,ithasbeenfoundthatthose

prop-ertiescanbeimprovedwithspecialmorphologies,shapes,sizes

andcrystallinityofZnOnanostructures[1–6].Thus,thedesigned

andcontrollablefabricationsofZnOwithspecificmorphologiesand

structureshavebeenexploredtogainsuperiorpropertiesinrecent

years[7–10]

Three-dimensionalhierarchicalZnOexhibitedexcellentoptical

andcatalyticproperties.Primaryroutesforthree-dimensional

hier-archicalZnOsynthesisincludevapor–liquid–solid(VLS)growthat

relativelyhightemperature,electrochemicalandsolution-based

methodsforself-assemblyofhierarchicalZnO[11,12].Amongthese

synthesismethods,thehydrothermalmethodisasimple,facileand

∗ Corresponding author at: Key Laboratory of Semiconducter Nanocomposite

Materials, Ministry of Education Department of Physics, School of Physics and

Elec-tronic Engineering, Harbin Normal University, Harbin 150025, PR China.

Tel.: +86 451 88060526; fax: +86 451 88060629.

∗∗ Corresponding author Tel.: +86 451 88060526; fax: +86 451 88060629.

E-mail addresses: xulingling hit@163.com (L Xu), zhaoyan516@126.com

(Y Zhao).

controllablewaytoobtainlargeyieldswithuniquemorphology ZnO canbeusedasa kindof photocatalyst,whichdecomposes organicpollutantswithultra-violetlightexcitation[2,4,13].The hierarchicalstructuresincreasedtheefficiencyofoptical absorp-tionandenhancedthephotocatalyticactivity.Tosynthesizethe hierarchical mesoporous ZnO, the multi-layeredbasic zinc car-bonate (LBZC) was reported to be used as a precursor in the ureaprecipitationorhydrothermalmethod[14,19].Severalreports aboutthefabricationofLBZChaveconcernedabouttheeffectsof surfactants

In the past decade, kinds of morphologies of ZnO can

be synthesized with different surfactant, like cetyltricetyl-trimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyethylene glycol (PEG) and so on [21,4,22,23] Usu-ally,theenvironmentally-friendly,low-costandeasily-obtainable sucroseisusedasfuelinthecombustionsynthesisprocedurefor ceramicmaterialfabrication[5,6,15–17].Also,itisreportedthat sucrose can play the role of chealtingagent after the hydroly-sation inacidsolution.Inthis work,sucrosewasintroducedin the urea hydrothermal procedure tofabricate hierarchical ZnO flowers as a chelating agent and fuel The annealing process

of sucrose added precursor wasperformed and moreheat and gases were released, resulting in the good crystallization and largereaction areas in ZnO flowers.Thephotocatalytic proper-tiesofZnOflowersdependentonthesucrosecontentwerealso discussed

0169-4332/$ – see front matter © 2012 Elsevier B.V All rights reserved.

2 Experimental

2.1 PreparationofZnOflowers

Allthechemicalswereanalyticalgradereagentsandwereused

withoutfurtherpurification.Firstly,0.002Mzincnitratesolution

waspreparedbydissolvingproperZn(NO3)2indeionizedwater.In

atypicalprocedure,0.006molureapowderwasaddedinto20mL

0.002MZn(NO3)2solutionwithvariablequantityofsucrose.After

acontinuousstirringfor15min,themixedsolutionwastransferred

intoa50mLTeflonbottleheldinastainlesssteelautoclave,which

waskeptat90◦Cfor2h.Thewhiteprecursorwaswashedfor

sev-eraltimeswithdeionizedwaterfollowedbydryinginairat75◦Cfor

12h.Furtherheat-treatedwascarriedouttoobtainthefinalZnOat

300◦Cfor2h.Thesampleswith0.08gand0gsucroseaddedwere

labeledasP0andS0.Inordertoassesstherelationshipbetween

theamountofsucroseandthephotocatalyticactivityofZnO,

vari-ableamountofsucroseaddedsampleswerepreparedthroughthe

similarprocess,labeledasP−2,P−1,P1andP2for0.04g,0.06g,0.1g

and0.25g,respectively

2.2 Characterization

Thethermaldecompositionprocessoftheprecursorswas

inves-tigatedbythermogravimetricanalysis/differentialthermalanalysis

(TGA–DTA)usingaTASDT2960instrument.Itwasperformedinair

from40to1000◦Cwithaheatingrateandflowrateof10◦Cmin−1

and 100mLmin−1,respectively PowderX-raydiffraction(XRD)

analysiswascarriedoutbyaRigakuD/Max-2550/pc

diffractome-terusingCu-K␣radiation.TheIRspectraofsucroseandsamples

before/after heat treatmentwere determined by Fourier

trans-forminfraredspectroscopy(FTIR,BrukerIFS66v/s)usingKBrdisc

method.TheratioofKBrtosampleswasabout300:1inweight

ThemorphologiesofZnOflowersobtainedwithvarioussucrose

amountswererevealedbyascanningelectronmicroscope(SEM,

HitachiS-4800).X-rayphotoelectronspectroscopy(XPS)

experi-mentsweremeasuredwithaK-Alpha (ThermofisherScienticfic

Company)X-rayphotoelectronspectrometerusingAlK␣radiation

(12kV,6mA).Thebindingenergiesofelementswerecalibratedby

takingcarbonC1s(285.06eV)asreference

2.3 Photocatalyticactivitiestests

Inthiswork,thephotocatalyticactivitiesofhierarchical

struc-turesZnOweretestedbyusingmethylorange(MO)asthemodel

pollutant.0.02gsamplewasaddedinto50mL,1.2×10−5MMO

solutionandmechanicallystirredindarkfor20mintoachievethe

adsorptionequilibriumofMOwithZnObeforetheUVirradiation

Inacoolwaterbath,themixturewasirradiatedbytwoUVlamps

(Philips,8W)withcontinuousstirring.Thesamplesweretakenout

fromthemixedsuspensionatevery20mintocheckthechangesof

MOconcentration.ToremovethecatalystsofZnO,centrifugation

wascarriedoutat10,000rpmfor10min.TheUV–visabsorption

spectraofthecentrifugedsolutionsweremeasuredontheHITACHI

UV/visspectrometer(U-3010)

Toinvestigatetheappropriatecalcinationstemperatureforthe

transformationoftheprecursortoZnO,thethermalanalysisinair

atmosphere wasconducted.TypicalTGA/DTA plotsfor the

pre-cursorofsampleP0isshowninFig.1.Atthebeginning,asmall

endothermicpeakwith5.4%weightlosscanbeobserved,which

ismainlyattributedtotheevaporationofwaterintheprecursors

Inthetemperaturerangeof100–400◦C,anobviousendothermic

Fig 1. TGA–DTA curves of the precursor of P 0

Fig 2.XRD patterns of the samples: (a) the precursor of P 0 , (b) P 0

peakscenteredat259.3◦CcanbefoundinDTAcurve Simultane-ously,afasterweightlossstage,claimedas25.8%canbeobservedin TGAcurve.Thethermaldecompositionprocessescanbeascribedto thedecompositionandoxidationoftheprecursorbythereleasing

ofwaterandcarbondioxide.Therefore,theannealingtemperature waschosenat300◦Ctoobtainthefinalproducts

ThepurityandcrystallinephaseofP0 andtheprecursorofP0 weredeterminedbyXRD.Fig.2(a)showedtheXRDpatternsof theprecursor.Asacomparison,theXRDpatternofZnOproduct (P0)aftercalcinationwasalsopresented(Fig.2(b)).Thediffraction peaksinFig.2(a)canbeidentifiedastheZn4(CO3)(OH)6H2O,which wasconsistentwithJCPDSCardNo.11-0287.While,thediffraction peaksofP0 canbeidentifiedaspurehexagonalZnO(JCPDSCard

No.36-1451).TheXRDpatternsofP0andtheprecursorare con-sistentwithourpreviousresultswithnosucroseaddedsynthesis procedure[19].Itshowsthatthesucroseascomplexingagentwill notinfluencetheformationoftheprecursor(Zn4(CO3)(OH)6H2O) andthefinalproductZnO

Inthesynthesisprocess,sucrosewasintroducedintotheurea hydrothermalprocedure Toclarifytherole ofsucroseactingin thecrystalgrowth,FTIRspectraweremeasuredtoverifythe possi-bleintermediateby-productsandtheresultswereshowninFig.3

Wefoundthatsucroseplayedtherolesofcomplexingagentand fuelinthesynthesisprocess.Inacidicsolution,thesucrosefirstly hydrolyzesintoglucoseandfructose,whichcanbefurtheroxidized intosaccharicacid,glycolicacidandtrihydroxy-butyricacidwitha largenumberof–COOHand–OHgroups.Furthermore,the COOH groupscaneasilycombinewithmetalionsinthesolution,which

isquitesimilartothecitricacidcomplexingmechanisms

Fig 3.FTIR spectra of samples (a) sucrose (b) the precursor of P 0 (c) calcined at

300 ◦ C (P 0 ).

Fig.3(a)shows theFTIR spectrumof sucrose andits typical

absorptionsareinagreementwiththespectrumindatabase[18]

Itisworthnoticingthatnoobviousabsorptionispresentbetween

1500cm−1and2500cm−1.While,thespectrumfortheprecursor

ofP0showninFig.3(b)clearlyshowsthecoordinated COO−

sym-metricstretchingwithbroadabsorptionaround1618cm−1,which

comesfromtheproductsofthesucrosehydrolyzation[20]

Consid-eringthecomplexingabilitymentionedabove,itcanbeidentified

thatthemetalionsarewellcomplexedbythe COOHgroups,

form-ingstable COOZn2+.Andinfact,noprecipitationwasobserved

duringthestirring.Moreover,thebroadabsorptionbandcentered

at3400cm−1canbeobservedduetothe OHstretchingvibration,

whichcanbeattributedtotheexistenceofcrystallizationwater

intheprecursor.Theabsorptionbandaround1385cm−1 is typ-icalasymmetricstretchingvibrationofNO3 −,whichcomesfrom

therawmaterialZn(NO3)2.Aftercalcinationat300◦C(Fig.3c),the chelatingcomplexesdecomposedandamassofgasesare gener-ated,whicharefavoredfortheformationofporousproduct.As curve(b)showed,ininfraredabsorptionspectraoftheprecursor, theabsorptionpeakat1048cm−1,830cm−1,711cm−1areascribed

toCO3 −latticevibrationinducedinfraredabsorption.Therefore,

the FTIR shows theprecursor is theZn4(CO3)(OH)6H2O,which

isconsistentwiththeXRDresults.Afterannealingat300◦C,the infraredabsorptionspectra(Fig.3(c))showsthatanewabsorption peakcenteredat474cm−1appears,indicatingtheformationofZnO andthecompletedecompositionoftheprecursors

Fig.4(a)showsthetypicalSEMimagesoftheproductsafter annealingat300◦C.Obviously,thehierarchicalstructurewas con-structedbylargequantitiesoffluffynanosheeteswithauniform sizedistributionofmicro-flowers.TheenlargeviewoftheP0in

Fig.4(b)showsthatthediameterofZnOflowersisabout10␮m The nanosheets petals are narrow in width and ended with a sharptip.Theabundanceofpetalswillgreatlyincreasethe con-tactareabetweenthecatalystsandorganicdyes.Moreover,thegap formedbytheadjacentnanosheetswouldenhancetheabsorption

ofexcitinglightandpromotethephotocatalyticactivitiesofZnO Theopticalabsorptionefficiencyincreasedbythediffuse reflec-tionhappensamongthepetals,asshownintheinsertedfigureof

Fig.4(b).Ontheotherhand,themicrostructureofthenanosheets petalsalsoshowsdifferencesbetweensucroseaddingsampleP0

andnosucroseaddingoneS0.ThehighmagnificationSEMimages

ofpetalsfromS0andP0wereshowninFig.4(c,d).Apparently,the poresonthenanosheetsarequitedistinguishedfromeachother Themicrostructureof S0 presentsthattheporesareembedded

inthepetals,likelargenumberofholesonaflatsurface.While, forthesucroseaddedsampleP0,theporeswereformedbythe

Fig 4.SEM images (a) Flower-like ZnO of P 0 (b) An enlarge view of P 0 The inserted shows the abridged general view of the possible light absorption in the sample P 0 (c)

Fig 5.Photodegradation of MO in the solution with S 0 and P 0 ZnO flowers.

connectionofagreatquantitiesofZnOnanoparticlespresenting

largersurfaceareascompared withS0.In fact,thereisno

obvi-ousdifferenceintheflowerlikestatusbetweentheprecursorsof

P0andS0,indicatingthatthesucroseeffectsonthemorphology

ofLBZC(Zn4(CO3)(OH)6H2O)isnotobvious.However,togainthe

finalZnOhierarchicalstructures,annealingprocesswascarriedout

andtheroleofsucrosewasactivatedduringthedecompositionof

LBZC.Intheprocessofsynthesis,thesucrosehydrolyzesintotwo

kindsofmonosaccharides,glucoseandfructosethatis

homodis-perseintheZn4(CO3)(OH)6H2Oandassistcombustionduringthe

annealing.Consideringthesucrosecanbeusedasfuelinthe

fab-ricationofoxides,thehightemperaturedecompositionprocessof

LBZCwithsucroseaddingcanbetreatedasamoreintensiveand

rapidcombustion,leadingtotheprecursorburningmuch more

sufficientlyandthecrystallinityofZnOparticlesimproved.Good

crystallinequalitycanbereflectedfromthemicrostructureof

sam-ples.Sphericalnanoparticlesconstitutingtheresultantnanosheets

wereformedbytheadditionalheatingfromtheaddedsucrose,

whichwouldbebeneficialtothephotocatalyticactivity.Toevaluate

thesucroseeffectsonthephotocatalyticactivity,theperformances

ofS0andP0wereinvestigatedbythedegradationofMOdyeunder

UVirradiation.Fig.5comparesthephotodegradationofMOasa

functionofirradiationtimefortheP0andS0samples.Asclearly

shown,afterirradiationfor100min,thephotocatalyticdegradation

ofMOonS0is80%.Infact,wehavediscussedthesuperior

photocat-alyticpropertiesofthemulti-layeredmesoporousZnOstructures

(S0)decomposingtheMO,whichshowedthesuperior

photocat-alyticactivitytothecommercialZnO19.Surprisingly,incomparison

withtheS0,asmallamountofsucroseaddingsampleP0displayed

muchhigherdecompositionefficiencywithadegradationrateof

nealy100%afterirradiationfor80min.Consideringthedifferences

inthesynthesizedprocedure,sucroseaddingplaysanimportant

roleinimprovingthephotocatalyticproperties

ThesurfacesensitivediagnostictestXPSwasconductedto

elu-cidatetheoxidationstatesofS0 andP0.Fig.6demonstratesthe

high-resolutionXPSspectraofO1sstatesofsampleS0andP0

Obvi-ously,theXPSspectraofO1speaksisasymmetricandbroadening,

whichcanberesolvedintotwopeaksbyaGaussiandistribution

fittingcenteredat530.1±0.2eVand531.7±0.2eV,respectively

Thefittingindicatesthatatleasttwooxygenspeciesarepresentin

thenear-surfaceregion

OAsignalpeaksarecenteredat530.1±0.2eVisduetooxygenin

thewurtzitestructureofZnO(latticeoxygen),andtheintensityof

thispeakisameasureoffullyoxidizedoxygenatoms[24].O signal

(a) S0

Bindin g Energ y (e V)

O1s Scan B O1s Scan A

(b) P0

Binding En ergy (eV)

O1s Scan A

O1s Scan B

Fig 6.The high-resolution XPS spectra of O1s states of sample S 0 (a) and P 0 (b).

peaksat531.7±0.2eVcorrespondstotheadsorbedoxygen,which

isascribedtothepresenceofadsorbedoxygen,includinghydroxyl andcarbonategroupsadsorbedonthematerialsurface.[25–28]The integratedintensityofpeakOAcanbecomparedwiththatofpeak

OBusingtheOA toOB integratedintensityratio“X,”whichwas approximately2.0and1.7forP0andS0,respectively.Apparently, thelatticeoxygeninthesucroseaddedsampleP0ishigherthan thatofsampleS0.Thisresultalsoindicatesthatthecrystallinityof

P0issuperiortoS0duetotheaddedsucroseprovidingwithmore energyduringannealing.Under theUVexcitation,electron-hole pairscarriedoutredoxreactionandmoresurfacedefectswillbe companiedwithhighercombinationprobabilityofsurfacestates andhole.However,thehighcrystallinitywoulddecreasesurface defectsandthecombinationprobabilityofsurfacestatesandholes thatcanenhancephotocatalyticactivity.[5,6]Consideringthe pho-tocatalyticactivityofP0 andS0,thesucroseinducedcrystallinity improvementisaneffectivetreatmenttoincreasethe photoactiv-ityofZnOphotocatalysts.Inordertofindtherelationshipbetween theamountofsucroseandthephotocatalyticactivityofZnO, vari-ableamountofsucroseaddedsampleswerepreparedthroughthe similarprocess.Fig.7showstheplotofthedecolorization efficien-ciesofMObytheZnOwithvariablesucroseafter40minreaction time.It canbeseenthatnosucroseaddedZnO S0 shownearly 60%decolorizationefficiency.Withthesucroseadded,ZnOsamples showedmuchbetterphotocatalyticactivityandthedecolorization efficienciesweregreatlyincreased.AsshowninFig.7,P0showsthe superiorphotocatalyticactivityanddecolorizationefficiencywas achieved95%.While,otherZnOsamplewithfewerormoresucrose

Fig 7.Photocatativity comparison of ZnO flowers after MO degradation for 40 min.

The sucrose contents of S 0 , P−2, P−1, P 0 , P 1 , P 2 were 0 g, 0.04 g, 0.06 g 0.08 g 0.1 g and

0.25 g, respectively.

addedshowlowerdecolorizationefficienciesduringthesame

reac-tiontime.Sincethesmallamountofsucroseaddedcanresultin

negligibleeffectsonthemorphology,thecrystallinityand

agglom-erationofphotocatalystsshouldbeconsidered.Insomecases,it

wasfoundthattheheatgeneratedduringthereactioncouldbe

moreprominenttocausesinteringoragglomerationofparticles,

resultingin graingrowthand lowphotocatalytic reactionsites

Therefore,theoptimizationofreactionconditionwasestablished

for0.08gsucroseaddedZnOflowers

Inthisstudy,hierarchicalstructuresZnOwassuccessfully

syn-thesized via a sucrose added urea hydrothermal method The

preparedZnOflowerswerecharacterizedbyTG-DTA,FTIR,XRD

andSEM.ThephotocatalyticactivitiesofZnOflowerswere

evalu-atedbythedegradationofMOandresultsshowthatthesucrose

addedsamplepresentssuperiordecolorizationefficiency.TheXPS

analysis reflected that the adding of sucrose can improve the

crystallizationofZnO TheZnO flowerssynthesizedviavariable

sucroseamountwerealso estimatedbythedecolorization

effi-ciencyofMOafter40minreactiontime.Itwasfoundthathigher

sucrose added would inducea slightly reduction effect onthe

photocatalyticactivitiesandtheoptimizedreactionconditionwas

estimated

Acknowledgments

ThisworkwaspartlysupportedbytheNationalNaturalScience FoundationofChina(No.51102069).Thisworkwasalsosupported

byHeilongjiangEducationDepartment(12511164)andInnovative TalentsFundofHarbin(2010RFQXG034)

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