Anatase tio2 mesocrystals with exposed (0 0 1) surface for enhanced

511 Kehua Street, Tianhe, Guangzhou 510640, China b Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus QLD, 4222, Australia c University of Chinese Academy of

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

Jiangyao Chena,c, Guiying Lia, Haimin Zhangb, Porun Liub, Huijun Zhaob, Taicheng Ana,∗

a State Key Laboratory of Organic Geochemistry, Guangdong Key Laboratory of Environmental Resources Utilization and Protection, Guangzhou Institute of

Geochemistry, Chinese Academy of Sciences, No 511 Kehua Street, Tianhe, Guangzhou 510640, China

b Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus QLD, 4222, Australia

c University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:

Received 20 July 2013

Received in revised form 16 October 2013

Accepted 25 October 2013

Available online 27 November 2013

Keywords:

Anatase TiO 2 mesocrystals

Solvothermal synthesis

High energy{00 1}facets

Photocatalytic activity

Volatile organic compounds.

a b s t r a c t AnataseTiO2mesocrystalswithexposed(001)surfacehavebeensuccessfullysynthesizedbyafacile one-stepsolvothermalmethodusingNH4Fasthestructureregulatoringlacialaceticacidenvironment.The exposed(001)surfaceoftheobtainedanataseTiO2mesocrystalwasconsistedofnumerousnanocrystals withexposed{001}facet.TheresultsindicatedthatboththeaddedamountofNH4Fand solvother-malreactiontimeplayedsignificantrolesintheformationofanataseTiO2mesocrystalswithexposed (001)surface.ApossibleformationmechanismofanataseTiO2mesocrystalwithexposed(001)surface wasproposedbasedontheexperimentaldata.AsUVactivephotocatalysts,theresultantanataseTiO2 mesocrystalswereevaluatedindetailbyphotocatalyticdecompositionofgaseousstyrene.Theresults demonstratedthattheanataseTiO2mesocrystalsfabricatedbysolvothermaltreatingamixtureof0.50g

NH4F,2.50mLTi(OC4H9)4and50mLofglacialaceticacidat210◦Cfor24hexhibitedthehighest pho-tocatalyticactivitytothedecompositionofstyrene.Thisisduetothesynergisticeffectsofexcellent crystallinity,highenergy{001}crystalfacets,relativelylargesurfacearea,enhancedbandgapenergy anduniquemesoporousstructure

©2013ElsevierB.V.Allrightsreserved

1 Introduction

Inthepastfewdecades,nanostructuredTiO2 materialshave

beenwidelyappliedtomanyemergingresearchfields,suchas

envi-ronmentalremediationandsolarenergyconversion[1–3].Studies

haveshownthattheperformanceofTiO2materialishighly

depend-entonitssize,surfacearea,crystalstructureandexposedcrystal

facet[4–6].Recently,thesynthesisofanataseTiO2withexposed

highenergy {001} facethasattracted intensiveresearch

inter-estbecauseboththeoreticalpredictionandexperimentalresults

indicatethat{001}crystalfacetismuchmorereactivethanother

thermodynamicallystablecrystalfacetsofanataseTiO2[7]

How-ever,anataseTiO2 crystalswithexposed(101)surfacearemore

easilyformedduetoitsmuchlowersurfaceenergythanthatof

{001}facetedsurface[8].Toobtainsuchahighlyreactivesurface,

itisnecessaryandchallengeabletodevelopaneffectivemethodto

reduce(001)surfaceenergy[1]

Inthisrespect,abreakthroughhasbeenmaderecentlybyYang

etal.who, usinga surfacefluorinationapproach, have

success-fullysynthesizedwell-definedanataseTiO2 singlecrystalswith

∗ Corresponding author Tel.: +86 20 85291501; fax: +86 20 85290706.

E-mail address: antc99@gig.ac.cn (T An).

47%exposed{001}facets[9].Afterthat,enormousresearchwork

on{001}facetedanataseTiO2hasbeendevelopedbasedonthis surfacefluorinationprinciple[10–14].However,itshouldbenoted thatmostreported{001}facetedanataseTiO2singlecrystalswere obtainedbywater-basedsyntheticstrategieswhichusuallydisplay irregularshapeandwidesizedistributionduetothefastnucleation andgrowthofTiO2crystals[7].Moreover,hydrofluoricacid(HF),

adangerousandenvironmentallydetrimentalfluorinesource,was frequentlyusedinwater-basedmethodbecauseHFplaysacritical roleinformationofexposed{001}crystalfacets[9].Inorderto avoiddirectlyusingHF,lessharmfulfluorinesourcessuchasNH4F [15,16],TiF4[17,18]andionliquidcontainingF[19,20]havebeen thenappliedinthefabricationofanataseTiO2withexposed{001}

facetsinwater-involvedsystem.Recently,solvothermalmethod withoutadditionofH2OandHFhasbeenanenvironmentalbenign approachtocontrollablygrowanataseTiO2crystalswithexposed

{001}facets[7].Asreported,aceticacidisaparticularlyinteresting candidateasastabilizingsolventandchemicalmodifieroftitanium alkoxidestolowerthereactivityofprecursorsbycontrollingthe hydrolysisoftitaniumprecursorsviaslowreleaseofH2Othrough theesterificationreactionbetweenaceticacidandalcohols[21,22] Moreover,owingtoitsstrongchelatingeffect[23],aceticacidmay leadtotheformationofuniqueintermediatesduringsolvothermal reactionanddifferentfinalcrystalswithspecialmorphology[22] 0920-5861/$ – see front matter © 2013 Elsevier B.V All rights reserved.

thetetrabutyltitanate–aceticacidreactionsystem[22].Asknown,

mesocrystalsarecharacterizedbyhighcrystallinity,highporosity,

subunitalignment,andsimilaritytohighlysophisticated

biomin-erals,makingthempromisingsubstitutesforsingle-crystallineor

porouspolycrystallinematerialsinmanyapplicationssuchas

catal-ysis,sensing,andenergystorageandconversion[22].However,the

obtainedTiO2crystalsdonotdisplaywell-defined{001}faceted

surface.Moreover,theformationmechanismisstilluncleardue

totheproductionofcomplexintermediatesduringsolvothermal

reaction[22].Todate,however,nomatterwhatsyntheticmethod

isused,thefabricated{001}facetedanataseTiO2 as

photocat-alystismainlyfocusedonphotocatalyticdegradationoforganic

pollutantsinwater[12,24],andlittleattentionhasbeengivento

usepureanataseTiO2crystalswithexposed{001}reactivefacets

asphotocatalystforphotocatalyticdegradationofvolatileorganic

compoundsinair[25,26]

Herein, submicron-sized anatase TiO2 mesocrystals with

exposed(001)surfaceweresuccessfullysynthesizedbya facile

solvothermalapproachinglacialaceticacidenvironment.These

squared-shaped mesocrystals were assembled with numerous

square-shapednanocrystals withexposed{001}facets,leading

tonumerousnanoporesformationandmuchhighersurfacearea

thanreportedTiO2singlecrystalswithexposed(001)surface[5]

Theeffects ofaddedamountofNH4Fandsolvothermalreaction

timeonthemorphologyandcompositionoftheresulting

prod-uctswereinvestigatedindetail.Further,theformedintermediates

duringsolvothermalreactionwerealsoidentifiedexperimentally,

andapossibleformationmechanismwasproposed.AsUVactive

photocatalysts,thephotocatalyticactivityofthepreparedanatase

TiO2mesocrystalsassembledwith{001}facetednanocrystalswas

evaluatedinacontinuousflow-throughreactorbyphotocatalytic

decompositionofgaseousstyreneinair

2 Material and methods

2.1 Synthesis

Acertainamountof NH4F(0,0.05,0.15, 0.30.0.50,0.80 and

1.20g) wasadded intoadriedTeflon-linedstainless-steel

auto-clavecontaining50mLofglacialaceticacid.Afterclearsolution

wasobtained,2.50mLofTi(OC4H9)4wasintroducedabovemixture

withstirring.Thentheautoclavewasheatedat210◦Cfor

desig-natedintervals(0.5,1.0,1.5,2.0,2.5,3.0,6.0,12.0and24.0h).After

solvothermalreaction,theproductswerecollectedbycentrifuge,

washedwithdistilledwaterthoroughlyandfinallydriedat80◦C

for8h.Thedriedsampleswerethencalcinatedat600◦Cfor90min

toremovethesurfacefluorine[11]

2.2 Characterizations

X-raydiffraction(XRD)patternsofthesampleswererecorded

on a Rigaku Dmax X-ray diffractometer The morphology and

microstructure of the prepared samples were characterizedby

scanning electron microscope (SEM, JSM-6330F) and

transmis-sionelectronmicroscope(TEM,JEM-2010).Theultraviolet–visible

(UV–vis) absorption spectra were recorded with a UV–visible

spectrophotometer(UV-2501PC).Nitrogenadsorption–desorption

isothermsofthesampleswereobtainedwithaMicromeriticsASAP

2020system

2.3 Adsorptionabilityandphotocatalyticactivity

Theadsorptioncapabilityandphotocatalyticactivityofthe

pre-paredsamplesweretestedbytheadsorptionanddegradationof

gaseousstyrenewithaninitialconcentrationof15±1ppmv oper-atinginasamecontinuousflow-throughmodeasreportedinour previousworks[6,27,28].Inatypicalexperimentalprocess,0.10g sampleasphotocatalystwasloadedinacubicquartzglass reac-torwithavolumeof1.0cm×1.0cm×0.5cm A365nmUV-LED spotlamp(ShenzhenLamplicScienceCo.,Ltd.)wasusedasalight sourcewhichwasfixedverticallytopofthereactorwithadistance

of6.0cm(UVintensitywascontrolledat70mWcm−2).Beforethe lampwasswitchedon,thegaseousstyrenewasallowedtoreach

agas–solidadsorptionequilibrium.Theconcentrationofgaseous styrenewasdirectlyanalyzedbyagaschromatograph(GC-900A) equippedwithaflameionizationdetector.Thetemperaturesof thecolumn,injectoranddetectorwere110◦C,230◦Cand230◦C, respectively.Gassampleswerecollectedatregularintervalsusinga gas-tightlockingsyringe(Agilent,Australia),anda200␮Lgas sam-plewasinjectedintothecolumnforconcentrationdetermination

inasplitlessmode

Theadsorptionefficiencyandphotocatalyticdegradation effi-ciencywerebothcalculatedforstyreneaccordingtothefollowing equation:efficiency=(1−C/C0)×100%,whereCisthe concentra-tionofresidualpollutant,andC0isitsoriginalconcentration

3 Results and discussion

3.1 Structuralcharacteristics Fig.1AshowstheXRDpatternsofthepreparedsamplewiththe addedamountof0.15gNH4F(curvea).Asshown,therearefour maindiffractionpeaksat2=25.3,37.8,48.0and55.1◦whichare correspondingtothe(101),(004),(200)and(211)crystalface

ofanataseTiO2.Andnootherdiffractionpeakscanbeobserved, indicatingthatthepreparedsampleisanataseTiO2 [29].Fig.1B andCshowstheSEMimagesofthepreparedanataseTiO2 sam-ple.Itcanbeseenthattheproductconsistsofevenlydistributed submicron-sizedparticle-likestructures(Fig.1B).Further observa-tionindicatesthattheseparticle-likestructuresaresquare-shaped withanaveragesidelengthofca.350nm(Fig.1C).TEM examina-tionclearlydemonstratedthattheanataseTiO2crystalisindeeda square-shapedstructurewithexposed(001)surface.Interestingly, the{001}facetedsurfaceoftheanataseTiO2crystalisassembled withnumeroussquare-shapednanocrystalswithexposed{001}

facetshavingsidelengtharound10–40nm(Fig.1D).Aselectedarea electrondiffraction(SAED)pattern(topinsetinFig.1D)indicates thatthesesquare-shapednanocrystalshavegoodcrystalnature Thezone axisis [001]and inturntheexposedfacetedsurface

isthe(001)surface[13].Thecorrespondinghigh-resolutionTEM image(bottominsetinFig.1D)shows theperpendicular lattice spacingof0.19nmrepresentingthe(200)and(020)atomicplanes

ofanataseTiO2,furtherconfirmingthattheexposedcrystalfacet

ofthenanocrystalisindeedthe{001}facet[30].Theseexposed

{001}facetedsurfacesofthenanocrystalsconstitutealarger(001) surfacedominatedanataseTiO2crystal

TheeffectoftheaddedamountofNH4Fonthemorphologyand compositionofthesampleisinvestigatedinthiswork.Figs.2and3 showtheSEMimagesandXRDpatternsofthesamplessynthesized

inareactionsolutioncontaining50mLglacialaceticacid,2.5mL Ti(OC4H9)4anddifferentaddedamountsofNH4Fwith solvother-malreactionat210◦Cfor24h,respectively.WhenNH4Fisabsent, spindle-shapedTiO2structureswithabout200nminlengthand

100nminwidthareobserved(Fig.2A).Ahigh-magnificationSEM image(insetin Fig.2A)revealsthatthesurface ofthe spindle-shapedstructureisrelativelyroughandcomposedoftiny nanopar-ticleswithdiametersaround10–20nm,whichisconsistentwith thereportedresultbyYeetal.[22].Intheirstudy,asimilarTiO2 structureishighlyorientedalong[001]direction[22],implying

Fig 1.(A) XRD patterns of prepared sample with the added amount of 0.15 g NH 4 F (curve a) and JCPDS No 21-1272 is used as a reference of bulk anatase TiO 2 (B) SEM image

of the prepared sample (low magnification) (C) SEM image of the prepared sample (high magnification) (D) TEM image, SAED pattern (top inset) and high-resolution TEM image (bottom inset) of the prepared sample.

that{001}exposedfacetsarenotdominant.Somespindle-shaped

structuresaretransformedtosquare-shapedstructuresresulting

inamixtureofspindle-andsquare-shapedstructureswhen0.05g

NH4Fisadded(Fig.2B).Underthiscondition,allobtained

struc-tures withsmooth surfaceshow largersize than thatobtained

withoutNH4F(insetinFig.2B).Thesizeincreaseofthese

struc-turesmaybeduetotheexistenceofF-,whichcanenhancethe

crystallizationof anatasephase and promote crystallitegrowth

[31].ThefabricatedsampleswerefurtherdeterminedbyXRD

tech-nique(Fig.3).Thediffractionpeaksofthesampleobtainedwith

0.05gNH4Fisobviously strongerandsharperthanthoseofthe

samplewithoutNH4F,implyingahighercrystallinityofthe

for-merandfurtherconfirmingF−roleinenhancingthecrystallization

ofanatasephase.WithincreasingNH4Famountto0.15g,

spindle-shapedstructuresdisappearandpurelysquare-shapedstructures

withsidelengthofca.350nmareobtained(Fig.1BandC)

Com-paredtothesamplesobtainedwith0and0.15gNH4F,theaddition

ofNH4Fcanefficientlyretardthegrowthofthecrystalstructure

along[001]direction,resultingintheformationofsquare-shaped

structureswithhigh{001} exposed facetedsurface.Whenthe

added amountof NH4F is furtherincreased to0.30g (Fig 2C),

spherical-likeanataseTiO2structuresareformedwhichthenmelt

togethertoformcluster-likeandfilm-likestructureswithfurther

increasingNH4Famountto0.50and0.80g,respectively(Fig.2D

andE).TheaboveresultsindicatethatNH4Fplaysanimportantrole

incontrollingthemorphologyoftheanataseTiO2,andhigh

qual-itysquare-shapedanataseTiO2crystalsassembledbynanocrystals

withexposed{001}facetscanbeonlysynthesizedwithanapt

amountofNH4F(0.15ginthiscase).Moreover,itisfoundthatthe

diffractionpeaklocatedat2=37.8◦ofthesamplebecomesmuch

weakerandwiderwithincreasingtheaddedamountofNH4Ffrom

0.15to0.80gthanthatofthesampleobtainedwith0.05gNH4F (Fig.3 revealingthatexcessive amountofNH4F (≥0.15g)can effectivelyretardtheorientedgrowthofanataseTiO2along[001] direction[32],leadingtosignificantlyincreased{001}exposed facets[33].When1.20gNH4Fisintroducedintothereaction sys-tem(Figs.2Fand3),bulkyNH4TiOF3particleswithanaveragesize

ofca.400nmareformed[34,35].Basedontheaboveresults,itcan

befoundthattheproductswithdifferentmorphology(from spin-dleshapetobulk)andcomposition(fromTiO2toNH4TiOF3)canbe easilyadjustedbysimplycontrollingtheaddedamountofNH4F WithanaptaddedamountofNH4F(0.15ginthisstudy), square-shapedsingle-crystal-likeanataseTiO2 structuresassembled by square-shaped nanocrystals withexposed {001} facets can be obtained

3.2 N2adsorption–desorptionisotherms

To further investigate the pore structure properties of the preparedsamples,theN2 adsorption–desorptionisotherms and correspondingporesizedistributioncurvesofthesamples pre-paredwithdifferentaddedamounts ofNH4F(0,0.15, 0.50 and 0.80g)areplotted(Fig.4).Itcanbeseenthatallisothermsareof type IV(IUPACclassification)witha typicalH3hysteresisloop, indicatingtheexistenceofmesoporousstructureandslit-likepores [6,36,37].Theaverageporesizesofthesamplespreparedwiththe addedamountof0,0.15,0.50and0.80gNH4Farecalculatedalmost thesameas12.2,10.6,11.2and12.0nm,respectively(theinset

ofFig.4).Again,thesepreparedsamplesareconfirmedas meso-porousmaterials.Previously,ithasbeenreportedthatthesample synthesizedwithoutadditionofNH4Fismesoporousmaterialdue

totheexistenceoftinynanoparticlesonthelargesizestructure

Fig 2. SEM images of the prepared samples fabricated with different added amounts of NH 4 F: (A) 0 g; (B) 0.05 g; (C) 0.30 g; (D) 0.50 g; (E) 0.80 g; and (F) 1.20 g.

[22] In this case, fromSEM and TEM resultsdisplayed above,

thesingle-crystal-likeanataseTiO2structuresarealsoassembled

bynanocrystalswithexposed{001}facets.Thus,similarasthe

reportedresult[22],mesoporousstructurescanbeformedbetween

thesenanocrystalsafterremovaloftheorganicresidualsby

calcina-tion.ThequantitativedetailsabouttheBrunauer–Emmett–Teller

(BET) surface areas, Barrett–Joyner–Halen (BJH) total pore

vol-umesandaverageporediameterarelistedinTable1.Clearly,with

increasingtheadded amountofNH4Ffrom0to0.80g,theBET

surfaceareaand thetotal porevolumeofthesamplesdecrease

from 97.4m2/g and 0.152cm3/g to 18.3m2/g and 0.041cm3/g,

respectively.Moreover,thesamplespreparedwiththeaddition

ofNH4FinthisstudyshowlowerBETsurfaceareasthanthatof

P25(surfaceareaof50m2/g).LargerBETsurfaceareaandbigger

totalporevolumemayleadtohigheradsorptioncapacityofthe

TiO2 sampleonpollutants which canbeverifiedby theresults

displayedinthefollowedadsorptionexperiments

Table 1

Effect of the amount of NH 4 F on structure properties of photocatalysts.

crystallinity

a The relative intensity of the diffraction peak from the anatase (1 0 1) plane (ref-erence = the sample prepared with the amount of 0 g NH 4 F).

3.3 UV–visanalysis Fig.5showstheUV–visabsorptionspectraandtheindirectband energyofthepreparedphotocatalysts.Allsamplesexhibiteda typ-icalabsorptionwithanintensetransitionintheUVregionofthe spectrum,whichwasattributedtotheelectrontransitionofTiO

20 30 40 50 60 70 80

NH 4 TiOF 3 Anatase

1.20 g

0.80 g

0.50 g

0.30 g

0.15 g

0.05 g

0 g

Fig 3.XRD patterns of the prepared samples fabricated with different added

amounts of NH 4 F.

0

20

40

60

80

100

10 20 30 40 50

3 g

-1 nm

-1 )

0 g NH4F 0.15 g NH4F 0.50 g NH4F 0.80 g NH4F

3 /g)

Relative pressure (P /P

0)

Pore diameter ( nm)

Fig 4.N 2 adsorption–desorption isotherm and pore-size distribution curves of

sam-ples prepared with added amounts of 0, 0.15, 0.50 and 0.80 g NH 4 F.

0.0

0.3

0.6

0.9

1.2

1.5

Photon energy (e V)

0 gNH 4 F 0.15 gNH

0.50 gNH

0.80 gNH

Fig 5.UV–vis absorption spectra of the photocatalysts obtained with addition of

different amounts of NH 4 F.

fromthevalencebandtotheconductionband[38].Theindirect bandgapenergiesofthepreparedsampleswereestimatedfrom

aplotof(˛hv)1/2versusphotonenergy(hv)(insetinFig.5).The interceptofthetangenttotheplotgaveagoodapproximationofthe indirectbandgapenergyofthefabricatedanataseTiO2.The absorp-tioncoefficient˛couldbecalculatedfromtheabsorbance.From theinsetinFig.5,theindirectbandgapenergiesestimatedfrom theinterceptofthetangentstotheplotsare3.09,3.08,3.14and 3.09eVforthepreparedphotocatalystsobtainedwiththeadded amountsof0,0.15,0.50and0.80gNH4F,respectively.Itcanbeseen thatawiderbandgapisclearlyobservedforthesampleobtained with0.50gNH4F,possiblyowingtothepresenceofsurfacetiny nanocrystals.ThelargebandgapmaymeanthatthefabricatedTiO2 samplehasstrongerredoxabilityduringphotocatalyticreaction [39]

3.4 Formationmechanism

Toinvestigatethegrowthmechanism, thegrowthprocesses

ofTiO2 submicron-sizedmesocrystalare examinedindetail by SEM and XRDtechniques Fig.6 shows the SEMimages ofthe as-synthesizedsampleswithdifferentreactiontimes(theadded amountof0.15gNH4Fforallinvestigatedsamples).Itcanbeseen thatwith0.5hofsolvothermaltreatment,theparticulateproducts withirregularshapehavingparticlesizerangingfrom50to150nm areformed(Fig.6A),whichthenmelttogethertoformfilm-like structureafter1.0hofsolvothermaltreatment(Fig.6B)and fur-thertransformtonanofiberswhenthereactiontimeprolongsto 1.5h(Fig.6C).Aftersolvothermaltreatmentof2.0h,amixtureof nanofibersandparticlescanbeobserved(Fig.6D),andtheparticles becomedominantafter2.5h(Fig.6E).Then,theformedparticles withirregularshapefurtherevolveintosquare-shapedstructures andtheco-existednanofibersaretotallytransformedintoparticle clusterswhenthereactiontimefurtherincreasesto3.0h(Fig.6F) Finally,theseparticleclustersdissolveandgrowslowlytoform square-shapedstructures,asshowninFig.6GandHwithextending thereactiontimecontinuouslyto6.0and12.0h,andhigh qual-itysquare-shapedTiO2submicron-sizedcrystalsareobtainedafter 24.0hofreactionandthenmesocrystalsaresynthesizedafter cal-cination

Besides the morphology evolution, the composition of the productsobtainedatdifferentreactiontimesiscorrespondingly investigated.Fig.7showstheXRDdataoftheas-prepared prod-uctsfromdifferentreaction times.Clearly,theXRD patternsof sampleobtainedwith0.5hofsolvothermalreactionisamixture

ofNH4TiOF3andanataseTiO2diffractionpeaks.Asthetreatment timeincreasesto1.0h,theintensityofNH4TiOF3diffractionpeaks weakens dramatically Moreover, the diffraction peak intensity

oftheNH4TiOF3 becomesweakerand weakerwiththereaction timefurtherincreasingandfinallydisappearswhenthereaction time reaches3.0h On thecontrary,theintensity ofdiffraction peaksforanataseTiO2becomemuchstrongerwhenthetreatment timeincreasesandtheXRDpatternsobtainedfromthesamples treatedfor3.0–24hcanbeindexedtopureanataseTiO2.TheXRD resultsfurthersupportthemorphologychangeofthefabricated samples,suggesting afour-stageformationprocessasshownin Scheme1:(1)formationofamixtureofNH4TiOF3andanataseTiO2 (0–0.5h)(stepA);(2)dissolutionandrearrangementofNH4TiOF3

toformanataseTiO2(0.5–3.0h)(stepsB–E);(3)growthof square-shapedanataseTiO2crystals(3.0–24h)(stepF)and(4)subsequent calcination to obtain TiO2 mesocrystal (step G) Therefore, a tentativeformation mechanismof TiO2 submicron-sized single-crystal-like mesocrystals obtained by solvothermal method is proposedandillustratedasfollows:firstly,Ti(OC4H9)4reactswith glacialaceticacidtoreleaseC4H9OH[22].TheproducedC4H9OH thenreactswiththeglacialaceticacidtoformwaterbyaslow

Fig 6. SEM images of the prepared samples fabricated with different reaction times: (A) 0.5 h; (B) 1.0 h; (C) 1.5 h; (D) 2.0 h; (E) 2.5 h; (F) 3.0 h; (G) 6.0 h; and (H) 12.0 h.

esterificationreaction(Eq.(1)).Subsequently,hydrolysisreaction

occursforTi(OC4H9)4 toformTi(OH)4 (Eq.(2)),furtherforming

TiO2growthseeds(Eq.(3)).Meanwhile,HFisgeneratedthroughthe

hydrolysisofNH4F(Eq.(4)).ThenOH−onthesurfaceofTiO2growth

seedswillbesubstitutedbyF− toformTiF − (Eq.(5)), further

producing[TiF3(OH)3]2−(Eq.(6)).Then,NH4TiOF3isformedafter reactionwithNH4 undertheacidicenvironment (Eq.(7))[35] Theformationoftheanataseinvolvesthedissolutionof ammo-niumandfluorideionsfromtheNH4TiOF3,followedbycollapse

toanatase[35].Finally,anataseTiO mesocrystalswithpreserved

20 30 40 50 60 70 80

NH

3

Anatase 0.5 h

1.0 h

1.5 h

2.0 h

2.5 h

3.0 h

6.0 h

12.0 h

2theta (degre e) Fig 7.XRD patterns of the prepared samples fabricated with different reaction

times.

morphologycanbeobtainedafterremovaloftheorganicresiduals

bysubsequentcalcination,accompaniedbyamoderateincreasein

thesizeofthenanocrystals[22]

C4H9OH+CH3COOH→CH3COOC4H9+H2O (1)

Ti(OC4H9)4+4H2O→Ti(OH)4+4C4H9OH (2)

TiO2+6H++6F−→2H++TiF26−+2H2O (5)

TiF2−6 +3H2O↔ [TiF3(OH)3]2−+3H++3F− (6)

[TiF3(OH)3]2−+H++NH+4 ↔NH4TiOF3↓+2H2O (7)

3.5 Adsorptioncapabilityandphotocatalyticactivity

Fig.8shows theadsorption,direct photolysisand

photocat-alytic decomposition curves of gaseous styrene by theanatase

TiO2 photocatalystsprepared withdifferentNH4F amounts.For

comparison, commercially availableP25 wasalso measuredby

0.0 0.2 0.4 0.6 0.8 1.0

UV on

UV off

C 0

0 g NH 4 F 0.05 g NH 4 F 0.15 g NH 4 F 0.30 g NH 4 F 0.50 g NH 4 F 0.80 g NH 4 F 1.20 g NH 4 F P25

Time ( min)

Ph otolys is

Fig 8.Adsorption, photolysis and photocatalytic degradation kinetic curves of styrene by P25 and photocatalysts prepared with different added amounts of NH 4 F.

photocatalyticdecompositionofgaseousstyrene.Before switch-ingon thelamp, theadsorption equilibrium experimentswere firstlyconducted FromFig 8,styrene is swiftly adsorbedonto allinvestigatedsamplesduringtheinitial20–30minuntilslow breakthroughoccurs(breakthroughpointisdefinedherewherethe outletconcentrationofstyreneisequalto5%oftheinletstyrene concentration).ForP25,thecompletebreakthrough(whenthe out-letandinletconcentrationsofstyreneareequal)isobservedafter

180minofadsorption.Similarresultcanbealsofoundforthe sam-plepreparedwiththeaddedamountof0.30gNH4F.Forsamples obtainedwiththeaddedNH4Famountof0.05,0.15,0.50and0.80g, thecompletebreakthroughtimeisthesame(150min)whichis shorterthanthatofP25.Moreover,thesynthesizedsampleswith theaddedNH4Famountof0(270min)and1.20g(120min)show thehighest andlowestadsorptioncapacitiestostyrene, respec-tively Clearly,the time for complete breakthrough followsthe order:1.20gNH4F<0.80gNH4F=0.50gNH4F=0.15gNH4F=0.05g

NH4F<P25=0.30gNH4F<0gNH4F.Thisresultindicatesthatall samplespreparedwiththeadditionofNH4Fshowsimilar adsorp-tioncapability asthat for P25due totheirunique mesoporous structure,whilethesampleobtainedintheabsentofNH4Fshows thehighest adsorptionabilitywhich is consistentwiththeBET surfacearearesults

Astheadsorptionequilibrium reached,thelampisswitched

on Firstly, the control experiment of direct photolysis is car-riedout, and less than1% of styreneis removedafter 180min

Scheme 1. Schematic illustration of the formation mechanism for the anatase TiO mesocrystals with exposed (0 0 1) surface.

of photolysis, indicating that only UV light cannot efficiently

decomposethestyrene.Whenphotocatalystispresent,different

removalefficienciesareobservedforvariousphotocatalysts.For

P25,a swiftremovalofstyrene(82.9%)canbediscernedinthe

first 30min However, as UV illumination goes on for another

60min,thedecompositionefficiencydecreasessharplyto62.6%,

whichcanbeascribedtotheblockageofphotocatalyticactivesites

bystableintermediatesontheTiO2surfaceduringthe

photocat-alyticremovalofstyrene,therebyleadingtothedecreaseofthe

photocatalyticefficiency[38,40].Finally,asteadystatewiththe

decompositionefficiencyofca.58.5%isachieved[41].Forall

pre-paredphotocatalysts inthis study,similardecompositioncurve

toP25isobtainedasreportedinourpreviousworks[6,28].After

180minofphotocatalyticreaction,thedecompositionefficiencies

followtheorder:0.05gNH4F(17.5%)<0.15gNH4F(17.9%)<0.03g

NH4F(23.4%)<0gNH4F(55.5%)<P25(58.5%)<1.20gNH4F(64.9%)

<0.80gNH4F(79.9%)<0.50gNH4F(85.9%).Apparently,the

sam-pleobtainedwiththeaddedamountof0.50gNH4Fshowsthebest

photocatalyticactivity

Thisenhancedphotocatalyticactivitymaybeattributedtothe

synergisticeffectsofseveralfactorssuchasrelativecrystallinity,

bandenergy,specificsurfacearea,highpercentageof{001}facets

andmesoporousstructure Relativecrystallinityisanimportant

factorinfluencingthephotocatalyticactivityofTiO2.Asshownin

Table1,therelativeanatasecrystallinityincreasessharplyfrom

1.00to1.50forthesamplespreparedwiththeincreaseoftheadded

amountofNH4Ffrom0to0.50g.Therelativeanatasecrystallinity

thendecreasesto0.56forthesamplespreparedwiththeadded

amountsof0.80gNH4F.Inthecaseofspecificsurfacearea,this

sampledoesnotdisplaythelargestspecificsurfaceareaamong

thepreparedsamples.However,ithasalreadybeenprovedthat

largesurfaceareaisnottheonlydecisivefactorforthe

enhance-mentofthephotocatalyticactivityofTiO2 photocatalystbecause

TiO2 withalargesurfaceareaisusuallyaccompaniedbyalarge

numberofcrystallinedefects,whichcouldactasthecentersfor

therecombinationofphotogeneratedelectronsandholes,leading

topoorphotoactivityofTiO2[39,42].Moreover,highpercentage

of{001}facetsalsoplaysanimportantrole.Previousstudyhas

shownthatthephotocatalyticefficiencyincreaseswiththeincrease

ofpercentageof{001}facetsforanataseTiO2[43].Furthermore,

theuniquemesoporousstructureofeachparticlewillfacilitatethe

transferanddiffusionofstyrenemoleculesinthecatalyst.Thus,the

photocatalystsamplepreparedwiththeaddedamountof0.50g

NH4Fshowsthehighestphotocatalyticactivityinthisstudydue

totherelativelylargesurfacearea, whichcanefficiently enrich

reactantmolecules,andgoodanatasecrystallinityaswellashigh

percentage of{001}facets,which canreducetheelectron and

holerecombination[43,44].Muchwiderbandenergycanresultin

thephotocatalystwithhighredoxcapabilityandtheunique

meso-porousstructurecanfacilitatethetransferanddiffusionofstyrene

molecules.Allthesefactorscombinedtogetherareresponsiblefor

thehighestphotocatalyticactivityofthesamplepreparedwiththe

addedamountof0.50gNH4F

4 Conclusions

TiO2submicron-sizedmesocrystalphotocatalystwithexposed

(001) surface has been successfully synthesized by a facile

solvothermalmethod.Theresultsrevealedthattheaddedamount

ofNH4Fandsolvothermalreactiontime playedboth significant

rolesin theformationoftheTiO2 submicron-sizedmesocrystal

structure.FormationofthemixedNH4TiOF3andanataseTiO2,

dis-solutionandtransformationofNH4TiOF3toanataseTiO2,growth

processofTiO2submicron-sizedcrystalassembledbynanocrystals

with{001}exposedfacetsandsubsequentcalcinationtoobtain

TiO2mesocrystalwerefoundtobefourmajorsynthesisstages.The photocatalyticdegradationresultsrevealedthatthephotocatalyst preparedwhen0.50gNH4Faswellas2.50mLTi(OC4H9)4added

to50mLofglacialaceticacidandthensolvothermallytreatedfor

24hat210◦C showedthehighestphotocatalytic activityin the decompositionofgaseousstyrene,duetothesynergisticeffectsof itsgoodanatasecrystallinity,highpercentageof{001}facets, rela-tivelylargesurfacearea,widebandenergyanduniquemesoporous structure

Acknowledgements

ThisiscontributionNo.1757fromGIGCAS.Thisworkwas par-tiallysupportedbytheScienceandTechnologyProjectof Guang-dongProvince,China(2011A030700003 and2012A032300017), the Cooperation Projects of the Chinese Academy of Sci-encewithFoshanGovernment(20121071010041),TeamProject

of Natural Science Foundation of Guangdong Province, China (S2012030006604) and National Nature Science Foundation of China(21077104)

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