FouadAraiedha,b,FranckDucosb,AmmarHouasa,c,NouariChaouib,∗
aUnitédeRechercheCatalyseetMatériauxpourl’EnvironnementetlesProcédés(URCMEP),UniversitédeGabès,CampusUniversitaire—CitéErriadh,6072 Gabès,Tunisia
bLaboratoiredeChimieetPhysique-ApprocheMulti-échelledesMilieuxComplexes(LCP-A2MC)UniversitédeLorraine,France cAlImamMohammadIbnSaudIslamicUniversity(IMSIU),CollegeofSciences,DepartmentofChemistry,Riyadh11623,SaudiArabia
a r t i c l e i n f o
Articlehistory:
Received12October2015
Receivedinrevisedform11January2016 Accepted16January2016
Availableonline25January2016
Keywords: Photocatalysis Opticalmicroscopy TiO2thinfilm Stearicacid a b s t r a c t
Reflected-lightdifferentialinterferencecontrastmicroscopywasusedtoobservethedisappearanceof stearicacidcrystal(B-polymorph)depositedontoaheterogeneousanatase/amorphoustitaniafilmupon ultravioletlightexposure.Microstructuralstudiesofthefilmsdemonstratetheformationofanatase microdomainswithsub-micrometricsizerandomlydistributedthroughouttheamorphoussurfaceof thefilm.Themicroscopyimagesrevealthatthedisappearanceofstearicacidcrystalisinitiatedinthe immediatevicinityofthesemicrodomainslocatedwithinthecrystalorclosetoitsedges.Thestearicacid disappearanceproceedsvialateralgrowthandcoalescenceofpitsinshapeofflattened-hexagons show-ingapreferentialorientationwithrespecttothestearicacidcrystalsymmetry.Thislatterfact,which isobservedforthefirsttimetothebestofourknowledge,isexplainedbythedependenceon crystal-lographicorientationoftheprogressionrateofthepitedges.Tojustifytheobservedphotodegradation mode,wefirstinvoketheultraviolet-inducedformationofradicalspeciesattheanatasemicrodomains andtheirdiffusiontowardsthepitsedges.Then,thegeometryandthepreferentialorientationofthe pitsarediscussedintermsofanisotropyofintermolecularinteractionswithinthecrystal.Theseresults suggestthattheenergybarrierseenbytheradicalspeciesreachingthepitedgesiscorrelatedtothe crystallographicorientationwithconsequencesonthereactionkinetics.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Titanium dioxide is well-known as a material of choice for self-cleaningsurfacesbecauseofitsfavorablephysicaland chem-icalproperties.Asa thin layer,thephotocatalyticperformances oftitaniaareoftenestimatedbymeasuringtheevolutionofthe concentrationoforganiccontaminantsinitiallydepositedontoits surfaceasafunctionofultravioletirradiationtime[1–4].Sincethe pioneerworkofPazandHeller[1],onethemostpopular contami-nantsusedisstearicacid(CH3(CH2)16COOH)becauseofitsstability underultravioletirradiationanditslow vaporpressure.Several studieshaveestablishedthatthedegradationmodeofstearicacid (SA,hereafter)ontitaniathinfilmsisstronglyinfluencedbythe structureandmorphologyof thesurface atvariousscales[5–7] butalsothemorphologyoftheSAdeposits[6,8].Sawunyamaetal.
∗Correspondingauthor.
E-mailaddress:nouari.chaoui@univ-lorraine.fr(N.Chaoui).
[6]comparedthedecompositionofapartialandfullmonolayer ofSAontitaniausinginsituatomicforcemicroscopy(AFM, here-after).Theirmeasurementsrevealedaninhomogeneousreactivity patternatthenanometerscaleattributedtorandomlydistributed activecenters.In apreviouswork[8]. wepioneeredtheuseof standardopticalmicroscopytoobservethephotocatalyzed degra-dationofSAislandsdepositedonacontinuousanatasecoatingat themicrometricscale.Weobservedthatthephotodecomposition ofindividualislandsproceededfromtheedgestowardsthecenter accordingtoazero-orderkineticswithrespecttoislandarea.These studieshaveshownthatthedirectobservationatvariousscalesof thedegradationmodeofSAdepositedontitaniacanprovidenew insightsinthestudyofthefactorsthatcontrolthekineticsand mechanismsofphotocatalysis.
Thishasmotivatedagreatpartofthisworkwhichconsidersan originalandinterestingSA/photocatalystconfiguration.We pre-paredbysolgelrouteheterogeneousanatase/amorphoustitania filmsaccordingto a protocolreported byBahtat et al. [9].The as-preparedfilm consistsofa non-photoactiveamorphous
tita-http://dx.doi.org/10.1016/j.apcatb.2016.01.039
sub-micrometricsize.Weusereflected-lightdifferential interfer-encecontrast(RL-DIC)tovisualizetheultravioletphotocatalytic degradationmodeofwell-crystallizedstearicacid(Bpolymorphs) initiallygrownonthefilmsurface.TheRL-DICmicroscopyis capa-ble ofproducing high-contrastimages(high-edge definition)of transparentmediumandisthereforeusefulfortheobservationof stearicacidontitaniasurface.Furthermore,thistechniqueallows themicroscopetoachievehigh-resolutionimagesatlargeaperture sizesenabling,atthesametime,theobservationoftheanatase microdomains(sub-micrometricscale).Weshowthatthedirect observationoftheTiO2photocatalyticsurfacereaction,using RL-DICmicroscopy,providesnewinsightsintothereactionkinetics and degradation mechanismsof SAonTiO2. Our resultsreveal thatthephotocatalyzeddegradationoftheSAcrystalsisinitiated atanatasemicrodomains,proceedsalongspecificcrystallographic directionsandismediatedviathediffusionofactivespecies.
2. Experimental
2.1. PreparationoftheTiO2thinfilms 2.1.1. PreparationofTiO2sol
Titaniumtetra-isopropoxide[Ti(iOC3H7)4] (Aldrich,97%)was usedasaprecursortosynthesizethetitaniasolviaanacid cat-alyzedsol-gelprocess.TheTiO2solwaspreparedaccordingtoa procedurereportedbyBahtatetal.[9].Briefly,4.65mlof propan-2-ol(Aldrich,99.8%)wasaddeddropbydrop,undermagneticstirring, in1.6mlofTTiP.Thesolutionwasleftunderclosedagitationunder heatingat60◦C.After15min,5.15mlofaceticacid(ACROS Organ-ics,99.5%)wasaddedfor10minunderheatingat60◦C.Finally, 12mlofmethanol(VWR,100%)waspouredandthesolutionwas furtherstirredfor2h.Thesolwasagedduring24h.Theobtained TiO2 solwastransparent,stableandveryfluidforseveralweeks. ItsresultingpHwasintherange2–3.
2.1.2. Dipcoating
Priortocoating,theglasssubstrateswerecleanedinultrasonic bathwithdetergentandsuccessivelyrinsedwithdistilledwater andabsoluteethanolgrade.Thesolwasdipcoatedontoborosilicate glasssubstrates(64mm×22mm×0.5mm).Thesolwasdeposited at a withdrawal speed of 100mm/min. and theprocedurewas repeatedeighttimesinordertoobtainafilmthicknessofabout 120–150nm.Then,thefilmwasthendriedat100◦Cduringone hourandwasfinallycalcinedat450◦Cfortwohours.
2.2. Characterizationofthefilms
ThecrystalstructureandcrystallinityoftheTiO2catalystswere determined by micro-raman spectrometry (Jobin Yvon Horiba, LabRAM HR evolution). The excitation laser wavelength was 633nm. Thepower usedwasvoluntarilykeptat theminimum inordertobewellbelowthepowerlevelneededtoinduce crys-tallization.ThetopographyofTiO2thinfilmswasinvestigatedby atomicforcemicroscopy(AFM,PacificNanotechnology)witha sil-iconnitridetipintappingmode.
2.3. Photocatalyticexperiment 2.3.1. Stearicaciddeposition
Stearicacidfilmwasdip-coatedontothetitaniaphotocatalyst froma0.06Mstearicacidmethanolicsolution.Thesamplewas dippedtwiceatawithdrawalspeedof100mm/mininthesolution at30◦Canddriedfortwohoursinambientatmosphere.
The photodegradation of the SAlayer was performedusing theultravioletlightfroma200Wmercury-xenonlamp simulat-ingsolarradiationequippedwitha0.5mlengthopticalfiber.The power densityin the UVA was2.5mW/cm2,as measuredby a photo-radiometer(DeltaOhmDO9021)equippedwithUVAhead. 2.3.3. Reflectedlight-differentialinterferencecontrast(RL-DIC) microscopy
The samples were observed with a reflected-light differen-tial interference contrast microscope (Axio Imager A1m from Zeiss)equippedwithahigh-resolutioncamera(AxioCamMRc5). The principle of this technique is basedon theinterference of veryclosely-spacedandlinearlycross-polarizedbeamsthat trans-formthicknessgradientorrefractiveindexgradientorbothinan imagewithshadow-castappearance.Thiseffectenablesacontrast enhancementofedgesandtheobservationofminutefeaturesin transparentobjects(SAcrystalhere)withhighcontrastandhigh resolution.
3. Results
3.1. Characterizationofthetitaniafilm
Fig.1(a)showsaRL-DICmicroscopyimageofthesol-geltitania filmdip-coated(eightlayers)onborosilicateglassandheattreated at450◦Cfor2h.Thesurfaceofthefilmshowsaconstellationofdark spotssurroundedbywhitehalosonahomogenousbackgroundand uniformlydistributed onthefilmsurface.Theirdiametervaries froma fewhundredsnanometertoonemicrometer.Exceptfor theseblackspots,therestofsurface appearsfeaturelessatthis scale.
ArepresentativeAFMimageofthesurfaceoftheTiO2 filmis showninFig.1(b).TheAFMimagesrevealasurfacewithclearly visible microdomainswithsizerangingfrom300nmuptoone micrometeremergingfromthesurfaceofthefilm.Theheightsof thesemicrodomainswithrespecttothesurfaceofthefilm typi-callyvaryfromafewtenstoahundredofnmasshownbythe imageprofiles.Acloseinspectionoftheflatregionsalsoreveals minutehollowsstructures(surroundedinwhite)withdiameters rangingfromtenstoafewhundredsofnanometresand approx-imatelytennmdeepasshownbytheimageprofile.Itisstriking howtheformsof themicrodomainsandthehollows,whatever theirsizes,arealikestronglysuggestingacommonorigin.Onecan alsoappreciateapreferentialorientationofbothmicrodomainsand hollows.Fig.1(c)presentsrepresentativemicro-ramanspectraofa microdomainandaflatregionofthetitaniafilm.Thespotsizeofthe laseronthefilmsurfacewasabout1mindiameter.Thepowerof thelaserusedwasvoluntarilykeptattheminimuminordertobe wellbelowthepowerlevelneededtoinducecrystallization.This explainstheoveralllowsignal/noiseratiointherecordedspectra. TheRamanspectraofthemicrodomainsshowasharpandstrong lineat142cm−1andfourothermuchweakerlinesat196,398,515 and638cm−1whicharecharacteristicoftheanatasephase[10]. TheRamansignaloftheflatregionatleast10mawayfromany visiblemicrodomainisconsiderablyreducedcomparedwiththatof themicrodomainssothatonlyaweaklineat196cm−1isobserved. Thisstronglysuggeststhatthedegreeofcrystallinityoftheflat regionismuch weakerand supportsthefactthattheobserved hollowsmaybeattributedtofineanatasegrainsembeddedinan amorphousmatrix.
Inourstudy,thefilmswerepreparedinaccordancewiththe procedureproposedbyBahtatetal.[9]exceptforthedurationof thethermaltreatmentat450◦Cwhichwas2hinourworkinstead of15min.Theseauthorsobservedsphericalnanocrystalsofabout
Fig.1.(a)RL-DICmicroscopyimageofthesol-geltitaniafilmdipcoatedon borosil-icateglassandheat-treatedat450◦Cfor2hinair.(b)AFMimageoffilmsurface andrepresentativeprofilesofthemicrodomainsandhollows(c)Representative micro-Ramanspectraofthemicrodomainsandtheflatregionofthefilm.
5nmindiameterembeddedinanamorphousmatrix.The observa-tionofmicrodomainswithdiameterrangingfromafewhundreds ofnanometresuptoonemicrometreinFig.1(b)isconsistentwith theworkofExarhosandAloi[11]whostudiedtheisothermal crys-tallizationofsol-geldepositedamorphoustitaniafilm.According totheseauthors,thecrystallizationoccursatnucleationcentres whicheitherpre-existintheamorphousphaseorareformed dur-ing the treatment.After several hoursof isothermal annealing at350◦C,themicrostructureoftheirfilmshowedadistribution ofroundsphericalanataseparticleswith100–200nmdiameters embeddedinafine-grainedamorphousmatrixandsurroundedbya thinvoidshell.Thisstronglysuggeststhattheobservedhollowsare
growthand/orcoalescetoformlargertheobservedmicrodomains. 3.2. Photocatalyticactivities
3.2.1. RL-DICmicroscopyobservationofSAcrystalspriorto ultravioletexposure
Fig.2(a) presentsRL-DIC microscopy imagesof a SAcrystal grownonthetitaniafilmpriortoultravioletexposure.Asshown bythecentralpanelofFig.2(a),theSAcrystalisflat, rhombus-shapedandpresentsanobtuseangleof107◦ (andthusanacute angleof73◦).TheidentificationofSApolymorphscanbeeasily per-formed,carriedoutbyobservingthecrystalshape[12–15].Both BandC polymorphsformplaterhombus-shapedcrystalswhich canbeclearlydistinguishedfromtheiracuteanglewhichisabout 74◦fortheBandcloseto54◦fortheCpolymorph.Therefore,the measuredangleisconsistentwiththeB-polymorphofSA.
Thecolourcontrastbetweenthecrystal(palebrown)andthe background(lightblue)observedinFig.2(a)isamanifestationof thestronginteractionbetweenthepolarizedlightandtheSA crys-tal[16]thatisbirefringent.Duetothethicknessofthecrystaland itsorientationintheplanofthemicroscopestage,phase differ-enceisgeneratedbetweentheordinaryandextraordinarybeams, inherenttoitsbirefringence,explainingtheobservedcolour con-trast.ThecolourvariationontheRL-DICmicrographcanbethen correlatedwiththethicknessofthecrystal.Asaresult,theregions ofthecrystalshowinghomogenouspalebrowncolourpresenta constantthickness.
Themagnitudeoftheimage(×1000)enablestheresolutionof submicrometricfeaturesandmakespossibletheobservationofthe previouslydescribedmicrodomainswhichappearasminutedark pointssurroundedbyawhitehaloontheimagebackground.It isalsoworthnotingthatseveralmicrodomainsareverycloseto crystal edgeas indicatedby theblack arrowsontheleft panel ofFig.2(a).Theirdistancetothecrystaledgeis oftheorderof 1m.Ontheotherhand,acloseinspectionofthepicturereveals anatasemicrodomainsdistributedwithinthecrystalasseenbythe magnificationoftheimageintherightpanelofFig.2(a).Someof themaresurroundedbyabluehalosuggestingtheexistenceofa gapbetweenthemicrodomainandthecrystal.AnAFMimageof arepresentativeanatasemicrodomainwithinaSAcrystalshown inFig.2(b)confirmsthisassumption.Asexpected,thebluehalois relatedtothepresenceofadipinwhichthemicrodomainisalmost centered.Thedippresentsaflattened-hexagonshapeandisabout 1mwideand3mlong.Theprofileofthedipallowsan esti-mationofthethicknessoftheSAcrystalwhichisabout30–40nm thick.Apossibleexplanationfortheformationofsuchadipisthat thepreventionofcrystalgrowthduetothemicrodomains. 3.2.2. RL-DICmicroscopyobservationofSAcrystalsupon ultravioletirradiation
Fig.3presentsRL-DICmicroscopyimagesofaSAcrystalgrown onthetitaniafilmuponultravioletirradiation.Itisobservedthe pitswidenastheexposuretimeisincreased(Fig.3(a)–(d))strongly suggesting aphotocatalyzed degradationof theSAcrystal.This assumptionissupportedbythefactthatthedisappearanceofSA issystematicallyinitiatedintheimmediatevicinityofananatase microdomainasshownbythemagnificationsinFig.3(a)–(d).It shouldalsobenotedthatthedipspresent amoreorless regu-larflattened-hexagonshapeandshowapreferentialorientation towards the rhombus big axis. As the irradiation time is fur-therincreased,thelateralgrowthofthepitsleadstocoalescence (Fig.3(e))whichresultsinpercolationofthehexagons(Fig.3(f)) andfinally,thetotaldisappearanceoftheSAcrystal(Fig.3(g))as shownbytheremainingbackgroundcolour.
Fig.2. (a)RL-DICmicroscopyimageofstearicacidcrystal(B-polymorph)depositedonthetitaniafilmbeforeultravioletirradiation.Theleftpanelisamagnification(×10) oftheedgesofthecrystalshowingamicrodomainattheedgeandtwoothersinitsimmediatevicinity.Theleftpanelisamagnification(×5)ofaregionthecrystalshowing microdomainswithinthecrystal.(b)AFMimageofamicrodomainatthecentreofadipwithinthecrystalandprofileofthedip.
Table1
CrystallographicdatafortheB-polymorphsofstearicacid(fromRef.[15]).
Polytype Structure Spacegroup a(Å) b(Å) c(Å) (deg)
Bm monoclinic P21/cZ=4 43.95 7.397 5.598 90.31
Bo orthorhombic PbcaZ=8 7.408 5.587 87.69 90
Fromtheflattened hexagonorientationandthecrystal sym-metry,itispossibletodeterminethecrystallographicorientation oftheedgesofthegrowingpits.Table1presentsthe crystallo-graphicdataoftheB-polymorphs.TheB-polymorphofSApresents twopolytypes:oneismonoclic(Bm)crystalizesintheP21/cspace groupwhiletheotheroneisorthorhombic(Bo)andcrystalizesin thePbcaspacegroup.
Fig.4presentstherelationshipbetweenthecrystalstructure, thegeometryandtheorientationofthepits.Forsimplicity,wehave onlyconsideredthecaseofthemonoclinicB-polymorphsbutthe followingdescriptionscanbeeasilytransposedtothe orthorhom-bicformoftheB-polymorphs.Fig.4(a)showsaRL-DICmicroscopy imageofadiponwhichareindicatedthecharacteristicanglesof theB-polymorph.Fig.4(b)showstherespectiveorientationsand morphologiesoftheSAcrystalandpitwithrespecttotheunit-cell whileFig.4(c)presentstheprojectionoftheunit-cellbasalplane (bcplane)ofthemonoclinicBformadaptedfromRefs.[13,15]. Thebasalplaneofthecrystalcorrespondstothe{100}faces.The geometricalcharacteristicofthepits(flattenedhexagons)isalso consistentwiththeB-form.Thefouredgesofthepitthatareparallel tothefoursidesoftherhombuscoincidewiththe{h11}facesand thetwootherscorrespondtothe{h10}faces.Theanglebetween
(011)and [01 ¯1]crystallographicdirections asmeasuredin the (100)basalplaneis73◦andthatbetween(011)and[0 ¯11]is107◦in goodagreementwiththecharacteristicanglesoftheB-polymorph. Thegapdistancesg{h11}andg{h10}betweentheactivedomain andthepitsedgesaccordingto{h11}and{h10}facesrespectively, havebeenplottedasafunctionofirradiationtimeinFig.5.From5 to40min.,g{h10}andg{h11}increaselinearlywithirradiationtime toreach2.8mand 4mat40min,respectively.The displace-mentratesoftheedgesR{h11}andR{h10}accordingtothenormal directionstothe{h11}and{h10}faces,werefoundtobe0.069 and0.057mmin−1,respectively.Thefactthattheinterceptsof bothregressionlineshaveapositivevalueisconsistentwiththe preexistenceofthepitspriortotheUV–visirradiation(Fig.2).
4. Discussion
Thepresentedresultsprovidefurtherevidencethatmolecules wellseparatedfromaphotoactivetitaniadomaincanbe photode-graded.Asearlierreported[17–19].thisremotephotodegradation ismainlyduetothesurfacediffusionofactivespecieslike hydrox-ide radicals (OH•) or even peroxide (H2O2) from the titania microdomain.Theseactivespeciescandiffuseatrelativelylarge
Fig.3.RL-DICmicroscopyimageofstearicacidcrystal(B-polymorph)depositedonthetitaniafilmuponultravioletirradiationafter(a)5min,(b)10min,(c)15min,(d) 20min,(e)30min,(f)40minand(g)50minexposuretime.
distance,uptoseveraltensmandreactwithanchoredmolecules [17–19].Unliketheabovecitedworks,wheretheactivedomains presenta linearedgewithindefinitelengthmakingthespecies todiffusetowardsthenormaldirection,theactivemicrodomains describedinourworkareverylimited(afewhundrednm)sothat theycouldalmostbeconsideredaspointdiffusionsources.Asa consequence,theactivespeciescandiffuseonthesurfaceinall thedirectionsaroundtheanatasemicrodomains.Thesurface dif-fusioncoefficientoftheactivespeciesonamorphoustitaniabeing isotropic,theisoconcentrationlinesofthediffusingspeciesaround themicrodomainshouldalmostbecircular.Thiscontrastswiththe anisotropicevolutionofthepitsedgesstronglysuggesting,atfirst glance,thatthephotodegradationkineticsisgovernedbythe reac-tionattheedgeswhichisrelatedtheanisotropicpropertiesofthe SAcrystal.Thisisalsoclearlyevidencedbythelinearevolutionsof thegapdistancesasafunctionofirradiationtimeobservedinFig.4
[20].Inthefollowing,wetrytocorrelatethemorphologyofthepits withthecrystalproperties.