stearic acid microcrystal grown on an amorphous titania surface
scattered with anatase microdomains
FouadAraiedha,b,FranckDucosb,AmmarHouasa,c,Nouari Chaouib,*
a
UnitédeRechercheCatalyseetMatériauxpourl'EnvironnementetlesProcédés(URCMEP),UniversitédeGabès,CampusUniversitaireCitéErriadh,6072, Gabès,Tunisie
bLaboratoiredeChimieetPhysique-ApprocheMulti-échelledesMilieuxComplexes(LCP-A2MC),UniversitédeLorraine,France
cAlImamMohammadIbnSaudIslamicUniversity(IMSIU),CollegeofSciences,DepartmentofChemistry,Riyadh,11623,SaudiArabia
ARTICLE INFO
Articlehistory:
Received26October2017
Receivedinrevisedform4December2017 Accepted4December2017
Availableonline9December2017
Keywords: Photocatalysis Titaniumdioxide Anatase Stearicacid ABSTRACT
Thephotodegradationofastearicacid(SA)microcrystalonanamorphoustitaniafilmsurfacescattered withsubmicrometricanatasedomainsisobservedusingaReflectedLight–DifferentialInterference Contrast(RL-DIC)microscopy.TheSAmicrocrystal,grownfromasaturatedsolution,isflatand rhombus-shapedwithacuteangleof55inagreementwiththeC-polymorphofSAcrystal.Weobservethatthe photodegradationinitiatesattheanatasemicrodomainsunderthemicrocrystalandcreatesSAholesin theshapeoftruncated-rhombusorelongated-hexagonshavingthesameorientationandsymmetryas theSA microcrystal.The expansionofthe SAholes is ascribedtoaphotodegradation mechanism mediatedbythephotogenerationandsurfacediffusionofradicalspeciesacrossthegapbetweenthe anatasemicrodomainandtheedgesoftheSAholes.AdetailkineticstudyoftheSAholesexpansion showsthatthesurfaceareaoftheSAholes scalesalmostlinearlywithUVexposuretimeingood agreementwithanapparentzero-orderkinetics.Themorphologyandtheexpansionkineticsofthese holesarerationalizedthroughtheorientationdependenceofthereactivityofthemoleculeswiththe radicalspecies.
©2017ElsevierB.V.Allrightsreserved.
1.Introduction
SincethepioneerworkofPazetal.[1],thephotodegradationof long-chain carboxylic acids on TiO2 surfaces has attracted tremendousinterestfrombothappliedand fundamentalpoints ofview[1–12].Inparticular,theuseofstearicacid(SA)asamodel pollutant quickly became a popular method to determine the activityofphotocatalystthinfilmsforapplicationsintheareaof environmentalremediationandself-cleaningsurfaces.Atamore fundamental level, the kinetics models and related processes involvedinthephotodegradationofsolid organicdepositsona TiO2 surface differin numerousaspects fromthose involvedin liquid-orgasphaseandremainnotfullyunderstood.
Thephotocatalyticreactionofanoxidizablesolidphaseis,in numerouscases, attributedtothestrong oxidation potential of activeoxygenspecies,suchasthehydroxylradicalsOH,thatare
photogenerated on the UV-illuminated TiO2 surface [13]. The bimolecularreactionbetweentheseactivespeciesandtheorganic co-reactantisresponsibleforinitiatingthephotocatalytic degra-dation process. Furthermore, there is a body of evidence supporting thefact that this bimolecular reaction can even be mediatedviathediffusionofOHradicals[14–17].Asregardsthe degradationkinetics, thereaction ratesstrongly dependonthe respectivestructure and morphologyof bothphotocatalyst and organic layer. D. Ollis [11] examined different photocatalyst/ organiclayerconfigurationsandproposedsimplekineticmodels associatedtoeachcase.Recently,wehighlightedthedependence of theinitialmorphologyof theSAdeposit onthedegradation kineticsofSAwhichislargelyinfluencedbythedepositionmethod andconditions[10].SAdepositsareusuallypreparedbysolution castingmethodssuchasdip–[2,10]orspin-coating[1,3,7]butalso by more sophisticated Langmuir–Blodgett film method [5,8,9]. Whilethelatterallowsthedepositionofoneormoremonolayers ofSAwithveryaccuratethickness,theformersleadinmostcases totheformationofmicroscopicislandswithirregularshapesand presenting a distribution in sizes and heights [10,12]. The
*Correspondingauthor.
E-mailaddress:nouari.chaoui@univ-lorraine.fr(N.Chaoui). https://doi.org/10.1016/j.jphotochem.2017.12.006
1010-6030/©2017ElsevierB.V.Allrightsreserved.
ContentslistsavailableatScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
processwhich is influencedby numerousparameters including solutionconcentrationand temperature[18].By varyingtheSA depositionconditions,itispossibletogrowfacetedislandsonthe photocatalystsurfaceandevenwell-definedSAmicrocrystals[19]. Twomainpolymorphic forms ofSAcrystal canbegrown from solution:theBpolymorphwhichcrystalizesasaflatand rhombus-shapedcrystalwithanacuteangleof74 and theC-polymorph whichisalsoflatandrhombusshapedbutwithanacuteangleof 54[19,20].ThesetwoformsofSAcrystalscanbethereforeeasily distinguishedbythemeasureoftheiracuteangles.Inaveryrecent report [21], using Reflected Light Differential Interference Contrast(RL-DIC)microscopy,weobservedthephotodegradation
mode of the B-polymorph of SA microcrystals grown on an
amorphous titania film surface scattered with submicrometric anatasemicrodomains.Interestingly,thisstudyrevealedthatthe photodegradation of these microcrystals proceeded via the appearanceattheanatasemicrodomainsandwideningofholes in shape of flattened hexagons within the microcrystal which exhibitthesamesymmetryand orientationasthemicrocrystal. Themorphologyoftheseholeswasrelatedtotheanisotropyofthe crystalstructureandtherelatedorientationsofthemoleculesin thedifferentcrystallographicdirectionsofthecrystal.Inthiswork, westudythephotodegradationoftheC-polymorphofSA micro-crystals initially grown on an amorphous titania film surface containing submicrometric anatase microdomains. This report providesfurtherinsightsintotheformationoftheseSAholesand their relationship with the crystal morphology and structure. Emphasisisputontheholesexpansionkineticswhichisdiscussed onthebasistheanisotropyofthereactionkinetics.
2.Experimentalsection
2.1.PreparationoftheTiO2thinfilms
TheTiO2solwaspreparedaccordingtoaprocedurereportedby Bahtat et al. [22].Propan-2-ol (Aldrich, 99.8%) and acetic acid (ACROS Organics, 99.5%) were used as solvent and catalyst, respectively.Thesolwasdepositedontoborosilicateglassslides (D263M,Schott)Theglassslidewasdipped8timesinthesolwith atawithdrawalspeedof100mm/minandfinallydriedat100C for1handheat-treatedfor2hinairat450C.Furtherdetailsare giveninref.[21].
2.2.Photocatalyticexperiment
SA microcrystals were grown onto the titania films surface according to a recently published procedure [21]. Briefly, the titaniafilmsweredippedtwiceina0.06Mmethanolicsolution maintainedat30C ata withdrawalspeed of100mm/min and driedduring5minbetweeneachdipandfinallyfurtherdriedfor 2h.
The photocatalytic degradation of the SA microcrystals was measured as a function of UV-exposure time. The films were irradiatedbya200Wmercury-xenonlamp(LotOrielinstruments) simulatingsolarradiation equippedwitha 0.5mlength optical
fiber.ThepowerdensityintheUVAwas25Wm 2,asmeasuredby a photo-radiometer (Delta Ohm DO 9021) equippedwith UVA head.ThephotodegradationoftheSAmicrocrystalswasobserved as a function of UV exposure time with a Reflection Light DifferentialInterferenceContrastmicroscope(AxioImagerA1m, Zeiss),equippedwithahigh-resolutioncamera(AxioCamMRc5). The enhanced contrast and resolution inherent to RL-DIC microscopy enables the observation of minute details on the surfaceandisthenparticularlyappropriateforthesimultaneous observation of the microcrystal edges and the submicrometric
imageswasperformedusingImagejsoftware[23]. 3.Resultsanddiscussions
3.1.RL-DICmicroscopyobservationoftheSAmicrocrystalafter
variousUVexposuretimes
Fig.1 presents the RL-DIC images of the C-polymorph of a representative SAmicrocrystalgrownonthetitaniafilmbefore UV-illumination(Fig.1(a))andaftervariousUV exposuretimes (Fig.1(b)–(h)). The TiO2 film consists of an amorphous titania matrix withembeddedanatasenanocrystals[22] andscattered withanatasemicrodomains[21].OntheRL-DICimagetheanatase microdomains appearas darkspotsuniformlydispersedonthe
filmsurface.Theirdiametervariesfromafewhundredsofnmto aboutonemicrometer[21].Exceptfortheseblackspots,therestof surfaceappearsfeaturelessatthisscale.
Aspreviouslymentioned,theSApolymorphsareknowntobe easily identified from the crystal morphology and their angle values.Theas-grownSAmicrocrystalisrhombus-shapedwithan acuteangleof55ingoodagreementwiththeC-polymorphofSA crystalswhichis54[19,20].Notethesharpcontrastbetweenthe crystal(lightbrown)andthetitaniafilmsurface(lightgray)and thewell-definededgesofthecrystal.Thehigh-resolutionachieved byRL-DICmicroscopyalsoenablestheobservationoftheanatase microdomainswithsub-micrometricsizewhichappearasminute blackdotsonthelightgraybackground.Anatasemicrodomainsare alsovisiblewithinthecrystalsurfaceasindicatedbythearrowon themagnifiedimages(Fig.1(a),rightpanels).Thelightgraycolor aroundthemicrodomainrevealstheexistenceofagapbetween themicrodomainandthemicrocrystalasalreadyreportedinour previousstudyonthephotodegradationmodeoftheB-formofSA [21].Thisgapisoftheorderofonemicrometerorless.Apossible explanationfortheexistenceofthisgapisanincidentalexposure ofthesampletoUVlightwhichcouldcomefromthemicroscope illumination.WeusedaZeissmicroscopeequippedwitha100W halogenlampwhichhasaradiationspectrumofablackbodyatthe temperatureof3300K.Atthistemperature,thelampemitsatiny fractionofUVradiation.WealsousedNeofluarobjectiveswhich transmita bitintheUVAdomain.Priortothe(intentional) UV exposure, we spent some time to examine in detail the SA microcrystals grown on the TiO2 film and depending on the selectedobjectivetheirradiancecanreach0.6Wm 2intheUVA range.Therefore,itcannotbeexcludedthattheholesobservedat t=0 were obtained during these preliminary observations. However, it should benoted that during the photodegradation experiments,the observation and imagerecordingof theholes afterUVexposureswereperformedinfewminutes.Therefore,the contributionofthemicroscopelight tothephotodegradation of theSAmicrocrystalisstillnegligible.
AttheearlystageoftheUVexposureFig.1(b),severalholes appearintheSAcrystalandacloseinspection(seemagnifications onrightpanels)showsthateachholeinitiatesatthelocationofan anatase microdomain.As the UV exposure is furtherincreased (Figs.1(c)–(f)),theholeswidenandbecometruncatedrhombusor elongatedhexagonalinshape withthesame orientationasthe microcrystal(Fig.1(d)).Theholesexhibitapreferentialorientation alongthemajoraxisofthecrystal.Itisalsoworthnotingthatthe lateraledgesoftheholesareparalleltothoseofthemicrocrystal. After 48min.UV exposure, theholes merge (Fig.1(g))and the microcrystal totally disappears (Fig. 1(h)) after 56min. UV exposure.
TheexpansionoftheSAholesasafunctionofUVexposuretime is explained bythe well-documentedremote photodegradation mechanism[13–16]involvingactiveoxygenspeciessuchasOH
Fig.1.RL-DICmicroscopyimagesoftheC-polymorphofaSAmicrocrystalgrownonthetitaniafilmbefore(a)andaftervariousUVexposuretimes(b)–(h).Therightpanels showmagnifiedimagesoftworepresentativeSAholes.Theblackrowsinthemagnifiedimages(a)and(b)indicatethelocationoftheanatasemicrodomain.Thebigrhombus indashedlinesinimages(a),(d)and(f)representstheinitialsizeoftheSAmicrocrystalwhilethesmallonesinthemagnifiedimages(d)helptocomparethesymmetryofthe microcrystaltothatoftheSAholes.
radicals.Theseoxygenspeciesarephotogeneratedattheanatase microdomains,thendiffuseonthesurfacetowardstheedgesofthe holesandreactwiththeSAmolecules.
Itisalsoobservedthattheexternaledgesofmicrocrystalrecede ataverylowratewithUVexposuretime.Thisisclearlyevidenced when comparing thesize of the microcrystal after 24min. UV exposure(Fig.1(d))tothatbeforeUVexposure(Fig.1(a))whichis representedbytherhombusindashedlinesintheleftimageof Fig. 1(d). The photodegradation of the external edges of the microcrystalmayberelatedtotheweakactivityoftheamorphous region of the film due to the presence of embedded anatase nanocrystalsbutcanalsobeexplainedbythecontributionofthe anatasemicrodomainsinthevicinityofthecrystaledge[21].These imagesalsorevealthatthemorphologyoftheSAholesafteralong exposuretimeisclearlyrelatedtothatobservedattheearlystage oftheprocess.Thisisclearlydemonstratedbyacomparisonofthe magnified imagesafter 8 (Fig.1(b)) and 40min. (Fig.1(f)) UV exposuretime.Thisstatementisvalidwhatevertheinitialshapeof thehole.
3.2.Relationshipbetweenthecrystalhabit,thecrystallographic
orientationsandtheshapeoftheholes
Table1presentsthecrystallographicdataoftheC-polymorphs ofSA.ThispolymorphismonoclinicandcrystallizesintheP21/c spacegroup.Fromthemorphologyof theholes andthecrystal symmetry, it is possible to determine the crystallographic orientation of the edges of the holes. Fig. 2(a) displays the projectionoftheunit-cellbase(bc)oftheC-formofstearicacid adaptedfromrefs.[19,20]Fig.2(b)and(c)showmorphologiesand orientationsoftheSAmicrocrystal(Fig.2(b))andholeedgesinthe (bc)planeoftheunitcell(Fig.2(c)).
Thebigaxisofthecrystalandtheholecoincidewiththe[001] crystallographic direction. The angle between [011] and [011] crystallographicdirectionsasmeasuredinthe(100)basalplanis 55andthatbetween[011]and[011]is125ingoodagreement withthe characteristic angles of theC-polymorph [19,20]. The {100} faces are by far the most developed in the crystal
SA crystal suggest that the lateral faces of the crystal are in decreasingorderofimportancethe{h11},{h01}and{110}[19].In goodagreementwiththecrystalmorphology,the{h11}facesare well-developedandrepresentthemajorcontributiontothehole perimeterwhereasthe{h01}facesarealsoratherwell-developed ontheholebutnotonthemicrocrystal.
3.3.KineticstudyoftheSAholesexpansion
To obtain a more quantitative understanding of the holes expansionprocess,thesurfaceareaofseveralholeswasplotted as a functionof UV exposuretimein Fig. 3(a).Because of the resolution limits of themicroscope image, thesurface area of theholeswasplottedstartingfrom10min.Themeasurements are also limited by the merging of the holes for UV exposure timeabove50min.AsshowninFig.3(a),theevolutionsofthe holes areasas a functionof UVexposure timeshowthe same behavior. This canbe appreciated from theweak scatteringof the plots. This means that the degradation rate is poorly influenced by the size or shape of theanatase microdomains. Theevolutionofthesurfaceareaisalmostlinearindicatingthat the SA removal is almost proportional to UV exposure time. Assuming the thickness of the crystal to be locally constant (about40nm),thereactionkineticsisthereforeconsistentwith anapparentzero-orderwithrespecttoSAmassasobservedfor mono- multilayer carboxylic acid films [5,6,10] deposited on continuous anatase films. The regression line of the averaged area values has a slope of about 1.2mm2min 1. These results suggestthatthereactionkineticsis notlimitedbydiffusion of the oxygen radicals but rather by the reaction between the radicals andthe SAmoleculesat the edges.
InFig.3(b)and(c),thegapdistancesg{h01}andg{h11}between theanatasemicrodomainandtheholeedgescorrespondingtothe four{h11}andthetwo{h01}faces,respectively,areplottedasa function of UV exposure time. The presented data are raw measurements from a representative hole (hole 2 shown in Fig.1).Above15–20min.UVexposure,theevolutionsofg{h01}and g{h11} (Fig. 3(b) and (c)) are almost linear and present similar slopes.Thisis clearlyevidenced bythecalculatedvalues ofthe displacementratesR{h01}andR{h11}obtainedfromtheslopeofa linearfittingintherange20–48minandgatheredinTable2.These values are indeed found to be similar whatever the crystallo-graphicdirection.
Fig.2.Projectionoftheunit-cellbase(bc)oftheC-polymorphofstearicacid(a)adaptedfromrefs.[19]and[20].MicroscopyimagesoftheSAmicrocrystal(b)andofaSAhole (c)showingtheircrystallographicorientationsinrelationwiththeirmorphologiesinthe(bc)planeoftheunitcell.
CrystallographicdatafortheC-polymorphsofstearicacid(datafromref.[19]).
Spacegroup a(Å) b(Å) c(Å) a(deg) b(deg)
Inanalogywithgrowththeoryofcrystals[19],themorphology of a hole could be determined by the difference between the displacementsratesofitsdifferentfaces[21].Thoseshowingthe fastestdisplacementratesvanishwhereasthoseslowlygrowing dominatetheshapeofthehole.ThefactthatR{h01}andR{h11}are similar is consistent with the aforementioned (Fig. 2 and 3) conservationoftheinitialmorphologyoftheholes.Inourprevious workonthephotodegradationoftheB-polymorphofSA[21],the displacementrateoftheedgesoftheholeswasputinrelationwith theanisotropyofthecrystalwhichisgovernedbytheanisotropic molecules interactions along the different crystallographic
thattheholegrowsfasteralongthedirectionscorrespondingtothe strongest interaction, i.e. the hydrogen bonds, between the molecules.Inthiswork,thefactthatR{h01}andR{h11}aresimilar maysuggestthattheinteractionsbetweentheOHradicalsandthe SA molecules in the normal direction of these faces are comparable.
Itshouldalsobenotedthattheearlystageoftheholeevolution, below15–20minexposuretime,somewhatdeviatesfromalinear behavior.ThisisespeciallyobservedinFig.3(c).Inthistimerange, thedisplacementrateoftheedgesisestimatedtobetwicethan thatmeasuredinthe20–48minrange.Thistransitionmayindicate a changein thephotodegradationmechanisms which couldbe reasonablyascribedtotheproximityoftheedgeswiththeanatase microdomainintheearlystageoftheprocess.Asmentionedabove, the photodegradation of the SA microcrystal proceeds via the photogeneration and diffusion of OH radicals from the titania microdomaintowardstheSAedges.TheOHradicalscanevenbe transportedthroughthegasphase toreactwithremoteorganic molecules[14,15].Therefore,theobservedfastdisplacementrate below20minexposurecouldbeascribedtoajointcontributionof theOHradicalsreachingtheSAmoleculesbeyondtheedgesofthe holesviathegasphaseandthosereachingthemoleculesonthe edgesoftheholesviasurfacediffusionacrossthegap.Inthistime range, the gap distance varies from 1 up to about 3mm. The observedlinearevolutionabove15–20minprocesscouldthenbe exclusivelyascribedtothephotodegradationinvolvingthesurface diffusionoftheOHradicalsacrossthegap.
Besides,thegeometryofremotephotodegradationexperiment usuallyobservedintheliteratureinvolvetwoparallellinearedges: that of a wide titania domain and that of the organicfilm (or adsorbedmolecules),facingoneanotherandseparatedbyagap [16,17]. The photogenerated radicals from the titania domain diffusetowardstheorganicfilmincreasingthegapasafunctionof exposuretime.Inthiswork,thetitaniadomainisoflimitedextent and can be considered as a point source of radical species
surroundedbytheedgesoftheorganicmolecules.Inthiscase,the
radicalspeciesdiffuseradiallytowardstheedgesoftheorganic
film.Sinceinsuchaconfigurationthediffusionfluxdensityevolves in theinverse proportionto gapdistance,it is quitenatural to expectdiffusion-limitedreactionkineticsatlongerexposuretime. Theroundingoftheanglesoftheholesobservedinthetwolast imagesofFig.1maybecausedbytheonsetofsuchatransition. Unfortunately,themergingoftheholesatabout50minprevents