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Applied
Surface
Science
j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c
Tuning
the
surface
morphology
of
self-assembled
graphene-like
thin
films
through
pH
variation
Michela
Alfè
a,
Valentina
Gargiulo
a,
Roberto
Di
Capua
b,∗aIstitutodiRicerchesullaCombustione,ConsiglioNazionaledelleRicerche(IRC)-CNR,p.leV.Tecchio,80,80125Napoli,Italy bDipartimentodiFisica,UniversitàdiNapoli“FedericoII”,andCNR-SPINUOS-Napoli,viaCintia,80126Napoli,Italy
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received22February2015
Receivedinrevisedform19June2015 Accepted19June2015
Availableonline24June2015 Keywords: Graphene-like(GL)layers Zetapotential pH AFM
a
b
s
t
r
a
c
t
Graphene-like(GL)layerswerepreparedthroughatwostepsoxidation/reductionmethodstartingfrom ahighsurfacecarbonblack,andpHoftheGLlayersinwatersuspensionwasvaried.TheeffectofpHof suchsuspensiononthemorphologyofself-assembledGLfilmshasbeenstudied.Zetapotentialsofthe watersuspensionsweremeasuredtoestimatethestabilityofthesuspensionatseveralpHvaluesand toselectthesamplesfordeeperinvestigationbyatomicforcemicroscopy(AFM).AFMmeasurements onfourdifferentsamplesarethendescribedanddiscussed.Thereportedresultsshowhowthesurface roughnessandmorphologyareaffectedbythepHinthepreparationprocess:inparticular,thelowest pHsampleexhibitsagranularsurface,whileathigherpHmoreregularmorphologiesareproduced, withinterestingobservationsasconcernsthethicknessofsomesurfacefeatures.Theobservationsare interpretedintermsoftheforcesactinginwatersuspensionandoftheroleofhydrophobicorhydrophilic behaviors.TheresultsdemonstratethepossibilitytotunethesurfacepropertiesofGLfilmsbysimply actingonthepHofthesuspensionduringthefabrication,andhelptounderstandthemicroscopicphysical mechanismsinvolvedinthefilmassembly.
©2015ElsevierB.V.Allrightsreserved.
1. Introduction
Therapiddevelopmentandupgradingofoptoelectronicdevices (liquidcrystaldisplays,organiclightemittingdiodes,organic pho-tovoltaiccellsandtouchpanels)hasinducedagrowingdemand of transparent conductive films. Films have to be conductive and transparent but also flexible, cheap, and compatible with largescalemanufacturingmethods(currentoptoelectronicdevices are normally assembled on hard substrates such as glass) [1]. Organic-conjugatedmolecules(aromatichydrocarbonsasacenes andfusedarenesandsubstitutedderivates)haveattractedgreat attention in nanodevices fabrication [2,3] due to the presence of delocalized electrons inside their structure, which offer the developmentofproperconductivepathways.Numerousreports addressingtheuseofcarbonnanotubes,reducedgraphiteoxideand graphenethinfilmsasidealtransparentelectrodesfor optoelec-tronicdeviceshaveappearedinthepastyears[4–6].Carbon-based filmsaddressabroadrangeofcoatingapplications[7],andsome
∗ Correspondingauthor.Tel.:+39081676915.
E-mailaddress:roberto.dicapua@na.infn.it(R.DiCapua).
recentstudiesshowedtheirpossibleapplicationsformedical appli-cations[8],andphotocatalysis[9,10].
The production of carbon-based films simultaneously with highstability,controlledthickness,andtunableperformancesstill remainsaninterestingchallenge.Theset-upofversatileand inex-pensivecoatingmethodsforthedepositionofcarbon-basedfilms withcontrollablemorphologyandthicknessisstillagoalin mate-rialscienceresearch.
Anewapproach for producinggraphene-like(GL) thin films hasbeenrecentlyreported[11].IntheproposedmethodGLlayers wereproducedinmildconditionsandinaqueousenvironments with highyields (55% mass yield) through a two steps oxida-tion/reductionmethodstartingfromananostructuredcarbonblack (CB).TheproposedapproachformakingGLthinfilmsis environ-mentallyadvantageousbecauseallproceduresareperformedin aqueousmedia,anditishighlycompatibletoanindustrialscale-up process.Thankstotheirgoodstabilityinwater,drivenby residu-alsoxygenfunctionalgroupsontheGLlayersedge,GLappearsasa veryversatilenanomaterial,suitableforthepreparationinaqueous environmentofcompositeswithtunablechemical/physical prop-ertiessuchasmicroporosity,conductivity,catalyticperformances andbiocompatibility[12–14].
TheGLlayersundergotoself-assemblinginthinfilmon sur-facesafterdrying,asshownbytransmissionelectronmicroscopy http://dx.doi.org/10.1016/j.apsusc.2015.06.117
andatomicforcemicroscopy(AFM)[11],thankstothe instaura-tionofhydrophobicinteractionsbetweenthegrapheniclayers,as typicallyobservedinreducedgraphiteoxide[15].
Foraconvenientexploitationoftheprocessandfilmthickness controlitisfundamentaltocarefullyexploreeachaspectofthefilm preparation.DuetotheresidualfunctionalgroupsontheGLlayers edge,thequalityofthethinfilmisexpectedtobestrongly depend-entonthepHofthegraphene-likelayerssuspension.Thisstudy clarifiestheeffectofthepHonthegraphene-likefilmqualityand itscapabilityindeterminingthesurfaceproperties,andintroduces asimplepictureoftheprocesstodescribetheinvolvedphysical mechanisms.
2. Materialsandmethods 2.1. Samplespreparation
Allthechemicals(analyticalgrade)werepurchasedfromSigma Aldrichandusedasreceived.FurnaceCBtypeN110wasobtained bySidRichardsonCarbonCo.NaClandNaOHsolutionsfor poten-tiometricstudywerepreparedbydissolvingtheCarloErbaRPE productsinwater. NaOHsolutions weretitratedwithHClstock solutionsusingaphenolphthaleinindicator.HClstocksolutionwas analyzedbyKHCO3,usingamixedmethylred–bromocresolgreen
indicator,withareproducibilityof0.1%.
GLlayersin water suspensionwereobtainedthrougha two stepsoxidation/reductionmethod[11]startingfromaCB.Briefly, 500mgofCBpowder(15–20nmprimaryparticlesdiameter, spe-cific BETarea139m2/g) wasoxidized with10mLof nitricacid
(67wt.%) at 100◦C under stirring for 90h. The oxidative step destroys the CB backbone providing hydrophilic nanoparticles functionalizedwithoxygenfunctionalgroups(mainlycarboxylic). Thistreatmentaffects theedgeof theCBgraphitic layers leav-inguntouchedthebasalplane.Theoxidizedcarbonnanoparticles wererecoveredbycentrifugationandwasheduntilacidtraceswere successfullyremoved.Followingtheoxidationstep,the nanoparti-cles(20mg)weredispersedin20mLdistilledwaterandtreated with450Lof hydrazine hydrate (50%) at 100◦C underreflux for24h.Thistreatmentreducesthehydrophiliccharacterofthe nanoparticlesandpromotesself-assemblingphenomenaasa con-sequence ofthe hydrophobicinteractionbetween thegraphitic planes[11].Attheendofthereactiontheexcessofhydrazinewas neutralizedwithnitricacid(4M).Thesolidwaswashedwith dis-tilledwater,recoveredbycentrifugation(3000rpm,30min)three timesinordertoremove tracesofunreactedreagentsand acid andnamedGLlayers.ThepHoftheas-preparedGLlayerswater suspensionwas3.70.ThedriedGLlayersareinsolubleinwater andin themostcommonorganicsolvents, bothpolarand apo-lar (ethanol, N-methylpirrolidinone, dichloromethane, heptane, dimetilformammide).
2.2. Characterizationprocedure
ThedeterminationofGLlayerssurfaceacidicfunctionalitieswas performedadaptingthetestreportedbyVisentinetal.[16]for oxi-dizedcarbonnanotubes.ThetotalamountofGLlayersusedforeach assaywasestimatedtobe0.01mg.GLlayerswatersuspensions weretreatedwithanethanolicsolutionofthioninacetate(THA, 1.5mL,4.3M)for30minwithconstantstirringatroom tempera-ture.Thesuspensionwasfilteredusinga0.02mAnotop25filter (Whatman),andtheabsorbancewasmeasuredat604nm[17]with aHP8453DiodeArrayspectrometer.TheTHAthathadnot inter-actedwiththematerialwasestimatedthroughacalibrationcurve. ThetotalamountofTHAthathadinteractedwiththeGLwas esti-matedandcorrectedfortheunspecificinteractionbysubtracting
theamountofTHAinteractingwithareferencecarbonaceous mate-rial(CB)exhibitingnoacidicfunctionalities(5.28×10−2mmol/g). Thequantificationoftheacidicsiteswasperformedbyconsidering a1:1stoichiometrybetweenthecationicTHAandthecarboxylic functionalities.
Theacid–basebehaviorofthecarboxylicfunctionalgroupon surfaceofGLlayershasbeenstudiedinNaCl0.25M,measuring thepotentialofaglasselectrodesensitivetoprotonactivity.The intervalofpHinvestigatedwas2.7<pH<7.Theglassmembrane electrodesaswellastheautomaticburettes“640MultiDosimat” weresuppliedbyMetrohm.Measurementswerecarriedoutintoan airthermostat.Thetemperaturewaskeptat25.00±0.02◦C (mea-suredusingaPt100TERSIDthermocouple).Coulometricvariations ofthesolutioncompositionhavebeencarriedoutusingaHewlett PackardDC Power Supply.Thecurrent intensitywasaccurately determinedbyreadingthepotentialdropattheendsofacalibrated resistancecoil,connectedinseriestothecoulometricdevice.
Thestudywasperformedbymeasuringtheelectromotiveforce (Emf)ofthecell(I):
G.E.|TS|R.E. (I)
inwhichG.E.indicatesaglassmembraneelectrodereversibleto protons;R.E.isanexternalreferenceelectrode,connectedtotest solution(TS)throughasaltbridge.Inthefirststepthecell con-stant(*Eg)andtheinitialnumberofmicromolesofH+(n
0)were
determined.Acoulometric–potentiometrictitrationwasthen per-formed:aweighedvolumeV0(50.00cm3)ofasolutionTS0=0.25M
NaClwasintroducedintothemeasuringvessel.Thehydrogenion concentration,H,wascoulometricallyvariedbymeansofthecircuit (II):
(II)
where A.E.: 025MNaCl/HgO(s)/Hg(s)(Pt)is an external aux-iliaryelectrode, connected toTS0 through a salt bridge and Pt
denotesaplatinumelectrode.Inordertoestimatetheamountof electrolysisproducedbycircuit(II),accordingtoreaction2H2O(l)
4H+(aq)+4e−+O
2(g),aftereachdeliveryofcurrentofintensityi(A),
foratimet(s),theEmfofcell(II),E(mV),wasmeasureduntil con-stancy.TheNernstpotentialofcell(I),E,canbewrittenasinEq. (1):
E=∗Eg+59.16logh+Ej (1)
where *Egis the glasselectrode constant and h representsthe equilibriumconcentrationofprotons.Foracidities,h,lowerthan 10−3M, the junction potential Ej, can be neglected. Similarly, thetermaccounting for theactivityfactorchanges canbe also neglected,becausethecompositionofTSdoesnotdiffer appre-ciablyfromthatoftheionicmedium.AccordingtoFaraday’slaw, thenumberofmicromolesofproducedH+ions,,isnumerically
equaltothemicrofaradaysgeneratedbythecircuit(II),aswritten intheEq.(2):
= (i∗t)
0.096487 (2)
Thesetsofexperimentaldata(Eg,)canbefurthertransformed intoalinearfunction,i.e.aGranplot[18],whichprovidesthevalue ofn0,aswellasE0,thatareindispensableforthenextstep.
Apotentiometric–volumetrictitrationof0.250gofsamplein NaCl0.25Mhasbeencarriedout.Titrationwasperformedby step-wiseadditions,bymeansofanautomaticburetissuingvolumesVT
Theexperimentaldata(Eg,VT)wereinterpretedbythesoftware
LETAGROP-ETITR[19].
ThezetapotentialsoftheGLsuspensionsweremeasuredusing aMalvernZetasizerNanoZSinstrument.Thezetapotential mea-surementswereperformedataGLconcentrationof0.05mg/mLasa functionofpHinthe2-12pHrange.EachpointofpHwasreachedby adding100LofNaOH0.1MtotheGLlayerssuspensionsandthen bysubsequentadditionsoftheproperamountofHCl(0.1M).The pHandthezetapotentialmeasurementswereperformedafterthe timenecessaryforthepHstabilization(approximately1hunder continuousstirring).
SamplesforAFMimagingwerepreparedbydrop-castingthe GLsuspensionsontofreshlycleavedmicasubstrates(gradeV-1, ElectronMicroscopySciences)whichwerethenallowedtodryin air.AFMimagesweretakenbymeansofanXE100Park instru-mentoperatinginnoncontactmode(amplitudemodulation,silicon nitridecantileverfromNanosensor)atroomtemperatureandin ambientconditions.
X-rayphotoemissionspectroscopy(XPS) wasperformedin a ultra high vacuum chamber (base pressure of 1×10−10mbar) equippedwithaSpecsPhoibos150electronanalyzer,usingAlK␣
(photonenergy:1486.6eV)X-raysource.
3. Resultsanddiscussion
GLlayersinwatersuspensionconsistofsmallflat nanoparti-clesdecoratedattheedgewithoxygen-containinggroups(mainly carboxylic/carbonylicandhydrazones)withgraphenicbasalplane untouched[11].The amountofcarboxylic groups estimatedby themodifiedTHAmethodresultedtobe0.16mmolCOOH/g,lower
withrespecttotheamountestimatedfortheunreducedGLlayers (0.46mmolCOOH/g).
The functional groups on the surface of GL were also investigated by coulometric–potentiometric titration in the pH range 2.7<pH<7. Two functional groups on thesurface in the carboxylateregion(pK2.0–5.0)[20]withpKa=3.40±0.05
(num-ber of sites=900±30mmol/g) and pKa=5.5±0.1 (number of
sites=240±30mmol/g) respectively, have been identified. The higherpKavalue(5.5)isascribabletothepresenceoflactonesand
carboxylicanhydridegroups thattendtohydrolyzeinthe pres-enceofacidsandbases[21].Itisnoteworthythatthenumberof sitesattributabletocarboxylicacidsitesissignificantlyhigherwith respecttothevalueestimatedbytheTHAmethodandmore con-sistentwithliteraturedatareferringtorelatedmaterialssuchas graphiteoxide[22].Thisdiscrepancycanberationalized consid-eringthatthemethodwithTHA,optimizedforthedetectionof lowconcentrationofcarboxylicsites(∼10−2mmol/g)astypically
detectedoncarbonnanotubes[16]andsoot afterreactionwith hydroxylradicals(∼10−4mmol/g[17]),probablyfailswhenapplied
tonanomaterialsfeaturingamoredensedistributionofcarboxylic sitesduetothesterichindranceofTHA.
ThestabilityoftheGLlayerswatersuspensionswasevaluated byzetapotentialanalyses.Themeasuredzetapotentials(Fig.1) werealwaysnegativeindicatingthepresenceofpartlypreserved anionic charge. The zeta potential of the GL dispersion is pH-dependentwhichisconsistentwiththefactthattheionizationof residualsofcarboxylicgroups,responsibleoftheanioniccharge, isstrongly related topH. GLwasfound tobestable in a wide rangeofpHvalues,namely3–12.Thezetapotentialwasbelow −30mVwhenthepHwasgreaterthan2anditreached−45mV whenthepHapproached12.Thezetapotentialvalueslowerthan −30mVareconventionallyconsideredasanindicationofsufficient mutualrepulsiontoensurethestabilityofdispersions[23].The filmsandtheeffectofthereductionprocesshavebeenalso investi-gatedbyRamanspectroscopyandUV–visiblespectroscopy,whose
Fig. 1.pH-dependent zeta potential curves of the GL layers suspensions (0.05mg/mL).
mainresultsarereportedelsewhere[11].Briefly,theshapeofthe Ramanspectraisconsistentwiththeoccurrenceofamultilayer structure.ThecomparableheightoftheDadGbandsisindicative ofsignificantstructuraldisorder,whilethefactthattheD-band totheG-bandratioisnearlythesameforthereducedGLlayers andtheunreducedonesindicatesthattheoxidationprocess pre-servesthepresenceofthegraphiticnetworkbeingthechemical functionalizationattheedgeandnotatthebasalplanesoftheGL layers.UV–visiblespectroscopyconfirmsthepreservationofthe graphiticnetworkinthereductionprocess;furthermore,the spec-tracollectedonthemareblue-shiftedcomparedtotheonesonCB, indicatingadecreaseinthesizeoftheconjugationdomains.
FourpHpointswereselectedforadeeperinvestigation,namely pH2.0,3.7,4.6and9.5,whichinthefollowingwillbeindicated asSampleA,B,C,D,respectively.ThefoursamplesandpHpoints correspondto:(1)GLlayersinacidsuspensionatthelimitofthe stabilityrangeofthesuspension(pH2.0,SampleA);(2)asprepared GLlayerssuspension(pH3.7,SampleB);(3)GLlayerssuspension withinthelimitofthestabilityrangeofthesuspension(pH4.6, SampleC);(4)GLlayersinbasicsuspension(pH9.5,SampleD).
TheGLlayerswaterdispersionsattheselectedpHpointswere drop-castedattheconcentrationof1mg/mLontofreshlycleaved micasubstrates,toensureanatomicallyflatsubstrate, driedat roomtemperature.Informationaboutthesurfacefeaturesofthe filmsobtainedafterGLdryingwereobtainedbyAFM.
Fig.2showsthetopographicimagesacquiredonthefour inves-tigatedsamples.Allimagesreporta5m×5mscan,acquired intheTrueNonContactTMmodeofoperationoftheXE-100Park
system,togetherwithalineprofilerepresentativeofthesurface morphology.
SampleA(Fig.2a),preparedinacidsuspensionatthelimitofthe stabilityrange,exhibitsaveryroughsurface,characterizedbythe presenceofgrainshavingwidthoftheorderofoneorfewhundreds nanometers,which leadtoasurfacepeak-to-peakroughnessof about50–100nanometers.
On the contrary,the samples prepared in suspensions with higherpHshowanextremelydifferentmorphology.
ThesurfacesofbothSampleBandSampleClookatomicallyflat overlargeareas(Fig.2bandc).However,onthesurfaceofSampleC, therearesomeisolated“islands”havingdifferentlateralsizes.For statisticalcomparison,onSampleAthegrainsresultinarootmean squareroughnessofabout43nm,whileonthecollectedimageon SampleBitislessthan1 ˚AandonsampleCitisabout9 ˚A.
Theselectedprofileonthelargestislandontheimagedsurface ofSampleCshowsstepsofverticalheightslightlymorethan4nm, havingthesameflatnessofthesurroundingsurface.Suchflatness suggeststhattheobservedislandsareactuallymadebythesame
Fig.2. AFMtopographicimageswithalineprofileforeachinvestigatedsample;allthereportedimagescorrespondtoascanareaof5m×5m.(a)SampleA;(b)Sample B;(c)SampleC(intheinset:thephasecontrastprofileonalinecrossingthelarge“island”onthetop-rightoftheimage);(d)SampleD(intheinset:histogramofthe distributionofmeasuredheightvaluesontheimage).
Fig.3.(a)ImagesofthedropcastedfilmscorrespondingtomeasuredSamplesA(pH2.0),B(pH3.7),C(pH4.6),D(pH9.5);(b)C1sXPSspectrumonGLsurface,andits deconvolutioninGaussianshapepeakscorrespondingtodifferentfunctionalgroups.
materialasthesurroundingcarbonsurface,andnotbyspurious phasesorunrelatedparticles.Thisisconfirmedbythephase con-trastimage:theinsetinFig.2cshowsaphaseprofiletakenacross theedgeoftheisland.ThephaseimageinthenoncontactAFMscan (repulsivevanderWaalsregime,amplitudemodulatedoscillation) isindeedabletoprovidequalitativeinformationaboutthechemical natureoftheimagedsurface,thephaseshiftoftheinduced oscil-lationbeingsensitivetothechemicalfunctionalitiesofthesurface itself(throughdifferencesinthemechanicalinteractionbetween thetipand thesurfaceitself).It hasbeenshowedongraphene nanosheetsproducedbychemicalreductionofgrapheneoxide[24] thatthedifferentdegreeofhydrophilicityandhydrophobicicityor differentlyoxidizedregionsarerevealedbyaphaseupper-shiftof theorderof2◦–3◦(suchphaseshiftsarewellabovethesensitivity ofourinstrument,cf.insetofFig.2c).Thereportedprofileshows thatnodifferencesaredetectedbetweenthephaseshiftonthe mainsurfaceandontheisland.
AsconcernsSampleD,representativeofsamplesobtainedin basicsuspension,an“uncompleted”layerappearsoveran under-lyingflatsurface:thetopographicimage(Fig.2d)exhibitsregions withlacksinthetopmostsurface.Thiscircumstanceisconfirmed byahistogram(intheinset)ofthemeasuredrelativeheights,which formaclearlybimodaldistribution.Theupperpeakisassociatedto theupmostlayer,whilethepresenceofthesecondpeakatalower valueindicatesthattheholesinthislayerhavebasicallythesame depth.Itisworthtonotethattheverticalseparationbetweenthese twolayersisbetween4and5nm,asshownalsointhereported topographicprofile.Thesamevalueordoubledis measuredon theisolated“brighter”spotonthesurfaces,alsoresemblingwhat alreadydescribedforthesurfaceofSampleC.Theseobservations suggestthattheuncompletedlayersofSampleD,aswellasthe largeisland imagedonthesurfaceof SampleC, couldbeseeds forfurtherlayers.Surprisingly,thereisapparentlya “fundamen-tal”verticalthicknessofmorethan4nm:inRef.[11],AFMscanson fracturedflatgraphene-likesamplesseemedtoshowstepshaving thesamebasicverticalunit.
ThetrendofthemorphologicalbehaviorasafunctionofthepH solutioncanbeinterpretedintermsofforcesactingintothewater suspensionandtheconsequentaggregationprocessesbetweenGL particlesandlayers.
TheincreaseofsolutionpHresultsinanincreaseofthecharge densitydrivenbytheprogressivedeprotonationoftheresidual car-boxylicfunctionalgroups(thecarboxylicgroupsareprogressively convertedintheanionicform COO−).Asaconsequence,theGL layershaveamorehydrophiliccharactergoingfromSampleAto SampleD,monotonically.Theincreaseofthehydrophilicitymakes
theformationofGL–waterinterfacesenergeticallyfavorable, lead-ingtoapositivespreadingcoefficient[25]andtotheconsequent formation of“wet” GL layers:inother words,theenergetically favorableinterfaceformationcorrespondstoalowsurfacetension betweenGLlayersandwater,andtotheconsequenttendencyto maximizethecontactsurface.ThissituationoccursinSamplesB, C,D,andtheflatlayersareformedinthesolution.Onthe con-trary,inSampleA,atlowerpH,acompletelydifferentregimetakes place:thesurfacechargetendstodecreaseasaconsequenceof thepresenceofthecarboxylicgroupsattheedgeoftheGL lay-ersintheneutralform( COOH).TheresultisthatGLlayersare forcedtoaggregatetominimizethecontactwiththepolarsolvent, producingthegrainsobservedintheAFMimage.
Avisualinspectionofthedriedfilmsqualitativelysupportsour identificationoftwofluidodynamicalregimes.Fig.3ashowshow thefilmsB,C,D,exhibitthesocalled“coffeeringstain”, characteris-ticofaqueouscolloidaldispersionsinwhichthedispersedmaterial caneasilyflowthroughthesolution[26];onthecontrary,SampleA hasamoreuniformaspect,whichcanbeascribedtotheformation ofthelargeobservedGLgrains.
Asconcernsthesurfacechemistryanditsroleindetermining theobservedbehavior,thepresenceofresidualcarboxylicgroups isalsoconfirmedbyX-rayPhotoemissionSpectroscopy(XPS) per-formedontheGLsurface.TheC1sXPSspectrumisreportedin Fig.3b.ThespectrumhasbeendeconvolvedusingGaussianpeak shape.AShirleybackgroundcorrectionhasbeenappliedbeforethe deconvolution,andthedeconvolution hasbeenperformedwith eachcomponentconstrainedtohavethesamefull-widthat half-maximum(resultingtobe1.5eV).TheC1sspectrumexhibitsthe mainpeak(atabindingenergyofabout284.5eV)correspondingto carboninvolvedinC CandC Hbonding,togetherwiththepeaks assignedtoC OH,C OandCOOHfunctionalgroups.Inparticular, astrongshoulderatabout289eVisobserved,characteristicofthe presenceofcarboxylic(COOH)groups.Suchshoulderisfittedbya Gaussiancontributionfromcarboxylicgroupwithaweightlarger thanwhatusuallyreportedinXPSmeasurementsontypical car-bonaceoussurfaces[27,28],suggestingthat,despitethereduction process,thereisstillarelativelylargeamountofsurfacecarboxylic functionalgroupsabletodrivethesurfaceaggregationinthe sus-pension.
From the point of view of microscopic forces between ele-mentaryconstituents,thecrossoverbetweenthetworegimesis determined,inthesimplestpicture,bythecompetitionbetween vanderWaals(vdW)andelectrostatic doublelayer(DL)forces. ThevdWattractionexhibitsatypicalpowerlawasafunctionof theseparation(longrangeinteraction)betweenchargedparticles
Fig.4.(a)QualitativepictureoftheinteractionenergybetweensuspendedparticlesasafunctionoftheirseparationL,resultingfromthecombinedactionofvdWattraction andDLrepulsion,fordifferentsurfacechargedensity.(b)SketchofonepossiblemechanismleadingtotheformationofGLlayersofselectedthickness.
inthesuspension, whiletherepulsiveDL contribution(itmust be specified that the repulsion arising from the DL forces has osmotic/entropicnature,ratherthanelectrostatic)ischaracterized byanexponentialdecayvs.separationwithacharacteristicdecay length(shortrangeinteraction),theDebyescreeninglengthdue, tothecounterionsinthesolution.Theircombinedactionresults ina total particle–particleinteractionpotentialasa functionof separationhhavingamaximumforagivenvalue(typicallyfew nanometers),i.e.apotentialbarrierwhichactsagainstaggregation (Fig.4a).However,whentheparticlesurfacechargeapproaches zeroorwhentheelectrolyteconcentrationincreasesabovesome concentration(criticalconcentrationcoagulation),andboth circum-stancesoccurinourcaseatlowerpH,theDLrepulsionhaslower intensityand shorter Debyelength, sothat thevdW attractive forcedominates:thepotentialprofilecanlosethebarrier(itmay approachtheshapeofapurevdWinteraction)andtheparticles inthesuspensioncaneasilycoagulate.Thesurfacemorphologies observedbyAFMindicatethatthisregimecharacterizessampleA. Intheothersamples,thehydrophilicforcesdrivetheaggregation processtothesurfacemaximizationdescribedabove.
InsamplesB,C,D,anidealmaximizationofexposedsurfaces wouldleadtoidealGLmonolayers.However,ourmeasurements indicatetheformationofthickerlayers,havingawelldefined thick-ness,i.e.composedbyagivennumberofmonolayers.Apossible explanationofthisinterestingphenomenoncanbegivenagainin termsofcompetitionbetweentheforcesactingintothesuspension, butactingbetweenextendedflatsurfacesinsteadofsmall parti-cles.Inthispicture,thevdWandDLforces(perunitsurface,i.e.the pressures)asafunctionofseparationLcanbewrittenrespectively as[29]: ˘vdW(L)= AH 6L3 (3) ˘DL(L)=− 2 2Z·exp(−L) (4)
whichcorrespondtointerfacialGibbsfreeenergiesperunitarea of−AH/12L2and(/2)e−Lrespectively.AHistheHamaker
con-stantofthetrilayerwater-GL-water,Zisaconstantdefiningthe strengthoftheDLinteraction,−1istheDebyescreeninglength. AquantitativeevaluationofsuchforcesandrelatedGibbs ener-giesiswellbeyondthepurposesofthiswork:inparticular,theZ andparametersarerelatedinanon-trivialwaytothepHofthe solution(i.e.tothecounterionsconcentration)andtothecharge (orpotential)ofthechargedsurfaces(whilevdWforcesarenearly independentonpH).Whatonecansay,isthat ourobservation
suggeststhatthevdW/hydrophilicattractionbetweenmonolayers drivesthemtoaggregateagainstDLrepulsion.
Thissimplepicturewellreconcilesthemeasured morphologi-caltrendwiththesurfacedeprotonationasaconsequenceofpH variation.Nevertheless,itcannotyetaccountforthe experimen-talindicationofaselectedthicknessinthemultilayerformation, attheleastinSamplesCandD:amechanismdependingonthe layer thickness (or,equivalently, on the number of aggregated monolayers)mustbeinvoked.We suggesttwopossiblegeneral mechanisms,providedthatfurtherexperimentalworkisneeded foraconclusiveanswer.
Afirstexplanationcanbegivenintermsofbalancebetween surfaceenergiesofthelayersandtheaggregationenergy(energy requiredforthecrystalformation),similartowhathappensinthe nucleationofpolymersinsolutions[30,31].Inthisframe,the vari-ationofGibbsfreeenergyassociatedtotheformationofalayerof averagethicknessdandsurfaceScanbewrittenas:
G=2S−FSd (5)
Intheexpressionabove,isthesurfacefreeenergyperunit area(onelayerexposestwosurfacesofareaS),andFisthebulk freeenergyofcrystallizationperunit volume(sinceweobserve layerthicknessesmuchsmallerthanthelateralsizes,weneglect thecontributionofthelateralsurfaceenergy).Thelayer forma-tionisenergeticallyfavorable(i.e.theformedlayerisstableagainst disaggregation)ifG<0,whichmeansif:
d> 2
F≡dmin (6)
TheGLlayerisstableonlyoveragivenminimumthicknessdmin
(whosevalueisindependentonthesurfaceS);afurther aggrega-tionoverdmincanbepreventedbythearisingDLrepulsion,atleast
inSamplesCandD,whereweobservedthepreferredthickness andwheretheDLrepulsioncanbeactuallyexpectedtobestronger (accordingtothetrendoftheinteractionvs.surfacechargedepicted inFig.4a;notethatastrongerDLrepulsioncouldalsocompetewith theaggregationprocessoccurringtominimizeG,andbetherefore responsibleofthehigherobserved“fragmentation”inSamplesC andDthaninSampleB).SinceFand arebasicallyrelatedto thematerialpropertiesandnottothepH(evenforintothe solu-tion:fromanenergeticallypointofview,theexcesssurfacecharge is compensated by a higher counterion concentrationnear the surface),thiswouldalsobeconsistentwiththefactthattheselected thicknessisnearlythesameinbothSampleCandD.
Fig.5. AFMcharacterization:NC-AFMtopographicimages,4m×4mand300nm×300nm,showingthepresenceofGLnanoparticlesonamicasubstrate;theprofiles takenonsomenanoparticlesshowthatsuchstructureshaveheightrangingfromlessthan1nmtofewnanometers.
Alternatively,asecond,thickness-dependent,mechanismcan befoundtakingintoaccounttheinteractionbetweentheopposite chargedsurfacesofalayer.TheDLpropertiesareusuallyderived bythinkingintermsofanisolatedchargedsurfaceinanelectrolyte. Inourcase,wehavetwochargedsurfacesseparatedbyathinGL layer:asaconsequence,thepropertiesofeachDLinsuchlayerare affectedbythepresenceofthechargeontheoppositesurface.Inthe simplestelectrostaticpicture,sketchedinFig.4b,a“close” oppo-sitechargedsurfaceproducesanon-negligibleelectrostaticfield whoseeffectistonarrowtheDLthickness;assuminganunchanged surfacechargedensity,thisresultsinalowerDLpotential (accord-ingtoGrahameequation[32]),andthereforeinalowerstability againstaggregation(asafirstintuitiveapproximation,theDLcan beassimilatedtoanidealcapacitorwithconstantcharge,inwhich thepotentialmustdecreasewhentheplatesapproacheachother). Whenagiventhicknessisreached,thiseffectislowenough(the fieldfromtheoppositesurfacemustbeactuallyseenasadipole fieldratherthanasolelysurfacefield)toallowtheDLrepulsionto preventfurtherlayeraggregation.
Finally,we wantedtodissipate thedoubtthat theobserved 4nmlayersarenotmadebysomewhatfundamentalconstituents, or to an artifact apparent thickness due to the interaction between AFM tip and GL surface (similarly to what observed, actually with much lower values, in graphene films grown on SiO2 [33]).Tothis purpose,a verydilutedGL water-suspension
(0.1g/mL),wasusedfordrop-castingontomicasubstrate.Such a low value was chosen in order to prevent the formation of aggregates on the substrate, allowing instead the observation of rather individual GL units. The concentration of water sus-pension,however, washighenoughtoallowthepresence of a certainnumber of nanoparticlesper unit areaof thesubstrate, sothattobeeasilydetectableinthescanningareaofthe micro-scope.
Fig.5showstheimagescollectedonthissample,characterized bythepresenceofnanoparticlesdepositedonthemicaflatsurface. Adetailedcharacterizationofsuchparticlesrevealsasize distri-bution,which canbeascribedtodifferentaggregationamounts ofelementaryconstituents.Selectedheightprofilescrossingthe particlesdemonstrate,firstofall,theflatnessofthesubstrateand itssuitabilityforareliablecharacterizationofthenanoparticles.
Fromsuchprofilesweobservedverticalsizesrangingfromabout 1nm or less tofew nanometers; thelarger lateraldimensions, a fewtens ofnanometers, confirms that atthechosen concen-trationofwatersuspensionsomeaggregationsarestilloccurring (thiscircumstancesalsorepresentsanindirectproofthatthe pri-maryaggregationprocessoccurstoformextendedthinplanes). Thesefindingssupporttheideathattheelementaryunitshave sub-nanometricverticalsize,whiletheirassemblingexhibitpreferred thicknessvalues.
4. Conclusions
Zetapotentialand AFMmeasurementsdemonstratethatthe qualityandthefeaturesoftheproducedself-assembledGLfilms arestronglyaffectedbythepHofthewatersuspensionfromwhich thefilmsthemselvesareprepared.Theobservedmorphologic dif-ferencesareconsistentwiththeinteractionsfeaturesexpectedfor surfaceshavingdifferenthydrophiliccharacterasaconsequence ofadifferentamountofdeprotonationofcarboxylicgroups:the observationofgrainsorofflatsurfacescanbeeasilyunderstoodin termsofconsequentsurfacetension(macroscopicpointofview) or,equivalently,intermsofmicroscopicinteractionbetweenthe fundamentalunitsinthewatersuspension.Besidessuchevident important difference in the quality of films surfaces, the mor-phological measurements seemto indicate theoccurrence of a preferredbasicthicknessfortheassembledlayers:this circum-stancecan bealso interpreted in terms of simple fundamental principlesbasedontheforcesconcurringinacolloidalsuspension. Ofcourse,theproposedmechanismsfortheobservedpreferred thicknessaresuggestedhypotheses,whichdeservefurther experi-mentalworktobeverifiedorimproved,forexamplebymeasuring thefilmsfeatureswhenvaryingfurtherpreparationparameter.On theotherhand,therole ofsurfacetensionandofthedescribed microscopicforcesindeterminingthetwoobserved morphologi-calregimes,theirrelationwiththecarboxylicgroupsonthefilm surface,aswellasthefactthat theobservedlayersareformed bysmallerfundamental constituents,arewellsupportedbythe measuredmorphologyandfurtherexperimentalobservation,and openthewaytothepossibilitytocontrolthesurfacefeatureof
GLlayers byproperlyacting onaccessible preparation parame-ters.
Acknowledgments
The workwaspartially financed by AccordoCNR-MSE “Uti-lizzopulitodeicombustibilifossiliaifinidelrisparmioenergetico” 2011–2012.Theauthorsgreatlyacknowledgeprof.CarlaManfredi (DipartimentoofChemistry,UniversitàdiNapoli“FedericoII”)for coulometric– potentiometrictitration. XPSmeasurementswere performedwithintheproposal20142026financedbyCERiC-ERIC: “StructuralcharacterizationofHKUST-1intercalatedwith conduc-tivegraphene-likelayers”.
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