A closer look at the synthesis and formation mechanism of hematite nanocubes

A

closer

look

at

the

synthesis

and

formation

mechanism

of

hematite

nanocubes

Vikash

Malik

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_Bernard

_Grobety

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_Veronique

_Trappe

_,

_Herve

_Dietsch

_,

Peter

Schurtenberger

a_{AdolpheMerkleInstitute(AMI)andFribourgCenterforNanomaterials(FriMat),UniversityofFribourg,CH-1723Marly,Switzerland} b_{DepartmentofGeosciences,UniversityofFribourg,CH-1700Fribourg,Switzerland}

c_{DepartmentofPhysics,UniversityofFribourg,CH-1700Fribourg,Switzerland} d_{PhysicalChemistry,DepartmentofChemistry,LundUniversity,SE-22100,Sweden}

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•We synthesized hematite cube-shapednanoparticlesusingdifferent synthesispaths.

•Ironchlorideconcentrationand aka-ganeiteconcentrationinfluence mor-phologyandparticlesize.

•We use electron microscopy and diffractionmethods tocharacterize theparticles.

•Controlling theakaganeite concen-trationandsizeiskeytotuneparticle sizeandcrystallinity.

•Wediscusspossibleformation mech-anismsofsingleandpolycrystalline hematitenanocubes.

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Wehavesynthesizedhematitecube-shapednanoparticlesusingdifferentsynthesispathstoexaminethe effectoftheintermediateproductakaganeite(␤-FeOOH)onthesizeandshapeofthefinalhematite par-ticles.Akaganeitespindlesandhematitenanocubesarepreparedusingforcedhydrolysisofironchloride (FeCl3)salt.Weuseacombinationoftransmissionelectronmicroscopy(TEM),highresolution

transmis-sionelectronmicroscopy(HR-TEM),selectedareaelectrondiffraction(SAED)anddynamiclightscattering (DLS).Akaganeiteparticlesarefoundtobespindles,withanaveragelengththatdependslinearlyonthe FeCl3concentration,whiletheiraspectratioishardlyaffectedbythesameparameter.Adjustingthe

akaganeiteconcentrationfrom0.01Mto0.10MequivalentofFeCl3inthesubsequenttransformation

tohematiteleadstosinglecrystalnanocubeswithsizesrangingfrom37to175nm.Thecombinationof akaganeiteconcentrationandsizeisfoundtobethekeyparametertocontrolthesizeandcrystallinityof theresultinghematiteparticles.Possibleformationmechanismsofsingleandpolycrystallinehematite nanocubesarediscussed.

∗ Correspondingauthor.Tel.:+46462228219;fax:+46462224413. E-mailaddress:peter.schurtenberger@fkem1.lu.se(P.Schurtenberger).

1. Introduction

Colloidalnanoparticlesareincreasinglyimportantfor applica-tionsinbiomedicine[1],imaging[2],DNAseparation[3],colloidal crystallization[4–6]and numerousotherareas.Controlled syn-thesistechniquesare required to obtainmonodisperse colloids

Published in &ROORLGVDQG6XUIDFHV$

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which should be cited to refer to this work.

withtunablesize,shape,physicalandchemicalproperties. Partic-ularlywell-knownandincreasinglystudiedclassesofparticlesare oxidesandhydroxidesofiron,forexamplehematite,magnetite, maghemite,andgoethite.Hematite(␣-Fe2O3)showsvery

interest-ingmagneticproperties,whichchangewithparticlesizeandshape. Nanosizedhematitehasbeensynthesizedinvariousshapessuchas cubes[7–9],rods[10],spheres[11,12],pseudocubes[12],platelets [13],ellipsoids[14,15],nanobeltsanddendritemicropines[16,17]. Thefinalhematiteparticlesizeandshapecanbetunedbyadjusting synthesis parameterssuchastheconcentrationofthereactants [7],durationofthereaction[18],ionicstrength[19,20],reaction temperature,surfactants[10],presenceofhematiteseed[21]and pHofthesolution[19].Theadditionoforganicadditivesto aque-ousreaction mixtureswasshown tohave anappreciableeffect onthegeometryofhematiteparticles[22].Changingtheratioof solventmixturesstronglyinfluencesthefinalshape,yielding spin-dle,rhombohedral,orcubicalgeometriesparticularlyinthecaseof ethylenediamine/ethanolmixtures[9].

Fine-tuningcanbeachievedbytheadditionofsulphateand phosphate saltstothereactionmixture.Thesesalts restrictthe growthofhematitenucleiinthedirectionperpendiculartothe c-axis[19,20].Theadsorptionofsulphateionshasbeenfoundto dependonthepHofthesolution,andwasreportedtodecrease withincreasingpH[19].Whendissolvedinwaterat100◦C,FeCl3

initiallyformsspindleshapedakaganeite(␤-FeOOH)particles[23]. Hematite particles are then grown via the dissolution and re-precipitationofthisintermediatemetastablephase.Inordertoget fullcontrolovertheformationofhematite,itisessentialto under-standtheformationandinterparticleinteractionsofakaganeite.In thiscontextitisalsoimportanttorealizethatakaganeitespindles formdiverseaggregatessuchasstacksdependingonthebalance betweenrepulsiveandattractiveforcesbetweenthem[24],which caninfluencetheresultingsize,shapeandstructureofhematite particlesformed.

Theformationmechanismsofdifferentlyshapedhematite par-ticles from akaganeite precursors were studied in the past by Baileyand Mercartney asa function oftheFeCl3 concentration

(0.018–0.45M)[12].Theydescribedinparticulartheformationof polycrystallinepseudocubicalhematiteparticles (i.e.cubeswith roundedges)athighFeCl3concentrationthroughathree-step

pro-cess:first,akaganeiterodsareformed,whichthen arrangeinto stacks,finallyleadingtopseudocubes[12].Animportant observa-tionmadewasthattheseparticleswerefoundtobepolycrystalline. Thepolycrystallinenatureofthepseudocubeswasrelatedtothe factthatnucleationofhematiteoccursattheakaganeiteinterface, andamodelfortheformationmechanismwasdevelopedbasedon Debye–Huckeltheory[25].Giventheimportanceofsinglecrystal nanoparticlesforvariousapplicationsandthefactthatthe char-acterizationoftypicalmaterialspropertiesisgenerallyeasierfor singlecrystalparticles[26–29],wethusstartedaninvestigation of theformationhematiteparticlesatlow FeCl3 concentrations

(0.01–0.1M).Moreover,inourattempttoachievefullcontrolover thenanostructureofhematiteparticlesweextendedthisstudyalso toaninvestigationoftheinfluenceofthesizeandconcentrationof themetastableakaganeiteprecursorparticlesonthecrystallinity andsizeofthefinalhematiteparticles.

2. Materialsandmethods

Reagent grade FeCl3 was purchased from Merck and used

without further purification. MilliQ water with a resistivity of 18.2Mcmwasusedforallthesyntheses.Carboncoatedgrids (300mesh)fromElectronMicroscopyServicesInc.wereusedfor thepreparationoftheTEMgridsandtheinvestigationsofthe par-ticlemorphology.

2.1. Characterizationtechniques

X-raypowderdiffraction(XRD)patternsoftheproductswere measuredonaX-raydiffractometer(XRD-IC100P)inthe2rangeof 10–80◦.Atransmissionelectronmicroscope(TEM-CM200,Philips) operatingat200keVwasusedforsizeandmorphologystudies, selectedareaelectrondiffraction(SAED)analysisandhigh reso-lutionTEM(HRTEM)imagingoftheindividualparticles.Atypical sampleforTEMwaspreparedbyconcentratingtheparticle dis-persionusingacentrifugeat5000rpmanddryingonedropletof 1vol%ofparticlesontoacarboncoated(300mesh)grid.Forthesize distributionofnanoparticlesinaliquidsuspension,dynamiclight scattering(3DDLS goniometersystemfromLSInstrumentsInc.) wasemployed.Intensitycross-correlationfunctionswereanalyzed usingasecondordercumulantexpansion.Nanoparticle suspen-sions (0.01vol% in water) were measured in a cylindrical DLS cuvette (10mmdiameter) atroom temperature.The scattering anglewaskeptat90◦.Wewerenotabletoaccuratelydetermine thehydrodynamicradiusofS1duetothefastsedimentationofthe largeparticlesintheDLStube.

2.2. Synthesis

Forthepreparationofakaganeite,solutionsofFeCl3with

con-centrations of 0.01, 0.02, 0.04, 0.08 and 0.10M were prepared by dissolving the appropriate amount of FeCl3 (162mg–1.62g)

into100mLofMilliQwater.Thesereactionmixtures werekept inapreheatedovenat98◦Cfor1handthenquenchedtoroom temperaturebyinsertingthesamplesintoawaterbathatroom temperature.

We followedthesynthesisofhematitein(1)aonestep pro-cess, starting from FeCl3 solutions at two initialconcentrations

(S1:0.1M;andS2:0.01M),and(2)inatwo-stepprocess,where thereactionwasquenchedaftertheproductionofthemetastable akaganeiteprecursor.

One-stepprocess:ForsampleS1(FeCl3 concentration0.10M),

1.62gFeCl3wasdissolvedin100mLwater,forsampleS2(FeCl3

concentration0.01M),356mgofFeCl3wasdissolvedinto220mL

ofwater.SampleS1wasdividedintofourglassbottlesofequal volume,whichwerekeptatthereactiontemperaturefor1h,4h, 24hand192h,respectively.SampleS2wasdividedintoeleven tightlyclosedglassbottlesofequalvolume,whichwereremoved fromtheovenafterreactiontimesrangingfrom5minupto192h. Allsampleswerequenchedintoawaterbathatroomtemperature afterthesedifferentreactiondurations.Theproductswerethen concentratedandcharacterizedbyTEM.

Two-stepprocess:Inthetwo-stepprocess,twostocksolutions ofrespectively0.01M(sampleS3)and0.10M(sampleS4)ofFeCl3

inwaterwereprepared.Theconcentrationofakaganeiteparticles wasthenadjustedandthereactionwasstartedagain(S3:initial FeCl3concentration0.01M,finalconcentration0.1M;andS4:

ini-tialFeCl3concentration0.1M,finalconcentration0.01M).

OneliterofS3solutionwaskeptinthepreheatedovenat98◦C for1handquenchedtoroomtemperaturetostopthereaction,then 90%(900mL)ofthewaterwasremovedfromthequenched aka-ganeitesolutionusingarotaryevaporatorat50◦Cunderreduced pressure.Theresultingsolutionwasconsideredtohavean equiv-alent iron chloride concentration of 0.10M. This solution was labelledas0hsolution.Itwasdividedintofivebatcheswithequal volume(20mL),keptinthepreheatedovenat98◦Candremoved afterdifferentperiodsoftime(1–192h).

Inparallel,fivebatchesof5mLofS4solutionwerekeptinthe preheated ovenfor 1h(labelledas0hsolution). Toeach batch 45mLofpreheated(98◦C)deionizedwaterwasaddedafter1hof reaction,dilutingallbatchesoftheS4startingsolutionbyafactor of10,whichleadstoasolutionwithanequivalentconcentrationof

Table1

Descriptionofinitial([FeCl3]ini)andfinal([FeCl3]f)concentrationofFeCl3.TEM

derivedandhydrodynamic(DLS)particlesizearealsoshownforallexperiments. Sample [FeCl3](M) [FeCl3]f(M) d(nm)from

TEM Rh(nm) fromDLS S1 0.1 0.1 925±230 – S2 0.01 0.01 103±35 71 S3 0.01 0.1 175±45 128 S4 0.1 0.01 37±13 65

0.01Mofironprecursor.Thesesolutionswerethenallowedtoreact fordifferentperiodsoftime(1–192h).Particlesizeandfinal mor-phologiesweredeterminedusingTEMandDLS.Theresultsfrom theseexperimentsaresummarizedinTable1.

3. Resultsanddiscussion

IndependentofinitialFeCl3concentration([FeCl3])and

subse-quenttreatmentofthesolution,allsynthesizedhematiteparticles exhibit a cube-like shape (Fig.1).Thisis somewhat surprising, asotherstudiesexploringtheeffectof[FeCl3],thoughunder

dif-ferentexperimentalconditions(0.02M),indicatedthattheactual shape of theparticles would sensitively dependon [FeCl3] [1].

However, although the shape of the particles remained essen-tially cube-like in all our experiments, the dimensions of the particles strangely varied with the experimental procedure, as summarizedinTable1.Theresultingparticlesizedistributionsall havepolydispersitiesaround20–30%,andtheaggregatesvisible intheTEMimagesareduetothedrying procedureusedinthe TEMsamplepreparationprocessanddonotexistinsuspension asindicatedbytheaveragehydrodynamicradiiRh measuredby

DLS.

ThereductionoftheinitialFeCl3concentration[FeCl3]inifrom

0.1M(S1, Fig.1a)to 0.01M (S2, Fig.1b) hasa dramatic influ-enceonthesizeoftheresultingfinalnanocubes,whichdecreases fromd=925±230nmforS1tod=103±35nmforS2.However, theFeCl3 concentrationis nottheonlyrelevant parameterthat

determinesthefinalproduct.ThisisdemonstratedwithsamplesS3 (Fig.1c)andS4(Fig.1d).IntheS1-andS2-experimentswe main-tainedtheinitialvolume(0.1Mand0.01M,respectively)forthe entirereactiontimeof7daysduringwhichessentiallyall start-ing materialswere converted intohematite. In contrast,in the S4-experiment,wedilutedthesampleafter1hto0.01M;atthis

pointofthereactionthestartingmaterialhasalreadybeen com-pletelyconvertedtoakaganeite,buttheamountofhematitewas stillnegligible.Afterthisdilutionstep,thesystemwasthenagain lefttofullyconverttohematitefor7days.Undertheseconditions theresultingnanocubesinS4withd=37±13nmarenowmuch smallerthanthoseobtainedwhenstartingdirectlywithanFeCl3

concentration[FeCl3]ini=0.01M.Similarly,whenwestartedwith

[FeCl3]ini=0.01M,and then quenched thereactionand

concen-tratedthesampleinarotavaptoanequivalentFeCl3concentration

of0.1Mbeforestartingthesynthesisagain,theresultingnanocube sizeafter7days,i.e.afterfullconversion,wasnowd=175nm.This valueisaboutafactorof1.7largerthanwhathasbeenfoundforS2 (same[FeCl3]ini),butaboutafactorof5.3smallerthanwhatwas

obtainedwithsampleS1([FeCl3]ini=[FeCl3]f).Thisclearlyindicates

that[FeCl3]isnottheonlysizedeterminingfactor,butthatthesize

andnumberdensityoftheakaganeiteparticlesasaprecursorphase alsoplayanimportantroleindeterminingthefinaldimensionsof thehematiteparticles.Inordertoexploretheroleoftheakaganeite precursorparticles,we thuscharacterizedtheshape andsizeof particlesformed1hafterthestartofthereactionusingdifferent initialFeCl3 concentrations.Asdescribedabove,atthispointthe

bulkofthestartingmaterialhasbeenconvertedtoakaganeitefor allsamplesinvestigated,whilenodetectableamountofhematite hasyetbeenproduced.Fortheinvestigatedconcentrationrange of[FeCl3]ini=0.01and0.1M,wefoundthatallakaganeiteparticles

werespindle-shaped,butofdifferentsizeand aggregationstate (Fig.2aandb).

Dependingon [FeCl3]ini, akaganeitespindles of varioussizes

wereobtainedduringthefirsthourofthesynthesisat98◦C(Fig.2a andb).Theaveragelengthdeterminedbymeasuringthelongaxisof 100particles(fromdifferentmeshesontheTEMgrid)increases lin-earlywith[FeCl3]ini(Fig.2c).Theaverageaspectratioappearstobe

almostindependentoftheinitialprecursorconcentration(Fig.2d), withvaluesalwaysremainingbetween4and5.Inadditionto spin-dles,somestarshapedand stackedstructuresofakaganeiteare observedafter1hofreactiontime(Fig.2aandb),wherethestar shapedstructuresaretwinnedakaganeite[30].Therelevant pre-cursorparticlestoconsiderwhentryingtounderstandthefinal hematiteparticlesshowninFig.1arethustheakaganeitespindles (Fig.2aandb).

Havingestablishedthesizeand shapeoftheakaganeite pre-cursor particles that are then converted to hematite in the synthesisroutesunderlyingsamplesS1–S4,wenextperformeda

Fig.1. TEMimagesofthefinalhematitecubesobtainedinsamples(a)S1,(b)S2,(c)S3and(d)S4.

Fig.2. PropertiesoftheprecursorakaganeitespindlesasafunctionoftheinitialFeCl3concentration.TEMimagesofakaganeitespindlessynthesizedwithdifferent[FeCl3]ini

valuesof(a)0.01M,(b)0.1M(scalebar=500nm).(c)Akaganeitespindlelength(longaxis)asafunctionof[FeCl3]ini.(d)Aspectratioofakaganeitespindles(longaxis/small

axis)vs.[FeCl3]ini.

Fig.3.(a)TEMimagesofsampleS1after(a)1h,(b)4h,(c)24hand(d)192h.

detailedinvestigationofthetimedependenceoftheconversion akaganeite–hematiteandthemorphologyoftheresultinghematite nanocubes.

In sample S1 obtained without subsequent dilution [FeCl3]ini=[FeCl3]f=0.1M, large hematite pseudocubical

par-ticles are formed (Fig. 3). Our experiments with S1 appear to be inaccordance withthe earlierfindings in Ref. [6]. Our data showthat after1hof reactiononly akaganeitespindles canbe observedwhich appeartohaveaggregatedintolargerstacksor rafts(Fig.3a).The spindleaxesare paralleltothec-axisof the ␤-FeOOHstructure.Diffractionpatternstakenwiththeaperture overanentireraftshowthatthec-axesofthespindlesforminga raftarewellaligned(Fig.4a).Moreover,theseraftsmaycontain morethanonelayerofspindles(Fig.4b).

Thehematiteparticlesnucleateepitaxiallyontheakaganeite rods(Fig.5),i.e.thecrystallographicorientationofthenucleiis controlledbytheakaganeitestructure.Thefollowingorientation relationshipisobserved:theb-axisofakaganeiteisparalleltothe

Fig.4. (a)SEADpatternfromanakaganeiteraft.Diffractionspotsfromtwodifferent rods(indexedspotsandarrowedspots)arevisible,thec-axesofbothrodsarewell aligned,howevertheb-axesarenotaligned.(b)TEMimageofanakaganeiteraft, whiteline:orientationofthec-axis;arrows1and2pointatthetworods,whichare eitheraboveorbelowthelongerlightercontrastedrods.

Fig.5.Detailedviewontheinitialconversionakaganeite–hematitefromHRTEMandTEM:ImagesareobtainedfromsampleS4after1hofreactiontime.(a)HRTEMimage depictingcrystallographicorientationofhematiteandakaganeiteparticles.(b)TEMimageofahematiteparticle,growingontopofaakaganeiteraft.

caxisofhematiteandthecaxisofakaganeiteisparalleltotheb axisofhematite.Thenucleigrowbyconsumingironinsolution fromalready dissolvedakaganeite.Severalhematitenucleimay formwithinthesamestack,andtheirmutualcrystallographic ori-entationisfixedaslongastheakaganeitespindlesareheldtogether inastack.

Tosomeextentthereseemtobealsotopotaxialreplacement, asindicatedbyembayments intheakaganeitesurfaceoccupied byhematite.Inthisprocess,hematitenucleatesina crystallogra-phicallycontrolledwayonthesurfaceoftheakaganeiterodsand growsintothelatterbyconsumingit.Asmultiplenucleiformona singleraft,thegrowingindividualhematitenanocubeseventually aggregate/fuseandformasinglepolycrystallinepseudocube.

NanocubefusionissupportedbyFig.6wherevarious crystal-lographic orientationscanbeobserved in a HRTEMimageof a largepseudocube.Furthermore,thesolutionchemistryenhances the cohesion betweenthe hematitenanocubes. Thepoint zero charge(pzc)increasesslightlywithparticlesizefrom7.8for12nm particlesto8.8for65nmparticlesasreportedbyHeetal.[31]. Thestability ratioasa measureofthecolloidalstability against aggregationfornanohematitesuspensionsdropsfrom1000to1 whenincreasingtheelectrolyteconcentrationfrom0.01Mto0.1M.

Fig.6.HRTEMimageofsampleS1.Shownisa[110]latticefringeimagefromthe boundarybetweentwosubcrystalstogetherwithtwoSAEDpatterns.Theupper SAEDpatternistakenwiththeaperturecenteredovertheupperlamella,whereas thelowerpatternistakenwiththeaperturecenteredovertheboundary.The boundaryisparalleltothe[−114]and[1−12]latticefringes(arrowsshowthe correspondingdiffractionspots).

Nanocubes,whichgetincontactwitheach other,willtherefore sticktogether.The growthof the individualcubes isenhanced bythepresenceofnon-aggregatedsinglespindleandsmall star-shapedaggregates,whichactasreservoirsthatenhancethegrowth ofhematitecrystalsbyOstwaldripening.Thisformationscenario forlargehematitepseudocubesisinlinewiththeproposedgrowth modelby Baileyetal. [12].They suggestedthat forthe forma-tionoflargepseudocubical particles,akaganeiterods ofsimilar lengthhavefirsttoaggregateintostacks.Nucleationofthehematite thentakesplaceattheinterfaceoftheakaganeiterodstacks.The hematitecrystalspresentinthestackcoalescethentoalarge poly-crystallinepseudocubicalhematiteparticles.

Therelativeorientationof thehematitesubcrystalswithina pseudocubecannotbeobservedinanormalTEMexperimentdueto thelargesizeoftheparticle.Inordertoexaminetheinternal nano-structureofthepseudocubicparticles,20nmthickslicesobtained bysandwichingthepseudocubesinanepoxyresinandcutusinga microtomewerestudiedwithTEM.Manysectionsofthehematite particleshaveanearlyquadraticshapeandalmostalledgesare rounded,butthecurvatureisnotthesameforeachedge(Fig.3d). Theinnerstructureofthesectionischaracterizedbylath-like sub-crystals,delineatedbychangesincontrast(usuallylinesofbright contrast)orcrackswhichradiateoutfromthecenterofthesection orwhichareparalleltooneexternalfaceofthecube,resembling theraft-likearrangementoftheprimaryakaganeitecrystal(Fig.7a). Thediffractionpatternstakenfromdifferentregionsofmanylarger crystalsareidentical,i.e.thecrystallographicorientationdoesnot changeacrosstheparticle(Fig.7b–d).Weakerdiffractionspotsare alsopresent,indicatingsomemisorientationofsmallersubgrains, partlyanartifactofmicrotoming.

The observed external shape and internal structure of the hematitecubesareverysimilartothefeaturesdescribedbyPark etal.[32],formonodispersedhematiteparticlessynthesizedfrom highly condensed hydroxide gels [13]. The pseudocubic parti-clesareboundby[012]faces.Theinternalstructureconsistsof columnarsub-grainsthatradiateinalldirections.Althoughthe ori-entationofthemorphologicallongaxisofthesub-grainschanges continuously,thecrystallographicorientationfromonesub-grain tothenextremainsthesame.Thepresenceofthesub-grainsand theconstantcrystallographicorientationacrosstheparticle indi-catesorientedaggregationofbasicparticles,inourcaseprimary hematiteparticleswhichgrewepitacticallyontoakaganeiterafts (Fig.5).

Havingobtainedadetaileddescriptionoftheformationprocess andtheresultingparticlemorphologyandinternalnanostructure ofthehematiteparticlesformedat[FeCl3]ini=0.1M,wenextlooked

attheparticleformationatlow[FeCl3]ini=0.01M,i.e.sampleS2.

Fig.7.DetailsoftheinteriorstructureofalargehematitenanocubeobtainedfromsampleS1:(a)TEMimageofamicrotomedlargehematitenanocube.(b)SAEDpattern fromregion1ofhematitenanocube(c)fromregion2and(d)fromregion3ofhematitenanocubes.Allthreediffractionpatternsshowthesamezoneaxisorientation.The slightstreakingisduetorotationaldisordercausedbymicrotoming.Thelongaxisofthesubcrystalinregion2isorientedat90◦_{withthelongaxisofthesubcrystalin}

region3.

AsshowninFigs.2aand8a,thetransformationtohematiteoccurs viamuchsmallerakaganeitespindles.Akaganeiteparticlesarein factformedduringthefirst5minofthereactionasevidentbyan XRDpowderdiffractionanalysis(Fig.2a,supplementarydata),and alsoestablishedbymeasuringtheevolutionofthepHduringthe synthesis(datanotshown).Incontrasttothesynthesiswith0.10M initial[FeCl3]concentrations,nolargeraftsarevisibleafter1h,and

theakaganeitespindlesappeartoexistmostlyasisolatedspindles andstar-liketwinnedparticles.Thelowerelectrolyteconcentration relativetoS1preventstheakaganeitespindlestoaggregate.When comparedtothetemporalevolutionoftheparticlesinS1(Fig.3), inS2asignificantnumberofhematitenucleiandsmaller nanopar-ticlesarealreadyvisibleafter4h(Fig.8b).After24hofreaction, cubic and rhomboid-shaped individual hematite crystals are

Fig.8.TEMimagesofsampleS2after(a)1h,(b)4h,(c)24hand(d)192h.

Fig.9. Selectedareaelectrondiffraction(SAED)patternsobtainedfromhematitenanoparticlesthroughthreedifferentroutes:(a)S2,(b)S3and(c)S4.

predominant andonly a smallamountofremainingsmall aka-ganeitedebrisisleft.Theakaganeitedebriscompletelydisappear after 192h of reaction, indicating the full transformation of akaganeite into hematite. The cubic morphology of hematite nanoparticleswasalsoconfirmedusingSEM(Fig.1,supplementary data)

Theinfluence oftheinitialFeCl3 concentrationand thusthe

ionicstrengthappearstobedramatic.Weobtainlarge polycrys-talline hematitepseudocubes inS1and small nanocubesin S2. Moreover,hematitenanoparticlesobtainedinS2singlecrystalsas demonstratedbyselectedareaelectrondiffraction(SAED) experi-ments(Fig.9a).InthespiritofthemodelproposedbyBaileyetal., itistemptingtolinkthedifferentfinalsizeandmicrotextureto thesizeofthetransientakaganeitecrystalsandtheiraggregation state.Akaganeiterodsformedat0.01M[FeCl3]iniaresmaller,but

morepolydispersethanthoseformedat0.1M[FeCl3]ini.Thelack

ofaggregationandstackformationcanbeexplainedbythe solu-tionchemistryandthesurfacechemistryofakaganeite.Thepoint ofzerocharge(pzc)ofakaganeiteinwaterisbetween7and8,and forpHvaluesbelowthepzcthesurfaceofthenanorodsispositive. ThecalculatedpHvaluesforallsolutionsusedinthesynthesisof akaganeitehaveindeedamuchlowerpH.Thestabilityofthecharge stabilizedsuspensionswillthusdependontheelectrolyte concen-tration. At0.1Minitial[FeCl3]ini concentrationandwitha high

concentrationofchlorineanionsthesurfacechargeofakaganeite willbecomeeffectivelyscreenedandaggregationpromoted. Sub-sequentdilutionwithwaterwillsignificantlyreduceaggregation. Thenon-aggregatedspindlesserveasasubstrateforthenucleation

ofhematite,whichwillgrowindependentlyofeachother,slowly consumingthesmallerakaganeitestructures.Thismechanismis distinctlydifferentfromthatproposedathigh[FeCl3]ini.

Appar-entlytheexistenceofstacksisapre-requisiteforthesubsequent formationoflargerhematitepseudocubes.

However, it is not only the resulting size and morphology thatdistinguishesS2fromS1.AcomparisonofFigs.3and8also revealsthatthetransformationkineticsat0.01M[FeCl3]iniismuch

faster,whichatfirstseemscounterintuitive.Ironhydroxidephases suchasakaganeiteandlepidocrocitearemetastablephases,which subsequentlytransformintoastablephase suchashematiteor magnetite.Thedegreeofmetastabilitymainlydependsontwo fac-tors,surfacechargeandsolid–liquidsurfacefreeenergy.Whilewe arenotchangingthesurfacechargewhenchanging[FeCl3]ini,the

differencesinakaganeiteparticlesizeresultinasignificant varia-tionofthesurfacefreeenergy.Ifwetreattheakaganeiteparticlesas cylindersoffinitelength(L)andradius(r),thesurfacefreeenergy perunitvolumeforS2akaganeiterodsishigherbyoneorderof magnitudethanthatofS1,andthelargeakaganeiteparticleswill thustakemuchmoretimetodissolveandtransformintohematite. TheresultspresentedsofarshowthatinS2theformationof hematiteisalsoatwo-stepprocess;first␤-FeOOHisformed,which thenenablesthenucleationandgrowthofhematite.Theresulting hematiteparticlessizeandthetransformationkineticsforbothS1 andS2aredirectlylinkedtothestateoftheprecursorakaganeite particles(size,aggregate state).However,itremains tobeseen whether[FeCl3]iniistheonlyrelevantcontrolparameter.We

there-foreperformedtwomore experimentalseriesusing atwo-step

Fig.10.TEMimagesofthesampleS3after(a)0h,(b)4h,(c)24hand(d)192h.

Fig.11. TEMimagesofthesampleS4after(a)0h,(b)4h,(c)24hand(d)192h.

synthesisproceduretodecoupletheroleoftheprecursorparticle sizeandthetotalamountofprecursormaterialavailable.

InthesetwoadditionalseriesS3andS4,westudythe trans-formation akaganeite–hematiteusinga precursorsuspensionof smallakaganeitespindles(preparedat[FeCl3]ini=0.01M)

concen-tratedbyafactorof10toahigheffectiveorfinalFe-concentration (denotedby[Fe]f)of0.1M(S3)andcompareitwitha

transfor-mationoflargeakaganeitespindles(preparedat[FeCl3]ini=0.1M)

dilutedbyafactorof10toalowfinalFe-concentration(denotedby [Fe]f)of0.01M(S4).Theresultingconversionkineticsandthesize

andshapeoftheformedparticlesaresummarizedinFigs.10and11. Whenwe firstcompare theeffectof thenumber densityor [FeCl3]fforthesmallprecursorspindles(S2andS3),weseethat

despitethe10-foldhigherparticleconcentrationtheaggregation behaviorappearsunchangedandtheappearanceofthesampleat earlystagesisstilldominatedbyindividualsmallakaganeite spin-dlesandafewtwins.Weobservethefirsthematitenanoparticles toappearafterabout4hofreaction(Fig.10b),quitesimilartothe situationencounteredforsampleS2(Fig.8b)ata10-foldlower particlenumberconcentration.Theakaganeitespindlesappearto haveconvertedcompletelytohematitenanoparticlesafter24hof reaction(Fig.10c),andnochangeinsizeandshapeofthehematite particlescanbeobserveduponfurtheragingat98◦C.Theparticle growthisthusacceleratedatthehigherconcentrationwhen com-paredtoFig.8,whereafter24hthereisstillasmallbutdetectable akaganeitepopulationpresent.AdditionalTEMexperiments(data notshown)showthatalreadyafter16hofreactionnoakaganeite canbedetectedanymoreinS3.Thereisnoevidencethatthefinal hematiteparticlesconsistofhematitesubunits,andtheelectron diffractionpatternsconfirmtheirsinglecrystallinenature(Fig.9b). Ourexperimentsthusindicateagrowthmechanisminagreement withnucleationand(diffusional)growthratherthananinitial for-mationofstacksofprecursorparticlesthatarethenconvertedinto hematitenanocubes.

It isinteresting tocomparethesizesof thefinalnanocubes. WhiletheprocedurefollowedforS2leadstohematitenanocubes withanaveragesizeof103nm,the10-foldhigherprecursor par-ticleconcentrationinS3leadstoananocubesizeof175nm.This correspondstoanincreaseinparticlevolumeofafactorof5.Itthus appearsasifthetotalnumberofparticleshasonlyincreasedbya factoroftwo,andmostoftheadditionalmaterialhasbeenusedto growlargerparticles,despitethefactthatthenumber concentra-tionofprecursorparticleshasincreasedbyafactorof10.Afirst

rather unexpectedresultoftheseinvestigationsisthusthatthe smallakaganeitespindlesactasnucleationsitesfortheformation ofhematitenanocubes,butthattheirnumberisnotdirectly cor-relatedwiththenumberofnuclei.Atwo-stepsynthesisprocedure asappliedinsampleS3couldthusbeusedasatooltotunethesize ofthefinalhematitenanocubes.

Whenwe nextcompare theeffect ofthenumberdensity or [FeCl3]fforthelargeprecursorspindles(S1andS4),westillobserve

thepresenceofraftswithwellalignedstacksofakaganeitespindles atlowertotalconcentration(Fig.11a).However,multiplehematite nucleiappeartogrownownotonlyonraftsbutalsoonstar-like twins.Incontrasttothelargehematitepseudocubicparticles syn-thesizedinsampleseriesS1,Fig.11dshowsthatthefinalparticles obtainedinS4arehematitenanocubeswithamuchsmallersize ofabout37nm.Moreover,SAED(Fig.9c)clearlyindicatesthatthe nanocubeparticlesaresinglecrystalinnature.Wethereforedonot observeaparticlesynthesismechanismalongthemodelinitially proposedforthehematiteparticleproductionathigh(0.10–0.45M) ironsaltconcentrationthroughanucleationandgrowthof multi-plenucleionraftsofakaganeiterods,wherelargepolycrystalline hematiteparticlesform.Instead,nucleiappearoneachaggregate orevenonindividuallargeakaganeiterods,andleadtothe for-mationofsmallsinglecrystallinenanocubes.Inthiscasealower concentrationnotonlysuppressesgrowthasseenforS2andS3, butatthesametimeappearstoenhancenucleation,leadingtothe formationofsmallcubesinS4.

Againitisinterestingtocomparetheinitialakaganeite precur-sorparticlesize(lengthaabout500nm,axialratiol/babout5)with thesizeofthefinalhematitenanocube(cubesizeabout37nm), which indicates thatevery precursorparticle providesmaterial foralargernumberofhematitenanocubes,inthiscase approx-imately 60.Thistwo-stepsynthesisprocessappearstofollowa LaMermodel-like mechanism.Accordingtothismodel, individ-ualnucleigrowbydissolutionandprecipitationofexistingspecies (akaganeite in this particular case) [33]. Hematite nuclei form in thesolutionand growby consumingakaganeite.Our exper-iments thus demonstrate that the presence of stacks of large akaganeiteprecursorparticlesdoesnotnecessarilyleadtothe for-mationoflargepolycrystallinepseudocubichematiteparticles.It is onlyin combinationwithahighmassconcentrationthat we find evidence for this scenario,whereas at lower particle con-centrationswestillobservetheformationofsmallsinglecrystal nanocubes.

4. Conclusion

Wehaveperformedadetailedanalysisoftheparticlegrowth in FeCl3 solutions under forced hydrolysis conditions at 98◦C,

wherefirstakaganeitespindlesformthatthenconvertinto cube-likehematiteparticleswithtime.Fortheakaganeitespindleswe observethattheaveragelengthdependslinearlyontheinitialFeCl3

concentration,whiletheanisotropyishardlyaffectedbythesame parameter.Studyingthesubsequentconversionintohematite,we find thatdepending upon theinitialFeCl3 concentration either

small,singlecrystalhematitenanocubesorlarger,pseudocubical polycrystallinehematiteparticlesform.Atfirstitseemstemptingto linkthefinalsize,shapeandcrystallinityofthehematiteparticlesto thedifferentsizesandstateofaggregationoftheintermediate aka-ganeiteparticles.However,ourexperimentsclearlydemonstrate thatboththeparticlesizeandconcentrationoftheakaganeite pre-cursorhaveasignificanteffectontheresultinghematiteparticles, theirsizeandcrystallinity.

Althoughwefindrationalargumentsthatallowusfor exam-pleto interpretthedifferences in conversionkineticsfor small and largeprecursor particlesin terms oftheirdifferentsurface free energy,orthevariationsseenfor differentfinal concentra-tionsasduetoaconcentration-enhancedgrowth,welackamodel that providesa consistentdescriptionof thefindings.However, while the findings made duringour studypose newquestions withrespecttotheformationmechanism ofcube-like particles thatwillrequiremoredetailedinvestigationsofthenucleationand growthprocessesusinghigh-resolutiontechniques,they neverthe-lessoffernewpathwaystowardsbettersizecontrolforhematite nanoparticles.Following a two-stepsynthesis procedure, single crystalhematitenanocubeswithtunablesizesrangingfrom37nm to175nmcanbeobtainedbyproperlychoosingtheakaganeite precursordispersionintermsofitsparticlemorphologyand con-centration.Itwillcertainlybeinterestingtoseewhetherwecan generalizeourfindingsandextendthemtoothersystems,oruse theresultingnanocubesastemplatesforcreatingmoreadvanced structures forexample throughan additionalcoating stepwith silica.

Acknowledgments

The authors would like to thank Laurent Mirolo for his help with the XRD measurements. We gratefully acknowledge financial support from the Swiss National Science Founda-tion and the Adolphe Merkle Foundation. We would also like to thank Kitty van Gruijthuijsen for valuable discussions and suggestions.

AppendixA. SupplementaryData

Supplementary data associated with this article can be found,intheonlineversion

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