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Synthesis of submicrometric dendritic iron particles in an Electrochemical and Vibrating Hele-Shaw cell: study of the growth of ramified branches

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O

pen

A

rchive

T

OULOUSE

A

rchive

O

uverte (

OATAO

)

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To link to this article : DOI:

10.1016/j.electacta.2017.08.023

URL :

http://dx.doi.org/10.1016/j.electacta.2017.08.023

To cite this version :

Iranzo, Audrey and Chauvet, Fabien and

Tzedakis, Théodore Synthesis of submicrometric dendritic iron

particles in an Electrochemical and Vibrating Hele-Shaw cell: study

of the growth of ramified branches. (2017) Electrochimica Acta, vol.

250. pp. 348-358. ISSN 0013-4686

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Synthesis

of

submicrometric

dendritic

iron

particles

in

an

Electrochemical

and

Vibrating

Hele-Shaw

cell:

study

of

the

growth

of

ramified

branches

A.

Iranzo,

F.

Chauvet

*

,

T.

Tzedakis

*

LaboratoiredeGénieChimique,UniversitédeToulouse,CNRS-INPT-UPS,Toulouse,France

Keywords: ironelectrodeposition sonoelectrochemistry ramifiedgrowth nucleation microstreaming ABSTRACT

Thepurposeofthisstudyistoexploreanewsynthesiswayfortheproductionofironnanoparticles exploitingthenanometricstructureoflongramifiedironbranchesformedbyelectrodepositionina Hele-Shawcell.Afterthegrowth,thesebranchesarefragmentedbytheactionofavibratingelement (piezoelectricdisk)integratedintothecell.Theemphasisisputonthegrowthoftheramifiediron brancheswhichisperformedbygalvanostaticelectrodepositioninastagnantelectrolyte(FeCl2)inside

the Hele-Shaw cell (50mm deep). The competition betweenthe co-formation of H2 bubbles (H+

reduction)andthegrowthoframifiedironbranches(FeII reduction)isanalyzedbyvaryingboththe applied current density j and the FeCl2 concentration. Two regimes, depending mainly on j, are

highlighted:belowathresholdcurrentdensityof8mA/cm2onlyH

2bubblesareformed,whileabovethis

threshold,ironbranchesgrowaccompaniedbytheformationofH2 bubbleswhichnucleateandgrowat

thetopofthebranchesduringtheirformation.TheH2 bubblesinfluencethebranchesgrowthespecially

atlowj(<24mA/cm2)whenthegrowthvelocityofthebranchesislowcomparedtothegrowthrateof

thebubbles.Athigherj(>24mA/cm2),thebranchesfollowacolumnargrowthwithaconstantfront

velocity,wellpredictedbythetheory.ScanningElectronMicroscopy(SEM)oftheironbranchesshowsa dendriticstructureconstituted ofnanometric crystallites,whosesizedependsonthelocal growth velocity:increasingthegrowthvelocityfrom3.6mm/sto40mm/sleadstoadecreaseinthecrystallites size,from!1mmto!10nm.Usingtheacousticvibrations(4kHz)ofthepiezoelectricdisk,thesefragile branches are successfully fragmentedinto submicrometricfragments of dendrites exhibiting high specificsurfacesS/V(equivalenttotheS/Vofnanoparticlesof30nmdiameter).Advantages/Drawbacks comparedtoothersynthesiswaysaswellastheoptimizationoftheproposedsynthesisarediscussed.

1.Introduction

Metallicironnanoparticlesattractsignificantinterestbothfor theirmagneticandcatalyticpropertiesinvariousfields(medical, energy,andenvironment).Inthemedicalfield,evenifironoxide nanoparticleshavebeeninvestigatedfortheirmagnetic proper-ties,forMagneticResonanceImaging(MRI)[1]orcancertreatment

[2], they appear to be less efficient when compared to iron nanoparticleswhichexhibitenhancedmagneticproperties[3].In addition, ironnanoparticlesare increasinglyinvestigated inthe treatment of contaminated watersand soils [4–6]due totheir reductiveproperties. Indeed,due tothe lowstandard electrode

potential of the Fe2+/Fe0 system (E"=#0.44V/SHE), their high

specific surface and their porous iron oxide/hydroxide outside layer,theseironnanoparticlesshowahighreactivityandallowthe removal of pollutants [5] such as ions of heavy metals or chlorinatedorganiccompounds[7].

Conceptually,nanoparticles canbe synthesizedthrough two globalapproaches,top-downandbottom-up.Thefirstoneconsists of using a large-sized material (micrometric particles for examples) which is chemically or physically cut off until the nanometricsizeisreached.Thebottom-upapproachconsistsof the reverse process; small building blocks (like atoms) are assembled to form nanoparticles. Based on these two global approaches, various physical or chemical syntheses have been exploredtoproduceironnanoparticles(ball-milling[8],thermal reduction of iron salt [9,10], iron salt wet-chemical reduction

[11,12]).

* Correspondingauthor

E-mailaddresses:chauvet@chimie.ups-tlse.fr(F. Chauvet),

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Among the wet-chemical syntheses, theproduction of iron nanoparticlesbyreductionofanironsaltbyaborohydridesalt (colloidal synthesis) is a method commonly employed at the laboratory scale [7]. Nevertheless, this synthesis requires specificconditions,firstbecauseoftheriskofgaseoushydrogen production(causedbyborohydrideoxidation)andsecondlydue tothe toxicity, corrosiveness,andflammability of the borohy-dride reductant [9,13]. Additionally, a step of purification is required to separate the produced nanoparticles from the remaining reacting species and byproducts (dialysis, ion exchange resin, centrifugal or filtration processes). This is required for several reasons: enhancement of the colloidal stability, to avoid the presence of salt crystals during dry characterization of the particles and to avoid contamination/ pollution effects by residues of the synthesis in the final application.

Another technique to produce metallic nanoparticles, here called“sonoelectrochemical synthesis”, combines electrochem-icaland ultrasonicationprocesses. Itconsists inthe electrode-position of iron nanoparticles (nuclei) on a cathode surface whichis subjectedtopowerultrasound(20kHz).The propaga-tionof theultrasonic wavesinducescavitation bubbleswhich, during their violent collapses, create strong enough fluid motion to detach the iron nanoparticles from the electrode surface (Fig. 1a)). Both processes, electrodeposition and ultra-sound,canbeappliedeithersimultaneously[14]orsequentially

[15]. This technique appears to be a promising alternative, avoiding the use of a reductive chemical agent, but: i) the purification step is, here again, required (removal of the supporting electrolyte and remaining metallic precursor) and ii)theultrasoundprocesssuffersfromalowenergeticyield(a large part of the mechanical energy is consumed by the cavitation and streaming generated far from the electrode surface,and so useless for the depositdispersion).

Onthebasisofthislasttechnique,weproposetoexploreinthe presentpaperanewsynthesisrouteaimingtobecosteffective, safeandimplyingalimitednumberofsteps.Asinthecaseofthe sonoelectrochemicalsynthesis, it consistsin producing electro-chemicallythemetallicironparticlesbut,insteadoflimitingthe irongrowthtoobtainnucleilyingontheelectrodesurface,the electrodepositionisdrivenforlongertimesgivingriseto“long” ramified ironbrancheswhich are thenfragmented viaacoustic vibrations,Fig.1b).

Tothatend,an“ElectrochemicalandVibratingHele-Shawcell” hasbeendesigned.ItconsistsofaHele-Shawcell(athingapcell,

50

mm

deep)integratingalow-frequencyacousticdevice (piezo-electric disk, PZT, resonance frequency=4kHz) to induce the vibrations(Fig.2).

Theuseofsuchaconfinedgeometryallowsthecontrolofthe iron growth and the generation of ramified deposits. When a currentisappliedtothecell,themetalliccationsMz+arereduced

intometalM0inducingthedecreaseoftheMz+concentrationat

thecathodeinterface(nofluidcirculationintothecell).Whenthe interfacialMz+concentrationreacheszero,themetalhastogrow

undertheformofaporousdeposit(ramifiedbranches)througha successionof nucleation/growthevents (orthewaterreduction takesplace)forthecurrenttokeepflowingthroughthecell[16]. Dependingontheoperatingparameters(applied currentorcell voltage and precursor concentration), the resulting ramified depositsexhibitoneofthethreemainmorphologies(arrangement of the branches): fractal, columnar and dendritic [17,18]. The branchesareknowntobeconstitutedofsmallcrystalliteswhich couldbenanometric(asshownin thecaseof theformationof ramifiedcopper branches[19]).Thus,theidea,proposedinthis work,istoexploitthesmallgranularstructureofthebranches,for the case of an iron deposit, to produce a suspension of iron nanoparticlesviathefragmentationofbranchesinducedbythe PZTvibrations.Thesizeofthecrystallitesthatconstitutetheiron branchesbeingdependentontheappliedcurrentdensity[19],this synthesis route offers an external control of the produced nanoparticles,atleastintermofsize,viatheoperatingparameters (appliedcurrentandironprecursorconcentration).Additionally,in

Fig.1.Schematicrepresentationof:a)theclassicalsonoelectrosynthesiswhichistheremovalbyultrasoundofironnanoparticleselectrodepositedonanelectrodesubstrate, b)thenewsynthesisproposedinthisstudywhichconsistsofthefragmentationunderacousticvibrationsoframifiedironelectrodeposits.

Fig.2. SchematicrepresentationoftheElectrochemicalandvibratingHele-Shaw cellusedinthisstudy.

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contrastwiththesonoelectrochemicalsynthesis,theintegrationof thePZTtotheHele-Shawcellallowslocalizingandconcentrating themechanicalactiononthemetalliccrystallitesassemblies.Last but notthe least,in this microfluidic-like device, thebranches couldberinsedbeforetheirfragmentation,whichshouldavoidthe finalpurificationstep.

Asindicatedinapreviouswork[20]andalsoin[21],onaniron electrode,thereductionofFeII(FeCl

2,(NH4)2Fe(SO4)2andFeSO4for

pH<!4)intoFe0isaccompaniedbythereductionoffreeprotons

H+.Therefore,hydrogenbubblesformationisexpectedduringthe

electrodepositionexperimentsintheHele-Shawcell(asshownin S. Bodea et al. [22]). This issue is specifically studied in the following.

The integration of the low-frequency acousticdevice in the Hele-Shaw cell is inspired by its use in microfluidic chips to enhancemixing[23–25].Intheseworks,ithasbeenestablished that the presence of bubbles, trapped into specially designed microchannels,isneeded toallowanefficientmixing[23].The vibrationof thePZTinduces theoscillationof thebubblesthat generatesmicrostreaming(astationaryandasymmetricfluidflow aroundthebubbles[26]).Weproposeheretotakeadvantageofthe microstreaminggeneratedbytheco-producedH2bubblesforthe

fragmentationofthebranches.

ThisstudyfocusesontheeffectofboththeFeCl2concentration

andtheappliedcurrentdensityoni)thegrowthoframifiediron branchesin theHele-Shawcelland ii) theinfluenceof the

co-Fig.3.Syntheticresultsofvariouselectrolysescarriedoutatvariouscurrentdensitiesandforvariousconcentrationsoftheironprecursor(FeCl2)withoutsupporting

electrolyte.ForeachFeCl2concentration,differentcurrentdensitiesareapplied,andthephenomenaoccurringintheHele-Shawcellareshownina);greencircles:onlyH2

bubbles,yellowsquares:mainlyH2bubbleswithsomeironbranches,greydiamonds:H2bubblesandironbrancheswhichfillthespaceuniformly.Foreachcase,animageofa

typicaldeposit,obtainedunderthecorrespondingconditions,isshowninb)0.02M,4mA/cm2,c)0.06M,12mA/cm2andd)0.1M,80mA/cm2.(Forinterpretationofthe

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producedH2bubbles.SEMandTransmissionElectronMicroscopy

(TEM) are used to observe respectively the structure of the branchesaswellastheparticlesresultingfromtheironbranches fragmentation(thefragmentationmechanismwillbedetailedin anotherpaper).Thepurityoftheobtainedaqueoussuspensionof ironparticles,intermofelectrolytesaltcontent(i.e.massfraction ofthemetalliciron),isdeterminedandtheresultiscomparedto theoneobtainedwiththecolloidalandthesonoelectrochemical syntheses.

2.Experimental 2.1.Chemicals

Iron(II)saltsolutionsarepreparedusingnormapursolidFeCl2

(Sigma-Aldrich)inultrapurewater(18.2MV.cm);nosupporting electrolyteisused.The naturalpHoftheFeCl2 solutionsvaries

from3.9to3.3dependingontheFeCl2concentrationsused(from

0.02to0.1Mrespectively).ThepresenceofdissolvedO2mustbe

avoidedtopreventitsco-reductionduringtheelectrodeposition and to limit the Fe0 corrosion. All the solutions are deaerated

(Argon,1bar),during15min(!30mL)beforebeingcollectedbya gastight syringe(Hamilton 1mL,1001LT) and injected intothe Hele-Shawcell.

2.2.Experimentalset-upandmethods

The Hele-Shaw cell is constituted of two iron plates (the electrodes)of a thickness of50

mm

(purity$99.5%),which are 9mmapartand sandwichedbetweenaglassplateandthePZT (ABT-441-RC,Radiospare).Thelengthoftheelectroactivezoneis 2.5cm,Fig.2.

Thefaceoftheglassplateexposedtothechannelisentirely coveredbytransparentlaboratoryparafilmsheetactingasagasket toavoidleakageswhilethesurfaceof thePZTis coveredbyan adhesive tape ensuring its protection against corrosion. The contoursoftheHele-Shawcellareclosedbyapplyinganadhesive paste.Twoclamps,pressingontheglassplates,areusedtoholdthe assembly.Thecellisfilledwiththeferrousionsolutionusingthe gastightsyringeviatwomicrotubes(PTFE)connectedtothecellby fluidicconnectionsmadebydrillingholesintheglassplateandby gluing Nanoport connectors (Idex-hs) on them. Special care is takentoavoidtheintroductionofatmosphericoxygeninsidethe channel. The cell is maintained horizontally to avoid natural convectionwhichcouldinfluencethebranchesgrowthandsotheir structure[27,28].Thecellcaneasilybedismantledtobecleaned,or torecovertheirondepositsforSEMobservations.Theelectrodes aremanuallypolishedusingapapergrid(P1200)tooperatewitha reproduciblestateofthesurfaceoftheelectrodes.Theelectrolyses areperformedwithapotentiostat(AutolabPGSTAT100N). Experi-mentsarecarriedoutatroomtemperature(18<T("C)<22).

A typical experiment consists in filling the cell with the electrolyte, then a constant current is applied between the electrodesandthegrowthoftheironbranches,onthecathode, isvisualizedbyacamera.Theoxidationoftheironmadeanode preventstheproductionofbothO2(bubblesneverobserved)and

FeIII.Aftertheirformation,themetallicbranchesarerinsedbya

flow of deaerated ultrapure water using a syringe pump. A sufficientlylowflowrate,!100

mL/min,

isappliedtoavoidboth damagingthe iron branches and the removal of the hydrogen bubbles.Besides,nochangeinthebubblessizeisobserved,dueto boththelowsolubility(!0.8mM)ofH2inwaterandalsoduetothe

lowsurfaceareaoftheliquidgasinterfacesofthecrushedbubbles in this confined geometry (the smallest apparent diameter of bubblesbeingequalto100

mm).

ThePZTisthenactivatedduring !15s to fragment the branches. The fragmented particles are

collected by pushing them throughthe cell applyinga flow of deaeratedultrapurewater.

2.3.Observationsoftheobtainedironelectrodeposits

2.3.1.Opticalobservationofthegrowthoftheramifiedironbranches The growth of the ramified iron branches is observed by reflectionusingafiberopticilluminatorandacameraPCOpixelfly connectedwitha105mmmacrolens(fieldofview%5mm&5 mm).Apictureofthedepositistakenevery5secondstomonitor theevolutionofitsgrowth.

2.3.2.SEMobservationoftheramifiedironbranchesandpreparation ofthesamples

Toobservethestructureoftheobtaineddeposit,SEMpictures of the ramified iron branchesare taken. The iron depositsare fragile,andtherecoveryoftheramifiedstructurerequiresspecial caution.Aftertherinsingphase,thecellisopenedwhichinduces strong modifications of the branches pattern, but without damaging the microstructure. The deposit is recovered on an adhesivecarbontapeanddriedunderatmosphericconditions.The structureoftheironbranches(beforefragmentation)isobserved bySEMwithaJEOLJSM7100FTTLSoraJEOLJSM7800F Prime-EDS.

2.3.3.TEMobservationoftheparticlessuspensionandpreparationof thesamples

Adropofthesuspension,containingthefragmentedparticles, islefttodryonaTEMgridbeforeanalysiswithJEOLJSM 2100F-EDS.

3.Resultsanddiscussion

3.1.Growthoframifiedironbranchesaccompaniedwiththeformation ofH2bubblesintheHele-Shawcell

Todeterminetheconditionsallowingthegrowthof theiron branches, in the Hele-Shaw cell used, several galvanostatic electrolyses are performed using a stagnant FeCl2 solution, by

varyingtheconcentrationc,from0.02Mto0.1M(thenaturalpH varyingfrom3.9to3.3)andtheappliedcurrentdensityfrom4to 240mA/cm2 (relative to the initial geometrical surface of the

electrode). The obtained results can be classified into three different groups (see Fig. 3), depending mainlyon thecurrent densityjandremarkablyindependentlyoftheconcentration(in theexaminedrange):

– j<!8mA/cm2:H

2bubblesareproduced,andnoironbranches

areobserved(greencirclesinFig.3a)andFig.3b)) – !8<j (mA/cm2)

<!16: large H2 bubbles are produced, and

some partially broken ramified iron branches are observed (yellowsquaresinFig.3a)andFig.3c))

– j>!16mA/cm2:smallH

2bubblesnucleateandgrowatthetop

oftheironbranches,andtheyarecontinuouslyleftbehindthe movingfront(graydiamondsinFig.3a)andFig.3d)).

These results, in agreement with those of Heresanu [29], highlight that thereis a thresholdcurrent density (%8mA/cm2

here,mainlydevotedtotheH+reduction)toovercomeinorderto produceironbranches.

Foranappliedcurrentdensityj<8mA/cm2,theinitialpotential

isnotsufficientlycathodictostartthereductionofFeII.Duetothe

decreaseintheinterfacialconcentrationofH+,thepotentialofthe

cathodedecreasesuntilitreachesthepotentialenablingtoinduce thereductionoftheFeIIions.Anadditionaltimeisthenrequiredto

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thetimenecessarytofullydepletetheFeIIatthecathodesurface [30].Theexperimentalresultssuggestthatthislastconditionis neverobtainedforj<8mA/cm2.

Foranappliedcurrentdensityj>8mA/cm2,theinitialpotential

issufficientlycathodictoinducethesimultaneousreductionofFeII

and H+.Duetoboththehighvalueof thecurrentand thelow

concentrationofH+(comparedtoFeIIconcentration),thefraction

ofthecurrentusedfortheH+reductionislowcomparedtotheone

usedfortheFeIIreduction.Thisshortensthedepletiontimeforthe

FeIIandmakesthegrowthofironbranchespossible,asobservedin

the experiments when appliedcurrent densities >8mA/cm2. A

modelbasedonthecouplingbetweenthetransientsemi-infinite diffusionprofilesofH+andFeIIwithButler-Volmerequationswill

beprovidedinanotherpaper.

However,thetransitionbetweenthesetwolimitingcasesisnot easy to model since thegrowth of H2 bubbles at thecathode

surfacecanchangetheelectrolysisconditionslocally:

– H2 bubbles cover the electroactive surface of the cathode

implyingtheincreaseofthecurrentdensityimposedtothefree surface,andthen,couldfavorthestartofabranchgrowth. – H2bubblescanalsototallyisolatesomeoftheironbranches

fromthesolutionandconsequentlyblocktheirgrowth.

BothcasesarerespectivelyhighlightedinFig.4a)andb). Even if H2 bubbles are produced whatever the operating

conditions,thedepositgrowthappearstobelessaffectedbytheir presence at high current density (for j>16mA/cm2). This is

because,fortheseconditions,thegrowthvelocityofthebranches ishigherthanthebubblesgrowthrate,leadingtotheformationof smaller bubbles, less disruptive than those produced at lower currentdensity.

3.2.MorphologiesoftheironelectrodepositsintheHele-Shawcell Themorphologiesoftheobtainedironelectrodeposits,atthe macroscopicscale(arrangementofthebranches),arereportedin

Fig.5asafunctionoftheappliedcurrentdensityandtheprecursor concentration.TheobjectivehereistoexaminetheeffectoftheH2

bubbles,electro-generatedsimultaneouslywiththeiron deposi-tion,onthemorphologytransitionsandtocomparewiththemain depositspatterns,andtheassociatedtransitions,usuallyobserved inabsenceofbubbles(fractal-columnar-dendritic)[17,18].

TwomainmorphologiesaredistinguishedinFig.5:thefractal andthecolumnar;nodendriticmorphologiesareobservedatthis macroscopicscale(fieldofview%5mm&5mm).

EvenifitisnotvisibleinFig.5(especiallyatthehighercurrent densities),theformationofbubbles,duringthegrowthoftheiron

Fig.4.OpticalimagestakenduringgalvanostaticelectrolysesintheHele-Shawcellfora)0.02MFeCl2,j=12mA/cm2andb)0.1MFeCl2,j=16mA/cm2.Withthesevaluesof

appliedcurrentdensity,theH2bubblesprogressivelyblocktheelectrodesurface,thatcouldfavorthestartofthegrowthofabrancha)butalsoitsstopb)(seethebranchin

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branches,hasbeenconfirmedbymagnifiedvisualizationsforall thedeposits.

Thecolumnarmorphology,whichconsistsofalargenumberof branchesregularlyspaced and growing atthesame velocity, is obtainedmainlyforthecurrentdensitieshigherthan24mA/cm2,

whatever the precursor concentration. Increasing the current density causes the distance between the branches todecrease whichleadstoapparentlydenserdeposits.

Lower current densities, typically a value of 12mA/cm2,

produce deposits with fractal-like morphology, which exhibit openstructureswithfewerbranches,andtheirarrangementisalso lessregularthantheoneobtainedforcolumnardeposits.

Suchatransition,fromthefractaltothecolumnar morphol-ogieswhenthepolarizationmagnitudeisincreased,hasoftenbeen reported for the electrochemical growth of ramified metals withoutH2 bubblesco-formation(zinc [18,31]copper [30–33]),

however the mechanism of this transition is not yet fully understood.

One convincing explanation is the relatively low electrical conductivityoftheporousbrancheswhichregulatestheheightsof thebranchesandthus stabilizesthegrowthfront, especiallyat highappliedcurrentdensities [34].Notethat electroconvective phenomena havealso beenproposed toexplain this transition

[33].

Thedendriticmorphologycorresponds toanorderedcrystal growth(offewmaintrunkswithsecondarybranchestiltedwitha well-definedangle);itisobtainedwhenthegrowth ofmetallic crystalsisfavoredcomparativelytonucleationevents,generallyat thehighestappliedcurrents(thedepositmorphologygoesfrom fractaltocolumnarandthentodendriticwhentheappliedcurrent

increases) [18]. In the present study (Fig. 5) no dendrites are observed at the macroscopic scale, the columnar morphology predominates even at high applied currents. However, as it is shown in the next section (Fig. 7), themorphology is actually dendriticata smallerlengthscale.Itmeansthatthereis stilla competition between nucleation and growth events when the appliedcurrentishigh;thedendritesgrowthisregularlyandearly stoppedbynucleationevents.Thiscouldbeduetotheco-produced H2 bubbles around the iron branches which act as “insulating

shields”thatlimitthelateralgrowthofsecondarybranchesofthe dendritesandensureaconstantseparationdistancebetweenthe branches.

To sum up, the co-formation of H2 bubbles doesnot affect

significantly the transition between the main patterns usually observedforothersystemsinabsenceofH2bubbles.Thebubbles

can only be suspected of affecting the transition between the columnarandthedendritic morphologiesbyalateral shielding effectwhichlimitsthedendritesgrowthathighappliedcurrents. 3.3.Faradaicyieldoftheironelectrodeposition

Asindicatedinthesection3.1,thedepositgrowthappearstobe lessaffectedbytheco-productionofH2bubblesathighcurrent

density, and so for columnar deposits (section 3.2). For this particular case, the faradaic yield of theiron electrodeposition processisestimatedfromthemeasurementofthegrowthfront velocityandthetheorydevelopedin[16].

When metallic branches grow without the formation of H2

bubbles(caseofcopperorzinc),duringgalvanostaticelectrolyses, thecorrespondingcolumnardepositsareboundedbyaflatfront

Fig.5.OpticalimagesofthedifferentdepositmorphologiesobtainedusingtheHele-Shawcellforthegalvanostaticelectrolyses:8'j'240mA/cm2and0.02'FeCl

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whichadvancesataconstantandpredictablevelocityvg[16].The

concentration profile of theelectroactive species, ahead of the growthfront,isthenstationaryanditisadvectedwiththefront. Withoutsupportingelectrolyte,themasstransferproblemcanbe reducedtoagenericdiffusion-advectionproblem([35])thathas been used in [16] to obtain the following modeling of the concentrationprofilecðx;tÞaheadofthefront:

cðx;tÞ c1 ¼1#exp xfðtÞ#x Ld ! " ; ð8Þ

withc1thebulkconcentration,x

fðtÞ thegrowthfrontlocation,x

thecoordinateperpendiculartothefrontanddirectedtowardsthe growth direction and Ld¼D=vg the diffusion length, D¼ zþuþDþ

#z#u#D#

uþ#z#u# being the diffusion coefficientof theunsupported

electrolyte (where z+, z#and u+, u# are the valences and the

mobilities of thecationicand anionic speciesof theelectrolyte respectively)[35].

Combining Equation (8) withtheboundary conditionatthe frontforthecurrentdensity(assumingthatthemetal electrode-position is quantitative) zþjF¼1#tDþ @c @xjxf (where @c @xjxf¼c 1 Ld, tþ¼uþ

uþ#z#u# is thetransference numberand Fis theFaraday’s

constant) [35], the growth velocity of the columnar deposits (withoutH2bubblesformation)isgivenby[16]:

vg¼

jð1# tþÞ

Fc1 ð9Þ

In Fig.6,forseveral columnarirondepositsproducedinthe presentstudy,themeasuredgrowthfrontvelocityvexpisplottedas

afunctionoftheratio2Fcj1,assumingafaradaicyieldof100%(jis

thecurrentapplied).Aspredicted,vexpdependslinearlyon2Fcj1and

the proportionality factor is equal to 0.56. Thus the t+ value

determinedviathisexperimentalapproach,assumingafaradaic yieldof100%,is0.44.Bycomparingthisexperimentalt+valueto

thetheoreticalone(tþ¼uþ

uþ#z#u#Þ,theactualfaradaicyieldofthe

ironelectrodepositioncanbeestimated.

Accordingto[36],forchlorideionsconcentrationlowerthan 1mol/kg, Fe2+ ions are not complexed with the chloride ions.

Thereforehere,itisassumedthatFeCl2isfullydissociatedintoFe2+

andCl#withouttheformationofcomplexes.TheH+concentration

beingalwayswelllowerthanFe2+orCl#concentrations,themain

cations and anions to consider for the determination of the theoreticalt+valuearetheFe2+andtheCl#ions.Atheoreticalt+

valueof0.41isdetermined(assuminganinfinitedilutionofthe species, u.=D./RT, R=8.314J/(mol/K),T=293K, D+=7.19

&10#10

m2/s[37]andD#=2.03&10#9m2/s[38])whichisslightlylower

thanthet+valuedeterminedwiththeexperimentaldata(Fig.6):

0.44.Thisdifferencebetweenexperimentaldataandtheorycanbe interpretedasaconsequenceoftheH+co-reduction.Indeed,the

wholecurrentdensityisnotusedtotheironelectrodepositionand thus,intheexperimentscarriedout,thedepositfrontadvancesa littlemoreslowlythanexpectedbythetheory,assumingafaradaic yield of 100%. Therefore, the actual faradaic yield for the electrodepositionofthecolumnardepositsisestimatedat95%.

These results show that, during the electrodeposition of columnardeposits,analmostnegligiblepart(5%)oftheapplied currentdensityjisallottedtotheH+reduction.Nevertheless,even

iflittleH2gasisgenerated,thebubblesarewellvisiblesincethey

arehighlycrushedintheverythinHele-Shawcellused. 3.4.Small-scalestructureoftheramifiedironbranches

Withtheobjectiveofusingthemicrostructureoftheramified deposits,toproduceasuspensionofironnanoparticles,thesmall scalestructureoftheironbranches,obtainedintheHele-Shawcell, isobservedbySEM.

Among the various deposits obtained in this study, five deposits,showingmorphologiesgoingfromcolumnartofractal (8'j'24mA/cm2),havebeenobservedbySEMandthepictures

arepresentedinFig.7:thedepositsshowninFig.7a),b)andc)are obtainedusinga0.02MFeCl2solutionatcurrentdensitiesof24,12

and8mA/cm2respectively;thedepositsshowninFig.7d)ande)

areobtainedusinga0.1MFeCl2solutionatcurrentdensitiesof12

and8mA/cm2 respectively.Tocompare thelargeandthesmall

scalestructuressimultaneously,threedifferentmagnificationsare providedinFig.7.

Forthedepositsobtainedwithaconcentrationof0.02Mandat currentdensitiesrangingfrom24to8mA/cm2(Fig.7a),b)andc)) thesmallscalestructure(picturesinthemiddle)reveals dendritic-likebranches(maintrunkswithsecondarybrancheshighlighting preferred growth orientation) which consist, however, at the nanometricscale(zoomontheright),ofsmallcrystallites.Thesize ofthesecrystallitesvariesfromonedeposittoanother.Thedeposit showninFig.7a)(24mA/cm2,0.02M)appearstoconsistofseveral

rowsofregularcrystallites,withasizeofabout20nm,whilefor thedepositshowninFig.7b)(12mA/cm2,0.02M),thecrystallites

sizeliesintherangeof50–100nm.

Fleurycorrelatesthisfinegranularstructuretotheoscillatory characterofthenucleationkineticsandclaimsthatthecrystallites size is dictated by the growth velocity, and so by the current density(fortheramifiedgrowthofcopper[19]):thehigherthe growthvelocity,thesmallerthecrystallitesare.

Regardingcomparativelytheapparentgrowthvelocityandthe crystallites size v24mA=cm2

; 0:02M

ð Þ % 40

mm/s

!10 ' dcrystallitesðnmÞ' 30; v 12mA=cm2

;0:02M

ð Þ % 20

mm/s

!50 ' dcrystallitesðnmÞ '100)itcanbeconcludedthattheseobservations,

for theiron case, are consistentwith theaffirmation of Fleury ([19]).

In addition, our results highlight that the influence of the growthvelocityonthecrystallitessizehasadirectimpactonthe regularityof thedeposit structure.Thefractal deposit obtained with8mA/cm2(0.02M),Fig.7c), showsa smallscale structure

(pictureinthemiddle)lessregularthanthedepositsobtainedwith highercurrentdensities,24and12mA/cm2(0.02M),Fig.7a)and

b).Thisdepositexhibits,atthenanometricscale(pictureonthe right)bigcrystallites ofvarioussizesrangingbetween200and

Fig.6.Averagegrowthvelocityofcolumnardepositsasafunctionoftheratio j

2Fc1.

Squares: experimental data; line: best linear fit, vexp¼0:56& 2Fcj1

# $

þ4:3&10#6; concentrations range from 0.02 to 0.1M

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Fig.7.SEMpicturesofironelectrodepositsobtainedintheHele-Shawcellatvariouscurrentdensitiesandusingvariousprecursorconcentrations.a),b),c)0.02MFeCl2using

24,12and8mA/cm2respectively;d),e):0.1MFeCl

2using12and8mA/cm2respectively.Millimetric(leftcolumn),micrometric(middlecolumn)andnanometric(right

(10)

500nm.Thisbroadrangeofcrystallitessize,inthesamedeposit,is explained by the fractal morphology. Indeed, contrary to the columnardeposit,forafractaldeposit,thegrowthvelocityisnot thesameforallthebranches,thegrowthofsomebranchescan slow down and stop in favor of the growth of others. The estimationofthegrowthvelocityofsomebranchesleadstovalues varying from 12

mm/s

to 19

mm/s

(Fig. 7c) on the left) which explainsthedispersionofcrystallitessizesobserved.

The deposits shown in Fig.7d) and e) are obtainedusinga higher concentration of0.1M,at two current densities,12 and 8mA/cm2respectively.Atthelargescale(picturesontheleft),the

depositobtainedat12mA/cm2(Fig.7d)),showsthickerbranches thantheotherdeposits.Atthesmallscale,thestructureisordered and thesizeofthecrystallites,composingthebranches,ranges between 500nmand 1

mm

(picturesin themiddle and onthe right).Thisisconsistentwiththelowgrowthvelocityestimatedfor mostofthebranchesofthisdeposit%3.6

mm/s;

thislastvalueis aboutfourtimesslowerthanthevelocityvaluemeasuredinthe previouscases.

Finally,thedepositobtainedat8mA/cm2(Fig.7e))shows,atthe

largescale,fractalmorphology,butitcanbenoticedthatlargeH2

bubbleshavebeenformedduringitsgrowth.RegardingtheSEM pictures(Pictureinthemiddle),itcanbeobservedthatthedeposit shows areallycomplexsmall-scale structure.Awide varietyof structures isobserved(thin branches,roughstructures,smooth iron blocks...), probably because the generation of H2 bubbles

disturbesstronglythedepositgrowth.Forexample,thegrowthof thebranchlabeled“1”inFig.7e),hasbeenstoppedbythepresence ofanH2bubble.Consideringthebranchlabeled“2”inFig.7e),the

observationsmadeduringtheelectrolysis,showthatthegrowthof therightbranchisstoppedforthebenefitoftheleftbranch.Then, the leftbranchgrowthis stoppedbythepresenceof a bubble, whichreactivatesthegrowthoftherightbranch.Thus,thegrowth ofonebranchisachievedinseveralstepswithdifferentgrowth velocities.

Tosumup,theSEMobservationshaveconfirmedtheFleury’s work[19]fortheironcase:theironbranchesconsistofcrystallites whose size decreases withincreasing theirgrowth velocity. In

addition,the regularity of thecrystallites size, observed atthe nanometricscale,isrelatedtothelarge-scalemorphologyofthe deposits. Fractal deposits consist of polydispersed crystallites whilecolumnardepositsshowcrystallitesofalmostuniformsize. ThedepositshomogeneityisevenmoreaffectedwhenH2bubbles

hinderthedepositgrowth.

Inconclusion,intheprospectofthefragmentationoftheiron branches to produce regular iron nanoparticles, the operating conditionsallowingthegrowth ofcolumnardepositsshouldbe favored.

3.5.Characterizationoftheproducedparticlesafterfragmentation Aftertheformationofacolumnardeposit(80mA/cm2,0.1Mof

FeCl2)anditsrinsing,thePZTisactivated(squareelectricalsignal:

peak topeak voltage=250V, frequency=4kHz, duration=15s). The characterization by TEM of the collected particles (Fig. 8) revealsthattheyareinfactfragmenteddendritesofvarioussizes, from few micrometers (Fig. 8a), b) and c)) to hundreds of nanometers(Fig.8e)andf)).Thesefragmentsofdendrites,shown inFig.8a),b)andc),wereprobablyinitiallysecondarybranchesof maintrunkswhichhavebeencutoffattheirroot.InFig.8a)andb), the micrometric fragments seem to be unaffected by the fragmentationprocess,while in Fig.8c)the fragment seemsto havebeenpeeledoffonitsrightside.Thissuggeststhatsmaller fragments(!200nm)arebrokenbythemechanicalstressinduced bybubblesoscillations.Apileofdendritesalongwithagroupof smallerentitiescanbeseeninFig.8d).Theyconsistofneedle-like fragmentswhichappeartobethesmallbranchesofthesecondary branches.

It can be concluded, that coupling the electrochemical formation of ramified iron branches with their fragmentation, usingtheelaborated “ElectrochemicalandVibrating Hele-Shaw cell”,enablestoobtainasuspensionofdendriticparticlesofsizes varying from hundreds of nanometers to few micrometers. However,themain interestof usingnanoparticlesis theirhigh specific surface, and even if the obtainedfragments are about 1

mm,

theirdendriticshapegivesthemahighsurfacetovolume

Fig.8.TEMpicturesoffragmentsofirondendritesobtainedafterthefragmentationofthecolumnardeposit(j=80mA/cm2,0.1MFeCl

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ratio.Thankstoanimageprocessing,aperimetertosurfaceratioof 1.3&108m#1 has been determined, which corresponds to a

nanoparticle of about 30nm in diameter. Therefore, these fragments should show a high catalytic activity that will be studiedshortly.

3.6.Purityoftheproducedsuspension

Theparticlesofinterestbeinginitiallythebuildingelementsof theironbranches,theyare,atfirst,immobilizedinsidethe Hele-Shawcell(inthebranches).Thisoffersthepossibilitytorinsethe particles(beforethedepositfragmentation) bya simpleflowof deaerated water in order to decrease theconcentration of the remainingelectrolyte(usedfortheirondepositproduction)inthe cell. Consequently, contrary to other syntheses, the produced suspension is purified in-situ and therefore no additional purificationstepshouldberequired.

However,since theelectrolytecannotbetotallyremoved,its residual concentration is measured, here for a particular case (columnardeposit,formedusingacurrentdensityof80mA/cm2

and 0.1MofFeCl2), andthe purityof theproducedsuspension

(definedhereasthemassfractionofmetallicironexcludingthe solventH2O),isalsoestimated.

Aftertheformationofthedeposit,1mLofdeaeratedultrapure water(!90timesthevolumeoftheHele-Shawcell)isinjectedinto the cell at a flow rate of !100

mL/min

(!10min). After the fragmentation,!700

mL

oftheproducedsuspensioniscollectedby aflowofdeaeratedultrapurewater.Themassconcentrationofthe remainingFe2+andCl#inthis volumeis respectively!1.11&10 #2g/Land!1.42&10#2g/L(titrationofchlorideionsbyAgNO

3).The

mass concentration of metallic iron is equal to 1.23&10#1g/L

(estimated by the passed charge during the electrolysis and assumingthatalltheparticlesarecollected).Therefore,thepurity oftheproducedsuspensionishighandequalto83%.Thisvalueof purity obtainedis nowcompared totheone reached withthe colloidalandthesonoelectrochemicalsyntheses.

Inthecolloidalsynthesisofironnanoparticles,thecommonly employed concentration of both the metal precursor and the reducingagentisintherange1–10#2M[12,39]thatleads,atthe

endofthesynthesis,tothesamerangeofbyproducts concentra-tion(NaCl,B(OH)3...).Consideringthetypicalsynthesisreaction [11,40]:

FeCl3+3NaBH4+9H2O!Fe0+3NaCl+3B(OH)3+10.5H2, (10)

and, assuming a stoichiometric initial composition and a total reaction, the purity of the produced suspension could be estimated:13%.

In the sonoelectrochemical synthesis, the useof supporting electrolyteinaconcentrationrange!0.5Mpreventsreachinghigh purityvalues.Infact,forthissynthesisway,thepuritydependson boththeexperimentalconfiguration(especiallytheratiobetween thesurfaceoftheelectrodeandthevolumeofthereactor)andthe productionduration.

Therefore,intheproposedsynthesis,eveniftheelectrolytewas nottotallyremovedduringtherinsing phase,thepurity ofthe produced suspension is well higher than in the colloidal and sonoelectrochemicalsyntheses. Evenhigherpurity and concen-trationofironparticlescanbeachievedbyoptimizingtheoutletof thedeviceusingsmallerandshortertubes.

4.Conclusions

Theobjectiveofthisworkistoexploreanewsynthesisroutefor the production of iron nanoparticles using a simple aqueous ferroussolution(FeCl2)inanelectrochemicalandvibrating

Hele-Shawcell.Theuseofthisconfinedcell(50

mm

deep),allowsthe growthoframifiedbrancheswithagranularmicrostructure.The growthofthebranchesis accompaniedbytheformationof H2

bubbleswhich,duetotheiroscillationsduringthevibrationphase (activation of the PZT), allow fragmenting the branches. The influenceoftheoperatingconditions(appliedcurrentdensityand FeCl2concentration)ontheelectrodepositionisstudied.Thefocus

is on thegrowth of H2 bubbles and on theobtained branches

patternandmicrostructure.Columnardeposits(ensuringregular branches growth), withembedded H2 bubbles (only5% of the

applied current is used for their generation), are obtained for sufficientlyhighappliedcurrentdensities(>!24mA/cm2).SEM imagesofthebranchesrevealadendriticstructureconsistingof crystallitesofalmostuniformsize.Thissizedependsonthelocal branch growth velocity. Increasing this growth velocity from !20

mm/s

to !40

mm/s

causes the average diameter of the crystallites to decrease from the range of 50–100nm to 10– 30nm.TEMimagesoftheparticles,obtainedafteractivationofthe PZT, have revealed that their sizes range from hundreds of nanometerstofewmicrometers.So,thisprocesshasnotallowed producingmonodispersednanoparticles.However,theproposed synthesiswayhasthefollowingadvantages:

– theproducedsubmicrometricirondendriteshaveahighspecific surface(perimetertosurfaceratioof1.3&108m#1comparable

toananoparticleof30nmdiameter),theyareconsequentlywell suitedforcatalysisapplications

– thepurityoftheproducedsuspensioniswellhigher(!83%)than the one obtained with other wet-chemical synthesis ways (colloidalandsonoelectrochemical),thepurificationstepcould besuppressedifitisnotrequiredintheapplication

– theinitialsolutioncontainsonlyacheapferroussalt

Consequently, this synthesis way could be used for cost effective and rapid production of “readyto use” iron particles mainlyforcatalysispurposes.

This work is the first step for the development of a new synthesisway,thedeviceandthemethodcouldbeoptimizedon severalpoints.Concerningtheelectrodepositionphase,itiswell knownthatthemorphologyofthedepositedmetaldependsonthe counter-ion and on the presence of additives (surfactants, polymers,chlorides, nitrates,...)andthusthestructureof the particlescouldbetunedbythesolutioncomposition.Thiscould leadtotheproductionoftailoredparticlesaswellastoeasierto fragmentdeposits.Concerningthefragmentationphase,here,the oscillationsofthenaturallyco-producedH2bubbles,inducedby

thevibrationsofthePZT,areexploited.Microfluidic-likestrategies couldbeemployedtocontrolpreciselythelocationofoscillating bubbles inside the channel to improve the efficiency of the fragmentationphaseandthusreachevensmallerparticles.Finally, this synthesis could be extended to other metals, the only prerequisitebeingthatthemetalhastobeabletoformramified branches.

Acknowledgements

The authors are very grateful to M. L. de Solan-Bethmale (Laboratoire de Génie Chimique), S. Le Blond du Plouy and L. Weingarten(CentredemicrocaractérisationRaimondCastaing)for SEMandTEMobservations.

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Figure

Fig. 2. Schematic representation of the Electrochemical and vibrating Hele-Shaw cell used in this study.
Fig. 3. Synthetic results of various electrolyses carried out at various current densities and for various concentrations of the iron precursor (FeCl 2 ) without supporting electrolyte
Fig. 4. Optical images taken during galvanostatic electrolyses in the Hele-Shaw cell for a) 0.02 M FeCl 2 , j = 12 mA/cm 2 and b) 0.1 M FeCl 2 , j = 16 mA/cm 2
Fig. 7. SEM pictures of iron electrodeposits obtained in the Hele-Shaw cell at various current densities and using various precursor concentrations
+2

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