University of Groningen Controlling vacancies in ... · converted to a mixture of pyrrhotite (Fe 1-xS) and either pyrite (FeS 2) or sulfur when heated in air between 180 to 200 °C,

University of Groningen

Controlling vacancies in chalcogenides as energy harvesting materialsLi, Guowei

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Chapter 3

CHAPTER3

HighpurityFe3S4greigitemicrocrystalsformagneticandelectro-chemicalperformance2

In this chapter, we investigated the fundamental properties of a rarelyreportedironsulfide:greigite(Fe3S4).HighpurityFe3S4isfirstsynthesizedbya simple hydrothermal method. The as-prepared material has the largestsaturation magnetization than of all reported values and showed no lowtemperature transitionbetween5K-300K.Ramanspectraofgreigiteweremeasured both in air and vacuum, and were in good agreement withtheoreticalcalculations.Theapplicationofthesynthesizedcrystalsasanodematerials of lithium ion batteries is also investigated. The results showedexcellent initial capacity and possible reactions occurring at the electrodearealsodiscussedhere.

2ThischapterisbasedonG.Li,B.Zhang,F.Yu,A.ANovakova,M.SKrivenkov,T.YKiseleva,L.Chang,J.Rao,A.OPolyakov, G. R Blake, R. de Groot, T. T.M. Palstra, High-Purity Fe3S4 greigitemicrocrystals formagnetic andelectrochemicalperformance,Chem.Mater.,2014,26(20),5821–5829.

HighPurityFe3S4GreigiteMicrocrystalsforMagneticandElectro-chemicalPerformance

38

3.1Introduction

Greigite (Fe3S4) was discovered in silt and clay sediments in 1964 in California.1 Itsformationaccompanies thedecompositionoforganicmatter in thebiogeochemical andbacterial sulphate reduction process.2 However, greigite was initially believed to bepalaeomagneticallyunimportantdue to its thermodynamicmetastabilityand lackofanylastingrecordongeologicaltimescales.3Thistraditionalviewpointchangedwhengreigitewas discovered in the sediments of Loch Lomond, indicating greater thermodynamicstabilitythanpreviouslyproposed.4,5Interestingly,greigiteprovidescluesinthesearchforlifeonMars.InthemartianmeteoriteALH84001,whichisproposedtohavecrystallizedfrommolten rock 4 billion years ago, an elongatedmulticrystalline core of greigitewasfoundinsideanorganicenvelope.6,7Thismaybefromthefossilremainsofmartianbiota.Greigite is also of interest in modern material science. Recent band structurecalculationsrevealedthatFe3S4showsacomplexFermisurfacewithauniqueinfluenceofrelativisticeffects:twosheetsoftheFermisurface(dis)appeardependingonthedirectionofanappliedmagnetization.Thisenablesspintronicsonthe levelofasinglecompound,rather than using traditional heterostructure devices.8 Fe3S4 is also a potential anodematerial in lithium-ion batteries (LIBs). The use of Fe3O4 in LIBs has been researchedextensively because of its high theoretical capacity.9 As discussed later, Fe3S4 has atheoreticalcapacityof785mAh/g,twotimeshigherthantheconventionalanodematerialgraphite (372mAh/g). Considering that greigite is nontoxic and abundant, it is an idealmaterialforhighperformanceLIBs.Furthermore,greigitealsohaspotentialapplicationsinhydrogenstorage,cancerhyperthermia,andmagneticguideddeliveryofdrug.10-12Greigite is an iron thiospinel and has the same inverse spinel structure as its oxide

counterpartmagnetite,Fe3O4.ThecrystallographicstructureofFe3S4isdisplayedinFigure1.TheunitcellconsistsofeightFe3S4moieties(spacegroup:Fd-3m).TheSatomsformaface-centered-cubic lattice, inwhich 1/8 of the tetrahedralA-sites are occupied by Fe3+and1/2oftheoctahedralB-sitesareequallyoccupiedbyFe2+andFe3+.Aneutronpowderdiffractionstudyindicatedacollinearferromagneticstructureinwhichtheironmomentson the tetrahedral and octahedral sites are antiparallel.13 No spin canting or significantcationvacancyconcentrationwasobservedforeithersublattice.14Itisgenerallyacceptedthatthemagneticeasyaxisofgreigite is[100]atalltemperatures,ratherthanthe[111]directionofmagnetite.However,thereisnoexactexperimentalconfirmation.15

Despite the studies referred to above, greigitehas receivedmuch less attention thanwell-studied Fe3O4due to itsmetastablenature. It hasbeendemonstrated that Fe3S4 isconvertedtoamixtureofpyrrhotite(Fe1-xS)andeitherpyrite(FeS2)orsulfurwhenheatedinairbetween180to200°C,andfinallytoFe3O4andmaghemite(γ-Fe2O3).Althoughitisrelativelystableinargongas,Fe3S4stilldecomposesabove250°C.16Theabsenceofpuresamples of either natural or synthetic greigite has thus far hindered precisedeterminationsofitsphysicalandchemicalproperties,includingsaturationmagnetization,Curie temperature Tc, the first anisotropy constant K1, and the electrical conductivity.Severalsynthesismethodshaverecentlybeenreported.Akhtaretal.17preparedprecursordithiocarbamato Fe3+ complexes and then performed thermolysis in oleylamine atdifferent temperatures. Greigite was always the dominant product but often coexistedwith a lower concentration of FeS. Zhang et al.18 prepared relatively pure Fe3S4

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nanoparticlesbyheatingaFe(Ddtc)3(Ddtc=diethyldithiocarbamate)precursorinanoleicacid/oleylamine/1-octadecenesolvent.Hydrothermalmethods,whicharecarriedoutathigh pressure (>2MPa) and low temperature (<300 °C) have been widely used in thesynthesisofnanoarchitecturedmaterials.19,20Forexample,ahydrothermalmethodinanexternal magnetic field allowed either greigite or marcasite (FeS2) to be selectivelysynthesizedintheformofmicrorods.21Usingasimilarmethod,Changetal.22synthesizedpolycrystalline greigite. It is difficult to obtain pure synthetic greigite because mostprocedures simultaneously produce other iron sulfides such as mackinawite (FeS) andpyrite(FeS2).ThisisoftenapparentinpreviousreportsfrompoorqualityX-raydiffractionpatternsthatcontainimpuritypeaksandbroadgreigitepeaksthatimplypoorcrystallinity.Lowvaluesof thesaturationmagnetization,Ms,also implypoorqualitysamples,with inmostcasesMs<2.5μB/fucomparedtotheexpected4μB/fu(seesummaryinRef.24).23-25

Figure 3.1 Crystal structure of Fe3S4 with the (001) and (111) planes outlined in blue and black,respectively.Sulfuratoms(yellowspheres)formacubicclose-packedlattice:1/8ofthetetrahedralAsitesareoccupiedbyFe3+(bluespheres)and½oftheoctahedralBsitesareoccupiedbyFe2+andFe3+(redspheres)equally.ThemagneticmomentsontheAandBsitesareantiparallelandalignedalongthe[100]crystallographicaxis(indicatedbyarrows).

Herein,a simplehydrothermalmethod isdeveloped tosynthesizehighpuritygreigite.By carefully controlling the reaction temperature, reaction time, and the quantity ofsurfactant,greigitemicrocrystalswitha(truncated)octahedralshapeandsize~1μmcanbesynthesized.Theas-preparedproducthasalargersaturationmagnetizationandlowerresistivity thanallpreviousreports.TheperformanceofhighpuritygreigiteasananodematerialinLIBsisalsostudied.Ahighcapacityismaintainedupto100cycles,makingitanexcellentprospectiveelectrodematerial.

3.2Method

3.2.1Synthesis


40

Nitrogen gaswas bubbled for at least 30minutes through 35mL of H2O to remove alldissolvedoxygenbeforethesynthesis.Inatypicalexperiment,(0.6mmol,0.2187g)ofthesurfactant cetrimonium bromide (CTAB) was dissolved in the water under continuousstirringtoformasolution.After10minutes,(3mmol,0.365g)ofL-cysteineand(2mmol,0.2535 g) of FeCl2were added to the solution. The solutionwas stirred for another 10minutes, and then transferred into a 50mL Teflon-sealed autoclave. The autoclavewaskeptat165°Cfor40hbeforebeingcooledtoroomtemperatureinair.Ablackprecipitatewas collected and washed with distilled water and ethanol three times. Finally, theproductwasheatedat60°Cundervacuumfor8h.

3.2.2Characterization

Room temperature power X-ray diffraction (XRD) datawere recordedwith a Bruker D8AdvancediffractometerequippedwithaCuKαsource(λ=0.15406nm).XRDdataat20Kwere collected using a Huber G670 diffractometer operating with Cu Kα radiation andequipped with a closed-cycle refrigerator. The morphology and crystal structure wereexamined using a Philips XL 30 scanning electron microscope (SEM) and a JEM 2010Ftransmission electronmicroscope (TEM) operated at an accelerating voltage of 200 KV.The magnetization was measured using a Quantum Design MPMS-XL7 SQUIDmagnetometer. Mössbauer spectra were measured at room and liquid nitrogentemperatures using a constant acceleration spectrometer MS-1104M with a 57Co(Rh)radiation source. For measurement of the electronic properties, a rectangular shapedpolycrystallinesamplewaspreparedbypressingFe3S4particlesunder3×10

7Papressurefor 15 min. The electrical contacts were made using Pt wire (0.05 mm in diameter)connected to the sample by silver paint. The measurements were performed using acommercial Quantum Design Physical Properties Measurement System (PPMS) and anAgilent 3458a multimeter. Raman spectra were measured in a backscatteringconfigurationusingaliquidnitrogencooledchargedcoupleddevice(CCD)connectedtoathree grating micro-Raman spectrometer (T6400 Jobin Yvon). The incident laser powerwas limited to 0.5 mW to avoid oxidation of the sample usingexcitationwavelengthsof632.8nm.DetailsofphononcalculationsrelatedtotheRamanspectroscopyaredescribedintheSupportingInformation.

3.2.3Electrochemicalmeasurements

Tofabricatetheanodeofacoincellbattery,theas-obtainedFe3S4powderwasmixedwithacetylene black and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 in N-methyl-2pyrrolidinone(NMP).Theobtainedslurrywascoatedontocopperfoil,driedat120°Cfor12h,andthenpunchedintoroundplatesofdiameter12.0mmtoformanodeelectrodes. Finally, the prepared anode, a Celgard2400 separator (diameter 16.0mm), alithium cathode, and an electrolyte consisting of 1M LiPF6 in ethylene carbonate (EC) /diethylcarbonate(DEC)/ethylmethylcarbonate(EMC)(1:1:1vol.%)wereassembledintoa coin cell (CR2032) in an argon filled glove box (H2O and O2<1ppm). The coin cellspreparedwere kept at room temperature for tenminutes at 4.2V during charging andexamined using a Maccor Series 4200 standard battery test system at various

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charge/dischargeratesbetween0.01-3V.Cyclicvoltammetry(CV)wascollectedusinganAutolabPGSTAT30electrochemicalworkstation.

3.3Resultsanddiscussion

3.3.1Phaseandmorphologycharacterization

Fig.3.2Observed(blackdatapoints),calculated(redline)anddifference(lightblueline)XRDpatternsoftheas-preparedsampleatroomtemperature.

Observed,calculatedanddifferencepowderXRDprofilesoftheobtainedproductatroomtemperature are presented in Fig. 3.2. All the peaks can be indexed in the cubic Fe3S4spinel structurewithspacegroupFd-3m(PDFcardNo.16-0713).Nopeaksbelonging tosulfur or to other iron sulfides or oxides were observed, implying that no crystallineimpuritieswerepresentatmorethan1%byweight.Therefinedlatticeparameterwasa=9.8719 (1) Å, which is in good agreement with the reported value for greigite.26 Therefinedatomic coordinateof sulfuron the32e sitewasx=y= z=0.2546(1), inperfectagreement with calculations.8 Other than a contraction of the unit cell, the refinedstructureat20Kwasessentiallythesameasatroomtemperature;detailsaregivenintheSupporting Information Figure S1. The molar ratio of Fe to S was 2.99:4 according toenergy-dispersive X-ray spectroscopy (EDS) analysis (Figure S2), consistent with astoichiometric Fe3S4 sample. It should be noted here that the synthesis of pure,stoichiometric greigite requires carefully optimized experimental parameters. A smalldifferenceinthereactiontemperature,thequantityofCTAB,andthepurityofthestartingmaterials (in particular FeCl2) will lead to the formation of second phases such as S orFe2O3.


42

Figure 3.3 (a) SEM image of Fe3S4 crystals synthesized in this work: three typical octahedra arepresented in the upper left inset, and the corresponding elementalmaps are shown in the right-handpanel (yellowforFe,andblueforS). (b)TEMimageof twoFe3S4crystalsandcorrespondingSAEDpattern(inset).(c)HRTEMlatticeimageoftheedgeandcorner(inset)ofatypicalcrystal.ThemorphologyofthesamplewascharacterizedbySEMandTEM,asshowninFigure3.The SEM images in Figure 3.3a reveal that the product consists of well dispersedmicrocrystals with uniform (truncated) octahedral shapes. These (truncated) octahedraarefullydevelopedandcomposedofeight27planeswithameanedge lengthof~1μm.Elemental mapping of the crystals indicates that the S and Fe are distributedhomogeneously. It has been demonstrated that greigite is very sensitive to oxygen andcanbeoxidizedto ironoxide,especially inwetconditions.16However,wedidnotdetectoxygenonthesurfaceofthecrystals,whichimpliesthathighpuritygreigiteismorestablethan expected. In accordance with the SEM results, TEM images (Fig. 3.3b) show themorphologyof the crystals inmoredetail. The selectedareaelectrondiffraction (SAED)patternrecordedalongthe[1-34]zoneaxisdirectioncanbeuniquelyindexed.Althoughsomestacking faults canbe identified in thehigh-resolutionTEM (HRTEM) image inFig.3.3c(indicatedbyarrows),theclearlatticefringesconfirmthattheentireoctahedronisasinglecrystal.Thespacingofthefringes is0.57nm,whichcorrespondswelltothe(111)interplanardistanceingreigite.

3.3.2Mössbauerspectra

Fig.3.4ashowstheroomtemperature(RT)57FeMössbauerspectrumofoursample.Thecorrespondingspectrumat80KisshowninFigureS3.Bothspectrawerebestfittedwiththree sextets corresponding to one tetrahedral (A site) and two magnetically non-equivalentoctahedral (Bsites).Thehyperfine interactionparametersextractedfromthefitsarelistedinTableS1.Wedidnotdetectsignalsbelongingtoanyparamagneticphase(FeS2)oroxidationproduct(Fe3O4orFe2O3),incontrasttopreviousstudies.

28Theisomer-shiftsfortheAsiteare0.27and0.37mm/satRTand80K(FigS3),respectively,whichareattributedtothehighspinFe3+state.29ThehyperfinefieldoftheAsiteis3.14TatRT,andincreasesslightlyto3.19Tat80K.ThefitunambiguouslyindicatesthatonlyFe3+occupiesthetetrahedralsiteinthistemperaturerange.Thisimpliesthatgreigitehasafullyinversespinelstructure.AsshowninTableS1,theisomershiftsoftheBaandBbsextetsare0.53

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and0.54mm/satRT,and0.67and0.65mm/sat80K.(BaandBbrepresentthetwonon-equivalentB sites, the detailedmeaning of which is explained below.) Thus, both siteshavethesameisomershiftwithinexperimentalerror.However,thehyperfinefieldoftheBa site is always larger than that of theBb site (3.14 T versus 3.05 T at RT, and 3.29 Tversus 3.19 T at 80 K). The same result was recently obtained in a study of greigitenanoparticlesofdifferentsizes,butherethethirdsextetwasattributedtothehexagonalsmythite phase.30 Smythite was first assigned the same chemical formula as greigite,Fe3S4,

31butthiswassubsequentlyrevisedtoFe9S11.32Thedifferenceinsymmetrybetween

smythiteandgreigitemeansthattheycaneasilybedistinguishedbyXRD.Consideringthehighpurityofour sample, it is reasonable toassume that the third sextet is intrinsic togreigite.

Fig.3.4(a)Roomtemperature57Fe-MössbauerspectrumoftheFe3S4microcrystalsinzerofield.Oneoctantofthecubicunitcellofgreigiteisshown(inset),wheretheyellowspheresaresulfurandtheredandgreenspheresdenotetwonon-equivalentoctahedralsiteironcations.(b)57Fe-Mössbauerspectruminamagneticfieldof1T.

ThevalueoftheisomershiftshowsthatvalencestateoftheB-siteironionsisbetween2+and3+,asit isformagnetiteduetofastelectronhopping.WealsonotethatHhypfortheA site increasedwithappliedexternalmagnetic field,whereasHhyp fortheBaandBbsitesdecreasedbynearlythesamevalues(seeFigure4BandTableS1).Bothobservations– the same isomer shifts and the same reaction to the externalmagnetic field, confirmthatthethirdcomponentofthespectracorrespondstotheB-siteironions.Thenatureoftwonon-equivalentB-sitepositionsformagnetite isdiscussed inRef.29andwetendtothesameexplanationforgreigite.For a dominatingmagnetic hyperfine interaction, theMössbauer resonance lines areshifted according to first-order perturbation theory by an amount proportional to


44

(3cos7 θ − 1),whereθ is the angle between the local hyperfine fieldHhyp and the localsymmetryaxis(the[111]axesfortheB-site).33Ifweassumethatthedirectionoftheeasymagnetizationforgreigiteis[001]asgenerallyaccepted,thentheanglebetweenthemainaxisoftheEFG(electricfieldgradienttensor)andHhypwouldbethesameforallfourFeions.ThisimpliesthatonlyonesextetwouldbeobservedfortheBsite,whichcontradictsour experimental data. If instead the easy magnetization direction is along [111] as inmagnetite,thenθ=90°for¾oftheB-sitecations(redspheresinFigure4a)andθ=0fortheremaining¼oftheB-sitecations(greensphereinFigure4a).ThiswillyieldtwoB-sitesubspectrawithrelative intensitiesof3 :1.Forourdatatheratio is2.6 :1at300Kbutonly 1.8 : 1 at 80 K. Two B-site sextets with an intensity ratio of 3 : 1 have also beenobserved inMössbauer spectra of Fe3O4 above the Verwey transition.

33-35 Furthermore,ferromagnetic resonance (FMR) spectroscopy on greigite indicates a negativemagnetocrystallineanisotropyconstantK1,thesameasinmagnetite.36Inacubiccrystal,thelowestordertermsinthemagnetocrystallineanisotropyenergycanbewrittenas

E/V=K1(α2β2+β2γ2+α2γ2)+K2α

2β2γ2,where α, β and γ are the direction cosines of themagnetization. Thus, the easy axis isdeterminedbybothK1andK2ifK2isnotassumedtobezero.TheworkofWinklhoferetal.indicatesasimilarvalueofK2/K1of+0.30to0.33fordifferentanisotropymodels.37Thisclearlyindicatesaneasyaxisinthe[111]direction.Thus,wecanconcludethatgreigitehasan easy magnetization axis along the [111] direction, rather than the [100] directionreportedinpreviouswork.WeconcludethatthedirectionofHhypwithrespecttotheeasyaxis is themost likely explanation for the two B-site sextets in greigite. The significantdifferencefromthetheoreticalintensityratioof3:1at80Kmightindicatethattheeasyaxis moves away from [111] at low temperature; for example, a 1 : 1 ratio would beexpected foraneasyaxisof [110].33Theapplicationofanexternalmagnetic fieldalignsthespinsinonedirection.Indeed,thisratioincreasedto2.8:1whenamagneticfieldof1Tparalleltotheγbeamwasappliedat300K(Fig.3.4bandTableS1).WenotethatHhypfortheAsiteincreasedby8kOewithappliedfield,whereasHhypfor

theBaandBbsitesdecreasedby9kOeand11kOe,respectively.ThisalsoprovesthattheFeionsontheAandBsublatticesareantiferromagneticallycoupledviasuperexchange.

3.3.3Magneticproperty

Magnetichysteresis loops at 5K and300Kofour greigite sample arepresented in Fig.3.5a and Figure S4, respectively. Typical ferrimagnetic behavior is observed in themeasured temperature range, with a coercive field Hc = 92 Oe and a remanentmagnetizationMr=0.4μB/f.u.at5K.Considering thatFe3S4 isa softmagneticmaterialaccordingtothehysteresisloopsandthatthecrystaldimensionisinthemicrometerscale,wecanapproximatethesaturationmagnetizationwith:

M=Ms(1–aH-1–bH-2)+cH1/2,

where M is the magnetization, Ms is the saturation magnetization, a, b, and c areconstants that describe the structural inhomogeneity within the sample, the magneticanisotropy energy and the paraeffect caused by the external field, respectively.38 Thefitting of the magnetization curves in the first quadrant for the applied fieldH >Hc isdisplayedinFig.3.5a,whichshowsexcellentagreementwiththeexperimentaldata.The

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magnetizationcurveisdeterminedbymicrostructuralinhomogeneitiesinthelowexternalfieldrange.ByplottingtheMversusH-1,a linearrelationshipisobtainedbelow1.2T,asshown in Fig. 3.5b. The slope of 0.1146 Oe indicates that these microstructuralinhomogeneitiesactasstresscentersinthespinalignmentaroundthem.Thedeviationofthelinearrelationshipathighfieldcanbeattributedtotheparaprocess.Thiscanbeseenin the variationofMs as functionofH

1/2(Fig. 3.5b) for fields > 3.2 T.Unfortunately,wecannot compare the level of inhomogeneity and the paraprocess in Fe3S4with previousdataintheliterature.Nevertheless,wecanestablishprecisevaluesofMsof3.74μB(70.56emu/g)at5Kand3.51μB(67.16emu/g)atRT.Thesevaluesaresignificantlylargerthaninpreviousreportsofgreigite(thehighestreportedvaluewasMs=3.4μBat5K

13).ThehighMs allows easy separation of the greigite crystals from the solution when a magnet isplacedneartheglassbottle(seeinsettoFig.3.5a).Coeyetal.predictedassumingapurelyionicmodel thatMs should be 4μB.

34 Themagnetizationmeasured in our currentworkapproachesthisvalueandisanindicatorofthehighqualityofthesample.However,itisimportanttoaddressthedifferencebetweenourexperimentalandexpectedvalues.Itisprobable that the moment is lowered from 4 μB due to covalency: our recent bandstructurecalculationspredictedamagnetizationof3.38μBingreigite.

8Similarresultshavealsobeenreportedbyothergroups.23,24FirstprinciplecalculationsbasedontheGGA+UmodelwithUeff=1.16eVgivesub-latticemagnetizationsofmA=3.05μBandmB=3.25μB,bothofwhicharesignificantlydecreasedcomparedwiththepurelyionicmodel.Generally,the increase in covalency is causedby theoverlapofwave functionsbetween Fe and Sions,whichfromthebandpointofviewcorrespondstoahigherdegreeofhybridizationbetween the sulfur 3p and iron 3d bands. Thus, the ordered moment is lowered. Theincreasedcovalencyeffectisalsosuggestedbythelowerhyperfinefieldsingreigitethanmagnetite,39 with values in the range 30.5-31.4 kOe in greigite and 45.8-49.2 kOe inmagnetite.29


46

Fig. 3.5 (a) Magnetization versus field loop measured at 5 K. The inset shows the magneticseparation of Fe3S4 crystals from aqueous solution using an external magnet. (b) MagnetizationversusH-1(black)andH1/2(blue)fortheloopsat5K.(c)FORCdiagramofthecrystals.

Anothercontributiontothereductionofthesaturationmagnetizationcouldbecausedby surface spins. Previous research identified the existence of a core of aligned spinssurroundedbyashellwithmomentsinclinedtothedirectionofthenetmagnetizationinbothCoFe2O4andNiFe2O4 inversespinelnanocrystalswithsinglemagneticdomains.Wenotethatasmallproportionofparticlesinoursamplehavesizessmallerthan10nm(asindicatedbytheblackarrowinFig.3.3bandFigureS5).AloweredmagneticmomentwasalsoobservedinNiFe2O4coatedwithanorganicsurfactant(oleicacid)duetosurfacespincanting.40-42Ifwemagnifyourmagnetizationloopsat5K,itisclearlyvisiblethattheloopis open (inset to Fig. 3.5a) for both positive and negative field sweeps up to 6 T. Thisopeningindicatesthatsomeofthemagneticspinshavea“switchingfield”largerthan6T.A similar phenomenonwas also observed inmilledNiFe2O4 spinel.

43We recorded first-order reversal curve (FORC)diagrams,44which are able toprobedomain states and theextent of magnetostatic interactions (Fig. 3.5c). The as-prepared samples have FORC

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distributionswithconcentricinnercontourswithhighcoercivityandstrongmagnetostaticinteractions. This is characteristic for pseudo single-domain (PSD) greigite.45 It is wellknownthatthespinsarecompletelyalignedbyexchangeinteractionsinsufficientlysmallPSD crystals, and that the rotation barriers induced by magnetocrystalline andmagnetoelasticanisotropycantrapparticlesintwoormoremetastableorientations.43Weshould thus carefully consider the role of CTAB in the synthesis. It has been shown inpreviousworkthatCTABcanbondtometalcationsviathepolarendofthemolecule.Inourcase,the(C19H42)N

+groupinCTABwillinteractwiththeFe2+orFe3+onthe(111)facesofthegrowingcrystals(Fig.3.3).Thesurfaceenergyofthesefacesisthusdecreasedandfinallypreservedafterthehydrothermalreaction.46Thisresultsinoctahedralcrystalswitheight27facesasshownintheSEMimages.Theinfluenceofthesurfactantisstillpresentinthe final product, because complete removal of the surfactant from the crystals isdifficult.47 TheCTABgroupswill interactwith the Fe2+ or Fe3+ at the surface, and thusresultinasurfacespincantingandalowerMs.

47ThereductioninMsis4.2%inoursample,muchsmallerthanthereductionofnearly20%innanocrystallineNiFe2O4butcomparablewiththatinsurfactant-coatedFe3O4.

42

3.3.4Ramanspectraofgreigite

Fig.3.6RoomtemperatureRamanspectraofFe3S4measuredinvacuum(black)andair(red).

Thelatticedynamicsofgreigitehasbeenlittlestudied;thereiscurrentlyonlyonereportonRamanspectroscopyofgreigiteatroomtemperature.48Thissamplewascontaminatedby mackinawite (FeS) and precise information on the positions of the greigite Ramanpeaksisstilllacking.Agrouptheoryanalysisoninverse-spinelgreigite(spacegroupFd-3mwithareducedunitcell(Fe6S8)thatcontains14atoms)predictsfiveRamanactivemodes:A1g,Eg,andthreeT2g.

49


48

A1g Eg T2g1 T2g2 T2g3

Exp. 365 181 350 252 140

Cal. 327 180 312 238 142

Ionsinvolved S S S, Fetera S, Fetetra S, Fetetra

Table3.1Measuredandcalculatedzonecenterphononfrequencies

The measured Raman spectrum of greigite in vacuum is shown in Fig. 3.6, and thecalculatedandexperimental frequenciesof theRamanactivemodesareshown inTable3.1. TheA1gmode represents the stretching of S atoms towards the tetrahedral site Featom.Thecalculatedfrequencyisunderestimatedby10%comparedtotheexperimentalvalue. The Eg mode represents the bending of S – Fetetra – S bonds. The calculatedfrequencyagreeswellwith theexperimentalvalue.ThreeT2gmodes thatoriginate fromtheasymmetricbendingofFe–Owerealsoobserved.For theT2g1andT2g2modes, thecalculated frequencies are underestimated by 11% and 5.5%. For the T2g3 mode, thecalculatedfrequencyagreeswellwithexperiment. Interestingly,whenthemeasurementwascarriedout inair,only threeRamanmodeswith frequenciesof223cm−1,290cm−1,and405cm−1werefound.ThesecanbeassignedtotheT2g3,EgandT2g2modesofFe3O4,respectively.50Becausegreigiteismorecovalentthanmagnetite,thefrequenciesofallthegreigitemodesarereducedbyapproximately60%withrespecttomagnetite.TheRamanlinesofgreigitenearlydisappearedinair,whichindicatesthattheincidentlaserinducedoxidationofthesample.

3.3.5Electricaltransportproperties

Thehighpurityofoursampleoffers theuniqueopportunity to investigate theelectricaltransport properties in Fe3S4. Four-probe resistancemeasurements were performed onpressed bar-shaped polycrystalline samples as illustrated in the inset to Fig. 3.7b. AsshowninFig.3.7b,thelinearI-VcurvesindicatethatOhm’slawisobeyedfrom20KtoRT,implying good contacts between the sample and the electrodes. The temperaturedependenceoftheresistivitywasmeasuredfrom5KtoRTandisshowninFig.3.7b.Theresistivityincreasesfrom10.5mΩcmat5Kto11.2mΩcmat100K,whichischaracteristicbehaviorforametal.Theresistivitythendecreasesonfurtherheatingandreaches~10.8mΩcm at RT, suggesting that there is a cross-over frommetallic to semiconductor-likebehavior at ~100 K. This may originate from the localization of carriers or a change inhopping mechanism with varying temperature.51 Nevertheless, the resistivity is in thepoor-metal range and is 40 times smaller than the values reportedbyCoeyet al.31 andPaolella et al.,51 and 7000 times smaller than the value measured on a single greigitemicrorod.21Thelowerresistivitycanbeattributedtothehighpurityandhighcrystallinityofthemicrocrystals.Itshouldbenotedthatthereisnosharpchangeinresistivityoverthetemperature range measured. The magnetization also changes continuously withtemperature,asshowninFigureS6.Therefore,wecanconcludethatgreigiteexhibitsnoanalogofthemagnetiteVerweytransitionat~120K.

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Fig. 3.7 (a) Four-probe I-Vmeasurements at 20, 100, 180, and 300 K. (b) Resistivity of greigitebetween5Kand300Kandthecorrespondingcontactgeometry(inset).

It has been demonstrated that Fe3O4 is a half metal where the electronic density ofstates is 100% spin polarized at the Fermi level. This has been confirmedby both bandstructure calculations and experiments.52-54 However, our recent electronic structurecalculationsusingthepreviouslyreportedroomtemperaturecrystalstructureofgreigiteshow that it is a good metal.8 We performed analogous calculations using the 20 Kstructureofgreigite.AsseeninFigureS7,threebandsintersecttheFermienergyforboththemajorityandminority-spindirections.Thebanddispersionissomewhatlargerforthemajority-spindirection.Thisbandstructureindicatesthatgreigiteisalsoagoodmetalat20K.Interestingly,wenotethatastudyofepitaxialFe3O4(100)filmsgrownonaW(100)singlecrystalindicatedthatthespinpolarizationattheFermileveldramaticallydecreasedin comparison with bulk samples.55 This implies that the Fe3O4 (100) surface showsmetallic behavior rather than the half metallic behavior exhibited by bulk samples.Considering themicron size and surfactant-coated surfaces of our greigite crystals, it isreasonabletoconcludefromthecurrentevidencethatoursamplesarerepresentativeofbulkgreigiteandthatitshowsmetallicbehavior,especiallyatlowertemperatures.

3.3.6Electrochemicalproperties

Fig.3.8(a)Galvanostaticcharge-dischargecurvesofFe3S4inthevoltagerange0.005-3.0V(versusLi)atacurrentof100mAg-1.(b)Cyclicvoltammogramoftheas-preparedFe3S4electrodeatascanrateof0.1mV•s-1intherange0.01-3.0V(versusLi+/Li).

Usingourgreigite-containingcoincells,weperformedupto100charge-dischargecyclesbetween0.005-3Vatacurrentdensityof100mA/gandatroomtemperature.Theopen-circuitvoltage(OCV)ofthecellsis~3.0VandcurvesforselectedcyclesareshowninFig.


50

3.8a. In the firstcycle thevoltagedecreasessharply to1.7Vandreachesaplateauatacapacityof~150mAh/g.Asecondvoltageplateauisobservedat~1.4Vuptoacapacityof~600mAh/g,followedbyasteadydecreasetothecutoffvoltageof0.01V.Thisindicatesthat thedischargeprocess involvesa two-phase reaction.56Forourgreigitecell the firstintercalationcyclegivesadischargecapacityof1161mAh/gandacorrespondingchargecapacity of 1139 mAh/g. This capacity is 10 times larger than previously reported forbattery anodes comprisedof greigite nanoparticles.51 The first chargeprofile has a longplateauat~1.87Vfollowedbyashorteroneat~2.5V,againconsistentwithatwo-phasereaction.Itisinterestingtonotethattheinitialdischargecapacityismuchhigherthanthetheoreticalvalueof785mAh/g.57ThisphenomenonhasalsobeenreportedforFe3O4andFe2O3andwasascribedtothereversibleformationanddecompositionofapolymericgel-like film on the particle electrode surface. This film is formed by kinetically governedelectrolytedegradationdrivenbyactivemetal(Fe)nanoparticles.58,59Inthesecondcyclethedischargecapacitydecreasesto903mAh/g(Fig.3.8a),whilethechargecapacityis960mAh/g.Thesevaluesfurtherdecreaseto674mAh/gand554mAh/ginthefifthcycle.Thereductionincapacityresultsfromlargevolumechangesofthegreigitemicrocrystalsafterlithium insertion, resulting in disintegration of the crystals and loss of the connectionbetween the electrode materials and the current collector.60 The capacity stopsdecreasingafter~20cyclesandthenincreasesgraduallyto563mAh/ginthe100thcycle(also seeFig. S8).This is stillmuch larger than thecapacityof cellswithgraphiteas theelectrodematerial,320-340mAh/g.59Thereasonforthestabilityofourgreigitecellisnotclear.Wesuggest that thenanostructuredFe0-amorphousLi2Scomposite formedduringthe first discharge reaction needs several cycles to form a stable solid electrolyteinterphase(SEI) film,allowingthegreigitecrystals topercolatethroughoutandestablishintimatecontactwiththecurrentcollector.Inordertounderstandtheelectrochemicalprocess,weperformedcyclicvoltammetry(CV)atascanningrateof0.1mVs-1asshowninFig.3.8b.Inthefirstcycle,thereductionpeakat~1.18Vwitha shoulderat~1.51V indicates the insertionof Li followedby thereduction of both Fe3+and Fe2+ions to Fe0, forming the Fe0-Li2S composite as describedabove.61Thereductionpeakat0.72VisconsistentwiththeformationofaSEIfilm.Thisisalso the main reason for the irreversible capacity during the discharge process. In thesubsequentoxidationscan,thematerialisconvertedtoLi2FeS2at~1.97VandthentoFeSat~2.52Vby the reactionof Li2FeS2with the resultingFe.Thesepeaksnearly coincidewith the twovoltageplateaus in thegalvanostatic charging curve in Figure8A.The firstandsecondCVprofilesaredifferent,indicatingachangeinmechanisminthebattery.Thesharpanodicpeakat~1.97Vremainswhereasthepeakat2.52Vdisappears.Inaddition,thecathodicpeaksareshiftedtomorepositivepotentialsfromthesecondcycleonwardsduetostructuralmodificationafterthefirstcycle.Thereducedpolarizationinthisprocessindicatesbetter reversibility. Thedetailed reaction in the seconddischarge cycle canbedescribedasfollows:

1.87V(1)2FeS+2Li++2e-↔Li2FeS2+Fe1.40V(2)Li2FeS2+2Li+2e

-↔Fe+2Li2SThe cycling performance of the Fe3S4 electrode at a current density of 100 mA/g isdisplayedinFigureS8.Thedischargecapacitydecreasescontinuouslyfrom1161mAh/gat

Chapter3

51

thefirstcycleto310mAh/gatthe25thcycle.Therateofdecreaseincapacityismaximumbetweenthe1stand2ndcyclesandbecomesmuchflatterduringcycles10-25.Theinitialloss of capacitymay result froman incomplete conversion reaction and the irreversibleloss of Li ions due to the formation of a SEI layer as discussed above. The subsequentcapacity losses are perhaps caused by defects in the greigite crystals, the structure ofwhich is shown inFig.3.3c.SmallnumbersofLi ionsmightbe trapped in thesedefects,thus inducing the irreversible capacity.62 Interestingly, the discharge capacity increasesfrom the 25th cycle onwards and reaches 563mAh/g after 100 cycles. This increase ofcapacitymightbesustainedovermorecyclesifthecharge/dischargeprocessisrepeated.Similarresultshavebeenreportedinmanyotherstudies,especiallythosewithsulfidesastheelectrodematerial.62-64Reasonsforthisphenomenonarestillunclear,butitmightbelinked to an activation process in the electrode. As the number of cycles increases, theincreaseincapacityisalwaysaccompaniedbyadecreaseintheelectrodeimpedance.Thecapacitanceof theelectrode / electrolyte interfaceswas alsoobserved to increasewithcycling for somesulfides.64Defectsandvacancies in the crystals,which trap Li ions,willtend tobecomemoreextendedand facilitate the insertionofmore Li ionswith furthercycling. In addition, the high crystallinity and low resistivity of our sample can alsopromote the uniform and stable delivery of electrons, further facilitating thedeintercalation/intercalationofLi ions.Consequently, the transferofelectronsandLi+ ismoreeffectiveattheinterfaceoftheactivematerialsandtheelectrolyte.65The octahedral shape of our crystals, comprising eight (111) surfaces,might be a key

factor in the high electrochemical performance. Figure 1 illustrates the atomicconfigurationsof the (001)and (111) surfaces. The (111)planecontainsamuchgreaterdensity of Fe3+/Fe2+ cations than the (001) plane. Research on the charge/dischargemechanismofspinelstructureshasshownthattheredoxreactionofMn+/M0(whereMisatransitionmetal)isrelatedtothedischargecapacity.66,67Themorphologyofourgreigitecrystals,whichiscontrolledbythesurfactant,willthusfacilitatefastFe3+(Fe2+)/Fe0redoxreactions,resultinginexcellentcyclingperformanceaswellasahighcapacity.

3.4Conclusion

Wehavesynthesizedhighlypure,monodispersegreigitemicrocrystalsusingasurfactant-basedhydrothermalmethod.Wemeasureasaturationmagnetizationof3.74μB,closetotheexpectedvalueof4μB(assumingapurelyionicmodel)andaresistivitythatis40timeslowerthanallpreviousreportsongreigite,indicatingthehighqualityofoursamples.Thesaturation magnetization is slightly lower than 4 μB due to the appreciable degree ofcovalencyinvolvedinFe-Sbonding,aswellasthepossiblecantingofsurfacespinscausedby the presence of surfactantmolecules bonded to the surface. Greigite is different tomagnetite in that it does not exhibit a Verwey transition down to 5 K, the lowesttemperatureinvestigatedhere.However,incontrasttopreviousreports,greigiteissimilartomagnetiteinthatbothmaterialshavea[111]magneticeasyaxis.AsananodematerialforLi-ionbatteries,greigiteexhibitsahighinitialcapacityof1161mAh/gand562.9mAh/gafter 100 cycles. This excellent performance, combined with the fact that greigite isabundant and environmentally friendly,makes it a candidate to replace generally usedgraphite.


52

Supplementaryinformation

Figure S1 Observed (black data points), calculated (red line) and difference (light blue line) XRD

patterns of the as-prepared sample at 20 K.Greigite at 20 K adopts space groupFd-3mwith a =

9.8426(2)Å;therefinedatomiccoordinateofsulfuronthe32esiteisx=y=z=0.2540(5).

FigureS2EnergyDispersiveX-RaySpectroscopy(EDS)spectrumoftheas-synthesizedsample

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53

Figure S3 Fitted 57Fe-Mössbauer spectrum of the Fe3S4 microcrystals at 80 K.

T=300K 80K 300K(with10kOefield)Parameter A Ba Bb A Ba Bb A Ba Bb

δ,mm/s(+-0.01) 0.27 0.53 0.54 0.37 0.67 0.65 0.27 0.54 0.51H,kOe(+-1) 314 314 305 319 329 319 322 305 294S,%(+-2%) 36 46 18 32 44 24 36 47 17

TableS1Hyperfineparametersobtainedfromfittingthe57Fe-MössbauerspectraatT=300K(zero

appliedfield),80K(zerofield),and300K(withanexternalmagneticfieldof10kOe):Histhemagnetichyperfinefieldatthe57Fenuclei,δistheisomershiftrelativetoFe,andSistheareaofthe

spectralcomponent.


54

FigureS4Magnetizationversusfieldloopsmeasuredat300K.Theinsetshowsthehysteresisinmoredetail.

FigureS5TEMimagesshowingtheexistenceofasmallfractionofFe3S4nanoparticleswithsizes<10nm

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55

FigureS6Zerofieldcooled(ZFC)andfieldcooled(FC)magnetizationcurvesofgreigitemeasuredinanappliedfieldof100Oe

FigureS7Calculatedbandstructuresforthemajority(a)andminority(b)spindirectionsofgreigite.


56

FigureS8.CyclingperformanceoftheFe3S4electrodeupto100cycles.

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