[Back]

G. Parkinson:
"Iron oxide surfaces";
Surface Science Reports, 71 (2016), 272 - 365.

Thecurrentstatusofknowledgeregardingthesurfacesoftheironoxides,magnetite(Fe3O4), maghemite(γ-Fe2O3), haematite(α-Fe2O3), and
wüstite(Fe1 xO) isreviewed.Thepaperstartswithasummaryofapplicationswhereironoxidesurfacesplayamajorrole,includingcorrosion,
catalysis,spintronics,magneticnanoparticles(MNPs),biomedicine,photoelectrochemicalwatersplittingandgroundwaterremediation.Thebulk
structureandpropertiesarethenbriefly presented;eachcompoundisbasedonaclose-packedanionlattice,withadifferentdistributionand
oxidationstateoftheFecationsininterstitialsites.Thebulkdefectchemistryisdominatedbycationvacanciesandinterstitials(notoxygen
vacancies)andthisprovidesthecontexttounderstandironoxidesurfaces,whichrepresentthefrontlineinreductionandoxidationprocesses.Fe
diffusesinandoutfromthebulkinresponsetotheO2 chemicalpotential,formingsometimescomplexintermediatephasesatthesurface.For
example, α-Fe2O3 adoptsFe3O4-likesurfacesinreducingconditions,andFe3O4 adoptsFe1 xO-likestructuresinfurtherreducingconditionsstill.It
is arguedthatknownbulkdefectstructuresareanexcellentstartingpointinbuildingmodelsforironoxidesurfaces.
The atomic-scalestructureofthelow-indexsurfacesofironoxidesisthemajorfocusofthisreview.Fe3O4 is themoststudiedironoxidein
surface science,primarilybecauseitsstabilityrangecorrespondsnicelytotheultra-highvacuumenvironment.Itisalsoanelectricalconductor,
which makesitstraightforwardtostudywiththemostcommonlyusedsurfacesciencemethodssuchasphotoemissionspectroscopies(XPS,
UPS) andscanningtunnelingmicroscopy(STM).Theimpactofthesurfacesonthemeasurementofbulkpropertiessuchasmagnetism,the
Verwey transitionandthe(predicted)half-metallicityisdiscussed.
The bestunderstoodironoxidesurfaceatpresentisprobablyFe3O4(100);thestructureisknownwithahighdegreeofprecisionandthe
majordefectsandpropertiesarewellcharacterised.AmajorfactorinthisisthataterminationattheFeoct-O planecanbereproducibly
preparedbyavarietyofmethods,aslongasthesurfaceisannealedin10 7 10 5 mbar O2 in the finalstageofpreparation.Such
straightforwardpreparationofamonophaseterminationisgenerallynotthecaseforironoxidesurfaces.Allavailableevidencesuggeststhe
oft-studied (√2 √2)R451 reconstruction resultsfromarearrangementofthecationlattice intheoutermostunitcellinwhichtwooctahedral
cations arereplacedbyonetetrahedralinterstitial, a motifconceptuallysimilartowell-knownKoch-CohendefectsinFe1 xO. Thecation
deficiency resultsinFe11O16 stoichiometry,whichisinlinewiththechemicalpotentialinultra-highvacuum(UHV),whichisclosetothe borderbetweentheFe3O4 andFe2O3 phases.TheFe3O4(111)surfaceisalsomuchstudied,buttwodifferentsurfaceterminationsexistclose
in energyandcancoexist,whichmakessamplepreparationanddatainterpretationsomewhattricky.BoththeFe3O4(100)andFe3O4(111)
surfacesexhibitFe-richterminationsasthesampleselvedgebecomesreduced.TheFe3O4(110)surfaceformsaone-dimensional(1 3)
reconstructionlinkedtonanofaceting,whichexposesthemorestableFe3O4(111)surface. α-Fe2O3(0001)isthemoststudiedhaematite
surface,butdifficulties preparingstoichiometricsurfacesunderUHVconditionshavehamperedadefinitivedeterminationofthestructure.
There isevidenceforatleastthreeterminations:abulk-liketermination attheoxygenplane,aterminationwithhalfofthecationlayer,anda
terminationwithferrylgroups.Whenthesurfaceisreducedtheso-called "bi-phase" structureisformed,whicheventuallytransformstoa
Fe3O4(111)-liketermination.Thestructureofthebi-phasesurfaceiscontroversial;alargelyacceptedmodelofcoexistingFe1 xO and α-
Fe2O3(0001)islandswasrecentlychallengedandanewstructurebasedonathin film ofFe3O4(111)on α-Fe2O3(0001)wasproposed.The
meritsofthecompetingmodelsarediscussed.The α-Fe2O3(1102) "R-cut" surfaceisrecommendedasanexcellentprospectforfuturestudy
given itsapparenteaseofpreparationanditsprevalenceinnanomaterial.
In thelattersectionstheliteratureregardingadsorptiononironoxidesisreviewed.First,theadsorptionofmolecules(H2, H2O, CO,CO2, O2,
HCOOH, CH3OH, CCl4, CH3I, C6H6, SO2, H2S, ethylbenzene,styrene,andAlq3) isdiscussed,andanattemptismadetorelatethisinformation
to thereactionsinwhichironoxidesareutilizedasacatalyst(water-gas shift,Fischer-Tropsch, dehydrogenationofethylbenzenetostyrene)or
catalyst supports(COoxidation).Theknowninteractionsofironoxidesurfaceswithmetalsaredescribed,anditisshownthatthebehaviouris
determined bywhetherthemetalformsastableternaryphasewiththeironoxide.Thosethatdonot,(e.g.Au,Pt,Ag,Pd)prefertoformthree-
dimensional particles,whiletheremainder(Ni,Co,Mn,Cr,V,Cu,Ti,Zr,Sn,Li,K,Na,Ca,Rb,Cs,Mg,Ca)incorporatewithintheoxidelattice.
The incorporationtemperaturescaleswiththeheatofformationofthemoststablemetaloxide.Aparticulareffortismadetounderlinethe
mechanisms responsiblefortheextraordinarythermalstabilityofisolatedmetaladatomsonFe3O4 surfaces, andthepotentialapplicationofthis
model systemtounderstandsingleatomcatalysisandsub-nanoclustercatalysisisdiscussed.Thereviewendswithabriefsummary,anda
perspective isofferedincludingexcitinglinesoffutureresearch.
& 2016 ElsevierB.V.Allrightsreserved.