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Title:
Atomically-precise colloidal nanoparticles of cerium dioxide
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Nature Commun. 2017, 8, 1445
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Christou, George
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Nature Communications
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Synthesis of truly monodisperse nanoparticles and their structural characterization to atomic precision are important challenges in nanoscience. Success has recently been achieved for metal nanoparticles, particularly Au, with diameters up to 3 nm, the size regime referred to as nanoclusters. In contrast, families of atomically precise metal oxide nanoparticles are currently lacking, but would have a major impact since metal oxides are of widespread importance for their magnetic, catalytic and other properties. One such material is colloidal CeO2 (ceria), whose applications include catalysis, new energy technologies, photochemistry, and medicine, among others. Here we report a family of atomically precise ceria nanoclusters with ultra-small dimensions up to ~1.6 nm (~100 core atoms). X-ray crystallography confirms they have the fluorite structure of bulk CeO2, and identifies surface features, H+ binding sites, Ce3+ locations, and O vacancies on (100) facets. Monodisperse ceria nanoclusters now permit investigation of their properties as a function of exact size, surface morphology, and Ce3+:Ce4+ composition.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by George Christou.

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ARTICLEAtomically-precisecolloidalnanoparticles ofceriumdioxideKylieJ.Mitchell1,KhalilA.Abboud1&GeorgeChristou 1Synthesisoftrulymonodispersenanoparticlesandtheirstructuralcharacterizationtoatomic precisionareimportantchallengesinnanoscience.Successhasrecentlybeenachievedfor metalnanoparticles,particularlyAu,withdiametersupto3nm,thesizeregimereferredto asnanoclusters.Incontrast,familiesofatomicallyprecisemetaloxidenanoparticlesare currentlylacking,butwouldhaveamajorimpactsincemetaloxidesareofwidespread importancefortheirmagnetic,catalyticandotherproperties.Onesuchmaterialiscolloidal CeO2(ceria),whoseapplicationsincludecatalysis,newenergytechnologies,photochemistry, andmedicine,amongothers.Herewereportafamilyofatomicallypreciseceriananoclusters withultra-smalldimensionsupto~1.6nm(~100coreatoms).X-raycrystallographycon rms theyhavethe uoritestructureofbulkCeO2,andidenti essurfacefeatures,H+bindingsites, Ce3+locations,andOvacancieson(100)facets.Monodisperseceriananoclustersnow permitinvestigationoftheirpropertiesasafunctionofexactsize,surfacemorphology,and Ce3+:Ce4+composition. DOI:10.1038/s41467-017-01672-4 OPEN 1DepartmentofChemistry,UniversityofFlorida,Gainesville,FL32611-7200,USA.Correspondenceandrequestsformaterialsshouldbeaddressedto G.C.(email: christou@chem.u .edu )NATURECOMMUNICATIONS|8: 1445 |DOI:10.1038/s41467-017-01672-4|www.nature.com/naturecommunications1 1234567890

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Sinceitsintroductionin1976asanoxygen-storagecomponenttoensuretheef cientactivityofthenoblemetalsused inthree-waycatalysisinautomobileexhausts1 – 3,cerium (IV)dioxide(CeO2,ceria)hasbecomeofconsiderableutilityasa catalystorco-catalystinindustrial,petrochemicalandenvironmentalprocesses2 – 7.Inaddition,CeO2-containingmaterialsare oftenusedinoxidefuelcells8,precisionpolishingmaterials9 10, UV lters10,corrosionprevention11,andotherapplications1 12 13. ThiswidespreaduseofCeispartiallyduetoitssigni cant abundance(0.0046%byweightoftheEarth ’ scrust)andits Ce3+/Ce4+redoxcouple,whichiscrucialtomanyapplications byfacilitatingtheformationofCeO2 x,containinghighlyreactive defectsitescomprisingOvacanciesandattendantCe3+ions1 12 14.Ceriacanthusactasanef cientoxygenbuffer, assistedbyoxygenmobilitywithinitslayered uoritestructure.In fact,bulkcerianaturallycontainsrelativelyfewCe3+/O-vacancy defectsitesatambienttemperatures,buttheirnumberincreases athighertemperatureswhereCe4+reductionandoxygenrelease arefavoured.Catalysisbybulkceriaisthereforenormallycarried outattemperatures > 450C. Inthelastdecade,interestinceriananoparticles(CNPs)has seenexplosivegrowthduetotheirmuchgreaterreactivityand increasedcatalyticef cienciesatlowertemperatures9 12. Signi cantCNPactivityatornearroomtemperaturehasalso beenestablished15,ashasfacet-dependentreactivity12 16. Forexample,appreciableoxygenstoragecapacityisobservedat 150Conthecubic(100)faceofnanoceriacrystals,whichis ~250Clowerthanforirregularlyshapednanoceriaorthebulk material17.CNPsarealsounderinvestigationasphotovoltaic materialsinsolarcellswhereasbulkCeO2hasnophotovoltaic response18.UsingCNPsinsteadofaceriumoxidesupport increasesbytwoordersofmagnitudetheactivityofaAucatalyst fortheselectiveoxidationofCO19.Inaddition,thehigher reactivityofCNPsatambienttemperaturesispermittingmany importantbiomedicalapplicationstobedeveloped,suchas scavengingofreactiveoxygenspecies(ROS)12 20 – 22.TheCNP activityandtoxicitytolivingtissueclearlydependonparticlesize andsurfacecomposition(e.g.,Ce3+/Ce4+ratios),butasis normallythecaseinallareasofnanoparticlescience,the problemsofpolydispersity,agglomeration,andsurfacevariations haveplagueddetailedstudyoftheseparameters10 23 24.ForCNPs, itisparticularlychallengingtodeterminetheconcentration andlocationsofCe3+,attendantOvacancies,andprotonatedO (i.e.,OH,H2O)speciesonthesurface,andtherelationship ac b df e gi h Fig.1 Structuresofceriananoclusters. a – c showthecompletestructuresof 1 (Ce24), 2 (Ce38),and 3a (Ce40),respectively.Hatomshavebeenomittedfor clarity.AtomsizesofC,N,andOaremadesmalltoemphasizeCelocations.Colourcode:CeIVgold,CeIIIskyblue,Ored,Nblue,Cgrey. d – f show theirCe/Ocoresfromthesameviewpoint(includingcarboxylateOatomsthatarebridging)usingthesamecolourcodeexceptthatprotonatedOatoms (i.e.,OHions)areindicatedinpurple. g – i showthecoresagain,fromapproximatelythesameviewpointbutwithsurfacefacetscolour-coded:(100)facets areblue;(110)facetsareviolet;(111)facetsaregreen.OnlycarboxylateOatomsthatarebridgingareincluded ARTICLENATURECOMMUNICATIONS|DOI:10.1038/s41467-017-01672-42NATURECOMMUNICATIONS|8: 1445 |DOI:10.1038/s41467-017-01672-4|www.nature.com/naturecommunications

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betweenthem10.Amorecontrolledapproachtomonodisperse CeO2nanoclustersandnanoparticlesisclearlyneeded,especially attheultra-small,sub-20nmsizesthatareofgrowingimportance,particularlyforbiomedicalapplications. Wenowdescribedevelopmentofproceduresusing simpleCe4+saltsandorganicreagentsthatyieldafamilyof monodisperseceriananoclustersofdifferentsizesdependingon thecarboxylicacidemployed.Suchanapproachwasrecently accomplishedformonodispersemetalnanoclusters,primarily ofAu,stabilizedbythiolateligands25 26.Inourwork,theligandsof choiceformetaloxidenanoclustersarecarboxylates,especiallysince oleicandsimilaracidsarecommonsurfactantsinmetaloxide nanoparticlesyntheses20 27.Thesolubilityandmonodisperse natureoftheproductsweobtainallowsmolecularcrystalstobe grown,leadingtostructuralcharact erizationofthenanoclustersand theirsurfacefeaturestoatomicprecisionbysingle-crystal X-raydiffractometry.Thenanoclustersare[Ce24O28(OH)8(PhCO2)30(py)4]( 1 ;Ce24),[Ce38O54(OH)8(EtCO2)36(py)8]( 2 ;Ce38) and[Ce40O56(OH)2(MeCO2)44(MeCO2H)2(py)4]/[Ce40O56(OH)2(MeCO2)44(MeCN)2(py)4]( 3a/b ;Ce40),wherepyispyridine. 3a / 3b denotethetwoindependentnanoclustersintheasymmetricunitof 3 ,whichdifferslightlyintheorganicligation. 2 alsocontainstwo independentnanoclusters( 2a / 2b ),buttheyhavethesame formulation. Results Nanoclusterstructures .Severalpertinentpointsaboutthe completestructuresof 1 – 3 (Fig. 1 a – c)willbesummarizedto allowforconvenientcomparisons.TheyallcompriseCexOycores (excludingcarboxylateOatoms)withmetal( x )andtotal( x + y ) atomcountsof24/60,38/100and40/98for 1 – 3 ,respectively, andtheyexhibitthesame uoritestructureasbulkCeO2, i.e.,alternatinglayersof8-coordinatecubicCe4+and 4-coordinatetetrahedralO2 ions.SomesurfaceCe4+ionsare 7-or9-coordinate(videinfra).FromtheviewpointofFig. 1 d – f, thecoresconsistof veCelayersinanA:B:C:B:Apattern ( 1 A = 2, B = 6, C = 8; 2 A = 4, B = 9, C = 12; 3 A = 4, B = 10, C = 12),leadingtotheCe38coreof 2 beingessentiallyspherical (1.121.121.12nm)whereasthoseofCe24( 1 ,0.751.10 1.10nm)andCe40( 3 ,1.131.131.61nm)areellipsoidal.The Ce38canalsobedescribedasa ‘ truncatedoctahedron ’ ,astructure thatisoneofthoserecentlypredictedbyDFTstudiestobe favouredforCexOyfragmentsofCeO2inthissizerange28. 2 containsonlyCe4+,but 1 and 3 eachalsocontaintwo 10-coordinateCe3+ionsatoppositeendsofthecores,as suggestedbyDFTcalculationsonCe3+inCexOyfragmentsof CeO228 29.TheCeoxidationstateswerecon rmedbybond valencesum(BVS)calculations(SupplementaryTable 2 )andthe detectionofCe3+( S = )EPRspectrafor 1 and 3 .Thelatterwere measuredonmicrocrystallinepowdersat5K(Supplementary Figs. 1 and 2 )andarecomparablewiththefewCe3+EPRspectra reportedforCeO2nanoparticlesorCe3+dopedintopolymeric species30 – 32.Nanoclusters 1 – 3 arelargeenoughtodisplay multiplefacets(Fig. 1 g – i),asdoCNPs;thedifferentfacesfor CeO2andCe40arede nedinSupplementaryFig. 3 1 and 2 displayonly(100)and(111)facets,whereas 3 exhibitsthese andalso(110)facets.Finally,thecoresareenvelopedwithin monolayerorganicshellsofcarboxylateandpygroups,which exhibitonlyminorpositionaldisorderinsomeCandOatoms (Fig. 1 a – c). a 2b2c2: 2d3eCe38fgCe38Ce40 Fig.2 Ligandbindingmodesonthesurfaceofceriananoclusters.Thedifferentbindingmodesofsurfacecarboxylateandpyridinegroupsin 1 – 3 : a chelating ( 2); b 2-bridging; c 2-chelatingand 2-bridging; d 3-bridging; e Ce38( 2 )showingterminalpyridines ‘ capping ’ (bindingtothecenterof)the(111) hexagons,and 2-carboxylatesbridgingedgesjoiningtwo(111)facets; f Ce38( 2 )showing 2: 2-carboxylatesatedgesjoining(100)and(111)facets; and g Ce40( 3 )showing 2: 2-and 3-carboxylatesonedgesof(110)facets,and 2-carboxylatesbridgingedgesjoining(110)and(111)facets.Colourcode: CeIVgold,CeIIIsky-blue,Ored,Nblue,Cgrey,(100)facetsdarkblue;(110)facetsviolet;(111)facetsgreen NATURECOMMUNICATIONS|DOI:10.1038/s41467-017-01672-4ARTICLENATURECOMMUNICATIONS|8: 1445 |DOI:10.1038/s41467-017-01672-4|www.nature.com/naturecommunications3

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Thestructuralresultsthusstronglysupportthedescriptionof 1 – 3 asatomicallypreciseceriananoclustersintheultra-smallsize rangecorrespondingtothesmallestCNPssynthesizedtodate, andstabilizedtoagglomerationbytheorganicmonolayers.CNPs atthissub-20nmsizearebeingheavilytargetedforuseinvarious applications,especiallyinthebiomedical eldbecausetheyshow enhancedcatalyticactivityandregenerativeproperties21 33 34. 1 – 3 arelargerthanthefewpreviouslyknownCe/Omolecularspecies, mostofwhichareCe6species35 36andsomewithtridentate amino-alcoholN,O,O-chelates37.Itshouldbenotedthatthelarge familyofmonodisperse,crystallinepolyoxometalates,somewith veryhighmetalnuclearitiesandsizesapproaching4nm,have beenknownformanydecades,buttheydonotpossessthe structureofbulkmetaloxidesandthereforecannotbedescribed astheirnanoparticles. Surfacefeatures .X-raycrystallographyhasallowedde nitionto atomicresolutionofthesurfaces,whicharecrucialtoCNP reactivity.TheoverallquestionishowthegeometryandenvironmentsofsurfaceCeandOatomsdifferfromthoseofbody atoms.Indeed,surfaceCe4+geometriesin 1 – 3 differmarkedly fromthe8-coordinatecubicofbodyCe4+ions.Eventhosestill8coordinatearesigni cantlydistorted,whilemanyare 9-coordinateandthereisevenrare7-coordinationforCe12/Ce22 in 3a/3b ,respectively;coordinationnumbersarelistedforall CeatomsinSupplementaryTable 2 .Thisvarietyre ectsboththe greaterdegreesoffreedomatthesurfaceandatthecarboxylate ligation.Nevertheless,allbodyandsurfaceCeatomsareessentiallyatthepositionstheywouldoccupyinbulkCeO2,asshown bytheoverlaysinSupplementaryFig. 4 .Thelargernanoclusters Ce38( 2 )andCe40( 3 )showverylittledeviationofCeandO atomsfromtheirpositionsinbulkceria;thesmallest, Ce24( 1 ),appearsmorepliablebyshowinggreaterdeviation,butit isstillsmall.Thus,theCexOycoresof 1 – 3 reallycanbedescribed asfragmentsofbulkceria,stabilized/passivatedbythemonolayer ofcarboxylateandpyridineligands. Therearefourtypesofcarboxylatebindingin 1 – 3 (Fig. 2 a – d): chelating( 2)andthreedoublyortriplybridgingmodes,allowing for exibilityandversatilityinbindingtoone,two,orV-shaped setsofthreesurfaceCeions.Thecarboxylatescanthus accommodatethemulti-facetedsurfacestructure,including pointsofhighcurvature(SupplementaryFig. 5 ),withterminal pygroupscompletingligationwherenecessary.Bothtypesof 2-carboxylatesoccurinallthreenanoclustersandbridgeCe2edgesjoiningtwofacets,oneofwhichisalwaysa(111)facet (Table 1 andFig. 2 e – g).Interestingly,the 2: 2modeisfound onlyat(100)(111)and(110)(111)edges,whereasthe 2mode isfoundonlyat(111)(111)and(110)(111)edges.Incontrast, 3-carboxylatesoccuronlyin 3 ,bridgingaV-shapededgeofthe (110)facets.The 2-chelatingmodeisalsoonlyfoundin 3 ,always boundtooneCeofa(100)Ce4square(videinfra).Terminalpy ligandsoccurinallthreenanoclusters,always ‘ capping ’ a(111) hexagon,i.e.,attachedtoitscentralCe(Fig. 2 e).Thetwo independentCe38nanoclustersin 2 areidenticalinformulaand structure,butthetwoCe40unitsin 3 providethebene tof slightlydifferingorganicmonolayershells,revealingonewaythe lattercanvaryforagivennanoclustercore.Thus,thechelating carboxylates(Fig. 2 a)ontwoCe4+ions(Ce9)in 3a areeach replacedbyaterminalMeCN(onCe32)in 3b ,converting 9-coordinateCe9into8-coordinateCe32. TherearetwodistinctCesurfacesubunitsin 1 – 3 resultingfrom the uoritestructure,Ce3trianglesandCe4squares,andthesewill bedescribedinturn.Ce3trianglesareverycommonsurfaceunitsin (111)and(110)facetsandarebridgedprimarilybypyramidal 3O2-ions(Table 1 ),fromtetrahedralbodyO2 ionsnowbinding onelessCe.Somein 1 and 2 areinsteadbridgedby 3-OHions (Fig. 3 a):Thefourin 1 areobviousfromtheirO-H Nhydrogen bondingtolatticepymolecules(O N = 2.7 – 2.9),whichthus anchorstheH+onO15andO16andgivestheexpectedOBVSof 1.21(SupplementaryTable 3 ).Incontrast,thetwo 3-OHin 2 are disorderedsincethereisnoreasonforH+tofavourparticular 3O2 ionswhensomanyareessentiallyequivalent.Slightlylowered BVSvalues(1.52 – 1.72)forthefour 3-O2 ionsatO18/O39and thefouratO49/O60in 2a and 2b ,respectively(Supplementary Table 4 ),suggestthatthe2H+arerandomlydistributedprimarily amongthesepositionstogivepartial 3-OHoccupancies. InbodyCe4squares,eachedgeisoxide-bridged,butatthe surfacetheedgesarecarboxylate-bridged.Thesearethe(100) facets(Fig. 1 g – i)andoccurinthreeslightlydifferentforms.The sixseparatedCe4+4squaresin 2 (Fig. 3 b),thetwoCe4+3Ce3+Ce4+3V-shapeddouble-squaresfusedataCe3+cornerin 1 (Fig. 3 f),and twoCe4+3Ce3+squaresin 3 (Fig. 3 c)areallbridgedbya 4-OHionwithraretetragonalpyramidalgeometry(theOis0.7 – 0.8 abovetheCe4plane).All 4-OHionshavesimilarOBVSvalues of0.52 – 0.71(SupplementaryTables 3 – 5 ),intermediatebetween thoseofOHandH2O.In 1 (butnot 2 or 3 ),the 4-OHprotons (H12andH14)wereobservedindifferenceFouriermaps, con rmingthem(andbyextensionthosein 2 and 3 )tobe OH,notH2O.TheCe4+ OHandCe3+ OHdistancesare extremelylong(2.7 – 3.0;SupplementaryTable 7 )andsuggest minimalCe-Obonding;forcomparison,Ce4+3-O2 = 2.2 – 2.3, Ce4+4-O2 = 2.3 – 2.35,andCe4+3-OH= 2.3 – 2.45.The very-longCe 4-OHdistancessuggestanessentiallyfreeOHionactingasaweaklydocked ‘ lid ’ ontheCe4surface(andthus rationalizingitssmallBVS).Spacellingrepresentations(SupplementaryFig. 6 )showtheOHtobeencapsulatedbythe surroundingcarboxylatesandcannotmovefromits 4central positiontobecomemorestronglybound 2or 3. 3 alsocontainsplanardouble-squareunits(Fig. 3 d,e),and thesedonotcontain 4-OHions.Instead,thosein 3a (Fig. 3 d) have 2-carboxylatesattachedtooneCethatactaslids,tilting inwardssothatoneOatomapproachesthemid-pointofeach square;thethreeresultingCe Oseparations(~3.0)indicate extremelyweakcontacts(SupplementaryTable 8 ).In 3b ,one 2carboxylateofeachdouble-squareisreplacedbyanMeCN, asdescribedabove,andthisagaintiltsoverthecenterofthe squaretoactasalid,givingaveryunusualbentbindingmode. ThethreeresultingCe Nseparations( > 3.0)againindicate onlyveryweakcontacts.Interestingly,theseplanardouble squaresin 3 areeachfusedattheirCe3+cornerstothe 4-OH-bridgedCe4+3Ce3+squares(Fig. 3 c)describedabove,so that 3 containstwoasymmetricL-shaped(86.1)triplesquares withtheCe3+lyingattheinnerpointoftheL.Forchargebalance, 3a mustalsocontaintwoadditionalH+.SincetheOBVSvalues indicatetheyarenotonsurface 3-O2 ions,wesuspectedthem tobeonligandgroups.Indeed,threecarboxylateOatoms Table1Typeofsurfaceligandsinnanoclusters1 – 3TypeBindingmodeFoundSurfacelocation O2 3-bridging 1 – 3 (111)or(110)Ce3triangle OH3-bridging 1 2 (111)Ce3triangle OH4-bridging 1 – 3 Lidon(100)Ce4square pyterminal 1 – 3 Cappingof(111)hexagon MeCNterminal 3b Lidon(100)Ce4square RCO22-chelating 3 Lidon(100)Ce4square RCO22: 2-chel/brid 1 – 3 Ce2edgejoining(100)(111) 3 Ce2edgejoining(110)(111) RCO22-bridging 1 – 3 Ce2edgejoining(111)(111) 3 Ce2edgejoining(110)(111) RCO23-bridging 3 V-shapedCe3edgeof(110) ARTICLENATURECOMMUNICATIONS|DOI:10.1038/s41467-017-01672-44NATURECOMMUNICATIONS|8: 1445 |DOI:10.1038/s41467-017-01672-4|www.nature.com/naturecommunications

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attachedtoCe3+,namelyO40,O40 andO83,formatriangleand allshowloweredBVSvaluesof1.78,1.78and1.65,respectively, suggestinganH+iscappingeachofthetwotrianglesin 3a by interactingwiththeOatomsinatrifurcatedfashion(SupplementaryTable 5 ).Theformulationof 3b cannowberationalized asresultingfromlossofsomeofthechelatingMeCO2groupsin 3a ,assistedbyprotonationtoMeCO2HbytheseH+,and replacementbyMeCNsolventmoleculesin 3b ThetwoCe3+eachin 1 and 3 arethusallatsurfacesites,as suggestedtoalsobethecaseinCNPs10 38.ThelowerCe3+charge favorsfewerO2 ligandsthanCe4+andthusdisfavorsbodysites. Incontrast, 2 containsnoCe3+.Interestingly,allCe3+occur within(100)Ce4+3Ce3+squarefacets.In 3 (Fig. 3 c),the Ce3+ OHandCe4+ OHdistancesareidentical(~2.7),again supportingweakcontactsbythe 4-OH.In 1 ,the V-shapeddouble-squarejoinedattheCe3+corner(Fig. 3 f)hasa Ce3+ OHdistanceof~2.7ononeside,butthiscausesa longerCe3+ OHontheother(~3.0;SupplementaryTable 7 ). ItisalsoextremelyinterestingthatwhenCe3+ionsarepresent, thesurfaceH+(i.e.,OHions,andH+hydrogenbondingto carboxylategroups)arelocatedveryclosetothem.Thepresence andpositionsofH+onnanoparticlesareextremelychallenging todetermine39,butinnanoclusters 1 and 3 mostofthemare directlyobservedandclearlyaccumulateonOatomsnearCe3+(Fig. 1 d,f).Theeffectislikelysynergistic,i.e.,thelowerCe3+chargefavorsaccumulationofH+nearby,whichinturnmollify theO2 andcarboxylatechargesandstabilizethelowerCe3+charge.Incontrast,withnoCe3+in 2 ,theH+spreadoutoverthe surface(Fig. 1 e),althoughtheyagainfavorCe4squares.H+are expectedtobemobileonthenanoclustersurfaces,asrecentwork hasconcludedfromstudiesofhydrogenmobility( ‘ hopping ’ )on surfaceOatomsofCeO2thin lms40.Doubleprotonationofan O2 anddesorptionofsurfaceH2Owassuggestedasthemeansof formingOvacancies. Discussion Wehaveshownthatabottom-upsyntheticapproachinsolution atambienttemperaturesusingreadilyavailablereagentscanbe successfullyappliedtoobtainafamilyofmonodispersemetal oxidenanoparticlesofultra-smalldimensions.Thisthusachieves formetaloxideswhatwaspreviouslyaccomplishedforthedistinctlydifferentareaofmetalnanoparticles,particularlyofAu.In thepresentwork,monodisperseCeO2nanoclusterswiththe uoritestructureandmonolayerorganicligandshellscanbe synthesizedandstructurallycharacterizedtoatomicresolution. Theyexhibitmultifacetedstructuresconsistingmainlyof(100) and(111)facets,but 3 alsohas(110)facetsgivingnoticeable surfacekinks/edges/trenches.ThesurfacelocationofanyCe3+ionsandtheH+positionson 3-and 4-OHgroups,aswellas ligandgroups,areparticularlywelcometoknow.The 4-OHare weaklyattachedwithlongCe Odistancestothe(100)facets, actingaslidsonCe4squares,asalsodoO(carboxylate)andN (MeCN)lidsonother(100)facets.Suchsurfacefeaturesarelikely ofgreatrelevancetoCNPreactivity:Underheterogeneouscatalysisconditions,orinsolutionorcolloidalsuspension,onecan abc d e f Fig.3 Structuralfeaturesonthesurfaceofceriananoclusters. a 3-OHon(111)CeIV3triangle; b 4-OHona(100)CeIV4square; c 4-OHona(100) CeIIICeIV3square; d 2-carboxylatesin 3a actingaslidsonadjacent(100)CeIV4andCeIIICeIV3squares; e theanalogoussituationin 3b tothatin d ,withan MeCNligandreplacingone 2-carboxylateaslid;and f 4-OHlidsonaV-shaped(100)CeIIICeIV3double-squarein 1 linkedattheCeIIIcorner.Colorcode: CeIVgold,CeIIIsky-blue,Ored,OHpurple,Nblue,Cgrey.Hatomshavebeenomittedforclarity NATURECOMMUNICATIONS|DOI:10.1038/s41467-017-01672-4ARTICLENATURECOMMUNICATIONS|8: 1445 |DOI:10.1038/s41467-017-01672-4|www.nature.com/naturecommunications5

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envisagethereadylossor ‘ opening ’ ofsuchweaklyinteractinglids (e.g.,byprotonationofOH,detachmentofMeCN,ortilting awayofthechelatingcarboxylate,perhapsbybecomingmonodentate)exposingCe4squarefacesforreaction.Wethuspropose theseweaklyliddedCe4sitesasrestingstatesofsomeofthe catalyticallyhighlyreactive,surfaceO-vacancysitesinCNPs.In addition,whenCe3+ionsarepresent,theirlocationsin 1 and 3 corner-linkingtwo(100)Ce3+Ce4+3squares,andtheconcomitant accumulationnearbyofmobileH+,on 3-OH, 4-OHand/or ligandgroups,togetherofferapossiblepictureforthehighcatalyticactivityofsurfaceCe3+inCNPs.Similarly,thekinks/edges/ trenchesassociatedwiththe(110)facetsin 3 suggestadditional sitesofincreasedreactivity,asseenfor(110)facetsofCNPs,and theyhavealsobeenidenti edinCNPsasnucleationsitesfor heterometals41 42,aprocesswearetryingtomimicwith 3 .We notethatthereisageneralconsensusthatthe(111)facetofCNPs isthemostthermodynamicallystablewhilethe(100)facetis highlyreactivedueitslowerstabilityandisthereforeaproposed siteforOvacanciesandCe3+ions17 29 43,observationsthatare consistentwiththesurfacefeatureswehaveidenti edin 1 – 3 Evenonthebasisofonlythethreenanoclustersdescribed herein,itisalreadyapparenthowCNPswithsimilarsizescan haveverydifferentpropertiesandreactivities.Although 2 and 3 areessentiallythesamesizeandmetalnuclearity,theydiffer signi cantlyintheiroverallshape,thevarietyoffacetsthey exhibit,theresultingsurfacemorphology,andtheirCe3+content. Ontheotherhand,theavailabilitynowofsamplesofidentical, monodispersenanoclustersmakespossiblethestudyofactivity vs.exactsize,surfacemorphologyandCe3+content.Inaddition, whiledispersionsofCNPsinwaterareoftenunstable,leadingto agglomerationthatcanaffecttheirtransport,distributionand reactivity,particularlyforultra-smallCNPsinbiomedicalstudies, 3 iscompletelywatersolubleandaffordstheopportunitytostudy reactivityinbiologicallyrelevantmedia27 44. Finally, 1 – 3 containeitherCe4+44-OHorCe3+Ce4+34OH(100)squares,orboth,andthisvariationmayalsobe responsiblefortherecognizedredox-statedependentROSscavengingabilityandtoxicityofCNPswithdifferentamounts ofsurfaceCe3+ 21 22 45 46.Incontrast,recentsuggestionsthat 1.1 – 3.5nmCNPsshouldhaveadefectuoritestructureanda largesurfaceCe3+:Ce4+ratioarenotsupportedby 1 – 347 48.Ce3+iscertainlyonthesurface,butnocorrelationbetweensizeandthe numberofCe3+isseen,withCe24andCe40havingtwoeach,but Ce38none.Infact,giventhatCe3+ionsin 1 – 3 alwaysoccuratthe centreofV-shapeddouble-squaresubunits(Fig. 3 fandsimilar), wehypothesizethatthehighersymmetry,essentiallyspherical 2 containsnoCe3+becauseitssurfacestructurecontainsnosuch doublesquares.Furthermore,theconventionalwisdomthat smallerCNPshavemoresurfaceCe3+mayre ectthegreater numberofsuchV-shapedunitspresentinsmallernanoparticles oflowersymmetryasaresultoftheirincreasednumberofpoints ofhighcurvature.Wearecurrentlyseekingtoextendthefamily tolargernanoclustersandhigherCe3+:Ce4+ratios,andexploring thereactivityof 1 – 3 withROS. MethodsSyntheses .[Ce24O28(OH)8(PhCO2)30(py)4]( 1 )waspreparedbythereactionof (NH4)2[Ce(NO3)6]andPhCO2Hina1:2molarratioinpyridineatroomtemperature.Thegolden-yellowsolutionwasstirredfor30mins,dilutedwith2 volumesofMeCN,andmaintainedundisturbedfor1week.Theresultingyellow squareplatesof 1 9pywerecollectedby ltration,washedwithMeCN,anddriedin vacuum.Theyieldwas14%basedonCe.Anal.Calcd(Found)fordried 1 2py (C240H188Ce24N6O96):C,35.79(35.63);H,2.35(2.00);N,1.04(0.98). [Ce38O54(OH)8(EtCO2)36(py)8]( 2 )and[Ce40O56(OH)2(MeCO2)44(MeCO2H)2(py)4]/[Ce40O56(OH)2(MeCO2)44(MeCN)2(py)4]( 3 )werepreparedbythe reactionof(NH4)2[Ce(NO3)6],thecorrespondingRCO2H,andNH4Iina1:4:1 molarratioinpyridine/H2O(10:1v/v)atroomtemperature.Thegolden-yellow solutionswerestirredfor30min,dilutedwithtwovolumesofMeCN,and maintainedundisturbedfor4weeks.Theresultingyellowsquareplates ( 2 16MeCN)orrods( 3 48MeCN)werecollectedby ltration,washedwithMeCN, anddriedinvacuum.Theyieldswere49%and35%for 2 and 3 ,respectively.Anal. Calcd(Found)for 2 7H2O(C148H242Ce38N8O141):C,18.30(17.90);H,2.51(2.36); N,1.15(1.05).Anal.Calcd(Found)for 3 8H2O(C112H178Ce40N5O156):C,13.88 (13.79);H,1.85(1.78);N,0.72(0.76).Theindicatedatomiccompositionof 3 is calculatedusingtheaverageofthe 3a and 3b formulas. Fullerdetailsofthethreesynthesesandinfra-redspectraldatafor 1 – 3 are availableinSupplementaryMethods. X-raycrystallography .Single-crystalX-raydiffractionstudiesat 173Cwere performedonaBrukerDUOdiffractometerusingMoK( = 0.71073)orCuK( = 1.54178)radiation(fromanImuSpowersource),andanAPEXIICCDarea detector(SupplementaryMethodsandSupplementaryTable 1 ).Themetric parametersofthere nedstructureswereusedtodeterminetheCeoxidationstates andtheOprotonationlevelsbybondvalencesum(BVS)calculations(SupplementaryTables 2 – 5 ). 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WethankJ.GoodsellforhisassistancewiththeEPRmeasurements.AuthorcontributionsK.J.M.carriedoutallthesynthesesandcharacterizations,andco-wrotethepaper.K.A.A. performedthesingle-crystalX-raydiffractometrystudies.G.C.supervisedtheresearch andco-wrotethepaper.AdditionalinformationSupplementaryInformation accompaniesthispaperatdoi: 10.1038/s41467-017-01672-4 Competinginterests: Theauthorsdeclarenocompeting nancialinterests. Reprintsandpermission informationisavailableonlineat http://npg.nature.com/ reprintsandpermissions/ Publisher'snote: SpringerNatureremainsneutralwithregardtojurisdictionalclaimsin publishedmapsandinstitutionalaf liations. 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