Atomic origins of the high catalytic activity of(2)

¯]directionwitha(111)aconvexregionviewedalongthe[011

surfaceplane.ForGPA,predefineddirections(xandy)werechosenparallelandperpendiculartothesurfaceplane,respectively.Strainanalysissuggestsnegligiblein-planestrain(Fig.4b)but～10%out-of-planepositivestrain(Fig.4c)withinaregion～1nm(3–4atomiclayers)fromthesurface.Thefeaturelessin-planestrainmapnearthesurface(Fig.4b)indicatesthattheout-of-planepositivestraincannotbegeneratedbyedgeartefactsresultingfromGPA.

¯]observationdirection,weselectedaconcaveUsingthesame[011

regionwitha(100)terraceattheapex.Again,thereisanegligiblein-planestrain(SupplementaryFig.S2b),butanout-of-planepositivestrainof～5%(SupplementaryFig.S2c).Wealsoobserved

¯]direction(SupplementaryFig.(100)surfaceplanesfromthe[011

S2d–f).Frombothconcaveandconvexcurvatures,thereisnomeasurablein-planestrainbut～5%out-of-planepositivestrainnormaltothesurfaces.Theout-of-planestrainisalsoconfirmedbycoherentnanobeamelectrondiffraction,whichshowsstreakeddiffractionspotsperpendiculartothesurfaces(SupplementaryFigsS3andS4).Moreover,weusedpeak-pairsanalysisbasedonthedetectionofpeakintensitiesinrealspace23asanalternativeaccuratemeasurementofthestrainfieldtoensurethevalidityoftheimageprocessing(SupplementaryFig.S5).

Weperformedabinitiocalculationstodeterminewhetherthesurfacestrainoriginatesfromtheintrinsicnatureofthecurvedsurfacesorfromextrinsiceffectssuchaslatticedefects.Webuiltastructuralmodel,inspiredbyFig.4a,asarepresentativeexampleofaconvexregionconsistingof(111)surfacesteps.TheperiodicmodelisshowninFig.4d.Otherthanstraightsteps,thesurfacemodeldoesnotcontainanylatticedefects(forexample,kinks,vacancies,adatomsorAgimpurities).Afterstructuralrelaxation,wefoundoutwarddisplacementofsurfaceatomsnormaltothesurfaceplaneinthesimulatedHRTEMimage,asshowninFig.4e.Thecalculatedstrains,relativetothe(111)latticespacingofbulkgold,were7.6%,7.5%,7.8%and5.8%fortheinter-planarspacings,correspondingto1–4surfaceatomiclayersinFig.4e,consistentwiththeexperimentalobservationsandrecentmoleculardynamicssimulations24.Wealsonoticedthat,althoughthestructuralmodelassumesstraightsteps,moregeneralsteporientationscanbebuiltfromsegmentsofstraightstepsseparatedby6-coordinatedstepkinksandtheactuallocalstrainscouldbehigherthan8%.WealsoconfirmedthattheHRTEMphasecontrastofsurfacekinkssculpturedfromperfectAulatticesdoesnotshowsignificantpositivestrainingintheHRTEMimage(SupplementaryFig.S6).TheHAADF-STEManalysisdescribedaboveshowsthattheinternalsurfacesofNPGinvolveacomplexarrangementoflow-andhigh-indexsurfacesandahighdensityofatomicsteps.Theresultalsosuggeststhatasignificantnumberofkinksappearwithinsurfacesteps.Associatedwiththiscomplexstructureisahighconcentrationoflow-coordinationatoms(forexample,6-and7-coordinatedkinksandstepsites,comparedwith8-and9-coordinatedclose-packedsurfacesites).Thepresenceofahighconcentrationoflow-coordinationsurfaceatomshasbeen

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Ag-Lα/Au-Mα

a

b

c

de

Figure2|SurfaceatomicstructureofNPG.a,TEMimageofa[01¯1]nanopore.Thediffractionpattern(inset)showsthattheincidentdirectionis[01¯1].ThelabelledsquaresindicatetheareasimagedbyHAADF-STEMinthefollowingobservations.b,HAADF-STEMimageneara(100)surfaceplanewithatomickinks.Theintensitypro le(inset)alongthedottedlinerepresentsasteppedsurface.c,HAADF-STEMimagefromareacina.Inthe atregion,amonoatomiclayerislandonthe(111)planecanbeobserved.d,Convex(111)terracesandneighbouringatomickinks.(e)Concave(111)terracesandsteppedsurfacealongtheedge.

predictedtobeoneoftheimportantoriginsofthehighcatalyticactivityofsmallgoldnanoparticles1,25–27.Thisisbecauselow-coordinationatomsinteractmorestronglywithmoleculesowingtotheirmodifiedlocalelectronicstructure(inparticular,alocalup-shiftandnarrowingofthed-band26),leadingtoreducedreactionbarriersrelativetoclose-packedgoldsurfaces27.However,simplycomparingspecificsurfaceareastoassesstherelativeactivityofNPGandgoldnanoparticlescanbemisleading,asitdoesnotdescribedifferencesintheconcentrationofvariousactivesurfacesites28.Inthisstudy,weemployaphenomenologicalmodeltoobtaininitialestimates.

Theatomic-scalesurfacestructureofNPGconsistsofclose-packedterracesseparatedbyatomicsteps.Weconsiderthattheprincipalclose-packedsurfacesinNPGare(111)and(100),withtotalareasininverseproportiontotheirsurfaceenergiesγ(111)(1.52Jm 2)andγ(100)(1.80Jm 2),accordingtotheWulffconstruction.Theatomicstepsthatseparatetheterracesmaygenerallyhavearangeofcrystallographicorientations.However,

¯-onthermodynamicgrounds,weassumethatthemoststable110and[001]-orientedstepsarethedominantones.Givenanaverageseparationbetweenstepsl,wecanestimatetherelativeproportionofstepatomswithcoordinationnumber7ontheinternalsurfaceofNPG,denotedf7C(seeSupplementaryInformation).OnthebasisofquantitativeHAADF-STEManalysis,wemeasuredthedistributionofthesurfacestepseparations.Theaveragesteplengthis1.7nm(seeSupplementaryFig.S7),whereasthegoldligamentsareapproximately30nminsize.Forl=2nmforestimation,wecalculatef7C=13.4%forNPG.Forl=1nm,weestimatef7C=26.8%.Forcomparison,similarestimatescanbereadilymadeforgoldnanoparticleswithdifferentsizesandstructures.Forexample,fora6nmgoldnanoparticle,f7C=13.2%,andf7Cincreasesto21.9%forasmaller(3nm)nanoparticle29.Therefore,althoughthespecificsurfaceareaofthecoarseNPGissmallerthanthatofgoldnanoparticles,thenumberofactivesurfacesitesperunitareaiscomparable,assupportedbytheturnoverfrequencymeasurements(seeSupplementaryInformation).

TounderstandthecorrelationofsurfaceatomicstructurewithcatalyticactivityofNPG,inadditiontothestaticstructurecharacterization,weperformedinsituenvironmentalHRTEMobservationsofNPGcatalysingCOoxidation.InaCO/airmixtureatroomtemperature,NPGcontaining1.2at.%Agbeginstoshowcatalyticactivityalongwithsignificantsurfacereconstructionatafewpascals(seesupplementaryMovieS1).{111}facetingdynamics