Engineering surface atomic structure of

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ARTICLEReceived16May2016|Accepted9Aug2016|Published21Sep2016DOI:10.1038/ncomms12876OPENEngineeringsurfaceatomicstructureofsingle-crystalcobalt(II)oxidenanorodsforsuperiorelectrocatalysisTaoLing1,2,*,Dong-YangYan1,*,YanJiao2,*,HuiWang3,YaoZheng2,XueliZheng1,JingMao1,Xi-WenDu1,ZhenpengHu4,MietekJaroniec5&Shi-ZhangQiao1,2Engineeringthesurfacestructureattheatomiclevelcanbeusedtopreciselyandeffectivelymanipulatethereactivityanddurabilityofcatalysts.Herewereporttuningoftheatomicstructureofone-dimensionalsingle-crystalcobalt(II)oxide(CoO)nanorodsbycreatingoxygenvacanciesonpyramidalnanofacets.TheseCoOnanorodsexhibitsuperiorcatalyticactivityanddurabilitytowardsoxygenreduction/evolutionreactions.Thecombinedexperimentalstudies,microscopicandspectroscopiccharacterization,anddensityfunctionaltheorycalculationsrevealthattheoriginsoftheelectrochemicalactivityofsingle-crystalCoOnanorodsareintheoxygenvacanciesthatcanbereadilycreatedontheoxygen-terminated{111}nanofacets,whichfavourablyaffecttheelectronicstructureofCoO,assuringarapidchargetransferandoptimaladsorptionenergiesforintermediatesofoxygenreduction/evolutionreactions.Theseresultsshowthatthesurfaceatomicstructureengineeringisimportantforthefabricationofefficientanddurableelectrocatalysts.1TianjinKeyLaboratoryofCompositeandFunctionalMaterials,KeyLaboratoryforAdvancedCeramicsandMachiningTechnologyofMinistryofEducation,InstituteofNew-Energy,SchoolofMaterialsScienceandEngineering,TianjinUniversity,Tianjin300072,China.2SchoolofChemicalEngineering,TheUniversityofAdelaide,Adelaide,SouthAustralia5005,Australia.3KeyLaboratoryofAerospaceMaterialsandPerformance(MinistryofEducation),SchoolofMaterialsScienceandEngineering,BeihangUniversity,Beijing100191,China.4SchoolofPhysics,NankaiUniversity,Tianjin300071,China.5DepartmentofChemistryandBiochemistry,KentStateUniversity,Kent,Ohio44242,USA.*Theseauthorscontributedequallytothiswork.CorrespondenceandrequestsformaterialsshouldbeaddressedtoS.-Z.Q.(email:s.qiao@adelaide.edu.au)ortoX.-W.D.(email:xwdu@tju.edu.cn)ortoZ.H.(email:zphu@nankai.edu.cn).NATURECOMMUNICATIONS|7:12876|DOI:10.1038/ncomms12876|www.nature.com/naturecommunications1 ARTICLENATURECOMMUNICATIONS|DOI:10.1038/ncomms12876hegrowingconcernsoverclimatechangeandenergyimmobilizeddirectlyonacarbonfibrepaper(CFP)substratesecurityhavestimulatedarapiddevelopmentinthe(Fig.1a–c)viaasimpleandwell-controlledcationexchangeTgenerationofcleanenergy1.Electrocatalystsplayakeymethod(SupplementaryFig.1).Thissynthesiswasaccomplishedroleinsustainableenergyproduction,includingfuelcells2,byconvertingSCzincoxide(ZnO)NRs(SupplementaryFig.2)tometal-airbatteries3–5andwatersplitting6.Currently,nobleCoONRs(SupplementaryFig.3)viatheaforementionedcationmetalsandtheircomplexesarethemostefficientcatalysts7–9,exchangereactioningasphase43.AcontrolledfabricationofCoObuttheirhighcostandscarcitygreatlyrestrictscommercialNRswithtailorablelengthwasachievedbyprecisetuningtheapplications10.Transitionmetaloxides(TMOs)areconsideredaslengthofZnONRsfromdozennanometrestoseveralmicronsthemostprobablealternativestonoblemetal-basedcatalysts(SupplementaryFig.4).duetotheirlowcost,nontoxicityandhighstability3,11–19.Themicrostructureofas-synthesizedCoONRswasinvesti-Nevertheless,thereisstillanurgentneedtofurtherimprovetheirgatedbyscanningelectronmicroscopy(SEM)andtransmissionactivitiesandmakethemhighlycompetitiveincomparisonelectronmicroscopy(TEM).AsdisplayedinFig.2a,theentirewiththeirnoble-metalcounterpartsandusefulforpracticalsurfaceofCFPisuniformlycoveredwithCoONRs,whichareapplications.SCs(Fig.2c,inset).Interestingly,aftercationexchangereaction,Inthelightoftheabovediscussion,thesurfacestructurenumerousnanoporeswithsizesof5–20nmarevisibleontheengineeringofTMOcatalystsinvolvingbetterexposureofactivesurfaceandacrossNRs(SupplementaryFig.5).Surprisingly,sitestopromotetheirelectrocatalyticperformancebecomesofthesurfaceofCoONRsbecomesratherroughasevidencedbyparamountsignificance.Inthepastdecade,fundamentalresearchtooth-likegrowthswithsizesofabout5nm(Fig.2b).Atomiclevelhasdemonstratedthattherationaldesignofactivefacetswithhigh-angleannulardarkfield-scanningTEM(HADDF-STEM)favourableatomicarrangementandcoordinationisthemostimageshowsthatthesegrowthsaresharplyterminatedwith{111}promisingroutetocontroltheatomicstructureofnoblemetals7,8nanofacets(Fig.2c).Notably,thegradualcontrastvariationinandmetaloxide20–24particulatecatalysts,andachievehighthesesingletooth-likegrowthssuggestsaprogressivevariationincatalyticactivity.Furtherinvestigationssuggestthatthesurfacetheirthickness(Fig.2c).Simulationofexperimentalimagewasdefects25–28cangreatlyinfluencetheelectronicstructureandthusperformedtoaccuratelydeterminethethree-dimensionalatomicthesurfacechemistryoffacetedcatalysts29–36.Hence,aproperarrangementintheaforementionedgrowths.Aspeculatedmanipulationofdefectsonthedesiredfacetsofcatalystshasnanopyramidalstructurewithexposed{100}and{111}facetsreceivedaconsiderableattentionandbroughtsomeexcitingwasconstructedasshowninFig.2d,e.Figure2f,gindicatesagoodbreakthroughs.Forinstance,introductionofdefectson{100}agreementbetweentheexperimentalandsimulatedimages.facetshasbeeneffectivelyusedtotailorthebandstructureofMoreover,theintensityprofilealongtheterminated{111}facetintitaniumdioxide(TiO2)tomakeitsuitableforphotocatalytictheexperimentalimage(Fig.2h)closelyresemblesthatinthehydrogenevolutionundervisiblelightillumination30,33.simulatedone(Fig.2i).AgoodmatchbetweenexperimentalandEngineeringsulfurvacanciesonthebasalplanesofsimulatedHADDF-STEMimagesclearlydemonstratesthatthemolybdenumdisulfide(MoS2)nanosheetscanbeusedtofinelysurfaceofSCCoONRsissurroundedbynanopyramidsandtunetheadsorptionfreeenergyofhydrogentoachievethepreferentiallyexposed{111}facets.Thesurfaceareaofexposedhighestactivityoftheaforementionednanosheetstowards{111}facetsisestimatedtobe46%ofthetotalsurfaceareaofSChydrogenevolutionreactionamongvariousMoS2-basedCoONRs(SupplementaryFig.6andSupplementaryNote1).catalysts31.However,theapplicationoffacetedTMOcatalystsNotably,forCoOthesurfaceenergyof{111}ismuchhigherthaninelectrocatalysisisinitsinfancy37.Also,engineeringfavourablethatofotherlow-indexedfacets44.Suchhighpercentageof{111}defectsonthedesiredfacetsandunderstandingtheirroleinfacetswithoutforeignstabilizerisratherdifficulttoachieveviaelectrocatalysisattheatomiclevelisstilllacking.thermodynamicallycontrolledsynthesis44.However,inourVeryrecently,one-dimensional(1D)nanoarraysdirectlykinetics-governedcationexchangestrategy,facetswithhighgrownonthecurrentcollectorshaveattractedalotofattentionsurfaceenergyanddefects(discussedlater)areforcedtobeinelectrocatalysis16,38–42becausetheir1Dmorphologyassuresexposedtofacilitatetheionexchangeprocess45,assuringadequatediffusionofreactantsandrapidchargetransport.theformationofalargeamountofcleananddefect-rich{111}Althoughagreatprogresshasbeenachievedinelectrocatalysis,facetsonthesurfaceofSCCoONRs,whichiscertainlymuchlesshasbeendonetowardsengineeringthesurfaceatomichighlypreferableforcatalysis.Furthermore,itshouldbestructureoftheaforementionednanoarraystoexploretheirfullnotedthat{111}facetsofthebulkCoOarepolar,eitherpotential.Herein,wereportthesurfacestructureengineeringofterminatedbyoxygen(O)orCo-atomiclayer46.Adetailedsingle-crystal(SC)CoOnanorods(NRs)throughcreatingdesiredX-rayphotoelectronspectroscopy(XPS)analysisindicatesfacetsanddefects(Fig.1).Ourexperiments,microscopicandthattheexposed{111}facetsonCoONRsshouldbespectroscopiccharacterizationandthedensityfunctionaltheoryO-terminated(SupplementaryFig.7,SupplementaryTable1(DFT)computationstudiesdemonstratethattheO-vacanciesandSupplementaryNote2).presentonthepyramidalnanofacetsofCoONRscanbeeffectivelyusedtotailortheelectronicstructureofNRs,whichAnalysisofO-vacancy-richpyramidalnanofacets.Tofurtherresultsinrapidchargetransferandfavourableenergeticsforbothprobethelocalchemicalandelectronicenvironmentontheoxygenreductionreaction(ORR)andoxygenevolutionreactionsurfaceofSCCoONRs,XPSandsynchrotron-basedX-ray(OER)asevidencedbyexcellentactivityanddurabilityofCoOabsorptionnearedgefinestructure(XANES)spectroscopyNRstowardsbothreactions.Significantly,theirORRactivitymeasurementswereperformed.TheXPSO1sspectrumsuggestsapproachesthatofplatinum(Pt)catalystsandtheirOERactivityanenrichmentofO-vacanciesonthesurfaceofSCCoONRsexceedsthatofrutheniumdioxide(RuO2)catalysts;theiroverall(SupplementaryFig.7a).FurtherevidencecomesfromacloseactivityiscomparabletothatofthebestbifunctionalORR/OERinspectionofthefinestructureoftheO-KedgeofXANEScatalysts.spectrum(Fig.3a),inwhichthepeakatB536.0eVassignedtoOdeficiency47,48inSCCoONRsismuchhigherthanthatofResultsreferenceCoO.ThisisalsoconsistentwiththeobservationofaSynthesisandcharacterizationofSCCoONRs.First,wereportnoticeablepeakshiftinCo-L2,3edgetowardslowphotonenergythesynthesisofSCCoONRswithtexturedpyramidalnanofacetsandCo2pXPSspectrumtowardslowbindingenergyofSCCoO2NATURECOMMUNICATIONS|7:12876|DOI:10.1038/ncomms12876|www.nature.com/naturecommunications NATURECOMMUNICATIONS|DOI:10.1038/ncomms12876ARTICLEabCoONRarraysoncarbonfiberSingle-crystalOOH–2dcOERORRCoOActivevacancy-richTexturednanopyramidsO-terminated{111}nanofacetsFigure1|SchematicillustrationofengineeringthesurfaceofSCCoONRs.(a)SCCoONRsfabricateddirectlyoncarbonfibresubstrate.(b)NumerousnanoporespresentonthesurfaceandacrossSCNRs.(c)ThesurfaceofSCCoONRscoveredwithtexturednanopyramids.(d)Thedominantexposedfacetsofnanopyramidsareelectrochemicallyactivevacancy-richO-terminated{111}facets.NRs(Fig.3bandSupplementaryFig.8),whichisanindicativeofbetterdurabilityofactivereactionsitespresentonSCCoONRselectrontransferfromO-vacanciestoCodband.Thus,a(SupplementaryFigs12andSupplementaryFig.13).Moreover,combinationofXANESandXPSresultsprovidesacrucialevenafter3,000cyclecatalytictestswithacceleratedscanrateevidenceforthepresenceofabundantO-vacanciesonthesurfaceof100mVs1,SCCoONRsstillretainedtheirstructureofSCCoONRs,thequantityofwhichisevenhigherthanthaton(SupplementaryFig.14).Thisexcellentdurabilityoriginatesthesurfaceofpolycrystalline(PC)CoONRswiththreefoldlargerfromdirectgrowthofSCCoONRsontheCFPsubstratetosurfacearea(SupplementaryTable1).OurDFTcomputationsavoidaggregatinganddetachingproblems,whichareusuallyindeedrevealthattheO-vacancyformationenergyontheencounteredinotherfacetedcatalysts49.O-terminated{111}facets(hereafter,referredtoas‘{111}-OAsregardstheOERactivity(Fig.4d),SCCoONRsdeliverafacet’)isby3eVlowerthanthecorrespondingvaluesof{100}currentdensityof10.0mAcm2(EJ¼10)at1.56VRHEandaand{110}facets(Fig.3c).Clearly,suchasignificantreductioninTafelslopeof44mVperdecade.SuchexcellentOERactivityisthevacancyformationenergyresultsinlargerconcentrationofbetterthanthatofthecommercialRuO2catalyst.Importantly,theequilibriumO-vacancieson{111}-OfacetsofSCCoONRs.ItSCCoONRsexhibitanoutstandingoverallelectrodeactivityindicatesthatthesurfacedefectscanbetunedandstabilizedasindicatedbylowervalue(DE¼0.71V)ofthedifferencethroughfacetengineering.betweentheORRandOERmetrics(DE¼EJ¼10E1/2)16,50,outperformingthemostofthereportedhighlyactivereversibleoxygencatalysts(SupplementaryTable2).ThisvalueisalsoActivityanddurabilityofSCCoONRstowardsORR/OER.As-comparabletothevalueobtainedforCo3O4NCsdepositedonfabricatedSCCoONRswithlengthof1.6mmonCFPwere11N-dopedgraphene(DE¼0.71V),whichisconsideredasthedirectlyusedastheworkingelectrodesforbothORRandOERmostefficientbifunctionalcatalyst.(SupplementaryFig.9),andtheirperformanceswerecomparedwithanalogouselectrodespreparedbyusingPCCoONRs(SupplementaryFig.10),thestate-of-artPtandRuO2catalystsEnhancementofelectronicconductivityofSCCoONRs.HighsupportedonCFP.Thepolarizationcurveswererecordedwith-activityanddurabilityofSCCoONRsclearlydemonstratesthatoutiRcorrection.AsregardsORR(Fig.4a,bandSupplementarytheas-synthesizedSCCoONRsarehighlyversatileandefficientFig.11a),PCCoONRsexhibitlowactivity,whileSCCoONRselectrocatalyststowardsbothORRandOER.Oneprerequisiteinshowanonsetpotentialof0.96Vversusreversiblehydrogenthedesignofahighlyefficientelectrocatalystisarapidelectronelectrode(VRHE),ahalf-wavepotential(E1/2)of0.85VRHEandatransfer16.ItisacknowledgedthattheelectronicconductivityofTafelslopeof47mVperdecade,whichapproachthevaluesTMOsisrelativelypoor15,greatlylimitingtheirelectrocatalyticmeasuredforPtcatalysts.Thesevaluesarebetterthanthoseofactivities.InthecaseofSCCoONRs,alargequantityofthewell-developedcobaltoxidenanocrystals(NCs)coupledwithO-vacancieslocalizedonthe{111}-Ofacets,aswellastheSCcarbonmaterials(SupplementaryTable2).Moreover,SCCoOnatureinheritedfromtheZnONRsconsiderablyenhancetheNRsshowhighselectivitytowardsORRwithstrongmethanolcarrierconcentrationinSCCoONRs,whichisoneorderhighertolerance(Fig.4a,inset).BesidesanextraordinaryactivitythanthatinPCCoONRs(SupplementaryFig.15andtowardsORR,SCCoONRsalsodemonstrateanexcellentSupplementaryNote3).Moreover,thenucleationandgrowthdurability.AsshowninFig.4c,SCCoONRsretain97%oftheofSCCoONRsdirectlyonCFPalsoassuresarapidcollectionofinitialORRcurrentafter10hcontinuoustesting,whereasPtcharges.Theaforementionedthreeimportantcharacteristicscatalystlostmorethan26%ofitsinitialcurrent,confirmingmuchremarkablyenhancetheelectronicconductivityofSCCoONRs.NATURECOMMUNICATIONS|7:12876|DOI:10.1038/ncomms12876|www.nature.com/naturecommunications3 ARTICLENATURECOMMUNICATIONS|DOI:10.1038/ncomms12876abc{111}Nanofacets–––0.25nm(111)(111)dfExperimentgSimulation(001)–––(111)(111)(100)hie{111}NanofacetsExperimentSimulation–(111)––11)(1Intensity(a.u.)Intensity(a.u.)[110]0.00.51.01.52.02.53.03.5050100150200250300350Distance(nm)Distance(pixel)Figure2|StructuralcharacterizationofSCCoONRs.(a)Top-viewSEMimageofSCCoONRsfabricateddirectlyonCFP.Scalebar,10mm.TheinsetinashowsmorphologyofSCCoONRs.Scalebar,1mm.(b)HighmagnificationTEMimageofanindividualSCCoONRwithsaw-likeedges.Scalebar,20nm.(c)High-resolutionHAADF-STEMimagetakenfromtheoutermostsurfaceofasingleSCCoONRrevealingtheexposed{111}nanofacets(indicatedbyorangeandgreenarrows),withinsetshowingthecorrespondingselectedareaelectrondiffractionpatterntakenfrom[110]zoneaxis.Scalebar,2nm.(d)Atomicmodelofananopyramidenclosedwith{100}and{111}facets,and(e)theprojectionofthispyramidalstructurealong[110]zoneaxis.(f,g)ExperimentalandsimulatedHADDF-STEMimagesofthepyramidalstructure,respectively.Scalebarinf,1nm.Notethatinelasticandneutronscatteringswerenotconsideredinthesimulation,whichcontributetothebackgroundinh.(h,i)Theintensityprofilestakenfromorangeandgreylinesinfandg,respectively.abc5OKSCCoONRsCoL2,3SCCoONRs536.0eVReferenceCoOReferenceCoO40.4eV781.5eV32Intensity(a.u.)Intensity(a.u.)Vacancyformationenergy(eV)1781.7eV0530535540545550555560770780790800810(100)(110)(111)-OPhotonenergy(eV)Photonenergy(eV)Figure3|AnalysisofO-vacanciesonthepyramidalnanofacetedsurfaceofSCCoONRs.(a,b)O-KedgeandCo-L2,3edgeXANESspectraofSCCoONRsandreferenceCoO,respectively.InathepeakatB536eVofSCCoONRsisassignedtoOdeficiency.Inbthepeaksat781.5andB800eVofSCCoONRsshifttowardslowphotonenergyrelativetothereferenceCoO,indicatingthetransferofelectronsfromO-vacanciestoCodband.(c)O-vacancyformationenergieson{100},{110}and{111}-OfacetsofCoOshowingasignificantreductioninthevacancyformationenergyon{111}-Ofacets.4NATURECOMMUNICATIONS|7:12876|DOI:10.1038/ncomms12876|www.nature.com/naturecommunications NATURECOMMUNICATIONS|DOI:10.1038/ncomms12876ARTICLEab300.42ORRORR–8.4Methanoladdition)0.3920–2–9.0Pt–9.6(mAcm)J0.36–210PCCoONRs–10.2SCCoONRs73mVdec–1–10.80.33Pt(mAcm04008001,2001,600J–1Time(s)45mVdec0Overpotential(V)0.30SCCoONRsSCCoONRs–10PCCoONRs47mVdec–1Pt0.27Carbonfiberpaper0.60.70.80.91.01.11.2–0.4–0.20.00.20.40.60.8Potential(VversusRHE)LogJ(mAcm–2)cd800.40ORROER1000.36SCCoONRs60PCCoONRs0.32–162mVdec90)0.28–2SCCoONRsOverpotential(V)044mVdec–1/J80400.24JRuO265mVdec–1(mAcmJPt0.2070–0.6–0.4–0.20.00.20.40.60.8LogJ(mAcm–2)20SCCoONRs60PCCoONRsRuO2Carbonfiberpaper50002468101.31.41.51.61.7Time(h)Potential(VversusRHE)Figure4|BifunctionalORR/OERperformanceofSCCoONRs.(a)ORRlinear-sweepvoltammograms(LSVs)ofSC,PCCoONRsandPtcatalystsdirectlydepositedonCFPinO2-saturated1MKOHsolutionatscanrateof0.5mVs1withoutiRcorrection,withORRchronoamperometricresponsetomethanoladditionshownininset.(b)ORRTafelplotsofSC,PCCoONRsandPtcatalysts.(c)ORRchronoamperometricresponseofSCCoONRsandPtcatalystsataconstantvoltageof0.60VRHE.(d)OERLSVsofSC,PCCoONRsandcommercialRuO2catalystsdirectlydepositedonCFPinO2-saturated1MKOHsolutionatscanrateof0.5mVs1withoutiRcorrection,withthecorrespondingOERTafelplotsshownininset.abcd200.16120.10ORRORROEROER100.0815)0.12))–28–2)–2–20.06100.086(mAcm(mAcm(mAcmk(mAcm0.04JJ4k,specificJspecific5J0.040.02200.0000.00SCNCsPCNCsSCNCsPCNCsSCNCsPCNCsSCNCsPCNCsFigure5|IntrinsicORR/OERactivityofSCCoONCsincomparisonwiththeactivityofPCCoONCs.(a,b)JkandJk,specificforORRat0.6VRHE,respectively.(c,d)JandJspecificforOERat1.65VRHE,respectively.NATURECOMMUNICATIONS|7:12876|DOI:10.1038/ncomms12876|www.nature.com/naturecommunications5 ARTICLENATURECOMMUNICATIONS|DOI:10.1038/ncomms12876aO-OdistanceO1.24Å1.35Å1.23Å1.26Å2OCo{100}{110}{111}-O{111}-Ovb0=1.23V3URHE{100}ORR2{111}-O1{110}{111}-Ov(eV)0GΔOOOHOH2–1O–2OHOER–3ReactionpathwaycPristineCoOEFPDOSTotalO-2pCo-3dCo-3sdCoOwithO-vacanciesPDOSTotalO-2pEFCo-3dCo-3s–4–3–2–1012Energy(eV)Figure6|OriginofORR/OERactivityonvariousfacetsofCoO.(a)AtomicconfigurationsofO2moleculeson{100},{110},{111}-Oand{111}-OVfacets.Notably,on{110}and{111}-Ovfacets,theO–ObondofadsorbedO2isremarkablyelongated(theO–OdistanceofO2is1.23Å),suggestinganeffectiveactivationofO2inORR.(b)ThecalculatedORR/OERfreeenergydiagramattheequilibriumpotentialondifferentfacets.(c)Theprojecteddensityofstates(PDOS)onpristineCoOand(d)CoOwithO-vacancies.Thearrowindpointsnewelectronicstates,whichappearneartheFermilevelinCoOwithO-vacancies,responsiblefortheadsorptionofintermediatesontheO-vacancies.IntrinsicactivityofSCCoONCstowardsORR/OER.TospecificORRkineticcurrentdensity(Jk,specific)andthespecificdecoupletheenhancedactivityofSCCoONRsfromtheOERcurrentdensity(Jspecific),respectively(seedefinitionincontributionofadvanced1Dnanoarrayarchitecture,theSupplementaryTable3).AsshowninFig.5a,b,forORR,JkandintrinsicORR/OERactivityofSCCoONCswithsizesofJk,specificofSCCoONCsat0.6VRHEareabout4.2and7.2timesB50nm(SupplementaryMethod)wasevaluatedincompar-greaterthanthoseofPCCoONCs,respectively.AsregardsisontothatofPCCoONCs(SupplementaryFigs16–18andOER,JandJspecificofSCCoONCsat1.65VRHEareaboutSupplementaryNote4)andotherwell-developedparticulate1.5and2.6timeslargerthanthoseofPCCoONCs,respectivelycobaltoxidecatalysts(SupplementaryTable6).TheORR(Fig.5c,d).ThesecollectiveresultsclearlydemonstratethatthekineticcurrentandOERcurrentarenormalizedbytheintrinsicORR/OERactivityofCoOisstronglydependentonelectrochemicallyactivesurfaceareaofcatalysttoobtainthethesurfacestructure.6NATURECOMMUNICATIONS|7:12876|DOI:10.1038/ncomms12876|www.nature.com/naturecommunications NATURECOMMUNICATIONS|DOI:10.1038/ncomms12876ARTICLEORR/OERfreeenergydiagramandelectronicstructure.AnanoarrayelectrodescanpaveanewavenueforthefabricationofseriesofDFTcomputationswasconductedtogetafundamentalefficientanddurableTMO-basedelectrochemicaldevices.understandingofthecorrelationbetweenthesurfaceatomicstructureofCoOandtheORR/OERactivity.Fig.6aclearlyMethodsrevealsthattheatomicarrangementondifferentfacetsSynthesisofSCCoONRsonCFPsubstrate.ZnONRswithtailorablelengthsignificantlyaffectsadsorptionsitesandconfigurationofthe(SupplementaryFigs4and20)weregrownonCFPunderhydrothermalconditionsreactant,thatis,O2,inORR.Moreover,theoverallORR/OERandfinallyconvertedintoCoONRsusingacationexchangeprocessingasphasepathwaywascalculated,andthefreeenergydiagramatthe(SupplementaryFig.1).Specifically,theCFPloadedwithZnONRswasplacedin0thecentreofaquartztubeandcobaltchloride(CoCl2)powderwasplaced2.5cmequilibriumpotential(URHE¼1.23V)areshowninFig.6b.Asupstreamfromthetubecentre.Afterthequartztubewasoutgassedundervacuum,reportedbyNørskovetal.51–53,bothORRandOERinvolvefourargon(Ar)gasflow(50s.c.c.m.)wasintroducedintothesystem.Thefurnacewaselementaryreactionsteps,inwhichORRproceedsthroughtheheatedtoandkeptat600,650or700°Cfor30min,andthencooleddowntoroomformationofHOO*fromadsorbedO2,followedbyitsfurthertemperature.ItisfoundthatthecationexchangetemperaturecanconsiderablyaffecttheconcentrationofO-vacancies(SupplementaryTable7),andthusthereductiontoO*andHO*,whileOERproceedsinthereverseelectrocatalyticperformanceofSCCoONRs(SupplementaryFigs21–23anddirection.ForbothORRandOER,theidealthermodynamicfreeSupplementaryNote5).Theoptimalexchangetemperatureis600°CandusedinenergychangeoftheintermediatesshouldbejDGOOHj¼jthisstudyunlessspecificallynotified.Theloadingmassofas-synthesizedSCCoODGj¼jDGj¼0(refs51,52),indicatingnoenergywouldNRswasB0.19mgcm2.OOHbewastedtoactivatethereactions.AsillustratedinFig.6b,forORRalargejDGOOHjonthesurfaceof{100}and{111}-OCharacterization.SEMandTEMimagesweretakenonaHitachiS-4800SEMandfacetsindicatesthatthefirstelectrontransfersteptoreducetheaJOEL2100TEM,respectively.HAADF-STEMimageswerecollectedusingaadsorbedO2toOOH*isendothermic,whichisconsistentwithJEOLARM200FmicroscopewithSTEMaberrationcorrectoroperatedat200kV.HADDF-STEMimagesimulationwascarriedoutusingasoftwarepackageobservationsforthewell-developedmetalandmetaloxideMacTempasX.Theconvergentsemiangleandcollectionanglewere21.5andcatalysts17,53.Besides,thelargenegativeDGOandDGOHon200mrad,respectively.Theaberrationcoefficient(C)usedwasequalto1mm.s{110}and{100}facetsindicatethatthechemicaladsorptionofO*Inelasticandneutronscatteringswerenotconsideredinthesimulation.TheandOH*,respectively,istoostrong,whichisalsounfavourablesynchrotron-basedXANESmeasurementswerecarriedoutusingthesoftX-rayspectroscopybeamlineattheCanadianSynchrotron.XANESspectrawereforthesubsequentelectrocatalyticreactions.However,whenrecordedinthesurfacesensitivetotalelectronyieldwithuseofspecimencurrent.O-vacancyiscreatedonthesurfaceof{111}-O(hereafter,Allsampleswerescannedfrom750to820eVandfrom510to580eVin0.1eVreferredtoas‘{111}-OVfacet’),theformationofOOH*issteps,whichencompassestheCo-L2,3andO-Kabsorptionedges,respectively.facilitated,andalljDGOOHj,jDGOjandjDGOHjexhibitthelowestvaluesamongthefourfacets,suggestingthemostElectrochemicalcharacterization.ElectrochemicalmeasurementswerefavourableORRkineticsonthe{111}-OVfacets.Asregardsperformedinathree-electrodeelectrochemicalcellusinganHg/HgOelectrodeinOER,asimilaranalysisofthediagramforthereversereactionsaturatedKClsolutionasthereferenceelectrode,Ptplateasthecounterelectrodeshowsthatthe{111}-OvsurfaceoutperformstheotherthreeandtheCFPelectrodeastheworkingelectrode(SupplementaryFig.9).AflowofO2wasmaintainedovertheelectrolyte(1.0MKOH)duringmeasurementstofacets.Overall,the{111}-OvsurfaceexhibitsamediatedensuretheO2/H2Oequilibriumat1.23VRHE.adsorption–desorptionbehaviour(jDGOOHjjDGOjjDGOHj!0),whichisbeneficialfortheoverallORR/OER.Thus,thetheoryandexperimentareinanexcellentagreement,DFTcalculations.AllDFTcomputationswereperformedusingViennaAb-initioSimulationPackage.AneffectiveUvalueof3.7eVwasappliedforCo3dstates.suggestingthattheORR/OERactivityissuccessfullyenhancedTheprojectoraugmentedwavepseudopotentialwiththePerdew–Burke–Ernzerhofthroughatomicstructureengineering.Ourresultsdemonstrateexchange-correlationfunctionalwasusedinthecomputations.Therelevantdetails,thatthereisastrongcorrelationbetweenactivityandatomicreferencesanddataaregivenintheSupplementaryMethodssectionandinstructureofCoO;thatis,theORR/OERactivityofCoOincreasesSupplementaryTables4and5.inthefollowingorder{100}o{110}o{111}-Ov.Toourknowledgethisatomicscalestructure–functionrelationshiphasDataavailability.ThedatathatsupportthefindingsofthisstudyareavailablenotbeenconsideredforanyotherTMOsurfacesintheanalysisoffromthecorrespondingauthoronrequest.electrocatalysts.ThenatureoftheO-vacanciesonthe{111}-OfacetsisfurtherReferencesrevealedthroughinvestigationoftheirelectronicstructure1.Lewis,N.S.&Nocera,D.G.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