Resultsanddiscussion解读

3.ResultsanddiscussionCyclicvoltammogram(CV)ofPtelectrodepositiononGCelectrodein19.3mMH2PtCl6/DESssolutionat80◦CisshowninFig.1.Fromthisfigure,inthenegative-goingpotentialscan,itcanbeclearlyobservedthattwocurrentpeaksofreductionoccurredatnear−0.93Vand−1.29V(vs.Pt),incomparisonwiththevoltammogram(insetofFig.1)recordedonthesameGCsubstrateinDESs.X-rayphotoelectronspectra(XPS)resultsdemonstratedthatthesetworeductionpeaksarecorrespondingtotheelectrochemicalreductionofPt(IV)toPt(II)andPt(II)toPt(0),respectively(Fig.S1andTableS1insupplementaryinformation).ThisisconsistentwiththeelectrodepositionbehaviorsofPtinhydrophilic1-n-butyl-3-methylimidazoliumtetrafluoroborate(BMIMBF4)andhydrophobic1-n-butyl-3-methylimidazoliumhexafluorophosphate(BMIMPF6)room-temperatureionicliquids[36].Fig.2adisplaysthetypicalSEMimageofPtnanoflowerselectrodepositedonGCbyusingCVmethodin19.3mMH2PtCl6/DESssolutionat80◦C.ItcanbeseenthatthePtnanoflowerswithsharppetalswerehomogeneouslyformed,andtheirsizewasabout200nm.ThecrystalstructureofPtnanoflowerswasfurtherinvestigatedbyhighresolutiontransmissionelectronmicroscopy(HRTEM).Fig.2bshowstheTEMimageofasinglePtnanoflower,andtheinsetisthecorrespondingselectedareaelectrondiffraction(SAED)pattern,whichindicatesthatthepetalsofas-preparednanoflowerspossessthesingle-crystallinestructure.TheHRTEMimageofapetalmarkedinFig.2bisdisplayedinFig.2c.Thecontinuousfringepatternfurtherverifiesthesinglecrystallinepropertyofthepetal.Thelatticespacingof0.23nmagreeswiththedistancebetweentwo{111}planesofPt.AscomparedtootherPtnanoflowersandnanothornassembliesreportedpreviously[16–19,37],theuniquecharacteristicofas-preparedPtnanoflowersistheformationofhighdensityofatomicstepsattheedgeofthepetals(Fig.2c),whicharecrucialfortheenhancedactivityofPtnanoflowerstowardethanolelectrooxidation.TheenergydispersiveX-rayspectroscopy(EDX)analysisofPtnanoflowersconfirmsthepresenceofonlyPt,CandOelements(Fig.2d),indicatingnoDESsresidueonthesurfaceofPtnanoflowers.Theeffectofdepositionconditions,namely,theprecursorconcentration,CVnumberofcycle,scanrateandtemperatureonthesizeandmorphologyofPtnanostructureselectrodepositedinDESswasexamined.Fig.3showstheSEMimagesofPtnanostructurespreparedbyusingthedifferentconcentrationsofH2PtCl6.Wecanseethat,attheH2PtCl6concentrationof1.93mM,thequasisphericalPtNPswereformed(Fig.3a),andtheirsizerangedfrom45to95nm.WhentheH2PtCl6concentrationwas5mM,theflowerlikePtNPswithoutsharppetalsandseveralcubicPtNPsappeared(Fig.3b).FurtherincreasingtheH2PtCl6concentrationto10mM,thesharppetalsstartedtoappearattheedgeofthePtnanoflowers(Fig.3c).Finally,theperfectPtnanoflowerswithsharppetalswerehomogeneouslyformedattheH2PtCl6concentrationof19.3mM(Fig.3d).TheconcentrationdependenceoftheabovePtnanostructuresmayberesultsfromthehighviscosityofDESs[25,26],whichdecreasedthemasstransportationofreactivespeciesinDESs,leadingtothedifficultformationofPtnanoflowerswithsharppetalsatthelowerH2PtCl6concentration.Amongalldepositionconditions,theCVnumberofcycleexertsaleadinginfluenceontheprocessofparticlegrowth.DifferentPtnanostructuresgeneratedbyvariousCVnumberofcyclewereobtained,asshowninFig.4.Someirregularquasi-sphericalnanoparticleswithlowsurfacecoveragewereproducedinthelowerCVnumberofcycle(Fig.4aandb),whichactedasthenucleiforsubsequentlyproducingPtnanoflowers[17,19,38].Sincethenucleationprocessisrelativelyslowandirreversible,newlydepositedPtfavorsgrowingonthesmallPtcoresinsteadofgeneratingmorenewnuclei[39].IncreasingtheCVnumberofcyclewouldresultincomplexmonodispersenanoflowerswithmoresharppetalsandlargersize(Fig.4c).FurtherincreasingtheCVnumberofcycleto80cycles,theperfectPtnanoflowerswithsharppetalswereformedontheGCsubstrate(Fig.3d).WhentheCVnumberofcyclewasincreasedto100cycles,theaggregationphenomenonofPtnanoflowerswasobserved(Fig.4d).Inaddition,thescanrateplaysimportantrolesintheelectrodepositionofPtowingtoitspossibleinfluenceontheanisotropicgrowthofnanoparticles.Atthelowerscanrateof1mVs−1,theroughnanosheetsformedonthetopofPtnanostructures(Fig.S2insupplementaryinformation).FurtherincreasingthescanratewouldresultinthegrowthofPtnanoflowerswithsharppetals.Itisworthwhilenotingthatallelectrodepositionprocessesdiscussedabovewerecarriedoutat80◦C,whenthelowerorhigherdepositiontemperaturewasused,thesharppetalsintheresultantPtNPswerepoorlydeveloped(Fig.S3insupplementaryinformation).ThereasonforsuchanobviousdifferenceinsizeandshapeofPtNPspreparedatdifferenttemperaturescouldbethevariationofDESsviscositywiththetemperature[25],whichinfluencesthediffusionofreactivespeciesinDESsandthusthenucleation/growthprocessofPtonGCsubstrate.ThePtnanoflowersofvariousshapescanbealsoobtainedbyadjustingthedepositionpotentialofthepotentiostaticreduction.TypicalSEMimagesofPtnanoflowerspreparedatdifferentpotentialsareshowninFig.5.Atthedepositionpotentialof−0.95V,manypricksappearedandextrudedfromthesurfacesofthestuddedparticles(Fig.5a).Whenthedepositionpotentialwas−0.90V,thenanosheetassemblieswereformedonthetopofPtnanostructures(Fig.5b).Asthedepositionpotentialbecamemoreposit