Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(5)

SynthesisofMesoporousR-Fe2O3NanostructuresFigure6.MagnetichysteresisloopsofmesoporousR-Fe2O3measuredatdifferenttemperatures:5,70,and300K.

momentofoursampleat1Tand200Kisabout0.6emu/g.Hematitesinglecrystalhasmagneticsusceptibilityat200Kforparallel-6andperpendiculardirectiontotheeasyaxisofabout3×10and18×10-6emu/(gOe),respectively.43Thisresultsinamagneticsusceptibilityforpowderwithrandomlyorientedcrystalsof13×10-6emu/(gOe).Takingintoaccountthisantiferromagneticsusceptibility,surfacespinscontribute0.47emu/gtothemagneticmomentat200Kand1T(Figure5).Thisissubstantiallymorethantheadditional～0.15emu/gcontributedbytheMorintransitionjustabove200K.TheArtman-typemagneticmomentinmacroscopichematitecrystalsis0.4emu/g,43whichissubstantiallyhigherthantheincreaseby0.15emu/gattheMorintransitionforoursample.Thisisprobablybecauseoursampleconsistsofsomecrystallitessmallerthantheparticularsized0atwhichTMvanishesiftheparticlesizesaresmallerthand0.Thevalueofd0canbeestimatedfromthereportbyZysleratal.tobeabout10nm.41TheMorintemperatureTMdecreaseswithparticlesizeandstrainsintheparticles.44Therefore,particlessmallerthand0donotundergoMorintransitionatthemeasuredtemperatures.Instead,theseparticleshavetheArtman-typeferromagneticcomponentatallmeasuredtemperaturesasshownbyBodkeratal.45Asaresult,theincreaseofthemagneticmomentuponMorintransition,normalizedtothetotalsamplemass,issmallerthanforthesampleswherealloftheparticlesundergoMorintransition.Therefore,thesesub-d0particlescontributetothesuperparamagneticbehaviorbecauseoftheirArtman-typemagneticmoment.Therearealsounpairedsurfacespins.Forparticlessmallerthand0,thesesurfacespinswillcontributetosuperparamagneticbehaviorbecausetheyarecoupledtothecantedArtman-typebulkmagneticmomentatalltemperatures.Couplingoftheunpairedsurfacespinstotheantiferromagneticbulkorderisalsolikelytocontributebecausethiscouplingisdisturbedbythesurfacedisorderforparticlesofanysizes.Thus,bothsurfacespinsandArtman-typemagneticmomentofsub-d0particlescontributetothemagneticmomentof0.47emu/gjustbelow200Kat1T.ThemesoporousR-Fe2O3seemstobeinsuperparamagneticstatefor50K<T<200Kasindicatedbymagnetichysteresisloops(Figure6).TheunpairedsurfacespinsandtheArtman-typecontributionofsub-d0particlesareresponsibleforthissuperparamagneticstate.ThereisastrongincreaseofmagneticirreversibilityforT<50K(Figure5)indicatingyetanothermagnetictransition.AsimilareffectwasalsoreportedbyJiaoetal.28

Mostlikelythiscanbeascribedtotheblockingtransitionat50K,wheresuperparamagneticspinsfreeze-inuponcoolingandgiveanalmostconstantmagneticmoment.Thismomentiszeroifcoolingisinzero eld.WithwarmingupinZFCWmeasurements,themagneticmomentriseswithtemperatureas

J.Phys.Chem.C,Vol.114,No.44,2010

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Figure7.Temperaturedependenceofthecoercive eldformesopo-rousR-Fe2O3.

anincreasingnumberofspinsgetsunfrozenbythermalexcitationandgetsalignedinthedirectionofthe eld.Finally,aboveblockingtemperature,superparamagneticsystemoccurs.MeasurementsinFigure5areconsistentwiththispictureasZFCWmagneticmomentincreasesfromalowvalueat5Kuptoitsmaximumvalueattheblockingtemperatureof50K.Above50K,thesampleisinsuperparamagneticstateforH)1Tshowingaverysmallcoercive eldandS-shapedhysteresisloopstypicalofsuperparamagneticparticles(Figures6and7).Thistransitiontemperatureis eld-dependent.Itincreasesfrom53Kat1Tto96Kat0.01T.ThisisalsoconsistentwiththepictureofunfreezingspinsasthetemperatureincreasesinZFCWmeasurements.Ahighermagnetic eldexertslargerforceonmisalignedspins,andalowerenergyofthermalexcitationisrequiredtoaligntheminthedirectionofthe eld.Withthelower eld,higherthermalexcitationsarerequiredforthesameeffect.

TheZFCW,FCC,andFCWcurvesdonotoverlapabove200K.ThereasonforthiscouldbetheirreversibilitiesassociatedwiththeorderingofsurfacespinsthatarestillnotsuppressedatthesetemperaturesaswellasArtman-typeferromagneticorderaboveTM.However,thereisalsoapossibilityofmagneticnanoparticlesrotatinguponthermalcyclingwithintheagglomeratesinadifferentwayforFCC,FCW,andZFCWmeasurements.Thischangestheinteractionbetweenthenanoparticlesandresultsinadditionalirreversibili-tiesthatdependonthewaythemeasurementsareperformed.41AgoodindicationforthiseffectisthatthemagneticmomentforT>200KinFigure5measureduponwarmingdecreaseswithtemperaturewiththesamerateforZCFWandFCWmeasurements.However,magneticmomentmeasureduponcooling(FCC)inthesametemperaturerangeandthesame elddecreaseswithtemperatureatahigherrate.

Thecoercive eldofournanoparticlesatroomtemperatureismuchlargerthanforbulkcrystals(Figure7).Thecoercive eldcanbestronglyaffectedbydistributionofsizesofnanoparticlesaswellasbytheirinteractionwithintheag-glomerates.Hcofsimilarmagnitudetooursampleswasobtainedforhematitenanoparticlesthatweredenselypackedintoagglomerates.Looselypackednanoparticlesgave～10timeslowerHc.42Thissupportsourargumentontheimportanceofnanoparticleinteractiontotheirmagneticproperties.

Magnetichysteresisloopsareinagreementwiththeabovedescription(Figure5).Thehysteresisloopmeasuredat5Kstronglydiffersfromtheonesmeasuredabove50K.Ithasmuchlargercoercive eld,Hc,thanthehysteresisloopsmeasuredbetween50and200K(Figures6and7).Thehysteresisloopsbetween50and200KaredistinctlyS-shapedwithvanishinglylowcoercive eldasexpectedforsuperparamagneticparticles.HcstartsincreasingwiththeonsetofArtman-typeferromag-