Mesoporous α-Fe2O3 Nanostructures for Highly Sensitive Gas S(6)

18758J.Phys.Chem.C,Vol.114,No.44,2010netismfornanoparticleslargerthand0at～200K(Figure7).SuperparamagneticbehaviorseemstobesuppressedforT>200KbecauseofanadditionalmagneticmomentarisingfromArtman-typeofordering.Theinteractionbetweenthenanopar-ticleswithinthemesoporousstructuresprobablyplaysanimportantroleintheseprocesseseventhoughitsmagnitudeandtheexactmechanismsarestillunknown.TheincreaseofHcandtheappearanceofferromagneticorderattemperatureshigherthantheblockingtemperaturemaysoundconfusing.However,therearetwofamiliesofspinscontributingtothemeasuredmagneticmoment:unpairedsurfacespinsandbulkspinsduetoArtman-typetransition.Whenbothofthemareorderedferromagnetically,thenetmagneticmomentmaybelargeenoughtogivetheobservedferromagneticbehavior.Nanoparticleagglomerationprobablyalsoplaysaroleherethroughmagneticcouplingofnanoparticles.

Therefore,surfacespinsandArtman-typemagneticmomentofsub-d0nanoparticlesdominatethemagneticbehaviorofmesoporousR-Fe2O3below200K.Thereisevidenceforsuperparamagneticbehaviorofthissystemwithblockingtemperaturearound50Kat1T.Morintransitionofparticleslargerthand0occursat200Kgivingadditionalferromagneticcontributionabove200K.Themagneticmomentofthesurfacespinsandsub-d0particleswithArtman-typeorderingisabout3timeshigherthantheoneduetoMorintransitionofparticleslargerthand0at200Kand1T.Thereseemstobeaneffectoforientationofthenanoparticleswithinthemesoporousstructuresonthenetmagneticmoment.Asthisorientationchangeswith eldandtemperature,sodoesthemagneticinteractionbetweenthenanoparticlesgivingamagneticmomentthatdependsonhistoryofthermalcyclinginmagnetic eld.

3.3.Gas-SensingPerformance.Thegas-sensingperfor-manceoftheas-preparedmesoporousR-Fe2O3wasinvestigatedtowardavarietyof ammable,toxic,andcorrosivegasessuchasethanol,acetone,gasoline,heptane,formaldehyde,aceticacid,1-butanol,and2-propanol.Figure8adisplaysthereal-timesensingresponsecharacteristicstowardethanolofasensorbasedontheas-preparedmesoporousR-Fe2O3ataworkingtemper-atureof150°Cand30%relativehumidity(RH).Itcanbeseenthatthevalueoftheoutputvoltageincreasedabruptlyaftertheinjectionofethanolandrecovereditsinitialvalueafterreleaseofthegas.Theresponseandrecoverytimes(de nedasthetimerequiredtoreach90%ofthe nalequilibriumvalue)oftheas-preparedmesoporousR-Fe2O3basedsensorwereonly1-3seachindicatingtheirrapidresponseandgoodreversibility.FromOhm’slaw,theelectricresistanceofthesensorunderwentadecrease(increase)whentheethanolwasinjected(released),whichisthetypicalsensorbehaviorofn-typesemiconductorsensors.Intheambientenvironment,n-typeR-Fe2O3isexpectedtoadsorbbothoxygenandmoisture.TheadsorbedO2-andOH-groupstrapelectronsfromtheconductionbandoftheR-Fe2O3nanocrystalsandincreasetheresistanceofthesensor.Whenthesensorisexposedtoethanol,thetestgasmoleculesarechemiadsorbedattheactivesitesonthesurfaceoftheas-preparedmesoporousR-Fe2O3.Thesemoleculeswillbeoxidizedbytheadsorbedoxygenandthelatticeoxygen(O2-)ofR-Fe2O3atthesensorworkingtemperature(150°C).Inthisprocess,electronswillbetransferredtothesurfaceoftheas-preparedmesoporousR-Fe2O3tolowerthenumberoftrappedelectronsinducingadecreaseintheresistance.10Themagnitudeoftheresponseforasensorbasedontheas-preparedmesoporousR-Fe2O3improveddramaticallywithincreasingtheconcentra-tionoftheethanolfrom5ppmto1000ppmsuggestingthattheas-preparedmesoporousR-Fe2O3hasgoodsensitivitytoward

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Figure8.(a)Real-timeethanolsensingcharacterizationofsensorsbasedontheas-preparedmesoporousR-Fe2O3.(b)Sensitivityofthesensorsbasedonas-preparedmesoporousR-Fe2O3towarddifferentgasesorvapors.

ethanol.Aftereightcyclesbetweentheethanolandfreshair,theoutputvoltagescouldstillrecovertotheirinitialstatesindicatingexcellentreversibility.Figure8bpresentsthesensitiv-ityofasensorbasedontheas-preparedmesoporousR-Fe2O3towardavarietyofgasesatdifferentconcentrations.Thegassensitivity(S)isde nedastheratioofthestationaryelectricalresistanceofthesensorinair(Rair)toitsresistanceinthetestgas(Rgas),thatis,S)RR-Feair/Rgas.Theas-preparedmesoporous2O3exhibitedaveryimpressivesensitivitytowardethanolathighconcentration(1000ppm),whichisbetterthantheperformanceofitsnanosphereandnanowirecounterpartsaswepreviouslyreported.9,10Theimprovementofthesensingper-formanceoftheas-preparedmesoporousR-Fe2O3maybeattributedtothemesoporousstructure.Thehighlyporousstructureleadstohighspeci csurfacearea,whichismeasuredtobe128m2/gresultinginmoreactivesitesforgaschemi-sorption.Theabundantopenporesdistributedinthethree-dimensionalspacecanfacilitatethediffusionofthetestgasandcanimprovethekineticsofboththereactionofthetestgaswithsurfaceadsorbedoxygenandthereplacementofthelatterfromthegasphase.FromFigure8b,wecanalsonotethattheas-preparedmesoporousR-Fe2O3sensoralsohashighsensitivitytowardsomeother ammableandexplosivegasessuchas92#gasoline,2-propanol,1-butanol,heptane,andacetone.However,thesensorcouldbarelydetectheptaneevenathighconcentration(e.g.,1000ppm).Forapplicationinairqualitymonitoring,wealsoinvestigatedthesensingperformanceoftheas-preparedmesoporousR-Fe2O3tosometoxic,corrosive,