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Room temperature gas sensor based on tin dioxide-carbon nanotubes composite films

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Room temperature gas sensor based on tin dioxide-carbon nanotubes composite films
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  SensorsandActuatorsB 190 (2014) 227–233 ContentslistsavailableatScienceDirect Sensors   and   Actuators   B:   Chemical  journalhomepage:www.elsevier.com/locate/snb Room   temperature   gas   sensor   based   on   tin   dioxide-carbon   nanotubescomposite   films Frank   Mendoza a , b , ∗ ,   Dionne   M.   Hernández a , c ,   Vladimir   Makarov a , b ,   Emmanuel   Febus a , b ,BradR.   Weiner a , c ,   Gerardo   Morell a , b a InstituteforFunctionalNanomaterials,UniversityofPuertoRico,PR00931,USA b DepartmentofPhysics,UniversityofPuertoRico,SanJuan,PR00936,USA c DepartmentofChemistry,UniversityofPuertoRico,SanJuan,PR00936,USA a   r   ti   c   l   e   i   n   f   o  Articlehistory: Received29December2012Receivedinrevisedform10August2013Accepted14August2013 Available online xxx Keywords: HF-CVDprocessGassensorCarbonnano-materials a   b   s   t   r   a   c   t Chemical   sensors   based   ontin   dioxide-carbon   nanotubes   (SnO 2 -CNT)   composite   films   were   fabricatedby   hot   filament   chemical   vapor   deposition   (HF-CVD)   technique.   The   composite   films   consist   of    SnO 2 nanoparticles   highly   dispersed   on   the   CNTs   surface.   Their   resistivity   is   highly   sensitive   to   the   presenceof    adsorbates,   which   become   easily   attached   or   detached   atroom   temperature   and   ambient   pressuredepending   ontheir   gas   phase   concentration.   We   systematically   studied   the   sensitivity   of    the   SnO 2 -CNTcomposite   films   for   ethanol,   methanol   and   H 2 S.   The   results   were   also   compared   to   those   for   SnO 2 and   CNTsseparately.   Itisshown   that   the   SnO 2 -CNT   composite   films   can   detect   ethanol,   methanol   and   H 2 Sdownto   ppm   levels   below   OSHA’s   permissible   exposure   limits   at   room   temperature   and   ambient   pressure.Moreover,   they   self-recover   within   1minwithout   requiring   any   heating   or   energy   source. © 2013 Published by Elsevier B.V. 1.Introduction Tindioxide(SnO 2 )hasbeenstudiedbydifferentresearchgroupsduetoitscapacitytoadsorbmoleculesinthegasphase[1–3]andLi- ionsinrechargeablebatteries[4–7],improvingtheperformanceof  bareCNTs.SnO 2 isan-typesemiconductormetaloxidewithabandgapof3.6eV[8],makingitafavorablematerialforsensingdevices [9–11].Ithasapplicationsinenvironmentalproblemscausedbytheemissionofpollutantsfromassortedsources,suchasCO,NO  x ,SO 2 [12]andH 2 S[13],whichrepresentagreatconcernforpublicsafety. Hydrogensulfideiswellknownintheoilandfoodindustry,andinorganicwasteinareassuchassewers.Inthecaseofalcohols,theyhaveimportantusesinthepharmaceuticalandfoodindustry.SnO 2 iswidelyusedforalcoholdetection,especiallyethanol[14].AlthoughundopedSnO 2 gassensorshavelowsensitivityandselec-tivity[15],theycanbeimprovedbycombiningwithothermaterials [16].Studiesdemonstratethatcarbonnano-materials(CNMs)[17,18](i.e.,carbonnanotubes,nanobelts,nanowires)aregoodcandi-datesaspotential“dopants”ofSnO 2 .Ingeneral,CNMsinfluencethematerials’chargetransfer,electrostaticenvironment,surfacearea,andmolecularadsorption[19],thusimprovingtheproper- tiesrequiredforgassensing[20].GassensorsbasedonCNMs ∗ Correspondingauthor. E-mailaddresses: frank.w.mendoza@uprrp.edu,frankmendoza1@gmail.com(F.Mendoza). intheirbareform(withoutusingdopantsorhybridmaterials)havebeenshowntodetectnitrogen[21],hydrogen[22],ammo- nia[23],andothergasandvapors[24].Moreover,theirsensitivity canbeenhancedbyblendingwithactivatedmaterials[18,25,26].However,thesesensorsconventionallyoperateattemperaturesabove ∼ 200 ◦ Ctoobtainreasonableresponses[18,27,28].Further- more,thebaselinegasesusedandanalyteconcentrationaffecttheresponseandrecoverytimes[29]ofthesensors.Wei   etal.[19]combinedSnO 2 withsinglewallcarbonnanotubestodevelopagassensorforNO 2 atroomtemperature.TheyreportedenhancedsensitivitiesofthehybridsensorcomparedtopureSnO 2 ,buttheresponsetimeislong,intheorderofminutes.Chenetal.[30]pre- paredCNT/SnO 2 core/shellstructurestomeasureethanolgasatvariousconcentrationswithaworkingtemperatureof300 ◦ C.Thecore/shellconfigurationandworkingtemperatureenhancedthesensitivityupto24.5atlowgasconcentrations.Weherebyreporttheroom-temperaturesensorcharacteristicsoftindioxide-carbonnanotubescompositefilms.Theycandetectethanol,methanolandH 2 Sdowntoppmlevels,withexcellentresponseandrecoverytimesatroomtemperatureandambientpressure. 2.Experimental  2.1.Materialsandmethods Fig.1depictsaflowchartofthemethodusedforthesynthesisof tindioxide-carbonnanotubescompositefilmsandarepresentative 0925-4005/$–seefrontmatter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.snb.2013.08.050  228 F.Mendozaetal./SensorsandActuatorsB 190 (2014) 227–233 Fig.1. SchematicofthesynthesisprocessforSnO 2 -CNTscomposite. electronmicroscopyimageoftheproduct.Thefilmsweregrownon14-mmdiameter,0.5mmthickcoppersubstrates(GoodFel-low,99.9%pureCu).Thesesubstratesweremechanicallypolishedwithsandpapersdownto2500gritandcleanedbysonicationinmethanol.A2:1ethanolsolutionofSnO 2 nanopowder(Nanostruc-tureandAmorphousMaterials,Inc.,99.5%)andNiOpowder(AlfaAesar,99%)wasdepositedovertheheatedsubstrateuntilthesur-facewascompletelycoveredandthesolventwasevaporated.ThecoatedsubstrateswerethenintroducedinacustommadeHF-CVDchamberwhichhasbeenpreviouslydescribedindetailelsewhere[31].Thegasmixtureconsistedof2.0%methanedilutedinH 2 .Itwasflownat100sccmand35Torr,andkeptconstantduringthedepositiontime.Thecompositefilmsweregrownat ∼ 700 ◦ Cfor5min.Thisdepositiontimewasselectedinordertoachieveafullycoveredfilmthatdidnotexperiencepeelingordelamination.Thesurfaceandnanostructureofthecompositefilmswereanalyzedbyhighresolutionscanningelectronmicroscopy(HRSEM)withaJEOL  JSM-7500F.  2.2.Gassensingprocedure ThesetupemployedforgasdetectionisshowninFig.2.Nitro- genwasusedascarriergasataflowrateof100sccm.Mixturesof 50,100,150and200ppmwerepreparedinadvanceforeachgas:H 2 S,C 2 H 5 -OHandCH 3 -OH.Thecyclesconsistedof100sofwaittimefollowedby100sofgasdetection.Standardgastestingwasperformedatroomtemperature,atmosphericpressure,andinN 2 atmospherewithnegligiblerelativehumidity.Nonetheless,testsdoneinopenairinthepresenceofambientvapor(relativehumid-ity ∼ 57%)showednosignificantchangesinthesensors’response,inthe3–5%range.Notealsothatthedeviceswereexposedtoopenairpriortotestingandwedidnotapplyanyspecialproceduretoremovetheadsorbedwatervapor.Thedeviceusedisdescribedindetailelsewhere[32].Briefly, thedeviceconsistsofaplasticplateof2cm × 2cm,   towhich4pol-ishedrectanglealuminumelectrodesareattachedby4stainlesssteelscrews.Sampleswereplacedbetweentheplasticplateandaluminumelectrodeswhilethebacksidesubstratesurfacewasattachedtoplastictheplatesurface.Fig.3showsageneralrep-resentationofthesensordeviceconnections. R r  isthereferenceresistanceand V  m isthemeasuredvoltage.Abiasvoltage( V  s )of3Vwasconstantlyappliedtothesensor,whichbehavesasaresistance.Themostcommonmodel[13,33,34]ofgassensingmechanismformetaloxide-basedsensorsinvolvesadsorption,reactiongas-surface,anddesorption.Inordertounderstandthebehaviorofoursensorswemeasuredtheirsensitivitytowardtoanalytes,followingreference[19],astheratiobetweenthechangeinrelativeresistance Fig.2. Diagramofthegassensingexperimentalsetup.  F.Mendozaetal./SensorsandActuatorsB 190 (2014) 227–233 229 Fig.3. Deviceconfigurationforgassensing. (  R )andthechangeintheconcentration(  C  )ofthedesiredgas(analyte),usingthefollowingequation:Sensitivity = R analyte / R line Concentration = RC  (1)where R line and R analyte representtheelectricalresistanceofthedeviceinpureN 2 andinthemixtureofN 2 plustheanalyte,respec-tively.Thevaluesof  R line and R analyte areobtainedthroughbythefollowingrelationships: R analyte = R r  × V  s − V  analyte V  analyte and R line = R r  × V  s − V  line V  line (2)wherethemeasuredvoltage( V  m = V  s − V  (analyte , line) )istakendirectlyfromthesignalwhenitreachessaturation(signalceiling)asafunctionoftime.We   appliedthisalgorithmtoallthedevicestestedinthisstudy.We   testedsensorswithSnO 2 -CNTcompositeastheactivematerialand,forcomparisonpurposes,we   alsotestedsensorswithbareCNTsandbareSnO 2 . 3.Resultsanddiscussion  3.1.Morphologyoftheactivematerial TheHRSEMimages(Fig.4)showthattheSnO 2 -CNTcompositefilmsconsistofrandomlyorientedcarbonnanostructuresembed-dedwithSnO 2 .EachpanelinsideFig.4showsdifferentaspectsofthenanocomposite:(a)clustersofSnO 2 particlesembeddedinCNTs,(b)sporadicpresenceofCunanoparticles( ∼ 0.3atwt.%estimatedbyenergydispersivespectroscopy),(c)SnO 2 parti-clesoverCNTs,and(d)bamboo-shapedCNTsdecoratedwithSnO 2 nanoparticles.Thefilmsareuniformlydenseoverthewholesub-strate(1.4cmdiameter).Thecarbonnanostructuresmainlyconsistofbamboo-likecarbonnanotubeswithdiametersbetween100and200nmandmorethan10 ␮ moflength.TheSnO 2 nanoparticlesarefoundrandomlydistributedovertheCNTssurface.Theyformclustersaround30–100nm,   andtheycanalsobefoundisolatedinsizesaround5–20nm.   Cunanoparticlescanalsobefound,whichcomefromthesubstrate.TheSnO 2 -CNTnanocompositeresultsinanetworkstructurewithaveryhighsurface-to-volumeratio.Transmissionelectronmicroscopyimages(Fig.5)confirmthatthe resultinghybridmaterialconsistsofCNTsfreeofcatalyticmetal-licparticles(Fig.5a),notcovered,butpartiallydecoratedwith SnO 2 nanoparticles(Fig.5bandc).Thishybridstructureisadirect consequenceoftheonestepgrowthprocessherebyemployed,asopposedtosputtering,coatingorpaintingSnO 2 ontotheCNTs.  3.2.PerformanceofSnO  2 -CNTnanocompositeindetecting alcoholvapors TheresponseofSnO 2 -CNTnanocompositetomethanolandethanolisshowninFig.6aandb,respectively.Theyshowthe Fig.4. Highresolutionscanningelectronmicroscopeimagesof:(a)clustersofSnO 2 particlesembeddedinCNTs,(b)SnO 2 particlesoverCNTs,(c)sporadicpresenceofCuparticles,and(d)bamboo-shapedCNTsdecoratedwithSnO 2 nanoparticles.  230 F.Mendozaetal./SensorsandActuatorsB 190 (2014) 227–233 Fig.5. Transmissionelectronmicroscopyimagesofbamboo-likeCNTsshowingthat:(a)theyarefreeofcatalyticmetallicparticles,(b)theyarepartiallydecoratedwithtindioxidenanoparticles,and(c)close-upviewofthetindioxidenanoparticlesontheCNTs. behaviorofthevoltagesignal(response)asafunctionofthetime,whiletheactivesensormaterialswereexposedtodifferentalco-holconcentrationsatroomtemperature.Theindividualresponseof SnO 2 andCNTsisalsoshownforcomparison.NoticethatSnO 2 aloneessentiallyshowsnoresponse,whereasCNTsshowasignificantresponsetobothtypesofalcohols.Nonetheless,theintegrationofSnO 2 intoCNTsenhancestheirresponseby1.8and2.4times,formethanolandethanol,respectively.Moreover,theSnO 2 -CNTnanocompositekeepsthegoodrepeatabilityandreversibilityofthe Fig.6. Voltageresponseofthesensorstoalcoholvaporsundercyclicexposureto:(a)methanol,and(b)ethanol. hostCNTmaterial.We   observethatwhiletheanalyteisreleasedintothetestchamber(gasON),theresponsetimeisaround45 ± 5s.Analogously,whentheanalyteisremovedfromthetestchamber(gasOFF),theresponsedecreasesquicklywitharecoverytimeof 35 ± 5s.Theresponsetimeandrecoverytimearedefinedhereasthetimerequiredforthesignal(voltage)toreach90%ofthetotalsignal,ordecaythetotalsignalby90%,respectively.Fig.7showsthealcoholsensitivityvaluesofSnO 2 -CNT,CNTandSnO 2 obtainedfromEq.(1).Theplotsindicatethatthereisalin- earrelationbetweentherelativeresistance(  R )andthealcohol Fig.7. Relativeresistanceresponseofthesensorsto:(a)methanol,and(b)ethanol.  F.Mendozaetal./SensorsandActuatorsB 190 (2014) 227–233 231  Table   1 SummaryoftheevolutionofthedevelopmentofgassensorsbasedonSnO 2 mixedwithCNTs(N/R,noreport;R-T,roomtemperature).Ref.AnalytegasLowdetectionlimit(ppm)ResponsetimeRecoverytimeOperatingtemperatureSensitivity S  =  R /  C  [13]H 2 S50 ∼ 5min   ∼ 1min   ∼ 140–260 ◦ CN/R [39]C 2 H 5 OH 10N/RN/R355 ◦ C<4[30]C 2 H 5 OH10 ∼ 1s ∼ 10s300 ◦ C<10[40]C 2 H 5 OH100<10s<10s210–400 ◦ C<2[41]C 2 H 5 OH250 ∼ 100sN/R190 ◦ C1.58 × 10 − 2 [18]CH 3 OH100N/RN/R200 ◦ CN/R [18]C 2 H 5 OH100N/RN/R300 ◦ CN/R [37]H 2 S ∼ 20 ∼ 10min   ∼ 1min   30–90 ◦ CN/R [42]C 2 H 5 OH 100N/RN/RR-T4 × 10 − 3 ThisworkCH 3 OH30<1min   <1minR-T13 × 10 − 2 ThisworkC 2 H 5 OH30<1min   <1minR-T16 × 10 − 2 ThisworkH 2 S ∼ 9<1min   <1minR-T1.5 × 10 − 2 vaporsconcentration(  C  ).Thesensitivityvaluesobtainedwithmethanol(Fig.7a)are13 × 10 − 2 ,7.55 × 10 − 2 ,and1 × 10 − 2 ,forSnO 2 -CNT,CNTandSnO 2 ,respectively.Similarly,thesensitivityvaluesobtainedforethanol(Fig.7b)are15 × 10 − 2 ,6 × 10 − 2 ,and1 × 10 − 2 ,respectively.Inbothcases,thesensitivityofthenanocom-positematerialissomewhatimprovedincomparisontoCNTsalone.Thecorrelationcoefficientis0.94 ± 0.03forallthecurvefitsofMeOHandEtOHinFig.7.Theperformanceofcarbonnanomaterialsinthegasdetec-tionprocesshasbeenexplainedinvariousstudies.Vargheseetal.[34],forexample,reportedthatthecarbonnanotubesundergoreversibleadsorption.Duringgasexposure,twokindsofadsorptioncantakeplace:molecule-CNTinteractionviaintermolecularforces,andchargetransferthroughvalenceforcesinducingachangeintheCNTSsurfaceconductance(undoped,n-typeorp-type,sincebothmaterialsshareasimilarworkfunction)[23,35].Otherstud- ies[18,36]show,fromanexperimentalpointofview,thatmixing CNTsintoSnO 2 hasgreatinfluenceonitsdetectionperformancedependingtheamountanddiameteroftheCNTs.Largeamountsof multi-walledcarbonnanotubes(MWCNTs>15%)mixedwithSnO 2 changethelinearrelationbetweentheresponseandtheanalytegasconcentrationdecreasingtheperformanceofthematerial.More-over,VanHieuetal.[25]f oundthatCNTsofdiametersbelow10nm producelowerresponsesthanthoseof100nmandexplainedthatthiseffectmay   bestronglyrelatedtothesizeofthegasmoleculeadsorbedonthematerial.  3.2.1.ResponseofSnO  2  /CNTtoH   2 S  SnO 2 iswellknowntohavelimitedsensitivitytotoxicgasesatroomtemperature[28],butwehavealsoseenthattheSnO 2 -CNTnanocompositehasanenhancedresponsecomparedtoSnO 2 alone.Accordingly,theSnO 2 -CNTnanocompositewas   testedforthedetectionoftoxicgasH 2 Sinthiswork.Fig.8showsthatthesensitivityofthenanocompositetoH 2 Sisof1.5 × 10 − 2 from ∼ 9ppmto200ppm.Thecorrelationcoeffi-cientis0.81forH 2 SinFig.8,whichislowerthanthatofMeOH andEtOH.Thismay   beduetothefactthatH 2 Sisweaklypolarandtheinteractionwiththesensormaterialishencerelativelyweak.TheseresultscorroborateandextendthoseofDaietal.[37],who foundgoodsensitivitytoH 2 Satroomtemperature,butforconcen-trationsabove20ppm.ThemechanismthattheyproposedisalsoapplicablefortheRTsensorherebydescribed.TheyexplainedthatH 2 Sisareductantgas,losingelectronsthatareacceptedbySnO 2 -CNTnanocomposite(n–pheterojunction),whichcompensatefreeholesthuscausingaresistancedecrease.AlthoughsomerolemightalsobeassignedtotheCupresentinthehybridsensormaterial( ∼ 0.3atwt.%Cu)duetosurfaceelectronicstates,suchCueffectwouldrequirehightemperatures,asdiscussedforthecaseofH 2 byGaldikasetal.[38].Inourcasethemeasurementsaredoneat RT,thusweassumethatCuhasanegligibleroleinthesensitivity.TheheterojunctionsensingmechanismofthehybridSnO 2 -CNTnanocompositethusappearstobemuchmoreeffectivethanthechemisorptionmechanismofSnO 2 andtheadsorptionmechanismofCNTsalone.TheenhancedsensitivityofSnO 2 -CNTnanocompositetohydro-gensulfideherebyshownenablesitsusetomonitorthecompliancewiththerecommendationsoftheNationalInstituteforOccu-pationalSafetyandHealth(NIOSH)andtheexposurelimitsestablishedbytheOccupationalSafetyandHealthAdministra-tion(OSHA).NIOSHrecommends10ppm(15mg/m 3 )for10min   asexposurelimit,whileOSHAestablished20ppmasthepermissibleexposurelimitand50ppmfor10min   asmaximumpeak.Table1showstheevolutionofthedevelopmentofgassen-sorsbasedonSnO 2 mixedwithCNTsfrom2003untilthispieceofresearch.Collectively,thereportsquotedinthetableconfirmthatthecombinationofCNTsandSnO 2 significantlyimprovesthesensorresponsetoalcoholsandhydrogensulfide.Noticethatmostpreviousreportsspecifytheapplicationofheattothesensorsasarequirementforoperation.Thetemperaturesrequiredvar-iedbetween30and400 ◦ C.Unlikethosesensors,thegassensorsherebydescribedoperateatroomtemperature,therebyreducingtheoperatingcostsandhardwarebulk,whileimprovingthedevicereliabilitythroughthesimplificationoftheoperatingrequirements.Ingeneral,atrade-offisobservedbetweenoperatingtemperatureandsensitivity,forthoseauthorswhoreportedthesensitivityval-ues.However,aslongasthetargetdetectionlimitis ≥ 30ppmforalcoholsand ≥ 10ppmforhydrogensulfide,theroom-temperaturegassensorbasedonSnO 2 -CNTcompositefilmsrepresentanadvan-tageoverpreviouslyreportedsensors. Fig.8. RelativeresistanceresponseofthesensorstoH 2 S.
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