Preparation, characterization and gas sensitivity ofpolypyrrole/g-Fe2O3hybrid materials

Preparation, characterization and gas sensitivity ofpolypyrrole/ g -Fe 2 O 3 hybrid materials § Lina Genga,* , Shihua Wub a Department of Chemistry, Hebei Normal University, Shijiazhuang

Preparation, characterization and gas sensitivity of

polypyrrole/ g -Fe 2 O 3 hybrid materials §

Lina Genga,* , Shihua Wub

a

Department of Chemistry, Hebei Normal University, Shijiazhuang 050016, People’s Republic of China

b Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China

1 Introduction

Organic–inorganic hybridmaterials composedof oxides and

conducting polymers that can synergize or complement the

properties of pure organic and inorganic materials have been

usedinmanyapplications,suchasinelectronics,optics,coating

andcatalysis[1–5].Polymer/ferricoxidehybridsinparticularhave

superiorpropertiestothoseofpurepolymerandferricoxide,with

both magneticand polymerproperties,and havedemonstrated

wideusesinmedicine,biochemistryandindustry[6,7]

Gassensorshavebeendevelopedtomeasuregasconcentration,

monitoremissionsincombustionprocessesandprovidefeedback

control[8].Thestudy oforganic–inorganichybridmaterialsfor

applicationingassensorsisacurrentresearchhotspot,asthese

hybrids can compensate for the drawbacksof single inorganic

sensorswithhighoperatingtemperaturesandlowselectivityand

oforganic sensorswithpoor processability and longresponse–

recoverytime [9,10].Itohetal [11]developeda (PNMA)xMoO3

hybridthinfilmandfoundinevaluatingitsVOC(volatileorganic

compound)– sensingpropertiesthat theselectivityof organic/

MoO3 hybrids can be controlled by modifying the organic components Nardis et al [12]reported that cobalt porphyrin/ tindioxidehassuperiorselectivitytomethanolvaporandlower workingtemperaturesthan pureSnO2.Meanwhile,Hosono and Matsubara [13,14] synthesized a PPy/MoO3 thin film and PPy/ MoO3 pressed pellet and found that PPy/MoO3 materials have better selectivity compared with polar VOCs Suri et al [15]

reported on a PPy/iron oxide material that is sensitivity to humidityandtoN2,O2,CO2andCH4gasesatdifferentpressures Our previousexperimentsconfirmedthatPPy/ZnO,PPy/WO3

andPAni/SnO2hybridsaresuperiortosinglepolymerandoxide sensingmaterialintermsofselectivityandworkingtemperature

[16–18] In this work, PPy/g-Fe2O3 hybrids were prepared by simultaneous gelation and polymerization processes and then characterized by FT-IR, XRD, TG–DTA and HRTEM The gas sensitivities of PPy/g-Fe2O3 hybrids compared topure PPy and

g-Fe2O3underCO,H2,NH3,ethanolandacetoneatmosphereatlow operating temperatures (<1008C) wereevaluated The sensing mechanismofpolypyrrole/g-Fe2O3isalsodiscussed

2 Experimental 2.1 PreparationandcharacterizationofPPy/g-Fe2O3

Pyrrole monomers were distilled under reduced pressure, placedinadesiccatorandstoredat48Cuntiluse.Methoxyethanol was added to Fe(NO3)39H2O in a 100ml round bottom flask

A R T I C L E I N F O

Article history:

Received 7 April 2013

Received in revised form 1 July 2013

Accepted 7 July 2013

Available online 15 July 2013

Keywords:

A Composites

B Sol–gel chemistry

C Differential scanning calorimetry (DSC)

C Thermogravimetric analysis (TGA)

A B S T R A C T Polypyrrole (PPy)/g-Fe2O3 hybrid materials were prepared bysol–gel polymerization in situ and characterizedbyFouriertransforminfrared(FT-IR),X-raypowderdiffraction(XRD),thermogravimetric and differential thermal analysis (TG–DTA) and high-resolution transmissionelectron microscope (HRTEM).ThegassensitivitiesinCO,H2,NH3,ethanoloracetoneatmospheresweredeterminedat308C,

608Cand908C.FT-IRandXRDpatternssuggestthatferricoxideinthehybridswasg-Fe2O3,witha diameterofapproximately5nm.TG–DTAandHRTEManalysesshowedthatdifferentreactantmolar ratios of pyrrole monomer: Fe(NO3)39H2O resulted in different microstructures of g-Fe2O3 and molecularweightsofPPy.AnincreasedamountofFe(NO3)39H2Oincreasedthedegreeofuniformityof themolecularweightofPPyandresultedinachangeofg-Fe2O3microstructurefromgranulartostick particles.TheresultsofgassensitivitiesshowedthatthePPy/g-Fe2O3hybridsexhibitedhighsensitivity

to NH3 at mild operating temperature (<1008C) Furthermore, the sensing mechanism was also discussed

ß2013TheAuthors.PublishedbyElsevierLtd.Allrightsreserved

§

This is an open-access article distributed under the terms of the Creative

Commons Attribution-NonCommercial-No Derivative Works License, which

permits non-commercial use, distribution, and reproduction in any medium,

provided the original author and source are credited.

* Corresponding author Tel.: +86 311 80787400; fax: +86 311 87881815.

E-mail address: genglina0102@126.com (L Geng).

ContentslistsavailableatSciVerseScienceDirect

j our na l ho me pa g e : w ww e l se v i e r com / l oca t e / m a tr e sbu

0025-5408/$ – see front matter ß 2013 The Authors Published by Elsevier Ltd All rights reserved.

containing a magnetic stir bar, and then distilled pyrrole was

addeddropwisetothestirredsolutionforthemolarratioofpyrrole

monomer:Fe(NO3)39H2O=1:3or1:4(theproductsaredenoted

asS1andS2).Thesolutionwascontinuouslystirredandheatedata

slowratetoevaporatethesolvent,andablackpowderprecipitate

wasobtained

Thechemicalreactionequationswereasfollows:

FeðNO3Þ3þ3CH3OCH2CH2OH ¼FeðOCH2CH2OCH3Þ3þ3HNO3

MðORÞnþH2O ¼MðORÞxðOHÞnxþðnxÞHOR

½FeðOCH2CH2OCH3Þ3isabbreviatedasMðORÞn

MOHþHOM¼MOMþH2O

MOR þROM¼MOMþROH

After washing withwaterfollowed byethanol, theproducts

weredriedinanovenandthenannealedatdifferenttemperatures

of1008C,1308C,1508Cor1808C

ThepropertiesofthePPy/ferricoxidehybrids wereanalyzed

using several structural methods: FT-IR (Avatar 360 FT-IR

spectrophotometer), XRD (DMAX-2500 diffractometer with Cu

Karadiationat40kVand100mA),TG–DTA(ZRY-2PSimultaneous

ThermalAnalyzer)andHRTEM(PhilipsT20ST,operatedat200kV)

2.2 Determinationofgassensingcharacteristics

CO,H2,NH3,ethanolandacetonewereselectedfortestingthe

gassensitivityofthematerials.Thedetectionsystemandelectric

circuithave been described in ourpreviously studies [15–17]

Briefly,thematerialswerefabricatedonanaluminumtubewith

Auelectrodesandplatinumwires.ANi–Cralloythroughthetube

wasusedas a heating filament Thevoltage ofthe sensor was

measuredindirectlybyanexternalresistorinthetestingcircuit

Gassensitivity is defined asS=Vg/Va, where Va and Vg are the

voltagesofthesensorinclearairandinthetestgas,respectively

60%

3 Resultsanddiscussion The FT-IR spectra of S1 and S2 annealed at 1508

C were comparedwiththatofPPyintherangeof400–4000cm1(Fig.1

ThecharacteristicbandsofPPywereobservedat1560,1398,1298,

1211,1047,930and790cm1,whichwereclosetothosereported

intheliterature[21]:stretchingvibration(1560cm1)oftheC55C bond,stretchingvibration(1298cm1)oftheC55Cbond,stretching vibration(1211cm1)oftheC–Nbond,andthepyrroleringbonds (1407,1398,1047,930,790cm1).IntheS1(150 8C)andS2(150 8C)

spectra,characteristicpeaksofPPywerealsofoundat1407,1398,

1047,930and790cm1,andtheg-Fe2O3specificbandsappeared

in 681, 578 and 468cm1 In the spectra, the C55O bond in pyrrolidoneatabout1700cm1duetotheoveroxidationofPPy was clearly seen The obvious absorptionpeak at 1390cm1

correspondedtoKBr

TheXRDpatternsrevealedthatthediffractionpeaksoftheS1 andS2samplesannealedatdifferenttemperaturesappearedatthe samecrystalface(Fig.2 Thesepeakswereconsistentwiththose fromtheJointCommitteeonPowderDiffractionStandards(JCPDS) data file (25-1402) and, along withthe FT-IR spectra analysis, indicatedtheironoxideinS1andS2wasg-Fe2O3.Thediffraction

0 10 20 30 40 50 60 70

S2

S1

PPy

Wavelength (nm-1)

Fig 1 FT-IR spectra of PPy, S1 (150 8C) and S2 (150 8C)

0

500

1000

1500

2000

2500

3000

4 3 2 1

2 Theta (deg.)

1.100oC 2.130oC 3.150oC 4.180oC

0 500 1000 1500 2000 2500 3000

4 3 2 1

2 Theta (deg.)

1.100oC 2.130oC 3.150oC 4.180oC

intensity increased with increasing annealing temperature,

andtheparticle sizesof S1and S2samplesannealedat 1508C

were5.3nmand4.7nm,respectively,accordingtotheScherrer

formula

TheS1andS2samplesannealedat1008Cwereheatedatthe

rate of 108C/min The TG–DTA curve for S1 showed three

exothermic and twoweight lossprocesses in therange of 20–

5508C(Fig.3a).ThetwopointsofweightlossesintheTGcurve

correspondedtothefirstandsecondexothermicprocessesinthe

DTA curve, while thethird exothermic process had no quality

change.Thesetwoexothermicpeakscoupledwithweightlosses

near2198Cand3088CwerecausedbythedegradationofPPy,as

themolecularweightofPPyinS1wasnotuniform,thesmallforms

degraded first, while the larger onesdegraded later The total

weightlosspercentageofS1(100 8C)was31.9%.Thethird

exother-micpeaknear4418Cwasthecrystalphasetransitionofg-Fe2O3to

a-Fe2O3andthereforecausednoweightloss

Thetwoexothermicpeaksandonepointofweightlossinthe

TG–DTA curve of S2(100 8C) occurred in the range of 20–6008C

(Fig.3b).Thebaseofthefirstexothermicpeaknear2618Cwas

wide,whichspannedthetemperaturerangeofthetwoexothermic

processesnear2198Cand3088CofS1(100 8C).Thisresultcouldbe

explainedbythemolecularweightsofthePPyspeciesinS2(100 8C)

beingclose,andthereforedegradationofPPyappearedcontinuous

duringheating.ThetotalweightlosspercentageofS2(100 8C)was

29%, similar to that of S1(100 8C) As withS2(100 8C), the second

exothermicpeak of S2(100 8C) near 4588Cwasthecrystal phase

transitionofg-Fe2O3toa-Fe2O3,whichdidnotresultinweight loss

Although the percentages of weight loss of S1(100 8C) and S2(100 8C) were similar, the number of exothermic peaks were differentduetodifferencesinmolecularweightsofPPyinthetwo samples (Fig 3a and b) In addition, the phase-transition temperatureofS2(1008C)was178ChigherthanthatofS1(1008C) TheseresultscouldbeexplainedfurtherfromtheTEMandHRTEM micrographs Fig 4a shows the polymer characteristics (i.e., amorphous particles and blurry boundaries) of S1(150 8C), even thoughitwasthehybridofPPyandg-Fe2O3.Thecrystallinesofg

-Fe2O3werenotobviousevenwithHRTEM(Fig.4b),whichwasdue

totheg-Fe2O3beingenwrappedbyPPy(Fig.5

From the TEM and HRTEM micrographs of S2(150 8C), the amorphous polymer,granularandstick g-Fe2O3 particlescould

beseen,and thelengthand widthofthestickformwasabout

200nmand15nm,respectively.Onlyg-Fe2O3diffractionpeaks appearedintheXRDpatternsofS2,indicatingthatthegranular andstickparticleswereallg-typeferricoxide.Thisresultindicates thatthedifferentmolarratiosofpytoFe(NO3)39H2Ocanaffectthe morphologyofironoxide.BrezoiandIon[22]hadreportedthatthe amountofpycouldinfluencethecrystalphaseofironoxideinPPy/ ironoxidehybrids.XiaandWang[23]andHe[24]allreportedthat polymer conformation does influence the crystal shape of inorganic oxide However, these authors did not discuss the specific effectof proportion ofreactants on themorphology of inorganicoxideinthecrystalphase

Fig 3 TG–DTA curves of sample S1 (100 8C) (a) and S2 (100 8C) (b).

sensors, and their sensitivities for CO, H2, NH3, ethanol and

acetonegasesweretestedat308C,608Cand908C.PPy/g-Fe2O3

hybridspreparedwithreactantsattwodifferentratiosandfour

annealing temperatures all showed no gas sensitivity to

3000ppmCO,H2,ethanolandacetone at308C,608Cor908C,

butshowed good response to 2000ppmNH3 under thethree

operatingtemperatures.Inaddition,S1andS2showedsimilar

sensitivitycharacteristics

Theresponse–recoverycurvesofS1(150 8C)(Fig.6a)andS2(150 8C)

(Fig.6b)showedthattheyhadgoodreversibleandquickresponse– recoverytimesto2000ppmNH3attheoperatingtemperaturesof

308C,608Cand908C(Fig.6 Theresponseandrecoverytimesof S1(150 8C) were 12–36s and 20–22s, respectively, at different workingtemperatures,and thoseofS2(150 8C) were17–40sand 20–23s.TheresultsshowninFig.6alsosuggestthatthetesting voltageincreasedwhentheNH3gaswasinputted,whichwasdue

to the increase in resistances and decreased conductivities of

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

gas in

3 2

1

Time (s)

1.30oC 2.60oC 3.90oC

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32

gas out

gas in

3 2

1

Time (s)

1.30oC 2.60oC 3.90oC

(b) (a)

Fig 6 Response–recovery curves of S1 (150 8C) (a) and S2 (150 8C) (b) to 2000 ppm NH 3 at different working temperatures.

Fig 5 TEM (a) and HRTEM (b) micrographs of sample S2 (100 8C)

S1(150 8C)andS2(150 8C)intheNH3 atmosphere.Thus,thePPy/g

-Fe2O3hybridsshowedcharacteristicsofann-typesemiconductor,

althoughtheycontainedbothp-andn-typesemiconductors.This

findingmaybeattributedtotherelativelyhighcontentofg-Fe2O3

inthehybrids.However,thepureg-Fe2O3preparedasdescribedin

reference [25] showed no gas sensitivity at the operating

temperaturesof308C,608Cor908C,whichwasduetog-Fe2O3

beinganinsulatoratnormaltemperatures

ThesensitivitiesofS1andS2annealedat1008C,1308C,1508C

and1808CunderdifferentconcentrationsofNH3gasesat908C

increasedlinearlywithincreasingconcentrationsofNH3(Fig.7

ThesensitivitycurvesofS1andS2testedat308Cand608Cwere

similarwiththatat908C(datanotshown).Theseresultssuggest

that S1 and S2 based sensors can be used in low operating

temperatures(<1008C)todetecta widetesting rangeNH3 gas

concentrations.OurpreviousstudiesreportedthatPPy/ZnOand

PPy/WO3hadgoodselectivitytoNOxandH2Srespectively,buthad

no sensitivity to NH3 at the high concentration of 2000ppm

[16,26] Therefore, PPy/g-Fe2O3 hybrids can be developed in

furtherapplicationsasNH3selectivitysensors

4 Conclusions

The reactant ratio of pyrrole monomer: Fe(NO3)39H2O and

annealingtemperatureofPPy/g-Fe2O3hybridspreparedbysol–gel

polymerizationinsituwereshown hereto influencetheir

micro-structureandgassensitivity.IncreasingamountsofFe(NO3)39H2O

increased the degree of PPy uniformity and resulted in the

microstructure change of g-Fe2O3 from granular to stick particle

form Furthermore, the PPy/g-Fe2O3 hybrids were all selectively

sensitive to NH3 gas at low temperatures (<1008C) and could

overcometheshortcomingsofthelongresponsetimeofPPyandhigh

operatingtemperatureofg-Fe2O3.Therefore,thehybridspresented

importantandpracticalfeaturesformanufactureofgassensors

Acknowledgment

ThisworkwassupportedbygrantsfromtheNationalNatural

ScienceFoundationofChina(31201305)

AppendixA Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.materresbull.2013 07.020

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