Para tornar ainda mais completo o estudo, foi realizada uma busca de patentes usando o banco de dados do Escritório Europeu de Patentes (EPO, www.epo.org). Essa base, contendo mais de 90 milhões de documentos de patente de diferentes países, permite o acesso ao texto completo de grande parte destes documentos, inclusive pedidos depositados no Brasil.
Uma primeira busca com o termo polipirrol (polypyrrole or PPy) no espacenet (plataforma mantida pelo EPO) retornou 690 resultados, em que os 500 primeiros resultados em função do ano são mostrados na Figura 6.4. Note que, corroborando com a Figura 6.1, há um aumento exponencial nos últimos anos, a partir de 2005, do número de patentes registradas utilizando esse polímero.
Figura 6.4 Patentes registradas com o termo polipirrol de 1983 a 2017
Desses 690 registros, apenas 24 utilizam dióxido de titânio e 9 destes possuem como finalidade aplicações em fotocatálise. Quando se trata de óxido de zinco esse número é ainda menor, possuindo apenas 4 registros e não havendo nenhuma aplicação em processos de degradação. Dessa maneira, a utilização de compósitos a base de polipirrol e óxidos metálicos como TiO2 e ZnO possuem grandes possibilidade
de patentes no que diz respeito à degradação de corantes em água.
No que diz respeito a atividade antibacteriana, também foi encontrado poucas patentes com esse fim. Dos 690 registros que usam polipirrol, apenas 15 estão baseados em compósitos com prata, e desses, apenas 1 é aplicado para remover bactéria, no caso E. coli, da água. Assim, aplicações bactericidas desse compósito possuem bastante espaço tecnológico, podendo ser foco de novas patentes e aplicações em escala industrial.
1985 1990 1995 2000 2005 2010 2015 0 20 40 60 80 100 N ú m e ro d e pa te n te s Ano
7 Conclusões gerais
O uso adequado de surfactantes, como o CTAB e o DBSA, durante o processo de eletropolimerização do polipirrol resulta em um material altamente ramificado com propriedades elétricas superiores, tornando-o uma matriz com grande potencial de aplicação em fotocatalizadores e agentes bactericidas.
Quando na forma do nanocompósito PPy/TiO2, a área superficial do material,
além do seu bandgap, diminuem quando comparado ao polímero puro, indicando a adsorção do semicondutor na matriz polimérica, e sendo responsável pela fotodegradação direta da rodamina B em 85%.
Não obstante ao compósito PPy/TiO2, a incorporação de óxido de zinco à matriz
polimérica também permite uma adequada separação de pares elétron-lacuna sem que haja uma taxa de recombinação elevada desses pares. Isso possibilita uma geração de fortes radicais responsáveis pela fotodegradação de rodamina B e rodamina 6G. Esse processo de degradação direta das moléculas de rodamina pode ser usado em associação com outros sistemas de remoção de corantes.
Além de propriedades fotocatalíticas, o polímero polipirrol altamente ramificado pode ser usado como agente antimicrobiano, seja a matriz pura, ou na forma de nanocompósito com nanopartículas de prata. Esse processo consiste de duas etapas, em que a adesão polimérica é seguida pela permeação de nanopartículas metálicas na célula bacteriana. Este processo é favorecido pela interação eletrostática estabelecida entre o polipirrol e a parede celular bacteriana.
Dessa maneira, o polipirrol ramificado e sua combinação na forma de compósitos com óxidos metálicos representam um potencial material para aplicações futuras. Sua forma ramificada e baixa solubilidade sugerem um filtro antibacteriano eficiente ou uma esponja para fotodegradação de corantes de fácil remoção do meio tratado.
Synthesis
and
characterization
of
branched
polypyrrole/titanium
dioxide
photocatalysts
EricleitonR.Macedoa,b,PatríciaS.Oliveiraa,Helinando P.de Oliveiraa,*
a
LaboratóriodeEspectroscopiadeImpedânciaeMateriaisOrgânicos,InstitutodePesquisaemCiênciadosMateriais,UniversidadeFederaldoValedoSão Francisco,Juazeiro,Bahia48902-300,Brazil
b
InstitutoFederaldeEducação,CiênciaeTecnologiadoSertãoPernambucano,Petrolina,56302-320Pernambuco,Brazil
ARTICLE INFO Articlehistory:
Received23January2015
Receivedinrevisedform14April2015 Accepted19April2015
Availableonline21April2015 Keywords: Polypyrrole RhodamineB Electrochemicalsynthesis Composite ABSTRACT
Polymercompositessynthesizedbyfractalgrowthofpolypyrroleinthepresenceoftitaniumdioxide
nanoparticleswereinvestigatedaspotentialphotocatalysts.Thesebranchedstructuresarecharacterized
byhighsurfaceareaandimprovedactionasphotocatalystinthedegradationofrhodamineB. The
resultingabsorptionofphotonsinthevisibleregionandlowrateofelectron–holerecombinationwith
adequategenerationofradicalsforsubsequentN-deethylationofrhodamineBrepresentimportant
aspectswhichcorroboratepotentialapplicationofnewcompositeasefficientphotocatalyst.
ã2015ElsevierB.V.Allrightsreserved.
1.Introduction
Conductingpolymers(CPs)haveattractedinterestduetotheir superior electrical and optical properties which contribute to developmentofpromisingdevicessuchasmicrobialfuelcells[1– 3],supercapacitors[4–6],strain/stresssensors[7],biosensorsand immunosensors[8–9], photocatalysts [10–12] and electroactive actuators[13].
The production of conducting polymer-metal oxide nano- compositesincorporateshighconductivityofCPs withintrinsic propertiesofinorganicsemiconductors(highsurfacearea,photo- stabilityandlowdensity)[10–12,14].
Particularly,themodificationinducedbypolypyrroleonsurface oftitaniumdioxideimprovestheactionofresultingcompositeas photocatalystduetothetunableabsorptionbandofpolypyrrolein associationwithsemiconductorresponse[11].
Yangetal.[15]reportedthatadequatecouplingofPPyandTiO2
enhancesthephotocatalyticactivityofresultingmaterialunder visible light excitation. The disposition of lowest unoccupied molecular orbital (LUMO) above the conduction band of TiO2
providestheabsorptionofphotonsinthevisibleregionfollowed byrapidcharge separationassociated withslowrecombination
[15].
Branched polymeric structures are characterized by high densityofinternalcavitiesandavailablesitesforfunctionalization
[16,17],afavorableconditionforapplicationssuchasdrugdelivery, tissuetargeting(duetothepermeabilityacrossbiologicalbarriers) andenhancedluminescenceinlanthanides[18,19].
AsreportedbyDasetal.[17],theproductionofhighlybranched conductingpolymer(fractalgrowthofpolymericmatrix)depends oncombinationofsurfactant,electricfield,monomersconcentra- tion and time of reaction.Using these considerations,we have introduced thenovelty related with production of new photo- catalystsbasedonnanocompositesofbranchedpolypyrroleand TiO2.
2.Experimental
Sodiumdodecylsulphate–SDS(Aldrich),cetyltrimethylam- moniumbromide–CTAB(Aldrich),rhodamineB(Vetec),sodium dodecylbenzenesulfonate–DBSA(Aldrich)andtitaniumdioxide (anataseparticlesizeinnanopowderof21nm,purityof99.5%and bandgapof3.2eV(Aldrich))wereusedasreceivedwhilepyrrole (Aldrich)wasdistilledbeforetheuse.
The morphology of composites was analyzed by scanning electron microscopy (Hitachi TM1000) with an accelerating voltageof20kVwhileoverlaid imageswithEDSofpolypyrrole/ TiO2complexwereperformedbySEMVega3XMH-Tescan.
The structure of composite was characterized by Fourier transforminfraredspectrum(KBrmethod)usinganIRPrestige- 21FouriertransforminfraredspectrometerShimadzu.
*Correspondingauthor.Tel.:+558721016795;fax:+558721016795. E-mailaddress:[email protected](H.P. deOliveira). http://dx.doi.org/10.1016/j.jphotochem.2015.04.013
1010-6030/ã2015ElsevierB.V.Allrightsreserved.
Journal
of
Photochemistry
and
Photobiology
A:
Chemistry
Allofexperimentswereconductedat25C.
Brunauer–Emmett–Teller (BET) surface area measurements were performed by Micromeritics ASAP 2420 surface area analyzer.
Electrical impedance measurements (Nyquist plot) were performed using a Potentiostat/Galvanostat Metrohm Autolab AUT302Ninassociationwithasampleholdersolartron12962A.
Results were fitted by equivalent circuit (modified Randles circuit)byassociationofabulkresistanceinserieswithaparallel combinationofbulkcapacitorandWarburgelementinserieswith achargetransferresistance.
2.1.Synthesisofbranchedfibersofpolypyrrole
64-sampleswerepreparedusingallofpossiblecombinationof low (#) and high (") values of six parameters viz. DBSA concentration– parametera(#) 0mM and (") 50mM; voltage level–parameterb–(#)4.5V and(")9.0V;Timeofreaction– parameterc (#)30minand (")60min;pyrrole concentration– parameter d (#) 50mM and (") 250mM; SDS concentration – parametere(#)25mMand(")125mMandCTABconcentration– parameterf(#)12.5mMand(")300mM.Thecompletedescription of samples (corresponding variation of low and high level of parameters (a, b, c, d, e and f) is shown in the Table 1. The identificationofsamplesiscomposedbycombinationofsixletters (a, b,c,d, eand f). The presence of specificletter in a sample indicatesthatcorrespondingparameterassumesmaximumvalue (e.g.,sampleadewaspreparedusingmaximumconcentrationof DBSA(a),pyrrole(d)andSDS(e)andminimuminparametersb,c andf).
Surfactants(atrelativeconcentrationdescribedintheTable1) weredispersedin50mLofmilli-Qwaterandkeptunderintense stirring(300rpm)untilcompletedispersionofreagents.Afterthis step,pyrrole was introducedin theresultingsolutionand kept under stirring during additional 2min. An aliquot of 17mL of solutionwasintroducedinareactorinwhichanaluminumcircular cathode (25mm-diameter) is disposed at 25mm from anode (metallicwire)atair-liquidinterface.Electricfieldisestablished between electrodes and electrochemical polymerization takes placeduringfixedintervaloftime(accordinglowandhighvalues of parameterc). The resulting material is washedwith milli-Q waterforsurfactanteliminationanddriedat60Cduring1h.
maintained under stirring during 30min. 340mg of titanium dioxidewasintroducedintheresultingsolutionandstirredduring additional30minforcompletedispersionofnanoparticles.310mL of pyrrole wasadded in thesolutionfollowedbyan additional period of 2min of stirring. After this step, the solution was transferred to reactor and electrochemical reaction established during1hwithexternalDCvoltageof9V.Theresultingmaterial was washedwithwaterfor surfactantelimination anddried at 60Cduring1h.
2.3.Preparationofpellets
50mgofresultingpolymericmatrixwascompressedinapress machine (20kN) for production of pellets with thickness of 300mmand13mmofdiameter.
2.4.Photodegradationmeasurements
14mgofresultingPPy/TiO2nanocompositewasimmersedin
5mL of aqueous solution of rhodamine B (1mM) under dark condition.Theexposureofresultingsolutiontowhitelightwas establishedwiththeuseofhalogenlight(60W)duringstandard intervaloftimeof4h.Aliquotsofsolution(3.5mL)wereremoved fromreactorinfixedintervalof30minandanalyzedintermsof UV–vis absorbance and fluorescence in order todetermine the kinetics of photodegradation. Comparisonwithdirect action of neat TiO2 nanoparticleswasprovided bysimilarexperiment, in
which corresponding concentration of TiO2 (0.02g/L) was dis-
persedinaqueoussolutionofdye(1mMofRhB).
2.5.Determinationofopticalbandgapofresultingcomposites Thebandgapofpolypyrrole(polypyrrole/TiO2)wasdetermined
fromabsorptionspectraandTaucrelation(Eq.(1))[20,21]
ahv¼BðhvEgapÞm (1)
whereaistheabsorptioncoefficient,hnisthephotonenergy,and m=1/2 for direct bandgap material.Ghobadi [22] described a directmethodforfittinganddeterminationofbandgapusingTauc relation.AftersubstitutionsinEq.(1)(describedinRef.[22]),we canwritethat
Abs: l 1=m ¼B 1 ll1gap (2) wherel isthewavelengthandAbs.thecorrespondingvalueof measured absorbance. lgap can be easily obtained from curve
(Abs./l)1/mvs.1/latcondition(Abs./l)1/m=0.Thebandgapvalue
isobtainedfromrelationEgap=1239.83/lgap.
Inordertoavoidsaturationinthemeasuredabsorbance,we havepreparedthinfilmsofbranchedpolypyrroleandpolypyrrole/ TiO2 under the same experimental conditions than bulk film.
Aluminum electrodewas substitutedbytransparentconductive glass slide (ITO) and deposition (under previously described conditions)wasestablishedonthesurfaceofelectrodeduring10s. 3.Resultsanddiscussion
3.1.Optimizationofbranchedpolymericmatrix
Thestructureofsynthesizedsamplesandaverageofgrainsize are shown in the SEM images of Fig. 1. Sample bd (low concentrationofthreedifferentsurfactants,highvoltageandhigh
Table1
Descriptionofsamplespreparedwithpossiblecombinationofsixparameters(a– DBSA concentration, b – Voltage level, c – Time of reaction, d – Pyrrole concentration,e–SDSconcentrationandf–CTABconcentration).
Samplename
Correspondinglevel(a,b,c,d,e,andf) I ###### b #"#### c ##"### d ###"## e ####"# f #####" bc #""### bd #"#"## be #"##"# bf #"###" cd ##""## ce ##"#"# cf ##"##" de ###""# df ###"#" ef ####"" bcd #"""## bce #""#"# bcf #""##" bde #"#""# bdf #"#"#" bef #"##"" cde ##"""# cdf ##""#" cef ##"#"" def ###""" bcde #""""# bcdf #"""#" bcef #""#"" bdef #"#""" cdef ##"""" bcdef #""""" a "##### ab ""#### ac "#"### ad "##"## ae "###"# af "####" abc """### abd ""#"## abe ""##"# abf ""###" acd "#""## ace "#"#"# acf "#"##" ade "##""# adf "##"#" aef "###"" abcd """"## abce """#"# abcf """##" abde ""#""# abdf ""#"#" abef ""##"" acde "#"""# acdf "#""#" acef "#"#"" adef "##""" abcde """""# abcdf """"#" abcef """#"" abdef ""#""" acdef "#"""" abcdef """"""
concentrationofpyrrole)ischaracterizedbyhighconcentrationof grainswhilesamplebdf(highconcentrationofCTABinadditionto samplebd)introducesreasonabledegreeofporosityinbranched open-porestructureswithhighsurfaceareaandlowweight.
The influence of SDS on morphology of samples (from comparison of sample bd and bde) can be characterized by increaseinthedegreeofsmoothnessofgrains.Asexpected,the
sample bdef (mutual inclusion of CTAB and SDS) presents an intermediatedegreeofrugosityincomparisonwithsamplesbdf (maximum degree of rugosity) and bde (minimum degree of rugosity).
Optimal conditions for efficient polymeric production were determinedfromcalculationofparameterimportance(PI)which considerstheaverageofdifferencesbetweenweightsofsamples
Fig.1.SEMofsamplebd(maximuminthevoltagelevelandpyrroleconcentration),samplebdf(maximuminthevoltagelevel,pyrroleandCTABconcentration),samplebde (maximuminthevoltagelevel,pyrroleandSDSconcentration)andsamplebdef(maximuminthevoltagelevel,pyrrole,CTABandSDSconcentration).
Resultsofparameterimportancebasedontheweightof64- samples(asshownintheFig.2)indicatethatparametersb,d,eand ccontributepositivelytoproductionofpolymericsamples.Onthe other side, a and f are less representative parameters in the optimizationofpolymericproduction.Inspiteoflowimportance ofparameterf,aninterestingaspecttobereportedconcernstothe negativeinfluenceofCTABonproductionofpolypyrroleduetothe reduction in the weight,in agreement with SEM images (high concentrationofpores).
Theinfluenceofsurfactantsonstructureofresultingmaterial was characterized by FTIRspectrum. For comparison, we have studied the response of isolated and combined surfactants on polymerization(usinghighlevelofconcentrationforeachone,as previouslydefined).
FTIRspectrum of differentsystems (shown in theFig. 3)is characterizedbytypicalpeaksofpolypyrroleviz.NHstretching vibration of nitrogenin pyrrole ring, C¼C and CN stretching vibration and CH in-plane vibration band observed at 3450, 1543, 1463 and 1030cm1, respectively, in an indication that reasonable degree of polymerization is established in all of samples[23–26].
Peaksat2918and2849cm1areattributedtothestretching vibrationmodeofmethyleneinPPystructure,sinceSDSandDBSA actasdopantofPPystructure[26].
Peakat656cm1characterizestypicalvibrationofDBSA[24]
whilestrongpeakat1164cm1characterizesthedopingstateof polypyrrole[25].Aswecansee,thesepeaksareclearlyidentified insampleswithinclusionofDBSAduringsynthesis,asaresultof dopinglevelinducedbyadditive.
Electrical characterization of resulting polymeric matrix (shownintheNyquist plot–Fig.4)indicatesthat DBSAactas anefficientdopant,providingreductionintheimpedancelevel,in agreementwithdatafromFTIRspectra.
Thefittingofexperimentaldata(linesintheFig.4)indicates thathighestvalueofchargetransferresistanceisestablishedfor samplePPy+SDS.TheincorporationofDBSA(samplePPy+DBSA+ SDS) provides reduction in the diameter of characteristic semicirclein theNyquistplotdue tothedopingeffectinduced bymutualincorporationofsurfactants.
Moreover, the inclusion of CTAB and DBSA (sample PPy+ CTAB+DBSA)resultsin a materialwith lowestcharge transfer
resistance, due to the association of high degree of porosity inducedbyCTABanddopingeffectofDBSAonpolypyrrole. 3.2.Applicationofoptimizedpolymericsupportasphotocatalyst
Basedonresults,thenanocompositePPy+DBSA+CTAB(sam- ple abcdf) was considered the most adequate matrix for TiO2
incorporationduetotheassociationofsuperiorelectricalresponse andhighdegreeofpolymerization.Theincorporationofsemicon- ductornanoparticlesduringfractalgrowthofpolypyrrolereduces thebandgapofresultingcomposite,asshownintheFig.5.The measuredbandgapofneatPPyisinorderof1.93eVandaslight shiftisverifiedwithprogressiveincorporationoftitaniumdioxide (at0.02g/L thebandgapisinorderof1.91eVand1.88eVwith incorporationof2g/LofTiO2).CorrespondingvaluesofBETsurface
areaandadsorptionaverageporewidtharesummarized inthe
Table2.
As we can see, branchedpolypyrrole presentssuperior BET surfacearea(97.38m2/g)incomparisonwithreportedvaluesfor
chemicallysynthesizedpolypyrrole(24.4m2/g)[6].Theincorpo-
ration of TiO2 nanoparticles takes place with introduction of
nanoparticlesinavailablesitesofbranchedstructure.Asaresult,
Fig.3.FTIRofsamplesPPy+SDS,PPy+DBSA,PPy+CTAB+DBSAandPPy+SDS+ DBSA.
Fig.4. RXdiagramofpelletsofsamplesPPy+SDS,PPy+DBSA,PPy+CTAB+DBSA andPPy+SDS+DBSA.
Fig.5. Plotof(Abs/l)2
vs.1/lforPPy+DBSA+CTABandPPy+DBSA+CTAB+TiO2at differentconcentrationofsemiconductor.
reductioninthesurfaceareaisestablishedwithincreaseinthe semiconductorconcentration(88.73m2/gfor0.02g/LofTiO
2).
SEM image of PPy+DBSA+CTAB+TiO2 reveals the homoge-
neousdistributionofTiO2(overlaidEDSimageintheinsetofFig.6
–reddotsindicatethepresenceoftitanium).XRDdataofresulting materialshows a broad bandaround 2u=20 attributedtothe amorphous behavior of purepolypyrrole [14] and sharp peaks identifiedaccordingJCPDScard#89-4921whichareassignedto characteristicsignatureoftitaniumdioxide(anatase).
TheactionofPPy+DBSA+CTAB+TiO2complexinorganicdye
aqueoussolution under continuous excitationwith white light (spectrumofexcitationshownintheFig.7)wasmonitoredatfixed
intervalof30minfromabsorbanceandfluorescenceofresulting dyeaqueoussolution.
As wecan see in theFig. 8,the continuous illuminationof aqueoussolution ofrhodamine Bin thepresence of polymeric composite (PPy+DBSA+CTAB+TiO2) reduces the fluorescence
intensity(insetofFig.8)ofresultingsolutionasaconsequence ofprogressivedegradationofdye,accomplishedbyreductionin thedyecharacteristicpeak.
Themeasurementofphotodegradationkinetics(asshownin theFig.9)indicatesthatefficiencyofdegradation(after210minof reaction)measuredbyreductioninthedyeconcentrationinduced by PPY+DBSA+CTAB+TiO2 composite is in order of 81.2%.
Photodegradation provided by PPy+DBSA+TiO2 is in order of
85%, PPy+SDS reduces the dye concentration in 66% while PPy+SDS+DBSA+TiO2providesareductioninthedyeconcentra-
tioninorderof67%.
ResultsofdirectinteractionofrhodamineBwithTiO2(0.02g/L)
undercontinuous excitationwithwhitelight returnsnegligible degradation, in an indication that polypyrrole/TiO2 composite
introducesanimportantroleinthephotodegradationofrhoda- mineB.
Basedontheseresults,typicalmechanismsofphotodegrada- tioncanbeestablishedaccordingspectrumofilluminationsource (seeFig.7)inwhichdominantradiationiscenteredinthevisible region(mainsourceof energy)withresidual energyin theUV region:
-Undervisible-lightillumination
Rhodamine Band polypyrrole interact withradiation in the visible light region and absorb photons. Excited rhodamine B molecules(Eq.(3))donateelectronstoconductionbandofTiO2
(Eq.(4)).TheinteractionofTiO2(e)withoxygenmoleculesresult
in theformationof O2radical(Eq. (5)).Subsequent reactions
(Eqs.(6)–(8))resultintheformationofradicalOH.
RhB+hn!RhB* (3) RhB*+TiO 2!RhB++TiO2(e) (4) TiO2(e)+O2!TiO2+O2 (5) TiO2(e)+ O2+H+!HO2 +TiO2 (6) HO2 +H+!H2O2 (7) H2O2+e !OH+OH (8)
Theincorporation ofpolypyrrole improves theabsorptionof lightinthevisibleregionduetotheelectrontransferfromground stateofPPytotheexcitedstateandsubsequentlytotheconduction bandofTiO2[10,11,15](incorrespondencewithmechanisminthe
Eq.(4)), allowingtheproductionof strongradicals(similarlyto stepsdescribedbyEqs.(5)–(8)).
PPy+DBSA+CTAB 97.38 39.52
PPy+DBSA+CTAB+TiO2(0.02g/L) 88.73 42.34
PPy+DBSA+CTAB+TiO2(2g/L) 51.99 42.92
Fig.6. X-raydiffractionofcomposite(PPy+DBSA+CTAB)+TiO2.Theinsetshowsan overlaidEDSonSEMimageofcompositeindicatinghomogeneousdispersionof titaniumdioxideparticles.(Forinterpretationofthereferencestocolourinthetext, thereaderisreferredtothewebversionofthisarticle.)
-UnderUV-lightillumination
Thedirectexcitationof TiO2 withUVilluminationresultsin
electron–holepairseparation(Eq.(9)).Theoxidativereactionwith watergeneratesOHradical(Eq.(10)).
TiO2+hn!TiO2(hnb+)+TiO2(ecb) (9)
H2O+TiO2(hnb+)!nOH+H++TiO2 (10)
The reaction of rhodamine B with OH (generated from interactionof PPy/TiO2 and rhodamineBwithvisible light and
TiO2withUVlightillumination)resultsintheN-deethylationof
rhodamine B [27,28]. Rhodamine has been considered as an intermediateproductofN-deethylationofrhodamineBwhilefinal productsofdegradationareestablishedascarbondioxide,water andmineralacids.Thetypicalsignatureofrhodamineisverified from blue shift in the spectrum of rhodamine B. However, as reportedbyWilhelmandStephan[28],theabsenceofblueshift