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Surface potentials of polyaniline lb films
ELSEVIER
Synthetic Metals 101 (1999) 658-689
SURFACE POTENTIALS OF POLYANILINE
LBFILMS
C.J.L. Con&&no, A. Dhanabalan,A. Riul Jr., O.N. Oliveira Jr., Instituto de Fisica de SLoCarlos, Universidade de Slo Paulo, CP 369,13560-970, SgoCarlos,SP, Brazil Abstract The surface potential (Vra) of Langmuir-Blodgett (LB) films of pure emeraldine base polyaniline (PANi) and mixties of PANi and cadmium stearate(CdSt) was investigated.ProtonatedPANi LB films producedfrom acidic subphasesdisplayed higher Vr,s than the nonprotonated films transferredfrom monolayersspreadon pure water. For the mixed LB films, VLBincreasedwith the amount of PANi, and it was always positive regardlessof whether an odd or even mmber of layers were deposited,in contrastto the switch-over from positive to negative potential when going from an odd to an even number of layers for pure cadmium stearateLB films. The results have been analyzed by taking account of the negative contribution from the substrate/film interface. Keywords: Langmuir-Blodgett Films, Polyaniline, Surfacepotential Introduction The surface potential technique has long been used to characterizeLangmuir monolayers and Langmuir-Blodgett (LB) films, though much less work has been done with the latter. For Langmuir monolayersof simple aliphatic materials, a quantitative analysis can be madein terms of the dipole momentsinherent in the monolayer-forming molecules and of the dielectric constants of the media in which these dipoles are embedded [l]. The interpretation of surface potential of LB films is more complex, despite the successfnl attempts to explain data for specific materials [2-4]. The main difficulty lies in the role of the substrate-film interface, which provides an important contribution to the surface potential in addition to that from the molecular dipole moments.It has been found that the surface potential of LB films obtained from unionized monolayers is generally lower than the corresponding monolayer potential, mainly owing to a negative contribution from the substrate-film interface [5]. ln the caseof LB films formed from ionized monolayers,the analysis is complicated because it depends on whether the double-layer contribution for the monolayer potential is replaced by a corresponding one in the transferredLB films. In this work, we addressthese issues by analyzing surface potential data of LB films from polyaniline (PANi). Owing to their polymeric nature, one cannot ascribe dipole momentsto the PANi molecules, and therefore the measurementof the monolayer surface potential (AV,) serves only to probe the influence of subphase composition, as in the change of the degree of protonation [6]. Nevertheless,measuring the surface potential of transferred LB films (V& may allow a better understanding of electrostatic phenomenain metal-insulator/semiconductorsLB fis [3,4]. Here, surface potential measurementsare presented for the first time for Langmuir-Blodgett films obtained from pure, doped PANi - in the emeraldine base form, and also from composite films of PANi and cadmium stearate(Cd%). For comparison,the VLBof pure CdSt has also been obtained.
Experimental The LB films from PANi and from composite PANi-CdSt were produced according to the methods reported elsewhere [7,8]. For pure PANi monolayers, a mixture of polyaniline and camphor sulfonic acid in chloroform was spread on a pure water surface(pH = 6.0) or on an aqueoussolution containing HCl (PH = 2.0). For producing compositePANi-CdSt and pure CdSt films, the subphase contained cadmium chloride and the pH was maintained at ca. 6.0. Monolayer studiesand LB depositionswere performed with a KSV-5000 systemhousedin a clean room. Thin gold-coated glass plates were used as substrates.Pure and mixed polyaniline monolayers were transferred as Z- and Y-type LB films, respectively, with a transfer ratio close to unity for the transfer process.Different numbers of layers were deposited-on the samesubstrateand the reproducibility of such procedure has been confirmed by measurementswith duplicate ftis. Surface potential measurementswere carried out with a Trek-320K electrostatic voltmeter at room tempxature in air. Results and discussion Earlier UV-vis studies have indicated that polyaniline is protonated when an acidic su@hGe isemployed-fG spreading the monolayer [7j, in contrast to the LB film obtained jrom- a monolayer spreadon a neutral subphasef8], even though in both cases PANi was doped in the spreading solution. The surface potential, AVr,, is slightly higher for the protonated PANi monolayer on an acidic subphase,most probably becauseof the positive contribution from the double-layer potential 163.The difference, however, is less than the double-layer contribution to the doped monolayer, which is around +lOOmV, This meansthat the dipole moment contribution was higher for the undoped monolayer, possibly indicating a different chain organization. Such a finding is not surprising since doping is known to affect the polymer chain conformations. When transferred onto solid substrates,these doped LB films exhibit a surface potential, VLB, that is only 50-60 mV lower than the monolayer potential, AVL. Assuming that the monolayer does not reorganize, upon transfer one may expect three types of change in the surface potential: i)
0379~6779/99/$- see front matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: SO379-6779(98)00798-X
C.J.L.
Constantino~cr
al.
I
Syniheric
loss of the double layer contribution (for ionized monolayers), which may be compensatedby ii) contribution from a charged layer that induces opposite charges on the substrate, and iii) charge injection from the metal substrate.The latter is responsible for the lower VLBwhen comparedto AV, for unionized films [5] and is generally 100-200 mV (negative). Therefore, one may conclude that for doped PANi LB films, factor ii) must have compensated the loss of the double-layer contribution, and furthermore the negative contribution from charge injection cannot be large. With regard to the undopedLB PANi films, V~B is lower than AVL of the corresponding monolayer, again by some 50-60mV. This decreasecan only be attributed to electron injection from the substrate. It is lower than the usual value reported in ref. [5], as also occurred for doped PANi films. Vra for dopedPani LB films is higher than that of non-dopedPani LB film, regardless of the number of deposited layers. While the contribution (i) may not be significanf this difference could arise either from contribution (ii) which can appearonly in the doped films, or due to differences in the contribution due to electron injection (iii). For the ionization potential of the doped PANi is likely to be lower than of its undoped counterpart, and therefore electron injection from gold would be more efficient in the undoped PANi films, leading to a higher negative contribution to VLB.
Table 1 shows Vra for doped (pH=2.0) and undoped PANi LB films with different numbers of layers. Results for lower numbers are not shown becauseVra increases for the fast few layers until reaching a reasonably constant value. Furthermore, for PANi ftis, VLB doesnot dependon whether an odd or even number of layers was deposited.For pure CdSt, however, VLB is negative for an even number of layers, becausenow the dipole moments of adjacent layers are canceled out in these centrosymmetric LB films. Then, only the negative contribution from charge injection remains. It must be stressedthat the CdSt LB films with an even number of layers are less stable since the hydrophilic head groups are directed towards the air. Taken together these results show that a unique polar axis is not established in the Z-type PANi ftis, in spite of the noncentrosymmetric nature of the deposition, which may be related to the poor stability of Z-type films and/or the random organization of the polymer chains in each individual layer. In all cases,Vra varied by at most rtlOmV while scanning the surface of the whole fti, which is close to the sensitivity limit of the measuring measurement,indicating good film uniformity at least at the macroscopiclevel. Table-l: VLB of LB films of polyaniline and cadmium stearate containing different numbers of layers Number of layers 7 9 8 6
VLBPANi (pH = 6.0) + 250 mV + 240 mV + 200 mV + 240 mV
vLB PANi (pH = 2.0) + 340 mV +300mV + 330 mV + 340 mV
VLBCdSt + 130 mV + 130mV - 120 mV -4OmV
For the mixed LB ftis, Vra was measuredfor 20 and 80% weight percentagesof PANi (see Table 2). It increaseswith the amount of PANi in the fti, as expected because the PANi monolayers - even the undoped ones - display a higher AVL than C&St. Vrs for the mixed films has an intermediate value between those of pure PANi and CdSt LB films. The striking feature in
659
iLletuls 101 (1999) 68&559
Table 2 is that Vrs is practically independent of the number of depositedlayers for the SO:20PANi:CdSt composition, including the ftis with an even number of layers. Moreover, in subsidiary experiments in which VLB was monitored through several days, we noticed that these ftis are stable, despitethe even number of layers, unlike their counterparts of pure CdSt. Therefore, the PANi molecules appearto have a sort of “stabilizing” effect on the CdSt molecules. Such an effect is obviously less important when the PANi amount is decreasedconsiderably. Indeed, for the 2080 PANi:CdSt ratio, Table 2 shows a VLB that is appreciably lower for the LB films with an even number of layers, although the inversion in the sign is not observed,in contrast to the pure CdSt LB films. The latter observationindicates the dominant role of PANi even in the films which containedonly 20% of PANi. Table-2: Vrs of composite LB films of different weight percentagesof polyaniline and cadmium stearate and different numbersof layers Number of layers 7 9 8 6
VLBPAN~-CdSt (20:80) + 190 mV + 210 mV + 160 mV + 120 mV
Vrs PANi-CdSt (80:20) + 230 mV + 220 mV + 210 mV + 210 mV
Earlier X-ray diffraction (XRD) studies of mixed LB ftis revealed that both PANi and CdSt are present as separate domains and that the domain structure of CdSt was destroyed upon increasing the amount of PANi in the fti [8]. On the basis of the results presentedhere, it seemsthat in the composite LB films, the ordered CdSt domains are distributed randomly in the disorganizedPANi matrix. Conclusions The surfacepotentials of LB films from doped and undoped PANi were investigated. They are lower than the corresponding monolayer potential due to electron injection from the substrate. In composite PANUCdSt films, the polymer had a stabilizing effect on the CdSt molecules when their head groups were exposedto air. Acknowledgements The authors thank FAPESPand CNPq for the fmancial support. References 1. O.N. Oliveira Jr. and C. Bonardi, Langmuir, 13 (1997) 5920. 2. R.H. Tredgold and G.W. Smith, J.Phys.D:Appl. Phys., 14 (1981)L193. 3. E. Itoh, A. Fukuda, M. Iwamoto, J. Elctrostatics, 33 (1994) 147. 4. E. Itoh, H. Kokubo, S. Shouriki, M. Iwamoto, J. Appl. Phys., 83 (1998) 372 (and referencestherein). 5. C.J.L. Constantino,L.P. Juliani, V.R. Botaro, D.T. Balogh, M.R. Pereira, E.A. Ticianelli, A.A.S. Curvelo, O.N. Oliveira Jr., ThinSolidFilms,284-285(1996)191. 6. S.V. Mello, A. Riul Jr., L.H.C. Mattoso, R.M. Faria and O.N. Oliveira Jr., Synth. Met., 84 (1997) 773. 7. A. Riul Jr., L.H.C. Mattoso, G.D. Telles,P.S.P. Herrmann, L.A. Colnago, N.A. Parizotto, V. Bamnauskas,R.M. Faria and O.N. Oliveira Jr., Thin Solid Films, 284-285 (1996) 177. 8. A. Dhanabalan,A. Riul Jr., O.N. Oliveira Jr., Supramolecular Sci., 5 (1998) 75.
Synthetic Metals 101 (1999) 658-689
SURFACE POTENTIALS OF POLYANILINE
LBFILMS
C.J.L. Con&&no, A. Dhanabalan,A. Riul Jr., O.N. Oliveira Jr., Instituto de Fisica de SLoCarlos, Universidade de Slo Paulo, CP 369,13560-970, SgoCarlos,SP, Brazil Abstract The surface potential (Vra) of Langmuir-Blodgett (LB) films of pure emeraldine base polyaniline (PANi) and mixties of PANi and cadmium stearate(CdSt) was investigated.ProtonatedPANi LB films producedfrom acidic subphasesdisplayed higher Vr,s than the nonprotonated films transferredfrom monolayersspreadon pure water. For the mixed LB films, VLBincreasedwith the amount of PANi, and it was always positive regardlessof whether an odd or even mmber of layers were deposited,in contrastto the switch-over from positive to negative potential when going from an odd to an even number of layers for pure cadmium stearateLB films. The results have been analyzed by taking account of the negative contribution from the substrate/film interface. Keywords: Langmuir-Blodgett Films, Polyaniline, Surfacepotential Introduction The surface potential technique has long been used to characterizeLangmuir monolayers and Langmuir-Blodgett (LB) films, though much less work has been done with the latter. For Langmuir monolayersof simple aliphatic materials, a quantitative analysis can be madein terms of the dipole momentsinherent in the monolayer-forming molecules and of the dielectric constants of the media in which these dipoles are embedded [l]. The interpretation of surface potential of LB films is more complex, despite the successfnl attempts to explain data for specific materials [2-4]. The main difficulty lies in the role of the substrate-film interface, which provides an important contribution to the surface potential in addition to that from the molecular dipole moments.It has been found that the surface potential of LB films obtained from unionized monolayers is generally lower than the corresponding monolayer potential, mainly owing to a negative contribution from the substrate-film interface [5]. ln the caseof LB films formed from ionized monolayers,the analysis is complicated because it depends on whether the double-layer contribution for the monolayer potential is replaced by a corresponding one in the transferredLB films. In this work, we addressthese issues by analyzing surface potential data of LB films from polyaniline (PANi). Owing to their polymeric nature, one cannot ascribe dipole momentsto the PANi molecules, and therefore the measurementof the monolayer surface potential (AV,) serves only to probe the influence of subphase composition, as in the change of the degree of protonation [6]. Nevertheless,measuring the surface potential of transferred LB films (V& may allow a better understanding of electrostatic phenomenain metal-insulator/semiconductorsLB fis [3,4]. Here, surface potential measurementsare presented for the first time for Langmuir-Blodgett films obtained from pure, doped PANi - in the emeraldine base form, and also from composite films of PANi and cadmium stearate(Cd%). For comparison,the VLBof pure CdSt has also been obtained.
Experimental The LB films from PANi and from composite PANi-CdSt were produced according to the methods reported elsewhere [7,8]. For pure PANi monolayers, a mixture of polyaniline and camphor sulfonic acid in chloroform was spread on a pure water surface(pH = 6.0) or on an aqueoussolution containing HCl (PH = 2.0). For producing compositePANi-CdSt and pure CdSt films, the subphase contained cadmium chloride and the pH was maintained at ca. 6.0. Monolayer studiesand LB depositionswere performed with a KSV-5000 systemhousedin a clean room. Thin gold-coated glass plates were used as substrates.Pure and mixed polyaniline monolayers were transferred as Z- and Y-type LB films, respectively, with a transfer ratio close to unity for the transfer process.Different numbers of layers were deposited-on the samesubstrateand the reproducibility of such procedure has been confirmed by measurementswith duplicate ftis. Surface potential measurementswere carried out with a Trek-320K electrostatic voltmeter at room tempxature in air. Results and discussion Earlier UV-vis studies have indicated that polyaniline is protonated when an acidic su@hGe isemployed-fG spreading the monolayer [7j, in contrast to the LB film obtained jrom- a monolayer spreadon a neutral subphasef8], even though in both cases PANi was doped in the spreading solution. The surface potential, AVr,, is slightly higher for the protonated PANi monolayer on an acidic subphase,most probably becauseof the positive contribution from the double-layer potential 163.The difference, however, is less than the double-layer contribution to the doped monolayer, which is around +lOOmV, This meansthat the dipole moment contribution was higher for the undoped monolayer, possibly indicating a different chain organization. Such a finding is not surprising since doping is known to affect the polymer chain conformations. When transferred onto solid substrates,these doped LB films exhibit a surface potential, VLB, that is only 50-60 mV lower than the monolayer potential, AVL. Assuming that the monolayer does not reorganize, upon transfer one may expect three types of change in the surface potential: i)
0379~6779/99/$- see front matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: SO379-6779(98)00798-X
C.J.L.
Constantino~cr
al.
I
Syniheric
loss of the double layer contribution (for ionized monolayers), which may be compensatedby ii) contribution from a charged layer that induces opposite charges on the substrate, and iii) charge injection from the metal substrate.The latter is responsible for the lower VLBwhen comparedto AV, for unionized films [5] and is generally 100-200 mV (negative). Therefore, one may conclude that for doped PANi LB films, factor ii) must have compensated the loss of the double-layer contribution, and furthermore the negative contribution from charge injection cannot be large. With regard to the undopedLB PANi films, V~B is lower than AVL of the corresponding monolayer, again by some 50-60mV. This decreasecan only be attributed to electron injection from the substrate. It is lower than the usual value reported in ref. [5], as also occurred for doped PANi films. Vra for dopedPani LB films is higher than that of non-dopedPani LB film, regardless of the number of deposited layers. While the contribution (i) may not be significanf this difference could arise either from contribution (ii) which can appearonly in the doped films, or due to differences in the contribution due to electron injection (iii). For the ionization potential of the doped PANi is likely to be lower than of its undoped counterpart, and therefore electron injection from gold would be more efficient in the undoped PANi films, leading to a higher negative contribution to VLB.
Table 1 shows Vra for doped (pH=2.0) and undoped PANi LB films with different numbers of layers. Results for lower numbers are not shown becauseVra increases for the fast few layers until reaching a reasonably constant value. Furthermore, for PANi ftis, VLB doesnot dependon whether an odd or even number of layers was deposited.For pure CdSt, however, VLB is negative for an even number of layers, becausenow the dipole moments of adjacent layers are canceled out in these centrosymmetric LB films. Then, only the negative contribution from charge injection remains. It must be stressedthat the CdSt LB films with an even number of layers are less stable since the hydrophilic head groups are directed towards the air. Taken together these results show that a unique polar axis is not established in the Z-type PANi ftis, in spite of the noncentrosymmetric nature of the deposition, which may be related to the poor stability of Z-type films and/or the random organization of the polymer chains in each individual layer. In all cases,Vra varied by at most rtlOmV while scanning the surface of the whole fti, which is close to the sensitivity limit of the measuring measurement,indicating good film uniformity at least at the macroscopiclevel. Table-l: VLB of LB films of polyaniline and cadmium stearate containing different numbers of layers Number of layers 7 9 8 6
VLBPANi (pH = 6.0) + 250 mV + 240 mV + 200 mV + 240 mV
vLB PANi (pH = 2.0) + 340 mV +300mV + 330 mV + 340 mV
VLBCdSt + 130 mV + 130mV - 120 mV -4OmV
For the mixed LB ftis, Vra was measuredfor 20 and 80% weight percentagesof PANi (see Table 2). It increaseswith the amount of PANi in the fti, as expected because the PANi monolayers - even the undoped ones - display a higher AVL than C&St. Vrs for the mixed films has an intermediate value between those of pure PANi and CdSt LB films. The striking feature in
659
iLletuls 101 (1999) 68&559
Table 2 is that Vrs is practically independent of the number of depositedlayers for the SO:20PANi:CdSt composition, including the ftis with an even number of layers. Moreover, in subsidiary experiments in which VLB was monitored through several days, we noticed that these ftis are stable, despitethe even number of layers, unlike their counterparts of pure CdSt. Therefore, the PANi molecules appearto have a sort of “stabilizing” effect on the CdSt molecules. Such an effect is obviously less important when the PANi amount is decreasedconsiderably. Indeed, for the 2080 PANi:CdSt ratio, Table 2 shows a VLB that is appreciably lower for the LB films with an even number of layers, although the inversion in the sign is not observed,in contrast to the pure CdSt LB films. The latter observationindicates the dominant role of PANi even in the films which containedonly 20% of PANi. Table-2: Vrs of composite LB films of different weight percentagesof polyaniline and cadmium stearate and different numbersof layers Number of layers 7 9 8 6
VLBPAN~-CdSt (20:80) + 190 mV + 210 mV + 160 mV + 120 mV
Vrs PANi-CdSt (80:20) + 230 mV + 220 mV + 210 mV + 210 mV
Earlier X-ray diffraction (XRD) studies of mixed LB ftis revealed that both PANi and CdSt are present as separate domains and that the domain structure of CdSt was destroyed upon increasing the amount of PANi in the fti [8]. On the basis of the results presentedhere, it seemsthat in the composite LB films, the ordered CdSt domains are distributed randomly in the disorganizedPANi matrix. Conclusions The surfacepotentials of LB films from doped and undoped PANi were investigated. They are lower than the corresponding monolayer potential due to electron injection from the substrate. In composite PANUCdSt films, the polymer had a stabilizing effect on the CdSt molecules when their head groups were exposedto air. Acknowledgements The authors thank FAPESPand CNPq for the fmancial support. References 1. O.N. Oliveira Jr. and C. Bonardi, Langmuir, 13 (1997) 5920. 2. R.H. Tredgold and G.W. Smith, J.Phys.D:Appl. Phys., 14 (1981)L193. 3. E. Itoh, A. Fukuda, M. Iwamoto, J. Elctrostatics, 33 (1994) 147. 4. E. Itoh, H. Kokubo, S. Shouriki, M. Iwamoto, J. Appl. Phys., 83 (1998) 372 (and referencestherein). 5. C.J.L. Constantino,L.P. Juliani, V.R. Botaro, D.T. Balogh, M.R. Pereira, E.A. Ticianelli, A.A.S. Curvelo, O.N. Oliveira Jr., ThinSolidFilms,284-285(1996)191. 6. S.V. Mello, A. Riul Jr., L.H.C. Mattoso, R.M. Faria and O.N. Oliveira Jr., Synth. Met., 84 (1997) 773. 7. A. Riul Jr., L.H.C. Mattoso, G.D. Telles,P.S.P. Herrmann, L.A. Colnago, N.A. Parizotto, V. Bamnauskas,R.M. Faria and O.N. Oliveira Jr., Thin Solid Films, 284-285 (1996) 177. 8. A. Dhanabalan,A. Riul Jr., O.N. Oliveira Jr., Supramolecular Sci., 5 (1998) 75.