Synthetic Metals 100 Ž1999. 249–259
Comparative studies on Langmuir–Schaefer films of polyanilines Manoj Kumar Ram a
a,)
, Manuela Adami a , Marco Sartore a , Marco Salerno a , Sergio Paddeu b, Claudio Nicolini b,c
Polo Nazionale Bioelettronica, Parco Scientifico e Tecnologico dell’Elba, Via Roma 28, 57030 Marciana (LI), Italy b Elba Foundation, Via A. Moro 15, 57033 Marciana Marina (LI), Italy c Institute of Biophysics, UniÕersity of Genoa, Corso Europa 30, 16132 Genoa, Italy Received 6 July 1998; received in revised form 10 November 1998; accepted 11 January 1999
Abstract Langmuir isotherms of polyaniline ŽPANI., polyŽ o-toluidine. ŽPOT., polyŽ o-anisidine. ŽPOAS. and polyŽ o-ethoxy aniline. ŽPEOA. were investigated at aqueous subphase of pH 1, where doping during monolayer formation appeared as an essential step for high quality of the film. The effect of substituent groups in polyanilines plays a prominent role for the formation of Langmuir films. The area per unit repeat molecule was shown to increase by an increment of the substituent groups in polyanilines. Ultra-thin films of PANI, POT, POAS and PEOA were engineered by Langmuir–Schaefer ŽLS. technique. The uniformity of the deposited polyanilines LS films was verified by atomic force microscopy ŽAFM.. The electrochemical properties of polyanilines LS films were investigated by cyclic voltammetry and current transient measurements, and the electrical characteristics were investigated by depositing the films on interdigitated electrodes. The electrochromic switching response time and diffusion coefficient of such LS films were also estimated by electrochemical surveyings. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polyanilines; PANI; POT; POAS; PEOA; Langmuir–Schaefer ŽLS.; Isotherm
1. Introduction In recent years, polyanilines have received great attention due to their environmental stability, ease in preparation, exciting electrochemical, optical and electrical properties w1–3x. This class of conducting polymers has also been postulated as potential candidates for numerous applications in electrochromic displays, rechargeable batteries, microelectronics devices, biosensors, protective coatings and chemical sensors w4,5x. For many technological applications, it is desirable to have polyanilines in thin films structure, preferably with known thicknesses and molecular packings w5x. Langmuir–Blodgett ŽLB. or Langmuir– Schaefer ŽLS. technique offers a unique control over architecture, thickness and molecular orientation of the films w6,7x. Recently, LB films of polyaniline were obtained by exploiting the solubility of polyaniline in N-methyl-2-pyrrolidinonerCHCl 3 mixture w8–11x. It was shown that even if polyaniline ŽPANI. is not a typical amphiphilic molecule,
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still stable Langmuir monolayer could be obtained and subsequently transferred as LB films w12x. The development of surface irregularities during multilayer deposition of PANI films by LB technique resulted in poor redox kinetics in the films after a certain number of layers w12x. The substituted polyanilines are considered as processable conducting polymers, which can be appropriate for application in various technologies comprising electronic and optical devices w13–15x. The effect of substituent groups Ž –CH 3 , –OCH 3 , –OC 2 H 5 , etc.. in the monomer or the polymeric chain of polyanilines appear to enhance potentiality by displaying a significant increase in electronic localization and a simultaneous decrease in conductivity, but an excellent solubility into a number of organic solvents w16–20x. The solubility of substituted polyanilines in CHCl 3 enabled to assemble such class of conducting polymers into ultra-thin films at the molecular level with a high degree of order w16–19x. Recently, ultra-thin films of polyŽ o-toluidine. ŽPOT., polyŽ o-anisidine. ŽPOAS. and polyŽ o-ethoxy aniline. ŽPEOA. were manufactured by LB technique, which introduced a stabilized shape of voltammetry and a stable current response behaviour in comparison to that of the electrochemical films. The electrochemi-
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 0 2 4 - 7
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cal behaviour of POT LB films was found stronger than that of its electrochemical or dip-coated films w20x. However, comparative studies about the fabrication and characterization of LS films of various types of polyanilines have not been shown in literature. The influence of subphase condition and compression speed etc., on the pressure–area Žp –A. isotherm of polyanilines plays an important role for the stable Langmuir monolayers w21–26x. In our earlier investigation, the effect of subphase Žat various pH values. on the formation of Langmuir monolayer and the orientation of POAS molecules at air–water interface were shown w21,22x. POAS revealed a decrease in the area per molecule of the Langmuir monolayer fabricated at pH ranging from 6.4 to 1. In particular, the effect of subphase for the formation of the Langmuir monolayers was shown, and LB films of polyanilines were fabricated. We have also observed the linearity in thickness of POAS LS films for multilayer deposition besides the fast electron transfer process in POAS LS films. With these considerations, we used polyanilines, i.e., PANI, POAS, POT and PEOA. The Langmuir monolayers of such polyanilines were studied at deionized water and pH 1. The deposited LS films were examined by using cyclic voltammetry in HCl medium in order to assess the redox characteristics of various polyanilines at identical conditions. The surface morphology and uniformity of polyaniline LS films were investigated by atomic force microscopy ŽAFM., and later the uniformity in each 15 monolayers LS of various polyanilines was investigated by such technique. The current–voltage Ž I–V . characteristics were measured by depositing films on interdigitated electrodes. The electrochromic switching response time and the diffusion coefficient of polyanilines LS films were also estimated by electrochemical investigations. 2. Experimental section
times, and later in methanol and diethyl ether in order to eliminate the polymers of low molecular weight and oligomers Žviolet in colour.. Each emeraldine form of polyanilines was heated at 1008C. The green powder thus obtained was the emeraldine salt ŽES.. Each ES form of polyanilines was subsequently treated by using aqueous ammonia for 24 h. Further emeraldine base ŽEB. form of polyanilines was washed in distilled water and acetone for several times, and then dried for 6 h at a temperature of 1008C. It should be remarked that POAS and PEOA were washed using acetone during the synthesis, which is very similar to the preparation of polyaniline w23x. It was found that POT, POAS and PEOA were soluble in chloroform. The powder thus obtained was an EB form Ždark blue in colour. of each type of polyanilines. 2.2. Formation of polyanilines LS films The spreading solutions of POT, POAS and PEOA were simply prepared by dissolving in chloroform, whereas polyaniline was initially dissolved in N-methyl-2-pyrrolidinione ŽNMP. before the use of CHCl 3 . Different concentration of polyanilines in CHCl 3 was prepared and subsequent isotherms were recorded. It was shown that, when the concentration was kept to a minimum level, no immediate change in collapse pressure was observed w8,9x. A stock solution was prepared by dissolving 5 mg of polyaniline in 2 ml of NMP and 20 ml of CHCl 3 for the immediate use. The resulting solution was filtered with a solvent resistant filter Ž0.5 ml.. The precipitation of POT solution occurred in 1 to 2 days, while POAS and PEOA did not show any precipitation for a prolonged period. POT, POAS and PEOA, 0.2 mgrl solutions, were made in CHCl 3 , and 100–150 ml of each solution were spread onto two types of aqueous subphases, i.e., pH 1 using HCl and deionized water. Fig. 1 shows the general structure of the
2.1. Synthesis of polyanilines Monomers of aniline, o-toluidine, o-anisidine and oethoxy aniline, ammonium perdisulphate wŽNH 4 . 2 S 2 O 8 .x as oxidizing agents and various reagents were obtained from Sigma for the synthesis of polyanilines. Polyanilines were chemically synthesized by oxidative polymerization of the monomers by using wŽNH 4 . 2 S 2 O 8 x under controlled conditions w23,27–29x. Distilled aniline, toluidine, oanisidine and o-ethoxy aniline Ž0.215, 0.215, 0.219 and 0.220 M. were separately used, for the synthesis of PANI, POT, POAS and PEOA, respectively. Each monomer was added slowly in a 200-ml solution of HCl containing 0.05 M Ž11.5 g. ammonium perdisulphate pre-cooled to 48C in an ice bath. The reaction was continued for 12 h for each polyanilines. A dark green precipitate recovered from the reaction vessel was filtered, and then washed by using 1 M HCl to remove the oxidant and oligomers. Each precipitate was subsequently washed in deionized water for several
Fig. 1. Chemical structure of polyanilines.
M.K. Ram et al.r Synthetic Metals 100 (1999) 249–259
polyanilines, where y s 1r2, 1 and 0 gives rise to emeraldine, leucoemeraldine and pernigraniline form, respectively. Fig. 1 also shows the ES form of polyaniline class. After the isotherms were recorded, it appeared that the film formation at pH 1 showed higher collapse pressure than if made at deionized water. Thus, pH 1 was used as the subphase for the deposition of LS films for polyanilines. LS films were formed in a LB trough, 240 = 100 mm2 in size and 300 ml in volume ŽMDT, Russia. having a compression speed of 1.667 mmrs Ž100 cm2rmin. for all the polyanilines. The different compression speed was utilized for the recording of POAS isotherm. Optimum compression speed, 100 cm2rmin, was optimized from POAS p –A isotherm curves. Different number of monolayers were transferred onto glass, platinum, glass indium– tin-oxide ŽITO. plates and interdigitated electrode substrates Žcontaining chromium electrodes, which were previously cleaned with ethanol and chloroform. by LS technique. The stability of Langmuir monolayers were checked at various surface pressures for each polyanilines, and 20, 22, 25 and 18 mNrm for PANI, POT, POAS and PEOA were optimized for the deposition of their LS films. Such deposition pressures were also shown in literature w21,22x.
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When such surface pressures were maintained for 2 h, the area per monomer remained constant. The feedback Žgain. for each deposition was kept to be 5 mNrm. 2.3. Atomic force microscope The AFM was a home-built instrument ŽPolo Nazionale Bioelettronica. working in contact mode in air at constant contact force w30x. The probes used were triangular shaped, gold-coated Si 3 N4 microlevers by Park Scientific Instruments, with tips of standard aspect ratio Žabout 1:1. and spring constant of 0.03 Nrm. The contact force was 0.1 nN, while the images acquired were 256 = 256 pixel maps. The row scanning frequency was 4 Hz during acquisition, i.e., a physical tip–sample motion speed of 8–4–2 mmrs in the 2–1–0.5 mm scan size images, respectively. All images are standard top-view topographic maps, where brightness is proportional to the quote of the sample features. The raw data images tilt have been corrected by removal of a first order Ži.e., plane. fitted background, and later smoothed with a 3 = 3 kernel low-pass filter in order to remove high frequency noise. The shown pictures are
Fig. 2. Ža. Pressure–area isotherm of Langmuir monolayer in deionized water: Ž1. PANI, Ž2. POT, Ž3. POAS and Žd. PEOA. Žb. Pressure–area isotherm of Langmuir monolayer in aqueous subphase at pH 1: Ž1. PANI, Ž2. POT, Ž3. POAS and Ž4. PEOA.
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representative of the samples, since same looking images were found in four different regions of the samples, at the vertices of a 4 mm square, centred at the specimen. 2.4. Electrical and electrochemical The electrical characterization was performed by using an electrometer ŽKeithley model 6517.. I–V characteristics were obtained by potential step of 0.05 V. Each strip of the interdigitated electrodes was spaced 50 mm and 40 nm in thickness. The electrochemical measurements were made by PotentiostatrGalvanostat ŽEG & G PARC model 163., through M270 supplied software. A standard three electrodes configuration was used, where LS films on glass ITO plate acted as a working electrode, platinum as a counter and AgrAgCl as a reference electrode. Cyclic voltammograms PANI, POT, POAS and PEOA LS films in 1 M HCl were measured. The electrochromic response time for each of the polyanilines LS films was also studied. 3. Results and discussions 3.1. Pressure–area isotherm The p –A isotherms of polyanilines in deionized water are shown in Fig. 2a. The stability of Langmuir monolayers for PANI Žcurve 1., POT Žcurve 2. and POAS Žcurve 3. was found to be associated to a high collapse pressure in the condensed phase. The p –A isotherms show an increase in the pressure of the condensed phase, and that it could be possible to obtain the yielding Žbreaking. points at various surface pressures for each polyanilines. Wrin-
kles on the aqueous subphase could be observed, when the pressures were 42, 40, 39 and 28 mNrm for PANI Žcurve 1., POT Žcurve 2., POAS Žcurve 3. and PEOA Žcurve 4., respectively. PANI shows a higher collapse pressure in comparison to that of the other polyanilines, which may be linked to the NMP Ži.e., NMP is springly soluble in water and helps in compacting the Langmuir monolayer., as shown in Fig. 2a. An effect of substituent groups in the polyanilines can be observed at the air–water interface pertaining to the large change in area per molecule Žrepeat unit, r.u.. at the condensed phase for PANI, POT, POAS and PEOA. The molecular area at condensed phase was estimated by using 1 r.u. of each polyaniline, as shown in Fig. 1. The obtained area per molecule for PANI, POT, POAS and PEOA at the air–water interface was estimated by extrapolating the p –A isotherm curves of Fig. 2a, and ˚ 2rr.u. The cross estimated to be 26, 45, 55 and 62 A sectional area of the aniline r.u. at the air–water interface ˚ 2 w8,9x. In the light of these was estimated to be 20 A results, it could be predicted that molecules of substituted polyanilines are not oriented and occupy larger area than polyaniline at air–water interface w31x. The stability of the Langmuir film at air–water interface depends strongly on the selection of suitable subphase w4,5,8,9,21–26x. p –A isotherms of polyanilines at the aqueous subphase of pH 1 using HCl are shown in Fig. 2b. Doping appears to be an important factor for the stability of the monolayers, which is probably associated to the ordering, that is introduced in the polymer at air–water interface at pH 1. So, an attempt was made to estimate molecular area of substituted polyanilines at the air–water interface by extrapolating p –A isotherm curves as shown in Fig. 2b. The area per molecule was estimated to be
Fig. 3. Pressure–area isotherm of Langmuir monolayer in aqueous subphase at pH 1, as function of compression speed. Various compression speed magnitudes are shown in the figure.
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˚ 2 for PANI, POT, POAS and about 20, 25, 21 and 45 A PEOA, respectively. Such, values support the concept that the EB form of polyanilines is changed to ES at pH 1 w8,9x. The bigger value in the area per molecule, found for the r.u. molecule of PEOA in curve 4 of Fig. 2b, can be linked to the bulky group Ž –OC 2 H 5 . of PEOA. Such bulky groups could have caused different arrangements at the air–water interface, though the film was formed at pH 1 and possibly behaved little differently than PANI, POT or POAS conducting polymer. Fig. 3 reveals the change in p –A isotherm curves as a function of compression speed. It can be seen that the relative change in the mean molecular area at various compression speed could be experienced. When the compression speed was maintained at 6 mmrs, there was a big shift of the mean area of the molecule and the sudden collapse of the area was noticed, along with the two type of solid phase condensation. When the compression speed was kept at 3 mmrs, the isotherm curve showed the stable behaviour. Further decrease of the speed showed the mean molecular area compatible to the area occupied by the one r.u. of POAS molecule. The low speed showed the high collapse pressure but one could see the wrinkle on the subphase. The stability of the films was further checked at different pressure with compression speed of 1.67 mmrs of PANI, POT, POAS and PEOA Žfigures are not shown.. Various pressures Ži.e., 20, 22, 25 and 18 mNrm. were estimated for PANI, POT, POAS and PEOA based on the stability of Langmuir films. Such deposition pressures were also reported in literature w4,8,9,21,22x. 3.2. Atomic force microscopy This technique was used to investigate the morphology of multilayer LS films of polyanilines. Initially, we used the AFM pictures of polyaniline as a function of monolayers. The equal grain size was shown up to 3 to 15 monolayers of polyaniline films and later it showed the different bundles of polyaniline in AFM image as shown in Fig. 4. So, the AFM images of size 1 = 1 mm2 for the intermediate Ž15 monolayers. films of PANI, POT, POAS and PEOA LS on Mica are shown in Fig. 5. The images reveal a fine grained structure of PANI, POT and POAS and show an extremely fine grain size of PEOA LS films. Moreover, these images show the uniformity in the polyanilines LS films. It attributes to the fact that polyanilines LS films are smooth, complete and continuous till 15 monolayers w32x. Table 1 summarizes the results of atomic force microscopic pictures of each polyanilines. All forms of substituted polyanilines exhibit microscopic structure varying from small to bigger grain size. An interesting feature, which can be noticed is that PANI shows the bigger grain size, which can be linked to N-methyl-2-pyrrolidinone, whereas a decrease in grain size was observed for POT, POAS and PEOA respect to PANI. The brighter intensity of the light visualised at some places in AFM
Fig. 4. Atomic force microscope pictures of 3 Žimage a., 20 Žimage b. and 25 Žimage c. monolayers of polyaniline having dimension of 1=1 mm2 .
pictures is related to the overlapping of some molecules during the drying process by the flux of nitrogen. The size of the grains depends upon the nature of the polyanilines molecules. It should be noted that though PEOA is a larger molecule, nevertheless it shows smaller grains than the other studied polyanilines. 3.3. Electrical characterization We performed the electrical measurements of polyanilines LS films on interdigitated electrodes with the perspectives as: Ž1. the interdigitated chromium electrode does not react in the polyanilines films, Ž2. there is no pinning effect of conducting polymer films, which generally occurs in the sandwiched Žmetal–LB–metal. type of
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Fig. 5. Atomic force microscope pictures of substituted polyanilines: PANI, POT, POAS and PEOA, the image dimension is 1 = 1 mm2 .
configuration, and Ž3. the interdigitated electrodes have fixed size and equal separation from one tract to another w21–23x. The I–V characteristics study as a function of monolayers of POAS LS films were performed on the interdigitated electrodes. Fig. 6a shows the stability in the I–V characteristics for HCl doped POAS LS films, i.e., measured immediately after the films formation and after a period of one month. It shows the stable electrical behaviour of HCl doped POAS LS films. Fig. 6b shows behaviour of the current vs. number of monolayers for
POAS LS films at a measured potential of 0.6 V. It reveals that the current flowing through the POAS LS film increases with an increment in number of monolayers, i.e., the electrical conductivity increases in the successive deposition, and was found to be 0.1 Srcm for 40 monolayers of POAS LS films, where the thickness of each single layer ˚ w15x. Fig. 6c shows the I–V was estimated to be 24 " 1 A characteristics of the polyanilines films measured at a scan rate of 20 mVrs for PANI Žcurve 1., POT Žcurve 2., POAS Žcurve 3. and PEOA Žcurve 4., respectively. The
Table 1 The results of atomic force microscopy Material
Grain density ŽNrmm2 .
Grain lateral size B, m " s Žnm.
The distribution of the grains height Žnm.
Total grain scale corrugation Žnm.
RMS roughness Žnm.
PANI POT POAS PEOA
53 34 27 19
73 " 20 82 " 24 68 " 17 82 " 14
5–15 5–25 5–20 5–25
57 65 50 51
6.9 10.2 7.2 5.0
The numerical shown in the table is the representative of selected images.
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Fig. 6. Ža. Current–voltage characteristics of 40 monolayers POAS LS films deposited on interdigitated electrodes. Žb. Current–voltage characteristics of POAS LS films as a function of monolayers. Žc. Current–voltage characteristics of 40 monolayers polyanilines LS films deposited on interdigitated electrodes: Ž1. PANI, Ž2. POT, Ž3. POAS and Ž4. PEOA.
non-ohmic behaviour at low potential value could be related to the possible redox reaction of the interdigitated electrodes with HCl during the preparation of LS films, or to some potential barrier created between the electrodes and highly doped Ždegenerately doped. conducting polyaniline w15,21,22x. The PANI LS films Žcurve 1. show the higher magnitude of current for each measured poten-
tial. Whereas, the decrease in the current magnitude of POT LS films can be related to the decrease in interchain order due to the –CH 3 substituent in aniline. POAS shows that the decrease in conductivity may be linked to an increase in electronic localization. Interestingly, PEOA Žcurve 4. reveals a delineate relationship in I–V characteristics, and shows minimum magnitude of current for the
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3.4. Electrochemical inÕestigation
Fig. 7. Cyclic voltammograms of 30 monolayers polyanilines on deposited on glass ITO plates in 1 M HCl at a scan rate of 50 mVrsec: Ža. PANI, Žb. POT, Žc. POAS and Žd. PEOA.
LS films. The diminishing of the current Žor conductivity. value can be due to the larger substituents group ethoxy in PEOA, and to the charge localization along the polymer chains.
The electrochemistry of substituted polyanilines LS films was investigated by cyclic voltammetry. The cyclic voltammogram ŽCV. of 30 layers of each of the polyanilines LS films prepared at pH 1 in 1 M HCl medium is shown in Fig. 7 Žcurves a–d.. The bias potential was swept from y0.3 to 1.0 V at a scan rate of 50 mVrs. The shape of the CV in Fig. 7a–d shows the confined surface species. These CVs exhibit redox features characteristic of the individual polyanilines. Table 2 shows the potential peaks values of the polyanilines as derived from Fig. 7a–d. There is a gradual decrease in the redox potentials as a function of substituents in polyanilines. The positive shift in the position of peak 1 is associated to the electron transfer, which implies that the substituent groups can induce some non-polar conformations, which in fact decrease the conjugation along the polymer backbone and is somewhat responsible for the oxidation potential peak. In fact, the blue shift in the oxidized potential peak can be related to the decrease in the number of polaronsrbipolarons or charged states due to an increase of substituents in polyanilines. Additionally, the lower value of electrochemical reduction potential for the LS films of POT Ž662 mV., POAS Ž637 mV. and PEOA Ž465 mV. is in close agreement to that of the polyaniline films Žat 780 mV. w33x. The change in the oxidation potential Žshown in Table 2. is linked to the higher electronic density states due to the substitution in the aromatic ring, which facilitates the protonation and the oxidation of the amine group. Similar results were observed for the electrochemically grown polyanilines films w24x. These processes are related to the interconversion reaction between different oxidation states Žpeak 1: leucoemeraldine to emeraldine and peak 3: emeraldine to pernigraniline and also to the protonation degree from undoped base form to the doped salt form.. The middle peak has been variously interpreted, but there is evidence for the formation of quinoid-like species, especially when the potential is brought above 0.9 V w13,34x. It has been shown that the CV tends to show the stable response after a given number of cycles. Small variation on the values of peak potential can probably be due to the size and chemical nature of the anion, which leads to the differences in its diffusion through the film. Further, interesting aspects such as the electrochromic effects of the films were also observed. The elec-
Table 2 The electrochemical parameters of 15 monolayers polyanilines LS films Material
Oxidation potential ŽmV.
Reduction potential ŽmV.
Response time Žms.
PANI POT POAS PEOA
0.78, 320 662, 531.8, 304 707, 506, 282, 563.4, 374.4, 100.5
130, 710 627.4, 499.3, 165, 23.54 680, 410, 144, y5.91 401.1, 262
180 240 230 280
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Fig. 8. Oxidation and reduction current response time of 15 monolayers PANI LS films. Inset: oxidation current vs. time Žcurve 1., current transient response of 15 monolayers of LS films of PANI Žcurve 2..
Fig. 9. Cyclic voltammograms of 40 monolayers PANI films at a scan rate of 100 mVrs as a function of time: Ž1. native films, Ž2. after 10 4 cycles, Ž3. 10 5 cycles and Ž4. 3 = 10 5 cycles.
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trochromism was studied at a scan rate of 100 mVrs. The colour was noted to change from yellow to green and later to violet, as the potential was swept from y0.2 to 0.9 V for each polyanilines conducting polymer LS films. In order to verify the redox processes, diffusion coefficient and transport mechanism of PANI LS films, the oxidation and reduction current transient were studied by sweeping from y0.2 to 0.9 V, at different scan rate. Fig. 8 shows the redox current vs. time for 15 monolayers of polyaniline films: there is the nearly equal oxidation and reduction system, when the potential is swept from y0.2 to 0.9 V. The redox changes are associated with the electronic resonant structure of the polymer backbone caused by the oxidation and reduction processes of polyaniline films. The redox switching of the PANI conducting polymer may be dependent on the redox ionic conductivity of the polymer matrix. Fig. 8 Žinset. shows the plot of oxidation current vs. time and reduction current vs. time plot for 15 multilayers PANI films. The slope obtained for polyaniline shows that redox processes are diffusion controlled. The value of the slope was obtained to be 0.64 and 0.57. To investigate the ion diffusion process on switching time, we considered the current transient for the redox switching process. The diffusion coefficient was estimated by using the Bulfer–Volmer equation, where two electrons participated in the reaction mechanism of the polyaniline system. The diffusion coefficient for the oxidation process was estimated to be 4.8 = 10y5 cm2rs, whereas the diffusion coefficient for reduction has been shown to be 1.27 = 10y5 cm2rs. The oxidation and reduction response time was found to be 180 ms. Similar studies of oxidation and reduction were performed for each polyaniline system, and the response time was given in Table 2. Interestingly, the response time of POAS was found shorter than that of POT and PEOA, but longer than that of PANI films. Later, the lifetime of the PANI films were estimated by applying the pulse continuously at 100 mVrs. The CV was recorded in Fig. 9 to understand the mechanism and lifetime of PANI films. There is a change in the peaks potential after 10 4 as well as 10 5 cycles but the shape of the CV always follows the Nerst equation Žclosed and reversible to up 3 = 10 5 cycles.. It shows the close shape of CV after 10 5 cycles, which diffuses ions and maintains the electroactivity in close comparison to electrochemical films w24–26x. The lifetime of PANI LS films was shown to be greater than 10 5 cycles.
4. Conclusions LS technique has been successfully applied, obtaining ultra-thin films of various polyanilines. The performed investigations emphasised the inclusion of HCl dopants in the polymer, which occurs during the film formation and causes a different organization of polyanilines molecules at the air–water interface. The area per r.u. Žmolecule. occu-
pied increases as a function of substituents in polyanilines, either studied in deionized water or pH 1. Virtually all forms of polyanilines exhibit a microscopic structure formed by small, nanometer-scale grains or bundles, which fall in the range of 10–80 nm. The LS films of various polyanilines reveal stable cyclic voltammetric response. The PANI LS films were found to display better storage charge capability than the other substituted polyanilines. The measured lifetime of such LS film was shown to be greater than 10 5 cycles.
Acknowledgements We are thankful to Dr. Victor Erokhin and Mr. P. Faraci for their interesting discussions during the preparation of the manuscript. Thanks are due to Mrs. D. Nardelli and Mr. A. Sardi for their help in carrying out the experiments. Financial supports from EL.B.A. Foundation and Polo Nazionale Bioelettronica are gratefully acknowledged.
References w1x J.J. Langer, Synth. Met. 36 Ž1990. 35. w2x A.J. Epstein, A.G. MacDiarmid, in: H. Kuzmany, M. Mehring, S. Roth ŽEds.., Electronic Properties of Conjugated Polymer, Springer Verlag, Berlin, 1989. w3x E.M. Paul, A.J. Ricco, M.S. Wrighton, J. Phys. Chem. 89 Ž1985. 1441. w4x J.H. Cheung, M.F. Rubner, Thin Solid Films 244 Ž1994. 990. w5x N.E. Agbor, M.C. Petty, A.P. Monkman, H. Harris, Synth. Met. 55–53 Ž1993. 3789. w6x G.G. Roberts ŽEd.., Langmuir–Blodgett Films, Plenum, New York, 1990. w7x M.F. Rubner, T.A. Skotheim, in: J.L. Bredas, R. Silbey ŽEds.., Conjugated Polymers, Kluwer, Amsterdam, 1991, pp. 363–403. w8x M.K. Ram, N.S. Sundaresan, B.D. Malhotra, J. Phys. Chem. 97 Ž1993. 11580. w9x M.K. Ram, R. Gowri, B.D. Malhotra, J. Appl. Polym. Sci. 13 Ž1997. 141–145. w10x J.H. Cheung, E. Punkka, M. Rikukawa, M.B. Rosner, A.J. Rorappa, M.F. Rubner, Thin Solid Films 210–211 Ž1992. 246. w11x A. Dhanabalan, R.B. Dabke, N. Prasant Kumar, S.S. Talwar, S. Major, R. Lal, A.Q. Contractor, Langmuir 13 Ž16. Ž1997. 4395–4400. w12x A. Riul Jr., L.H.C. Mattoso, G.D. Telles, P.S.P. Herrmann, L.A. Colnago, N.A. Parizotto, V. Baranauskas, R.M. Faria, O.N. Oliveira Jr., Thin Solid Films 284–285 Ž1996. 177. w13x S.V. Mello, L.H.C. Mattoso, R.M. Faria, O.N. Oliveira Jr., Synth. Met. 71 Ž1995. 2039–2040. w14x A. Riul Jr., L.H.C. Mattoso, S.V. Mello, G.D. Telles, O.N. Oliveira, Synth. Met. 71 Ž1995. 2060. w15x M.K. Ram, S. Carrara, S. Paddeu, C. Nicolini, Thin Solid Films 302 Ž1997. 89–97. w16x M. Leclerc, J. Guay, L.H. Dao, Macromolecules 22 Ž1989. 649. w17x R.V. Gregory, W.C. Kimbrell, H.H. Kuhn, Synth. Met. 28 Ž1989. C823. w18x J. Pellerino, R. Radebaugh, B.R. Mattes, Macromolecules 29 Ž1996. 4985. w19x E.M. Scherr, A.G. MacDiarmid, S.K. Manohar, J.G. Masters, Y. Sun, S. Tang, M.A. Druy, P.J. Glatwski, V.B. Cajipe, J.E. Fischer,
M.K. Ram et al.r Synthetic Metals 100 (1999) 249–259
w20x w21x w22x w23x w24x w25x w26x
K.R. Cromack, M.E. Jozefowicz, J.M. Ginder, R.P. MacCall, A.J. Epstein, Synth. Met. 25 Ž1989. 649. M.K. Ram, M. Joshi, M. Mehrotra, S.K. Dhawan, S. Chandra, Thin Solid Films 304 Ž1997. 65–66. M.K. Ram, S. Paddeu, S. Carrara, E. Maccioni, C. Nicolini, Langmuir 13 Ž10. Ž1997. 2760–2765. S. Paddeu, M.K. Ram, S. Carrara, C. Nicolini, Nanotechnology 9 Ž1998. 228–236. S. Paddeu, M.K. Ram, C. Nicolini, J. Phys. Chem. B 101 Ž1997. 4759–4766. M.K. Ram, E. Maccioni, C. Nicolini, Thin Solid Films 303 Ž1997. 27–33. M.K. Ram, N.S. Sundaresan, B.D. Malhotra, J. Mater. Sci. Lett. 13 Ž1994. 1490–1493. M.A. Habib, S.P. Maheshwari, J. Electrochem. Soc. 113 Ž1989. 1050.
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259
w27x S.P. Armes, J.F. Miller, Synth. Met. 13 Ž1986. 193. w28x H. Bleier, J. Finter, B. Hilti, W. Hofherr, C.W. Mayer, E. Minder, H. Hediger, J.P. Anssermet, Synth. Met. 55–57 Ž1993. 3605–3610. w29x A.G. MacDiarmid, J.C. Chiang, A.F. Richter, N.L.D. Somasiri, A.J. Epstein, in: L. Alcacer ŽEd.., Conducting Polymers, Riedel, Dordrecht, Holland, 1987, pp. 105–120. w30x G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56 Ž1986. 930. w31x M. Ando, Y. Watanabe, T. Iyoda, K. Honda, T. Shimidzu, Thin Solid Films 178 Ž1989. 373. w32x T.L. Porter, D. Thompson, M. Bradley, Thin Solid Films 288 Ž1996. 268–271. w33x E.W. Paul, A.J. Ricco, M.S. Wrighton, J. Phys. Chem. 89 Ž1985. 1441. w34x M.K. Ram, G. Mascetti, S. Paddeu, E. Maccioni, C. Nicolini, Synth. Met. 55–57 Ž1997. 3611–3616.
