Chem. Mater. 1993,5, 1474-1480
1474
Growth of Electropolymerized Polyaniline Thin Films A. Guiseppi-Elie,' S. R. Pradhan, and A. M. Wilson AAI-ABTECH, P.O. Box 376, Yardley, Pennsylvania 19067-4032
D. L. Allara,? P. Zhang,? R. W. Collins,$ and Y.-T. Kimtps Department of Materials Science and Department of Chemistry, and Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802 Received April 30, 1993. Revised Manuscript Received August 16, 199P
Electroactive polyaniline films were grown by potentiodynamic electropolymerization at planar gold electrodes from deaerated solutions of aniline (1.0 M) in aqueous HCl(2.0 M). Films were produced by repeated cycling over the range -0.20 t o +0.65 V vs SCE to six different levels of electropolymerization charge density. At each level, the separate films were analyzed by cyclic voltammetry a t a 50 mV/s scan rate in deaerated 0.2 M HC1 at 20 "C, by ex situ spectroscopic ellipsometry over the range 1.5-4.0 eV a t an incidence angle of 70Oand by Rutherford backscattering spectroscopy (RBS) using a 4He ion beam with an energy of about 1.8MeV. The anodic charge density measured a t 50 mV/s over the range -0.20 to +0.80V vs SCE varied from C/cm2. Measured film thicknesses, determined by spectroscopic 8.6 X lo4 to 1.2 X ellipsometry, varied from 100 to 1600 A. The anodic charge density was found t o correlate linearly with film thickness over this range, consistent with surface confined electroactivity. The molar concentration of oxidizable equivalents in the polymer film is a remarkably constant 4.3 X 103 M equiv/cm3 over the thickness range studied. RBS analysis of the polymer film indicates that within the limit of the depth resolution of the RBS instrument the C1 species are evenly distributed throughout the thickness of the film and that these species include both polymer cation associated chloride ion and trapped HC1.
Introduction
voirs for the controlled release12 or concentrationl3 of anionic but electroinactive species. Polyaniline (PAN),its redox state variants, and its arylElectropolymerization offers the capabilityto grow these substituted derivatives have attracted considerable new films by precise control of the electropolymerization attention, for both scientific interest and commercial charge, to direct the film to targeted conducting or application, since the recent report of ita electropolymersemiconductingsites of complex device s t r u ~ t u r e sand ~~J~ ization at an inert electrode.' The attention is justified to achieve composition and materials property control by by the relative ease with which this polymer and its electrophoretic blending with other non-redox-active copolymers may be chemically2J or electrochemically p ~ l y m e r s . ~Two ~ J ~approaches ~~ are possible for the ~ y n t h e s i z e d its , ~ ~excellent ~ environmental chemical staelectropolymerization of electroactive polymers while bility,S*6the reversibility of the redox reactions,7*8and the under potential control-the potentiostatic18method and very large and dramatic change in electrical conductivity the potentiodynamicmethod.1g-21Both methods produce that occurs upon simple oxidation and r e d u ~ t i o n .The ~ ~ ~ ~ ~the growth of polymer films by the initiation and discharge commercial appeal is found in the use of electroactive of redox active monomer at a conductor or semiconductor1 polymers, such as polyaniline, as chemoresistive transsolution interface. In the potentiodynamic method, the ducers1° and as redox mediators'' in various forms of film is grown by repeated cycling over a potential range chemical sensor devices and as voltage-stimulated reserselected to induce charge-transfer initiation of the monot Department of Materials Science and Department of Chemistry. 8 Department of Physics. I Current address: Los Alamos National Laboratories, Los Alamos,
NM. e Abstract published in Advance ACS Abstracts, October 1, 1993. (1) D i u , A. F.; Logan, J. A. J. Electroanul. Chem. 1980, 111, 111. (2) Angelopoulos, M.; Ray, A.; MacDiarmid, A. G. Synth. Met. 1987, 21, 21. (3) Wei, Y.; Hariharan, R.; Patel, S. Macromolecules 1990,23, 758. (4) Zotti, G.; Cattarin, S.; Comisso, N. J. Electroanal. Chem. 1987, 2.25. 2.59. __ -, __ -.
(5) LaCroix, J.-C.; Dim, A. F. J.Electrochem. SOC.1988, 135, 1457. (6) Wei, Y.; Hsueh, K. F. J.Polym. Sci.: Polym. Chem. 1989,27,4351. (7) Focke, W. W.; Wnek, G.E.;Wei,Y. J.Phys. Chem. 1987,91,5813. (8) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441. (9) Cowan, D. L.; Priest, V.; Marrero, T. R.; Slaughter, D. W. J.Phys. Chem. Solids 1990,51 (4), 307. (10) (a) Guiseppi-Elie, A. In Progress in On-line Metabolic Measurement Techniques; Wang, N. S., Ed.; Academic Press: New York, in press. (b) Chidsey, C. E. D.; Murray, R. W. Science 1986, 231, 25.
0897-4756/93/2805-1474$04.00/0
(11) (a) Guiseppi-Elie, A.; Wilson, A. M. In Abstracts of the h o c . 64th Colloid and Surf. Sci. Symp.: ACS Division of Colloid and Surf. Sci.;Lehigh University, 1990. (b) Hepel, T. In Abstracts of the R o c . 64th Colloid and Surf. Sci. Symp.: ACS Division of Colloid and Surf. Sci.; Lehigh University, 1990. (c) Hale, P. D.; Boguelavsky, L. I.; Inagaki,T.; Karan, H. I.; Lee, H. S.;Skotheim, T. A. ;Okamoto, Y. Anal. Chem. 1991, 63, 677. (12) Miller, L. L. Mol. Cryst. Lip. Cryat. 1988,160, 297. (13) Oyama, N.; Ohsaka, T.; Shimizu, T. Anal. Chem. 1985,57,1526. (14) Chao, S.; Wrighton, M. S. J. Am. Chem. SOC.1987, 109, 6627. (15) Umana, M.; Waller, J. Anal. Chem. 1986,58, 2979. (16) Dong, S.; Sun, Z.; Lu, Z. J. Chem. SOC.,Chem. Commun. 1988, 993.
(17) Reynolds, J. R.; Baker, C. K.; Gieselman, M. Polym. P r e p . (Am. Chem. SOC.,Diu. Polym. Chem.) 1989, 30 (1) 151. (18) Konig, U.; Schultze, J. W. J. Electroanal. Chem. 1988,242,243. (19) Zotti, G.; Cattarin, S.; Comisso, N. J.Electroanal. Chem. 1987, 239, 387. (20) Focke, W. W.; Wnek, G. E. J.Electroanal. Chem. 1988,256,259. (21) Wei, Y.; Tang, X.; Sun,Y.; Focke, W. W. J.Polym. Sci.: Polym. Chem. 1989,27, 2385.
0 1993 American Chemical Society
Electropolymerized Polyaniline Thin Films
mer, growth, and deposition of the polymer film, as well as repeated oxidation and reduction of the electropolymerized polymer. In the potentiostatic method, the film is grown at a fixed potential selected to induce charge transfer initiation of the monomer as well as growth and deposition of the polymer film. However, since the polymer generally has an oxidation potential that is lower than that of its monomer, the polymer is consequently grown under continuously oxidizing conditions. Paul et aL8 have measured the thickness of potentiostatically (+0.90 V vs SCE) electropolymerized PAn films using SEM and surface profilometry. The films studied ranged from 0.25 to 26.5 pm and were found to correlate poorly with the electropolymerization charge as well as with the anodic charge density measured by cyclic voltammetry. Stillwell and Park22have also measured the thickness of PAn films, and for films of micron thicknesses they have provided the following relationship: T (pm) = (7.2 f 1.5)Qa (C/cm2) - (0.08 f 0.16). Wei et al.23have measured the mass of potentiodynamically (-0.20 to 0.80 V vs SCE) electropolymerized PAn and found this to be linear with the anodic peak current of the first redox reaction, ipal (Epa1 = ca. 0.17 V vs SCE),up to a peak current density of 12 mA/cm2. Possible variation in density perpendicular to the plane of polymer film growth limits the value of the madcurrent relationship. Furthermore, for chemosensor and biosensor applications of these polymer films, it is the thickness that must be controlled. In this paper we report on our investigation of the relationship between the electrochemically determined anodic charge density and the thickness of electropolymerized polyaniline films as measured by spectroscopic ellipsometry. We confirm a relationship of the form provided by Stillwell and Park22T (A) = 1.39 X 10'Qa (C/cm2) - 34.32, measured over the range 100-1600 A. Using the measured film thickness, we have determined the molar concentration of electroactive moieties within the electropolymerized polymer film and have found this to be a constant 4.3 X 10-3 M equiv/cm3 over the range of film thicknesses studied. Rutherford backscattering spectroscopy (RBS) has been used to characterize the distribution of chloride species within the PAn films. This information in turn has been used to characterize the distribution of the electroactive moieties within the polymer film. The distribution of chloride species suggests a uniform electroactivity throughout the PAn film; although thicker films possess more chloride than thinner films. This we believe to be the result of HC1 entrapment within the polymer. XPS analysis supports the presence of molecularly bonded chloride within thicker PAn films.
Chem. Mater., Vol. 5, No. 10, 1993 1475
Instrumentation. Electropolymerizationof aniline and cyclic voltammetric characterization of polyaniline films were carried out using an EG&GPAR 173Potentiostat/Galvanostat outfitted with a PAR 179 digital coulometer. Where needed, potentiodynamic sweeps were accomplished by interfacing the PAR 173 to a PAR 175 universal programmer. Cyclic voltammograms were recorded on an Esterline Angus XYY' 540 recorder. Spectroscopicellipsometrywas performed on a custom-designed unitu consisting of a Xe arc source, rotating (12.5 Hz) polarizer, sample stage, fixed analyzer, a single prism spectrometer, and a cooled (-15 OC) optical multichannel analyzer as detector.
Materials. Aniline (C&NHa) was supplied by Aldrich and used after distillation under reduced nitrogen pressure. Acetone, 2-propanol, and Omnisolve triply distilled water were used as supplied. Phosphate buffered potassium chloride, pH 7.2 buffer solution, was prepared in the standard way.% Pt foil electrodes were fabricated in these laboratories, and saturated calomel electrodes (SCE)were supplied by Fisher. Electropolymerization of PAn was done on planar gold electrodes (PGE, Model AU-118, AAI-ABTECH). The PGEs are suitable for a variety of characterizations of polymer thin films. The PGE is fabricated from 800 8, of magnetron sputtered gold over 200 8, of adhesion promotingchromiumon a chemically resistant, electronicsquality Schott D263 Borosilicate glass substrate. The total analytical area of the AU-118 device was 1.18 cm2. For RBS analysis, different Cr/Au thickness values and substrate materials were found to be more convenient, so for these measurements polymer filmswere electropolymerizedon gold-coatedsilicon wafers using 18008, of gold with 90 8, of chromium as the adhesion promoter. RBS spectral simulation prior to actual measurements showed that when a low-energy ( I calculated in this manner has the limiting value of 4.3 x 103M equiv/cm3over the range of f i i thicknesses studied. It is important to point out that the anodic charge density, Qa, measured by the present CV standard method, whether in situ or ex situ to the polymerization solution, is a measure of the concentration of electroactive moieties within the electropolymerized polymer film. The relationship between this value and the concentration of electropolymerized polyaniline formed in the electropolymerization reaction has not been established. Distribution of the C1 Species from RBS Analysis. Spectral simulations were performed to assist the interpretation of the experimental RBS spectra. The samples were modeled assuming the Si/Cr/ Au/PAn structure with sharp layer boundaries and a PAn density of 1.2 g/cm3. The film thicknesses of Cr and Au were determined during metal deposition and these values stayed fixed in the simulation. Polyaniline film thicknesses obtained from spectroscopic ellipsometry measurements were used as an I I I initial try, and the thickness of the films were then varied .* in order to obtain the best fit to the experimental data. 12 To determine the thickness of PAn films from RBS spectra, J it is necessary to know the correct film den~ity.3~ This .density can also be arrived at by taking the weighted :lot > average of the elemental densities of the constituent PAn film with the composition being C6NH6Clz. Since the Cr layer is buried deeply, the energy straggle of the He ions can be expected to be a significant factor in analyzing this peak so the Cr peak was corrected for energy ~ t r a g g l i n g . ~ ~ The experimental RBS spectra of the thickest polyaniline film (1524A from SE measurements) is shown in Figure 7 together with simulated spectra for three C1 concentrations with an assumption of uniform Cl distri180 190 200 210 220 230 bution throughout the film. Similar spectral simulations Chonnel were performed on other PAn films with different thickFigure 8. Simulated RBS spectra assumin that the chlorine nesses. The general agreement of the simulated with species are concentrated in the outermost 300 and in the bottom experimental spectraclearly indicates that, within the limit 300 A of the thickest polyaniline film.The composition was assumed to be CaHeC1, (x = 0.52)in C1 Concentrated layer. 0, of the depth resolution of the instrument, the C1 species experimental; A, simulated with C1 being concentrated on the are evenly distributed throughout the thickness of the surface; +,simulated with C1being concentrated a t the interface. film. Two extreme cases were also examined using the RBS spectral simulation and are presented in Figure 8 for Table 111. Compositions of the Polyaniline Films with the thickest PAn film. In one case, the C1 species are Different Thicknesses Based on the RBS Spectral Simulations assumed to be concentrated in the outermost 300 A of the film and in the other, in the innermost or bottom 300 A, thickness (nm) Composition i.e., at the polymer/gold interface. The composition of SEa RBSb C H N c1 the C1 concentrated layer was assumed to be C ~ N H ~ C ~ Z , 6 6 1 0.52 f 0.05 152.4 150 where x = 0.52. The poor fits of the simulated to 6 6 1 0.45 0.05 105.6 130 experimental spectra clearly excludes the possibility of 6 6 1 0.30 0.05 79.1 100 high surface or interface concentrations of C1 species in 17.0 40 6 6 1 0.20 0.10 the polyaniline films, at least to the extent of the depth a SE spectroscopic ellipsometry. * RBS Rutherford backscatresolution of the RBS instrument. tering spectroscopy. The compositions of the PAn films with different listed in Table 111. The film thicknesses derived from the thicknesses based on the RBS spectral simulations are best fit of the RBS spectral simulation are listed also in Table111 together with thicknesses obtained from SE (32) Huang,W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. measurements. It should be pointed out that the RBSSoc., Faraday Trans. 1 1986,82, 385. (33) Bagliin,J.E. E.; Williams,J.S. InZonBeamforMaten'a[sAnalysis; derived thickness values might be significantly larger than Bird, J. R., Williams, J. S., E&.; Academic Press: New York, 1989. those listed in Table 111,since the film has been assumed (34)Feldman, L. C.; Mayer, J. W. In Fundamentals of Surface and to be densely packed in our RBS simulation. I t has been Thin Film Analysis; North-Holland: New York, 1986.
i
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1480 Chem. Mater., Vol. 5, No. 10,1993
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shows that a minimum of three peaks are required to fit the spectrum. The analysis of the spectrum shown in Figure 9 results in the appearance of peaks at the binding energies of 197.4,198.9,and 200.6eV. The dominant peak at the lower binding energy of 197.4eV has been attributed to the contribution from C12rm,2 originated from C1 atoms associated with alkali metal chlorides.36~3~ The intermediate peak at the binding energy of 198.9 eV can be attributed to the combination of contributions from ClzPu2 in alkali metal chlorides and from C12ps,2in molecular HC1, while the third peak is appropriate for a contribution from ClzPu2in HCl.3e The increased porosity and hence the surface area of the thicker film can certainly increase the possibility of a molecular type of chlorine species, presumably HC1 from the electrolyte, being absorbed inside the thicker polyaniline films.
Binoing E n e r g y eV
Figure 9. X-ray photoelectron spectrum in the C~Z, region of a 1524-Apolyaniline films.
well-known that polyaniline films obtained from so-called class 2 acid solutions such as H2S04 and HC1 tend to be very porous.19 Finally, Table I11 also shows the chloride atomic composition inferred from the RBS simulation. There is clearly an increase in chloride content in thicker PAn films. Polyaniline films prepared by potentiodynamic electropolymerization from 2.0 M HC1 and analyzed ex situ by RBS following air equilibration are likely the halfoxidized, diprotonated emeraldineeHC1salt. This salt has been reported, based on quantitative 13CNMR anal~sis,~5 to contain one C1 ion for every two aniline repeat units of the polymer backbone. The measured C1content in thicker films corresponds well with this expected result. When compared to thinner films, however, it seems to contradict the trend of the concentration of electroactive species shown in Figure 6. It should be realized, however, that due to the insensitivity of backscattering measurements to the nature of chemical bonding, the C1 concentration measured from RBS will include all C1 species. Such species may include chloride ion, as well as covalently bonded chloride. Thus, while the concentration of electroactive moieties within the films remains essentially constant, the total chloride content does appear to increase. In fact, our XPS measurements of the polyaniline films clearly indicates the existence of covalently bonded C1 species in the polyaniline film. Figure 9 shows a typical ClzP spectrum, in this case obtained from a polyaniline film with a thickness of 1524 8, as measured from spectroscopic ellipsometry. Ideally, if only C1ionic species existed in the film, one should observe Clap spin-orbit splitting doublets with the intensity of the higher binding peak being half of the intensity of the lower binding energy peak. However, exhaustive analysis of the ClzPspectrum (35) Kaplan, S.; Cornwell, E. M.; Richter, A. F.; MacDiarmid, A. G. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1988, 29 (l),58.
Summary And Conclusions Potentiodynamic electropolymerization of aniline follows a second-order polynomial dependence upon time. There exists a simple linear relationship between the amount of charge discharged in the potentiodynamic electropolymerization of aniline and the anodic charge density of the electrodeposited polyaniline film. This relationship supports a model for electropolymerization within which monomer initiation and propagation at the electrified interface is followed by quantitative electrophoretic deposition of the electropolymerizedpolymer onto the electrode. The electrochemical oxidation of electropolymerized polyaniline is a reversible surface confined reaction in which the redox active species are confined within the polymer film. The counter anions required to maintain electroneutrality upon oxidation are drawn from the bulk solution and must gain access by diffusion to redox-active sites within the polymer. The thickness of PAn films, measured by spectroscopic ellipsometry, increases linearly with the anodic charge density of the polymer films. The concentration of electroactive moieties in the polyaniline film therefore, as determined by CV, shows a remarkable constancy with film thickness. For thicknesses over the range 100-1600 A, RBS analyses support a uniform distribution of chloride species throughout the electropolymerized PAn film. However, while the concentration of electroactive moieties within the polymer film remains essentially constant, there is an increased abundance of chloride species in thicker PAn films. The presence of covalently bonded chloride indicated by XPS suggests that there may be HCl entrapment within the thicker polymer films. (36) Uvdal, K.; Hasan;M. A.; Nilson,J. 0.; Salaneck,W. R.;Lundetrom, I.; MacDiarmid,A. G.;Ray, A.; Angelopoulos, A.. InElectronicProperties of Conjugated Polymers; Kuunany, H.,Mehring, M.; Roth, S., Eds.; Springer-Verlag: New York, 1987. (37) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G . Polymer 1991,32, 1354.
