Polypyrrole films functionalized with pendant titanocene dichloride complexes: Ellipsometric study of the electropolymerization process

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Electrochimica Acta 53 (2007) 1195–1205

Polypyrrole films functionalized with pendant titanocene dichloride complexes: Ellipsometric study of the electropolymerization process J.P. Correia a,∗ , M. Graczyk b , L.M. Abrantes a , M.A. Vorotyntsev b a

CQB, Faculdade de Ciˆencias da Universidade de Lisboa, Lisboa, Portugal b LSEO-UMR 5188 CNRS, Universit´ e de Bourgogne, Dijon, France

Received 16 November 2006; received in revised form 22 January 2007; accepted 31 January 2007 Available online 13 February 2007

Abstract Electrochemical and ellipsometric methods have been used to study the electropolymerization process of a functionalized monomer, Tc3Py (in which pyrrole is covalently bonded with titanocene dichloride complex) on platinum electrode from a dilute monomer solution in acetonitrile. The deposition has been performed in the potentiodynamic regime. A new method for the acquisition and treatment of ellipsometric data has been proposed which allowed us to analyze the evolution of the film properties at each potential. The model of a single uniform layer was unable to describe the ellipsometric experimental observations with the necessary precision. A proper fitting of the data has been achieved considering a model of two layers having identical values of the refractive index but different values of the extinction coefficient (higher absorption for the outer layer). The analysis of the calculated values of the optical parameters of these layers has led to the assumption that both layers represent morphologically the same medium (i.e. that the whole polymer film is uniform) while the higher extinction coefficient of the external layer was due to a penetration into it of solute oligomers possessing an intensive absorption at the wavelength of the observation, 633 nm. The dependences of the thicknesses of each layer and of the whole film on the cycle number have been established, the increment of the growth (5.5 nm per cycle) being constant within the whole deposition procedure. The variation of the optical constants of the layers during the potential cycling was determined and interpreted. The calculated values for the extinction coefficients and the layer thicknesses have been used to estimate how the film absorbance changes during the potential cycling. Its comparison with the same characteristic measured directly in a spectroelectrochemical cell for a film deposited at the ITO electrode surface has provided an extra evidence in favor of the above hypothesis on the film being a morphologically uniform material, with incorporated solute oligomers in its outer layer. © 2007 Published by Elsevier Ltd. Keywords: Functionalized polypyrrole; Titanocene complex; Electropolymerization; Ellipsometry; Acetonitrile; Absorption spectra

1. Introduction Electron-conducting polymers in the form of a thin film deposited at the electrode surface represent promising materials for applications in (electro)catalysis [1,2], chemo- and biosensoring [3,4], biologically active materials [5,6], separating membranes and biomembranes [7], batteries [8–15] and supercapacitors[16–22], micro- and nanoelectronic elements [23], displays, mirrors, windows, light-emitting diodes [24], photovoltaic [25] near-infrared devices, electrochromic [26,27] and protective layers [28–32]. A recent tendency in this area is the use of basic conducting polymers (PPy, PAni, PTh, etc.)

∗

Corresponding author. Tel.: +351 217 500000; fax: +351 217 500088. E-mail address: [email protected] (J.P. Correia).

0013-4686/$ – see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.01.078

derivatized with various functional groups [33,34] and, in particular, transition metal complexes [1,35–41]. The first examples of such complexes containing “early transition metals” (groups 4–6) have been published, in which a titanocene dichloride complex, Tc (Cp2 TiCl2 , Cp = C5 H5 ), is linked by an aliphatic chain to either a polypyrrole (PPy) [42,43] or a PEDOT matrix [44]. These films have been characterized by conventional electrochemical techniques combined with EQCM, XPS, SEM and STM-AFM [42–45] which revealed their uniform and flat surfaces, with structural elements on the scale of 10–50 nm. Most of the estimations of the film thickness [42] have been based on electrochemical data, with a great uncertainty for the proportionality coefficient between the deposition charge density and the film thickness. Another open question has been the uniform or heterogeneous character of the film in the normal

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direction to the electrode surface. An indirect indication in favor of the former possibility was given [45] by a proportionality between the film mass and the polymerization charge since a change of the structure of a new layer in the course of the deposition should hypothetically be accompanied by a change of the polymerization yield. Direct information on the above-mentioned properties of the film may be accessed with the use of ellipsometry. In this technique the changes of the polarization state of a collimated monochromatic light beam caused by the specular reflection from the electrode are monitored. From the measured ellipsometric parameters (phase shift, Δ, and azimuth angle, Ψ ) both the refractive index, n, and the extinction coefficient, k (i.e. the real and imaginary parts of the complex refraction index) as well as the film thickness can be derived. Since the mathematical relationships correlating these quantities cannot be solved analytically, the determination of n, k and film thickness is usually done by a fitting procedure of experimental data with simulated results for a given model of the system under study. As ellipsometry is very sensitive both to the electrode surface coverage and to the changes of the dielectric constants of the probed material, the data recorded during the potentiodynamic synthesis of a conducting polymer carry information not only on the thickness increase caused by the reactions leading to the polymer growth but also on the structural transformations involved in the redox conversion of the polymeric film. Providing a relatively fast data acquisition, numerous data points can be measured during each growth/redox conversion cycle allowing one to analyze the polymer properties, at the corresponding values of potential, at different stages of film formation. On the basis of collected data, namely for the film thickness and the extinction coefficient, one can calculate the absorbance of the film for the normal passage of the beam. The latter parameter can be determined by an independent measurement at the same wavelength of the absorbance of the film deposited on the surface of an optically transparent electrode. This paper describes such in situ studies for titanocenepolypyrrole films, PTc3Py, which possess a polypyrrole matrix, with titanocene dichloride complexes (Tc) linked covalently (via one of the Cp ligands of Tc) to the nitrogen of each pyrrole unit by a propyl aliphatic chain. A new way to determine the variation of the film optical constants and thickness in the course of the potentiodynamic polymer deposition is proposed. 2. Experimental Acetonitrile (AN), HPLC grade (Carlo Erba, water content <0.02%) was used as received, TBAPF6 (Aldrich, electrochemical grade) was stored permanently at 80 ◦ C and dried under vacuum at 80 ◦ C for 2 h prior to use. ITO plates (Merck) were separately (in order to avoid scratches if two plates are placed together) cleaned in ethanol with the use of ultrasonic bath for 15 min before use. Electrochemical measurements were controlled by a potentiostat, AUTOLAB PGSTAT30 (Ecochimie, The Netherlands) or WENKING POS 73 (Bank Electronik). The studies of the film deposition and its characterization were performed in

1 mM monomer (Tc3Py) acetonitrile (AN) solution with 0.1 M TBAPF6 with the use of three different cells: (1) Single compartment cell for the film deposition on a small-size Pt disk working electrode (diameter about 1.1 mm, surface area 0.0095 cm2 found by calibration with Fc reaction in AN) or on the “ITO electrode” (thin conducting indium–tin oxide layer at the glass surface, immersed surface area was about 1.8 cm2 ). RE: double junction (with two frits) Ag/0.01 M AgNO3 in AN + 0.1 M TBAPF6 . CE: Pt foil, 1 cm2 ; (2) Homemade two-compartment Teflon cell with two optical windows at 20◦ from the bottom plane; the working electrode was a “specpure” platinum disk with an exposed area of 0.196 cm2 , its potential being controlled against the saturated calomel electrode, aqueous SCE, for ellipsometric studies; (3) Hermetic spectroelectrochemical cell (“homemade” on the basis of a standard 10 mm quartz cuvette, Hellma), for optical absorbance measurements with film/ITO/glass as WE in 0.1 M TBAPF6 + AN solution versus Ag wire (RE) and Pt wire (CE). After the deposition of the film at ITO surface in cell 1 the electrode was rinsed with AN, then transferred into cell 3. The spectra were registered in the deoxygenated conditions ensured by dry Ar passage through the solution before the measurements. Potential was changed as a series of steps; the spectra were measured in the middle of the potential step (8 s after potential step) and registered by means of a VARIAN spectrophotometer Cary 50. Potentials of Ag wire and of double junction (with 0.1 M TBAPF6 + AN solution between two frits) Ag/0.01 M AgNO3 + 0.1 M TBAPF6 + AN are −0.05 V and 0.32 V versus SCE, respectively. All potential values in the paper (if not specified) are given versus SCE. For the ellipsometry experiments, the working electrode was hand-polished in successively finer grades of aqueous suspension of alumina (down to 0.3 ␮m), sonicated in Millipore water for 15 min, flame-annealed until the platinum attained a yellowish-orange color and cooled under high purity nitrogen flux. The solution was thoroughly deoxygenated directly in the cell with nitrogen bubbling (purity >99.9997) for 20 min. The ellipsometric data were collected with a SENTECH SE 400 ellipsometer working in PCSA mode, fitted with a He–Ne laser (λ = 632.8 nm) at 70◦ incident angle programmed to a sampling speed of 1 s−1 . At the above-mentioned electrode surfaces, the polymer films, PTc3Py, were deposited potentiodynamically (with scan rate of 100 mV/s) within the same potential limits, −0.2 V, 1.4 V versus SCE from 1 mM deoxygenated monomer solution in AN + 0.1 M TBAPF6 . The number of deposition cycles was adjusted to attain the same current density for the anodic maximum of the polymer redox response, about 0.15 mA/cm2 , which corresponds to about the same film thickness in all cases. 3. Results and discussion 3.1. Potentiodynamic deposition of polymer films at a small- and large-size Pt electrodes The synthesis of the titanocene dichloride-pyrrole monomer, Tc3Py (Cl2 TiCpC5 H4 (CH2 )3 NC4 H4 ), as well as its anodic

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Fig. 1. Potentiodynamic deposition (25 cycles) of PTc3Py film at the small Pt disk electrode (S = 0.0095 cm2 ).

electropolymerization in AN solution have been described elsewhere [42,43]. Compared to these references, in the present work a higher anodic potential limit, 1.4 V versus SCE, has been used for the polymer potentiodynamic deposition which resulted in a fast film growth. The corresponding voltammograms, for the small-area Pt disk electrode, is given in Fig. 1. The cycling leads to a gradual increase of the polymer matrix response, without a change of the shape of the anodic or cathodic branches and with the same current peak potentials. It gives indirect evidence in favor of a uniform deposition of the film layer-by-layer (in the meaning that about the same amount of the polymer material is deposited in each cycle). The CV also implies that the electronic conductivity of the film is sufficiently high to consider the ohmic potential drop inside the film as negligible (otherwise, the current peaks would shift in the opposite directions in the course of the increase of the film resistance and its total redox capacitance). The thickness of the deposited film can be estimated with the use of the redox charge (per unit of surface area, S) in the range of the polymer matrix response (limited to about 0.7 V Ag/0.01 M Ag+ + AN, i.e. about 1.0 V SCE) to avoid the film degradation or solute monomer oxidation) [42, Eq. (8)]: d (nm) = bqred (mC/cm2 )fpor ,

b = 100–120

(1)

Here, qred = Qred /S, redox charge density, fpor > 1, a factor reflecting the “film porosity”, i.e. a greater volume per monomer unit in the film due to both real pores and steric restrictions for the compactness of the monomer unit packing which should be significantly lower than in the monomer crystal, because of chemical bonds between the neighboring monomer units. This “extra volume” is partially due to a low density of the polymer itself (compared to the monomer crystal) as well as due to the space occupied by solvent molecules and ions. An equivalent estimate of the film thickness can also be found from the current density of the anodic maximum of the polymer response (ipa ∼ = 0.15 mA/cm2 in Fig. 1). The shape of the voltammetric response of the matrix is close to a rectangle so that the anodic redox charge density for the standard interval in Eq. (1), qred , may be estimated as ipa tscan , tscan being about 6 s. It leads

Fig. 2. Cyclic voltammograms recorded during the electrodeposition of PTc3Py on the large area Pt electrode (S = 0.196 cm2 ). Shown cycles 2, 6, 10, 15, 20, 25, 31, and 35.

to an approximate formula: d (nm) = b ipa (mA/cm2 )fpor ,

b = 600–700

(2)

For the films corresponding to Fig. 1, both formulas lead to the estimation of the thickness as about 100 nm × fpor . One can only speculate on the value of this “porosity factor”, fpor , which should depend on the state of the film (dried or in contact with solution, its oxidation degree, etc.). One of the goals of this paper will be to clarify this point owing to the ellipsometric study. The voltammograms of the film deposition within the identical conditions but for the large-surface Pt electrode, in the ellipsometric cell, are shown in Fig. 2. Similar to Fig. 1, the wave related to the monomer oxidation is clearly visible for electrode potentials higher than E ≈ 1.10 V. The effectiveness of this procedure for polymer formation is revealed by the development of anodic and cathodic current waves, corresponding to the redox conversion of the polypyrrole matrix. In the final (35th) growth cycle the anodic and cathodic current maximums are located at E = 0.82 V and 0.78 V, respectively. Contrary to the CV curves in Fig. 1, one can notice an anodic shift of the peak potentials of the polymer redox processes in the course of the polymerization, likely due to a background current caused by an ohmic resistance at the electrode contact, originating slightly tilted voltammograms. 3.2. In situ ellipsometry in the course of the film deposition Ellipsometric information collected during the electrosynthesis procedure shown in Fig. 2 represents the time evolution of the azimuth angle (Ψ ) and the phase shift (Δ) recorded every 1 s (Fig. 3).

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Fig. 3. Evolution of the ellipsometric parameters, azimuth (a) and phase (b), with time during the potentiodynamic polymerization of Tc3Py, registered simultaneously with the current variation in Fig. 2.

At the beginning of first anodic potential scan, the collected data correspond to the characteristic ellipsometric parameters for the bare platinum surface in this solution, being approximately constant during the first nine seconds of the experiment. When the electrode potential reaches 1.1 V, at the 10th second of the first anodic scan, the observed change in Ψ and Δ indicates the beginning of the deposition. Two important features can be retrieved from the curves representing the evolution of the ellipsometric parameters with time. One concerns the formation and subsequent thickening of a new material at the electrode surface, causing the progressive

modification of Ψ and Δ with time, during the whole experiment. In particular, the initial progression of the azimuthal angle to more positive values (first 600 s) is a sound indication of the formation of a low absorbing and relatively dense material; Ψ curve follows then the sinusoidal-like evolution as predicted by the arctan dependence of the exponential of the film thickness. Another characteristic feature of those curves is the small-amplitude variation of the ellipsometric signals within each potentiodynamic cycle, revealing that the modification of the film optical properties as a function of the electrode potential, namely upon polymer redox conversion, are not very strong.

Fig. 4. Δ vs. Ψ data recorded in the course of the polymer growth at several potentials during the anodic (a) and cathodic (b) scans. Comparison of the data monitored at the same potentials in the forward and backward scans (c).

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Since the above described procedure for the acquisition of the ellipsometric parameters provides data for the same set of potentials within each of the 35 cycles, it is possible to analyze the modification in the optical signals as a function of the cycle number, at each fixed value of potential, i.e., to correlate these two angles along the series of cycles (for each fixed value of potential). Fig. 4 displays Δ versus Ψ for selected electrode potentials along the anodic (Fig. 4a) and cathodic (Fig. 4b) potential scans. At each potential, the sequence of points correspond to the thickening of the deposited film, cycle after cycle. The observed similar paths of Δ versus Ψ advancement for E ≤ 0.8 V versus SCE in the anodic potential sweep (Fig. 4a) indicate that no important structural changes take place until the polymer oxidation peak potential has been reached. At higher potential values, covering both the regions of polymer anodic conversion and monomer oxidation, modifications of the optical properties of the material are noticeable. For the reverse potential scan (Fig. 4b), at potentials where the monomer oxidation still occurs, the data points follow an analogous trajectory revealing no important structural differences in this potential range; in contrast, distinct routes of Δ versus Ψ progression are observed for lower electrode potential domains. Therefore, the optical constants of the polymer in the potential region of polymer redox conversion depend not only on the electrode potential but also on the direction of the potential sweep. This feature is clearly illustrated in Fig. 4c where the data points collected for the same given potentials in the forward and backward scans are contrasted. The observed small difference among the anodic and cathodic peak potentials of the polymer redox conversion does not explain the extent of the hysteresis in the optical properties; it must be attributed to differences in the kinetics of the structural rearrangements where processes other than the purely electrochemical ones must be considered (e.g. solvent incorporation/expulsion). 3.3. Modeling the film structure for all electrode potentials: single- and two-layer description In order to estimate the optical properties and the thickness of the film (at each potential value), the usual procedure

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consisting in the fit of these experimental data to Δ versus Ψ curves generated from a certain model [46–48] was employed. As a first approach, the simplest model of the growth of a single homogeneous layer on the electrode surface was used. The procedure is based on finding (for each fixed value of potential) the complex refractive index (˜n = n − ik, n being the real part of the refractive index and k the extinction coefficient) of the material that produces the Δ versus Ψ theoretical curves for the thickening of the film with a better adjustment to the experimental points. As an illustration, Fig. 5 shows a set of theoretical curves for several values of the complex refractive index and the experimental data, for two extreme values of potential, 0.2 V and 1.4 V. The values of the complex refractive index found for these electrode potentials reveal an increase in the absorption coefficient of the film (directly proportional to the value of k) from the reduced to the oxidized state of the matrix, which is a common feature in electronically conducting polymers [49] due to the generation of interband states upon doping, responsible for the light absorption. Although the curves generated by using the model of the growth of a single homogeneous layer on the electrode surface are able to reproduce globally the experimental data suggesting the formation of a relatively uniform film, the deviation between the experimental and best theoretical curves (up to several degrees) exceeds significantly the measurement precision. It was also observed that approximately the same fitting quality could be achieved with markedly different sets of the model parameters, rendering impossible to extract unambiguously the thicknesses and optical constants of the layer. Thus, for a correct interpretation of the present experimental data a more elaborated model has been explored. As shown in Fig. 6, a satisfactory fitting has been obtained assuming the polymer formed by two distinct homogeneous phases. The inner layer is well described by the formation of a uniform material till the 15th growth cycle at every potential value in both scan directions, but the simulated curve deviates from the experimental points thereafter (Fig. 6a). To fit the data recorded for the subsequent cycles, a second layer must be considered in the model.

Fig. 5. Δ vs. Ψ data points recorded at 0.2 V (a) and 1.4 V (b) and sets of theoretical curves corresponding to the growth of homogeneous films for several different values of the complex refractive index.

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Fig. 6. Δ vs. Ψ data points recorded at 0.2 V (a), 0.7 V (b) in the anodic (open circles) and cathodic potential scan (solid circles) and 1.4 V (c), together with theoretical curves corresponding to the gradual increase of the thickness of a film composed of two homogeneous layers.

3.4. Variation of the optical properties and of the thickness of the layer in the course of the film deposition At E = 0.2 V the complex refractive indices of these layers are equal to n˜ inner = 1.645 − 0.035i and n˜ outer = 1.645 − 0.063i, respectively. An interesting feature is the coincidence of the real parts of the refractive index while the extinction coefficient of the outer layer is noticeably higher. This situation (similar n and different k) is very unusual for a two-layered system since different light absorption properties in the two regions are usually due to a distinct chemical/structural nature or to dissimilar porosities of the probed phases, which should also lead to a change of the real part of the refractive index. Possible interpretations of this result will be discussed in a later section of this manuscript. From the simulated curves it is possible to determine both the complex refractive indices and the thicknesses of the layers, as

well as the total film thickness at every potential value for each growth cycle. On the basis of the above-mentioned treatment, till the 15th cycle the external layer is absent, while for subsequent cycles the parameters of the internal layer remain unchanged (for each potential value). In Fig. 7 the total thickness calculated for E = 0.2 V (cathodic limit of the potential scan) is plotted against the cycle number, revealing a regular increase of the film thickness, 5.5 nm per cycle, during the whole deposition process, both before and after 15th cycle (growth of the internal or external layer, correspondingly). It is worth to note that this regularity has been observed in the treatment based on the two-layer model. This result gives an additional support to the interpretation of the polymer formation as a uniform material whose optical heterogeneity is originated from solute species into the pores of the film. In this context it is worthwhile to mention a similar observation for another film

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Fig. 7. Thickness change with the cycle number. Calculated for E = 0.2 V considering the growth of the film having the complex refractive indices, n˜ inner = 1.645 − 0.035i and n˜ outer = 1.645 − 0.063i.

property, a monotonic increase of the film mass in the course of the polymer formation, as it was observed in electrogravimetric measurements of the growth of the same polymer [45]. The analysis of the dependence of the calculated thickness of the film on the potential in the course of last deposition cycles reveals a superposition of two tendencies: an increase of the thickness due to the deposition of a new material (in the range of the monomer oxidation) and a slight change of the film thickness (about 3–4%) as a function of the its oxidation degree (swelling for the polymer charging and shrinkage for its discharging). The latter tendency is in conformity with our previous observations with EQCM experiments [45]. 3.5. Refractive indices and extinction coefficients of the layers: dependence on the electrode potential The variation of the real and imaginary parts of the refractive index is shown in Fig. 8 for a complete polymerization cycle. The potential dependence of the real part, n (Fig. 8a), identical for both layers, resembles the behavior generally exhibited by other conducting polymers [50–52]; the values of n start dropping at 0.7 V in the anodic scan which means a decrease of the optical density of the material on polymer oxidation. This feature should not be ascribed to an increase of film volume but to the electronic modifications that take place in the polymeric segments (and influx of doping ions) that occur in the course of the polymer oxidation [50,52]. The diminution of the refractive index continues up to the anodic limit of the scan. It reaches a minimum soon after the beginning of the cathodic scan (i.e. still within the range of high anodic potentials, about 1.2 V). Then, it starts increasing in the cathodic scan, to return to the initial n value at E = 0.4 V. The absolute values of n found for PTc3Py are comparable with those published for other electronically conducting polymers [49,53–58] but the magnitude of its variation during a potential cycle is smaller [49–51]. This may be attributed to the fact that the thickness modification of the film during the redox conversion process is small and then the value of the refrac-

Fig. 8. Variations of the film refractive index (a) and extinction coefficients (b) of the inner (䊉) and outer () layers of PTc3Py in the course of a complete potential cycle. Also shown the voltammogram corresponding to the 35th growth cycle.

tive index reflects mainly the electronic structure of the polymer matrix rather than changes in its morphology. Fig. 8b presents the variation of the extinction coefficient of the inner layer of the film for a complete polymerization cycle. A general tendency is an increase of this parameter for more positive potentials, similar to other conducting polymers [49–51]. This effect is due to the generation of mobile electronic charge carriers (called usually “polarons” and “bipolarons”) possessing absorption bands in the visible and/or NIR frequency ranges, owing to their delocalization over several pyrrole units in the polymer chain [59]. The values of k computed for the outer layer are higher than those of the internal layer but reveal the same trend with the electrode potential which is an additional indication that the film can be regarded as structurally similar across the thickness, its external region being more absorbing likely due to the incorporation of oligomers within the polymer pores. 3.6. Discussion: single- versus two-layer structure of the film An unambiguous interpretation of this result requires a further study involving potentiostatic and potentiodynamic preparation with various anodic potential limits, currently under progress. Here, two possible interpretations are discussed, both being based on the assumption that this coincidence of the values of the real part of the refractive index, n, for all electrode

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potentials (despite a noticeable variation of n in the course of cycling, see Fig. 8a) is not occasional, i.e. that both layers are formed by the polymer with identical optical density. The former explanation assumes that after reaching a certain thickness of the film, the outer part of the deposited polymer would not have enough time to be fully reduced in the course of potential scanning and may remain more doped than the inner layer, due to a poorer conductivity of the modified polymer as compared to the parent one, polypyrrole. This interpretation does not contradict to the analysis of the light absorption measurements below since they were performed in an independent experiment with a stepwise potential variation (the signal being registered after a waiting time) in the negative-to-positive direction so that the polymer should remain doped/undoped uniformly throughout the whole film, its optical parameters (including the extinction coefficient, k), being identical to those of the inner layer, in accordance with the result discussed below. However, there are several arguments against this interpretation: • One should expect that the variation (as a function of E, i.e. the polymer oxidation degree) of not only k but also of n for the external layer is different compared to that of the internal layer since these layers would have different oxidation levels in the course of the potential scanning, contrary to observations in Fig. 8a and b. • The oxidation degree of the external layer for the anodic limit of the potential scan (E = 1.4 V) must be equal or lower (due to its slower relaxation) than that of the internal layer. Since the extinction coefficient, k, of the polymer varies as a function of the oxidation level (hypothetically, in the same way for both layers) the values of k at/near the anodic limit cannot be higher than the maximum of k of the inner layer for the same potential range. This conclusion of the model is at variance with experimental data for the extinction coefficients of both layers in Fig. 8b. It allows us to exclude this explanation, at least for the potential range near the anodic limit, while it may play a certain role for low charging levels of the polymer where the electronic conductivity of the film drops drastically, making the discharge of the external layer especially slow. • More generally, the values of k for the external layer should vary between the maximum and minimum values of k for the inner layer. The relation between these curves in Fig. 8b is obviously quite different: the curve for the external layer is shifted up with respect to that for the inner layer within the whole potential range and for both directions of the scan. The latter observation may represent an indication to the search for a reason of this phenomenon: it looks as if there is a factor which increases the light absorption by the external layer in the course of the film deposition (without influencing the refractive property of the film) and which is independent of the potential. The latter excludes its attribution to the polymer film itself so that one has to look for an “external” factor. A tentative hypothesis is to relate this effect to solute (shortchain) oligomers near the external and internal (inside the pores

of the matrix) surfaces of the film. The monomer itself does not absorb the light in this wavelength range (633 nm), its Nsubstituted pyrrole unit absorbing in the near-UV interval. The monomer oxidation leads to generation of various oligomers which are soluble up to a certain chain length (6–10). Their oxidation gives cationic forms with delocalized electronic states which shift the maximum of their absorption band into the visible range. Therefore, one can speculate that some of them may be intensively absorbing the light at 633 nm used in our measurements. Then, they may increase markedly the absorption property of the whole system in the course of the film deposition, despite of their relatively low concentrations, while their effect on the real part of the refractive index is practically not visible. This phenomenon is only noticeable after a certain time from the beginning of the synthesis, due to a gradual increase of the oligomer concentration in the solution. Since these charged oligomers rest in the solution their absorption properties are not affected immediately by the change of the oxidation level of the polymer in the course of the potential scanning, in accordance with the relation of the curves, k(E), for both layers in Fig. 8b. Within the framework of this interpretation, the polymer matrix itself can be regarded as a uniform material, its optical parameters being characterized by those of the inner layer that is free from oligomers, while the properties of the external section of the polymer during the film deposition are perturbed owing to the accumulation of colored solute oligomer species. 3.7. Film deposition at a transparent (ITO) electrode: absorbance measurements A conventional methodology to get information on absorbance properties of conducting polymer films is a direct measurement of its transmission in the visible range. However, this procedure requires the film deposition on an optically transparent electrode and it is well known that the nature of the substrate may affect significantly both the growth and the final properties of the film. Notwithstanding, the data collected by this technique have been used to appraise if the information obtained using optically transparent electrodes is consistent with that on platinum. For this goal, the same quantity – absorbance of the film for the light beam normal to the electrode surface, A – should be calculated on the basis of the extinction coefficients and thicknesses for all layers, found by ellipsometry, e.g. for the two-layer model: A = −log T = 4π(2.3λ)−1 (k1 d1 + k2 d2 )

(3)

where T is the light transmission, λ = 633 nm (light wavelength), k1,2 extinction coefficients and d1,2 thicknesses of internal and external layers, correspondingly. Coefficient 2.3 is related to the decimal logarithm in the definition of A [60]. Films of the polymer, PTc3Py, were deposited at the surface of a thin ITO layer at glass with the use of the CV procedure with the same parameters as for ellipsometric experiments (Fig. 9). The number of scans (40 in Fig. 9) was adjusted to reach the same

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Fig. 9. CV deposition at the surface of an ITO/glass electrode (cycles from 10th to 40th). S = 1.8 cm2 .

current density of the polymer matrix response, 0.15 mA/cm2 , i.e. a similar thickness. After polymerization the film-coated electrode was transferred into the spectroelectrochemical cell with the background solution, AN + 0.1 M TBAPF6 . The electrode was subject to a series of potential steps with 100 mV amplitude, the values of these potentials being identical to those for which the ellipsometric data were obtained, but with an upper limitation to 1.1 V SCE, to avoid the film degradation (Fig. 10). In the middle of the time interval between the subsequent steps the spectrum was registered. 3.8. Comparison of absorbance data from direct measurements and calculated from ellipsometric parameters The values of absorbance, A, at 633 nm, measured with the potential-step procedure (Fig. 10) are given in Fig. 11 as a function of potential. Then, Eq. (3) was used to calculate the theoretical curve with the use of the values for extinction coefficients and thicknesses of both layers found from the ellipsometric data. Note that it implies an implicit consideration of

Fig. 11. Absorbance (at 633 nm) of the film deposited on the ITO surface () as a function of potential. Comparison with theoretical curves calculated on the basis of the ellipsometric data, with the use of Eq. (3) () or Eq. (4) (䊉).

this optical parameter for both layers as a characteristic of the polymer itself (see a discussion above). Both plots in Fig. 11 reveal the same tendency as functions of potential: higher absorbance values for more positive potentials, in accordance with expectations for higher charging levels of the polymer matrix. However, the absolute values of A for the direct measurement are noticeably lower within the whole potential range. This deviation may originate from different conditions of these measurements. The absorbance spectra were registered in the background solution while the ellipsometric study was realized in the course of the film deposition. As it has been pointed out above, the difference in the extinction coefficients of the internal and external layers is expectedly related to accumulation of solute colored oligomeric species inside the outer layer (or near its surface) in the course of the film deposition. In the absorbance measurement such solute oligomers are absent. Therefore, a proper curve for the comparison with absorbance data should be constructed with the use of the ellipsometric data “cleaned” from the effect of solute oligomers, i.e. assuming the whole film to possess the properties of the internal layer: A = 4π(2.3λ)−1 k1 (d1 + d2 )

Fig. 10. Series of potential steps with 0.1 V amplitude from 0.3 V till 1.1 V. After each step the potential was kept constant for 16 s. The spectra were measured at each potential, 8 s after the step.

1203

(4)

The comparison of the curve obtained in this manner with direct spectral measurements (Fig. 11) shows a much closer proximity. The agreement is quite satisfactory for the potential interval between 0.6 V and 1.1 V for the anodic branch of the potential scan in ellipsometry. For lower potentials the spectral data are located below the ellipsometric points. The reason of this deviation is probably related to “memory effects” [59–69]. The spectra were measured after holding the film at each potential for 8 s which is accompanied by a gradual loss of the electronic charge of the matrix (which is left “trapped” in the film in the course of the cyclic voltammetry used in ellipsometry) and by a diminution of the film absorbance at the potentials where the matrix is discharged almost completely.

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The above reasoning indicates also that the above comparison should be made with the anodic branch of the ellipsometric curve. Its cathodic branch is markedly perturbed by the passage through high anodic potentials in the course of the polymerization cycle (up to 1.4 V) leading to an extra increase of the film absorbance, as it has been discussed above. 4. Conclusions Our comparison of films having the same thickness but deposited potentiodynamically at the surfaces of three different electrodes (small- and large-size Pt disks and ITO-glass plate) and studied by electrochemical, ellipsometric and UV–vis absorption techniques has resulted in a conclusion that the polymerization of these films is realized by regular deposition of a polymer layer of the same thickness (per cycle) and with the same optical density within the whole procedure. It means that the deposited solid material is uniform in the normal direction to the electrode surface after the end of the polymerization process. However, during the film deposition this system demonstrates a difference of its absorption properties between its internal and outer regions: starting from a certain thickness the film looks in ellipsometric experiments as being composed of two layers having identical refractive indices but different extinction coefficients. A higher light absorption in the outer region of the film during the deposition has been attributed to an effect of solute oligomers which are accumulated in pores of this layer or/and near the external surface of the film. Acknowledgements We express our gratitude to Evelyne Pousson for technical assistance in electrochemical experiments. This study has been performed owing to financial support of FCT (project POCTI/QUI/46019/2002) and of the bilateral Portugal-France collaborative programs (GRICES-EGIDE and CRUP-CPU). References [1] A. Malinauskas, Synth. Met. 107 (1999) 75. [2] G. Inzelt, M. Pineri, J.W. Schultze, M.A. Vorotyntsev, Electrochim. Acta 45 (2000) 2403. [3] M.M. Alam, J. Wang, H.-R. Tseng, PMSE Preprints 92 (2005) 662. [4] S. Ivanov, V. Tsakova, V.M. Mirsky, Electrochem. Commun. 8 (2006) 643. [5] P.R. Bidez, S. Li, A.D. Macdiarmid, E.C. Venancio, Y. Wei, P.I. Lelkes, J. Biomater. Sci. Polym. Ed. 17 (2006) 199. [6] K. Ramanathan, M.A. Bangar, M. Yun, W. Chen, N.V. Myung, A. Mulchandan, J. Am. Chem. Soc. 127 (2005) 496. [7] N.-H. Kwon, M.A. Rahman, M.-S. Won, Y.-B. Shim, Anal. Chem. 78 (2006) 52. [8] S. Panero, P. Prosperi, F. Bonino, B. Scrosati, A. Corradini, M. Mastragostino, Electrochim. Acta 32 (1987) 1007. [9] B. Scrosati, Mater. Sci. Eng. B 12 (1992) 369. [10] D. Delabouglise, Synth. Met. 51 (1992) 321. [11] A.P. Chattaraj, I.N. Basumallick, J. Power Sources 45 (1993) 237. [12] K. Gurunathan, A. Vadivel Murugan, R. Marimuthu, U.P. Mulik, D.P. Amalnerkar, Mater. Chem. Phys. 61 (1999) 173. [13] A. Lisowska-Oleksiak, K. Kazubowska, A. Kupniewska, J. Electroanal. Chem. 501 (2001) 54.

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