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Photoelectrochemical and in situ atomic force microscopy studies of films derived from o-methoxyaniline solution on gallium arsenide (100) photoelectrode
Thin Solid Films 424 (2003) 191–200
Photoelectrochemical and in situ atomic force microscopy studies of films derived from o-methoxyaniline solution on gallium arsenide (100) photoelectrode ´ Emil Wierzbinski, Marek Szklarczyk* Laboratory of Electrochemistry, Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093, Warsaw, Poland Received 14 March 2002; received in revised form 15 November 2002; accepted 24 November 2002
Abstract Organic films derived from o-methoxyaniline water solution were deposited on GaAs electrodes in an adsorption process. The photoelectrochemical behavior of the modified electrodes was examined by potentiodynamic and potentiostatic methods. The film morphology dependencies on the monomer concentration in the bulk of solution, electrode potential and oxidation time were monitored by in situ atomic force microscopy. The monitored morphology changes were correlated with the changes in the photocurrent. The differences in the onset potentials and photocurrent magnitude were explained in terms of the formation of a continuous organic film and on the basis of the p–n junction creation at the GaAsyorganic film interface. It is suggested that the organic film mediates transfer of the photocharge to the solution. Based on morphological changes in the deposited film and the spectrophotometric data, the formation of the leucoemeraldine, a reduced form of poly (o-methoxyaniline) is proposed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: o-Methoxyaniline; Polymers; GaAs; In situ atomic force microscopy
1. Introduction Conjugated polymers have been the subject of intense interest in surface science w1x. In particular, more than a century-old polyaniline (PANI) w2–4x and its derivatives have attracted significant attention thanks to straightforward methods of their preparation (e.g. w5– 9x), good stability of the conductive form in aqueous solution w10,11x and multiple potential applications w12,13x. Considering the large number of papers devoted to research on deposition of conducting polymers on metals and on electrochemistry of such deposited materials, there have been relatively less studies on deposition of polymers on semiconductors w14–19x. PANI can be deposited onto semiconductor surface either electrochemically in dark deposition process performed above 0.4 V or in photoreaction in a potential range 0.0–0.4 V w16x. One purpose of PANI film deposition on the surface of semiconductor photoanodes was to protect *Corresponding author. Tel.: q48-22-822-0211; fax: q48-22-8225996. E-mail address: [email protected] (M. Szklarczyk).
them against photocorrosion w15,16,20x. Enhanced photocurrent stability was reported in the presence of PANI films on n-type semiconductor electrodes, including Si, CdSe, GaP and GaAs. This was explained by a rapid removal of a photogenerated charge from the surface before it was allowed to react with the electrode material. The charge transfer could be mediated by a polymeric film w16x. Photoelectrochemical studies of the polymeric films deposited onto non-semiconductive electrodes (e.g. platinum, gold, indium-tin oxide electrodes) have resulted in the development of the new electrochemistry field, i.e. photoelectrochemistry of polymeric films. The photocurrent for PANI films deposited on metals has been reported w21–26x. Similarly, the photocurrent for polymers obtained from aniline derivatives solution was also observed w27–29x. It was shown that the reduced form of polymeric film generates cathodic photocurrent and no evidence for a photocurrent response while film was in the emeraldine form in the anodic potential range was found w23x. However, a photoresponse of PANI films over a very wide range of potentials has also been
0040-6090/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 1 1 2 2 - 7
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reported in other work w22x. In general, all types of the photoresponse are observed for PANI or its derivatives. P-type or n-type response of the polymer film sample depends on the way of its preparation, polymer oxidation state and the type of the solution w21–29x. In this paper we report the results of the influence of organic films deposited from an acidic aqueous solution of o-methoxyaniline on the photoactivity of an n-GaAs photoelectrode. The study was carried out in the potential range from y0.25 to 0.3 V vs. normal hydrogen electrode (NHE). The application of such potential range let us study the formation of the organic deposit onto the bare n-GaAs surface (potentials from y0.25 to f0 V) and on the oxidized surface (potentials from 0 to 0.3 V). Furthermore, it allows to assume that the formed film will be in the reduced form and therefore will exhibit the p-type semiconductive properties w23x. Obtaining the p-type semiconductive film could enable the formation of a p–n junction between n-GaAs sample and an organic film. On the other hand, the applied potential range makes electrochemical determination of the deposited film nature difficult, because the well known and well defined current peaks attributed to aniline polymerization reaction on metals are observed above 0.45 V vs. NHE (e.g. Ref. w30x). The deposition of polymers on semiconductive materials differs from that observed on metals. On the whole, in water solutions, the redox current peaks were not observed even up to q0.8 V vs. NHE w31x in dependence on the polymer (PANI, polypyrrole, polystyrene) and semiconductor type (n-GaAs, n-Si, n-CdSe) w16,31,32x. Contrary to water solution, in organic electrolytes current peaks can be observed at the potential as low as 0.1 V vs. NHE w33x in dependence on the type of the polymer (polypyrrole, poly-3-methyl-tiophene) and semiconductive materials (n-GaAs, n-Si) w33–35x. Such a behavior could be explained firstly: by the cathodic potential range applied in the experiments carried out in water solution, and secondly: by the over imposition of the current due to polymerization reaction by higher current due to oxygen evolution. To prove the presence and to characterize the properties of the film we employed UV–Vis spectrophotometry and contact atomic force microscopy (AFM) techniques. The first AFM image of PANI in water was presented as early as in 1989 w36x. The newer AFM or STM (scanning tunneling microscopy) studies on the PANI or poly (o-methoxyaniline), POMA, morphology were carried out mostly as ex situ experiments w37–40x. In this paper, in situ AFM studies of the deposit morphology as a function of the electrode potential, time and monomer concentration are presented as part of a research project aimed at establishing the impact of polymeric film on the ability of semiconductor electrodes to resist photocorrosion.
2. Experimental details 2.1. Electrodes The working electrodes were made of a single GaAs (100) crystal supplied by the Institute of Electronic Materials Technology, Warsaw, Poland. A Te doped n-type material with a resistivity of 0.012 V cm (1.65=1017 cmy3) and a Zn doped p-type material with resistivity of 0.25 V cm (1.25=1017 cmy3) were used in photoelectrochemical and AFM microscopic studies, respectively. It was assumed that surface activity was the same in both cases, that is, irrespective of whether electrons were created by light in an electron–hole pair phenomenon (n-type material) or in thermal activation from donor levels (p-type material). The working surface of the crystals were polished with alumina, down to 1.0 mm, etched in the mixture of H2SO4, H2O2 and H2O (5:1:1), chemically polished in NaClO solution and finally chemically etched in ammonia solution. An ohmic contact was prepared by either the vacuum evaporation of indium or by the deposition of gallium– indium alloy. The working area was circular in shape, approximately 4 mm in diameter. Counter electrodes were made of PtyPt gauze or wire. Electrode potential was measured against an AgyAgCl electrode and then expressed in the hydrogen scale. 2.2. Solutions All solutions were prepared from Millipore-Q water. The supporting electrolyte, 0.5 M H2SO4, was prepared from a reagent grade concentrated sulphuric acid and stored under argon atmosphere in a glass flask. The o-methoxyaniline monomer was obtained from Merck and was distilled prior to use. The transfer of the supporting electrolyte form storage flask to the photoelectrochemical cell was carried under Ar atmosphere. All experiments were carried out at ambient temperature, 22"2 8C. 2.3. Equipment and experimental procedures A three-electrode cell with a quartz window was used for photoelectrochemical studies. The photocurrent– voltage behavior was monitored with programmed potentiostat (KSP, Poland). The light source was a 1 kW halogen lamp (LH-21, Polam, Poland). A water filter was placed between the cell and the light source to filter out the long-wavelength part of radiation and to avoid excessive heating of the solution. The presence of o-methoxyaniline in solution was found not to affect the halogen light intensity. The absorption bands of o-methoxyaniline were detected in the UV range, i.e. below 300 nm, which is outside of the halogen lamp radiation range. The absorption of light by o-methoxy-
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aniline solution of different concentration was measured by using UV–Vis spectrophotometric technique (Double beam Lambda 12, Perkin-Elmer spectrophotometer). A spectrophotometric technique was used to monitor the presence as well as the chemical state of organic deposits. For this purpose a reflection technique was used. The spectra were recorded at 308 angle. As the background, the spectrum of n-GaAs was used. A Nanoscope III AFM microscope, a commercial AFM electrochemical cell and a potentiostat (Digital Instruments, USA) were used in imaging the surface of studied samples. The silicon nitrides cantilevers with a spring constant 0.58 N my1 (manufacturer data) were used. Calibration of the microscope was achieved by imaging calibration gratings supplied by the manufacturer. The images presented in this work are the height type images, but in some cases the section diagrams are also shown for better characterization of the morphology details. AFM images were examined for artifacts and reproducibility was made in the usual way, i.e. by changing a semiconductor sample or the AFM cantilever, and by either moving (during the experiment) the sample in the X or Y direction or by varying the scanning angle and frequency. In order to determine the influence of organic layer on the surface morphology, the bare GaAs electrode was first monitored in air and then in the supporting electrolyte solution under the same conditions as later used in the experiments with the o-methoxyaniline. 3. Results 3.1. Photoelectrochemical studies The photocurrent–potential (iph –E) dependencies for n-GaAs electrode in the absence (line 1) and in the presence of o-methoxyaniline (lines 2–4) in the bulk of the supporting electrolyte solution are shown in Fig. 1. In these experiments o-methoxyaniline aliquot was added to the photoelectrochemical cell at the constant electrode potential of y0.21 V and iph –E plots were registered after a 20 min adsorption. The plots are dependent on the concentration of o-methoxyaniline in the solution. Relatively to bare gallium arsenide (Fig. 1, line 1) the iph –E dependence recorded with the lowest used concentration of o-methoxyaniline (10y8 M, Fig. 1, line 2), is positively shifted by approximately 70 mV, at a half photocurrent point. The iph –E dependence shifts back in the negative direction at higher concentration of o-methoxyaniline (Fig. 1, line 3) and finally at the highest concentration, 10y2 M (Fig. 1, line 4) it becomes more cathodic than that recorded with a bare gallium arsenide (Fig. 1, line 1). Independently of these changes, for any concentration of o-methoxyaniline in the solution, the onset potential is more negative than that observed in the absence of monomer in the photo-
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Fig. 1. Cyclic voltammograms of illuminated n-GaAs electrode in 0.5 M H2SO4 solution in the absence of monomer in the bulk of solution (line 1) and after addition of o-methoxyaniline to the cell to the concentration 10y8 M, 3=10y5 M and 10y2 M, lines 2, 3 and 4 respectively. o-Methoxyaniline was added at the electrode potential y0.21 V and sweeping was started after 20 min of adsorption. Sweep rate: 50 mV sy1.
electrochemical cell (Fig. 1). Furthermore, the observed saturation photocurrent is higher in the presence of o-methoxyaniline in the solution than that in the o-methoxyaniline-free solution (Fig. 1). To discriminate between the effect that bulk monomer and the organic film have on the photocurrent magnitude, the cell was subjected to a so-called washing procedure. In this procedure, 20 min after the addition of the o-methoxyaniline to the photoelectrochemical cell the solution containing o-methoxyaniline was replaced by a fresh supporting electrolyte. This procedure was repeated six times in order to remove organics completely from the photoelectrochemical cell. No influence of the rinse on the photocurrent magnitude was observed, which indicates that the observed differences in photocurrent (Fig. 1) are caused by the presence of organic film on the GaAs surface. The photocurrent–time (iph –t) dependencies for n-GaAs photoelectrode at the constant potential 0.23 V, are shown in Fig. 2. Line 1 in this figure corresponds to o-methoxyaniline-free solution while line 2 represents the system response in the presence of the organic film obtained from the 3=10y5 M solution of o-methoxyaniline. The o-methoxyaniline was added to the photoelectrochemical cell at the potential y0.21 V. After 20 min of adsorption the washing procedure was applied in order to remove monomer from the bulk of solution, and then the potential was changed to 0.23 V. As expected, the photocurrent observed with bare gallium arsenide surface decreased with time. At the presence of an organic film on the electrode surface the photocurrent first slightly increased during first 20 min (cf. insert in Fig. 2) and then became stable for approximately 6.5 h. Subsequently, it decreased slowly. This
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Fig. 2. The dependence of iph on time for bare GaAs electrode, line 1, and for electrode covered with the organic layer deposited from 3=10y5 M o-methoxyanilineq0.5 M H2SO4 solution, line 2, at the electrode potential 0.23 V. o-Methoxyaniline was added at the electrode potential y0.21 V and this potential was changed to 0.23 V after 20 min of adsorption and after washing procedure.
behavior was independent of the presence of o-methoxyaniline monomer in the solution, which was checked in the experiments without washing procedure application. The current oscillation in Fig. 2 was due to the formation and later desorption of oxygen bubbles, easily observable during experiments. The reproducibility of described iph –E and iph –t dependencies viewing their shape and position on the potential or time scale was very good. The only observable variation worth mentioning was tied with the maximum photocurrent magnitude, which varied between GaAs samples "15%. 3.2. In situ AFM studies A typical image taken for the electrode placed in the supporting electrolyte is shown in Fig. 3. The surface consists of distinguishable solid bumps of different length, ranging from approximately 100 to 400 nm. The average height of these bumps is approximately 4 nm. The bumps are ordered in one direction (Fig. 3). The presented image was found independent of time and potential in the studied potential range. The AFM data for the organic layer deposited from 3=10y5 M monomer solution at a potential of y0.21 V are presented in Fig. 4a–c. The presented image was registered within 10 min after pure supporting electrolyte solution (0.5 M H2SO4) had been replaced with a 3=10y5 M monomer-containing solution in the AFM electrochemical cell. The deposit consists of several trapezoidal bulky blocks, which are better defined in three dimensional image presented in Fig. 4c. Another layer of blocks can be observed below the topmost layer of blocks. The continuity of the deposited film cannot be established on the basis of the height mode images (Fig. 4a and c) but it is noticeable in the section diagram
presented in Fig. 4b. The section analysis presented in Fig. 4b was made along the black line shown in Fig. 4a. Two arrows show two characteristic features of the deposited layer. The left arrow shows that under the topmost layer of slabs there are other underneath (bumps on the section diagram). The right arrow points that slabs are overlaying one another building a layer that looks like a fish scale (Fig. 4c). The average dimensions of slabs are approximately 180=300 nm2. The observed surface angles in trapezoids are 838"48 and 1018"38. An effect on the monomer concentration on the organic deposit morphology is given in Fig. 5. The presented images were taken at the y0.21 V over time as that used in the experiments shown in Fig. 4. At a monomer concentration of 3=10y5 M, the organic deposit is composed of the rounded blocks (Fig. 5a). This morphology fades away with an increase in the monomer concentration and becomes messy and disordered at the concentration 10y2 M (Fig. 5b). The changes in the organic film morphology at a more positive potential of 0.12 V, are shown in Fig. 6a– d. At this potential, the massive blocks and grains between the scales start to disappear (Fig. 6a and c, height view and section analysis view, respectively). These changes are very slow. The shown pictures were registered after 50 min since the potential had been changed from y0.21 V (Figs. 4 and 5a) to 0.12 V (Fig. 6a and c). When the potential is lowered back to a negative value, y0.25 V, the organic film structure returns to the previous form within 10 min (Fig. 6b and d, height view and section analysis, respectively). The section analysis presented in Fig. 6d was intentionally made through the visible deep cavity in the right lower
Fig. 3. In situ AFM image (top view) of GaAs bare electrode surface taken in 0.5 M H2SO4 solution at the electrode potential y0.2 V. Scan size 0.5=0.5 mm2. The bare placed on the right side of the image shows the scale in the vertical to the surface direction. The roughness factor, root mean square (RMS) value for the presented surface is 0.321 nm.
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resemble small hills (Fig. 7a). The next 15 min of oxidation brings dramatic changes in the film morphology, which takes the shape of long chains with clearly distinguishable rounded substructures (Fig. 7b). This change is irreversible, i.e. returning to negative potential has no effect on the organic film morphology. 3.3. Spectrophotometric studies The spectrophotometric results are shown in Fig. 8. The shown absorbance dependence was registered for the deposit formed at the y0.21 V potential. In the studied potential range the features of spectra were independent of the electrode potential. The presented spectra display single absorption peak at approximately 350 nm, shoulders in the approximately 400–500 nm range, and broad maximum at approximately 730 nm. The similarity of the absorbance dependencies for the
Fig. 4. In situ AFM data taken in 3=10y5 M o-methoxyanilineq0.5 M H2SO4 solution at the electrode potential y0.21 V. The image was registered within 10 min from the moment when the procedure of replacing basic electrolyte solution (0.5 M H2SO4) for 3=10y5 M monomer solution in the AFM electrochemical cell started. (a) topographic image. (b) section analysis along the line shown in (a). (c) three dimensional representation of the surface morphology. Scan size 4=4 mm2 and 1.2=1.2 mm2 for (a and c), respectively.
corner in order to show that the organic deposit slabs are presented there (bumps on the diagram). An influence of the oxidation time on the organic film morphology is shown in Fig. 7 for the deposit formed in the 10y2 M monomer solution. After 5 min of oxidation at 0.2 V a disordered structure presented in Fig. 5c disappears and the polymer surface starts to
Fig. 5. In situ AFM images (the height mode) showing morphology of organic film deposited at the electrode potential y0.21 V. Images presented in (a and b) were taken for electrode immersed in 3=10y5 M or 10y2 M o-methoxyanilineq0.5 M H2SO4 solution, respectively. The presented image was registered within 10 min from the moment when the procedure of replacing basic electrolyte solution (0.5 M H2SO4) for monomer solution in the AFM electrochemical cell started. Scan size 2=2 mm2.
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Fig. 6. In situ AFM data showing morphology of organic film after 50 min of oxidation at electrode potential 0.12 V, (a and c). Data presented in (b and d) show the film morphology after changing the electrode potential to y0.25 V for 10 min. (a and b) were taken in the height mode. (c and d) section analysis along the lines shown in (a and b). Organic film prior to oxidation was formed at y0.21 V for 10 min from 3=10y5 M o-methoxyanilineq0.5 M H2SO4 solution. Scan size 2=2 mm2.
films derived from aniline and o-methoxyaniline has already been reported w41x. On this basis, the spectra presented here for deposits derived from o-methoxyaniline can be ascribed to leucoemeraldine form of phenyly phenyl tetramer or of poly (o-methoxyaniline) film, and the maximum at 340 nm can be ascribed to p™p* transition observed for benzoid units w42–44x. The classification of the broad maximum at approximately 730 nm is rather intricate, especially, because its intensity was varied between the experiments. A comparison with other results indicates that the peak may be due to absorbance of either free electronic states w42x, or salt form of leucoemeraldine w43x. Some electronic transitions in the GaAs sample between the states induced by organic deposit may possibly be considered as the source of the broad maximum as well. 4. Discussion The observation described thus far can be summarized as follows: (i) an addition of the o-methoxyaniline monomer solution to the supporting electrolyte solution strongly influences the observed photocurrent (Figs. 1 and 2) and (ii) addition of the o-methoxyaniline monomer to the bulk of solution strongly influences observed surface morphology (Figs. 3–7).
These effects cannot result from the changes of the GaAs surface itself with time, because the photocurrent on the bare electrode should decrease rather than increase or remain stable (cf. w16x and also Fig. 2, line 1). Therefore, the observed photocurrent changes must be attributed to either the effect of the deposited organic film (the adsorbed layer of monomers or oligomers, or the polymeric film), or to the electrode reaction of the o-methoxyaniline monomer from the bulk of the solution. The latter explanation appears unlikely in view of the fact that no change in the photocurrent was observed after washing procedure was being applied (cf. Section 3.1). Thus, it can be concluded that the observed current changes are likely due to the altering of the surface phenomenon by deposited organic film, ultimately, due to reaction of the film, rather than the monomer, with the electrode surface. The differences in surface morphology observed by means of AFM microscope (Figs. 4–7) presented in this work can be assigned to the organic film morphology changes. It is because the presented image for bare GaAs surface was stable in time and with the change of the electrode potential (Fig. 3) and because they were observed only after replacing the solution o-methoxyaniline-free for the solution containing this monomer.
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monomers are present at the electrode surface. These forms are usually identified with the leucoemeraldine form of phenylyphenyl tetramer or of polymeric film w42–44x. The presented in situ AFM images show significant changes of the organic deposit morphology when electrode potential is changed from the adsorption one, y0.21 V (Fig. 4) to a more positive potential, 0.12 or 0.2 V, Figs. 6 and 7, respectively. The potential shift changes the conditions at the electrode surface from the oxygen free to the oxygen presence at the electrode surface. The evolving oxygen would moderate the process of the polymeric film formation (Figs. 6 and 7) from the oligomeric one (Fig. 4), which would explain observable changes in the morphology of the organic deposit. Greater morphological changes with the increase in the anodic potential (Figs. 6 and 7, respectively) well correlate with the increase in the oxygen concentration at electrode surface, which comes from the photocurrent increase with the electrode potential (Fig. 1). Some additional experimental evidence of the polymer formation under studied condition is a small increase of the photocurrent during early 20 min under potentiostatic condition (Fig. 2, line 2), when the electrode potential was changed from the adsorption to the oxygen evolution potential. The time fairly agree with the slow reconstruction of the deposited film (cf. AFM images presented in Fig. 4a and c with those presented in Fig. 7, respectively). So, this current increase can be ascribed to the polymer formation. The slow formation of polymer observed in this work can result from the relatively low concentration of monomer, the high con-
Fig. 7. In situ AFM images (the height mode) showing organic film morphology after 5 min of oxidation at electrode potential 0.2 V, (a), and after next 15 min of oxidation, (b). Organic film was formed at y0.21 V for 10 min from 10y2 M o-methoxyanilineq0.5 M H2SO4 solution. Scan size 1.2=1.2 mm2 and 1.5=1.5 mm2 for (a and b), respectively.
An attempt of the identification of the organic deposit chemical state may be made on the basis of the spectrophotometric and AFM results. The obtained organic film may consist of adsorbed monomers, or adsorbed oligomers, or polymer chains. In the potential range (y0.2 to q0.3 V) studied, the formation of the polymer cannot be expected in agreement with the electrochemical mechanism. The known proof of the electrochemical polymerization process is the occurrence of the well defined peaks, which are observed above 0.45 V (e.g. Ref. w30x). In our studies we did not observe these peaks because the electrochemical potential was too low. However, the detected spectrophotometric peak at 340 nm (Fig. 8) indicates that the forms higher than the
Fig. 8. UV–Vis spectrum of organic film deposited from 10y2 M omethoxyanilineq0.5 M H2SO4 solution. Adsorption time was 20 min, and potential of adsorption was y0.21 V.
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centration of supporting electrolyte (0.5 M) w45x and the low oxidation potential. The polymerization process discussed here for which electrochemical peaks are not observable can be identified with the electroless (no charge transfer through the electrodeysolution interface) polymerization mechanism, which has already been described w40x. According to this mechanism, an electrode surface plays a role of polymerization catalysts in the presence of oxygen on the surface. In those studies w40x the highly oxidized polymer was obtained by immersing platinum or palladium foil in the oxygen saturated acidic aniline solution. According to the described reaction mechanism, aniline monomer was first protonated in the acidic medium, and then catalytically oxidized to anilinium radical cation on the metal surface. Further oxidation leads to the formation of the anilinium radical by transferring the proton from the nitrogen group to the water or aniline molecules from the solution. The formed anilinium radicals and anilinium radical cations couple to form dimers or higher oligomers. Further oxidation led to the formation of polymer of different oxidation state, even as high as 75%, i.e. the nigraniline was formed w40x. The oxidation state of the polymer in the cited studies was monomer concentration, acid concentration and oxygen concentration dependent. In the studies presented in this paper the observed oxidation state of the obtained deposit is a leucoemeraldine form, i.e. the reduced POMA form (cf. Fig. 8). This difference can be explained firstly by better catalytic properties of platinum surface compared with GaAs surface, and secondly by different electrochemical condition applied in Ref. w40x in comparison with these applied in this work. In the studies reported in Ref. w40x open circuit conditions were used. Under such circumstances, the electrochemical potential at the platinumyacidic solution interface is equal to approximately 0.8 V, while in these studies electrochemical potential did not exceed 0.3 V, so the oxidative strength of the interface used in this work was much lower. Taking into account the spectrophotometric results (Fig. 8) and the cited electroless polymerization process w40x the formation of leucoemeraldine form of tetramer during adsorption can be suggested. Next, when electrode potential has been shifted to a potential where oxygen starts to evolve, above 0.0 V, tetramers start to polymerize, and leucoemeraldine form of POMA is created at the electrode surface. This last process can be observed by the film reconstruction monitored by AFM imaging (Figs. 6 and 7 vs. Fig. 4). The changes in the observed photocurrent after deposition of the organic film onto n-GaAs surface can be summarized as follows: (i) the onset potential in the presence of organics on the surface is negatively shifted; (ii) the saturation photocurrent increases in the presence of an organic film on the surface; (iii) at a given
potential, the photocurrent increases with films obtained in the solution of the higher monomer concentration, and finally becomes higher than the photocurrent for the bare gallium arsenide in the entire studied potential range. In order to explain these observations, it is necessary to take into account all processes first, which could potentially lead to the photocurrent, i.e. oxygen evolution, surface oxidation and possible oxidation of the surface organic film itself. If present, the latter effect would result in an increase in the currents by approximately 0.1 mA (e.g. Ref. w23x) that is significantly less than the increase reported in this work (Figs. 1 and 2). Surface oxidation, i.e. photodegradation of the GaAs photoelectrode is known to be responsible for the photocurrent decrease w16,20,46x. Such a behavior can be seen in Fig. 2, line 1. The observed photocurrent stabilization or its increase (Fig. 1, lines 2–4, Fig. 2, line 2) suggests that the degradation process is either stopped or at least inhibited in the presence of organic deposit on the n-GaAs surface. Thus, the observed photocurrent is likely to be caused by the oxygen evolution reaction. It can be attributed to the photocharge transfer to the solution mediated by organic film. Such a process is expected to be faster than the reaction of photocharge with the semiconductor lattice w16x. This means that the activation energy of electron transfer through the solutionyorganic film interface and organic filmysemiconductor interface must be lower than at the semiconductorysolution interface. Another explanation for the inhibition of the photocorrosion process at nGaAs surface could be in terms of the compact organic film (Fig. 4) forming a mechanical shield, protecting the semiconductor surface against oxidation due to the separation of the semiconductor surface from the solution. It is also possible that both phenomena could occur at the same time. The slow photocurrent decrease observed in the potentiostatic measurement after approximately 6.5 h (Fig. 2, line 2) is probably tied with a certain penetration of the polymeric film by oxygen and slow oxidation of the GaAs surface. The observed increase in the saturation current and the cathodic shift of the onset potential (Fig. 1) in the presence of the deposited layer on the electrode surface indicates a rise in the photocharge carrier concentration available for oxygen evolution. An increased availability of photocharge carrier can be explained on the basis of semiconductive properties of the obtained organic layer. Photoconductive p-type behavior of PANI had been previously detected and reported w21–23,25,26,28x. Assuming similar semiconductive properties for POMA w22x, the formation of p–n junction between the polymeric film and GaAs surface can be proposed. The creation of p–n junction would greatly increase the electrical field across the GaAsyPOMA interface, which explains faster transfer of photogenerated charges. Consequently, the concentration of these charges at the
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POMAysolution interface would increase and this phenomenon could be responsible for the photocurrent increase at the GaAsyPOMA interface. The shift of iph –E dependencies with the addition of monomer to the solution (Fig. 1) is likely to be caused by the mediation of the photocharge transfer to the solution by the organic film (see above). However, the magnitude of these shifts, which varies from 0.08 V in the positive direction for the lowest concentration to 0.04 V in the negative direction for the highest concentration (Fig. 1), can be influenced by other factors, too. It has to be emphasized that the observed displacement for the deposit formed in the solution with the lowest concentration of the monomer (10y8 M) is positive, as expected for the surface poisoned by adsorbed organics. At a higher monomer concentration the shift changes direction to negative, which is related to an increase in the surface activity. At the lowest monomer concentration the electrode surface is covered with deposit, which is in the form of islets that do not sufficiently protect the semiconductor against photodegradation. The surface becomes oxidized and the photocurrent decreases. With the increase in the monomer concentration, the formed deposit layer becomes continuous and its thickness increases (Fig. 5b) resulting in the photocurrent increase because of better surface protection and the increase thickening of the polymeric semiconductive space charge region. With all these changes the region of the GaAsyPOMA interface, where the p–n junction is formed, becomes bigger, leading to an enhancement in the photocurrent. The different morphology of the films obtained from the monomer solution of different concentration (Fig. 5a and b) is probably the reason for obtaining different structures as the result of the film reconstruction (Fig. 6aFig. 7a and b). The chain like structure shown in Fig. 7b provides an additional argument for the thesis that the films formed under low positive potential are polymeric. The morphology of the film presented in Fig. 7b agrees well with the fractal model of PANI film growth w47x. According to this model, the branching density between the main linear chains for polymers obtained from ortho-derivatives of PANI should be low, just as seen in the AFM image presented (Fig. 7b). 5. Conclusion The films deposited on gallium arsenide surface from aqueous solution of o-methoxyaniline help to protect the electrode surface against photocorrosion and lead to an enhanced photoactivity. The obtained GaAsyorganic film photoelectrode shows good stability for approximately 6.5 h. The observed phenomena are explained by the formation of the p–n junction at the GaAsyorganic film interface. The results show that a photocurrent depends on the morphology of the deposited film. The effect can
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be explained by the polymeric mediation of the photocharge transfer from GaAs electrode to the solution and by the increase in the thickness of the space charge region in the organic layer with the increase in the monomer concentration in the solution. On the basis of the evident morphology changes and spectrophotometric results, the formation of poly (o-methoxyaniline) is proposed. The observed structure of the film is in good agreement with the fractal model of the PANI structure. Acknowledgments The authors gratefully acknowledge the support of this work by the Department of Chemistry, Warsaw University under the Grant Nos. BW-1453y1y1999 and BW-1483y1y2000. References w1x J.F. Rubinson, H.B. Mark Jr., Conducting polymer films as electrodes, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, Inc, New York, 1999. w2x H. Letherby, J. Chem. Soc. 15 (1862) 161. w3x A.G. Green, A.E. Woodhead, J. Chem. Soc. 97 (1910) 2388. w4x A.G. Green, A.E. Woodhead, J. Chem. Soc. 101 (1912) 1117. w5x D.M. Mohilner, R.N. Adams, W.J. Argersinger, J. Am. Chem. Soc. 84 (1962) 3618. w6x R. deSurville, M. Josefewicz, L.-T. Yu, J. Perichon, R. Buvet, Electrochim. Acta 12 (1968) 1451. w7x A.G. MacDiarmid, S.L. Mu, N.L. Somasiri, W. Wu, Mol. Cryst. Liq. Cryst. 121 (1985) 187. w8x A. Volkov, G. Tourillon, P.-C. Lacaze, J.-E. Duois, J. Electroanal. Chem. 115 (1980) 279. w9x S. Gottesfeld, A. Redondo, S.W. Feldberg, J. Electrochem. Soc. 134 (1987) 271. w10x W.S. Huang, B.D. Humprey, A.G. MacDiarmid, J. Chem. Soc. Faraday Trans. 86 (1986) 2385. w11x C.Y. Yang, Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 53 (1993) 293. w12x B. Johnstone, East. Econ. Rev. 17 (1988) 78. w13x B. Scrosati, Application of Electroactive Polymers, Chapman & Hall, New York, 1993. w14x M.S. Wrighton, R.G. Austin, A.B. Bocarsly, J.M. Bolts, O. Haas, K.O. Legg, L. Nadjo, M.C. Palaggoto, J. Am. Chem. Soc. 100 (1978) 1602. w15x E.M. Genies, A. Boyle, M. Lapkowski, C. Tsinativis, Synth. { Met. 36 (1990) 139. w16x R. Noufi, A.J. Nozik, J. White, L.F. Warren, J. Electrochem. Soc. 129 (1982) 2261. w17x H.H. Kim, T.M. Miller, E.H. Westerwick, Y.O. Kim, E.W. Kwock, M.D. Morris, M. Cerullo, J. Lightwave Technol. 12 (1994) 2107. w18x X. Zhou, J. He, L.S. Liao, M. Lu, Z.H. Xiong, X.M. Ding, X.Y. Hou, F.G. Tao, C.E. Zhou, S.T. Lee, Appl. Phys. Lett. 74 (1999) 609. w19x F. Wunsch, ´ ¨ J.-N. Chazawiel, F. Ozanam, P. Sigaud, O. Stephan, Surf. Sci. 489 (2001) 199. w20x H. Gerisher, Solar electrolysis with semiconductor electrodes, in: B.O. Seraphin (Ed.), Topics in Applied Physics, vol. 31, Verlag, Berlin, 1979, p. 115.
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w21x M. Kaneko, H. Nakamura, J. Chem. Soc. Commun. 346 (1985). w22x E.M. Genies, M. Lapkowski, { Synth. Met. 69 (1988) 24. w23x M. Kalaji, L. Nyholm, L.M. Peter, A.J. Rudge, J. Electroanal. Chem. 310 (1991) 113. w24x Z. Li, S. Dong, Electrochim. Acta 37 (1992) 1003. w25x F.L.C. Miquelino, M.-A. De Paoli, E.M. Genies, Synth. Met. 68 (1994) 91. w26x P.A. Kilmartin, G.A. Wright, Electrochim. Acta 43 (1998) 3091. w27x W.A. Gazotti Jr., R. Faez, M.-A. De Paoli, J. Electroanal. Chem. 415 (1996) 107. w28x P.A. Kilmartin, G.A. Wright, Synth. Met. 104 (1999) 145. w29x S. Mergulhao, ˜ R.M. Faria, G.F. Leal Ferreira, J. Sworakowski, Synth. Met. 101 (1999) 692. w30x W.-C. Chen, T.-C. Wen, C.-C. Hu, A. Gopalan, Electrochim. Acta 47 (2002) 1305. w31x R. Noufi, A.J. Frank, A.J. Nozik, J. Am. Chem. Soc. 103 (1981) 1849. w32x K. Rajeshwar, M. Kaneko, A. Yamada, J. Electrochem. Soc. 130 (1983) 38. w33x T.A. Skotheim, O. Inganas, J. Electrochem. Soc. 132 (1985) 2116. w34x G. Horowitz, G. Tourillon, F. Garnier, J. Electrochem. Soc. 131 (1984) 151.
w35x H. Yoneyama, K. Wakamoto, H. Tamura, J. Electrochem. Soc. 132 (1985) 2414. w36x B. Drake, C.B. Prater, A.L. Weisenhorn, S.A.C. Gould, T.R. Albrecht, C.F. Quate, D.S. Cannell, H.G. Hansma, P.K. Hansma, Science 243 (1989) 1586. w37x M. Mazur, P. Krysinski, ´ Electrochim. Acta 46 (2001) 3963. w38x S.Y. Vassiliev, K. Jackowska, A. Frydrychevicz, G.A. Tsirlina, O.A. Petrii, Electrochim. Acta 46 (2001) 4043. w39x E.C. Venancio, C.A.R. Costa, S.A.S. Machado, A.J. Motheo, Electrochem. Comm. 3 (2001) 229. w40x Y. Chen, E.T. Kang, K.G. Neoh, Appl. Surf. Sci. 185 (2002) 267. w41x J.E. Albuquerque, L.H.C. Mattoso, D.T. Balogh, R.M. Faria, J.G. Masters, A.G. MacDiarmid, Synth. Met. 113 (2000) 19. w42x D.E. Stilwell, S.-M. Park, J. Electrochem. Soc. 136 (1989) 427. w43x J.Y. Shimano, A.G. MacDiarmid, Synth. Met. 123 (2001) 251. w44x R. Patil, Y. Harima, K. Yamashita, K. Komaguchi, Y. Itagaki, M. Shiotani, J. Electroanal. Chem. 518 (2002) 13. w45x J. Widera, W. Grochala, K. Jackowska, J. Bukowska, Synth. Met. 89 (1997) 29. w46x R. Noufi, D. Tench, L.F. Warren, J. Electrochem. Soc. 128 (1981) 2596. w47x J.L. Langer, Synth. Met. 113 (2000) 263.
Photoelectrochemical and in situ atomic force microscopy studies of films derived from o-methoxyaniline solution on gallium arsenide (100) photoelectrode ´ Emil Wierzbinski, Marek Szklarczyk* Laboratory of Electrochemistry, Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093, Warsaw, Poland Received 14 March 2002; received in revised form 15 November 2002; accepted 24 November 2002
Abstract Organic films derived from o-methoxyaniline water solution were deposited on GaAs electrodes in an adsorption process. The photoelectrochemical behavior of the modified electrodes was examined by potentiodynamic and potentiostatic methods. The film morphology dependencies on the monomer concentration in the bulk of solution, electrode potential and oxidation time were monitored by in situ atomic force microscopy. The monitored morphology changes were correlated with the changes in the photocurrent. The differences in the onset potentials and photocurrent magnitude were explained in terms of the formation of a continuous organic film and on the basis of the p–n junction creation at the GaAsyorganic film interface. It is suggested that the organic film mediates transfer of the photocharge to the solution. Based on morphological changes in the deposited film and the spectrophotometric data, the formation of the leucoemeraldine, a reduced form of poly (o-methoxyaniline) is proposed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: o-Methoxyaniline; Polymers; GaAs; In situ atomic force microscopy
1. Introduction Conjugated polymers have been the subject of intense interest in surface science w1x. In particular, more than a century-old polyaniline (PANI) w2–4x and its derivatives have attracted significant attention thanks to straightforward methods of their preparation (e.g. w5– 9x), good stability of the conductive form in aqueous solution w10,11x and multiple potential applications w12,13x. Considering the large number of papers devoted to research on deposition of conducting polymers on metals and on electrochemistry of such deposited materials, there have been relatively less studies on deposition of polymers on semiconductors w14–19x. PANI can be deposited onto semiconductor surface either electrochemically in dark deposition process performed above 0.4 V or in photoreaction in a potential range 0.0–0.4 V w16x. One purpose of PANI film deposition on the surface of semiconductor photoanodes was to protect *Corresponding author. Tel.: q48-22-822-0211; fax: q48-22-8225996. E-mail address: [email protected] (M. Szklarczyk).
them against photocorrosion w15,16,20x. Enhanced photocurrent stability was reported in the presence of PANI films on n-type semiconductor electrodes, including Si, CdSe, GaP and GaAs. This was explained by a rapid removal of a photogenerated charge from the surface before it was allowed to react with the electrode material. The charge transfer could be mediated by a polymeric film w16x. Photoelectrochemical studies of the polymeric films deposited onto non-semiconductive electrodes (e.g. platinum, gold, indium-tin oxide electrodes) have resulted in the development of the new electrochemistry field, i.e. photoelectrochemistry of polymeric films. The photocurrent for PANI films deposited on metals has been reported w21–26x. Similarly, the photocurrent for polymers obtained from aniline derivatives solution was also observed w27–29x. It was shown that the reduced form of polymeric film generates cathodic photocurrent and no evidence for a photocurrent response while film was in the emeraldine form in the anodic potential range was found w23x. However, a photoresponse of PANI films over a very wide range of potentials has also been
0040-6090/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 1 1 2 2 - 7
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reported in other work w22x. In general, all types of the photoresponse are observed for PANI or its derivatives. P-type or n-type response of the polymer film sample depends on the way of its preparation, polymer oxidation state and the type of the solution w21–29x. In this paper we report the results of the influence of organic films deposited from an acidic aqueous solution of o-methoxyaniline on the photoactivity of an n-GaAs photoelectrode. The study was carried out in the potential range from y0.25 to 0.3 V vs. normal hydrogen electrode (NHE). The application of such potential range let us study the formation of the organic deposit onto the bare n-GaAs surface (potentials from y0.25 to f0 V) and on the oxidized surface (potentials from 0 to 0.3 V). Furthermore, it allows to assume that the formed film will be in the reduced form and therefore will exhibit the p-type semiconductive properties w23x. Obtaining the p-type semiconductive film could enable the formation of a p–n junction between n-GaAs sample and an organic film. On the other hand, the applied potential range makes electrochemical determination of the deposited film nature difficult, because the well known and well defined current peaks attributed to aniline polymerization reaction on metals are observed above 0.45 V vs. NHE (e.g. Ref. w30x). The deposition of polymers on semiconductive materials differs from that observed on metals. On the whole, in water solutions, the redox current peaks were not observed even up to q0.8 V vs. NHE w31x in dependence on the polymer (PANI, polypyrrole, polystyrene) and semiconductor type (n-GaAs, n-Si, n-CdSe) w16,31,32x. Contrary to water solution, in organic electrolytes current peaks can be observed at the potential as low as 0.1 V vs. NHE w33x in dependence on the type of the polymer (polypyrrole, poly-3-methyl-tiophene) and semiconductive materials (n-GaAs, n-Si) w33–35x. Such a behavior could be explained firstly: by the cathodic potential range applied in the experiments carried out in water solution, and secondly: by the over imposition of the current due to polymerization reaction by higher current due to oxygen evolution. To prove the presence and to characterize the properties of the film we employed UV–Vis spectrophotometry and contact atomic force microscopy (AFM) techniques. The first AFM image of PANI in water was presented as early as in 1989 w36x. The newer AFM or STM (scanning tunneling microscopy) studies on the PANI or poly (o-methoxyaniline), POMA, morphology were carried out mostly as ex situ experiments w37–40x. In this paper, in situ AFM studies of the deposit morphology as a function of the electrode potential, time and monomer concentration are presented as part of a research project aimed at establishing the impact of polymeric film on the ability of semiconductor electrodes to resist photocorrosion.
2. Experimental details 2.1. Electrodes The working electrodes were made of a single GaAs (100) crystal supplied by the Institute of Electronic Materials Technology, Warsaw, Poland. A Te doped n-type material with a resistivity of 0.012 V cm (1.65=1017 cmy3) and a Zn doped p-type material with resistivity of 0.25 V cm (1.25=1017 cmy3) were used in photoelectrochemical and AFM microscopic studies, respectively. It was assumed that surface activity was the same in both cases, that is, irrespective of whether electrons were created by light in an electron–hole pair phenomenon (n-type material) or in thermal activation from donor levels (p-type material). The working surface of the crystals were polished with alumina, down to 1.0 mm, etched in the mixture of H2SO4, H2O2 and H2O (5:1:1), chemically polished in NaClO solution and finally chemically etched in ammonia solution. An ohmic contact was prepared by either the vacuum evaporation of indium or by the deposition of gallium– indium alloy. The working area was circular in shape, approximately 4 mm in diameter. Counter electrodes were made of PtyPt gauze or wire. Electrode potential was measured against an AgyAgCl electrode and then expressed in the hydrogen scale. 2.2. Solutions All solutions were prepared from Millipore-Q water. The supporting electrolyte, 0.5 M H2SO4, was prepared from a reagent grade concentrated sulphuric acid and stored under argon atmosphere in a glass flask. The o-methoxyaniline monomer was obtained from Merck and was distilled prior to use. The transfer of the supporting electrolyte form storage flask to the photoelectrochemical cell was carried under Ar atmosphere. All experiments were carried out at ambient temperature, 22"2 8C. 2.3. Equipment and experimental procedures A three-electrode cell with a quartz window was used for photoelectrochemical studies. The photocurrent– voltage behavior was monitored with programmed potentiostat (KSP, Poland). The light source was a 1 kW halogen lamp (LH-21, Polam, Poland). A water filter was placed between the cell and the light source to filter out the long-wavelength part of radiation and to avoid excessive heating of the solution. The presence of o-methoxyaniline in solution was found not to affect the halogen light intensity. The absorption bands of o-methoxyaniline were detected in the UV range, i.e. below 300 nm, which is outside of the halogen lamp radiation range. The absorption of light by o-methoxy-
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aniline solution of different concentration was measured by using UV–Vis spectrophotometric technique (Double beam Lambda 12, Perkin-Elmer spectrophotometer). A spectrophotometric technique was used to monitor the presence as well as the chemical state of organic deposits. For this purpose a reflection technique was used. The spectra were recorded at 308 angle. As the background, the spectrum of n-GaAs was used. A Nanoscope III AFM microscope, a commercial AFM electrochemical cell and a potentiostat (Digital Instruments, USA) were used in imaging the surface of studied samples. The silicon nitrides cantilevers with a spring constant 0.58 N my1 (manufacturer data) were used. Calibration of the microscope was achieved by imaging calibration gratings supplied by the manufacturer. The images presented in this work are the height type images, but in some cases the section diagrams are also shown for better characterization of the morphology details. AFM images were examined for artifacts and reproducibility was made in the usual way, i.e. by changing a semiconductor sample or the AFM cantilever, and by either moving (during the experiment) the sample in the X or Y direction or by varying the scanning angle and frequency. In order to determine the influence of organic layer on the surface morphology, the bare GaAs electrode was first monitored in air and then in the supporting electrolyte solution under the same conditions as later used in the experiments with the o-methoxyaniline. 3. Results 3.1. Photoelectrochemical studies The photocurrent–potential (iph –E) dependencies for n-GaAs electrode in the absence (line 1) and in the presence of o-methoxyaniline (lines 2–4) in the bulk of the supporting electrolyte solution are shown in Fig. 1. In these experiments o-methoxyaniline aliquot was added to the photoelectrochemical cell at the constant electrode potential of y0.21 V and iph –E plots were registered after a 20 min adsorption. The plots are dependent on the concentration of o-methoxyaniline in the solution. Relatively to bare gallium arsenide (Fig. 1, line 1) the iph –E dependence recorded with the lowest used concentration of o-methoxyaniline (10y8 M, Fig. 1, line 2), is positively shifted by approximately 70 mV, at a half photocurrent point. The iph –E dependence shifts back in the negative direction at higher concentration of o-methoxyaniline (Fig. 1, line 3) and finally at the highest concentration, 10y2 M (Fig. 1, line 4) it becomes more cathodic than that recorded with a bare gallium arsenide (Fig. 1, line 1). Independently of these changes, for any concentration of o-methoxyaniline in the solution, the onset potential is more negative than that observed in the absence of monomer in the photo-
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Fig. 1. Cyclic voltammograms of illuminated n-GaAs electrode in 0.5 M H2SO4 solution in the absence of monomer in the bulk of solution (line 1) and after addition of o-methoxyaniline to the cell to the concentration 10y8 M, 3=10y5 M and 10y2 M, lines 2, 3 and 4 respectively. o-Methoxyaniline was added at the electrode potential y0.21 V and sweeping was started after 20 min of adsorption. Sweep rate: 50 mV sy1.
electrochemical cell (Fig. 1). Furthermore, the observed saturation photocurrent is higher in the presence of o-methoxyaniline in the solution than that in the o-methoxyaniline-free solution (Fig. 1). To discriminate between the effect that bulk monomer and the organic film have on the photocurrent magnitude, the cell was subjected to a so-called washing procedure. In this procedure, 20 min after the addition of the o-methoxyaniline to the photoelectrochemical cell the solution containing o-methoxyaniline was replaced by a fresh supporting electrolyte. This procedure was repeated six times in order to remove organics completely from the photoelectrochemical cell. No influence of the rinse on the photocurrent magnitude was observed, which indicates that the observed differences in photocurrent (Fig. 1) are caused by the presence of organic film on the GaAs surface. The photocurrent–time (iph –t) dependencies for n-GaAs photoelectrode at the constant potential 0.23 V, are shown in Fig. 2. Line 1 in this figure corresponds to o-methoxyaniline-free solution while line 2 represents the system response in the presence of the organic film obtained from the 3=10y5 M solution of o-methoxyaniline. The o-methoxyaniline was added to the photoelectrochemical cell at the potential y0.21 V. After 20 min of adsorption the washing procedure was applied in order to remove monomer from the bulk of solution, and then the potential was changed to 0.23 V. As expected, the photocurrent observed with bare gallium arsenide surface decreased with time. At the presence of an organic film on the electrode surface the photocurrent first slightly increased during first 20 min (cf. insert in Fig. 2) and then became stable for approximately 6.5 h. Subsequently, it decreased slowly. This
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Fig. 2. The dependence of iph on time for bare GaAs electrode, line 1, and for electrode covered with the organic layer deposited from 3=10y5 M o-methoxyanilineq0.5 M H2SO4 solution, line 2, at the electrode potential 0.23 V. o-Methoxyaniline was added at the electrode potential y0.21 V and this potential was changed to 0.23 V after 20 min of adsorption and after washing procedure.
behavior was independent of the presence of o-methoxyaniline monomer in the solution, which was checked in the experiments without washing procedure application. The current oscillation in Fig. 2 was due to the formation and later desorption of oxygen bubbles, easily observable during experiments. The reproducibility of described iph –E and iph –t dependencies viewing their shape and position on the potential or time scale was very good. The only observable variation worth mentioning was tied with the maximum photocurrent magnitude, which varied between GaAs samples "15%. 3.2. In situ AFM studies A typical image taken for the electrode placed in the supporting electrolyte is shown in Fig. 3. The surface consists of distinguishable solid bumps of different length, ranging from approximately 100 to 400 nm. The average height of these bumps is approximately 4 nm. The bumps are ordered in one direction (Fig. 3). The presented image was found independent of time and potential in the studied potential range. The AFM data for the organic layer deposited from 3=10y5 M monomer solution at a potential of y0.21 V are presented in Fig. 4a–c. The presented image was registered within 10 min after pure supporting electrolyte solution (0.5 M H2SO4) had been replaced with a 3=10y5 M monomer-containing solution in the AFM electrochemical cell. The deposit consists of several trapezoidal bulky blocks, which are better defined in three dimensional image presented in Fig. 4c. Another layer of blocks can be observed below the topmost layer of blocks. The continuity of the deposited film cannot be established on the basis of the height mode images (Fig. 4a and c) but it is noticeable in the section diagram
presented in Fig. 4b. The section analysis presented in Fig. 4b was made along the black line shown in Fig. 4a. Two arrows show two characteristic features of the deposited layer. The left arrow shows that under the topmost layer of slabs there are other underneath (bumps on the section diagram). The right arrow points that slabs are overlaying one another building a layer that looks like a fish scale (Fig. 4c). The average dimensions of slabs are approximately 180=300 nm2. The observed surface angles in trapezoids are 838"48 and 1018"38. An effect on the monomer concentration on the organic deposit morphology is given in Fig. 5. The presented images were taken at the y0.21 V over time as that used in the experiments shown in Fig. 4. At a monomer concentration of 3=10y5 M, the organic deposit is composed of the rounded blocks (Fig. 5a). This morphology fades away with an increase in the monomer concentration and becomes messy and disordered at the concentration 10y2 M (Fig. 5b). The changes in the organic film morphology at a more positive potential of 0.12 V, are shown in Fig. 6a– d. At this potential, the massive blocks and grains between the scales start to disappear (Fig. 6a and c, height view and section analysis view, respectively). These changes are very slow. The shown pictures were registered after 50 min since the potential had been changed from y0.21 V (Figs. 4 and 5a) to 0.12 V (Fig. 6a and c). When the potential is lowered back to a negative value, y0.25 V, the organic film structure returns to the previous form within 10 min (Fig. 6b and d, height view and section analysis, respectively). The section analysis presented in Fig. 6d was intentionally made through the visible deep cavity in the right lower
Fig. 3. In situ AFM image (top view) of GaAs bare electrode surface taken in 0.5 M H2SO4 solution at the electrode potential y0.2 V. Scan size 0.5=0.5 mm2. The bare placed on the right side of the image shows the scale in the vertical to the surface direction. The roughness factor, root mean square (RMS) value for the presented surface is 0.321 nm.
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resemble small hills (Fig. 7a). The next 15 min of oxidation brings dramatic changes in the film morphology, which takes the shape of long chains with clearly distinguishable rounded substructures (Fig. 7b). This change is irreversible, i.e. returning to negative potential has no effect on the organic film morphology. 3.3. Spectrophotometric studies The spectrophotometric results are shown in Fig. 8. The shown absorbance dependence was registered for the deposit formed at the y0.21 V potential. In the studied potential range the features of spectra were independent of the electrode potential. The presented spectra display single absorption peak at approximately 350 nm, shoulders in the approximately 400–500 nm range, and broad maximum at approximately 730 nm. The similarity of the absorbance dependencies for the
Fig. 4. In situ AFM data taken in 3=10y5 M o-methoxyanilineq0.5 M H2SO4 solution at the electrode potential y0.21 V. The image was registered within 10 min from the moment when the procedure of replacing basic electrolyte solution (0.5 M H2SO4) for 3=10y5 M monomer solution in the AFM electrochemical cell started. (a) topographic image. (b) section analysis along the line shown in (a). (c) three dimensional representation of the surface morphology. Scan size 4=4 mm2 and 1.2=1.2 mm2 for (a and c), respectively.
corner in order to show that the organic deposit slabs are presented there (bumps on the diagram). An influence of the oxidation time on the organic film morphology is shown in Fig. 7 for the deposit formed in the 10y2 M monomer solution. After 5 min of oxidation at 0.2 V a disordered structure presented in Fig. 5c disappears and the polymer surface starts to
Fig. 5. In situ AFM images (the height mode) showing morphology of organic film deposited at the electrode potential y0.21 V. Images presented in (a and b) were taken for electrode immersed in 3=10y5 M or 10y2 M o-methoxyanilineq0.5 M H2SO4 solution, respectively. The presented image was registered within 10 min from the moment when the procedure of replacing basic electrolyte solution (0.5 M H2SO4) for monomer solution in the AFM electrochemical cell started. Scan size 2=2 mm2.
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Fig. 6. In situ AFM data showing morphology of organic film after 50 min of oxidation at electrode potential 0.12 V, (a and c). Data presented in (b and d) show the film morphology after changing the electrode potential to y0.25 V for 10 min. (a and b) were taken in the height mode. (c and d) section analysis along the lines shown in (a and b). Organic film prior to oxidation was formed at y0.21 V for 10 min from 3=10y5 M o-methoxyanilineq0.5 M H2SO4 solution. Scan size 2=2 mm2.
films derived from aniline and o-methoxyaniline has already been reported w41x. On this basis, the spectra presented here for deposits derived from o-methoxyaniline can be ascribed to leucoemeraldine form of phenyly phenyl tetramer or of poly (o-methoxyaniline) film, and the maximum at 340 nm can be ascribed to p™p* transition observed for benzoid units w42–44x. The classification of the broad maximum at approximately 730 nm is rather intricate, especially, because its intensity was varied between the experiments. A comparison with other results indicates that the peak may be due to absorbance of either free electronic states w42x, or salt form of leucoemeraldine w43x. Some electronic transitions in the GaAs sample between the states induced by organic deposit may possibly be considered as the source of the broad maximum as well. 4. Discussion The observation described thus far can be summarized as follows: (i) an addition of the o-methoxyaniline monomer solution to the supporting electrolyte solution strongly influences the observed photocurrent (Figs. 1 and 2) and (ii) addition of the o-methoxyaniline monomer to the bulk of solution strongly influences observed surface morphology (Figs. 3–7).
These effects cannot result from the changes of the GaAs surface itself with time, because the photocurrent on the bare electrode should decrease rather than increase or remain stable (cf. w16x and also Fig. 2, line 1). Therefore, the observed photocurrent changes must be attributed to either the effect of the deposited organic film (the adsorbed layer of monomers or oligomers, or the polymeric film), or to the electrode reaction of the o-methoxyaniline monomer from the bulk of the solution. The latter explanation appears unlikely in view of the fact that no change in the photocurrent was observed after washing procedure was being applied (cf. Section 3.1). Thus, it can be concluded that the observed current changes are likely due to the altering of the surface phenomenon by deposited organic film, ultimately, due to reaction of the film, rather than the monomer, with the electrode surface. The differences in surface morphology observed by means of AFM microscope (Figs. 4–7) presented in this work can be assigned to the organic film morphology changes. It is because the presented image for bare GaAs surface was stable in time and with the change of the electrode potential (Fig. 3) and because they were observed only after replacing the solution o-methoxyaniline-free for the solution containing this monomer.
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monomers are present at the electrode surface. These forms are usually identified with the leucoemeraldine form of phenylyphenyl tetramer or of polymeric film w42–44x. The presented in situ AFM images show significant changes of the organic deposit morphology when electrode potential is changed from the adsorption one, y0.21 V (Fig. 4) to a more positive potential, 0.12 or 0.2 V, Figs. 6 and 7, respectively. The potential shift changes the conditions at the electrode surface from the oxygen free to the oxygen presence at the electrode surface. The evolving oxygen would moderate the process of the polymeric film formation (Figs. 6 and 7) from the oligomeric one (Fig. 4), which would explain observable changes in the morphology of the organic deposit. Greater morphological changes with the increase in the anodic potential (Figs. 6 and 7, respectively) well correlate with the increase in the oxygen concentration at electrode surface, which comes from the photocurrent increase with the electrode potential (Fig. 1). Some additional experimental evidence of the polymer formation under studied condition is a small increase of the photocurrent during early 20 min under potentiostatic condition (Fig. 2, line 2), when the electrode potential was changed from the adsorption to the oxygen evolution potential. The time fairly agree with the slow reconstruction of the deposited film (cf. AFM images presented in Fig. 4a and c with those presented in Fig. 7, respectively). So, this current increase can be ascribed to the polymer formation. The slow formation of polymer observed in this work can result from the relatively low concentration of monomer, the high con-
Fig. 7. In situ AFM images (the height mode) showing organic film morphology after 5 min of oxidation at electrode potential 0.2 V, (a), and after next 15 min of oxidation, (b). Organic film was formed at y0.21 V for 10 min from 10y2 M o-methoxyanilineq0.5 M H2SO4 solution. Scan size 1.2=1.2 mm2 and 1.5=1.5 mm2 for (a and b), respectively.
An attempt of the identification of the organic deposit chemical state may be made on the basis of the spectrophotometric and AFM results. The obtained organic film may consist of adsorbed monomers, or adsorbed oligomers, or polymer chains. In the potential range (y0.2 to q0.3 V) studied, the formation of the polymer cannot be expected in agreement with the electrochemical mechanism. The known proof of the electrochemical polymerization process is the occurrence of the well defined peaks, which are observed above 0.45 V (e.g. Ref. w30x). In our studies we did not observe these peaks because the electrochemical potential was too low. However, the detected spectrophotometric peak at 340 nm (Fig. 8) indicates that the forms higher than the
Fig. 8. UV–Vis spectrum of organic film deposited from 10y2 M omethoxyanilineq0.5 M H2SO4 solution. Adsorption time was 20 min, and potential of adsorption was y0.21 V.
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centration of supporting electrolyte (0.5 M) w45x and the low oxidation potential. The polymerization process discussed here for which electrochemical peaks are not observable can be identified with the electroless (no charge transfer through the electrodeysolution interface) polymerization mechanism, which has already been described w40x. According to this mechanism, an electrode surface plays a role of polymerization catalysts in the presence of oxygen on the surface. In those studies w40x the highly oxidized polymer was obtained by immersing platinum or palladium foil in the oxygen saturated acidic aniline solution. According to the described reaction mechanism, aniline monomer was first protonated in the acidic medium, and then catalytically oxidized to anilinium radical cation on the metal surface. Further oxidation leads to the formation of the anilinium radical by transferring the proton from the nitrogen group to the water or aniline molecules from the solution. The formed anilinium radicals and anilinium radical cations couple to form dimers or higher oligomers. Further oxidation led to the formation of polymer of different oxidation state, even as high as 75%, i.e. the nigraniline was formed w40x. The oxidation state of the polymer in the cited studies was monomer concentration, acid concentration and oxygen concentration dependent. In the studies presented in this paper the observed oxidation state of the obtained deposit is a leucoemeraldine form, i.e. the reduced POMA form (cf. Fig. 8). This difference can be explained firstly by better catalytic properties of platinum surface compared with GaAs surface, and secondly by different electrochemical condition applied in Ref. w40x in comparison with these applied in this work. In the studies reported in Ref. w40x open circuit conditions were used. Under such circumstances, the electrochemical potential at the platinumyacidic solution interface is equal to approximately 0.8 V, while in these studies electrochemical potential did not exceed 0.3 V, so the oxidative strength of the interface used in this work was much lower. Taking into account the spectrophotometric results (Fig. 8) and the cited electroless polymerization process w40x the formation of leucoemeraldine form of tetramer during adsorption can be suggested. Next, when electrode potential has been shifted to a potential where oxygen starts to evolve, above 0.0 V, tetramers start to polymerize, and leucoemeraldine form of POMA is created at the electrode surface. This last process can be observed by the film reconstruction monitored by AFM imaging (Figs. 6 and 7 vs. Fig. 4). The changes in the observed photocurrent after deposition of the organic film onto n-GaAs surface can be summarized as follows: (i) the onset potential in the presence of organics on the surface is negatively shifted; (ii) the saturation photocurrent increases in the presence of an organic film on the surface; (iii) at a given
potential, the photocurrent increases with films obtained in the solution of the higher monomer concentration, and finally becomes higher than the photocurrent for the bare gallium arsenide in the entire studied potential range. In order to explain these observations, it is necessary to take into account all processes first, which could potentially lead to the photocurrent, i.e. oxygen evolution, surface oxidation and possible oxidation of the surface organic film itself. If present, the latter effect would result in an increase in the currents by approximately 0.1 mA (e.g. Ref. w23x) that is significantly less than the increase reported in this work (Figs. 1 and 2). Surface oxidation, i.e. photodegradation of the GaAs photoelectrode is known to be responsible for the photocurrent decrease w16,20,46x. Such a behavior can be seen in Fig. 2, line 1. The observed photocurrent stabilization or its increase (Fig. 1, lines 2–4, Fig. 2, line 2) suggests that the degradation process is either stopped or at least inhibited in the presence of organic deposit on the n-GaAs surface. Thus, the observed photocurrent is likely to be caused by the oxygen evolution reaction. It can be attributed to the photocharge transfer to the solution mediated by organic film. Such a process is expected to be faster than the reaction of photocharge with the semiconductor lattice w16x. This means that the activation energy of electron transfer through the solutionyorganic film interface and organic filmysemiconductor interface must be lower than at the semiconductorysolution interface. Another explanation for the inhibition of the photocorrosion process at nGaAs surface could be in terms of the compact organic film (Fig. 4) forming a mechanical shield, protecting the semiconductor surface against oxidation due to the separation of the semiconductor surface from the solution. It is also possible that both phenomena could occur at the same time. The slow photocurrent decrease observed in the potentiostatic measurement after approximately 6.5 h (Fig. 2, line 2) is probably tied with a certain penetration of the polymeric film by oxygen and slow oxidation of the GaAs surface. The observed increase in the saturation current and the cathodic shift of the onset potential (Fig. 1) in the presence of the deposited layer on the electrode surface indicates a rise in the photocharge carrier concentration available for oxygen evolution. An increased availability of photocharge carrier can be explained on the basis of semiconductive properties of the obtained organic layer. Photoconductive p-type behavior of PANI had been previously detected and reported w21–23,25,26,28x. Assuming similar semiconductive properties for POMA w22x, the formation of p–n junction between the polymeric film and GaAs surface can be proposed. The creation of p–n junction would greatly increase the electrical field across the GaAsyPOMA interface, which explains faster transfer of photogenerated charges. Consequently, the concentration of these charges at the
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POMAysolution interface would increase and this phenomenon could be responsible for the photocurrent increase at the GaAsyPOMA interface. The shift of iph –E dependencies with the addition of monomer to the solution (Fig. 1) is likely to be caused by the mediation of the photocharge transfer to the solution by the organic film (see above). However, the magnitude of these shifts, which varies from 0.08 V in the positive direction for the lowest concentration to 0.04 V in the negative direction for the highest concentration (Fig. 1), can be influenced by other factors, too. It has to be emphasized that the observed displacement for the deposit formed in the solution with the lowest concentration of the monomer (10y8 M) is positive, as expected for the surface poisoned by adsorbed organics. At a higher monomer concentration the shift changes direction to negative, which is related to an increase in the surface activity. At the lowest monomer concentration the electrode surface is covered with deposit, which is in the form of islets that do not sufficiently protect the semiconductor against photodegradation. The surface becomes oxidized and the photocurrent decreases. With the increase in the monomer concentration, the formed deposit layer becomes continuous and its thickness increases (Fig. 5b) resulting in the photocurrent increase because of better surface protection and the increase thickening of the polymeric semiconductive space charge region. With all these changes the region of the GaAsyPOMA interface, where the p–n junction is formed, becomes bigger, leading to an enhancement in the photocurrent. The different morphology of the films obtained from the monomer solution of different concentration (Fig. 5a and b) is probably the reason for obtaining different structures as the result of the film reconstruction (Fig. 6aFig. 7a and b). The chain like structure shown in Fig. 7b provides an additional argument for the thesis that the films formed under low positive potential are polymeric. The morphology of the film presented in Fig. 7b agrees well with the fractal model of PANI film growth w47x. According to this model, the branching density between the main linear chains for polymers obtained from ortho-derivatives of PANI should be low, just as seen in the AFM image presented (Fig. 7b). 5. Conclusion The films deposited on gallium arsenide surface from aqueous solution of o-methoxyaniline help to protect the electrode surface against photocorrosion and lead to an enhanced photoactivity. The obtained GaAsyorganic film photoelectrode shows good stability for approximately 6.5 h. The observed phenomena are explained by the formation of the p–n junction at the GaAsyorganic film interface. The results show that a photocurrent depends on the morphology of the deposited film. The effect can
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be explained by the polymeric mediation of the photocharge transfer from GaAs electrode to the solution and by the increase in the thickness of the space charge region in the organic layer with the increase in the monomer concentration in the solution. On the basis of the evident morphology changes and spectrophotometric results, the formation of poly (o-methoxyaniline) is proposed. The observed structure of the film is in good agreement with the fractal model of the PANI structure. Acknowledgments The authors gratefully acknowledge the support of this work by the Department of Chemistry, Warsaw University under the Grant Nos. BW-1453y1y1999 and BW-1483y1y2000. References w1x J.F. Rubinson, H.B. Mark Jr., Conducting polymer films as electrodes, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, Inc, New York, 1999. w2x H. Letherby, J. Chem. Soc. 15 (1862) 161. w3x A.G. Green, A.E. Woodhead, J. Chem. Soc. 97 (1910) 2388. w4x A.G. Green, A.E. Woodhead, J. Chem. Soc. 101 (1912) 1117. w5x D.M. Mohilner, R.N. Adams, W.J. Argersinger, J. Am. Chem. Soc. 84 (1962) 3618. w6x R. deSurville, M. Josefewicz, L.-T. Yu, J. Perichon, R. Buvet, Electrochim. Acta 12 (1968) 1451. w7x A.G. MacDiarmid, S.L. Mu, N.L. Somasiri, W. Wu, Mol. Cryst. Liq. Cryst. 121 (1985) 187. w8x A. Volkov, G. Tourillon, P.-C. Lacaze, J.-E. Duois, J. Electroanal. Chem. 115 (1980) 279. w9x S. Gottesfeld, A. Redondo, S.W. Feldberg, J. Electrochem. Soc. 134 (1987) 271. w10x W.S. Huang, B.D. Humprey, A.G. MacDiarmid, J. Chem. Soc. Faraday Trans. 86 (1986) 2385. w11x C.Y. Yang, Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 53 (1993) 293. w12x B. Johnstone, East. Econ. Rev. 17 (1988) 78. w13x B. Scrosati, Application of Electroactive Polymers, Chapman & Hall, New York, 1993. w14x M.S. Wrighton, R.G. Austin, A.B. Bocarsly, J.M. Bolts, O. Haas, K.O. Legg, L. Nadjo, M.C. Palaggoto, J. Am. Chem. Soc. 100 (1978) 1602. w15x E.M. Genies, A. Boyle, M. Lapkowski, C. Tsinativis, Synth. { Met. 36 (1990) 139. w16x R. Noufi, A.J. Nozik, J. White, L.F. Warren, J. Electrochem. Soc. 129 (1982) 2261. w17x H.H. Kim, T.M. Miller, E.H. Westerwick, Y.O. Kim, E.W. Kwock, M.D. Morris, M. Cerullo, J. Lightwave Technol. 12 (1994) 2107. w18x X. Zhou, J. He, L.S. Liao, M. Lu, Z.H. Xiong, X.M. Ding, X.Y. Hou, F.G. Tao, C.E. Zhou, S.T. Lee, Appl. Phys. Lett. 74 (1999) 609. w19x F. Wunsch, ´ ¨ J.-N. Chazawiel, F. Ozanam, P. Sigaud, O. Stephan, Surf. Sci. 489 (2001) 199. w20x H. Gerisher, Solar electrolysis with semiconductor electrodes, in: B.O. Seraphin (Ed.), Topics in Applied Physics, vol. 31, Verlag, Berlin, 1979, p. 115.
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