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Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes A. El Guerraf a,⇑, S. Ben Jadi b, M. Bouabdallaoui a, Z. Aouzal a, M. Bazzaoui b, J. Aubard c, G. Lévi c, E.A. Bazzaoui a a
LCM, Faculté des Sciences, Département de Chimie, Université Mohammed 1er, 60 000 Oujda, Morocco LME, Faculté des Sciences, Université Ibn Zohr, 80 000 Agadir, Morocco c ITODYS, 15 Rue Jean Antoine de Baïf, 75013 Paris, France b
a r t i c l e
i n f o
Article history: Received 20 June 2019 Accepted 5 August 2019 Available online xxxx Keywords: X-ray photoelectron spectroscopy Raman spectroscopy Electrosynthesis Adherence Polythiophene/aluminium interface
a b s t r a c t The mechanism by which an electrochemically synthesized polymeric coating adheres to the surface of the metallic working electrode is complex and poorly known. The determination of the factors responsible for adherence requires a thorough investigation of the interfacial zone. In this context, we have undertaken to analyze by X-ray photoelectron (XPS) and Raman spectroscopies the contact layers of polythiophene with the aluminium working electrode surface. The decomposition of the XPS signals revealed the existence of additional C 1s and S 2p components of low-binding energies, probably due to the establishment of real covalent bonds between the polymer film and the metal substrate, which would explain the strong adherence of polythiophene on aluminium. Moreover, the investigation of the interface PT/Al with Raman spectroscopy has demonstrated the existence, in addition to the normal vibrational modes, of peaks assigned to structural defects in the polymer chains. These defects are close to the surface of the electrode and serve as anchoring sites, thus contributing to the strong adherence of the polymer to the aluminium substrate. Because of the high affinity of aluminium for heteroatoms, it is probable that the plans of the thiophene units are in the ‘‘standing” position with the sulfur atoms directed towards the surface of the metal. Ó 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Congress: Applied Materials for the Environment CIMAE-2018.
1. Introduction Since their discovery in the late 70 s, the range of applications of conducting polymers (CPs) has continued to expand [1–3]. In this context, the use of PCs for the protection of oxidizable metals against corrosion requires a thorough knowledge of the mechanisms that govern the adherence of the polymeric coating [4]. The material and the surface condition of the working electrode are important parameters that play a decisive role in the electropolymerization reaction. In the case of easily oxidizable metals with relatively low oxidation potentials, the dissolution reaction of the working electrode is likely to inhibit the formation of the
⇑ Corresponding author. E-mail address: [email protected] (A. El Guerraf).
radical-cation and the propagation of the electropolymerization process. Thiophene being a heterocyclic monomer well suited to functionalization reactions, particularly in b position, offers more possibilities of electrosynthesis of coatings with various and controllable properties. However, its relatively high oxidation potential (l.8 V) [5], in comparison with those of other monomers such as pyrrole (0.9 V) [6,7], is a serious impediment that needs to be overcome to perform the electropolymerization of thiophene on low oxidation potential metallic substrates. The solution to this problem is therefore to find experimental conditions that can slow down or inhibit the process of anodic dissolution of the electrode without preventing the electropolymerization reaction from occurring. In this sense, we had extensively examined in a previous paper [8,9] the effect of the solvent, the supporting electrolyte and the surface treatment on the growth
https://doi.org/10.1016/j.matpr.2019.08.084 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Congress: Applied Materials for the Environment CIMAE-2018.
Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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A. El Guerraf et al. / Materials Today: Proceedings xxx (xxxx) xxx
of homogenous and adherent polythiophene films on oxidizable metals. In the present investigation, we are interested in the case of aluminium electrodes because of the exceptional properties of the polythiophene coating electrochemically elaborated on this substrate, including its strong adherence. However, it is necessary to measure the difficulty of the problem considering that thiophene electropolymerizes at a potential of 1.8 V/ENH while the Al usually oxidizes at ca. 1.66 V/ENH, which represents a potential gap of about 3.5 V between the two redox systems. In this work, various electrochemical techniques have been used to optimize the electrosynthesis conditions of polythiophene on aluminium electrodes. Furthermore, in order to determine the parameters and mechanisms responsible for the strong adherence of the polymeric coating to the aluminium surface, PT films of different thicknesses have been prepared by the galvanostatic method which has proved particularly suitable for the obtaining of homogeneous and perfectly adherent films. The samples were then analyzed by XPS and Raman spectroscopies which are better adapted to the study of PT/Al interfacial zone. 2. Materials and methods Thiophene (99%, Aldrich) was purified by distillation under vacuum and stored under argon before use. Anhydrous dichloromethane (99%, Sigma-Aldrich) was used as received. Tetrabutylammonium hexafluorophosphate (98%, Aldrich) was dried under vacuum before use. The electropolymerization curves and the electrochemical behavior of the Al electrode were recorded using a PGZ 301 Potentiostat/Galvanostat controlled with Volta master4 analysis software. The electrosynthesis was carried out in a one-compartment cell equipped with the well-known three electrode system, aluminium as working electrode, Ag/AgCl/Cl- as reference electrode and the auxiliary electrode was platinum plate. XPS analysis was performed with a Vacuum Generators VG Scientific Escalab 200 A spectrometer equipped with a double X-rays sources (Mg Ka : 1253.6 eV and Al Ka : 1486.6 eV). Raman spectra were recorded using a green excitation wavelength ke = 514.5 nm (Spectra-Physics Model 165 argon ion laser) with 5 mW laser power. The scattered beam, collected at 180° from the incident beam (retro-Raman), was focused onto the entrance slit of a Dilor XY spectrometer. The spectrometer consists of a double monochromator used in substractive mode followed by a spectrograph to complete the dispersion and by a 1024 256 JobinYvon CCD matrix multichannel detector, cooled by the thermoelectric effect.
The adherence of the obtained films was measured using the standard sellotape test. The film was cut into small squares, and by sticking the tape and then stripping it, the percentage of adherence can be calculated (ratio between the number of adherent film squares remaining and their total number). 3. Results and discussion 3.1. Electrosynthesis of polythiophene on aluminium electrode The electropolymerization of thiophene on aluminium is carried out in dichloromethane (CH2Cl2) in the presence of N(Bu)4PF6 as supporting electrolyte. This solvent has proven particularly favorable by leading to a significant slowdown of the electrode dissolution. 3.1.1. Cyclic voltammetry Beforehand, the electrochemical behavior of the aluminium electrode in the absence of the monomer, recorded by cyclic potential scanning between 0 and 2 V in CH2Cl2 + 0.1 M N(Bu)4PF6 medium, presents an anodic peak at ca. 1.3 V separating the activation and passivation domains (Inset, Fig. 1). From the second scan, the electrode shows a strong and fast passivation for which the current density takes very low values. By analogy with several reported studies dealing with other metal substrates [10], the observed passivation could be due to the formation of a protective layer of aluminium oxide and fluoride. The involvement of the supporting electrolyte in the passivation process was confirmed by XPS analyzes which revealed the presence of fluoride at the electrode surface after thorough rinsing and drying. In the presence of 0.5 M thiophene, the voltammetric curves obtained on aluminium, very similar to those recorded under the same conditions on Pt are characteristic of the formation of an electroactive polymer (Fig. 1). It should be noted that from the first potential sweep, there is almost total disappearance of the oxidation peak observed in the absence of the monomer and associated with the metal substrate. Apparently, the oxidation of thiophene begins at 1.8 V, and leads to the formation of an electroactive deposit which, however, does not cover the surface homogeneously, but actually forms islands of polythiophene randomly dispersed on the metal surface. The distribution of these islands is denser at the periphery of the electrode, which seems to indicate that the growth of the polymer is made from germination active centers whose number is a function of the local current density. It is also interesting to note that, contrary to the case of platinum, the first voltammogram recorded on Al is marked by an abrupt change in slope and a steep rise of the current density at
Fig. 1. Cyclic voltammograms of thiophene electropolymerization on Al (a) and Pt (b) working electrodes in CH2Cl2 + 0.1 M N(Bu)4PF6 + 0.5 M thiophene with 100 mV s Scan rate. Inset: Al electrode behavior in the same conditions without monomer.
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Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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1.8 V, probably corresponding to punctual breaks of passivation, which locally induce high current densities, and which has the advantage of keeping constant the overall kinetics of the electropolymerization process. On platinum electrode the beginning of the electropolymerization reaction is more gradual in the vicinity of l.6 V. 3.1.2. Galvanostatic mode Different current densities were imposed to the Al electrode in the same electrolytic medium. As expected, the potential-time curves vary considerably depending on the applied current densities (Fig. 2(a)). For low current densities (j < 0.5 mA.cm 2), the potential stabilizes at 0.2 V, which corresponds to the dissolution of the metal. When j = 0.5 mA.cm 2 is applied, there is a gradual variation of potential from 0.15 to 2.6 V, corresponding to a transition between the dissolution of the metal and the electropolymerization of thiophene, resulting in the formation of PT islands, comparable to those previously observed with cyclic voltammetry. For the larger values of current density, the potential variation is much faster, and is fixed almost instantaneously at a high potential value corresponding to the electropolymerization of thiophene. Thus, for j = 1 mA.cm 2, the potential of the Al electrode is kept constant at 2.6 V, and a homogeneous and very adherent film, perfectly covering the aluminium surface is formed. 3.1.3. Potentiostatic mode The behavior of the j = f (t) curve strongly changes between 1.72 and 1.74 V (Fig. 2(b)). For E = 1.72 V, j decreases with time and no PT deposit is observed, whereas for E = 1.74 V, a rapid decrease of the current is followed, after a few seconds, by a gradual rise and a plateau of 0.96 mA.cm 2 in which an inhomogeneous film of PT is formed. When E = 1.8 V the value of the plateau is 1.3 mA.cm 2 and a very homogeneous film is obtained. Considering these results, the PT/Al samples for XPS and Raman analyses will be prepared by the galvanostatic technique, which seems much better adapted to achieve the electropolymerization of thiophene on aluminium, leading to more homogeneous and adherent films. 3.2. Characterization of PT/Al interface The synthetized films were extremely adherence in the Al surface. The adherence percentage was estimated at 100% based on the standard sellotape test. To determine the origin of this strong adherent, the following part is devoted to the investigation and analysis of the PT/Al interface. For this purpose, three PT films of
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different thicknesses (25, 100 and 1000 nm) were prepared on a pre-polished aluminium plates and then analyzed by XPS. Mainly, the carbon signal C 1s (Fig. 3) consists of three peaks. However, in the case of thin PT films a fourth component is observed. The main peak at 284.8 eV is assigned to the C–H and C–C of the thiophene rings. The two other peaks at 286.0 and 287.5 eV are due to C-OH and C = O groups respectively and may have several origins, particularly the inevitable contamination or lateral groups that attach to the polymer skeleton during its electrosynthesis. It should be noted that the intensity of these later components increases with the oxidation rate and reach a maximum value in the overoxidation zone of PT [11]. In the case of thin of PT films, a low-intensity band rises on the low-binding energy side of the XPS signal at 283.1 eV and corresponds to a negatively charged carbon which is bound to aluminum. In the same way, after decomposition, the signal of sulfur S 2p (Fig. 3) consists of three doublets of which the one of lowbinding energy is observed only in the case of thin films. The main doublet at (164.8 eV, 165.9 eV) corresponds to neutral sulfur of the thiophene rings [12,13]. A second doublet of weak intensity, located at (166.4 eV, 167.5 eV), is assigned to an oxidized species of sulfur [12,14,15,16]. The third component S 2p at (162.9 eV, 164.0 eV) due to a negatively charged sulfur results from an aluminum sulfide which is formed due to the high affinity of aluminium for heteroatoms, especially sulfur. The low-binding energy C 1s and S 2p components disappear when the film thickness increases, indicating that these polymermetal interactions are limited to the interface. These results are consistent with those of Brédas and al. [17], who reported that after evaporating a thin metal layer of Al on polythiophene, there was formation of real covalent bonds S-Al and C-Al [18–23]. Moreover, to gain additional information, Raman spectra of polythiophene films electrodeposited on aluminum electrodes were recorded with 514.5 nm excitation wavelength. The global spectrum (Fig. 4) with assignments of the characteristic bands are well known and have been widely described in literature [24]. Therefore, in what follows, we will be interested in the two spectral zone centered at 700 and 1450 cm 1 that provide a wealth of information about the nature of PT/Al interface. It must be highlighted that the Raman analysis of the PT deposited on Al can be carried out with high laser powers up to 500 mW on the sample without any degradation associated with local destruction of the PT coating. The high thermal conduction of aluminum, capable of rapidly dissipating the local heating, does not seem sufficient to explain such thermal stability. It must also be considered that there is a very strong adhesion of the film on the aluminum surface causing its stabilization. The results of the XPS
Fig. 2. Galvanostatic (a) and potentiostatic (b) curves recorded during polythiophene electropolymerization on Al in CH2Cl2 + 0.1 M N(Bu)4PF6 + 0.5 M thiophene.
Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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Fig. 3. C 1s and S 2p XPS signals of PT films of different thicknesses electrodeposited on Al working electrode. a) 25 nm, b) 100 nm and c) 1000 nm.
Fig. 4. Global Raman spectra of PT electrodeposited on Al, recorded with 514.5 nm excitation wavelength and 5 mW laser power.
Fig. 5. Decomposition of the spectral domains centered at (a) 1450 cm
1
analyzes effectively showed the presence of C-Al and S-Al bonds in agreement with the adherence measurements which proved that the PT films electrodeposited on Al was strongly adherent to the metal surface. We have therefore carried out a detailed analysis of the spectral zone centered around 1450 cm 1 (Fig. 5(a)), by decomposing it into three bands: m1 (antisymmetric stretching of the C = C double bonds of the nucleus), m2 (symmetrical stretching of the C = C double bonds of the nucleus) and m3 (stretching of the C–C simple bonds of the nucleus). The width of the m1 line reflects the distribution of the conjugation length within the polymer chains [25,26]. This line is sharper compared to that observed in the case of PT electrodeposited on Pt [24], which indicates that the distribution of the conjugation length is less spread in the films deposited on Al. Sauvajol and al. [27] reported the presence of a band denoted D1, related to a structural defect overlapping with the m1 band, and whose intensity increases with the oxidation degree of PT. In a previous surface-enhanced Raman scattering study of thin PT film electrodeposited on roughened silver electrodes, and based on the SERS selection rules applied to m3 mode, we have shown
and (b) 700 cm
1
into three and four components of Voigt profiles respectively.
Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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that in the inner PT layers directly in contact with the metal substrate, the plans of the thiophene rings are almost perpendicular to the electrode surface [28]. It is likely that in the case of Al the thiophene units also adopt the ‘‘standing” position with the sulfur atoms directed towards the metal surface with which they form true covalent bonds. The 700 cm 1 spectral zone (Fig. 5(b)) includes low intensity bands, in particular the bands m6 and m7 at 740 and 700 cm 1 assigned to the stretching and deformation modes of C-S and CS-C bonds respectively, and the bands D4 and D5 at 681 and 651 cm 1 originating from structural defects. The intensity ratios of the defects’ bands D4 and D5 compared to the m6 and m7 normal modes are used to evaluate the disorder in the PT films [25,27,29,30]. Like what we proved by SERS [31], these structural defects are close to the metal surface and serve as anchoring points for the PT film to the surface of the aluminium electrode, thus participating in the strong adherence of the coating.
4. Conclusion Homogeneity and adherence are the two most important criteria required for the application of a conducting polymer in any field where it acts as a coating of common metals. In this work, after optimization of the electrosynthesis conditions of homogeneous and adherent polythiophene films on aluminum electrodes, we strived to look for the mechanisms that are at the origin of their strong adherence. We have therefore used XPS and Raman spectroscopies, which have proved adequate for the elemental and structural investigation of the inner layers that form the PT/Al interface. The analysis showed that in the interfacial zone, the thiophene units that form the polymer chains adopt the ‘‘standing” position by establishing true covalent bonds with the metal substrate and highlighted the presence of structural defects that act as anchoring points of the PT film on the aluminium surface.
Acknowledgements This work was supported by the MESRSFC and CNRST (Morocco) under grant No. PPR/30/2015.
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Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes A. El Guerraf a,⇑, S. Ben Jadi b, M. Bouabdallaoui a, Z. Aouzal a, M. Bazzaoui b, J. Aubard c, G. Lévi c, E.A. Bazzaoui a a
LCM, Faculté des Sciences, Département de Chimie, Université Mohammed 1er, 60 000 Oujda, Morocco LME, Faculté des Sciences, Université Ibn Zohr, 80 000 Agadir, Morocco c ITODYS, 15 Rue Jean Antoine de Baïf, 75013 Paris, France b
a r t i c l e
i n f o
Article history: Received 20 June 2019 Accepted 5 August 2019 Available online xxxx Keywords: X-ray photoelectron spectroscopy Raman spectroscopy Electrosynthesis Adherence Polythiophene/aluminium interface
a b s t r a c t The mechanism by which an electrochemically synthesized polymeric coating adheres to the surface of the metallic working electrode is complex and poorly known. The determination of the factors responsible for adherence requires a thorough investigation of the interfacial zone. In this context, we have undertaken to analyze by X-ray photoelectron (XPS) and Raman spectroscopies the contact layers of polythiophene with the aluminium working electrode surface. The decomposition of the XPS signals revealed the existence of additional C 1s and S 2p components of low-binding energies, probably due to the establishment of real covalent bonds between the polymer film and the metal substrate, which would explain the strong adherence of polythiophene on aluminium. Moreover, the investigation of the interface PT/Al with Raman spectroscopy has demonstrated the existence, in addition to the normal vibrational modes, of peaks assigned to structural defects in the polymer chains. These defects are close to the surface of the electrode and serve as anchoring sites, thus contributing to the strong adherence of the polymer to the aluminium substrate. Because of the high affinity of aluminium for heteroatoms, it is probable that the plans of the thiophene units are in the ‘‘standing” position with the sulfur atoms directed towards the surface of the metal. Ó 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Congress: Applied Materials for the Environment CIMAE-2018.
1. Introduction Since their discovery in the late 70 s, the range of applications of conducting polymers (CPs) has continued to expand [1–3]. In this context, the use of PCs for the protection of oxidizable metals against corrosion requires a thorough knowledge of the mechanisms that govern the adherence of the polymeric coating [4]. The material and the surface condition of the working electrode are important parameters that play a decisive role in the electropolymerization reaction. In the case of easily oxidizable metals with relatively low oxidation potentials, the dissolution reaction of the working electrode is likely to inhibit the formation of the
⇑ Corresponding author. E-mail address: [email protected] (A. El Guerraf).
radical-cation and the propagation of the electropolymerization process. Thiophene being a heterocyclic monomer well suited to functionalization reactions, particularly in b position, offers more possibilities of electrosynthesis of coatings with various and controllable properties. However, its relatively high oxidation potential (l.8 V) [5], in comparison with those of other monomers such as pyrrole (0.9 V) [6,7], is a serious impediment that needs to be overcome to perform the electropolymerization of thiophene on low oxidation potential metallic substrates. The solution to this problem is therefore to find experimental conditions that can slow down or inhibit the process of anodic dissolution of the electrode without preventing the electropolymerization reaction from occurring. In this sense, we had extensively examined in a previous paper [8,9] the effect of the solvent, the supporting electrolyte and the surface treatment on the growth
https://doi.org/10.1016/j.matpr.2019.08.084 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Congress: Applied Materials for the Environment CIMAE-2018.
Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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of homogenous and adherent polythiophene films on oxidizable metals. In the present investigation, we are interested in the case of aluminium electrodes because of the exceptional properties of the polythiophene coating electrochemically elaborated on this substrate, including its strong adherence. However, it is necessary to measure the difficulty of the problem considering that thiophene electropolymerizes at a potential of 1.8 V/ENH while the Al usually oxidizes at ca. 1.66 V/ENH, which represents a potential gap of about 3.5 V between the two redox systems. In this work, various electrochemical techniques have been used to optimize the electrosynthesis conditions of polythiophene on aluminium electrodes. Furthermore, in order to determine the parameters and mechanisms responsible for the strong adherence of the polymeric coating to the aluminium surface, PT films of different thicknesses have been prepared by the galvanostatic method which has proved particularly suitable for the obtaining of homogeneous and perfectly adherent films. The samples were then analyzed by XPS and Raman spectroscopies which are better adapted to the study of PT/Al interfacial zone. 2. Materials and methods Thiophene (99%, Aldrich) was purified by distillation under vacuum and stored under argon before use. Anhydrous dichloromethane (99%, Sigma-Aldrich) was used as received. Tetrabutylammonium hexafluorophosphate (98%, Aldrich) was dried under vacuum before use. The electropolymerization curves and the electrochemical behavior of the Al electrode were recorded using a PGZ 301 Potentiostat/Galvanostat controlled with Volta master4 analysis software. The electrosynthesis was carried out in a one-compartment cell equipped with the well-known three electrode system, aluminium as working electrode, Ag/AgCl/Cl- as reference electrode and the auxiliary electrode was platinum plate. XPS analysis was performed with a Vacuum Generators VG Scientific Escalab 200 A spectrometer equipped with a double X-rays sources (Mg Ka : 1253.6 eV and Al Ka : 1486.6 eV). Raman spectra were recorded using a green excitation wavelength ke = 514.5 nm (Spectra-Physics Model 165 argon ion laser) with 5 mW laser power. The scattered beam, collected at 180° from the incident beam (retro-Raman), was focused onto the entrance slit of a Dilor XY spectrometer. The spectrometer consists of a double monochromator used in substractive mode followed by a spectrograph to complete the dispersion and by a 1024 256 JobinYvon CCD matrix multichannel detector, cooled by the thermoelectric effect.
The adherence of the obtained films was measured using the standard sellotape test. The film was cut into small squares, and by sticking the tape and then stripping it, the percentage of adherence can be calculated (ratio between the number of adherent film squares remaining and their total number). 3. Results and discussion 3.1. Electrosynthesis of polythiophene on aluminium electrode The electropolymerization of thiophene on aluminium is carried out in dichloromethane (CH2Cl2) in the presence of N(Bu)4PF6 as supporting electrolyte. This solvent has proven particularly favorable by leading to a significant slowdown of the electrode dissolution. 3.1.1. Cyclic voltammetry Beforehand, the electrochemical behavior of the aluminium electrode in the absence of the monomer, recorded by cyclic potential scanning between 0 and 2 V in CH2Cl2 + 0.1 M N(Bu)4PF6 medium, presents an anodic peak at ca. 1.3 V separating the activation and passivation domains (Inset, Fig. 1). From the second scan, the electrode shows a strong and fast passivation for which the current density takes very low values. By analogy with several reported studies dealing with other metal substrates [10], the observed passivation could be due to the formation of a protective layer of aluminium oxide and fluoride. The involvement of the supporting electrolyte in the passivation process was confirmed by XPS analyzes which revealed the presence of fluoride at the electrode surface after thorough rinsing and drying. In the presence of 0.5 M thiophene, the voltammetric curves obtained on aluminium, very similar to those recorded under the same conditions on Pt are characteristic of the formation of an electroactive polymer (Fig. 1). It should be noted that from the first potential sweep, there is almost total disappearance of the oxidation peak observed in the absence of the monomer and associated with the metal substrate. Apparently, the oxidation of thiophene begins at 1.8 V, and leads to the formation of an electroactive deposit which, however, does not cover the surface homogeneously, but actually forms islands of polythiophene randomly dispersed on the metal surface. The distribution of these islands is denser at the periphery of the electrode, which seems to indicate that the growth of the polymer is made from germination active centers whose number is a function of the local current density. It is also interesting to note that, contrary to the case of platinum, the first voltammogram recorded on Al is marked by an abrupt change in slope and a steep rise of the current density at
Fig. 1. Cyclic voltammograms of thiophene electropolymerization on Al (a) and Pt (b) working electrodes in CH2Cl2 + 0.1 M N(Bu)4PF6 + 0.5 M thiophene with 100 mV s Scan rate. Inset: Al electrode behavior in the same conditions without monomer.
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1.8 V, probably corresponding to punctual breaks of passivation, which locally induce high current densities, and which has the advantage of keeping constant the overall kinetics of the electropolymerization process. On platinum electrode the beginning of the electropolymerization reaction is more gradual in the vicinity of l.6 V. 3.1.2. Galvanostatic mode Different current densities were imposed to the Al electrode in the same electrolytic medium. As expected, the potential-time curves vary considerably depending on the applied current densities (Fig. 2(a)). For low current densities (j < 0.5 mA.cm 2), the potential stabilizes at 0.2 V, which corresponds to the dissolution of the metal. When j = 0.5 mA.cm 2 is applied, there is a gradual variation of potential from 0.15 to 2.6 V, corresponding to a transition between the dissolution of the metal and the electropolymerization of thiophene, resulting in the formation of PT islands, comparable to those previously observed with cyclic voltammetry. For the larger values of current density, the potential variation is much faster, and is fixed almost instantaneously at a high potential value corresponding to the electropolymerization of thiophene. Thus, for j = 1 mA.cm 2, the potential of the Al electrode is kept constant at 2.6 V, and a homogeneous and very adherent film, perfectly covering the aluminium surface is formed. 3.1.3. Potentiostatic mode The behavior of the j = f (t) curve strongly changes between 1.72 and 1.74 V (Fig. 2(b)). For E = 1.72 V, j decreases with time and no PT deposit is observed, whereas for E = 1.74 V, a rapid decrease of the current is followed, after a few seconds, by a gradual rise and a plateau of 0.96 mA.cm 2 in which an inhomogeneous film of PT is formed. When E = 1.8 V the value of the plateau is 1.3 mA.cm 2 and a very homogeneous film is obtained. Considering these results, the PT/Al samples for XPS and Raman analyses will be prepared by the galvanostatic technique, which seems much better adapted to achieve the electropolymerization of thiophene on aluminium, leading to more homogeneous and adherent films. 3.2. Characterization of PT/Al interface The synthetized films were extremely adherence in the Al surface. The adherence percentage was estimated at 100% based on the standard sellotape test. To determine the origin of this strong adherent, the following part is devoted to the investigation and analysis of the PT/Al interface. For this purpose, three PT films of
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different thicknesses (25, 100 and 1000 nm) were prepared on a pre-polished aluminium plates and then analyzed by XPS. Mainly, the carbon signal C 1s (Fig. 3) consists of three peaks. However, in the case of thin PT films a fourth component is observed. The main peak at 284.8 eV is assigned to the C–H and C–C of the thiophene rings. The two other peaks at 286.0 and 287.5 eV are due to C-OH and C = O groups respectively and may have several origins, particularly the inevitable contamination or lateral groups that attach to the polymer skeleton during its electrosynthesis. It should be noted that the intensity of these later components increases with the oxidation rate and reach a maximum value in the overoxidation zone of PT [11]. In the case of thin of PT films, a low-intensity band rises on the low-binding energy side of the XPS signal at 283.1 eV and corresponds to a negatively charged carbon which is bound to aluminum. In the same way, after decomposition, the signal of sulfur S 2p (Fig. 3) consists of three doublets of which the one of lowbinding energy is observed only in the case of thin films. The main doublet at (164.8 eV, 165.9 eV) corresponds to neutral sulfur of the thiophene rings [12,13]. A second doublet of weak intensity, located at (166.4 eV, 167.5 eV), is assigned to an oxidized species of sulfur [12,14,15,16]. The third component S 2p at (162.9 eV, 164.0 eV) due to a negatively charged sulfur results from an aluminum sulfide which is formed due to the high affinity of aluminium for heteroatoms, especially sulfur. The low-binding energy C 1s and S 2p components disappear when the film thickness increases, indicating that these polymermetal interactions are limited to the interface. These results are consistent with those of Brédas and al. [17], who reported that after evaporating a thin metal layer of Al on polythiophene, there was formation of real covalent bonds S-Al and C-Al [18–23]. Moreover, to gain additional information, Raman spectra of polythiophene films electrodeposited on aluminum electrodes were recorded with 514.5 nm excitation wavelength. The global spectrum (Fig. 4) with assignments of the characteristic bands are well known and have been widely described in literature [24]. Therefore, in what follows, we will be interested in the two spectral zone centered at 700 and 1450 cm 1 that provide a wealth of information about the nature of PT/Al interface. It must be highlighted that the Raman analysis of the PT deposited on Al can be carried out with high laser powers up to 500 mW on the sample without any degradation associated with local destruction of the PT coating. The high thermal conduction of aluminum, capable of rapidly dissipating the local heating, does not seem sufficient to explain such thermal stability. It must also be considered that there is a very strong adhesion of the film on the aluminum surface causing its stabilization. The results of the XPS
Fig. 2. Galvanostatic (a) and potentiostatic (b) curves recorded during polythiophene electropolymerization on Al in CH2Cl2 + 0.1 M N(Bu)4PF6 + 0.5 M thiophene.
Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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Fig. 3. C 1s and S 2p XPS signals of PT films of different thicknesses electrodeposited on Al working electrode. a) 25 nm, b) 100 nm and c) 1000 nm.
Fig. 4. Global Raman spectra of PT electrodeposited on Al, recorded with 514.5 nm excitation wavelength and 5 mW laser power.
Fig. 5. Decomposition of the spectral domains centered at (a) 1450 cm
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analyzes effectively showed the presence of C-Al and S-Al bonds in agreement with the adherence measurements which proved that the PT films electrodeposited on Al was strongly adherent to the metal surface. We have therefore carried out a detailed analysis of the spectral zone centered around 1450 cm 1 (Fig. 5(a)), by decomposing it into three bands: m1 (antisymmetric stretching of the C = C double bonds of the nucleus), m2 (symmetrical stretching of the C = C double bonds of the nucleus) and m3 (stretching of the C–C simple bonds of the nucleus). The width of the m1 line reflects the distribution of the conjugation length within the polymer chains [25,26]. This line is sharper compared to that observed in the case of PT electrodeposited on Pt [24], which indicates that the distribution of the conjugation length is less spread in the films deposited on Al. Sauvajol and al. [27] reported the presence of a band denoted D1, related to a structural defect overlapping with the m1 band, and whose intensity increases with the oxidation degree of PT. In a previous surface-enhanced Raman scattering study of thin PT film electrodeposited on roughened silver electrodes, and based on the SERS selection rules applied to m3 mode, we have shown
and (b) 700 cm
1
into three and four components of Voigt profiles respectively.
Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084
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that in the inner PT layers directly in contact with the metal substrate, the plans of the thiophene rings are almost perpendicular to the electrode surface [28]. It is likely that in the case of Al the thiophene units also adopt the ‘‘standing” position with the sulfur atoms directed towards the metal surface with which they form true covalent bonds. The 700 cm 1 spectral zone (Fig. 5(b)) includes low intensity bands, in particular the bands m6 and m7 at 740 and 700 cm 1 assigned to the stretching and deformation modes of C-S and CS-C bonds respectively, and the bands D4 and D5 at 681 and 651 cm 1 originating from structural defects. The intensity ratios of the defects’ bands D4 and D5 compared to the m6 and m7 normal modes are used to evaluate the disorder in the PT films [25,27,29,30]. Like what we proved by SERS [31], these structural defects are close to the metal surface and serve as anchoring points for the PT film to the surface of the aluminium electrode, thus participating in the strong adherence of the coating.
4. Conclusion Homogeneity and adherence are the two most important criteria required for the application of a conducting polymer in any field where it acts as a coating of common metals. In this work, after optimization of the electrosynthesis conditions of homogeneous and adherent polythiophene films on aluminum electrodes, we strived to look for the mechanisms that are at the origin of their strong adherence. We have therefore used XPS and Raman spectroscopies, which have proved adequate for the elemental and structural investigation of the inner layers that form the PT/Al interface. The analysis showed that in the interfacial zone, the thiophene units that form the polymer chains adopt the ‘‘standing” position by establishing true covalent bonds with the metal substrate and highlighted the presence of structural defects that act as anchoring points of the PT film on the aluminium surface.
Acknowledgements This work was supported by the MESRSFC and CNRST (Morocco) under grant No. PPR/30/2015.
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Please cite this article as: A. El Guerraf, S. Ben Jadi, M. Bouabdallaoui et al., Investigation of the geometry and anchoring mode of conducting polythiophene films electrosynthesized on aluminium working electrodes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.08.084