Next Article in Journal
Material Structure and Mechanical Properties of Silicon Nitride and Silicon Oxynitride Thin Films Deposited by Plasma Enhanced Chemical Vapor Deposition
Next Article in Special Issue
Time-Resolved X-ray Absorption Spectroscopy in (Photo)Electrochemistry
Previous Article in Journal / Special Issue
On the Contrasting Effect Exerted by a Thin Layer of CdS against the Passivation of Silver Electrodes Coated with Thiols
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Surfaces
Volume 1
Issue 1
10.3390/surfaces1010005
surfaces-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Diazonium Salts: Versatile Molecular Glues for Sticking Conductive Polymers to Flexible Electrodes

by
Momath Lo
1,2,3,
Rémi Pires
3,
Karim Diaw
1,
Diariatou Gningue-Sall
1,*,
Mehmet A. Oturan
2,
Jean-Jacques Aaron
2,* and
Mohamed M. Chehimi
3,*
1
Faculté des Sciences, Université Cheikh Anta Diop, BP 5005 Dakar-Fann, Sénégal
2
Laboratoire Géomatériaux et Environnement, Université Paris-Est, 5 Bd. Descartes, 77454 Marne-la-Vallée CEDEX 2, France
3
Université Paris Est, CNRS, ICMPE (UMR 7182), 2-8 rue Henri Dunant, 94320 Thiais, France
*
Authors to whom correspondence should be addressed.
Surfaces 2018, 1(1), 43-58; https://doi.org/10.3390/surfaces1010005
Submission received: 20 June 2018 / Revised: 20 July 2018 / Accepted: 22 July 2018 / Published: 8 August 2018
(This article belongs to the Special Issue Electrochemical Surface Science: Basics and Applications)
Browse Figures
Versions Notes

Abstract

:
Adhesion of polymers to surfaces is of the upmost importance in timely applications such as protective coatings, biomaterials, sensors, new power sources and soft electronics. In this context, this work examines the role of molecular interactions in the adhesion of polypyrrole thin films to flexible Indium Tin Oxide (ITO) electrodes grafted with aryl layers from various diazonium salts, namely 4-carboxybenzenediazonium (ITO-CO2H), 4-sulfonicbenzenediazonium (ITO-SO3H), 4-N,N-dimethylbenzenediazonium (ITO-N(CH3)2), 4-aminobenzenediazonium (ITO-NH2), 4-cyanobenzenediazonium (ITO-CN) and 4-N-phenylbenzenediazonium (ITO-NHPh). It was demonstrated that PPy thin layers were adherent to all aryl-modified surfaces, whereas adhesive failure was noted for bare ITO following simple solvent washing or sonication. Adhesion of polypyrrole was investigated in terms of hydrophilic/hydrophobic character of the underlying aryl layer as probed by contact angle measurements. It was found that sulfonic acid-doped polypyrrole (PPy-BSA) thin films were preferably deposited on the most hydrophobic surfaces. More importantly, the redox properties and electrochemical impedance of PPy were closely related to the hydrophobic character of the aryl layers. This work demonstrates that diazonium compounds are unique molecular glues for conductive polymers and permit to tune their interfacial properties. With robust, diazonium-based architectured interfaces, one can design high performance materials for e.g., sensors, printed soft electronics and flexible thermoelectrics.

Graphical Abstract

1. Introduction

In the recent decades, conductive organic polymers have attracted a great deal of interest due to their salient features, namely electric, thermoelectric, electronic, optical, ion exchange, and reinforcing properties, to name but a few [1,2,3]. Particularly, polypyrrole (PPy) constitutes an excellent material for building electronic devices [4], because of its ease of preparation by a variety of chemical [5], electrochemical [6], biocatalytic [7], and radiation-induced [8,9] methods. In addition, it is possible to tune its electrical conductivity upon doping, copolymerization, blending and nanostructuring [10,11].
Several studies have been focused on the effects solvent nature [12], electrolyte [13], potential or current density [14,15], concentration of monomer [16], composition of working electrode (metals, glassy carbon, ITO) [17,18], on the electrochemical deposition and mechanical properties of PPy films. However, the performances as well as the long-term stability of polymer thin films is often limited by various factors, including poor adhesion to the substrate and migration of metal contamination from the substrate to the polymer [19]. To overcome these adhesion problems, coupling agents such as organosilanes and aryl layers from diazonium salts proved to efficiently anchor PPy [20,21,22,23] and other insulating polymers [24,25,26] to a large variety of materials surfaces. These coupling agents not only impart strong adhesive bonding to the conductive polymer top layer, but also limit or suppress the diffusion in the film of elements leached from the substrates [21].
While organosilanes are the main coupling agents used in adhesion science and technology [27,28,29], studies of diazonium salts as adhesion promoters remain sparse. This is due to the fact that the diazonium interface chemistry is still incipient. The interest of diazonium salts lies in their fast reaction with practically all surfaces, triggered by chemicals, reactive surfaces or radiations [30]. In addition, diazonium compounds are versatile as they impart functionalities to the materials surfaces depending on the nature of the substituent borne by the benzene ring in either para-, meta- or ortho- position of the diazonium group. In this way, one can design new surfaces possessing a plethora of properties such as hydrophilic/hydrophobic character [31], electrocatalytic activity [32], initiation of surface radical or oxidative polymerization [33,34,35], resistance to biofouling [36], sensing [37,38], reactivity towards polymer matrices [39,40].
However, to the best of our knowledge, no systematic study comparing the effect of several functional groups on the adhesion of diazonium-modified surfaces has been undertaken. Therefore, the objective of this work is to examine the propensity of a series of diazonium compounds to provide ultrathin adhesive layers to conductive polymer top coatings and to check to which extent the said aryl adhesive layers govern the physicochemical properties of the polymer top coatings. To illustrate the actual study, polypyrrole was selected as a reactive and functional polymer coating, and flexible ITO as a substrate which is generally difficult to coat with conductive polymers for obtaining long-term robust adhesion.
Herein, we investigated the electrografting of a series of diazonium compounds of the general formulae N2-C6H4-R, with R in para position =CO2H, SO3H, N(CH3)2, NH2, CN and NH-Ph. The hydrophobic/hydrophilic character of aryl-modified ITO electrodes was probed by means of contact angle measurements of water drops. The modified flexible ITO sheets further served as platforms for the electrodeposition of benzene sulfonic acid-doped polypyrrole (PPy-BSA), and the electrochemical properties were interrogated by cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical properties of PPy-BSA coatings will be discussed in terms of surface composition and wettability of the underlying flexible aryl-modified ITO sheets.

2. Materials and Methods

2.1. Reagents

4-Aminobenzoic acid, 4-aminobenzene sulfonic acid, N,N-dimethyl-1,4-phenylenediamine, 1,4-phenylenediamine, 4-aminobenzonitrile, N-phenyl-4-phenylenediamine, were purchased from Sigma Aldrich. Sodium nitrite (≥99.0%, Sigma-Aldrich, St. Louis, MO, USA), chloric acid (37%), ferrocene (Sigma-Aldrich) and potassium chloride were used without further purification. Pyrrole (purity ≥ 98%, Aldrich) and benzene sulfonic acid were refrigerated prior to synthesis. All aqueous solutions were prepared using Milli-Q ultrapure water (MQ 18.2 MΩ·cm). ITO-coated polyethylene naphtalate (PEN) sheets (Peccel, Yokohama, Japan) were cut to 5 × 10 mm with scissors and served as working electrodes.

2.2. Diazonium Modification of Flexible ITO

The diazonium cations were generated in situ from the parent aromatic amines. The aromatic amines and the corresponding modified electrodes ITO-R (R = functional group) were the following: 4-aminobenzoic acid (ITO-CO2H), 4-aminobenzene sulfonic acid (ITO-SO3H), N,N-dimethyl-1,4-phenylenediamine (ITO-N(CH3)2), 1,4-phenylenediamine(ITO-NH2), 4-aminobenzonitrile (ITO-CN), N-phenyl-p-phenylenediamine (ITO-NH-Ph). The ITO surface was cleaned by sonication in a mixture of water/ethanol 80:20 (v/v) for 2 min. After sonication, the electrode was washed with DI water. The electrografting of aryl layers from the diazonium salts generated in situ was performed using a three-electrode system, including an ITO-coated polyethylene naphthalate (PEN) working electrode (area~5 × 5 mm), a stainless steel grid as counter electrode and a Ag/AgCl (saturated KCl) reference electrode.
The diazonium salt was produced by mixing 0.1 mmole of aromatic amine (equivalent to 13.7, 17.3, 13.6, 10.8, 11.8, or 18.4 mg of 4-aminobenzoic acid, 4-aminobenzene sulfonic acid, N,N-dimethyl-1,4-phenylenediamine, 1,4-phenylenediamine, 4-aminobenzonitrile, or N-phenyl-4-phenylenediamine, respectively) with 90 mL of 0.5 M HCl in a glass bath for 30 min, before adding 10 mL of 10 mM NaNO2 aqueous solution (this 10 mL solution contains 0.1 mmol NaNO2). The resulting mixture was stirred for 1 h. The electrografting was then carried out in potentiostatic conditions (−0.8 V/SCE for 45 s), using a Biologic SP 150 potentiostat (Seyssinet-Pariset, France), computer-controlled with an EC-lab software. The blocking effect of the bare and modified ITO electrodes was investigated in a 1 mM Fe(CN) 3−/4− redox solution by cyclic voltammetry (CV) in the −0.4 and 0.6 V/SCE range at a scan rate of 50 mV·s−1. The relative electroactivity of the modified electrode, which is inversely related to its blocking effect properties, was quantified by CV in the presence of the redox probe species. The Parameter Irel is defined as:
I r e l = I pa   w i t h   a   f i l m I pa   for   a   bare   ITO × 100
where Ipa is the intensity of the anodic peak current. Electrochemical impedance spectroscopy measurements were performed at open circuit potential in 1 mM K3Fe(CN)6 and 0.1 M KCl.

2.3. Preparation and Electrochemical Characterization of Polypyrrole Films on Diazonium-Modified ITO

Prior to the electropolymerization, diazonium-modified ITO electrodes were sonicated in a water/ethanol 80:20 (v/v) mixture for 2 min. The PPy on the ITO modified aryl layer was prepared by electropolymerization in a 0.01 M BSA + 0.1 M pyrrole aqueous solution (20 mL) by CV in a potential range of −1.0 to 1.2 V. The fresh PPy films were characterized by CV under the experimental conditions indicated in Section 3.
The impedance spectra were recorded in the 0.1 Hz to 100 kHz frequency range.

2.4. Surface Analysis

The X-ray photoelectron spectra (XPS) were recorded using a K Alpha apparatus (Thermo Fisher Scientific, East Grinsted, UK) fitted with a monochromatic Al X-ray source (hν = 1486.6 eV; spot size = 400 µm). A flood gun was used for static charge compensation.
An Easy Drop Krüss instrument (Hamburg, Germany) was used to determine the contact angles of water drops deposited on diazonium-modified ITO and on PPy samples.

3. Results and Discussion

3.1. Brief Description of the Strategy and the Objectives of the Work

Six diazonium compounds of the general formulae N2-C6H4-R, with R = CO2H, SO3H, N(CH3)2, NH2, CN and NH-Ph, were selected for the preparation of aryl layers on flexible ITO surfaces. Electrochemistry was selected as a means of attaching aryl monolayers, because it is a simple and straightforward method which can be conducted in aqueous medium. Diazonium compounds were generated in situ from the commercially-available parent aromatic amines; this adds up to the versatility of chemical surface engineering with diazonium compounds. The choice of the functional group in para position of the diazonium depends on two criteria: the electron-donor or electron-acceptor character of the functional group, on the one hand, and its propensity to govern the wetting of ITO surfaces, on the other hand.
Figure 1 depicts the chemical structures of the in situ generated diazonium cations (Upper panel) and the simple strategy adopted to attach PPy in two steps (Lower panel). Attachment of an aryl adhesive layer was followed by electrodeposition of PPy. It is to note that no physicochemical study will be provided on the adhesion of PPy on bare ITO, which is known to be very poor, as previously reported [22]. Instead, we will concentrate the present investigation on the diazonium susbstituent effect on the PPy adhesion and its redox behaviour.
Hereafter, the study is split into three main sections: (i) attachment and properties of aryl layers, (ii) electropolymerization and behaviour of the PPy topcoat, and (iii) adhesion aspects of PPy top coat to modified ITO sheets.

3.2. Electrochemical Modification of Flexible ITO Sheets with Diazonium Salts

Aryl layers were electrografted by chronoamperometry by setting the potential to −0.8 V/SCE for 45 s. This electrografting period of time was sufficient for good polypyrrole adhesion without inducing any major blocking effect that hampers the deposition of the polymer [22,41]. The blocking effect of the aryl layers was assessed using Fe(CN)63−/4−, as depicted in Figure 2. The quasi-reversible redox wave obtained with bare ITO (Figure 2A(a), peaks at 0.25 and 0.1 V) suffered a decrease in the intensity for ITO-N(CH3)2 (Figure 2A(b), peaks at 0.38 and 0.1 V), but disappeared quasi-completely in the case of ITO-SO3H (Figure 2A(c)), indicating a blocking effect of the aryl layer as reported by several authors [22,42,43,44,45,46,47]. The relative Fe(CN)63−/4− redox peak intensity (Irel) decreased in the order ITO > ITO-N(CH3)2 > ITO-NH2 > ITO-NH-Ph > ITO-CN > ITO-CO2 H > ITO-SO3H (see Table 1). The influence of the aryl grafting to ITO electrodes is thus straightforward since the response of Fe(CN)63−/4− is flattened for modified ITO plates compared to bare ITO strips. From measurements similar to those of Figure 2A, Irel values were computed and reported in Table 1 for all aryl layers. The ITO-N(CH3)2, ITO-NH-Ph and ITO-NH2 surfaces exhibit a blocking effect for Fe(CN)63−/4− redox system reactions with Irel values of 49, 46.7, 23.6%, respectively. The highest blocking effects correspond to ITO-CN, ITO-CO2H and ITO-SO3H, with Irel values of 15.7, 11.2 and 3.9%, respectively. Under the same grafting conditions, the blocking effect appear to be more important for the ITO-SO3H modified electrode [48].
The Nyquist plots for ITO-SO3H and ITO-N(CH3)2 electrodes (Figure 2B) present typical semicircle shapes in the high-frequency domain, characteristic of an interfacial charge-transfer mechanism. The surfaces exhibit straight lines in the low-frequency regime, characteristic of a semi-infinite diffusion phenomenon. ITO-N(CH3)2 displays the lowest charge transfer resistance (Rct), ca. 7.15 kΩ/cm2 compared to 68.8 kΩ/cm2 for ITO-SO3H, a trend that is in agreement with previously-reported studies on glassy carbon electrodes [48]. Table 1 reports Rct values for all diazonium-modified ITO surfaces and clearly indicates the general following trend: low values for electron-donor aryl groups (7.15–9.32 kΩ cm2) and ~5–10-fold higher Rct values (44.0–68.8 kΩ cm2) for electron-acceptor aryl groups.
Figure 3 shows images of water drops deposited on the various ITO-R surfaces. For R = N(CH3)2, NH2 and NH-Ph, the contact angles were found to be 136.7 ± 0.1, 116 ± 0.4 and 103.1 ± 0.3°, respectively. Taking into account the accepted rule of thumb, the water contact angle (θ) values above 90° indicated hydrophobic surfaces. In contrast, more wetting was observed for the electron-acceptor groups R = CN, COOH and SO3H, the water contact angles of which are 81.2 ± 0.3, 70.7 ± 0.4 and 48.7 ± 0.5°, respectively. These surfaces can thus be ranked as relatively hydrophilic (θ < 90°). As expected, the layers with more ionizable groups COOH and SO3H produced the lowest contact angles and thus led to the highest hydrophilicity. It is thus possible to control the hydrophobic/hydrophilic character of flexible ITO surfaces by simply changing the radical R in para position of the diazonium group.
Figure 4 displays plots of the electrochemical characteristics of the aryl layers versus their wetting property, expressed by the water drop contact angle values. For the hydrophilic surfaces, the barrier to the Fe(CN)63−/4− probe is most probably due to the favourable water-aryl interfacial interactions. Under such wetting conditions, the probe molecules hardly reach the aryl-modified ITO surfaces and undergo redox reactions. Higher Irel values indicate weaker barrier, and the probe interact more favorably with the surfaces compared to water.
As far as Rct is concerned, this physical property decreases for hydrophobic aryl-modified ITO surfaces (θ > 90°). These results are due to the steric hindrance or the physical barrier role played by the free pair of the organic molecules [48].
All in one, the results can be discussed in terms of hydrophobic/hydrophilic effects [49]. Indeed, Fe(CN)63−/4− redox probe is relatively hydrophilic [49] and can rather interact with water instead of the aryl-modified surface. In addition, the hydrophilic aryl groups are negatively charged which may add up electrostatic repulsions at the ITO-aryl/probe interface.

3.3. Electrodeposition of Polypyrrole on ITO

3.3.1. Electrochemical Characterization of Poly(pyrrole-benzene sulfonic acid) Films on Aryl-Modified ITO

The aryl layers served as a primer for the electrodeposition of PPy-BSA. We achieved the electropolymerization on aryl-modified ITO strips by performing ten potential scanning cycles between −1 and 1.2 V/ECS at 100 mV·s−1 in an aqueous solution of 0.1 M of pyrrole and 0.01 M BSA (Figure 5). Table 2 reports the Epox and Epred values for the electropolymerization process together with the oxidation peak density (Jpox). Highly uniform and adherent PPy-BSA films were easily obtained on all modified electrodes, regardless the nature of the diazonium compound. The films obtained on the hydrophobic surfaces (ITO-N(CH3)2, ITO-NH2, ITO-NH-Ph) were much more conductive than those obtained on the hydrophilic surfaces (ITO-CO2H, ITO-SO3H, ITO-CN), as deduced from the corresponding high current in the CV curves.
As a matter of fact, the oxidation peak is centered at 0.25, 0.27, and 0.2 V/SCE for ITO-N(CH3)2, ITO-NH2 and ITO-NH-Ph electrodes, respectively. These peaks are positioned near those obtained on the hydrophilic surfaces ITO-CN and ITO-SO3H (0.25 and 0.24 V/SCE, respectively). In contrast, on ITO-CO2H the oxidation peak is centred at a higher potential value (0.41 V/SCE) and the reduction peak at −0.59 V/SCE. This is the largest potential difference for the electropolymerization of pyrrole on the actual series of diazonium-modified ITO electrodes. The density of Jpox of PPy-BSA on electrodes materials takes the maximum values for hydrophobic surface (972 µA/cm2 on ITO-N(CH3)2), hence the highest conductivity achieved for electropolymerization of pyrrole on the actual electrode materials.
The results obtained so far show that thin films of PPy were preferentially deposited on hydrophobic surfaces, in line with the literature [50,51].
Figure 6 shows the blocking effect and EIS results of PPy films on all aryl-modified ITO electrodes. The blocking effect of PPy on all aryl layers was assessed with Fe(CN)63-/4-, as depicted in Figure 6A. The quasi-reversible redox couple (located at 0.25 V and 0.1 V) obtained on the hydrophobic surfaces was characterized by a higher current, which accounted for a more conductive surface. It is worth to note that the PPy coating on ITO-N(CH3)2, ITO-NH2, ITO-NH-Ph (Figure 6B) exhibited the lowest charge transfer resistance (0.15, 0.36, 0.6 kΩ/cm2, respectively). Moreover, the Rct increased for PPy on ITO-SO3H, ITO-CO2H, and ITO-CN; the impedance curves were significantly broadened and the semi-circle diameters were about 28.59, 21.44 and 10 kΩ/cm2 respectively. These results indicated that ITO-SO3H had the largest obstruction, which resulted in reducing electron-transfer rate or increasing resistance to the flow of electrons. Furthermore, EIS showed that charge transfer resistance of ITO-N(CH3)2-PPy was much lower compared to others.
The results reported so far stress the paramount importance of the hydrophobic character of the substrate, here imparted by the aryl layers. As a matter of fact, PPy is relatively hydrophobic, and preferentially binds to hydrophobic surfaces [52]. Conversely, the strength of (macro) molecule-polypyrrole molecular interactions are maximized in water [53].

3.3.2. XPS Characterization of PPy Top Coats and Reference Materials

Figure 7 displays survey regions of ITO, ITO-SO3H, ITO-N(CH3)2, ITO-N(CH3)2-PPy samples in the 100–600 eV range. S2p, C1s, N1s, and In3d/Sn3d doublets are centered at 168, 285, 400, 445–453 and 487–496 eV, respectively.
Bare ITO (Figure 7a) exhibits a quasi-horizontal background in the 160–285 eV spectral region and a sharp In3d doublet together with Sn3d peaks. Attachment of an aryl monolayer screened ITO (Figure 7b,c) therefore resulting in an increase of the relative C1s peak intensity, and in the appearance of an S2p peak at ~168 eV for ITO-SO3H (Figure 7b) and of a N1s peak at 400 eV for ITO-N(CH3)2 (Figure 7c). After electropolymerization, the survey region resembled that of pure polypyrrole with effective screening of the characteristic In3d and Sn3d features from ITO (Figure 7d).
All apparent compositions (in at. %) are reported in Table 2 for the main elements. The carbon and nitrogen contents increased in the order ITO < ITO-aryl < ITO-aryl-PPy. Sulfur content is due to the sulfonate group from ITO-SO3H, and to all PPy-BSA topcoats. This sulfur content is invariably higher for PPy-BSA than for ITO-SO3H. In contrast, there is a clear decrease for the O at. %, as one shifts from ITO to ITO-aryl, then PPy. Indium and tin atoms are detected for bare and aryl-modified ITO surfaces only. After electropolymerization, In and Sn could not be detected since XPS is surface specific technique (about 12–15 nm depth) while PPy topcoat thickness is in the micrometer scale range (~3 µm, SEM image not shown). This is a much too large thickness for a topcoat to possibly detect the buried layers using XPS (10–12 nm sampling depth for organic coatings).
Table 3 provides also the In4d/In3d intensity ratio for bare and aryl-modified ITO sheets. Invariably, this ratio is higher for the modified ITO sheets. Indeed, an aryl layer screens ITO, but at a different extent for the In4d and In3d core electrons. The sampling depth of In4d is about 12.6 nm higher than that of In3d (10.8 nm) [47]. It follows that In3d peak attenuation is larger, hence In4d/In3d intensity ratio increases upon aryl grafting. All aryl layers have nanometer scale thickness well below 10.8 nm (the sampling depth of In3d).
As the blocking effect can be due to the thickness or grafting density of the top layer, we were tempted to correlate Irel (in Table 1) to I4d/I3d XPS intensity ration (in Table 3). The correlation is poor and the data corresponding to ITO-NH2 (I4d/I3d = 0.159; Irel = 46.6%) differs markedly from the trend given by the 5 other surfaces. As a matter of fact, Baranton and Bélanger [48] clearly indicated that blocking effect cannot be simply related to the film thickness or the surface coverage. Herein, we do confirm such an observation previously reported [48] and confirm that the hydrophilic/hydrophobic effects operate at the interface, which gives full credit to Downard and Prince [49]. It follows that barrier properties cannot be interpreted simply in terms of coverage or thickness; herein we demonstrate that barrier properties are very likely to be correlated to the hydrophobic/hydrophilic character of the ITO-aryl surfaces as shown in Figure 4a,b.

3.4. Practical Adhesion Aspects of Polypyrrole Thin Layers

Practical adhesion of PPy thin layers has been qualitatively investigated by solvent washing, ultrasonication and bend stress. Simple solvent washing using a laboratory wash bottle demonstrates the efficiency of the diazonium adhesive layer in anchoring PPy to ITO. This is shown in a video available in the Supplementary Material SM Video1. This video shows that without any aryl adhesive layer, PPy is simply and easily removed upon washing with deionized water. The middle part of the video (starting at 0.48 min) shows that another PPy film grown on bare ITO is very easily delaminated using tweezers. In the last part of the video (starting at 1.07 min), prolonged washing of PPy topcoat of ITO-NH2 electrode does not induce any adhesion failure. We have then compared adhesion of PPy to bare ITO and ITO-NH2 using ultrasonication (video not shown): again, the aryl layer prevents adhesion failure of the PPy, whereas on bare ITO the film is removed previously reported [22]. Actually, the removal of PPy from ITO-NH2 was only possible through scratching. Such robust adhesion of polypyrrole to aminophenyl-grafted glassy carbon electrode found by Patterson and Ignaszak [23] confirms our previous findings on ITO-NH2 and supports the actual work which generalizes the strong adhesion of polypyrrole to flexible ITO and other electrodes.
One of the important aspects of polymer adhesion is to define the type of adhesion bonding. In a previous publication, Samanta et al. [47] have demonstrated that photochemically-prepared PPy/silver nanocomposite film was removed using adhesive tape from bare ITO, whereas no failure occurred in the case of pyrrole-functionalized ITO sheets. A failure within the polymer film is defined as “cohesive failure”. Herein, ultrasonication of ITO-PPy resulted in the delamination of the conductive polymer topcoat. XPS inspection of the ITO fracture surface, after ITO-PPy adhesion failure, brought strong supporting evidence for a neat ITO surface without any attenuation of the characteristic peaks from ITO (see Survey region in Figure 8a). It is worthwhile to note that the C1s peak is weakly intense and is partly due to unavoidable adventitious contamination during the transfer from atmosphere to the vacuum chamber of the XPS apparatus. The spectrum displayed in Figure 8a is a clear sign of an “adhesive failure”, ca exactly at the ITO-PPy interface. Both materials are split apart upon ultrasonication.
We also examined the propensity of the ITO-NH2-PPy hybrid electrodes to withstand bending stress. The ITO-NH2-PPy test electrode was prepared as described above and stuck on the outer wall of a tube for 24 h bending stress; the bending radius was 3.2 mm as measured using an electronic digital caliper (Fowler, model S225; Newton, MA, USA). This is a small radius and accounts for an important stress [20]. Compared to the unbent sample, the PPy film withstood the very high bending stress and remained stuck to the underlying ITO-NH2 substrate. However, we found a slight increase in the In/C and Sn/C atomic ratios, which probably resulted from a possible diffusion of In and Sn in the top coat. Indeed, SEM inspection of the ITO-NH2-PPy surface after bending showed a crack in the ITO layer (Figure 8b).
From these qualitative tests, clearly the conductive polymer films strongly adhere to the ITO surface due to the molecular interactions that operate at interfaces between PPy and all diazonium-modified ITO flexible electrodes. The aryl groups were shown to be bridged to the oxide surface through metal-O-C bonds [54,55]. We have selected functional groups in para position, that undergo either electrostatic interactions or electron-donor/acceptor bonds (Figure 9). Although not covalent in nature, these interfacial interactions are sufficiently strong to keep PPy anchored to the ITO via the aryl layer. The deposition of PPy on aryl-modified ITO is simple, fast, and provides strong adhesion that withstands severe conditions, even under high mechanical stress. This contrasts with the poor adhesion of polypyrrole to bare ITO, as previously reported in the literature [19].

4. Conclusions

In this work, flexible ITO sheets were grafted using in situ-generated diazonium salts bearing electron acceptor (COOH, SO3H and CN) or electron-donor (NH2, N(CH3)2, NH-Phenyl) groups. We have carefully grafted monolayers which were thin enough to prevent passivation but which permitted to prepare adherent polypyrrole top coatings by electropolymerization. The aryl layers were characterized by contact angle measurements and by XPS. Interestingly, the blocking effects and charge transfer resistance of the aryl layers were correlated with their hydrophobic character. All aryl layers served as remarkable coupling agents for the polypyrrole top coats which withstand sonication, solvent washing and bend stress. On bare ITO, PPy could be removed by simple washing with water or using tweezers, without any trace of polymer left on the surface as judged from XPS analysis of the fracture surface. This accounts for adhesive failure of the coating. In contrast, on all aryl-modified ITO sheets no delamination took place, and the polymer could be only removed by scratching the polypyrrole film. The adherent polypyrrole exhibited a better redox activity in terms of high current output on the most hydrophobic surfaces obtained with aminated, electron-donor groups, thus indicating the possibility to tune both the adhesion strength and redox properties of the conductive polymer. Adhesion of polypyrrole to ITO via aryl layers, is likely to be due to interfacial electrostatic interactions between the electron acceptor groups COOH/COO−, SO3H/ SO3H and CN/C+ = N− and the polypyrrole positively charged backbone. As far as the nitrogen-containing electron-donor groups, such as NH2, N(CH3)2 and NH-Phenyl, are concerned, the adhesion is attributed to n-σ* electron-donor-acceptor interactions (the pyrrole N-H bond being the σ* acceptor).
This work sheds a new light on the use of diazonium salts to make adhesive primer layers for electroactive polymers and other organic coatings. As silanes in the 20th century, so diazonium salts could be regarded as magic molecular glues for tuning interfacial properties of next generation advanced materials of relevance to sensors, printed soft electronics and flexible thermoelectrics to name but these timely remarkable applications.

Supplementary Materials

The following are available online at https://www.mdpi.com/2571-9637/1/1/5/s1.

Author Contributions

Conceptualization of the research work by M.L., M.M.C., D.G.-S. and K.D.; Methodology by M.L. and M.M.C.; Validation by M.M.C., K.D., D.G.-S., J.-J.A.; Formal Analysis by M.L., R.P., M.M.C.; Writing of Original Draft was done by M.L. and M.M.C.; Writing: Review and Editing, M.L., M.M.C., J.-J.A. and M.A.O.; Supervision, M.M.C. and D.G.-S.; Funding Acquisition, J.-J.A., M.A.O. and D.G.-S.

Funding

One of us (Momath Lo) gratefully thanks the Cooperation and Cultural Action Service of the French Embassy in Senegal for a PhD grant.

Acknowledgments

All authors thank S. Chehimi (EFREI, Villejuif, France) for the video making presented as Supplementary Material.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. MacDiarmid, A.G. Polyaniline and polypyrrole: Where are we headed? Synth. Met. 1997, 84, 27–34. [Google Scholar] [CrossRef]
  2. George, P.M.; Lyckman, A.W.; LaVan, D.A.; Hegde, A.; Leung, Y.; Avasare, R.; Testa, C.; Alexander, P.M.; Langer, R.; Sur, M. Fabrication and biocompatibility of polypyrrole implants suitable for neural prosthetics. Biomaterials 2005, 26, 3511–3519. [Google Scholar] [CrossRef] [PubMed]
  3. Bharti, M.; Singh, A.; Samanta, S.; Debnath, A.K.; Aswal, D.K.; Muthe, K.P.; Gadkari, S.C. Flexo-green Polypyrrole—Silver nanocomposite films for thermoelectric power generation. Energy Convers. Manag. 2017, 144, 143–152. [Google Scholar] [CrossRef]
  4. Sun, X.; Zhang, H.; Zhou, L.; Huang, X.; Yu, C. Polypyrrole-coated zinc ferrite hollow spheres with improved cycling stability for lithium-ion batteries. Small 2016, 12, 3732–3737. [Google Scholar] [CrossRef] [PubMed]
  5. Omastová, M.; Trchová, M.; Kovárová, J.; Stejskal, J. Synthesis and structural study of polypyrroles prepared in the presence of surfactants. Synth. Met. 2003, 138, 447–455. [Google Scholar] [CrossRef]
  6. Sadki, S.; Schottland, P.; Brodie, N.; Sabouraud, G. The mechanisms of pyrrole electropolymerization. Chem. Soc. Rev. 2000, 29, 283–293. [Google Scholar]
  7. Cruz-Silva, R.; Amaro, E.; Escamilla, A.; Nicho, M.E.; Sepulveda-Guzman, S.; Arizmendi, L.; Romero-Garcia, J.; Castillon-Barraza, F.F.; Farias, M.H. Biocatalytic synthesis of polypyrrole powder colloids and films using horseradish peroxidase. J. Colloid Interface Sci. 2008, 328, 263–269. [Google Scholar] [CrossRef] [PubMed]
  8. Saad, A.; Cabet, E.; Lilienbaum, A.; Hamadi, S.; Abderrabba, M.; Chehimi, M.M. Polypyrrole/Ag/mesoporous silica nanocomposite particles: Design by photopolymerization in aqueous medium and antibacterial activity. J. Taiwan Inst. Chem. Eng. 2017, 80, 1022–1030. [Google Scholar] [CrossRef]
  9. Hamouma, O.; Oukil, D.; Omastová, M.; Chehimi, M.M. Flexible paper@carbonnanotube@polypyrrole composites: The combined pivotal roles of diazonium chemistry and sonochemicalpolymerization. Colloids Surf. Physicochem. Eng. Asp. 2018, 538, 350–360. [Google Scholar] [CrossRef]
  10. Omastová, M.; Mičušík, M. Polypyrrole coating of inorganic and organic materials by chemical oxidative polymerization. Chem. Pap. 2012, 66, 392–414. [Google Scholar] [CrossRef]
  11. Baibarac, M.; Gómez-Romero, P. Nanocomposites based on conducting polymers and carbon nanotubes: From fancy materials to functional applications. J. Nanosci. Nanotechnol. 2006, 6, 289–302. [Google Scholar] [CrossRef] [PubMed]
  12. Lee, S.; Sung, H.; Han, S.; Paik, W. Polypyrrolefilm formation by solution-surface electropolymerization: Influence of solvents and doped anions. J. Phys. Chem. 1994, 98, 1250–1252. [Google Scholar] [CrossRef]
  13. Ferreira, C.A.; Aeiyach, S.; Delamar, M.; Lacaze, P.C. Electropolymerization of pyrrole on iron electrodes: Influence of solvent and electrolyte on the nature of the deposits. J. Electroanal. Chem. Interfacial Electrochem. 1990, 284, 351–369. [Google Scholar] [CrossRef]
  14. Kaplin, D.A.; Qutubuddin, S. Electrochemically synthesized polypyrrolefilms: Effects of polymerization potential and electrolyte type. Polymer 1995, 36, 1275–1286. [Google Scholar] [CrossRef]
  15. Raso, M.A.; González-Tejera, M.J.; Carrillo, I.; Sanchez De La Blanca, E.; García, M.V.; Redondo, M.I. Electrochemical nucleation and growth of poly-N-methylpyrrole on copper. Thin Solid Films 2015, 19, 2387–2392. [Google Scholar] [CrossRef]
  16. Sharifirad, M.; Omrani, A.; Rostami, A.A.; Khoshroo, M. Electrodeposition and characterization of polypyrrolefilms on copper. J. Electroanal. Chem. 2010, 645, 149–158. [Google Scholar] [CrossRef]
  17. Alfaro-López, H.M.; Aguilar-Hernandez, J.R.; Garcia-Borquez, A.; Hernandez-Perez, M.A.; Contreras-Puente, G.S. Electropolymerization of polypyrrole films in aqueous solution with side-coupler agent to hydrophobic groups. Interface Control. Org. Thin Films 2009, 129, 73–78. [Google Scholar]
  18. Wang, Y.; Northwood, D.O. An investigation into the nucleation and growth of an electropolymerized polypyrrole coating on a 316L stainless steel surface. Thin Solid Films 2008, 516, 7427–7432. [Google Scholar] [CrossRef]
  19. Castro-Beltran, A.; Dominguez, C.; Bahena-Uribe, D.; Sepulveda-Guzmana, S.; Cruz-Silva, R. Effect of non-electroactive additives on the early stage pyrrole electropolymerization on indium tin oxide electrodes. Thin Solid Films 2014, 566, 23–31. [Google Scholar] [CrossRef] [Green Version]
  20. Singh, A.; Salmi, Z.; Joshi, N.; Jha, P.; Kumar, A.; Lecoq, H.; Lau, S.; Chehimi, M.M.; Aswal, D.K.; Gupta, S.K. Photo-induced synthesis of polypyrrole-silver nanocomposite films on N-(3-trimethoxysilylpropyl)pyrrole-modified biaxially oriented polyethylene terephthalate flexible substrates. RSC Adv. 2013, 3, 5506–5523. [Google Scholar] [CrossRef]
  21. Bouktit, B.; Salmi, Z.; Decorse, P.; Lecocq, H.; Jouini, M.; Aswal, D.; Singh, A.; Chehimi, M.M. Polypyrrole/Ag nanocomposite films on diazonium salt modified indium tin oxide substrate. J. Colloid Sci. Biotechnol. 2013, 2, 200–210. [Google Scholar] [CrossRef]
  22. Lo, M.; Diaw, A.K.D.; Gningue-Sall, D.; Aaron, J.-J.; Oturan, M.A.; Chehimi, M.M. The role of diazonium interface chemistry in the design of high performance polypyrrole-coated flexible ITO sensing electrodes. Electrochem. Commun. 2017, 77, 14–17. [Google Scholar] [CrossRef]
  23. Patterson, N.; Ignaszak, A. Modification of glassy carbon with polypyrrole through an aminophenyl linker to create supercapacitive materials using bipolar electrochemistry. Electrochem. Commun. 2018, 93, 10–14. [Google Scholar] [CrossRef]
  24. Mevellec, V.; Rousse, S.; Tessier, L.; Chancolon, J.; Mayne-L’Hermite, M.; Deniau, G.; Viel, P.; Palacin, S. Grafting polymers on surfaces: A new powerful and versatile diazonium salt-based one-step process in aqueous media. Chem. Mater. 2007, 19, 6323–6330. [Google Scholar] [CrossRef]
  25. Viel, P.; Le, X.T.; Huc, V.; Bar, J.; Benedetto, A.; Le Goff, A.; Filoramo, A.; Alamarguy, D.; Noël, S.; Baraton, L.; et al. Covalent grafting onto self-adhesive surfaces based on aryldiazonium salt seed layers. J. Mater. Chem. 2008, 18, 5913–5920. [Google Scholar] [CrossRef] [Green Version]
  26. Alageel, O.; Abdallah, M.N.; Luo, Z.Y.; Del-Rio-Highsmith, J.; Cerruti, M.; Tamimi, F. Bonding metals to poly(methyl methacrylate) using aryldiazonium salts. Dent. Mater. 2015, 31, 105–114. [Google Scholar] [CrossRef] [PubMed]
  27. Watts, J.F.; Rattana, A.; Abel, M.-L. Interfacial chemistry of adhesives on hydrated aluminium and hydrated aluminium treated with an organosilane. Surf. Interface Anal. 2004, 36, 1449–1468. [Google Scholar] [CrossRef]
  28. Jesson, D.A.; Abel, M.-L.; Hay, J.N.; Smith, P.A.; Watts, J.F. Organic-inorganic hybrid nanoparticles: Surface characteristics and interactions with a polyester resin. Langmuir 2006, 22, 5144–5151. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  29. Tashiro, H.; Nakaya, M.; Hotta, A. Enhancement of the gas barrier property of polymers by DLC coating with organosilane interlayer. Diam. Relat. Mater. 2013, 35, 7–13. [Google Scholar] [CrossRef]
  30. Bélanger, D.; Pinson, J. Electrografting: A powerful method for surface modification. Chem. Soc. Rev. 2011, 40, 3995–4048. [Google Scholar] [CrossRef] [PubMed]
  31. Wang, J.; Firestone, M.A.; Auciello, O.; Carlisle, J.A. Surface functionalization of ultrananocrystalline diamond films by electrochemical reduction of aryldiazonium salts. Langmuir 2004, 20, 11450–11456. [Google Scholar] [CrossRef] [PubMed]
  32. Mirkhalaf, F.; Graves, J.E. Nanostructured electrocatalysts immobilised on electrode surfaces and organic film templates. Chem. Pap. 2012, 66, 472–483. [Google Scholar] [CrossRef]
  33. Bakas, I.; Yilmaz, G.; Ait-Touchente, Z.; Lamouri, A.; Lang, P.; Battaglini, N.; Carbonnier, B.; Chehimi, M.M.; Yagci, Y. Diazonium salts for surface-confined visible light radical photopolymerization. J. Polym. Sci. Part A Polym. Chem. 2016, 54, 3506–3515. [Google Scholar] [CrossRef]
  34. Stockhausen, V.; Nguyen, V.Q.; Martin, P.; Lacroix, J.-C. Botom-up electrochemical fabrication of conjugated ultrathin layers with tailored switchable properties. ACS Appl. Mater. Interfaces 2017, 9, 610–617. [Google Scholar] [CrossRef] [PubMed]
  35. Wang, Y.; Fantin, M.; Park, S.; Gottlieb, E.; Fu, L.; Matyjaszewski, K. Electrochemically mediated reversible addition−fragmentation chain-transfer polymerization. Macromolecules 2017, 50, 7872–7879. [Google Scholar] [CrossRef] [PubMed]
  36. Gui, A.L.; Luais, E.; Peterson, J.R.; Godding, J.J. Zwitterionic phenyl layers: Finally, stable, anti-biofouling coatings that do not passivate electrodes. ACS Appl. Mater. Interfaces 2013, 5, 4827–4835. [Google Scholar] [CrossRef] [PubMed]
  37. Cao, C.; Zhang, Y.; Jiang, C.; Qi, M.; Liu, G. Advances on aryldiazonium salt chemistry based interfacial fabrication for sensing applications. ACS Appl. Mater. Interfaces 2017, 9, 5031–5049. [Google Scholar] [CrossRef] [PubMed]
  38. Guselnikova, O.; Postnikov, P.; Elashnikova, R.; Trusova, M.; Kalachyova, Y.; Libansky, M.; Barek, J.; Kolska, Z.; Švorčík, V.; Lyutakov, O. Surface modification of Au and Ag plasmonic thin films via diazonium chemistry: Evaluation of structure and properties. Colloids Surf. A Physicochem. Eng. Asp. 2017, 516, 274–285. [Google Scholar] [CrossRef]
  39. Jlassi, K.; Chandran, S.; Poothanari, M.A.; Benna-Zayani, M.; Thomas, S.; Chehimi, M.M. Clay/polyaniline hybrid through diazonium chemistry: Conductive nanofiller with unusual effects on interfacial properties of epoxy nanocomposites. Langmuir 2016, 32, 3514–3524. [Google Scholar] [CrossRef] [PubMed]
  40. Sandomierski, M.; Strzemiecka, B.; Chehimi, M.M.; Voelkel, A. Reactive diazonium-modified silica fillers for high-performance polymers. Langmuir 2016, 32, 11646–11654. [Google Scholar] [CrossRef] [PubMed]
  41. Jacques, A.; Chehimi, M.M.; Poleunis, C.; Delcorte, A.; Delhalle, J.; Mekhalif, Z. Grafting of 4-pyrrolyphenyldiazonium in situ generated on NiTi, an adhesion promoter for pyrrole electropolymerisation. Electrochim. Acta 2016, 211, 879–890. [Google Scholar] [CrossRef]
  42. Kullapere, M.; Mirkhalaf, F.; Tammeveski, K. Electrochemical behaviour of glassy carbon electrodes modified with aryl groups. Electrochim. Acta 2010, 56, 166–173. [Google Scholar] [CrossRef]
  43. Jacques, A.; Devillers, S.; Delhalle, J.; Mekhalif, Z. Electrografting of in situ generated pyrrole derivative diazonium salt for the surface modification of nickel. Electrochim. Acta 2013, 109, 781–789. [Google Scholar] [CrossRef]
  44. Shul, G.; Weissmann, M.; Bélanger, D. Electrochemical characterization of glassy carbon electrode modified with 1,10-phenanthroline groups by two pathways: Reduction of the corresponding diazonium ions and reduction of phenanthroline. Electrochim. Acta 2015, 162, 146–155. [Google Scholar] [CrossRef]
  45. Seck, S.M.; Charvet, S.; Fall, M.; Baudrin, E.; Geneste, F.; Lejeune, M.; Benlahsen, M. Functionalization of amorphous nitrogenated carbon thin film electrodes for improved detection of cadmium vs. copper cations. J. Electroanal. Chem. 2015, 738, 154–161. [Google Scholar] [CrossRef] [Green Version]
  46. Jiang, C.; Moraes Silva, S.; Fan, S.; Wu, Y.; Tanzirul Alam, M.; Liu, G.; Gooding, J.J. Aryldiazonium salt derived mixed organic layers: From surface chemistry to their applications. J. Electroanal. Chem. 2017, 785, 265–278. [Google Scholar] [CrossRef]
  47. Samanta, S.; Bakas, I.; Singh, A.; Aswal, D.K.; Chehimi, M.M. In situ diazonium-modified flexible ITO-coated PEN substrates for the deposition of adherent silver–polypyrrole nanocomposite films. Langmuir 2014, 30, 9397–9406. [Google Scholar] [CrossRef] [PubMed]
  48. Baranton, S.; Belanger, D. Electrochemical derivatization of carbon surface by reduction of in situ generated diazonium cations. J. Phys. Chem. B 2005, 109, 24401–24410. [Google Scholar] [CrossRef] [PubMed]
  49. Downard, A.J.; Prince, M.J. Barrier properties of organic monolayers on glassy carbon electrodes. Langmuir 2001, 17, 5581–5586. [Google Scholar] [CrossRef]
  50. Wu, J.-S.; Gu, D.-W.; Huang, D.; Shen, L.-J. Chemical in situ polymerization of polypyrrole nanoparticles on the hydrophilic/hydrophobic surface of SiO2 substrates. Synth. React. Inorg. Met. Org. Nano Met. Chem. 2012, 42, 1064–1070. [Google Scholar] [CrossRef]
  51. Wang, P.C.; Huang, Z.; MacDiarmid, A.G. Critical dependency of the conductivity of polypyrrole and polyaniline films on the hydrophobicity/hydrophilicity of the substrate surface. Synth. Met. 1999, 101, 852–853. [Google Scholar] [CrossRef]
  52. Perruchot, C.; Chehimi, M.M.; Delamar, M.; Cabet-Deliry, E.; Miksa, B.; Slomkowski, S.; Khan, M.A.; Armes, S.P. Chemical deposition and characterization of thin polypyrrole films on glass plates: Role of organosilane treatment. Colloid Polym. Sci. 2000, 278, 1139–1154. [Google Scholar] [CrossRef]
  53. Azioune, A.; Chehimi, M.M.; Miksa, B.; Basinska, T.; Slomkowski, S. Hydrophobic protein−polypyrrole interactions:  The role of van der Waals and Lewis acid−base forces as determined by contact angle measurements. Langmuir 2002, 18, 1150–1156. [Google Scholar] [CrossRef]
  54. AïtAtmane, Y.; Sicard, L.; Lamouri, A.; Pinson, J.; Sicard, M.; Masson, C.; Nowak, S.; Decorse, P.; Piquemal, J.-Y.; Galtayries, A.; et al. Functionalization of aluminum nanoparticles using a combination of aryl diazonium salt chemistry and iniferter method. J. Phys. Chem. C 2013, 117, 26000–26006. [Google Scholar] [CrossRef]
  55. Jacques, A.; Devillers, S.; Arrotin, B.; Delhalle, J.; Mekhalif, Z. Polyelectrolyte multilayers deposition on Nitinol modified by in situ generated diazonium in gentle conditions. J. Electrochem. Soc. 2014, 161, G55–G62. [Google Scholar] [CrossRef]
Surfaces 01 00005 g001 550
Figure 1. Upper panel: Chemical structures of the diazonium compounds under test. Lower panel: electrodeposition of benzene sulfonic acid-doped polypyrrole on diazonium-modified flexible ITO electrodes.
Figure 1. Upper panel: Chemical structures of the diazonium compounds under test. Lower panel: electrodeposition of benzene sulfonic acid-doped polypyrrole on diazonium-modified flexible ITO electrodes.
Surfaces 01 00005 g001
Surfaces 01 00005 g002 550
Figure 2. Electrochemical characterization of diazonium-modified ITO electrodes determined in aqueous solutions of 1 mM K3Fe(CN)6 and 0.1 M KCl. (A) Typical cyclic voltammograms (a) bare ITO, (b) ITO-N(CH3)2, and (c) ITO-SO3H. (B) Electrochemical impedance spectroscopy plots recorded with (a) ITO-N(CH3)2, and (b) ITO-SO3H.
Figure 2. Electrochemical characterization of diazonium-modified ITO electrodes determined in aqueous solutions of 1 mM K3Fe(CN)6 and 0.1 M KCl. (A) Typical cyclic voltammograms (a) bare ITO, (b) ITO-N(CH3)2, and (c) ITO-SO3H. (B) Electrochemical impedance spectroscopy plots recorded with (a) ITO-N(CH3)2, and (b) ITO-SO3H.
Surfaces 01 00005 g002
Surfaces 01 00005 g003 550
Figure 3. Contact angles of water drops gently deposited on diazonium-modified ITO: (a) ITO-N(CH3)2, (b) ITO-NH2, (c) ITO-NH-Ph, (d) ITO-CN, (e) ITO-CO2H, (f) ITO-SO3H.
Figure 3. Contact angles of water drops gently deposited on diazonium-modified ITO: (a) ITO-N(CH3)2, (b) ITO-NH2, (c) ITO-NH-Ph, (d) ITO-CN, (e) ITO-CO2H, (f) ITO-SO3H.
Surfaces 01 00005 g003
Surfaces 01 00005 g004 550
Figure 4. Correlations between the electrochemical and wetting properties of diazonium-modified flexible ITO electrodes: (a) Irel-vs-water contact angle; and (b) Rct-vs-water contact angle.
Figure 4. Correlations between the electrochemical and wetting properties of diazonium-modified flexible ITO electrodes: (a) Irel-vs-water contact angle; and (b) Rct-vs-water contact angle.
Surfaces 01 00005 g004
Surfaces 01 00005 g005a 550Surfaces 01 00005 g005b 550
Figure 5. Electrosynthesis of PPy-BSA films by cyclic voltammetry on (A) ITO-SO3H; (B) ITO-CO2H; (C) ITO-CN; (D) ITO-N(CH3)2; (E) ITO-NH-Ph; and (F) ITO-NH2.
Figure 5. Electrosynthesis of PPy-BSA films by cyclic voltammetry on (A) ITO-SO3H; (B) ITO-CO2H; (C) ITO-CN; (D) ITO-N(CH3)2; (E) ITO-NH-Ph; and (F) ITO-NH2.
Surfaces 01 00005 g005aSurfaces 01 00005 g005b
Surfaces 01 00005 g006 550
Figure 6. PPy-BSA film on (a) ITO-SO3H, (b) ITO-CN, (c) ITO-CO2H, (d) ITO-NH-Ph, (e) ITO-NH2, (f) ITO-N(CH3)2. (A) Blocking effect of the PPy-ABS films on the ITO electrodes modified by the different diazonium cations. (B) Impedance measurements recorded for PPy-ABS coatings on modified ITO sheets.
Figure 6. PPy-BSA film on (a) ITO-SO3H, (b) ITO-CN, (c) ITO-CO2H, (d) ITO-NH-Ph, (e) ITO-NH2, (f) ITO-N(CH3)2. (A) Blocking effect of the PPy-ABS films on the ITO electrodes modified by the different diazonium cations. (B) Impedance measurements recorded for PPy-ABS coatings on modified ITO sheets.
Surfaces 01 00005 g006
Surfaces 01 00005 g007 550
Figure 7. XPS survey regions of (a) ITO, (b) ITO-SO3H, (c) ITO-N(CH3)2 and (d) ITO-N(CH3)2-PPy.
Figure 7. XPS survey regions of (a) ITO, (b) ITO-SO3H, (c) ITO-N(CH3)2 and (d) ITO-N(CH3)2-PPy.
Surfaces 01 00005 g007
Surfaces 01 00005 g008 550
Figure 8. Adhesion testing of polypyrrole thin films on (a) bare ITO using XPS; and (b) ITO-NH2 using SEM and XPS. Inset of 8b shows the bar graph of In/C atomic.
Figure 8. Adhesion testing of polypyrrole thin films on (a) bare ITO using XPS; and (b) ITO-NH2 using SEM and XPS. Inset of 8b shows the bar graph of In/C atomic.
Surfaces 01 00005 g008
Surfaces 01 00005 g009 550
Figure 9. Possible molecular interactions at aryl-polypyrrole interfaces.
Figure 9. Possible molecular interactions at aryl-polypyrrole interfaces.
Surfaces 01 00005 g009
Table
Table 1. Irel, Rct and water drop contact angles (θ) obtained for aryl-modified ITO.
Table 1. Irel, Rct and water drop contact angles (θ) obtained for aryl-modified ITO.
SurfaceIrel (%)Rct (kΩ/cm2)θ (°)
ITO-SO3H3.9068.848.7 ± 0.5
ITO-CO2H11.247.370.7 ± 0.4
ITO-CN15.744.081.2 ± 0.3
ITO-NH-Ph23.69.32103.1 ± 0.3
ITO-NH246.68.97116 ± 0.4
ITO-N(CH3)248.67.15136.7 ± 0.1
Table
Table 2. Electrochemical characteristics of electrodeposition of polypyrrole top layers: Eox, Ered and Jpox values for the 6 ITO-aryl electrode materials.
Table 2. Electrochemical characteristics of electrodeposition of polypyrrole top layers: Eox, Ered and Jpox values for the 6 ITO-aryl electrode materials.
Electrode MaterialEox (V)Ered (V)Jpox (µA/cm2)
ITO-N(CH3)20.25−0.53972
ITO-NH20.27−0.52960
ITO-NH-Ph0.2−0.5936
ITO-CN0.25−0.58208
ITO-CO2H0.41−0.59908
ITO-SO3H0.24−0.43228
Table
Table 3. XPS-determined apparent surface chemical composition of ITO, ITO-aryl and ITO-aryl-PPy samples.
Table 3. XPS-determined apparent surface chemical composition of ITO, ITO-aryl and ITO-aryl-PPy samples.
MaterialsCNSOInSnIn4d/In3d
ITO32.5--39.125.43.060.155
ITO-SO3H45.20.771.0030.020.52.430.167
ITO-SO3H-PPy73.99.291.3815.4--
ITO-COOH58.61.68-25.111.33.340.174
ITO-COOH-PPy78.710.51.139.71--
ITO-CN71.44.91-16.85.881.040.179
ITO-CN-PPy75.29.232.1213.5--
ITO-NH-Ph49.21.97-29.616.23.050.171
ITO-NH-Ph-PPy75.59.831.7112.9--
ITO-NH242.91.31-33.116.26.550.159
ITO-NH2-PPy77.312.22.228.22--
ITO-N(CH3)252.71.13-28.215.52.480.181
ITO-N(CH3)2-PPy76.56.941.1815.4--

Share and Cite

MDPI and ACS Style

Lo, M.; Pires, R.; Diaw, K.; Gningue-Sall, D.; Oturan, M.A.; Aaron, J.-J.; Chehimi, M.M. Diazonium Salts: Versatile Molecular Glues for Sticking Conductive Polymers to Flexible Electrodes. Surfaces 2018, 1, 43-58. https://doi.org/10.3390/surfaces1010005

AMA Style

Lo M, Pires R, Diaw K, Gningue-Sall D, Oturan MA, Aaron J-J, Chehimi MM. Diazonium Salts: Versatile Molecular Glues for Sticking Conductive Polymers to Flexible Electrodes. Surfaces. 2018; 1(1):43-58. https://doi.org/10.3390/surfaces1010005

Chicago/Turabian Style

Lo, Momath, Rémi Pires, Karim Diaw, Diariatou Gningue-Sall, Mehmet A. Oturan, Jean-Jacques Aaron, and Mohamed M. Chehimi. 2018. "Diazonium Salts: Versatile Molecular Glues for Sticking Conductive Polymers to Flexible Electrodes" Surfaces 1, no. 1: 43-58. https://doi.org/10.3390/surfaces1010005

Article Metrics

Back to TopTop