Article pubs.acs.org/JPCC
Grafting π‑Conjugated Oligomers Incorporating 3,4Ethylenedioxythiophene (EDOT) and Thiophene Units on Surfaces by Diazonium Electroreduction Verena Stockhausen, Gaelle Trippé-Allard, Nguyen Van Quynh, Jalal Ghilane, and Jean-Christophe Lacroix* Interfaces, Traitements, Organisation et Dynamique des Systèmes, Université Paris 7-Denis Diderot, CNRS-UMR 7086, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France S Supporting Information *
ABSTRACT: The electrochemical reduction of diazonium salts, generated in situ, from 2-(4-aminophenyl)-3,4-ethylenedioxythiophene and two new amino functionalized π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) and thiophene units, has been investigated. It coats the electrodes (glassy carbon (GC), gold or ITO) with an ultrathin organic layer (less than 10 nm thickness). The X-ray photoelectron spectroscopy (XPS) investigations confirm the presence of the starting oligomers deposited on the surface. As an important result, EDOT-based oligomer grafting was achieved on those surfaces which may be of general use as adhesion primer layers in all devices using PEDOT type materials. Furthermore, the coating is electroactive and the electrochemical investigations exhibit redox signal at potentials close to that obtained for short oligoEDOT in solution. The electrochemical responses of the modified GC electrodes were further studied in the presence of various reversible redox probes, showing that the attached layer acts as a conductive switch. The switching potential of the generated layer depends on the configuration of the starting oligomers and more precisely on the relative location of the EDOT unit. Such layer behave as a barrier to electron transfer when the standard redox potential of the redox probe is below the layer switching potential; in this case, a positive potential shift of the probe oxidation peak and a diode-like behavior are observed. However, for redox probes with redox potentials above the switching potential of the grafted film, the layer is transparent toward electron transfer, and no barrier effect is observed.
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molecule devices.33−38 Electrical doping of COs in field effect transistors is the basis of their increasing use in low-cost plastic electronics.39 COs and CPs can be deposited from solution (dip coating, self-assembly), from the vapor phase, or by electrochemical oxidation of a monomer. In such cases, the bond between the substrate and the deposited oligomers is not covalent and the interface is often ill defined. As a consequence, in order to achieve efficient organic or molecular electronic devices with long lifetime, progress must be made in controlling the interface between COs units and the metallic electrodes. In other words, covalent grafting of conjugated oligomers with well-defined metal/oligomer interface retaining reversible on/ off switching capabilities, is crucial for molecular or plastic electronic devices, including light emitting diodes, organic field effect transistor; photovoltaic devices (bulk heterojunction and
INTRODUCTION Intense research efforts are being devoted on developing simple and reliable procedures to functionalize surfaces by organic molecules, and to create novel interfaces. Such interfaces are used in the fabrication of molecular,1−5 or plastic6,7 and bioelectronic devices,8,9 anticorrosion coatings,10−14 and smart surfaces.15−19 Conjugated polymers (CPs) and/or oligomers (COs) are key systems in such fields. Such materials transport and transfer charge at the molecular level along the πconjugated backbone. This property is less attenuated with distance than in saturated molecular systems or through vacuum. As a consequence, many experimental20−23 and theoretical studies,24−26 have been devoted to understand their charge transport and transfer properties in various configurations from thin layer systems to single molecule devices. Another important property of COs and CPs is their switching behavior between two states with different electron transport properties, upon electrochemical or electrical doping,. Electrochemical switching proves to be an easy means of controlling the properties of grafted molecules27−29 and of metallic nanoparticles,30−32 and is used in redox-gated single© XXXX American Chemical Society
Received: June 8, 2015 Revised: July 23, 2015
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DOI: 10.1021/acs.jpcc.5b05456 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Scheme 1. 2-Aminophenyl (oligoEDOT-Thiophene) Compounds under Study and Their Corresponding Diazoniums
graft covalently EDOT oligomers on surfaces and to generate well-defined metal/oligo(EDOT) interface and ultrathin films retaining reversible redox on/off switching at lower potential than that obtained with oligo(thiophene)-based materials. The present paper uses new diazonium cations, derived from 2-(4-aminophenyl)-3,4-ethylenedioxy-thiophene and two amino- functionalized π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) and thiophene units, as depicted in Scheme 1, and study their electrochemical reduction on surfaces at a glassy carbon (GC), gold or ITO electrodes. In doing so we wish to investigate the formation of ultrathin deposit of oligo(EDOT)-based material. Such layers are studied using XPS and electrochemistry in the presence of different redox couples in solution in order to evidence their composition and their switching behavior.
Dyes sensitized solar cells), biosensing systems, anticorrosion coatings, and smart surfaces. Among the many reactions that lead to strong attachment of organic moieties to a surface,40,41 the electroreduction of a substituted phenyldiazonium salt42 is of wide interest. It produces radicals (and release N2), which bond covalently to the electrode.43 This reaction can be performed on various materials, such as metallic surfaces,44−47 carbon,45 and silicon48 and in various media.49,50 It yields ultrathin organic layers consisting of oligo(substituted-phenyl) grafted on the surface since the deposit is insulating in the potential range required for further electrochemical reduction of the diazonium salt. As a consequence, electrodeposition generally stops when films of 2 to 5 nm are obtained. Such films are not well-defined single monolayer, as the growth generally generates multilayer films,51−55 even though progress has been recently reported in order to generate monolayers using diazonium reduction.56−59 They are less organized than self-assembled monolayer and not better organized than films generated by physisorption (spin coated or electropolymerized). The covalent bond between the aryl groups and the electrode surface has been demonstrated for carbon,60 iron surfaces47 and more recently on gold.61 Based on these findings and on the strong mechanical adherence of the layer, it is now widely accepted that covalent bonding between the organic layer and the electrode occurs.43 Conductive oligomers and ultrathin layers based on diazonium electroreduction have been initially developed separately. It is only recently that these two fields of research have come to a mutual enrichment.62 The latter brings some control of the interface between metal and oligomers; the former adds to grafted ultrathin organic layers new properties based on conductance switch, charge transfer and charge transport properties. Ultrathin junctions based on oligothiophene,63,64 with a well-defined metal/oligomer interface and reversible on/off switching capabilities, have now been demonstrated. The redox switching of such films is controlled by the electrochemical properties of the oligomers grafted on the surface and as a consequence remains close to that of polythiophene and above 0.5 V versus SCE. Poly(3,4ethylenedioxythiophene) (PEDOT) and oligoEDOT are widely used thiophene-based polymer65−68 and oligomers characterized by a lower oxidation potential than OT, by a smaller intrinsic band gap and better processability.69−71 EDOT incorporation into a π-conjugated chain enhances the π-donor ability and extends the properties of this class of material. EDOT and PEDOT based layers are integrated in an impressive amount of devices. It is thus of wide interest to
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EXPERIMENTAL SECTION Chemicals. Dissymmetric π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units and bearing nitro and amino end-groups were synthesized in good yields through Pd-catalyzed Suzuki coupling reactions and direct C− H bond arylation according to a published procedure.72,73 All chemical reagents were used as received. Ferrocene (Fc), decamethylferrocene (DmFc), N,N-dipropyl phenylenediamine (DPPDA), p-aminodiphenylamine (ADPA), and thianthrene (Th) were purchased from Aldrich. Perchloric acid (HClO4, 17 M) was purchased from Acros Organics, and sodium nitrite (NaNO2) was obtained from Fluka. Lithium perchlorate (LiClO 4 ) and tetrabutylammonium tetrafluoroborate (Bu4NBF4) from Aldrich were used as supporting electrolyte at 0.1 M concentration in acetonitrile (ACN). In Situ Preparation of the Diazonium Cation. The in situ formation of the diazonium cation from aminophenyl compounds74 is generally performed with NaNO2 in the presence of HClO4.75 We have used a procedure allowing in situ formation of diazonium but based on acid free solution using tert-butyl nitrite (tBuO-NO): An acetonitrile solution containing amino compound (5 or 0.5 mM) and Bu4NBF4 (0.1 M) as supporting electrolyte was prepared and degassed for at least 10 min. Following that, various amounts of tBuO-NO was added to the solution. As reaction between amine function and tert-butyl nitrite is relatively slow, the latter was added in excess. The necessary amount was determined for each compound by absorption spectroscopy, monitoring the emerging diazonium peak. Spectroscopic evidence, for conversion of R-NH2 to RN2+, are given in the Supporting Information (Figure SI-1). As the solution generally evolved less rapidly than those in the B
DOI: 10.1021/acs.jpcc.5b05456 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
observed during the reduction of in situ generated aryldiazonium cations.43,76 It is attributed to the progressive modification of the electrode by the formation of an insulating organic film, which blocks the surface in the 0.3 to −1.0 V potential range (Scheme 2). The electrochemical reduction of the other molecules based on intercalated EDOT and thiophene units, TEB and ETB diazonium salts, behaves similarly, and the CVs of their reduction on a GC electrode are given in the SI (Figure SI-2). AFM Investigation of the Modified Electrode. A convenient way of characterizing the surface morphology of organic layers attached to a surface is AFM investigation. To do so, a film was deposited on a 1 cm2 ITO electrode. Electroreduction of the diazonium salts was performed as for the GC electrode using 5, 10, and 20 polarization cycles between 0.3 V and −0.8 V (see Figure SI-3). The modified electrodes were sonicated in acetonitrile for 20 min to remove any weakly adsorbed species. Topography and thickness measurements of one of the EB layer are shown in Figure 2. The thickness of the film was measured using a scratch experiment made by the AFM tip. The topography image shows that whatever the number of cycles used during diazonium electroreduction, the obtained film appears homogeneous with no visible pinholes (at the AFM resolution). The comparison of the topography images for films generated with 5, 10, or 20 cycles shows an average roughness of 1, 1.5, and 2 nm, respectively. Following that, a square scratch was performed, which permits measuring the film thickness. Figure 2 shows the cross section through the hole generated by the AFM tip. An average thickness of 7 nm is found on the EB layer on ITO after electrochemical grafting in ACN using 10 and 20 cycles, whereas average thickness of 4.5 nm was observed after grafting using 5 cycles. Overall, AFM experiments indicate that an ultrathin organic layer has been generated on the ITO surface. (note that the exact film thicknesses also depend on the used substrates and the shape of the electrode). XPS Investigation of the Modified Electrode. XPS analysis was also used to analyze the surface composition of the generated films. To do so, we deposited the various films on a 1 cm2 gold electrode. Figure 3 displays the XPS survey spectrum (Figure 3a) and the high-resolution XPS signal for S 2p (Figure 3b) and C 1s (Figure 3c) obtained on the EB modified surface. Several major differences can be observed in the XPS spectra before and after electrochemical grafting. First, the carbon and sulfur signals increase after the reduction of diazonium, in comparison with the bare substrate, which indicates the formation of an organic layer (the S component on the bare Au substrate is negligible, and the carbon seen before grafting is attributed to surface contamination). Second, the detection of the Au XPS signals, in the survey spectrum (Figure 3a), is an indication of rather thin film (
