In Situ Diazonium-Modified Flexible ITO-Coated PEN Substrates for

Article pubs.acs.org/Langmuir

In Situ Diazonium-Modified Flexible ITO-Coated PEN Substrates for the Deposition of Adherent Silver−Polypyrrole Nanocomposite Films Soumen Samanta,*,†,‡ Idriss Bakas,‡ Ajay Singh,† Dinesh K. Aswal,*,† and Mohamed M. Chehimi*,‡,§ †

Technical Physics Division, Bhabha Atomic Research Centre (BARC), Mumbai 400085, India Université Paris Diderot, CNRS UMR 7086, ITODYS, Sorbonne Paris Cité, 15 rue J-A de Baïf, 75013 Paris, France § Université Paris-Est Créteil, CNRS UMR 7182 ICMPE, 2-8 rue Henri Dunant, 94320 Thiais, France ‡

ABSTRACT: In this paper, we report a simple and versatile process of electrografting the aryl multilayers onto indium tin oxide (ITO)-coated flexible poly(ethylene naphthalate) (PEN) substrates using a diazonium salt (4-pyrrolylphenyldiazonium) solution, which was generated in situ from a reaction between the 4-(1H-pyrrol-1-yl)aniline precursor and sodium nitrite in an acidic medium. The first aryl layer bonds with the ITO surface through In−O−C and Sn−O−C bonds which facilitate the formation of a uniform aryl multilayer that is ∼8 nm thick. The presence of the aryl multilayer has been confirmed by impedance spectroscopy as well as by electron-transfer blocking measurements. These in situ diazonium-modified ITO-coated PEN substrates may find applications in flexible organic electronics and sensor industries. Here we demonstrate the application of diazonium-modified flexible substrates for the growth of adherent silver/polpyrrole nanocomposite films using surface-confined UV photopolymerization. These nanocomposite films have platelet morphology owing to the template effect of the pyrrole-terminated aryl multilayers. In addition, the films are highly doped (32%). This work opens new areas in the design of flexible ITO-conductive polymer hybrids.

1. INTRODUCTION In recent years, there has been growing interest in the field of flexible organic electronics, such as solar cells,1 thin film transistors,2 sensors,3 and light-emitting diodes4 owing to their mechanical flexibility, low cost, and light weight, easy fabrication. Flexible electronics require coatings of various conducting polymers on the flexible polymer sheets, such as PET, PMMA, polyimide, ITO-coated PEN, and PET. Unfortunately, conducting polymers do not form adherent films with the plastic substrates as there is no chemical bonding between the substrate surface and the conducting polymer. Adhesion is indeed challenging and needs to be addressed for durable systems. Therefore, in order to prepare adherent conducting polymer films, it is imperative to modify the plastic substrates using appropriate coupling agents. In fact, in the literature, several coupling agents, such as silanes,5 thiols,6 carboxylates and phosphonates,7 zirconates and titanates,5 and diazonium8 have widely been employed. Among these, diazonium coupling agents, employed first to deposit aryl layers onto glassy carbon via the electroreduction of diazonium salts,9 are versatile and capable of anchoring a variety of polymers8 using different methods (e.g., electrochemical,8,10 chemical,11 photochemical,12 or spontaneous reduction13) onto a large panel of surfaces, such as glassy carbon,14 metals,15 metal oxides,16 ITO,17−19 clay,20 polymers,21,22 and semiconductors.10,23 The fundamental and applied aspects pertaining to the diazonium reduction processes have recently been summarized and discussed in some review papers24,25 and an edited book.8 © 2014 American Chemical Society

In most of these processes, the diazonium salts are prepared separately and stored at low temperatures (−18 °C) prior to the grafting onto the surfaces. However, from the viewpoint of industrial scale-up, a simple synthesis process is desirable in which the in situ generation of diazonium salt as well as its grafting on a suitable substrate takes place in a single pot. In this one-pot process, diazonium salts are generated in situ from a reaction between amines and nitrosonium ions (NO+).26 In the literature, NO+ has been generated using organic nitrite or NaNO2 with different choices of acids, such as HCl, H2SO4, HClO4, CH3COOH in water, and ACN.27−29 The aryl layers obtained from the reduction of diazonium salts have been widely employed for the coupling of molecules, macromolecules, polymers, nanoparticles, and nanotubes for different applications. In particular, the nonconjugated polymers (e.g., vinylic polymers) are grafted onto diazoniummodified substrates by two methods: (i) in situ polymerization (grafting from method) on surface-tethered initiators provided by diazonium grafting on the substrates and (ii) by reaction with preprepared polymers with reactive groups present on the diazonium-modified surface (grafting onto method).30 However, these processes have not been fully explored for the deposition of conjugated polymers, which are very important for application in flexible electronics. The reported literature in Received: May 19, 2014 Revised: July 15, 2014 Published: July 16, 2014 9397

dx.doi.org/10.1021/la501909r | Langmuir 2014, 30, 9397−9406

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Figure 1. Schematic of in situ diazonium generation and their electroreduction onto an ITO-coated flexible PEN substrate. performed using a three-electrode system having ITO-coated PEN as the working electrode (area ∼1.5 cm2), a platinum mesh as the counter electrode, and saturated calomel (SCE) as the reference electrode.The 0.1 M TBAB was used as the supporting electrolyte. For comparison, a reference sample (ITO/Py-A) was also prepared by employing identical electrografting conditions except NaNO2. The electrografting was carried out in cyclic voltammetry mode (a minimum of 20 cycles in each experiment) in the potential range between 0.5 and −0.7 V at a scan rate of 30 mV/s using a Biologic potentiostat (model SP-150). After the grafting, substrates were washed in ACN and cleaned ultrasonically in ethanol (for 15 min) to remove any unreacted and physisorbed species. The diazoniummodified flexible substrates were dried under a flow of Ar and were used for the deposition of the silver−polypyrrole nanocomposite films. 2.3. Preparation of Silver−Polypyrrole Nanocomposite Films on Diazonium-Modified ITO-Coated PEN. The photopolymerization of pyrrole on ITO/Py-D as well as on bare ITO was done in a glass bottle containing a fixed amount of distilled pyrrole (0.5 M) and silver nitrate (0.5 M) in 10 mL of water. The resultant solution was placed under a UV lamp (Spectrolinker, XL-1500UV cross-linker) set at a wavelength of 365 nm with a UV source to sample distance of ∼13 cm, an energy density of ∼120 mJ cm−2, and a power density of ∼5 mW cm−2 for periods ranging between 60 and 120 min. After UV exposure the samples were thoroughly cleaned ultrasonically using deionized water and ethanol to remove the unreacted species. It may be noted that composite films deposited on the unmodified surface peeled off during the ultrasonication (owing to poor adhesion), while those deposited on diazonium-modified substrates were found to be adherent. Finally the samples were dried at 70 °C for 4 h in an oven to remove any trapped solvent. The average thickness of the films was estimated to be 2 to 3 μm. 2.4. Characterization Techniques. The morphology of the samples was imaged by a Zeiss scanning electron microscope (SEM). The imaging of samples was carried out at a low excitation voltage (2.5 kV) with a small sample−detector distance (6 mm) to avoid charging effects. Surface chemical analysis was performed using a Thermo VG Escalab 250 X-ray photoelectron spectrometer (XPS) fitted with an Al monochromatic X-ray source (hν = 1486.6 eV; spot size = 500 μm). The samples were stuck onto the sample holder using double-sided adhesive tape. Binding-energy positions were calibrated against the C− C/C−H C 1s peak position set at 285.0 eV. Elemental atomic concentrations were calculated from the XPS peak areas, and the corresponding Scofield sensitivity factors were corrected for the analyzer transmission work function. XRD patterns were recorded on an X’pert Pro diffractometer (Panalytical) operating at 40 kV and 40 mA, with an anode using Co Kα as the radiation source (λ = 1.7902 Å). The blocking of the electrochemical activity of bare ITO and ITO/ Py-D surfaces was studied by cyclic voltammetry using the ferrocene/ ferrocenium (Fc/Fc+) redox couple (2 mM) prepared using ACN and 0.1 M TBAB as the supporting electrolyte. The experiments were carried out with a classical three-electrode system having platinum mesh as the counter electrode, saturated calomel (SCE) as the

this direction is limited and includes the attachment of conductive polymers on glassy carbon by means of electropolymerization31,32 and in situ chemical polymerization of aniline onto diazonium-modified MWCNTs.29 However, the deposition of adherent conducting polymer or metal/ conducting polymer nanocomposites on transparent conducting ITO-coated plastic substrates modified using in situgenerated diazonium salts is needed for flexible electronic devices (e.g., organic solar cells and organic light-emitting diodes) and hence forms the motivation for the current investigation. In this paper, we report for the first time the surface modification of an ITO-coated flexible poly(ethylene naphthalate) (PEN) substrate using the electrografting of an in situgenerated pyrrole derivative diazonium salt (Py-D) and the subsequent use of Py-D for the deposition of an adherent silver/polypyrrole (PPy-Ag) nanocomposite film via UV photopoymerization. Py-D was generated in situ by reacting 4-(1H-pyrrol-1-yl)aniline (Py-A) with NaNO2 in the presence of perchloric acid. The modified surface (ITO/Py-D) has been investigated by impedance spectroscopy, electrochemical blocking of the ferrocene/ferrocinium (Fc/Fc+) redox couple, and Xray photoelectron spectroscopy (XPS). In addition, the role of the aryl multilayer in the adhesion of the silver/polypyrrole (PPy-Ag) nanocomposite films has been investigated.

2. EXPERIMENTAL SECTION 2.1. Material and Reagents. Absolute ethanol (VWR Prolabo), chloroform (VWR), acetonitrile (Chem-Lab,HPLC grade), sodium nitrite (Sigma-Aldrich, ≥99.0%), tetrabutyl-ammonium tetrafluoroborate (TBAB) (Aldrich, 99%), 4-(1H-pyrrol-1-yl)aniline (SigmaAldrich, 97%), perchloric acid (Vel, 70% p.a.), ferrocene (SigmaAldrich), and silver nitrate (Sigma-Aldrich) were used without further purification. Pyrrole (Sigma-Aldrich, purity ≥98%) was refrigerated in the dark prior to synthesis. Prior to use, pyrrole was passed through a basic alumina powder (Merck, size 63 mm)-filled column to remove the impurities. All aqueous solutions were prepared using ultrapure Milli-Q water (18.2 MΩ cm). The ITO (thickness, 150 nm; sheet resistance, 30−50 Ω/square)-coated PEN substrates (thickness, 25 μm) were purchased from Sigma-Aldrich. These sheets were cut into a 20 mm × 10 mm size using a pair of scissors to conduct the surfacemodification experiments. 2.2. Electrografting of in Situ-Generated Diazonium Salt onto ITO-Coated PEN. Prior to the surface-modification process, ITO-coated PEN substrates were ultrasonically washed using chloroform, DI water, and ethanol. Diazonium salt was generated by mixing 5 mM Py-A with 5 mM NaNO2 and 10 mM HClO4 in 60 mL of ACN. The resulting solution was stirred for 1 h under a continuous flow of Ar. The electrografting of in situ-generated diazonium salts was 9398


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reference electrode, and the surface under investigation as the working electrode. The cyclic voltammograms are recorded from −0.1 to 0.65 V at a scan rate of 10 mV/s. The impedance spectra were recorded in the frequency range between 1 Hz and 100 kHz at the midpotential of the reduction and oxidation potential of the Fc/Fc+ redox couple. The amplitude of the alternating voltage was 5 mV. The obtained results are presented in Bode plots (i.e., log |Z| vs log f ).

aryl layer is grafted in the eighth cycle. The aryl layer, being a prominent insulator, prevents the effective electron transfer to the surface that inhibits further polymerization. This inference is also supported by the gradual reduction of the background current as the grafted insulating aryl layers block the electron transfer between the working electrode and the electrolyte.34,35 In order to further confirm that the reduction peak is due to the anchoring of the free aryl radical on ITO, cyclic voltammogram under identical conditions were recorded without NaNO2, and the results are shown in Figure 2b. No reduction peak is observed, which is expected as no diazonium is generated in the solution without NaNO2. This result also shows that free monomers do not anchor to the ITO substrate. However, there is a possibility that some Py-A molecules get physisorbed onto the ITO surface, and henceforth these samples are referred to as ITO/Py-A. 3.2. Characterization of the Modified Surface. 3.2.1. Electrochemical Impedance Study. The electrochemical impedance study is a very effective way to study the surface modification. We have studied the impedance response of the ferrocene/ferrocenium (Fc/Fc+) redox probe in acetonitrile with TBAB as the supporting electrolyte on bare ITO, ITO/PyD, and ITO/Py-A surfaces, and the typical results are plotted in Figure 3a in the Bode representation, which is a plot of

3. RESULTS AND DISCUSSION 3.1. In Situ Diazonium Modification of ITO-Coated PEN Substrates. Figure 1 schematically shows the concept of the in situ diazonium salt generation process and its electroreduction onto an ITO-coated flexible PEN substrate using a single solution. This in situ process is envisaged to take place in two steps: (i) the diazonium salt (Py-D) is generated in situ from the reaction between the amine (Py-A) precursor and NaNO2 in the presence of HClO4 and (ii) Py-D is reduced by taking an e− from the ITO and producing active free aryl radicals, which subsequently bond covalently to ITO (i.e., ITO/ Py-D). The concept of Figure 1 was experimentally realized by performing cyclic voltammetry. Typical recorded cyclic voltammograms are shown in Figure 2a. It is noted that the

Figure 2. Cyclic voltammograms obtained using flexible ITO-coated PEN substrates as the working electrode in the solution containing in situ-generated diazonium salt (a) and without diazonium salt due to the deliberate absence of NaNO2 (b). TBAB was used as the supporting electrolyte, and scans were recorded at a rate of 30 mV/s.

Figure 3. (a) Bode plots obtained from the impedance analyses for the three surfaces, namely, ITO, ITO/Py-A, and ITO/Py-D. The empty circles represent the experimental data, while the solid lines represent the fitting of experimental data using the equivalent circuit shown in the inset. (b) Cyclic voltammetry recorded for ITO, ITO/Py-A, and ITO/Py-D surfaces using the 2 mM ferrocene/ferrocenium (Fc/Fc+) redox couple in ACN solution at a scan rate of 10 mV/s. TBAB (0.1 M) is used as the supporting electrolyte.

first cycle contains an irreversible reduction peak at 0.56 V (vs SCE), which suggests the covalent bonding of the free aryl radical onto ITO, concomitant with the release of N2. The position of this reduction peak, however, would depend upon the acidic conditions, nature of the substrates,33 and monomer used for the in situ generation of the diazonium salt. The intensity of the reduction peak decreases gradually with an increasing number of cycles and disappears after the eighth cycle. This implies that the number of free sites available on ITO substrates for the covalent grafting of the aryl radical decreases with the number of cycles, and a completely uniform

log(frequency) vs log(impedance). The obtained results are fitted with Randle’s equivalent circuit shown in the inset of Figure 3a. The circuit elements in the equivalent circuit include the ohmic resistance of the electrolyte solution (Rs), the Warburg impedance (Zw), the double layer capacitance (Cdl), and the electron transfer resistance (Rct). It should be noted that to account for the local inhomogeneity the capacitance is 9399


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replaced with the constant phase element with n being very close to unity. It is very clear from Figure 3a that the experimental results are in good agreement with the theoretically fitted model data, and the values of the parameters are tabulated in the Table 1.

ITO/Py-A and ITO/Py-D surfaces, and the results are shown in Figure 3b. It is evident from Figure 3b that bare ITO exhibits clear anodic and cathodic peaks at 228.0 and 597.8 mV, respectively, with respect to the SCE reference electrode for the ferrocene/ ferrocenium (Fc/Fc+) redox couple. There is a slight increase in the peak splitting (ΔEP = 390.7 mV) for the ITO/Py-A surface with respect to bare ITO (ΔEP = 369.8 mV), and also a slight decrease in the current density (Irel = 96.9%) is observed. The slight change may be due to the physisorption of Py-A. On the other hand, a huge decrease in the current density (Irel = 9.1%) is observed for the ITO/Py-D surface as compared to the bare ITO. Due to the strong inhibition there is no clear cathodic peak observed although a small anodic peak is observed at 44.9 mV. Together the shift in the anodic peak position and huge inhibition confirm the presence of an aryl multilayer on the flexible ITO surface, which is in agreement with our impedance data. 3.2.3. XPS Surface Characterization. The survey spectrum as well as the high-resolution C 1s, N 1s, O 1s, and In 3d5/2 spectra for bare ITO, ITO/Py-A, and ITO/Py-D are presented in Figure 4. From these spectra, the presence of various peaks, their attributions, and their atomic percentages are summarized in Table 2. Before we discuss the interpretation of XPS data, it is worthwhile to estimate the thickness of the electrografted aryl layer on ITO, which was determined from the attenuation of the intensities of characteristic In 3d and In 4d peaks. According to the Beer−Lambert law, the peak intensities (peak areas) are expressed by

Table 1. Summary of Different Electrical Parameters Obtained from the Fitting of Experimental Data Using an Equivalent Circuit Shown in the Inset of Figure 3a surface

Rs (Ω)

Cdl (μF)

Rct (Ω)

Zw (Ω s1/2)

ITO ITO/Py-A ITO/Py-D

108.2 ± 0.2 112.3 ± 0.2 111.7 ± 0.3

4.84 ± 0.03 4.92 ± 0.03 5.66 ± 0.04

167.5 ± 0.5 172.5 ± 0.5 5314.5 ± 0.9

78.0 ± 0.3 79.1 ± 0.4 127.4 ± 0.5

Ideally, the ohmic resistance of the electrolyte (Rs) and the Warburg impedance (Zw) are related to the bulk resistance of the electrolyte and the diffusion of the redox probe in solution and therefore are not expected to change upon modification of the surface. On the other hand, the double-layer capacitance (Cdl) and electron-transfer resistance (Rct) are closely related to the electron-transfer process at the electrode/electrolyte interface, and thus they are very sensitive to any changes in the electrode surface. It is clear from Figure 3a that at high frequency (∼10 kHz) all of the samples show the same behavior because the impedance in this region is controlled by the frequency-independent ohmic resistance of the electrolyte (Rs). The value of Rs is almost same for all three samples. The contributions from Cdl and Rct are prominent in the frequency range of 1000 to 1 Hz. From the fitted values of the parameters it is clear that there is a marginal increase in the values of Rct and Cdl for the ITO/Py-A surface as compared to that of bare ITO. This confirms that there is no attachment of aryl layers to the ITO/Py-A surface, which is in agreement with the inference drawn from the cyclic voltammograms, as shown in Figure 2b. However, the marginal increase is attributed to the physisorption of Py-A onto the ITO surface. In the case of the ITO/Py-D surface, a large change in the charge-transfer resistance is observed, indicating that the grafted aryl multilayers act as an insulating layer which inhibits the charge transfer. Similar increases in the charge-transfer resistances are reported for the silane deposited on ITO,36 silane on silicon,35 and alkenethiols on gold.37 From the consideration of the physical process, the Warburg impedance should remain unchanged upon surface modification. However, a slight increase in the Warburg impedance is observed for the ITO/Py-D surface. This may be due to the different diffusion rate in the aryl multilayer as compared to that in the electrolyte. The increase in the Warburg impedance upon surface modification was previously observed in the case of self-assembled monolayers (SAM) deposited on ITO and was explained on the basis of a different charge-transfer rate within the organic layer.38 3.2.2. Blocking Effect of the Ferrocene Redox Couple. The study of the redox property of an electroactive species in solution is an elegant way to probe the surface modification of the electrode surface. This method has been successfully applied to Au, glassy carbon, and ITO electrodes modified with aryl layers.17,39−41 Generally, a decrease in the peak current density and an increase in the peak splitting for oxidation and reduction peaks are the signature of a modified surface. We have studied the redox property of the ferrocene/ferrocenium (Fc/Fc+) redox couple by cyclic voltammetry for bare ITO and

I4d = I4d° exp( −d /λ4d sin θ)

(1)

and I3d = I3d° exp( −d /λ3d sin θ)

(2)

where I is the peak intensity of core-level electron 3d or 4d recorded for grafted ITO, while I° corresponds to the peak intensity obtained from bare ITO. In 3d and In 4d intensity ratios can be calculated for bare and coated ITO plates: R ° = I4d°/I3d°

(3)

and R = I4d /I3d

(4)

Combining eqs 1−4 leads to the analytical expression for assessing the thickness: d = sin θ /[(λ4d)−1 − (λ3d)−1] ln(R °/R )

(5)

The analyses were performed at an emission angle of 90°, so sin θ = 1. The attenuation lengths were estimated using the Seah and Dench equation42 established for an organic coating λ ≈ 0.11(E K )1/2 (mg/m 2)

(6)

where EK is the kinetic energy of core-level electron In 3d (∼1041 eV) or In 4d (∼1468 eV). It follows that to determine the thickness in nanometers one has to divide λ in mg/m2 by the overlayer density in g/cm3, which is set here to 1 because we are dealing with an organic layer. Equation 6 permits us to estimate attenuation lengths of 4.22 and 3.55 nm for In 4d and In 3d, respectively. Applying these values in eq 5 yields an aryl layer thickness of 7.9 nm, which is at the higher end of the 9400


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increase considerably, suggesting the grafting of the aryl layer. In addition, the shakeup satellite at 291.3 eV (shown in the inset of Figure 4b), a signature of an aromatic ring, confirms the presence of the aryl layer. As expected, no N 1s peak (Figure 4c) was observed for the bare ITO surface. In the case of the ITO/Py-A surface, the N 1s spectrum is fitted with two components, which are assigned to nitrogen in the amine group of aniline (NNH2) and nitrogen present in pyrrole (NPy), respectively. As seen from Table 2, the NPy/NNH2 ratio is ∼1 (which is the theoretical value for PyA molecules), suggesting that PyA precursor molecules are physisorbed on the surface. However, the N 1s spectrum for the ITO/Py-D surface is fitted with three components corresponding to NNH2 and/or the azo group (Nazo) due to the formation of multilayers, NPy, and protonated N (N+) within the aryl layers. In our case the thickness of the aryl layer is ∼8 nm, which indicates the formation of a multilayer via azo groups. From Table 2, it is noted that the NPy/(NNH2 + Nazo) ratio is >6.3, which clearly suggests the formation of a uniform aryl layer on the ITO substrates. The O 1s XPS spectra (Figure 4d) for ITO are fitted with three components at 530, 531.5, and 532.8 eV assigned to oxygen in ITO, oxygen in hydroxide, or oxyhydroxides and oxygen in adventitious contaminants, respectively.45 For the ITO/Py-D surface, three components are assigned to the metal−O bond, M−O−C, and/or M−O−H and surface contamination, respectively. The middle peak testifies to the effective grafting of the aryl groups. Indeed, various metal−O− C chemical environments are known to yield an O 1s peak in the 531.5−533 eV range.46−49 Compared to the chemical environment of C−O−C for which the BE range is 533−535.4 eV, the reported trend for M−O−C is lower as one replaces C by a metal whose electronegativity is well below 2.5 (for carbon). Therefore, the electron-withdrawing effect experienced by oxygen is less and hence has a lower binding energy. By analogy to other metals, the peak at 531.5 eV is indisputably assigned to In−O−C and presumably to Sn−O−C. Nevertheless, any remaining metal hydroxides would be lumped with the M−O−C component. The In 3d5/2 spectrum for ITO (Figure 4e) is fitted with two components centered at 443.8 and 444.8 eV and assigned to In in In2O3 and In(OH)3, respectively.45 The In(OH)3/In2O3 intensity ratio is found to be ∼0.34. After the attachment of the aryl layer, the peak positions of these two In 3d5/2 components remain the same, but the intensity ratio has now increased to 1. This can be explained on the basis of the fact that the aryl radical is bound to both In2O3 and In(OH)3 via In−O−C bonds. As the peak position of In in In−O−C is at ∼444.8 eV, the conversion of In2O3 and In(OH)3 to In−O−C bonds leads

Figure 4. (a) XPS survey spectra of ITO, ITO/Py-A, and ITO/Py-D surfaces. (b−e) High-resolution XPS spectra for ITO, ITO/Py-A, and ITO/Py-D surfaces for C 1s, N 1s, O 1s, and In 3d5/2, respectively. (Inset of b) Magnified version of the π−π* shake-up peak.

reported range.43 Such a layer would be defined as a medium thickness layer as pointed out by Pinson.44 The C 1s spectra in Figure 4b are fitted with three components centered at 285.0, 286.5, and 288.8 eV, which are assigned to C−C/C−H, C−O/C−N, and O−CO, respectively. In the case of bare ITO, the presence of carbon is attributed to unintentional carbon contamination. As seen in Table 2, the carbon composition of the ITO/Py-A surface is similar to that of bare ITO, except for a marginal increase in the C−N component, which is attributed to the physisorption of Py-A molecules. On the other hand, for the ITO/Py-D surface the contributions due to C−C/C−H and C−N components

Table 2. Surface Composition in Atomic % of Various Elements as Determined from the XPS Analysesa C (%) C ls

a

surfaces

285.0 C−C; C−H

286.5 C−O; C−N

288.8 O−OO

399.1 NNH2; Nazo

ITO ITO/Py-A ITO/Py-D

20.9 22.2 47.3

2.5 4.0 15.1

2.7 1.6 2.8

0.14 0.65

N (%)

O (%)

In (%)

Sn (%)

N ls

O ls

In 3d5/2

Sn 3d5/2

400.6 NPy 0.16 4.10

402.4 N+

530.0 In2O3 24.1

0.82

3.7

531.5 OH; M−O−C 13.5 40.4 14.7

532.8 contaminants

443.8 ln2O3

3.3

22.1

444.8 ln(OH)3; In−O−C

486.5 SnO2; Sn(OH)2

7.5

3.4 3.3 1.0

28.2 3.0

2.8

2.8

The binding-energy position is in eV. 9401


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Figure 5. (a) Schematic of the photopolymerization process for pyrrole with AgNO3 as the photoinitiator. (b) Schematic of the photopolymerization of pyrrole with AgNO3 as the photoinitiator on modified ITO-coated flexible PEN substrates. (c, d) Digital photographs of the PPy-Ag nanocomposite (ITO/Py-D/PPy-Ag) under bent and flat conditions. (e) Digital photograph of the PPy/Ag nanocomposite (ITO/PPy-Ag) on bare ITO.

Figure 6. (a, b) SEM images of polypyrrole grown on bare ITO for growth periods of 1 and 2 h, respectively. (c, d) SEM images of polypyrrole grown on a modified substrate for growth periods of 1 and 2 h, respectively. The inset shows a high-resolution image for the respective cases.

polymerization is shown in Figure 5a. In the first stage of the process, Ag+ goes into the excited state and acts as an electron acceptor. The excited Ag+ takes an e− and oxidizes the pyrrole monomer. This process eventually results in metallic Ag and radical pyrrole. Two reactive pyrrole radicals react to form a pyrrole dimer which initiates the polymerization, and subsequently polymer chains grow and finally lead to polymer formation. In the present study, the diazonium modification leaves the pyrrole-terminated surface which we have used as the template for the growth of polypyrrole. In the presence of the modified surface, polymerization initiates from the pyrrole attached to the substrate, leading to the formation of the nanocomposite films (Figure 5b). This process leads to the good adhesion of the film to the substrate. Figure 5c,d shows digital photographs

to an increase in its intensity and a decrease in the intensity of In2O3 (i.e., at 443.8 eV). We have observed the same situation with Sn 3d5/2 (spectra not shown), which suggests that the attachment of aryl groups results in both In−O−C and Sn−O− C. 3.3. Silver−Polypyrrole Nanocomposite Films on Diazonium-Modified Surfaces. We have adopted the elegant photopolymerization approach to prepare silver/ polypyrrole nanocomposite thin films on a flexible ITO/Py-D surface (ITO/Py-D/PPy-Ag), which is an interesting extension of our previous work on free-standing flexible polypyrrole− silver nanocomposite films at the air−water interface.50 For comparison we have prepared a nanocomposite film under identical conditions on the bare ITO surface (ITO/PPy-Ag). We have used AgNO3 as a photoinitiator. The schematic of the 9402


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Figure 7. (a) XRD pattern of the PPy-Ag nanocomposite prepared on modified and bare ITO. (b) XPS survey spectra of the PPy-Ag nanocomposite prepared on modified and bare ITO. (c, d) High-resolution N 1s XPS spectra for the PPy-Ag nanocomposite deposited on unmodified and diazonium-modified ITO-coated PEN substrates, respectively.

Table 3. Surface Composition in Atomic % of Various Elements as Determined from the XPS Analyses for PPy-Ag Nanocomposite Films on Modified and Bare ITO Surfacesa C (%)

N (%)

C 1s

a

N 1s

surfaces

285.0

397.9 CN

ITO/PPy-Ag ITO/Py-D/PPy-Ag

68.6 65.5

2.5 0.9

O (%)

Ag (%)

In (%)

O 1s

Ag3d5/2

In3d5/2

400.0 N

401.8 N+

406.1 NO−3

ratio

532.0

368.2

444.2

C/N

Ag/N

N+/N

NO3−/N

NCN/N

8.3 7.6

1.6 4.0

1.5 3.7

15.8 17.7

0.4 0.6

1.3

5.55 5.25

0.030 0.045

0.13 0.32

0.12 0.30

0.20 0.07

The binding-energy position is in eV.

of the films prepared on the ITO/Py-D surface under bent and flat conditions. These films are found to be very adherent because they did not peel off, despite making several cycles of the bending. Figure 5e shows a photograph of nanocomposites film deposited on bare ITO. It can be seen that part of the film is peeled off even under flat conditions. Subjecting these films to any kind of bending resulted in their complete peeling off. Thus, these results indicate that the in situ diazonium modification of ITO is essential to preparing adherent nanocomposites films. 3.4. Characterization of Silver−Polypyrrole Nanocomposite Films. Figure 6 shows the morphology evolution of PPy-Ag films grown on bare ITO and ITO/Py-D surfaces. In the case of the bare ITO substrate, during the initial stages the polymer grows randomly in granular form (Figure 6a). The granular formation is attributed to the hydrophobic interaction between the hydrophilic ITO surface (contact angle ∼24°) and hydrophobic PPy-Ag nanocomposite. As the polymerization time increases, random granular growth occurs both in the plane as well as in the vertical direction of the substrates, which results in the nonuniform morphology (Figure 6b). On the other hand, for ITO/Py-D substrates, the polymer films (Figure 6c) initially grow in the platelet morpohology as the pyrrole moiety present at the surface of the substrate (Figure 5b) act as

a template. This template effect even remains at the higher thickness (Figure 6d), where large platelets of polymer islands in the plane of the substrates are observed. In addition, the aryl layer on the modified surface makes ITO hydrophobic (contact angle ∼74°), which improves its wettability. It has been reported that hydrophobic interactions are essential to the adhesion of polypyrrole to the substrate.51 It may be noted that silver particles are not visible in these images as they are at low concentration (0.6%) and are uniformly embedded in the polymer matrix. The grazing angle XRD patterns for the ITO/Py-D/PPy-Ag and ITO/PPy-Ag films are presented in Figure 7a. The XRD pattern confirms the presence of nanocomposites in both films. As shown in Figure 7a, a broad, low-intensity reflection centered at 2θ ≈ 25.6° is observed, which is a characteristic of doped PPy.52 The interchain separation corresponding to this peak is calculated to be 3.5 Å. This value of interchain separation is in accordance with a previously reported value for highly conjugated and ordered PPy films prepared by chemical polymerization.53 It may be noted that a relatively high intensity, sharper peak is observed for the films prepared on the modified substrate as compared to that prepared on bare ITO. This suggests better structural ordering of PPy chains on the modified substrate. Five other sharp diffraction peaks at 2θ = 9403


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for highly conducting pyrrole, the typical doping level reported is 36%.58,59 The highly doped nanocomposite flexible films make them suitable candidates for the sensing of toxic gases and is therefore a subject of independent research.

38.1, 44.3, 64.4, 77.4, and 81.5° correspond to (111), (200), (220), (311), and (222) planes of metallic Ag, which are in good agreement with the reported data.54 This reveals the presence of Ag in the PPy/Ag nanocomposite films. The crystallite size (DAg) of Ag is estimated using the Scherrer formula as55 DAg =

0.94λ β1/2 cos θ

4. CONCLUSIONS We have demonstrated an in situ process of electrografting diazonium layers onto ITO-coated flexible PEN substrates. The diazonium salt (4-pyrrolylphenyldiazonium) solution employed for the electografting was in situ generated from a reaction between an aniline precursor (1H-pyrrol-1-yl) and sodium nitrite in an acidic medium. The XPS studies revealed that the grafted aryl layer bonds with the ITO surface through In−O−C and Sn−O−C bonds, and the thickness of the grown multilayer is ∼8 nm. The impedance spectroscopy and electron-transfer blocking measurements conclusively proved the presence of the aryl multilayers. We have employed diazonium-modified flexible substrates for the growth of unique hybrid materials, i.e., silver/polypyrrole nanocomposite films, which have remarkable physicochemical attributes. The grown films are adherent and highly doped (32%) and contain few structural defects because the aryl multilayer acts as a template for platelet growth. Apart from these particular results, we have demonstrated that aryl layers can easily be electrografted onto flexible conducting surfaces via the chemistry of in situgenerated diazonium salts, providing a unique platform for the covalent attachment of conductive polymer/silver nanocomposite coatings. They open new avenues in flexible organic electronics, gas sensors, and biomedical applications, to name but a few.

(7)

where λ is the radiation wavelength of cobalt X-rays, β1/2 is the full width at half-maximum (FWHM) of the diffraction peak, and θ is the peak position. Using the fwhm of highest-intensity peak (111) centered at 38.1°, the size of Ag was calculated to be ∼95 nm for both films. Figure 7b shows XPS survey spectra for ITO/Py-D/PPy-Ag and ITO/PPy-Ag surfaces. In both spectra, four prominent peaks corresponding to C 1s (285 eV), the Ag 3d doublet (368.2 and 374.2 eV), N 1s (400 eV), and O 1s (532 eV) are observed. The surface composition in atomic % obtained from XPS analyses is summarized in Table 3. It is interesting that despite the film thickness being on the order of micrometers, the In peak (1.3%) is seen at the surface of the film deposited on bare ITO (XPS samples only 5−10 nm in depth). There are two possible explanations for this: (i) the film being peeled off of the XPS analysis area and/or (ii) the diffusion of In from ITO to the surface of the films, as In is known to exhibit high solubility with the polymer matrix.56,57 On the other hand, no trace of In is observed in the XPS spectrum of PPy films grown on aryl-modified ITO, which confirms that the aryl layer hinders the In diffusion. For both films, the binding energy of Ag 3d5/2 is found to be 368.2 eV, which indicates the presence of Ag in metallic form. As seen in Table 3, the value of C/N is ∼5, indicating that the contributions of C and N are purely due to the polypyrrole. Figure 7c,d shows the high-resolution XPS spectra of the N 1s peak for ITO/PPy-Ag and ITO/Py-D/PPy-Ag surfaces, respectively. N 1s spectra can be fitted with three different types of nitrogen: 397.9 eV (CN), 400.0 eV (neutral nitrogen atoms in the pyrrole ring), and 401.8 eV (positively charged nitrogen attached to the pyrrolium cation). The presence of the 397.9 eV peak is attributed to the deprotonated or imine-like nitrogen atoms (CN), which is basically a structural defect in the sample.50,58 From the ratio of deprotonated nitrogen to total nitrogen one can estimate the extent of structural defects present in the sample. This ratio for the nanocomposites films deposited on bare ITO and ITO/PyD is found to be 0.2 and 0.07, respectively (Table 3), indicating that the latter films contain relatively fewer structural defects. It may be noted that in our studies we have employed AgNO3 as an oxidant; therefore, NO3− acts as the counterion in the films. The presence of the N 1s peak at 406.1 (not shown here) confirmed the presence of NO 3 − . These NO 3 − counterions on one hand maintain the charge neutrality and on the other hand dope the polymer, which enhances its conductivity. The level of doping can be estimated in two different ways: (i) from the ratio of N+ in oxidized pyrrole to the total amount of nitrogen or (ii) from the ratio of nitrogen in NO3− to the total amount of nitrogen present. These ratio as presented in Table 3 are in very good agreement. A doping of 32% has been obtained for the nanocomposite films grown on the modified substrate, which is significantly greater than for that grown on bare ITO substrates (13%). It may be noted that

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the Indo-French Centre for the Promotion of Advanced Research (IFCPAR) for financial support through the All Polymer Flexible Gas Sensors project (no. 4705-2). This work was also supported by a DAE-SRC Outstanding Research Investigator Award (2008/21/05-BRNS) granted to D.K.A. Dr. P. Decorse, Ms. H. Lecocq, and Ms. S. Nowak are acknowledged for their assistance with XPS, SEM, and XRD characterizations, respectively.

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REFERENCES

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