Electrochemical Grafting of Reduced Graphene Oxide with

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Electrochemical Grafting of Reduced Graphene Oxide with Polydiphenylamine Doped with Heteropolyanions and Its Optical Properties I. Smaranda,† A. M. Benito,‡ W. K. Maser,‡ I. Baltog,† and M. Baibarac*,† †

National Institute of Materials Physics, Lab. Optical Processes in Nanostructured Materials, P.O. Box MG-7, Bucharest, R077125, Romania ‡ Instituto de Carboquímica ICB-CSIC, C/Miguel Luesma Castán 4, E-50018 Zaragoza, Spain ABSTRACT: Electropolymerization of diphenylamine (DPA) onto a reduced graphene oxide (RGO) film was carried out to obtain a corresponding RGO/ polydiphenylamie (PDPA) composite doped with phosphotungstic acid (H3PW12O40) heteropolyanions (PT). The synthesis was performed in the absence of light, since UV−vis spectroscopy and photoluminescence (PL) studies on RGO/DPA blends irradiated by UV light revealed a partial transformation of the DPA monomer into oligomers of PDPA. Raman scattering demonstrates that the electropolymerization of DPA in the presence of H3PW12O40 (PTA) and RGO leads to the formation of PDPA covalently bonded to the RGO sheets (RGO/PDPA:PT). The presence of heteropolyanions in the PDPA matrix (PDPA:PT) is detected by FTIR spectroscopy. Comparing the PL excitation spectra of PDPA:PT and the RGO/PDPA:PT composite highlights an upshift of the band gap that is accompanied by a change in the composition of the PL spectrum in the spectral range of 2.25−3.54 eV. These changes originate in a charge transfer that takes place at the interface of nongrafted RGO and PDPA:PT. The gradual increase of the PL intensity of RGO covalently grafted with PDPA:PT reveals photochemical reactions under UV irradiation. These involve the C−C stretching vibrational mode in the benzene ring of PDPA and indicate the transformation of an RGO/PDPA:PT composite containing HPW12O402− anions into an RGO/PDPA:PT composite stabilized by PW12O403− anions. These results not only provide important insights on the interactions between RGO, conjugated polymers, and stabilizing dopant ions but also impact on the synthesis conditions.

1. INTRODUCTION In the last two decades, composite materials based on conjugated polymers (CPs), heteropolyacids, and carbon nanotubes have been considered as promising active materials in the field of energy storage.1,2 However, the intensive use of these composite materials in different applications is restricted by the high cost of carbon nanotubes. Alternatively, less expensive graphene-based materials have gained increased interest for the development of functional composites. Special attention is given to reduced graphene oxide (RGO) and CPs, such as polyaniline,3−6 poly(3-hexylthiophne) (P3HTh),7,8 poly(p-phenylenevinylene),9 poly[2-methoxy-5(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV),10 poly(o-phenylenediamine),11 and polydiphenylamine (PDPA),12 which have demonstrated applications in the field of supercapacitors,3,4,9 fuel cells,5 and nanometer-scale electronics.7 Two methods were used for the synthesis of the CPs/RGO composites: in situ oxidative polymerization5,6 and layer-by-layer electrochemical synthesis.2 The difference between the deposition of CPs onto RGO sheets1 and noncovalent5,6 or covalent grafting of RGO with CPs13,14 has been investigated using different techniques: UV−vis and FTIR spectroscopy, Raman scattering, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning © XXXX American Chemical Society

electron microscopy (SEM). In the case of composites based on RGO and conjugated polymers of the polyaniline (PANI) family, it has been demonstrated that RGO can be both an acceptor of electrons and a dopant agent for polyaniline flakes prepared by chemical polymerization of aniline.3 In the case of derivative compounds of PANI, such as polydiphenylamine (PDPA), the FTIR spectroscopy and XPS studies have shown that the chemical polymerization of diphenylamine in the presence of a sulfuric acid solution and ammonium peroxydisulfate used as oxidant agent leads to a PDPA noncovalently grafted RGO composite.12 To design different molecular structures, the knowledge of the chemical interactions between reactants, the influence of the UV light on the reaction medium as well as the stability of the resulted composites under UV irradiation are important factors for the synthesis of these hybrid materials. This work reports new results obtained by photoluminescence and vibrational spectroscopic studies on reduced graphene oxide/polydiphenylamine composites doped with phosphotungstic acid (H3PW12O40) heteropolyanions (RGO/PDPA:PT) synthesized electrochemically by cyclic voltammetry. The interest Received: July 22, 2014 Revised: October 6, 2014

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dx.doi.org/10.1021/jp507324r | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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UV−vis spectra of the solution of DPA and the RGO/DPA blend 1 in DMF were recorded using as the reference beam DMF. The UV−vis spectra of films of PDPA in the undoped and doped states, electrosynthesized onto ITO supports, with stopping at the potentials of +100 and +960 mV vs Ag/AgCl, respectively, were recorded using on the reference beam an ITO support. All UV−vis spectra were recorded using a PerkinElmer spectrometer (Lambda 950 model). FTIR spectra of DPA and RGO/DPA blend 2, as well as attenuated total reflection infrared (ATR-IR) spectra of PDPA:PT and RGO/PDPA:PT composites on Au supports, were recorded using a Bruker FTIR spectrophotometer (Vertex 70 model). Raman spectra of RGO, PDPA:PT and RGO/PDPA:PT composites at an excitation wavelength of 1064 nm were measured using an FT Raman spectrophotometer (RFS100S model) from Bruker. Studies of the resonant Raman spectroscopy on samples of RGO, PDPA:PT, and RGO/PDPA:PT were carried out at the excitation wavelengths of 457.9, 514.5, and 647.4 nm using a Horiba Jobin Yvon spectrophotometer (T64000 model). The photoluminescence (PL) spectra of DPA, the RGO/ DPA blend 2, PDPA:PT and RGO/PDPA:PT composites were recorded in the right-angle geometry at room temperature (RT) using a Horiba Jobin Yvon Fluorolog-3 spectrometer (model FL 3-22). Scanning electron microscopy (SEM) images of samples of RGO and RGO/PDPA:PT were recorded using a microscope SEM Tescan.

for the electrochemical polymerization of DPA in the presence of RGO is to use the films of RGO grafted with PDPA doped with heteropolyanions as electrode active materials in supercapacitors and lithium rechargeable batteries. In order to understand and optimize the synthesis conditions, first, the interactions of RGO with the reactants had to be studied. While information is available for RGO suspended in dimethylformamide (DMF),15 RGO/PT in 2-propanol,16 and RGO in HCl,17 to the best of our knowledge, no report describes the interaction between RGO and DPA itself. Thus, for the first time, we show that UV irradiation causes significant photochemical reactions of DPA in the presence of RGO. Consequently, in order to gain a high reproducibility in the electrochemical polymerization of DPA on RGO, the corresponding syntheses had to be carried out in the absence of light. Subsequently, the spectroscopic studies on the novel electrochemically synthesized RGO/PDPA:PT composite material reveal in detail the interaction of RGO with PDPA and the intercalated PT heteropolyanion dopants. They also show that an important photochemical transformation of the composite can take place upon UV irradiation affecting the oxidation state of the stabilizing H3PW12O40 heteropolyanions and the overall interaction with PDPA and RGO.

2. EXPERIMENTAL SECTION The compounds DPA, DMF, H3PW12O40 (PTA), and HCl were purchased from Sigma-Aldrich. RGO films were prepared according to a procedure described in detail by Vallés et al.18 In brief: first, an aqueous dispersion of exfoliated GO sheets (0.45 mg/mL) was prepared. Vacuum-assisted flow-filtration resulted in free-standing GO films of 10 μm in thickness. Exposure to vapors of hydrazine monohydrate and subsequent thermal annealing under argon at 700 °C resulted in freestanding RGO films. Two types of RGO/DPA blends were prepared: (i) 1 mg of RGO was dispersed into 2 mL solution of 0.5 M DPA in DMF, and the mixture was ultrasonicated for 30 min. This sample is labeled RGO/DPA blend 1. (ii) 1 mg of RGO was grinded into 0.199 g of DPA until the white color turned gray. This sample is labeled RGO/DPA blend 2. Electropolymerization of DPA was performed by cyclic voltammetry in a conventional three-electrode, one-compartment cell, using as the working electrode a blank Au plate or a gold support coated with RGO and as the counter electrode a spiral Pt wire. The RGO coating was prepared by dispersing 2 mg of RGO into 2 mL of ethanol and drop-casting this dispersion onto the gold support. A commercial Ag/AgCl electrode (3 M KCl) was used as reference electrode. The three electrodes were immersed into a solution consisting of 10−2 M DPA, 10−3 M PTA, and 1 M HCl in DMF:H2O (1:1 volumetric ratio).2 The cyclic voltammograms were recorded in the potential range from +100 to +960 mV vs Ag/AgCl with a sweep rate of 50 mV s−1 using a Radiometer Analytical potentiostat/ galvanostat, VOLTALAB 80 model. The resulting samples are labeled as PDPA:PT and RGO/PDPA:PT, with PT as the heteropolyanion dopant of PDPA. Elemental analysis was performed using a PerkinElmer 2400 Series II CHNS/O elemental analyzer. The values found for PDPA:PT are 45.2% C, 2.22% H, 4.01% N, 0.52% P, 39.2% W, and 8.85% O. These are in agreement with the formula for C12H9N(PW12O40)0.057, for which the calculated values are 43.49% C, 2.72% H, 4.23% N, 0.53% P, 38.01% W, and 11.02% O. The doping level of PDPA with heteropolyanions is approximately 2%.

3. RESULTS AND DISCUSSION 3.1. Chemical Interaction of RGO with DPA. Figure 1, spectra a and b, shows the UV−vis spectra of the solutions of DPA and RGO/DPA blend 1 in DMF. The UV−vis spectrum of DPA shows an intense absorption band peak at 325 nm assigned to the π−π* transition in benzene rings19 and an absorption band of lower intensity at 260 nm attributed to the π−n transition characteristic of aniline compounds.20 In the case of the RGO/DPA blend, an additional absorption band appears in the spectral range of 500−800 nm, with a maximum at 600 nm. Its intensity increases with the aging time of the solution. After an aging time of 24 h, the colorless solution turns green and exhibits a high stability (Figure 1e). In this context, it should be noted that the absorption band of RGO in DMF with its maximum at 271 nm15 does not alter within the time frame of 24 h. Spectra c and d in Figure 1 confirm the presence of the band situated in the spectral range of 500−800 nm when PDPA, in undoped and doped states, was electrochemically synthesized using stopping potentials of +100 and +960 mV vs Ag/AgCl, respectively. The evolution of this band in the case of the RGO/DPA blend 1 solution most likely can be attributed to photochemical reactions, which are revealed more clearly by photoluminescence (PL) studies. Figure 2 shows the PL spectra of DPA and the DPA/RGO blend 2 in the initial state and after a UV irradiation time of 112 min. Comparing panels c1 and d1 in Figure 2, one observes that the presence of RGO in DPA leads to a decrease of the global intensity of the complex PL band of DPA while its maximum shifts from 3.58 to 3.66 eV. Such an upshift in the PL spectrum should be correlated with changes in the vibration spectrum of DPA. Figure 3 depicts the FTIR spectra of DPA and the DPA/ RGO blend 2. The absorption bands of DPA at 680−746, 876, B



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Figure 1. UV−vis spectra of the DPA in DMF (a), RGO/DPA blend 1 in DMF (b), the PDPA and PDPA:PT films electrochemically synthesized onto ITO supports at stopping potentials of +100 mV (c) and +960 mV (d), respectively. Photo of solutions of DPA and RGO/DPA blend 1 after an aging time of 24 h (e).

Figure 2. PL spectra recorded at room temperature under λexc = 275 nm of DPA (a) and DPA/RGO blend 2 (b) under UV irradiation time of 112 min (black curves with solid and dashed lines correspond to the initial state and the intermediary state; red curves correspond to the final state after irradiation). Insets a1 and b1 show the PL decay as a function of the UV irradiation time. Panels c1 and d1 depict the deconvoluted PL spectra at the initial state of DPA and RGO/ DPA blend 2, respectively. Panels c2 and d2 show the deconvoluted PL spectra after 112 min of UV irradiation of DPA and RGO/DPA blend 2, respectively. C



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1173, 1220−1244, 1317, 1416, 1456−1518, and 1595 cm−1 are assigned to the following vibrational modes: out-of-plane vibration of the benzene ring, benzene ring deformation, C−H bending, C−N stretching, C−H bending, N−H bending + C−H bending, C−C stretching + C−H + N−H bending, and C−C stretching vibration of the benzene ring.21 In the case of the DPA/RGO blend, clearly the appearance of a new absorption band at 1632 cm−1 can be seen. This band originates from the formation of new C−C covalent bonds between different carbon nanoparticles and polymers via the transformation of aromatic rings of carbon nanotubes into orthodisubstituted cyclic hydrocarbon rings.22 The formation of new C−C covalent bonds indicates a covalent grafting of RGO with DPA. A closer look back at the PL spectra provides further additional support for this hypothesis. Here, it is worthwhile mentioning that, with an exposure time of 112 min at the excitation wavelength of 275 nm, both samples, DPA and DPA/RGO blend, show a gradual decrease of global PL intensity under UV irradiation of approximately 35% and 60%, respectively (Figure 2a,b). The PL quenching effect induced by the irradiation with light of 275 nm for 112 min is accompanied by (i) a down shift of the complex PL bands of DPA from 3.58 and 3.35 eV to 3.41 and 3.17 eV, respectively (Figure 2c2,c1), and (ii) the appearance of new PL bands with maxima at 3.12, 2.92, and

Figure 3. FTIR spectra of DPA (a) and the DPA/RGO blend after UV irradiation (b).

Scheme 1. (a) The Transformation of DPA into Dimers or Oligomers under UV Irradiation. (b) The Reaction of RGO with DPA and the Transformation of DPA Covalently Grafted RGO into Dimers or Oligomers Covalently Grafted RGO under UV Irradiation

D



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Figure 4. 25th, 50th, and 100th cyclic voltammograms recorded with a sweep rate v = 50 mV s−1 on the blank Au support (from bottom to top: a3, a2, a1) and the gold plate covered with RGO film (from bottom to top: b3, b2, b1), upon immersion of the two working electrodes in a semiaqueous solution of 2 × 10−3 M DPA + 1 M HCl + 10−2 M PTA.

2.76 eV (Figure 2d1,d2) in the case of the RGO/DPA blend 2, which indicates the development of chemical reactions under UV irradiation. These PL bands at 3.12, 2.91, and 2.76 eV are also found in the PL spectrum of PDPA.21 These photochemical reactions concern the homolytic breaking of the C−H bond in DPA, which results in the formation of two very instable free radicals that react rapidly with themselves, leading to the formation of dimers and oligomers in the case of DPA. In the case of the DPA/RGO blend, the homolytic breaking of the C−H bond leads to a covalent grafting of RGO with monomers that are transformed into oligomers. The chemical mechanism of the transformation of monomers into dimers or oligomers and RGO covalently grafted with DPA oligomers is presented in Scheme 1. Altogether, the new absorption FTIR band with a maximum at 1632 cm−1 (Figure 3) assigned to the new C−C covalent bonds established by the grafting of RGO with DPA as well as the appearance of the new PL bands with maxima at 3.12, 2.92, and 2.76 eV (Figure 2d1,d2), which characterizes the PL spectrum of PDPA and its oligomers,21 confirms that significant photochemical reactions take place under UV irradiation of the RGO/DPA blend. These lead to the formation of RGO covalently grafted with PDPA oligomers, as shown in Scheme 1.

As a direct consequence, in order to avoid the grafting of DPA oligomers on RGO and to gain enhanced reproducibility, it is mandatory that the electrochemical polymerization of DPA in the presence of RGO is carried out in the absence of light. 3.2. Electrochemical Synthesis of RGO Covalently Grafted with PDPA Doped with H3PW12O40 Heteropolyanions and Its Optical Properties. Figure 4 shows the 25th, 50th, and 100th cyclic voltammograms recorded on the blank Au electrode and the Au plate covered with RGO film. In the case of the Au electrode, two oxidation maxima in the potential ranges of 0.34−0.55 V and 0.6−0.86 V vs Ag/AgCl are observed in Figure 4 a1,a2,a3. These oxidation peaks are attributed to the formation of DPA cation radicals (DPA+.), N,N′-diphenylbenzidine (DPB), and N,N′-diphenylbenzidine dications (DPB2+). The last two compounds are obtained during the coupling process of the DPA cation radicals with themselves or with the parent molecules (DPA).23−25 A reduction peak is revealed in the potential range of 0.3−0.6 V vs Ag/AgCl during cathodic scanning, when the electrochemical polymerization of DPA in the presence of the semiaqueous solution of HCl and H3PW12O40 occurs on the blank Au electrode. This peak is associated with the reduction of the DPA2+ dications and/or the nondimerized DPA+. cation radicals.23−25 E



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spectral range of 1000−1700 cm−1, the Raman spectrum of RGO (Figure 5, spectrum a) shows the characteristic D and G bands of graphite materials located at 1294 and 1593 cm−1.27 The main Raman lines of the PDPA:PT are situated at 1176, 1342, 1369, 1496, 1581, and 1614 cm−1, and they are attributed to the following vibration modes: C−H bending in the benzene ring, Caromatic−N in the entities of the type N,N′-diphenylbenzidine radical cations, C−C stretching (B) + C−H bending (B), CN stretching, CC stretching in the quinoid ring, and C−C stretching in the benzene ring, respectively.21 The Raman line of lower intensity localized at 1001 cm−1 belongs to the H3PW12O40 heteropolyanions and is assigned to the P−O vibration mode in the PO4 sites.28 The inset of Figure 5 presents the stretching vibration mode of the P−O bond in PO4 sites of the PTA, which is situated at 1010 cm−1. According to our previous work,2 the down shift of the stretching vibration mode of the P−O bond in PO4 sites is assigned to the insertion of the PTA heteropolyanions in the PDPA

According to Hua et al.,26 the repeating units of macromolecular chains of PDPA may contain different entities of quinoid rings and N,N′-diphenylbenzidine in the case of PDPA in the undoped state as well as N,N′-diphenylbenzidine radical cations and N,N′-diphenylbenzidine dications in the case of PDPA in the doped state. In the case of the Au electrode covered with RGO film, significant changes in the shape and current densities of the cyclic voltammograms are found. An upshift of the two oxidation maxima from 0.45 and 0.74 V (Figure 4a1−a3) to 0.50 and 0.84 V (Figure 4b1−b3), accompanied by a down shift of the reduction peak from 0.38 V (Figure 4a1−a3) to 0.36 V vs Ag/AgCl (Figure 4b1−b3), is reported for the electrochemical polymerization of DPA in the presence of H3PW12O40, when the blank Au working electrode was replaced with the RGO film coated Au plate. In comparison with the blank Au support, the use of a gold electrode coated with RGO film leads to an approximately 2-fold increase in the current density of the oxidation peak situated in the potential range of 0.34−0.55 V vs Ag/AgCl, as is observed in Figure 4. This experimental observation, most likely, is a direct consequence of covalent grafting of RGO with the macromolecular chain of PDPA doped with the PTA heteropolyanions. Raman light scattering, FTIR spectroscopy, and photoluminescence provide further evidence for this hypothesis. Figure 5 shows the Raman spectra of

Figure 5. Raman spectra recorded at λexc = 1064 nm of the RGO film deposited onto the Au support (a) before and after electrochemical synthesis of PDPA during 25 (b), 50 (c), and 100 (d) cyclic voltammograms conducted in the potential range (+100; +960) mV vs Ag/AgCl with stopping at the potential of +960 mV. Curve e corresponds to the Raman spectrum of PDPA:PT on a blank Au support after 100 cyclic voltammograms. The inset shows the Raman spectrum of PTA. Figure 6. ATR-IR spectra of PDPA:PT electrochemically synthesized on the Au electrode covered with RGO film after 5 (a), 25 (b), 50 (c), and 100 (d) cyclic voltammograms in the potential range (+100; +960) mV vs Ag/AgCl. Panel e shows the ATR-IR spectrum of PDPA:PT synthesized on the blank Au support. The inset in panel a corresponds to the FTIR spectrum of PTA.

PDPA doped with H3PW12O40 heteropolyanions (PDPA:PT), as well as the RGO film in the initial state and after the electrochemical polymerization of DPA, which was performed with 25, 50, and 100 cyclic voltammograms in the potential range of (+100; +960) mV vs Ag/AgCl. In the F



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Figure 7. Raman spectra recorded at λexc = 457.9 nm (a) and 514.5 nm (b). RGO film deposited onto the Au support before (a1, b1) and after electrochemical synthesis of PDPA during 25 (a2, b2), 50 (a3, b3), and 100 (a4, b4) cyclic voltammograms, and PDPA film deposited onto the blank Au electrode after 100 cyclic voltammograms (a5, b5). Stopping potentials of +960 mV were applied.

macromolecular chain. As expected, upon increasing the number of cycles conducted on the gold electrode covered with RGO film, one observes the appearance of the typical PDPA Raman lines. Spectra b−d in Figure 5 show that, regardless of the number of cycles, no Raman line is observed in the spectral range of 900−1050 cm−1. Two hypotheses may account for this experimental observation. One refers to a process of intercalation of the PDPA macromolecular chain doped with PTA heteropolyanions in-between the RGO sheets. The other suggests that PDPA is doped with Cl− ions. To support one or the other of the two hypotheses, Figure 6 shows the ATR-IR spectra of the electrochemically synthesized RGO/PDPA:PT

composite. The inset of Figure 6a, corresponding to H3PW12O40, shows four FTIR bands situated at 800, 893, 984, and 1080 cm−1, which are assigned to the W−Oc−W stretching mode (Oc is corner oxygen atoms), the W−Oe−W stretching mode (Oe is edge oxygen atoms), the WO bond, and the P−O−W stretching mode, respectively.29 Figure 6e reveals an upshift of the W−Oc−W stretching mode that occurs simultaneously with a down shift of the vibration modes WO and P−O−W in the case of PDPA:PT. As established in our former work,2 these variations were attributed to the incorporation of the H3PW12O40 heteropolyanions into the macromolecular chain of PDPA. G



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The main absorption bands of PDPA situated at 692, 746, 1022, 1155, 1240, 1310, 1430, 1468, 1489, 1591, and 1652 cm−1 are associated with the vibrational modes of interring deformation, benzene ring (B) deformation, quinoid ring (Q) deformation, C−H bending in Q, C−C stretching in B deformation, C−H bending in B, C−C stretching in B + C−H bending in B, CN stretching + C−H bending in B, CN stretching, C−C stretching in B + CC stretching in Q, and CC stretching in −NH+QQNH+−, respectively.21,30 In the case of the electrochemical synthesis of PDPA onto the Au electrode covered with RGO film, the presence of all infrared features of PTA, as well as the down shift of the vibration modes WO and P−O−W, which are evidenced in Figure 6a−d, confirms that PDPA doped with PT heteropolyanions was synthesized on the RGO surface. The absence of the Raman line at 1001 cm−1 in spectra b−d in Figure 5 is a consequence of both a shielding effect resulting from the intercalation of PDPA:PT in-between the RGO sheets and the conditions of nonresonant excitation of Raman spectra of the polymer in the doped state. In agreement with an early resonant Raman study concerning the electrochemical oxidation of DPA,31 the resonant excitation wavelengths for recording of Raman spectra of PDPA in the doped state are 457.9, 514.5, and 647 nm. Figures 7 and 8 show Raman spectra of RGO, PDPA:PT, and the RGO/PDPA:PT composite, recorded under resonant conditions for the polymer in the doped state, namely, at the excitation wavelengths of 457.9, 514.5, and 647 nm, when an increase in the relative intensities of Raman lines of PDPA:PT is expected to be observed as well as the appearance of other new Raman lines. In the case of the excitation wavelength of 457.9 nm, the Raman spectra of PDPA:PT and the RGO/ PDPA:PT composite are accompanied by a significant PL background that hinders to evidence the Raman line at 1001 cm−1 during the first 50 cyclic voltammograms. However, this Raman line is obtained when the Raman spectrum of the RGO/PDPA:PT composite is recorded after 100 cyclic voltammograms. A notably different situation is encountered at the excitation wavelengths of 514.5 and 647 nm. Regardless of the number of cycles performed, here the Raman spectra (Figures 7b1−b5 and 8) show clearly the presence of the Raman line with the maximum at ∼1000−1006 cm−1. Other changes observed in Figures 7 and 8 that are worthwhile to be mentioned are: (i) The appearance of new Raman lines with maxima at 614−616 cm−1 (Figure 8), 885−888 cm−1 (Figure 7), 894 cm−1 (Figure 8), 1205 and 1328 cm−1 (Figure 7, curves b1−b5) both in the case of PDPA:PT and the RGO/PDPA:PT composite; these Raman lines have been assigned to PDPA repeating units having entities of the type N,N′-diphenylbenzidine radical cations.31 (ii) In the case of PDPA:PT electrochemically synthesized on the blank Au electrode by 100 cyclic voltammograms, the ratios between the relative intensities of the Raman lines situated in the spectral range of 1500−1650 cm−1 and those having maxima peaked at 615, 886, 1203, and 1327 cm−1 have the values equal with I1500−1650/I615 = 8.4, I1500−1650/I886 = 3.9, I1500−1650/I1205 = 21.7, and I1500−1650/ I1328 = 3.1. (iii) In the case of PDPA:PT electrochemically synthesized on the Au electrode covered with RGO film after 100 cyclic voltammograms, a significant increase in the

Figure 8. Raman spectra recorded at λexc = 647 nm. RGO film deposited onto the Au support before (a) and after electrochemical synthesis of PDPA during 25 (b), 50 (c), and 100 cyclic voltammograms (d), and PDPA film deposited on the blank Au electrode after 100 cyclic voltammograms (e). Stopping potentials of +960 mV were applied.

Figure 9. SEM pictures of the RGO film before (a) and after the grafting with PDPA:PT (b).

relative intensities of the Raman lines assigned to PDPA repeating units having entities of the type N,N′diphenylbenzidine radical cations should be noted. A consequence of this fact is the change of the values of the H



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Scheme 2. Molecular Structure of RGO Covalently Grafted with PDPA Doped with PW12O403− Heteropolyanion

Scheme 3. Reactions of the Nongrafted RGO with the PDPA Doped with Heteropolyanions H2PW12O40− and HPW12O402−, Respectively

ratios I1500−1650/I615, I1500−1650/I886, I1500−1650/I1205, and I1500−1650/I1328 as follows: 2, 2.4, 1.1, and 1.4, respectively. All of these changes originate in both the intercalation process of PDPA:PT in-between the RGO sheets and the molecular structure of the samples resulting from the electrochemical process. Figure 9 shows SEM images of RGO before and after intercalation and grafting of RGO with PDPA:PT. Having a closer look at Figure 6, the intercalation process can be better revealed by comparing

the ratio between the relative intensities of the vibrational modes of inter-ring deformation (692−685 cm−1), benzene ring (B) deformation (746−741 cm−1), and C−H bending in B (1310 cm−1) with those assigned to the vibration CN stretching + C−H bending in B (1468 cm−1). The intensity ratios I1468/I685, I1468/I741, and I1468/I1310 (Figure 6d,e) are double in value, when comparing PDPA:PT with RGO/PDA:PT for the same number of polymerization cycles (Table 1). I



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Figure 10. Photoluminescence (PL) spectra of PDPA synthesized by cyclic voltammetry using as working electrode a blank Au electrode (a1), and a gold plate covered with a RGO film (a2) at the excitation wavelength of 275 nm. Curves in black, red, and blue correspond to the samples resulting from 5, 25, and 50 cyclic voltammograms, respectively. The spectral composition of the emission of the samples obtained on a blank Au electrode and a gold plate covered with RGO film after 5 and 50 cyclic voltammograms is shown in panels b1−b2 and c1−c2, respectively. Panel d shows the PL excitation spectra of PDPA:PT obtained on a Au support (black curve) and RGO covalently grafted with PDPA:PT (red curve).

Table 1. Value of the Ratios I1468/I685, I1468/I741, I1468/I1310, and I1468/I1591 as a Function of the Number of Cycles Performed on the Blank Au Support and the Au Electrode Covered with RGO Film working electrode

number of cycles

I1468/I685

I1468/I741

Au electrode RGO/Au electrode RGO/Au electrode RGO/Au electrode RGO/Au electrode

100 100 50 25 5

1.63 3.43 9.73 17.89 21.45

1.76 2.97 10.93 12.26 24.54

ring deformation in the case of PDPA:PT electrochemically synthesized onto Au electrode covered with RGO film in comparison with the sample of PDPA:PT deposited on the Au support indicates an intercalation of the macromolecular chain between the RGO sheets. Other variations in the FTIR spectra of PDPA:PT electrochemically synthesized on a Au electrode covered with RGO film in comparison with the PDPA:PT deposited on a Au support consist of (i) a gradual down shift of the vibrational mode C−C stretching in B + C−H bending in B from 1430 cm−1 (Figure 6e) to 1415 cm−1 (Figure 6a); (ii) a change in the relative intensities of the two absorption bands situated in the spectral range of 1500−1700 cm−1, namely, ∼1590 and ∼1660 cm−1, assigned to the vibrational modes of C−C stretching in B + CC stretching in Q and CC stretching in −NH+QQNH+− from 2.5:1 (Figure 6a) to ∼0.9:1 (Figure 6e), respectively; and (iii) the absence of the

I1468/I1310 I1591/I1655 2 2.4 4.2 4.4 32.6

2.5 1.27 0.8 0.9 0.9

This difference is even larger for the samples obtained after the 25th and 5th cyclic voltammograms, respectively. The low relative intensity of the absorption bands assigned to the vibrational modes of the inter-ring deformation and benzene J



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Figure 11. Variation of the PL intensity of PDPA:PT, electrochemically synthesized (50 cycles) on the Au electrode (a) and on the Au plate covered with RGO film (b), depending on the irradiation time at a 275 nm excitation wavelength. Black curves with solid and dashed lines in panels a and b correspond to the initial state and the intermediary state, while the red curves correspond to the final state after irradiation. Insets a1 and b1 show the deconvolution of PL spectra of the PDPA:PT/RGO composite in the initial state and after the UV irradiation time of 40 min. Panel c shows the variation of the PL intensity of the PDPA:PT and the PDPA:PT/RGO composite under UV irradiation.

absorption band at 1489 cm−1, for the samples prepared by 25 and 5 cyclic voltammograms in the potential range (+100; +960) mV vs Ag/AgCl. Evidently, these changes must be correlated with the molecular structure of the samples resulting from the electrochemical process. As shown above by the use of the Au electrode, the electrochemical polymerization of DPA in the presence of PTA leads to the formation of PDPA doped with PT heteropolyanions. In this context, we note that PDPA macromolecular chains doped with heteropolyanions H2PW12O40−, HPW12O402−, and PW12O403− can be obtained during electrochemical polymerization of DPA in the presence of PTA. According to Sadakane et al.,32 the most stable PT heteropolyanions are those obtained after stepwise multielectron redox reactions, namely, PW12O403−. The molecular structure of RGO covalently grafted with PDPA doped with a PW12O403− heteropolyanion is shown in Scheme 2. In the case of the samples prepared by 5 and 25 cyclic voltammograms in the potential range (+100; +960) mV vs Ag/AgCl using the Au electrode covered with RGO film, the absence of the absorption band at 1489 cm−1, assigned to the vibration mode of CN stretching, indicates that a charge

transfer between the nongrafted RGO and the PDPA doped with heteropolyanions H2PW12O40− and HPW12O402− takes place in the early stage, according to Scheme 3. As the number of cycles increases, the amount of the nongrafted RGO decreases, and the charge transfer invoked between nongrafted RGO and PDPA doped with heteropolyanions H2PW12O40− and HPW12O402− does not occur, the fact which explains the presence of the absorption band at 1489 cm−1 in samples obtained by 100 cyclic voltammograms on the Au plate covered with RGO film (Figure 6d). The reaction products resulting from the electropolymerization of DPA on the Au electrode covered with RGO film are shown in Schemes 2 and 3. Additional arguments for producing a charge transfer between nongrafted RGO and PDPA:PT in the samples obtained by recording 5 cyclic voltammograms are revealed by photoluminescence (PL) studies. Figure 10a1,a2 shows the PL spectra of PDPA:PT synthesized on the blank Au support and on the Au plate covered with RGO film, respectively. Regardless of the cycle numbers, the samples synthesized onto the Au plate covered with RGO film are characterized by a K



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Scheme 4. Photochemical Reaction of the RGO/PDPA:PT Composite Material with the A3 Molecular Structure under UV Irradiation

lower PL intensity in comparison with that reported for the samples obtained on the blank Au electrode; in other words, the presence of RGO induces a quenching effect on PDPA:PT PL. To date, such a behavior was reported only for the composites RGO/P3HTh8 and RGO/MEH-PPV.10 In addition, significant differences are seen in the shape of the PL spectra of samples of PDPA:PT electrochemically synthesized on the Au electrode and the sample prepared on the Au plate covered with RGO film. According to Figure 10b1, the PL spectrum of the sample PDPA doped with H3PW12O40 heteropolyanions prepared by 5 cyclic voltammograms can be deconvoluted into three Gaussian components with maxima at 2.65, 2.95, and 3.2 eV, which are assigned to different entities of the macromolecular chain,33 namely, PDPA doped with heteropolyanions PW12O403−, HPW12O402−, and H2PW12O40−, respectively. Figure 10b2 shows the PL spectrum of PDPA:PT synthesized on the Au electrode covered with RGO film. Comparing panels b1 and b2 in Figure 10, one observes the disappearance of the component at 3.2 eV and the upshift of the components at 2.65 and 2.95 eV to 2.77 and 3.04 eV, respectively. In our opinion, the disappearance of the component at 3.2 eV is the result of the charge transfer between nongrafted RGO and the PDPA doped with heteropolyanions H2PW12O40− and HPW12O402−, which contributes to a band-gap widening, as is shown in Figure 7d. This is also the origin of the upshift of the PL bands at 2.65 and 2.95 eV. Such a modification of the band gap was also observed in other conjugated polymers and is determined by the length of π-conjugation, polymerization degree, or steric hindrance effects that induce changes in the molecular architecture.32 As shown in Figure 10c1,c2, the increase of the number of cycles in the preparation of PDPA:PT on the blank Au support or Au plate covered with RGO film leads to similar PL spectra, which can be explained by a full covalent grafting of RGO with PDPA:PT. Besides, different behavior is reported under UV irradiation of PDPA:PT and RGO covalently grafted with polymer. In comparison with the polymer alone, the relative intensity of the PL spectrum of RGO covalent grafted

with PDPA:PT increases under UV irradiation, as is evidenced in Figure 11. An analysis of the spectral composition of the PL spectra recorded in the initial state and after 40 min of UV irradiation reveals an upshift of the PL band from 2.72 to 2.75 eV. Relating to this modification, it is interesting to note that the energy of separation between the two PL bands situated in the lowenergy range, namely, 2.55 and 2.75 eV, is 0.2 eV. This value corresponds to the activation of the Raman line at 1613 cm−1 assigned to the vibrational mode C−C stretching in the benzene ring. This fact indicates that the UV irradiation induced photochemical reactions are correlated with the vibrational mode of C−C stretching in the benzene ring. Taking into account that the most stable heteropolyanion of PTA is PW12O403−, we assume that a transformation of RGO grafted with PDPA doped with HPW12O402− heteropolyanions into RGO grafted with PDPA doped with PW12O403− ions occurs, as is suggested in Scheme 4.

4. CONCLUSIONS This work reports the electrochemical polymerization of diphenylamine (DPA) on reduced graphene oxide (RGO), resulting in a corresponding RGO/PDPA composite. Novel insights concerning the interaction of RGO with DPA and PDPA doped with phosphotungstic acid heteropolyanions (PDPA:PT) are revealed by UV−vis spectroscopy, FTIR spectroscopy, Raman light scattering, and photoluminescence studies. The following results may be highlighted: (i) UV−vis spectroscopy and photoluminescence (PL) on blends of RGO/DPA evidence the partial transformation of the DPA monomers into oligomers of polydiphenylamine (PDPA) upon UV irradiation. Therefore, the absence of light is a critical condition for the electrochemical synthesis of RGO/PDPA composites. (ii) Raman scattering and FTIR spectroscopy reveal a covalent grafting of RGO with PDPA:PT. A PDPA:PT PL quenching effect is reported to be induced by RGO that is accompanied by a charge transfer that takes place L



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at the interface of the two constituents, nongrafted RGO and PDPA:PT. (iii) Important variations in PL properties of RGO sheets covalently grafted with PDPA:PT are reported under UV irradiation of this composite material. The increase in the relative intensity of the PL spectrum of RGO covalently grafted with PDPA:PT reveals photochemical reactions under UV irradiation that involve the transformation of the RGO/PDPA:PT composite containing HPW12O402− anions into an RGO/PDPA:PT composite stabilized by PW12O403− anions. This work provides new insights on the interactions between RGO, conjugated polymers, and stabilizing dopant ions. Moreover, it provides valuable feedback on the synthesis conditions, which should be applied to achieve respective composites with reproducible physicochemical properties.

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 40 21 3690170. E-mail: barac@infim.ro (M.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Romanian National Authority for Scientific Research (project PN09-450103), Spanish Ministry MINECO (project MAT2010-15026), CSIC (project 201080E124, Ph.D. grant JAEPre09-01155 for J.D.N.), and the Regional Government of Aragon and European Social Fund DGA-ESF (T66 Grupo Consolidado) are gratefully acknowledged. The authors thank Mr. I. Mercioniu for SEM pictures.

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Electrochemical Grafting of Reduced Graphene Oxide with

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