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Synthesis of reduced graphene oxide-poly(phenyleneethynylene) hybrids. A supramolecular and photophysical analyses
Polymer 122 (2017) 174e183
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Synthesis of reduced graphene oxide-poly(phenyleneethynylene) hybrids. A supramolecular and photophysical analyses nez-Barrera, Gleb Turlakov, Eduardo Arias*, Ivana Moggio**, Rosa M. Jime lez-Morones, Salvador Ferna ndez, Oliverio Rodríguez, Carlos Avila-Orta, Pablo Gonza Ronald F. Ziolo n en Química Aplicada, Boulevard Enrique Reyna 140, 25294 Saltillo, Coahuila, Mexico Centro de Investigacio
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2017 Received in revised form 23 June 2017 Accepted 24 June 2017 Available online 27 June 2017
Functionalization of graphene oxide (GO) by esterification or amidation reactions with a series of conjugated phenyleneethynylene (PPEs) copolymers bearing different electron-donating and/or withdrawing groups was achieved by microwave irradiation in just 90 min. Copolymers as well as GO-PPE hybrids were characterized by Raman, UVeVis, static and dynamic fluorescence spectroscopy, transmission electron microscopy, X-rays diffraction and scanning tunneling microscopy. Effective interaction of the copolymers with GO is evidenced by a strong quenching of the PPE's fluorescence quantum yield (f), and by an increase of the non-radiative decay constant (Knr); features that suggest energy transfer from PPEs copolymers to GO. HRTEM shows that the molecules of copolymers self-assemble in blocks resembling bricks or board-like shapes that can be classified as sanidic LCs materials as a general term to identify their mesomorphism. Functionalization is mainly carried out at the edges of the GO sheets, so the GO-sheets are not totally covered, even by increasing the PPE-vs-GO ratio because of the trend of the PPEs in to self-assemble in an “edge-on” conformation, meaning that the conjugated backbones are parallel to the GO surface, rather than in “face-on”, where the backbones are flat-lying on the GO surface. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Microwave Graphene oxide Phenyleneethynylenes
1. Introduction Functionalization of GO with conjugated polymers (GO-CPs) is an attractive research line in electronic and optoelectronic area, in particular for photovoltaic applications from the point of view of i) the development of electrodes, i.e. dispersed GO-PCs layers can be deposited on glass slides by techniques such as Langmuir-Blodgett, dipping, etc. The deposited layers can further be submitted to a GO reduction process increasing and/or modulating the hole or electron mobility of the deposited layers, ii) or as buffer layers or iii) even as active layers in organic heterojunction photovoltaic devices. The key parameters for efficient organic photovoltaic devices are fast charge separation and efficient transport to the electrodes, so it is of crucial importance to assure that materials present a good interpenetrating donor-acceptor phase without segregation.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E. Arias), [email protected]. mx (I. Moggio). http://dx.doi.org/10.1016/j.polymer.2017.06.068 0032-3861/© 2017 Elsevier Ltd. All rights reserved.
Generally, the active layer in organic bulk heterojunction solar cells is composed of an electron donor conjugated polymer and electron acceptor material, e.g. fullerene, carbon nanotube or graphene. There are many reports where graphene or graphene oxide were used as electron acceptor for solar cells applications [1e3]. Generally, two strategies are used to obtain graphene and graphene oxide interaction with conjugated polymers. The first one consists of p-p interactions between conjugated parts of graphene with the polymer backbone [4]. The second one is based on the direct covalent connection [5] of the functional conjugated polymers on the GO by means of esterification, amidation, click chemistry, nitrene chemistry, radical addition, etc. The esterification reaction seems to be a practical via for attaching a conjugated polymer to graphene oxide, but the polymer must bear hydroxyl groups in the side chains. In this respect, the Sonogashira polycondensation reaction supports a large variety of functional groups including the hydroxyls in monomers without being disturbed, but carboxylic acids of GO require activation with a proper catalyst. In what concerns to the functionalization, Salavagione [6] suggests that it is important to use an excess of esterified macromolecule owing to the lack of data about the quantity of carboxylic groups on
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GO's surface. A typical example is the esterification of hydroxyl terminated poly(3-hexyl thiophene) with GO (4:1 w/w) [7]. However, the main drawback of the GO functionalization is still the long reaction times due to the difficulty to exfoliate the GO sheets for esterification reactions. Microwave irradiation (MI) as a suitable and rapid heating method was used for preparing exfoliated graphite [8,9] and recently, it was applied for performing graphene oxide exfoliation and reduction [10] including the heating of its suspensions in aqueous or organic media [11]. It has been demonstrated that MI promotes not only the GO exfoliation by interpenetrating the wave between GO sheets, but also the carboxylic acid groups are activated. In this work a series of phenyleneethynylene (PPEs) copolymers were synthesized bearing hydroxyl terminated chains in order to esterify the GO, a particular copolymer bearing hydrazide in the main chain (EV) was also evaluated in order to carry out reactions with the epoxy groups that usually are found in the GO plane. The GO chemical functionalization by MI was carried out in 90 min, instead of the typical 6 days applied by thermal chemical solution via. Functionalized GO-PPE's hybrids were characterized by Raman spectroscopy, where the D (1350 cm1), G (1585 cm1), and the ethynylene (~2200 cm1) vibration bands were clearly identified. Energy transfer process from the PPE's to the GO was also determined, since the fluorescence of the PPE's with quantum yield e.g. of 63% for EV is drastically shouted down to 5.0%, for its GO-EV hybrid.
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radiation at a wavelength of 0.1542 nm. STM characterization was performed by using a tip mechanically cut from Pt/Ir wire (80/20, diameter 0.25 mm, Nanoscience) and another one prepared from tungsten by electrochemical etching in KOH solution using an applied voltage of 4 V. The photophysical characterization was carried out in spectroscopic grade (Aldrich) toluene, THF and CH2Cl2. UVeVis and fluorescence spectra were recorded on an Agilent 8453 spectrophotometer and a Perkin Elmer LS50B spectrofluorimeter or Horiba Quantamaster 8075e11, respectively. Fluorescence quantum yield (f) was obtained according to literature method [13] with quinine sulphate (0.1 M in H2SO4) as standard. Three different solutions with optical density at the excitation wavelength <0.1 were analyzed at 25.0 ± 0.3 C by using a circulating water bath and the values were averaged. The excitation wavelength is reported in the tables. Lifetimes were obtained by TCSPC (Time-correlated single photon counting) with a Tempro Horiba equipment with a 370 nm nanoLED. A 0.01% suspension of Ludox AS40 (Aldrich) in ultrapure water was used for the prompt signal. Calibration of the equipment was realized with a POPOP [1,4-Bis(4-methyl-5-fenil-2-oxazolyl)benzene] methanol solution (optical density <0.1 and lifetime of 0.93 ns [14]). Data were fit in the software DAS6 available with the equipment. 2.2. Synthesis
2. Experimental section
Experimental procedures, chemical and physicochemical characterization of each compound are given in the supplementary information section.
2.1. Equipment and methods
3. Results and discussion
The microwave copolymerizations were carried out in close quartz vials in a Microwave Apparatus Monowave 300 from Anton Parr at 300 Watts of constant irradiation power, during 90 min at 190 C and 600 rpm. 1H (300 MHz), 13C NMR (75.4 MHz) spectra were obtained at room temperature with a Jeol Eclipse spectrometer using CDCl3 as solvent and internal reference. The molecular weights were determined by GPC on an HP 1100 HPLC using PS standards and refractive index as detector. A series of three HP PLGel columns were used: 103, 105 and 106 Å and THF as the mobile phase at 40 C and 1 mL/min. Raman spectra were recorded on a Horiba Xplora equipment, focusing the sample as powders on microscopic slide with a 10 objective. The excitation wavelength was 785 nm and the Nanoled power was 25% of total laser power (25 mW). Spectra were acquired with 10s acquisition time and 5 cycles, spectral resolution of 2 cm1. For electron microscopy studies, all of the materials were deposited by casting from CHCl3 (2 mg/mL) solutions on Lacey carbon grids and were examined by SAED and HRTEM techniques in an FEI-TITAN-200-300 kV field emission gun microscope, which has a symmetrical condenserobjective lens type S-TWIN (with an spherical aberration Cs ¼ 1.25 mm). All of the STM images were obtained at ambient conditions using a compact STM AA5000 Scanning Probe Microscope from Angstrom Advanced Inc. Calibration of the scanner tube was performed by means of atomic resolution images obtained from commercial HOPG sample, exfoliated before each measurement by the adhesive tape method. All measurements were performed in the constant-current mode. Details on the experiment bias and current set-point are given below the images. The raw and FFT images were processed from WSxM 5.0 [12] and Gwyddion software with the aim to reduce noise and normally observed drift. The samples were prepared by directly depositing a drop (~1e2 mL) of diluted solutions (104 e105 M) of copolymers in phenyloctane. Small and wide angle X-ray scattering patterns were obtained with an Anton Paar SAXSess mc2 SWAXS instrument using CuKa
3.1. Copolymers synthesis and characterization One semi-rigid copolymer hereafter named EV and three rigid rod-like copolymers named EC, LC and MJ were synthesized according to the chemical routes given in Scheme 1 and Scheme 1S (supporting information). In general the pathway involved only two basic reactions: i) amidation of the 4-iodo benzoic acid 1 with hydrazine hydrate 2 under the EDAC/DMAP-mediated protocol in CH2Cl2, to give 4iodo-N'-(4-iodobenzoyl)benzohydrazide 3 in 53% yield, and the esterification of 2,5-dibromobenzoic acid 1S with three different bromoalkanes 3S, 4S, 5S by the action of DBU, Scheme 1S (supporting information). Then, ii) the Sonogashira Pd/CuI crosscoupling polymerization between the corresponding diiodo or dibromo terminated monomer 3, 6, 6S, or 8S with the corresponding ethynylene 4, 5 or 7 to afford the copolymers EV in 57%, LC 70%, MJ 85% and EC in 67% yield. The relatively moderate yields of EV and EC are due to the fact that DMF was used as a co-solvent for the polymerization, it is well known that co-solvents highly increase the molecular weights giving rise to insoluble aggregates. After polymerization, each copolymer was submitted to a metal decomplexation process by precipitation once in a methanol sodium dithiocarbamate solution, and twice with clean methanol. The shortest oligomers were eliminated by preparative SEC column (Biorad, Bio-Beads SX1) using toluene as eluent. The proton resonance peaks of PPEs are broader than those of the corresponding monomers; it was observed that the acetylenic protons disappeared and in contrast, the protons near the iodine or bromine could be identified, suggesting that all of the chains are mainly halogen terminated, Fig. 1S (supporting information). The calculated average molecular weight (Mw), number average molecular mass (Mn), and polydispersity index (PI) using PS standards and refraction index as detector are: Mw ¼ 33729, Mn ¼ 25668, PI ¼ 1.31 for MJ; Mw ¼ 11006, Mn ¼ 4909, PI ¼ 2.24 for LC;
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Scheme 1. Reagents and conditions: (a) EDAC, DMAP, THF, 0 C to reflux, 17 h, 53% yield; (b) [(C6H5)3P]2PdCl2 (3.0% mol), CuI (1.5% mol), Et3N, 60 C, 36 h 60e75% yield.
Mw ¼ 6591, Mn ¼ 1866, PI ¼ 3.5 for EV. All of the PPEs are soluble in CH2Cl2, CHCl3, THF, toluene and mostly precipitate in methanol, acetone and DMF. In contrast, gelation was observed for EC at r.t. being soluble in a mixture of hot DCB:DMF, likely due to the organogelator property of the steroid cholesteryl group [15]. 3.2. Microwave functionalization of GO with copolymers In this work, a two-step oxidation approach was applied to generate more carboxylic acid groups on graphene oxide than epoxy and/or hydroxyl groups by a modified Hummers method. This consisted in pre-oxidizing the graphite by using a mixture of sulfuric acid, K2S2O8 and P2O5. Then, the pre-oxidized graphite was oxidized with a mixture of KMnO4, NaNO3, H2SO4 and H2O2. The GO was characterized by WAXD, Raman and UVeVis spectroscopy. The results of these two latter techniques will be discussed in the GOPPEs section, as term of comparison. The X-ray diffraction pattern showed a shift of the typical sharp (002) peak from 2q ¼ 26.3 of pristine graphite to 2q ¼ 13.88 for GO, confirming the exfoliation, with an interlayer space of d ¼ 0.64 nm, Fig. 2S (supporting information). We observed that GO always contained traces of Nitrogen, probably from the NaNO3, that can be removed by continuous dialysis and if GO is always maintaining in solution and changing the water monthly, thus pure GO can be obtained. Esterification of the GO with the four PPEs to form the corresponding hybrids, Scheme 2, was carried out by using the DCC protocol, but with the addition of DPTS/4-Py [16] in DMF to promote a better activation of the highly steric carboxylic acids. DMF is a good solvent for dispersing GO, but not for PPEs, which tend to precipitate. In order to avoid PPEs aggregation during the GO-PPEs
functionalization, and after having proved different alternatives of reaction, we decided to sonicate DMF dispersion, and then add the rest of the reactants in dichlorobenzene. We note that the dark solutions showed fluorescence (by exciting at 365 nm), which completely disappeared at the end of the reaction. Since the p-p GO-PPEs interaction is very strong, the workup after reaction consisted in successive washings with hot solvents with different polarities to obtain non fluorescent solutions. It is worth to be mentioned that the microwave copolymerization was achieved in just 90 min contra 6 days by conventional solution reaction. The GO-PPEs hybrids were maintained in 1,2-diclorobenzene. 3.3. Vibrational properties Fig. 1a shows the Raman spectra of the PPEs copolymers obtained by exciting at 785 nm wavelength, out of their absorption range, to avoid the strong fluorescence of the materials in the visible region. The band at around 2200 cm1 associated with the -C≡C- stretching vibrational modes gives information on the pelectrons delocalization along the conjugated backbone, while that at 1592 cm1 is associated with the aromatic (-C¼C-) stretching vibrations [17]. A shift to lower frequencies of the ethynylene group is found for MJ (n ¼ 2195 cm1) with respect to EC (2202 cm1), LC (n ¼ 2204 cm1), and EV (n ¼ 2204 cm1). The higher frequency associated to the triple bond stretching in MJ is a result of its large polarizability in agreement with the electron donor character of the 2,5-bis dodecanoxy phenyl moiety. This moiety is also part of the EV copolymer, however its conjugation is interrupted by the hydrazide segment. Fig. 1b shows the spectra of GO-PPEs hybrids. The structural
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Scheme 2. Reagents and conditions: (a) (a) DCC/DPTS/4-PPy, DMF/DCB, 300 W, 4.75 GHz, 600 rpm, 90 min, 150 C.
Fig. 1. Raman spectra of (a) PPE copolymers and (b) GO-PPEs hybrids.
changes that occur in graphene oxide-based materials are evidenced by shifts or intensity changes of the D band (~1345 cm1,
breathing mode of A1g, C sp3 atoms) and G band (~1586 cm1; the in-plane bond-stretching motion of pairs of C sp2 atoms, E2g mode).
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The D band gives information about the presence of defects and the intensity ratio ID/IG is used to distinguish the quality of graphene and to determine the quantity of graphene layers [18,19] The Raman spectra of the hybrids present bands attributed to the copolymer (-C≡C- stretching vibrational mode, around 2200 cm1) and those typical of graphene oxide. The G band of GO-PPEs: GO-LC (1585 cm1), GO-EV (1598 cm1), GO-MJ (1592 cm1), GO-EC (1596 cm1), shows a broadening and upshifting relative to GO at 1586 cm1. According to Qi et al. [20], a shift to higher frequencies is generated by electron acceptor macromolecules, i.e. n-doping process, which in our case is consistent with the electronwithdrawing character of the ester or hydrazide groups of the PPEs. Accordingly, the D band also upshifts from 1345 cm1 (GO) to ~1350 cm1 for GO-PPEs. We note that the intensity ratio ID/IG, which is usually a measure of the disorder of the sp3/sp2 carbons of GO, for the GO-PPEs is noticeably reduced from 1.03 for GO, e.g. to 0.65 for GO-MJ. The reduction may be attributed to both, reduction of the GO layers during the microwave reaction and to the increase of the sp2 C atoms due to the PPEs presence. 3.4. Supramolecular order 3.4.1. HRTEM In order to verify the presence of the copolymers on the GO, we carried out a study of the supramolecular order of both, the PPEs copolymers and of the GO-PPEs hybrids by TEM, SWAX and STM. Fig. 2aeb shows the HRTEM image of EC copolymer. We recently reported that a series of oligomers of both: (cholesteryl) [19] or (diethylene glycol methyl ether) [21] benzoateethynylene and of their homologue polymers, when characterized by HRTEM, shows that what is seen in planar view corresponds to the benzoateethynylenes backbones, with the cholesteryls or the diethylene glycol methyl ether lengthened perpendicularly. The calculated periodicity of c.a. 0.33 nm coincided with the distance between conjugated benzoates. In this respect: 1) similar observations were found for the PPEs, Fig. 2 and Fig3S (supporting information), which show periodicity (DL) of 0.32 nm for (EC), 0.34 nm for (MJ), 0.29 nm for (EV) and of 0.23 nm for (LC), indicating that they are standing perpendicular to the substrate as is sketched in Fig. 2c. We also observed, that the molecules are packed in random blocks, as confirmed by their selected area electron diffraction patterns (SAED), where many diffraction spots with the same periodicity can be seen e.g. for MJ in Fig. 3S. The type of lateral chain increases or decreases this distance, for instance cholesteryl presence increases
it to 0.32 nm for EC, while 11-undecanol chains decreases it to 0.23 nm for LC. Additionally, we found a direct correlation of this DL value with the materials solubility; the higher the value, the more soluble is the copolymer. In contrast for low values, insoluble aggregates are formed. HRTEM of the GO-PPEs hybrids showed in general that GO sheets are not totally covered by the copolymers; certain zones are denser (zone a-b, Fig. 3), while others are practically uncovered (zone c, Fig. 3), a periodicity of ca. 0.33 nm was found in all of the hybrids (zone a, Fig. 3) with the exception of copolymer MJ where an additional periodicity of 1.03 nm was also observed (zone b, Fig. 3). Multiple diffraction spots were observed on the FFT of the HRTEM images, which we attributed to blocks of copolymer randomly oriented, such as edge-on, in which the p-p stacking is perpendicular to the substrate and face-on in which the p-p stacking is parallel to the substrate [22], or in a transitional state between them, as is sketched in Fig. 3, some spots correspond to the graphene oxide lattice. 3.4.2. STM We and others had found that phenyleneethynylene macromolecules are difficult to be analyzed by scanning tunneling microscopy, mainly due to the strong p-p interaction that governs in the conjugated backbones and that gives rise to the formation of fibrils once the solution is deposited on substrates. The previous HRTEM data suggests that MJ deposits on GO in two different supramolecular assemblies: parallel and perpendicular. We analyzed all of the copolymers at the molecular level by STM, and found that effectively only MJ gave evidence of molecular order. The freshly cleaved HOPG was used to calibrate the STM, Fig. 4. The lighter regions represent high points on the surface or areas of larger electron density, while the darker regions represent lower points on the surface or lower electron density. The bright and dark areas on the image correspond to alternate atoms. The variation of electron density on the surface is caused by variations in the delocalized p-orbitals. It is well known that the six carbon atoms in each hexagon of the topmost graphene layer form two groups designated as a and b atoms [23]. The former ones are located atop of the carbon atoms of the underlying graphene layer and the latter ones right in the hollow sites of the underlying carbon hexagons of the second carbon layer as depicted in the model of Fig. 4b. The calculated distance between two maxima is 0.223 nm, (i.e. 2.009/9), Fig. 4c, while that reported in literature [24] is of 0.246 nm; this difference is attributed to the thermal drift of the STM tip.
Fig. 2. (a) HRTEM image of a casting film of EC copolymer deposited on a lacy carbon grid showing a periodicity of 0.32 nm and corresponding to the distances between conjugated back bones, (b) HRTEM image of the same film and (c) sketch of the molecular arrangement seen from a lateral view.
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Fig. 3. HRTEM image of a casting film of GO-EC hybrid deposited on a lacy carbon grid. The sketch gives a visual description of the different molecular assemblies of the copolymer on the GO surface according to their calculated periodicity; edge-on (region b) in which the conjugated backbone are assembled in parallel to the GO, and face-on (region a) for their assembly perpendicular to the GO; region c represents the unattended GO.
Fig. 4. (a) Filtered STM image of HOPG; at capture conditions: Vbias ¼ 0.1 V, Itunnelling ¼ 1 nA, (b) proposed molecular packing of graphite layers and (c) line profile along the line indicated in figure (a), the distance between two maxima is 0.223 nm.
Fig. 5 presents the large scale and high-resolution images of copolymer MJ on HOPG. We note that the molecular images correspond predominantly to short macromolecular segments, while single layers of molecules were not observed, but rather as a package, with enough thickness to allow their characterization by tunneling current characterization. One of these packages shows levels of contrast as visualized in Fig. 5aeb. The bright lines can be attributed to unsaturated hydrocarbon represented by the copolymer backbones (due to a higher tunneling current at the STM tip with the p electrons), while the dark ones are related to interdigitated lateral chains. Fig. 5a is composed of two segments: the ordered structure of copolymers and the amorphous part. Individual segments of conjugated chains of MJ are clearly seen at high resolution, Fig. 5c. This image showing PPE backbone folding, indicated by green arrows, corroborates previously reports, that the PPE backbone rods are elastic [25], in fact the FFT of the corresponding zone, Fig. 5d, shows diffraction spots, and we attribute one spot to one block of molecules, where in
one of these blocks an intercalant and folded molecule was identified. For these particular blocks, molecules appear flat-lying (faceon) and densely packed allowing to see the rods on the HOPG surface and following the substrate directions, likely due to strong p-p interactions. The distance obtained from the profile is 2.48 nm corresponding to the end-to-end length of two neighbor conjugated chains, where the alkyl chains are interdigitated as sketched in Fig. 5e. This observation is in good agreement with the 2.16 nm distances determined by X-ray (vide supra) and with similar values and proposed models for molecules reported in the literature [26]. 3.4.3. SWAXS Since the HRTEM showed that all of the PPEs are not amorphous, but rather randomly organized in blocks, they were analyzed by small-wide angle X-ray scattering. Fig. 6 shows the SWAX spectra of the copolymers at room temperature. It is to point out that the four copolymers showed strong birefringence under polarized light as a signature of their mesomorphism, but without exhibiting any
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Fig. 5. STM images obtained at the HOPG-1,2,4-trichlorobenzene (TCBM) interface of MJ copolymer: a) large-scale and b) high-resolution image of the self-assembled MJ conjugated macromolecules; imaging conditions: a) 1000 V, 0.3 nA; b) 1200 V, 79 pA. The high resolution image (c) shows a folded phenyleneethynylene molecule from the marked region in image a). d) Is the corresponding FFT pattern; white arrows indicate the two observed spots; e) proposed schematic molecular conformation of MJ on HOPG, where lateral chains are interdigitated giving a phenyl-to-phenyl distance of 2.48 nm (green arrows correspond to the fold in the structure). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Room temperature X-ray scattering patterns of EV, MJ and EC. The sketches represent the possible molecular packing deduced from the SWAXS patterns.
known texture. The MJ diffraction pattern shows a wide peak at the low-angle region 2q ¼ 4.09 assigned to a first-order reflection with a distance of d1 ¼ 2.16 nm, however two other wide bands of low intense diffractions at 2q ¼ 7.64 , d ¼ 1.16 nm and 2q ¼ 12.56 , d ¼ 0.70 nm giving a ratio of ca. 1:2:3 in 2q, which is indicative of a disordered lamellar system in a staggered molecular packing in order to fulfill the maximum occupied space of volume. We believe that a smectic instead of a nematic mesophase is more reasonable in this case, since the birefringent pasty showed no fluidity when pressed between two glass slides at room temperature. At wide angles, two peaks are observed; a wide halo at 2q ¼ 21 ; d2 ¼ 4.34 Å of the side-to-side distances between packed lateral chains, and at 2q ¼ 23 ; d3 ¼ 3.85 Å originated from the layer-to-layer p-p stacking distance between the adjacent polymer molecules. In its more extended conformation, MJ has a theoretical length of the dodecanoxy-phenyl-dodecanoxy chains of 3.68 nm [19], while that of the benzoate-to-11-undecanol is of 1.93 nm [27]. The interplanar distance matches well when the 11-undecanol chains are interdigitated, as is represented in the sketch accompanying the spectrum. In contrast, the EV copolymer shows fluidity at room temperature and its X-ray pattern exhibits only diffuse bands in both, the small and wide angle regions, which are features of a likely nematic phase. In contrast, the X-ray patterns of EC and LC exhibit very similar features; i) a very sharp peak at low angles 2q ¼ 1.53 , d ¼ 5.77 nm for EC and at 2q ¼ 1.80 , d ¼ 4.90 nm for LC, ii) and a very diffuse
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band at high angles 2q ¼ 19.40 , d ¼ 0.46 nm for EC and at 2q ¼ 22.7, d ¼ 0.39 nm for LC. In its more extended conformation, EC has a theoretical length benzoate-to-cholesteryl of 2.23 nm and that of the benzoate-to-diethylene glycol methyl ether is of 1.73 nm, while for LC the theoretical length benzoate-to-diethylene glycol methyl ether is of 1.49 nm and that of the benzoate-to11undecanol is of 2.16 nm. This suggests that both copolymers self-assemble in bilayers into the lamellae in a typical “comb-like” fashion, where the side chains are oriented in an opposite sense in a back-to-back comb like “style” as is depicted in the side sketch of Fig. 6. 3.5. Photophysical properties All of the photophysical properties of the copolymers and hybrids are collected in Tables 1 and 2. Fig. 7 shows the absorption and fluorescence spectra of the PPEs in chloroform. The absorption spectra present a main peak with maximum at 420 nm (MJ), 386 nm (LC), 380 nm (EC) and 382 nm (EV), which is attributed to the p-p* (HOMO/LUMO) electronic transitions of the conjugated system, in agreement with previous experimental and theoretical studies [17,21,27e29]. EV and MJ also exhibit a peak at around 325 nm that has been previously ascribed to HOMO-1/LUMO electronic transitions in 2,5-dialcoxy substituted phenylenethynylenes [17,28]. Among these two copolymers, the absorption maximum of MJ is red shifted with respect to that of EV, which suggests a larger electronic delocalization. EV presents electron donor dodecanoxy groups in the conjugated unit contra to the benzoate electron withdrawing unit of MJ, however the non-conjugated benzo-hydrazide moiety of EV likely interrupts the electronic delocalization, in agreement with the Raman results. In comparison, both LC and EC present glycolic type benzoate units alternated with (11-undecanol)benzoate (LC) or (cholesteryl) benzoate (EC). The position of their maximum is in good agreement with the maximums observed for their respective homopolymers. For example, the poly((diethylene glycol methyl ether) benzoateethynylene) I-nBEgly [21] showed an absorption maximum at 375 nm in CH2Cl2, the poly((11-undecanol)benzoateethynylene) pPEn [28] is at 380 nm in CHCl3, and the poly((cholesteryl)benzoateethynylene (Br-nPEBzCol) [15] lmax ¼ 380 nm (in CHCl3). In general, the Eg of the four copolymers is in the semiconducting range.
Fig. 7. Normalized absorption and fluorescence (inserted) spectra in CHCl3 of MJ (stars), EC (squares), EV (continous line) and LC (circles).
The fluorescence spectra of MJ, LC and EC show excitonic features with a main peak and a shoulder as reported for other PPEs [15,27]. The emission band of EV is on the contrary broader similarly to what was found for other dodecanoxy sequenced copolymers [30,31]. The trend in the emission maxima is similar to that of the absorption spectra so that the fluorescence spectra of LC and EC are at almost the same wavelength and blue shifted with respect to MJ. EV emission is however more red shifted presenting also the largest Stoke's shift. This feature and the broadness of the fluorescence band suggests that a stronger geometrical change from the fundamental to the excited state occurs for this specific copolymer, likely because of its semirigid structure. In general, the fluorescence decay is mono-exponential with lifetimes of around 0.5e1 ns, Fig. 8, quantum yields f are of moderate value for MJ and EV and low for EC and LC. Non-radiative processes as internal conversion are expected; although Stokes shift (Dn) values are in the range for molecules that undergo a geometry change from aromatic to a more stable planar quinoidal structure after excitation, and being the largest values for EC and EV benzoateethynylene copolymers. However, there is not a clear match between the trend in the f values with those of the Stokes shift (Dn), suggesting that intersystem crossing process are also affecting the fluorescence quantum yield, and in particular for
Table 1 Photophysical properties of PPEs in CHCl3. Eg (eV)
ε X 102 (gL1cm1)b
lem/lexc
(nm)
(nm)
(cm1)
MJ LC EC
327, 420a 386a 380a
2.61 2.79 2.66
1.7 4.4 5.0
465/410 429/376 426/370
2309 2597 2846
EV
325, 382a
2.70
3.9
450/372
3956
Dn
F
PPE
a b
labs
Dn
F
t (ns)
Krad (ns1)
Knr (ns1)
0.50 0.31 0.15
0.72 0.45 0.69
0.69 0.69 0.22
0.69 1.53 1.23
0.53
1.17
0.45
0.40
Q
t (ns)
Krad (ns1)
Knr (ns1)
0.39 0.70 0.18 0.79
0.04 0.06 0.19 0.06
2.53 1.37 5.37 1.20
Maximum absorption. Calculated for the maximum absorption.
Table 2 Photophysical properties of GO-PPE hybrids in DMF. Hybrid GO-MJ GO-LC GO-EC GO-EV
labs
Eg (eV)
lem/lexc (nm)
(nm) 334, 450 455 409 312, 405
2.15 1.55 1.64 1.56
470/410 517/376 500/370 529/372
(cm1) 922 2638 4450 5790
0.014 0.041 0.034 0.050
35.7 7.6 4.4 10.6
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Fig. 10. Fluorescence spectra of GO-MJ in DMF and MJ in CHCl3. Inserted image shows the fluorescence quenching by passing from the MJ copolymer to the GO-MJ hybrid, under 365 nm excitation. Fig. 8. Time resolved decay profiles of the MJ copolymer and GO-MJ hybrid.
the cholesteryl copolymer. In this respect, we expected an intersystem crossing mechanism due to the presence of halogens as termini groups, which are known to decrease the energy of the triplet state. In addition for EC, the cholesteryl substituent enhances this effect due to steric hindrance, as previously reported [15]. The photophysical properties of PPEs strongly change after functionalization with GO. i) When GO is functionalized with PPEs, the absorption spectra present in general the absorption features of the copolymers overlapped with a marked baseline due to GO scattering. Fig. 9 shows the absorption spectrum of GO-MJ hybrid and its precursors GO and MJ, as example. The absorption spectra of the other hybrids are collected in the supporting material. The absorption maximums are red shifted with respect to the pure copolymers and the absorption bands are broader. ii) A similar change is observed in the fluorescence spectra that losses the excitonic features and are bathochromically shifted. These findings suggest an increase in the effective conjugation as a consequence of the functionalization with GO as found in other works [4,7]. Another possible explanation could be that, even in the dilute solutions used for the photophysical studies, the copolymers supramolecularly assemble onto graphene forming a layer, in agreement with the morphological characterization. In fact, it is well known that solid state effects yield to red shift of both absorption and emission spectra. iii) the copolymers fluorescence is strongly quenched in the hybrids, Fig. 10. The quenching (Q) calculated as
Fig. 9. Normalized and comparative UV-Vis spectra of MJ (stars) in CHCl3, GO (triangles) and GO-MJ hybrid (crosses) in DMF.
the ratio of the fluorescence quantum yield between the emission of the copolymer and the GO hybrids ranges between 4.4 and 35.7. Quenching effect is a very common process in graphitic materials [32] and has been ascribed to different de-activation processes: reabsorption or scattering effects, photoinduced energy or electron transference [33]. In our hybrids, scattering is evident from the absorption spectra. However, particularly for GO-MJ and GOEV, the quenching is very strong and likely indicates photoinduced energy transfer. This is also in agreement with the electron donor character of these PPEs and the electron acceptor character of graphene. iv) The fluorescence decay (Fig. 8 shows the fluorescence decay for MJ and GO-MJ as example) changes from monoexponential for PPEs to multi-exponential for GO-PPEs. The decay of GO-MJ shown in Fig. 8 is clearly formed by a short (t1 ¼ 1.55 ns) and long (t2 ¼ 0.19 ns) component, which is a characteristic feature of hybrid carbon based materials, where electronic communication between graphene and the conjugated polymer occurs [33]. v) The radiative Krad constant decreases in agreement with the quenching of the fluorescence, while the non-radiative constant Knr increases, with the exception of GO-EC. This deviation from the general trend can be likely inferred to the decrease in the Stoke’ shift, which suggests a lower contribution of internal conversion for this hybrid, among the other non-radiative decay processes. 4. Conclusions The functionalization of graphene oxide with rigid rod-like phenyleneethynylene via microwave assist synthesis to form GOPPE hybrids was successful achieved in very short reaction times (90 min) relative to those needed to non-irradiated solution via (6 days). The usual NOx remaining contaminants from the Hummers synthesis in GO are easily eliminated by previous microwave treatment (e.g. 5 min). Raman and photophysical studies confirm the covalent functionalization of graphene oxide, which significantly improves its solubility and dispersion stability as hybrids in common organic solvents. The hybrids are characterized by fluorescence quenching and by an increase of the non-radiative constant, due to a possible photoinduced energy or electron transference process from the conjugated copolymers to the GO. However, we also conclude that microscopy studies are necessary to determine where and how the PPEs are deposited and/or if GO sheets are totally covered. HRTEM, STM and SWAXS studies reveled that in particular benzoateethynylene like copolymers due to both, their tendency to self-assemble in bilayers and to their strong p-p intramolecular interaction give rise to the formation of blocks of molecules resembling bricks, but in disordered arrays.
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Functionalization by esterification is mainly carried out at the edges of the GO sheets that presumably are not totally covered, even by increasing the PPE-vs-GO ratio in the reaction. These observations support the tendency of PPEs to self-assemble in an edge-on conformation, meaning that the conjugated backbones are parallel to the GO surface, rather than in face-on assembly, where the backbones lie flat on the GO surface. The fabrication of GO-PPEs solar cells is in progress to investigate how this peculiar supramolecular organization may affect the energy transfer in solid state and the device performance. Acknowledgments This work was supported by the Mexican National Council for Science and Technology [grant numbers CB-2015: 256716 and 256709; National Laboratory for Graphene, Infrastructure 2016: 268326] and the U.S. Air Force Office of Scientific Research (AFOSR) [grant FA9550-15-1-0143]. Authors also thank Rosario Rangel for her technical assistance. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2017.06.068. References [1] Z. Yin, J. Zhu, Q. He, X. Cao, C. Tan, H. Chen, Q. Yan, H. Zhang, Graphene-based materials for solar cell applications, Adv. Energy Mater 4 (2014) 1e19. pez-Naranjo, W. Soboyejo, Y. Meas-Vong, [2] A. Manzano-Ramírez, E.J. Lo B. Vilquin, A review on the efficiency of graphene-based BHJ organic solar cells, J. Nanomat. (2015) 15. Article ID 406597. [3] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S.K. Banerjee, L. Colombo, Electronics based on two-dimensional materials, Nat. Nanotechnol. 9 (2014) 768e779. [4] S.-H. Oh, K.-R. Kim, J.-M. Yun, P.H. Kang, Graphene oxide and water-soluble polymer composite materials as efficient hole transporting layer for high performance organic solar cells, Phys. Status Solidi A 212 (2015) 376e381. [5] X. Xu, J. Chen, X. Luo, J. Lu, H. Zhou, W. Wu, H. Zhan, Y. Dong, S. Yan, J. Qin, Z. Li, Poly(9,90 -diheylfluorene carbazole) functionalized with reduced graphene oxide: convenient synthesis using nitrogen-based nucleophiles and potential applications in optical limiting, Chem. Eur. J. 18 (2012) 14384e14391. mez, G. Martínez, Polymeric modification of gra[6] H.J. Salavagione, M.A. Go phene through esterification of graphite oxide and poly(vinyl alcohol), Macromolecules 42 (2009) 6331e6334. [7] D. Yu, Y. Yang, M. Durstock, J.-B. Baek, L. Dai, Soluble P3HT-grafted graphene for efficient bilayer-heterojunction photovoltaic devices, ACS Nano 4 (2010) 5633e5640. [8] B. Tryba, A.W. Morawski, M. Inagaki, Preparation of exfoliated graphite by microwave irradiation, Carbon 43 (2005) 2417e2419. [9] T. Wei, Z. Fan, G. Luo, C. Zheng, D. Xie, A rapid and efficient method to prepare exfoliated graphite by microwave irradiation, Carbon 47 (2008) 337e339. [10] Y. Zhu, S. Murali, M.D. Stoller, A. Velamakanni, R.D. Piner, R.S. Ruoff, Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors, Carbon 48 (2010) 2118e2122. [11] H.M.A. Hassan, V. Abdelsayed, E.A. Rahman, A.E.R.S. Kheder, K.M. AbouZeid, J. Terner, M.S. El-Shall, S.I. Al-Resayes, A.A. El-Azhary, Microwave synthesis of graphene sheets supporting metal nanocrystals in aqueous and organic media, J. Mater. Chem. 19 (2009) 3832e3837. mez-Rodríguez, J. Colchero, J. Go mez-Herrero, [12] I. Horcas, R. Fern andez, J.M. Go A.M. Baro, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705 (1-8).
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Synthesis of reduced graphene oxide-poly(phenyleneethynylene) hybrids. A supramolecular and photophysical analyses nez-Barrera, Gleb Turlakov, Eduardo Arias*, Ivana Moggio**, Rosa M. Jime lez-Morones, Salvador Ferna ndez, Oliverio Rodríguez, Carlos Avila-Orta, Pablo Gonza Ronald F. Ziolo n en Química Aplicada, Boulevard Enrique Reyna 140, 25294 Saltillo, Coahuila, Mexico Centro de Investigacio
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2017 Received in revised form 23 June 2017 Accepted 24 June 2017 Available online 27 June 2017
Functionalization of graphene oxide (GO) by esterification or amidation reactions with a series of conjugated phenyleneethynylene (PPEs) copolymers bearing different electron-donating and/or withdrawing groups was achieved by microwave irradiation in just 90 min. Copolymers as well as GO-PPE hybrids were characterized by Raman, UVeVis, static and dynamic fluorescence spectroscopy, transmission electron microscopy, X-rays diffraction and scanning tunneling microscopy. Effective interaction of the copolymers with GO is evidenced by a strong quenching of the PPE's fluorescence quantum yield (f), and by an increase of the non-radiative decay constant (Knr); features that suggest energy transfer from PPEs copolymers to GO. HRTEM shows that the molecules of copolymers self-assemble in blocks resembling bricks or board-like shapes that can be classified as sanidic LCs materials as a general term to identify their mesomorphism. Functionalization is mainly carried out at the edges of the GO sheets, so the GO-sheets are not totally covered, even by increasing the PPE-vs-GO ratio because of the trend of the PPEs in to self-assemble in an “edge-on” conformation, meaning that the conjugated backbones are parallel to the GO surface, rather than in “face-on”, where the backbones are flat-lying on the GO surface. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Microwave Graphene oxide Phenyleneethynylenes
1. Introduction Functionalization of GO with conjugated polymers (GO-CPs) is an attractive research line in electronic and optoelectronic area, in particular for photovoltaic applications from the point of view of i) the development of electrodes, i.e. dispersed GO-PCs layers can be deposited on glass slides by techniques such as Langmuir-Blodgett, dipping, etc. The deposited layers can further be submitted to a GO reduction process increasing and/or modulating the hole or electron mobility of the deposited layers, ii) or as buffer layers or iii) even as active layers in organic heterojunction photovoltaic devices. The key parameters for efficient organic photovoltaic devices are fast charge separation and efficient transport to the electrodes, so it is of crucial importance to assure that materials present a good interpenetrating donor-acceptor phase without segregation.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (E. Arias), [email protected]. mx (I. Moggio). http://dx.doi.org/10.1016/j.polymer.2017.06.068 0032-3861/© 2017 Elsevier Ltd. All rights reserved.
Generally, the active layer in organic bulk heterojunction solar cells is composed of an electron donor conjugated polymer and electron acceptor material, e.g. fullerene, carbon nanotube or graphene. There are many reports where graphene or graphene oxide were used as electron acceptor for solar cells applications [1e3]. Generally, two strategies are used to obtain graphene and graphene oxide interaction with conjugated polymers. The first one consists of p-p interactions between conjugated parts of graphene with the polymer backbone [4]. The second one is based on the direct covalent connection [5] of the functional conjugated polymers on the GO by means of esterification, amidation, click chemistry, nitrene chemistry, radical addition, etc. The esterification reaction seems to be a practical via for attaching a conjugated polymer to graphene oxide, but the polymer must bear hydroxyl groups in the side chains. In this respect, the Sonogashira polycondensation reaction supports a large variety of functional groups including the hydroxyls in monomers without being disturbed, but carboxylic acids of GO require activation with a proper catalyst. In what concerns to the functionalization, Salavagione [6] suggests that it is important to use an excess of esterified macromolecule owing to the lack of data about the quantity of carboxylic groups on
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GO's surface. A typical example is the esterification of hydroxyl terminated poly(3-hexyl thiophene) with GO (4:1 w/w) [7]. However, the main drawback of the GO functionalization is still the long reaction times due to the difficulty to exfoliate the GO sheets for esterification reactions. Microwave irradiation (MI) as a suitable and rapid heating method was used for preparing exfoliated graphite [8,9] and recently, it was applied for performing graphene oxide exfoliation and reduction [10] including the heating of its suspensions in aqueous or organic media [11]. It has been demonstrated that MI promotes not only the GO exfoliation by interpenetrating the wave between GO sheets, but also the carboxylic acid groups are activated. In this work a series of phenyleneethynylene (PPEs) copolymers were synthesized bearing hydroxyl terminated chains in order to esterify the GO, a particular copolymer bearing hydrazide in the main chain (EV) was also evaluated in order to carry out reactions with the epoxy groups that usually are found in the GO plane. The GO chemical functionalization by MI was carried out in 90 min, instead of the typical 6 days applied by thermal chemical solution via. Functionalized GO-PPE's hybrids were characterized by Raman spectroscopy, where the D (1350 cm1), G (1585 cm1), and the ethynylene (~2200 cm1) vibration bands were clearly identified. Energy transfer process from the PPE's to the GO was also determined, since the fluorescence of the PPE's with quantum yield e.g. of 63% for EV is drastically shouted down to 5.0%, for its GO-EV hybrid.
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radiation at a wavelength of 0.1542 nm. STM characterization was performed by using a tip mechanically cut from Pt/Ir wire (80/20, diameter 0.25 mm, Nanoscience) and another one prepared from tungsten by electrochemical etching in KOH solution using an applied voltage of 4 V. The photophysical characterization was carried out in spectroscopic grade (Aldrich) toluene, THF and CH2Cl2. UVeVis and fluorescence spectra were recorded on an Agilent 8453 spectrophotometer and a Perkin Elmer LS50B spectrofluorimeter or Horiba Quantamaster 8075e11, respectively. Fluorescence quantum yield (f) was obtained according to literature method [13] with quinine sulphate (0.1 M in H2SO4) as standard. Three different solutions with optical density at the excitation wavelength <0.1 were analyzed at 25.0 ± 0.3 C by using a circulating water bath and the values were averaged. The excitation wavelength is reported in the tables. Lifetimes were obtained by TCSPC (Time-correlated single photon counting) with a Tempro Horiba equipment with a 370 nm nanoLED. A 0.01% suspension of Ludox AS40 (Aldrich) in ultrapure water was used for the prompt signal. Calibration of the equipment was realized with a POPOP [1,4-Bis(4-methyl-5-fenil-2-oxazolyl)benzene] methanol solution (optical density <0.1 and lifetime of 0.93 ns [14]). Data were fit in the software DAS6 available with the equipment. 2.2. Synthesis
2. Experimental section
Experimental procedures, chemical and physicochemical characterization of each compound are given in the supplementary information section.
2.1. Equipment and methods
3. Results and discussion
The microwave copolymerizations were carried out in close quartz vials in a Microwave Apparatus Monowave 300 from Anton Parr at 300 Watts of constant irradiation power, during 90 min at 190 C and 600 rpm. 1H (300 MHz), 13C NMR (75.4 MHz) spectra were obtained at room temperature with a Jeol Eclipse spectrometer using CDCl3 as solvent and internal reference. The molecular weights were determined by GPC on an HP 1100 HPLC using PS standards and refractive index as detector. A series of three HP PLGel columns were used: 103, 105 and 106 Å and THF as the mobile phase at 40 C and 1 mL/min. Raman spectra were recorded on a Horiba Xplora equipment, focusing the sample as powders on microscopic slide with a 10 objective. The excitation wavelength was 785 nm and the Nanoled power was 25% of total laser power (25 mW). Spectra were acquired with 10s acquisition time and 5 cycles, spectral resolution of 2 cm1. For electron microscopy studies, all of the materials were deposited by casting from CHCl3 (2 mg/mL) solutions on Lacey carbon grids and were examined by SAED and HRTEM techniques in an FEI-TITAN-200-300 kV field emission gun microscope, which has a symmetrical condenserobjective lens type S-TWIN (with an spherical aberration Cs ¼ 1.25 mm). All of the STM images were obtained at ambient conditions using a compact STM AA5000 Scanning Probe Microscope from Angstrom Advanced Inc. Calibration of the scanner tube was performed by means of atomic resolution images obtained from commercial HOPG sample, exfoliated before each measurement by the adhesive tape method. All measurements were performed in the constant-current mode. Details on the experiment bias and current set-point are given below the images. The raw and FFT images were processed from WSxM 5.0 [12] and Gwyddion software with the aim to reduce noise and normally observed drift. The samples were prepared by directly depositing a drop (~1e2 mL) of diluted solutions (104 e105 M) of copolymers in phenyloctane. Small and wide angle X-ray scattering patterns were obtained with an Anton Paar SAXSess mc2 SWAXS instrument using CuKa
3.1. Copolymers synthesis and characterization One semi-rigid copolymer hereafter named EV and three rigid rod-like copolymers named EC, LC and MJ were synthesized according to the chemical routes given in Scheme 1 and Scheme 1S (supporting information). In general the pathway involved only two basic reactions: i) amidation of the 4-iodo benzoic acid 1 with hydrazine hydrate 2 under the EDAC/DMAP-mediated protocol in CH2Cl2, to give 4iodo-N'-(4-iodobenzoyl)benzohydrazide 3 in 53% yield, and the esterification of 2,5-dibromobenzoic acid 1S with three different bromoalkanes 3S, 4S, 5S by the action of DBU, Scheme 1S (supporting information). Then, ii) the Sonogashira Pd/CuI crosscoupling polymerization between the corresponding diiodo or dibromo terminated monomer 3, 6, 6S, or 8S with the corresponding ethynylene 4, 5 or 7 to afford the copolymers EV in 57%, LC 70%, MJ 85% and EC in 67% yield. The relatively moderate yields of EV and EC are due to the fact that DMF was used as a co-solvent for the polymerization, it is well known that co-solvents highly increase the molecular weights giving rise to insoluble aggregates. After polymerization, each copolymer was submitted to a metal decomplexation process by precipitation once in a methanol sodium dithiocarbamate solution, and twice with clean methanol. The shortest oligomers were eliminated by preparative SEC column (Biorad, Bio-Beads SX1) using toluene as eluent. The proton resonance peaks of PPEs are broader than those of the corresponding monomers; it was observed that the acetylenic protons disappeared and in contrast, the protons near the iodine or bromine could be identified, suggesting that all of the chains are mainly halogen terminated, Fig. 1S (supporting information). The calculated average molecular weight (Mw), number average molecular mass (Mn), and polydispersity index (PI) using PS standards and refraction index as detector are: Mw ¼ 33729, Mn ¼ 25668, PI ¼ 1.31 for MJ; Mw ¼ 11006, Mn ¼ 4909, PI ¼ 2.24 for LC;
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Scheme 1. Reagents and conditions: (a) EDAC, DMAP, THF, 0 C to reflux, 17 h, 53% yield; (b) [(C6H5)3P]2PdCl2 (3.0% mol), CuI (1.5% mol), Et3N, 60 C, 36 h 60e75% yield.
Mw ¼ 6591, Mn ¼ 1866, PI ¼ 3.5 for EV. All of the PPEs are soluble in CH2Cl2, CHCl3, THF, toluene and mostly precipitate in methanol, acetone and DMF. In contrast, gelation was observed for EC at r.t. being soluble in a mixture of hot DCB:DMF, likely due to the organogelator property of the steroid cholesteryl group [15]. 3.2. Microwave functionalization of GO with copolymers In this work, a two-step oxidation approach was applied to generate more carboxylic acid groups on graphene oxide than epoxy and/or hydroxyl groups by a modified Hummers method. This consisted in pre-oxidizing the graphite by using a mixture of sulfuric acid, K2S2O8 and P2O5. Then, the pre-oxidized graphite was oxidized with a mixture of KMnO4, NaNO3, H2SO4 and H2O2. The GO was characterized by WAXD, Raman and UVeVis spectroscopy. The results of these two latter techniques will be discussed in the GOPPEs section, as term of comparison. The X-ray diffraction pattern showed a shift of the typical sharp (002) peak from 2q ¼ 26.3 of pristine graphite to 2q ¼ 13.88 for GO, confirming the exfoliation, with an interlayer space of d ¼ 0.64 nm, Fig. 2S (supporting information). We observed that GO always contained traces of Nitrogen, probably from the NaNO3, that can be removed by continuous dialysis and if GO is always maintaining in solution and changing the water monthly, thus pure GO can be obtained. Esterification of the GO with the four PPEs to form the corresponding hybrids, Scheme 2, was carried out by using the DCC protocol, but with the addition of DPTS/4-Py [16] in DMF to promote a better activation of the highly steric carboxylic acids. DMF is a good solvent for dispersing GO, but not for PPEs, which tend to precipitate. In order to avoid PPEs aggregation during the GO-PPEs
functionalization, and after having proved different alternatives of reaction, we decided to sonicate DMF dispersion, and then add the rest of the reactants in dichlorobenzene. We note that the dark solutions showed fluorescence (by exciting at 365 nm), which completely disappeared at the end of the reaction. Since the p-p GO-PPEs interaction is very strong, the workup after reaction consisted in successive washings with hot solvents with different polarities to obtain non fluorescent solutions. It is worth to be mentioned that the microwave copolymerization was achieved in just 90 min contra 6 days by conventional solution reaction. The GO-PPEs hybrids were maintained in 1,2-diclorobenzene. 3.3. Vibrational properties Fig. 1a shows the Raman spectra of the PPEs copolymers obtained by exciting at 785 nm wavelength, out of their absorption range, to avoid the strong fluorescence of the materials in the visible region. The band at around 2200 cm1 associated with the -C≡C- stretching vibrational modes gives information on the pelectrons delocalization along the conjugated backbone, while that at 1592 cm1 is associated with the aromatic (-C¼C-) stretching vibrations [17]. A shift to lower frequencies of the ethynylene group is found for MJ (n ¼ 2195 cm1) with respect to EC (2202 cm1), LC (n ¼ 2204 cm1), and EV (n ¼ 2204 cm1). The higher frequency associated to the triple bond stretching in MJ is a result of its large polarizability in agreement with the electron donor character of the 2,5-bis dodecanoxy phenyl moiety. This moiety is also part of the EV copolymer, however its conjugation is interrupted by the hydrazide segment. Fig. 1b shows the spectra of GO-PPEs hybrids. The structural
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Scheme 2. Reagents and conditions: (a) (a) DCC/DPTS/4-PPy, DMF/DCB, 300 W, 4.75 GHz, 600 rpm, 90 min, 150 C.
Fig. 1. Raman spectra of (a) PPE copolymers and (b) GO-PPEs hybrids.
changes that occur in graphene oxide-based materials are evidenced by shifts or intensity changes of the D band (~1345 cm1,
breathing mode of A1g, C sp3 atoms) and G band (~1586 cm1; the in-plane bond-stretching motion of pairs of C sp2 atoms, E2g mode).
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The D band gives information about the presence of defects and the intensity ratio ID/IG is used to distinguish the quality of graphene and to determine the quantity of graphene layers [18,19] The Raman spectra of the hybrids present bands attributed to the copolymer (-C≡C- stretching vibrational mode, around 2200 cm1) and those typical of graphene oxide. The G band of GO-PPEs: GO-LC (1585 cm1), GO-EV (1598 cm1), GO-MJ (1592 cm1), GO-EC (1596 cm1), shows a broadening and upshifting relative to GO at 1586 cm1. According to Qi et al. [20], a shift to higher frequencies is generated by electron acceptor macromolecules, i.e. n-doping process, which in our case is consistent with the electronwithdrawing character of the ester or hydrazide groups of the PPEs. Accordingly, the D band also upshifts from 1345 cm1 (GO) to ~1350 cm1 for GO-PPEs. We note that the intensity ratio ID/IG, which is usually a measure of the disorder of the sp3/sp2 carbons of GO, for the GO-PPEs is noticeably reduced from 1.03 for GO, e.g. to 0.65 for GO-MJ. The reduction may be attributed to both, reduction of the GO layers during the microwave reaction and to the increase of the sp2 C atoms due to the PPEs presence. 3.4. Supramolecular order 3.4.1. HRTEM In order to verify the presence of the copolymers on the GO, we carried out a study of the supramolecular order of both, the PPEs copolymers and of the GO-PPEs hybrids by TEM, SWAX and STM. Fig. 2aeb shows the HRTEM image of EC copolymer. We recently reported that a series of oligomers of both: (cholesteryl) [19] or (diethylene glycol methyl ether) [21] benzoateethynylene and of their homologue polymers, when characterized by HRTEM, shows that what is seen in planar view corresponds to the benzoateethynylenes backbones, with the cholesteryls or the diethylene glycol methyl ether lengthened perpendicularly. The calculated periodicity of c.a. 0.33 nm coincided with the distance between conjugated benzoates. In this respect: 1) similar observations were found for the PPEs, Fig. 2 and Fig3S (supporting information), which show periodicity (DL) of 0.32 nm for (EC), 0.34 nm for (MJ), 0.29 nm for (EV) and of 0.23 nm for (LC), indicating that they are standing perpendicular to the substrate as is sketched in Fig. 2c. We also observed, that the molecules are packed in random blocks, as confirmed by their selected area electron diffraction patterns (SAED), where many diffraction spots with the same periodicity can be seen e.g. for MJ in Fig. 3S. The type of lateral chain increases or decreases this distance, for instance cholesteryl presence increases
it to 0.32 nm for EC, while 11-undecanol chains decreases it to 0.23 nm for LC. Additionally, we found a direct correlation of this DL value with the materials solubility; the higher the value, the more soluble is the copolymer. In contrast for low values, insoluble aggregates are formed. HRTEM of the GO-PPEs hybrids showed in general that GO sheets are not totally covered by the copolymers; certain zones are denser (zone a-b, Fig. 3), while others are practically uncovered (zone c, Fig. 3), a periodicity of ca. 0.33 nm was found in all of the hybrids (zone a, Fig. 3) with the exception of copolymer MJ where an additional periodicity of 1.03 nm was also observed (zone b, Fig. 3). Multiple diffraction spots were observed on the FFT of the HRTEM images, which we attributed to blocks of copolymer randomly oriented, such as edge-on, in which the p-p stacking is perpendicular to the substrate and face-on in which the p-p stacking is parallel to the substrate [22], or in a transitional state between them, as is sketched in Fig. 3, some spots correspond to the graphene oxide lattice. 3.4.2. STM We and others had found that phenyleneethynylene macromolecules are difficult to be analyzed by scanning tunneling microscopy, mainly due to the strong p-p interaction that governs in the conjugated backbones and that gives rise to the formation of fibrils once the solution is deposited on substrates. The previous HRTEM data suggests that MJ deposits on GO in two different supramolecular assemblies: parallel and perpendicular. We analyzed all of the copolymers at the molecular level by STM, and found that effectively only MJ gave evidence of molecular order. The freshly cleaved HOPG was used to calibrate the STM, Fig. 4. The lighter regions represent high points on the surface or areas of larger electron density, while the darker regions represent lower points on the surface or lower electron density. The bright and dark areas on the image correspond to alternate atoms. The variation of electron density on the surface is caused by variations in the delocalized p-orbitals. It is well known that the six carbon atoms in each hexagon of the topmost graphene layer form two groups designated as a and b atoms [23]. The former ones are located atop of the carbon atoms of the underlying graphene layer and the latter ones right in the hollow sites of the underlying carbon hexagons of the second carbon layer as depicted in the model of Fig. 4b. The calculated distance between two maxima is 0.223 nm, (i.e. 2.009/9), Fig. 4c, while that reported in literature [24] is of 0.246 nm; this difference is attributed to the thermal drift of the STM tip.
Fig. 2. (a) HRTEM image of a casting film of EC copolymer deposited on a lacy carbon grid showing a periodicity of 0.32 nm and corresponding to the distances between conjugated back bones, (b) HRTEM image of the same film and (c) sketch of the molecular arrangement seen from a lateral view.
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Fig. 3. HRTEM image of a casting film of GO-EC hybrid deposited on a lacy carbon grid. The sketch gives a visual description of the different molecular assemblies of the copolymer on the GO surface according to their calculated periodicity; edge-on (region b) in which the conjugated backbone are assembled in parallel to the GO, and face-on (region a) for their assembly perpendicular to the GO; region c represents the unattended GO.
Fig. 4. (a) Filtered STM image of HOPG; at capture conditions: Vbias ¼ 0.1 V, Itunnelling ¼ 1 nA, (b) proposed molecular packing of graphite layers and (c) line profile along the line indicated in figure (a), the distance between two maxima is 0.223 nm.
Fig. 5 presents the large scale and high-resolution images of copolymer MJ on HOPG. We note that the molecular images correspond predominantly to short macromolecular segments, while single layers of molecules were not observed, but rather as a package, with enough thickness to allow their characterization by tunneling current characterization. One of these packages shows levels of contrast as visualized in Fig. 5aeb. The bright lines can be attributed to unsaturated hydrocarbon represented by the copolymer backbones (due to a higher tunneling current at the STM tip with the p electrons), while the dark ones are related to interdigitated lateral chains. Fig. 5a is composed of two segments: the ordered structure of copolymers and the amorphous part. Individual segments of conjugated chains of MJ are clearly seen at high resolution, Fig. 5c. This image showing PPE backbone folding, indicated by green arrows, corroborates previously reports, that the PPE backbone rods are elastic [25], in fact the FFT of the corresponding zone, Fig. 5d, shows diffraction spots, and we attribute one spot to one block of molecules, where in
one of these blocks an intercalant and folded molecule was identified. For these particular blocks, molecules appear flat-lying (faceon) and densely packed allowing to see the rods on the HOPG surface and following the substrate directions, likely due to strong p-p interactions. The distance obtained from the profile is 2.48 nm corresponding to the end-to-end length of two neighbor conjugated chains, where the alkyl chains are interdigitated as sketched in Fig. 5e. This observation is in good agreement with the 2.16 nm distances determined by X-ray (vide supra) and with similar values and proposed models for molecules reported in the literature [26]. 3.4.3. SWAXS Since the HRTEM showed that all of the PPEs are not amorphous, but rather randomly organized in blocks, they were analyzed by small-wide angle X-ray scattering. Fig. 6 shows the SWAX spectra of the copolymers at room temperature. It is to point out that the four copolymers showed strong birefringence under polarized light as a signature of their mesomorphism, but without exhibiting any
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Fig. 5. STM images obtained at the HOPG-1,2,4-trichlorobenzene (TCBM) interface of MJ copolymer: a) large-scale and b) high-resolution image of the self-assembled MJ conjugated macromolecules; imaging conditions: a) 1000 V, 0.3 nA; b) 1200 V, 79 pA. The high resolution image (c) shows a folded phenyleneethynylene molecule from the marked region in image a). d) Is the corresponding FFT pattern; white arrows indicate the two observed spots; e) proposed schematic molecular conformation of MJ on HOPG, where lateral chains are interdigitated giving a phenyl-to-phenyl distance of 2.48 nm (green arrows correspond to the fold in the structure). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Room temperature X-ray scattering patterns of EV, MJ and EC. The sketches represent the possible molecular packing deduced from the SWAXS patterns.
known texture. The MJ diffraction pattern shows a wide peak at the low-angle region 2q ¼ 4.09 assigned to a first-order reflection with a distance of d1 ¼ 2.16 nm, however two other wide bands of low intense diffractions at 2q ¼ 7.64 , d ¼ 1.16 nm and 2q ¼ 12.56 , d ¼ 0.70 nm giving a ratio of ca. 1:2:3 in 2q, which is indicative of a disordered lamellar system in a staggered molecular packing in order to fulfill the maximum occupied space of volume. We believe that a smectic instead of a nematic mesophase is more reasonable in this case, since the birefringent pasty showed no fluidity when pressed between two glass slides at room temperature. At wide angles, two peaks are observed; a wide halo at 2q ¼ 21 ; d2 ¼ 4.34 Å of the side-to-side distances between packed lateral chains, and at 2q ¼ 23 ; d3 ¼ 3.85 Å originated from the layer-to-layer p-p stacking distance between the adjacent polymer molecules. In its more extended conformation, MJ has a theoretical length of the dodecanoxy-phenyl-dodecanoxy chains of 3.68 nm [19], while that of the benzoate-to-11-undecanol is of 1.93 nm [27]. The interplanar distance matches well when the 11-undecanol chains are interdigitated, as is represented in the sketch accompanying the spectrum. In contrast, the EV copolymer shows fluidity at room temperature and its X-ray pattern exhibits only diffuse bands in both, the small and wide angle regions, which are features of a likely nematic phase. In contrast, the X-ray patterns of EC and LC exhibit very similar features; i) a very sharp peak at low angles 2q ¼ 1.53 , d ¼ 5.77 nm for EC and at 2q ¼ 1.80 , d ¼ 4.90 nm for LC, ii) and a very diffuse
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band at high angles 2q ¼ 19.40 , d ¼ 0.46 nm for EC and at 2q ¼ 22.7, d ¼ 0.39 nm for LC. In its more extended conformation, EC has a theoretical length benzoate-to-cholesteryl of 2.23 nm and that of the benzoate-to-diethylene glycol methyl ether is of 1.73 nm, while for LC the theoretical length benzoate-to-diethylene glycol methyl ether is of 1.49 nm and that of the benzoate-to11undecanol is of 2.16 nm. This suggests that both copolymers self-assemble in bilayers into the lamellae in a typical “comb-like” fashion, where the side chains are oriented in an opposite sense in a back-to-back comb like “style” as is depicted in the side sketch of Fig. 6. 3.5. Photophysical properties All of the photophysical properties of the copolymers and hybrids are collected in Tables 1 and 2. Fig. 7 shows the absorption and fluorescence spectra of the PPEs in chloroform. The absorption spectra present a main peak with maximum at 420 nm (MJ), 386 nm (LC), 380 nm (EC) and 382 nm (EV), which is attributed to the p-p* (HOMO/LUMO) electronic transitions of the conjugated system, in agreement with previous experimental and theoretical studies [17,21,27e29]. EV and MJ also exhibit a peak at around 325 nm that has been previously ascribed to HOMO-1/LUMO electronic transitions in 2,5-dialcoxy substituted phenylenethynylenes [17,28]. Among these two copolymers, the absorption maximum of MJ is red shifted with respect to that of EV, which suggests a larger electronic delocalization. EV presents electron donor dodecanoxy groups in the conjugated unit contra to the benzoate electron withdrawing unit of MJ, however the non-conjugated benzo-hydrazide moiety of EV likely interrupts the electronic delocalization, in agreement with the Raman results. In comparison, both LC and EC present glycolic type benzoate units alternated with (11-undecanol)benzoate (LC) or (cholesteryl) benzoate (EC). The position of their maximum is in good agreement with the maximums observed for their respective homopolymers. For example, the poly((diethylene glycol methyl ether) benzoateethynylene) I-nBEgly [21] showed an absorption maximum at 375 nm in CH2Cl2, the poly((11-undecanol)benzoateethynylene) pPEn [28] is at 380 nm in CHCl3, and the poly((cholesteryl)benzoateethynylene (Br-nPEBzCol) [15] lmax ¼ 380 nm (in CHCl3). In general, the Eg of the four copolymers is in the semiconducting range.
Fig. 7. Normalized absorption and fluorescence (inserted) spectra in CHCl3 of MJ (stars), EC (squares), EV (continous line) and LC (circles).
The fluorescence spectra of MJ, LC and EC show excitonic features with a main peak and a shoulder as reported for other PPEs [15,27]. The emission band of EV is on the contrary broader similarly to what was found for other dodecanoxy sequenced copolymers [30,31]. The trend in the emission maxima is similar to that of the absorption spectra so that the fluorescence spectra of LC and EC are at almost the same wavelength and blue shifted with respect to MJ. EV emission is however more red shifted presenting also the largest Stoke's shift. This feature and the broadness of the fluorescence band suggests that a stronger geometrical change from the fundamental to the excited state occurs for this specific copolymer, likely because of its semirigid structure. In general, the fluorescence decay is mono-exponential with lifetimes of around 0.5e1 ns, Fig. 8, quantum yields f are of moderate value for MJ and EV and low for EC and LC. Non-radiative processes as internal conversion are expected; although Stokes shift (Dn) values are in the range for molecules that undergo a geometry change from aromatic to a more stable planar quinoidal structure after excitation, and being the largest values for EC and EV benzoateethynylene copolymers. However, there is not a clear match between the trend in the f values with those of the Stokes shift (Dn), suggesting that intersystem crossing process are also affecting the fluorescence quantum yield, and in particular for
Table 1 Photophysical properties of PPEs in CHCl3. Eg (eV)
ε X 102 (gL1cm1)b
lem/lexc
(nm)
(nm)
(cm1)
MJ LC EC
327, 420a 386a 380a
2.61 2.79 2.66
1.7 4.4 5.0
465/410 429/376 426/370
2309 2597 2846
EV
325, 382a
2.70
3.9
450/372
3956
Dn
F
PPE
a b
labs
Dn
F
t (ns)
Krad (ns1)
Knr (ns1)
0.50 0.31 0.15
0.72 0.45 0.69
0.69 0.69 0.22
0.69 1.53 1.23
0.53
1.17
0.45
0.40
Q
t (ns)
Krad (ns1)
Knr (ns1)
0.39 0.70 0.18 0.79
0.04 0.06 0.19 0.06
2.53 1.37 5.37 1.20
Maximum absorption. Calculated for the maximum absorption.
Table 2 Photophysical properties of GO-PPE hybrids in DMF. Hybrid GO-MJ GO-LC GO-EC GO-EV
labs
Eg (eV)
lem/lexc (nm)
(nm) 334, 450 455 409 312, 405
2.15 1.55 1.64 1.56
470/410 517/376 500/370 529/372
(cm1) 922 2638 4450 5790
0.014 0.041 0.034 0.050
35.7 7.6 4.4 10.6
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Fig. 10. Fluorescence spectra of GO-MJ in DMF and MJ in CHCl3. Inserted image shows the fluorescence quenching by passing from the MJ copolymer to the GO-MJ hybrid, under 365 nm excitation. Fig. 8. Time resolved decay profiles of the MJ copolymer and GO-MJ hybrid.
the cholesteryl copolymer. In this respect, we expected an intersystem crossing mechanism due to the presence of halogens as termini groups, which are known to decrease the energy of the triplet state. In addition for EC, the cholesteryl substituent enhances this effect due to steric hindrance, as previously reported [15]. The photophysical properties of PPEs strongly change after functionalization with GO. i) When GO is functionalized with PPEs, the absorption spectra present in general the absorption features of the copolymers overlapped with a marked baseline due to GO scattering. Fig. 9 shows the absorption spectrum of GO-MJ hybrid and its precursors GO and MJ, as example. The absorption spectra of the other hybrids are collected in the supporting material. The absorption maximums are red shifted with respect to the pure copolymers and the absorption bands are broader. ii) A similar change is observed in the fluorescence spectra that losses the excitonic features and are bathochromically shifted. These findings suggest an increase in the effective conjugation as a consequence of the functionalization with GO as found in other works [4,7]. Another possible explanation could be that, even in the dilute solutions used for the photophysical studies, the copolymers supramolecularly assemble onto graphene forming a layer, in agreement with the morphological characterization. In fact, it is well known that solid state effects yield to red shift of both absorption and emission spectra. iii) the copolymers fluorescence is strongly quenched in the hybrids, Fig. 10. The quenching (Q) calculated as
Fig. 9. Normalized and comparative UV-Vis spectra of MJ (stars) in CHCl3, GO (triangles) and GO-MJ hybrid (crosses) in DMF.
the ratio of the fluorescence quantum yield between the emission of the copolymer and the GO hybrids ranges between 4.4 and 35.7. Quenching effect is a very common process in graphitic materials [32] and has been ascribed to different de-activation processes: reabsorption or scattering effects, photoinduced energy or electron transference [33]. In our hybrids, scattering is evident from the absorption spectra. However, particularly for GO-MJ and GOEV, the quenching is very strong and likely indicates photoinduced energy transfer. This is also in agreement with the electron donor character of these PPEs and the electron acceptor character of graphene. iv) The fluorescence decay (Fig. 8 shows the fluorescence decay for MJ and GO-MJ as example) changes from monoexponential for PPEs to multi-exponential for GO-PPEs. The decay of GO-MJ shown in Fig. 8 is clearly formed by a short (t1 ¼ 1.55 ns) and long (t2 ¼ 0.19 ns) component, which is a characteristic feature of hybrid carbon based materials, where electronic communication between graphene and the conjugated polymer occurs [33]. v) The radiative Krad constant decreases in agreement with the quenching of the fluorescence, while the non-radiative constant Knr increases, with the exception of GO-EC. This deviation from the general trend can be likely inferred to the decrease in the Stoke’ shift, which suggests a lower contribution of internal conversion for this hybrid, among the other non-radiative decay processes. 4. Conclusions The functionalization of graphene oxide with rigid rod-like phenyleneethynylene via microwave assist synthesis to form GOPPE hybrids was successful achieved in very short reaction times (90 min) relative to those needed to non-irradiated solution via (6 days). The usual NOx remaining contaminants from the Hummers synthesis in GO are easily eliminated by previous microwave treatment (e.g. 5 min). Raman and photophysical studies confirm the covalent functionalization of graphene oxide, which significantly improves its solubility and dispersion stability as hybrids in common organic solvents. The hybrids are characterized by fluorescence quenching and by an increase of the non-radiative constant, due to a possible photoinduced energy or electron transference process from the conjugated copolymers to the GO. However, we also conclude that microscopy studies are necessary to determine where and how the PPEs are deposited and/or if GO sheets are totally covered. HRTEM, STM and SWAXS studies reveled that in particular benzoateethynylene like copolymers due to both, their tendency to self-assemble in bilayers and to their strong p-p intramolecular interaction give rise to the formation of blocks of molecules resembling bricks, but in disordered arrays.
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Functionalization by esterification is mainly carried out at the edges of the GO sheets that presumably are not totally covered, even by increasing the PPE-vs-GO ratio in the reaction. These observations support the tendency of PPEs to self-assemble in an edge-on conformation, meaning that the conjugated backbones are parallel to the GO surface, rather than in face-on assembly, where the backbones lie flat on the GO surface. The fabrication of GO-PPEs solar cells is in progress to investigate how this peculiar supramolecular organization may affect the energy transfer in solid state and the device performance. Acknowledgments This work was supported by the Mexican National Council for Science and Technology [grant numbers CB-2015: 256716 and 256709; National Laboratory for Graphene, Infrastructure 2016: 268326] and the U.S. Air Force Office of Scientific Research (AFOSR) [grant FA9550-15-1-0143]. Authors also thank Rosario Rangel for her technical assistance. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2017.06.068. References [1] Z. Yin, J. Zhu, Q. He, X. Cao, C. Tan, H. Chen, Q. Yan, H. Zhang, Graphene-based materials for solar cell applications, Adv. Energy Mater 4 (2014) 1e19. pez-Naranjo, W. Soboyejo, Y. Meas-Vong, [2] A. Manzano-Ramírez, E.J. Lo B. Vilquin, A review on the efficiency of graphene-based BHJ organic solar cells, J. Nanomat. (2015) 15. Article ID 406597. [3] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S.K. Banerjee, L. Colombo, Electronics based on two-dimensional materials, Nat. Nanotechnol. 9 (2014) 768e779. [4] S.-H. Oh, K.-R. Kim, J.-M. Yun, P.H. Kang, Graphene oxide and water-soluble polymer composite materials as efficient hole transporting layer for high performance organic solar cells, Phys. Status Solidi A 212 (2015) 376e381. [5] X. Xu, J. Chen, X. Luo, J. Lu, H. Zhou, W. Wu, H. Zhan, Y. Dong, S. Yan, J. Qin, Z. Li, Poly(9,90 -diheylfluorene carbazole) functionalized with reduced graphene oxide: convenient synthesis using nitrogen-based nucleophiles and potential applications in optical limiting, Chem. Eur. J. 18 (2012) 14384e14391. mez, G. Martínez, Polymeric modification of gra[6] H.J. Salavagione, M.A. Go phene through esterification of graphite oxide and poly(vinyl alcohol), Macromolecules 42 (2009) 6331e6334. [7] D. Yu, Y. Yang, M. Durstock, J.-B. Baek, L. Dai, Soluble P3HT-grafted graphene for efficient bilayer-heterojunction photovoltaic devices, ACS Nano 4 (2010) 5633e5640. [8] B. Tryba, A.W. Morawski, M. Inagaki, Preparation of exfoliated graphite by microwave irradiation, Carbon 43 (2005) 2417e2419. [9] T. Wei, Z. Fan, G. Luo, C. Zheng, D. Xie, A rapid and efficient method to prepare exfoliated graphite by microwave irradiation, Carbon 47 (2008) 337e339. [10] Y. Zhu, S. Murali, M.D. Stoller, A. Velamakanni, R.D. Piner, R.S. Ruoff, Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors, Carbon 48 (2010) 2118e2122. [11] H.M.A. Hassan, V. Abdelsayed, E.A. Rahman, A.E.R.S. Kheder, K.M. AbouZeid, J. Terner, M.S. El-Shall, S.I. Al-Resayes, A.A. El-Azhary, Microwave synthesis of graphene sheets supporting metal nanocrystals in aqueous and organic media, J. Mater. Chem. 19 (2009) 3832e3837. mez-Rodríguez, J. Colchero, J. Go mez-Herrero, [12] I. Horcas, R. Fern andez, J.M. Go A.M. Baro, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705 (1-8).
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