polyaniline nanocomposites via sacrificial surfactant-stabilized reduced graphene oxide

Colloids and Surfaces A 589 (2020) 124415

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New approach for the development of reduced graphene oxide/polyaniline nanocomposites via sacrificial surfactant-stabilized reduced graphene oxide

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Tereza Bautkinováa, Adam Siftona, Edith Mawunya Kutorgloa, Marcela Dendisováb, Dušan Kopeckýc, Pavel Ulbrichd, Petr Mazúra, Abdelghani Laachachie, Fatima Hassounaa,* a

Department of Chemical Engineering, University of Chemistry and Technology Prague, Technická 3, 166 28, Prague 6, Czech Republic Department of Physical Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28, Prague 6, Czech Republic c Department of Computing and Control Engineering, University of Chemistry and Technology Prague, Technická 5, 166 28, Prague 6, Czech Republic d Department of Biochemistry and Microbiology, University of Chemistry and Technology Prague, Technická 3, 166 28, Prague 6, Czech Republic e Materials Research and Technology Department, Luxembourg Institute of Science and Technology (LIST) – 5, Rue Bommel, ZAE Robert Steichen, L-4940, Hautcharage, Luxembourg b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyaniline Reduced graphene oxide Colloidal stability Sacrificial surfactant Electrical conductivity

An aggregation of graphene oxide (GO) during its reduction to reduced graphene oxide (rGO) limits its performance in nanocomposites. The use of rGO stabilized by a sacrificial surfactant should overcome this limitation and it yields nanocomposites with enhanced properties. A new, simple and cost-effective approach for the synthesis of polyaniline (PANI) nanocomposites based on rGO stabilized by a sacrificial surfactant was developed. Two routes of synthesis, the in situ and ex situ reduction, were compared. The former involved the reduction of GO already coated by PANI, forming rGO/PANIin whereas the latter involved the reduction of GO in the presence of a sacrificial surfactant to well-exfoliated rGO sheets followed by polymerization of aniline, forming rGO/PANIex. Differences in morphology and physical properties between rGO/PANIin and rGO/PANIex correlated with their chemical structure were raised. Accordingly, rGO/PANIin exhibited higher thermal stability but lower electrical conductivity (0.01 S·cm−1) compared to neat PANI (0.11 S·cm−1), while rGO/PANIex demonstrated thermal properties comparable to those of PANI and remarkable electrical conductivity (∼ 280 S·cm−1). Mechanistic insights into the interactions between rGO and PANI are proposed.

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Corresponding author. E-mail address: [email protected] (F. Hassouna).

https://doi.org/10.1016/j.colsurfa.2020.124415 Received 10 September 2019; Received in revised form 22 November 2019; Accepted 2 January 2020 Available online 03 January 2020 0927-7757/ © 2020 Published by Elsevier B.V.

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1. Introduction

The polymerization of aniline in the presence of rGO showed other limitations associated with the aggregation of rGO during the reduction step of GO, and the subsequent lack of colloidal stability in aqueous solution [23,26]. Functionalization by surfactants was used to prevent the aggregation of rGO, while retaining its intrinsic properties [25,27–29]. Unlike covalent functionalization of rGO, non-covalent (adsorption) approaches ‒ involving electrostatic, hydrogen bonding and π-π stacking interactions ‒ represent promising alternatives since electronic properties of rGO should be maintained without severe deterioration [25,27–29]. Recently, Mensing et al. [29] reported a new route of synthesis, which involves the combination of surfactant-stabilized graphene with PANI nanofibers. Exfoliation of the graphite rods was performed in the presence of an electrolyte which contains sodium dodecylbenzenesulfonate. A significant improvement of the overall electrical performance of the nanocomposite with respect to PANI was recorded. Meanwhile, Rajagopalan et al. [28] reported the elaboration of graphene/PANI nanocomposites by the manganese dioxide templateaided oxidative polymerization of aniline on surfactant-stabilized graphene sheets. The specific capacitances of the obtained nanocomposite were higher than those of neat PANI. Moreover, enhanced uniformity of the composite morphology was important for successful electrolyte transport at the electrode-electrolyte boundaries [27,28,30]. These findings pave the way for further development of conductive nanocomposites using simple and inexpensive approaches based on colloidal stabilization of rGO in aqueous solution. The ability to elaborate homogeneous nanocomposites while retaining the intrinsic structures and properties of both rGO and PANI is imperative for fabricating high performance materials. In this respect, the goal of this work is to prepare rGO/PANI nanocomposites from well-exfoliated rGO. To attain better synergetic effects, it is crucial to maintain the structure and intrinsic properties of both rGO and PANI. Therefore, a straightforward and inexpensive route for synthesis of rGO/PANI nanocomposites based on rGO stabilized by sacrificial surfactant, with controllable morphologies and properties was developed. This approach enables to overcome some recurrent limitations when preparing nanocomposites based on reduced graphene oxide, i.e.: i) colloidal instability of GO under acidic conditions of the polymerization reaction of aniline monomers, and ii) aggregation of rGO during the step of reduction of GO and its subsequent lack of colloidal stability in aqueous solution. Moreover, an exchange between the surfactant and PANI at the surface of rGO during aniline polymerization via surfactant leaching into the aqueous medium, allows eliminating the surfactant from the nanocomposite, and therefore allows conserving the electronic structure and intrinsic features of both rGO and PANI. To the best of our knowledge, this is a first attempt to tackle all the issues above mentioned using a so-called sacrificial surfactant. For the sake of comparison and understanding of the structureproperty-relationships, two routes were investigated. The first route is named in situ reduction. It involves the polymerization of aniline monomer in the presence of GO sheets stabilized by sacrificial surfactant in acidic medium and subsequent reduction of GO in rGO/PANIin nanocomposite. The second route, so-called ex situ reduction, involves the reduction of GO to rGO in the presence of sacrificial surfactant and the subsequent polymerization of aniline in the presence of rGO. The relationship among the processing conditions, chemical structure, morphology and final properties of the nanocomposites is established. Finally, their potential for application in electronic devices is evaluated through measurement of their electrical conductivity.

Graphene has received significant attention since its discovery in 2004. The reason lies in its outstanding features, including excellent thermal and electrical conductivity, flexibility, mechanical strength and large specific surface area [1–5]. Consequently, graphene is a promising material for various applications including nanoelectronic [6] and optoelectronic devices [7], batteries [5] and supercapacitors [8]. In particular, its large surface-to-volume ratio makes it a suitable nano-filler for preparation of polymer-based composite materials [9]. Though graphene is mostly produced from graphite, which is inexpensive and abundant, synthesis of true graphene monolayers provides substantial challenges and can be extremely costly. Solution-based approaches involving an oxidized derivative of graphene, i.e. graphene oxide (GO), have become popular due to their relative ease of preparation. Reduction of GO offers potential to produce single-layered graphene sheets, i.e. reduced graphene oxide (rGO), in high amounts. It also offers the possibility for processing of advanced graphene-based materials [9,10]. Functional groups, such as epoxy, hydroxyl, carboxyl and carbonyl, assure the homogeneous dispersion of rGO sheets in a polymer matrix. Moreover, they also allow interfacial interactions such as hydrogen bonding, electrostatic forces and π-π stacking between rGO and the polymer matrix [9,11]. Appropriate interactions between the highly conjugated structure of graphene and the conjugated polymers are of particular pertinence. Polyaniline (PANI) belongs to the most extensively studied conducting polymers, as it possesses unique properties. PANI occurs in three different oxidation states, i.e. entirely reduced leucoemeraldine, half-oxidized emeraldine and entirely oxidized pernigraniline. The emeraldine proceeds the salt - base transition under acidic or alkaline conditions, respectively. The emeraldine salt with protonated imine nitrogen exhibits the highest electrical conductivity [12]. PANI has several advantages such as the high specific capacitance, good electrical conductivity [12,13], stretchability, lightweight, inexpensiveness, biocompatibility, good environmental stability and relative ease of synthesis [14–16]. Owing to these features, PANI has found applications in conversion devices and energy storage, like supercapacitors, fuel cells and batteries [12]. Nevertheless, PANI suffers from several drawbacks (e.g. poor cycle stability) as a result of a swelling, shrinkage and cracking sustained during the process of PANI doping/dedoping [12]. Moreover, PANI is prone to over-oxidation at relatively high potentials, thus limiting the working potentials of PANI electrodes [12]. In this respect, combining the unique properties of graphene with the advantages of PANI aims to improve the performance of the resulting nanocomposites and to broaden the range of applications in flexible plastic and wearable electronics [4,9,12,17,18]. Nowadays, various methods have been developed to elaborate graphene-based nanocomposites with various functional properties [9,12,17–19]. Significant improvements of their properties were achieved due to the synergistic effect between the graphene derivatives and PANI ‒ primarily through the noncovalent mixing/adsorption route [12,18,20]. The majority of reported studies adopted in situ polymerization methods of aniline monomer in the presence of GO or rGO [17,19,21–24]. However, it did not lead to enhancement of the electrical conductivity of the nanocomposites. To overcome this problem, reduction of the nanocomposite is carried out, which simultaneously converts GO to the conductive rGO and PANI to the non-conductive form. This procedure is followed by a reoxidation step, which aims to recover PANI from emeraldine base to the emeraldine salt. Though the reoxidation step can partially recover the electrical conductivity of the nanocomposites, it drastically affects the chemical and electronic structure of both PANI and rGO concurrently [12,17,25]. Another issue, which needs to be solved to successfully polymerize aniline on the surface of GO, is the colloidal instability of GO under acidic conditions in the polymerization reaction. This concern is not discussed in the literature and it needs to be addressed.

2. Experimental 2.1. Materials Expanded graphite was supplied by ECOPHIT G (GFG5) (real density 2.25 g·cm−3, mean diameter d50 5e7 mm). Ammonium persulfate 2

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(APS, 98 %), polyvinylpyrrolidone (PVP with Mw = 40 000 g·mol−1), aniline (ANI, ACS reagent, > 99.5 %), 2-acrylamido-2-methyl-1-propanesulfonic acid (AAMPSA, 99 %), sulfuric acid (H2SO4, 95–98 %), potassium permanganate (KMnO4, 327 mesh, 97 %), hydrogen peroxide (H2O2, p.a. ≥ 30 %, RT) and hydrazine hydrate (50–60 %) were all purchased from Sigma Aldrich. Fuming hydrochloric acid (HCl, 37 %) was purchased from Merck. All chemicals were used as received without any further purification.

aggregation of the nanofillers. The mixture is then heated at 80 °C followed by addition of hydrazine hydrate. The reduction of GO to rGO runs for 6 and 24 h. Subsequently, aniline is polymerized in the presence of well-washed rGO, following the same procedure as in case of in situ reduction. The obtained nanocomposite is washed with DI water and then dried at 60 °C. The final nanocomposites are identified as rGO/PANIex-6 and rGO/PANIex-24. 2.3. Characterizations

2.2. Preparation of nanocomposites 2.3.1. Transmission electron microscopy Nanocomposites morphology is investigated using transmission electron microscope (TEM model JEM-1010 JEOL, Ltd; Japan) and visualized by a CCD camera MegaView III (Olympus Soft Imaging Systems, Germany). Approximately 10 μl of the nanocomposite dispersed in water is deposited onto a carbon coated electron microscopic grid followed by contrasting of the sample using ethanol solution of 1 % uranyl acetate. The grid is left for a few minutes to dry and subsequently it is inserted into the electron microscope column for the sample analysis. Measurements are performed under 80 kV of accelerating voltage.

GO is synthesized following modified Hummers method described in Hassouna et al. [31] (see Supporting Information). To obtain graphene/PANI nanocomposites, two different routes are explored as described later. Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA) is used as polymeric dopant of PANI via polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid (AAMPSA). PAAMPSA offers some benefits during aniline polymerization, including the maintenance of pH in the polymerization medium around 2. This pH is essential for formation of electrically conductive form of PANI (emeraldine salt) [32–34]. PANI is a rigid and brittle polymer because of the conjugated nature of its molecules. Hence, incorporation of PAAMPSA, which exhibits lower glass transition temperature, can make the final material more ductile [35]. Moreover, the presence of polymeric dopant in PANI, should aid in conserving the electrical properties of the composites, as it has been previously stated that small dopant molecules such as chlorides may migrate out of the material resulting in an electrical conductivity loss during long-term use [36].

2.3.2. Scanning electron microscopy Scanning electron microscope (SEM) is employed to investigate the structure of the prepared materials on a microscopic level. The samples are sputter coated with 5 nm gold layer. Subsequently, morphology of the samples is analyzed using termoautoemission SEM Mira 3 LMH (Tescan Orsay; Czech Republic) under 3 kV of accelerating voltage.

2.2.1. Preparation of GO/PANI nanocomposite The first step consists of mixing GO dispersion with PVP to prevent aggregation of GO sheets in acidic medium. The ratio GO:PVP is 10:1, followed by 30 min of stirring. The final concentration of solid content is kept constant at 1 wt. % (amount of DI water / final solid content: 100 / 1). AAMPSA is dissolved in small volume of water and then mixed with ANI in an ANI:AAMPSA ratio equal to 1:5.56. This mixture is stirred for 15 min and then mixed with the prepared GO/PVP dispersion in a cooled round flask (0 °C). A GO:ANI ratio of 1:3 is used. APS is used as an initiator of oxidative polymerization of aniline. It is dissolved in a small volume of DI water and then injected drop-wise into the mixture. The system is stirred for 24 h at 0 °C. After the polymerization is complete, unreacted monomer, PVP and side products of the polymerization are removed from the product through filtration and repeated washing with DI water. The final nanocomposite is dried in the vacuum oven at 60 °C overnight.

2.3.3. Atomic force microscopy Atomic force microscopy (AFM) topographies of the elaborated nanocomposites were obtained in the tapping mode with an atomic force microscope MFP 3D Infinity (Asylum Research) at a frequency of 1 Hz and with a resolution of 256 × 256 pixels over a surface area of 2 μm × 2 μm. Probes used were AC 160 R3 from Asylum Research, whose resonance frequency and force constant were approximately 300 kHz and 26 N/m, respectively. A suspension of 1 mg of sample and 0.1 ml of ethanol is prepared by sonication for 30 min. Resulting suspension is drop casted onto a mica plate. 2.3.4. Raman spectroscopy Dispersive Raman spectrometer with the microscope InVia Reflex (Renishaw, United Kingdom) is used for chemical characterization of different samples. Excitation wavelength 785 nm with laser power of 2 mW was used. Ten accumulations taking 30 s are recorded for one acquisition and final spectrum is average of ten acquisitions.

2.2.2. Preparation of rGO/PANIin: in situ reduction route The washed GO/PANI nanocomposite is re-dispersed in DI water. The reactor is then heated under reflux to 80 °C. After stabilization of temperature, hydrazine hydrate is added (1 ml of hydrazine hydrate for every 10 mg of GO). The reduction time is set for 6, 12 and 24 h. The subsequent product is filtered and washed repeatedly with DI water and then dried in the vacuum oven at 60 °C overnight. Nanocomposites prepared in the same manner are identified as rGO/PANIin-6, rGO/ PANIin-12 and rGO/PANIin-24. For better understanding of the impact of GO reduction in presence of PANI, neat PANI is also reduced under the same conditions as GO/PANI for a period of 24 h. The sample is named rPANI. The nanocomposite rGO/PANIin-24 is exposed to oxidative environment of APS. The resulting nanocomposite is named rGO/PANIin24-reox.

2.3.6. UV–vis spectroscopy Dried samples are grinded and sonicated in DI water to obtain suspensions. The suspensions are then analysed by ThermoScientific Evolution 220 UV–vis spectrophotometer (UV–vis), equipped with single xenon flash lamp with an operating range 190−1100 nm.

2.2.3. Preparation of rGO/PANIex: ex situ reduction route The first step of the ex situ reduction route consists of chemical reduction of GO. Similarly, as in the case of in situ reduction, GO is primarily mixed with PVP (ratio of GO:PVP equal to 5:8) to prevent

2.3.7. X-ray diffractometry X-ray diffraction (XRD) measurements are performed with a θ-θ powder diffractometer X'Pert3Powder at room temperature by means of wavelength CuKα radiation (λ =1.5418 Å, I =30 mA, U =40 kV). To

2.3.5. Fourier transform infrared spectroscopy Infrared measurements are performed with Fourier transform infrared (FTIR) spectroscopy in transmission mode using Nicolet FTIR spectrometer Nexus 670 from Thermo Scientific. Spectra are collected with OMNIC Software. To obtain the IR spectra, KBr pellets are prepared from the powder materials. The globar is used as source of mid infrared radiation, KBr is used as beam splitter and DTGS as detector, 128 scans with resolution of 4 cm−1 were collected for each spectrum.

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thinning of rGO lamellae. In fact, when looking at TEM pictures (Fig. 1G), in situ reduction route does affect the morphology of PANI, as a discontinuous PANI coating in the form of droplets is clearly formed at the surface of the rGO layers. Compared to rGO/PANIin-24, TEM picture of rGO/PANIex-24 (Fig. 1F) shows the formation of homogeneous PANI coating on the surface of rGO due to the strong interactions between PANI and rGO [39,40]. This layered structure was also observed by AFM analysis (Figure SI 5A). It is also important to emphasize that the morphology of rGO/ PANIex also depends on the reduction time of GO. Indeed, while SEM micrographs of rGO/PANIex-24 show a compact structure in which it becomes hard to identify rGO lamellae, rGO/PANIex-6 displays an intermediate morphology between GO/PANI and rGO/PANIex-24 (Figure SI 4). In fact, GO is only partially reduced after 6 h of hydrazine treatment, thereby leading to an intermediate behavior between GO and fully reduced GO. For effective reduction of GO to rGO with hydrazine, 24 h of reduction time is required. Interestingly enough, AFM images depicted in Fig. 2 shows that all GO/PANI, rGO/PANIin-24 and rGO/PANIex-24 exhibit layered morphology, in which the surface of GO or rGO sheets is coated with a thin layer of PANI, thus confirming the observations made by TEM. In addition, rGO/PANIex-24 exhibits a sandwich-like structure of lamellae in which the layers of rGO coated with PANI are overlapping. This assembly and ordering at a nanoscale level shown by TEM (Fig. 1F) and AFM (Fig. 2B) can explain the distinctive compact structure of rGO/ PANIex-24 observed by SEM (Fig. 1A). Unlike rGO/PANIex-24, GO/ PANI displays a random distribution of the layers at a nanoscale level (Fig. 1H and Fig. 2B). As a result, a porous micro-structure is formed (Fig. 1C). Overall, the morphological discrepancies noticed among GO/PANI, rGO/PANIin-24 and rGO/PANIex-24 highlight differences in the chemical structures and, thus, in the mode of interaction between the polymer and the nanofiller. For better understanding of the physical structure of the nanocomposites, XRD analysis was carried out (Fig. 3). XRD diffractograms of PANI, GO/PANI and rGO/PANIex show peaks at 2θ = 23.6ᵒ and 26.3ᵒ corresponding to the (100) and (110) reflections, respectively, from the crystalline plates of PANI [41]. The presence of peaks at 23.6ᵒ and 26.3ᵒ in rGO/PANIex indicates that the unit structures of PANI were preserved after the formation of the nanocomposite with rGO. Furthermore, the intense and sharp peak centered at 2θ = 11.44ᵒ (001), is attributed to the interplanar spacing of GO sheets in GO/PANI nanocomposite. In rGO/PANIex, this peak disappeared and a new peak at 25.5ᵒ corresponding to (200) appeared instead which shows that most of GO is reduced to rGO. This observation implies that rGO sheets are fully used as substrates of PANI to produce the nanocomposite hybrid. However, X-ray pattern of rGO/PANIin (though modified) still indicates the presence of crystalline regions of PANI after 6 h of exposure to hydrazine hydrate at elevated temperatures (Figure SI 5). Indeed, two new broad peaks centered at 2θ = 14.4ᵒ and 20.2ᵒ and one intense peak at about 25.3ᵒ arose which correspond to the crystal planes (011), (020), and (200), respectively. They are practically identical to that of neat PANI [30]. Above 12 h of reduction time, only one very weak diffraction peak at 2θ = 20ᵒ remains, attesting for amorphization of PANI (Figure SI 5 and Fig. 3). Contrary, rPANI (PANI exposed to hydrazine hydrate under the same conditions as rGO/PANIin) retained its crystalline regions. Nevertheless, significant differences in the relative intensity of peaks characteristics for PANI can be noticed, indicating that the crystalline structure of rPANI is affected by the reducing agent. The crystal domains of rPANI can be visualized by SEM (Fig. 1E). rPANI exhibits micro crystals with undefined shape instead of nanofibrillar structure. This particular morphology is caused by the rearrangement of rPANI chains as a result of the reduction process. The structural differences observed between rPANI and reduced PANI in rGO/PANIin-24 may point to the fact that the mechanism of PANI reduction by hydrazine hydrate is influenced by the presence of GO. In other words, the loss of

scan the data, an ultrafast detector 1D PIXcel with a step size of 0.039° (2θ) and a counting time of 175.185 s per step is used. Evaluation of the data is performed on HighScorePlus 4.0. 2.3.8. X-ray photoelectron spectroscopy X-ray photoelectron analysis are carried out with Omicron Nanotechnology X-ray photoelectron spectroscopy (XPS) composed of monochrome radiation of Al lamp (1486.7 eV) in constant analyzer energy mode. Evaluation of collected spectra are done using CasaXPS software. 2.3.9. Thermogravimetric analyses Thermogravimetric analyses (TGA) are performed in StantonRedcroft TG 750 under nitrogen atmosphere (20 ml·min−1) from room temperature to 800 °C at a rate of 10 °C min−1. 2.3.10. Measurement of electrical conductivity Measurements were made by four-electrode arrangement on powdered samples (sample weight around 200 mg) of known geometrical dimensions under constant contact pressure of 3903 kPa [37]. Measurements of electrical current and voltage passing through the sample were done using HP 34,401 multimeter. HP E3631A served as an electrical current source. 3. Results and discussion 3.1. Preparation and characterization of rGO/PANI nanocomposites At the beginning, GO was obtained by chemical oxidation of expanded graphite. GO was fully characterized using several techniques including Raman spectroscopy, XRD, FTIR and XPS (see Figure SI 1, Figure SI 2, Figure SI 3 and Table SI 1 in Supporting Information). Then, two routes of preparation of rGO/PANI nanocomposites were studied. A comparative study between the properties of rGO/PANIin and rGO/ PANIex is imperative to better understand the complex interactions occurring between PANI and rGO. Results reveal the relationship among the morphology, chemical structure, and physical properties of rGO/PANI nanocomposites. 3.1.1. Morphology and structural properties of rGO/PANI nanocomposites As aforementioned, the preparation of rGO/PANI reported here includes two routes. Both routes are expected to produce GO coated with a layer of PANI in form of the emeraldine salt (intermediate product in in situ reduction route) and rGO sheets coated with PANI (ex situ reduction route). Both GO and rGO can play a role of effective support on which nucleation and polymerization of aniline occurs [17,23,38]. Fig. 1 and Figure SI 1 gather representative morphologies at micro- and nano-scale of the prepared nanocomposites. Both SEM and TEM pictures of GO/PANI (Fig. 1C and H) reveal a layered morphology, in which the surface of well-exfoliated GO sheets is coated with a thin layer of PANI. Hence, GO works as a template for PANI polymerization. It is worth mentioning that polymerization of PANI under the same conditions in absence of GO leads to fairly uniform crystalline nanofibrilar morphology, due to the macromolecular structure of PAAMPSA (Fig. 1D). Layered morphology of GO/PANI was confirmed by AFM analysis as depicted in Fig. 2A. In situ reduction of GO in GO/PANI, i.e. rGO/PANIin does not affect the layered morphology of the nanocomposite, even after 24 h of reduction time (Fig. 1B and Figure SI 4). This morphology was confirmed by AFM analysis. On the other hand, Figure SI 4 gathers SEM micrographs of rGO/PANIin for different reduction times showing evidence of a morphology change from layeredlike to layered along with the reduction time. In fact, at higher reduction times, layers of rGO lamellae are better distinguished and become thinner. This phenomenon could be attributed to the fact that during in situ reduction, PANI which is responsible for the thickening of GO lamellae undergoes chemical modifications, thus leading to the further 4

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Fig. 1. SEM images of prepared samples: A) rGO/PANIex-24, B) rGO/PANIin-24, C) GO/PANI, D) PANI, E) rPANI and TEM images of prepared samples F) rGO/ PANIex-24, G) rGO/PANIin-24, H) GO/PANI.

corresponding to C]C stretching of quinonoids/benzenoids and to polaronic bands was noticed in neat rPANI, highlighting discrepancies in the degree of protonation/deprotonation. The structural differences observed between rPANI and reduced PANI in rGO/PANIin confirm the theory that the process of PANI reduction by hydrazine hydrate is affected by the presence of GO as it was suggested above. FTIR spectroscopy in transmission mode of neat PANI, rPANI and PANI based nanocomposites depicted in Fig. 5 A and B confirmed all observations made by Raman spectroscopy. In fact, similarly to Raman results, FTIR spectra of PANI and rPANI are identical. The absorption band at 3100 cm−1 assigned to hydrogen bonding of eNH and NeeH+= groups is observed, thus indicating that there is a good intermolecular interaction between the polymer chains, which is crucial for effective charge transfer [45,46]. Moreover, a broad signal pattern at approximately 2000 cm−1 typical for emeraldine salt is noticed. Two peaks assigned to benzenoid and quinonoid ring stretching are at 1474 cm−1 and 1560 cm−1, respectively [43,47], and a very pronounced signal of eNH+= vibrations is at 1140 cm−1. A peak at 1302 cm−1 corresponding to delocalization of π-electrons can be also observed. The presence of PAAMPSA is highlighted by the appearance of peaks at 1030 cm−1 attributed to SeO3H vibration bonds, and also at 1652 cm−1, attributed to COe] stretching [48]. Both PANI and rPANI are clearly based on emeraldine salt, thereby confirming the observations made by Raman spectra. More complex spectra are recorded for the nanocomposites GO/ PANI and rGO/PANIex-24, which show very similar chemical structures. In both nanocomposites, the characteristic peaks of PANI at 1560, 1474, 1305 and 1224 cm−1 are observed. The high intensity of the absorption band at 3100 cm−1 in rGO/PANIex (Figure SI 4B) suggests a good interaction between the fillers and PANI enabling easy passage of current, which should make these nanocomposites well conductive. Interestingly enough, the absorption band at 1140 cm−1 corresponding to vibrations of-NH+= disappears in the spectra of both nanocomposites, which might be explained by selective interactions between PANI and the fillers restricting the eNH+= vibrations. FTIR analysis also confirmed that the structure of rGO/PANIin-24 exhibits emeraldine base form; this observation is evidenced by: i) the decrease of the intensity of the peak at 1140 cm−1 corresponding to protonated nitrogen atoms; ii) the decrease of the peak at 1305 cm−1 assigned to CNe stretching of secondary aromatic amine in protonated

crystallinity of PANI is connected with the simultaneous reduction of GO and PANI. The morphological and structural changes observed in rGO/PANIin and rGO/PANIex nanocomposites are revealed by spectroscopic techniques (Raman, FTIR and UV–vis). Fig. 4 depicts Raman spectra of neat PANI, rPANI and PANI based nanocomposites. First, it is important to mention that Raman spectra of all nanocomposites, i.e. GO/PANI, rGO/PANIex and rGO/PANIin, indicates the absence of peaks corresponding to PVP surfactant. This finding suggests that an exchange between the surfactant and PANI takes place on the surface of GO or rGO during aniline polymerization, resulting in PANI coating and surfactant leaching into aqueous solution. Characteristic peak of neat PANI in form of emeraldine salt is observed at 1170 cm−1, assignable to CH in-plane bending vibrations in benzenoids and quinonoids [e17]. The position of this vibrational peak indicates the oxidative form of PANI, and in this case, it designates the presence of bipolarons [42]. Other characteristic peaks of neat PANI at 1252, 1318, 1340, 1490, 1520, 1586 and 1627 cm−1 correspond to CN stretching, CN in presence of polarons, CNeee%+ stretching of delocalized polarons, CN of quinonoids, NH]e vibrations of semi-quinonoids, C]C stretching of quinonoid rings, and CeC stretching of benzenoid rings, respectively [42,43]. Similar peaks are noticed in GO/ PANI and rGO/PANIex-24 Raman spectra, suggesting complete coverage of the nanofillers with PANI in its emeraldine salt form. In contrast, upon in situ reduction, spectroscopic characteristics of rGO/PANIin indicate the presence of PANI in form of emeraldine base, evidenced by the appearance of CeH bending peak at 1155 cm−1 and CNe stretching peak at 1219 cm−1. Surprisingly, the peak at 1490 cm−1 corresponding to CN] (imine) stretching of quinoids, which is usually strong in emeraldine base form, exhibits very weak intensity. This can be explained by the formation of mixture of emeraldine and leucoemeraldine base, since pure leucoemeraldine base does not contain imine groups [17]. The presence of deprotonated PANI in rGO/ PANIin is also confirmed by the shift of peaks corresponding to C]C stretching of quinonoids/benzenoids (1586 cm−1) and to polaronic bands (1318 cm−1) to 1593 and 1312 cm−1, respectively [43]. It leads to their overlapping with rGO absorption bands. In fact, rGO exhibits characteristic peaks at 1593 and 1307 cm−1 attributed to G-band (vibration of sp2 hybridized carbon) and to the D-band (defect or edge areas), respectively [31,44]. Unlike rGO/PANIin, no shift of the peaks 5

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Fig. 2. AFM images of A) GO/PANI, B) rGO/PANIex-24 and C) rGO/PANIin-24.

respect to rPANI may find its origin in the morphology of PANI and in the mode of interactions established between GO and PANI [21]. XPS is employed to gain a better understanding of the chemical changes occurring during the synthesis of the nanocomposites at their surface. The full survey XPS analysis of the PANI, GO/PANI, rGO/ PANIin-24 and rGO/PANIex-24 is displayed in Figure SI 6. The atomic proportion of elements presented at the surface of the prepared samples are summarized in Table 1. XPS analysis indicates the presence of four main peaks at binding energies of about 285, 400, 532 and 168 eV, attributed to carbon (C(1 s)), nitrogen (N(1 s)), oxygen (O(1 s)) and

polymer; and iii) up-shift of the benzenoid and quinonoid ring stretching in emeraldine base forms. Considering the significant difference of intensities of benzenoid and quinonoid ring stretching peaks, intermediate oxidation state between emeraldine and leucoemeraldine base can be envisaged [17]. Similarly to Raman observations, none of the above mentioned changes in the structure of rGO/PANIin were detected in the FTIR spectrum of rPANI, confirming the influence of the presence of GO on the reduction of PANI during the simultaneous reduction process. Stronger deprotonation phenomena occurring in rGO/PANIin with 6

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Fig. 3. XRD patterns of neat PANI, rPANI, GO and PANI based nanocomposites.

Fig. 4. Raman spectra of neat PANI, rPANI and PANI based nanocomposites.

sulphur (S(1 s)) atoms, respectively, in PANI, GO/PANI and rGO/PANIex. A further striking observation is that rGO/PANIin-24 which also contains C(1 s), N(1 s) and O(1 s) atoms, does not bear S(1 s) in its surface. The absence of sulphur atoms in rGO/PANIin-24 might suggest that PAAMPSA underwent reaction with hydrazine hydrate, hence leading to the loss of its sulphate groups. This finding is in agreement with the observations made by spectroscopic techniques, where presence of PANI in the form of emeraldine base in rGO/PANIin was confirmed, as PAAMPSA is no longer able to provide protonation to the polymer chains. Besides, quantitative XPS analysis of C(1 s), N(1 s), O(1 s) and S(1 s) atoms reveals that while PANI and rGO/PANIex have a similar proportion of each atom, rGO/PANIin-24 exhibits lower proportion of oxygen atoms and higher proportion of carbon atoms. This proves again that ex situ reduction route leads to an efficient coating of rGO sheets with PANI in form of the emeraldine salt, whereas strongly reduced PANI in the form of emeraldine base covers rGO sheets in rGO/ PANIin. Deconvolution of the C(1 s) core level spectra for PANI, GO/PANI, rGO/PANIin-24 and rGO/PANIex-24 (Fig. 6 and Table 1) reveals the presence of four different overlapping peaks at 284.8, 285.5, 286.6 and 288.5 eV, which are attributed to (eCC and CC), (CN), (CO), and (COeee]eeeeeeee]), respectively. Generally, all the nanocomposites present similar types of carbon functionalities, among which the graphitic carbon is the major component. Furthermore, quantitative evaluation of the C(1 s) groups reveals that rGO/PANIin exhibits the lowest number of carbonyl groups, as a consequence of a simultaneous reduction of GO and PANI with a considerable loss of the dopant PAAMPSA. The chemical states of nitrogen functionalities were further

Fig. 5. FTIR spectra of neat PANI, rPANI and PANI based nanocomposites. Table 1 Summary of XPS data of as-synthesized materials. rGO/PANIex-24 Proportion of C, O, N and O atoms C 55 % O 26 % N 9% S 10 % Proportion of C1 s carbons CeC; C]C 62 % eCO 10 % CeNe 20 % C]O; OCe]O 8% Proportion of N1 s nitrogens N]C eeNH 32 % eNH+% 38 % ]NH+ 30 %

rGO/PANIin-24

GO/PANI

PANI

85 % 5% 10 % 0%

45 31 11 13

% % % %

58 % 23 % 9% 10 %

62 % 15 % 20 % 3%

54 16 20 10

% % % %

64 % 12 % 17 % 7%

28 % 16 % 56 %

35 % 30 % 35 %

30 % 70 %

evaluated by fitting the N(1 s) core spectra (Fig. 6 and Table 1). The deconvolution of N(1 s) core level spectrum for PANI-PAAMPSA shows the existence of three main peaks at binding energies of 399.6, 400.4 and 401.5 eV attributed to secondary amine (-NH-), protonated radical cation nitrogen (NH+), and protonated nitrogen (]NH+), respectively

7

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Fig. 6. C(1 s) and N(1 s) core-level spectra for A) PANI, B) GO/PANI, C) rGO/PANIex-24 and D) rGO/PANIin-24.

recently showed that the interactions between PANI and graphene derivatives, which are usually due to strong van der Waals interaction, can be modified by the charge-transfer dipolar attraction. They also showed that as the protonation degree increases, the binding energy increases too, due to the amplified dipolar attraction. This is likely the case in GO/PANI. The spectrum of rGO/PANIex exhibits slightly

[17,48,49]. Unlike PANI, GO/PANI spectrum shows higher proportion of the protonated nitrogen at 401.5 eV. It can be interpreted in terms of a higher doping of PANI due to the extra protonation of the latter by the carboxylic groups on GO sheets (besides its protonation by PAAMPSA), via possible generation of charge transfer complex. Carboxylate anions of GO would play a role of counterions of PANI. Wang et al. [40] 8

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Fig. 7. UV–vis spectra of A) PANI, B) GO/PANI, C) rGO/PANIex-24, D) rGO/PANIin-24, E) rGO/PANIin-24-reox, F) rPANI.

UV–vis spectrum by the change of intensity of absorption bands at 338 nm and 432 nm, which can be explained by the change in the degree of protonation [50,51]. Traver et al. [51] recently reported the mechanism of reduction of PANI-PAAMPSA in presence of hydrazine hydrate. They showed that hydrazine hydrate allows conversion of PANIPAAMPSA from its as-spun electrically conductive form, i.e. emeraldine salt, to its fully reduced neutral form, i.e. leucoemeraldine base [51]. Conversion of PANI by reduction process from its emeraldine salt to leucoemeraldine base is achieved by elimination of its electrostatic interactions with PAAMPSA, thus allowing chain relaxation of the released PAAMPSA through plasticization by moisture [51]. The difference observed between UV–vis spectra of rPANI and rGO/PANIin indicates again that the mechanism of reduction of PANI by hydrazine hydrate is influenced by the presence of GO, which may have also an impact on the structural properties of PANI. In fact, as above mentioned, the first aspect resides in the morphology of PANI. As highlighted by SEM, TEM and AFM analysis, GO/PANI exhibits a layered morphology, in which the surface of well-exfoliated GO sheets are covered by a thin layer of PANI. This template effect allows probably better diffusion of hydrazine hydrate in PANI coating during the reduction process and restricts the rearrangement of reduced PANI chains into microcrystals, as observed in neat rPANI. The second aspect to be considered is the interactions between GO and PANI via doping of PANI with carboxylic groups of GO. This means that during the reduction of GO in GO/PANI, de-doping of PANI in the sites of interactions with GO occurs. Another aspect which should be also taken into account is the

different proportions of the three nitrogen groups with respect to PANI, mainly reflected by an increase in the proportion of the species eNH+ and a decrease of the species N%%]H+, indicating that the overall oxidation state of PANI in rGO/PANIex is lower. The most striking observation concerns rGO/PANIin spectrum which shows: i) quasi-total disappearance of both peaks corresponding to the protonated nitrogen atoms, ii) appearance of new peak at 398.6 eV corresponding to quinoid imine, and iii) significant increase of the proportion of secondary neutral amine (70 %). All of these spectral changes are consistent with the mechanism of dedoping of PANI emeraldine salt as a consequence of simultaneous reduction of GO and PANI by hydrazine hydrate, resulting in the emeraldine base. In order to complete the chemical characterization of the nanocomposites, UV–vis analysis was performed and the absorption spectra are depicted in Fig. 7. UV–vis spectra of neat PANI shows the existence of three main peaks. The high polaron band at 442 nm and polaron band at 845 nm indicates the presence of PANI in form of emeraldine salt. The third peak at 353 nm is attributed to π-π* electron transition of benzenoid rings [17]. Polymerization of PANI as emeraldine salt in presence of GO or rGO leading to GO/PANI and rGO/PANIex-24, respectively, is confirmed by UV–vis spectra. However, unlike GO/PANI and rGO/PANIex-24, sample rGO/PANIin-24 displays a broad absorption band with maximum at 782 nm and noticeable shoulder at 616 nm, while the polaron peak at about 440 nm is absent, suggesting rather the presence of emeraldine base form [50]. Note that reduction reaction of PANI (rPANI) is reflected in 9

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Fig. 8. TGA curves for neat PANI, rPANI and PANI based nanocomposites.

intrinsic properties of GO/rGO, i.e. high thermal conductivity, which could potentially influence the kinetic of deprotonation process of PANI during the reduction of GO and PANI, which occurs simultaneously. A formation of intermediate forms of GO during its reduction by hydrazine hydrate, which can accelerate the reduction of PANI, cannot be totally excluded. To date, the mechanism of simultaneous reduction of both PANI and GO is remains obscure. Nevertheless, it is well-established that specific interactions between GO and PANI take place via electrostatic interaction, π-π stacking and hydrogen bonding [17,23,24]. These interactions are expected to inevitably influence the mechanism of reduction of PANI in presence of reducing agent.

Fig. 9. Electrical conductivities of PANI, rPANI and PANI based nanocomposites.

droplets is clearly formed at the surface of rGO layers in rGO/PANIin24. Thus, rGO/PANIin-24 allows better contact and interactions between rGO sheets and hence better thermal stability thanks to the high thermal conductivity feature of rGO [52]. 3.1.3. Electrical conductivity and electronic structure of the nanocomposites The electrical conductivities of the nanocomposites were measured to ascertain the possibility to apply them in electronic technologies. Fig. 9 shows a summary of electrical conductivity analyses of the prepared materials. PANI doped with PAAMPSA displays a conductivity of 0.11 S·cm−1 and dry GO has a very low conductivity of 9 × 10-7 S·cm−1. GO/PANI exhibits conductivity of 0.18 S·cm−1, which is only slightly higher than that of PANI. GO is an insulating material [53,54] hence, this increase can be explained only by a supplementary doping of PANI with some carboxylic acid groups of GO [55]. A dramatic loss of the electrical conductivity is observed in all in situ reduced materials, i.e. 2 × 10-5 and 0.01 S·cm−1 for rPANI and rGO/PANIin, respectively. In addition, the electrical conductivity decreases as the reduction time increases. The loss of the conductivity of rPANI and rGO/PANIin is due to the dedoping of PANI evidenced by the spectroscopic techniques and the morphology changes of the nanocomposites during the reduction process [19]. Interestingly, reoxidation of rGO/PANIin-24 allows partial restoration of its lost conductivity up to 0.27 S·cm−1 which is slightly higher than that of neat PANI. Such enhancements of conductivity can be attributed to the partial reprotonation of PANI, as highlighted by UV–vis analysis (Fig. 7). In fact, UV–vis spectrum of rGO/PANIin-24reox exhibits a small shoulder at around 440 nm, suggesting the presence of polaronic structures, responsible for enhancement of electrical conductivity of the nanocomposite [17]. In addition, establishment of π-π stacking interactions between the conductive form of PANI chains and rGO sheets is highly probable as higher electrical conductivity is recorded for rGO/PANIin-24-reox (σ = 0.27 S·cm−1) with respect to pure PANI (σ = 0.11 S·cm−1) [19]. Unlike rGO/PANIin-24, the nanocomposite prepared by ex situ reduction route, i.e. rGO/PANIex-24, shows an extraordinary increase of electrical conductivity from 0.18 S·cm−1 for GO/PANI to 76 S·cm−1. The value 76 S·cm−1 is comparable to electrical conductivity of nanostructured conducting polymers e.g. polypyrrole (approx. 90 S·cm−1) [56]. The electrical conductivity of rGO/PANIex can be further improved up to 3.7 times more than the original recorded value, i.e. 280 S·cm−1, by milling the material. This electrical conductivity value is higher in comparison with previously reported values in the literature on rGO/PANI nanocomposites prepared by chemical routes [57,24,30]. The outstanding enhancement of

3.1.2. Thermal properties Structural and morphological changes observed in the nanocomposites prepared by both aforementioned routes are expected to have an impact on the final properties of the nanocomposites. Thermal properties are examined by means of thermal gravimetric analysis (TGA). The TG curves for neat PANI, rPANI and PANI based nanocomposites are presented in Fig. 8. PANI exhibits three weight loss stages due to its hygroscopic nature. A first stage of weight loss of about 7 % from 20 to 110 °C is caused by the release of moisture. The second weight loss stage of PANI in the temperature range 200–400 °C, can be likely attributed to the degradation of the polymeric dopant PAAMPSA and oligomers and a breakdown of the main chains in the polymer [41]. The third stage occurs at a decomposition onset temperature of 590 °C, where the weight loss may be connected with further degradation of polymer chains. The overall degradation of PANI up to 800 °C reaches a total weight loss of 84 wt. % in nitrogen atmosphere. However, GO, besides the weight loss corresponding to moisture release, shows a significant weight loss from 200 to 270 °C caused by the decomposition of oxygen-containing functional groups [19]. In all PANI based nanocomposites, a slight mass loss from room temperature to 110 °C due to moisture release is detected. While GO/PANI and rGO/PANIex-24 show very similar thermal behavior to neat PANI, rGO/PANIin-24 displays much higher thermal stability. rGO/PANIin-24 only shows 11 wt.% mass loss from 23 to 500 °C and a total of 44 wt.% at 800 °C, stating that majority of the oxygen containing functional groups were eliminated during the simultaneous chemical reduction of PANI and graphene sheets. At lower reduction time, i.e. rGO/PANIin-6 (Figure SI 7), thermal stabilization of PANI is less effective than in rGO/PANIin-24 since a weight loss of 37 % from 23 to 500 °C and of 54 wt.% when reaching 800 °C is observed. Relatively larger weight loss of rGO/PANIin-6 as compared with rGO/PANIin-24 is due to the incomplete reduction of GO after 6 h. The unexpected discrepancies observed in terms of thermal stability between rGO/PANIex-24 and rGO/PANIin-24 can be correlated with the structural and morphological changes in the nanocomposite. Indeed, while a homogeneous coating of PANI is formed at the surface of rGO in rGO/PANIex-24, a discontinuous PANI coating in the form of 10

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polymerization reaction of aniline, and ii) aggregation of reduced graphene oxide during the step of reduction of graphene oxide and its subsequent lack of colloidal stability in aqueous solution. Furthermore, the exchange between the surfactant and polyaniline at the surface of reduced graphene oxide during aniline polymerization, via surfactant leaching into the aqueous medium, allows eliminating the surfactant from the nanocomposite to restore the electronic structure and the interactions between polyaniline and reduced graphene oxide. To the best of our knowledge, this is the first time such a behaviour of a surfactant is reported, as it is usually very difficult to desorb the surfactant from the surface of reduced graphene oxide. In this regard, two routes of synthesis of reduced graphene oxide/doped-polyaniline nanocomposites based on sacrificial surfactant-stabilized graphene oxide/reduced graphene oxide are developed. The first route named “in situ reduction” consists of in situ polymerization of aniline monomers in the presence of sacrificial surfactant stabilized-graphene oxide suspension followed by reduction of graphene oxide sheets coated with polyaniline. The second route named “ex situ reduction” consists of colloidal stabilization using sacrificial surfactant and reduction of graphene oxide followed by in situ polymerization of aniline monomers in the presence of reduced graphene oxide. Interestingly, chemical structure of all nanocomposites highlighted the occurrence of an exchange between the surfactant and polyaniline on the surface of graphene oxide or reduced graphene oxide during aniline polymerization. It results in polyaniline coating and surfactant leaching into the aqueous medium. Morphological characterization revealed different morphological structures between in situ and ex situ reduced composite. While the former exhibits a discontinuous polyaniline coating in the form of droplets, the latter displays a homogeneous thin coating of polyaniline on the surface of reduced graphene oxide sheets. Meanwhile, X-ray diffraction patterns indicate a loss of crystallinity of polyaniline in in situ reduced composite associated with the simultaneous reduction of graphene oxide and polyaniline, whereas the crystalline structure of polyaniline is retained in ex situ reduced composite. The morphological and physical structure discrepancies noticed between in situ reduced composite and ex situ reduced composite highlight differences in their chemical structures as shown by infrared, Raman, X-ray photoelectron spectroscopy and UV–vis, and, thus, differences in the mode of interaction between polyaniline chains and reduced graphene oxide sheets, which in turn affects the related properties. While the simultaneous in situ reduction approach induces transition of polyaniline from the emeraldine salt to the emeraldine base, polymerization of polyaniline at the surface of reduced graphene oxide sheets viaex situ reduction approach allows conserving the emeraldine salt form. As a consequence, in situ reduced composite exhibits much higher thermal stability (shift of onset

electrical conductivity observed in rGO/PANI nanocomposite can be attributed to the particular morphology of this latter obtained under well-controlled processing conditions. In fact, the formation of a thin homogeneous coating of emeraldine salt on the surface of rGO and the resulting sandwich like-structure in which the sheets of coated-rGO are overlapping allows establishment of the optimum interactions between PANI and rGO. Three different features can be taken into consideration. Firstly, thanks to the excellent colloidal stability of rGO in water achieved via addition of optimum amount of PVP to GO, a large surface area of rGO sheets are formed. Thus, there is higher contact area between PANI and rGO leading to strong interactions at the contact interface. It is worth emphasizing that PVP, which plays both roles of electrically inactive surfactant and viscosity enhancer during the reduction step of GO, does not affect the final electrical conductivity of the nanocomposite. The most of PVP is leached out from the surface of rGO during the polymerization of aniline and subsequently removed during the washing step. Secondly, thin PANI coating grown at the surface of rGO sheets is achieved using lower amount of rGO as compared to aniline monomers (ratio 0.25/0.75 : rGO/ANI) thanks to its very high exfoliation degree into mono-sheets (Fig. 3). Thirdly, as a consequence of the composition and the particular morphology of rGO/ PANIex-24, strong interactions between rGO and PANI take place including π-π stacking. However, this type of interaction is not sufficient to explain the measured conductivities. Additional interactions associated with the structures and the intrinsic properties of rGO and PANI are highly probable. By considering the fact that rGO is an exceptional electron acceptor and that aniline is an excellent electron donor, charge-transfer complexes could be formed during the synthesis of PANI on the surface of rGO [17,23]. Finally, rGO may act as a chemical dopant of PANI through its residual carboxylic acid groups. All these reasonably proposed interactions based on the experimental observations can lead to highly conductive nanocomposites. The proposed mechanism of interactions between PANI and rGO is presented in Fig. 10. Therefore, ex situ reduction methodology can be considered as a promising chemical route for elaboration of highly electrically conductive rGO/PANI nanocomposites.

4. Conclusions In this study, a simple and cost-effective approach for the synthesis of reduced graphene oxide/polyaniline nanocomposites based on sacrificial surfactant-stabilized reduced graphene oxide was developed. This approach enables us to overcome some recurrent limitations when preparing nanocomposites based on reduced graphene oxide, e.g.: i) colloidal instability of graphene oxide under acidic conditions of the

Fig. 10. Schematic representation of the proposed mechanism of interaction between rGO and PANI. 11

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financial support.. The authors are grateful to prof. Miroslav Šoóš from University of Chemistry and Technology Prague for the fruitful discussions. The authors would like to thank also Dr. Ladislav Fišer from University of Chemistry and Technology Prague for the electrical conductivity measurements.

decomposition temperature by about 300 °C) and poor electrical conductivity (σ = 0.01 S·cm−1) with respect to graphene oxide/polyaniline (σ = 0.18 S·cm−1) and to neat polyaniline (σ = 0.11 S·cm−1). Reoxidation of in situ reduced composite allows partial restoration of its lost conductivity up to 0.27 S·cm−1, which is fairly higher than that of neat polyaniline. In contrast, ex situ reduced composite displays thermal properties comparable to those of neat polyaniline and superior electrical properties (σ = 76 S·cm−1), which can be further improved up to 3.7 times more than the original recorded value, i.e. 280 S·cm−1, by milling the material. Elaboration of ex situ reduced composite with such electrical conductivity features is possible only thanks to the excellent colloidal stability of reduced graphene oxide in water. The colloidal stability is achieved via effective stabilization of graphene oxide during the reduction process carried under well-controlled processing conditions, and to the subsequent morphology of ex situ reduced composite. Using this novel approach, the selected surfactant played a key role in preventing the aggregation of reduced graphene oxide, followed by its removal during the aniline polymerization. This step is crucial in restoring the electronic structure and the interactions between polyaniline and reduced graphene oxide. To the best of our knowledge, this electrical conductivity value is the highest measured for reduced graphene oxide/polyaniline nanocomposites prepared by chemical route. In the future, here presented nanocomposite can be used for energy storage applications such as a material for supercapacitor electrodes.

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Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Contribution of the authors Tereza Bautkinová and Adam Sifton carried out the experimental work and the acquisition and interpretation of the data. The contribution of Tereza Bautkinová is more important. Edith Mawunya Kutorglo carried out the preliminary experiments of this study and she participated to the revision of the manuscript. Marcela Dendisová carried out Raman analysis and participated to the revision of the manuscript. Dušan Kopecký carried out SEM analysis and participated to the revision of the manuscript. Pavel Ulbrich performed TEM analysis and participated to the revision of the manuscript. Petr Mazúr was a specialist consultant in this project and he contributed to the analysis and interpretation of the data. Moreover, he participated to the revision of the manuscript. Abdelghani Laachachi performed the new AFM experiments and he contributed to their interpretation. Fatima Hassouna designed/conceptualized and supervised the whole study. She supervised the students Tereza Bautkinová, Adam Sifton and Edith Mawunya Kutorglo who carried out the experimental work. Moreover, she wrote the article. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank Czech Science Foundation (GACR) grant (16-22997S) for the financial support. The authors would like to thank also European Regional development Fund-Project 'Organic redox couple based batteries for energetics of traditional and renewable resources' (ORGBAT) No.CZ.02.1.01/0.0/0.0/16_025/0007445 for the 12

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