Article pubs.acs.org/Langmuir

Self-Assembled Multilayer Films of Sulfonated Graphene and Polystyrene-Based Diazonium Salt as Photo-Cross-Linkable Supercapacitor Electrodes Zhiyuan Xiong, Tonghan Gu, and Xiaogong Wang* Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: Photo-cross-linkable multilayer films composed of sulfonated reduced graphene oxide (SRGO) and polystyrene-based diazonium salt (PSDAS) were fabricated by electrostatic layer-by-layer (LbL) self-assembly. Polystyrene with narrow molecular weight distribution was synthesized by atom transfer radical polymerization (ATRP), and cationic PSDAS was prepared through nitration, reduction, and diazotization reactions. Negatively charged SRGO nanosheets were prepared through prereduced by NaBH4, modified by diazonium salt of sulfanilic acid, and then further reduced by hydrazine. The multilayer films were obtained by alternately dipping substrates in the PSDAS solution and SRGO dispersion in acidic conditions. The cross-linking between the components occurred during the multilayer formation process and was further achieved by the UV light irradiation after the film preparation. The assembling process and surface morphology of LbL multilayer films were monitored by UV−vis spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The cross-linking between SRGO and PSDAS was verified by attenuated total reflectance FTIR (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle measurement. The graphene nanosheets were found to be homogeneously distributed in the cross-linked network of the films. The large accessible surface area of graphene nanosheets and the cross-linking structure afforded the LbL films with high specific capacitance and excellent cyclic stability when used as supercapacitor electrodes. At a sweeping rate of 10 mV/s, the film with nine bilayers exhibited a specific capacitance of 150.4 F/g with ideal rectangular cyclic voltammogram. Large capacitance retention of 97% was observed after 10 000 charge−discharge cycles under the scanning rate of 1000 mV/s. This new approach for preparing graphene-containing multilayer films can be used to develop supercapacitor electrodes and other functional devices.

1. INTRODUCTION Since the pioneering work by Novoselov et al., graphene has sparked enormous research interest in scientific and industrial fields.1−3 The excellent electrical, electronic, and mechanical properties as well as extremely large surface areas of graphene make it a competitive candidate for many applications. One of the promising applications is to be used in the electrochemical devices for the low-cost and environmentally friendly energy storage.4 Electrochemical capacitors (ECs), also known as supercapacitors, have been widely used in energy storage fields.5 ECs can combine the high power performance of traditional capacitors and large energy density of batteries, which have played an important role complementary to batteries in the energy storage field. Depending on the charge storage mechanism, ECs can be classified into two categories, namely electrochemical double-layer capacitors (EDLCs) and pseudocapacitors.6 Pseudocapacitors are fabricated by using Faradaic pseudocapacitor materials, such as conducting polymers and various transition metal oxides. The energy storage is realized by fast and reversible electrochemical redox reactions between electroactive electrode materials and ions in suitable potential windows. On the other hand, EDLCs mainly © 2013 American Chemical Society

use the active carbon with high surface areas as electrode materials and employ charge separation at the electrode/ electrolyte interface to store energy.7 Graphene as a twodimensional material with an extremely high surface area is considered to be promising as the electrode material for EDLCs.8−10 As the chemically converted reduced graphene oxide can be produced with the ton-scale yield at a low cost, this easy availability makes it possible for graphene to be used as practical electrode materials.11 At the current stage, the benefits of the graphene for EDLC applications have not been fully reaped. Theoretically, EDLCs derived from graphene can reach a high capacitance of 550 F/g if the surface areas of graphene can be made full of use.12 However, it is almost inevitable for graphene to undergo the phase separation and irreversible agglomeration in the devices, which results in the loss of its surface areas as well as the reduction of the specific capacitance.13,14 Layer-by-layer (LbL) self-assembly is a versatile and cost-effective approach to Received: October 7, 2013 Revised: December 7, 2013 Published: December 30, 2013 522

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manufacture thin films with highly controlled nanoscale structures and extended functionality.15 Through the charge compensation mechanism, multilayer films can be formed on the substrates by alternately dipping them in two oppositely charged solutions. The LbL technique can incorporate various species, such as polymers, nanosheets, nanotubes, and nanoparticles, which shows advantages to combine the properties of each component.16 Under suitable conditions, phase separation of nanocomponents can be effectively avoided by the LbL process. The free-standing LbL hybrid films composed of single-wall carbon nanotube (SWNT) and polyelectrolyte have been obtained by Kotov et al.17 Even when the content of SWNT reaches as high as 50%, the film keeps highly structural homogeneity without phase separation and is thus found to be exceptionally strong. Recently, this method has been used to prepare self-assembled LbL films of graphene with charged polymers, metal nanoparticles, and carbon nanotubes (CNTs) to obtain transparent conductive films or biosensors.18−21 The ultrathin film approach and LbL technique have been extended to the energy storage applications.22−26 Because of the effective separation by the polyelectrolyte in the LbL film, the surface areas of graphene can be fully exploited. Graphene/CNT LbL film has been explored for the electrochemical energy-storage application.27 In a recent article, we have reported the fabrication of multilayer films of graphene and azo polyelectrolyte by electrostatic LbL self-assembly and their application as supercapacitor electrodes.28 However, as the graphene and polyelectrolyte interact with each other through electrostatic attraction, the stability of the multilayer films cannot be entirely satisfactory. Polymeric diazonium salts, also called diazo-resins, are polymers containing cationic diazonium groups.29,30 Upon UV light irradiation, the photosensitive diazonium groups will release N2 and be converted to phenyl cations or free radicals depending on the polarity of the surrounding medium.31 The phenyl cations can react with a variety of nucleophilic reagents or groups such as H2O, carboxyl group, or sulfonic group to form the corresponding phenol, carboxylate ester, and sulfonate ester.29,32 Using the photochemical reaction, covalently attached LbL films have been produced by exploiting diazo resins as polycations and other polyelectrolytes containing nucleophilic groups as polyanions.33,34 Because of the transformation of multilayer film structure from ionic to covalent upon UV irradiation, the as-prepared films show high stability even toward polar solvents. In the past decades, diazonium salts have been used as a component to produce covalently stable LbL films with sulfonated polyaniline, poly(acrylic acid), poly(sodium styrenesulfonate), carbon nanotube, and others.33,34 In the recent years, aryldiazonium salts have been used to functionalize graphene, which reacts via an electron transfer process.35,36 Similar reaction can occur between the graphene and polymeric diazonium salt during the multilayer fabrication process. In principle, the cross-linked LbL films can be used as electrode materials of supercapacitors to significantly enhance the cyclic stability of the electrode. However, to our knowledge, stable LbL film composed of photoreactive polymeric diazonium salts and graphene has not been reported in the literature. In this work, we fabricated stable multilayer films of polystyrene-based diazonium salts (PSDAS) and graphene modified with p-phenyl-SO3H groups through the electrostatic LbL adsorption method. The performance of the films as electrode materials of EDLCs was evaluated after UV light

irradiation. The photosensitive PSDAS was obtained from polystyrene (PS) through nitration, reduction, and diazotization steps. The sulfonated reduced graphene oxide (SRGO) was selected as the negative-charged graphene because of its stability in an acidic solution. Because an acidic condition is necessary for the diazonium salts to avoid possible decomposition, the dipping solution of SRGO should have similar acidity. Owing to the charge interaction, the agglomeration of the graphene was avoided in the dipping process. Upon UV light irradiation, the diazonium groups of PSDAS were found to be reacted with sulfonic groups as well as the conjugated carbon atoms of SRGO to form cross-linked structure. The as-prepared films were electrochemically stable in aqueous and organic solutions. The supercapacitors using these films as electrodes showed excellent performances and good cyclic stability.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. The graphite powder used in the experiments was purchased from Sinopharm Chemical Reagent Co. Ltd. Before use, graphite powder with the size of 300−400 mesh was sieved out. Methyl 2-bromoisobutyrate and 2,2-dipyridyl were purchased from Sigma-Aldrich. CuBr was washed with acetic acid and ethanol for a few times and stored with argon protection before use. Deionized water (resistivity >18 MΩ·cm) was supplied by a Milli-Q water purification system and used for all experiments. Other chemicals and solvents were all commercially purchased and used as received without further purification. UV−vis spectra were recorded by using an Agilent 8453 UV−vis spectrophotometer. 1H NMR spectra were measured on a JEOL JNMECA 300 spectrometer. The molecular weights and molecular weight distributions were obtained by using gel permeation chromatography (GPC) equipped with a refractive index (RI) detector (Wyatt Optilab rEX). The measurements were carried out at 35 °C using a PLgel 5 μm mixed-D column, which was calibrated with polystyrene standards. THF was used as the eluent and the flow rate was 1.0 mL/min. Fourier transform infrared (FTIR) and attenuated total reflectance (ATR) measurements were carried out on a Nicolet 560-IR spectrophotometer. X-ray diffraction (XRD) patterns of graphene samples were recorded at room temperature by a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (1.5406 Å). The Raman spectra were measured on a Renishaw 1000 microspectrometer using an excitation wavelength of 514.5 nm. The bonding energy of elements was measured by an X-ray photoelectron spectrometer (ESCALAB250Xi) using a monochromatized Al Kα X-ray source of 1486.6 eV under normal incidence, and the binding energy of XPS peaks was standardized by the C 1s peak at 284.6 eV. For the peak fitting, a standard Shirley background and fixed Gaussian−Lorentzian line shape with 20% Lorentzian were used for all sample spectra. The atom force microscope (AFM) images of the film surfaces were obtained by using a Nanoscope-IIIa scanning probe microscope in the tapping mode. The cross-section SEM images of the films were obtained on a field-emission scanning electron microscope (JEOL-JSM-6700F). Electrochemical properties of the PSDAS/SRGO multilayer films as the supercapacitor electrodes were measured in a three-electrode cell, which used an Ag/AgCl electrode and Pt wire as the reference and counter electrodes. The PSDAS/SRGO multilayer films absorbed on indium−tin oxide coated glasses (ITO glasses) were used as the working electrode in 1 M Na2SO4 or 1 M H2SO4 solution. Cyclic voltammetry (CV) was measured in the potential range between 0 and 0.8 V versus Ag/AgCl at room temperature at various scan rates from 10 to 1000 mV/s on a CHI660B bipotentiostat. The galvanostatic charge−discharge test was conducted on a commercial instrument (Arbin Instruments Corp., Model SCTS) at room temperature. Electrochemical impedance spectroscopic (EIS) measurement was performed by using a commercial instrument (Biological Science Instrument, VSP-300), measured in the frequency range from 4000 kHz to 100 mHz at an ac amplitude of 10 mV in 1.0 M Na2SO4 solution. 523

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Scheme 1. Multilayer Film Prepared by Layer-by-Layer Deposition and Post-Preparation Cross-Linking

2.2. Preparation of Polystyrene-Based Diazonium Salt (PSDAS). PSDAS was prepared from polystyrene (PS) through nitration, reduction, and diazotization reactions.30 PS with a moderate molecular weight and narrow distribution, Mn = 15 500, Mw/Mn =1.25 (GPC), was obtained by atom transfer radical polymerization (ATRP). Poly(4-nitrostyrene) (PS-NO2) was obtained by nitration of PS through the reaction with nitric acid in coexistence with sulfuric acid. The degree of functionalization was estimated to be 100% by 1H NMR analysis. Poly(4-aminostyrene) (PS-NH2) was then obtained by reduction of PS-NO2 with tin powder. The conversion of this reaction was about 100% as verified by 1H NMR. The preparation and characterization details for the PS, PS-NO2, and PS-NH2 are given in the Supporting Information. For preparing PSDAS, HCl (2.8 mL) was added into a solution of PS-NH2 (1 g) in deionized water (20 mL) cooling with an ice bath. NaNO2 (1.2 g) dissolved in water (10 mL) was added dropwise into the PS-NH2 solution. After reaction with the ice bath cooling for 6 h, the insoluble impurity was removed by filtration to afford the dark red diazonium salt solution, which was used for the self-assembly after dilution. 2.3. Preparation of Graphite Oxide (GO). Graphite oxide (GO) was prepared via a modified Hummers method,37 which is briefly presented as follows. Graphite (3 g), K2S2O8 (2.5 g), P2O5 (2.511 g), and H2SO4 (12 mL) were added into a 50 mL flask. After reaction at 80 °C for 4.5 h, the mixture was poured into excess of cold deionized water. The preoxidized graphite was collected by filtration and then redispersed in H2SO4 (120 mL). Under the ice bath cooling, KMnO4 (15 g) was added very slowly into the dispersion. After being treated at 35 °C for 2 h, the dispersion was diluted with the deionized water (250 mL) and stirred for 4 h. H2O2 (12 mL) was then slowly added into the dispersion to remove the excessive oxidant. Crude graphite oxide was collected by filtration using a 0.45 μm filter membrane. The crude product was then washed with HCl (1 M, 1 L), water (1 L) sequentially and separated by centrifugation. The GO was obtained by drying at 45 °C for 24 h to afford the brown graphite oxide. 2.4. Preparation of Sulfonated Reduced Graphene Oxide (SRGO). SRGO was synthesized through three continual steps, i.e., prereduced with NaBH4, modified with diazonium salt of sulfanilic acid, and reduced with hydrazine.38 Graphite oxide (75 mg) was dispersed in water (75 mL) by strong sonication. A few drops of Na2CO3 water solution (5 wt %) were added to adjust pH = 9−10. NaBH4 (0.6 g) dissolved in deionized water (15 mL) was then dropped into the above mixture with strong stirring. After the reaction at 80 °C with reflux for 1 h, the solution gradually turned from brown to black with the appearance of agglomeration. The precipitate was filtrated with a 0.45 μm filter membrane and washed with water several times. After being redispersed in deionized water (75 mL), the

diazonium salt prepared from sulfanilic acid (138 mg) was added to the dispersion with the ice bath cooling. After reaction for 24 h, the dispersion became a uniform black solution, which was caused by the linkage of negatively charged sulfo groups to enhance the dispersion stability. The product was collected by high-speed centrifugation and washed with water several times to remove the small molecules. The solid was then dispersed in deionized water (75 mL) again. To remove the remaining oxygen-containing functional groups on graphene nanosheets, hydrazine (80%, 1 mL) was added into the dispersion. After reacting at 90 °C for 2 h under reflux, the final sulfonated graphene was collected by high-speed centrifugation, washed with water, and dried under vacuum for 24 h. ATR-FTIR (SRGO suctionfiltrated film): 3259 cm−1 (stretching vibration of O−H); 2919, 2814 cm−1 (stretching vibration of −CH2−); 1643, 1606 cm−1 (skeletal vibrations of unoxidized graphitic domains and bending vibration of O−H in the absorbed water); 1296 cm−1; 1171 cm−1 (symmetric stretching vibration of OSO); 1035 cm−1 (stretching vibration of SO); 1007 cm−1, 838 cm−1 (in-plane bending of C−H and out-ofplane hydrogen wagging vibrations of p-disubstituted phenyl group). 2.5. Fabrication of LbL Films. SRGO was dispersed in water to form a uniform dispersion with the solid content of 0.04 mg/mL. A few drops of diluted acetic acid solution were added into the dispersion to adjust pH to 5. After strong sonication and low-speed centrifugation, the homogeneous black dispersion was collected and kept at 2 °C for the LbL self-assembly. The PSDAS solution prepared as above was diluted to a concentration of 1 mg/mL and kept at 0 °C for the self-assembly. The quartz slides and silicon wafers as substrates were pretreated by sequentially washing with hot H2SO4/H2O2 (7:3) and H2O/H2O2/NH3 (5:1:1). ITO glass substrates did not need to be pretreated in this way. The LbL self-assembly was carried out in 2 °C surroundings by alternately dipping the substrates in the PSDAS solution for 30 min and in the SRGO dispersion for 1 h. After each dipping, the substrates were washed with excessive deionized water and blown dry with cold air. The dipping cycle was repeated until the required bilayer number was reached, and the LbL growth processes were monitored by UV−vis spectroscopy. For UV light irradiation, a high-pressure Hg lamp, equipped with quartz jacket condenser cooled with running water, was used as the light source. The self-assembled films were irradiated with the UV light (365 nm, 50 mW/cm2) for 30 min.

3. RESULTS AND DISCUSSION The multilayer films were fabricated from polystyrene-based diazonium salts (PSDAS) and sulfonated reduced graphene oxide (SRGO) through the electrostatic LbL deposition 524

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method. The LbL approach is used to guarantee the homogeneity and separation of the graphene sheets in the matrix, which will make a full use of the high surface areas of the materials. Scheme 1 illustrates the preparation procedure of the multilayer films and cross-linking reactions taking place upon light irradiation. The performance of the multilayer films as EDLC electrodes was tested and the electrode stability was evaluated. 3.1. Materials for Multilayer Fabrication. Polystyrenebased diazonium salt (PSDAS) was synthesized from PS through the nitration, reduction, and diazotization steps,30 as illustrated by Figure S1 in the Supporting Information. The PS with suitable molecular weight and narrow molecular weight distribution was obtained by atom transfer radical polymerization (ATRP). The number-average molecular weight of the PS was 15 500 with the polydispersity index of 1.25. The nitration and reduction reactions were carried out by routine procedures and characterized by spectroscopic methods (in the Supporting Information). Both 1H NMR and FTIR analyses confirmed that poly(4-aminostyrene) (PS-NH2) with the high degree of functionalization was obtained after nitration and reduction. After converting PS-NH2 to PS-N2+ by diazotization, the polycationic PSDAS was obtained and kept at 0 °C for the LbL deposition. It is critically important to find a suitable graphene containing anionic groups for the LbL adsorption. The solution of the polycationic PSDAS is acidic as a significant amount of HCl was added into the solution during the preparation process. This low-pH condition is necessary to stabilize PSDAS as diazonium salt will decompose in a neutral or alkaline water solution.31 Therefore, the electrostatic LbL assembly has to be carried out under this acidic condition. On the other hand, a chemically reduced graphene oxide is usually unstable in an acidic condition.3 Under the common conditions, the reduced graphene oxide is prepared by the selective reduction of graphene oxide in the presence of weak base, in which the remaining carboxyl groups are negatively charged.3 The repulsion between the negative charges is important for preventing agglomeration of graphene nanosheets, which is easy to occur due to the large surface area. In order to obtain graphene suitable for the LbL self-assembly with PSDAS under the acidic conditions, sulfonated graphene was prepared according to the previous report.38 The sulfo groups, which remain negatively charged even in a lower pH solution, are bonded to the graphene nanosheets through phenyl groups. Such-prepared graphene can then ensure the high stability in an acidic solution (pH = 5) and is suitable for the fabrication of multilayer films. SRGO stable in the acidic solution was prepared from GO through three steps, i.e., prereduction by NaBH4, reaction with the diazonium salt of sulfanilic acid, and reduction by hydrazine.38 Both SRGO and GO were characterized by XRD, Raman spectroscopy, and AFM. After the oxidation in the Hummers process, the 2θ peak of XRD of graphite shifts from 26.6° to 10.9° (Figure 1). It indicates the increased interplanar distance in GO, which is caused by the introduction of the oxygen-containing groups on the nanosheets. The diffraction peak of the graphite almost disappears due to the high degree of the oxidation. The obtained GO could be easily dispersed in water because of the existence of the large amount of the functional groups. For SRGO, a broader peak at 2θ ∼ 25° can be seen from the XRD spectrum (Figure 1). The backshifted diffraction peak indicates that the conjugated nano-

Figure 1. XRD curves for graphite, graphite oxide (GO), and sulfonated graphene (SRGO).

sheets partly regain the stack structure, although it is not tightly packed as the graphite. The water dispersion of the sulfonated graphene nanosheets after sonication was dropped on the silicon wafer for AFM observation. Figure 2 shows the typical AMF images of exfoliated SRGO nanosheets in comparison with that of single-layered GO nanosheets on the substrates. The thickness of the SRGO nanosheets is about 1.40 nm, which is slightly thicker than that of GO (1.24 nm). It can be attributed to the existence of the p-phenyl-SO3H groups on the facets, which increases the thickness measured by AFM. The result confirms that the completely exfoliated SRGO nanosheets can be obtained by sonication under the experimental conditions. The restoration of the π-conjugation in SRGO compared to GO was studied by Raman spectroscopy. The ratio of D and G peak intensities in Raman spectrum is inversely proportional to the in-plane crystalline sizes.39 Compared to GO, the I(D)/I(G) ratio of SRGO increases from 0.86 to 1.07 as shown in the Raman spectrum given in Figure 3. As the π-conjugation systems formed from the reduction have the smaller crystalline sizes, the I(D)/I(G) increase is one evidence for the restoration of the πconjugation. The restoration of the π-conjugation in SRGO can also be seen by comparing the UV−vis spectra of GO and SRGO (Figure S3). The maximum absorption band shifts from 232 to 270 nm as a result of the restoration of the πconjugation.3,28 3.2. Multilayer Fabrication and Properties. The multilayer films were prepared by electrostatic adsorption through alternately dipping the substrates in the PSDAS and SRGO aqueous solutions. As diazonium salt is usually unstable when 525

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Figure 2. Tapping-mode AFM images, (a) GO and (b) SRGO together with the height profiles for the cross-section analysis.

to the absorption of both SRGO and PSDAS, also shows a linear increase. The linear relationship of the absorbance versus bilayer number evidences the LbL growth of the multilayer films on the substrates. Figure 5a shows the photographic image of the multilayer films on the quartz slides. As the absorption increases in the visible range, the films gradually become blacker with the increase of the bilayer numbers. Figure 5b shows the surface morphology of the multilayer film with 12 bilayers observed by AFM. It can be seen that the graphene nanosheets with the irregular shapes ∼200 nm dimensions densely packed on the surface. The root-mean-square roughness measured from the AFM image is about 7 nm. Some degree of the surface roughness is beneficial for the easier permeation of the electrolyte, which can increase the usage ratio of graphene and enhance the performance of corresponding supercapacitor. Figure 5c gives the optical microscopic image of a piece of the free-standing multilayer film. The film was carefully separated from the substrate in a sodium hydroxide solution and then transferred onto the glass slide. Figure 5d shows the crosssectional SEM image of the multilayer film of 15 bilayers. The thickness of this multilayer film is 54 ± 6 nm estimated by AFM, which is consistent with the SEM result. The thickness is in the similar scale as other graphene-containing LbL films reported before.24−26 One unique property of polymeric diazonium salts is their ability to form cross-linked LbL films with other components.29,34 After postpreparation UV-light irradiation, the

Figure 3. D and G bands in Raman spectra for GO and SRGO.

exposed to light,31 the dipping process was carried out at 2 °C in the dark. Because of the large sizes of the graphene nanosheets, a longer dipping time was required for them to diffuse to and adsorb on the surfaces. The growth of the multilayer films on quartz slides was monitored by UV−vis spectroscopy, and the surface morphology was observed by AFM. Figure 4a shows a series of the UV−vis spectra for the multilayer films with different numbers of bilayers. The gradual increase of the UV−vis absorption indicates the growth of the multilayer films. Figure 4b shows the absorbance at 650 and 270 nm as a function of the bilayer number. As PSDAS has very weak absorbance at 650 nm, the linear increase of the absorbance at this wavelength indicates a linear growth of the graphene layers. The absorbance at 270 nm, which is attributed 526

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Post-preparation UV light irradiation can cause the self-crosslinking of PSDAS through the diazonium groups if they are not completely reacted during the multilayer assembling process. To verify this point, the contact angles of water on spin-coated PSDAS film surface were measured before and after UV light irradiation for 5 min under argon air protection (Figure S4 in the Supporting Information). The PSDAS film is hydrophilic before the light irradiation, which has a contact angle of 69.8 ± 0.9°. After UV-irradiation, the contact angle changes to 92 ± 1.3°, which is even larger than the contact angle on PS surface (88 ± 2°).42 After the light irradiation, the PSDAS film can no longer be etched by either water or organic solvents as a result of the cross-linking. Besides the above two reactions, this study shows that the diazonium groups of PSDAS can undergo a more specific reaction with sulfo groups on the SRGO surfaces, which can cause photochemical cross-linking between PSDAS and SRGO. The evidence for this reaction was obtained by ATR-FTIR and XPS. To carry out the ATR measurement, a mixture of PSDAS and SRGO in deionized water (pH = 3) was obtained by mixing the solution and dispersion of these two components. Thin film was prepared from the dispersion by suction filtration. The film was then freeze-dried in the dark and exposed to UV light (365 nm, 50 mW/cm2) for 30 min. Figure 6 shows the vacuum ATR-FTIR spectra of the film before and after the UV light irradiation. The ATR-FTIR spectrum of SRGO was also obtained for the absorption band assignment and comparison purpose (Figure S5 in the Supporting Information). Before the UV light irradiation, the absorption bands at 2253 and 1079 cm−1 of the SRGO/PSDAS film are caused by the stretching vibration of −N2+ and C−N from polymeric diazonium salt.43 Compared with SRGO, the absorption band of OSO of the −SO3H groups shifts from 1175 to 1171 cm−1 due to the ion-pair interaction (−N2+SO3−). After UV light irradiation, the 2253 and 1079 cm−1 bands of the film completely disappear, which indicate a complete decomposition of the diazonium groups. Meanwhile, the symmetric stretching band of OSO shifts from 1171 to 1167 cm−1, which indicates the conversion of the ionic bond (−N2+SO3−) to covalent bond (−SO2−O−) upon the UV exposure. This proves the conversion from the ion pairs between diazonium and sulfo groups to ester linkages after UV irradiation. This reaction and quantitative conversion ratio were further investigated by the XPS measurement carried out on the LbL film with 9 bilayers. The S(2p) core level feature of −SO3− group has been reported in previous publications.44 Because of the spin−orbit coupling, the sulfur atom will give rise to a spinsplit doublet, S(2p1/2, 2p3/2), which show the fixed energy splitting (1.18 eV) and relative intensities of 1:2. Figure 7 shows S(2p) XPS spectrum of the PSDAS/SRGO LbL multilayer films after UV irradiation in comparison with that of the casting SRGO film. For the casting SRGO film, where the sulfur atoms mainly exist in the −SO3−H+ form, the spinsplit peak of SRGO can be best fitted by a doublets with the S(2p1/2) maximum at 169.35 eV (Figure 7a). The full width at half-maximum (fwhm) of the S(2p) spectrum for the LbL films is broader than that of the SRGO film (Figure 7), which indicates the existence of different forms of the sulfur atoms. The XPS spectrum of the LbL self-assembling film can be best fitted by two spin-split doublets corresponding to the sulfur atoms in two forms, which are −SO3−H+ and −SO2−O−, respectively. The higher binding energy constituent of the

Figure 4. (a) UV−vis spectra of SRGO/PSDAS multilayer films assembled on a quartz slide. (b) Plot of absorbance at 270 and 650 nm versus the number of bilayers.

PSDAS/SRGO multilayer films showed high stability against the solvents. The stability of the multilayer films was tested by immersing the films into different solvents such as DMF, THF, water, and others. After dipping the films in the solvents for 12 h, no sign of material loss was detected as monitored by UV− vis spectroscopy. Only when the film on quartz slide was exposed to an alkaline solution, the film was peeled off from the substrate as the reaction of the quartz with alkali. This enhanced stability can be attributed to the reactions of PSDAS which occurred during the multilayer growth process and induced by postpreparation UV-light irradiation. The UV light irradiation will change the ionic groups into covalent bonds to further enhance the stability of the films. The chemical crosslinking in the multilayer films can occur through several type reactions at different stages as discussed below. 3.3. Photochemical Reaction and Cross-Linking. The direct reaction between diazonium and graphene can occur during the multilayer assembling process. It has been reported that small molecular diazonium salts react with surfactantdispersed graphene and CVD graphene.40,41 In the process, diazonium cations attack the conjugated carbon atom of graphene to form covalent bond after eliminating N2 under mild conditions. As mentioned above, this reaction was also used in this study for the preparation of SRGO by the reaction of graphene with the diazonium salt of sulfanilic acid. The reaction between diazonium ions and single-wall carbon nanotubes (SWNT) has been observed in the LbL assembly of a cationic diazo resin and poly(sodium 4-styrenesulfonate)functionalized SWNT.34 Therefore, a similar reaction will occur between SRGO and the polymeric diazonium salt during the multilayer build-up process. 527

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Figure 5. (a) Optical image of the LbL films on quartz slides with 3, 6, 9, and 12 bilayers from left to right. (b) AFM image (20 μm × 20 μm) of the LbL film with the root-mean-square roughness about 7 nm. (c) Optical microscope image of the free-standing multilayer film with 9 bilayers. (d) Cross-section SEM image of the film with 15 bilayers assembled on ITO glass.

Comparing the peak areas shown in Figure 7b, the conversion of −SO3−H+ to form the −SO2−O− bond is approximately 60% after the UV light irradiation. From the results of ATR-FTIR and XPS, it can be concluded that the UV light irradiation causes the transformation of the ion bonds of −N2+SO3− to the covalent bonds of −SO2−O−. This observation is consistent with the previous report on the photochemical reaction between diazonium ions and sulfonate ions on carbon nanotubes.34 Above results indicate that this reaction induced by UV-light irradiation and others are the main cause for the enhanced interaction between the components in the PSDAS/SRGO multilayer films. 3.4. Electrochemical Characterization. To test the performance of the multilayer film as the EDLC electrode, the multilayer films with a size of 2 cm × 2 cm and different numbers of bilayers were prepared on ITO glass slides. The self-assembled LbL multilayer films were used as the work electrodes of supercapacitors and characterized by cyclic voltammetry (CV), galvanostatic charge−discharge (CD), and electrochemical impedance spectroscopy (EIS). All electrochemical measurements were conducted in a three-electrode cell, where a platinum wire and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. The CV curves were recorded within the potential range from 0 to 0.8 V by varying the scanning rate from 10 to 1000 mV/s.

Figure 6. ATR-FTIR spectra of SRGO/PSDAS film before and after UV irradiation. The film was prepared by suction filtration at pH = 3.

S(2p1/2) peaks, centered at 169.71 eV, is associated with −SO2−O−. As the two doublets have a small energy difference of 0.36 eV, the overlap leads to the broadening of the fwhm. 528

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curves for the LbL film (Figure S6 in the Supporting Information). This observation rules out the possibility of the redox reaction of SRGO. Therefore, the redox reaction could only be related to the PSDAS or its complex formed with SRGO. Detailed explanation for this redox reaction will be explored in future study. Compared with the double-layer capacitance, the faradaic pseudocapacitance of the PSDAS/ SRGO multilayer film is minor in the contribution to the overall energy storage. The EDLC performance of the multilayer film with 9 bilayers was tested under different scanning rates from 10 to 1000 mV/ s. The film was treated with UV irradiation (365 nm, 50 mW/ cm2) for 30 min to increase its stability. As shown in Figure 9a, their CV curves exhibit typical EDLC behavior, where the approximate rectangular shape can be seen at all scan rates from 10 to 1000 mV/s. The gravimetric specific capacitance was calculated by using the formula47 Cg =

1 2mΔV

∫V

Vfinal

initial

I dV dV /dt

where m is the mass of the electrode material, ΔV is the potential window, Vinitial and Vfinal are the starting and end potential in one cycle, I is the instantaneous current at a given potential, and dV/dt is the potential scanning rate. In this study, the m in above formula refers to the amount of graphene in the multilayer film. This value was calculated from the absorbance of UV−vis spectroscopy of a film prepared on quartz slide (Figure S7 in the Supporting Information). The estimated amount of SRGO is 9.1 μg in the multilayer film, i.e., 2.28 μg/ cm2 (mass in per square centimeter film). Under the scanning rate of 10 mV/s, Cg is calculated to be 150.4 F/g (342.2 μF/ cm2), while at the high rate of 1000 mV/s, Cg decreases to 75 F/g (Figure 9b). This corresponds to about 50% retention of capacitance at the high rate, which is consistent with the previous reports.17 Figure 9c gives the representative galvanostatic charge/ discharge curves of the film with 9 bilayers obtained at different current densities. The curves are nearly linear and symmetric even at a high current density of 26.22 A/g. The slight asymmetric profile indicates the existence of some irreversible faradaic processes.48 The CD curves verify the result obtained from the CV measurement, which reveals the supercapacitive properties of the multilayer films. The specific capacitance under constant current in single electrode system can be calculated according to the equation48,49

Figure 7. S(2p) core-level of X-ray photoelectron spectroscopy (XPS): (a) SRGO film made by suction filtration at pH = 3; (b) LbL self-assembly films with 9 bilayers after UV irradiation.

As shown in Figure 8a, the CV curves for films with different numbers of bilayers show a nearly quasi-rectangular shape at a sweep rate of 10 mV/s, which is a typical characteristic of EDLC. As the number of bilayers of the LbL films increases, the capacitances of the supercapacitor show a linear increase (Figure 8b). Some pseudocapacitance is observed during the testing, where the two redox peaks appears at 0.25 and 0.15 V. When testing the multilayer films in 1 mol/L HCl solution, the peaks shift to 0.6 and 0.5 V, which implies the involvement of H+ in the reaction (Figure 8c). The change of pH value from 0 to 7 gives rise to a shift about 0.35 V for the redox potential. The varying surface functionality of carbon nanotube or graphene may cause the occurrence of pseudo-faradaic capacitor.45,46 To clarify this point, a multilayer film was prepared from poly(diallyldimethylammonium chloride) and SRGO and tested under the same measurement conditions. In this case, we did not find pseudocapacitive behavior in the CV

Figure 8. (a) CV curves and (b) normalized capacitance of the LbL multilayer films with the different bilayer numbers in the 1.0 M Na2SO4 solution. (c) CV curves of LbL multilayer films with 9 bilayers in the 1.0 M Na2SO4 solution and 1.0 M HCl solution. The scanning rate was 10 mV/s. 529

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Figure 9. Electrochemical properties of the multilayer film as the electrode in 1.0 M Na2SO4 solution. (a) CV curves, (b) specific capacitance of the multilayer film with 9 bilayers under different scanning rates, (c) galvanostatic charge/discharge curves at different current densities, (d) specific capacitance retention of the multilayer film with 9 bilayers versus the discharge current density, (e) cyclic test of the multilayer film with 9 bilayers at the scanning rate of 1000 mV/s, and (f) Nyquist plot for the multilayer film with 12 bilayers in the frequency range from 4000 kHz to 100 mHz at an ac amplitude of 10 mV.

Cgt = I Δt /mΔV

electrode/electrolyte system is shown by the curve with the lower left portion corresponding to the higher frequency. The Nyquist plot of multilayer film shows an incomplete arc in the high-frequency region, related to electronic resistance of the electrode. From the intersection of the curve at X-axis, the internal resistance of the cell is estimated to be 29.12 Ω. The near vertical line at lower frequency region in Figure 9f demonstrates a pure capacitive behavior of the multilayer film.48,49 The slope of 45° portion in the plot corresponds to the Warburg resistance, which is a result of the frequency dependence of ion diffusion/transport in the multilayer.49 The transition frequency between the Warburg region and vertical region is 3 Hz, which indicates the maximum frequency for maintaining the capacitive behavior.48,49 The above study shows that the cyclic stability can be significantly improved by using polymeric diazonium salt as one of the components owing to the cross-linking reactions discussed in section 3.3. On the other hand, as no charge conductivity, the PSDAS moieties in multilayer films could deteriorate the capacitive properties of graphene. The above electrochemical characterization indicates that the multilayer films as the electrode show large specific capacitance, high charging/discharging rate, and high-frequency response. The results can be rationalized by considering the high graphene density in the multilayer films, which is much higher than the percolation threshold to form the conducting network. This point is supported by the morphological images shown in Figure 5 and also consistent with the previous reports.17,28 The large lateral size of the nanosheets and high content in LbL film

where I is the constant current, Δt is the discharging time, m is the mass of the electrode material, and ΔV is the voltage drop during the discharging process. The multilayer film shows the specific capacitance of 93.0 F/g under the current density of 5.12 A/g. The rate performance of the electrode was evaluated by the charge/discharge capacitance measured at the different current densities. The multilayer film shows excellent rate performance with 60% capacitance retention under the high current density 260 A/g (Figure 9d). As illustrated in Figure 9e, the cyclic stability of the films with 9 bilayers was evaluated at 1000 mV/s up to 10 000 cycles. Even after charge−discharge 10 000 cycles, the cross-linked LbL film shows the high capacitance retention about 97%. Because of the change of the minor pseudocapacitance, the capacitance shows a small decrease of 3% in initial 2000 cycles, and then it remains almost unchanged in the following cycles. The result shows that the photo-cross-linking structure formed in the multilayers can afford excellent cyclic stability. This superior capacitance retention behavior can be attributed to the cross-linking between the components in the self-assembly films. Further understanding of the supercapacitive performance was obtained by the electrical impedance spectroscopy (EIS). The EIS data were analyzed by the Nyquist plot of the asprepared film with 12 bilayers (Figure 9f). In a Nyquist plot, the imaginary component (Z″) of the impedance is plotted versus the real component (Z′). The frequency response of the 530

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make it easy to form electron conductive network. The role of PSDAS moiety is to effectively separate the graphene nanosheets in the multilayer. The large Warburg region from 8064 to 3 Hz indicates that the ions can diffuse into the interior of the multilayer films and approach the graphene nanosheets through different pathways. It means that the PSDAS moieties are not densely packed and have subnanoscopic pores between them. These factors warrant the high efficiency for store and release energy through the microscopic charge separation at graphene/electrolyte interface. Therefore, the LbL method using PSDAS as one component can improve cyclic stability of the energy storage devices and maintain the excellent capacitive properties of graphene at the same time.

ASSOCIATED CONTENT

* Supporting Information S

More results about the synthesis, characterization, and properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.W.). Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS In summary, self-assembled multilayer films composed of SRGO and PSDAS were prepared by electrostatic LbL adsorption. The cross-linking between the components could occur during the multilayer assembling process and also be introduced by the UV light irradiation after film preparation. The conversion from the ion interaction to covalent bonds in the multilayer films was verified by ATR, XPS, and contact angle measurement. By this method, SRGO nanosheets were efficiently dispersed and fixed in the multilayer films. The aggregates of SRGO sheets were effectively avoided, which could ensure the full use of the huge surface area of SRGO. The multilayer film as electrodes were characterized by CV measurement and displayed a nearly rectangular shape composed of a major double-layer capacitance and a minor faradaic pseudocapacitance with the voltage-sweeping rate from 10 to 1000 mV/s. The specific capacitance of cross-linked films with 9 bilayers was measured to be 150.4 F/g in 1.0 M Na2SO4 under the scanning rate of 10 mV/s. Because of the cross-linked structure, this electrode material exhibited superior cyclic stability with a 97% capacitance retention even after 10 000 charge−discharge cycles. The cross-linked network structure of self-assembled films and high double-layer capacitance of SRGO nanosheets make the multilayer films a promising material for supercapacitor electrodes. This new approach for fabricating multilayer films with well-controlled structure can be used for energy storage and other applications.

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ACKNOWLEDGMENTS

This work was supported by National Basic Research Program of China (973 Program, 2012CB933402) and NSFC under Project 91027024. 531

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