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Electrophoretic deposition of graphene, carbon nanotubes and composite films using methyl violet dye as a dispersing agent
Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 97–103
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Electrophoretic deposition of graphene, carbon nanotubes and composite films using methyl violet dye as a dispersing agent Y. Su, I. Zhitomirsky ∗ Department of Materials Science and Engineering, McMaster University, 1280 Main St., West Hamilton, Ontario L8S 4L7, Canada
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Methyl violet (MV) films were • • • •
obtained by cathodic electrophoretic deposition. MV allowed efficient dispersion of carbon nanotubes and graphene in water. Electrophoretic deposition of carbon nanotubes and graphene was achieved using MV. Composite carbon nanotube–graphene film were fabricated using MV dispersant. Composite films showed good capacitive behavior for application in supercapacitors.
a r t i c l e
i n f o
Article history: Received 28 April 2013 Received in revised form 11 June 2013 Accepted 11 June 2013 Available online xxx Keywords: Electrophoretic deposition Methyl violet Carbon nanotubes Graphene Composite Film
a b s t r a c t Cathodic electrophoretic deposition (EPD) of methyl violet (MV) was performed from aqueous solutions. The film microstructure and deposition mechanism were studied using scanning electron microscopy (SEM) and cyclic voltammetry (CV). MV allowed efficient dispersion of multiwalled carbon nanotubes (MWCNT) and graphene in aqueous suspensions. Thin films of MWCNT and graphene were obtained by cathodic EPD using MV as a charging, dispersing and film forming agent. The deposition yield was varied by variation of MV concentration in the suspensions and deposition voltage. The possibility to deposit both MWCNT and graphene using MV as a dispersing and charging agent allowed the fabrication of MWCNT–graphene composites by EPD. The film microstructures and advantages of cathodic EPD were discussed. The films were investigated for application in electrochemical supercapacitors (ES). Electrochemical investigation showed capacitive behavior of the films in 0.5 M Na2 SO4 electrolyte. The specific capacitance (SC) of ∼130 F g−1 was obtained at a scan rate of 2 mV s−1 , and the capacitance retention in the range of 2–100 mV s−1 was ∼50%. The impedance spectroscopy data were in agreement with simulation results, obtained using equivalent circuit model. The composite films showed improved capacitive behavior compared to the films of individual components. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene and CNT are under investigation for energy storage devices [1,2] due to their unique electrical, thermal, mechanical
∗ Corresponding author. Tel.: +1 905 525 9140 x23914. E-mail address: [email protected] (I. Zhitomirsky). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.06.024
properties and extremely large specific surface area [3–5]. The composite films of CNT and graphene are of special interest [6,7] for applications in ES [8,9], biomedical [10,11], photovoltaic [12,13], electrochemical [14,15] and optical [16] devices. In the composite materials, CNT were inserted between the layers of graphene and prevented the aggregation and restacking of the layers. It was shown that 3D pillared architecture, with CNT supported graphene layers, is very promising for hydrogen storage [17]. The open
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nanostructures, containing CNT as spacers between the graphene layers, provided extremely high specific area for many applications, such as electrodes of ES [9,18] and catalysts for fuel cell [19]. Many investigations were focused on the development of new methods for the fabrication of graphene–CNT composites. Chemical vapor deposition (CVD) is widely used to deposit layers of CNT and graphene sequentially [9,18,20]. However, this method is expensive, complex, and not suitable for large area substrate processing [21,22]. Despite the impressive progress achieved in the fabrication of graphene–CNT composites, there is a need in the development of simple and versatile techniques, suitable for large area applications. EPD offers many benefits, such as film uniformity, rigid control of film thickness and deposition rate and the possibility of film formation on relatively large substrates [23–25]. Comparing with anodic EPD, cathodic EPD showed many advantages [26]. Anodic EPD could cause oxidation or dissolution of non-noble electrodes, which, in turn, can result in increased interfacial resistance at the film–substrate interface and reduced adhesion of deposited film to the substrate [15]. Cathodic EPD can be combined with other electrochemical techniques, such as electroplating of metals and electrogenerated based method for the fabrication of composite films [15,26]. For film formation by EPD, the CNT and graphene must be dispersed in a solvent to form a stable suspension. Different methods were invented for the dispersion of CNT and graphene. Chemical oxidation of CNT or graphene in strong acids allowed their improved dispersion by the formation of surface polar or ionizable groups [27,28]. However, such treatment results in material degradation and reduction of the electronic, thermal and mechanical properties [29–31]. Other methods, based on the use of polyelectrolytes [32] and surfactants [33–35], could also achieve stable suspensions of CNT or graphene. However, polyelectrolytes often resulted in the formation of CNT bundles and graphene stacking [28,36]. Relatively large amounts of surfactants were necessary for the CNT dispersion [33,34]. The concentration range of CNT suspension stability, achieved by the use of surfactants was relatively narrow [34,37]. It was also found that metal salts could be used for charging of CNT and graphene because of the adsorption of metal ions on their surfaces [6,38]. However, metal ions could be deposited with CNT and graphene during EPD, resulting in deposit contamination [26]. The adding of metal salts can reduce the stability of suspension, as the DLVO theory predicted [39]. Therefore, there is a need in the development of efficient dispersants for dispersing and charging of CNT and graphene and their efficient deposition. The goal of this investigation was the fabrication of MWCNT–graphene composites by EPD. The important finding was the possibility of efficient dispersion, charging and EPD of MWCNT and graphene using cationic MV, which exhibited film forming properties. The method offers advantages of cathodic EPD. The fabrication of MWCNT–graphene composites was achieved using MV as a charging and dispersing agent for both graphene and MWCNT. The MWCNT–graphene composites are promising materials for application in electrodes of ES.
2. Experimental procedures MV (Aldrich), MWCNT (Arkema) and graphene (Graphene Supermarket) were used as starting materials without further purification. The deposition of MV was performed at constant voltages in the range of 5–20 V from aqueous 0.1 to 1 g L−1 MV solutions. The deposition process was studied using CV at a scan rate of 20 mV s−1 . Aqueous MWCNT and graphene suspensions for EPD, containing dissolved MV as dispersant, were ultrasonicated for 30 min to achieve homogeneous dispersions. EPD
was performed at deposition voltages of 1–11 V and 20–50 V for MWCNT and graphene suspensions, respectively, from their aqueous suspensions with concentrations varying in the range of 0.1–1 g L−1 . Stainless steel foils (2.5 × 5 cm) were polished, washed with distilled water and used as substrates for EPD. The distance between the substrate and platinum counter electrodes was 15 mm. A constant voltage EPD was performed using a power supply (EPS 2A200, Amersham Biosciences). The deposits were dried in air for 24 h. Deposition yield of MWCNT and graphene was studied versus MV concentration and deposition voltage. All deposition experiments were performed using fresh solutions. Three samples were prepared for each deposition condition and the error for each experiment was less than 5%. The deposited material was removed from the stainless steel substrates for the Fourier transform infrared spectroscopy (FTIR) using Bio-Rad FTS-40 instrument. The film microstructures were investigated using JEOL JSM-7000F SEM. Electrochemical studies were performed using a potentiostat (PARSTAT 2273, Princeton Applied Research). Surface area of the working electrode was 1 cm2 . The counter electrode was a platinum gauze, and the reference electrode was a standard calomel electrode (SCE). Capacitive behavior and electrochemical impedance of the films were investigated in 0.5 M Na2 SO4 aqueous solutions. CV studies were performed within a potential range of -0.4 to 0.6 V versus SCE at scan rates of 2–100 mV s−1 . The SC was calculated using half the integrated area of the CV curve to obtain the charge (Q), and subsequently dividing the charge by the film mass (m) and width of the potential window (V): C=
Q mV
(1)
Electrochemical impedance spectroscopy (EIS) investigations were performed in the frequency range of 0.1 Hz–100 kHz, the amplitude of the applied voltage was 5 mV. The ZSimpWin (Princeton Applied Research) software was used for the analysis of the impedance data using equivalent circuit of the thin film electrodes. 3. Results and discussion Fig. 1 shows a chemical structure of the MV dye, which includes 3 aromatic rings and sp2 hybridized central carbon atom (Fig. 1A). The cationic properties of MV are attributed to the NH+ group. It was found that electrodeposition from 0.1 to 1 g L−1 MV solutions resulted in the formation of films on the cathodic substrates. The suggested deposition mechanism is based on the literature data on behavior of MV in basic solutions [40]. It is known that cationic MV molecules react with hydroxide groups in basic environment as shown in Fig. 1A. This reaction results in charge neutralization and the changes from sp2 to sp3 hybridization of the central carbon atom. It is suggested that electric field provided electrophoresis of cationic MV toward the cathode. The following cathodic reaction resulted in the pH increase at the cathode surface: 2H2 O + 2e− → H2 + 2OH−
(2)
The charge neutralization (Fig. 1A) in the high pH region at the cathode promoted MV film formation. Fig. 2A, C shows the microstructures of deposited MV films for different magnification at 5 V, which were porous and contained MV particles with an average diameter of ∼100 nm. The increase in the deposition voltage from 5 to 20 V resulted in reduced porosity (Fig. 2B, D). The reduction in porosity was attributed to faster deposition at higher deposition voltage, which led to the necks formation between the particles and filling voids between the particles. The deposition process was studied by potentiodynamic cycling. Fig. 3 shows the CVs in the 0.1 g L−1 MV aqueous solution in the potential range from −0.9 to 0 V versus SCE. Fig. 3 shows a peak
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Fig. 3. CV data for Pt electrode in 0.1 g L−1 MV solution at a scan rate of 20 mV s−1 for cycles 1–3.
Fig. 1. (A) Cathodic reaction of MV with hydroxide groups, (B) schematic of MV adsorption on the surface of MWCNT.
at ∼−0.55 V versus SCE which was attributed to the electrochemical reduction (Fig. 1A) of MV molecules. Thin film formation was observed after the first cycle. The decrease of current with cycle number indicated the formation of insulating film on the cathodic substrate. MV was investigated for the dispersion of MWCNT and graphene in aqueous suspensions. The suspensions of MWCNT and graphene
in water were unstable and showed rapid sedimentation immediately after ultrasonic agitation. No EPD was achieved from such suspensions. The addition of MV to the suspensions of MWCNT and graphene resulted in improved suspension stability. The suspensions were stable for more than 2 months. It is suggested that MV was adsorbed on MWCNT and graphene and provided a positive charge for electrostatic stabilization. The adsorption of MV on MWCNT and graphene was governed by – interactions. Fig. 1B shows a schematic of adsorption of polyaromatic MV molecule on the MWCNT surface. The adsorbed MV imparted a positive charge to the MWCNT and graphene and allowed their EPD. In this investigation, the EPD kinetics was studied by the analysis of deposition yield measurements due to limitations of zeta potential concept for application to particles, containing adsorbed organic molecules [26,41]. The deposition yield of MWCNT was recorded to analyze the influences of both MV concentration (Fig. 4A) and deposition voltage (Fig. 4B). Under a constant deposition voltage of 5 V, the deposition yield of MWCNT increased with MV concentration as shown in Fig. 4A. The data indicated that adsorbed MV allowed charging and deposition of MWCNT, even at MV concentration as low as 0.1 g L−1 in the 1 g L−1 MWCNT suspensions. The increase in
Fig. 2. SEM images of cathodically deposited MV films from 1 g L−1 MV solutions at (A, C) 5 V and (B, D) 20 V.
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Fig. 4. Deposit mass for films prepared from 1 g L−1 MWCNT suspension at a deposition time of 2 min versus (A) MV concentration at the deposition voltage of 5 V and (B) deposition voltage at CV concentration of 0.5 g L−1 . Lines are the guide for the eye.
MV concentration resulted in increasing adsorption of cationic MV molecules on MWCNT surface, and increased MV charge, which, in turn, resulted in higher deposition rate. The results indicated that deposition yield can be varied by variation of MV concentration. The deposition yield increased with increasing deposition voltage (Fig. 4B). Nearly linear relationship dependence was observed in agreement with Hamaker equation [42]. Similar dependences were observed for graphene suspensions, where the deposition yield increased with the MV concentration (Fig. 5A) due to increased charge of the graphene particles, containing adsorbed MV molecules [43]. However, a higher deposition voltage was required for the deposition of graphene due to the larger mass of the graphene particles. A linear dependence of the deposition yield versus the deposition voltage was observed in the range of 0–30 V, and significant deviation from the linear dependence was observed at higher voltage range. The linear part is in agreement with Hamaker equation [42]. The deviation from the linear dependence can be attributed to different factors, such as deposit spalling, increased voltage drop in thick deposit and corresponding reduction in electric field in the bulk of the suspensions, electrode reactions and other factors discussed in the literature [23]. The results indicated that the deposition yield for MWCNT films and graphene films can be controlled by changing MV concentration and deposition voltage. FTIR studies were performed in order to confirm the MV adsorption. Fig. 6 compares the FTIR spectra of MV, MWCNT, graphene and deposited materials. The absorption assignments are summarized in Table 1. In comparison with the spectra of pristine MWCNT and graphene, the spectra of deposited materials (Fig. 6) showed additional absorptions at 1582, 1514, 1472, 1360 and 1168 cm−1 for deposited MWCNT and at 1582, 1517, 1360 and 1168 cm−1 for deposited graphene. Taking into account that similar absorptions were observed in the spectra of MV (Table 1), it was concluded that deposited MWCNT and graphene contained adsorbed MV.
Fig. 5. Deposit mass for films prepared from 1 g L−1 graphene suspension at a deposition time of 2 min versus (A) MV concentration at the deposition voltage of 30 V and (B) deposition voltage at CV concentration of 0.5 g L−1 . Lines are the guide for the eye.
Fig. 6. FTIR spectra of (a) MV, (b) pristine MWCNT, (c) deposited MWCNT, (d) pristine graphene, and (e) deposited graphene.
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Table 1 Band assignments for MWCNT, graphene, and MV. MV (cm−1 )
Pristine MWCNT (cm−1 )
Deposited MWCNT (cm−1 )
Pristine graphene (cm−1 )
Deposited graphene (cm−1 )
Band assignments
1632
1632 1582 1472 1514 1385 1127 1360 1168
1634
1630 1582
C CC stretch in CNT [52] C C in aromatic ring [53]
1517 1385 1127 1360 1168
N H stretch [53] C O stretch [54]
1582 1479 1520 1385 1127 1359 1168
1385 1127
Fig. 7 shows the SEM images of MWCNT (Fig. 7A), graphene (Fig. 7B), and MWCNT–graphene composite (Fig. 7C). Fig. 7A showed a continuous and uniform MWCNT film with an average pore size of ∼100 nm. As shown in Fig. 7B, graphene was dispersed from particles to sheets, and the substrate was fully covered. The possibility of deposition of MWCNT and graphene paved the way
Fig. 7. SEM images of (A) MWCNT, (B) graphene and (C) composite films, deposited from (A) 1 g L−1 MWCNT and 0.5 g L−1 MV suspensions at 5 V, (B) 1 g L−1 graphene and 0.5 g L−1 MV suspension at 30 V, and (C) 1 g L−1 MWCNT, 1 g L−1 graphene and 1 g L−1 MV suspension at 20 V.
C H in CH3 [53] C N stretch [53]
for the co-deposition of both materials from one suspension using MV as a charging and dispersing agent for both materials. Literature data [44,45] indicates that mixed suspensions of CNT and graphene exhibit improved stability, compared to the suspensions of individual materials. The results of sedimentations tests of mixed MWCNT–graphene suspension are in agreement with the literature data and indicate that MWCNT–graphene suspensions without MV were stable for 3–4 h. In contrast, MWCNT–graphene suspensions containing MV were stable for more than 2 months. Mixed suspensions, containing MWCNT, graphene and MV dispersant were used for the formation of composite MWCNT–graphene films by EPD. The SEM image of the composite material shows graphene and MWCNT (Fig. 7C). As pointed out above, the MWCNT–graphene composites have potential applications in many fields, including ES [14] and batteries [46], due to improved electrolyte access to graphene layers, separated by MWCNT. The capacitive behavior of the composite films was studied in the potential range between −0.4 and 0.6 V versus SCE. The SC was calculated from the CV data and plotted versus scan rate in Fig. 8A. At the scan rate of 2 mV s−1 , a SC of ∼130 F g−1 was achieved
Fig. 8. (A) SC versus scan rate and (B) corresponding CV at 10 mV s−1 for (a) composite film, prepared from a suspension containing 1 g L−1 MWCNT, 1 g L−1 graphene and 1 g L−1 MV (b) MWCNT film, prepared from 1 g L−1 MWCNT suspension, containing 0.5 g L−1 MV and (C) graphene film, prepared from 1 g L−1 graphene suspension, containing 0.5 g L−1 MV.
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4. Conclusions
Fig. 9. Nyquist plot of a complex impedance Z = Z − iZ in the frequency range of 0.1 Hz–100 kHz for (a) composite, (b) MWCNT, and (c) graphene films; the suspensions for film preparation are the same as in Fig. 8. The inset shows the equivalent circuit used for the simulation.
for a composite film with the mass loading of 0.04 mg cm−2 . With the increase of the scan rate, the SC decreased to ∼63 F g−1 at 100 mV s−1 . This decrease was attributed to diffusion limitations of the electrolyte in the pores of the composite material. The graphene and MWCNT film with the same mass loading exhibited SC of ∼55 F g−1 and ∼81 F g−1 , respectively, at a scan rate of 2 mV s−1 . Fig. 8B shows corresponding CVs at 10 mV s−1 scan rate for composite, MWCNT and graphene films. The CV of composite film has larger area, compared to the CV area of MWCNT and graphene films, due to higher capacitance. Therefore, the composite films showed improved capacitive behavior, compared to the MWCNT and graphene films. It is suggested that MWCNT incorporated between graphene layers during EPD and prevented their restacking in the composite films. Such structures allowed improved electrolyte access to the graphene surface. As a result, the high surface area of graphene was better utilized in the composite materials for application in ES. The graphene, MWCNT, and composite films were studied by EIS and the data was shown in Fig. 9. The equivalent circuit, presented in the inset of Fig. 9, was used for the analysis of the EIS data. It included two RQ transmission lines, describing the porous electrode [47,48]. The diffusion resistance of electrolyte inside the pores was represented by the Warburg impedance (W) [49,50]. In this circuit, R1 element represented the electrolyte resistance, while R2 represented the film resistance. Q elements represented the double-layer capacitance and pseudocapacitance of the composite film, with the consideration of microscopic roughness of the surface and capacitance dispersion of interfacial origin [14]. To simulate an impedance spectrum using an equivalent circuit, a minimum set of model parameters was used and good agreement of the experimental data and the results of simulation was achieved. By comparing the EIS of three samples, it was found that composite film had the smallest film resistance, especially at low frequency range. It is suggested that MWCNT acted as spacers between the graphene layers and improved electrolyte access to the graphene layers, resulting in lower resistance of the composite material. The lower resistance allowed improved capacitive behavior. EPD offers many processing advantages, compared to other methods for the fabrication of MWCNT–graphene composites. The EPD method is simple and suitable for controlled deposition of well dispersed MWCNT and graphene on high surface area substrates. This method is especially attractive for application in ES. The SC of composite films prepared by this method is comparable with SC of the MWCNT–graphene composites synthesized by other methods [45,51]. The method developed in this investigation gives an alternative route to achieve nanoscale fabrication of MWCNT–graphene composites, utilizing processing advantages of EPD technology.
Cathodic EPD method has been developed for the fabrication of MV films, which were deposited by a constant voltage EPD or potentiodynamically. The deposition mechanism involved electrophoresis of cationic MV, pH increase at the cathode due to the electrode reactions, charge neutralization of the cationic MV at the electrode surface and film formation. MV allowed efficient dispersion, charging and cathodic EPD of MWCNT and graphene at relatively low dispersant concentrations. The deposition yield can be varied by variation of dispersant concentration and deposition voltage. FTIR data proved the adsorption of MV molecules on MWCNT and graphene surface. The use of MV as a codispersing agent for MWCNT and graphene allowed the fabrication of MWCNT–graphene composite films for application in electrodes of ES. The composite films showed capacitive behavior. The SC of ∼130 F g−1 was obtained at a scan rate of 2 mV s−1 . The composite films showed improved capacitive behavior compared to the films of individual components. The MWCNT–graphene composites are promising materials for ES. Acknowledgement The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. References [1] Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, Y. Chen, Supercapacitor devices based on graphene materials, J. Phys. Chem. C 113 (2009) 13103–13107. [2] S. Vivekchand, C. Rout, K. Subrahmanyam, A. Govindaraj, C. Rao, Graphenebased electrochemical supercapacitors, J. Chem. Sci. 120 (2008) 9–13. [3] Y. Sun, Q. Wu, G. Shi, Graphene based new energy materials, Energy Environ. Sci. 4 (2011) 1113–1132. [4] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitors based on flexible graphene/polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963–1970. [5] S.H. Aboutalebi, A.T. Chidembo, M. Salari, K. Konstantinov, D. Wexler, H.K. Liu, S.X. Dou, Comparison of GO, GO/MWCNTs composite and MWCNTs as potential electrode materials for supercapacitors, Energy Environ. Sci. 4 (2011) 1855–1865. [6] A.R. Boccaccini, J. Cho, J.A. Roether, B.J.C. Thomas, E. Jane Minay, M.S.P. Shaffer, Electrophoretic deposition of carbon nanotubes, Carbon 44 (2006) 3149–3160. [7] Y. Wang, I. Deen, I. Zhitomirsky, Electrophoretic deposition of polyacrylic acid and composite films containing nanotubes and oxide particles, J. Colloid Interface Sci. 362 (2011) 367–374. [8] J. Cai, M. He, Y. Gu, L. Kang, Z. Lei, Z. Yang, Z.H. Liu, Assembling fabrication and capacitance of manganese oxide nanosheets and functionalized carbon nanotubes hybrid material, Colloids Surf. A 429 (2013) 91–97. [9] Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian, F. Wei, A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors, Adv. Mater. 22 (2010) 3723–3728. [10] A.R. Boccaccini, S. Keim, R. Ma, Y. Li, I. Zhitomirsky, Electrophoretic deposition of biomaterials, J. R. Soc. Interface 7 (2010) S581–S613. [11] J. Du, R. Yue, Z. Yao, F. Jiang, Y. Du, P. Yang, C. Wang, Nonenzymatic uric acid electrochemical sensor based on graphene-modified carbon fiber electrode, Colloids Surf. A 419 (2013) 94–99. [12] G. Zhu, L. Pan, T. Lu, T. Xu, Z. Sun, Electrophoretic deposition of reduced graphene–carbon nanotubes composite films as counter electrodes of dyesensitized solar cells, J. Mater. Chem. 21 (2011) 14869–14875. [13] Y. Sun, Y. Wang, I. Zhitomirsky, Dispersing agents for electrophoretic deposition of TiO2 and TiO2 –carbon nanotube composites, Colloids Surf. A 418 (2013) 131–138. [14] Y. Su, I. Zhitomirsky, Electrophoretic assembly of organic molecules and composites for electrochemical supercapacitors, J. Colloid Interface Sci. 392 (2012) 247–255. [15] Y. Su, I. Zhitomirsky, Cataphoretic assembly of cationic dyes and deposition of carbon nanotube and graphene films, J. Colloid Interface Sci. 399 (2013) 46–53. [16] B. Cichy, A. Gorecka-Drzazga, J.A. Dziuban, Field-emission light sources utilizing carbon nanotubes and composite phosphor made of SiO2 nanospheres covered with Y2 O3 :Eu, J. Vac. Sci. Technol. B 27 (2009) 757–760. [17] E. Yoo, J. Kim, E. Hosono, H.-s. Zhou, T. Kudo, I. Honma, Large reversible Li storage of graphene nanosheet families for use in rechargeable Lithium Ion batteries, Nano Lett. 8 (2008) 2277–2282. [18] F. Du, D. Yu, L. Dai, S. Ganguli, V. Varshney, A.K. Roy, Preparation of tunable 3D pillared carbon nanotube–graphene networks for high-performance capacitance, Chem. Mater. 23 (2011) 4810–4816.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Electrophoretic deposition of graphene, carbon nanotubes and composite films using methyl violet dye as a dispersing agent Y. Su, I. Zhitomirsky ∗ Department of Materials Science and Engineering, McMaster University, 1280 Main St., West Hamilton, Ontario L8S 4L7, Canada
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Methyl violet (MV) films were • • • •
obtained by cathodic electrophoretic deposition. MV allowed efficient dispersion of carbon nanotubes and graphene in water. Electrophoretic deposition of carbon nanotubes and graphene was achieved using MV. Composite carbon nanotube–graphene film were fabricated using MV dispersant. Composite films showed good capacitive behavior for application in supercapacitors.
a r t i c l e
i n f o
Article history: Received 28 April 2013 Received in revised form 11 June 2013 Accepted 11 June 2013 Available online xxx Keywords: Electrophoretic deposition Methyl violet Carbon nanotubes Graphene Composite Film
a b s t r a c t Cathodic electrophoretic deposition (EPD) of methyl violet (MV) was performed from aqueous solutions. The film microstructure and deposition mechanism were studied using scanning electron microscopy (SEM) and cyclic voltammetry (CV). MV allowed efficient dispersion of multiwalled carbon nanotubes (MWCNT) and graphene in aqueous suspensions. Thin films of MWCNT and graphene were obtained by cathodic EPD using MV as a charging, dispersing and film forming agent. The deposition yield was varied by variation of MV concentration in the suspensions and deposition voltage. The possibility to deposit both MWCNT and graphene using MV as a dispersing and charging agent allowed the fabrication of MWCNT–graphene composites by EPD. The film microstructures and advantages of cathodic EPD were discussed. The films were investigated for application in electrochemical supercapacitors (ES). Electrochemical investigation showed capacitive behavior of the films in 0.5 M Na2 SO4 electrolyte. The specific capacitance (SC) of ∼130 F g−1 was obtained at a scan rate of 2 mV s−1 , and the capacitance retention in the range of 2–100 mV s−1 was ∼50%. The impedance spectroscopy data were in agreement with simulation results, obtained using equivalent circuit model. The composite films showed improved capacitive behavior compared to the films of individual components. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphene and CNT are under investigation for energy storage devices [1,2] due to their unique electrical, thermal, mechanical
∗ Corresponding author. Tel.: +1 905 525 9140 x23914. E-mail address: [email protected] (I. Zhitomirsky). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.06.024
properties and extremely large specific surface area [3–5]. The composite films of CNT and graphene are of special interest [6,7] for applications in ES [8,9], biomedical [10,11], photovoltaic [12,13], electrochemical [14,15] and optical [16] devices. In the composite materials, CNT were inserted between the layers of graphene and prevented the aggregation and restacking of the layers. It was shown that 3D pillared architecture, with CNT supported graphene layers, is very promising for hydrogen storage [17]. The open
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nanostructures, containing CNT as spacers between the graphene layers, provided extremely high specific area for many applications, such as electrodes of ES [9,18] and catalysts for fuel cell [19]. Many investigations were focused on the development of new methods for the fabrication of graphene–CNT composites. Chemical vapor deposition (CVD) is widely used to deposit layers of CNT and graphene sequentially [9,18,20]. However, this method is expensive, complex, and not suitable for large area substrate processing [21,22]. Despite the impressive progress achieved in the fabrication of graphene–CNT composites, there is a need in the development of simple and versatile techniques, suitable for large area applications. EPD offers many benefits, such as film uniformity, rigid control of film thickness and deposition rate and the possibility of film formation on relatively large substrates [23–25]. Comparing with anodic EPD, cathodic EPD showed many advantages [26]. Anodic EPD could cause oxidation or dissolution of non-noble electrodes, which, in turn, can result in increased interfacial resistance at the film–substrate interface and reduced adhesion of deposited film to the substrate [15]. Cathodic EPD can be combined with other electrochemical techniques, such as electroplating of metals and electrogenerated based method for the fabrication of composite films [15,26]. For film formation by EPD, the CNT and graphene must be dispersed in a solvent to form a stable suspension. Different methods were invented for the dispersion of CNT and graphene. Chemical oxidation of CNT or graphene in strong acids allowed their improved dispersion by the formation of surface polar or ionizable groups [27,28]. However, such treatment results in material degradation and reduction of the electronic, thermal and mechanical properties [29–31]. Other methods, based on the use of polyelectrolytes [32] and surfactants [33–35], could also achieve stable suspensions of CNT or graphene. However, polyelectrolytes often resulted in the formation of CNT bundles and graphene stacking [28,36]. Relatively large amounts of surfactants were necessary for the CNT dispersion [33,34]. The concentration range of CNT suspension stability, achieved by the use of surfactants was relatively narrow [34,37]. It was also found that metal salts could be used for charging of CNT and graphene because of the adsorption of metal ions on their surfaces [6,38]. However, metal ions could be deposited with CNT and graphene during EPD, resulting in deposit contamination [26]. The adding of metal salts can reduce the stability of suspension, as the DLVO theory predicted [39]. Therefore, there is a need in the development of efficient dispersants for dispersing and charging of CNT and graphene and their efficient deposition. The goal of this investigation was the fabrication of MWCNT–graphene composites by EPD. The important finding was the possibility of efficient dispersion, charging and EPD of MWCNT and graphene using cationic MV, which exhibited film forming properties. The method offers advantages of cathodic EPD. The fabrication of MWCNT–graphene composites was achieved using MV as a charging and dispersing agent for both graphene and MWCNT. The MWCNT–graphene composites are promising materials for application in electrodes of ES.
2. Experimental procedures MV (Aldrich), MWCNT (Arkema) and graphene (Graphene Supermarket) were used as starting materials without further purification. The deposition of MV was performed at constant voltages in the range of 5–20 V from aqueous 0.1 to 1 g L−1 MV solutions. The deposition process was studied using CV at a scan rate of 20 mV s−1 . Aqueous MWCNT and graphene suspensions for EPD, containing dissolved MV as dispersant, were ultrasonicated for 30 min to achieve homogeneous dispersions. EPD
was performed at deposition voltages of 1–11 V and 20–50 V for MWCNT and graphene suspensions, respectively, from their aqueous suspensions with concentrations varying in the range of 0.1–1 g L−1 . Stainless steel foils (2.5 × 5 cm) were polished, washed with distilled water and used as substrates for EPD. The distance between the substrate and platinum counter electrodes was 15 mm. A constant voltage EPD was performed using a power supply (EPS 2A200, Amersham Biosciences). The deposits were dried in air for 24 h. Deposition yield of MWCNT and graphene was studied versus MV concentration and deposition voltage. All deposition experiments were performed using fresh solutions. Three samples were prepared for each deposition condition and the error for each experiment was less than 5%. The deposited material was removed from the stainless steel substrates for the Fourier transform infrared spectroscopy (FTIR) using Bio-Rad FTS-40 instrument. The film microstructures were investigated using JEOL JSM-7000F SEM. Electrochemical studies were performed using a potentiostat (PARSTAT 2273, Princeton Applied Research). Surface area of the working electrode was 1 cm2 . The counter electrode was a platinum gauze, and the reference electrode was a standard calomel electrode (SCE). Capacitive behavior and electrochemical impedance of the films were investigated in 0.5 M Na2 SO4 aqueous solutions. CV studies were performed within a potential range of -0.4 to 0.6 V versus SCE at scan rates of 2–100 mV s−1 . The SC was calculated using half the integrated area of the CV curve to obtain the charge (Q), and subsequently dividing the charge by the film mass (m) and width of the potential window (V): C=
Q mV
(1)
Electrochemical impedance spectroscopy (EIS) investigations were performed in the frequency range of 0.1 Hz–100 kHz, the amplitude of the applied voltage was 5 mV. The ZSimpWin (Princeton Applied Research) software was used for the analysis of the impedance data using equivalent circuit of the thin film electrodes. 3. Results and discussion Fig. 1 shows a chemical structure of the MV dye, which includes 3 aromatic rings and sp2 hybridized central carbon atom (Fig. 1A). The cationic properties of MV are attributed to the NH+ group. It was found that electrodeposition from 0.1 to 1 g L−1 MV solutions resulted in the formation of films on the cathodic substrates. The suggested deposition mechanism is based on the literature data on behavior of MV in basic solutions [40]. It is known that cationic MV molecules react with hydroxide groups in basic environment as shown in Fig. 1A. This reaction results in charge neutralization and the changes from sp2 to sp3 hybridization of the central carbon atom. It is suggested that electric field provided electrophoresis of cationic MV toward the cathode. The following cathodic reaction resulted in the pH increase at the cathode surface: 2H2 O + 2e− → H2 + 2OH−
(2)
The charge neutralization (Fig. 1A) in the high pH region at the cathode promoted MV film formation. Fig. 2A, C shows the microstructures of deposited MV films for different magnification at 5 V, which were porous and contained MV particles with an average diameter of ∼100 nm. The increase in the deposition voltage from 5 to 20 V resulted in reduced porosity (Fig. 2B, D). The reduction in porosity was attributed to faster deposition at higher deposition voltage, which led to the necks formation between the particles and filling voids between the particles. The deposition process was studied by potentiodynamic cycling. Fig. 3 shows the CVs in the 0.1 g L−1 MV aqueous solution in the potential range from −0.9 to 0 V versus SCE. Fig. 3 shows a peak
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Fig. 3. CV data for Pt electrode in 0.1 g L−1 MV solution at a scan rate of 20 mV s−1 for cycles 1–3.
Fig. 1. (A) Cathodic reaction of MV with hydroxide groups, (B) schematic of MV adsorption on the surface of MWCNT.
at ∼−0.55 V versus SCE which was attributed to the electrochemical reduction (Fig. 1A) of MV molecules. Thin film formation was observed after the first cycle. The decrease of current with cycle number indicated the formation of insulating film on the cathodic substrate. MV was investigated for the dispersion of MWCNT and graphene in aqueous suspensions. The suspensions of MWCNT and graphene
in water were unstable and showed rapid sedimentation immediately after ultrasonic agitation. No EPD was achieved from such suspensions. The addition of MV to the suspensions of MWCNT and graphene resulted in improved suspension stability. The suspensions were stable for more than 2 months. It is suggested that MV was adsorbed on MWCNT and graphene and provided a positive charge for electrostatic stabilization. The adsorption of MV on MWCNT and graphene was governed by – interactions. Fig. 1B shows a schematic of adsorption of polyaromatic MV molecule on the MWCNT surface. The adsorbed MV imparted a positive charge to the MWCNT and graphene and allowed their EPD. In this investigation, the EPD kinetics was studied by the analysis of deposition yield measurements due to limitations of zeta potential concept for application to particles, containing adsorbed organic molecules [26,41]. The deposition yield of MWCNT was recorded to analyze the influences of both MV concentration (Fig. 4A) and deposition voltage (Fig. 4B). Under a constant deposition voltage of 5 V, the deposition yield of MWCNT increased with MV concentration as shown in Fig. 4A. The data indicated that adsorbed MV allowed charging and deposition of MWCNT, even at MV concentration as low as 0.1 g L−1 in the 1 g L−1 MWCNT suspensions. The increase in
Fig. 2. SEM images of cathodically deposited MV films from 1 g L−1 MV solutions at (A, C) 5 V and (B, D) 20 V.
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Fig. 4. Deposit mass for films prepared from 1 g L−1 MWCNT suspension at a deposition time of 2 min versus (A) MV concentration at the deposition voltage of 5 V and (B) deposition voltage at CV concentration of 0.5 g L−1 . Lines are the guide for the eye.
MV concentration resulted in increasing adsorption of cationic MV molecules on MWCNT surface, and increased MV charge, which, in turn, resulted in higher deposition rate. The results indicated that deposition yield can be varied by variation of MV concentration. The deposition yield increased with increasing deposition voltage (Fig. 4B). Nearly linear relationship dependence was observed in agreement with Hamaker equation [42]. Similar dependences were observed for graphene suspensions, where the deposition yield increased with the MV concentration (Fig. 5A) due to increased charge of the graphene particles, containing adsorbed MV molecules [43]. However, a higher deposition voltage was required for the deposition of graphene due to the larger mass of the graphene particles. A linear dependence of the deposition yield versus the deposition voltage was observed in the range of 0–30 V, and significant deviation from the linear dependence was observed at higher voltage range. The linear part is in agreement with Hamaker equation [42]. The deviation from the linear dependence can be attributed to different factors, such as deposit spalling, increased voltage drop in thick deposit and corresponding reduction in electric field in the bulk of the suspensions, electrode reactions and other factors discussed in the literature [23]. The results indicated that the deposition yield for MWCNT films and graphene films can be controlled by changing MV concentration and deposition voltage. FTIR studies were performed in order to confirm the MV adsorption. Fig. 6 compares the FTIR spectra of MV, MWCNT, graphene and deposited materials. The absorption assignments are summarized in Table 1. In comparison with the spectra of pristine MWCNT and graphene, the spectra of deposited materials (Fig. 6) showed additional absorptions at 1582, 1514, 1472, 1360 and 1168 cm−1 for deposited MWCNT and at 1582, 1517, 1360 and 1168 cm−1 for deposited graphene. Taking into account that similar absorptions were observed in the spectra of MV (Table 1), it was concluded that deposited MWCNT and graphene contained adsorbed MV.
Fig. 5. Deposit mass for films prepared from 1 g L−1 graphene suspension at a deposition time of 2 min versus (A) MV concentration at the deposition voltage of 30 V and (B) deposition voltage at CV concentration of 0.5 g L−1 . Lines are the guide for the eye.
Fig. 6. FTIR spectra of (a) MV, (b) pristine MWCNT, (c) deposited MWCNT, (d) pristine graphene, and (e) deposited graphene.
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Table 1 Band assignments for MWCNT, graphene, and MV. MV (cm−1 )
Pristine MWCNT (cm−1 )
Deposited MWCNT (cm−1 )
Pristine graphene (cm−1 )
Deposited graphene (cm−1 )
Band assignments
1632
1632 1582 1472 1514 1385 1127 1360 1168
1634
1630 1582
C CC stretch in CNT [52] C C in aromatic ring [53]
1517 1385 1127 1360 1168
N H stretch [53] C O stretch [54]
1582 1479 1520 1385 1127 1359 1168
1385 1127
Fig. 7 shows the SEM images of MWCNT (Fig. 7A), graphene (Fig. 7B), and MWCNT–graphene composite (Fig. 7C). Fig. 7A showed a continuous and uniform MWCNT film with an average pore size of ∼100 nm. As shown in Fig. 7B, graphene was dispersed from particles to sheets, and the substrate was fully covered. The possibility of deposition of MWCNT and graphene paved the way
Fig. 7. SEM images of (A) MWCNT, (B) graphene and (C) composite films, deposited from (A) 1 g L−1 MWCNT and 0.5 g L−1 MV suspensions at 5 V, (B) 1 g L−1 graphene and 0.5 g L−1 MV suspension at 30 V, and (C) 1 g L−1 MWCNT, 1 g L−1 graphene and 1 g L−1 MV suspension at 20 V.
C H in CH3 [53] C N stretch [53]
for the co-deposition of both materials from one suspension using MV as a charging and dispersing agent for both materials. Literature data [44,45] indicates that mixed suspensions of CNT and graphene exhibit improved stability, compared to the suspensions of individual materials. The results of sedimentations tests of mixed MWCNT–graphene suspension are in agreement with the literature data and indicate that MWCNT–graphene suspensions without MV were stable for 3–4 h. In contrast, MWCNT–graphene suspensions containing MV were stable for more than 2 months. Mixed suspensions, containing MWCNT, graphene and MV dispersant were used for the formation of composite MWCNT–graphene films by EPD. The SEM image of the composite material shows graphene and MWCNT (Fig. 7C). As pointed out above, the MWCNT–graphene composites have potential applications in many fields, including ES [14] and batteries [46], due to improved electrolyte access to graphene layers, separated by MWCNT. The capacitive behavior of the composite films was studied in the potential range between −0.4 and 0.6 V versus SCE. The SC was calculated from the CV data and plotted versus scan rate in Fig. 8A. At the scan rate of 2 mV s−1 , a SC of ∼130 F g−1 was achieved
Fig. 8. (A) SC versus scan rate and (B) corresponding CV at 10 mV s−1 for (a) composite film, prepared from a suspension containing 1 g L−1 MWCNT, 1 g L−1 graphene and 1 g L−1 MV (b) MWCNT film, prepared from 1 g L−1 MWCNT suspension, containing 0.5 g L−1 MV and (C) graphene film, prepared from 1 g L−1 graphene suspension, containing 0.5 g L−1 MV.
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4. Conclusions
Fig. 9. Nyquist plot of a complex impedance Z = Z − iZ in the frequency range of 0.1 Hz–100 kHz for (a) composite, (b) MWCNT, and (c) graphene films; the suspensions for film preparation are the same as in Fig. 8. The inset shows the equivalent circuit used for the simulation.
for a composite film with the mass loading of 0.04 mg cm−2 . With the increase of the scan rate, the SC decreased to ∼63 F g−1 at 100 mV s−1 . This decrease was attributed to diffusion limitations of the electrolyte in the pores of the composite material. The graphene and MWCNT film with the same mass loading exhibited SC of ∼55 F g−1 and ∼81 F g−1 , respectively, at a scan rate of 2 mV s−1 . Fig. 8B shows corresponding CVs at 10 mV s−1 scan rate for composite, MWCNT and graphene films. The CV of composite film has larger area, compared to the CV area of MWCNT and graphene films, due to higher capacitance. Therefore, the composite films showed improved capacitive behavior, compared to the MWCNT and graphene films. It is suggested that MWCNT incorporated between graphene layers during EPD and prevented their restacking in the composite films. Such structures allowed improved electrolyte access to the graphene surface. As a result, the high surface area of graphene was better utilized in the composite materials for application in ES. The graphene, MWCNT, and composite films were studied by EIS and the data was shown in Fig. 9. The equivalent circuit, presented in the inset of Fig. 9, was used for the analysis of the EIS data. It included two RQ transmission lines, describing the porous electrode [47,48]. The diffusion resistance of electrolyte inside the pores was represented by the Warburg impedance (W) [49,50]. In this circuit, R1 element represented the electrolyte resistance, while R2 represented the film resistance. Q elements represented the double-layer capacitance and pseudocapacitance of the composite film, with the consideration of microscopic roughness of the surface and capacitance dispersion of interfacial origin [14]. To simulate an impedance spectrum using an equivalent circuit, a minimum set of model parameters was used and good agreement of the experimental data and the results of simulation was achieved. By comparing the EIS of three samples, it was found that composite film had the smallest film resistance, especially at low frequency range. It is suggested that MWCNT acted as spacers between the graphene layers and improved electrolyte access to the graphene layers, resulting in lower resistance of the composite material. The lower resistance allowed improved capacitive behavior. EPD offers many processing advantages, compared to other methods for the fabrication of MWCNT–graphene composites. The EPD method is simple and suitable for controlled deposition of well dispersed MWCNT and graphene on high surface area substrates. This method is especially attractive for application in ES. The SC of composite films prepared by this method is comparable with SC of the MWCNT–graphene composites synthesized by other methods [45,51]. The method developed in this investigation gives an alternative route to achieve nanoscale fabrication of MWCNT–graphene composites, utilizing processing advantages of EPD technology.
Cathodic EPD method has been developed for the fabrication of MV films, which were deposited by a constant voltage EPD or potentiodynamically. The deposition mechanism involved electrophoresis of cationic MV, pH increase at the cathode due to the electrode reactions, charge neutralization of the cationic MV at the electrode surface and film formation. MV allowed efficient dispersion, charging and cathodic EPD of MWCNT and graphene at relatively low dispersant concentrations. The deposition yield can be varied by variation of dispersant concentration and deposition voltage. FTIR data proved the adsorption of MV molecules on MWCNT and graphene surface. The use of MV as a codispersing agent for MWCNT and graphene allowed the fabrication of MWCNT–graphene composite films for application in electrodes of ES. The composite films showed capacitive behavior. The SC of ∼130 F g−1 was obtained at a scan rate of 2 mV s−1 . The composite films showed improved capacitive behavior compared to the films of individual components. The MWCNT–graphene composites are promising materials for ES. Acknowledgement The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. References [1] Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, Y. Chen, Supercapacitor devices based on graphene materials, J. Phys. Chem. C 113 (2009) 13103–13107. [2] S. Vivekchand, C. Rout, K. Subrahmanyam, A. Govindaraj, C. Rao, Graphenebased electrochemical supercapacitors, J. Chem. Sci. 120 (2008) 9–13. [3] Y. Sun, Q. Wu, G. Shi, Graphene based new energy materials, Energy Environ. Sci. 4 (2011) 1113–1132. [4] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitors based on flexible graphene/polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963–1970. [5] S.H. Aboutalebi, A.T. Chidembo, M. Salari, K. Konstantinov, D. Wexler, H.K. Liu, S.X. Dou, Comparison of GO, GO/MWCNTs composite and MWCNTs as potential electrode materials for supercapacitors, Energy Environ. Sci. 4 (2011) 1855–1865. [6] A.R. Boccaccini, J. Cho, J.A. Roether, B.J.C. Thomas, E. Jane Minay, M.S.P. Shaffer, Electrophoretic deposition of carbon nanotubes, Carbon 44 (2006) 3149–3160. [7] Y. Wang, I. Deen, I. Zhitomirsky, Electrophoretic deposition of polyacrylic acid and composite films containing nanotubes and oxide particles, J. Colloid Interface Sci. 362 (2011) 367–374. [8] J. Cai, M. He, Y. Gu, L. Kang, Z. Lei, Z. Yang, Z.H. Liu, Assembling fabrication and capacitance of manganese oxide nanosheets and functionalized carbon nanotubes hybrid material, Colloids Surf. A 429 (2013) 91–97. [9] Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian, F. Wei, A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors, Adv. Mater. 22 (2010) 3723–3728. [10] A.R. Boccaccini, S. Keim, R. Ma, Y. Li, I. Zhitomirsky, Electrophoretic deposition of biomaterials, J. R. Soc. Interface 7 (2010) S581–S613. [11] J. Du, R. Yue, Z. Yao, F. Jiang, Y. Du, P. Yang, C. Wang, Nonenzymatic uric acid electrochemical sensor based on graphene-modified carbon fiber electrode, Colloids Surf. A 419 (2013) 94–99. [12] G. Zhu, L. Pan, T. Lu, T. Xu, Z. Sun, Electrophoretic deposition of reduced graphene–carbon nanotubes composite films as counter electrodes of dyesensitized solar cells, J. Mater. Chem. 21 (2011) 14869–14875. [13] Y. Sun, Y. Wang, I. Zhitomirsky, Dispersing agents for electrophoretic deposition of TiO2 and TiO2 –carbon nanotube composites, Colloids Surf. A 418 (2013) 131–138. [14] Y. Su, I. Zhitomirsky, Electrophoretic assembly of organic molecules and composites for electrochemical supercapacitors, J. Colloid Interface Sci. 392 (2012) 247–255. [15] Y. Su, I. Zhitomirsky, Cataphoretic assembly of cationic dyes and deposition of carbon nanotube and graphene films, J. Colloid Interface Sci. 399 (2013) 46–53. [16] B. Cichy, A. Gorecka-Drzazga, J.A. Dziuban, Field-emission light sources utilizing carbon nanotubes and composite phosphor made of SiO2 nanospheres covered with Y2 O3 :Eu, J. Vac. Sci. Technol. B 27 (2009) 757–760. [17] E. Yoo, J. Kim, E. Hosono, H.-s. Zhou, T. Kudo, I. Honma, Large reversible Li storage of graphene nanosheet families for use in rechargeable Lithium Ion batteries, Nano Lett. 8 (2008) 2277–2282. [18] F. Du, D. Yu, L. Dai, S. Ganguli, V. Varshney, A.K. Roy, Preparation of tunable 3D pillared carbon nanotube–graphene networks for high-performance capacitance, Chem. Mater. 23 (2011) 4810–4816.
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