Reduced Graphene Oxide@Polyaniline for

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Facile Synthesis of MoS2/Reduced Graphene Oxide@Polyaniline for High-Performance Supercapacitors Xuan Li,†,⊥ Chaofeng Zhang,‡,⊥ Sen Xin,‡ Zhengchun Yang,§ Yutao Li,*,∥ Dawei Zhang,‡ and Pei Yao*,† †

School of Material Science and Engineering, Center for Analysis and Tests, Tianjin University, Tianjin 300072, P.R. China School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R. China § Tianjin Key Laboratory of Film Electronic & Communication Devices, Advanced Materials and Printed Electronics Center, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China ∥ Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ‡

S Supporting Information *

ABSTRACT: The molybdenum disulfide/reduced graphene oxide@polyaniline (MoS2/RGO@PANI) was facilely and effectively prepared through a two-stage synthetic method including hydrothermal and polymerized reactions. The rational combination of two components allowed polyaniline (PANI) to uniformly cover the outer face of molybdenum disulfide/reduced graphene oxide (MoS2/RGO). The interaction between the two initial electrode materials produced a synergistic effect and resulted in outstanding energy storage performance in terms of greatest capacitive property (1224 F g−1 at 1 A g−1), good rate (721 F g−1 at 20 A g−1), and cyclic performance (82.5% remaining content after 3000 loops). The symmetric cell with MoS2/RGO@PANI had a good capacitive property (160 F g−1 at 1 A g−1) and energy and power density (22.3 W h kg −1 and 5.08 kW kg−1). KEYWORDS: MoS2, polyaniline, symmetric supercapacitor, synergistic effect, electrochemistry

1. INTRODUCTION Supercapacitors with high power density and long cycle life are important energy storage devices, and pseudocapacitors have higher specific capacitances than those of the electrical double layer capacitors, which attracted extensive interest of scientific research.1−7 In terms of pseudocapaitive materials, several types were widely implemented in supercapacitor electrodes, including transition metal oxides,8−11 metal hydroxides,12−14 and conducting polymers.15−18 Polyaniline (PANI) has several advantages over other pseudocapacitive materials, such as cost-effectiveness, high theoretical capacitance, and flexibility. However, two major defects found in bulk PANI dealing with the low practical capacitance and poor cycle stability limit its application in supercapacitors.19−21 Bulk PANI is inadequate for exposure to electrolytes because it increases the inactive electrochemical surface area, which effectively reduces its specific capacitance. In addition, the volumetric change induced during the charge−discharge processes gives rise to a loss in the electrode material and unstable structural stability, resulting in poor supercapacitor performances. To overcome the above problems, a number of studies dealing with architectural design have been performed using two-dimensional nanosheets as the substrates for growing nanosized PANI.22−24 The former is expected to provide larger electroactive surface areas and fast electron transport. Furthermore, the synergistic effect created by the combined components is expected to effectively improve the practical supercapacitive properties of the electrodes. © 2016 American Chemical Society

Among the existent two-dimensional nanomaterials, molybdenum disulfide (MoS2) is the favorite candidate for combination with PANI to yield improved electrochemical performances.25−27 Many studies have used the substrate of MoS2 nanosheets to disperse the PANI nanowires for enhancing the supercapacitive performance.19,28,29 However, the facile aggregation and low electronic conductivities of MoS2 limit the application of substrate using for increasing the supercapacitive performance. Reduced graphene oxide (RGO) is used in this study as a template to grow MoS2 nanosheets and improve the dispersion capability and electronic conductivity of MoS2. The hybridization of both the RGO and MoS2 nanosheets produces an interesting two-dimensional material that could be applied as a substrate to grow nanosized PANI and yield supercapacitor composite materials with improved characteristics. The formation process of the MoS2/RGO@PANI is shown in Scheme 1. Typically, the highly dispersive MoS2 nanosheets are first grown on the GO nanosheets via a hydrothermal route to produces the precursor MoS2/RGO. In this hydrothermal reaction, the GO is reduced to RGO with the functional group decreasing. Afterward, in the presence of the APS and HCl, the aniline monomers react with the remaining functional group to form corner-like polyaniline on the MoS2/RGO. The obtained MoS2/RGO@PANI composite displays the great supercapacitive performance. Received: June 6, 2016 Accepted: July 27, 2016 Published: July 27, 2016 21373

DOI: 10.1021/acsami.6b06762 ACS Appl. Mater. Interfaces 2016, 8, 21373−21380

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ACS Applied Materials & Interfaces Scheme 1. Fabrication Process of MoS2/RGO@PANI Composite

2. EXPERIMENTAL SECTION 2.1. Synthesis of MoS2/RGO Composite. Graphene oxide (GO), aniline (ANI), hydrochloric acid (HCl), ammonium persulfate (APS), ammonium molybdatete trahydrate ((NH4)6Mo7O24·4H2O), and thiourea were used as the starting materials. Typically, GO (40 mg) was added to 40 mL deionized water and sonicated for 1 h to form a GO suspension. (NH4)6Mo7O24·4H2O (0.211 g) and thiourea (0.182 g) were dispersed in 20 mL deionized water and stirred for 1 h. The above mixed solution was slowly instilled into the previous GO dispersion, ultrasonically shook for 30 min, and finally poured into a 100 mL enclosed stainless steel high pressure reactor kept at 200 °C for 24 h. when the reaction finished, the precursor was centrifuged, rinsed with water and forced-air-dried at 60 °C for 10 h to obtain the precursor MoS2/RGO. 2.2. Synthesis of MoS2/RGO@PANI Composite. Typically, the precursor MoS2/RGO (125 mg) was ultrasonically shaken in 40 mL deionized water for 1 h to form a suspension of MoS2/RGO. Aniline (1.14 mL) and HCl (3.86 mL) were blended in 17 mL deionized water for 30 min. The mixture was slowly added dropwise to the MoS2/RGO suspension and stirred ultrasonically for 1 h to make sure the aniline well dispersed on the MoS2/RGO nanosheets. Subsequently, a solution of APS (0.1 M, 13 mL) was quickly poured into the aniline-MoS2/RGO reaction system and continually agitated with a magnetic stirrer at 4 °C for 4 h. After the reaction completing, the MoS2/RGO@PANI composite was collected from the solution and freeze-dried for 24 h. Thereafter, the individual PANI nanowires were also synthesized with same procedures without adding MoS2/RGO. 2.3. Characterization. The Schottky thermal field emission scanning electron microscope (STFE-SEM, FEI Nanosem430) were applied to survey the macro profiles of the electrode materials. Transmission electron microscopy (TEM, Tecnai F20) were afforded to demonstrate the internal nature and architecture of the electrode materials. The chemical constitutions were drew out by means of energy dispersive spectroscopy (EDS) integrated in to a TEM system and VG Scientific ESCALAB 220i-XL X-ray photoelectron spectroscopy (XPS) device. RigakuDmax 2500 system was used to collect X-ray diffraction (XRD) dates and analyze the crystal composition for 2θ ranging from 10° to 80° with 4°/min rotational speed. Renishawin ViaReflex spectroscopy equipment was employed to explain the chemical bonds of the electrode materials by using 633 nm laser transmitter. The ASAP-2020 surface area analyzer was used to examine the N2 adsorption/desorption measurements. The PANI sample

Figure 1. (a) SEM image of the MoS2/RGO composite. A low (b) and high (c) magnification SEM image of MoS2/RGO@PANI. (d) TEM image of the MoS2/RGO@PANI composite.

was pretreated at 120 °C for 6h before the BET measurement. Moreover, the MoS2/RGO and MoS2/RGO@PANI composites were degassed at 200 °C for 6h before the BET measurement. The weight loss of the obtained materials was tested by using the TG 209 in air. 2.4. Electrochemical Measurements. The three-electrode configuration cell connected to the electrochemical workstation (CHI 660E, Zahner-Elektrik IM6e and LAND 2001 Cell) was demanded to analyze the electrochemical performance through a series of electrochemical measuring techniques: cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). This cell included the electrolyte solution (1 M H2SO4), counter electrode (Platinum plate), reference electrode (AgCl/Ag electrodes), and the working electrode (mass: 1.05 mg cm−2). The capacitance of the working electrode was evaluated with eq 1: C = I Δt /mΔV

(1)

−1

where C (F g ) and m (mg) is the capacitance and mass, respectively. I (mA), ΔV (V), and Δt (s) represent the current, voltage window, and time during discharge process, respectively. In a two-electrode system, the symmetric configuration cell included the two working electrodes (MoS2/RGO@PANI, 2.1 mg), the separator (a filter paper), and electrolyte solution 21374


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ACS Applied Materials & Interfaces

3. RESULTS AND DISCUSSION Figure 1 shows the morphologies and structures of obtained electrode materials. Figure 1a reveals that MoS2 nanosheets grown on the GO nanosheets are well-scattered and interlaced and compose a multistage structure. MoS2/RGO composites provide better structural morphologies for the growth of PANI nanowires than pure MoS2 nanosheets. The rough surfaces of as-synthesized MoS2/RGO composite fills with nanowires after loading with aniline (Figure 1b and c). Additional structural information on the MoS2/RGO@PANI composite is obtained with TEM characterization (Figure 1d). The lattice distance is estimated to 0.62 nm stand for the existence of the MoS2 phase. In addition, the PANI nanowires are uniformly anchored into the surface of MoS2/RGO composite,

(1 M H2SO4). The supercapacitive performance of symmetric configuration cell was respectively estimated using eqs 2−4: Ct = I Δt /M ΔV

(2)

E = Ct ΔV 2/2 × 3.6

(3)

P = (3600E)/Δt

(4)

where Ct (F g−1) and M (mg) is the capacitance and mass of the symmetric configuration cell, respectively. E (W h kg−1), P (kW kg−1), I (mA), ΔV (V), and Δt (s) represent the energy density, power density, current, voltage window, and time during discharge process, respectively.

Figure 2. Nitrogen adsorption−desorption isotherms and pore diameter distribution of (a and b) PANI, (c and d) MoS2/RGO, and (e and f) MoS2/RGO@PANI composite. 21375


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ACS Applied Materials & Interfaces confirming results from SEM. In the MoS2/RGO@PANI composite, the morphology of PANI nanowires displays cornerlike structures, which are smaller than those of pure PANI nanowires (Figure S1). The uniform distribution of C, N, Mo, and S elements across the entire area from the EDS mapping in Figure S2 suggest that PANI nanowires are uniformly covered on MoS2/RGO composite. The weight percent of supercapacitive materials is tested by TGA measurement (Figure S3). The thermal stability of PANI is obviously increased by the introduction of the MoS2/RGO nanosheets. The weight of PANI, MoS2/RGO, and MoS2/RGO@PANI remain 36.7%, 97.7%, and 76.3%, respectively. Thus, can reason out the weight percent of PANI in the ultimate product is 35%. Meanwhile, the BET technique effective proposes the surface structure change of the surpercapacitive materials. Figure 2 depicts that the isotherm curves exhibit type IV characteristics, pointing out to the existence of mesoporous structure. The specific surface areas of MoS2/RGO@PANI (53.95 m2 g−1) lies between the values obtained with PANI (31.2 m2 g−1) and MoS2/RGO (130.1 m2 g−1). Moreover, MoS2/RGO@PANI displays relatively narrow pore size distribution with the smallest average pore diameters (9.7 nm). High specific surface areas of MoS2/RGO can provide additional actives sites for reaction of PANI and reduce the aggregation of the materials to yield uniformly dispersed cornerlike PANI nanowires. The cornerlike structure of PANI nanowires enables larger internal surface areas, prompts more regular pore size of the composite, and expands the contact space between the electrolyte solution and surpercapacitive material. Figure 3 shows the crystal characteristics of PANI, MoS2/RGO, and MoS2/RGO@PANI. The diffraction peaks of MoS2/RGO

Figure 4. Raman spectra of PANI, MoS2/RGO, and MoS2/RGO@ PANI composite.

correspond to D band (disorderly defect structure) and G band (sp2-bonded carbon atoms) of RGO,35 respectively. After coating with PANI, several new peaks appear and reveal the existence of PANI nanowires. The signals at 415 and 518 cm−1 are assigned to out-of-plane PANI ring bend.36 The 580 and 810 cm−1 signals are characteristic of the benzenoid ring deformation and out-of-plane vibrations of aromatic rings,37 respectively. Other signals of 1169, 1223, 1257, 1336, 1486, 1589, and 1620 cm−1 are related with C−H vibration, C−N vibration, C−N+ vibration, CN of the di-imine units, CC band, and C−C band,21,35,37 respectively. The obvious presence of the PANI peaks in MoS2/RGO@PANI supports the idea that PANI nanowires are uniformly grown to coat the entire surface of the MoS2/RGO composite. Further structural properties and composition of MoS2/ RGO@PANI composites are obtained by XPS analysis and the spectra are displayed in Figure 5. The C 1s spectrum can be

Figure 3. XRD spectra of PANI, MoS2/RGO, and MoS2/RGO@ PANI composite.

composite recorded at 2θ of 14.1°, 33.1°, 36.7°, 39.6°, 53.1°, and 58.9° can be assigned to (002), (100), (102), (103), (106), and (110) crystal faces of MoS2.30,31 The diffraction peak at 26.5° can be ascribed to the (002) plane of RGO. The diffraction peaks of PANI located at 2θ of 14.8°, 20.3°, and 25.3°correspond to (011), (020), (200) planes of PANI.19,32 The peaks of both MoS2/RGO and PANI are clearly present in the XRD spectrum of the MoS2/RGO@PANI composite, revealing that PANI cornerlike nanowires are formed on the outer face of the MoS2/RGO composite. The structures of PANI, MoS2/RGO, and MoS2/RGO@PANI further confirmed by Raman spectroscopy are shown in Figure 4. The characteristic Raman peaks of MoS2/RGO composite detected at 375 and 408 cm−1 are ascribed to the E2g1 and A1g modes of MoS2.33,34 The other two peaks at 1326 and 1595 cm−1

Figure 5. XPS spectra of MoS2/RGO@PANI composite: (a) C 1s, (b) N 1s, (c) Mo 3d, and (d) S 2p.

categorized into four main peaks located respectively at 284.5 (the graphitic carbon), 285.4 (C−N), 286.5 (C−O), and 288.2 eV (CO).38 The relatively recorded small peaks at 286.5 and 288.2 eV suggest that GO has been reduced. The registered N 1s peak of MoS2/RGO@PANI might be divided 21376


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shows the characteristic CV curve of electrochemical Faradaic reactions having two conspicuous groups of redox peaks. The redox peaks are generated from the doping state changes of PANI.40 In addition, it could be noted that surface areas below the CV curves of MoS2/RGO@PANI are larger than those of both the PANI and MoS2/RGO. This proposes that MoS2/RGO@PANI has enhanced capacitance if compared to PANI and MoS2/RGO. On the other hand, careful analysis of the data revealed that the electrochemical capacitance of MoS2/RGO@PANI is mainly derived from PANI and the improvements in capacitance performances seen with MoS2/ RGO@PANI are relying mostly on the import of MoS2/RGO that produces a synergistic effect between MoS2/RGO and PANI. As shown in the Figure 6b, the specific peak current and response have improved and maintained the shape of the CV with the scan rate increasing. The latter demonstrates that the

into four separate signals: 398.3 (pyridinic N), 399.5 (C−N), 400.4 (pyrrolic N), and 401.2 eV (interaction between MoS2 and N atom of PANI).39 The Mo 3d spectrum depict strong peaks at 229.8 (Mo4+ 3d5/2), 233.2 (Mo4+ 3d3/2), and 236.4 eV (Mo6+ 3d5/2),38 respectively. Additionally, the S 2p spectrum depicts strong signals at 162.0 and 163.2 eV on behalf of S 2p3/2 and S 2p1/2.38 All of the data above strengthen the idea that PANI nanowires can uniformly coat the surface of MoS2/RGO. The interaction between PANI nanowires and MoS2/RGO not only provides excellent interfacial contact but also it improves the electrochemical performances of MoS2/RGO@PANI. The supercapacitor performances of the obtained samples are first assessed through a series of electrochemical measuring techniques (CV, GCD and EIS). In the Figure 6a, MoS2/RGO composite displays the typical CV curve of electric-double-layer capacitor. After coating with the PANI, MoS2/RGO@PANI

Figure 6. (a) CV curves of PANI, MoS2/RGO, and MoS2/RGO@PANI composite tested in 1 M H2SO4 electrolyte at a scan rate of 5 mV s−1. (b) CV curves of MoS2/RGO@PANI composite tested in 1 M H2SO4 electrolyte at different scan rates of 2, 5, 10, 20, 50, and 100 mV s−1. (c) Charge−discharge curves of PANI, MoS2/RGO and MoS2/RGO@PANI composite at a current density of 1 A g−1. (d) Charge−discharge curves of MoS2/RGO@PANI composite at current densities of 1, 2, 5, 10, 15, and 20 A g−1. (e) Specific capacitances of PANI, MoS2/RGO, and MoS2/RGO@PANI composite at different current densities of 1, 2, 5, 10, 15, and 20 A g−1. (f) Cycle performance of MoS2/RGO@PANI composite obtained over 3000 cycles at 10 A g−1. 21377


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indicating that the conductivity is increased upon the introduction of MoS2/RGO. The cyclic performance of supercapactive material is a significant indicator of the real applications. Figure 6f shows that MoS2/RGO@PANI retained over 82.5% of the starting value after 3000 loops at 10 A g−1 and Coulombic efficiency close to 100%. This indicates a relevant structural stability of PANI cornerlike nanowires dispersed on the MoS2/RGO. MoS2/RGO@PANI has improved electrochemical performance compared with the reportedly advanced materials in the previous literature (Table S1).41−43 The combination of MoS2/RGO and PANI nanowires is shown to dramatically increase the electrochemical properties because of the synergistic effect. The MoS2/RGO composite provides additional active sites for the reaction of PANI nanowires by preventing their aggregation and facilitating cornerlike structures. This, in turn, adequately exposes the composite material to the electrolytes for optimal electrochemical performances. The distribution of PANI cornerlike nanowires on the surface of MoS2/RGO increases the accessible active surface area, produces smaller and regular pore size and reduces the diffusion length for the electrolyte ions. These features yield improved specific capacitances and capability rates. Meanwhile, the MoS2/RGO composite as the mechanical support and electronic conductor also enhance the cycle and rate performance. The symmetric cell was constructed with MoS2/RGO@PANI and tested with a series of supercapacitive measuring techniques (Figure 7). The CV results (Figure 7a) of symmetric cell have a

MoS2/RGO@PANI composite has a good rate and cycle performance. The capacitive property of each sample is determined by GCD measurement, and the obtained results are presented in Figure 6c. The addition of PANI makes the nearly symmetric shape of MoS2/RGO change to asymmetric type, suggesting that the capacitance behavior of the final product is dominated by Faradaic charge transfer processes. The synergistic effect between two interoperable materials inside the MoS2/RGO@ PANI makes its capacitance (1224 F g−1) far surpass the sum of PANI (774 F g−1) and MoS2/RGO (216 F g−1), which estimated by eq 1 with 1 A g−1 current density. At the same time, as Figure 6d implies, the synergistic effect also brings up the fantastic electrochemical capacitive behavior, as well as elevated charge−discharge Coulombic efficiency. According to the estimated results of the Figure 6e, the capacitance of MoS2/RGO@PANI stays around 58.9% and lies between PANI (46.5%) and MoS2/RGO (64.8%). The rate capability of MoS2/RGO@PANI shows notable increase upon the introduction of MoS2/RGO. For better understanding the electrochemical reaction kinetics of PANI, MoS2/RGO, and MoS2/RGO@PANI materials, the electrodes were analyzed by EIS as shown in Figure S4 (inset represents an enlarged view of the high-frequency portion). The imperfect semicircular and linear tail in the high and low frequency portion are considered to be the charge transfer and ion diffusion resistance,14 respectively. It is clear that the above resistances of MoS2/RGO@PANI significantly reduce,

Figure 7. (a) CV curves of the symmetric cell at different scan rates of 2, 5, 10, 20, 50, and 100 mV s−1. (b) Charge−discharge curves of the symmetric cell at current densities of 1, 2, 3, 4, 5, 6, 8, and 10 A g−1. (c) Specific capacitances of symmetric cell at different current densities of 1, 2, 3, 4, 5, 6, 8 and 10 A g−1. (d) Ragone plots of the symmetric cell. 21378


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couple of peaks with the pseudocapacitive characteristics, in accordance with other published reports.32,44 Meanwhile, the symmetric cell has excellent reversibility and obviously potential drops, suggesting the elevated interface resistance exists in the cell (Figure 7b). When the current density increases from 1 to 10 A g−1, the capacitance of symmetric cell is reduced from 160 to 64 F g−1. In the above current range, the greatest energy and power density is calculated using eqs 3 and 4 to be 22.3 W h kg−1 and 5.08 kW kg−1, respectively (Figure 7c and d).

4. CONCLUSION With the introduction of MoS2/RGO in the matrix, MoS2/ RGO@PANI composite was prepared by a facile and effective route. The introduction of the MoS2/RGO in the matrix did not only induce a uniform dispersion of PANI nanowires on the surface but also result in a synergistic effect between both components. The latter yields a MoS2/RGO@PANI composite with improved electrochemical property and elevated supercapacitive performance. These findings suggest the promising applicability of the MoS2/RGO@PANI composite in supercapacitor devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06762. EDS mapping analysis of MoS2/RGO@PANI composite, SEM and TEM analyses of PANI nanowires, EIS plots of PANI, MoS2/RGO, and MoS2/RGO@PANI composite, and the comparison of the electrochemical performance of MoS2/RGO@PANI with those in previous reports (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (P.Y.). Author Contributions ⊥

X.L. and C.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 21403050, 51472070). REFERENCES

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Reduced Graphene Oxide@Polyaniline for

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