Sonochemical preparation of a ytterbium oxide/reduced graphene oxide nanocomposite for supercapacitors with enhanced capacitive performance

Abstract

Decoration of graphene with different nanostructures can result in fundamental advancements in versatile technologies, especially in the fast growing fields of catalysts, sensors and energy storage. In this study, we have synthesized ytterbia (Yb₂O₃) nanoparticles through a facile sonochemical process. These nanoparticles have been anchored on the surface of reduced graphene oxide (RGO) via a self-assembly approach. We investigated the supercapacitive behavior of the nanocomposites as electrode materials with cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy techniques. These nanocomposites exhibit a specific capacitance of 240 F g⁻¹ in 0.5 M Na₂SO₄ electrolyte at a scan rate of 2 mV s⁻¹. Additionally, the specific capacitance of the nanocomposite electrode is 222 F g⁻¹ at the current density of 1 A g⁻¹ in the galvanostatic charge–discharge measurements. These excellent electrochemical performances can stem from the synergism between the properties of Yb₂O₃ nanoparticles and RGO sheets, high charge mobility of them and good flexibility of the graphene sheets. Furthermore, the nanocomposite electrode presents excellent cycling durability with 96.5% specific capacitance restored after 4000 cycles. Our present work introduces a novel procedure to fabricate Yb₂O₃/RGO nanocomposites as a promising candidate in high performance energy applications.

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Introduction

Reduced graphene oxide (RGO) provides the possibility of large-scale production of graphene through a low-cost solution based process which has increased the potentials of using the material in a wide range of applications.¹ Recently, decoration of RGO with various nanoparticles has attracted huge interest as it allows one to gain a new class of hybrid materials with properties that are different from those of each individual component.^1–5 These properties can be altered through changing the extent of loading, as well as the nature of the loaded material.^1–5 The use of metal oxides (e.g. MnO₂, Mn₃O₄, RuO₂, CeO₂ and SnO₂), in this area has been found to prevent the loss of surface area of RGOs which happens due to the restacking of the graphene sheets and making the graphene surface more accessible, through acting as spacers among the graphene sheets.^6,7 On the other hand, RGO as a support of metal oxides can prevent agglomeration of nanoparticles which preserve their accessible surface area.⁸

Metal oxides decorated RGOs have variety of applications in sensing, catalysis and energy storage.^4,9–13 Use of advanced energy storage devices like supercapacitors, is a necessity for handling the incremental demand for energy in today's modern world.¹⁴ Some of the advantages of supercapacitors over conventional capacitors and batteries include their high considerable energy density, their capability to charge or discharge faster, and their prolonged life cycles.^12,15,16 Two functioning mechanisms have been used in the development of super capacitors, and hence the devices are classified into the two major categories of pseudocapacitors and electrical double-layer capacitors (EDLCs). In pseudocapacitors the electrodes are made of electroactive materials like oxides of noble and transition metals, as well as conducting polymers. EDLCs, on the other hand, majorly use carbon based materials like carbon nanotubes (CNTs) or graphene to build the electrodes.^16,17

The open space between graphene layers in decorated RGOs significantly reduces the internal resistance and facilitates the diffusion of electrolyte into the electrode.¹⁸ These modifications in the structure of the carbonous material lead to improved properties in terms of energy density and capacitance in EDLCs.^{7,12,16,19–21}

Lanthanoid oxides, also known as rare earth oxides (REOs), have received considerable attention in recent years as they possess high potential for applications in versatile fields such as glass industry, heterogeneous catalysis, electronics and fuel cells.^22–26 Based on the principle properties of lanthanoids, REOs undergo two major categories of chemical processes: the first category of reactions has been related to their redox chemistry while the second including the acid–base processes.^22,27 Due to the electrochemical redox characteristics of lanthanoids.²² REOs can be considered as potential candidates for pseudocapacitors. Among lanthanoids, ytterbium is one which shows both +2 and +3 oxidation states.^22,28

Herein, we demonstrate a simple and green method to fabricate Yb₂O₃ nanoparticles and Yb₂O₃/RGO hybrid materials under mild sonochemical conditions. As the size of the Yb₂O₃ is reduced down to the nanometer scale, the redox activity is much promoted because of the enlarged surface area. In order to better understand the effect of composition on the supercapacitive properties of as-prepared nanocomposite, three different ratios of the Yb₂O₃/RGO have been prepared and investigated.

Experimental

Materials

Graphite flakes (cat #332461), ytterbium(III) nitrate pentahydrate (Yb(NO₃)₃·5H₂O), phosphoric acid (H₃PO₄), hydrazine hydrate (N₂H₄) and ammonium hydroxide (NH₄OH) were purchased from Sigma-Aldrich Co. Potassium permanganate (KMnO₄), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and hydrogen peroxide (H₂O₂) were all purchased from Merck Chemical Co. All reagents were used without any further modification.

Graphene oxide (GO) preparation

We prepared GO from graphite flake powder based on the Tour's method.²⁹ In summary, 18 g of KMnO₄ was added gradually to the mixture containing 400 mL of concentrated H₂SO₄/H₃PO₄ with a volume ratio of 9 : 1 and 3 g of graphite flake powder. This mixture was stirred for 12 h at 50 °C. The color of the mixture turned to bright yellow with pouring it onto the ice and H₂O₂. The product was centrifuged and washed several times with 5% HCl solution and distilled water. The final neutralized product was dried in an oven at 50 °C. At the final step, a brown suspension of GO prepared by sonicating the graphite oxide in distilled water using an ultrasonic probe.

Yb₂O₃–RGO nanocomposites preparation

We prepared Yb₂O₃–RGO nanocomposites via an easy sonochemical procedure (by means of an ultrasonic bath) involving three steps: Yb₂O₃ nanoparticles synthesis; as-synthesized Yb₂O₃ nanoparticles deposition on GO; and decorated GO reduction into RGO (Scheme 1). A 32% ammonia solution was added gradually into the solution containing Yb(NO₃)₃·6H₂O while it is sonicated in an ultrasonic bath (FALC instruments ultrasonic). After 66 min sonication, the resulting suspension was mixed with GO suspension and further sonicated for 22 min to anchor nanoparticles on the surface of GO sheets. This followed by adding N₂H₄ to reduce GO to RGO.

Scheme 1 Schematic illustration for the synthesis of the Yb₂O₃–RGO nanocomposite.

After washing the black precipitate, it was dried at 60 °C for 24 h designated as YbRGO. We prepared the pure Yb₂O₃ and RGO via a same procedure. Table 1 summarizes the details of the various amounts of RGO and Yb₂O₃ in YbRGO samples.

Table 1 Theoretical ratios of Yb₂O₃ to RGO for various Yb₂O₃–RGO nanocomposites

Nanocomposite	Yb^3+ (mmol)	Theoretical mass ratio Yb2O3/GO
YbRGO1	0.35	2/1
YbRGO2	0.175	1/1
YbRGO3	0.0875	1/2

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Characterizations

We characterized the crystallinity of the samples by X-ray diffraction (XRD) technique on a Philips PW-1730 X-ray diffractometer using Cu K_α radiation (λ = 1.5405 Å). Field-emission scanning electron microscopy (FE-SEM) on a Zeiss SIGMA VP with gold coating was used for characterizing size and morphology of products. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a BRUKER EQUINOX 55 spectrophotometer.

Electrochemical study

We prepared the samples for the electrochemical tests as follows: a mixture of each of as-prepared materials (i.e. RGO, Yb₂O₃, and YbRGO nanocomposites), acetylene black, and poly(tetrafluoroethylene) with the mass ratio of 80 : 15 : 5 was prepared and dispersed in ethanol. The homogeneous paste was pressed onto a piece of stainless steel current collector. Each electrode contained about 1 mg of electroactive material and had a geometric surface area of about 1 cm². Finally, the fabricated electrode was dried at 80 °C for 4 h in a vacuum oven. Cyclic voltammetry (CV) tests, galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) were carried out by an Autolab PGSTAT30 type electrochemical workstation. We employed a custom-made device to perform continues cyclic voltammetry (CCV) measurements described in our previous works.^6,30,31

Results and discussion

Nanocomposite characterization

XRD. As illustrated in Fig. 1, the XRD pattern of GO shows the diffraction peak at 2θ = 12.2° which is the most evident diffraction peak attributed to the (001) reflection of graphite oxide.² The deficiency of this definitive diffraction peak in the pattern of the RGO, indicating good reduction of GO to RGO.² Furthermore, the RGO's pattern shows two diffraction peaks around 25° and 43° assigned to (002) and (100) planes of graphite-like structure.³² The XRD pattern of the as-prepared Yb₂O₃ nanoparticles is in good agreement with earlier studies of ytterbium oxide.^33–37 The peak broadening is attributed to the small size of the nanoparticles and also low crystallinity of Yb₂O₃. This pattern is almost repeated for YbRGO nanocomposites, which implies the successful attachment of nanoparticles on the surface of the RGO.

Fig. 1 X-ray diffraction patterns of GO, RGO, Yb₂O₃ and Yb₂O₃–RGO nanocomposites.

FE-SEM. Fig. 2a reveals layered and winkled morphology of RGO. This special morphology of RGO as well as the anchored Yb₂O₃ nanoparticles can be seen in FE-SEM images of YbRGO nanocomposites (Fig. 2c–e). Particle size and morphology of Yb₂O₃ nanoparticles are same in its pure and composite materials, although the nanoparticles are aggregated in the pure Yb₂O₃ (Fig. 2b). These imply that not only this composition prevented the Yb₂O₃ nanoparticles to agglomerate but also these nanoparticles restrained RGOs from restacking.²¹

Fig. 2 FE-SEM images of (a) RGO, (b) ytterbia, (c) YbRGO1, (d) YbRGO2, and (e) YbRGO3.

FT-IR study. Fig. 3 exhibits the FT-IR spectra of GO, RGO, Yb₂O₃ and Yb₂O₃–RGO nanocomposites. The bands at 1735 cm⁻¹ and 1050 cm⁻¹ in the case of GO have been assigned to the COOH group (C=O and C–OH stretching vibrations). The band at 1621 cm⁻¹ has been attributed to the adsorbed water molecules (bending vibration) and the contribution of the sp² bonds. The –OH functional group caused peaks around 1397 cm⁻¹ and 3400 cm⁻¹ due to deformation and stretching vibrations, respectively.

Fig. 3 FT-IR spectra of GO, RGO, Yb₂O₃ and Yb₂O₃–RGO nanocomposites.

Furthermore, a band at 1225 cm⁻¹ is corresponded to the epoxy groups. In the FT-IR spectra of RGO and Yb₂O₃–RGO nanocomposites, the characteristic absorption band of GO at 1735 cm⁻¹ is vanished, indicating the successful reduction of GO. Furthermore, these spectra exhibit the characteristic bands of graphene at 1624 cm⁻¹ and 1578 cm⁻¹.²¹ The FT-IR spectrum of Yb₂O₃ exhibits a band at 675 cm⁻¹ which has been assigned to a deformation mode of the Yb–O–H of the Yb₂O₃·xH₂O phase. Furthermore, a shoulder at 1622 cm⁻¹ corresponding to the bending mode of molecular water can be seen in this spectrum.³⁸ The peak at 1384 cm⁻¹ in the spectra of Yb₂O₃ and Yb₂O₃–RGO nanocomposites is assigned to the adsorbed nitrosyl (N–O)⁻.²⁵ In the spectra of Yb₂O₃–RGO nanocomposites, the band around 620 cm⁻¹ can be assigned to the Yb–O vibration of the strong interaction between Yb₂O₃ nanoparticles and RGO which can describe anchoring of nanoparticles on RGO.

XPS. Both of chemical composition and bonding configuration of the YbRGO1 nanocomposite and results from Yb 4p, and C 1s core levels of the above sample were examined using XPS (Fig. 4). The spectrum in Fig. 4a shows the peaks of Yb (4s, 4p_3/2, 4p_1/2, 4d, 5s and 5p), O 1s, and C 1s that revealing the composites consist of RGO (C) and Yb₂O₃ (Yb). Also, in Fig. 4b, the Yb 4d_5/2 peak is centered at 199.9 eV and Yb 4d_3/2 peak at 185.4 eV, which are related the characteristics of Yb³⁺.³⁹ The Yb 4d spectrum is rather complex and cannot be fitted using standard spin–orbit splitting parameters. We note that the 4d spectrum is even more complex and, therefore, cannot be readily deconvoluted. The complexity of these spectra results from plasmon loss structures.⁴⁰ Nevertheless, the most intense peaks at (199.9 ± 0.5) eV and (185.4 ± 0.5) eV are due to emissions from the 4d_3/2 and 4d_5/2 levels of the Yb atoms, respectively, and those 6–7 eV higher in binding energy than the main peaks are shakeup lines.⁴¹ Fig. 4c shows the deconvoluted C 1s spectra of the YbRGO1 nanocomposite. The peaks at 284.4, 286.6, 288.2, and 289.2 eV were attributed to the C–C (sp² carbon in the graphene basal plane), C–O, C=O, and O–C=O groups, respectively.⁴² In the XPS spectra, the full width at half maximum (FWHM) of an element reflects the multiplicity of molecular adsorption states.

Fig. 4 XPS spectra of YbRGO1 nanocomposite.

Electrochemical studies

CV and specific capacitances. We investigated the supercapacitive behaviors of the as-prepared RGO, Yb₂O₃, and Yb₂O₃–RGO nanocomposites electrodes by means of a three-electrode system in 0.5 M Na₂SO₄. We did these measurements under ambient conditions in the optimized potential range of −0.9 V to 0.1 V (vs. Ag/AgCl) at various scan rates. We used CV as a potentiodynamic electrochemical technique for calculating the specific capacitances of the electrodes as well as the current responses of the electroactive materials under applied potentials. The CV results of the Yb₂O₃ and YbRGO electrodes are shown in Fig. 5.

Fig. 5 (a) CV curves of Yb₂O₃ and YbRGO at a scan rate of 50 mV s⁻¹, (b) CV curves of the YbRGO electrodes with various RGO contents at scan rate of 50 mV s⁻¹, (c) CV curves of YbRGO with different scan rates (10–200 mV s⁻¹), and (d) variation of specific capacitance as a function of scan rate.

The specific capacitance (SC) of the electrodes can be calculated from cyclic voltammograms according to the following equation.⁴³

where dV is the voltage difference, ν is the scan rate (V s⁻¹), m is the mass of the electrode material, V₁ is the initial voltage (V), and V₂ is the final voltage. Fig. 5a shows the CV curves of the RGO, Yb₂O₃, and YbRGO1 in 0.5 M Na₂SO₄ electrolyte, at a scan rate of 50 mV s⁻¹. The CV curve of the YbRGO1 electrode shows approximate mirror images, respect to the zero-current line. In fact, observing a rapid current response to voltage reversal at the end of each potential indicates the ideal pseudocapacitive behavior of the constructed composites. SC values of RGO, Yb₂O₃, and YbRGO1 electrodes were 75 F g⁻¹, 66 F g⁻¹ and 147 F g⁻¹ at the scan rate of 50 mV s⁻¹, respectively. The obtained SC of the YbRGO1 electrode is 96% higher than that of the pure RGO. Such enhancement in SC can be attributed not only to the synergistic effects of each pristine component in the composite but also to the existence of smaller particles on the electrode surface. The later one provides a high specific surface area which cause an increase in the number of available active sites and thus boost the amount of energy that may store in the supercapacitor electrodes.⁴⁴ Fig. 5b illustrates CV curves of YbRGO electrodes with various mass ratios at scan rate of 50 mV s⁻¹. In the case of the YbRGO1 electrode, the area of the CV loop in is, significantly, larger than that of the other YbRGO electrodes.

Fig. 5c shows the CV curves of the YbRGO1 electrodes at various scan rates (10–200 mV s⁻¹). These curves are quasi-rectangular showing ideal pseudocapacitance behavior and fast charging/discharging process characteristic. Fig. 5d illustrates the change in the SC as a function of the scan rates for the YbRGO electrode with various mass ratios. As shown, SC of YbRGO1 decreases from 240 to 108 F g⁻¹ with scan rate, from 2 to 200 mV s⁻¹. Actually, at low scan rates, the ions of the electrolyte (Na⁺ or H⁺) have enough time to enter into material's pores. This provides more available surface for effective redox reactions rather than that at high scan rates, which only the outer surface of the material is available. So, increasing the scan rate, caused a decrease in the total SC.

CCV. The cycling stability of RGO and YbRGO electrodes with various mass ratios at the scan rate of 200 mV s⁻¹ for 4000 cycles was carried out by CCV technique.^6,31 As shown in Fig. 6, among YbRGO electrodes with different mass ratios, YbRGO1 exhibited exceptionally high cyclic stability. The SC of YbRGO1 electrode decreased about 3.3% after 3000 cycles and then kept a stable value and finally showed 3.5% decrease in SC after 4000 cycles. The SC of YbRGO2 electrode also showed slight increase after 100 cycles and then decreased to 93.5% retention after 4000 cycles. The increase in the SC during the first 100 cycles may be due to the electrode activation which can increase the number of available active sites and improves electrolyte accessibility. Also RGO and YbRGO3 electrodes showed 3.9% and 3.6% decrease in SC after 4000 cycles. These results prove that the amount of RGO has considerable impact on the cyclic stability of nanocomposite electrodes and it must be optimized to fabricate the RGO with improved cyclic stability. The high stability of RGO-based electrodes at high scan rates suggests that these electrodes are suitable for fast charging applications.

Fig. 6 (a) Variation of the specific capacitance of the RGO and YbRGO electrode with different mass ratios as a function of time measured at 200 mV s⁻¹, and (b) 3D-CCV curves of YbRGO1 electrode measured at 200 mV s⁻¹.

Charge/discharge. As a dependable technique, we carried out galvanostatic charge/discharge analyse to evaluate the supercapacitive performance of the electrodes under controlled conditions. We used a two-electrode system and applied a potential window of −0.9 to 0.1 V (vs. Ag/AgCl) to provide complementary measurements. Fig. 7a shows the charge/discharge curves of RGO and YbRGO electrodes at a current density of 1.0 A g⁻¹. As can be seen, the curves' shapes are triangular, linear, symmetric and very sharp. The equal durations of charging and discharging for each electrode imply a reversible behavior, a high columbic efficiency, and an ideal capacitor performance. We also calculated the SC from charge/discharge data through the following equation:
where I is the charge/discharge current (A), Δt is the discharge time (s), and V is the potential drop during discharge (V).

Fig. 7 Charge/discharge curves of RGO and YbRGO electrodes with different RGO contents at charge/discharge current density of 1.0 A g⁻¹ (a), and charge/discharge curves of the YbRGO1 electrodes at different current densities (1–16 A g⁻¹) (b), and the Ragone plot (power density vs. energy density) of YbRGO electrodes with different RGO contents (the energy and power densities were derived from the charge/discharge curves at various current densities) (c).

The YbRGO1 electrode shows the largest specific capacitance, 193 F g⁻¹ when the current density is 1 A g⁻¹. Also, there is a less of a decrease in specific capacitance with increasing current density for the YbRGO1 electrode than other composite electrodes. This indicates the higher accessible surface area in YbRGO1 electrode. This specific capacitance is in good agreement with CV data.

Fig. 7b shows the charge/discharge curves of YbRGO1 electrode in the potential region of −0.9 to 0.1 V at different current densities, ranging from 1 to 16 A g⁻¹. Equilateral triangle shapes for all the curves suggest an ideal capacitive behavior and a good reversibility during the charge/discharge processes. We attribute this capacitive behavior to the following features: (1) the valuable electrical conductivity of RGO caused a reduction in the internal resistance of Yb₂O₃ nanoparticles, (2) fast redox reactions due to facilitated charge transport from Yb₂O₃ as a function of uniform distribution of Yb₂O₃ nanoparticles on the RGO sheets.⁴⁵

The better power performance of the YbRGO1 electrode over other composite electrodes has been further verified from the Ragone plot (power density versus energy density) in Fig. 7c. The energy and power densities were derived from charge/discharge analysis at the various current densities. The maximum energy density obtained for the YbRGO1 electrode with a value of 26.8 W h kg⁻¹ at the power density of 500 W kg⁻¹ which is higher than other composite electrodes. This value is considerably higher than other same electrodes in aqueous electrolyte.^46–48 Due to value of the energy density and power density, it could be stated that YbRGO electrodes are suitable materials for supercapacitor electrodes.

EIS studies. In order to compare the electrode materials' internal resistance and the resistance between electrode and electrolyte, we employed the EIS. For RGO and YbRGO electrodes, the EIS was measured at −0.4 V vs. Ag/AgCl in the range of 0.1–10⁶ Hz (Fig. 8). The charge transfer between electrode and electrolyte causes a semicircle at the high frequency region in Nyquist plots which is followed by a tail in low frequency due to the diffusion of the ions into the electrode. In other words, the electrochemical reaction impedance affects the size of a semicircle; the smaller semicircle radius, the smaller charge transfer resistance.⁴⁹ We analyzed the obtained EIS curves by using complex nonlinear least square (CNLS) fitting method⁵⁰ based on the equivalent circuit given in Fig. 8 (inset). As shown, the equivalent circuit includes five elements: R_s, R_ct, C_dl, Z_W, and C_F. By definition, R_s, here, is the internal resistance (including the intrinsic resistance of active material, the resistance of the bulk electrolyte solution, and the ionic resistance of the electrolyte at the interface between current collector and electrode).⁵¹ R_ct is an electrode/electrolyte interfacial charge transfer resistance. C_dl represents the electrical double layer capacitance at the interface between electrode and electrolyte while C_F describes the pseudocapacitance accounted for the faradic reaction.

Fig. 8 Nyquist plots of the impedance spectra experimentally measured on YbRGO electrodes with different ratios measured at an applied potential of −0.4 V (versus Ag/AgCl).

Finally, Z_W is Warburg resistance that shows the frequency dependency of ion diffusion/transport to the surface of the electrode.⁵² The obtained values of equivalent circuit are presented in Table 2. As illustrated in Table 2, both of R_s and R_ct of the YbRGO1 electrode are smaller than those of other composite electrodes.

Table 2 Calculated values of R_s, C_dl, R_ct, Z_W, and C_F by CNLS fitting of the experimental impedance spectra based on the proposed equivalent circuit

	Rs (ohm)	Rct (ohm)	Cdl (mF)	CF (mF)	ZW
YbRGO1	2.2	8.1	3.0	115	101
YbRGO2	2.7	8.5	2.1	83	56
YbRGO3	2.9	10.6	3.2	65	51

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The low value of the R_ct for YbRGO1 electrode indicates that the electrochemical reaction at the interface between electrode and electrolyte is more facile for this electrode material. In general, the YbRGO1 electrode shows better electrochemical performance than those of other composite electrodes. Furthermore, the YbRGO1 electrode shows a more ideal Warburg resistance; where a more vertical line indicates that the electrode is closer to an ideal capacitor.⁵³ Indeed, the EIS data prove that the synergistic effect due to the combination of Yb₂O₃ and RGO sheets reached to the maximum in the YbRGO1 composite material. This data is consistent with CVs and chronopotentiograms data.

Conclusions

In summary, we prepared ytterbia/RGO nanocomposites by a novel and simple sonochemical procedure. The addition of RGO improved the conductivity through a decline in the ionic mass-transfer resistance. Consequently, we could expect an increase in the power as well as the energy density of the electrode. The results showed that the YbRGO electrodes are better than pure RGO and Yb₂O₃ electrodes for the SC, stability, and energy density. Also, we found that the YbRGO1 electrode with the 2 : 1 mass proportion of Yb₂O₃ to RGO has the best composition for the supercapacitive performance. The SC of this electrode was approximately 240 F g⁻¹ at the scan rate of 2 mV s⁻¹; higher than that of pure RGO and Yb₂O₃ electrodes. The stability of YbRGO electrodes was studied by the CCV technique at high scan rate (200 MV s⁻¹). The CCV study exhibited that the SC of the YbRGO1 electrode retained approximately 96.5% of its initial value after 4000 cycles. The EIS and galvanostatic charge/discharge studies confirm the CV studies. Based on these results, we suggest the YbRGO nanocomposites as a promising material to improve high performance supercapacitor electrodes. Furthermore, we hope to extend the present synthetic strategy to other metal oxides and carbon material such as CNTs and mesoporous carbons in order to improve materials for supercapacitor technology.

Acknowledgements

The authors would like to thank the University of Tehran (6102027) for financial support of this work as grants.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02943d

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