Porous materials of nitrogen doped graphene oxide@SnO2 electrode for capable supercapacitor application

The porous materials of SnO2@NGO composite was synthesized by thermal reduction process at 550 °C in presence ammonia and urea as catalyst. In this process, the higher electrostatic attraction between the SnO2@NGO nanoparticles were anchored via thermal reduction reaction. These synthesized SnO2@ NGO composites were confirmed by Raman, XRD, XPS, HR-TEM, and EDX results. The SnO2 nanoparticles were anchored in the NGO composite in the controlled nanometer scale proved by FE-TEM and BET analysis. The SnO2@NGO composite was used to study the electrochemical properties of CV, GCD, and EIS analysis for supercapacitor application. The electrochemical properties of SnO2@NGO exhibited the specific capacitance (~378 F/g at a current density of 4 A/g) and increasing the cycle stability up to 5000 cycles. Therefore, the electrochemical results of SnO2@NGO composite could be promising for high-performance supercapacitor applications.

carbon composite was synthesized by hydrothermal process for electrochemical supercapacitor, sensors, and solar cells [25][26][27][28][29] . The SnO 2 and RuO 2 mixtures are used in the electrode materials for storage properties with an excellent cyclic stability 30 . The SnO 2 /graphene composite reported the increasing electrochemical performance and cyclic stability via microwave synthesis 31 . The construction of Ni/SnO 2 composite shows an excellent capacitance and cyclic retention reported in the literature 32 . The hierarchical SnO 2 composite displays the specific capacitance of ~188 F/g with 2000 cycles 33 . In the present study, SnO 2 @NGO composite was synthesized and it's electrochemical properties investigated by CV analysis. The SnO 2 @NGO composite was analyzed by using Raman, XRD, XPS, BET, SEM, EDS, and HR-TEM analysis. Furthermore, the SnO 2 @NGO composite was studied by CV, GCD, and EIS techniques with 6 M KOH electrolyte.

Results and Discussion
Structural and surface morphology of Sno 2 @nGo. The schematic illustration of SnO 2 @NGO synthesis via thermal reduction process is depicted in Fig. 1. The Raman spectral analysis used to study the carbon-based materials and its defect structure. Figure 2(a) represent the Raman results of graphene oxide (GO) and NGO obtained by a thermal reduction reaction. The properties of graphene materials are represented at 1,580 cm −1 to the E 2g peaks of sp 2 C atoms, and D band at 1,350 cm −1 , which was ascribed to the breathing modes of the A 1g symmetry 34 . These peaks provide the information of local defects and disorder behavior of NGO by Raman spectroscopy (Fig. 2a). The Raman peaks represented that the D band at 1358 cm −1 and G bands 1597 cm −1 in the NGO structure. Moreover, the D/G intensity of NGO decreased when comparted to GO composite. The intensity change is may be due to the reduction of the NGO materials by thermal reduction process in the sp 2 carbon structure. The peak position at lower wave numbers represent the different vibration modes of SnO 2 nanoparticles in the NGO composite 35,36 .
The XRD studies of GO indicated the peak at about 2θ = 10.80° with d -spacing 0.89 nm. The typical XRD peak of GO confirmed the well exfoliated carbon sheets in graphite structure has been reported previously [37][38][39] . The peak position at 2θ = (9.84° and 19.7°), corresponds to the (002) and (100) planes of GO materials. This may be due to the thermal reduction of GO to graphitic crystal structure in presence of high temperature. In addition, the diffraction pure SnO 2 was studied in previous reports [40][41][42][43] and SnO 2 @NGO diffraction peaks are represented in Fig. 2 and (202), respectively. The structure of tetragonal confirmed the PDF file no: JCPDS 41-1445. Therefore, the SnO 2 @NGO composite, which might be due to the exfoliation of NGO sheets at 550 °C by thermal reduction process. Figure 3 represent the XPS peaks of SnO 2 @NGO composite. The survey spectrum (Fig. 3d) indicates that C 1s (285), O 1s (532), and Sn3d (487) eV, which complete the effective adornment of SnO 2 nanoparticles onto the NGO surface. The Fig. 3c of Sn3d shows the main peaks of (3d 5/2 ) and 3d 3/2 corresponds to the binding energies of 487.0 eV and 495.5 eV, respectively. The binding energy difference between Sn3d 5/2 and Sn3d 3/2 almost ~8.7 eV. These results confirmed the identical to the binding energies of SnO 2 and compared to the previous reports [44][45][46] .   The SnO 2 @NGO composite was studied by cyclic voltammetric analysis results are shown in Fig. 8 and Table 1. The composite results indicate the ideal capacitive nature showing the rectangular profile owing to outstanding trend for supercapacitor. The trend of CV curves of composite electrodes, rectangular properties and corresponding increase of current than that of pristine SnO 2 . These properties of composite shows the increasing the specific capacitance because of the combined influence from EDLC and pseudo capacitance of the composite. Figure 8a represent the CV results of SnO 2 @NGO composite electrodes with the scanning rate from (10-100) mV s −1 in presence of 6 M KOH aqueous electrolyte. The electrochemical properties were studied the potentials range between (−0.2 and 0.8 V), it can be seen that the increasing the capacitance behaviors between the electrode and electrolyte. The scan rates increases, the current density also increases, because of the anodic and cathodic current change towards the reversible reaction. This fast redox or reversible reactions occurring between the electro-active material/electrolyte interfaces in presence 6 M KOH. Because of the CV represent a slight alteration with certain number of functional groups react in the NGO and SnO 2 nanoparticles. The specific capacitance of SnO 2 @NGO electrodes are intended from the CV curves. The cyclic voltammetry results are shown Fig. 8a. In this CV experiments, the scan rate increased from 10 to 100 mV s −1 and also increases the electrochemical supercapacitor properties. The enhancement of electrochemical properties of the composite is mainly denoted to the more electroactive sites via EDLC and pseudo capacitance arises in the SnO 2 @NGO composite. Further, www.nature.com/scientificreports www.nature.com/scientificreports/ the SnO 2 nanoparticles at the NGO matrix successfully decorated and low internal resistance with high electrical conductivity of the composite for electrochemical reversible reaction 26,[47][48][49] .
Furthermore, the SnO 2 @NGO composite for charge-discharge test from the GCD curves represented in Fig. 8(b). The GCD curves of composite electrodes are increases the current density of 4, 8.5 and 12.6 A/g by using 6 M KOH solution. The GCD results evidently designates the triangular shape of the composite materials and the contribution of EDLC and pseudo capacitance properties from CV analysis. The composite electrodes are confirmed the higher discharge time than that of pristine SnO 2 composite represent the increasing specific capacitance with stronger electrolytes 26,[47][48][49] .
The specific capacitance values are ~378 F/g, 345 F/g, and 230 F/g at the current density of 4, 8.5, and 12.6 A/g, respectively. The Fig. (8c,d) represent the cyclic retention and corresponds to the current density (vs) its specific capacitance values. The trend of the specific capacitance values was decreased, as the current density increased 4, 8.5, and 12.6 A/g. Because of the diffusion of the electrolyte ions, depends on the morphology, surface of the materials and concentration of the SnO 2 and NGO components. The electrochemical properties of the SnO 2 @ NGO nanoparticles was compared to the previous reports for supercapacitor applications [50][51][52] .
In addition, the SnO 2 @NGO composite, energy density and power density results are shown in Fig. 8(e,f). The cyclic stability is the important property of the electrode for practical applications in the supercapacitors.  www.nature.com/scientificreports www.nature.com/scientificreports/ Figure 8(c) shows the GCD analysis at a current density of 4 A/g for 5,000 cycles. After 2,000 cycles, the composite electrode maintained 89% of its original performance, which signifies of electroactive material has better cyclic stability 49,53 and reversibility of the GCD process.
Furthermore, the EIS results were estimated at open circuit potential by relating the various ac voltages in the frequency range from 0.1 Hz to 100 kHz. Figure 8(d) represent the EIS result of Nyquist plots for SnO 2 @ NGO composite, which is indicate that the real and imaginary parts of EIS results, respectively. First, the smaller semi-circular loop at higher frequencies is attributed to the Faradaic reactions in presence of the 6 M KOH electrolyte. The lower frequency region of the EIS increases due to the capacitive nature of SnO 2 @NGO composite. The phase angle of EIS of composite electrode was perceived to be higher than 45° and low frequencies demonstrating the electrochemical capacitive nature of the composite materials. In this regard, the charge transfer resistance (Rct) of the NGO and composite electrodes are ~36 and 38, respectively. In this lower value of Rct represent the stable electrochemical performance. The low-frequency region of the impedance analysis results are called Warburg resistance of diffusion behavior in presence of 6 M KOH with in the electrodes. The vertical slope of Warburg curves specifies the rapid development of an electric double-layer in the composite because of quick ion diffusion in presence of 6 M KOH electrolyte for supercapacitor applications 49,54 . The porous material of SnO 2 @NGO composite synthesized by thermal reduction process and studied their electrochemical characteristics towards high-performance supercapacitors. The Raman, X-ray diffraction, and photoelectron spectroscopy analysis reveals the successful formation of SnO 2 @NGO composite. The specific capacitance of the SnO 2 @NGO composite displayed the capacitance ~378 F/g at a current density of 4 A/g in the 6 M KOH solution. Moreover, the composite electrode exhibited an excellent cycling stability at 4 A/g. The composite electrode maintained 89% of its original performance after 5000 cycles in presence of 6 M KOH electrolyte, there by representing the plausible applicability for energy storage applications. Graphene oxide synthesis (GO). The graphene oxide (GO) materials were developed by Hummer's technique in the previous reports [27][28][29] . The graphite (5 g), H 2 SO 4 (400 mL), H 3 PO 4 (50 mL), and KMnO 4 (18 g) were mixed in the three-neck flask using a magnetic stirrer at 40 °C and continually stirred for 48 h to achieve the complete conversion from graphite. After that, the reaction mixture was changes from dark purple to greenish brown color and the calculate amount 20 mL of H 2 O 2 was added to complete the conversion of GO. The GO was purified by using 1 M of HCl or ethanol and then purified the oven at 80 °C for 12 h. N-Doped graphene oxide synthesis (NGO). The calculated amount of 0.5 g of GO was distributed in the 300 mL of distilled water followed by ultra-sonication and the solution becomes the brownish GO suspension. The GO suspension and required amount of 20 mL of excess of water is added and stirred for 4 h and filtered/dried in the vacuum oven at 90 °C for 4 h. Then the calculated amount of 1 g of urea and ammonia and excess of 20 mL of ammonia was added and continuously stirred at 90 °C for 12 h. Finally, the GO product was dried in the oven at 200 °C for 12 h, and purified by using ethanol solvent. Sno 2 @nGo composite synthesis. Briefly, 0.2 g of GO was distributed in 150 mL of water and sonication for 1 h to become the homogeneous solution. Then the calculated amount of 1.2 g of stannous chloride, 25% of 10 mL ammonia solution was added to the GO solution to maintain the basic medium. Afterwards, the reaction GO solution was refluxed at 200 °C for 8 h by using three neck flask with condenser. Then, the reaction becomes www.nature.com/scientificreports www.nature.com/scientificreports/ changed to black color product of SnO 2 @NGO and dried in the vacuum oven at 180 °C for 12 h. Further, the SnO 2 @NGO sample was calcination at 550 °C for 8 h and collected the product for further characterization.

Materials
preparation of electrochemical analysis. The CV experiment was studied in the regular three-electrode system connected through Autolab PGSTAT302N (Metrohm, Netherlands). The SnO 2 @NGO composite (working electrode), carbon black, and PVDF in the stoichiometric of 75: 15: 10 and dispersed in the n-methyl 2-pyrrolidone. The resulted black paste was then covered onto a nickel wire collector and dried at 110 °C for 12 h. The mass loading of the SnO 2 @NGO composite is around 1.5 mg cm −2 . In this experiment, platinum wire (counter electrode) and Ag/AgCl (reference electrode) were fabricated in the CV analysis. The synthesized SnO 2 @NGO composite and its electrochemical properties were determined by cyclic voltammetry analysis. The CV curves were documented at various scan rates (10,20,40,60,80, and 100) mV s −1 in a potential range of (−0.2 to 0.8) V. The GCD curves were acquired at various current densities (4, 8.5 and 12.6) A/g and EIS results in the frequency range of (0.1 Hz to 100 kHz). The capacitance of SnO 2 @NGO composite was designed from the CV Eq. (1), and GCD curves Eq. (2) were shown in the previous reports [30][31][32][33] .