Introduction

In recent years, transition metal oxide nanomaterials have gained great attention in various promising technological applications due to their fascinating electrical,optical, and chemical properties. Consequently, these metal oxides are extensively used in various fields such as solar cells, gas sensors, Li-ion batteries, hydrogen evolution reactions and supercapacitor electrode applications [1, 2]. Supercapacitors, specifically, have gained more attention among researchers due to their application in power devices. It has unique advantages including high specific power, long cycle life, fast charge/discharge rate and superior reversibility [3, 4]. Its applications are not just limited to electrical vehicles, portable electronics, energy management, industrial energy and so on [5]. Based on the charge storage mechanism, it has been classified into two categories: electric double layer capacitor (EDLC) and pseudocapacitor. Generally, the EDLC delivers low capacitance, whereas pseudocapacitor electrodes achieve higher specific capacitance due to their fast reversible Faradic redox reactions [3, 6, 7].

Among various metal oxides considered for SCs electrode materials, RuO2 possesses superior capacitance, but owing to its toxicity and high cost, researchers are now focusing on alternative, economic and less toxic supercapacitive electrode materials with better specific capacity. Due to this, various alternative metal oxides such as Fe3O4, CoO3, MnO2, NiO, WO3 and SnO2 have been widely investigated for pseudocapacitor electrode applications [8, 9]. From the aforesaid metal oxides, SnO2 has gained considerable attention due to its unique electro-optical properties and wide bandgap in nature. Also, it has been considered a co-material with RuO2 for pseudocapacitor electrode applications due to its chemical stability, non-toxicity, high conductivity and inexpensive material [10, 11]. Nevertheless, the registered specific capacitance values of pure SnO2 is still low due to its poor transportation property of electrolytic ions within the SnO2 matrix and its low electrical conductivity. To overcome this issue, different strategies were adopted to improve the capacitive properties, such as size reduction, structural doping and carbon coating etc., [12,13,14]. The 2-D materials of graphene consist of single-layer carbon atoms organised in a honeycomb lattice structure. Its other two derivative structures are Graphene Oxide (GO) and reduced Graphene oxide (rGO). In particular, rGO is a promising material for various application possibly because of its interesting properties such as good conductivity, a large surface area and the presence of functional groups on its surface [15, 16]. Henceforth, rGO has gained more attention among researchers for the synthesis of nanocomposites with metal oxide nanoparticles. Also, various synthesis methods have been adopted to prepare nanocomposite materials for numerous practical applications. Among these methods, the hydrothermal process is more popular than any other nanocomposite synthesis process due to its advantages in obtaining smaller particles, different structures, crystallinity, simple and high purity of the resultant product [17].

In this report, a simple hydrothermal method has been adopted to prepare the nanocomposite of SnO2/rGO. Further, the structural, surface morphological and optical characterization studies of the synthesised samples were characterised using XRD, FE-SEM, HR-TEM, UV-vis and Raman spectroscopy. Furthermore, the capacitive performance and cyclic stability of the resultant electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge studies and impedance spectra (EIS) using electrochemical workstation.

Experimental Details

Hydrothermal Synthesis of SnO2 and SnO2/rGO

All analytical grade chemicals were purchased and used without any further purification. For the synthesis of SnO2/rGO composite, 0.3 g of stannous chloride dihydrate (SnCl2 2H2O, SRL, India) was dissolved in 75 ml of ethanol and the mixture was stirred for 30 min to get a transparent solution. Simultaneously, 50 mg of rGO (Shilpent, India) was dispersed in 20 ml of ethanol in a separate beaker and stirred for 30 min until a homogeneous mixture was obtained. Further, 0.1 gram of hexamethylene-tetramine (C6H12N4, Sigma Aldrich) was added to the above mixture. The obtained solution was then transferred into a Teflon container and sealed inside a stainless steel autoclave. To initiate the hydrothermal reaction, the autoclave was maintained at a temperature of 170 ℃ for 16 h, followed by cooling to room temperature. The sample was then rinsed with deionised water and ethanol several times and the final product was collected by centrifugation. For the synthesis of SnO2, the above procedure was carried out without the addition of rGO. The final product was annealed at 500 ℃ for an hour for further characterization and electrode preparation for electrochemical studies.

Material Characterization

The crystalline phase of the sample was identified using the Malvern PANalytical Empyrean diffractometer with Cu-Kα radiation with a wavelength of \(\lambda =1.5418 \text{\AA }\). The elemental composition and surface morphology was examined by FESEM (ZEISS SmartSEM, Germany) and high-resolution TEM (JEOL JEM 2010 running at 200 kV). The optical properties of the samples were measured by using UV-vis spectrometer (Drawell-DU-8200,China) and a Confocal Raman microscope (WITec alpha 300 RA, Ulm, Germany) using a green laser (excitation operating at 532 nm).

Electrochemical Measurements

All electrochemical measurements were conducted at 3 M KOH electrolyte solution using a GAMRY (Interface1010E, USA) electrochemical work station at room temperature. The working electrode was obtained by pressing the consistent slurry of prepared active material, porous carbon and PVDF in a mixing weight ratio of 80:10:10 respectively, in the NMP solvent and onto a cleaned Ni foam. The electrodes were dried at 80 ℃ for 15 h and further used as the working electrode. For a conventional three-electrode test, platinum foil served as the counter electrode and Ag/AgCl as the reference electrode. The electrochemical behaviour and specific capacitance of the resultant electrodes were established by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements.

Structural, Morphological and Optical Studies

Fig. 1
figure 1

X-ray diffraction patterns of SnO2 and SnO2/rGO nanocomposite

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The XRD measurement was conducted to examine the crystallinity and phase structure of the hydrothermally synthesized samples. Figure 1 shows the typical XRD patterns of SnO2 and SnO2 nanocomposite calcined at 500 °C for an hour in an open atmosphere. The diffraction peaks at 26.6, 33.8, 37.9, 38.9, 42.6, 51.7, 54.7, 57.8, 61.9, 64.6, 65.8, 71.3 and 78.7ºcorresponds to the reflection planes of (110), (101), (200), (111), (210), (211), (220), (002), (310), (112), (301), (202) and (321), respectively. The reflection planes well matched with the JCPDS card no: 88–0287 and were found to be the tetragonal crystal structure of SnO2. The absence of other impurity peaks in the spectrum confirms the high purity of the synthesized SnO2. The average crystallite size was calculated by using Sherer’s formula and was found to be 16 nm for SnO2, whereas 9 nm for the SnO2/rGO nanocomposite. The disappearance of rGO peaks in the XRD spectrum might be due to destroyed regular stacks, limited amounts of rGO in the SnO2/rGO nanocomposites, and the crystallinity of SnO2 [18,19,20]. However, it is clearly evident that the broadening of peaks for the SnO2/rGO composite indicates a smaller size of the particle. This might be due to the inclusion of rGO sheets, which reduces the availability of SnO2 precursors [21].

The surface morphological and elemental features of the synthesized materials were carried out by FE-SEM and EDX measurements. Figure 2 shows the FE-SEM image of SnO2 and SnO2/rGO composite with different magnifications. The SnO2 particles have a high degree of agglomeration with the majority of them in a spherical shape. The agglomeration of SnO2 particles might have been triggered by mutual interactions between the neighboring particles due to certain forces like Vander-waals forces, electrostatic forces and capillary forces [22]. The distribution arrangement of nanoparticles and size particularly depends upon the relative rates of nucleation and growth process, as well as the extent of agglomeration. Further, the chemical composition of the synthesized samples was investigated by Energy Dispersive X-ray spectrum (EDX). Figure 3 (a) and (b) show the EDX spectrum of the prepared SnO2 and SnO2/rGO composite. The presence of tin and oxygen elements are well confirmed by the EDX spectrum. From the EDX spectrum results, the atomic percentage of oxygen component is around 67.37%, almost double the percentage of the tin component (32.43%), thus confirming the formation of SnO2. The existence of carbon content in the EDX spectrum indicates the presence of rGO in the nanocomposite (Fig. 3(b)).

Fig. 2
figure 2

FE-SEM images of SnO2 (a and b) and SnO2/rGO nanocomposite (c and d) with different magnifications

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Fig. 3
figure 3

(a) EDX spectrum of SnO2 nanoparticle and (b) SnO2/rGO nanocomposite

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TEM measurement is a powerful technique to study material properties such as shape, size and structure of the particles. Figure 4 (a) and (b) show lower magnification TEM images of the synthesized SnO2 with different magnifications, which confirms as-prepared SnO2 particles have a quasi-spherical morphology consisting of clusters with a diameter range of around 12–19 nm. Figure 4(c) depicts HR-TEM image of the SnO2 nanoparticles possessing lattice fringes with a spacing of 0.36 nm, which agrees with the interplanar distance of \(\left(110\right)\) planes of cassiterite tetragonal SnO2.

Fig. 4
figure 4

(a-c) shows TEM images with different magnifications and HR-TEM images of SnO2.

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The UV–visible absorption spectroscopy is an effective tool to determine the optical property of nanomaterials. Figure 5 shows the absorption spectrum (inset) and tauc’s plot of SnO2 and SnO2/rGO nanocomposite. The nature of electronic transition from the absorption of photon energy is determined by using the relation [23, 24],

$$\alpha =\frac{A{(h\nu -{E}_{g})}^{n}}{h\nu }$$

Where, α is the absorption coefficient, A is an energy independent constant, n is a constant whose value depends upon the transition nature, and Eg is the energy band gap of the sample. The optical energy band gap is refered to the minimum energy required to excite an electron from the valance band to the conduction band by an allowed optical transition. The optical band gap energy is calculated by extrapolating the plot between \({\left(\alpha h\upsilon \right)}^{2}\)and \(h\nu\). The Tauc’s plot of the samples is shown in Fig. 5.

Fig. 5
figure 5

UV-vis absorption spectrum (Inset) and Tauc’s plot of SnO2 and SnO2/rGO nanocomposite

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The optical band gap of the synthesised tin oxide nanoparticle was found to be 3.1 eV, whereas 2.9 eV for SnO2/rGO nanocomposite. In general, based on quantum confinement effect, the bandgap value increases from bulk, as size of the particle approaches the Bohr exciton radius, which is around 2.4 nm for SnO2. However, in this case, the redshift from the bulk might be due to the formation of defects during the synthesis process [25].

Fig. 6
figure 6

shows the room temperature Raman spectrum of SnO2 and SnO2/rGO nanocomposite

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Raman analysis is an important tool to study the oxygen vacancies, crystallinity, stacking defects and size effects of the nanomaterials. SnO2 has a tetragonal rutile structure with 18 vibrational modes from six-unit cell atoms. The Raman optic vibrational modes of SnO2 are given by \({\Gamma }={{\Gamma }}_{1}^{+}({A}_{1g})+{{\Gamma }}_{2}^{+}({A}_{2g})+{{\Gamma }}_{3}^{+}({B}_{1g})+{{\Gamma }}_{4}^{+}({B}_{2g})+{{\Gamma }}_{5}^{-}({E}_{g})\)+\({{\Gamma }}_{1}^{-}({A}_{2u})+2{{\Gamma }}_{4}^{-}({B}_{1u})+3{{\Gamma }}_{5}^{+}({E}_{u})\). Among these modes, the vibrations of \({A}_{1g}\), \({B}_{1g}\), \({B}_{2g}\)and \({E}_{g}\) represents the Raman active modes while \({A}_{2u}\) and \(3{E}_{u}\) modes arise from infrared active region. The other two acoustic modes of \({A}_{2g}\) and \({B}_{1u}\) are inactive modes of SnO2 [26, 27]. Figure 6 shows the room temperature Raman spectrum of the SnO2 and SnO2/rGO nanocomposite. The peaks at 487, 635 and 776 cm–1 are assigned to\({E}_{g}\), \({A}_{1g}\) and \({B}_{2g}\) modes, respectively, which confirms the tetragonal structure of SnO2 [28]. The low intense peaks at 1337 and 1587 cm–1 for the SnO2/rGO nanocomosite represent the D and G bands of rGO.

Electrochemical Studies

Fig. 7
figure 7

(a) Cyclic voltammetry curves of SnO2 and (b) SnO2/rGO nanocomposite at different scan rates in 3 M KOH.

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The electrochemical properties of the synthesized SnO2 and SnO2/rGO nanocomposite samples were measured by using 3 M KOH aqueous electrolyte solution in a three-electrode system. The capacitive behavior of electrode materials is commonly investigated by cyclic voltammetry measurements [29, 30]. Figure 7 (a and b) shows the cyclic voltammetry results of SnO2 and SnO2/rGO samples with a voltage window of -0.2 to 0.4 V. The CV curves of SnO2 and SnO2/rGO electrodes possess predominant redox peaks, signifying the existence of reversible faradaic reactions. Further, the anodic and cathodic peak currents increase with an increase in the scan rate from 2 to 150 mV s–1, representing the good reversibility of the prepared electrodes. In addition, the CV curves are nearly symmetrical even with the scan rate increasing up to 150 mV s–1, confirming the excellent capacitive behavior and rate capability. Also, the anodic and cathodic peaks are correlated with the intercalation/deintercalation of electrolytic ions in the SnO2 matrix. The possible redox mechanism on the surface of the SnO2 nanostructure is given by the following relation [31].

$$SnO2+{M}^{+}+{e}^{-}\leftrightarrow SnOOM$$

Where, \({M}^{+}\)is the proton or electrolyte ion. The specific capacitances of the SnO2 and SnO2/rGO electrodes were evaluated using CV curves. The capacitance values registered for SnO2 and SnO2/rGO nanocomposite are 330, 265, 232, 201, 182, 169, 156, 146 F g–1 and 446, 414, 403, 380, 366, 334, 298, 254 F g–1, respectively, for the scan rates of 2, 5, 10, 30, 60, 90, 120 and 150 mV s–1. The measured capacitance for the SnO2/rGO electrode is higher than the SnO2 electrode, which might be due to the inclusion of rGO into SnO2 nanostructures. It is believed that the rGO provides a better pathway for ion intercalation and also acts as a conductivity enhancer in the composite electrode. The decreasing specific capacitance with increasing scan rate is possibly attributed to the electrolytic ions approaching only at the outer surface of the active materials and at lower scan rates, the electrolytic ions freely diffuse into the inner surface of the active materials in all available pores, resulting in an adequate insertion reaction to get a high specific capacitance [32].

Figure 8 (a) and (b) show typical galvanostatic charge/discharge curves of SnO2 and SnO2/rGO electrodes at constant current density. The shape of the charge/discharge curves of both electrodes are asymmetric in nature, which confirms the typical pseudocapacitive behaviour [33]. Consequently, the registered specific capacitance was mainly governed by the faradic charge-transfer reaction in both electrodes. Also, the iR drop in the charge/discharge curve for SnO2 is higher than the SnO2/rGO nanocomposite, which indicates higher capacitance with lower resistance [34].

Fig. 8
figure 8

Galvanostatic charge/discharge curves of (a) SnO2 and (b) SnO2/rGO nanocomposite at different current densities

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Further, to confirm the specific capacitance values of the electrodes, charge/discharge curves were calculated from the relation as below [35,36,37,38,39]:

$$\text{C}=\frac{\text{I}\times \varDelta \text{t}}{\text{m}\times \varDelta \text{V}}$$

Where, I is the discharge current, ∆t is the discharge time, m is the mass of the active material and ∆V is the potential range. Figure 9 (a) and (b) shows the specific capacitance values by charge/discharge curves and retention stability test of SnO2 and SnO2/rGO nanocomposite at different current densities. The calculated specific capacitance value of SnO2 nanoparticle was 267 F g–1 at the current density of 0.95 A g–1, whereas rGO based nanocomposite was at 346 F g–1. The contribution boosted up the capacitive performance of rGO based nanocomposite electrode, due to the synergistic effect of faradaic and non-faradaic reactions, higher conductivity and higher surface area of the rGO sheets, which helps the proper penetration of electrolytic ions deeply into the nanocomposite matrix. The retention stability test was conducted for both electrodes for 3000 cycles at a current density of 5.71 F g–1 as shown in Fig. 9 (b). The SnO2/rGO composite electrode retains 92% of capacitance after 3000 cycles, whereas SnO2 maintained only 86% from the initial capacitance. Table 1 lists the findings of this study in comparison to reports in the literature on similar SnO2 and SnO2/rGO-based systems.

Fig. 9
figure 9

(a) Specific capacitance values at different current densities of SnO2 and SnO2/rGO nanocomposite, and (b) retention stability test at 5.71 F g–1

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Table 1 A comparison of the SnO2 and SnO2/rGO-based electrode materials for supercapacitor applications reported in the literature
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Figure 10 depicts the Nyquist plots of SnO2 and SnO2/rGO composite in aqueous 3 M KOH. In the figure, the high frequency region contains a depressed semicircle attributed to the double layer phenomenon of coated material KOH and the low frequency region consists of a slanted line, which might be due to the supercapacitance of the coated materials. Further, the experimentally obtained data are well fitted with the proposed circuit as shown in the inset figure. The calculated solution resistance (R1) of both SnO2 and SnO2/rGO are about 11.5 Ohm and the charge transfer resistances (R2) of SnO2 and SnO2/rGO are 2 and 2.5 Ohm, respectively. The deviation in charge transfer resistance is attributed to the hydrophobicity of rGO. The obtained adsorption resistances (R3) of SnO2 and SnO2/rGO are calculated to be 2350 and 1988 Ohm/cm2. It is noticed that the adsorption resistance of SnO2/rGO is slightly reduced when compared with SnO2, which might be due to the good adsorptive and electrical conductivity of rGO [46]. Further, the calculated adsorption capacitance (Q3) (supercapacitance) of SnO2 and SnO2/rGO are 0.00125 and 0.00162 Fcm–2,respectively. The result suggested that the SnO2/rGO composite exhibits better supercapacitance behavior than that of bare SnO2, probably due to the addition of rGO to SnO2 NPs.

Fig. 10
figure 10

Nyquist plots of SnO2 and SnO2/rGO electrodes and the equivalent circuit of electrode materials (inset diagram)

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The two most important parameters that determine the operational performance and efficiency of electrochemical supercapacitors are the energy density (ED) and power density (PD). To investigate the aforesaid properties, a symmetrical device is made for the SnO2/rGO composite electrode, which has shown improved capacitive performance in the three-electrode system. SnO2/rGO was used as the positive and negative electrodes, polyethylene served as the separator, and stainless steel served as the current collectors in the symmetrical device. Figure 11 (a–c) displays the CV curves, charge/discharge data, and capacitance values derived from the charge/discharge curves for symmetric devices between − 0.4 and 1.0 V in a 3 M KOH electrolyte. The ED and PD values were calculated by using the following Eqs. [47, 48]:

$$ED=\frac{0.5\times {C}_{s}\times \left({V}_{max}^{2}-{V}_{min}^{2}\right)}{3.6}$$
$$PD=\frac{ED\times 3600}{\varDelta t}$$

where C is the capacitance, Vmax and Vmin, respectively, are the maximum and minimum potentials during charge and discharge processes, and\(\varDelta t\) is the discharge time. The Ragone plot for symmetrical SnO2/rGO//SnO2/rGO devices is shown in Fig. 11(d). The energy density values of the SnO2/rGO electrode are evaluated as 22, 18.5, 13.6, 11.4, 7.7, and 5.6 Wh kg− 1 while the power density is found as 97.2, 194.4, 388.9, 583.3, 777.8, and 972.2 W kg− 1 at current densities of 0.5,1, 2, 3, 4 and 5 A g− 1, respectively. These experimental results also show that the SnO2/rGO nanocomposite has good supercapacitive behavior.

Fig. 11
figure 11

CV results of the (a) SnO2/rGO//SnO2/rGO symmetrical device at different scan rates (the inset figure shows a Swagelok cell, SnO2/rGO coin electrodes and PE separator) and (b) GCD plot of a symmetrical device at different current densities; (c) specific capacitance as a function of current density; and (d) energy density vs. power density (Ragone plot)

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Conclusion

We have successfully synthesized SnO2 and SnO2/rGO nanocomposite via a simple hydrothermal method for pseudocapacitor electrode applications. The phase and elemental analysis of the synthesized samples were confirmed by XRD and EDX measurements, respectively. The redshift of the band gap energy from bulk SnO2 might be due to the formation of defects during the synthesis process. To evaluate the electrochemical performance of the fabricated electrodes, cyclic voltammetry, charge/discharge and EIS tests were conducted. It was found that SnO2/rGO nanocomposite electrode showed an excellent specific capacitance of 446 F g–1 at the scan rate of 2 mV s–1 whereas SnO2 was at 330 F g–1. EIS results showed that rGO based nanocomposite electrode possess lower resistance as compared to SnO2 electrode. Based on the electrochemical performance, SnO2/rGO nanocomposite electrode proved as a potential electrode material in the field of energy storage devices.