Introduction

Metal oxides perform a vital role in the fields such as chemistry, material science, energy science, and bio-electronics [1]. Supercapacitors have a high power density (~ 10 kW kg−1), good cycle stability and fast charge and discharge speed, but their energy density is very low (~ 5 Wh kg−1). While lithium-ion batteries have a high energy density (~ 150–210 Wh kg−1), their power density and cycle stability are relatively poor [2]. Hence, the supercapacitor materials may consider the promising energy storage devices with quick charging and discharging capacity when their energy density is improved comparing to other storage devices such as batteries [2]. The energy storage devices like supercapacitors are utilized for quick charge/discharge cycles rather than long-term compact energy storage especially for many automotive machines, classically the combination of conductive polymers and transition metal oxides/hydroxides/sulfides/nitrides are the most excellent materials for faradic charge storage device [3,4,5,6]. The transition metal oxide (TMO)-based electrode materials such as RuO2 [7], MnO2 [8], NiO [9], Co3O4 [10], and Fe2O3 are extensively studied [11]. Similarly, the semiconducting materials like ZnO, SnO2 nanomaterials have been extensively explored for large number of energy storage devices and applications. For example, nanostructured SnO2 and its nanocomposite with different materials are used innumerous applications such as gas sensors [12,13,14,15,16], electrocatalytic materials [17, 18], biosensors [19], and in nonlinear optics applications. Different synthesis mechanisms for oxide and various SnO2 nanostructures have been configured for supercapacitor applications and their electrochemical properties have been studied in detail [20]. Mishra et al. have studied the LPG-sensing properties of Zn-doped and undoped SnO2 nanoparticles, and a comparison have been made to understand the role of dopant in gas sensing applications [21]. Similarly, Zn-doped SnO2 nanorods have been synthesized by adopting the solvothermal method and its ferromagnetic properties were investigated in detail. The study shows that the synthesized nonmagnetic Zn ion-doped SnO2 nanorods exhibit obvious room-temperature ferromagnetism, and their saturated magnetic moment is sensitive to the concentration of Zn dopant. Based on d0 ferromagnetism, the observed ferromagnetic ordering of Sn1 − xZnxO2 nanorods could be related to the doping-induced VSn defects [22]. Mohamed Rashad and his collaborators describe the decomposition of methylene blue on transition metal-doped SnO2 nanoparticles and the detailed studies explained the following points clearly that Zn2+-doped SnO2 (Zn0.3Sn0.7O2) was remarked the highest photo catalytic activity for the MB photo degradation. The improvement in the photo catalytic activity of Zn0.3Sn0.7O2 is endorsed to hetero-junctions of ZnO/SnO2. The effect of doping Ni, Co, and Mn ions on the photo catalytic properties are insignificant in the photo catalytic activity of SnO2. The recycling tests indicated that ZnO/SnO2 was quite stable and there is no decrease in photo catalytic activity after five-time repetition, indicating a promising catalyst for commercial applications [23]. Vignesh and his group demonstrated the photo catalytic performance of Ag-doped SnO2 nanoparticles modified with curcumin in their work. The photocatalytic activity was measured by the degradation of rose Bengal and Ag and curcumin modification enhanced the photocatalytic activity of SnO2. In the same way, Cur–Ag–SnO2 is also showed antifungal activity against Candida alb cans [24]. Another work on thermoluminescence response and dose-linearity of SnO2 nanoparticles have been improved on 1.0% Ag doping, and subsequent thermal annealing. However, a higher Ag content causes the formation of Ag clusters, reducing both the thermoluminescence and photo luminescence responses of the nanoparticles [25]. In addition, Kumar and his co-workers adopted the dip-coating method to prepare ceria electrode material made by uniform doping of silver and platinum nanoparticles for high performance supercapacitors applications [26]. The silver doping revealed 1017 Fg−1 specific capacitance at 1 Ag−1 current density, a fivefold increase, while platinum doping revealed a tenfold increase in specific capacitance, i.e., 1987 Fg−1 at 1 Ag−1 current density [26]. Similarly, the doping of nickel and graphite can increase the electrical conductivity and the specific surface area of nano-amorphous TiO2 [27]. The initial capacity of the composite electrode material TiO2-NiO-C was 13.4 mAh/g, and the value dropped slightly to 12.88 mAh/g after 500 cycles [27]. Moreover, Tung and his co-workers explored the electrode position method to synthesis Fe- and Co-doped manganese oxides composed of nanofiber-like structure with 5–20 nm radius and studied its electrochemical properties [28]. The specific capacitance value of deposited manganese oxide reaches a maximum of 175.3 F/g for Co doping and 244.6 F/g for Fe doping. The obtained thin films retained about 84% of the initial capacity even after 500 cycles of charge–discharge test [28]. Dong and his co-workers discussed clearly about enhanced supercapacitors performance of Mn3O4 nanocrystals by doping transition-metal ions and evaluated the specific capacitance of Cr-doped Mn3O4 nanocrystals which exhibits 272 F g−1 at a current density of 0.5 A g−1 [29].

Even though SnO2-based materials have been used for several related applications in wide spectrum of area, the study on the effect of transition metal doping (Zn and Ag) on SnO2 nanoparticles for different electrochemical supercapacitors applications was rarely studied. Hence, in the present study, we try to understand and establish the impact of Zn and Ag dopants on the SnO2 nanoparticles as electrode for supercapacitor applications. Further, the role of high conductive silver (Ag) dopant and low electrical conductive zinc (Zn) dopant on the SnO2 nanoparticles will be investigated based on their electrochemical properties for pseudocapacitive electrode applications. Hence, in the present study, SnO2, Zn-SnO2, Ag-SnO2 nanostructures have been successfully synthesized and investigated its electrochemical properties as pseudocapacitive electrode applications.

Experimental methods

In this work, we implement hydrothermal synthesis procedure to obtain SnO2 nanoparticles with different molar concentration of precursor. 0.758 g tin chloride (SnCl2) salt is mixed with ethanol. The dissolved precursor slats in a beaker were stirred for 30 min to attain homogeneity of solution. Five millimolars CTAB as surfactant was added to the above solution. The homogenous precursor solution was transferred to 100 ml teflon-coated autoclave and maintained at 160 °C for 12 h. Then, the autoclave is allowed to cool down to room temperature naturally. The obtained powder were cleaned and washed by distilled water and ethanol for three times to remove the untreated salts precursors and impurities. Then, the cleaned products were dried in hot air oven at 80 °C for overnight and named as SF1. The same procedure was followed for other two products by introducing the metal dopant of 2 mM ZnCl2 solution added with tin chloride solution precursor and the final product was named as SJ1. Similarly, 2 mM AgCl2 solution was added with tin chloride solution precursor and final product was named as SK1. The electrochemical properties of obtained product were analyzed by employing CH660D instrument with three electrode cell configuration. Ag/AgCl, platinum wire and SnO2, Ag-SnO2 and Zn-SnO2 coated on Ni-foam were acted as references, counter and working electrodes respectively. The working electrode was prepared by dissolving Ni-Foam in 3 M of HCl for 30 min under sonication to remove the oxide layer in Ni-foam. After that the slurry was prepared by taking active material, activated carbon (AC) and Polyvinylidene fluoride (PVDF) in the ratio of 80:15:5 with n-methly2-pyyrolidone (NMP) as solvent. The mass of active material loaded on the Ni-foam can be estimated by measuring the weight of the Ni-foam before and after coating the active materials. In this work, the measured mass of active materials was about 2 mg. The slurry was coated on to the Ni-foam of area 1 cm × 1 cm and dried at 80 °C in a hot air oven for 12 h.

Results and discussion

The X-ray diffraction method is adopted to understand the crystallinity of obtained nanoparticles. Figure 1a, b, c reveals the crystalline structure formation of synthesized SnO2, Zn-SnO2, and Ag-SnO2 nanoparticles. The observed peaks at 26°, 33.3°, 37.4°, 51.2°, 54.2°, 57.3°, 61.4°, 64.3°, 65.4°, 70.7°, and 78.1° can be associated with (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321) planes matched with JCPDS No:88-0287. The synthesized SnO2 nanoparticles encompass the tetragonal crystal structure belongs to P42/mnm (136) space group with lattice parameter a = 4.737, c = 3.186, volume of unit cell = 71.51. The observed XRD pattern for Zn-SnO2 and Ag-SnO2 shows a slight shift from SnO2 which inhibits the incorporation of dopant ions (Zn and Ag) onto the lattice of SnO2 nanoparticles. The decrease in intensity and shift in the diffraction peak probably may due to the dopant effect of Zn2+ and Ag2+ ions on to the lattice of tetragonal crystal structure of SnO2 nanoparticles [30,31,32]. The Scherer formula for determination of crystalline size was used as follows,\( \boldsymbol{Dp}=\frac{\mathbf{0.94}\ast \boldsymbol{\lambda}}{\boldsymbol{\beta} \boldsymbol{Cos}\boldsymbol{\theta }} \), where Dp is the crystallite size, λ the wavelength of the X-ray, β1 the wavelength of full width half maximum, and θ the diffraction angle [33,34,35,36,37,38,39]. The evidently distinctive peaks suggesting the formation of high-quality crystalline structure with average crystalline size of sample SF1, SJ1, and SK1 as 36.63 nm, 44.39 nm, and 36.63 nm.

Fig. 1
figure 1

XRD pattern a SnO2, b Zn-SnO2, c Ag-SnO2 nanoparticles

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The obtained Raman spectra for the obtained products (SF1, SJ1, SK1) exhibited the similar peaks at 441, 628, and 896 cm−1 which corresponds to the vibrations of Sn–O modes (Fig. 2a, b, c). The observed extra peak at 441 cm−1 exhibited for SJ1 can be attributed to the Zn–O vibration modes and predicts the dopant Zn2+ ions exactly placed on the lattice planes of SnO2 nanoparticles. Correspondingly, the appearance of board peak around the 475 cm−1 may be the contribution of metal oxide bond vibration due to the dopant role of Ag2+ ions and was exactly placed on the lattice plane of SnO2 nanoparticles. The peak observed at 628 and 896 cm−1 can be attributed to the vibration modes of O–Sn–O, especially 628 cm−1 corresponds to A1g modes of Sn–O vibrations and peak at 896 can be attributed to the B2g vibration modes of Sn–O [40,41,42]. The other small peaks at 1110 and 1364 cm−1 may be corresponds to the secondary Raman peaks for SnO2 nanoparticles.

Fig. 2
figure 2

Raman spectra a SnO2, b Zn-SnO2, c Ag-SnO2 nanoparticles

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The vibrational properties of constituent metal oxides and the functional groups present in the synthesized nanomaterials can be investigated by performing InfraRed analysis (FTIR). We have performed Mid-FTIR studies typically range from 400 to 4000 cm−1. This range can be classified into two regions: 400–1000 cm−1 as finger print region and 1000–4000 cm−1 as functional group region. In Fig. 3a, b, c, for all obtained products SF1, SJ1, and SK1, the peaks appeared at 532 and 617 cm−1 falls on the finger print region corresponds to the vibration mode of LO phonon due to E║ to the C-axis [43] and the antisymmetric vibration of Sn–O–Sn mode corresponding to Eu vibration modes of TO phonon [44,45,46] respectively. A small peak observed at 1630 can be readily indexed to the C–O and C=O vibration modes [47].

Fig. 3
figure 3

IR spectra a SnO2, b Zn-SnO2, c Ag-SnO2 nanoparticles

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The SEM micrograph shows the nanoparticles formation of all the three products as shown in the Fig. 4a, b, c. Nevertheless, the experimental parameter conditions are same for all products expect the addition of dopant, the results seem similar in the formation of nanoparticles; it is evident from the Fig. 4a, b, c. All obtained products SF1, SJ1, and SK1 have same uniform distribution of the spherical nanoparticles synthesized at the optimized experimental conditions. The EDS studies displayed in Fig. 5a, b, c confirms the addition of dopant Zn2+ and Ag2+ ions on to the SnO2 nanoparticles. The uniform formation of nanoparticles is facilitating from the CTAB surfactant role in the nucleation mechanism formation which can be rightly evidenced from the SEM micrographs.

Fig. 4
figure 4

SEM images a SnO2, b Zn-SnO2, c Ag-SnO2 nanoparticles [200 nm scale]

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Fig. 5
figure 5

EDS spectra a SnO2, b Zn-SnO2, c Ag-SnO2 nanoparticles

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To understand the electrochemical behavior of the synthesized products cyclic voltammetry (CV), galvanostatic charging–discharging (GCD) studies has been performed to evaluate the capacitance abilities. Figure 6a, b, c shows the cyclic voltammetry curves for SF1, SJ1, and SK1 having similar curves pattern with redox peaks, one in positive potential and another in the negative region of potential window 0–0.6 V. The CV curve for pure SnO2 (SF1) exhibits smaller curve area with uniform increase in the oxidation and reduction voltage as the scan rate increases. Similarly, the shift in the redox peaks are also observed in both the positive and negative region due to the fact that, the high scan rate results the less time to interact OH− with the active material of the working electrode. The same pattern can also be evidenced in all the CV curves of the obtained products. The different scan rate is adopted for the CV studies as 10, 20, 30, 50, 80, 100 mV/s and its corresponding specific capacitance can be estimated from the following expression \( \boldsymbol{C}=\int \frac{\boldsymbol{IdV}}{\boldsymbol{ms}\ \left(\boldsymbol{V}\mathbf{2}-\boldsymbol{V}\mathbf{1}\right)}, \) where C is the specific capacitance of active material, IdV the integral sweep area by CV curve, m the mass of the active material coated on Ni-foam, s the scan rate, and (V2 − V1) the applied potential window of electrolyte. The estimated specific capacitances for all the obtained products (SF1, SJ1, SK1) are tabulated in Table 1. The calculated specific capacitance value reveals that the dopant zinc ions (Zn) and silver ions (Ag) plays a significant role in the capacitance ability of the SnO2 nanoparticles. The role of different dopant Zn2+- and Ag2+-doped onto the lattice of the SnO2 nanoparticles could be easily revealed form the specific capacitance values. The observed results shows that the metal dopant Ag2+ on SnO2 (Ag-SnO2) gives rise to high conductivity than the semiconductor dopant Zn2+ on SnO2 (Zn-SnO2). Though the variation in specific capacitance seems small for the different dopant, the role of the active sites in the working electrode with corresponding dopant has been the cause for capacitive performances.

Fig. 6
figure 6

SnO2 nanoparticles a–c CV curve for SF1, SJ1, and SK1 at different scan rates d EIS spectra for SF1, SJ1, and SK1

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Table 1 Specific capacitance of synthesized SnO2 nanoparticles (SF1, SJ1, and SK1) at different scan rates
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In order to evaluate the capacitive behavior in a detailed manner, generally electroimpedance analysis have been instituted and its character is estimated for all obtained products for the AC frequency range 10 mHz−100 kHz. Figure 6d reveals the EIS spectra for all three SnO2, Zn-SnO2, and Ag-SnO2 nanoparticles and the high conductivity for the Ag-SnO2 nanoparticles can be explained in the terms of the presence of metal ions in the place of SnO2 lattice enhances its capacitive nature. The contact resistance (Rct) arises from the electrode and electrolyte interaction and the solution resistance (Rs) at high frequency of applied AC field has been investigated from the Fig. 6d. The measured Rct values such as 1.23 Ω, 1.06 Ω, and 0.61 Ω for SnO2 (SF1), Zn-SnO2 (SJ1), and Ag-SnO2 (SK1) nanoparticles indicates that SK1 have demonstrated the low contact resistance when compared to SJ1 and SF1 along with a small semi-arc with high spike overtone and revealed the good capacitive nature which reflects in the specific capacitance values. At low frequency range, the Warburg resistance also seems similar trend, in which the SK1 have greater slope than the SJ1 and SF1 which can help to achieve high capacitance.

In addition, the galvanostatic charging and discharging studies have been employed for all three SnO2, Zn-SnO2, and Ag-SnO2 nanoparticles to understand its charging capacity. Fig. 7a, b, c, d shows the GCD studies with symmetry curves and explains the good capacitive nature without any IR drop in the discharging region. This feature enables the SnO2, Zn-SnO2, and Ag-SnO2 nanoparticles as suitable candidate for positive electrode for supercapacitor applications. For comparison, the GCD curves for all products at current density 0.5 A/g is displayed in Fig. 7d. Ag-SnO2 nanoparticles perform well with superior specific capacitance. The specific capacitance of the all products is measured by adopting the following equation \( \mathrm{Cs}=\frac{\boldsymbol{I}\mathbf{\nabla}\boldsymbol{t}}{\boldsymbol{m}\mathbf{\nabla}\boldsymbol{V}} \), where I is the current applied to cathode, ∇t the discharging time, m the mass of the active material, and ∇V the potential window [48]. The evaluated specific capacitance for SnO2, Zn-SnO2, and Ag-SnO2 nanoparticles for different current densities 0.5, 1, 2, 3, 5, and 10 A/g are tabulated in Table 2. It explains the same pattern as cyclic voltammetry studies that, the Ag-SnO2 nanoparticles exhibit the superior specific capacitance presumably due to the presence of metal ions in lattice of tin oxide which enables the access to more active sites than the tin oxide nanoparticles. Moreover, the observed FESEM images of the obtained product clearly revealed that all the three products (SF1, SJ1, SK1) possess same uniform spherical nanoparticles since synthesized at same experimental procedure and conditions. Hence, the variation or an increase in the specific capacitance of Ag-SnO2 was mainly contributed from the dopant Ag2+ and that can be supported by many reports [29, 49]. In charging and discharging profile, all three obtained products have been limited to 0.35 V due to the potential window limitation of synthesized materials. In the CV test, the electrode potential window is 0.6 V, but in GCD measurement, the potential window is reduced to 0.35 V. It is due to fact that in CV studies the different ranges of potential window have been explored for obtaining information on redox reaction process and finally 0–0.6 V window is demonstrated. In GCD studies, initially 0–0.5 V potential window has been explored for SnO2 nanoparticles. However, at low current density (0.5 A/g and 1 A/g), symmetrical GCD curve has not been achieved with low columbic efficiency in charging and discharging process. In order to execute and reveal its symmetrical and high columbic efficiency, the optimized potential window for SnO2 nanoparticles has been opted as 0–0.35 V. Further, as the current density increased as 1, 2, 3, 5, and 10 Ag−1, the GCD curves displays very good symmetrical charging and discharging pattern which results in the good coulomb efficiency of all three obtained products.

Fig. 7
figure 7

GCD studies of SnO2 nanoparticles a–c CV curve for SF1, SJ1, SK1 at different current density d comparative GCD curve for SF1, SJ1, and SK1 at 0.5 A/g

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Table 2 Specific capacitance of synthesized SnO2 nanoparticles (SF1, SJ1, and SK1) at different current density
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The calculated specific capacitance of the synthesized Ag-SnO2 (SK1) nanoparticles can be compared with already reported results as shown in Table 3. Pusawale et al. has adopted an expensive chemical route to synthesis nanocrystalline SnO2 thin films and tested for supercapacitor applications. The observed results revealed the specific capacitance of 66 Fg−1 in 0.5 Na2SO4 electrolytes at 10 mVs−1 scan rate [50]. Liu et al. also demonstrated the hierarchical SnO2 nanostructures synthesized by employing facile hydrothermal process and its electrochemical studies reveals 187.7 Fg−1 specific capacitance at 1 Ag−1 current density and also can be efficiently used as photo catalyst, gas sensing materials [51]. Polyaniline/SnO2 nanocomposite exhibits the superior specific capacitance of 305.3 Fg−1 with a specific energy density of 42.4 Wh kg−1 and a coulombic efficiency of 96%. These results explain the synergistic effect of PANI/SnO2 which complements the properties of both materials [52]. The Ni/SnO2 nanoflowers exhibited enhanced capacitance of 410 mF cm−2 at discharge currents of 1 mAcm−2 and capacity retention keeping 91% when the discharge current density increased from 1 mAcm−2 to 10 mAcm−2 [53]. Moreover, the effective interaction between the metal ions (Ag2+) in the working electrode with OH− ions from electrolyte and quick kinetics of the OH− ions in the form of intercalation and deintercalation between active sites enables the superior capacitive characteristics of the Ag-SnO2 nanoparticles [54, 55]. The reason behind the superior capacitive nature might be the uniform distribution of Ag-SnO2 nanoparticles with uniform spherical nanoparticles configuration which enables more number of the active sites due to the presence of Ag2 + ions and hence initiates the faradic kinetics in the electrolyte (Fig. 8).

Table 3 The comparison of SnO2 by different literature on storage device applications
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Fig. 8
figure 8

Specific capacitance Vs Current density a SnO2, b Zn-SnO2, c Ag-SnO2

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To understand the solidity of the pseudocapacitive materials, the cyclic stability test has been investigated in order to acquire an insight into the performance of nanomaterials for large number of galvanostatic charging and discharging cycles. Figure 9 depicts the GCD cyclic stability of SK1 at a current density of 2 Ag−1 for 5000 cycles. The GCD stability curves explored that the first and last few cycles of the stability test clearly reveals the superior materials permanence for large GCD cycles; especially SK1 maintains the 67% specific capacitance after 5000 cycles at current density 2 Ag−1. Owing to the superior cyclic stability, Ag-SnO2 can be considered as an efficient electrode for pseudo capacitance applications in near future. The specific capacitance 308.2 Ag−1 of Ag-SnO2 nanoparticles at current density 0.5 A/g withstand for higher current density and will guaranteed as an appropriate positive electrode for supercapacitor device applications.

Fig. 9
figure 9

Cyclic stability GCD curve of Ag-SnO2 for 5000 cycles at current density of 2 A/g

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Conclusions

In this work, we have adopted hydrothermal method to synthesis SnO2, Zn-SnO2, Ag-SnO2 nanoparticles at suitable conditions of temperature, pH, and reaction time with CTAB as capping agents. Ag-SnO2 nanoparticles synthesized at 160 °C for 12 h exhibited better electrochemical performance. The presence of dopant (Ag, Zn) and morphology of synthesized Zn-SnO2 and Ag-SnO2 nanoparticles was examined by FESEM-EDS analysis and revealed the uniform spherical configuration of all three obtained nanoparticles. The synthesized Ag-SnO2 (SK1) nanoparticles in uniform spherical nanoparticles configuration enables large number of the active sites due to the presence of Ag2+ ions which initiates the faradic kinetics in electrolyte. The specific capacitance 308.2 Ag−1 of Ag-SnO2 nanoparticles at current density 0.5 A/g can be considered as suitable candidate as positive electrode for supercapacitor device applications.