Synthesis of surfactant-free SnS nanorods by a solvothermal route with better electrochemical properties towards supercapacitor applications

Abstract

We demonstrate a simple, low cost, and eco-friendly synthesis of surfactant free tin monosulfide (SnS) nanorods by a solvothermal route for applications in supercapacitor devices with high specific capacitance. The as-synthesized SnS nanorods, consisting of an intrinsic layered structure, were thoroughly characterised by XRD, TEM, HRTEM, SEM, EDAX and BET techniques to determine their crystal structure, size, morphology and surface area. To explore potential applications for supercapacitors, the nanocrystals were used to fabricate a two electrode system without adding any binder, large area support or conductive filler, and the system was characterised by cyclic voltammograms, galvanostatic charge–discharge and electrochemical impedance spectroscopy measurements in aqueous 2 M Na₂SO₄ electrolyte. These SnS nanorods exhibit enhanced supercapacitor performance with specific capacitance, energy density and power density values of ∼70 F g⁻¹, 1.49 W h kg⁻¹ and 248.33 W kg⁻¹, respectively, which are found to be two times higher than those of SnS–carbon composites, and thus make SnS nanorods a better alternative source for energy storage devices.

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1. Introduction

Supercapacitors, a family of electrochemical capacitors, are considered to be one of the most promising energy-storage devices because of their many advantages, including faster charge–discharge processes, high power density, long cycle life and relatively low cost.^1–4 Pseudocapacitors along with electric double layer capacitors (EDLCs) create a supercapacitor, which can store charge by redox-based faradaic reactions,^5–8 and thus can have higher capacitance values than electric EDLCs; moreover, they can have higher power densities than secondary batteries. In recent trends, attempts have been made to enhance the specific capacitance values of pseudocapacitors by the use of the emerging fields of nanoscience and nanotechnology. Different allotropes of carbon,^9,10 metal oxide and chalcogenide nanoparticles^11–13 or their nanocomposites^14,15 have been explored as potential materials for pseudocapacitors. In these cases, for faradaic energy storage, the fast redox reactions make charging and discharging considerably faster than batteries mainly due to superior crystal structures, large area support and/or conductive fillers. However, Sn-based chalcogenides, such as SnS, SnSe, SnS₂ and SnSe₂, which may be termed as multifunctional materials due to their potential applications, have not been widely investigated as supercapacitor (pseudocapacitor) materials, although they are extensively used as anodic materials for lithium ion batteries due to the large volume expansions of these layered materials. They are also applied as light absorbers in solar cells, photodetectors and photocatalysts.^16–21

Among various tin chalcogenides, tin monosulfide (SnS) is important from electrochemical lithium ion storage as well as a photovoltaic point of view. Tin monosulfide, also called herzenbergite, was first reported by the German mineralogist R. Herzenberg; it exists in the GeS crystal form with each Sn atom coordinated by six S atoms, which results in a distorted octahedral geometry system.²² SnS possesses two layers per unit cell, with one long distance S atom between the neighbouring layers. This type of layer structure is of particular interest owing to the arrangement of the cations and anions within the structural lattice, where the layers of cations are separated by weak van der Waals forces. This layer structure of tin sulphides facilitates the intercalation of ions (Na⁺ and Li⁺) and makes them a more favourable alternative anodic material with improved electrochemical performance. Bulk SnS have an indirect band gap of 1–1.2 eV and a direct optical band gap of 1.2–1.5 eV with a high solar conversion efficiency and a high absorption coefficient of α > 10⁴ cm⁻¹.²³ Particularly, its high absorption coefficient, dual carrier nature (p-type and n-type), low cost and non-toxicity have made it a promising candidate for potential applications. There have been several reports in recent years on the size and shape controlled synthesis of SnS nanostructures, for instance narrow size quantum dots, nanorods, ultrathin nanoribbons and nanosheets,^18,23–26 and their unusual optical, electronic, and mechanical properties have been extensively investigated. To the best of our knowledge, there are only a few reports where SnS has been introduced as a new electrode material for electrochemical capacitors; however, these reports examined nanocomposites with carbon powder,^27,28 which had high surface area and lower specific capacitance.

In this report, for the first time, we are introducing SnS nanorods (NRs) as a supercapacitor (pseudocapacitor) material, synthesized by a simple, low cost and eco-friendly solvothermal method, which is considerably novel in tin sulphide chemistry. We synthesised surfactant free (without the use of any surfactant or stabilizing agent) SnS nanoparticles with a NRs-like morphology by a solvothermal approach, as shown in Scheme 1, in ethylene glycol (EG) solvent at 180 °C for 10 h in the presence of HMDS as a shape moderator.¹⁹ Stannous chloride and sodium sulphide were first reacted in EG to obtain smaller SnS particles due to the adsorption of polar EG solvent molecules on their surfaces.^29,30 In the present case, however, the addition of the shape moderator HMDS directs the growth of the particles in an anisotropic way in 1D, leading to a nanorod-like morphology. The high surface area and two-dimensional layered characteristics of these lamellar as-synthesized SnS NRs are advantageous for the intercalation phenomena of alkali metal; therefore, these can be proposed as good energy storage devices due to the high exchange of ions at the interface.³¹ As shown in Scheme 1(c and d), a two electrode cell has been developed without the addition of any binder, large area support and/or conductive filler, and the cell has high specific capacitance, energy density and power density values of ∼70 F g⁻¹, 1.49 W h kg⁻¹ and 248.33 W kg⁻¹, respectively; furthermore, these SnS NRs have been studied for the first time using cyclic voltammograms, electrochemical impedance spectroscopy and charge–discharge studies. These parameters are accountably higher than those described in the previous reports on SnS nanoparticles and their respective carbon composites.^27,28

Scheme 1 Schematic view of synthesis, as-obtained particles and device fabrication (two electrode system) for tin monosulphide NRs.

2. Experimental details

2.1 Materials

Stannous chloride (SnCl₂·2H₂O, SRL-India, 99%), sodium sulfide (Na₂S·xH₂O, Thomas Baker-India, 85%), hexamethyldisilazane (HMDS, Sigma Aldrich-USA, 99%), and ethylene glycol (EG, SRL-India, 99%) were used without any further purification.

2.2 Synthesis of SnS NRs

In a typical synthesis process, 1 mmol of SnCl₂·2H₂O was dissolved in a minimum amount of water, and then 20 mL of EG and 2 mL of HMDS were added to the solution and stirred for 30 min. Another well-stirred solution of 5 mmol sodium sulphide (dissolved in a minimum amount of water) and 20 mL EG were added to the above mentioned solution drop-wise and stirred for another 30 min. This reaction mixture was transferred to a 50 mL Teflon-lined stainless steel hydrothermal vessel, and the vessel was sealed and maintained at 180 °C for 10 h. On the completion of the reaction, the as-obtained black product was washed and finally dispersed in 1 mL of ethanol for use as a stock solution for further characterization and study.

2.3 Instrumentation

Powder X-ray diffraction (XRD) patterns of the as-synthesized products were collected at room temperature using a Bruker D8 Advance diffractometer system employing a monochromatized Cu Kα radiation (λ = 1.54056 Å) source. Concentrated nanocrystal solutions were spread on the top of a glass substrate, after which the sample was allowed to dry and then measured in reflection geometry. Data were collected at a fixed incident angle of about 1°. Optical absorption measurements were carried out using a Perkin Elmer Lambda 35 UV-visible spectrophotometer. A diluted, well dispersed solution of NRs in absolute ethanol was used for the absorption study. Transmission electron microscopy (TEM) and phase-contrast high resolution TEM (HRTEM) measurements were performed with a FEI Technai G²-20 transmission electron microscope operating at an accelerating voltage of 200 kV. Samples suitable for TEM observation were prepared by applying one drop of dilute SnS NRs dispersion in ethanol on a carbon coated Cu grid and the solvent was allowed to slowly evaporate at room temperature. Scanning electron microscopy (SEM) and electron diffraction for X-ray analysis (EDAX) measurements were performed with a JEOL JSM 6610 at 20 kV, with width distance 10 mm and spot size 30. EDAX was performed at a resolution of 135.2 eV. For SEM/EDAX measurements, a dilute and well-dispersed solution of SnS NRs in ethanol was spread on the top of a 1 × 1 cm² glass substrate and dried at room temperature.

2.4 Electrochemical analysis

To prepare electrodes for supercapacitors, a dilute and well-sonicated dispersion of SnS in ethanol was spin coated on graphite sheets (250 μm thick, Nickunj. Eximp. Entp. India) and dried in an oven at ∼50 °C overnight. The area of cell assembly was ∼1 cm², and the weight of the electrode material (1.5 mg cm⁻²) deposited on the graphite sheet was determined by weighing the graphite sheet before and after the spin coating of the electrode material. An aqueous solution of 2 M Na₂SO₄ was used as the electrolyte for the device, as shown in Scheme 1 (panel d), and a two-electrode geometry was chosen for the characterization of the capacitor cells. The two electrode super/pseudo-capacitor cell configuration using SnS NRs as the electrode material (without binder) and 2 M Na₂SO₄ as the electrolyte is schematically given as follows:

Cell: SnS_NRs//SnS_NRs

The electrochemical characterization of the capacitor cells was performed using cyclic voltammetry (CV), galvanostatic charge–discharge tests and electrochemical impedance spectroscopy (EIS). The CV and EIS responses were recorded with an electrochemical analyzer (660E, CH Instruments, USA) and the charge–discharge tests were performed at constant current using a charge–discharge unit (BT-2000, Arbin Instruments, USA).

3. Results and discussion

The powder XRD pattern of the as-synthesised sample was refined to determine the crystal structure of the sample, which was found to be well indexed with the herzenbergite SnS structure with an orthorhombic (pseudo-tetragonal distorted NaCl) unit cell (a = 4.329 Å, b = 11.190 Å, c = 3.983 Å; JCPDS no: 39-0354, space group: D_2h: Pnma) without the presence of any impurity. The powder XRD pattern of the sample is compared with the bulk XRD pattern of herzenbergite SnS in Fig. 1. No other phase of Sn–S (such as Sn₂S₃ and SnS₂) from their phase diagram³² was found in the XRD pattern. The only detected effect is the huge increase in the intensity of the (040) peak with respect to the (111) peak (see inset of Fig. 1a). The interesting fact observed from the XRD pattern of the as-synthesized SnS is that the peak intensity ratio of the (040)/(111) planes was higher than that of bulk SnS. The peak intensity ratio of (040)/(111) in bulk SnS is 0.66, whereas the observed ratio for our SnS sample is 1.18. The vertical stretching in the peak intensity can be explained by the larger domain of the SnS particles along the 040 direction with respect to the bulk sample. These preliminary results suggest a greater exposure of the (040) surfaces relative to the (111) surface in the present SnS NRs as compared to the bulk SnS particles and plausible growth of the expected NRs in the 〈040〉 direction.

Fig. 1 (a) Comparison of the XRD patterns of the as-synthesised SnS NRs (red curve) and bulk orthorhombic herzenbergite SnS (black curve). (b) Optical absorption spectrum of the as-synthesized SnS NRs in ethanol dispersion (inset: dispersed solution). (c and d) Determination of the direct and indirect band gaps, respectively, following the Tauc and Davis–Mott model.

Because a true optical band gap is an indication of a pure phase of a material, we performed optical absorption measurements and determined the band gap of our as-synthesized SnS NRs, and the results are shown in Fig. 1b–d. To determine the optical band gap, the Tauc and Davis–Mott model was followed:

(αhν)ⁿ = A*(hν − E_g) (1)

where hν is the photon energy, n is an index characterizing the type of optical transition, A* is a certain frequency independent constant, E_g is the optical band gap and α is the absorption coefficient defined by the Beer–Lambert law as follows:

α = −ln A/l (2)

where A is the absorbance and l is the optical path length. To determine the optical band gap and the nature of the transition, we plotted (Ahν)ⁿ vs. hv, where n = 2 for direct transition and n = 1/2 for indirect transition. A linear portion near the absorption edge was observed for both the transitions, i.e., for n = 2 and n = 1/2, as shown in Fig. 1c and d, respectively, depicting the presence of the direct and indirect band gaps of the as-synthesized SnS NRs. A direct band gap value of 1.20 eV was determined from the extrapolated intercept with the energy (hν) axis (Fig. 1c). However, an indirect inter-band transition gap of 1.1 eV was determined from Fig. 1d. Both the direct and indirect band gap values are consistent with earlier reports for SnS samples,³³ depicting a pure phase of our as-synthesized SnS NRs. However, we did not observe the predicted blue shift in the band gap possibly due to the larger size of the particles.

The morphology, topology and crystal structure of the as-synthesized samples were investigated by TEM and HRTEM, and the images are summarized in Fig. 2. The low magnified TEM images, at scales of 500 nm, 100 nm and 50 nm respectively, in panel a–c from different grid areas show the anisotropic 1D nature of the SnS. All the as-obtained particles were found to have nanorod morphology; however, the particle size distribution was large. On plotting histograms for the length and diameter separately (Fig. 2d and e), the average dimension of the SnS NRs was found to be 60 × 450 nm². This observed wide particle size distribution, or polydispersity, is related to the surfactant-free synthesis conditions of the NRs, where the absence of any surface or facet selective surfactant leads to the unhindered growth of the NRs. However, with the help of HRTEM analyses, we tried to determine the preferential growth direction of the NRs, and the reason behind that preference. The lattice-resolved HRTEM image in Fig. 2f shows the interplanar spacings of 0.40 and 0.29 nm for the adjacent (110) and (040) planes of SnS, respectively, depicting its pure orthorhombic phase. Furthermore, the fast Fourier transform (2D-FFT, zone axis [0,0,4]) was performed from the HRTEM image, and is shown in Fig. 2g. The clear spots in the FFT also correspond to the (110) and (040) planes, corroborating the interlamellar spacings of 0.40 and 0.28 nm, respectively. As shown in the depiction of the planar crystal structure of SnS, the (110) and (040) planes are seen under the [0,0,4] zone axis; moreover, from the data obtained from the HRTEM analyses, it could be clearly reasoned that the growth of the SnS nuclei after their formation in the solvothermal process takes place in the 〈110〉 and 〈040〉 directions. Because the synthesis does not contain any surfactant, at 180 °C the growth is probably favoured thermodynamically in the stated two directions, and results in a wide distribution of particles. In addition, the experimental results suggest that the dihedral angle between these two sets of fringes is 46°, which matches well with the angle between the Miller planes, i.e. 45°, as shown in Fig. 2f and h. The topography and elemental composition were further analysed with SEM and EDAX, as shown in Fig. 3. A clear view of the NRs, as seen in the SEM images, shows dimensions similar to those established from the TEM measurements. A single nanorod of SnS was chosen for elemental analysis on the line alignment along its length (inset of panel a), which gives a Sn : S ratio of nearly 1 : 1 within the instrumental error.

Fig. 2 Characterization of SnS NRs. (a–c) Low magnification TEM images at different scales, (d and e) particle size distributions of the as-synthesized SnS NRs in the lateral and longitudinal directions, (f) HRTEM image showing lattice fringes belonging to the (110) and (040) planes, (g) calculated 2D-FFT pattern from panel ‘f’ and (h) depiction of the layer crystal structure of SnS seen from the [0,0,4] zone axis.

Fig. 3 (a) SEM image of the as-synthesized sample showing a 3-dimensional view of the SnS NRs. Inset: SEM image of a single SnS nanorod taken for EDAX. (b) EDAX spectrum from the elemental analysis.

3.1 Surface area study

A high surface area (>10 m² g⁻¹) makes nanoparticles more suitable as supercapacitor electrodes.³⁴ Therefore, the specific surface area of SnS NRs was determined from the Brunauer–Emmett–Teller (BET) plots using the multipoint BET equation. A high specific surface area value (∼221 m² g⁻¹) was obtained for SnS NRs, observed under the repeated measurements of different batches of samples. The N₂ adsorption–desorption measurement indicates the porous texture of the as-synthesised SnS NRs, as shown in Fig. 4, which is referred to as a Type-IV isotherm with H3-type hysteresis according to IUPAC classification. The predominant mesoporosity in the material could be indicated from the slowly increasing isotherm and the hysteresis loop. The sudden jump at around 3 nm and the broad pore size distribution between 15 and 25 nm (inset of Fig. 4) indicate that two different sized mesopores are present in the SnS NRs. The mesopore volume was estimated from the isotherm and was found to be ∼0.26 cm³ g⁻¹. The H3-type hysteresis loop indicates that the material basically contains slit-shaped pores at the SnS nanorod surface. In particular, nanostructured tin sulphides with controlled morphologies possess larger surface areas, greater accessibility to the electrolytes, faster transportation of ions, and accelerated phase transitions³⁵ due to their intrinsic layer crystal structures. Thus, we can expect a large capacitance value due to the observed high surface area of the as-synthesized layered SnS NRs. Chemically, the origin of pseudocapacitance in electroactive materials such as SnS is mainly due to intercalation redox reactions, which strongly depend on the particle size and morphology of the nanoparticles.³⁴ Thus, larger numbers of intercalating anions between the layers of herzenbergite SnS are available for redox reactions, which is the origin of the pseudocapacitance in the present case.

Fig. 4 N₂ adsorption–desorption isotherm of the SnS nanorod sample at 77 K. Inset: pore volume distribution vs. pore radius of the as-synthesized nanorods.

3.2 Electrochemical studies

The CV studies were performed at different scan rates in the symmetric potential window of −0.4 V to +0.4 V, and also in the potential range of 0.1 V to 1.0 V, as used for galvanostatic charge–discharge cycling (presented later). Fig. 5a and b depicts scan rate dependent voltammograms of the capacitor cell, which are close to rectangular shapes even at higher scan rates. This is an important characteristic of supercapacitors, indicating their high rate capability, i.e., fast charging and discharging of the devices. Furthermore, the recyclability of the capacitor cells with electrodes of as prepared tin sulfide nanoparticles was tested by CV cycling at a scan rate of 50 mV s⁻¹ in the potential window of 0.1 V–1.0 V, as shown in Fig. 5c. The test indicates the excellent stability of the SnS NRs as electrode materials for many cycles.

Fig. 5 Cyclic voltammograms of the capacitor cell: (a) CV in the symmetric window of −0.4 V to +0.4 V at different scan rates, (b) CV in the asymmetric potential window of 0.1 V–1.0 V, (c) CV for various number of cycles at a scan rate of 50 mV s⁻¹, (d) galvanostatic charge–discharge profile of the cell in the potential range from 0.1 V to 1.0 V at the current density of 0.5 mA cm⁻² (the inset shows a few cycles), (e) comparison of the charge–discharge profiles for various cycles and (f) plot of capacitance vs. number of cycles to determine the cycling stability of SnS nanorods as a supercapacitor material.

In general, capacitors employing active redox electrodes such as SnS (as in the present case) have the combined effects of double layer capacitance and pseudocapacitance owing to the fast redox reactions at the electrode–electrolyte interfaces. In the present system, the possible reactions involved to obtain pseudocapacitive behavior at the SnS-nanorod electrode–electrolyte interface are as follows:³⁶

Redox reaction: Sn^IIS + Na⁺ ↔ Sn^ISNa + e⁻ (3)

or

SnS + 2Na⁺ + 2e⁻ ↔ Sn + Na₂S (4)

It may be noted that the CV patterns show almost rectangular shapes till the scan rate of 50 mV s⁻¹, whereas a small deviation in the curves has been found at higher scan rates, which is primarily due to the resistive components of the cell. It may be further noted that the nanostructured tin sulphides, i.e. SnS NRs, possess higher surface areas than bulk SnS, and moreover due to their unique layered structure, they have potential to enhance the electrochemical performance as supercapacitor electrode materials. The enhanced supercapacitive behaviour of the SnS NRs over bulk SnS (discussed above) is mainly due to the intercalation of Na⁺ ions in the van der Waals gap of the S–Sn–S layers. Because the large surface area of these nanostructure tin sulphides greatly reduces the diffusion length over which both ion and electron transfer take place during the charge–discharge process, a higher capacitance value is obtained for the nanostructures. This observation is also in agreement with the capacitance obtained for the layered SnS nanostructures, which shows greater compatibility and the formation of excellent capacitive interfaces with the SnS NRs electrode compared to the bulk SnS electrode.²⁷

Fig. 5d shows the charge–discharge characteristics of the capacitor cell in the potential range from 0.1 to 1.0 V as CV, recorded at a constant current density of 0.5 mA cm⁻². The cells are observed to have a capacitive nature, as characterized by the almost linear discharge patterns. The discharge capacitance has been evaluated from the discharge characteristic regions using the following expression:^6,37

where i is the discharge current, and Δt/ΔV is the inverse of the slope of the discharge curve. The specific capacitance of the electrode material is obtained using the expression C_sp = 2 × C_cell/m, where m is the mass of the single electrode. The capacitance value for the SnS NRs based cell was evaluated from eqn (5) and was found to be ∼70 F g⁻¹, which is greater than or even twice the reported values of the capacitors based on electrodes using bulk SnS and even SnS–carbon nanocomposites.^27,28 Because SnS NRs have a layered structure, better capacitance behaviour is expected due to more encapsulation of Na⁺ ions between the two layers of tin sulphide; both the layers are orthogonal lattices of SnS. Charge–discharge profile shows a decrease in capacitance value as the number of cycles increase. The capacitance C_sp decreases as electrolyte ions block the pores of electrode materials; in the present case, the electrolyte ions Na⁺ and SO₄²⁻ may be trapped in the layers of SnS NRs, and they are prevented from participating in further redox reactions.

Fig. 5e depicts the galvanostatic charge–discharge behavior of the two electrode cells of SnS at different cycles (up to 100 cycles) in a potential window of 0.1 to +1.0 V. Fig. 5f shows the variation of capacitance (C_sp) with cycles; it was found that up to 150 cycles, the capacitance value is reduced by 20%, and the C_sp retains 60% of its initial value until 500 cycles. This observation might be possibly due to the blockage of the mesopores by SO₄²⁻ (from Na₂SO₄). The efforts are still being made to improve the capacitance by adding graphene or other carbon active material in the future work.

The other important parameters, namely, specific energy and power density, have also been evaluated using the following expressions:³⁸

where C_cell is the capacitance of the cell (in F cm⁻²), V is the voltage, excluding the equivalent series internal resistance (ESR), occurring at the beginning of the discharge profile of the cell, and M is the mass of both the capacitor electrodes. The ESR value of the cell was observed to be ∼215 Ω cm². The specific energy and power density of the capacitor cell have been evaluated to be ∼1.5 W h kg⁻¹ and ∼248 W kg⁻¹, respectively.

The EIS pattern, i.e., the plot between the real (Z′) and imaginary parts (Z′′) of the impedance (Z), of the SnS-nanorod-based capacitor cell is shown in Fig. 6a, indicating its capacitive behaviour as reflected by the steeply rising pattern in the lower frequency range.³⁹ This capacitive impedance response is accompanied with a high-frequency semicircular feature mainly showing the bulk properties of tin sulphide and the electrode–electrolyte interfaces, as shown in the expanded representation in the high frequency range (Fig. 6a, inset). A depressed semicircular arc has been plotted based on the experimental points in the high frequency region (Fig. 6a, inset). The arrows marked on the arc indicate the bulk electrolyte resistance R_b, and R_b + R_ct (R_ct being charge-transfer resistance). The values of R_b, R_ct and the overall resistance at 10 mHz (R) have been found to be ∼1.17, ∼11.4 and ∼375 Ω cm², respectively. Followed by the semicircular arc, the EIS plot shows a linear pattern with an ∼45° angle slope; in addition, a steeply rising capacitive pattern can be observed in the low frequency region. The specific capacitance (C) has been evaluated from the expression C = 2/(2πfmZ′′) (where m is the mass of the single SnS electrode, and Z′′ is the imaginary part of the impedance at frequency f), which was found to be 30.3–34.7 F g⁻¹ at the frequency of 10 mHz. This value is lower than the specific capacitance observed from the charge–discharge studies, which has been discussed earlier.

Fig. 6 (a) EIS plot, recorded in the frequency range from 10 mHz to 100 kHz; the expanded representation of the EIS plot in the high to mid-frequency range is shown in the inset, and (b) Bode plots of the cell.

The rate capability of the supercapacitor has also been evaluated in terms of knee frequency and response time from the EIS studies.⁴⁰ Below the knee frequency, the impedance pattern shows a steeply rising capacitive response, whereas the response time (τ_o) is the reciprocal of a frequency f_o at which the imaginary and real parts of the impedance with respect to the applied frequency (Bode plots) are equal.⁴¹ The knee frequency of the present capacitor cell was found to be ∼15.8 Hz (inset of Fig. 6a). Fig. 6b depicts the Bode plots of the supercapacitor, showing f_o at ∼908 mHz and a corresponding τ_o of ∼1.10 s. The values of τ_o and the knee frequency indicate the high rate performance of the capacitor cell containing nanostructured SnS electrodes.

4. Conclusion

In this report, we introduced the enhanced supercapacitor behaviour of bare (surfactant free) SnS NRs synthesized by a simple, eco-friendly and low-cost solvothermal method. The SnS NRs were found to grow in the 〈110〉 and 〈040〉 directions due to the presence of a shape moderator as well as thermodynamically favourable crystal structures. Superior supercapacitor behaviour, with a capacitance of ∼70 F g⁻¹, using a two-electrode system for the as synthesised bare SnS NRs was observed, which is found to be considerably greater than that of the reported SnS–carbon composites; thus, the SnS NRs can be applied as a promising pseudocapacitor material for energy storage devices.

Acknowledgements

H.C. thanks UGC-India for providing a junior research fellowship. SD acknowledges financial support from CSIR-India (01(2773)/14/EMR-II) and the University of Delhi (DRCH/R&D/2013-14/4155). The authors thank USIC-DU and AIIMS EM facility for sample characterization.

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