SnO thin film electrodes deposited by spray pyrolysis ...

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SnO2 thin film electrodes deposited by spray pyrolysis for electrochemical supercapacitor applications Abhijit A. Yadav 1,* Phone +919975213852 Email [email protected] 1 Thin Film Physics Laboratory, Department of Physics, Electronics and Photonics, Rajarshi Shahu Mahavidyalaya (Autonomous), Latur, Maharashtra, 413512 India

Abstract The present paper reports the growth and structural, morphological and electrochemical characterization of SnO2 thin films by spray pyrolysis. XRD results showed that films are polycrystalline with tetragonal crystal structure. The band gap energy is found to be 3.88 eV for film deposited at 500 °C. The electrochemical performance of the supercapacitor was characterized using a three-electrode configuration, and cyclic voltammetry curve recorded at a scan −1 rate of 10 mV s was pseudo-rectangular. All samples show good supercapacitance in 1 M KOH electrolyte with highest specific capacitance of −1 119 F g for film deposited at 500 °C. Nyquist and Bode plots show the ideal capacitive behavior. The present study signifies successful application of SnO2 thin films as supercapacitor electrode. These findings demonstrate that substrate temperature is an effective parameter to improve the performance of pseudo-capacitive metal oxides.

1. Introduction Supercapacitors have been known for decades and are studied as one of the potential energy storage systems. Electrochemical capacitors (ECs), often described electrical double-layer capacitors (EDLCs), supercapacitors,

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ultracapacitors, gold capacitors, power capacitors or power caches, have attracted universal research interest because of their potential applications as energy storage devices in many fields [ 1 – 4 ]. Supercapacitors have desirable characteristics such as high power density, faster charging/discharging rate, superior cycling stability and wide operating temperature range, which make supercapacitors quite competitive among various types of electrochemical energy storage devices [ 5 – 8 ]. In recent times, low-cost transition-metal oxides, such as SnO2, MnO2, Ni(OH)2 or CuOx, Co3O4 [ 9 – 12 ], have been investigated as possible supercapacitor electrode materials. Among these, SnO2 with the advantages of low cost, efficient semiconducting nature and environmental safety has been applied extensively [ 13 – 16 ]. However, the capacitance property of SnO2 has been hardly investigated. Extensive work on SnO2 thin films has been reported. Various review articles are now available with up-to-date data on these materials. SnO2 thin films can be deposited by several methods such as co-precipitation [ 17 ], pulsed laser deposition [ 18 ], sputtering [ 19 ], SILAR [ 20 ], electrodeposition [ 21 ], chemical bath deposition [ 22 ], and spray pyrolysis [ 14 – 16 , 23 – 25 ]. Spray pyrolysis technique is preferred among these techniques since it is easy in controlling and tailoring the film properties for desired application by changing spray conditions. Also the spray pyrolysis offers advantages including high degree of control, high reproducibility and good uniformity over a large area for the film growth. Pusawale et al. [ 26 ] has synthesized nanocrystalline thin films of SnO2 by chemical route. The films showed agglomeration of nanograins with porous morphology and hydrophilic nature with the average particle size of 5–10 nm. −1 The maximum specific capacitance of 66 F g in 0.5 M Na2SO4 electrolyte was observed. Gao and coworkers [ 27 ] has investigated SO4/SnO2 as a potential electrode material for supercapacitors. The films are characterized by XRD, FT-IR and an electrochemical analyzer system. The specific capacitance of −1 −1 SO4/SnO2 is 51.95 F g at 5 mV s scan rate compared to the SnO2 value of −1 only 15.61 F g . SO4/SnO2 maintained 92.15 % of its maximum capacitance after 2000 cycles. From these and other studies [ 9 – 13 ] it is well understood that SnO2 thin film electrode is one of promising materials for the fabrication of supercapacitors. Therefore in present study, the SnO2 thin films are deposited at various substrate temperatures by using spray pyrolysis. The effect of substrate

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temperature on the film properties, like crystal structure, optical, electrical and electrochemical properties has been studied. The results obtained are compared and discussed.

2. Experimental SnO2 thin films were deposited by using computerized chemical spray pyrolysis technique discussed elsewhere [ 28 ]. For deposition of SnO2 thin films, 1 M −1 solution of SnCl4:5H2O was prepared. Spray rate employed was 3–4 cc min and kept constant throughout the experiment. The carrier gas pressure was −2 1.8 kg cm . The substrate temperature was varied from 450 to 525 °C in the interval of 25 °C by keeping all other parameters at optimized values. The distance between the substrate to nozzle was 28 cm. The amorphous glass micro slides (7.5 cm × 2.5 cm × 0.13 cm) and FTO were used as substrates. Thickness of SnO2 thin films is computed from transmission data. The structural characterization of the spray deposited SnO2 thin films was carried out by using a Philips X-ray diffractometer model PW-1710 (λ = 1.5406 Å for Cu-Kα radiation). The surface morphology was studied by using JEOL-JSM-6360A analytical scanning electron microscope. Atomic force microscopy (AFM) was carried out in air at ambient condition (300 K) using Nanoscope III from Veeco. In order to estimate the band gap of the SnO2 thin films, optical absorption of the films have been studied by Systronic make UV–Vis spectrophotometer (model 119) at room temperature in the wavelength range of 200–850 nm. Hall Effect setup supplied by Scientific Equipment’s, Roorkee was used for measurements sheet resistance (Rs) at room temperature. A typical three-electrode glass cell equipped with a working electrode, a platinum foil counter electrode, and an Ag/AgCl reference electrode was used for electrochemical measurements. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy measurements were performed using an electrochemical workstation (CHI606D, CH Instruments, USA) in a 1 M KOH aqueous solution.

3. Results and discussion Thin films of SnO2 are prepared by the reaction, ( 1)

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According to Arai [ 29 ] if this reaction is completed the resulting SnO2 would become an insulator. Since the films obtained by pyrolytic decomposition are semiconducting, the expected reactions are,

The analogous type of reaction mechanism is reported previously by Chacko et al. [ 30 ] for spray pyrolytically grown nanocrystalline SnO2 thin films.

3.1. Thickness calculation Thicknesses of SnO2 thin films deposited at various substrate temperatures are computed from transmission data and by using following formula [ 15 ]; ( 4)

where, ( 5)

with n0 = 1 the R.I. of air, n1 = refractive index of thin film, n2 = refractive index of the substrate, d = thickness of thin film, T = transmittance and λ = wavelength of incident radiation. By fitting the observed transmittance data with the theoretical data given by Eq. ( 4 ), one can search for a pair of thickness and refractive index [ 31 ]. The thickness values determined by using above mentioned procedure are found to be 0.65, 0.87, 0.98, and 0.78 µm. The variation of film thickness with substrate temperature is shown in Fig. 1 , relatively higher thickness is found to be 0.98 µ m for film deposited at 500 °C. From figure, it is found that the film thickness increases with increase in substrate temperature reaches the maximum at 500 °C, decreases further with increase in substrate temperature. This can be explained as follows: Initially, at lower substrate temperatures e.g., 450 °C, the temperature may not be sufficient to decompose 4+ the sprayed droplets of Sn ions from the solution and this therefore, results in a

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low thickness. At a particular substrate temperature 500 °C, decomposition occurs at the optimum rate resulting in the terminal thickness being attained. A noticeable decrease of the film thickness with increasing the substrate temperature is observed after substrate temperature 500 °C. This decrease may be attributed to re-evaporation of film material after deposition or to thermal convection of the sprayed droplet during the deposition process or both. Another cause which may account for thickness decrease is water loss or removal of interlayer water with consequent formation of the compact film. The decrease in film thickness at higher substrate temperatures may also be due to a higher evaporation rate of the initial ingredients of the solution [ 25 ]. Fig. 1 Variation in the film thickness with substrate temperature for spray deposited SnO2 thin films

3.2. X-ray diffraction The crystal structure of as deposited SnO2 thin films was revealed by X-ray diffraction with CuKα radiation (1.5406 Å). The range of 2θ angle was from 20° to 80°. Figure 2 is a typical X-ray diffractogram of SnO2 thin film deposited at 500 °C. X-ray diffraction patterns reveal that the SnO2 film deposited by spray pyrolysis technique is polycrystalline in nature. A matching of the observed and the standard‘d’ values confirms that the deposited films are of SnO2 having

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tetragonal crystal structure [ 32 ]. The diffraction peaks correspond to (110), (101), (200), (211), (310), (301) planes, which are indexed to the tetragonal routile crystal structure of the SnO2 phase. The lattice parameters of SnO2 thin films are estimated using the standard relation [ 25 ], ( 6)

where h, k, l are the lattice planes and ‘d’ is the interplanar spacing determined using Bragg’s equation. The lattice parameters of SnO2 films are a = b = 4.765 Å and c = 3.197 Å; which are higher than standard JCPDS values a = b = 4.7324 Å and c = 3.1831 Å [ 32 ]. Fig. 2 X-ray Diffraction pattern of spray deposited SnO2 thin film at substrate temperature of 500 °C

3.3. Scanning electron microscopy and atomic force microscopy The surface morphology of SnO2 thin film was studied by using Analytical scanning electron microscope (JEOL-JSM-6360 at accelerating voltage of 20 kV). Figure 3 shows the surface morphology of SnO2 deposited at substrate

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temperature of 500 °C at 10,000× magnifications. The films are without cracks, uniform and continuous with the continuous distribution of grains. The presence of big and faceting polyhedron-like grains observed in the image present the fact that the crystallites are formed by coalescence type of growth. The grains are randomly distributed giving rise to a scattering effect, thereby reducing transmittance. Fig. 3 Typical SEM micrograph at 10000 × magnifications of SnO2 thin film prepared at optimized substrate temperature of 500 °C

Atomic force microscopy (AFM) is a well-known characterization technique commonly used to study surface morphology with very high, sometimes even molecular or atomic resolution. The AFM system used in these studies is the Nanoscope III multimode scanning probe microscope, produced by the Digital Instruments, Veeco Metrology Group, USA. The detailed analysis of all obtained AFM data was performed with the help of the WSxM 4.0 (Nanotec electronica S.L.) software. This software allows us to extract the basic topographic information. The surface morphology was studied by AFM; root mean square (RMS) roughness of the films was extracted from AFM data. Figure 4 shows a typical two-dimensional (2D) and three-dimensional (3D) AFM image of 2 2 × 2 µm size of the SnO2 thin film deposited at a substrate temperature of 500 °C. The surface roughness is 25.85 nm. The surface roughness is unavoidable due to three-dimensional growth of the film [ 33 – 35 ].

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Fig. 4 Two-dimensional (2D) and three-dimensional (3D) AFM image of SnO2 film prepared at substrate temperature of 500 °C

3.4. Optical absorption In order to estimate the band gap of the SnO2 thin film, optical absorption of the films deposited on quartz substrate have been studied at room temperature in the wavelength range of 200–850 nm. The optical absorption data are analyzed for near edge optical absorption of semiconductor by using the following relationship [ 36 – 38 ], ( 7) where α is absorption coefficient, A is an energy-independent constant and Eg is the optical band gap. The exponent n depends on the nature of the transition, n = ½, 2, 3/2 or 3 for allowed direct, allowed in-direct, forbidden direct or forbidden in-direct transitions, respectively. The optical absorption coefficient is 4 −1 2 of the order of 10 cm . Figure 5 shows the variation of (αhν) with hν for spray deposited SnO2 thin film deposited at 500 °C, it has a straight line portion indicating that transition involved is direct allowed type. The direct bandgap, 2 determined by extrapolating the straight portion to the energy axis to (αhν) = 0, is found to be 3.88 eV, which is slightly higher than the value of Eg = 3.57 eV

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reported for single crystal SnO2 [ 39 ]. Fig. 5 2

Variation of (αhν) versus hν for SnO2 thin film spray deposited at substrate temperature of 500 °C

3.5. Sheet resistance Figure 6 shows variation of sheet resistance (Rs) with substrate temperature for spray deposited SnO2 thin films. The sheet resistance (Rs) is found to decrease with increasing film substrate temperature initially but then increased for higher substrate temperatures. It is seen that sheet resistance has its minimum value for the film deposited at 500 °C temperature, and it increases for further values of deposition temperature. This is due to relatively higher thickness and crystallite size of the deposits at 500 °C than other temperatures. The sheet resistance was −2 found to decrease from 159 to 25 Ω cm for the increase in substrate temperature from 450 to 500 °C. Fig. 6 Variation of sheet resistance with substrate temperature for spray deposited SnO2 thin films

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3.6. Cyclic voltammetry The electrochemical properties of spray deposited SnO2 thin films at various substrate temperatures were studied using cyclic voltammetry. Cyclic voltammetry (CV) is a powerful tool in the field of electrochemistry for initial screening of materials for electrochemical capacitor applications. It has been used extensively to characterize the performance of various energy storage devices. In these applications, the charged electrodes are typically immersed in the electrolyte solution. Electric double layers form at the electrode/electrolyte interfaces which are accessible to ions present in the electrolyte. Figure 7 a shows the cyclic voltammograms of the SnO2 thin films deposited at various −1 substrate temperatures in aqueous 1 M KOH electrolyte at scan rate of 10 mV s in the potential range −0.22 to −0.8 V versus Ag/AgCl. All the curves are pseudo rectangular shape and exhibit mirror image characteristics, which indicate that the Faraday redox reactions are electrochemically reversible as well as an ideal electrochemical capacitive behavior [ 7 ]. CV curves were used to calculate the specific capacitance of SnO2 films deposited at various substrate temperatures using following equation: ( 8)

−1

−1

where, Cs is the specific capacitance (F g ), v is the potential scan rate (mV s ), (Vc–Va) is an operational potentional window, I is the current response (mA) of

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2

the SnO2 electrode for unit area (1 cm ) dipped in 1 M KOH electrolyte and m is 2 deposited mass of SnO2 on 1 cm surface of FTO coated glass substrate. In this investigation, cyclic voltammetry measurement at various substrate temperature 450, 475, 500 and 525 °C showed the specific capacitance of 99, 111, 119 and −1 92 F g respectively. The variation of specific and interfacial capacitance with substrate temperature is shown in Fig. 7 b. It is observed that both the specific and interfacial capacitance values increase with increase in substrate temperature −1 −2 reaches a maximum 119 F g and 0.16 F cm respectively at 500 °C and decrease further with increase in substrate temperature. The SnO2 electrode −1 exhibited the supercapacitance of 119 F g in 1 M KOH electrolyte, which is −1 greater than recently reported value i.e. 66 F g for SnO2 prepared by chemical −1 route in 0.5 Na2SO4 electrolytes at 10 mV s scan rate [ 26 ]. Fig. 7 a The cyclic voltammograms of spray deposited SnO2 thin film electrode at various substrate temperatures in the 1 M KOH electrolyte, in the working potential window of −0.22 to −0.8 V (vs. Ag/AgCl). b Variation of specific and interfacial capacitances with substrate temperatures for spray deposited SnO2 thin film electrodes

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3.7. Electrochemical impedance spectroscopy studies In order to further evaluate the ion transport kinetics and electrode conductivity, electrochemical impedance spectroscopy measurement of the SnO2 thin film electrode deposited at 500 °C, were performed in the frequency range from 1 Hz to 1 MHz. As shown in Nyquist plots in Fig. 8 a, the straight line nearly parallel to the imaginary axis showed the ideal capacitive behavior of the device. The intercept of the Nyquist curve on the real axis is about 47 Ω, manifesting the good conductivity of the electrolyte and very low internal resistance of the electrode. The EIS data can be fitted by an equivalent circuit illustrated in the inset of Fig. 8 b. The elements in the equivalent circuit include the solution resistance (Rs), the double-layer capacitance (Cdl), the charge-transfer resistance

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(Rct), the Warburg diffusion element (Wo), and the pseudocapacitance element (Cpseud). The values of Rs and Rct obtained from the Nyquist plots are 47 and −2 85 Ω cm respectively. Figure 9 displays the Bode plots of the SnO2 thin film supercapacitor. It can be observed that the capacitor response frequency, f0, at the phase angle of 45° was about 17 Hz, as expected which is even comparable to the values for supercapacitors with aqueous electrolytes [ 26 ]. Fig. 8 a Nyquist plot of the supercapacitor device with SnO2 thin film electrode spray deposited at 500 °C in the 1 M KOH electrolyte, b its equivalent circuit, respectively

Fig. 9 Bode plots of the supercapacitor device with SnO2 thin film electrode spray deposited at 500 °C in the 1 M KOH electrolyte

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4. Conclusions The structural, morphological, optical, electrical and electrochemical properties of SnO2 thin films deposited by spray pyrolysis have been studied. XRD studies confirmed that films are polycrystalline with tetragonal crystal structure. SnO2 films are highly oriented along (200) direction. The direct bandgap is found to be decreased from 3.88 eV for the SnO2 thin film deposited at 500 °C. The sheet −2 resistance was found to decrease from 159 to 25 Ω cm for the increase in substrate temperature from 450 to 500 °C. The specific and interfacial −1 −2 capacitance values are found to be 119 F g and 0.16 F cm respectively for film deposited at 500 °C. Nyquist and Bode plots show the ideal capacitive behavior of the SnO2 thin film electrode. These results demonstrate the potential of developing the SnO2 electrode material for high performance supercapacitors.

Acknowledgments Dr. A. A. Yadav is grateful to the Science and Engineering Research Board, Department of Science and Technology, New Delhi, India for the financial assistance through the Project under SERC Fast Track Scheme for Young Scientist (File No. SB/FTP/PS-068/2013).

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