Microstructure and Supercapacitor Properties of V2O5 Thin Film Prepared by Thermal Evaporation

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Microstructure and Supercapacitor Properties of V2O5 Thin Film Prepared by Thermal Evaporation Method 1 M. Dhananjaya1, N. Guru Prakash1, G. Lakshmi Sandhya1, A. Lakshmi Narayana1, O.M. Hussain1 1 – Thin Film Laboratory, Dept. of Physics, Sri Venkateswara University, Tirupati, India a – hussainsvu@gmail.com DOI 10.2412/mmse.88.66.781 provided by Seo4U.link

Keywords: Vanadium pentoxide thin films, thermal evaporation, structure and electrochemical properties.

ABSTRACT. Transition metal oxide based supercapacitors perform excellent charge storage capability and long life time stability. Among transition metal oxides, vanadium pentoxide is one of the best suited materials for supercapacitve applications, because it has wide range of oxidation states, layered structure, high energy density (theoretical capacity of 440 mAhg−1), and low cost. The nano structured vanadium pentoxide thin films are deposited onto Ni substrates at various substrate temperature by thermal evaporation technique. The prepared V2O5 films at TS = 300 ˚C exhibited characteristic peaks with predominant (0 0 1) orientation signifying orthorhombic V2O5 phase with space group of Pmmn (59), and the calculated crystallite size is 25 nm. Raman studies confirmed the formation of V2O5 phase. The average grain size of the deposited film is about 148 nm. The films deposited at TS = 300 ˚C exhibited a high rate pseudo capacitance of 730 mFcm-2 at 1mAcm-2 of current density. The electrochemical impedance analysis revealed the films have a lower charge transfer resistance, resulting better capacitance.

Introduction. In recent decades, electrochemical capacitors have been considered as one of the prime candidate for the next generation energy storage devices due to their higher power densities (5 kW kg-1) with longer cycling life (105 cycles) than the batteries and higher energy density (100-200 W h kg-1) than conventional dielectric capacitors [1] . These outstanding properties made them as excellent candidates for hybrid electric vehicles, computers, electric mobile devices, camera-flash equipment, navigational devices and other applications. According to the charge storage mechanism, the electrochemical capacitors are classified into two types, viz electric double-layer capacitors (EDLCs) and pseudo capacitors. In EDLCs no electron transfer takes place between the electrodeelectrolyte interface during charge storage process (non-faradic), while in pseudo capacitors, charge storage process involve a reversible faradaic redox reaction at the electrode-electrolyte interface. Till to date, most extensively used electrode materials for super capacitors are carbon materials such as activated carbon fiber cloth, CNTs, carbon aerogels, conducting polymers, and transition metal oxides or hydroxides. One major disadvantage of carbon based EDLC is lower specific energy storage. Most of the available commercial products have a specific energy below 10 Wh/kg, whereas the lowest numeral for batteries is 35-40 Wh/kg. Transition metal oxides present an attractive alternative electrode materials because of high specific capacitance at low resistance, probably making it easier to construct high energy, high power super capacitors. Recently, oxide materials such as CuO, MnO2, NiO2, TiO2, V2O5, SnO2 etc which have been studied as electrode material for super capacitor. Among transition metal oxides, vanadium pentoxide (V2O5) has been studied as the active material for electrochemical pseudo capacitor applications because of its broad range of oxidation states, layered structure, high energy density, low cost and capability of fast response during charge-discharge process . Especially, V2O5 is interesting in the form of thin film for the possibility of integration into micro-electronic circuitry and its applications in electrochromic devices. The V2O5 thin films can be prepared by various types of techniques including RF/DC magnetron sputtering [2], pulsed laser 1

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

ablation [3], e-beam evaporation [4]-[5], plasma-enhanced chemical vapor deposition [6], electrodeposition [7]-[8], hydrothermal [9], sol-gel deposition [10] etc. Hence in present work vanadium oxide thin films were deposited by using thermal evaporation technique (12" vacuum coating unit model-12A4D), and studied the effect of the substrate temperature on electrochemical behavior for supercapacitor applications.

Experimental details. Thin films of V2O5 were prepared by thermal evaporation technique using 12" vacuum coating unit model-12A4D. Target material such as commercially available V2O5 powder, of purity 99.995% was subjected to a pressure of 8MPa in air at room temperature to make pellets 12 mm diameter and 2 mm thickness with specific gravity of 2.2116 g cm-3. Nickel substrates were used for the formation of vanadium oxide thin films at ambient temperature. Initially the system is evacuated to a base pressure of 1x 10 -6 mbar with a diffusion pumping system backed by rotary pump. The source material has been thermally evaporated from a molybdenum boat while keeping the deposition pressure at 1x 10 -4 mbar and the source-substrate distance at 13 cm. The depositions were carried out by varying substrate temperature from 100 C to 400 C. The microstructural properties of the as-deposited films have been ascertained by X-ray diffraction (XRD), and was performed in a Siefert X-ray diffractometer (model 3003TT) with Cu K radiation source(1.54 A ).

The angular (2) range was studied from 10 to 70 . The Raman spectra were recorded at room temperature with a Horiba Jobin Yvon LabRAM HR800UV Raman spectrometer using a 532 nm as an excitation wavelength from He-Ne laser. The surface topology of the films were observed by Scanning Electron Microscopy (SEM), (Carl Zeiss EVO50 Scanning Electron Microscope). The elemental composition have been analyzed with Energy Dispersive Spectrometer (INCA, Oxford instrumental EDS).The effects of stoichiometry on the electrochemical properties of vanadium oxide thin films were investigated using a three-electrode cell with V2O5 thin films on Ni-substrate as working electrode, platinum foil as a counter electrode and Ag/AgCl electrode as reference electrode.

The electrochemical properties were carried out using a CHI 600C electrochemical analyzer.

Results and discussion Surface Morphology. The physical properties of materials are strongly dependent on the microstructure such as grain size, grain boundaries, and orientation distribution of grains. Figure 1 illustrates the SEM images of V2O5 thin films deposited at various substrate temperatures. The surface topography shows the uniform distribution of nano grains. The grain size of the prepared films increased with increase in the substrate temperature. The average grain size of the prepared samples at substrate temperature of 200,250 and 300 C are 100 nm, 106 nm and 148 nm respectively.

(a)

(b)

(c)

Fig. 1. SEM images of V2O5 thin films at various substrate temperatures (a) 200 (b) 250 (c) 300 C. X-ray diffraction studies. The X-ray diffraction (XRD) spectra of the vanadium oxide thin films deposited at substrate temperatures of 200, 250, and 300 C is shown in Figure 2. The films prepared at 200 C and 250 C exhibited (2 0 1) orientation. However at 300 C, a predominant (0 0 1) MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

orientation is observed to be predominant at 300 C, which indicates a c-axis oriented structure. The XRD pattern of the films prepared at 300 C can be indexed as V2O5 phase with orthorhombic structure and is in good agreement with JCPDS no: 772418 [11]. The average crystallite size calculated using Scherrer’s equation for films deposited at 300 C is 25.34 nm.

Fig. 2. The X-ray diffraction spectra of V2O5 thin films with various substrate temperatures.

Raman studies. The prepared V2O5 thin films are characterized by Raman spectra in the wavelength range from 100-1200 cm-1 , as shown in figure 3. Raman spectroscopic measurements can be discussed using the shape and frequency of the 21 ( = 7Ag+3B1g+7B2g+4B3g ) allowed modes located in the high- and low-wavenumber regions corresponding to the internal and external modes, respectively [12]. The internal modes consists of V-O stretching vibrations in the range of 500-1000 cm-1 and external modes includes V-O-V bending vibrations in the range of 200-500 cm-1 [10] .The prepared samples were exhibited eight raman active modes and one raman inactive (infraredactive) mode (=4Ag(R)+2B1g(R)+B2g(R)+B3g(R)+B3u(IR)), corresponding nine obvious peaks that are located at 147, 200, 284, 310, 403, 532, 700, 837, 980 cm-1 respectively [13], [14]. In internal modes, the high-frequency Raman peak around 1000 cm-1 corresponds to vanadyl oxygen streching mode (V=OV ). The peaks exhibited at 977 and 980 cm-1 correspond to the terminal oxygen stretching mode which consequences from the unshared oxygen. The second peak at 700 cm-1 is corresponds to doubly coordinated oxygen (V2OB) stretching mode which results from corner oxygen, which is common to two pyramids. The third peak at 532 cm-1 is associated with the triply coordinated oxygen (V3OC). In external modes, the predominant low-wavenumber peak at 147 cm-1 corresponds to the skeleton bent vibration, which is a characteristics of the layer-type structure of V2O5. The peaks at 200 and 284 cm-1 are associated to the bending vibrations of the OCVOB bond.

The peak located at 310, and 403 cm-1 are assigned to the triply coordinated oxygen (V3OC) and the bending vibration of the VOBV bonds respectively [13], [14].

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Fig. 3. Raman spectra of V2O5 thin films with various substrate temperatures.

Electrochemical studies. The electrochemical measurements of V2O5 are carried using threeelectrode cell. Figure 4 shows a characteristic CV curves of deposited V2O5 thin films at various substrate temperature within the potential range from -0.6 to 0 V in 1M of LiSO4 solution at a scan rate of 10 mV s-1. The electrochemical Li+ ion insertion into and extraction from the layered framework of V2O5 can be expressed as follows: V2O5xLi+xeâ&#x;şLixV2O5

(1)

where x – is the mole fraction of inserted Li+ ions.

The specific capacitance of these films can be derived from the CV curves by following equation: âˆŤ đ??ź đ?‘‘đ?‘‰

CS = đ?‘¤âˆ†đ?‘‰đ?‘Ł

(2)

where I – represent the applied working current; đ?‘‘đ?‘‰ – is the potential difference; w – is the total electrode area; ∆đ?‘‰ – is potential window; đ?’— – is scan rate [15].

The film deposited on Ni substrates exhibited specific capacitance of 95.3, 205.9 mF cm-2, and 241 mF cm-2 at substrate temperature of 200, 250, and 300 ď‚°C respectively. The process of intercalation and de-intercalation of the Li+ ions into V2O5 nano structured frames increased with increase the substrate temperature for prepared films , due to the presence of large number of ( 0 0 1) orientation planes and good crystallinity offered by the electrode. The galvanostatic charge-discharge (GCD) studies are also used to study the specific capacitance of V2O5 thin films. The CD profiles collected in 1M LiSO4 of electrolyte at current density of 1 mAcm-2 as shown in figure 5. These nonlinear charge/discharge curves indicate a significant contribution of pseudo capacitance from vanadium oxides. The potential drop is decreases for the films prepared at 300ď‚°C due to the presence of large MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

number of ( 0 0 1) orientation planes and good crystallinity offered by the electrode. These results reveals that the films prepared at 300ď‚°C have much lower internal resistance, which is of great interest in fabricating higher specific capacitance super capacitors. The specific capacitance of the prepared films can be derived based on the following equation: đ?‘‘đ?‘Ą đ??ź

C = đ?‘‘đ?‘‰ đ??´

(3)

where I – represent the applied working current; dV – is the potential range, dt – is the discharging time A – represents active area of the electrode material.

The specific capacitance of these films obtained at current density of 1 mAcm-2 are 388, 244 mFcm - 2 and 730 mFcm-2 at substrate temperature of 200,250 and 300 ď‚°C. The electrochemical impedance spectroscopic (EIS) measurements of V2O5 thin films are studied in the frequency range from 1 Hz to 0.1 M Hz in Li2SO4 solution, and corresponding Nyquist plots are shown in figure 6. The Nyquist plots of the prepared films presented semicircle in high frequency region from electrochemical reaction impedance of the electrodes. The charge transfer resistance of the films were 8.4, 8.1, 7.5 ohms acquired from the Nyquist plots of the prepared sample at substrate temperature of 200, 250 and 300 ď‚°C. The series resistance assessed from the Nyquist plot is found to be decreased as the substrate temperature increases. Hence, the electrochemical impedance analysis revealed the films deposited at 300 0C have a lower charge transfer resistance, resulting better discharge capacity.

Fig. 4. CV curves of the prepared V2O5 thin films at different substrate temperature

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Fig. 5. CP curves of the prepared V2O5 thin films at different substrate temperature.

Fig. 6. Nyquist plots of the prepared V2O5 thin films at different substrate temperature.

Summary. The nano structured vanadium pentoxide thin films have been deposited onto Ni substrates at maintained 1 × 10−4 m.bar of base pressure with different substrate temperature by thermal evaporation technique. The XRD, Raman Spectroscopy and SEM analysis revealed the films prepared at 300 C have orthorhombic structure, increased crystallainity and increased average grain size with increase the substrate temperature. The electrochemical studies such as cyclic voltammetry, Galvanostatic Charge-Discharge, electrochemical impedance spectroscopic revealed the electrochemical performance of the prepared electrode films increased with increase the substrate temperature. Among all conditions the optimized was 300 C substrate temperature, which have (0 0 1) orientation peak of orthorhombic structure, average crystallite size is 25 nm from XRD, average grain size is 148 nm from SEM. And also the charge transfer resistance is 7.5 ohms and resulting better specific capacitance is 730 mFcm-2 at 1 mA cm-2 of current density.

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