Appl. Phys. A (2016)122:128 DOI 10.1007/s00339-015-9588-z

Growth, microstructure and supercapacitive performance of copper oxide thin films prepared by RF magnetron sputtering B. Purusottam Reddy1 • K. Sivajee Ganesh1 • O. M. Hussain1

Received: 11 November 2015 / Accepted: 28 December 2015  Springer-Verlag Berlin Heidelberg 2016

Abstract The supercapacitive performance of copper oxide thin film electrodes mainly relies on micro structure, phase, surface area and conductivity which in turn depend on the deposition technique and process parameters during growth. In the present study, thin films of copper oxide were prepared by RF magnetron sputtering on stainless steel substrates keeping O2-to-Ar ratio at 1:11 and RF power at 250 W and varying the substrate temperature. The microstructure and the induced phase changes in copper oxide films are observed to be strongly influenced by the substrate temperature since the relaxation time, surface diffusion and surface structural changes are thermally activated. The XRD and Raman studies reveal that the films deposited at low substrate temperature (\200 C) exhibited CuO, while the films deposited at substrate temperature [200 C exhibited Cu2O phase. The films prepared at 350 C exhibited reflections correspond to cubic Cu2O with predominant (111) orientation. The estimated maximum grain size from AFM studies was 72 nm with surface roughness of 51 nm. These films exhibited a highest areal capacitance of 30 mF cm-2 at scan rate of 5 mV s-1. The galvanostatic charge–discharge studies demonstrated high specific capacitance of 908 F g-1 at 0.5 mA cm-2 current density with 80 % of its initial capacity retention even after 1000 cycles.

& O. M. Hussain [email protected] 1

Thin Film Laboratory, Department of Physics, S.V. University, Tirupati 517 502, India

1 Introduction Over the past few decades, the impacts of the consumption of non-renewable energy resources on both mankind and environment have been increasingly driving the world towards the harvesting of clean and sustainable energy [1]. The development in the field of energy storage devices is particularly important for efficient storage of harvested energy and to manage the gap between energy production and requirement. Supercapacitors or electrochemical capacitors (ECs) are a sub class of energy storage devices which utilize electrical double layer phenomena to store energy. Since supercapacitors store energy on the surface of the active material, the charging–discharging occurs at faster rates which results in high power densities and high cycling stability [2]. These unique features positioned supercapacitors as one of the most promising and potential energy storage devices in addition to batteries. Supercapacitors are again classified into electrical double layer capacitors (EDLCs) and pseudocapacitors based on whether the charge transfer is non-faradaic or faradaic, respectively [3]. EDLCs store charge in electric double layer that is formed at interface between the electrode and electrolyte, while pseudocapacitors store the charge utilizing Faradic redox reactions as well as non-faradic electric double layer formation which allows pseudocapacitors to store higher energy than that of EDLCs [4]. Although pseudocapacitors are ahead of EDLCs in the context of energy density, the cycling stability still is far behind that of EDLCs which restricts their evolution as a prime energy source for future technologies. In this perspective, the study and exploration of electrode materials for pseudocapacitors are important in order to compete with batteries. Among all materials under study, metal oxides such as amorphous hydrous ruthenium oxide [5–8] and iridium

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oxide [9, 10] are widely accepted as ideal electrode material for pseudocapacitor application due to their high cycling stability, high theoretical capacity and large potential window; however, the high cost and toxicity of these noble metals restrict their application. Therefore, nowadays research has been focused on alternative, abundant and cost-effective electrode materials such as manganese oxides (MnO2) [11, 12], nickel oxide (NiO2) [13– 15] and copper oxide (CuxO) [16–18]. Due to its low cost, chemical stability, abundance and high theoretical capacity of 690 F g-1, copper oxides have considered as alternative electrode materials for pseudocapacitors. Both physical and chemical deposition techniques are employed for the preparation of copper oxide thin films. For example, Pierson et al. [19] synthesized three well-defined Cu–O binary systems by varying oxygen partial pressure using RF reactive magnetron sputtering. Dubal et al. [20] prepared copper oxide nanosheets using chemical bath deposition (CBD) technique which exhibited a specific capacitance of 346 F g-1 at 5 mV s-1 scan rate. Shinde et al. [21] reported a specific capacitance of 498 F g-1 at 5 mV s-1 for 3D-nanoflower-like CuxO architecture prepared by chemical bath deposition method. Endut et al. [22] synthesized vertical nanoflakes of copper oxide electrodes using chemical bath oxidation technique which exhibited a specific capacity of 190 F g-1 at 2 mA cm-2 current density. A high-rate pseudocapacitance of 569 F g-1, with columbic efficiency higher than 93 %, was reported by Wang et al. [23] for CuxO nanosheet arrays synthesized by template free growth method. Yu et al. [24] using wet chemical process prepared 3D porous gear-like CuxO nanostructure on a Cu substrate, with specific capacitance as high as 348 F g-1 at a discharge current density of 1 A g-1. Although studies reveal that the copper oxides are good candidates for supercapacitor electrode materials, the practically achievable specific capacitance of copper oxides is still far behind that of RuO2. The reasons are manifold. The difficulty in phase formation and low electrical conductivity are the two important factors which lagging their application towards supercapacitors. The performance of the pseudocapacitors mainly relies on microstructure, phase, specific surface area and electrical conductivity which in turn depend on the deposition technique and process parameters employed. Various physical vapour deposition techniques offer control over process parameters which enable us to tune the phase and microstructure [19, 25–31]. Among all physical vapour deposition techniques employed for the growth of copper oxide thin films, RF magnetron sputtering technique is one of the industrially practiced techniques for the preparation of uniform films with uniform thickness and with required chemical composition [32]. The substrate temperature employed during the preparation of thin films plays an

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important role in sputtering, since the relaxation time to dissipate kinetic energy of sputtered species, surface diffusion, surface structural changes, etc. are thermally activated processes. Besides that, the optimization of substrate temperature is of great importance in preparation of thin films as it directly related to the phase changes induced in copper oxides and also causes notable changes in electrochemical properties of copper oxides. Therefore, in the present investigation, copper oxide thin films are prepared by RF sputtering technique and the structural, morphological, electrochemical properties of films are studied as a function of substrate temperature in the range 30–400 C keeping the optimized O2-to-Ar ratio at 1:11 and RF power at 250 W [33, 34].

2 Experimental Copper oxide thin films of about 0.4 lm thick are deposited from a three-inch copper target of 99.99 % purity using RF magnetron sputtering technique onto wellcleaned stainless steel substrates. The distance between the target and the substrate is maintained constant at 8.0 cm. During the depositions, the substrate temperature (Ts) is varied from 30 to 400 C and the sputtered gas (O2:Ar) composition was maintained at 1:11. The RF power maintained during sputtering was 250 W, and the pressure inside the chamber while in the process of sputtering was about 9.4 9 10-3 mbar. The grown copper oxide thin films were characterized by studying their structural, morphology, electrical and electrochemical properties. The XRD spectra were taken by using Seifert X-ray diffractometer, in the 2h range 20– 60 with step width of 0.02/s. The peak positions were precisely determined by using REYFLEX-Analyze software, and the lattice parameter was calculated using the standard formula. The modes of vibration were studied by Horiba LabRam HR800 Raman spectrophotometer, in the wavenumber region 100–800 cm-1, using an excitation wavelength of 532 cm-1 (Nd:YAG laser). The morphological studies were carried out using NT-MDT SOLVER NEXT atomic force microscope. The grain size and surface rough were analysed using Nova PX software. Standard four-probe technique was employed to study the electrical properties. The electrochemical measurements, cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) studies were performed by designing a prototype three-electrode aqueous electrochemical cell using CHI 608C (CH Instruments Inc., USA) electrochemical analyser. The RFsputtered CuxO thin film coated on well-cleaned stainless steel substrate is employed as working electrode. A platinum counter electrode, which acts as a reversible source

Growth, microstructure and supercapacitive performance of copper oxide thin films prepared…

and sink of ions, and Ag/AgCl (3 M KCl) as reference electrode, by which the electrochemical analysis is calibrated in the presence of a phosphate-buffered aqueous solution (PBS) of pH 7 as electrolyte, are employed.

3 Results and discussion 3.1 Structure and morphology Figure 1 shows the X-ray diffraction patterns of as-deposited copper oxide thin films prepared at different substrate temperatures. The absence of diffraction peaks in the XRD pattern for the films prepared below 150 C reveals the amorphous nature of thin films. The copper oxide thin films prepared at 150 C showed a weak reflection at  35.55, which corresponds to the (111) orientation of monoclinic CuO phase (JCPDS Card No: 89-5897). As the substrate temperature increased from 150 to 200 C, the  (111) reflection corresponds to CuO phase gradually disappeared and films exhibited two feeble diffraction peaks

Fig. 1 XRD spectra of copper oxide thin films deposited at different substrate temperatures

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at 36.44 and at 42.32 corresponding to the (111) and (200) (JCPDS Card No: 78-2076) planes of cubic Cu2O, respectively. This clearly indicates that a phase change was occurred at the substrate temperature of 200 C. A further increase in substrate temperature from 200 to 400 C, no phase conversion was observed and all the films exhibited reflections corresponds to cubic Cu2O with predominant (111) orientation. Besides that, the XRD pattern reveals that the films prepared at higher temperatures above 150 C are nanocrystalline. As can be seen from XRD pattern, the relative intensity of (111) plane compared to that of (200) plane of Cu2O gradually increases as the substrate temperature increases from 200 to 350 C and attains a maximum at 350 C. A further increase in substrate temperature decreased the relative intensity of (111) orientation. The average crystallite sizes were calculated using Scherrer’s equation and are tabulated in Table 1. The observed crystallite size was found to be increased upon increasing substrate temperature from 150 to 350 C and then decreases with further increase in substrate temperature. At low substrate temperatures (\150 C), the ad-atom experiences a large potential barrier at the substrate surface which opposes the ad-atom diffusion to the substrate surface and also processes lower ad-atom mobility. And hence the species landing at the substrate may be stuck at the respective landing sites and coalescence does not take place, leading to the amorphous nature of the films [35]. As the substrate temperature increases, the substrate potential barrier becomes flatten and the ad-atom diffusion takes place easily and also the mobility of ad-atoms increases on surface and coalescence takes place easily resulting an improvement in the crystallite size. In XRD, both the (111) and (200) reflections were observed and were used to calculate the lattice parameter. The evaluated lattice parameter for the films prepared at 200 C is 0.417 nm and continuously increased with the increase in growth temperature up to 350 C and then decreased with further increase in substrate temperature. At 350 C, the films exhibited relatively better crystallite size of 29 nm and lattice parameter a = 0.423 nm which is very nearer to the bulk (0.426 nm) indicating the stability

Table 1 Variation in crystallite size, lattice parameter, grain size, roughness and resistivity as a function of temperature Substrate temperature (C)

Crystallite size from XRD (nm)

Lattice parameter (nm)

Grain size from AFM (nm)

Roughness (nm)

Resistivity (ohm cm)

150

7

–

38

25

15.26

200

17

0.417

50

31

1.24

300

22

0.421

65

36

3.1 9 10-2

350

29

0.423

72

51

1.02 9 10-3

400

17

0.422

58

43

9.2 9 10-3

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Fig. 2 Variation strain and dislocation density of Cu–O thin films with substrate temperature

of films. The dislocation density (d) and strain values are calculated by using the relations  d ¼ 1 D2 ð1Þ e ¼ b cos h=4

ð2Þ

The variation in strain and dislocation density with substrate temperature is shown in Fig. 2. The lower grain size at lower substrate temperatures yields large number of grain boundaries which creates excess free volume in the film and causes radial hydrostatic pressure which is responsible for high stress values and high dislocation densities resulting lower lattice parameter (a) [36, 37]. As the substrate temperature increases from 150 to 350 C, the grain size gets improved resulting in less free volume which in turn yields lower stress and increased ‘a’ value in the films. As the substrate temperature increased further from 350 to 400 C, the creation of oxygen vacancies and decrease in grain size lead to distortion in local symmetry and the bond length which results increased stress values in the films. Figure 3 shows the Raman spectra of copper oxide thin films prepared at various substrate temperatures in the wave number range 100–800 cm-1. The Raman spectra of the prepared thin films exhibited characteristic peaks correspond to CuO and Cu2O. The Raman spectra of the copper oxide thin films prepared at RT and 150 C show a strong peak at 298 cm-1 corresponding to Ag Raman-allowed mode of CuO phase. The Raman spectra of thin films prepared at substrate temperatures in the range of 200–400 C exhibited characteristic peaks which corresponds to Cu2O phase. According to the group theoretical analysis, the Raman spectrum of a perfect Cu2O crystal should exhibit one one-phonon Raman signal only, that of the threefold-degenerate T2g mode [38]. However, a typical

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Fig. 3 Raman spectra of Cu–O thin films deposited at different substrate temperatures

Raman spectrum of cuprous oxide is much richer with a multitude of Raman signals that have been assigned to different one-phonon scattering processes in addition to the background due to two-phonon scattering. This is because of violation of selection rules due to the presence of local symmetries caused by the point defects which reduce crystal symmetry. Thus, vibrational modes that are Raman forbidden in a perfect crystal may become Raman allowed due to the reduction in symmetry caused by the point defects. In general, the band positions and relative intensities are very sensitive to the oxidation state of copper oxide films and also depend on different scattering geometries and polarization conditions, different excitation conditions (i.e. in-resonance and off-resonance with the excitonic transitions of Cu2O), surface treatment or ionimplantation of the samples [39, 40]. The various peaks observed in Raman scattering spectroscopy are assigned as follows. The peak located at 511 cm-1 can be attributed to T2g Raman-allowed mode. The peak at 218 cm-1 originated from the second-order Eu Raman-allowed mode of the Cu2O crystals. The peak at 148 cm-1 may be attributed to Raman scattering from phonons of T1u symmetry of Cu2O. The weak peak at 612 cm-1 is attributed to the infrared-allowed mode of Cu2O phase [38]. Apart from the features of CuO, the Raman spectrum of copper oxide thin films prepared at lower substrate temperature (\200 C) showed characteristic peaks that corresponds to Cu2O which were not detected in XRD spectra indicating coexistence of CuO and Cu2O. The appearance of Raman vibrational modes corresponding to Cu2O only for the films deposited in the substrate temperature range 200–400 C established the phase purity of the films. A shift in the peak position for Eu (218 cm-1) Raman-allowed mode observed

Growth, microstructure and supercapacitive performance of copper oxide thin films prepared…

in the Raman spectra is because of the induced stress in the films. A lower phase shift was observed in the Raman spectra of thin films prepared at 350 C which is in correlation with the XRD results. The surface topography of CuxO thin films was studied by AFM measurements. Figure 4 displays the AFM images of CuxO thin films deposited at different substrate temperatures. The grain size and surface roughness values measured from AFM are presented in Table 1. At low substrate temperature, the sputtered species may not have sufficient energy to overcome the potential energy of the

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nucleation sites of the substrate, and hence, low temperature grown films are observed to have amorphous nature. Well-defined grains and grain boundaries are observed with increase in substrate temperature. The development in the grain size and surface roughness was observed with the increase in substrate temperature from 200 to 350 C. A maximum grain size of 72 nm and surface roughness of 51 nm were observed for the films prepared at 350 C. At a substrate temperature of 350 C the films exhibited open structure which allows greater surface-to-volume ratio which results in higher specific capacitance values. A

Fig. 4 AFM images of copper oxide thin films deposited at various substrate temperatures

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further increase in substrate temperature leads to decrease in grain size and roughness of the films. At moderate substrate temperature, the mobility of ad-atoms enhances on the surface, which leads to overcome the potential energy of the nucleation sites on the substrate with net increase in the diffusion distance. In addition to this, the collision process initiates the nucleation and favours the island formation in order to grow continuous film with larger grain size. A further increase in substrate temperature from 350 to 400 C and the decrement in grain size were observed. This may be due to increase in probability of escaping particles from substrate surface caused by higher thermal energy. 3.2 Electrical properties The supercapacitive and sensing properties of the copper oxide films mainly depend on the resistivity offered by the films, and hence, the electrical properties of the films were also studied. The variation in electrical resistivity with substrate temperature is presented in Table 1. The electrical resistivity of the films decreases from 44 to 1.02 9 10-3 ohm cm as the substrate temperature increases from 30 to 350 C and then decreases to 9.2 9 10-3 ohm cm as the temperatures increased to 400 C. At low substrate temperatures, as evident from XRD and Raman studies, the films exhibited CuO phase which provides insulating behaviour to the films. As the substrate temperature increased from 200 to 400 C, the films exhibited Cu2O phase which is generally p-type semiconductor. The p-type conduction in Cu2O arises due to the formation of acceptor levels due to copper vacancy created in valance band (VB) and is responsible for the p-type conductivity in Cu2O. The electrical conductivity and hole density of p-type Cu2O films vary with copper vacancy density, which act as shallow acceptors. The electrical properties of Cu2O also relied on grain dimensions, grain boundary, film thickness, specific phase and dopants that are present in the films. As the substrate temperatures increased from 200 to 350 C, the grain size of the films increased which decreases the electrical resistance. 3.3 Electrochemical properties The supercapacitive performance of the RF-sputtered copper oxide electrodes prepared at different substrate temperatures was investigated by using cyclic voltammetry (CV) in a phosphate-buffered saline aqueous solution with a three-electrode electrochemical cell. Figure 5 shows the cyclic voltammograms (CVs) of Cu–O thin films prepared at various substrate temperatures recorded in a potential window of 0.0 to -1.0 V with respect to Ag/AgCl reference electrode at different scanning rates ranging from 1 to

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10 mV s-1. The CV results show that a Faradic charge transfer processes occurred for the Cu–O films, indicating a pseudocapacitance effect. Obviously, the CV strongly depends on microstructure which in turn depends on the substrate temperature employed during the growth of the films. It was observed that the peak potentials shift towards higher anodic potentials and cathodic potentials with increasing scan rate. The increase in CV response at different scan rates explains that the electrochemical kinetics of copper oxide electrodes is controlled by a diffusion mechanism. The copper oxide electrodes prepared at lower substrate temperature (in the range 30–150 C) do not show any redox peaks, and hence, these films are not suitable for supercapacitor application. The CV of films prepared in the temperature range of 200–400 C exhibited two distinct anodic peaks near to -0.4 and -0.8 V and two distinct cathodic peaks at -0.8 and -0.22 V. The anodic peak at -0.8 V is attributed to the formation of Cu(III) species. The anodic peak at -0.45 V and corresponding cathodic peak at -0.22 V correspond to the Cu(I)/Cu(II) redox couple and formation of Cu(OH)2 which can be written as [41, 42]: CuO þ H2 O , Cu(OH)2 

Cu2 O þ 2OH , 2CuO þ H2 O þ 2e

ð3Þ 

ð4Þ

The variation in areal capacitance with substrate temperature at different scan rates is shown in Fig. 6. The plot reveals that the areal capacitance of the films increased with increase in substrate temperature from 200 to 350 C and then decreased with increase in temperature. A highest areal capacitance of 30 mF cm-2 at 5 mV s-1 was observed for the films prepared at a substrate temperature of 350 C which is comparable to the most commonly used supercapacitor materials such as molybdenum oxide, vanadium oxide and ruthenium oxide films (17–38 mF cm-2) [43–45]. From the XRD, AFM and CV results, it is observed that the specific capacitance of the films mainly depends on two factors, one is the intensity of (111) planes and other is roughness and grain size of the samples which in turn depend on the substrate temperature. According to density functional theory and thermodynamic calculations on Cu2O, it is confirmed that the [111] planes correspond to the Cu2O phase and are most stable when compared to Cu2O [100] planes which are more robust [46]. Hence, in aqueous solutions in order to minimize the surface energy, the (200) planes of Cu2O oxidize to form CuO2- which then converts to non-reducible CuO upon application of voltage. As a result, these planes become non-conductive and reduce the active number of sites contributing to electrochemical process, whereas the Cu2O (111) planes are stable and convert to reducible CuO on the application of voltage. On the other hand, the films with

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Fig. 5 Cyclic voltammetry profiles for copper oxide thin films at different scan rates

high surface roughness increase the active area which increases the available number of active sites for electrochemical redox process. The films prepared at 350 C exhibited higher surface roughness with predominant (111) planes and with high areal capacitance compared to other films. This may be due to the high electronic conductivity of material which provides easy transportation of redox species and increases the overall performance of the electrode. GCD studies were also used to study the specific capacitance performance of copper oxide thin films. The GCD profiles of copper oxide thin films at 0.5 mA cm-2 current density in PBS solution (pH of 7.0) are shown in Fig. 7. The specific capacitance of the prepared copper oxide thin films was calculated using the formula Fig. 6 Variation in areal capacitance as a function of substrate temperature at different scan rates

Cs ¼

I ðDV=DtÞw

ð5Þ

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Fig. 7 Charge and discharge of Cu–O thin films deposited at different substrate temperatures at current density of 0.5 mA cm-2

where ‘Cs’ is the specific capacitance in F g-1, I is the current in A, DV is the potential drop during discharge in ‘V’, Dt is the total discharge time in ‘s’ and w is the weight of the material in ‘g’. The film prepared at 200, 300, 350 and 400 C exhibited specific capacitance values of 183, 271, 908 and 475 F g-1, respectively, at a current density of 0.5 mA cm-2. From GCD studies, it is revealed that most of the charging of thin films was observed in the potential region of -0.4 to 0.5 V, while discharging a large potential drop for all the electrodes, except the electrodes prepared at 350 C, was observed due to polarization resistance or equivalent series resistance (ESR) offered by the electrode. The decrease in IR drop for the films prepared at 350 C may be due to the presence of large number of (111) planes and high surface roughness offered by the electrode. Besides that, these films exhibited nearly a flat plateau correspond to Cu(I)/Cu(II) redox couple which barely contributes to large specific capacitance of the electrodes. This result reveals that the films prepared at 350 C have much lower internal resistance which is of great interest in fabricating higher specific capacitance supercapacitor. Figure 8 represents the GCD profiles of Cu2O thin films prepared at substrate temperature of 350 C carried out at different current densities. A maximum specific capacitance of 908, 503, 356 and 225 F g-1 was observed at 0.5, 1, 2 and 5 mA cm-2 current densities, respectively. The decrease in specific capacitance with increase in discharge current density may be due to large potential drop and less utilization of active material. The electrochemical impedance spectroscopic (EIS) measurements of Cu–O thin films were taken in the frequency range from 0.01 Hz to 1 MHz in PBS solution, and corresponding Nyquist plots are shown in Fig. 9. The Nyquist plots of the films exhibited depressed semicircle

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Fig. 8 GCD profiles of Cu–O thin films deposited at substrate temperature of 350 C at different current densities

Fig. 9 Nyquist plots of Cu–O films prepared at different substrate temperatures

in high frequency region from which electrochemical reaction impedance of the electrode can be known. The series resistance (Rct) evaluated from the Nyquist plot was found to be decreased as the substrate temperature increased from 200 to 350 C and then increased with substrate temperature. This may be due to high surface roughness of the films which provide the increased amount of active area, thus improving the energy density per unit area. Moreover, efficient pathways for ion and electron transport through the entire electrode architecture are improved. As a result, the time constant ‘t’ for ion diffusion is evidently shortened. At lower frequencies, the films exhibited a nonlinear branch corresponding to diffusion resistance. The films prepared at 350 C exhibited low diffusion resistance compared to all other electrodes and also exhibited lower imaginary part of impedance

Growth, microstructure and supercapacitive performance of copper oxide thin films prepared…

Fig. 10 Cycling stability of Cu–O thin films prepared at different substrate temperatures

values even at higher frequencies due to the availability of more number of thermodynamically stable (111) planes. This result is in good agreement with the XRD, Raman and CV studies. Figure 10 shows the cycling stability of Cu–O films prepared under various substrate temperatures at 0.5 mA cm-2 current density. The films prepared at lower substrate temperatures (RT and 150 C) do not show capacitance behaviour due to high resistance offered by the CuO phase present in the electrodes. The other samples prepared at higher substrate temperatures exhibited good charge discharge characteristics even after 1000 cycles. A sharp decrease in specific capacitance was observed during early cycles which may be due to dissolution of material and irreversible electrode reactions taking place at the surface of the electrodes. After 200 cycles, all the thin films get stabilized and the films able to deliver constant specific capacitance values. The films prepared at 350 C exhibited a high specific capacitance of 908 F g-1 and decreased to 724 F g-1 after 200 cycles. The films retain 80 % of its initial capacity even after 1000 cycles. Thus, the open nanostructure, high surface roughness and high relative intensity of (111) planes strongly enhance the reversible redox reaction process which helps in obtaining high specific capacitance and good cycle ability of the electrode.

4 Conclusion Copper oxide thin films were prepared using RF magnetron sputtering technique at various substrate temperatures ranging from 30 to 400 C on stainless steel substrates keeping other process parameters constant. The induced phase changes in copper oxide thin films as a function of

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substrate temperature and their relative electrochemical properties are reported in detail. The Raman and XRD studies of copper oxide thin films prepared at 350 C confirm the growth of single-phase Cu2O with predominant (111) orientation. The estimated maximum grain size from AFM studies was 72 nm with surface roughness of 122 nm. A maximum areal capacitance of 30 mF cm-2 at 5 mV s-1 was observed for these films. The charge–discharge profile of copper oxide thin films demonstrated a superior specific capacitance value of 908 F g-1 at 0.5 mA cm-2 current density. It is also observed that 80 % of the specific capacity of these films retained after 1000 cycles. Thus, the open structure, high surface roughness with large number of thermodynamically stable (111) planes allows greater specific capacitance values for copper oxide thin films prepared at 350 C. The greater specific capacitance of these copper oxide thin film electrodes showed that copper oxide has a promising future as potential electrode material for pseudocapacitors. Acknowledgments This research work was supported by DST under Promotion of University in Research Science and Excellence (PURSE) programme.

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