Indian J Phys DOI 10.1007/s12648-017-1072-5

ORIGINAL PAPER

RF-sputter deposited flexible copper oxide thin films for electrochemical energy storage B Purusottam Reddy1, K Sivajee Ganesh2, S-H Park1 and O M Hussain2* 1

Department of Electronics Engineering, College of Engineering, Yeungnam University, Gyeongsan-si, Gyeongsangbuk-do 38541, Republic of Korea 2

Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Received: 18 October 2016 / Accepted: 23 May 2017

Abstract: Flexible Cu–O thin films were grown on different metal coated non rigid polyimide substrates by the RFmagnetron sputtering and their microstructure and supercapacitive properties were studied. The Raman and XRD studies confirms the formation of a single Cu2O phase with predominant (111) orientation. Surface topography observations revealed that as the percentage of lattice mismatch of Cu2O lattice and substrate lattice is greater the average grain size tends to decrease. The Cu2O films deposited on Ti-Kapton, Ni-Kapton and Pt-Kapton substrates exhibited maximum specific capacitances of 255, 273 and 350 F g-1 correspondingly at a constant current density of 1 A g-1. The observed maximum specific capacitance for CuxO thin films deposited on Pt-Kapton substrates is due to the lower lattice mismatch, high work function of the metal, availability of (111) planes and highly available active area. Keywords: Sputtering; Flexible CuxO thin films; Supercapacitor PACS Nos.: 81.15.Cd; 68.60.Bs; 88.80.fh; 88.85.jp

1. Introduction In past two decades, the world has witnessed a revolution in the field of non-rigid electronic devices, such as flexible displays, smart watches, e-paper, flexible sensor devices and smart products which often utilize input power. As a result, a continuous demand has been created for flexible, cheap, highly efficient and recyclable energy storage devices, such as non-rigid supercapacitors (FSCs) and batteries [1, 2]. Among these energy storage devices, flexible batteries, which offer a high energy density, are commonly used as primary energy sources for these devices. However, at startup, these devices require a large amount of input power which cannot be drawn from flexible batteries. On the other hand, due to their high power density, flexible supercapacitors (FSCs), which can provide an ample power for a short interval of time, can be used at startup. Apart from their high energy density, FSCs also offer a high cycle life, which make them promising energy

*Corresponding author, E-mail: [email protected]

storage devices to use along with flexible batteries [3, 4]. Although FSCs possess a remarkable power density, they are still far behind their rigid counterparts. To address this problem, a systematic understanding of mechanical, electrochemical, and interfacial properties of the flexible electrodes is crucial. Recently, polyimide Kapton flexible substrates with improved conductivities were obtained by depositing thin metallic layers and have been used as supporting structures for the deposition of active materials [5, 6]. Various metallic layers, such as platinum (Pt), titanium (Ti) and nickel (Ni) have been widely used as support structures in FSCs, owing to their high conductivity and ductility, high work function and simplicity in growth. Instead, the selection of the active material is also important in the fabrication of FSCs. Due to the practically achievable high specific capacitance and the existence of multiple valance states motivated researchers to focus on the transition Metal Oxides (TMOs), for instance Ru2O [7–10], MnO2 [11], Co3O4 [12], NiO [13–15], and CuxO [16, 17] as active materials for pseudo capacitors. Among all of the TMOs under study, copper oxides have been of particular research interest as the active material for supercapacitors, due to their cost effectiveness, non-

Ó 2017 IACS

B Purusottam Reddy et al.

toxicity, generous resources and great theoretical capacitance of 690 F g-1. Both physical vapour deposition techniques and chemical routes are employed for the preparation of Cu–O thin films [18–24]. Among these techniques, sputtering which is plasma based thin film deposition technique and is widely accepted industrial technique for the preparation of homogeneous thin films with good adhesion. Furthermore, RF sputtering facilitates the deposition of electroactive materials with increased energy density and flexibility on flexible substrates by tuning the various process parameters such as substrate temperature, O2:Ar ratio and RF power. However, the growth of thin films on non-rigid substrates for cuttingedge technologies is significant and new research area. Hence, the present investigation aims to deposit thin films of copper oxide on polyimide Kapton flexible substrates coated with different metals (Pt, Ni and Ti) using the RF magnetron sputtering technique using the optimized O2:Ar ratio (1:11), RF power (250 W) [25, 26] and substrate temperature of 300 °C and study their microstructural and electrochemical properties.

2. Experimental details Firstly a thin metallic layer (Pt, Ni and Ti) was deposited on to the well cleaned polyimide substrates (Kapton) using the thermal evaporation technique. The Cu–O thin films were grown via RF sputtering technique from a Cu target on to Pt, Ni and Ti coated polyimide Kapton substrates using the optimized sputtered gas (O2/Ar) composition of 1:11 and RF power of 250 W while keeping the substrates at a temperature of 300 °C. The depositions were carried out for 10 min while maintaining the sputter pressure at 9.1 9 10-3 mbar. The micro structural properties of films were analyzed by using XRD technique (Siefert-model 3003 TT) in the 2h range of 10° to 60° with a step width of 0.03°/s and the peak positions were precisely determined by using REYFLEX-Analyze software. The crystallite size was determined by using the Scherrer equation. The vibrational modes were studied in the wavenumber region of 100–800 cm-1 using Raman spectrophotometer (Horiba Labram HR800) at 532 cm-1 excitation wavelength. The surface topographical features of the films were probed by atomic force microscope (NT-MDT SOLVER NEXT) and scanning electron microscope (Carl Zeiss EVO MA 15). Electrochemical studies were done by means of a CHI 608C electrochemical analyzer (CH Instruments Inc., USA) to understand the electrochemical behavior of the Cu–O thin film electrodes. Electrochemical studies were done by employing three electrode configuration where Ag/AgCl used as reference electrode, a Cu–O flexible

electrode used as the working electrode, a thick foil of platinum having higher dimensions than the working electrode used as counter electrode, and Phosphate Buffer solution (PBS) at pH = 7 was used as the electrolyte.

3. Results and discussion 3.1. Structure and morphology The XRD profiles of the Cu–O thin films grown on the metal coated (Ti, Pt and Ni) Kapton substrates were shown in Fig. 1. The films displayed distinctive features with preferred (111) orientation corresponding to cubic Cu2O  symmetry. Generally, in sputtering structure of Pn3m process, the growth direction and film formation density mainly depends on the RF power, O2:Ar ratio and substrate temperature employed during the sputtering, as well as on the substrate chosen to deposit the films. In the case of flexible substrates, the crystal structure, deformation and augmentation of the films across the substrate surface are limited by the rupture of the polymer substrate, as described in a previous study [27]. Moreover, the adatom mobility in the case of flexible substrates across the surface is limited, and the rate of downward funneling is comparatively larger. As a result the process of coalescence of

Fig. 1 XRD spectra of Cu–O on different metal coated flexible substrates

RF-sputter deposited flexible copper oxide thin films

Fig. 2 Raman spectra of Cu–O on different metal coated flexible substrates

islands does not take place and large crystallite centers are formed, resulting in smaller grain size. In the present case, the films deposited on the Pt coated Kapton (Pt-Kapton) substrate exhibited a high intensity of (1 1 1) planes when compared to the films deposited on the Ni coated Kapton (Ni-Kapton) and Ti coated Kapton (Ti-Kapton), indicating high crystalline and thermodynamic stability of the films. The lattice parameter was calculated by considering the FWHM value of preferred (111) orientation and were observed to be 0.413, 0.407 and 0.403 nm for the films grown on the Pt-Kapton, Ni-Kapton and Ti-Kapton substrates, respectively. The observed lattice parameters of the films are comparatively low with respect to bulk Cu2O (0.427 nm), indicating inherent compressional stresses in the films. Generally, the growth rate and direction mainly depend on the predominant orientation and lattice parameter of the substrate [28, 29]. In our case, the Ni-Kapton and Ti-Kapton substrates exhibited predominant (111) and (110) orientations with estimated lattice parameters of 0.347 and 0.327 nm, respectively, which greatly differ from the lattice parameters of Cu2O with lattice mismatches of 18 and 24%, respectively. Hence, the films grown on these substrates have low crystallinity, while the film grown on the Pt-Kapton substrate exhibited a (1 1 1) orientation with lattice parameter of 0.383 nm with the lowest lattice mismatch of 7%. Hence, these films exhibited good crystallinity with a (1 1 1) preferred orientation. It was also observed that the peak position of the (111)

Fig. 3 AFM topography of Cu–O films on different Metal coated flexible substrates

orientation shifts towards lower angles, indicating the presence of compressive stress, probably due to the mismatch between substrate and Cu–O lattices. Of the three substrates, the Pt-Kapton substrate exhibited less stress, indicating the higher stability of the films. It was observed that as the degree of (111) orientation increases, the distortion in the space lattice parallel to the (111) orientation decreases, resulting in a reduction in the micro stress. Raman scattering spectroscopy was used to examine the vibrational modes of as-grown Cu–O thin films and corresponding spectra are shown in Fig. 2. The Raman Spectra of all of the samples exhibited representative peaks corresponding to Cu2O phase. Cu2O crystallizes in the cuprite structure with two formula units per unit cell resulting eighteen vibrational modes with symmetries F2u ? Eu ? 3F1u ? A 2u ? F2g, of which only the F2g mode is Raman active (gerade mode) [30–32]. Apart from the characteristic feature

B Purusottam Reddy et al. Fig. 4 SEM image of Cu–O films on Pt-Kapton substrates (inset (a) As prepared Cu2O Thin films on Pt-Kapton Substrate (b) 3D view of Cu–O films on Pt-Kapton substrates constructed using ImajeJ software

of F2g (located at 511 cm-1), the Raman scattering showed additional features which are forbidden in pure crystals. This intrinsic violation of the selection rules is due to the breakdown of the crystal symmetry. The Raman scattering features sited at 148 and 218 cm-1 are ascribed to F1u symmetry and the second-order Eu symmetry of Cu2O crystals. The broad peak centered at 612 cm-1 is originated from IR allowed mode of Cu2O phase. The surface features of as-deposited Cu–O thin films were probed by AFM and the results are shown in Fig. 3. The grains are distributed randomly throughout the surface without any cracks. Cu–O films deposited on the PtKapton substrates exhibited spherical grains with high surface roughness, while the films coated on the NiKapton and Ti-Kapton substrates exhibited spherical grains with a smaller number of drains, resulting in their lower surface to volume ratio. The grain sizes estimated by AFM topography are 65, 79 and 93 nm for the films deposited on the Ti-Kapton, Ni-Kapton and Pt-Kapton substrates, respectively. This observation reveals that the grain size depends on the percentage of lattice mismatch of the Cu2O lattice and substrate lattice and it is found that as the mismatch increases, the grain size decreases. The RMS surface roughness values estimated by AFM are 20, 28 and 55 nm for Cu–O films grown on Ti-Kapton, Ni-Kapton and Pt-Kapton substrates, respectively. The increased grain size and roughness of the Cu–O films deposited on the Pt-Kapton substrates were attributed to their lower lattice mismatch and cube to cube lattice

growth. The SEM topography of the Cu–O thin film coated on the Pt-Kapton substrates (Fig. 4) reveals a uniform grain distribution throughout the surface without any cracks. The inset shows a 3D view obtained by SEM topography. The grain size and roughness values calculated using ImajeJ software were in good relation with AFM results. 3.2. Electrochemical properties Figure 5 describes the CV’s (cyclic voltammetry curves) of Cu–O thin films deposited on Ti-Kapton, Ni-Kapton and Pt-Kapton substrates with reference to an Ag/AgCl electrode at various scan rates after reaching the equilibrium condition. The CV’s of Cu–O films reveals that a Faradic charge transfer processes indicating a pseudo-capacitance. The CV of the as-deposited films exhibited two distinct oxidation and reduction peaks (A and B) corresponding to the Cu(I)/Cu(II) redox couple and the conversion of CuO to Cu(OH)2 which can be explained by the following reactions [33, 34] CuO þ H2 O , CuðOHÞ2 

Cu2 O þ 2OH , 2CuO þ H2 O þ 2e

ð1Þ 

ð2Þ

The CV results reveal that at a particular scan rate the maximum charge transfer that is responsible for Cu(I)/Cu(II) redox couple (peak A) and the area enclosed by the curve are maximum for the films deposited on the Pt-Kapton substrates. This may be attributed to high surface

RF-sputter deposited flexible copper oxide thin films

Fig. 6 GCD profiles of Cu–O thin films deposited on (a) Pt-Kapton (b) Ni-Kapton (c) Ti-Kapton flexible substrates at different current densities

Fig. 5 CV profiles for copper oxide thin films deposited on (a) PtKapton (b) Ni-Kapton (c) Ti-Kapton flexible substrates at different scan rates

roughness, less strain and large number of available (111) planes in the films. The Galvanostatic charge–discharge (GCD) profiles of Cu–O thin films at different current densities in PBS

solution are shown in Fig. 6. From GCD profiles the specific capacitance (Cs) was obtained using the formula: Cs ¼

I ð AÞ  DtðsÞ DV ðV Þ  wðgÞ

ð3Þ

The films deposited on the Pt-Kapton substrates exhibited Cs values of 350, 163 and 80 F g-1 at current densities of 1, 2 and 5 A g-1, respectively, while those deposited on the NiKapton substrates exhibited Cs values of 273, 147 and 59 F g-1 and those deposited on the Ti-Kapton substrates

B Purusottam Reddy et al.

Fig. 7 Nyquist plots of Cu–O films on flexible substrates

Fig. 8 Cycling stability of Cu–O thin films on Pt-Kapton substrate at 1 A g-1 current density (inset SEM image of Cu–O thin films on PtKapton substrate after 1000 cycles)

exhibited Cs values of 255, 113 and 53 F g-1 at the same current densities, respectively. All the electrodes displayed a potential drop during discharge due to the polarization resistance afforded by the electrodes. The large IR drop of the films may be due to the large compressive strain persistent in the films, which increases their resistivity. Compared to the films deposited on the Ni-Kapton and TiKapton substrates, the films deposited on the Pt-Kapton substrates exhibited a smaller IR drop. This is likely caused by two factors, the polarization and work function of the metal surface. Since Pt has a higher work function (5.64 eV) than Ni (5.35 eV) and Ti (4.33 eV), the films deposited on the Pt-Kapton substrate offers less ohmic resistance, resulting in a smaller potential drop. The discharge profile exhibited a nearly smooth plateau corresponding to the Cu(I)/Cu(II) redox couple, which is significantly contributes

in achieving the high specific capacitance of the electrodes. The considerable fall in capacitance values with increase of the current density is due to increased polarization resistance and the reduced usage of the active material for electrochemical reaction. Figure 7 shows Nyquist plots of Cu–O thin films carried out in the frequency range from 0.01 Hz to 1 MHz in PBS solution. The impedance results showed the large charge transfer resistance for the films deposited on the Ti-Kapton substrates, which results in the large IR drop observed in the charge–discharge curves. This may be due to the large compressive strain present in the films. While, the films grown on the Pt-Kapton and Ni-Kapton substrates displayed lower charge transfer resistances, which are likely caused by their high surface roughness, which increases the active electrode area for electrochemical reactions to takes place and less strain present in the films. At lower frequencies, the films exhibited a nonlinear branch corresponding to the diffusion resistance. The films deposited on the Pt-Kapton substrates exhibited lower diffusion resistances compared to other films, which may be due to the presence of a greater number of thermodynamically stable (111) planes offering more ion conduction paths. The cycling permanence and columbic efficiency are two important factors for studying supercapacitor electrodes. Figure 8 displays the cycling permanence of the Cu–O films deposited on the Pt-Kapton substrates at constant current density of 1 A g-1. The films exhibited a sudden fall in Cs values during initial cycles due to the irreversible reactions occurring at the electrode surface and dissolution of the active material of the electrodes. The inset Fig. 8 shows SEM image of Cu2O thin films after 1000 cycles. From SEM analysis it is obvious that there were no observable cracks appeared in the films even after 1000 cycles. The features (grains) that are observed at initial stages were disappeared after 1000 cycles and the films become amorphous. This may be due to phase change from Cu2O to CuO which is preferably in amorphous state and loss of active material during cycling tests. The as-deposited films exhibited an initial Cs value of 350 F g-1 and after 200 cycles which decreased to 225 F g-1. The films retain 67% of their initial capacity even after 1000 cycles.

4. Conclusion Flexible Cu–O thin films were prepared using the RF sputtering technique on metal coated (Ti, Pt and Ni) flexible polyimide Kapton substrates, while keeping the substrate temperature and RF power at 300 °C and 250 W, respectively, in the presence of O2 and Ar gases at a flow ratio of 1:11. The structural studies confirms the formation of single phase Cu2O with preferred (111) orientation. The grain sizes

RF-sputter deposited flexible copper oxide thin films

estimated by AFM topography are 65, 79 and 93 nm for the films deposited on the Ti-Kapton, Ni-Kapton and Pt-Kapton substrates, respectively. The films deposited on the Ni-Kapton substrates exhibited specific capacitance values of 273, 147 and 60 F g-1 and those deposited on the Ti-Kapton substrates exhibited specific capacitance values of 255, 113 and 53 F g-1 at current densities of 1, 2, 5 A g-1, respectively, while the films deposited on the Pt-Kapton substrates exhibited specific capacitance values of 350, 163 and 80 F g-1 respectively, at the same current densities. The high specific capacitances of the CuxO thin films deposited on the PtKapton substrate resulted from their lower lattice mismatch, high work function of the metal, availability of stable (111) planes and high surface roughness. At a constant current density of 1 A g-1, these films displayed an initial Cs value of 350 F g-1 that decreased to 225 F g-1 after 200 cycles and retained 67% of their capacity even after 1000 cycles.

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