CeO2 spherical nanostructures for magnetic and ...

PCCP PAPER

Cite this: Phys. Chem. Chem. Phys., 2017, 19, 4396

Ni–CeO2 spherical nanostructures for magnetic and electrochemical supercapacitor applications Ramachandran Murugan,a Ganesan Ravi,*a Gandhi Vijayaprasath,a Somasundharam Rajendran,a Mahalingam Thaiyan,a Maheswari Nallappan,b Muralidharan Gopalanb and Yasuhiro Hayakawac The synthesis of nanoparticles has great control over the structural and functional characteristics of materials. In this study, CeO2 and Ni–CeO2 spherical nanoparticles were prepared using a microwaveassisted method. The prepared nanoparticles were characterized via thermogravimetry, X-ray diffraction (XRD), Raman, FTIR, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM) and cyclic voltammetry (CV). The pure CeO2 sample exhibited a flake-like morphology, whereas Ni-doped CeO2 showed spherical morphology with uniform shapes. Spherical morphologies for the Ni-doped samples were further confirmed via TEM micrographs. Thermogravimetric analyses revealed that decomposition varies with Ni-doping in CeO2. XRD revealed that the peak shifts towards lower angles for the Ni-doped samples. Furthermore, a diamagnetic to ferromagnetic transition was observed in Ni-doped CeO2. The ferromagnetic property was attributed to the introduction of oxygen vacancies in the CeO2 lattice upon doping with Ni, which were confirmed by Raman and XPS. The pseudo-capacitive properties of pure and Ni-doped CeO2 samples were evaluated

Received 4th December 2016, Accepted 9th January 2017

via cyclic voltammetry and galvanostatic charge–discharge studies, wherein 1 M KOH was used as the electrolyte. The specific capacitances were 235, 351, 382, 577 and 417 F g1 corresponding to the pure

DOI: 10.1039/c6cp08281e

1%, 3%, 5% and 7% of Ni doped samples at the current density of 2 A g1, respectively. The 5% Ni-doped

rsc.li/pccp

sample showed an excellent cyclic stability and maintained 94% of its maximum specific capacitance after 1000 cycles.

1. Introduction Energy conversion and storage is one of the main challenges in modern society, and in order to overcome this problem, portable energy storage devices are required. Nowadays, nanomaterials attract much interest due to their potential use in portable device applications such as supercapacitors, dilute magnetic oxides (spintronics devices), batteries, sensors, solar cells and biomedical applications for antibacterial and anticancer treatments.1–6 Among energy storage technologies, supercapacitors have received special attention, as they bridge the gap between batteries and conventional capacitors, and result in specific capacitances of six to nine orders of magnitude larger, higher energy densities, higher power densities, low equivalent series resistance and a long charge–discharge cycle life, compared to those of

a

Department of Physics, Alagappa University, Karaikudi-630 003, Tamil Nadu, India. E-mail: [email protected]; Fax: +91 4565-225202; Tel: +91 4565-230251 b Department of Physics, Gandhigram Rural Institute, Gandhigram-Deemed University, Tamilnadu, India c Research Institute of Electronics, Shizuoka University, Hamamatsu-432-8011, Japan

4396 | Phys. Chem. Chem. Phys., 2017, 19, 4396--4404

conventional capacitors.7 Nanoelectrodes are used in electrochemical technologies because of their high charge–discharge rates, which result from a high surface to volume ratio and short path length for electron and ion transport. Due to their high charge/discharge rates and long life, supercapacitors are used for practical applications in electric vehicles, laptops, cell phones, flashlights, and memory cards.8–10 In general, depending on the nature of the interfacial processes, supercapacitors can be classified as electrochemical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs are charged by the reversible adsorption of ions on the electrolyte–electrode interface of carbon-based materials, such as activated carbon, carbon nanotubes, aerogels and graphene, with high surface areas. On the other hand, pseudocapacitors are charged by redox reactions taking place on the surface of electrodes made of metal oxides or conducting polymers. Various materials, such as transition metal oxides, carbonaceous materials and conducting polymers, have been investigated for supercapacitor applications.11–17 Among these materials, RuO2 and IrO2 have been studied extensively due to their high intrinsic capacitance. However, their high cost, toxic nature and poor abundance limit their commercialization. To overcome these

This journal is © the Owner Societies 2017

Paper

problems, cheaper and better materials are needed to replace RuO2. Nowadays, much research is being devoted to carbonbased materials and transition metal oxides for supercapacitor applications.11 There is vast literature available on highperformance transition metal-based supercapacitors. NiO nanoparticles have been successfully synthesized by a single-source precursor method, which is very important for improving the manufacturing process of energy storage devices. Despite that, only few reports have been published on the effect of calcination on the structural and electrical properties of NiO nanoparticles.18,19 Likewise, only a limited amount of research work has been published in the field of rare earth oxide-based materials for energy storage applications. Consequently, there is a newly found interest in developing rare earth oxide materials with different morphologies for supercapacitor applications. There have been few very interesting reports on these recently developed class of nanomaterials, whose unique photophysical properties are helping to create a new generation of devices in the field of photonics and microelectronics.20–22 Due to their incomplete 4f shell, the trivalent and divalent rare earth ions provide very interesting optical and magnetic properties. Different valence states exist in rare earth oxides. It has been reported that rare earth ions enhance the specific capacitance and cyclic stability. Particularly, CeO2 is a promising rare earth oxide material that is widely used in several technological applications.8,23 Defects, such as oxygen vacancies in CeO2 play an important role in catalytic, magnetic and electrochemical properties, and the oxygen vacancies, can be easily formed and removed. Oxygen vacancies in CeO2 are highly reactive with transition metal, graphene–CeO2 nanocomposite, which could enhance the photocatalytic, magnetic and electrochemical properties. Despite this, transition metal-doped rare earth-based nanoparticles have rarely been investigated for their magnetic and electrochemical properties. In the present study, Ni–CeO2 nanoparticles were synthesized with different weight percentages of Ni. The prepared nanoparticles were characterized via various analytical techniques to determine their structural, optical, compositional, morphological and magnetic properties. The electrochemical performance of the Ni–CeO2 nanoparticles was evaluated by cyclic voltammetry and galvanostatic charge–discharge tests, and the results are discussed in detail.

2. Experimental In a typical synthesis of CeO2 and Ni–CeO2 nanoparticles, 0.2 M of cerium nitrate hexahydrate was dissolved in 40 ml of distilled water and stirred for 5 min at room temperature. Subsequently, 0.1 M of citric acid were added as a capping agent. Simultaneously, in order to keep the pH around 10, 3 ml of ammonia was added dropwise to this aqueous solution under vigorous stirring. Then, the solution was transferred to a polypropylenelined autoclave bottle and irradiated by microwaves with a power of 300 W for 10 min in a microwave oven (Samsung CE1031LFB), which works at 2.45 GHz and a maximum power of 900 W. The system was then allowed to cool to room temperature.


PCCP

The resultant products, at the bottom of the vessel, were collected via centrifugation and the precipitates were rinsed several times with distilled water and finally with methanol to remove soluble ions. Product was subsequently dried at 80 1C in a hot air oven for 12 h and then calcined at 500 1C for 3 h. To obtain Ni-doped CeO2 nanoparticles of different compositions, the same procedure was followed with different concentrations of Ni (1, 3, 5 and 7 wt%), and products obtained were named as CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7, respectively. Thermogravimetric analyses were conducted with a Perkin Elmer Pyris Diamond analyzer. The structural properties of nanoparticles were studied via X-ray diffraction (XRD) with a Cu-Ka (l = 0.154 nm) radiation source (X’ pert Pro PANalytical) over a 2y scan range of 10–701. Raman spectra were recorded using a micro-Raman spectrometer (Acton Spectra Pro 2500i, Princeton Instruments, Acton Optics & Coatings). The functional characterization was performed via Fourier transform infrared (FT-IR) spectroscopy using a Thermo Nicolet 380 by the KBr pellet method at room temperature in the range of 4000–400 cm1. The morphologies of the prepared samples were studied using SEM (FEIQUANDA 200F Field Emission SEM (FESEM) operating at 30 kV) and TEM (JEOL JEM 2100 (200 kV) system). XPS analyses were performed under ultra high vacuum with an Al Ka (1486.6 eV) radiation source on a Carl Zeiss instrument. The magnetic properties of the nanoparticles were studied by Vibrating Sample Magnetometry (Lake Shore, Model: 7404). The electrochemical properties of Ni–CeO2 nanoparticles were studied with a using CHI-660D electrochemical workstation. The working electrode was prepared by separately mixing 80% of the nanoparticles (pure and doped with different percentages of Ni), as the active material, with a 15% of activated carbon, a 5% of polyvinylidene fluoride, as the binder, and a few drops of ethanol as the solvent. A slurry was formed and deposited (3 mg of active material) onto a nickel foam electrode (1 cm 1 cm). The prepared electrodes were dried at 80 1C for 10 h. The electrodes composed of either non-doped CeO2 or Ni-doped CeO2 with different percentages of Ni were employed as the working electrode, a saturated calomel electrode was used as the reference electrode and a platinum wire was used as the counter electrode. As the electrolyte, 1 M aqueous KOH solution was used.

3. Results and discussion Thermo gravimetric analyses (TGA) were conducted for pure and Ni-doped CeO2 nanoparticles to study the material behavior during annealing, and the results are shown in Fig. 1a. The first weight loss, of approximately 9.15%, occurs before 216 1C and corresponds to the loss of physically adsorbed water.24 The second weight loss, of 28.32%, in the range of 216–330 1C, is related to the loss of structural water molecules.25,26 The last weight loss occurs from 330 1C to 400 1C and is ascribed to the decomposition of metal nitrate groups in the prepared nanoparticles.27 There is no major weight loss observed at higher temperatures, which indicates the absence of additional phases or other structural changes. Compared with the pure CeO2, Ni–CeO2 shows a

Phys. Chem. Chem. Phys., 2017, 19, 4396--4404 | 4397

PCCP

Paper

Fig. 1 (a) Thermo gravimetric analyses for non-doped and Ni-doped CeO2; (b) powder X-ray diffraction patterns for non-doped and Ni-doped CeO2 (with different percentages of Ni) nanoparticles; (c) Raman spectra for non-doped and Ni-doped CeO2 (with different percentages of Ni) nanoparticles (the inset shows an enlarged portion of defects); (d) FTIR spectra for pure and Ni-doped CeO2 (with different percentages of Ni).

considerable weight loss up to 234 1C, which is characteristic of the loss of weakly bonded water on Ni–CeO2. Then, a gradual weight loss was observed up to 446 1C. Thus, TGA results confirm that the thermal stability of CeO2 is greatly enhanced by the presence of Ni. Fig. 1b shows the XRD spectra of pure CeO2 and Ni-doped CeO2 (with different percentages of Ni) nanoparticles. The diffraction peaks at the 2y values of 28.61, 33.11, 47.41, 56.51, 59.11, 69.61, 76.81 and 79.11 are ascribed to the crystal planes of (111), (200), (220), (311), (222), (400), (331) and (420), respectively, with a space group of Fm3% m (225). It is observed that there are no diffraction peaks corresponding to the doped material. A peak shift towards lower angle side was observed with the increase of Ni content in doped CeO2. The peak broadening is due to the presence of smaller crystallite size of CeO2 nanoparticles. As the Ni content increases, the intensity of the XRD peaks decreases, showing the decrease in crystallinity due to the generation of crystal defects around the dopants, resulting in a charge imbalance arises.28 The crystallite size, microstrain and dislocation densities of pure and Ni-doped CeO2 nanoparticles were estimated based on relevant relations.29 It is clear from the calculated parameters that CeO2 crystallite size decreases from 18 to 9 nm when Ni content increases from 0% to 5% and then increases for 7% of Ni. The data shows that the presence of Ni ions in CeO2 hinders the growth of crystal grains and slows the motion of grain boundaries due to the interruption by Zener pinning.30 When the moving boundaries are pinned by the interstitial cerium atoms and dopant Ni ions, they develop a retarding force on the boundaries. If the generated retarding force is more than the driving force for grain growth, then the


particles cannot grow further. For 7% Ni-doped CeO2 nanoparticles, the crystallite size increases, which may be due to the possible presence of some Ni atoms in the cubic interstitial sites, which in turn is due to the strong preference of Ni for cubic coordination with oxygen. The phase purity of the synthesized samples was further examined via Raman spectroscopy. This is an effective technique to detect the possible secondary phases, which cannot be observed below the detection limits of XRD. Furthermore, it gives information about structural defects caused by the dopants, as defects lead to a shift in Raman peaks. Fig. 1c displays the Raman spectra of non-doped and Ni-doped CeO2 nanoparticles. A well defined Raman band is observed at 447 cm1, which corresponds to the first order F2g mode of the cubic fluorite structure of CeO2. The observed Raman band has a large shift compared to bulk CeO2 (the F2g mode for bulk CeO2 is at 465 cm1). The F2g mode is very sensitive to any disorder/defects in the oxygen sublattice.31 Hence, a large shift in the F2g mode may be linked to the presence of Ce3+ ions and oxygen vacancies in the samples.32–34 The only differences between the Raman spectra of pure and Ni-doped CeO2 is the reduction in the peak intensity and the shift towards higher wavenumber for Ni-doped CeO2. The reduction in peak intensity is associated with lattice defects such as oxygen vacancies.35 For Ni–O, Raman peaks appear in the range of 570–645 cm1, which are displayed in the inset of Fig. 1c. Fig. 1d shows the FTIR spectra of pure and Ni-doped CeO2 nanoparticles calcined at 500 1C. Two broad absorption bands positioned at 3418 cm1 and 1532–1630 cm1 are associated to the symmetrical stretching (n OH) and bending modes (d OH)


Paper

of internally bonded water molecules, respectively. A band at 2363 cm1 is observed for the Ni-doped nanoparticles, which confirms additional CO2 absorption at the surface. Furthermore, the O–C–O stretching band is also observed in the 1335 cm1 region. The bending mode of Ce–O–C (d) is observed well below 650 cm1, confirming the formation of CeO2 without impurities.36,37 Finally, the peak positioned at 431 cm1 is attributed to the O–Ce–O stretching mode of vibration.38 The morphology of pure and Ni-doped CeO2 samples was investigated via FESEM. Fig. 2(a–e) shows the FESEM images of CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7, respectively. The growth mechanism of pure and Ni-doped CeO2 nanoparticles is represented in Fig. 3. It can be clearly seen from Fig. 2 and 3 that non-doped CeO2 nanoparticles are evenly distributed and have a flake-like morphology. For 1% of Ni doped sample, nanoflakes are aggregated and do not exhibit any particular shape. Upon further increasing the dopant percentage to 3%, 5% and 7%, the agglomerated mountains split to yield well-defined spherically-shaped particles that cover the entire surface. It can be clearly observed in Fig. 2 that the agglomeration is almost restricted to Ni-doped CeO2 particles, which exhibit a uniform shape and narrow size distribution. Moreover, some porosity is found in the background portion. The presence of larger particles for the samples with higher percentages of Ni might be attributed to the aggregation or overlapping of smaller particles. Fig. 4(b–d) shows the TEM images of CeO2 N5 nanoparticles with different magnifications and the selected area energy diffraction (SAED) pattern of the TEM micrograph is presented in the inset of Fig. 4d. Most grains exhibit sizes between 5–8 nm

PCCP

Fig. 3

Growth mechanism of pure and Ni-doped CeO2 nanoparticles.

Fig. 4 (a) FESEM image of CeO2 N5 nanoparticles; (b–d) TEM images of CeO2 N5 nanoparticles at different magnifications. Inset of Fig. 4d shows the SAED pattern for CeO2 N5 nanoparticles.

Fig. 2 FESEM images of (a) pure, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5 and, (e) CeO2 N7 nanoparticles.


and the average size is close to 6 nm. The polycrystalline nature of the prepared nanoparticles is further confirmed from the SAED pattern. These results agree well with the XRD data. The elemental composition and oxidation states in CeO2 N5 nanoparticles were examined using X-ray photoelectron spectroscopy (XPS) technique. The survey spectrum of CeO2 N5 nanoparticles shows (Fig. 5) sharp peaks corresponding to C 1s, O 1s, Ce 3d and Ni 2p. The Ce 3d, Ni 2p and O 1s spectra were fitted using the Gaussian fitting. Eight peaks were observed for the Ce 3d spectrum and these peaks correspond to different oxidation states of Ce3+ and Ce4+. The characteristic signals of Ce3+ due to the 3d5/2 and 3d3/2 electron states are observed at 878 and 896 eV (Fig. 6a), respectively, with a separation of 18 eV. The additional satellite signal observed at 919 eV is due to the 3d3/2 electron state of Ce4+. These peaks are denoted as v, v 0 , v00 and v 0 0 0 for Ce 3d5/2 and u, u 0 , u00 and u 0 0 0 for the Ce 3d3/2 spin orbit states. The main peaks observed at 878 and 919 eV represent the relative amounts of Ce3+ and Ce4+ in the sample, respectively. From the area of the respective peaks, it is clear that the concentration of Ce3+ is relatively higher in the samples, which confirms the


PCCP

Fig. 5

Paper

XPS wide survey of the CeO2 N5 nanoparticles.

Fig. 6 (a) Ce 3d, (b) Ni 2p and (c) O 1s deconvoluted XPS spectra of CeO2 N5; (d) VSM spectra for non-doped and Ni-doped CeO2 (with different percentages of Ni).

presence of oxygen vacancies in CeO2.39 The Ni 2p spectrum shows the two characteristic spin–orbit doublets of Ni3+ and Ni2+, with two shake-up satellites (Fig. 6b). On deconvolution, the O 1s spectrum shows three components at 531.19, 531.95 and 533.34 eV, corresponding to metal–oxygen bonds, surface hydroxyl or oxyhydroxide functional groups, and adsorbed metal atoms at the surface, respectively (Fig. 6c). The binding energy values of Ce, Ni and O atoms are in good agreement with those of previous reports.40,41 The prepared non-doped and Ni-doped CeO2 nanoparticles were characterized for their magnetic properties using vibrating sample magnetometry. Ni-doped CeO2 nanoparticles are ferromagnetic, whereas non-doped CeO2 exhibits a diamagnetic character. Semiconducting systems develop ferromagnetic behaviour when doped with 3d transition metal ions. For Ni, which has 3d8 + 4s2 valence electrons, the low spin state of Ni2+ shows that 3d8 electrons are not compensated, resulting in a spin state of S = 1/2. Hence, Ni is often used for synthesizing magnetic semiconductors. Ferromagnetism is due to the exchange interaction between unpaired electron spins arising from oxygen


vacancies formed at the surface of the nanoparticles.42 As the Ni concentration increases, the probability of forming oxygen vacancies on the surface of the nanoparticles also increases. The increased number of oxygen vacancies is also observed in the PL and Raman spectra. The uncompensated spins on the surface of the nanoparticles induce ferromagnetism, while the compensated spins at the inner core of the particle still exhibit the diamagnetic behavior.42 The surface to volume ratio of the nanoparticles system plays an important role in determining the magnetic properties of the prepared nanoparticles, such that there is a competition between diamagnetic and ferromagnetic behavior. Enhanced surface to volume ratio is used to the increase magnetization under high magnetic fields for the as-prepared nanoparticles. Fig. 6d shows the ferromagnetic properties of the as-prepared Ni-doped CeO2 nanoparticles. The increase in the dopant concentration of Ni2+ ions in the host increases the ferromagnetic property. Saturation magnetization considerably increases up to 5 wt% Ni and slightly decreases for 7 wt% doped samples, which can be attributed to the variation in crystallite size, as calculated from the XRD results. The capacitance of pure and Ni-doped CeO2 with various percentage of nickel was studied via cyclic voltammetry (CV) at different scan rates. Fig. 7 shows the CV curves of (a) pure CeO2, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5 and (e) CeO2 N7 in 1 M KOH solution, as the electrolyte. The electrodes coated with non-doped and Ni-doped CeO2 nanoparticles store charges at the electrode/electrolyte interfaces. As it can be seen from the CV curves, redox peaks are present for all electrodes at current densities of 2 A g1. Furthermore, the current densities of the Ni-doped CeO2 electrodes indicate an increased capacitance when compared to that of the pure CeO2 electrodes. These curves

Fig. 7 CV curves of the Ni foam electrode coated with the prepared nanoparticles: (a) pure, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5, and (e) CeO2 N7 nanoparticles at scan rates from 2 to 100 mV s1.


Paper

PCCP

are different from the rectangular CV curves of traditional electric double layer capacitors. CeO2 exhibits prominent redox peaks, indicating that the sample has typical faradic pseudocapacitance characteristics, attributed to the redox reaction between Ce3+ and Ce4+.43,44 Compared with the other electrodes, the CeO2 N5 electrode shows a higher current response in the potential range of 0 to 0.45 V, as shown in Fig. 7. The cyclic behavior and stability were studied, and slow-charge and fast-discharge tests were carried out using the chronopotentiometry technique.45 The charge–discharge measurements were carried out in 1 M KOH between 0 to 0.45 V at different current densities from 2 to 20 A g1. Fig. 8(a–f) shows the charge– discharge curves of (a) non-doped CeO2, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5 and (e) CeO2 N7, respectively. The specific capacitance (Cs) was calculated using the following formula:46 Cs ¼

IDt mDv

where I is the discharge current (mA), Dt is the discharging time (s), m is the mass of active material (g), and Dv is the potential window (V). The Cs values of pure CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7 samples are 235, 351, 382, 577 and 417 F g1, respectively at a current density of 2 A g1. As the Ni content increases (up to 5%), as shown in the figure the specific capacitance also increases. However, with excessive Ni content, as in CeO2 N7, the specific capacitance decreases due to structural disintegration and self-aggregation of nickel ions. The excessive self-aggregation of nickel can produce higher solution resistance (Rs), which reduces the capacitance output.

Fig. 8 Charge–discharge curves of the Ni foam electrode coated with the prepared nanoparticles: (a) pure, (b) CeO2 N1, (c) CeO2 N3, (d) CeO2 N5, (e) CeO2 N7 nanoparticles at different current densities; (f) variation in the specific capacitance for non-doped and Ni-doped CeO2 (with different percentages of Ni) nanoparticles at different current densities.


The variation in specific capacitance of non-doped and Ni-doped CeO2 at different current densities are presented in Fig. 8d. Among all samples, CeO2 N5 exhibits the highest specific capacitance. A Cs value of 167 F g1 has been reported by Padmanathan et al.47 for the NiO–CeO2 binary oxide, calcined at 700 1C. Rudra Kumar et al.48 reported a Cs value of 78 F g1 for CeO2 at a current density of 2 A g1 by the galvanostatic charge–discharge method. Among these reported values, CeO2 N5 exhibits the highest specific capacitance of 577 F g1 at a current density of 2 A g1. The charge storing mechanism in Ni-doped CeO2 can be represented using the following equation: Ce

IV

O2 + K+ + e 2 Ce

III

OOK

In order to explore the lifetime of the capacitor for practical applications, the stability of the electrode was tested by cyclic charge/discharge tests. Fig. 9 shows the specific capacitance for CeO2 N5 as a function of the number of cycles. The capacitance retention after the first and last few cycles is shown in the upper part of Fig. 9. There is gradual degradation observed throughout 1000 cycles. Furthermore, we observed that 94% of the maximum capacity was retained even after 1000 cycles. Recently, Dongyang Deng et al.49 achieved specific capacitance retention of 86% for graphene decorated with cerium oxide nanoparticles after 500 cycles. Zhang et al.41 reported a capacitance retention of 93.9% after 1000 charge/discharge cycles for MnO2/CeO2 composites. Vijayakumar et al.50 observed a capacitance retention of 88% after 1000 cycles for a NiO electrode. Compared to these results, our studies demonstrate increased capacitance retention even after 1000 cycles. As supercapacitors are power devices in an electrode, for supercapacitor applications, it is preferred to have a low electrochemical resistance. The impedance plots of the prepared CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7 electrodes in the frequency range from 0.01 to 100 kHz at the bias potential of 0.4 V are shown in Fig. 10. The inset of Fig. 10 shows the equivalent fitting circuit with pure and Co-doped electrodes. Rs is the solution resistance, Rct is the charge transfer resistance, Cdl is the double layer capacitance and W is the Warburg impedance.

Fig. 9 Charge/discharge curves indicating the cyclic stability of the CeO2 N5 electrode at 20 A g1.


PCCP

Paper

References

Fig. 10 Impedance plots of pure and Ni-doped CeO2 (with different percentages of Ni) electrodes. Inset shows the equivalent fitting circuit.

The semicircle portion represents the charge transfer resistance, which is 9.52, 7.12, 5.78, 4.21 and 6.34 O for the CeO2, CeO2 N1, CeO2 N3, CeO2 N5 and CeO2 N7 electrodes, respectively. The CeO2 N5 electrode gives the lower Rct value, which is due to the enhanced diffusivity of the OH ions at the porous electrode. The diffusive resistance of the OH ions within the CeO2 N5 electrodes (known as the Warburg resistance), is represented by the straight line situated in the lower frequency range.

4. Conclusion In summary, pure and Ni-doped CeO2 nanoparticles were prepared using a microwave-assisted method. The prepared nanoparticles were less than 20 nm in size, as confirmed from our XRD and TEM analyses. Oxygen vacancies were present in Ni-doped CeO2 samples, which was confirmed through Raman and XPS analyses. Furthermore, oxygen vacancies lead to a ferromagnetic behavior in the samples, as confirmed through VSM analysis. Electrochemical analyses showed that Ni-doped CeO2 electrodes exhibit significantly higher specific capacitances than that of the non-doped counterpart. A specific capacitance of 577 F g1 was achieved for the CeO2 N5 electrode. A capacitance retention of 94% was observed after 1000 cycles. These studies demonstrate that microwaves are suitable for elemental doping and that elemental doping is an effective way to improve the performance of pseudo-capacitive metal oxides, thus enabling the fabrication of active electrode materials for supercapacitor applications.

Acknowledgements The authors R. Murugan and S. Rajendran gratefully acknowledge UGC, New Delhi for awarding UGC-BSR (Grant No. F.25-1/ 2013-14 (BSR)/7-14/2007 (BSR)/30th May 2014.) and Emeritus fellowship respectively. G. Ravi acknowledge Japan Society for the Promotion of Science (JSPS), Japan, for the financial support under Invitation fellowship programme (Award Ref.: JSPS/236 ID No. S15167).


1 G. Anandha Babu, G. Ravi, T. Mahalingam, M. Kumaresavanji and Y. Hayakawa, Influence of microwave power on the preparation of NiO nanoflakes for enhanced magnetic and supercapacitor applications, Dalton Trans., 2015, 44, 4485–4497. 2 R. Murugan, G. Vijayaprasath, T. Mahalingam and G. Ravi, Enhancement of room temperature ferromagnetic behavior of rf sputtered Ni-CeO2 thin films, Appl. Surf. Sci., 2016, 390, 583–590. 3 Y. Sun, N. Liu and Y. Cui, Promises and challenges of nanomaterials for lithium-based rechargeable batteries, Nat. Energy, 2016, 1, 16071. 4 R. Senthilkumar, T. Mahalingam and G. Ravi, Studies on growth and characterization of heterogeneous tungsten oxide nanostructures for photoelectrochemical and gas sensing applications, Appl. Surf. Sci., 2016, 362, 102–108. 5 Y.-M. Sung, Y.-C. Lai, M.-F. Tsai, H.-H. Hsieh, M.-H. Yang, P. P. Wei, C.-S. Yeh, F.-C. Hsu and Y.-F. Chen, Broad band plasmonic nanomaterials for high performance solar cells, J. Mater. Chem. C, 2016, 4, 513–520. 6 K. Kasinathan, J. Kennedy, M. Elayaperumal, M. Henini and M. Malik, Photodegradation of organic pollutants RhB dye using UV simulated sunlight on ceria based TiO2 nanomaterials for antibacterial applications, Sci. Rep., 2016, 6, 38064, DOI: 10.1038/srep38064. 7 K. Wang, J. Huang and Z. Wei, Conducting Polyaniline Nanowire Arrays for High Performance Supercapacitors, J. Phys. Chem. C, 2010, 114, 8062–8067. 8 N. Maheswari and G. Muralidharan, Hexagonal CeO2 nanostructures: an efficient electrode material for supercapacitors, Dalton Trans., 2016, 45, 14352–14362. 9 M. F. El-Kady, M. Ihns, M. Li, J. Y. Hwang, M. F. Mousavia, L. Chaney, A. T. Lech and R. B. Kaner, Engineering threedimensional hybrid supercapacitors and microsupercapacitors for high-performance integrated energy storage, PNAS, Early Ed., 2015, 112, 4233–4238. 10 P. Bujlo, G. Pasciak, J. Chmielowiec and A. Sikora, Experimental Evaluation of Supercapacitor-Fuel Cell Hybrid Power Source for HY-IEL Scooter, J. Energy, 2013, 162457. 11 X. Long, Z. Wang, S. Xiao, Y. An and S. Yang, Transition metal based layered double hydroxides tailored for energy conversion and storage, Mater. Today, 2016, 19, 213–226. 12 C. Ramirez-Castro, C. Schutter, S. Passerini and A. Balducci, Microporous carbonaceous materials prepared from biowaste for supercapacitor application, Electrochim. Acta, 2016, 206, 452–457. 13 X.-L. Wu and A.-W. Xu, Carbonaceous hydrogels and aerogels for supercapacitors, J. Mater. Chem. A, 2014, 2, 4852–4864. 14 M. Aghazadeh, A. Bahrami-Samani, D. Gharailou, M. G. Maragheh, M. R. Ganjali and P. Norouzi, Mn3O4 nanorods with secondary plate-like nanostructures; preparation, characterization and application as high performance electrode material in supercapacitors, J. Mater. Sci.: Mater. Electron., 2016, DOI: 10.1007/s10854-016-5239-1.


Paper

15 M. Aghazadeh, A. Rashidi, M. R. Ganjali and M. G. Maragheh, Nickel oxide Nano-Rods/Plates as a High Performance Electrode Materials for Supercapacitors; Electrosynthesis and Evolution of Charge Storage Ability, Int. J. Electrochem. Sci., 2016, 11, 11002–11015. 16 M. Aghazadeh, M. G. Maragheh, M. R. Ganjali, P. Norouzi and F. Faridbod, Electrochemical preparation of MnO2 nanobelts through pulse baseelectrogeneration and evaluation of their electrochemical performance, Appl. Surf. Sci., 2016, 364, 141–147. 17 M. Aghazadeh, R. Ahmadi, D. Gharailou, M. R. Ganjali and P. Norouzi, A facile route to preparation of Co3O4 nanoplates and investigation of their charge storage ability as electrode material for supercapacitors, J. Mater. Sci.: Mater. Electron., 2016, DOI: 10.1007/s10854-016-4882-x. 18 K. Kaviyarasu, E. Manikandan, J. Kennedy, M. Jayachandran, R. Ladchumananandasiivam, U. Umbelino De Gomes and M. Maaza, Synthesis and characterization studies of NiO nanorods for enhancing solar cell efficiency using photon upconversion materials, Ceram. Int., 2016, DOI: 10.1016/ j.ceramint.2016.02.054i. 19 K. Kaviyarasu, E. Manikandan, J. Kennedy, M. Jayachandran and M. Maaza, Rice husks as a sustainable source of high quality nanostructured silica for high performance Li-ion battery requital by sol-gel method – a review, Adv. Mater. Lett., 2016, 7, 684–696. 20 K. Kaviyarasu, E. Manikandan, J. Kennedy and M. Maaza, A comparative study on the morphological features of highly ordered MgO:AgO nanocube arrays prepared via a hydrothermal method, RSC Adv., 2015, 5, 82421–82428. 21 K. Kaviyarasu, E. Manikandan, P. Paulraj, S. B. Mohamed and J. Kennedy, One dimensional well-aligned CdO nanocrystal by solvothermal method, J. Alloys Compd., 2014, 593, 67–70. 22 K. Kaviyarasu, E. Manikandan, J. Kennedy and M. Jayachandran, Quantum confinement and photoluminescence of well-aligned CdO nanofibers by a solvothermal route, Mater. Lett., 2014, 120, 243–245. 23 T. Saravanan, M. Shanmugam, P. Anandan, M. Azhagurajan, K. Pazhanivel, M. Arivanandhan, Y. Hayakawa and R. Jayavel, Facile synthesis of graphene-CeO2 nanocomposites with enhanced electrochemical properties for supercapacitors, Dalton Trans., 2015, 44, 9901–9908. 24 M. Aghazadeh, B. Arhami, A.-A. M. Barmi, M. Hosseinifard, D. Gharailou and F. Fathollahi, La(OH)3 and La2O3 nanospindles prepared by template-free direct electrodeposition followed by heat-treatment, Mater. Lett., 2014, 115, 68–71. 25 F. Khosrow-pour, M. Aghazadeh, B. Sabour and S. Dalvand, Large-scale synthesis of uniform lanthanum oxide nanowires via template-free deposition followed by heat-treatment, Ceram. Int., 2013, 39, 9491–9498. 26 M. Aghazadeh, A.-A. M. Barmia, H. M. Shiri and S. Sedaghat, Cathodic electrodeposition of Y(OH)3 and Y2O3 nanostructures from chloride bath. Part II: Effect of the bath temperature on the crystal structure, composition and morphology, Ceram. Int., 2013, 39, 1045–1055.


PCCP

27 M. Aghazadeh, M. Hosseinifard, B. Sabour and S. Dalvand, Pulse electrochemical synthesis of capsule-like nanostructures of Co3O4 and investigation of their capacitive performance, Appl. Surf. Sci., 2013, 287, 187–194. 28 G. Tian, F. X. Kober, U. Lewandrowski, A. Sickmann, W. J. Lennarz and H. Schindelin, The catalytic activity of proteindisulfide isomerase requires a conformationally flexible molecule, J. Biol. Chem., 2012, 283, 33630–33640. 29 H. P. Klug and L. E. Alexander, X-Ray Diffraction Procedures, John Wiley & Sons Inc., 2nd edn, 1974, pp. 687–703. 30 F. J. Humphreys and M. G. Ardakani, Grain boundary migration and Zener pinning in particle-containing copper crystals, Acta Mater., 1996, 44, 2717–2727. 31 X. Chen, L. Liu, P. Y. Yu and S. S. Mao, Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals, Science, 2011, 331, 746–749. 32 L. G. Cancado, M. A. Pimenta, B. R. A. Neves, M. S. S. Dantas and A. Jorio, Influence of the atomic structure on the Raman spectra of graphite edges, Phys. Rev. Lett., 2004, 93, 247401. 33 L. Wang and F. Meng, Oxygen vacancy and Ce3+ ion dependent magnetism of monocrystal CeO2 nanopoles synthesized by a facile hydrothermal method, Mater. Res. Bull., 2013, 48, 3492–3498. 34 S. Colis, A. Bouaine, G. Schmerber, C. Ulhaq-Bouillet, A. Dinia, S. Choua and P. Turek, High-temperature ferromagnetism in Co-doped CeO2 synthesized by the coprecipitation technique, Phys. Chem. Chem. Phys., 2012, 14, 7256–7263. 35 V. D. Araujo, W. Avansi, H. B. de Carvalho, M. L. Moreira, E. Longo, C. Ribeirod and M. I. B. Bernardi, CeO2 nanoparticles synthesized by a microwave-assisted hydrothermal method: evolution from nanospheres to nanorods, CrystEngComm, 2012, 14, 1150–1154. 36 M. G. Sujana, K. K. Chattopadyay and S. Anand, Characterization and optical properties of nano-ceria synthesized by surfactant-mediated precipitation technique in mixed solvent system, Appl. Surf. Sci., 2009, 254, 7405–7409. 37 H. M. Dos Santos, G. Simmonds and A. J. Krause, A stem-loop structure in the wingless transcript defines a consensus motif for apical RNA transport, Development, 2008, 135, 133–143. 38 W. J. Roth, W. Makowski, B. Marszalek, P. Michorczyk, W. Skuza and B. Gil, Activity enhancement of zeolite MCM-22 by interlayer expansion enabling higher Ce loading and room temperature CO oxidation, J. Mater. Chem. A, 2014, 2, 15722–15725. 39 Y. Chen, B. H. Song, H. S. Tang, L. Lu and J. M. Xue, One-step synthesis of hollow porous Fe3O4 beads–reduced graphene oxide composites with superior battery performance, J. Mater. Chem., 2012, 22, 17656–17662. 40 X. Wang, X. Li, D. Liu, S. Song and H. Zhang, Green synthesis of Pt/CeO2/graphene hybrid nanomaterials with remarkably enhanced electrocatalytic properties, Chem. Commun., 2012, 48, 2885–2887. 41 H. Zhang, J. Gu, J. Tang, Y. Hu, B. Guan, B. Hu, J. Zhao and C. Wang, Hierarchical porous MnO2/CeO2 with high performance for supercapacitor electrodes, Chem. Eng. J., 2016, 286, 139–149.


PCCP

42 S. Saravanakumar, S. Sasikumar, S. Israel, G. R. Pradhiba and R. Saravanan, Structural, magnetic and charge-related properties of nano-sized cerium manganese oxide, a dilute magnetic oxide semiconductor, Mater. Sci. Semicond. Process., 2014, 17, 186–193. 43 H. Chen, J. Jiang, L. Zhang, D. Xia, Y. Zhao, D. Guo, T. Qi and H. Wan, In situ growth of NiCo2S4 nanotube arrays on Ni foam for supercapacitors: maximizing utilization efficiency at high mass loading to achieve ultrahigh areal pseudocapacitance, J. Power Sources, 2014, 254, 249–257. 44 Y. Wang, C. X. Guo, J. H. Liu, T. Chen, H. B. Yang and C. M. Li, CeO2 nanoparticles/graphene nanocomposites based high performance supercapacitor, Dalton Trans., 2011, 40, 6388–6391. 45 S. Vijayakumar, S. Nagamuthu and G. Muralidharan, Supercapacitor studies on NiO nanoflakes synthesized through a microwave route, ACS Appl. Mater. Interfaces, 2013, 5, 2188–2196.


Paper

46 N. Wang, C. Wu, J. Li, G. Dong and L. Guan, Binder-free manganese oxide/carbon nanomaterials thin film electrode for supercapacitors, ACS Appl. Mater. Interfaces, 2011, 3, 4185–4189. 47 N. Padmanathen and S. Selladurai, Mesoporous MnCo2O4 spinel oxide nanostructure synthesized by solvothermal technique for supercapacitor, Ionics, 2014, 20, 409–420. 48 R. Kumar, A. Agarwal, R. K. Nagarale and A. Sharma, High Performance Supercapacitors from Novel Metal Doped Ceria Decorated Aminated Graphene, J. Phys. Chem. C, 2016, 120(6), 3107–3116. 49 D. Deng, N. Chen, X. Xiao, S. Du and Y. Wang, Electrochemical performance of CeO2 nanoparticle-decorated graphene oxide as an electrode material for supercapacitor, Ionics, DOI: 10.1007/s11581-016-1812-0. 50 S. Vijayakumar, S. Nagamuthu and G. Muralidharan, Porous NiO/C nanoacomposites as electrode material for electrochemical supercapacitors, ACS Sustainable Chem. Eng., 2013, 1, 1110–1118.
