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Urchin-like NiCo2O4 microsphere by hydrothermal route: Structural, electrochemical, optical and magnetic properties
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Urchin-like NiCo2O4 microsphere by hydrothermal route: Structural, electrochemical, optical and magnetic properties Priyambada Nayak∗, M. Sahoo, Saroj K. Nayak School of Basic Science, Indian Institute of Technology, Bhubaneswar, Odisha, 752050, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Urchin-like structure Specific capacitance UV–visible spectroscopy M-H loop Density functional theory
Urchin-like microsphere of NiCo2O4/NIC sample was synthesized by hydrothermal method, followed by post annealing treatment at 400 °C for 3 h. The phase confirmation and surface morphology was analyzed by XRD, FESEM and Raman spectroscopy, which shows that cubic spinel structure with Fd3m space group supporting the microstructure. The elemental mappings were characterized by EDS analysis. The electrochemical performance revealed that the prepared NIC sample exhibits specific capacitance of 295Fg-1 at 20 mV−1 owing to its high surface area; consist of interconnection of nanopores between the subunit of nanoneedle inside the urchin like microsphere. The optical band gap energy was estimated by using Tauc plot, are 2.17eV and 3.5eV respectively. This result well consistent with the theoretical investigation by using DFT calculation. Furthermore a significant M-H loop revealed that, NIC system shows ferromagnetic behavior at RT. Hence all the combined experimental and theoretical results provide to design synergetic microstructure of NIC system for multifunctional application.
1. Introduction With exponentially growing demand on energy material in portable electronic devices (PED) and hybrid electric vehicles (HEV), intensive research have been devoted worldwide to exploring the high-performance energy-storage material for novel device fabrication. Compared with Li-ion batteries and polymer dielectric materials, supercapacitors/ electrochemical capacitors, emerged as new class of energy source devices that have received most attraction by the researcher because of their excellent properties such as superior power density (> 10 kW kg−1), long life cycles (> 105 cycle) and rapid charge/discharge capability (within second) in a broad temperature range with long stability [1,2]. Therefore recently, massive research has been mainly focused to the electrode materials with desirable performances. For example, carbon based materials, highly conducting polymers and transition metal oxides. Among the investigated material, for practical application point of view transition metal oxides like RuO2, V2O5, MnO2, Co3O4 and NiO have been successfully established as potential electrode materials owing to their excellent pseudo-capacitance properties caused by a fast redox reaction with a low cost and act as a cathode material for energy source [3]. However both low contact and poor intrinsic conductivity hinders them to reach high energy density for large scale application. For an instance the recent work by Y. Zhu et al. demonstrated that RuO2 and its composite system exhibits
∗
outstanding supercapacitance properties (specific capacitance 700Fg-1 of RuO2) under fast charge-discharge characteristics with excellent stability [4]. But extremely high cost limits its practical application. Therefore, intensive research work has been carried out to develop such new alternative electrode material with low-cost and better electro chemical properties. Spinel nickel cobaltite (NiCo2O4) in sort NIC has opened up great attention in current years because it is cheap, abundant and environmental friendly. Specially its ultrahigh theoretical capacitance value (> 3000 F g−1), superior conductivity (higher than two order magnitude) as compare to that of NiO and Co3O4 with excellent stability makes it for new generation supercapacitor material [5]. In addition, the open literature depicts that NIC was also an excellent electrode reaction material for O2/Cl2 evolution, O2/H2O2 reduction reaction, methanol oxidation etc. [6–8]. Notwithstanding this, NIC was also used in other applications such as photocatalysis, electrical transport, sensors, Li-ion batteries and in very recently for magnetic devices such as spintronics [9]. The versatile applications of NiCo2O4/NIC are mainly due to its crystal structure which is associated with the mixed valence states i.e. +2 and + 3 of both Ni and Co-cation. In particular, half cobalt ion can occupy in the Td-site (tetrahedral), whereas the other half of cobalt/nickel ions can be occupied in the Oh-sites (octahedral) to form nickel cobaltite (NIC) and possess higher redox chemistry compare to pure NiO and Co3O4 material [10]. However, the evidence of Ni in
Corresponding author. E-mail address: [email protected] (P. Nayak).
https://doi.org/10.1016/j.ceramint.2019.10.105 Received 13 April 2019; Received in revised form 4 October 2019; Accepted 10 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Priyambada Nayak, M. Sahoo and Saroj K. Nayak, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.105
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spectrometry (Renisha winVia) was used to study the compositional variation and vibrational mode of the as–synthesized sample. The electro-chemical performance of the material was measured by using PG16125 potentiostat electrochemical work station with a typical three electrode system. The optical study was carried out by using JASCO V666 UV–visible spectrometer. The Magnetic field dependence magnetization (M(H)) at RT was measured by using VSM (vibrating sample magnetometer, Lakeshore-model no.7410).
the Td site has also been reported which indicates that the exchange of Ni(Oh) ↔ Co(Td) may takes place [11]. The nonmagnetic characteristics of Co(Oh) ion and antiparallel spins of Co(Td), Ni(Oh) ions results NIC system to a ferromagnetic material, better spintronic functionalities (for example: spin injection, filtering etc.), and large p-type conductivity. But, it is well known that, different synthesis condition influences the electrochemical and magnetic properties owing to its coupling between two metal species and lowering the conductivity and surface area. Meanwhile, the electro-catalytic performance of an electrode material mainly depends on the geometric factors, such as: crystal structure, surface area and morphology, electronic conductivity, oxidation state of the ions etc. Therefore much research has been address to improve the performance of NIC material by constructing nanostructures including 2D-nanotube, nanoplatelet, and nanosheets [12–14]. Even, the volume change density and fast aggregation and/pulverization (structural deterioration) of NiCo2O4 nanostructures couldn't be evade during repeated charge-discharge process. Therefore the diverse morphologies such as 2D-nanoparticles, nanoflowers, nanospheres, nanoneedles, and 3D coral-like NiCo2O4 has been synthesized by different routes such as microwave-assisted, solvo-thermal, hydrothermal and electrochemical deposition [15–17]. In the present work our motivation is to synthesize urchin-like morphologies of NiCo2O4 by using the hydrothermal route with postannealing treatment at 400 °C for 3 h and we believe that this method exhibits more advantages like simplicity, low cost and low energy consuming. Furthermore, we also tried to investigate the complete structural and morphological properties of the prepared sample by, XRD, SEM, EDS and Raman analyses. As expected the prepared urchin like morphologies of NiCo2O4 electrodes exhibits good Faradic capacitance value of in a three-electrode system. This indicates that the morphology of NiCo2O4 provide a large surface area to make active electrolyte materials for electrochemical performance. On the other hand an extensive simulation by using density functional theory (DFT) has also been carried out to find out the electronic properties with onsite Hubbard U-terms on the transition metal d-state in order to support the UV-result analysis. Additionally, we have studied the magnetic behavior of NiCo2O4 samples. All these results offer that the combining property of NIC makes it to be a new way for preparing multi-functional device designing application.
3. Result and discussion 3.1. Formation of NiCo2O4 microsphere and structural analysis by XRD and FESEM The schematic diagram of growth mechanism of NiCo2O4 urchin like microspheres is presented in Fig. 2. As discussed in the experimental section, 1:2 ratio of Ni(NO3)2·6H2O, Co(NO3)2·6H2O and urea mixer has been taken. In accordance to the chemical reaction OH− is produced from the decomposition of Co(NH2)2 given in equations (1) and (2). In Step I, Ni2+ and Co2+ react with the anion OH−, released from urea to generate Ni-Co bimetallic-hydroxide nano particles as illustrate with chemical equation (3). In step II, according to oriented mechanism, NiCo2O4 nanoneedles were developed with high surface energy [18]. In step III, under the promotion of NO3−, the self-assembly NiCo2O4 microsphere is formed which is composed of needle-like subunits in all direction by different reaction state as given with equation (4). Finally after calcinations process, metal hydroxide has been completely decomposed to metal oxide and the corresponding equations are given below: CO(NH2)2 → C3H6 N6+ 6NH3 + 3CO2 NH3 + H2O → NH4 +OH +
Ni
2+
+ Co
2+
−
−
+6OH → NiCo2(OH)6
2NiCo2(OH)6+ O2→ 2NiCo2O4 + 6H2O
(1) (2) (3) (4)
The above result signifies that the possibly mechanism for the formation of urchin-like structure is mainly depends on two factors. First, the reaction temperature under hydrothermal process (i.e. a number of forces acted upon it, such as Vander Waals forces, hydrophobic attraction, hydrogen bonding, crystal field attraction, crystal contraction, electrostatic, and Ostwald ripening etc. Second, kinetic aspects. Initially, under homogeneous precipitation conditions, each precursor particle must have sufficient free space to form nanosized particles. Further, these nano particles coalesces, undergo Ostwald repining processes and their growth mechanism along particular direction is modified by adsorbing the modifiers like H2O, surfactants, anions (OH− and CO32−) and cations (Ni2+, Co2+) and finally urchin-like microsphere is formed [19,20]. The structural investigation and phase detection of the prepared sample has been determined by using XRD analysis, as shown in Fig. 3. The XRD patterns of the sample exhibits Bragg's diffraction peaks at 37.2, 43.3, 62.9, 75.4 and 79.4°, which corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes respectively and are well matched with the cubic spinel structure of NIC system [21]. No characteristic peaks regarding the secondary phase were observed, revealed that high purity of the material and the sharp intense peak again indicates well crystallization of the sample. The cubic cell parameter was calculated to be 0.809 nm by using Chekcell software, which corresponded with a = 0.811 nm as per JCPDS card No. 020-0781, with space group: Fd3m. The average crystalline size was calculated by using well known Deby-Scherrer formula, 0.9λ = βcosθ and it is found to be 23.25 nm, where β is the FWHM (full width with half maxima), λ = wave length (1.54 Å), θ = diffraction angle [22]. Fig. 4 depicts the morphological image of the prepared NIC sample employed by FESEM measurement at different magnification. From the
2. Experimental 2.1. Materials synthesis Fig. 1(a) shows the schematic representation for the fabrication of NiCo2O4 (NIC) sample by hydrothermal route. Firstly, 1 mmol of Ni (NO3)2·6H2O (Nickel nitrate hexahydrate) and 2 mmol of Co (NO3)2·6H2O(Cobalt nitrate hexahydrate) were taken and dissolved in an appropriate amount of deionized water under constant magnetic stirring. Then, 60 mmol of urea was then added in the above solution and further stirred. The solution turns to pink transparent color after stirring for 45min and then moved into a Teflon-lined autoclave of 40 mL capacity. After hydrothermal treatment at 90° for 12 h, the product was transferred in to a beaker (a digital camera image is also show in Fig. 1(b), filtered and washed many times with deionized water and ethanol. The precipitate was dried in a vacuum oven at 80 °C for 4 h and finally the as-synthesized powder was annealed at 400 °C under air for 3 h to obtain urchin porous like NiCo2O4 structure. 2.2. Characterizations The structural characterization by XRD has been examined by Bruker D8 Advanced X-Ray diffractrometer at RT using Cu Kα-radiation (λ~1.54 Å) with scan rate 0.02°/min. The surface morphology image was analyzed by field emission scanning electron microscope (FESEM, Zeiss Pvt. Ltd.), X-ray energy dispersive spectroscopy (EDS), and Raman 2
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Fig. 1. (a) Schematic representation of formation of NiCo2O4/NIC urchin like microsphere (b) a digital camera image of the synthesized urchin solution.
Fig. 2. Schematic diagram of growth mechanism of NiCo2O4 microsphere.
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3.2. Raman study Fig. 6 depicts the Raman spectrum of urchin like structure of NiCo2O4 in spectral range of 100–800 cm−1. Five raman active vibrational modes namely A1g + Eg + 3F2g were clearly observed which was well agreement with the polarization selection rules and theoretical calculation of spinel Fd3m structure [24]. The peaks marked at (P1) at 208 cm−1, (P2) 451 cm−1, (P3) 503 cm−1, (P4) 593 cm−1 and (P5) 651.98 cm−1are related to F2g (1), Eg, F2g (2), F2g (3), and A1g modes of the NiCo2O4, respectively, which are well consistent with the previous reported results by Bhojane et al. [25]. These optical phonon raman active bands are mainly assigned to the vibration of Co–O and Ni–O bond respectively, whereas no peak regarding OH–group was observed. This result indicates that after final thermal treatment, nickel/cobalt hydroxides [Co(OH)2 and Ni(OH)2] are completely decomposed. The peak at 651.98 cm−1 is assigned to A1g mode corresponding to stretching of oxygen atoms with respect to the Co3+ ion in the tetrahedral site. The peak at 451 cm−1, 503 cm−1 were attributed because of the combined vibrational effect of tetrahedral and octahedral oxygen atoms in the lattice.
Fig. 3. XRD pattern of NiCo2O4 sample post annealed at 400 °C for 3 h.
figure we can see that the prepared NIC sample showed uniform microspheres of diameter of 3–4 μm and composed of nanoneedles (with diameters of ~200 nm) on surfaces, with a length of 1 μm, just like sea urchins. It may be considered that these nanoneedles consist of various nanoparticles, which are agglomerate with each other to form porous structure with high specific surface area. This type of porous structure may be formed due to the release of gases at different stage of thermal treatment, which makes possible to adsorption of oxygen and provides enough active sites [23]. The average diameter of nanoparticle was ~24 nm and was well consistent with the XRD result. Fig. 5(a) shows the compositional variation of the sample verified by EDS mapping. The EDS spectrum data signifies Ni, Co and O-elements respectively. The molar ratio of Ni: Co is 22.29: 48.57 i.e.⁓ 1:2, which revealed that the formula of NiCo2O4 holds good. Again the EDS mapping images of NiCo2O4-demonstrates uniform distribution of Ni, Co, and O within the whole hollow NiCo2O4 microspheres as shown in Fig. 5(b).
3.3. Electro chemical observation A typical three-electrode setup was used to characterize the electrochemical performance of the sample, which consist of glassy carbon electrode (GCE) as the working one, Ag/AgCl as reference electrode and platinum wire as counter electrode. Before chemical measurement the surface of the glassy carbon electrode was polished with alumina powder slurry (Al2O3 of 0.3 μm). After that the electrode was rinsed thoroughly with DI water, sonicated for 5 min to obtain a shiny mirror like surface and dried for 1–2 h in a vacuum desiccators. 1 mg of the prepared sample was taken and then added to 100 μL solutions which contain 95 μL ethanol and 5 μL nafion to achieve a homogenous dispersion. Then 2.5 μL of the above mixer has been drop casted onto the polished GCE surface and kept it for dry. Finally the detailed
Fig. 4. Low and High resolution FESEM image of NiCo2O4sample. 4
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Fig. 5. EDS images of the as prepared NiCo2O4 sample.
Csp =
IΔt mΔV
(6)
Where I = discharge current (A), m is the mass of used active material (g), ‘v ’ is the scan rate (mV s−1), ΔV is the potential window (V), and ‘Δt’ refers as the discharge time (s). Again, Energy density (EWh/kg) and power density (PW/Kg) can be calculated using eqns (7) and (8) [27].
E= E= P=
electrochemical properties were performed in an electrochemical-glasscell with 3 M aqueous KOH solution as the electrolyte and the potential window was fixed at 0–0.5 V. The specific capacitance (Csp, F g−1) was computed from the CV and GCD measurements in accordance to the following relation [26]:
1 IΔVt 2 m } E t
(8)
NiCo2 O4 + OH− + H2 O ↔ NiOOH + 2CoOOH + e−1 } CoOOH + OH−CoO2 + H2 O + e−
∫ I (dV ) mvΔV
(7)
Here, I = response current (A), m = mass of active material (kg), ΔV = potential window (V), and t = discharge time (h). Fig. 7(a and b) demonstrates the CV and GCD (galvanostatic-chargedischarge) curve of the synthesized NiCo2O4 sample with scan rates of 5–100 mV s−1. From Fig. 7(a), following characteristics are observed: (i) Appearance of peak clearly shows pseudo-capacitive feature corresponding to redox transitions of Ni2+/Ni3+ and Co3+/Co4+ [28]. The redox reaction may possibly due to the following equation
Fig. 6. Raman spectra of NiCo2O4sample.
Csp =
1 CSP ΔV 2 2
(9)
(ii) The different scan rate CV curve revealed same type behavior, which demonstrates reversibility of the redox reaction (iii) Both oxidation and reduction peaks were slightly moved towards the anodic and cathodic direction with increase of scan rate. For an example, with
(5)
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Fig. 8. Optical band gap energies of NiCo2O4 sample by using Tauc plots (insert figure shows absorption spectra).
As a result, faradaic reaction takes place even at very high current densities. Again for electrochemical performance of supercapacitor material, power density (PD) and energy density (ED) are plays an important role which was calculated by using equations (7) and (8). The ED stored 10.247.65Whkg−1 corresponding to PD of 362Wkg-1 at a current density of 20Ag-1, whereas at current density of 60 Ag-1, ED and PD values are 7.65Whkg−1 and 720 Wkg-1 respectively. This result depicts that present prepared NIC sample gives excellent supercapacitance properties. 3.4. UV–visible spectroscopy The optical band gap (Eg), which is responsible for the crystallite size and surface morphology, studied by using UV–vis diffuse reflectance spectroscopy and as shown in Fig. 8. The insert figure shows absorption spectra of the NIC sample. The band gap energy value (Eg) was calculated by using the following standard Tauc expression [33].
Fig. 7. Electrochemical performance of NiCo2O4 sample under three-electrode system. (a) CV curve at the different scan rates (b) GCD curve at different current density.
(αhυ)n=K(hυ-Eg)
increase of scan rate from 5 to 100 mVs−1 the anodic peak potential increased from 0.257 to 0.363 V and correspondingly, Ipa-anodic peak current value increased from 7.982 to 71.293Ag-1. This result indicates lower resistance value of the electrode material with excellent electrochemical reversibility. The insert of Fig. 7(a) shows specific capacitance vs. scan rate, which was measured by using equation (5). These results are remarkable among most of the earlier report in the literatures [29]. To further confirm the supercapacitance properties of the prepared NIC material, we have performed the Galvanostatic charge/discharge measurement (GCD) in the same potential range (0–0.5 V) (Fig. 7(b)). The specific capacitance from the GCD curve of NiCo2O4 electrodes can be calculated by using equation (6). The specific capacitance are 295, 250, 237 and 225 Fg-1 at current densities of 20, 30, 40, 60 Ag−1and it is high of 295 at low current density of 20 mV−1, which value is slightly high compare to other preparation method such as hydrothermal method [29], and co-precipitation method and electrochemical deposition method [30,31] as reported in the literature. The possibly reason for the electrochemical performance stated as mainly (i) the unique urchin like structure(ii) The whole NiCo2O4 microsphere consists of interconnected nanoneedle, which provides large number of active sites for redox reaction and possess high surface area. Meanwhile, the characteristics of the thin nanoneedle consist of large number of nanoparticle as discussed in the surface morphology section, ensure that majority of electro active materials are occupied in the redox reactions and efficiently contribute superior capacitance [32]. The interconnected nanopores between the nanoneedle provide appropriate diffusion paths to enhance the diffusion kinetics within the electrode.
(10)
Here, ‘α’ referred as coefficient of absorption, ‘hν’ is the photon energy, ‘K’ is the relative material constant, and n values are either 1/2 or 2, for direct transition n = 2 and for indirect transition n = 1/2. The optical band gap energies can be determined by using Tauc plots of equation (10) with the linear fit for n = 2. Meanwhile no linear relation can be obtained for n = 1/2. Therefore we confirmed that the present NIC system showing semiconducting behavior with direct allowed transitions. By extrapolating the linear portion of the graph to (αhυ) 2 = 0, two band gap energy are observed i.e. 2.17 eV and 3.5 eV. It was relevant for the spinel oxides that, the obtained band gap energies are associated with the transition of electron from O-2p valence band to the transition metals-3d (t2g, eg) conduction band [34]. Further, two band gap energies are attributed due to the co-existence of high and low spin states of Co3+ ion. Also it is well reported that the band gap energy and crystallite size are inversely related. In the present case, XRD result reveals that the crystallite size of urchin-like NiCo2O4 was found to be ~24 nm and well consistent with the UV-analysis. Again the band gap energy is directly related to the theoretical calculation obtained from DFT analysis. 3.5. DFT calculation To support the experimental findings, theoretical investigation was also performed, that gives information regarding the interaction and charge-transfer mechanism of NIC system through electronic structure calculations. Literature report reveals that electronic conductivity of NIC system is high and argued due to the formation of σ*(eg) band via 6
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Fig. 9. (a) Projected density of state (PDOS) of NiCo2O4, calculated using DFT + U (b,c) Majority and minority spin states.
empty eg-state was formed, which leads to a hole at the top or just above the valence band. Hence in both the cases empty states are formed at the top of the minority spin valence band and results high conductivity of NIC sample. The calculated band gap energy are found to be 2.1 eV, 3.5 eV (Fig. 9(b and c)) and are well agreement with the literature report. Again the densities of states of Ni-Oct and Co-Tet crossed the Fermi level in the spin-down channel (Fig. 9(c)). Therefore in between Ni-Oct and Co-Tet site exchange interaction takes place, which influences the conductivity of NIC system and enhances the specific capacitance properties.
intervening oxygen ion through strong covalent interaction between low spin Co3+–Ni3+ in the octahedral site assuming the distribution Co2+[Ni3+Co3+]O4 behaves as a semimetal [35]. Therefore we use density functional theory (DFT) incorporating with on-site Coulomb repulsion (DFT + U) to carry out the first principle calculations. 3.5.1. Computational details All the calculations were performed within Spin-polarized DFT framework using VASP package. The Exchange correlation terms were explained by using Perdew−Burke−Ernzerh of (PBE) function by adopting the U term on Co and Ni 3 d electron. The U values are determined from linear response theory and ULR(Ni) = 6.6 eV for nickel, and ULR(Co) = 4.4 and 6.7 for Co ions at Td site and Oh sites, respectively. The valence electrons O 2s, 2p; Co 3 d, 4s and Ni 3d, 4s states are taken for ultra soft pseudo-potential. The used cutoff KE (kinetic energy) was 50 Ryd for wave function and 500 Ryd for augmented density, respectively. Optimization of structural information was accepted by relaxing all atoms and calculation was performed by using 56- atom conventional cubic cell containing 8 formula units, with a 3 × 3 × 3 Monkhorst−Pack k-point mesh integration Brillouin zone. Fig. 9 shows the density of states (DOS) of NIC sample. The optimized lattice parameter was found to be 8.09 Å, which was well agreement with the XRD result. We can see that the present system shows half metal in which majority spin channel remains insulating while the minority channel becomes conducting. In both the spin channels valence band shift toward the Fermi level, which indicate that Ni2+ in place of Co3+ at an Oh site acts as p-type dopant and the result well matched with other experimental outcomes. The presence of Ni2+ at the octahedral site causes formation of a Co(Td)3+, which has only one occupied e-orbital in the minority spin channel. A hole is created on top of the valence band due to other empty e-orbital of Co(Td). In the similar manner, Ni3+ at the octahedral site results Co(Td)2+ and an
3.6. Magnetic behavior Fig. 10 shows the RT (300 K) the magnetic behavior (magnetic moment ‘M’ vs. magnetic field ‘H’ loop) of the prepared NIC sample investigated by VSM system. From the figure it is observed that the present system shows typical ferromagnetic characteristics, however the value of remnant magnetization (Mr~1.18emu/g) and coercivity (Hc~241.32Oe) was very small (owing to the weak ferromagnetic behavior at RT). Low temperature experimental data may give quite well result, but due the temperature limitation of the experimental setup used, we do not observe low temperature saturation magnetization result. It is well know that the magnetic behavior of the NiCo2O4 samples mainly depends on shape, size of the particle, super-exchange interactions and mostly on the surface morphology followed by the synthesis procedure [36]. In the present case, small value of saturation magnetization (Ms) may be correlated to the surface spin disorder which depends on the urchin-like surface morphology. Also literature report states that small value of Hc may possibly due to two reasons. (i) From magneto crystalline anisotropy called as shape anisotropy, i.e. magnetic disorder on the surface (ii) AFM-FM (anti ferromagnetic to ferromagnetic) exchange coupling, in which the ferromagnetic mainly 7
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Banks, Spinel NiCo2O4 for use as a high-performance supercapacitor electrode material: understanding of its electrochemical properties, J. Power Sources 67 (2012) 888–900. [23] E. Umeshbabu, G. Rajeshkhanna, P. Justin, G. Ranga Rao, Magnetic, optical and electrocatalytic properties of urchin and sheaf-like NiCo2O4 nanostructures, Mater. Chem. Phys. 165 (2015) 235–244. [24] P. Bhojane, S. Sen, P.M. Shirage, Enhanced electrochemical performance of Mesoporous NiCo2O4 as an excellent supercapacitive alternative energy storage material, Appl. Surf. Sci. 377 (2016) 376–384. [25] Y. Chen, Y. Li, Z. Hai, Y. Li, S. Jiamin Chen, X. Chen, S. Zhuiykov, D. Cui, C. Xue, Facile-synthesized NiCo2O4@MnMoO4 with novel and functional structure for superior performance supercapacitors, Appl. Surf. Sci. 452 (2018) 413–422. [26] S. Ramesh, D. Vikramanb, H. Kim, H. Soo Kim, Joo-H. Kim, Electrochemical performance of MWCNT/GO/NiCo2O4 decorated hybrid nanocomposite for supercapacitor electrode materials, J. Alloy. Comp. 765 (2018) 369–379. [27] A.E. Elkholy, F. E-TaibHeakal, Nageh K. Allam, Nanostructured spinel manganese cobalt ferrite for high-performance supercapacitors, RSC Adv. 7 (2017) 51888–51895. [28] Y. Duan, Y. Huo, Y. Qi, Li Lu, Q. Wu, C. Wang, Z. Su, Uniform NiCo2O4/NiFe2O4 hollow nanospheres with excellent properties for Li-ion batteries and upercapacitors, J. Alloy. Comp. 767 (2018) 223–231. [29] L. Lu, Y. Xie, Z. Zhao, Improved electrochemical stability of NixCo2x(OH)6x/ NiCo2O4 electrode material, J. Alloy. Comp. 731 (2018) 903–913. [30] W. Chen, L. Wei, Z. Lin, Q. Liu, Y. Chen, Y. Lin, Z. Huang, Hierarchical flower-like NiCo2O4@TiO2 hetero-nanosheets as anodes for lithium ion batteries, RSC Adv. 7 (2017) 47602–47613. [31] J. Wu, P. Guo, R. Mi, X. Liu, H. Zhang, J. Mei, H. Liu, W.M. Lau, L.M. Liu, Ultrathin NiCo2O4 nanosheets grown on three-dimensional interwoven nitrogen-doped
Fig. 10. M-H hysteresis loop of NiCo2O4 sample at room temperature (RT).
originates from the oxygen vacancy or from the defects at interfaces [37]. In accordance with the FCBMP theory (F-center mediated bound magnetic polaron) or so called bound magnetic polaron (BMP) theory, oxygen vacancies are created nearby lattice of Ni2+ due to exchange of Ni(Oh) ↔ Co(Td) [38]. An electron is trapped in the oxygen vacancy (F center) couple with the magnetic spins of the nearest Ni2+ ions within the radius of the hydrogen like orbit of the F-center and forms a bound magnetic polaron, which influence the ferromagnetic behavior. Therefore, contribution of magnetic behavior in the present system is due to both the surface morphologies (shape anisotropy) and interface defects (oxygen vacancy). This result well supports the electronic properties of this system using DFT + U calculation. 4. Conclusion In summary, we have prepared urchin-like structure of spinel NiCo2O4 via hydrothermal route and its structural, morphological study were analyzed by using XRD, FESEM, EDS and Raman spectroscopy. The Electrochemical analysis revealed that the present sample shows excellent supercapacitive property. Such good capacitive performances are mainly attributed to the high specific surface area due to the urchin like structure composed of nanoneedle. The optical band gap from UV–visible spectroscopy was found to be 2.17 eV and 3.5 eV, which was well consistent with the theoretical calculation by using DFT. Again from the result we conclude that NIC behaves as a semimetal and can be used as spintronic application. The magnetic property of the material was confirmed from M-H loop. In particular weak ferromagnetic nature with small coercivity (Hc)~241.32Oe corresponding to remnant magnetization (Mr)~1.18emu/g was observed. Moreover, all the characterization states that the present spinel system may be used as novel multi-functional application, especially energy storage as well as magnetic devices etc. Declaration of competing interest No other conflicts to declare. Acknowledgement We acknowledge Ministry of Human Resource Development (MHRD), INDIA for the financial support of the project RP074. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] R.A. Huggins, Review—a new class of high rate, long cycle life, aqueous electrolyte battery electrodes, J. Electrochem. Soc. 164 (2017) A5031–A5036. [3] S.K. Meher, G.R. Rao, Ultralayered Co3O4 for high-performance supercapacitor
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Co) spinel nanocrystals—DFT+U and TEM screening investigations, J. Phys. Chem. C 118 (2014) 19085–19097. [36] M. Cabo, E. Pellicer, E. Rossinyol, M. Estrader, A. Lopez-Ortega, J. Nogues, O. Castell, S. Surinacha, M. Dolors Baroa, Synthesis of compositionally graded nanocast NiO/NiCo2O4/Co3O4 mesoporous composites with tunable magnetic properties, J. Mater. Chem. 20 (2010) 7021–7028. [37] Y.Z. Su, Q. ZhiXu, G.F. Chen, H. Cheng, N. Li, Z.Q. Liu, One dimensionally spinel NiCo2O4 nanowire arrays: facile synthesis, water oxidation, and magnetic properties, Electrochim. Acta 174 (2015) 1216–1224. [38] X. Yang, X. Yu, Q. Yang, D. Zhao, K. Zhang, J. Yao, G. Li, H. Zhouc, X. Zuo, Ceramics International, Controllable synthesis and magnetic properties of hydrothermally synthesized NiCo2O4 nano-spheres, 43 (2017) 8585–8589.
carbon nanotubes as binder-free electrodes for high-performance supercapacitors, Mater. J. Mater. Chem. A. 3 (2015) 15331–15338. J. Liu, T. Chen, P. Jian, L. Wang, X. Yan, Hollow urchin-like NiO/NiCo2O4 heterostructures as highly efficient catalysts for selective oxidation of styrene, J. Colloid Interface Sci. 526 (2018) 295–301. P. Nayak, K. Mitra, S. Panigrahi, Electrical and optical properties of four-layered perovskite ferroelectric ABi4Ti4O15 (with A= Sr, Ba, Ca), Mater. lett. 216 (2018) 54–57. Y. Bitla, Y. Chin, J. C. Lin, C. N. Van, R. Liu,Y. Zhu, H. J. Liu, Q. Zhan, H. J. Lin, C. T. Chen, Y. H. Chu, Q. He, Origin of metallic behavior in NiCo2O4 ferrimagnet, Sci. Rep.. | 5:15201 | DOI: 10.1038/srep15201. F. Zasada, J. Grybos, P. Indyka, W. Piskorz, J. Kaczmarczyk, Z. Sojka, Surface structure and morphology of M[CoM']O4 (M = Mg, Zn, Fe, Co and M′ = Ni, Al, Mn,
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Urchin-like NiCo2O4 microsphere by hydrothermal route: Structural, electrochemical, optical and magnetic properties Priyambada Nayak∗, M. Sahoo, Saroj K. Nayak School of Basic Science, Indian Institute of Technology, Bhubaneswar, Odisha, 752050, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Urchin-like structure Specific capacitance UV–visible spectroscopy M-H loop Density functional theory
Urchin-like microsphere of NiCo2O4/NIC sample was synthesized by hydrothermal method, followed by post annealing treatment at 400 °C for 3 h. The phase confirmation and surface morphology was analyzed by XRD, FESEM and Raman spectroscopy, which shows that cubic spinel structure with Fd3m space group supporting the microstructure. The elemental mappings were characterized by EDS analysis. The electrochemical performance revealed that the prepared NIC sample exhibits specific capacitance of 295Fg-1 at 20 mV−1 owing to its high surface area; consist of interconnection of nanopores between the subunit of nanoneedle inside the urchin like microsphere. The optical band gap energy was estimated by using Tauc plot, are 2.17eV and 3.5eV respectively. This result well consistent with the theoretical investigation by using DFT calculation. Furthermore a significant M-H loop revealed that, NIC system shows ferromagnetic behavior at RT. Hence all the combined experimental and theoretical results provide to design synergetic microstructure of NIC system for multifunctional application.
1. Introduction With exponentially growing demand on energy material in portable electronic devices (PED) and hybrid electric vehicles (HEV), intensive research have been devoted worldwide to exploring the high-performance energy-storage material for novel device fabrication. Compared with Li-ion batteries and polymer dielectric materials, supercapacitors/ electrochemical capacitors, emerged as new class of energy source devices that have received most attraction by the researcher because of their excellent properties such as superior power density (> 10 kW kg−1), long life cycles (> 105 cycle) and rapid charge/discharge capability (within second) in a broad temperature range with long stability [1,2]. Therefore recently, massive research has been mainly focused to the electrode materials with desirable performances. For example, carbon based materials, highly conducting polymers and transition metal oxides. Among the investigated material, for practical application point of view transition metal oxides like RuO2, V2O5, MnO2, Co3O4 and NiO have been successfully established as potential electrode materials owing to their excellent pseudo-capacitance properties caused by a fast redox reaction with a low cost and act as a cathode material for energy source [3]. However both low contact and poor intrinsic conductivity hinders them to reach high energy density for large scale application. For an instance the recent work by Y. Zhu et al. demonstrated that RuO2 and its composite system exhibits
∗
outstanding supercapacitance properties (specific capacitance 700Fg-1 of RuO2) under fast charge-discharge characteristics with excellent stability [4]. But extremely high cost limits its practical application. Therefore, intensive research work has been carried out to develop such new alternative electrode material with low-cost and better electro chemical properties. Spinel nickel cobaltite (NiCo2O4) in sort NIC has opened up great attention in current years because it is cheap, abundant and environmental friendly. Specially its ultrahigh theoretical capacitance value (> 3000 F g−1), superior conductivity (higher than two order magnitude) as compare to that of NiO and Co3O4 with excellent stability makes it for new generation supercapacitor material [5]. In addition, the open literature depicts that NIC was also an excellent electrode reaction material for O2/Cl2 evolution, O2/H2O2 reduction reaction, methanol oxidation etc. [6–8]. Notwithstanding this, NIC was also used in other applications such as photocatalysis, electrical transport, sensors, Li-ion batteries and in very recently for magnetic devices such as spintronics [9]. The versatile applications of NiCo2O4/NIC are mainly due to its crystal structure which is associated with the mixed valence states i.e. +2 and + 3 of both Ni and Co-cation. In particular, half cobalt ion can occupy in the Td-site (tetrahedral), whereas the other half of cobalt/nickel ions can be occupied in the Oh-sites (octahedral) to form nickel cobaltite (NIC) and possess higher redox chemistry compare to pure NiO and Co3O4 material [10]. However, the evidence of Ni in
Corresponding author. E-mail address: [email protected] (P. Nayak).
https://doi.org/10.1016/j.ceramint.2019.10.105 Received 13 April 2019; Received in revised form 4 October 2019; Accepted 10 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Priyambada Nayak, M. Sahoo and Saroj K. Nayak, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.105
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spectrometry (Renisha winVia) was used to study the compositional variation and vibrational mode of the as–synthesized sample. The electro-chemical performance of the material was measured by using PG16125 potentiostat electrochemical work station with a typical three electrode system. The optical study was carried out by using JASCO V666 UV–visible spectrometer. The Magnetic field dependence magnetization (M(H)) at RT was measured by using VSM (vibrating sample magnetometer, Lakeshore-model no.7410).
the Td site has also been reported which indicates that the exchange of Ni(Oh) ↔ Co(Td) may takes place [11]. The nonmagnetic characteristics of Co(Oh) ion and antiparallel spins of Co(Td), Ni(Oh) ions results NIC system to a ferromagnetic material, better spintronic functionalities (for example: spin injection, filtering etc.), and large p-type conductivity. But, it is well known that, different synthesis condition influences the electrochemical and magnetic properties owing to its coupling between two metal species and lowering the conductivity and surface area. Meanwhile, the electro-catalytic performance of an electrode material mainly depends on the geometric factors, such as: crystal structure, surface area and morphology, electronic conductivity, oxidation state of the ions etc. Therefore much research has been address to improve the performance of NIC material by constructing nanostructures including 2D-nanotube, nanoplatelet, and nanosheets [12–14]. Even, the volume change density and fast aggregation and/pulverization (structural deterioration) of NiCo2O4 nanostructures couldn't be evade during repeated charge-discharge process. Therefore the diverse morphologies such as 2D-nanoparticles, nanoflowers, nanospheres, nanoneedles, and 3D coral-like NiCo2O4 has been synthesized by different routes such as microwave-assisted, solvo-thermal, hydrothermal and electrochemical deposition [15–17]. In the present work our motivation is to synthesize urchin-like morphologies of NiCo2O4 by using the hydrothermal route with postannealing treatment at 400 °C for 3 h and we believe that this method exhibits more advantages like simplicity, low cost and low energy consuming. Furthermore, we also tried to investigate the complete structural and morphological properties of the prepared sample by, XRD, SEM, EDS and Raman analyses. As expected the prepared urchin like morphologies of NiCo2O4 electrodes exhibits good Faradic capacitance value of in a three-electrode system. This indicates that the morphology of NiCo2O4 provide a large surface area to make active electrolyte materials for electrochemical performance. On the other hand an extensive simulation by using density functional theory (DFT) has also been carried out to find out the electronic properties with onsite Hubbard U-terms on the transition metal d-state in order to support the UV-result analysis. Additionally, we have studied the magnetic behavior of NiCo2O4 samples. All these results offer that the combining property of NIC makes it to be a new way for preparing multi-functional device designing application.
3. Result and discussion 3.1. Formation of NiCo2O4 microsphere and structural analysis by XRD and FESEM The schematic diagram of growth mechanism of NiCo2O4 urchin like microspheres is presented in Fig. 2. As discussed in the experimental section, 1:2 ratio of Ni(NO3)2·6H2O, Co(NO3)2·6H2O and urea mixer has been taken. In accordance to the chemical reaction OH− is produced from the decomposition of Co(NH2)2 given in equations (1) and (2). In Step I, Ni2+ and Co2+ react with the anion OH−, released from urea to generate Ni-Co bimetallic-hydroxide nano particles as illustrate with chemical equation (3). In step II, according to oriented mechanism, NiCo2O4 nanoneedles were developed with high surface energy [18]. In step III, under the promotion of NO3−, the self-assembly NiCo2O4 microsphere is formed which is composed of needle-like subunits in all direction by different reaction state as given with equation (4). Finally after calcinations process, metal hydroxide has been completely decomposed to metal oxide and the corresponding equations are given below: CO(NH2)2 → C3H6 N6+ 6NH3 + 3CO2 NH3 + H2O → NH4 +OH +
Ni
2+
+ Co
2+
−
−
+6OH → NiCo2(OH)6
2NiCo2(OH)6+ O2→ 2NiCo2O4 + 6H2O
(1) (2) (3) (4)
The above result signifies that the possibly mechanism for the formation of urchin-like structure is mainly depends on two factors. First, the reaction temperature under hydrothermal process (i.e. a number of forces acted upon it, such as Vander Waals forces, hydrophobic attraction, hydrogen bonding, crystal field attraction, crystal contraction, electrostatic, and Ostwald ripening etc. Second, kinetic aspects. Initially, under homogeneous precipitation conditions, each precursor particle must have sufficient free space to form nanosized particles. Further, these nano particles coalesces, undergo Ostwald repining processes and their growth mechanism along particular direction is modified by adsorbing the modifiers like H2O, surfactants, anions (OH− and CO32−) and cations (Ni2+, Co2+) and finally urchin-like microsphere is formed [19,20]. The structural investigation and phase detection of the prepared sample has been determined by using XRD analysis, as shown in Fig. 3. The XRD patterns of the sample exhibits Bragg's diffraction peaks at 37.2, 43.3, 62.9, 75.4 and 79.4°, which corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes respectively and are well matched with the cubic spinel structure of NIC system [21]. No characteristic peaks regarding the secondary phase were observed, revealed that high purity of the material and the sharp intense peak again indicates well crystallization of the sample. The cubic cell parameter was calculated to be 0.809 nm by using Chekcell software, which corresponded with a = 0.811 nm as per JCPDS card No. 020-0781, with space group: Fd3m. The average crystalline size was calculated by using well known Deby-Scherrer formula, 0.9λ = βcosθ and it is found to be 23.25 nm, where β is the FWHM (full width with half maxima), λ = wave length (1.54 Å), θ = diffraction angle [22]. Fig. 4 depicts the morphological image of the prepared NIC sample employed by FESEM measurement at different magnification. From the
2. Experimental 2.1. Materials synthesis Fig. 1(a) shows the schematic representation for the fabrication of NiCo2O4 (NIC) sample by hydrothermal route. Firstly, 1 mmol of Ni (NO3)2·6H2O (Nickel nitrate hexahydrate) and 2 mmol of Co (NO3)2·6H2O(Cobalt nitrate hexahydrate) were taken and dissolved in an appropriate amount of deionized water under constant magnetic stirring. Then, 60 mmol of urea was then added in the above solution and further stirred. The solution turns to pink transparent color after stirring for 45min and then moved into a Teflon-lined autoclave of 40 mL capacity. After hydrothermal treatment at 90° for 12 h, the product was transferred in to a beaker (a digital camera image is also show in Fig. 1(b), filtered and washed many times with deionized water and ethanol. The precipitate was dried in a vacuum oven at 80 °C for 4 h and finally the as-synthesized powder was annealed at 400 °C under air for 3 h to obtain urchin porous like NiCo2O4 structure. 2.2. Characterizations The structural characterization by XRD has been examined by Bruker D8 Advanced X-Ray diffractrometer at RT using Cu Kα-radiation (λ~1.54 Å) with scan rate 0.02°/min. The surface morphology image was analyzed by field emission scanning electron microscope (FESEM, Zeiss Pvt. Ltd.), X-ray energy dispersive spectroscopy (EDS), and Raman 2
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Fig. 1. (a) Schematic representation of formation of NiCo2O4/NIC urchin like microsphere (b) a digital camera image of the synthesized urchin solution.
Fig. 2. Schematic diagram of growth mechanism of NiCo2O4 microsphere.
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3.2. Raman study Fig. 6 depicts the Raman spectrum of urchin like structure of NiCo2O4 in spectral range of 100–800 cm−1. Five raman active vibrational modes namely A1g + Eg + 3F2g were clearly observed which was well agreement with the polarization selection rules and theoretical calculation of spinel Fd3m structure [24]. The peaks marked at (P1) at 208 cm−1, (P2) 451 cm−1, (P3) 503 cm−1, (P4) 593 cm−1 and (P5) 651.98 cm−1are related to F2g (1), Eg, F2g (2), F2g (3), and A1g modes of the NiCo2O4, respectively, which are well consistent with the previous reported results by Bhojane et al. [25]. These optical phonon raman active bands are mainly assigned to the vibration of Co–O and Ni–O bond respectively, whereas no peak regarding OH–group was observed. This result indicates that after final thermal treatment, nickel/cobalt hydroxides [Co(OH)2 and Ni(OH)2] are completely decomposed. The peak at 651.98 cm−1 is assigned to A1g mode corresponding to stretching of oxygen atoms with respect to the Co3+ ion in the tetrahedral site. The peak at 451 cm−1, 503 cm−1 were attributed because of the combined vibrational effect of tetrahedral and octahedral oxygen atoms in the lattice.
Fig. 3. XRD pattern of NiCo2O4 sample post annealed at 400 °C for 3 h.
figure we can see that the prepared NIC sample showed uniform microspheres of diameter of 3–4 μm and composed of nanoneedles (with diameters of ~200 nm) on surfaces, with a length of 1 μm, just like sea urchins. It may be considered that these nanoneedles consist of various nanoparticles, which are agglomerate with each other to form porous structure with high specific surface area. This type of porous structure may be formed due to the release of gases at different stage of thermal treatment, which makes possible to adsorption of oxygen and provides enough active sites [23]. The average diameter of nanoparticle was ~24 nm and was well consistent with the XRD result. Fig. 5(a) shows the compositional variation of the sample verified by EDS mapping. The EDS spectrum data signifies Ni, Co and O-elements respectively. The molar ratio of Ni: Co is 22.29: 48.57 i.e.⁓ 1:2, which revealed that the formula of NiCo2O4 holds good. Again the EDS mapping images of NiCo2O4-demonstrates uniform distribution of Ni, Co, and O within the whole hollow NiCo2O4 microspheres as shown in Fig. 5(b).
3.3. Electro chemical observation A typical three-electrode setup was used to characterize the electrochemical performance of the sample, which consist of glassy carbon electrode (GCE) as the working one, Ag/AgCl as reference electrode and platinum wire as counter electrode. Before chemical measurement the surface of the glassy carbon electrode was polished with alumina powder slurry (Al2O3 of 0.3 μm). After that the electrode was rinsed thoroughly with DI water, sonicated for 5 min to obtain a shiny mirror like surface and dried for 1–2 h in a vacuum desiccators. 1 mg of the prepared sample was taken and then added to 100 μL solutions which contain 95 μL ethanol and 5 μL nafion to achieve a homogenous dispersion. Then 2.5 μL of the above mixer has been drop casted onto the polished GCE surface and kept it for dry. Finally the detailed
Fig. 4. Low and High resolution FESEM image of NiCo2O4sample. 4
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Fig. 5. EDS images of the as prepared NiCo2O4 sample.
Csp =
IΔt mΔV
(6)
Where I = discharge current (A), m is the mass of used active material (g), ‘v ’ is the scan rate (mV s−1), ΔV is the potential window (V), and ‘Δt’ refers as the discharge time (s). Again, Energy density (EWh/kg) and power density (PW/Kg) can be calculated using eqns (7) and (8) [27].
E= E= P=
electrochemical properties were performed in an electrochemical-glasscell with 3 M aqueous KOH solution as the electrolyte and the potential window was fixed at 0–0.5 V. The specific capacitance (Csp, F g−1) was computed from the CV and GCD measurements in accordance to the following relation [26]:
1 IΔVt 2 m } E t
(8)
NiCo2 O4 + OH− + H2 O ↔ NiOOH + 2CoOOH + e−1 } CoOOH + OH−CoO2 + H2 O + e−
∫ I (dV ) mvΔV
(7)
Here, I = response current (A), m = mass of active material (kg), ΔV = potential window (V), and t = discharge time (h). Fig. 7(a and b) demonstrates the CV and GCD (galvanostatic-chargedischarge) curve of the synthesized NiCo2O4 sample with scan rates of 5–100 mV s−1. From Fig. 7(a), following characteristics are observed: (i) Appearance of peak clearly shows pseudo-capacitive feature corresponding to redox transitions of Ni2+/Ni3+ and Co3+/Co4+ [28]. The redox reaction may possibly due to the following equation
Fig. 6. Raman spectra of NiCo2O4sample.
Csp =
1 CSP ΔV 2 2
(9)
(ii) The different scan rate CV curve revealed same type behavior, which demonstrates reversibility of the redox reaction (iii) Both oxidation and reduction peaks were slightly moved towards the anodic and cathodic direction with increase of scan rate. For an example, with
(5)
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Fig. 8. Optical band gap energies of NiCo2O4 sample by using Tauc plots (insert figure shows absorption spectra).
As a result, faradaic reaction takes place even at very high current densities. Again for electrochemical performance of supercapacitor material, power density (PD) and energy density (ED) are plays an important role which was calculated by using equations (7) and (8). The ED stored 10.247.65Whkg−1 corresponding to PD of 362Wkg-1 at a current density of 20Ag-1, whereas at current density of 60 Ag-1, ED and PD values are 7.65Whkg−1 and 720 Wkg-1 respectively. This result depicts that present prepared NIC sample gives excellent supercapacitance properties. 3.4. UV–visible spectroscopy The optical band gap (Eg), which is responsible for the crystallite size and surface morphology, studied by using UV–vis diffuse reflectance spectroscopy and as shown in Fig. 8. The insert figure shows absorption spectra of the NIC sample. The band gap energy value (Eg) was calculated by using the following standard Tauc expression [33].
Fig. 7. Electrochemical performance of NiCo2O4 sample under three-electrode system. (a) CV curve at the different scan rates (b) GCD curve at different current density.
(αhυ)n=K(hυ-Eg)
increase of scan rate from 5 to 100 mVs−1 the anodic peak potential increased from 0.257 to 0.363 V and correspondingly, Ipa-anodic peak current value increased from 7.982 to 71.293Ag-1. This result indicates lower resistance value of the electrode material with excellent electrochemical reversibility. The insert of Fig. 7(a) shows specific capacitance vs. scan rate, which was measured by using equation (5). These results are remarkable among most of the earlier report in the literatures [29]. To further confirm the supercapacitance properties of the prepared NIC material, we have performed the Galvanostatic charge/discharge measurement (GCD) in the same potential range (0–0.5 V) (Fig. 7(b)). The specific capacitance from the GCD curve of NiCo2O4 electrodes can be calculated by using equation (6). The specific capacitance are 295, 250, 237 and 225 Fg-1 at current densities of 20, 30, 40, 60 Ag−1and it is high of 295 at low current density of 20 mV−1, which value is slightly high compare to other preparation method such as hydrothermal method [29], and co-precipitation method and electrochemical deposition method [30,31] as reported in the literature. The possibly reason for the electrochemical performance stated as mainly (i) the unique urchin like structure(ii) The whole NiCo2O4 microsphere consists of interconnected nanoneedle, which provides large number of active sites for redox reaction and possess high surface area. Meanwhile, the characteristics of the thin nanoneedle consist of large number of nanoparticle as discussed in the surface morphology section, ensure that majority of electro active materials are occupied in the redox reactions and efficiently contribute superior capacitance [32]. The interconnected nanopores between the nanoneedle provide appropriate diffusion paths to enhance the diffusion kinetics within the electrode.
(10)
Here, ‘α’ referred as coefficient of absorption, ‘hν’ is the photon energy, ‘K’ is the relative material constant, and n values are either 1/2 or 2, for direct transition n = 2 and for indirect transition n = 1/2. The optical band gap energies can be determined by using Tauc plots of equation (10) with the linear fit for n = 2. Meanwhile no linear relation can be obtained for n = 1/2. Therefore we confirmed that the present NIC system showing semiconducting behavior with direct allowed transitions. By extrapolating the linear portion of the graph to (αhυ) 2 = 0, two band gap energy are observed i.e. 2.17 eV and 3.5 eV. It was relevant for the spinel oxides that, the obtained band gap energies are associated with the transition of electron from O-2p valence band to the transition metals-3d (t2g, eg) conduction band [34]. Further, two band gap energies are attributed due to the co-existence of high and low spin states of Co3+ ion. Also it is well reported that the band gap energy and crystallite size are inversely related. In the present case, XRD result reveals that the crystallite size of urchin-like NiCo2O4 was found to be ~24 nm and well consistent with the UV-analysis. Again the band gap energy is directly related to the theoretical calculation obtained from DFT analysis. 3.5. DFT calculation To support the experimental findings, theoretical investigation was also performed, that gives information regarding the interaction and charge-transfer mechanism of NIC system through electronic structure calculations. Literature report reveals that electronic conductivity of NIC system is high and argued due to the formation of σ*(eg) band via 6
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Fig. 9. (a) Projected density of state (PDOS) of NiCo2O4, calculated using DFT + U (b,c) Majority and minority spin states.
empty eg-state was formed, which leads to a hole at the top or just above the valence band. Hence in both the cases empty states are formed at the top of the minority spin valence band and results high conductivity of NIC sample. The calculated band gap energy are found to be 2.1 eV, 3.5 eV (Fig. 9(b and c)) and are well agreement with the literature report. Again the densities of states of Ni-Oct and Co-Tet crossed the Fermi level in the spin-down channel (Fig. 9(c)). Therefore in between Ni-Oct and Co-Tet site exchange interaction takes place, which influences the conductivity of NIC system and enhances the specific capacitance properties.
intervening oxygen ion through strong covalent interaction between low spin Co3+–Ni3+ in the octahedral site assuming the distribution Co2+[Ni3+Co3+]O4 behaves as a semimetal [35]. Therefore we use density functional theory (DFT) incorporating with on-site Coulomb repulsion (DFT + U) to carry out the first principle calculations. 3.5.1. Computational details All the calculations were performed within Spin-polarized DFT framework using VASP package. The Exchange correlation terms were explained by using Perdew−Burke−Ernzerh of (PBE) function by adopting the U term on Co and Ni 3 d electron. The U values are determined from linear response theory and ULR(Ni) = 6.6 eV for nickel, and ULR(Co) = 4.4 and 6.7 for Co ions at Td site and Oh sites, respectively. The valence electrons O 2s, 2p; Co 3 d, 4s and Ni 3d, 4s states are taken for ultra soft pseudo-potential. The used cutoff KE (kinetic energy) was 50 Ryd for wave function and 500 Ryd for augmented density, respectively. Optimization of structural information was accepted by relaxing all atoms and calculation was performed by using 56- atom conventional cubic cell containing 8 formula units, with a 3 × 3 × 3 Monkhorst−Pack k-point mesh integration Brillouin zone. Fig. 9 shows the density of states (DOS) of NIC sample. The optimized lattice parameter was found to be 8.09 Å, which was well agreement with the XRD result. We can see that the present system shows half metal in which majority spin channel remains insulating while the minority channel becomes conducting. In both the spin channels valence band shift toward the Fermi level, which indicate that Ni2+ in place of Co3+ at an Oh site acts as p-type dopant and the result well matched with other experimental outcomes. The presence of Ni2+ at the octahedral site causes formation of a Co(Td)3+, which has only one occupied e-orbital in the minority spin channel. A hole is created on top of the valence band due to other empty e-orbital of Co(Td). In the similar manner, Ni3+ at the octahedral site results Co(Td)2+ and an
3.6. Magnetic behavior Fig. 10 shows the RT (300 K) the magnetic behavior (magnetic moment ‘M’ vs. magnetic field ‘H’ loop) of the prepared NIC sample investigated by VSM system. From the figure it is observed that the present system shows typical ferromagnetic characteristics, however the value of remnant magnetization (Mr~1.18emu/g) and coercivity (Hc~241.32Oe) was very small (owing to the weak ferromagnetic behavior at RT). Low temperature experimental data may give quite well result, but due the temperature limitation of the experimental setup used, we do not observe low temperature saturation magnetization result. It is well know that the magnetic behavior of the NiCo2O4 samples mainly depends on shape, size of the particle, super-exchange interactions and mostly on the surface morphology followed by the synthesis procedure [36]. In the present case, small value of saturation magnetization (Ms) may be correlated to the surface spin disorder which depends on the urchin-like surface morphology. Also literature report states that small value of Hc may possibly due to two reasons. (i) From magneto crystalline anisotropy called as shape anisotropy, i.e. magnetic disorder on the surface (ii) AFM-FM (anti ferromagnetic to ferromagnetic) exchange coupling, in which the ferromagnetic mainly 7
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Fig. 10. M-H hysteresis loop of NiCo2O4 sample at room temperature (RT).
originates from the oxygen vacancy or from the defects at interfaces [37]. In accordance with the FCBMP theory (F-center mediated bound magnetic polaron) or so called bound magnetic polaron (BMP) theory, oxygen vacancies are created nearby lattice of Ni2+ due to exchange of Ni(Oh) ↔ Co(Td) [38]. An electron is trapped in the oxygen vacancy (F center) couple with the magnetic spins of the nearest Ni2+ ions within the radius of the hydrogen like orbit of the F-center and forms a bound magnetic polaron, which influence the ferromagnetic behavior. Therefore, contribution of magnetic behavior in the present system is due to both the surface morphologies (shape anisotropy) and interface defects (oxygen vacancy). This result well supports the electronic properties of this system using DFT + U calculation. 4. Conclusion In summary, we have prepared urchin-like structure of spinel NiCo2O4 via hydrothermal route and its structural, morphological study were analyzed by using XRD, FESEM, EDS and Raman spectroscopy. The Electrochemical analysis revealed that the present sample shows excellent supercapacitive property. Such good capacitive performances are mainly attributed to the high specific surface area due to the urchin like structure composed of nanoneedle. The optical band gap from UV–visible spectroscopy was found to be 2.17 eV and 3.5 eV, which was well consistent with the theoretical calculation by using DFT. Again from the result we conclude that NIC behaves as a semimetal and can be used as spintronic application. The magnetic property of the material was confirmed from M-H loop. In particular weak ferromagnetic nature with small coercivity (Hc)~241.32Oe corresponding to remnant magnetization (Mr)~1.18emu/g was observed. Moreover, all the characterization states that the present spinel system may be used as novel multi-functional application, especially energy storage as well as magnetic devices etc. Declaration of competing interest No other conflicts to declare. Acknowledgement We acknowledge Ministry of Human Resource Development (MHRD), INDIA for the financial support of the project RP074. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] R.A. Huggins, Review—a new class of high rate, long cycle life, aqueous electrolyte battery electrodes, J. Electrochem. Soc. 164 (2017) A5031–A5036. [3] S.K. Meher, G.R. Rao, Ultralayered Co3O4 for high-performance supercapacitor
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