Microwave-Assisted Coprecipitation Synthesis and Local Structural Investigation on NiO, β-Ni(OH)2/Co3O4 Nanosheets, and Co3O4 Nanorods Using X-ray Absorption Spectroscopy at Co-Ni K-edge and Synchrotron X-ray Diffraction.

Developing the most straightforward, cheapest, and eco-friendly approaches for synthesizing nanostructures with well-defined morphology having the highest possible surface area to volume ratio is challenging for design and process. In the present work, nanosheets of NiO and β-Ni(OH)2/Co3O4, and nanorods of Co3O4 have been synthesized at a large scale via the microwave-assisted chemical coprecipitation method under low temperature and atmospheric pressure. X-ray absorption spectroscopy (XAS) measurements, which comprises both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, have been carried out at Co and Ni K-edges to probe the electronic structure of the samples. Also, the local atomic structural, chemical bonding, morphological, and optical properties of the sample were systematically investigated using XAS, synchrotron X-ray diffraction (SXRD), Raman spectroscopy, FTIR, transmission electron microscopy (TEM), and UV-visible spectroscopy. The normalized XANES spectra of the β-Ni(OH)2/Co3O4 nanosheets show the presence of Ni2+ and a mixed oxidation state of Co. The disorder factor decreases from β-Ni(OH)2/Co3O4 to Co3O4 with increasing Co-O bond length. The SXRD pattern analyzed using Rietveld refinement reveals that NiO has a face-centered cubic phase, Co3O4 has the standard spinal structure, and β-Ni(OH)2/Co3O4 has a mixed phase of hexagonal and cubic structures. TEM images revealed the formation of nanosheets for NiO and β-Ni(OH)2/Co3O4 samples and nanorods for Co3O4 samples. FTIR and Raman spectra show the formation of β-Ni(OH)2/Co3O4, which reveals the fingerprints of Ni-O and Co-O.

Introduction

The shape, size, and surface area of metal oxides at the nanoscale have a significant impact on their structural, electrical, chemical, and catalytic properties. Exploring low-cost and eco-friendly approaches to inculcate the development of metal oxide nanomaterials for desired structures and well-defined facets would be a significant step in the progress and development of research and industrial applications.[1] The method presented here to develop nanosheets and nanorods by simple microwave-assisted chemical coprecipitation methods may be one of the promising approaches. Nowadays, various nanomaterials have been studied due to their unique properties at the nanoscale compared to the bulk materials. Among them, two-dimensional (2D) nanomaterials like graphene and inorganic nanosheets have been receiving great attention due to their fascinating physical and chemical properties for various applications.[2,3] This great importance of 2D nanomaterials in catalysis[4,5] and energy devices is due to their fewer ion/electron diffusion path distance, significant electrochemical activity, high electronic conductivity, and improved structural stability. They can also provide surface-dependent electrochemical performance for next-generation batteries and supercapacitor applications.[6,7]

In particular, a high interest in the preparation of transition metal–metal oxide heterojunction 1D and 2D nanomaterials such as nickel oxide/cobalt oxide (NiO/Co3O4), nickel/nickel oxide (Ni/NiO), nickel hydroxide/cobalt oxide (Ni(OH)2/Co3O4), and α-Ni(OH)2 has been seen for their applications in supercapacitors,[8,9] solar cells,[10] ferro-fluids, catalysis,[11] magnetic materials,[12] gas sensors,[13,14] and as efficient anodes in lithium batteries.[15] It is well known that CoO occurs in 6 different oxidation states, such as Co, CoO2, Co2O3, CoO(OH), CoO, and Co3O4.[16,17] Nickel hydroxide Ni(OH)2 has received great attention due to its high theoretical capacitance for supercapacitor applications and high performance in battery applications.[18−24] NiO and Co3O4 are p-type antiferromagnetic semiconductors with direct band-gap energy in the range of 3.6–4.3 eV.[23] The structure of NiO is similar to that of NaCl, with octahedral Ni2+ and O2– sites having face-centered cubic (fcc) structure (rock salt structure), whereas Ni has a hexagonal hcp and fcc structure. The stoichiometric ratio (1:1) of NiO shows a green color, and non-stoichiometric NiO appears black.[23,24] There are many methods available for the synthesis of 1D and 2D nanomaterials; among them, microwave synthesis methods are very fast. Tian et al.[21] and Bazgir et al.[22] report on the microwave synthesis of nanoball-like mesoporous α-Ni(OH)2 as a precursor for NiO for supercapacitor applications and Co3O4 nanorods for photocatalytic degradation of methylene blue under visible light irradiation, respectively. A variety of high-cost methods are involved in the synthesis of morphologically essential nanomaterials.[25]

In this report, a simple microwave-assisted chemical coprecipitation method was implemented to synthesize NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorod samples. Herein, β-Ni(OH)2/Co3O4 nanosheets were synthesized with a 1:1 molar ratio of nickel nitrate and cobalt nitrate chemicals. For an in-depth investigation of the local structural study of samples by as-synthesis, we combined XAS and SXRD patterns. XAS is the combination of X-ray near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES is used to investigate charge transfer, orbital occupancy, and symmetry. The Fourier transform of EXAFS was used to obtain the bond length, and coordination details were extracted from the synthesized samples. The elemental specific nature and high sensitivity of the local chemical structure of the XAS technique make it an ideal tool to investigate the electronic structural properties and interatomic environment.[26] Rietveld refinement of the synchrotron X-ray diffraction data reflects the formation of NiO with face-centered cubic phase, Co3O4 with spinal cubic phase, and β-Ni(OH)2/Co3O4 with hexagonal and spinal cubic mixed phases. Again, the structural and morphological properties were investigated by TEM using SAED patterns of the as-synthesized samples.

Results and Discussion

Crystallographic Study

The synchrotron X-ray diffraction patterns shown in Figure were used to obtain the detailed crystallography of the as-synthesized samples. Structural analysis of samples from SXRD data and Rietveld refinement was carried out using FULLPROF software.[2,30,31]Figure a–c displays the SXRD patterns of samples of NiO, Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods, where the red, black, and blue curves indicate the observed pattern, calculated pattern, and the difference between the observed and calculated pattern, respectively. In the SXRD patterns of Figure a NiO exhibits sharp peak reflections at positions 2θ = 19.78, 22.90, 32.49, and 38.43, which can be readily indexed to (111), (200), (220), and (311), respectively. These reflections can be indexed as a face-centered cubic (fcc) structure with space group Fm3̅m for cubic NiO (JCPDS card no. 03-065-2901) at lattice parameter a = 4.19420 ± 0.044 Å without any trace of additional impurity. The obtained lattice parameters from the SXRD data of the NiO nanosheets are well in agreement with the reported literature.[10]Figure b shows the diffraction pattern of the β-Ni(OH)2/Co3O4 sample. Figure b shows five diffraction peaks (blue color) at angles 2θ = 9.90, 17.14, 19.48, 27.11, and 34.69 in the SXRD pattern, which can, respectively, be assigned to the (001), (100), (011), (110), (012), and (200) reflection indexes of β-Ni(OH)2. The refined lattice parameters for the Ni(OH)2/Co3O4 sample are found to be a = 3.1506 ± 0.038 Å and c = 4.7123 ± 0.0541Å (JCPDS no. 14-0117), and β-Ni(OH)2 crystallized as a hexagonal structure with a space group of (164) P3̅m1 of the β-Ni(OH)2/Co3O4 sample. The other nine diffraction peaks from Figure b can be attributed to the Co3O4 phase at 9.95, 16.59, 19.13, 19.98, 23.12, 28.41, 30.17, 32.92, and 36.94, indexed by (111), (220), (311), (222), (400), (422), (333), (440), and (620) for the cubic spinel structure of space group (277) Fd3̅m (JCPDS no. 43-1003) at the lattice parameter a = 8.1195 ± 0.14 Å.[32,33] The lattice constants calculated from the SXRD data of the NiO nanosheets are well in agreement with the reported data.[9] Also, the diffraction patterns of the Co3O4 sample given in Figure c evidence the cubic spinel phase of the samples with space group (277) Fd3̅m at the reflection indexes of (111), (220), (311), (222), (400), (331), (422), (511), (440), and (531), respectively, for angles 2θ = 10.14, 16.29, 19.48, 2.36, 23.55, 25.70, 28.94, 30.74, 33.55, and 35.13 for a lattice constant a = 8.0868 Å (JCPDS no. 43-1003).[33] An additional minute peak (indicated by the star) appears in the Co3O4 sample’s SXRD as shown in Figure c, which is due to the splitting of the (111) peak.

Figure 1

(a–c) SXRD patterns of samples of NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods; (d) cubic structure of the Co3O4 sample.

Gaussian function fitting[34] was used to find out the full width at half-maximum (fwhm) values from the diffraction peaks of the samples. The average crystallite size was estimated from the Scherrer formula D = kλ/β cos θ, where β is the fwhm of the diffraction peak, k is the Scherrer constant (0.94), θ is the diffraction angle, and λ is the incident X-ray wavelength (0.82521 Å). It is found that the average crystallite sizes of NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods are 5.5 ± 0.87, 3.42 ± 0.33, and 11.22 ± 1.07 nm, respectively. The SXRD results are in accordance with the TEM and SAED patterns.

X-ray Absorption Spectroscopic Study

The X-ray absorption spectroscopy (XAS) measurements, which comprise both X-ray near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, were carried out on Co and Ni K edges to probe the local structure. The XAS measurements were carried out at the Energy-Scanning EXAFS beamline (BL-9) at the Indus-2at RRCAT, Indore, India.[33,34]

The normalized XANES spectra at the Ni K-edge and Co K-edge are shown in Figures and 3, respectively. They provide detailed information about the oxidation state and coordination environment of the Ni and Co atoms. The position of the absorption edge depends on the oxidation state of the absorbing atom. From Figure , the Ni-edge[35,36] positions of NiO and β-Ni(OH)2/Co3O4 nanosheets are found to be at the same energy, indicating +2 oxidations for both the samples. There is no significant change observed between NiO and β-Ni(OH)2/Co3O4 nanosheets except for a small increase in the white -line peak intensity (8351 eV) of β-Ni(OH)2/Co3O4 and a phase shift in EXAFS oscillations. This indicates a small alteration in the structure of β-Ni(OH)2/Co3O4 compared to NiO. The normalized XANES spectrum at the Co K-edge is shown in Figure along with CoO and Co3O4 standards. The absorption edge position and XANES profile of Co3O4 and β-Ni(OH)2/Co3O4 samples match with those of the Co3O4 standard or bulk material, clearly indicating that the local electronic structure of Co3O4 is present in these two samples. In Figure , the normalized XANES spectra of the Co K-edge energy indicate the oxidation state of cobalt ions in +2 to +3, with a slight change in intensity from Co3O4 to β-Ni(OH)2/Co3O4 samples, whereas Figure represents the presence of Co-edge and Ni-edge in the β-Ni(OH)2/Co3O4 samples. This is also corroborated by the EXAFS analysis part below. There is a small decrease in the white-line peak intensity (7729 eV) observed for β-Ni(OH)2/Co3O4 compared to the Co3O4 sample.

Figure 2

Normalised XANES spectra at Ni K-edge along with standard samples.

Figure 3

Normalized XANES spectra at the Co K-edge along with standard samples.

Figure 4

Normalized XANES spectra of β-Ni(OH)2/Co3O4 samples at the Co K-edge and Ni-edge.

The local structure around the absorbing atom is obtained from the quantitative analysis of the EXAFS spectra.[37,38] In order to take care of the oscillations (Figures and 6) in the absorption spectra, μ(E) was converted to the absorption function χ(E). The set of EXAFS data analysis programs available within the Demeter software package have been used for EXAFS data analysis.[38] This includes background reduction and Fourier transform to derive the χ(R) versus R plots from the absorption spectra (using ATHENA software), generation of the theoretical EXAFS spectra starting from an assumed crystallographic structure, and finally, fitting of the experimental data with the theoretical spectra using ARTEMIS software.

Figure 5

EXAFS oscillation obtained after background subtraction at the Ni K-edge.

Figure 6

EXAFS oscillation obtained after background subtraction at the Co K-edge.

The χ(R) versus R plots generated using Fourier transform of χ(E) versus E spectra by following the methodology described above are shown in Figure at the Ni K-edge and Figure c,d at the Co K-edge. The initially guessed structure of NiO is used for Ni K-edge fitting to generate theoretical scattering paths. The Fourier transform spectrum of NiO and β-Ni(OH)2/Co3O4 samples shows two strong coordination peaks (Figure ) at ∼1.56 and ∼2.60 Å, respectively. Note that the Fourier transform spectrum shown here is not phase corrected; hence, the coordination peaks will appear at the slightly lower R side compared to the actual bond lengths. The first peak is fitted with the Ni–O coordination path, and the second peak is fitted with the Ni–Ni coordination path. The bond distances, coordination numbers, and Debye–Waller factors (σ2), which give the mean square fluctuations in the distances, and their best fitted results are shown in Tables and 2 for Ni K-edge and Co K-edge measurements. The β-Ni(OH)2/Co3O4 sample shows a small decrease in Ni–O bond length (0.03 Å) and Ni–Ni bond length (0.07 Å) with a small increase in the coordination number. We can also see the decrease in the Fourier transform spectrum intensity in the region of 3–5 Å (Figure ) for β-Ni(OH)2/Co3O4 compared to NiO. Figure c,d shows the Fourier transform EXAFS spectrum at the Co K-edge. Figure a,b shows EXAFS fits with k-space fitting and also shows two coordination peaks at ∼1.47 and ∼2.48 Å. The first peak at ∼1.47 Å is fitted with the bond length of ∼1.88 Å, which is much lower than the Co–O bond length (∼2.04 Å). Then we compared the CoO and Co3O4 commercial standards with Co EXAFS data (Figure ) and found that it exactly matches with the Co3O4 nanorod sample, which is also corroborating the XANES observation discussed above. We used the Co3O4 structure to fit the EXAF spectrum of Co3O4 and β-Ni(OH)2/Co3O4 at the Co K-edge, and the fitting results are shown in Table . There are two equivalent sites (tetrahedral and octahedral) in Co3O4. However, because of the existing data range, which limits the maximum number of independent parameters that can be varied during the fitting, we carried out fitting using three paths only and varied their bond length, coordination number, and disorder factor. The results obtained from the XAS study of the Ni and Co K-edge is well in agreement with the reported literature.[12,39]

Figure 7

(a) Fourier-transformed EXAFS spectra of the NiO sample and (b) β-Ni(OH)2/Co3O4 sample at the Ni K-edge along with the best fit. The experimental spectra are represented by scatter points, and the theoretical fits are represented by the solid line.

Figure 8

EXAFS oscillation and fit of the (a) Co3O4 sample, (b) β-Ni(OH)2/Co3O4 sample, and the corresponding Fourier-transformed EXAFS spectra of the (c) Co3O4 sample and (d) β-Ni(OH)2/Co3O4 sample at the Co K-edge along with the best fit. The experimental spectra are represented by Scatter points and the theoretical fits are represented by the solid line.

Table 1

Bond Length, Coordination Number, and Disorder Factor Obtained by EXAFS fitting at the Ni K-Edge

path	parameter	NiO	β-Ni(OH)2/Co3O4
Ni-O	R (Å)	2.068 ± 0.009	2.036 ± 0.006
	N	5.5 ± 0.4	5.3 ± 0.3
	σ2	0.0069 ± 0.0015	0.0081 ± 0.0011
Ni-Ni	R (Å)	3.033 ± 0.009	2.962 ± 0.005
	N	11.1 ± 0.8	11.5 ± 0.5
	σ2	0.0146 ± 0.0013	0.0133 ± 0.0007

Table 2

Bond Length, Coordination Number, and Disorder Factor Obtained by EXAFS Fitting at the Co K-Edgea

path	parameter	Co3O4	β-Ni(OH)2/Co3O4
Co-O	R (Å)	1.90 ± 0.01	1.91 ± 0.01
	N	5.28 ± 0.36	4.56 ± 0.42
	σ2	0.0018 ± 0.0009	0.0024 ± 0.0014
Co-Co	R (Å)	2.85 ± 0.02	2.88 ± 0.02
	N	5.28 ± 0.92	5.84 ± 1.21
	σ2	0.0052 ± 0.0017	0.0079 ± 0.0025
Co-Co	R (Å)	3.35 ± 0.02	3.37 ± 0.02
	N	11.28 ± 0.42	4.64 ± 0.43
	σ2	0.0112 ± 0.0028	0.0097 ± 0.0067

The parameter is kept fixed.

Figure 9

Fourier transformed EXAFS spectra of at the Co K-edge along with CoO and Co3O4 standards.

Morphological Investigation

Figure shows the transmission electron microscopy (TEM) micrograph of NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorod samples. In particular, TEM micrographs of Figure a,b and d,e reveal the formation of nanosheets of NiO and β-Ni(OH)2/Co3O4, and those of Figure g,h reveal the formation of nanorods of Co3O4. These formations have taken place by the simple microwave-assisted chemical coprecipitation method. The thicknesses of NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods are found to be ∼7.6, ∼3.5, and ∼33 nm, respectively. The nanorods’ lengths, as evaluated from the TEM micrographs of Co3O4, are found to be ∼283 nm. The d-spacing measured using SAEDs matches well with the d-spacing of SXRD analysis. Figure c,f shows a typical SAED pattern corresponding to the NiO and β-Ni(OH)2/Co3O4 nanosheets consisting of three distinct concentric spotty rings. The d-spacings of NiO nanosheets as measured from the SAED pattern in Figure c are 0.498, 0.326, 0.152, and 0.140 nm, and are indexed, respectively, to (111), (200), (220), and (311). Figure f reveals the SAED pattern of Ni(OH)2/Co3O4; the corresponding rings for Ni(OH)2 give d values of 0.4712 and 0.2361 nm, appearing from the (001) and (011) planes. The corresponding rings for Co3O4 for d values at 0.4787, 0.2343, 0.202, and 0.143 nm appear from planes (111), (222), (400), and (440), respectively. The Co3O4 nanorods also show the d values at 0.4687, 0.2343, 0.202, and 0.143 nm arising from (111), (222), (400), and (440), respectively. All of the SAED patterns show the mixed, polycrystalline, and textured nature of agglomeration. These obtained d values match well with the values measured using SXRD.

Figure 10

(a, b) and (d, e) show TEM micrographs of clusters of NiO and β-Ni(OH)2/Co3O4 nanosheets, and the corresponding SAED patterns given in (c, f) show the mixed, polycrystalline, and textured nature of agglomeration, (g, h) show Co3O4 nanorods and (i) their SAED pattern. The nanorods are mostly randomly aggregated.

FTIR, Raman Shift, and UV–vis Spectroscopic Study

The FTIR spectra are given in Figure for the as-synthesized samples. They were used to investigate the functional groups present in the samples and were recorded in the range of 400 to 4000 cm–1 at room temperature. The strong wide bands at 3336, 3353, and 3396 cm–1 indicate the O–H ν(H2O) stretching vibration and that at ∼1624 cm–1 indicate the δ(H2O) bending vibration characteristics of the water molecules for NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods, respectively. However, the peak at 521 cm–1 indicates Ni–O bonds in the fingerprint region for the NiO nanosheet sample.[40,41] The absorption bands at 527.7 and 642.3 were assigned to Co–O and Ni–O stretching vibration modes, indicating Co–O and Ni–O bonds in the fingerprint region for the β-Ni(OH)2/Co3O4 nanosheets. For Co3O4 nanorods having sharp bands at 549.61 and 653.2 cm–1 are associated for O–Co and Co-O–Co lattice vibration, respectively. The results of FTIR matched well with the reported literature and also indicate the presence of Ni and Co lattices in the samples.[40−42]

Figure 11

FTIR spectra of NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods.

The Raman spectra of NiO, Co3O4, and β-Ni(OH)2/Co3O4 samples taken using 532 nm wavelength of visible LASER are shown in Figure . In Figure a, the Raman peaks of the NiO nanosheets at 492, 709, and 1044 cm–1 were assigned to the first-order longitudinal optical (LO) phonon modes, second-order transverse optical 2TO, and longitudinal optical 2LO phonon scattering of NiO, respectively.[24]Figure b shows the spectra of the synthesized Co3O4 sample with Co2+ (3d7) and Co3+ (3d6) situated at tetrahedral and octahedral sites, respectively. The Co3O4 sample crystallizes in cubic spinel structures of space group Fd3m with symmetry Γ = A1g (R) + Eg (R) +F1g (IN) + 3F2g (R) + 2A2u (IN) +2Eu (IN) + 4F1u (IR) + 2F2u (IN), where R, IR, and IN represent Raman active vibrations, infrared-active vibrations, and inactive modes, respectively.[43,44]Figure b reveals the five active Raman modes at 180.2, 450.9, 494.7, 591, and 652 cm–1, which are in agreement with the values of the pure Co3O4 spinel structure in the reported literature.[45] The band at 180 cm–1 (F2g) exhibits CoO6 vibration, and the other bands at 450.9, 494.7, 591, and 652 cm–1correspond to Eg, 2F2g, and A1g, which exhibit Co–O symmetric stretching vibration.[45,46]

Figure 12

(a) Raman spectra of NiO, (b) β-Ni(OH)2/Co3O4 nanosheets, and (c) Co3O4 nanorods.

Figure c displays Raman peaks at 177, 455, 502, 643, and 1050 cm–1 corresponding to the β-Ni(OH)2/Co3O4 sample.[9] It can be observed that the Raman broad band between 450 and 519 cm–1 corresponds to the Ni–OH/Co–OH Ni–O/Co–O stretching modes, matching with the literature.[9,44−47] Hall et al.[44] reported on the β-Ni(OH)2 A1g mode of the O–H stretching at 1067 cm–1; herein it is observed that the broadened peak at 1050 cm–1 in Figure c corresponds to the A1g mode of the same O–H stretching of β-Ni(OH)2 of the sample β-Ni(OH)2/Co3O4.[9,44−46] The peak at 1050 cm–1 is also noted for the 2P mode of Ni–O.[47]

Figure a reflects the UV–Vis absorption spectra of the synthesized samples in the wavelength range of 200–900 nm. The strongest absorption peak is displayed at 249 and 330 nm for the NiO and β-Ni(OH)2/Co3O4 sample, respectively, and three peaks appear at 226, 436, and 748 nm for the Co3O4 sample. These three distinct absorption band gaps at around 226, 436, and 748 nm are assigned to the charge transfer from O2– to Co3+, octahedral coordinated Co3+, and transition of Co2+ in tetrahedral coordination in well-defined-order Co3O4 respectively.[48−50] Hence, the UV–vis result is consistent with Co3O4 as the phase assignment of the Raman spectra. The energy band gap of the samples was calculated by Tauc eq , with the plots displayed in Figure b.where h is Planck’s constant, ν is the photon’s frequency, Eg is the band gap, A is a constant, and n is a factor depending on the nature of the electron transition. n = 1/2 is used in eq for direct allowed band gaps.[50] The energy band gap of bulk NiO, β-Ni(OH)2, and Co3O4 is 4, 3.6, and less than 3 eV, respectively.[48−51] The energy band gap was estimated from Tauc plots, and it was found to be 2.29 and 3.6 eV for Co3O4 and 4.22 and 3.58 eV for β-Ni(OH)2/Co3O4 and NiO samples, respectively, which is well in agreement with the reported literature.[48−53]

Figure 13

(a) UV–vis absorbance spectra and (b) Tauc’s plot of NiO, Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods.

Conclusions

Samples of NiO, β-Ni(OH)2/Co3O4, and Co3O4 were synthesized using the microwave-assisted chemical coprecipitation method. Local structure was evaluated from XAS and SXRD data. SXRD, TEM, and Raman shift were used to confirm the structure of the as-synthesized samples. The structural parameters were well matched with the Rietveld refinement of SXRD of the samples. It is confirmed from the SXRD patterns that the NiO, β-Ni(OH)2/Co3O4, and Co3O4 samples were crystallized with fcc, hexagonal/cubic spinel, and cubic spinel structure, respectively. The TEM micrograph clearly reveals the formation of nanosheets of NiO, β-Ni(OH)2/Co3O4, and Co3O4 nanorods. FTIR and Raman results show the Co–O, Ni–O, and O–H of β-Ni(OH)2 vibration bands. The energy band gap of β-Ni(OH)2/Co3O4 nanosheets is increased as compared to NiO and Co3O4 samples. The β-Ni(OH)2/Co3O4 nanosheet sample shows different properties than the NiO and Co3O4 samples, which indicates its promising applications in the field of supercapacitors.

Experimental Details

In the present work, the microwave-assisted chemical coprecipitation method has been implemented for the synthesis of NiO, β-Ni(OH)2/Co3O4 nanosheets, and Co3O4 nanorods. For the synthesis of samples, analytical-grade nickel nitrate hexahydrate (Ni(NO3)26H2O) and cobalt nitrate hexahydrate (Co(NO3)26H2O), NaOH, and N,N-dimethylformamide (DMF) were used without any further purification. A typical reaction was carried out for NiO and Co3O4 as follows: the appropriate amount of nickel nitrate hexahydrate (1 mmol), cobalt nitrate hexahydrate (1 mmol), and NaOH (2 mmol) were dissolved separately with 50 mL of DMF and stirred for 1 h. For synthesis of β-Ni(OH)2/Co3O4 nanosheets, the appropriate amount (1:1 ratio) of nickel nitrate hexahydrate (0.5 mmol), cobalt nitrate hexahydrate (0.5 mmol), and NaOH (1 mmol) were dissolved separately in beakers with 50 mL of DMF and stirred for 1 h. Then, NaOH solution was added to a mixture of precursors dropwise in all of the above-mentioned solutions; the beakers were placed inside a microwave oven for heating through microwave irradiation for 20 s for 5 cycles at 800 W output power and 2.45 GHz. After that, the solution was again stirred for 2 h at 40 °C till a clear precipitate was formed. The obtained precipitate was filtered, and washed with deionized water and ethanol several times. Finally, the obtained sample powder was dried at 100 °C overnight. Then, the obtained products were characterized.

Characterization of the Samples

Synchrotron X-ray diffraction measurements were performed at Angle Dispersive X-ray Diffraction Beamline, BL-12, at INDUS-2 Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.[27] This beamline is a bending magnet-based beamline and consists of an Si(111)-based double-crystal monochromator (DCM). The SXRD was recorded on the image plate area detector (MAR-345 dtb) experimental station in transmission mode using a monochromatic wavelength of 0.82521 Å. The sample to detector distance and the actual wavelength were accurately calibrated by measuring the SXRD of the LaB6 NIST standard sample. Fit2D software was used to convert the image data into Intensity-2θ form. The XAS measurements (consisting of both X-ray absorption near-edge structure and extended X-ray absorption fine structure) of the as-synthesized samples were carried out at the Energy Scanning EXAFS beamline (BL-9 Indus-2 at source 2.5 GeV, 100 mA) in transmission mode at RRCAT, Indore, India. This beamline operates in the energy range of 4–25 KeV.[28,29] The beamline optics consists of an Rh/Pt-coated collimating meridional cylindrical mirror, and the collimated beam reflected by the mirror is monochromatized by a Si(111) (2d = 6.2709 Å)-based double-crystal monochromator (DCM). The set of EXAFS data analysis software available within the IFEFFIT package was used for the EXAFS data analysis. This includes background reduction and Fourier transform to derive the w(R) versus R spectra from the absorption spectra (using ATHENA software), generation of the theoretical EXAFS spectra starting from an assumed crystallographic structure, and finally, fitting of the experimental data with the theoretical spectra using ARTEMIS software. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns of the samples were recorded at 200 kV accelerating voltage using TEM Tecnai-20 G2. Fourier transform infrared spectroscopy (FTIR, Bruker Tensor II), Raman shift (Horiba), and UV–vis (JASCOV-750) spectroscopic techniques were used for the optical study of the samples.