Off-Stoichiometric Nickel Cobaltite Nanoparticles: Thermal Stability

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Off-Stoichiometric Nickel Cobaltite Nanoparticles: Thermal Stability, Magnetization and Neutron Diffraction Studies Seema Verma, Amit Kumar, Dhanapal Pravarthana, Aparna Deshpande, Satishchandra B. Ogale, and Seikh M Yusuf J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp504538y • Publication Date (Web): 04 Jul 2014 Downloaded from http://pubs.acs.org on July 7, 2014

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The Journal of Physical Chemistry

Off-Stoichiometric Nickel Cobaltite Nanoparticles: Thermal Stability, Magnetization and Neutron Diffraction Studies Seema Verma, †, * Amit Kumar, § D. Pravarthana, † Aparna Deshpande, ‡ Satishchandra B. Ogale, ‡,* and S. M. Yusuf §,* †

Department of Chemistry, Indian Institute of Science Education and Research, Pune,900

NCL Innovation Park, Dr Homi Bhabha Road, Pune - 411 008, India ‡

Physical and Materials Chemistry Division, National Chemical Laboratory, Dr. Homi

Bhabha Road, Pune-411008 §

Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

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ABSTRACT In the present investigation, we report a detailed examination of the effect of offstoichiometry introduced in NiCo2O4 by adding excess cobalt. Thus, we compare and analyse the structural and magnetic properties of the Ni0.75Co2.25O4 and NiCo2O4 cubic systems. A low temperature combustion method was utilized to synthesize stoichiometric (NiCo2O4) and off-stoichiometric (Ni0.75Co2.25O4) nanoparticles on a large scale. The x-ray diffraction pattern for the sample annealed at high temperature (773 K) shows the presence of much less intense NiO phase (~ 2-5 %) in Ni0.75Co2.25O4 as compared to that in the case of NiCo2O4 sample (~ 15-20%). The Ni 2p and Co 2p XPS spectra reveal coexistence of Ni2+, Ni3+, Co2+ and Co3+ species on the surface of both the NiCo2O4 and Ni0.75Co2.25O4 samples in differing proportions. In addition to the basic magnetic characterizations using PPMS, these were also analysed by neutron diffraction. The off-stoichiometric Ni0.75Co2.25O4 sample shows an interesting magnetic phase conversion from frustrated dipolar system to an enhanced magnetic ordering upon annealing. Local moments on the lattice sites of NiCo2O4 and Ni0.75Co2.25O4 samples are further compared by neutron diffraction confirming stronger ordered moments and enhanced structural and thermal stability for the Ni0.75Co2.25O4 sample.

Keywords: Off-Stoichiometry, Metal Oxide, Neutron Diffraction, Spinel, Cation Distribution, Phase Stability 2 ACS Paragon Plus Environment

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INTRODUCTION Nickel cobaltite is a promising transparent conducting oxide material which has been extensively studied as an electrocatalyst for anodic oxygen evolution, optical limiters and switches, infrared transparent conducting electrodes for flat panel displays, and chemical sensors.1-5 Recently, this material has attracted much attention as a cost effective, environment-friendly and scalable alternative to the supercapacitor material RuO2.6-10 Nickel cobaltite, NiCo2O4, generally acquires an inverse spinel crystal structure. However, there is significant evidence to suggest that NiCo2O4 adopts an unusual magnetic structure in comparison to other inverse spinels owing to the variation in cation distribution.11-13 While it is generally accepted that Ni ions prefer to occupy octahedral sites, and Co ions are distributed in both octahedral and tetrahedral sites, a detailed description of the cation distribution remains a matter of investigation.14-16 The deviation in the cationic distribution is governed by the synthetic protocol employed and its elucidation depends on the characterization techniques involved.17-19 Also, the fact that NiCo2O4 phase is thermally and structurally unstable above 673 K imposes the need for low temperature synthesis of the material. Indeed, the thermal history is likely to profoundly affect the cationic distribution which in turn governs the structural and magnetic behaviour of the material. NiCo2O4 exhibits interesting magnetic behaviour with a Curie temperature TC of approximately 673 K. Bulk NiCo2O4 samples have been widely studied by utilizing X-ray and neutron diffraction, magnetization, Mössbauer spectroscopic technique etc.13-15,17,18 However, not many studies on the variation of the structural and magnetic behaviour of nickel cobaltite nanoparticles are available in the literature. Earlier, we reported synthesis of fairly mono-dispersed nickel cobaltite nanoparticles at 473 K by a combustion route utilizing glycine as a fuel and nitrate as the oxidizer under nearly smouldering combustion conditions.20 Upon annealing such nanoparticles, we observed an interesting evolution of the 3 ACS Paragon Plus Environment


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magnetic phase exhibiting unusual magnetic properties.20 This behaviour was attributed to the appearance of anti-ferromagnetic NiO predominantly on the surface of the nanoparticles on one hand, and the concurrent formation of off-stoichiometric cobalt-rich NiCo2O4 nanoparticles on the other. Incomplete compensation of the spins among the coordinated NiO induces spin-glass type of behaviour.20 In view of our earlier findings and the prevailing issues about cation distributions; an important question which remains unaddressed is the effect of off-stoichiometry on the unusual structural and magnetic properties. As reported earlier, the nickel cobaltite system Ni1-xCoxOy (x varying from 0.67 to 1 and y = 4/3) forms an interesting example of spinel structure.21,22 With x = 1, the oxide system ends with the Co3O4 phase, whereas with x = 0.67 as Ni replaces Co in the spinel lattice, the site occupancy distribution of the cations favours ideal stoichiometric Ni0.33Co0.67O4/3 (NiCo2O4) phase. However, when x is less than 0.67, the spinel structure is less stable and it forms NaCl-type cubic phase with y = 1. With the substitution of Co for Ni in Ni (1-x)CoxOy lattice (x varying from 0 to 0.25 and y = 1) NaCl type phase forms almost extensively with a predominant NiO phase for x = 0.16 A literature review of the research on Ni(1-x)CoxOy system suggests that no effort has been expended on the induced off-stoichiometry by introducing excess Co into stoichiometric NiCo2O4 spinel structure. Despite numerous studies on structural, electrical and magnetic properties on bulk NiCo2O4 particles, the effect of induced off-stoichiometry on bulk system or nanoparticles remain an important area to be explored. In the present investigation, we report a detailed examination of the effect of induced off-stoichiometry by introducing excess Co to NiCo2O4 cubic system. This was motivated by our previous study wherein NiO was seen to precipitate at higher temperature in nanosystem NiCo2O4. In order to preserve the phase stability we conjectured that excess cobalt may be helpful, hence this study.


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EXPERIMENTAL METHODS The reagents cobalt nitrate (AR, Merck), nickel nitrate (extrapure, Merck), glycine (AR, Spectrochem) were used as raw materials to prepare stoichiometric (NiCo2O4) and Co-excess off-stoichiometric (e.g. Ni0.75Co2.25O4, Ni0.88Co2.12O4 etc.) nickel cobaltite nanoparticles. Previous studies have shown that 0.5 mole of glycine per mole of metal is required for the synthesis of spinel phase nickel cobaltite nanoparticles.20 Based on this, an appropriate molar ratio of metal nitrates and glycine were mixed to obtain 10-15 g of stoichiometric and offstoichiometric naoparticles. The resultant homogeneous solutions were slowly evaporated on a water bath to form a viscous gel. The gels were allowed to undergo rapid combustion reaction in a preheated furnace at 473 K. The samples were kept at the same temperature for 4h. Although we looked at different off-stoichiometric cases, we chose to discuss the case of Ni0.75Co2.25O4 in several details in the manuscript because of the minimum content of NiO noted in this case in the annealed state and the attendant stability. As an example, we have presented the data for one other case, namely Ni0.88Co2.12O4, in the supporting information (See Figures S1 and S2). The as-prepared samples were named as A-473 and B-473 for NiCo2O4 and Ni0.75Co2.25O4 samples, respectively. The as-prepared samples were further annealed at 573 K and 773 K for 4 h and were named as A-573, A-773 for NiCo2O4 samples and B-573, B-773 for Ni0.75Co2.25O4 samples, respectively (see Table 1 for sample codes). The samples were characterized for their phase purity and crystallinity by powder X-ray diffraction (XRD) measurements (Pan-Analytical Xpert Pro) with Cu Kα radiation using Ni as a filter. XPS studies were carried out using a VG Scientific ESCA-3 MK spectrometer at a base pressure of better than 1 × 10

-9

Torr. The exciting radiation was Mg Kα X-rays (1253.6 eV) and the

spectrometer was operated in the constant analyser energy mode (CAE) at a pass energy of 50 eV yielding an overall resolution of ~1.1 eV. Temperature dependent (300 - 5 K) neutron 5 ACS Paragon Plus Environment


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diffraction measurements were recorded using the Powder Diffractometer -II (λ = 1.2443 Å) at Dhruva reactor, Mumbai. The dc magnetization measurements were carried out using a Quantum Design PPMS (Physical Properties Measurement system). The measurements were made between 5 and 300 K using zero-field-cooling (ZFC) and field-cooling (FC) protocols at 50 Oe, and the hysteresis loops were obtained in a magnetic field varied from + 4 to - 4 T.

RESULTS AND DISCUSSION Figure 1 compares the powder X-ray diffraction patterns of the as-prepared and annealed stoichiometric NiCo2O4 and off-stoichiometric Ni0.75Co2.25O4 samples. The XRD patterns of A-473 and B-473 samples show the formation of a pure homogeneous spinel phase (Figure 1(a), and Figure 1(b)), which corroborates well with the simulated pattern for NiCo2O4 (a = 8.11 Å, PCPDF No. 20-0781). Samples (both A and B) when annealed at 573 K show pure spinel phase without having any additional reflections corresponding to NiO or CoO phases (Figure 1 (c) and Figure 1 (d)). On further annealing to higher temperature 773K, additional reflections corresponding to NiO phase appear which has a contribution of ~ 15-20% (nominally based upon the relative intensity) as seen from the Figure 1(e) and Figure 1 (f). However, for B-773, much less contribution (~2-5%) of the NiO phase is observed. From the charge and valence distribution of the cations, it is possible to perceive the concurrent precipitation of the Co3O4 phase. However, due to the extremely small difference between the powder diffraction patterns for NiCo2O4 (a = 8.11 Å) and Co3O4 (a = 8.08 Å) phases it is hard to ascertain this aspect. The lattice parameters, obtained from the XRD patterns of A-473 and A-573, are 8.13 ± 0.002 Å and 8.12 ± 0.004 Å, respectively, and are slightly larger than the value reported earlier for the bulk NiCo2O4 (8.11 Å) sample.11, 14 The reason for this is attributed to the nanocrystalline nature of these particles.20 For A-773, a small shift of lattice parameter value by 0.12% (8.10 ± 0.004 Å) is observed. This implies 6 ACS Paragon Plus Environment

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that the formation of the NiO phase may be accompanied by the formation of cobalt-rich off stoichiometric NiCo2O4 nanoparticles, resulting into a lattice distortion from the ideal spinel structure.20 The derived lattice parameter values for the B-473 and B-573 samples are 8.12 ± 0.004 Å, and 8.11 ± 0.005 Å, respectively, which are very close to the value 8.11 Å for the bulk NiCo2O4 samples. It is interesting to note that unlike A-773, the lattice parameter value for B-773 is 8.11 ± 0.004 Å which is comparatively closer to that of the bulk NiCo2O4 sample. This suggests that the excess incorporated Co ions in the lattice do not relax (or strain) the spinel structure and the crystalline phase in the off-stoichiometric sample is very close to that of the stoichiometric sample. Based on the XRD data analysis, one could argue that for the B-773 case at high temperature the precipitated NiO and excess Co ions may be combining and undergoing a thermal reaction to form a stoichiometric NiCo2O4 phase simply by an oxidation reaction in air, thereby converting a larger fraction of precipitated NiO phase to the NiCo2O4 phase. However, in order to demonstrate the enhanced thermal stability of the off-stoichiometric Ni0.75Co2.25O4 phase, other experimental evidences are required to further elucidate these aspects. The average crystallite sizes were derived using the Scherrer's equation, d = (0.9λ/β cosθ), where d is the diameter in Angstroms, β is the half-maximum line width, and λ is the wavelength of x-rays. The average crystallite size for A-473 was found to be 7 (±1) nm. Powder XRD studies indicate that the average crystallite size of the as-prepared sample is increased after the heat treatment at 573 and 773 K. Upon annealing the as-prepared sample, the crystallite size grows to 8 (±1) nm and 19 (±1) nm for the A-573 and A-773 samples, respectively, following the diffusion mechanism. 23


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Intensity (arb. unit)


* *

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f e

*

d c b a s3

311 111

220

222 400 111 200

30

511 440 220

111 200

20

422

s2

220

40 50 60 2θ (degrees)

s1

70

80

Figure 1. XRD patterns of (a) A-473, (b) B-473, (c) A-573, (d) B-573, (e) A-773 and (f) B773 powders. Simulated patterns of (s1) NiO, (s2) CoO and (s3) NiCo2O4 phases; an asterisk indicates the NiO phase.

Interestingly, the calculated average crystallite sizes obtained for the off-stoichiometric B473, B-573 and B-773 samples are comparable to that of the stoichiometric samples (see Table 1) confirming that the induced off-stoichiometry has no role on the diffusion mechanism. 23


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Figure 2. Panels A and B: TEM images of A-773 and B-773 samples, respectively. Panels C and D: The corresponding SAED patterns from the collection of nanoparticles. Panels E and F: respective HRTEM images; insets: the lattice fringes of square marked nanoparticles. Scale bars: (A, B) 50 nm, (C, D) 5 (1/nm), (E, F) 10 nm.

Figure 2, panels A and B show the TEM images of A-773 and B-773 samples. These clearly B

show the formation of nearly monodispersed faceted crystalline nanoparticles with mean 9 ACS Paragon Plus Environment


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particle sizes of 25 (± 1) nm and 23 (± 1) nm, respectively, which are slightly larger than the crystallite sizes estimated by XRD. Panels C and D of Figure 2, illustrate the selected area electron diffraction (SAED) patterns acquired from the collection of nanoparticles. The diffused ring with spotty structure indicates that each crystallite exists as a monolith which diffracts coherently. It is clear that for B-773, all the rings were contributed by the characteristic crystal planes of a spinel structure whereas for A-773, some of the rings were indexed with the characteristic crystal planes of NiO, implying the presence of NiO phase, consistent with the XRD results. Panels E and F of Figure 2 show the HRTEM images for A773 and B-773 samples, respectively. The inter-planar distances of 0.244 nm and 0.286 nm corresponding to {311} and {220} planes, respectively, were observed for the samples A-773 and B-773. This clearly indicates the presence of spinel structure (see JCPDS file no. 200781) for annealed nickel cobaltite nanoparticles. Insets to panels E and F correspond to the lattice fringes of regions marked with square. A careful lattice fringe analysis of A-773 reveals presence of inter-planar distance of 0.208 nm. It is close to the lattice spacing of the {200} plane of NiO phase (see JCPDS file no. 897130) confirming the presence of segregated NiO phase in the annealed stoichiometric samples. In order to get the compositional information and the oxidation states of the metal ions near A

B

the surface region, x-ray photoelectron spectroscopic (XPS) studies were conducted on the C

D

stoichiometric, NiCo2O4 and off-stoichiometric, Ni0.75Co2.25O4 samples. Ni 2p spectra obtained from the samples were best fitted to two spin - orbit doublets, Ni 2p3/2 and Ni 2p1/2 and two strong satellite peaks. Co 2p spectra of the samples were also fitted in the similar manner with two spin - orbit doublets and two weak satellite peaks. The satellites were fitted considering only one broad peak. The corresponding spectral parameters are represented in the Tables 2 and 3, respectively, for A-473 and B-473 samples. Figure 3 represents the Ni 2p and Co 2p XPS spectra of the as-prepared A-473 and B-473 samples. As it is seen for both 10 ACS Paragon Plus Environment

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the samples, the shape and peak positions of the spectra are almost identical. Results from the fit of Ni 2p XPS spectra suggest that in both the samples, Ni2+ and Ni3+ species coexist and these results are consistent with the earlier report.24 The Ni 2p3/2 and Ni 2p1/2 binding energies values for the A-473 and B-473 samples are comparable with spin - orbit doublet separation of 17.4 eV and 17.5 eV, respectively. The lower binding energy values of 854.5 eV (spin orbit doublet separation of 17.76 eV) and 854.2 eV (spin - orbit doublet separation of 17.9 eV), respectively, for A-473 and B-473 are comparable to that of the octahedral Ni2+ species found in NiO. Therefore, part of nickel species on the surface of nickel cobaltite particles is found as Ni2+ in the octahedral sites.25, 26 Higher binding energy values of 855.7 eV (spin orbit doublet separation of 17.8 eV) and 855.7 (spin - orbit doublet separation of 17.9 eV), respectively, for A - 473 and B - 473 samples are attributed to the presence of Ni3+ species.13,17,27 Strong satellite peaks at 861.3 eV, 879.2 eV for the A - 473 samples, and 861.4 eV, 879.4 eV for the B-473 samples indicate the presence of both Ni2+ and Ni3+ species. It is, however, interesting to note that the spectral Ni2+/Ni3+ areas ratio for the B-473 samples is ~0.67 which is slightly greater than the value ~0.54 obtained for A-473 samples suggesting that the surface of the B-473 sample could be enriched with the coordinated octahedral NiO nanoparticles. Co 2p XPS data obtained from A-473 and B-473 samples were identical and they resemble spectra recorded for the Co3O4 sample.2,27 Results obtained from the fit of Co 2p spectral lines indicate the presence of both Co2+ and Co3+ species. The asymmetry in the Co 2p peak is the result of the overlapping of more intense Co3+ line with that of weaker Co2+ line. The binding energy values of the main peak Co 2p are 779.7 eV, 779.67 eV (FWHM = 2.5 eV, 2.4 eV) with doublet separation of 15.3 eV and 15.0 eV, respectively, for the A-473 and B-473 samples. Presence of broad and weak satellite structures at 788.2 eV (FWHM = 6 eV) and 803 eV (FWHM = 5 eV) for the NiCo2O4 sample indicate that there are only few Co2+ species in the octahedral sites and most of the low spin Co3+ species occupies the



octahedral sites.28 It is interesting to note that the nature of Co 2p peak including that of satellite structure remains same for Ni0.75Co2.25O4 phase (B - 473) indicating that the surface composition with respect to the cobalt species does not change by introducing excess Co to the spinel structure.

10.0 A-473

10.4

(c)

B-473

4

Counts(X10 )

4

Counts (X10 )

(a)

9.6 9.2 8.8 Ni-2p 890

880 870 860 850 Binding Energy (eV)

9.6 A-473

9.6

Ni-2p 880 870 860 850 Binding Energy (eV)

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(b)

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Co-2p

810

800 790 780 Binding Energy (eV)

770

9.6 8.8 8.0 Co-2p 810

800

790

780

770

Binding Energy (eV)

Figure 3. (a) Ni 2p and (b) Co 2p XPS data for the A-473 sample. (c) Ni 2p and (d) Co 2p XPS data for the B-473 sample.

In order to investigate the variation of surface composition on annealing, XPS studies were conducted on the A-773 and B-773 samples. Figure 4 represents Ni 2p 3/2 and Co 2p 3/2 XPS spectra of the A-773 and B-773 samples. In the case of A-773 and B-773 samples the Ni2+/Ni3+ relative area ratio are 2 and 1.38, which clearly reveal that the surface composition of NiCo2O4 and Ni0.75Co2.25O4 samples changes upon annealing. Though the surface layer of 12 ACS Paragon Plus Environment

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both the samples appears to be enriched with nickel species, majority of the nickel is clearly present as Ni2+ for the A-773 sample, which may be interpreted as a consequence of the formation of NiO phase. Results obtained from the fit of Co 2p spectral line match well with those reported earlier.13 However, it is interesting to note that Co3+/Co2+ area ratio is ~1.3 for both the A-773 and B-773 samples, indicating an identical surface composition with respect to cobalt species. Figure 5 represents the O1s XPS spectra for the as-prepared (A-473, B-473) and the annealed (A-773, B-773) samples. All the samples show the asymmetry of the O1s peak on its higher binding energy side. As-prepared samples (A-473 and B-473) show O1s binding energy of 529.5 eV, which is typical of metal-oxygen bonds.28 In most of the reports, higher binding energies at 531.4 eV have been generally assigned to the presence of surface hydroxylation 2 and oxide defect states with low oxygen coordination.17 Still higher binding energies at 532.6 and 533.0 eV, respectively, for the A-473 and B-473 are attributed to the presence of physio and chemisorbed water at and within the surface of the samples.28 As expected, the annealed samples (A-773 and B-773) do not show the presence of peaks around 533 eV, however, the presence of strong 531.2 eV peaks (36% in A-773 and 50% in B-773) clearly indicates that surface hydroxylations and oxide defects are predominantly present on the surface of the B773 sample.



11.2

(a)

A-773

B-773

10.8

10.8

10.4 Ni-2p

Ni-2p 890

880 870 860 850 Binding Energy (eV) (b)

4

9.6 9.0 Co-2p 8.4 810


10.0 B-773 4

10.2 A-773

890

Counts (X 10 )

10.4

(c)

4

Counts (X10 )

4

Counts(X10 )

11.2

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(d)

9.6 9.2 8.8 8.4 Co-2p 810


Figure 4. (a) Ni 2p and (b) Co 2p XPS data for annealed A-773 sample. (c) Ni 2p and (d) Co 2p XPS data for annealed B-773 sample.


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A-473

7

(a)

6

B-473

(c)

6

4

Counts (X10 )

4

Counts (X10 )

7

5 O-1s

4 3 535

530

525

5

3

520

O-1s

4 535

Binding energy (eV) (b)

A-773

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Counts (X10 )

2.1 1.8 O-1s 1.5 540


530 525 Binding energy (eV)

520

(d)

B-773

4

Counts (X10 )

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2.1 1.8

O-1s

1.5

520

540


520

Figure 5. O-1s XPS data for (a) A-473, (b) A-773, (c) B-473 and (d) B-773 nickel cobaltite samples.

Figure 6 illustrates the room temperature magnetic behaviour of the as-prepared (A-473, B473) and annealed (A-773, B-773) nickel cobaltite samples investigated using a quantum design physical properties measurement system (PPMS). As is evident from Figure 6, the M-

H characteristics for both A-473 and B-473 samples are typical of a superparamagnetic behaviour.29,30 For the annealed samples, although no saturation is achieved till the high magnetic field of 40 kOe, thin hysteresis loops with coercive force of 31.3 Oe and 75.2 Oe, respectively, are observed for A-773 and B-773 (see lower insets in Figure 6). It is possible that in the A-773 sample larger fraction of small particles and smaller fraction of big particles



are present. On the other hand, in the sample B-773 with slightly higher coercivity, the presence of larger fraction of bigger particles is expected.

1 0 1 2 3 4 5 4 1/H (X 10 kOe) 0.8

300 K -1

A-773

0.4

1

M (emu/g)

A-473

2 A-473

0

3

B-773

2

B-473 1 0 1 2 3 4 5 4 1/H (X 10 kOe)

A-473

-40

-0.8 -0.8 0.0 0.8 H (kOe)

-20

0 H (kOe)

B-773

0.4 0.0

B-473

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-0.4

-2

B-473 0.8

300 K -1

0.0

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M (emu/g)

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M (emu/g)

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M (emu/g)

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20

40

-2 -40

-0.8 -0.8 0.0 0.8 H (kOe)

-20

0 H (kOe)

20

40

Figure 6. Field - dependent magnetization behaviour of A-473, A-773, B-473 and B-773 nickel cobaltite samples. The upper insets: Corresponding extrapolation of M versus 1/H curves to the limit 1/H → 0. The lower insets: Corresponding magnetization data at an enhanced scale.

The room temperature saturation magnetization values are obtained by the extrapolation of M versus 1/H curves to the limit 1/H → 0 (see upper insets in Figure 6). These saturation magnetization values of ~ 3.2 emu/g and ~ 2.7 emu/g are obtained for A-473 and B-473 samples, respectively. Interestingly for annealed A-773 samples with much bigger particles and a high coercivity, similar saturation magnetization value of ~ 3.3 emu/g is observed. On the other hand for B-773 sample a small increase in saturation magnetization value to 3.6 emu/g is obtained. This is due to the presence of fine segregated NiO nanoparticles 16 ACS Paragon Plus Environment

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predominantly on the surface of A-773 particles.20 The result supports the high temperature combination reaction of segregated NiO and coordinated excess cobalt species at octahedral site to form an ordered NiCo2O4 magnetic structure.

0.0

TMax1

A-473

0 -10

10 M (emu/g)

10

50

5 0

5K

-4 -2 0 2 4 H (kOe)

5

10

5K -20

0 H (kOe)

5K

-4 -2 0 2 4 H (kOe)

20

B-773

A-773

0.0 0

A-473 B-473

-5

1.5 50 Oe 1.0

20

0

-10

2.0

B-473 10

-5

-10

-20 -40

(c)

A-473

ZFC FC

0.5

ZFC B-473 FC 100 150 200 250 300 T (K)

M (emu/g)

0

20

TMax2

50 100 T (K)

0.2 0.0

(b)

50 20

0

10

100 150 200 250 300 T (K)

B-773

0 5K

-10 -20

-10

-4 -2 0 2 4 H (kOe)

-40

20 10

B-773

0

-10 -20

-20

40

A-773

(d)

A-773

M (emu/g)

0.6 50 Oe 0.4

3.0 2.5

0.2

M (emu/g)

M (emu/g)

0.8

B-473

0.4

M (emu/g)

M (emu/g)

(a)

M (emu/g)

1.0

M (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5K -20

5K

-4 -2 0 2 4 H (kOe)

0 20 H (kOe)

40

Figure 7. (a) and (b): ZFC and FC magnetization of A-473, B-473, A-773 and B-773 nickel cobaltite samples. Inset (a): Corresponding ZFC data at the enhanced scale. (c) and (d): Corresponding field dependent magnetization behaviour measured at 5K. Insets (c) and (d): Corresponding magnetization data at the enhanced scale.

For an assembly of non-interacting particles with random distribution of anisotropy axis, zero field cooled (ZFC) and field cooled (FC) dc magnetization measured in a small magnetic field show a distinct effect. Usually, superparamagnetic particles are characterized by the presence of a maximum (Tmax) in the ZFC magnetization curve.20, 17 ACS Paragon Plus Environment

29-31

The bifurcation


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between ZFC and FC magnetization curves is due to the existence and distribution of energy barriers of the magnetic anisotropy and the slow relaxation of the nanoparticles below Tmax. The bifurcation between the ZFC and FC magnetization starts at irreversible temperature (Tirr), which is related to the blocking of the largest particles.32 Tmax obtained from the ZFC maximum is directly proportional to the average blocking temperature (TB) that depends upon the type of size distribution. Therefore, the difference between Tmax and Tirr provides a qualitative measure of the width of blocking temperature distribution which can further be correlated to the particle size distribution.33 Figures 7 (a) and (b) compares the ZFC and FC magnetization of the stoichiometric (A-473, A-773) and off-stoichiometric (B-473, B-773) samples. As seen from Figure 7(a), the maximum in ZFC magnetization for A-473 occurs at ~132 K. However, for B-473 the ZFC magnetization curve shows an interesting behaviour. The magnetization initially increases rapidly till the temperature increased to Tmax1 ~ 27 K and then increases slowly to reach to broad maxima at Tmax2 ~ 77 K implying the sample may be comprised of mixed magnetic phases of different anisotropy energy barriers (see Figure 7 inset (a)). The nature of initial ZFC and FC magnetization curves indicates the presence of inter-particle interactions. 31 The orientation disorder of the dipole may lead to frustration. It is possible that the excess octahedral cobalt ions push the Ni ions to the boundary which remains coordinated to the surface leading to frustrated dipolar system. It is also evident from the XPS data analysis of B-473 (see Figure 3(b)), which indicated that the surface is enriched with coordinated NiO phase. This in turn results in the core-shell type of structure with a nickel deficientNi1-xCo2+xO4 core and a magnetically dead shell originating from uncompensated antiferromagnetically coupled nickel oxide phase coordinated to the surface of each particles. Therefore, with increase in the thermal agitation, the frustration in the dipolar system tends to further increase thereby decreasing the magnetization. However, this behaviour is largely in 18 ACS Paragon Plus Environment

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competition with the blocking behaviour induced by the core of the particles which results in a overall slow increase of the magnetization till Tmax2. Above Tmax2, the particles start relaxing faster and hence the magnetization further decreases. Similar magnetization behaviour was also observed for the sample Ni0.88Co2.12O4 indicating that the incorporation of excess Co ions results in the formation of a mixed magnetic phase (see supporting information Figures S3 and S4). Figure 7 (b) represents the ZFC and FC curves for the annealed (A-773, B-773) samples. In general, the Tmax is expected to increase with increasing particle sizes. However, for A-773, having bigger particle size (~19 nm) and room temperature coercive field of 31.3 Oe, instead of showing larger TB value, it showed a lower value of ~ 38 K. The cause for this has been explained earlier on the basis of the precipitation of antiferromagnetic NiO phase and the simultaneous formation of cobalt rich off-stoichiometric NiCo2O4 nanoparticles.20 Excess cobalt lattice location disorders lead to lattice distortion and thereby a substantial loss in magnetocrystalline anisotropy results in the lowering of Tmax. It is however interesting to note that for B-773 the magnetization curves show results opposite to that of A-773 and an expected increase in Tmax to ~ 129 K. In spite of having excess cobalt ions in B-773, enhanced Tmax value clearly indicates the increase in magneto-crystalline anisotropy energy suggesting that the magnetic spin structure of this sample is different from that of A-773. From the previous investigation, 34 it is known that the observation of λ-shaped magnetization curves is a signature of the magnetically inhomogeneous system. In addition, recent research35 has proposed that the magnetic inhomogeneity may be attributed to the presence of either spin glass or cluster glass state. One of the most important features of the canonical spin glass system is the presence of almost constant FC magnetization values below TB.

35

It

is noteworthy that in the present investigation, the ZFC and FC magnetization curves of the annealed off-stoichiometric sample (B-773) show similar λ-shape magnetization curve.



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However, the gradual increase of FC magnetization curve below TB indicates a frustrated dipolar state induced by the developing NiO-type coordination on the surface and may be classified as a cluster glass state at low temperature. 35 Figures 7 (c) and (d) represent the field dependent magnetization behavior at 5 K. The expected behavior of the opening of the hysteresis loop is observed for A-473 and B-773 samples. It is also interesting to note that the nature of the field dependent magnetization curves for B-473 and A-773 are similar. An asymmetric hysteresis loop with a kink at low field clearly indicates the presence of mixed magnetic phases in B-473 and A-773 samples. Contrary to this, annealed off-stoichiometric sample (B-773) displays high coercivity value of 1130 Oe and is slightly higher than the value 1116 Oe for the as-prepared stoichiometric NiCo2O4 sample indicating that the magnetic structures of A-473 and B-773 is very close. It is interesting to note that the off-stoichiometric B-773 sample shows a significant enhancement in coercivity value to 1130 Oe at 5K as compared to the value 75.2 Oe, at 300 K. Similar enhancement in the coercivity and their monotonic temperature dependence was recently reported for the manganite bilayers composed of ferromagnetic (FM) and antiferromagnetic (AFM) layers, where there is a clear evidence of exchange coupling at the FM/AFM interface.36-39 This type of coupling induces frustration of FM ordering in AFM layer resulting in the spin glass state and a significant coercivity enhancement. In the present study, it is possible that the segregated NiO and coordinated cobalt species combine at high temperature to form a stoichiometric NiCo2O4 phase, thereby further resulting in a bilayer heterostructure comprised of ferrimagnetic NiCo2O4 coupled to the coordinated antiferromagnetic NiO layers. Though, an enhanced coercivity value and a systematic increase in the blocking temperature for the annealed off-stoichiometric sample clearly indicate the presence of stronger exchange interaction, much further work is needed to elucidate the possibility of interfacial exchange coupling in this nanosystem.


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In order to further compare the magnetic structure and to probe the local moments on the lattice sites of the stoichiometric and the off-stoichiometric samples, neutron diffraction measurements were carried out at different temperatures. Figure 8 illustrates the typical neutron diffraction profiles for the as-prepared stoichiometric (A-473) and the offstoichiometric (B-473) samples recorded at 300 K and 5 K. We carried out the Rietveld

300 K

3

2.8 2.1 1.4 0.7 20 3.5

40 60 80 Scattering angle (deg)

100

A-473 5K

2.8 2.1 1.4 0.7 20

40 60 80 scattering angle (deg)

Neutron Counts X10 (arb. units)

A-473

3

3

3.5


3


refinement of all the powder diffraction patterns using the FullProf program.40


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

B-473

2.0

300 K 1.5 1.0 0.5 20


2.0

100

B-473 5K

1.5 1.0 0.5 20


100

Figure 8. Fitted neutron diffraction profiles showing observed (circle), calculated (solid line) and difference (solid line at bottom) data for as-prepared nickel cobaltite nanoparticles at 300 K and 5 K.

A consolidated value depicting the refinement results is given in Table 4. The Table also enlists the magnetic moment obtained from the Rietveld refinement. The cation distribution



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obtained from neutron diffraction measurements clearly indicates that the tetrahedral sites are predominantly occupied by Co. A small amount of mixing of Co/Ni ions (9% in A-473 and 8% in B-473) is observed and this result is similar to that reported earlier.10 However, for the B-473 sample, the excess Co ion occupies octahedral sites of the spinel structure replacing Ni ions. As seen from the XRD pattern (Figure 1b) and from the derived lattice parameter value of 8.149(1) Ǻ, B-473 forms a spinel structure. From the refinement of neutron diffraction data for A-473, the ordered magnetic moments on the tetrahedral and octahedral sites are found to be 1.5(2) µB and –0.6(1) µB respectively. The overall moment calculated from the neutron diffraction data at 300 K is 0.3(2) µB/f.u. which is higher than the dc magnetization data value of ~ 0.14 µB/f.u. at 300 K. On the other hand for B-473, the overall moment calculated from neutron diffraction data at 300 K is 0.4(3) µB/f.u. that is again much higher than the value ~ 0.12 µB/f.u. observed from dc magnetization data at 300K. The net moments derived from neutron diffraction at 5 K are 0.5(2) and 0.4(3) µB/f.u. for A-473 and B-473 samples, respectively, which is found to be somewhat smaller than the values 0.7 µB/f.u. and 0.78 µB/f.u., respectively, derived from the dc magnetization data. In order to understand how the magnetic structure changes on annealing these samples, we carried out neutron diffraction measurements on the annealed A-773 and B-773 samples. Figure 9 shows the typical neutron diffraction profiles for annealed samples (A-773, B-773) recorded at 300 K and 5 K. The refined data measured at 300 K are represented in the Table 5. All the data represented here are refined with 4 phases namely two ferrite phases (nuclear and magnetic) and two NiO phases (nuclear and magnetic). The detailed analysis for the annealed samples A-773 reveals 84(1) % of spinel phase with 16(1) % precipitation of NiO phase. The mean ordered magnetic moment at tetrahedral and octahedral sites at 300 K are 1.1(3) µB and -0.5(1) µB, respectively. Net magnetic moment derived from neutron diffraction 22 ACS Paragon Plus Environment

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at 300 K is 0.1(2) µB/f.u. which is close to the value 0.14 µB/f.u. obtained from the dc magnetization data. As expected, the ordered moment at the tetrahedral and octahedral sites increases to 2.2(1) µB and -0.6(1) µB respectively, when measured at 5 K. The net moment obtained from neutron diffraction at 5 K is 1.0(2) µB/f.u., which is very close the magnetization value 0.90 µB/f.u obtained from dc magnetization studies. Using the neutron diffraction analysis on the annealed stoichiometric sample (A-773) at 300 K, the cation

3

3

A-773 300 K

1 0 20


100

A-773 2

3

3

5K

Neutron counts X10 (arb. units)

3

3

2



distribution was approximated to be (Ni0.11Co0.89)[Ni0.52Co1.52] O4.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0 20


100

4 B-773 3

300 K

2 1 0 -1

20


100

4 B-773 3

5K

2 1 0 -1

20


100

Figure 9. Fitted neutron diffraction profiles showing observed (circle), calculated (solid line) and difference (solid line at bottom) data for annealed NiCo2O4 and Ni0.75Co2.25O4 samples at 300 and 5 K.



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It is interesting to note that for B-773 there is 94% (0.002) occupancy of the tetrahedral sites by cobalt showing Néel ferrimagnetism with relatively much higher ordered moment of 0.6(4) µB and -0.4(1) µB respectively for tetrahedral and octahedral sites at 300 K. Net moment derived from neutron diffraction at 300 K is 0.2(2) µB/f.u. which is close to the value 0.16 µB/f.u. observed in dc magnetization data at 300 K. As expected, the net moment obtained from neutron diffraction increases to 0.4(2) µB/f.u at 5K. The result obtained matches with the value of 0.7 µB/f.u. obtained from dc magnetization data at 5 K. The cation distribution for the off-stoichiometric annealed sample (B-773) is approximated to be (Ni0.06Co0.94)[Ni0.53Co1.47] O4 which shows the presence of 93 (1) % of spinel phase and 7 (1) % of NiO phase.

CONCLUSIONS In the present investigation we report a detailed examination of the effect of induced offstoichiometry on structural, thermal and magnetic properties of nickel cobaltite, NiCo2O4 nanoparticles. It is seen that the excess cobalt ions stabilize the nickel cobaltite structure even up to the temperature of 773 K and has interesting consequences on the magnetic structure and properties. Enhanced thermal stability, improved structural and magnetic properties of the off-stoichiometric sample is evident from the magnetic and neutron diffraction studies. Off-stoichiometry in nanosystems may thus offer a novel route to new materials with interesting properties.

Acknowledgements S.V. gratefully acknowledges IISER Pune and DST nanoscience unit of IISER, SRNM/NS42/2009 for research support.


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REFERENCES (1) Haenen, J.; Visscher, W.; Barendrecht, E. Characterization of NiCo2O4 Electrodes for O2 Evolution: Parts I - III. Non-Electroochemical Characterization of NiCo2O4 Electrodes. J.

Electroanal. Chem. 1986, 208, 273-341. (2) Roginskaya, Y. E.; Morozova, O. V.; Lubnin, E. N.; Ulitina, Y. E.; Lopukhova, G. V.; Trasatti, S. Characterization of Bulk and Surface Composition of CoxNi1-xOy Mixed Oxides for Electrocatalysis. Langmuir 1997, 13, 4621-4627. (3) Windisch, C. F., Jr.; Exarhos, G. J.; Ferris, K. F.; Engelhard, M. H.; Stewart, D. C. Infrared Transparent Spinel Films with p-type Conductivity. Thin Solid Films 2001, 398-399, 45-52. (4) Windisch, Jr. C. F.; Exarhos, G. J.; Sharma, S. K. Influence of Temperature and Electronic Disorder on the Raman Spectra of Nickel Cobalt Oxides. J. Appl. Phys. 2002,

92(9), 5572-5574. (5) Windisch, Jr. C. F.; Ferris, K. F.; Exarhos; G. J. Synthesis and Characterization of Transparent Conducting Oxide Cobalt–Nickel Spinel Films. J. Vac. Sci. Technol. A 2001, 19

(4), 1647-1651. (6) Cui, B.; Lin, H.; Li, J-B.; Li, X.; Yang, J.; Tao, J. Core–Ring Structured NiCo2O4 Nanoplatelets: Synthesis, Characterization, and Electrocatalytic Applications. Adv. Funct.

Mater. 2008, 18, 1440-1447. (7) Wang, H.; Gao, Q.; Jiang, L. Facile Approach to Prepare Nickel Cobaltite Nanowire Materials for Supercapacitors. Small 2011, 7(17), 2454-2459



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8) Xiao, J.; Yang, S. Sequential Crystallization of Sea Urchin-Like Bimetallic (Ni, Co) Carbonate Hydroxide and its Morphology Conserved Conversion to Porous NiCo2O4 Spinel for Pseudocapacitors. RSC Adv. 2011, 1, 588-595. (9) Yuan, C.; Li, J.; Hou, L.; Zhang, X.; Shen L.; Lou, X. W. (David). Ultrathin Mesoporous NiCo2O4 Nanosheets Supported on Ni Foam as Advanced Electrodes for Supercapacitors.

Adv. Funct. Mater. 2012, 22, 4592-4597. (10) Yuan, C.; Li, J.; Hou, L.; Yang, L.; S. Laifa, Zhang, X. Facile Template-Free Synthesis of Ultralayered Mesoporous Nickel Cobaltite Nanowires towards High-Performance Electrochemical Capacitors. J. Mater. Chem. 2012, 22, 16084- 16090. (11) Knop, O.; Reid, K. I. G.; Sutarno,; Nakagawa, Y. Chalkogenides of the Transition Elements. VI. ' X-Ray, Neutron, and Magnetic Investigation of the Spinels Co3O4, NiCo2O4, Co3S4, and NiCo2S4. Can. J. Chem. 1968, 46, 3463- 3476. (12) F. K. Lotgering, Ferrimagnetism of Sulfides and Oxides. III. Oxygen and Sulfur Spinels Containing Cobalt (MCo2O4 and MCo2S4). Philips. Res. Rep. 1956, 11, 337-350 (13) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rios, E. I.; Palmer, H. M.; Greaves, C.; Berry, F. J. Cation Distribution and Magnetic Structure of the Ferrimagnetic Spinel NiCo2O4. J. Mater. Chem. 2001, 11, 3087-3093. (14) Battle, P. D.; Cheetham, A. K.; Goodenough, J. B. A Neutron Diffraction Study of the Ferromagnetic Spinel NiCo2O4. Mater. Res. Bull. 1979, 14(8), 1013-1024. (15) Lenglet M.; Guillamet, R.; Durr, J.; Gryffroy, D.; Vandenberghe, R. E. Electronic Structure of NiCo2O4 by XANES, EXAFS and 61Ni Mössbauer Studies. Solid State Commun.

1990, 74(10), 1035-1039.


Page 26 of 33

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Windisch, Jr. C.F.; Exarhos, G. J.; Owings, R. R. Vibrational Spectroscopic Study of the Site Occupancy Distribution of Cations in Nickel Cobalt Oxides. J. Appl. Phys.2004, 95(10), 5435-5442. (17) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rios, E.; Berry, F. J. Characterization of Nickel Cobaltite, NiCo2O4, Prepared by Several Methods: An XRD, XANES, EXAFS and XPS Study. J. Solid State Chem. 2000, 153, 74-81. (18) Cabo, M.; Pellicer, E.; Rossinyol, E.; Castell, O.; Surinach, S.; Baro, M. D. Mesoporous NiCo2O4 Spinel: Influence of Calcination Temperature over Phase Purity and Thermal Stability. Crystal Growth & Design 2009, 9, 4814-4821. (19) Lapham, D. P.; Tseung, A. C. C. The Effect of Firing Temeperature, Preparation Technique and Composition on the Electrical Properties of the Nickel Cobalt Oxide Series NixCo1-xOy. J. Mater. Sci. 2004, 39(1), 251-264. (20) Verma, S.; Joshi, H.M.; Jagadale, T.; Chawla, A.; Chandra, R.; Ogale, S. Nearly Monodispersed Multifunctional NiCo2O4 Spinel Nanoparticles: Magnetism, Infrared Transparency, and Radiofrequency Absorption. J. Phys. Chem. C 2008, 112, 15106-15112. (21) Windisch. Jr., C. F.; Ferris , K. F.; Exarhos, G. J.; Sharma, S. K. Conducting Spinel Oxide Films with Infrared Transparency. Thin Solid Films 2002, 420-421, 89-99. (22) Nydegger, N. W.; Couderc, G.; Langell, M. A. Surface Composition of Cox Ni1-x O Solid Solutions by X-Ray Photoelectron and Auger Spectroscopies. Appl. Surf. Sci. 1999,

147, 58-66. (23) Morais, P. C.; Neto, K. S. Spectroscopic Characterization of Superparamagnetic Particles of Thermolytic Products of Ferric Sulphate Hydrates. Polyhedron 1983, 2(9), 875880.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(24) King, W. J.; Tseung, A. C. C. The Reduction of Oxygen on Nickel-Cobalt Oxides-II: Correlation between Crystal Structure and Activity of Co2NiO4 and Related Oxides.

Electrochim Acta 1974, 19(8), 493-498. (25) Van Veenendaal, M. A.; Sawatzky, G. A. Nonlocal Screening Effects in 2p X-Ray Photoemission Spectroscopy Core-Level Line Shapes of Transition Metal Compounds. Phys.

Rev. Lett. 1993, 70(16), 2459-2462. (26) Thissen, A.; Ensling, D.; Madrigal, F. J. F.; Jaegermann, W.; Alcantara, R.; Lavela, P.; Tirado, J. L. Photoelectron Spectroscopic Study of the Reaction of Li and Na with NiCo2O4.

Chem. Mater. 2005, 17, 5202-5208. (27) Chung, T. J.; Brundle, C. R.; Rice, D. W. Interpretation of the X-Ray Photoemission Spectra of Cobalt Oxides and Cobalt Oxide Surfaces. Surf. Sci. 1976, 59(2), 413-429. (28) Kim, J. –G.; Pugmire, D. L.; Battaglia, D.; Langell, M. A. Analysis of the NiCo2O4 Spinel Surface with Auger and X-Ray Photoelectron Spectroscopy. Appl. Surf. Sci. 2000,

165, 70-84. (29) Hansen, M. F.; Morup, S. Estimation of Blocking Temperatures from ZFC/FC Curves.

J. Magn. Magn. Mater. 1999, 203, 214-216. (30) Yusuf, S. M.; Sahana, M.; Dorr, K.; Roßler, U. K.; Mueller, K.-H. Effect of Ga Doping for Mn on the Magnetic Properties of La0.67Ca0.33MnO3. Phys. Rev. B 2002, 66, 064414. (31) Mukadam, M. D.; Yusuf, S. M.; Sharma, P.; Kulshreshtha, S. K.; Dey, G. K. Dynamics of Spin Clusters in Amorphous Fe2O3. Phys. Rev. B 2005, 72, 174408. (32) Cannas, C.; Musinu, A.; Piccaluga, G.; Fiorani, D.; Peddis, D.; Rasmussen, H. K.; Morup, S. Magnetic Properties of Cobalt Ferrite–Silica Nanocomposites Prepared by a SolGel Autocombustion Technique. J. Chem. Phys. 2006, 125, 164714.


Page 28 of 33

Page 29 of 33

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(33) Gittleman, J. L.; Abeles, B.; Bozowski, S. Superparamagnetism and Relaxation Effects in Granular Ni-SiO, and Ni-Al Films. Phys. Rev. B 1974, 9(9), 3891- 3897. (34) Li, X. G.; Fan, X. J.; Ji, G.; Wu, W. B.; Wong, K. H.; Choy, C. L.; Ku, H. C. Field – Induced Crossover from Cluster-Glass to Ferromagnetic State in La0.7Sr0.3Mn0.7Co0.3O3. J.

Appl. Phys. 1999, 85(3), 1663-1666. (35) Lourembam, J.; Ding, J.; Bera, A.; Lin, W.; Wu, T. Asymmetric Electroresistance of Cluster Glass State in Manganites. Appl. Phys. Lett. 2014,104, 133508. (36) Tian, Y. F.; Ding, J. F.; Lin, W. N.; Chen, Z. H.; David, A.; He, M.; Hu, W. J.; Chen, L.; Wu, T. Anomalous Exchange Bias at Collinear/Noncollinear Spin Interface. Sci. Rep.2013, 3, 1094. (37) Ding, J. F.; Tian, Y. F.; Hu, W. J.; Lin, W. N. Wu, T. Exchange Coupling and Coercivity Enhancement in Cuprate/Manganite Bilayers. Appl. Phys. Lett. 2013, 102, 032401. (38) Ding, J. F.; Lebedev, O. I.; Turner, S.; Tian, Y. F.; Hu, W. J.; Seo, J. W.; Panagopoulos, C.; Prellier, W.; Tendeloo, G. V.; Wu, T. Interfacial Spin Glass State and Exchange Bias in Manganite Bilayers with Competing Magnetic Orders. Phys. Rev. B 2013, 87, 054428. (39) Tian, Y. F.; Lebedev, O. I.; Roddatis, V. V.; Lin, W. N.; Ding, J. F.; Hu, S. J.; Yan, S. S.; Wu, T. Interfacial Magnetic Coupling in Ultrathin All-Manganite La0.7Sr0.3MnO3TbMnO3 Superlattices. Appl. Phys. Lett. 2014, 104, 152404. (40) Rodriguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Physica B 1993, 192, 55-69.



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Table 1. Sample Code, Annealed Temperature and XRD Crystallite Sizes of Nickel Cobaltite Samples Off-Stoichiometric nickel cobaltite Stoichiometric nickel cobaltite (A) NiCo2O4 (B)Ni0.75Co2.25O4 Sample Code A-473 A-573 A-773

Annealed Temperature (K) 473 573 773

XRD crystallite size (±1 nm) 7 8 19

Sample Code B-473 B-573 B-773

Annealed Temperature (K) 473 573 773

XRD crystallite size (±1 nm) 6 7 19

Table 2: Results of the Fit of the Ni2p X-ray Photoelectron Spectra for As-Prepared NiCo2O4 (A-473) and Ni0.75Co2.25O4 (B-473) Samples sample Parameter Spin – Orbit Doublet I Spin – Orbit Doublet II S I S II Ni 2p3/2 Ni 2p1/2 Ni 2p3/2 Ni 2p1/2 A-473

B-473

Eg (eV) FWHM (eV)

854.5 3

871.9 3

Area rel (%)

35 %

Assignment

Ni2+

Eg (eV) FWHM (eV)

854.2 3

855.7 3

40 %

Assignment

Ni2+

861.3 5.8

879.4 6.8

861.4 6.2

879.4 6.1

65 % Ni3+

871.7 3

Area rel (%)

873.5 3

855.7 3

873.6 3 60 % Ni3+


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Table 3: Results of the Fit of the Co 2p X-ray Photoelectron Spectra for As-Prepared NiCo2O4 (A-473) and Ni0.75Co2.25O4 (B-473) Samples sample Parameter Spin – Orbit Doublet I Spin – Orbit Doublet II S I S II Co 2p3/2 Co 2p1/2 Co 2p3/2 Co 2p1/2 779.7 795.0 781.5 797.0 788.2 803 A-473 Eg (eV) FWHM (eV) 2.5 2.5 2.4 2.4 6 5 Area rel (%) Assignment

B-473

Eg (eV) FWHM (eV)

60 % Co3+

779.7 2.4

40 % Co2+

794.7 2.4

Area rel (%) Assignment

781.5 2.5

62 % Co3+

796.5 2.5

787.1 6

38 % Co2+

Table 4: Refined Neutron Diffraction Parameters for A-473 and B-473 Samples Measured at 300 K A-473 (Space group = F d -3 m), a = 8.157(1) Ǻ Atoms Co/Ni (T)* Co/Ni (O)* O

x 0. 125 0.5 0.261(1)

y 0. 125 0.5 0.261(1)

z

Occupancy

0. 125 0.91(1)/0.09(1) 0.5 1.09(1)/0.91(1) 0.262(1) 4

µ/ µB 1.5 (2) -0.6(1)

B-473 a = 8.149(1) Ǻ Atoms

x

y

Co/Ni (T)* 0. 125 0. 125 Co/Ni (O)* 0.5 0.5 O 0.259(1) 0.259(1) *T = tetrahedral, O = octahedral

z

Occupancy

µ/ µB

0. 125 0.5 0.259(1)

0.92(1)/0.08(1) 1.33(1)/0.67(1) 4

1.4 (2) -0.5(1)


804 5


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Page 32 of 33

Table 5: Refined Neutron Diffraction Parameters for A-773 and B-773 Samples Measured at 300 K A-773 a (NiCo2O4) = 8.1171(6) Ǻ, a (NiO) = 4.1722(9) Ǻ, Phase fraction: NiCo2O4 = 84(1)% and NiO = 16(1)% Bragg R factor (AB2O4 phase) = 8.5, Bragg R factor (NiO phase) = 6.4, χ2 = 2.27 Magnetic R factor (AB2O4 phase) = 13.6 Atoms x y z Occupancy µ/ µB Co/Ni (T)* Co/Ni (O)* O

0. 125 0.5 0.2612(3)

0. 125 0.5 0.2612(3)

0. 125 0.89(2)/0.11(2) 0.5 1.42(3)/0.58(3) 0.2612(3) 4

1.1 (3) -0.5(1)

B-773 a (Ni0.75Co2.25O4) = 8.1098(5) Ǻ, a (NiO) = 4.171(1) Ǻ, Phase fraction: Ni0.75Co2.25O4 = 93(1)% and NiO = 7(1)% Bragg R factor (AB2O4 phase )= 11.8, Bragg R factor (NiO phase) = 12.5, χ2 = 3.72 Magnetic R factor (AB2O4 phase) = 10.9 Atoms x y z Occupancy µ/ µB Co/Ni (T)* 0. 125 0. 125 Co/Ni (O)* 0.5 0.5 O 0.2622(2) 0.2622(2) *T = tetrahedral, O = octahedral

0. 125 0.94(2)/0.06(2) 0.5 1.47(3)/0.53(3) 0.2622(2) 4


0.6 (4) -0.4(1)

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For Table of Contents (TOC) Only

Ni0.75Co2.25O4 M (arb.units)

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NiCo2O4 as-prep annealed 0

100

200 T (K)

300


Off-Stoichiometric Nickel Cobaltite Nanoparticles: Thermal Stability

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