Electrical and Magnetic Properties Modification in Heavy Ion Irradiated

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Article

Electrical and Magnetic Properties Modification in Heavy Ion Irradiated Nanograin NiCo O Films x

(3-x)

4

John Stuart McCloy, Weilin Jiang, Wendy Bennett, Mark H. Engelhard, Jeffrey Lindemuth, Narendra Singh Parmar, and Gregory J. Exarhos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06406 • Publication Date (Web): 10 Sep 2015 Downloaded from http://pubs.acs.org on September 15, 2015

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The Journal of Physical Chemistry

Electrical and Magnetic Properties Modification in Heavy Ion Irradiated Nanograin NixCo(3-x)O4 Films John S. McCloy,1* Weilin Jiang,2 Wendy Bennett,2 Mark Engelhard,3 Jeffrey Lindemuth,4 Narendra Parmar,1 Gregory J. Exarhos2

1

School of Mechanical and Materials Engineering and Materials Science & Engineering Program, Washington State University, Pullman, WA 99164, USA

2

Pacific Northwest National Laboratory, Richland, Washington 99352, USA

3

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA

4

Lake Shore Cryotronics, Inc., 575 McCorkle Blvd., Westerville, Ohio 43081, USA

ACS Paragon Plus Environment


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ABSTRACT Reactively sputtered NixCo(3-x)O4 films (x = 1.5, 1.0, and 0.75) were grown and subsequently irradiated with 5.5 MeV Si+ ions to investigate effects of lattice-site and charge state distribution.

Films were characterized before and after

irradiation by x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectroscopy, electric resistivity measurements, and temperaturedependent AC and DC magnetometry. Results indicate that ion irradiation induces oxygen loss, partial reduction of nickel, and an increase in both low temperature ferrimagnetism and room temperature conductivity. Frequency dependent AC magnetic susceptibility measurements indicate a spin-glass like transition at low temperature which moves to higher temperature after irradiation. Significance of the charge transfer for magnetism and conduction in a mixed spinel with Co2+, Co3+, Ni2+, and Ni3+ in tetrahedral and octahedral sites is discussed.

Keywords Spinel, multivalent transition metal, cobalt, nickel, polaron hopping, catalysis


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1. INTRODUCTION Nickel cobalt oxides are candidate materials for supercapacitors,1-4 electrocatalysis,5;

6

and

infrared transparent electrodes.7 For these applications, understanding and tailoring of cation site occupancy and valence is required.7-9 NiCo2O4 is a ferromagnetic spinel with reported Curie temperature ~450 K,10;

11

though this value may highly depend on the cation valence and

occupancy as recently shown.12 Recent work has shown that single crystal epitaxial films of NiCo2O4 grown on insulating substrates change their site distribution and magnetic character depending on growth temperature.13 Epitaxial films grown by pulsed laser deposition at high temperatures have the inverse spinel structure and are insulating and non-magnetic at room temperature.13 Raman measurements suggest that growth at temperatures 0.9999. 3.4. Magnetic properties Magnetic properties showed dramatic changes with irradiation (see Table III). The peak in real (χ') and imaginary (χ'') contributions to the AC susceptibility are frequency dependent and shift slightly with field cooling. Irradiation results in an increase in low temperature coercivity (Hc) in all cases. Hysteresis curves at 10 K are shown in Figure 6. Substrate effects have been removed.

Most results showed a ferromagnetic component superimposed on a strong

diamagnetic component from the Si substrate, with the exception of x = 1.5 (irrad) and x = 0.75 (irrad) which showed a small residual paramagnetic background on the ferromagnetic component. Temperature dependent AC susceptibility measurements show that ion irradiation shifts the χ' peak to higher temperatures and increases the value by more than an order of magnitude (Figure 6). A corresponding peak in the χ'' imaginary component exists at similar temperatures to the χ' peak, but the peak of the DC ZFCW curve (Tmax,ZFC, see Table III) is 20–60 K lower in temperature, where the ZFCW-FCC branches split (Figure 7). All χ' peaks exhibit a frequency dependence, with the higher frequencies shifted to higher temperatures and lower intensities, thus showing relaxation phenomena similar to spin or cluster glasses.39; 40 4. DISCUSSION 4.1. Mixed spinel structures For stoichiometric NiCo2O4 high spin (HS) Co2+(d7) is normally dominant in tetrahedral sites,25 with some HS Ni3+(d7) present.22 Charge transfer may occur within the octahedral sites, changing from low spin (LS) Co3+ (d6) and LS Ni3+ (d7) to HS Co2+ (d7) and LS Ni4+ (d6).22 End-



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member Co3O4 is p-type semiconductor with a normal spinel structure, AB2O4, represented as {Co2+}[Co3+]2O4, where {tetrahedral} and [octahedral] sites are indicated. As Ni is added to Co3O4, Ni2+ displaces Co3+ due to its larger octahedral site preference energy,41 and Co3+ goes to the tetrahedral sites.7 When Ni>Co (i.e., more Ni-rich than Ni1.5Co1.5O4) the system normally phase separates into NiO plus an inverse spinel {B}[AB]O4, observed at lower Ni concentrations in solution-derived than sputtered films.18 There is some potential indication of NiO in the unirradiated samples, based on the XRD patterns of the higher Ni (x = 1 and x = 1.5) samples, and XPS (see section 4.3) cannot easily distinguish since Ni2+ in NiO and likely in NiCo2O4 is HS Ni2+ in octahedral sites. The pure Ni3O4 spinel end member has also been reported, usually as a surface state of NiO.42-45 As previously stated, single crystal films grown at high temperatures are insulating and nonmagnetic at RT and possess the inverse spinel structure, suggesting {Co3+HS}[Ni2+HS,Co3+LS]O4, while higher temperature growths result in conduction and ferrimagnetism due to a mixed spinel.13 Magnetization versus temperature measurements on insulating materials suggest they may still be ferromagnetic, though having quite low Néel temperature 300 K and shows a frequency dependence suggesting a spin glass-like behavior due to cation disorder, and possibly structural disorder (normal and inverse spinel) as well. It is not clear without additional data from other techniques, such as depth profiled Auger emission spectroscopy, that surface composition and oxidation state are the same as in the bulk, or that multiple clusters with different compositions might exist, particularly for Ni-Co-O.15; 22 Regardless, it appears that ion irradiation is indeed a way of enhancing desirable electrical and magnetic properties in Ni-Co-O films. It remains to be seen whether this type of treatment could enhance other properties, such as catalytic activity, as well.

ACKNOWLEDGEMENTS This work was supported in part by the Laboratory Directed Research and Development (LDRD) program at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the U.S. Department of Energy (DOE) by Battelle under Contract DE-AC05-76RL01830. Some of the research was performed using facilities within the Environmental Molecular Sciences Laboratory (EMSL) at PNNL, sponsored by the DOE's Office of Biological and Environmental Research. The authors thank Saehwa Chong, Chuck Henager, Alex Rettie, and Scott Chambers for helpful comments.


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TABLES Table I. XRD (a) and RBS (O/(Ni+Co) molar ratio) data for unirradiated (U) and irradiated (I) samples. XPS data shown is prior to Ar+ sputtering. RBS-1 assumes Ni/Co as the target, and RBS-2 assumes Ni/Co as determined at the surface from XPS. a (Å) NixCo(3-x)O4 Ni1.5Co1.5O4 NiCo2O4 Ni0.75Co2.25O4

x 1.50 1.00 0.75

U 8.376 8.313 8.149

I 8.178 8.154 8.173

Ideal spinel 1.333 1.333 1.333

O/(Ni+Co) RBS-1 RBS-2 U U 1.700 1.567 1.900 1.750 1.873 1.722

XPS U 1.099 1.185 1.157

XPS I 1.311 0.971 0.956

RBS-1 I 1.633 1.883 1.840

RBS-2 I 1.500 1.667 1.699

Table II. Comparison of RT acquired electrical data from E (Ecopia, DC), M (MMR, DC), and L (Lakeshore, AC) measurements, for U (unirradiated), and I (ion irradiated) samples.

ρ (x 10-3 Ω-cm) U

µ (cm2/Vs) I

U

n (x 1020 cm-3) I

U

x

E

M

L

E

M

L

E

L

E

L

1.50

2.25

2.24

2.17

1.45

1.42

1.42

0.14

0.77

49.8

33.9

1.00

6.05

5.27

5.97

7.73

7.99

6.93

0.24

0.06

0.10

0.082

0.75

11.2

12.41

11.5

3.94

3.99

3.89

0.03

0.0017

0.41

0.56

I

E

L

E

L

175 (p-type) 74 (p-type)

37.2 (n-type) 165 (n-type)

0.862 (p-type) 141 (p-type)

1.30 (n-type) 110 (n-type)

394 (p-type)

3090 (p-type)

510 (p-type)

28.7 (n-type)

Table III. Summary of magnetic properties of unirradiated (U) and irradiated (I) samples, including real (χ') and imaginary (χ'') components of AC susceptibility, peak in ZFC DC magnetization (Tmax,ZFC), irreversibility temperature between ZFC and FC (Tirr), and low temperature coercivity (Hc). The maxima in χ' and χ'' are indicated for the ZFCC experiment, 100 Hz. n/m indicates that these values were not discernable from the experimental data.

x 1.50 1.00 0.75

χ' max (K) U I 120 ≥300 30 90 50 170

χ'' max (K) U I None ≥300 None 90 50 160

Tmax,ZFC (K) U I 80 270 30 50 30 110

Tirr (K) U I 90-130 ~245 n/m ~55 n/m ~120

Hc (10 K) (kOe) U I 2.9 4.2 0.7 1.03 1.08 1.88


Ms (10 K) (emu/g, µB/f.u.) U I 8.2, 0.35 21.8, 0.94 3.7, 0.16 17.6, 0.76 3.2, 0.14 20.6, 0.89

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FIGURE CAPTIONS Figure 1. Normalized GIXRD data for all samples. Intensities have been normalized to each maximum and offset for clarity. Standard powder diffraction files (PDF) for Ni1.71Co1.29O4 (401191), NiCo2O4 (73-1702), Co3O4 (74-1656), and NiO (71-1179), are shown for comparison.

Figure 2. Typical cross-sectional microstructure obtained by helium ion microscopy (HIM), in this case of the x=1.0 ion irradiated sample.

Figure 3. Ni2p, Co2p, and O1s XPS spectra. Legend is shown in the O1s plot and is the same for all plots. Irradiated curves are not shown for O1s but are identical to the x = 0.75 unirradiated spectrum with no high BE component.

Figure 4. Example of RBS fit for x = 1.5, unirradiated: data (points), simulation assuming Ni/Co is 1.0 (solid line), simulation assuming Ni/Co per XPS (dashed line).

Figure 5. Fits for temperature dependent resistivity for Mott 3D Variable Range Hopping, for T ~80 – 200 K.

Figure 6. Magnetic properties of NixCo(3-x)O4. Hysteresis at 10 K (a)-(c) with all linear effects of the substrate removed; dotted lines are irradiated samples in each case. Real part of the AC magnetic susceptibility (d)-(f) taken in zero DC field while cooling (ZFCC). Pre- (solid) and post-irradiated (dashed) data are on different y-axes.



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Figure 7. Magnetic properties of NixCo(3-x)O4. ZFCW-FCC magnetization curves versus T. Pre(solid) and post-irradiated (dashed) data. FIGURES


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Figure 1. Normalized GIXRD data for all samples. Intensities have been normalized to each maximum and offset for clarity. Standard powder diffraction files (PDF) for Ni1.71Co1.29O4 (401191), NiCo2O4 (73-1702), Co3O4 (74-1656), and NiO (71-1179), are shown for comparison.



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Figure 2. Typical cross-sectional microstructure obtained by helium ion microscopy (HIM), in this case of the x=1.0 ion irradiated sample

Figure 3. Ni2p, Co2p, and O1s XPS spectra. Legend is shown in the O1s plot and is the same for all plots. Irradiated curves are not shown for O1s but are identical to the x = 0.75 unirradiated spectrum with no high BE component.


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Figure 4. Example of RBS fit for x = 1.5, unirradiated: data (points), simulation assuming Ni/Co is 1.0 (solid line), simulation assuming Ni/Co per XPS (dashed line).

Figure 5. Fits for temperature dependent resistivity for Mott 3D Variable Range Hopping, for T ~80 – 200 K.



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Figure 6. Magnetic properties of NixCo(3-x)O4. Hysteresis at 10 K (a)-(c) with all linear effects of the substrate removed; dotted lines are irradiated samples in each case. Real part of the AC magnetic susceptibility (d)-(f) taken in zero DC field while cooling (ZFCC). Pre- (solid) and post-irradiated (dashed) data are on different y-axes.


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Figure 7. Magnetic properties of NixCo(3-x)O4. ZFCW-FCC magnetization curves versus T. Pre(solid) and post-irradiated (dashed) data.



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TABLE OF CONTENTS IMAGE


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Normalized GIXRD data for all samples. Intensities have been normalized to each maximum and offset for clarity. Standard powder diffraction files (PDF) for Ni1.71Co1.29O4 (40-1191), NiCo2O4 (73-1702), Co3O4 (74-1656), and NiO (71-1179), are shown for comparison. 106x128mm (300 x 300 DPI)



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Typical cross-sectional microstructure obtained by helium ion microscopy (HIM), in this case of the x=1.0 ion irradiated sample. 270x296mm (96 x 96 DPI)


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Ni2p, Co2p, and O1s XPS spectra. Legend is shown in the O1s plot and is the same for all plots. Irradiated curves are not shown for O1s but are identical to the x = 0.75 unirradiated spectrum with no high BE component. 170x59mm (300 x 300 DPI)



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Example of RBS fit for x = 1.5, unirradiated: data (points), simulation assuming Ni/Co is 1.0 (solid line), simulation assuming Ni/Co per XPS (dashed line). 77x70mm (300 x 300 DPI)


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Fits for temperature dependent resistivity for Mott 3D Variable Range Hopping, for T ~80 – 200 K. 73x56mm (300 x 300 DPI)



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84x161mm (300 x 300 DPI)


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80x34mm (300 x 300 DPI)


Electrical and Magnetic Properties Modification in Heavy Ion Irradiated

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