Synthesis, structural characterization, and magnetic

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8.112. 12.3. 11.7. The FTIR spectra of MnxNi1-xCo2O4 (x=0, 0.5, 1) prepared by calcination at 450 °C are shown in Figure 2. Generally, spinel oxides show two ...
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ScienceDirect Materials Today: Proceedings 5 (2018) 10488–10495

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AEMC_2017

Synthesis, structural characterization, and magnetic properties of mixed ternary spinel-type Mn-Ni-Co oxides Tarekegn Heliso Dollaa, Karin Pruessnerb, Dave G. Billingc, Charles Sheppardd, Aletta Prinslood, Patrick Ndungua* a

Department of Applied Chemistry, Centre for Nanomaterials Science Research, Energy, Sensors and Multifunctional Nanomaterials Research Group, University of Johannesburg, Doornfontein Campus, South Africa b School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban, South Africa c DST-NRF Centre of Excellence in Strong Materials and Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa d Chromium Research Group, Department of Physics, University of Johannesburg, Auckland Park, South Africa

Abstract Mixed ternary spinel-type oxides MnxNi1-xCo2O4 (x = 0, 0.5, 1) were synthesized using a citrate sol-gel route. The morphologies and structures of the mixed oxide nanoparticles were characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FTIR). Structural analysis from XRD and FTIR confirm the formation of single phase cubic spinels. TEM and SEM images show the formation of interconnected nanoparticles. The particle sizes obtained from TEM and XRD are consistent with each other. It was observed that the lattice constant increases with increasing Mn concentration from x=0 to x=0.5 and then decreases with further increase in Mn concentration to x=1. Magnetization measurements were carried out using Vibrating Sample Magnetometery (VSM), which showed that the oxides have weak ferromagnetic behavior with the presence of an antiferromagnetic component. The oxide with the highest Mn substitution was observed to be superparamagnetic at room temperature. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 1st Africa Energy Materials Conference. Keywords: Citrate sol-gel; Mixed metal oxides; Magnetic properties; Spinel

* Corresponding author. Tel.: +27 (0) 11 559 6148 E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 1st Africa Energy Materials Conference.

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1. Introduction Spinels based on transition metals (AB2O4, A, B = Mn, Fe, Co, Ni, Cu, Zn and other transition metals, A≠B) are an important class of metal oxides having diverse applications due to their thermal, electronic, magnetic, optical and catalytic properties amongst others [1-3]. The range of applications includes catalysis [4], energy storage devices [5], thermistors [6], and next generation electronic devices [6]. Ternary spinel-type Mn-Ni-Co oxides are the most extensively studied mixed metal oxide systems, due to their unique magnetic, electrical, and optical properties [2, 6, 7]. The reduction of antiferromagnetic behaviour of these type of materials to weak ferromagnetic has been an interesting phenomenon and has resulted in a number of diverse applications [2,7]. A key aspect of controlling the properties of nanoscale metal oxides is judicious selection and control of the synthesis parameters used. Different synthesis strategies have been used, such as solvothermal, hydrothermal, hard template synthesis, and sol-gel methodologies, amongst others [8-10]. Depending on the synthesis method used, the shape, porosity, particle size, and crystallite size can be altered and controlled to some extent, and as a result, the magnetic, optical and electrical properties can be tailored towards various applications [8-10]. In this study, spinel type oxides of MnxNi1-xCo2O4 (x=0, 0.5, 1) were synthesized by adapting a citrate sol-gel route. The method made use of citric acid and ethylene glycol as cation chelating agents, metal acetate precursors and was able to control structure and particle size during the formation of polymeric gels. Duran et. al. (2005) were able to prepare pure phase CoxNiMn2-xO4 (0.2≤x≤1.2) spinel-type powders by auto-combustion of ethylene glycol–metal nitrate polymerized gel precursors and by subsequent calcination at 600–700 °C [10]. Wang et al. (2007) also have synthesized nanocrystalline Ni1Co0.2Mn1.8O4 powders via a gel auto-combustion process of nitrate–citrate gels and got single spinel phase at 500 °C [11]. The majority of work reported on the citrate sol–gel method uses metal nitrate precursors because of the role of the counter nitrate ion as an oxidant during the combustion. Nevertheless, other types of metal precursors like acetates are also used. For example, Zhang et al. have synthesized Mn1.5-xCo1.5NixO4 nanoparticles by the sol–gel method using citric acid and metal acetate precursors [12]. However, synthesis of ternary mixed spinel-type oxides MnxNi1-xCo2O4 (with x=0, 0.5 and 1) have not been reported previously. The oxides prepared here were characterized and investigated for their structural and magnetic properties. 2. Experimental 2.1 Synthesis Ternary spinel-type oxides of MnxNi1-xCo2O4 (x=0, 0.5, 1) were prepared through a citrate sol-gel method. Co(CH3COO)2·4H2O (AR), Ni(CH3COO)2·4H2O (98%) and Mn(CH3COO)2·4H2O (≥99%) were purchased from Sigma Aldrich (South Africa) and were used as the starting materials. The complexing agent was made up from citric acid monohydrate, C6H8O7·H2O (SAR-CHEM) and ethylene glycol (VWR) and were used as received. A stoichiometric amount of cobalt, nickel, and manganese precursors were dissolved in 40 mL of distilled water and continuously stirred (magnetic stirrer bar) until a homogenous metal acetate solution was formed. During stirring, the citric acid monohydrate was slowly added into the metal acetate solution with a target molar ratio of citric acid to metal ions of 1.5:1. The pH value of the solution was adjusted to ~7 by adding ammonia solution (ACE, 25%) and then 40 mL of ethylene glycol was added to the mixture. The solution was then heated on a hot plate with stirring at 80-90 °C for 6 hours until a purple gel was obtained. This gel was dried at 100 °C overnight in an oven. The dried gel was then calcined at a temperature of 450 °C with a ramp rate of 2 °C min-1 in air for 5 h in a tube furnace. The product was a black powder. 2.2 Characterization The crystal structure of the prepared spinel-type oxides were characterized by powder X-Ray Diffraction (XRD; Bruker D2 phaser, Co-Kα radiation, λ=1.78060Å) with a step size of 0.026° over the 2θ range of 15 to 110°. The crystallite size (D) estimated using Scherrer’s equation and the lattice constants (a) were obtained using Retiveld refinements using TOPAS 4.2 software [13]. Fourier-Transform Infrared (FTIR) spectra were recorded in the range

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of 400–4000 cm−1 using a Shimadzu 8400 FTIR spectrometer employing potassium bromide (KBr) pellets at room temperature. The morphology and chemical composition were characterized by Scanning Electron Microscopy (SEM; S-4800 operated at 20 kV) equipped with an Energy-dispersive X-Ray Spectroscope (EDS), Transmission Electron Microscopy (TEM), which was performed on a JEM-2010 instrument at 200 kV acceleration voltage. Magnetic measurements were performed using a cryogenic measurement platform equipped with a Vibrating Sample Magnetometer (VSM) insert. The VSM measurements were done in zero field cooled (ZFC) and field cooled (FC) modus of measurement in a temperature range from 2 to 300K. Hysteresis loops were performed in a field range of +2.5 to -2.5T at 300K. 3. Results and Discussion 3.1. Structural Characterizations Figure 1 shows the XRD patterns of the prepared mixed ternary spinel-type oxides MnxNi1-xCo2O4 (x=0, 0.5, 1) obtained after calcination at 450 °C. All peaks can be indexed to a cubic spinel structure, with a space group of Fd3m (PDF card no. 01-073-1702). The absence of any significant impurity from the nickel oxide or manganese oxide phases indicates that high purity crystalline MnxNi1-xCo2O4 was synthesized. In reference to NiCo2O4 (x=0), the diffraction peak of (311) shifts initially to smaller angles (2) and then moves slightly to higher angles (2) with increasing Mn concentration shown as an inset in figure 1. In addition, the intensity of the diffraction peaks increases and the peaks become narrower with decreasing Mn concentration which indicated an increase in crystallite size.

Fig. 1. XRD pattern of the spinel oxides of MnxNi1-xCo2O4 prepared by using citrate sol-gel route and calcined at 450 ℃. The inset is the XRD peaks of (311) plane shift.

The average crystallite size obtained from XRD and the particle size from TEM are shown in Table 1. The lattice parameter increases from 8.113 Å to 8.160 Å for the Mn concentration range between 0 and 0.5. When the Mn concentration increases further, the lattice parameter decreases to 8.112 Å (x=1). The increase in the lattice

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parameter from x=0 to x=0.5 may be due to the substitution of smaller Ni ions by the larger Mn ions. When x is greater than 0.5, the Mn (and/or Ni) cations might be forced to move from tetrahedral (A) site to octahedral (B) site which might cause the decrease in the lattice parameter from 8.160 Å (x=0.5) to 8.112 Å (x=1) [14]. The average particle size obtained from XRD data for the spinel oxides range from 11 to 24 nm, which is in close agreement with the result from TEM measurements (Figure 3). Table 1. Lattice parameter (a) and mean crystal size (D) of the MnxNi1-xCo2O4 nanoparticles MnxNi1-xCo2O4

a (Å)

x=0 x=0.5 x=1

8.113

D (nm) XRD 23.6

D (nm) TEM 22.8

8.160

11.6

12.2

8.112

12.3

11.7

The FTIR spectra of MnxNi1-xCo2O4 (x=0, 0.5, 1) prepared by calcination at 450 °C are shown in Figure 2. Generally, spinel oxides show two distinguished absorption bands in the wave numbers between 400 and 700 cm-1 [15]. The strong peaks at ~555-563 cm-1 and ~653-655 cm-1 correspond to the M-O vibration frequency of metals (M= Ni, Mn or Co) at the octahedral and tetrahedral sites, respectively. The broad band centered around 1430 cm-1 is assigned to the H–O–H bending vibrations of the free or adsorbed water. The carboxylate anion (COO-) stretching is also shown by the appearance of an absorption band at 1383-1425 cm-1 which may be due to some traces of acetates and citric acid left undecomposed.

Fig. 2. FTIR spectra of MnxNi1-xCo2O4 (x=0, 0.5, 1) spinel oxides

The very weak band around 2342 cm-1 could be due to traces of adsorbed or atmospheric CO2 [15]. The absorption band at 2854-2924 cm-1 is attributed to the stretching vibration mode of the CH2 groups from ethylene glycol [16]. The bands observed at ∼3422 and ∼1612-1630 cm-1 show the presence of adsorbed water on the surface of the cobaltite nanoparticles. These FTIR absorption data confirms the successful formation of phase pure spinel MnxNi1-xCo2O4 in agreement with the XRD studies.

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3.2 Morphology Figure 3 shows the TEM images with the corresponding particle size distribution of the prepared metal oxide nanoparticles. It revealed that the metal oxides formed mostly spherical and some facetted nanoparticles and some agglomeration. The size distributions of the nanoparticles were observed to slightly change with stoichiometry of the metal oxides. Narrower size distribution was observed for NiCo2O4 compared to the other oxides.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3. TEM images (a, c, e) and particle size distribution (b, d, f) of the NiCo2O4, MnCo2O4, and Mn0.5Ni0.5Co2O4 respectively obtained by calcining at 450 °C.

Furthermore, to assess the bulk morphology of the nanoparticles and their composition, SEM studies and EDS analysis was done on all of the samples, and a representative image of a typical sample, Mn0.5Ni0.5Co2O4, is presented in Figure 4. The morphology can be viewed as agglomerates of nanoparticles, consistent with the TEM observation. EDS analysis confirmed the presence of Mn, Ni, Co and O atoms.

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O

(a)

Co

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(b)

Co Mn

10 m

Ni Mn

Ni Ni

Fig. 4. SEM image (a) and EDS spectrum (b) of the Mn0.5Ni0.5Co2O4 nanoparticles.

3.3 Magnetic Measurements

Fig. 5. M–T curves of the MnxNi1-xCo2O4 samples

The temperature dependence of zero field cooled (ZFC) and field cooled (FC) magnetization of the ternary spinel MnxNi1-xCo2O4 oxides measured in a magnetic field of 500 Oe at a temperature range of 2 K to 300 K is depicted in figure 5. The samples were cooled from 300 K to 2 K without any external magnetic field (ZFC), and the

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magnetization was recorded while warming the sample in an applied field of 500 Oe. With the measured temperature decreasing from 300 K to 2 K, the ZFC curves rise dramatically and reach a maximum peak at a certain temperature, namely the blocking temperature (TB), except for x=0.5 which shows anomalous behaviour. The blocking temperature (TB) is the temperature required to overcome the energy barrier by thermal activation whereby the ZFC magnetization reaches its maximum. The ZFC finally begin to drop while the FC magnetization curve keeps rising below the TB. The big difference between ZFC and FC data indicates possible existence of spin-glass like state. The blocking temperature (TB) value for NiCo2O4 samples was determined to be 66.58 K. There was no blocking temperature observed for Mn0.5Ni0.5Co2O4 even at a temperature of 300 K. No thermal irreversibility was observed, which indicates that the blocking temperature of the sample is above room temperature and the sample remains ferromagnetic in the temperature range. Two blocking temperatures were observed for MnCo2O4 at TB1=38.6 K and TB2=109 K. This type of behaviour was also observed for other mixed ternary spinel-type oxides and can be related to a ferrimagnetic state and some specific ordering [2, 17]. Magnetic hysteresis loops were measured for all the samples at 300 K in a field range of -25,000 Oe to 25,000 Oe (Figure 6). A hysteresis loop was observed for x=0 and x=0.5 indicating the ferromagnetic nature of the samples. For x=1, an almost linear variation of M-H which is typical of superparamagnetic materials was found.

Fig. 6. Room temperature M-H hysteresis loops of the various MnxNi1-xCo2O4 samples. The inset shows the magnified portion of the hysteresis loops. Table 2. Magnetic properties of MnxNi1-xCo2O4 Composition

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

x=0

2.62

0.163

229

x=0.5

4.40

1.647

1416.5

x=1

-

-

-

No saturation magnetization was observed up to a field of 25,000 Oe, which might be due to the magnetic surface disorder effects often observed at reduced dimensions and the presence of antiferromagnetics component along with the ferromagnetic phase [18, 19]. A broader hysteresis loop was observed for Mn0.5Ni0.5Co2O4 . The remnant magnetization (Mr), saturation magnetization (Ms) and coercive field (Hc) for the samples are given in table 2. The highest coercivity and remnant magnetization is found for Mn0.5Ni0.5Co2O4 while MnCo2O4 has almost zero coercivity and remnant magnetization.

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4. Conclusions Single-phase crystalline nano-sized mixed ternary spinel-type oxides MnxNi1-xCo2O4 (x= 0, 0.5, 1) were successfully obtained from acetate precursors via a citrate–ethylene glycol method at a relatively low temperature (450 °C). XRD studies combined with FTIR data confirm a cubic spinel structure with an Fd3m space group and a crystallite size between 12.3 and 23.6 nm. These XRD results agree well with the TEM-based size estimate, which is between 11.7 and 22.8. While NiCo2O4 and Mn0.5Ni0.5Co2O4 show ferromagnetic behavior at room temperature, MnCo2O4 nanoparticles show superparamgnetic behavior. The highest Hc value was obtained for Mn0.5Ni0.5Co2O4. More detailed studies of the magnetic properties and the structure and chemical composition of the prepared ternary spinel-type oxides are needed to better understand their behavior. Acknowledgements The authors would like to acknowledge the School of Chemistry, University of the Witwatersrand, the Department of Applied Chemistry and the Department of Physics, the Faculty of Science, University of Johannesburg (UJ), for support and access to research facilities. This work is based on research supported by the National Research Foundation (South Africa). (Grant Numbers: 93620, 88080, 98719:2015 and 103239:2016). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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