Mn-substituted cerium oxide nanostructures and their magnetic properties

Accepted Manuscript Title: Mn-substituted Cerium Oxide Nanostructures and their Magnetic Properties Authors: S.K. Alla, P. Kollu, Sher Singh Meena, H.K. Poswal, R.K. Mandal, N.K. Prasad PII: DOI: Reference:

S0025-5408(17)32605-3 https://doi.org/10.1016/j.materresbull.2018.04.008 MRB 9946

To appear in:

MRB

Received date: Revised date: Accepted date:

6-7-2017 23-2-2018 5-4-2018

Please cite this article as: Alla SK, Kollu P, Meena SS, Poswal HK, Mandal RK, Prasad NK, Mn-substituted Cerium Oxide Nanostructures and their Magnetic Properties, Materials Research Bulletin (2010), https://doi.org/10.1016/j.materresbull.2018.04.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mn-substituted Cerium Oxide Nanostructures and their Magnetic Properties

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S. K. Alla1, P. Kollu2, Sher Singh Meena3, H. K. Poswal4, R. K. Mandal1, N. K. Prasad1*

Department of Metallurgical Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India

Department of Physics, University of Hyderabad, Hyderabad - 500046, India

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

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High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre,

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Mumbai 400085, India

* Corresponding author: [email protected], Phone: 91-5422369346, Mobile: 91-9956629843, Fax:

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91-5422369478.

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MnxCe1-xO2 (0.1≤ x ≤0.5) nanostructures were prepared by one step microwave refluxing

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Highlights

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Graphical abstract

method.

The saturation magnetization values for the samples rose with increased Mn content.

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The oxygen vacancies inherently played crucial role in increasing the magnetization

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values.

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Abstract In this study, we report the magnetic properties of nanostructured Mn xCe1-xO2 (0 .1 ≤ x ≤ 0.5) materials, synthesized via microwave refluxing technique. Phase purity of the samples was

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investigated using the structural characterization techniques such as X-ray diffraction (XRD) and transmission electron microscopy (TEM). These analyses indicated the substitution of Ce-ions with Mn-ions in the lattice. The progressive increase in the oxygen vacancies with increased dopant concentration was demonstrated by Raman, UV-Vis and photoluminescence spectroscopic studies. X-ray photoelectron spectroscopy analysis further supported the presence of Mn2+, Ce3+

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and Ce4+ ions as well as oxygen vacancies in the samples. The Mn substituted CeO2 nanostructures

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are found to exhibit room temperature ferromagnetism (RTFM). There was a continuous

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improvement in the saturation magnetization (MS) values with increased Mn concentration. This

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might be due to the prevalence of surface oxygen vacancies and the systematic rise in their

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concentration with increased dopant content . Keywords: MnxCe1-xO2 nanostructures; X-ray

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diffraction; defect states; saturation magnetization.

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1. Introduction Nanostructured CeO2 materials have been extensively utilized as a catalyst over decades in the field of environmental catalysis [1–4]. The higher oxygen storage capacity and the presence of surface defects i.e. oxygen vacancies are responsible for its excellent catalytic behavior. On the other hand, due to the emergence of surface oxygen vacancies, the CeO2 nanostructures 3

display

the room temperature ferromagnetic (RTFM) behavior.

The ferromagnetic and

semiconducting properties of these materials could be suitable for spintronic applications. Like other nanocrystalline magnetic oxide semiconductors (e.g. TiO2, ZnO, SnO2, Cu2O, In2O3 and

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HfO2), doping with a small amount of other ions could influence the intrinsic magnetic properties of CeO2 [5–22]. In fact, Co-doped CeO2 display comparatively large magnetic moment with Curie temperature (TC) around 875 K. The doping of CeO2 with various elements viz. Fe, Co, Ni, Cr, Cu, Mg, Ca, Pr shows alteration of the saturation magnetization (MS) values [9,10,12–14,17,21,22]. Notably, the doped CeO2 materials with doping elements such as Fe,

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Co, Cu, Ca, Cr or Ni have shown improved MS values whereas, the others have shown converse

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effect i.e. suppress the MS values. These observations indicated a strong dependence of MS value

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on the type of dopant. However, there is a limitation of dopant concentration beyond which MS

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value tends to diminish. Thus, the types of dopants as well as their concentrations are decisive parameters for the MS values [8,13,16,20,23]. In addition, the MS values may also be tailored

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by varying the shape or size of CeO2 nanostructures [24,25]. Nevertheless, the size, shape as well

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as dopant concentration in CeO2 vary with the synthesis protocols which finally modify its

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magnetic properties [20, 22, 33]. There are large numbers of synthesis protocols for the preparation of undoped and doped CeO2 nanostructures viz. co-precipitation [26], electrochemical

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deposition [25], composite-hydroxide-mediated (CHM) approach [27], Sol – gel [28], hydrothermal [29], solution-combustion [10], pyrolysis [30], solid-state reaction [31] and

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microwave-assisted method [32]. The RTFM behavior of nanostructured CeO2 samples are intrinsically related to the

presence of surface defects or oxygen vacancies [6,34–36].

However, beyond a particular

concentration of oxygen vacancies, the MS value tends to decrease [36]. In contrast, Liu et al.

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[37] argued that surface oxygen vacancies might not be responsible for RTFM in nanocrystalline CeO2. Recently, Coey et al. [38] demonstrated that the MS value of CeO2 does not depend only on the oxygen vacancies but also on the surface configuration of neighbouring particles. They

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believed that the saturation magnetization in CeO2 emerged due to giant orbital paramagnetism.

There are a few reports on magnetic properties of Mn doped CeO2 nanostructures. For example, Xia et al. [27] reported that Mn substituted CeO2 (16.5 at. % or x = 0.5) synthesized via CHM approach show RTFM behavior with MS value ~ 0.012 Am2/kg. In contrast, thin film

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of Mn doped CeO2 (1.06 at. % or x = 0.032) grown on SiO2 / Si (001) substrate shown weaker

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RTFM behavior [39]. The series of nanocrystalline MnxCe1-xO2 (0 ≤ x ≤ 0.1) samples were

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synthesized by co-precipitation method which displayed RTFM behavior [40] with significantly

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higher MS values viz. 0.12 Am2/kg (for x = 0.03 sample). However, beyond this concentration, a

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systematic decrease in the MS value was noticed. Further, Al-Agel et al. [41] reported that

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nanocrystalline MnxCe1-xO2 (0.01 ≤ x ≤ 0.1) samples, produced by hexamethylene triperoxide diamine (HMTD) assisted solvothermal method, have also shown RTFM behavior. The MS value

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(~ 4.48 Am2/kg) for x = 0.08 sample was substantially higher than the other samples [41]. It is also reported that the MS value tend to rise upto the concentration of 2.64 at. % of Mn (x = 0.08)

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and then it decreases with increased doping. Similar findings were also observed for MnxCe1-xO2 (x = 0.01, 0.03, 0.05, 0.07, 0.09, 0.11 and 0.13) thin films which were grown on LaAlO3 (001)

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substrate [42]. The highest MS value of 1.75 μB / Mn was observed for x = 0.07 sample. Beyond this concentration, a gradual decrease in the MS value was noticed. These observations indicate that the critical concentration to obtain maximum MS value for Mn doped CeO2 nanostructures may be less than 3.3 at. % (x = 0.1).

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The reported MS value for a particular concentration of Mn in CeO2 nanostructures is observed to be different. Further, threshold concentration of Mn to obtain highest MS value is also varying. This inconsistency in MS values might be due to the variation of the synthesis protocol.

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Therefore, synthesis protocol was also playing a key role in the alteration of MS value for Mn doped CeO2. Nevertheless, the reports on the effects of higher Mn doping (x > 0.1) on the magnetic properties of CeO2 are limited. In addition, the systematic study on correlation between defect density and magnetic properties of Mn doped CeO2 nanostructures is also lacking in the literature. Hence, in the present investigation, we have synthesized nanostructured MnxCe1-xO2 (0 .1 ≤ x ≤

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0.5) samples using single step microwave refluxing technique and their magnetic properties are

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analyzed systematically.

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2. Experimental Details

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The precursor of Ce(NO3)3.6H2O was obtained from Loba Chemie, Mumbai, India. The other precursors such as MnCl2.6H2O, NaOH pellets and ethylene glycol were purchased from

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Merck, Mumbai, India. The detailed synthesis protocol is reported in our previous work [17]. In brief, the metallic salts were dissolved initially in a solution of 40 mL of DI water and 50 mL of

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Ethylene Glycol. An aqueous solution of NaOH was added dropwise for attaining pH value of

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the solution nearly 12. This solution was further stirred for 3 h and then irradiated to microwaves for 20 min using a domestic microwave oven. The precipitate was then washed several times with DI water and finally with absolute ethanol. The obtained products are dried in an oven at 100 °C for overnight (12 h). The concentration of Mn was x = 0.1, 0.3 and 0.5 in MnxCe1-xO2. Further,

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Mn0.1Ce0.9O2 sample was heat-treated at 500 °C for 2 h in open air to understand the effect of oxygen vacancies on the optical and magnetic properties of CeO2. X-ray diffraction analysis for Mn substituted CeO2 nanostructures was carried out for the

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identification of the phases and estimation of the crystallite size. Phillips X’pert Pro Advanced powder X-ray diffractometer with Cu-Kα radiation (λ = 0.15431 nm) was employed for recoding the XRD patterns of these samples. The crystallite size of the samples was calculated using Scherrer’s equation. The size and morphology of the Mn doped CeO2 nanostructures are observed by transmission electron microscopy (TEM, FEI TECHNAI G2) operating at an accelerating

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voltage of 200 kV. The compositional analysis of the samples were carried out using FE-SEM,

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Curl Zeiss, Supra 40. Raman spectra in the range of 200 to 800 cm-1 were recorded using a triple

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stage confocal micro-Raman spectrograph (JY T6400) with a 514.5 nm Ar+ laser excitation source.

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UV-Vis absorption spectra for Mn doped CeO2 were obtained by Cary 60 UV-Vis spectrometer

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(Agilent Technologies). Photoluminescence studies were conducted by LS-45 Fluorescence

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spectrometer (Perkin Elmer) under an excitation wavelength of 330 nm. X-ray photoelectron spectra (XPS) for Mn doped CeO2 were recorded by PHI 5000 Versaprobe II photoelectron

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spectrometer (ULVAC-PHI) using Al-Kα X-ray beam. Magnetic measurements in the temperature range from 2 to 300 K are performed using SQUID (MPMS-XL, Quantum Design) magnetometer.

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3. Results and Discussion

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3.1 XRD analysis Fig. 1 shows XRD patterns for MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples recorded in the

range of 20 to 90 degrees. All the patterns confirmed the formation of single phase which is similar to that of face centered cubic fluorite type structure of CeO2 (JCPDS NO. 43–1002, space group: Fm3m). The peaks were indexed as (111), (200), (220), (311), (222), (400), (331) and (420). The

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diffraction patterns showed no traces of other impurity phases. The calculated lattice parameter values are presented in table 1. The continuous decrease in the lattice constant with increased Mn content may be due to induced strain field associated with ionic radii (radius of Mn2+ = 0.66 Å,

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Ce4+ = 0.97 Å and Ce3+ = 1.14 Å) difference in the corresponding samples as well as presence of oxygen vacancies [14, 43]. The single phase nature as well as a systematic decrease in the lattice parameter with increased Mn content, were evidence for the incorporation of Mn-ions into CeO2 lattice. The estimated average crystallite size from XRD patterns for x = 0.1, 0.3 and 0.5 samples were ~ 9, 8 and 8 nm respectively.

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The XRD pattern for heat-treated Mn0.1Ce0.9O2 sample was similar to that of as synthesized one

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(Fig. 1). The increased intensity and decreased broadening of the peaks for heat-treated sample

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ascribed to the grain growth of the particles as well as decrease in the lattice strain upon heat-

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Fig. 1: XRD patterns of nanocrystalline MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples and heattreated Mn0.1Ce0.9O2 sample.

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Table 1: The lattice parameter, at. % of Mn and saturation magnetization values of Mn substituted CeO2 nanostructures. at.% of Mn

Saturation Magnetization

Sample

(Å)

(obtained from SEM)

(x10-3 Am2/kg)

Mn0.1Ce0.9O2

5.423 ± 0.0010

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Mn0.3Ce0.7O2

5.417 ± 0.0012

9.2

Mn0.5Ce0.5O2

5.416 ± 0.0014

15.7

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5.419 ± 0.0015

--

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Heat-treated

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Lattice parameter

8.5

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Mn0.1Ce0.9O2

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3.2 TEM analysis

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Fig. 2 shows TEM bright field image for Mn0.1Ce0.9O2 sample at different magnifications.

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The selected area electron diffraction (SAED) pattern (inset image of

Fig. 2 a) shows

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polycrystalline rings which could be indexed as (111), (200), (220) and (311). This indicates face centered cubic type structure for Mn0.1Ce0.9O2 sample. Fig. 2 (a) & (b) display the presence of

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both spherical and rod shaped particles. The average size of the spherical particles was ~ 4 ± 1 nm whereas; the average length of the rods was estimated to be ~ 34 ± 5 nm with an aspect ratio

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of ~ 4. Furthermore, the HRTEM image (Fig. 2 c) shows the width of lattice fringes for both

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spherical and rod shaped particles which is ~ 0.31 nm. This fringe width corresponds to an

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interplanar spacing of (111) plane of CeO2.

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Fig. 2: (a & b) TEM-BF image with corresponding SAED pattern (inset of Fig. 2 (a)) and (c) HRTEM image of Mn0.1Ce0.9O2 sample.

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The TEM bright field images for Mn0.5Ce0.5O2 sample shown in Fig. 3 (a) & (b), also confirm the

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presence of both spherical and rod shaped nanoparticles. Relatively, the concentration of rod

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shaped particles in this sample was more than that of Mn0.1Ce0.9O2 sample. The average aspect ratio for nanorods increased to ~ 7 and the average length was ~ 45 ± 7 nm. The size of the

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spherical particles was ~ 5 ± 1 nm. HRTEM image (Fig. 3c) display two sets of lattice fringes for both spherical and rod shaped particles. Their fringe widths were ~ 0.31 and ~ 0.27 nm. These

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correspond to d-spacing of (111) and (200) planes of CeO2. These observations further support the single phase nature of Mn substituted CeO2 nanostructures.

In addition, a rise in the

concentration of rod shaped particles upon increased Mn substitutions could improve the defect density in the sample [24]. The chemical compositions of elements present in the samples are displayed in table 1 which are obtained by SEM-EDS analysis. 10

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Fig. 3: (a & b) TEM-BF image with corresponding SAED pattern (inset of Fig. 3 (a)) and (c) HRTEM image of Mn0.5Ce0.5O2 sample.

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3.3 Raman Spectroscopy Analysis

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Raman spectra for MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples identify two distinct Raman

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active bands as shown in Fig. 4a. They are positioned at frequencies around 460 and 600 cm-1.

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These bands are derived from triply degenerate Raman active mode and non-degenerate LO mode

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[44]. The main band near 460 cm-1 can be assigned to the F2g mode of fluorite structure whereas the other band is associated with the defect states present in the sample [44]. It is known that these

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bands are sensitive to doping as well as particles size [45]. The main band at 460 cm-1 shifted

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towards lower energy with increased Mn concentration. This suggests the incorporation of Mnions into CeO2 lattice, which was also supported by our XRD measurements (decrease in the

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lattice parameter with increasing Mn substitutions). Besides, the band at around 600 cm-1 became prominent with increased concentration of Mn. This indicates an enhanced defect density by

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augmented dopant concentration. This may be due to the progressive increase of rod shaped particles with increased Mn content [24].

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Fig. 4: Raman spectra for (a) nanocrystalline MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples and (b) heat-treated Mn0.1Ce0.9O2 sample.

The Raman spectrum of heat-treated Mn0.1Ce0.9O2 sample is shown in Fig. 4b. The Raman

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active F2g mode at ~ 460 cm-1 for the sample shifted towards higher energy side. In addition, a decrease in broadening indicates the grain growth of the particles and decrease in the oxygen

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vacancies. The disappearance of Raman band at around 600 cm-1 also supports the diminished

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oxygen vacancy content upon heat-treatment.

3.4 UV and PL spectroscopy analysis The typical optical absorption spectra (Fig. 5a) for MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples have shown blue shift of absorbance with increased Mn content. The optical band gap (Eg) can be estimated by the equation [20] 12

Eg = 1240/λabs. edge where λabs. edge is the onset absorption edge in nanometers and Eg is in eV. The estimated band gap energy values were ~ 2.62, ~ 2.96 and ~ 2.84 eV for x = 0.1, 0.3 and 0.5

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samples respectively. These values were calculated using their corresponding onset absorbance edges which lied at 473, 418 and 436 nm for the respective samples. The variation in the

bandgap values is evident for the formation of defects states between Ce 4f conduction band and O 2p valence band upon Mn substitutions. The similar trend of increased bandgap values was

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also observed for Cr or Co doped CeO2 nanoparticles [12,20]. Besides, the augmentation of

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bandgap values represents the defect density modulation with increased Mn concentration.

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Fig. 5: (a) UV-vis and (b) PL spectra of MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples.

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The variations of defect concentration with Mn substitutions were also observed in PL

spectra. Fig. 5 (b) presents room temperature PL spectra for Mn substituted samples. The spectra for each sample displayed emission bands between 370 to 500 nm which corroborate the presence of defect states between Ce 4f states and O 2p states as well as the oxygen vacancies [46]. With increased Mn content, the intensity of the PL emission bands continuously decreased. In general, 13

the decreased PL intensity for undoped CeO2 is governed by suppression of defect states in the sample. However, at high dopant concentration, such decrease in the intensity could be because of the population of non-radiative defect states between valence and conduction bands of CeO2

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[47]. 3.5 XPS Analysis

X-ray photoelectron spectroscopy was utilized for the identification of oxidation states

for Ce and Mn elements in Mn0.3Ce0.7O2 sample. Fig. 6 shows deconvoluted XPS spectra of Ce 3d, Mn 2p and O 1s respectively. The main peaks located at 898.7 and 917.0 eV ascribed to

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Ce4+ 3d5/2 and 3d3/2 respectively (Fig. 6a) whereas, the peaks at 882.5 and 900.3 eV correspond

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to Ce3+ 3d5/2 and 3d3/2 respectively. The other peaks could be attributed to the satellite peaks

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for Ce 3d3/2 and 3d5/2 [48]. From this observation, it is clear that Ce was present in +3 and +4

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states in Mn0.3Ce0.7O2 nanostructures. The relative concentration of Ce3+ ions ([Ce3+] / [Ce3+ + Ce4+]) was estimated from the respective peak areas. It is was found to be ~ 0.5 and which also

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indicates the presence of oxygen vacancies in the sample. Fig. 6b displays the peaks for Mn2+

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2p1/2 (652.5 eV) and Mn2+ 2p3/2 (640.9 eV) [27]. These indicate the oxidation state for Mn to

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be +2. O 1s deconvoluted core level spectra as shown in fig. 6c displays the peaks at 530 and 532.7 eV [49, 50]. The first peak was due to lattice oxygen and the second one is could be

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attributed to the adsorbed oxygen species.

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Fig. 6: XPS spectra of (a) Ce 3d core level spectra (b) Mn 2p core level spectra and (c) O 1s core level spectra for Mn0.3Ce0.7O2 sample.

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3.6 Magnetic Measurement

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RTFM behavior for MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples was realized from their M

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vs. H curves as shown in Fig. 7. Each sample had both para- and ferromagnetic components. The MS values obtained after the removal of paramagnetic component were 0.012, 0.014 and 0.021

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Am2/kg for x = 0.1, 0.3 and 0.5 samples respectively. To the best of our knowledge, such

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continuous rise in the MS values for x > 0.1 samples has not been reported in the literature. The paramagnetic component became prominent with increased substitution of Mn. The systematic

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rise in the defect concentration with increased Mn content was evidenced from Raman and PL

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spectroscopic analyses (Figs. 4a & 5b). There were no secondary phases in the samples (Figs. 1 & 2) and even the presence of antiferromagnetic Mn or Mn-oxide (e.g. MnO, MnO2, Mn2O3 and

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Mn3O4) clusters could not improve the MS values [51]. Therefore, these observations acclaim that the continuous rise in the MS value could be due to the modulation of defect density with increased Mn substitutions. It can be presumed that the incorporation of Mn-ions into CeO2 lattice rise the concentration of surface oxygen vacancies as the substitution of one Ce4+ ion by one Mn2+ ion enables the removal of one O2- ion to maintain charge neutrality . In addition, the augmented 15

nanorods with increased Mn content evidently enhanced the intrinsic oxygen vacancies. Subsequently, these oxygen vacancies established the ferromagnetic interactions between the cations which could be explained by F-center exchange mechanism [52]. However, it can be

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possible that the formation of ferromagnetic clusters could also enhance the MS values of these Mn

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doped CeO2 nanostructures.

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Fig. 7: M vs. H curves (a) before (b) after subtraction of linear background for MnxCe1-xO2 (x = 0.1, 0.3 and 0.5) samples. Magnetization (M) vs. temperature (T) curves were recorded at 50 mT to understand the

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effect of Mn substitution on the intrinsic magnetic properties of CeO2 (Fig. 8). The obtained ZFC and FC curves for all the samples revealed that the paramagnetic component dominates the

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ferromagnetic one. The separation between ZFC and FC curves for x = 0.1 sample provides

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evidence for the occurrence of ferromagnetic clusters in the sample and which contributed to its larger coercivity (Fig. 8). The MS value for x = 0.1 sample was less than that for x = 0.3 and 0.5 samples. The difference between the MS values for x = 0.1 and 0.3 samples was relatively smaller as compared to the difference between x = 0.3 and 0.5 samples. This may be due to the presence of strong ferromagnetic interactions in x = 0.1 sample which may substantially improve the MS 16

value. It is noteworthy to mention that surface oxygen vacancies progressively increase with increased Mn content. In addition, nanorod structures possess high defect concentration than the spherical particles [24,25]. Therefore, the progressive increase of MS values for these samples

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may be attributed to the enhanced oxygen vacancies. However, to confirm the noticeable rise in the MS value for Mn0.1Ce0.9O2 sample due to the presence of strong ferromagnetic interactions, Mn0.1Ce0.9O2 sample was heat-treated at 500 °C for 2 h in open air atmosphere and their magnetic

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properties were compared.

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Fig. 8: M vs. T curves at 50 mT for (a) Mn0.1Ce0.9O2 (b) Mn0.3Ce0.7O2 and (c) Mn0.5Ce0.5O2 samples.

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The MS value for Mn0.1Ce0.9O2 sample decreased to 0.0085 Am2/kg and the paramagnetic component slightly enhanced upon heat-treatment (Fig. 9a). On the other hand, the bifurcation

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between ZFC and FC curves decreased (Figs. 8a & 9c) which may be associated with the suppression of ferromagnetic interactions. Therefore, it can be assumed that the decrease in the

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MS value may predominantly be due to the suppression of ferromagnetic clusters rather than decreased oxygen vacancies. Therefore, these observations suggest that the RTFM behavior and systematic increase in the MS values for as synthesized Mn substituted samples were inherently due to the surface oxygen vacancies.

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Fig. 9: M vs. H curve for as synthesized and heat-treated Mn0.1Ce0.9O2 sample (a) before (b) after subtraction of linear background and (c) M vs. T curves for heat-treated sample.

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4. Conclusions

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Spherical and rod shaped Mn substituted CeO2 nanostructures have been synthesized by

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microwave refluxing method. XRD, TEM and Raman spectroscopy analyses demonstrated the

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single phase nature for the samples. The formation of defect states or oxygen vacancies increases

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with increased Mn content which are corroborated by Raman and PL measurements. All the

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samples exhibited RTFM behavior which can be due to the presence surface oxygen vacancies or defects. The continuous rise in the MS values for Mn substituted CeO2 samples may be attributed

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to the enhanced surface oxygen vacancies with increased dopant concentration.

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