Structural, dielectric, optical and magnetic studies of dysprosium doped iron oxide nanostructures

Journal Pre-proof Structural, Dielectric, Optical and Magnetic Studies of Dysprosium Doped Iron Oxide Nanostructures

Ruqiya Bhat, Mubashir Qayoom, Ghulam Nabi Dar, Basharat Want PII:

S0254-0584(20)30143-7

DOI:

https://doi.org/10.1016/j.matchemphys.2020.122764

Reference:

MAC 122764

To appear in:

Materials Chemistry and Physics

Received Date:

29 June 2019

Accepted Date:

03 February 2020

Please cite this article as: Ruqiya Bhat, Mubashir Qayoom, Ghulam Nabi Dar, Basharat Want, Structural, Dielectric, Optical and Magnetic Studies of Dysprosium Doped Iron Oxide Nanostructures, Materials Chemistry and Physics (2020), https://doi.org/10.1016/j.matchemphys. 2020.122764

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Structural, Dielectric, Optical and Magnetic Studies of Dysprosium Doped Iron Oxide Nanostructures Ruqiya Bhat1, Mubashir Qayoom1, Ghulam Nabi Dar*1, Basharat Want2 1Nanophysics

2Solid

*Corresponding

Research Laboratory, Department of Physics, University of Kashmir, Srinagar-India

State Research Laboratory, Department of Physics, University of Kashmir, Srinagar-India

author :Tel: 9086486160, fax: +91-194-2421357 E-mail address: [email protected]

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Abstract Lanthanide-doped iron oxide nanomaterials are potential candidates of current interest for many advanced applications. In this paper, we report the synthesis of pure and dysprosium (Dy3+) ion doped iron oxide nanostructures, such as ⍺-DyxFe2-xO3 by employing a sol-gel auto combustion method. The pure and doped iron oxide nanostructures have been characterized by employing Xray diffraction (XRD), transmission electron microscopy (HR-TEM), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. The synthesized nanostructures were studied for dielectric, optical and magnetic properties. Dy3+ ion doped ⍺-Fe2O3 nanostructures shows enhancement in the dielectric properties as compared to pure ⍺-Fe2O3. Temperature-dependent conductivity follows Motts law, which confirms the mechanism of variable range hopping in them. Band gap and photoluminescence increases with increase in concentration of Dy3+ ion doping in ⍺-Fe2O3 system. Saturation magnetization increases significantly in doped systems. It was observed that doped systems saturate at low fields (10 kOe) as compared to pure ⍺-Fe2O3 system, which does not saturate up to the maximum applied field (20 kOe). High value of saturation magnetization at low applied magnetic fields, large band gap, enhanced dielectric and efficient photoluminescent properties makes the Dy3+ ion doped iron oxide nanostructures possibly the potential candidates for device applications. Keywords: Iron Oxide Nanostructures, Dysprosium, Motts Law, Luminescence, Magnetization

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1.

Introduction

Nanostructures (NS) of metal oxides as functional materials have been largely studied due to their outstanding applications in material as well as biological sciences [1-4]. Iron oxide NS represents new class of materials due to their superior electric, magnetic and optical properties [5-8]. Mainly iron oxide exists in the forms such as Fe3O4, FeO and Fe2O3. Further, ferric oxide (Fe2O3) exists in five phases-α-Fe2O3, β-Fe2O3, γ-Fe2O3, ε-Fe2O3 and ζ-Fe2O3 [9, 10]. α-Fe2O3 is most stable and has been widely studied due to its unique properties such as n-type semiconducting behavior, high resistance to corrosion, low-cost synthesis, biocompatibility, environment-friendly and non-toxic nature [11]. Study of α-Fe2O3 is not limited to only academic interest, though, it has been widely investigated for its use as pigments, catalysts, sensors, lithium-ion batteries, biomedical and magnetic materials and many more [11-15]. In order to achieve advanced applications of such materials, it is imperative to modify their properties. This can be achieved by incorporating foreign cations into the host matrix which leads to the change in its properties [16, 17]. Many studies have been carried out by doping divalent (transition metal) or trivalent (lanthanide) ions as foreign cations independently into iron oxide NS [18-20]. Modifying the structural arrangement of iron oxide NS leads to a substantial change in its properties including electric, magnetic, and optical properties. Metal ions doping in iron oxide NS have been widely investigated [19-22]. Incorporation of trivalent lanthanide (Ln3+) ions in iron oxide NS results in the enhancement of magnetic and optical properties because of their high magnetic moment and luminescent properties [22]. The unique optical properties of Ln3+ ions are attributed to its intra-configurational 4f-4f transitions [23, 24]. Among various Ln3+ ions, doping of Nd3+, Eu3+, Ho3+ and La3+ ions into ⍺-Fe2O3 NS have been studied [25-29]. In this study, we have chosen Dy3+ ion doping in ⍺-Fe2O3 because of the high magnetic moment

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and unique optical property associated with Dy3+ ions and this indeed motivates a detailed discussion. We have successfully synthesized pure and Dy3+ ion doped ⍺-Fe2O3 NS using sol-gel auto combustion method. Structural, dielectric, optical and magnetic properties of Dy3+ ion doped α-Fe2O3 NS have been thoroughly studied and discussed. 2.

Experimental Details

2.1.

Materials

Dysprosium nitrate hexahydrate (Dy(NO3)3.6H2O), Ferric nitrate nonahydrate (Fe(NO3)3.9H2O), glycine (C2H5NO2), and ammonia were purchased from Sigma-Aldrich and were used without further purifications. 2.2.

Synthesis

Iron oxide NS with general formula ⍺-DyxFe2-xO3 (x = 0.00, 0.01, 0.02, 0.03, 0.05, and 0.07), denoted as D0, D1, D2, D3, D5 and D7, respectively, were synthesized by sol-gel method. Solutions of Dy(NO3)3.6H2O, Fe(NO3)3.9H2O and glycine (C2H5NO2) were prepared separately using distilled water as a solvent. A homogeneous mixture was prepared by mixing the solutions of Fe(NO3)3.9H2O and Dy(NO3)3.6H2O. To this homogeneous solution, C2H5NO2 solution was added in equal amounts, which acts as a fuel. The resulting homogeneous solution was stirred for 2 h at 1000 rpm on a magnetic stirrer at 25⁰C. Ammonia solution was added to attain the pH 7. The reaction mixture gets transformed to gel upon heating at 70⁰C on a hot plate. Temperature was increased to 120⁰C to ensure complete combustion of the organic residues and the solid material obtained was final product. To attain the homogeneity, the solid product was ground for 1 h. For the proper ⍺-DyxFe2-xO3 phase formation, the obtained powdered sample was calcined at 600⁰C for 6 h. For pellet formation, a binder polyvinyl alcohol was mixed with the powder obtained after calcination and was ground for 1 h. A pressure of 120 kg/cm2 at a constant rate

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was applied to this powder sample from KBr press to obtain pellets. In order to make these pellets dense, these were sintered at 600⁰C for 2 h. A flowchart of the reaction scheme is shown in Fig. 1.

Fig. 1 Flowchart for the synthesis of D0, D1, D2, D3, D5 and D7.

2.3.

Methods

The phase confirmation and crystallinity of D0, D1, D2, D3, D5 and D7 were confirmed by using a Siemens Diffractometer (Cu K⍺; λ = 1.54 Å). The X-ray diffraction (XRD) was obtained over 2θ values of 20-80⁰ in the steps of 0.02⁰ with a scanning rate of 6 s per step. The particle size was analyzed by using high resolution transmission electron microscopy (HR-TEM, JSM). The microstructure and chemical composition were analyzed by field emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray spectroscopy (EDS) of FEI (Nova Nano). IR spectra were recorded using FTIR, MAGNA550. The phonon modes were identified from

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Raman spectra using a Raman microscope (Renshaw RL633). The dielectric constant (ε') and dielectric loss (tanδ) with the variation of frequency (20 Hz to 3 MHz) at different temperatures were analyzed using LCR Agilent Precision meter (4284A). The ε' and tanδ variation with temperature in the range of 170-400 K at different frequencies were analyzed by LCR meter which is connected to a furnace fitted with two terminal sample holder. The temperature dependent conductivity measurement was done by using two probe method (Keithley 2612A Source meter). The optical band gap was obtained from the diffuse reflectance spectroscopy (UV-3101, UV-Vis Spectrophotometer). Photoluminescence spectra were obtained by using Horiba Jobin Yvon Fluorolog 3-22 Spectrofluorometer. The various magnetic parameters including remanence (Mr),

coercivity (Hc), saturation magnetization (Ms), squareness ratio

(Mr/Ms) and magnetic anisotropy (K) were determined from M-H hysteresis loops at room temperature by using a vibrating sample magnetometer (Micro Sense EZ9 VSM, USA). 3.

Results and Discussion

3.1.

Crystal Structure and Phase Composition

In order to investigate the structural parameters of D0, D1, D2, D3, D5 and D7, the XRD data was Rietveld refined by using the FullProf suite software package [30, 31]. The Rietveld refined XRD patterns are depicted in Fig. 2 and the observed peaks in the diffraction patterns have been indexed. During the refinement process, various factors including background, the scale factor, half-width parameters, zero correction, thermal parameters, lattice parameters positional parameters, and interfacial angles were taken into consideration. The refined XRD patterns provide clear evidence of ⍺-Fe2O3 phase (JCPDS card # 86-0550) and confirmed the hexagonal crystal structure with R-3c space group in the synthesized samples. From the refined XRD patterns, it is clear that no additional phase is formed by incorporating Dy3+ ions into ⍺-Fe2O3

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lattice [29]. The strong, sharp and narrow diffraction peaks confirm that the synthesized NS are well crystallized. Apart from this, peak shifting was observed for the most intense peak towards lower 2θ for all the doped samples D1, D2, D3, D5 and D7 as depicted in Fig 3. This could be due to the lattice expansion by the substitution of larger radii Dy3+ (0.94 Å) ions for smaller radii Fe3+ (0.69 Å) ions [27]. However, for the higher dopant concentration (D5 and D7), the peak shifting was observed towards higher 2θ value as compared to lower concentration doped samples [32]. This could be attributed to the fact that at higher dopant concentrations that is beyond x = 0.03, all the Dy3+ ions may not replace Fe3+ ions at octahedral site, however, they may occupy interstitial octahedral sites and cause lattice contraction. Lattice contraction causes peak shifting towards higher 2θ value. The refined structural parameters Rb, Rf, χ2 (goodness of fit), obtained from Rietveld refinement are shown in Table I, where Rb is Bragg factor, and Rf is the crystallographic factor. The low value of χ2 confirms a high degree of refinement. Furthermore, the Debye-Scherrer equation (eq. 1) given below was employed for the calculation of crystallite size (Table II). Kλ

D = βcos θ

(1)

where D represents the diameter of the crystallite, K = 0.89 represents the shape factor, β denotes the full width half maximum (FWHM) intensity, λ = 1.54 Å represents the wavelength (Cu Kα) and θ denotes the Bragg’s angle [27]. The crystallite size obtained for the synthesized NS was observed to decrease by incorporating Dy3+ ions into the host matrix. Also, the lattice constants were observed to increase which could be the reason for the reduction in crystallite size. The increase in lattice constants could be ascribed to the substitution of larger size Dy3+ ions for

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smaller size Fe3+ ions causing dilation in the host system [33]. The dislocation density (δ) of these synthesized NS was calculated using the below equation [34]: 1

(2)

δ = D2

The dislocation density was observed to increase for all the doped samples as compared to pure sample, which indicates modified disorder in these systems after doping with Dy3+ ions. Moreover, the strain (S), porosity (γ), bulk density (ρB), and X-ray density (ρx) were determined using the following equations [35, 36]: S=

βcos (θ) 4 M

(3) (4)

ρB = V

M ′Z

ρx = VcellNa ρB

γ = 1 ― ρx

(5) (6)

where M represents the mass and V represents the volume of the pellet, M′ represents the molecular weight, Vcell represents the volume of the unit cell, Z represents the number of molecules per unit cell, and Na = 6.023 × 1023 is the Avogadro’s number. The calculated values of S, ρB, ρx and γ for D0, D1, D2, D3, D5 and D7 are listed in Table II. The density was observed to increase by the inclusion of Dy3+ ions into ⍺-Fe2O3 lattice which can be also visualized by FESEM micrographs (Section 3.3).

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Fig. 2 XRD patterns with Rietveld refinement (a) D0, (b) D1, (c) D2 (d) D3, (e) D5 and (f) D7.

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Fig. 3 Peak shift of (b) D1, (c) D2, (d) D3, (e) D5 and (f) D7 compared to (a) D0. Table I: Goodness of fit (χ2), Crystallographic factor (Rf) and Bragg factor (Rb)

Sample

χ2

Rf

Rb

D0

2.86

5.04

5.98

D1

1.89

7.10

4.36

D2

1.53

8.09

3.79

D3

1.33

9.43

2.50

D5

1.61

7.44

5.28

D7

1.92

10.30

9.88

Table II: Lattice constants, the crystallite size (D), dislocation density (δ), strain (S), bulk density (ρB), X-ray density (ρx) and porosity (γ)

Sample

D0

a=b

c

D

δ

S

ρB

ρx

γ

Å

Å

(nm)

( Å )-2

(%)

(g/cm3)

(g/cm3)

(%)

5.033

13.749

17.17

3.392×10-5

0.0050

4.10

5.280

22.34

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3.2.

D1

5.034

13.750

15.39

4.133×10-5

0.0050

4.19

5.298

20.91

D2

5.036

13.752

14.12

5.776×10-5

0.0048

4.37

5.344

18.22

D3

5.037

13.755

12.84

6. 065×10-5

0.0047

4.45

5.369

17.12

D5

5.035

13.753

13.87

5. 195×10-5

0.0052

4.80

5.445

11.54

D7

5.034

13.751

13.89

5.179×10-5

0.0055

4.97

5.519

9.95

High Resolution Transmission Electron Microscopy

HR-TEM micrographs of samples D0, D3, D5 and D7 are shown in Fig. 4, and it appears from the micrographs that the synthesized NS are composed of aggregated grains. With the incorporation of Dy3+ ions into ⍺-Fe2O3 lattice, the average particle size decreases in D3, D5 and D7 compared to D0. However, average particle size increases in D5 and D7 as compared to D3 as shown in Table III. The increase in average particle size is due to lattice contraction at higher dopant concentration. The distribution profile of the average particle size is presented in Fig. 5. This decrease in average particle size supports the average crystallite size decrease (section 3.1).

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Fig. 4 HR-TEM micrographs (a) D0, (b) D3, (c) D5 and (d) D7.

Fig. 5 Average particle size distribution profiles (a) D0, (b) D3, (c) D5 and (d) D7.

3.3.

Microstructure and Chemical Composition

FE-SEM micrographs of D0, D3, D5 and D7 are shown in Fig. 6. Density was found to increase in D3, D5 and D7 as compared to D0 supported by the decreased average grain size in doped samples as compared to pure sample. The average grain size was also observed to increase beyond x = 0.03 dopant concentration which is again because of the lattice contraction at higher dopant concentrations. The calculation of average grain size was done by IMAGE-J software and the distribution profile is shown in Fig. 7. The EDS spectrograms of D0, D3, D5 and D7 are shown in Fig. 8. The EDS spectrum of D0 displays the presence of iron and oxygen only; however, the presence of dysprosium along with iron and oxygen was observed in EDS spectra of D3, D5 and D7. The calculated values and observed values of percentage elemental compositions of these NS agree well with each other, shown in Table III.

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Fig. 6 FE-SEM micrographs (a) D0, (b) D3, (c) D5 and (d) D7.

Fig. 7 Average grain size distribution profiles (a) D0, (b) D3, (c) D5 and (d) D7.

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Fig. 8 EDS spectrograms (a) D0, (b) D3, (c) D5 and (d) D7. Table III: Particle size, Grain size, experimental weight percentage and calculated weight percentage of Fe, O and Dy. Sample

D0

D3

D5

D7

Particle size

Grain size

(nm)

(𝝻m)

47

0.035

28

28.93

30.60

0.024

0.025

0.027

Element

Experimental weight

Calculated weight

(%)

(%)

Fe

66.74

69.982

O

33.26

30.017

Fe

64.77

67.537

O

32.47

29.460

Dy

2.76

2.993

Fe

62.46

65.987

O

32.65

29.088

Dy

4.89

4.923

Fe

58.39

64.477

O

35.22

28.717

Dy

6.39

6.805

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3.4.

IR Analysis

IR spectra of D0, D3, D5 and D7 recorded from 400 to 4000 cm-1, are shown in Fig. 9. Absorption bands at 3422, 1637, 544 and 431 cm-1 displayed in the spectra are consistent with the literature [37]. Bands observed at 3422 and 1637 cm-1 could be assigned to the stretching and bending vibrations of O-H respectively, while as the bands at 544 and 431 cm-1 could be due to Fe-O stretching vibrational modes in ⍺-DyxFe2-xO3 NS [38, 39]. Absence of 2924 cm-1 band confirms the organic moieties have been completely removed from the surface of these systems upon calcinations at 600⁰C [40]. In addition, a slight shifting of absorption bands for D3, D5 and D7 confirms doping in these NS [41].

Fig. 9 IR spectra (a) D0, (b) D3, (c) D5 and (d) D7.

3.5.

Raman Spectroscopy

Room temperature Raman spectra of D0, D3, D5 and D7 were recorded upon excitation at 632 nm and are shown in Fig. 10. α-Fe2O3 system corresponds to the space group of R-3c and

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exhibits D63d symmetry. Seven Raman active modes including two A1g and five Eg phonon modes exist in the Raman spectrum of these systems [42, 43]. From the spectra, the modes observed at 227, 246, 295, 413, 491, and 612 cm-1 are assigned as A1g (1), Eg (2), Eg (3) + Eg (4), Eg (5), A1g (6) and Eg (7) respectively [42, 44]. The obtained results indicate that all the synthesized NS retain ⍺-Fe2O3 phase, thereby supporting XRD results (section 3.1). Moreover, the intensity of all the observed phonon modes was enhanced with the incorporation of Dy3+ ions into ⍺-Fe2O3 lattice system, which agrees well with the literature [28]. The increase in the intensity of phonon modes observed for D3, D5 and D7, as compared to D0 may be due to the strong electronphonon interaction [45]. The magnon mode for the synthesized NS was observed to be around 1325 cm-1 which agrees with the literature [46].

Fig. 10 Raman spectra (a) D0, (b) D3, (c) D5 and (d) D7.

3.6.

Dielectric Measurement

The values of dielectric constant (ε′) were obtained using the below equation: Cd

ε′ = Aϵo

(7)

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where C represents the capacitance measured from impedance analyzer, d and A represents the thickness and surface area of the pellet, and εo is the electric permittivity of free space (8.85 × 10−12 F/m ) [47]. The dielectric parameters at selected temperatures and frequencies are given in Table IV and Table V respectively. 3.6.1. Frequency-Dependent Dielectric Measurement Frequency dependent (20 Hz to 3 MHz) ε′ at selected temperatures (180, 220, 260 and 300 K) for D0, D3, D5 and D7 are displayed in Fig. 11. From the figure, it is apparent that at lower frequencies, higher values of ε′ are observed. With the increase in frequency, ε′ is found to decrease for all the synthesized systems. Above a certain frequency, ε′ has a weak dependence on the applied frequency which is consistent with the literature [48-51]. The observed behavior of ε′ is assigned to the Maxwell and Wagner-type polarization and is supported by Koop’s theory [5254]. Moreover, the dielectric behavior observed in these NS can be associated with the exchange interaction energy between Fe3+ and Fe2+ ions. In the lower frequency region, the dipoles Fe3+ ↔ Fe2+ in these NS align with the applied frequency and hence higher ε′ values are observed. However, an abrupt decrease was observed in ε′ at higher frequencies which can be assigned to the fact that the electron exchange between dipolar Fe2+ and Fe3+ ions does not follow the applied frequency [55]. By the inclusion of Dy3+ ions in ⍺-Fe2O3 lattice, ε′ was observed to increase for D3 as compared to D0, however, a decrease in ε′ was observed for D5 and D7, which may be attributed to the defects created by a high concentration of Dy3+ ions. Furthermore, with the decrease in temperature from 300 to 180 K, a decrease in ε′ was observed. This could be because of the fact that the motion of dipoles ceases at low temperatures, so the exchange interaction between the dipolar Fe2+ and Fe3+ ions is reduced, thereby resulting in a decrease in ε′ [56].

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The frequency dependent (20 Hz to 3 MHz) dielectric loss (tanδ) at selected temperatures (180, 220, 260 and 300 K) exhibited similar behavior as ε′ and is displayed in Fig. 12. In the lower temperature region, the higher value of tanδ could be assigned to space charge polarization [47]. However, in the higher frequency region, the accumulation of charges is negligible at the interfaces, so tanδ is small. In other words, at lower frequencies more energy is needed for electron exchange between the dipolar Fe2+ and Fe3+ ions and hence the values of tanδ were observed to be high, however at higher frequencies, small energy is needed for electron exchange between the dipolar Fe2+ and Fe3+ ions and therefore lower tanδ values were observed. Moreover, with the decrease in temperature, an appreciable increase was observed in tanδ. This is attributed to the Mott conductivity, which is dominant at the low temperatures [57]. The confirmation of Mott conductivity at low temperatures has been done in section 3.7.

Fig. 11 Variation of ε' with frequency (a) D0, (b) D3, (c) D5 and (d) D7 at 180, 220, 260 and 300 K.

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Fig. 12 Variation of tanδ with frequency (a) D0, (b) D3, (c) D5 and (d) D7 at 180, 220, 260 and 300 K.

3.6.2. Temperature-Dependent Dielectric Measurement Temperature dependent (170 to 400 K) ε′ at selected frequencies (100 Hz and 1 kHz) for D0, D3, D5 and D7 are shown in Fig. 13. It was observed that there is no significant change in ε′ up to 300 K. However, above 300 K ε′ was found to increase abruptly by increasing temperature for both the frequencies, which could be because of the thermally activated Fe3+ ions at octahedral sites [57]. By doping Dy3+ ions in ⍺-Fe2O3 lattice system, ε′ was observed to increase for D3. This may be attributed to the fact that at lower concentration (x = 0.03), all the Dy3+ ions occupy the octahedral sites, which are thermally activated by the increase in temperature. This leads to dielectric polarization, and hence enhances ε′ [33]. For higher dopant concentration, all the incorporated Dy3+ ions may not occupy octahedral sites, however, these may occupy interstitial sites in the lattice and may create hindrance in the movement of thermally activated ions at octahedral sites and hence reduces ε′ for D5 and D7. Apart from this, a sharp increase of ε′ was

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observed at 100 Hz compared to 1 kHz which could be because of space charge polarization effect at lower applied frequencies [58]. The temperature dependent tanδ at selected frequencies (100 Hz and 1 kHz) for D0, D3, D5 and D7 are shown in Fig. 14. The tanδ increases by increasing the temperature which may be attributed to more thermal fluctuations of Fe3+ ions in these NS. In addition, with the incorporation of Dy3+ ions into ⍺-Fe2O3 lattice, tanδ was observed to increase for D3 as compared to D5 and D7. The increase in tanδ for D3 could be due to more thermal fluctuations of thermally activated ions at octahedral sites for D3, as compared to D5 and D7, consistent with the ε′ behavior.

Fig. 13 Variation of ε' with temperature (a) D0, (b) D3, (c) D5 and (d) D7 at 100 Hz and 1 kHz.

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Fig. 14 Variation of tanδ with temperature (a) D0, (b) D3, (c) D5 and (d) D7 at 100 Hz and 1 kHz. Table IV: Dielectric parameters at 180, 220, 260 and 300 K

Sample

180 K

220 K

ε′

tanδ

20

3

260 K

ε′

20

3

tanδ

20

3

300 K

ε′

20

3

tanδ

20

3

ε′

20

3

tanδ

20

3

20

3

Hz

MHz

Hz

MHz

Hz

MHz

Hz

MHz

Hz

MHz

Hz

MHz

Hz

MHz

Hz

MHz

D0

23.15

6.55

1.79

0.03

27.22

9.38

2.29

0.02

74.50

9.44

3.35

0.02

434.15

3.82

2.12

0.19

D3

49.89

10.89

4.05

0.03

87.97

14.31

4.36

0.04

115.73

11.07

4.25

0.03

1237.06

6.55

3.60

0.19

D5

17.66

2.40

1.58

0.01

15.29

4.45

1.53

0.04

21.19

4.73

1.28

0.01

341.76

5.92

1.92

0.01

D7

8.72

1.6

0.55

0.01

7.52

0.73

1.10

0.01

16.65

2.85

0.90

0.01

198.99

4.75

0.84

0.12

Table V: Dielectric parameters at 100 Hz and 1 kHz Sample

100 Hz ε′

D0

1 kHz tanδ

ε′

tanδ

170 K

400 K

170 K

400 K

170 K

400 K

170 K

400 K

8.23

380.57

0.15

2.25

4.87

130.85

0.03

1.43

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3.7.

D3

30.34

956.21

0.17

2.36

6.15

499.32

0.04

2.05

D5

6.43

273.34

0.10

1.78

4.08

113.04

0.02

1.07

D7

4.74

96.75

0.05

1.16

2.12

34.73

0.01

0.77

Conductivity, Activation Energy and Motts Law

The conductivity measurements were done to explore the conduction mechanism and to analyze the activation energies of the synthesized NS. The variation of conductivity (σ) with temperature ranging from 170-400 K at selected frequencies (100 Hz and 1 kHz) for D0, D3, D5 and D7 are shown in Fig. 15. In the lower temperature region (170 to 300 K), there is no significant change in conductivity at both the frequencies. However, above 300 K, the conductivity was increased abruptly by increasing the temperature due to the increase in the mobility of thermally activated Fe3+ ions [57].The value of activation energies was determined from the slope of ln(σ) versus 1000/T using the below relation:

σ = σ° e

―E° kBT

+ σ1e

―E1 kBT

+ σ2e

―E2 kBT

+…

(8)

where, σ is the conductivity, σO, σ1, σ2,…are constants, T represents the temperature, kB represents Boltzmann’s constant, Eo represents the activation energy needed for intrinsic conduction and E1, E2,... represents the activation energies for hopping conduction [59, 60]. In the temperature range (400-250 K), the calculated activation energies at 100 Hz and 1 kHz denoted by Ea1 and Eb1 respectively, are of the order 0.5 eV as depicted in Fig. 16 and Fig. 17. However, in the temperature region (250-170 K), the activation energies at 100 and 1 kHz are denoted by Ea2 and Eb2 respectively and are in the range of 0.02 eV. The activation energies Ea1 and Eb1 confirm the semiconducting behavior of all NS at high temperatures (400-250 K). On the other hand, the calculated activation energies Ea2 and Eb2 in lower temperature region (250-170

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K), revealed the metallic nature of the synthesized NS which agrees with our earlier reports [29] and with systems derived from iron oxides [61]. The decrease in activation energy value in the low-temperature region could be attributed to small polaron hopping. The lower values of conductivity for D0, D3, D5 and D7 at both the frequencies indicate that the dominant conduction mechanism is the hopping conduction [62]. The hopping conduction occurs by nearest neighbour hopping or by variable range hopping. In the synthesized NS, the hopping conduction mechanism was supported by the Mott theory [63]. According to this theory, the conductivity varies with temperature given in the form below depicted in Fig. 18.

(―

σ = σ°e

1 T° 4 T)

(9)

From Fig. 18, it appears that all the synthesized systems obey Motts law, and confirming variable range hopping mechanism. Moreover, conductivity was observed to decrease beyond x = 0.03 dopant concentration at both the frequencies, which is consistent with the dielectric behavior. Apart from this, an increase in conductivity was observed at 1 kHz, as compared to 100 Hz. This is well supported by the decrease in activation energy at 1 kHz as compared to 100 Hz.

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Fig. 15 Variation of conductivity with temperature (a) D0, (b) D3, (c) D5, and (d) D7 at 100 Hz and 1 kHz.

Fig. 16 Plot of ln (σ) versus 1000/T (a) D0, (b) D3, (c) D5 and (d) D7 at 100 Hz.

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Fig. 17 Plot of ln (σ) versus 1000/T (a) D0, (b) D3, (c) D5 and (d) D7 at 1 kHz.

Fig. 18 Plot of ln (σ) versus (T)-1/4 (a) D0, (b) D3, (c) D5 and (d) D7 at 100 Hz and 1 kHz.

3.8.

UV-vis Spectroscopy

UV-vis spectroscopy was employed to determine the energy band gap. In iron oxide based systems, different types of electronic transitions are allowed by its structure which gives rise to a

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variety of colors in these systems. Other factors including particle size, shape, adsorbed impurities, and crystal defects (oxygen vacancies/interstitials) also significantly affect the color in these systems. Strong absorption in blue and UV spectral regions and strong reflection in red and infrared regions has been observed in the case of iron oxides [64]. The room temperature diffuse reflectance spectra of D0, D3, D5 and D7 in the range 200-900 nm are displayed in Fig. 19. From the figure, a nearly linear dependence of reflectivity from 200-500 nm and a shoulder near 620 nm was observed. An increase in the intensity of reflectivity occurs rapidly at about 500 nm with a maximum near 750 nm [27]. From 500 to 800 nm, D3, D5 and D7 display higher reflectance compared to D0, indicates lower absorption for Dy3+ ion doped systems compared to the pure system in this region. By doping Dy3+ ions into ⍺-Fe2O3 system, absorption edge was shifted to lower wavelength which possibly indicates an increase in optical band gap [65]. The Kubelka-Munk (KM) relation given below was employed to determine the optical band gap of the synthesized NS. F(R∞) =

(1 ― R∞)2 2R∞

K

=S

(10)

where F(R∞) represents the remission function, R∞ represents the limiting reflectance. S represents the scattering function and K represents the absorption function. The scattering function S becomes independent of wavelength if the material scatters in a perfectly diffused manner, and the KM function is proportional to the absorption coefficient (α) as given below: F(R∞) ∝ α ∝

(hν ― Eg)n hν

(11)

where n is 2 and ½ for allowed indirect band gap and allowed direct band gap transitions respectively [66, 67]. A plot of [F(R∞)hv]2 versus hv gives the allowed direct band gap as displayed in Fig. 20, wherein the extrapolation of straight-line to zero estimates the value of the

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band gap (Eg). The calculated band gap for the system D0 is 2.10 eV and is small compared to the bulk ⍺-Fe2O3 system (2.20 eV) [37, 65, 68]. In addition, the band gaps of D3, D5 and D7 were found to be 2.02, 2.24, and 2.37 eV respectively. The band gap of Dy3+ ion doped ⍺-Fe2O3 system increased for higher dopant concentrations (x = 0.05 and 0.07) due to the decrease in grain size for these doped systems as compared to undoped ⍺-Fe2O3 system [69]. However, an unusual decrease in band gap was observed for lower dopant concentration (x = 0.03) which has even the smallest grain size among D0, D3, D5 and D7 samples. This unusual behaviour could be attributed to the fact that at lower dopant concentrations, the intentional defects (oxygen vacancies) are created. These oxygen vacancies in the systems can exist as neutral (Vo), single (Vo-) or double (Vo2­) charged vacancies and give rise to several donor levels within the forbidden energy gap [70]. While as, in case of higher dopant concentrations all Dy3+ ions are not replacing Fe3+ ions, rather occupying interstitial sites and do not contribute to the defect levels within energy band gap. All these band gap values lie in the range of band-gap of semiconductors, confirming the semiconducting nature. As the optical band gap for visible light is 1-3 eV, the band gap of synthesized systems falls in the visible light (1-3 eV), suggesting that these nanomaterials can serve as photo-sensing material [71].

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Fig. 19 Diffuse reflectance spectra (a) D0, (b) D3, (c) D5 and (d) D7.

Fig. 20 KM band gap determination (a) D0, (b) D3, (c) D5 and (d) D7.

3.9.

Photoluminescence (PL)

Room temperature photoluminescence spectra of D0, D3, D5 and D7 are shown in Fig. 21. In αFe2O3 system, the luminescence mainly originates from electronic transitions allowed by its

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structure. The lowest exciton in this system arises from the excitation from the valence band to the conduction band. The valence band contains the hybrid orbitals of iron (3d) and oxygen (2p) states, and the conduction band contains iron (4s) states [37]. Upon excitation at 350 nm, an electron excites from the valence band to the conduction band and leaves a hole in the valence band. The movement of electron and hole in the conduction and valence band forms an exciton. The electron and hole recombination in the exciton yields a red emission. The emission intensity increases upon incorporating Dy3+ ions into iron oxide host lattice. This could be due to inherent luminescence observed in Dy3+ which arises from the intra-configurational 4f-4f transitions and is consistent with the literature [27, 64]. Luminescence of α-Fe2O3 is greatly enhanced by Dy3+ ion doping, therefore Dy3+ ion doped α-Fe2O3 NS can serve as an excellent luminescent material for optoelectronic devices.

Fig. 21 PL spectra (a) D0, (b) D3, (c) D5 and (d) D7.

3.10.

Magnetic Studies

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The factors on which the magnetic properties of materials depend include the morphology and crystal structure of the materials (including impurities or substitutions) [72]. As the size of ⍺Fe2O3 particles decrease to the nano range, they can reveal strange behavior of magnetism, quite different from the bulk ⍺-Fe2O3 system [73]. Fig. 22 shows the magnetization versus applied magnetic field (M-H) hysteresis loops of D0, D1, D2, D3, D5 and D7 at room temperature. The various magnetic parameters including remanence (Mr), coercivity (Hc), saturation magnetization (Ms), squareness ratio (Mr/Ms) and magnetic anisotropy (K) are summarized in Table VI. From M-H loops, it is clear that D0 (shown in the inset of the figure) does not saturate up to the maximum field strength of 20 kOe which is consistent with the literature [29, 74, 75]. To saturate the pure ⍺-Fe2O3 system, a minimum field of 60 kOe is required [76]. To our surprise we were able to attain saturation at very low applied fields with Dy3+ ion doping in ⍺-Fe2O3 system. With the incorporation of Dy3+ ions into ⍺-Fe2O3 system, all the doped systems attain saturation at an applied field of around 10 kOe. This decrease in the saturating field could be due to the decrease in antiferromagnetic coupling strength which originates due to the larger Dy-O-Fe bond length compared to Fe-O-Fe bond length [77, 78]. The Ms was observed to increase for D1, D2, D3, D5 and D7 as compared to the maximum magnetization of D0. The increase in magnetization with Dy3+ ion doping could be due to the replacement of lesser magnetic moment Fe3+ (5.92 �b) ions with higher magnetic moment Dy3+ (10.65 �b) ions in the ⍺-Fe2O3 system. However, the value of Ms decreased beyond x = 0.03 concentration of Dy3+ ions in ⍺-Fe2O3 system. This decrease at higher concentration of dopant ions in the system could be due to the decrease in super-exchange interaction between Fe3+ ions at octahedral sites which occurs because of the increase in bond length due to higher ionic radii of Dy3+ ions as compared to Fe3+ ions. Decrease in super exchange interaction causes canting spins or disordered spins on the surface of nanoparticles that

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prevent the core spins from aligning along the field direction resulting in decrease of the saturation magnetization at the surface of nanoparticles [79]. Apart from Ms, the Hc for smaller size samples were observed to be less as compared to the larger size samples. The reduced coercivity in doped samples could be due to the reason that the grains in the system subdivide into multi-domains supported by squareness ratio. Since the system is multi-domain, so the magnetic anisotropy constant (K) was determined by using the ‘Law of Approach’ analysis. As per the Law of Approach, near the saturation magnetization Ms, the magnetization M can be written as below [80]: b

M = Ms[1 ― H2]

(12)

K2

8

where, b = 105 × μ 2 M 2 , M represents the magnetization, Ms represents the saturation °

s

H magnetization, μ° = 4π × 10 ―7 M is the permeability of free space and K is the magneticanisotropy constant. The M-H curves at high field (> 10 kOe) depicted in Fig. 23 (D0) and Fig. 24 (D1, D2, D3, D5 and D7) were fitted to equation (12) to obtain the parameters b and Ms, and then the magnetic anisotropy constant ‘K’ listed in Table VI was calculated using the equation below: K = μ°Ms

105b 8

(13)

In comparison to D0, magnetic anisotropy was observed to increase for D1, D2, D3, D5 and D7.

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Fig. 22 Room temperature M-H loops (a) D0, (b) D1, (c) D2, (d) D3, (e) D5 and (f) D4.

Fig. 23 LA Fit (a) D0.

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Fig. 23 LA Fit (b) D1, (c) D2, (d) D3, (e) D5 and (f) D7. Table VI: Various magnetic parameters at room temperature

Sample

Hc (Oe)

Mr (emu/g)

Ms (emu/g)

Mr/Ms

K (erg/cm3)

D0

2644.44

0.052

0.371

0.14

1.25×104

D1

286.28

1.358

5.212

0.26

4.35×104

D2

251.80

1.661

6..158

0.27

5.29×104

D3

244.44

3.226

7.416

0.43

5.71×104

D5

288.88

1.973

4.911

0.40

5.44×104

D7

355.55

1.391

3.841

0.36

5.12×104

Conclusions Pure and Dy3+ ion doped ⍺-Fe2O3 NS were successfully synthesized using sol-gel auto combustion method. XRD reveals hexagonal crystal structure of the synthesized materials is retained. Decrease in average particles size was observed upon Dy3+ ion doping. FE-SEM micrographs display an increase in density with Dy3+ ion doping. IR spectral bands confirm the phase purity of these NS. Raman spectral features determine two A1g and five Eg modes in the

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synthesized NS with no structural change. At lower concentrations of Dy3+ ions into ⍺-Fe2O3 lattice, an increase in ε' was noticed. Above room temperature, the conductivity increases in all the synthesized systems which confirm semiconducting behavior. At higher dopant concentrations the decrease in conductivity occurs and the hopping conduction mechanism is dominant and was confirmed using Motts law. Band gap increases at higher Dy3+ ion concentrations and shows the semiconducting nature of the synthesized NS. Photoluminescence increases in Dy3+ ion doped ⍺-Fe2O3 systems. Doping of Dy3+ ions in ⍺-Fe2O3 NS makes these materials highly magnetized. Dy3+ ion doped ⍺-Fe2O3 NS attain high saturation magnetization at low applied fields. This study signifies that Dy3+ ion doped ⍺-Fe2O3 NS will inherit most of the properties of Dy3+ ions and transfers them to the host ⍺-Fe2O3 system. Doped ⍺-Fe2O3 NS have outstanding dielectric, optical and magnetic properties at a critical doping concentration of x = 0.03. Therefore, Dy3+ ion doped ⍺-Fe2O3 NS may possibly find their use in many technological applications such as microwave, optoelectronic and medical applications. Acknowledgment R. B. acknowledges UGC, Govt. of India, for Maulana Azad National Fellowship (MANF) vide reference number F1-17.1/2015-16/MANF-2015-17-JAM-49627. G. N. D. thanks the DST, Govt. of India, for financial support vide reference number DST/TM/WTI/2K16/248 (G). References [1]

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Highlights  Dielectric constant significantly increased at x = 0.03 in ⍺-DyxFe2-xO3 system.  Luminescence was observed to enhance with Dy3+ ion doping in ⍺-Fe2O3 system.  A considerable increase in magnetization was observed at x = 0.03 concentration.  Dy3+ ion doped systems were saturated at low fields (10 kOe) compared to pure system.