Structural, EPR, optical and magnetic properties of α-Fe2O3 nanoparticles

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 512–518

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Structural, EPR, optical and magnetic properties of a-Fe2O3 nanoparticles A.A. Jahagirdar a, N. Dhananjaya b, D.L. Monika c, C.R. Kesavulu d, H. Nagabhushana c,⇑, S.C. Sharma c, B.M. Nagabhushana e, C. Shivakumara f, J.L. Rao d, R.P.S. Chakradhar g a

Department of Chemistry, Ambedkar Institute of Technology, Bangalore 560 056, India Department of Physics, B.M.S. Institute of Technology, Bangalore 560 064, India c C.N.R. Rao Centre for Advanced Materials Research, Tumkur University, Tumkur 572 103, India d Department of Physics, Sri Venkateswara University, Tirupathi 517 502, India e Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore 560 054, India f Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India g CSIR-National Aerospace Laboratories, Bangalore 560 017, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" Dumbbell shaped

a-Fe2O3 nanoparticles with rhombohedral phase have been prepared. " Spectroscopic investigations on aFe2O3 nanoparticles were carried out. " From TL glow peaks trapping parameters were evaluated and discussed.

Packing diagram of a-Fe2O3 nanoparticles.

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 29 April 2012 Received in revised form 18 September 2012 Accepted 22 September 2012 Available online 2 October 2012 Keywords: Nanostructures Chemical synthesis Crystal structure Electron microscopy Luminescence

a-Fe2O3 nanoparticles were synthesized by a low temperature solution combustion method. The structural, magnetic and luminescence properties were studied. Powder X-ray diffraction (PXRD) pattern of

a-Fe2O3 exhibits pure rhombohedral structure. SEM micrographs reveal the dumbbell shaped particles. The EPR spectrum shows an intense resonance signal at g  5.61 corresponding to isolated Fe3+ ions situated in axially distorted sites, whereas the g  2.30 is due to Fe3+ ions coupled by exchange interaction. Raman studies show A1g (225 cm1) and Eg (293 and 409 cm1) phonon modes. The absorption at 300 nm results from the ligand to metal charge transfer transitions whereas the 540 nm peak is mainly due to the 6 A1 + 6A1 ? 4T1(4G) + 4T1(4G) excitation of an Fe3+–Fe3+ pair. A prominent TL glow peak was observed at 140 °C at heating rate of 5 °C s1. The trapping parameters namely activation energy (E), frequency factor (s) and order of kinetics (b) were evaluated and discussed. Ó 2012 Elsevier B.V. All rights reserved.

Introduction

⇑ Corresponding authors. Tel.: +91 9945954010. E-mail address: [email protected] (H. Nagabhushana). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.09.069

In recent years, the synthesis of oxide nanoparticles have received considerable attention due to its unique electrical, optical and magnetic properties [1,2] and has become the focus of modern

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materials science. Haematite (a-Fe2O3) is the most stable iron oxide and environmentally friendly semiconductor (band gap of 2.1 eV) having different forms viz., a-Fe2O3, maghemite (c-Fe2O3) and magnetite (Fe3O4). Among this a-Fe2O3 has the corundum structure, while the other two have the cubic structure [3]. aFe2O3 has special properties due to the potential applications in catalysts, gas sensors, high density magnetic recording media, printing ink, ferro fluids, negative temperature coefficient of resistance, high resistivity to corrosion, magnetic resonance imaging and especially biomedical field [4–9]. Various methods have been reported for the synthesis of oxide nanostructures [10–16]. Among the available chemical methods, self-sustaining combustion synthesis is simple in process, low cost and time saving and energy consumption in comparison to the techniques mentioned above. Further, combustion synthesized powders are generally more homogeneous than the powders prepared by conventional solid-state method. This process utilizes the exothermicity of the redox reaction between an oxidizer and a fuel to directly crystallize the oxide phase from the molecular mixture of the precursor solution. In this study, we report a simple procedure to synthesize aFe2O3 nanoparticles by a low temperature solution combustion method. The nanoparticles are well characterized for their structural and magnetic properties by various spectroscopic techniques such as electron paramagnetic resonance (EPR), Raman spectroscopy, UV–Visible, photoluminescence (PL) and thermoluminescence (TL) techniques. From the TL glow curves the trap parameters such as activation energy (E), frequency factor (s) and order of kinetics (b) were evaluated and discussed. To the best of our knowledge, very limited reports are available on the synthesis, thermoluminescence and photoluminescence properties of aFe2O3 nanoparticles.

Characterization The phase purity of the nanoparticles have been characterized by powder X-ray diffractometer (PANalytical X0 Pert Pro) using Cu Ka (k = 1.541 Å) radiation with a nickel filter. The surface morphology of the powders was examined by scanning electron microscope (SEM), using a JEOL (JSM-840A). The infrared spectrum was recorded using a Perkin–Elmer spectrometer with KBr pellet. Raman studies was carried out on Renishaw In-via Raman spectrometer with 633 nm He–Cd laser and a Leica DMLM optical microscope equipped with 50 objective, thus providing a laser spot of 2 lm in diameter. The UV–Vis absorption of the samples was recorded on SL 159 ELICO UV–Vis Spectrophotometer. The magnetic properties were investigated using vibrating sample magnetometer (VSM). Magnetic hysteresis measurement was carried out at room temperature in an applied magnetic field sweeping from 10,000 to 10,000 Oe. The photoluminescence (PL) measurements were performed on a Shimadzu Spectroflourimeter (Model RF 510) equipped with 150 W Xenon lamp as an excitation source. EPR spectra were recorded on an EPR spectrometer (JEOL-FE1X) operating in the X-band frequencies with a field modulation of 100 kHz. Microwave frequency was kept at 9.205 GHz. Thermoluminescence (TL) measurements were carried out on a TL Reader (model Nucleonix) with a heating rate of 5 °C s1 in the temperature range from 0 to 250 °C. Before measurement, 0.025 g powder samples were pressed into pellets (8 mm diameter and 1 mm thickness), then exposed for 5–30 min to a standard UV lamp peak at 254 nm with a power of 15 W. All the measurements were carried out at room temperature (RT). Results and discussion X-ray diffraction

þ 5N2 ðgÞ 10:0mol of gases liberated=mol ofFe2 O3

where B (FWHM in radian) is measured for different XRD lines corresponding to different planes, e is the strain developed and D is the grain size. The equation represents a straight line between 4 sin h

1800 1200 600

(024)

(104)

2400

(116) (018) (214) (300)

! Fe2 O3 ðsÞ þ 2CO2 ðgÞ þ 3H2 O ðgÞ

ð2Þ

(113)

2FeðNO3 Þ3 ðaqÞ þ C2 H6 N4 O2 ðaqÞ

B cos h ¼ eð4 sin hÞ þ k=D

(110)

The starting chemicals used were of analar grade Ferric nitrate (Fe(NO3)39H2O) and oxalyl dihydrazide (C2H6N4O2; ODH). The ODH was used as a fuel, which is prepared in our laboratory. The detailed synthesis procedure was given elsewhere [17]. An aqueous solution containing stoichiometric amounts of ferric nitrate Fe(NO3)39H2O and ODH in 2:1 ratio were taken in a Petri dish of approximately 300 ml capacity. The excess water is allowed to evaporate by heating over a hot plate until a wet powder is left out. Then the Petri dish is introduced into a pre heated muffle furnace maintained at 300 ± 10 °C. The reaction mixture undergoes thermal dehydration and ignites at one spot with liberation of gaseous products such as oxides of nitrogen and carbon. The combustion propagates throughout the reaction mixture without further need of any external heating, as the heat of the reaction is sufficient for the decomposition of the redox mixture. In low temperature solution combustion synthesis process, ODH acts as a fuel and it is oxidized by nitrate ions, the exothermic reaction can be expressed as follows:

(012)

Synthesis of a-Fe2O3

Fig. 1 shows the powder X-ray diffraction pattern of as-prepared Fe2O3 sample at room temperature. All the diffraction peaks were readily indexed to a pure rhombohedral phase [space group: R-3c (167)] of a-Fe2O3 (JSPDS card No. 87-1165) with a = 5.035 Å and c = 13.749 Å. The diffracted patterns are well matched with the literature [18]. No impurity peaks were observed. Further, the strong and sharp diffraction peaks confirm the high crystallinity of the products. It is known that X-ray diffraction line broadening is influenced by the crystallite size and the internal strains. The crystallite size was calculated by Scherrer’s formula [19] and is found to be approximately 44 nm. The grain size was also estimated from the powder X-ray diffraction line broadening (B) using the analysis described by Williamson and Hall (W–H) method [20]:

Intensity (a.u)

Experimental

JCPDS 87-1166

0

ð1Þ 10

For combustion method, it is well known that the morphological characteristics of the product are strongly dependant on number of moles of gases liberated.

20

30

40

50

60

70

2θ (degree) Fig. 1. PXRD pattern of a-Fe2O3 nanoparticles.

80

90

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60 3400

% Transmittance

70

50

40

536

462

30

20

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 2. Rietveld analysis of a-Fe2O3 nanoparticles.

Fig. 3. FTIR spectrum of a-Fe2O3 nanoparticles.

(X-axis) and B cos h (Y-axis). The slope of the line gives the inhomogeneous strain (e) and intercept (k/D) of this line on the Y-axis gives grain size (D). The crystallite size and strain evaluated by W–H method are found to be 48 nm and 2.5134  104 respectively. The crystallite size determined from W–H method is slightly different from those calculated using Debye–Scherer’s method. The small variation in the values is because in Scherrer’s formula strain component is assumed to be zero and observed broadening of diffraction peak is considered because of reducing crystallite size only. In order to estimate the actual cell parameters Rietveld refinement was performed on the a-Fe2O3 nanoparticles and is shown in Fig. 2. The Rietveld refinement is a method in which the profile intensities obtained from step-scanning measurements of the powders allow to estimate an approximate structural model for the real structure. In our work, the Rietveld refinement was performed through the FULLPROF program [21]. We utilize the pseudo-Voigt function in order to fit the several parameters to the data point: one scale factor, one zero shifting, four back ground, three cell parameters, five shape and width of the peaks, one global thermal factors and two asymmetric factors. A typical analysis of the a-Fe2O3 nanoparticles in Fig. 2 presents the experimental and calculated PXRD patterns obtained by the Rietveld refinement. The packing diagrams of corresponding a-Fe2O3 nanoparticles after Rietveld refinement are shown in Fig. S1. The refined parameters such as occupancy and atomic functional positions of the aFe2O3 nanoparticles are summarized in Table 1. The fitting parameters (Rp, Rwp and v2) indicate a good agreement between the refined and observed PXRD patterns for the a-Fe2O3 nanoparticles.

strongly dependent on the heat and gases generated during the complex decomposition. Large amount of gases are suitable for preparation of tiny particles while the heat released is an important factor for crystal growth. The agglomeration of nanoparticles is usually explained as a common way to minimize their surface free energy; however, some authors have reported that the agglomeration is assigned to the presence of organic radicals that act as binders. The voids and pores present in the sample are due to large amount of gases produced during the combustion synthesis [22]. Fourier transform infrared (FTIR) and Raman spectroscopy The formation of a-Fe2O3 nanoparticles was further confirmed by FTIR spectroscopy. Fig. 3 shows FTIR spectrum of as formed aFe2O3 sample at room temperature. The spectrum shows two absorption bands of Fe–O at 536 and 450 cm1. The absorption band at 3400 cm1 is due to water [23]. Fig. 4 shows the Raman spectrum of a-Fe2O3 recorded at room temperature using 514 nm 6 excitation wavelength. a-Fe2O3 belongs to the D3 d crystal space group and seven phonon lines are expected in the Raman spectrum, namely two A1g phonon modes and five Eg phonon modes [24]. The peak at 225 cm1 is associated with the A1g phonon mode while the peaks at 293 and 409 cm1 are related to Eg phonon modes. Similar results have been reported in a-Fe2O3 nanoparticles [25]. Xu et al. [26] recorded Raman spectrum of a-Fe2O3 nano leaves synthesized by oxygenating pure iron. They observed Ag (225, 498 cm1), Eg (252, 293, 411, 612 cm1).

Scanning electron microscopy

UV–Vis spectroscopy

Fig. S2 shows the SEM micrograph of combustion-derived aFe2O3 nanoparticles, which show voluminous, weakly agglomerated dumbbell shape. For combustion method, it is well known that the morphological features of the prepared powders are

Nanostructured materials usually exhibit blue-shift phenomena in optical properties compared with the bulk counterparts. To reveal the electronic structure and size effect, UV–Vis absorption measurement of the as-prepared a-Fe2O3 nanoparticles was car-

Table 1 Rietveld refined structural parameters for a-Fe2O3 nanoparticles. Atoms

Fe1 O1

Oxidation State

+3 2

Wyckoff notation

12c 18e

Positional parameters x

y

z

0.0000 0.3113(9)

0.0000 0.0000

0.3551(2) 0.2500

Biso

Occupancy

0.10 0.50

1 1

Crystal system = Rhombohedral, Lattice parameter, a = 5.037(5) Å, c = 13.757(9) Å, Cell volume = 302.36(4) Å3. Space group = R  3c (167) RFactor; Rp = 0.824, Rwp = 0.12, RBragg = 0.035, RF = 0.029.

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where ‘a’ is optical absorption co-efficient, (hm) is the photon energy, and Ed is the band gap energy for direct transitions. The plot of (ahm)2 vs photon energy is shown in Fig. 5 (inset). The direct optical band gap was found to be Ed = 2.22 eV. This value is in good agreement with those reported in literature [26].

612

0.08

0.04

Magnetic hysteresis measurement

499

293

409

0.06

Magnetic hysteresis measurements were carried out for aFe2O3 nanoparticles at room temperature in an applied magnetic field sweeping from 10,000 to 10,000 Oe. Fig. 6 shows the hysteresis loop of the a-Fe2O3 nanoparticles. It can be seen that saturation was observed around 3000 Oe with a magnetic moment of 7.5 emu g1 and a coercivity of 240 Oe at room temperature (300 K). The magnetic properties of materials are influenced by many factors, such as size, crystallinity and surface structure [31]. The inset of this Fig. 6 shows a magnified view of the hysteresis loop recorded for the a-Fe2O3 nanoparticles.

225

Raman Intensity (a.u)

0.10

0.02 200

300

400

500

600 -1

Wavenumber (cm ) Fig. 4. Raman spectrum of a-Fe2O3 nanoparticles.

Photoluminescent studies ried out. According to the literature [27,28], two absorption regions are expected between 200 and 600 nm. The absorption at 250– 350 nm centered at 300 nm (region 1) and the broad absorption at 400–600 nm with peak at 540 nm (region 2) are observed and is shown in Fig. 5. The first region mainly results from the ligand to metal charge transfer transitions and partly from the contribution of the Fe3+ ligand field transitions 6A1 ? 4T1(4P). In the second region the absorption peaks are mainly due to the 6 A1 + 6A1 ? 4T1(4G) + 4T1(4G) excitation of an Fe3+–Fe3+ pair, possibly overlapped the contributions of 6A1 ? 4E, 4A1(4G) ligand field transition and the charge-transfer band tail. Xu et al. [26] reported similar results in a-Fe2O3 nano leaves synthesized by oxygenating pure iron. Further, Zhang et al. [29] recorded the absorption spectrum of a-Fe2O3 nanoparticles suspended in ethanol. The absorption bands at 310 and 414 nm are assigned to the 6 A1 ? 4T1(4P) and 6A1 ? 4T2, while the absorption bands in the visible region near 580 nm and 681 nm are assigned to the 6 A1 + 6A1 ? 4T1(4G) + 4T1(4G) double exciton process (DEP) and 6 A1 ? 4T2(4G) ligand field transitions of Fe3+ respectively. The direct band gap energy (Ed) of a-Fe2O3 nanoparticles was estimated using Tauc relation [30]:

aa

ðhm  Ed Þ1=2 hm

ð3Þ

Fig. S3 shows the PL studies on a-Fe2O3 nanoparticles excited with Ar ion laser (514.5 nm) beam. A broad and intense emission band centered at 760 nm and a weak shouldered peak at 886 nm was observed. a-Fe2O3 does not show PL emission in its bulk form due to the local d-band transition nature and efficient energy relaxation [32]. In nanosized particles the increase of Fe–O bonding separation, resulting in enhancement of the magnetic coupling of the neighboring Fe3+, which is responsible for PL [33]. A similar PL spectrum has been reported for iron oxide nanoparticles prepared by laser oxidation of Fe catalysts in carbon nanotubes [34]. Zou and Volkov [35] investigated the optical spectral characteristics of the chemically synthesized various iron oxide nanocrystals with different sizes. They observed the different optical responses in different energy and time scales, from local d–d transitions to delocalized pair excitations and charge-transfer transitions by the femtosecond time-resolved absorption spectra. They observed that the emission peak position and the PL intensity are significantly affected by the nanocrystal size and surface morphology. Zhang et al. [29] observed PL spectra from the as prepared aFe2O3 nanoparticles excited by 514 nm laser. A broad and intense emission band centered at 750 (1.65 eV) and a weak shoulder band at 890 nm were observed. Fei et al. [36] observed a dominating band edge emission at 580 nm under 514 nm excitation; the

10

(a)

11

1.2x10

10

8.0x10

10

4.0x10

Ed= 2.22 eV 0.0 1

0.5

2

3 hυ (eV)

4

5

0 -1

11

1.6x10

Magnetic moment (emug )

1.0

Magnetic moment : 7.5 emug Coercivity : 240 Oe

540 650 Magnetic moment (emu/g)

1.5

-1

300

(αhυ)2 (eV cm -1 )2

Absorbance (a.u)

2.0

-5

5

5

0

-5

-1500

-1000

-500

0

500

1000

1500

Applied field (Oe)

200

400

600

800

1000

Wavelength (nm) Fig. 5. UV–Vis absorption spectrum of a-Fe2O3 nanoparticles. (Inset: (a) Direct band gap.)

-10 -10000

-5000

0 Applied field (Oe)

5000

10000

Fig. 6. Room temperature magnetic hysteresis loop of a-Fe2O3 nanoparticles. (Inset: Magnified view of the hysteresis loop.)

A.A. Jahagirdar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 512–518

emission band has a large redshift to750 nm in 20 nm sized particles. Therefore, the observed emission peaks in the present study believed to be responsible for the unique structure of a-Fe2O3 due to quantum confinement and surface state effect in nanoscale [37]. Electron paramagnetic resonance studies (EPR) Fig. S4 shows the EPR spectrum of a-Fe2O3 nanoparticles observed at room temperature. The EPR spectrum exhibits an intense resonance signal at g  5.61 and g  2.30. The Fe3+ ions belong to d5 configuration with 6S as ground state in the free ion, and there is no spin-orbit interaction [38]. The g value is expected to lie very near the free-ion value of 2.0023. However, g value very much greater than 2.0 often occurs and the large g values arise when certain symmetry elements are present. The theory of these large g values is usually expressed by the spin-Hamiltonian [39]:

H ¼ gbBS þ D½S2z  fSðS þ 1Þ=3g þ EðS2x  S2y Þ

ð4Þ

where S = 5/2. Here D and E are the axial and rhombic structure parameters [40]. When Fe3+ impurity complexes are situated in crystal field with a large axial component, the free ion 6S state splits into three Kramers doublets |±1/2i, |±3/2i and |±5/2i. In the present study, the EPR spectrum exhibits intense resonance signals at g  5.61 and g  2.30. Goldfarb et al. [40] attributed the signals at g  4.3 and g > 6 which arise from Kramers doublets |±1/2i transition of isolated Fe3+ sites in strong rhombic or axial distortion. The EPR spectra of Fe3+ ions exhibit a series of resonance signals around g = 6.0 [41]. The resonance signal at g  5.6 arises from middle Kramers doublet [42]. A similar signal is reported by Jajoo et al. [43] and Berrier et al. [44]. The resonance signal around g = 6.0 is characteristic for isolated Fe3+ ions situated in axially distorted sites [45]. The signal around g = 2.30 arises from pairs of small clusters of exchange coupled Fe3+ ions [46]. It is reported in literature [47] that if the Fe2O3 particles are in the few nanometer range a broad line at g  2.3 will be observed. Piazzesi et al. [47] reported for pure iron oxide a resonance signal at g  2.3. Therefore, the observed resonance signal at g  2.3 in this study justifies that our prepared Fe2O3 is in nanometer range. The number of spins (N) participating in resonance for g  5.61 resonance signal can be determined by numerical double integration method with the help of a reference (CuSO45H2O) using the formula [48]:

N¼

Ax ðScanx Þ2 Gstd ðBm Þstd ðg std Þ2 ½SðS þ 1Þstd ðPstd Þ1=2 Astd ðScanstd Þ2 Gx ðBm Þx ðg x Þ2 ½SðS þ 1Þx ðPx Þ1=2

½Std

ð5Þ

The EPR spectra were recorded at different temperatures and the number of spins were calculated as a function of temperature. A graph is drawn between log N versus 1/T and is shown in Fig. S5a. It is observed that as temperature is lowered, the population of spin levels increases and a linear relationship between log N and 1/T is observed. This is in accordance with the Boltzmann law. The activation energy can be calculated from the slope of the straight line. The activation energy thus calculated is found to be 0.015 eV. The magnetic susceptibility (v) of the paramagnetic ion Fe3+ has been calculated at different temperatures using the expression [49]:

v¼

Ng 2 b2 JðJ þ 1Þ 3kB T

ð6Þ

where N is the population of spin levels/kg, which can be calculated from Eq. (5), g = 5.61 and J = 5/2. Fig. S5b shows the reciprocal of susceptibility against absolute temperature. With increasing temperature, the susceptibility of the sample decreases obeying the

Curie–Weiss law. From this graph the Curie, constant was evaluated and is found to be 367  103 emu mol1. Thermoluminescence studies (TL) The dose response of a-Fe2O3 nanoparticles was studied as a function of irradiation time between 5 and 30 min using UV irradiation. Fig. S6 shows some of the selected glow curves of a-Fe2O3 nanoparticles after different UV dose levels. All the samples were recorded at room temperature with a heating rate of 5 °C s1. It is seen from the glow curve that the structure remains constant without any observable change of the glow curves for different periods. It was reported in literature [50,51], that the luminescence process in semiconductor nanoparticles is very complex phenomena and it was attributed that the luminescence of semiconductor nanoparticles is generally arising from the deep traps of surface states whose energy levels lie within the band gap of semiconductor. In nanoparticles, the ions at the surface of samples quickly increase as the particle size is decreased. The excited electrons and holes from the surface ions are easily trapped at the surface states. So, the trapped carriers at the surface states are released by heating the sample and they recombine with each other and gives out luminescence. The variation in TL intensity as a function of UV dose was plotted and is shown in Fig. 7. It is observed that TL glow peak intensity increases with UV dose. This is due to the number of surface ions and surface states increase due to the increase in the surface to volume ratio with decreasing size of the particles; so the number of trapped charged particles at surface state increases; thus TL efficiency is increased [52]. Therefore, the TL of a-Fe2O3 nanoparticles may be related with the surface states. The influence of different heating rates between 2 and 15 °C s1 on TL response has been investigated on Fe2O3 sample, UV irradiated for a dose of 5 min and results are shown in Fig. S7. It is found that with the increase in heating rates the TL peak intensity and area under the peak decreases (inset Fig. S7), however, peak temperature shifts towards higher temperature side. The glow peak (Tm) is shifted from 142 to 152 °C with increase of heating rate from 2 to 15 °C s1. The decrease in intensity with increase in heating rate is a phenomenon frequently observed in the practice of TL and it has been suggested to thermal quenching [53]. Analysis of trapping parameters (E, b, s) The TL kinetic parameters of a-Fe2O3 nanoparticles were calculated by the peak shape method. It is known that the geometrical factor lg of a TL peak is determined using [54]:

250

TL Intensity (a.u)

516

200 150 100 50 0 5

10

15

20

25

30

UV- exposure (min) Fig. 7. Variation of TL glow peak intensity with respect to UV-exposure (5–30 min) of a-Fe2O3 nanoparticles.

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Peak

Peak temperature Tm (°C)

Geometrical form factor lg

Order of kinetics (b)

Activation energy Eave (eV)

Frequency factor (s) (s1)

5

1 2 3

101 156 149

0.52 0.51 0.51

2 2 2

0.58 0.28 0.55

5.4  1007 5.6  1002 2.0  1006

10

1 2 3

113 151 188

0.51 0.52 0.51

2 2 2

0.53 0.75 0.68

3.7  1006 7.0  1008 2.2  1007

15

1 2 3

121 151 183

0.51 0.52 0.52

2 2 2

0.45 0.58 0.84

2.9  1005 6.5  1006 2.0  1009

20

1 2 3

94 148 221

0.51 0.51 0.52

2 2 2

0.54 0.41 2.03

2.5  1007 3.6  1004 8.4  1020

25

1 2 3

88 138 201

0.52 0.52 0.51

2 2 2

0.71 0.43 1.84

7.7  1009 1.1  1005 6.3  1019

30

1 2 3

82 134 206

0.51 0.52 0.51

2 2 2

0.61 0.64 1.20

5.3  1008 7.2  1007 5.3  1012

lg ¼

ðT 2  T m Þ ðT 2  T 1 Þ

ð7Þ

Here, T1, Tm and T2 represent the temperature of half-intensity at low temperature side, peak temperature and temperature of half-intensity at high-temperature side of TL peak, respectively. In Fig. S8, the T1, Tm and T2 are 100, 136 and 171 °C respectively for peak 2. Using above values in Eq. (7), the geometrical factor is found to be 0.5, indicating that it obeys the second-order kinetics. Therefore, the activation energy (E) can be estimated with the following equation: 2

E ¼ cc ðkT m =cÞ  bc ð2kT m Þ

ð8Þ

Here c stands for s, d, or x which are respectively determined by low temperature half-width (s = Tm  T1), high-temperature halfwidth (d = T2  Tm) and full width (x = T2  T1); k is the Boltzmann constant. For second-order kinetics, the values of the cc depending on s, d, or x are 1.81, 1.71 and 3.54 respectively; and the values of the bc depending on s, d, or x are 2.0, 0 and 1.0 respectively. Therefore, according to Eq. (8), the value of the activation energy (E) can be calculated, further the value of frequency factor ‘s’ can be obtained using E, the known values of b (2 for second-order kinetics) and b (heating rate, 5 °C s1) into the following equation:

    2kT m E ¼ s 1 þ ðb  1Þ exp  2 kT m E kT m bE

evaluated from the EPR data at various temperatures. PL measurement shows two emission bands at 760 nm and 890 nm. The 300 nm peak in the UV–Vis spectrum can be attributed to the ligand to metal charge transfer transitions and partly from the contribution of the Fe3+ ligand field transitions 6A1 ? 4T1(4P) and 540 nm peak is mainly due to the 6A1 + 6A1 ? 4T1(4G) + 4T1(4G) excitation of an Fe3+–Fe3+ pair. The dose response was studied as a function of UV irradiation time from 5 to 30 min, at the heating rate of 5 °C s1. The influence of different heating rates from 2 to 15 °C s1 has been studied. The glow peak (Tm) is shifted from 142 to 152 °C with increase of heating rate. Further decrease in intensity with increase in heating rate. The kinetic parameters E, s and b are calculated by Chen’s method. Acknowledgments The author HN thanks Department of Science and Technology (DST), New Delhi, India for providing financial assistance under Project No. SR/NM/NS-48/2010. Prof. J.L. Rao is highly thankful to University Grants Commission (New Delhi) for the award of Emeritus Fellowship. The authors are grateful to TEQIP Lab (Chemistry) of M.S. Ramaiah Institute of Technology, Bangalore for providing facilities for preparation of materials.

ð9Þ

The calculated values of E, s and b are given in Table 2.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.09.069.

Conclusions Dumbbell shaped a-Fe2O3 nanoparticles with rhombohedral phase have been prepared by a low temperature solution combustion method. The average crystalline size calculated by Debye– Scherer’s and Williamson–Hall plots was found to be 44 and 48 nm respectively. The Raman spectrum shows A1g and Eg type phonon modes. The EPR spectrum shows resonance signals at g  5.61 and g  2.30. The resonance signal at g  5.61 can be attributed to isolated Fe3+ ions situated in axially distorted sites, whereas the resonance signal at g = 2.30 arises from pairs of small clusters of exchange coupled Fe3+ ions. It is observed that the variation of N with temperature obeys Boltzmann law. A linear relationship is observed between 1/v and T in accordance with Curie–Weiss law. The paramagnetic susceptibilities (v) were

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