Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Author’s Accepted Manuscript Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ballmilling technique Chandana Roy Chaudhury, Anirban Roychowdhury, Anusree Das, Dipankar Das www.elsevier.com/locate/jpcs

PII: DOI: Reference:

S0022-3697(16)30014-2 http://dx.doi.org/10.1016/j.jpcs.2016.01.014 PCS7713

To appear in: Journal of Physical and Chemistry of Solids Received date: 10 November 2015 Accepted date: 18 January 2016 Cite this article as: Chandana Roy Chaudhury, Anirban Roychowdhury, Anusree Das and Dipankar Das, Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2016.01.014 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 galley proof before it is published in its final citable 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.

Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique Chandana Roy Chaudhurya, Anirban Roychowdhuryb, Anusree Dasb, Dipankar Dasb,* a

Department of physics, Kalna College, Kalna-713409, Burdwan, India

b

UGC-DAE Consortium For Scientific Research, III/LB-8, Bidhannagar, Kolkata-98, India

Abstracts Magnetic-fluorescent nanocomposites (NCs) with 10 wt. % of α-Fe2O3 in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe2O3 and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 hr. XRD analysis confirms presence of α-Fe2O3 and ZnO in pure form in all the NCs. UV-vis study of the NCs shows a continuous blue shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect- related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc

and

ac

magnetization

measurements

show

presence

superparamagnetic (SPM) α-Fe2O3 particles in the NCs.

57

of

weakly interacting

Fe-Mössbauer study confirms

presence of SPM hematite in the sample milled for 30 hr. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies. Keywords: Nanocomposites; hematite; zinc oxide; Mössbauer spectroscopy; optical studies *

Corresponding author’s e-mail: [email protected] (D. Das); Tel.: +91 33 2335 1866;

Fax: +91 33 2335 7008. 1

1. Introduction Multifunctional materials having multiple properties e.g. dielectric and magnetic, magnetic and optical, magnetic and elastic etc. united in the same phase have attracted tremendous research interest because of their diverse technological applications [1]. Nanocomposites (NCs) belong to this class of materials and are widely studied due to their exotic properties and potential applications in different fields. With reduction of particle size to nanometer regime physical, mechanical, optical, magnetic and electrochemical properties of a material drastically change and many interesting phenomena like quantum confinement, quantum tunnelling, etc. become significant. In recent decades, various semiconductor based hetero-nanostructures e.g. ZnO/SnO2 [2], SnO2/α-Fe2O3 [3] and TiO2/SnO2 [4] have been synthesized and shown to have interesting photocatalytic applications. Progress has also been made in synthesis of other composite nanostructures, such as SnO2/α-Fe2O3 [3], ZnO/α-Fe2O3 [5], CuO/SnO2 [6], TiO2/ZnO [7], ZnO/Mn2O3 [8], for applications in gas sensing, magnetics, optics, solar cells and supercapacitors. Alpha iron oxide (α-Fe2O3) also known as hematite is one of the most studied iron based oxides because of its wide applications as photoelectrodes, gas sensors and anti-corrosive agents [9]. Alpha Fe2O3 is strongly antiferromagnetic below the Morin transition temperature (TM= 260 K) and weakly ferromagnetic above that. ZnO is a wide band gap semiconductor (Eg ~3.37 eV) having high excitonic binding energy (60 meV) and shows very strong optical properties in the UV-visible range and is extensively used for spintronic, catalytic, electrical, optoelectronic and photochemical applications. It also shows antibacterial, anticorrosive, antifungal and UV filtering properties and hence is used considerably in medicine and ceramic industries [10] Synthesis of nanocomposites comprising α-Fe2O3 and ZnO is an interesting way to combine optical properties of ZnO and 2

magnetic properties of α-Fe2O3 in a single material. Moreover, it has been reported by J Zhang et al. [11] that gas sensing properties of α-Fe2O3 gets enhanced when it is associated with ZnO to form a nanocomposite. Hence a systematic investigation on α-Fe2O3@ZnO nanocomposites is worth pursuing. In the present paper, α-Fe2O3/ ZnO NCs with 10 at wt. % of α- Fe2O3 has been prepared by the high energy ball milling method. The duration of milling was varied to obtain NCs with different sizes of the constituents. Structural, optical, magnetic and hyperfine properties of the NCs are studied by XRD, UV-vis spectroscopy, photoluminescence (PL), SQUID magnetometry, 57Fe-Mössbauer spectroscopy and Positron annihilation lifetime spectroscopy (PALS). The present work demonstrates that nanocomposites of α-Fe2O3/ ZnO having both magnetic and optical properties can be prepared in bulk amount using a simple ball milling technique. The presence of different kind of defects in the nanocomposites are shown to modify their optical properties significantly whereas magnetic properties are found to depend strongly on nature of interparticle interactions. 2. Experimental 2.1 Materials and method For the preparation of α-Fe2O3@ZnO NCs, hematite (α-Fe2O3) and white zinc oxide (ZnO) powders of average particle size of 60 nm procured from Merck-Germany were used. Both the chemicals were of analytical grade and were used without further purification. Nanocomposite samples were prepared by using a Fritsch Pulverisette 7 planetary ball mill. Calculated amount of α-Fe2O3 (10 atomic wt. %) and ZnO powders (90 atomic wt. %) were taken in an 80 cc stainless-steel vial along with required number of 10 mm stainless-steel ball and the mixture was milled in argon atmosphere without any additives (dry milling). The milling was carried out at a speed of 300 rpm for 2, 5, 10, 20 and 30 hour keeping ball to powder mass ratio 10:1. After every 1 hour milling the machine was left to rest for half an 3

hour for dissipation of heat generated during milling and fresh argon gas was purged in the vial every time before resumption of milling. Samples were marked according to milling duration i.e. FZ2 denotes the sample that was milled for 2 hour. 2.2 Characterization techniques Phase purity and structural properties of the milled samples were checked by a Bruker D8 Advance x-ray diffractometer using the Cu-Kα radiation of wavelength 1.5406Å (step-size= 0.02o and scan duration = 2 sec/step). Rietveld powder structure refinement analysis of the xray diffraction data was done by using the JAVA based software program MAUD to obtain structural refinement parameters through a least-squares method. Morphological properties of the samples were determined by a FEI-Inspect F50 field emission scanning electron microscope (FESEM). Optical properties of the nanocomposites were investigated by UVvisible spectroscopy (UV-1601PC Shimadzu, Japan) and photoluminescence (PL) spectroscopy (Perkin-Elmer LS-55). Dc and ac magnetic properties were measured by a SQUID magnetometer (MPMS XL 7, Quantum Design, USA).

57

Fe Mössbauer spectra at

room temperature were recorded in the transmission geometry with a 10 mCi Co-57 source in Rh matrix. The spectrometer was calibrated with a high purity iron foil of thickness 12 m. The spectra thus obtained were fitted using the LGFIT2 programme [12]. Positron annihilation lifetime measurements were carried out using a fast-fast coincidence system consisting of two 1 inch tapered off BaF2 scintillators coupled to XP 2020Q photo multiplier tubes. The time resolution obtained using a 60Co source with start and stop channels gates set at 1.27 and 0.511 MeV (prompt and annihilation gamma ray energies of respectively was 290 ps. A 8 Ci

22

22

Na source)

Na positron source (deposited on nickel foil) has been

used for lifetime measurements and data obtained was analysed by the PATFIT 88 programme after incorporating necessary source corrections [13].

4

3. Results and discussions 3.1 Structural analysis XRD patterns of the α-Fe2O3@ZnO NC samples ball-milled for different durations are shown in Fig-1. All the Bragg reflection peaks of the samples were identified and indexed according to the JCPDS file No. 750576 for hexagonal ZnO and the JCPDS file No. 840311 for hexagonal α-Fe2O3. Diffraction peaks at 31.8o, 34.5o, 36.4o, 47.7o, 56.7o, 63.1o, 68.1o and 69.3o are attributed to reflections from (100), (002), (101), (102), (110), (103), (112) and (201) planes respectively of the ZnO structure. The peaks at 24.3o, 33.4 o, 35.9 o, 41.2 o, 49.9 o, 54.5

o

and 62.9

o

are assigned to (012), (104), (110), (113), (024), (116) and (214) planes

respectively of α-Fe2O3. These diffraction lines confirm the formation of pure phase αFe2O3@ZnO NCs. The lattice parameters and average crystallite sizes of the constituents of all the NCs samples were obtained by Reitveld analysis and are tabulated in table-1.A systematic reduction of crystallite size was clearly seen with increasing milling duration. Fig-2 displays FESEM micrographs of the NC samples. It is seen from the figure that particles are mostly spherical in shape and average particle size decreases with increase of milling duration. In all the samples quite a good fraction of the grains are of nanometer size which agrees with the results obtained from XRD analysis. 3.2 Optical studies Fig-3 shows the UV-vis absorption spectra of all the α-Fe2O3@ZnO NCs. The absorption peak was seen to be blue shifted continuously in samples milled for longer duration. Bandgap energy (EG) of the NCs were calculated from the absorption peaks using the equation, EG=hc/λ

(1);

where ‘h’ is Planck’s constant, ‘c’ is velocity of light, ‘λ’ is absorption wavelength. Bandgap energy was found to be 3.21 eV (387 nm) for the pristine ZnO whereas for the NC samples

5

milled for 2, 5, 10, 20 and 30 hr, band gap values were 3.23 eV (384 nm), 3.24 eV (383 nm), 3.30 eV (376 nm), 3.33 eV (373 nm ) and 3.36 eV (370 nm) respectively. The steady enhancement of band gap in NC samples with increase in ball milling duration is attributed to gradual reduction of size of the optically active ZnO component with increasing milling duration. [14]. It may also be noted from fig-2 that absorbance is better and peak is sharper in samples milled for longer duration. As average particle sizes of the NCs reduce with increasing milling duration, light scattered from the non-optical component hematite also reduces and hence optical absorbance improves and the peak becomes sharper due to lower background. Fig-4 exhibits photoluminescence emission (PLE) spectra of all α-Fe2O3@ZnO NCs excited at a wavelength of 330 nm. For nanostructured materials, PL emission is very sensitive to the stoichiometry of the sample studied and its surface states [15]. A strong emission peak at 391 nm (UV region) and three weak emission bands at 421 nm (violet), 445 nm (blue) and 485 nm (green) in the visible region are seen in all the samples. The emission band at 391 nm in the UV region has been assigned to the near band edge emission (NBE) in ZnO. This UV emission (NBE) peak originates due to transition of electron from the bottom of conduction band to the top of valence band and other weak emission bands observed in the visible range are assigned to electronic transitions involving different defect- related energy levels e.g. VZn (zinc vacancy), Zni (zinc interstitial) and VO (oxygen vacancy) in ZnO nanostructures. The violet emission band at 421 nm (2.95 eV) is attributed to electronic transition from Zni level to the top of the valence band, whereas the blue emission band at 445 nm (2.79 eV) is due to electronic transition from the defect level corresponding to Zni to the VZn level [16, 17]. The green emission around 485 nm is assigned to electronic transition from the bottom of the conduction band to the VO level [16, 18] though there is no consensus in the literature on the origin of the green emission [19, 20]. A few interesting features noted

6

in PL spectra of the NCs are: (i) a clear and prominent red shift of the NBE peak with respect to the pristine ZnO for the samples FZ2 and FZ5 (ii) a blue shift of the NBE emission peak with respect to the pristine ZnO for the samples milled for longer duration (FZ10, FZ20 and FZ30). The red-shift seen in the samples FZ2 and FZ5 is attributed to overlap of bands of ZnO and α-Fe2O3 as the composite forms. Since the band gap of α-Fe2O3 is smaller than that of ZnO, a red shift is observed in the NBE emission of ZnO in the composite. This red shift i.e. reduction of band gap energy in FZ2 and FZ5 confirms the formation of the nanocomposite. Average particle sizes of ZnO in these composites are 47 nm (in FZ2) and 33 nm (in FZ5), therefore quantum confinement effect that shifts the NBE emission to shorter wavelength (blue shift) is negligible. With higher milling duration the average size of ZnO particles becomes smaller and due to quantum confinement effect the NBE peak shifts towards shorter wavelength offsetting the red shift due to band overlap as seen in the samples FZ2 and FZ5. This explains the blue shift observed in the samples FZ10, FZ 20 and FZ 30. It can also be observed that the intensity of the visible emission gradually decreases with increase of milling time in the NCs. This could be due to presence of hematite, a non optical material in close proximity of ZnO in the nanocomposites. 3.3 Magnetic studies Magnetic properties of the prepared nanocomposite samples have been measured by a SQUID magnetometer in the temperature range 5-300 K. Fig-5 shows typical M-H loops recorded for the sample FZ30 at 300, 150 and 10 K. At all the temperatures typical ‘S’ like ferromagnetic patterns with low magnetization value were seen that confirm presence of ferromagnetic interaction in the samples. The magnetization was found to increase monotonically with field and did not saturate even at 5 Tesla indicating presence of an antiferromagnetic component as well in the system. The coercivity (HC) values determined

7

from the M-H loops of the sample FZ30 at 300, 150 and 10K are 200, 210 and 370 Oe respectively. At 10 K, the increase of magnetization and HC is attributed to blocking of spins of single domain particles and freezing of uncompensated surface spins. To have more insight into the magnetic properties, variation of magnetization with temperature has been studied. Fig-6 shows M-T data of the sample FZ30 recorded in zerofield-cooled (ZFC) and field-cooled (FC) conditions at 200 Oe field. In the ZFC curve, a broad maximum is observed around 150 K that is assigned to superparamagnetic (SPM) blocking of finer α-Fe2O3 particles present in the sample. The broadening of the peak indicates that there is a distribution of particle size that results in distribution of anisotropy energy barrier. It may be noted from the ZFC curve that there is no sudden drop of magnetization as the temperature is lowered from 300 K to about 25 K. This indicates that Morin transition (~260 K for pure hematite) is suppressed for the hematite (size~20 nm) present in the nanocomposite. It has been reported that hematite particles having size 20 nm or less do not show Morin transition [21]. The increasing trend of magnetization seen below 25 K in the ZFC curve is attributed to freezing of surface magnetic moment at low temperature and formation of a spin-glass-like (SGL) state [22]. The bifurcation of the ZFC and FC curves near room temperature indicates presence of some kind of relaxing spin moments in the samples. To get more information on magnetic relaxation and nature of interparticle interaction among the hematite particles in the nanocomposite ac susceptibility measurements were carried out. Fig-7 shows temperature dependent magnetic susceptibility (real (χ) and imaginary (χ) components) data of the NC FZ30 taken at 3 Oe ac field with different frequencies (1, 100 and 1000 Hz). The real component (χ) of ac susceptibility data exhibits similar behavior as was observed for the dc magnetization measurements (Fig-5). The real part of susceptibility shows a frequency dependence in the temperature range 25 to 300K and 8

the broad peak observed shifts to higher temperature as frequency increases. It is well-known that in the case of noninteracting SPM nanoparticles, the peak (Tm) of χ-T plot shows substantial frequency dependence. On the other hand this frequency dependence of Tm is very negligible for spin-glass (SG) or interacting SPM particles [23, 24]. A useful parameter that represents relative shift of the blocking temperature per a frequency decade is given by =Tm/(Tmlog10 f)

(2)

where Tm is the difference between the blocking temperatures measured in log10 f and f is the ac magnetic field frequency [22]. For noninteracting superparamagnetic particles, value is 0.10, whereas for magnetically interacting as well as for spin-glass, the values of the parameter  is below 0.010. The obtained value of the parameter  for the FZ30 NC is 0.04. This value suggests the presence of weak interparticle interaction in the sample. So, the hump (Fig-6 a) is related to the weakly interacting superparamagnetic nanoparticles present in the sample [22, 25]. The imaginary component of susceptibility (χ) data also showed a shift of the broad hump to higher temperature with increasing frequency of the applied ac field. This also suggests presence of weakly interacting SPM NPs in the sample [26]. 3.4 Hyperfine studies To check the chemical state of iron and iron-bearing phases present in the nanocomposites, the samples were examined by

57

Fe-Mössbauer spectroscopy which is an

efficient probe to investigate the local environment of Fe nuclei of a compound [18, 25, 26]. Fig-8 shows Mössbauer spectra of α-Fe2O3@ZnO NCs recorded at room temperature. The spectra of the samples milled up to 20 hr are fitted with one typical six- finger pattern and the spectrum of the sample milled for 30 hr was fitted with a sextet pattern and a doublet. The fitting was done with a least squares programme assuming Lorentzian line shapes [12]. The

9

obtained hyperfine parameters are tabulated in table-2. The isomer shift values (0.28 – 0.39 mm/s) indicate that only Fe3+ ions in the high spin configuration is present in all NC samples [27]. The hyperfine parameters and the observation of a single sharp sextet in each NC with large internal magnetic field (~517 kOe) confirm that all the nanocomposites contain only αFe2O3 as the Fe- bearing component; formation of ferrite phase as an impurity did not take place even after milling the mixture for 30 hour. The isomer shift of α-Fe2O3 has been found to increase slightly (from 0.37 to 0.39 mm/s) as the milling duration is increased. This indicates a decrease in the s-electron density at Fe nuclei in the samples milled for longer duration. The s-electron density may be altered due to change in Fe3+-O2- distance in α-Fe2O3 caused by crystal distortion or change of cationic environment. It is also seen from table-2 that quadrupole splitting (Q. S.) value slightly increases (from 0.16 to 0.19 mm/s) in samples milled for longer duration. This is attributed to higher electric field gradient in samples milled for longer duration. With increase in milling hour the average crystallite size decreases and due to generation of lattice strain, the Q. S. value increases. In the samples with higher milling duration the hyperfine field was found to decrease as expected. The average hyperfine field (Hint) is related to the particle volume V, by Hint = HB[1-(kBT/2KV)], where ‘HB’ is average hyperfine field of the bulk sample and ‘K’ is anisotropy constant [28]. With reduction of particle size, volume reduces and also the anisotropy constant decreases, hence the hyperfine field also decreases according to the above equation. The doublet observed in the central region of the spectrum of the sample milled for 30 hr is assigned to weakly interacting finer hematite particles in the sample undergoing superparamagnetic relaxation [29] which was also confirmed by ac magnetization measurements. The broadened lines seen in XRD pattern of this sample also support the presence of finer particles. 3.4 Positron annihilation lifetime studies

10

Positron annihilation lifetime spectroscopy (PALS) is an important tool to investigate different types of defects (mainly cation vacancies) present in a sample [30]. Zinc oxide is known to have different types of inherent cation vacancies [31] that strongly affects its optical properties [32]. Since the NCs studied presently contain 90 wt. % of ZnO, PALS has been used to characterize vacancy- type defects present in the samples. Positron annihilation spectra of the NCs were fitted with three lifetime components τ1, τ2 and τ3 with corresponding intensities I1, I2 and I3. The fitted parameters are listed in Table-3. It is well known that in nanomaterials the shortest lifetime τ1 does not represent the bulk lifetime of the material because positron diffusion length is more than the grain size of the material studied [32]. From Table-3 it can be seen that the value of τ1 of the pristine ZnO ((~157.2 ps) and that of the NC milled for 2 hr (~158.6 ps) are very close to the reported bulk lifetime of positron in ZnO [33]. Therefore, τ1 of the pristine ZnO and the sample FZ2 correspond to annihilation of positrons with free electrons residing at grain boundaries. It may be noted that positron lifetime for Zn mono-vacancy in ZnO is 237 ps [34]. Hence observed τ1 values (160-165 ps) for the samples milled for 5 hr and above are admixture of lifetimes corresponding to positron annihilation with free electrons and electrons at monovacancies residing at the grain boundaries [32]. The intermediate lifetime component τ2 has been assigned to positron annihilation at larger vacant space e.g in the triple junctions [16]. The longest lifetime component τ3 has been attributed to the peak off annihilation of o-Ps atoms formed inside the larger voids. Fig.9 shows variation of τ1 and τ2 and the corresponding intensities I1 and I2 with milling duration. Since intensity of the third lifetime component is very low compared to others, the variation of the first two lifetime components are only discussed. From the figure it is clear that both τ1 and τ2 increase up to milling time 20 hr and then decreases slightly on milling for longer duration. The increase of τ1 with milling duration indicates formation of divacancies

11

as monovacancies agglomerate in samples with finer grain size. The reduction of the corresponding intensity I1 with increase of milling time also supports this. The increase of τ2 and I2 with milling time indicates increase of the size and number of triple junctions which most probably is due to morphological changes in the nanocomposites with prolonged milling. The slight reduction of both τ1 and τ2 after 20 hr milling could be due to tunnelling of electrons from adjacent hematite nanoparticles in the NCs that increases average electron density in both the annihilation sites of ZnO. Tunnelling of electrons between the components of a nanocomposite becomes significant when the particle size becomes smaller [35]. In the present case after 20 hr milling the sizes of ZnO and hematite becomes smaller than 16 and 23 nm respectively and hence electron tunnelling is facilitated. The variation of average defect density with milling time may be seen from the dependence of average lifetime with duration of milling hour (Fig-8(c)). The average lifetime of positron (τav) was calculated using the relation: av = (I11+ I22+ I33)/( I1+ I2+ I3)

(3)

It is clear from Fig-8(c) that the average defect density which is proportional to the average lifetime (av) increases steadily up to 20 hr. milling. As the surface to volume ratio increases with decreasing particle size increase of average defect density is also expected because most of the vacancy-type defects seen by positrons are located at the surface. The slight reduction of av after 20 hr. milling could be due to filling up of lager voids in ZnO nanostructure by ultrafine hematite nanoparticles. 4. Conclusions α-Fe2O3@ZnO NCs with different particle sizes were successfully prepared by the high energy ball-milling technique and phase purity of the constituents was confirmed by XRD analysis. A continuous blue shift of the UV-vis absorption line was observed with increasing milling duration that is attributed to increase of band gap energy of ZnO with 12

reduction of particle size. PL emission spectra of the NCs showed emission peaks in the visible region due to vacancy and interstitial type defects present in ZnO nanostructures. Dc magnetization measurements reveal the presence of a weakly ferromagnetic state along with an antiferromagnetic component at room temperature and a spin glass like phase below 25 K. Ac susceptibility and Mössbauer measurements confirm presence of superparamagnetic hematite particles in the sample milled for longer duration. Positron annihilation lifetime studies indicate enhancement of average defect density in the NCs with increasing milling duration. Acknowledgments Authors thank Dr. A. Saha, UGC-DAE CSR Kolkata for providing instrumental facilities for optical studies and Dr. S. Kumar, Jadavpur University, Kolkata for FESEM measurements. The magnetization measurements were done at the high magnetic field facility at UGC-DAE CSR, Kolkata created under the DST project no. IR/S2/PU-0006/2006. References [1] S. A. Corr, Y. P. Rakovich, Y. K. Gun’ko, Nanoscale Res. Lett. 3 (2008) 87-104. [2] W. W. Wang, Y. J. Zhu, L. X. Yang, Adv. Funct. Mater. 17 (2007) 59-64. [3] M. Niu, F. Huang, L. F. Cui, P. Huang, Y. L. Yu,Y. S. Wang, ACS Nano 4 (2010) 681688. [4] C. Xiong, J. Kenneth, J. Balkus, J. Phys. Chem. C 111 (2007) 10359-10367 [5] C. L. Zhu, Y. J. Chen, R. X. Wang, L. J. Wang, M. S. Cao, X. L Shi. Sensors Actuators B 140 (2009) 185. [6] X. Y. Xue, L. L. Xing, Y. J. Chen, S. L. Shi, Y. G. Wang, T. H. Wang, J. Phys. Chem. C 112 (2008) 12157. [7] H. Y. Yang, S. F. Yu, S. P. Lau, X. Zhang, D. D. Sun, G. Jun, Small 5 (2009) 2260.

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Figure captions: Fig. 1. XRD patterns of all α-Fe2O3/ZnO nanocomposites. Fig. 2. FESEM images of (a) FZ2, (b) FZ5, (c) FZ10, (d) FZ20 and (e) FZ30. Fig. 3. UV-vis absorption spectra of the pristine ZnO and all α-Fe2O3@ZnO nanocomposites. Fig. 4. PL emission spectra of pristine ZnO and all α-Fe2O3@ZnO nanocomposites; inset: Rescaled PL emission spectra of the NCs. Fig. 5. Magnetic hysteresis curves of FZ30 NC at 300, 150 and 10 K. The inset shows magnified view of the central part of the M-H loops. Fig. 6. Temperature dependent magnetization curve of FZ30 NC at 200 Oe applied field. Fig. 7. Temperature dependent ac susceptibility data (a) χ vs T; (b) χ vs T taken at 1, 100 and 1000 Hz frequencies for the FZ30 NC. Fig. 8. Room temperature Mössbauer spectra of all α-Fe2O3@ZnO nanocomposites. Fig. 9. Positron annihilation lifetime plots of α-Fe2O3@ZnO nanocomposites.

Table captions: Table 1. Fitting parameters of XRD data for all the α-Fe2O3@ZnO nanocomposites. Table 2. Room temperature Mössbauer parameters of α-Fe2O3@ZnO nanocomposites. Table 3. Positron annihilation lifetime parameters of the pristine ZnO and all α-Fe2O3@ZnO nanocomposites.

16

Table-2: Room temperature Mössbauer parameters of α-Fe2O3@ZnO nanocomposites. Sample

Nature of

Isomer

Quadrupole

Line-

Internal

ID

absorption

shift (I.S.)

splitting (Q.S.)

width

magnetic

spectra

(mm/s)

(mm/s)

(L.W.)

field Hint

(mm/s)

(kOe)

% area

FZ2

Sextet

0.37

0.16

0.27

520.0

100

FZ5

Sextet

0.38

0.17

0.28

521.4

100

FZ0

Sextet

0.38

0.18

0.30

518.4

100

FZ20

Sextet

0.39

0.18

0.30

517.6

100

FZ30

Sextet

0.38

0.19

0.37

516.2

90

Doublet

0.28

0.72

0.64

-

10

17

Table-3: Positron annihilation lifetime parameters of pristine ZnO and all α-Fe2O3@ZnO nanocomposites. Sample

τ1 (ns)

τ2 (ns)

τ3 (ns)

I1 (%)

I2 (%)

I3 (%)

τav (ns)

0.1572

0.3129

6.1510

54.6042

45.3070

0.0887

0.2331

±0.0021

±0.0049

±1.3503

±2.4655

±2.4637

±0.0104

0.1586

0.3147

4.53106

54.9055

45.0710

0.1235

±0.0023

±0.0055

±0.2419

±2.4425

±2.3221

±0.0255

0.1600

0.3227

2.6106

55.4004

44.3733

0.2263

±0.0022

±0.0057

±0.3219

±2.5586

±2.5446

±0.0298

0.1623

0.3250

4.1782

44.0986

55.8190

0.0824

±0.0028

±0.0038

±1.0732

±2.0018

±1.9972

±0.0144

0.1648

0.3349

8.1099

43.5988

56.2840

0.1172

±0.0032

±0.0040

±1.4281

±2.2415

±2.2399

±0.0108

0.1621

0.3299

3.8691

42.9165

56.9707

0.1127

±0.0034

±0.0041

±0.7724

±2.3234

±2.1175

±0.0164

Name ZnO

FZ2

FZ5

FZ10

FZ20

FZ30

18

0.2345

0.2377

0.2564

0.2699

0.2619

Table-1: Fitting parameters of XRD data for all the α-Fe2O3@ZnO nanocomposites. Lattice parameters (in Å)

Crystallite size

Sample

a-value of

c-value of

a-value

c-value of

α-Fe2O3

ZnO

Average size

ID

α-Fe2O3

α-Fe2O3

of ZnO

ZnO

(nm)

(nm)

(nm)

T2

5.0418

13.7670

3.2537

5.2128

65.0

47.00

56.0

T5

5.0449

13.7642

3.2545

5.2128

36.0

32.6

34.3

T10

5.0471

13.7677

3.2556

5.2143

27.5

25.0

26.2

T20

5.0479

13.7637

3.25643

5.2135

23.0

16.4

19.7

T30

5.0472

13.75575

3.2571

5.2104

20.2

14.8

17.5

19

20

21

22

23

24

25

26