Investigation of dielectric and optical properties of

Ceramics International 42 (2016) 11447–11452

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Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Investigation of dielectric and optical properties of structurally modified bismuth ferrite nanomaterials Alina Manzoor a, A.M. Afzal a,n, N. Amin a, M. Imran Arshad a, M. Usman b, M.N. Rasool c, M.F. Khan d a

Department of Physics, GC University, Faisalabad 38000 Pakistan Department of Physics, Quid-e-Azam University, Islamabad 44000 Pakistan c Department of Physics, Isalmia University, Bahawalpur 63100 Pakistan d Department of Physics & Astronomy and Graphene Research Institute, Sejong University, Seoul 05006 Republic of Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 8 January 2016 Received in revised form 22 March 2016 Accepted 15 April 2016 Available online 16 April 2016

We present the effects of size and oxygen defects on electrical and optical properties of nano crystalline bismuth ferrite (BiFeO3) with rhombohedral symmetry synthesized by solution evaporation method. The effects of oxygen vacancies on the behavior of system have been studied by annealing all samples under two different environments, air and oxygen. Single phase crystal structure was confirmed by X-Ray diffraction (XRD). The crystalline size calculated by Scherrer's formula falls in the range of 11–21 nm for air and 20–29 nm in case of oxygen annealing atmosphere. The crystalline size was confirmed by Transmission electron microscope (TEM). Dielectric response reveals that both dielectric constant and dissipation factor (tanδ) decrease with increase of frequency without depicting any peak. Optical studies showed that the system possesses an indirect energy gap whose value increases with increasing particle size. There is an evidence for the presence of intra band states in this system arising from defect states. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Solution evaporation method Multiferroic Nanoparticles Dielectric properties Oxygen vacancies Optical properties

1. Introduction Multiferroics have been known as materials exhibiting ferromagnetic and ferroelectric properties at the same time. In multiferroic materials the magnetic and the electric properties modulate each other. They have a wide range of potential applications in information storage, spin valves, spintronic devices, quantum electromagnets, microelectronic devices and sensors [1–6]. It has been realized however that ferromagnetism and ferroelectricity do not in general co-exist due to conflicting structural symmetry requirements for the two systems. Hence strong effort is needed to understand and explore the few multiferroics that exist and use this understanding to develop other and better systems. Bismuth ferrite (BiFeO3) with a rhombohedrally distorted perovskite structure having lattice parameter ar ¼5.63 Å, αr ¼ 59.350 or hexagonal parameter ahex ¼5.58 Å, Chex ¼13.87 Å is a typical n

Corresponding author. E-mail addresses: [email protected] (A. Manzoor), [email protected] (A.M. Afzal), [email protected] (N. Amin), [email protected] (M.I. Arshad), [email protected] (M. Usman), [email protected] (M.N. Rasool), [email protected] (M.F. Khan). http://dx.doi.org/10.1016/j.ceramint.2016.04.083 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

multiferroic in which ferroelectric (FE) and antiferromagnetic properties exist simultaneously [7]. BiFeO3 has been attracted much attention being a single phase magnetoelectric material exhibiting antiferromagnetic and ferroelectric properties simultaneously. Reported study of bismuth ferrite nanoparticles points out that the multiferroic properties are stronger at small crystalline size and magnetic properties disappeared at higher size (d»65 nm) [8,9]. It is the magnetic moment of this weak ferromagnetic state, coexisting alongside the main antiferromagnetic state, which make it one of the prime candidates for room-temperature magnetoelectric (ME) application. A. Manzoor et al. investigated the role of size and oxygen defects on multiferroic properties of BiFeO3 nanoparticles [10]. Bismuth ferrite is a narrow bandgap metal oxide semiconductor. This material is useful for visible light active photocatalysis. In present work, we report the comparative study of frequency dependent dielectric response and optical properties of BiFeO3 nanoparticles annealed under two different environments; air (A) and oxygen (O). These results propose that bismuth ferrite has broader applications in photo catalyst and opotoelectric devices especially under visible light waveband. Bismuth ferrite nanoparticles have been synthesized by the solid state method,

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Fig. 1. (a) XRD pattern of air (A) annealed samples at different temperatures. (b) XRD pattern of oxygen (O) annealed samples at different temperatures.

mechanochemical method, solution chemistry method, sol-gel method, hydrothermal method and sonochemical method [11–16].

2. Experimental procedure In this study, Single phase bismuth ferrite was prepared by solution evaporation method (SEM). Stoichiometric amounts of metal nitrates and tartaric acid were used. Equimolar amounts (0.01 M) of iron nitrate nonahydrate Fe(NO3)3 9H2O and bismuth nitrate pentahydrate Bi(NO3)3 5H2O were added into the dilute nitric acid-2N HNO3 (65% Merck). Tartaric acid in 1:1 M ratio with respect to nitrates of metal was entered into the solution. The solution was placed on hot plate and heated at temperature 150 °C–160 °C under constant stirring until all liquid evaporated out from beaker. The remaining solution was changed into fluffy green powder. The samples were annealed at different temperatures for 2 h in air and oxygen atmospheres. We used the XRD model JDX-11 of Joel Company Ltd., Japan, performed at 35 kV and 30 mA and transmission electron microscope (TEM), (Zeiss Libra 120) to investigate the size and structure of bismuth ferrite nanoparticles. This is a powder type X-ray diffractrometer and x-ray have wavelength CuKα (1.5418 Å). All spectra were taken in the range between 200–800 nm on Lemda-950 Perkin-Elmer spectrometer with integrating sphere attachment and Spectralon reflectance standards.

3. Results and discussion 3.1. X-ray diffraction analysis (XRD) From the XRD results of samples annealed in air and oxygen atmosphere at different temperatures, it was observed that the

bismuth ferrite peaks became visible at 450 °C. The peaks became sharp and prominent at 500 °C. At this temperature the XRD pattern shows the pure phase perovskite structure of bismuth ferrite without any detectable impurity. At higher annealing temperature above 600 °C, the impurity peaks developed such as Bi2O3. Annealing in oxygen atmosphere was observed to lead to better crystallinity and larger grain size as compared to the sample annealed in air at the same temperature. The XRD pattern of samples annealed in air and oxygen environment was shown in Fig. 1(a and b) respectively. The average crystallite size from the XRD peak was calculated by using Scherrer's formula.

D =

Kλ β cos θ

where D is the crystalline size, K is a constant (0.94), ʎ is wavelength of the radiation source (1.59 Å, β is full width at half maximum and θ is Bragg's angle.

3.2. TEM analysis The TEM images of some sample of bismuth ferrite nanoparticles annealed in both air and oxygen atmosphere are shown in Fig. 2(a, b and c). It was perceived that most of the particles had spherical shape. The mean size of nanoparticles was 30 nm and 18 nm in case of oxygen annealing and 27 nm in case of air annealing at 500 °C and 450 °C respectively. The TEM images display that the crystallite size calculated by scherrer's formula is generally less than the particle size obtained from TEM. The selected area electron diffraction (SAED) images are shown in Fig. 2(d). From SAED image it is clear that the particles are well crystalline in case of Oxygen annealing.


Fig. 2. TEM images (a, b and c) and SAED (d) of different samples annealed at different temperatures in air (A) and oxygen (O) atmosphere for 2 h.

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Fig. 3. (a). Frequency dependence of real part of dielectric constant (έ) for air annealed samples at room temperature. (b). Frequency dependence of real part of dielectric constant (έ) for oxygen annealed samples at room temperature. Fig. 4. (a). Frequency dependence of dissipation factor (tanδ) for air annealed samples at room temperature. (b). Frequency dependence of dissipation factor (tanδ) for oxygen annealed samples at room temperature.

3.3. Dielectric analysis The behavior of dielectric constant and dissipation factor with respect to frequency variation (1KHz–1MHz) was measured at room temperature for both air and oxygen annealed samples. It can be seen from Fig. 3(a) and (b) that dielectric constant (έ) decreases continuously with increasing frequency. This type of behavior depicts the dispersion due to Maxwell Wagner type interfacial polarization [17]. The phenomenon of electron transfer between Fe2 þ and Fe3 þ ions results into local displacement of charges in the direction of an applied electric field, which consequently determines the polarization in typical ferrites. The decrease of the dielectric constant with frequency is also related to the effects of the hopping process. At lower frequencies electrons involved in hopping between Fe2 þ 2Fe3 þ ions are locally aligned in the direction of applied electric field and define polarization process which means high dielectric constant. But with increasing frequency, the polarization decreases and eventually reaches a constant value when the frequency of electron hopping between Fe2 þ 2Fe3 þ ions cannot follow the fast-changing alternating field. This is because of with increasing frequency the access time decreases (time taken by the field to switch its polarity) ultimately becoming less than the response time of the electrons. Hence the electrons cannot respond to the applied field. At high enough

frequencies (4 105Hz) the dipoles due to inertia could not align themselves in the direction of applied field so ε' becomes almost constant [17,18]. The enhancement of dielectric constant at low frequency is also associated with a high dissipation factor (tanδ). Fig. 4(a) and (b) show the frequency dependence of dissipation factor (tanδ) for both air and oxygen annealed samples respectively which tend to increase without showing any peak. According to Koop's phenomenological model [18], the decrease of tanδ is explained by the fact that at lower frequencies, where the resistivity is high and the grain boundary effect is dominant, more energy is needed for the exchange of electrons between Fe2 þ and Fe3 þ ions located at grain boundaries, i.e. energy loss (tanδ) is high. At high frequencies, when the resistivity is low and grains themselves have a dominant role, little energy is needed for hopping of electrons between the Fe2 þ and Fe3 þ ions located in a grain, and therefore tanδ is also small. 3.4. Optical analysis Optical spectroscopy is developed as an important experimental tool for the determination of energy band of semiconductors. Optical


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Fig. 5. (a). Diffuse reflectance spectra of 2 h air annealed samples. (b). Diffuse reflectance spectra of 2 h oxygen annealed samples.

Fig. 6. (a). Indirect band gap measurements for 2 h air annealed samples. (b). Indirect band gap measurements for 2 h oxygen annealed samples.

characterization of BiFeO3 samples was carried out by measuring the reflectance spectroscopy at room temperature. All spectra were taken in the range between 200–800 nm on Lemda-950 Perkin-Elmer spectrometer with integrating sphere attachment and Spectralon reflectance standards. The diffuse reflectance spectra are analyzed using Kubelka-Munk function. This function F(R) is related to the diffuse reflectance, R, of the sample by the relation [19].

for larger particle size is close to the reported of 2.18 eV for BiFeO3 nanoparticles [21]. In BiFeO3 the valence band is primarily formed from the O2p states hybridized with Fe 3d states while the conduction band comprises the Fe 4s states and Bi 6p states. The Fe 3d states lie close to the edge of the bottom of the conduction band [22]. The fitted curves for energy band gap of oxygen annealed samples are shown in Fig. 6(b). Table 1 shows a comparison between band gaps of air and oxygen annealed samples. The error calculated here is the error induced in the indirect band gap values while fitting a straight line. The instrumental error induced with the wave length step size of 1 nm is about 15 meV. The values of indirect band gap for air annealed samples are larger as compared to oxygen annealed samples. As we move towards smaller sizes, band gap values decreased indicating a red-shift which may be attributed to intra gap defect states. The variation of energy band gap with average crystallite size for air and oxygen annealed sample is shown in Fig. 7. It is now well established that electronic band structure of semiconducting solids is size dependent. From Fig. 6(a and b), a clear red-shift in indirect band gap with decrease of crystallite size can be observed for both air and oxygen annealed samples. This red-shift may be attributed to the presence of intra gap defects states whose density increases with decreasing particle size.

F (R)=(1−R)2 /2R Here R΄ is the absolute value of reflectance. The fundamental absorption edge of BiFeO3 nanoparticles was calculated from their diffuse-reflectance spectra by plotting the square root of the Kubelka-Munk function F(R) 1/2 versus energy in electron volts. The tangent line which is extrapolated to F(R) 1/2 ¼0 indicates the indirect band gap energy of BiFeO3 nanoparticles [20]. Fig. 5(a) and (b) show the diffuse reflectance spectra of various air and oxygen annealed samples. Fig. 6(a) shows the band gap measurements for air annealed samples. It is clear from Fig. 6(a) that all samples show the similar shape. The indirect band gap decreases with decreasing the particle size and the photo absorption edge exhibits a red-shift. This red-shift may attribute due to the defects in the intra gap states of BiFeO3 nanoparticles. The value of indirect band gap for largest particle size is 2.1 eV, while for smallest one is 1.88 eV. The value

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Table 1 Variation of bang gap with av. crystallite size for both air and oxygen annealed samples. Sample name

Av. crystallite size (nm)

Indirect band gap for air annealed samples (eV)

BFO-600 (A) BFO-550 (A) BFO-500 (A) BFO-450 (A) Sample name BFO-600 (O) BFO-550 (O) BFO-500 (O)

21 18 13 11 Av. crystallite size (nm) 29 25 20

2.17 0.028 2.03 7 0.045 1.88 70.032 1.577 0.009 Indirect band gap for oxygen annealed samples (eV) 2.06 70.023 2.02 7 0.013 1.85 70.008

Fig. 7. Variation of energy bang gap with av. crystallite size for air annealed samples (inset for oxygen annealed sample).

4. Conclusion We have successfully synthesized the phase pure multiferroic nano powders of BiFeO3 via soft chemical route. The effects of oxygen vacancies on the behavior and phase of the system have been studied by annealing samples under two different environments, air and oxygen. Oxygen annealing has proved to stabilize the phase of the system. The room temperature, frequency dependent behavior of both real part of dielectric constant and tanδ show a decreasing trend with increasing frequency without showing any peak. Incidentally, the enhancement of dielectric constant at low frequency for both samples is also associated with a high dielectric loss. Diffuse reflectance spectroscopy measurements of both series of different sizes (11–29 nm) indicate a consistent red shift in the fundamental band gap (indirect band gap) with the decrease in particle size. This observed red-shift may be due to the presence of intra gap states whose density increases with the decrease in particle size.

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