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Syntheses of new spinels Zn1-xFexAl2O4 nanocrystallines structure: Optical and magnetic characteristics
Journal of Alloys and Compounds 795 (2019) 114e119
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Syntheses of new spinels Zn1-xFexAl2O4 nanocrystallines structure: Optical and magnetic characteristics A. Abu El-Fadl a, M.I. Abd-Elrahman a, *, Noha Younis a, b, N. Afify a, A.A. Abu-Sehly a, M.M. Hafiz a a b
Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt Faculty of Computers and Information Technology, National Egyptian E-Learning University, 12611 Giza, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2019 Accepted 1 May 2019 Available online 2 May 2019
Nanocrystalline of ZnAl2O4 spinel was synthesized by employing the microwave combustion method. Different new spinel compositions of formula Zn1-xFexAl2O4, (0.0 x 1.0) were prepared by substitution of Zn ions with Fe ions. The as-synthesized resultant material ZnAl2O4 with spinel structure as well as Zn1-xFexAl2O4 compositions were characterized by Energy Dispersive X-ray and X-ray diffraction. Morphology and particle size evaluation of the nanoparticles were characterized by transmission electron microscopy images. The optical absorption spectra and the magnetization were measured at room temperature for each Fe ratio. The substitution effect of Fe ions on the optical and magnetic properties of the original spinel was studied. The optical absorbance showed that the optical band gap decreased with increasing Fe ratio. © 2019 Elsevier B.V. All rights reserved.
Keywords: Zinc iron aluminate Nanocrystalline Optical gap Magnetic properties
1. Introduction Interest in the synthesis of nanoparticles has increased due to their different properties when compared to the corresponding bulk material. Spinel-type oxides AB2O4, where A represent the divalent cations which in normal spinel occupying tetrahedral sites in a face centred cubic (FCC) lattice of oxygen anions and B represents trivalent cations occupying the octahedral sites of the cubic crystal structure [1]. These materials are suitable for a wide range of applications, such as magnetic materials, catalysis and ceramics [2]. Spinel nanoparticles can be synthesized by several techniques such as solegel [3], co-precipitation [4], microwave combustion [5,6], hydrothermal [7], Penchini method [8,9]. Aluminum-based spinels are intriguing class of oxide ceramics which have important technological applications. In spinel ZnAl2O4 structure, Znþ2 and Alþ3 cations occupy the tetrahedral and octahedral in FCC lattice of oxygen anions respectively. The ZnAl2O4 spinel has attracted considerable attention since it possesses desirable properties such as high thermal stability, high mechanical resistance, better diffusion and ductility, low temperature sinterability, good resistance to acids and bases and excellent optical
* Corresponding author. E-mail address: [email protected] (M.I. Abd-Elrahman). https://doi.org/10.1016/j.jallcom.2019.05.008 0925-8388/© 2019 Elsevier B.V. All rights reserved.
transparency [10e16]. According to those properties, the ZnAl2O4 spinel was used as ceramic materials [10,11], electronic and optoelectronic devices [12e14], catalyst and catalystic support of transition metal [15,16]. Consistent with all of the applications, ultrafine ZnAl2O4 spinel nanoparticles with a narrow particle size distribution and a low degree of agglomeration are desirable. Completely replacement of Zn by Fe in ZnAl2O4 results in FeAl2O4 spinel (hercynite) which is mineral with the spinel structure. Hercynite, which is a representative antiferromagnetic spinel, shows a magnetization regardless of the normal spinel [17]. This work aims to synthesize and characterize Zn1-xFexAl2O4, (0.0 x 1.0) nanoparticles as new spinels by the combustion method. Also, the effect of Fe ions substituted Zn ions on the optical and magnetic properties of ZnAl2O4 nanoparticles was studied. 2. Experimental 2.1. Samples synthesizing In the typical microwave combustion method for preparation of pure ZnAl2O4 spinel, all starting chemicals used in this study were pure grade Zn(NO3)2$6H2O and Al(NO3)3$9H2O. These chemicals were dissolved in double distilled water with glycine (NH2CH2COOH) as a combustion fuel at a fixed ratio 1:2:0.3 (according to their weights) to obtain a homogeneous solution. The
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solution was placed in a microwave oven type [SHARP R-241R (W)] to improve the full reaction occurs as the water evaporates and the combustion takes place. After the combustion, a white dry foamy was generated and the resultant powder was collected to perform further analyses. The synthesis of the composition Zn1-xFexAl2O4 (0.0 x 1.0) took almost the same steps as before but the starting materials were Zn(NO3)2$6H2O, Al(NO3)3$9H2O and Fe(NO3)3$9H2O using also glycine (NH2CH2COOH) as a combustion fuel at a fixed ration (1-x): x:2:0.3. The resultant powders were colored graduate from white to brown with increasing the ratio of Fe. 2.2. Characterization methods The chemical composition of prepared samples was check by Energy-Dispersive X-Ray (EDX) Spectroscopy technique using scanning electron microscope model QUANTA FEG 250 (Field Emission Gun) attached with EDX unit with accelerating voltage 30 kV. X-ray diffraction (XRD) measurements were performed on Philips X-ray diffractometer model PW. 1710 with CuKa radiation (l ¼ 1.5405 Å) with an operating applied voltage 40 kV and current 30 mA. Scanning rate was maintained at 0.06 per minute in the range of 2q from 4 to 70 . The diffraction patterns of the prepared samples were compared with those reported in the database of the Powder Diffraction File (PDF) maintained in the International Centre for Diffraction Data (ICDD). The morphology of the prepared nanoparticles was investigated by high resolution transmission electron microscope (HRTEM-JEOL JEM 2100). Infrared (IR) transmittance spectra of the samples were measured by Nicolet Fourier transformation infrared (FTIR) 6700 spectrometer using the KBr pellet method in the range 400e4000 cm1. Optical absorbance was measured for the powder suspension of the samples using Thermo Evolution 300 UVeVisible Spectrophotometer. The optical measurements were carried out between 200 and 900 nm. A Vibrating Sample Magnetometer (VSM, LakeShore 7400) was used to investigate the magnetic properties of Zn1-xFexAl2O4 spinels at room temperature.
Fig. 1. Energy Dispersive X-ray (EDX) spectrum of Zn1-xFexAl2O4 nanoparticles.
3. Results and discussion 3.1. Energy dispersive X-ray analysis The chemical composition of prepared samples was check by Energy-Dispersive X-Ray (EDX) Spectroscopy technique. The results in Fig. 1 show the spectral distribution of ZnAl2O4 spinel sample and those of substituted with Fe ions. The spectral distribution of substituted samples indicates that the samples are consisted of O, Al, Zn and Fe only. No other elements were observed.
Fig. 2. X-ray diffraction patterns of (a) as prepared ZnAl2O4 nanoparticles and (b) reported ZnAl2O4 nanoparticles in the ICDD PDF files.
3.2. Structure and particle size X-ray powder diffraction measurements were collected for the synthesized materials without any heat treatment. The diffraction patterns of the as prepared ZnAl2O4 and FeAl2O4 were illustrated in Figs. 2a and 3a. Comparing the diffraction patterns of Figs. 2a and 3a with those reported in the ICDD PDF files of Figs. 2b and 3b confirms the spinel cubic structure of synthesized ZnAl2O4 and FeAl2O4. However, the sharpness of the diffraction peaks in Figs. 2a and 3a indicates the good degree of crystallinity of the synthesized spinels. The diffraction data of Fe substitution samples were illustrated in Fig. 4 showing that the samples were crystallized in preferential orientations indicated with different peaks. These orientations were along seven planes. The formation of cubic spinel aluminate structure was confirmed by the existence of diffraction planes of
(220), (311), (400), (331), (422), (511) and (440). In particular, the presence of (440), (400) and (311) diffraction planes is an evidence for face-centred cubic spinel crystal. The strong XRD peaks of (220), (311) and (440) planes indicate these planes are the most preferential orientations. The assignment of the planes was determined on the basis of best agreement between the experimentally observed of inter-planar spacing and diffraction angles with those of ICDD. The average particle size (Dav) in nm can be calculated using the Scherrer equation [18]:
Dav ¼
kl b cos q
(1)
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Where l is the X-ray wavelength, b is the full width at half maximum (FWHM) in radians, q is Bragg's angle and k is constant which assigned to 0.9. The calculated Dav values for observed crystalline phases for each Fe ratio are listed in Table 1. Average lattice constant (a) is calculated as:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l2 l a¼ 2 sin q
(2)
The dislocation density (d) which represents the amount of defects in a given sample is defined as the length of dislocation lines per unit volume of the crystal; and it was calculated by the following equation [19]:,
d¼
1
(3)
D2av
The average internal lattice strain (ε) was estimated using the following relation [20]:
ε¼
b cos q
(4)
4
The theoretical density (r) for any material can be calculated from the XRD data based on the number of molecules per unit cell (Z) as follows [21]: Fig. 3. X-ray diffraction patterns of as prepared (a) as prepared FeAl2O4 nanoparticles and (b) reported FeAl2O4 nanoparticles in the ICDD PDF files.
r¼
ZM NA a3
(5)
where Z ¼ 8 for aluminates and ferrite spinel, M is the molecular weight of the material, NA is Avogadro's number and a is the lattice constant. The calculated values of Dav, d, ε and r were listed in Table 1. From this Table, one notes that the value of r decreases with increasing the ratio of Fe ions. That is because the atomic weight of the substituted iron (55.845 gm/mol) which is lower than that for zinc (65.38 gm/mol). 3.3. TEM analyses
Fig. 4. X-ray diffraction patterns of Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles.
High resolution transmission electron microscope (HR-TEM) was used to study the morphology of the prepared nanoparticles and to evaluate the crystallite grains microstructure. Fig. 5 shows the HR-TEM images of pure ZnAl2O4, Zn0$5Fe0$5Al2O4 and FeAl2O4 at magnification of 50 nm. The TEM images indicated the particles sizes were within the nanoscale range. The micrographs also showed irregular shaped particles with particle size distribution below 50 nm. The average particle size, Dav, was estimated from the TEM images by the so-called image j viewer software [22]. The calculated Dav was found to be 13.55, 13.25 and 10.84 nm for ZnAl2O4, Zn0$5Fe0$5Al2O4 and FeAl2O4, respectively, which in agree with X-ray results. The corresponding selected area of electron diffraction (SAED) patterns was shown in Fig. 6. The ring patterns of
Table 1 Average particle size (Dav), lattice constant (a), dislocation density (d), internal lattice strain (ε) and theoretical density (r) for Zn1-xFexAl2O4. Fe content
Dav (nm)
a (Å)
Vol. (Å3)
d 103 (nm)2
ε 103
r (gm/cm3)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
13.70 12.32 14.09 13.06 15.12 12.86 12.27 10.80 11.26 9.56 9.44
8.119 8.129 8.121 8.123 8.131 8.141 8.153 8.127 8.132 8.145 8.171
535.19 537.17 535.59 535.98 537.57 539.55 541.94 536.77 537.76 540.35 545.54
5.323 6.588 5.037 5.863 4.374 6.047 6.642 8.573 7.887 10.942 11.222
2.645 2.939 2.571 2.779 2.406 2.824 2.967 3.381 3.235 3.817 3.889
4.554 4.512 4.502 4.475 4.438 4.398 4.355 4.374 4.343 4.298 4.234
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Fig. 5. HR-TEM images of Zn1-xFexAl2O4 nanoparticles at magnification of 50 nm for (a) x ¼ 0.0, (b) x ¼ 0.5 and (c) x ¼ 1.0.
Fig. 6. The selected area of electron diffraction (SAED) of Zn1-xFexAl2O4 nanoparticles for (a) x ¼ 0.0, (b) x ¼ 0.5 and (c) x ¼ 1.0.
SAED indicated the crystalline nature of the investigated samples. However, the observed of many spots contained by each ring was ascribed to the presence of large number of nanoparticles. 3.4. Infrared spectroscopy The FTIR spectra for the samples were examined to study their functional groups. Fig. 7(a) shows the FTIR spectra of the Zn1-xFexAl2O4 in the range (400-4000 cm1) and Fig. 7(b) shows the spectra in the range 450e850 cm1 which is the active region for the metal oxide bands. The spinel type structure can be confirmed by the bands at low energy (below 1000 cm1), which are related to the stretching and bending modes of AleO bonds. If Alþ3 ions are in the octahedral sites of the six-coordinated AlO6 groups, the structure subsequently is normal spinel and the stretching and bending vibrations are expected at 500e700 cm1 [23e26]. The observed three strong absorption bands at 499, 554, and 665 cm1 were assigned to the characteristic vibration bands of zinc aluminate spinel structure [25], in which, Alþ3 cations occupy the octahedral sites of a cubic lattice. The absorption bands at 1381 cm1 present in all composition can be attributed to the vibration modes of the groups originating from the organic compounds [25]. In all spectra, the observed bands at 1630 cm1 and 3447 cm1 were assigned to H2O deformation vibration and the OeH stretching vibration of water molecules, respectively [25], which was due to the absorption of water from the atmosphere by the nanosized sample having high surface area [27]. 3.5. Absorbance and optical gap Significant information about the mechanism of the electronic
transition and the optical band gap of different materials can be obtained by measuring the absorption spectra in the UVeVis regions. The measured absorbance (A) for Zn1-xFexAl2O4 (0.0 x 1.0) was shown in Fig. 8. The absorbance decreased with increasing the wavelength (l) of the incident photons for all samples. The absorption coefficient (a) was calculated from the absorbance, A, according to Beer-Lambert law [28]:,
a¼
A cl
(6)
where c and l are the concentration and the length of the solution, respectively. The type of the optical transition and the optical band gap (Eg) can be determined using Tauc's equation [29]:
ahv ¼ Bðhv EÞm
(7)
Where hn is the energy of the incident photons, B is a constant and m is an exponent characterizes the type of the optical transition. The exponent m can take four values 1/2, 2 and 1/3, 3 for direct, indirect allowed transition and direct, indirect forbidden transition, respectively. Most semiconductors exhibit an allowed direct transition mechanism. For present dependent of a on hn in Fig. 9, it is noticed that the exponential like behavior and the linear portion that characterizes the optical band edge are clearly shown at the value of m ¼ 1/2. The intercept of the fitting straight line with the x-axis reveals the Eg value as shown in the inset of Fig. 9. For pure ZnAl2O4, Eg is 3.28 eV which is little lower than that previously reported where Eg was 3.80 eV [30]. The dependence of Eg on the Fe ion content in the studied compositions is shown in Fig. 10. The optical energy gap decreases linearly with increasing Fe content in the original spinel according to the empirical equation:
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Fig. 9. The relation between (ahn)2 and (hn) for Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles. The inset shows the fitting for calculating the optical energy gap.
Fig. 7. FTIR spectra of Zn1-xFexAl2O4 nanoparticles, (a) in the range (400-4000 cm1) and (b) in the range (450-850 cm1).
Fig. 10. The energy band gap dependence on the composition of Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles with the value of the adjusted R-squared.
Eg ¼ ð 0:6846 ± 0:0300Þx þ ð3:2596 ± 0:0178Þ;
(8)
reaching to 2.62 eV for pure FeAl2O4 in agreement with that reported by Halenius el al. [31]. It has been observed that when the metal oxide ion of low Eg added to other material of high Eg, the produced new energy band gap will have a value between the two materials [32]. The observed decrease in Eg with adding of Fe ions could be attributed to the formation of new states at lower energy due to defects (vacancies) extant in the sample [33]. 3.6. Magnetic properties
Fig. 8. UVeVis absorbance of Zn1-xFexAl2O4 nanoparticles.
The magnetization curves for the Zn1-xFexAl2O4 (0.1 x 1.0) system at room temperature are shown in Fig. 11. From the figure it is cleared that the magnetization curves do not exhibit hysteresis behavior and the increasing Fe content leads the zinc aluminates to possess magnetic property. This means, an introducing of iron in the matrix of zinc aluminate leads to converse diamagnetic zinc
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successfully prepared by microwave auto combustion method using glycine as a fuel. EDX and XRD analyses indicated the asprepared powder have the spinel structure without any other impurities. HR-TEM revealed that the particle size of samples was in the range of nanoparticles which accorded with X-ray results. The absorption bands between 500 and 700 cm1 in FTIR spectra were assigned to the characteristic vibration bands of spinel structure. The results of absorption spectra in UVeVis regions showed the energy band gap for pure ZnAl2O4 is 3.28 eV and decreases with increasing the ratio of Fe. VSM study showed that an introducing of iron in the matrix of zinc aluminate leads to converse diamagnetic zinc aluminate to superparamagnetic material. References
Fig. 11. Magnetic moment of Zn1-xFexAl2O4 (0.1 x 1.0) nanoparticles. The solid lines are guides to the eye.
Table 2 Magnetic properties for Zn1-xFexAl2O4 (0.1 x 1.0) nanoparticles. Fe content
Hc (G)
Mr 103 (emu/gm)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
271.37 48.28 135.69 17.61 4.98 1.57 26.26 20.68 24.15 26.04
1.33 0.86 1.35 1.40 0.41 0.13 6.16 4.44 8.86 13.75
aluminate to superparamagnetic material. The measured coercivity (Hc) and retentivity (Mr) are given in Table 2. The low values of Hc and Mr are attributed to the characteristic of magnetic nanoparticles where thermal fluctuations are sufficient to overcome the anisotropy energy barrier, thus allowing the magnetization to spontaneously revers the direction [34]. The magnetization curves of Fig. 11 can be divided into two parts: linear parts and curvature parts [35]. The linear parts are interpreted by the antiferromagnetic phase of ZnAl2O4 [36], while the curved parts may be interpreted by magnetic nature of Feþ2. Further adding of Fe cations, the curved part increases and that can be attributed to iron magnetism behavior which has high magnetic moments instead of the lower magnetic moments of zinc in the Zn1-xFexAl2O4 spinel. 4. Conclusions New spinels of Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles were
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Syntheses of new spinels Zn1-xFexAl2O4 nanocrystallines structure: Optical and magnetic characteristics A. Abu El-Fadl a, M.I. Abd-Elrahman a, *, Noha Younis a, b, N. Afify a, A.A. Abu-Sehly a, M.M. Hafiz a a b
Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt Faculty of Computers and Information Technology, National Egyptian E-Learning University, 12611 Giza, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2019 Accepted 1 May 2019 Available online 2 May 2019
Nanocrystalline of ZnAl2O4 spinel was synthesized by employing the microwave combustion method. Different new spinel compositions of formula Zn1-xFexAl2O4, (0.0 x 1.0) were prepared by substitution of Zn ions with Fe ions. The as-synthesized resultant material ZnAl2O4 with spinel structure as well as Zn1-xFexAl2O4 compositions were characterized by Energy Dispersive X-ray and X-ray diffraction. Morphology and particle size evaluation of the nanoparticles were characterized by transmission electron microscopy images. The optical absorption spectra and the magnetization were measured at room temperature for each Fe ratio. The substitution effect of Fe ions on the optical and magnetic properties of the original spinel was studied. The optical absorbance showed that the optical band gap decreased with increasing Fe ratio. © 2019 Elsevier B.V. All rights reserved.
Keywords: Zinc iron aluminate Nanocrystalline Optical gap Magnetic properties
1. Introduction Interest in the synthesis of nanoparticles has increased due to their different properties when compared to the corresponding bulk material. Spinel-type oxides AB2O4, where A represent the divalent cations which in normal spinel occupying tetrahedral sites in a face centred cubic (FCC) lattice of oxygen anions and B represents trivalent cations occupying the octahedral sites of the cubic crystal structure [1]. These materials are suitable for a wide range of applications, such as magnetic materials, catalysis and ceramics [2]. Spinel nanoparticles can be synthesized by several techniques such as solegel [3], co-precipitation [4], microwave combustion [5,6], hydrothermal [7], Penchini method [8,9]. Aluminum-based spinels are intriguing class of oxide ceramics which have important technological applications. In spinel ZnAl2O4 structure, Znþ2 and Alþ3 cations occupy the tetrahedral and octahedral in FCC lattice of oxygen anions respectively. The ZnAl2O4 spinel has attracted considerable attention since it possesses desirable properties such as high thermal stability, high mechanical resistance, better diffusion and ductility, low temperature sinterability, good resistance to acids and bases and excellent optical
* Corresponding author. E-mail address: [email protected] (M.I. Abd-Elrahman). https://doi.org/10.1016/j.jallcom.2019.05.008 0925-8388/© 2019 Elsevier B.V. All rights reserved.
transparency [10e16]. According to those properties, the ZnAl2O4 spinel was used as ceramic materials [10,11], electronic and optoelectronic devices [12e14], catalyst and catalystic support of transition metal [15,16]. Consistent with all of the applications, ultrafine ZnAl2O4 spinel nanoparticles with a narrow particle size distribution and a low degree of agglomeration are desirable. Completely replacement of Zn by Fe in ZnAl2O4 results in FeAl2O4 spinel (hercynite) which is mineral with the spinel structure. Hercynite, which is a representative antiferromagnetic spinel, shows a magnetization regardless of the normal spinel [17]. This work aims to synthesize and characterize Zn1-xFexAl2O4, (0.0 x 1.0) nanoparticles as new spinels by the combustion method. Also, the effect of Fe ions substituted Zn ions on the optical and magnetic properties of ZnAl2O4 nanoparticles was studied. 2. Experimental 2.1. Samples synthesizing In the typical microwave combustion method for preparation of pure ZnAl2O4 spinel, all starting chemicals used in this study were pure grade Zn(NO3)2$6H2O and Al(NO3)3$9H2O. These chemicals were dissolved in double distilled water with glycine (NH2CH2COOH) as a combustion fuel at a fixed ratio 1:2:0.3 (according to their weights) to obtain a homogeneous solution. The
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solution was placed in a microwave oven type [SHARP R-241R (W)] to improve the full reaction occurs as the water evaporates and the combustion takes place. After the combustion, a white dry foamy was generated and the resultant powder was collected to perform further analyses. The synthesis of the composition Zn1-xFexAl2O4 (0.0 x 1.0) took almost the same steps as before but the starting materials were Zn(NO3)2$6H2O, Al(NO3)3$9H2O and Fe(NO3)3$9H2O using also glycine (NH2CH2COOH) as a combustion fuel at a fixed ration (1-x): x:2:0.3. The resultant powders were colored graduate from white to brown with increasing the ratio of Fe. 2.2. Characterization methods The chemical composition of prepared samples was check by Energy-Dispersive X-Ray (EDX) Spectroscopy technique using scanning electron microscope model QUANTA FEG 250 (Field Emission Gun) attached with EDX unit with accelerating voltage 30 kV. X-ray diffraction (XRD) measurements were performed on Philips X-ray diffractometer model PW. 1710 with CuKa radiation (l ¼ 1.5405 Å) with an operating applied voltage 40 kV and current 30 mA. Scanning rate was maintained at 0.06 per minute in the range of 2q from 4 to 70 . The diffraction patterns of the prepared samples were compared with those reported in the database of the Powder Diffraction File (PDF) maintained in the International Centre for Diffraction Data (ICDD). The morphology of the prepared nanoparticles was investigated by high resolution transmission electron microscope (HRTEM-JEOL JEM 2100). Infrared (IR) transmittance spectra of the samples were measured by Nicolet Fourier transformation infrared (FTIR) 6700 spectrometer using the KBr pellet method in the range 400e4000 cm1. Optical absorbance was measured for the powder suspension of the samples using Thermo Evolution 300 UVeVisible Spectrophotometer. The optical measurements were carried out between 200 and 900 nm. A Vibrating Sample Magnetometer (VSM, LakeShore 7400) was used to investigate the magnetic properties of Zn1-xFexAl2O4 spinels at room temperature.
Fig. 1. Energy Dispersive X-ray (EDX) spectrum of Zn1-xFexAl2O4 nanoparticles.
3. Results and discussion 3.1. Energy dispersive X-ray analysis The chemical composition of prepared samples was check by Energy-Dispersive X-Ray (EDX) Spectroscopy technique. The results in Fig. 1 show the spectral distribution of ZnAl2O4 spinel sample and those of substituted with Fe ions. The spectral distribution of substituted samples indicates that the samples are consisted of O, Al, Zn and Fe only. No other elements were observed.
Fig. 2. X-ray diffraction patterns of (a) as prepared ZnAl2O4 nanoparticles and (b) reported ZnAl2O4 nanoparticles in the ICDD PDF files.
3.2. Structure and particle size X-ray powder diffraction measurements were collected for the synthesized materials without any heat treatment. The diffraction patterns of the as prepared ZnAl2O4 and FeAl2O4 were illustrated in Figs. 2a and 3a. Comparing the diffraction patterns of Figs. 2a and 3a with those reported in the ICDD PDF files of Figs. 2b and 3b confirms the spinel cubic structure of synthesized ZnAl2O4 and FeAl2O4. However, the sharpness of the diffraction peaks in Figs. 2a and 3a indicates the good degree of crystallinity of the synthesized spinels. The diffraction data of Fe substitution samples were illustrated in Fig. 4 showing that the samples were crystallized in preferential orientations indicated with different peaks. These orientations were along seven planes. The formation of cubic spinel aluminate structure was confirmed by the existence of diffraction planes of
(220), (311), (400), (331), (422), (511) and (440). In particular, the presence of (440), (400) and (311) diffraction planes is an evidence for face-centred cubic spinel crystal. The strong XRD peaks of (220), (311) and (440) planes indicate these planes are the most preferential orientations. The assignment of the planes was determined on the basis of best agreement between the experimentally observed of inter-planar spacing and diffraction angles with those of ICDD. The average particle size (Dav) in nm can be calculated using the Scherrer equation [18]:
Dav ¼
kl b cos q
(1)
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Where l is the X-ray wavelength, b is the full width at half maximum (FWHM) in radians, q is Bragg's angle and k is constant which assigned to 0.9. The calculated Dav values for observed crystalline phases for each Fe ratio are listed in Table 1. Average lattice constant (a) is calculated as:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l2 l a¼ 2 sin q
(2)
The dislocation density (d) which represents the amount of defects in a given sample is defined as the length of dislocation lines per unit volume of the crystal; and it was calculated by the following equation [19]:,
d¼
1
(3)
D2av
The average internal lattice strain (ε) was estimated using the following relation [20]:
ε¼
b cos q
(4)
4
The theoretical density (r) for any material can be calculated from the XRD data based on the number of molecules per unit cell (Z) as follows [21]: Fig. 3. X-ray diffraction patterns of as prepared (a) as prepared FeAl2O4 nanoparticles and (b) reported FeAl2O4 nanoparticles in the ICDD PDF files.
r¼
ZM NA a3
(5)
where Z ¼ 8 for aluminates and ferrite spinel, M is the molecular weight of the material, NA is Avogadro's number and a is the lattice constant. The calculated values of Dav, d, ε and r were listed in Table 1. From this Table, one notes that the value of r decreases with increasing the ratio of Fe ions. That is because the atomic weight of the substituted iron (55.845 gm/mol) which is lower than that for zinc (65.38 gm/mol). 3.3. TEM analyses
Fig. 4. X-ray diffraction patterns of Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles.
High resolution transmission electron microscope (HR-TEM) was used to study the morphology of the prepared nanoparticles and to evaluate the crystallite grains microstructure. Fig. 5 shows the HR-TEM images of pure ZnAl2O4, Zn0$5Fe0$5Al2O4 and FeAl2O4 at magnification of 50 nm. The TEM images indicated the particles sizes were within the nanoscale range. The micrographs also showed irregular shaped particles with particle size distribution below 50 nm. The average particle size, Dav, was estimated from the TEM images by the so-called image j viewer software [22]. The calculated Dav was found to be 13.55, 13.25 and 10.84 nm for ZnAl2O4, Zn0$5Fe0$5Al2O4 and FeAl2O4, respectively, which in agree with X-ray results. The corresponding selected area of electron diffraction (SAED) patterns was shown in Fig. 6. The ring patterns of
Table 1 Average particle size (Dav), lattice constant (a), dislocation density (d), internal lattice strain (ε) and theoretical density (r) for Zn1-xFexAl2O4. Fe content
Dav (nm)
a (Å)
Vol. (Å3)
d 103 (nm)2
ε 103
r (gm/cm3)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
13.70 12.32 14.09 13.06 15.12 12.86 12.27 10.80 11.26 9.56 9.44
8.119 8.129 8.121 8.123 8.131 8.141 8.153 8.127 8.132 8.145 8.171
535.19 537.17 535.59 535.98 537.57 539.55 541.94 536.77 537.76 540.35 545.54
5.323 6.588 5.037 5.863 4.374 6.047 6.642 8.573 7.887 10.942 11.222
2.645 2.939 2.571 2.779 2.406 2.824 2.967 3.381 3.235 3.817 3.889
4.554 4.512 4.502 4.475 4.438 4.398 4.355 4.374 4.343 4.298 4.234
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Fig. 5. HR-TEM images of Zn1-xFexAl2O4 nanoparticles at magnification of 50 nm for (a) x ¼ 0.0, (b) x ¼ 0.5 and (c) x ¼ 1.0.
Fig. 6. The selected area of electron diffraction (SAED) of Zn1-xFexAl2O4 nanoparticles for (a) x ¼ 0.0, (b) x ¼ 0.5 and (c) x ¼ 1.0.
SAED indicated the crystalline nature of the investigated samples. However, the observed of many spots contained by each ring was ascribed to the presence of large number of nanoparticles. 3.4. Infrared spectroscopy The FTIR spectra for the samples were examined to study their functional groups. Fig. 7(a) shows the FTIR spectra of the Zn1-xFexAl2O4 in the range (400-4000 cm1) and Fig. 7(b) shows the spectra in the range 450e850 cm1 which is the active region for the metal oxide bands. The spinel type structure can be confirmed by the bands at low energy (below 1000 cm1), which are related to the stretching and bending modes of AleO bonds. If Alþ3 ions are in the octahedral sites of the six-coordinated AlO6 groups, the structure subsequently is normal spinel and the stretching and bending vibrations are expected at 500e700 cm1 [23e26]. The observed three strong absorption bands at 499, 554, and 665 cm1 were assigned to the characteristic vibration bands of zinc aluminate spinel structure [25], in which, Alþ3 cations occupy the octahedral sites of a cubic lattice. The absorption bands at 1381 cm1 present in all composition can be attributed to the vibration modes of the groups originating from the organic compounds [25]. In all spectra, the observed bands at 1630 cm1 and 3447 cm1 were assigned to H2O deformation vibration and the OeH stretching vibration of water molecules, respectively [25], which was due to the absorption of water from the atmosphere by the nanosized sample having high surface area [27]. 3.5. Absorbance and optical gap Significant information about the mechanism of the electronic
transition and the optical band gap of different materials can be obtained by measuring the absorption spectra in the UVeVis regions. The measured absorbance (A) for Zn1-xFexAl2O4 (0.0 x 1.0) was shown in Fig. 8. The absorbance decreased with increasing the wavelength (l) of the incident photons for all samples. The absorption coefficient (a) was calculated from the absorbance, A, according to Beer-Lambert law [28]:,
a¼
A cl
(6)
where c and l are the concentration and the length of the solution, respectively. The type of the optical transition and the optical band gap (Eg) can be determined using Tauc's equation [29]:
ahv ¼ Bðhv EÞm
(7)
Where hn is the energy of the incident photons, B is a constant and m is an exponent characterizes the type of the optical transition. The exponent m can take four values 1/2, 2 and 1/3, 3 for direct, indirect allowed transition and direct, indirect forbidden transition, respectively. Most semiconductors exhibit an allowed direct transition mechanism. For present dependent of a on hn in Fig. 9, it is noticed that the exponential like behavior and the linear portion that characterizes the optical band edge are clearly shown at the value of m ¼ 1/2. The intercept of the fitting straight line with the x-axis reveals the Eg value as shown in the inset of Fig. 9. For pure ZnAl2O4, Eg is 3.28 eV which is little lower than that previously reported where Eg was 3.80 eV [30]. The dependence of Eg on the Fe ion content in the studied compositions is shown in Fig. 10. The optical energy gap decreases linearly with increasing Fe content in the original spinel according to the empirical equation:
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Fig. 9. The relation between (ahn)2 and (hn) for Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles. The inset shows the fitting for calculating the optical energy gap.
Fig. 7. FTIR spectra of Zn1-xFexAl2O4 nanoparticles, (a) in the range (400-4000 cm1) and (b) in the range (450-850 cm1).
Fig. 10. The energy band gap dependence on the composition of Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles with the value of the adjusted R-squared.
Eg ¼ ð 0:6846 ± 0:0300Þx þ ð3:2596 ± 0:0178Þ;
(8)
reaching to 2.62 eV for pure FeAl2O4 in agreement with that reported by Halenius el al. [31]. It has been observed that when the metal oxide ion of low Eg added to other material of high Eg, the produced new energy band gap will have a value between the two materials [32]. The observed decrease in Eg with adding of Fe ions could be attributed to the formation of new states at lower energy due to defects (vacancies) extant in the sample [33]. 3.6. Magnetic properties
Fig. 8. UVeVis absorbance of Zn1-xFexAl2O4 nanoparticles.
The magnetization curves for the Zn1-xFexAl2O4 (0.1 x 1.0) system at room temperature are shown in Fig. 11. From the figure it is cleared that the magnetization curves do not exhibit hysteresis behavior and the increasing Fe content leads the zinc aluminates to possess magnetic property. This means, an introducing of iron in the matrix of zinc aluminate leads to converse diamagnetic zinc
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successfully prepared by microwave auto combustion method using glycine as a fuel. EDX and XRD analyses indicated the asprepared powder have the spinel structure without any other impurities. HR-TEM revealed that the particle size of samples was in the range of nanoparticles which accorded with X-ray results. The absorption bands between 500 and 700 cm1 in FTIR spectra were assigned to the characteristic vibration bands of spinel structure. The results of absorption spectra in UVeVis regions showed the energy band gap for pure ZnAl2O4 is 3.28 eV and decreases with increasing the ratio of Fe. VSM study showed that an introducing of iron in the matrix of zinc aluminate leads to converse diamagnetic zinc aluminate to superparamagnetic material. References
Fig. 11. Magnetic moment of Zn1-xFexAl2O4 (0.1 x 1.0) nanoparticles. The solid lines are guides to the eye.
Table 2 Magnetic properties for Zn1-xFexAl2O4 (0.1 x 1.0) nanoparticles. Fe content
Hc (G)
Mr 103 (emu/gm)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
271.37 48.28 135.69 17.61 4.98 1.57 26.26 20.68 24.15 26.04
1.33 0.86 1.35 1.40 0.41 0.13 6.16 4.44 8.86 13.75
aluminate to superparamagnetic material. The measured coercivity (Hc) and retentivity (Mr) are given in Table 2. The low values of Hc and Mr are attributed to the characteristic of magnetic nanoparticles where thermal fluctuations are sufficient to overcome the anisotropy energy barrier, thus allowing the magnetization to spontaneously revers the direction [34]. The magnetization curves of Fig. 11 can be divided into two parts: linear parts and curvature parts [35]. The linear parts are interpreted by the antiferromagnetic phase of ZnAl2O4 [36], while the curved parts may be interpreted by magnetic nature of Feþ2. Further adding of Fe cations, the curved part increases and that can be attributed to iron magnetism behavior which has high magnetic moments instead of the lower magnetic moments of zinc in the Zn1-xFexAl2O4 spinel. 4. Conclusions New spinels of Zn1-xFexAl2O4 (0.0 x 1.0) nanoparticles were
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