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A new approach to the synthesis of nanostructured Fe3Al alloy and aluminum doped iron oxide material
Advanced Powder Technology 23 (2012) 839–844
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
A new approach to the synthesis of nanostructured Fe3Al alloy and aluminum doped iron oxide material Mohammad Bamdad a,⇑, Mohammad Yeganeh Ghotbi b a b
Physics Department, Faculty of Science, Borujerd Branch, Islamic Azad University, Borujerd, Iran Nanomaterials and Nanotechnology Program, Ceramic Engineering Department, Faculty of Engineering, University of Malayer, Malayer, Iran
a r t i c l e
i n f o
Article history: Received 3 October 2011 Received in revised form 19 November 2011 Accepted 28 November 2011 Available online 13 December 2011 Keywords: Nanostructure Intermetallics Oxide materials Amorphous materials
a b s t r a c t Aluminum doped iron oxide nanostructured material with hematite phase has been produced by the heat-treatment process of an amorphous precursor under air atmosphere. The nano sized precursor was synthesized by using a co-precipitation method. The heat-treated product of the precursor under reduction atmosphere was Fe3Al alloy with particle sizes around 200 nm. Powder X-ray diffraction pattern and Fourier transform infrared spectroscopy confirmed the formation of the products. That is, the intermetallic alloy, Fe3Al has really produced via a wet chemical route for first time. Magnetic measurements indicated that all the samples are soft magnets with different magnetization. Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction The iron oxide based nanostructured materials have been used for applications in various industrial and medical areas. This is due to their magnetic, electric and optoelectronic properties leading to the use of the materials in water purification [1,2], magnetic refrigeration [3], thermoelectric devices [4], sensors and biosensors [5,6], deliver useful functional materials [7], tissue targeting [8,9], magnetic resonance imaging [10], etc. Furthermore, iron in its oxide structure can be replaced by other elements to promote or modify the metal oxide properties for various practical applications. For example, the replacement of iron by aluminum improves the use of the material in water purification [2] and catalysis applications [11,12]. As a result, the aluminum doped iron oxide has been prepared using co-precipitation [2] and sol–gel [13] methods. However, the material has not been produced from an amorphous iron–aluminum precursor obtained by a wet chemical synthesis method in our knowledge. On the other hand, iron–aluminum alloy, namely Fe3Al is an important intermetallic material that has been used in magnetoelectronic devices, anti-wear, carbon adsorption and structural applications [14–19], etc. This is due to its high strength/density ratio and excellent corrosion resistance in various oxidizing, sulfurizing and carbonizing environments. The material is produced either by mechanochemical process using high energy ball milling or by co-melting method [19–23]. ⇑ Corresponding author. Tel.: +98 662 3518015; fax: +98 662 3518016. E-mail addresses: [email protected], [email protected] (M. Bamdad).
In the co-melting method, high temperatures are necessary in addition to high vacuum conditions (10 4 Pa). The use of highfrequency induction heating method decreases the temperature to 1000 K at the same vacuum [21,24,25]. However, this method requires expensive equipment leading to the expensive product. On the other hand, the preparation of the material by using high energy ball milling also needs expensive apparatus due to the use of the milling media (hardened chromium steel or tungsten carbide balls and vials). Furthermore, high vacuum or high purity argon is necessary for the sample preparation. Another disadvantage of this method is time wasting [19,20]. This study deals with the synthesis and characterization of an amorphous iron–aluminum precursor and the heat-treatment of the material in oxidation and reduction atmospheres to produce aluminum doped iron oxide and iron–aluminum metallic alloy nanomaterials, respectively. Results from the powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), surface area studies and also magnetic measurements are discussed. 2. Experimental All solutions were prepared using de-ionized water. The initial sample was synthesized by the precipitation method from an aqueous solution of iron and aluminum salts. FeCl36H2O and Al(NO3)39H2O were dissolved in de-ionized water with initial Fe to Al molar ratio in the mother liquor (Fe/Al) of R = 3 and ([Fe] + [Al] = 1 M). Forty milliliters of this solution was added to 200 mL of urea solution (6 M) under vigorous stirring. The reaction
0921-8831/$ - see front matter Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2011.11.005
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mixture was heated to 90 °C and maintained at this temperature for 3 h. The precipitate was centrifuged, washed with de-ionized water and ethanol several times and finally dried in an oven, overnight at 80 °C. The synthesized precursor sample, (labeled as AIA) was heated at 300, 500, 700 and 900 °C in an electric furnace at air atmospheric for 2 h at a rate of 5 °C/min. The precursor, AIA was also heat-treated at the same temperatures in a tubular furnace under H2/Ar (10/90%) atmosphere at a flow rate of 100 ml/min for 2 h at a rate of 5 °C/min. Since the heat-treated products under reduction atmosphere were not completely reduced below 700 °C and due to the comparing the oxidation and reduction products, further analyses were done for the products heated at 700 °C for both atmospheres. The heat-treated product at 700 °C under oxidation atmosphere was labeled as OIA and the product heated under reduction atmosphere at the same temperature was labeled as RIA. Powder X-ray diffraction patterns were recorded on a GNR MPD 3000 powder diffractometer unit using CuKa (k = 1.5418 Å) at 40 kV and 30 mA. Fourier transform infrared (FTIR) spectra were recorded using a BRUKER (VECTOR 33) spectrophotometer in the range of 400–4000 cm 1. Inductively coupled plasma-atomic emission spectrophotometer (ICP-AES), a Labtest Equipment Co. Model 710 Plasmascan, was used to determine the mole fraction ratios of metal ions ([Fe]/[Al]) in the precursor, AIA and in its heat-treated products, OIA and RIA samples. A scanning electron microscope (VEGA II TESCAN) was used to study the surface morphology of AIA, OIA and RIA samples. Transmission electron microscopy (TEM) observation was taken by a PHILIPS CM120 electron microscope. The surface area was determined with a BELSORP measuring instrument (BELSORP-mini, Japan, Inc.) using nitrogen gas adsorption–desorption technique at 77 K. The magnetic measurements were done using a vibrating sample magnetometer. 3. Results and discussion 3.1. Powder X-ray diffraction Fig. 1 shows powder X-ray diffraction (PXRD) patterns for the initial precursor (AIA), its heat-treated product under oxidation (OIA) and under reduction (RIA) atmospheres. As shown in the figure, the lack of diffraction peaks for the precursor confirms the amorphous nature of the material (Fig. 1a). The heat-treated sample in air atmosphere, OIA (Fig. 1b) shows the characteristic peaks due to the formation of pure hematite (a-Fe2O3) with rhombohedral structure (JCPDS Card 01-1053) [1,26,27]. The lack of additional reflections due to Al2O3 reveals that the incorporation of aluminum ions within the structure of hematite does not change
the material structure. It means that the aluminum ions are well replaced with iron ions in its lattice positions. Since, crystal ionic radii for Fe3+ (0.69 Å) and Al3+ (0.675 Å) are similar; they have almost the same ionic potentials leading to similar electrostatic interactions in their lattice positions, resulting in the same diffraction planes [28]. As a result, iron oxide and aluminum doped iron oxide have the same diffraction patterns [28]. However, incorporation of aluminum ions into the host hematite lattice leads to decrease in intensity of all reflections of the doped hematite in compression with the pure hematite [28,29]. Fig. 1c shows PXRD pattern of Fe3Al alloy (JCPDS Card 06-0695) with cubic structure, RIA obtained by the heat-treatment of the amorphous precursor in a reduction atmosphere [20,25]. The obtained pure phase alloy confirms the eligibility of the method to produce the intermetallic alloy. The mean crystallite sizes estimated according to Debye– Scherrer’s method for OIA sample by using 104, 110 and 113 planes and for RIA sample by 111, 200 and 220 planes were 42.1 and 69.2 nm, respectively. 3.2. FTIR study The chemical bonding characteristics of the as-prepared precursor, AIA and its heat-treated products, OIA and RIA were evaluated by IR spectroscopy. Fig. 2a displays the characteristic vibration modes of precursor synthesized using urea decomposition. As observed, a low intensity band at 475 cm 1 and a band with higher intensity around 614 cm 1 are due to metal–oxide and/or metal– hydroxide bonds in the material [26,29]. The intense broad band around 1117 cm 1 can be attributed to the cyanate ions, OCN [30]. Two bands at 1654 cm 1 asymmetric and at 1423 cm 1 symmetric vibrations show the presence of C@O groups of the urea [30–32]. A shoulder around 3214 cm 1 is attributed to stretching vibration of –NH2 group [30,32]. OCN , C@O and –NH2 groups are produced by decomposition of urea during the synthesis, which are adsorbed onto the surface of the as-prepared amorphous material [33,34]. The broad absorption band around 3425 cm 1 is related to the O–H stretching of physically adsorbed water molecules [29]. FTIR spectrum of OIA, the product of the heat-treated precursor in oxidation atmosphere is shown in Fig. 2b. As shown in the figure, only metal oxide bands around 475 and 540 cm 1 can be seen. It means that the adsorbed anions onto the surface of the precursor are burnt out completely at 700 °C. With the disappearing of intensities for these decomposed groups the M–O vibrational absorption bands have increased obviously [29]. FTIR spectrum feature of OIA is the same with that of hematite iron oxide phase [26]. The bands at 1631 and 3424 cm 1 are ascribed to water bending and
214
116
024
40
400
220
113
104 200
012 111 20
110
intensity/a.u.
(a)
60
(b)
(c) 80
2θ/degrees Fig. 1. PXRD patterns of the amorphous precursor, AIA (a), the heat-treated product of the precursor under oxidation media, OIA (b) and the heat-treated product of the precursor under reduction media, RIA (c).
Fig. 2. FTIR spectra for the amorphous precursor, AIA (a), the heat-treated product of the precursor under oxidation media, OIA (b) and the heat-treated product of the precursor under reduction media, RIA (c).
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stretching vibrations of physically adsorbed water molecules, respectively [29]. One can expect the absence of any absorption bands in the FTIR spectrum of RIA sample. This is due to the fact that there are no any chemical bonds in a metallic alloy as displayed in Fig. 2c. Only adsorbed water molecule vibrations can be observed in this spectrum feature. Therefore, FTIR evaluation together with PXRD pattern studies confirm the formation of aluminum doped iron oxide and aluminum–iron alloy materials by using the heat-treatment of an amorphous precursor under oxidation and reduction atmosphere, respectively. Elemental analysis showed that the mole fraction ratio of iron and aluminum ions in the mother liquor and in the precursor (AIA) and also in its heat-treated products OIA and RIA donot have remarkable differences.
Table 1 Obtained data from surface analyses of AIA, OIA and RIA samples.
a
Sample
Surface areaa (m2/g)
Total pore volume,a Vt (cm3/g)
Average pore diametera (nm)
AIA OIA RIA
12.19 60.85 15.84
0.1295 0.4989 0.1822
42.50 32.79 45.99
Calculated by BET method.
15.0 7.5
(a)
0
3.3. Surface property study
-7.5
Fig. 3 shows the nitrogen adsorption–desorption isotherms for amorphous precursor (AIA), iron–aluminum oxide material (OIA) and iron–aluminum alloy (RIA). Furthermore, Brunauer– Emmett–Teller (BET) surface areas, total pore volumes and average pore diameters are given in Table 1. As indicated in Fig. 3, the adsorption–desorption isotherms for all the samples have a type II
-15.0 -10000
0
10000
M (emu/g)
0.8 0.4
(b)
0 -0.4 -0.8 -10000
0
10000
100 50
(c)
0 -50 -100 -10000
0
10000
H (Oe) Fig. 4. Magnetization versus magnetic field for AIA (a), OIA (b) and RIA (c) samples.
Fig. 3. Adsorption–desorption isotherms of nitrogen gas for AIA (a), OIA (b) and RIA (c).
classification and a type H3 hysteresis loop, which is typically observed for nonporous or macroporous materials following the IUPAC nomenclature [35]. This behavior is characterized with slow gas uptake in low relative pressure and very rapid near the saturated pressure with no any limiting adsorption at high p/p° [35]. The increase in the adsorption amount around high relative pressure would be due to the voids in aggregates of particles. In fact, all the samples are composed of large aggregates of particles (Fig. 5). In addition, aggregates of smaller particles generally offer smaller voids, which agrees well with the trend seen from the isotherms in Fig. 3. Regarding to data shown in Table 1, the surface area for AIA is 12.2 m2/g and this increases for its heat-treated products under both oxidation and reduction media. The surface area has increased to 60.85 and 15.84 m2/g for for OIA and RIA samples, respectively. It can be assigned to evaporation of some functional groups that are consisting within precursor as indicated in its FTIR spectrum.
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Fig. 5. SEM image of the amorphous precursor, AIA (a), TEM micrograph of the same sample (b) (scale bar is 50 nm), SEM images of the heat-treated product of the precursor under oxidation media, OIA (c) and the heat-treated product of the precursor under reduction media, RIA (d).
3.4. Magnetic measurements In order to study the magnetic behavior of the samples, magnetization measurements were performed. As can be observed in Fig. 4, the room-temperature magnetization curves (M–H loop) show narrow hysteresis loops for all the samples, amorphous precursor and its heat-treated products, indicating they are soft magnets [36,37]. The saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and squareness (Sr = Mr/Ms) values obtained from the magnetization curve are listed in Table 2. The saturation magnetization values were 10.20, 0.544 and 94.42 emu/g for AIA, OIA and RIA, respectively. As shown in the table, Ms increased for RIA and it decreased for OIA sample in comparison with that of the parent material, AIA. Although, AIA shows amorphous nature, some vague peaks can be seen in its PXRD pattern [38,39]. Those are at around 30° and 36° which can be attributed to 206 and 313 planes of maghemite (c-Fe2O3), respectively [38,39]. Thus, AIA may be formed from high defective structure of maghemite with high Ms for its bulk material (73.5 emu/g) and it is converted to hematite phase at the temperatures higher than 500 °C [26,27]. The maghemite and hematite
Table 2 Data obtained from magnetic measurements of AIA, OIA and RIA samples. Sample
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
Sr
AIA OIA RIA
10.20 0.544 94.42
2.41 0.072 1.43
92.9 177.2 58.4
0.235 0.132 0.015
are the materials with ferromagnetic and antiferromagnetic properties, respectively [38,39]. At room temperature nano-hematite behaves like a weak ferromagnet with low saturation magnetization [38]. The decrease in Ms for OIA sample compared to that of the precursor, AIA can be due to the formation of the crystalline hematite iron oxide phase [27,36,40]. Saturation magnetization can not be observed up to the maximum applied magnetic field (8500 Oe) [26,27]. This is a characteristic of hematite iron oxide material with weak ferromagnetic behavior [26,27,41]. The substitution of iron ions by aluminum ions within the hematite structure causes to the enhancement of the Ms and Hc for the doped material, OIA in comparison with those of the pure hematite material [28]. This enhancement can be emanated from distribution of aluminum
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ions in preferred A-site [28,42]. In the antiferromagnetic material, the net magnetization is the difference between A and B sub lattice magnetizations [42]. The replacement of iron ions by aluminum ions in the A sub lattice leads to decrease in magnetic moment of A sub lattice and this causes to enhance in the total magnetic moment [28]. For RIA sample the saturation magnetization has been increased in comparison with that of the precursor, although Mr and Hc have been decreased for this alloy. RIA magnetic hysteresis loop feature has been observed in previous research with higher value for Ms (200 emu/g) [43]. The decrease in Ms for RIA is due to the decrease in the particle size of RIA particles. For smaller particles, the surface area and the amorphous structures especially in the nanoparticle surfaces are increased and it therefore results in the decline of the effective magnetic moments and the decrease of Ms [44,45]. Generally, this very thin hysteresis loop feature recommends that the sample is a soft magnet with nearly superparamagnetic type. Superparamagnetism occurs for the materials with very small crystallites. In the case, the magnetic particles have basically single domains. That is, in a particle all magnetic moments are aligned in one direction [37,39,46]. 3.5. Surface morphology study Fig. 5a and b illustrate SEM and TEM images of the amorphous precursor, AIA. As exhibited in the figure, seed-like nanoparticles with the sizes around 80 nm can be observed. SEM image shows that the nanoparticles are agglomerated intensively. Fig. 5c shows the heat-treated product of the precursor under oxidation media (OIA), the particles are also agglomerated with the sizes bigger than those of the parent material, AIA. The heat-treated product of the precursor under reduction media (RIA) shows the particles with sizes around 200 nm. This increasing in the particle sizes for the products occurs as a result of the heat-treatment process. The different between the mean crystallite sizes calculated using the Debye– Scherrer’s formula for the samples (OIA and RIA) and the sizes obtained by SEM reveals that the samples are poly crystals [47,48]. 4. Conclusions An amorphous aluminum–iron compound was produced by the co-precipitation method in the presence of urea. Aluminum doped iron oxide with pure phase hematite was obtained by the heattreatment of the amorphous precursor under oxidation atmosphere. Fe3Al alloy nanomaterial was also produced when the heat-treatment process of the precursor was done under reduction atmosphere. The results confirmed the formation of an intermetallic alloy, Fe3Al via a wet chemical route for first time. Magnetic measurements revealed that the amorphous precursor, Aluminum doped iron oxide and also Fe3Al alloy are all soft magnets. The results demonstrated that this easy to make and large scale producing method can be used to produce the pure phase doped oxides. Furthermore, other alloys can be produced by this method with cheap and nonpoisonous chemicals. Acknowledgement This study was supported by Islamic Azad University, Borujerd Branch, Iran. The authors would like to acknowledge staffs of university. References [1] N. Chen, Z. Zhang, C. Feng, D. Zhu, Y. Yang, N. Sugiura, Preparation and characterization of porous granular ceramic containing dispersed Aluminum and Iron oxides as adsorbents for fluoride removal from aqueous solution, J. Hazard. Mater. 186 (2011) 863–868.
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
A new approach to the synthesis of nanostructured Fe3Al alloy and aluminum doped iron oxide material Mohammad Bamdad a,⇑, Mohammad Yeganeh Ghotbi b a b
Physics Department, Faculty of Science, Borujerd Branch, Islamic Azad University, Borujerd, Iran Nanomaterials and Nanotechnology Program, Ceramic Engineering Department, Faculty of Engineering, University of Malayer, Malayer, Iran
a r t i c l e
i n f o
Article history: Received 3 October 2011 Received in revised form 19 November 2011 Accepted 28 November 2011 Available online 13 December 2011 Keywords: Nanostructure Intermetallics Oxide materials Amorphous materials
a b s t r a c t Aluminum doped iron oxide nanostructured material with hematite phase has been produced by the heat-treatment process of an amorphous precursor under air atmosphere. The nano sized precursor was synthesized by using a co-precipitation method. The heat-treated product of the precursor under reduction atmosphere was Fe3Al alloy with particle sizes around 200 nm. Powder X-ray diffraction pattern and Fourier transform infrared spectroscopy confirmed the formation of the products. That is, the intermetallic alloy, Fe3Al has really produced via a wet chemical route for first time. Magnetic measurements indicated that all the samples are soft magnets with different magnetization. Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction The iron oxide based nanostructured materials have been used for applications in various industrial and medical areas. This is due to their magnetic, electric and optoelectronic properties leading to the use of the materials in water purification [1,2], magnetic refrigeration [3], thermoelectric devices [4], sensors and biosensors [5,6], deliver useful functional materials [7], tissue targeting [8,9], magnetic resonance imaging [10], etc. Furthermore, iron in its oxide structure can be replaced by other elements to promote or modify the metal oxide properties for various practical applications. For example, the replacement of iron by aluminum improves the use of the material in water purification [2] and catalysis applications [11,12]. As a result, the aluminum doped iron oxide has been prepared using co-precipitation [2] and sol–gel [13] methods. However, the material has not been produced from an amorphous iron–aluminum precursor obtained by a wet chemical synthesis method in our knowledge. On the other hand, iron–aluminum alloy, namely Fe3Al is an important intermetallic material that has been used in magnetoelectronic devices, anti-wear, carbon adsorption and structural applications [14–19], etc. This is due to its high strength/density ratio and excellent corrosion resistance in various oxidizing, sulfurizing and carbonizing environments. The material is produced either by mechanochemical process using high energy ball milling or by co-melting method [19–23]. ⇑ Corresponding author. Tel.: +98 662 3518015; fax: +98 662 3518016. E-mail addresses: [email protected], [email protected] (M. Bamdad).
In the co-melting method, high temperatures are necessary in addition to high vacuum conditions (10 4 Pa). The use of highfrequency induction heating method decreases the temperature to 1000 K at the same vacuum [21,24,25]. However, this method requires expensive equipment leading to the expensive product. On the other hand, the preparation of the material by using high energy ball milling also needs expensive apparatus due to the use of the milling media (hardened chromium steel or tungsten carbide balls and vials). Furthermore, high vacuum or high purity argon is necessary for the sample preparation. Another disadvantage of this method is time wasting [19,20]. This study deals with the synthesis and characterization of an amorphous iron–aluminum precursor and the heat-treatment of the material in oxidation and reduction atmospheres to produce aluminum doped iron oxide and iron–aluminum metallic alloy nanomaterials, respectively. Results from the powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), surface area studies and also magnetic measurements are discussed. 2. Experimental All solutions were prepared using de-ionized water. The initial sample was synthesized by the precipitation method from an aqueous solution of iron and aluminum salts. FeCl36H2O and Al(NO3)39H2O were dissolved in de-ionized water with initial Fe to Al molar ratio in the mother liquor (Fe/Al) of R = 3 and ([Fe] + [Al] = 1 M). Forty milliliters of this solution was added to 200 mL of urea solution (6 M) under vigorous stirring. The reaction
0921-8831/$ - see front matter Ó 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2011.11.005
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mixture was heated to 90 °C and maintained at this temperature for 3 h. The precipitate was centrifuged, washed with de-ionized water and ethanol several times and finally dried in an oven, overnight at 80 °C. The synthesized precursor sample, (labeled as AIA) was heated at 300, 500, 700 and 900 °C in an electric furnace at air atmospheric for 2 h at a rate of 5 °C/min. The precursor, AIA was also heat-treated at the same temperatures in a tubular furnace under H2/Ar (10/90%) atmosphere at a flow rate of 100 ml/min for 2 h at a rate of 5 °C/min. Since the heat-treated products under reduction atmosphere were not completely reduced below 700 °C and due to the comparing the oxidation and reduction products, further analyses were done for the products heated at 700 °C for both atmospheres. The heat-treated product at 700 °C under oxidation atmosphere was labeled as OIA and the product heated under reduction atmosphere at the same temperature was labeled as RIA. Powder X-ray diffraction patterns were recorded on a GNR MPD 3000 powder diffractometer unit using CuKa (k = 1.5418 Å) at 40 kV and 30 mA. Fourier transform infrared (FTIR) spectra were recorded using a BRUKER (VECTOR 33) spectrophotometer in the range of 400–4000 cm 1. Inductively coupled plasma-atomic emission spectrophotometer (ICP-AES), a Labtest Equipment Co. Model 710 Plasmascan, was used to determine the mole fraction ratios of metal ions ([Fe]/[Al]) in the precursor, AIA and in its heat-treated products, OIA and RIA samples. A scanning electron microscope (VEGA II TESCAN) was used to study the surface morphology of AIA, OIA and RIA samples. Transmission electron microscopy (TEM) observation was taken by a PHILIPS CM120 electron microscope. The surface area was determined with a BELSORP measuring instrument (BELSORP-mini, Japan, Inc.) using nitrogen gas adsorption–desorption technique at 77 K. The magnetic measurements were done using a vibrating sample magnetometer. 3. Results and discussion 3.1. Powder X-ray diffraction Fig. 1 shows powder X-ray diffraction (PXRD) patterns for the initial precursor (AIA), its heat-treated product under oxidation (OIA) and under reduction (RIA) atmospheres. As shown in the figure, the lack of diffraction peaks for the precursor confirms the amorphous nature of the material (Fig. 1a). The heat-treated sample in air atmosphere, OIA (Fig. 1b) shows the characteristic peaks due to the formation of pure hematite (a-Fe2O3) with rhombohedral structure (JCPDS Card 01-1053) [1,26,27]. The lack of additional reflections due to Al2O3 reveals that the incorporation of aluminum ions within the structure of hematite does not change
the material structure. It means that the aluminum ions are well replaced with iron ions in its lattice positions. Since, crystal ionic radii for Fe3+ (0.69 Å) and Al3+ (0.675 Å) are similar; they have almost the same ionic potentials leading to similar electrostatic interactions in their lattice positions, resulting in the same diffraction planes [28]. As a result, iron oxide and aluminum doped iron oxide have the same diffraction patterns [28]. However, incorporation of aluminum ions into the host hematite lattice leads to decrease in intensity of all reflections of the doped hematite in compression with the pure hematite [28,29]. Fig. 1c shows PXRD pattern of Fe3Al alloy (JCPDS Card 06-0695) with cubic structure, RIA obtained by the heat-treatment of the amorphous precursor in a reduction atmosphere [20,25]. The obtained pure phase alloy confirms the eligibility of the method to produce the intermetallic alloy. The mean crystallite sizes estimated according to Debye– Scherrer’s method for OIA sample by using 104, 110 and 113 planes and for RIA sample by 111, 200 and 220 planes were 42.1 and 69.2 nm, respectively. 3.2. FTIR study The chemical bonding characteristics of the as-prepared precursor, AIA and its heat-treated products, OIA and RIA were evaluated by IR spectroscopy. Fig. 2a displays the characteristic vibration modes of precursor synthesized using urea decomposition. As observed, a low intensity band at 475 cm 1 and a band with higher intensity around 614 cm 1 are due to metal–oxide and/or metal– hydroxide bonds in the material [26,29]. The intense broad band around 1117 cm 1 can be attributed to the cyanate ions, OCN [30]. Two bands at 1654 cm 1 asymmetric and at 1423 cm 1 symmetric vibrations show the presence of C@O groups of the urea [30–32]. A shoulder around 3214 cm 1 is attributed to stretching vibration of –NH2 group [30,32]. OCN , C@O and –NH2 groups are produced by decomposition of urea during the synthesis, which are adsorbed onto the surface of the as-prepared amorphous material [33,34]. The broad absorption band around 3425 cm 1 is related to the O–H stretching of physically adsorbed water molecules [29]. FTIR spectrum of OIA, the product of the heat-treated precursor in oxidation atmosphere is shown in Fig. 2b. As shown in the figure, only metal oxide bands around 475 and 540 cm 1 can be seen. It means that the adsorbed anions onto the surface of the precursor are burnt out completely at 700 °C. With the disappearing of intensities for these decomposed groups the M–O vibrational absorption bands have increased obviously [29]. FTIR spectrum feature of OIA is the same with that of hematite iron oxide phase [26]. The bands at 1631 and 3424 cm 1 are ascribed to water bending and
214
116
024
40
400
220
113
104 200
012 111 20
110
intensity/a.u.
(a)
60
(b)
(c) 80
2θ/degrees Fig. 1. PXRD patterns of the amorphous precursor, AIA (a), the heat-treated product of the precursor under oxidation media, OIA (b) and the heat-treated product of the precursor under reduction media, RIA (c).
Fig. 2. FTIR spectra for the amorphous precursor, AIA (a), the heat-treated product of the precursor under oxidation media, OIA (b) and the heat-treated product of the precursor under reduction media, RIA (c).
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stretching vibrations of physically adsorbed water molecules, respectively [29]. One can expect the absence of any absorption bands in the FTIR spectrum of RIA sample. This is due to the fact that there are no any chemical bonds in a metallic alloy as displayed in Fig. 2c. Only adsorbed water molecule vibrations can be observed in this spectrum feature. Therefore, FTIR evaluation together with PXRD pattern studies confirm the formation of aluminum doped iron oxide and aluminum–iron alloy materials by using the heat-treatment of an amorphous precursor under oxidation and reduction atmosphere, respectively. Elemental analysis showed that the mole fraction ratio of iron and aluminum ions in the mother liquor and in the precursor (AIA) and also in its heat-treated products OIA and RIA donot have remarkable differences.
Table 1 Obtained data from surface analyses of AIA, OIA and RIA samples.
a
Sample
Surface areaa (m2/g)
Total pore volume,a Vt (cm3/g)
Average pore diametera (nm)
AIA OIA RIA
12.19 60.85 15.84
0.1295 0.4989 0.1822
42.50 32.79 45.99
Calculated by BET method.
15.0 7.5
(a)
0
3.3. Surface property study
-7.5
Fig. 3 shows the nitrogen adsorption–desorption isotherms for amorphous precursor (AIA), iron–aluminum oxide material (OIA) and iron–aluminum alloy (RIA). Furthermore, Brunauer– Emmett–Teller (BET) surface areas, total pore volumes and average pore diameters are given in Table 1. As indicated in Fig. 3, the adsorption–desorption isotherms for all the samples have a type II
-15.0 -10000
0
10000
M (emu/g)
0.8 0.4
(b)
0 -0.4 -0.8 -10000
0
10000
100 50
(c)
0 -50 -100 -10000
0
10000
H (Oe) Fig. 4. Magnetization versus magnetic field for AIA (a), OIA (b) and RIA (c) samples.
Fig. 3. Adsorption–desorption isotherms of nitrogen gas for AIA (a), OIA (b) and RIA (c).
classification and a type H3 hysteresis loop, which is typically observed for nonporous or macroporous materials following the IUPAC nomenclature [35]. This behavior is characterized with slow gas uptake in low relative pressure and very rapid near the saturated pressure with no any limiting adsorption at high p/p° [35]. The increase in the adsorption amount around high relative pressure would be due to the voids in aggregates of particles. In fact, all the samples are composed of large aggregates of particles (Fig. 5). In addition, aggregates of smaller particles generally offer smaller voids, which agrees well with the trend seen from the isotherms in Fig. 3. Regarding to data shown in Table 1, the surface area for AIA is 12.2 m2/g and this increases for its heat-treated products under both oxidation and reduction media. The surface area has increased to 60.85 and 15.84 m2/g for for OIA and RIA samples, respectively. It can be assigned to evaporation of some functional groups that are consisting within precursor as indicated in its FTIR spectrum.
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Fig. 5. SEM image of the amorphous precursor, AIA (a), TEM micrograph of the same sample (b) (scale bar is 50 nm), SEM images of the heat-treated product of the precursor under oxidation media, OIA (c) and the heat-treated product of the precursor under reduction media, RIA (d).
3.4. Magnetic measurements In order to study the magnetic behavior of the samples, magnetization measurements were performed. As can be observed in Fig. 4, the room-temperature magnetization curves (M–H loop) show narrow hysteresis loops for all the samples, amorphous precursor and its heat-treated products, indicating they are soft magnets [36,37]. The saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and squareness (Sr = Mr/Ms) values obtained from the magnetization curve are listed in Table 2. The saturation magnetization values were 10.20, 0.544 and 94.42 emu/g for AIA, OIA and RIA, respectively. As shown in the table, Ms increased for RIA and it decreased for OIA sample in comparison with that of the parent material, AIA. Although, AIA shows amorphous nature, some vague peaks can be seen in its PXRD pattern [38,39]. Those are at around 30° and 36° which can be attributed to 206 and 313 planes of maghemite (c-Fe2O3), respectively [38,39]. Thus, AIA may be formed from high defective structure of maghemite with high Ms for its bulk material (73.5 emu/g) and it is converted to hematite phase at the temperatures higher than 500 °C [26,27]. The maghemite and hematite
Table 2 Data obtained from magnetic measurements of AIA, OIA and RIA samples. Sample
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
Sr
AIA OIA RIA
10.20 0.544 94.42
2.41 0.072 1.43
92.9 177.2 58.4
0.235 0.132 0.015
are the materials with ferromagnetic and antiferromagnetic properties, respectively [38,39]. At room temperature nano-hematite behaves like a weak ferromagnet with low saturation magnetization [38]. The decrease in Ms for OIA sample compared to that of the precursor, AIA can be due to the formation of the crystalline hematite iron oxide phase [27,36,40]. Saturation magnetization can not be observed up to the maximum applied magnetic field (8500 Oe) [26,27]. This is a characteristic of hematite iron oxide material with weak ferromagnetic behavior [26,27,41]. The substitution of iron ions by aluminum ions within the hematite structure causes to the enhancement of the Ms and Hc for the doped material, OIA in comparison with those of the pure hematite material [28]. This enhancement can be emanated from distribution of aluminum
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ions in preferred A-site [28,42]. In the antiferromagnetic material, the net magnetization is the difference between A and B sub lattice magnetizations [42]. The replacement of iron ions by aluminum ions in the A sub lattice leads to decrease in magnetic moment of A sub lattice and this causes to enhance in the total magnetic moment [28]. For RIA sample the saturation magnetization has been increased in comparison with that of the precursor, although Mr and Hc have been decreased for this alloy. RIA magnetic hysteresis loop feature has been observed in previous research with higher value for Ms (200 emu/g) [43]. The decrease in Ms for RIA is due to the decrease in the particle size of RIA particles. For smaller particles, the surface area and the amorphous structures especially in the nanoparticle surfaces are increased and it therefore results in the decline of the effective magnetic moments and the decrease of Ms [44,45]. Generally, this very thin hysteresis loop feature recommends that the sample is a soft magnet with nearly superparamagnetic type. Superparamagnetism occurs for the materials with very small crystallites. In the case, the magnetic particles have basically single domains. That is, in a particle all magnetic moments are aligned in one direction [37,39,46]. 3.5. Surface morphology study Fig. 5a and b illustrate SEM and TEM images of the amorphous precursor, AIA. As exhibited in the figure, seed-like nanoparticles with the sizes around 80 nm can be observed. SEM image shows that the nanoparticles are agglomerated intensively. Fig. 5c shows the heat-treated product of the precursor under oxidation media (OIA), the particles are also agglomerated with the sizes bigger than those of the parent material, AIA. The heat-treated product of the precursor under reduction media (RIA) shows the particles with sizes around 200 nm. This increasing in the particle sizes for the products occurs as a result of the heat-treatment process. The different between the mean crystallite sizes calculated using the Debye– Scherrer’s formula for the samples (OIA and RIA) and the sizes obtained by SEM reveals that the samples are poly crystals [47,48]. 4. Conclusions An amorphous aluminum–iron compound was produced by the co-precipitation method in the presence of urea. Aluminum doped iron oxide with pure phase hematite was obtained by the heattreatment of the amorphous precursor under oxidation atmosphere. Fe3Al alloy nanomaterial was also produced when the heat-treatment process of the precursor was done under reduction atmosphere. The results confirmed the formation of an intermetallic alloy, Fe3Al via a wet chemical route for first time. Magnetic measurements revealed that the amorphous precursor, Aluminum doped iron oxide and also Fe3Al alloy are all soft magnets. The results demonstrated that this easy to make and large scale producing method can be used to produce the pure phase doped oxides. Furthermore, other alloys can be produced by this method with cheap and nonpoisonous chemicals. Acknowledgement This study was supported by Islamic Azad University, Borujerd Branch, Iran. The authors would like to acknowledge staffs of university. References [1] N. Chen, Z. Zhang, C. Feng, D. Zhu, Y. Yang, N. Sugiura, Preparation and characterization of porous granular ceramic containing dispersed Aluminum and Iron oxides as adsorbents for fluoride removal from aqueous solution, J. Hazard. Mater. 186 (2011) 863–868.
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