Synthesis, characterization and magnetic behavior of Mg–Fe–Al mixed oxides based on layered double hydroxide

Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

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Synthesis, characterization and magnetic behavior of Mg–Fe–Al mixed oxides based on layered double hydroxide Angélica C. Heredia a,n, Marcos I. Oliva b,c, Ulises Agú a,c, Carlos I. Zandalazini c,d, Sergio G. Marchetti e, Eduardo R. Herrero a, Mónica E. Crivello a a

Universidad Tecnológica Nacional, Facultad Regional Córdoba-CITeQ, Maestro López esq. Cruz Roja Argentina, Ciudad Universitaria, 5016 Córdoba, Argentina IFEG, Universidad Nacional de Córdoba, Córdoba, Argentina c CONICET, Argentina d INFIQC, FCQ Universidad Nacional de Córdoba, Córdoba, Argentina e CINDECA, UNLP, Buenos Aires, Argentina b

art ic l e i nf o

a b s t r a c t

Article history: Received 22 November 2012 Received in revised form 12 April 2013 Available online 25 April 2013

In the present work, Mg–Al–Fe layered double hydroxides were prepared by coprecipitation reaction with hydrothermal treatment. The characterization of precursors and their corresponding calcinated products (mixed oxides) were carried out by X ray diffraction, X-ray photoelectron spectroscopy (XPS), termogravimetric analysis and differential scanning calorimetry, diffuse reflectance UV–vis spectroscopy, specific surface area, Mössbauaer and magnetic properties. The Fe3+ species were observed in tetrahedrally and octahedrally coordination in brucite layered. The XPS analysis shows that the Fe3+ ions can be found in two coordination environments (tetrahedral and octahedral) as mixed oxides, and as spinel-structure. Oxides show a decrease in the specific surface areas when the iron loading is increased. The magnetic and Mössbauaer response show that MgAlFe mixed oxides are different behaviours such as different population ratios of ferromagnetic, weak-ferromagnetic, paramagnetic and superparamagnetic phases. The better crystallization of spinel structure with increased temperature, is correlated with the improved magnetic properties. & 2013 Elsevier B.V. All rights reserved.

Keywords: Layered doubled hydroxides Coprecipitation Mixed oxides Magnetic properties

1. Introduction Mixed oxides with iron obtained by thermal decomposition of layered double hydroxides (LDH) have been attracting interest because of their exclusive properties and potential applications in numerous technological fields. In particular, they are mainly important in medicine, as adsorbents, anion exchangers, and most importantly as basic catalysts. Besides, if iron is used, the final compounds show interesting magnetic properties [1–5]. LDH are 3+ represented by the general formula [M2+ (OH)2] (An−x/n)
(1−x) Mx mH2O], where the divalent ion may be Mg2+, Co2+, Zn2+, Ni2+, Mn2+, and the trivalent ion may be Al3+, Fe3+, Cr3+. Their structure consists of brucite-type layers, where the substitution of M2+ with M3+ cations results into a net positive charge, compensated by interlayer anions (A). There is also water crystallization into the interlayer region [6,7]. The interlayer distance depends on both, the nature of anions and the state of hydration. Because of the large variety of anions that can be incorporated between the

n

Corresponding author. Tel.: +543514690585. E-mail addresses: [email protected], [email protected] (A.C. Heredia). 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.04.057

brucite-like layers, the high anionic exchange capacity of these materials, and the large specific surface area values, LDH can be successfully used as matrices for tailoring specific organic–clay hybrid structures with new potential applications in pharmaceuticals or as new biocompatible materials [8–10]. It is possible to synthesize LDH containing more of two cations in the layers. To use LDH precursors produces an excellent dispersion of metals compounds on a matrix of magnesium and aluminum. Thermal decomposition of LDH containing iron start at 300 1C and leads to mixed oxides. The final decomposition product is a mixture of spinel and oxides depending of M2+/M3+ ratio and synthesis method. This allows obtaining a wide range of magnetic properties, according to the variable cation distribution in the crystalline structure because magnetic properties are strongly dependent of spin structure. Thus, magnesium-ferrite aluminate is the first system where canting of Fe3+ spins on A-site has been observed when Fe3+ spins on B-site has a collinear spin structure [11]. Recent studies have indicated interesting magnetic properties for novel drugs–anionic clay structures, strongly dependent on synthesis conditions, internal structure and the drug nature stability [12,13]. Qi et al. [14] reported the synthesis of Co0.9Mn0.1Fe2O4 using the LDH precursors, which displayed magnetostrictive properties that were similar to the traditional ceramic method.

A.C. Heredia et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

Zhang et al. [15] investigated the effects of the high magnetic field on products obtained by calcinations of Co–Fe LDH precursors under the applied field (10 T) at different temperatures. In the last years, some oxides mixed have been successfully prepared using LDH as precursors [16,17], but they were synthesized varying the temperature, cations ratio or pH value. In this work, we report that mixed oxides can be prepared from Mg2+/Fe3+/Al3+–LDH precursors. They were synthesized by coprecipitation at 60 1C under ambient atmosphere, the pH value was 9 and hydrothermal method was used. The content of iron was changing in order to assess this effect on the structural properties of the LDH and magnetic behaviors of the calcinations materials. Due to the properties of these materials, they can be used as catalysts in reactions of ethylbenzene dehydrogenation [18], photocatalysis [19], catalysis for remediating the contaminated waters with organic compounds in solution [20,21].

2. Experimental 2.1. Sample preparation The Mg, Al, Fe LDH precursors were prepared by coprecipitation at low supersaturation method at constant pH (9 70.5), with M2+/M3+ ¼3 M ratio constant [22] while the molar ratios Al3+/Fe3+ was changed in order to assess this effect on the structural and magnetic properties of the LDH and their calcinations products. Two solutions, A and B, were prepared. In order to obtain A solution, Mg(NO3)2
6H2O, Al(NO3)3
9H2O and Fe(NO3)3
9H2O were dissolved together in distilled water. The amounts of nitrates were selected to obtain the total cation concentration of 0.7 M. The B solution contains 0.085 M of Na2CO3 (according to the relation of [CO32−]¼ 0.5 [M3+]); both solutions were added simultaneously to 30 mL of distilled water at drip rate of 50 mL/h, the solution was continuously magnetically stirred, while the pH was maintained constant by adding NaOH 2 M. The coprecipitation was carried out at 60 1C. The gel was transferred into Teflon-lined stainless-steel autoclave and kept in an oven at 200 1C for 18 h, then was separated by centrifugation at 2800 rpm, and washed with distilled water until a sodium content lower than 0.13 wt%. Finally, the solid was dried overnight at 90 1C in air. All samples were calcined in atmosphere of air at 550 1C for 9 h. The HT100 sample was calcined at three different temperatures to analyze the different structures formed. The LDH precursors will be called HT100, HT75, HT50, HT25 and HT0, according to the iron loading (keeping in mind only cations M3+), and the calcined solids will be named CHT100, CHT75, CHT50, CHT25 and CHT0. Compositions of the samples are presented in Table 1. 2.2. Sample characterization The XRD powder patterns were collected on a Rigaku diffractometer, using monochromatized Cu Kα radiation (λ ¼1.54 Ǻ) at a scan speed of 1/41 min in 2θ and interfaced to a DACO-MP data Table 1 Chemical composition of the studied LDH. LDH sample

HT0 HT 25 HT 50 HT 75 HT 100

M3+

Al Al–Fe Al–Fe Al–Fe Fe

M2þ M3þ

%Fe3þ ¼

Al

Fe3þ þFe3þ

3þ

 100

Theoretical

ICP

Theoretical

ICP

3 3 3 3 3

2.82 3.03 3.22 3.09 2.42

0 25 50 75 100

0 23.51 45.02 70.69 100

39

acquisition microprocessor provided with Diffract/AT software. The diffraction pattern was identified by comparing with those included in the JCPDS (Joint Committee of Powder Diffraction Standards) data base. Inductively coupled plasma (ICP) optical emission spectroscopy was used for the determination of the metal content in the oxides. The measurements were performed with a Varian Spectra AA. Diffuse reflectance UV–vis (DRUV-vis) spectroscopic measurements of the LDH precursors were recorded using an Optronics OL 750-427 spectrometer in the wavelength range 200–600 nm. Termogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out in a TA instrument SDT Q600 (0–1500 1C) instrumentation apparatus in a flowing air atmosphere. Approximately 30 mg of sample were loaded and heated at a rate of 10 1C/min up to 550 1C. X-ray photoelectron spectroscopy (XPS) analyses were carried out using an ESCA (VG microtech) spectrometer with a nonmonochromatic Mg Kα radiation (υ¼ 1253.6 eV) as the excitation source. High-resolution spectra were recorded in the constant pass energy mode at 20 eV, using a 720 mm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV FWHM (full width at half maximum) at a binding energy (BE) of 84.0 eV. The spectrometer energy scale was calibrated using Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7, 368.3 and 84.0 eV, respectively. Charge referencing was done against adventitious carbon (C 1s, 284.8 eV). The pressure in the analysis chamber was maintained lower than 10–9 Torr. PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Specific surface area was determined by the BET method, which was recorded on a Micromeritics ASAP 2000 instrument. The precursors were degassed at 200 1C and the calcined materials at 390 1C, both for 60 min. The magnetic measurements were performed in a commercial vibrating sample magnetometer (Lakeshore 7300) at room temperature varying the applied field between 0 and 15000 Oe. The powder samples were compacted applying 5 Tn to make a disc shaped sample of 5 mm diameter and 2 mm height to measure magnetization as function of applied field (M vs H). Hysteresis loops were well fitted using the function:  MðHÞ ¼ M s 

  h i BH ð1−αÞ  1−eð−H=Hs Þ þ α  L T

where LððB  HÞ=TÞ is the Langevin's function [23], T is the system temperature, α is the paramagnetic and/or superparamagnetic fraction, (1−α) is the ferromagnetic fraction, Hs is the ferromagnetic saturation magnetic field, Ms is the total magnetization of saturation. The B factor is an adjustable parameter, B ¼μ/k where μ is the dipolar magnetic moment per paramagnetic molecule [11] and k is the Boltzmann's constant. The parameters α, Ms y Hs are also obtained from the fitting. The SEM images were obtained using JEOL JSM-6380 LV (accelerating voltage: 20 kV) and a Zeiss Supra 40. The Mössbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performed against a 12-μm-thick α-Fe foil. All isomer shifts (δ) mentioned in this paper are referred to this standard at 298 K. The Mössbauer spectra were evaluated using a commercial program with constraints named Recoil [24]. Lorentzian lines with equal widths were considered for each spectrum component. The spectra were folded to minimize geometric effects. In order to consider the existence of different iron environments, hyperfine magnetic field distributions were used.

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3. Results and discussion 3.1. X-ray diffraction

(110) (113)

(018)

(015)

(009)

(006)

(003)

By X-ray diffraction, the hydrotalcite phase was detected in all precursor samples. Most of the peak positions were matched with the ICDD (International Centre for Diffraction Data) PCPDFWIN data. Powder X-ray diffraction patterns of precursors with different (Al3+/Fe3+) molar ratio are shown in Fig. 1. The hydrotalcite phase (PCPDFWIN 70-2151) was observed in all samples. Two peaks located at 2θ of 11.61 and 23.41 are associated to diffraction by (0 0 3) and (0 0 6) planes characteristics of hydrotalcite phase. The secondary phase MgCO3 (magnesium carbonate PCPDFWIN 831761) was observed only in HT75. After calcinations at 550 1C, the (0 0 3) and (0 0 6) reflections disappeared (Fig. 2). The MgO phase (periclase PCPDFWIN 78-

HT0

Intensity (a.u)

HT25

HT50

HT75

HT100

10

20

30

40

50

60

70

2θ Fig. 1. X-ray diffraction patterns of the LDH precursors with different iron percentage. (▲) Magnesium Carbonate MgCO3.

0430) was detected in all samples. The XRD patterns showed that Fe+3 species were crystallized in two structures: MgFe2O4 (Magnesium Iron oxide PCPDFWIN 71-1232) and α-Fe2O3 (Hematite PCPDFWIN 79-1741). The transformation of the phases produced after calcinations are also demonstrated by the analysis of the correspondent XRD patterns. Fig. 3 displays the XRD patterns of HT100 sample heated in air at different temperatures. From this XRD patterns, three peaks are observed at 35.41, 431, 62.21 for samples calcined corresponding to characteristic diffractions of MgFe2O4, MgO and Fe2O3. As the calcination temperature further increases, the intensities of these peaks are increased together with the peaks at 301 and 571 of MgFe2O4 spinel structure, indicating that this phase is enhanced at high temperature [25]. 3.2. DRUV–vis spectroscopy The DRUV–vis spectra of the LDH precursors are shown in Fig. 4. All precursor exhibited band at ∼210 and ∼275 nm. They could be assigned to Fe3+ in tetrahedrally and octahedrally [26,27] coordination in brucite layered. The band between∼350 and 550 nm is assigned to octahedral Fe3+ present in small clusters, isolated species and large particles [27]. The temperature during of the coprecipitation (60 1C) and the aging by hydrothermal method in the autoclave, can promote the clusters production and particles of great size outside the laminar structure [7]. Fig. 5 show the diffuse reflectance UV–vis spectra of mixed oxides. The two characteristic bands at ∼210 nm and ∼275 nm present in LDH lose their definition and intensity, when the laminar structure disappears in the calcined samples. In all samples the band at ∼290 nm was observed, indicating the presence of Fe3+ species. The peak around 350 nm is assigned to isolated Fe3+ in either periclase Mg(Fe, Al)O or MgFe2O4 spinel in mixed oxide [27]. Between 450 and  550 nm is observed a band, which can probably be assigned to the aggregated Fe3+ oxide clusters. 3.3. Measurement of specific surface areas The specific surface areas of precursor and calcined samples were determined by BET method. The values obtained are summarized in Table 2. The specific surface area is inversely related with the iron content.

850 °C

CHT0

Intensity (a.u)

Intensity (a.u)

CHT25

CHT50

700 °C

CHT75 550 °C

CHT100

10

20

30

40

50

60

70

80

2θ Fig. 2. X-ray diffraction patterns of calcined samples. (●) spinel MgFe2O4, (□) periclase MgO, and (+) hematite Fe2O3.

30

40

50

60

70

80

2θ Fig. 3. X-ray diffraction patterns of HT100 at different temperatures. (□) periclase MgO, (●) spinel MgFe2O4, (+) α-Fe2O3.

A.C. Heredia et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

41

increase of the specific surface area. In agreement with this concept, Table 2 shows that the largest area was obtained in the CHT0, while in the sample without iron the area was only 48 m2/g.

Fe3+ Octahedral Fe3+ Tetrahedral

Absorbance (a.u)

3.4. Thermal analyses (TGA and DSC) HT100 HT75

HT50 HT25

200

250

300

350

400

450

500

550

600

Wavelength (nm)

Absorbance (a.u)

Fig. 4. UV–vis diffuse reflectance spectra of precursors with different iron percentages.

CHT100

CHT75 CHT50

3.5. XPS analysis

CHT25

200

250

300

350 400 450 Wavelength (nm)

500

Thermal properties of the samples have been assessed by TGA and DSC. These studies were carried out in air. To understand the decomposition procedure of the Mg–Al–Fe LDH the TGA–DSC profiles were analyzed in detail. Two weight loss stages can be observed on the TGA curve, corresponding to the two endothermic peaks on DSC profile, demonstrating that the decomposition proceeded in two steps. In the TGA curves (Fig. 6), all samples show a weight loss below ∼100 1C, due to the water physically adsorbed, which corresponds to a shoulder in the DSC profiles (Fig. 7), after this step starts the decomposition proceeds. In the region between 100 and ∼300 1C the TGA profiles show a weight loss which corresponds to the loss of interlayer water, with a first maximum endothermic peak in the DSC profiles centered between 119 and 217 1C (Table 3). The dehydroxylation of the brucite-like sheets and the loss of carbonates take place between 300 and 410 1C. This weight loss is accompanied by a maximum endothermic peak in the DSC profile centered between 345 and 396 1C approximately (Fig. 7). TGA curves showed a decrease in the total weight loss when the iron content is increased due the lower expulsion of CO2 and H2O from the LDH. This is consistent with a decrease of LDH phase production observed by DRX and the decrease of the specific surface area. Besides, the shift to lower temperature of dehydroxylation of the brucite-like sheets with an increase in the Fe content was observed, indicating a weakening of the brucite-like structure. Final thermal decomposition products have shown to be metal oxides as well as mixed oxides and spinel-like species.

550

600

Fig. 5. UV–vis diffuse reflectance spectra of the oxides with different iron percentages.

The XPS analysis has been done in order to obtain information about the surface composition of the Mg–Al–Fe mixed oxides derived from LDH precursors. Fig. 8 shows the Fe 2p spectra for the oxides. All spectra show the main Fe 2p3/2 peak at BE of around 710.15 70.27 eV, accompanied by a satellite line visible at BE of around 718.87 70.40 eV, only indicating the presence of Fe3+ cations [29–31]. Whereas the contribution at 723.9 eV is assigned to Fe 2p1/2. [32]. 100

Table 2 Specific surface area of the samples.

0 25 50 75 100

Specific surface area (m2/g) Precursor

Oxide

48 20 19 18 17

179 150 119 96 48

Upon calcination at 550 1C for 9 h the surface areas of the oxides are higher than their corresponding precursors. This increase can be attributed to the formation of micro and mesopores due to expulsion of CO2 and H2O from the LDH [28]. The samples calcined showed a decrease in the areas when the iron content is increased. This result is attributed to the decrease of aluminum content, since the Al2O3 amorphous phase produces an

Weight (%)

% Fe (M+3)

90

80

70 HT100 HT75 HT50 HT25 HT0

60

50 35

100

165

230

295

360

425

490

555

620

Temperature (°C) Fig. 6. Precursor TGA curves with different iron percentages.

685

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A.C. Heredia et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

Table 3 Results of the TGA–DSC for different samples. Sample temperature range (1C)

Observed weight loss (%)

DSC maximum (1C) 7 0.01 1C

Fe3+

Satellite Peak II

45.22 261.9 395.03

loss

1.92 15.6 25.19 42.71

64.41 206.62 378.73

loss

1.57 13.84 25.14 40.55

64.33 151.97 373.43

loss

1.19 8.99 26.62 36.8

102.59 179.34 363.37

loss

1.87 5.56 22.94 30.37

119.36 120 345

loss

1.53 4.08 15.86 21.47

Fe3+

Fe2p1/2

CHT100

Intensity (a.u)

HT0 Until 100 100–300 300–550 Total weight HT25 Until 100 100–300 300–550 Total weight HT50 Until 100 100–300 300–550 Total weight HT75 Until 100 100–300 300–550 Total weight HT100 Until 100 100–300 300–550 Total weight

Fe2p3/2

CHT75

CHT50

CHT25

700

705

710

715 BE (eV)

720

725

730

Fig. 8. XPS of the Fe 2p regions in the oxides.

-1

O1s

OI

-3

OII

-7

-9

HT100 HT75 HT50 HT25 HT0

-11

Intensity (a.u)

-5

Heat Flow (W/g)

CHT100

CHT75

CHT50

-13 35

100

165

230

295

360

425

490

555

620

685

Temperature (°C) Fig. 7. Precursor DSC curves with different iron percentages.

CHT25

The signal at 710.1570.27 eV can be decomposed in two contributions; which indicate that the Fe3+ species exist in more than one chemical state. Most probably, the two chemical states may be related to different coordination environments of the Fe3+: tetrahedral (A sites) at higher binding energy and octahedral (B sites) at lower binding energy [31,33]. The peak at 714.027 0.28 eV (peak II) could be related to the coordination environment of the Fe3+ cations in spinels phase (tetrahedral sites) [29]. Fig. 9 shows the O1s spectra for the surface oxides. The signal to 529.5 70.2 eV can be deconvoluted in two contributions, OI and OII, representing two different kinds of surface oxygen species. There is general agreement between the literature and the present results such that the OI with the lower oxygen bound to metal cations of the structure, while OII with the higher BE al ca. 531.1 eV

526

528

530 532 BE (eV)

534

536

Fig. 9. XPS of the O1s regions in the oxides.

belongs most likely to surface oxygen, including mainly oxygen species of hydroxyl group [34]. 3.6. Magnetism The room temperature hysteresis loops (only the first quadrant is shown in order to get a better comprehension) for all samples

A.C. Heredia et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

CHT100

10

5

0

-5

550°C

700°C

-10 850°C

-15 -15

-10

-5

0 H [KOe]

5

10

15

Fig. 11. Hysteresis curves and their corresponding fittings for CHT100 at different calcinations temperatures.

CHT25

CHT75

Fit Ec (1)

3.5

15

M [emu/g]

are displayed in Fig. 10. The magnetization curves of these mixed oxides are well fitted by equation (1), supposing a paramagnetic and/or superparamagnetic contribution and a ferromagnetic one. From these fittings the saturation magnetizations Ms, saturation fields Hs, and the B parameter, were calculated and they are shown in Table 4. The saturation magnetization of sample with 100% Fe content, is lower than reported values (10.6–24.1 emu/g) for MgFe2O4 produced by calcination of the intercalated LDH at 750–1100 1C for 2 h [35]. This result would indicate that the precursor is not fully transformed in MgFe2O4 after calcination at 550 1C for 9 h. It is in total agreement with both XRD analysis and calorimetric studies. As expected, the substitution of Fe+3 cations by Al+3 affect these magnetic parameters. There are different behaviors with the incorporation of aluminum, given that MgFe2O4 is almost an inverse spinel, and MgAl2O4 is a normal spinel, then when we replace Fe by Al, to obtain compounds of the type MgAlxFe2−x O4, the Fe3+ ions, that occupy A sites, could be replaced by Al3+ ions or by the migration of Mg2+ ions from site B to site A [11]. This phase was not detected by XRD in the samples; this could be due to their small size. In any case, if a magnetic ion is substituted in any of the sub-lattices of the spinel by a diamagnetic one, the magnetic interactions between networks of the spinel will be changed. These phenomena can be explained by ramdon canting model [36,37] and for a chemically disordered system such as MgAlxFe2−x O4, Modi et al. [38] explained that the canting is no uniform shown a locally dependent upon non-magnetic neighboring ions distribution. Also the magnetic behavior of this compound depends on procedures and synthesis conditions [11]. However, a correlation between the magnetic properties and aluminum content was not observed. These results would indicate that the samples are

43

3.0 CHT100

2.0

Transmission (a.u)

M [emu/g]

2.5

CHT50

1.5 1.0

CHT25

CHT50

CHT75

0.5 0.0 0

2500

5000

7500 H [Oe]

10000

12500

15000

Fig. 10. First quadrant of the room temperature hysteresis loops whit fitting curve using equation (1).

CHT100 Table 4 Saturation magnetization, saturation field, B parameter and paramagnetic percentage fraction (α) resulting from fitting Eq. (1) to the curves in Fig. 10. % Fe

Ms [emu/g]

Hs [Oe]

B¼μ/k

%α

100 75 50 25

2.93 3.88 1.33 1.13

7900 7100 7700 6800

7.7 5.6 4.1 4.1

26.6 43.4 57.9 65

-12 -10

-8

-6

-4

-2 0 2 4 Velocity (mm/s)

6

8

Fig. 12. Mössbauer spectra of the mixes oxides samples.

10

12

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A.C. Heredia et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

complex, with different population ratios of ferrimagnetic (MgFe2O4), weak-ferromagnetic (α-Fe2O3), paramagnetic and superparamagnetic phases depending on the aluminum content. Fig. 11 shows the curves of hysteresis and their corresponding fittings for CHT100 for different calcination temperatures. The magnetization of saturation increases with the calcination temperature, corresponding to the observed increase of the ferrimagnetic phase and the crystallinity appreciated in X-ray diagrams. 3.7. Mössbauer Fig. 12 shows the Mössbauer spectra of mixed oxides at room temperature and their hyperfine parameters are listed in Table 5. The spectra were fitted with one doublet and three sextets, except the CHT100 spectrum in which only two sextets were used. The sextet of higher hyperfine magnetic field (H≅51.5 T) present in all samples corresponds to α-Fe2O3 with all hyperfine parameters consistent with this species, with a hyperfine field value slightly decreased in comparison with the “bulk” value [39]. This decrease in H can be attributed to crystal size effects due to the process of collective magnetic excitations [40], and/or to isomorphic replacement of Fe3+ ions by Al3+[39]. The second sextet of the sample CHT100 corresponds to the sum of the two sites, tetrahedral (A sites) and octahedral (B sites) occupied by Fe3+ions in MgFe2O4 spinel [41]. The presence of a large distribution of neighbors would produce a broadening of the signals. Therefore, is not possible to distinguish between both sites. The other two sextuplets present in the samples with iron content between 25 and 75% have hyperfine parameters assigned to Fe3+ ions located in tetrahedral sites (sites A) and octahedral (B sites) of MgFe2O4 [42]. The presence of A and B sites occupied by Fe3+ ions in the spinel structure is consistent with that observed by XPS surface analysis. Another aspect is that the magnetic hyperfine field values of the two sites are significantly diminished compared to a pure spinel MgFe2O4 (between 14 and 15% for A site and between 11 and 9% for B site). The isomorphic Fe3+ substitution by diamagnetic Al3+ ions would explain this result. This would produce different environments for Fe3+ ions depending on the nature and number of nearest neighbors which is reflected in a broadening of

the hyperfine magnetic field distributions of both sites. However, it can not completely ruled out the existence of collective magnetic excitations. Finally, the doublet present in all samples could be attributed to several species: – isolated paramagnetic Fe3+ions dissolved in MgO matrix. – superparamagnetic α-Fe2O3 (as very small crystals) – susperparamagnétic MgFe2O4 with crystallite sizes below 12 nm [43] – MgFexAl2−xO4 with x o1, since, the presence of a high concentration of diamagnetic cations (Al3+) in the oxide, causes the formation of small magnetic “clusters” inside the particles (for this reason the MgFexAl2−xO4 phase was not detected by XRD), which show superparamagnetism. If the diamagnetic ions concentration is very high, these “clusters” can be completely isolated, and the spectrum shows a doublet even at low temperatures [44].

3.8. Scanning electron micrograph (SEM) Fig. 13(a)–(d) shows the micrographs of MgAlFe LDH with different iron content. The images were taken with secondary electrons; all images show the presence of lamellar phase. When the iron content is increased, smaller and exfoliated sheets are produced. The layered double hydroxide structure is distinguished in all samples except in the HT100. Fig. 14(a) shows the image of the CHT75 sample. It was formed by secondary electrons indicating a rough surface, and the presence of small clusters generated by the calcination. When this micrograph is compared with the corresponding LDH (Fig. 13(c)) it is possible to see that has a greater porosity and dispersion of the particles as well as the breakdown of the laminar structure. This observation is consistent with the increased specific surface area, when precursors are calcined. The image of the CHT100 sample (Fig. 14(b)) was formed with secondary electron and backscattered. Note the presence of a smooth surface roughness and small clusters. This can be attributed to sintering of the oxides, in absence of the amorphous phase (Al2O3). This phase and the MgO are the cause of the iron oxides dispersion [45]. Comparing the precursor area (17 m2/g)

Table 5 Oxides Mössbauer parameters at 298 K. Species

Parameters

HT25

HT50

HT75

HT100

α-Fe2O3

H (T) δ (mm/s) 2ε (mm/s) % H (T) δ (mm/s) 2ε (mm/s) % H (T) δ (mm/s) 2ε (mm/s) % H (T) δ (mm/s) 2ε (mm/s) % Δ δ %

51.487 0.08 0.37 70.01 −0.21 70.02 297 6 — — — — 397 1 0.27n −0.017 0.06 257 11 447 1 0.35n 0.047 0.06 247 11 0.85 7 0.03 0.32 7 0.02 227 4

51.46 70.06 0.38 70.01 −0.217 0.02 24 72 — — — — 39.8 70.8 0.27n 0.017 0.04 217 4 45.2 70.2 0.35n 0.02 70.02 297 3 0.777 0.01 0.337 0.01 267 2

51.5 70.3 0.39 70.05 −0.20 7 0.09 57 1 — — — — 39.5 70.4 0.27n 0n 14 72 45.3 70.2 0.35n 0n 257 2 0.707 0.01 0.357 0.01 567 2

51.3 7 0.1 0.38 70.02 −0.20 7 0.03 317 2 42.8 70.7 0.32 70.06 0.17 0.1 157 2 — — — — — — — — 0.687 0.01 0.31 70.01 54 7 2

Fe3+ ions in sites (A+B) of MgFe2O4

Fe3+ ions in A sites of MgFe2O4

Fe3+ ions in B sites of MgFe2O4

Fe3+ paramagnetic and/or superparamagnetic Species

H: hyperfine magnetic field in Tesla; δ: isomer shift (all the isomer shifts are referred to α-Fe at 298 K); 2ε: quadrupole shift; Δ: quadrupole splitting. n

Parameters held fixed in fitting.

A.C. Heredia et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 38–46

2 µm

5 µm

45

2 µm

2 µm

Fig. 13. Micrographs of MgAlFe LDH with different iron content. (a) HT0, (b) HT50, (c) HT75, (d) HT100.

1 µm

1 µm

Fig. 14. Micrographs of mixed oxides. (a) CHT75, (b) CHT100.

with its oxide (48 m2/g) is not observed a significant increase after calcination.

4. Conclusions A series layered double hydroxides (LDH) as potential mixed oxides precursors containing Mg2+, Al3+ and Fe3+ cations in the brucite-like layers were obtained. The precursors have been prepared by coprecipitation method at 60 1C with hydrothermal treatment and a M2+/M3+ ¼3 constant molar ratio. Powder X-ray diffraction shows the presence of the hydrotalcite phase. It was observed that the change the aluminum by iron, the layer structure not was modified. On the other hand, when the samples were calcined at 550 1C, MgO, MgFe2O4 and α-Fe2O3 were detected by DRX; but by Mössbauer and magnetism the MgFexAl2−xO4 phase could be assigned.

The formation of MgFe2O4 spinel structure is enhanced at high temperature. By UV–vis-DRS were observed Fe3+ in tetrahedrally and octahedrally coordination in brucite layered. In the calcined samples was observed the presence of isolated Fe3+ in either periclase Mg(Fe, Al)O or MgFe2O4 spinel in mixed oxide and Fe3+ oxide clusters. A lower expulsion of CO2 and H2O of the interlayer when the iron content is increased could be observed by a decrease in the total weight loss from the LDH and in the specific surface areas in the oxides. The XPS analysis shows that the Fe3+ ions can be found in two coordination environments (tetrahedral and octahedral) as mixed oxides, and as spinel-structure. The combined magnetic and Mössbauer results analysis of the complete series show that the samples are complex, with different population ratios of ferrimagnetic (MgFe2O4), weak-ferromagnetic (α-Fe2O3), paramagnetic and superparamagnetic phases depending

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on the aluminum content. Magnetic properties were significantly improved when the calcination temperature was increased as a consequence of a better crystallization of spinel structure. Due to the magnetic phase (spinel) in the materials calcined at temperatures above 550 1C, they could be used as catalysts in reactions of photocatalysis and dehydrogenation of ethylbenzene. Acknowledgements This work was supported by the UTN-FRC of Argentina. We thank UTN for doctoral fellowship for Angélica C. Heredia. We thank geol. Julio D. Fernández (UTN-FRC, Córdoba, Argentina) for help in recording specific surface area data and to Dra. Valentinuzi of LAMARX for SEM photografies. References [1] J. Roelofs, A. van Dillen, Y.K. de Jong, Catalysis Today 60 (2000) 297. [2] K. Simeonidis, S. Mourdikoudis, M. Moulla, I. Tsiaoussis, C. Martinez-Boubeta, M. Angelakeris, C. Dendrinou-Samara, O. Kalogirou, Journal of Magnetism and Magnetic Materials 316 (2007) e1–e4. [3] V. Rives, O. Prieto, A. Dubey, S. Kannan, Journal of Catalysis 220 (2003) 161. [4] F. Prinetto, D. Tichit, R. Teissier, B. Coq, Catalysis Today 55 (2000) 103. [5] S. Murcia-Mascarós, R. Navarro, L. Gómez-Sainero, U. Costantino, M. Nocchetti, J.L.G. Fierro, Journal of Catalysis 198 (2001) 338. [6] M. Crivello, C. Pérez, E. Herrero, G. Ghione, S. Casuscelli, E. RodríguezCastellón, Catalysis Today 107–108 (2005) 215. [7] F. Cavani, F. Trifiro, A. Vaccari, Catalysis Today 11 (1991) 173. [8] S. Kwak, W. Kriven, M. Wallig, Biomaterials 25 (2004) 5995. [9] H. Nakayama, N. Wada, M. Tsuhako, International Journal of Pharmaceutics 269 (2004) 469. [10] G. Carja, H. Chiriacb, N. Lupu, Journal of Magnetism and Magnetic Materials 311 (2007) 26. [11] M.D. Sundararajan, A Narayanasamy, T. Nagarajan, L. Haggstrom, C.S. Swamy, K.V. Ramanujachary, Journal of Physics C: Solid State Physics 17 (1984) 2953. [12] H. Zhang, K. Zou, H. Sun, X. Duan, Journal of Solid State Chemistry 178 (2005) 3485. [13] P.M. Forster, M.M. Tafoya, A.K. Cheetham, Journal of Physics and Chemistry of Solids 65 (2004) 11. [14] X. Qi, D. Wu, Journal of Magnetism and Magnetic Materials 320 (2008) 666. [15] X. Zhang, D. Wang, S. Zhang, Y. Ma, W. Yang, Y. Wang, S. Awaji, K. Watanabe, Journal of Magnetism and Magnetic Materials 312 (2010) 3023. [16] J.M. Fernandez, M.A. Ulibarri, F.M. Labajos, V. Rives, Journal of Materials Chemistry 8 (1998) 2507. [17] Qinghong Xu, Yabo Wei, Yao Liu, Xuemei Ji, Lan Yang, Mingguang Gu, Solid State Sciences 11 (2009) 472.

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