Hyperfine Interact (2005) 165: 203–208 DOI 10.1007/s10751-006-9266-9
Mössbauer studies on ultraporous Fe-Oxide/SiO2 aerogel A. Lančok & K. Závěta & M. Popovici & C. Savii & M. Gich & A. Roig & E. Molins & K. Barčová
Published online: 10 November 2006 # Springer Science + Business Media B.V. 2006
Abstract Magnetic aerogels with very low volume density of ∼0.2 g/cm3 were prepared by sol-gel method and supercritical drying. The resulting materials were monolithic and displayed high surface area. By X-ray diffraction and Mössbauer spectroscopy the crystalline phase formed inside the mesopores of the SiO2 matrix was identified as a spinel iron oxide. Comparison of the magnetic measurements with Mössbauer spectra at various temperatures contributed to the elucidation of the magnetic state of this nanocomposite system with restricted magnetic interactions, in particular its transition to a superparamagnetic state. Key words aerogels . Fe2O3 . nanoparticles . superparamagnetic state . Mössbauer spectroscopy
1 Introduction Nanostructured materials have been a subject of intense research during the past 10 years. The nanocomposites that contain a reinforcement component in the form of one or more ultrafine phase with dimensions less than 100 nm, have been used to improve various
A. Lančok (*) : K. Závěta Joint Laboratory of Low Temperatures, V Holešovičkách 2, 180 00 Prague, Czech Republic e-mail:
[email protected] M. Popovici : C. Savii Institute of Chemistry Timisoara of Romanian Academy, 24 Mihai Viteazul Blvd., 300223 Bucharest, Romania M. Gich : A. Roig : E. Molins Institut de Ciència de Materials de Barcelona (CSIC), Esfera UAB, 08193 Bellaterra, Catalonia, Spain K. Barčová Institute of Physics, Technical University of Ostrava, 17. listopadu 15, 70833 Ostrava, Czech Republic
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material characteristics, including mechanical, chemical, structural, optical and electric/ magnetic properties. Porous amorphous matrices play an active role in shaping the physical properties of intergrown nanoparticles in addition to providing a means of their confinement and dispersion [1]. These matrices are easily obtained by sol-gel method which offers the opportunity to produce not only ultra-homogeneous materials but also heterogeneous or nanocomposite materials [2]. Sol-gel process consists of hydrolysis and condensation reactions of an alkoxide resulting in formation of a wet gel, a solid network containing liquid phase entrapped inside the pores. Materials called aerogels are obtained by supercritical drying process of the wet gels and consist of a continuous polymeric inorganic network comprising open pores in the mesoporous range. These materials are of interest due to the combination of high porosity with small pore size [3] that assures physical and chemical properties difficult to obtain by other synthesis methods at low temperatures. Despite the growing interest in magnetic nanoparticles, due to their peculiar properties such as superparamagnetism [4], quantum tunnelling of the magnetization [5], and the broad range of applications, from medicine to magnetic recording [6, 7], relatively few studies on the magnetic properties of nanocomposite aerogels [8, 9] have been published. The iron oxide/silica aerogels obtained by us as monolithic samples by sol-gel process using the impregnation method, but with considerably higher bulk density than in the present work, are described in [8]. The aim of the present work is to prepare and characterize aerogels containing iron oxide nanoparticles with increased spatial separation and thus suppressed magnetic interactions. Their magnetic state, in particular the transition to superparamagnetic behaviour, has been investigated by magnetometry and Mössbauer spectroscopy in the temperature range from 5 to 300 K.
2 Experimental Silica gel matrices were prepared by sol-gel synthesis in neutral medium via hydrolysis and condensation of tetraethoxysilane (Aldrich 98%) in hydroethanolic solution. All the gels, obtained after six days at room temperature, were transparent monolithic slabs, and were washed several times with pure ethanol to remove residual water; then they were kept in closed vessels. Anhydrous ferrous acetylacetonate (Aldrich 99.95%) was used as iron source for the magnetic nanoparticles. The impregnation of silica gels with a supersaturated ethanolic solution of iron (II) acetylacetonate took place during four days. Finally, ethanol supercritical drying was carried out in an autoclave at 260°C and 131 bar. The whole drying cycle lasted about 24 h. The iron content of the samples was determined by chemical analysis with flame atomic absorption spectrophotometry. Samples were characterized by X-ray diffraction (XRD) with the crystallite sizes estimated from the width of the diffraction peaks using the Scherrer formula. Using an ASAP 2000 equipment, the pore-size distribution was derived from N2 adsorption/desorption isotherms by the Barrett–Joyner–Halenda (BJH) method and surface area was determined by the Brunauer–Emmett–Teller (BET) method. Hysteresis loops were measured by an extraction method in a PPMS facility by Quantum Design at 5, 130, and 300 K with applied fields up to 5 T. Mössbauer spectra (MS) were obtained at the same temperatures using a conventional spectrometer with a 57Co/Rh source and constant acceleration and fitted by means of the NORMOS program [10] using Voigt line profiles for the sextets.
Mössbauer studies on ultraporous Fe-Oxide/SiO2 aerogel Table I Textural parameters of the aerogels
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Sample
Surface area BET (m2/g)
Bulk density (g/cm3)
A AFe
717±50 774±50
0.19±0.02 0.21±0.02
Figure 1 Pore-size distribution from BJH method.
0.06 -1
0.02
d(Vp)/d(Dp) [cm g nm ]
3 -1
AFe 0.04
0.00 0.10
0
20
40
60
80
100
120
A 0.05 0.00
0
20
40
60
80
100
120
diameter [nm]
3 Results and discussion The textural parameters for the pure silica matrix A, and iron oxide-impregnated silica AFe are given in Table I. There are no large differences between the density and surface area of the A and AFe samples with rather large BET surface area of 774 m2/g for the latter case. The iron content was estimated at 6 wt.% by chemical analysis. The wide pore-size distribution determined from the adsorption/desorption isotherms at 77 K is shown in Figure 1. It indicates a high volume porosity in both samples which is also evident from the nitrogen adsorption mainly occurring at high relative pressure. From the XRD pattern of the AFe nanocomposite, see Figure 2, the main diffraction lines of cubic iron oxide spinel phase are clearly seen though it is not possible to decide whether they belong to maghemite or magnetite phases. The Scherrer diameter of the particles was estimated to 5.7±2 nm taking into consideration the instrumental broadening. The magnetization curves of the AFe sample are displayed in Figure 3. The saturated magnetizations mS(T ) were estimated from the extrapolation of the Langevin-type function to high fields. The reduced magnetizations m/mS are plotted against H/T in the inset. The curves for 300 and 130 K practically merge, which, together with negligible coercivity, indicates mainly superparamagnetic (SPM) behaviour. The rather different curve with a coercivity of ∼14 mT points to dominating blocked moments at 5 K. The Mössbauer spectra of the AFe sample acquired at 300, 130, and 5 K are shown in Figure 4 and the parameters of their fits are given in Table II. The 300 and 130 K spectra were fitted by one doublet and a narrow (S1) and “broad” (S2) sextets. The widths of the S1 sextets were ∼4 T while those of the S2 sextets were comparable with the hyperfine field itself. From the relative areas of the narrow and broad sextets, the ratio of particles with blocked and fluctuating moments, in the time window relevant to Mössbauer effect, was roughly estimated [11]. The 5 K spectrum could be fitted by two doublets and two relatively narrow sextets S1 (the widths ∼4 T) that were definitely different from the
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Figure 2 X-ray diffraction pattern.
γ -Fe2O3 Fe3O4
80
Intensity [counts]
70
AFe
60
γ-Fe O 2 3 Fe3O4
γ-Fe O 2 3 Fe3O4
50 40 30 20 10 20
30
40
50
60
70
80
2θ degree
5K 130 K 300 K
1.0 0.8
0 0.6
mred
magnetization [emu/g]
8
0.4 0.2
-8 0.0 0.00
-3
0 Field [T]
0.01
0.02 0.03 H/T [T/K]
0.04
3
Figure 3 Magnetization curves of aerogel.
magnetite spectrum. Taking into account the distribution of the particle sizes, the effect of SPM relaxation, and the large relative volume of the nanoparticle surfaces, all of them decreasing the mean hyperfine field [11, 12], we may most probably ascribe these sextets to spinel-like oxide of three-valent Fe, i.e., maghemite. It is worth noting that the relative areas of the doublets in our tentative fit do not essentially change with temperature and only the relative areas of the resolved and broad sextets are affected. This is in contrast to [13], where the SPM relaxation of slightly larger γ-Fe2O3 nanoparticles lead to a higher share of the doublets at the expense of the resolved sextets.
Mössbauer studies on ultraporous Fe-Oxide/SiO2 aerogel
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Figure 4 Mössbauer spectra at various temperatures.
Intensity
300 K
130 K
5K
-10
-5
0
5
10
Velocity (mm/s)
Table II Parameters of Mössbauer spectra 300 K Bhf (T)
130 K QUA ISO r.a. (mm/s) (mm/s) (%)
Bhf (T)
5K QUA ISO r.a. (mm/s) (mm/s) (%)
42.0 −0.05
0.34
24.2 47.5 0.03
0.41
Sextet 2 24.6 −0.54 Doublet 1 −0.68 Doublet 2
0.03 0.37
60.6 19.9 −0.49 15.2 −0.45
0.53 0.29
Sextet 1
Bhf (T)
QUA ISO r.a. (mm/s) (mm/s) (%)
51.4 47.0 0.19 46.5 −0.78 31.1 17.5 0.08 0.41
0.52 0.35
67.3 17.6
0.46 0.27
12.6 2.4
4 Conclusions Dispersed nanoparticles of iron oxide embedded in silica aerogels have been obtained by impregnation of the wet silica gel with an anhydrous iron(II) acetylacetonate salt and subsequent supercritical drying without post annealing. The nanocomposite has a rather low bulk density of ∼0.2 g/cm3, large surface area and the pore distributions of the matrix exhibit a maximum around 40 nm. The size of the iron oxide particles, most probably γ-Fe2O3, is in the nanometer range (∼6 nm) and we may assume that their interactions are effectively avoided. From the magnetization curves follows that at 130 K most of the particles are already in the SPM state, while in the time window seen by the Mössbauer effect the transition to the SPM behaviour is not complete even at 300 K.
Acknowledgements The discussions and help of J. Nogués are gratefully acknowledged. M. Popovici thanks the Marie Curie Program for fellowship No. HPMT-CT-2000-0006. This work was supported by Ministerio de Educación y Ciencia of Spain (MAT2003-01052) and by the Grant Agency of the Czech Republic under grants Nos. 202/05/2111 and 202/03/P158; the participation of K.B. on this conference was enabled by IGS-VSB-2005-516/1. Our respectful thanks go to the referee, whose remarks considerably helped to improve our text.
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