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On the Nature and Location of Nanoparticulate Iron Phases and Their Precursors Synthetized within a Sepiolite Matrix Antonio Esteban-Cubillo,†,‡ Jose´ F. Marco,§ Jose´ S. Moya,† and Carlos Pecharroma´ n*,† Instituto de Ciencia de Materiales de Madrid CSIC, C/ Sor Juana Ine´ s de la Cruz no. 3, 28049 Madrid Spain ReceiVed: September 6, 2007; In Final Form: NoVember 21, 2007
In this work, we report on the characterization by Mo¨ssbauer spectroscopy, X-ray diffraction, and transmission electron microscopy of nanoparticulate iron phases and their precursors synthesized by a wet chemical route within the framework of a silicate matrix (sepiolite). These results show that the nature and location of the different iron species within the sepiolite structure are dependent on the conditions of the wet chemical preparation and, specifically, on the pH value of the acidic treatment to which the sepiolite powder was subjected prior to the impregnation with the iron(III)-containing solution. A subsequent thermal treatment at 500 °C (2 h) of the iron-containing sepiolites in a reducing H2/Ar atmosphere leads to the formation of different iron-reduced species. The nature of these reduced species is also dependent on the initial acidic treatment of the sepiolite. So, in the case of the iron-containing materials produced after treatment in strong acidic conditions of the original sepiolite, the reducing conditions bring about the formation of a significant amount of metallic iron nanoparticles. These metallic iron nanoparticles have been shown to be strongly resistant to oxidation at high temperatures. In the case of milder acidic initial conditions, the reduced samples do not contain metallic iron particles but mainly nonstoichiometric Fe3O4 nanoparticles.
Introduction Iron magnetic nanoparticles1-3 present huge potential in multiple applications as high-density recording media, magnetic and magnetooptical sensors, and in biotechnology4-6 (NMR imaging, hyperthermia, drug targeting, separation, and selection). Thus, considerable effort has been devoted to the synthesis of magnetic nanoparticles by different methods to obtain particles with controlled surface chemistry, size, and shape distribution. However, the strong tendency of nanoparticles to coalesce, the limited production rate of nanomaterials, the chemical instability against oxidation,7 and health problems related with the production and manipulation of nanoparticles8,9 have hindered their application on a large scale. In a previous work,10 some of the authors have described a novel procedure to produce large quantities of metallic nanoparticles supported on a magnesium silicate (sepiolite) matrix. This method has revealed to be versatile because metallic nanoparticles can be obtained either on the surface or inside the magnesium silicate microparticles depending on the preparation conditions. Furthermore, embedded metallic nanoparticles show an extreme resistance against oxidation. Sepiolite (Mg8Si12O30(OH)4(H2O)4‚8H2O) presents a structure of needle-like particles, which can be described as a quincunx arrangement of talc-type layers separated by parallel channels with dimensions of 3.6 × 10.6 Å2 running along the axis of the particle11 (Figure 1). Sepiolite presents a high specific surface (300 m2/ g) and special structural characteristics based on the folding of * Corresponding author. E-mail:
[email protected]. † Instituto de Ciencia de Materiales de Madrid CSIC, C/ Sor Juana Ine ´s de la Cruz no. 3, 28049 Madrid Spain. ‡ Tolsa S.A. R.&D. Department, Ctra Vallecas-Mejorada del Campo, km 1,6, 28031 Madrid, Spain. § Instituto de Quı´mica-Fı´sica “Rocasolano” CSIC, C/ Serrano 119, 28006 Madrid, Spain.
the crystal structure due to the loss of four water molecules12 per unit cell. However, neither the nature nor the nucleation mechanism of the metallic nanoparticles embedded in a sepiolite matrix have been unambiguously established. The interesting property of resistance against corrosion of the metallic nanoparticles when they are completely covered by a silicate matrix, however, complicates their study at a great extent because most spectroscopic probes (based on electronic interactions) are not able to access to the nanoparticles or because it is difficult to separate the signal belonging to the nanoparticles from that belonging to the silicate matrix. In this respect, Mo¨ssbauer spectroscopy appears to be a quite appropriate technique to determine the chemical nature and the local environment of iron species in a sepiolite matrix. Although the ability of Mo¨ssbauer spectroscopy in the characterization of iron-containing silicates is wellestablished,13 this technique has been scarcely used in the study of nanoparticulate iron phases synthesized within a sepiolite matrix. Recent works have been focused mainly on the characterization of iron and iron oxide nanoparticles encapsulated in different zeolitic structures14,15 or silica matrices.16,17 Therefore, the present work is aimed at the chemical and structural characterization of monodispersed nanoparticulate iron phases synthesized in a silicate (sepiolite) matrix, either embedded or deposited on its surface,10 and having a narrow size distribution. Experimental Methods The starting material was a sepiolite powder (TOLSA, Spain), purified and micronized by a wet process to obtain a particle size below 1 µm. Sepiolite powder was dispersed at 10 wt % concentration in water using high shear mixing, and then the suspension was acidified with HCl at two pH levels, pH ) 0 and pH ) 2. In previous works, it has been stated that a fraction
10.1021/jp077173w CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008
Nanoparticulate Iron Phases and Their Precursors
Figure 1. (a) 3D model of sepiolite crystalline structure: silica tetrahedral appears in red, internal magnesium octahedral in blue, external (at the edges of the magnesium layers) in yellow and zeolitic water in green. (b) 3D model of Mg2+ leached sepiolite structure: blue spheres represent the proposed silanol positions.
of magnesium cations are leached from the silicate matrix by the acid treatment, forming silanol groups in the octahedral vacancies.18-20 Afterward, 1.5 L of both types of treated sepiolite suspensions were mixed with 1.0 L of aqueous solution of FeCl3‚6H2O so that the final relative metal concentration into sepiolite was 15 wt %. As a result, in the sepiolite suspension, the following chemical reactions take place:
Mg8Si12O30(OH)4(H2O)4‚8H2O + 2xH+ a Mg8-xH2xSi12O30(OH)4(H2O)4‚8H2O + xMg2+ Mg8-xH2xSi12O30(OH)4(H2O)4‚8H2O + 2 xFe3+ a Mg8-xFe(2/3)xSi12O30(OH)4(H2O)4‚8H2O + 3 2xH+ After some time to homogenize the suspension (1 h), the pH was raised by the addition of NaOH up to 7.5. According to the potential-pH equilibrium diagrams, iron cations are not stable in aqueous solution within the pH range 3 < pH < 6, while magnesium cations remain stable in solution up to pH 8. The precipitation was carried out under high shear mixing. Finally, the dispersion was vacuum-filtered, washed with distilled water, and dried at room temperature. The corresponding iron-containing samples, obtained from the initial sepiolite powders treated at pH ) 0 and pH ) 2, were labeled SFe0 and
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Figure 2. XRD powder patterns corresponding to sepiolite (i), SFe0 (ii), and SFe2 (iii) samples (a); and anhydrous sepiolite (i), reduced SFe0 (ii), and reduced SFe2 (iii) samples (b) (square and arrows indicate Fe3O4 and R-Fe peaks, respectively).
SFe2, respectively. A fraction of the obtained precursors was reduced at 550 °C in a 10 vol % H2/Ar atmosphere for 2 h using a H2/Ar at a volumetric flow of 0.20 L/m2. The reduced samples were labeled as SFe0R and SFe2R. X-ray diffraction patterns were recorded in a Bruker D8 diffractometer using Cu KR radiation. Transmission electron microscopy (TEM) images were taken with a JEOL microscope model FXII operating at 200 kV. Mo¨ssbauer spectra were recorded at different temperatures in the transmission mode using a constant acceleration spectrometer equipped with a 57Co (Rh) source and a closed-cycle He refrigerator. All of the spectra were computer-fitted, and the isomer shifts were referred to the centroid of the spectrum of R-Fe at room temperature. Results The X-ray diffraction patterns recorded from the SFe0 and SFe2 samples (Figure 2a) are very similar to that of sepiolite with the exception of a moderate peak broadening, probably associated to a loss of crystallinity due to the acid attack. After the reduction process, only anhydrous sepiolite and metallic iron (R-Fe) could be identified in the X-ray diffraction pattern of the SFe0R sample (Figure 2bi). A similar result was obtained for SFe2R, but instead of metallic iron some small peaks and a shoulder, tentatively assigned to magnetite (Fe3O4), were observed in the anhydrous sepiolite pattern (Figure 2bii).
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Figure 3. TEM micrographs corresponding to SFe0R (a) and SFe2R (b) samples.
In Figure 3, TEM micrographs corresponding to the SFe0R and SFe2R samples are shown. As can be observed in Figure 3a, the SFe0R sample presents monodispersed nanoparticles (with a narrow size distribution ranging from 3 to 10 nm) along the sepiolite fibers, which, in view of the XRD data, we assign to R-Fe. In the case of the SFe2R sample (Figure 3b), magnetite nanoparticles are deposited on its surface with sizes ranging from 5 to 15 nm. To gain more specific information on the nature of the iron species incorporated into the sepiolite framework, we studied all of the samples by Mo¨ssbauer spectroscopy. Figure 4 shows the Mo¨ssbauer spectra recorded at different temperatures from the original iron-containing sepiolite sample SFe0. The room-
temperature Mo¨ssbauer spectrum (Figure 4, top) consists of a broad paramagnetic doublet with parameters (Table 1) characteristic of Fe3+ in octahedral oxygen coordination. The parameters are very similar to those reported for many superparamagnetic or amorphous Fe3+ oxyhydroxides (they are compatible, for example, with the presence of superparamagnetic goethite, lepidocrocite, ferrihydrite, or a mixture of all of them21). In fact, because the spectrum is broad it can be regarded as a simplified representation of an essentially continuous distribution of quadrupole splittings. The possibility that a fraction of the Fe3+ could be bounded to the sepiolite framework cannot be discarded either from the room-temperature Mo¨ssbauer spectrum.
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Figure 6. Mo¨ssbauer spectrum recorded at room temperature from the SFe0R reduced sample.
Figure 4. Mo¨ssbauer spectra taken at various temperatures from the SFe0 sample.
Figure 7. Mo¨ssbauer spectra taken at various temperatures from the SFe2R reduced sample.
Figure 5. Mo¨ssbauer spectra taken at various temperatures from the SFe2 sample.
The spectrum recorded at 85 K continued to show a paramagnetic doublet. However, the spectrum recorded at 51 K showed, besides the paramagnetic doublet, the presence of incipient magnetic relaxation, which was simulated by introducing a very broad band in the fit of the spectrum. The spectrum recorded at 30 K showed already a very broad magnetic
component. This magnetic component is much better defined in the spectrum recorded at 16 K, where a broad sextet component can be observed. Besides that, inspection of the central part of the spectrum indicated that a small paramagnetic doublet is still present at this low temperature. The 16 K spectrum was best-fitted considering a discrete sextet (S), a hyperfine magnetic distribution (SD), and a doublet (D). The results of this kind of fit are presented in Table 1. The Mo¨ssbauer parameters of the discrete magnetic sextet could be assigned to ferrihydrite.5 This assignment is made mainly on the basis of the hyperfine magnetic field (48.7 T) and the quadrupole shift value (-0.13 mms-1) (see Table 2, which compiles literature values for the Mo¨ssbauer parameters at 4.2 K of different Fe3+ oxyhydroxides). The values obtained for the broad hyperfine magnetic field distribution (width of the distribution 12 T) could be compatible with the presence of poorly crystalline ferrihydrite because the maximum of the hyperfine magnetic field distribution appears at 46.5 T, a value that is higher than that characteristic of lepidocrocite (45.5 T)22,23 or with ferrihydrite-like environments for some ferric ions substituting Mg2+ ions at the edges of the sepiolite channels (we will come on this point later). The small quadrupole doublet observed at 16 K could correspond to a small fraction of Fe3+ incorporated in the octahedral sites of the sepiolite framework. The Mo¨ssbauer spectra recorded at temperatures between 16 and 298 K from sample SFe2 are shown in Figure 5. The roomtemperature Mo¨ssbauer spectrum is very similar to that recorded from sample SFe0 although it is characterized by a slightly smaller quadrupole splitting (Table 3). As in the spectra recorded
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TABLE 1: Mo1 ssbauer Parameters Obtained from the Fit of the 298 and 16 K Spectra Recorded from the SFe0 Sample temp (K)
a
component
δ (mms-1)
∆ (mms-1)
2 (mms-1)
H (T)
area (%)
298
D
0.35 ( 0.01
0.76 ( 0.02
100
0.40 ( 0.03 0.45 ( 0.01 0.44 ( 0.01
0.71 ( 0.20
16
D S SD
48.7 ( 0.1 46.5 ( 0.1a
5(2 36 ( 1 59 ( 1
-0.13 ( 0.02 -0.02 ( 0.02
Maximum of the hyperfine magnetic field distribution.
TABLE 2: 4,2 K Mo1 ssbauer Parameters for Different Fe3+ Oxyhydroxides (Taken from Refs 21-24) compound goethite
δ (mms-1)
2 (mms-1)
H (T)
0.48
-0.25
50.6
lepidocrocite
0.43
0.02
45.5
ferrihydriteb
0.47 0.49 0.49
-0.02 -0.07 -0.07
46.5c 49.3c 50.0c
WHFDa
7.6 6.0 4.7
a Width of the hyperfine field distributon. b Values depend on crystallinity: the lower the values of H, the lower the crystallinity. c Maximum of the hyperfine magnetic field distribution.
from the previous sample, magnetic order appears to start very incipiently at 50 K. However, in this case, the magnetic component observed in the 30 K spectrum has a larger intensity than that in the corresponding spectrum of the SFe0 sample and the 16 K spectrum shows narrower magnetic sextets and the absence of paramagnetic components. The results of the fit of the 16 K spectrum to a discrete sextet and a hyperfine magnetic distribution are collected in Table 3. Similar to the latter case, the discrete sextet can be assigned to ferrihydrite. However, in the SFe2 sample the width of the hyperfine magnetic distribution is much narrower (9.9 T) and the maximum of the distribution appears at 45.0 T. This, and the evolution of the spectra with temperature, which indicate that a given temperature the magnetic components are much less developed in the SFe0 sample than in the SFe2 one, strongly suggests that, in the latter, the hyperfine magnetic field distribution is mainly due to superparamagnetic lepidocrocite.22,23 Figure 6 depicts the room-temperature Mo¨ssbauer spectra recorded from the reduced SFe0R sample. The spectrum is dominated by an intense sextet (S0) with parameters characteristic of metallic iron (Table 4). The spectrum also shows a small magnetic component (ca. 5%) having a larger hyperfine magnetic field, which, in fact, could be fitted to two sextets (S1 and S2) with parameters characteristic of Fe3O4 (Table 4). Additionally, the spectrum shows several paramagnetic components in the center of the spectrum. We tried two different models to fit that central part. In the first model, a singlet with an isomer shift near zero or slightly negative (which could account for the presence of superparamagnetic Fe0 particles17,24) and two Fe2+ doublets, were used. In the second model, we considered the presence of one Fe3+ doublet and two Fe2+ doublets. Although the differences were small, the second model gave a lower χ2 value. Besides that, the Mo¨ssbauer parameters obtained for the Fe2+ doublets in the second model were more similar to those shown by the SFe2R reduced sample (see below), and they were more consistent with the values obtained for those doublets at 16 K (both the isomer shifts and the quadrupole splittings of the Fe2+ doublets increase with decreasing temperature as should be expected, while, in the first model, this variation does not occur). It is also worth commenting that the intensity of the R-Fe sextet does not increase in the 16 K spectrum with respect to the value observed in the room temperature one, as should be expected if the possible R-Fe
superparamagnetic particles contributing at the singlet in the first model were already blocked at 16 K. Therefore, we adopt a more probable model comprising a doublet characteristic of Fe3+ (D) and two doublets with parameters of Fe2+ (D1 and D2), see Table 4. The Mo¨ssbauer parameters of the Fe3+ doublet are characteristic of Fe3+ in distorted octahedral oxygen coordination, whereas those of the Fe2+ doublets are characteristic of high spin Fe2+ cations in octahedral coordination. Spectra taken at temperatures between 16 and 298 K (not presented) did not show any additional magnetic component that could be assigned to iron oxides or oxyhydroxides but the same components as those observed at room temperature. The room-temperature Mo¨ssbauer spectrum recorded from the reduced sample SFe2R did not show any magnetic components but only paramagnetic doublets (Figure 7, top). It was best-fitted to one Fe3+ doublet and two Fe2+ doublets. The parameters of the Fe2+ doublets (Table 5) are almost identical, within the experimental error, to those observed in the spectrum of the previous sample. However, because a room-temperature Mo¨ssbauer spectrum gives quite unspecific information about the nature of the Fe3+ species contributing to those doublets, a series of low-temperature Mo¨ssbauer spectra were recorded (Figure 7). The spectra recorded between 125 and 16 K show the coexistence of the Fe3+ paramagnetic contributions and magnetic components down to 30 K, in a clear indication of superparamagnetic behavior. The two magnetic components appearing in the 101 K spectrum are characteristic of nonstoichiometric Fe3O4. Therefore, we assume that the doublet is due to small Fe3O4 particles that are above their blocking temperature. At 16 K, there is no evidence of the Fe3+ doublet and the spectrum only shows magnetic components, which we associate with the presence of Fe3O4, and the doublets corresponding to the Fe2+ species that remain in the paramagnetic state down to that temperature. The corresponding Mo¨ssbauer parameters obtained from the fit of the 16 K spectrum are collected in Table 5. Discussion The use of three complementary techniques, X-ray diffraction, TEM, and Mo¨ssbauer spectroscopy, has allowed us to give a precise picture of the substitution and location of the iron cations during the synthesis process and the iron species formed after reduction. The obtained results indicate that the nature of the iron species incorporated within the sepiolite matrix is clearly dependent on the acid treatment used on the original silicate. In fact, the amount of available vacancies able to incorporate foreign cations depends on the magnesium cations leached from sepiolite during the acid treatment.25 In a previous work, we have observed that a moderate acid treatment, (pH ) 2 for 1 h) only removes the magnesium cations corresponding to the particle surface, while a stronger acid treatment, (pH ) 0 for 1 h) is able to leach all of the Mg2+ cations located at the edges of the octahedral layer into the channels and at the particle surface11,12 (Figure 1a). The characteristics of the acid attack, and therefore the fraction of magnesium cations leached by acid treatment, appears to be crucial because it has been shown that
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TABLE 3: Mo1 ssbauer Parameters Obtained from the Fit of the 298 and 16 K Spectra Recorded from the SFe2 Sample temp (K) 298 16 a
component D S SD
δ (mms-1)
∆ (mms-1)
0.35 ( 0.01 0.48 ( 0.01 0.42 ( 0.01
0.67 ( 0.02
2 (mms-1)
H (T)
area (%)
-0.05 ( 0.02 -0.06 ( 0.02
48.0 ( 0.1 45.9 ( 0.1a
100 42 ( 2 58 ( 2
Maximum of the hyperfine magnetic field distribution.
TABLE 4: Mo1 ssbauer Parameters Obtained from the Fit of the RT Spectrum Recorded from the Reduced Sample SFe0R component D D1 D2 S0 S1a S2a a
δ (mms-1)
∆ (mms-1)
2 (mms-1)
0.37 ( 0.04 1.02 ( 0.01 1.05 ( 0.03 0.00 ( 0.01 0.30 0.62
0.95 ( 0.07 1.47 ( 0.03 2.29 ( 0.06
0.00 ( 0.01 -0.02 0.00
H (T)
32.9 ( 0.1 49.0 46.0
area (%) 19 ( 1 8(2 9(2 59 ( 1 5(2
The Mo¨ssbauer parameters (except the area) of sextets S1 and S2 were kept fixed during the fitting procedure.
TABLE 5: Mo1 ssbauer Parameters Obtained from the Fit of the 298 and 16 K Spectrum Recorded from the SFe2 Reduced Sample temp (K)
component
δ (mms-1)
∆ (mms-1)
298
D D1 D2 D1 D2 M1 M2
0.35 ( 0.02 1.01 ( 0.02 1.05 ( 0.08 1.22 ( 0.01 1.25 ( 0.02 0.44 ( 0.01 0.48 ( 0.01
0.82 ( 0.05 1.42 ( 0.05 2.38 ( 0.13 1.45 ( 0.01 2.59 ( 0.06
16
a
2 (mms-1)
0.00 ( 0.01 0.02 ( 0.02
H (T)
area (%)
50.8 ( 0.1 48.0 ( 0.1a
82 ( 5 10 ( 2 8(2 9(1 4(1 35 ( 1 52 ( 1
Maximum of the hyperfine magnetic field distribution.
a weak treatment (pH ) 2 or pH ) 0 for 30′) is unable to remove enough Mg2+ cations located at distal positions while a stronger one (pH ) 0 for 2 h.) completely removes all of the Mg2+ cations from the sepiolite framework to transform into porous silica.20 In this sense, during the raising of the pH process, iron cations are incorporated into the sepiolite magnesium vacancies26,27 forming a variety of stable iron-containing sepiolites similar to the naturally occurring mineral known as ferricsepiolite or xylotile.28 Once all of the magnesium vacancies have been occupied and in case the aqueous solution would contain an excess of iron cations, they would precipitate on the sepiolite surface as poorly crystallized iron oxides or oxyhydroxides. In the case of the strong acid treatment (pH ) 0), Mo¨ssbauer spectroscopy has revealed the presence of three different components in the 16 K spectrum of sample SFe0. We have associated the narrower sextet with the presence of poorly crystalline ferrihydrite, which is probably located inside the channels of the sepiolite structure. The Mo¨ssbauer parameters of the second magnetic component, characterized by a broad hyperfine magnetic distribution, are, in principle, compatible with the presence of even more poorly crystallized ferrihydrite. However, if we take into account the synthesis conditions (use of a strong acid leaching) all of the Mg2+ ions located at the edges of the octahedral layer have probably been removed. Therefore, there is a strong probability that an important fraction of the Fe3+ cations are substituting Mg2+ at the edges of the sepiolite internal channels (Figure 1a). For crystalline sepiolite, the Mg2+-O octahedra located at the edges of the internal channels present a crystallographic environment constituted by silica tetrahedra, internal magnesium octahedra, and water molecules (Figure 1b). The charge excess due to the substitution of Mg2+ by Fe3+ can be compensated by a OH- group. In that case, the local environment of those Fe3+ cations would become very unsymmetrical and similar to that of a poorly crystallized ferrihydrite. Therefore, it is quite plausible that these ferrihydrite-
like environments are responsible for the hyperfine magnetic distribution observed in the 16 K spectrum. This would also explain the large line width observed at that really low temperature because the presence of Si4+ ions in the local environment of the Fe3+ would reduce the strength of the superexchange interactions significantly and consequently the magnitude of the hyperfine magnetic field. Because there are probably many possible local configurations around those ferric ions located at the edges of the octahedral layer, we observe a broad distribution of hyperfine magnetic fields. Finally, the small Fe3+ paramagnetic contribution present in the 16 K spectrum would correspond to a small fraction of Fe3+ ions occupying nondistal octahedral sites within the sepiolite framework. The reduction process on this sample from 500 to 600 °C simultaneously reduces the Fe3+ cations and collapses the sepiolite structure. As a result, according to the Mo¨ssbauer spectra recorded from the SFe0R sample, most of the Fe3+ ions (59%) are reduced to R-Fe. This result is consistent with the XRD and TEM results, which have also shown the presence of metallic iron in the reduced samples. It is worth noting that, according to the TEM results, the size of the R-Fe particles ranges between 3 and 10 nm. Because in the Mo¨ssbauer spectrum we have observed only an R-Fe magnetic sextet with an hyperfine magnetic field of 32.9 T and no singlet that can be associated with superparamagnetic iron particles, it is plausible to think that the particle size distribution is balanced to the larger values. In this respect, we would mention that it has been reported recently that in the Mo¨ssbauer spectrum of 7.3 nm R-Fe particles confined inside submicrometer spherical silica particles, 70% of the spectral area corresponds to the characteristic sextet of R-Fe.17 Our results have also shown that an important fraction of the Fe3+ has been reduced to Fe2+. The parameters of those Fe2+ species are very similar to those observed in related systems, and they can be assigned to the interface between the metallic iron particles and the sepiolite framework, where the Fe2+ ions can exist in an octahedral
2870 J. Phys. Chem. C, Vol. 112, No. 8, 2008 environment similar to that occurring in many Fe2+-containing silicates.16,17 This would imply that the iron nanoparticles are, therefore, totally embedded within the silicate matrix. It is interesting that the Mo¨ssbauer data show the presence of a quite intense (ca. 20%) Fe3+ doublet. There are several possible explanations to account for the presence of this paramagnetic Fe3+ component in the 16 K spectrum of the reduced SFe0R sample. In principle, it is unlikely that this Fe3+ component is due to very small particles of ferrihydrite because it would be expected that these should have been reduced quite easily under the reduction conditions used. Thus, this Fe3+ component might be due to a fraction of Fe3+ occupying octahedral sites in the sepiolite framework and having a coordination and/or location comfortable enough as to not undergo reduction. The line width associated with these Fe3+ ions is quite broad, which indicates the existence of several environments for iron having slightly different coordination geometries. If the Fe3+ belongs to the sepiolite framework, then the magnetic superexchange interaction among the Fe3+ ions, which presumably should also be quite far one from each other, would be completely weakened due to the presence of Si4+. This would explain the lack of Fe3+ magnetic components in the 16 K spectrum. Another possible explanation is that the reduction process has led to the formation of extremely small particles of Fe3O4 with a blocking temperature lower than 16 K (this is unlikely because magnetite nanoparticles as small as 6 nm show already magnetic ordering above 80 K32). Finally, a third explanation could be the following: if the reduction treatment had led to the partial destruction of the sepiolite framework, then the possibility exists that, as a result of that, an Fe3+ amorphous silicate could have been formed, or, alternatively, some Si4+ ions could have been incorporated into the Fe3+ oxide or oxyhydroxide lattice. Finally, the results indicate that some small amount of iron (5%), probably from the reduction of Fe3+ cations precipitated on the surface, is partially oxidized to magnetite. In the case of a weak acid attack (pH ) 2), only a small fraction of the surface Mg2+ cations are expected to be leached from the sepiolite structure. These vacancies then become nucleation centers for Fe3+ precipitation, and ferrihydrite (42%) and lepidocrocite (58%) are formed according to Mo¨ssbauer spectroscopy. It should be pointed out that because of their very small particle size these species were not observed by X-ray diffraction. In addition, the X-ray fluorescence of iron atoms hinders the detection of poorly crystallized nanoparticles by this technique. After the thermal reduction process, 5-15 nm nanoparticles appear to be deposited on the sepiolite surface particles as can be observed by TEM (Figure 3b). Taking into account the X-ray diffraction and Mo¨ssbauer results, we associate these nanoparticles with magnetite (Figure 2b). Because of their small particle size, these magnetite nanoparticles present a clear superparamagnetic behavior. Regarding the identification of Fe3O4 by Mo¨ssabuer spectroscopy, we would comment the following. It is well known that below the Verwey transition (119 K) the electron hopping occurring between the Fe(II) and Fe(III) ions in the B sites of Fe3O4 is inhibited and the Mo¨ssbauer spectrum changes noticeably.21 The magnetic components observed in the spectrum recorded at 16 K (that is, well below the Verwey transition) did not resemble those shown by Fe3O4 at such low temperature.29 However, it is also well known that poor crystallinity, nonstoichiometry, and incorporation of foreign cations (for example Si4+) can affect the electron exchange in the B sites changing (or even suppressing) the Verwey transition.21,29 This is most probably the scenario that we have at 16 K in this sample because the
Esteban-Cubillo et al. magnetic components of that spectrum are quite broad, indicating that magnetic relaxation is still important at this temperature. Anyway, and given the difficulty in distinguishing nonstoichiometric29 Fe3O4 from γ-Fe2O3 and the complexity of the present system, we cannot discard, with the present data, the presence of some γ-Fe2O3 contribution. Similar to the sample treated in stronger acidic conditions, it seems that the reduction treatment results in a partial collapse of the sepiolite structure and formation of some small amount of Fe2+ in an octahedral environment, similar to that occurring in Fe2+-containing silicates. The different iron species observed in the SFe0R and SFeR2 samples are obtained because, in the case of the SFe0R sample, pO2 is very small, in the neighborhood of metallic particles. This is because sepiolite acts as a barrier decreasing the oxygen diffusion. This fact does not occur in sample SFe2R because the nanoparticles are deposited on the sepiolite surface From an application point of view, the most important result of this study is the ability to control the deposit of iron nanoparticles in a sepiolite matrix. In this case, depending on the acid treatment of the sepiolite suspension, it is possible to obtain, after a treatment under reducing conditions, superparamagnetic magnetite nanoparticles on the surface or stable R-Fe nanoparticles, completely covered by a silicate matrix. The later result is very relevant because it may become a new procedure to synthesize large amounts of magnetic iron nanoparticles. The second important advantage of this synthesis method is the extreme resistance of the produced R-Fe nanoparticles to corrosion and oxidation. That is, ordinary R-Fe nanoparticles are not stable in air atmosphere because they suffer a highly exothermic oxidation. Therefore, they must be passivated by a controlled and partial oxidation of their surface or protected with a coating that totally blocks the oxygen diffusion.30-35 This latter mechanism seems to work in the case of iron nanoparticles in sepiolite (sample SFe0R). In fact, we have proven that even after a heating cycle of 250 °C for 2 h in air, a considerable fraction of R-Fe nanoparticles (>80%) remain unaltered.10 This large range of stability would allow the use of this ferromagnetic material for applications in extreme conditions (high temperatures and oxidizing environments). Conclusions Iron and iron oxide monodispersed nanoparticles have been obtained with a narrow size distribution (3-10 nm) embedded into the sepiolite structural channels or deposited on its surface depending on the initial acidic treatment conditions used on the original sepiolite. At pH ) 0, metallic iron nanoparticles, with sizes ranging from 3 to 6 nm, appear completely embedded into the sepiolite particles. The covering is so effective that iron nanoparticles are stable even after a 250 °C heating in air. In the sample treated at pH ) 2, superparamagnetic magnetite (515 nm) appears to be deposited on the sepiolite surface. Acknowledgment. This work has been supported by the Spanish Ministry of Education and Science under projects MAT2003-04199-C02 and PTR1995-0832-OP. A.E.C. is thankful for financial support of the I3P grant by CSIC and European Social Fund (ESF). Financial support from the Comunidad Auto´noma de Madrid under the project S-0505/MAT/0194 is also gratefully acknowledged. References and Notes (1) Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. MRS Bull. 2001, 26, 985.
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