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Formation of γ-Fe2O3 Isolated Nanoparticles in a Silica Matrix F. del Monte,† M. P. Morales,*,† D. Levy,† A. Fernandez,‡ M. Ocan˜a,‡ A. Roig,§ E. Molins,§ K. O’Grady,| and C. J. Serna† Instituto Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049-Madrid, Spain, Instituto Ciencia de Materiales de Sevilla, CSIC, Avda Americo Vespucio s/n, 41092-Sevilla, Spain, Instituto Ciencia de Materiales de Barcelona, CSIC/LEA SIMAP, Campus UAB, 08193-Bellaterra, Spain, and School of Electronic Engineering and Computer Systems, University of Wales, Bangor, LL57 1TU, U.K. Received January 6, 1997. In Final Form: April 17, 1997X Isolated nanometric particles (D < 30 nm) of γ-Fe2O3 in a silica matrix have been prepared by heating at 400 °C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)3‚9H2O and tetraethylorthosilicate (TEOS). However, when FeCl3‚6H2O was used as precursor, well-developed hematite particles were obtained in the final composite. This different behavior was already manifest in the initial gels. Thus, the gel obtained from iron nitrate salt shows a compact appearance as a result of its higher degree of network connectivity (polymeric gel) whereas the one from the iron chloride appears more loose and highly hygroscopic (colloidal gel). In addition, small superparamagnetic nuclei are formed during the hydrolysis and condensation of the gel obtained from the iron nitrate salt. The γ-Fe2O3 nanoparticle formation takes place through a reduction-oxidation reaction which occurs during the burning of the organic species trapped inside the gel pore. The growth mechanism of the γ-Fe2O3 nanoparticles in the silica network has been studied as well as the optimum conditions for their preparation. Thus, γ-Fe2O3 nanocomposites with different particle sizes and distributions can be prepared by adequate modification of the initial gel microstructure through different gelation times, salt concentrations, and mechanical treatment. Superparamagnetic behavior has been found in all nanocomposites at room temperature, meanwhile at 70 K, a transition from superparamagnetic to ferrimagnetic behavior is observed as the particle size increases. In all cases, the variation in particle size observed by X-ray diffraction corresponds well with changes in the saturation magnetization for the γ-Fe2O3 nanocomposites. Similar size effects are also found via the coercivity values at 70 and 5 K.
Introduction There is a great deal of interest in finding new synthetic routes and understanding the behavior of nanoscale magnets which could have important technological applications in biology and chemistry.1-3 Very peculiar properties are shown by these small particle systems in comparison with bulk materials which could provide new ways of using magnetic structures in technology. The problem is that the traditional methods of synthesis from material science are not able to produce uniform and reproducible particles in the nanometer size range. Also, the tendency of magnetic particles having nanometer dimensions to agglomerate makes the study of their behavior difficult. One possible synthetic route involves the use of preorganized biomolecular networks as chemically and spatially confined environments for the growth of inorganic clusters or nanoparticles. These systems are useful in biomineralization processes in which inorganic crystals are incorporated in an organized fashion into living organisms. In this way, iron oxide particles have been synthesized in situ in the nanodimensional cavity of horse spleen ferritin4 and in polymer organic matrices.1,5,6 In
spite of the major potential application of these nanocomposites in information storage,3 color imaging,1 magnetic refrigeration,7 and as model systems, few papers have been found in the bibliography reporting the synthesis of iron oxide in an inorganic matrix.8-12 The porous nature of the matrix formed by sol-gel provides the sites for nucleation of the iron oxide particles, minimizes their aggregation, and imposes an upper limit on their size. However, the influence of the preparation conditions on the iron oxide phase obtained and on the properties of the final composite is not clear. In this work, an effort has been made to clarify the mechanism of formation of γ-Fe2O3 nanoparticles in a silica network prepared “in situ” by the sol-gel method. A detailed study of the influence of the preparation variables, such as the nature and concentration of the iron salts and thermal treatment of the gels, has been carried out in order to determine the optimum conditions to obtain pure gamma iron oxide particles with a precise size. The reaction was tracked by different techniques such as X-ray diffraction, differential thermal analysis (DTA), and Mo¨ssbauer spectroscopy. The magnetic properties of the final composites have been studied from the hysteresis
†
Instituto Ciencia de Materiales de Madrid. Instituto Ciencia de Materiales de Sevilla. § Instituto Ciencia de Materiales de Barcelona. | University of Wales. X Abstract published in Advance ACS Abstracts, June 1, 1997. ‡
(1) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russel, M. W.; Huffman, D. R. Science 1992, 257, 219. (2) Yong-Nam Jun; Dabbs, D. M.; Aksay, I. A.; Erramilli, S. Langmuir 1994, 10, 3377. (3) Awschalom D. D.; DiVincenzo, D. P. Phys. Today 1995, 43. (4) Meldrum, F. C.; Heywood, B. R.; Mann, S. Science 1992, 257, 522. (5) Kroll, E.; Winnik, F. M.; Ziolo, R. F. Chem. Mater. 1996, 8, 1594.
S0743-7463(97)00022-X CCC: $14.00
(6) da Costa, G. M.; De Grave, E.; de Bakker, P. M. A.; Vandenberghe, R. E. J. Solid State Chem. 1994, 113, 405. (7) McMichael, R. D.; Shull, R. D.; Swartzendruber, L. J.; Bennett, L. H.; Watson, R. E. J. Magn. Magn. Mater. 1992, 111, 29. (8) Shull, R. D.; Ritter, J. J.; Shapiro, A. J.; Swartzendruber, L. J.; Bennett, L. H. Mater. Res. Soc. Symp. Proc. 1988, 132, 179. (9) Masayuki Nogami, Asuha, J. Mater. Sci. Lett. 1993, 12, 1705. (10) Niznansky, D.; Rehspringer, J. L.; Drillon, M. IEEE Trans. Magn. 1994, 30 (2), 821. (11) Niznansky, D.; Viart, N.; Rehspringer, J. L. J. Sol-Gel Sci. Technol. 1997, 8, 615. (12) do Carmo Rangel, M.; Galembeck, F. J. Catalysis 1994, 145, 3647.
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loops at room and low temperature measurements using a vibrating sample magnetometer (VSM). Preparation and Characterization Iron oxide-hydroxide polymers trapped in a silica solgel matrix have been prepared by adding 3.36 mL of tetraethylorthosilicate (TEOS) to 2.5 mL of a 1.5 M solution of an iron salt in ethanol. Thermal treatment at 45 °C for 7 days was used to promote the hydrolysis and condensation of the TEOS resulting in a gel (sample C). The water required for the reaction was supplied only by the iron salt. Finally, this initial gel was heated in air at 400 °C for 10 h to obtain iron oxide nanoparticles inside the silica matrix (sample C-400). This procedure will be referred to here after as standard procedure. Different preparation conditions (solution composition, treatment temperatures, etc.) were varied with respect to this standard procedure to study their effects on the nature of the resulting composites. Thus, two different salt precursors, Fe(NO3)3‚9H2O and FeCl3‚6H2O, were used at different molar concentrations to obtain percentages of Fe/Si between 6.5 and 45%. In addition to the study of heated monolithic gels, some samples were ball milled before the heat treatment and the nature of the final powder composite was studied for comparison. The mechanism of iron nanoparticle growth was studied in an initial gel prepared following the standard procedure described above, after heating at 200, 300, and 400 °C. The iron oxide phases obtained in the composites were identified by X-ray diffraction, in a Philips 1710 diffractometer and the particle size (DRX) was calculated from the full width at half maximum of the (311) reflection using Scherrer’s equation. This technique has insufficient resolution to determine the presence of iron oxide particles in the silica matrix at low concentration or in the first stages of the formation, when the particles are very small. In these cases, Mo¨ssbauer spectroscopy was found to be more suitable. A conventional transmission Mo¨ssbauer spectrometer, operating in constant acceleration mode, with a 57Co source in Rh was used to study the effect of the nature and concentration of the salt precursor on the formation mechanism of the iron particles in the silica network. Spectra were recorded from room temperature to 70 K. The calibration was undertaken using a 25 µm thick R-Fe foil. The Mo¨ssbauer spectral parameters are given relative to this standard at room temperature. The data were folded, plotted, and fitted by a computer procedure using a single doublet for superparamagnetic samples and a doublet plus a distribution of hyperfine fields for the partially split spectra. The magnetic behavior of the nanocomposites was studied from their hysteresis loops at room temperature and 70 K recorded using a vibrating sample magnetometer (VSM). Also, the first stages in the formation of the iron oxide-hydroxide polymers in the gel were studied from initial susceptibility curves. To measure the susceptibility, the samples were first cooled to 5 K and then magnetization values at 20 Oe were recorded as the samples were progressively warmed up from 5 to 300 K. Transmission electron micrographs (TEM) were recorded in a Philips CM 10 microscope operating at 100 kV. The samples were scratched with a diamond pencil, and the powder obtained was dispersed in ethanol and dropped on a conventional carbon-coated copper grid. High-resolution TEM images were obtained in a Philips CM 200 FEG microscope working at 200 kV. d-spacing values were determined by conventional Fourier Transformation procedures. TGA of the samples was carried out to determine the phase changes as well as the weight loss of the samples
Figure 1. X-ray diffraction patterns for the samples C-400 obtained from iron nitrate and chloride salts by the standard procedure.
using a Stanton STA-781 thermal analyzer. The temperature was increased at a rate of 10 °C min-1 under a flowing air atmosphere. Results Nature of the Nanocomposites: Effect of the Salt Precursor. First, it should be noted that a different physical appearance was exhibited by the two gels prepared by the standard procedure from the different salts, Fe(NO3)3‚9H2O and FeCl3‚6H2O. Those obtained in the presence of iron nitrate appeared more dense, probably due to a higher degree of network connectivity, than those obtained from the chloride salt. In the first case, the hydration water from the salt allows the total hydrolysis and subsequent condensation of the TEOS giving rise to a compact gel (polymeric gel). However, the chloride salt retains the water and leaves few molecules to promote the TEOS hydrolysis and condensation, resulting in an colloidal gel. In fact, a much greater weight lost is found by TGA for the chloride gel (40%) than for the nitrate gel (30%) on heating to 400 °C. X-ray diffraction patterns for the nanocomposites obtained by heating the initial gels from both nitrate and chloride salts at 400 °C are shown in Figure 1. Together with the broad peaks corresponding to amorphous SiO2 (2θ ) 23-27°), the peaks of the diffractogram obtained when the precursor salt is Fe(NO3)3‚9H2O can be identified as Fe3O4 or γ-Fe2O3 spinel phase. The broad peaks are characteristic of small particles with a mean crystallite diameter, estimated from the (311) reflection, of 15 nm.13 However, when FeCl3‚6H2O is used as a precursor, the iron oxide obtained is R-Fe2O3 as shown by the X-ray data set. The diffractogram in this case contains intense and narrow Bragg scattering peaks which get wider near the base line. The mean crystallite size calculated is about 40 nm, but the broadening of the peaks near the base line suggests that a significant fraction of the sample consists of smaller particles (Figure 1). Mo¨ssbauer spectra for both nanocomposites at room temperature are characterized by the presence of a large central doublet with an isomer shift of 0.35 mm/s superimposed on a multiple line spectrum (Figure 2). The spectral parameters are given in Table 1. In the case of (13) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedure; Wiley and Sons: New York, 1954; Chapter 9.
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Figure 2. Mo¨ssbauer spectra at room temperature and 80 K for the samples C-400 obtained from iron nitrate and chloride salts by the standard procedure. Table 1. Mo1 ssbauer Spectra Parameters for the Materials Prepared by the Standard Procedurea sample δ/Fe C-400 T (K) (mm/s) NO3-
300 80
Cl-
300 80
∆Eq (mm/s)
Γ (mm/s)
0.35 (2) 0.83 (2) 0.77 (2) 0.37 (4) 0.03 (4) 0.48 (2) 0.85 (7) 0.80 (8) 0.44 (2) -0.02 (1) 0.35 (1) 1.6 (2) 0.61 (2) 0.38 (2) -0.17 (3) 0.42 (4) 0.45 (2) 1.6 (2) 0.63 (1) 0.48 (2) 0.28 (3) 0.56 (4)
〈BHF〉 (T) 28.7 (6) 46.3 (4)b 51.5 (2) 53.7 (2)
area (%) 66 44 18 82 77 23 78 24
χ2 1.3 2.1 1.4 2.0
a δ/Fe, ∆E , and Γ are the isomer shift (relative to R-Fe), the q quadrupolar splitting, and the full width at half maximum respectively. BHF is the hyperfine field in tesla and corresponds to the median field of the distribution for C-400 (NO3-) and to a discrete field for C-400 (Cl-). The area of each subspectrum is given in relative percentage. The numbers within brackets represent the errors of the given values for the last digit. b 46.3 T is the median field value of the distribution. A sharp maximum appears at about 50 T.
the nitrate precursor at 80 K, the spectrum is almost totally split with a negligible quadrupole splitting and a median field of the distribution of 46.3 T, showing a sharp maximum at about 50 T. From these data and the fact that particles of 10 nm of Fe3O4 give a completely split sextet at room temperature,14 we conclude that the nanocomposite consists of γ-Fe2O3 particles. It should be noted that Fe3O4 transforms rapidly in air to γ-Fe2O3 at temperatures as low as 100 °C. In fact, the brown color showed by the nanocomposites will also favor this conclusion. The composite obtained from the chloride salt gives a spectrum at 80 K, for the split component, with ∆Eq ) 0.28 and BHF ) 53.8 T values, characteristic of R-Fe2O3. For this sample, the fraction of blocked particles remains constant with decreasing temperature. Therefore, together with R-Fe2O3 particles of 40 nm (calculated from X-ray diffraction) responsible for the split component of the spectrum, there are particles of a small size with blocking temperatures lower than 80 K which give rise to the doublet in the Mo¨ssbauer spectrum at this temperature (Figure 2). As described before, these small particles are also responsible for the X-ray peak broadening near the base line of the diffractogram (Figure 1). These differences in the resulting iron oxide phases could originate partially during the hydrolysis of the iron salts inside the silica pores. It is well-known that the composition and structure of iron(III) (hydrous) oxide formed in water depend on the preparation conditions, such as Fe3+ concentration, the nature of the anion present, pH, etc.15 Hydrolysis of the iron salt proceeds by the formation of monomers and dimers of iron(III) ions, followed by the (14) Mørup, S.; Topsøe, M.; Lipka J. J. Phys. Colloq. 1976, C6, supplement 12, Tome 37, 287. (15) Flynn, C. M., Jr. Chem. Rev. 1984, 84, 31.
condensation of polymeric species. The polymers formed in the case of nitrates are presumed not to include the nitrate ion in the polymer chain, whereas it has been suggested by several studies15,16 that the polymers formed in the chloride solution contain some chloride ions in place of the hydroxyl ions with the formation of larger particles than those in nitrate solutions. The next step in the precipitation process is the formation of oxybridges and the development of R-FeOOH or β-FeOOH structures. In the presence of chloride ions, β-FeOOH is produced initially which later converts to R-Fe2O3 by dissolutionrecrystallization or heating.17 From nitrate solutions, R-FeOOH is usually obtained which transforms to R-Fe2O3 by heating, but, it has also been reported that γ-FeOOH can be precipitated from nitrate solutions at low Fe concentration and a low OH-/Fe ratio,18 which directly transforms by heating to γ-Fe2O3. The later explanation seems very unlikely in our case since the iron concentration in the silica pores should not be so low and γ-Fe2O3 particles can also been obtained by the standard procedure in an acidic medium (data not shown). Therefore, γ-Fe2O3 nanocomposites should be preferentially formed through the reduction-oxidation of initially precipitated iron oxide-hydroxide polymeric material. However, it is clear that the nature of our nanocomposites formed in an organic medium (ethanol) depends on the salt precursor as it has been reported in aqueous solutions.15 The different nature of both nanocomposites is also manifested under the scanning electron microscope (Figure 3). Whereas the γ-Fe2O3 nanocomposite appears to have a very smooth surface (characteristic of a polymeric gel), that giving R-Fe2O3 exhibits a microstructure composed of spherical particles of about 100 nm in diameter (characteristic of a colloidal gel) as a result of the different hydrolysis and condensation behavior already suggested for both initial gels. It should be noted that although at first sight the gel appears homogeneous under SEM (Figure 3), when observed at higher magnification under the TEM (Figure 4) there is a wide size distribution of the γ-Fe2O3 particles dispersed in the silica matrix. The particles appear spherical in shape with average sizes smaller than 30 nm. The crystalline structure of these particles was confirmed by high-resolution images in which d-spacing values of 4.8 (111), 2.95 (220), and 2.5 Å (311) have been clearly identified and assigned to the γ-Fe2O3 phase. γ-Fe2O3 Nanocomposites: Concentration Effects. In Figure 5, X-ray diffraction patterns for γ-Fe2O3 nanocomposites prepared at different concentrations of iron nitrate salt are presented. Samples with less than a 15% Fe/Si molar concentration give only broad peaks, characteristic of amorphous silica (data not shown). For samples of concentration between 15% and 40%, the X-ray diffraction patterns show the presence of γ-Fe2O3 with a small particle size. The average size of the γ-Fe2O3 particles determined through the (311) reflection of the spinel structure is shown in Table 2. It should be noted that the average particle size increases with Fe/Si concentration from 6 nm (15%) to a maximum of 15 nm for the sample with 25% Fe/Si. At higher concentrations of Fe/Si, a decrease in the particle size is observed (Table 2). The hysteresis loops of the samples with different concentrations of iron oxide in silica measured at 70 K are (16) Music, S.; Ve´rtes, A.; Simmons, G. W.; Czako´-Nagy, I.; Leidheiser, H., Jr. J. Colloid Interface Sci. 1982, 85, 256. (17) Morales, M. P.; Gonza´lez-Carren˜o, T.; Serna, C. J. J. Mater Res. 1992, 7, 2538. (18) Murphy, P. J.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 312.
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Figure 3. Scanning electron micrographs of the samples C-400 obtained from iron nitrate and chloride salts by the standard procedure.
Figure 4. Transmission electron micrograph and high-resolution image of the γ-Fe2O3 nanocomposite (sample C-400).
shown in Figure 6. Apparent superparamagnetic behavior is always observed for all samples at room temperature (data not shown). The saturation magnetization values at 70 K increase up to a maximum of 40 emu/g obtained for samples with 20-25% Fe/Si concentration (Table 2). Larger concentrations of iron in silica lead to a decrease in the apparent saturation magnetization of the composite. The saturation magnetization value is, in all cases, far from the reported value for bulk γ-Fe2O3 (74 emu/g)19 but it is in fairly good agreement with the values measured
in γ-Fe2O3 particles of similar size.19-21 Surface and finite size effects have been reported as being responsible for the decrease in the magnetic properties of nanoparticles.20,21 Spin canting at the surface and/or in the interior of the particles gives rise to a decrease in saturation (19) Berkowitz, A. E.; Schuele, W. J.; Flanders, P. J. J. Appl. Phys. 1968, 39 (2), 1261. (20) Coey, J. M. D. Phys. Rev. Lett. 1971, 27, 1140. (21) Martinez, B.; Roig, A.; Obradors, X.; Mollins, E.; Rouanet, A.; Monty, C. J. Appl. Phys. 1996, 79 (5), 2580.
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Figure 6. Hysteresis loops at 70 K for the γ-Fe2O3 nanocomposites with different Fe/Si molar concentration.
Figure 5. X-ray diffraction patterns for γ-Fe2O3 nanocomposites with different Fe/Si molar concentration. Table 2. Crystallite Size and Magnetic Parameters of γ-Fe2O3 Nanocomposites Prepared by the Standard Procedure with Different Concentrations of Iron in Silica
sample
theoretical molar concn % Fe/Si
particle size DRX (311) (nm)
Msa (emu/g) 70 K
Hc (Oe) 70 K/5 K
A-400 B-400 C-400 D-400 E-400
15 20 25 30 40
6.2 7.4 15 8 7.3
20 40 40 30 25
30/480 90/525 150/100/550 110/475
a Magnetization values are given in terms of grams of γ-Fe O 2 3 assuming that all Fe in the samples is in this form.
magnetization. Therefore, the variation in particle size observed by X-ray diffraction (Figure 5) corresponds to the changes in saturation magnetization since the presence of other phases was not detected. On the contrary, γ-Fe2O3 nanocomposites, prepared from nitrate at temperatures higher than 400 °C which were reported to contain R-Fe2O3 as a minor constituent,9 show a decrease in the magnetization due to the presence of the nonmagnetic iron oxide. A similar variation with concentration is also exhibited by other magnetic parameters. It can be seen in Table 2 that the coercivity values at 70 and 5 K for the samples A, B, C, and D vary with the particle size. Both effects, the decrease in the coercivity values with the decreasing crystallite size and the sharp increase of this magnetic parameter at lower temperatures, indicate superparamagnetic behavior in the samples.18 For γ-Fe2O3 cubic particles, magnetic single domain behavior is occurring in the range of crystallite size between 25 and 40 nm with a maximum in the coercivity according to a coherent rotation of the magnetization. Smaller and larger
Figure 7. X-ray diffraction patterns of the initial gel (sample C) heated at different temperatures.
crystallite sizes give rise to a decrease in the coercivity due to superparamagnetic and multidomain behavior.22 Mechanism of Formation of γ-Fe2O3 Nanocomposites. The evolution with temperature of the initial gels was studied in detail in an attempt to understand the formation of the γ-Fe2O3 nanoparticles in gels obtained from the iron nitrate. In Figure 7 we show the X-ray diffraction patterns of the initial gel obtained by the standard procedure heated at different temperatures. At 200 °C, broad peaks which are difficult to assign are (22) Luborsky, F. E.; Morelok, C. R. J. Appl. Phys. 1964, 35, 2058.
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Figure 8. Mo¨ssbauer spectra at 80 K of the initial gel (sample C) and after heating at different temperatures. Table 3. Crystallite Size and Magnetic Parameters of a Sample Prepared by the Standard Procedure and Its Evolution with the Temperature from the Initial Gel to the Final γ-Fe2O3 Nanoparticles at 400 °C theoretical gelation heat particle molar concn temp treatment size DRX sample % Fe/Si ( °C) ( °C) (311) (nm) C C-200 C-300 C-400
25
45
0 200 300 400
4.5 7.0 15
Figure 9. Hysteresis loops at 70 K of the evolution with temperature from the initial gel to the final γ-Fe2O3 nanocomposite.
Hc (Oe) 70 K superparam. superparam. 100 150
observed whereas γ-Fe2O3 is clearly observed at 300 °C with subsequent growth with increasing temperature up to 800 °C as suggested by the sharpening of the reflection peaks (Figure 7 and Table 3). Temperatures higher than 500 °C give rise to the partial transformation of maghemite into hematite and Fe silicates (Fayalite) due to a reaction with the matrix. Thus, 400 °C is the highest temperature at which only the γ-Fe2O3 phase is obtained, giving the maximum particle size. This is the reason why this temperature was selected as the standard preparation procedure. Information on the mechanism of formation of the nanocomposite was obtained from Mo¨ssbauer spectroscopy (Figure 8). The spectrum of the initial gel (sample C) at 80 K shows a quadrupole doublet associated with a paramagnetic phase and an incipient magnetic splitting component associated with superparamagnetic particles. As the sample is heated at higher temperatures, the intensity of the doublet gradually decreases while correspondingly a gradual increase in the sextet component takes place. A well-resolved sextet component appears after heating at 400 °C; however, the doublet component does not completely disappear at 80 K, even for this sample (C-400), indicating a wide particle size distribution in agreement with the observations by TEM (Figure 4). Hysteresis loops at 70 K (Figure 9) show a drastic change in shape from the initial gel, which is paramagnetic, to that of the sample heated at 200 °C, which is clearly superparamagnetic at this temperature. An increase in the initial susceptibility (slope M/H) with the temperature takes place up to 400 °C indicating a gradual increase in the particle size (Figure 9). From 300 to 400 °C, the sample exhibits hysteresis at 70 K (Table 3) and both coercivity and remanent magnetization increase with the annealing temperature. This type of behavior is entirely consistent with a model of particle growth in the system in such a
Figure 10. Variation with temperature of the initial susceptibility for the inital gel (sample C) and after annealing at 200 °C.
way that the differences in the magnetic parameters are associated with changes in particle size.19 Further information concerning the initial stage of the growth process in our nanoparticles is illustrated in Figure 10 which shows the variation of the initial susceptibility with temperature for the initial gel (sample C) and for the sample annealed at 200 °C. For sample C, it is observed that the initial susceptibility at very low temperature falls until a temperature around 40 K is reached. This drop in susceptibility is entirely consistent with classic Curieor Curie-Weiss-like behavior of a paramagnetic sample, and it is in agreement with earlier studies where the hydrolysis and condensation of the TEOS were carried out at room temperature.23 However, from 30 to 60 K the initial susceptibility remains flat and then commences to fall again at a much slower rate than in the case of low temperature. However, the initial susceptibility in the heated gel exhibits dramatically different behavior rising from a low value at low temperatures to a pronounced (23) Shull, R. D.; Ritter, J. J.; Shapiro, A. J.; Swartzendruber, L. J.; Bennett, L. H. J. Appl. Phys. 1990, 67 (9), 4490.
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Figure 11. Mo¨ssbauer spectra for the initial gel from room temperature to 70 K.
Figure 12. Differential thermal analysis for the initial gel and after washing with water.
peak in the region of 55 K whereafter, the susceptibility falls at a slow rate consistent with the rate of change of susceptibility above 70 K in the initial gel. This peak in susceptibility is classic behavior of superparamagnetic material passing through its blocking transition and has been observed in a range of fine particles and other disordered materials.24-26 Hence, our interpretation of these results is that the sample annealed at 200 °C exhibits classic superparamagnetic behavior in which all the iron has been converted into the form of fine grains leading to exclusively superparamagnetic behavior. However, for the initial gel, mixed behavior is observed where a superparamagnetic peak occurs at about 40 K superimposed on top of some paramagnetic behavior which gives rise to the steep fall in susceptibility at temperatures below 30 K. Thus, in this sample, two phases of magnetic material are seen to coexist due to the low annealing temperature (45 °C) being insufficient to convert all the iron salt into an oxide form. Data obtained from Mo¨ssbauer spectroscopy for the initial gel (sample C) at temperatures between 300 and 70 K (Figure 11) also show the presence of these small particles at the first stage of the reaction. In addition to the central doublet corresponding to Fe3+ paramagnetic species, there exists a significant fraction of superparamagnetic particles which are blocked at 70 K giving a multiple spectrum superimposed onto the doublet. From these data we cannot specify the nature of the superparamagnetic particles present in the initial gel. It is entirely possible that this behavior could be exhibited by any iron oxide-hydroxide grains of a sufficiently small size such that perfect spin compensation is not produced and leads to a net uncompensated moment on the particle which then behaves as a classic superparamagnetic. This latter explanation could also account for the observed low value of saturation moment in these samples (Figure 9) although with the low fields used in our study an unambiguous conclusion cannot be drawn. Also, due to the fact that the nature of the magnetic species is not well defined, and the fact that the value of the anisotropy constant for grains of this size is difficult to ascertain, it is not possible to specify a grain size from the data.
However, from previous published data,26 we would conclude that the grains in the initial gel would have a particle size much less than 10 nm whereas the grains in the sample annealed at 200 °C could be expected to have grains of about this size. Further magnetic measurements on these samples are currently being undertaken in order to define more closely the nature of the distribution of blocking temperatures in these materials and also to measure the saturation magnetization to high fields. The results suggest that small superparamagnetic nuclei appear at temperatures lower than 100 °C during the hydrolysis and condensation of the initial gel. γ-Fe2O3 particles are formed as the temperature is increased by a partial reduction of Fe3+ ions to magnetite due to the carbonaceous species coming from the organic compounds and later maghemite is formed by oxidation. A similar process has been reported in the preparation of γ-Fe2O3 particles from iron salts in organic solution by the spray pyrolysis technique.27 In this case, γ-Fe2O3 is formed in one single step by combustion of an iron (III) salt in air; nevertheless when the pyrolysis is carried out in the presence of a nitrogen atmosphere, Fe3O4 is obtained.28 Clear evidence of the mechanism of formation of γ-Fe2O3 nanoparticles in the silica matrix was obtained by comparison of differential thermal analysis data (DTA) for the initial gel and after it was washed with water (Figure 12). The washed gel was obtained by immersing the initial gel in water during 6 h in order to remove organic compounds, and then it was dried at room temperature. In both cases, the sample shows a low-temperature endothermic peak, 80-100 °C, which can be assigned to the loss of volatile components such as alcohol and water molecules. However, an important difference between both gels is observed at temperatures greater than 200 °C where an exothermic peak, associated with the loss of organic species and magnetite formation,12 disappeared completely when the gel is washed. X-ray diffraction analysis of the samples after heating at 400 °C gives only γ-Fe2O3 in the first case whereas the washed sample results in a completely amorphous material. In agreement with this interpretation, a significant effect is observed in the γ-Fe2O3 nanocomposites when the initial gel is milled prior to the heat treatment (Figure
(24) Gittleman, J. I.; Abelas, B.; Bozowski, S. Phys. Rev. B 1974, 9, 2891. (25) Guy, C. N. J. Phys. F: Metal Phys. 1977, 7 (8), 1505. (26) El-Hilo, M.; O’Grady, K.; Chantrell, R. W. J. Magn. Magn. Mater. 1992, 114, 295 and 307.
(27) Gonza´lez-Carren˜o, T.; Morales, M. P.; Gracia, M.; Serna, C. J. Mater. Lett. 1993, 18, 151. (28) Vallet-Regı´, M.; Caban˜as, M. V.; Labeau, M.; Gonza´lez-Calbet, J. M. J. Mater. Res. 1993, 8, 2694.
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Figure 14. Schematic diagram of the mechanism of formation of γ-Fe2O3 nanocomposite in the silica matrix.
to a little shrinkage which allows the particles to grow in the pore.29b The size of the final particle increases as the molar concentration of iron salt in the initial solution is increased. But when the Fe/Si molar concentration is higher than 25%, several nucleations take place in a pore producing more particles of smaller size. Figure 13. X-ray diffraction pattern for the γ-Fe2O3 nanocomposites obtained by milling the initial gel before heating at 400 °C.
13). The γ-Fe2O3 particles decrease in size as the milling proceeds. Here, the kinetics of formation differ from that of the monolithic gel due to a more rapid evaporation of the organic components, causing the pores of the milled gels to collapse by the action of the capillary forces during drying.29a This result is in good agreement with the proposed mechanism of formation of the nanoparticles of γ-Fe2O3 in the silica matrix where the iron particle size is changed by the microstructure of the silica gel. A schematic diagram of the proposed mechanism for the formation of γ-Fe2O3 nanoparticles in the silica matrix is presented in Figure 14. TEOS is hydrolyzed by the hydration water of the iron salt and condenses at temperatures lower than 80 °C, giving rise to the initial gel. The pore of the gel constitutes the ideal environment for the nucleation of some iron oxide-hydroxide polymers which show superparamagnetic behavior. Later heat treatment up to 400 °C promotes the formation and growth of γ-Fe2O3 nanoparticles on the preformed nuclei. At this temperature the release of organic species is associated (29) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physic and Chemistry of Sol-Gel Processing; Academic Press, Inc.: San Diego, CA, 1990; (a) Chapter 11, p 718; (b) Chapter l6, p 367.
Conclusions In summary, isolated γ-Fe2O3 nanoparticles in a silica matrix have been obtained by heating a gel obtained from a solution of nitrate salt and TEOS in ethanol at 400 °C. A wide range of γ-Fe2O3 particle sizes within the nanometer scale are obtained by this method. By changing the preparation conditions such as the concentration of iron salt and the treatment temperature, particles with an average diameter from 5 to 15 nm can be prepared. The presence of organic species inside the gel pore has been determined as essential to obtain the magnetic iron oxide phase, whose mechanism of formation takes place through a reduction-oxidation process. The magnetic properties of these nanocomposites indicate that this material has good potential for important technological applications. Further efforts are being conducted to narrow the particle size distribution. Acknowledgment. A. Ferna´ndez thanks the Spanish Research Council and the Max-Planck Society for a grant to carry out high-resolution TEM at the Fritz-HaberInstitut der Max-Planck Society. The authors wish to express their thanks to Miss. M. Blanco-Manteco´n for her help in carrying out the magnetic measurements. This research was supported by Project PB95-0002. LA9700228
