Magnetic iron oxide-silica nanocomposites. Synthesis and characterization Corinne ChanCac, Elisabeth Tronc and Jean Pierre Jolivet Chimie de la Matih-e Condensie, URA-CNRS 1466, Universiti Pierre et Marie Curie, T54, E 5, 4 Place Jussieu, 75252 Paris Cedex 05, France Composite materials containing nanoparticles of maghaemite (y-Fe203) dispersed in a silica matrix have been made by polymerizing a silica precursor (triethoxysilane or silicic acid) inside an aqueous sol of maghaemite particles. After gelation, the examination of xerogels by electron microscopy does not reveal noticeable aggregation of particles. The structure and composition of the silica matrices were deduced from ,'Si MAS NMR spectroscopy. Thermal analysis, FTIR and NIR spectroscopic studies showed that the particles and silanol groups of the matrix remain solvated in the composite materials.No Si-0-Fe bonds are formed in the xerogels and the dispersion of particles in the matrix seems to result from the mutual solvation of particle surfaces and remaining silanol groups, as indicated by strongly associated hydrogen-bonded water molecules. Nanophase materials are interesting for various technological applications because of the specifically size-related properties (mechanical, electronic, optical, magnetic, etc.) of crystalline domains or particles.lY2 Nanoparticles of ferrimagnetic oxides are typically characterized by the superparamagnetic relaxation phenomenon, which is strongly dependent on the particle size and shape, on the magnetic interactions between particles and on various surface effect^.^ The synthesis of calibrated and well dispersed particles into a rigid matrix is consequently a pre- requisite for the study of such phen~mena.~ Composites formed by nanoparticles of maghaemite, y-Fe203, a spinel iron oxide, dispersed in silica have been tentatively synthesized by heating a mixture of iron nitrate and silicon alkoxide between 700 and 900°C.5*6 The size and the dispersion of the maghaemite particles formed during the thermal treatment seem to be determined by the porosity of the silica matrix.It is, however, difficult to obtain only the spinel phase in the form of calibrated and well dispersed particles. Other attempts consisted of making multiple coatings of silica on more or less oxidized magnetite particles peptized in alkaline medium, but the aggregation of particles always seems to OCCU~.~*~ We showed previously that composites with well dispersed y-Fe,03 particles can be obtained by in situ polymerization of a silica precursor in a sol of well dispersed y-Fez03 particles, and we focused on the behaviour of the composites at high temperatures.' Here, we report the synthesis of composites from a different silica precursor and we focus on the charac- terization of the interactions between particles of varying mean size and the silica matrices by IR spectroscopies and thermal analysis.Experimental Syntheses y-Fe,03 aqueous sols. y-Fe203 nanoparticles result from the oxidation of nanoparticles of stoichiometric magnetite, Fe,O,.They were prepared as described previously," by coprecipi- tation of Fe( NO3), and FeC1, (Fe"/Fe"' =0.5) in alkaline medium under vigorous stirring. The control of the precipi- tation conditions [pH, nature of the base (NaOH or NH,), ionic strength imposed by a salt, e.g. NaNO,] allowed us to prepare calibrated particles with a mean size in the range 4-10 nm.'l,12 The black precipitate immediately formed was first decanted by magnetic settling on a permanent magnet and then isolated and treated with a concentrated HClO, (3 mol 1-l) solution. The Fe" ions released in solution13 were eliminated after centrifugation. The precipitate was treated repeatedly with the perchloric acid solution until the Fe"/Fe"' ratio in the solid was ca.0.01. After the last separation by centrifugation, the particles were dispersed in pure water, giving a stable sol of y-Fe203 particles (iron concentration, 1 mol 1-l) at pHz2. Under such conditions, the aggregation of the particles, which are electrostatically positively charged (C104- counter ions), is at a minimum because of the high surface charge density (ca. 0.9 C mV2) and the low ionic strength.', Dispersion in a silica matrix. Two precursors of silica glasses, triethoxysilane and silicic acid, were used to prepare iron oxide-silica nanocomposites. In acidic aqueous medium, hydrolysis and condensation of the precursors yield a gel which leads, after drying, to a transparent monolithic gla~s.'~,~~ By mixing the precursor with the aqueous maghaemite sol, hydrolysis and condensation of the precursor take place in situ, the gelation being catalysed by the acidity of the sol.Such a procedure leads to homogeneous solid composites. Triethoxysilane precursor. Acidic water contained in the maghaemite solution was used for hydrolysis and condensation of triethoxysilane according to the rea~tion.','~ HSi(OCH,CH,), +3/2 H20-+HSiOl.,+3 CH,CH,OH The alkoxysilane was added to the sol in a proportion corre- sponding to a molar ratio H,O/Si>2. In every case, under vigorous stirring, an emulsion formed initially, then a homo- geneous solution and finally a gel after 30-45 mn at room temperature. The gelation time seems to be independent of the particle concentration. After drying at room temperature, a brown monolith was obtained.Samples with an Fe/Si ratio varying between 0.03 and 1.6 were thus prepared. Fe/Si=O.O3 corresponds to H,O/Si =2 using a sol with an Fe concentration of 1moll-l. Fe/Si=1.6 corresponds to the amount of precur- sor just needed to cover particles of diameter 10nm with a monolayer of silica. Silicic acid precursor. Na,Si03 (1 mol 1-l) was exchanged on a Dowex 50 W X2 resin in H-form and a solution of silicic acid (ca. 0.45 mol l-l, pH M 3) was collected. The pH was adjusted to 2 by adding perchloric acid. For preparing com- posites with Fe/Si 20.1, the silicic acid solution was introduced directly into the sol of y-Fe20, particles. For low particle concentrations (Fe/Si <O.l), the same procedure leads to a fast flocculation of the particles.In order to prevent flocculation, J. Muter. Chem., 1996, 6(12), 1905-1911 1905 it is necessary to control the pH during the mixing The silicic acid solution was thus introduced drop by drop into the sol and the pH of the mixture was kept at 2 by adding perchloric acid We prepared samples with Fe/Si varying between 003 and 0 7 In all cases, homogeneous gels were obtained after a few days at 50 "C, the gelation time decreasing with increasing particle concentration Drying at room temperature yielded homogeneous brown monoliths Aggregation phenomena observed for low Fe/Si ratios pre- sumably result from the acidic catalysis of the condensation of silicic acid l6 This reaction initially consumes protons from solution as it is observed when hydrochloric acid is mixed with silicic acid Thus, when a small quantity of sol (pH=2) is introduced into a large excess of silicic acid (pH32 5, Fe/Si <0 l), the reaction of silicic acid at the particle surface rapidly lowers the surface charge density and leads to the flocculation'' before the proton adsorption equilibna at pH 32 5 take place As aggregates are embedded into silicic acid polymers, the surface cannot be recharged On the other hand, if protons are supplied by perchloric acid addition, the flocculation is prevented If the concentration of particles is higher (Fe/Si >0 l),the amount of protons introduced is larger This allows the protonation of silicic acid with a lesser relative decrease in surface charge density Consequently, the stability of the colloids is not affected significantly and no aggregation occurs Techniques Transmission electron microscopy (TEM) experiments were performed using a JEOL 100 CX I1 apparatus Observations of the particles alone were achieved by evaporating a drop of the very dilute aqueous sols onto a carbon-coated grid The composites were examined in the form of thin sections, 80-100 nm thick X-Ray diffraction (XRD) patterns were recorded using a powder diffractometer (Philips PW 1830) operating in the reflection mode with Cu-Ka radiation The average particle size was deduced from the line broadening by applying the Scherrer formula assuming Gaussian profiles for experimental and instrumental broadenings Quasi-elastic light scattering measurements were performed on aqueous sols with an Amtec SM200 apparatus equipped with a Brookhaven BI2030 correlator These were used to control the lack of significant aggregation in the sols Thermogravimetry and differential thermal analyses (TG-DTA) were effected using a Netzsch STA 409 apparatus IR spectroscopic studies were performed in the range 1800-400 cm-' using a FTIR Nicolet 550 spectrometer Samples dehydrated at room temperature were pelleted with dned KBr (1% by mass) Near-IR spectroscopic studies in the wavelength range 2500-800 nm were performed in a closed cell using a Vanan Cary 5E spectrometer with a 240 nm min-l scanning rate 29S1 NMR spectra of silicic acid solutions were obtained using a Bruker AM 250 spectrometer (49 6 MHz) The probe signal was avoided by the use of a Hahn echo sequence with a delay of 30ms The silica xerogels were studied using a Bruker MSL 300 spectrometer (59 6 MHz) with standard single-phase or cross-polanzation techniques under magic angle spinning (MAS) conditions Results and Discussion Fig la shows the XRD patterns of y-Fe20, particles of different mean sizes The lattice parameter is 0 835 nm, independent of the particle size The line broadening is essentially due to the size effect The average sizes, deduced from the full width at half maximum, are consistent with the mean sizes deduced 1906 J Muter Chem , 1996, 6(12), 1905-1911 A 81 I I 1 I I I I bI C 10 20 30 40 50 60 70 80 2Bldeg rees Fig.1 X-Ray diffraction patterns of samples dried at room tempera- ture a, y-Fe,O, particles of different average diameter, D, composites made from b, triethoxysilane (D = 10 nm) and c, silicic acid (D=7 4nm) from TEM observations In the composites, the average particle size remains unaltered (Fig lb,c) Fig 2a and b show micrographs of composites made from triethoxysilane and silicic acid, respectively In each case, the particles appear to be well separated The condensation of silicic acid is much slower than that of the hydrolysis products of triethoxysilane In both cases, the polymerization of the precursors inside the sol allows particles to be trapped without significant aggregation, and allows the preparation of dispersed materials in a silica matrix over a large range of iron oxide concentration However, the two matrices are chemically very distinct which leads to different effects on the particles during thermal treatment of the composites (see above) The dispersability of the particles in the silica matrices may result from various types of interactions covalent, through Si-0-Fe bond formation, electrostatic, between negatively charged Sir0 terminal ligands and positively charged groups on the particle surface, or by hydrogen-bond interactions between hydration layers of silanol groups and the particle surface In order to characterize the interactions between the particles and the silica matrices, we studied separately the pure silica xerogels and the particles in various states in the initial aqueous sol, after drying at room temperature and in the composites Maghaemite particles For particles with an average diameter of 5 nm, as deduced from XRD and TEM, the average hydrodynamic diameter measured by light scattering on a dilute sol at pH=2 is ca 8 nm The large difference in the two sizes shows that a large amount of water is taken up by the particles in their thermal motion in the sol, which results from the solvation of the positively charged surface groups Of course, a rapid exchange between water molecules in the solvation layer and the bulk of the sol must occur Fig* TEM images Of Y-Fe203 in matrix Obtained: a,from triethoxysilane (10 nm y-Fe203 particles, molar ratio Fe/Si = 0.03); b, from silicic acid (7.4 nm y-Fe203 particles, molar ratio Fe/Si =0.07) Fig.3a shows typical DTA-TG curves of 5 nm y-Fe203 particles. They indicate a loss of water up to the transformation into a-Fe203, at ca. 400-500°C depending on the sample. The water hydration retained by the particles after drying depends strongly on the mode of dessication. As shown in Fig. 4 the thermal treatment of powders obtained by drying the sol at pH =2 for a few days at room temperature (RT)proceeds with three successive mass losses, which represent in all ca. 12% by mass for 10 nm particles (Fig. 4a) and 25% for 5 nm particles (Fig. 4b); they take place from ca.RT to 180 "C, from 200 to 240°C and beyond 250"C, respectively. The first mass loss corresponds to the elimination of hydration water which is relatively weakly bonded (physisorbed) to the particles. For both particle sizes, this hydration represents ca. three water layers around each particle, taking into account the mean volume of the water molecules (3 xlop2nm3) in the liquid phase. The second mass loss corresponds to the removal of the perchlorate counter ions which are always present because the particles remain electrostatically charged after drying. The Fig. 3 DTA-TG curves: a, of 5 nm y-Fe203 particles; b, of composites from silicic acid (molar ratio Fe/Si =0.7) 0. -2 ' -4 -8 h -6j8-101v h $-12C.,, , ,,.,.I.., ,,.., , v uJo8 -4'-8 -12 -16 -20 -24 0 100 200 300 400 T1"C Fig. 4 Thermogravimetry curves: a, 10 nm y-Fe203 particles, pure silica xerogel from triethoxysilane and composites [SiHo *O, 5(OH)0(O), b, 5 nm y-Fe203 Fe/Si=O.17 (m),Fe/Si=1.6 (O), y-Fe203 (O)]; particles, pure silica xerogel from silicic acid and composites [SOl 75(OH)o (O),Fe/Si =0.03 (W), Fe/Si =0.7 (O),y-Fe,03 (O)].The y-Fe,03 particles were isolated from aqueous sols (pH =2) and dried at temperature. presence of perchlorate was shown by its characteristic IR absorption bands (see below), which are still present for samples heated at 180 "C but are absent for samples heated at 240 "C. Such a temperature range for the removal of perchlorate as perchloric acid corresponds well to the boiling point (200 "C) of hydrated perchloric acid." The amount of perchlorate remaining after drying of the sample is, however, smaller than that expected from the surface electrostatic charge of the particles determined in s01ution.l~ This is an effect of the drying process.The drying of particles, involving their progress- ive concentration, induces a relaxation phenomenon of the double layer between the oxide surfaces." The increase of the electrostatic interactions between the particle surfaces leads to a partial desorption of protons from charged surface groups as perchloric acid, which evaporates during the drying. Above 250"C, the mass loss results from the removal of strongly adsorbed water and from the dehydration of surface hydroxy ligands.A sudden small mass loss is systematically observed (Fig. 3a) when the y-Fe20, +a-Fe203 transformation occurs. As the transformation begins by the sintering of aggregated J. Muter. Chem., 1996,6(12), 1905-1911 1907 particles,20 water trapped inside the aggregates is suddenly removed during the recrystallization of the aggregates When the drying conditions are harsher (at 50°C or at RT in the presence of P205),the total mass loss, of 5% for 10 nm particles and 7 5% for 5 nm particles, represents only one water layer around each particle Perchlorate is still present, but in a smaller amount, and the extent of hydration is comparable with that existing on particles flocculated at pH=7 (near the point of zero charge) and then dried FTIR spectra of the particles dried at RT (Fig 5a,b) show one band in the range 630-580 cm-' (denoted +) correspond-~-~ing to the stretching vibration v~ of tetrahedral iron atoms Perchlorate groups present as counter ions exhibit two bands at 1110 and 630 cm-' (denoted *) 21 Silica ma trices The matnx has a very different chemical composition depending on the precursor For tnethoxysilane-based silica, the relative amounts of various sites (Table 1) were deduced from 29S1 MAS NMR spectra and refined by 29S1 CP-MAS NMR using MAS data The NMR studies of this pure silica matrix show that, under the hydrolysis conditions employed, 80% of the Si-H bonds of the precursor are preserved The elimination of ethoxy groups is quasi-complete and the silicium atoms are in different environments The high proportion of trifunctional silicium atoms (HSiO, groups) suggests a rela- tively low degree of crosslinking of the matrix of mean composi- tion SiH, 5(OH), The condensation of pure silicic acid was studied by 29S1 NMR spectroscopy in solution One hour after the exchange, no monomer was observed The monomers condense rapidly to form oligomers which aggregate to form a in which 67% of the silicon atoms carry at least one Si-OH terminal bond After gelation and drying at room temperature, the relative proportions of the different groups in the xerogel (Table2) deduced from the 29S1 MAS NMR study, lead to a mean composition of SiO, 75(OH)0 In view of the chemical compositions, the matnx appears more crosslinked and more hydroxylated than that formed by triethoxysilane hydrolysis The xerogel formed from triethoxysilane is therefore more flexible than the xerogel obtained from silicic acid The pure xerogel obtained from triethoxysilane is thermally * I I I I 1*1 1 I I I I I 191 J 1800 1400 lo00 600 vlcm Fig.5 FTIR spectra of pure xerogels, y-Fe,O, particles and nanocom- posites a, from ethoxysilane, b, from silicic acid (symbols see Table 3) 1908 J Muter Chem, 1996, 6(12), 1905-1911 Table 1 Distribution of the different groups into a pure silica matrix formed by hydrolysis of tnethoxysilane (H,O/Si =2, pH =2) 6 YO OH I -0-Sl-O-I -76 6 13 5 H I 0 -0-Sl-O--85 1 66 2 IH OH I -0-SI-0 -I -101 5 0 I -0-SI-0 --111 1 13 5 I 0 Table 2 Distnbution of the different groups into a pure silica matrix formed by condensation of silicic acid (pH z 2, Ca=0 45 mol I I) 6 % OHI -0-SI-OHI -91 5 710I OHI -0-SI-0 --101 1 38 5 0 I I0 -O-SI-O--1100 54 4 I 0 I stable up to 350°C17 and exhibits only a slight mass loss (<2%) at ca 100 "C (Fig 4a) corresponding to the elimination of hydration water Above 350"C, the cleavage of the Si-H bond occurs, up to the crystallization of the silica network into cristobalite near 1400 "C Up to 350 "C the pure silica xerogel obtained from silicic acid dned at RT (Fig 4b) shows similar behaviour 22 However, the water loss at ca 150°C reaches 17% by mass showing greater hydration of the xerogel in agreement with the NMR analysis Beyond 450"C, a con- tinuous thermal condensation of the Si-OH groups, rep- resented by a slight mass loss (2-3%), occurs up to the transformation of the xerogel into glass The IR absorption bands of the silica networks obtained from tnethoxylsilane or from silicic acid in the range 1800-400 cm-' (Fig 5a,b) are listed in Table 3 on the basis of the absorption spectra of conventional vitreous silica l5 23 24 Silanol group vibrations appear at 3680-3650 cm-' if the SiO-H group is isolated and at 3400 cm-' if it is hydrogen bonded" However, in silica xerogel spectra, these bands are very broad and are not distinguishable from one another because of superimposition of stretching vibrations of hydro- gen-bonded water molecules adsorbed on the surface 25 The Table 3 Assignments of the absorption bands (in cm-') in pure SiH, 8015(OH)0 and SiO, 75(OH)0 xerogels SiHO 8(OH)0Zol 5 Si(OH)O 5Ol 75 v,, (Sl-0-S1) v 1154 (LO) 1205" 1065 (TO) 1082' V, (Si-0-S1) A 824 797 v (Si-OH) 0 936 959 -6 (Si-H) 0 880 6 (Si-0-Sl) 0 452 459 " Longitudinal mode.'Transverse mode. contributions of various silanol species and water molecules to the envelope of vibrations near 3400cm-' may be deter- mined using near-IR spectro~copy.'~ Near-IR studies have been performed in the wavelength range 2500-800nm where the main groups of absorption bands of Si-OH and H20 have been a~signed.~~,~~ In prin- ciple, it is possible to separate free or hydrogen-bonded silanol groups2, and isolated or associated H20 molecules.The spec- trum of the pure xerogel obtained from silicic acid (Fig. 6) essentially shows the vibrations of the silanol groups and water molecules.29 Three main domains can be distinguished. In the 2400-2250 nm region only the stretching vibrations of Si- OH groups and a contribution from the bulk of the matrix arise. In the 2050-1850nm region, there are the combinations of stretching and deformation vibrations of water, and in the range 1600-1300 nm are the overtones of the stretching fre- quencies of silanol groups and water molecules. For each vibration, the band is asymmetrical.At low energy, the main band corresponds to isolated groups, free silanol or isolated water hydrogen-bonded to a silanol group. At higher energy, the shoulder corresponds to hydrogen-bonded groups, silanol groups or hydrogen-bonded water molecules.30 Composites Fig. 3b shows typical TG-DTA curves for a silicic acid-based composite. The exothermic peak characteristic of the y+a-Fe203 transformation (Fig. 3a) is no longer observed. XRD patterns of composites after heating at various temperatures up to 1400°C are shown in Fig. 7. It is clear that the y-Fe203 particles are stabilized in the composite up to at least 1000"C. At 1200"C, is the major iron oxide phase and a-Fe203 appears only beyond 1200 "C. The matrix containing the particles acts as an antisintering agent, and stabilizes the spinel structure.In composites with low particle concentration, structural transformation of the iron oxide occurs only when the matrix crystallizes into cristobalite. Similar features were observed with the triethoxysilane-based composites in an oxid- izing atmosphere.' Below ca. 400"C, all composite materials exhibit the com- bined thermal behaviour of the maghaemite particles and the pure matrix (Fig. 4a,b). This suggests that the formation of Isolaud H20 ,- - +Bonded SiOH IsolatedH70 0.3 . Q,0c ([I4! 0.2 . 8n I ([I 0.1 1 I I I I 1 1200 1400 1600 1800 2000 2200 2400 Alnm Fig. 6 Reflectance NIR spectrum of SiO, 75(OH)* xerogel dned at 100°C for 2 h Cnstobi E-Fe,03 + a-Fe203I II,1 10 20 30 40 50 70 80 2Bldeg rees Fig.7 X-Ray diffraction patterns of a silicic acid-based composite (Fe/Si =0.07) after heating at different temperatures: a, 25 "C; b, 750°C; c, 1000°C; d, 1200°C; e, 1400°C nanocomposites involves neither the dehydration of the par- ticles nor that of the matrix. The FTIR spectra of RT-dried nanocomposites obtained with the highest and intermediate particle concentrations [Fe/Si= 1.6 and 0.17 for SiH, ,O, 5(OH), matrix and Fe/ Si=0.7 and 0.07 for SiOl 75(OH)o matrix] are shown Fig. 5. The absorption bands due to the matrix and those due to the iron oxide are not shifted. No additional band characteristic of Si-0-Fe bonds appears, especially around 900 or 680 cm-', as is observed for Fe-substituted silicalite in ZSM5 zeolites33 or ferrisilicates with the sodalite structure.34 The spectra of the composites can thus be described as superim- posed spectra of the iron oxide and of the silica matrix.In Fig. 8 are shown FTIR spectra of composites after heating at high temperature and characterization using the XRD patterns given in Fig. 7. The continued polycondensation of the silica matrix manifests itself (Fig. 8) by the decreasing intensity of the 970cm-' band assigned to Si-OH vibrations and the evolution of Si-0-Si vibration bands at 1100, 790 and 500 cm-I is in agreement with the structural transformation of the silica network. After heating at 1200 and 1400"C, well resolved bands appear at 580-620 cm-', due to Fe-0-Fe bonds in E-and a-Fe,03.35,36 In these spectra, there is no band which can reasonably be attributed to Si-0-Fe bonds.In order to prove the existence of weak interactions between the surface of the particles and the matrix, it is important to examine the spectral range 3800-3000 crn-', characteristic of the stretching vibration of SiO- H and water molecules. Near-IR investigations on silicic acid-based composites were carried out on materials dried for 4 days at various tempera- tures up to 450°C and corresponding to different degrees of dehydration. The spectra (Fig. 9) of all samples, pure Si(OH), 501 xerogel and composites, show bands located 75 between 2100 and 1250 nm characteristic of both isolated and l...I.V.l...l...l...I 1600 1400 1200 loo0 800 600 400 v1crn-l Fig.8 FTIR spectra of a silicic acid-based composite (Fe/Si = 0.07) heated at different temperatures: a, 25°C; b, 750°C; c, 1000°C; d, 1200°C; e, 1400°C. J. Muter. Chern., 1996, 6(12), 1905-1911 1909 2.5 I 1 0.8 0.6 0.4 0.2 n $j 0.8 -0a 0.8 A 0.6 0.6 0.4 0.4 0.2 0.2 ,......IL 7200 1400 1600 lsoo 2000 Xlnm Fig. 9 NIR spectra of silicic acid-based samples thermally treated for 4 days at different temperatures: A, 25°C; B, 100°C; C, 250°C; D, 450 "C. a, Pure xerogel; composites with 7.4 nm y-Fe,O, particles and molar ratios b, Fe/Si=O.O3 and c, Fe/Si=O.O7; d, 4.5 nm y-Fe203 particles and Fe/Si =0.07. associated water molecules. After dehydration above 100 "C, the band at 1900nm becomes more defined.The hydrogen- bonded water molecules are eliminated progressively as shown by the relative decrease of the shoulder near 1950nm and some isolated water molecules attached to Si-OH groups are still present. Simultaneously, free silanol groups appear as shown by the sharp peak at 1360 nm. After thermal treatment at 25O-45O0C, the spectra are practically the same for all samples. This is in good agreement with the quasi-complete dehydration of the samples as shown by the TG curves (Fig. 4b). The near-IR spectra (Fig. 9) do not allow us to differentiate clearly the hydration of the matrix from that of the particles. However, for the pure silica matrix, free silanols are the main groups remaining after prolonged drying at 100°C and no evolution is observed after heating to 250°C (Fig.9a). In the composites, (Fig. 9b,c,d), the band near 1450 nm, characteristic of associated water molecules, is still present after treatment at 250 "C and such hydration water probably forms multilayers between the particles and the matrix, as shown schematically in Fig. 10. This also suggests that the surface of the iron oxide particles is more strongly solvated than the matrix and the remaining water is in fact essentially a solvation layer of the particles. This could indicate that the dispersal of the particles into the silica matrix results from solvation of silanol groups Multilayers ,,-HIk..._ 6 Fig. 10 Schematic representation of the interactions between the surface of the y-Fe20, particles and the silica matrix 1910 J.Muter. Chem., 1996, 6(12), 1905-1911 of the matrix by the associated water layers around the colloids, without other chemical surface interactions. Very similar results are obtained for the composites made from ethoxysilane in the same thermal treatment range. An advantage given by the ethoxysilane precursor is the rapidity of the gelation. This matrix is much less hydrated than the matrix formed from silicic acid. It is also less crosslinked which is likely to favour particle dispersal. The behaviour of these matrices becomes very different at high temperatures in an inert atmosphere when the cleavage of the Si-H bond leads to the reduction of the iron oxide particles to U-F~.~ Preliminary Miissbauer spectroscopy investigations of the same y-Fe203 particles dispersed, at the same concentration, in silica matrices or in polyvinylic alcohol4 indicate no signifi- cant changes either in the low-temperature spectra or in their temperature dependence, Since the superparamagnetic relax- ation rate is, in principle, very sensitive to surface effect^,^' these observations support the results of the IR spectroscopic studies, namely the presence of only weak interactions between the particle surface and the matrix, and the absence of Fe-0-Si bonds in our materials.This is in contrast with the conclusions of Jung3* for silane-coated spinel iron oxide particles. Conclusion In this study, we showed that the polymerization of silica precursors in an aqueous dispersion of iron oxide nanoparticles allows the formation of nanocomposites free from aggregation of particles.This evidently requires control of the particles dispersion in the starting aqueous sol. Note that the inter- actions between the surface of particles and the matrix are very weak and probably involve only the solvation layers. References 1 H. Gleiter, Nanostruct. Muter., 1995,6, 3. 2 Nanophase Materials. Synthesis, Properties, Applications, ed. G. C. Hadjipanayis and R. W. Siegel, NATO ASI Ser. E, Applied Sciences, Kluwer Academic Publishers, Dordrecht, 1993, vol. 260. 3 J. L. Dormann, D. Fiorani and E. Tronc, Adu. Chem. Phys., 1996, 98, in press. 4 E.Tronc, P. Prene, J. P, Jolivet, F. D'Orazio, F. Lucari, D. Fiorani, M. Godinho, R. Cherkaoui, M. Nogubs and J. L. Dormann, Hype$. Interact., 1995, 95, 129; J. L. Dormann, F. DOrazio, F. Lucari, E. Tronc, P. Prene, J, P. Jolivet, D. 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