Multifunctional iron and iron oxide nanoparticles in silica

Materials Chemistry and Physics 130 (2011) 1026–1032

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Multifunctional iron and iron oxide nanoparticles in silica ˇ Irena Simkien e˙ a , Marius Treideris a , Gediminas Niaura a,∗ , Ritta Szymczak b , Pavlo Aleshkevych b , ˙ a , Irmantas Kaˇsalynas a , Virginijus Bukauskas a , Gintautas J. Babonas a Alfonsas Reza a b

Center for Physical Sciences and Technology, Goˇstauto 11, LT-01108 Vilnius, Lithuania Institute of Physics, PAS, Al. Lotnikow 32/46, PL-02-668 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 18 April 2011 Received in revised form 21 July 2011 Accepted 16 August 2011 Keywords: Iron oxide nanoparticles Silica matrix Raman spectroscopy Magnetic properties

a b s t r a c t Iron and iron oxide nanoparticles in silica layers deposited by sol–gel techniques on Si wafers were formed and studied. It was shown that multifunctional nanoparticles of different iron oxides possessing various physical properties can be fabricated by means of post-growth annealing of (SiO2 :Fe)/SiO2 /Si samples in various atmospheres. The hematite, maghemite, and iron nanoparticles were found to be dominant upon annealing the samples in air, argon, and hydrogen atmosphere, respectively. The physical properties of produced hybrid structures were studied by Raman and FT-IR spectroscopy, spectroscopic ellipsometry, AFM, and magnetic measurements. The sol–gel technique with subsequent annealing procedure is demonstrated to be an effective method for the formation of multifunctional hybrid structures composed of iron or iron oxide nanoparticles in silica matrix. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, the studies of magnetic nanoparticles have been widely carried out for fundamental scientific interest caused by the difference in the physical properties of bulk and nano-sized materials [1–3] as well as for many technological applications, among others, magnetic storage media [4], catalysis [5], medical applications such as drug delivery [6], magnetic resonance imaging [7], hyperthermia [8], miniaturized analysis [9], and manipulation systems also called lab-on-a-chip [10]. The composition-dependent properties of Fe/Fe oxide nanoparticles (NPs) have stimulated great efforts towards the controllable synthesis of functional nanocomposites. However, the preparation of pure single-phase monodisperse-sized NPs of Fe oxides presents some difficulties because of the different metal oxidation states leading to the contemporary presence of various oxides such as magnetite (Fe3 O4 ), maghemite (␥-Fe2 O3 ), hematite (␣-Fe2 O3 ) and wüstite (FeO). Several strategies have been proposed to control the size, shape, stability and composition of iron oxide NPs. The formation of core–shell NPs with coatings of biomolecules, polymers [11], and silica [12] was an effective technique in development of the structures for biological applications. Iron oxide–silica nanocomposite materials ranging in shape from nanocrystals and pod-like NPs [13] to nanorods [14] have been also fabricated. It should be emphasized that silica surface can be easily modified for further functionaliza-

tion to perform biolabeling and drug conjugation [14]. Both Fe2 O3 [11,15] and Fe3 O4 [8,16] core–shell type NPs were formed and investigated. Iron oxide NPs can be also embedded in an inert, transparent and temperature-resistant silica matrix produced, e.g., by sol–gel technique. In the latter case, the gelation process determined the size and the phase of NPs formed inside the silica matrix [1]. For instance, the size of the iron oxide NPs was controlled by Fe precursor concentration and preparation procedure [17]. The morphology and physical properties of Fe-containing NPs were efficiently changed by varying the porosity of silica [5,18,19]. In particular, the magnetic [20] and optical properties [21] of sol–gel processed Fe-containing silica films were found to be strongly dependent on the annealing procedure. In this work the hybrid structures composed of iron oxide NPs formed inside silica films deposited by spinning on Si substrates have been investigated. A particular attention was paid to the influence of ambient during the post-growth annealing procedure of the samples prepared by sol–gel technique. Samples were characterized by AFM, Raman, infrared spectroscopy, spectroscopic ellipsometry and magnetic properties measurements. The properties of the samples annealed in various atmospheres were compared and the difference in the structure and physical properties was revealed and systematized. 2. Experimental 2.1. Synthesis of hybrid samples

∗ Corresponding author. Tel.: +370 5 2729642; fax: +370 5 2729373. E-mail address: [email protected] (G. Niaura). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.08.025

The multilayered hybrid structures (SiO2 :Fe)/SiO2 /Si were fabricated by sol–gel spin-on technique using the procedure described elsewhere [20]. For deposition of

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Table 1 Characteristics of investigated (SiO2 :Fe)/SiO2 /Si structures (n is the ratio of FeCl3 and TEOS solutions in coating solution). Sample

Multilayered structure

n

Annealing atmosphere

S33 S38 S41 S42 S43 S47 S48 S49 S57 S58 S59

(SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si SiO2 /SiO2 /SiO2 /SiO2 /Si (SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si (SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si (SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si (SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si (SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si (SiO2 :Fe)/(SiO2 :Fe)/SiO2 /SiO2 /Si (SiO2 :Fe)/Si (SiO2 :Fe)/Si (SiO2 :Fe)/Si

1:1 0:1 1:1 1:1 1:1 1:3 1:3 1:3 1:3 1:3 1:3

Ar Ar Air Ar H2 Air Ar H2 Air Ar H2

SiO2 layers, tetraethoxysilane (TEOS) colloidal solution (pH 1.96) of composition Si(C2 H5 O)4 (20 ml):C2 H5 OH (40 ml):H2 O (4 ml):HCl (0.1 ml) has been prepared. A uniform silica sol was formed in 24 h at room temperature. The TEOS sol was spincasted on cleaned Si wafer at 2500 rpm for 30 s and dried in air at 100 ◦ C for 1 h. The second SiO2 layer was subsequently formed. The layered structure was annealed in air at 300 ◦ C for 1 h. As a result of this procedure [22], the SiO2 layer of thickness 200–300 nm and porosity of 5–6% was fabricated. For deposition of the Fe-containing silica layer, the saturated at 20 ◦ C water solution of FeCl3 was prepared. The mixture of FeCl3 solution and TEOS colloidal solution at the ratio 1:1 (pH 2.5) or 1:3 (pH 5) was spin-casted for 60 s on silicacoated Si substrate and dried in air at 100 ◦ C for 30 min. The second Fe-containing silica layer was subsequently deposited and dried. In a second series of samples, one Fe-containing SiO2 layer was formed on a bare Si substrate without the pure silica layer. At the final stage of the sample preparation process, multilayered structures were annealed at 550 ◦ C for 2 h in various atmospheres: air, Ar or H2 . The characteristics of the samples under investigation are summarized in Table 1. Thus, the studies carried out on a series of hybrid (SiO2 :Fe)/SiO2 /Si samples prepared by the same technology allowed us to reveal a specific influence of the postgrown thermal treatment at various atmospheres. It is reasonable to assume that the accepted sample preparation process is useful for the formation of Fe-containing NPs, the physical properties of which are controlled by the conditions of post-growth thermal treatment.

2.2. Apparatus Atomic force microscopy (AFM) images were obtained making use of scanning probe microscope Dimension 3100/Nanoscope IVa (Veeco Metrology Group). The scanning electron microscope (SEM) images were obtained with SUPRA 35 instrument with resolution from 2.5 nm at 1 kV to 1.7 nm at 15 kV. Raman spectra were recorded with Horiba Jobin Yvon spectrometer LabRam HR800 equipped with 600 grooves/mm grating and liquid nitrogen cooled CCD detector. The 632.8 nm emission of a Spectra Physics He–Ne laser was used to excite the spectra. The laser power at the sample was adjusted to 1 mW. Raman spectra were taken by using 100×/0.90 NA objective lens. The hole size was set to 200 ␮m and the integration time was 30 s. Each spectrum was recorded by accumulation of 10 scans. Fourier transform infrared (FT-IR) measurements in transmittance mode have been carried out on the Nicolet 8700 FT-IR spectrometer. Multiple scans of 20 runs were taken for each sample to ensure the repeatability of the spectra. Spectroscopic ellipsometry measurements were performed in the 250–850 nm spectral range by means of a photometric ellipsometer with rotating analyzer. The corrections due to wondering of light spot on the photo-cathode and the nonlinearity of photo-detector were made. In the spectral range under consideration the ellipsometric angles  and  were measured with an accuracy of 0.02◦ .

Magnetic properties of hybrid structures were studied using a commercial SQUID magnetometer (MPMS-5, Quantum Design). The magnetization was measured in the temperature range from 5 to 300 K in magnetic fields up to 10 kOe. The magnetic field was oriented in the plane of the films.

3. Results A series of investigations have been carried out on the hybrid samples to determine the main regularities in the physical properties and to reveal a particular importance of annealing process. Structural, optical and magnetic properties have been studied in order to determine the presence and to define the formation mechanism of iron and iron oxide NPs in dielectric media. 3.1. Structure and morphology Particular features in surface morphology are clearly seen in AFM images. The microstructure of the sample surfaces was visualized in a tapping mode. Fig. 1 illustrates the surface structure for (SiO2 :Fe)/SiO2 /Si samples fabricated by identical preparation process and annealed in various atmospheres. As it is seen, the grains of the size in the range from 30 to 50 nm are formed in the samples thermally treated in air (Fig. 1a). It should be noted that simulation of AFM measurements has shown [23] that the width of the nanoparticles depends on the probe shape but the height does not. Therefore, a more reliable estimation of the NPs size follows from the height profile measurements. From the profile trace it is reasonable to assume that the grains observed in AFM images are composed by NPs of size 10–15 nm. The frame-like structure with wall thickness 40–80 nm is typical for the surface of samples annealed in Ar (Fig. 1b). The grains of size 30–50 nm are observed on the surface of samples annealed in H2 atmosphere (Fig. 1c). The edge-on SEM images of the samples under investigation are presented in Fig. 2. As it is seen, the resulting SiO2 :Fe layer is not uniform. Most probably, due to local internal strain, at some places the silica layer (Fig. 2a) or the Fe-containing part is slivered from the substrate or pure silica, respectively, in the (SiO2 :Fe)/SiO2 /Si

Fig. 1. AFM micrographs of (SiO2 :Fe)/SiO2 /Si structures annealed in (a) air (sample S41), (b) Ar (sample S42), and (c) H2 (sample S43) atmosphere.

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Band position (cm−1 )

Assignment

S41

226 s

A1g , ␣-Fe2 O3 , hematite [25,26] Eg , ␣-Fe2 O3 , hematite [25,26] Eg , ␣-Fe2 O3 , hematite [25,26] Eg , ␣-Fe2 O3 , hematite [25,26] Eg , ␣-Fe2 O3 , hematite [25,26] A1g , ␣-Fe2 O3 , hematite [25,26] Eg , ␣-Fe2 O3 , hematite [25,26] (Eu ) ␣-Fe2 O3 , hematite, disorder-induced band [26–28] ␥-Fe2 O3 , maghemite [27] Si–O–Si stretching, silica [30] (2 Eu ) 2nd order band, ␣-Fe2 O3 , hematite [25,28] ␤-FeOOH, akaganeite [25] ␥-Fe2 O3 , maghemite [27] ␥-Fe2 O3 , maghemite [27] ␥-Fe2 O3 , maghemite [27] Carbon [31] Carbon [31]

245 w 293 vs 300 w, sh 411 s 499 w 612 m 660 w

714 m, br 1078 w, br 1316 m, br

S42

307 w 351 w 707 s, br 1370 vw, br

S43

1349 s, br 1589 s, br

Abbreviations: s, strong; vs, very strong; m, middle, w, weak, vw, very weak; br, broad; sh, shoulder.

lines have been also noticed. While in the samples annealed in H2 (S43), metallic iron was indicated together with Fe-containing silicates like greenalite Fe3 Si2 O5 (OH)4 and silicides Fe5 Si3 and Fe2 Si. The structural data on chemical composition of iron oxides in variously thermally treated (SiO2 :Fe)/SiO2 /Si samples were confirmed by investigations of physical properties. Below, the results of optical and magnetic measurements are presented and discussed. 3.2. Raman spectroscopy

Fig. 2. Cross-sectional SEM images of (SiO2 :Fe)/SiO2 /Si structures annealed in (a) air (sample S41), (b) Ar (sample S42), and (c) H2 (sample S43) atmosphere.

samples. The Fe-containing inclusions are seen at the top part of porous silica layer (Fig. 2b) annealed in air or Ar atmosphere. Previous studies by electron probe microanalysis (EPMA) technique have shown an increased Fe concentration in the micrometer-sized clusters formed in the samples annealed in Ar atmosphere [20]. In the samples annealed in H2 atmosphere, the NPs are formed on the top of silica layer (Fig. 2c). A small content of Fe and Fe oxides in the investigated structures (SiO2 :Fe)/SiO2 /Si makes difficult to determine directly the chemical composition of formed NPs. Nevertheless, the X-ray diffraction (XRD) data have shown the presence of hematite (␣-Fe2 O3 ) as the dominant phase of Fe-oxides in the samples (S41) annealed in air. In the samples annealed in Ar (S33), the maghemite (␥-Fe2 O3 ) and hematite phases were indicated though weak magnetite (Fe3 O4 )

For several decades, Raman and Fourier transform infrared spectroscopies have been used to characterize iron oxides (see e.g., [24]). Based on characteristic band frequencies and relative intensities various iron oxides and hydroxides can be easily differentiated [24–33]. In addition to assignment of the oxide phases, these techniques allow one to estimate the degree of crystallinity by analysis of the parameters of the phonon modes [34]. In the recorded spectra, the intense band at 520.7 cm−1 along with a lower-intensity broad peak near 941 cm−1 belong to Si substrate (1st and 2nd order peaks, respectively). To obtain pure spectra of sol–gel prepared silica–Fe compounds, the difference Raman spectra were constructed by subtracting the Si contribution. The position and assignment of Raman bands are presented in Table 2 along with the reference data [25–31]. Fig. 3 shows the difference Raman spectra of studied compounds. Raman spectrum observed for sample S41 exhibits a set of clear and strong bands typical for hematite (␣-Fe2 O3 ) (Fig. 3a and Table 1) [25–27]. Strong and well-defined peaks in the frequency region between 200 and 300 cm−1 indicate an ordered layered structure [25]. Intense band in the vicinity of 660 cm−1 indicates

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Fig. 4. Transmittance FT-IR spectrum of sample S41 (resolution 4 cm−1 ) smoothed over 10 experimental points and normalized with respect to the spectrum of sample S38.

Fig. 3. Difference Raman spectra of (SiO2 :Fe)/SiO2 /Si structures annealed in (a) air (sample S41), (b) Ar (sample S42), and (c) H2 (sample S43) atmosphere. Excitation wavelength is 632.8 nm (1 mW).

disordering of the ␣-Fe2 O3 lattice structure, which might be associated with incorporation of Si into the lattice [27]. The broad peak near 714 cm−1 cannot be attributed to ␣-Fe2 O3 . Based on previous Raman analysis of oxygenated Fe species [25–28], we associate this band to maghemite (␥-Fe2 O3 ). Drastic changes take place in the spectra of the sample annealed in Ar atmosphere instead of air (Fig. 3b). Bands due to ␣-Fe2 O3 structure are no longer visible; instead the intense and broad peak near 707 cm−1 due to ␥-Fe2 O3 compound dominates the spectrum. By using Raman spectroscopy, we were not able to identify clearly any oxygenated Fe compounds for sample annealed in H2 atmosphere (Fig. 3c). However, broad peaks near 1349 and 1589 cm−1 show the presence of nanostructured carbon at interface [31]. These bands are resonantly enhanced, so the actual amount of carbon species might be relatively low. 3.3. FT-IR spectroscopy The experimental FT-IR spectrum of (SiO2 :Fe)/SiO2 /Si sample annealed in air is presented in Fig. 4. The transmittance of Fe-containing sample was normalized with respect to reference sample S38 in order to minimize the influence of silica contribution to IR spectra. In a perfect agreement with Raman data, the fine structure of transmittance spectra is well interpreted by the phonon modes typical of hematite ␣-Fe2 O3 . The major part of the dips in transmittance spectrum corresponds to the frequencies of IR-active TO phonon modes for hematite [24]. For instance, the absorbance peaks at 225, 275 and 440 cm−1 are in agreement with corresponding peaks at 227, 286 and 437 cm−1 assigned to Eu symmetry mode. The peaks at 300 and 526 cm−1 coincide well with two TO phonon modes of A2u symmetry. The latter peak overlaps with TO mode of Eu symmetry which manifests itself the absorption band at 524 cm−1 [24].

It should be noted that, on the one hand, the surface modes are responsible for the features observed for ␣-Fe2 O3 thin films at frequencies between TO and LO modes [34]. In addition, a greater contribution from the LO component has been attributed to an increase in the crystallinity of hematite sample. Basing on these considerations, the absorbance peak at 385 cm−1 in hematite was assigned to A2u phonon mode [24]. On the other hand, it is well established that IR spectra are dependent on the size and shape of the NPs as well as on the nature of the matrix, in which the NPs are embedded. For instance, the increase of ␣-Fe2 O3 NPs size ranging from 18 to 120 nm shifted the Eu band with two components near 440 and 475 cm−1 towards higher wavenumbers [34]. From this point of view, the mode at 485 cm−1 , which corresponds to the peak observed in this work, was well interpreted, when homogeneous aggregation of spherical NPs was taken into account [35]. Thus, the IR transmittance spectra of (SiO2 :Fe)/SiO2 /Si samples annealed in air correspond to those typical for hematite ␣-Fe2 O3 . However, the other samples were not reliably interpreted, most probably, because of a smaller amount of Fe-compounds. 3.4. Ellipsometric studies Spectroscopic ellipsometry presents the other non-destructive optical technique using which the composition and structure of composite multilayer samples can be efficiently investigated [36]. The layer thickness and dielectric function of materials can be determined by analysis of the optical response of a complex structure [37]. In particular, the variable angle spectroscopic ellipsometry is useful for analysis of complex structures. In experimental ellipsometric studies, the complex reflectance () was determined and analyzed in terms of ellipsometric parameters  () and (): =

rp = tan  exp(i), rs

(1)

where rp and rs are Fresnel reflectance coefficients for light polarized parallel (p) and perpendicular (s) with respect to light incidence plane. Fig. 5 illustrates the experimental ellipsometric data and calculation results for the (SiO2 :Fe)/SiO2 /Si sample annealed in air. The spectra of ellipsometric parameters were simulated by a transfer-matrix technique. The inverse problem was solved for three effective layers on Si substrate. The contribution of two upper layers was calculated in the effective media approximation [36] with two constituent compounds, SiO2 and Fe2 O3 . A surface layer

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Fig. 5. Experimental (points) and calculated (curves) spectra of ellipsometric parameters  (a) and  (b) for sample S41 at various angles of light incidence .

of thickness ∼11 nm due to surface roughness and thickness nonuniformity of ∼60% have to be introduced to obtain a reasonable agreement of simulated spectra with experimental ones (Fig. 5). The analysis of ellipsometric data has shown that Fe2 O3 is responsible for the major part of contribution to the optical response of the samples annealed in air. The ellipsometric studies of the (SiO2 :Fe)/SiO2 /Si samples annealed in H2 have been carried out previously [17]. It was determined that the optical response of these particular samples has been dominated by the contribution of surface plasmon resonance in Fe metal-like NPs. 3.5. Magnetic studies As it was determined in our earlier studies [20], the magnetic properties of hybrid (SiO2 :Fe)/SiO2 /Si samples depend critically on the annealing procedure. The magnetization of the samples increased in the series of samples annealed in air, argon and hydrogen. At room temperature, the magnetization of samples S41, S33 and S41 was about 1.0 × 10−5 , 2.0 × 10−5 and 2.4 × 10−4 emu, respectively. In the present work we have paid the main attention to the hydrogen-annealed samples, which have shown the enhanced magnetization. It should be noted that in the structures under investigation the characteristics of magnetic properties are influenced by many factors such as polydispersity, structural defects, interparticle interactions, etc. However, it is reasonable to analyze the experimental data in a simple model in order to reveal particular features of the samples annealed in hydrogen. The magnetic field dependence of magnetization at different temperatures is shown in Fig. 6a for sample S43. In all temperature range from 5 K to 300 K the magnetization shows a hysteresis

Fig. 6. (a) Field dependence of magnetization at various temperatures for sample S43; (b) the coercitivity field HC versus temperature. Inset in (b) shows the coercitivity versus square root of temperature.

loop, which is broader for samples annealed in H2 as compared to that for samples annealed in Ar [19]. The temperature dependence of the coercitivity field HC as a function of temperature is shown in Fig. 6b. In the theory of superparamagnetism, the coercivity of the sample has the following temperature dependence below the blocking temperature [38,39]:

HC =1− HC0



T , TB

(2)

where HC0 is the coercivity at 0 K, and TB is the blocking temperature. In inset of Fig. 6b, the coercivity with respect to T1/2 is shown. From the linear fitting according to Eq. (2), it is found that the blocking temperature TB = 260 ± 60 K with HC0 = 580 Oe. The determined value of TB correlates well with experimental temperature dependence of magnetization (Fig. 7). As seen, in magnetic field 600 Oe, at cooling the divergence between zero-field cooled (ZFC) and field cooled (FC) magnetization begins at about 260 K (see Fig. 7). It is reasonable to consider the sample annealed in hydrogen as the system of non-interacting superparamagnetic clusters with ferromagnetic interaction inside the cluster at room temperature. Assuming that each particle is characterized by: (i) magnetic moment  = MS V, where MS is the spontaneous magnetization and V is the volume of NP; (ii) uniaxial anisotropy K with easy axis randomly oriented in the sample, the field dependence of

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superparamagnetic model is pretty close to spontaneous magnetization of bulk iron at room temperature (∼1400 Gs). The analysis of field and temperature dependence of magnetization has shown that in hybrid (SiO2 :Fe)/SiO2 /Si samples annealed in hydrogen, the NPs are composed of metal iron. 4. Discussion On the one hand, the mechanism of the formation of silica layers by sol–gel technique using the condensation of alcosilanes is well known [42,43]. For this purpose, ethanol solution of TEOS with a small amount of water and acid catalyzer is used. A hydrolization process is characterized by the following reaction: Si(OC2 H5 )4 + H2 O → Si(OC2 H5 )3 (OH) + C2 H5 OH. Fig. 7. Temperature dependence of magnetization of sample S43 at H = 600 Oe in ZFC (open circles) and FC (closed circles) regimes.

magnetization in this particular case could be described as following [40]: 1 M = Np kB T 2





0

∂ ln Z d(cos ˛), ∂H

(3)

where Np is the number of NPs per unit of sample mass, kB is the Boltzmann constant, ˛ is the angle between H and K, and Z is the partition function of the particles:







Z =

exp 0

× I0



KV cos2  + MS VH cos ˛ cos  kB T

MS VH sin ˛ sin  kB T





d(cos ),

(4)

where I0 is the modified Bessel function to order zero and  is the angle between K and MS . On the basis of this model, it is possible to evaluate the volume of NP by knowing the blocking temperature because of [41]: TB =

KV . 25kB

(5)

Taking anisotropy for bulk iron K ≈ 6 × 105 erg cm−3 and TB = 260 K, we obtain V ≈ 1.5 × 10−18 cm3 . The latter value indicates that NPs observed by AFM can be considered as a composition of several mono-domain particles. The least square fitting of Eq. (3) to experimental dependence at room temperature is shown on Fig. 8 by solid line with the following parameters: Msat = Np MS V = 17.4 ± 0.6 emu g−1 , MS = 850 ± 150 Gs. The given value of MS estimated in the accepted

(6)

In a solution, the condensation and polymerization reactions occur simultaneously: ≡ Si–OH + ≡ Si–OH → ≡ Si–O–Si ≡ + H2 O,

(7)

≡ Si–OC2 H5 + ≡ Si–OH → ≡ Si–O–Si ≡ + C2 H5 OH.

(8)

Small amount of water and ethanol released during the condensation are vaporized at the drying process. The porosity of homogeneous silica layers formed by this technique can be controlled by annealing procedure [22]. It should be noted that the silicon oxide matrix produced by sol–gel technique presents the media favorable for the formation of iron oxide or metal iron NPs with a reduced tendency of aggregation. On the other hand, it is well known [44] that the composition and structure of iron oxide depend on the preparation conditions, such as Fe3+ concentration, the nature of the anions present, the acidity of solution, and other factors. During hydrolysis of iron salt, monomers and dimers of Fe3+ ions form followed by the condensation of polymeric species [45]. When Cl− ions are present in a solution, the ␤-FeOOH structures form primarily. Annealing in air results in a transfer of ␤-FeOOH to ␣-Fe2 O3 [45]. In addition, redox reactions lead also to the formation of ␥Fe2 O3 and Fe3 O4 [46] depending on the difference between the rates of polycondensation and solution vaporization. As the layers were deposited by spinning technique with successive drying, both processes occur quite quickly. As a result, not all the ethoxy-groups of TEOS are hydrolized and present in silica pores. In this case, NPs of ␥-Fe2 O3 and Fe3 O4 can be formed due to the redox reactions of iron ions with ethoxy-groups. Thus, annealing in various ambient allows one to control the chemical composition of NPs. Annealing in nitrogen atmosphere of bulk iron oxide–silica samples has led to the formation of maghemite NPs in silica matrixes [47]. FeO NPs annealed in Ar atmosphere converted to ␥-Fe2 O3 and Fe3 O4 [48]. Therefore, it is reasonable to propose that annealing of the (SiO2 :Fe)/SiO2 /Si samples in inert Ar atmosphere with oxygen deficiency leads to the occurrence of ␥-Fe2 O3 and Fe3 O4 in porous silica layers. Annealing of composite structures (SiO2 :Fe)/SiO2 /Si in H2 atmosphere is expected to cause a reduction of iron oxides to metallic iron: 3Fe2 O3 + H2 → 2Fe3 O4 + H2 O; 3Fe3 O4 + 12H2 → 9Fe + 12H2 O. However, at such thermal treatment conditions silicon oxide might also experience partial reduction. An occurrence of Si can contribute to reduction of iron oxide [6] and can lead to the formation of silicides. 5. Conclusions

Fig. 8. Experimental field dependence of magnetization at 300 K (points) described by Langevin function (3) (solid line) for sample S43.

Hybrid structures composed of iron and iron oxide nanoparticles formed inside the SiO2 film were deposited by sol–gel technique on Si substrate. The annealing procedure favors the formation of various iron-containing nanoparticles in silica matrix.

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Based on Raman, infrared spectroscopy, and magnetic properties studies it was demonstrated that the dominant nanoparticles of hematite (␣-Fe2 O3 ), maghemite (␥-Fe2 O3 ), and metallic iron were formed at annealing in air, argon, and hydrogen atmosphere, respectively. Thus, a post-growth treatment of (SiO2 :Fe)/SiO2 /Si composite structures allows one to functionalize the specifically formed nanoparticles for particular applications. Acknowledgements We gratefully acknowledge the Department of Bioelectrochemistry and Biospectroscopy at the Institute of Biochemistry of Vilnius University for the possibility to use the LabRam HR800 spectrometer. The authors thank Dr. J. Sabataityte˙ and A. Kindurys for assistance in SEM studies and IR measurements. References [1] S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander Elst, R.N. Muller, Chem. Rev. 108 (2008) 2064. [2] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [3] X. Batlle, N. Pérez, P. Guardia, O. Iglesias, A. Labarta, F. Bartolomé, L.M. Garcia, J. Bartolomé, A.G. Roca, M.P. Morales, C.J. Serna, J. Appl. Phys. 109 (2011) 07B524. [4] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [5] Y. Wang, Res. Chem. Intermed. 32 (2006) 235. [6] I. Chourpa, L. Douziech-Eyrolles, L. Ngaboni-Okassa, J.-F. Fouquenet, S. CohenJonathan, M. Soucé, H. Marchais, P. Dubois, Analyst 130 (2005) 1395. [7] S. Boutry, S. Laurent, L. Vander Elst, R.N. Muller, Contrast Med. Mol. Imaging 1 (2006) 15. [8] K.C. Souza, N.D.S. Mohallem, E.M.B. Sousa, J. Sol–Gel Sci. Technol. 53 (2010) 418. [9] M.A.M. Gijs, F. Lacharme, U. Lehmann, Chem. Rev. 110 (2010) 1518. [10] L.N. Kim, S.-E. Choi, J. Kim, H. Kimb, S. Kwon, Lab Chip 11 (2011) 48. [11] P.D. Stevens, J. Fan, H.M.R. Gardimalla, M. Yen, Y. Gao, Org. Lett. 7 (2005) 2085. [12] S.L.C. Pinho, G.A. Pereira, P. Voisin, J. Kassem, V. Bouchaud, L. Etienne, J.A. Peters, L. Carlos, S. Mornet, C.F.G.C. Geraldes, J. Rocha, M.-H. Delville, ACS Nano 4 (2010) 5339. [13] W. Zhu, X. Cui, L. Wang, T. Liu, Q. Zhang, Mater. Lett. 65 (2011) 1003. [14] S.A. Corr, Y.K. Gun’ko, A.P. Douvalis, M. Venkatesan, R.D. Gunning, P.D. Nellist, J. Phys. Chem. C 112 (2008) 1008. [15] A. Teleki, M. Suter, P.R. Kidambi, O. Ergeneman, F. Krumeich, B.J. Nelson, S.E. Pratsinis, Chem. Mater. 21 (2009) 2094. [16] X. Liu, J. Xing, Y. Guan, G. Shan, H. Liu, Colloids Surf. A 238 (2004) 127.

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