Effect of the Support on the Formation of FeCo Alloy Nanoparticles in

Article pubs.acs.org/JPCC

Effect of the Support on the Formation of FeCo Alloy Nanoparticles in an SBA-16 Mesoporous Silica Matrix: An X-ray Absorption Spectroscopy Study D. Carta,† G. Mountjoy,‡ R. Apps,‡ and A. Corrias*,† †

Dipartimento di Scienze Chimiche e Geologiche and INSTM, Università di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Cagliari, Italy ‡ School of Physical Sciences, Ingram Building, University of Kent, Canterbury, CT2 7NH, U.K. S Supporting Information *

ABSTRACT: A series of nanocomposites consisting of FeCo alloy nanoparticles supported on a three-dimensional cubic mesoporous silica matrix (SBA-16) were prepared by wet impregnation of the matrix with a solution of Fe and Co nitrates. FeCo alloy nanoparticles were obtained by heat treatment at 800 °C in reducing atmosphere of the impregnated SBA-16 previously calcined at 500 °C. Three different SBA-16 types were used as a support of the nanophase. The influence of the matrix on the absorption of Fe and Co ions was investigated using X-ray diffraction and X-ray absorption spectroscopy. In particular, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at the Fe and Co K-edges were used in order to identify the intermediate products before reduction and to study the formation of the bcc FeCo alloy, which cannot be assessed unambiguously using X-ray diffraction. An important influence of the matrix has been observed in the phases formed before reduction, in the size of nanoparticles, and in the oxidation of the FeCo alloy nanoparticles.

1. INTRODUCTION Transition metal nanoparticles have been an important subject of research in recent years, for their important applications in catalysis and as magnetic materials. In particular, FeCo alloys nanoparticles dispersed into an highly porous matrix have gained significant interest due to the improved catalytic performance and due to their size-dependent soft magnetic properties.1,2 The dispersion in a matrix is important in order to avoid agglomeration and coalescence of nanoparticles, that would lead to a reduction in saturation magnetization and in catalytic properties. Dispersion of nanoparticles in a matrix is important in catalysis where a high surface area porous matrix influences the accessibility of the active nanoparticles, improving selectivity and activity of the catalyst. Dispersion of nanoparticles is also important for magnetic applications. FeCo alloy nanoparticles show high values of saturation magnetization, permeability, and low coercitivity. However, due to their high conductivity, they are not suitable for highfrequency applications. Dispersion in an insulating matrix such as a silica makes them suitable also for these applications.3 Among the variety of the potential matrices, ordered mesoporous silicas have been reported as ideal supports thanks to the uniform mesopore dimension ranging from 20 to 500 Å, the regularly arranged pore structure, and the high specific surface area.4,5 In particular, cubic mesoporous silica, having a cubic array of mesopores developing in three dimensions (3D), has the potential to be highly efficient in the fields of catalysis, sensors, separation membranes, electronic and magnetic devices, biology, and nanotechnology.6,7 Among the cubic mesoporous © 2012 American Chemical Society

silicas, the so-called SBA-16, formed by an arrangement of spherical empty cages having a body-centered cubic symmetry (Im3m), is considered the most interesting. SBA-16 is usually prepared via sol−gel using a surfactant-templated synthetic procedure often using amphiphilic block copolymers as structure-directing agents. The most common way, described by Zhao et al.,8 is to use a nonionic triblock polymer such as poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), Pluronic, as a surfactant due to its low-cost, availability, and biodegradability. Block-copolymers form self-assembled micellar aggregates in solution. Monomeric and oligomeric species which arise from hydrolysis of a silica precursor interact with the surface of the micelles by van der Waals or electrostatic forces and then condense on the surface of the micelles leading to the final silica. After removal of the organic template, the silica maintains the “negative” structure of the organic template, with an ordered pore structure generated by the calcination of the micelles. However, SBA-16 can only be prepared in a narrow range of conditions and the synthesis is difficult to control. A similar but improved synthetic method was described by Kleitz et al.;9 according to this method the addition of butanol to a triblock copolymer-based mesoporous silica synthesis at low acidic concentration enables better control of the synthesis and tailoring of the pore lattice type and symmetry. SBA-16 can also be obtained using a templated-gelation method described by Received: March 27, 2012 Revised: May 11, 2012 Published: May 14, 2012 12353

dx.doi.org/10.1021/jp302927t | J. Phys. Chem. C 2012, 116, 12353−12365

The Journal of Physical Chemistry C

Article

500 °C with a heating step of 1 °C/min and kept at the final temperature of 500 °C for 6 h. The three matrices differ for surface area, pore volume, pore size, and degree of microporosity. In particular, G-SBA-16 matrix has a surface area of 252 m2 g −1 and pore volume of 0.21 cm3 g −1, whereas S-SBA-16 and B-SBA-16 have much larger surface areas, 952 m2 g −1 and 746 m2 g −1, respectively, and pore volume of about 0.60 cm3 g −1. S-SBA-16 has the largest pore size (4.6 nm), B-SBA-16 intermediate (3.8 nm), and G-SBA-16 the smallest (3.5 nm). Moreover, the S-SBA-16 has a high degree of microporosity. Nanocomposites were prepared by wet impregnation of 0.3 g of the three types of calcined SBA-16 with an aqueous solution of Fe(NO3)3·9H2O (Aldrich, 98%) and Co(NO3)2·6H2O (Aldrich, 98%), vigorously stirred for 24 h. Different nanocomposites were prepared varying the total metal molar concentrations of Fe and Co ions (0.2, 0.4, and 0.8 M) and the Fe:Co ratio (1:1 and 2:1). The impregnated SBA-16 were separated from the solution by centrifugation and then calcined in air at 500 °C, in order to promote the decomposition of the metal nitrates. The formation of FeCo alloy nanoparticles in the nanocomposites was achieved by treating the samples in H2 flux with a heating step of 10 °C/min up to 800 °C and held at 800 °C for 2 h. The products calcined at 500 °C and reduced at 800 °C will be hereafter indicated as A_FexCoy_Z_500 and A_FexCoy_Z_r800, respectively, where A is the type of matrix (S, B, G), x:y is the Fe:Co ratio, and Z indicates the overall metal molar concentration. A list of all nanocomposites investigated is presented in Table 1. 2.2. XRD. Wide-angle X-ray diffraction (XRD) patterns were recorded on a Panalytical Empyrean diffractometer equipped with a graphite monochromator on the diffracted beam and a X′Celerator linear detector. The scans were collected within the range of 15−85° (2ϑ) using Cu Kα radiation. 2.3. X-ray Absorption Data Collection (XANES and EXAFS). The X-ray absorption spectroscopy experiments were carried out at beamline 11.1 (XAFS) at the ELETTRA synchrotron (Trieste, Italy) and at beamline A1 at the DORIS III synchrotron (HASYLAB, Germany), on a selection of the samples, as indicated in Table 1, and on several reference compounds. Data were collected at room temperature using a Si(111) monochromator. Three ion chambers were used to measure the incident, transmitted, and reference beam intensities, respectively. 5 μm Fe and Co foils were placed between the second and third ion chambers so that the absorption spectrum of the foil was recorded simultaneously, for energy scale calibration. The energy of the first inflection point for Fe and Co foils was taken to be 7112 and 7709 eV, respectively. The acquisition steps were 0.1 eV in the XANES region and 2.0 eV in the EXAFS region with a kmax of 12.5. Harmonic rejection was carried out when required by detuning the second crystal of the monochromator until 50% of the intensity was reached. Samples with a suitable and highly uniform optical thickness were prepared from powders of the nanocomposites and of some reference compounds. The powders of the reference compounds were dispersed in an inert solvent and then filtered onto polyethylene supports. The powders of the samples were diluted in polyvinylpyrrolidone (PVP) and pressed as pellets. 2.4. XANES Data Analysis. The XANES spectra were processed in the usual way to obtain normalized absorbance.19 XANES at the K-edge involves the excitation of a 1s photoelectron into low-lying empty states at the central atom with

Costacurta et al.10 through a slow and controlled solvent evaporation in the presence of a triblock copolymer as the structure-directing agent. The advantage of this method is that SBA-16 can be obtained as a bulk monolith, membranes, and thin films. In the present work, bulk composites containing nanoparticles of FeCo alloys dispersed in the SBA-16 matrix were prepared by wet impregnation of the matrix with a solution of Fe and Co nitrates. Three different SBA-16 matrixes were previously prepared according to the three methods described above. After removal of the volatiles by calcination, Fe and Co are present as oxidized phases dispersed in the matrix. The FeCo alloy nanoparticles are formed only after a reduction treatment in hydrogen. Understanding the phases present after calcination and before reduction is an important factor for the successful preparation of the resulting nanocomposite. Moreover, it is important to prove the formation of the FeCo alloy nanoparticles and its degree of oxidation. X-ray diffraction (XRD) can be used to study the phases formed before reduction and to check the formation of the FeCo alloy phase. However, the nature of the products formed before and after reduction cannot be completely clarified using XRD. The limitation in the use of XRD is mostly due to the low percentage of nanophase with respect to the silica matrix, to the disordered nature of the nanophase especially at low calcination temperatures, and to the similar X-ray scattering factors of Fe and Co. In order to obtain a detailed study on the evolution of the nanocomposite with the temperature, on the FeCo alloy formation, and on the oxidation of nanoparticles, X-ray absorption spectroscopy techniques (XAS), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were used. EXAFS and XANES have been shown to be powerful tools for the structural study of metal, metal oxide, alloy nanoparticles, and nanocomposites prepared by the sol−gel process.11−15 Since these techniques are element specific and sensitive to the local structure,16 they are ideal for studying multicomponent dilute and disordered materials. In particular, XAS analysis of FeCo alloy nanoparticles dispersed silica and alumina xerogels and aerogels11,12 and in mesoporous silica17,18 have shown that the possibility of studying separately the Fe and Co environment is essential in identifying the intermediate products before reduction and in proving the formation of the FeCo alloy nanoparticles and the presence of additional phases, such as oxides.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Three different host systems, all SBA-16 type, were used for the Fe and Co nitrate impregnation. All three types of SBA-16 matrix were prepared by using a surfactant-templated synthetic procedure. The first one, labeled S-SBA-16, was obtained using the conventional precipitation method described by Zhao et al.8 The second one, labeled B-SBA-16, was obtained according to an improved synthetic method, which makes use of butanol a cosurfactant, described by Kleitz et al.9 The third one, labeled G-SBA-16, was obtained using the gelation method described by Costacurta et al.10 All three types of SBA-16 matrixes were prepared by using the Pluronic F127 (P127, Aldrich) block copolymer surfactant, formed by a sequence of poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (EO106-PO70EO106) units with high molecular weight (Mav = 12600) and high EO/PO ratio, as a structure directing agent. In order to remove the surfactant, all pure SBA-16 were calcined in air at 12354



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Table 1. List and Characteristics of the Prepared Nanocomposites acronym

support

Fe:Co ratio

thermal treatment

ions conc (M) of impregnating solution

XAS measurements

S_Fe2Co1_0.4_500 S_Fe2Co1_0.4_r800 S_Fe1Co1_0.4_500 S_Fe1Co1_0.4_r800 S_Fe1Co1_0.8_500 S_Fe1Co1_0.8_r800 B_Fe2Co1_0.2_500 B_Fe2Co1_0.2_r800 B_Fe1Co1_0.2_500 B_Fe1Co1_0.2_r800 B_Fe1Co1_0.4_500 B_Fe1Co1_0.4_r800 G_Fe2Co1_0.4_500 G_Fe2Co1_0.4_r800 G_Fe2Co1_0.8_500 G_Fe2Co1_0.8_r800

S-SBA-16 S-SBA-16 S-SBA-16 S-SBA-16 S-SBA-16 S-SBA-16 B-SBA-16 B-SBA-16 B-SBA-16 B-SBA-16 B-SBA-16 B-SBA-16 G-SBA-16 G-SBA-16 G-SBA-16 G-SBA-16

2:1 2:1 1:1 1:1 1:1 1:1 2:1 2:1 1:1 1:1 1:1 1:1 2:1 2:1 2:1 2:1

calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C calcined in air at 500 °C reduced in H2 at 800 °C

0.4 0.4 0.4 0.4 0.8 0.8 0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4 0.8 0.8

yes yes yes yes yes yes no yes yes yes yes yes no yes yes yes

two edges are less than 1 KeV apart and the oscillations at the Fe K-edge might slightly superimpose to the Co K-edge. The same k range was used at the Co edge in order to achieve similar resolution. Fourier transform (FT) of EXAFS data corrected for phaseshift shows peaks corresponding to local atom correlations. The positions of the peaks (R) correspond to bond distances between the central and the backscatterers atoms, while the amplitudes are related to the coordination number (N) and to the static and thermal disorder (σ) of the atoms around the absorber. Theoretical parameters, | f i(k,Ri)|, φi(k,Ri), δ(k), and λ(k), were calculated using the von Barth potential for ground states, the Hedin−Lundquist exchange potential for excited states,23 and the relaxed approximation for the core-hole.24 In DL_EXCURV the k-independent parameter AFAC takes the place of S0(k)2 in eq 1. AFAC was determined to be 0.9 from fitting to the reference samples. The parameter EF, which is a correction to E0, was free to vary in all fitting. The structural parameters were obtained by nonlinear least-squares fitting in k-space with a k3 weighting of experimental EXAFS spectra to emphasize the high-energy part of the spectrum. The errors in the fit parameters were obtained from the 95% confidence level, as calculated in EXCURV98. The number of fitted parameters was always less than the number of statistically independent data points, as estimated in the standard way. In particular, the number of statistically independent data points was between 30 and 35, depending on the range of the FT showing significant peaks, while the number of fitted parameters ranged between 5 and 15. The quality of the fit can be judged from the normalized sum of residuals

p-type symmetry. The K-edge XANES spectra in transition metals has a gradually sloping main absorption edge, with a pronounced step on the low energy side, a rounded main absorption edge peak, and approximately constant intensity following the edge. In contrast, transition metal oxides have sharply rising main absorption edge, with main absorption edge peak(s) of high intensity, and a notable drop in intensity after the main absorption edge peak. In addition, oxides may show a small pre-edge peak if the excited atom site has a lack of centrosymmetry. In both metals and oxides, oscillations in intensity occurring up to approximately 30 eV beyond the absorption edge are due to strong multiple-scattering or shape resonance around the excited atom site. The XANES spectra have been analyzed using the “fingerprint” method, by comparing spectra from samples with those from reference compounds. 2.5. EXAFS Data Analysis. The program Viper was used to sum the data, identify the beginning of the absorption edge, Eo, fit pre- and postedge backgrounds, μtpre and μtpost, respectively, and hence to obtain the normalized absorbance χ = (μt − μtpost)/(μtpost − μtpre) as a function of the modulus of the photoelectron wavevector k.20 The modular package DL_ EXCURV,21 based on the EXCURV98 code, was used in the final stage of data processing to model the experimental χ(k) in order to extract structural information. Fast curved wave theory was used22 where χ (k) = ΣiS0 2 (k)(Ni /kR i 2)|fi (k , R )| sin(2kR i + 2δ(k) + φi(k , R )) exp( −2σi 2k 2) exp(− 2R i /λ(k))

(1)

and Ri, Ni, and 2σi2 are the distance, coordination number, and Debye−Waller term (static and thermal disorder) for the ith shell of neighboring atoms. The additional parameters in eq 1 are the effective curved wave backscattering amplitude f(k,Ri) of the scatterer, the phase shift due to the absorbing atom potential 2δ(k), the phase shift due to the scatterer φi(k,R), and the inelastic mean free path of the photoelectron λ(k). Equation 1 is valid for single scattering of the photoelectron. The fitting was carried out in k space using the range 2.5− 12 Å−1, where 12 Å−1 is the highest accessible value at the Fe K-edge due to the presence of Co K-edge. It should be pointed out that previous studies on FeCo alloy nanoparticles have shown that detailed information can still be obtained even if the

R ‐factor = Σn|kn 3χexpt (kn) − kn 3χfit (kn)|/Σn|kn 3χexpt (kn)|×100 (2)

Reasonable EXAFS fits of single shells typically have values of R-factor of around 20%. However, when the fit is performed on the total EXAFS spectra, higher values of R-factor can still correspond to good fits especially if the fit is not extended to peaks at high R. 12355



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spinel phase of either γ-Fe2O3 or CoFe2O4. The patterns of G_Fe2Co1_0.4_500 and G_Fe2Co1_0.8_500 (Figure 1C) show sharp Bragg peaks due to α-Fe2O3 (PDF Card No. 33-664), which are more intense in the more concentrated G_Fe2Co1_ 0.8_500 and a broad shoulder at 2ϑ ∼ 37°, that is difficult to assign. 3.1.2. XANES. More detailed information of the phases present on the nanocomposites calcined at 500 °C can be inferred from the analysis of the XANES spectra at both Fe and Co K-edge. XANES spectra at the Fe K-edge are reported in Figure 2A along with those of F2L, γ-Fe2O3, CoFe2O4, α-Fe2O3, and

3. RESULTS 3.1. Nanocomposites Calcined at 500 °C. 3.1.1. XRD. The X-ray diffraction patterns of the nanocomposites prepared using the S-SBA-16, B-SBA-16, and G-SBA-16 supports and calcined in air at 500 °C are reported in Figure 1A−C,

Figure 2. XANES spectra from experiment at the Fe K-edge (A) and Co K-edge (B): (A) (a) S_Fe2Co1_0.4_500; (b) S_Fe1Co1_0.4_500; (c) S_Fe1Co1_0.8_500; (d) B_Fe1Co1_0.2_500; (e) B_Fe1Co1_0.4_500; (f) G_Fe2Co1_0.8_500; (g) F2L; (h) γ-Fe2O3; (i) CoFe2O4; (l) α-Fe 2 O 3 ; (m) FePO 4 . (B) (a) S_Fe2Co1_0.4_500; (b) S_Fe1Co1_0.4_500; (c) S_Fe1Co1_0.8_500; (d) B_Fe1Co1_0.2_500; (e) B_Fe1Co1_0.4_500; (f) G_Fe2Co1_0.8_500; (g) Co3O4; (h) CoFe2O4; (i) CoO. Figure 1. Wide angle XRD of samples calcined at 500 °C: (A) (a) S_Fe2Co1_0.4_500; (b) S_Fe1Co1_0.4_500; (c) S_Fe1Co1_0.8_500; (B) (a) B_Fe2Co1_0.2_500; (b) B_Fe1Co1_0.2_500; (c) B_Fe1Co1_0.4_500; (C) (a) G_Fe2Co1_0.4_500; (b) G_Fe2Co1_0.8_500.

FePO4 reference compounds. Details of the pre-edge peaks of nanocomposites and reference compounds are reported in Figure S1, as Supporting Information. The edge position of all the samples corresponds to iron in the oxidation state +3. The spectrum profile of S_Fe2Co1_0.4_500 and S_Fe1Co1_ 0.4_500, which did not show any detectable Bragg peak in the XRD patterns, are similar to each other but different from all the other samples. They have a pre-edge peak similar to γ-Fe2O3 but slightly higher, suggesting that a higher fraction of Fe is in a tetrahedral coordination compared to γ-Fe2O3, a high prepeak being typical of a highly non-centrosymmetric environment. However, the pre-edge peak is lower than that of FePO4, which has 100% Fe3+ in tetrahedral sites.26,27 It has to be noted that Fe3+ in tetrahedral coordination has been observed in several composites systems, such as iron containing mesoporous silica. In fact, the postedge XANES features of our sample are very similar to those observed for 4-folded iron in mesoporous silica.28−30 The spectrum profiles of S_Fe1Co1_ 0.8_500, B_Fe1Co1_0.2_500, and B_Fe1Co1_0.4_500 are similar to F2L, while that of G_Fe2Co1_0.8_500 is consistent with α-Fe2O3. XANES spectra at the Co K-edge for the same nanocomposites calcined at 500 °C are reported in Figure 2B, along with those of the Co3O4, CoFe2O4, and CoO reference compounds. Details of the pre-edge peaks of nanocomposites and reference compounds are reported in Figure S2, as Supporting

respectively. The intensity and the broadening of the peaks in the XRD patterns vary strongly as a function of the support indicating that the latter influences the impregnation of the ions solutions. In particular, peaks are more intense in the samples obtained with the B-SBA-16 matrix, indicating that a higher loading of dispersed phase is obtained. Moreover, the type of matrix has an influence on the phases and on the dimensions of the nanoparticles. Broad peaks are present in the samples obtained with the S-SBA-16 and B-SBA-16 supports, whereas by using the G-SBA-16 matrix sharp peaks can be observed indicating formation of bigger particles. All the patterns exhibit the broad halo at 2ϑ ∼ 20° due to the silica matrix. Patterns of S_Fe2Co1_0.4_500 and S_Fe1Co1_ 0.4_500 (Figure 1A) do not show any intense Bragg peak, while the more concentrated S_Fe1Co1_0.8_500 show peaks which can be ascribed to Co3O4 (PDF Card No. 42-1467) and either 2-line ferrihydrite (F2L), a very poorly crystalline Fe(III) oxyhydroxide,25 or a spinel phase, such as γ-Fe2O3 (PDF Card No. 25-1402) or CoFe2O4 (PDF Card No. 79-1744). The patterns of all the samples prepared with the B-SBA-16 matrix (Figure 1B) show weak peaks that can be ascribed to Co3O4 and to an additional phase which could be either F2L or a 12356



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At the Fe K-edge, S_Fe2Co1_0.4_500 and S_Fe1Co1_ 0.4_500 have the weakest EXAFS oscillations, that are different from those of all the other samples. EXAFS oscillations of S_Fe1Co1_0.8_500, B_Fe1Co1_0.2_500, and B_Fe1Co1_ 0.4_500 are similar to those of F2L, in agreement with XANES results, and oscillations of G_Fe2Co1_0.8_500 are similar to those of α-Fe2O3, also in agreement with XANES and XRD results. At the Co K-edge, the EXAFS oscillations of S_Fe1Co1_0.4_500 and S_Fe2Co1_0.4_500 are different from all the others, whereas oscillations of S_Fe2Co1_ 0.8_500 are higher and similar to those of Co3O4. Oscillations of B_Fe1Co1_0.2_500, B_Fe1Co1_0.4_500, and G_Fe2Co1_0.8_500 are similar to each other and also similar to those of Co3O4. EXAFS results are confirmed by the analysis of the FTs at the Fe and Co K-edges, reported in Figure 4.

Information. The edge position varies depending on the sample indicating the presence of cobalt in different oxidation states. The position of the edge of S_Fe2Co1_0.4_500 and S_Fe1Co1_0.4_500 indicate that only cobalt +2 is present, differently from all the other nanocomposites. Moreover, the pre-edge peaks and postedge profiles are similar to each other but different from all the reference compounds, being compatible with the presence of a mixture of CoO and CoFe2O4. On the other hand, the pre-edge peak and the postedge profile of S_Fe1Co1_0.8_500 are very similar to those of Co3O4. In B_Fe1Co1_0.2_500 and B_Fe1Co1_0.4_500 the position of the edge is indicating the presence of Co +2 and +3, and the postedge profiles are compatible with a spinel structure. The prepeak are very similar for the two samples, and they are lower than Co3O4 and higher than CoFe2O4 and CoO. This suggests the presence of a mixture of Co3O4 and CoFe2O4. Also in G_Fe2Co1_0.8_500 the edge position and the spectrum profile is similar to Co3O4, containing cobalt +2 and +3 in a spinel structure. 3.1.3. EXAFS. The analysis of the EXAFS results allows one to gather additional information. In Figure 3, the k3χ(k)

Figure 4. Fourier transforms of k3χ(k) spectra at the Fe K-edge (left) and Co K-edge (right) from experiment (−) and fit results (···): (S1) (a) F2L; (b) γ-Fe2O3; (c) CoFe2O4; (d) α-Fe2O3. (S2) (a) Co3O4; (b) CoFe2O4. (A) (a) S_Fe2Co1_0.4_500; (b) S_Fe1Co1_0.4_500; (c) S_Fe1Co1_0.8_500. (B) (a) B_Fe1Co1_0.2_500; (b) B_Fe1Co1_0.4_500. (C) (a) G_ Fe2Co1_0.8_500.

Figure 3. k3χ(k) spectra at the Fe K-edge (left) and Co K-edge (right) from experiment (−) and fit results (···): (S1) (a) F2L; (b) γ-Fe2O3; (c) CoFe2O4; (d) α-Fe2O3. (S2) (a) Co3O4; (b) CoFe2O4. (A) (a) S_Fe2Co1_0.4_500; (b) S_Fe1Co1_0.4_500; (c) S_Fe1Co1_0.8_500. (B) (a) B_Fe1Co1_0.2_500; (b) B_Fe1Co1_0.4_500. (C) (a) G_ Fe2Co1_0.8_500.

At the Fe K-edge, the FT of S_Fe2Co1_0.4_500, S_Fe1Co1_0.4_500 show only a first prominent peak, the second being very weak. This confirms that Fe in these samples is present in a highly disordered environment. FT of S_Fe1Co1_0.8_500 show a first prominent peak and a small and broad second peak consistent with the poor crystallinity of F2L. FTs of B_Fe1Co1_0.2_500 and B_Fe1Co1_0.4_500 are similar to that of S_Fe1Co1_0.8_500 suggesting also in this case the presence of F2L. FT of of G_Fe2Co1_0.8_500 shows two well-defined peaks, in accordance with the presence of the well-defined structure of α-Fe2O3. As observed at the Fe K-edge, the FTs of S_Fe2Co1_0.4_500 and S_Fe1Co1_0.4_ 500 at the Co K-edge appear to indicate a highly disordered environment because just a single well-defined coordination

functions at the Fe and Co K-edge for the nanocomposites calcined at 500 °C are shown along with those of F2L, γ-Fe2O3, α-Fe2O3, Co3O4, and CoFe2O4 reference compounds. Oscillations of all nanocomposites at both edges are quite weak and do not appear up to high k. This is an indication that the samples calcined at 500 °C have either a low degree of symmetry/ crystallinity or very small nanoparticles. It has to be noted that the oscillations at the Co K-edge for all samples (apart from S_Fe2Co1_0.4_500) are higher than at the Fe K-edge. 12357



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R-factor = 49% R*-factor = 36%

2σ2 (Å2) 0.024(3) 0.019(2) 0.027(3) N 5.3 4.0 8.0

R-factor = 48% R*-factor = 31% R-factor = 53% R*-factor = 35% R-factor = 46% R*-factor = 35%

Ni was kept fixed for the fitting of S_Fe1Co1_0.8_500. Ri, 2σi2 were left free to vary in all fittings.

O Fe Fe Fe

a


N 5.1 0.8 2.4 1.5 2σ2 (Å2) 0.026(2) N 4.3(1) R (Å) 1.90(1) 2σ2 (Å2) 0.024(2) N 4.5(2)

S_Fe2Co1_0.4_500

R (Å) 1.90(1)

S_Fe1Co1_0.4_500

R (Å) 1.95 (1) 2.87(4) 3.00(2) 3.45(7)

S_Fe1Co1_0.8_500

2σ2 (Å2) 0.028 (2) 0.020 (1) 0.020 (5) 0.016(2)

O Co Co


S_Fe1Co1_0.8_500

R (Å) 1.92(1) 2.85(1) 3.35(1) 2σ2 (Å2) 0.029(4) N 6.0(1)

S_Fe1Co1_0.4_500

R (Å) 1.99(3) 2σ2 (Å2) 0.029(1) N 6.0(4)

S_Fe2Co1_0.4_500

R (Å) 2.06(1)

Co K-edge

shell is observed, the second one being very weak. In the FTs of S_Fe1Co1_0.8_500, B_Fe1Co1_0.2_500, B_Fe1Co1_ 0.4_500, and G_Fe2Co1_0.8_500, two well-defined peaks are observed confirming the presence of the well-defined structure of Co3O4. 3.1.4. EXAFS Fitting. The results of the fitting of the nanocomposites calcined at 500 °C are shown in Figures 3 and 4 and the best fit parameters are reported in Tables 2 and 3. At the Fe K-edge, S_Fe2Co1_0.4_500 and S_Fe1Co1_ 0.4_500 show very weak EXAFS oscillations and a single prominent peak in the FTs. Therefore the fitting was done using a single Fe−O distance. Since the EXAFS oscillations of S_Fe1Co1_0.8_500 are very similar to those of F2L, the same parameters as F2L where used for the fitting, that is, one shell of O and three shells of Fe. The same also applies to the fitting of B_Fe1Co1_0.2_500 and B_Fe1Co1_0.4_500, which also showed strong similarity to those of F2L. The EXAFS oscillations of G_Fe2Co1_0.8_500 and the corresponding two well-defined peaks in the FTs were fitted with distances agreeing with the α-Fe2O3 crystallographic data. In particular, the best fitting parameters include a Fe−O distance of 1.94 Å, corresponding to the first FT peak and 2 Fe shells, corresponding to the second double peak. As observed at the Fe K-edge, the FTs of S_Fe2Co1_ 0.4_500, S_Fe1Co1_0.4_500 show only a single prominent shell at the Co K-edge. Therefore, these samples were fitted using a single Co−O shell. On the other hand, since the oscillations of the S_Fe1Co1_0.8_500 sample, impregnated with a more concentrated nitrate solution, are very similar to those of Co3O4, this sample was fitted with parameters in agreement with this structure. Co3O4 has a normal spinel structure, that is, with Co2+ in tetrahedral sites (B) and Co3+ in octahedral sites (A). In accordance with Co3O4 crystallographic data,11 the best fit parameters present a single O shell with an average distance of 1.93 Å arising from the two distances CoA−O (1.92 Å) and CoB−O (1.94 Å) due to tetrahedrally and octahedrally coordinated Co. The double peak in the region 2.5−4 Å is due to the overlapping of the CoA−CoA distances (2.85 Å) and of several contributions such as CoB−CoB, CoA− CoB, CoB−O, CoA−O (∼ 3.4 Å). A similar approach was used for B_Fe1Co1_0.2_500, B_Fe1Co1_0.4_500, and G_Fe2Co1_0.8_500 since they also showed similarity to Co3O4, and fitted distances are in agreement with the crystallographic data. 3.2. Nanocomposites Reduced at 800 °C. 3.2.1. XRD. The X-ray diffraction patterns of the nanocomposites prepared using the S-SBA-16, B-SBA-16, and G-SBA-16 support after reduction in hydrogen at 800 °C are reported in Figure 5 A−C, respectively. All the spectra exhibit the broad halo at 2ϑ ∼ 20° due to the silica matrix. Patterns of S_Fe2Co1_0.4_r800 and S_Fe1Co1_0.4_r800 show a broad peak at 2ϑ ∼ 45°; additional reflections at 2ϑ ∼ 65° and 2ϑ ∼ 82° can also be seen in the more concentrated S_Fe1Co1_0.8_r800 sample (Figure 5A). Similarly, in B_Fe1Co1_0.2_r800 only the peak at 2ϑ ∼ 45° is evident, whereas in B_Fe2Co1_0.2_r800 and B_Fe1Co1_0.4_r800 the additional reflections at 2ϑ ∼ 65° and 2ϑ ∼ 82° can also be seen (Figure 5B). Unlike the other samples, peaks are quite sharp in the samples obtained with the G-SBA-16 matrix, indicating the formation of larger crystals. Patterns of both G_Fe2Co1_0.4_r800 and G_Fe2Co1_0.8_r800 show a sharp peak at 2ϑ ∼ 45° and additional reflections at 2ϑ ∼ 65° and 2ϑ ∼ 82° that are more evident in G_Fe2Co1_0.8_r800

Fe K-edge

Table 2. Interatomic Distances (R), Coordination Numbers (N), and Debye-Waller Factors (σ) Obtained by Fitting the Experimental EXAFS Spectra of S_Fe2Co1_0.4_500, S_Fe1Co1_0.4_500, and S_Fe1Co1_0.8_500 at the Fe and Co K-Edgea

Article



2σ2 (Å2) 0.028(3) 0.020(2) 0.027(2) N 5.3 4.0 8.0


O Co Co


G_Fe2Co1_0.8_500

R (Å) 1.93 (1) 2.85 (1) 3.36(1) 2σ2 (Å2) 0.025(2) 0.026(3) 0.033(2) N 5.3 4.0 8.0 R (Å) 1.93(1) 2.85 (1) 3.36(1)

B_Fe1Co1_0.4_500

Article

2σ2 (Å2) 0.026(2) 0.030(3) 0.035(3) N 5.3 4.0 8.0

B_Fe1Co1_0.2_500

R (Å) 1.92 (1) 2.85 (1) 3.32(1)

Co K-edge

N 6.0 4.0 3.0


(Figure 5C). The reflections at 2ϑ ∼ 45°, 65° and 82° are typical of bcc FeCo alloy phase (PDF Card No. 44-1433) but also of pure bcc α-Fe (PDF Card No. 6-696). Therefore, the formation of the FeCo alloy cannot be unambiguously proved by only using XRD. However, XRD can give us useful information on the crystallite sizes. To this end, the Scherrer formula, t = 0.91 λ/ (B cos ϑ), where t is the crystallite size, λ is the incident radiation wavelength, ϑ is the Bragg angle, and B is the full-width at half-maximum of the diffraction peak (after correction for instrumental broadening), was used. The particle size is similar for samples in the S and B series (around 7 nm) but significantly bigger for samples in the G series (around 40 nm). 3.2.2. XANES. The XANES spectra at the Fe K-edge for the nanocomposites reduced at 800 °C are reported in Figure 6A along with that of a bcc Fe foil. In the latter the main absorption edge is at about 7130 eV, and a contribution at 7140 eV due to multiple scattering can be observed. The main absorption edge of all samples is also at about 7130 eV, and their profiles are similar to that of bcc Fe. However, the bcc multiple scattering peak is not evident in all the samples. In particular, a reduction of this peak along with a negative gradient over the region 7130−7160 eV indicates a significant contribution arising from an oxide phase. In the S series two of the samples show a certain degree of oxidation: S_Fe2Co1_ 0.4_r800 and, to a lesser extent, S_Fe1Co1_0.8_r800, while the XANES profile of S_Fe1Co1_0.4_r800 is different from all the others. Also in the B series a different degree of oxidation

a

O Fe Fe Fe


Ni were kept fixed for all fitting.


2σ2 (Å2) 0.031(3) 0.030(6) 0.022(6) 0.023(6) N 5.1 0.8 2.4 1.5 R (Å) 1.96(1) 2.87(2) 2.98(3) 3.45(7)

B_Fe1Co1_0.4_500

R (Å) 1.94(2) 2.95(1) 3.38(1)

G_Fe2Co1_0.8_500

2σ2 (Å2) 0.032(3) 0.023(2) 0.017(3)

Figure 5. Wide angle XRD of samples reduced at 800 °C: (A) (a) S_Fe2Co1_0.4_r800; (b) S_Fe1Co1_0.4_r800; (c) S_Fe1Co1_0.8_r800. (B) (a) B_ Fe2Co1_0.2_r800; (b) B_Fe1Co1_0.2_r800; (c) B_Fe1Co1_0.4_r800. (C) (a) G_Fe2Co1_0.4_r800; (b) G_Fe2Co1_0.8_r800.

2σ2 (Å2) 0.036(3) 0.032(1) 0.023(4) 0.023(1) N 5.1 0.8 2.4 1.5

B_Fe1Co1_0.2_500

R (Å) 1.95(1) 2.87(4) 2.98(2) 3.45(2)

Fe K-edge

Table 3. Interatomic Distances (R) and Debye-Waller Factors (σ) Obtained by Fitting the Experimental EXAFS Spectra of B_Fe1Co1_0.2_500, B_Fe1Co1_0.4_500 and G_Fe2Co1_0.8_500 at the Fe and Co K-Edgea


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Figure 6. XANES spectra from experiment at the Fe K-edge (A) and Co K-edge (B): (A) (a) S_Fe2Co1_0.4_r800; (b) S_Fe1Co1_ 0.4_r800; (c) S_Fe1Co1_0.8_r800; (d) B_Fe2Co1_0.2_r800; (e) B_Fe1Co1_0.2_r800; (f) B_Fe1Co1_0.4_r800; (g) G_ Fe2Co1_0.4_r800; (h) G_Fe2Co1_0.8_r800; (i) bcc Fe. (B) (a) S_Fe2Co1_0.4_r800; (b) S_Fe1Co1_0.4_r800; (c) S_Fe1Co1_ 0.8_r800; (d) B_Fe2Co1_0.2_r800; (e) B_Fe1Co1_0.2_r800; (f) B_Fe1Co1_0.4_r800; (g) G_Fe2Co1_0.4_r800; (h) G_Fe2Co1_ 0.8_r800; (i) fcc Co.

is observed depending on the sample composition. In B_Fe2Co1_0.2_r800 and to a greater extent in B_Fe1Co1_ 0.2_r800, the negative gradient over the region 7130−7160 eV is evident, suggesting quite a high degree of oxidation. However, in B_Fe1Co1_0.4_r800 the multiple scattering peak at 7140 eV is still evident, and only a slight negative gradient can be observed suggesting that the bcc phase is just slightly oxidized. Similar observations can be made for the sample G_Fe2Co1_0.4_r800 for which a similar low degree of oxidation is expected. Finally, the XANES profile of G_Fe2Co1_0.8_r800 is very similar to that of the Fe foil and shows the multiple scattering peak at 7140 eV, indicating the presence of a pure bcc phase. The XANES spectra at the Co K-edge for the same nanocomposites reduced at 800 °C are reported in Figure 6B along with that of the fcc Co reference compound. The profile of all the spectra are more similar to bcc Fe than to fcc Co which is expected if a bcc FeCo alloy is formed. The XANES spectra of most of the samples do not show any sign of oxidation. Only the XANES profiles of B_Fe2Co1_0.2_r800 suggest some oxidation, while the profile of S_Fe1Co1_0.4_ r800 is different from all others. 3.2.3. EXAFS. The k3χ(k) at the Fe K-edge and Co K-edge for nanocomposites reduced at 800 °C along with those of the bcc Fe and the fcc Co foils are shown in Figure 7. At Fe K-edge, the frequencies of the oscillations are very similar to those of bcc Fe, whereas the amplitudes of the oscillations are smaller. The small oscillations of S_Fe2Co1_0.4_r800, S_Fe1Co1_ 0.4_r800, B_Fe2Co1_0.2_r800, and B_Fe1Co1_0.2_r800 suggest the presence, together with the bcc phase, of a significant amount of a phase which does not give rise to strong oscillations, such as an iron oxide. At the Co K-edge the oscillations of all the samples are more similar to bcc Fe than to fcc Co, with smaller amplitudes. In particular, the weakness of the oscillations of S_Fe1Co1_0.4_r800 and to a less extent B_Fe2Co1_0.2_r800 suggests the presence of an oxidized phase, together with the bcc phase. These results are confirmed by the analysis of the FTs at the Fe and Co K-edges for the same samples, reported in Figure 8. It is evident that the FTs shapes of all samples are similar to

Figure 7. k3χ(k) spectra at the Fe K-edge (left) and Co K-edge (right) from experiment (−) and fit results (···): (A) (a) S_Fe2Co1_ 0.4_r800; (b) S_Fe1Co1_0.4_r800; (c) S_Fe1Co1_0.8_r800. (B) (a) B_Fe2Co1_0.2_r800; (b) B_Fe1Co1_0.2_r800; (c) B_Fe1Co1_0.4_r800. (C) (a) G_Fe2Co1_0.4_r800; (b) G_ Fe2Co1_0.8_r800.

that of bcc Fe. In particular, at the Co K-edge no similarities are observed with the fcc Co FT profile. Distances corresponding to a bcc structure are observed at both Fe and Co K-edges. However, in some samples, a shoulder on the left-hand side of the first peak on the FTs can be observed, in particular, at the Fe K-edge. This contribution must arise from a metal−oxygen distance indicating that a fraction of the metal is present in the form of an oxide. At the Fe K-edge, this contribution, around 1.90 Å, is evident in S_Fe2Co1_0.4_r800, S_Fe1Co1_ 0.4_r800, B_Fe2Co1_0.2_r800, and B_Fe1Co1_0.2_r800. The contribution is less evident in S_Fe1Co1_0.8_r800, B_Fe1Co1_0.4_r800, and G_Fe2Co1_0.4_r800, whereas G_Fe2Co1_0.8_r800 does not seem to have any contribution from an oxide phase. At the Co K-edge the contribution, around 2.05 Å, is evident only in S_Fe1Co1_0.4_r800 and to a lesser extent in B_Fe2Co1_0.2_r800. 3.2.4. EXAFS Fitting. The qualitative comparison of EXAFS and XANES oscillations of the nanocomposites with those of bcc Fe and fcc Co reference compounds were used as a starting point for a more detailed analysis, performed by fitting of the EXAFS spectra. The fitting was performed on the basis of the known crystalline structures of bcc FeCo.11 Fitting of samples was performed by keeping Ni fixed and allowing small variations of Ri, while 2σi2 and EF were left free to vary. The results of the fitting of the nanocomposites reduced at 800 °C are presented in Figures 7 and 8. The best fit parameters at the Fe and Co K-edges are reported in Tables 4, 5, and 6. 12360



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% Co oxide = 40(2) 2.05(1) 6.0 0.011(1)


R-factor = % 30 R*-factor = % 23

2σ2 (Å2) 0.022(1) 0.026(2) 0.030(2) 0.042(7) 0.018(1) N 8.0 6.0 12.0 24.0 8.0


2σ2 (Å2) 0.028(1) 0.028(2) 0.032(6) 0.050(1) 0.020(1)

Ni were kept fixed for all fitting. a

R-factor = 43% R*-factor = 41% R-factor = 36% R*-factor = % 30


O % Fe oxide = 60(1) 1.88(1) 6.0 0.024 % Fe oxide = 40(2) 1.92(1) 6.0 0.022(2) O

2σ2 (Å2) 0.030(1) 0.035(1) 0.035(2) 0.058(1) 0.023(1) Fe Fe Fe Fe Fe

% Fe oxide = 20(2) 1.90(1) 6.0 0.011(2)

2σ2 (Å2) 0.022(1) 0.026(1) 0.030(3) 0.042(1) 0.018(1)

Co Co Co Co Co

R (Å) 2.48(1) 2.83(1) 4.03(2) 4.74(1) 4.96(1)

S_Fe1Co1_0.8_r800

R (Å) 2.48(1) 2.82(1) 4.02(1) 4.72(4) 4.95(1) 2σ2 (Å2) 0.027(1) 0.035(1) 0.035(1) 0.058(3) 0.023(1) N 8.0 6.0 12.0 24.0 8.0

2σ2 (Å2) 0.024(1) 0.028(2) 0.032(2) 0.045(4) 0.020(1)

% alloy = 60(2) R (Å) N 2.48(1) 8.0 2.83(1) 6.0 4.02(2) 12.0 4.73(7) 24.0 4.94(1) 8.0

S_Fe1Co1_0.4_r800 S_Fe2Co1_0.4_r800

Co K-edge

S_Fe1Co1_0.8_r800

% alloy = 80(2) R (Å) N 2.48(1) 8.0 2.82(1) 6.0 4.02(2) 12.0 4.72(5) 24.0 4.95(1) 8.0 % alloy = 40(1) R (Å) N 2.48(1) 8.0 2.83(1) 6.0 4.02(4) 12.0 4.74(5) 24.0 4.96(2) 8.0

S_Fe1Co1_0.4_r800 S_Fe2Co1_0.4_r800

For all nanocomposites, at both Fe and Co K-edges, the FT shows three main peaks. In accordance with the bcc structure, the best fitting parameters corresponding to the first peak include two O shells at about 2.48 Å and 2.83 Å, the second peak was fitted with a shell at about 4.03 Å, and the third was fitted with two shells at about 4.75 and 4.96 Å. Atoms of the first (2.48 Å) and fifth shells (4.96 Å) are collinear. Therefore, the strong multiple scattering contribution from these paths was taken into account during the fitting. The results of the fitting at the Fe and Co K-edges indicate that FeCo alloy is present in all samples. However, in order to take into account the contribution around 1.92 Å, which is observed in all samples, with the exception of G_Fe2Co1_0.8_r800, at the Fe K-edge, a shell of O was added at the typical Fe−O distance, in addition to the FeCo alloy contributions. In the same way, a contribution around 2.05 Å was added in B_Fe2Co1_0.2_r800 and S_Fe1Co1_0.4_r800 at the Co K-edge, which can be ascribed to a typical Co−O distance in one of the cobalt oxide phases. In all these cases, the fitting was performed considering the FeCo alloy and the iron/cobalt oxide as two separate sites. An estimation of the relative amounts of FeCo alloy and metal oxide was obtained by fitting coordination numbers of the central atom in the two clusters. However, due to the correlation between coordination numbers and Debye−Waller factors, the fitting procedure has to be done with caution. Considering that the presence of oxide appears to be lower at

% alloy = 60(2) R (Å) N 2.48(1) 8.0 2.83(1) 6.0 4.03(2) 12.0 4.74(2) 24.0 4.96(1) 8.0

Figure 8. Fourier transforms of k3χ(k) spectra at the Fe K-edge (left) and Co K-edge (right) from experiment (−) and fit results (···): (A) (a) S_Fe2Co1_0.4_r800; (b) S_Fe1Co1_0.4_r800; (c) S_Fe1Co1_ 0.8_r800. (B) (a) B_Fe2Co1_0.2_r800; (b) B_Fe1Co1_0.2_r800; (c) B_Fe1Co1_0.4_r800. (C) (a) G_ Fe2Co1_0.4_r800; (b) G_ Fe2Co1_0.8_r800.

Fe K-edge

Table 4. Interatomic Distances (R) and Debye−Waller Factors (σ) Obtained by Fitting the Experimental EXAFS Spectra of S_Fe2Co1_0.4_r800, S_Fe1Co1_0.4_ r800, and S_Fe1Co1_0.8_ r800 at the Fe and Co K-Edgea

Article


the Co K-edge, the fit has been done first at the latter edge, and then the parameters obtained for the FeCo alloy were transferred to the Fe K-edge and variation of the amount of alloy was allowed. Fitting of the Fe K-edge indicates that around 10% of iron is oxidized in G_Fe2Co1_0.4_r800, whereas no oxide has been found in G_Fe2Co1_0.8_r800. In the B series, the amount of iron oxide increases from 12% in B_Fe1Co1_0.4_r800, to 40% in B_Fe1Co1_0.2_r800, and to 55% in B_Fe2Co1_0.2_r800. Finally, in the S series, the amount of iron oxide increases from 20% in S_Fe1Co1_ 0.8_r800 to 40% in S_Fe2Co1_0.4_r800 to 60% in S_Fe1Co1_0.4_r800. At the Co K-edge, B_Fe2Co1_ 0.2_r800 has 20% of cobalt oxide and S_Fe1Co1_0.4_r800 has 40% of cobalt oxide. It has to be noted that the intensity of EXAFS oscillations and FTs is consistent with the oxide amount resulting from the fitting at both Fe and Co K-edges. Therefore, at the Fe K-edge, EXAFS oscillations and intensity of FTs of G_Fe2Co1_0.8_r800 are higher than those of G_Fe2Co1_0.4_r800. In the B series, those of B_Fe1Co1_ 0.4_r800 are higher than those of B_Fe1Co1_0.2_r800 and B_Fe2Co1_0.2_r800, and in the S series intensity decrease from S_Fe1Co1_0.8_r800 to S_Fe2Co1_0.4_r800 to S_Fe1Co1_0.4_r800, in agreement with an increasing contribution from oxide phase. At the Co K-edge, the samples with an oxide contribution, S_Fe1Co1_0.4_r800 and B_Fe2Co1_0.2_r800 have the lowest oscillations and FTs intensities.


2σ2 (Å2) 0.018(2) 0.018(2) 0.023(2) 0.026(4) 0.013(2) N 8.0 6.0 12.0 24.0 8.0 R (Å) 2.47(1) 2.83(1) 4.03(1) 4.74(2) 4.95(1)

R-factor = 27% R*-factor = 20% R-factor = 29% R*-factor = 22%

% Co oxide = 20(1) 2.05(1) 6.0 0.006(3) O 0.021(3)

a


R-factor = 29% R*-factor = 17% R-factor = 29% R*-factor = 25% R-factor = 39% R*-factor = 34%

2σ2 (Å2) 0.026(2) 0.027(2) 0.030(5) 0.041(7) 0.021(1)

4. DISCUSSION XRD analysis of nanocomposites calcined in air at 500 °C shows that there is a strong influence of the support on the absorption of the Fe and Co nitrate solution. In particular, if we compare the XRD pattern of S_Fe1Co1_0.4_500, obtained by impregnating the matrix S-SBA-16, and that of B_Fe1Co1_0.4_500, obtained by impregnating the matrix B-SBA-16, with an identical Fe/Co nitrate solution, we observe that only in B_Fe1Co1_0.4_500 peaks corresponding to a nanophase are detected, whereas in S_Fe1Co1_0.4_500 no evidence of specific phases is observed. On the other hand, using the G-SBA-16 matrix, much bigger particles are obtained. The identification of the phases present in the nanocomposites after calcination in air at 500 °C is not an easy task. The small amount of the dispersed phase and the low crystallinity make the identification quite difficult. XRD provides valuable information to identify the phases present in G_Fe2Co1_0.8_500 (α-Fe2O3 and Co3O4) where peaks are relatively sharp. However, the identification of the phases present at this stage is more ambiguous in B_Fe1Co1_0.2_500, B_Fe1Co1_0.4_500, and S_Fe1Co1_0.8_500, where peaks are less defined. For all these samples, in addition to the peaks ascribed to Co3O4, other reflections can be observed that could be ascribed to various iron oxide phases with spinel structure (e.g., γ-Fe2O3 or CoFe2O4) or to ferrihydrite, F2L. Finally, XRD does not give indications of the phases present in S_Fe2Co1_0.4_500 and S_Fe1Co1_0.4_500 since the patterns do not show any Bragg peak. Therefore, XRD provides very limited information on the analysis of the phases present in the nanocomposites calcined at 500 °C. EXAFS and XANES prove to be much more powerful, since they can be used to obtain independent and detailed information on both the Fe and Co environments in the nanocomposites calcined at 500 °C. The EXAFS and XANES results have shown that most

6.0 % Fe oxide = 40(3) 1.92(1) 6.0 0.026(3) % Fe oxide = 55(2) 1.92(1) 6.0 0.032(3) O

% Fe oxide = 12(3) 1.90(1)

Co Co Co Co Co 2σ2 (Å2) 0.018(2) 0.018(2) 0.023(2) 0.026(5) 0.013(2) N 8.0 6.0 12.0 24.0 8.0 2σ2 (Å2) 0.022(1) 0.025(2) 0.034(2) 0.035(6) 0.022(1) Fe Fe Fe Fe Fe

B_Fe1Co1_0.4_r800

% alloy = 88(3) R (Å) 2.47(1) 2.83(1) 4.03(1) 4.74(2) 4.95(1) 60(3) N 8.0 6.0 120 24.0 8.0 % alloy = R (Å) 2.48(1) 2.83(1) 4.03(1) 4.74(3) 4.96(1) 45(2) N 8.0 6.0 12.0 24.0 8.0

B_Fe1Co1_0.2_r800

% alloy = R (Å) 2.48(1) 2.83(1) 4.03(2) 4.74(3) 4.96(1)

B_Fe2Co1_0.2_r800

Article

2σ2 (Å2) 0.021(1) 0.026(3) 0.034(2) 0.035(4) 0.023(4) N 8.0 6.0 12.0 24.0 8.0

B_Fe1Co1_0.4_r800 B_Fe1Co1_0.2_r800

R (Å) 2.48(1) 2.83(1) 4.03(1) 4.74(2) 4.96(2) 2σ2 (Å2) 0.024(2) 0.027(2) 0.030(3) 0.041(6) 0.021(2) 80(1) N 8.0 6.0 12.0 24.0 8.0

B_Fe2Co1_0.2_r800

% alloy = R (Å) 2.48(1) 2.82(1) 4.02(1) 4.74(3) 4.96(1)

Co K-edge Fe K-edge

Table 5. Interatomic Distances (R) and Debye−Waller Factors (σ) Obtained by Fitting the Experimental EXAFS Spectra of B_Fe2Co1_0.2_r800, B_Fe1Co1_0.2_r800, and B_Fe1Co1_0.4_r800 at the Fe and Co K-Edgea


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Table 6. Interatomic Distances (R) and Debye−Waller Factors (σ) Obtained by Fitting the Experimental EXAFS Spectra of G_Fe2Co1_0.4_r800 and G_Fe2Co1_0.8_r800 at the Fe and Co K-Edgea Fe K-edge G_Fe2Co1_0.4_r800

Fe Fe Fe Fe Fe

% alloy = 90(2) R (Å) N 2.48(1) 8.0 2.83(1) 6.0 4.03(1) 12.0 4.75(3) 24.0 4.96(1) 8.0

O

% Fe oxide = 10(2) 1.92(2) 6.0

2σ2 (Å2) 0.023(2) 0.023(2) 0.032(3) 0.044(8) 0.016(2)

G_Fe2Co1_0.8_r800 R (Å) 2.47(1) 2.83(1) 4.03(1) 4.72(2) 4.95(1)

N 8.0 6.0 12.0 24.0 8.0

G_Fe2Co1_0.4_r800 2σ2 (Å2) 0.018(1) 0.018(1) 0.023(2) 0.035(3) 0.010(1)

Co Co Co Co Co

R (Å) 2.48(1) 2.83(1) 3.98(1) 4.75(3) 4.96(1)

N 8.0 6.0 12.0 24.0 8.0

G_Fe2Co1_0.8_r800

2σ2 (Å2) 0.025(1) 0.028(3) 0.036(4) 0.051(7) 0.022(2)

R (Å) 2.47(1) 2.82(1) 4.03(1) 4.74(3) 4.94(1)

N 8.0 6.0 12.0 24.0 8.0

2σ2 (Å2) 0.022(1) 0.028(2) 0.039(3) 0.041(6) 0.021(2)

0.010(6)

R-factor = 28% R*-factor = 18% a

Co K-edge





Table 7. Fraction of Oxide Present in the Reduced Samples, As Determined from the EXAFS Fitting

of the samples at this stage contain two separate phases: one phase containing cobalt, where Co has an oxidation state either +2 or intermediate between +2 and +3 and one phase containing iron, with Fe always with oxidation state +3. Moreover, the EXAFS and XANES results indicate that, even though the three kinds of mesoporous silica matrixes are quite similar to each other, different phases are obtained as a function of the matrix and as a function of the Fe:Co ratio and of the concentration of the nitrate solution. In G_Fe2Co1_0.8_500, the only sample prepared with the G matrix and calcined at 500 °C for which EXAFS and XANES data have been collected, the identified phases are Co3O4 and α-Fe2O3, in accordance with the XRD results. In the two samples prepared using the B matrix, B_Fe1Co1_0.2_500, B_Fe1Co1_0.4_500, the same Co phase is observed while Fe is present in a much more disordered phase, that is, ferrihydrite. The same phases, Co3O4 and ferrihydrite, are also observed in the sample obtained impregnating the matrix S-SBA-16 with the more concentrated nitrate solution with Fe:Co ratio 1:1, S_Fe1Co1_0.8_ 500. The other two samples obtained impregnating the same matrix with a less concentrated solution (0.4 M) and with 2 different Fe:Co ratios 1.1 and 2:1 show weaker EXAFS oscillations in agreement with their XRD patterns that do not show any Bragg peak, indicating a lower degree of crystallinity. However, even if EXAFS oscillations and XANES profiles of these two nanocomposites appear to be different from all the other samples and reference compounds at both the Fe and Co K-edge, some information on the Fe and Co environment can be obtained. In particular, a shell of O around the Fe3+ with a coordination number of 4.3/4.5 and a shell of O around Co2+ with a coordination number of 6 were identified. The presence of a large amount of tetrahedrally coordinated Fe is in agreement with XANES data, since the prepeak at the Fe K-edge indicates more tetrahedral Fe than in γ-Fe2O3, but less than FePO4, which has 100% Fe3+ in tetrahedral sites. It has been reported that under certain conditions when Fe3+ is adsorbed on silica and heated at temperature higher than 400 °C Fe3+ assumes a tetrahedral coordination.31 EXAFS and XANES have also shown to be very powerful tools for the analysis of the nanocomposites after reduction at 800 °C. Even in this case XRD is not conclusive for the identification of all the phases present. The reflections observed

acronym

% Fe oxide

S_Fe2Co1_0.4_r800 S_Fe1Co1_0.4_r800 S_Fe1Co1_0.8_r800 B_Fe2Co1_0.2_r800 B_Fe1Co1_0.2_r800 B_Fe1Co1_0.4_r800 G_Fe2Co1_0.4_r800 G_Fe2Co1_0.8_r800

40(2) 60(1) 20(2) 55(2) 40(3) 12(3) 10(2)

% Co oxide 40(2) 20(1)

in the XRD patterns can be ascribed either to bcc FeCo alloy or to pure bcc α-Fe, since due to their very similar cell parameter it is not possible to distinguish the two bcc phases by only using XRD. On the other hand, the inspection of the EXAFS data of the reduced samples can give a straightforward indication of the formation of the bcc FeCo alloy, since the EXAFS oscillations and the corresponding FT of the bcc Fe foil are significantly different from those of the fcc Co foil. In fact, the data at both the Fe and Co K-edges are very similar to those of bcc Fe, confirming the formation of the FeCo alloy. Moreover, EXAFS and XANES are very sensitive for identifying the oxidation of the nanoparticles. Results show that Fe is more oxidized than Co, which is expected due to the higher tendency of Fe to be oxidized. The degree of oxidation of the nanocomposites varies with the matrix and with the concentration of the nitrate solution used for the impregnation and with the Fe:Co ratio. Since most likely the oxide is present on the surface of the nanoparticles, the relative amount should change with nanoparticle size. In fact, the G_Fe2Co1_0.8_r800 and G_Fe2Co1_0.4_r800 samples show sharp peaks in the XRD pattern corresponding to an average size of around 40 nm and the nanoparticles are very little or not oxidized. The formation of large nanoparticles in these samples seem to be correlated to the presence in the sample treated at 500 °C of two intermediate phases which are already well crystallized. On the other hand, the nanoparticle sizes of the nanocomposites obtained with the other two mesoporous silicas are quite similar (6.9−7.5 nm), but differences in the amount of iron oxide are observed in the two series of samples. Therefore, the different 12363



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particle sizes cannot be the only factor influencing the oxidization of the nanoparticles. In particular, it can be noticed that B_Fe1Co1_0.4_r800 and S_Fe1Co1_0.4_r800, which differ only in the matrix, contain 12% and 60% of iron oxide, respectively. Therefore, it seems that the textural properties of the matrix can play an important role in influencing the oxidation of nanoparticles, as summarized in Table 7, where the fraction of oxidized metal, as determined from the EXAFS fitting, is reported.

framework of the SEED project “NANOCAT”, funded by the Italian Institute of Technology (IIT), the project “Porous catalyst for the production of carbon nanotubes with tailored features”, funded by the Italian Ministry for Education, University and Research (MIUR) through the PRIN 2009 call, and with the contribution of “Ministero degli Affari Esteri, Direzione Generale per la Promozione del Sistema Paese”.

5. CONCLUSIONS Different cubic mesoporous silica matrices (SBA-16) were used to prepare FeCo-SiO2 nanocomposites by wet impregnation with a solution of Fe and Co nitrates followed by calcination in air and reduction in hydrogen flow. The choice of the matrix greatly influences both the kind and the amount of the nanophase which is loaded after impregnation and calcination; in particular, the matrix prepared by using butanol as a cosurfactant turns out to have an improved absorption of the metal ions compared to the matrix obtained using a conventional precipitation method. Moreover, in the matrix prepared by the gelation method much bigger particles are obtained. EXAFS and XANES allow determination with great accuracy of which phases are present after calcination and after the final reduction treatment. Results have shown that most of the samples after calcination in air contain two separate phases: one phase containing cobalt, where Co has an oxidation state either +2 or intermediate between +2 and +3, and one phase containing iron, always with an oxidation state +3. After the reduction treatment, FeCo alloy nanoparticles are formed in all samples; however, they show a certain degree of oxidation that varies with the matrix and with the concentration of the nitrate solution used for the impregnation and with the Fe:Co ratio. In particular, nanoparticles embedded in the matrix obtained by the gelation method are very little or not oxidized; a higher degree of oxidation is observed in the nanoparticles embedded in the matrix prepared by using butanol as a cosurfactant and to a higher extent in those embedded in the matrix prepared using the conventional precipitation method. Results show that in all samples Fe is more oxidized than Co, which is expected due to the higher tendency of Fe to be oxidized.

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ASSOCIATED CONTENT

S Supporting Information *

XANES pre-edge peaks details at the Fe K-edge and Co K-edge of nanocomposites and reference compounds are presented in Figures S1 and S2. Moduli and imaginary parts of the Fourier Transforms are reported in Figures S3−S10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the EU funded ELISA scheme for data collection at Hasylab, and the XAFS beamline scientists at Elettra, Dr L. Olivi and Dr A. Cognigni, for assistance during data collection. This work has been carried out within the 12364

REFERENCES



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