J Nanopart Res (2010) 12:3097–3103 DOI 10.1007/s11051-010-9905-6
RESEARCH PAPER
High-temperature XRD study of thermally induced structural and chemical changes in iron oxide nanoparticles embedded in porous carbons M. A. Schettino Jr. • J. C. C. Freitas • M. K. Morigaki • E. Nunes • A. G. Cunha E. C. Passamani • F. G. Emmerich
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Received: 21 July 2009 / Accepted: 17 March 2010 / Published online: 3 April 2010 Ó Springer Science+Business Media B.V. 2010
Abstract Magnetic carbon-based nanomaterials have promising applications in many fields owing to their biocompatibility and thermal/mechanical stability. This study describes a high-temperature X-ray diffraction (XRD) study of the chemical and structural transformations suffered by superparamagnetic iron oxide nanoparticles embedded in porous carbons. The nanoparticles were prepared from the decomposition of iron pentacarbonyl over porous carbons, resulting in nanometer-sized iron oxides homogeneously dispersed into the carbon matrix. The thermally induced changes in these materials were followed by in situ high-temperature XRD, using synchrotron radiation. The growing of the nanoparticles and of the carbon crystallites were first observed, followed by the reduction of the iron oxides to form a-Fe (at temperatures as low as 400 °C in some cases) and c-Fe(C). The temperatures at which these chemical reactions occurred were dependent on the total time spent on heating and on the nature of the iron oxides formed in the as prepared
M. A. Schettino Jr. J. C. C. Freitas (&) E. Nunes A. G. Cunha E. C. Passamani F. G. Emmerich Departamento de Fı´sica, Universidade Federal do Espı´rito Santo, 29075-910 Vito´ria, ES, Brazil e-mail:
[email protected] M. K. Morigaki Departamento de Quı´mica, Universidade Federal do Espı´rito Santo, 29075-910 Vito´ria, ES, Brazil
materials. A noticeably large thermal expansion coefficient was also observed for the iron oxide nanocrystals. The formation of austenitic iron, stabilized by the presence of carbon, was found to be only partially reversible upon cooling. Keywords Nanoparticles Iron oxide Porous carbons X-ray diffraction In situ analysis Nanocomposites Magnetism
Introduction The preparation of magnetic iron-carbon nanocomposites consisting of iron-based nanoparticles embedded into porous carbons has been reported with frequency in recent years. These materials have promising applications in many different fields, including catalysis, hydrogen storage, magnetic separation, magnetic recording, water cleaning, and magnetic resonance imaging (Fuertes and Tartaj 2006; Hayashi et al. 2001; Kouprine et al. 2006; Lee et al. 2005; Oliveira et al. 2002; Sajitha et al. 2004; Xu and Teja 2008). Among the reasons that stimulate the interest in magnetic carbon-based nanomaterials, one can cite the good mechanical properties, the outstanding thermal stability of the materials under non-oxidizing conditions and the biocompatibility of the nanocomposites.
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This study describes the use of iron pentacarbonyl, Fe(CO)5, for the preparation of iron-carbon nanocomposites, using activated carbon (AC) as the support material. Fe(CO)5 is an commonly used iron precursor, since it contains iron in the zero-valence state (Fe0) and the activation energy for breaking the Fe–CO bonds is low (Phillips et al. 1980; Dyakonov et al. 1997; Ja¨ger et al. 2006). Furthermore, this substance is a liquid with high vapor pressure at room temperature, being thus easily decomposed to form iron compounds dispersed into the AC matrix. The composition and the structure of the iron-based nanocomposites were investigated as a function of temperature by in situ X-ray diffraction (XRD), using high-intensity synchrotron radiation to achieve wellresolved XRD patterns in short times. Mo¨ssbauer spectroscopy, scanning electron microscopy (SEM), thermogravimetry (TG), chemical analysis, and magnetic measurements were also used, allowing a thorough characterization of the chemical and structural transformations suffered by the nanocomposites.
Experimental methods Commercial AC powder (Merck, with specific surface area of 1,232 m2/g) and Fe(CO)5 (Aldrich) were used as received. The materials were mixed either under argon (samples labeled as ‘‘ARG samples’’)—using the Schlenk technique (Davis and Curran 2007)—or under ambient atmosphere with abundant exhaustion (samples labeled as ‘‘AMB samples’’). A volume of 4.0 cm3 of Fe(CO)5 was added to the carbon powder (2.0 g). More details about the synthesis methods and basic characterization of the products are presented elsewhere (Schettino Jr. 2009). Next, the samples were heat-treated under nitrogen flow in the XRD chamber of the X-ray powder diffraction beamline of the Brazilian National Synchrotron Light Laboratory ˚ . The (LNLS), with an X-ray wavelength of 1.746 A heat-treatments were performed at temperatures up to 900 °C. The X-ray diffractograms were recorded either under isothermal conditions (after 1 h plateau at each desired temperature) or with continuous heating (with constant temperature only for recording the X-ray diffractograms). The other characterization techniques employed were: elemental analysis (Leco CHNS932/VTF900); TG under N2 or O2 flow up to 1,000 °C (Shimadzu TGA-50H); Mo¨ssbauer
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spectroscopy, with 57Co:Rh radioactive source; SEM (Shimadzu SS-550), coupled to energy dispersive X-ray spectroscopy (EDX) (SEDX-500 instrument); and measurements of magnetic properties (Quantum Design PPMS).
Results and discussion The elemental analysis and TG results showed that the AMB and ARG samples presented Fe contents of 31 and 19 wt%, Fe/C atomic ratios of 0.16 and 0.07, and Fe/O atomic ratios of 0.33 and 0.30, respectively. SEM images and EDX analysis showed that the ironbased nanoparticles were homogeneously dispersed through the porous structure of the carbon matrix within the length scale of the observation (*1 lm). The room temperature Mo¨ssbauer spectrum (Fig. 1) indicated that the AMB sample was constituted of Fe3? oxides in the superparamagnetic state, giving rise to a doublet with isomer shift of 0.34 mm/s (relative to a-Fe) and quadrupole splitting of 0.77 mm/s. This finding was confirmed by XPS analysis (Schettino Jr. 2009). On the other hand, the room temperature Mo¨ssbauer spectrum of the ARG sample exhibited the presence of two doublets, with isomer shifts around 0.27 and 0.50 mm/s, associated with contributions from Fe3? and also Fe2? sites, respectively. It is important to stress that all doublets used for fitting the Mo¨ssbauer spectra had large linewidths, reflecting the disordered nature of the Fe chemical and/or structural environment and the contribution of Fe atoms located at the surface of the nanoparticles. Thus, the hyperfine parameters must be interpreted as mean values representative of these environments. From the magnetic point of view, both analyzed samples exhibited typical superparamagnetic behavior at room temperature, as detected by magnetization measurements performed as a function of the applied magnetic field. This is consistent with the doublets observed by Mo¨ssbauer spectroscopy (Fig. 1). Figure 2 shows the sequence of X-ray diffractograms recorded at several constant temperatures during the heat treatments of the samples prepared from the decomposition of Fe(CO)5 over the AC powder. To record these data, the sample was first heated at a constant heating rate of 5 °C/min up to the desired temperature; the temperature was held constant for 1 h and then the X-ray diffractogram was
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Fig. 1 Room temperature Mo¨ssbauer spectra of the samples prepared under ambient (bottom) or argon (top) atmosphere
recorded. Next, the same sample was heated again to the subsequent temperature, with the process being repeated for all the temperatures shown in Fig. 2. In the case of the samples prepared under ambient atmosphere (Fig. 2a), the dominant phases detected at room temperature (30 °C) were turbostratic carbon (big halo around 27°) and a-Fe2O3; the presence of Fe3O4 as a minor component was also recognized by the large relative intensity of the peak close to 40° (which occurs roughly in the same position for both Fe2O3 and Fe3O4). Upon heating under the oxygenfree atmosphere of the XRD chamber, the a-Fe2O3 phase was progressively reduced; at 400 °C, Fe3O4 became the dominant phase. At 600 °C, metallic a-Fe was formed, being the only crystalline phase observed at this temperature. At 900 °C, the austenitic iron phase c-Fe(C) was the dominant one, probably associated with the incorporation of some carbon into fcc iron. Less intense diffraction peaks associated with a-Fe and free carbon were also detected at 900 °C. Upon cooling again to room temperature (patterns shown in Fig. 3a), the c-Fe(C) reverted only partially to a-Fe. However, the detection of austenitic iron in this case suggested that the incorporation of carbon atoms contributed to the stabilization of such phase at room temperature. The XRD patterns of the samples prepared under argon atmosphere (Fig. 2b) showed an important difference when compared to the samples prepared in
ambient conditions. The X-ray diffractograms recorded at room temperature and at temperatures up to 400 °C showed the occurrence of only one iron oxide phase (together with the turbostratic carbon). Although the diffraction peaks were broad and the angular positions of the peaks corresponding to c-Fe2O3 and Fe3O4 are very close (Cornell and Schwertmann 1996), the observed phase was identified as magnetite (Fe3O4) in the present case. This identification was justified by the better agreement between the angular positions of the observed peaks and those of Fe3O4 for the XRD patterns recorded at several temperatures. Also, the Mo¨ssbauer spectra recorded for the heat-treated samples (not shown) supported this finding, as the hyperfine magnetic fields of the observed sextets in the Mo¨ssbauer spectra matched well with the values expected for magnetite. For temperatures from 600 °C upward, similar reduction processes were observed for the samples prepared under argon as compared to the samples prepared under ambient atmosphere, with the presence of a-Fe and of c-Fe(C) as the dominant phases at 600 and 900 °C, respectively. Also, upon cooling a large amount of c-Fe(C) was still observed at room temperature (Fig. 3b), together with a-Fe and of free carbon. It is noteworthy that the peaks corresponding to austenitic iron appear at room temperature with a quite large relative intensity in this case, as compared to the AMB sample.
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3100 Fig. 2 X-ray diffractograms recorded in situ at different temperatures during the heat-treatments of the samples prepared under (a) ambient or (b) argon atmosphere. The symbols (whose height is proportional to the relative intensity of each peak) indicate the expected angular positions for the corresponding crystalline bulk phases at each temperature
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Fig. 3 X-ray diffractograms recorded at 900 °C and at room temperature (after cooling), for the samples prepared under (a) ambient or (b) argon atmosphere. The symbols (whose height is proportional to the relative intensity of each peak) indicate the expected angular positions for the corresponding crystalline bulk phases at each temperature
Another particular feature of the samples prepared under argon atmosphere was the observation of the formation of a-Fe from the reduction of Fe3O4 at a temperature as low as 400 °C (see Fig. 2b). This process was clearly a kinetically controlled one. In a different experiment, the XRD patterns were recorded as a function of the time at different constant temperatures (Fig. 4). For total heating times higher than ca. 30 min at 400 °C, the growth of the most intense peak associated with the a-Fe phase was easily detected at 50.7°. It is also interesting to note the appearance of the wu¨stite phase (Fe1 - xO), identified from a broad diffraction peak at 47.7° and a shoulder around 40.6° at 550 °C. This phase is known to be thermodynamically stable above *570 °C (Cornell and Schwertmann 1996; Jozwiak et al. 2007). In the present case, the wu¨stite coexisted with magnetite and metallic iron for certain time in the temperature range 500–550 °C. In the case of the AMB sample (patterns not shown), the presence of wu¨stite was identified for short times at 600 °C (up to ca. 30 min.), which was the temperature
required for the complete formation of a-Fe in this case. From the XRD patterns shown in Fig. 2, the average crystallite sizes associated with each identified phase were computed, using the inverse linewidth of the most intense peaks and the Scherrer equation. The iron oxides present in the as prepared samples were found to have average crystallite sizes of 4 nm (6 nm) for the sample prepared under argon (ambient) atmosphere. This value increased to 10 nm (26 nm) at the temperature of 400 °C. The a-Fe phase formed at 600 °C exhibited an average crystallite size of 37 nm (61 nm), whereas the c-Fe(C) phase formed at 900 °C exhibited an average crystallite size of 53 nm (75 nm). The turbostratic carbon phase showed an Lc value of 7 nm at 900 °C for the sample prepared under argon atmosphere, and 9 nm for the sample prepared under ambient atmosphere. These results show that it is possible to tune to a specific iron-containing phase and a specific size range by suitably choosing the temperature/time of heat
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Fig. 4 X-ray diffractograms recorded in situ at different temperatures while continuous heating the sample prepared under argon atmosphere. The time values indicated to the right correspond to the total time spent on the heat-treatment, including the heating ramp and the isothermal plateau. The symbols (whose height is proportional to the relative intensity of each peak) indicate the expected angular positions for the corresponding crystalline bulk phases at each temperature
treatment, as well as the atmosphere of preparation of the Fe–O–C nanocomposites. The thermal expansion of the nanocrystalline iron oxides were investigated from the temperature variation of the interplanar spacing associated with the most intense peak of each phase identified in the XRD patterns. For magnetite, with cubic symmetry, the results obtained for the samples prepared under argon atmosphere are shown in Fig. 5. A significantly large
linear thermal expansion coefficient was found for nanocrystalline Fe3O4, with a value of 21.1 9 10-6 °C-1, compared to the value of 6.1 9 10-6 °C-1 expected for bulk Fe3O4 (Fei 1995). A similar behavior was found for nanocrystalline hematite, with a linear thermal expansion coefficient of 13.1 9 10-6 °C-1 as compared to the bulk value of 8.0 9 10-6 °C-1 (Fei 1995). This behavior is common to many nanocrystalline materials, as a consequence of the large surface to volume ratio of the nanocrystals, implying in a large fraction of atoms at the surface of the nanoparticles (Freitas et al. 2006).
Conclusions
Fig. 5 Thermal expansion of the Fe3O4 nanocrystals observed in the samples prepared under argon atmosphere, compared to the behavior expected for bulk Fe3O4
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Nanocomposites made of iron oxide nanoparticles dispersed into the pores of a carbon matrix were prepared from the decomposition of Fe(CO)5 over an AC powder. Depending on the atmosphere of preparation, Fe2O3 or Fe3O4 nanocrystals were produced, with average crystallite sizes in the range 4–6 nm. High-temperature XRD measurements were used to investigate the thermal changes suffered by these materials. The iron oxide nanocrystals revealed an enhanced thermal expansion coefficient, as a consequence of their large surface to volume ratio. Upon
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heating under an oxygen-free atmosphere, the iron oxides were progressively reduced to form a-Fe, starting at a temperature as low as 400 °C in some cases. At higher temperatures (900 °C), austenitic iron was formed; this transformation was partially reversible after cooling the samples down to room temperature. These results showed the possibility of monitoring the size and the nature of the iron-based nanoparticles dispersed into the AC matrix, by suitably choosing the atmosphere of preparation and the temperature, the time and the atmosphere of heattreatment. Acknowledgments This study has been supported by the Brazilian Synchrotron Light Laboratory (LNLS) under proposal D10B - XPD 7639. The support of CAPES, CNPq, FAPES, and FINEP is also gratefully acknowledged.
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