Ultrasmall Î±-Fe2O3 Superparamagnetic Nanoparticles with High

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Ultra-small #-FeO Superparamagnetic Nanoparticles with High Magnetization Prepared by Template-Assisted Combustion Process Khachatur V. Manukyan, Yong-Siou Chen, Sergei Rouvimov, Peng Li, Xiang Li, Sining Dong, Xinyu Liu, Jacek Furdyna, Alexei Orlov, Gary H. Bernstein, Wolfgang Porod, Sergey Roslyakov, and Alexander S. Mukasyan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp504733r • Publication Date (Web): 27 Jun 2014 Downloaded from http://pubs.acs.org on June 30, 2014

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Ultra-small α-Fe2O3 Superparamagnetic Nanoparticles with High Magnetization Prepared by Template-Assisted Combustion Process *

Khachatur V. Manukyan1† , Yong-Siou Chen2, Sergei Rouvimov3, Peng Li3, Xiang Li4, Sining Dong4, Xinyu Liu4, Jacek K. Furdyna4, Alexei Orlov3, Gary H. Bernstein3, Wolfgang Porod3, Sergey Roslyakov5, Alexander S. Mukasyan1

1

*

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States

2

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States 3

Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, United States 4

Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States 5

Center of Functional Nano-Ceramics, National University of Science and Technology, "MISIS", Moscow, 119049, Russia

* To whom correspondence should be addressed by: [email protected] (A.M.) and [email protected] (K.M.) † Current address, Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, United States

1 ACS Paragon Plus Environment


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ABSTRACT

A template-assisted combustion-based method is developed to synthesize the ultra-small (below 5 nm) α-Fe2O3 nanoparticles. The iron and ammonium nitrides are used as oxidizers, glycine as a “fuel” and mesoporous silica (SBA-15) as a template. Because of the ultra-low sizes and high crystallinity, the combustion-derived α-Fe2O3 nanoparticles exhibit superparamagnetism in the temperature range of 70-300 K. The high specific surface area (132 m2/g) of α-Fe2O3 indicates the important role of surface magnetic spins resulting in remarkably high magnetization (21 emu/g) at 300 K. KEYWORDS: α-Fe2O3, ultra-small nanoparticles, template-assisted synthesis, combustion processing, superparamagnetism.


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INTRODUCTION Hematite (α-Fe2O3), an n-type semiconductor (Eg=2.1 eV), is the most thermodynamically stable phase of iron trioxide. Due to its nontoxicity and low processing cost, it has been extensively investigated for applications in various fields, including energy conversion and storage technologies1,2,3,4,5,6,7,8, catalysts9,10 and pigments11,12, gas sensors5,13, optical devices14,15,16 and water purification17,18. The magnetic properties of the α-Fe2O3 also have attracted much attention19,20,21,22. Crystalline α-Fe2O3 is antiferromagnetic below ~260 K (Morin temperature), and exhibits weak ferromagnetism between 260 and 950 K. Numerous recent efforts have been directed toward the fabrication of nanoscale α-Fe2O3 to enhance its performance in currently existing applications. To date, well-defined nanostructures of iron oxides with different dimensionalities such as nanoparticles, nanorods, nanowires, nanotubes, nanorings, nanobelts, nanocubes, as well as hollow and porous nanostructures have been successfully obtained by a variety of solution chemistry routes14,15,16,17,23,24,25,26,27,28,29,30 as well as vapor-phase processes1,6,31,32,33,34. The solution chemistry routes provide a vast variety of morphologies of hematite that can be tuned by changing the composition of solution, temperature, surfactants, etc. In vapor-phase synthesis, αFe2O3 nanostructures form through the controlled oxidation of iron, or through deposition of iron oxide aerosols. Those processes mostly provide the synthesis of well aligned one- or twodimensional nanostructures and porous hierarchic nano-architectures. Among various synthesis methods, template-assisted approaches have also been extensively investigated. For example, spherical carbon nanoparticles35,36 or carbon nanotubes37 have been used as templates for the synthesis of hollow spheres and nanotubes of hematite. In these cases, precursor composites of iron pentacarbonyl or nitrate with carbon templates are



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prepared first using solution chemistry routes. Subsequent annealing (750 K for 4 h) of precursors forms crystalline hematite with the shapes of the initial templates. To prepare hematite nanotubes with different diameters and aspect ratios, the thermal decomposition of iron nitrate5, iron (III) hydroxide38 and tris(acetylacetonato)iron(III)39 within anodic alumina or polycarbonate membranes at 600-850 K for 5 h was suggested. Recently, iron-containing metalorganic frameworks8 and ferritin proteins40 were proposed as templates to prepare clusters of hematite nanoparticles through controlled pyrolysis. Different template-assisted approaches to create mesoporous hematite (with pore diameter of 3 to 30 nm) using a triblock copolymer41 or silica42 templates were also investigated. Those approaches involve solution-based synthesis of hematite in the presence of nanosized templates followed by a high temperature (650-850 K) and long-time (4-6 h) calcination step. It is well recognized that combustion-based processes allow energy-efficient routes for synthesis of binary and complex oxides, refractory compounds, metals, intermetallics and atomically thin crystals43,44,45,46. The heat that self-generates during combustion reactions provides optimum synthesis conditions, and eliminates the need for external energy sources. Previously, we reported the synthesis of nanostructured iron oxides by a solution combustion route using iron nitrate as oxidizer and glycine (or citric acid, hydrazine) as fuel46,47. In that process, an aqueous solution of reactants preheated to the boiling point of water results in a viscous gel formation. Upon continuous heating to a critical temperature, the entire gel selfignites forming iron oxides. Alternatively, local preheating of the gels prepared in previous step ignites an exothermic reaction that propagates through the reactive media in the form of a rapidly moving (~1 mm/s) combustion front. Using this approach, pure α-Fe2O3, Fe3O4, and mixtures of


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α-Fe2O3 and γ-Fe2O3 have been prepared. The particles sizes of oxides were varied between 2050 nm by changing the fuel/oxidizer ratio. Here, we report a new strategy for preparation of ultra-small (below 5 nm) superparamagnetic hematite nanoparticles with remarkably high magnetization. We have successfully integrated a combustion approach with template-assisted synthesis. In the present study, a simple redox-type combustion route is applied to produce hematite nanoparticles using iron (Fe(NO3)3) and ammonium (NH4NO3) nitrides as oxidizers, glycine (C2H5NO2) as a “fuel,” and mesoporous silica (SBA-15) as a template. Besides the benefit of energy savings, which may lead to low-cost large-scale production technologies, template-assisted combustion-based strategy offers several unique characteristics that cannot be achieved in conventional schemes. In the self-sustained combustion process, the precursors are converted to hematite within a very short time period (on the order of seconds). The short-duration of the high-temperature stage favors the formation of ultra-small nanoparticles and diminishes the interaction between reaction media and the template. Moreover, the synthesis process efficiently removes organic impurities without coke formation.

EXPERIMENTAL METHODS Synthesis of Materials The new route to prepare ultra-small α-Fe2O3 particles includes the self-sustained combustion reaction that propagates along mesoporous silica (SBA-15) template impregnated with ironcontaining reactive solutions (Figure 1). Synthesis of template followed a literature report47 with some modifications. 4 g of Pluronic P123 triblock copolymer (EO20PO70EO20, Mav = 5800,



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Aldrich) was dissolved in 120 mL of 2 M HCl aqueous solution in a polypropylene bottle under stirring at room temperature. 30 mL of deionized water and 8.5 g of tetraethyl orthosilicate (TEOS, 98%, Aldrich) were added subsequently. The bottle was then placed in a preheated (318 K) water bath for 24 h under stirring. The color of solution changed from transparent to milky white indicating the formation of silica. The solution-containing silica was then transferred to a glass beaker and heated at 423 K for 24 h. The resulted materials were washed with water and ethanol through vacuum filtration followed by drying at 353 K for 8 h. The P123 was removed by heating at 823 K (with 1 K/min heating rate) for 6 hours under continuous air flow. BET surface area and pore volume of as-prepared template was 780 m2/g and ~1 cm3/g, respectively. The template-assisted combustion synthesis procedure of α-Fe2O3 was as follows: 0.5 g of SBF-15 was impregnated with 3 ml of solution containing 1.1g of Fe(NO3)3·9H2O (Alfa Aesar, 99%), 0.35 g of glycine (Alfa Aesar, 98.5%) and 3.5 of NH4NO3 (Alfa Aesar, 95%). Impregnation was performed by insipient wetness method for 6 times. In every step 0.5 ml of solution is mixed with template followed by vacuum drying for 1 h at room temperature. After complete impregnation, the powder was cold pressed to a pellet with 10 mm diameter. Combustion process was initiated by the local preheating (spot of ∼1 mm3) of sample in air by a resistively heated tungsten wire. Two control samples using conventional calcination of iron nitrate impregnated into SBA-15 template were also prepared. In this case 0.5 g of SBF-15 was impregnated with 1 ml of solution containing of 1.1g of Fe(NO3)3·9H2O (Alfa Aesar, 99%). Impregnation was performed by insipient wetness method twice. Calcination of resulting material was conducted at 1000 K for 2 and 6 h.


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Produced materials were ground

to fine powder and then subjected to leaching of

template by concentrated (2 M) solution of sodium hydroxide at 350 K for 12 h. α-Fe2O3 then recovered by centrifugation and washed for thee times with deionized water. Combustion diagnostic. The temperature-time history of the combustion processing was recorded by 100 µm K-type thermocouples inserted inside the reactive mixtures. The output signal of the thermocouple is transformed to data storage by a data acquisition system (Data Translation Inc.) and recorded with 1 kHz frequency using Quick DAC software. A high-speed infrared camera (FLIR Systems, SC6000) was also used to monitor combustion wave characteristics. The latter provides in situ two-dimensional maps for the temperature-time history of the process. The thermal videos were captured over several different temperature ranges which include temperatures as high as 2300 K and frame rates upward of 1000 frames per second with spatial resolution of 5 µm. The velocity of the combustion wave propagation was calculated by data obtained from different thermocouples located at known distances along the direction of the reaction front. A TGA−DSC device (Mettler-Toledo) coupled with a mass spectrometer (Pfeiffer Vacuum) was used to perform in situ thermo-gravimetric and gas-phase analysis of reactive mixtures during external heating conditions. In these experiments, 0.05 g of mixture was heated to 730 K at a heating rate of 50 K/min in an atmosphere of air. The sampling rate for in situ gas analysis was 10 data points per second. Characterization methods. A Titan 80-300 (FEI, USA) transmission electron microscope with resolution of 0.136 nm in STEM mode and about 0.1 nm information limit in HRTEM mode was used to characterize microstructure and the atomic structure of the materials. To measure the specific surface area of the products, the nitrogen adsorption-desorption analysis 7 ACS Paragon Plus Environment


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(at 77 K) was performed using an ASAP 2020 instrument (Micromeritics). Prior to this analysis, the samples were vacuum degassed at 370 K for 12 hours. The infrared optical properties were measured on a Bruker Tensor 27 spectrophotometer using a KBr pellet technique with 4 cm-1 resolution over a scanning range of 400 - 4000 cm-1. The Raman spectroscopy (NRS-5100, Jasco Analytical Instruments) of materials was performed using green (532 nm) laser excitation. Magnetic measurements at low temperatures were carried out with Quantum Design SQUIDVSM dc magnetometers, and at 300 K using Microsense EV 7 VSM. RESULTS AND DISCUSSION

We were able to precisely control the quantity of self-generated energy, and thus the synthesis temperature, through fine tuning of the content of ammonium nitride in the reactive mixtures. The results (Figure 2A) of differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) have shown that an optimized SBA-15/(Fe(NO3)3+C2H5NO2+NH4NO3) reactive mixture exhibits one strong and one weak exothermic peak at 450 K and 560 K, respectively. The intense peak coincides with the abrupt mass loss (~55%) in the TGA curve, followed by a gradual mass loss of about 33% between the two peaks. The total energy released during the first reaction step is 440 J/g, and only about 40 J/g for the second. The in situ mass-spectroscopy analysis data (Figure 2B) suggest that the rapid release of CO2, H2O and nitrogen oxides (N2O, NO, NO2) takes place at 450 K. Our recent research50 indicates that combustion in metal nitrate + glycine systems is triggered by the highly exothermic reaction of nitrogen oxides (e.g. N2O) and ammonia formed during the decomposition of metal nitride and glycine, respectively. In the Fe(NO3)3 + C2H5NO2 + NH4NO3 system, ignition temperature (450 K) coincides with both the onset temperature of decomposition for Fe(NO3)351 and the melting point of NH4NO352. The


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gradual mass loss between two exothermic peaks is due to the slow oxidation of solid organic residues (glycylglycine and 2,5-piperazinedione) formed during the decomposition of glycine53 by molten ammonium nitride. Figure 2A also indicates that no essential mass loss is observed after the second exothermic peak at 560 K. This means that organic residues are completely oxidized during this stage, which coincides with the decomposition temperature of ammonium nitride. The video frames of high-speed infrared imaging (Supporting Information, Figure S1) illustrate the combustion reaction front propagation along a cylindrical sample compacted from an SBA-15 template impregnated by reactive mixture. It can be seen that after local initiation from the top, by using an electrically heated wire, an exothermic reaction wave moves through the reactive media with a velocity of ~0.4 mm/s. A temperature-time profile suggests (Figure S1) that the temperature rapidly (200 K/s) increases at the combustion front, reaching 1000 K, followed by relatively slow (~50 K/s) cooling stage. Those results highlight the uniqueness of the conditions occurring during the template-assisted combustion that allow the rapid synthesis of materials using simple equipment, without the need for an external source of energy. Transmission electron microscopy (TEM) of products (Figure S2) indicates that prolonged (2-6 h) calcination promotes significant growth of iron oxide particles. It is important to note that the intense growth of iron oxide particles destroys the thin walls of channels in the template. In contrast, no damage of the template channels is observed after the short-term combustion process (Figure S2), allowing synthesis of the ultra-small particles. A TEM image (Figure 3A), as well as particle size distribution (insert in Figure 3A), of combustion products after leaching of the template clearly indicates that ~95% of hematite



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particles are smaller than 5 nm, with an average size of ~3.5 nm. High-resolution TEM (HRTEM) images (Figure 3B,C) provide information on the atomic structure and morphology of α-Fe2O3 nanoparticles. The interplanar spacing of 0.27 nm corresponds to the d104 spacing between (104) atomic planes of the crystal structure. Electron diffraction and fast Fourier transform analysis (inset in Figure 3C) indicate the presence of (104) and (110) reflections with a d-spacing of 0.27 nm and 0.25 nm, respectively33. These data prove that the particles are indeed crystalline α-Fe2O3. The TEM analysis of materials prepared by calcination with 2 and 6 h duration are summarized in Figure 4 and Figure 5, respectively. Based on TEM data, the calcination process yields polydisperse particles with sizes ranging from 2 to 25 nm (Figures 4A and 5A). Statistical analysis indicates that the fraction of particles with diameters below 5 nm is only 30 and 22 % for 2 and 6 h of calcination, respectively (inserts in Figure 4A and 5A). The mean diameter of particles prepared by calcination is 6-8 nm. Typical HRTEM images of small α-Fe2O3 particles in the calcination product are shown in Figures 4B and 5B. Both particles are in the (4,4,-1) crystallographic projection, where both (104) and (110) atomic planes are visible in the image with interplanar distances of 0.27 nm and 0.25 nm, respectively. The specific surface area of combustion-derived hematite calculated from the nitrogen adsorption/desorption isotherm (Figure S3) is 132 m2/g. Note that the surface area of oxides prepared by calcination for 2 and 6 h are 94 and 60 m2/g, respectively. Those data confirm the results of TEM analysis that the template-assisted combustion approach allows the production of ultra-small, high surface area hematite particles that are much finer than those produced under optimized conventional conditions.


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The vibrational properties of prepared nanocrystals were studied by Fourier transform infrared (FTIR) spectroscopy (Figure 6A). Theoretical analysis predicts that the α-Fe2O3 lattice should have six infrared active modes54, among which two infrared active modes are associated with the vibrations parallel to the c-axis, while others are perpendicular to the c-axis. According to the literature55, perpendicular modes of the Fe-O stretching vibrations in the α-Fe2O3 structure appear at about 480 and 540 cm-1, while peaks at 630 and 440 cm-1 have been associated with vibrations parallel to the c-axis. In Figure 4A, the peak at 528 cm-1 can be ascribed to the perpendicular mode of the Fe-O stretching vibration in α-Fe2O3, while the peaks at 630 and 435 cm-1 are associated with the modes that have polarization parallel to the c-axis. Note that the predicted peak at 480 cm-1 corresponding to the Fe-O stretching vibration perpendicular to the caxis is not observed. Our FTIR data confirm the existence of the α phase, and we see no evidence of the γ phase. This is in contrast to the calcination products that do show the existence of some γ phase, as discussed further in the Supporting Information (Figure S4). Combustion-derived α-Fe2O3 was also characterized by Raman spectroscopy. The Raman spectrum (Figure 6B) exhibits five lines of α-Fe2O3 at about 216, 280, 292, 491 and 585 cm-1. The literature data56 indicate that the Raman spectrum of nanostructured hematite has seven bands at 226, 245, 293, 298, 412, 500 and 612 cm-1. It is obvious that two bands at about 245 and 412 cm-1 are missing in the Raman spectrum of combustion-derived hematite, due to the small particle sizes. On the other hand, observed peaks are significantly broadened and shifted towards lower wavenumbers, which is also related to the quantum size effects. Thus, the above results reveal that ultra-small α-Fe2O3 particles prepared by combustion process show vibrational characteristics that are distinct from those of the calcination products.



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Considering high surface area, small size and distinctive vibrational characteristics of the combustion-derived ultra-small α-Fe2O3 particles, it is important to investigate the magnetic properties to see how they differ from hematite prepared by calcination. Magnetic properties of ultra-small α-Fe2O3 nanoparticles were therefore investigated as a function of temperature and magnetic field (H). We first present the temperature dependence of magnetization (M), and then discuss magnetization curves obtained at high (300 K) and low (5 K) temperatures. The temperature dependence of magnetization is shown in Figure 7A. Blue dots correspond to zero-field cooling (ZFC), where the sample is first cooled to 5 K in zero magnetic field, and then the data are acquired as the sample is warmed up in an applied field of 1.0 kOe. Red dots are obtained in the field-cooled (FC) mode, with the sample cooled in 1.0 kOe, and the same applied field of 1.0 kOe used in the measurements during the warming process. In the range of 70-300 K the results are essentially identical, with the shape of the curve M(T) indicative of typical paramagnetic behavior. Indeed, the decrease of magnetization with increasing temperature can be readily understood in terms of the magnetization within the nanoparticles affected by thermal fluctuations, similar to atomic spins in paramagnetic materials. The FC and the ZFC curves are, however, strikingly different at low temperatures, below ~70 K, which we identify as the blocking temperature. In the case of the FC curve, the nanoparticles were cooled in a finite magnetic field that has aligned their magnetic moments, which “freeze” at the blocking temperature. Upon warming from 5 K the magnetization stays frozen i.e. varies very insignificantly until the blocking temperature is reached, where magnetic moments of the particles become unfrozen and exhibit paramagnetic behavior. In the ZFC case, on the other hand, the magnetic moments of each particle is also frozen below the blocking temperature, but are randomly oriented in a way typical for a cluster spin glass57, so that net


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magnetization of the ensemble approaches zero as T → 0 K. Upon warming, the magnetization of the ZFC ensemble of randomly oriented magnetic moments increases as the thermal energy of the particles allows their moments to increasingly align with the applied field. Finally, as the blocking temperature is exceeded, the magnetization exhibits the same paramagnetic behavior as was observed in the FC experiment. Figures 7B and 5C show field dependences of the magnetization at 300 K and at 5 K, respectively. The magnetization observed at room temperature, Figure 7B, reveals two notable features. First, an extremely small hysteresis loop with the coercive field (~0.02 kOe) and weak remnant magnetization (~0.3 emu/g), characteristic of superparamagnetic behavior, is observed. Second, the observed magnetization value reaches 21 emu/g at 10 kOe. This value is conspicuously larger than the magnetization typically reported in larger α-Fe2O3 nanoparticles (above 10 nm), which usually does not exceed 1 emu/g15,58. Furthermore, this value is also larger than that observed in larger Fe2O3 nanoparticles prepared by calcination (Figure 8). The large value of magnetization in ultra-small nanoparticles is to our knowledge the largest net magnetization ever observed in hematite. This fact, along with its predominately superparamagnetic behavior at room temperature, thus holds promise for new applications, particularly where superparamagnetic properties (near-zero remanent magnetization and nearlinear response to external magnetic field) are advantageous over ferromagnetic properties59. In contrast to the behavior above the blocking temperature, at 5 K we observe a hysteresis loop with a significant coercive field (~0.65 kOe) and a significant remanent magnetization of ~6 emu/g (Figure 7C), resembling weak ferromagnetic behavior due to freeze out of spins below the blocking temperature.



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We can understand the record high magnetization in these ultra-small nanoparticles as follows. The magnetic properties of combustion-derived α-Fe2O3 nanoparticles can be viewed as consisting of two components. One comes from an antiferromagnetically ordered core, characteristic of bulk hematite, which, similarly to the bulk hematite, does not bring a significant contribution to the magnetization. The second contribution comes from the shell of uncompensated surface spins surrounding the core, which tend to interact ferromagnetically. However, due to a very small particle size, the magnetization of the shell randomly flips under the influence of thermal fluctuations, while exhibiting high magnetization values in an external magnetic field, a behavior characteristic of superparamagnetics60,61. This ability of surface spins to respond to the applied field is possible, since surface spins are not surrounded by antiferromagnetically-coupled nearest neighbors on all sides, and magnetic constraints on their alignment are thus very weak. It is to be expected that the contribution of such spins to the total magnetization will increase with reduction of nanoparticle size, both because of the increased surface-to-volume ratio and, in ultra-small particle limit, of the reduction of the size of the interior antiferromagnetic core. Such increase of magnetic moment per unit volume was reported in many other systems62,63. These characteristics are further seen in the shape of the magnetization curves. In the case of combustion-derived ultra-small particles, in which the uncompensated surface spins are dominant, the magnetization curve in Figure 7B exhibits a very narrow hysteresis loop with weak remanent magnetization. In contrast, nanoparticles prepared by calcination (where both antiferromagnetic α-Fe2O3 and ferromagnetic γ-Fe2O3 phases are present) the magnetization curve shows a typical two-phase behavior (Figure 8). Moreover, it is very significant in the present context that the magnitude of magnetization is much larger for the ultra-small particles reported here (Figure 7B) than in the data for the larger nanoparticles


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prepared by calcination, despite the presence of the ferromagnetic γ-Fe2O3 phase in the latter system. This clearly suggests that the fraction of the spins contributing to superparamagnetic magnetization increases dramatically with decreasing size.

CONCLUSIONS In summary, we have successfully integrated a combustion approach with template-assisted synthesis of α-Fe2O3 nanoparticles, which may be easily generalized for the synthesis of wide variety of materials. Compared to the calcination process, the template-assisted combustion synthesis enables the production of ultra-small hematite nanoparticles with narrow particle size distribution and without detectable amounts of the γ-Fe2O3 phase. Combustion-derived α-Fe2O3 nanoparticles exhibit distinctive vibrational properties and superparamagnetic behavior with unusually high net magnetization. Such unique vibrational and magnetic properties make this material a highly promising candidate for a variety of applications including photocatalysts for environmental protection, solar water splitting reactions, magnetic separation, data storage, etc.

ASSOCIATED CONTENT Supporting Information. Additional details of characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This research was supported by the Notre Dame SEI Program and National Science Foundation Grant DMR10-05851. The authors also gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase 15 ACS Paragon Plus Environment


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Competitiveness Program of NUST «MISiS» (№ К2-2014-001). This work was partially supported by Notre Dame Integrated Imaging Facility (NDIIF), Materials Characterization Facility (MCF) and Center for Environmental Science and Technology (CEST).

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Figure captions Figure 1 Schematic representation of the template-assisted combustion synthesis of α-Fe2O3: initial template (A) is impregnated by reactive solution (B) followed by initiation of reaction, which self-propagates (C) forming nanoparticles inside channels (D) followed by leaching of template and obtaining α-Fe2O3 nanoparticles (E). Figure 2 The results of DSC/TGA analysis (A) and in-situ mass-spectroscopy (B) during linear heating of SBA-15/(Fe(NO3)3 + NH4NO3 + C2H5NO2) reactive mixture Figure 3 Microstructure (A) and particle sized distribution (insert in A), as well as highresolution TEM images (B and C) and Fourier transform (insert in C) of ultra-small αFe2O3 nanoparticles after leaching of SBA-15 Figure 4 Microstructures (A) and particle size distribution (insert in A), a high resolution TEM image and Fourier transform (insert in B) α-Fe2O3 prepared by template-assisted calcination at 1000 K for 2 h Figure 5 Microstructures (A) and particle size distribution (insert in A), a high resolution TEM image and Fourier transform (insert in B) α-Fe2O3 prepared by template-assisted calcination at 1000 K for 6 h Figure 6 FTIR (A) and Raman (B) spectra of ultra-small α-Fe2O3 Figure 7 Temperature dependence of magnetization for ultra-small α-Fe2O3 nanoparticles cooled H=0 and H=1.0. kOe (A), magnetic field dependence of magnetization at 300 K (B) and at 5 K (C) Figure 8 Magnetic field dependence of magnetization for α−Fe2O3 nanoparticles prepared by calcination for 2 h (A) and 6 h (B) at 300 K


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Figure 1 Schematic representation of the template-assisted combustion synthesis of α-Fe2O3: initial template (A) is impregnated by reactive solution (B) followed by initiation of reaction, which self-propagates (C) forming nanoparticles inside channels (D) followed by leaching of template and obtaining α-Fe2O3 nanoparticles (E). 33x9mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2 The results of DSC/TGA analysis (A) and in-situ mass-spectroscopy (B) during linear heating of SBA-15/(Fe(NO3)3 + NH4NO3 + C2H5NO2) reactive mixture 273x209mm (300 x 300 DPI)


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Figure 2 The results of DSC/TGA analysis (A) and in-situ mass-spectroscopy (B) during linear heating of SBA-15/(Fe(NO3)3 + NH4NO3 + C2H5NO2) reactive mixture 273x209mm (300 x 300 DPI)



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Figure 3 Microstructure (A) and particle sized distribution (insert in A), as well as high-resolution TEM images (B and C) and Fourier transform (insert in C) of ultra-small α-Fe2O3 nanoparticles after leaching of SBA-15 117x81mm (300 x 300 DPI)


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Figure 4 Microstructures (A) and particle size distribution (insert in A), a high resolution TEM image and Fourier transform (insert in B) α-Fe2O3 prepared by template-assisted calcination at 1000 K for 2 h 83x166mm (300 x 300 DPI)



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Figure 5 Microstructures (A) and particle size distribution (insert in A), a high resolution TEM image and Fourier transform (insert in B) α-Fe2O3 prepared by template-assisted calcination at 1000 K for 6 h 83x166mm (300 x 300 DPI)


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Figure 6 FTIR (A) and Raman (B) spectra of ultra-small α-Fe2O3 104x80mm (300 x 300 DPI)



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Figure 6 Figure 6 FTIR (A) and Raman (B) spectra of ultra-small α-Fe2O3 104x80mm (300 x 300 DPI)


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Figure 7 Temperature dependence of magnetization for ultra-small α-Fe2O3 nanoparticles cooled H=0 and H=1.0. kOe (A), magnetic field dependence of magnetization at 300 K (B) and at 5 K (C) 273x209mm (300 x 300 DPI)



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Figure 8 Magnetic field dependence of magnetization for α-Fe2O3 nanoparticles prepared by calcination for 2 h (A) and 6 h (B) at 300 K 273x209mm (300 x 300 DPI)


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Figure 8 Magnetic field dependence of magnetization for α-Fe2O3 nanoparticles prepared by calcination for 2 h (A) and 6 h (B) at 300 K 273x209mm (300 x 300 DPI)



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Ultrasmall Î±-Fe2O3 Superparamagnetic Nanoparticles with High

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