Size-Dependent Phase Map and Phase Transformation Kinetics for

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A Size-Dependent Phase Map and Phase Transformation Kinetics for Nanometric Iron(III) Oxides (##### Pathway) Seungyeol Lee, and Huifang Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05287 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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The Journal of Physical Chemistry

A Size-Dependent Phase Map and Phase Transformation Kinetics for Nanometric Iron(III) Oxides (γεα pathway) Seungyeol Lee and Huifang Xu* NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706

* Corresponding author: Prof. Huifang Xu Department of Geoscience University of Wisconsin-Madison 1215 West Dayton Street, A352 Weeks Hall Madison, Wisconsin 53706 Tel: 1-608-265-5887 Fax: 1-608-262-0693 Email: [email protected]

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ABSTRACT: Nanometric iron(III)-oxide has been of great interest in a wide range of fields due to its magnetic properties, eminent biochemical characteristics, and potential for technological applications. Among iron-oxides, ε-Fe2O3 is considered as a remarkable phase due to its giant coercive field at room temperature and ferromagnetic resonance capability. Here we present the first size-dependent phase map for ε-Fe2O3 via a γεα pathway together with the activation energies for the phase transformations, based on X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). HRTEM images of ε-Fe2O3 nanocrystals show the inversion and pseudo-hexagonal twins, which are fundamentally important for understanding the correlation between its nanostructure and magnetic properties. Two activation energies for γ-Fe2O3 α-Fe2O3 phase transformations are 186.37 ± 9.89 kJ mol-1 and 174.58 ± 2.24 kJ mol-1, respectively. The results provide useful information about the size, crystal structure, and transformation of the nanometric iron-oxide polymorphs for applications in areas such as designing engineered materials.


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1. INTRODUCTION Nanometric iron-oxides are important functional materials due to their magnetic properties, eminent biochemical characteristics, and technological applications.1-5 To date, five crystalline polymorphs of Fe2O3 are known: (i) α-Fe2O3 (i.e., hematite), (ii) β-Fe2O3, (iii) γ-Fe2O3 (i.e., maghemite), (iv) ε-Fe2O3, and (v) ζ-Fe2O3, all of which have different morphologies, various size, and magnetic properties.3-6 Among ferric oxide, the ε-Fe2O3 is considered as a remarkable phase due to its giant coercive field (Hc) at room temperature, coupled magneto-electric properties, and ferromagnetic resonance capability that is a distinctive properties in other metal oxides.1,5,7 The ε-Fe2O3 was also reported in ancient blackglazed Chinese Jian wares, which formed by heating iron-bearing clays at ~1300 °C.8 The natural εFe2O3 (luogufengite) is discovered from vesicles’ surfaces of basaltic scoria. The ε-Fe2O3 is a dark brown nanocrystal that has an orthorhombic structure with the Pna21 space group. The structure of ε-Fe2O3 consists of triple chains of Fe-octahedra sharing edges with corner sharing Fe-tetrahedra that run parallel to the a direction, which is isomorphous with AlFeO3 and GaFeO3.5,9 The ε-Fe2O3 is considered as an intermediate polymorph between γ-Fe2O3 and α-Fe2O3. It is difficult to synthesize large ε-Fe2O3 crystals as a pure phase, because ε-Fe2O3 is considered thermodynamically unstable.3,5,10 The γ-Fe2O3 → ε-Fe2O3 → α-Fe2O3 pathway is widely recognized as a simple way to prepare ε-Fe2O3 nanoparticles.3-5,10 Synthetic procedures lead to a mixture of ε-Fe2O3 with α-Fe2O3 and γ-Fe2O3 in varying contents depending on the reaction temperature, supporting silica matrix, and presence of other metal ions (e.g., Al3+, Ba2+, and Ca2+).1,3,7,8,11,12 Among the synthetic methods, Kelm and Mader13 proposed a synthesis route via thermal decomposition of an Fe(III)dominated clay mineral of nontronite (~(Na0.3-2x,Cax)Fe2(Al,Si)4O10(OH)2•nH2O, an iron(III) end member of smectite). The method can produce almost pure ε-Fe2O3 nanoparticles since SiO2 matrix increases the stability of ε-Fe2O3 nanocrystals.13 In this article, we present the first quantitative phase map for polymorphs of Fe2O3 via a γεα pathway, based on high-resolution transmission electron microscopy (HRTEM) and X-ray powder ACS Paragon Plus Environment

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diffraction (XRD). This map is important because the development of nanotechnologies requires control of the size, structure, and growth conditions of individual components. Furthermore, we report a detailed description of the crystal structure and kinetic properties of the nanometric Fe2O3polymorphs.

2. EXPERIMENTAL METHODS 2.1. Synthesis. Iron(III)-oxide nanoparticles were prepared through the thermal decomposition of clay mineral nontronite (S. W. Bailey XRD Laboratory collection, University of Wisconsin-Madison) from Quincy, Washington, USA. We followed a procedure similar to the synthesis method of Kelm and Mader.13 The crushed nontronite was inserted at alumina boat in the furnace from 800 °C to 1100 °C. After heating, the samples were quenched in cold water. Isolation of the ferric oxides was performed from the amorphous SiO2 by dissolving in the 10M NaOH solution at 80 °C for 2 days. After washing with distilled water five times, iron(III)-oxide powder was obtained. 2.2. Measurements. XRD patterns were collected using a Rigaku Rapid II XRD system (Mo-Kα radiation) in the Geoscience Department at the University of Wisconsin-Madison. Diffraction data were recorded on a 2-D image-plate detector. The original two dimensional diffraction rings were then converted to produce conventional 2θ vs. intensity patterns using Rigaku’s 2DP software. Percentages of mineral phases in the sample were calculated using the Rietveld method. Input structure models of hematite and maghemite for Rietveld analysis are from Blake et al.14 and Shmakov et al.15. A pseudoVoigt method was used for fitting the peak profiles. High-resolution TEM images and selected-area electron diffraction (SAED) analyses were carried out using a Philips CM200-UT microscope operated at 200 kV in the Materials Science Center at the University of Wisconsin-Madison. Chemical analyses obtained by using X-ray energy-dispersive spectroscopy (EDS) method (spot size 5 with a beam diameter of ~50 nm) with minerals of fayalite, anorthite, forsterite, and titanite as standards for quantifying elements of Fe, Al, Ti, and Ca.


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2.3. Consecutive reactions.

The modeling was carried out using the first order consecutive

reactions. An initial reactant A reacts to make some intermediate B (k1), which then reacts to make a final product C (k2). By setting up the change in concentration of A, B, and C, these differential phase can be integrated to get equations for three time-dependent concentrations: A = A0exp(-k1t) B = A0[exp(-k1t) - exp(-k2t)] / (k2 - k1) C = A0[k2(1 - exp(k1t)) – k1(1 - exp(-k2t))] / (k2 - k1) When the concentration of substrate (A), intermediate (B), and final product (C) are determined by time, we can calculate the reaction rate constants (k1 and k2). More information on the model is found in references.16,17

3. RESULTS AND DISCUSSIONS 3.1. Polymorphous Transformations of Nanometric Iron-Oxide. The Fe2O3 polymorphous transformations of γ-Fe2O3 to α-Fe2O3 via ε-Fe2O3 are observed in a range from 850 °C to 1000 °C (Figure 1). The reaction rates of the γεα transformation progressively increase as the reaction temperature increases (Figure 1). Sharpness of the diffraction peaks increase as annealing time increases, indicating the growth of the crystals (Figure 1). Bright-field TEM images and SAED analyses identity the ferric oxide nano-crystals (γ, ε, and α) and 15Å-nontronite (Figure 2). The natural nontronite shows continuous SAED rings, indicating the turbostratic disorder in stacking (Figure 2a). Figure 2b shows the roughly rod shapes of ε-Fe2O3 inside the silica matrix. The SiO2 matrix serves as an anti-sintering agent, which extends the life of the metastable ε-Fe2O3.13,18 The size of ferric oxide nanoparticles is significantly influenced by the annealing temperatures and times (Figures 2c-2f). The ε-Fe2O3 is typically ~50 nm and ranges from ~10 nm to ~ 200 nm. Overall, αFe2O3 crystals are larger than ε-Fe2O3 crystals, whereas γ-Fe2O3 crystals are smaller than ε-Fe2O3


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crystals (Figures 2e-2f). The size variation is important because the phase transformations of iron-oxide nanoparticles take place once they reach a certain critical size.3,19,20


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Figure 1. Powder X-ray diffraction patterns of Fe2O3 polymorphs from thermal decomposition of nontronite heated at different temperatures (800 to 1100 °C) and annealing times. Mole fractions of the Fe2O3 polymorphs were calculated using the Rietveld method. Diffraction peaks from the reference minerals of ε-Fe2O3, α-Fe2O314 and γ-Fe2O315 are also illustrated at the bottom.

Figure 2. Bright-field TEM images and selected-area electron diffraction (SAED) analyses of ironoxide nanocrystals: (a) 15Å-nontronite; (b) Iron-oxide nanocrystals in matrix of amorphous SiO2; (c-f) Iron-oxide crystals from the different heating temperatures and reaction times after leaching amorphous silica using 10M NaOH solution. Scale bar = (a-d) 200 nm, (e) 50 nm, and (f) 500 nm.

The

average

chemical

formulas

for

γ-Fe2O3

(Fe1.67Al0.18Mg0.12Ca0.03O3),

ε-Fe2O3

(Fe1.82Al0.10Mg0.04Ca0.03Ti0.01O3), and α-Fe2O3 (Fe1.93Ti0.04Al0.03O3) are calculated from X-ray energy dispersive (EDS) spectra, respectively (shown in Figure S1). It was suggested that the alkaline earth


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ions (Ca and Mg) stabilize the metastable ε-Fe2O3, increasing the size of nanoparticles.4,21 In addition, cation substitution may affect the average particle size of Fe-oxide nanoparticles.1,21-25 High-resolution TEM images show that ε-Fe2O3 commonly displays (00 1 ), (011), (01 1 ), (0 1 1), and (0 1 1 ) faces (Figures 3a-3c). The shape of ε-Fe2O3 is generally elongated along a-axis with combinations of sphenoid, pinacoid and pedion forms (Figures 3a-3c). Figure 3b shows the inversion twinning of ε-Fe2O3, further illustrated in Figure 4a. The pseudo-hexagonal twin relationships are frequently observed in the ε-Fe2O3 nanocrystals.26 The ε-Fe2O3 has ( 1 10) II (010) and (100) II (130) in a (110) twinning relationship (Figure 3d) due to pseudo-hexagonal symmetry, which is illustrated in Figure 4b. Figure 3e shows the ~60° rotation orientation relationship between crystals with [100]ε and [110]ε zone-axes sharing interface of (001). The pseudo-hexagonal twinning operation is the same as in that the isostructural phases κ-Al2O3.27


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Figure 3. HRTEM images and fast Fourier transform (FFT) patterns of iron-oxide nanoparticles. (a) The single ε-Fe2O3 nanocrystal along the [100]-zone-axis. (b) The inverse twinning relationship of εFe2O3. (c) Rod-shape of ε-Fe2O3 elongated along a-axis. (d) The composition plane boundaries are ( 1 10) II (010) and (100) II (130) of ε-Fe2O3 in a (110) twinning relationship. (e) The (001) twin boundaries between [100]ε and [110]ε with ~60° rotation relationship. (f) The (001) crystal interface of [110]α and [110]ε. Scale bar = (a, b, d, e, f) 10 nm and (c) 20 nm. The twinning in ε-Fe2O3 can make it difficult to understand the correlation between the microstructure and magnetic properties. The nonzero orbital momentum in the ε-Fe2O3 results from the uncompensated magnetic moment of tetrahedral Fe3+ and octahedral Fe3+ positions.28 The inversion or rotational twin operations are also able to change the Fe3+ magnetic moments to influence the net magnetization of ε-Fe2O3, leading to an unexpectedly significant-orbit coupling phenomenon.23 A HRTEM image shows the crystal interface of ε-Fe2O3 and α-Fe2O3 along the c-axis (Figure 3f). Both Fe-oxides have the hexagonal arrangement of oxygen atoms on their (001) surfaces. It is proposed that the pseudo-hexagonal packing of ε-Fe2O3 (001) surface could serve as the substrate/interface for αFe2O3, which is illustrated in Figure 4c. The unusual small hematite crystal (Figure 3f) may be as a result of quenching during the synthetic procedure.29

Figure 4. Structure models showing the twin relationships and crystal interfaces: (a) The inversion twin ACS Paragon Plus Environment

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relationship in Figure 3b, (b) The (011) twinning relationship in Figure 3d, and (c) The crystal interfaces between ε-Fe2O3 and α-Fe2O3 along the c-axis in Figure 3f.

Rietveld refinement analyses were performed to quantify concentrations of Fe-oxide polymorphs (shown in Figure S2). The lattice parameters of the ε-Fe2O3 were refined on the basis of the determined space group Pna21. The Al3+ prefers to occupy the tetrahedral Fe site in the ε-Fe2O3 because of its smaller ionic radius.5,30 Calculated fractional coordinates and occupancies for the structure are listed in Table S1. The unit cell parameters of ε-Fe2O3 are lower than those of Al-bearing ε-Fe2O3 heated at 1050 °C

30

(Figure 5) because the low annealing temperature may result in the ordering of Al in the

tetrahedral sites. The unit cell volume of the ε-Fe2O3 increases as the temperature increases (Figure 5).

Figure 5. Comparisons of unit cell parameters of synthetic ε-Fe2O3 (blue symbols) and Al-substituted εFe2O3 synthesized at 1050 °C (red squares)30.

3.2. A Size-Dependent Phase Map. A size-dependent phase map of nanosized iron(III)-oxide is illustrated based on maximum sizes of γ-Fe2O3 and ε-Fe2O3 nanocrystals (Figure 6). The TEM images show that the particle size of nanometric iron-oxide is dependent on annealing temperature and time. The γ-Fe2O3 to ε-Fe2O3 phase transition takes place once the γ-Fe2O3 nanoparticles reach a certain critical size between ~9 nm at 1100 ºC and ~14 nm at 800 ºC (Figure 6). Similarly, Tronc et al.11 obtain the ε-Fe2O3 nanoparticles from 10 nm of γ-Fe2O3 particles in silica xerogel by using the thermal ACS Paragon Plus Environment

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transformation. We sort the maximum size of the ε-Fe2O3 into two types: single crystals and twinned crystals (Figure 6). The critical size of ε-Fe2O3 to α-Fe2O3 phase conversion is approximately between ~150 nm at 1100 ºC and ~80 nm at 800 ºC (Figure 6). Twined crystals have larger sizes than single crystal in general. Pure ε-Fe2O3 phase generally converts to the α-Fe2O3 polymorph if the particle size exceeds a value of ~ 30 nm without silica matrix.5 However, the large ε-Fe2O3 nanocrystals, with sizes up to 200 nm, can be stable within silica matrix.12,13,18 It is also affected by alkaline earth metals (Mg and Ca) and Al concentration in the reaction system.1,5,21,31

Figure 6. A size-dependent phase map of nanometric iron(III) oxide (γεα pathway) based on the TEM observations. Blue dots: the maximum size of γ-Fe2O3 crystals, dark brown dots: the maximum size of twinned ε-Fe2O3 crystals, and brown circles: the maximum size of ε-Fe2O3 single crystals.

Due to the slow decomposition rate of nontronite and extreme long annealing time, we cannot accomplish the experiment below 800 ºC. Direct γ-Fe2O3 to α-Fe2O3 phase transformation is reported at ACS Paragon Plus Environment

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temperatures ranging from 370 to 600 °C in the absence of a silica matrix,20,32 in which the phase transition temperature decrease as the particle size decreases, due to a reduction in the activation energy of the system.32 The triple point is set at ~ 700 °C, because cation substitution for Fe could result in the γ-Fe2O3 α-Fe2O3 phase transformation to ~700 °C.33 Navrotsky et al.34 reported that γ-Fe2O3 nanocrystals become thermodynamically stable with respect to α-Fe2O3 at sizes of about 16 nm under dry conditions, which is close to dashed line between γ-Fe2O3 and α-Fe2O3 phases in our phase map (Figure 6). The phase boundary between maghemite and hematite is also controlled by relative humidity.35 As observed in soils, the size of maghemite decreases as the humidity increases.36 3.3. Kinetics of Nanometric Iron-Oxide Polymorphs (γεα pathway). In all the experiments, the Fe2O3 polymorphous transformations (γεα pathway) are observed between 850 °C to 1000 °C (Figure 1). The mole fractions of the Fe2O3 polymorphs are calculated from XRD patterns by using Rietveld refinement method (shown in Table S2).


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Figure 7. The first-order consecutive reaction rates of the (γεα pathway) determined by Rietveld analyses for the samples annealed at (a) 1000 °C, (b) 900 °C, and (c) 850 °C, respectively. Solid lines are calculated based on best fitting parameters, and dots are experiments data (green: γ-Fe2O3; brown: εFe2O3; red: α-Fe2O3).

Because all the three polymorphs exist in the reactions, a consecutive reaction method is used to calculate the phase transformation kinetics, in which initial γ-Fe2O3 reacts to make some intermediate εFe2O3, which then reacts to make a final product of α-Fe2O3. The kinetics of the transformation of γεα polymorphs occurring at 1000 °C, 900 °C, and 850 °C are calculated using the first order ACS Paragon Plus Environment

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consecutive reaction model (Figure 7). The first order kinetic reaction is also used in previous studies for the γε transition.19,20 Both rate constants (k1 and k2) increase as annealing temperature increases, and the γε transformation is faster than the εα transition at all three temperatures due their differences in boundary sizes (Figure 7).

Figure 8. Arrhenius plot used to determine the activation energies of the γε and εα phase transition as a function of annealing temperature.

The γε and εα transition activation energies are 186.37 ± 9.89 kJ mol-1 and 174.58 ± 2.24 kJ mol-1, respectively, based on Arrhenius equation: k = k0exp(-Ea/RT) (Figure 8). We believe this is the first reported activation energy for the ε-Fe2O3 to α-Fe2O3 phase transformation. Two difference activation energies for γ-Fe2O3 α-Fe2O3 were reported to the 176 ± 31 kJ mol-1 and 152 ± 17 kJ mol-1, respectively.19,20 However, it is difficult to calculate the restricted activation energy of the nanocrystals because the activation energy is also size-dependent.19,31

4. CONCLUSION ACS Paragon Plus Environment

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We have presented the first size-dependent phase map for nanosized Fe2O3 polymorphs (γ, ε, and α) with their phase transformation activation energies, based on HRTEM and quantitative XRD. Combining the phase map with their kinetic properties predicts that stability regime of the nanosized Fe2O3 as the function of crystal size, temperature, and annealing times. As it is challenge to synthesize pure phase ε-Fe2O3 nanocrystal the proposed size-dependent phase map will help to improve controlled synthesis of ε-Fe2O3 nanocrystal, a promising material for many future applications.4,5 In addition, the method will be useful for determining size-dependent phase maps in other systems.

ACKNOWLEDGMENTS This study was supported by the NASA Astrobiology Institute (N07-5489). The authors thank Mr. Franklin Hobbs for reading the manuscript and two anonymous reviewers for providing many helpful suggestions. Supporting Information Available: Further details on the TEM-EDS spectra, XRD pattern (Mo-Kα), atomic coordinates of synthetic ε-Fe2O3, and the experiment results of the γ/ε and ε/α transition. This material is available free of charge via the Internet at http://pubs.acs.org.


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(11) Tronc, E.; Chaneac, C.; Jolivet, J. P. Structural and Magnetic Characterization of ε-Fe2O3. J. Solid State Chem. 1998, 139, 93-104. (12) Popovici, M.; Gich, M.; Niznansky, D.; Roig, A.; Savii, C.; Casas, L.; Molins, E.; Zaveta, K.; Enache, C.; Sort, J.; de Brion, S.; Chouteau, G.; Nogues, J. Optimized Synthesis of the Elusive ε-Fe2O3 Phase via Sol-gel Chemistry. Chem. Mater. 2004, 16, 5542-5548. (13) Kelm, K.; Mader, W. Synthesis and Structural Analysis of ε-Fe2O3. Z. Anorg. Allg. Chem. 2005, 631, 2383-2389. (14) Blake, R. L.; Hessevic.Re; Zoltai, T.; Finger, L. W. Refinement of Hematite Structure. Am. Mineral. 1966, 51, 123-129. (15) Shmakov, A. N.; Kryukova, G. N.; Tsybulya, S. V.; Chuvilin, A. L.; Solovyeva, L. P. Vacancy Ordering in γ-Fe2O3 - Synchrotron X-Ray Powder Diffraction and High-Resolution ElectronMicroscopy Studies. J. Appl. Crystallogr. 1995, 28, 141-145. (16) Shen, J.; Cheng, W. D.; Wu, D. S.; Huang, S. P.; Hu, H.; Xie, Z. First-Principles Determinations and Investigations of the Electronic Absorption and Third-Order Polarizability Spectra of Electron Donor-Acceptor Chromophores Tetraalkylammonium Halide/Carbon Tetrabromide. J. Phys. Chem. A 2007, 111, 9249-9254. (17) Ball, D. W. Kinetics of Consecutive Reactions: First Reaction, First Order; Second Reaction, Zeroth Order. J. Chem. Educ. 1998, 75, 917-919. (18) Gich, M.; Roig, A.; Frontera, C.; Molins, E.; Sort, J.; Popovici, M.; Chouteau, G.; Marero, D. M. Y.; Nogues, J. Large Coercivity and Low-temperature Magnetic Reorientation in ε-Fe2O3 Nanoparticles. J. Appl. Phys. 2005, 98, 044307. (19) Schimanke, G.; Martin, M. In situ XRD Study of the Phase Transition of Nanocrystalline Maghemite (γ-Fe2O3) to Hematite (α-Fe2O3). Solid State Ionics 2000, 136, 1235-1240. (20) Belin, T.; Millot, N.; Bovet, N.; Gailhanou, M. In situ and Time Resolved Study of the γ/αFe2O3 Transition in Nanometric Particles. J. Solid State Chem. 2007, 180, 2377-2385.


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Size-Dependent Phase Map and Phase Transformation Kinetics for

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