Control of the Crystal Phase Composition of FexOy Nanopowders

Article pubs.acs.org/crystal

Control of the Crystal Phase Composition of FexOy Nanopowders Prepared by CO2 Laser Vaporization Christian Stötzel,† Heinz-Dieter Kurland,† Janet Grabow,† Silvio Dutz,‡ Eberhard Müller,§ Marek Sierka,† and Frank A. Müller*,† †

Otto-Schott-Institute of Materials Research (OSIM), Friedrich-Schiller-University of Jena, Löbdergraben 32, 07743 Jena, Germany Institute of Photonic Technology (IPHT), Albert-Einstein-Straße 9, 07745 Jena, Germany § Institute of Electronic and Sensor Materials, TU Bergakademie Freiberg, Gustav-Zeuner-Straße 3, 09596 Freiberg, Germany ‡

S Supporting Information *

ABSTRACT: Crystalline magnetic iron oxide nanopowders are prepared by CO2 laser vaporization (LAVA) of a hematite (α-Fe2O3) raw powder. Condensation at normal pressure leads to maghemite (γ-Fe2O3) as the main phase in the nanopowders. With an increasing oxygen partial pressure in the zone of condensation, an increasing amount of Fe2O3 polymorph ε-Fe2O3 is found. The LAVA-prepared Fe 2O3 nanopowders are characterized by X-ray diffraction, transmission electron microscopy, and chemical analysis and with respect to their magnetic properties. A mechanism for the initial nucleation is proposed to explain the formation of ε-Fe2O3 with an increasing oxygen content in the condensation atmosphere. The model is based on the evidence of ozone in oxygen-rich condensation atmospheres. Density functional theory calculations indicate that ozone facilitates the formation of 6-fold oxygen-coordinated Fe ions acting as building units for the emerging crystal structure during the solidification of the nanoparticles. This insight into early nucleation stages will be useful for the functional design and crystal engineering of either isostructural materials like alumina (Al2O3) or, more generally, vapor phase synthetic routes for ceramic materials.

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INTRODUCTION Iron oxide (FexOy) nanoparticles exhibit unique magnetic properties such as particle size-dependent ferrimagnetism or superparamagnetism, high coercivity, low Curie temperatures, and high magnetic susceptibility.1,2 These versatile features lead to a widespread application spectrum ranging from data storage and magnetic filtering to a variety of promising biomedical uses, e.g., drug targeting, cell separation, magnetic resonance imaging, hyperthermic cancer therapy, and immunoassay applications.3−5 Convenient methods for synthesizing iron oxide nanoparticles are wet chemical precipitation reactions with a variety of additives to control the size, shape, and superficial properties6,7 as well as gas phase processes such as flame spray pyrolysis,8 laser ablation,9 and plasma synthesis.10 In general, gas phase processes require elaborate laboratory setups and large amounts of energy but offer interesting features that make them a topic of intense scientific research.11 They can be scaled for industrial applications, can be operated continuously, and allow reproducibility, and in the case of laser ablation, no specially designed precursors are required. Different groups utilized laser ablation into a condensation gas to prepare metallic and ceramic nanoparticles.12,13 Ablation of various ceramic materials under the same experimental conditions revealed a striking similarity of the size distributions and the agglomeration behavior of the obtained nanoparticles.14 Generally, it was found that the dynamics of particle formation is strongly influenced by the condensation atmosphere.15 In the © XXXX American Chemical Society

case of iron oxide, it was found that the size distribution of the prepared nanoparticles depends on the gas pressure,16,17 the laser pulse energy,17 and the target porosity.18 Recently, the influence of the condensation gas itself on the phase composition of the resulting iron oxide nanopowders was described.16,19 Beyond that, there are several studies examining the early stages of particle formation during the condensation process. It was found that the composition of iron oxide clusters correlates with the Fe2O3 stoichiometry.20 Thermodynamic bond energies and heats of formation for a variety of possible iron oxide clusters were experimentally and theoretically determined.21,22 It was found that the stability of oxygenrich clusters increases with an increasing number of incorporated iron ions.23,24 In this study, the CO2 laser vaporization (LAVA) technique was used for the preparation of iron oxide nanopowders. Basic investigations already revealed that this method is well suited for the synthesis of FexOy nanoparticles.23,24 A detailed description of the LAVA process and its versatility is given by Popp et al.25 and Kurland et al.26 Our previous work revealed a close connection between the iron oxide phase composition and the oxygen content of the condensation atmosphere.19,27 To understand this finding, in the work presented here, FexOy Received: July 12, 2013 Revised: September 3, 2013

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dx.doi.org/10.1021/cg4010533 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

complete vacancy ordering, lowering the symmetry to tetragonal space group P43212.30 Maghemite is metastable at room temperature and converts to hematite at elevated temperatures (572−650 K).1 Like magnetite, it shows ferrimagnetism with a theoretical net magnetic moment of 2.5 μB per formula unit and a measured saturation magnetization (MS) of 76 A m2 kg−1 (2.39 μB).28 ε-Fe2O3 is a rare Fe2O3 polymorph with no natural abundance. It was first synthesized by Schrader and Büttner31 and structurally resolved in 1998 by Tronc et al.32 They proposed an orthorhombic unit cell (a = 0.5095 nm, b = 0.8789 nm, c = 0.9437 nm, and VM = 31.8 cm3 mol−1) with space group Pna21. The anion arrangement is a double hexagonal stacking (ABAC) in which the tetrahedral interstices are filled with one-quarter of the Fe3+ ions and the octahedral sites with three-quarters of the Fe3+ ions. These structural features, i.e., the higher content of FeO6 octahedra in comparison to that of maghemite, render the ε-Fe2O3 phase an intermediate between α- and γ-Fe2O3.33 Magnetic structure determination is still in the focus of research, with recent evidence that ε-Fe2O3 might be a collinear ferrimagnet at room temperature.34 It exhibits a very high coercive field (HC ≈ 1600 kA m−1) because of the large magneto-crystalline anisotropy and a relatively small saturation magnetization (MS) of 15−25 A m2 kg−1.35

nanopowders were prepared by the LAVA method applying condensation gases with systematically varied oxygen content. In-depth morphological, structural, and chemical analyses of the synthesized iron oxide nanopowders were conducted. Magnetic properties of the powders were measured by using vibrating sample magnetometry (VSM). Finally, on the basis of the results of density functional theory (DFT) calculations, a mechanism is proposed for the formation of initial nucleation stages during the gas phase condensation, which explains the dependence of the crystal phase composition of the nanopowders on the oxygen content of the condensation gas. Fe2O3 Polymorphs and Fe3O4: Crystal Structure and Magnetic Properties. The common iron oxides are hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4). The most stable Fe2O3 polymorph, α-Fe2O3, is isostructural with corundum (α-Al2O3).1 Its unit cell shows hexagonal symmetry with the following values: a = 0.5034 nm, c = 1.375 nm, and molar volume VM = 30.39 cm3 mol−1. The crystal lattice consists of a hexagonal close packing of oxygen anions in which two-thirds of the octahedral interstices are filled with Fe3+ ions. These FeO6 octahedra share edges in the (001) plane and faces along the [001] direction. This causes a deviation from the ideal cation position due to the electrostatic interaction between these two closely neighboring Fe3+ ions. At room temperature, α-Fe2O3 is a weak ferromagnet and becomes paramagnetic at the Neel temperature (TN) of 950 K. Magnetite is not a Fe2O3 polymorph as its stoichiometry can be written as Fe2O3·FeO. It has an inverse spinel structure with a face-centered cubic unit cell (a = 0.839 nm, and VM = 44.56 cm3 mol−1). The oxygen anions are stacked in a cubic close packaging along the [111] direction. In this packaging, oneeighth of the tetrahedral interstices are occupied by Fe3+ ions and one-half of the octahedral interstices by Fe3+ and Fe2+ ions in equal parts. Therefore, the chemical formula of magnetite is often formally written as Fe3+[Fe2+Fe3+]O4, with the brackets denoting the octahedral sites. This structure gives rise to ferrimagnetism because the tetrahedrally and octahedrally coordinated iron ions form two magnetic sublattices that are aligned in an antiparallel fashion. The net magnetic moment is essentially the magnetic moment of Fe2+ that amounts to 4 μB (Bohr magneton μB = 9.27 × 10−24 J T−1) because the magnetic moments of the octahedrally and tetrahedrally coordinated Fe3+ ions cancel each other out. The measured saturation magnetization (MS) at room temperature is 92 A m2 kg−1.28 Spherical magnetite nanoparticles 99% pure, grain sizes of 6000 K), we can assume that only neutral species contribute to the nanoparticle formation process that is expected to take place at lower temperatures. Introducing oxygen into the plasma leads to the formation of O radicals, O2, and O3 molecules in the condensation zone. The rate of formation of O3 in a laser breakdown plasma depends on the partial pressure of oxygen.57,58 In addition, it has been shown that the rapid mixing of plasma with an oxygen-containing quench gas facilitates the formation of O3.57 Once formed, O3 shows decay times significantly longer than the nucleation time scale14,59 even at temperatures as high as 2000 K.57 Indeed, in the oxygen-enriched atmospheres, appreciable ozone content has been detected in the proximity of the plasma flame of the LAVA process. To shed light on the possible role of ozone in the mechanism of nanoparticle formation, DFT calculations of several neutral FeOn gas phase species were performed. Such molecules, shown in Figure 5, are expected to be relevant for the initial nucleation stages acting as precursors and nucleation “monomers”.14 Table 5 shows calculated reaction energies and Gibbs energies of various formation channels of the species involving O, O2, and O3. For the atomization energy of O2 (Table 5, reaction 1), the calculated value of 505.2 kJ mol−1 is in good agreement with the experimental value of 498 kJ mol−1.60 However, the energy of decomposition of O3 into O and O2 (Table 5, reaction 2, 65.3 kJ mol−1) is underestimated by ∼36 kJ mol−1 compared to the experimental estimate of 101 kJ mol−161 and more accurate ab initio results.62 Therefore, the stability of ozone-containing FeOn species is expected to be somewhat underestimated in our calculations. The results presented in Table 5 demonstrate that Fe and ozone form remarkably stable gas phase complexes. The energy of binding of ozone to Fe (302 kJ mol−1) is >40 kJ mol−1 higher than that of O2 (269.1 kJ mol−1) and only ∼110 kJ mol−1 lower than the binding energy of FeO (411.6 kJ mol−1). Our calculations predict that Fe(O3) and Fe(O3)2 gas phase species are stable with respect to the dissociation to Fe and O3 at temperatures as high as 2500 K. In fact, the calculated values of the change in the Gibbs free energy, ΔG (Table 5), indicate that Fe complexes with ozone are formed at temperatures higher than those at which complexes with O2 form. F



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Fe−O bond lengths of the monomers determine the size of the coordination sphere and therefore the number and spatial arrangement of oxygen ligands in the vicinity of Fe atoms within the nanoparticles. In maghemite and other iron−oxygen compounds, the Fe−O bond lengths of Fe in the tetrahedral coordination (1.84−1.88 Å) are significantly shorter than in the octahedral coordination (1.89−2.15 Å).63 Therefore, in the absence of ozone, the rapid formation of a crystal lattice starts with weakly coordinated FeO and FeO2 precursors with short Fe−O bond lengths, which statistically favors formation of 4fold coordinated Fe atoms. A metastable phase emerges with a considerable amount of occupied tetrahedral interstices. This is maghemite with an octahedral:tetrahedral Fe ratio of 5:3. This consideration is supported by Ostwald’s rule of steps: it is not the thermodynamically stable polymorph that crystallizes at first but the one whose crystallographic structure resembles the local structure of the melt. In contrast, when O 3 is present at higher oxygen concentrations, stable Fe(O3), FeO(O3), and Fe(O3)2 molecules are formed (ΔG < 0 for reactions 5, 8, and 11 in Table 5). The formation of FeO2(O3) cannot be excluded because the ΔG value of 17.2 kJ mol−1 (Table 5, reaction 10) is certainly within the accuracy of the method. In all these O3 complexes, the Fe−O bond lengths of bonds between Fe and O3 are significantly longer than those in FeO and FeO2 (cf. Figure 5). Therefore, the ozone complexes provide precursors that not only contain Fe in a higher coordination state but also possess increased space within the first coordination sphere. This increases the number of more highly coordinated Fe atoms in the condensed droplets, which, upon rapid solidification, increases the probability of developing a crystal structure with an elevated number of octahedrally coordinated Fe atoms. Because of the higher octahedral:tetrahedral Fe ratio of ε-Fe2O3 (3:1) compared to that of maghemite (5:3), there is a higher probability of developing the ε-Fe2O3 polymorph. Therefore, an increasing oxygen concentration in the condensation atmosphere leads to a higher concentration of O3 and, subsequently, increases the mass fraction of ε-Fe2O3 in the LAVA-prepared nanopowders. Because no particles were found to consist of domains with different crystal structures, the higher oxygen concentration also implies an increasing number of ε-Fe2O3 nanoparticles in the powders. This model is confirmed by the description of the early discovery of ε-Fe2O3 by Schrader and Büttner.31 They synthesized ε-Fe2O3 in a DC arc burning between iron electrodes in an oxidizing

Figure 5. Atomic structures of FeOn species and calculated bond distances (in angstroms). Spin multiplicities are given as superscripts.

On the basis of the results of the calculations as well as thermodynamic and steric considerations, we propose a model for initial nucleation stages of the nanoparticles that explains the increasing ε-Fe2O3 content with an increasing oxygen partial pressure. The nucleation begins with the formation of precursors (monomers) consisting of small gas phase molecules.14,59 When the temperature falls below a critical value, the monomers form a supersaturated vapor from which homogeneous nucleation of nanoparticle droplets occurs. For Fe2O3, the condensation temperature is expected to be around 1838 K, which is the melting and decomposition point of hematite.60 The composition of the droplets reflects the content of the precursors in the supersaturated vapor. On the basis of the calculated Gibbs reaction energies (Table 5) in the absence of ozone, FeO and FeO2 are the only stable gas phase species at ∼2000 K (ΔG < 0 for reactions 3, 4, and 6 in Table 5). These molecules contain Fe atoms in a low coordination state with short Fe−O bond lengths of ∼1.60 Å (cf. Figure 5). Upon rapid condensation and subsequent solidification, the

Table 5. Calculated Reaction Energies ΔE0a and Gibbs Reaction Energies ΔG (2000 and 2500 K, 1 atm) for FeOn Species 1 2 3 4 5 6 7 8 9 10 11 a

reactionb

ΔE0 (kJ mol−1)

ΔG (2000 K) (kJ mol−1)

ΔG (2500 K) (kJ mol−1)

O + 3O → 3O2 O + 3O2 → 1O3 5 Fe + 3O → 5FeO 5 Fe + 3O2 → 5FeO2 5 Fe + 1O3 → 5Fe(O3) 5 FeO + 3O → 5FeO2 5 FeO + 3O2 → 5FeO3 5 FeO + 1O3 → 3FeO(O3) 5 FeO2 + 3O2 → 5Fe(O2)2 5 FeO2 + 1O3 → 5FeO2(O3) 5 Fe(O3) + 1O3 → Fe(O3)2

−505.2 −65.3 −411.6 −269.1 −301.9 −362.7 −150.6 −284.0 −56.8 −211.1 −322.5

−334.9 122.5 −253.3 −97.4 −105.0 −179.0 60.0 −58.4 192.7 17.2 −56.5

−287.9 171.3 −210.3 −53.7 −57.4 −131.3 109.2 −7.5 249.5 66.3 2.6

3 3

Electronic energy and zero-point vibrational energy contribution. bSpin multiplicities are given as superscripts. G



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gas phase condensation and particle crystallization processes. It also opens the possibility of predetermining an emerging crystal lattice by the vapor phase metal coordination. The gained insight into these processes will be useful for the functional design of not only isostructural materials like alumina (Al2O3) but also in general ceramic powders and layers prepared by vapor phase synthesis.

atmosphere. It is legitimate to assume that ozone is also formed under these conditions because a similar process is used for the industrial production of ozone.64 Furthermore, the proposed model may explain why even with oxygen as a condensation gas only mixtures of ε- and γFe2O3 arise instead of pure ε-Fe2O3. Because of the elevated temperature and continuous gas flow, the overall ozone concentration within the condensation zone cannot exceed a certain threshold. This limits the concentration of iron−ozone complexes that act as precursors for octahedrally coordinated Fe atoms during the condensation and subsequent crystallization of the FexOy nanoparticles. This in turn limits the amount of ε-Fe2O3 formed. The introduction of oxygen into the condensation gas not only affects the composition of the iron oxide nanopowders but also has a considerable impact on their magnetic properties. Saturation magnetizations MS of the samples prepared in an oxygen-depleted atmosphere are 72.2 A m2 kg−1 (sample FexOy-O0) and 71.2 A m2 kg−1 (sample FexOy-O14). These values agree well with the saturation magnetization of 74 A m2 kg−1 for fine-grain maghemite powders (grain sizes of 100−170 nm) used in magnetic recording tapes.28 A decrease in relative remanence MR/MS from a theoretical value of 0.87 to the determined values of approximately 0.26−0.27 (Table 4) is attributed to a superparamagnetic mass fraction in the nanopowders that relaxes fast after magnetization because of thermal energy.65 With an increase in the oxygen content of the condensation atmosphere, the saturation magnetization of the obtained iron oxide nanopowders decreases while their coercivity increases. This is due to the increasing fraction of ε-Fe2O3 with its comparatively small saturation magnetization of 15−23 A m2 kg−1 and its giant coercive field (HC = 1.6 × 106 A m−1).35 This conclusion is in agreement with findings reached by Osipov et al., who prepared iron oxide nanopowders by the ablation of a sintered hematite target in air at varied pressures using pulsed-periodic CO2 laser radiation.16 They also found that the content of ε-Fe2O3 in the obtained nanopowders is a function of the oxygen partial pressure during ablation.

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ASSOCIATED CONTENT

S Supporting Information *

High-resolution TEM micrographs of iron oxide nanoparticles with crystal imperfections, crystallographic data of ε-Fe2O3 and maghemite, the quality of the Rietveld refinement, and hysteresis measurements. 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 This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant MU1803/8-2. We thank Professor Peter Schaaf (TU Ilmenau, Ilmenau, Germany) for very helpful discussions and the Thüringer Landesanstalt für Umwelt und Geologie (TLUG) for providing the ozone analyzer.

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REFERENCES

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CONCLUSION Crystalline magnetic iron oxide nanopowders with maghemite as the main phase were successfully prepared by CO2 LAVA. It was found that the oxygen partial pressure in the condensation atmosphere has a strong influence on the structural and, thus, magnetic characteristics of the iron oxide nanoparticles. Ozone that is produced in the presence of oxygen within the laserinduced plasma plays an essential role in the formation of the additional ε-Fe 2O3 phase. If ozone is present in the condensation atmosphere, remarkably stable iron−ozone complexes are formed, which upon rapid condensation and solidification of the nanoparticles act as precursors for octahedrally coordinated Fe3+. As a consequence, higher concentrations of ozone in oxygen-rich atmospheres lead to an increasing level of formation of ε-Fe2O3 with a higher number of octahedrally coordinated Fe3+ ions. In contrast, under oxygen-poor conditions and resulting low ozone concentrations, maghemite is formed, which contains fewer Fe3+ ions in octahedral positions. The presented nucleation model for the formation of different iron oxide phases in the LAVA process allows the preparation of Fe2O3 nanopowders with adjustable saturation magnetization and coercive field solely via the control of the oxygen content of the condensation atmosphere. The nucleation model also sheds light on general H



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Control of the Crystal Phase Composition of FexOy Nanopowders

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