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Strain-Engineered Cube Nanocrystals Ce1−yFeyO2 That Brought Forth Abnormal Structural and Magnetic Properties Liping Li* and Xiuqi Li Key Lab of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China ABSTRACT: Elucidating the relationship between internal strain and structural modification of metal oxide nanostructures expands our ability to fine tune their physical properties. However, this aspect of strain engineering is still challenging. Here, we employed two-step solution chemistry and prepared cube nanostructures Ce1−yFeyO2 with high sample uniformity. The solubility limit for Fe3+ ions doped in CeO2 nanostructures is determined to be around y = 0.15. Below the solubility limit, cube nanostructures showed a pronounced tensile strain that increased almost linearly with the doping levels. This tensile strain did not destroy the fluorite structure of parent CeO2 nanostructures but resulted in the smallest lattice constant ever reported for all CeO2-based solid solutions. In spite of a large change in lattice parameters, Raman phonon modes for these solid solutions did not shift at all with the strain, which differs from those under external compression or size effects. Further increasing the strain led to a magnetic transition of cube nanocrystals Ce1−yFeyO2 from the very weak paramagnetic to coexistence of paramagnetic and ferromagnetic components. Results reported in this work demonstrate that strain engineering is the base for intrinsic control over the structures and magnetic properties of metal oxide nanostructures and therefore may find many applications in spintronics and other devices.

1. INTRODUCTION Metal oxide nanomaterials have witnessed promising applications in numerous fields such as spintronics,1,2 fenton-like heterogeneous catalysts,3 photocatalysts,4,5 photovoltaic cells,6,7 and lithium batteries.8,9 A major obstacle in developing metal oxide nanomaterials is the current inability to produce nanocrystals with high sample uniformity and tunable internal strain. Strain engineering is especially essential for property tuning since it is capable of altering the lattice vibrations (or physically, phonons) in desirable and controllable ways to tailor electron−phonon, photon−phonon, or even magnetic exchange interactions. However, strain engineering is very difficult to achieve for oxide nanocrystals, because all metal oxides have rigid frameworks characterized by strong metal− oxygen bonds. Furthermore, when made at the nanoscale, metal oxides such as CeO2 will exhibit structural stability much higher than their bulk counterparts due to their strengthened surface metal−oxygen bonds and hardened shell surface.10,11 Therefore, understanding the relationship between internal strain and structural modifications of oxide nanocrystals to effectively tune their physical properties is crucial for developing a broad class of new nanomaterials and other sciences. Extensive theoretical and experimental investigations have demonstrated that structural modifications of metal oxide nanocrystals (especially by foreign ion doping) are highly useful in creating internal strain necessary for rational tuning of physical properties.12−14 Nevertheless, doping foreign ions in nanostructures generally leads to apparent changes in particle sizes and morphologies of parent compounds.15,16 These changes introduce uncertainties in identifying specific causes © 2013 American Chemical Society

for internal strain. It is well established that internal strain is strongly dependent on lattice dimension, particle size, and dimensionality (or morphologies). One may thus speculate whether strain engineering can be achieved solely through manipulating the occupation of dopants in oxide nanostructures without changing the particle sizes or morphologies. Motivated by this, cube nanostructures Ce1−yFeyO2 were taken as a model compound for study under the considerations that (i) CeO2 represents a prototype metal oxide with a closely packed crystal structure, in which a cerium atom is enclosed by 8 oxygen atoms, while each oxygen atom forms a 4-fold coordination with cerium atoms. All Ce−O8 polyhedra are held together through sharing edges to form a face cubic fluorite structure that yields outstanding physical and chemical properties. (ii) CeO2 and its derivatives have many types of fluorite-analogous materials17−20 including δ-Bi2O3, Y-stabilized ZrO2, CaF2, and pyrochlore Gd2Ti2O7 with a defect fluorite structure. Results of relevant investigations about Ce1−yFeyO2 may have broad implications. (iii) As for almost all doped CeO2 nanostructures, the majority of dopants are limited to those (e.g., Bi3+, Eu3+, Gd3+, Tb3+, etc.)21−23 with similar or compatible ionic sizes to Ce4+. These dopants tend to occupy Ce4+ sites of the CeO2 framework. However, Fe3+ ions have a much smaller ionic size, which enables Ce1−yFeyO2 nanostructures to be highly compacted. Further, small Fe3+ ions tend to occupy the lattice sites and interstitial sites simultaneously, probably giving rise to Received: May 2, 2013 Revised: June 27, 2013 Published: July 12, 2013 15383

dx.doi.org/10.1021/jp4043649 | J. Phys. Chem. C 2013, 117, 15383−15393

The Journal of Physical Chemistry C

Article

Scheme 1. Illustration of Two-Step Synthetic Processes for Fe3+-Doped CeO2 Cube Nanocrystals

Figure 1. TEM and HRTEM images of the samples Ce1−yFeyO2 at y = 0.0 and 0.10.

formation reaction. All reagents employed in experiments were analytical grade and used without further purification. All samples were synthesized according to a hydrothermal chemistry method described in our previous work24 but with some modifications. Namely, two-step formation reaction processes were adopted. The first step involves a roomtemperature reaction in which 0.12 − y mol of Ce(NO3)3·6H2O and y mol of Fe(NO3)3·9H2O were dissolved in 55 mL of water at room temperature to form a mixed solution that contains both Ce3+ and Fe3+ ions. A 15 mL amount of 5 M NaOH was gradually added into the above mixed solution with stirring to form a suspension mixture. The second step involves hydrothermal crystallization into the final products. The suspension obtained in the first step along with the mother liquid was transferred to 100 mL Teflon-lined stainless steel autoclaves. After hydrothermal reactions at 220 °C for 12 h, autoclaves were cooled to room temperature naturally. Residual solutions were all colorless, indicating that the majority of Fe3+ ions were probably doped in the final products during the second reaction. Precipitates were separated by centrifugation and washed with deionized water

a valid strain control. Consequently, preparing Ce1−yFeyO2 nanostructures with high sample uniformity and identifying their internal strain dependence on structures can be expected to have many implications in understanding and tuning the physical properties of metal oxide nanostructures for applications in spintronics and other devices. In this work, cube nanostructures Ce1−yFeyO2 with high sample uniformity were prepared using two-step solution chemistry. Both the particle size and the cube shape of nanostructures did not change with doping. The impacts of strain on the structure and magnetic properties of cube nanostructures Ce1−yFeyO2 were further investigated. It is shown that the strain almost linearly increased with doping, which has led to an abnormal stiffing of the Raman phonon mode and a transition from very weak paramagnetism to coexistence of paramagnetism and ferromagnetism.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Starting chemicals for sample syntheses were Ce(NO3)3·6H2O and Fe(NO3)3·9H2O. NaOH was used as the mineralizer for accelerating the rate of 15384



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fluorite structure, nanoscale solid solutions Ce1−yFeyO2 thus formed would be thermally stable to show a cube morphology. Morphology and phase structures of the samples were monitored, respectively, by TEM and XRD. As demonstrated by TEM images in Figure 1, both compositions y = 0 and 0.10 showed a similar cube shape and high crystallinity, and the corresponding particle sizes were distributed around 23 nm. The slight difference is that the particles for doping at y = 0.10 were more uniform than that for y = 0, as indicated by the size distribution in the inset of Figure 1. Further, the d spacing for y = 0 was 2.728 nm, which corresponds to the plane (200) of the CeO2 lattice, while that of 3.128 nm observed for y = 0.10 was attributed to the plane (111). Formation of the cube shape could be beneficial from the strong oxidization and alkalinity under the current hydrothermal reactions since (i) CeO2(111) surface is stable under oxidized condition,28 while the Fe3+ species involved in the present conditions has a very strong oxidation ability, (ii) the surface of CeO2 has a stable configuration of water bonding with the cerium site that involves two O−H bonds of hydrogen and oxygen atoms at the surface,29 and (iii) ceria nanoshape is explicitly determined by the reactivity of specific OH groups.30 For the present system, there are sufficient OH groups because of the high alkalinity (pH > 14). By contrast, when Fe3+-doped CeO2 was prepared using other methodologies (e.g., coprecipitation,31−33 combustion,34,35 or electrochemical deposition36), cube shape cannot be obtained but smaller rounded grains ( 0.05. It appears that the magnetism for the present nanocubes differs from those previously reported for undoped and doped CeO2 nanocrystals.79,80 For instance, with the help of a deep reduction that gives more oxygen vacancies, CeO2:Fe2+/Fe3+ nanocrystals exhibit a ferromagnetic component superimposed on a paramagnetic background.81 Low-valence nonmagnetic doping also gives rise to a ferromagnetic behavior at room temperature as for Ce1−2xZnxCoxO2−δ nanoparticles79,80,82 and 10 nm CeO2 doped with Ca2+ due to the abundant oxygen vacancies.25 Our cube nanocrystals Ce1−yFeyO2 cannot be single-domain particles with perfect magnetic ordering because of the presence of a paramagnetic component. We assume that the small dopants Fe3+ might be incorporated into the near surface layer lattice of cube nanocrystals to form core−shell-like particles in which the shell could be enriched by oxygen vacancies and dopants Fe3+. The bulk lattice (core) with longterm ordering was mostly paramagnetic, while near surface lattice (shell) exhibits ferromagnetic interactions, resulting in an evident open hysteretic loop. Since the mass ratio of the shell to core is low as indicated by a relatively large particle size around 20 nm, our cube nanocrystals exhibited a small saturated magnetization when compared to the undoped or doped ceria nanoparticles reported previously.83

Figure 6. Magnetization curves measured at room temperature for Ce1−yFeyO2 nanocubes at y < 0.15: (a) raw experimental data and (b) magnetization data after subtracting the linear paramagnetic component.

paramagnetism while doping at y > 0 led to a strengthened paramagnetic behavior. After subtracting the linear paramagnetic background, an evident open hysteretic loop was observed for y = 0.05 and 0.15 (Figure 6b), while for other samples, magnetic moment was very low. For example, the saturated moment (M) value for y = 0 at H = 5kOe was smaller than 1 × 10−5 emu/g. Increasing Fe3+ content to y = 0.05, the M value increased to 2 × 10−3 emu/g, 2 orders of magnitude higher than that for y = 0. However, when the dopant content varied in between y = 0.075 and 0.125, the magnetic moment reduced back to the low level of 1 × 10−5 emu/g. Different from the trend of saturated moment, coercive force (Hc) increased with the dopant content and reached a maximum 170 Oe at y = 0.10. Room-temperature ferromagnetism of doped semiconductors is generally induced by two primary factors: secondary magnetic phases and oxygen vacancies.75−78 Concerning the secondary magnetic phases, no traces of Fe2O3 were detected at y ≤ 0.10 by XRD (Figure 2) and Raman spectra (Figure 5a). Therefore, iron oxide impurities cannot be the primary cause for the room-temperature ferromagnetic component. Further, if secondary phase Fe2O3 was present at y ≤ 0.10, there would be a large coercive force of several hundred oersteds,50 which appears far beyond what we observed for y ≤ 0.10. When 15390



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4. CONCLUSIONS Ce1−yFeyO2 nanocubes were prepared by a two-step solution chemistry. With this preparation, dopants Fe3+ showed a relatively high solubility limit of about 15% in the CeO2 lattice. Incorporation of Fe3+ results in a large internal strain that significantly increased with doping. Further, the strain is also followed by a heavy lattice contraction that obeys a linear relationship with doping. The lattice parameters for the cube nanocrystals are all surprisingly smaller than those ever reported for CeO2-based solid solutions or even for Fe3+doped CeO2 prepared by other methods. Owing to the large internal strain, the Raman phonon mode became stiffened and the oxygen vacancies associated with Fe3+ doping were compensated by the presence of interstitial Fe3+. Eventually, cube nanocrystals Ce1−yFeyO2 showed a magnetic transition from very weak paramagnetism to coexistence of paramagnetic and ferromagnetic components.

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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 financially supported by NSFC (91022018, 21271171, and 21025104) and NBRP of China (2011CBA00501 and 2013CB632405).

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REFERENCES

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