Interchange core/shell assembly of diluted magnetic semiconductor CeO 2 and ferromagnetic ferrite Fe 3 O 4 for microwave absorption

Core/shell-structured CeO 2 /Fe 3 O 4 and Fe 3 O 4 /CeO 2 nanocapsules are prepared by interchange assembly of diluted magnetic semiconductor CeO 2 and ferromagnetic ferrite Fe 3 O 4 as the core and the shell, and vice versa, using a facile two-step polar solvothermal method in order to utilize the room-temperature ferromagnetism and abundant O-vacancies in CeO 2 , the large natural resonance in Fe 3 O 4 , and the O-vacancy-enhanced interfacial polarization between CeO 2 and Fe 3 O 4 for new generation microwave absorbers. Comparing to Fe 3 O 4 /CeO 2 nanocapsules, the CeO 2 /Fe 3 O 4 nanocapsules show an improved real permittivity of 3–10% and an enhanced dielectric resonance of 1.5 times at 15.3 GHz due to the increased O-vacancy concentration in the CeO 2 cores of larger grains as well as the O-vacancy-induced enhancement in interfacial polarization between the CeO 2 cores and the Fe 3 O 4 shells, respectively. Both nanocapsules exhibit relatively high permeability in the low-frequency S and C microwave bands as a result of the bi-magnetic core/shell combination of CeO 2 and Fe 3 O 4 . The CeO 2 /Fe 3 O 4 nanocapsules effectively enhance permittivity and permeability in the high-frequency Ku band with interfacial polarization and natural resonance at ∼ 15 GHz, improving absorption with a large reﬂection loss of -28.9 dB at15.3 GHz. Experimental theoretical comparisons 2 and Fe 3 O 4 nanoparticles are also O the an improved The of the of The results are also experimentally and theoretically with their nanoparticle


I. INTRODUCTION
The integration of microwave absorbers in electronic devices and systems has become an essential strategy in minimizing electromagnetic (EM) radiation and improving EM compatibility. 1 Core/shellstructured nanocapsules, typically in the form of a magnetic nanoparticle core coated by a dielectric shell of nanometer size, have attracted great interest in recent years because of their generally strong absorption, small thickness, and low density in the S, C, X, and/or Ku bands of microwaves covering the 2-18 GHz range. 2,3 Many of the recent reports have focused on preserving strong absorption while extending the frequency and thickness ranges by minimizing the interface reflection caused by the EM mismatch between permittivity and permeability. 4,5 Besides the traditional magnetic/dielectric combination for the core/shell structure, several different and interesting combinations involving magnetic cores and piezoelectric, ferroelectric, multiferroic, semiconducting, insulating/conducting, conducting/insulating, etc. shells have been proposed in the core/shell and even core/double-shell structure. [4][5][6][7][8][9] In addition to combining different core and shell material phases, the effect of interchange core and shell on the microwave absorption properties has ignited new research interest and direction in more recent years. 4,5,7 From a physical perspective, the distinct dielectric, electrical, and/or magnetic properties of these shells will lead to characteristically different interfacial effects and hence microwave absorption properties. 9 Diluted magnetic semiconductors and oxides, especially for those without doping of magnetic elements, possess versatile physical properties, and so are scientifically interesting and technologically important for magnetoelectronics. 10 CeO 2 is an interesting diluted magnetic semiconducting oxide of such kind. It exhibits ferromagnetism at room temperature and contains abundant O-vacancies. [10][11][12] By combining these unique features in CeO 2 with the large natural resonance in an appropriate ferromagnetic ferrite (e.g., Fe 3 O 4 ), it is likely to observe interestingly strong absorption with broad absorption bandwidth and absorber thickness range underpinned by new physics in such a bi-magnetic core/shell structure. Moreover, the space charges at the interface between CeO 2 and Fe 3 O 4 are capable of inducing an enhancement in interfacial polarization by the abundant O-vacancies in CeO 2 .
In this work, we report the use of a facile two-step polar solvothermal method to prepare two different types of core/shell-structured bi-magnetic nanocapsules by interchange assembly of diluted magnetic semiconductor CeO 2 and ferromagnetic ferrite Fe 3 O 4 as the core and the shell (i.e., CeO 2 /Fe 3 O 4 nanocapsules) and vice versa (i.e., Fe 3 O 4 /CeO 2 nanocapsules). Our aim is to utilize the room-temperature ferromagnetism and abundant O-vacancies in CeO 2 , the large natural resonance in

II. EXPERIMENTS
A facile two-step polar solvothermal method with PVP-assisted interface activation was utilized to prepare CeO 2 /Fe 3 O 4 and Fe 3 O 4 /CeO 2 core/shell-structured nanocapsules. In a typical reaction for CeO 2 /Fe 3 O 4 nanocapsules, Ce(NO 3 ) 3 of 1.5 mmol was dissolved in distilled water of 30 ml, and NaOH of 2 g was rapidly added into the solution under magnetic stirring. The stirring process was maintained until a uniform precursor was formed. The precursor was transferred into a stainless steel autoclave of 50 ml with Teflon line before being heated in an oven at 200 • C for 6 h. The CeO 2 product was collected by centrifugation and treated ultrasonically in 5 g/L PVP aqueous solution for 12 h. After further centrifugation, the CeO 2 product was dispersed in EG of 30 ml with FeCl 3 of 1.5 mmol and PEG of 0.15 g under magnetic stirring. The mixture was sealed in the autoclave and heated in the oven at 200 • C for 4 h. The CeO 2 /Fe 3 O 4 product was obtained after being washed by distilled water and ethanol and then dried in a vacuum at 60 • C for 6 h. Fe 3 O 4 /CeO 2 nanocapsules were prepared using the reverse steps of the above with slightly different solvents and heating conditions. In a typical reaction, Fe 3 O 4 cores were prepared by dissolving FeCl 3 of 1.5 mmol and PEG of 0.15 g in EG of 30 ml, and the mixture was solvothermally treated at 200 • C for 8 h. The Fe 3 O 4 product was added into the CeO 2 precursor formed by Ce(NO 3 ) 3 of 1.5 mmol, ammonia hydroxide of 1.5 ml, and ethanol of 30 ml after it was ultrasonically treated in 5 g/L PVP aqueous solution for 12 h. The mixture was solvothermally treated in the autoclave at 120 • C for 6 h. The Fe 3 O 4 /CeO 2 product was washed by distilled water and ethanol, collected by centrifugation, and dried in a vacuum at 60 • C for 6 h.
The Fe 3 O 4 nanoparticles were also fabricated into paraffin-bonded CeO 2 and Fe 3 O 4 nanoparticle composites for comparison and further discussion. The complex relative permittivity ε r = ε r − jε r and permeability µ r = µ r − j µ r of the composites were measured by a transmission/reflection coaxial line method in the 2-18 GHz microwave range using a vector network analyzer (Agilent 5244A).
The frequency (f ) and thickness (d) dependence of reflection loss (RL) was determined using 4,5 where Z in = Z 0 (µ r /ε r ) 1/2 tanh j (2πfd/c) (µ r ε r ) 1/2 is the input impedance of composite, Z 0 ∼377 Ω is the characteristic impedance of air, c = 3×10 8 m/s is the velocity of light, and d is the thickness of composite.   Figure 1(b) illustrates the TEM image of a typical CeO 2 /Fe 3 O 4 product. The CeO 2 core is in spherical shape of ∼160 nm diameter. The core surface is coated with many smaller sized Fe 3 O 4 nanoparticles to form the Fe 3 O 4 shell. The HRTEM image in Fig. 1(c) displays the interface of the CeO 2 /Fe 3 O 4 product in Fig. 1(b). Fe 3 O 4 nanoparticles of ∼10 nm diameter (with an image of periodic stripes) are closely packed on the surface of the CeO 2 core as the Fe 3 O 4 shell. The concentric rings in the SAED pattern (the inset of Fig. 1(c)) suggest that the CeO 2 core is also a closely packed cluster of many CeO 2 nanoparticles. Figure 1(d) gives the TEM image of a typical Fe 3 O 4 /CeO 2 product. The Fe 3 O 4 core is in cubic shape of ∼300 nm length. The HRTEM image in Fig. 1(e) reveals a close-packing of CeO 2 nanoparticles of ∼5 nm diameter on the surface of the Fe 3 O 4 core to form the CeO 2 shell. The periodical spots and concentric rings in the SAED pattern (the inset of Fig. 1(e)) infer that a Fe 3 O 4 /CeO 2 nanocapsule consists of a single-crystal Fe 3 O 4 core coated by closely packed CeO 2 nanoparticles as the shell.  Fig. 2(b) and described by the Debye theory as follows: 16

III. RESULTS AND DISCUSSION
where ε ∞ and ε s are the optical and stationary permittivity constants, respectively. By contrast, the  In order to investigate the effect of interchange core/shell assembly on the microwave absorption properties, the complex effective permittivity and permeability of a particulate composite having the spherical CeO 2 and Fe 3 O 4 nanoparticles with a molar ratio of 1:1 bonded by the same 60 wt.% of paraffin were calculated using the measured ε r and µ r data of Fe 3 O 4 and CeO 2 composites, as shown in Figs. 2(a) and 2(c), respectively, in the Bruggeman's effective-medium theory described as follows: 5 where ε i and ε eff are the complex permittivity of components and complex effective permittivity of the particulate composite, respectively; µ i and µ eff are the corresponding permeabilities; p i is the volume fraction of component in the composite. The calculated ε eff and µ eff are plotted as the dotted lines in Figs. 2(a) and 2(c), respectively. It is seen that the calculated ε eff in Fig. 2(a) Fig. 2(c) show similar trends and levels with the measured µ r and µ r . µ r shows a reduction in the S and C band of 2-8 GHz and an enhancement in the X and Ku band of 8-18 GHz, while µ r displays a slight reduction in the S, C, and X bands and an enhancement in the Ku band. The magnetic attenuation of nanocapsules in the Ku band is elevated by the bi-magnetic core/shell assembly, making µ r and µ r more steadily in the whole f range. Figure 3 shows

IV. CONCLUSION
We have prepared CeO 2 /Fe 3 O 4 and Fe 3 O 4 /CeO 2 nanocapsules in order to impart the roomtemperature ferromagnetism and abundant O-vacancies of CeO 2 , the large natural resonance of Fe 3 O 4 , and the O-vacancy-enhanced interfacial polarization between CeO 2 and Fe 3 O 4 in the nanocapsules as well as to study the effect of interchange assembly of CeO 2 and Fe 3 O 4 on their phase, lattice parameters, morphology, interface nanostructure, and microwave absorption properties. We have also performed experimental and theoretical comparisons with their CeO 2 and Fe 3 O 4 nanoparticle counterparts. We have found that the use of CeO 2 as the core and Fe 3 O 4 as the shell (CeO 2 /Fe 3 O 4 nanocapsules) can increase both permittivity and permeability at the high-frequency microwave Ku band of 12-18 GHz due to the increased O-vacancy concentration in the CeO 2 cores of larger grains, the O-vacancy-induced enhancement in interfacial polarization between the CeO 2 cores and the Fe 3 O 4 shells at ∼15 GHz, and the natural resonance at ∼15 GHz. As a result, a strong absorption with RL = -28.9 dB at f = 15.3 GHz and d = 7.8 mm, together with broad absorption f range of