Article pubs.acs.org/JPCC
Core/Shell Metal/Heterogeneous Oxide Nanocapsules: The Empirical Formation Law and Tunable Electromagnetic Losses X. F. Zhang,*,†,‡ H. Huang,† and X. L. Dong† †
School of Materials Science and Engineering, Dalian University of Technology, Dalian, Liaoning 116024, P. R. China National Research Council of Canada, 75 Boul. de Mortagne, Boucherville, Québec J4B 6Y4, Canada
‡
ABSTRACT: We report an empirical thermodynamic law for the synthesis of core/shell metal/heterogeneous oxide nanocapsules, such as Fe/SiO2, Ni/SiO2, Fe/B2O3, Ni/B2O3, Fe/ Al2O3, Fe/MnO2, Fe/Y2O3, Fe/CeO2, and Fe/La2O3, by arc-discharge evaporating a compressed mixture of Fe (or Ni) powders and the oxide powders of shell components. By integrating such dielectric oxide shells and magnetic metal cores in each particle, the electromagnetic losses can be artificially manipulated at the whole S-band, C-band, X-band, and Ku frequency ranges. The microscopic origins of the controllability of electromagnetic losses are ascribed to the specific matching and interfacial coupling between the magnetic cores and dielectric shells.
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INTRODUCTION Recently, serious electromagnetic interference (EMI), which caused from mobile communication devices (0.8−1.5 GHz), local Internet (2.45, 5.0, 19.0, 22.0, and 60.0 GHz), and radar systems (11.7−12.0 GHz), has been arousing considerable attention.1−9 The potential EMI harm has been driving to develop suitable electromagnetic wave absorbents for human safety, particularly toward the absorption at specific frequency for various work conditions. Among the candidates, magnetic metal nanoparticles have attracted great interest with merits of steady permeability (μr = μ′ + iμ″) and high Snoek’s limit in the gigahertz frequency range. However, one important factor which should be taken into account is how to optimize the permeability and permittivity (εr = ε′+iε″), which represent the magnetic and dielectric loss capabilities of electromagnetic wave energy, respectively.10−13 Magnetic metal nanoparticles (Fe, Co, and Ni) with extremely chemical instability have been thought as one of critical issues in practical applications, and therefore they usually have to be selfpassivated by an epitaxially homogeneous oxide shell.12−15 Such structure could suppress their intrinsic merits of metal cores and result in undesirable electromagnetic matching between the shell and the core. Integrating heterogeneous components into one particle could create more significant characteristics than their counterparts. Many studied have thus been focused on the postmodifications of nanoparticles to form heterogeneous core/ shell structure.16−22 However, the postformation process of shells could unavoidably result in either undesirable oxidation of metal cores in reaction environment or relatively complex approaches, limiting the controllability of both structure and properties. We herein developed an arc-discharging strategy for the in situ synthesis of various heterogeneous core/shell metal/oxide nanocapsules, for example, metal/SiO2 and metal/B2O3/H3BO3.23−27 © 2013 American Chemical Society
On the basis of abundant experiments, we summarize a universal formation law for such nanostructures, defined as the stoichiometric oxygen-assisted vapor−liquid−solid (SOA-VLS) mechanism. By integrating various dielectric oxide shells and/or magnetic metal cores into one particle, the relatively complex permittivity and permeability can be artificially manipulated at expected frequency over a moderate wide-band range, which thus exhibits the competitive potentials as electromagnetic wave absorbents. We also convince that the SOA-VLS mechanism could be extended to synthesize other various core/shell systems that are not explored currently, probably some of which have additional advantages in meeting specific fields.
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EXPERIMENTS The arc-discharge method was described elsewhere.23,24 The metal (Fe or Ni) and oxide (SiO2, B2O3, Al2O3, MnO2, Y2O3, CeO2, and La2O3) micrometer powders were homogeneously mixed in a weight ratio of 5:1 and then pressed into bulk as the anode. The arc discharge was triggered between a tungsten rod that served as the cathode and the anode under a mixture of H2 and Ar. The as-made samples were collected after about 4−6 h. To measure the electromagnetic properties, the nanocapsules were mechanically mixed with the same weight of wax paraffin and then cut into a toroidal shaped specimen of 7.00 mm in outer diameter, 3.00 mm in inner diameter, and 1 mm in thickness. The measurement was carried out at 2−18 GHz by using an Agilent 8722ES vector network analyzer (VNA) with a sweep oscillator and an S-parameter test set. The VNA was calibrated for the full two-port measurements of reflection and Received: February 12, 2013 Revised: March 25, 2013 Published: March 28, 2013 8563
dx.doi.org/10.1021/jp4015417 | J. Phys. Chem. C 2013, 117, 8563−8569
The Journal of Physical Chemistry C
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Article
RESULTS AND DISCUSSION
The VLS mechanism was first proposed to explain the silicon nanowires growth by catalytic assistance of a liquid gold droplet placed upon a silicon substrate.28 In the latest decades, it has been widely extended to synthesize different kinds of nanowires using various metals or alloys catalysts. The nanowires growth is generally performed out under very mild chemical vapor deposition condition, in which the catalytic droplet can steadily adsorb a vapor to supersaturating levels and then separate it out. Differently, the arc-discharding approach reported here is a nonequilibrium process consisting of instantaneous evaporation and condensation, associated with a nucleation burst and subsequent fast fusion of nucleus, which thus limits the continuous growth and evolution of microstructure with insufficient feedstock. As shown in Figure 1a, the mixture of Fe and SiO2 micropowders with a mass ratio of 5:1 is evaporated to be vapor by arc discharge. Because of a tremendous cooling gradient, Fe would be rapidly condensed to liquid droplets and subsequently adsorb silicon and oxygen atoms. Meanwhile, they grow by adsorbing relatively smaller Fe droplets and silica droplets by Ostwald ripening.29 As further decreasing the temperature, supersaturated silica components are separated out and finally deposited on the surface of solidified Fe particles, forming a dense shell. Inspired from the Fe/SiO2 nanocapsuels, this strategy can also be extended to synthesize various heterogeneous core/shell metal/oxide nanocapsules. We also obtained Ni/SiO2, Fe/B2O3, and Ni/B2O3 nanocapsules by evaporating a compressed mixture of Fe (or Ni) powders and the corresponding oxides of shell components. In all the experiments, the mass ratio of metal to oxide is set at 5:1 in order to make sure the compressed mixture conductive for starting the arc discharge. Further increasing the metal ratio would result in a small
Figure 1. (a) Illustration for synthesizing the oxide-coated metal nanocapsules by the SOA-VLS mechanism. (b) Room-temperature Gibbs free energies of various oxides used in this study. (c) Fe−Si−O phase diagram at 1600 °C.
transmission at each port and then connect to the coaxial line sample holder.
Figure 2. TEM images of various metals/oxides nanocapsules supported on an ultrathin amorphous carbon-coated TEM grid. 8564
dx.doi.org/10.1021/jp4015417 | J. Phys. Chem. C 2013, 117, 8563−8569
The Journal of Physical Chemistry C
Article
Figure 3. High-resolution TEM images (left), iFFT images (middle), and the measured spacing distances (right) of Fe/Y2O3 nanocapsules: (a) shell and (b) core.
part of naked metal nanoparticles without heterogenerous shells. Evaporating the metal and oxide into vapor, it is unclear why the oxide precursor can be remained during the condensation process, rather than forming other oxide with metal atoms such as iron/nickel oxides. This could be related to the origin of the formation of heterogeneous core/shell. We herein propose a possible mechanism based on the fundamental thermodynamics by comparing Gibbs free energies of these oxides that presented in our study, as shown in Figure 1b. It is noted that B2O3 have lower Gibbs free energies than that of FeO, Fe2O3, Fe3O4, NiO, and Ni2O3. During the evaporation and cooling processes, the oxides are easier to be re-formed, and therefore they are finally stabilized as outer shells after experiencing the nucleation−supersaturation−diffusion process. While Gibbs free energy of SiO2 is higher than that of Fe2O3, Fe3O4, and Ni2O3, implying that latter ones should be formed, evidently which is controversy to our experimental observations. This is because, from the view of fundamental thermodynamics, oxygen fugacity is related to the metal oxide equilibrium reaction. In the present study, the SiO2 with traces of stoichiometric oxygen in the precursor is a critical factor for impeding the formation of Fe2O3, Fe3O4, and Ni2O3. The phase diagram of bulk Fe−Si−O components at 1600 °C shown in Figure 1c confirms the predicted formation process of the core/shell architecture, caused from the solubility and diffusion of SiO2 in Fe, at which composition there is indeed no any iron oxide. Transmission electron microscope (TEM) images of all the nanocapsules are shown in Figure 2. All the nanoparticles are essentially spherical in shape and microstructure and have a size distribution of 50−100 nm by direct statistic observation. The core/ shell interfaces are clearly distinguished, presenting that the thickness of shells are 5−20 nm. The microstructures of a part of nanocapsules have been characterized and reported elsewhere.23−27 Herein, we summarize the empirical formation law to synthesize the Fe/Y2O3, Fe/CeO2, and Fe/La2O3 nanocapsules, all of which have a desirable Gibbs energy differences between the core and the shell materials. The high-resolution TEM images of the core and shell regions confirm the heterogeneous structures of all the nanocapsules. For example, Figures 3a and 3b show the detailed microstructures of the shell and the core of a Fe/Y2O3 nanocapsule, respectively, in which we can clearly see the atomic plane arrangements. By inverse
Figure 4. XRD patterns of Fe/La2O3, Fe/Y2O3, and Fe/CeO2 nanocapsules and the raw materials of microsized Fe, La2O3, Y2O3, and CeO2 powders. 8565
dx.doi.org/10.1021/jp4015417 | J. Phys. Chem. C 2013, 117, 8563−8569
The Journal of Physical Chemistry C
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Figure 5. (a, c) Complex permittivity and (b, d) complex permeability of Fe/SiO2, Ni/SiO2, Fe/B2O3, and Ni/B2O3 nanocapsules. All the measured samples are uniformly mixed with the same weight of wax paraffin.
aroused from a large proportion of microscopic interfaces.16,30−33 To further reveal the capacity of magnetic loss and dielectric loss of these nanoparticles with various components, we compare the values of tan δM/tan δE, as shown in Figure 6, which is usually defined as the electromagnetic matching. The results show the relative capabilities of dielectric and magnetic losses for metal/SiO2 and metal/B2O3 nanocapsules in the whole frequency range, evidently indicating that a highly component correlation to the electromagnetic characteristics. The efficiency and specific frequency of electromagnetic wave absorption are mainly affected by both the magnetic loss and the dielectric loss and in particular their matching. In cases of metal/SiO2 and metal/B2O3 nanocapsules, such electromagnetic matching can be effectively adjusted by heterogeneous core/shell components, providing a possibility of controlling electromagnetic wave absorption at expected frequency. To prove the characteristics of electromagnetic wave absorption, the reflection loss R (dB) of metal/SiO2 and metal/B2O3 nanocapsules was calculated according to the transmit-line theory. The normalized input impedance (Zin) of electromagnetic absorption layer for reflection and transmission at the boundary air-absorber backed by a metallic reflector is given by
fast Fourier transform (iFFT) images of the selected regions, one can confirm the (400) planes of Y2O3 shell with a spacing distance of 0.265 nm and the (100) planes of α-Fe core region with a spacing distance of 0.208 nm. XRD patterns of the Fe/Y2O3, Fe/CeO2, and Fe/La2O3 nanocapsules are shown in Figure 4, together with the raw materials as references. It is evident that there is not new formed phase appearing in the products, reconfirming the microstructures and the assistance role of the stoichiometric oxygen in the precursor. We herein report the electromagnetic properties of Fe/SiO2, Ni/ SiO2, Fe/B2O3, and Ni/B2O3 nanocapsules, each of which has a specific absorption frequency across the whole S-band, C-band, X-band, and Ku frequency ranges. Figure 5 shows the frequency dependences of complex permittivity and permeability consisting of the real part (ε′) and imaginary part (ε″) at 2−18 GHz. The SiO2 shells results in a strong frequency dependence of complex permittivity of Fe/SiO2 and Ni/SiO2 nanocapsules, while Fe/B2O3 and Ni/B2O3 nanocapsules are almost constant over the 2−18 GHz range. Obviously, the differences of dielectric characteristics between metal/SiO2 and metal/B2O3 are consequences of its intrinsic dielectric loss of shells and the interfacial coupling and matching between the cores and shells.16,30−32 The hybrid core/shell architectures not only presents variable dielectric behaviors but also combines with the magnetic behavior that depends on the kinds of magnetic metal cores, as shown in Figure 5b,d. The real parts and imaginary parts of complex permittivity and permeability correlate to the energy storage and loss of electromagnetic wave in materials, respectively, which can be quantitatively expressed by the dielectric loss tangent (tan δE= ε″/ε′) and magnetic loss tangent (tan δM= μ″/μ′).10 They could be effectively manipulated by the compositing various components, for example, either by mechanical mixing or periodic construction. Furthermore, integration of heterogeneous components into one single particle at nanoscale is significant because it probably provides additionally cooperative phenomena such as coupling and/or polarization that
Z in =
μr εr
tanh i
2πf c
με d r r
(1)
where the μr and εr are respectively the relatively complex permeability and permittivity of absorbers, c is the velocity of light in the free space, f is the frequency of the electromagnetic wave, and d is the thickness of the absorber. The reflection loss R (dB) of the incident electromagnetic wave normal to the single layer absorber can be expressed as R (dB) = 20 log 8566
Z in − 1 Z in + 1
(2)
dx.doi.org/10.1021/jp4015417 | J. Phys. Chem. C 2013, 117, 8563−8569
The Journal of Physical Chemistry C
Article
Figure 6. Ratios of the magnetic loss factor to the dielectric loss factor of (a) Fe/SiO2 and Ni/SiO2 and (b) Fe/B2O3 and Ni/B2O3 nanocapsules.
Figure 7. Microwave reflection losses (R) of Fe/SiO2, Ni/SiO2, Fe/B2O3, and Ni/B2O3 nanocapsules.
On the basis of the eqs 1 and 2, it is known that the best electromagnetic matching for zero reflection of electromagnetic wave should be tan δM = tan δE. However, the existing materials could not be tan δM = tan δE at the whole frequency, and thus microwave absorption of absorbers is frequency-dependent. The minimum reflection loss can be expressed as eq 3:
Figure 7 shows that the maximum reflection loss of the Fe/ SiO2 nanocapsules reaches −14.5 dB at 15.4 GHz, and the absorption band less than −10 dB is from 8 to 18 GHz with a thickness range of 1.46−2.87 mm. Replacing the Fe cores, the maximum reflection loss of Ni/SiO2 nanocapsules shifts to lower frequency range with the optimized value of −25 dB at 6.6 GHz, and the absorption band less than −10 dB is at 2−10.8 GHz. Evidently, such manipulation of electromagnetic wave absorption is ascribed to the different magnetic cores of nanocapsules, which are related to the complex permeability (magnetic losses). Furthermore, Fe/B2O3 and Ni/B2O3, nanocapsules exhibit the maximum reflection losses of −32 dB at 11 GHz and −43 dB at 14.4 GHz, and the absorption band (
