Ag3PO4 nanoparticle-decorated Ni/C nanocapsules with tunable electromagnetic absorption properties

Core/shell-structured nickel/carbon (Ni/C) nanocapsules with Ag3PO4 nanoparticle decoration (Ag3PO4@Ni/C) are prepared by an arc-discharge process and an ion-exchange process. The Ag3PO4@Ni/C nanocapsules show a clear decoration of Ag3PO4 nanoparticles of 4–20 nm diameter on the C shell of the Ni/C nanocapsules of ∼60 nm diameter. The amount of Ag3PO4 nanoparticles that can be decorated on the Ni/C nanocapsules depends on the volume of Na2HPO4 reactant used in the ion-exchange process. The Ag3PO4@Ni/C nanocapsules demonstrate interestingly high and tunable electromagnetic absorption properties with different amounts of Ag3PO4 nanoparticle decoration in the paraffin-bonded composites over the 2–18 GHz microwave range. The nanocapsules prepared with 100 ml Na2HPO4 exhibit much enhanced dielectric and magnetic losses for an improved electromagnetic impedance match. These result in a large reflection loss (RL) of -31.4 dB at 12.3 GHz for a small absorber thickness of 2.6 mm in conjunction with a very wide eff...


I. INTRODUCTION
The rapid development and utilization of electronic and communication devices have caused serious electromagnetic (EM) radiation and interference problems in our society. 1 High-performance EM absorbers, featuring simultaneously strong absorption, wide absorption bandwidth, small thickness, and low density in the gigahertz microwave range, have become increasingly important and demanding. [2][3][4] Core/shell-structured nanocapsules, comprising a magnetic nanoparticle core and a dielectric shell of nanometer size, have attracted considerable attention because of their synergetic effects between dielectric and magnetic losses for absorbing EM waves. [1][2][3][4][5][6][7] Among various types of core/shell-structured nanocapsules, nickel/carbon (Ni/C) nanocapsules are regarded as a highpotential candidate due to their simple material type, simple preparation process, interesting dielectric and magnetic properties, good EM impedance match, and high EM absorption properties, etc. 1,2,6 For instance, a Ni/C nanocapsule absorber with 2.0 mm thickness exhibits a large reflection loss (RL) of ∼-32 dB at ∼13 GHz and a wide effective absorption bandwidth (for RL<-10 dB) of ∼4.3 GHz (i.e.,11.2-15.5 GHz). 6 However, the major obstacle to widespread use of the core/shell-structured nanocapsules in EM absorbers is their almost untunable EM absorption properties constrained by the limited material phase variety in the single core/shell structure. 1 In fact, once the material phases are fixed for the core and the shell in such a nanocapsule (e.g., Ni/C nanocapsule), its EM absorption properties are unable to be further tuned.
Semiconductor materials are known to have complex and rich electronic transport properties. 3 By introducing appropriate semiconductor nanoparticles onto/into the core/shell-structured nanocapsules, it can modify their EM absorption properties and lead to new physics. Ag 3 PO 4, which has complex and rich electronic transport properties with a wide bandgap of 2.45 eV, is regarded as a simple and important type of photocatalytic semiconductor materials. 2 It was recently reported to possess a high relative permittivity of ∼20 in the 100 Hz-10 MHz range at room temperature due to the existence of a strong space charge polarization. 3 By introducing controlled amounts of Ag 3 PO 4 nanoparticles onto/into the Ni/C nanocapsules, it is likely to tune/fine-tune their EM absorption properties and to realize tunable EM absorbers.
In this paper, we aim to study the effect of decoration of Ag 3 PO 4 nanoparticles on the EM absorption properties of the core/shell-structured Ni/C nanocapsules. Accordingly, Ni/C nanocapsules decorated with different amounts of Ag 3 PO 4 nanoparticles (denoted as Ag 3 PO 4 @Ni/C) are prepared by an arc-discharge process followed by an ion-exchange process. The amount of Ag 3 PO 4 nanoparticle decoration is adjusted using different volumes of Na 2 HPO 4 reactant in the ion-exchange process. The phase, morphology, microstructure, composition, and charge state of Ni element of the Ag 3 PO 4 @Ni/C nanocapsules are investigated, while their EM absorption properties are evaluated in the corresponding paraffin-bonded composites with 50 wt.% Ag 3 PO 4 @Ni/C over the 2-18 GHz microwave range. Interestingly high and tunable EM absorption properties are observed and discussed at different amounts of Ag 3 PO 4 nanoparticle decoration to provide a new prospective for realizing tunable EM absorbers in general and for fine-tuning their properties to meet application requirements in specific.

II. EXPERIMENTS
Ni/C nanocapsules were prepared by an arc-discharge process. 1,8 A Ni ingot of 99.9% purity was placed in a water-cooled copper crucible as the anode, while a tungsten needle of 5 mm diameter was employed as the cathode. After evacuating the arc-discharge chamber to 5 mPa, ethanol of 40 ml was introduced into the chamber as the C source. Meanwhile, Ar gas of 20 kPa and H 2 gas of 10 kPa were added into the chamber as the plasma source. An arc-discharge current of 110 A was applied to the chamber for ∼30 min to ensure a sufficient evaporation of the Ni ingot. The residual gas was then pumped out, and the black powders formed on the inner surface of the chamber were collected as the Ni/C product after they were passivated in N 2 gas for 10 h. Ag 3 PO 4 nanoparticles were prepared on the surface of the as-prepared Ni/C product using an ionexchange process. 8 Ni/C product of 0.25 g was dispersed into Na 2 HPO 4 hydrate solution of controlled volume under mechanical stirring. AgNO 3 solution of 12 mol/ml was added into the mixture of Ni/C product and Na 2 HPO 4 hydrate solution of 3 mol/ml using an AgNO 3 :Na 2 HPO 4 volume ratio of 1:2 at a titration rate of 100 ml/h under vigorous stirring for 1 h. The precipitates were collected by centrifugation, washed several times with distilled water and absolute ethanol, and dried in a vacuum at 60 • C for 12 h as the Ag 3 PO 4 @Ni/C product. The amount of Ag 3 PO 4 nanoparticles being decorated on the Ni/C product was adjusted using different volumes of Na 2 HPO 4 reactant. For ease of discussion, the Ag 3 PO 4 @Ni/C products (or nanocapsules) prepared in 100, 200, and 300 ml Na 2 HPO 4 are denoted as NC 100 , NC 200 , and NC 300 , respectively.
The phase analysis of NC 100 , NC 200 , and NC 300 was performed by an X-ray diffractometer (XRD, Bruker D8 Advance) with monochromatic Cu − Kα radiation (λ = 1.54 Å). Their morphology and microstructure were evaluated by a transmission electron microscope (TEM, JEOL 2010) at an emission voltage of 200 kV. Their compositions were analyzed using an energy dispersive spectrometer (EDS) integrated with the TEM. The charge state of Ni element in the cores was examined using an X-ray photoelectron spectroscope (XPS, ESCALAB-250) after the products were etched for 20 s. To enable measurements of the EM absorption properties of NC 100 , NC 200 , and NC 300 , 50 wt.% of NC 100 , NC 200 , and NC 300 was respectively mixed with 50 wt.% of EM wave-transparent liquid paraffin and hexane by ultrasonic waves, 1 and the mixtures were pressed into toroidal composites with an outer diameter of 7.00 mm, an inner diameter of 3.04 mm, and a thickness of 2.00 mm after the evaporation of hexane. For convenient discussion, the composites fabricated with NC 100 , NC 200 , and NC 300 are denoted as PC NC100 , PC NC200 , and PC NC300 , respectively. The complex relative permittivity (ε r = ε r − jε r ) and permeability (µ r = µ r − j µ r ) of PC NC100 , PC NC200 , and PC NC300 were measured by a transmission/reflection coaxial line method in the 2-18 GHz microwave range covering the full S, C, X, and Ku bands of microwaves using a vector network analyzer (Anritsu 37269D) with short, open, load, and thru calibrations. The frequency ( f ) dependence of reflection loss (RL) was determined from the measured ε r and µ r spectra using 1 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 shows the XRD patterns of typical NC 100 , NC 200 , and NC 300 . The Ni phase is identified by the characteristic diffraction peaks of Ni (JCPDS card No. 04-0850), while the other diffraction peaks match well with the body-centered cubic phase of Ag 3 PO 4 (JCPDS card No. 06-0505). The observation suggests a constitution of Ni and Ag 3 PO 4 in the Ag 3 PO 4 @Ni/C products. The intensity ratio of Ni (111) peak to Ag 3 PO 4 (210) peak in NC 100 , NC 200 , and NC 300 is found to be 1.34, 1.00, and 0.55, respectively, indicating an increase in the amount of Ag 3 PO 4 nanoparticle decoration with increasing the volume of Na 2 HPO 4 reactant used for preparing NC 100 (100 ml), NC 200 (200 ml), and NC 300 (300 ml) in the ion-exchange process. The weaker Ag 3 PO 4 diffraction peaks in NC 100 and then NC 200 compared to NC 300 implies the existence of a lower degree of crystallization in Ag 3 PO 4 for NC 100 and NC 200 than for NC 300 . No diffraction peaks in relation to Ni oxides are detected in all products so that the C shells are effective in protecting the Ni cores from oxidation. 1,7 Also, no diffraction peaks are observed for C due to the breaking of the translational symmetry along the radial direction. 1,7 Figures 2(a), 2(b), and 2(c) illustrate the TEM images of typical NC 100 , NC 200 , and NC 300 , respectively. It is clear that NC 100 , NC 200 , and NC 300 consist of different amounts of small spherical nanoparticles of 4-20 nm diameter decorated on the surface of the relatively larger spherical nanoparticle of ∼60 nm diameter. Our in-situ TEM observation found that the amount of small nanoparticle decoration increases from 12 in NC 100 to 25 in NC 200 and then 35 in NC 300 (not shown), so that it can be effectively controlled by the volume of Na 2 HPO 4 reactant adopted in the ion-exchange process. The diameter of the small nanoparticles was observed to be 4-6 nm in NC 100 , 15-20 nm in NC 200 , and 8-15 nm in NC 300 . The reason why NC 300 has smaller particle sizes than NC 200 may be   Fig. 2(a). It is seen that a small spherical nanoparticle of ∼5 nm diameter is firmly grown on the shell of a relatively larger spherical nanoparticle having a typical core/shell structure in which the nanoparticle core is encapsulated by an onion-like shell of ∼2 nm thickness. The variation of onion-like shell thickness was found to have no obvious correlation with the nanoparticle core size based on statistical approach. Nonetheless, the lattice plane spacing of the decorated small nanoparticles is found to be ∼0.24 nm. This value agrees with the d-spacing of {211} planes of Ag 3 PO 4 , confirming the successful preparation of Ag 3 PO 4 nanoparticles of 4-20 nm diameter on the C shell of the Ni/C nanocapsules of ∼60 nm diameter. Figure 2(e) gives a typical XPS spectrum of the Ni cores after etching NC 100 for 20 s. The two XPS peaks observed at 582.5 and 870.0 eV, corresponding to the Ni 2p3/2 and Ni 2p1/2 electron states, respectively, indicate the existence of zero charge state of Ni element in the Ni cores. Figure 2(f) provides a typical EDS pattern of NC 100 . The detection of Ag, P, O, Ni, and C elements further evidences the co-existence of Ag 3 PO 4 and Ni/C in our Ag 3 PO 4 @Ni/C products. Figure 3(a) shows the measured f dependence of ε r = ε r − jε r of PC NC100 , PC NC200 , and PC NC300 . Recalling that ε r and ε r represent the amount of polarization and the level of energy dissipation, respectively. 9 ε r and ε r of all composites demonstrate similar quantitative decreasing trends with increasing f in the 2-6 GHz range, except for the broad dielectric resonance in the 6-18 GHz range. The weakening of the ε r and ε r responses is mainly caused by the lagging of dipole polarization response with respect to the change in electric field at elevated frequencies. 7,10 This is the typical behavior of a dielectric material outside the vicinity of a dielectric resonance. Following the effective-medium theory of three-phase inclusions, the dielectric resonance behavior originates mainly from the special geometries and structures of the inclusions of Ag 3 PO 4 @Ni/C nanocapsules, the intrinsic permittivity of each component in the composite (e.g., Ni, C, Ag 3 PO 4 , and paraffin), and the dispersion of the nanocapsules. 1,11 Nevertheless, the values of ε r and ε r in PC NC100 are generally larger than those of PC NC300 and PC NC200 . According to the free electron theory, 12 ε r ≈ 1/2π ρf , where ρ is the resistivity. A small ε r implies a high ρ. The largest ε r in PC NC100 compared to PC NC300 and then PC NC200 indicates the presence of the lowest ρ in PC NC100 than in PC NC300 and then in PC NC200 . As the effective ρ depends on the distance between Ni/C nanocapsules, the size of Ag 3 PO 4 nanoparticles shall have the determinative effect on the effective ρ. Therefore, the use of smaller sized Ag 3 PO 4 nanoparticles can induce a lower ρ and hence a larger ε r . 3. Measured f dependence of (a) ε r = ε r − jε r and (b) µ r = µ r − jµ r as well as calculated f dependence of (c) tan δ e = ε r /ε r and (d) tan δ m = µ r /µ r for PC NC100 , PC NC200 , and PC NC300 . Figure 3(b) shows the measured f dependence of µ r = µ r − j µ r of PC NC100 , PC NC200 , and PC NC300 . µ r of all composites shows similar quantitative decreasing trends in the 2-18 GHz range due to the relaxation of magnetic moment procession. 6,7 However, µ r of all composites increases slightly in the 2-4.3 GHz range and then decreases in the 4.3-18 GHz range because the magnetic resonance peaks at ∼4 GHz. It is noted that µ r of PC NC100 is smaller than that of PC NC200 and PC NC300 , especially for f >10 GHz. This is because the amount of Ag 3 PO 4 nanoparticle decoration in PC NC100 is relatively few (i.e., 12 for NC 100 , 25 for NC 200 , and 35 for NC 300 ) and the decorated Ag 3 PO 4 nanoparticles are not strong enough to protect the magnetic Ni cores in the 10 GHz range. However, µ r of PC NC100 is larger than that of PC NC200 and PC NC300 since the decoration of Ag 3 PO 4 nanoparticles can dilute the effective magnetic response in the gigahertz range. In a general sense, the magnetic loss in a magnetic material can be attributed to domain-wall displacement, eddy-current loss, natural resonance, and/or exchange resonance. 1 The domain-wall resonance occurs only in multi-domain materials in the 1-100 MHz range. 9 The eddy-current loss is usually excluded because of the size effect. 1 The exchange resonance happens in the high-frequency microwave range in excess of 10 GHz in accordance with Ahsroni's theory. 9 Thus, the magnetic resonance observed at ∼4 GHz is mainly due to natural resonance. Figures 3(c) and 3(d) show the calculated f dependence of dielectric loss tangent (tan δ e = ε r /ε r ) and magnetic loss tangent (tan δ m = µ r /µ r ) of PC NC100 , PC NC200 , and PC NC300 based on the data in Figs. 3(a) and 3(b), respectively. These loss tangents are the main loss contributors to the absorption ability of an EM absorber. 7 The higher the value, the larger the loss is. It should be noted that tan δ e of all composites in Fig. 3(c) exhibits similar quantitative trends with their respective ε r in Fig. 3(a). This similarity supports our previous discussion that the size of Ag 3 PO 4 nanoparticles has a crucial role in determining ρ and hence ε r and tan δ e . On the other hand, tan δ m in Fig. 3(d) decreases steadily with increasing the amount of Ag 3 PO 4 nanoparticle decoration so that it can be effectively tuned by the amount of Ag 3 PO 4 nanoparticle decoration. Both tan δ e and tan δ m are generally larger in PC NC100 than in PC NC200 and PC NC300 , suggesting the existence of a higher EM absorption ability in PC NC100 .  6 Recalling that the EM absorption performance of an absorber relies on the degree of EM impedance match, which can be characterized by the delta function expressed as ∆ = sinh 2 (Kfd) M , where K and M depends on ε r andµ r . 10 A smaller ∆ value implies a higher degree of EM impedance match. Figure 5 presents the calculated 3D ∆-f -d mapping plots of PC NC100 , PC NC200 , and PC NC300 . The ∆ values in PC NC100 are generally less than 0.6 (blue areas) ( Fig. 5(a)), reflecting the presence of a high degree of EM impedance match. The degree of EM impedance match is comparatively lower in PC NC300 and then in PC NC200 (Figs. 5(b) and 5(c)). It is noted that these 3D ∆-f -d mapping plots show very similar tendency with the 3D RL-f -d mapping plots in Fig. 4. The high similarity confirms that the enhancement in EM absorption performance in PC NC100 is a result of the enhanced dielectric and magnetic losses for an improved EM impedance match via the decoration of an appropriate amount of Ag 3 PO 4 nanoparticles onto the Ni/C nanocapsules by using an appropriate volume of Na 2 HPO 4 reactant in the ion-exchange process. Ag 3 PO 4 nanoparticles may play a dual role in this case. First, they adjust the impedance between permittivity and permeability to make more EM waves traveling into the absorber. Second, they introduce additional interfaces to enhance dielectric and magnetic losses in the absorber.

IV. CONCLUSION
We have applied an arc-discharge method and an ion-exchange method to prepare three different types of Ag 3 PO 4 nanoparticle-decorated, core/shell-structured Ni/C nanocapsules (Ag 3 PO 4 @Ni/C nanocapsules, denoted as NC 100 , NC 200 , and NC 300 ) having three different amounts of Ag 3 PO 4 nanoparticles (12, 25, and 35) using three different volumes of Na 2 HPO 4 reactant (100, 200, and 300 ml) in the ion-exchange process. We have also investigated the phase, morphology, microstructure, composition, and charge state of Ni element of NC 100 , NC 200 , and NC 300 in addition to evaluating their EM absorption properties in the corresponding paraffin-bonded composites (PC NC100 , PC NC200 , and PC NC300 ) in the 2-18 GHz microwave range. The Ag 3 PO 4 @Ni/C nanocapsules have been found to have a clear decoration of Ag 3 PO 4 nanoparticles of 4-20 nm diameter on the C shell of the Ni/C nanocapsules of ∼60 nm diameter as well as an interestingly high and tunable EM absorption properties with different amounts of Ag 3 PO 4 nanoparticle decoration. Among them, the one prepared with 100 ml Na 2 HPO 4 (NC 100 or PC NC100 ) has exhibited much enhanced dielectric and magnetic losses for an improved EM impedance match, giving rise to a large RL of -31.4 dB at 12.3 GHz and 2.6 mm as well as a very wide effective absorption bandwidth (for RL<-10 dB) of 14 GHz (4-18 GHz) at a wide d range of 1.4-5.0 mm. This tunable effect has been discussed to provide a new prospective for realizing tunable EM absorbers.