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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 15104−15111
Excellent NiO−Ni Nanoplate Microwave Absorber via Pinning Effect of Antiferromagnetic−Ferromagnetic Interface Wenbin You and Renchao Che* Laboratory of Advanced Materials, Department of Materials Science, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, 220 Handan Road, Shanghai 200433, China
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S Supporting Information *
ABSTRACT: Materials with strong magnetic property that can provide excellent microwave absorption performance are highly desirable, especially if their dielectric and magnetic properties can be easily modulated, which make minimal thickness and ultrawide bandwidth become achievable. The magnetic properties of ferromagnetic (FM) and antiferromagnetic (AFM) composite materials are closely related to their ratio of composition, size, morphology, and structure. AFM−FM composites have become a popular alternative for microwave absorption; however, the controllable design and preparation need to be urgently optimized. Here, we have successfully prepared a series of platelike NiO−Ni composites and demonstrated the potential of such composites for microwave absorption. Strong magnetic coupling was found from NiO−Ni nanoparticles by electron holography, which makes NiO−Ni composites a highly efficient microwave absorber (strong reflection loss: −61.5 dB and broad bandwidth: 11.2 GHz, reflection loss < −10 dB). Our findings are helpful to develop a strong microwave absorber based on magnetic coupling. KEYWORDS: microwave absorption, AFM−FM composite, platelike, magnetic coupling, electron holography
1. INTRODUCTION The electromagnetic (EM) pollution problem has lately become worse due to the popularity of telecommunications equipment, digital systems, and electronic devices.1,2 Microwave absorption materials directly convert electromagnetic (EM) wave energy into thermal energy or dissipate EM wave via interference.3,4 Magnetic metal-based absorbers have been attracting intense attention because of their high efficiency and easy convenience.5 An effective absorber must both sufficiently reduce microwave reflection and effectively work in different antidetection wavebands.6,7 The primary target of absorber design is impedance matching, which makes microwaves penetrate into materials without surface reflection.8 The classical method is to combine a special ratio of dielectric materials (e.g., SiO2,8 TiO2,9 carbon10) and magnetic materials (e.g., Fe3O4,11 Ni,12 Co13) via core−shell structure, multilayer design, and carrier loading. The secondary important point of structure design is to provide internal scattering14 via void15 or interface16 for microwave energy dissipation. However, the equally important modulation to the intrinsic dielectric and magnetic properties of the absorbing material itself has been less studied, which can actually adjust the microwave absorption performance.17 Especially, achieving strong absorption performance via magnetism modification still remains a great challenge. In essence, the material magnetization arises mainly from the movement or displacement of electric charge. Previously, tuning of the magnetic property of an absorber could be © 2018 American Chemical Society
realized by optimizing the synthesis parameters to fabricate materials with special morphology and suitable grain size.18,19 Controlling the magnetic core size is another way of tuning the magnetic property of an absorber with a core−shell structure. However, the above routines are usually too complicated to facilely produce lightweight, low-cost absorbers.2,15 Remarkably, transition metals conjugated with their oxides used as ferromagnetic (FM) and antiferromagnetic (AFM) materials have become a popular alternative for microwave absorption20,21 because the magnetic property of AFM−FM composites can be easily controlled by tuning the composition ratio, size, morphology, and structure.22,23 In addition, many interesting phenomena are exhibited in the heterojunction interface of AFM−FM, such as the exchange bias effect and giant magnetoresistance effect, which finally exhibit unexpected magnetism behavior in materials23,24 and may benefit microwave absorption. The usual procedures followed to obtain AFM−FM nanocomposite absorbers are creating an oxide shell out of FM nanoparticles (e.g., Fe, Co, and Ni) by different techniques,25−28 such as thermal decomposition and chemical reduction. Among all transition metals, nickel29 has attracted extensive interests because its oxide (NiO) shows a high Néel temperature (523 K of bulk materials), which makes it AFM Received: March 3, 2018 Accepted: April 13, 2018 Published: April 13, 2018 15104
DOI: 10.1021/acsami.8b03610 ACS Appl. Mater. Interfaces 2018, 10, 15104−15111
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) SEM image of Ni(OH)2 nanoplate and insets are its TEM image; (b) HRTEM image of the nanoplate and inset curve is its XRD pattern; (c−e) TEM images of Ni(OH)2@polydopamine precursor with different polymer layer thicknesses: (c) ∼1 nm, (d) ∼2 nm, and (e) ∼3 nm. (C8H11NO2·HCl) were obtained from Sigma-Aldrich. Sodium hydroxide (NaOH), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), and ethanol (C2H5OH) were ordered from Shanghai Chemical Reagent Co. Ltd. All chemicals were used directly at analytical grade without further purification. Deionized water, which came from MilliQ system (Millipore, Bedford, MA), was used in all experiments. 2.2. Preparation of Ni(OH)2 Hexagonal Plates. Uniform hexagonal plate Ni(OH)2 was synthesized through a common hydrothermal synthesis. Typically, 30 mL of 0.5 M NaOH solution was added into 10 mL of 2 M Ni(NO3)2·6H2O solution; then, the mixture was mixed with vigorous stirring, which was diverted to a Teflon-lined stainless steel autoclave (50 mL), and maintained at 180 °C in an electric oven for 8 h. The reactor was cooled down naturally to room temperature finally. The precipitate was washed with distilled water and ethanol several times and collected by centrifugation with a centrifuge and then dried in a vacuum oven 60 °C overnight. 2.3. Synthesis of Ni(OH)2@Polydopamine Hexagonal Plates. As-prepared Ni(OH)2 (400 mg) and C8H11NO2·HCl (200 mg) were successively dispersed into the 200 mL Tris buffer solution (10.0 mM, pH = 8.5) with magnetic stirring for 40, 80, and 120 min at room temperature (25 °C) for different thicknesses of polydopamine (PDA) layer. The fulvous Ni(OH)2@polydopamine core−shell nanocomposite was collected after rinsing and centrifugation and then dried in a vacuum oven for 12 h. 2.4. Synthesis of Ni, NiO, and NiO−Ni Plates. The NiO−Ni nanocomposites were obtained after a high-temperature reduction of the precursors in a furnace under a flow of pure N2 atmosphere for 3, 8, and 15 h at 700 °C for the precursor with different thicknesses of polydopamine layers. These NiO−Ni nanocomposites were denoted P-2, P-3, and P-4 following the different layer thicknesses of PDA. Unadulterated NiO was obtained when nanoplates Ni(OH)2 calcine in a furnace under a flow of air atmosphere at 550 °C for 2 h, whereas pure Ni was obtained by reducing as-prepared NiO in a furnace under a flow of H2/N2 (5% H2) atmosphere at 500 °C for 1.5 h. 2.5. Characterization. Bruker D8 X-ray diffractometer (Germany) equipped with Ni-filtered Cu Kα radiation (40 kV, 40 mA) was used to characterize the phase content of the products. Morphologies of the samples were analyzed by a field-emission scanning electron microscope (Hitachi S-4800, acceleration voltage of 1 kV) and a field-emission transmission electron microscope (TEM, JEM-2100F, acceleration voltage of 200 kV). Magnetic property was determined
under room temperature.30 Moreover, compared to the nanoparticles of Fe31 and Co32 in air, the Ni nanoparticles are more stable and less likely to be oxidized spontaneously. In addition, the Ni−NiO nanocomposites can exhibit high Hc value due to the existence of NiO,33 which can offset the paramagnetism of Ni nanoparticles and ensure potential magnetic loss.23 At the same time, NiO works not only as an AFM material but also as a dielectric material, which ensures the impedance matching with Ni.20 However, conventional synthesis methods of Ni−NiO composites usually obtain an uncertain quantity of NiO and no uniformity between different particles.34 The unstable magnetic property of previous Ni− NiO composites makes it difficult to be used for microwave absorption. As far as we know, the absorption mechanism of Ni−NiO composite absorber has been rarely reported. Herein, we report a facile and controllable process for the fabrication of Ni−NiO nanocomposites, which exhibit excellent microwave absorption performance. The Ni−NiO heterojunction was obtained onside the surface of the NiO nanoplate by the reduction of Ni(OH)2@polydopamine (PDA). The content of Ni was controlled by tuning the polymer layer thickness of the Ni(OH)2@polydopamine precursor. The electromagnetic parameters display the influence of magnetic microstructure and the content of Ni (FM) on the magnetic properties of the AFM−FM system by our electron holography analysis. Our Ni−NiO nanocomposites have additional advantages such as easy processing, strong reflection loss (RL) (−61.5 dB, coating thickness = 3.24 mm at 6.5 GHz), and broad bandwidth (11.2 GHz, RL < −10 dB, coating thickness as thin as 2 mm). These findings pave the way for the microstructure design strategy of high-performance microwave absorbers and shed insights on the magnetic property research of an AFM−FM system.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Tris(hydroxymethyl)aminomethane (C 4 H 11 NO 3 ) and dopamine hydrochloride 15105
DOI: 10.1021/acsami.8b03610 ACS Appl. Mater. Interfaces 2018, 10, 15104−15111
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a, c) TEM image of pure NiO nanoplate and pure Ni nanoplate; (b, d) HRTEM image of pure NiO nanoplate and pure Ni nanoplate; (e, f, h) TEM image of NiO−Ni nanoplate corresponding to the precursor with different polymer layer thicknesses after reduction: (e) ∼1 nm, (f) ∼2 nm, and (h) ∼3 nm; and (g) higher magnification TEM image of (f).
Figure 3. (a) XRD patterns of five nanoplate samples and (b) corresponding components of the five samples. with electron holography (JEM-2100F) and a superconducting quantum interference device magnetometer (MPMS VSM, Quantum Design Company). Electromagnetic parameters were measured with an Agilent N5230C vector network analyzer, and more details about electromagnetic measurements can be found in the Supporting Information (Section S1).
(101) planes, respectively, of the ophrastite phase (JCPDS card #14-0117). Briefly, our product can be reasonably defined as a truncated hexagonal prism, with a compressed structure along the [001] orientation. A polydopamine (PDA) sacrificial layer is homogeneously coated on the surface of Ni(OH)2 hexagonal plates at room temperature used for the transition from Ni(OH)2 template to NiO−Ni nanocomposite. Specifically, the PDA layer thickens (≈1, ≈2, and ≈3 nm) with the change in polymerization reaction time from 40 and 80 to 120 min (Figure 1c−e). After a high-temperature reduction of the precursors in a furnace under a flow of N2 atmosphere, a slight morphology distortion and particle aggregation of the hexagonal plate could be observed from the TEM images (Figure 2e,f,h). Some nanoparticles appear on the surface of plates. With the PDA layer becoming thicker, the reduced Ni particles become much larger. Nanoparticle lattice spacing (inset circle of Figure 2g) is measured to be 0.20 and 0.12 nm, which could be, respectively, indexed to the (111) and (220) lattice planes of fcc Ni (Figure 2d) and those of the substrate (inset square of Figure 2g) are 0.24 and 0.15 nm, respectively (Figure 2b), in agreement with the fcc NiO. These NiO−Ni nanocomposites were denoted P2, P-3, and P-4 following the different layer thicknesses of PDA.
3. RESULTS AND DISCUSSION The Ni(OH)2 hexagonal plates were synthesized as templates via a simple hydrothermal method following the reported literature procedure with minor modifications.35 From the scanning electron microscopy (SEM) image, the specimen can be observed to have a truncated symmetric hexagonal shape (Figure 1a), which is more distinct in the representative TEM images with an edge length of 80 nm and a thickness of 30 nm (inset of Figure 1a). The high-resolution TEM (HRTEM) viewed along the c axis of the specimen (Figure 1b) provides information that it is the hcp crystal structure with a lattice distance of 0.27 nm matching with the directions of [1000], [0100], and [0010]. Crystallographic structures of the assynthesized plates were further verified by powder X-ray diffraction (XRD), which showed a typical nickelous hydroxide without any impurities. The sharp diffraction peaks of the assynthesized samples (inset of Figure 1b) located at 19.26, 33.06, and 23.54 can be well-indexed to the (001), (100), and 15106
DOI: 10.1021/acsami.8b03610 ACS Appl. Mater. Interfaces 2018, 10, 15104−15111
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a−c) Off-axis electron holography of a single nanoplate: (a) P-1, (b) P-3, and (c) P-5. (d) Magnetic coupling between several neighbor NiO−Ni nanocomposites (A1−A6 corresponding to P-3). The insets show the corresponding TEM images and the phase amplification is 5.
field of 500 Oe due to no maximum observed in ZFC magnetization (Figure S2). In this experiment, the nanocomposites without treatment of field-cooling from the Néel temperature do not exhibit exchange bias effect at room temperature. However, the pinning effect of the antiferromagnetic layer to the ferromagnetic layer still exists. To further study the influence of Ni nanoparticles to magnetization on NiO surface, off-axis electron holography (Figure 4) was carried out to analyze the original magnetic properties of the assynthesized samples, P-1, P-3, and P-5 (more details in the Supporting Information, Section S2). For the reconstructed phase images of holography with high resolution can provide more magnetic detail, which can correspond to the magnetic flux distribution outer the magnetite particle. P-1 (Figure 4a) exhibits no external magnetic stray field due to the antiferromagnetic NiO phase, which means less possibility of magnetic interaction with other NiO particles. However, the sparse magnetic contours around P-5 (Figure 4c) demonstrates the paramagnetism of the Ni nanoplate. Remarkably, P-3 (Figure 4b), which is supposed to be poorly magnetic compared to P-5, exhibits a stronger external magnetic field (the even denser distribution of magnetic flux lines). These magnetic flux lines may originate from Ni nanoparticles in the edge and side face of the nanoplate. The pinning effect of antiferromagnetic NiO makes paramagnetic Ni present some degree of ferromagnetism. Moreover, the stronger external magnetic field of P-3 also matches with the greater interference with the neighbor nanoplates (double-sided arrow in Figure 4d) compared to that of P-5. The frequency-dependent equivalent complex relative permeability (real part μ′ and imaginary part μ″) and
Both NiO (Figure 2a, denoted P-1) and Ni (Figure 2d, denoted P-5) nanoplates were obtained when Ni(OH)2 was, respectively, calcined under a flow of air and flow of H2/N2 (5% H2) atmosphere. Some pores appear in the NiO plates after calcination, whereas a slight morphology distortion happens in reduced Ni. The powder XRD patterns of P-1 to P-5 (Figure 3a) can be accurately identified. P-1 can be assigned to fcc NiO (JCPDS card #47-1049) with five peaks at 2θ = 37.25, 43.28, 62.86, 75.40, and 79.39 corresponding to planes of reflection (111), (200), (220), (311), and (222), respectively. P-5 matches with fcc Ni (JCPDS card #04-0850) containing three peaks at 2θ = 44.491, 51.851, and 76.381 corresponding to planes of reflection (111), (200), and (220), respectively. In addition, eight mixed XRD peaks were detected from P-2, P-3, and P-4, which confirms their hybrid phase character. With the PDA layer thickness increasing from 1 to 3 nm, the intensity of Ni peaks gradually enhances. The relative contents of Ni and NiO are given in the table of Figure 3b, which was calculated by the Rietveld refinement of XRD patterns. Researchers have found that the magnetic property of Ni and NiO mixture, such as saturation magnetization, mainly depends on the ferromagnetic Ni content.23 However, if Ni and NiO are bonded with each other (e.g., Ni film on a NiO substrate), the hysteresis loops may shift after field-cooling (FC) lower than the Néel temperature of antiferromagnetic NiO,27 which was defined as the exchange bias effect. This is attributed to the pinning effect of the antiferromagnetic layer to the ferromagnetic layer. The temperature dependence (from 300 to 5 K) of the magnetization curves of P-3 during both zerofield-cooling (ZFC, in white) and field-cooling (FC, in black) implies that the blocking temperature is higher than 300 K in a 15107
DOI: 10.1021/acsami.8b03610 ACS Appl. Mater. Interfaces 2018, 10, 15104−15111
Research Article
ACS Applied Materials & Interfaces
Figure 5. Frequency dependence of the real and imaginary parts of complex permittivity (ε) and permeability (μ) of five samples (a) μ′, (b) μ″, (c) ε′, and (d) ε″. All of the parameters were measured at 300 K.
permittivity (real part and ε′ imaginary part ε″) were characterized on a network analyzer to better understand the performances of the five samples in a microwave field (Section S1). The μ′ value of pure NiO (P-1) obviously decreases in the 2−8 GHz range and then almost stabilizes in the 8−18 GHz range (Figure 5a). In addition, the μ″ value does not exhibit an obvious peak at 8 GHz (Figure 5b). A similar analysis can be used for pure Ni (P-5), except for an even sharply decreasing tendency. However, for P-3, the μ′ value is almost unchanged in 2−18 GHz, which can be attributed to the quite stable pinning effect of the AFM phase (NiO) to FM phase (Ni). In addition, the μ′ values of P-2 and P-4 show a decreasing tendency due to both their contents of phases and interfacial areas of AFM−FM. Moreover, the μ′ values of nanocomposites (P-2, P-3, P-4) are larger than that of the pure Ni corresponding to their strong external magnetic stray. However, the μ″ values of nanocomposites exhibit complex variation tendencies compared with each other because of the existence of interactions between neighbor particles and interfaces between AFM and FM. Furthermore, the variations of both ε′ and ε″ of the five samples, especially the nanocomposites, are negligibly small (Figure 5c,d) and the higher values of P-1 and P-5 can be attributed to their highly conductive nature with less interfaces. Apparently, the excellent micromagnetic performance and higher μ′ and μ″ values of nanocomposites promise the enormous potential of magnetic loss contribution from the samples to the microwave absorption. The three-dimensional images (Figure 6) of the samples at the applied frequency (2−
18 GHz)-dependent reflection loss (RL) value of three samples with a coating layer (thickness 2−5 mm) reveal the microwave absorption properties of the as-synthesized samples (Section S1). As for the pure NiO (P-1, Figure 6a) and Ni (P-5, Figure 6e) nanoplates, they exhibit poor performance of microwave absorption in the range of 2−18 GHz from thickness 2 to 5 mm. Better performances are obtained only with a rather thicker coating layer (5 mm) at low microwave frequency. In comparison, the nanocomposites (P-2, P-3, P-4) show both excellent absorption strength and effective absorption bandwidth (
