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The porous 3-Dimensional flower-like Co/CoO and its excellent electromagnetic absorption properties Hualiang Lv, Xiaohui Liang, Guangbin Ji, Haiqian Zhang, and Youwei Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b01654 • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 17, 2015
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ACS Applied Materials & Interfaces
The Porous 3-Dimensional Flower-like Co/CoO and Its Excellent Electromagnetic Absorption Properties Hualiang Lv†, Xiaohui Liang†, Guangbin Ji*,†, Haiqian Zhang†, and Youwei Du‡ †
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, Peoples R China ‡ Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, Peoples R China
ABSTRACT: The porous 3-Dimensional flower structures assembled by numerous ultra-thin flakes were favor for strengthen electromagnetic absorption capability. However, it still remains a big challenge to fabricate such kind of materials. In this study, an easy and flexible two-step method consisting of hydrothermal and subsequent annealing process have been developed to synthesize the porous 3-D flower-like Co/CoO. Interestingly, we found that the suitable heat treatment temperature played a vital role on the flower-like structure, composition, and electromagnetic absorption properties. In detailed, only the composite treated with 400 o C can we gain the porous 3-D flower structure. If the annealing temperature was heated at 300 o C, the Co element was unable to generate. Moreover, when the annealing temperature increased from 400 o C to 500 o C, these flower-like structures was unable kept because the enlarged porous diameter would wreck the flower frame. Moreover, these 3-D porous flower-like structures presented outstanding electromagnetic absorption properties. For example, such special structure enabled an optimal reflection loss value of -50 dB with the frequency bandwidth ranged from 13.8-18 GHz. The excellent microwave absorption performance may attribute to the high impedance matching behavior and novel dielectric loss ability. Additionally, it can be supposed that this micrometer size of flower structure was more beneficial to scatter the incident electromagnetic wave. Meanwhile, the rough surface of the ultrathin-flake is apt to increase the electromagnetic scatting among the leaves of the flower due to their large spacing and porous feature. KEYWORDS: porous 3-Dimensional flower structure, Co/CoO, electromagnetic absorption properties, impedance matching, lightweight
1. INTRODUCTION Arising from the rapid development of electronic devices, wireless communication tools, local area networks, and electromagnetic pollution (EMI) have great threaten our healthy and disturbed various commercial or industrial equipments.1-3 Thus, the electromagnetic absorbent is urgent demand over the past years. It is well known that the electromagnetic absorbent is a type of functional materials which is able to absorb incidence of electromagnetic wave effectively and then convert it into thermal energy. 4-5 The ideal absorbing materials must meet the requirements of lightweight, strong absorption, thin thickness as well as broad frequency. 6 There are several factors, such as morphology, geometry and microstructure, playing an crucial role in determining the electromagnetic absorption properties.7 Currently, different morphologies of absorbents have been fabricated by various methods, containing one-dimensional (nanowires, nanoparticles),8-11 two-dimensional (flake, nanodisks)12-13 and three-dimensional(tubes, sphere, urchinlike structure).14-18 For example, SiC nanowires with 20-80 nm in diameter, several tens of micrometers in length have
been developed by reducing the multiwall carbon nanotubes (MWCNTs) and silicon vapor from molten salt medium. The minimum reflection loss value (RLmin) reached as high as 17.4 dB at a coating thickness of 4 mm.19 Meanwhile, the well-defined hexagonal flake-shape iron with a thickness about 100-500 nm and edge length of 5 µm has been produced by hydrogen reduction of α-Fe2O3 microflakes, and the optimal reflection value was up to 15.3 dB.20 Sun et al. had compared the electromagnetic absorption performance of CoO nanobelts, submicrometer spheres and bulks, and found that the nanobelts have the highest reflection loss value of -12.3 dB at a thickness of 3 mm.21 Therefore, we may conclude that these shapes of absorbent with different sizes and crystal structures pose an vital influence on the electromagnetic absorption which may result from electronic polarization, magnetic loss and so on. Currently, considered attention has been focused on the mono-dispersed flower shape of absorbing materials due to its fascinating advantages. In generally, most of the microwave absorbents have difficulties in improving the permeability in high frequency regions because of its large eddy current (the strong magnetic field inducted by the eddy current can cancel the external magnetic field and cause the low value of permeability and poor
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microwave absorption properties).22-23 However, some reports pointed out that the absorbing materials with the flake-like structure can efficiently suppress the eddy current due to its high resonance peak at high frequency range.24 The 3-D flower structure consisting by numerous of flake-shape has been regarded as an promising microwave absorbing materials. More importantly, the special 3-dimensional structure may scatter the electromagnetic wave among the layers of flower and increase the microwave absorption properties. Nowadays, various of flower shape absorbing materials have been successfully synthesized, such as Ni with a RLmin value of -17 dB at 13 GHz,25 FeNi@C with a RLmim value of -47.6 dB at 3.17GHz. 26 But, these flower-shaped absorbing materials are always consisted by high density of Cobalt or Nickel which is not benefit for lightweight microwave absorbing material. In order to solve this problem, porous materials are regarded as an effective way to meet the demand of light absorption. 27-28 In this study, we designed a novel porous 3D-flower like Co/CoO structure as a new type of microwave absorption materials. These well-defined flower structures consist of numerous large ultrathin flakes which are attributed to exceed Snoek limitation. At the same time, the porous structure with a lower density can be used as lightweight microwave absorption material. In additional, the layer spacing of these flakes in each flower structure is relatively favor for the electromagnetic scattering.
2. EXPERIMENTAL SECTION 2.1. Materials. Cobalt acetate (Co(OAc)2), urea and polyvinyl-pyrrolidone (PVP, K30) were purchased from Nanjing Chemical Reagent Co. Methanol were purchased from the Sinopharm Chemical Reagent Co. All of the chemical regents used in this reagents were analytically pure and without further purification. 2.2. Preparation of porous flower-shaped Co/CoO composite The flower-shaped Co/CoO composite was prepared by a simple solvethermal approach. The typical experimental procedure is as follow: 0.5 g of polyvinyl-pyrrolidone (PVP) was dissolved in 50 mL methanol under magnetic stirring 10 min. Then, 0.5 g cobalt acetate and 0.25 g urea were added into the solution and ultrasonic treatment 20 min to form a clearly red solution. Subsequently, the mixed solution was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL. The temperature of hydrothermal process was carried out at 200 °C for 6 hours and then allowed to cool to ambient temperature. The final products were washed with ethanol. Finally, the product was dried in a vacuum with the temperature of 60 °C. In the second stage, 0.1 g of the asprepared precursor was heat at 400 °C / 500 °C ( marked in S1 and S2) for 2 hours with a slow increase ratio of 1°C /min and the nitrogen gas (N2) as the protective gas.
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scanning current. A Hitachi S4800 type scanning electron microscopy (operating at an acceleration voltage of 3.0 kV and equipping with an energy dispersive X-ray spectroscopy) were used to observe the morphology features and sizes. The morphology of composite was further characterized by transmission electron microscope (TEM, JEM JEOL 2100). The magnetic properties including coercive force and saturation magnetization were performed by a vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series) at room temperature. The XPS spectrum was measured in a PHI 5000 VersaProbe systems using an Al Kα X-ray source and operating at 150 W. The Brunauer-Emmett-Teller (BET, ASAP2020) was utilized to analysis the specific surface and pore size. The S parameter including S11, S12, S21 and S22 will be tested by an Agilent PNA N5224A vector network analyzer using the coaxial-line method which the samples prepared by homogeneously mixing the paraffin wax and sample ( mass ratio was 1:1) and then pressing into toroidalshaped samples (Φout:7.0 mm, Φin:3.04 mm). Afterward, software which has been installed in Agilent PNA can calculate the ε′, ε′′, µ′, µ′′ values. Finally, the RL value with different thickness can be calculated by using the following formulas. 30-31
Z in = Z 0 ur εr tanh[ j ( 2πfd / c )] urεr RL ( dB ) = 20 log ( Zin − Z 0 ) ( Zin + Z 0 )
(1) (2)
Where Zin is the input impedance of the absorber, f is the frequency of electromagnetic wave, d is the coating thickness of the absorber while c is the velocity of electromagnetic wave in free space. Where εr (εr=ε′-jε′′) and µr (µr =µ′-jµ′′) are the complex permittivity and permeability of the absorber
3. RESULTS AND DISCUSSION 3.1 Morphology and microstructure of precursor During the solvethermal process, the precursor salt (Co(OAc)2) can be divided into two reaction process under alkaline solution, as expressed in the following: Co(CH3COO)2+ 2OH- = Co(OH)2 + 2CH3COO(1) Co(CH3COO)2 = CoC2O4 + C2H6 (2) It is worth pointing out that for reaction (2), decomposition of Co(CH3COO)2 should satisfy the conditions of high temperature and high pressure. During the solvethermal process, the sealed autoclave with a high temperature of 200 o C may generate the high pressure and thus result in the occurring of reaction (2).
2.2. Characterizations. Phase analysis was tested by the powder X-ray diffraction (XRD) patterns (Bruker D8 ADVANCE X-ray diffractometer) using Cu Kα radiation (λ=0.154178 nm with 40 kV scanning voltage, 40 mA
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intensity (a.u.)
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Subsequently, the precursor is tested by the FT-IR spectrum as displayed in Figure 1c. Obviously, the wave-numbers at 1065, 1160~1460 and 1630 cm-1 belong the C-C, O-C= and C=O bands, suggesting the presence of CoC2O4. Thus, we may deduce that the as-prepared precursor is composed of Co(OH)2 and CoC2O4. In additional, the wave-numbers around at 586 cm-1 is attributed to the characterization vibrating peak of Co(OH)2 and is consistent with the XRD result. The morphology feature of the precursor was displayed in Figure 2. It can be seen that the precursor presents 3-D flower-like shape and the size of monodisperse flower structure is ranged in a narrow region of 2-4 µm. Further magnifying the flower reveals that the flower-like structure consists of numerous smooth ultrathin-sheet structures with uniformly thickness of about 20 nm. From the element mappings of Figure 2(c-f), we can observe that the C, O and Co is uniformly distributed in the flower structure. Whereas, the density of C is less dense than that of Co and O element, meaning the mass ratio of CoC2O4 is less than Co(OH)2.
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Figure 1. XRD patterns (a) EDS (b) and FT-IR spectra (c) of the asprepared flower-like precursor.
Derived from the XRD patterns analysis of the precursor, it is found from the Figure 1a that these strong diffraction peaks at 22.2°, 32.7°, 33.3°, 37.7°, 45.1°, 50.9° and 58.2° can be easily ascribed to the crystal planes of Co(OH)2 (JCPDS card No: 02-0925).32 In fact, the color of pure Co(OH)2 presents pink while the as-prepared precursor appearing in this study is slight dark (inset of Figure 1a), demonstrating that the precursor is not single component and may still exist the CoC2O4. Nevertheless, there is still no distinct evidence to prove the presence of crystalline CoC2O4 just from the XRD patterns due to its probable amorphous state. Herein, further measurements are carried out including Energy Disperse Spectroscopy (EDS), Fourier Transform Infrared (FT-IR) to analyze the composition. Firstly, the EDS spectrum shows the presence of Co, O and C elements, which proves the probable presence of CoC2O4 (Figure 1b). (Before the EDS tested, the sample was put in the single-crystal Si substrate in order to rule out the C element coming from the conductive adhesive).
Figure 2. SEM images (a~b) and elements mapping(c-f) of the flowerlike precursor.
3.2 Morphology and microstructure of the Co/CoO The Co/CoO composite is prepared by a simple heat treatment process under the flowing nitrogen gas, and the temperature is set at 300 to 500 °C for 2 hours, respectively. As can be seen from Figure 3, there are six mainly diffraction peaks located at 36.5°, 42.6°, 44.2°, 51.1°, 61.6° and 75.8° in S1 and S2, which can be assigned to the (111), (200), (220) crystal planes of CoO (JCPDS card No: 65-2902) and (111), (200) (211) of Co (JCPDS card No: 15-0806), respectively. However, it should be pointed out that the Co diffraction peaks are not present in the sample prepared at 300 °C. The Co diffraction peaks of S2 are obviously strong. As a result, we deduce that the apparent increase of Co diffraction peaks intensity may be
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caused by the increase mass of Co in the composite. Such a phenomenon can be explained in the following: When the precursor under the heat treatment process, a series of reactions is acquired as expressed in the following equations: Co(OH)2 = CoO + H2O (3) CoC2O4=CoO+CO+CO2 (4) CoO+CO =Co+CO2 (5) It is clearly observed that the metal Co is resulted from the decomposition of CoC2O4. For reaction (4), it is an endothermic process, so the equilibrium constant K increases with the temperature constantly heightening and it is favor for generating more and more CO as the temperature from 400 to 500 oC. Afterward, the CoO is reducted to Co. Such a result is consistent with XRD patterns and the magnetic properties (see below). However, for reaction (5), it is still need a relative high temperature although it is an exothermal process (∆H=45.1 kJ/mol
