ARTICLE pubs.acs.org/JPCC
Morphology-Controlled Synthesis and Novel Microwave Absorption Properties of Hollow Urchinlike R-MnO2 Nanostructures Min Zhou, Xin Zhang, Jumeng Wei, Shuli Zhao, Long Wang, and Boxue Feng* School of Physical Science and Technology and Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China
bS Supporting Information ABSTRACT: Three types of hollow urchinlike R-MnO2 nanostructures, namely, columnar nanorod clusters, tetragonal nanotube clusters, and tetragonal nanorod clusters, have been synthesized through a facile hydrothermal method. The microstructure and morphologies of the resulting materials were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction, and the microwave absorption properties of these nanostructures were investigated in terms of complex permittivity and permeability. The results indicate an obvious magnetic loss in the manganese oxide/paraffin wax composites. The tetragonal nanorod clusters exhibit enhanced microwave absorption properties compared with columnar nanorod clusters and tetragonal nanotube clusters, which result from proper electromagnetic impedance matching. These urchinlike manganese oxide nanostructures are considered to have great potential applications as microwave absorbents.
’ INTRODUCTION Microwave absorption materials are of crucial importance in solving the expanding electromagnetic interference problems caused by the development of wireless communications and high-frequency circuit devices in the gigahertz range. Conventional microwave absorbents, such as magnetic ferrites, usually have high densities, which limits their widespread applications. In addition, carbon nanotubes1,2 and conducting polymers3,4 display good abilities for microwave absorption, but the fabrication of these materials involves complex processes. Therefore, it is desirable to exploit new microwave absorption materials that are lightweight and easy to synthesize and exhibit strong absorption in a wide range. Recently, considerable research attention has been focused on nanostuctured transition-metal oxides,5-8 which show highly efficient microwave absorption properties because of their strong dielectric loss. Microwave absorption properties of transitionmetal oxides often depend strongly on their morphologies. In particular, enhanced microwave absorption properties can be obtained from hierarchical nanomaterials with complicated geometrical morphologies. For instance, Cao et al. found that cagelike ZnO nanostructures exhibit relatively strong microwave absorption in the X band, compared with ZnO nanoparticles.7 In another example, the microwave absorption performance of ZnO dendritic nanostructures was found to be better than that of ZnO nanowires.8 Manganese oxides have attracted great interest because of their wide applications in energy storage,9 molecular sieves,10 and catalysts.11 A few recent studies showed that manganese oxides could also be used as microwave absorption materials. Yan et al. reported that γ-Mn3O4 nanoparticles with diameters of about r 2010 American Chemical Society
25 nm display a strongest absorption peak of -27.1 dB at 3.1 GHz.12 Duan's group has studied the microwave absorption properties of MnO2 nanomaterials prepared under different conditions.13-16 Manganese oxides with hierarchical nanostructures might be good candidates for microwave absorbents, yet their microwave absorption properties have not been extensively studied. Moreover, morphology-controlled synthesis of manganese oxide nanostructures might offer an avenue to understanding the role that morphology plays in affecting the corresponding microwave absorption performance. As one of the most attractive hierarchical architectures, on the other hand, urchinlike manganese oxide nanostructures have displayed enhanced physical and chemical properties.17-20 However, the synthesis of urchinlike nanostructures that consist of morphology-controlled one-dimensional nanomaterials through a simple route is still a challenge. Herein, we report a facile route for preparing three types of hollow urchinlike R-MnO2 nanostructures, namely, columnar nanorod clusters, tetragonal nanorod clusters, and tetragonal nanotube clusters. The microwave absorption properties of these nanostructures with different morphologies were investigated in terms of complex permittivity and permeability. Manganese oxide urchins consisting of tetragonal nanorods exhibit the best microwave absorption performance among the three products. Furthermore, magnetic loss was found to be important for the loss mechanisms. Received: July 17, 2010 Revised: December 5, 2010 Published: December 21, 2010 1398
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’ EXPERIMENTAL METHODS Synthesis. All chemicals were of analytical grade and were used without further purification. To prepare hollow urchinlike R-MnO2 columnar nanorod clusters, a 30 mL aqueous solution containing 2 mmol of KMnO4 and 10 mL of 1.0 M HCl aqueous solution was transferred into a 46 mL Teflon-lined stainless steel autoclave and kept at 150 °C for 6 h. The product was filtered, washed with distilled water and ethanol, vacuum dried at 80 °C for 6 h, and labeled as sample A. By decreasing the volume of HCl aqueous solution to 3 mL, hollow urchinlike R-MnO2 tetragonal nanotube clusters were formed and labeled as sample B. Hollow urchinlike R-MnO2 tetragonal nanorod clusters were synthesized by altering the volume of HCl aqueous solution to 3 mL and the hydrothermal temperature to 120 °C and labeled as sample C. Characterization. The as-synthesized products were characterized by X-ray powder diffraction (XRD) on a Rigaku D/Max-2400 diffractometer using Ni-filtered Cu KR1 irradiation. Scanning electron microscopy (SEM) measurements were obtained on a Hitachi S-4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) measurements were carried out on a JEM-2010 transmission electron microscope. The composite samples used for measurements of relative permittivity and permeability were prepared by mixing the products and paraffin wax in a mass ratio of 1:1. The mixtures were then pressed into toroidal-shaped samples (jout = 7.00 mm, jin = 3.04 mm). The complex permittivity (εr = ε0 - jε00 ) and permeability (μr = μ0 - jμ00 ) of the mixtures in the 0.1-18 GHz frequency range were recorded on an Agilent E8363B vector network analyzer. The reflection loss was calculated according to the transmission line theory,21 expressed as follows
RL ¼ 20 logjðZin - Z0 Þ=ðZin þ Z0 Þj
ð1Þ
Zin ¼ Z0 ðμr εr Þ1=2 tanh½jð2πfd=cÞðμr εr Þ1=2
ð2Þ
Figure 1. XRD patterns of samples (a) A, (b) B, and (c) C.
where f is the frequency of the electromagnetic wave, d is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of free space, and Zin is the input impedance of the absorber.
’ RESULTS AND DISCUSSION The phase and purity of the resulting materials were tested by XRD. For sample A, all of the diffraction peaks from Figure 1a can be indexed to the tetragonal phase of R-MnO2 (JCPDS 44-0141, a = 9.784 and c = 2.863 Å). The products obtained with decreased solution acidity (samples B and C) were also found to be R-MnO2 but with less purity (Figure 1b,c, respectively). Because poorly crystallized δ-MnO2 is formed in the initial step of the reaction and a higher concentration of Hþ is favorable for the phase transition from δ- to R-MnO2,17 therefore, the broad peaks at 2θ angles between 20° and 30° in the patterns of samples B and C indicate the presence of δ-MnO2, which is not dissolved. The details of the formation process will be discussed later. The morphologies of the products prepared under different conditions were examined by SEM. Figure 2a shows that sample A exhibits an urchinlike shape with the diameter of about 6 μm and consists of straight and radially grown nanorods. Further observation (Figure 2b) reveals that these nanorods have a quasicolumnar shape. For sample B, well-defined urchins with diameters of 4-5 μm are observed, as shown in Figure 2c. Figure 2d reveals that these urchinlike nanostructures consist of
Figure 2. SEM images of samples (a,b) A, (c,d) B, and (c,d) C.
tetragonal nanotubes with tetragonal open ends. Decreasing the temperature of hydrothermal treatment to 120 °C (sample C) leads to the formation of urchinlike nanorod clusters, as shown in Figure 2e. The clear tetragonal cross section of a nanorod in sample C is observed in Figure 2f. The TEM images of samples A (Figure 3a), B (Figure 3c), and C (Figure 3e) indicate that they have homogeneous hollow structures. It can be noticed that sample B (Figure 3d) consists of one-dimensional nanotubes, whereas samples A (Figure 3b) and C (Figure 3f) consist of nanorods. These results agree well with those of the SEM studies. The SAED pattern (Figure 3g) of a single nanorod from sample C suggests it to be singlecrystalline. HRTEM analysis (Figure 3h) shows a lattice spacing of about 0.5 nm, which corresponds to the interplanar distance of (200) planes. The SAED and HRTEM patterns demonstrate that the nanorods grow along the [001] axis. The SAED and HRTEM patterns of samples A and B (not shown) are rather 1399
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Figure 3. TEM images of samples (a,b) A, (c,d) B, and (e,f) C. (g) SAED pattern and (h) HRTEM image of sample C.
similar to those of sample C, implying they have the same growth direction. On the basis of our observations of the time-dependent evolutions of morphology (Figure S1, Supporting Information) and crystallinity (Figure S2, Supporting Information), the mechanisms by which the hollow urchinlike R-MnO2 nanostructures were formed can be explained using the Ostwald ripening process.19,20 In the initial stage, a large number of flowerlike δ-MnO2 microspheres were formed. Then, R-MnO2 nanorods grew along [001] on the microspheres. Meanwhile, because δ-MnO2 is a metastable phase and the inner core has a higher surface energy, the δ-MnO2 microspheres were dissolved from inside to outside. The rapid growth of nanorods and the dissolution of δ-MnO2 microspheres led to the formation of the hollow urchinlike nanorod clusters. For samples B and C, δ-MnO2 was not completely dissolved because of the lower acidity of the solutions, as suggested above. Moreover, the evolution process of the nanotubes in sample B (Figure S3, Supporting Information) reveals that the formation of the nanotubes can be proposed as “etching” of the nanorods from the tips toward the interior along the [001] axis.22,23 Because the top areas of the nanorods are polar metastable (001)
Figure 4. Reflection loss curves of different composites at different thicknesses, consisting of mixed paraffin wax with samples (a) A, (b) B, and (c) C.
surfaces, it is more stable for the product to form hollow nanotubes with reduced metastable top areas. The microwave absorption properties of these R-MnO2 nanostructures were investigated by mixing the samples and paraffin wax in a mass ratio of 1:1. Figure 4 shows the reflection loss (RL) data for the sample/paraffin wax composites. The values of minimum RL for samples A-C are -36 dB at 2.9 GHz with a thickness of 3.8 mm, -21 dB at 4.9 GHz with a thickness of 3.0 mm, and -41 dB at 8.7 GHz with a thickness of 1.9 mm, respectively. The minimum RL of -41 dB obtained from sample C is much better than that of other manganese oxides reported before.12,13 Moreover, by adjusting the thickness of the absorber, the absorption bandwidths with RL lower than -10 dB (90% absorption) of samples A-C are up to 4.6, 5.4, and 8.7 GHz. It is worth noting that sample C displays enhanced microwave absorption properties in terms of both the minimum RL value and the absorption bandwidth compared with samples A and B. To understand the possible microwave absorption mechanisms, the real (ε0 ) and imaginary (ε00 ) parts of the relative permittivity 1400
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Figure 5. (a) Real and (b) imaginary parts of relative permittivity and (c) dielectric loss tangents of the sample/paraffin wax composites.
Figure 6. (a) Real and (b) imaginary parts of relative permeability and (c) magnetic loss tangents of the sample/paraffin wax composites.
of different composites were investigated in the frequency range of 0.1-18 GHz, as shown in Figure 5a,b. The ε0 curve of each sample (Figure 5a) exhibits a decrease from 0.1 to 18 GHz, with a small fluctuation in the range of 13-15 GHz. Meanwhile, the values of ε00 (Figure 5b) exhibit abrupt decreases at low frequency and then increase along with the frequency. In addition, the ε00 curves of both samples A and C display obvious fluctuations in the range of 14-18 GHz, suggesting the occurrence of strong resonance. Figure 5c shows the dielectric loss tangents (tan δε = ε00 /ε0 ) of samples A-C, in which the maximum values of tan δε are 0.88, 0.82, and 0.26, respectively. The relatively high values of ε00 and tan δε imply that the hollow urchinlike R-MnO2 nanostructure/paraffin wax composites exhibit intense dielectric losses, which should be attributed to such mechanisms as dominant dipolar polarization, interfacial polarization and associated relaxation phenomena.24 Furthermore, we introduced a microcircuit model to explain the different dielectric loss performances among different samples. The inner walls of the urchinlike nanostructures can be considered as microloops,
whereas the actinomorphic one-dimensional manganese oxides can be considered as numerous antennas that convert electromagnetic waves into vibrating microcurrent. Hence, microcurrent can be produced in the microloops, which leads to dielectric resonant peaks in the ε00 curves. Compared with nanorods, nanotubes, which have a smaller cross-sectional area, exhibit a feebler microcurrent intensity, thus resulting in weaker dielectric losses in the nanotube clusters. The presence of poorly crystallized δ-MnO2 might deteriorate the dielectric properties of the composites. As a result, sample A exhibits the best dielectric loss values whereas sample B exhibits the worst among the three of them. Generally, MnO2 nanostructures are antiferromagnetic materials, in which magnetic loss is negligibly small and dielectric loss dominates the loss mechanisms.5 In this work, however, sample C exhibits largely enhanced microwave absorption properties compared with sample A despite the fact that the latter has higher ε00 and tan δε values. According to eqs 1 and 2, the contribution of magnetic loss to the loss mechanisms thus should be taken into account. To confirm this assumption, the real (μ0 ) and imaginary (μ00 ) parts of the relative permeability and the magnetic loss 1401
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The Journal of Physical Chemistry C tangents (tan δμ = μ00 /μ0 ) of all samples are presented in Figure 6. The μ0 values (Figure 6a) of all samples are about 1.0 and exhibit a slow decline. The μ00 values (Figure 6b) fluctuate around zero at low frequency, and strong peaks can be observed in the curves of samples A and C in the range of 14-18 GHz. The maximum tan δμ values of samples A and C (Figure 6c) are 0.22 and 0.27, respectively, which are much higher than those of sample B and other manganese oxides reported in the literature.12,13 Previous reports showed that magnetic behavior can be disturbed by the dielectric behavior at microwave frequency.25,26 Furthermore, one can clearly see that the locations of the strong peaks in the μ00 curves for samples A and C are rather similar to those of ε00 . Thus, the strong peaks in the μ00 curves are believed to be related to the resonances in ε00 . The magnetic loss is supposed to indicate that magnetic energy is transferred into electric energy and finally dissipates in the composites. More experimental work is needed to verify this proposal. Therefore, we suggest that the loss mechanisms of the hollow urchinlike R-MnO2 nanostructures consist of both dielectric and magnetic losses. Because the electromagnetic impedance matching requires that the magnetic loss and the dielectric loss be equal, compared with sample A, impedance matching is more satisfied in sample C, because of its relatively low tan δε values and high tan δμ values. The enhanced microwave absorption performance of sample C thus results from a proper electromagnetic impedance matching.
’ CONCLUSIONS In summary, a facile hydrothermal process was developed to synthesize hollow urchinlike R-MnO2 nanostructures with controllable morphologies. Columnar nanorod clusters, tetragonal nanorod clusters, and tetragonal nanotube clusters could be obtained by tuning the reaction conditions. Excellent microwave absorption performances were observed in the R-MnO2 nanostructure/ paraffin wax composites. The tetragonal nanorod clusters display enhanced microwave absorption properties compared with columnar nanorod clusters and tetragonal nanotube clusters. Moreover, for the first time, we discovered that not only dielectric loss, but also magnetic loss contributes to the loss mechanisms in manganese oxides. A proper electromagnetic impedance matching leads to the enhanced microwave absorption of the tetragonal nanorod clusters. Considering the low-cost and facile synthesis process of manganese oxide urchins, the present study offers promising materials for microwave absorption. ’ ASSOCIATED CONTENT
bS
Supporting Information. Time-dependent evolutions of morphology and crystallinity of the hollow urchinlike R-MnO2 nanostructures and evolution process of the nanotubes. This material is available free of charge via the Internet at http:// pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86-931-8912719. Fax: þ86-931-8913554. E-mail: fengbx@ lzu.edu.cn.
’ ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (Grants 60536010 and 61006001). The authors appreciate Xuhui Xu and De Yan for useful discussions. 1402
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