rXXXX American Chemical Society A dx.doi.org/10.1021/jp106652x | J. Phys. Chem. C XXXX, XXX, 000–000

ARTICLE

pubs.acs.org/JPCC

Morphology-Controlled Synthesis and Novel MicrowaveAbsorption Properties of Hollow Urchinlike R-MnO2

NanostructuresMin 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 nano-structures, namely, columnar nanorod clusters, tetragonal nano-tube clusters, and tetragonal nanorod clusters, have beensynthesized through a facile hydrothermal method. The micro-structure and morphologies of the resulting materials wereinvestigated by X-ray diffraction, scanning electron microscopy,transmission electron microscopy, and selected-area electrondiffraction, and the microwave absorption properties of thesenanostructures were investigated in terms of complex permittivityand permeability. The results indicate an obvious magnetic lossin the manganese oxide/paraffin wax composites. The tetragonal nanorod clusters exhibit enhanced microwave absorption propertiescompared with columnar nanorod clusters and tetragonal nanotube clusters, which result from proper electromagnetic impedancematching. These urchinlike manganese oxide nanostructures are considered to have great potential applications as microwave absorbents.

’ INTRODUCTION

Microwave absorption materials are of crucial importance insolving the expanding electromagnetic interference problems causedby the development of wireless communications and high-frequencycircuit devices in the gigahertz range. Conventional microwaveabsorbents, such as magnetic ferrites, usually have high densities,which limits their widespread applications. In addition, carbonnanotubes1,2 and conducting polymers3,4 display good abilitiesfor microwave absorption, but the fabrication of these materialsinvolves complex processes. Therefore, it is desirable to exploitnew microwave absorption materials that are lightweight andeasy to synthesize and exhibit strong absorption in a wide range.

Recently, considerable research attention has been focused onnanostuctured transition-metal oxides,5-8 which show highlyefficient microwave absorption properties because of their strongdielectric loss. Microwave absorption properties of transition-metal oxides often depend strongly on their morphologies. Inparticular, enhanced microwave absorption properties can beobtained from hierarchical nanomaterials with complicated geom-etrical morphologies. For instance, Cao et al. found that cagelikeZnO nanostructures exhibit relatively strong microwave absorptionin theXband, comparedwithZnOnanoparticles.7 In another example,the microwave absorption performance of ZnO dendritic nano-structures was found to be better than that of ZnO nanowires.8

Manganese oxides have attracted great interest because oftheir wide applications in energy storage,9 molecular sieves,10 andcatalysts.11 A few recent studies showed that manganese oxidescould also be used as microwave absorption materials. Yan et al.reported that γ-Mn3O4 nanoparticles with diameters of about

25 nm display a strongest absorption peak of -27.1 dB at3.1 GHz.12 Duan's group has studied the microwave absorptionproperties of MnO2 nanomaterials prepared under different con-ditions.13-16 Manganese oxides with hierarchical nanostructuresmight be good candidates for microwave absorbents, yet theirmicrowave absorption properties have not been extensively studied.Moreover, morphology-controlled synthesis of manganese oxidenanostructures might offer an avenue to understanding the rolethat morphology plays in affecting the corresponding microwaveabsorption performance.

As one of the most attractive hierarchical architectures, onthe other hand, urchinlike manganese oxide nanostructures havedisplayed enhanced physical and chemical properties.17-20 How-ever, the synthesis of urchinlike nanostructures that consist ofmorphology-controlled one-dimensional nanomaterials througha simple route is still a challenge. Herein, we report a facile routefor preparing three types of hollow urchinlike R-MnO2 nano-structures, namely, columnar nanorod clusters, tetragonal nano-rod clusters, and tetragonal nanotube clusters. The microwaveabsorption properties of these nanostructures with differentmorphologies were investigated in terms of complex permittivityand permeability. Manganese oxide urchins consisting of tetrag-onal nanorods exhibit the best microwave absorption perfor-mance among the three products. Furthermore, magnetic losswas found to be important for the loss mechanisms.

Received: July 17, 2010Revised: December 5, 2010

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’EXPERIMENTAL METHODS

Synthesis. All chemicals were of analytical grade and wereused without further purification. To prepare hollow urchinlikeR-MnO2 columnar nanorod clusters, a 30 mL aqueous solutioncontaining 2 mmol of KMnO4 and 10 mL of 1.0 M HCl aqueoussolution was transferred into a 46 mL Teflon-lined stainless steelautoclave and kept at 150 �C for 6 h. The product was filtered,washed with distilled water and ethanol, vacuum dried at 80 �Cfor 6 h, and labeled as sample A. By decreasing the volume of HClaqueous solution to 3 mL, hollow urchinlike R-MnO2 tetragonalnanotube clusters were formed and labeled as sample B. HollowurchinlikeR-MnO2 tetragonal nanorod clusters were synthesized byaltering the volume of HCl aqueous solution to 3 mL and thehydrothermal temperature to 120 �C and labeled as sample C.Characterization. The as-synthesized products were charac-

terized by X-ray powder diffraction (XRD) on a Rigaku D/Max-2400diffractometer using Ni-filtered Cu KR1 irradiation. Scanningelectron microscopy (SEM) measurements were obtained on aHitachi S-4800 field-emission scanning electron microscope.Transmission electron microscopy (TEM) and selected-areaelectron diffraction (SAED) measurements were carried out ona JEM-2010 transmission electron microscope.The composite samples used for measurements of relative

permittivity and permeability were prepared by mixing theproducts and paraffin wax in a mass ratio of 1:1. The mixtureswere then pressed into toroidal-shaped samples (jout = 7.00mm,jin = 3.04 mm). The complex permittivity (εr = ε0 - jε00) andpermeability (μr = μ0 - jμ00) of the mixtures in the 0.1-18 GHzfrequency range were recorded on an Agilent E8363B vectornetwork analyzer. The reflection loss was calculated according tothe transmission line theory,21 expressed as follows

RL ¼ 20 logjðZin -Z0Þ=ðZin þZ0Þj ð1Þ

Zin ¼ Z0ðμrεrÞ1=2 tanh½jð2πfd=cÞðμrεrÞ1=2� ð2Þwhere f is the frequency of the electromagnetic wave, d is the thick-ness of the absorber, c is the velocity of light, Z0 is the impedanceof free space, and Zin is the input impedance of the absorber.

’RESULTS AND DISCUSSION

The phase and purity of the resulting materials were tested byXRD. For sample A, all of the diffraction peaks fromFigure 1a canbe indexed to the tetragonal phase of R-MnO2 (JCPDS 44-0141,a = 9.784 and c = 2.863 Å). The products obtained withdecreased solution acidity (samples B and C) were also found tobe R-MnO2 but with less purity (Figure 1b,c, respectively).Because poorly crystallized δ-MnO2 is formed in the initial stepof the reaction and a higher concentration of Hþ is favorable forthe phase transition from δ- to R-MnO2,

17 therefore, the broadpeaks at 2θ angles between 20� and 30� in the patterns of samplesB 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 differentconditions were examined by SEM. Figure 2a shows that sampleA exhibits an urchinlike shape with the diameter of about 6 μmand consists of straight and radially grown nanorods. Furtherobservation (Figure 2b) reveals that these nanorods have aquasicolumnar shape. For sample B, well-defined urchins withdiameters of 4-5 μm are observed, as shown in Figure 2c.Figure 2d reveals that these urchinlike nanostructures consist of

tetragonal nanotubes with tetragonal open ends. Decreasing thetemperature of hydrothermal treatment to 120 �C (sample C)leads to the formation of urchinlike nanorod clusters, as shown inFigure 2e. The clear tetragonal cross section of a nanorod insample C is observed in Figure 2f.

The TEM images of samples A (Figure 3a), B (Figure 3c), andC (Figure 3e) indicate that they have homogeneous hollowstructures. It can be noticed that sample B (Figure 3d) consistsof one-dimensional nanotubes, whereas samples A (Figure 3b)and C (Figure 3f) consist of nanorods. These results agree wellwith those of the SEM studies. The SAED pattern (Figure 3g)of a single nanorod from sample C suggests it to be single-crystalline. HRTEM analysis (Figure 3h) shows a lattice spacingof about 0.5 nm, which corresponds to the interplanar distanceof (200) planes. The SAED and HRTEM patterns demonstratethat the nanorods grow along the [001] axis. The SAED andHRTEM patterns of samples A and B (not shown) are rather

Figure 1. XRD patterns of samples (a) A, (b) B, and (c) C.

Figure 2. SEM images of samples (a,b) A, (c,d) B, and (c,d) C.

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similar to those of sample C, implying they have the same growthdirection.

On the basis of our observations of the time-dependentevolutions of morphology (Figure S1, Supporting Information)and crystallinity (Figure S2, Supporting Information), the mecha-nisms by which the hollow urchinlike R-MnO2 nanostructures wereformed can be explained using the Ostwald ripening process.19,20

In the initial stage, a large number of flowerlike δ-MnO2 micro-spheres were formed. Then, R-MnO2 nanorods grew along[001] on the microspheres. Meanwhile, because δ-MnO2 is ametastable 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 nano-rod clusters. For samples B and C, δ-MnO2 was not completelydissolved because of the lower acidity of the solutions, as sug-gested above. Moreover, the evolution process of the nanotubesin sample B (Figure S3, Supporting Information) reveals that theformation of the nanotubes can be proposed as “etching” of thenanorods from the tips toward the interior along the [001] axis.22,23

Because the top areas of the nanorods are polar metastable (001)

surfaces, it is more stable for the product to form hollow nano-tubes with reduced metastable top areas.

The microwave absorption properties of these R-MnO2 nano-structures were investigated by mixing the samples and paraffinwax in a mass ratio of 1:1. Figure 4 shows the reflection loss (RL)data for the sample/paraffin wax composites. The values ofminimum RL for samples A-C are -36 dB at 2.9 GHz witha thickness of 3.8 mm, -21 dB at 4.9 GHz with a thickness of3.0 mm, and -41 dB at 8.7 GHz with a thickness of 1.9 mm,respectively. The minimum RL of-41 dB obtained from sampleC is much better than that of other manganese oxides reportedbefore.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 isworth noting that sample C displays enhanced microwaveabsorption properties in terms of both the minimum RL valueand 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

Figure 3. TEM images of samples (a,b) A, (c,d) B, and (e,f) C. (g) SAEDpattern and (h) HRTEM image of sample C.

Figure 4. Reflection loss curves of different composites at different thick-nesses, consisting ofmixedparaffinwaxwith samples (a) A, (b) B, and (c) C.

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of different composites were investigated in the frequency rangeof 0.1-18 GHz, as shown in Figure 5a,b. The ε0 curve of eachsample (Figure 5a) exhibits a decrease from 0.1 to 18 GHz, witha small fluctuation in the range of 13-15 GHz. Meanwhile,the values of ε00 (Figure 5b) exhibit abrupt decreases at lowfrequency and then increase along with the frequency. Inaddition, the ε00 curves of both samples A and C display obviousfluctuations in the range of 14-18 GHz, suggesting the occurrenceof strong resonance. Figure 5c shows the dielectric loss tangents(tan δε = ε00/ε0) of samples A-C, in which the maximum valuesof tan δε are 0.88, 0.82, and 0.26, respectively.

The relatively high values of ε00 and tan δε imply that thehollow urchinlike R-MnO2 nanostructure/paraffin wax compos-ites exhibit intense dielectric losses, which should be attributedto such mechanisms as dominant dipolar polarization, interfacialpolarization and associated relaxation phenomena.24 Further-more, we introduced amicrocircuit model to explain the differentdielectric loss performances among different samples. The innerwalls of theurchinlike nanostructures canbe considered asmicroloops,

whereas the actinomorphic one-dimensional manganese oxidescan be considered as numerous antennas that convert electro-magnetic waves into vibrating microcurrent. Hence, micro-current can be produced in the microloops, which leads to dielectricresonant peaks in the ε00 curves. Compared with nanorods, nano-tubes, which have a smaller cross-sectional area, exhibit a feeblermicrocurrent intensity, thus resulting in weaker dielectric lossesin the nanotube clusters. The presence of poorly crystallizedδ-MnO2 might deteriorate the dielectric properties of the com-posites. As a result, sample A exhibits the best dielectric loss valueswhereas 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 lossdominates the loss mechanisms.5 In this work, however, sampleC exhibits largely enhanced microwave absorption propertiescompared with sample A despite the fact that the latter has higherε00 and tan δε values. According to eqs 1 and 2, the contributionof magnetic loss to the loss mechanisms thus should be taken intoaccount. To confirm this assumption, the real (μ0) and imaginary(μ00) parts of the relative permeability and the magnetic loss

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.

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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 exhibita slow decline. The μ00 values (Figure 6b) fluctuate around zero atlow frequency, and strong peaks can be observed in the curves ofsamples A and C in the range of 14-18 GHz. The maximum tanδμ values of samples A and C (Figure 6c) are 0.22 and 0.27,respectively, which are much higher than those of sample B andother manganese oxides reported in the literature.12,13 Previousreports showed that magnetic behavior can be disturbed by thedielectric behavior at microwave frequency.25,26 Furthermore,one can clearly see that the locations of the strong peaks in the μ00curves for samples A andC are rather similar to those of ε00. Thus,the strong peaks in the μ00 curves are believed to be related to theresonances in ε00. The magnetic loss is supposed to indicate thatmagnetic energy is transferred into electric energy and finallydissipates in the composites. More experimental work is neededto verify this proposal.

Therefore, we suggest that the loss mechanisms of the hollowurchinlike R-MnO2 nanostructures consist of both dielectric andmagnetic losses. Because the electromagnetic impedance match-ing requires that themagnetic loss and the dielectric loss be equal,compared with sample A, impedance matching is more satisfiedin sample C, because of its relatively low tan δε values and hightan δμ values. The enhanced microwave absorption perfor-mance of sample C thus results from a proper electromagneticimpedance matching.

’CONCLUSIONS

In summary, a facile hydrothermal process was developedto synthesize hollow urchinlike R-MnO2 nanostructures with con-trollable morphologies. Columnar nanorod clusters, tetragonalnanorod clusters, and tetragonal nanotube clusters could be obtainedby tuning the reaction conditions. Excellent microwave absorp-tion performances were observed in theR-MnO2 nanostructure/paraffin wax composites. The tetragonal nanorod clusters displayenhanced microwave absorption properties compared withcolumnar nanorod clusters and tetragonal nanotube clusters.Moreover, for the first time, we discovered that not only dielectricloss, but also magnetic loss contributes to the loss mechanisms inmanganese oxides. A proper electromagnetic impedance match-ing leads to the enhancedmicrowave absorption of the tetragonalnanorod clusters. Considering the low-cost and facile synthesisprocess of manganese oxide urchins, the present study offerspromising materials for microwave absorption.

’ASSOCIATED CONTENT

bS Supporting Information. Time-dependent evolutions ofmorphology and crystallinity of the hollow urchinlike R-MnO2

nanostructures and evolution process of the nanotubes. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

’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 Foundationof China (Grants 60536010 and 61006001). The authors appreciateXuhui Xu and De Yan for useful discussions.

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