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Materials Chemistry and Physics 130 (2011) 1191– 1194

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

rowth process and microwave absorption properties of nanostructured �-MnO2

rchins

in Zhou, Xin Zhang ∗, Lei Wang, Jumeng Wei, Long Wang, Kangwei Zhu, Boxue Fengey Laboratory for Magnetism and Magnetic Materials of MOE, and School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China

r t i c l e i n f o

rticle history:eceived 29 March 2011eceived in revised form 20 August 2011ccepted 27 August 2011

a b s t r a c t

A low-temperature hydrothermal method was developed to synthesize urchinlike �-MnO2 nanostruc-tures. Time-dependent evolutions of morphology and crystallinity were investigated to explore thegrowth mechanism of the �-MnO2 urchins. The results show that the growth process of the �-MnO2

urchins occurs in two main stages, which are the generation of �-MnO2 microspheres and the following

eywords:. Nanostructures. Oxides. Epitaxial growth. Dielectric properties

epitaxial growth of �-MnO2 nanoneedles on the surface of the initial microspheres. Microwave absorp-tion properties of the urchinlike �-MnO2 nanostructures were studied in terms of complex permittivityand permeability. An effective absorption bandwidth (reflection loss lower than −10 dB) of 8.8 GHz wasachieved from the �-MnO2/paraffin wax composite.

© 2011 Elsevier B.V. All rights reserved.

canning electron microscopy

. Introduction

Nowadays serious electromagnetic interference problemsaused by the ever-growing applications of wireless communi-ations and high-frequency circuit devices have highlighted theignificance of microwave absorbing materials. Generally, materi-ls used for microwave absorption are supposed to be lightweightnd easy to synthesize and exhibit strong absorption in a wideange. However, the properties of most existing microwave absorb-ng materials are not very satisfying [1,2]. Considerable researchttention thus was paid to develop novel microwave absorbingaterials.Large varieties of one-dimensional (1D) nanomaterials have

een synthesized in the past decade owing to their unique physicalnd chemical properties resulting from intrinsic anisotropicature [3]. Recent focus is shifting to the architecture ofhree-dimensional (3D) nanostructures, which, consisting ofD nanoscale building blocks, are expected to provide enhancedroperties and novel applications of nanomaterials [4]. ManyD hierarchical nanostructures have proved to be promis-

ng microwave absorbing materials [5–7]. In addition, variousanganese oxides, such as �-MnO2 nanowires [8], �-MnO2

anorods [9], �-MnO2 microspheres [10,11], and Mn3O4 nanopar-icles [12], have attracted considerable attention as microwavebsorbing materials. Our previous work revealed that hollow

∗ Corresponding author. Tel.: +86 931 8912719; fax: +86 931 8913554.E-mail address: zhangxin@lzu.edu.cn (X. Zhang).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.08.056

urchinlike �-MnO2 3D nanostructures display excellentmicrowave absorption properties [13]. However, microwaveabsorption properties of �-MnO2 nanourchins have been lessreported.

In this paper, we report a facile low-temperature route forpreparing 3D urchinlike �-MnO2 nanostructures, neither templatenor surfactant was introduced in the reaction. Time-dependentexperiments were carried out to explore the evolution process ofthe �-MnO2 urchins, and a rational growth mechanism was pro-posed. Microwave absorption properties of the �-MnO2 urchinswere investigated in terms of complex permittivity and perme-ability, which indicated a potential application of the as-preparedmaterials.

2. Materials and methods

All chemicals were of analytical grade and were used without further purifi-cation. In a typical synthesis of �-MnO2 urchins, 2 mmol MnCl2·4H2O and 4 mmol(NH4)2S2O8 were dissolved in 30 mL distilled water to form a homogeneous solu-tion. The mixed solution was then transferred into a 46 mL Teflon-lined stainlesssteel autoclave and kept at 90 ◦C for 24 h. The product was filtered, washed withdistilled water and ethanol, vacuum dried at 60 ◦C for 6 h.

The resulting materials were characterized by X-ray powder diffraction (XRD)on a Rigaku D/Max-2400 diffractometer using Ni-filtered Cu K�1 irradiation. Scan-ning electron microscopy (SEM) measurements were obtained on a Hitachi S-4800field-emission scanning electron microscope. The composite samples used for mea-

surements of relative permittivity and permeability were prepared by mixing theproducts and paraffin wax in a mass ratio of 1:1. The mixtures were then pressedinto toroidal-shaped samples (ϕout: 7.00 mm; ϕin: 3.04 mm). The complex permit-tivity (εr = ε′ − jε′′) and permeability (�r = �′ − j�′′) of the mixtures in the 2–18 GHzfrequency range were recorded on an Agilent E8363B vector network analyzer.

1192 M. Zhou et al. / Materials Chemistry and Physics 130 (2011) 1191– 1194

XRD

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WotnaeF

Fig. 1. (a–c) SEM images and (d)

. Results and discussion

.1. Microstructure and growth process

The SEM images of the resulting materials are shown in Fig. 1a–c.ell-defined urchinlike nanostructures with size of about 3 �m are

bserved in Fig. 1a. Further observations in Fig. 1b and c revealshat these nanourchins consist of radially grown one-dimensional

eedlelike nanostructures. The diameter of these nanoneedles isbout 100 nm. The crystalline structure of the nanourchins wasxamined by XRD, as shown in Fig. 1d. All of the diffraction peaks inig. 1d can be indexed to the orthorhombic phase of �-MnO2 (JCPDS

Fig. 2. SEM images of the products obtained after differen

pattern of �-MnO2 nanourchins.

NO. 14-0644), the reported lattice constants of which are a = 6.36 A,b = 10.15 A, and c = 4.09 A. No additional peaks corresponding toother impurities are observed, indicating the resulting materialcrystallized as a pure phase. Manganese dioxides have differentcrystalline structures, such as �-, �-, �-, and �-types, depending onhow [MnO6] octahedrons are connected. �-MnO2 is considered tobe an intergrowth of (1 × 1) and (1 × 2) tunnels constructed fromthe [MnO6] octahedrons [14], which are the basic structure units

of MnO2.

Time-dependent experiments were carried out to explore theevolution process of the �-MnO2 urchins. Fig. 2a–d shows represen-tative SEM images of the products obtained after various reaction

t reaction time: (a) 3 h, (b) 6 h, (c) 12 h, and (d) 24 h.

M. Zhou et al. / Materials Chemistry an

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the range of 10–12 and 15–17 GHz. Generally, the dielectric losses

Fig. 3. XRD patterns of the products obtained after different reaction time.

imes. The products collected at 3 h are microspheres consisting ofanoplates. Prolonging the reaction time to 6 h, ultrathin nanonee-les with diameter of about several nanometers are observed,hich grew from the outmost surface of the nanoplates. With the

eaction going on, the nanoneedles grew up to about 100 nm iniameter. In addition, all of the products collected at different stagesave been demonstrated to be pure �-MnO2 crystals by the corre-ponding XRD patterns displayed in Fig. 3. No obvious difference isbserved from the XRD patterns among different samples obtainedfter different reaction time, indicating the crystallinity of the ini-ial microspheres and the radially grown nanoneedles are ratherimilar.

The growth mechanism of the �-MnO2 nanourchins wasationalized based on our observations of the time-dependent evo-utions of morphology and crystallinity. The synthesis of �-MnO2

Fig. 4. Formation process for

Fig. 5. (a) Real and imaginary parts of relative permittivity and dielectric loss

d Physics 130 (2011) 1191– 1194 1193

is dependent on the reaction between ammonium persulfate andmanganese chloride. The chemical reaction can be described asfollows:

Mn2+ + S2O82− + 2H2O → MnO2 + 4H+ + 2SO4

2− (1)

The basic units of MnO2 were produced in a very short time fromthe redox reaction between S2O8

2− and Mn2+ in the solution. Theconcentration of the MnO2 units in the solution is considered to becritical for the formation of the �-MnO2 urchins. In the initial stage,a high concentration of the MnO2 units was present. These unitstended to aggregate to form spherical particles that minimized theoverall energy of the system. The MnO2 units were consumed con-tinuously, the concentration of which therefore was decreasing.When the concentration of the MnO2 units was lower than a specificvalue, the whole system was then transferred to a thermodynam-ically stable environment. After that, �-MnO2 nanoneedles beganto grow due to the 1D growth habit of �-MnO2 [14], and the out-most surface of the spheres might serve as nucleation seeds for thegrowth. At last, the ultrathin nanoneedles grew larger and formedthe nanoneedles constructed �-MnO2 urchinlike nanostructures.On the basis of the proposition discussed above, the growth stagesof the urchinlike �-MnO2 nanostructures are illustrated in Fig. 4. Asimilar process was also observed in the formation of the urchinliketungsten oxide nanostructures [15].

3.2. Microwave absorption properties

Fig. 5a shows the real (ε′) and imaginary parts (ε′′) of the relativepermittivity and the dielectric loss tangents (tg �� = ε′′/ε′) of the �-MnO2/paraffin wax composite. Both of the ε′ and ε′′ values decreaseslowly at low frequency, and then approach to be constant as 10 and5, respectively. The tg ıε value is about 0.5 with two fluctuations in

of bulk transition-metal oxides are negligibly small. However,nanostructured transition-metal oxides exhibit favorable dielectriclosses contributed from dominant dipolar polarization, interfacial

�-MnO2 nanourchins.

tangent, and (b) reflection loss of the sample/paraffin wax composite.

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194 M. Zhou et al. / Materials Chemis

olarization and associated relaxation phenomena [16]. In addi-ion, the antenna model was widely used to interpret the dielectricosses of transition-metal oxides with hierarchical nanostructures17,18]. Here, in our work, the random oriented nanoneedles cane considered as isotropic antennas that convert electromagneticaves into vibrating microcurrent. The microcurrent then dissi-ates in the �-MnO2/paraffin wax composite, which results inhe energy attenuation consequently. The real (�′) and imaginary�′′) part of complex permeability are about 1.0 and 0.0, and the

agnetic loss tangent (tg ı�=�′′/�′) is about 0.0 (not shown). Theagnetic loss of the urchinlike �-MnO2 nanostructures is unlike

hat of hollow urchinlike �-MnO2 nanostructures reported previ-usly [13], in which magnetic loss was found to be important to theoss mechanism.

The reflection loss of the �-MnO2/paraffin wax composite wasalculated according to the transmission line theory [19], expresseds follows:

L = 20 log∣∣∣Zin − Z0

Zin + Z0

∣∣∣ (2)

in = Z0(�rεr)1/2 tanh[

j(

2�fd

c

)(�rεr)1/2

](3)

here f is the frequency of the electromagnetic wave, d is the thick-ess of the absorber, c is the velocity of light, Z0 is the impedancef free space, and Zin is the input impedance of the absorber. Fig. 5bhows the reflection loss data for the �-MnO2/paraffin wax com-osite. The reflection loss peak shifts to a low frequency along with

ncreased thickness of the absorber, and the minimum value ofeflection loss is −18.8 dB at 7.7 GHz with a thickness of 2.75 mm.y tuning the thickness of the absorber, the absorption bandwidthf the �-MnO2/paraffin wax composite with reflection loss lowerhan −10 dB (90% absorption) is up to 8.8 GHz. Our results indi-ate that urchinlike �-MnO2 nanostructures are good candidatesor microwave absorbing materials.

. Conclusions

Urchinlike �-MnO2 nanostructures were prepared via a low-

emperature, template-free hydrothermal route. On the basis ofhe observations, the evolution process of these �-MnO2 urchinss proposed to involve two main stages: (i) formation of �-MnO2

icrospheres and (ii) epitaxial growth of the nanoneedles from the

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d Physics 130 (2011) 1191– 1194

surface of the microspheres. Furthermore, microwave absorptionproperties of the as-prepared �-MnO2 urchins were investigatedin details, which showed a minimum reflection loss of −18.8 dB,as well as an absorption bandwidth of 8.8 GHz correspondingto reflection loss below −10 dB. Our results might promote theunderstanding of growth process, and provide novel applicationof �-MnO2 hierarchical nanostructures.

Acknowledgments

This work was supported by the National Science Foundationof China (Grants 61006001 and 60536010). The authors appreciateBitao Liu and Xuhui Xu for useful discussions.

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