Appl Phys ADOI 10.1007/s00339-013-7988-5

Synthesis and significantly enhanced microwave absorptionproperties of hematite dendrites/polyaniline nanocomposite

Hao Wu · Liuding Wang · Hongjing Wu · Qian Lian

Received: 25 June 2013 / Accepted: 14 September 2013© Springer-Verlag Berlin Heidelberg 2013

Abstract Hematite dendrites/polyaniline (HDs/PANI)nanocomposite, i.e. coating HDs with conductive PANI,has been successfully synthesized through a two-step poly-merization of aniline monomers on the surface of pre-synthesized HDs for the first time. It is discovered thata lower concentration of HCl (0.02 mol L−1) has only aslight adverse effect on the dendritic structures of the coatedHDs, while a higher concentration of HCl (0.04 mol L−1)results in severe damage to the sub-branches of the HDs.The morphology, composition, structure, static magnetic,and spectral properties of the as-prepared pristine HDs andHDs/PANI composites were thoroughly characterized byvarious physicochemical techniques. Moreover, the electro-magnetic and microwave absorbing properties of the HDsand HDs/PANI wax composites were compared in detail. Itwas found that the frequency of absorption obeys a quarter-wavelength model for both of them, and the HDs/PANI waxcomposite exhibits far superior microwave absorption prop-erties. This phenomenon can be attributed to the improveddielectric loss abilities and the complementary behaviors re-sulting from the PANI coatings and fractal-structured HDs.

Electronic supplementary material The online version of this article(doi:10.1007/s00339-013-7988-5) contains supplementary material,which is available to authorized users.

H. Wu · L.D. Wang · H.J. Wu (B) · Q. LianDepartment of Applied Physics, Northwestern PolytechnicalUniversity, Xi’an 710072, P.R. Chinae-mail: wuhongjing@mail.nwpu.edu.cnFax: +86-29-88431664

H. Wue-mail: wuwanghaoygqx@mail.nwpu.edu.cn

H.J. WuInstitute for the Study of Nanostructured Materials (ISMN)-CNR,Palermo 90146, Italy

1 Introduction

In order to solve the increasingly deteriorated electromag-netic interference (EMI) problem arising from the ever-accelerated development of electronic systems and telecom-munications in the gigahertz (GHz) range, considerable the-oretical and experimental efforts have been devoted to ex-ploit effective microwave absorbing materials with the char-acteristics of broad absorption frequency, strong absorp-tion ability, low density, and high resistivity. Among manynew microwave absorbers, conductive polymers, such aspolyaniline (PANI), have provided the possibility of beinggood microwave absorbers because of their controllable di-electric loss ability, low density, low cost, ease of synthe-sis, and corrosion resistance [1]. However, pure conductivePANI would possess too high permittivity and negligiblepermeability, resulting in the mismatch between the permit-tivity and permeability, thereby inducing poor electromag-netic (EM) wave absorption [2]. Thus, it is very importantto balance the complex permittivity (εr = ε′ − jε′′) and per-meability (μr = μ′ − jμ′′) effectively. Recently, some PANI-related EM absorbers [1–5] with good impedance matchinghave been fabricated, indicating that coating the magneticconstituents with conductive PANI could be an effective wayto optimize the EM parameters and enhance the EM absorb-ing performances.

In the last decade, direct fabrication of hierarchicallystructured metal oxides has attracted extensive researchattention because of their fundamental scientific impor-tance and potential technological applications [6, 7]. Asan important class of magnetic metal oxides, iron ox-ides (e.g. hematite α-Fe2O3, maghemite γ -Fe2O3, andmagnetite Fe3O4) as well as their hybrids with diversethree-dimensional (3D) hierarchical morphologies, such asspheres [8], flowers [9], dendrites [10], and urchins [11],

H. Wu et al.

have already been synthesized, and their various applica-tions have been investigated including photocatalysts, sen-sors, water splitting, drug delivery, and anode materials forlithium batteries [12, 13]. In addition, the 3D hierarchicaliron oxides can also be utilized as microwave absorbing ma-terials due to their unique dielectric and magnetic proper-ties. For example, Sun et al. [14] first reported that the hi-erarchical dendrite-like materials of Fe3O4, γ -Fe2O3, andFe prepared by a hydrogen reduction method exhibited ex-cellent microwave absorption abilities. Yu et al. [15] alsofound that the 3D dendritic α-Fe synthesized by an electricfield-induced and electrochemical reduction method showedboth high absorption efficiency and a broad absorption band.Many other hierarchical iron oxide based composites, suchas porous Fe3O4/C core/shell nanorods [16], nanosizedurchin-like α-Fe2O3 and Fe3O4 [17], and monodispersedhollow Fe3O4 nanospheres [18], also exhibited good EMdissipation abilities. Nevertheless, to the best of our knowl-edge, no research work has been reported on the synthesisof hematite dendrites/polyaniline (HDs/PANI) nanocompos-ite, i.e. uniformly coating the pre-formed hematite dendrites(HDs) with a layer of conductive PANI coating. Moreover,although many researches have been conducted to developiron oxide based composites, very little attention has beenpaid toward the microwave absorption performances of theHDs or HDs-based composites, which is probably due to thevery weak ferromagnetic property above the Morin transi-tion temperature (about −13 °C) for α-Fe2O3, resulting inpoor microwave absorption.

In this paper, considering the merits of HDs: (1) lowcost, non-toxicity, high chemical stability, and ease of syn-thesis, which are beneficial to commercial applications;(2) high resistivity, which can make the electromagneticwaves enter effectively; (3) very large anisotropy field,which can be expected to show a high-frequency magneticresonance; (4) unique fractal architectures, which can endowthe HDs with new fascinating properties, such as large shapeanisotropy, high specific surface area, and characteristics ofleft-handed materials (LHMs) [19], when compared with itsbulk counterpart, we successfully prepared the HDs/PANIcomposite while still maintaining the fractal morphology ofHDs perfectly, and compared the EM and microwave ab-sorption properties of HDs and HDs/PANI composites com-prehensively (with the same filling factor of 50 wt%). Par-ticularly, the morphology-related static magnetic propertieswere investigated, and the possible microwave absorptionmechanisms are discussed in detail.

2 Experimental

2.1 Materials

Potassium hexacyanoferrate (III) K3[Fe(CN)6], sucrose,and ethanol were purchased from Sinopharm Chemical

Reagent Co., Ltd. All the chemicals were of analytical gradeand used without further purification. Distilled water wasused in all experiments.

2.2 Preparation of hematite dendrites (HDs)

HDs with fractal structure were successfully prepared bya hydrothermal reaction, which is similar to the synthe-sis procedure reported by Cao et al. [10] Typically, 1.98 gK3[Fe(CN)6] was dissolved in 150 mL distilled water toform a yellow–green transparent solution, then the solutionwas transferred into a 200 mL Teflon-sealed autoclave andhydrothermally treated at 138 °C for 48 h. The final redprecipitate was filtrated and rinsed with distilled water andethanol repeatedly, and dried in an oven at 80 °C for 10 h.

2.3 Preparation of hematite dendrites/polyaniline(HDs/PANI) nanocomposite

The HDs/PANI nanocomposite was prepared by the two-step polymerization of aniline initiated by FeCl3 and am-monium persulfate (APS) successively [2]. Typically, 0.24 gHDs was dispersed into 50 mL of stock solution containing0.30 mol L−1 FeCl3 and 0.02 mol L−1 HCl, then 160 µL ani-line monomer was added. The solution was stirred in an ice–water bath for 6 h before adding 10 mL of the pre-made APSaqueous solution (0.44 mol L−1). Then, another 8 h poly-merization process with mechanical stirring was carried outat 0 °C. Finally, the precipitated powder was filtrated andrinsed with water and ethanol until the filtrate became col-orless, then dried in an oven at 60 °C overnight. For com-parison, the HDs/PANI composite was also prepared with ahigher concentration of HCl (0.04 mol L−1) in the stock so-lution, following the same procedures. All the characteriza-tion of the HDs/PANI composite was assigned to the sampleprepared with a lower concentration of HCl unless otherwisespecified.

2.4 Characterization

The surface morphology and in-situ chemical compositionof the samples were examined by a field-emission scan-ning electron microscope (FESEM, FEI Quanta 600FEG)equipped with an energy dispersive X-ray analyzer (EDX,IncaPentaFETx3). During the measurement, the HDs sam-ples were fixed by using carbon conductive tape, whereasthe HDs/PANI samples were directly deposited on Cu–Znspecimen stages from ultrasonically processed ethanol so-lutions of the products. The crystalline structures of the as-prepared samples were analyzed by powder X-ray diffrac-tion (PXRD, PANalytical X’Pert PRO MPD) with Cu Kα

radiation. Fourier-transform infrared (FT-IR) spectroscopyanalysis was carried out on a Nicolet Is10 spectropho-tometer with KBr pellets. UV–Vis absorption spectra were

Synthesis and significantly enhanced microwave absorption properties of hematite dendrites/polyaniline nanocomposite

Fig. 1 FESEM images of (a–c)HDs and (d–f) HDs/PANIcomposites at differentmagnifications, thecorresponding (g, h) EDXspectra, and (i) XRD patterns ofthe two composites

recorded on a UNICO UV-2802PCS spectrophotometer bydispersing the samples into ethanol (approx. 0.1 mg mL−1).The static magnetic properties of the prepared sampleswere measured by a vibrating sample magnetometer (VSM,Lakeshore 7304) at room temperature.

In order to investigate the microwave absorption proper-ties of the composites, the as-prepared products were uni-formly dispersed into paraffin matrix, which is transparentto EM waves, with a weight ratio of 50 wt%, and then themixture was pressed into a toroidal-shaped compact with anouter diameter of 7.00 mm, inner diameter of 3.04 mm, andthickness of about 3.00 mm. The relative complex permit-tivity and permeability of the obtained wax composites weretested on a HP vector network analyzer (HP 8720B) over 2–18 GHz. The reflection losses (RL) of the composites werecalculated from the measured EM parameters at given fre-quency and layer thickness according to the transmissionline theory [20], which can be expressed as follows:

RL = 20 log∣∣(Zin − Z0)/(Zin + Z0)

∣∣, (1)

Zin = Z0(μr/εr)1/2 tanh

[

j (2πf d/c)(μr/εr)1/2], (2)

where Zin is the input impedance of the absorber, Z0 is theimpedance of free space, f is the frequency of the incidentEM wave, d is the thickness of the absorber, and c is thevelocity of light in free space.

3 Results and discussion

3.1 The morphology and structure of the preparedcomposites

Figure 1a–f display the representative FESEM images ofthe HDs and HDs/PANI composites at different magnifica-tions. The low-magnified image (Fig. 1a) gives an overviewof the HDs. It is found that almost all the HDs have similardendrite-like architectures, indicating that a good uniformityand high yield have been achieved. The high-magnified im-ages (Fig. 1b and c) present a more detailed morphologyof a single dendrite in two opposite directions. They revealan exquisite dendritic fractal structure consisting of a cen-tral trunk with hierarchical branches and sub-branches, andthe branches at either side of the trunk are parallel to eachother with a uniform angle of about 60◦ between the trunkand the branches. The length of the dendrite trunks is about5–8 µm, and that of the branches and sub-branches rangesfrom 100 nm to 2 µm. Moreover, the protrusion direction ofthe dendrite trunk and those of the branches are opposite.Figure 1h presents the EDX spectrum of the HDs, whichconfirms the sole presence of Fe and O in the dendritic struc-ture, except for the C signal from the carbon conductive tape.

The FESEM images of the HDs/PANI are also shown inFig. 1d–f. From the low- and high-magnified images, weclearly find that the pre-formed HDs are uniformly coatedwith a rough layer of PANI coating. Comparing with themorphology of the pristine HDs carefully, it can be noted

H. Wu et al.

that after the two-step polymerization process, not only thedendritic fractal morphology but also the lengths of the trunkand branches are perfectly maintained with only a slight ad-verse impact to the dendritic structures of HDs. The typi-cal EDX spectrum of the HDs/PANI composite is shown inFig. 1g, which proves the coexistence of Fe, O, N, C, andCl elements in the composite, except for the Cu and Zn sig-nals from the specimen stage of the scanning electron mi-croscope. The Fe and O signals are mainly from the HDs,and the N, C, and Cl signals are definitely from the PANIcoatings. Besides, the FESEM images of the HDs/PANIcomposite, which was prepared by the same procedure butwith a higher concentration of HCl (0.04 mol L−1), are pre-sented in Fig. S1 (see Electronic supplementary material).As shown in the low-, medium-, and high-magnified FE-SEM images, the pre-formed HDs are uniformly coveredby a relatively smooth layer of PANI coating, and the den-dritic shape of the precursor is also well preserved. Unfor-tunately, the sub-branches of the HDs endured somewhatsevere damage during the polymerization process, which isobviously caused by the higher concentration of HCl. As theconcentration of HCl is twice as high as that used previously,the nanoscaled sub-branches would undergo a faster corro-sion rate (even though the HDs have the corrosion-resistantcorundum structure); thereby, the sizes of the sub-branchesare largely reduced or even completely dissolved by the HClduring the polymerization process, causing the formation ofnanorods. It can be concluded that too high a concentrationof HCl would bring about severe damage to the fractal ar-chitectures of the HDs, whereas too low a concentration ofHCl cannot make sure that the PANI coatings were dopedby photonic acid (HCl) sufficiently. So, there must be a bal-ance between the concentration of HCl and the morphologyof HDs, which demands further research work in order toachieve an optimal result.

Based on the FESEM images, a possible synthesis pro-cess of the HDs/PANI composite is schematically illustratedin Fig. 2. During the first polymerization stage, the oxi-dant Fe3+ can induce the formation of aniline oligomersas well as a fraction of PANI in the solution [2], and onlysmall amounts of them are deposited or grown on the sur-face of HDs. After the addition of APS, the pre-formed ani-

Fig. 2 Schematic illustration for the possible synthesis process ofHDs/PANI composite

line oligomers will be further polymerized into PANI, andthen the resultant PANI gradually assembles into larger ag-gregates and fully covers the surface of most HDs, formingthe HDs/PANI nanocomposite. In fact, we have to admit thatnot all the aniline monomers can be transformed into PANIdue to the weak acidic medium. The total mass of HDs andaniline monomers in the initial solution is about 0.4 g, butthe mass of resultant HDs/PANI composite (0.02 mol L−1)is only about 0.32 g after the polymerization process. Con-sidering that the dissolution of HDs is negligible, one canestimate that the PANI content in the composite is about25 wt%, and the yield of PANI is only 50 %.

To determine the crystallinity, phase, and purity of the as-prepared samples, the typical powder XRD patterns of HDsand HDs/PANI composites are shown in Fig. 1i. It can beobserved that both samples exhibit almost the same patternsincluding the diffraction peak locations and their relative in-tensities. All the diffraction peaks could be unambiguouslyindexed to a pure rhombohedral phase of α-Fe2O3 with lat-tice parameters of a = 5.036 Å and c = 13.747 Å, in goodagreement with the JCPDS card no. 33-0664. The narrowsharp peaks suggest that the HDs are highly crystalline. Noother impurities such as β-FeOOH, γ -Fe2O3, or Fe3O4 canbe detected.

3.2 The FT-IR and UV–Vis spectra of the preparedproducts

Figure 3 shows the FT-IR spectra of the HDs and HDs/PANIcomposites. For the HDs/PANI sample (black line), somecharacteristic peaks of PANI at around 1589, 1505 (the C=Cstretching deformation of quinoid and benzenoid rings, re-spectively), 1299 (the C–N stretching of secondary aromaticamine), 1173, 1034 (the aromatic C–H in-plane bending),and 834 cm−1 (the out-of-plane deformation of C−H in the1,4-disubstituted benzene ring) can be observed, confirmingthe existence of PANI in the composite [2, 21]. On the otherhand, both the HDs and HDs/PANI samples present threepeaks at 629, 527, and 448 cm−1, respectively, which are

Fig. 3 The typical FT-IR spectra of the HDs and HDs/PANI compos-ites

Synthesis and significantly enhanced microwave absorption properties of hematite dendrites/polyaniline nanocomposite

Fig. 4 The typical UV–Vis absorption spectra of the HDs andHDs/PANI composites

attributed to the characteristic lattice vibrations of α-Fe2O3

[22]. The additional peak at 1631 cm−1 for HDs may orig-inate from the surface-absorbed H2O or the water of hydra-tion.

The UV–Vis absorption spectra of the HDs and HDs/PANI composites were also recorded at room temperature.As shown in Fig. 4, three broad absorption peaks increas-ing sequentially from left to right can be observed at about305, 446, and 572 nm for the HDs sample (red line), indi-cating that the HDs may be used as effective photocatalystsunder visible-light irradiation. Interestingly, the wavelengthof 572 nm, where the strongest absorption peak is located,corresponds to the energy gap between the conduction andvalence bands of α-Fe2O3 (Eg = 2.1 eV), suggesting the n-type semiconductor characteristic for the prepared HDs. Forthe HDs/PANI sample, besides the absorption peak at about572 nm, which comes from the HDs, another two absorptionpeaks in the vicinity of 310 and 661 nm can also be detected,which may be attributed to the π–π∗ transition of the ben-zenoid ring and the benzenoid–quinoid excitonic transition,respectively, identical to conventional PANI powders [2].

3.3 The static magnetic properties of the synthesizedcomposites

Figure 5 demonstrates the magnetic hysteresis (M–H ) loopsof the HDs and HDs/PANI composites measured at 27 °C.The M–H loops confirm the weak ferromagnetic behaviorfor the HDs and HDs/PANI composites at room tempera-ture, with the applied field sweeping from −10 to +10 kOe.It is obvious that the magnetization of the HDs/PANI com-posite is smaller than that of HDs, which can be attributedto the non-magnetic PANI coatings. Interestingly, the mag-netization of both the HDs and HDs/PANI composites doesnot reach saturation when increasing the external field upto maximum. Close inspection of the M–H curves revealsthat the coercivity (Hcj) values of HDs and HDs/PANI com-posites are about 2.01 and 1.65 kOe at 27 °C, respectively,which are both larger than the previously reported value of

Fig. 5 Magnetic hysteresis (M–H ) loops of as-prepared (a) HDs and(b) HDs/PANI composites at room temperature

Fig. 6 Reflection loss (RL) curves of (a) HDs and (b) HDs/PANI waxcomposites at different thicknesses over 2–18 GHz

1.51 kOe [10]. The large Hcj values of the two compositesare believed to result from the shape anisotropy and highintrinsic magneto-crystalline anisotropy of HDs [14, 23].Besides, it is evident that Hcj of the HDs/PANI compos-ite is smaller than that of HDs, which may be due to theslight adverse effect to the dendritic structures of HDs dur-ing the polymerization process. The results prove that themorphology-related shape anisotropy could greatly influ-ence the coercive field, and subsequently determines themagnetic resonance position described below [24].

3.4 A comparison of the microwave absorbingperformances for the HDs and HDs/PANI waxcomposites

Figure 6 presents a comparison of the calculated reflectionloss (RL) curves in the frequency range of 2–18 GHz for theas-prepared HDs and HDs/PANI paraffin composites at dif-ferent thicknesses. It can be seen that the HDs wax samplewithout PANI coatings exhibits weak EM dissipation abil-ity when the layer thickness is varied from 1.9 to 5.3 mm,and the minimum RL value is only −6.2 dB at 14.6 GHzwith a layer thickness of 2.9 mm. However, the microwave

H. Wu et al.

Fig. 7 Reflection loss (RL) maps of (a) HDs and (b) HDs/PANI wax composites over 2–18 GHz when the matching thickness varies continuously;(c) the corresponding selected area RL map (RL < −10 dB) and odd numbers of λ/4 thickness lines for HDs/PANI wax composite

absorption performance of the HDs/PANI wax composite issignificantly enhanced. All the RL curves in Fig. 6b exhibitthe absorption ranges with RL values less than −10 dB whenthe layer thickness is increased from 1.9 to 5.3 mm. Espe-cially, when the matching thickness is fixed at 5.3 mm forthe HDs/PANI wax sample, not only does the minimum RLvalue reach about −54.0 dB at 4.6 GHz, but three signifi-cant RL peaks present in the RL curve with total bandwidth(RL < −10 dB) of 6.5 GHz, which are located at the 3.8–5.7, 12.8–13.9, and 14.5–18 GHz ranges, respectively.

As the matching thickness of the absorber is one ofthe crucial parameters which could determine the incidentimpedance, it is necessary to explore the influence of thick-ness on the absorption properties in detail. The color mapsof RL are plotted, which are shown in Fig. 7. From Fig. 7aand b, it is obvious that the minimum RL moves towardthe lower frequency region with increasing thickness, whichmay be explained by the quarter-wavelength model [25]. Inorder to verify the assumption, the HDs/PANI wax samplewas studied as an example. The areas with RL value lessthan −10 dB in the RL map of the HDs/PANI wax sam-ple are selected particularly for clarity, and two odd num-bers of λ/4 thickness lines (3λ/4 and 9λ/4) are also plot-ted, which are demonstrated in Fig. 7c. Just as expected,the frequency of the microwave absorption peak obeys thequarter-wavelength model very well except for some devia-tions over 15–18 GHz. To make a further comparison, it canbe observed that the absorption band of RL < −10 dB isobtained in the whole X-band for the HDs/PANI wax sam-ple with a thickness of 2.0–2.9 mm (Fig. 7b and c), whereasthe HDs wax sample does not show any effective absorp-tion (RL < −10 dB) with increasing thickness from 1.0 to7.5 mm (Fig. 7a). As shown in Fig. 7c, one can also tune therange of absorption frequency by adjusting the thickness.In a word, the HDs/PANI wax composite exhibits greatlyenhanced microwave absorption properties when comparedwith the HDs case, which are due to (1) the obviously en-hanced dielectric loss and appropriate conductivity; (2) the

complementary behavior between the dielectric and mag-netic losses discussed below.

3.5 The dynamic EM properties and the microwaveabsorption mechanisms for the prepared composites

The real part (ε′) and imaginary part (ε′′) of the relative com-plex permittivity for HDs and HDs/PANI wax compositesare shown in Fig. 8a and b. Obviously, when the pre-formedHDs are covered by conductive PANI coatings, the ε′′ valueof the HDs/PANI wax sample is larger than that of theHDs wax sample over almost the whole measured frequencyrange. According to the free-electron theory, ε′′ ≈ 1/2πε0ρ,where ρ is the resistivity. It can be speculated that the largerε′′ of the HDs/PANI wax sample indicates a higher conduc-tivity with respect to the HDs wax sample, which apparentlyoriginates from the conductive PANI coatings on the sur-face of the insulating HDs. Generally, too low a conductiv-ity could not attenuate EM waves efficiently and too higha conductivity could lead to too much front-face reflectionof EM waves. Thus, the PANI coatings may be preferred toadjust the dielectric behavior of the hierarchical composite,endowing the hierarchical composite with proper conduc-tivity. Moreover, the presence of a distinct relaxation peakwith a value of 3.3 at 14.1 GHz in the ε′′ curve for theHDs/PANI wax sample is apparently attributed to the PANIcoatings. On the other hand, the changing trend of ε′ forthe HDs/PANI wax sample is quite different from that ofthe HDs wax sample, which further proves that the PANIcoatings can play a predominant role in determining the di-electric behavior of the resulting composite. Besides, the ε′′values of the HDs/PANI wax composite are negative overthe 17–18 GHz range. Since the negative values of ε′′ arevery close to zero, it is reasonable to consider the negativevalues of ε′′ as noise. Nevertheless, the negative ε′′ valuesmay imply that the HDs/PANI composites have potential asLHMs.

Synthesis and significantly enhanced microwave absorption properties of hematite dendrites/polyaniline nanocomposite

Fig. 8 Real part (ε′) andimaginary part (ε′′) of complexpermittivity as a function offrequency for (a) HDs and(b) HDs/PANI wax composites

In general, the possible dielectric polarization in mate-rials can arise from ionic, electronic, atomic, dipolar, andinterfacial polarizations. Since ionic, electronic, and atomicpolarizations often work at about THz and PHz [26], thedielectric resonance of HDs and HDs/PANI wax compos-ites can only come from dipolar and interfacial polariza-tions. To investigate the dipolar polarization of the hierar-chical composites, the ε′ versus ε′′ curves, which are usu-ally defined as Cole–Cole semicircles, are shown (Fig. S2,Electronic supplementary material). It is worth noting thatthe HDs wax sample presents only one clear segment of asemicircle while the HDs/PANI wax sample exhibits two,suggesting that the former possesses only one Debye dipo-lar relaxation while the latter possesses two. The additionalsemicircle for the HDs/PANI wax sample is believed to re-sult from the PANI coatings, which can explain the distinctrelaxation peak in the ε′′ curve in Fig. 8b. Besides, the in-terfacial polarization, which occurs when the neighboringphases differ from each other in the dielectric constant, con-ductivity, or both, can also give rise to the permittivity dis-persion [27]. For both the HDs and HDs/PANI wax compos-ites, a large area of interfaces can be easily achieved betweenthe filler and the paraffin matrix due to their hierarchicaldendrite-like structures. Therefore, the interfacial polariza-tion could be significantly enhanced when the EM field isapplied. Particularly, the HDs/PANI wax sample has an ad-ditional interface between the HDs and the PANI coatings,which would be favorable to increasing the microwave ab-sorption ability. Additionally, other dielectric mechanismssuch as the surface defects of the PANI coatings and resis-tivity loss may also contribute to the dielectric loss of theHDs/PANI wax composite [28].

The real part (μ′) and imaginary part (μ′′) of the rela-tive complex permeability for the HDs and HDs/PANI waxcomposites are shown in Fig. 9a and b. It can be seen thatboth the μ′ curves fluctuate significantly near 1.0 over 2–18 GHz, which is due to the very weak ferromagnetic prop-erties of HDs. Meanwhile, the μ′′ curves present broadmulti-resonance peaks over 2–6 GHz, with maxima of 0.7at 2.6 GHz and 0.5 at 2.5 GHz, respectively. Besides, the

HDs wax sample exhibits a single relaxation peak, while theHDs/PANI case presents two relaxation peaks and a deepvalley with negative μ′′ values between the two relaxationpeaks. Now, it is necessary to figure out the possible originsfor the multi-resonance peaks and negative values of μ′′.For the former, the multi-resonance peaks (defined as ex-change resonance modes), which are often a consequenceof a small size effect of the nanocrystalline structure, a sur-face effect, and spin-wave excitations [29], are believed to beclosely related to the hierarchical dendrite architectures andnanoscaled sub-branches of the composites. For the latter,combined with the ε′′ curve of the HDs/PANI wax sample(Fig. 8b), it is surprisingly found that the frequency range(13.8–14.5 GHz) with negative μ′′ values is just where thestrong dielectric relaxation peak of ε′′ occurs, which indi-cates a complementary behavior between the permeabilityand permittivity. Similarly, the negative μ′′ values of theHDs/PANI wax sample may arise from the radiation and/ortransfer of energy [30, 31]. The negative μ′′ values may alsoindicate that the HDs/PANI composite can be a candidatefor LHMs.

Conventionally, the natural resonance frequency (fr) canbe calculated by the equation 2πfr = γHa, where γ isthe gyromagnetic ratio and Ha is the magneto-crystallineanisotropy field. For the Hcj value proportional to the Ha

value [24], and consequently proportional to fr according tothe equation, this means that a larger value of Hcj will leadto a blue shift of fr. So, it can be expected that the unusuallylarge values of Hcj for the HDs and HDs/PANI composites(above 1.6 kOe) could make the natural resonance frequen-cies of them shift to the GHz range. As anticipated, the posi-tions of fr for HDs and HDs/PANI wax samples are locatedat 2.6 and 2.5 GHz, respectively. The results also agree verywell with the measured Hcj values of HDs and HDs/PANIwax samples, i.e. the larger Hcj value corresponding to thehigher fr value.

In our work, the magnetic hysteresis loss and domain-wall displacement can be excluded because of the weak ap-plied field and the testing frequency in the GHz range [28].Thus, the magnetic loss in the present HDs and HDs/PANI

H. Wu et al.

Fig. 9 Real part (μ′) andimaginary part (μ′′) of complexpermeability as a function offrequency for (a) HDs and(b) HDs/PANI wax composites

paraffin composites can only come from eddy currentloss, magnetic natural resonance, and exchange resonance.To further investigate the magnetic loss, the values ofμ′′(μ′)−2f −1 versus frequency curves for HDs and HDs/PANI wax samples are plotted (see Electronic supplemen-tary material). As shown in Fig. S3, the two μ′′(μ′)−2f −1

versus frequency curves exhibit similar characteristics. Bothfirst decrease sharply over 2–6.1 GHz, and then keep nearlyconstant as the frequency is increased, except for one weakpeak at 14.5 GHz for the HDs wax sample and two slightpeaks at 13.4 and 15.1 GHz for the HDs/PANI case. If theobserved magnetic loss only results from eddy current loss,the value of μ′′(μ′)−2f −1 should be constant with increas-ing frequency. Accordingly, the contribution of eddy currentloss to the magnetic loss is significant over 6.1–18 GHz, andthe magnetic, natural, and exchange resonances are the maincontributors over 2–6.1 GHz.

In order to evaluate the contributions of dielectric andmagnetic losses to microwave absorption comprehensively,Fig. 10 shows the dielectric loss tangent (tan δε = ε′′/ε′)and magnetic loss tangent (tan δμ = μ′′/μ′) of the twoas-prepared composites. It is found that tan δμ of theHDs/PANI wax sample is comparable to that of the HDswax sample, whereas tan δε of the HDs/PANI wax sampleis greatly improved over almost the whole frequency rangethan that of the HDs case. Consequently, the dielectric lossmakes more contribution to the microwave absorption forthe HDs/PANI wax sample due to the PANI coatings, ascompared with the HDs case. As shown in Fig. 10b, wecan obviously observe that the change trend of tan δμ isalmost inverse to that of tan δε for the HDs/PANI wax sam-ple, which can be explained by the LRC equivalent circuitmodel [30]. However, the complementary behavior betweenthe dielectric and magnetic losses is negligible for the HDswax sample. The significant complementary behavior maythrow light on the strong coupling effect between the fractal-structured HDs and the dielectric PANI coatings, endow-ing the HDs/PANI wax composite with a better impedancematching, and thereby enhancing the microwave attenuationcapability largely.

Fig. 10 Magnetic loss tangent (tan δμ) and dielectric loss tangent(tan δε) of (a) HDs and (b) HDs/PANI wax composites in the rangeof 2–18 GHz

4 Conclusions

The conclusions in this study are summarized as follows.

1. The nanocomposite of HDs coated with a layer of PANIwas successfully realized by a two-step polymerizationprocess, maintaining the dendritic shape of the coatedHDs perfectly. Nevertheless, further work is still de-manded to explore the balance between the concentrationof doped HCl and the morphology of HDs.

2. Both the pristine HDs and the HDs/PANI compositeswere characterized thoroughly by different techniques.Particularly, the coercivity values of as-prepared HDsand HDs/PANI composites are 2.01 and 1.65 kOe, re-spectively, which contribute to the blue shift of the natu-ral resonance frequency (fr).

3. Compared with the pristine HDs wax composite, themicrowave absorption properties of the HDs/PANI caseare significantly improved. The absorption band (RL <

−10 dB) could cover the whole X-band when the thick-ness ranges from 2.0 to 2.9 mm. Moreover, an optimalreflection loss of −54.0 dB at 4.6 GHz and a broad ab-sorption bandwidth of about 6.5 GHz (RL < −10 dB) areobtained when the thickness is 5.3 mm.

4. Compared with the HDs wax composite, the HDs/PANIcase exhibits quite a different electromagnetic behavior,

Synthesis and significantly enhanced microwave absorption properties of hematite dendrites/polyaniline nanocomposite

such as the presence of negative ε′′ and μ′′, which isthe characteristic of LHMs, the enhanced conductivityand dielectric loss due to the PANI coatings, and thedistinct complementary behavior between the dielectricand magnetic losses, which all contribute to the signifi-cantly enhanced microwave absorption properties of theHDs/PANI wax composite.

Acknowledgements This work was supported by the National Nat-ural Science Foundation of China (Nos. 50771082 and 60776822) andthe Graduate Starting Seed Fund (No. Z2013155) of NorthwesternPolytechnical University. Hongjing Wu thanks the Excellent DoctorateFoundation, the Doctorate Foundation of Northwestern PolytechnicalUniversity, and the Scholarship Award for Excellent Doctoral Studentsgranted by the Ministry of Education.

References

1. X.L. Dong, X.F. Zhang, H. Huang, F. Zuo, Appl. Phys. Lett. 92,013127 (2008)

2. C.K. Cui, Y.C. Du, T.H. Li, X.Y. Zheng, X.H. Wang, X.J. Han, P.Xu, J. Phys. Chem. B 116, 9523 (2012)

3. Z.F. He, Y. Fang, X.J. Wang, H. Pang, Synth. Met. 161, 420(2011)

4. Z.Z. Wang, H. Bi, J. Liu, T. Sun, X.L. Wu, J. Magn. Magn. Mater.320, 2132 (2008)

5. Y.-E. Moon, J. Yun, H.-I. Kim, J. Ind. Eng. Chem. 19, 493 (2013)6. J.S. Hu, L.S. Zhong, W.G. Song, L.J. Wan, Adv. Mater. 20, 2977

(2008)7. S. Liu, Y.F. Yang, Y.Z. Jin, J.Y. Huang, B.H. Zhao, Z.Z. Ye, Appl.

Phys. A 100, 57 (2010)8. D.B. Yu, X.Q. Sun, J.W. Zou, Z.R. Wang, F. Wang, K. Tang, J.

Phys. Chem. B 110, 21667 (2006)9. L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan,

Adv. Mater. 18, 2426 (2006)10. M.H. Cao, T.F. Liu, S. Gao, G.B. Sun, X.L. Wu, C.W. Hu, Z.L.

Wang, Angew. Chem. Int. Ed. 44, 4197 (2005)

11. L.P. Zhu, H.M. Xiao, X.M. Liu, S.Y. Fu, J. Mater. Chem. 16, 1794(2006)

12. X.L. Hu, J.C. Yu, J.M. Gong, J. Phys. Chem. C 111, 11180 (2007)13. M. Zhang, X.M. Yin, Z.F. Du, S. Liu, L.B. Chen, Q.H. Li, H. Jin,

K. Peng, T.H. Wang, Solid State Sci. 12, 2024 (2012)14. G.B. Sun, B.X. Dong, M.H. Cao, B.Q. Wei, C.W. Hu, Chem.

Mater. 23, 1587 (2011)15. Z.X. Yu, Z.P. Yao, N. Zhang, Z.J. Wang, C.X. Li, X.J. Han, X.H.

Wu, Z.H. Jiang, J. Mater. Chem. A 1, 4571 (2013)16. Y.J. Chen, G. Xiao, T.S. Wang, Q.Y. Ouyang, L.H. Qi, Y. Ma, P.

Gao, C.L. Zhu, M.S. Cao, H.B. Jin, J. Phys. Chem. C 115, 13603(2011)

17. G.X. Tong, W.H. Wu, J.G. Guan, H.S. Qian, J.H. Yuan, W. Li, J.Alloys Compd. 509, 4320 (2011)

18. F.L. Wang, J.R. Liu, J. Kong, Z.J. Zhang, X.Z. Wang, M. Itoh, K.I.Machida, J. Mater. Chem. 21, 4314 (2011)

19. L. Qiao, X.H. Han, B. Gao, J.B. Wang, F.S. Wen, F.S. Li, J. Appl.Phys. 105, 053911 (2009)

20. H.J. Wu, L.D. Wang, S.L. Guo, Z.Y. Shen, Appl. Phys. A 108, 439(2012)

21. L.R. Kong, X.F. Lu, E. Jin, S. Jiang, X.J. Bian, W.J. Zhang, C.Wang, J. Solid State Chem. 182, 2081 (2009)

22. X.L. Hu, J.C. Yu, Adv. Funct. Mater. 18, 880 (2008)23. D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8, 1770 (1996)24. S. Ohkoshi, S. Kuroki, S. Sakurai, K. Matsumoto, K. Sato, S.

Sasaki, Angew. Chem. Int. Ed. 46, 8392 (2007)25. A.N. Yusoff, M.H. Abdullah, S.H. Ahmad, S.F. Jusoh, A.A. Man-

sor, S.A.A. Hamid, J. Appl. Phys. 92, 876 (2002)26. Z. Ma, Q.F. Liu, J. Yuan, Z.K. Wang, C.T. Cao, J.B. Wang, Phys.

Status Solidi B 249, 575 (2012)27. T. Liu, P.H. Zhou, J.L. Xie, L.J. Deng, J. Appl. Phys. 110, 033918

(2011)28. H.J. Wu, L.D. Wang, Y.M. Wang, S.L. Guo, Z.Y. Shen, J. Alloys

Compd. 525, 82 (2012)29. X.F. Zhang, X.L. Dong, H. Huang, B. Lv, J.P. Lei, C.J. Choi, J.

Phys. D: Appl. Phys. 40, 5383 (2007)30. X.L. Shi, M.S. Cao, J. Yuan, X.Y. Fang, Appl. Phys. Lett. 95,

163108 (2009)31. X.F. Zhang, P.F. Guan, X.L. Dong, Appl. Phys. Lett. 97, 033107

(2010)