Materials Research Bulletin 47 (2012) 602–607

Solvothermal synthesis of monodispersed CoZr4(PO4)6 microspheres and theirapplication as microwave absorber

Tingting Chen a, Genban Sun a,*, Shulan Ma a, Xiaojing Yang a, Changwen Hu b,*a College of Chemistry, Beijing Normal University, Beijing 100875, PR Chinab Key Laboratory of Cluster Science, Ministry of Education of China and Department of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China

A R T I C L E I N F O

Article history:

Received 24 July 2011

Received in revised form 31 October 2011

Accepted 27 December 2011

Available online 4 January 2012

Keywords:

A. Inorganic compounds

A. Microporous materials

B. Chemical synthesis

C. Electron microscopy

D. Microstructure

A B S T R A C T

Monodispersed CoZr4(PO4)6 microspheres with a diameter of 40 mm were achieved via a combining

solvothermal and calcination route. The crystallinity of the calcined microspheres with shell structure

was improved, while the monodisperse property and morphologies remained. The possible formation

mechanism of the porous CoZr4(PO4)6 microspheres with nanoshell was proposed. The samples were

characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform

infrared spectra (FT-IR) technologies, thermal analysis (TG and DSC), nitrogen adsorption–desorption

isotherms and network analyzer. The sample calcined at 900 8C shows a strongest absorbability in the

radar-wave absorbability test.

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1. Introduction

Colloidal microspheres have been regarded as ideal theoreticaland practical research objects because their intrinsic propertiesmight be finely tuned by simply changing their parameters, such asthe diameter, the chemical composition, the morphology and thecrystallinity. The hollow or porous microspheres with a highsurface area and high pore volume can be used as functionalmaterials in the fields of magnetics, optical devices and catalysis.Moreover, they can also be used as low-density structuralmaterials for microwave absorbing materials and capsules forthe controlled release of medications and dyes [1–4]. In the pastfew years, many studies on preparing the hollow microspheres ofmetal-oxide have been reported, and hydrothermal assisted sol–gel process followed by thermal treatment for crystallization areproved to be a beneficial method for controlling their morphol-ogies and crystal structures [3–5].

Double metal phosphates represent an important class ofinorganic compound with potential applications as heterogeneouscatalysts, low thermal expansion ceramic materials, ion exchan-gers and biocompatible coatings for metal endoprostheses. Inrecent years, some catalysts based on complex frameworkphosphates of Zr and Co or Mn were found to be very efficientin the oxidative dehydrogenation of propane into propylene at a

* Corresponding authors. Tel.: +86 10 62242577; fax: +86 10 58802075.

E-mail address: gbsun@bnu.edu.cn (G. Sun).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.12.037

short contact time [6]. Cobalt-containing framework zirconiumphosphates with the NaZr2(PO4)3 structure appear to be veryattractive for systematic studies of those factors, since they possessa flexible lattice allowing the realization of aliovalent (hetero-valent) substitution both in cation and anion positions without thedestruction of their structures [7–10]. Materials based upon thosesystems possess a high cationic conductivity, a high radiation andcorrosion stability, as well as ultralow thermal expansion [11–13].Aliovalent substitution allows us to tune into a broad range of theredox and acid–base properties of framework zirconium phos-phates, which make them promising as catalysts for the processesof acid–base [14–16] and redox [17] types.

So far, large numbers of simple inorganic compound micro-spheres such as Co hollow mesospheres [18], BiTeI submicrometerhollow spheres [19], ZnSe hollow microspheres [20], Fullerene-likeBN hollow spheres [21], CuO and ZnO dandelions [22,23] wereprepared by a method of self-assemble or via a hydro-/solvothemalroute in many research groups. Therefore, the hydro-/solvothemalprocess, offering unique advantages for the preparation ofhomogeneous phosphates, would be a method suitable to obtainCoZr4(PO4)6 with the morphology of microspheres. Recently,Ohkoshi et al. synthesized a new series of e-GaxFe2�xO3

(0.10 � x � 0.67) nanoparticles by the combination of reversemicelle and sol–gel techniques or only the sol–gel method andfound that the materials can absorb electromagnetic wave in awide range between 35 GHz and about 190 GHz [24]. Some effortsfocused on electromagnetic materials such as hierarchicalcobalt phosphate microarchitectures [25], CoO nanobelts and

Fig. 1. XRD patterns of CoZr4(PO4)6 dried at (a) 40 8C, calcined at (b) 500 8C, (c)

700 8C, and (d) 1000 8C.

Fig. 2. FT-IR spectra of CoZr4(PO4)6 dried at (a) 40 8C; (b) calcined at 900 8C.

T. Chen et al. / Materials Research Bulletin 47 (2012) 602–607 603

submicrometer spheres [26], and iron-based dendritic micro-structured materials [27] as vivid microwave absorption materialshave been done in our group. However, to the best of ourknowledge, until now, there have been few reports on thepreparation of double metal phosphates porous microsphereswith shell structure and no research on the radar-wave absorb-ability of double metal phosphates micro-/nanostructures. In thispaper, monodispersed CoZr4(PO4)6 microspheres were achievedvia a mild solvothermal method. After calcined at differenttemperatures from 500 to 1000 8C, the crystallinity of the spheresgot improved and their morphologies changed from hard-corespheres to porous spheres with shell structure while no damage tothe monodispersity. The possible formation mechanism and theradar-wave absorbability of the hollow microspheres werediscussed in detail.

2. Experimental

2.1. Chemicals

ZrOCl2�8H2O, Co(NO3)2�6H2O, H3PO4, NH4F and HO(CH2)2OHwere of analytical purity. CH3NH2 were of chemical purity. All ofthe reagents were used without further purification, which werepurchased from Beijing Chemical Reagents Co., Ltd.

2.2. Synthesis

In a typical synthesis, 1.61 g of ZrOCl2�8H2O and 1.46 gCo(NO3)2�6H2O were mixed in 15 mL of HO(CH2)2OH and formeda solution, to which 0.41 mL of H3PO4 (85%), 0.74 g of NH4F and0.64 mL of CH3NH2 water solution (25.0–30.0%) were added. Thereaction mixture was stirred with a magnetic stirrer for 120 minand then transferred into a Teflon-lined stainless steel autoclave toa fill fraction of 60%. The crystallization was carried out underautogenous pressure at 180 8C for 5 days. The purple product wasfiltered and washed with deionized water and ethanol and thendried in air at 40 8C. The sample synthesized via the solvothermalmethod was calcined in air at temperatures in the range of 500–1000 8C for 2 h.

2.3. Characterization

The composition and phase purity of the samples were analyzedby XRD with monochromatized Cu Ka (l = 1.54178 A) incidentradiation by a Shimadzu XRD-6000 operated at 40 kV voltage and50 mA current. XRD patterns were recorded from 58 to 608 (2u)with a scanning step of 0.028. The size distribution andmorphologies of the samples were characterized by a JEOL JSM-6700F field-emission SEM. FT-IR spectra were recorded on aNicolet MAGNA-IR 560 spectrometer with a wide-band MCTliquid-nitrogen-cooled detector and a KBr beam splitter. TG andDSC curves were recorded on a PERKIN-ELMER Pyris-1 and aNETZSCH thermal analyzer with a calefactive speed of 10 8C/min ina N2 atmosphere, respectively. The nitrogen adsorption–desorp-tion isotherm and Barrett–Joyner–Halenda (BJH) methods wereanalyzed on an Automated Surface Area and Pore Size Analyzer(model: QUDRASORB SI). The radar-wave absorbabilities of the as-prepared products were tested on a Network analyzer (AgilentE8362B, 8–12 GHz).

3. Results and discussion

The phase purity and crystallinity of the samples are supportedby powder X-ray diffraction (XRD). Fig. 1 shows the XRD patternsof the samples. As shown in Fig. 1a, a halo was observed, thusevidencing the amorphous nature of the sample dried at 40 8C.

After calcined at 500–1000 8C (Fig. 1b–d), the crystalline phase wasdetected. When the calcination temperature was 500 8C, thecharacteristic peaks of zirconium phosphate and cobalt phosphatewere detected, which indicate that there are zirconium phosphateand cobalt phosphate in the amorphous sample and they becamecrystalline state firstly (Fig. 1b). The increase of the calcinationtemperature up to 700 8C can transform and improve thecrystallinity of the samples, which is reflected in the narrowingof the diffraction peaks, their shifts, and intensity redistribution(Fig. 1c). From Fig. 1c, we can also conclude that a small quantity ofzirconium phosphate and cobalt phosphate were also observed inthe as-prepared sample. But when T was 1000 8C, the peaks can bewell indexed to the monoclinic primitive CoZr4(PO4)6 (JCPDS card,No. 45-0014) with lattice parameters of a = 8.853 A, b = 8.924 A,c = 12.43 A and b = 90.358. No characteristic peaks of the impuri-ties were observed (Fig. 1d).

FT-IR spectra of the samples synthesized by the solvothermalmethod then directly dried in air at 40 8C and calcined at 900 8C areshown in Fig. 2, respectively. The spectral peaks providevibrational information of the functional groups, which is veryimportant for characterizing the structure and composition of thematerials. From Fig. 2a, it can be concluded that CoZr4(PO4)6 havetwo characteristic peaks: the symmetric stretching vibration of the

Fig. 3. SEM images of CoZr4(PO4)6 dried at 40 8C.

Fig. 4. SEM images of CoZr4(PO4)6 calcined at (a and b) 500 8C, (c and d) 700 8C, (e and f) 900 8C, and (g and h) 1000 8C.

T. Chen et al. / Materials Research Bulletin 47 (2012) 602–607604

P–O (900–1200 cm�1) and the finger peaks of the lattice vibrationmodes of Co–O (400–900 cm�1). Besides, the stretching vibrationof the C–H (2900 cm�1) and the asymmetrical stretching vibrationof the C–H (1425 cm�1), the vibration of the N–H (3300–3500 cm�1) can be also observed, which indicate there is CH3NH2

in the sample. In addition, the FT-IR spectrum of the samplecalcined at 900 8C was also provided (shown in Fig. 2b). FromFig. 2b, many peaks became weak or disappeared such as thevibration of the N–H (3300–3500 cm�1), the stretching vibration ofthe C–H (2900 cm�1) and the asymmetrical stretching vibration ofthe C–H (1425 cm�1), we can concluded that most organic elementdisappeared by calcination at 900 8C.

Fig. 3 shows the SEM images of the samples synthesized by thesolvothermal route and dried at 40 8C. Monodispersed CoZr4(PO4)6

microspheres with the diameter of 40 mm can be obtained (Fig. 3a).As shown in Fig. 3b, the surface of the sphere is smooth, whichconfirmed the microsphere was composed of a single nuclear.

Fig. 4 shows the SEM images of the samples obtained via thesolvothermal route and calcined at 500, 700, 900 and 1000 8C for2 h. As shown in Fig. 4a and b, the morphology of the samplecalcined at 500 8C is same to that dried in air at 40 8C and thesurface of the sphere is still smooth. After calcined at 700 8C, the

surface of sphere becomes rough (Fig. 4c) and the hollow structurecan be observed in the sample after grinding (Fig. 4d). Fig. 4d showsan individual broken shell of one sphere and the shell thickness ofthe microsphere is about 15 mm. Fig. 4e shows the morphology ofmicrosphere retained after calcined at 900 8C, but the surface of thesphere becomes rough or granular (see the inset of Fig. 4e). Fig. 4fshows the sample calcined at 900 8C followed by grinding. It can beseen from Fig. 4f that the interior of the sphere has turned intoporous structure but the shell is remained. Fig. 4f also reveals thatthe shell thickness of the microspheres is about 200 nm. Aftercalcined at 1000 8C, the surface of the sphere becomes coarse pored(Fig. 4g). Fig. 4h shows the structure of the shell has been destroyedby grinding and the shell was porous different from the samplecalcined at 900 8C. Based on the above discussions and the XRDresults of all the samples, we concluded that the calcinationtemperature can enhance the grade of the samples’ crystallinityand obtain the porous microspheres with the nanoscale shell.

It is effective to prepare porous microspheres with the shellstructure by the template via the solvothermal method. Based onthese observations, an aggregation mechanism to formCoZr4(PO4)6 microspheres is proposed, with reference to thepreparation of monodispersed ZnSe microspheres [20]. At the

Fig. 5. Schematic representation of the formation mechanism of the microspheres.

T. Chen et al. / Materials Research Bulletin 47 (2012) 602–607 605

beginning of the reaction, the reactant particles and the molecularsof methylamine disperse randomly in the glycol solution (Fig. 5a).In the process of chemical reaction, as illustrated in Fig. 5b, themicrospheres of CoZr4(PO4)6 monomers form gradually around themethylamine moleculars. After calcined at a high temperature,methylamine decomposed into carbon dioxide, carbon monoxideand nitrogen oxide, thus the porous CoZr4(PO4)6 microsphereswere finally obtained (Fig. 5c).

Thermal analysis experiments including TG and DSC wereemployed to further characterize the thermal stability and theevolution of the as-prepared microspheres in combination withXRD data and FT-IR spectra. TG curve for the sample obtained viathe solvothermal method protected by N2 atmosphere ispresented in Fig. 6. The process for weight loss consists of tworemarkable steps in the TG curve from room temperature up toca. 900 8C, including the desorption of water physisorbed on theexternal surface of the crystallites and the removal of water andorganic compounds in the interior of the microspheres,respectively. From TG curve in Fig. 6, it can be clearlyseen that the crystallization water and physisorbed water inthe sample stripped down at 147.3 8C and the weight loss was1.1 wt%. The process from 316 to 590.2 8C corresponds to the lossof water and organic compounds in the interior ofthe microspheres and the weight loss was about 4.0 wt%. FromDSC curve in Fig. 6, there are four remarkable endothermic peaks.The broad endothermic peak at 447 8C should be ascribed to thedesorption of water physisorbed on the external surface of thecrystallites and the removal of water and organic compounds in

Fig. 6. TG and DSC curves of the sample obtained via a solvothermal method and

dried at 40 8C.

the interior of the microspheres. The small endothermic peak at651 8C was attributed to the evolution of a small quantity ofzirconium phosphate and cobalt phosphate. The endothermicpeak at 872 8C was ascribe to the evolution of CoZr4(PO4)6 from asmall quantity of zirconium phosphate and cobalt phosphate.The small endothermic peak at 960 8C should be attributed to thetransformation of CoZr4(PO4)6. All the data is well consistentwith the XRD patterns obtained at different temperatures.

The isotherms of nitrogen adsorption and desorption at 77 K forthe samples obtained via the solvothermal method followed bydried at 40 8C and calcined at 500 8C, 900 8C and 1000 8C are plottedin Fig. 7. All the isotherms can be categorized as type IV, withdistinct hysteresis loops observed in the range of 0.05–0.97p/p0

(Fig. 7a), 0.05–0.91p/p0 (Fig. 7b), 0.1–0.96p/p0 (Fig. 7c) and 0.65–0.98p/p0 (Fig. 7d), respectively. Using these isotherms, it is possibleto calculate the values of the BET surface area (SBET); SBET values ofthe samples dried at 40 8C and calcined at 500 8C, 900 8C and1000 8C are 6.12, 7.25, 19.37 and 14.72 m2/g, respectively. The poresize distribution analysis of the samples dried at 40 8C and calcinedat 500 8C show the pore diameters are around 10 nm (inset ofFig. 7a and b), which are attributed to the boundary spaces ofmicrospheres. The inset of Fig. 7c and d indicate the pore diametersof the samples obtained at 900 8C are mainly around 3 nm and10 nm and the samples obtained at 1000 8C are mainly around5 nm and 11 nm, which should be ascribed to the aggregation ofsmall nanoparticles and the existence of structural porosity.

The samples obtained via the solvothermal method followed bydried at 40 8C and calcined at 500 8C, 900 8C and 1000 8C were feltedby paraffin and pressed in the wave-guide flange of 3 cm. The radar-wave absorbability of the samples was tested and the results areshown in Fig. 8. When the frequency is 10.3, 8.39, 8.49 and 8.5 GHz,the maximum of wave-absorbability of the samples dried at 40 8C,calcined at 500 8C, 900 8C and 1000 8C can be observed. Themaximum of the wave-absorbability are 9.17, 29.6, 31.8 and29.2 dB, respectively. Therefore, the wave-absorbability property ofthe sample calcined at 900 8C is the strongest in the as-obtainedsamples. The wave-absorbability property relates to the shellnanostructure of the microsphere, since the number of surfaceatoms with unsaturated bonds will greatly increase as the sizedecreases, resulting in an increase of the dipoles. The dipolepolarizations can contribute to the dielectric loss. According theformula [25–27]:

Zin ¼ffiffiffiffime

rtanh � j

2p fd

c

� � ffiffiffiffiffiffiffimep� �

R ¼ 20 logZin þ 1

Zin � 1

��������

Fig. 7. Nitrogen isotherms and pore sizes of the samples obtained at (a) 40 8C, (b) 500 8C, (c) 900 8C, and (d) 1000 8C. Inset of the figures are pore sizes distribution, respectively.

T. Chen et al. / Materials Research Bulletin 47 (2012) 602–607606

the crystallinity and the morphology of porous microspheres with

shell nanostructure of the sample can mainly influence theparameter of conductivity e, which significantly contributes towave-absorbability property. In addition, the tremendous surfacearea and the hollow nanostructure of the microspheres calcined at900 8C (Fig. 7c and 4f) can be propitious to absorb the energy ofelectromagnetic wave due to their diffuse reflection in the cavitiesof each CoZr4(PO4)6 microsphere.

Fig. 8. The radar wave-absorbability of the as prepared products.

4. Conclusions

Monodispersed CoZr4(PO4)6 microspheres with the diameter of40 mm were achieved via a mild solvothermal method. Thecrystallinity of the microspheres was improved by a simplecalcination, while the monodisperse property and morphologiescan be remained. We demonstrated that the radar-wave absorb-ability of CoZr4(PO4)6 microspheres calcined at 900 8C is strongestat 8.5 GHz.

Acknowledgments

This work was supported by the Natural Science Foundation ofChina (NSFC, No. 20901010).

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