PPy layer on Fe3O4 聚吡咯包覆四氧化三铁

Magnetic and conducting particles:preparation of polypyrrole

layer on Fe3O4nanospheres

Wei Chen a,Xingwei Li a,Gi Xue a,*,Zhaoqung Wang a,Wenqing Zou b

a Department of Polymer Science,State Key Laboratory of Coordination Chemistry,Nanjing University,Nanjing210093,PR China

b National Laboratory of Solid State Microstructures,Nanjing University,Nanjing210093,PR China

Received9February2003;received in revised form14April2003;accepted14April2003

Abstract

We reported here the preparation of Fe3O4–polypyrrole(PPy)nanocomposites with both magnetic and conducting properties. The magnetic properties of the resulting composites were investigated by vibrating sample magnetometer(VSM).The Fe3O4–PPy nanocomposites had coercive force from98.4to116.3Oe and saturation magnetization from0.268to9.23emu/g with the increasing Fe3O4content.The electrical conductivities ranged from10à5to10à2S/cm,depending on the Fe3O4content.The average size of Fe3O4–PPy nanocomposites with core-shell structure was about50nm.Structural characterizations by Fourier transform infrared(FTIR)and thermogravimetric analysis(TGA)proved the interaction between Fe3O4and PPy chains.

Keywords:Core-shell structure;Polypyrrole;Nanocomposite

1.Introduction

There is currently immense interest in the materials having both electrical and magnetic properties for the potential application as batteries,non-linear optics, electrochemical display devices,molecular electronic, electrical and magnetic shields,and microwave absorbing materials[1–6].Earlier studies indicated the presence of ferromagnetic interactions among some spins in some conducting polymers at low temperatures(<30K),but it was obviously not sui-table for practical applications[7].Compared with organic polymer ferromagnets,conducting polymer–inorganic ferromagnet composites are considered to be easier to prepare and more ready to be put into use.Bidan et al.had reported an electrochemical method to include nanomagnetic particles into polypyrrole(PPy) based on the use of ferro?uids in which a magnetic core is surrounded by an anionic complexing shell[8]. Recently,a conducting polypyrrole–ferromagnet com-posite?lm was prepared by means of the technique anodic oxidation in our laboratory[9].However, because the quantum of the product of the composites is limited by the electrochemical method,it is still interesting to synthesize PPy composites with both conducting and ferromagnetic behaviors by a chemi-cal method.Blending inorganic ferromagnetic powder with conducting polymer powder is obviously a sim-ple route to endow the conducting polymer with ferromagnetic properties.Liu and Wan et al.had reported in situ polymerization of pyrrole and aniline using FeCl2á4H2O and FeCl3á6H2O as oxidants,fol-lowed by treatment with KOH aqueous[10,11].Suri et al.had prepared nanocomposites of PPy and

iron

Applied Surface Science218(2003)215–221

*Corresponding author.Tel.:t86-253-592569;

fax:t86-253-317761.

E-mail address:xuegi@https://www.360docs.net/doc/902876971.html,(G.Xue).

doi:10.1016/S0169-4332(03)00590-7

oxide using iron nitrate and2-methoxy ethanol by simultaneous gelation of the oxide and PPy[12,13]. Although PPy–iron oxide composites had been suc-cessfully prepared by the above two approaches,their structure and properties are dif?cult to control due to synthetic method.Recently,Peng’s group had pre-pared a nanocomposite of Fe3O4-cross-linked-polya-niline with core-shell structure[14].The resulting composites had low coercive force(H c?2–8Oe). The conductivity of the composites at room tempera-ture was in a range of10à3–10à2S/cm.This method produced composites with core-shell structure whose electric and magnetic properties could be controlled easily by modifying ratios of the starting materials. Their research encouraged us to synthesize double functional magnetic-conducting composites with high conductivity and coercive force.

The fabrication of core-shell particles with unique and tailored properties was proved to be a useful means to combine the properties of both the core and the shell materials[15].Coatings based on intrinsi-cally conducting polymers generally show electrical conductivity of the composite surface[16].In this article,we present a novel approach to synthesize a core-shell Fe3O4–PPy nanocomposites,where Fe3O4 is the magnetic core and PPy is the conducting shell. PPy was chosen as the coating material for its high conductivity and stability in air.The Fe3O4nano-particles were primarily prepared by precipitation–oxidation method.Pyrrole monomer was chemically polymerized with Fenton’s reagent in the presence of polyethylene glycol(PEG Mw?4000)as surfactant and Fe3O4nanoparticles.The effects of iron oxide content in the composite with respect to the elec-trical and ferromagnetic properties of PPy composite were investigated.The origin of their electrical and ferromagnetic properties were also discussed based on the structural characterization including transmis-sion electron micrograph(TEM),Fourier transform infrared(FTIR),X-ray diffraction(XRD),thermo-gravimetric analysis(TGA),and vibrating sample magnetometer(VSM).

2.Experimental section

Fe3O4nanoparticles were prepared by precipita-tion–oxidation method as shown in reference[14].Coating experiments were carried out in250ml round bottom?ask in aqueous solution containing Fe3O4 nanoparticles and PEG.The pyrrole monomer was polymerized using Fenton’s reagent as oxidant at room temperature for12h under magnetic stirring. The resulting composites were washed with distilled water and vacuum oven dried.

The conductivities of resulting composites were measured by using the standard four-probe method. The magnetization measurements were performed at room temperature using Lakeshore VSM.The mor-phologies of the resulting composites were examined on a JEM-200CX transmission electron microscope. The crystal structure of the composites was examined by a Rigaku D/MAX-RC diffractometer using a Cu K a at35keV.FT-IR spectra were measured on an IFS66V vacuum-FTIR spectrophotometer.All spectra were recorded using KBr pellets.Thermal stability of the resulting composites was investigated by a SDT2960 thermogravimetric analyzer with nitrogen or oxygen as pure gas at a?ow rate of50ml/min.The heating rate was108C/min.

3.Results and discussion

3.1.Electrical and magnetic properties

As shown in Table1,values for the room conduc-tivities of Fe3O4–PPy composites are signi?cantly affected by the concentration of Fe3O4.When the concentration of Fe3O4increased from34to67%, the values for the room conductivity decreased from 10à2to10à5S/cm.This should be attributed to the partly exposure of insulating Fe3O4core which inhi-bits electron translation between PPy chains.

Table1

The conductivities and quantities of the starting materials Sample

No.

Fe3O4

(g)

Pyrrole

(m l)

Fe3O4Content

(wt.%)

Conductivity

(S/cm)

10.11091.18a

20.12083.79a

30.15067.41 3.66?10à5 40.110050.84 1.54?10à3 50.120034.08 1.15?10à2 602000 5.00?10à2

a Too low to be measured.

216W.Chen et al./Applied Surface Science218(2003)215–221

A typical magnetization curve of Fe 3O 4–PPy com-posite (containing 34wt.%Fe 3O 4)with the applied magnetic ?eld at room temperature is shown in Fig.1.A hysteresis loop for the composites was observed,which proved that those Fe 3O 4–PPy composites exhibited ferromagnetic behavior.The saturation magnetization (M s )of the composite is 0.268emu/g (electromagnetic unit per gram)and the coercive force is 98.4Oe.In our experiments,all the resulting Fe 3O 4–PPy composites with different contents of Fe 3O 4had hysteresis loop.The magnetic properties of the composites depend on the Fe 3O 4content.Increasing Fe 3O 4content from 34to 67%,the M s values increased from 0.268to 9.23emu/g distinc-tively,and the coercive force also increased from 98.4to 116.3Oe.These magnetic properties of the com-posites are quiet different from bulk Fe 3O 4particles with saturation magnetization of 84emu/g and coer-cive force of 500–800Oe [17].The reduced saturation magnetization is caused by the low content of Fe 3O 4.According to the report,the low coercive force may be resulted from the small size of Fe 3O 4cores [9].Since Fe 3O 4–PPy composites have core-shell stru-cture,it is expected to improve the capability of corrosion protection of iron oxide.For comparison,both the Fe 3O 4–PPy composites (containing 34wt.%Fe 3O 4)and Fe 3O 4nanoparticles were soaked into the 0.001M HCl solution for 24h.The capability of corrosion protection was measured by comparing cor-responding saturation magnetization of the samples before and after soaking.The ferromagnetic properties of the Fe 3O 4–PPy composites kept steady,while some Fe 3O 4nanoparticles were eroded which could be proved from the distinct decrease of the saturation magnetization.So,the core-shell structure of the com-posites provides advantages in corrosion protection of the core material.The use of magnetic particles allows,in a single chemical step,the preparation of a doubly functionalized composite exhibiting the magnetic properties of the magnetic core and the conducting properties of the PPy shell.Such systems could be easily tuned by modifying ratios of starting materials,which makes the doubly functionalized Fe 3O 4–PPy composites to have controllable electric and ferromag-netic properties.The Fe 3O 4–PPy composites can be used as electrical and magnetic shields and microwave absorbing materials since they have wide range of electrical conductivity and saturation

magnetization.

Fig.1.Dependence of the applied magnetic ?eld on the saturation magnetization of Fe 3O 4–PPy composite.

W.Chen et al./Applied Surface Science 218(2003)215–221217

3.2.Structure characterization

The morphology of the resulting Fe 3O 4–PPy com-posites (containing 34wt.%Fe 3O 4)is shown in Fig.2a ,while that of Fe 3O 4nanoparticles is used in comparison (Fig.2b ).The Fe 3O 4nanoparticles had average diameter of 10–20nm.Some of them form multiparticle aggregates,presumably because of the magneto dipole interparticle interactions.The Fe 3O 4–PPy composites were polydispersed with average diameter of 50nm.Since the Fe 3O 4–PPy composites have core-shell structure,we can obtain the thickness of PPy shell layer by calculation.The thickness of PPy shell layer in the resulting Fe 3O 4–PPy composites is about 15–20nm.

Fig.3a –e show FTIR spectra of the corresponding Fe 3O 4–PPy composites with Fe 3O 4content of 34,51,67,84,and 91%,respectively.When the Fe 3O 4con-tent is greater than 50%,the FTIR spectra of the Fe 3O 4–PPy composites are almost the same as that of normal PPy sample [18].For instance,the bands at 1541and 1456cm à1are corresponding to typical pyrrole rings vibration;the bands at 1300,1089,and 1170cm à1are corresponding to ?C –H in plane vibration and the bands at 784and 903cm à1are corresponding to ?C –H out-of-plane vibration.As shown in the ?gure,with the increase of Fe 3O 4con-tent,the intensity of the band at 570cm à1correspond-ing to Fe 3O 4[19]increased distinctively,and those bands corresponding to PPy characteristics,such as 1541,1170,903,784cm à1,shift to higher wavenum-bers.This indicates that there is some interaction between PPy and Fe 3O 4nanoparticles.

Fig.4shows the XRD patterns of Fe 3O 4–PPy composite (containing 34wt.%Fe 3O 4)as well as Fe 3O 4nanoparticles.The main peaks at 2y ?35:58,56.98,62.38are characteristic of Fe 3O 4[20].A broad peak at about 2y ?258is a characteristic peak of PPy [21].Following the Scherrer ’s formula [22]D ?

K l b cos y

a crystallite size (D )of particle can be estimated.Here

the X-ray wavelength of Cu K a radiation l is 1.54A

?,and K is the shape factor,which can be assigned a value of 0.89if the shape is unknown,cos y is the cosine of the Bragg angle and b is the half-height of angle diffraction in radians.When the re ?ecting peak at 2y ?35:58,which is the [311]characteristic peak of Fe 3O 4,is chosen to calculate the average diameter,the average sizes of Fe 3O 4particles are about 20nm,which is consistent with the result of TEM.

The TGA results in nitrogen or air of Fe 3O 4–PPy composite with the Fe 3O 4content of 34%from room temperature are shown in Fig.5,where those of normal PPy are compared.The TGA patterns obtained in nitrogen clearly indicate a better thermal stability of the Fe 3O 4–PPy composite than the normal PPy.The normal PPy began decomposing at 2148C while the Fe 3O 4–PPy composite decomposed at a higher tem-perature of about 2508C.The improved thermostabil-ity should be attributed to the interactions between Fe 3O 4and PPy chains.The TGA patterns obtained

Fig.2.The TEM photographs of (a)the Fe 3O 4–PPy composite and (b)Fe 3O 4particles.

218W.Chen et al./Applied Surface Science 218(2003)215–221

Fig.3.FTIR spectra of Fe 3O 4–PPy composites (a)with 34%Fe 3O 4content,(b)with 51%Fe 3O 4content,(c)with 67%Fe 3O 4content,(d)with 84%Fe 3O 4content,(e)with 91%Fe 3O 4

content.

Fig.4.XRD patterns of (a)Fe 3O 4–PPy composite and (b)Fe 3O 4particles.

W.Chen et al./Applied Surface Science 218(2003)215–221219

air condition show that for the case of composites the PPy is removed and the core is left behind at about 5008C.

4.Conclusion

Fe 3O 4–PPy composites with core-shell structure were successfully synthesized.The resulting compo-

sites have both ferromagnetic and electric properties.The electrical conductivities range from 10à5to 10à2S/cm,depending on the Fe 3O 4content.The saturation magnetization increased distinctively with an increase of Fe 3O 4content.Accordingly,it provides an easy way to control the ferromagnetic and electric properties by modifying ratios of starting materials.Structure characterization of the resulting composites was carried out by TEM,FTIR,TGA,and XRD.

TEM

Fig.5.TGA patterns of (a)Fe 3O 4–PPy composite and (b)normal PPy particles in (A)nitrogen and (B)air condition.

220W.Chen et al./Applied Surface Science 218(2003)215–221

and XRD results proved that the Fe3O4particles with the size of10–20nm might be responsible for the low coercive force.FTIR and TGA results indicate that there are some interactions between the Fe3O4cores and PPy shells.

Acknowledgements

The authors are grateful for the support from the National Natural Science Foundation of China(No. 90205015).

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