J. Phys. Chem. C 2007, 111, 12603-12608
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Synthesis and Characterization of Novel Coralloid Polyaniline/BaFe12O19 Nanocomposites Ping Xu,† Xijiang Han,*,† Jingjing Jiang,† Xiaohong Wang,‡ Xuandong Li,† and Aihua Wen† Chemistry Laboratory Center, Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China, and Beijing Institute of Aeronautical Materials, Beijing 100095, China ReceiVed: May 19, 2007; In Final Form: June 20, 2007
By supplying inorganic ferrite nanoparticles of different morphologies as nucleation sites, PANI/ferrite nanocomposites with novel coralloid structures were synthesized successfully through a simple, conventional, and inexpensive one-step in situ polymerization method without the aid of any surfactant, organic dopant, or template. As shown by XRD, FT-IR and UV-vis, there is no obvious chemical interaction between PANI and BaFe12O19 (BF) nanoparticles; that is, ferrite nanoparticles served only as the nucleation centers for the growth of PANI nanofibers, whereas the nanoparticles have an effect in reducing the diameters of the produced PANI nanofibers. PANI/BF nanocomposites are hard magnetic properties with alternative electrical conductivities and magnetic properties. The reflection loss of BF nanoparticles in 2-18 GHz was essentially enhanced upon PANI coating, and the frequency relating to maximum reflection loss shifts to a higher value with the increase in BF content because of the higher anisotropy field. With controllable electrical, magnetic, and electromagnetic properties, the prepared nanocomposites may have potential applications in chemical sensors, gas separation, catalysis, microwave absorbing, and magnetoelectric devices.
Introduction Conventional conducting polymers, of great technological and scientific interest, are being applied and/or investigated for many applications including in batteries, sensors, catalysts, electromagnetic shielding, separation membranes, and electrochromic devices. One-dimensional (1D) polyaniline (PANI) nanostructures have generated much interest in nanotechnology, with the expectation that such materials will possess the advantages of both low-dimensional systems and organic conductors.1 A large number of polymerization approaches, such as “hard and soft” templates,1d,2 interfacial,3 nanofiber seeding,4 oligomer-assisted,5 electrochemical,6 radiolytic,7 and dilute polymerization techniques8 have been applied to synthesize 1D PANI nanofibers or nanotubes. M-type barium ferrite with hexagonal molecular structure (BaFe12O19) is a promising material for permanent magnet, advanced recording, and microwave absorbing because of its fairly large magnetocrystalline anisotropy, high Curie temperature, relatively large magnetization, excellent chemical stability, and corrosion resistivity,9 whereas its magnetic and electric properties should be modulated to satisfy different applications. Organic-inorganic composites with organized structures usually provide a new functional hybrid, with synergetic or complementary behavior between organic and inorganic materials, which have attracted considerable attention for their potential applications. Multifunctionalized PANI nanotubes or nanowires have also been prepared by blending with inorganic electrical, optical, and magnetic nanoparticles to form composite nanostructures.7,10 However, it is noticed that without the aid of complex dopants with bulky side groups1b,11 surface modification of inorganic phases,12 or radiolysis7 nanoparticles with core/ * Corresponding author. Tel: +86-451-86413702. Fax: +86-45186418750. E-mail:
[email protected]. † Harbin Institute of Technology. ‡ Beijing Institute of Aeronautical Materials.
shell structures are usually obtained,13 instead of 1D PANI nanofibers or nanotubes containing inorganic nanoparticles. In this paper, we describe a simple, inexpensive, and environmentally friendly in situ polymerization method to synthesize bulk quantities of uniform coralloid PANI nanocomposites containing BaFe12O19 (BF) nanoparticles in one step without large organic dopants, surfactants, and any template. HCl was used as the dopant, and ammonium persulfate (APS) was used as the oxidant. The concentration of the inorganic phase can affect the morphology and physicochemical properties of the resulting PANI/BF nanocomposites significantly. The formation mechanism is discussed from the classical nucleation theory. These findings may have applications in morphological control in the polymerization reactions and preparation of organic-inorganic composite materials for various applications. Experimental Section Synthesis of Ferrite Nanoparticles and PANI/Ferrite Nanocomposites. Barium hexaferrite, BaFe12O19, nanoparticles were prepared by a reverse microemulsion technique as reported in a previous work.14 Spinel NiFe2O4 nanoparticles were synthesized by the same method. The PANI/ferrite nanocomposites were synthesized by in situ polymerization in the presence of ferrite nanoparticles without an external template. A typical preparation process for PANI/ BF nanocomposites is as follows: BF nanoparticles were added to 0.2 M HCl under ultrasonication for 30 min to obtain a uniform suspension, and then aniline was added under ultrasonication for another 30 min to form an aniline/HCl mixture containing BF. The mixture was cooled in an ice-water bath for 1 h before adding precooled APS aqueous solution for oxidative polymerization for 24 h. The precipitated powder was centrifuged and washed with distilled water and methanol until the filtrate became colorless and then dried in a vacuum drying cabinet at 70 °C for 48 h. Throughout the experiment, the molar ratio of aniline to both HCl ([An]/[HCl]) and APS ([An]/[APS])
10.1021/jp073872x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
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Figure 3. Schematic illustration of the nucleation and growth of PANI nanofibers with ferrite nanoparticles as nucleation sites and the formation of coralloid nanostructures.
Figure 1. SEM images of PANI/BF nanocomposites synthesized by a simple in situ polymerization method from different mass ratios of aniline monomer and BF nanoparticles [An/BF]: (a) 9:1, (b) 4:1, (c) 2:1, (d) 1:1, and (e) 1:2. The average diameter of the outward branches is in the range of 20-50 nm. (f) SEM image of BaFe12O19 nanoparticles prepared from a reverse microemulsion technique.
pure PANI nanofibers were prepared under the same conditions without the inorganic nanoparticles. Characterization. Scanning electron microscope (SEM) measurements were carried out on the S-4800 (Hitachi) scanning microscope to study the morphology of the samples, and the samples were mounted on aluminum studs using adhesive graphite tape and sputtercoated with gold before analysis. The characteristics of the crystallite structure of the prepared samples were determined using an XRD-6000 X-ray diffractometer (Shimadzu) with a Cu KR radiation source (λ ) 1.5481 Å, 40.0 kV, 30.0 mA). The FT-IR spectroscopy was measured on Nicolet Avatar 360 FT-IR Spectrometric Analyzer with KBr pellets. UV-vis spectra were recorded on a Shimadzu UV3100 sepctrophotometer by dissolving the samples in N,N-dimethyl formamide to form saturated solutions. The electrical conductivity of compressed rods at room temperature was measured by a standard four-probe method using a Keithley 2400 System digital sourcemeter. The surface area of the nanofibers was measured by nitrogen adsorption-desorption isotherms using the Brunauer-Emmett-Teller (BET) method (Quantachrome, Autosorb-1). The samples were degassed under vacuum at 100 °C before measurement. The magnetic properties (intrinsic coercivity, saturation, and remanent magnetization) were measured using a vibrating sample magnetometer (VSM, Lake Shore 7307). A HP-5783E vector network analyzer was applied to determine the relative permeability and permittivity in the frequency range of 2-18 GHz for the calculation of reflection loss. A sample containing 50 wt % ferrite particles was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for microwave measurement in which paraffin wax was used as the binder. Results and Discussion
Figure 2. SEM images of (a) PANI nanofibers synthesized by in situ polymerization without inorganic materials as nucleation sites (average diameter: 200 nm); (b) PANI/BF nanocomposite prepared from [An/ BF] ) 1:9, without coralloid morphologies; (c) coralloid PANI/NiFe2O4 nanocomposites prepared from [An/NiFe2O4] ) 9:1; and (d) NiFe2O4 nanoparticles prepared from the reverse microemulsion technique.
was retained both at 1:1. The effect of the concentration of BF nanoparticles on the morphology, structure, and physicochemical properties of the resulting PANI/BF nanocomposites was studied by modulating mass ratios of aniline monomer to BF nanoparticles [An/BF] at 1:9, 1:2, 1:1, 2:1, 4:1, and 9:1, respectively. PANI/NiFe2O4 nanocomposites were synthesized by the same preparation process with [An/NiFe2O4] controlled at 9:1, and
The SEM images of all of the polyaniline powders containing BF nanoparticles show coralloid morphologies with outward extending branches having an average diameter in the range of 20-50 nm (Figure 1a-e). It is important to notice that the concentration of BF nanoparticles plays a critical role in the growth of the coralloid nanocomposites. With the increase in the content of the inorganic phase, the growth of the branches of the coralloid structure is restrained. In the synthesis of polyaniline nanofibers by “nanofiber seeding”, it was suggested that unseeded reactions or reactions seeded with particulate or nanospherical powder yielded only emeraldine‚HCl precipitate having nonfibrous, particulate, or nanospherical morphologies.4 Here, with flaky BF nanoparticles prepared by a reverse microemulsion technique as polymerization seeds (10-20 nm in thickness and 50-100 nm in size, Figure 1f), PANI/BF
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Figure 4. UV-vis (a) and FT-IR (b) spectra of PANI and PANI/BF nanocomposites from different mass ratios of aniline monomer and BF nanoparticles [An/BF].
Figure 5. Powder XRD patterns of BF nanoparticles, PANI, and PANI/ BF nanocomposites from different mass ratios of aniline monomer and BF nanoparticles [An/BF].
Figure 6. Effect of BaFe12O19 concentration on the room-temperature electrical conductivities and specific surface areas of PANI, BF, and PANI/BF nanocomposites (mass fractions of BF (10%, 20%, 33.3%, 50%, and 66.7% correspond to different mass ratios of aniline monomer and BF nanoparticles [An/BF] ) 9:1, 4:1, 2:1, 1:1 and 1:2).
nanocomposites can be obtained through the simple in situ polymerization method, without the aid of any further surfactants, organic phases, and extra assisting conditions such as radiolysis. We believe that the growth of nanofibers is intrinsic to the polymerization of aniline in aqueous solution,1g,8 because dendritic PANI nanofibers can also be produced for the sample without containing inorganic phases, with an average diameter
of about 200 nm (Figure 2a). Therefore, the BF nanoparticles played the role of the nucleation sites for the radial growth of oriented PANI nanofibers. The nucleation and growth of PANI nanofibers on ferrite nanoparticles and the formation of coralloid nanostructures are depicted in Figure 3. In polymerization, PANI nanofibers grew toward different directions from the BF nucleation centers after first polymerization at inorganic surfaces, similar to the nucleation and growth of polyaniline nanowires on a colloidal particle.6b During further polymerization, the radial growth of PANI nanofibers overlap and form 3D interconnected polymer networks;6b thus, coralloid morphologies would finally be produced. However, with excessive nucleation sites, the growth at the inorganic surfaces consumes large amount of aniline monomers, resulting in the decreasing growth of PANI nanofibers (Figure 1d and e). A continuing increase in the content of BF nanoparticles would finally lead to the disappearance of the coralloid morphology of PANI/BF nanocomposites (Figure 2b). With the same preparation conditions, coralloid PANI/NiFe2O4 nanocomposites with average diameters of about 50 nm can also be produced by offering particulate (quasi-spherical) NiFe2O4 nanoparticles (80-100 nm in diameter), as shown in Figure 2c and d. The above analyses indicate that by supplying inorganic nanoparticles as nucleation sites, coralloid PANI nanostructures can be obtained in our in situ polymerization method despite the morphology of inorganic materials. Moreover, besides morphology changes (dendritic to coralloid), the diameter of the outward branches of the produced coralloid nanostructures can be reduced in the polymerization of aniline by supplying nucleation sites with inorganic nanoparticles, as compared to PANI nanofibers polymerized without inorganic nucleation sites. The UV-vis absorption spectra (in N,N-dimethyl formamide) of the resulting powders of PANI/BF nanocomposites in Figure 4a are identical to conventional nonfibrillar PANI powders.3a,13a,c The nanocomposites have a stronger absorbance at both absorption peaks, which is attributed to the π-π* transition of the benzenoid ring and the benzenoid-quinoid excitonic transition, respectively, than emeraldine‚HCl except for the sample with the highest concentration of BF nanoparticles ([An/BF] ) 1:2). No shifts in the absorption wavelength were detected for PANI/BF nanocomposites. In Figure 5, powder XRD patterns of the produced PANI/BF nanocomposites not only have the characteristic diffraction peaks of the HCl-doped PANI (two broad peaks centered at 2θ ) 21° and 25°, ascribed to the periodicity parallel and perpendicular to the polymer chain,
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Figure 7. Field-dependent magnetization curves of BF, PANI/BF nanocomposites, and PANI (inset) at 300 K.
respectively1b,15) but also those of intrinsic diffraction peaks for the standard pattern for M-type hexagonal BaFe12O19 crystals (JCPDS no. 27-1029). The intensities of the diffraction peaks corresponding to HCl-doped PANI become weaker with the increase in the content of BF nanoparticles, resulting from the decrease in polymerization degree (crystallinity). Similar results are given by FT-IR spectra (Figure 4b), where PANI/BF nanocomposites have both the characteristic vibrations for emeraldine‚HCl powders and the tetrahedral and octahedral sites of Fe-O bonds of the hexagonal barium ferrite structure. As described above, it can be concluded that there is no obvious chemical interaction between PANI and BF nanoparticles in the PANI/BF nanocomposites, which is consistent with the phenomenon discovered for the prepared PANI-β-NSA/TiO2 nanotubes.1b As shown in Figure 6, higher BET-specific surface areas are discovered for PANI/BF nanocomposites with longer branches of the coralloid nanostructures, which may permit easier addition of surface functionalities compared with traditional highly agglomerated PANI powders. However, inclusion of a mass fraction of BF nanoparticles higher than 33.3% will reduce the BET-specific surface areas of the nanocomposites dramatically, originating from the restricting growth of outward extending branches of the coralloid nanostructures, as shown in Figure 1d and e. The electrical conductivities (σ) of PANI/BF nanocomposites have a first-order exponential decay relationship as a function of the mass fraction of BF nanoparticles (x) as σ ) 0.342 exp(-x/21.15), due to reduction in doping degree of PANI and the insulating behavior of BF (Figure 6); however, the conductivities are still comparable to some PANI-metal nanocomposites.7 Both increase in charge carrier scattering from the embedment of BF nanoparticles in the PANI matrix and increased charge carrier trapping, either by the nanoparticles themselves or by morphological change and defects induced by them, will lead to increase the resistivity.16 In other words, electrical conductivity of the BF nanoparticles can be enhanced by the conductive PANI coating. Compared to BF nanoparticles, coercivities of PANI/BF nanocomposites decrease slightly due to reduction in surface anisotropy upon coating17 but are qualitatively larger than that of PANI, resulting in the production of hard magnetic materials (Figure 7). Magnetization and coercivity exhibit monotone increasing functions with the
content of BF nanoparticles in PANI/BF nanocomposites. According to Stoner-Wohlfarth theory,18 the coercivity, HC, of nanoparticles is determined by magnetocrystalline anisotropy constant K and saturation magnetization MS:
HC )
2K µ oM S
(1)
where µo is the universal constant of permeability in free space, 4π × 10-7 H/m. Thus, K can be calculated combining the product of HC and MS, and it can be easily deduced that the value of K is also a monotone increasing function with the content of BF nanoparticles in PANI/BF nanocomposites, that is, more PANI coating on the BF nanoparticles produces samples with smaller K. In M-type barium ferrites, lowering the anisotropy field results in lower natural resonance frequency.19 Because the observed magnetic properties of nanoparticles are a combination of many anisotropy mechanisms, the PANI coating on the BF nanoparticles will likely affect the contributions of the surface anisotropy, shape anisotropy, and interface anisotropy to the net anisotropy, K. Therefore, in polymerization of aniline with BF nanoparticles, hard magnetic materials with alternative conductivities and magnetic properties can be produced. The normalized input impedance, Zin, of a metal-backed microwave-absorbing layer is given by19
Zin )
x
µr 2π tan h j r c
[ ( ) xµ fd] r r
(2)
where µr and r are the relative permeability and permittivity, respectively, of the composite medium, c is the velocity of electromagnetic waves in free space, f is the frequency of microwaves, and d is the thickness of the absorber. The reflection loss is related to Zin as
Zin - 1 R(dB) ) 20 log| | Zin + 1
(3)
Figure 8a shows the reflection loss of the prepared PANI/BF nanocomposites and BF nanoparticles with a thickness of 2 mm in the frequency range of 2-18 GHz, which demonstrates that
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Figure 8. Reflection loss for (a) PANI/BF nanocomposites and (b) mechanical mixtures of PANI nanofibers and BF nanoparticles, with a sample thickness of 2 mm in the frequency range of 2-18 GHz. (c) Dependence of reflection loss on the thickness of PANI/BF nanocomposites with [An/BF] ) 2:1 in the frequency range of 2-18 GHz.
the reflection loss characteristics are sensitive to the content of BF nanoparticles. The microwave absorption of PANI/BF nanocomposites was enhanced substantially compared to that of pure BF nanoparticles, which hardly exhibit any microwave absorption in the 2-18 GHz range because its resonance frequency is 47.6 GHz.20 The frequency relating to maximum reflection loss shifts to higher value with the increase in BF content, in accordance with the analyzed results of decreasing K; however, the increased reflection loss at higher frequency ranges is accompanied by the attenuation at lower frequency bands. PANI/BF nanocomposites from [An/BF ) 2:1] has the largest microwave absorption of 19.7 dB (∼98% absorption) at 14.6 GHz, which may arise from the obtained best-matched characteristic impedance of free space when BF nanoparticles were coated with the proper amount of conductive PANI. The effect of BF content on the frequency shift for the maximum reflection loss of the mixtures of PANI nanofibers and BF nanoparticles is similar to that of PANI/BF nanocomposites (Figure 8b); however, the maximum reflection loss shifts to a lower frequency and is more or less attenuated when PANI nanofibers and BF nanoparticles are simply mechanically mixed. Figure 8c displays the reflection loss for PANI/BF nanocomposites [An/BF ) 2:1] of different thicknesses as a function of frequency. Interestingly, the maximum attenuation of the incident wave with different thicknesses is nearly identical, whereas the corresponding frequency is an inverse function of thickness, which is in good agreement with one of the parameter requirements for zero-reflection as21
f)
c 2πµ′′r d
(4)
where µ′′r is the imaginary part of relative permeability µr . The same phenomena were found for other PANI/BF nanocomposites synthesized from different [An/BF] ratios. Therefore, the microwave absorbing properties can be modulated simply by manipulating the thickness of the prepared PANI/BF nanocomposites for application in different frequency bands (Ku band f X band f C band). Conclusions We demonstrate a facile synthesis of PANI nanostructures containing inorganic ferrite nanoparticles with novel coralloid morphologies through a simple, conventional, and inexpensive one-step in situ polymerization method, without the aid of any surfactant, organic dopant, or template. Coralloid nanostructures can be formed regardless of the morphology of the inorganic nanoparticles, flaky or quasi-spherical; however, the growth degree of the coralloid nanostructures can be significantly affected by the concentration of the inorganic nanoparticles.
There is no evident chemical interaction between PANI and ferrite nanoparticles, where inorganic nanoparticles serve only as the nucleation sites for the growth of PANI nanofibers and have effects in reducing the diameters of the resulting outward extending branches. PANI nanocomposites containing BaFe12O19 (BF) are hard magnetic properties with alternative electrical conductivities and magnetic properties. PANI coating on the BF nanoparticles substantially reinforced the reflection loss of barium ferrite in 2-18 GHz and the frequency relating to maximum reflection loss shifts to higher value with the increase in BF content because of the higher anisotropy field. Microwave absorbing properties can be modulated simply by controlling the thickness of the samples for the required frequency bands. With the controllable specific surface areas, electrical, magnetic and electromagnetic properties, the prepared PANI/BF nanocomposites may have potential applications in chemical sensors, gas separation, catalysis, microwave absorbing, and magnetoelectric devices. Acknowledgment. We gratefully acknowledge helpful discussions with Philip Cox, advanced engineer Xinrong Liu, Zushun Lv, and financial support from the National Natural Science Foundation of China (no. 20676024). References and Notes (1) (a) Sawall, D. D.; Villahermosa, R. M.; Lipeles, R. A.; Hopkins, A. R. Chem. Mater. 2004, 16, 1606. (b) Zhang, L.; Wan, M. J. Phys. Chem. B 2003, 107, 6748. (c) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (d) Martin, C. R. Science 1994, 266, 1961. (e) Nikiforov, M.; Liu, H.; Craighead, H.; Bonnell, D. Nano Lett. 2006, 6, 896. (f) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077. (g) Huang, J.; Kaner, R. B. J. Mater. Chem. 2006, 367-376. (2) (a) Wu, C. G.; Bein, T. Science 1994, 264, 1757. (b) Jiang, J.; Yoon, H. Chem. Commun. 2003, 720. (c) Qiu, H.; Wan, M.; Matthews, B.; Dai, L. Macromolecules 2001, 34, 675. (d) Choi, S. J.; Park, S. M. AdV. Mater. 2000, 12, 1547-1549. (e) Wei, Z. X.; Zhang, Z. M.; Wan, M. X. Langmuir 2002, 18, 917-921. (f) Yang, Y. S.; Wan, M. X. J. Mater. Chem. 2002, 12, 897. (3) (a) Haung, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (b) Haung, J.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. (c) Hopkins, A. R.; Sawall, D. D.; Villahermosa, R. M.; Lipeles, R. A. Thin Solid Films 2004, 469-470, 304. (d) Virji, S.; Haung, J.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491. (4) Zhang, X.; Goux, W. J.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 4502. (5) Li, W.; Wang, H. L. J. Am. Chem. Soc. 2004, 126, 2278. (6) (a) Verghese, M. M.; Ramanathan, K.; AShraf, S. M.; Kamalasanan, M. N.; Malhotra, B. D. Chem. Mater. 1996, 8, 822. (b) Liang, L.; Liu, J.; Windisch, C. F.; Exarhos, G. J.; Lin, Y. H. Angew. Chem., Int. Ed. 2002, 41, 3665. (c) Wang, J.; Chan, S.; Carlson, R. R.; Ge, G.; Ries, R. S.; Heath, J. R.; Tseng, H. Nano Lett. 2004, 4 (9), 1693. (d) Hoang, H. V.; Holze, R. Chem. Mater. 2006, 18, 1976. (7) Pillalamarri, S. K.; Blum, F. D.; Tokuhiro, A. T.; Story, J. G.; Bertino, M. F. Chem. Mater. 2005, 17, 227. (8) Chiou, N. R.; Epstein, A. J. AdV. Mater. 2005, 17, 1679. (9) (a) Paul, K. B. M. Physcia B 2007, 388, 337. (b) Yu, H. F.; Huang, K. C. J. Magn. Magn. Mater. 2003, 260, 455. (c) Wang, S.; Ng, W. K.;
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