Fe2O3 Nanocomposite Fibers

transmission electron microscopy and X-ray diffraction showed that the magnetic Fe2O3 nanoparticles with a mean diameter of 18 nm were uniformly dispe...
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J. Phys. Chem. C 2008, 112, 4538-4544

Structure and Properties of Cellulose/Fe2O3 Nanocomposite Fibers Spun via an Effective Pathway Shilin Liu,† Lina Zhang,*,† Jinping Zhou,† and Ruixin Wu‡ Department of Chemistry, Wuhan UniVersity, Wuhan, 430072, China, and Department of Electronic Science and Engineering, Nanjing UniVersity, Nanjing, 210093, China ReceiVed: December 4, 2007; In Final Form: January 6, 2008

Nanocomposite fiber is one of the most fascinating materials with broad applications. In the present work, nanocomposite fibers were prepared by using a low-cost, simple, and “green” process as follows. Regenerated cellulose (RC) fibers were spun from cellulose dope in 7 wt % NaOH/12 wt % urea aqueous solution precooled to -12 °C, and magnetic nanocomposite fibers were fabricated by introducing in situ synthesized iron oxide (Fe2O3) nanoparticles into the wet cellulose fibers spun via a small-scale pilot machine. The results from transmission electron microscopy and X-ray diffraction showed that the magnetic Fe2O3 nanoparticles with a mean diameter of 18 nm were uniformly dispersed in the cellulose matrix. The composite fibers exhibited a higher mechanical strength than RC fibers, as well as a strong capability to absorb UV rays, superparamagnetic properties, and a relatively high dielectric constant. FT-IR results indicated that there is a strong interaction between cellulose and Fe2O3 in the fibers, leading to the formation and stabilization of the novel magnetic materials. The nanocomposite fibers will be important for the development of functional fabrics and protective clothing for ultraviolet radiation or microwaves.

1. Introduction Polymer nanocomposite fibers have attracted much attention in the past decade because of their unique properties, such as mechanical,1-2 thermal,3 electrical,4-5 and magnetic properties,6 as well as antibacterial activities,7 and so forth.8-10 There are various viable methods for fabricating hybrid organic-inorganic fibers, such as blending,11 electrospinning,12-15 and coating.16 It is worth noting that highly conducting composite fibers have been obtained by spinning gel-state carbon nanotubes into aqueous polyethyleneimine (PEI) solution followed by conversion into solid fibers.17 However, these approaches are often difficult when they come into producing stable composite fibers on a large scale, because of the aggregation of nanoparticles in the polymer matrix. Stimulated by the novel properties of magnetic nanoparticles, much effort has been devoted to the preparation of magnetic composite fibers, because of their fascinating applications that range broadly over both military and civilian spheres.18-19 Usually, the key step for fabricating these functional fibers is the blending of metal powders with polymer solutions or melts before spinning.20-21 However, the mixing of the hydrophobic polymer and hydrophilic inorganic nanoparticles often leads to the aggregation of the magnetic additives, leading to discontinuity and unintended or defective functionalities in the fibers. By dip-coating the surfaces of polymeric fibers, magnetic composite fibers have been prepared with the sol-gel method.22-23 The utilization of a simple method for the preparation of magnetic nanocomposite fibers is important to nanoscience and nanotechnology. In our laboratory, we have developed a novel solvent system for cellulose, that is, a 7 wt % NaOH/12 wt % urea aqueous * Corresponding author. Tel.: +86-27-87219274. Fax: +86-2768754067. E-mail: [email protected]. † Wuhan University. ‡ Nanjing University.

solution precooled to -12 °C, in which the dissolution of cellulose could be achieved rapidly at ambient temperature, leading to the formation of a transparent cellulose solution.24 Moreover, novel cellulose multifilaments have been spun from this dope.25 Encouraged by our recent work, nanocomposite cellulose fibers were fabricated from the cellulose solution. To avoid aggregation of the magnetic Fe2O3 nanoparticles in the fiber matrix, we used a template nanomanufacturing process by in-situ synthesizing Fe2O3 nanoparticles into the scaffolds of the nanostructured cellulose fibers spun from the cellulose solution in the aqueous NaOH/urea system. The main advantage of this method is its relative simplicity and low cost, as well as an environmentally friendly process for the preparation of magnetic nanocomposite fibers. In the present study, we demonstrated a new route for the fabrication of magnetic nanocomposite fibers, and their structure and properties have been investigated. 2. Experimental Section 2.1. Materials. Cotton linter pulp (R-cellulose >95%) was provided by Hubei Chemical Fiber Group Ltd. (Xiangfan, China). Its viscosity-average molecular weight (Mη) was determined by using an Ubbelohde viscometer in LiOH/urea aqueous solution at 25 ( 0.05 °C and calculated from the equation26 [η] ) 3.72 × 10-2Mw0.77 to be 8.1 × 104. Other chemical regents were purchased in China with analytical grade and used without further purification. 2.2. Preparation of Fibers. A solution with NaOH/urea/H2O of 7:12:81 by weight was precooled to -12 °C. Then cotton linter pulp (cellulose) in the desired amount was immediately dispersed into the solvent system (3 L) under vigorous stirring for 30 min at ambient temperature to obtain a transparent cellulose dope with concentration of 4.8 wt %. The fibers were made by wet spinning on a small-scale spinning machine

10.1021/jp711431h CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008

Cellulose/Fe2O3 Nanocomposite Fibers

J. Phys. Chem. C, Vol. 112, No. 12, 2008 4539 was measured with a UV-vis spectroscope at a wavelength of 510 nm; then a graph relating the absorption value to the number of micrograms of iron oxide was constructed. 2.4. Characterization. Fourier-transform infrared spectroscopy (FT-IR) of the cellulose fibers and composite fibers was conducted on an FT-IR spectroscope (model 1600, Perkin-Elmer Co.). The samples were prepared by the KBr-disk method. Wide-angle X-ray diffraction (XRD) measurement was carried out on an XRD diffractometer (D8-Advance, Bruker). The patterns with Cu KR radiation (λ ) 0.15406 nm) at 40 kV and 30 mA were recorded in the region of 2θ from 5 to 70°. Samples were ground into powders and dried in a vacuum oven at 60 °C for 48 h. The crystallinity χc (%) of the cellulose fibers and composite fibers were estimated by Rabek’s method,27 using the following relationship:

χc ) Figure 1. SEM imagines of surface (a) and cross-section (b) for a single swollen RC fiber and the surface (c) and cross-section (d) of the composite fibers (F01) in a dry state.

manufactured by Hubei Chemical Fiber Group Ltd. The spinneret cylinder was immersed directly into the first coagulation bath with 9.9% H2SO4/10% Na2SO4 at 19 °C, and the resulting gelation fibers solidified in the first coagulation bath were taken up on the first roller and then drawn to the second roller. In order to give a jet stretch, the second roller with a speed of 35.6 m/min was faster than the first roller with a speed of 33.9 m/min. Subsequently, the fibers were drawn from the second roller into the second coagulation bath with 5 wt % H2SO4. The fibers, after washing by running water, were drawn into a container with a different concentration of FeCl3 solution and kept for 12 h. They were then removed to another container containing 2 M NaOH and treated for 20 min. The treated fibers were washed with deionized water and then rolled up and dried at ambient temperature. The cellulose fibers that were not treated with FeCl3 solution were coded as RC fiber, and the composite cellulose fibers treated with FeCl3 solution with different concentrations of 0.01, 0.1, 0.5 M were coded as F001, F01, and F05, respectively. 2.3. Detection of Fe2O3 Nanoparticles. To determine the dependence of Fe2O3 nanoparticles removed from composite fibers on immersing time, 2 g of composite fibers (F05) were immersed into 200 mL of water, and the temperature was kept at 30 °C. Then 10 mL of the solution was transferred to a 25 mL volumetric flask at different time intervals; 3 mL of hydroxylamine hydrochloride (50 g/L), 1 mL of 1,10-phenanthroline monohydrate (1.5 g/L) solution, 2 mL of hydrochloric acid (10% v/v), and an amount of sodium citrate solution (1.0 mol/L) were added and then diluted with water to 25 mL. The solution was allowed to stand for 0.5 h. Then the absorption values of the test and blank solutions were measured with a UV-vis spectroscope (Shimadzu UV-160A, Japan) at a wavelength of 510 nm using 4 cm cells. The content of the removed Fe2O3 nanoparticles from cellulose fibers was calculated by the calibration graph established as follows. Ammonium ferric sulfate solution (standard iron solution, 0.001 mol/L) corresponding to 0.00, 0.02, 0.04, 0.06, 0.10, 0.12, and 0.16 µg of iron oxide was added into a series of 100 mL volumetric flasks. A volume of 2 mL of hydroxylamine hydrochloride (50 g/L), 2 mL of 1,10-phenanthroline monohydrate (1.5 g/L) solution, and an amount of sodium citrate solution (1.0 mol/L) were added and then diluted with water to 100 mL. The solution was allowed to stand for 0.5 h, and the absorption value of the test solutions

Sc × 100 Sc + S a

where Sc and Sa are the area of crystal and amorphous diffraction peaks of samples, respectively. The contribution of the Fe2O3 nanoparticles in composite fibers on the crystallinity χc (%) of the cellulose matrix was not taken into account. X-ray photoelectron spectroscopy (XPS) was conducted on a XSAM 800 instrument (KRATOS Product, Britain). An Mg KR target at 1253.6 eV and 16 mA × 12.5 kV was used in the experiment. The sample was detected under 2 × 10-7 Pa. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2010 (HT) electron microscope, using an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was performed on a FESEM (SEM, SIRION TMP, FEI) by using an accelerating voltage of 20 kV. The cellulose fibers in the wet state were frozen in liquid nitrogen, snapped immediately, and freeze-dried by using lyophilizer (CHRIST Alpha 1-2, Germany); the composite fibers were dried at ambient temperature. All the fibers were coated with gold for the SEM observation. Nitrogen adsorption-desorption measurements were performed by using an ASAP 2020 (Micromeritics, USA) volumetric adsorption apparatus. Thermal gravimetric analysis (TGA) was carried out using thermogravimetric analysis equipment (Netzsch, German). The fibers were ground into powder, and about 5 mg of the powder was placed in a platinum pan and heated from 20 to 600 °C at a rate of 10 k min-1 in air atmosphere. The magnetic properties of the composite fibers were studied with a superconducting quantum interference device (SQUID, MPMS XL-7, QUANTUM DESIGN, USA) at 25 °C, and the hysteretic loop was obtained in a magnetic field that varied from -7 to +7 T. The fibers used for electromagnetic measurements were prepared by dispersing the cellulose and composite fibers ground into powder into epoxy resin with a concentration of 1:10 (vol %). Electromagnetic measurements were conducted with an E8368A Network Analyzer. A series of cellulose fibers and composite fibers were ground into powder and formulated with a concentration of 10 vol % in epoxy resin. The tested samples were toroidal with an inner diameter of 3.04 mm and an outer diameter of 7 mm. The complex relative permittivity (r ) ′- j′′, where ′ and ′′ are the real and imaginary parts of the relative complex permittivity) and permeability (µr ) µ′ - jµ′′, where µ′ and µ′′ are the real and imaginary parts of the relative complex permeability, respectively, were measured by using a transmission/reflection method on an E8368A Network Analyzer within the range of 2-18 GHz. Mechanical properties testing of the fibers was carried out on a universal tensile tester

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Figure 2. Photo of the F01 composite fibers (a) and its EDS spectrum (b) from SEM.

(CMT 6503, Shenzhen SANS Test machine Co. Ltd, China) according to ASTMD6614-2000 with a speed of 5 mm min-1 at room temperature. 3. Results and Discussion 3.1. Morphology and Structure of the Fibers. The nanocomposite fibers containing Fe2O3 were fabricated via a facile pathway by wet spinning. The morphologies of the RC fibers and composite fibers are shown in Figure 1. The RC fibers have a diameter of about 45 µm in the swollen state (Figure 1a), and the cross section exhibits an interpenetrating microporous structure with a mean pore diameter of about 150 nm (Figure 1b). This unique structure is due to phase separation during the wet-spinning process, where the solvent-rich regions contribute to the pore formation.28 Figure 1, parts c and d, shows the SEM images of the composite fibers treated with 0.1 mol/L FeCl3 solution. Clearly, the surface and cross section of the composite fibers (Figure 1, parts c and d) are denser than that of the RC fibers, and the diameter in the dry state is about 27 µm. When the cellulose fibers were soaked in FeCl3 solution, Fe3+ could be readily impregnated into the cellulose fibers through the pores. The incorporated Fe3+ ions could be bound to the cellulose macromolecules via electrostatic interaction, because the electron-rich oxygen atoms of polar hydroxyl of cellulose are expected to interact with electropositive transition-metal cations.29 When the fibers were treated with NaOH aqueous solution and rinsed with water, hydrolysis and condensation reactions occurred. Thus, Fe2O3 nanoparticles were synthesized in situ in the micropores of the cellulose fibers to obtain the composite fibers containing Fe2O3. Figure 2 shows the picture of the F01 composite fibers prepared from 0.1 M FeCl3 solution. Clearly, the colorless RC fibers have transformed into reddish-brown (Figure 2a), as a result of the strong absorption of the Fe2O3 component on cellulose. It is worth noting that the color of the composite fibers changes from light yellow to brown with increasing concentration of FeCl3 solution from 0.01 to 0.5 M. The energy-dispersive spectrum (EDS) from SEM indicates that there are only C, O, and Fe elements in the composite fibers, further proving that the inorganic nanoparticles have been synthesized in the cellulose fibers. Figure 3a shows the concentration of Fe2O3 nanoparticles removed from the F05 composite fibers as a function of immersion time in water. The composite fibers were immersed into water for about 70 h, and the content of the Fe2O3 nanoparticles removed from cellulose fibers into water has been determined to be negligible (15µg/L). Figure 3b shows the FT-

Figure 3. The concentration of Fe2O3 nanoparticles removed from F05 composite fibers as a function of the immersion time in water (a), FTIR spectrum of the cellulose fibers and F01 composite fibers (b).

IR spectrum of the cellulose fibers and F01 composite fibers. The peaks at 3300-3450 cm-1 corresponding to stretching vibrations of hydroxyl groups of cellulose in F01 moved to higher wavenumbers and became broader, indicating a strong interaction between the groups of cellulose and Fe2O3 nanoparticles through hydrogen bonding. Iron oxides are often precipitated in situ in fibers with in-fiber formation of pigments for dyeing of fibers,30 because of the high stability of iron oxides in fibers. The stability of Fe2O3 nanoparticles in cellulose fibers is very important to the properties of the composite fibers. X-ray diffraction and X-ray photoelectron spectroscopy (XPS) were used to study the structure and interaction of the inorganic

Cellulose/Fe2O3 Nanocomposite Fibers

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TABLE 1: Mechanical Properties of the Regenerated Cellulose Fibers and Fe2O3-Nanoparticle-Incorporated Composite Fibers

a

samples

crystallinity (%)

tensile strength (cNdtex-1)

elongation at break (%)

linear densitya (dtex)

Fe2O3 contentb (%)

RC fiber F001 F01 F05

51 47 46 39

1.22 ( 0.33 1.84 ( 0.24 1.40 ( 0.41 0.93 ( 0.17

2.34 ( 0.51 3.97 ( 0.38 2.17 ( 0.27 1.42 ( 0.30

1.36 ( 0.09 1.37 ( 0.11 1.38 ( 0.14 1.52 ( 0.13

0.17 1.07 11.08

The linear density of the fiber is defined as the weight per unit length, 1dtex corresponds to 100 mg/km. b Results obtained from TG analysis.

Figure 4. Powder X-ray diffraction pattern of the RC fibers and composite fibers (a), Fe 2p XPS spectrum of the composite fiber (F01) (b).

nanoparticles in cellulose fibers. Figure 3a shows the XRD pattern of the cellulose fibers and composite fibers. The cellulose fibers exhibit three peaks at 2θ ) 12.4, 20.2, and 22.2°, assigned to the (11h0), (110), and (200) planes of crystalline cellulose II.31 It is noted that, in addition to the peaks of cellulose II, the composite fibers display some distinct peaks at 2θ values of 24.0, 29.8, 33.2, 35.6, 40.9, 49.5, 54.2, 62.4, and 64.0°. This indicates that the crystalline phase of Fe2O3 exists as (R + γ) Fe2O3 (Powder Diffraction file, JCPDS card no. 89-2810 and 39-1346) in the composite fibers. The incorporated Fe2O3 nanoparticles lead to a decrease of the crystallinity χc (%) of the cellulose fibers as shown in Table 1. The reducing of the crystallinity of the cellulose fibers containing Fe2O3 nanoparticles suggests that a strong interaction between Fe2O3 nanoparticles and the cellulose matrix exists in F001-F05. The XPS of the Fe 2p region for the F01 composite fibers is shown in Figure 2b. The Fe 2p3/2 and 2p1/2 photoelectron peaks have been observed at 710.7 and 724.6 eV, respectively. There is a shakeup satellite of γ-Fe2O3 observed at 718.4 eV. Compared to the shape of main Fe (2p3/2) line of pure R- Fe2O3 and that of γFe2O3, these features indicate that the crystalline phase in the composite fibers includes (R + γ) Fe2O3.32-33 This further supports the result obtained from WAXRD. TEM images of the composite fibers are shown in Figure 4. It is clear that the Fe2O3 nanoparticles of the mean diameter of

Figure 5. TEM images and the size distribution of the nanoparticles for the composite fibers (a-c) and Fe2O3 nanoparticles after removal of cellulose matrix by calcination (d-f). The samples are F001(a, d), F01(b, e), and F05 (c, f); and the insert is an HRTEM image of F05.

18 nm are uniformly dispersed in the cellulose matrix (Figure 4, parts a-c). With an increase of FeCl3 concentration from 0.01 to 0.5 M, the mean particle size of the incorporated Fe2O3 nanoparticles increases slightly. The high-resolution TEM (HRTEM) image (insert in Figure 4c) indicates that the Fe2O3 nanoparticles in the cellulose fibers have been well crystallized, and the lattice plane distance of 0.18 nm is in good agreement with the separation between the (024) lattice plane of R-Fe2O3 (Powder Diffraction file, JCPDS card no. 89-2810). Interestingly, after removal of the cellulose matrix by calcination under air atmosphere at 600 °C, the particle sizes of the resulting Fe2O3 nanoparticles change only slightly, as shown in Figure 4, parts d-f. This suggests that the concentration of FeCl3 solution hardly changes the particle size of Fe2O3 nanoparticles. However, the effect of the concentration of FeCl3 on the content of Fe2O3 nanoparticles in the cellulose fibers is obvious as shown in Table 1. Moreover, the nanoparticle size distribution is relatively narrow. Therefore, this work provides a facile and green method for the preparation of inorganic nanoparticles/ cellulose fibers with nanoparticle size and a narrow size distribution. The content of the incorporated Fe2O3 nanoparticles in cellulose fibers could be controlled by the diffusion equilibrium of Fe3+ between the cellulose matrix and the solution.

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Figure 6. Nitrogen gas adsorption-desorption isotherms of the RC fibers and composite fibers with different content of Fe2O3 nanoparticles (solid symbol is for desorption, open is for adsorption).

When the concentration of FeCl3 solution increases, the Fe3+ incorporated into the cellulose matrix increases as well. It has been previously reported that noble metal nanoparticles with a narrow size distribution have been synthesized by using porous native cellulose fibers as nanoreactor and particle stabilizer.34 In our finding, the porous structure and hydroxyl groups of the regenerated cellulose fibers may play an important role in the improvement of dispersion and stabilization of Fe2O3 nanoparticles. Moreover, the micropores of the cellulose fibers that can serve as templates to prevent Fe2O3 nanoparticles from aggregating. 3.2. Mechanical Properties of the Fibers. The mechanical properties of the RC fibers and composite fibers are summarized in Table 1. The tensile strength of the nanocomposite cellulose fibers is stronger than that of the RC fibers, when the content of the incorporated Fe2O3 nanoparticles ranged from 0.17 to 1.07 wt %. Moreover, the cellulose fibers containing 0.17% Fe2O3 nanoparticles exhibit the maximum in tensile strength. The enhanced tensile strength of the composite fibers could be a result of the strong interactions between the Fe2O3 nanoparticles and the cellulose. Figure 6 shows the nitrogen gas adsorption-desorption isotherms of cellulose fibers and composite fibers. It indicates that the interior structure of the RC fibers has been destroyed after the loading of Fe2O3 nanoparticles. It has been reported that higher loading of carbon nanotubes into cellulose fibers may destroy the interior structure, leading to the decreased mechanical properties of the composite fibers.35 It is worth noting that the linear density of the cellulose fibers increases slightly with incorporation of Fe2O3 nanoparticles (Table 1), indicating that for composite fibers with good mechanical strength, a relatively lightweight feature can be

Figure 7. Optical absorption spectra of RC fibers and composite cellulose fibers.

integrated into the current fibril and woven-mesh manufacturing systems for the preparation of functional fabrics for other applications. 3.3. Multifunction of the Composite Fibers. Figure 7 shows the optical absorption ability of the RC fibers and composite fibers at wavelengths from 200 to 800 nm. The RC fibers do not show prominent UV absorption in the region. Interestingly, there is an obvious UV-ray absorption in the composite fibers, and the absorption gradually increases and shifts to higher wavelength region with an increase of the content of Fe2O3 nanoparticles. This characterization is very important to materials used for protection against UV.

Cellulose/Fe2O3 Nanocomposite Fibers

Figure 8. Magnetic hysteresis loop of the composite fibers at 298 K.

The magnetic property is one of the most important characteristics of the composite fibers. Figure 8 shows the magnetization of the composite fibers as a function of applied magnetic field at 298 K. The magnetization of the composite fibers increases with an increase of the applied magnetic field. However, the magnetization of all composite fibers is weak and shows lack of saturation, and they exhibit an extremely small hysteresis loop and low coercivity. The lack of hysteresis and coercivity are typically characteristic of superparamagnetic particles or some single-domain particles.36-37 It is well-known that magnetic particles, smaller than some critical particle diameter, can be called single domains. As the particle size continues to decrease below the single-domain value, the

J. Phys. Chem. C, Vol. 112, No. 12, 2008 4543 particles exhibit superparamagnetic properties, that is, no hysteresis and coercivity. The weak magnetization and lack of saturation have often been observed in the Fe2O3 nanoparticles of similar sizes due to the surface and size effects. The discontinuity of the super-exchange between the iron cations near or on the nanoparticle surfaces leads to the formation of nonlinear spins. The noncollinear spin structure reduces the total magnetic moment of the nanoparticles and gives rise to the decrease in the magnetization of the nanocomposite fibers. The monodispersity of Fe2O3 nanoparticles in the cellulose matrix and the persistence of superparamagnetic relaxation of the nanoparticles lead to the nonsaturation of the magnetization.38-39 It is worth noting that these magnetic composite fibers with Fe2O3 nanoparticles fillers have the ability to yield significant dielectric properties,40 making this kind of fiber more attractive in the application as magnetic shielding materials for electromagnetic wave absorbance, reflection, and so forth. The permittivity (′) and permeability (µ′) of the fibers as a function of microwave frequency are shown in Figure 9. Within the range of the measured frequencies, the ′ value of the composite fibers decreases with increasing the frequency at first (Figure 9a). When the frequency is further increased, ′ starts to rise and shows two peaks at frequencies of about 13 and 17 GHz. According to the work of Hsu et al.,41 there are many factors contributing to the dielectric properties, such as dielectric relaxation, resonance, the motion of conduction electrons, defects of materials, length and diameter, and so forth. Moreover, the ′ of the fibers increases with the incorporation of Fe2O3 nanoparticles, so a higher content of the incorporated Fe2O3 nanoparticles results in a higher dielectric constant. The permeability (µ′)-frequency patterns of the cellulose fibers and composite fibers are shown in Figure 9b. The permeability-

Figure 9. Permittivity (a, c) and permeability (b, d) of the RC fibers and composite fibers (F001, F01, and F05) as a function of microwave frequency, (a, b) the ratio of RC fibers or composite fibers within epoxy resin is 10 vol %, and (c, d) the ratio of F05 composite fibers within epoxy ranges from 10 to 55 vol %.

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frequency patterns of the composite fibers are different from that of the RC fibers, and many peaks at microwave frequency from 2 to 18 GHz can be observed in the composite fibers. As shown in Figure 4, TEM results have indicated that the Fe2O3 nanoparticles with small particle size were mono-dispersively incorporated in the cellulose matrix. Moreover, the magnetic measurement suggests that the nanoparticles exist as singledomain particles. Thus, only spin rotation would contribute to the magnetization mechanism of the Fe2O3 nanoparticles in the cellulose matrix. Therefore, the peaks in permeability-frequency patterns may result from the spin resonance of Fe2O3 nanoparticles. It must be noted that when the content of the composite fibers reaches more than 50% in the epoxy resin matrix, ′ increases from about 3 to 3.7, as shown in Figure 9c. There are also many resonance peaks in the permeability-frequency spectra (Figure 9d). It is perceptible that for the polymer matrix with little or no metal loading, the dielectric constant may vary little at microwave frequencies. Usually, ′ is relative to the microwave absorption ability. The dielectric constant of epoxy resin and cellulose fibers composites is about 3. Therefore, the increased ′ of the composite fibers may result mainly from the contribution of the incorporated magnetic Fe2O3 nanoparticles. When the composite fibers are exposed to an electromagnetic field, an ac electric field will be produced through the motion of polarized charges generated by the cellulose macromolecules under the ac field. According to the Maxwell equations, a magnetic field can be induced by the ac electric field. Therefore, the energy of the electromagnetic wave can be radiated out by the spin resonance of the Fe2O3 nanoparticles, leading to the increase of the ′ of the cellulose fibers. This functional composite fiber has potential applications in the electromagnetic interference shielding and magnetic materials with both commercial shielding and defense purposes.

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4. Conclusion

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We have successfully fabricated magnetic composite cellulose fibers through an effective pathway by in situ synthesis of magnetic Fe2O3 nanoparticles in the cellulose fibers, providing a new method for the nanocomposite fibers. The magnetic Fe2O3 nanoparticles with a mean diameter of 18 nm were uniformly dispersed in the cellulose matrix. The composite fibers exhibited higher mechanical strength than RC fibers, as well as a strong capability to absorb UV rays, superparamagnetic properties, and arelatively high dielectric constant. Moreover, the interactions between Fe2O3 nanoparticles and the cellulose matrix are strong and could prevent Fe2O3 nanoparticles from being removed from cellulose fibers at a wet state. The method is versatile, easy to perform, suggesting that a wide range of functional cellulose fibers can be routinely fabricated by a template incorporating electrical, optic, and other functional nanoparticles into this structured fiber, which may open a new application range for cellulose. Acknowledgment. This work was supported by the National Supporting Project for Science and Technology (2006BAF02A09), and the National Natural Science Foundation of China (20674057 and 20474048).

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