Facile Synthesis of Size-Controlled Silver Nanoparticles Using Plant

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Facile Synthesis of Size-Controlled Silver Nanoparticles Using Plant Tannin Grafted Collagen Fiber As Reductant and Stabilizer for Microwave Absorption Application in the Whole Ku Band Junling Guo,†,‡ Hao Wu,† Xuepin Liao,*,‡ and Bi Shi*,†,‡ †

Department of Biomass chemistry and Engineering, and ‡National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, P. R. China ABSTRACT: Size-controlled silver nanoparticles (AgNPs) were facilely synthesized on collagen fiber (CF), in which bayberry tannin (BT), a natural plant polyphenol, was grafted on CF to act as a reductant and stabilizer without any additional reagent (surfactant, template, and capping agent) or treatment (heat and photoirradiation) needed. The as-synthetic AgNPsBT@CF was well characterized via combined techniques including SEM, HTEM, XRD, XPS, and FT-IR. The particle diameter and size distribution of AgNPs in BT@CF matrix are feasible to adjust by varying the grafting degree of BT on the CF surface. When the grafting degree of BT was 0.4, the particle size of AgNPs is as small as 5.2 ( 1.9 nm. Furthermore, the complex permittivity, and complex permeability of the AgNPs-BT@CF were also investigated in detail. It was found that with the increase of grafting degree of BT on CF the imaginary part (ε00 ) of complex permittivity was dramatically increased, whereas the real part (μ0 ) and imaginary part (μ00 ) of complex permittivity were not obviously changed. The reflection loss (RL) of AgNPs-BT@CF exceeding
10 dB was achieved in the whole Ku band (12.5
18 GHz).

’ INTRODUCTION With the rapid development of the electronics industry, the dominant frequency range of communication devices has shifted toward a higher range in order to enhance the data transfer rates. There have been an increasing number of applications of electromagnetic (EM) waves in the Ku band (12.5
18 GHz), such as radar, broadcast services, and satellite digital data transmission. Consequently, much attention has been devoted to microwave absorbing materials concentrating on the Ku band.1 Considerable efforts have been made toward the developments of lightweight and effective microwave absorption materials, among which metallic magnetic compounds, carbon-nanotube based composites, and metal-nanoparticle based composite are the exciting research fields. Metallic magnetic compounds, such as BaFe12O19(BaM)/ferroelectric Ba0.5Sr0.5TiO3(BST),2 α-Fe/SmO,3 and Fe/Fe3B/Y2O3,4 have been extensively studied. These microwave absorption materials exhibit microwave absorption property in the X-band range (8
12 GHz). However, almost all of the present research about magnetic loss absorbing materials still required introduction of strict reaction conditions (high temperature and high vacuum, etc.).5 Carbon-nanotube (CNT) based composites also attracted great interest in the development of microwave absorption materials. Chen et al. successfully fabricated a CNT-polystyrene composite that performs a maximum reflection loss (RL) of
12 dB in the 1
1000 MHz range.6 However, the complex synthesis of CNT in conjugation with magnetic nanoparticles is not favorable for practical applications.7,8 r 2011 American Chemical Society

More recently, another kind of interesting microwave absorption materials, metal-nanoparticle based composites, has emerged.9
13 Ravindran et al. prepared aluminum oxide layers embedded with silver nanoparticles (AgNPs) by multilayer reactive electron-beam evaporation method. It was found that this AgNPs/Al2O3 composite shows a substantially enhanced permittivity (εr) in the range of 1
1000 MHz, suggesting that it has good microwave absorption property in this MHz range.9 However, even though all of these attractive microwave absorption materials exhibit satisfactory absorbing performance, their disadvantages of multistep complex synthesis procedures restrict their practical uses. In addition, the synthesis steps of these microwave absorbers often involve addition of chemical reducing agents such as sodium borohydride, sodium citrate, hydrazine, or organic solvents like N,N-dimethylformamide (DMF) and glycol. All of these chemicals are highly reactive and have potential environmental and biological risks, which could be a problem for large-scale production and their subsequent applications. Therefore, the demands to develop novel microwave absorption materials with more facile strategy, such as one- or two-step synthesis and room temperature method, are ever increasing. Collagen fiber (CF), one of the most abundant renewable biomass in nature, mainly comes from the skin of high-ranking Received: July 28, 2011 Revised: October 25, 2011 Published: October 26, 2011 23688

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Figure 1. Schematic outline of the synthesis of silver nanoparticles/ plant tannin grafted collagen fiber (AgNPs-BT@CF) composite.

vertebrate.14 It is well documented that collagen has a high level of molecular organization.15 The collagen molecule is composed of three polypeptide chains with triple helical structure. The staggered triple helix arrangement leads to the formation of collagen fibrils that further assemble into collagen fibers. Therefore, it is feasible that the hierarchical structure of collagen is of benefit in good microwave absorption. In addition, owing to a great number of electric dipoles and molecular bounded charges stored in the collagen molecule, collagen is actually a biological electret and has the properties of polarization in electromagnetic field.16 Consequently, the wealthy merits of collagen fiber, such as hierarchical structure, lightweight, fibrous morphology, naturally occurring electric dipoles, and biological origin, make the fabrication of collagen fiber based microwave absorption materials more fascinating. However, to the best of our knowledge, the use of collagen for the reduction of metal ions still required an introduction of extra chemical reductants (sodium borohydride and citrate) or physical approaches (heat treatment, γ-ray, and UV irradiation).17,18 Additionally, although collagen itself contains some stabilizing groups for nanoparticles, our previous investigation indicated that AuNPs stabilized by collagen lacked sufficient stability.19 Quite recently, we found that grafting of epigallocatechin-3gallate (EGCG) onto the surface of a collagen fiber (CF) could significantly improve the dispersion and stabilization of Au nanoparticles (AuNPs) on CF support,20 which prompted us to investigate the capability of the polyphenol-modified collagen serving as an efficient support for preparing stable AgNPs. Plant tannin, a natrual polyphenol widely distributed in plants, is introduced in the present study to achieve a facile, reductantfree, and size-controlled synthsis of stable AgNPs. According to the chemical structures of tannins, they are generally classified into condensed tannins and hydrolyzable tannins. As shown in Figure 1, the molecular structures of condensed tannins are mainly polymerized products of flavan-3-ols and/or flavan-3,4diols which contains a large number of phenolic hydroxyls. Our previous study found that condensed tannins can serve as reducing agents due to their unique reducing properties of the ortho-phenolic hydroxyls which are able to gradually donate the electrons, thus showing a mild and stepwise reduction ability.21 In addition, the molecular backbone of condensed tannins consists of rigid aromatic rings, which provide steric effect to prevent NPs from aggregation and thus act as an effective stabilizer.22 Indeed, plant tannins have been traditionally used

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as an effective tanning agent in leather manufacture to increase the thermal stabilization of collagen.15 All of these facts suggest that plant tannins could be very useful component in the preparation of metal nanoparticles/collagen fiber composite. Herein, we explore a facile approach to synthesize sizecontrolled AgNPs on collagen fiber, in which the bayberry tannin (BT, a typical condensed tannin) was grafted on CF to serve as reductant and stabilizer. The synthesis processes were completely carried out in aqueous solution at room temperature without any extra reagents or treatments, which is compatible with green chemistry principles. The complex permittivity (εr) and complex permeability (μr) of the resultant AgNPs-BT@CF were also investigated. The effects of grafting degree of BT on the particle diameter and size distribution of AgNPs were studied, and the microwave absorption property of these CF-supported AgNPs composites were also investigated.

’ EXPERIMENTAL SECTION Preparation of Bayberry Tannin Grafted Collagen Fiber (BT@CF). Collagen fiber (CF) was prepared from cattle skin

according to the procedures in our previous work.23 A certain amount of BT was dissolved in 50 mL of deionized water and then mixed with 1.0 g of CF. The mixture was stirred at 298 K for 2 h. Then, 0.5 mL of glutaraldehyde solution (2.0 wt %, used as cross-linking agent between collagen and tannin) was added into the mixture and stirred at 318 K at pH 6.5 for 6 h. Subsequently, the BT-grafted CF (BTx@CF, x: grafting degree of BT on CF) were collected after filtrating and fully washing with deionized water. The concentration of BT in the reaction solution before and after grafting reaction was analyzed by ultraviolet
visible spectra (UV
vis, Shimadzu UV-3600). The grafting degree of BT on BTx@CF was defined as: x = amount of BT grafted on CF (g)/amount of CF (g). Herein, the grafting degree of bayberry tannin on the BTx@CF prepared was approximately 0.05, 0.2, and 0.4, respectively. Preparation of Silver Nanoparticles/Bayberry Tannin Grafted Collagen Fiber Composites (AgNPs-BTx@CF). The resultant BT@CF was suspended in 100 mL of AgNO3 aqueous solution, where the concentration of Ag(I) was 1 g L
1. Then the color of BT@CF gradually became darker and darker and finally change to black within 24 h at room temperature. After filtrating and fully washing with distilled water, the dark product of AgNPsBT@CF was obtained. Based on the measurements of inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Optima 2100 DV), the Ag loading amounts of AgNPsBT0.05@CF, AgNPs-BT0.2@CF, and AgNPs-BT0.4@CF were determined to be 1.44%, 1.65%, and 1.88% (w/w), respectively. Characterization. The structural morphology of AgNPsBTx@CF was observed by scanning electron microscopy (SEM, JEOL LTD JSM-5900LV, Japan). TEM images, HAADFSTEM image and EDS point analysis of AgNPs-BTx@CF composites were observed by using Field Transmission Electron Microscope (FE-TEM, 200 kV, Tecnai G2 F20, FEI, Netherlands). Wide-angle X-ray diffraction patterns of the samples were recorded by using Cu Kα X-radiation (XRD, Philips X0 Pert Pro-MPD diffractometer). X-ray photoelectron spectra (XPS, Shimadzu ESCA-850, Japan) of the samples were recorded by employing Mg Kα X-radiation (hv = 1253.6 eV) and a pass energy of 31.5 eV. Peaks from all of the high-resolution core spectra were fitted with XPSPEAK 4.1 software, using mixed Gaussian
Lorentzian functions. FT-IR spectra were measured 23689

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Figure 2. SEM images of AgNPs-BT@CF at different magnifications (a) and (b), exhibiting the ordered fibrous morphology.

Figure 4. TEM images and the corresponding particle size distribution of AgNPs-BTx@CF with various BT/CF initial mass ratios: x = (a) 0.05, (b) 0.2, and (c) 0.4. Figure 3. X-ray diffraction (XRD) patterns of CF, BT0.4@CF, and AgNPs-BT0.4@CF.

by Fourier transforms infrared spectrometer (FT-IR, Thermo Nicolet iS10, USA) equipped with a DTGS detector. For the measurements of microwave absorption properties, the experiments were carried out based on the transmission line technique, where AgNPs-BTx@CF composites were pressed into toroidal shaped mold (jout = 7.00 mm; jin = 3.00 mm) for the measurements of complex permittivity (εr = ε0
jε00 ) and complex permeability (μr = μ0
jμ00 ) by using a vector network analyzer (VNA, Agilent E8363B, U.S.A.) in the frequency range of 0.5 - 18.0 GHz. Actually, the complex permeability and complex permittivity were deduced from the scattering parameters, reflection coefficient (S11) and transmission coefficient (S21), respectively, using a typical Nicholson
Ross algorithm.24 The reflection loss of microwave (RL) has been calculated by using Matlab software (a registered trademark of The MathWork, Inc.) based on the measurements of complex permittivity and complex permeability using transmission line theory.

’ RESULTS AND DISCUSSION Structural Characterization. As presented in Figure 2, SEM images indicate that the architecture of AgNPs-BT@CF composites is still in ordered fibrous state with approximately 1.0
4.0 μm in diameter, which is assigned with the diameter of the bundle of natural collagen fiber. In other words, the well-defined hierarchical fibrous morphology of CF is well reserved even after BT grafting and Ag loading reactions. Compared with monolithic absorbing counterparts, it has been proved that the hierarchical structures can possess more complicated interfaces which lead to the interface dielectric loss properties25 and form a unique multiple scattering absorption characteristic which leads to the improved values of microwave absorption.26 Therefore, the hierarchical fibrous structure of collagen fibers is benefit to the preparation of microwave absorption material.

Figure 3 shows the typical XRD patterns of CF, BT0.4@CF, and AgNPs-BT0.4@CF. It can be found that all of these samples exhibit a broad signal at about 2θ = 23°, which is attributed to the amorphous polymer phase of collagen fiber. As expected, there is no crystal diffraction peaks in the CF and BT0.05@CF. Meanwhile, the XRD pattern of AgNPs-BT0.4@CF reveals the crystal structure of the AgNPs. The narrow peaks at 2θ = 37.7°, 43.82°, and 63.71° are consistent with the (111), (200), and (220) crystalline plane diffraction peak of face-centered-cubic (fcc) silver (JCPDS-4784), which indicates that ionic Ag+ was reduced into crystal Ag0, and the AgNPs are highly dispersed onto the BT@CF. Figure 4 shows the TEM images of AgNPs-BTx@CF composites and the corresponding histogram of particle size distribution. Figure 4, panels a1 and a2, shows the TEM images of AgNPsBT0.05@CF at different magnifications. The distinctive D-period formed by the ‘‘quarter stagger’’ arrangement of collagen molecules is clearly observed at low magnification (Figure 4a2).27 Moreover, Ag particles, as dark dots ringed by white circle mark, are homogeneously arrayed onto the outer surface of the collagen fiber. The mean particle diameter of the roughly spherical AgNPs is 9.6 ( 2.9 nm, and some aggregations of Ag particles are also observed, as indicated by white arrows (Figure 4a1). Hence, we inferred that BT is able to reduce Ag+ to form AgNPs, but the AgNPs size distribution of AgNPs-BT0.05@CF is in a wide range, which suggests that BT with relatively low concentration is not efficient enough for the stabilization and of dispersion of AgNPs. Smaller size of AgNPs with slightly narrower size distribution 6.8 ( 2.1 nm are formed if the grafting degree of BT increases to 0.2, as shown in Figure 4b1. Markedly, the mean particle diameter of AgNPs drastically decreases when the grafting degree of BT was 0.4. It is found that the particle size of AgNPs further decreases to 5.2 ( 1.9 nm, and their size distribution is mainly in the range of 2.0
8.0 nm (Figure 4c1). The inset diagram in Figure 4c2 shows high resolution transmission electron microscope (HR-TEM) micrographs of an individual AgNPs below 5.0 nm. The closer observation of AgNPs indicates that they are crystalline with 23690

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Figure 5. HAADF-STEM image and EDS point analysis of AgNPsBT0.4@CF.

Figure 7. FTIR spectra of CF (a), BT0.2@CF (b), AgNPs-BT0.05@CF (c), AgNPs-BT0.2@CF (d), and AgNPs-BT0.4@CF (e).

Figure 6. O1s and N1s core levels XPS spectra for CF (a1 and a2), BT0.2@CF (b1 and b2), and AgNPs-BT0.2@CF (c1 and c2).

visible lattice fringes with diameter of 0.294 nm, which can be assigned to the fcc structure of Ag(111).28 This direct-viewing result is in good agreement with the XRD characterization analysis. The EDS point analysis for black region and bright dot in HAADF-STEM image of AgNPs-BT0.4@CF (Figure 5) demonstrates that the AgNPs are high-dispersive arrayed onto the surface of collagen fiber, and there is no signal of silver at blank area. These analyses suggest that BT is effective in reducing Ag+ to form AgNPs at room temperature without any additional reductant, and the diameter and size distribution of AgNPs can be facilely controlled by varying the grafting degree of BT on CF. In order to further understand the particular stabilization effect of BT to AgNPs, we performed XPS and FT-IR analysis. As a surface-specific technique, the curve-fitted high-resolution XPS spectra of O1s and N1s core levels for CF, BT0.2@CF, and AgNPs-BT0.2@CF are presented in Figure 6. As shown in Figure 6a1, there is only one peak of the O1s signal for CF at 531.6 eV, which is attributed to the OdC groups on CF. However, for BT0.2@CF (Figure 6b1), there is a new peak with higher intensity that appeared at 532.9 eV, which should be assigned to the oxygen atoms of hydroxyl groups of BT.27 Moreover, it was found that, with further increasing the grafting degree of BT on collagen fiber (Figure 6c1), the intensity of this

O
C peak became higher than that in the case of low BT grafting degree. This trend is also observed in the feature of the N1s. As shown in Figure 6, panels a2, b2, and c2, there is a small increase in the binding energy of N1s signal (N
CdO) when the grafting degree of BT was increased, indicating a decrease of electronic density around N atom, probably due to the hydrogen bonding interactions between N atoms in polypeptide chains and H atoms in hydroxyl groups of BT (C
N 3 3 3 HO
C).27 These results indicated that BT with multiple orthophenolic hydroxyls (C
OH) was successfully grafted onto collagen fiber. As shown in Figure 6, panels c1 and d1, the peak of HO
C group in O1s spectra was dramatically decreased in intensity and shifts from 532.9 eV to a higher binding energy of 534.4 eV, and no considerable change is observed for another peak at 531.6 eV (OdC). Moreover, the slight change of binding energy of N1s (Figure 6, panels c2 and d2) before and after the loading of AgNPs indicates that the amino groups of CF almost have not participated in the interaction with Ag ions and/or AgNPs. The results demonstrates that AgNPs are predominantly bonded with the oxygen atoms in orthophenolic hydroxyls of BT.27 Figure 7 presents the FT-IR spectra of CF (a), BT0.2@CF (b), AgNPs-BT0.05@CF (c), AgNPs-BT0.2@CF (d), and AgNPsBT0.4@CF (e). It can be seen that the adsorption band around 3390 cm
1 appears to be broadened when BT was grafted on CF, mainly due to the strong hydrogen bond interaction between the phenolic hydroxyl groups of BT and the amino/amide groups of CF.29 Meanwhile, the appearance of a new adsorption peak at 1120 cm
1 should be ascribed to the C
O
H stretching vibration of phenolic hydroxyls in BT.29 As for the AgNPsBT0.05@CF (c), AgNPs-BT0.2@CF (d), and AgNPs-BT0.4@CF (e), however, the stretching vibration peak of phenolic hydroxyls of AgNPs-BT@CF at 3390 cm
1 appears to be relatively narrow than that of BT@CF, which should be attributed to the interactions of phenolic hydroxyls with AgNPs. Additionally, the inplane deformation vibration (1400 cm
1) and the C
O
H stretching vibration (1120 cm
1) of phenolic hydroxyls are both weaker than that of BT@CF, which indicated that the reduction of silver ions could be coupled to the oxidation of phenolic hydroxyls of BT.30 Microwave Absorption Property of AgNPs-BT@CF Composites. To study the microwave absorption property and the possible absorbing mechanisms of AgNPs-BT@CF composites, their real and imaginary parts of complex permittivity (εr) and permeability (μr) were measured. As shown in Figure 8a, it can be seen that the real part of complex permittivity (ε0 ) of the CF is about 4.0. Similarly, the real part (ε0 ) of BTx@CF is still about 4.0, which imply that the grafting of BT on CF cannot change the 23691

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Figure 9. Variation of real (μ0 ) and imaginary (μ00 ) part of complex magnetic permeability of CF (a), BT@CF (b) and AgNPs-BTx@CF with various grafting degrees of BT: (c) 0.05, (d) 0.2, and (e) 0.4.

Figure 10. Schematic illustration of possible microwave absorption mechanisms for the AgNPs-BT@CF heterogeneous system.

0

00

Figure 8. Frequency dependence of (a) real (ε ) and (b) imaginary (ε ) parts of relative complex permittivity and the corresponding dielectric loss tangents (tanδ) for AgNPs-BTx@CF with various grafting degrees of BT.

dielectric storage ability of CF. The real parts (ε0 ) of AgNPsBTx@CF composites show a complex behavior, where the real parts ε0 are enhanced when the grafting degree of BT increase from 0.05 to 0.4, suggesting that the AgNPs supported on BT@CF can effectively increase the charge storage ability. Subsequently, as for the imaginary parts of the complex permittivity (ε00 ), there are some small resonance peaks in the CF, which are assigned to the dielectric relaxation of naturally occurring electric dipoles of CF (Figure 8b). The imaginary part (ε00 ) of BT@CF is relatively higher in contrast with that of CF, which implies that the formation of hydrogen bonds in BT@CF probably leads to the interfacial polarization. As expected, the values of imaginary parts ε00 of AgNPs-BTx@CF composites are increased compared with that of BT@CF, and four absorption peaks at 4.5, 7.8, 9.7, and 15.3 GHz are found to be electromagnetism resonances.31 Moreover, it is worth noticing that the dramatic increase of imaginary part (ε00 ) appears in the high frequency band (12.5
18 GHz) when the grafting degree of BT was 0.2 and 0.4. The dielectric loss tangent (tan δ) is presented as a function of frequency, which is shown in Figure 8c. The loss tangent, called the dissipation value, is the ratio of the imaginary part (ε00 ) to the real part (ε0 ) of relative complex permittivity, indicating the loss of dielectric. It can be observed that the values of loss tangent of CF, BT@CF, and AgNPs-BTx@CF are all on the order of magnitude of 10
1, suggesting they are dielectric materials.

The real and imaginary parts of the complex magnetic permeability (μ0 and μ00 ) of CF, BT@CF, and AgNPs-BTx@CF with various grafting degrees of BT are shown in Figure 9. It was found that the real part (μ0 ) is near to “1”, whereas the imaginary part (μ00 ) is near “0”. According to the normalization method, the complex magnetic permeability of AgNPs-BTx@CF can be expressed as μr = 1
j0, which suggests that AgNPs-BTx@CF have no magnetic loss ability. It should be pointed out that the noise level in the measurements of dielectric and magnetic parameter is as small as 0.03 dB, which in terms of the value of ε00 is about (0.2. When compared with the amplitude of these small peaks in the ε00 , the noise level is so small that it does not affect the establishment of genuine peaks. To get a better understanding of the effect of BT molecule on the adjustment of the imaginary part (ε00 ) of complex permittivity of AgNPs-BTx@CF, the proposed mechanism is depicted in Figure 10, in which two microwave loss mechanisms are involved. The first is the dipole polarization at the AgNPs surface, where surface charge polarization and the associated relaxation phenomena constitute the loss mechanisms. It has been reported that the surface charge density of metal nanoparticles is affected by the particle size and morphology.32 As presented before, the TEM analysis indicated that the particle size of AgNPs can be facilely controlled by varying the grafting degree of BT. When the grafting degree of BT increases, the size of AgNPs becomes smaller, which leads to the increase of surface charge density. Therefore, the high surface charge accumulated on AgNPs would act as dipoles that will be tuned with incident microwaves and contribute to strong absorption performance. In addition, the AgNPs-BT@CF is a heterogeneous system, where additional dielectric interfaces and more polarization charges on the interface between the AgNPs and BT@CF make the behaviors of 23692

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The Journal of Physical Chemistry C dielectric relaxation more complex.31 As such, the second microwave loss mechanism is the interfacial polarization and associated relaxation. The presence of the BT@CF matrix results in the formation of more interfaces, where space charge accumulates at the interface between AgNPs and BT@CF. Based on the XPS and FTIR analysis presented before, two bonding sites contribute to the interface polarization: one is the synergetic anchoring interaction between the AgNPs and the oxygen atoms in orthophenolic hydroxyls of BT and the other is the interaction between the AgNPs and the nitrogen atoms in amino groups of CF. In these two bonding sites, the mobility of bound charges (dipoles) is restricted. When the frequency of the electrical field is increased, the interfacial dipoles cannot reorient themselves fast enough to respond to the electrical field, leading to the distortion of valence charge density in these bonding sites. Therefore, with the increase of frequency, the imaginary parts ε00 of AgNPs-BT@CF composites are enhanced, exhibiting a strong peak from 13.0 to 18.0 GHz. Based on the discussion above, it is logical to explain the unique microwave absorption mechanism of AgNPs-BT@CF composites. With further increasing the grafting degree of BT, smaller AgNPs with higher surface charge density and much more interface with BT@CF matrix lead to the accumulation of more space charge, and stronger interface polarization in two bonding sites, which consequently leads to the improved values of microwave absorption. According to transmission line theory,24 the reflection loss of electromagnetic radiation, RL (dB), under normal wave incidence at the surface of a single-layer material backed by a perfect conductor can be defined by 
 Zin
Z0 RL ¼ 20 log Zin þ Z0 where Z0 is the characteristic impedance of free space rffiffiffiffiffi μ0 Z0 ¼ ε0 Zin is the input impedance at free space and material interface 
  rffiffiffiffi μr 2πfd pffiffiffiffiffiffiffiffi tanh j Zin ¼ Z0 ð μr εr Þ c εr where f is the frequency of the microwave, d is the thickness of the microwave absorption material, and c is the velocity of light. μr and εr are the complex permeability and permittivity of the microwave absorption material, respectively. Thus, the microwave reflection loss (RL) of the AgNPs-BTx@CF with various grafting degrees of BT have been calculated from a computer simulation using transmission line theory, assuming d = 2.0 mm. The calculated microwave reflection losses of AgNPs-BTx@CF are shown in Figure 11. It should be noted that the RL equation used here is the same with the equation presented by Rizzi.33 As expected, the AgNPs-BT@CF exhibited strong microwave absorption behaviors. It can be seen that the reflection loss of AgNPs-BT0.05@CF (in blue color) is relatively low and only a single absorbing peak at 16.0 GHz with value of
10.0 dB (absorbing about 90.0%). However, as for the sample of AgNPsBT0.2@CF (olive color), the microwave absorption is evidently improved and exhibits two absorbing peaks at 15.5 and 17.5 GHz with values of
14 and
12 dB (absorbing about 96.0 and 93.7%), separately. A substantially enhanced absorbing behavior

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Figure 11. (a) Simulation of reflection loss of AgNPs-BTx@CF composite with various grafting degrees of BT in thickness of 2.0 mm. (b) Three-dimensional representations of reflection loss of AgNPsBT0.4@CF with different thicknesses from 1.6 to 8.8 mm.

can be observed in the sample of AgNPs-BT0.4@CF (red color). This sample shows the maximum reflection loss of
20.0 dB (absorbing about 99.0%) with two absorbing peaks at 15.5 GHz and 17.5 GHz, separately. Figure 11b shows simulations of reflection loss of AgNPs-BT0.4@CF composite with different thicknesses. It is noted that the RL values exceeding
10 dB are achieved in the whole Ku band (12.5
18 GHz) when the thickness is 2.4 mm, which are broader frequency ranges than those of previously reports.34 In addition, the RL value of AgNPsBT0.4@CF do not change dramatically in the thicknesses range of 2.4
3.0 mm for the RL values exceeding
10 dB, which is important for the practical application. The excellent broadband microwave absorbing performance is attributed to the special hierarchical fibrous nanostructure of collagen fiber which can produce a unique multiple scattering absorption characteristic leading to the microwave absorption in a broad frequency band. Herein, it is worth pointing out that the weight of our AgNPsBT0.4@CF composite is only 1/10
1/5 as compared with those inorganic wave absorbing materials at same thickness.

’ CONCLUSIONS In summary, we developed a simple and facile strategy for the synthesis of size-controlled silver nanoparticles/collagen fiber composites, being effective and lightweight microwave absorption materials, in which the bayberry tannin was grafted on collagen fiber to serve as reductant and stabilizer. The mean diameter and size distribution of AgNPs can be facilely controlled by varying the grafting degree of BT on CF. Furthermore, these AgNPs-BTx@CF composite exhibited good microwave absorption properties. With further increasing the grafting degree of BT, the imaginary parts (ε00 ) of AgNPs-BT@CF composites are enhanced, and the microwave absorbing behaviors of AgNPsBTx@CF composites are stronger. Based on TEM, XPS, and 23693

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (X.P.L.); [email protected] (B.S.). Tel.: +86-28-85405508. Fax: +86-28-85400356.

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (20976111, 21176161) and A Foundation for the Author of National Excellent Doctor Dissertation of China (FANEDD200762). We especially thank Prof. L. Chen in the Engineering Research Center of Stealth Materials and Technology in State Key Laboratory of Electronic Thin Films and Integrated Devices for the microwave adsorption measurements.

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dx.doi.org/10.1021/jp207194a |J. Phys. Chem. C 2011, 115, 23688–23694