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Surface Plasmon Resonances, Optical Properties, and Electrical Conductivity Thermal Hystersis of Silver Nanofibers Produced by the Electrospinning Technique Nasser A. M. Barakat,*,†,‡ Kee-Do Woo,§ Muzafar A. Kanjwal,| Kyung Eun Choi,⊥ Myung Seob Khil,| and Hak Yong Kim*,| Chemical Engineering Department, Faculty of Engineering, El-Minia UniVersity, El-Minia, Egypt, Center for Healthcare Technology DeVelopment, DiVision of AdVanced Materials, RCIT, and Department of Textile Engineering, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea, and Department of Practical Art Education, Jeonju National UniVersity of Education, Chonju, 560-757, Republic of Korea ReceiVed July 3, 2008. ReVised Manuscript ReceiVed August 14, 2008 In the present study, silver metal nanofibers have been successfully prepared by using the electrospinning technique. Silver nanofibers have been produced by electrospinning a sol-gel consisting of poly(vinyl alcohol) and silver nitrate. The dried nanofiber mats have been calcined at 850 °C in an argon atmosphere. The produced nanofibers do have distinct plasmon resonance compared with the reported silver nanoparticles. Contrary to the introduced shapes of silver nanoparticles, the nanofibers have a blue-shifted plasmon resonance at 330 nm. Moreover, the optical properties study indicated that the synthesized nanofibers have two band gap energies of 0.75 and 2.34 eV. An investigation of the electrical conductivity behavior of the obtained nanofibers shows thermal hystersis. These privileged physical features greatly widen the applications of the prepared nanofibers in various fields.
1. Introduction Recently, noble metals nanostructures such as silver and gold have attracted a great deal of attention because of their superior electrical, optical, mechanical, and catalytic properties.1-5 Surface plasmon resonance (SPR) is one of the most interesting features of these nanoparticles (NPs). SPR is a phenomenon that occurs when light is reflected off thin metal films or NPs. A fraction of the light energy incident at a sharply defined angle can interact with the delocalized electrons in the metal surface (plasmon), thus reducing the reflected light intensity.6 In more detail, when small metallic NPs are illuminated, the oscillating electric field causes the conduction electrons to oscillate coherently. In particular, silver and gold metals are the most popular materials used in this technique;7 however, silver is more commonly used8 because its d-s band gap is in the UV region and does not damp out the plasmon mode as strongly as does gold.9 Many applications * Corresponding authors. (N.A.M.B.) E-mail:
[email protected]. (H.Y.K.) Tel: +82 63 270 2351. Fax: +82 63 270 2348. E-mail: khy@ chonbuk.ac.kr. † El-Minia University. ‡ Center for Healthcare Technology Development, Chonbuk National University. § Division of Advanced Materials, RCIT, Chonbuk National University. | Department of Textile Engineering, Chonbuk National University. ⊥ Jeonju National University of Education. (1) Bockrath, M.; Liang, W.; Bozovic, D.; Hafner, J. H.; Lieber, C. M.; Tinkham, M.; Park, H. Q. Science 2001, 291, 283–285. (2) Cui, Y.; Wei, Q. Q.; Park, H. Q.; Lieber, C. M. Science 2001, 293, 1289– 1292. (3) Torres, D.; Lopez, N.; Illas, F.; Lambert, R. M. J. Am. Chem. Soc. 2005, 127, 10774–10775. (4) Williams, F. J.; Bird, D. P. C.; Palermo, A.; Santra, A. K.; Lambert, R. M. J. Am. Chem. Soc. 2004, 126, 8509–8514. (5) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R. R.; Sun, Y. G.; Xia, Y. N.; Yang, P. D. Nano Lett. 2003, 3, 1229–1233. (6) Bohrem, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley-Interscience: New York, 1983. (7) Nikolajsen, T.; Leosson, T.; Salakutdinov, K.; Bozhevolnyi, S. Appl. Phys. Lett. 2004, 85, 5833–5835. (8) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 7426–7433. (9) Hodak, J. H.; Martini, I.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 6958–6967.
based on SPR features have been reported, including medical diagnostics and therapeutics,10,11 chemical and biochemical sensors,12,13 substrates for enhanced spectroscopy,14 and light transmission in the subwavelength regime.15 Another advantageous feature of surface plasmon excitations in metallic nanostructures is that the electromagnetic fields propagating in the form of surface plasmons are not diffraction-limited, which can be used to create fast optical devices with significantly reduced dimensions. It has recently been reported that the sensitivity and the tunability of the resonance wavelength maximum are closely related to the geometry of the nanoparticles.6,13,16 Both calculations and experiments proved that 1D metal NPs reveal a much greater local field enhancement and therefore more significant applications.17,18 Moreover, the relative ratio of the long axis to short axis for the 1D NPs has a special effect.19 Therefore, many reports have been introduced with regard to the production of silver nanorods.20-23 (10) El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Nano Lett. 2005, 5, 829–834. (11) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549–13554. (12) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057–1062. (13) Cognet, L.; Tardin, C.; Boyer, D.; Choquet, D.; Tamarat, P.; Lounis, B. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 11350–11355. (14) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964–9972. (15) Dionne, J. A.; Sweatlock, L. A.; Atwater; H. A.; Polman, A. Phys. ReV. B 2006, 7335407-135407-9. (16) Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkov, N.; Bein, T.; Nichtl, A.; Kurzinger, K. Nano Lett. 2004, 4, 1853–1857. (17) Wang, D.-S.; Kerker, M. Phys. ReV. B 1981, 24, 1777–1790. (18) Nikoobakht, B.; Wang, J.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17–23. (19) Kyeong-Seok, L; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 19220– 19225. (20) Kadir, A.; Zoya, L.; Joseph, R. L.; Chris, D. G. J. Phys. Chem. B 2005, 109, 3157–3162. (21) Xin, H.; Xiujian, Z.; Yunxia, C.; Jinyang, F.; Zhenya, S. J. Solid State Chem. 2007, 180, 2262–2267. (22) Gil, J. L.; Seung, I. S.; Young, C. K.; Seong, G. O. Mater. Chem. Phys. 2004, 84, 197–204.
10.1021/la802084h CCC: $40.75 2008 American Chemical Society Published on Web 09/24/2008
SilVer Nanofibers Produced by Electrospinning
Beside the importance of the plasmon resonance feature of the metal NP colloids, there is special interest in the band gap energy in spectroscopy24 and optical switch25 applications. Also, the optical properties depends on the particle shape, and the 1D form reveals a high band gap energy.26 Among the 1D NPs, nanofibers possess an unlimited axial ratio that greatly enhances the physical properties. Therefore, silver nanofibers are expected to have special features. In this study, we have prepared silver metal in a nanofibrous form for the first time. Silver nanofibers have been prepared by utilizing the electrospinning process for a sol-gel consisting of silver nitrate and poly(vinyl alcohol). The recently reported interesting physical features of the NPs have been investigated for the obtained nanofibers and include plasmon resonance, band gap energy, and electrical conductivity thermal hystersis.
2. Experimental Details 2.1. Materials. Silver nitrate (99.8 assay) and poly(vinyl alcohol) (PVA, MW ) 65 000 g/mol) were obtained from Showa Co., Japan and Dong Yang Chem. Co., South Korea, respectively. These materials were used without any further treatment. Distilled water was used as the solvent. 2.2. Experimental Work. The sol-gel was prepared by mixing a 15 wt % aqueous silver nitrate solution and a 10 wt % PVA aqueous solution in a ratio of 2:5. The obtained solution was placed in a plastic capillary. A carbon pin connected to a high-voltage generator was inserted into the solution, and the solution was kept in the capillary by adjusting the angle of inclination. A ground iron drum covered with a polyethylene sheet served as a counter electrode. A voltage of 20 kV was applied to this solution. The formed nanofiber mats were initially dried for 24 h under vacuum and then calcined at 850 °C for 5 h in an argon atmosphere at a heating rate of 2.3 °C/min. 2.3. Characterization. Surface morphology was studied by scanning electron microscopy (SEM, JSM-5900, JEOL, Japan) and field-emission scanning electron microscopy (FESEM, Hitachi S-7400, Hitachi, Japan). Information about the phase and crystallinity was obtained by using a Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu KR (λ ) 1.540 Å) radiation over the Bragg angle ranging from 10 to 90°. High-resolution images and selected-area electron diffraction patterns were obtained via transmission electron microscopy (TEM, JEM-2010, JEOL, Japan) operated at 200 kV. The thermal properties have been studied with a thermal gravimetric analyzer (TGA, Pyris1, PerkinElmer). A colloidal solution has been prepared by adding the obtained nanofibers to distilled water; the solution was sonicated for 1 h. The optical properties have been studied using UV-visible spectroscopy (HP 8453 UV visible spectroscopy system, Germany), and the spectra obtained were analyzed by the HP ChemiStation software 5890 series. However, the electrical conductivity was investigated with EC meter CM 40 G version 1.09 (DKK, TOA Co., Japan).
3. Results and Discussion Electrospinning is a process by which to make nanofibers with fiber diameters in the range of about 10 to several hundred nanometers from a polymer solution through electrostatic force. This technique involves the use of high voltage to charge the surface of a polymer solution droplet and thus to induce the ejection of a liquid jet through a spinneret. Because of bending (23) Fu, K. L.; Pei, W. H.; Yu, C. C.; Chu, J. K.; Fu, H. K.; Tieh, C. C. J. Cryst. Growth 2005, 273, 439–445. (24) Zai, R.; Selker, M. D.; Brongersma, M. L. Phys. ReV. B 2005, 71, 1654311–165431-9. (25) Hochberg, M.; Baehr-Jones, T.; Walker, C.; Scherer, A. Opt. Express 2004, 12, 5481–5486. (26) Barakat, N. A. M.; Khil, M. S.; Sheikh, F. A.; Kim, H. Y. J. Phys. Chem. C 2008, 112, 12225–12233.
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instability, the jet is subsequently stretched many times to form continuous, ultrathin fibers. Recently, the electrospinning technique has been exploited to prepare some metal nanofibers.27-29 Figure 1 shows the SEM and FESEM images of the silver nitrate/PVA nanofiber mats after and before calcination at 850 °C for 5 h. As shown in Figure 1A,B in the SEM images of the dried silver nitrate/PVA nanofiber mats, the electrospinning process produced relatively smooth nanofibers with an almost 350 nm average diameter. Beads or agglomerated nanofibers cannot be observed in the obtained mats. Figure 1C,D shows the SEM images of the obtained nanofibers after the calcination process. As shown in these Figures, calcination of the original nanofiber mats in an argon atmosphere did not strongly affect the nanofibrous morphology, with the final product being nanofibers. FESEM of the final powder obtained is shown in Figure 1E. As shown in this Figure, the surface morphology of the obtained nanofibers is acceptable; the average diameter of the obtained nanofibers was about 250 nm. The polymer is an essential constituent in the sol-gel to carry out the electrospinning process. PVA does have wide utilization in producing metallic nanofibers via electrospinning methodology for its novel chemical and thermal characteristics. The hydroxyl groups deployed on the PVA chains have the capability to generate hydrogen bonds with many anions (especially those having oxygen atoms), which enhances the solubility of the metal salt in PVA solution. Moreover, PVA has a low decomposition temperature, so eliminating PVA from the obtained electrospun nanofiber mats is an easy task.30 Likewise, silver nitrate decomposes easily by heat.31 Accordingly, one can say that at the aforementioned calcination temperatures PVA has been completely eliminated and silver nitrate has decomposed to silver. The typical XRD pattern of the calcined powder at the utilized calcination temperatures is presented in Figure 2. The strong diffraction peaks at 2θ values of 38.25, 44.45, 64.60, and 77.65° corresponding to (111), (200), (220), and (311) crystal planes indicate the formation of purely crystalline silver metal (JCDPS, card no 04-0783). Standard silver does have cubic crystals with a cell parameter of 0.4086 nm (JCDPS, card no 04-0783). Figure 3 shows the TEM image of the obtained silver nanofibers. The top inset in this Figure shows the HRTEM image; the distance between two successive planes almost matches the standard value of 0.41 nm, which indicates good crystallinity of the obtained nanofibers. The bottom inset reveals the SAED pattern. As shown in this inset, the SAED pattern reveals good crystallinity at the utilized calcination temperature. There are no dislocations or imperfections observed in the lattice planes, which indicates good crystallinity of the synthesized nanofibers. Actually, the black dots that appear on the nanofiber in Figure 3 demonstrate silver nanoparticles attached to the surface of the nanofiber (as can also be observed in Figure 1C,D). The darkness in color can be expounded on as the high crystallinity of these nanoparticles compared with that of the nanofiber as a result of the small size. Figure 4 shows the thermal gravimetric analysis for silver nitrate/PVA nanofiber mats in an argon atmpsphere, along with (27) Hui, W.; Rui, Z.; Xinxin, L.; Dandan, L.; Wei, P. Chem. Mater. 2007, 19, 3506–3511. (28) Michael, B.; Mathias, B.; Martin, G.; Werner, M.; Joachim, H. W.; Andreas, S.; Dirk, W.; Andre, B.; Armin, G.; Andreas, G. AdV. Mater. 2006, 18, 2384– 2386. (29) Graeser, M.; Bognitzki, M.; Massa, W.; Pietzonka, C.; Greiner, A.; Wendorff, J. H. AdV. Mater. 2007, 19, 4244–4247. (30) Fernandes, D. M.; Hechenleitner, A. A. W.; Pineda, E. A. G. Thermochim. Acta 2006, 441, 101–119. (31) Hui, W.; Dandan, L.; Rui, Z.; Wei, P. Chem. Mater. 2007, 19, 1895– 1897.
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Figure 1. Scanning electron microscope images for silver nitrate/PVA nanofiber mats (A, B) and the obtained nanofibers after calcination in an Ar atmosphere at low and high magnifications (C, D). Field-emission scanning electron microscope image for the obtained nanofibers after calcination (E).
Figure 3. TEM image for the obtained silver nanofibers. The upper inset show the high-resolution TEM image, and the lower one represents the selected-area electron diffraction pattern.
Figure 2. XRD data for the obtained nanofibers after calcination at 850 °C in an Ar atmosphere, where (111), (200), (220), (311), and (222) represent the main crystal planes in a pure silver crystals.
the plotted first derivative. As shown in the first derivative, there are some peaks denoting a decrease in the weight of the sample. The peak at around 50 °C is due to the evaporation of the physically absorbed water in the sample. Another sharp, high-depth peak appears at ∼200 °C, which is based on the thermal properties of PVA and its mass content in the original nanofiber mats; this peak can be characterized by the decomposition of PVA. Silver
is a noble metal, so it tends to stay in the zero oxidation state. However, it is well known that the thermal decomposition depends upon the environment. Therefore, the small peak at around 240 °C might be due to the partial decomposition of the silver nitrate according to this reaction:
2AgNO3 f Ag2O + 2NO2 + 0.5O2
(1)
The last broad peak at ∼340 °C reveals the formation of silver metal from the inorganic compounds remaining in the sample as follows:
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Ag2O f 2Ag + 0.5O2
(2)
AgNO3 f Ag + NO2 + 0.5O2
(3)
It is noteworthy to mention that the last reaction is common and is widely cited in the literature.31 As previously mentioned, the thermal decomposition of the silver nitrate/PVA nanofiber mat behavior mainly depends on the calcination atmosphere. Figure 5 shows the thermal decomposition of the prepared nanofiber mats in an oxygen atmosphere. As shown in this Figure, a harsh, sudden decrease in the weight occurred at ∼200 °C, which can be explicated as the rapid decomposition of PVA due to the presence of oxygen. According to a previous study31 on silver nitrate, a sudden, sharp decrease in the weight at ∼317 °C reveals the decomposition of silver nitrate to a pure silver compound according to reaction 2. After such a temperature, only silver metal is present in the sample; no weight change was seen because of the noble properties of the silver metal. An important observation can be made by comparing the rate of weight loss in the case of argon and oxygen environments (Figures 4 and 5). In an argon atmosphere, the rate of weight loss is very slow (Figure 4); however, in an oxygen environment, a sharp, sudden decreases in the weight can be observed at almost 200 and 317 °C (Figure 5). Actually, these sharp decreases in the weight in an oxygen atmosphere destroy
Figure 6. Surface plasmon absorption spectra of silver nanofibers (A) and the corresponding photon energy (B).
Figure 4. Thermal gravimetric analysis in an argon atmosphere and the corresponding first derivative of silver nitrate/PVA nanofiber mats.
Figure 7. Plot of (REPhoton)2 vs EPhoton. The intersections of the dashed lines with the abscissa represent the band gap energies of the synthesized silver nanofibers.
Figure 5. X3/PVA nanofiber mats and the nanofibers obtained after calcination in an argon atmosphere at 700 and 850 °C.
the nanofibrous morphology (Figure 4 in Supporting Information, which represents the SEM images of the powders obtained after calcination of the silver nitrate/PVA nanofiber mats in air at different temperatures). However, in an argon atmosphere, lowrate weight degradation maintains the nanofibrous morphology, so silver nanofibers can be obtained as the final product (Figure 1, panels C, D and E). Actually, most of the polymers do have similar thermal decomposition behavior in the presence of oxygen, which we think is the main reason for the difficulty facing
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infrared (near-IR) region along with high sensitivity. All of the reported silver and gold nanoparticles possess surface plasmon resonance at wavelength higher than 450 nm. Figure 6A shows the UV-vis spectrum for the obtained silver nanofibers colloids (in water). As shown in this Figure, the prepared nanofibers possess surface plasmon at 330 nm, which is considered to be an interesting result because it is much lower than for any reported silver NPs. It is noteworthy to mention that this maximum wavelength value is not temperature-dependent; almost the same value was obtained at many temperatures (Figure 3 in Supporting Information). We think that the main reason for obtaining such a blue-shifted wavelength is the very high axial ratio of the prepared nanofibers compared with that of the nanorods. Consequently, this result supports the formation of the nanofibrous shape because the plasmon resonances of NPs or nanorods are obtained toward the red. Figure 6B shows the photon energy. Also, the energy of the photon corresponding to the maximum wavelength in the case of silver nanofibers is higher than that of any other reported NPs. Many approaches have been introduced to theoretically estimate the surface plasmon resonance. Classical electrodynamics has proven to be a valuable tool for rationalizing in an accurate way the plasmon behavior of metal nanostructures. For simple shapes such as spheroids in the quasi-electrostatic regime, the resonance condition for a dipole resonance can be stated as the frequency at which the following relation is fulfilled
(ω) ) -fm
Figure 8. Thermal hysteresis by electrokinetic properties of the silver nanofibers (A) and the experimental electrical conductivity vs the calculated values (B).
researchers in producing silver nanofibers not only by using the electrospinning process but also by any other technique based on polymeric substrates. To ensure that all PVA polymers have been eliminated and silver nitrate has been decomposed, we have performed thermal analyses in an oxygen atmosphere for two nanofiber products obtained by calcination of AgNO3/PVA nanofiber mats in an argon atmosphere at 700 and 850 °C. The obtained results were demonstrated in Figure 5. As shown in this Figure, at a calcination temperature of 700 °C, a slight decrease in the weight around the decomposition temperature of silver nitrate is observed. This draws our attention to the fact that calcination at 700 °C is not sufficient to produce pure silver metal. However, as also shown in this Figure, this trivial decrease in the weight almost disappears at a calcination temperature of 850 °C; this result supports XRD and TEM data revealing that the nanofibers obtained at this temperature consist of pure silver metal. As mentioned in the Introduction, the plasmon resonance feature strongly depends on the geometry of the nanoparticles. Many reports have been introduced to study the effect of particle shape.32-34 The main conclusion that can be drawn according to these studies is that the axial ratio has a distinct effect. Therefore, the longer nanorods exhibit plasmon resonances in the near(32) Benjamin, J.; Wiley, S. H. I.; Zhi, Y. L.; Joeseph, M.; Andrew, S.; Younan, X. J. Phys. Chem. B 2006, 110, 15666–15675. (33) Lee, K. S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 19220–19225. (34) Ezequiel, R. E.; Coronado, E. A. J. Phys. Chem. C 2007, 111, 16796– 16801.
(4)
where (ω) stands for the complex, frequency-dependent, dielectric function of the particle and m stands for the real part of the dielectric constant of the surrounding media (assumed to be nonabsorbing). The f factor is a geometrical factor that depends only on particle shape, that is, the ratio of the major to minor axis (aspect ratio R) and the incident field polarization. This factor has been estimated for many nanostructural shapes. For instance, in the case of spheres, the f value is 2, independent of the illumination direction. For prolate or oblate spheroids, f is greater than 2 and increases with R when the polarization is along the major axis. An opposite trend is found for polarization along the minor axis; that is, f is smaller than 2 and decreases with R.34 Recently, Encina and Coronado34 reported a simple graphical method to calculate the f factor. According to this strategy and after the mathematical extrapolation process, the value of the f factor in the case of the produced silver nanofibers is almost 1. UV-visible absorption spectra were carried out to characterize the optical absorbance properties of the synthesized silver nanofibers. For semiconductor materials, the quantum confinement effect is expected if the semiconductor dimension becomes smaller than the Bohr radius of the excitation, and the absorption edge is shifted to higher energy.35-37 For a semiconductor, the absorbance in the vicinity of the onset due to the electronic transition is given by the following equation38,39
R)
K(hν - Eg)n hV
(5)
where R is the absorption coefficient, K is a constant, Eg is the band gap, and n is a value that depends on the nature of the (35) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119–8123. (36) Gu, F.; Wang, S. F.; Lu, M. K.; Cheng, X. F.; Liu, S. W.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Cryst. Growth 2004, 262, 182–185. (37) Gu, F.; Li, C. Z.; Hu, Y. J.; Zhang, L. J. Cryst. Growth 2007, 304, 369– 373. (38) Dare-Edwards, M. P.; Goodenough, A. H.; Hammett, A.; Trevellick, P. R. J. Chem. Soc., Faraday Trans. 1983, 9, 2027–2041. (39) Xu, R.; Zeng, H. C. Langmuir 2004, 20, 9780–9790.
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transition (1/2 for a direct allowed transition or 2 for an indirect allowed transition). In this case, n is equal to 1/2 for this direct allowed transition. The band gap can be estimated from a plot of (Rhυ)2 versus photon energy. The linear regions in this plot can be invoked to estimate the band gap energies by calculating the intersection points between these regions and the abscissa (x axis). Previous work has indicated that the optical properties of NPs mainly depend on the particle shape and size.35,36,39 Figure 7 shows the relationship between (REPhoton)2 and EPhoton for the synthesized silver nanofibers. Extrapolation of the linear regions in the obtained graph can give two Eg values for the silver nanofibers obtained at 0.75 and 2.34 eV. The optical band gap energy difference is 1.59 eV. Thermal hystersis is a phenomenon in which a physical quantity depends not only on the temperature but also on the preceding thermal history. In other words, thermal hystersis is said to occur if the behavior of such a property is different when the material is heated through a given temperature range from when it is cooled through the same temperature range. We have study the thermal hystersis of the synthesized nanofibers for two physical properties: the wavelength corresponding to the surface plasmon and the electrical conductivity. A temperature range of room temperature to 80 °C was used. In the case of surface plasmon resonance, we have found that the silver nanofibers have surface plasmon resonance at approximately 330 nm and this value was not affected by either the temperature or the thermal history (Figure 3 in Supporting Information). Therefore, one can say that the synthesized nanofibers have no plasmon resonance thermal hystersis. Electrical conductivity is an important physical feature of the nanostructure; it strongly affects the use of the nanostructure in the nanodevices. Consequently, this parameter has been investigated for the prepared nanofibers, and the thermal hystersis phenomenon has also been studied. We have found that with increasing temperature the value of the conductivity increases and decreases with cooling within different paths as shown in Figure 8A. Moreover, it is observed that the conductivity reaches a maximum and reverts to a lower value close to the initial (i.e., at 25 °C) via a hystersis loop. We have built a mathematical model to represent this loop as follows:
C ) RTβ
(6)
where C is the electric conductivity (mS/m), T is the temperature (°C), and R and β are constants. The values of these constants
Table 1. Empirical Equation Constants (r and β) in Both Cooling and Heating Paths parameter
heating path
cooling path
R β
0.625 0.8633
0.7643 0.7034
are given in Table. 1. To verify the validity of the designed empirical equation, the relationship between the experimental and calculated values of the electric conductivity was plotted and is represented in Figure 8B. As shown in this Figure, all of the data points are located around the 45° line, which indicates good accuracy for the empirical model.
4. Conclusions Heating nanofiber mats composed of silver nitrate/PVA in an argon atmosphere preserves the nanofibrous morphology. The absence of oxygen gas from the heating environment reduces the rate of polymer decomposition, which gives the silver NPs a chance to form silver nanofibers. The obtained nanofibers possess blue-shift plasmon resonance as a result of the high axial ratio. The plasmon resonance occurs at a wavelength of 330 nm, and it is a temperature-independent variable. Also, the optical properties study reveals two band gap energies for the synthesized nanofibers. However, the electrical conductivity of the obtained nanofibers possesses thermal hystersis behavior. The electrical conductivity increases with increasing the temperature and does not return via the same pathway during the cooling process. Acknowledgment. This work was supported by the Korean Research Foundation Grant founded by the Korean Government (MOEHRD) (The Center for Healthcare Technology & Development, Chonbuk National University, Jeonju 561-756, Republic of Korea) and Korea Science and Engineering Foundation (KOSEF) grant no. 2008-022, Republic of Korea. We thank Mr. T. S. Bae and J. C. Lim, KBSI, Jeonju branch, and Mr. JongGyun Kang, Centre for University Research Facility, for taking high-quality SEM and TEM images, respectively. Supporting Information Available: Standard experimental setup and a photograph for the utilized electrospinning experiment. UV spectra of the prepared silver nanofibers at many temperatures and SEM images of the nanoparticles obtained after the calcination of silver nitrate/PVA nanofiber mats in air at 350, 450, and 500 °C. This material is available free of charge via the Internet at http://pubs.acs.org. LA802084H
