Preparation And Applied Study Of Organic/Inorganic Nanofibers

Polymer/inorganic nanocomposites are of great interest in recent years because of not only the novel properties of the nanocomposite materials but also continuously growing demand for further miniaturization of electronic components, optical detectors, chemical and biochemical sensors and devices.Within the past decade, dispersion of silver nanoparticles in polymer matrixhave particularly received extensive attention due to the possibility of fabricating suitable materials for applications as catalysts, drug and wound dressings, information storage and surface enhanced Raman scattering etc. Many different approaches have been used to prepare Ag/polymer nanocomposite. Conventionally, silver nanoparticles are formed firstly and then polymer and the nanoparticles are mixed to prepare composites in ex-situ methods. However, it is difficult to disperse silver nanoparticles homogeneously into polymer matrix by ex-situ methods because of the easy agglomeration of nanoparticles. So convenient and effective ways to prepare Ag nanoparticles into polymer materials are still strongly demanded.In recent years, researchers have rediscovered and continues to develop a technology called electrospinning. Simply stated, electrospinning is a process by which a suspended droplet of polymer solution is charged to high voltage to produce fibers with diameter ranging from 10nm to 500nm. When a voltage is sufficient to overcome surface tension forces, fine jets of polymer solution shoot out toward a grounded collector. The jet is stretched and elongated before it reaches the collector, dries and is collected as a non-woven nanofiber film. It is a simple and low cost method for fabrication of ultrafine polymer fibers. This novel nanofiber spinning technique has been explored mainly to prepare pure polymer nanofibers in past years.It is known that Polyvinyl pyrrolidone (PVP) is one of attractive polymers to immobilize metal nanoparticles. Some metal and semiconductor nanoparticles have been successfully prepared under the existence of PVP in solution. As so far, however, few convenient approaches can homogeneously disperse metal nanoparticles in solid PVP substrates. In this study, we present an efficient and economical method to prepare PVP nanofiber film homogeneously dispersed silver nanoparticles with diameter of 8nm, which includes two main steps. In the first one, silver nanoparticles are prepared through the reduction of AgNO3 by ethanol under refluxing condition in PVP ethanol solution. In the following one, the solution after reaction is spun directly to prepare PVP nanofiber film dispersed silver nanoparticles by electrospinning.In preparing the silver nanoparticles, the procedure was as follows. A known quantity of silver nitrate was added to 100ml of ethanol containing a predetermined quantity of PVP. The molar ratio of silver nitrate to repeating unit of PVP was 1:10. The ethanol solution containing silver nitrate and PVP was refluxed at 360K and kept stirring. At different time during reaction, 2 ml of solution was taken out for UV spectrum measurement. The reaction reached the end point when UV absorption increased no more.We used home-made electrospinning setup which contained basically three components: a high voltage supplier, a glass tube with a tip diameter of 0.8mm and an Al-foil collector to fabricate Ag/PVP nanocomposite. The process is that the Ag/PVP solution was loaded into glass capillary with 0.8mm of diameter, which as an anode was 15cm distant from cathode stuck on a drum whose rotation speed can be controlled. An aluminum sheet was circled on the drum to collect the Ag/PVP nanocomposite under a voltage of 15.0KV.Reduction of AgNO3 by ethanol in the presence of PVP resulted in nice colloidal solution with yellowness. Results displayed the absorption spectra of the solution before refluxing and after refluxing. Before refluxing, the absorption of the solution had hardly any absorption band as shown. After refluxing, an absorption band arose with a sharp maximum at 410 nm. The absorption corresponded well with the known spectral behavior of silver nanoparticles. Silver nanoparticles exhibit a high optical absorbency due to the existence of discrete energy levels and particularly of specific surface states. The absorption band in figure 1b is ascribed to the plasma resonance of silver. Silver nanoparticles with diameters below 5 nm have a rather high absorption band with a maximum at about 400nm. This band is broadened and shifted toward higher wavelength with increasing particle diameter. Based on the maximum absorption peak at 410 nm, it is estimated that the nanoparticles of silver with diameters of about 10 nm are formed during refluxing of ethanol solution containing silver nitrate and PVP.XRD pattern of Ag/PVP nanocomposite film was done. The XRD pattern revealed that silver nanoparticles were in the form of face-centred cubic (f.c.c.) crystalline in PVP nanofiber film, as indicated by the diffraction peaks with 2θvalues of 38.2°, 44.3°, 64.5°and 77.6°, corresponding to the crystal faces of (111), (200), (220) and (311) of f.c.c. crystalline silver.TEM images of the PVP nanofiber containing Ag nanoparticles were mesured. The average diameters of the nanofiber are 80nm. The result shows further magnified TEM image of the single nanofiber. It is observed that very small silver particles are dispersed homogenously in PVP nanofiber. The average diameter of Ag nanoparticles is 8nm, which is consistent with the result of UV absorption spectrum.In our study, so small silver nanoparticles dispersed homogeneously in PVP nanofibers can be explained as follows. PVP has a structure of a polyvinyl skeleton with polar groups. PVP donates lone pair electrons of oxygen and nitrogen atoms to sp orbits of silver ions, and thus the coordinative complex of silver ions and PVP in ethanol solution. PVP promotes the nucleation of metallic silver because the Ag ions-PVP complex is more easily reduced by ethanol than the pure Ag ions owing to Ag ions receiving more electronic clouds from PVP. PVP can effectively decrease the surface energy of silver particles and keep them from aggregation. Moreover, Steric effect of PVP also help to prohibit silver grain growth. It is well known that SERS spectroscopy has been recognized as one of the most sensitive tools for studying the interfacial properties of carbonaceous materials embedded silver nanoparticles. Its spectra can provide information on the crystalline perfection of graphite-based materials. Single-crystal graphite and highly oriented pyrolytic graphite show a single band at 1582cm-1(G peak). Less ordered carbon materials show an additional strong band at about 1360cm-1(D peak). Figure 4 shows the SERS spectra of pure PVP nanofiber film and Ag/PVP nanofiber film. There is no signal in the spectrum of pure PVP nanofiber film. But it is amazing that D peak and G peak are observed in spectrum of Ag/PVP nanocomposite film at room temperature. This spectrum is identical to our previously reported SERS spectra of Ag/PAN nanocomposite in-situ prepared by electrospinning. The result indicates that the structure of PVP has been changed after PVP is doped Ag nanoparticles.we also present a convenient and effective way to dope Ag nanoparticles into PAN nanofiber film. UV spectrum and TEM studies have been done in order to reveal the structural properties of the Ag/PAN nanocomposite film. SERS spectrum is employed to investigate the interaction between silver nanoparticles and PAN.The absorption spectrum of Ag/PAN nanocomposite film was mesured. It revealed that an absorption band with a sharp maximum at 425nm arises. The result corresponds very well with the spectral behavior of silver nanoparticles. Small metallic particles exhibit a high optical absorbance due to the existence of discrete energy levels of electron and particularly of specific states. Silver particles with diameters below 5nm have a rather high absorbance band with a maximum at about 400nm. This band is broadened and shifted toward higher wave lengths with increasing particle diameter. Silver particles with diameters of ~10nm exhibit absorption bands at 410~450nm depending on their chemical environment. It is believed, based on the maximal absorbance at 425nm caused by Ag /PAN film, that small spherical particles of silver with a diameter of 10nm or less are formed in PAN film in the process of reduction of silver ions in N2H5OH aqueous solution.A typical TEM image of the PAN nanofibrous film containing Ag nanoparticles was mesured. It reveals that the nanocomposite film exhibits high porosity and that the average diameter of PAN nanofiber embedded Ag nanoparticles is about 100nm. Further magnified TEM image of the single nanofiber shown hat quasi-spherical-shaped silver particles are dispersed very homogenously on and in PAN nanofiber. Silver particles are a little larger on PAN nanofiber surface than those in PAN nanofiber. The average diameter of Ag nanoparticles observed is about 10nm, which is consistent with the result of UV absorption spectrum.It has generally been accepted that there is coordination between silver ions and cyano nitrogen of PAN .Cyano nitrogen can donate its lone-pair electron from occupied 2p orbitals to empty orbitals of silver ions to formδ-bond. The back-donation of electron density from occupied d orbitals of silver ions into the emptyπ*-2p antibonding orbitals of cyano nitrogen leads to the formation ofπ-bonds. The coordination between cyano nitrogen and silver ion makes PAN an ideal carrier of silver ion, which provides an excellent precursor to synthesize in-situ silver nanoparticles. Since silver ions are coordinated with cyano groups, silver nanoparticles can be prevented from aggregation in the process of reduction in N2H5OH aqueous solution and dispersed very homogenously in PAN nanofibrous film. Further, such a small size of silver particle prepared in PAN nanofiber may imply that the coordination of silver ion with cyano group plays an important role in particle size.SERS spectroscopy has been recognized as one of the most sensitive tools for studying the interfacial properties of carbonaceous materials embedded silver nanoparticles. Its spectra can provide information on the crystalline perfection of graphite-based materials. Single-crystal graphite and highly oriented pyrolytic graphite show a single band at 1582cm-1(G peak). Less ordered carbon materials show an additional strong band at about 1360cm-1(D peak). Figure 3 shows the SERS spectra of pure PAN nanofiber film and PAN nanofiber film embedded silver nanoparticles. There is no signal in pure PAN nanofiber film. But it is amazing that D peak and G peak are observed in spectrum of Ag/PAN nanocomposite film at room temperature. The result indicates that the structure of PAN has been changed after PAN is doped Ag nanoparticles. This spectrum is exactly identical to the reported Raman spectra of PAN-based carbon fiber.It is well known that PAN undergoes structural changes in the presence of oxygen at high temperature. Its full graphitization usually needs a temperature of 2000oC. An ex situ infrared reflection-absorption study of PAN film on a nickel substrate showed that cyclization had been completed after 24h at 200oC and dehydrogenation started well above 300oC. The temperature is much lower than that at which PAN undergoes structural changes without interaction with metals. In our study, graphite peak is observed even at room temperature in SERS spectrum of the PAN/Ag nanocomposite. It is probable that the graphitization of PAN at room temperature is caused by the function of Ag nanoparticle as a catalyst for dehydrogenation of hydrocarbon compound.