Characterization and application of electrospun alumina nanofibers
© Kim et al.; licensee Springer. 2014
Received: 27 August 2013
Accepted: 18 December 2013
Published: 27 January 2014
Alumina nanofibers were prepared by a technique that combined the sol–gel and electrospinning methods. The solution to be electrospun was prepared by mixing aluminum isopropoxide (AIP) in ethanol, which was then refluxed in the presence of an acid catalyst and polyvinylpyrolidone (PVP) in ethanol. The characterization results showed that alumina nanofibers with diameters in the range of 102 to 378 nm were successfully prepared. On the basis of the results of the XRD and FT-IR, the alumina nanofibers calcined at 1,100°C were identified as comprising the α-alumina phase, and a series of phase transitions such as boehmite → γ-alumina → α-alumina were observed from 500°C to 1,200°C. The pore size of the obtained γ-alumina nanofibers is approximately 8 nm, and it means that they are mesoporous materials. The kinetic study demonstrated that MO adsorption on alumina nanofibers can be seen that the pseudo-second-order kinetic model fits better than the pseudo-first-order kinetic model.
In recent years, ceramic with nanostructures has attracted a lot of attention and is being used in the fields of electronics, information technology, and communications . It has found wide application in other areas as well, including the mechanical and chemical sciences and electrical, optical, and electrochemical energy sectors as effective electrode materials [2, 3]. Among various chemical or physical synthetic methods, the electrospinning method is a popular one and involves the use of an electrically charged jet of polymer solution to form the nanofibers. The method can be described as follows. A high voltage is applied to the ceramic material solution with a polymer, and an electric field is generated between the tip of the syringe containing the solution and the collector. The solution is ejected in the form of a jet by electrical repulsion onto the collector, and fibers of nanoscaled diameters with inorganic precursor are formed . The precursor nanofibers at high temperature are calcined to remove the polymers, and ceramic phase is obtained. This technique has been applied for the preparation of various metal oxide and ceramic nanofibers as well [5, 6], which included TiO2, ZnO , SnO2, BaTiO3, and Al2O3[2–6, 11].
Alumina (Al2O3) is one of the most important types of ceramic and is applied to the areas of catalysis, reinforcing components, electronic device fabrication, microelectronics, optics, and fire protection . Most recently, alumina has been explored as effective electrode material for electrochemical energy storage device [13–15]. Al2O3 has specific physical, chemical, and mechanical properties, and during the process of forming the stable α-Al2O3, gibbsite is transformed to boehmite and then to a variety of metastable intermediate structures such as χ-, γ-, κ-, δ-, θ-alumina, depending on the temperature [16, 17].
The main objective of the study is to investigate the calcination conditions on morphological appearance and crystal structure of the resulting alumina and the adsorption property of alumina calcined at different temperatures. Therefore, we investigated the synthesis of alumina nanofibers using a technique that combined the sol–gel and electrospinning methods using aluminum isopropoxide (AIP), an organometallic compound, as the precursor and polyvinylpyrolidone (PVP) polymer solution. The formation, morphology, and crystallinity of the electrospun alumina nanofibers were determined through thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy, Gas Chromatograph (Shimadzu GC-2010 Plus AF) and the alumina nanofiber samples synthesized were evaluated by nitrogen adsorption/desorption analysis. In addition, different phase alumina nanofibers were applied for the adsorption of methyl orange dye (MO) solution.
The PVP (MW = 1,300,000; Kanto, Japan), AIP (C9H21O3Al) (>97.0%; Sigma-Aldrich Corporation, St. Louis, MO, USA), ethanol (94.0%; Daejung, Korea), and nitric acid (60%; Daejung, Korea) were obtained commercially and used as received without further purification. All the equipment used in the study was thoroughly cleaned prior to the experiments. A typical synthesis run was as follows: A certain amount of nitric acid and 10 mmol of the aluminum precursor AIP were added to 20 mL of ethanol, and the solution was stirred vigorously. The final composition of the mixed solution was such that the molar ratio of AIP/nitric acid/ethanol was 1:m:34, where m (=2.57) is the molar ratio of the acid (HNO3) to the alkoxide . The mixture was covered with polyethylene (PE) film and then stirred vigorously at room temperature for at least 5 h. The PVP solution (10 wt.%) was prepared by dissolving the PVP polymer powder in ethanol under constant and vigorous stirring. The weight ratio of the polymer to the aluminum precursor was maintained at 3:1. The AIP and PVP solutions were then mixed, and the resulting AIP/PVP solution was loaded into a 10-mL syringe (SGE LL type) that was fitted with a metallic needle. The positive terminal of a variable high-voltage power supply was connected to the metallic needle and the negative terminal to a rotating collector (speed = 200 rpm) that was covered with the aluminum foil and served as the counter electrode. During a typical procedure, the voltage and the feeding rate were kept at 18 kV and 1.5 mL/h, respectively. The distance between the needle tip and the collector was maintained at 18 cm.
After the electrospinning was complete, the as-electrospun nanofibers were dried at 80°C for 24 h. Some of the dried nanofibers were used for the characterization by TGA, SEM, energy-dispersive X-ray spectroscopy (EDX), FT-IR spectroscopy, XRD, gas chromatography (Shimadzu GC-2010 Plus AF, Nakagyo-ku, Kyoto, Japan), and Brunauer-Emmett-Teller (BET) analysis. The remaining as-spun AIP/PVP composite nanofibers were calcined at different temperature (500°C to 1,200°C) for 2 h each at a heating rate of 5°C/min in order to obtain alumina nanofibers. Also, calcined alumina nanofibers were used for the characterization analysis and adsorption properties. As mentioned previously, the morphology of the fibers was examined by SEM (S4800, Hitachi Ltd., Tokyo, Japan). The diameters of the nanofiber were calculated from the SEM images using the Image J (National Institutes of Health, USA) software. The X-ray diffraction data was obtained with an X'Pert PRO MPD (PANalytical, B.V., Almelo, The Netherlands) diffractometer using Cu Kα radiation. FT-IR spectroscopy was performed on the samples using a NICOLET6700 (Thermo Scientific, Waltham, MA, USA) spectrometer that had a KBr beam splitter (operational wavelength range = 7,800 to 350 cm−1). TGA (STARSW, Mettler) was conducted up to 1,000°C with a heating rate of 5°C/min under a nitrogen and air atmosphere to evaluate the thermal behavior of the component nanofibers. The specific surface area and pore volume of the prepared alumina nanofibers were measured using the BET equation and the Horvath-Kawazoe (HK) method (ASAP2020, Micromeritics) after preheating the samples to 150°C for 2 h to eliminate adsorbed water. The pore size distributions were obtained by applying the HK method (micro-pore) to the nitrogen adsorption isotherms at 77 K using the software ASAP 2020.
Results and discussion
Kinetic parameters for the adsorption of MO on alumina nanofibers
Calcination temperature (°C)
Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
k2(g mol−1 min−1)
Alumina nanofibers were prepared by combining the sol–gel and electrospinning methods using AIP as an alumina precursor. The thus-produced alumina nanofibers were characterized by TGA, SEM, XRD, FT-IR spectroscopy, and nitrogen adsorption/desorption analysis. It was found from the SEM images of the various samples that the fiber-like shape and continuous morphology of the as-electrospun samples were preserved in the calcined samples. The diameters of the fabricated alumina nanofibers in this study were small and in the range of 102 to 378 nm with thinner and narrower diameter distributions. On the basis of the results of the XRD and FT-IR analysis, the alumina nanofibers calcined at 1,100°C were identified as comprising the α-alumina phase. In addition, a series of phase transitions such as boehmite → γ-alumina → α-alumina were observed from 500°C to 1,200°C. Adsorption kinetic data were analyzed by the first- and second-order kinetic equations. The adsorption property of MO of the α- and γ-alumina nanofibers was confirmed on the basis of the pseudo-second-order rate mechanism. Mesoporous alumina nanofibers prepared by electrospinning method can be successfully applied for the removal of dye pollutant from aqueous solutions.
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