Novel hollow α-Fe2O3 nanofibers via electrospinning for dye adsorption
© Gao et al.; licensee Springer. 2015
Received: 6 November 2014
Accepted: 21 March 2015
Published: 14 April 2015
Nanomaterials such as iron oxides and ferrites have been intensively investigated for water treatment and environmental remediation applications. In this work, hollow α-Fe2O3 nanofibers made of rice-like nanorods were successfully synthesized via a simple hydrothermal reaction on polyvinyl alcohol (PVA) nanofiber template followed by calcination. The crystallographic structure and the morphology of the as-prepared α-Fe2O3 nanofibers were characterized by X-ray diffraction, energy dispersive X-ray spectrometer, and scanning electron microscope. Batch adsorption experiments were conducted, and ultraviolet-visible spectra were recorded before and after the adsorption to investigate the dye adsorption performance. The results showed that hollow α-Fe2O3 fiber assembles exhibited good magnetic responsive performance, as well as efficient adsorption for methyl orange in water. This work provided a versatile strategy for further design and development of functional nanofiber-nanoparticle composites towards various applications.
In recent decades, magnetic iron oxide has developed into a kind of nanomaterial with the property of magnetic targeting [1,2]. α-Fe2O3 has attracted considerable attention due to its widely applications, such as catalysis [3-5], batteries [6,7], and gas sensors [8,9]. Recently, much effort has been devoted to the design and controllable synthesis of one-dimensional (1D) nanostructure α-Fe2O3 due to the novel properties of nanoscale materials. Liu et al. synthesized α-Fe2O3 nanotubes by a templating method . Jiang’s group and Gou’s research group have fabricated α-Fe2O3 nanofibers via electrospinning, respectively [11,12].
Electrospinning is a simple method for producing nanofibers and nonwovens for various applications [13-16]. Electrospinning is advantageous to fabricate not only solid nanofibers but also hollow nanofibers. The main strategies adopted for hollow nanofiber synthesis via electrospinning are as follows: (1) Coaxial electrospinning involves the use of two coaxial capillaries in a spinneret containing different solutions to generate core-shell composite fibers that results in hollow fibers via removal of core fibers by extraction or calcination at high temperature [17,18]. (2) Single nozzle co-electrospinning: this process involves two immiscible polymers dissolved in solvent that results in phase separation during electrospinning owing to the intrinsic polymer properties, yielding core-shell composite fibers or hollow fibers after suitable core removal [19,20]. However, inorganic hollow fiber with hierarchical structure is hard to be achieved via the two mentioned methods.
Nanoscale α-Fe2O3 has been intensively investigated for water treatment and environmental remediation applications [21-23]. Song et al. reported that flowerlike α-Fe2O3 nanoparticles can remove the heavy metal ions from the waste water . Grätzel et al. employed nanostructured α-Fe2O3 films for azo-dye adsorption . Yu et al. investigated the methyl orange degradation performance by using α-Fe2O3 nanocrystals . To the best of our knowledge, there are no reports in the literature dealing with interconnected α-Fe2O3 hollow fibers for dye adsorption in the waste water. In this work, we have synthesized a interconnected 1D hollow structure of α-Fe2O3 nanofibers made of rice-like nanorods by annealing electrospun polyvinyl alcohol (PVA)-Fe3O4 composite fibers and investigated its potential applications in removal of noxious dye from wastewater.
99.9% hydrolyzed PVA samples (DP = 3,200) were provided by Kuraray Co. Ltd., Tokyo, Japan. Acetic acid, FeCl3·6H2O, FeCl2·4H2O, and sodium hydroxide were purchased from Wako, Osaka, Japan. All the reagents were used as received without further purification.
Preparation of PVA nanofibers
PVA solutions were prepared by dissolving PVA in acetic acid aqueous solution at 90°C with constant stirring for at least 4 h. The electrospinning setup (Kato Tech, Kyoto, Japan) used in this study consists of a syringe with a flat-end metal needle (1.20-mm inner diameter, NN-1838 N, Terumo, Tokyo, Japan), a syringe pump for controlled the feeding rate, a grounded cylindrical stainless steel mandrel, and a high-voltage DC power supply. In a typical electrospinning process, PVA solution was transferred into a syringe and delivered to the tip of the syringe needle by the syringe pump at a constant feed rate (1.0 ml/h). A 12-kV positive voltage was applied to the PVA solution via the stainless steel syringe needle. The subsequently ejected polymer fiber was collected on the rotating cylindrical stainless steel mandrel, which was rotated during the electrospinning process (150 rpm). The distance between the tip of the needle and the surface of the mandrel was about 14 cm. The PVA nanofibers were vacuumed at room temperature for 24 h and thermal-treated at 180°C for 5 min.
Preparation of hollow α-Fe2O3 nanofibers
FeCl3·6H2O and FeCl2·4H2O were dissolved in 8 ml of distilled water, and the aqueous solution was degassed by N2. The PVA nanofiber mat (0.5 mg) was immersed within the degassed aqueous solution of ferrous and ferric ions and stood for 1 h. After 1 h, NaOH aqueous solution (0.5 ml) was added slowly and the mixture was heated to 70°C for 60 min. After cooling to room temperature, the PVA-Fe3O4 composite mat was washed with water and dried in vacuo for 12 h.
Fiber morphology of the electrospun fibers was characterized using scanning electron microscopy (SEM) (SU1510, Hitachi Co. Ltd., Tokyo, Japan) and field-emission scanning electron microscopy (FE-SEM) (JSM-6700 F, JEOL, Akishima-shi, Japan) with an energy-dispersive X-ray spectrometer (EDS). Differential scanning calorimeter (DSC) (Q200, TA Instruments Inc., New Castle, USA) was used to characterize the thermal properties of the electrospun PVA mats. A piece of PVA mat (2 to 5 mg) was placed in an aluminum sample pan and heated from 30°C to 350°C at 10°C/min under N2. Weight loss behavior was tested by thermogravimetric (TG) analysis (SDT Q600, TA Instruments Inc., New Castle, USA) (air, 10°C/min). The chemical structure of nanofibers was conducted with a Fourier transform infrared (FT-IR) reflection spectroscopy (NICOLET, Thermo Fisher Scientific, Waltham, USA), and a X-ray powder diffractometer (XRD) (D8 Advance, Bruker, Karlsruhe, Germany) operated in the reflection mode with Cu-Kα radiation in the 2θ range of 10° to 80° with a rate of 4°/min. Batch adsorption experiments were conducted and recorded by ultraviolet-visible spectra (U-3010, Hitachi Co. Ltd., Tokyo, Japan).
Results and discussions
Deposition of Fe3O4 nanoparticles on PVA nanofibers
The melting point and enthalpy of PVA nanofibers
Δ H (J/g)
After thermal treatment
PVA-Fe3O4 composite fibers
Hollow α-Fe2O3 nanofibers
The geometrical parameters of PVA nanofibers, such as the average diameter of nanofibers, the porosity, and the thickness of fiber assembles, would significantly influence the resultant morphology of α-Fe2O3 nanofibers. Furthermore, the hydrothermal reaction conditions, such as temperature and ion concentration, should have similar effects. Here, the effect of ion concentration on the resultant morphology of α-Fe2O3 was investigated. Other parameters will be reported in the following full paper.
Ion concentration in the hydrothermal synthesis
α-Fe 2 O 3
FeCl 3 ·6H 2 O (μmol)
FeCl 2 ·4H 2 O (μmol)
Magnetic response and absorption for dyes
As expected, hollow α-Fe2O3 nanofibers made of rice-like nanorods which combined the porous structure and the magnetic performance demonstrated efficient adsorption for organics and fast magnetic separation property. Methyl orange (MO; 2 × 10−5 M) was employed for typical organic pollutants in adsorption test. Figure 8 indicates that the adsorption capacity of MO of hollow α-Fe2O3 nanofibers was 93% for 10 min and could achieve almost complete adsorption of MO in 15 min (insert image of Figure 8) while solid α-Fe2O3 fibers revealed much slower adsorption rate. Moreover, hollow α-Fe2O3 nanofibers after adsorption could be separated facilely using an external magnet without any tedious separation process, which is of great importance for real applications.
Adsorption rate constant k 2 at different initial MO concentration and temperature
Initial MO concentration/ppm (at 20°C)
Temperature/°C (at 100 ppm)
Novel hollow α-Fe2O3 nanofibers made of rice-like nanorods were successfully synthesized via a simple hydrothermal reaction on PVA nanofiber template followed by calcination. The crystallographic structure and the morphology of the as-prepared α-Fe2O3 nanofibers were conformed by XRD, EDS, and FE-SEM. Moreover, hollow α-Fe2O3 fiber assembles exhibited magnetic responsive performance, as well as efficient adsorption for methyl orange in water which follows Lagergren pseudo-second-order kinetics. This work provided a versatile strategy for further design and development of functional nanofiber-nanoparticle composites towards various applications.
We acknowledge financial support from the National High-tech R&D Program of China (863 Program, Grant Number: 2012AA030313), State Key Laboratory of Molecular Engineering of Polymers(Fudan University) (K2015-23), and Fundamental Research Funds for the Central Universities of China (No. JUSRP11444).
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