Facile Preparation of Porous Inorganic SiO 2 Nanofibrous Membrane by Electrospinning Method

We presented a straightforward method to fabricate porous inorganic SiO2 nanofibrous membrane by one-step calcination of electrospun nanofibers, which encapsulated with carbon nanospheres as template for nanopore generation. The structure, morphology, and composition of the as-spun fibers (PVA/SiO2/C) and porous SiO2 were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).Thedetective results indicated that the carbonnanosphereswere uniformly encapsulated inside the PVA/SiO2/C nanofibers. After calcination, PVA polymer was removed and high flexible inorganic nanofibrous membrane composed of amorphous SiO2 was obtained. Simultaneously, carbon nanosphere template was decomposed and uniform nanopores were generated inside the SiO2 nanofibers. This new method is simple and of low cost and hence is suitable to prepare other porous inorganic nanofibers with high surface area for practical application.


Introduction
Electrospinning is a simple and versatile technique capable of generating ultrafine continuous fibers from a variety of materials [1][2][3][4][5].Electrospun fibrous membranes possess several fascinating features that make them very attractive for separation and adsorption of contaminants, such as a large surface-to-volume ratio, high porosity, interconnected open pore structure, and high permeability [6][7][8][9].In particular, porous electrospun nanofibers have been proposed to enhance the performance with higher absorbance capacity and separation efficiency [10].To this purpose, several methods based on phase separation and template sacrifice have been developed for the preparation of porous nanofibers [11][12][13][14].For example, Wu et al. reported superhydrophobic and superoleophilic polystyrene fibrous sorbent films by controlling the vapor-induced phase separation [15]; McCann and coworkers fabricated highly porous titania nanofibers by calcining fibers from coaxial electrospinning technology [16].Moreover, Lee and colleagues produced macroporous carbon fibers with macroscopic surface openings by carbonization of polymethyl methacrylate (PMMA) colloid incorporated polyacrylonitrile (PAN) fibers [17].Recently, we fabricated nanoporous poly(vinyl alcohol) (PVA) nanofibers by removal of Fe 3 O 4 particle template and cross-link of PVA fibers simultaneously [18].However, the above porous fibers are still limited for wide application because of the following several shortages.First, the obtained organic fibers suffer from the chemical and thermal stability.Second, routine inorganic materials are usually fragile to break into segments, which makes it difficult to achieve separation and recycle conveniently.Therefore, development of flexible porous inorganic membranes is of great interest for practical applications.
Inorganic SiO 2 nanofibrous membrane has both flexible and high-heat-resistant properties, which can be fabricated by a combination of electrospinning with sol-gel process, followed by calcination [19,20].Previously, hollow SiO 2 nanofibers have been prepared by controlling the ratios of H 2 O/C 2 H 5 OH or TEOS/PVP in the precursor solution [21,22].The above methods show feasibility to produce hollow fibers based on phase separation process; however, it is still a challenge to produce porous silica nanofibers.To the best of our knowledge, there are few reports on porous SiO 2 nanofibers.
In this work, we present a novel method to fabricate porous inorganic SiO 2 nanofibrous membranes by electrospinning using carbon nanosphere as templates.The carbon nanospheres were first synthesized by hydrothermal method and then mixed with silica sol-gel as precursor solution.The porous SiO 2 nanofibrous membrane was directly fabricated by electrospinning and postcalcinations.After calcinations, spherical nanopores were left at the position where carbon nanosphere templates were located.Importantly, polycondensation of SiO 2 and decomposition of carbon nanosphere and PVA can be accomplished in one step during the calcination, simplifying the operation and saving time to form porous SiO 2 nanofibers.

Synthesis of Carbon
Nanospheres.Carbon nanospheres were synthesized by using hydrothermal method as described elsewhere [23].Briefly, 8 g of glucose was dissolved in 40 mL of deionized (DI) water to form a transparent solution, which was then transferred into a Teflon-lined autoclave (50 mL) and sealed.The autoclave was maintained at 180 ∘ C for 5 h in an electrical oven.After naturally cooling down to room temperature, the carbon nanospheres were isolated and rinsed by centrifugation at 12000 rpm/min for 15 min for several cycles with DI water and then dried at 60 ∘ C.

Fabrication of Porous SiO 2 Nanofibers.
Porous SiO 2 nanofibers were fabricated by electrospinning and postcalcinations.TEOS was used as an alkoxide precursor of SiO 2 while PVA served as both a carrying polymer for TEOS and a precursor of PVA nanofiber.A PVA solution (11 wt%) was prepared by dissolving PVA powders in DI water at 80 ∘ C with vigorous stirring.A silica gel was prepared by hydrolysis and polycondensation of TEOS assisted by the dropwise addition of H 3 PO 4 with stirring at room temperature overnight, in which the molar ratio of TEOS : H 2 O : H 3 PO 4 was 1 : 11 : 0.02.Next, certain amounts of carbon nanospheres were added to 1 g of silica gel, and the mixture was sonicated for 30 min to obtain a homogeneous solution.Finally, this silica gel containing carbon nanospheres was mixed with 1 g of PVA solution and stirred for 6 h to obtain the precursor solution for electrospinning.For electrospinning process, the precursor solution was loaded into a plastic syringe and extruded through a needle equipped on the syringe at a feeding speed of 10 L/min.Electrospinning was performed at an applied potential of 12 kV and a distance of 15 cm between the needle and the negative foil collector.The collected fibrous membrane (PVA/SiO 2 /C) was dried at 80 ∘ C overnight and then calcined at 800 ∘ C for 2 h with heating rate of 4 ∘ C/min in air.The porous fibrous membranes obtained with carbon sphere concentrations of 20, 50, and 75 mg/mL were denoted as SiO 2 NF-20, SiO 2 NF-50, and SiO 2 NF-75, respectively.

Characterization. Morphology of fibrous membranes
was observed by scanning electron microscopy (SEM, Hitachi 4800) and transmission electron microscopy (TEM, Tecnai-G2-F30).X-ray diffraction (XRD) measurements were performed on a Shimadzu D6000.Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) spectra were recorded using Bruker tensor II spectrometer within the wavenumber range of 4000-400 cm −1 .X-ray photoelectron spectroscopy (XPS) was performed with a PHI-5702 Xray photoelectron spectrometer.Thermogravimetric analysis (TGA) was obtained at the heating rate of 10 ∘ C/min in air by the 1090B TGA instrument (DuPont Inc.).

Results and Discussion
The procedure for fabricating porous SiO 2 nanofibrous membrane is illustrated in Figure 1.Electrospinning was used to prepare carbon nanosphere incorporated electrospun fibers (PVA/SiO 2 /C), which were then calcined at high temperature for the generation of porous SiO 2 nanofibers.Herein, carbon nanospheres were chosen to serve as templates to generate nanopores inside the SiO 2 nanofibers for two reasons.First, carbon nanospheres are readily decomposed together with PVA polymers in one step, as well as the SiO 2 condensation during the high temperature treatment.Besides, the surface of the carbon nanospheres synthesized by hydrothermal method has numerous polar terminal moieties, such as hydroxyl (-OH) and carboxylic acid (-COOH), which facilitate the dispersion of carbon nanospheres in the hydrophilic electrospinning solution [23]. Figure 2 presents the representative SEM images of carbon spheres, PVA/SiO 2 /C fibers formed with various carbon nanosphere concentrations of 0, 50, and 75 mg/mL, respectively.The carbon nanospheres used for the electrospinning were synthesized by hydrothermal method and had an average diameter in the range of 90-130 nm (Figure 2(a)).As seen in Figure 2(b), the random nanofiber surface without carbon nanospheres is smooth and uniform with an average diameter of 530 nm.When 50 mg/mL of carbon nanospheres is introduced into the precursor solution, certain protuberances are observed on the fiber surface and the fiber diameter is in the wide range of 0.8∼1.1 m (Figure 2(c)).Compared with fibers in Figure 2(b), the increase of the diameter may be attributed to the higher viscosity of the electrospinning solution, which is caused by the interaction between carbon nanospheres and the PVA and hydrolyzed silica gel in the electrospinning solution.At a carbon nanosphere concentration of 75 mg/mL, the diameter of nanofibers increases to above 1.1 m, and the protuberances on the fibers are more obvious perhaps due to the aggregation of nanospheres at a high concentration (Figure 2(d)).Inset of Figure 2(d) is a magnified SEM micrograph of the fiber cross section in Figure 2(d).It can be clearly seen that the carbon nanospheres are wrapped in the polymer shell other than on the fiber's exterior surface, which is attributed to the effect of solution surface tension [24].The results in Figure 2 suggest that the carbon nanospheres can be easily encapsulated in the silica and PVA shell to form uniform core-shell structure nanofibers.
The disappearance of the organic PVA and occurrence of inorganic SiO 2 nanofibers after the calcination were evidenced by ATR-FTIR.Figure 3  are found at 1700 cm −1 , which is attributed to the stretching vibration of C=O [23].Besides, absorption peak at 956 cm −1 is assigned to the Si-OH bond.The peaks at 1078 cm −1 and 790 cm −1 are observed in the case of the Si-O-Si bond, which indicates that TEOS has been partially hydrolyzed to silicic acid and SiO 2 during the preparation of electrospun fibers.FTIR spectrum of silica fibrous membrane after calcination at 800 ∘ C for 2 h is exhibited in Figure 3(b).Compared with the plot of Figure 3(a), the peaks at 3350 cm −1 , 2938 cm −1 , and 1700 cm −1 disappeared, indicating that the PVA and carbon nanospheres have been completely decomposed.Meanwhile, the disappearance of Si-OH band at 956 cm −1 reveals the existence of silica by further polycondensation after high temperature calcination.In addition, the peaks at 1078 cm −1 and 790 cm −1 are significantly enhanced, which is attributed to the formation of inorganic silica fibers mainly containing Si-O-Si.As a result, the calcination process has removed organic polymer and carbon particles completely and left the inorganic SiO 2 fibers only.
To clarify the influence of the calcination on the crystalline phase, XRD patterns from 10 ∘ to 50 ∘ of PVA/SiO 2 /C and calcined SiO 2 fibrous membranes are displayed in Figure 4.The broad diffraction peak around 21.5 ∘ suggests the nanofibers are in amorphous phase of silica, which can be indexed to the (100) lattice plane of the tridymite (JCPDS number 75-0638) (Figure 4(a)) [26].No other obvious diffraction peak illustrates that the incorporated carbon spheres synthesized by the hydrothermal method are in amorphous phase.The diffraction peak is not significantly enhanced after calcination because of an amorphous phase of the silica and the porous structure of the fiber membrane resulting in a weak signal (Figure 4(b)).
Thermogravimetric analysis (TGA) was performed to understand the thermal behavior of as-spun fibers.Figure 5 shows the TGA spectra from 100 to 800 ∘ C of PVA, carbon spheres, and electrospun fibers without carbon nanosphere addition (PVA/SiO 2 ) and PVA/SiO 2 /C samples.It is noted that the weight losses are near 100% for the PVA and carbon spheres before 500 ∘ C and 550 ∘ C, respectively, indicating that they can be decomposed completely by the calcination temperature of 800 ∘ C used in this work.Weight loss around 100∼250 ∘ C resulted from the moisture desorption [26].The PVA is decomposed in a two-step process: dehydration of the polymer side chains at 250∼350 ∘ C and C-C bond cleavage of the polymer chain from 400 to 500 ∘ C. The decomposition of carbon nanospheres was accomplished at 350∼550 ∘ C [26].Both as-spun PVA/SiO 2 and PVA/SiO 2 /C fibers have similar weight loss tendencies that mainly occurred in the low temperature region.Incorporation of carbon nanospheres has no obvious influence on the total weight loss (44%) of the fiber membranes, which is due to the small weight ratio of the carbon spheres in the electrospinning solution.Furthermore, a slightly slower weight loss of the PVA/SiO 2 /C fibers compared to PVA/SiO 2 fibers might be attributed to the interaction between carbon nanospheres and polymer chain.No obvious weight loss of the porous SiO 2 fibers at high temperature shows their thermal stability, which means that the porous SiO 2 fibers can be easily recycled by calcination avoiding secondary pollution.
To investigate the morphology and size of nanofibers after calcination, the porous SiO 2 nanofibers by calcination of PVA/SiO 2 /C fibers electrospun from a precursor solution with carbon sphere concentration of 50 mg/mL were examined by SEM and TEM measurements.From the SEM photographs and diameter distribution in Figure 6(a), it can be noted that the fiber diameter of the randomly oriented fibers is in the range of 0.7-0.9m, which is slightly smaller fibers by TEM characterization.Due to the density difference, the nanopore structures can be observed inside the fiber with lighter color compared with SiO 2 fiber component with deeper color.Besides, the round shape nanopores uniformly and independently are distributed throughout the fiber, suggesting that the carbon nanoparticles were uniformly distributed inside the SiO 2 nanofibers during the electrospinning process.Moreover, all the nanopores are included inside the nanofibers other than opening pores, which is accordant with SEM results in Figure 2(d).
The electrospinning and subsequent calcination process were also accompanied with a color change of the fibrous membrane from brown (Figure 6(e)) to white (Figure 6(f)), due to the incorporation of carbon nanoparticles in the PVA/SiO 2 /C fibers and removal of the carbon nanospheres of the porous SiO 2 fiber.More importantly, no obvious break was observed when the porous SiO 2 fiber membrane was folded and curled, as shown by the inset optical graphs in Figure 6(f), which suggests that the porous SiO 2 fiber membrane still maintains its flexibility after calcination like the pure SiO 2 fiber mats.These results in Figure 6 demonstrate that porous SiO 2 nanofibers with uniform internal nanoporous can be fabricated after one-step calcination of the electrospun nanofibers encapsulated with carbon nanosphere template.
To further investigate the influence of the added carbon spheres on the porous nanostructures, N 2 adsorption-desorption isotherms at 77 K SiO 2 NF-20, SiO 2 NF-20, and SiO 2 NF-75 fiber membranes were carried out.The curves in Figure 7 illustrate type IV with a hysteresis loop, revealing characteristics of mesopores within the porous SiO 2 fiber mats [26].The inset of Figure 7 displays the BET surface areas and pore volumes of the different porous fibrous membranes, which are ranked in the order of SiO 2 NF-20 < SiO 2 NF-20 < SiO 2 NF-75.Notably, the more incorporated the carbon spheres are, the larger the BET surface areas and pore volumes are.These results suggested that the BET surface area and the pore volume of the porous membrance could be adjusted by turning the quantities of the carbon spheres.
The typical high resolution XPS spectra were acquired to further investigate the surface composition and chemical state of porous SiO 2 nanofibers.The XPS spectrum of Si2p region is displayed in Figure 8(a) with the characteristic peak at the binding energy of 103.7 eV corresponding to Si2p , indicating the state of Si 4+ in porous SiO 2 nanofibers [27].The peak located at binding energy of 532.8 eV can be assigned to the O 2− in the porous SiO 2 nanofibers (Figure 8(b)).The XPS results demonstrate the formation of SiO 2 after calcination of PVA/SiO 2 /C, which is in good accordance with the results of XRD analysis.

Conclusions
We proposed a simple method for the rapid preparation of porous inorganic SiO 2 nanofibrous membrane with carbon nanospheres as templates.By treating PVA/SiO 2 /C electrospun nanofibers at high temperature, carbon nanosphere template was removed and porous SiO 2 nanofibers were produced in one step.The obtained porous SiO 2 nanofibrous membrane shows high thermal stability and flexibility with uniform nanopores inside the fibers.BET surface area and the pore volume of the porous membrance could be controlled by turning the quantities of the carbon spheres.As calcination is a basic process in the preparation of inorganic nanofibers by electrospinning and the carbon nanospheres have high chemical stability and surface compatibility, therefore the method can also be applied to the preparation of other porous inorganic nanofibers for practical application.

Figure 2 :
Figure 2: SEM images of the carbon spheres (a), PVA/SiO 2 /C nanofibers with different concentrations of carbon nanospheres: 0 (b), 50 (c), and 75 mg/mL (d); inset of (d) is the cross section SEM image of fibers in (d); the inset graphs are the diameter distribution.
(a)  shows the FTIR spectrum of the as-spun PVA/SiO 2 /C nanofibers.It can be seen that there is a wide absorption band at 3350 cm −1 , which is assigned to the hydrogen bonded or H-O stretching vibration in water.The broad peak at 2938 cm −1 corresponds to the C-H stretching vibration and the in-plane bending vibration in -CH 2 -[25].The absorption features of carbon nanospheres
Figure 7: Nitrogen physisorption isotherms of SiO 2 NF-20, SiO 2 NF-50, and SiO 2 NF-75 fibrous membranes.The "solid circles" and "empty circles" represent the adsorption and desorption curve, respectively.thanthediameter of the PVA/SiO 2 /C fibers due to the removal of polymer and further polycondensation of the silica by calcination.Apparent nanopores can be observed in the cross section image of the fibers (Figure6(b)), indicating that carbon nanospheres are removed and nanoporous SiO 2 fibers are obtained after calcinations.The average size of the nanopores is 70 nm smaller than the diameter of the carbon nanosphere templates, revealing that the oxidation of carbon and shrinkage of nanofibers were carried out simultaneously.