Nanofibrous Membrane with Superb Flexibil - ACS Publications

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Hierarchical porous structured SiO2/SnO2 nanofibrous membrane with superb flexibility for molecular filtration Haoru Shan, Xueqin Wang, Feihao Shi, Jianhua Yan, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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ACS Applied Materials & Interfaces

Hierarchical porous structured SiO2/SnO2 nanofibrous membrane with superb flexibility for molecular filtration Haoru Shan†, Xueqin Wang‡, Feihao Shi§, Jianhua Yan†ǁ, Jianyong Yu*,ǁ, and Bin Ding*,†,‡,ǁ

†

Key Laboratory of Textile Science & Technology, Ministry of Education, College of

Textiles, Donghua University, Shanghai 201620, China ‡

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,

College of Materials Science and Engineering, Donghua University, Shanghai 201620, China §

College of Chemistry, Chemical Engineering and Biotechnology, Donghua

University, Shanghai 201620, China ǁ

Nanofibers Research Center, Modern Textile Institute, Donghua University, Shanghai

201620, China * Corresponding author: Prof. Jianyong Yu (E-mail: [email protected]); Prof. Bin Ding (E-mail: [email protected])

1

ACS Paragon Plus Environment


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ABSTRACT The separation and purification of chemical molecules from organic media under harsh chemical environments is of vital importance in the fields such as water treatment, biomedical engineering, and organic recycling. Herein, we report the preparation of a flexible SiO2/SnO2 nanofibrous membrane (SiO2/SnO2 NFM) with high surface area and hierarchical porous structure by selecting poly (vinyl butyral) as pore-forming agent and embedding crystalline phase into amorphous matrix without using surfactant as sacrificial template. Benefiting from the uniform micropore size on the fibers and negatively charged properties, the membranes exhibit a precise selectivity toward molecules based on electrostatic interaction and size exclusion, which could separate organic molecule mixtures with same electrostatic charges and different molecular sizes with a high efficiency of more than 97%. Furthermore, the highly tortuous open-porous structures and high porosity give rise to a high permeate flux of 288,000 L m-2 h-1. In addition, the membrane also displays excellent stability and can be reused for ten consecutive filtration-regeneration cycles. The integration of high filtration efficiency, large permeate flux, good reutilization, and easy to industrialization provides the SiO2/SnO2 NFM for potential applications in practical molecular purification and separation science.

Keywords: electrospinning, flexible, porous SiO2/SnO2 nanofibrous membrane, molecular filtration, high permeate flux

2


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1. INTRODUCTION The filtration and separation of organic molecules from complex solutions under harsh chemical environments, including biological and pharmaceutical molecules purification, play critical important roles in the fields such as water treatment, food processing, biomedical engineering, and organic recycling.1-3 The traditional separation and purification techniques including evaporation and distillation are considered energy-intensive and require a huge amount of spaces, making the separation process account for overwhelming majority of both capital and operating costs.1 In contrast, molecular filtration technology, based on the membrane’s high selectivity, could be applied to separate mixtures with similar chemical and physical properties at a molecular level, which is considered to be cheap, environmentally friendly, low energy consumption, and space saving.4 Currently, the majority of separation membranes are on the basis of polymeric materials, such as cellulose, cyclodextrins, polyamide, and polysulfone, which have advantages of flexibility, simple preparation process, and relatively low cost.1,

5-8

However, polymeric

membranes are only limited to the mild operating conditions ascribing to their poor tolerance to high temperatures, oxidants, strong acidic/alkaline regents, or organic solvents.9

Therefore,

the

molecular

filtration

membranes

with

adequate

chemical/thermal stability for operation under different conditions are highly desired.9-10 Inorganic porous materials, possessing large surface area, high chemical and thermal stability, have proven to be good candidates to be used in molecular filtration under severe operating conditions.3 However, routine inorganic porous materials in a powder form can hardly filtrate the molecules, and are difficult to recycle due to the suspended dispersive properties in water.11 Thus membrane-based inorganic porous materials are highly desired to fulfil the need for molecular 3



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filtration.3, 12 Electrospinning technology provides a straightforward and versatile method to produce continuous non-woven membranes composed by continuous organic, inorganic, or organic-inorganic fibers with dimensions in the range of microscale to nanoscale.13-15 Recently, electrospun inorganic porous nanofibrous membranes with high specific surface area, various compositions and porous structures have been widely reported, including tunable interconnected porous silica-titanium dioxide,16 porous tin oxide,17 mesoporous silica,18-19 multiscale porous titanium dioxide,20 hollow zinc oxide,21 and aluminium oxide microtubes,22 which show potential applications in molecular filtration, photocatalysis, gas detection, etc. However, despite these advances, most of the existing inorganic porous fibrous membranes are generally fragile and easy to fracture under bending deformation, which is the key problem need to be solved urgently to meet the demands of practical applications.9, 11, 19

Herein, we report a novel flexible porous SiO2/SnO2 nanofibrous membrane (SiO2/SnO2 NFM) fabricated via electrospinning technology by selecting poly (vinyl butyral) as the pore-forming template and embedding the crystalline phase SnO2 into the amorphous SiO2 matrix to regulate the pore size distribution without using surfactant as sacrificial template, which to the best of our knowledge have not been reported. The as-prepared SiO2/SnO2 NFM with hierarchical porous structures and enhanced mechanical strength presents high efficiency for removing organic molecules from the solutions. Furthermore, benefiting from the uniform slit micropore size on the fibers, the porous nanofibrous membranes exhibit a precise selectivity toward chemical molecules based on electrostatic interactions and molecular size exclusion. More importantly, the SiO2/SnO2 NFM with highly tortuous open-porous 4


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structures between nanofibers and high porosity give rise to significantly high permeate molecular filtration flux of 288,000 L m-2 h-1 and maintain high filtration efficiency even after 10 filtration-regeneration cycles. The successful fabrication of such material may provide a new approach to manufacture molecular filters for high selectivity, high filtration efficiency and high solvent permeate flux towards organic molecules in the fields of practical molecular purification and separation science.

2. MATERIALS AND METHODS 2.1. Materials. Poly (vinyl butyral) (PVB, Mw = 170000~250000), stannic chloride hydrated (SnCl4·5H2O, 99%), absolute ethanol (EtOH, 99%), acetone (99.5%), oxalic acid (C2H2O4, 98%), methylene blue (MB), methyl orange (MO, 96%), basic fuchsin (BF), rhodamine B (RhB), and solvent yellow 2 (SY2) were purchased from Shanghai Aladdin Reagent Co., Ltd., China. Basic red 2 (BR2, 85%) and neutral red (NR) were bought from Shanghai Fortuneibo-tech Co., Ltd., China. Tetraethyl otrhosilicate (TEOS) was supplied by Shandong Wan Cheng Chemical Products Co., Ltd., China. Distilled water was produced by a Heal-Force system. All chemicals were used as obtained without further refinement. 2.2. Preparation of SiO2/SnO2 NFM. The SiO2/SnO2 NFM were fabricated by combining the electrospinning technique with the subsequent high temperature calcination, which were described briefly as below. Tin precursor solution was obtained by adding SnCl4·5H2O into EtOH with a molar ratio of SnCl4·5H2O: EtOH = 1: 19 at 70 oC and stirred for 30 min. The silica sol was generated via hydrolysis and polycondensation by adding C2H2O4, EtOH, and H2O into TEOS with a molar ratio of TEOS: H2O: EtOH: C2H2O4 = 1: 3.57: 0.71: 0.016 with continued stirring for 8 h. A 10 wt% PVB/EtOH solution was obtained by adding PVB powder to EtOH and stirring for 5 h. The resultant tin and silica precursor solution were separately mixed 5



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with various molar ratios (SiO2:SnO2) of 10:0, 9:1, 8:2, and 6:4 with stirring at room temperature for another 2 h. Subsequently, an equivalent weight of 10 wt% PVB/EtOH was added into the as-prepared tin and silica precursor solution to obtain a homogeneous solution and then delivered into a syringe (10 mL) with a syringe needle. The solutions were fed at a controllable propulsion velocity of 1 mL h-1. Following, electrospinning was implemented by utilizing a DXES-1 spinning equipment supplied with the voltage of 15 kV, and the receiving distance of 15 cm. When the voltage is applied, the electrical charges would be induced on the surface of the suspending drop by the electric field. Once the electrostatic forces overcome the surface tension of precursor solution, the continuous liquid jet from the syringe nozzle would be ejected, accompanied by continuous stretching and elongating through electrostatic repulsive forces, and thus forming a long and thin thread. During the spinning process, the solvent rapidly evaporated and the diameter of the thread continually decreased, leading to the solid organic-inorganic precursor fibers collected on nonwoven substrate.23-25 The entire spinning process was performed under the ambient operating temperature and humidity of 25 oC and 40%, respectively. The pure SiO2/SnO2 nanofibers were obtained by thermal decomposition of PVB under air atmosphere with a calcination temperature of 800 oC and heating rate of 5 oC min-1. The resultant membranes with various SiO2:SnO2 molar ratios of 10/0, 9/1, 8/2, and 6/4 were denoted as SiO2/SnO2-0, SiO2/SnO2-1, SiO2/SnO2-2, and SiO2/SnO2-4, respectively. 2.3. Characterization. The solution properties of the spinning solution including the electrical conductivity, viscosity and values of surface tension were studied respectively by conductivity gauge (FE30, Mettler-toledo Group, Switzerland), viscosity tester (SNB-1A, Fangrui Co., Ltd., Shanghai, China), as well as surface 6


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tension instrument (QBZY-1, Fangrui Instrument Co., Ltd., Shanghai, China). Morphologies of the relevant SiO2/SnO2 NFM were characterized by scanning electron microscope (SEM, Vega 3, Tescan Ltd., Czech). The crystal phase structures of SiO2/SnO2 NFM with various molar ratios of SiO2 and SnO2 were represented by powder X-ray diffraction (XRD, D8 Advance, Bruker AXS, Karlsruhe, German) with Cu radiation and the wavelength of 1.5406 Å. In order to assess the flexibility of the resultant membranes, the tensile strength and fracture toughness of the membranes (with a size of 5 × 0.3 cm2 and thickness of 100 ± 5 µm) were tested by the fiber tensile strength tester (XQ-1C, Shanghai New Fiber Instrument Co., Ltd., China) based on the international standard named ISO 1798:2008, and the bending rigidity evaluation of the SiO2/SnO2 NFM (with a size of 10 × 10 cm2 and thickness of 60 ± 5 µm) was investigated via three-point bending method by a bending rigidity instrument (RRY-1000, Hangzhou Qingtong & Boke Automation Technology Co., Ltd., China) based on the American standard ASTM D 2923-01. Field emission transmission electron microscope (FE-TEM, JEM-2100F, JEOL Ltd., Japan), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) were employed to provide insight into the microstructure and composition of the obtained SiO2/SnO2 NFM. Nitrogen (N2) adsorption-desorption isotherms were tested by an ASAP 2020 surface area analyzer (Micromeritics Co., USA), and on the basis of which, Brunauer-Emmett-Teller (BET), Horvath-Kawazoe (HK), and nonlocal density functional theory (NLDFT) models were applied to calculate the specific surface area, micropore and mesopore size distribution, respectively. The electrical potential values of the SiO2/SnO2 NFM towards different pH values were performed via the Zeta Sizer Nano-ZS90 (Malvern Instruments Ltd., UK). Thermal analysis of the samples were measured by the simultaneous SDT Q600 TG-DSC analyser (TA Instruments-Waters 7



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LLC, USA) with a heating rate of 5 oC min-1 under air atmosphere. The porous structures were analysed by a capillary flow porometer (Porous Materials Inc. CFP-1100AI). Typically, 1 mL nanofiber/water dispersion with a concentration of 1 wt% was poured into the disposable capillary cell and each sample was measured for 10 times. 2.4. Molecular Filtration Measurements. For the sake of exploring the selective adsorption and dynamical molecular filtration performance of the as-prepared SiO2/SnO2 NFM, seven kinds of organic molecules with different electrostatic properties and molecular sizes, including positively charged molecules MB, BR2, RhB, NR, and BF with different sizes and electrostatic charges, negatively charged molecule MO, as well as non-charged molecule SY2 were selected as organic molecule models. Water was used to dissolve the above water soluble organic molecules, including MB, BR2, RhB, NR, BF, and MO, at the same time acetone was introduced to dissolve the water insoluble non-charged molecule SY2. The high selectivity towards organic molecules is crucial to the filtration performance, which were in brief detected by immersing 5 mg membranes separately into the above seven kinds of solutions with an initial concentrations of 40 mg L-1. Furthermore, the molecular filtration experiments were carried out by using a silica sand filtration unit. Six mixtures of organic molecules including MB/MO, MB/RhB, MB/BR2, MB/BF, MB/NR, and MB/SY2 were mixed up with the initial concentrations of 5 and 15 mg L-1, respectively. A piece of SiO2/SnO2 NFM was cut into a round shape with a diameter of 20 mm and an approximate mass of 20 mg and then placed in the unit. Following, the mixed dye solution of MB/MO, MB/RhB, MB/BR2, MB/BF, MB/NR, and MB/SY2 was respectively forced to flow through the SiO2/SnO2-2 NFM under the average pressure of 15 kPa by using a vacuum pump. The concentration of the 8


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molecules in solution was determined by a PG 2000 UV-vis spectrophotometer. And the separation efficiency (η) is used to evaluate the filtration ability by the equation:26 η (%) =

[OM]F × 100% [MB]F + [OM]F

(1)

where OM represents the organic molecules including MO, RhB, BR2, BF, NR, and SY2, [MB]F and [OM]F are the concentrations of MB and other six organic molecules in the filtrate, respectively. After the above molecular filtration towards the mixture dyes MB/OM, the MB-loaded nanofibrous membrane was dried at 80℃ in the vacuum oven for 4 h to remove the solvent. Subsequently, the membrane was put in the muffle furnace under air atmosphere to raise the temperature from 200 to 800 oC with a heating rate of 5 oC min-1 to completely decompose the adsorbed MB molecules for regeneration.11 The regenerated membrane was again used to perform the above separation experiment for ten continuous runs to evaluate the reversibility of the resultant SiO2/SnO2 NFM.

3. RESULTS AND DISCUSSION 3.1. Morphologies and Structures of SiO2/SnO2 NFM. The purpose of this work was to fabricate flexible porous inorganic nanofibrous membranes with high selectivity and high permeate flux as a molecular filter for organic molecule filtration. Following this, the membranes were designed on the basis of following three criteria: (1) the membranes should possess strong size exclusion or host-guest interaction between the nanofibers and organic molecules to obtain better performance in precise molecular sieving, (2) the membranes should possess highly tortuous open-cell porous structure between fibers and high porosity to enhance the permeate flux of the solvents, and (3) the inorganic porous fibrous membranes should be mechanically robust enough for dynamic molecular filtration and reutilization upon high applied 9



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pressure to satisfy the requirement of favorable reusability in practical application. The first two principles were satisfied by a facile and scalable strategy to fabricate the flexible porous SiO2/SnO2 NFM with negatively charged, high specific surface area, and interconnected porous structures among fibers via electrospinning technology. The third requirement was satisfied by embedding crystalline phase into amorphous matrix to enhance the mechanical properties of the porous inorganic nanofibrous membranes.

Figure 1. Morphology, crystal structure and mechanical properties of SiO2/SnO2 NFM. SEM images of the SiO2/SnO2 NFM with various SiO2:SnO2 molar ratios of (a) 10/0, (b) 9/1, (c) 8/2, and (d) 6/4. (e) XRD spectra, (f) tensile strength, fracture toughness, and (g) bending rigidity of SiO2/SnO2 NFM. The representative SEM images of the SiO2/SnO2 NFM acquired by altering SiO2:SnO2 molar ratio were shown in Figure 1a-d, indicating that the fibers arranged randomly to form the three-dimensional macroporous structures, which are beneficial to transport solvent with high flux.5, 8 Interestingly, the average diameter of SiO2/SnO2 NFM exhibited a gradually decreasing trend of 339, 308, 107, and 97 nm with increasing contents of SnO2, as demonstrated in Figure S1 (Supporting Information). This phenomenon was ascribed to the increasing electrical conductivity and 10


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decreasing viscosity of the spinning solution which induced the irregular whipping and larger stretching of the jet during electrospinning (Figure S2).27-28 Considering that with increasing content of the easy crystallization of SnO2 in the SiO2/SnO2 composite nanofibers, the microstructure of the membrane would be influenced tremendously, which was appraised by the characterization of XRD, TEM, EDX, and STEM analysis. As can be seen from XRD patterns in Figure 1e, the membranes retained amorphous without phase-change with decreasing the SiO2:SnO2 molar ratio gradually from 10/0 to 8/2; whereas with further decreasing molar ratio to 6/4, the crystallization peaks appeared and centered at 2θ values of 26.59o (110), 33.89o (101), 51.786o (211), which were generally in accordance with the standard XRD patterns of tetragonal SnO2 (JCPDS No. 72-1147). The representative TEM images (Figure 2a-d) provided intuitive insight into the microstructure of the SiO2/SnO2 NFM. Notably, the pristine SiO2 nanofiber (Figure 2a) was smooth without any obvious cracks or crystalline grains, which was in good consistent with the results obtained from XRD (Figure 1e) and previous studies.29 As observed from Figure 2b-d and Figure S3, SnO2 nanocrystals were homogeneously dispersed both inside and outside the surface of composite nanofiber, endowing the SiO2/SnO2 matrix with a hierarchical rough structure; whereas conflicting with XRD results for SiO2/SnO2-1 and SiO2/SnO2-2. Such phenomenon is most possibly attributed to the homogeneous dispersion and small grain size of the crystalline-phase SnO2 in the composite matrix, which was below the detection limit of XRD.30 Moreover, the corresponding HRTEM image (Figure 2e), revealed that some crystalline SnO2 randomly embedded inside the SiO2 matrix, and the lattice fringe distance originating from the crystal face (110) of SnO2 was 0.33 nm. As can be roughly seen from the EDX data presented in Table S1 that the Si:Sn molar ratio of SiO2/SnO2-2 was about 4/1, which was nearly in 11



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accordance with the composition of the membrane. The STEM images in Figure 2g-i verified that the Si, Sn, and O elements were dispersed homogenously in the nanofibers matrix in nanometer level.

Figure 2. TEM images of the SiO2/SnO2 NFM with various SiO2:SnO2 molar ratios of (a) 10/0, (b) 9/1, (c) 8/2, and (d) 6/4. (e) HR-TEM image of a selected area in (d). (f) EDX spectrum of a selected area in SiO2/SnO2-2. (g-i) Elemental mapping images of Si, Sn, and O on a single fiber of SiO2/SnO2-2. Owing to the transformation of fibers morphology and crystal structure, mechanical performance of the SiO2/SnO2 NFM would be significantly influenced, thus the tensile stress, fracture toughness, and bending rigidity of the membranes were studied. As observed from Figure 1f, the tensile stress dramatically increased from 0.89 to 4.15 MPa with increasing contents of SnO2, suggesting that the mechanical strength of the SiO2/SnO2 NFM was seriously affected by the SnO2 nanocrystals, which could be ascribed to the reduced deformability of the inside atoms, the gradually decreased 12


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of fiber diameters, and the increased fraction between fibers.31 Further decreasing the molar ratio to 6/4, whereas resulted in the tensile strength decreased to 2.48 MPa, which is mainly attributable to the agglomeration of nanocrystals that produced excessive defects inside the fibers resulting in a significant stress concentration.32 More interestingly, the fracture toughness (appraised by calculating the energy to break) of the membranes decreased notably from 0.056 to 0.016 MJ m-3 with gradually decreasing SiO2:SnO2 molar ratio from 10/0 to 6/4. This could mainly attribute to the decreasing of the fiber slippage during the tensile process, resulting in drastically decrease of rupture elongation, as shown in Figure S4. At the meantime, bending rigidity of the membranes gradually increased from 22 to 36 mN with molar ratio decreasing (Figure 1g), ascribing to the inhibition of the slippage of fibers during stretching process and then resulting in the decrease of flexibility.33 3.2. Quantitative Analysis of Porous Structures. Considering that the molecular filtration performance is highly relies on the effective surface area as well as pore structure of fibers, thus the effect of the varied SnO2 contents on the porous structure of the SiO2/SnO2 NFM was systematically investigated via N2 adsorption-desorption method. The corresponding N2 adsorption-desorption isotherms presented in Figure 3a exhibited a typical type I isotherms with a rapid monolayer adsorption and capillary condensation of the nitrogen molecules inside the pores of fiber. Most of the N2 adsorption concentrated in the region of P/P0 < 0.1 and an adsorption platform presented at the central part of the relative pressure, indicating that a large amount of micropores existed in the SiO2/SnO2 NFM.34 The isotherms curves located in the region of P/P0 > 0.1 without obvious hysteresis loops, demonstrated that the micropores presents in the fibers were slit-like shaped. This phenomenon may be ascribed to the decomposition of organic components PVB during the calcination 13



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process and the extrusion of gases, which would result in the pores difficult to be completely filled in the fibers.35 With gradually decreasing molar ratio from 10/0 to 6/4, N2 adsorption quantity of the relevant membranes progressively increased and the corresponding BET surface area were presented in Table 1, revealing a significant effect of the SnO2 nanocrystals on regulating the hierarchical porous structures of the membranes.

Figure 3. (a) N2 adsorption-desorption isotherms, (b) HK pore size distribution curves, and (c) NLDFT pore size distribution curves of the relevant SiO2/SnO2 NFM. (d) Plots of ln(V/Vmono) against ln(ln(P0/P)) reconstructed from (a). Table 1. Pore structure parameters of the resultant SiO2/SnO2 NFM. Samples

Si/Sn (molar ratio)

SBETa (m2 g-1)

Vtotalb (cm3 g-1)

Vmicroc (cm3 g-1)

PVFmicrod (%)

De

SiO2/SnO2-0

10/0

154.34

0.0851

0.0665

78.2

2.99

SiO2/SnO2-1

9/1

225.45

0.1281

0.0963

75.2

2.98

SiO2/SnO2-2

8/2

215.70

0.1344

0.0781

58.1

2.97

SiO2/SnO2-4

6/4

219.54

0.1398

0.0724

51.8

2.96

Note: a Total surface area was calculated by the BET method. b Total pore volume was 14


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calculated at P/P0= 0.99.

c

Vmicro was calculated by the HK method.

d

PVFmicro

indicates the pore volume fraction of microspores. e D indicates the fractal dimension determined from the N2 adsorption analysis. Moreover, typical HK model and NLDFT method were separately employed to further explore the micropore and mesopore size distribution of the as-prepared membranes.36 The micropore sizes of the membranes were chiefly centered at between 0.3 nm and 0.7 nm, as presented in Figure 3b. Combining total pore volume of the membranes calculated at P/P0 = 0.99 with the cumulative pore volume of HK model, the micropore volume fraction could be obtained, which was 78.2%, 75.2%, 58.1%, and 51.8%, respectively (Table 1), revealing the decreasing trend of the micropore volume fraction. This may be attributable to the difference of thermal expansion coefficient between the crystal phase SnO2 and amorphous phase SiO2 as presented in Figure 4d.37 The mesopore size distribution via NLDFT model presented in Figure 3c indicated that the SiO2/SnO2 NFM contained a spot of mesopores located in the range of 2-20 nm. Notably, the number of the mesopores located in 2-6 nm of the sample SiO2/SnO2-2 and SiO2/SnO2-4 were much more than that of the sample SiO2/SnO2-0 and SiO2/SnO2-1, which is in accordance with the results obtained from Table 1. Additionally, the hierarchical textures and geometric complexity of the as-prepared SiO2/SnO2 NFM was further quantified through fractal analysis according to the Frenkel-Halsey-Hill (FHH) theory, which is calculated on the basis of the nitrogen adsorption isotherm.38 (see more details in Supporting information) Figure 3d presented the corresponding FHH plots of the relevant SiO2/SnO2 NFMs, in which the straight line slopes located in the high coverage region were nearly the same; and the calculated fractal dimension (D) were 2.99, 2.98, 2.97, and 2.96, respectively, which further verifying that the porous structures of as-prepared membranes were 15



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chiefly composed by micropores accompanying by small amount of mesopores.38-39 3.3. Mechanism for Flexibility of Porous SiO2/SnO2 NFM. Hierarchical porous structure and crystal defects inside the ceramics would always turn into stress concentration point, accelerate the rapid extension of cracks across nanofiber and finally result in fracture of the nanofibers when subjecting to external bending stress, as it was widely reported in previous studies.40-41 To our surprise, the as-prepared porous SiO2/SnO2 NFM exhibited an intriguing softness and robust durability, which could be severely folded without any detectable fractures after being bended to a radius below 50 µm and regained the original state rapidly upon removing the loading, as presented in Figure 4a and Movie S1. Further observation on the deformation of single nanofiber (Figure 4c) showed that the single SiO2/SnO2 nanofiber was able to bear large bending deformation and bent to a curvature radius of less than 2 µm without generating any cracks, revealing outstanding bending properties of porous SiO2/SnO2 nanofibers.

Figure 4. (a-c) SEM images of a folded free-standing SiO2/SnO2 NFM at different magnifications. (d) Sketch of the bent fiber and zoomed-in section of the selected area. (e) Sketch of Si-O bond deformation in the selected area of (d) under the bending 16


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stress. (f) Sketch map showing the change for the SiO4 tetrahedra under tensile stress (up) and compression stress (down), respectively. On the basis of systematical analysis towards the nanostructure and hierarchical porous structure of SiO2/SnO2 NFM, a plausible flexibility mechanism for the flexible porous membranes was shown in Figure 4d-f. Benefiting from the interpenetrating fibrous networks composed by the freely movable nanofibers, once the bending stress loaded onto the membrane would immediately lead to the slippage between fibers and the accompanied bending of fibers as has been reported in previous studies.42 The key point to the flexible of SiO2/SnO2 nanofiber is the amorphous atomic structure lacking microstructural defects, which act as a “lubrication region” that could dissipate external stress and yield large deformation.41,

43

It is widely acknowledged that

amorphous silica is primarily composed of an open network of SiO4 tetrahedra, in which the Si-O bonds are consists of partial covalent bond and partial electrovalent bond. The atomic structures in the network composed by SiO4 tetrahedrons are relatively flexible, moreover the Si-O-Si bond angles θ have a widely distribution ranging from 120o to 180o with a maximum probability of 144o and the neighboring atoms O-Si-O would possess a mean value of 109.7o.42, 44-45 Under bending stress, deformation of amorphous region SiO2 inside the SiO2/SnO2 nanofiber could be divided into three zones: i) the tetrahedral in the surface tolerating tensile stress would be deformed by stretching the -Si-O-Si- bonds and increasing the bond angles; ii) the neutral strain axis establish zero strain with no deformation of the SiO4 tetrahedral; iii) the network of tetrahedral under compression stress pressure would be compacted together by decreasing the -Si-O-Si- bond angle θ and the bond lengths between neighboring atoms, as shown in Figure 4e and f.45-46 Therefore, the deformation of the SiO2/SnO2 nanofiber could be realized by changing the Si-O-Si bond angle and bond 17



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length. 3.4. Molecular Filtration Performance. Based on the aforementioned results, the as-prepared membranes with large surface area, hierarchical porous structure, favorable flexibility and enhanced mechanical strength enable SiO2/SnO2 NFM to be a very promising candidate for molecular filtration. In order to systematically evaluate the molecular filtration properties of the as-prepared membranes, adsorption activity towards the organic chemical molecules were firstly conducted. Since the fiber surface and pore channels in the SiO2/SnO2 NFM are covered with a layer of silanol groups (Si-OH), and thus became negatively charged due to the deprotonation of Si-OH at pH higher than its pka.3 Thus, we selected the positively charged molecule MB as an organic model to conduct the following experiment. In brief, 5 mg membranes were immersed into 10 mL MB solution with an initial concentration of 40 mg L-1 to detect the molecule removal capacities. Interestingly, the saturated adsorption capacities of SiO2/SnO2-0 and SiO2/SnO2-1 were 5.18 and 9.14 mg g-1, respectively; whereas the capacities of SiO2/SnO2-2 and SiO2/SnO2-4 could obtain 68 and 55 mg g-1, as calculated from Figure 5a. This may be attributable to the improved electrical potential with increasing contents of SnO2 in the composite fiber, which could enhance the electrostatic interaction between fiber matrix and MB molecules (Figure S5). On the other hand, considering that the MB molecular with a three-dimension volume of 1.43 × 0.61 × 0.40 nm3 could hardly to be fully extended in the tiny micropore which is smaller than 1.43 nm, and thus the diffusion was dramatically hindered by size exclusion.11,

47

As can be seen from

Figure S6, the BET surface area of SiO2/SnO2 NFM (taking SiO2/SnO2-2 as an example) after saturation adsorption dramatically decreased from 215.70 to 53.89 m2 g-1 accompanied by the increasing pore size. This phenomenon could be due to the 18


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fact that the molecules could enter into the approachable pores on the fibers and block small micropores to inhibit further mass transportation.47-48 Notably, the micropore volume fraction of the SiO2/SnO2-0 and SiO2/SnO2-1 were 78.2% and 75.2%, which were much bigger than that of SiO2/SnO2-2 and SiO2/SnO2-4, thus the pore channels blockage together with the long transmission path in the nanofiber matrix would be probably result in the inefficient adsorption capacity for SiO2/SnO2-0 and SiO2/SnO2-1.48 In view of the maximum adsorption capacity and tensile stress, SiO2/SnO2-2 was selected to conduct the following experiments. Effects of pH values on the adsorption performance of SiO2/SnO2-2 NFM were further investigated to understand the adsorption mechanism. The pH values ranging from 2 to 12 were regulated by dropwise adding 0.1 M NaOH or 0.1 M HCl solutions. As can be seen from Figure 5b, adsorption capacity gradually increased from 3.6 to 78.6 mg g-1 with increasing the pH values from 2 to 10, whereas the capacity dramatically decreased to 32 mg g-1 with pH further increased to 12, which has the same trend with the electric potential presented in Figure 5c. The membranes have a negative potential in the variation ranges of pH values from 2 to 12 and isoelectric point around pH 2, ascribing the ionization degree of the residual Si-OH on fiber surface were impressionable by pH values.3 It is therefore believed that electrostatic attraction could exist between negatively charged SiO2/SnO2 NFM and positively charged molecules.

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Figure 5. (a) The adsorption performance of SiO2/SnO2 NFM towards MB. (b) Adsorption performance of SiO2/SnO2-2 NFM towards MB at different pH values. (c) Zeta potential versus pH value of SiO2/SnO2-2 NFM. (d) Pseudo-first-order and pseudo-second-order kinetics plots of MB adsorption onto SiO2/SnO2 NFM.

Moreover, in order to understand the characteristics of molecule removal process, the kinetics of molecule adsorption onto the SiO2/SnO2 NFM were studied by utilizing the pseudo-first-order and pseudo-second-order adsorption kinetic models (see more details in Supporting information). As presented in Figure 5d and Table S2, the pseudo-first-order kinetic model with a higher correlation coefficient (R2> 0.999) possessed a better applicability, indicating that the adsorption mode between MB molecule and SiO2/SnO2 fibers was physisorption process. In addition, Figure S7 and Table S3 presents the adsorption isotherms including Langmuir and Freundlich isotherm models. Evidently, the Langmuir isotherm model with a higher correlation coefficient R2 value of 0.995 than that of Freundlich isotherm model (0.980) owned a 20


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better applicability for the adsorption process, indicating the monolayer adsorption of MB onto the homogeneous fiber surface rather than heterogeneous surface adsorption.

Figure 6. (a) Adsorption capacity of the SiO2/SnO2-2 on six representative water soluble dyes. The UV-Vis spectra of (b) MB/MO, (c) MB/RhB, (d) MB/BF, (e) MB/BR2, and (f) MB/NR before and after molecular filtration process and the inset photographs show the colors of the mixed aqueous solutions and after filtration, respectively.

Molecular filtration based on the selective adsorption and dynamical filtration of organic molecules is more attractive and challenging, which possesses high practical applied value for chemical purification.26 To validate whether the as-prepared inorganic porous nanofibrous membranes possess the ability to separate different chemical molecules, the dye molecules with different size and electrostatic charges were selected as models for the experiment. Here, positively charged molecules including MB, RhB, BF, BR2, and NR with different molecular sizes, negatively charged dye MO, as well as non-charged dye SY2 were selected to detect the molecular selectivity of the SiO2/SnO2 NFMs. The three-dimensional molecular structures and parameters of the organic molecules were presented in Figure S8 and 21



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Table S4, respectively. Typically, the as-prepared membranes were separately added into MB, BR2, NR, BF, RhB, and MO aqueous solution under stirring to achieve the adsorption equilibrium. As shown in Figure 6a, the equilibrium adsorption capacities towards MB, BR2, NR, BF, and RhB are 78.6, 21.0, 16.4, 5.4, and 4.7 mg g-1, respectively; whereas almost no equilibrium adsorption for MO. The different adsorption capacities towards positively charged molecules MB, BR2, BF, and RhB would be attributed to the difference in three-dimensional molecular volume, which was in the following order: MB < BR2 < BF < RhB.49 The large molecular volume would be steric hindered to be adsorbed on the active surface sites on SiO2/SnO2 NFM, furthermore aggregating and blocking the orifice of pores, and thus resulting in the decrease of adsorption capacity. Furthermore, positively charged dye NR has the similar molecular size as MB, whereas the charged group on the NR molecules is smaller than that of MB, resulting in the decrease of the electrostatic interaction between the fibers and dye molecules.49 The reason for little uptake capacity towards negatively charged dye MO, which possess the small molecular size to be able to ingress into the pores, could be ascribe to the electrostatic repulsion between dye molecules and the SiO2/SnO2 NFM.26, 49 Additionally, the adsorption capacity towards the water insoluble non-charged molecule SY2 was also carried out. As can be seen from Figure S9a that there were almost no uptakes toward the non-charged dyes SY2 over a period of time, indicating that the non-charged molecules could hardly be adsorbed by SiO2/SnO2 NFM. The significant difference of the equilibrium adsorption capacities towards various kinds of organic dyes indicate that the as-prepared SiO2/SnO2 NFM possess the ability to selectively transport molecules with different charges and sizes. To further demonstrate the molecular filtration of the membranes towards the 22


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mixture of different organic molecules with different electrostatic properties and molecular sizes, MB and MO with a similar molecular size and different electrostatic charges were firstly performed. As can be seen from Figure S10 and Movie S2, the dark green color of the mixture MB/MO changed to the orange color of MO in 20 s with a flux of approximately 288,000 L m-2 h-1 under the average pressure of 15 kPa, while the white color of SiO2/SnO2 NFM turned into the blue color of MB, indicating that the membranes successfully filtrate MB from the mixture of MB/MO with high efficiency. The separation efficiency of MB/MO after filtration was detected by the UV-vis spectra as presented in Figure 6b, suggesting that the concentration of MO in the filtrate decreased slightly from the initial 15 to 14.95 mg L-1, while the concentration of MB decreased dramatically from the initial 5 to 0.12 mg L-1. It means that the purity of MO in the filtrate is about 99.2% after filtration, and the purity of MB captured by the SiO2/SnO2 NFM was almost 100%. Furthermore, the mixtures of dye molecules with the same electrostatic charges and different molecular sizes, including MB/RhB, MB/BR2, MB/BF, and MB/NR, were also selected to perform this experiment. Once the aqueous solutions of dye mixture pass through the SiO2/SnO2 NFM, the color changed dramatically from the mixed colors to the pure color of RhB, BR2, BF, and NR, which has nearly the same phenomenon with MB/MO. As can be seen from the UV/Visible absorption spectra, the absorption peaks of MB all disappeared quickly, just leaving the characteristic absorption peaks of RhB, BR2, NR, and BF, respectively, and the filtration efficiency for the four mixtures were 99.49%, 97.66%, 99.75%, and 99.61% respectively. This could indicate that the porous SiO2/SnO2 NFM could filtrate the chemical molecules with the same electrostatic charges and different sizes. In addition, the molecular filtration experiment was also performed between the non-charged and positively charged 23



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organic molecule. Due to the water insoluble non-charged molecule SY2, the mixed solution of water and acetone with a mass ratio of 1:1 was selected to dissolve the mixture MB/SY2. As can be seen from Figure S9b, the green color of the mixture MB/SY2 changed dramatically to the yellow color of SY2 after filtration and the adsorption peaks of MB disappeared quickly, leaving the respective characteristic absorption peaks of SY2. This could indicate that the SiO2/SnO2 NFM could separate positively charged and non-charged organic molecules.

Figure 7. Schematic illustration of the molecular filtration process towards mixture of organic molecules by SiO2/SnO2 NFM.

The mechanism of molecular filtration towards various kinds of molecules with different electrostatic properties and molecular sizes was presented in Figure 7. It was electrostatic repulsion and size exclusion between charged organic molecules and the pores on the fibers play important roles in molecular filtration.2, 10 The fiber surface are covered with negative charges, which enables the pore channels on the fiber display charge permselectivity. Namely, the negatively charged and non-charged organic molecules will be expected to permeate through the membrane, whereas the positively charged ones will be excluded from the nanochannels by electrostatic 24


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forces.9-10 Furthermore, the micropore and mesopore on the fibers could block or slow down the permeation of molecules with larger size significantly, considering its size approaches that of nanochannels according to the hindered theory. Additionally, the mean macropore size of the SiO2/SnO2 NFM between nanofibers was ~ 0.93 µm (Figure S11), which could dramatically enhance the convection mass transport through interfiber pores in the nanofibrous membrane, endowing the membranes with a high permeate flux. Hence, the limited pore size distribution of the nanofibers and the open-cell interfiber porous structures with high porosity of the membranes promoted the high permeation flux and high filtration efficiency towards organic molecules. 3.5. Recycling Performance of SiO2/SnO2 NFM. The reusability of the molecular filter is of vital to long-term use in practical application, whereas it remains a big challenge to reserve high filtration performance after several cycles of separation and regeneration. In order to provide an insight into the reusability and hierarchical porous structural stability of the as-obtained SiO2/SnO2-2 during the entire process, the 10-cycle filtration-regeneration were performed. According to the thermal analysis of pure MB and MB loaded SiO2/SnO2 NFM presented in Figure S12, the used SiO2/SnO2 NFM was regenerated for the cyclic molecular filtration by using the appropriate calcination parameter as mentioned to decompose MB in the Materials and Methods section. As can be seen from Figure 8a, after 10 filtration-regeneration cycles the BET surface area of the SiO2/SnO2 NFM was slightly decreased from 215.70 to 187.99 m2 g-1 ascribed to the altered porous structure throughout the heating process. At the meantime, the separation efficiency decreased by less than 5% and the membranes still kept good fibrous morphology upon 10 cycles (Figure 8b), revealing the good reversibility and molecular filtration performance of the obtained 25



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membranes. More importantly, the SiO2/SnO2-2 NFM could also be easily scaled up to 60 × 60 cm2 (inset of Figure 8b) by multiple nozzles electrospinning equipment and are highly possible to realize industrialization by further expanding the spinning equipment. Thus, due to the high filtration and separation performance of the as-obtained flexible SiO2/SnO2 NFM in removing organic molecules, it might be fabricated as a molecular filter for chemical purification.

Figure 8. (a) BET surface area of SiO2/SnO2-2 after ten cycles of filtration separation experiments. Inset is SEM image of the regenerated SiO2/SnO2 membrane after 10 continuous filtration and separation experiments. (b) Separation efficiency by the regenerated SiO2/SnO2 NFM at different recycle times. The inset photograph shows a large scale membrane with size of 60 cm ×60 cm. 26


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4. CONCLUSION In summary, we presented a robust methodology for preparing a flexible SiO2/SnO2 NFM with large specific surface area, adjustable hierarchical porous structure, and enhanced mechanical strength via electrospinning technology by selecting PVB as pore-forming template and embedding the crystalline phase into the amorphous matrix. Benefiting from the large surface area, hierarchical porous structure, and enhanced negatively potential, the SiO2/SnO2 NFM presents high removal efficiency towards the positively charged molecule MB through electrostatic interaction. Furthermore, the SiO2/SnO2 NFM possess a good selective adsorption property towards various kinds of organic molecules with different molecular sizes and electrostatic charges. More significantly, the resultant flexible SiO2/SnO2 NFM exhibit a molecular filtration abilities towards organic molecules, with a high flux of approximately 288,000 L m2 h-1, high separation efficiency of more than 97%, and fine reusability, which could meet the requirements for chemical molecular purification. The successful fabrication of such flexible porous inorganic nanofibrous membranes with large surface area and enhanced mechanical strength holds great promise for the selective adsorption and molecular filtration in practical chemical molecules purification and separation science.

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ASSOCIATED CONTENT Supporting Information. Details on the calculation of fractal dimension based on the FHH equation, adsorption kinetic models, and adsorption isotherm models. Histogram showing the fiber diameter distribution of SiO2/SnO2 NFM with various SiO2:SnO2molar ratios of 10/0, 9/1, 8/2, and 6/4 (Figure S1). Conductivity, viscosity, and surface tension of SiO2/SnO2 NFM with various SiO2:SnO2 molar ratios (Figure S2). TEM images of the SiO2/SnO2 NFM with various molar ratios: 9/1, 8/2, and 6/4 (Figure S3). Stress-strain curves of the SiO2/SnO2 NFM (Figure S4). Zeta potential of the SiO2/SnO2 NFM with various molar ratios (Figure S5). N2 adsorption-desorption isotherms and NLDFT pore size distribution curves of SiO2/SnO2-2 after adsorption of MB (Figure S6). Adsorption isotherm curves for MB adsorption onto SiO2/SnO2 NFM (Figure S7). Three-dimensional molecular structures of representative dyes, i.e. MB, MO, RhB, BF, BR2, NR, and SY2 (Figure S8). The adsorption performance of SiO2/SnO2 NFM towards SY2, and the UV-Vis spectra of MB/SY2 before and after molecular filtration process and the inset photographs show the colors of the mixed solutions and after filtration (Figure S9). Equipment for molecular filtration and photographs before (left) and after (right) filtering MB/MO solution, and the inset photograph shows the corresponding morphology of SiO2/SnO2-2 NFM (Figure S10). Pore size distribution analysis of the SiO2/SnO2 NFM using the bubble point method (Figure S11). Thermogravimetric analysis of MB powder and MB loaded SiO2/SnO2 NFM was conducted from 200 to 800 °C in air (Figure S12). Typical EDX results of SiO2/SnO2-2 (Table S1). Kinetic parameters of MB adsorption onto SiO2/SnO2 NFM (Table S2). Isotherm parameters for MB adsorption onto SiO2/SnO2 NFM (Table S3). Characteristic parameters of the representative organic molecules (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org. 28


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AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51673037 and 51473030), the Shanghai Committee of Science and Technology (No. 15JC1400500), the “DHU Distinguished Young Professor Program”, the Fundamental Research Funds for the Central Universities (NO. 16D310105), and “111 Project” Biomedical Textile Materials Science and Technology, China (NO. B07024).

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Nanofibrous Membrane with Superb Flexibil - ACS Publications

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