Research Article www.acsami.org
Amphiphobic Polytetrafluoroethylene Membranes for Efficient Organic Aerosol Removal Shasha Feng, Zhaoxiang Zhong,* Feng Zhang, Yong Wang, and Weihong Xing* State Key Laboratory of Materials-Oriented Chemical Engineering, National Engineering Research Center for Special Separation Membrane, Nanjing Tech University, Nanjing 210009, China S Supporting Information *
ABSTRACT: Polytetrafluoroethylene (PTFE) membrane is an extensively used air filter, but its oleophilicity leads to severe fouling of the membrane surface due to organic aerosol deposition. Herein, we report the fabrication of a new amphiphobic 1H,1H,2H,2H-perfluorodecyl acrylate (PFDAE)-grafted ZnO@PTFE membrane with enhanced antifouling functionality and high removal efficiency. We use atomic-layer deposition (ALD) to uniformly coat a layer of nanosized ZnO particles onto porous PTFE matrix to increase surface area and then subsequently graft PFDAE with plasma. Consequently, the membrane surface showed both superhydrophobicity and oleophobicity with a water contact angle (WCA) and an oil contact angle (OCA) of 150° and 125°, respectively. The membrane air permeation rate of 513 (m3 m−2 h−1 kPa−1) was lower than the pristine membrane rate of 550 (m3 m−2 h−1 kPa−1), which indicates the surface modification slightly decreased the membrane air permeation. Significantly, the filtration resistance of this amphiphobic membrane to the oil aerosol system was much lower than the initial one. Moreover, the filter exhibited exceptional organic aerosol removal efficiencies that were greater than 99.5%. These results make the amphiphobic PTFE membranes very promising for organic aerosol-laden air-filtration applications. KEYWORDS: amphiphobic, PTFE membrane, atomic layer deposition, organic aerosol, ZnO, plasma
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INTRODUCTION It is well-known that oil fumes have severe effects on human health because they contain many chemical compounds such as benzene, toluene, xylene, naphthalene, methylnaphthalene, and other similar compounds,1−3 which are carcinogenic. Thus, it is of great importance to efficiently remove the oily aerosol from the fumes. However, the oil viscosity is usually high and blocks most filter materials available in the market. Polytetrafluoroethylene (PTFE) is a specialized polymer widely used in clothing fabric, medical implants, advanced dielectric materials, and in industrial flue gas filtration.4−7 PTFE generally presents strong hydrophobicity due to its highly symmetrical nonpolar linear configuration, where a skeleton constructed of carbon atoms is surrounded by fluorine atoms (F2). In industrial flue gas filtration, hydrophobic PTFE membranes are used as the membranes to repel the liquid contained in the flue gas, allowing the air to pass through the membrane at a high flow rate. Also, the hydrophobicity of the membranes prevents the liquid from being trapped within the membrane, which improves its filtration efficiency. However, PTFE is oleophilic and is not suitable for filtering flue gas containing oily compounds. When used, the oily aerosol adheres on the surface and the pore channels of the membrane, which causes membrane fouling and increased filtration resistance. © XXXX American Chemical Society
Previous research concerning the PTFE membrane was mainly focused on the hydrophilic modification of the membrane for wastewater treatment.8,9 Various approaches including the high-energy radiation grafting method,10 atomic layer deposition (ALD),11 chemical vapor deposition,12,13 and plasma treatment14 were employed to improve the PTFE membrane hydrophilic performance. Among the few techniques, ALD has been known as an efficient technique for depositing thin films conformally on various substrates with precise atomic layer control by using sequential surface reactions. The self-limiting growth mechanism of the reactions as well as the reaction in the gas phase enable the deposition of uniform films on the surface of substrates with porous or hierarchical structures.15 In addition, it plays a role in improving the structural strength of some flexibility materials.16 Wang et al.11,17 used ALD to coat TiO2 or Al2O3 on PTFE membranes. The approach improved the hydrophilicity of the membrane, increasing the water permeation through the membrane. Corn et al.18 prepared nanocone array on PTFE surfaces to achieve superhydrophobicity for self-cleaning. Up to now, the related organic amphiphobic membranes are mainly focus on poly(εReceived: November 23, 2015 Accepted: March 22, 2016
A
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Schematics of the modification process of the membrane: (a) pristine PTFE fibrils; (b) ZnO film coating; and (c) monomer grafted under plasma.
gen, Germany) and used as received. The ALD reactants were diethyl zinc (DEZ, 98%, MO source center of Nanjing University) and deionized H2O, which were used as the Zn and O precursors, respectively. N2 with high purity (99.99%) was used as the precursor carriers and purging gas. 1H,1H,2H,2H-perfluorodecyl acrylate (PFDAE) (97%, J&K, China) was used as the grafting monomer. Atomic Layer Deposition of ZnO on PTFE Membranes. The dried PTFE membranes were placed in the chamber of a commercialized ALD reactor (SavannahS100, Cambridge Nano-Tech) and dried at the operating temperature for 30 min under vacuum (∼1 Torr). Both the DEZ and H2O were kept in the storage cylinders at room temperature. The generation of ZnO (in ALD process) is expressed by the equation: Zn(C2H5)2 + H2O = ZnO + 2C2H6. In ALD, the substrate is exposed to the DEZ and H 2O reactants alternatively, and the produced film is formed in a stepwise and very digital fashion. The ALD was performed at 130 °C or different numbers of cycles (up to 150) with a steady N2 flow rate of 20 sccm. In a typical ALD cycle, the DEZ and water vapor were sequentially pulsed into the reactor for 0.03 s. Immediately after each pulse of precursors, the system was purged with nitrogen for 30 s to sweep off the excess precursor. To investigate the influence of ALD cycles, we used cycle times of 70, 100, 120, and 150. Grafting PFDAE on ZnO@PTFE Membranes. Plasma treatment of ZnO@PTFE membrane was performed by first immersing the PTFE membranes in the PFDAE solution for 5−30 s in a low-temperature glow discharge reactor (HPD-280, Corona-lab Nanjing). The membranes were placed on a glass plate in the middle of the chamber, and the chamber was vented and vacuumed to a base pressure of −99.8 kPa. The plasma reactor was then initiated at 40 W for 5−10 min. After this process, the membrane was dried in an oven at 60 °C for 2 h. Characterization. Surface morphology and microstructure were analyzed using field emission scanning electron microscopy (FESEM, HitachiS-4800). Prior to SEM analysis, the samples were sputter-coated with a thin layer of gold and palladium alloy. The chemical groups and chemical compositions were determined by means of Fourier transform infrared spectroscopy (FT-IR, Nicolet 8700) in thin film mode by using
caprolactone) (PCL), poly(methyl methacrylate) (PMMA), and poly urethane (PU).19−21 There are some industrial oleophobic membranes; for example, the oleophobic membranes from Gore are used for car headlights sticker, acoustic ventilation products, and some other small electron devices (from www.gore.com). Pall Versapor membrane is mainly used as syringe-driven filter in water system. However, few membranes are available on applying modified porous PTFE filters to oil fume cleaning. Badyal et al.22−24 prepared an oleophobic PTFE plate by depositing low-surface-energy plasma polymer layers onto microroughened PTFE substrates, which increased the contact angle of decane liquid on the modified substrate. However, due to the chemical inertness of PTFE, the binding force between the monomer and the PTFE was weak. In particular, for porous PTFE, it is difficult to improve its oleophobic property without compromising its hydrophobicity, high permeation rate, and uniform pore structure. Commercial oleophobic PTFE membrane used in industry oily bearing system is practically negligible. Therefore, there is a need to search for an alternative approach to produce a stable membrane effective for oil-fume filtration. Herein, we propose a novel approach to fabricate amphiphobic (i.e., hydrophobic and oleophobic) 1H,1H,2H,2H-perfluorodecyl acrylate (PFDAE)-grafted ZnO@PTFE membrane for oil aerosol filtration as shown in Figure 1. A layer of ZnO film was first deposited on the pristine membrane via ALD to enhance the binding strength between PFDAE and PTFE matrix. In addition, 3D ZnO shell wrapping PTFE fibrils helps retain the pore structure shape. Then, PFDAE, a low surface free energy molecule, was grafted on the membrane by plasma treatment in a low-pressure air atmosphere. The PFDAE-grafted ZnO@PTFE composite membrane was characterized by means of water and oil contact angles and gas permeation tests. The oil aerosol filtration performance of the modified membrane was also carried out and compared with that of the pristine membrane and PFDAEgrafted ZnO@PTFE composite membrane.
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EXPERIMENTAL SECTION Materials. Porous PTFE membranes with a mean pore diameter of 5 μm in the form of circular disc (diameter: 47 mm; thickness: 100 μm) were purchased from Sartorius (GoettinB
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Oil-smoke filter performance test rig.
Figure 3. SEM images of the surface morphologies of PTFE membranes treated by different ways. (a) PTFE membrane; (b) PFDAE-grafted membrane; (c) ZnO-ALD PTFE membrane; and (d) PFDAE-grafted ZnO@PTFE membrane.
a total reflection method and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250; fixed accelerating voltage: 30 eV). The XPS samples were irradiated with the monochromatic Al−Kα line (1486.6 eV). Also, the surface wettability was characterized by contact angle measurements (DropMeterA100P). A total of 5 μL of hexadecane or water were dropped on different regions of the membranes triplicated for averaging and standard deviation values, and each droplet stayed on the samples for 30 s before the test. The composition and thermal property of the membranes were characterized by thermogravimetric analysis (TGA, STA 449F3). The operation temperature was set from 20−800 °C with a ramp rate of 10 °C/min in air atmosphere. The X-ray diffraction patterns of the samples were obtained using a X-ray diffractometer (XRD MiniFlex 600) with Cu Kα radiation (λ = 0.154 nm) at a generator voltage of 40 kV and a generator current of 15 mA. The scanning speed and the step were 5°/min and 0.02°/min, respectively. To investigate the bonding strength of monomers with membrane, we ultrasonicated the modified membranes in ethyl alcohol for 3 h and then heated them in an oven at 90 °C for another 3 h. The FT-IR results of the treated membranes were compared with the original samples. Filtration Performances. The gas permeation rates of the membranes were measured by an in-house air filtration test rig
(Figure 2). The gas was supplied by an air compressor toggle and then entered into a steel tank, which was used to keep the gas pressure stable. The inlet pressure was kept at 500 kPa. The whole process was carried out at ambient temperature. The membrane test area was 7.07 cm2 (φ = 3 cm). The oily aerosol was generated by burning incense. The burning incense contained a variety of pollutant gases, including CO, CO2, NO2, SO2, and also volatile organic compounds, such as benzene, toluene, xylenes, aldehydes, and polycyclic aromatic hydrocarbons.25 The absorption flask was employed to create an atmosphere for the incense flaming and to adjust the smoke concentration. The gas flow rate was kept at 1 L/min by controlling the intake valve. The pressure drop was gathered by using a differential pressure gauge, and the smoke concentration was detected by the dust concentration detector (Casella CEL-712) at the test ports before and after the position of the membrane.
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RESULTS AND DISCUSSION Surface Morphologies and Microstructure Evolution of Membranes. Figure 3a shows that PTFE membrane filters manufactured by a stretching process are highly porous and exhibit a netlike structure composed of interconnected and smooth nanofibrils. The free spaces between the nanofibrils C
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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m−2 h−1 kPa−1). PFDAE-grafted membrane showed an increase in gas permeation rate of 15.5% to 635 (m3 m−2 h−1 kPa−1) as the monomers infiltrated into the nanofibrils leading to the formation of large pores. The gas permeation of the PFDAEtreated ZnO@PTFE membrane was 6.7% decreased compared to pristine membrane. This was because of the additional ZnO deposition and monomer grafting step, which reduced the pore size and the gas permeation rate (see Figure S1). In addition, the criss-cross interlinkage in PFDAE-grafted ZnO@PTFE membrane could also increase the permeate resistance. Crystal Structure and Thermal Behaviors of the Membranes. Successful ZnO deposition and monomer grafting on the membrane was confirmed by XRD and TG analysis. The peak centered at 18.06° in the XRD spectrum of different samples shown in Figure 5a was assigned to the crystalline PTFE, and additional weak peaks were observed at 31.7° and 36.8°.26 Using the Bragg equation, the interchain dspacings were calculated to be 4.9, 2.8, and 2.4 Å, respectively. These results confirm that the PTFE membrane is a semicrystalline material in a randomly moving coil state with high mobility, which allows it to effectively interact with the metal oxide precursor to form a robust metal oxide bond.27 The ZnO@PTFE membrane and PFDAE-grafted ZnO@PTFE membrane have the same peaks marked by virtual vertical lines, which are also seen in the pure ZnO XRD spectrum (Figure 5a-5). Similar peaks were observed for samples with or without grafting (Figure 5a-2, 5a-4) due to the noncrystallinity nature of PFDAE. Figure 5b is an XRD pattern of the same samples between the diffraction angles of 16−20°. In the spectrum of Figure 5b, curves (2), (3), and (4) show a detectable left-shift relative to curve (1). The shift was due to the increase of the interplanar distance.28 A new element (Zn) was inserted between the broken C−C bonds or C−F bonds, causing lattice deformation. The lattice deformation leads to the shift of the main peak. Infiltrated Zn is likely to build − Zn−O−Zn− bridging units or − Zn−F groups at both the defect sites and chain termini.29 Therefore, Zn atoms connecting helical chains in the amorphous phase of PTFE presumably function as impurities, which can noticeably reduce or hinder the flexibility or mobility of single chains as well as the whole chain bundles. Such significantly reduced flexibility or hindered mobility by Zn infiltration is believed to lead to a considerable mechanical-property enhancement of the resulting Zn−PTFE hybrid under uniaxial tensile force. Thermogravimetric analysis (TGA) was performed on the pristine PTFE membrane and modified PTFE membrane (Figure 6). As shown in Figure 6, pristine PTFE starts to lose material at 500 °C and completely degrades at 700 °C. ZnO@ PTFE membrane has 20% of the initial weight remaining after the high-temperature degradation process, which can be ascribed to the remaining deposited ZnO because of its high thermal stability. The TG curves suggest that the ZnO deposition does not have a noticeable effect on the thermal stability of the PTFE membranes because both pristine and ZnO@PTFE membranes initially degraded at 500 °C. However, PFDAE-grafted ZnO@PTFE membrane starts to degrade at ∼30 °C higher than does PFDAE-grafted membrane (at 110 °C). Also, the same membrane has a higher initial mass loss compared to PFDAE-grafted membrane. This is likely due to the higher specific surface area, which is supplied by the ZnO-ALD process, increasing the PFDAE monomer grafted amount and also the heat resistance. The PFDAE-grafted membrane without ZnO-ALD process decomposed completely,
define the pores in the PTFE filter. A pristine PTFE membrane was immersed in the monomer liquid and then placed in the plasma device chamber to grafted PFDAE directly without the ZnO-ALD process. Figure 3b shows that the adjacent fibrils of obtained membrane were bound to each other, and the yellow dotted line on the picture indicates their binding trends. This process happened while the membrane was dipped into the monomer solution, in which the low surface free energy monomers quickly infiltrated into the pore tunnels, introducing the shrinking and bending of fibrils due to the liquid surface tension. The adjacent fibrils eventually combined into one bundle, leading to some pores disappearing and generating larger pores. The inset in Figure 3b shows the large pore distribution at a low magnification. After the ALD treatment, the original smooth surface of the nanofibrils in ePTFE was uniformly covered by densely arranged nanoparticles, as clearly shown by Figure 3c. Energy dispersive X-ray spectroscopy (EDX) analysis revealed the relatively uniform distribution of Zn throughout the surface of the filter and the approximately 1:1 atomic ratio of zinc and oxygen, which indiated that ZnO was successfully deposited on the ePTFE substrate. Figure 3d shows a membrane that had undergone ZnO-ALD and PFDAE monomer grafting by plasma in air atmosphere. The fibrils appeared smoother than the ZnO-ALD sample because the membrane was grafted with a layer of PFDAE polymers. The Figure 3d inset shows a low magnification image of the same sample. Compared with that shown in the Figure 3b inset, PFDAE-grafted ZnO@PTFE membrane possessed more uniform pores than the PFDAE grafting without the ZnO-ALD membrane. This was because ZnO nucleates in the near-surface region and grows on the surface of the PTFE nanofibrils, forming an interconnected ZnO layer wrapping the PTFE nanofibrils. The ZnO structure interwoven with the PTFE substrate may provide an additional spatial stabilizing force for the pore structure of PTFE membrane, which avoided the appearance of large pores during PFDAE grafting. Clean air permeability tests were conducted on the pristine and modified ePTFE filters. The range of operating pressure drop was from 0 to 1.5 kPa. The error bars represent a standard deviation of gas flux within five measurements. Figure 4 shows that the gas permeation for the pristine membrane was 550 (m3
Figure 4. Gas permeation of different samples. (a) PTFE membrane; (b) PFDAE-grafted membrane; (c) PFDAE-grafted ZnO@PTFE membrane (error bars represent a standard deviation of gas flux within five congeneric samples). D
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. (a) XRD pattern of different samples (1) pristine PTFE; (2) ZnO@PTFE membrane; (3) PFDAE-grafted PTFE membrane; (4) PFDAEgrafted ZnO@PTFE membrane; (5) XRD spectrum of pure ZnO. Panel b is a high-magnification image of a part of panel a at the range of 2τ 16−20 degree.
expected due to the lower percentage composition of ZnO in the sample. Influence of PFDAE-Plasma on the Membranes. The PTFE membranes for the different samples were investigated using FT-IR spectroscopy in the mid-infrared region (2000− 600 cm−1) (Figure 7a). Characteristic peaks for C−F (i.e. 1208 cm−1 (symmetric C−F stretch), 1152 cm−1 (asymmetric C−F stretch), and 640 cm−1 (CF2 rocking)) appear upon the pristine PTFE membrane.23,30 In comparison, PFDAE (the structural formula of PFDAE is shown in Figure S2) exhibited strong absorption at 740 and 1735 cm−1, which are values that are assigned to CF3 stretching deformation and CO stretching vibration, respectively. At the 1340 and 1375 cm−1 position, the values were assigned to C−H flexural vibration and CH3 flexural vibration, respectively (Figure 7a-2). These five bands at 740, 1340, 1375, 1640, and 1735 cm−1 existed on the other modified samples (see Figure 7a-3−5). In addition, because the boiling point of PFDAE monomers is 90 °C, sample a-5 was treated in a water bath over 90 °C to investigate the binding force between monomer PFDAE and PTFE, but no obvious decrease of the five new peaks was observed, indicating that the grafting of the PFDAE on the membrane was stable. To evaluate the adhesion between PFDAE and PTFE further, we
Figure 6. TGA of different samples, pristine PTFE membrane; ZnO@ PTFE membrane; PFDAE-grafted PTFE membrane; PFDAE-grafted ZnO@PTFE membrane.
and the PFDAE-grafted ZnO@PTFE has about 12% weight left at 800 °C. The lower weight percentage left in PFDAE-grafted ZnO@PTFE as compared to the 20% of the ZnO@PTFE was
Figure 7. FT-IR of (a): (1) pristine PTFE; (2) PFDAE; (3) PFDAE-grafted PTFE membrane; (4) PFDAE-grafted ZnO@PTFE membrane; (5) sample, treated in a water bath at 90 °C; (b): (1) PFDAE-grafted membrane; (2) PFDAE-grafted ZnO@PTFE membrane; (3) acetone liquid ultrasound treatment PFDAE-grafted PTFE membrane; (4) acetone liquid ultrasound treatment PFDAE-grafted ZnO@PTFE membrane. E
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces immersed the membranes in acetone and treated them with a strong ultrasound oscillation for 6 h at a power of 200 W and an ultrasonic frequency of 40 kHz. FTIR spectra in the mode of attenuated total reflectance were then collected for these treated membranes. An obvious decrease of the transmittance peaks at 1735 and 1640 cm−1 originated from PFDAE (Figure 7b-3) indicates the exfoliation of grafted polymer of PFDAEgrafted membrane. In contrast, for the grafted ZnO@PTFE membrane, the intensity of this peak only slightly decreased. Therefore, there is a stronger adhesion with the presence of nano-ZnO layer. X-ray photoelectron spectroscopy (XPS) was employed to analyze the C 1s binding energy of different samples (Figure 8).
Table 1. Percentage of Elements Existing on the Surface of the Samples elements
C 1s
F 1s
O 1s
Zn 2p3
O/C
pristine PTFE PFDAE-grafted PTFE PFDAE-grafted ZnO@PTFE
27.7 35.23 35.22
72.3 56.62 54.27
− 8.15 9.62
− − 0.89
− 0.23 0.27
and O were discovered in the PFDAE-grafted PTFE membrane, and Zn was found in PFDAE-grafted ZnO@ PTFE membrane. The percentage of each element indicates that monomers grafted on the surface of the membrane. The ratio of F/C for pristine membrane was 2.6:1. However, when it is modified by monomer plasma, the F/C ratio was 1.6:1. The decreased F/C ratio was attributed to the F/C monomer ratio (1.3:1). Table 1 shows that the O/C ratio increased from 0.23 to 0.27 from the PFDAE-grafted PTFE membrane to the PFDAE-grafted ZnO@PTFE membrane because the ZnO@ PTFE process supplied an additional “O” element to increase the O/C ratio. Amphiphobic Property of the Membranes. The hydrophobic and oleophobic properties of the membrane were determined by liquid contact-angle goniometry. As shown in Figure 9, the contact angles of pristine membrane and the modified membrane were measured by water and hexadecane, respectively. A water drop exhibited a high contact angle on the pristine membrane with a measured contact angle of 122° (Figure 9a). With the ZnO-ALD treatment, the membrane hydrophobicity decreased (Figure 9b). When hexadecane was used to measure the oil contact angle, it instantaneously infiltrated into the pristine membrane and ZnO@PTFE membrane (Figure 9e,f). Even though the surface free energy of the membrane was low, the oil easily infiltrated into the membrane due to the lower hexadecane free energy and the membrane porosity made. Consequently, the membrane is effortlessly blocked. In the oil bearing system filtration, and the oleophilic property is a vital weakness of the membrane. By grafting the low-surface-energy monomer (such as PFDAE), we enhanced the hydrophobicity and oleophobicity of the membrane surface, with a measured maximum contact angles of 155° and 130°, respectively (Figure 9c,d and g,h). This result indicates that grafting a low-surface-energy monomer can reduce the surface energy of the membrane. In addition, ZnO nanoparticles contributed to the surface roughness.29 As for a rough surface, its contact angle can be interpreted by Wenzel’s model, in which the surface roughness is considered as an important factor that influences the wettability.39,40 Thus, the water or oil contact angle of the modified membrane is increased along with the increase of roughness. Figure S3 shows the influence of ZnO-ALD cycles on the water−oil contact angle. Photographs of water and oil droplets suspended on the surface of PTFE and PFDAE-grafted ZnO@PTFE membrane are shown in Figure 9i,j. The Performance of Organic Aerosol Laden Air Filtration. The filtration performance of PTFE and PFDAEgrafted ZnO@PTFE membrane were tested with an in-house filtration test rig (Figure 10). The inlet pressure was 50 kPa, the gas flow rate was kept at 1 L/min by controlling the valve. The curve shows the relationship between time and pressure drop of different membranes. The pristine membrane filtration pressure drop increased sharply from 0.5 kPa exponentially to 3.3 kPa in 350 s. In contrast, the PFDAE-grafted ZnO@PTFE membrane displayed only a slight increase in pressure, with lower initial
Figure 8. XPS spectra for PTFE membrane: (a) pristine PTFE membrane; (b) PFDAE-grafted PTFE membrane; and (c) PFDAEgrafted ZnO@PTFE membrane.
The XPS survey spectrum of pristine PTFE shows two peaks that can be assigned to CF2 and CF3 in Figure 8a with the binding energy of 291.5 and 293.7 eV.31,32 Figure 8b,c also shows this at a lower intensity. CF3 appeared in the modified PTFE,23,33−35 which is also observed in Figure 8c-1. The plasma-modified PTFE membrane showed binding energies of 293.7, 288.9, and 285.4 eV, which are ascribed to CF3, COOR, and C-OR, respectively.36−38 However, an additional peak was observed in the PFDAE-grafted ZnO@PTFE spectrum compared with that of PFDAE-grafted membrane. The arrow “7” shows the position of the C−Zn binding energy.29 Moreover, XPS measurements were performed to evaluate the chemical composition of the samples. The pristine PTFE membrane has two chemical elements (C and F) with atomic percentages of 27.7% and 72.3%, respectively (Table 1). C, H, F
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 9. Water−oil contact angle; (a and e) original PTFE membrane; (b and f) ZnO@PTFE membrane; (c and g)PFDAE-grafted PTFE membrane; (d and h) PFDAE-grafted ZnO@PTFE membrane; (i) photograph of water−oil droplet on pristine PTFE membrane; (j) photograph of water−oil droplet on PFDAE-grafted ZnO@PTFE membrane.
Figure 10. Panel a shows the relationship between filter pressure drop and filter time of pristine PTFE and PFDAE-grafted ZnO@PTFE membrane; panel b is the oil fume filtration result of PFDAE-grafted ZnO@PTFE membrane manifested by oil fume rejection rate and the oil concentration change between upstream and downstream.
Figure 11. Oil aerosol filtration schematic. Panel a is the oil aerosol filtration process of pristine PTFE membrane; panel b is the oil aerosol filtration process of PFDAE-grafted ZnO@PTFE membrane.
but also exhibited good filtering performance in the oil bearing system. Figure 11 is a schematic representation of the oil aerosol filtration process of pure PTFE membrane (Figure 11a) and PFDAE-grafted ZnO@PTFE membrane (Figure 11b). Because of the oleophilicity of the pristine PTFE membrane, the adhesive force between the oil layer and membrane surface was
filter pressure of ∼0.3 kPa, which was increased to ∼0.4 kPa. The initial feed concentration of ∼1100 mg/m3 was reduced to 5 mg/m3 after 5 min of filtration (Figure 10b). PFDAE-grafted ZnO@PTFE showed a rejection of the oil-solid smoke of 99.5% after 5 min. When the inlet concentration was 100 mg/ m3, the export concentration of 0.5 mg/m3 was achieved. The modified membrane not only showed consistent air flow rate G
DOI: 10.1021/acsami.5b11315 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces stronger than the gravitational force. This causes the oil droplets to strongly adhere on the surface, gradually forming a thicker oil layer that increases the permeation resistance (see Video S1). However, the deposition of aerosol particles resulted in a modified membrane with amphiphobic surface. During the oil aerosol filtration process, a fouling layer is slowly formed on the membrane surface. As the filtration proceeds, more aerosol particles deposit and the fouling layer becomes thicker. With the growth of the fouling layer, the gravity force surpasses the adhesive force between the fouling layer and membrane surface, and the fouling layer gradually slips down from the filter (see Video S2). Thus, the fouled membrane is regenerated, and the gas flow rate was recovered. The modified membrane with amphiphobic surface can maintain its initial low filtration resistance and high oil rejection simultaneously. Consequently, the PFDAE-grafted ZnO@PTFE membrane can be used in an oil-bearing system effectively. Videos S1 and S2 show the oil infiltration process and the oil resistance process of the two membranes mentioned above, respectively. The surface morphologies difference between pristine membrane and PFDAE-grafted ZnO@PTFE membrane after oil aerosol filtration was indicated by SEM. The oil composition adhered on the membrane surface and turned into a layer of oil film (Figure S5a). For the PFDAE-grafted ZnO@PTFE membrane, the pore structure was retained after the filtration(Figure S5(b)).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial supports were provided by the National Natural Science Foundation of China (Nos. 21125629, 21276124, 21306079), the Jiangsu Province Scientific Supporting Project (Nos. BE2014717, BE2015695) and the Innovative Research Team Program by the Ministry of Education of China (No. IRT13070)
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REFERENCES
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CONCLUSIONS We have successfully fabricated a PFDAE-grafted ZnO@PTFE membrane via ALD and PFDAE grafting suitable for oil-aerosol filtration. ALD of ZnO enhances the integrity of the PTFE fibers and retains the pore structures to prepare the membrane for PFDAE grafting. The interconnected ZnO layer-wrapped PTFE nanofibrils structures of the ZnO@PTFE membrane also increase the specific surface area of the membrane, providing more sites for PFDAE grafting. The grafting step after ZnO ALD creates an additional oleophobic coating on the inherently hydrophobic membrane surface with low surface free energy, producing an amphiphobic microporous PTFE membrane suited for oil-bearing-system filtration. The modified membrane showed excellent oil rejection (>99.5%) and a stable filtration pressure drop (
