J Polym Res (2016) 23:183 DOI 10.1007/s10965-016-1063-9
ORIGINAL PAPER
Atomic layer deposition of TiO2 film on a polyethersulfone membrane: separation applications Javed Alam 1 & Mansour Alhoshan 1,2 & Lawrence Arockiasamy Dass 1 & Arun Kumar Shukla 1 & M. R. Muthumareeswaran 1 & Mukhtar Hussain 3 & Abdullah S. Aldwayyan 3
Received: 30 March 2016 / Accepted: 18 July 2016 # Springer Science+Business Media Dordrecht 2016
Abstract In the current study, a titanium dioxide (TiO2) nanostructured film was grown on a polyethersulfone (PES) substrate membrane using atomic layer deposition (ALD) with the aim of tailoring the membrane surface properties to be suitable for desalination applications. Scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA) and zeta potential measurements and a tensile meter were used to characterize the membrane morphology, surface properties and mechanical stability, respectively. In addition, the separation performance of all of the prepared membranes was evaluated in terms of water flux and salt rejection. The results showed that the TiO2 nanostructured film deposited-PES membrane exhibited excellent performance with a rejection of ≥90 % at room temperature for NaCl, which is four times greater than that of a PES membrane alone. It is interesting to note that the deposition of the TiO2 film resulted in a marginal decrease in the water flux from 60 ± 2 Lm−2 h−1 to 47 ± 2 Lm−2 h−1 of the resulting membrane due to the TiO2 film’s nanometre-scale thickness. Moreover, the ALD of the TiO2 film enhanced the mechanical strength of the membrane as it tightly wrapped the skeleton of the membrane.
* Javed Alam
[email protected]
1
King Abdullah Institute for Nanotechnology, King Saud University, P.O. Box- 2455, Riyadh 11451, Kingdom of Saudi Arabia
2
Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Kingdom of Saudi Arabia
3
Department of Physics & Astronomy, College of Science, King Saud University, P.O. 2454, Riyadh 11541, Saudi Arabia
Keywords Polyethersulfone . Nanostructured film . Atomic layer deposition . Separation applications . Mechanical strength
Introduction Polyethersulfone, PES, is one of the most extensively investigated membrane materials in the literature for preparing UF membranes because of its desirable thermal, mechanical, chemical and dimensional stability; biocompatibility; and outstanding film-forming properties [1, 2]. However, the hydrophobicity of the PES limits its application in membrane separation processes. To overcome this limitation, PES membranes are often modified with various approaches, such as bulk modification of the PES polymer (such as copolymerization and hydrophilic groups functionalization; surface modification by coating and grafting techniques; blending PES with hydrophilic polymers, such as polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG) and nanomaterials to develop mixed matrix membranes [3, 4]. In addition, the deposition of organic/inorganic thin films on the PES substrate membrane intended to develop a thin film composite (TFC) membrane have experienced tremendous development over the past decade [5–12]. TFC membranes were not only used dominantly in the desalination industry but also showed huge potential to be used in other applications including gas separation, pervaporation, forward osmosis, etc. [13–16]. The research into improving TFC membranes is focused on three main areas: (1) top active layer improvements using new monomers and/or dispersing nano-scale particles within the top active layer with materials such as TiO2, carbon nanotubes (CNTs), zeolites; (2) the development of chemically and mechanically stable substrate membranes; and (3) the modification of the conventional interfacial polymerization (IP)
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technique [16–27]. This paper is focused on producing an innovative top active layer of titanium dioxide using an atomic layer deposition (ALD) technique onto a PES substrate membrane intended to develop a new generation of TFC membranes. ALD is powerful tool for depositing functional films with nanometre-thick precision on a wide variety of substrate membranes, and the deposited films are strongly adhered to the surface [28–34]. Many recent published reports have demonstrated its potential in surface modification of microfiltration and ultrafiltration (MF/UF) membranes to produce TFC membranes intended for nanofiltration (NF) and reverse osmosis (RO) applications [35–41]. In the current study, a TiO2 thin film was grown on a polyethersulfone (PES) substrate membrane using ALD with TiCl4 and deionized water as precursors. The aim was to upgrade the PES membrane surface properties to make them suitable for desalination applications. To the best knowledge of the author, ALD of TiO2 film with ≤100 nm thicknesses has not been yet explored for the modification of PES membranes. TiO2 was chosen in this work because it has a much stronger chemical resistance to acids and bases, has a low cost and is readily available compared to other metal oxides currently reported in the literature, such as Al2O3, and SiO2 [30, 42, 43]. In addition, TiO2 is distinct in its ability to function as a photocatalyst, which is applicable to self-cleaning, destruction of bacteria, etc. [44, 45]. During our investigation, we observed that the salt rejection and the permeate flux of the TiO2 thin film membranes were significantly improved showing that ALD of TiO2 is a suitable procedure for upgrading PES membrane separation performance.
Experimental Materials Polyethersulfone was kindly supplied by Solvay, Advanced Polymers, USA. Analytical grade N-methylpyrrolidone (NMP; 99.5 %), glycerol, sodium chloride, potassium chloride, acetone, and ethanol (99.8 %) were purchased from the Sigma-Aldrich and used as received. Titanium tetrachloride, TiCl4, and deionized water have been used as precursors for the deposition of the TiO2 thin films. De-ionized water used in all experiments was produced by a Milli-Q unit (Millipore, USA) with a resistivity of 18.2 MΩ cm. and sodium dodecyl sulphate (SDS) were used as the external coagulation bath medium. PES membrane fabrication PES substrate membrane was prepared using the wet-phase inversion method using N, N-methylpyrrolidone (NMP) as solvent and water and SDS as the coagulation medium [46].
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Briefly, 3.5 g of pre-dried (12 h in an oven at 45 °C) PES powder was mixed with 18.7 ml of the NMP solvent contained in a round bottle flask. The polymer dope mixture was then placed in a 55 °C isothermal water bath and continuously stirred using an overhead stirrer (IKA® WERKE) at 100 rpm for 48 h. When a clear homogeneous solution was obtained, the polymer solution was degassed at 25 °C for at least 24 h to remove air bubbles. Subsequently, the solution was cast on a glass plate using an automatic film casting machine. The casted polymer was then immersed into a nonsolvent bath, in which phase inversion started, and the membrane was formed. Finally, the membrane was dried by placing it between two sheets of filter paper, and this drying process was repeated three times to remove the water from the membrane. The membrane was then stored in a tank containing a 60 % glycerol aqueous solution. Deposition of the TiO2 film on the PES membrane surface Prior to the ALD of the TiO2, the membrane sample was inspected using a light box to detect defects, such as thin spots, and shrinkage, as well as rigidified polymer. Briefly, the TiO2 film on the PES membrane surface was grown in a lowpressure flow-type ALD reactor using TiCl4 and deionized water as precursors. To achieve this, a pulsing scheme was used with TiCl4/N2/H2O/N2 pulsing times of 0.1 s/2 s/ 0.3 s/ 3 s. The flow rate of the carrier gas at the deionized water and at the TiCl4 line flow was adjusted to 200 sccm and 150 sccm with pressures of 8 hPa and 9 hPa, respectively. The deposition was carried out through a series of cyclic operations and each cycle was further divided into two half cycles. In the first half cycle, the TiCl4 pulse was carried out in the reaction chamber using a pure nitrogen (99.9999 %) carrier gas, and self-terminating reactions occurred on the substrate. The residual, non-reacted reactant and the byproduct were removed from the chamber by purging. In the remaining half cycle, deionized water was pulsed through the reaction chamber by the carrier gas, which causes the self-terminating reaction and the by-product to be purged. The total number of cycles used for the deposition of the TiO2 film was 1000. Membrane characterization Microscopic examinations Scanning electron microscopy (SEM) JEOL, Japan was used to examine the surface and the cross-section morphology of the membrane samples. The cross-sections were obtained by fracturing the membrane in liquid nitrogen. Prior to imaging, the samples were sputtered with platinum to a thickness of approximately 10 nm at 20 mA for 50 s. The SEM studies were conducted at an acceleration voltage of 5 and 10 kV and a working distance of 8 mm using various magnifications.
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Atomic force microscopy (AFM, Veeco MultiMode SPM with a Nanoscope V controller) Bruker, United States was used to analysis the surface topography of the membranes. For the AFM study, the membrane samples were cut into small square pieces (0.5 × 0.5 cm2) and pasted on a metal substrate. AFM was performed in tapping mode using a silicon tip with a radius of ~10 nm at a scan frequency of ~70 kHz. The roughness parameters, which were the average roughness, Ra, and the root-mean-square roughness, Rq, were determined using the Nanoscope software. The measured areas of the membrane were 5 × 5 μm and 1 × 1 μm. Surface properties analysis A contact angle meter (Model Atension; MAC 200, Netherland) was used to measure the surface hydrophilicity using the sessile drop method at 25 °C. For the experiment, the membrane were first dried under a gentle stream of instrument grade air, and then, the sample with a size of about 3 × 1 cm and a thickness of 0.165 mm was attached to a glass slide and was mounted on the tensiometer. Then, 3 μL deionized water droplets were dropped on the surface at five random locations for each membrane sample. The contact angle was measured on both sides of the droplet. Droplet images were recorded using a digital camera and were analysed using computer software by adjusting the diameter of the drop and the contact angle with the surface. The average of five contact angle values was reported. An electro-kinetic analyser (SurPASS, Anton Paar GmbH, Austria) was used to determine the zeta (ζ) potential of the membrane samples by using tangential streaming potential measurement. The experiment was performed at 25 °C by circulating 1 mM electrolyte solution of potassium chloride through an adjustable gap cell containing the square shape membrane samples (10 × 20 mm) with a pressure ramp from 0 to 300 mbar. The pH measurements were based on the original pH of the electrolyte solution (5.8), which was decreased stepwise (0.5–1 units) by titration with a 0.05 M hydrochloric acid (HCl) solution until a pH of 2.5 was achieved. The experiment provides the charge characteristics and iso-electric point (IEP) of the membrane surface. In past authors [47] have examined Helmholtz–Smoluchowski (H-S) correlation following equation to determine the zeta potential value. ζ¼
ΔU η L Δp ε ε0 D
ð1Þ
where, ΔU and ΔP – streaming potential (mV) and applied pressure (Pa); L & D were channel length (m) and cross
section area (m2); η - Viscosity of the solvent in the bulk (cP); ε & ε 0 −Dielectric permittivity (C 2 . J−1.m −1 ) and Permittivity of free space (8.854 × 10−10 C2.J−1.m−1). Tensile test A tensile test was conducted on a Lloyd-LR5KPlus (USA) with a load cell capacity of 100 N. The tensile properties including the tensile stress (MPa) and strain (%) were measured using the NEXYGENPlus software. Three samples were tested for each membrane to obtain the average value. The operating parameters were the following: the length of the specimen in the gauge section was 25 mm; the width was 7 mm; the thickness was 0.20 mm; the applied stroke speed was 15 mm/min; the measuring range was 0–10 MPa; and the test temperature was 25 °C. Water flux and rejection tests Water flux and salt rejection tests of the prepared membrane samples were performed using a CF042 Crossflow Cell (Sterlitech, USA). This filtration cell (CF042, Sterlitech) had an effective membrane area and a thickness of 42 cm2 and 0.165 ± 0.02 mm, respectively. To accurately measuring the water flux, membrane compaction was first performed in a constant transmembrane pressure (TMP) 7 bar up to 3 h for steady-state flux. Finally, the membrane water flux was calculated at room temperature using the following equation: Jw ¼
Q A:t
ð2Þ
where Jw (l/m2 h) is the pure water flux, Q (l) is the volume of the permeate, t (h) is the permeation time, and A (m2) is the effective membrane area. For the rejection experiment, a 500 mg L−1 of an aqueous NaCl solution was used as the feed solution. The salt concentrations at the feed and permeate solutions were measured using an atomic absorption spectroscopy (AAS). The rejection (%) was calculated using the following equation: Cp Rejection ð%Þ ¼ 1− 100% ð3Þ Cf where Cf and Cp are the salt concentrations in the feed and permeate solutions.
Results and discussion Membrane morphology Figure 1 shows the SEM images of the surface and the cross-section of the pristine PES membrane and the TiO2
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Fig. 1 Cross-section and surface SEM images of the pristine PES membrane and the TiO2 film deposited − PES membrane
film deposited-PES membrane. As seen from Fig. 1a and b, the PES membrane exhibited an asymmetric structure containing a top layer and a porous middle layer full of uniform finger-like macrovoids, and the support layer had a sponge-type structure with dense and interconnecting pores over its entire thickness. After the ALD of the TiO2 film, the membrane morphology was nearly identical to the pristine PES membrane except for the deposited ultra-thin TiO2 film on the top active layer, which can be clearly seen in the SEM image in Fig. 1c. It is a very evident that the asgrown TiO 2 film was continuous, uniform and dense (Fig. 1d), and the thickness of the film was approximately 100 nm, and the grown film tightly wrapped the skeleton of the PES membrane. In the AFM analysis, as shown in Fig. 2a, the pristine PES membrane had a surface with a layer having a Bridge-and-valley^ structure distributed throughout the plane, which is the typical morphology of the substrate PES membranes [1, 10]. After the ALD of the TiO2, the membrane surface texture changed, as shown in Fig. 2b and c. Most ridge-and-valleys were roofed by the deposited TiO2 film. In addition, the deposition of TiO2 film showed a significant impact on the membrane surface roughness. The membrane with ALD-grown film showed less rough surface than a pure PES membrane, as shown in Table 1.
Mechanical properties Figure 3 shows the stress-strain curves of the PES membrane and the TiO2 film deposited −PES membrane. As seen in the stress-strain curves in Fig. 3, the deposition of the TiO2 film efficiently enhanced the mechanical properties of the resulting membrane and increased the tensile break from 4.28 MPa to 4.50 MPa. This result is due to the strong interphase interaction between the TiO2 film and the PES substrate membrane, which reduces the stress concentration point when a tensile load is applied to the membrane. In addition, such an evident increase in the tensile strength is attributed to the dense TiO2 film tightly wrapped with the skeleton of the PES membrane and acted as a cushion to enhance the mechanical strength of the membrane. However, the membrane with the deposited TiO2 film was fractured at a low strain, which demonstrates that the TiO2 nanostructured film deposited − PES membrane is less flexible than the PES membrane. Contact angle The contact angle, θ, is the most commonly used parameter to describe the membrane surface’s hydrophilicity/hydrophobicity. A hydrophilic membrane surface means that the surface has the ability to interact with polar molecules, such as water.
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Fig. 2 AFM 2D and 3D images of the pristine PES membrane and the TiO2 film deposited − PES membrane
On a hydrophilic membrane, the contact angle is less than 90o, whereas on a hydrophobic membrane, the contact angle exceeds 90o. Figure 4 shows the contact angle values of the modified and unmodified PES membranes. As presented in Fig. 4, the contact angle of the PES membrane was 62 ± 1o. The contact angle decreased for the TiO2 film deposited − PES membrane. The contact angle was 55 ± 1°, which demonstrates that the TiO2 film deposited − PES membrane has a greater ability to interact with water. The decrease in the contact angle of the ALD modified membrane is related to the influence of the TiO2 film deposited on the membrane surface as TiO2 has high affinity for water molecules [44, 48]. In addition, decreasing the contact angle value of the TiO2 film deposited − PES membrane can be accredited to the surface roughness. Overall, the hydrophilicity of the PES membrane was found to clearly increase upon the deposition of TiO2 film, which means that the TiO2 film has a significant impact on the membrane hydrophilicity.
Table 1
Zeta potential Figure 5 shows the zeta (ζ) potential as a function of the pH for the PES membrane and the TiO 2 film deposited − PES membrane. As seen in the results, for a pH range 2.0 to 3.6, the PES membrane showed a positive zeta potential, and its isoelectric point (the pHiep) appeared to be around pH 3.6. The zeta potential increased with more negative charges at alkaline pH values due to proton desorption and hydroxyl anion adsorption [47]. As with the TiO2 film deposited − PES membrane, the (ζ) potential was also positive at low pH values and negative at high pH values. The increasing negative charge at high pH values can result from adsorption of anions. Moreover, the iso-electric point (IEP) of the TiO2 film-PES membrane was found to be at pH 4.4, which is greater than that of the PES membrane, which demonstrates that the TiO2 film-PES membrane has more negatively charged groups on the its surface.
Surface roughness parameters of all prepared membranes
Membranes
Surface Roughness Parameters Ra (nm)
Rq (nm)
Membrane performance
Pure PES TiO2 film deposited − PES
18.2 14.5
24.4 18.1
The membrane performance characteristics were evaluated by measuring the rejection % of NaCl and the water flux. The results are shown in Figs. 6 and 7. From the
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Fig. 3 Stress–strain curves of the pristine PES membrane and the TiO2 film deposited − PES membrane
results, it can be clearly seen that the deposition of the TIO2 film boosted the membrane performance compared w i t h t h e p u r e P E S m e m b r a n e . T h e Ti O 2 f i l m deposited − PES membrane showed a NaCl rejection of ≥90 %, which is four times greater than that of the PES membrane alone. This abrupt increase the salt rejection occurred with the TiO2 film deposited membrane due to the thin and dense skin TiO2 layer. With respect to the water flux, it is interesting to note that the deposition of the thin TiO 2 film resulted in a marginal Fig. 4 Contact angle value of pure PES and TiO2 film deposited − PES membrane
decrease in the water flux due to the greater hydrophilicity of the TIO2 caused by the greater affinity of TiO2 to water. In addition, results showed that the salt rejection increased with increasing of TiO2 film thickness. This is accredited to the increasing TiO 2 content, resulting in a greater negative change on the membrane surface, thus membrane repulse more anions. On the other hand, water flux was decreased with increasing of TiO2 film thickness. A small reduction in the flux, as seen with the modified membrane, can also be
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Fig. 5 The zeta (ζ) potential as a function of the pH for the pristine PES membrane and TiO2 film deposited − PES membrane
connected to the thickness of the nanoscale as-deposited TiO2 film, which is known to give very little resistance to the flow of water. Moreover, a decrease in the water flux can be partly caused by the decreased surface roughness of the TiO2 film-PES membrane, as seen in the AFM images. In summary, the deposition of the TiO2 thin film upgrades the PES membrane to a greater degree in terms of the water flux and salt rejection efficiency.
Fig. 6 NaCl rejection of the pristine PES membrane and the TiO2 film deposited − PES membrane
Conclusion In the current study, we demonstrated that a titanium dioxide (TiO2) thin film grown by ALD is a simple and effective method to upgrade the separation performance of a PES membrane by improving the membrane surface hydrophilicity and roughness. The research findings revealed that the TiO2 film deposited − PES membrane resulted in a salt rejection increase of ≥33 ± 2 % compared with the pristine PES membrane. In
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Fig. 7 Water flux of the pristine PES membrane and the TiO2 film deposited − PES membrane
addition, a marginal decrease in the water flux from 60 ± 2 Lm−2 h−1 to 47 ± 2 Lm−2 h−1 was observed with the deposition of the TiO2 film. This result was likely due to the combined effects of (1) the greater hydrophilicity of the TiO2 due to the greater affinity of TiO2 to water and hydrolysis with hydroxyl groups; (2) the thickness of the nanoscale as-deposited TiO2 film, which imparted very little resistance to the flow of water; and (3) the decreased surface roughness, as it provided less membrane surface area to contract water molecules. The ALD of TiO2 was found to result in an increase in the mechanical strength of the resulting membrane as it tightly wrapped the skeleton of the PES membrane.
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10. Acknowledgments The authors are thankful to the financial support from the King Abdullah Institute for Nanotechnology, Deanship of Scientific Research, King Saud University; Riyadh, Saudi Arabia.
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