Effect of templating agents on the properties and membrane distillation performance of TiO2-coated PVDF membranes

Journal of Membrane Science 450 (2014) 48–59

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Effect of templating agents on the properties and membrane distillation performance of TiO2-coated PVDF membranes Suwan Meng a, Jaleh Mansouri a,b,n, Yun Ye a, Vicki Chen a a b

UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia Cooperative Research Centre for Polymers, Notting Hill, VIC 3168, Australia

art ic l e i nf o

a b s t r a c t

Article history: Received 13 May 2013 Received in revised form 16 July 2013 Accepted 28 August 2013 Available online 3 September 2013

Porous polyvinylidene fluoride membranes were coated with TiO2 and 1H,1H,2H,2H-perfluorododecyltrichlorosilane (FTCS) to create superhydrophobic surfaces for membrane distillation. The effect of the templating agents used in the sol–gel for TiO2 deposition was investigated. Results indicated that the use of a templating agent resulted in porous, multi-level microstructures. The final microstructure, nanostructure, wettability and desalination performance of the membranes were affected by both the physical and chemical properties of the templating agent. Coatings which did not use a templating agent resulted in greater deposition of TiO2, but lacked the hierarchical structure of their templated counterparts, leading to lower overall hydrophobicity. All modified membranes displayed a significant improvement in salt rejection, indicating a substantial reduction in pore wetting. Membranes templated with polyethylene glycol were found to have the optimum properties for desalination via membrane distillation. & 2013 Elsevier B.V. All rights reserved.

Keywords: Direct contact membrane distillation Superhydrophobic Composite membrane Titania sol–gel

1. Introduction Water scarcity around the world has seen an increasing amount of interest in novel desalination processes in recent years. Membrane distillation (MD) is a promising candidate, where a hot feed stream containing non-volatile species is passed across a hydrophobic membrane [1]. A cooling stream on the other side of the membrane creates a vapour pressure difference across the membrane, which causes the feed liquid to evaporate and pass through the pores into the permeate stream. However, a big setback in the industrial implementation of membrane distillation is the low initial permeate flux and the rapid decline in flux over time compared to conventional processes, such a reverse osmosis and multi-stage flash distillation, which is partly caused by mass transfer inefficiencies. In simple MD configurations, vapour moving across the membrane is subject to mass resistance by air trapped in the pores, and by the physical structure of the pores [2]. To overcome this resistance, the permeate stream can be placed under vacuum [3,4]. Although the permeate flux increases, the risk of pore wetting also increases. Penetration of the feed water into the membrane severely compromises the efficiency of the process. The main cause of pore wetting is exceeding the liquid pressure of the membrane. This pressure is n Corresponding author at: UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia. Tel.: þ 61 293854328. E-mail address: [email protected] (J. Mansouri).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.08.036

dependent on several membrane properties; most notably the membrane pore size, hydrophobicity, surface roughness, and chemical resistance to the feed solution [5]. However, when the operating pressure is lower than the liquid entry pressure, pore wetting may still occur via partial pore wetting. Partial wetting of the membrane is generally caused by changes in the membrane properties during the process, particularly due to fouling or scaling [6]. Deposition of foulants or crystals on the membrane surface causes the pores adjacent to the deposit to be filled with liquid. The degree of partial wetting can be controlled by using an appropriate material. Experiments conducted by Srisurichan et al. using humic acid and salt indicated that when a highly hydrophobic membrane was used, deposition of these two compounds occurred on the surface only, not within the pores, therefore decreasing the occurrence of wetting [7]. It stands to reason that the use of a superhydrophobic surface on the feed side would further reduce the possibility of pore wetting. At the present time, the reported membrane distillation performance of superhydrophobic membranes is extremely scarce. However, existing literature have indicated that the use of superhydrophobic membranes improved the long-term salt rejection and flux recovery after fouling [8–10]. A surface is considered superhydrophobic when the static water contact angle exceeds 1501 and the contact angle hysteresis is less than 101 [11]. Various methods have been developed to fabricate superhydrophobic coatings, all of which involve surface roughening or the use of a low energy material. But only a fraction of these methods is suitable for polymeric membranes. In many

S. Meng et al. / Journal of Membrane Science 450 (2014) 48–59

studies, the surface of an intrinsically hydrophobic polymer is etched to increase the roughness [12,13]. However, not only is this process energy intensive, it is not practical for porous membranes due to the irregular topography. Other studies have overcome this issue by depositing surface modified particles onto the polymer substrate [14,15]. Despite achieving superhydrophobicity at lower temperatures, this method requires an additional preparation step prior to deposition to reduce the surface energy of the particles. This lacks the simplicity of more facile methods, such as sol–gel synthesis. Blending or grafting a low energy material to the membrane has also become a popular approach to achieving superhydrophobicity [16–18]. However, this can impede further modifications of the membrane for optimising membrane distillation performance, such as the fabrication of a hydrophobic– hydrophilic composite membrane to improve both mass transfer and heat transfer [19–21]. Recently, Razmjou et al. combined the low surface energy of fluorinated silanes with the surface roughness created by TiO2 nanoparticles to fabricate superhydrophobic polyvinylidene fluoride membranes via a low temperature hydrothermal coating method [8]. The TiO2 coating layer on the membrane provided sites for covalent bonding with hydrolysed silane coupling agents, effectively inducing superhydrophobicity one side of the membrane via a self-assembled monolayer. The multi-level hierarchical structure of the TiO2 coating prior to flouro-silane deposition was found to play an active role in determining the final wettability of the surface. This structure was attributed to the templating agent, but further explanations were not given as to the exact correlation between template properties, nano-architecture and wettability of the coating. The use of organic additives, such as molecular surfactants or block copolymers, during sol–gel synthesis allows the organic precursor species to form liquid crystal structures around or within the organic species [22,23]. Removal of these organic species leaves behind a solid inorganic framework [24]. Conventionally, non-ionic surfactants such as polyethylene glycol, Brij 58, Triton X100, Pluronics F127 and Tween 20 have been used as pore-directing agents to increase coating porosity and contacting surface area [25]. Several studies on the photocatalytic properties of TiO2 films have established that using high molecular weight templating agents resulted in thicker, more porous (less dense) titania films [25–28]. Increasing the concentration of the templating agent saw a similar increase in the thickness and porosity of the coatings [29]. However, a critical concentration for each templating agent exists, where the uniformity of the coating is disrupted if that value is exceeded [29–31]. These templating agents have also been known to affect the size of the inorganic crystals [26], although the specific relationship has yet to be determined. Ionic surfactants, such as cetyltrimethylammonium bromide, have also been used as pore-generating templates for inorganic coatings [32–34]. Pan et al. asserted that ionic surfactants have better interaction with inorganic sol compared to block copolymers, although these lower molecular weight surfactants may cause pore-wall collapse during calcination of transition metal oxides [35]. Similar to their non-ionic counterparts, an increase in the templating agent concentration increases the pore size [36],

49

although too high a content results in the collapse of the mesostructure during heat treatment [31]. Jiu et al. reported that further control of inorganic particles can be achieved by the combination of ionic and non-ionic templating agents, where self-modifications to the micelle structure results in different sized nanocrystals [36]. Despite much work already undertaken on the effect of templating agents in inorganic coatings, the majority of studies on templating agents in current literature have been conducted using inorganic substrates and high calcination temperatures [37,38], which are not suitable for polymers. Furthermore, these studies focused on thin films, where the structure is typically limited to two dimensions. Since templating agents have been mainly used to generate pores and increase the superficial area, inorganic coatings reported in literature are almost always hydrophilic [39], which consequently limits the applications of the coatings. Therefore, more research is required to examine the three dimensional effects of templating agents on inorganic coatings deposited on polymeric membrane substrates. The objective of this study is to investigate the effects of templating agents on the structure and properties of TiO2-coated polymer membranes, how this impacts superhydrophobicity, and the changes in performance when applied in membrane distillation. Four different templating agents were introduced into the TiO2 sol–gel, and coatings were prepared via the low temperature hydrothermal process [8,40]. The surface structure, chemistry and wettability of modified membranes were examined, and the desalination performance was tested in a direct contact membrane distillation system.

2. Experimental 2.1. Materials Commercial hydrophobic polyvinylidene fluoride (PVDF) flatsheet membranes (HVHP, Millipore, 0.45 mm) were used as the substrate for this study. The contact angle of the virgin membrane was measured to be 112 721. Titanium (IV) isopropoxide (97%), acetylacetone (99%), Pluronics F-127, polyethylene glycol (1000) and 1H,1H,2H,2H-perfluorododecyltrichlorosilane were purchased from Sigma Aldrich. Toluene, anhydrous ethanol and sodium chloride were supplied by Ajax Finechem. Perchloric acid (70%) was purchased from Chem-Supply. Silicone fluid IM-22 was purchased from Wacker, and cetyltrimethylammonium bromide was provided by BDH. 2.2. Coating and post-treatments TiO2 coatings were prepared using the procedure developed by Razmjou et al. [8,40]. Anhydrous ethanol, perchloric acid (HClO4), acetylacetone (AcAc), titanium (IV) isopropoxide (TTIP) and Milli-Q water were mixed at 650 rpm in room temperature to form a precursor sol. Another solution consisting of a templating agent in anhydrous ethanol was prepared and left to stir simultaneously with the precursor sol. Details of the four templating agents investigated are given in Table 1. After 1 h, the two solutions were

Table 1 Templating agents investigated in this study. Templating agent

Abbreviation

Formula

Mw (g/mol)

Classification

Polyethylene glycol Pluronics F-127 Wacker IM-22 Cetyltrimethylammonium bromide

PEG F127 IM22 CTAB

C2nH4n þ 2On þ 1 EO100PO65EO100 EO15PDMS15EO15 C19H42BrN

1000 12,600 2578 364

Hydrophilic polymer Amphiphilic block copolymer Amphiphilic block copolymer Cationic surfactant

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mixed together and left to stir for another hour to form a sol–gel. The final molar ratio of TTIP:HClO4:AcAc:H2O in the sol–gel was 4.76:1:0.5:0.5:0.45. The molar ratio of TTIP to templating agent was varied between 1:0.002 and 1:0.016. For ‘No TiO2’ samples, a sol–gel containing only ethanol, HClO4, AcAc and water was used. Membranes were dip coated in the sol–gel with a holding time of 8 s and a withdrawal speed of 0.2 mm/s. The coated membranes were dried in a 120 1C oven for 16 h, then rinsed and placed into a 90 1C water bath for 24 h. The membranes were rinsed again, and UV irradiated in water for 24 h. Finally, the membranes were left to dry in ambient temperature for 24 h. This process, labelled a ‘coating cycle’, was repeated up to 4 times for selected samples. To achieve superhydrophobicity, a toluene solution containing 5 wt % 1H,1H,2H,2H-perfluorododecyltrichlorosilane (FTCS) was filtered through modified membranes under low vacuum. The FTCS is believed to anchor to OH functional groups on the TiO2 coating via covalent bonding [8]. FTCS solutions were maintained below 0 1C to minimise surface energy [41]. A standard of 1 mL of FTCS solution per 1 cm2 of membrane was used. Filtered membranes were then placed into a 120 1C oven for 2 h, before backwashing with 30 vol% ethanol for 5 min at 100 kPa to remove any FTCS trapped within the pores. The overall process is highlighted in Fig. 1. 2.3. Characterisation 2.3.1. Structure and composition The surface morphology of the membrane was characterised by field emission scanning electron microscopy (Hitachi S900). Samples for FE-SEM were prepared by sputter coating a thin layer of chromium under vacuum to generate conductivity. The surface composition was determined using X-ray photoelectron spectroscopy (VG Scientific), where samples of approximately 10 mm  10 mm were irradiated by soft X-rays. Electrons ejected from the top 5 to 10 nm of the sample generate a spectrum and allow elemental quantification to be carried out. The surface chemistry of the membranes after desalination tests was analysed using energydispersive X-ray spectroscopy (Hitachi 3400) to check for the presence of salt. Samples for EDS were coated with a thin layer of carbon prior to analysis.

Fig. 1. Overview of the membrane modification process.

Fig. 2. Schematic of the DCMD setup used to test desalination performance.

To investigate the changes in the loading of TiO2, thermogravimetric analysis (TA instruments TGA5000) was carried out in air at a heating rate of 20 1C/min up to 1000 1C. Residual weight of the samples and its weight derivative as a function of temperature were recorded. Specific surface area of the membranes was determined using BET (Micromeritics TriStar 3000). Prior to analysis, membranes were dried in a vacuum oven at 150 1C for 3 h. 2.3.2. Wettability Static contact angles were measured using a contact angle goniometer (KSV CAM 200) by the sessile drop method. Reported values are the average of at least 4 measurements. For hydrophilic samples, 60 frames were captured for each water droplet, with a onesecond time interval between frames. Liquid entry pressure (LEP) of water for each membrane was measured using a dead end membrane filtration system. The LEP of selected samples were verified with a direct contact membrane distillation setup. The feed pressure was increased in steps, with intervals of 3 min. The pressure at which the first drop of permeate was observed is recorded as the LEP. 2.4. Direct contact membrane distillation The desalination performance of the membranes was tested in a direct contact membrane distillation (DCMD) system, shown in Fig. 2. A horizontal counter-current flow configuration was used with a membrane area of 14.6 cm2. The feed solution consisted of 10 wt% sodium chloride (Λ ¼166 mS) to model high concentration saltwater or brine, as well as to speed up the wetting process. The inlet feed temperature, pressure and flow velocity was set to 60 1C, 25 kPa and 0.2 m/s respectively. The permeate temperature, pressure and flow velocity was set to 25 1C, 23 kPa and 0.4 m/s respectively. A volume of 300 mL was used for the circulating cold water in the permeate stream. For each test, the permeate flux and conductivity were measured over 24 h.

3. Results and discussion 3.1. Effect on TiO2 deposition Each post treatment step (in Section 2.2) serves a different purpose. Heat treatment at 120 1C removes the solvent and other volatile chemicals in the sol–gel. It is likely that preliminary templating agent decomposition also occurs at this stage. Wet heat treatment at 90 1C allows hydrothermal reactions to occur, which produces nuclei that subsequently promote crystal growth [42]. In the presence of hot water, the templating agent begins to leach out from the coating as well. Final UV irradiation in water removes the majority of residual templating agents from the coating. The microstructure at each stage of post treatment is shown in Fig. 3. Free floating particulates observed in the water after the 90 1C treatment step for all coated membranes indicate the loss of weakly bonded TiO2 from the coating. Frothing was observed during movement of the water, signifying the additional removal of the templating agent. These simultaneous actions leave behind the general microstructure seen in Fig. 3C. Removal of the templating agent via leaching during UV irradiation increases porosity (Fig. 3D), resulting in the final micro- and nanostructure. For this reason, further discussions on structural and chemical differences of coatings refer to the coating after UV irradiation. 3.1.1. Microstructure After one coating cycle, a significant microstructural difference between templated and non-templated coatings was detected. Coatings which did not use a template (Fig. 4A) followed the structure of the PVDF substrate (Fig. 3A). Coatings templated with

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Fig. 3. SEM images of (A) uncoated PVDF membrane, (B) coated membrane using a TTIP:template molar ratio of 1:0.004 M ratio of F127 after 120 1C, (C) after 90 1C in water, and (D) after UV irradiation. Images are at 100k magnification.

Fig. 4. SEM images of TiO2 coatings using TTIP:template molar ratio of 1:0.016, 1 cycle and after UV irradiation using (A) no template, (B) F127, and (C) CTAB. Images are at 200k magnification.

Fig. 5. SEM images of TiO2 coatings using a TTIP:template molar ratio of 1:0.008 after 1 cycle, where the template is (A) PEG 1000, and (B) PEG 400. Images are at 200k magnification.

F127 produced a sharp microstructure (Fig. 4B). Similar microstructures were observed for coatings templated with PEG and IM22. Coatings templated with CTAB appeared spherical and with deeper porosity. The difference between the microstructures seen in Fig. 4B and C is partly attributed to the molecular weight or chain length of the templates. Using lower molecular weight templates in coatings has been known to result in smaller pores and narrower pore size distributions [43], which leaves behind a tighter structure with more TiO2 upon the removal of the template. This was verified by using PEG 400 (Fluka, Mw ¼380–420 g/mol) as a template and

comparing the final structure with coatings templated with PEG 1000. As these two templating agents are identical in structure and chemistry, differences in the resulting coating microstructure must be due to the chain length or molecular weight. It is evident in Fig. 5 that coatings templated with lower molecular weight PEG resulted in a denser microstructure, analogous to that seen in CTAB-templated coatings. However, there is still a significant difference in structure between PEG 400 and CTAB-templated coatings, indicating that the coating microstructure is also dependent on other factors, particularly the chemistry of the templating agent.

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Fig. 6. SEM images of TiO2 coatings at 1:0.016 M ratio, 4 cycles and after UV irradiation using (A) no template, (B) F127, and (C) CTAB. Images are at 200k magnification.

The structural differences for each template at different concentrations were considered negligible, although a slight increase in the apparent coating porosity and uniformity was seen with increasing template concentration. After 4 coating cycles, the general microstructures appear similar for all coatings (Fig. 6). Clusters are more evident compared to when only one coating cycle was used, and the coating porosity is more prominent. The main difference between the coatings is the cluster size. Coatings which did not use a template had larger clusters compared to templated coatings, indicating that more TiO2 may be present for non-templated coatings. Templated coatings had relatively similar cluster sizes for each coating cycle. 3.1.2. Nanostructure After four cycles, differences in the nanostructures were observed at all template concentrations. Coatings templated with IM22 and F127 (Fig. 7A and D) have similar nanostructures, although the clusters for F127 are slightly larger and with greater apparent porosity. This is most likely due to F127 having longer chains compared to IM22, resulting in the deposition of thicker films and deeper porosity when the template is removed [26]. Coatings using PEG have a sharper nanostructure compared to the other templates (Fig. 7C), which could be the resulting structure of hydrogen bonding at the non-micellar TiO2–PEG interface [44]. CTAB-templated coatings (Fig. 7B) have a similar nanostructure to coatings which did not use a template, which suggests that the nanostructural effect of CTAB is relatively low, although a more hierarchical structure is evident. Once again, this is attributed to the low molecular weight of CTAB. A distinctive nanostructure was not observed for coatings which did not use a templating agent. 3.1.3. Surface chemistry Based on XPS elemental quantification (Fig. 8), there is an evident increase in the amount of O1s after coating the membranes, indicating an increase in OH functional groups [45]. After one cycle, coatings deposited with and without a templating agent appear to have similar quantities of Ti and O. It should be noted that XPS analysis is generally limited to the top 10 atomic layers of the surface; therefore the values presented are not necessarily representative of the bulk of the coating. FTIR-ATR was carried out to detect changes in bonding; however the amount of coating relative to the amount of PVDF substrate was too low to pinpoint any significant difference. 3.1.4. FTCS deposition Although the TiO2 coated membranes had different structures prior to FTCS filtration, the final structure of the coating after FTCS

deposition (Fig. 9) appeared to be similar across all samples. No obvious difference in the micro- or nanostructures of the FTCS filtered coatings was observed in SEM imaging. However, a difference was seen in the wettability and membrane distillation performance of the modified membranes, indicating that the properties of the underlying TiO2 coating layer affected the FTCS coating layer. 3.2. Effect on wettability 3.2.1. Contact angle After UV irradiation, most coatings were found to be hydrophilic. This is in accordance with the results reported by Razmjou et al. [40], where hydrophilicity occurs as a result of capillary phenomena. Contact angles were found to decrease over time for most samples due to the spreading of water across the coating surface. In Table 2, coatings which did not use a template had the highest degree of hydrophilicity. This is partly due to higher amounts of TiO2 promoting water molecule adsorption onto the coating surface [46], which enhances the hydrophilicity of the coating. The lack of a templating agent also means that residual hydrophobic compounds were not present in the coating, thus there is less obstruction to the spreading of water. However, the use of a hydrophobic membrane substrate prevents full penetration of water into the membrane, which results in the eventual stabilisation of the contact angle above 01 at ‘t¼ 60 s’. The exception to this is non-templated membranes after four coating cycles, where an increase in the porosity and the amount of TiO2 present allows full spreading of water without penetration into the membrane. At equal templating agent concentration, an increase in the number of coating cycles increases the hydrophilicity for coatings which used PEG (Table 2). Similar results were seen for coatings which used CTAB and F127. This is due to increasing surface roughness as TiO2 content increases [47], which is evident in Fig. 7. Coatings which used F127 and PEG had relatively similar contact angle values for each cycle. Although following the same trend, coatings which used CTAB had higher overall contact angles, with the lowest at approximately 601. This may be the result of residual hydrophobic alkyl chains from CTAB, which did not leach out during post-treatment. Coatings which used IM22 remained hydrophobic regardless of the number of coating cycles (Table 2). Since the microstructure for coatings which used IM22 and PEG were relatively similar, the difference in wettability must be due to the chemistry of the coating. This points towards residual siloxane compounds in the coating, which are intrinsically hydrophobic. However, the presence

S. Meng et al. / Journal of Membrane Science 450 (2014) 48–59

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Fig. 7. SEM images of coatings after 4 cycles at a molar ratio of 1:0.002 using (A) IM22, (B) CTAB, (C) PEG, (D) F127, and (E) no templating agent. Insets indicate the illustrative nanostructure of each cluster.

of silicon was not seen in the XPS elemental quantification of IM22templated coatings (Fig. 10). Based on the highest concentration of IM22 used, the maximum residual silicon in the coating would be less than 0.01 at%, making it difficult to detect by XPS analysis. The molar ratio of TTIP:IM22 was increased to 1:0.16 to overcome this issue. Approximately 1 at% silicon was detected after the 120 1C heat treatment, but a Si 2p peak was not detected after UV irradiation. It is likely that residual silicon compounds are still present in the coating after UV irradiation; however the levels are less than 0.1 at% in the top 10 atomic layers.

An increase in template concentration does not appear to have an effect on the hydrophilicity of the coatings. As mentioned previously, an increase in the templating agent concentration had negligible effect on the structure of the coating; therefore there is no change in the strength of resulting capillary effect. The exception to this is IM22, where it can be seen in Table 2 that an increase in the IM22 content from 1:0.002 to 1:0.016 M ratio results in a decrease in the contact angle after one cycle. This is due to the structural changes of IM22, where an increase in the templating agent concentration results in a more porous structure (Fig. 11).

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After two or more cycles, the coating takes on the structure seen in Fig. 6B, and the wettability of IM22-templated membranes was once again dominated by the coating chemistry. After the deposition of FTCS on the TiO2 coating, all templated coatings were found to be superhydrophobic (Fig. 12). Although superhydrophobic, coatings which used IM22 had a lower overall contact angle compared to the other coatings. It is believed that the residual siloxane segments from IM22 obstructs some OH functional groups, resulting in less FTCS being able to bond with the titania coating, and thus reducing the overall hydrophobicity. There appears to be no significant difference in the contact angle when the templating agent concentration is increased. A non-linear change in the water contact angle was observed after each coating cycle (Fig. 13). Based on the XPS analysis in Fig. 10, increasing the number of coating cycles will increase the amount of OH functional groups. Theoretically, this would increase the amount of FTCS that bond with the titania coating, which results in a higher contact angle. However, Table 2 shows a reduction in hydrophobicity with increasing coating cycles prior to FTCS deposition, which may lower the contact angle after FTCS deposition. This means a trade-off exists between the number of available OH functional groups and the overall hydrophobicity. It can be seen in Fig. 13 that there is a different optimum coating cycle for each templating agent. For most modified membranes, one coating cycle is close to optimum. In Fig. 13 the water contact angle for ‘No template’ was roughly equal to that of ‘CTAB’, ‘PEG’ and ‘F127’ after one cycle. However, from TGA results in Table 3, coatings which did not use a template had a higher amount TiO2 deposition on the membrane surface. An increase in the amount of TiO2 should theoretically result in

more OH functional groups for FTCS to bond with; therefore the contact angle of non-templated coatings should be higher than templated coatings. The lower than expected hydrophobicity may be attributed to the lack of hierarchical structure for coatings Table 2 Static water contact angles for virgin and modified membranes after UV irradiation. Reported values are the average of three trials, with an average standard deviation of approximately 4.31. Sample

No TiO2 cycle 1a No TiO2 cycle 4a No template cycle 1 No template cycle 2 No template cycle 3 No template cycle 4 1:0.002 PEG cycle 1 1:0.002 PEG cycle 2 1:0.002 PEG cycle 3 1:0.002 PEG cycle 4 1:0.002 IM22 cycle 1 1:0.002 IM22 cycle 2 1:0.002 IM22 cycle 3 1:0.002 IM22 cycle 4 1:0.016 IM22 cycle 1 1:0.016 IM22 cycle 2 1:0.016 IM22 cycle 3 1:0.016 IM22 cycle 4 a

t¼ 0 s

t ¼ 10 s

t ¼60 s

81.73 25.88 30.48 18.14 84.16 36.95 35.94 33.53 110.95 118.25 117.78 115.31 85.65 118.46 113.61 118

97.68 96.60 80.19 18.23 21.69 7.06 58.94 29.64 25.78 25.15 109.74 117.40 117.95 114.17 68.13 118.9 112 117.1

74.88 13.99 11.85 0 51.92 20.78 13.42 16.86 107.20 115.86 115.54 113.34 56.63 115.86 111.13 116.57

Changes in the contact angle was not observed over time.

50

55

Virgin PVDF No Template IM22 CTAB PEG F127

45 40 35 30 25 20

Cycle 1 Cycle 4

40

At. %

50

At. %

Water contact angle (deg)

30

20

15 10

10 5

0

0 F1s

C1s

Ti2p

O1s

Fig. 8. XPS elemental quantification of virgin and modified coatings at a molar ratio of 1:0.016 after one cycle.

F1s

C1s

Ti2p

O1s

Fig. 10. XPS elemental quantification of coatings templated with IM22 at 1:0.016 after one and four cycles.

Fig. 9. SEM images of (A) coated membrane after UV irradiation, and (B) coated membrane after FTCS filtration.

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Fig. 11. SEM image of TiO2 coatings using IM22 as the template after UV irradiation for a molar ratio of (A) 1:0.002 and (B) 1:0.016.

Water contactangle (°)

170

Table 3 TGA residuals of virgin and coated membranes after UV irradiation. Reported values are the average of three trials, with an average standard deviation of 1.4 wt%.

160

150

1:0.002

Sample

1:0.004

Residual (wt%) after cycle #1 Residual (wt%) after cycle #4 1:0.002

1:0.016

1:0.002

1:0.016

1:0.008

140

1:0.016 130

120 IM22

CTAB

PEG

F127

Virgin PVDFa No templatea IM22 CTAB PEG F127

0 0 0 0 0 0

10 0 0 0 0

2.3 4.9 1.7 1.8

2.1 7.5 2.5 1.2

Templating agent Fig. 12. Water contact angles after FTCS deposition for coatings which used templating agents at different concentrations for one coating cycle.

Sample

170

Water contact angle (°)

Table 4 BET surface area of coated membranes after UV irradiation. Specific surface area (m2/g) 1:0.002 Cycle 1

160

150

Cycle 1 Cycle 2

140

Cycle 3 Cycle 4

130

IM22 CTAB PEG F127 No templatea

5.477 0.07 4.377 0.11 4.82 7 0.10 4.55 7 0.06

1:0.016 Cycle 1

4.40 7 0.08 3.497 0.03 3.93 7 0.06 3.687 0.05 8.40 70.22

1:0.016 Cycle 4 16.42 70.56 6.86 70.07 6.7770.11 5.56 70.11 20.2 70.27

a There is no template, so the molar ratio of TTIP:template in the heading should be disregarded.

120 Virgin PVDF

No Template

IM22

CTAB

PEG

F127

Templating agent Fig. 13. Water contact angles after FTCS deposition for virgin and modified membranes after different coating cycles. A molar ratio of 1:0.016 was used for the templating agents.

which did not use a templating agent, as seen in Fig. 4A, compared to coatings which did use a templating agent. This hierarchical structure in templated coatings increased the overall surface roughness of the FTCS layer, thus increasing the hydrophobicity. Coatings that use a template hold less TiO2 after UV irradiation compared to coatings without a template, which is complementary with the increase in apparent porosity of templated coatings. Out of the four templating agents, CTAB-templated coatings should theoretically have the highest residual amount of TiO2 due to its low molecular weight. This is confirmed in the results for 4 coating cycles. IM22-templated coatings have the second highest

residual weight for most concentrations and cycles, despite IM22 having the second highest molecular weight of the four templating agents. Once again, it is believed that the higher residual weight of coatings using IM22 is caused by residual siloxane segments, which were not removed during post-treatment. Oxidation of these segments at high temperatures causes the formation of SiO2 or a cross-linked PDMS compound [48], which consequently increases the residual weight. This deposition theory is further supported by specific surface area (SSA) values obtained from BET analysis. Results suggest a decrease in the amount of surface area after coating the membrane (Table 4). Virgin PVDF had an initial surface area of 12.02 70.07 m2/g, which was reduced by more than 50% after one coating cycle. SEM imaging of the membrane surface at low magnifications (Fig. 9A) and the membrane cross section (figure not provided) after one coating cycle did not show a significant decrease in the membrane pore size. Assuming the mass of PVDF remains constant across all membranes, the reduction in specific

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Table 5 Average liquid entry pressure for coated membranes after FTCS filtration. All values are in kPa, with an average standard deviation of approximately 5 kPa. Sample

Cycle1

Cycle 2

1:0.002 No TiO2a No templatea IM22 CTAB PEG F127 a

1:0.016

1:0.002

150 120 85 155 185 95

Cycle 3 1:0.016

1:0.002

140 100 100 120 190 120

100 130 125 140

Cycle 4 1:0.016

1:0.002

135 100 85 140 150 200

1:0.016 145 100

55 185 125 155

60 120 190 160

80 125 190 150

60 75 190 190

There is no template, so the molar ratio of TTIP:template in the heading should be disregarded.

surface area must therefore be largely due to the increase in the mass of TiO2. The specific surface area ratio is given in the following equation: ð1Þ

At higher template concentrations, more TiO2 appears to be deposited, resulting in a larger specific surface area. This is in agreement with previous studies on surfactant concentrations [36,49]. Yang et al. proposed that partially decomposed organic components of templating agents can ‘glue’ nanoparticles together [37]. Following this train of thought, the higher the template concentration, the more ‘glue’ exists to bind the TiO2 together during post-treatment. Therefore, the SSA at 1:0.016 M ratio is expected to be lower than at 1:0.002 M ratio. For coatings which were deposited without a template, a higher SSA was measured compared to coatings which did use a template. This indicates that either coatings without templates have higher surface areas or lower mass of TiO2. Based on the TGA results in Table 3, where coating without templates had the highest residuals, the former is more likely to be the case. It is believed that the lack of hierarchical structure seen in Fig. 4A leads to lower pore restriction, and consequently a higher overall surface area. After 4 cycles, there is an increase in the SSA of all coated membranes. Fig. 4 shows a visible increase in the apparent porosity of the coatings after 4 coating cycles, which suggests that the role played by the surface area in the SSA ratio grows over several coating cycles. Similar to one cycle, coatings which did not use a template had a higher SSA, indicating that non-templated coatings continue to have less pore restriction, and therefore lack hierarchical structure. Of the coatings which were templated, those that used IM22 had the highest SSA for all concentrations and coating cycles. As the apparent porosity of IM22-templated coatings appear to be equal, if not lower, to other templated coatings in Fig. 7, there must be less TiO2 deposited when IM22 is used as the template. CTAB-templated coatings produced the lowest SSA, which supports the high residual weight values obtained from TGA. 3.2.2. Liquid entry pressure Despite the similarity in contact angles after FTCS filtration seen in Fig. 12, the LEP of water varied with different templates (Table 5). According to the Young–Laplace equation, the LEP of the membrane should be proportional to the wetting angle [50]. This is shown in Eq. (2), where γ is the surface tension of the fluid, θ is the contact angle, and R is the radius of the pore LEP p

2γ cos θ R

ð2Þ

Since the contact angles at equal concentration or coating cycle were relatively similar for coated membranes, and Milli-Q water was used as the wetting fluid for all measurements, the differences

Virgin PVDF 1.6

Conductivity (mS)

surf ace area SSA ¼ mPVDF þ mcoating

1.8 No Template

1.4

PEG

1.2

IM22 CTAB

1

F127

0.8 0.6 0.4 0.2 0 0

2

4

6

8

10

12

14

16

18

20

22

24

Time (h) Fig. 14. Permeate conductivity of virgin and modified membranes. Modified membranes have gone through FTCS filtration after one coating cycle, using a TTIP:template molar ratio of 1:0.016.

in LEP arises due to changes in the pore size. After FTCS filtration, there is a significant reduction in the pore size, as well as an increase in the tortuosity (Fig. 9). These two factors will vary depending on the amount of FTCS that is able to bond to the coated membrane. Therefore a higher LEP suggests a greater overall amount of FTCS on the membrane. From Table 5, IM-22 templated coatings resulted in the lowest LEP at all concentrations and cycles, which is in accordance with the contact angles given in Fig. 12. PEG-templated coatings gave the highest LEP across most concentrations and cycles, indicating more FTCS may be present compared to other modified membranes. As PEG is a hydrophilic polymer, greater amounts of the organic compound would be removed after the 90 1C wet heat treatment and UV irradiation in water compared to other templating agents with hydrophobic segments. This would potentially expose more TiO2 at the surface, and consequently increase the amount of available OH functional groups for FTCS bonding. There is a general decrease in LEP seen for most modified membranes after several coating cycles, which matches the decrease in contact angle seen in Table 2. This is due to the decreasing contact angle, which results in a proportional decrease in the LEP as shown in Eq. (2). This is evident when comparing the ‘No TiO2’ and ‘No Template’ membranes which have both undergone FTCS filtration. ‘No Template’ samples had a lower LEP than ‘No TiO2’ samples, indicating that the TiO2 in the ‘No Template’ samples are less hydrophobic and therefore more likely to allow penetration of water after single point of failure. 3.3. Effect on membrane distillation performance Desalination performance was tested across coated membranes using templating agents at a molar ratio of 1:0.016 after one cycle. The permeate conductivity (Fig. 14) measured for virgin PVDF is

S. Meng et al. / Journal of Membrane Science 450 (2014) 48–59

57

Fig. 15. (A) SEM image of salt deposited on a 1:0.016 IM22-templated membrane (one cycle) after 24 h of desalination and (B) EDS surface map of sodium atoms.

20 Virgin PVDF No Template PEG IM22 CTAB F127

Flux (kg/m2h)

18 16 14 12 10 8 6 4 0

2

4

6

8

10

12

14

16

18

20

22

24

Time (h)

16

0.2

14

0.18 0.16

12

0.14 10 PEG Cycle 1 Flux

0.12

8

PEG Cycle 3 Flux

0.1

6

PEG Cycle 1 Conductivity

0.08

PEG Cycle 3 Conductivity

0.06

4

0.04 2

Conductivity (mS)

Fig. 16. Permeate flux of virgin and modified membranes. Modified membranes have gone through FTCS filtration after one coating cycle, using a TTIP:template molar ratio of 1:0.016.

Flux (kg/m2h)

considerably higher than all modified membranes after 4 h, reaching a plateau at 200 mS before rapidly wetting to 1700 mS after 24 h. For modified membranes, relatively low conductivities, ranging from 40 mS to 300 mS, was still observed after 24 h, indicating that superhydrophobic modification of the membrane surface does indeed reduce the chances of pore wetting. However, partial pore wetting is evident for modified membranes, particularly for membranes which did not use a templating agent. This partial wetting is mainly attributed to the deposition of salt in the membrane pores [51,52]. Evidence of salt deposition can be seen in Fig. 15, and peaks for sodium and chlorine were present in EDS analysis for all modified membranes. The permeate flux trends were similar across all membranes (Fig. 16). The gradual decrease in permeate flux is a result of various factors, most notably an increase in feed concentration [53], a decrease in the bulk temperature of the feed solution as a result of decreasing volume [1], and an increase in the permeate temperature over time [54]. Virgin PVDF had the highest initial permeate flux of 18 kg/m2 h due to a larger pore size; however the rate of flux decrease is faster than modified membranes. Since operating conditions were kept constant, the larger rate of flux decline is most likely due to a higher amount of salt deposition on the virgin membrane, where the lack of an air gap introduced by superhydrophobic modifications of the surface causes virgin membranes to be more susceptible to scaling [55], which ultimately hinders vapour transport across the membrane [56]. In Fig. 16, membranes which used templating agents had slightly different initial permeate flux values. As the starting process parameters remained the same for all membranes, the initial permeate flux of modified membranes would be mostly determined by the degree of pore restriction created by the FTCS layer. This, in turn, is proportional to the contact angle, where a higher amount of FTCS would increase the hydrophobicity. In Fig. 12, coatings which used IM22 gave the lowest contact angles due to a lower amount of FTCS bonded to the TiO2 coating. This resulted in less pore restriction, and thus a higher initial permeate flux of 15 kg/m2 h can be seen in Fig. 16. The same concept is seen for membranes which did not use a templating agent, where a lower initial permeate flux of 10 kg/m2 h was observed due to the greater amount of FTCS present. Based on contact angle and LEP results, as well as preliminary desalination performance, PEG-templated membranes were chosen as the optimum candidate in this study. Further desalination tests were carried out to investigate the changes in performance after three coating cycles for membranes which used PEG as the templating agent. Since XPS analysis shows an increase in the amount of O1s after several coating cycles, more FTCS should be able to bond to the coating after three cycles, allowing the superhydrophobic modification to become more robust. This is supported by the contact angles for PEG-templated coating in Fig. 13, where the contact angle increased from 1591 to 1611 after

0.02

0

0 0

2

4

6

8

10

12

14

16

18

20

22

24

Time (h) Fig. 17. Permeate flux of membranes using PEG as the templating agent. Membranes have gone through FTCS filtration, using a TTIP:template molar ratio of 1:0.016.

three coating cycles. Fig. 17 shows relatively similar permeate flux profiles, although the membrane using three coating cycles had a higher overall flux. This increase in flux is most likely caused by a reduction in scaling. However, the greater degree of hydrophilicity of the TiO2 layer after three coating cycles resulted in a higher permeate conductivity after partial pore wetting, indicating that a trade-off may exist between sustained performance and overall salt rejection. To further increase permeate flux, a PEG-templated membrane after three coating cycles with a lower molar ratio of 1:0.002 was used. BET and TGA results have indicated that less TiO2 is deposited when using a lower concentration of template, which would result in less restriction for vapour transport after FTCS filtration. Fig. 12 shows that the hydrophobicity of the membranes is not compromised when using a lower concentration of PEG. Therefore a similar conductivity profile, but higher permeate flux, is expected. The initial flux of 15 kg/m2 h in Fig. 18 is indeed higher for the lower concentration sample compared to a maximum

58

S. Meng et al. / Journal of Membrane Science 450 (2014) 48–59

16

1.8

Flux (kg/m2h)

1.6 1.4

12

1.2

10

1

8

0.8

6

0.6

4

0.4

2

0.2

0

Conductivity (mS)

Flux Conductivity

14

0 0

20

40

60

80

100

120

140

Time (h) Fig. 18. Permeate flux and conductivity after three coating cycles for a membrane using PEG as the templating agent. Membranes have gone through FTCS filtration, using a TTIP:template molar ratio of 1:0.002. Downward arrows indicate when the feed solution was refilled.

Superhydrophobicity was achieved for all modified membranes after the deposition of FTCS. However, contact angles varied depending on the templating agent used and the number of coating cycles. This was attributed to the changes in the TiO2 coating properties, which subsequently affected the bonding between TiO2 and FTCS. Among the templating agents, polyethylene glycol had the most promising properties, with optimum wettability, liquid entry pressure and desalination performance. While the overall permeate flux observed during membrane distillation was on par with virgin membranes, modified superhydrophobic membranes significantly improved salt rejection, and consequently extended the operating time.

Acknowledgements 2

permeate flux of 14 kg/m h in Fig. 17, however this higher flux is only maintained for approximately 2 h before declining to the same flux level as the 1:0.016 M ratio sample. To sustain the flux, further optimisation of the operating parameters is required, such as increasing the feed flow rate to reduce temperature and concentration polarisation effects [53]. The desalination performance of the 1:0.002 M ratio PEGtemplated membrane was observed over several days (Fig. 18). A feed volume of 1.8 L was passed through the system, giving a recovery ratio of approximately 68%. The permeate conductivity remained low and relatively stable at 30 mS until 50 h, after which the conductivity gradually increased to 270 mS. Regardless of the increase, the permeate conductivity is still significantly lower than that observed for virgin membrane, which reached 1700 mS in one-sixth of the operating time. The permeate flux appeared more unstable with increasing operating time. Several peaks in the flux profile were attributed to a reduction in the feed concentration when refilling the feed solution, as well as a decrease in the permeate temperature when the cooling tank water was replaced. However, the majority of the peaks occurred without external influence. The unstable flux profile is believed to be a result of heterogeneous crystallisation on the membrane. Approximately 25 g of solid NaCl was found deposited on the walls of the feed solution container at the end of the experiment. The final conductivity of the feed solution was measured to be 253 mS. Assuming the conductivity is directly proportional to the salt concentration, the final feed concentration is approximately 152 g/L. At 60 1C, the saturation limit of NaCl in water is roughly 375 g/L [57]. After factoring in the mass of crystallised salt, the feed concentration is still well below the saturation limit. Therefore, the precipitation of NaCl is due to heterogeneous crystallisation, which occurs on the membrane surface. This interaction on the membrane surface results in flux decline caused by the obstruction of vapour transport [58]. However, due to the air gap at the membrane–solution interface, the majority of the salt is only loosely deposited on the surface, and is eventually washed away by the feed stream. Further studies are currently being undertaken to verify this theory.

4. Conclusions Nanostructured TiO2 coatings were synthesised using sol–gel containing ionic and non-ionic templating agents. Coatings which did not use a templating agent were found to have a higher loading of TiO2, but lacked the hierarchical structure required for improved hydrophobicity. The use of templating agents resulted in porous coatings, with multi-level micro- and nanostructures after several coating cycles. The structure, wettability and desalination performance of the resultant coatings were dependent on the physical and chemical properties of the templating agent.

The authors gratefully acknowledge funding from the Australian Research Council. The authors would also like to thank Dr. Jason Scott for his help with BET measurements.

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