Strong Adhesion of Graphene Oxide Coating on Polymer Separation

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Strong adhesion of graphene oxide coating on polymer separation membranes Ruirui Hu, Yijia He, Meirong Huang, Guoke Zhao, and Hongwei Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02342 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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Strong adhesion of graphene oxide coating on polymer separation membranes

Ruirui Hu, Yijia He, Meirong Huang, Guoke Zhao, Hongwei Zhu* State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, C213 Yifu Technology and Science Building, Tsinghua University, Beijing 100084, China *

Corresponding authors. Email: [email protected]

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ABSTRACT Graphene oxide (GO) has been demonstrated as the most promising candidate to surface modification of polymer separation membranes for durable filtration applications. However, the adhesion between GO coating and polymer substrate, as the most essential issue for reliable applications, has been little explored. Herein, we developed a facile high pressure assisted deposition method to physically anchor GO sheets on microfiltration (MF) and reverse osmosis (RO) membranes, and established a tape test procedure for assessing the adhesion of GO coating to polymer substrates based on the ASTM D3359. Through regulating the GO sources and coating processing, we demonstrated that the adhesion depends sensitively on the GO flake size and deposition pressure, while the adhesion level dramatically improved from 0B to 5B with decrease of the lateral size of GO and increase of the coating deposition pressure. The strong GO coatings showed evidently higher water flux than that of weak counterparts. The underlying mechanism was further analyzed and verified. Nanosize of GO and high deposition pressure favor for forming the conformal morphologies of GO coatings on both MF and RO membranes, which allow strong interfacial van der Waals interaction due to the large contact areas and result in the strong GO coatings on membranes. These results potentially open up a versatile pathway to develop the strong graphene-based coatings on separation membranes.

Keywords: graphene oxide, coating, adhesion, polymer membrane


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INTRODUCTION Currently, membrane-based technology has become a leading approach in wastewater treatment [1], food production [2] and petrochemical industry [3], and the conventional polymeric membranes maintain the dominance for liquid separation applications owing to their feasible processing and excellent permselectivity [4]. According to the characteristic pore size, the present commercial membranes are divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes, which are used to sieve various solutes at different scales on the basis of distinguish retention mechanisms [5]. However, these polymer separation membranes generally suffer from some surface-related problems, such as fouling [6], scaling [7], microorganism deposition [8] and polyamide chlorination [9], which would deteriorate membrane performance and need to be overcome by surface modification [10, 11]. Graphene oxide (GO), the two-dimensional carbon sheets with various oxygen-containing functional groups decorated on basal planes and peripheries, can serve as one of the most promising candidates to functionalize membrane surface due to integrated properties for a desirable protective coating, such as unimpeded water permeation, superior hydrophilicity, bactericidal effect, chemical stability as well as planar structure [9, 12-15]. Up to now, GO sheets have been assembled on many kinds of polymer membranes, including polyamide thin film composites [16], polyvinylidene fluoride MF [17], polyethersulfone UF [18], and polyacrylonitrile porous membranes [19]. However, the extra protective coating layer generally increases the transport resistance to water and results in severe decline of flux. Besides, while most studies centered on the functionalities of GO coatings for durable applications of membranes, the interfacial adhesion between GO and underlying separation layer, one of the most essential issues for reliable applications, remained little explored due to lack of effective means for evaluation of interfacial adhesion performances [20-23]. In order to enhance the interfacial interaction between GO modifiers and polymer membranes, GO sheets were commonly immobilized on membrane surface by diverse chemical methods, including chemical cross-linking [24], layer-by-layer assembly [25] and UV grafting [26]. Apparently, chemical modification always required multi-step reaction and accurate control, causing a time- and effort- consuming process. Besides, the harsh reaction conditions 3 ACS Paragon Plus Environment

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exemplified by organic solvents or exposure to irradiation, generated undesired defects in polymer membranes [27]. As well, it was doubtful if the chemical modification is necessary since GO sheets are expected to have the strong adhesion with polymer membrane via physical van der Waals interaction due to its extreme flexibility, ultimate thickness and oxygen-rich functional groups [28]. As reported, an adhesion energy of ~0.45 J/m2 was measured between monolayer graphene and silicon oxide substrate, which was comparable to solid-liquid adhesion energies. The ultra-strong adhesion of graphene was attributed to the extreme flexibility, which allowed it to conform to the topography of substrates, and thus making the interaction more liquid-like than solid-like [28]. As such, GO sheets, as the prevalent graphene derivative, should interact with substrate more strongly, benefiting from the possible hydrogen bond provided by the extra oxygen-containing groups, besides the large van der Waals force induced by the sufficiently large contact area into the nanoscale regime. The theoretical results predicted that the physical adhesion energy of graphene on various substrates depended sensitively on the numbers of graphene layers and substrate surface roughness [29, 30]. However, in practical applications, it is hard to improve the interfacial adhesion by regulating graphene layers or substrate properties since the substrate and coating thickness are generally prescribed for designated applications. From the respective of process-structure-performance relationship, the interfacial interaction should be closely correlated with the physicochemical properties of GO as well the surface functionalization processing. Therefore, the strong adhesion of GO sheets on polymer separation membranes can be expected by controlling the properties and processing of GO coating. GO sheets can be synthesized by different recipes such as Brodie method, Hummers method and its modifications [15, 31], with significant difference in physical and chemical qualities of materials in terms of lateral size, oxygen-containing groups species and proportions, and the surface microchemical environment (oxidation debris) [32-34]. Although GO has been extensively explored to surface modified polymer separation membranes for durable applications [9, 35, 36], actually, the “GOs” were not completely identical, and it remained unknown that what the specification of GO is most suitable for protective coatings. Meanwhile, the fabrication techniques for physically adhesive GO coating commonly 4 ACS Paragon Plus Environment

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covered vacuum filtration, spinning or spray coating, wherein vacuum filtration acted as the most pervasive method due to the controllable coating quality and quantity. However, GO functionalization by suction filtration is restricted to the MF membranes, while it fails to deposit GO sheets on denser polymer membranes such as NF or RO membranes limited by the pressure ceiling of 0.1 MPa. In this study, we employed a fast high pressure assisted deposition method to physically anchor GO sheets on MF and RO membranes, and established a tape test procedure for assessing the adhesion of GO coatings to polymer substrates by applying and removing tape over GO film, referring to the ASTM D3359. Through regulating the GO sources and coating methods, the effects of physicochemical properties of GO sheets and the coating processing on interfacial adhesion between GO layers and MF, RO substrates were examined. As a result, we built a bridge connecting the GO qualities, processing craft and adhesion strength, and revealed the intrinsic mechanism of membrane structure: nanoscale GO sheets and high deposition pressure favored for forming the conformal morphology of GO coating on both MF and RO membranes, which allowed large van der Waals interaction at interface and resulted in the strong GO coatings on polymer separation membranes. At last, the filtration performance of GO coated membranes were evaluated and it was found that the strong GO coatings also showed evidently higher water flux than that of weak counterparts, demonstrating the promising potential as protective coatings.

EXPERIMENTAL Materials Expanded graphite (80 mesh) was provided by Teng Sheng Da Tan Su Ji Xie Co., Ltd. Qingdao, China. H2SO4, KMnO4, NaNO3, H2O2, HCl were supplied by Beijing Chemical Factory to prepare GO-1 aqueous suspension. GO-2 was purchased from Fangda Carbon New Material CO., Ltd, Chengdu, China. GO-3 was supplied by Nanjing XFNANO Materials Tech Co., Ltd, Nanjing, China. The mixed cellulose esters microfiltration membranes (MF) were provided by Shenghe Technology Co., Ltd, Beijing, China. The flat-sheet polyamide reverse osmosis (RO) membranes were purchased from Ande Membrane Separation Technology & Engineering Co., Ltd, Beijing, China. 5 ACS Paragon Plus Environment

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Preparation of GO GO-1 was prepared by chemical exfoliation of expanded graphite according to the modified Hummers’ method. The specific protocol of GO synthesis was described in our previous work [37]. GO-1 was used after ultrasonic treatment, aiming to enable the uniform dispersion of GO in deionized water. Characterizations of GO The lateral size and thickness of GO sheets were directly examined with atomic force microscope in tapping mode (AFM, MFP-3D infinity, Oxford, USA). GO samples for AFM observation were prepared by dropping the dilute GO aqueous solution of 30 µg/mL on silicon wafer. The silicon substrate was cleaned by ultrasonic using water for 10 min followed by another 10 min with acetone prior to use. The characteristic of GO sheets was further confirmed by X-ray diffractometer (XRD, Smartlab, Rigaku). The chemical composition of GO was investigated by X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher, USA) and Raman spectrometer (LabRAM HR Evolution, HORIBA, Japan). The GO bulk was used for XRD, XPS and Raman examination, which fabricated by freeze drying of GO suspensions for 24 h. Preparation of GO coatings on MF and RO membranes High pressure assisted deposition was developed to anchor GO nanosheets on MF and RO membranes surfaces by a Stirred Cell system (HP4750, Sterlitech, USA). The cell was originally a dead-end filtration equipment commonly used for evaluation of flux and retention of separation membranes. Herein, we employed it as an approach to coating processing on polymer separation membrane wherein GO solutions acted as the feed solutions. The high pressure (up to 6 MPa) provided by the N2 gas supply allows the deposition of GO sheets on dense membrane such as RO at a fast coating rate. 50 mL of GO suspension with 100 µg GO was filtrated at the pressure of 100 psi for preparing GO coatings on mixed cellulose esters membranes (MF-GO). The deposition procedure started immediately once the operation pressure was applied and then ended within several seconds, resulting from the low critical pressure of microfiltration membranes with relatively large pores. As a result, the maximal pressure was 100 psi during depositing GO coatings on MF membranes and the average pressure was less than 100 psi. For preparation of GO coatings on RO membranes (RO-GO), 6 ACS Paragon Plus Environment

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50 mL of GO aqueous suspension with 200 µg GO was exploited as feed solution and the deposition pressure was fixed at 1 MPa. To examine the effect of coating processing on interfacial adhesion of GO to polymer membranes, GO sheets were also deposited on MF membrane surface by vacuum filtration using the same GO content of 100 µg dispersed in 50 mL deionized water via a set of common suction filtration equipment. All the above experiment conditions maintained identical for GO-1, 2, and 3. The GO coated membranes were dried at room temperature for 24h before adhesion tests. Measurement of adhesion between GO and MF, RO membranes by tape test Given the complexities of the interfacial adhesion, it is reasonable to state that no tests can precisely assess the actual physical strength of an interfacial adhesion of coating to substrate. But it is possible to obtain an indication of relative adhesion performance. Some characterization methods that possess the potential for quantitative coating adhesion measurement, such as scratch testing [20], nanoindentation [22] and AFM [21], can’t be used to assess the adhesion of flexible GO coating on soft polymer substrate. Moreover, the test is affected by a series of intrinsic and extrinsic factors which are not adhesion-related and the results of test are usually regarded as semi-quantitative. Herein, referring to the ASTM D3359 that is designated as the standard test methods for measuring adhesion by tape test, we developed a method for assessing the adhesion of GO coatings to polymer substrates by applying and removing 3M tape over GO coatings: i) fix the membranes on a sheet of A4 paper with double-side adhesive to provide a sufficiently plane test area; ii) clean the GO surface with purge gas slightly to sweep the impurities; iii) place a piece of transparent 3M tape (Scotch, 600, 19 mm×60 mm) over the center of GO coating and rub the tape firmly with a finger to ensure good contact with membranes. The color under the tape acted as a useful indication of when good contact has been made; iv) within 60s of application, remove the tape rapidly by seizing the free end and stick the tape below membranes on the white A4 paper; v) inspect the center area or the tested tape for the removal of coating from the substrate by visual assessment. Rate the adhesion of GO to polymer substrates in accordance with the following scale: 5B: none of GO sheets is detached. 4B: less than 5% of the area is affected. 3B: the area affected 7 ACS Paragon Plus Environment

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is 5 to 15%. 2B: the area affected is 25 to 35%. 1B: the area affected is 35 to 65%. 0B: the detachment is worse that grade 1. The failure modes of GO coatings were further examined by SEM observation in the tested area. In addition, some commentary should be stated here to clarify the possible controversy about the tape test for adhesion evaluation in our experiments. Firstly, the transparent tape produced by Scotch 600 is a type of standard tape for adhesion test, and all the tests were performed using the same tapes. Secondly, the difference in adhesion performance among different samples was significant enough in our tests so that the influence of some subjective factors, such as the rate of tape removal and visual assessment, can be neglected, ultimately resulting in an excellent repeatability of results. Accordingly, it is reliable to gain the relative level of adhesion of GO coating to polymer membranes by the simple tape test. Pure water flux test Membrane flux was evaluated by filtrating 300 mL pure water on a dead end stirring cell (HP4750, Sterlitech, USA) at the fixed pressure of 1 MPa. The water flux of MF membranes was measured by weighing the permeated water in a time interval of 10 s due to the fast water permeation. In contrast, the RO membranes were pre-compacted for 30 min before recording the flux in the next 10 min. The water flux was calculated as follows: ௏

‫ = ܬ‬஺×௧

(1)

where J is water flux (LMH), V is the volume of permeated water (L), A is the effective membrane area (m2) and t is the filtration time (h). Herein, the effective permeation area was 14.6 cm2.

RESULTS AND DISCUSSION Characterizations of different GO sources GO has been extensively exploited as functional protective coatings for durable osmosis applications. However, the “GOs” may be home-made in the laboratory or supplied by various companies, which are synthesized by distinct recipes resulting in inevitable differences in physicochemical properties. In turn, the completely different functionalities may be generated although they are collectively called “GO” sheets. Herein, the physical 8 ACS Paragon Plus Environment

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(Figure 1) and chemical (Figure 2) characteristics of different GO sources were investigated, including GO-1, 2, 3 obtained from different sources. As shown in Figure 1a, GO solutions were prepared by dispersing three kinds of GO with the same concentration of 30 µg/mL in deionized water, and the similar pale yellow color and uniform dispersibility were observed, suggesting that one can’t discriminate these three GO samples according to their appearance of aqueous solution. GO macroscopic configurations were obtained by freeze drying of isometric solutions of three kinds of GO, and the distinct architectures were exhibited in Figure 1b. As one can see, GO-1 was fine powder. GO-2 showed a porous structure while this network seemed a little collapsed with some crumbs left in the surroundings. In contrast, GO-3 behaved as an integrated and strong porous foam exhibiting an intact skeleton. The distinguish macroscopic architectures of three GO samples indicated the possibly incremental lateral size from GO-1 to GO-3 because large-size GO sheets are more readily interconnected and assemble into the three-dimension bulk [38]. XRD results showed the characteristic peaks of GO laminates at 2θ=12o for all the GO samples (Figure 1c), indicating the alignment of GO sheets into lamellar structure with an interlayer spacing of 0.74 nm. However, comparing with the broad semi-crystalline peak of GO-1, the sharp peaks were observed for GO-2 and GO-3 while it’s significantly more intense for GO-3. This phenomenon suggested that the alignment degree of GO sheets increased gradually from GO-1 to GO-3, which can be rationalized by the importance of GO flake size for the interlayer alignment process: larger size of GO sheets favors for forming better layered structure ascribed to the stronger interlayer interactions between larger overlapping areas [39]. Therefore, XRD results further confirmed the difference of lateral size of GO samples that deduced from Figure 1b. In order to directly examine the GO flake size, AFM was performed and the results were shown in Figure 1d-f. As expected, GO-1 possessed the smallest lateral size of ~500 nm, and GO-2 exhibited a larger flake size of ~1.5 µm, and the largest sheet size of ~3 µm was observed for GO-3. Furthermore, indicated by the height profile, GO-2 and GO-3 were both well exfoliated to monolayer while part of GO-1 would aggregate together to generate two-layer or thicker GO sheets due to the small size. X-ray photoelectron C1s spectra of three GOs were shown in Figure 2a-c and the specific 9 ACS Paragon Plus Environment

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binding energies as well the plausible chemical species, determined from the deconvolution of C1s core level XPS spectra, were listed in Table 1. The proportions of various bonds were estimated from the integral area of corresponding functionality peaks. GO-1 had a relatively less oxidation with a high C-C component of 55.98% comparing with GO-2 and GO-3. Furthermore, different from the C=O (aldehyde/ketone) at binding energy of ~288.1 eV for GO-2 and GO-3, GO-1 exhibited an apparent shift to 288.8 eV, indicating the actual C=O in O=C-OH (carboxyl). Raman spectra showed the characteristic peaks of D band at 1350 cm-1 and G band at 1600 cm-1 for all GO samples (Figure 2d), which signified the structural disorders and sp2 hybridization of graphite lattice respectively. The obvious D band in Raman further confirmed the effective oxidation on graphene panels that would generate many disorders. However, the ID/IG of GO-1 (ID/IG=0.91) was slightly lower than that of GO-2 (ID/IG=1.06) and GO-3 (ID/IG=1.09), which was consistent with the relatively lower degree of oxidation of GO-1 indicated by XPS analysis. Morphologies of GO coated polymer membranes The surface morphologies of GO coated MF and RO membranes by high pressure assisted deposition using different GO sources were exhibited in Figure 3. The pristine MF membrane showed a characteristic porous structure with an average pore size of approximately 1 µm. After deposition of planar GO-1 sheets, it can be clearly seen that GO-1 can’t overlap with each other to form a continuous film but wrap around the pore walls even fall into the interior of micropores. Consequently, CO-1 coated MF membranes still exhibited the apparent pores on the surface while the outmost polymer was completely covered by flexible GO nanosheets. In contrast, the GO-2, with a larger flake size of about 1.5 µm, can shelter most of the pores on membrane surface while there still existed a few pores exposed due to the size distribution of GO-2. As for MF-GO-3 membrane, the surface pores of MF have been completely covered as the result of the largest lateral size of ~3 µm, and an integrated GO film was formed atop the membrane surface while the skeleton of polymer substrate was faintly visible. The virgin RO membrane showed a rough “ridge-and-valley” morphology with the distance of several hundreds of nanometers between adjacent peaks, which is the typical topography of polyamide RO membranes. Under the deposition pressure, GO-1 snugly adhered to 10 ACS Paragon Plus Environment

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membrane surface along the contour of rough architecture, with some filled in the valley and others attached on the ridge, resulting in a completely conformal morphology on RO membranes. In contrast, the surface structure of RO membranes was completely sheltered by GO-2, GO-3 sheets and the wrinkled morphology was observed, which was the characteristic topography of GO film. This can be attributed to the larger lateral size of GO sheets that made it difficult for GO to tuck into valley but only suspend relatively flat atop the ridge. Although there was a significant difference in the structure of polymer substrates, with a discontinuous porous morphology of MF membrane and a continuous rough topography of RO membrane, the GO coated polymer membranes with different GO sources actually exhibited a common feature that the morphology transition from conformal to non-conformal occurred with increasing the lateral size of GO sheets, while it is gradual for MF and sharp for RO depending on the match quality between the GO flake size and pore size/ridge distance of polymer membranes. Therefore, the membrane morphology sensitively depends on the lateral size of GO sheets and GO-1 tends to conform to the structure of polymer substrate while GO-2, 3 intended to maintain their intrinsic morphology. Adhesion performance of GO coatings to polymer membranes The adhesion of GO coatings to polymer membranes was evaluated by tape-and-peel method, and the significant difference was observed for different GO sources. As shown in Figure 4, GO-1 adhered most strongly to MF substrates without any visible detachment after the tape is pulled off, indicating the highest adhesion level of 5B between GO-1 and MF membrane based on the descriptive scale (see Experimental section). However, the apparent exfoliation was observed for GO-2 in tested region while there still remained a thin GO layer attaching on membrane surface. This indicated that the breaking of GO-2 coating maybe resulted from the delamination within GO multilayer system but not the separation in the interface of GO coating/MF membrane. Resultantly, a poor adhesion level of 1B was certified for GO-2 according to the rating scale. Meanwhile, it can be clearly seen that GO-3 exhibited the poorest adhesion to MF membranes with an intact film peeled cleanly by the tape, resulting in the lowest adhesion level of 0B. Microscopic examinations were also performed to more precisely assess the removal process of GO coatings from MF substrates. As shown in Figure 4d,g, the strong adhesion of GO-1 to MF membrane was further 11 ACS Paragon Plus Environment

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confirmed due to the identical morphology of the tested area with that of pristine GO-1 coatings, suggesting that there was no microscopic detachment of GO sheets from MF either. In contrast, for GO-2 and GO-3 coated membranes, an evident boundary between the tested region and virgin coatings was observed (Figure 4e,f) and the porous morphologies of pristine MF substrate have been completely exposed in the tested area (Figure 4h,i), indicating the severe removal of GO due to the weak combination between GO coatings and MF substrates. However, the peeling of GO-2 was slightly superior to that of GO-3 because there still existed some GO sheets on the porous skeleton of MF membrane (as indicated by the red arrow in Figure 4f), like the structure of GO-1 coatings (Figure 4g), probably resulting from the small GO sheets in GO-2. In contrast, the structure of GO-3 coatings in the tested area was the same as that of pristine MF membranes, further verifying the complete detachment of GO coating as indicated by the macroscopic assessment. The adhesion of GO coatings on RO membranes was explored in Figure 5 and GO-1 exhibited distinct adhesion performance from GO-2 and GO-3. As shown in Figure 5a,d,g, GO-1 could combine with RO substrate so strongly that cannot be removed by tape peeling verified from both macroscopic and microscopic examinations, resulting in the highest adhesion rating level of 5B. However, the delamination within RO substrate was observed as black arrow indicated in Figure 5a, which have been demonstrated by tape test of pristine RO membranes, as shown on the right of Figure 5a, where the easy detachment of polysulfone (PSF) layer from nonwoven substrate was revealed with peeling the tape. Furthermore, the cracks were exhibited in SEM images of the tested areas and they were regarded as the rupture of polyamide layer due to the strong tape adhesion force because the porous PSF layer has been exposed under the cracks (Figure 5d,g). Therefore, the adhesion of GO-1 coatings to RO membranes was strong enough to exceed the cohesive strengths of RO substrates, which could ensure the reliable stability of GO coatings in practical applications. In contrast, GO-2 and GO-3 exhibited the comparably poor adhesion to RO membranes with almost all the GO sheets debonded from RO membranes (Figure 5b,c), and thus the lowest adhesion level of 0B was rated based on the scale. Furthermore, as shown in Figure 5e-f, h-i, the exposure of ridge-and-valley structures of polymer substrates suggested the preferable detachment of GO sheets covering on the “ridges” where the small contact area 12 ACS Paragon Plus Environment

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caused the weak physical interaction. Mechanism analysis and verification Why there existed such dramatic difference in adhesion of GO to polymer membranes for different GO sources? By carefully comparing membrane morphologies and adhesion performances of different GO coatings on both MF and RO substrates, it was interestingly found that the adhesion properties changed in the same trend as the evolution of coating morphologies with altering GO sources. With the transition of conformal to non-conformal morphology from GO-1 to GO-3 coatings, the drastic drops of adhesion energies between GO coatings and polymer substrates have been observed, while the transition is gradual for MF but sharp for RO membranes respectively. This indicated that adhesion performance of GO coatings to polymer membranes was determined by the coating morphology in nature, because conformal topography indicated larger contact area that would lead to more intense van der Waals interaction and an ultimate stronger adhesion force. But how the different morphologies of GO coatings formed? As analyzed before, they were attributed to the difference in physical properties of lateral size between different GO sources. In addition, it was not common that the conformal morphology formed for GO coatings on MF membranes in previous reports. Therefore, we speculated that the high deposition pressure could also make a great contribution to strong adhesion of GO coatings which resulted from our developed high pressure assisted deposition technique. In order to verify the effect of GO flake size and coating processing on adhesion performance between GO coatings and polymer separation membranes, we conducted a series of experiments and the results were exhibited in Figures 6, 7. As shown in Figure 6, we inspected the importance of GO lateral size by comparing the properties and performances of GO-3 coatings with that of smashed GO-3 (GO-3-1) to remove the influences of other interference factors originating from different GO sources. As indicated by AFM results (Figure 6a,b), the lateral size of GO-3 sheets has been successfully reduced to nanoscale level of approximately 300 nm by physical ultrasonic treatment, an order of magnitude less than the original size of 3 µm (Figure 1f). Compared to the continuous film on MF and wrinkled surface on RO of GO-3 coatings (Figure 3d,h), GO-3-1 coated membranes exhibited significant differences while porous structures and rough 13 ACS Paragon Plus Environment

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ridge-and-valley architectures were observed for MF and RO, which was similar with that of GO-1 coatings, demonstrating the fact of GO flake size regulated coating morphologies. As shown in Figure 6e-h, it can be clearly seen that the adhesion performances of GO coatings to both polymer substrates remarkably improved after decreasing the lateral size of GO-3 sheets based on macroscopic and microscopic inspections, with the adhesion level increasing from 0B to the highest 5B. Therefore, it was confirmed that GO lateral size can adjust the adhesion performance of GO coatings to polymer membranes by regulating the membrane morphologies. In addition, it can be deduced that the difference in chemical properties seemed to have no impact on the adhesion performance because the strong GO coatings on polymer membranes were obtained simply by reducing the GO sheets size. The membrane morphology and adhesion performance of GO coatings on MF surfaces by vacuum filtration method was exhibited in Figure 7 for comparison with those fabricated by high pressure assisted deposition to verify the effect of coating processing on adhesion properties. As one can see, the porous structures of pristine MF have been completely covered by GO sheets for all GO sources, suggesting the non-conformal morphology of GO coating. Consequently, the lowest adhesion levels of 0B were obtained with intact detachment of GO film on both macroscopic and microscopic results. Especially, by comparing the GO-1 coatings on MF membranes by vacuum filtration with high pressure assisted deposition (Figure 7a,d,g,j) and Figure 4a,d,g), it was evident that high deposition pressure could significantly enhance the adhesion performance of GO coatings from 0B to the highest level of 5B by affecting the coating morphologies. The adhesion force has been also investigated in more details by varying the pressure in series via a set of low pressure filtration equipment (see Supporting Information Figure S1-S4). Therefore, we interpreted the strong adhesion of GO-1 coatings on MF and RO polymer membranes using the schematics illustrated in Figure 8. Considering a rough substrate surface, the interfacial van der Waals interaction tends to bring the GO coating conformal to the surface. However, corrugation of GO film increases the elastic strain energy due to bending. The competition between the GO-substrate adhesion energy and bending energy determines the equilibrium morphology of the membranes [29, 30]. In general, the membrane morphology can be categorized into two types. If the interfacial van der Waals interaction is 14 ACS Paragon Plus Environment

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stronger, the GO remains bonded to the compliant substrate to form the conformal morphology and the amplitude of GO corrugation is determined by minimizing the total free energy. On the contrary, if the GO-substrate adhesion energy cannot balance the strain energy, the GO coating debonds and remains nearly flat on the top of rough substrate to form a non-conformal morphology. In our MF-GO and RO-GO systems, high pressure makes GO sheets tend to attach on polymer substrate while nanoscale flake size favors for decreasing the strain energy. As a result, under the synergistic effect of high pressure and nanoscale size, the equilibrium conformal morphologies were obtained, where GO sheets wrap over the pore walls to form conformal porous skeleton for MF (Figure 8a) while GO corrugates to an amplitude up to that of RO architectures to form conformal “ridge-and-valley” structure (Figure 8c). Otherwise, the bending energy cannot be overcome which results in the non-conformal morphology where GO sheets suspend flat on membrane surface and the intrinsic morphology of polymer membranes were sheltered (Figure 8b,d). Ultimately, the membrane morphology can determine the adhesion performance between GO coatings and polymer substrates. The conformal structure facilitates the strong adhesion due to the strong van der Waals interaction resulted from the large contact areas. On the contrary, the non-conformal morphology causes the poor adhesion due to the limited contact regions. Membrane filtration performance As the protective coatings, it is expected that the pristine filtration performance, especially for water flux, cannot be compromised. The effect of GO coatings with different adhesion strength on the membrane performance of water permeation was assessed by comparing the pure water flux of pristine with that of GO-1, GO-3 coated MF, RO membranes. As shown in Figure 9, MF-GO-1 could maintain the comparable water flux with that of MF membrane at the same order of magnitude, with the flux decreasing from 46602 to 36247 LMH. In contrast, GO-3 coatings caused the serious water decline of 95%, as the flux reduced by an order of magnitude from 48822 to 2466 LMH. Similarly, the RO-GO-1 showed a slight decrease of water flux by 8.4% from 72.8 to 66.7 LMH, while the sever decline of 34.9% was revealed for GO-4 coatings from 76 to 51.5 LMH. The results can be well understood by the microscopic structures. The nanosized GO-1 coatings caused less transport resistance by forming conformal morphologies than that microsized GO-4 sheets, which tended to shelter 15 ACS Paragon Plus Environment

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the surface skeleton of pristine membranes. Therefore, one can conclude that strong GO coatings also showed evidently higher water flux than that of weak counterparts, demonstrating the promising potential as protective coatings.

CONCLUSION In summary, we prepared GO protective coatings on MF and RO separation membranes using different GO sources and depositing methods. The adhesion performance of GO coatings was evaluated by a modified tape test based on the ASTM D3359. The results were summarized in Table 2, which showed the optimized formula, including the specification of GO modifies and coating processing, for preparation of strong GO coatings with the highest adhesion level of 5B. As indicated by blue arrow, it can be clearly seen that the adhesion performance depends sensitively on the GO lateral size and coating methods, where high pressure and nanoscale size of GO can both significantly improve the adhesion level. The mechanism was further proposed and verified by inspecting the membrane morphologies: the transition of non-conformal to conformal morphologies occur with reducing GO flake size or increasing deposition pressure, which result in the dramatic improvement of adhesion performance. The strong GO coatings showed evidently higher water flux than that of weak counterparts. These results potentially open up a versatile pathway to develop the strong graphene-based coatings on separation membranes by simple control of modifier size and deposition pressure.


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Acknowledgements This work was supported by Beijing Natural Science Foundation (2172027). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.xxxxx.


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Figure 1. Physical properties characterizations of different GO sources. (a) Photographs of GO aqueous suspensions (30 µg/mL). (b) GO macroscopic configurations obtained by freeze drying of isometric aqueous solutions of three kinds of GO. (c) XRD patterns of GO sheets. (d-f) AFM images and the height profiles for GO-1, GO-2 and GO-3.

C-C C-O C=O

GO-1

(b)

C-C C-O C=O

GO-2

Intensity (a.u.)

Intensity (a.u.)

(a)

280

282

284

286

288

290

292

294

280

Binding energy (eV)

284

286

288

290

292

294

C-C C-O C=O

GO-3

(d)

GO-1 GO-2 GO-3

Intensity (a.u.)

(c)

282

Binding energy (eV)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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280

282

284

286

288

290

292

1000

294

1500

2000

2500

3000

Raman shift (cm-1)

Binding energy (eV)

Figure 2. Chemical composition and species characterizations of different GO sources. (a-c) XPS C1s spectra for GO-1, GO-2 and GO-3. (d) Raman spectra of GO sheets.


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Figure 3. Membrane surface morphologies. SEM images of (a) pristine MF, (b-d) GO coated MF membranes, (e) pristine RO, and (f-h) GO coated RO membranes.

Figure 4. Adhesion performance of GO coatings on MF membranes. (a-c) Photographs of GO coated MF membranes and adhesive tapes after the tape test. (d-f) SEM images of the virgin (on the left of images) and peeling areas (on the right of images) for GO-1, GO-2 and GO-3. (g-i) Magnified images of the areas within white box in (d-f). The red arrows indicate the GO sheets on membrane surfaces.


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Figure 5. Adhesion performance of GO coatings on RO membranes. (a-c) Photographs of pristine RO, GO coated RO membranes and adhesive tapes after the tape test. (d-f) SEM images of the peeling areas for GO-1, GO-2 and GO-3. (g-i) Magnified images of the areas within white box in (d-f). The red arrows indicate the polysulfone middle layer underneath the polyamide skin of RO membranes.


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Figure 6. Characterization and adhesion of GO-3-1 coatings on MF and RO membranes. (a, b) AFM image and height profile of GO-3-1 (smashed GO-3 samples by ultrasonic treatment). (c, d) SEM images of GO-3-1 coatings on MF and RO membranes. (e, f) Photographs of GO coated MF, RO membranes and adhesive tapes after the tape test. (g, h) SEM images of the peeling areas for GO-3-1 coated MF and RO membranes.


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Figure 7. Characterization and adhesion of GO coatings on MF membranes prepared by vacuum filtration method. (a-c) SEM images of GO coatings on MF membranes. (d-f) Photographs of GO coated MF membranes and adhesive tapes after the tape test. (g-i) SEM images of the virgin (on the left of images) and peeling areas (on the right of images) for GO-1, GO-2 and GO-3. (j-l) Magnified images of the areas within white box in (g-i).


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Figure 8. Schematic illustrations of the conformal morphologies of GO coatings on (a) MF, (c) RO membranes and non-conformal morphologies of GO coatings on (b) MF, (d) RO membranes, determined by GO flake size and deposition pressure.


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105

Water flux (LMH)

104

103

(c)

(b)

MF

90

105

104

103

MF-GO-1

(d)

80

70

70

60 50 40 30

MF-GO-3

RO

RO-GO-3

60 50 40 30

20

20

10

10

0

MF

90

80

Water flux (LMH)

Water flux (LMH)

(a)

Water flux (LMH)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

RO

RO-GO-1

Figure 9. The pure water flux of pristine and GO coated polymer separation membranes.


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Table 1. Chemical functional groups and proportions of different GO sources derived from XPS results.

C-C

C-O

C=O

Energy (eV)

Content (%)

Energy (eV)

Content (%)

Energy (eV)

Content (%)

GO-1

284.8

55.98

287.0

33.11

288.8

10.91

GO-2

284.8

46.87

286.9

38.78

288.2

14.35

GO-3

284.8

46.65

287.0

36.72

288.1

16.63

Table 2. Adhesion performances of GO coatings on MF and RO membranes with adjusted GO flake size and coating methods.

Polymer membrane

MF (Pore size ~1 µm)

RO (polyamide TFC)

GO lateral size

Preparation method

Adhesion level

~500 nm

Vacuum filtration

0B

~500 nm

Pressure deposition

5B

~1.5 µm

Vacuum filtration

0B

~1.5 µm

Pressure deposition

1B

~3 µm

Vacuum filtration

0B

~3 µm

Pressure deposition

0B

~500 nm

Pressure deposition

5B

~1.5 µm

Pressure deposition

0B

~3 µm

Pressure deposition

0B


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For Table of Contents Use Only


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High pressure & nanoscale size

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Low pressure / microscale size

MF

High pressure & nanoscale size

RO

MF

Low pressure & microscale size

RO

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Strong Adhesion of Graphene Oxide Coating on Polymer Separation

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