Selective Gas Permeation in Graphene OxideâPolymer Self

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Applications of Polymer, Composite, and Coating Materials

Selective gas permeation in graphene oxide-polymer self-assembled multilayers Davide Pierleoni, Matteo Minelli, Simone Ligi, Meganne Christian, Sebastian Funke, Niklas Reineking, Vittorio Morandi, Ferruccio Doghieri, and Vincenzo Palermo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01103 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 10, 2018

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ACS Applied Materials & Interfaces

Selective gas permeation in graphene oxidepolymer self-assembled multilayers

Davide Pierleoni, Matteo Minelli, Simone Ligi, Meganne Christian, Sebastian Funke, Niklas Reineking, Vittorio Morandi, Ferruccio Doghieri and Vincenzo Palermo*

D. Pierleoni, Dr. M. Minelli, Prof. F. Doghieri, Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), Alma Mater Studiorum - University of Bologna, via Terracini 28, Bologna, I-40131, Italy. Dr. M. Minelli, Prof. F. Doghieri, Interdipartimental Center for Industrial Research – Advanced Mechanics and Materials (CIRI-MAM), Alma Mater Studiorum - University of Bologna, via Terracini 28, Bologna, I-40131, Italy. Dr. S. Ligi, Graphene-XT srl, via D’Azeglio 15, I-40123, Bologna, Italy S. Funke, N. Reineking, Accurion GmbH, Stresemannstraße 30, DE-37079, Göttingen, Germany Dr. M. Christian, Dr. V. Morandi, Institute for Microelectronics and Microsystems (IMM), National Research Council of Italy (CNR), via Gobetti 101, Bologna, I-40129, Italy Dr. V. Palermo, a) Institute for Organic Synthesis and Photoreactivity (ISOF), National Research Council of Italy (CNR), via Gobetti 101, Bologna, I-40129, Italy; b) Department of Industrial and Materials Science, Chalmers University of Technology, Goteborg, Sweden. [email protected]; [email protected]

Keywords: graphene; gas separation; coatings; polymer materials; composite materials; nanomaterials; functional surfaces; membranes

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Abstract The performance of polymer-based membranes for gas separation is currently limited by the Robeson limit, stating that it is impossible to have high gas permeability and high gas selectivity at the same time. We describe the production of membranes based on graphene oxide (GO) and poly(ethyleneimine) (PEI) multilayers able to overcome such limit. The PEI chains acts as molecular spacers in between the GO sheets, yielding a highly reproducible, periodic multi-layered structure with constant spacing of 3.7 nm, giving a record combination of gas permeability and selectivity. The membranes feature a remarkable gas selectivity (up to 500 for He/CO2), allowing to overcome the Robeson limit. The permeability of these membranes to different gases depends exponentially on the diameter of gas molecule, with a sieving mechanism never obtained in pure GO membranes, in which a size cut-off and a complex dependence on the chemical nature of the permeant is typically observed. The tunable permeability, the high selectivity and the possibility to produce coatings on a wide range of polymers represent a new approach to produce gas separation membranes for large-scale applications.


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Introduction One of the peculiar properties of a single, perfect graphene sheet is its 2-dimensional shape, which provides a highly effective barrier to gas permeation.1 While a single graphene sheet is impermeable to even the smallest molecules such as helium and hydrogen, computational simulations demonstrated the capability of properly designed graphenic multilayer structures to exhibit a selective gas flux, thus acting as molecular sieves.2, 3, 4 Exciting results have been obtained in recent years by Geim et al., who showed the unique behavior of graphene layers towards ions and molecules, as the 2D materials can allow precise molecular sieving, selective proton transport and even separation of atomic isotopes.5,

6, 7

Other experimental

approaches exploited different assembly methods to achieve well-controlled channels or holes suitable for a fast and selective molecular transport of some probe molecules. As example, Shen et al. produced micrometer-thick graphene oxide (GO) membranes with sub-nanometer channels, obtaining excellent selectivity performances for H2/CO2 mixtures, while Liang et al. obtained graphene-based membranes with an appropriate size of channels between sheets, for water purification purposes.8, 9 Ultrathin graphene and GO coatings were deposited on porous substrates for the separation of gaseous mixtures, providing excellent selectivity.10, 11, 12 The separation of CO2-containing gaseous streams is a technological problem of growing interest for carbon capture and storage (CCS) applications, due to the need to reduce the impact of greenhouse gases on global climate. Pre-combustion CCS strategy exploits a gasification stage of a fossil energy source prior to its combustion, and the use of hydrogen as energy vector. The key step of such a method is the separation of H2/CO2 mixtures.13 There is thus a clear need for the development and optimization of membrane materials that can guarantee great performances as well as good resistance to the various gases and stability over time. In this work, we describe a procedure to fabricate graphene-based coatings on standard polymer substrates using the layer-by-layer (LbL) technique process. LbL is a bottom-up 3 ACS Paragon Plus Environment


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method already used successfully to fabricate thin layers of highly ordered graphene sheets.14, 15 16

It uses positive and negative electrolyte species that adsorb consecutively on a charged surface, yielding a multilayer structure;17 hence, the process requires polar substrate materials. The substrate is dipped into a polyelectrolyte solution to create the first charged layer, then alternate dips in two different solutions create layers that grow on top of each other thanks to electrostatic forces.18 The coatings we studied were composed of alternated layers of GO and poly(ethyleneimine) (PEI). Each GO-PEI bilayer (BL) was obtained by dipping solutions of negatively charged GO and positively charged PEI (Figure 1, see methods section for further details). At each dipping cycle, an additional BL was formed on both sides of the sample. We deposited our coatings on two industrially relevant substrates: A) polyethylene terephthalate (PET), a reference substrate largely employed in oxygen barrier packaging, and B) Matrimid, a commercial polyimide currently used for the fabrication of gas separation membrane modules.

Figure 1. a) Schematic procedure of the multilayer assembly to form a variable number of GO-PEI bilayers (BL). This is a simplified representation of the real system, where the real GO and PEI layers have defects and are partially overlapping (see main text for details); b,c) PET samples coated by different numbers of GO-PEI bilayers: 10+10 BL (b) and 5+5 BL (c).


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We studied this LbL process in a systematic way, varying the number of deposited GO and polymer layers, the properties and lateral size of the GO sheets, using different substrates and substrate pre-treatment steps. We then tested the obtained coatings as gas barriers to oxygen permeation, which is of major interest for industrial polymeric materials, as well as membranes for gas separations, for instance for the removal of CO2 from other gases.

Results Optical images of the coated samples are shown in figure 1b,c. Samples are named hereafter according to the number of BL on both sides, plus a code stating the GO material used: GO(L) for larger GO sheets with an average flake size of 40 µm (figure S1,S2 in SI), GO(C) smaller flakes with average flake size of 25 µm. The complete list of all the samples prepared and analyzed in this work is available in Table S1 in Supporting Information. Upon deposition, the transparency of the polymer decreased proportionally to the number of deposited layers, as visible also by naked eye (Figure 1 (b)). The measured transmittance at 550 nm was 86% for the bare 100 µm thick PET substrate; it decreased to 80% for the coating with 10 BL (5 BL per side), 68% for 20 BL (10 BL per side) BL, and 57% for 50 BL (25 BL per side), indicating a roughly linear behavior. All samples were produced at least twice, always giving highly reproducible optical transparency values. In all samples, the GO coatings were stable upon bending, twisting and other mechanical actions; PET coated samples were also tested for chemical stability, showing no degradation after immersion in water, acetone and chloroform. Comparative experiments were performed to verify the importance of alternating the GO and PEI dips in the formation of the LbL structure: multiple dips of clean PET in GO (omitting the 5 ACS Paragon Plus Environment


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dip in PEI) did not yield any GO deposition, and the obtained transparency was equal to that of the bare PET substrate. Conversely, one single dip of the substrate in PEI was enough to promote subsequent GO deposition. Significant amount of GO could be deposited by either multiple dips or one very long (24 h) immersion on a PET substrate previously coated by a single PEI layer, leading only to a slight decrease of light transmittance (85.3% and 85.5%, respectively). However, no layered structure could be obtained without performing alternated GO and PEI depositions. The same coating technique was also employed on Matrimid polyimide films, a technopolymer suitable for gas separation applications: this application of the LbL method, indeed, is general and can be employed for various polymeric substrates. We deposited the GO-PEI alternated layers also on silicon wafers, to evaluate with greater precision coating thickness and surface morphology. Figure 2a shows the scanning electron microscopy (SEM) micrograph of the cross-section of a 25 BL-sample, in which a compact and homogeneous coating layer is observed. The low roughness of the Si substrate allowed an accurate and reliable estimation of GO-PEI thickness. The value obtained for the 25 BL was 92±5 nm corresponding to 3.7±0.2 nm for each GO+PEI BL, consistent with values reported in previous works on LbL technique.14 The top surface of the LbL coating on PET was also analyzed by SEM, after gold sputtering (figure 2b), revealing a surface with uniform roughness, comparable to typical polymer substrates, confirming the absence of macro-agglomerates or uneven coverage. The higher magnification image (figure 2c) showed small wrinkles, indicative of the presence GO sheets on the substrate.


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Figure 2. SEM micrographs of GO based LbL coatings: (a) cross section of a 25BL GO-PEI sample on silicon wafer; (b) top view of 25BL GO-PEI coating on PET (sample 50GO(L) a); and (c), higher magnification of the same sample).

Unfortunately, the low conductivity of the two phases did not allow to obtain higher resolution images of the single GO or PEI layers, or to examine the inner stacking of the multilayers. Therefore, we used imaging ellipsometry to obtain an insight of the LbL coating structure at the nanoscale. Conventional ellipsometers perform point measurements, and thus would need a large amount of measurements to obtain information on the lateral uniformity of the layer’s thickness and optical parameters. Conversely, imaging ellipsometry uses as an imaging detector a CCDcamera on to map the experimental ellipsometric angles ∆ and Ψ on a mesoscopic area (more details are reported in the “methods” section). The ellipsometric angles ∆ and Ψ mapped are related to thickness and refractive index changes. ∆ is the phase difference and Ψ the amplitude ratio for p- and s- polarized light (linear and orthogonal components to the light incidence plane). With this setup, a mesoscopic field of 7 ACS Paragon Plus Environment


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view is imaged and mapped by the CCD-camera. Any structure or discontinuity larger than the lateral resolution of 2.5 µm will be resolved and measured simultaneously. In contrast, conventional ellipsometry gives only a single measurement, averaged on areas that are approx. 50 µm in size. Imaging ellipsometry allowed to identify the effective number of layers, and the average thicknesses of both PEI and GO domains. A 25 BL GO-PEI coating sample (on PET) was analyzed by imaging an area of 65 x 140 µm. On average, each layer was formed by 1.4±0.2 nm of GO and 2.3±0.2 nm of PEI, giving a BL thickness of 3.7±0.3 nm, repeated 25 times. The average coating thickness obtained by this method (92±8 nm) was equal to what observed by SEM (92±5 nm). The number of layers was calculated from the experimental data ∆ and Ψ. The calculation was performed with a standard model, see supplementary information section for further details. The best agreement with the experimental data was obtained assuming 25 bilayers (figure 3).


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a)

b)

WAVELENGTH [nm]

c)

WAVELENGTH [nm]

d)

Figure 3. Spectroscopic measurement on 25 BL of GO-PEI of the ellipsometric angles ∆ (a) and Ψ (b) for two angles of incidence (AOI). The solid lines show the optical model used; c,d) maps of the thickness of (c) the PEI layer and (d) the GO layer determined by imaging ellipsometry. The area measured is 65x140 µm.

For comparison, the fitting was performed also assuming a different number of layers, ranging from 23 to 26, and the different models were compared using their Mean Square Deviation (MSD) from experimental data. The results obtained demonstrated clearly the existence of 25 well-distinguished GO-PEI bilayers on top of the PET substrate (see Table S2), and that an alternating structure was in fact formed by the deposition process. Noteworthy, the average thickness of the GO and PEI layers showed minimal variations (9000 µm2), spanning 1.2-2.5 nm for PEI layers and 1.1-1.6 nm for GO layers respectively, confirming the excellent uniformity achievable by the LbL deposition (figure 3c,d).



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The lower limit of thickness range obtained for GO (1.1 nm) agreed well to what measured by atomic force microscopy (AFM) on GO single sheets (1.1 ± 0.2 nm). This indicates that the LbL technique is able to arrange the GO sheets with a high degree of order, creating basically single layers of GO monolayers, with a control at the nanoscale. The ellipsometry data showed a broader thickness range (1.1-1.6 nm) with respect to AFM, due to irregular sheet-tosheet border overlapping. The fraction x of overlapping GO nanosheets in each layer of the stack was estimated to be x 90% and > 80% for GO(L) and GO(C), respectively), resulting in very large aspect ratios. Furthermore, they were orderly distributed and closely packed together. Given this mesoscopic lateral size, the main component of gas permeation shall not be only due to molecules moving around the edges of each sheet, but also through holes and cracks typically present in each GO sheet.20 If, instead, the sheet size decreases to smaller values, gas transport around the sheets becomes comparable to that across the GO defects, leading thus to a permeability increase. To confirm this hypothesis, we prepared samples after reducing the lateral size of GO(C) by ultrasonication, decreasing the average sheet dimensions to < 1 µm (for a detailed analysis of GO lateral size reduction by ultrasonication see ref.21). The LbL coatings fabricated from sonicated GO dispersions showed lower barrier effect (Table S3), confirming that the sheet size becomes relevant for nanometric GO sheet dimensions. The intrinsic permeability of the GO-PEI coating (PGO-PEI) was evaluated from the penetrant transmission rate (TR), i.e. the molar flux per unit area rescaled by the pressure difference 11 ACS Paragon Plus Environment


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across the sample. We could remove the effect of the bare substrate (TRsubst) from that of the multilayer coated sample (TRML) by means of the series resistance Equation 1: l l 1 1 1 = + = nBL GO − PEI + subst TRML TRcoat TRsubst PGO − PEI Psubst

(1)

in which lsubst and lGO-PEI are, respectively, the thickness of the substrate and of a single GOPEI bilayer ( lGO-PEI ≈ 3.7 nm) determined from ellipsometry; nBL is the bilayer number; Psubst is the permeability measured on the bare substrate. A key parameter for gas separation in membrane processes is the selectivity, which can be calculated as the ratio of the permeability values of the two components, αi , j =

Pi

Pj . The

second crucial parameter for the evaluation of the separation performances is the absolute permeability of the more permeable component i , which should be as high as possible to increase productivity. A trade-off between these two parameters is well-known for polymeric membranes, with selectivity decreasing at increasing permeability, as proposed by Robeson in recent years.22 Permeability tests of other gaseous penetrants (He, H2, CO2) were carried out at 35 °C, in order to evaluate the selectivity of the GO-PEI coatings. The sieving effect was thus evaluated estimating selectivity values for a) H2/CO2, a gaseous mixture highly relevant for hydrogen purification and pre-combustion carbon capture, b) He/ H2, a critical separation in helium purification from natural gas, and c) He/CO2, typically reported in the technical literature for the evaluation of the sieving effect of a gas separation membrane, as a safer surrogate of a H2/CO2 mixture. The results obtained are reported in Table S4 in SI section. Figure 4a reports permeability values of He, H2, CO2 and O2 in samples with a 25 GO-PEI BLs per side coating on PET supports. As one can see, PGO-PEI values depend strongly on the dimension of the penetrant molecule, both for the bare PET substrate and for the GO-PEI coated sample. An appreciable reduction in permeability with respect to the clean substrate 12 ACS Paragon Plus Environment

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was observed for all gas molecules, and in particular for the largest ones. An exponential decrease was observed for GO-PEI permeability (calculated by Equation 1) of different gases

i with respect to their penetrant kinetic diameter kd,i23, as in the following Equation 2: PGO − PEI ,i = a exp ( −b k d ,i )

(2)

which confirmed the presence of a strong molecular sieving mechanism in the compact LbL coating, without specific cut-off in the dimensional range investigated. The tight dependence of permeability on molecular size demonstrated that the thin coatings did not contain defects at the macro or micro scale, in which the penetrant flux would be not selective at all, and that the permeation took place along the GO-PEI layers, through nanometric channels, with small species being much faster than larger ones. Each component of the composite membrane acts in a synergic way: the 2-dimensional GO sheets force the gases to diffuse across tortuous paths; each gap between stacked GO sheets is cramped with PEI polymer chains, which act as spacers between two GO sheets, allowing a significant gas transport but, at the same, time giving a high selectivity for smaller molecules.



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Figure 4. (a) Gas permeability values at 35°C in bare PET substrate (triangles), effective permeability in the multilayer sample with 25 GO-PEI BLs coating per side (50 BLs) (circles), as well as the values


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of intrinsic permeability of the GO-PEI coatings (squares), as a function of penetrant kinetic diameter. b) a schematic representation of the material structure, with oxygen molecules (in red) diffusing along the PEI molecules, constrained between GO nanosheets. The real system is likely much more disordered than that shown here. c) Intrinsic gas permeability of GO-PEI coatings of both 10+10 BLs (on Matrimid) and 25+25 BLs (on PET) at 35°C; data for the two bare substrates are also included for comparison sake.

A continuous decrease of permeability with respect to molecular size was previously observed only for nanoporous monolayer graphene deposited on porous alumina and partially etched, well described by a simple model obtained considering a log-normal distribution of the GO layer pore size.12 Though, in that case the sieving action was through holes and defects in a single monolayer of graphene, achieving only low selectivity values (up to 8 for the He/SF6 pair). The high number of layers and the tortuosity of the GO-PEI allow instead to reach selectivity values of 150 up to 500, outperforming standard gas sieving materials. The slope of the permeability curve vs. molecular size in the log scale (coefficient b in Eq. 2) represents the membrane sieving capability. Its value is typically high (between 40 and 70) for “rigid” and highly packed glassy polymers, in which the selective process is driven by diffusion (20-25),24and even higher in carbon molecular sieves (70-75).25The GO-PEI coatings showed a slope of 90, better than the one of both glassy polymers and carbon molecular sieves. The rigorous dependence of permeability on molecule size was not reported in previous works on pure GO membranes. Instead, a size cut-off and a complex dependence on the chemical nature of the permeant was typically observed;3,

6, 10, 11, 26

the sieving mechanism was

attributed to the capillary action of GO sheets, whose inter-layer distance varied from 0.8 nm to >1.6 nm due to the presence of water or other solvents.6, 26 In GO-PEI bilayers instead the GO sheets are forced to have a fixed, uniform spacing of 2.3 nm, much larger than the size of any of the molecules tested, thus ruling out any sieving due to GO spacing.



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Water transport in GO membranes was studied extensively in previous works, showing a unique mechanism still to be fully understood.

4, 27

Similar to what previously observed in

pure GO membranes, a peculiar behavior determined by presence of water molecules was observed also here for GO-PEI multilayers. Indeed, water vapor permeation tests performed on PET films coated by 50 BLs revealed a negligible barrier effect on the water flux, confirming that water represents an exception on barrier and size-selective effects here described. Water molecules are able to interact significantly with both GO and PEI layers and the diffusion is similar to that of helium atoms, as they have almost equal molecular size (kinetic diameters are equal to 265 and 260 pm for water and helium, respectively). The resulting water flux is thus very high, so that no appreciable flux reduction was observed after the coating deposition on PET film, and no reliable water permeability of the coating can be calculated. The selectivity of GO-PEI was compared to those of other benchmark materials for each binary mixture (Table S4 and S5).22A value of 150 for a reference He/CO2 separation was obtained for the 25+25 PET+GO-PEI multilayer, compared to 7.8 and 2.6 for the two substrate materials, PET and Matrimid, respectively. Furthermore, if the intrinsic selectivity of the 25+25 BLs GO-PEI coating was evaluated removing the contribution of the substrate, a very high selectivity (≈500) could be obtained for the He/CO2 couple. Although the 25+25 BLs showed a remarkable selectivity performance, the transmembrane flux achievable was still too low for a potential application in membranes for gas separation, and not comparable to those of benchmark polymeric materials. However, as already discussed above, a great advantage of the LbL technique is the strong control over the structure and coating thickness at the nano-scale that shall be achieved. Thinner coatings with 10+10 GO-PEI BL on Matrimid were thus prepared and analyzed, exploiting the much larger gas transmission rate in these systems.


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The results obtained are listed in Table S4 and S5, and illustrated in Figure 4c, which compares the permeability of various gases in GO-PEI coatings of both 10+10 and 25+25 BLs, and those of the bare PET and Matrimid substrates. The selectivity values obtained for a 10+10 BL coating resulted lower than those obtained for a 25+25 BL coating, with maximum selectivity ≈350 (for the He/CO2 gas pair). However, the transport of light gases was significantly enhanced: in particular, the He permeability increased by an order of magnitude when the coating decreased from 25 to 10 BL. The He/CO2 selectivity vs. He permeability is showed in the Robeson’s plot in Figure 5 for a 10+10 BLs coating, together with literature data on several available polymer membranes and the upper-bound limits estimated in 1999 and 2008.22 Remarkably, the results obtained for a GO-PEI 10+10 BLs deposited onto a 60 µm-thick Matrimid film (black symbols) are comparable or better than the actual upper-bound, demonstrating the potential of this material, able to combine the great selectivity of the LbL coating with the larger permeability of the Matrimid substrate.

Figure 5. He permeability and He/CO2 selectivity of Matrimid samples coated by 10+10 GO-PEI BLs, reported in a Robeson’s plot22 including some of the relevant reference data reported in the same source: black triangle is GO-PEI, 10 BL on Matrimid (60 µm), red triangle corresponds to GO-PEI 10 BL coating without any substrate (calculated value) and the orange triangle is GO-PEI, 10 BL coating



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on a reference Matrimid 1 µm thick (calculated value).The effect of a further heat treatment of each sample at 80°C for 24 h after conventional IR exposure is also illustrated (square symbols).

We used Equation (1) to calculate the intrinsic permeability and selectivity of a 10 BL coating (red symbols in Figure 5) with no Matrimid, and even of an “ideal” system composed by 10 BL on a 1 µm thin Matrimid (orange symbols in Figure 5). This configuration, indeed, resembles the possible structure of a real gas separation membrane, with a thin polymer active layer (1 micron), as typically the case of commercial modules, coated on one side only by the GO-PEI composite layer developed in this work. The performance of such system combines the intrinsic selectivity of GO-PEI (≈ 270) with a high He permeability of 3.3 · 10-16 mol m-1 s-1 Pa-1. The permeability and selectivity of the membranes did not degrade with temperature; conversely, a heat treatment at 80 °C under vacuum improved the separation performance of the membranes, leading to a two-fold increase of the intrinsic He permeability of the coating This was most likely related to a partial rearrangement of the coating structure and the GO sheets, promoted by the increase in temperature, which favored the membrane performance. Hence, thanks to the nanometric structure of the multilayer system, the composite structure described here combines the high selectivity of the GO-PEI and the good permeability of standard bulk substrates, allowing the 2008 Robeson’s limit22 to be overcome, and materials with exceptional performance in gas separation to be obtained.

Conclusions We produced GO layered composite materials using a self-assembly technique based on LbL, alternating GO monoatomic nanosheets with highly polar polymeric chains of PEI. A significant reduction in oxygen permeability was obtained for these nanometric, partially transparent and resistant coatings. The technique is robust and versatile, and was tested using different materials and processing parameters, obtaining good results with different GO types 18 ACS Paragon Plus Environment

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and different polymeric films as substrates. The permeability of the coatings could be tuned by simply increasing the number of layers. The multilayer structure obtained also showed a remarkable selectivity when used as a gas separation membrane. Gas permeability showed an exponential dependence on the radius of the penetrant, giving a sieving performance comparable to or better than other highperformance materials, in particular for He/CO2 and H2/CO2 mixtures. The ability to exfoliate and process a wide range of 2D materials has created great excitement in the scientific community, allowing to imagine new 2D “metamaterials” composed of different monoatomic layers stacked over each other, featuring properties not achievable with natural materials known up to date. Most of the research on such 2D composite materials is in the field of electronics, targeting as example BN-graphene-BN or graphene-MoS2 multilayers for high-performance computing. Our work targets a completely different system composed of 2D nanosheets and 1D polymer chains, in which graphene oxide is stacked with a thin layer of a selected polymer; the 2D nanosheets force molecules to diffuse along a tortuous path, enhancing in this way their interaction time with the PEI chains. This highly constrained but ordered structure, delimited in between GO layers, controls the diffusion of gas molecules in a way proportional to their diameter, and significantly different to what previously observed in similar systems (e.g GO membranes). Noteworthy, all production steps used to the GO-PEI multi-layered structure are compatible with large scale production, allowing to obtain high productivity and good selectivity for gas separation on industrial scale. Continuous, roll-to-roll surface treatments requiring multiple solution coating steps are common and efficient at industrial level (think as example to the printing of journals and magazines using CMYK four-color process). In the SI, we describe also how we tested a cheaper and faster procedure, which avoids the washing steps; this procedure gives as well working membranes even if with lower quality, but would allow to decrease by a factor 2 the 19 ACS Paragon Plus Environment


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cost for a future process scale up. A LbL continuous process was also recently described in literature by Chang et al. 28 Differently form CMYK printing, the process described here requires only 2 different types of deposition (GO and PEI). The 25 or 50 layers could be obtained using just 2 liquid reservoirs and a suitable mechanical loop, to guide in continuous thousands of linear meters of film in and out the liquid reservoirs. The high value added by our coatings to cheap polymeric films renders thus this process potentially attractive for industrial applications.

Experimental Section Materials: Two different GO aqueous dispersions were used for the fabrication of composite coatings, named GO(L) and (GO(C). GO(L) was produced in our laboratories by a modified Hummers' method, following the procedure already described by Treossi et al.29 The dispersion obtained was characterized by a solid content of 0.2 wt. %, with > 90% of monolayer sheets, C/O ratio around 1 and an average lateral sheet dimension of 70 µm (as measured by Atomic Force Microscopy analysis). GO(C) was also kindly provided by Graphene-XT srl (Bologna, Italy) as water dispersion, and used as received. The producer declared a C/O ratio of around 1, and a monolayer content greater than 80%, with an average lateral sheet dimension of 25 µm. Samples with smaller GO sheet size were prepared by ultrasonication (water bath, 25 kHz) of the GO dispersions. Two different sonication times were investigated, 3 and 6 hours).21 Sodium hydroxide (NaOH, reagent grade as anhydrous pellets), hydrochloric acid (HCl, reagent grade, 37% wt. aqueous solution), dichloromethane (> 99.8% purity, anhydrous, with stabilizers) and branched poly(ethylene imine) (PEI, 750kDa, 50% wt. aqueous solution) were purchased from Sigma-Aldrich S.r.l. (Milano, Italy). 20 ACS Paragon Plus Environment

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Matrimid® polyimide powder was kindly provided by Huntsman Advanced Materials. Polyethylene terephthalate (PET, Kemafoil®, thickness = 100 µm) films were kindly provided by Coveme S.p.A. (San Lazzaro di Savena, Bologna, Italy). Film thickness was measured by a digital micrometer (Mitutoyo, precision of ±0.5 µm).

Film Production and Pretreatments: Self-standing Matrimid films were fabricated by solvent casting: polymer powder was dissolved in dichloromethane and the solution stirred under ambient conditions; 1% wt. solution was used to produce a film by solvent casting on a glass Petri dish. The film obtained was then peeled off the glass and thermally treated at 200°C for 24 h under vacuum before the coating process and tests, in order to ensure complete solvent removal.30 The final thickness of the membranes prepared was 60 µm. The surface of PET and Matrimid films was treated to provide a hydrophilic character, required by the self-assembly method. In the absence of this surface activation, absent or uneven coverage was achieved. The treatment used was an alkaline activation method proposed by Chen and McCharty: clean films were dipped in NaOH 1 M overnight, then rinsed with HCl 0.1 M aqueous solution for 5 minutes at room temperature.31 Although the treatment was done at 60°C by different authors,14,

31

we performed it at room temperature to prevent any possible thermal

modification of the polymer matrix, while allowing for longer treatment times. Finally, polymeric films were rinsed in deionized water (DI), and dried at room temperature. This technique induced a nucleophilic behavior of the surface.

Samples Assembly: we prepared uniform graphene based coatings on top of various substrates using a dip coating LbL technique, following a 4-step procedure: 1) A clean, dry hydrophilic sample was dipped into a positively-charged PEI solution for 5 min, 2) the sample was rinsed by DI for 20 min and dried in air at room temperature for 60 min, to ensure the formation of a homogeneous layer at the nanoscale. 21 ACS Paragon Plus Environment


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3) the sample was dipped in a 0.01 % wt. GO aqueous dispersion for 5 min, 4) the sample was finally rinsed by DI water and dried. The deposition cycle of oppositely-charged components described above yields a single BL; the procedure can be subsequently repeated up to the desired number of BL coatings on the substrate surface. The washing protocol was developed to ensure the complete removal of any excess of solutions or dispersions, and in order to obtain a thin and homogeneous nano-layer. The dipping and rinsing procedures were carried out automatically, by means of a homemade robot, able to guarantee constant lift speed (about 1 mm/s), and programmed to stop when the desired number of BL was reached. Partially-charged PEI was used as a cationic polyelectrolyte species, alternated to the GO sheets. PEI shows a cationic charge when nitrogen atoms are protonated, so that the charge density is pH-dependent.32 The concentration and pH of solutions and dispersions were thus crucial parameters to obtain a reliable and homogeneous self-assembly process. For this reason, DI water was normalized to pH 7 (by means of NaOH) and PEI 0.1 wt. % aqueous solution was prepared at pH 10 aiming to work at a low cationic charge density, 14, 32whereas a GO 0.01 wt. % aqueous dispersion (pH ≈ 5) was used without normalization. GO is a negatively-charged species, as the presence of oxygen moieties (e.g. epoxy or hydroxyl groups) confers to GO sheets a nucleophilic behavior. The two different types of GO were tested in order to investigate the effect of the GO layers lateral size on the coating assembly and on the resulting transport properties. In this work, the polymeric substrates were coated on both sides; thus, the number of BL present on each sample corresponds to two times the number of dipping cycles. A single specimen was instead produced ad-hoc with only one sample surface coated, masking the other face by a removable adhesive tape. Alternatives LbL deposition methods were also tested to overcome the main bottlenecks for the industrial implementation of such a technique, i.e. the large number of dips required and 22 ACS Paragon Plus Environment

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the rinsing steps, eventually leading to a shortened overall protocol (see SI). Alternative procedures tested were: - one long-duration (24h) dip into the graphene solution; - a multilayer LbL dipping sequence without any rinsing steps.

Graphene oxide characterization (AFM): Atomic Force Microscopy (AFM) measurements were carried out using a Digital Instruments AFM (NT-MDT), Nanoprobe cantilevers (Model: RTESP, Material: 1-10 Ohm-cm Phosphorus (n) doped Si, f0: 27-309 kHz, k: 20-80 N/m; from Veeco) operating in tapping mode.

Sample Characterization (SEM): A Scanning Electron Microscopy (SEM) analysis was carried out on samples deposited on Si wafers to demonstrate the presence of a regular alternating coating structure produced by the self-assembly process. Observations were performed with a ZEISS LEO 1530 microscope equipped with a Schottky emitter, operated at 5 keV and collecting secondary electrons by means of an In-Lens detector. Plane view images were obtained after sputtering with gold to inhibit charging effects in the non-conducting samples, allowing high resolution of surface characteristics to be achieved, whereas no treatment was employed in the cross-section, in order to prevent any modification of the layered species. The specimen was first frozen in liquid nitrogen and then cut by a microtome.

Sample Characterization (Ellipsometry): Ellipsometry measures the change of polarization of light reflected from a sample and yields information about thin film layers, even thinner than the wavelength of the probing light itself. The change of amplitude and phase of the p and s components (respectively linear and orthogonal to the plane of incidence of the light) after the reflection from the sample is depending on film properties like thickness, refractive index and absorption. Ellipsometry measures such change in the amplitudes and phases in both components of polarized light by rotating polarization components, namely Ψ and ∆. These values are input to an optical model to calculate the thickness and morphology of the sample multilayer materials. 23 ACS Paragon Plus Environment


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Imaging ellipsometry combines microscopy and ellipsometry. The microscopy aspect allows the direct visualization of the sample with an ellipsometric contrast image with a lateral resolution as small as 1 micron as well as the measurement of the ellipsometric parameters ∆ and Ψ with the highest lateral ellipsometric resolution down to 1 micron. By using a camera as detector, the complete field of view is measured simultaneously. The resulting ∆ and Ψ maps allow to analyse homogeneities of thickness, optical constants within a lateral ellipsometric resolution of 1 µm.

Gas Transport Measurements: A manometric technique was used for the characterization of pure gas transport properties, according to the ASTM D-1434 norm,33 in which the penetrant flux through samples is evaluated from the pressure increase in a calibrated closed volume, starting from high vacuum conditions. The tests were carried out in a lab-made apparatus, already described elsewhere,34 whose main features are recalled here for the sake of clarity. A low-range capacitive gauge (Edwards Barocell, 0-10 mbar range, sensitivity 10-3 mbar) measured the gas pressure in the downstream compartment for the evaluation of the penetrant flux, while the upstream pressure was kept constant above atmospheric pressure to prevent any possible contamination of pure gas contamination, at approximately 1.2-1.5 bar (Druck, 0-10 bar). The whole experimental apparatus was enclosed in a thermostatic chamber (PID control, sensitivity 10-1 K). A complete system scheme and a typical output of this type of experiment are reported in the Supporting Information (Figure S3). Specimens were first treated overnight under high-vacuum and temperature, then placed in a specific sample holder (a Millipore filter cell, 2.2 cm2 permeation area), set across the two closed volumes. Before each run, a vacuum test was performed to ensure the absence of any leak. Each test was repeated at least two times to ensure the accuracy and reproducibility of data obtained.


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The penetrant transmission rate (TR), i.e. the molar flux per unit area rescaled by the pressure difference across the sample, is calculated as follows: TR =

Vd 1 J P  dp  = = d  ∆ p l  t  t → +∞ R T A ∆ p

(3)

in which J is the penetrant molar flux per unit area through the membrane, Δp is the pressure gradient, pd the downstream pressure, Vd the calibrated volume of the downstream side and A is the membrane area. The permeability P, a material property, can also be obtained conveniently by multiplying the TR by the sample thickness: in case of multilayer samples, this procedure only gives an “effective” permeability, dependent on the respective thicknesses of the two layers.

Acknowledgements Andrea Liscio and Emanuele Treossi, ISOF-CNR, are acknowledged for data analysis and for the support on building of the dipping robot, respectively. Alessandra Scidà, ISOF-CNR, is acknowledged for her support with optical transparency experiments. TizianoVollaro, Gabriele Ghirotti, and Giovanni Gnazzo, DICAM, University of Bologna, and Andrea Sardano, CIRI-MAM, University of Bologna, are acknowledged for their experimental support. We acknowledge the Operative Program FESR 2007–2013 of Regione Emilia-Romagna – Attività I.1.1. Partial support of CIRI-MAM Advanced Applications in Mechanical Engineering and Materials Technology Interdepartmental Center for Industrial Research (Alma Mater Studiorum-Università di Bologna) is also gratefully acknowledged. The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme (grant agreement n°696656 Graphene Flagship).

Supporting Information: the Supporting Information contains details on effect of heath treatment, rinsing steps, additional characterization data and complete tables of samples prepared.

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