Selective Gas Permeation in Graphene Oxide–Polymer Self

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Selective Gas Permeation in Graphene Oxide−Polymer SelfAssembled Multilayers Davide Pierleoni,† Matteo Minelli,†,‡ Simone Ligi,§ Meganne Christian,⊥ Sebastian Funke,∥ Niklas Reineking,∥ Vittorio Morandi,⊥ Ferruccio Doghieri,†,‡ and Vincenzo Palermo*,#,¶

Downloaded via UNIV OF CONNECTICUT on June 15, 2018 at 10:10:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Civil, Chemical, Environmental and Materials Engineering (DICAM) and ‡Interdipartimental Center for Industrial ResearchAdvanced Mechanics and Materials (CIRIMAM), Alma Mater StudiorumUniversity of Bologna, via Terracini 28, Bologna I-40131, Italy § Graphene-XT srl, via D’Azeglio 15, I-40123 Bologna, Italy ∥ Accurion GmbH, Stresemannstraße 30, DE-37079 Göttingen, Germany ⊥ Institute for Microelectronics and Microsystems (IMM) and #Institute for Organic Synthesis and Photoreactivity (ISOF), National Research Council of Italy (CNR), via Gobetti 101, Bologna I-40126, Italy ¶ Department of Industrial and Materials Science, Chalmers University of Technology, Goteborg 412 58, Sweden S Supporting Information *

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 the ability of graphene oxide (GO) and poly(ethyleneimine) (PEI) multilayers to overcome such a limit. The PEI chains act as molecular spacers in between the GO sheets, yielding a highly reproducible, periodic multilayered structure with a 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 the gas molecule, with a sieving mechanism never obtained in pure GO membranes, in which a size cutoff 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. KEYWORDS: graphene, gas separation, coatings, polymer materials, composite materials, nanomaterials, functional surfaces, membranes

■

INTRODUCTION One of the peculiar properties of a single, perfect graphene sheet is its two-dimensional (2D) shape, which provides a highly effective barrier to gas permeation.1 Although a single graphene sheet is impermeable even to 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.2−4 Exciting results have been obtained in recent years by Geim et al., who showed the unique behavior of graphene layers toward ions and molecules, as the 2D materials can allow precise molecular sieving, selective proton transport, and even separation of atomic isotopes.5−7 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, © 2018 American Chemical Society

Shen et al. produced micrometer-thick graphene oxide (GO) membranes with sub-nanometer channels, obtaining excellent selectivity performances for H2/CO2 mixtures, whereas 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.10−12 The separation of CO2-containing gaseous streams represents a technological problem of growing interest for carbon capture and storage (CCS) applications because of the need to reduce the impact of greenhouse gases on global climate. The Received: January 23, 2018 Accepted: March 9, 2018 Published: March 9, 2018 11242

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces

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, and a code states the GO material used: GO(L) for larger GO sheets with an average flake size of 40 μm (Figures S1 and S2 in the Supporting Information) and GO(C) smaller flakes with an average flake size of 25 μm. The complete list of all samples prepared and analyzed in this work is available in Table S1 in the Supporting Information. Upon deposition, the transparency of the polymer decreased proportionally to the number of deposited layers, as visible also by naked eye (Figure 1b). 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 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. A 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 techno-polymer 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 good precision coating thickness and surface morphology. Figure 2a shows the scanning electron microscopy (SEM) micrograph of the cross section of a 25 BLsample, in which a compact and homogeneous coating layer is

precombustion CCS strategy exploits a gasification stage of a fossil energy source prior to its combustion and the use of hydrogen as an 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 graphenebased coatings on standard polymer substrates using the layerby-layer (LbL) technique process. LbL is a bottom-up method already used successfully to fabricate thin layers of highly ordered graphene sheets.14−16 It uses positive and negative electrolyte species that are adsorbed 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, and then alternate dips in two different solutions, with intermediate rinsing steps, create layers that grow on top of each other thanks to the 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 the Experimental

Figure 1. (a) Schematic procedure of the multilayer assembly to form a variable number of GO−PEI BLs. 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 BLs: 10 + 10 BL (b) and 5 + 5 BL (c).

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. 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 pretreatment 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.

Figure 2. SEM micrographs of GO-based LbL coatings: (a) cross section of a 25 BL GO−PEI sample on silicon wafer; (b) top view of 25 BL GO−PEI coating on PET (sample 50GO(L) a); and (c) higher magnification of the same sample. 11243

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces

Figure 3. Spectroscopic measurement on 25 BL of GO−PEI of the ellipsometric angles Δ (a) and Ψ (b) for two angles of incidence. 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 65 × 140 μm.

observed. The low roughness of the Si substrate allowed an accurate and reliable estimation of the 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 the 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 macroagglomerates or uneven coverage. The higher magnification image (Figure 2c) showed small wrinkles, indicative of the presence of GO sheets on the substrate. 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 insights into 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 charge-coupled device (CCD) camera, to map the experimental ellipsometric angles Δ and Ψ on a mesoscopic area (more details are reported in the Experimental section). The ellipsometric angles Δ and Ψ mapped are related to thickness and refractive index changes. Δ is the phase difference and Ψ the amplitude ratio for pand s-polarized light (linear and orthogonal components to the light incidence plane). With this setup, a mesoscopic field of 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. By contrast, conventional ellipsometry gives only a single measurement, averaged on areas that are approx. 50 μm in size. Imaging ellipsometry allowed identifying 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 × 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 that observed by SEM (92 ± 5 nm). The number of layers was calculated from the experimental data Δ and Ψ. The calculation was performed with a standard model; see the Supporting Information section for further details. The best agreement with the experimental data was obtained assuming 25 BLs (Figure 3). For comparison, the fitting was performed also assuming a different number of layers, ranging from 23 to 26, and different models were compared using their mean square deviation from experimental data. The results obtained demonstrated clearly the existence of 25 well-distinguished GO−PEI BLs on top of the PET substrate (see Table S2) and demonstrated that an alternating structure was in fact formed by the deposition process. It is noteworthy that 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, confirming the excellent uniformity achievable by the LbL deposition (Figure 3c,d). The lower limit of the thickness range obtained for GO (1.1 nm) agreed well to that 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, because of irregular sheet-to-sheet border overlapping. The fraction x of overlapping GO nanosheets in each layer of the stack was estimated to be x < 25% based on a rule of mixture calculation. We used a simple weighted average on the GO thickness: tMEAS = t1(1 − x) + 2t1x, where t1 is the thickness of a monolayer (1.1 nm, from AFM) and tMEAS is the 11244

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces

difference across the sample. We could filter the effect of the bare substrate (TRsubst) from that of the multilayer coated sample (TRML) by means of the series resistance eq 1

average thickness of the GO phase in the composite (≈1.35 nm, obtained by ellipsometry). Gas Permeability. The addition of a thin GO−PEI coating on a polymer substrate changed dramatically the gas transport properties of the materials, measured by gas permeation tests at 35 °C (manometric method; see Figure S3). All samples were heated after coating deposition and before permeation measurements using an infrared (IR) lamp for 30 min, to completely remove possible trapped water (see discussion in the Supporting Information). First, the permeability of a relatively large molecule (O2) was analyzed revealing that a 25 GO−PEI BLs per side coating (50GO(C)) on PET gave a 96% reduction of the oxygen transfer rate (OTR), as illustrated in the Supporting Information section (see Figures S4−S6). Such a blocking effect was attributed to the highly ordered structure provided by the LbL deposition and the 2D nature of the composite, dictated by the presence of the GO layers.19 Pure PEI coatings were prepared for the sake of comparison and showed no barrier effect and no appreciable difference in OTR with respect to bare PET (Table S3). The effect of the number of BLs was measured and, as expected, more BLs enhanced the barrier effect (Figure S7) due to the increase of the barrier coating thickness. The LbL technique is indeed tunable and modular, allowing the desired gas transport properties only changing the number of dipping cycles. The effect of the nanosheet size on gas permeability was also tested. We prepared samples with two different types of GO with different lateral sizes: a laboratory made suspension (GO(L), average size 40 ± 20 μm) and a commercial one (GO(C), average size 25 ± 10 μm). Surprisingly, the O2 permeability obtained with these two GOs was similar (5.0 × 10−15 vs 5.6 × 10−15 mol m−2 Pa−1 s−1). It may be expected that large GO sheets would provide a more effective blocking layer toward large molecules such as O2, forcing the diffusing molecules to follow a highly tortuous path. In traditional nanocomposite systems in fact the higher the aspect ratio of the impermeable particles, the lower the OTR. However, in the coatings prepared in this work by LbL, the GO sheets had a very large lateral size, comparable to the macroscopic thickness of the polymer film used. Indeed, both GO types considered were characterized by quite high average lateral dimensions and monolayer content (>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 be due to molecules not only 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, thus leading 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 1.6 nm because of the presence of water or other solvents.6,26 In GO−PEI BLs, 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. Water transport in GO membranes was studied extensively in previous works, showing a unique mechanism still to be fully understood.4,27 Similar to that previously observed in pure GO membranes, a peculiar behavior determined by the 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 described here. Water molecules are expected to diffuse similarly to helium atoms, as they have almost equal molecular size (kinetic diameters are equal to 265 and 260 pm for water and helium, respectively), but water is able to interact significantly with both GO and PEI layers. The resulting water transport was thus very high, so that no appreciable flux reduction was observed after the coating deposition on the PET film, and no reliable water permeability of the coating can be calculated. The selectivity of GO−PEI (Tables S4 and S5) was compared to those of other benchmark materials for each binary mixture.22 A value of 150 for a reference He/CO2 separation was obtained for the 25 + 25 PET + GO−PEI multilayer, compared with 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 filtering the contribution of the substrate (eq 1), 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 nanoscale that shall be achieved. Thinner coatings with 10 + 10 GO−PEI BL on Matrimid were thus prepared and analyzed, exploiting the much larger gas TR in these systems. The results obtained are listed in Tables S4 and S5 and are 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 were 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 versus He permeability is shown in the Robeson’s plot in Figure 5 for a 10 + 10 BL coating, together with the literature data on several available polymer membranes and the upper-bound limits estimated in 1999 and 2008.22 Remarkably, the results obtained for GO−PEI 10 + 10 BLs deposited onto a 60 μm thick Matrimid film (black symbols) are comparable or better than the actual upperbound, demonstrating the potential of this material, able to

Figure 4. (a) Gas permeability values at 35 °C in bare PET substrate (triangles), effective permeability in the multilayer sample with 25 GO−PEI BL coating per side (50 BLs) (circles), as well as the values of intrinsic permeability of the GO−PEI coatings (squares), as a function of penetrant kinetic diameter. (b) 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.

monolayer graphene deposited on porous alumina and partially etched, and 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 selectivity values of 150 up to 500, outperforming standard gas sieving materials. The slope of the permeability curve versus 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 mainly driven by diffusion,24 and is even higher in carbon molecular sieves (70−75).25 The 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 the molecule size was not reported in previous works on pure GO membranes. Instead, a size cutoff and a complex dependence on the chemical nature of the permeant was typically 11246

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces

permeability showed an exponential dependence on the kinetic diameter of the penetrant, giving a sieving performance comparable to or better than other high-performance 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 imagining 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 boron nitride (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 GO 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 that previously observed in similar systems (e.g., GO membranes). It is noteworthy that all production steps used to the GO− PEI multilayered structure are compatible with large-scale production, allowing for obtaining 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 the industrial level (think as example to the printing of journals and magazines using CMYK four-color process). In the Supporting Information, 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 decreasing the cost by roughly a factor 2 for a future process scale up. A LbL continuous process was also recently described in literature by Chang et al.28 Different from 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 continuously thousands of linear meters of film in and out the liquid reservoirs. The high value added by our coatings to cheap polymeric films thus renders this process potentially attractive for industrial applications.

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 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).

combine the great selectivity of the LbL coating with the larger permeability of the Matrimid substrate. We used eq 1 to calculate the intrinsic permeability and selectivity of a 10 BL coating (red symbols in Figure 5) with no Matrimid and even those of an “ideal” system composed of 10 BLs 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 μm), 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 systems 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 twofold 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.

■

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 AFM 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 h).21 Sodium hydroxide (NaOH, reagent grade as anhydrous pellets), hydrochloric acid (HCl, reagent grade, 37 wt % aqueous solution),

■

CONCLUSIONS We produced GO-layered composite materials using a selfassembly technique based on LbL, alternating GO monoatomic nanosheets with 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 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 11247

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces dichloromethane (>99.8% purity, anhydrous, with stabilizers), and branched PEI (750 kDa, 50 wt % aqueous solution) were purchased from Sigma-Aldrich S.r.l. (Milano, Italy). Matrimid polyimide powder was kindly provided by Huntsman Advanced Materials. PET (Kemafoil, thickness = 100 μm) films were kindly provided by Coveme S.p.A. (San Lazzaro di Savena, Bologna, Italy). The film thickness was measured by a digital micrometer (Mitutoyo, resolution 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 was 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, 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 McCarthy: clean films were dipped in NaOH 1 M overnight, and then rinsed with HCl 0.1 M aqueous solution for 5 min 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, the 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 aqueous PEI solution (0.1 wt %) 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. (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 to obtain a thin and homogeneous nanolayer. 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 pHdependent.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,14,32 whereas 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 to investigate the effect of the GO layer 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.

Alternative LbL deposition methods were also tested to overcome the main bottlenecks for the industrial implementation of such a technique, that is the large number of dips required and the rinsing steps, eventually leading to a shortened overall protocol (see Supporting Information). Alternative procedures tested were • one long-duration (24 h) dip into the graphene solution; • a multilayer LbL dipping sequence without any rinsing steps. GO Characterization (AFM). AFM measurements were carried out using a Digital Instruments AFM (NT-MDT), Nanoprobe cantilevers (model: RTESP, material: 1−10 Ω cm phosphorus (n) doped Si, f 0: 27−309 kHz, k: 20−80 N/m; from Veeco) operating in tapping mode. Sample Characterization (SEM). A 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 nonconducting samples, allowing high resolution of surface characteristics to be achieved, whereas no treatment was employed in the cross section, 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 depends on the film’s properties like thickness, refractive index, and absorption. Ellipsometry measures such changes in the amplitudes and phases in both components of polarized light by rotating polarization components, namely, Ψ and Δ. These values are input to an optical model to calculate the thickness and morphology of the sample multilayer materials. 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 μm as well as the measurement of the ellipsometric parameters Δ and Ψ with the highest lateral ellipsometric resolution down to 1 μm. By using a camera as detector, the complete field of view is measured simultaneously. The resulting Δ and Ψ maps allow analyzing 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 Barocel, 0−10 mbar range, sensitivity 10−3 mbar) measured the gas pressure in the downstream compartment for the evaluation of the penetrant flux, whereas 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, and 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 the data obtained. 11248

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces The penetrant TR, that is, the molar flux per unit area rescaled by the pressure difference across the sample, is calculated as follows TR =

⎛ dp ⎞ Vd 1 J P = = ⎜ d⎟ Δp l ⎝ t ⎠t →+∞ RT AΔp

(5) Hu, S.; Lozada-Hidalgo, M.; Wang, F. C.; Mishchenko, A.; Schedin, F.; Nair, R. R.; Hill, E. W.; Boukhvalov, D. W.; Katsnelson, M. I.; Dryfe, R. A. W.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K. Proton Transport through One-Atom-Thick Crystals. Nature 2014, 516, 227− 230. (6) Yang, Q.; Su, Y.; Chi, C.; Cherian, C. T.; Huang, K.; Kravets, V. G.; Wang, F. C.; Zhang, J. C.; Pratt, A.; Grigorenko, A. N.; Guinea, F.; Geim, A. K.; Nair, R. R. Ultrathin Graphene-Based Membrane with Precise Molecular Sieving and Ultrafast Solvent Permeation. Nat. Mater. 2017, 16, 1198. (7) Lozada-Hidalgo, M.; Hu, S.; Marshall, O.; Mishchenko, A.; Grigorenko, A. N.; Dryfe, R. A. W.; Radha, B.; Grigorieva, I. V.; Geim, A. K. Sieving Hydrogen Isotopes through Two-Dimensional Crystals. Science 2015, 351, 68. (8) Shen, J.; Liu, G.; Huang, K.; Chu, Z.; Jin, W.; Xu, N. Subnanometer Two-Dimensional Graphene Oxide Channels for Ultrafast Gas Sieving. ACS Nano 2016, 10, 3398−3409. (9) Liang, B.; Zhang, P.; Wang, J.; Qu, J.; Wang, L.; Wang, X.; Guan, C.; Pan, K. Membranes with Selective Laminar Nanochannels of Modified Reduced Graphene Oxide for Water Purification. Carbon 2016, 103, 94−100. (10) Kim, H. W.; Yoon, H. W.; Yoon, S.-M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S.; Choi, J.-Y.; Park, H. B. Selective Gas Transport through Few-Layered Graphene and Graphene Oxide Membranes. Science 2013, 342, 91−95. (11) Li, H.; Song, Z.; Zhang, X.; Huang, Y.; Li, S.; Mao, Y.; Ploehn, H. J.; Bao, Y.; Yu, M. Ultrathin, Molecular-Sieving Graphene Oxide Membranes for Selective Hydrogen Separation. Science 2013, 342, 95− 98. (12) Boutilier, M. S. H.; Jang, D.; Idrobo, J.-C.; Kidambi, P. R.; Hadjiconstantinou, N. G.; Karnik, R. Molecular Sieving across Centimeter-Scale Single-Layer Nanoporous Graphene Membranes. ACS Nano 2017, 11, 5726−5736. (13) Scholes, C. A.; Smith, K. H.; Kentish, S. E.; Stevens, G. W. Co2 Capture from Pre-Combustion Processes-Strategies for Membrane Gas Separation. Int. J. Greenhouse Gas Control 2010, 4, 739−755. (14) Yang, Y.-H.; Bolling, L.; Priolo, M. A.; Grunlan, J. C. Super Gas Barrier and Selectivity of Graphene Oxide-Polymer Multilayer Thin Films. Adv. Mater. 2013, 25, 503−508. (15) Yu, L.; Lim, Y.-S.; Han, J. H.; Kim, K.; Kim, J. Y.; Choi, S.-Y.; Shin, K. A Graphene Oxide Oxygen Barrier Film Deposited Via a SelfAssembly Coating Method. Synth. Met. 2012, 162, 710−714. (16) Xiong, R.; Hu, K.; Grant, A. M.; Ma, R.; Xu, W.; Lu, C.; Zhang, X.; Tsukruk, V. V. Ultrarobust Transparent Cellulose NanocrystalGraphene Membranes with High Electrical Conductivity. Adv. Mater. 2016, 28, 1501−1509. (17) Dubas, S. T.; Schlenoff, J. B. Factors Controlling the Growth of Polyelectrolyte Multilayers. Macromolecules 1999, 32, 8153−8160. (18) Rajasekar, R.; Kim, N. H.; Jung, D.; Kuila, T.; Lim, J. K.; Park, M. J.; Lee, J. H. Electrostatically Assembled Layer-by-Layer Composites Containing Graphene Oxide for Enhanced Hydrogen Gas Barrier Application. Compos. Sci. Technol. 2013, 89, 167−174. (19) Pierleoni, D.; Xia, Z. Y.; Christian, M.; Ligi, S.; Minelli, M.; Morandi, V.; Doghieri, F.; Palermo, V. Graphene-Based Coatings on Polymer Films for Gas Barrier Applications. Carbon 2016, 96, 503− 512. (20) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Determination of the Local Chemical Structure of Graphene Oxide and Reduced Graphene Oxide. Adv. Mater. 2010, 22, 4467−4472. (21) Kouroupis-Agalou, K.; Liscio, A.; Treossi, E.; Ortolani, L.; Morandi, V.; Pugno, N. M.; Palermo, V. Fragmentation and Exfoliation of 2-Dimensional Materials: A Statistical Approach. Nanoscale 2014, 6, 5926−5933. (22) Robeson, L. M. The Upper Bound Revisited. J. Membr. Sci. 2008, 320, 390−400. (23) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids; McGraw-Hill: New York, NY, USA, 2000. (24) Kelman, S. D.; Matteucci, S.; Bielawski, C. W.; Freeman, B. D. Crosslinking Poly(1-Trimethylsilyl-1-Propyne) and Its Effect on

(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 material property permeability P 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.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01103. Effect of heat treatment, rinsing steps, additional characterization data, and complete tables of samples prepared (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Vincenzo Palermo: 0000-0002-0168-9693 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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. Tiziano Vollaro, 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. The 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 no. 696656 Graphene Flagship).

■

REFERENCES

(1) Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; van der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Impermeable Atomic Membranes from Graphene Sheets. Nano Lett. 2008, 8, 2458−2462. (2) Leenaerts, O.; Partoens, B.; Peeters, F. M. Graphene: A Perfect Nanoballoon. Appl. Phys. Lett. 2008, 93, 193107. (3) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving through Graphene Oxide Membranes. Science 2014, 343, 752−754. (4) Su, Y.; Kravets, V. G.; Wong, S. L.; Waters, J.; Geim, A. K.; Nair, R. R. Impermeable Barrier Films and Protective Coatings Based on Reduced Graphene Oxide. Nat. Commun. 2014, 5, 4843. 11249

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250

Research Article

ACS Applied Materials & Interfaces Solvent Resistance and Transport Properties. Polymer 2007, 48, 6881− 6892. (25) Hayashi, J.-i.; Yamamoto, M.; Kusakabe, K.; Morooka, S. Effect of Oxidation on Gas Permeation of Carbon Molecular Sieving Membranes Based on Bpda-Pp’oda Polyimide. Ind. Eng. Chem. Res. 1997, 36, 2134−2140. (26) Huang, L.; Li, Y.; Zhou, Q.; Yuan, W.; Shi, G. Graphene Oxide Membranes with Tunable Semipermeability in Organic Solvents. Adv. Mater. 2015, 27, 3797−3802. (27) Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K. Unimpeded Permeation of Water through Helium-Leak-Tight Graphene-Based Membranes. Science 2012, 335, 442−444. (28) Chang, S.; Slopek, R. P.; Condon, B.; Grunlan, J. C. Surface Coating for Flame-Retardant Behavior of Cotton Fabric Using a Continuous Layer-by-Layer Process. Ind. Eng. Chem. Res. 2014, 53, 3805−3812. (29) Treossi, E.; Melucci, M.; Liscio, A.; Gazzano, M.; Samorì, P.; Palermo, V. High-Contrast Visualization of Graphene Oxide on DyeSensitized Glass, Quartz, and Silicon by Fluorescence Quenching. J. Am. Chem. Soc. 2009, 131, 15576−15577. (30) Ansaloni, L.; Minelli, M.; Baschetti, M. G.; Sarti, G. C. Effect of Relative Humidity and Temperature on Gas Transport in Matrimie: Experimental Study and Modeling. J. Membr. Sci. 2014, 471, 392−401. (31) Chen, W.; McCarthy, T. J. Layer-by-Layer Deposition: A Tool for Polymer Surface Modification. Macromolecules 1997, 30, 78−86. (32) Elzḃieciak, M.; Zapotoczny, S.; Nowak, P.; Krastev, R.; Nowakowska, M.; Warszyński, P. Influence of Ph on the Structure of Multilayer Films Composed of Strong and Weak Polyelectrolytes. Langmuir 2009, 25, 3255−3259. (33) ASTM Standard D 1434, 1982; ASTM International, 2009. (34) Minelli, M.; De Angelis, M. G.; Doghieri, F.; Marini, M.; Toselli, M.; Pilati, F. Oxygen Permeability of Novel Organic-Inorganic Coatings: I. Effects of Organic-Inorganic Ratio and Molecular Weight of the Organic Component. Eur. Polym. J. 2008, 44, 2581−2588.

11250

DOI: 10.1021/acsami.8b01103 ACS Appl. Mater. Interfaces 2018, 10, 11242−11250