Subnanometer Two-Dimensional Graphene Oxide Channels for Ultrafast Gas Sieving Jie Shen,‡ Gongping Liu,‡ Kang Huang, Zhenyu Chu, Wanqin Jin,* and Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University (former Nanjing University of Technology), 5 Xinmofan Road, Nanjing 210009, P.R. China S Supporting Information *
ABSTRACT: Two-dimensional (2D) materials with atomic thickness and extraordinary physicochemical properties exhibit unique mass transport behaviors, enabling them as emerging nanobuilding blocks for separation membranes. Engineering 2D materials into membrane with subnanometer apertures for precise molecular sieving remains a great challenge. Here, we report rational-designing external forces to precisely manipulate nanoarchitecture of graphene oxide (GO)-assembled 2D channels with interlayer height of ∼0.4 nm for fast transporting and selective sieving gases. The external forces are synergistic to direct the GO nanosheets stacking so as to realize delicate size-tailoring of in-plane slit-like pores and plane-to-plane interlayer-galleries. The 2D channels endow GO membrane with excellent molecular-sieving characteristics that offer 2−3 orders of magnitude higher H2 permeability and 3-fold enhancement in H2/CO2 selectivity compared with commercial membranes. Formation mechanism of 2D channels is proposed on the basis of the driving forces, nanostructures, and transport behaviors. KEYWORDS: subnanometer channels, external forces, graphene oxide, two-dimensional, membranes, molecular sieving, gas separation
T
area and high-density perforation remains a technical challenge.18 In contrast to the top-down processing approach, bottom-up approach, involving assembly of 2D materials into well-defined macroscopic structures, opens up a more practical route for the facile and large-scale membrane production. For instance, the single-atom-thick with lateral dimensions as high as tens of micrometers structure makes graphene oxide (GO) highly stackable for laminate.19−21 Molecular transport through the 2D materials-assembled laminate occurs in in-plane slit-like pores and then plane-toplane intergalleries. Subsequent to the pioneer work of Geim and co-workers,22 more and more studies including our previous work11,23−25 have demonstrated that the intergalleries between GO sheets play a vital role in fast and selective molecular transport. That leads the successful application of laminar GO membranes for water purification. Researchers also developed physical or chemical approaches to modulate the intergalleries to improve water flux and/or selectivity.26−29 However, those methods are difficult to finely regulate the intergallery of GO laminate within subnanometer-size that is necessary for precise separation of small molecules, e.g., desalinating water and sieving gases. Another great challenge is developing strategies to avoid nonselective in-plane defects
he discovery of graphene has triggered great interest in studying two-dimensional (2D) materials in chemistry, physics, materials science, and related areas owing to their single-atom or single-polyhedral thicknesses and distinct properties from their bulk counterparts.1,2 2D materials, including nanosheets of graphene family materials,3 metal− organic frameworks (MOFs),4,5 or zeolites,6 offer an exciting opportunity for developing a new family of separation membranes featuring unique apertures structure. Selective mass transport of a synthetic membrane is enabled by intrinsic or predefined pores. Very few 2D MOF or zeolite nanosheets with intrinsic nanochannels were exploited for molecular separation membranes because of the difficulties in exfoliation without structural deterioration. The intrinsic defects of graphene layer are found to be selective permeation of ions7−9 and gases,10 whose implementation is however limited by the low density, random distribution and uncontrollable size of defects in the graphene nanosheets.11 Theoretical studies predicated that imparting nanopores in single-layer graphene is able to achieve highly selective passages of water,12,13 ions,14 and gases.15 Such ideal vision strongly depends on the techniques for fabricating perfect monolayer and drilling controlled holes in the graphene sheet. Recent experimental investigations, using elaborate oxidative etching and electron/ ion bombardment to realize physical perforation of graphene, demonstrated the potential of nanoporous graphene membranes for water purification.7,8,16,17 Nevertheless, precise, large© XXXX American Chemical Society
Received: November 19, 2015 Accepted: February 11, 2016
A
DOI: 10.1021/acsnano.5b07304 ACS Nano XXXX, XXX, XXX−XXX
Article
www.acsnano.org
Article
ACS Nano
Figure 1. Design and construction of 2D channels. (a) External force driven assembly approach for fabricating 2D channels. It involves threedimensional external forces in x, y, and z axes. Enlarged schematic shows force analysis for one 2D channel unit consisting of GO nanosheets and polymer chain. Three main types of forces are included: intrinsic force, “outer” external forces (compressive force, centrifugal force and shear force) which are applied outside the 2D channel unit and “inner” external force (GO−polymer molecular interactions) which are applied inside the 2D channel unit. (b) Hypothetical evolution of surface and cross section of GO-assembled 2D channels from intrinsic force induced disordered structure (left) to highly ordered laminar structures (right) driven by introduced synergistic external forces.
method32 (see Methods section, Figure 2a,b). As indicated by Fourier transform infrared (FTIR) (Figure S1) and Raman spectra (Figure S2), the oxidation process introduced hydroxyl and carboxyl groups on both surface and edges of the graphene layer. The arisen repulsive electrostatic interactions between the negatively charged carboxyl groups, on the one side, prevent GO aggregation caused by attractive van der Waals’ force and hydrogen bonds between GO nanosheets. Thus, GO nanosheets were easily dispersed in water solution with GO concentration of 0.1−2 mg/mL. On the other side, the stronger repulsive forces would hinder close packing of GO nanosheets during self-assembly process.33 That is supposed to give rise to local out-of-order accumulation and several large voids in GO laminate. A schematic of this disorder laminar structure driven by intrinsic forces of GO is given in Figure 1b. Advances in nanoscience have demonstrated that nanobuilding blocks often demand an input of energy or forces for engineering into particular structures to their thermodynamically lowest energy state.34 Here, we classify the introduced external forces (EFs) into two types: “outer” EFs and “inner” EFs. The “outer” EFs refer to compressive, centrifugal, and shear forces, which are applied outside GO laminate; the “inner” EFs refer to molecular interactions that are applied inside the laminate. As shown in Figure 1a, the concept of three-dimensional “outer” EFs was demonstrated by employing vacuum-spin technique for GO assembly process, in which the vacuum-suction provides EF along z axis while the centrifugal rotation supplied EFs along x and y axes. The negative effect of repulsive interactions in x, y, and z directions can be effectively suppressed by the introduced in-plane (along x and y axes) centrifugal force and air shear force, and plane-to-plane (along z
and random plane-to-plane stacking. They are prone to be generated during GO assembly caused by intrinsic repulsive electrostatic forces between the carboxyl groups of GO.30 The nonideal transport channels hampered success of laminar 2D materials membrane in exhibiting as excellent gas sieving properties as inorganic membranes. A recent breakthrough in physically perforation technology making porous graphene membranes was reported for gas separation.31 But the drilled pores cannot be smaller than 7.6 nm, which is far from the requirement of gas sieving membranes. Until now, it still has to confront the complex trouble of finely controlling the nanostructure of 2D channels. Herein, we propose a facile external force driven assembly approach to realize subnanometer 2D channels readily assembled by GO building blocks. As shown in Figure 1, the designed external forces are applied both outside and inside the GO laminate. They collaboratively overcome the intrinsic repulsive electrostatic interactions between GO, with the aim of eliminating nonselective stacking defects and stimulating highly ordered assembly of GO nanosheets. We also utilize the external forces to finely manipulate the 2D channels aperture for fast and selective transporting gases. The as-prepared ∼0.4 nm-height 2D channels enable development of high-permeable membranes with remarkable molecular-sieving properties, having much higher separation performance for H2/CO2 and H2/C3H8 gas pairs than state-of-the-art gas separation membranes.
RESULTS AND DISCUSSION Fabrication of 2D Channels. The starting 2D materials, single-layer GO nanosheets, were prepared by chemically oxidation of bulk graphite followed the modified Hummer’s B
DOI: 10.1021/acsnano.5b07304 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 2. Morphologies and fine structures of membranes with 2D channels. (a) Digital photo of GO aqueous dispersion with concentration of 1 mg/mL. (b) AFM image of the synthesized single-layer GO nanosheets. (c) Digital photos of blank Al2O3 substrate and EFDA−GO membrane. The rubber washer was used for GO protection during the permeation test. (d) Surface SEM images of EFDA−GO membrane with blank Al2O3 substrate inserted. (e) Surface SEM image of self-assembly GO membrane. (f and g) Cross-section SEM images of EFDA− GO and self-assembly GO membrane, respectively. (h−j) TEM images of (h) GO membrane prepared by “outer” EFs, (i and j) EFDA−GO membrane (PEI solution concentration of 0.25 wt %). The yellow dashed arrows are eye-guiding lines indicating the orientations of GO.
behavior was governed only by intrinsic forces of GO including van der Waal’s force, hydrogen bonding, and electrostatic interactions. (2) Dip-coating method was conducted for assembling GO nanosheets on a porous ceramic substrate, which provides one kind of external force−capillary force in z axis. (3) Centrifugal force and air shear force in x and y axes were applied for depositing GO nanosheets on the ceramic substrate via spin method. (4) Filtration, which is state-of-theart method for preparing 2D materials laminate, was adopted to provide compressive force in z axis for assembling GO nanosheets into laminate. Finally, (5) GO laminate was also prepared by vacuum-spin method that includes all the above “outer” EFs in x, y and z axes but “inner” EFs (molecular interactions) introduced by polymer molecules. Characterization of 2D Channels. Figure 2c shows typical digital photos of blank ceramic substrate and the asprepared EFDA−GO membrane. With the step-by-step deposition of GO nanosheets driven by EFs, the white disk with a diameter of 3 cm was entirely covered by a continuous brown GO layer. As displayed in Figure 2d, our homemade ceramic substrate has a porous surface with average pore size of ∼100 nm. GO layer was uniformly distributed on the substrate surface, and no in-plane defect was observed by scanning electron microscope (SEM) even under higher magnification (Figure S8). From the cross-sectional view (Figure 2f), we can
axis) compressive force. Consequently, the stacking of GO nanosheets begins to be ordered both in in-plane and plane-toplane directions (Figure S3). The quasi-ordered laminar structure offers a prerequisite for controlling the slit-like pores size (d) and interlayer height (h) for selective transporting molecules. To further weaken the negative intrinsic repulsive electrostatic forces, we added “inner” EFs into the assembly by means of building up molecular interactions via step-by-step depositing GO nanosheets and polymer molecules (see Methods section). The idea was demonstrated by choosing polyethylenimine (PEI) as the polymer, and then electrostatic interaction, covalent binding, and hydrogen bonding can be expected between GO and PEI. The synergic function of “outer” EFs and “inner” EFs would realize the highly ordered laminar structure (Figure 1b). By using the proposed external force driven assembly approach (EFDA), we deposited GO-assembled laminate with 2D channels onto the surface of porous ceramic (Al2O3) substrates to engineer composite membranes for practical separation (see Methods section). The formed membranes are denominated as EFDA−GO membrane. To study the force influence on GO assembly behaviors and 2D channels microstructure, we also prepared GO laminates via five controlled methods (Figures S4−S7). (1) GO nanosheets were assembled on the glass via drop-casting, in which assembly C
DOI: 10.1021/acsnano.5b07304 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano find a porous support layer corresponding to the ceramic substrate, providing sufficient mechanical strength while owning negligible transport resistance for gases, and a 1-μmthick well-stacked GO laminate layer is firmly adhered onto the porous substrate. Meanwhile, we compared the schematics, appearances, SEM and atomic force microscope (AFM) images of the controlled GO membranes (Figures S4−S7). The dropcasted GO laminate induced by intrinsic forces exhibits a visible heterogeneous surface with a poorly stacked and highly folded structure with large defects (Figure 2e,g). This morphology was improved by the capillary force of porous substrate, showing a less corrugated surface (Figure S6b). However, the disordered structure and tens of nanometer-sized defects were not eliminated. Defective laminar stacking and poor adhesion with the substrate can be seen in the spin-coated samples. It is probably because GO nanosheets can be easily swung out by the centrifugal force and air shear force while being without the influence of vertical pulling forces. The filtrated GO laminate has a more firmly stacked laminar structure. Nevertheless, the compressive stress in z axis is unable to assemble GO nanosheets into a well ordered alignment at horizontal direction; thus, several defects are still found in the surface. The GO laminate that resulted from the comprehensive threedimensional EFs shows a much smoother membrane surface without visible defects. It suggests that the “outer” EFs can stimulate GO nanosheets organized assembly both in in-plane and plane-to-plane directions, transforming the GO laminate from disordered to quasi-ordered structure (Figure S3). From the SEM surface and cross-sectional morphologies of GO laminate with (Figure 2d,f) and without (Figure S6e) applying “inner” EFs, we could hardly distinguish structural differences. Therefore, we further examined the fine nanostructures of GO laminates under transition electron microscope (TEM). Only driven by the “outer” EFs, GO nanosheets tend to assembly in several different orientations, indicating a relatively random packing manner (Figure 2h). This random packing could lead to tiny defects and disordered channel arrangement in the directions of in-plane and plane-to-plane. On the contrary, after tuning by additional “inner” EFs (GO−polymer molecular interactions), GO nanosheets were assembled in almost the same direction to form a highly ordered laminar structure with subnanometer spaces among them (Figure 2i,j). It implies formation of plenty of well-organized 2D channels in the GO layer. We attempted to take molecular simulation tool to visualize the structural transformation between GO monolayer and PEI molecule under an ideal situation (see Methods section). As displayed in Supporting Information Movie 1, GO nanosheet induced by the GO−polymer molecular interactions is in the trend of moving to PEI molecule. It is believed to enhance the firm and ordered structure, as well as inhibit structural defects. Meanwhile, this enables finely tuning the GO interlayer gallery spacing which offers an exciting opportunity for molecular transport and sieving.19 The average height of the 2D channels in GO membranes was characterized by X-ray diffraction (XRD) technique, which is a widely used tool for analyzing interlayer gallery spacing (namely d-spacing).28 First, we compared the XRD patterns of graphite, GO powder and assembled GO laminates without introducing molecular interactions (Figures S9 and S10). After the introduction of oxygen-containing groups by oxidation, the distance between the adjacent graphene sheets was increased to 0.82 nm. The enlarged intergallery spacing is of great
importance since it provides more spaces for molecules diffusion through the membrane prepared by GO than by bulk graphite.22,24 Only driven by intrinsic forces, the selfassembled GO laminate exhibits a slightly reduced d-spacing of 0.79 nm. By applying external forces (e.g., capillary, centrifugal, compressive) to promote the GO assembly, the d-spacing was effectively reduced, indicating GO nanosheets are closer to each other. Furthermore, the comprehensive “outer” EFs in three dimensions could achieve a highest laminar stacking order. Then, by further introducing “inner” EFs by molecular intercalation, we realized precise manipulation of the interlayer height of 2D channels by readily controlling the amount of polymer molecules. As shown in Figure 3, the characteristic
Figure 3. Analysis for the average interlayer height of 2D channels in membranes. XRD spectra of EFDA−GO membranes with PEI solution concentration of 0−1 wt %.
peak of pristine GO laminate (i.e., “outer” EFs sample) at 2θ = 12.2° indicates the initial d-spacing is about 0.72 nm. When the appropriate number of PEI molecules (determined by PEI concentration in solution) were intercalated into GO nanosheets, the peak shifted toward lower diffraction angle, yielding an enlarged d-spacing. We did not find obvious decline in laminate crystallinity, indicating a maintained ordered stacking structure. As the PEI concentration increased to 0.25 wt %, the d-spacing increased to 0.76 nm with the characteristic peak shifted to 2θ = 11.5°. Since graphene thickness (0.34 nm) accounts for a part of the d-spacing,24,35 the height of empty space (interlayer height, h) between GO nanosheets is calculated to be 0.42 nm (Figure 3). Much higher PEI concentration could further enlarge the GO interlayer gallery, but the crystallinity would be reduced, according to the larger full width at half-maximum (fwhm) and lower intensity for GO laminates using 0.5−1.0 wt % PEI. D
DOI: 10.1021/acsnano.5b07304 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 4. Mechanical properties of membranes with 2D channels. (a) Comparison of hardness and elasticity modulus of pristine GO (without “inner” EFs) and EFDA−GO membranes; (b) schematic of the nanoindentation/scratch test; (c) the scratching load-distance and frictiondistance curves for pristine GO and EFDA−GO membranes, respectively. (d) SEM images of scratch morphologies for pristine GO and EFDA−GO membranes.
good interfacial adhesion is also attributed to capillary force of the porous substrate, hydrogen and chemical bonding between GO and Al2O3,37 as well as electrostatic attraction between positively charged PEI and negatively charged Al2O3 surface. As it can be seen, after scratching, pristine GO membrane showed the exposure of substrate layer while EFDA−GO membrane did not (Figure 4d). It is worth pointing out that our robust EFDA−GO composite membrane is particularly preferred for practical application and long-term continuous operation. Gas Transport of 2D Channels. Gas permeation measurement was carried out to test gas sieving properties of the asprepared GO membranes with 2D channels. First, we used hydrogen (H2) as the small gas molecule, with kinetic diameter of 0.29 nm, and carbon dioxide (CO2) as the large gas molecule with kinetic diameter of 0.33 nm (only 0.04 nm larger than that of H2). As shown in Figure 5a, the as-prepared EFDA−GO membranes exhibit extraordinary molecular-sieving properties for H2/CO2 with H2 permeability of 840−1200 Barrer [1 Barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1] and H2/CO2 selectivity of 29−33. This performance clearly surpasses the permeability-selectivity upper-bound for polymeric membranes.38 In particular, the EFDA−GO membranes showed
Additionally, we measured mechanical properties of the GO membranes with 2D channels by nanoindentation/scratch technique (see Supporting Method 1).36 The mechanical strength and interfacial adhesive force were compared for the GO laminates with or without applying “inner” EFs. Generally, highly ordered laminar structure of paper-like GO is able to show good mechanical properties because of the combination of macroscopic flexibility and stiffness. However, less wellpacked structure can cause poor mechanical strength and even fracture. Figure 4a shows that the hardness of EFDA−GO laminate was about 1.3 times than that of pristine GO laminate and the elastic modulus was also increased by 45%. With the introduced molecular interactions, the GO laminate can exhibit self-reinforcing behavior along the load direction to resist the exterior forces.30 Also, it would become more pliable and sustain more deformation during the local bending tests because of the reinforcement of interlocking of plane-to-plane GO nanosheets. The adhesive force for EFDA−GO membrane is determined as 41.36 mN (Figure 4c), which is much higher than that of pristine GO membrane. We suppose that the inplane stiffness can be reinforced after intercalating polymer chains, so that GO layers were hard to be scratched out. The E
DOI: 10.1021/acsnano.5b07304 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Figure 5. Gas permeation and adsorption properties of membranes with 2D channels. (a) H2/CO2 separation performance of EFDA−GO membranes compared with state-of-the-art gas separation membranes; (b) H2/CO2 long-term operation test for EFDA−GO membrane; (c) gas permeation behaviors of EFDA−GO membrane and the comparison of H2/CO2 and H2/C3H8 selectivity with those of pristine GO membrane. The gas permeation tests of (a−c) were measured at 0.2 MPa and 25 °C, 1 Barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1. (d) CO2 and H2 adsorptions on pristine GO (without “inner” EFs) and EFDA−GO membranes at 25 °C, both of which employ single-site Langmuir models. α is the calculated adsorption selectivity of CO2/H2. Gas permeation results of EFDA−GO membrane under different transmembrane differential pressure. (e) Gas permeability and (f) selectivity for H2/CO2 and H2/C3H8.
microporosity (PIMs),40 carbon molecular sieves (CMS),41 MOFs42−44 and laminar GO and MoS2 membranes.45−47 The gas channels at subnanometer scale make our GO membrane ultrafast and highly selective in permeation of H2, making it promising for practical applications which need effective high
2−3 orders of magnitude higher H2 permeability than the commercialized polybenzimidazole (PBI) and polyimide (PI) membranes, accompanied by 3-fold increase of selectivity.39 And our membranes are also competitive compared with emerging advanced membranes including polymers of intrinsic F
DOI: 10.1021/acsnano.5b07304 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano throughput.48 They show great potential for industrial application especially for H2 purification (methanol reforming process) and CO2 capture. Furthermore, the membrane showed excellent steady performance during a 100-h continuous permeation test (Figure 5b). After the long-term operation, no defects were found in the membrane. The result demonstrates that the GO composite membranes with highly interlocked structure are able to deal with high volumetric gases flow of and streams. The excellent gas sieving property of the 2D channels was further probed by permeation of larger gas molecules: N2 (0.364 nm) and C3H8 (0.43 nm). As expected, high selectivity of H2/N2 and H2/C3H8 was obtained for EFDA−GO membrane, as shown in Figures 5c and S12. In particular, C3H8 permeability is significantly low (
