Abstract
Engineering different two-dimensional materials into heterostructured membranes with unique physiochemical properties and molecular sieving channels offers an effective way to design membranes for fast and selective gas molecule transport. Here we develop a simple and versatile pyro-layering approach to fabricate heterostructured membranes from boron nitride nanosheets as the main scaffold and graphene nanosheets derived from a chitosan precursor as the filler. The rearrangement of the graphene nanosheets adjoining the boron nitride nanosheets during the pyro-layering treatment forms precise in-plane slit-like nanochannels and a plane-to-plane spacing of ~3.0 Å, thereby endowing specific gas transport pathways for selective hydrogen transport. The heterostructured membrane shows a high H2 permeability of 849 Barrer, with a H2/CO2 selectivity of 290. This facile and scalable technique holds great promise for the fabrication of heterostructures as next-generation membranes for enhancing the efficiency of gas separation and purification processes.
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Introduction
Membrane-based hydrogen separation technologies have advantages over conventional processes1,2 in energy efficiency, capital cost, and footprint3,4,5. However, current commercial membranes, which are fabricated from a few polymeric materials, suffer from the trade-off between gas permeability and selectivity6. Laminar membranes fabricated from two-dimensional (2D) nanosheets7,8, such as graphene oxide (GO)9,10,11,12, MXene13,14 and molybdenum disulfide (MoS2)15, can outperform the current gas permeability-selectivity upper bound by enabling molecular transport through in-plane channels and plane-to-plane interlayer spacings11,13. Nonetheless, engineering the interlayer spacing between neighboring nanosheets as gas transport pathways is crucial to fast and selective molecular transport13,16,17,18,19. Various techniques have been proposed to regulate gas transport pathways, including chemical cross-linking12,20,21,22, mechanical compression11, and pore etching23,24. Yang and coworkers developed a thiourea covalently linked GO framework (TU-GOF) membrane with narrow and well-defined 2D channels of 3.7 Å due to the small linkers and the interaction between GO and TU20. Shen and coworkers applied an external force on a GO laminate to direct GO nanosheet stacking, realizing subnanometer 2D apertures with an interlayer height of 4.0 Å11. However, the random arrangement of nanosheets causes nonselective in-plane defects and plane-to-plane spacing when they are stacked into membranes11,13. It is very important to develop effective strategies for not only precise manipulation of the gas transport pathways but also for minimization of the defects caused by the random stacking of 2D nanosheets.
Recently, fabricating van der Waals heterostructures by stacking different 2D materials alternatingly25,26,27,1a). Then, the filtration-made membranes were pyrolyzed at 900 °C for 3 h in a horizontal quartz tube furnace in an argon atmosphere. The heating process is illustrated in Supplementary Fig. 1b. Chitosan was carbonized and converted to graphene at high temperature38, while boron nitride nanosheets (BN) with high thermal resistance dominated the scaffold of the BNG membrane.
Characterization methods
Scanning electron microscopy (SEM): The SEM images of all samples were recorded on a Magellan 400 FEG-SEM (FEI, USA) operating at an accelerating voltage of 5 kV. All mounted samples were sputter-coated with iridium. High-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS): HRTEM images and EELS spectroscopy were obtained on an FEI Tecnai G2 F20 S-TWIN TEM operated at an accelerating voltage of 200 kV. High-magnification high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were acquired using a HAADF detector. Selected area diffraction pattern (SEAD) images were obtained on a FEI Tecnai G2 T20 transmission electron microscope operating at an accelerating voltage of 200 kV. The TEM samples were prepared by ultramicrotomy to obtain the cross-section of the fabricated membrane. More specifically, the settled samples were slightly trimmed down (25 mm/s) to obtain the cutting faces by a Leica ultra-cut S equipped with a 35° diamond blade. The cross-sectional layers were collected by TEM carbon grids from the water trough. Raman spectra: Raman spectra were recorded using a confocal micro-Raman System (Renishaw RM 2000) equipped with a near-IR diode laser at a wavelength of 782 nm (laser power: 1.15 mW and laser spot size: 1 μm). The excitation wavelength was 514 nm. All Raman spectra were collected by fine-focusing a 50× microscope objective, and the data acquisition time was 10 s. X-ray photoelectron spectroscopy (XPS): XPS was measured on a Thermo Scientific Nexsa Surface Analysis System equipped with a hemispherical analyzer. The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 72 W (6 mA and 12 kV, 400 × 800 μm 2 spots). Survey (wide) and high-resolution (narrow) scans were recorded at analyzer pass energies of 150 and 50 eV and step sizes of 1.0 eV and 0.1 eV, respectively. Data processing was carried out using Avantage software, and the energy calibration was referenced to the mainline of C 1 s at 284.8 eV. X-ray diffraction (XRD): The XRD patterns were recorded by using a 2θ range of 2–60° and a scan rate of 2 °•min−1 at ambient temperature on a Miniflex 600 diffractometer (Rigaku, Japan) with a Cu Kα radiation source (15 mA, 40 kV). The samples were analyzed using a physisorption analyzer (Micromeritics Triflux, USA) for characterization of BET surface area, pore volume, and pore size distribution. Both nitrogen sorption at −196 °C and carbon dioxide sorption at 0 °C were performed. Prior to gas sorption measurements, the samples were degassed at 200 °C for 1440 min to ensure the removal of adsorbed gas from the micropores of the heterostructured membranes. The pore size distribution was calculated using the Horvath-Kawazon method from the carbon dioxide sorption isotherm. Positron annihilation lifetime spectroscopy (PALS, EG&G ORTEC fast-fast spectrometer): Free-standing BNG membrane and pure carbonized chitosan film were broken into pieces and stored in foil for PALS measurements. The samples were on both sides of the positron source (1.5 × 106 Bq of 22NaCl sealed in a Mylar envelope). At least five files, each containing 1 × 106 integrated counts, were recorded for every sample. Due to the conductivity of the samples, no long-lifetime annihilation events occurred, and the average pore size was estimated from the free-annihilation using tau2. Thermogravimetric analysis (TGA): TGA measurements were recorded by a TA Instrument STD 650 TGA/DSC analyzer (USA). The pristine FBN film, the precursor membrane and the carbonized heterostructure membrane (FBN: chitosan = 7:10) were packed and weighed. Then, they were heat treated (heating rate: 20 °C/min) under an air atmosphere (Instrument grade, BOC) to burn off the carbon-related content. Finally, the white residual layer (boron nitride nanosheets, as shown in Supplementary Fig. 19) was weighed. The gas flow rate was set to 100 mL/min. Gas chromatograph: mixed gas separation was tested by an Agilent 8860 with a thermal conductivity detector (TCD).
Gas permeation experiments
The single gas permeation of the fabricated membranes was measured using a constant-volume/variable-pressure apparatus (Supplementary Fig. 12). The gas permeation rate was recorded by fixing a piece of the prepared free-standing membrane (with alumina substrate with a diameter of 13 mm) on a stainless-steel sample holder using an Agilent Torr Seal vacuum sealant. Then, it was placed inside a Pyrex tube with feed gas flowing through and connected to an MKS 628B Baratron pressure transducer and a vacuum pump.
The gas permeance experiments were performed using the steady-state gases H2, N2, CO2, and CH4 at room temperature. To achieve steady-state permeation conditions, each single gas measurement on the permeate side of the membrane was degassed in a vacuum for half an hour.
The molar flow rate of the permeating gas was calculated from the linear pressure rise, and its coefficient was calibrated using a digital flowmeter (ADM2000, Agilent, California, USA). The feed gas is supplied at room temperature and atmospheric pressure. The effective membrane area and thickness were measured by a Vernier caliper and SEM. The gas permeance, Pi (mol·m−2·s−1·pa−1), is defined by the following equation:
where Ni (mol·s−1) is the permeate flow rate of component gas i, ∆Pi (Pa) is the transmembrane pressure difference of i, and A (m2) is the measured membrane area (0.16 cm−2). The ideal selectivity Si/j was calculated from the relation between the permeance of pure i and j gases.
Mixed-gas permeation was conducted by a gas mixture with a ratio of 50:50 applied at the feed side of the BNG membrane, and the total flow rate of the feed gas was maintained at 200 mL/min (each gas at 100 mL/min) with a feed pressure at 1 bar. The gas flow was controlled by a mass flow control system (ALICAT Scientific). The sweep gas flow was constantly controlled by another mass flow meter. A gas chromatograph system (8860, Agilent) was used to analyze the composition of the permeate gases.
The mixed selectivity (αi/j) of two components in the mixed-gas permeation experiment was calculated by:
where x and y ate the volumetric fractions of the corresponding component in the feed and permeate side, respectively.
MD simulation methods
The MD simulation system is composed of feed and permeate sides connected by a BN/graphene slit-like nanochannel (Supplementary Fig. 15a). The nanochannel width W can be adjustable, which was determined by subtracting 0.34 nm from the interlayer distance between graphene and BN nanosheets. The feed side is filled with a binary mixture of 0.5 bar H2 and 0.5 bar CO2 (or CH4). Under the force of pressure-driven flow, gas molecules in the feed side will travel through the nanochannel to the permeate side. During simulations, the feed side is replenished with new gas molecules, and the molecules flowing into the vacuum side are deleted to maintain constant pressure on both sides. The gas flux was given by the first derivative of the number of deleted molecules with respect to time (Supplementary Fig. 15b). We applied the periodic boundary conditions along the X and Y axes and a reflection boundary condition along the Z axis. The velocity-Verlet integrator was implemented to update the position of atoms with a 1 fs timestep. The Berendsen thermostat58 was employed to keep the temperature of gas molecules at 300 K. In simulations, the channel atoms are fixed. The nonbonded interactions between atoms that are within a cutoff distance of 12 Å are modeled using Lennard‒Jones (LJ) and Coulomb potentials, given by
where εij characterizes the interaction strength, σij represents the effective atomic diameter, rij shows the distance between two atoms, ε0 is the permittivity of the vacuum, and q denotes the charge. The LJ parameters among atoms of different types were calculated using the Lorentz-Berthelot mixing rule. The bond and angle energies were calculated with the following equations:
where Kb and Ka are the spring constants for the bond and angle, respectively, r is the bond length, θ is the angle value, and 0 is the corresponding equilibrium state. The CHARMM force field was used for CO259. The OPLS-AA force field was employed for CH460. The force field parameters for H261, graphene62, and BN63 were taken from the references. All MD simulations were carried out using LAMMPS64.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
Data availability
All data that support the findings of this study are available within the paper’s Supplementary Information or from the corresponding author upon request.
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Acknowledgements
This work was supported by the Australian Research Council through Huanting Wang’s Australian Laureate Fellowship (Project no. FL200100049). The authors acknowledge the use of the instruments and scientific and technical assistance at the Monash Centre for Electron Microscopy (MCEM), a Node of Microscopy Australia. The authors also thank Anthony De Girolamo of the Department of Chemical and Biological Engineering for his technical support. The authors acknowledge the use of the facilities and the assistance of Yvonne Hora and Jisheng Ma at the Monash X-ray Platform.
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R.W., X.C., and H.W. conceived the project. R.W. designed and performed the membrane synthesis and characterization and gas permeation experiments with inputs from Z.L. H.W. supervised the project. Y.C. performed the HRTEM, HAADF-STEM, EELS, and SEAD characterizations. C.M.D. and D.A. performed PALS characterization. H.M. contributed to the discussion of the results and materials synthesis. Z.X., M.R.H., and W.S. contributed to the discussion of results and revision of the manuscript. R.W., J.Q., Z.L., and H.W. wrote and revised the manuscript. J.Q., F.W., and H.A.W. performed modeling studies. All authors approved the final version of the manuscript. R.W. and J.Q. contributed equally.
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Wang, R., Qian, J., Chen, X. et al. Pyro-layered heterostructured nanosheet membrane for hydrogen separation. Nat Commun 14, 2161 (2023). https://doi.org/10.1038/s41467-023-37932-9
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DOI: https://doi.org/10.1038/s41467-023-37932-9
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