Direct Chemical Vapor Deposition Synthesis of Porous Single‐Layer Graphene Membranes with High Gas Permeances and Selectivities

Abstract

yield orders of magnitude higher gas permeances because of its atomic thickness and low cross-membrane transport resistance. [3,4] Because perfect single-layer graphene is almost impermeable to gases, [5,6] in-plane pores, which are vacancy defects in the graphene lattice, are necessary for gas permeation. To realize the enormous potential of porous graphene for gas separation, the areal pore density in graphene should be considerably high. Our group theoretically predicted that the pore density needs to exceed 10 14 m −2 for a graphene membrane to surpass the Robeson upper bound for polymers. [7,8] Further, to enable selective gas transport, the pore sizes in the graphene membrane should be precisely controlled such that they are commensurate with the gas molecular sizes. In fact, the pore sizes in porous graphene are typically widely distributed and fitted by a lognormal distribution, where a small fraction of larger pores determine the total gas permeance. [9][10][11][12] As a result, an even higher pore density is needed for porous graphene to achieve a high gas permeance with enough competitiveness. Etching away atoms from pristine graphene has been the most widely applied strategy to increase the pore density in a graphene membrane. High-energy ion or electron bombardment was used to perforate graphene in some early studies. [12][13][14][15] Later, chemical oxidative etching was developed as a more scalable graphene perforation method. [16][17][18] For example, He et al. used O 2 plasma to perforate as-synthesized graphene from chemical vapor deposition (CVD) and measured a H 2 /CH 4 selectivity > 15. [11] Zhao et al. exposed pristine graphene to O 2 plasma for a short pore nucleation burst, and then to mild O 3 etching for controllable pore expansion, in order to partially decouple the pore nucleation and growth and to obtain a narrow pore size distribution. [19] However, despite the efforts made to decouple the pore nucleation and growth, the correlation between them still exists for those etching-based methods. Because the nucleation and growth of the pores are both triggered by etching (e.g., O 2 plasma), one needs to raise the energy intensity of the etching reaction to increase the pore density, which in turn generates larger, less selective pores. This Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are ...

Gas Separations using Nanoporous Atomically Thin Membranes: Recent Theoretical, Simulation, and Experimental Advances

“…The oxidatively etched NATMs not only exhibited higher selectivities than The experimental results are categorized according to the perforation methods used, including ion beam bombardment (some followed by additional chemical etching), [150][151][152][153][154]156,157] oxidative etching, [143,[158][159][160][161][162][163][164][165][166] and intrinsic defect formation during CVD. [62,143,149,159,160,164,[167][168][169][170][171] The Robeson upper bounds for polymers are plotted assuming 1 µm thickness. [11] the Knudsen selectivities, but also surpassed the 2008 Robeson upper bound of polymeric membranes for H 2 /CH 4 separation and CO 2 /N 2 separation (assuming 1 µm thickness).…”

Demonstrating and Unraveling a Controlled Nanometer‐Scale Expansion of the Vacancy Defects in Graphene by CO2

“…Size and density of these defects are sensitive to the CVD environment (precursor, temperature, pressure, relative concentrations of precursors, etc.) and the catalytic substrate [11, 29, 31, 36, 44, 45] . Currently, there is a lack of characterization technique which can distinguish between the relative population of grain boundary and intragrain defects.…”

“…and the catalytic substrate. [11,29,31,36,44,45] Currently, there is a lack of characterization technique which can distinguish between the relative population of grain boundary and intragrain defects. This is mainly because these defects are present in a low density and it is extremely challenging to search them in highresolution mode in HRTEM.…”

Chemical vapor deposition of 2D materials: A review of modeling, simulation, and machine learning studies

“…Very recently, Yuan et al. reported the CVD synthesis of nanoporous graphene to produce monolayered graphene membranes demonstrating the highest hydrogen to methane permeation selectivity reported so far ( Yuan et al., 2021 ). The authors observed a larger number of nanopores at a growth temperature of 800°C, as compared to 900°C.…”