Bottom-up synthesis of graphene films hosting atom-thick molecular-sieving apertures

Abstract

Incorporation of a high density of molecular-sieving nanopores in the graphene lattice by the bottom-up synthesis is highly attractive for high-performance membranes. Herein, we achieve this by a controlled synthesis of nanocrystalline graphene where incomplete growth of a few nanometer-sized, misoriented grains generates molecular-sized pores in the lattice. The density of pores is comparable to that obtained by the state-of-the-art postsynthetic etching (1012 cm−2) and is up to two orders of magnitude higher than that of molecular-sieving intrinsic vacancy defects in single-layer graphene (SLG) prepared by chemical vapor deposition. The porous nanocrystalline graphene (PNG) films are synthesized by precipitation of C dissolved in the Ni matrix where the C concentration is regulated by controlled pyrolysis of precursors (polymers and/or sugar). The PNG film is made of few-layered graphene except near the grain edge where the grains taper down to a single layer and eventually terminate into vacancy defects at a node where three or more grains meet. This unique nanostructure is highly attractive for the membranes because the layered domains improve the mechanical robustness of the film while the atom-thick molecular-sized apertures allow the realization of large gas transport. The combination of gas permeance and gas pair selectivity is comparable to that from the nanoporous SLG membranes prepared by state-of-the-art postsynthetic lattice etching. Overall, the method reported here improves the scale-up potential of graphene membranes by cutting down the processing steps.

Gas Separation Membranes with Atom-Thick Nanopores: The Potential of Nanoporous Single-Layer Graphene

“…Recently, in this direction, we introduced an advanced image analysis to investigate the grain orientation in nanocrystalline nanoporous graphene samples by calculating the local FFT around each pixel of the HRTEM and using these data to produce an orientation map (Figure 8H−I). 42 While the advances mentioned above allow characterization of porous graphene lattice, it is currently extremely challenging to resolve functional groups around the nanopores, which tend to gasify during sample preparation and imaging. Ongoing developments in (i) atomic resolution using low electron accelerating voltages, 50 (ii) high-speed imaging by coupling ultrafast isolated electron pulses 53 with direct counting electron detectors, 54 (iii) in situ cleaning protocols, and (iv) scanning atomic electron tomography, will open venues for studying the dynamics of structural changes and the threedimensional position of atoms and functional groups at the edge of graphene nanopores.…”

“…We recently exploited this route to prepare porous graphene with a high density of intrinsic vacancy defects. Depositing a small amount of carbon precursor on the foil as a thin polymer film followed by a short heat treatment at 500 °C, produced porous nanocrystalline graphene (PNG) (Figure E) . The heat treatment resulted in the pyrolysis of the polymer, which built a C reservoir in the Ni matrix.…”

“…While it is crucial to develop sample preparation techniques that allow imaging of a large volume of nanopores per sample, it is equally important to develop computational methods capable of efficiently analyzing the increasingly large volume of data. Recently, in this direction, we introduced an advanced image analysis to investigate the grain orientation in nanocrystalline nanoporous graphene samples by calculating the local FFT around each pixel of the HRTEM and using these data to produce an orientation map (Figure H–I) …”

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

“…Other experimental work has also reported the presence of gas-sieving defects in large-area CVD-grown graphene ( Huang et al., 2018 ; Khan et al., 2019 ), with the latter work employing a benzene-based CVD process. Reduction in the CVD temperature has been successfully used to introduce defects into graphene ( Kidambi et al., 2018 ; Villalobos et al., 2021 ), by promoting polycrystalline growth. Despite these experimental advances, there is a lack of understanding about how nanopores are formed during the CVD process and how their shape and size may be controlled.…”

Kinetic Control of Angstrom-Scale Porosity in 2D Lattices for Direct Scalable Synthesis of Atomically Thin Proton Exchange Membranes

“…In this context, scalable cost-effective synthesis of 2D materials via CVD − ,, and related processes have typically focused on minimizing defects in the 2D lattice and/or minimizing grain boundaries by forming larger domains ,,− to yield high-quality continuous monolayers for electronic applications. Some studies have explored the synthesis of nanoporous graphene for size-selective membrane applications by using lower CVD synthesis temperature, , pyrolyzing polymers/sugars on Ni substrate, quenched hot Pt foils in hydrocarbons to form nanoporous graphene, synthesized monolayer amorphous carbon (MAC) via laser-assisted CVD, introduced N dopants into graphene . Notably, Griffin et al measured enhanced proton transport through micron-scale membranes of nanoporous graphene ∼2 S cm –2 and MAC ∼1 S cm –2 with H + /Li + selectivity ∼10 for both, while Zeng et al measured proton conductance of ∼1.4 × 10 5 S m –2 (1 M HCl) for N-doped graphene (1 min N 2 plasma treatment of graphene) with H + /Cl – selectivity ∼40 and H + /methanol selectivity ∼1–2 orders of magnitude higher than Nafion.…”