The single-layer graphene film, when incorporated with molecular-sized pores, is predicted to be the ultimate membrane. However, the major bottlenecks have been the crack-free transfer of large-area graphene on a porous support, and the incorporation of molecular-sized nanopores. Herein, we report a nanoporous-carbon-assisted transfer technique, yielding a relatively large area (1 mm2), crack-free, suspended graphene film. Gas-sieving (H2/CH4 selectivity up to 25) is observed from the intrinsic defects generated during the chemical-vapor deposition of graphene. Despite the ultralow porosity of 0.025%, an attractive H2 permeance (up to 4.1 × 10−7 mol m−2 s−1 Pa−1) is observed. Finally, we report ozone functionalization-based etching and pore-modification chemistry to etch hydrogen-selective pores, and to shrink the pore-size, improving H2 permeance (up to 300%) and H2/CH4 selectivity (up to 150%). Overall, the scalable transfer, etching, and functionalization methods developed herein are expected to bring nanoporous graphene membranes a step closer to reality.
Oxidation of graphitic
materials has been studied for more than
a century to synthesize materials such as graphene oxide, nanoporous
graphene, and to cut or unzip carbon nanotubes. However, the understanding
of the early stages of oxidation is limited to theoretical studies,
and experimental validation has been elusive. This is due to (i) challenging
sample preparation for characterization because of the presence of
highly mobile and reactive epoxy groups formed during oxidation, and
(ii) gasification of the functional groups during imaging with atomic
resolution, e.g., by transmission electron microscopy. Herein, we
utilize a low-temperature scanning tunneling microscope (LT-STM) operating
at 4 K to solve the structure of epoxy clusters form upon oxidation.
Three distinct nanostructures corresponding to three stages of evolution
of vacancy defects are found by quantitatively verifying the experimental
data by the van der Waals density functional theory. The smallest
cluster is a cyclic epoxy trimer. Their observation validates the
theoretical prediction that epoxy trimers minimize the energy in the
cyclic structure. The trimers grow into honeycomb superstructures
to form larger clusters (1–3 nm). Vacancy defects evolve only
in the larger clusters (2–3 nm) in the middle of the cluster,
highlighting the role of lattice strain in the generation of vacancies.
Semiquinone groups are also present and are assigned at the carbon
edge in the vacancy defects. Upon heating to 800 °C, we observe
cluster-free vacancy defects resulting from the loss of the entire
epoxy population, indicating a reversible functionalization of epoxy
groups.
A controlled manipulation of graphene edges and vacancies is desired for molecular separation, sensing and electronics applications. Unfortunately, available etching methods always lead to vacancy nucleation making it challenging to control etching. Herein, we report CO2‐led controlled etching down to 2–3 Å per minute while completely avoiding vacancy nucleation. This makes CO2 a unique etchant for decoupling pore nucleation and expansion. We show that CO2 expands the steric‐hindrance‐free edges with an activation energy of 2.71 eV, corresponding to the energy barrier for the dissociative chemisorption of CO2. We demonstrate the presence of an additional configurational energy barrier for nanometer‐sized vacancies resulting in a significantly slower rate of expansion. Finally, CO2 etching is applied to map the location of the intrinsic vacancies in the polycrystalline graphene film where we show that the intrinsic vacancy defects manifest mainly as grain boundary defects where intragrain defects from oxidative etching constitute a minor population.
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