Molecular Mechanisms of CO₂ Adsorption in Diamine-Cross-Linked Graphene Oxide

Graphene oxides (GOs) are layered carbon materials composed of a mixture of oxygenated functional groups that can react with different molecules. Alkyl diamine molecules (ethylenediamine, butanediamine, or hexanediamine) can insert into the layers of GO and systematically expand its interlayer spacing from 0.762 nm to over 1.030 nm. In this work, we found that CO2 uptake maximizes at the interlayer spacing of 0.860 nm in GOs cross-linked by diamines. We characterized the subtle chemical and compositional differences among samples by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and solid-state NMR. In situ 13C NMR was applied at variable temperatures to investigate the adsorption mechanisms of CO2, which revealed both physically and chemically adsorbed CO2 in diamine-cross-linked GO matrices. In particular, we observed the unique state of CO2 that are physically trapped in GO layers with 0.860 nm spacing by van der Waals interactions. This state of CO2 was not observed in GOs with either smaller or larger interlayer spacings.