Graphene-based CO₂ sensing and its cross-sensitivity with humidity

We present graphene-based CO₂ sensing and analyze its cross-sensitivity with humidity. In order to assess the selectivity of graphene-based gas sensing to various gases, measurements are performed in argon (Ar), nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), and air by selectively venting the desired gas from compressed gas bottles into an evacuated vacuum chamber. The sensors provide a direct electrical readout in response to changes in high concentrations, from these bottles, of CO₂, O₂, nitrogen and argon, as well as changes in humidity from venting atmospheric air. From the signal response to each gas species, the relative graphene sensitivity to each gas is extracted as a relationship between the percentage-change in graphene's resistance response to changes in vacuum chamber pressure. Although there is virtually no response from O₂, N₂ and Ar, there is a sizeable cross-sensitivity between CO₂ and humidity occurring at high CO₂ concentrations. However, under atmospheric concentrations of CO₂, this cross-sensitivity effect is negligible-allowing for the use of graphene-based humidity sensing in atmospheric environments. Finally, charge density difference calculations, computed using density functional theory (DFT) are presented in order to illustrate the bonding of CO₂ and water molecules on graphene and the alterations of the graphene electronic structure due to the interactions with the substrate and the molecules.