Identifying and clearing individual oxygen impurities on graphene through the use of NO₂ as a radical scavenger

We present a novel method to detect and eliminate oxygen adatoms on a graphene basal plane, which can be observed as individual single-electron transfer (SET) events through graphene Hall measurements. The central idea, supported by first-principles calculations, is to use NO₂ as a radical scavenger, effectively targeting and removing oxygen impurities on graphene. The reaction between NO₂ and the oxygen adatom produces a negatively charged NO₃⁻ species and creates a hole carrier in graphene. Subsequently, NO₃⁻ reacts with NO₂ to form N₂O₅, accompanied by an electron back-donation to graphene. The thermal decomposition of N₂O₅ produces neutral NO₃, which can accept an electron from graphene and return to the NO₃⁻ state. Under conditions of low oxygen impurity levels and NO₂ partial pressures, these SET reactions induce intermittent, step-like changes in the graphene's Hall resistivity ρ_xy at room temperature. Notably, the simulated ρ_xy patterns during NO₂ adsorption closely resemble previously observed patterns [Schedin et al., Nat. Mater. 6 (2007) 652–655], suggesting a connection to the proposed SET reactions for unintentionally introduced oxygen impurities. Furthermore, we find that NO₂ also reacts with hydroxyl and peroxide groups on the graphene basal plane, removing them by forming HNO₃ and N₂O₅, respectively. Our findings offer a promising approach to remove oxygen-containing functional groups from graphene, paving the way for obtaining high-quality graphene from reduced graphene oxide.