For oxidation, the reactor temperature was lowered to the oxidation temperature, and the H2/Ar atmosphere was exchanged with O3. Samples were exposed to O3 for a given period of time, after which they were cooled down to room temperature. O-clusters are formed on graphene during this step. The extent of oxidation was increased by either increasing the reaction time, temperature, or O3 velocity (the latter discussed in the last section). To incorporate pores in the O-cluster, the oxidation with O3 was stopped, and the reactor was heated to 150 ºC to facilitate the gasification of the clusters.