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Fluorination of graphene sheets with xenon difluoride leads to the formation of the widest bandgap Gr derivative, namely, fluorographene. Accurate experimental observations distinguish two stages of mechanism in the fluorination procedure: the half-fluorination stage, wherein one side of the Gr sheet is rapidly fluorinated, and the full-fluorination stage, involving much slower fluorination of the opposite side of the sheet [R. J. Kashtiban et al., Nat. Commun. 5, 5902 (2014)]. Here, we perform comprehensive density functional calculations to illustrate accurate microscopic insights into the much slower rate of the full-fluorination stage compared with the half-fluorination one. The calculated minimum energy paths for the half- and full-fluorination processes demonstrate much enhanced fluorine adsorption after the half-fluorination stage, which sounds inconsistent with the experimental picture. This ambiguity is explained in terms of significant chemical activation of the graphene sheet after half-fluorination, which remarkably facilitates the formation of chemical contaminants in the system and, thus, substantially slows down the full-fluorination procedure. After considering the binding energy and durability of the relevant chemical species, including hydrogen, oxygen, and nitrogen molecules and xenon atom, it is argued that oxygen–fluorine ligands are the most likely chemical contaminants opposing the complete fluorination of a graphene sheet. Then, we propose an oxygen desorption mechanism to carefully explain the much enhanced rate of the full-fluorination procedure at elevated temperatures. The potential photocatalytic application of the pristine and defected samples in water splitting and carbon dioxide reduction reactions is also discussed. [ABSTRACT FROM AUTHOR]