Isolated　graphene,　a　single　atomic　layer　two-dimensional　analog　of　fullerenes　and　carbon　nanotubes,　has　sparked　enormous　excitement　because　of　its　fascinating　properties　and　great　potential　for　various　applications.　While　pristine　graphene　is　desirable　for　applications　that　require　a　high　electrical　conductivity,　many　other　applications　require　modified　or　functionalized　forms　of　graphene　materials,　such　as　graphene　oxide,　reduced　graphene　oxide　(rGO),　or　other　functionalized　forms.　Structurally　modifying　graphene　through　chemical　functionalization　reveals　the　numerous　possibilities　for　tuning　its　structure　and　properties.　In　this　chapter,　we　focus　on　basic　surface　science　issues　of　graphene　oxide,　rGO,　and　pristine　graphene.　The　excellent　mechanical,　electronic,　and　adsorption　properties　and　photocatalysis　applications　of　the　related　materials　are　highlighted.　Some　challenges　and　opportunities　for　future　exploration　of　functionalized　graphene–based　materials　are　critically　discussed.　©　2020　Elsevier　Ltd