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(a) Graphite flakes (GF) were used as raw material to prepare improved GO (IGO), Hummers GO (HGO), and Hummers modified (HGO+) using three different methods. Reprinted with permission from Ref. [71]. Copyright 2010, Elsevier. (b) Schematic diagram of GO preparation by spontaneous expansion using expanded graphite as a precursor. Reprinted with permission from Ref. [72]. Copyright 2013, Elsevier.
