Variation in the bandgap by gradual reduction of GOs with different oxidation degrees: A DFT analysis

Abstract

The bandgap of two-dimensional graphene (G), graphene oxide (GO), and reduced graphene oxide (rGO) materials is modified by the gradual incorporation and removal of functional groups on the surface of the material. In this work, using Density Functional Theory (DFT) with BIOVIA Materials Studio software, the variation of the bandgap of GO and rGO as a function of the O/C ratio has been studied. The direct relationship between the bandgap and the number of epoxide functional groups in the GO is demonstrated. A gradual increase in the bandgap from 0.211 to 3.58 eV was observed by varying the O/C ratio from 0.06 to 0.50. In the range of O/C ratios from 0.17 to 0.28, the bandgap decreased, estimating the lowest value of 0.0211 eV at O/C = 0.22 despite the increased functional groups in the material. This phenomenon is explained by the balance between the number of sp2 conducting states and the loss of linearity of the sheets due to the incorporation of functional groups. On the other hand, the GOs were reduced by gradually removing their functional groups at different O/C ratios, ranging from 0.50 to the total elimination of the epoxide functional groups. This study showed that the bandgap of the now-called rGOs varies according to two parameters: (i) the number of functional groups to be eliminated in a GO (reduction process) and (ii) the number of functional groups present in the GO to be reduced. This theoretical study has calculated the existence of multiple bandgaps in these materials, finding the most significant difference in the O/C ratio=0.33, estimating bandgaps of 0.50 eV and 1.82 eV, for rGO and GO, respectively, thus raising the need to be more specific in their denomination. Finally, the use of a nomenclature based on subscripts a, b, c, and d, describing the number of layers, the type of functional group, the number of functional groups added (in GO), and the number of functional groups removed in rGO, respectively, has been proposed.