New insight into the photo-transformation mechanisms of graphene oxide under UV-A, UV-B and UV-C lights

Abstract

Photo-transformation dominates the fate of graphene oxide (GO) in the environment. However, the photo-transformation mechanisms of GO under different UV bands remain unclear. Our results showed that UV bands played a crucial role in sunlight-induced GO transformation. UVA and UVB induced significant photo-reduction of GO as indicated by decreasing surface O/C ratio, which could be explained by an O2-independent electron-hole pair-mediated mechanism (Mechanism I), and an O2-dependent reactive oxygen species (ROS)-mediated reduction mechanism (Mechanism II). Mechanism II accounted for 62.7 % and 33.3 % of total GO photo-transformation under UVA and UVB, respectively. Different from UVA and UVB, UVC led to GO reduction under anaerobic condition via Mechanism I and Mechanism III (direct decarboxylation). However, under aerobic condition, UVC caused significant oxidation of GO, which was the combined effect of Mechanisms I-III and the oxidation of graphitic structure on GO with the assistance of O2 (Mechanism IV). Moreover, it was demonstrated that the environmental factors (e.g., dissolved organic matter, phosphate) significantly enhanced the photo-transformation of GO in natural water. The information in the present work is useful for better understanding the fate of GO in aquatic environments.