Photochemical Transformation of Graphene Oxide in Sunlight Wen-Che Hou,* ,†,‡ Indranil Chowdhury, † David G. Goodwin, Jr., § W. Matthew Henderson, ⊥ D. Howard Fairbrother, § Dermont Bouchard, ⊥ and Richard G. Zepp* ,⊥ † National Research Council Associate, National Exposure Research Laboratory, Ecosystems Research Division, U.S. Environmental Protection Agency, Athens, Georgia 30605, United States ‡ Department of Environmental Engineering, National Cheng Kung University, Tainan City 70101, Taiwan § Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States ⊥ National Exposure Research Laboratory, Ecosystems Research Division, U.S. Environmental Protection Agency, Athens, Georgia 30605, United States * S Supporting Information ABSTRACT: Graphene oxide (GO) is promising in scalable production and has useful properties that include semiconducting behavior, catalytic reactivity, and aqueous dispersibility. In this study, we investigated the photochemical fate of GO under environmentally relevant sunlight conditions. The results indicate that GO readily photoreacts under simulated sunlight with the potential involvement of electron-hole pair creation. GO was shown to photodisproportionate to CO 2 , reduced materials similar to reduced GO (rGO) that are fragmented compared to the starting material, and low molecular-weight (LMW) species. Kinetic studies show that the rate of the initially rapid photoreaction of GO is insensitive to the dissolved oxygen content. In contrast, at longer time points (>10 h), the presence of dissolved oxygen led to a greater production of CO 2 than the same GO material under N 2 -saturated conditions. Regardless, the rGO species themselves persist after extended irradiation equivalent to 2 months in natural sunlight, even in the presence of dissolved oxygen. Overall, our findings indicate that GO phototransforms rapidly under sunlight exposure, resulting in chemically reduced and persistent photoproducts that are likely to exhibit transport and toxic properties unique from parent GO. ■ INTRODUCTION Graphene oxide (GO) is an important precursor to graphene, a one-atom thick, two-dimensional nanomaterial made of sp 2 - hybridized carbon that has received unprecedented attention as a result of its fundamental properties and broad applications. 1,2 The hydrophobic nature and absence of a bandgap, however, limit graphene’s utility in water-based applications (e.g., biomedicines), as well as in light-emitting electronics and photocatalysis. 3,4 GO is a structural analog to graphene with added functionalities such as epoxy, hydroxyl, carbonyl, and carboxyl groups covalently bound on the basal planes (for epoxy and hydroxyl groups) or the edges (for carbonyl and carboxyl groups). 4,5 Beyond structural variation, however, GO has other promising characteristics distinct from graphene: for example, in addition to enhanced aqueous dispersibility, GO is more likely to be made in large batches due to its scalable production process involving oxidative exfoliation of graphite in aqueous solutions containing concentrated acids and oxidants. 4 The oxygen groups are the basis for further covalent attachment of smaller molecules or polymers to GO. 4 Although extensive oxidation destroys the sp 2 -hybridized carbon network, rendering GO electrically insulating, GO can be readily reduced by chemical, thermal, electrochemical, photocatalytic, or microbial reactions to produce rGO for recovery of electron- conducting capabilities. 6-9 Recent studies examined potential risks to ecosystems posed by graphene and GO. Several studies have shown that graphene Received: September 25, 2014 Revised: February 8, 2015 Accepted: February 11, 2015 Published: February 11, 2015 Article pubs.acs.org/est © 2015 American Chemical Society 3435 DOI: 10.1021/es5047155 Environ. Sci. Technol. 2015, 49, 3435-3443