The direct and indirect (with H2O2 added to serve as a hydroxyl radical source) photoreactions of graphene oxide (GO) were examined under sunlight exposure. Results indicate that GO photoreacts rapidly under both conditions, leading to significant alterations in GO's physicochemical properties.  In particular, photoreactions in the presence of H2O2 produce a significant decrease in GO nanosheet size, the loss of ~85% aqueous organic carbon content, and color bleaching.  Similar effects were observed to a lesser extent in the absence of H2O2, except that the color of GO oppositely increased. GO photoproducts exposed to sunlight at various time points were analyzed using XPS and ATR-IR. A large (approximately 20%) reduction in the C-OH/C-O-C group concentration was observed for both direct and indirect phototransformation processes after 11 h of irradiation. In the absence of H2O2, the rate of oxygen loss from GO was found to follow first-order kinetics with respect to the oxygen content. In the presence of H2O2, hydroxyl radical-mediated oxidation processes occurred in competition with photoreduction. The results suggest that sunlight photoreaction is likely a key process controlling the environmental persistence of GO and that the photoproducts could have transport and biological properties distinct from the parent material.

Direct and indirect phototransformations of graphene oxide in sunlight

248th National Meeting (2014)

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