ULTRATHIN FILMS OF GRAPHENE OXIDE - EVALUATION OF PHOTOREDUCTION PROCESSES

摘要

Approaches to graphene-based electronic materials and devices frequently use graphene oxide (GO), combined with a suitable reduction process to remove the oxygen-containing groups. Photoreduction of GO films can generate suitable structures of reduced graphene oxide (RGO) by means of irradiation masks or laser direct writing techniques. However, the photoreduction mechanisms are not fully resolved yet as the distinction between photothermal and photochemical reaction pathways still needs fundamental clarification. We have prepared ultrathin films of GO by single and multiple spin-coating from aqueous solutions, irradiated them with monochromatic light obtained from a high-pressure mercury arc combined with narrow band filters, and a femtosecond laser system at 800 nm. We used UV-Vis-NIR absorption spectroscopy to characterize the conversion and reaction rate of the photoreduction quantitatively. The reduction yield is evaluated by means of the ratio R = A_(400)/A_(217) where A_(400) and A_(217) refer to the absorbances at 400 nm after irradiation and at 217 nm afore, respectively. The impact of the light intensity I on the reaction yield R indicates the kind of photoreaction. Photothermal reactions show a threshold behavior, i.e., a minimum sample heating is required. Photochemical reactions can occur via single-photon absorption indicated by R ～ I which is independent on the magnitude of I, or via two-photon-absorption which is proven by the R ～ I~2 relationship. We verify pure photochemical photoreduction of GO by single-photon-absorption at wavelengths even up to 546 nm irradiation wavelengths, and two-photon induced photoreduction with high-intensity laser pulses at 800 nm.