Ultrafast time-resolved electron diffraction revealing the reductionmechanism of graphene oxide through the photon and thermalmodes

摘要

Graphene oxide (GO) is a derivative of graphene that is produced by chemicaloxidization of graphite and exhibits a variety of applications in the fields of electronics,chemistry,biology,and pharmaceuticals.The high popularity of GO can be ascribed to thefacts that the atomic frame-work and physical properties of its reduced form (rGO) aresimilar to those of graphene and GO is highly dispersible in water and polar organicsolvents and is easily functionalized by chemical methods.The dispersibility of GO in wateris attributed to the presence of oxygenated functional groups such as,hydroxyl (-OH),epoxy (-O-),carbonyl (-C=O),and carboxyl (-COOH) ones.Notably,graphene has a flatstructure featuring two-dimensionally spread six-membered carbon rings,whereas thepresence of the above functional groups in GO and rGO results in the formation ofquasi-two-dimensional non-uniform networks.Although the reduction of GO to rGO,accompanied by the partial removal of the oxygenated functional groups,can be easilyachieved by exposing GO to reducing agents,high temperature,or ultraviolet (UV) light,the reduction mechanism of GO have not been fully clarified yet.Here,a novel technique isurgently required for elucidating the reduction mechanism of GO,and a reasonable solutionis thought to be provided by the use of ultrashort electron pulses (duration of ~1 ps)1.Thesepulses generate heat on the timescale of several hundred picoseconds to nanoseconds thatimmediately escapes from the sample through the substrate before the arrival of the nextpulse,which makes electron diffraction analysis employing ultrashort pulsed electron apowerful means of observing the structures of reactive molecules.Herein,we applied theabove technique to GO during UV-photoexcitation or thermal treatment to directly observeits reduction process.The obtained structural dynamics of GO to rGO were compared withthose determined by time-resolved mid-infrared (mid-IR) vibrational spectroscopy andtime-dependent density functional theory (TD-DFT) calculations.