Ultrafast Spectroscopic Study of Insulator–Semiconductor–SemimetalTransitions in Graphene Oxide and Its Reduced Derivatives

In graphene oxide (GO)-based compound photocatalysts, where GO is a novel cocatalyst, the mainly photophysical processes are the charge separation/transfer between GO and other nanocatalysts. However, the understanding on electronic structures of GO and reduced GO (rGO) is still unclear, despite the fact that it could be one of the most significant keys to improve the photocatalytic performances of GO-based compound photocatalysts. Here, we discuss the relationship of oxygen content of GO and its electronic structure by femtosecond transient absorption spectroscopy and X-ray photoelectron spectroscopy. We find that as-prepared GO and rGO samples with the oxygen content of similar to 35% are still "insulators". When the oxygen content slightly decreases to similar to 34%, the rGO samples begin to exhibit semiconductor-like properties. Moreover, the band gap of those semiconductor-like rGO samples would not continue red shift as further reduction of oxygen content. When the oxygen content of rGO is further reduced to similar to 29%, rGO begins to possess semimetal-like material characteristic, presenting a two-phase (semiconductor/semimetal) coexistence state due to the nature of inhomogeneity of rGO. Thus, the insulator-semiconductor-semimetal transitions in GO and its reduced derivatives is directly observed in transient spectroscopy, demonstrating that those transitions initially occur in such a relatively narrow range of oxygen content. Especially, for the semimetal-like state of rGO, our findings explain their dual roles in photoelectron conversion (semiconductor part) and electrical transport (semimetal part) applications. It will be helpful for understanding the photophysical properties and related functional information for GO-based optoelectronic devices.