  The pH dependence of emission from graphene oxide is believed to be due to
the protonation of surface functional groups. In this study we use transient
absorption spectroscopy to study the sub-picosecond charge dynamics in graphene
oxide over a range of pH values, observing dynamics consistent with an excited
state protonation step for pH < 9.3. The timescale of this process is ~ 1.5 ps,
and a corresponding change in recombination dynamics follows. A broad
photo-induced absorption peak centred at 530 nm associated with excited state
protonation is also observed.
