  Two-dimensional materials represented by graphene and transition metal
dichalcogenides undergo charge transfer (CT) processes and become hole-doped in
strong mineral acids. Nonetheless, their mechanisms remain unclear or
controversial. This work proposes and verifies two distinctive CT channels in
sulfuric acids, respectively driven by oxygen reduction reaction involving
O2/H2O redox couples and reduction of bisulfate or related species.
Acid-induced changes in the charge density of graphene were in-situ quantified
as a function of oxygen content using Raman spectroscopy. At acid
concentrations lower than 6 M, the former channel is operative, requiring
dissolved O2. Above 6 M, the degree of CT was even higher because the former is
cooperated with by the latter channel, which does not need dissolved oxygen.
The mechanism revealed in this study will advance our fundamental understanding
of how low-dimensional materials interact with chemical environments.
