The excellent photocatalytic activity of the graphene/TiO2 composite
prepared via the single-step nonionic surfactant strategy
combined with the solvothermal treatment technique is primarily
attributed to the following three reasons: (i) the incorporation of
graphene with a suitable loading within the anatase TiO2 can result
in an enhanced quantum efficiency. This enhancement contributes
to the electron-accepting and electron-transporting properties of
graphene. Graphene functions as the acceptor of photogenerated
electrons from the anatase TiO2 and effectively suppresses the
recombination of e−–h+ pairs. Thus, more photogenerated holes
are generated to form reactive species and the photodegradation
of aqueous POPs is facilitated (Scheme 1). On the other hand,
the surface defects of the anatase TiO2 lattice can also enhance
the quantum efficiency of the photocatalyst (Fig. 7). These surface
defects are beneficial to the photocatalytic activity because
they reduce the probability of e−–h+ recombination. Surface defects
prolong the charge lifetime not only by trapping charge carriers
with defects, but also by transferring the trapped charges to
the reactants. However, for the graphene/TiO2 composite with a
high graphene loading and for the graphene/TiO2-0.91% (no P123)
samples, the aggregation of graphene nano-sheets and TiO2 nanoparticles
is unfavorable to the separation of e−–h+ pairs. Thus, the
photocatalytic activity is reduced (Scheme 1). In addition, the lightshielding
effect of graphene for the graphene/TiO2 composite with
a high graphene loading also causes a decrease in photocatalytic
activity [48]; (ii) the band gap of the graphene/TiO2 composites narrowed
compared with pure TiO2. The decrease in band gap resulted
in an increase in the photocatalytic activity of TiO2 (Fig. 5). This