According to the point of zero charge (pzc), the surface
charge property of TiO2 changes with solution pH. The reported
pHpzc for TiO2 is in the range 6.25–6.90. Thus, the TiO2 surface
is positively charged in acidic media (pH < pHpzc) via the protonation
depicted by Eq. (3) and negatively charged under alkaline
conditions (pH > pHpzc) via the proton abstraction by hydroxide
ion depicted by Eq. (4) [40]. Therefore, it is expected that at pH
below pHpzc, TiO2 particles at the FT sheet surface would bear a
positive charge.