Preparation of the TA-G
Graphene oxide (GO) was synthesized from natural graphite
powder by a modified Hummers and Offeman method [19]. Then
GO was dispersed in water and a ultrasonic exfoliated process was
performed to yield a yellow-brown dispersion in a ultrasonicator
for 2 h, followed by centrifugation at 3000 rpm to remove any
unexfoliated GO to obtain the homogeneous exfoliated graphene
oxide (EGO) dispersion. Subsequently, 4 g tannin acid was added
into a three-necked flask, which had contained 100.0 mL of the
homogeneous EGO dispersion and stirred for 30 min until the
tannin acid (TA) had resolved completely. The resulting mixture
was then heated to 95 ◦C and allowed to react for 9 h under stirring.
Finally, the black tannin acid functionalized graphene (TA-G)
was obtained by three filtration and water washing cycles, and
then dried in vacuum. Furthermore, as a comparison in a control
experiment, pure graphene was also prepared using hydrazine
hydrate to reduce the EGO without adding any functionalized
reagent.