Page 95 - Multifunctional Photocatalytic Materials for Energy

84 Multifunctional Photocatalytic Materials for Energy

the simultaneous reduction of GO into rGO and the formation of TiO 2 nanoparticles,
resulting in situ loading onto graphene through chemical bonds (TiOC bond) to
yield a 2D sandwich-like nanostructure.
In another study, rGO-TiO 2 nanocomposites were synthesized by a simple and en-
vironmentally benign one-step hydrothermal method using GO and TiCl 4 , as the tita-
nia precursor [62]. While conventional approaches mostly utilize multistep chemical
methods using strong reducing agents, this method provides the notable advantages
of a single-step reaction without employing toxic solvents or reducing agents, thereby
providing a novel green synthetic route to produce the nanocomposites of rGO and
TiO 2 . The fabrication of high-quality GO-TiO 2 nanorod composites (GO-TiO 2 NRCs)
on gram scale has also been reported by a two-phase assembly method, exhibiting
an improved photocatalytic performance by the effective charge antirecombination
on GO [63]. In another report [64], graphene-wrapped anatase TiO 2 was synthetized
through one-step hydrothermal GO reduction and TiO 2 crystallization from GO-
wrapped amorphous TiO 2 . Graphene-TiO 2 nanoparticles exhibited a red-shift of the
band-edge and a significant reduction of the band gap up to 2.80 eV.
Recently, the synthesis of high energy (001) facet-exposed crystalline TiO 2 on
graphene using hydrothermal [42] and solvothermal methods [43] has also attracted
much interest. Graphene-modified TiO 2 nanosheets with exposed (001) facets (sam-
ple G1.0) were prepared using a microwave-hydrothermal treatment of GO and TiO 2
nanosheets in an ethanol-water solvent [65]. Because of the interaction between the
hydrophilic functional groups (e.g., OH, COOH) on GO and the hydroxyl groups on
TiO 2 , the TiO 2 particles were well dispersed on the GO sheets with face-to-face orien-
tation (Fig. 5.3). The corresponding high-magnification TEM image (Fig. 5.3D) clearly
shows the lattice fringes, which are parallel to one of the edges of the TiO 2 nanosheets.
Another procedure for the preparation of graphene-TiO 2 composites that has
been reported by our group involves the liquid phase deposition method (LPD) us-
ing (NH 4 ) 2 TiF 6 and H 3 BO 3 as precursors, followed by a thermal post-treatment in
an N 2 atmosphere [56]. Graphene-based TiO 2 composites were prepared using GO
and different chemical rGO samples to assess the effect of the nature and number of
oxygen-containing surface groups on the photocatalytic performance of the composite
photocatalysts under near-UV/Vis and visible irradiations. The results showed that the
presence of the oxygenated groups mediates the efficient and uniform assembly of the
TiO 2 nanoparticles on the graphene-derivative materials, as shown in Fig. 5.4 [66].
Similarly to TiO 2 , other inorganic metal oxides, such as ZnO [67–70], Cu 2 O
[71–74], WO 3 [75–77], Ag 3 PO 4 [78–80], Fe 2 O 3 [81–83], BiVO 4 , [84,85] and MnO 2
[86,87], have been used successfully to fabricate other graphene-based composite
photocatalysts via different synthesis procedures.

5.3.2 Synthesis of other graphene-based semiconductor
photocatalysts
The synthesis of other graphene-based photocatalysts includes the use of several metal
sulfides, such as CdS [88–91], ZnS [92], ZnIn 2 S 4 [93], In 2 S 3 , MoS 2 [94], metal-free
photocatalysts like graphitic carbon nitride (g-C 3 N 4 ) [95,96], and nonmetal-doped
materials [90,97–101]. Both GO and rGO have been used as precursors in most