Page 94 - Multifunctional Photocatalytic Materials for Energy

Graphene photocatalysts 83

TiO 2 have been studied extensively because of the significant enhancement of pho-
tocatalytic activity. In the synthesis of graphene-TiO 2 composite photocatalysts, TiO 2
has been synthetized from different precursors, such as inorganic titanium salts—for
example, titanium(IV) sulfate, Ti(SO 4 ) 2 [41,42]; titanium(IV) fluoride, TiF 4 [43–45];
ammonium hexafluorotitanate(IV), (NH 4 ) 2 TiF 6 [46–48]; and titanium alkoxides that
can hydrolyze easily in aqueous solution (e.g., tetrabutyl titanate, Ti(BuO) 4 [49–51];
and titanium(IV) isopropoxide, Ti[OCH(CH 3 ) 2 ] 4 [23,52]). On the other hand, a GO
aqueous suspension is normally used as precursor instead of graphene on its own be-
cause of the presence of hydrophilic oxygen-containing surface groups on GO sheets
that can be used to facilitate the anchoring of semiconductors. These groups are ben-
eficial for the dispersion of GO layers in water and the heterogeneous nucleation and
growth of the TiO 2 particles, which are needed to develop highly efficient photocat-
alysts [53,54]. During the preparation method, GO can be reduced to rGO via the
hydrothermal/solvothermal process or by UV light irradiation [20,55–57].
Different TiO 2 semiconductors with well-defined morphologies have been con-
structed on graphene sheets, for example, zero-dimensional TiO 2 nanospheres [41],
one-dimensional TiO 2 nanorods [58], two-dimensional TiO 2 nanosheets, and three-
dimensional macro-/mesoporous TiO 2 [59–61]. In general, these TiO 2 nanoarchitec-
tures can be fabricated and then anchored onto graphene by carefully controlling the
synthesis conditions, such as the additives and hydrothermal parameters, and using
titanium salts as precursors [54]. The epoxy and hydroxyl functional groups on GO
sheets can act as heterogeneous nucleation sites for anchoring TiO 2 nanoparticles,
leading to the formation of well-dispersed mesoporous TiO 2 nanospheres on the
graphene sheets via a template-free self-assembly process [41]. These functionalities
(such as epoxy and hydroxyl groups) mediate the efficient and uniform assembly of
the TiO 2 nanoparticles on the GO sheets, thus avoiding agglomeration and subse-
quently increasing the surface area of the resulting materials. During the photocatalyst
preparation, TiO 2 nanoparticles are produced and interact with the surface chemis-
try of GO by means of hydrogen bonds, resulting in the formation of well-dispersed
mesoporous TiO 2 nanospheres on the GO sheets. The hydroxyl and epoxy groups are
linked to TiO 2 particles and should not function as active sites during photocatalysis.
Moreover, these functionalities are stable during the photocatalytic process because of
the formation of TiOC bonds.
TiO 2 nanorods were stabilized by oleic acid and self-assembled on GO sheets at the
water/toluene interface [58]. The two-phase, self-assembling procedure is simple and
reproducible, and it can be widely and easily used for self-assembling other nonpolar
organic soluble nanocrystals on GO sheets. Mesoporous graphene-TiO 2 nanocompos-
ites have been synthesized via two successive steps of hydrothermal/hydrolysis using
Ti(SO 4 ) 2 and an acidic GO solution, followed by UV-assisted photocatalytic reduc-
tion of GO [59]. On the other hand, hierarchical macro/mesoporous graphene-TiO 2
composites have been prepared by a simple one-step hydrothermal method using GO
and tetrabutyl titanate as the titanium precursor [60]. A novel simultaneous reduction-
hydrolysis technique in a binary ethylenediamine/H 2 O solvent was used in the syn-
thesis of a graphene-TiO 2 2D sandwich-like nanostructure using GO nanosheets and
titanium(IV), bis(ammonium lactato)dihydroxide [61]. The technique was based on