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94 Multifunctional Photocatalytic Materials for Energy

Moreover, the presence of graphene derivatives in the composite photocatalysts
could also greatly enhance the adsorption capacity of CO 2 [90]. A close linear rela-
tionship was found between the CO 2 adsorption capacity of the composite photocat-
alysts and the rGO content, which was independent of the specific surface areas. The
CO 2 adsorption sites can also function as the active sites for CO 2 photoreduction, thus
facilitating the direct activation of adsorbed CO 2 and the enhancement of photocata-
lytic activity for CO 2 photoreduction. Furthermore, the CO 2 adsorption capacity and
catalytic activity of carbon co-catalysts could be further enhanced by nitrogen doping.
It was recently found that N dopants play a crucial role in the photoactivity and stabil-
ity of GO-TiO 2 composites for the photoreduction of CO 2 [117]. N-rGO with an ap-
propriate N quantity and N-bonding configuration acted as a dual-function promoter,
simultaneously enhancing CO 2 adsorption on the photocatalyst surface and facilitating
electron-hole separation, and eventually boosted the photocatalytic performance. This
work may inspire some new ideas for designing nanocarbon composite co-catalysts
with improved CO 2 adsorption capacity and catalytic activity for CO 2 photoreduction
via coupling graphene derivatives.

5.5 Conclusions and outlook

Graphene-based photocatalytic composites are robust materials with the potential to
solve the world’s increasing energy demand. In this chapter, we summarized recent
accounts of the synthesis and energy applications of graphene-based photocatalysts,
particularly those prepared with GO, rGO, and heteroatom-doped graphene. The high
morphological and electronic versatility of graphene materials offers the possibility
of designing novel photocatalytically active materials for solar fuels, including pho-
tocatalytic water splitting to H 2 and photocatalytic reduction of CO 2 to hydrocarbons.
The incorporation of graphene derivatives into various semiconductor photocat-
alysts has demonstrated that this approach can improve photocatalytic performance
because of a combination of several factors: (i) suppressed photogenerated carrier
recombination; (ii) increased adsorption capacity; (iii) enhanced photostability; and
(iv) enhanced light absorption.
Despite the considerable, rapid progress, several challenges remain in the synthesis
and application of graphene-based photocatalyst composites for highly efficient solar
fuel generation. First, improvements need to be made in the large-scale production of
graphene-based photocatalysts with controlled morphologies and compositions as well
as with an intimate contact interface. In this regard, more efficient synthesis methods
to achieve enhanced performance of graphene-derivative materials and graphene-based
semiconductor composites are required. Second, the efficiencies of solar fuel generation
by photocatalysis are far from being optimal and considerable breakthroughs must be
made before this method can be considered as a viable economical process. Finally, cur-
rent studies using graphene-based materials focus mostly on solar fuel generation in the
presence of sacrificial agents. Therefore the development of graphene-based materials
with improved photocatalytic efficiency using natural renewable resources like water
and sun is highly encouraged if we are to achieve clean and renewable energy.