**2.5. Nanocarbon modified TiO<sup>2</sup>**

The design and preparation of graphene-based composites containing metal oxides and metal nanoparticles have attracted attention for photocatalytic performances. For example, Tan et al. [89] prepared a novel graphene oxide-doped-oxygen-rich TiO2 (GO–OTiO<sup>2</sup> ) hybrid heterostructure and evaluated its activity for photoreduction of CO<sup>2</sup> under the irradiation of low-power energy-saving daylight bulbs. It was found that the photostability of O2 –TiO2 was significantly improved by the addition of GO, at which the resulting hybrid composite retained a high reactivity. The photoactivity attained was about 1.6 and 14.0 folds higher than that of bare O2 –TiO2 and the commercial Degussa P25, respectively. This high photocatalytic performance of GO–OTiO<sup>2</sup> was attributed to the synergistic effect of the visible-light-responsiveness of O2 –TiO2 and an enhanced separation and transfer of photogenerated charge carriers at the intimate interface of GO–OTiO<sup>2</sup> heterojunctions. This study is reported to have opened up new possibilities in the development of novel, next generation heterojunction photocatalysts for energy and environmental related applications. Lin et al. [90] also investigated photoreduction of CO<sup>2</sup> with H2 O vapor in the gas-phase under the irradiation of a Xe lamp using TiO2 /nitrogen (N) doped reduced graphene oxide (TiO<sup>2</sup> /NrGO) nanocomposites. They found that the quantity and configuration of N dopant in the TiO<sup>2</sup> /NrGO nanocomposites strongly influenced the photocatalytic efficiency, and the highest catalytic activity was observed for TiO2 /NrGO nanocomposites with the highest N doping content. Moreover, modified TiO<sup>2</sup> / rGO demonstrated a synergistic effect, enhancing CO<sup>2</sup> adsorption on the catalyst surface and promoting photogenerated electron transfer that resulted in a higher CO<sup>2</sup> photoreduction rate of TiO2 /NrGO. Qu et al. [91] prepared the graphene quantum dots (GQDs) with high quantum yield (about 23.6% at an excitation wavelength of 320 nm) and GQDs/TiO<sup>2</sup> nanotubes (GQDs/ TiO2 nanoparticles) nanocomposites and the photocatalytic activity was tested towards the degradation of methyl orange. It was found that the GQDs deposited on TiO<sup>2</sup> nanoparticles can expand the visible light absorption of TiO2 nanoparticles and enhance the activity on photocatalytic degradation of methyl orange under UV-vis light irradiation (ʎ = 380–780 nm). Furthermore, the photocatalytic activity of GQDs/TiO<sup>2</sup> nanoparticles was approximately 2.7 times as higher than that of bare TiO2 nanoparticles. Tian et al. [92] reported the preparation of N, S co-doped graphene quantum dots (N, S-GQDs)-reduced graphene oxide- (rGO)-TiO<sup>2</sup> nanotubes (TiO2 NT) nanocomposites for photodegradation of methyl orange under visible light irradiation. It was found that the S-GQDs+rGO + TiO<sup>2</sup> nanocomposites simultaneously showed an extended photoresponse range, improved charge separation and transportation properties. Moreover, the apparent rate constant of N, S-GQDs+rGO + TiO<sup>2</sup> NT is 1.8 and 16.3 times higher compared to rGO + TiO<sup>2</sup> NT and pure TiO<sup>2</sup> NT, respectively. Suggesting that GQDs can improve the utilization of solar light for energy conversion and environmental therapy.
