**2.2. TiO2-graphene composite**

Graphene, a two-dimensional carbonaceous material, can be used in many applications due to its unique and remarkable properties such as high conductivity, large surface area, and good chemical stability [56–59]. Tremendous interest is devoted to fabricating numerous graphene–semiconductor composites to aid charge separation and migration and improve the performance of the photocatalysts [60–64]. Graphene works as an electron acceptor or transporter to induce electron transfer, leading to an efficient charge separation. Thus, an appropriate integration of graphene and TiO2 could give rise to a nanocomposite that combines the desirable properties of graphene and TiO2, e.g., the photocatalytic activity of TiO2 can be improved. In the past few years, there were some reports about graphene– TiO2 composites [13–17].

Recently, Liu et al. synthesized the graphene oxide–TiO2 nanorod composites [65, 66]. Wu et al. reported the synthesis and application of graphene–TiO2 nanorod hybrid nanostructures in microcapacitors [67]. As shown in Fig. 3, the assembling of TiO2 nanocrystalline with exposed {001} facets on graphene sheets reported in our previous work showed a higher photocatalytic activity than the other normal TiO2/graphene composites [68]. In another work (Fig. 4), graphene/rod-shaped TiO2 nanocomposite was synthesized by the solvothermal method [69]. In a one-pot system, the rod-shaped TiO2 can be homogeneously dispersed on the surface of graphene sheets by syngraphenization strategy. Owing to the combination of graphene and rod-shaped TiO2, the graphene/rod-shaped TiO2 nanocomposite shows a significant enhancement in the photocatalytic performance compared with that of the gra‐ phene/spherical TiO2 nanocomposite, which can be attributed to the high electronic mobility of graphene, higher Brunauer-Emmett-Teller (BET) surface area, and rod-shaped structures of TiO2. In our recent work [70], a series of B-doped graphene/rod-shaped TiO2 nanocomposites were synthesized via one-step hydrothermal reaction. The photocatalytic activity of the obtained nanocomposites for the oxidative photodestruction of NO*x* gas showed better photocatalytic properties than pure TiO2 and graphene/TiO2 nanocomposites. This work provides new insight into the fabrication of TiO2–carbon nanocomposites as high-performance photocatalysts and facilitates their application in addressing environmental protection issues.
