**2. Supported photocatalysts onto graphene oxide (GO)**

Supported photocatalysts are a properly evolved concept in imparting progressed exposure of the catalysts to reactants and is common in industrial catalytic technologies. In this type of configuration, however, the nature of the photocatalyst-support interactions is important. For durable overall performance, a strong chemical bond is necessary, however, the influences of bonding on photocatalytic mechanisms are considered. The application of TiO2 in suspension is effective in capturing sunlight due to the fact suspended TiO<sup>2</sup> powders have a high particular surface area within the range from 50 to 300 m2 g−1, which in turn helps in keeping off mass transfer limitation, ensuing in a high photocatalytic activity. Moreover, TiO<sup>2</sup> alone showed a very low photocatalytic activity because of the rapid recombination of conduction band (CB) electrons and valence band (VB) holes. However, a light transport limitation appears with excessive catalyst loading. Besides, it is difficult to separate the small TiO<sup>2</sup> particles from the water after the remedy. To overcome this, the catalyst particles can be immobilized on a surface. In addition, this may lower the oxidation capacity in keeping with volume of water as compared to the suspension of solid particles system, due to the mass transfer difficulty and moderate transport limitation because of (i) a diminished catalyst surface-to-volume ratio, (ii) the presence of substrate that absorbs light and deteriorate its distribution, and (iii) a loss of movement of particles.

There are different kinds of materials that have been used as a support to fix photocatalysts. Among the different supports, GO is an excellent substrate material for many reasons due to its high specific surface area and superior electron mobility. There have been many efforts to immobilize TiO2 photocatalyst over diverse structures of supports together with increasing the surface/volume ratio concurrently, which subsequently improves the photocatalytic oxidation efficiency. However, the surface area can only be productive if it allows efficient absorption of light. There is extensive attention to the preparation methods of GO-based material nanocomposites. GO-based material nanocomposites can be synthesized by numerous methods and approaches including the hydrothermal method, electrochemical co-deposition, in situ polymerization, microwave-assisted method, vacuum impregnation, and sol-gel technique. In the GO-based nanocomposite, GO presents either as a functional component or as a substrate for immobilizing the other components.
