**7. Conclusion**

Photocatalysts alone almost showed a very low photocatalytic activity because of the rapid recombination of CB electrons and VB holes. The chemical bonding and associated charge transfer at the interface between the photocatalyst and GO support can be used to fine-tune the electronic and chemical properties of the active sites. GO can act as a common platform for more than one active site to produce enhanced heterostructure for photocatalytic activity.

GO is an excellent supporting material due to its high specific surface area and superior electron mobility. GO plays the role of an electron acceptor that accelerates the interfacial electron-transfer process from photocatalysts materials, which strongly hindering the recombination of charge carriers and thus improving the photocatalytic activity. The spread of the oxygenated functional groups on its surface facilitates the process of planting photoactive spots on its surface.

The band gap of GO is tunable by just varying the oxidation level. Fully oxidized GO act as an electrical insulator and partially oxidized GO can act as a semiconductor. Introducing more oxygen enlarges the band gap, and the VBM gradually changes from the p-orbital of graphene to the 2p orbital of oxygen; the π\* orbital remains as the CBM.

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Aluminates, ferrites, niobates, tantalates, titanates, tungstates, and vanadates are examples of the metallates, which are incorporated with GO and revealed an enhanced photocatalytic activity on the degradation of organic dyes under visible light due to charge transfer channel of GO.

The heterojunction between p-type GO and n-type semiconductors functioned as the separator for the photo-generated electron-hole pairs. These semiconductors could be excited by visible light with wavelengths longer than 510 nm for the degradation of methyl orange. In most cases, GO served as an electron sink to facilitate separation and store the separated electrons.

Surface modification by photocatalyst grafting provides a very promising route to the fabrication of high-performance photocatalytic membranes for sustainable water treatment.

Some of the outstanding properties of GO-based photocatalysts in CO2 reduction processes have been shortlisted: (i) block carrier recombination, (ii) Improving specific surface areas, (iii) strong π–π interaction with CO<sup>2</sup> , (iv) enhancing either photocatalyst mechanical and chemical stability, (v) improving nanoparticles dispersion, (vi) intensifying light absorption.
