**4. Modification of TiO2 with graphene oxide**

The incorporation of highly conductive carbon materials can also enhance the electron–hole pair separation of the photocatalyst. Graphene oxide commonly known as GO, is one such high conductive carbon materials that can be employed as a dopant/hybrid. Graphene can be regarded as the origin of all graphitic forms and can be curled, rolled or stacked to shape buckyball fullerenes, carbon nanotubes or graphite [58–60]. With free-standing 2-dimensional (2D) crystal and one-atom thickness properties, it has emerged with wide applications in several fields but employed mostly in nanotechnology for the improvisation of materials chemistry [61]. The unique single atom-thick planar sheet of sp2 hybridized carbon atoms contribute to efficient storing and shuttling of electrons [61]. Moreover, it attracted the scientific community tremendously because of its distinctive electronic properties, superior chemical stability and soaring specific surface area [61].

The exfoliated graphene sheets employ a theoretical surface area of around 2600 m2 /g, and as a result graphene appears as an attractive high-surface area 2D photocatalyst support [62]. Besides it has potential ideal electron sinks or electron transfer bridges [63]. This was attributed to its exceptional structure that allow ballistic transport, in which electrons can travel without scattering at mobilities exceedingly approximately 15,000 m2 /V/s at room temperature [63]. They are also foremost responsive for chemical doping, adsorbed or bound species and structure distortion [64]. Further incorporation of inorganic materials with modified graphene enormously improves their electronic, electrocatalytic and photocatalytic characteristics [32]. Thus, it proves the potential to enhance the fast electron transfer that highly benefits photocatalysis [32].

Recently modifying TiO2 surface with carbonaceous materials propounded to induce visible-light responsive property. Few types of carbonaceous materials such as graphitic or coke-like carbon [50, 65], or carbonate structural fragments bonding with titanium were employed for this purpose. Graphene oxide supported TiO2 is expected to create synergistic effect that enhances the solar photocatalytic activity. The synergistic effect attributes to its unique separation efficiency of electrons and holes between TiO2 and graphene oxide [32, 66]. The photo-reductions initiated in the transformation of graphene oxide to graphene lays a platform for continuous electron conducting network through cross-surface charge percolation and permitted graphene to act as an efficient exciton sink [66].

Nguyen-Phan et al., [32] adopted a simple one-step colloidal blending method as an environmentally friendly that preserves the TiO2 properties and combines the advantages of graphene oxide. The prepared composites showed superior adsorptivity and photocatalytic activity under both UV and visible light [32]. This was proved through a photocatalytic degradation study by adopting MB as model pollutant excited under artificial solar energy. The study indicated that graphene oxide acted as an adsorbent, electron acceptor and photosensitizer in the process of accelerating photodecomposition [32].

Recently Hu et al., 2012 also reported that graphene oxide/TiO2 hybrid (GOT) demonstrated an excellent adsorption and photocatalysis performance under visible radiation by degrading MB dye under solar irradiation. The phenomenon was due to the electron sink in GOT that contributed for the photoactivity [67].
