## 2 Carbon Nanostructures as Templates for Titania Nanomorphologies  
The efficiency of titania photo-activity can be significantly improved through nanostructuring and heterostructuring with carbon nanomaterials, allowing the enhanced use of the solar spectrum and better charge separation [87]. The addition of carbon nanostructures can lead to an increase of the adsorption capacity, of the absorption of visible light, and of the lifetime of photogenerated electron-hole pairs. In particular, CNTs have been widely used as supports for the growth of nanosized titania. The advantages of interfacing TiO\({}_{2}\) with CNTs include not only improved structural stability, but also electronic communication between the two phases, whereby charge carriers (electrons or holes) may be transferred through the phase boundaries through several proposed mechanisms. For instance, a general view is that CNTs can efficiently scavenge the titania photoexcited electrons, thus retarding the electron-hole recombination rates and resulting in enhanced photocatalytic activities [88]. It is worth noting, however, that alternative hypotheses have been advanced; for example, on the basis of transient absorption experiments, where the photogenerated holes are transferred from the TiO\({}_{2}\) to the CNT [89], confirming the complexity of the electron transfer dynamics, which for example could depend on the level of functionalization (in particular, with oxygenated groups) of the CNT surface. Other reports have suggested that CNTs can act as photosensizers, with the electrons excited and injected from the CNTs into the TiO\({}_{2}\) conduction band, with subsequent electron transfer from the titania valence band into the CNTs, thus yielding a charge separation state (electron-hole) on the metal oxide. However, such sensitization effects assume a semiconducting character of the CNT, which cannot always be taken for granted. In general, the micro-structure of the CNT and its functional group distribution, doping, and method of preparation deeply affect the photochemical response. Furthermore, Ti-C and Ti-O-C bonds can be formed during calcination, thus resulting in a \"doping\" effect with the formation of intragap states within the titania and a consequent improved absorption of visible light [88]. The use of carbon nanomaterials to template titania with nanomorphological control was reviewed in 2016 [90]; thus, we focus here on the progresses made over the last five years. An overview of selected examples is provided in Table 1. A summary of the physicochemical properties of carbon nanostructures after decoration with titania, including the type of surface functionalization used to anchor the metal oxide, is provided in Table 2.  
Carbon nanodots are not present in the table since they are typically not used as templates; rather, they are added onto preformed titania, for instance on anodized nanotube arrays for uses in photocatalysis [91] and sensing [92]. Readers with a particular interest in graphene quantum dots incorporated in titania nanostructures are referred elsewhere [93]. Furthermore, studies that used commercially available titania nanoparticles are not included, since our focus is on the influence of carbon nanomaterials on their formation.