## 2 Carbon Nanostructures as Templates for Titania Nanomorphologies
### Nano-Onions  
Carbon nano-onions consist of concentric fullerenes and have been used usually to template nanostructured titania by means of sol-gel methods. X-ray diffraction analyses confirmed that the nanocarbons were anchored onto anatase, whose microscopy images suggested a certain level of control against the particle agglomeration exerted by the nano-onions, relative to a reference without the template, despite the limited morphological control over the titania particles. The nano-onions enhanced the specific surface area, average pore size, and pore volume of the composite leading to the adsorption of pollutants, as well as its visible-light absorption, overall leading to a more efficient photocatalytic dye degradation as demonstrated on rhodamine B. Finally, the paramagnetic nature of the nano-onions with a magnetic core allowed for the easy recovery of the composite through magnetic separation [95].  
Carbon nano-onion/anatase hybrids have been proposed as innovative replacements for graphite anodes in lithium-ion batteries. Titania was formed with a sol-gel method onto the nanocarbon template, exerting a certain level of both morphological and size control over the nanocrystals, which appeared to be smaller than 10 nanometers in diameter. Furthermore, the presence of the conductive carbon nanostructure overall significantly enhanced the electrochemical properties of the final material relative to the reference without the nano-onions [96].  
Carbon nano-onions/anatase materials were also obtained by a hydrothermal route, with various mass compositions, ranging from an excess of the nano-onion to an excess of the titania, as confirmed by thermogravimetric analyses. The best results within the series were obtained with compositions featuring approximately 80-90 wt% nano-onions. In particular, high-resolution transmission electron microscopy (HR-TEM) images (Figure 4) revealed an intimate contact between the inorganic and the carbon components, with both the graphitic walls of the nano-onions and the lattice of the anatase nanocrystals being clearly visible. These materials demonstrated the highest capacitance and an enhanced electrochemical performance overall, which was rationalized through a cooperative effect of both the electrochemical double layer capacitance from the nano-onions and non-capacitive Faradaic storage regarded as pseudocapacitance from titania [97]. Carbon nano-onions have been proposed for use in dye-sensitized solar cells, thanks to a number of advantageous features, including their limited cost, ease of dispersibility that does not require the use of binders--in contrast with other carbon nanomaterials--and the high level of optical transparency of the final device [179].  
Figure 4: HR-TEM images of carbon nano-onion/anatase nanocrystals with different ratios of the two components, consisting of either (**a**) 89 wt%/11 wt% or (**b**) 83 wt%/17 wt%, respectively. White-dotted circles highlight the nano-onion cores; yellow-dotted circles indicate the interface between the two components. The nano-onion graphitic walls area visible with the typical 0.34 nm-distance, as is the lattice spacing of 0.35 nm, corresponding to the (101) plane of anatase. Reprinted from [97], Copyright © 2017, with permission from Elsevier.