**3.4 Three-dimensional nanostructures**

Owing to the large specific surface area and other superior properties over the bulk counterparts arising from the quantum size effect, three-dimensional nanostructures have attracted considerable research interest, and many threedimensional nanostructures have been synthesized in the past decade (**Figure 3d**). It is well-known that the surface area, shape, size, dimensionality, and morphologies of the nanostructures are key factors to obtain better performance of the device when they are envisaged. As these materials offer higher surface area, they can supply enough absorption sites for all involved molecules in a small space. On the other hand, such materials with higher porosity can lead to a better transportation of dye molecules. A typical three-dimensional nanostructured such as nanocoils, nanocones, nanoflowers, and nanoballs (dendritic structures) are on a great demand [21].
