**3.7. Alignment of the pores**

As previously mentioned, the pores of the thin films must be accessible. Indeed, many potential applications of mesoporous thin films are relying on the accessibility of the pores from the surface. Owe to interfacial orientation preferences between the substrate and the material, the 2D hexagonal mesoporous thin films present their pore channels parallel to the surface [25]. For this reason, many research groups have struggled to synthesize cubic mesoporous thin films, which present, regardless of the orientation, pore openings from their surfaces owe to their intrinsic geometry [44, 49–53]. Often, the solution resides in the optimization of the composition of the precursor solution performed by varying the ratios between quantity of surfactant and inorganic precursor [25, 54, 55]. In this context, Koganti et al. [56] showed that a modification of the substrate surface with a Pluronic block copolymer could lower the surface energy and induce the channels to tilt away from the substrate plane. Other groups have varied the aging temperature to form cubic structures [57]. Lee et al. [26] reported the preparation of MTTFs with vertical pores using TiCl<sup>4</sup> as source of titanium, F127 as surfactant, and by applying a specific heat treatment to direct the formation of the 3D network in a cubic phase. Zhou et al. [7] formed MTTFs with vertical pores, by calcination of the film at 450°C, so that the 3D hexagonal mesostructure was transformed to a grid-like mesostructure with quasi-perpendicular porosity through sintering-diffusion and pore merging along the c-axis. Richman et al. [30] used another approach: they synthesized mesoporous films through nanometer-scale epitaxy. They prepared a titania thin film with cubic symmetry and used it as a patterned substrate to direct a hexagonal top silica mesostructured with vertically oriented pore channels. Finally, Yamauchi et al. applied a very strong magnetic field (10T) in mesoporous silica films to align the channels, but the alignment was not homogeneously perfect, and such method remains rather impractical for large areas [58].
