Hierarchical porous TiO2 thin films by soft and dual templating: A quantitative approach of specific surface and porosity

• Hierarchical porous thin films are prepared by soft and dual templating.
• A geometrical model predicts the total specific surface of bimodal porous materials.
• The model is validated by dye-loading experiments.

Hierarchical porous structures, with different pore sizes, including pores larger than 10 nm, constitute an important field of research for many applications such as selective molecule detection, catalysis, dye-sensitized solar cells, nanobiotechnology and nanomedecine. However, increasing the pore size logically results in the decrease of specific surface. There is a need to quantify and predict the resulting porosity and specific surface.We have prepared hierarchical porous TiO2 thin films either by surfactant templating (soft) or dual surfactant/nanospheres templating (soft/hard). They all show narrow, bimodal distribution of pores.Soft templating route uses a modified sol–gel procedure by adding a swelling agent (polypropylene glycol) to a precursor solution containing Ti alkoxide and block-copolymer surfactant. This scheme leads to very thin films showing high specific surface and bimodal porosity with diameters of 10 nm and 54 nm.Dual templating route combines a precursor solution made of Ti alkoxide and block-copolymer surfactant with polystyrene (PS) nanospheres (diam. 250 nm) in a one-pot simple process. This gives thicker films with a bimodal distribution of pores (8 nm and 165-200 nm). The introduction of PS nanospheres in the surfactant–Ti system does not interfere with the soft templating process and results in a macroporosity with a pore diameter 20–30% smaller than the original beads diameter.The dye loading of hierarchical films is compared to pure surfactant-templated TiO2 films and shows a relative decrease of 29% for soft templating and 43% for dual templating.The microstructure of bimodal porous films is characterized by several techniques such as transmission and scanning electron microscopy, X-ray diffraction, profilometry and ellipsometry. Finally, a geometrical model is proposed and validated for each system, based on the agreement between calculated specific surfaces and experimental dye loading with N719 dye.