Mesocrystal TiO₂ films: in situ topotactic transformation and application in dye-sensitised solar cells

Thin film ceramics and semiconductors play an important role in energy- and environment-related areas such as photovoltaics, energy storage and water purification. The morphology and structure of materials significantly affect their properties and performance in applications. Mesocrystal materials with a hierarchical structure and designable overall shape possess not only the properties from nanosized building-blocks but also collective functions from the crystallographically ordered assembly, meeting the criteria of high performance candidates in various applications. In this study, a facile and versatile method was developed to prepare mesocrystal films by simply making a printable paste of the topotactic precursor, followed by in situ topotactic transformation of printed films. Using TiO2 as the model material, mesocrystal TiO2 films made from NH4TiOF3 paste possess high surface area, crystallographic orientation of anatase nanoparticles and overall large particle size, performing well in dye-sensitised solar cells (DSSCs) as either single-layer photoanodes or additional scattering layers.