Formation of ordered mesostructured TiO2 thin films: a soft coarse-grained simulation study
Ordered mesostructured TiO2 thin films are employed in diverse applications ranging from catalysis and sensing, to photovoltaic and lithium-ion batteries. Experimentally these mesostructured thin films are fabricated via a sol–gel process coupled with evaporation-induced self-assembly of a supramolecular template, where the concentration of hydrogen chloride (HCl) and water play vital roles. We employ a soft, coarse-grained model of the amphiphilic template Brij58 and spherical particles, representing titanium–oxo clusters, to study the role of HCl and water in the formation of mesostructured TiO2 thin films. The template–cluster and cluster–cluster interactions are reflected in the interaction terms δNBP and εPP, respectively. The results show that a decrease in HCl (increase in εPP) leads to the formation of large mesopores due to the strong attraction between particles, giving rise to a high dispersity index (low order) of the thin films. However, a decrease in water (increase in δNBP) will compensate for the entropic attraction between particles, resulting in thin films with low dispersity index (high order). The variation of the dispersity index in the δNBP–εPP plane provides an intuitive understanding that the slow evaporation of HCl could drive the film towards a uniform mesoporous state, whereas fast evaporation pushes the film through a non-uniform phase. These results indicate that even if the mass proportion of the surfactants Brij58 and titanium precursors is the same in the initial solution, the final mesoporous structures could be diverse, which was confirmed by the controlled experiments. We also confirm the post-processing-towards-order strategy by making the particle rearrangement available by weakening the εPP. The outlined procedure paves the way for soft, coarse-grained models to understand the complex co-assembly of transition metal clusters and amphiphilic surfactants towards the rational design of highly ordered mesoporous structures.