Correlating Photocatalytic Performance with Microstructure of Mesoporous Titania Influenced by Employed Synthesis Conditions

Ordered mesoporous anatase films can be prepared at low temperature by evaporation-induced self-assembly of a microemulsion used to preform nanocrystals of titania. Here, we study the impact of storage time of the reaction solution on the structure and photocatalytic properties of such films. The formation of anatase crystallites was found to initiate and proceed during storage of the reaction solution prior to the preparation of the titania films as expected. However, in contrast to current understanding, we find that the main part of the anatase formation occurs during aging of the prepared films in a controlled relative humidity as a consequence of the rapidly increased concentration of precursor caused by the solvent evaporation during film preparation. Ordered mesoporous films were thus obtained for reaction solutions stored up to 48 h, and the crystallinity of the pore walls was found to increase with increased storage time. Interestingly, the anatase crystallite size was found to remain between 2 and 5 nm for all films prepared from reaction solutions stored up to 48 h showing that the increased crystallinity is due to an increased number density of crystallites. Films prepared from reaction solutions stored for longer times than 48 h were found to be mesoporous but with a disordered pore arrangement. These films contained crystallites with sizes up to 10 nm, apparently too large to arrange in a meso-ordered fashion in the liquid crystalline template. The photocatalytic activity of the films was evaluated for phenol degradation and found to be higher for films prepared from reaction solutions with longer storage times of the reaction solution ascribed to their larger degree of crystallinity and larger crystallite size. Interestingly, although the degree of mesoporosity is important in providing access to high specific surface area and facile transport of reactants and products to the catalyst surface, the degree of order of the mesopores was found to be of less importance for the photocatalytic performance. In addition, this work provides new generic insight into the formation mechanism of low-temperature surfactant-templated synthesis of ordered mesoporous and crystalline materials.