2D-SAXS in Situ Measurements as a Tool to Study Elusive Mesoporous Phases: The Case of p6mm TiO2

Abstract

Mesoporous titania thin films (MTTF) with p6mm pore array are currently attracting a great interest because of their 2D-hexagonal arrangement of close-packed cylindrical pores, but they are difficult to synthesize. This work seeks to understand such apparent elusiveness of the p6mm phase for MTTF. To that end, a wide variety of MTTF were prepared using different precursors, templates, aging and stabilization conditions, according to procedures reported in the literature and several variations thereof. Initially, the obtained MTTF were cross-characterized by ex situ transmission electron microscopy, two-dimensional small-angle X-ray scattering (2D-SAXS), and grazing incidence SAXS, with no clear evidence of a well-organized p6mm pore structure in the final system. In view of the difficulty of obtaining the desired pore array, the mesophase evolution was monitored during the synthesis, aging, stabilization, and calcination stages, resorting to in situ 2D-SAXS experiments. The performed experiments indicate that the key parameters that affect the ordering in these systems are the humidity of conservation but, mainly, the thermal treatment. Among the different strategies rehearsed, the desired mesophase was finally obtained by a combination of high humidity aging, exposure to NH3 vapors, and a thermal treatment at low temperature, followed by ethanol extraction of the template. However, even resorting to such mild treatments, the obtained MTTF presented an ordered p6mm structure combined with disordered regions, as determined by electron microscopy and porosimetry measurements. Thus, the obtained results stress that considering the mesophase obtained only at initial or intermediate synthesis stages as the one that survives in the template-free system is not a safe assumption. Moreover, complementary experimental techniques should be used to determine mesoporous structures in the final systems. Finally, this work shows the strength of in situ 2D-SAXS measurements as a technique to design new synthetic procedures, in order to obtain specific properties of the final material.