Understanding the Supramolecular Self-Assembly of Zirconium Titanate Mesophases Formed from the Poly(ethylene oxide) Surfactant Brij-58

The evaporation-induced self-assembly of mesoporous zirconium titanium oxide thin films prepared from precursor solutions with the composition ZrxTil-xCl4:40 EtOH:0.005 Brij 58:h H2O (where x = 0.28, h = 10, and Brij 58 = diblock copolymer C16H33PEO20) by dip coating has been studied as a function of the mole fraction of zirconium (x), the relative humidity (RH), and the drying conditions. Extremely well-ordered mesophases and compositional homogeneity on the atomic scale were consistently obtained when the films were prepared at relative humidities above 60%, under which conditions a cubic phase was identified. The influence of humidity was monitored using a combination of in situ small-angle X-ray diffraction (SAXD) and small-angle X-ray scattering (SAXS). These techniques showed the existence of nanocrystalline phases during the initial drying process for films prepared from precursor solutions with x in the range of 0.20-0.60. The nanocrystalline phases were evidenced as multiple X-ray reflections above 8 degrees 2 theta. These reflections were ascribed to relatively crystalline nano building units of the mesophase and displayed only a transient stability in humid air, disappearing after similar to 1 h of film conditioning in any humidity. A clear dependence of the primary d-spacing observed in the SAXD on composition for films dried at 65% RH, stabilized at 200 degrees C, and calcined at 350 degrees C provided strong evidence of the ability to form a complete solid solution of Zr and Ti in the oxide mesophase prepared from the precursor solutions. This compositional homogeneity was confirmed by transmission electron microscopy and further corroborated using Ti K-edge X-ray absorption spectroscopy, which probed the local chemical environment of the embedded Ti atoms. The data provided here are in stark contrast to studies of zirconium titanium mixed oxide film systems using similar preparation procedures but triblock copolymer porogens such as F-127, A hypothesis is tendered to explain the dramatic difference in the products obtained using these distinct but related copolymer templates.