Striving for order and compositional homogeneity in bulk mesoporous zirconium titanium mixed metal oxides from triblock copolymers and metal chlorides

Under consideration in this work is the prospect for the preparation of ordered, thermally stable, compositionally homogenous bulk zirconium titanate mesophases using a triblock copolymer template (F-I 27). As a starting point, well established evaporation-induced self-assembly procedures for the production of thin films have been adapted to the preparation of bulk materials. The composition of the precursor solution was fixed at ZrxTi1-xCl4:40 EtOH:0.005 F127:h H2O (x = 0.28, h = 10) and ambient humidity was varied in a systematic and precise fashion. It has been demonstrated that while it was possible to prepare materials with localized order at humidities exceeding about 20% relative humidity, these materials showed macroscopic phase segregation with very little zirconium entering the mesophase structure instead precipitating as zirconium oxide. This phase segregation resulted in poor thermal stability at 500 degrees C. In contrast, at humidities below about 20%, more thermally stable worm-like materials with compositional homogeneity on the transmission electron microscope length scale could be prepared. For comparison with bulk materials, thin films were also prepared from the similar precursor solutions. The results of these preparations were unexpectedly very similar to those of the bulk preparations. It has been clearly demonstrated that attempts to incorporate about 30 mol% zirconium resulted in destruction of long range mesoscopic ordering under the conditions used. An additional important result of this study is the observation that the pore size distribution (PSD) was shifted from about 2.5 nm to about 6.5 nm for an increase in relative humidity from 0% to 20% and from 6.5 to about 7.6 nm for relative humidity increase from 20% to 70% for a calcination temperature of 300 degrees C. At this temperature all materials remained stable although similar overall trends were observed in the pore diameter with humidity when the calcination temperature was increased to 500 degrees C even though there was evidence of phase segregation and pore collapse at this temperature especially for samples prepared at the higher humidities. The present results suggest therefore that regulation of humidity during evaporation step affords a certain level of control over porosity. Crown Copyright (C) 2008 Published by Elsevier Inc. All rights reserved.