From template-assisted mesoporous titania powders to thin films: Differences and similarities

Abstract

A huge variety of template-assisted mesoporous powder syntheses are known in literature, but the conditions are not directly transferable to thin films. The present study discusses the differences and similarities between the titania powder and thin film synthesis for the preparation of mesoporous titania materials by means of evaporation induced self-assembly. Kr adsorption at −186°C is a sufficient method to analyze the porosity of thin films and is traceable to the N 2 adsorption porosity data obtained at −196°C. Next, problems concerning the macroscopic homogeneity of the thin film are discussed as well as the deposition of the porous layer. Finally, the thermal stabilization and calcination process are compared. In contrast to the powder sample, no nanocrystal formation is observed during the thermal stabilization of the thin film. And thus the average pore size of the thin film is independent of the thermal stabilization in the here investigated range and is in all cases 3.4 nm. Furthermore, the thin film synthesis leads to a better organized and more uniform pore structure as compared to the powder synthesis. Similar to the powder synthesis, uncontrolled crystallization during calcination may lead to a structural collapse of the mesostructure. Although the crystal size after calcination is much smaller for thin films, it seems that the critical crystal size for a structural collapse is smaller for the thin films as compared to the powders. The present study facilitates the transition from optimal powder to optimal thin film synthesis conditions, which could be useful for example in the synthesis of membranes with uniform pores starting from much more extended literature on solid powders certainly with respect to synthesis-structural property correlation.

Porosimetry for Thin Films of Metal–Organic Frameworks: A Comparison of Positron Annihilation Lifetime Spectroscopy and Adsorption‐Based Methods

“…[118] Similarly, at 87 K, pore size analysis of micro-and mesoporous thin films in the pore size range from 1-10 nm has been reported. [119][120][121]…”

The Use of Different Templates for the Synthesis of Reproducible Mesoporous Titania Thin Films and Small Pore Ultrafiltration Membranes

“…Initial solutions were prepared in a similar way as previous research based on the work of the Sanchez group . The final molar ratio of a typical solution was titanium tetra‐isopropoxide:1, surfactant:X, 2‐propanol (IPA):40, water:10 and hydrogen chloride:2.…”

“…In previous research, it was determined that optimal EISA conditions for mesostructured titania thin films, starting from optimal synthesis conditions for titania powder . Although synthesis conditions of mesoporous powders can be applied, some important differences have to be taken into account with respect to the underlying mechanisms, due to differences in evaporation of thin coated layers versus powder synthesis.…”

“…For instance, in contrast to the powder sample, no nanocrystal formation is observed during the thermal stabilization of the thin film, rendering the average pore size of the thin film independent of thermal stabilization (up to 140 °C) but at the same time also more sensitive to calcination in a later step. Nevertheless, the same calcination regime can be applied for the thin films and the powder without a structural collapse due to uncontrolled crystallization …”