title: Mesoporous and macroporous brookite thin films having a large thermal stability range.
abstract: A sol-gel chemistry approach is employed to generate mesoporous and macroporous brookite thin films using Ag ions as dopant species whose thermal stability is well above previously reported literature values for thin films. The Ag ions not only induce the formation of brookite but also participate in its enhanced thermal stability. Despite brookite being metastable in nature, which renders it a challenge to synthesize, it has been prescribed as a potential competitor to anatase. We have used a layer-by-layer approach to generate a mesoporous Ag-doped brookite structure at 500 °C with 95% composition by XRD. This tightly packed mesoporous structure can be described as striated grains of brookite protruding from the surface to form an interlocked network whose thermal stability spans up to 800 °C. The open structure of brookite makes it an apt host for the intercalant Ag species, whose inclusion within the brookite framework is improved by the presence of a stabilizing agent. Both the morphology of the surface and the presence of a stabilization agent for Ag contribute to enhancing its thermal stability. This is in contrast to the thermal stability of the macroporous brookite thin film, which was found to be lower (<700 °C) than that of the mesoporous brookite thin film. The reagents are deliberately chosen to produce a macroporous film in the absence of a stabilizing agent. Ag nodules are observed to be formed at 700 °C, which implies their limited intercalation into the brookite structure, thus rendering them relatively less stable. Moreover, the macroporous film being relatively more relaxed is more susceptible to phase transformation at a higher calcination temperature. Our results provide a platform that paves the way toward better control, thereby leading to a broader technological application of brookite.
authors.affiliation: Institute for Materials & Surface Technology (IMST), University of Applied Science , Grenzstrasse 3, 24149 Kiel, Germany.
authors.forename: Kevin R
authors.initials: KR
authors.lastname: Moonoosawmy
authors.forename: Hannelore
authors.initials: H
authors.lastname: Katzke
authors.forename: Martha
authors.initials: M
authors.lastname: Es-Souni
authors.forename: Matthias
authors.initials: M
authors.lastname: Dietze
authors.forename: Mohammed
authors.initials: M
authors.lastname: Es-Souni