title: Mesoporous film architectures and step gradient formation
Creators: name: Stanzel, Mathias
Publisher: name: UNSPECIFIED
Year: 2022

description: The development of novel devices along with technological progress of our time requires miniaturization and compartmentalization, and with this nanotechnology. For example, future innovative solutions are urgently pursued for life-inspired sustainable water and energy management. Nanoscale pores and channels, as one field within nanotechnology, hold great potential in mimicking the outstanding transport phenomena of their biological paragons. Such transport properties, as observed in biological pores and channels, originate from complex architectures and are influenced by pore geometry, surface charge distribution, chemical composition, and wettability. However, desired transport properties in advanced applications require enhanced control of surface functionalization in nanoscale pores and channels along with nanoporous material architecture design. In this regard, mesoporous silica thin films represent suitable model materials for nanoporous material architecture design providing ordered nanoscale pores and nanoscale film thicknesses. In this work, mesoporous silica thin films were investigated to create mesoporous step gradient architectures with respect to pore size, surface wettability, and surface charge. This work was divided into three main sections: i) generating a material library allowing step gradient design, ii) the fabrication of mesoporous architectures, and iii) (nano)local polymer placement into such multilayer architectures. To create mesoporous architectures, a material library was built in the first place. Thereby, the ionic pore accessibility of hydrophilic mesoporous silica thin films was investigated in dependence of preparation parameters, i.e. the template removal. Hydrophobic mesoporous silica thin films with tunable surface wettability were developed using co-condensation of tetraethylorthosilicate and methylated silica precursors resulting in mesoporous (organo)silica thin films. As a side note, an enhanced chemical stability in basic environment was observed for hydrophobic thin films. To replace petro-based templating macromolecules, hydroxypropyl cellulose was successfully applied as bio-based structure directing template for the generation of mesoporous silica thin films with permselective ionic pore accessibility. Mesoporous step gradient architectures were prepared by applying the developed material library combining mesoporous layers with orthogonal properties. Examples are: the fabrication of hydrophilic pore size step gradients, and the combination of layers with different wettability. Interestingly, investigation of mesoporous wettability step gradient films with respect to the ionic pore accessibility in dependence of the hydrophobic top layer’s thickness showed an overcoming of the hydrophobic layer through electrostatic attraction of the hydrophilic bottom layer in case of the thinnest hydrophobic top layer. Regarding local polymer placement, multilayer step gradient mesoporous film formation turned out to be advantageous, too. For example, the layer-selective polymer functionalization of hydrophilic double layered mesoporous silica thin films was achieved by predisposition of a single layer, followed by selective iniferter binding. Layer-selective polymer grafting was achieved resulting in step gradients with charge density control. To further investigate the limits of polymer placement in mesoporous film architectures, plasmonic metal nanoparticles were incorporated into mesoporous silica thin films. These particles served as nanoscopic plasmonic light source and were combined with photopolymerizations. Investigation of the prepared mesoporous composite materials allowed precise placement of the nanoparticles in mesoporous silica thin films with tunable density. The concept of nanolocal polymer placement using plasmonic metal nanoparticles in combination with photopolymerization was demonstrated for two distinct polymerization approaches. Due to the sensitivity of the nanoparticle’s surface plasmons on the surrounding refractive index, such mesoporous composite materials further demonstrated application as sensing unit allowing to detect local refractive index changes, e.g. in consequence of nanolocal polymer placement.