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Photo: Fröba / Mascotto / Huber

Synthesis

Nanoporous matrices: Tailor-made synthesis

There are still many unanswered questions about how water behaves under spatially very limited conditions and why. However, through the cooperation with and in subprojects 1-3, it will be possible to produce a wide range of nanoporous matrices for water and aqueous solutions and then to systematically and comparatively analyze the various factors influencing the properties of water. At first, the respective basic characterization of the finished materials will be carried out in-house, using the methods available there. Subsequently, these materials, or the water incorporated into them, will be investigated in detail and on different length and time scales in the further subprojects. 

Within a pore (e.g. one of the nanoporous matrices to be investigated here), the properties of the water are very strongly determined by the spatial extension of the geometric restriction on the one hand and by the locally present interactions with the surface of the pore wall on the other hand. While the former leads to a confinement effect only occurring at pore diameters below 10 nm, the interactions determine the way in which the properties of water or electrolyte solutions are changed. Within this research group not only different pore sizes and geometries but also different polarities, basicities, acidities and morphologies (powder vs. monoliths) will be realized and investigated.

Subproject 1: Water and aqueous electrolytes in periodic mesoporous organosilicas (PMOs) with different surface chemistries

Principal Investigator: Michael Fröba/UHH

So-called periodic mesoporous organosilicas (PMOs) have proven to be ideal model compounds for investigating confinement effects. The Fröba working group has been working on them for more than 15 years. These materials are nanoporous organic-inorganic hybrid materials whose organic component can be varied so that a specific variation of the surface chemistry is possible.

The synthesis is carried out with the aid of an organosilica precursor compound of the (R'O)3Si-R-Si(OR')3 type in the presence of a supramolecular liquid crystalline template, which, depending on size and shape, makes it possible to produce pore diameters in the 1-10 nm range with very narrow size distributions and various pore shapes (e.g. cylindrical, spherical, ink-bottle -like).

The organic bridging function R can be varied, e.g. to have a more hydrophobic or hydrophilic effect or to form hydrogen bonds. With the help of extensive investigations it could be shown, for example, that purely hydrophobic bridges R do not lead to water wetting of the organic areas, whereas the presence of amine functions allows H-bridges to the water in the pore and thus the pore wall is completely wetted. Such modulated surface polarities also have a clear effect on the diffusion behavior of the water within the pores.

In this subproject, PMOs with cylindrical pores and uniform pore diameters in the range between 1-7 nm and different bridging functions R (including charged ones) are to be synthesized and then act as nanoporous host structures for water and electrolyte solutions. This is to ensure that, in addition to the geometric restriction, the surface chemistry also varies and thus the confinement effect can be systematically investigated.

Subproject 2: Confinement of water in mesoporous functional metal oxides

Principal Investigator: Simone Mascotto/UHH

Due to the central role of water in most catalysis and energy conversion processes, it is of fundamental importance to know the interactions between water and metal oxide surfaces. Due to the increasing societal demand for a sustainable energy supply through renewable energy sources, there has been an increased focus in recent years on the development of metal oxide-mediated photo- and electrocatalytic processes in which water is reduced (hydrogen evolution reaction), oxidized (oxygen evolution reaction) or even generated (oxygen reduction reaction).

In order to improve the efficiency of such oxide-based catalysts, an increase in the specific surface area is necessary, which can be achieved by using appropriate mesoporous metal oxides. For the development of such sustainable processes the understanding of water-metal oxide interactions is a necessary step. While these interactions have been studied intensively for volume systems, almost nothing is known about the corresponding effects in geometrically confined systems.

In subproject 2 it is planned to gain a fundamental understanding of the water-metal oxide interface in ordered mesoporous titanium dioxide (anatase). Special attention will be paid to the role of surface acidity and surface defect chemistry (e.g. oxygen vacancies) and their interactions with water. These effects can be produced in a controlled manner by doping the titanium dioxide lattice with other cations (e.g. Al3+, Cr3+) in the range between 0.1 and 5 mol%.

It has already been shown that the presence of oxygen vacancies on planar anatase surfaces is advantageous for water dissociation and thus also for the photoinduced hydrogen evolution reaction. The confinement of the metal oxide should therefore have a significant influence on this interaction due to the geometrical limitation and the different defect structures of the pore surfaces.

Subproject 3: Water and aqueous electrolytes in mesoporous solids: Phase behaviour, hydrodynamic transport and dielectric behaviour

Principal Investigator: Patrick Huber / TUHH

For more than fifteen years, the Huber group has been working experimentally in the field of condensed matter in confined geometries, focusing on the atomic structure, thermodynamics, molecular dynamics, but also on the macroscopic transport properties of molecular condensates, especially water and aqueous electrolytes.

She also has great expertise in the electrochemical synthesis of monolithic mesoporous silicon and silica with parallel cylindrical pores. Hereby, porous media with well-defined pore geometry and pore wall chemistry will be produced for the planned dielectric and hydrodynamic experiments. The static dielectric constant for water and aqueous electrolytes is predicted, especially theoretically, to be significantly different from that of the bulk material. The same applies, for example, to the shear viscosity.

Because of its single crystal matrix structure, high porosity (up to 70%) and ordered pore structure (tubular pores along the <100> direction), mesoporous silicon is, for example, particularly suitable for static and dynamic X-ray scattering experiments. The monolithic silica obtained by thermal oxidation allows dielectric studies of water condensates in the pore space. The materials are therefore also of great interest for the experiments of the cooperating research groups.

More detailed information about the dielectric and hydrodynamic experiments in subproject 3 can be found in scope 2 - Thermodynamics.