Oxide nanostructures often show modified cell parameters, composition, or morphology, providing access to new materials with novel properties. Among the important aspects are size-dependent geometric and electronic properties, resulting in modified transport properties. Also, inert oxides can become highly reactive when applied in highly dispersed form. Guided by insights into local interaction mechanisms (funCOS 1, funCOS 2, and funCOS 3 and funCOS 6) and structure formation on planar surfaces (funCOS 4 and 6), funCOS 5 aims at transferring the obtained knowledge to different types of oxide nanostructures.
The participating workgroups have exceptional expertise with a broad range of preparation techniques for nanostructured oxide materials, including anodic oxidation of metal surfaces, FEBIP, or CVS. Self-organized TiO2 nanotube layers on metal substrates can be obtained by electrochemical etching and represent a unique model system for monolayer attachment of organic molecules. The use of electron beams represents an excellent and particularly clean tool to locally modify the properties of oxide materials. Localized electron probes such as SEM have made it possible to apply electron-induced processes at the nanometer scale and will be used to generate nanostructures with lithographic control. Finally, CVS at reduced pressure is particularly well suited for the production of metal oxide nanocrystals with small size and narrow size distribution. The comparison between vacuum sublimation and solution deposition of organic molecules will reveal the key mechanisms of binding and structure formation. Comparison of differently prepared systems with different surface structure will provide information on specific molecule–substrate interactions. Moreover, investigations of particle–particle interaction effects and the effect of the reaction environment will allow for a more general understanding of organic/oxide surface chemistry under realistic conditions.
funCOS 5 aims at the transfer of atomic level insights obtained by surface science techniques on plane and atomically clean surfaces (funCOS 1–3) to 3D high surface area materials, which—in terms of structure and chemical composition—are significantly more complex. Oxide nano-structures—with and without supporting substrates—will be established as complementary model systems for funCOS adsorption studies. The major goals of funCOS 5 are
- the fabrication of oxide nanostructures with increasing complexity by different synthesis and preparation routes for metal oxide substrates of defined shape and controlled spatial arrangement;
- the investigation of the adsorption behavior of functional molecules on oxide nanostructures;
- the design of complex molecule–oxide interface structures including the generation of radiation induced defects as means to control adsorption.
Systems and strategy
Morphologically different MgO, TiO2 and CoOx nanostructures will be employed. For their fabrication different routes such as anodic oxidation of metal surfaces, FEBIP or CVS in conjunction with deposition approaches to generate textured nanocrystal layers will be applied. Different morphologies and microstructures will be employed. These will range from nanocrystal ensembles at various stages of aggregation and texturing, through nanotubular metal oxide arrangements of adjustable pore orientation and connectivities to compact metal oxide films deposited under UHV conditions. It is the starting point of funCOS 5 to make use of synthesis and preparation routes that are already established in the other groups to further develop them for the generation of thin films and particle based nanostructures of controlled spatial arrangement.
The adsorption behavior of these structures toward functional molecules as well as the impact of defects thereon will be of central interest. We will search for preferred adsorption sites on the above-mentioned nanostructures and will compare differences in the adsorption of liquids in vacuum as well as in liquid phases. Adsorption-induced changes in the electronic, magnetic and catalytic properties will be addressed with spectroscopy. This part of the project will also include the characterization of photo-electrochemical reactivity and photoluminescence properties of the as synthesized as well as of adsorbate-covered TiO2, MgO and CoOx nanostructures. As an extension to nanostructure fabrication we will study the effect of radiation of different energy and nature on nanostructures as means to increase control over nanostructure growth and functionalization. For this purpose, we will employ sample exposure to photons, electrons and ions (three-beams concept) in order to generate selected surface elements as adsorption sites for molecules in the funCOS Molecular Toolbox. The radiation induced generation of defects, e.g. point defects that arise from local oxygen deficiencies or vacancy interstitial pairs, will be of interest for the introduction of new adsorption sites on the surface of oxide nanostructures. Ultimately, we aim at the rational design of oxide nanostructures, i.e. the establishment of optimized synthesis and engineering protocols to generate nanostructured templates for the site selective adsorption of functional molecules from the funCOS molecular toolbox.