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Organisationseinheit der BAM
Eingeladener Vortrag
- nein (6)
The synthesis and properties of a series of new structure-directing triblock copolymers with PEO-PB-PEO structure (PEO = poly(ethylene oxide) and PB = polybutadiene) and their application as superior pore-templates for the preparation of mesoporous titania coatings are reported. Starting from either TiCl4 or from preformed TiO2 nanocrystalline building blocks, mesoporous crystalline titanium oxide films with a significant degree of mesoscopic ordered pores are derived, and the pore size can be controlled by the molecular mass of the template polymer. Moreover, the triblock copolymers form stable micelles already at very low concentration, i.e., prior to solvent evaporation during the evaporation-induced self-assembly process (EISA). Consequently, the thickness of pore walls can be controlled independently of pore size by changing the polymer-to-precursor ratio. Thus, unprecedented control of wall thickness in the structure of mesoporous oxide coatings is achieved. In addition, the micelle formation of the new template polymers is sufficiently distinct from that of typical commercial PPO-PEO-PPO polymers (Pluronics; PPO = poly(propylene oxide)), so that a combination of both polymers facilitates bimodal porosity via dual micelle templating.
Catalysis, energy storage, and light harvesting require functional materials with tailored porosity and nanostructure. However, common synthesis methods that employ polymer micelles as structure-directing agents fail for zinc oxide, for cobalt oxide, and for metal carbonates in general. We report the synthesis of the oxides and carbonates of zinc, cobalt, and aluminum with micelle-templated structure. The synthesis relies on poly(ethylene oxide)-block-poly(butadiene)-block-poly(ethylene oxide) triblock copolymers and a new type of precursor formed by chemical complexation of a metal nitrate with citric acid. A general synthesis mechanism is deduced. Mechanistic insights allow for the prediction of optimal processing conditions for different oxides and carbonates based on simple thermogravimetric analysis. Employing this synthesis, films of ZnO and Co3O4 with micelle-controlled mesoporosity become accessible for the first time. It is the only soft-templating method reported so far that also yields mesoporous metal carbonates. The developed synthesis is generic in nature and can be applied to many other metal oxides and carbonates.
An overview is given on the synthesis of TiO2 nanoparticles with well-defined nonspherical shapes (platelet like, bipyramidal, and elongated), with the focus on controlled, reproducible synthesis, as a key requirement for the production of reference materials with homogeneous and stable properties. Particularly with regard to the nanoparticle shapes, there is a high need of certified materials,
solely one material of this type being commercially available since a few months (elongated TiO2). Further, measurement approaches with electron microscopy as the golden method to tackle the nanoparticle shape are developed to determine accurately the size and shape distribution for such nonspherical particles. A prerequisite for accurate and easy (i.e., automated) image analysis is the sample preparation, which ideally must ensure a deposition of the nanoparticles from liquid suspension onto a substrate such that the particles do not overlap, are solvent-free, and have a high deposition density. Challenges in the Synthesis of perfectly monodispersed and solvent-free TiO2 nanoparticles of platelet and acicular shapes are highlighted as well as successful measurement approaches on how to extract from 2D projection electron micrographs the most accurate spatial information, that is, true 3D size, for example, of the bipyramidal nanoparticles with different geometrical orientations on a substrate.
Analytical routines for a comprehensive in-depth morphological, structural, and chemical characterization of functionalized TiO2 films by using different state-of-the-art analytical techniques are presented and discussed with the main objective to identify potential reference TiO2 coating parameters able to be certified at a later stage. TiO2 films fabricated by two different synthetic procedures as representative for two main large-scale applications were selected: (i) pulsed d.c. magnetron sputtering for photocatalytic applications and (ii) screen printing from preformed anatase nanoparticles. The screen-printed films were further loaded with a sensitizing dye for application as a dye-sensitized solar cell. Film properties such as microstructure and crystallographic texture of pulsed d.c. magnetron sputtering synthesized films were systematically studied by means of scanning nanobeam electron diffraction in a transmission electron microscope and the surface and inner morphology by scanning electron microscopy. The dye distribution over the depth of screen-printed TiO2 layers was analyzed before and after dye-loading by means of energy dispersive X-ray spectroscopy at scanning electronmicroscope, Auger electron spectroscopy and time-of-flight secondary ion mass spectrometry. The long-term goal of the present study is the improvement of quality of the TiO2 film parameters as measured by using different types of reference TiO2 coatings having specific parameters certified.
One of the crucial characteristics of functionalized thin films is their porosity (i.e., the ratio between the pore volume and the volume of the whole film). Due to the very low amount of material per coated area corresponding to thin films, it is a challenge for analytics to measure the film porosity. In this work, we present an Approach to determine the porosity of thin films by means of electron probe microanalysis (EPMA) either by wavelength-dispersive X-ray spectrometry (WDX) or by energy-dispersive X-ray spectrometry (EDX) with a scanning electron microscope (SEM). The procedure is based on the calculation of the film mass deposition from electron-excited X-ray spectra. The
mass deposition is converted into film density by division of measured film thickness.
Finally, the film porosity is calculated from the measured film density and the density of bulk, nonporous film material. The general applicability of the procedure to determine the
porosity is demonstrated on thin templated mesoporous TiO₂
films, dip-coated on silicon
wafer, with controlled porosity in the range of 15 to 50%. The high accuracy of the mass deposition as determined from X-ray spectra was validated with independent methods (ICP-OES and weighing). Furthermore, for the validation of the porosity results, ellipsometry, interference fringes method (IFM), and focused ion beam (FIB) cross sectioning were employed as independent techniques. Hence, the approach proposed
in the present study is proven to be suited as a new analytical tool for accurate and relatively fast determination of the porosity of thin films.
Titanium dioxide (TiO₂) nanoparticles (NPs) are one of the main sources of the nanoparticulate matter to which humans are directly exposed and several studies have demonstrated their potential toxic effects. The in vivo detailed spatial distribution of TiO₂ NPs is investigated herein for the first time, using a 2D chemical imaging analysis based on confocal Raman spectroscopy. The invertebrate nematode C. elegans was employed as a prototypical model of living organisms. Rod, bipyramidal and quasispherical engineered TiO₂ NPs with different primary particle sizes and agglomeration states were prepared, characterized and then administered to nematodes. Exploiting the typical fingerprint of TiO₂ in the Raman spectrum, we monitored the biodistribution of NPs inside the worm using a non-invasive, label-free method. The high spatial resolution chemical imaging and the specificity of the Raman technique in the localization of TiO₂ NPs helped in the design of behavioral C. elegans studies aimed at elucidating the relationship among the size, shape, and agglomeration state of NPs and their ability to induce specific toxic effects. Rod-shaped NPs were the most toxic, greatly impairing pharyngeal function, reproduction and larval growth; this indicates that the rod shape, more than the bipyramidal and spherical shapes, enables NPs to interact with biological systems. These findings indicate that this Raman-nematode combined approach represents a step forward in the field of detection of NPs in living organisms, and being rapid and inexpensive enough, it can be applied as the first screening for the ability of NPs to biodistribute and exert toxicological properties in vivo.