Bernd Smarsly

Ordered Mesoporous Sb-, Nb-, and Ta-Doped SnO2 Thin Films with Adjustable Doping Levels and High Electrical Conductivity

This paper describes the synthesis and electrical properties of self-organized Sb-, Nb-, and Ta-doped SnO(2) thin films with adjustable doping levels. These transparent conducting oxides (TCOs) were prepared using a poly(ethylene-co-butylene)-b-poly(ethylene oxide) diblock copolymer as well as a novel polyisobutylene-b-poly(ethylene oxide) as organic templates. All samples are highly crystalline and have ordered cubic pore-solid architectures after removal of the polymer template by calcination; however, the electrical conductivity is not identical. The films are characterized by a combination of small- and wide-angle X-ray diffraction/scattering, SEM/TEM imaging, and X-ray photoelectron spectroscopy. Resistivity measurements conducted on the mesoporous frameworks show that the electrical properties strongly depend on both the degree of crystallinity and the elemental makeup. Considerable enhancements of the electrical properties result when the films are doped with antimony and treated in N(2) at elevated temperatures. Such TCO materials show electrical resistivities which are--despite the mesoporous morphology--only 1 order of magnitude higher than reported values for dense Sb-doped SnO(2) films.

From cyclodextrin assemblies to porous materials by silica templating.

[15] This is a high-concentration synthesis in which the solidified inorganic compound is a copy of the original phase structure, that is its structure is predetermined by the selection of the template phase. They used lyotropic liquid-crystalline phases, derived from surfactants or–later–block copolymers, [15±17]

Cavitation in Metastable Liquid Nitrogen Confined to Nanoscale Pores

We studied cavitation in metastable fluids drawing on the example of liquid nitrogen confined to spheroidal pores of specially prepared well-characterized mesoporous silica materials with mean pore diameters ranging from approximately 6 to approximately 35 nm. Cavitation was monitored in the process of evaporation/desorption from fully saturated samples with gradually decreasing vapor pressure at the isothermal conditions. The onset of cavitation was displayed by a sharp step on the desorption isotherm. We found that the vapor pressure at the onset of cavitation depended on the pore size for the samples with pores smaller than approximately 11 nm and remained practically unchanged for the samples with larger pores. We suggest that the observed independence of the cavitation pressure on the size of confinement indicates that the conditions of bubble nucleation in pores larger than approximately 11 nm approach the nucleation conditions in the bulk metastable liquid. To test this hypothesis and to evaluate the nucleation barriers, we performed grand canonical and gauge cell Monte Carlo simulations of nitrogen adsorption and desorption in spherical silica pores ranging from 5.5 to 10 nm in diameter. Simulated and experimental adsorption isotherms were in good agreement. Exploiting the correlation between the experimental cavitation pressure and the simulated nucleation barrier, we found that the nucleation barrier increased almost linearly from approximately 40 to approximately 70 k(B)T in the range of pores from approximately 6 to approximately 11 nm, and varied in diapason of 70-75 k(B)T in larger pores, up to 35 nm. We constructed the dependence of the nucleation barrier on the vapor pressure, which asymptotically approaches the predictions of the classical nucleation theory for the metastable bulk liquid at larger relative pressures (>0.6). Our findings suggest that there is a limit to the influence of the confinement on the onset of cavitation, and thus, cavitation of nanoconfined fluids may be employed to explore cavitation in macroscopic systems.

Recent progress in soft-templating of porous carbon materials

Mesoporous carbons synthesized via a soft-templating approach have attracted much attention due to their easy synthesis and facile control over the derived pore structure. In analogy to soft-templating approaches for mesoporous metal oxides, their synthesis is based on a sequence of forming supramolecular arrangements of precursor molecules with the soft templates, stabilization of the precursor framework by polymerization and finally the removal of the templates. Using micelles of amphiphilic block-copolymers as templates, facile control over the morphology and size of mesopores can be achieved by e.g. controlling size, composition, and concentration of the template polymers or composition and degree of polymerization of the precursor. Moreover, soft templating approaches can be extended to obtain also carbon materials with hierarchical meso- and macroporosity. The additional macroporosity either can result from templating by polymer latex or is induced via macrophase separation. In this review, we describe recent progress and examples in the synthesis and application of mesoporous carbon materials based on soft-templating approaches. Moreover, we reiterate fundamental principles of self-aggregation, highlight proposed synthesis mechanisms and present means of controlling pore size, also in hierarchical meso–macroporous carbon materials.

The interplay of colloidal organization and oxo-cluster chemistry : polyoxometalate-silica hybrids-materials with a nanochemical function

Polyoxometalates (POMs) of transition metals (e.g., molyb-denum, vanadium) have been a center of research interest for several decades, as they exhibit a great variety of structures and a rich diversity of fascinating properties. [1] The structures range from relatively simple, like the Keggin ion, [2] to extremely complex, such as the wheel-shaped [3±5] and ball-shaped molybdate clusters [6] prepared by Müller et al., of which the latter possess 154, 176, 248, or 132 metal centers per cluster. Their architectures are not only well-defined, relatively easy to produce, and beautiful, but these compounds also exhibit the electronic, electrochemical, magnetic, and optical qualities typical of mixed-valence molybdenum oxide compounds , which creates potential for many applications. In most cases, pure POMs are quite difficult to handle from a materials science point of view. For example, the problem of too high a solubility in water is encountered, together with low pH and redox stability. In addition, the cluster cavities of most POMs are filled with water of crystallization, which cannot be removed without destroying the structures. [9,10] There is also the inherent problem of lacking access to all inner sites of the clusters, which is necessary for application of these compounds, for example in catalysis. There are only a few examples of POMs which possess a permeable structure representing a pore system providing accessibility to the inner surface comparable to those of porous silica. [9±16] The reason for this is simply the fact that POMs are crystalline, and a sol-gel process similar to the synthesis of silica [17,18] is not yet known for the fabrication of molybdenum trioxide. The introduction of POMs into a sol-gel-derived silica matrix [7,19] or their immobilization in pre-formed porous materials [20,21] was used to solve these problems. In this publication we present the incorporation of supra-molecular molybdenum polyoxometalates into sol-gel-derived silica. Two types of molybdenum polyoxometalates were synthesized according to the preparation method published by Mül-ler et al. [4] The sol-gel process to produce the POM±sil-ica hybrid material produces amorphous, optically clear objects (monoliths) in variable size and form (see also Fig. 6a). Due to the presence of the polyoxometalate these monoliths are deeply colored. The higher electron density of POMs, compared with that of the silica matrix, enables X-ray scattering experiments to provide information about the structure as well as the POM cluster distribution in the hybrid material. The small-angle X-ray scattering (SAXS) diffraction pattern of the …

X-ray scattering of non-graphitic carbon: an improved method of evaluation

The wide-angle X-ray scattering (WAXS) of non-graphitic carbons shows relatively broad and diffuse interference maxima. In most cases, the corresponding line profiles cannot be separated unambiguously, so that an evaluation using profile analysis is difficult. In these cases, fitting procedures for the total scattering curve are more appropriate. This requires theoretical expressions for all interference maxima with a sufficient number of relevant structural parameters to ensure an acceptable fit. In this paper, such expressions are presented for the (hk) lines (intralayer scattering) and the (00l) lines (interlayer scattering), and the application of these expressions to the evaluation of selected WAXS curves is described. Furthermore, the effects of thermal motion, preferred orientation and Compton scattering are discussed.

The generation of mesostructured crystalline CeO2, ZrO2 and CeO2–ZrO2 films using evaporation-induced self-assembly

Mesostructured thin films of CeO2, ZrO2and CeO2–ZrO2 mixed oxides with highly crystalline pore walls and ordered arrays of mesopores were obtained by a straightforward fabrication process employing evaporation-induced self-assembly (EISA) and a well-designed temperature treatment, taking advantage of a novel type of amphiphilic block copolymer as template. The mesostructure and crystallinity were studied in detail using small-angle and wide-angle X-ray scattering and electron microscopy. The mesostructured CeO2 films are crack-free, possess a final pore size of ca. 10 nm, and the mesopores are surrounded by an almost completely crystalline matrix of nanoparticles of ca. 5–7 nm in size, as revealed by high-resolution electron microscopy. Additionally, the mesoscopic order (bcc structure) shows high thermal stability. The crystallization of the walls is usually accompanied by stresses and strong uniaxial structural shrinkage, which can, however, be significantly diminished by making mixed CeO2–ZrO2 mesostructured systems. Here, the crystallites represent “solid solutions” of both binary oxides and exhibit an even higher thermal stability, while the constituting nanocrystals are smaller compared to the pure CeO2.

Determination by x-ray reflectivity and small angle x-ray scattering of the porous properties of mesoporous silica thin films

Two-dimensional hexagonal silica thin films templated by a triblock copolymer were investigated by grazing incident small angle x-ray scattering (GISAXS) and x-ray reflectivity (XR) before and after removing the surfactant from the silica matrix. XR curves—analyzed above and below the critical angle of the substrate—are evaluated by the matrix technique to obtain the average electron density of the films, the wall thickness, the electron density of the walls, the radius of the pores, and subsequently the porosity of such mesoporous films. In combination with GISAXS, the surface area of the mesopores is ascertained, thereby providing a complete analysis of the porosity in thin films by x-ray scattering methods.

Influence of particle properties on the wall region in packed capillaries.

Analytical columns (4.6 mm i.d.) packed with core-shell particles have shown a significantly reduced eddy dispersion contribution to band broadening compared to conventional fully porous particles. It has been speculated if this is caused by the narrow particle size distribution (PSD) of the core-shell particles, as an intrinsic advantage, or by an improved packing structure that specifically reduces the transcolumn velocity biases caused by wall effects. A recent simulation study has pointed against the former proposition [A. Daneyko et al., Anal. Chem. 83 (2011) 3903]. It is more likely that the slurry packing process for core-shell particles results in bed morphologies with reduced wall effects compared to the fully porous particles with a wide PSD. To access the latter proposition experimentally we slurry packed capillary columns (100 μm i.d.) with different fully porous (wide PSDs) and core-shell (narrow PSDs) particles and imaged their bed structures three-dimensionally using confocal laser scanning microscopy. This allowed us to resolve and analyze the bed morphology in these columns locally on all length scales contributing to eddy dispersion. On the transcolumn scale we observed a systematic difference between core-shell and fully porous particles: In the vicinity of the column wall the core-shell particles packed denser (closer to the bulk packing densities) and with a higher regularity than the fully porous particles. The bulk regions of all packings were effectively indistinguishable. This provides experimental evidence that the reduced eddy dispersion contribution with core-shell packings should be attributed to a higher transcolumn homogeneity rather than to an improved bed morphology on smaller length scales, e.g., to a reduced short-range disorder.

Solid-state morphologies of linear and bottlebrush-shaped polystyrene-poly(Z-L-lysine) block copolymers

Abstract The solid-state structures of polystyrene–poly(Z- l -lysine) block copolymers were examined with respect to the polymer architecture and the secondary structure of the polypeptide using circular dichroism, quantitative small- and wide-angle X-ray scattering, and electron microscopy. Linear block copolymers exhibit a hexagonal-in-lamellar structure where folded and packed polypeptide α-helices form lamellae which extend over an exceptional broad range of the composition diagram. Star- or bottlebrush-shaped copolymers are able to stabilize a larger interface area than linear ones which promotes the formation of undulated lamellar mesophases. Depending on the secondary structure of polypeptide segments, plane lamellar, superundulated lamellar, or corrugated lamellar phases are formed. These results indicate the importance of a secondary structure and packing of polymer chains for the formation of new phases and ordering far from the ‘classical’ phase behavior.