Niklaus Kränzlin

Nonaqueous TiO2 Nanoparticle Synthesis: a Versatile Basis for the Fabrication of Self-Supporting, Transparent, and UV-Absorbing Composite Films

A successful strategy to obtain self-supporting (100 microm), UV-absorbing, and, in the visible region, highly transparent TiO2-poly(methyl methacrylate) (PMMA) films was developed. The 15 nm large anatase TiO2 nanocrystals were prepared in a nonaqueous sol-gel approach involving the mixing of Ti(O(i)Pr)4 and benzyl alcohol. The surfaces of the resulting particles were modified with minute amounts of organic ligands in order to make the particles easily dispersible in nonpolar media like xylene and dichloromethane and compatible with PMMA, a polymer of high optical transparency and considerable technical importance. The empirical optimization process of composite fabrication was supplemented by fundamental studies of the crystallization and growth mechanism of anatase particles in a nonaqueous medium. After the preparation of corresponding nanocomposites, the materials were investigated with respect to their UV absorption capability, optical transparency in the visible-wavelength region, and photodegradation.

Anatase–silica composite aerogels: a nanoparticle-based approach

Herein we present the synthesis of anatase–silica aerogels based on the controlled gelation of preformed nanoparticle mixtures. The monolithic aerogels with macroscopic dimensions show large specific surface areas, and high and uniform porosities. The major advantage of such a particle-based approach is the great flexibility in pre-defining the compositional and structural features of the final aerogels before the gelation process by fine-tuning the properties of the titania and silica building blocks (e.g., size, composition and crystallinity) and their relative ratio in the dispersion. Specific surface functionalization enables control over the interaction between the nanoparticles and thus over their distribution in the aerogel. Positively charged titania nanoparticles are co-assembled with negatively charged Stoeber particles, resulting in a binary aerogel with a crystalline anatase and amorphous silica framework directly after supercritical drying without any calcination step. Titania–silica aerogels combine the photocatalytic activity of the anatase nanoparticles with the extensive silica chemistry available for silica surface functionalization.

Microwave-assisted nonaqueous synthesis of WO3 nanoparticles for crystallographically oriented photoanodes for water splitting

Nanostructured WO3 photoanodes with crystallographic orientation along the [001] direction were fabricated via doctor blading nanoparticles synthesized through a microwave-assisted nonaqueous sol–gel route. Monoclinic WO3 platelets with a size ranging from 20 to 40 nm and a thickness of 3 nm were obtained after a short reaction time of 10 minutes under microwave irradiation. The films consisted of a porous network of nanoparticles and their photoelectrochemical activity was tested. After cathodic polarization of the photoanodes in the dark which led to a significant increase of 65% of the photocurrent, the films exhibited initially a maximum photocurrent of 2.7 mA cm−2 at 1 V vs. reversible hydrogen electrode (RHE) in 3 M H2SO4 under simulated AM 1.5 G illumination (100 mW cm−2) comparable to the best photocurrents reported for WO3 photoanodes. However oxygen evolution measurements showed that the faradaic efficiency dropped after the cathodic polarization and other products than O2 might be formed. In comparison to the chemical solution growth of films from molecular precursors, the use of preformed nanoparticles in the form of powders is not only more robust and easier to up-scale, but also offers many opportunities to improve the photoelectrochemical performance by tailoring the nanoparticle size, the shape, and their arrangement on the substrate.

Wet-Chemical Preparation of Copper Foam Monoliths with Tunable Densities and Complex Macroscopic Shapes

Macroscopic monoliths of copper foams have been prepared by a template-assisted wet-chemical process. The method offers subtle control over the pore size and size distribution, density and macroscopic size and shape of the metal foam. Uniaxial compression tests revealed different deformation behavior depending on the relative density. Non-vacuum-based and low-temperature routes are attractive for the cost-effective production of metal foams.

Controlled fabrication of porous metals from the nanometer to the macroscopic scale

Porous metals such as foams and sponges are produced by a wide variety of industrially applied techniques mainly for load bearing and structural applications. Their functional properties are much less explored, mainly due to the difficulty to simultaneously control the relative density, the pore structure and the macroscopic shape during their fabrication, which however would be required to control the functionality. Accordingly, to take full advantage of the mechanical and functional properties of porous metals, new synthesis methodologies have to be developed that address all structural features from the nanoscopic to the macroscopic scale. Thus, in this review we provide an overview of the different techniques to prepare porous metals with the focus on evaluating them with respect to their flexibility and ability to control and tailor the architecture over several length scales. We discuss the advantages and limitations of the different methods, correlate them with the properties of the resulting porous metals and present potential applications. In the last part of the review, we outline a wet-chemical deposition route to metallic copper, which can be applied to a wide variety of flat and spherical substrates, giving access to both copper sponges with defined porosity and relative density as well as complex macroscopic shapes and to copper line patterns on flexible substrates.

Matching the organic and inorganic counterparts during nucleation and growth of copper-based nanoparticles – in situ spectroscopic studies

Although syntheses in organic solvents provide access to a wide range of copper-based nanoparticles, the correlation between organic reactions in solution and nucleation and growth of nanoparticles with defined properties is not well understood. Here, we utilize the Multivariate Curve Resolution-Alternative Least Squares (MCR-ALS) methodology to examine spectroscopic data recorded in situ during the synthesis of copper-based nanoparticles. While earlier studies showed that depending on the temperature copper(II) acetylacetonate reacts with benzyl alcohol and forms either copper oxides or copper nanoparticles, we link the inorganic reaction with their organic counterparts. From X-ray Absorption Near Edge Spectroscopy (XANES) and Ultraviolet-visible spectroscopy (UV-vis) data we learn that copper(I) oxide forms directly from the solution and is the final product at low temperature of 140 °C. We observe in Fourier Transformed Infrared (FTIR) spectra an increasing concentration of benzyl acetate that co-occurs with the formation of a copper enolate and evolution of benzaldehyde, which accompanies the reduction of copper ions. We also record the interaction of organic species at the Cu2O surface, which inhibits a further reduction to metallic copper. When we raise the synthesis temperature to 170 °C it turns out that the Cu2O is just an intermediate species. It subsequently transforms by solid-state reduction to metallic copper accompanied by oxidation of benzyl alcohol to benzaldehyde.