Pascal Voepel

In-depth mesocrystal formation analysis of microwave-assisted synthesis of LiMnPO4 nanostructures in organic solution

In the present work, we report on the preparation of LiMnPO4 (lithiophilite) nanorods and mesocrystals composed of self-assembled rod subunits employing microwave-assisted precipitation with processing times on the time scale of minutes. Starting from metal salt precursors and H3PO4 as phosphate source, single-phase LiMnPO4 powders with grain sizes of approx. 35 and 65 nm with varying morphologies were obtained by tailoring the synthesis conditions using rac-1-phenylethanol as solvent. The mesocrystal formation, microstructure and phase composition were determined by electron microscopy, nitrogen physisorption, X-ray diffraction (including Rietveld refinement), dynamic light scattering, X-ray absorption and X-ray photoelectron spectroscopy, and other techniques. In addition, we investigated the formed organic matter by gas chromatography coupled with mass spectrometry in order to gain a deeper understanding of the dissolution–precipitation process. Also, we demonstrate that the obtained LiMnPO4 nanocrystals can be redispersed in polar solvents such as ethanol and dimethylformamide and are suitable as building blocks for the fabrication of nanofibers via electrospinning.

Tailoring the diameter of electrospun layered perovskite nanofibers for photocatalytic water splitting

Ba5Ta4O15 nanofibers are prepared with tailored diameter to investigate its influence on pure photocatalytic water splitting performance. The diameter of the resulting 111-layered perovskite nanofibers was solely adjusted by the electrospinning conditions, and no variation in temperature treatment was necessary. Fibers with diameters between 100 and 300 nm can be prepared by fine-tuning the viscosity of the spinning solution, depending on the amount of spinning polymer present. Diameter-dependent photocatalytic water splitting studies reveal for the first time a morphological influence, namely optimum fiber diameter, to be responsible for optimum photocatalytic activity. Ba5Ta4O15 nanofibers with an average diameter of 161 nm show the best performance in water splitting experiments.

Mesoporous niobium-doped titanium dioxide films from the assembly of crystalline nanoparticles: study on the relationship between the band structure, conductivity and charge storage mechanism

The charge storage mechanism of nanostructured intercalation materials such as anatase (TiO2) is still a matter of intense research, because it provides the basis for designing electrochemical energy storage and conversion materials. In this work, we addressed the relationship between conductivity and the charge storage mechanism exemplified with nanostructured Nb-doped TiO2. Nanoparticles of Nb-doped TiO2 were prepared by a novel two-step solvothermal process based on tert-amyl alcohol as the solvent. Several state-of-the-art techniques, including X-ray diffraction along with Rietveld refinement and Raman and X-ray photoelectron spectroscopy, were used to analyze the phase composition, the chemical oxidation state and the structure after doping with different niobium contents. Mesoporous Nb-doped TiO2 films with uniform pore sizes of 15 to 18 nm were produced by dip-coating employing these nanoparticles as building blocks. It is found that 5 at% Nb-doped TiO2 exhibits the highest conductivity. The analysis of the peak currents in the cyclic voltammetry of the mesoporous films indicates that the pseudocapacitive current contribution varies with the conductivity in the same trend. This correlation can be described qualitatively by the changes in the space charge layer at the interface with varying carrier concentration. Such a finding implies that doping not only affects the charge storage in the bulk due to changes of kinetic parameters including the chemical diffusion coefficient and electronic conductivity, but also influences the surface-related storage mechanism of a given nanostructured material. These results further provide a valuable insight in searching for and designing materials applied in pseudocapacitors.

Hydrophilic Ionic Liquid Mixtures of Weakly and Strongly Coordinating Anions with and without Water

With the aid of ab initio molecular dynamics simulations, we investigate an ionic liquid (IL) mixture composed of three components 1-butyl-3-methylimidazolium [C4C1Im]+, tetrafluoroborate [BF4]−, and chloride [Cl]− without and with water. In the pure IL mixture, we observe an already complex network of interactions between cations and anions, and addition of water to the system even extends the complexity. Observed number integrals show that the coordination number between cations and anions is reduced in the system with water compared to that in the pure system. Further studies show that the Coulombic network of the strongly coordinating anion [Cl]− is disturbed by water, while that of the weakly coordinating anion [BF4]− is not. These observations can also be confirmed by the Voronoi polyhedra analysis, which shows that the polar network of microheterogeneous IL collapses by the introduction of water. Hydrogen-acceptor interactions revealed that the [Cl]− anions are transferred from being situated in the IL to the water continuum, while [BF4]− is almost unperturbed; these effects mainly influence the interplay of the ionic liquid network.

Sustainable and surfactant-free high-throughput synthesis of highly dispersible zirconia nanocrystals

Herein, a cost-effective and tailored synthesis route for the preparation of cubic ZrO2 nanocrystals with high dispersibility (up to 45% by weight in H2O) is reported. The procedure is straightforward and produces uniform 2–3 nm particles of a high yield of up to 98% when applying microwave dielectric heating as a “green” method. Furthermore, it can be applied to a wide range of batch sizes (from 0.5 to 20 g ZrO2), which makes it interesting for industrial applications, and also lends itself to the preparation of yttria-stabilized ZrO2 nanocrystals with varying doping levels. Overall, the paper aims at unravelling all relevant reaction steps by means of nuclear magnetic resonance and gas chromatography-mass spectrometry. Two innovative synthesis routes are presented, which have not been considered in previous studies. Both the microstructure and chemical composition of the nanoparticles were analyzed via electron microscopy, X-ray diffraction and dynamic light scattering as well as Raman, X-ray absorption and X-ray photoelectron spectroscopy. Besides, it is demonstrated that alcoholic ZrO2 dispersions are highly suited for the preparation of nanoscale materials with different morphologies, including fibers as well as ordered mesoporous and macroporous thin films and powders. Collectively, this work provides a blueprint for the fabrication of high-quality nanoparticles and structured materials thereof and is likely to trigger further research in the field of solution-processed metal oxides.