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Type: Article × Subject: Nano-structuring ×

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    Towards uniform electrochemical porosification of bulk HVPE-grown GaN
    (Pennington, NJ : Electrochemical Society Inc., 2019) Monaico, E.; Moise, C.; Mihai, G.; Ursaki, V.V.; Leistner, K.; Tiginyanu, I.M.; Enachescu, M.; Nielsch, K.
    In this paper, we report on results of a systematic study of porous morphologies obtained using anodization of HVPE-grown crystalline GaN wafers in HNO3, HCl, and NaCl solutions. The anodization-induced nanostructuring is found to proceed in different ways on N-and Ga-faces of polar GaN substrates. Complex pyramidal structures are disclosed and shown to be composed of regions with the degree of porosity modulated along the pyramid surface. Depending on the electrolyte and applied anodization voltage, formation of arrays of pores or nanowires has been evidenced near the N-face of the wafer. By adjusting the anodization voltage, we demonstrate that both current-line oriented pores and crystallographic pores are generated. In contrast to this, porosification of the Ga-face proceeds from some imperfections on the surface and develops in depth up to 50 μm, producing porous matrices with pores oriented perpendicularly to the wafer surface, the thickness of the pore walls being controlled by the applied voltage. The observed peculiarities are explained by different values of the electrical conductivity of the material near the two wafer surfaces.
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    Curled cation structures accelerate the dynamics of ionic liquids
    (Cambridge : RSC Publ., 2021) Rauber, Daniel; Philippi, Frederik; Kuttich, Björn; Becker, Julian; Kraus, Tobias; Hunt, Patricia; Welton, Tom; Hempelmann, Rolf; Kay, Christopher W.M.
    Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to molecular liquids – slower dynamics. In previous studies we found that the dynamics of ionic liquids are significantly accelerated by the introduction of multiple ether side chains into the cations. However, the origin of the improved transport properties, whether as a result of the altered cation conformation or due to the absence of nanostructuring within the liquid as a result of the higher polarity of the ether chains, remained to be clarified. Therefore, we prepared two novel sets of methylammonium based ionic liquids; one set with three ether substituents and another set with three butyl side chains, in order to compare their dynamic properties and liquid structures. Using a range of anions, we show that the dynamics of the ether-substituted cations are systematically and distinctly accelerated. Liquefaction temperatures are lowered and fragilities increased, while at the same time cation–anion distances are slightly larger for the alkylated samples. Furthermore, pronounced liquid nanostructures were not observed. Molecular dynamics simulations demonstrate that the origin of the altered properties of the ether substituted ionic liquids is primarily due to a curled ether chain conformation, in contrast to the alkylated cations where the alkyl chains retain a linear conformation. Thus, the observed structure–property relations can be explained by changes in the geometric shape of the cations, rather than by the absence of a liquid nanostructure. Application of quantum chemical calculations to a simplified model system revealed that intramolecular hydrogen-bonding is responsible for approximately half of the stabilisation of the curled ether-cations, whereas the other half stems from non-specific long-range interactions. These findings give more detailed insights into the structure–property relations of ionic liquids and will guide the development of ionic liquids that do not suffer from slow dynamics.
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    Facile and scalable synthesis of sub-micrometer electrolyte particles for solid acid fuel cells
    (London : RSC Publishing, 2018) Lohmann-Richters, F.P.; Odenwald, C.; Kickelbick, G.; Abel, B.; Varga, Á.
    Nanostructuring fuel cell electrodes is a viable pathway to reach high performance with low catalyst loadings. Thus, in solid acid fuel cells, small CsH2PO4 electrolyte particles are needed for the composite powder electrodes as well as for thin electrolyte membranes. Previous efforts have resulted in significant improvements in performance when using sub-micrometer CsH2PO4 particles, but laborious methods with low throughput were employed for their synthesis. In this work, we present a simple, robust, and scalable method to synthesize CsH2PO4 particles with diameters down to below 200 nm. The method involves precipitating CsH2PO4 by mixing precursor solutions in alcohol in the presence of a dispersing additive. The influence of the concentrations, the batch size, the solvent, and the mixing process is investigated. The particle size decreases down to 119 nm with increasing amount of dispersing additive. Mixing in a microreactor leads to a narrower particle size distribution. The particle shape can be tuned by varying the solvent. The ionic conductivity under solid acid fuel cell conditions is 2.0 × 10-2 S cm-1 and thus close to that of CsH2PO4 without dispersing additive.