Comparing the electrical and protonic conductivity of mesoporous and nanocrystalline thin films of ceria-zirconia solid solutions

Due to the redox activity of the redox couple Ce3+/Ce4+, ceria-based solid solutions are typical mixed electronic and ionic conductors (MIECs) which are used e.g. as solid electrolytes in oxygen membranes or as electrode material in solid oxide fuel cells. CexZr1-xO2 (CZO) solid solutions not only show an increased thermal and mechanical stability compared to the corresponding binary oxides, but also exhibit an improved oxygen storage capacity making CZO a prominent material system for heterogeneous catalysis. Besides the control over composition, the defect chemistry of CZO may be optimized by nanostructuring. Here we present investigations of the electrical properties of mesoporous C0.8Z0.2O2 thin films prepared by solution phase coassembly of salt precursors with an amphiphilic diblock copolymer using an evaporation-induced self-assembly (EISA) process. The mesoporous thin films exhibit a regular pore network with a high surface to volume ratio making them an ideal model system to study the influence of surface effects on the transport properties. Structural characterization using SEM, WAXD, XRD, XPS and Raman spectroscopy reveal the high structural quality of the thin films with 24 nm diameter pores which are surrounded by a crystalline wall structure consisting of 3 to 15 nm grains. Nanocrystalline thin films were prepared using pulsed laser deposition and characterized by SEM and XRD. Using electrochemical impedance spectroscopy, the electrical properties of the mesoporous and nanocrystalline thin films were investigated in a temperature range from room temperature to 500 °C and under different oxygen partial pressures between 1 and 10-4 bar. Measurements under varying humidity show large differences between the mesoporous and nanocrystalline thin films. While a significant increase in the conductivity is observed for the nanocrystalline thin films at temperatures below 250 °C and high humidity conditions, the mesoporous samples show no contribution of protonic conductivity. As will be discussed, these results indicate that the high surface area of the mesoporous samples has either no or very little effect on the protonic transport properties in CZO. Please click Additional Files below to see the full abstract

Quantification of calcium content in bone by using ToF-SIMS-a first approach

The determination of the spatially resolved calcium distribution and concentration in bone is essential for the assessment of bone quality. It enables the diagnosis and elucidation of bone diseases, the course of bone remodelling and the assessment of bone quality at interfaces to implants. With time-of-flight secondary ion mass spectrometry (ToF-SIMS) the calcium distribution in bone cross sections is mapped semi-quantitatively with a lateral resolution of up to 1mum. As standards for the calibration of the ToF-SIMS data calcium hydroxyapatite collagen scaffolds with different compositions were synthesized. The standards were characterised by loss of ignition, x-ray diffractometry (XRD) and x-ray photoelectron spectroscopy (XPS). The secondary ion count rate for calcium and the calcium content of the standards show a linear dependence. The obtained calibration curve is used for the quantification of the calcium content in the bone of rats. The calcium concentration within an animal model for osteoporosis induction is monitored. Exemplarily the calcium content of the bones was quantified by XPS for validation of the results. Furthermore a calcium mass image is compared with an XPS image to demonstrate the better lateral resolution of ToF-SIMS which advances the locally resolved quantification of the calcium content

Elucidating the molecular landscape of the stratum corneum

Characterization of the molecular structure of skin, especially the barrier layer, the stratum corneum, is a key research priority for generating understanding to improve diagnostics, aid pharmaceutical delivery, and prevent environmental damage. Our study uses the recently developed 3D OrbiSIMS technique to conduct in situ analysis of ex vivo human skin tissue and reveals the molecular chemistry of skin in unprecedented detail, as a result of the step change in high mass resolving power compared with previous studies. This characterization exposes the nonhomogeneity of the stratum corneum, both laterally and as a function of depth. Chemical variations relating to fundamental biological processes, such as the epidermal cholesterol sulfate cycle, are visualized using in situ analysis. We are able to resolve the debate around the chemical gradients present within the epidermis, for example, whether palmitic acid is of sebaceous origin or a true component of the stratum corneum. Through in situ depth analysis of cryogenically preserved samples, we are able to propose that it is actually a component of both surface sebum and the intrinsic lipid matrix. This approach also suggests similarity between the epidermis compounds found in human and porcine skin as a function of depth. Since porcine skin is a widely used model for permeation testing this result has clinical relevance. In addition to using this technique for endogenous species, we have used it to demonstrate the permeation of a commercially important antiaging peptide into the human stratum corneum. Due to its chemical similarity to native skin components and exceptionally low effective concentration, this information was previously unattainable

Ceria - zirconia thin films : influence of nanostructure and moisture on charge transport properties

In the last decade a huge expansion of nano material applications being introduced into all kinds of markets is observed leading to a growing tendency to tailor the fabrication of materials with nano dimensions in order to tune their properties, optimizing them for a certain process. The same holds true for ceria based solid solutions. CeO2 - ZrO2 solid solutions are well established for the use as part of three-way catalytic converters in the exhaust gas cleaning system of combustion engines. Apart from that ceria is studied for the use in a wide range of applications, like solid oxide fuel cells, polymer exchange membrane fuel cells, both being almost fully developed to be introduced for a wide market. Next to others, currently emerging applications are the use of ceria as a catalyst for reforming processes, water-gas shift reaction or thermochemical water splitting. Due to the complexity of CeO2 - ZrO2- based materials the preparation of model systems has proven to be a highly versatile approach in order to gain a deeper understanding of specific phenomena. The same approach was used in the work presented here, following several preliminary studies on single crystalline material on the oxygen transport properties of ceria- zirconia solid solutions by means of surface analytical techniques and electrochemical impedance spectroscopy in the work group of Prof. Jürgen Janek. Thin films offer a more straightforward application of surface analysis techniques, as well as the possibility to tune the structural and electronic properties of the material under investigation. In this work CeO2 - ZrO2 thin films were deposited by means of pulsed laser deposition. The resulting Ce_{1-x}Zr_{x}O_{2} thin films of different morphology and composition (ranging from x=0-0.4) were characterized by surface analytical techniques as well as electrochemical impedance spectroscopy (EIS). EIS allows to precisely control the atmospheric conditions such as gas type and mixture or temperature and humidity using a respective experimental setup described in the methods section of this manuscript. The presented work focuses on the influence of grain boundaries and the surface, as they play an important role for thin film transport processes. Electrochemical impedance spectroscopy allows to deduce transport properties of the thin films under investigation and compare those to respective models. At moderate and very low temperatures down to room temperature water influences the conductivities determined during the experiments. In the respective literature which is going to be reviewed in the theoretical section of this work an ongoing discussion on the influence of water vapor in the surrounding atmosphere (humidity) during impedance measurements is revealed. The experimental results in this work are described in good agreement by a model combining thermodynamic adsorption models with the theory of percolating networks. These results reveal valuable information to the ongoing discussions in literature

Preparation of multilayer samples for scanning thermal microscopy examination

Thin film multilayer materials are very important for a variety of key technologies such as hard drive storage. However, their multilayered nature means it can be difficult to examine them after production and determining properties of individual layers is harder still. Here, methods of preparing multilayer samples for examination using scanning thermal microscopy are compared, showing that both a combination of mechanical and ion beam polishing, and ion beam milling to form a crater produce suitable surfaces for scanning thermal microscopy examination. However, the larger exposed surfaces of the ion beam milled crater are the most promising for distinguishing between the layers and comparison of their thermal transport properties

YSZ thin films with minimized grain boundary resistivity.

In recent years, interface engineering of solid electrolytes has been explored to increase their ionic conductivity and improve the performance of solid oxide fuel cells and other electrochemical power sources. It has been observed that the ionic conductivity of epitaxially grown thin films of some electrolytes is dramatically enhanced, which is often attributed to effects (e.g. strain-induced mobility changes) at the heterophase boundary with the substrate. Still largely unexplored is the possibility of manipulation of grain boundary resistivity in polycrystalline solid electrolyte films, clearly a limiting factor in their ionic conductivity. Here we report that the ionic conductivity of yttria stabilized zirconia thin films with nano-columnar grains grown on a MgO substrate nearly reaches that of the corresponding single crystal when the thickness of the films becomes less than roughly 8 nm (smaller by a factor of three at 500 °C). Using impedance spectroscopy, the grain boundary resistivity was probed as a function of film thickness. The resistivity of the grain boundaries near the film-substrate interface and film surface (within 4 nm of each) was almost entirely eliminated. This minimization of grain boundary resistivity is attributed to Mg(2+) diffusion from the MgO substrate into the YSZ grain boundaries, which is supported by time of flight secondary ion mass spectroscopy measurements. We suggest grain boundary "design" as an attractive method to obtain highly conductive solid electrolyte thin films

Imaging of Lipids in Native Human Bone Sections Using TOF–Secondary Ion Mass Spectrometry, Atmospheric Pressure Scanning Microprobe Matrix-Assisted Laser Desorption/Ionization Orbitrap Mass Spectrometry, and Orbitrap–Secondary Ion Mass Spectrometry

A method is described for high-resolution label-free molecular imaging of human bone tissue. To preserve the lipid content and the heterogeneous structure of osseous tissue, 4 μm thick human bone sections were prepared via cryoembedding and tape-assisted cryosectioning, circumventing the application of organic solvents and a decalcification step. A protocol for comparative mass spectrometry imaging (MSI) on the same section was established for initial analysis with time-of-flight secondary ion mass spectrometry (TOF-SIMS) at a lateral resolution of 10 μm to <500 nm, followed by atmospheric pressure scanning microprobe matrix-assisted laser desorption/ionization (AP-SMALDI) Orbitrap MSI at a lateral resolution of 10 μm. This procedure ultimately enabled MSI of lipids, providing the lateral localization of major lipid classes such as glycero-, glycerophospho-, and sphingolipids. Additionally, the applicability of the recently emerged Orbitrap–TOF-SIMS hybrid system was exemplarily examined and compared to the before-mentioned MSI methods

Synthesis and Physicochemical Characterization of Ce1-xGdxO2-δ: A Case Study on the Impact of the Oxygen Storage Capacity on the HCl Oxidation Reaction

This study reports the synthesis of high-surface-area Ce1−xGdxO2−δ (CGO) fibers that are used as catalysts for the oxidation of HCl. Special emphasis is put on the role of the oxygen storage capacity (OSC) of the CGO fibers on the catalytic performance. An in-depth physicochemical characterization of high-surface-area CGO was achieved by employing a multitude of dedicated spectroscopic techniques. The increasing OSC with Gd content is traced to the development of a space charge region with increased electron concentration as a result of the nano size of the CGO particles. The activity of CGO in the HCl oxidation reaction is shown to decrease with Gd concentration

YSZ thin films with minimized grain boundary resistivity

In recent years, interface engineering of solid electrolytes has been explored to increase their ionic conductivity and improve the performance of solid oxide fuel cells and other electrochemical power sources. It has been observed that the ionic conductivity of epitaxially grown thin films of some electrolytes is dramatically enhanced, which is often attributed to effects (e.g. strain-induced mobility changes) at the heterophase boundary with the substrate. Still largely unexplored is the possibility of manipulation of grain boundary resistivity in polycrystalline solid electrolyte films, clearly a limiting factor in their ionic conductivity. Here we report that the ionic conductivity of yttria stabilized zirconia thin films with nano-columnar grains grown on a MgO substrate nearly reaches that of the corresponding single crystal when the thickness of the films becomes less than roughly 8 nm (smaller by a factor of three at 500 °C). Using impedance spectroscopy, the grain boundary resistivity was probed as a function of film thickness. The resistivity of the grain boundaries near the film-substrate interface and film surface (within 4 nm of each) was almost entirely eliminated. This minimization of grain boundary resistivity is attributed to Mg(2+) diffusion from the MgO substrate into the YSZ grain boundaries, which is supported by time of flight secondary ion mass spectroscopy measurements. We suggest grain boundary "design" as an attractive method to obtain highly conductive solid electrolyte thin films