Low temperature defect chemistry of oxides

Defect chemistry at lower temperatures, e.g. room temperature, is systematically described, at which the oxygen exchange reaction is no longer reversible. Analytical relations, numerical results, case studies as well as technological and scientific relevance are highlighted. It is pointed out that the low temperature defect chemistry also offers a quantitative basis to manipulate charge carrier and defect concentrations and related physical properties of oxide electroceramics.     

Characterization of Nd1-xSrxMnO3±δ SOFC cathode materials

The crystal structure, thermal expansion behavior and electrical conductivity of Nd_1-xSr_xMnO_3±δ (x=0–0·5) perovskite oxides were investigated. The chemical compatibility of the compositions with 40 and 50 mol% Sr with Gd₂O₃ doped CeO₂ (CGO) electrolyte was also studied. An orthorhombic GdFeO₃-type symmetry (space group Pbnm, z=4) was identified for all perovskite oxides, and the lattice parameters were determined. As the level of Sr doping increases, the pseudo-cubic lattice constant decreases, and the thermal expansion coefficient increases. The electrical conductivity can be described by the small polaron hopping conductivity model. The conductivity increases on increasing Sr doping, while the activation energy decreases. The compositions with 40 and 50 mol% Sr show very good thermal expansion and chemical compatibility with CGO electrolyte and can be considered as candidate intermediate temperature solid oxide fuel cell cathode materials.     

Electrical properties of lithium intercalated p-type GaSe

We have studied the intercalation reaction of p-type GaSe with n-butyl lithium in hexane solution through the time dependence of the electrical resistivity, and by Hall effect measurements. The electrical properties as a function of temperature have also been examined. Li_xGaSe resistivity is increasing as x is increased. The large variation of the resistivity is mainly attributed to the variation of the free carrier concentration in the host material. Lithium intercalation causes a compensation of GaSe, and the semiconducting character is conserved.     

The influence of structural disorder on the luminescence of niobates: Scandium niobate (ScNbO4) and magnesium niobate (MgNb2O6)

The luminescence of niobates is strongly influenced by structural disorder. We report a low-temperature luminescence study of ScNbO₄ (and ScTaO₄) in order to explain the absence of efficient luminescence in this compound. Crystallographic disorder seems to be one of the main reasons why the luminescence efficiency is low. On the other hand we report the preparation of MgNb₂O₆ with a much higher efficiency than published before, which proves that earlier preparations yielded imperfect samples.     

Improved bi-functional ORR and OER catalytic activity of reduced graphene oxide supported ZnCo2O4 microsphere

Flower like ZnCo₂O₄ grafted onto the reduced graphene oxide (RGO) sheet, prepared by urea assisted solvothermal method acts as a bi-functional electrocatalyst for oxygen reduction and evaluation reaction in alkaline media. The structure, morphology and oxidation state of the catalysts are systematically studied by X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope and X-ray photoelectron spectroscopy. A nearly 4-electron assisted oxygen reduction is followed by ZnCo₂O₄ with onset potential at 0.81 V (vs. RHE). However, when the ZnCo₂O₄ flowers are embedded onto the RGO sheet, a nearly 4-electron pathway for oxygen reduction is achieved with onset potential at 0.95 V (vs. RHE). The formation of H₂O₂ is significantly reduced in RGO-ZnCo₂O₄ than bare ZnCo₂O₄ as detected at the tip in the scanning electrochemical microscope (SECM) during electrochemical reduction. The oxygen evolution study revealed that the overpotential of 0.30 V (vs. RHE) is required in RGO-ZnCo₂O₄ for overcoming the benchmark current density (10 mA/cm²). The SECM study enables to detect local depletion of oxygen at the SECM tip during the redox transition facilitating the in-situ observation of the initial steps of OER.     

Highly improved toluene gas-sensing performance of mesoporous Ag-anchored cobalt oxides nanowires

Mesoporous cobalt oxides nanowires (Co₃O₄ NWs) were synthesized by the nanocasting method, and then Ag nanoparticles with the different content were anchored on the surface of Co₃O₄ NWs. The experimental results indicate that Ag nanoparticles hardly affect the morphology and microstructure of Co₃O₄ NWs and actually exist on the surface of Co₃O₄ NWs. It is worth mentioning that Ag-loading greatly improves the toluene gas-sensing performance of Ag-anchored Co₃O₄ NWs. The operating temperature of Ag-anchored Co₃O₄ NWs sensors decreases from 210 °C for Co₃O₄ NWs sensor to 190 °C, while the response to 100 ppm toluene gas increases 3-folds. Ag_0.166-Co₃O₄ NWs sensor exhibits the best gas-sensing performance due to the optimal Ag-loading content. Ag nanoparticles not only provide more effective oxygen adsorption sites to reduce resistance in air, but also form metal–semiconductor heterojunctions at the Ag/Co₃O₄ interface to increase resistance in toluene gas. In this way, Ag-loading can further improve the gas-sensing performance of mesoporous Ag-anchored Co₃O₄ NWs sensors to toluene gas.     

Emitting area limitation via scattering control in phosphor film realizing high-luminance laser lighting

One major benefit of laser lighting is the possibility to achieve very high luminance. In phosphor-converted laser lighting systems, a blue (pump) laser can be focused into a very small spot. However, after excitation of the phosphor, the white-light-emitting area usually increases considerably, which reduces the luminance parameter substantially. Herein, we design and investigate a highly scattering YAG:Ce/glass composite film with a porous microstructure. Both the glass/phosphor interfaces and the introduced pores act as scattering centers, which can confine the emission area effectively. The relationship between the spot size and the microstructure (porosity, phosphor-particle size, thickness) is elucidated. Under excitation with blue laser, the composite film shows a uniform white-light emission with high luminous efficacy (230 lm/W) and high saturation threshold (> 40 W/mm²), thus achieving a high luminous exitance of ～1239 lm/mm². With above excellent properties, the designed composite films show great potential for use in high-luminance laser lighting.     

Near-infrared organic light-emitting diodes of pure fluorescence emission using small-molecule boron-dipyrromethene derivative

Near-infrared (NIR) light has various applications, such as in cameras and sensors. Organic light-emitting diodes (OLEDs), which are characterized by their thinness, lightweight, and flexibility, have potential application in various devices requiring light emission in the NIR region. In this study, we focused on the wavelength range of 700–1000 nm, which is called the “first optical window” in NIR light. Furthermore, we investigated the optical properties of a low-molecular-weight boron-dipyrromethene derivative (BODIPY-Ph), which has been reported to possess NIR absorption properties as well as the luminescence properties of BODIPY-Ph-based OLEDs in the NIR region. An acetone solution of BODIPY-Ph showed 52.3% photoluminescence quantum yield and the apparent external quantum efficiency of the OLED in the NIR region was high at 1.87%, which is one of the highest values when using OLEDs that are metal-free and use pure fluorescence emission without using triplet.     

Multiresponsive Emissions in Luminescent Ions Doped Quaternary Piezophotonic Materials for Mechanical-to-Optical Energy Conversion and Sensing Applications

Visualization and noncontact sensing can be achieved based on multimode luminescent materials, which is essential to flexible optoelectronics, information encryption, and infrastructure monitoring. However, the development of optical sensors is primally limited by developing high-performance luminescent functional materials with energy conversion. Here, by codoping transition metal and lanthanide ions into quaternary piezophotonic semiconductors MZnOS (M = Ca, Sr, Ba) microcrystals, the efficient multimode luminescent materials are successfully synthesized. They can simultaneously respond to ultraviolet, near-infrared and stress, and exhibit completely different optical characteristics. Specifically, both composite film and block are prepared by mixing luminescent particles and polymers for mechanical-to-optical energy conversion. The developed composite based on mechanoluminescence can be utilized for visualizing pressure distribution, even E-signature, and anti-counterfeiting systems. In addition, temperature detection is researched based on upconversion emissions. The results suggest that these materials have great potential applications in advanced optical multimode sensors, which is of significance for integrated optoelectronic devices.