41 research outputs found
Perovskite-related Oxynitrides in Photocatalysis
Get PDFOver the last decades photocatalytic water splitting has become of increasing importance for fundamental and applied research, since the direct conversion of sunlight into chemical energy via the production of H-2 has the potential to contribute to the world's energy needs without CO2 generation. One of the unsolved challenges consists of finding a highly efficient photocatalyst that is cheap, environmentally friendly, contains exclusively abundant elements, is (photo)chemically stable and absorbs visible light. Photocatalytic efficiency is closely connected to both structural properties like crystallinity, particle size and surface area and to electronic properties like the band gap and the quantum efficiency. Hence extensive control over a large parameter field is necessary to design a good photocatalyst. A material class where the structure-composition-property relations and the influence of substitution effects are well studied is the perovskite-type family of compounds. The perovskite-related oxynitrides belong to this very flexible compound family where many of the necessary characteristics for a photocatalyst are already given and some of the intrinsic properties like the band gap can be tuned within the same crystal structure by substitution. In this work we present materials' design concepts to improve the photocatalytic efficiency of a perovskite-type catalyst and describe their effects on the photocatalytic activity
An almost perfectly efficient light-activated catalyst for producing hydrogen from water
No full textConversion of LaTiO to LaTiON via Ammonolysis: An ab-initio Investigation
Get PDFPerovskite oxynitrides are, due to their reduced band gap compared to oxides,
promising materials for photocatalytic applications. They are most commonly
synthesized from {110} layered Carpy-Galy (ABO}) perovskites via
thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated
temperature. The conversion of the layered oxide to the non-layered oxynitride
must involve a complex combination of nitrogen incorporation, oxygen removal
and ultimately structural transition by elimination of the interlayer shear
plane. Despite the process being commonly used, little is known about the
microscopic mechanisms and hence factors that could ease the conversion. Here
we aim to derive such insights via density functional theory calculations of
the defect chemistry of the oxide and the oxynitride as well as the oxide's
surface chemistry. Our results point to the crucial role of surface oxygen
vacancies in forming clusters of NH decomposition products and in
incorporating N, most favorably substitutionally at the anion site. N then
spontaneously diffuses away from the surface, more easily parallel to the
surface and in interlayer regions, while diffusion perpendicular to the
interlayer plane is somewhat slower. Once incorporation and diffusion lead to a
local N concentration of about 70% of the stoichiometric oxynitride
composition, the nitridated oxide spontaneously transforms to a
nitrogen-deficient oxynitride
Improved persistent luminescence of CaTiO3:Pr by fluorine substitution and thermochemical treatment
Get PDFFluorine-substituted CaTiO3:Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH3). UV?Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr4+ in the pristine CaTi(O,F)3:Pr phosphor was partially reduced to Pr3+ under NH3 flow leading to an intensity improvement of ca. 450% compared to CaTiO3:Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH3 is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr4+ to Pr3+.Fil: Yoon, Songhak. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Otal, Eugenio Hernan. Eidgenössische Materialprüfungs- und Forschungsanstalt; Suiza. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; ArgentinaFil: Maegli, Alexandra E.. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Karvonen, Lassi. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Matam, Santhosh K.. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Ebbinghaus, Stefan G.. Martin Luther University Halle-Wittenberg. Institute of Chemistry; AlemaniaFil: Walfort, Bernhard. LumiNova AG; SuizaFil: Hagemann, Hans. Universidad de Ginebra; SuizaFil: Pokrant, Simone. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Weidenkaff, Anke. Eidgenössische Materialprüfungs- und Forschungsanstalt; Suiza. University of Stuttgart. Institute for Materials Science; Alemani
Crystal structure and band gap determination of HfO2 thin films
Get PDFValence electron energy loss spectroscopy (VEELS) and high resolution transmission electron microscopy (HRTEM) are performed on three different HfO2 thin films grown on Si (001) by chemical vapor deposition (CVD) or atomic layer deposition (ALD). For each sample the band gap (Eg) is determined by low-loss EELS analysis. The Eg values are then correlated with the crystal structure and the chemical properties of the films obtained by HRTEM images and VEELS line scans, respectively. They are discussed in comparison to both experimental and theoretical results published in literature. The HfO2 ALD film capped with poly-Si exhibits the largest band gap (Eg = 5.9±0.5?eV), as a consequence of its nanocrystallized orthorhombic structure. The large grains with a monoclinic structure formed in the HfO2 ALD film capped with Ge and the carbon contamination induced by the precursors in the HfO2 CVD film capped with Al2O3 are identified to be the main features responsible for lower band gap values (Eg = 5.25±0.5 and 4.3±0.5?eV respectively).Kavli Institute of NanoscienceApplied Science
‐reduced Graphene Oxide Composites for Improved Na‐ and Li‐ion Battery Cathodes
No full textDue to a growing demand for sustainable electrical energy storage alternatives to Li-ion batteries (LIBs), Na-ion batteries (SIBs) are of great interest because of the abundance of Na+. By modifying layered H2V3O8 by preintercalation and composite formation, improved electrochemical properties were obtained in LIBs. In analogy, a scalable soft chemistry synthesis is developed, to chemically presodiate H2V3O8 for the first time, in addition to a composite formation reaction with reduced graphene oxide (rGO). Crystal structure and morphology of all compounds are determined and their electrochemical properties as cathode are evaluated with respect to both Na+ and Li+ intercalation. The combination of preintercalation and composite formation leads to excellent initial capacities of 96 mAh ⋅ g−1 for SIBs and 371 mAh ⋅ g−1 for LIBs (58 % and 48 % higher than unmodified H2V3O8) at a practical current density of 100 mA ⋅ g−1, demonstrating that H2V3O8 is a promising cathode material for SIBs and LIBs.Daniela Söllinger, Thomas Berger, Günther J. Redhammer, Jürgen Schoiber, and Simone Pokran
ChemSusChem / Scaling Up electrodes for photoelectrochemical water splitting : fabrication process and performance of 40 cm2 LaTiO2N photoanodes
No full textA scalable process for fabrication of particle‐based photoanodes is developed. The electrodes are versatilely made of photocatalytically active semiconductor particles, in this case LaTiO2N, and optionally coated with cocatalysts and protecting components, all immobilized on a conducting substrate. The involved fabrication steps are restricted to scalable processes such as electrophoretic deposition, annealing in air, and dip coating. Special care is taken to ensure efficient charge transport in‐between particles and to the substrate by incorporating conducting connectors. By adapting the fabrication steps, the electrode geometrical dimension is increased from the size of a typical lab electrode of 1 to 40 cm2. The quality of the scale‐up process is characterized by comparing the photoanodes in terms of thickness, light‐absorption properties, and morphology. For several compositions, the electrochemical performance of both electrode sizes is assessed by measuring the photocurrents and faradaic efficiencies. The comparison revealed a complex upscaling behavior and showed that the photoelectrode size affects performance already on the 0.1 m scale
