Tungsten

About: Tungsten is a research topic. Over the lifetime, 35225 publications have been published within this topic receiving 456213 citations. The topic is also known as: W & element 74.

Oxidation of Water under Visible‐Light Irradiation over Modified BaTaO2N Photocatalysts Promoted by Tungsten Species

Abstract: In recent years, photocatalytic water splitting on illuminated semiconductor powder has attracted considerable attention as a potential way to produce the solar fuel H2 from renewable resources. Water oxidation is a particularly important step in artificial photosynthesis for solar fuel production not only for water splitting but also for CO2 reduction. [2] Because the main spectral component of sunlight is visible light (400< l< 800 nm), the development of heterogeneous water oxidation systems that operate under a wide range of visible light is currently a hot topic in chemistry. To effectively use solar energy, a narrow-gap semiconductor like (oxy)nitrides and oxysulfides is highly desirable. 3] However, narrowing the band gap of a photocatalyst decreases the driving force for redox reactions. This would become a more serious concern in water oxidation than in water reduction in terms of kinetics, because water oxidation involves a complicated four-electron process. It will also become difficult for a semiconductor to meet the thermodynamic requirement for water splitting; that is, the valence band maximum and the conduction band minimum should straddle the water-splitting potential, when the band gap energy decreases. While a photocatalyst having a band gap smaller than 2 eV and the ability to reduce and oxidize water is highly desirable for efficient solar energy conversion, however, such a photocatalyst had not been developed until very recently. We have experimentally demonstrated that the reduction and oxidation of water are both possible using perovskite BaTaO2N and the solid-solution materials with BaZrO3 (0 Zr/Ta 0.1) that have band gaps of 1.7–1.8 eV. This is the first example of producing H2 and O2 from water under visible-light irradiation using a semiconductor having a band gap lower than 2 eV. However, the BaZrO3-BaTaO2N solid solutions have a drawback in that the water oxidation activity is very low (about 0.03% apparent quantum yield, AQY, at 420 nm). The cause of the low activity for water oxidation is most likely due to the valence band potential being close to the water oxidation potential, as suggested by photoelectrochemical measurements. In such a situation, one may think that it is possible to tune the band-edge positions of a semiconductor by replacing the original constituent element with another. Taking BaTaO2N for example, it would be easy to expect that the valence band maximum shifts to more positive potential if the concentration of nitrogen, which forms the upper part of the valence band, is reduced. This is actually achievable by making a solid solution with a wide-gap oxide semiconductor, which is in general known as “band-gap enerineering”. From the viewpoint of efficient solar energy use, however, this may not be preferable because the reduction of the nitrogen concentration in BaTaO2N results in a blue shift of the absorption edge, downgrading the good light absorption capability. Thus, the ordinary strategy has a limitation, and a new way to enhance photocatalytic activity needs to be developed. Here we report that the activity of water oxidation of BaTaO2N photocatalyst can be enhanced by the introduction of pentavalent tungsten species, while maintaining the small band gap. In general, doping transition-metal cations having partly filled d orbitals into semiconductor photocatalysts contributes to a significant drop in photocatalytic activity. Such doped elements form a donor or accepter level in the forbidden band of the material, which may case by case act as a center for absorption at visible wavelengths. However, doping also obstructs prompt migration of photogenerated electrons or holes at the surface and in the material bulk, since the dopant frequently provides a discreet energy level rather than an energy band. Nevertheless, introducing pentavalent W species having a [Xe]4f5d electron configuration into BaTaO2N is exceptionally effective to enhancing the oxidation activity of water. [*] Prof. Dr. K. Maeda Department of Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550 (Japan) and Precursory Research for Embryonic Science and Technology Science and Technology Agency (JST) 4-1-8 Honcho Kawaguchi, Saitama 332-0012 (Japan) E-mail: maedak@chem.titech.ac.jp

Enhancement of dielectric characteristics in donor doped Aurivillius SrBi2Ta2O9 ferroelectric ceramics

Abstract: In this study, investigations have been made on the crystal structure, surface morphology, dielectric and electrical properties of tungsten doped SrBi 2 (W x Ta 1− x ) 2 O 9 (0.0 ≤ x ≤ 0.20) ferroelectric ceramics. Dielectric measurements performed as a function of temperature at 1, 10 and 100 kHz show an increase in Curie temperature ( T c ) over the composition range of x = 0.05–0.20. W 6+ substitution in perovskite-like units results in a sharp dielectric transition at the ferroelectric Curie temperature with the dielectric constant at their respective Curie temperature increasing with tungsten doping. The dielectric loss reduces significantly with tungsten addition. The temperature dependence of ac and dc conductivity vis-a-vis tungsten content shows a decrease in conductivity, which is attributed to the suppression of oxygen vacancies. The activation energy calculated from the Arrhenius plots is found to increase with tungsten content.