383 research outputs found
Sol-gel preparation of pure and doped TiOâ‚‚ films for the photocatalytic oxidation of ethanol in air
Stable sols of TiO2 were synthesized by a non-aqueous sol-gel process using titanium (IV) isopropoxide as precursor. The microstructure, optical and morphological properties of the films obtained by spin-coating from the sol, and annealed at different temperatures, were investigated using scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy and ellipsometry. The crystalline structure of the films was characterized by X-ray diffraction and their photocatalytic activity was evaluated for the oxidation of ethanol in air. The influence of the calcination temperature, pre-heat treatment and the number of layers was studied. Simultaneous thermo-gravimetric and differential thermal analysis measurements were carried out to ascertain the thermal decomposition behavior of the precursors. In order to obtain a higher photoresponse in the visible region, a series of vanadium-, niobium- and tantalum-doped TiO2 catalysts was synthesized by the same sol-gel method. For V doping two different precursors, a vanadium alkoxide and V2O5, were used. The effect on the crystallization and photocatalytic activity of the doped TiO2 films was investigated. Furthermore, to identify the effective composition of the samples, they were characterized by X-ray photoelectron spectroscopy and the surface area of the powders was measured by N-2 adsorption. The 10 wt.% doped catalysts exhibit high photocatalytic activity under visible light and among them the best performance was obtained for the sample containing Ta as dopant. The crystallite sizes are closely related to the photocatalytic activity
Influence of phase purity on photoluminescence properties of strontium oxynitride phosphor doped with Eu2+ ions
Rare earth doped core-shell particles as phosphor for warm-white light-emitting diodes
Strontium oxynitride phosphor doped with Eu2+ ions : phase purity/optical properties relationship
Site-selective mapping of metastable states using electron-beam induced luminescence microscopy
Metastable states created by electron or hole capture in crystal defects are
widely used in dosimetry and photonic applications. Feldspar, the most abundant
mineral in the Earth crust (>50%), generates metastable states with lifetimes
of millions of years upon exposure to ionizing radiation. Although feldspar is
widely used in dosimetry and geochronometry, the creation of metastable states
and charge transfer across them is poorly understood. Understanding such
phenomena requires next-generation methods based on high-resolution,
site-selective probing of the metastable states. Recent studies using
site-selective techniques such as photoluminescence (PL), and radioluminescence
(RL) at 7 K have revealed that feldspar exhibits two near-infrared (NIR)
emission bands peaking at 880 nm and 955 nm, which are believed to arise from
the principal electron-trapping states.
Here, we map for the first time the electron-trapping states in
potassium-rich feldspar using spectrally-resolved cathodoluminescence
microscopy at a spatial resolution of around 6 to 22 micrometer. Each pixel
probed by a scanning electron microscope provides us a cathodoluminescence
spectrum (SEM-CL) in the range 600-1000 nm, and elemental data from
energy-dispersive x-ray (EDX) spectroscopy. We conclude that the two NIR
emissions are spatially variable and, therefore, originate from different
sites. This conclusion contradicts the existing model that the two emissions
arise from two different excited states of a principal trap. Moreover, we are
able to link the individual NIR emission peaks with the geochemical variations
(K, Na and Fe concentration), and propose a cluster model that explains the
quenching of the NIR emission by Fe4+
Templated growth of textured and luminescent CaS : Eu thin films by a low-temperature solvothermal process
Water and ammonia abundances in S140 with the Odin satellite
We have used the Odin satellite to obtain strip maps of the ground-state
rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and
13CO(5-4) across the PDR, and H218O in the central position. A physi-chemical
inhomogeneous PDR model was used to compute the temperature and abundance
distributions for water, ammonia and CO. A multi-zone escape probability method
then calculated the level populations and intensity distributions. These
results are compared to a homogeneous model computed with an enhanced version
of the RADEX code. H2O, NH3 and 13CO show emission from an extended PDR with a
narrow line width of ~3 kms. Like CO, the water line profile is dominated by
outflow emission, however, mainly in the red wing. The PDR model suggests that
the water emission mainly arises from the surfaces of optically thick, high
density clumps with n(H2)>10^6 cm^-3 and a clump water abundance, with respect
to H2, of 5x10^-8. The mean water abundance in the PDR is 5x10^-9, and between
~2x10^-8 -- 2x10^-7 in the outflow derived from a simple two-level
approximation. Ammonia is also observed in the extended clumpy PDR, likely from
the same high density and warm clumps as water. The average ammonia abundance
is about the same as for water: 4x10^-9 and 8x10^-9 given by the PDR model and
RADEX, respectively. The similarity of water and ammonia PDR emission is also
seen in the almost identical line profiles observed close to the bright rim.
Around the central position, ammonia also shows some outflow emission although
weaker than water in the red wing. Predictions of the H2O(110-101) and
(111-000) antenna temperatures across the PDR are estimated with our PDR model
for the forthcoming observations with the Herschel Space Observatory.Comment: 13 pages, 14 figures, 10 tables. Accepted for publication in
Astronomy & Astrophysics 14 November 200
Physical Characteristics of a Dark Cloud in an Early Stage of Star Formation toward NGC 7538: an Outer Galaxy Infrared Dark Cloud?
In the inner parts of the Galaxy the Infrared Dark Clouds (IRDCs) are
presently believed to be the progenitors of massive stars and star clusters.
Many of them are predominantly devoid of active star formation and for now they
represent the earliest observed stages of massive star formation. Their Outer
Galaxy counterparts, if present, are not easily identified because of a low or
absent mid-IR background. We characterize the ambient conditions in the Outer
Galaxy IRDC candidate G111.80+0.58, a relatively quiescent molecular core
complex in the vicinity of NGC7538. We conduct molecular line observations on a
number of dense cores and analyze the data in terms of excitation temperature,
column and volume density, mass and stability. The temperatures (15-20K) are
higher than expected from only cosmic ray heating, but comparable to those
found in massive cores. Star forming activity could be present in some cores,
as indicated by the presence of warm gas and YSO candidates. The observed
super-thermal line-widths are typical for star forming regions. The velocity
dispersion is consistent with a turbulent energy cascade over the observed size
scales. We do not find a correlation between the gas temperature and the
line-width. The LTE masses we find are much larger than the thermal Jeans mass
and fragmentation is expected. In that case the observed lines represent the
combined emission of multiple unresolved components. We conclude that
G111.80+0.58 is a molecular core complex with bulk properties very similar to
IRDCs in an early, but not pristine, star forming state. The individual cores
are close to virial equilibrium and some contain sufficient material to form
massive stars and star clusters. The ambient conditions suggest that turbulence
is involved in supporting the cores against gravitational collapse.Comment: Accepted for publication in A&A -- 19 pages, 9 figures -- high
resolution available at
http://www.astro.rug.nl/~frieswyk/Data/Research/OGIRDC/index.htm
A new species of Colostethus (Anura, Dendrobatidae) from French Guiana with a redescription of Colostethus beebei (Noble, 1923) from its type locality
A new species of Colostethus, long mistaken for Colostethus beebei, is described from French Guiana. The new species can be distinguished from congeners by absence of median lingual process, first finger longer than second, third finger not distinctly swollen in males, differences in tadpole morphology, coloration and pattern (e.g. absence of dorsolateral stripe), bioacoustics, and reproductive behavior. A complete redescription of Colostethus beebei plus description of its tadpole and call is provided on the basis of recently collected topotypic specimens. The range of C. beebei is restricted to the Kaieteur plateau, Pakaraima Mountains, Guyana
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