RESUMO
In this work, we report an ultrasensitive hydrogen (H2) sensor based on tungsten trioxide (WO3) nanorods decorated with platinum (Pt) nanoparticles. WO3 nanorods were fabricated by dc magnetron sputtering with a glancing angle deposition (GLAD) technique, and decorations of Pt nanoparticles were performed by normal dc sputtering on WO3 nanorods with varying deposition time from 2.5 to 15 s. Crystal structures, morphologies, and chemical information on Pt-decorated WO3 nanorods were characterized by grazing-incident X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and photoelectron spectroscopy, respectively. The effect of the Pt nanoparticles on the H2-sensing performance of WO3 nanorods was investigated over a low concentration range of 150-3000 ppm of H2 at 150-350 °C working temperatures. The results showed that the H2 response greatly increased with increasing Pt-deposition time up to 10 s but then substantially deteriorated as the deposition time increased further. The optimally decorated Pt-WO3 nanorod sensor exhibited an ultrahigh H2 response from 1530 and 214,000 to 150 and 3000 ppm of H2, respectively, at 200 °C. The outstanding gas-sensing properties may be attributed to the excellent dispersion of fine Pt nanoparticles on WO3 nanorods having a very large effective surface area, leading to highly effective spillover of molecular hydrogen through Pt nanoparticles onto the WO3 nanorod surface.
RESUMO
Recent analyses by ion-exchange chromatography (IC) showed that, beside nitrate, the majority of the industrial-grade emulsion explosives, extensively used by most separatists in the southern Thailand insurgency, contained small traces of perchlorate anions. In demand for the faster, reliable, and simple detection methods, the portable detection of nitrate and perchlorate became the great interest for the forensic and field-investigators. This work proposed a unique method to detect the trace amount of perchlorate in seven industrial-grade emulsion explosives under the field tests. We utilized the combination of the portable Raman spectroscope, the developed surfaced-enhanced Raman substrates, and the sample preparation procedures. The portable Raman spectroscope with a laser diode of 785 nm for excitation and a thermoelectric-cooled CCD spectrometer for detection was commercially available. The SERS substrates, with uniformly distributed nanostructured silver nanorods, were fabricated by the DC magnetron sputtering system, based on the oblique-angle deposition technique. The sample preparation procedures were proposed based on (1) pentane extraction technique and (2) combustion technique, prior to being dissolved in the purified water. In comparison to the ion chromatography and the conventional Raman measurements, our proposed methods successfully demonstrated the highly sensitive detectability of the minimal trace amount of perchlorate from five of the explosives with minimal operating time. This work was therefore highly practical to the development for the forensic analyses of the post-blast explosive residues under the field-investigations.
RESUMO
By incorporation of an achromatic three-reflection quarterwave retarder to a spectroscopic ellipsometer and application of appropriate calibration and error correction procedures, it has been possible to characterize real thin-film fluoride optical coatings that are inhomogeneous. The refractive index and its dispersion with wavelengths greater than 300-700 nm as well as the depth profile of voids in the film have been determined for AlF(3), CeF(3), HfF(4), LaF(3), ScF(3) and YF(3) films on vitreous silica substrates.
RESUMO
Studies of dielectric materials by rotating-element spectroscopic ellipsometry (SE) are beset with a number of problems such as (1) low reflectance and hence low signal-to-noise ratio and (2) an almost zero (or 1800) change in the ellipsometric parameter A on reflection from the sample, which leads to significant errors in the measured parameters. These difficulties were overcome (1) by developing suitable procedures for correcting nonlinearity in the detection system and the deleterious effects of ambient light and (2) by incorporating an achromatic quarter-wave compensator in the SE system, respectively. A new rapid method of aligning and calibrating the compensator has also been developed. Test measurements with such an SE system on a vitreous silica sample revealed that the accuracy of measurements of Δ and Ψ are 0.03° and 0.015°, respectively, over the spectral range of 300-700 nm. The SE data were then analyzed by standard procedures with linear regression analysis to determine the optical function n(λ) (i.e., refractive index and its dispersion with wavelength) of vitreous silica and at the same time to characterize the microroughness of the surface layer of the sample. The refractive index of vitreous silica determined by this technique is within ±0.0004 of the best reported values in the literature over the spectral range of 300-700 nm. The SE data obtained on the vitreous silica sample revealed the presence of a 1.0-nm-thick microrough layer on the surface of the sample.
RESUMO
The optical constants of a thin inhomogeneous transparent film of LaF(3) on a transparent vitreous silica substrate have been determined by spectroscopic ellipsometry. To overcome the lack of accuracy in the spectroscopic-ellipsometry measurements of transparent samples, an achromatic compensator was used with a rotating-analyzer ellipsometer. As a result, we were able to determine the depth profile of the film and also determine the refractive index of LaF(3) and its dispersion in the near-UV-visible region to the third decimal place, even though the film thickness was only lambda/4 ( 150 nm).
