Photocatalytic reduction of levulinic acid using thermally modified niobic acid

After the discovery that commercial niobic acid (H0) is able to reduce the levulinic acid in value added molecules, H0 was thermally treated at 200 °C, 400 °C, and 600 °C, generating the niobium oxides H1, H2 and H3 and the photocatalytic improvement towards reduction was investigated. Although the higher temperatures significantly decreased the specific surface area, it was important to remove surface hydroxyl groups and create the T and TT-Nb2O5 phase mixture in H3 which were responsible for its best performance (36.4% of conversion and almost 99% of selectivity for reduced products). To further improve the H3 photoactivity, an identical synthesis was performed in H2 flow to produce oxygen vacancies in the structure of the new photocatalyst (H3OV). This simple modification method increased ∼7% of products yield, which is the best photocatalytic result obtained for pure niobium oxides so far, and proved that it is possible to significantly increase photocatalytic performance without laborious modifications. The electronic and structural differences between H3 and H3OV were investigated by XRD Rietveld refinement, EPR, HR-TEM, DRS and SAED analyses

Electron paramagnetic resonance signature of point defects in neutron-irradiated hexagonal boron nitride

© 2018 American Physical Society. Hexagonal boron nitride (h-BN) is an attractive van der Waals material for studying fluorescent defects due to its large band gap. In this work, we demonstrate enhanced pink color due to neutron irradiation and perform electron paramagnetic resonance (EPR) measurements. The point defects are tentatively assigned to doubly occupied nitrogen vacancies with (S=1) and a zero-field splitting (D=1.2GHz). These defects are associated with a broad visible optical absorption band and a near-infrared photoluminescence band centered at ∼490 and 820 nm, respectively. The EPR signal intensities are strongly affected by thermal treatments in the temperature range between 600 °C and 800 °C, where also the irradiation-induced pink color is lost. Our results are important for understanding of point defects in h-BN and their deployment for quantum and integrated photonic applications

Initialization and read-out of intrinsic spin defects in a van der Waals crystal at room temperature

Optically addressable spins in wide-bandgap semiconductors are a promising platform for exploring quantum phenomena. While colour centres in three-dimensional crystals such as diamond and silicon carbide were studied in detail, they were not observed experimentally in two-dimensional (2D) materials. Here, we report spin-dependent processes in the 2D material hexagonal boron nitride (hBN). We identify fluorescence lines associated with a particular defect, the negatively charged boron vacancy ([Formula: see text]), showing a triplet (S = 1) ground state and zero-field splitting of ~3.5 GHz. We establish that this centre exhibits optically detected magnetic resonance at room temperature and demonstrate its spin polarization under optical pumping, which leads to optically induced population inversion of the spin ground state-a prerequisite for coherent spin-manipulation schemes. Our results constitute a step forward in establishing 2D hBN as a prime platform for scalable quantum technologies, with potential for spin-based quantum information and sensing applications

Structural and magnetic characterization of colloidal CdMnS

This paper reports on the synthesis (chemical co-precipitation reaction) and characterization (X-ray diffraction, magnetization, and electron paramagnetic resonance) of nanosized Cd1-xMnxS particles with manganese concentration up to x = 0.73. Though the literature reports that nanosized (bulk) CdS can incorporate as much as 30% (50%) of manganese ion within its crystal structure we found manganese segregation at the nanoparticle surface at doping levels as low as 14%. We found that both XRD and magnetization data support the presence of the Mn3O4 phase (observed spin-glass transition around 43 K) at the high manganese doping levels whereas the EPR data strongly suggest preferential incorporation of manganese at the nanoparticle's surface, even at low manganese doping levels. Analyses of the experimental data strongly suggest the preparation of well-defined core/shell (Cd1-xMnxS/Mn3O4) structures at higher levels of manganese doping.CNPQ [503533/2003-3]CNP

Arsenic Antisite-Related Defects in Low-Temperature MBE Grown GaAs

GaAs layers grown by the molecular beam epitaxy (MBE) method at low temperatures (200-degrees-C) and also MBE samples grown at 300-degrees-C highly doped with Be or Si show high concentrations of As antisite-related defects in the optical absorption and in the magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation time is strongly reduced compared with that of EL2+. These differences may be related to their high concentrations, which are of the order of approximately 10(19)-10(20) cm-3. In MBE samples grown at higher temperatures (325-degrees-C, 400-degrees-C) a new As antisite-related defect was detected with a reduced As-75 hyperfine splitting as compared with that of EL2+. This new As antisite-related defect has properties very similar to those of another As antisite-related defect previously detected in horizontal Bridgman n-type GaAs. A comparison of four different As antisite-related defects with similar reduced As-75 hyperfine splittings is presented

Arsenic Antisite-Related Defects in Low-Temperature MBE Grown GaAs

GaAs layers grown by the molecular beam epitaxy (MBE) method at low temperatures (200-degrees-C) and also MBE samples grown at 300-degrees-C highly doped with Be or Si show high concentrations of As antisite-related defects in the optical absorption and in the magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation time is strongly reduced compared with that of EL2+. These differences may be related to their high concentrations, which are of the order of approximately 10(19)-10(20) cm-3. In MBE samples grown at higher temperatures (325-degrees-C, 400-degrees-C) a new As antisite-related defect was detected with a reduced As-75 hyperfine splitting as compared with that of EL2+. This new As antisite-related defect has properties very similar to those of another As antisite-related defect previously detected in horizontal Bridgman n-type GaAs. A comparison of four different As antisite-related defects with similar reduced As-75 hyperfine splittings is presented

Synthesis of [60]fullerene derivatives bearing five-membered heterocyclic wings and an investigation of their photophysical kinetic properties

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The preparation, characterization and photophysical properties of six new stable [6,6]-closed fullerene cycloadducts bearing five-membered heterocycles are described. The modified [60]fullerenes are obtained by a simple and rapid synthesis via a Bingel-type reaction with tetrazole and oxadiazole malonate derivatives. The photophysical kinetics of these new fullerene derivatives in toluene solution under ultraviolet illumination (375 nm, UVA) are studied by electron paramagnetic resonance and free-radical spin-trapping using alpha-phenyl-N-tert-butyl nitrone as a spin-trap. The results are compared with pure [60]fullerene and [6,6]-phenyl C-61 butyric acid methyl ester (C-60-PCBM). It is concluded that for all six new compounds as well as pure [60]fullerene and PCBM both superoxide and singlet oxygen are produced in the first stages of UVA illumination following the type I and II mechanisms, respectively. In all cases singlet oxygen is produced as the primary dominant species; however, the type I mechanism always occurs in parallel with type II. In the end, the superoxide is self-dismuted into hydroxyl radicals, thus yielding PBN-OH center dot spin adducts (g = 2.007 and a(hf) (N-14) = 1.54 mT). The kinetic reaction constants and their efficiencies in the production of reactive oxygen species at 375 nm and per mW of absorbed power are determined. The experimental results are consistent with an autocatalytic reaction model in which the system evolutes under UVA illumination, with superoxide catalyzing the conversion of singlet oxygen into more superoxide. (C) 2010 Elsevier B.V. All rights reserved.2171184190Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Institut de Nanotecnologia (MCT)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

New Material For Low-dose Brachytherapy Seeds: Xe-doped Amorphous Carbon Films With Post-growth Neutron Activated 125i

We report a novel material for use in 125I brachytherapy that consists of amorphous carbon films grown by ion-beam-assisted deposition and doped with Xe (5at%) by implantation. Samples of these films grown on Si substrates were irradiated with neutrons in a TRIGA-I nuclear reactor for the production 125Xe, and latter characterized by gamma spectroscopy. The results indicate that the 124Xe was efficiently converted into 125Xe, the precursor of 125I, and support the activity calculations for a model brachytherapy seed. © 2010 Elsevier Ltd.691118121Abdukayumov, M.N., Abdulakhatov, P., Abdusalyamov, N.N., Tillaev, T.S., Rikhsiev, A.Z., Yakushev, V.P., Optimization of the process for preparing high-purity carrier-free Na125I (2004) Radiochemistry, 46, pp. 287-289Belonoshko, A.B., Ahuja, R., Johansson, B., Molecular dynamics study of melting and fcc-bcc transitions in Xe (2001) Phys. Rev. Lett., 87, p. 165505(2000), CDTN, Manual de Operação do Reator TRIGA IPR-R1 (MO/TRIGA IPR-R1)Crook, J., Fleshner, N., Roberts, C., Pond, G., Long-term urinary sequelae following (125) iodine prostate brachytherapy (2007) J. Urol., 179, pp. 141-145Cynn, H., Yoo, C.S., Baer, B., Iota-Herbei, V., McMahan, A., Nicol, M., Carlson, S., Martensitic fcc-to-hcp transformation observed in xenon at high pressure (2001) Phys. Rev. Lett., 86, p. 4552Federmann, F., Bjorneholm, O., Beutler, A., Moller, T., Resonant X-ray absorption of neon clusters: the influence of cluster size on the character of core excited states (1994) Phys. Rev. Lett., 73, p. 1549Kakar, S., Bjorneholm, O., Weigelt, J., deCastro, A., Troger, L., Frahm, R., Moller, T., Size-dependent K-edge EXAFS study of the structure of free Ar clusters (1997) Phys. Rev. Lett., 78, p. 1675Knop, A., Wassermann, B., Ruhl, E., Site-specific excitation in free krypton clusters (1998) Phys. Rev. Lett., 80, p. 2302Krainyukova, N.V., Belan, V.G., Noble gas and metal clusters in carbon nanopores: from models to applications (2007) Eur. Phys. J. D, 43, pp. 169-172Lacerda, R.G., dos Santos, M.C., Tessler, L.R., Hammer, P., Alvarez, F., Marques, F.C., Pressure-induced physical changes of noble gases implanted in highly stressed amorphous carbon films (2003) Phys. Rev. B, 68, p. 054104Mélinon, P., Masenelli, B., Tournus, F., Perez, A., Playing with carbon and silicon at the nanoscale (2007) Nat. Mater., 6, pp. 479-490Neuville, S., Matthews, A., A perspective on the optimisation of hard carbon and related coatings for engineering applications (2007) Thin Solid Films, 515, pp. 6619-6653Parry, S.J., (2003) Handbook of neutron activation analysis, , Viridian PublishingPopov, C., Kulisch, W., Jelinek, M., Bock, A., Strnad, J., Nanocrystalline diamond/amorphous carbon composite films for applications in tribology, optics and biomedicine (2006) Thin Solid Films, 494, pp. 92-97Pritychenco, B., Herman, M.W., Mughabghab, S.F., Oblozinsky, P., Sonzogni, A.A., (2006), Nuclear reaction and structure databases of the national nuclear data centerRobertson, R., (1999), Method for making iodine-125 loaded substrates for use in radioactive sources. Patent CA2261444-A1Romanov, E.G., (2003), Research Center for Atomic Reactor. Chainsolver: A tool to calculate nuclear transmutations in a neutron fluxShultis, J., Faw, R., (2002) Fundamentals of Nuclear Science and Engineering, , Marcel DekkerSolberg, T., DeMarco, J., Hugo, G., Wallace, R., Dosimetric parameters of three new solid core I-125 brachytherapy sources (2002) J. Appl. Clin. Med. Phys., 3, p. 2Woolsey, J., Miller, N., Theodorescu, D., Permanent interstitial brachytherapy for prostate cancer: a current review (2003) World J. Urol., 21, pp. 209-219Zhu, H., Venkataraman, R., Mueller, W., Lamontagne, J., Bronson, F., Morris, K., Berlizov, A., X-ray true coincidence summing correction in Genie 2000 (2009) Appl. Radiat. Isot., 67, pp. 696-70

Selective methane photooxidation into methanol under mild conditions promoted by highly dispersed Cu atoms on crystalline carbon nitrides

Here we report a photocatalytic system based on crystalline carbon nitrides (PHI) and highly dispersed transition metals (Fe, Co and Cu) for controlled methane photooxidation to methanol under mild conditions. The Cu-PHI catalyst showed a remarkable methanol production (2900 µmol.g-1) in 4 hours, with a turnover number of 51 moles of oxygenated liquid products per moles of Cu. Up to date, this result is the highest value for methane oxidation under mild conditions (1 bar, 25 °C)