1,721,282 research outputs found
Photoactivity under visible light of defective ZnO investigated by EPR spectroscopy and photoluminescence
No full textThe photochemical and photophysical properties of a defective zinc oxide prepared by precipitation have been investigated using mainly electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. As already reported in the literature the band gap of the oxide contains intra band gap states related to the presence of point defects (mainly cation vacancies) in the structure. The concentration and the energy levels of such defects, a fraction of them being paramagnetic, are the basis of a mechanisms of double and multiple excitations allowing visible photons with energy lower than the band gap value (hν Eg. Remarkably, some of the paramagnetic defects present in the bulk of the as prepared materials derive from excitation of VB electrons induced by ambient light illumination.
The electron excitation from VB to CB has been observed upon irradiation under vacuum at 77 K with polychromatic light and applying four different filters to the emitted radiation with cut-off in the range between 400 nm and 495 nm. The process has been monitored by EPR as the photoexcitation ends up in the formation of trapped electron centers (signal at g = 1.96) and trapped hole centers (signal in the range 2.021 > g > 2.003) both clearly detected by this technique. A scheme of the energy state distribution in the band gap of the defective oxide, obtained coupling EPR and photoluminescence results, is proposed. Finally, a non-negligible fraction of the visible light generated carriers reach the surface of the nanocrystals, entailing the typical redox reactions of photocatalysis as indicated by the detection of hydroxyl radicals in solution performed by spin-trapping. For this reason, the wet chemistry prepared zinc oxide can be considered a visible light active (VLA) system
Avogadro Colloquia in Rome on “Vision and Opportunities of a Sustainable Hydrogen Society”
No full textThe fifth edition of Avogadro Colloquia entitled "From Water to Chemicals: Vision and Opportunities of a Sustainable Hydrogen Society", jointly organized by the Department of Chemical Sciences and Materials Technologies of the National Research Council (DSCTM CNR) and the Italian Chemical Society (SCI), was held at the CNR headquarters in Rome on December 15th-16th, 2022. This relevant periodic conference of SCI also was the first event for the celebrations of the Centenary of the Italian CNR. This edition of the Colloquia was focused on the technologies for hydrogen production and use in the so-called green transition towards a society with reduced CO2 emissions. Such technologies well fit the objectives recommended by the United Nations and by Mission EU2030 for the mitigation of the climate change. Greeting addresses to this event were given by the CNR president Maria Chiara Carrozza, the president of SCI Gaetano Guerra, the elected president of the European Chemical Society (EuChemS) Angela Agostiano, the IUPAC president Javier Garc & iacute;a Mart & iacute;nez and the director of DSCTM CNR Lidia Armelao. All the representatives of the national and international chemical societies underlined the key role of chemical sciences and innovative materials in the green transition
The Existence of Nitrate Radicals in Irradiated TiO2 Aqueous Suspensions in the Presence of Nitrate Ions
No full textEvidence of the existence of nitrate radical in irradiated aqueous TiO2suspensions in the presence of nitrate ions are reported for the first time. The joint use of UV/Vis and EPR spectroscopy showed that nitrate radicals are formed by hole induced oxidation of nitrate ions. Photocatalytic degradation of a model alkene compound allowed to highlight the presence of an intermediate organic nitrate deriving from nitrate radical attack to the double bond of the substrate. These results not only allow deeper understanding of photocatalytic processes, but open the route to new green photocatalytic syntheses initiated by nitrate radicals and to new insights in the field of atmospheric chemistry
Reversible adsorption of oxygen as superoxide ion on cerium doped zirconium titanate
Full text linkZirconium titanate (ZrTiO4) modified via cerium ions insertion in the oxide lattice has been successfully prepared via Sol-Gel synthesis forming solid solutions in the molar range between 0 and 10%. Cerium ions are hosted both as Ce4+ and Ce3+ ions whose ratio depends on the treatment undergone by the solid, with trivalent cerium always abundantly present even in strong oxidative conditions. Surface exposed Ce3+ ions are capable to adsorb O2 in the range of temperature between 273 K and 77 K, as superoxide anion (O2[rad]−), having side-on structure and characterized by complete surface-to-molecule electron transfer (about 98% of spin density on O2 molecule). Surprisingly such abundant adsorption is pressure dependent and fully reversible opening the perspective of applications of this system in the field of oxygen separation from gas mixtures
Photoactivity properties of ZnO doped with cerium ions: an EPR study
Full text linkIn the present study, we investigated the effect of cerium doping in zinc oxide matrix, used as photoactive material. Cerium ions into the matrix of ZnO can act like a 'trap' for the electrons, lowering the e−/h+ recombination rate and so increasing the photocatalytic efficiency of the ZnO.
We synthesised doped samples using a simple precipitation route. The amount of dopant used was, 1 and 10% molar. The samples have been studied via x-ray diffraction measurements for the structural characterisation; UV–visible diffuse reflectance was used for the optical analysis; Branauer–Emmett–Teller (BET) model for the measurement of the surface area. Finally, the samples have been analysed via electron paramagnetic resonance (EPR) spectroscopy for the electronic characterisation and for testing their photoactivity. The spin trapping technique was also use to measure the amount of stable radical adducts formed via reaction of OH• radicals with molecules of the DMPO (5,5-dimethyl-1-pyrroline-N-oxide) spin probe
Structural, electronic and photochemical properties of cerium-doped zirconium titanate
No full textMixed solid system involving cerium and zirconium titanate (ZrTiO4) have been prepared using the sol-gel technique. Both X-ray diffraction and DFT calculations firmly indicate that, till a doping level of 10 mol%, cerium ions are dissolved in the titanate matrix (which has the scrutynite structure, analogous to those of the main TiO2 polymorphs) occupying the cationic sites and progressively altering its cell parameters. Cerium is hosted in the matrix both in the form of Ce4+ and Ce3+ ions (XPS results). The trivalent state seems to be favoured even though the state of the dopant depends on the treatment undergone by the material. DFT calculations describe the intra-band gap states formed in both cases and the strong localisation of the single electron in the case of Ce3+ (4f1). Differently from the case of Ce doped ZrO2, that shows photoactivity in the visible light because of the presence of cerium, the doped titanate is inactive in the same conditions. Under UV–vis illumination charge separation occurs (EPR results) and the low-loading doped systems (0.5%, 1%) form OH radicals, detected by spin trapping, more efficiently than the pristine matrix. The absence of photoactivity in the visible range is interpreted in terms of the detrimental role (charge recombination) played by both the occupied intra-band gap states associated to Ce3+ and the corresponding oxygen vacancies formed in the lattice by charge compensation
A HYSCORE investigation of bimetallic titanium–vanadium microporous catalysts: elucidating the nature of the active sites
Full text linkVanadium and titanium bimetallic AlPO-5 molecular sieves have been synthesized and characterized by means of Electron Spin Echo detected EPR and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy. Direct evidence for framework substitution of redox-active Ti ions and VO2+ units at Al sites is provided through the detection of large P-31 hyperfine couplings
Reversible insulator-to-metal transition in p+-type mesoporous silicon induced by the adsorption of ammonia
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Zinc oxide hollow spheres decorated with cerium dioxide. The role of morphology in the photoactivity of semiconducting oxides
Full text linkThe photochemical activity of the recently proposed CeO2-ZnO photocatalytic material active under visible light has been improved by means of significant modifications of its morphology. A polymeric templating agent (Pluronic) has been used in the synthesis obtaining a particle morphology based on hollow spheres that is better defined in the case of high template concentration. The charge separation ability and the light-induced surface electron transfer under irradiation with visible polychromatic light in various ranges of wavelengths has been investigated by electron paramagnetic resonance. The reactivity of the photogenerated holes has been monitored by the spin trapping technique in the presence of DMPO. The hollow spheres morphology achieved through the synthesis here reported leads to systems with a higher photoactivity under visible irradiation than the same system displaying the classic platelets morphology. A parallel increase of the photocatalytic activity of this novel system in pollution remediation reactions is therefore predictable
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