Atomistic Mechanisms for the Nucleation of Aluminum Oxide Nanoparticles

A predictive model for nanoparticle nucleation has not yet been successfully achieved. Classical nucleation theory fails because the atomistic nature of the seed has to be considered. Indeed, geometrical structure as well as stoichiometry do not always match the bulk values. We present a fully microscopic approach based on a first-principle study of aluminum oxide clusters. We calculated stable structures of Al_<i>x</i>O_<i>y</i> and their associated thermodynamic properties. From these data, the chemical composition of a gas composed of aluminum and oxygen atoms can be calculated as a function of temperature, pressure, and aluminum to oxygen ratio. We demonstrate the accuracy of this approach in reproducing experimental results obtained with time-resolved spectroscopy of a laser-induced plasma from an Al₂O₃ target. We thus extended the calculation to lower temperatures, i.e., longer time scales, to propose a scenario of composition gas evolution leading to the first alumina seeds

Estimation of the Electron Thermalization Length in Ionic Materials

We report estimations of the thermalization length and the diffusion coefficient of photogenerated carriers in the insulator LiYF₄ as a function of their initial energy. Combining modeling of electron–phonon interaction and the detailed analysis of the kinetic response of fluorescent center Ce³⁺ under vacuum ultraviolet excitation, the thermalization length is obtained as a function of the initial kinetic energy of the electron. This parameter is essential for the description of the carrier recombination in the case of nonideal plasma conditions, where electrons and holes are strongly correlated. This approach also demonstrates the effect of a complicated structure of electronic band on the thermalization process, which impacts the complex nonproportionality response of materials under ionizing radiation excitation

Impact and Detailed Action of Sulfur in Syngas on Methane Synthesis on Ni/γ-Al₂O₃ Catalyst

Stability and deactivation phenomena are of utmost importance for metal nanocatalysts from both fundamental and industrial points of view. The presence of small amounts of sulfur at ppm and ppb levels in the synthesis gas produced from fossil and renewable sources (e.g., biomass, coal) is a major reason for deactivation of nickel catalysts for carbon monoxide hydrogenation. This paper addresses reaction pathways and deactivation mechanisms of alumina-supported nickel catalysts for methane synthesis from pure syngas and syngas containing small amounts of sulfur. A combination of SSITKA and operando FTIR is indicative of both reversible molecular and irreversible dissociative carbon monoxide adsorption on nickel nanoparticles under the reaction conditions. Methanation reaction involves irreversible carbon monoxide adsorption, dissociation, and hydrogenation on nanoparticle steps and edges. Hydrogenation of adsorbed carbon species leading to methane seems to be the reaction kinetically relevant step. Molecular forms of carbon monoxide reversibly adsorbed on nickel terraces are likely not to be involved in carbon monoxide hydrogenation. The results suggest a competition between sulfur and carbon monoxide for nickel surface sites. During methanation, sulfur preferentially adsorbs on the sites of reversible molecular carbon monoxide adsorption, whereas the low-coordinated nickel sites responsible for carbon monoxide dissociation and hydrogenation are affected to a lesser extent by sulfur poisoning. The active sites of carbon monoxide hydrogenation are poisoned much more rapidly by sulfur, when the catalyst has been exposed to small amounts of H₂S in the absence of methanation

Total Oxidation of Formaldehyde over MnO_<i>x</i>‑CeO₂ Catalysts: The Effect of Acid Treatment

The effect of acid treatment in mixed MnO_<i>x</i>–CeO₂ samples has been investigated in the catalytic total oxidation of formaldehyde. The acid treatment has no effect on the textural and redox properties of the materials when Mn is stabilized in a MnO_<i>x</i>–CeO₂ solid solution (Mn content below 50%). However, these properties were found to be highly altered by acid treatment when the solubility limit of Mn in the ceria was exceeded (Mn content above 50%). This enabled access to the primary porosity and oxidized the manganese species to a higher oxidation state via a Mn dismutation reaction. As a result, the catalytic activity of pure manganese oxide, after chemical activation, in the oxidation of formaldehyde is greatly improvedat 100 °C, the conversion of formaldehyde is increased by a factor of 5 and the corresponding intrinsic reaction rate by 1.4. Combined in situ surface analysis unambiguously identified formate species as a result of formaldehyde oxidation at room temperature on the chemically activated pure MnO_<i>x</i>. The evolution of various surface species was monitored by increasing the temperature and in situ FTIR, and XPS results provided direct evidence of the desorption of monodentate formate species into formaldehyde and the oxidation of bidentate-bridging formate species. Changes in the average oxidation state of surface manganese confirmed the participation of oxygen from MnO_<i>x</i> in the formation of formate species at room temperature and their transformation into CO₂ and H₂O when increasing the temperature

Radioluminescence Sensitization in Scintillators and Phosphors: Trap Engineering and Modeling

The role of charge carrier trapping in determining radioluminescence (RL) efficiency increase during prolonged irradiation of scintillators has been studied by using YPO₄:Ce,Nd as a model material. The Nd³⁺ ions act as efficient electron traps minimizing the role of intrinsic defects. Different Nd contents were considered in order to point out the correlation between the trap concentration and the detected RL efficiency dose dependence. RL measurements as a function of temperature clarified the role of the trap thermal stability in determining the shape and the magnitude of such effect. We propose also a model based on trap filling which is able to describe accurately the complex processes which are involved

The Influence of Oxygen Vacancies on Luminescence Properties of Na₃LuSi₃O₉:Ce³⁺

Oxygen vacancies play an important role in the luminescence processes of inorganic scintillator materials. In order to study the effects of oxygen vacancies on the luminescence properties of Na₃LuSi₃O₉:Ce³⁺, these phosphors were prepared using a high temperature solid-state reaction method under different atmosphere and raw materials. It was found that oxygen vacancy had great influence on the absorption, photoluminescence and decay curves of Na₃LuSi₃O₉:Ce³⁺. The luminescence intensity and peak position showed a regular change when synthesizing atmosphere changed. Na₃LuSi₃O₉:Ce³⁺ with more oxygen vacancies showed much stronger luminescence intensity at high temperature than that without vacancies. And it was also found that the decreasing of oxygen vacancies can quicken the photoluminescence decay of Ce³⁺ in Na₃LuSi₃O₉. The existence of oxygen vacancies in Na₃LuSi₃O₉:Ce³⁺ was confirmed by Zr⁴⁺ doping and thermoluminescence emission spectra. At last, emission bands of Ce³⁺ and oxygen vacancies were well distinguished under X-ray excitation and probable cause of the formation of oxygen vacancies was discussed

A₂B_<i>n</i>–1Pb_<i>n</i>I_3<i>n</i>+1 (A = BA, PEA; B = MA; <i>n</i> = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators

Quantum-well (QW) hybrid organic–inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications

A₂B_<i>n</i>–1Pb_<i>n</i>I_3<i>n</i>+1 (A = BA, PEA; B = MA; <i>n</i> = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators

Quantum-well (QW) hybrid organic–inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications

A₂B_<i>n</i>–1Pb_<i>n</i>I_3<i>n</i>+1 (A = BA, PEA; B = MA; <i>n</i> = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators

Quantum-well (QW) hybrid organic–inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications

A₂B_<i>n</i>–1Pb_<i>n</i>I_3<i>n</i>+1 (A = BA, PEA; B = MA; <i>n</i> = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators

Quantum-well (QW) hybrid organic–inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications