Eu³⁺-Doped ZnB₂O₄ (B = Al³⁺, Ga³⁺) Nanospinels: An Efficient Red Phosphor

This paper describes the synthesis of Eu­(III)-doped ZnB₂O₄ (B = Al­(III) or Ga­(III)) nanospinels with Eu­(III) concentrations varying between 1% and 15.6%. The synthesis was achieved through a microwave (MW) synthetic methodology producing 3 nm particles by the thermal decomposition of zinc undecylenate (UND) and a metal 2,4-pentanedionate (B­(acac)₃, B = Al³⁺ or Ga³⁺) in oleylamine (OAm). The nanospinels were then ligand exchanged with the β-diketonate, 2-thenoyltrifluoroacetone (tta). Using tta as a ligand on the surface of the particles resulted in soluble materials that could be embedded in lens mimics, such as poly­(methyl methacrylate) (PMMA). Through a Dexter energy transfer mechanism, tta efficiently sensitized the Eu­(III) doped within the nanospinels, resulting in red phosphors with intrinsic quantum efficiencies (QEs) and QEs in PMMA as high as 50% when excited in the UV. Optical measurements on the out of batch and tta-passivated nanospinels were done to obtain absorption, emission, and lifetime data. The structural properties of the nanospinels were evaluated by ICP-MS, pXRD, TEM, FT-IR, EXAFS, and XANES

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO₂) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/SiO₂ was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of Cr^VI first to Cr^IV, then to Cr^II. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization

Ligand-Mediated Modification of the Electronic Structure of CdSe Quantum Dots

X-ray absorption spectroscopy and ab initio modeling of the experimental spectra have been used to investigate the effects of surface passivation on the unoccupied electronic states of CdSe quantum dots (QDs). Significant differences are observed in the unoccupied electronic structure of the CdSe QDs, which are shown to arise from variations in specific ligand-surface bonding interactions

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO₂) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/SiO₂ was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of Cr^VI first to Cr^IV, then to Cr^II. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization