RESUMO
Control of phase separation of VO43- and rare earth precursors in reverse microemulsions afforded â¼35 nm YVO4 nanoparticles with functionalisable â¼7 ± 3 nm nanopores. Doping by Eu3+ allowed luminescent probing of interfacial crystallisation while xylenol orange absorption showed molecular encapsulation in particle cavities. This provides potential multifunctional systems combining UV-Vis-NIR luminescence and (photo)active molecules for optical sensing.
RESUMO
Terbium-doped YVO4 has been considered a nonluminescent solid since the first classic studies on rare-earth-doped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO4:Tb3+ nanoparticles overcomes the metal-metal charge transfer (MMCT) process which is responsible for the quenching of the Tb3+ luminescence. Tetragonal (Y1-xTbx)VO4 nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb3+ luminescence can be excited by VO43- â Tb3+ energy transfer and by direct population of the 5D4 state in YVO4. We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO4:Tb3+ can be reclassified as a UV-excitable luminescent material.
RESUMO
The synthesis of three coordination polymers of cerium(III) and the ligand pyridine-2,4,6-tricarboxylate (PTC) is reported. Two of the materials crystallise under hydrothermal conditions at 180 °C, with [Ce(PTC)(H2O)2]·1.5H2O, (1), being formed on extended periods of reaction time, 3 days or longer, and Ce(PTC)(H2O)3, (2), crystallising after 1 day. Both phases contain Ce(III) but are prepared using the Ce(IV) salt Ce(SO4)2·4H2O as reagent. Under solvothermal conditions (mixed water-N,N-dimethylformamide (DMF)), the phase [Ce(PTC)(H2O)(DMF)]·H2O (3) is crystallised. The structures of the three materials are resolved by single crystal X-ray diffraction, with the phase purity of the samples determined by powder X-ray diffraction and thermogravimetric analysis. (1) is constructed from helical chains cross-linked by the PTC linkers to give a three-dimensional structure that contains clusters of water molecules in channels that are hydrogen-bonded to each other and to additional waters that are coordinated to cerium. (2) also contains nine-coordinate cerium but these are linked to give a dense framework, in which water is directly coordinated to cerium. (3) contains corner-shared nine-coordinate cerium centres, linked to give a framework in which Ce-coordinated DMF fills space. Upon heating the material (1) in air all water is irreversibly lost to give a poorly crystalline anhydrous phase Ce(PTC), as deduced from X-ray thermodiffractometry and thermogravimetric analysis. The material (1), however, is hydrothermally stable, and is also stable under oxidising conditions, where immersion in 30% H2O2 gives no loss in crystallinity. Oxidation of around 50% of surface Ce to the +4 oxidation state is thus possible, as evidenced by X-ray photoelectron spectroscopy, which is accompanied by a colour change from yellow to orange. Photocatalytic activity of (1) is screened and the material shows effective degradation of methyl orange.
RESUMO
We show that particle size, morphology, nanocrystallinity, surface area, and defect density of (Y,Eu)VO4 structures can be tuned by one-pot colloidal conversion of rare earth hydroxycarbonates in water/ethylene glycol (EG) suspensions. Using small angle X-ray scattering, transmission electron microscopy and dynamic light scattering, we show how volume fractions of EG direct the amorphous to crystalline conversion at 1 atm/95 °C by controlling size and aggregation of hydroxycarbonate precursors. A template effect due to a Kirkendall-type conversion occurs for low EG contents, yielding solids with high densities of oxygen defects, as demonstrated by O2 uptakes in thermogravimetry and X-ray photoelectron spectroscopy profiles. Starting from small and aggregated hydroxycarbonates high-porosity (Y,Eu)VO4 nanoparticles were produced with expanded unit cells and short-range (<100 Å) crystalline ordering. We explored the effects of synthesis on the textural, microstructure, and defects of (Y,Eu)VO4 solids, which were further correlated to the spectroscopic profiles of Eu3+-activated samples. We show that the ratios between Eu3+ 5D0 internal quantum yields and particle diameters can be directly correlated to the particle surface areas, opening new perspectives for theoretical detailing of f-f luminescence in YVO4 solids, and enabling accurate tuning of structure and applicability of colloidal vanadate nanoparticles for sensing and catalysis applications.
RESUMO
1H and 13C NMR spectroscopy is used to investigate the organic constituents of metal complexes, MOFs and coordination compounds synthesised under solvothermal and precipitation conditions. The elucidation of the ligands in paramagnetic compounds bearing Cu2+ (d9), Gd3+ (f7), Eu3+ (f6), Fe3+ (d5), ions after treatment with a cationic exchange resin is possible. We prove the efficiency of two post-synthesis linker modifications on diamagnetic IRMOF-3 Zn2+ (d10) with ethyl isocyanate and benzyl bromide.
RESUMO
Metal-organic frameworks (MOFs) have emerged as an exciting class of porous materials that can be structurally designed by choosing particular components according to desired applications. Despite the wide interest in and many potential applications of MOFs, such as in gas storage, catalysis, sensing and drug delivery, electrical semiconductivity and its control is still rare. The use and fabrication of electronic devices with MOF-based components has not been widely explored, despite significant progress of these components made in recent years. Here we report the synthesis and properties of a new highly crystalline, electrochemically active, cobalt and naphthalene diimide-based MOF that is an efficient electrical semiconductor and has a broad absorption spectrum, from 300 to 2500 nm. Its semiconductivity was determined by direct voltage bias using a four-point device, and it features a wavelength dependant photoconductive-photoresistive dual behaviour, with a very high responsivity of 2.5 × 105 A W-1.