Shaohong Liu

Greatly enhanced Dy3+ emission via efficient energy transfer in gadolinium aluminate garnet (Gd3Al5O12) stabilized with Lu3+

Dy3+-doped and Lu3+-stabilized gadolinium aluminate garnet solid solutions of [(Gd1−xLux)1−yDyy]3Al5O12 (x = 0.1–1.0, y = 0–0.10) have been developed as efficient phosphors for simultaneously strong blue (∼483 nm, the 4F9/2 → 6H15/2 transition of Dy3+) and yellow (∼584 nm, the 4F9/2 → 6H13/2 transition of Dy3+) emissions. The efficient energy transfer from Gd3+ to Dy3+ produces an additional excitation band, being the strongest, at ∼275 nm that corresponds to the 8S7/2 → 6IJ intra-f–f transition of Gd3+. With the energy transfer, significantly stronger Dy3+ emission (roughly two-fold) was obtained through excitation of Gd3+ at 275 nm rather than direct excitation of Dy3+ at 352 nm (6H15/2 → 4I11/2 + 4M15/2 + 6P7/2 transition, the strongest intra-f–f transition of Dy3+). The quenching concentration of Dy3+ was determined to be ∼2.5 at%, and the quenching mechanism was suggested to be dipole–dipole interactions. At the optimal Dy3+ content of 2.5 at%, increasing Lu3+ substitution tends to weaken both the excitation and emission bands owing to the higher electronegativity of Lu3+. Comparative studies showed that the best luminescent [(Gd0.8Lu0.2)0.975Dy0.025]AG phosphor has an integrated emission intensity roughly 2.5 and 4 times those of its (Y0.975Dy0.025)AG and (Lu0.975Dy0.025)AG counterparts, respectively. The effects of processing temperature and Lu3+/Dy3+ contents on phase evolution, crystal structure, particle morphology, PLE/PL properties, and fluorescence lifetime of the phosphor are thoroughly investigated. Owing to its enhanced emission and high theoretical density, the (Gd,Lu)AG:Dy3+ phosphor developed in this work may potentially be used as a new type of photoluminescent and scintillation material.

Fluorine- and Iron-Modified Hierarchical Anatase Microsphere Photocatalyst for Water Cleaning: Facile Wet Chemical Synthesis and Wavelength-Sensitive Photocatalytic Reactivity

High photocatalytic efficiency, easy recovery, and no biological toxicity are three key properties related to the practical application of anatase photocatalyst in water cleaning, but seem to be incompatible. Nanoparticles-constructed hierarchical anatase microspheres with high crystallinity and good dispersion prepared in this study via one-step solution processing at 90 degrees C under atmospheric pressure by using ammonium fluotitanate as the titanium source and urea as the precipitant can reconcile these three requirements. The hierarchical microspheres were found to grow via an aggregative mechanism, and contact recrystallization occurred at high additions of the FeCl(3) electrolyte into the reaction system. Simultaneous incorporation of fluorine and iron into the TiO(2) matrix was confirmed by combined analysis of X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV-vis absorption spectroscopy. Surface structure and morphology changes of the microspheres induced by high-temperature annealing were clearly observed by field-emission scanning electron microscopy, especially for the phase-transformed particles. The original nanoparticles-constructed rough surfaces partially became smooth, resulting in a sharp drop in photocatalytic efficiency. Interestingly, iron loading has detrimental effects on the visible-light photocatalytic activity of both the as-prepared and the postannealed anatase microspheres but greatly enhances the photocatalytic activity of the as-prepared anatase microspheres under UV irradiation. No matter under UV or visible-light irradiation, the fluorine-loaded anatase microspheres and especially the postannealed ones show excellent photocatalytic performance. The underlying mechanism of fluorine and iron loading on the photocatalytic efficacy of the anatase microspheres was discussed in detail. Beyond photocatalytic applications, this kind of material is of great importance to the assembling of photoactive photonic crystal that can control light motion.

Effective lattice stabilization of gadolinium aluminate garnet (GdAG) via Lu3+ doping and development of highly efficient (Gd,Lu)AG:Eu3+ red phosphors

Abstract The metastable garnet lattice of Gd3Al5O12 is stabilized by doping with smaller Lu3+, which then allows an effective incorporation of larger Eu3+ activators. The [(Gd1−xLux)1−yEuy]3Al5O12 (x = 0.1–0.5, y = 0.01–0.09) garnet solid solutions, calcined from their precursors synthesized via carbonate coprecipitation, exhibit strong luminescence at 591 nm (the 5D0 → 7F1 magnetic dipole transition of Eu3+) upon UV excitation into the charge transfer band (CTB) at ∼239 nm, with CIE chromaticity coordinates of x = 0.620 and y = 0.380 (orange-red). The quenching concentration of Eu3+ was estimated at ∼5 at.% (y = 0.05), and the quenching was attributed to exchange interactions. Partial replacement of Gd3+ with Lu3+ up to 50 at.% (x = 0.5) while keeping Eu3+ at the optimal content of 5 at.% does not significantly alter the peak positions of the CTB and 5D0 → 7F1 emission bands but slightly weakens both bands owing to the higher electronegativity of Lu3+. The effects of processing temperature (1000–1500 °C) and Lu/Eu contents on the intensity, quantum efficiency, lifetime and asymmetry factor of luminescence were thoroughly investigated. The [(Gd0.7Lu0.3)0.95Eu0.05]3Al5O12 phosphor processed at 1500 °C exhibits a high internal quantum efficiency of ∼83.2% under 239 nm excitation, which, in combination with the high theoretical density, favors its use as a new type of photoluminescent and scintillation material.

The development of Ce3+-activated (Gd,Lu)3Al5O12 garnet solid solutions as efficient yellow-emitting phosphors

Abstract Ce3+-activated Gd3Al5O12 garnet, effectively stabilized by Lu3+ doping, has been developed for new yellow-emitting phosphors. The powder processing of [(Gd1−xLux)1−yCey]3Al5O12 solid solutions was achieved through precursor synthesis via carbonate precipitation, followed by annealing. The resultant (Gd,Lu)AG:Ce3+ phosphor particles exhibit typical yellow emission at ∼570 nm (5d–4f transition of Ce3+) upon blue-light excitation at ∼457 nm (the 2F5/2–5d transition of Ce3+). The quenching concentration of Ce3+ was determined to be ∼1.0 at% (y = 0.01) and the quenching mechanism was suggested to be driven by exchange interactions. The best luminescent [(Gd0.9Lu0.1)0.99Ce0.01]AG phosphor is comparative to the well-known YAG:Ce3+ in emission intensity but has a substantially red-shifted emission band that is desired for warm-white lighting. The effects of processing temperature (1000–1500 °C) on the spectroscopic properties of the phosphors, especially those of Lu3+/Ce3+, were thoroughly investigated and discussed from the centroid position and crystal field splitting of the Ce3+ 5d energy levels.

Significant improvement of critical current density in MgB2 doped with ferromagnetic Fe3O4 nanoparticles

Ferromagnetic Fe3O4-doped MgB2 bulks were first fabricated in this work by the hot pressing method. It was found that Fe3O4 does not react with Mg or B during the fabrication process. Peak Jc values of the 5 wt% Fe3O4-doped MgB2 are higher than 106 A cm−2 in the temperature range 5–30 K. Especially at 30 K, the peak Jc is 1.02 × 106 A cm−2 for the 5 wt% Fe3O4-doped MgB2, the highest values at 30 K found in the literature, and about seven times that of the 5 wt% SiC-doped MgB2 sample. The drop in Jc with increasing field for the Fe3O4-doped MgB2 is significantly slower than that of the SiC-doped MgB2 at 30 K. These results indicate that the Fe3O4-doped MgB2 is a potential superconductor to be used at temperatures greater than 25 K which is a critical temperature for large-scale practical applications.

Foamed single-crystalline anatase nanocrystals exhibiting enhanced photocatalytic activity

A novel and facile approach based on an aqueous reaction of ammonium fluotitanate with an alkaline substance was developed in this work to prepare foamed single-crystals (about 50 nm) of anatase TiO2 exhibiting excellent photocatalytic performances. It is shown that multicrystalline and rice-shaped anatase nanoparticles (around 100 nm in length, 30 nm in width), which have a foamed microstructure and F-dopants, were initially precipitated as a precursor, which can then be readily transformed by calcination at 500 °C into foamed anatase single crystals (about 50 nm in size) with significant amounts of entrapped nanopores (4–10 nm). A fluorine-assisted dehydration/assembly mechanism was believed to be responsible for the formation of such a peculiarly nano-structured precursor, and recrystallization may account for its transformation into the foamed single crystals. Both the precursor nanoparticles and the resultant single crystals were found to have narrowed bandgaps. Though the precursor has already shown good photocatalytic performance in the decomposition of methyl orange, the calcination derived single crystals exhibited even higher activity due to their better crystallinity, smaller size, single crystalline nature and hollow structure. The alkaline substance used for precipitation was found to influence the microstructure and consequently photoactivity of the anatase nanoparticles. It is also shown that, among all the obtained anatase nanomaterials, the foamed single-crystalline nanocrystals, precipitated with sodium hydroxide and having the highest pore density, exhibited photocatalytic activities far superior to Degussa P25, especially under simulated sunlight irradiation. The results obtained herein may shed light on the photoactivity improvement of anatase TiO2 and investigations of other foamed functional nanostructures.

Silver nanowires with rounded ends: ammonium carbonate-mediated polyol synthesis, shape evolution and growth mechanism

Under atmospheric conditions, silver nanowires were prepared via an (NH4)2CO3-mediated polyol process rather than the normally used metal salt-mediated polyol processes in order to improve the purity. The results revealed that the obtained silver nanowires had rounded ends rather than the conventionally believed {111} crystal planes. A growth mechanism was firstly proposed, in which a dynamic reversible deposition and dissolution process of Ag atoms occurred at the curved ends of the silver nanowires. The proposed mechanism could explain the effects of the titration rate, PVP and (NH4)2CO3 concentrations on the growth the of silver nanowires. Both melting and dissolution of Ag atoms contributed to the arcing of twin boundaries in multiply-twinned particles. The multiply-twinned particles with arcing twin boundaries finally grew into silver nanowires with rounded end shapes and higher aspect ratios. Lattice fringes of the twin structure were observed for the first time at the curved ends of the silver nanowires, and the addition of (NH4)2CO3 was found to facilitate the growth of the silver nanowires.

Preparation of transparent Y2O3 ceramic by slip casting and vacuum sintering

Abstract In the present work transparent Y 2 O 3 ceramics were made by slip casting and vacuum sintering of nanopowders with sodium polyacrylic acid (PAA-Na) as dispersant. The rheological properties of Y 2 O 3 nanopowder slurry were investigated using different amounts of dispersant and solid contents. The microstructures and transmittance of the sintered ceramics were also studied by means of scanning electron microscopy (SEM) and ultra-violet visible spectrometry. The results showed that rheological behaviors of the Y 2 O 3 nanopowder slurry were effectively promoted by sodium polyacrylic acid. Highly dispersive and stable slurries were obtained as the dispersant was added over 1.0 dwb% under the fixed conditions of pH 11 and 45 wt.% solid content. All the slip cast green bodies were sintered into highly dense ceramics after sintering at 1700 °C for 5 h in vacuum, wherein the sample added with 1.1% sodium polyacrylic acid exhibited the highest relative density of 99.36% and transmission of 30% at 800 nm wavelength.

Al2O3/yttrium compound core–shell structure formation with burst nucleation: a process driven by electrostatic attraction and high surface energy

A partial wet chemical route has been developed, in which aluminium oxide (Al2O3) nanoparticles in a Y(NO3)3 solution are induced to form a core–shell-structured yttrium aluminum garnet (YAG) precursor based on a burst nucleation synthesis. By simulating the reaction conditions with urea and ammonium bicarbonate as precipitants, a two-step mechanism involving different dynamic processes is proposed to describe the core–shell structure formation. During the majority of the process, the electrostatic attraction between the opposite surface charges of Al2O3 and nanoparticles of the Y-compound is regarded as the single driving force. However, the high surface energy of Y-compound nanoparticles originating from the burst nucleation process plays a key role in completing the formation of the core–shell structure. The precursor obtained can be transformed into pure YAG nanoparticles, which retain the morphology of the Al2O3 template, and these are of sufficiently high quality for the preparation of transparent ceramics. An understanding of the mechanism makes this a novel method for the synthesis of the nanostructured core–shell binary oxide precursor.

Synthesis and formation mechanisms of morphology-controllable indium-containing precursors and optical properties of the derived In2O3 particles

In2O3 particles with three distinctive morphologies of 1D rods, 2D disks and 3D cubes were converted from their respective precursors synthesized by a facile urea-based homogeneous precipitation method. Two kinds of precursor phases, including In(OH)3 and InOHSO4(H2O)2, were obtained. In the case of high urea concentration, mesocrystalline rod-like In(OH)3 particles were produced by oriented primary particle aggregation induced by the coordination of urea on {012} of the primary particles. In contrast, cube-like In(OH)3 was obtained by Ostwald ripening in low-concentration urea solution. The addition of K2SO4 facilitates the formation of an In–sulfate complex, and InOHSO4(H2O)2 precursor disks about 2 μm in diameter were formed. It is suggested that the adsorption of urea on the active growth sites of the precursor leads to the formation of round disks instead of hexagonal plates. Upon blue light excitation, the three types of In2O3 particles obtained from their respective precursors exhibited a morphology-dependent photoluminescence behavior (disks > cubes > rods), but did not differ greatly in the positions of the PLE and PL bands of the luminescence. The emission (in the range of 500–540 nm) from In2O3 is associated with oxygen vacancies and is highly dependent on the annealing atmosphere.