Liusai Yang

Preparation of cereal-like Y V O4:Ln3 + (Ln = Sm, Eu, Tb, Dy) for high quantum efficiency photoluminescence

In this work, preparation of cereal-like architectures Y V O(4) and Y V O(4):Ln(3 + ) (Ln = Eu, Sm, Dy, Tb) was initiated using a hydrothermal method. During the formation reaction, Na(3)C(6)H(5)O(7).2H(2)O was used to effectively adjust the concentration of Y(3 + ) species necessary for cereal-like architectures. Phase structure, surface chemistry, morphology, and photoluminescence were characterized by x-ray powder diffraction, Fourier transformed infrared spectra, scanning electron microscopy, transmission electron microscopy, and photoluminescence spectra. All samples crystallize in a tetragonal zircon structure, stably showing a homogeneous cereal-like morphology. This special morphology was constructed by self-assembly of tiny primary particles with a dimension of 31-32 nm. With increasing atomic number of Ln(3 + ), the lattice dimension of the cereal architectures became monotonously enlarged. This cereal-like architecture is proved unique in significantly improving the quantum efficiencies: the internal quantum efficiencies of (5)D(0) for Ln = Eu and (4)F(9/2) for Ln = Dy were 14.6% and 11.4%, respectively, which are all superior over those of the counterparts of nanoparticles reported in the literature. The average lifetime of the (5)D(0) level for Ln = Eu was calculated to be 98 micros, which is longer than that of 50 micros of the (4)F(9/2) level for Ln = Dy. The strong photoluminescence might be the consequence of the effective energy transfer due to the greatly reduced defect centers from this special self-assembly structure.

Solvent-driven room-temperature synthesis of nanoparticles BiPO4:Eu3+.

In this work, a novel solvent-driven room-temperature synthesis of BiPO(4):Eu(3+) nanoparticles was presented. By virtue of 11 solvents with different properties and function groups, phase structure and composition of BiPO(4):Eu(3+) can be systematically tailored. Hexagonal phase (HP) of BiPO(4):Eu(3+) was obtained in water and hydrophobic organic solvents such as arenes and cyclohexane, while low-temperature monoclinic phase (LTMP) was prepared in hydrophilic alcohols. In other solvents (i.e., hydrophilic ethers, aldehydes, ketones, and carboxylic acids), a mixture of HP and LTMP was formed, in which the relative content of LTMP gradually increased following the above solvent sequence. It is also found that particle sizes of BiPO(4):Eu(3+) nanoparticles were closely related to the phase structure: HP exhibited a comparatively larger particle size. The phase evolution processes for both polymorphs with varying solvents were investigated in details. Photoluminescence (PL) properties were sensitive to the phase structure and compositions of the final products. With increasing the phase content of LTMP, the lifetimes and quantum yields both increased. The methodology reported here is fundamentally important, which may give a novel insight into the polymorph-controlled synthesis for further optimized materials performance.

Is BiPO4 a Better Luminescent Host? Case Study on Doping and Annealing Effects

Metal phosphates have been popularly regarded as excellent luminescence hosts of lanthanide ions, while such an issue is challenged by the ignorance about the structural stability that may originate from the different chemical nature between the framework and the dopant lanthanide ions. Here, we choose BiPO(4) as a model compound to study. A detailed investigation of the effects of Eu doping and annealing on the structures and related luminescence properties of Bi(1-x)PO(4):Eu(x) (x = 0-0.199) has been carried out. A monoclinic phase (denoted as LTMP) was obtained for the undoped sample, which gradually transformed to a hexagonal phase (HP) with increasing doping level of Eu(3+) to x = 0.068. Further, it is also found that annealing had an obvious impact on the structures of the resulted samples. With increasing the annealing temperature up to 400 °C a phase transformation from HP to LTMP happens, which is opposite to that with doping. The above phase transformation behaviors were further confirmed by performing structural studies of doping with Dy(3+) ions and annealing undoped BiPO(4). The structural evolution had a great influence on the luminescent properties. Initially a significant decrease in Eu(3+) luminescence intensity and quantum efficiency was observed when LTMP transformed to HP. Afterward, a converse situation, increasingly enhanced luminescence performance, appeared when HP transformed to LTMP. Therefore, whether metal phosphates could be taken as better luminescence hosts must take into account their structural changes caused by the different chemical natures between framework and dopant ions, which may provide an important reference for designing new luminescent materials.

Preparation and polymorph-sensitive luminescence properties of BiPO4:Eu, Part I: room-temperature reaction followed by a heat treatment

In this work, BiPO4:Eu of three different polymorphs was prepared using a room-temperature reaction followed by a heat treatment. The formation mechanism, morphologies, structures, and luminescence properties of these three polymorphs were investigated in detail. It is found that a hexagonal phase (HP) was formed at room temperature, which transformed to a low-temperature monoclinic phase (LTMP) and then a high-temperature monoclinic phase (HTMP) when the treatment temperature was increased to 500 and 700 °C, respectively. With the phase transformation, the morphology changed from homogeneous rod-like shape to nearly spherical-like shape, while the lattice strain features varied from the compressive to tensile, and the symmetry of tetragonal PO43−groups decreased from pseudo-Td to C1. Such a change in the symmetry of PO43−groups also showed an impact on the local environment of the lattice sites for Eu3+ and moreover the luminescence properties. Judd–Ofelt parameters (Ω2) were estimated to understand the asymmetric nature of the dopant Eu3+ in these polymorphs. It is indicated that the Ω2 value for HP was 8.24 × 10−20 cm2, which is slightly increased to 8.42 × 10−20 cm2 for LTMP, and the quantum efficiency increased from 12.76% for HP to 70.77% for LTMP. These results demonstrate that rare-earth doped BiPO4 materials can be tailored to show optimum luminescence properties through kinetic control over the polymorphs.

Size-induced variations in bulk/surface structures and their impact on photoluminescence properties of GdVO4:Eu3+ nanoparticles

This work explores the size-induced lattice modification and its relevance to photoluminescence properties of tetragonal zircon-type GdVO(4):Eu(3+) nanostructures. GdVO(4):Eu(3+) nanoparticles with crystallite sizes ranging from 14.4 to 24.7 nm were synthesized by a hydrothermal method using sodium citrate as a capping agent. Regardless of the reaction temperatures, all samples retained an ellipsoidal-like morphology. Nevertheless, as the crystallite size reduces, there appears a tensile strain and lattice distortion, which is accompanied by a lattice expansion and a decreased symmetry of structural units. These lattice modifications could be associated with the changes in the interior chemical bonding due to the interactions of surface defect dipoles that have imposed an increased negative pressure with crystallite size reduction. Furthermore, crystallite size reduction also led to a significant increase in the amounts of surface hydroxyl groups and citric species, as well as the concentration of the surface Eu(3+) ions. When Eu(3+) was taken as a structural probe, it was found that the asymmetric ratio (I(02)/I(01)) of Eu(3+) gradually declined to show a remarkable decrease in color chromaticity as crystallite size reduces, which could be interpreted as due to the change of local environments of Eu(3+) ions from the interior to the surface of the nanoparticles.

Low-concentration donor-doped LiCoO2 as a high performance cathode material for Li-ion batteries to operate between −10.4 and 45.4 °C

The majority of electrode materials suffer from severe capacity fading on cycling at elevated temperatures or poor conductivity and diffusion of Li+ at low temperatures, which have made it very difficult for lithium-ion batteries to operate at low temperatures and/or elevated temperatures without loss of electrochemical performance. In this work, we report on a new strategy for tackling this issue through low-concentration donor-doping of higher valence Mn ions in LiCoO2, a typical commercial cathode for many lithium-ion batteries. Firstly, low-concentration Mn-doped LiCoO2 was successfully synthesized using a molten-salt method, in which solvent NaOH provides an alkaline environment that makes the reactant mixture uniform in reaction process and ensures the valence state of Mn ion at +4. Secondly, the chemical compositions for all samples were systematically tuned, while retaining the single-phase nature. The electrochemically inert Mn4+ was found to significantly enhance the structure stability, conductivity, and diffusion rate of LiCoO2. As a consequence, the cathode material with a composition of LiCo0.95Mn0.05O2 exhibited an excellent electrochemical performance in a temperature range from −10.4 to 45.4 °C. The finding reported in this work will be conducive to the applications of lithium-ion batteries under different temperature conditions.

Fabrication of assembled-spheres YVO4:(Ln3+, Bi3+) towards optically tunable emission

This work initiated a study on the preparation of assembled-spheres YVO4:(Ln3+, Bi3+) (Ln = Eu, Sm, Dy, Ho), in which Bi3+ co-doping of a lower content, aims to achieve emission tailored for many applications by supressing the serious changes in host structures when higher concentrations of Bi3+ are involved. The formation of assembled-spheres is beneficial from the simultaneous dissolution of the starting Bi and Ln salts with the use of ethylene glycol. As a result, all assembled-spheres are featured by low surface absorbents and little interior lattice defects, which decreased the nonradiative rate to give a luminescence performance superior to other morphologies like cereal-like architectures. Further, four paths were applied to achieve the tunable multicolour-emission in these assembled-spheres, which include: (i) selecting the dopant Ln3+ with characteristic emissions; (ii) varying the concentration of dopant Ln3+; (iii) co-doping Ln3+/Bi3+ with dual emissions from Bi3+ and Ln3+; and (iv) adjusting the excitation wavelength. These tunable multicolour-emissions were demonstrated to be the consequence of the energy transfer process from the VO43−group to Ln3+ in YVO4:Ln3+ or those from Bi3+ and the VO43−group to Ln3+ in the presence of lower Bi3+ content, as followed by a surprising red-shift of the absorption edge and extension of the luminescence lifetimes of Ln3+. The diverse approaches found in this work for emission tailoring are fundamentally important and may provide a general way to achieve multicolour-tunable emission for many applications.

Preparation and morphology-sensitive luminescence properties of Eu3+-doped YVO4: a defect chemistry viewpoint of study

YVO4:Eu3+ of four different morphologies was successfully synthesized by simply varying the pH under hydrothermal conditions. Systematic characterization indicates that by increasing the pH from 3 to 14, the morphology was changed from uniform hollow microspheres, potato-cakes, microhamburgers, to lichee-like architectures in sequence, which is followed by a colour variation from wine, red-purple, pink-purple, to red. Among all the morphologies, the lichee-like shape showed a superior luminescence performance as represented by a high quantum efficiency of 55.5% and long lifetime of 1.15 ms. Such a morphology-sensitive luminescence property was first interpreted in terms of the defect chemistry. The findings reported in this work are fundamentally important, which may give a deep insight into the origin of morphology-dependent properties necessary for exploring new luminescent materials.

Morphology-controllable growth of GdVO4:Eu3+ nano/microstructures for an optimum red luminescence

Chemically tailoring microstructures for an optimum red luminescence is a subject at the forefront of many disciplines, which still remains a challenge due to a poor knowledge about the roles of defects in structures. In this work, GdVO(4) :Eu(3+) nano/microstructures of different morphologies, including tomato-like, cookie-circle-like, and ellipsoidal-like nanoparticles, and microspheroids were synthesized via a simple hydrothermal route using trisodium citrate as a capping agent. During the growth processes, the types of vanadyl ions were adjusted by varying pH value to control the morphologies and nano/microstructures with the help of trisodium citrate. The possible mechanisms for the growth processes into diverse morphologies are presented. Further, a systematic study on defect characteristics pertinent to these diverse morphologies has been explored to achieve an optimum red luminescence. The ability is clearly shown to generate different nano/microstructures of diverse morphologies and varied defect concentrations, which provides a great opportunity for morphological control in tailoring the red luminescence property for many technological applications.

Exploring the unique electrical properties of metastable BiPO4 through switchable phase transitions

Metastable materials usually possess unique properties. How to acquire these properties is still a great challenge. In this work, we explored the electrical properties of metastable BiPO4 through switchable phase transitions. A metastable monoclinic phase (denoted as HTMP) was synthesized by a heat-treatment over a hexagonal phase (HP). It is found that there is a reversible phase transformation between HTMP and a low-temperature monoclinic phase (LTMP). Namely, HTMP gradually transformed to LTMP by a simple hand-grinding or ball milling, while LTMP transformed back to HTMP upon a heat treatment. Accompanying the transformation from HTMP to LTMP, the morphology varied from the cobblestone-like to spherical-like, and particle sizes changed from micrometre scale to several tens of nanometres, as followed by a decrease in the symmetry of tetragonal PO43− groups from Cs to C1. The reversible transformation was understood by taking into account several structural factors like the arrangement of PO4 tetrahedra and BiO8 polyhedra, unit cell volume (V/Z), and the symmetry of PO4. Finally, the electrical properties of the metastable HTMP were successfully acquired through a pellet-pressing technique. The temperature dependence of bulk conductivity indicates that the conductivities deviated from the Arrhenius law, but followed a simple temperature dependence, T−1/4, obeying a Mott variable range hopping conduction mechanism. The corresponding characteristic temperature of the bulk conduction for HTMP is estimated to be 2.3 × 109 K. These findings are fundamentally important, as they enable one to acquire the properties of many other metastable phases, meanwhile they pave a way for solving the controversies presented in metastable systems.