Zhiang Li

Synthesis of hexagonal phase Gd2O2CO3:Yb3+, Er3+ upconversion nanoparticles via SiO2 coating and Nd3+ doping

By SiO2 shell coating and Nd3+ doping, layer-structured hexagonal phase Gd2O2CO3:Yb3+, Er3+ nanoparticles were synthesized successfully through a homogeneous precipitation method. The detailed mechanism was investigated and the results indicate that the SiO2 shell and Nd3+ doping ions can effectively prevent hexagonal phase Gd2O2CO3 from decomposing into cubic phase Gd2O3 during heat treatment, because the SiO2 shell limits the diffusion of CO2 gas and the Nd3+ doping increases the decomposition temperature of Gd2O2CO3. Compared with non-layer-structured GdVO4:20%Yb3+, 2%Er3+, 4%Nd3+ particles with similar diameters and morphologies and lower phonon energies, layer-structured Gd2O2CO3:20%Yb3+, 2%Er3+, 4%Nd3+/SiO2 hexagonal phase particles show much a stronger upconversion emission, suggesting that layer-structured materials are more appropriate as upconversion hosts. Furthermore, the paramagnetic properties of Gd2O2CO3:Yb3+, Er3+, Nd3+/SiO2 nanoparticles were also demonstrated.

Hydrothermal synthesis and formation mechanism of Aurivillius Bi5Fe0.9Co0.1Ti3O15 nanosheets

As potential single-phase multiferroic materials, Aurivillius compounds have attracted much interest in recent years. In this paper, Bi5Fe0.9Co0.1Ti3O15 (BFCTO) nanosheets were first synthesized by a hydrothermal method; lateral length and thickness of them are ∼600 nm and ∼100 nm, respectively. The BFCTO nanosheets were assumed to form by a co-effect of the aggregate process, the oriented attachment of neighboring nanorods, and Ostwald ripening. Besides, an obvious hysteresis loop with 2Mr = ∼0.55 emu g−1 and 2Hc = ∼2600 Oe at 300 K was observed in the final products. Both dielectric permittivity and dielectric loss decrease with the increase of the frequency at room temperature. Our results could serve as guidance to realize a controllable synthesis of Aurivillius nanoparticles and will shed light on designing new nanodevices.

Morphology control of layered Bi11Fe2.8Co0.2Ti6O33 microcrystals: critical role of NaOH concentration and citric acid

In this article, morphology control of Aurivillius Bi11Fe3Ti6O33 (4.5-BFCTO) microcrystals was investigated in detail in the hydrothermal process, where NaOH concentration and citric acid play a critical role. When NaOH concentrations are 0.3–0.5 M, 0.6 M, and 0.7–1.5 M4.5-BFCTO microflowers, microsheets and truncated tetragonal bipyramid particles formed, respectively. For the 0.3–0.5 M-sample, (Bi2O2)2CO3(OH)2 intermediate product, rather than (Na/Bi)7FexCoyTi5−x−yO21−δ of 0.6 M and 0.7–1.5 M samples, acts as a template and is responsible for the morphology change due to the competition between OH− and CO32− ions originating from the decomposition of citric acid. For the 0.6 M and 0.7–1.5 M samples, the morphology was controlled by the Gibbs free energy along the [001] direction. Besides, the dependence of magnetic properties on the morphologies was also investigated. This research on the morphology control could serve as guidance to potentially synthesize larger BFCTO single crystals and shed light on further designing nanodevices.

Extended Near-Infrared Photoactivity of Bi6Fe1.9Co0.1Ti3O18 by Upconversion Nanoparticles

Bi6Fe1.9Co0.1Ti3O18 (BFCTO)/NaGdF4:Yb3+, Er3+ (NGF) nanohybrids were successively synthesized by the hydrothermal process followed by anassembly method, and BFCTO-1.0/NGF nanosheets, BFCTO-1.5/NGF nanoplates and BFCTO-2.0/NGF truncated tetragonal bipyramids were obtained when 1.0, 1.5 and 2.0 M NaOH were adopted, respectively. Under the irradiation of 980 nm light, all the BFCTO samples exhibited no activity in degrading Rhodamine B (RhB). In contrast, with the loading of NGF upconversion nanoparticles, all the BFCTO/NGF samples exhibited extended near-infrared photoactivity, with BFCTO-1.5/NGF showing the best photocatalytic activity, which could be attributed to the effect of {001} and {117} crystal facets with the optimal ratio. In addition, the ferromagnetic properties of the BFCTO/NGF samples indicated their potential as novel, recyclable and efficient near-infrared (NIR) light-driven photocatalysts.

Anisotropic electrical and magnetic properties in grain-oriented Bi4Ti3O12–La0.5Sr0.5MnO3

Aurivillius compounds have many fascinating properties such as ferroelectricity, magnetism, dielectricity, and piezoelectricity. Their structures and properties can be tuned flexibly, thus they have broad applications in FeRAM, spintronics, photocatalysts, capacitors, etc. We synthesized layer-inserted Aurivillius phase semiconducting Bi4Ti3O12–La0.5Sr0.5MnO3 (BIT–LSMO) nanoparticles by the hydrothermal method and subsequently obtained highly grain-oriented ceramics (Lotgering factor LF = 98.69% for muffle calcined samples and 99.87% for hot-pressed samples) by calcination. Significant electrical anisotropy at room temperature (the resistivity magnitude of the out-of-plane direction is about an order larger than that of the in-plane direction) and magnetic anisotropy at low temperature in the oriented ceramics were observed. Through the grain boundary conductivity estimation with the “brick layer” mode, we concluded that the anisotropy originates from the anisotropy within grain interiors for the Aurivillius layered structure rather than the contribution of grain boundary density difference. The transport path is “blocked” to some extent along the c-direction since hole hopping through the Mn4+–O2−–Mn3+ double exchange effect is the main conducting mechanism in our samples. The oxygen vacancies and element valence states of samples using different synthesis processes were investigated. The oxygen vacancies increase and the lattice shrinks during the sintering process due to the volatilization of bismuth element. The valence state of Ti is less than but near to +4, and the valence state of Mn is about +3.4. The electrical and magnetic anisotropies in BIT–LSMO provide an additional freedom in functional applications for layered complex oxides.