Pengfei Jiang

β-RE1–xBixB3O6 (RE = Sm, Eu, Gd, Tb, Dy, Ho, Er, Y): Bi3+ Substitution Induced Formation of Metastable Rare Earth Borates at Ambient Pressure

There emerge great interests in the syntheses of metastable polyborates; however, most are involved with the high-pressure technique. A facile method to synthesize metastable rare earth borates at ambient pressure is eagerly required for the large-scale production and property investigation. Here we demonstrate the critical role of Bi(3+) substitutions in the stabilization of metastable β-REB3O6 (RE = Sm, Eu, Gd, Tb, Dy, Ho, Er, and Y) at ambient pressure, where the Bi(3+)-to-RE(3+) substitutions would efficiently reduce the synthetic temperatures to 735-820 °C, well below the upper limit of thermodynamically stable window (840-980 °C). Partial solid solutions of β-RE1-xBixB3O6 were prepared, and the ranges of the solution were also studied experimentally. The thermal behaviors of β-RE0.8Bi0.2B3O6 were investigated by differential thermal analyses and powder X-ray diffraction, and they were divided into two categories; that is, β-RE0.8Bi0.2B3O6 (RE = Sm, Eu, Gd) transfers to α-RE0.8Bi0.2B3O6 with further increasing the temperature to 950 °C, while β-RE0.8Bi0.2B3O6 (RE = Tb, Dy, Ho, Er, and Y) decomposes into hexagonal REBO3 and B2O3. In particular, the allowed concentration of Bi(3+) in β-Gd1-xBixB3O6 was 0.10 ≤ x ≤ 0.25, and these samples show bright blue emissions under UV excitation, which suggests the high efficiency of light absorption and high potential as phosphors with further doping of other activators.

Ambient-Pressure Stabilization of β-GdB3O6 by Doping with Bi3+ and Color-Tunable Emissions by Co-Doping with Tb3+ and Eu3+: The First Photoluminescence Study of a High-Pressure Polymorph

Rare-earth borates are good candidates for optical applications. To date, however, the high-pressure/high-temperature technique has produced a large number of novel borates with optical properties that have rarely been investigated due to the severe problem of substantial defects. We targeted the high-pressure polymorph of β-GdB3 O6 and synthesized three solid solutions of β-Gd0.75-x Bi0.25 Tbx B3 O6 (0≤x≤0.75), β-Gd0.75-y Bi0.25 Euy B3 O6 (0≤y≤0.75), and β-Gd0.50-z Bi0.25 Tb0.25 Euz B3 O6 (0≤z≤0.05) by using typical solid-state reactions at 820 °C. Here, the function of Bi3+ is to stabilize the high-pressure phase by lowering the synthetic temperature and being the sensitizer to promote the green and red emissions of Tb3+ and Eu3+ . The multiple energy transfer paths were investigated by using lifetime decay experiments and photoluminescent spectra, and both efficiency and mechanism were determined. Eventually, color-tunable and white emissions were achieved by rational doping of Bi3+ , Tb3+ , and Eu3+ into β-GdB3 O6 , that is, the CIE chromaticity coordinate for β-Gd0.44 Bi0.25 Tb0.30 Eu0.01 B3 O6 is (0.318, 0.365) with a correlated color temperature of 6101 K.

Optimizing the performance of photocatalytic H2 generation for ZnNb2O6 synthesized by a two-step hydrothermal method

Semiconductor-based photocatalytic H2 generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNb2O6 is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve Nb2O5 with a highly basic aqueous solution and further react it with Zn2+ to form nanosized ZnNb2O6. The reaction time plays an important role on its morphology and photocatalytic performance in water reduction. The sample synthesized through 7 days of reaction was the optimal one with an appropriate crystallinity and a large specific surface area, however the severe surficial defects prohibited its photocatalytic activity in pure water. The H2 generation at a rate of 23.6(5) μmol h−1 g−1 emerged when 20 vol% methanol was used as the hole-sacrificial agent. Most remarkably, once metal or metal oxide cocatalysts, including Pt, Au, NiO, RuO2, Ag2O, and Pd/PdO, were loaded appropriately, the photocatalytic H2 generation rate ultimately achieved 3200(100) or 680(20) μmol h−1 g−1 with or without using methanol, respectively. Apparent quantum yields (AQYs) at 295 nm were investigated by changing the experimental parameters, and the optimal AQYs are 4.54% and 9.25% in water and methanol solution, respectively. Further post-modifications like bandgap engineering may be performed on this highly efficient nano-ZnNb2O6.

Chemical Substitution-Induced and Competitive Formation of 6H and 3C Perovskite Structures in Ba3–xSrxZnSb2O9: The Coexistence of Two Perovskites in 0.3 ≤ x ≤ 1.0

6H and 3C perovskites are important prototype structures in materials science. We systemically studied the structural evolution induced by the Sr2+-to-Ba2+ substitution to the parent 6H perovskite Ba3ZnSb2O9. The 6H perovskite is only stable in the narrow range of x ≤ 0.2, which attributes to the impressibility of [Sb2O9]. The preference of 90° Sb-O-Sb connection and the strong Sb5+-Sb5+ electrostatic repulsion in [Sb2O9] are competitive factors to stabilize or destabilize the 6H structure when chemical pressure was introduced by Sr2+ incorporation. Therefore, in the following, a wide two-phase region containing 1:2 ordered 6H-Ba2.8Sr0.2ZnSb2O9 and rock-salt ordered 3C-Ba2SrZnSb2O9 was observed (0.3 ≤ x ≤ 1.0). In the final, the successive symmetry descending was established from cubic (Fm3̅m, 1.3 ≤ x ≤ 1.8) to tetragonal (I4/m, 2.0 ≤ x ≤ 2.4), and finally to monoclinic (I2/m, 2.6 ≤ x ≤ 3.0). Here we proved that the electronic configurations of B-site cations, with either empty, partially, or fully filled d-shell, would also affect the structure stabilization, through the orientation preference of the B-O covalent bonding. Our investigation gives a deeper understanding of the factors to the competitive formation of perovskite structures, facilitating the fine manipulation on their physical properties.