Chemical Substitution-Induced and Competitive Formation of 6H and 3C Perovskite Structures in Ba_3–<i>x</i>Sr_<i>x</i>ZnSb₂O₉: The Coexistence of Two Perovskites in 0.3 ≤ <i>x</i> ≤ 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 <i>x</i> ≤ 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–Ba_2.8Sr_0.2ZnSb₂O₉ and rock-salt ordered 3C–Ba₂SrZnSb₂O₉ was observed (0.3 ≤ <i>x</i> ≤ 1.0). In the final, the successive symmetry descending was established from cubic (<i>Fm</i>3̅<i>m</i>, 1.3 ≤ <i>x</i> ≤ 1.8) to tetragonal (<i>I</i>4/<i>m</i>, 2.0 ≤ <i>x</i> ≤ 2.4), and finally to monoclinic (<i>I</i>2/<i>m</i>, 2.6 ≤ <i>x</i> ≤ 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

Y_1–<i>x</i>Sc_<i>x</i>BaZn₃GaO₇ (0 ≤ <i>x</i> ≤ 1): Structure Evolution by Sc-Doping and the First Example of Photocatalytic Water Reduction in “114” Oxides

“114” oxides have shown intriguing physical properties while their performance in photocatalysis has not yet been reported probably due to the instability in aqueous solution. YBaZn₃GaO₇ is an exception, which is stable and indeed shows observable photocatalytic H₂ evolution (∼2 μmol/h/g) in methanol aqueous solution under UV light. This activity was enhanced to 23.6 μmol/h/g by a full replacement of Y³⁺ by Sc³⁺. Optical absorption spectra and theoretical calculations show no significant difference upon Sc³⁺-doping. Instead, a systematic analysis of the structure evolution by Rietveld refinements for Y_1–<i>x</i>Sc_<i>x</i>BaZn₃GaO₇ (0 ≤ <i>x</i> ≤ 1) suggests that the increase of the catalytic activity is likely due to the decrease of the structural defects and thus the lower level of recombination rate of e^– and h⁺. In detail, Sc³⁺ substitution leads to a shrinkage of YO₆ octahedra, and successively the adjustment of the Zn²⁺/Ga³⁺ occupancy behaviors in tetrahedra sites. The photocatalytic H₂ evolution rate was further optimized to 118.2 μmol/h/g in methanol solution and 42.9 μmol/h/g in pure water for 1 wt % Pt-loaded ScBaZn₃GaO₇. Here, the relatively less investigated nonmagnetic “114” oxides were, for the first time, proved to be good candidates for photocatalytic water reduction

ZnGa_2–<i>x</i>In_<i>x</i>S₄ (0 ≤ <i>x</i> ≤ 0.4) and Zn_{1–2<i>y</i>}(CuGa)_<i>y</i>Ga_1.7In_0.3S₄ (0.1 ≤ <i>y</i> ≤ 0.2): Optimize Visible Light Photocatalytic H₂ Evolution by Fine Modulation of Band Structures

Band structure engineering is an efficient technique to develop desired semiconductor photocatalysts, which was usually carried out through isovalent or aliovalent ionic substitutions. Starting from a UV-activated catalyst ZnGa₂S₄, we successfully exploited good visible light photocatalysts for H₂ evolution by In³⁺-to-Ga³⁺ and (Cu⁺/Ga³⁺)-to-Zn²⁺ substitutions. First, the bandgap of ZnGa_2–<i>x</i>­In_<i>x</i>S₄ (0 ≤ <i>x</i> ≤ 0.4) decreased from 3.36 to 3.04 eV by lowering the conduction band position. Second, Zn_{1–2<i>y</i>}(CuGa)_<i>y</i>­Ga_1.7In_0.3S₄ (<i>y</i> = 0.1, 0.15, 0.2) provided a further and significant red-shift of the photon absorption to ∼500 nm by raising the valence band maximum and barely losing the overpotential to water reduction. Zn_0.7Cu_0.15­Ga_1.85In_0.3S₄ possessed the highest H₂ evolution rate under pure visible light irradiation using S^2– and SO₃^2– as sacrificial reagents (386 μmol/h/g for the noble-metal-free sample and 629 μmol/h/g for the one loaded with 0.5 wt % Ru), while the binary hosts ZnGa₂S₄ and ZnIn₂S₄ (synthesized using the same procedure) show 0 and 27.9 μmol/h/g, respectively. The optimal apparent quantum yield reached to 7.9% at 500 nm by tuning the composition to Zn_0.6Cu_0.2­Ga_1.9In_0.3S₄ (loaded with 0.5 wt % Ru)

Strong Lewis Base Ga₄B₂O₉: Ga–O Connectivity Enhanced Basicity and Its Applications in the Strecker Reaction and Catalytic Conversion of <i>n</i>‑Propanol

Heterogeneous solid base catalysis is valuable and promising in chemical industry, however it is insufficiently developed compared to solid acid catalysis due to the lack of satisfied solid base catalysts. To gain the strong basicity, the previous strategy was to basify oxides with alkaline metals to create surficial vacancies or defects, which suffers from the instability under catalytic conditions. Monocomponent basic oxides like MgO are literally stable but deficient in electron-withdrawing ability. Here we prove that a special connectivity of atoms could enhance the Lewis basicity of oxygen in monocomponent solids exemplified by Ga₄B₂O₉. The structure-induced basicity is from the μ₃-O linked exclusively to five-coordinated Ga³⁺. Ga₄B₂O₉ behaved as a durable catalyst with a high yield of 81% in the base-catalyzed synthesis of α-aminonitriles by Strecker reaction. In addition, several monocomponent solid bases were evaluated in the Strecker reaction, and Ga₄B₂O₉ has the largest amount of strong base centers (23.1 μmol/g) and the highest catalytic efficiency. Ga₄B₂O₉ is also applicable in high-temperature solid–gas catalysis, for example, Ga₄B₂O₉ catalyzed efficiently the dehydrogenation of <i>n</i>-propanol, resulting in a high selectivity to propanal (79%). In contrast, the comparison gallium borate, Ga-PKU-1, which is a Brönsted acid, preferred to catalyze the dehydration process to obtain propylene with a selectivity of 94%

Ga₄B₂O₉: An Efficient Borate Photocatalyst for Overall Water Splitting without Cocatalyst

Borates are well-known candidates for optical materials, but their potentials in photocatalysis are rarely studied. Ga³⁺-containing oxides or sulfides are good candidates for photocatalysis applications because the unoccupied 4s orbitals of Ga usually contribute to the bottom of the conducting band. It is therefore anticipated that Ga₄B₂O₉ might be a promising photocatalyst because of its high Ga/B ratio and three-dimensional network. Various synthetic methods, including hydrothermal (HY), sol–gel (SG), and high-temperature solid-state reaction (HTSSR), were employed to prepare crystalline Ga₄B₂O₉. The so-obtained HY-Ga₄B₂O₉ are micrometer single crystals but do not show any UV-light activity unless modified by Pt loading. The problem is the fast recombination of photoexcitons. Interestingly, the samples obtained by SG and HTSSR methods both possess a fine micromorphology composed of well-crystalline nanometer strips. Therefore, the excited e^– and h⁺ can move to the surface easily. Both samples exhibit excellent intrinsic UV-light activities for pure water splitting without the assistance of any cocatalyst (47 and 118 μmol/h/g for H₂ evolution and 22 and 58 μmol/h/g for O₂ evolution, respectively), while there is no detectable activity for P25 (nanoparticles of TiO₂ with a specific surface area of 69 m²/g) under the same conditions

Systematic Study of Cr³⁺ Substitution into Octahedra-Based Microporous Aluminoborates

Single crystals of pure aluminoborate PKU-1 (Al₃B₆O₁₁(OH)₅·<i>n</i>H₂O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the <i>R</i>3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by ²⁷Al NMR results. This distortion of Al3O₆ octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported <i>R</i>3̅ to the presently reported <i>R</i>3. We applied a pretreatment to prepare Al³⁺/Cr³⁺ aqueous solutions; as a consecquence, a very high Cr³⁺-to-Al³⁺ substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr³⁺ substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr³⁺. We also systematically investigated the thermal behaviors of PKU-1:<i>x</i>Cr³⁺ (0 ≤ <i>x</i> ≤ 0.50) by thermogravimetric–differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr³⁺-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ <i>x</i> ≤ 0.20, PKU-1:<i>x</i>Cr³⁺ transferred to the PKU-5:<i>x</i>Cr³⁺ (Al₄B₆O₁₅:<i>x</i>Cr³⁺) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al₄B₂O₉:<i>x</i>Cr³⁺. For <i>x</i> > 0.20 in PKU-1:<i>x</i>Cr³⁺, the heat treatment led to a composite of Cr³⁺-substituted PKU-5 and Cr₂O₃, so the doping upper limit of Cr³⁺ in PKU-5 structure is around 20 atom %

Open-Framework Gallium Borate with Boric and Metaboric Acid Molecules inside Structural Channels Showing Photocatalysis to Water Splitting

An open-framework gallium borate with intrinsic photocatalytic activities to water splitting has been discovered. Small inorganic molecules, H₃BO₃ and H₃B₃O₆, are confined inside structural channels by multiple hydrogen bonds. It is the first example to experimentally show the structural template effect of boric acid in flux synthesis

PKU-3: An HCl-Inclusive Aluminoborate for Strecker Reaction Solved by Combining RED and PXRD

A novel microporous aluminoborate, denoted as PKU-3, was prepared by the boric acid flux method. The structure of PKU-3 was determined by combining the rotation electron diffraction and synchrotron powder X-ray diffraction data with well resolved ordered Cl^– ions in the channel. Composition and crystal structure analysis showed that there are both proton and chlorine ions in the channels. Part of these protons and chlorine ions can be washed away by basic solutions to activate the open pores. The washed PKU-3 can be used as an efficient catalyst in the Strecker reaction with yields higher than 90%