Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2): site-selective doping, band structure engineering and photocatalytic overall water splitting.

Developing new photocatalysts and deciphering the structure-property relationship are always the central topics in photocatalysis. In this study, a new photocatalyst Ba3SnGa10O20 containing two d10 metal cations was prepared by a high temperature solid state reaction, and its crystal structure was investigated by Rietveld refinements of monochromatic X-ray powder diffraction data for the first time. There are 2 Ba, 4 metal cations and 6 O independent atoms in a unit cell. Sn4+ and Ga3+ co-occupy the octahedral cavities named M1 and M2 sites, and the other two metal sites are fully occupied by Ga3+. Rational In3+-to-Ga3+ substitution was performed to reduce the potential of the conduction band minimum and enhance the light absorption ability, which was indeed confirmed using UV-vis diffuse reflectance spectra and Mott-Schottky plots for Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2). Interestingly, In3+ exhibits site selective doping at M1 and M2 sites exclusively. With the light absorption ability enhanced, the photocatalytic overall water splitting activity was also improved, i.e. the photocatalytic H2 generation rate was 1.7(1) µmol h-1 for Ba3SnGa10O20, and the optimal catalyst Ba3SnGa8.5In1.5O20 loaded with 1.0 wt% Pd exhibited the H2 generation rate of 27.5(4) µmol h-1 and the apparent quantum yield at 254 nm was estimated to be 2.28% in pure water.

From Lewis Acid to Lewis Base by La3+-to-Y3+ Substitution in α-YB5O9: Local Structure Modification Induced Lewis Basicity.

Different from the common perspective of average structure, we propose that the locally elongated metal-oxygen bonds induced by La3+-to-Y3+ substitution to a Lewis acid α-YB5O9 generate medium-strength basic sites. Experimentally, NH3- and CO2-TPD experiments prove that the La3+ doping of α-Y1-xLaxB5O9 (0 ≤ x ≤ 0.24) results in the emergence of new medium-strength basic sites and the increasing La3+ concentration modifies the number, not the strength, of the acidic and basic sites. The catalytic IPA conversion exhibits a reversal of the product selectivity, i.e., from 93% of propylene for α-YB5O9 to ∼90% of acetone for α-Y0.76La0.24B5O9, which means the La3+ doping gradually turns the solid from a Lewis acid to a Lewis base. Besides, α-Y0.76RE0.24B5O9 (RE = Ce, Eu, Gd, Tm) compounds were prepared to consolidate the above conjecture, where the acetone selectivity exhibits a linear dependence on the ionic radius (or electronegativity). This work suggests that the substitution-induced local structure change deserves more attention.

Combined Analyses on Electronic Structure and Molecular Orbitals of d10 Bimetal Oxide In2Ge2O7 and Photocatalytic Performances for Overall Water Splitting and CO2 Reduction.

Semiconducting photocatalytic overall water splitting and CO2 reduction are possible solutions to the emerging worldwide challenges of oil shortage and continual temperature increase, and the key is to develop an efficient photocatalyst. Most photocatalysts contain the d0, d10 or d10ns2 metals, and a guiding principle is desired to help to distinguish outstanding semiconductors. Here, the d10 bimetal oxide In2Ge2O7 was selected as the target. First, density functional theory (DFT) calculations point out that the nonbonding O 2p orbitals dominate the valence band maximum (VBM), and In 5s-O 2s and Ge 4s-O 2s antibonding orbitals are the major components of conduction band minimum (CBM). Moreover, the molecular orbitals were analyzed to consolidate the DFT calculations and make it more understandable for chemists. Due to the very small specific surface area (0.51 m2/g) and wide band gap (4.14 eV), as-prepared In2Ge2O7 did not exhibit any overall water splitting activity; nevertheless, when loading with 1 wt% cocatalyst (i.e., Pt, Pd), the surficial charge recombination can be greatly eliminated and the overall water splitting activity is significantly improved to 33.0(4) and 17.2(7) µmol/h for H2 and O2 generation, respectively. The apparent quantum yield (AQY) at 254 nm is 8.28%. This observation is proof that the inherent electronic structure of In2Ge2O7 is beneficial for the charge migration in bulk. Moreover, this catalyst also exhibits an observable CO2 reduction activity in pure water, which is a competition reaction with water splitting, anyway, the CH4 selectivity can be enhanced by loading Pd. This is a successful attempt to unravel the structure-property relationship by combining the analyses on electronic structure and molecular orbitals and is enlightening to further discover good candidates to photocatalysts.

Transformation-Optics-Designed Plasmonic Singularities for Efficient Photocatalytic Hydrogen Evolution at Metal/Semiconductor Interfaces.

Inspired by transformation optics, we propose a new concept for plasmonic photocatalysis by creating a novel hybrid nanostructure with a plasmonic singularity. Our geometry enables broad and strong spectral light harvesting at the active site of a nearby semiconductor where the chemical reaction occurs. A proof-of-concept nanostructure comprising Cu2ZnSnS4 (CZTS) and Au-Au dimer (t-CZTS@Au-Au) is fabricated via a colloidal strategy combining templating and seeded growth. On the basis of numerical and experimental results of different related hybrid nanostructures, we show that both the sharpness of the singular feature and the relative position to the reactive site play a pivotal role in optimizing photocatalytic activity. Compared with bare CZTS, the hybrid nanostructure (t-CZTS@Au-Au) exhibits an enhancement of the photocatalytic hydrogen evolution rate by up to ∼9 times. The insights gained from this work might be beneficial for designing efficient composite plasmonic photocatalysts for diverse photocatalytic reactions.

First-Principles Calculations on Narrow-Band Gap d10 Metal Oxides for Photocatalytic H2 Production: Role of Unusual In2+ Cations in Band Engineering.

The d10 metal oxides with low effective mass and high mobility of photoexcited electrons have received much attention in photocatalytic water splitting. However, there are still challenges in practical application due to insufficient visible light absorption. Here, an unusual phenomenon of the In2+ cation in PtIn6(GeO4)2O and PtIn6(Ga/InO4)2 with a narrow band gap is systematically investigated using density functional theory calculations. According to chemical bond analysis, the final band edge structure results from the interaction between the empty In-5p orbitals and the occupied antibonding state of the In 5s-O 2p orbitals as well as the further hybridization of adjacent In cations in PtIn6 octahedrons. The unique bonding characteristic of In2+ cations endows them with a narrow band gap and visible light response ability. Moreover, the occupied antibonding state could weaken the strength of the In-O covalent bond and strengthen the orbital hybridization of the In-In bond, causing the conduction band minimum to be located in the electroactive In6 cavity. This work reveals the origin of the narrow band gap of PtIn6(GeO4)2O and PtIn6(Ga/InO4)2 in view of bond theory and shows that they are promising semiconductors for the application of photocatalytic H2 generation.

Deciphering the photocatalytic hydrogen generation process of Fresnoite Ba2TiGe2O8 by electronic structure and bond analyses.

In addition to enhancing the activity of already-known photocatalysts, developing new ones is always desired in photocatalysis, giving more opportunities to approach practical applications. Most photocatalysts are composed of d0 (i.e. Sc3+, Ti4+, Zr4+) and/or d10 (i.e. Zn2+, Ga3+, In3+) metal cations, and a new target catalyst is Ba2TiGe2O8 containing both. Experimentally, it exhibits a UV-driven catalytic H2 generation rate of 0.5(1) µmol h-1 in methanol aqueous solution, which could be enhanced to 5.4(1) µmol h-1 by loading 1 wt% Pt as the cocatalyst. Most interestingly, theoretical calculations together with analyses on the covalent network could help us to decipher the photocatalytic process. The electrons in O 2p non-bonding orbitals are photo-excited to either Ti-O or Ge-O anti-bonding orbitals. The latter interconnect with each other to form an infinite two-dimensional network for electron migration to the catalyst surface, while the Ti-O anti-boding orbitals are rather localized because of the Ti4+ 3d orbitals; thus, those photo-excited electrons mostly recombine with holes. This study on Ba2TiGe2O8 containing both d0 and d10 metal cations gives an interesting comparison, suggesting that a d10 metal cation is probably more useful to construct a favorable conduction band minimum for the migration of photo-excited electrons.

Temperature-driven order-disorder structural transition in the oxygen sub-lattice and the complex superstructure of the high-temperature polymorph of CaSrZn2Ga2O7.

Structural order-disorder plays a decisive role in the physical properties of materials, such as magnetism, second-order harmonic generation, and ionic conductivity, and it is thus widely utilized to manipulate the crystal structure and understand structure-property correlations. Herein, we report the structural polymorphism, complex crystal structure and temperature-driven irreversible order-disorder phase transition of the polar oxides (Sr1-xCax)SrZn2Ga2O7. The low-temperature (LT) structure crystallizes in Pna21 with partial Zn/Ga ordering. Upon heating, (Sr1-xCax)SrZn2Ga2O7 undergoes an irreversible phase transition from orthorhombic Pna21 to hexagonal P63. Interestingly, the high-temperature (HT) P63 structure possesses an unexpected 3/2-fold superstructure rather than a substructure of the low-temperature (LT) Pna21 structure, which is a rare structural phenomenon in solid-state chemistry. This new HT superstructure is the most complex one in this series of oxides with 21 crystallographically independent sites determined accurately by a combination of the maximum entropy method and Rietveld refinement against high-resolution neutron powder diffraction data. In terms of the mechanism, this is a temperature-driven order-to-disorder transition in the oxygen sublattice. A careful structural analysis revealed that the oxygen disordering mainly occurs in the [SrO3] layers of the HT structure and it can be understood as respective clockwise and anticlockwise rotations of distinct GaO4-tetrahedra along the c-axis. Alternating current electrochemical impedance spectroscopic analysis revealed that the oxygen disordering in the HT structure is incapable of giving rise to oxide ionic conductivity but does lead to increased electronic conduction compared to the LT structure. The optical properties of the CaSrZn2Ga2O7 and Sr2Zn2Ga2O7 representatives are also investigated in-depth via diffuse reflectance spectroscopy and theoretic calculations.