Synthesis and Structure of the Double-Layered Sillén-Aurivillius Perovskite Oxychloride La2.1Bi2.9Ti2O11Cl as a Potential Photocatalyst for Stable Visible Light Solar Water Splitting.

Exploring photocatalysts for solar water splitting is a relevant step toward sustainable hydrogen production. Sillén-Aurivillius-type compounds have proven to be a promising material class for photocatalytic and photoelectrochemical water splitting with the advantage of visible light activity coupled to enhanced stability because of their unique electronic structure. Especially, double- and multilayered Sillén-Aurivillius compounds [An-1BnO3n+1][Bi2O2]2Xm, with A and B being cations and X a halogen anion, offer a great variety in material composition and properties. Yet, research in this field is limited to only a few compounds, all of them containing mainly Ta5+ or Nb5+ as cations. This work takes advantage of the outstanding properties of Ti4+ demonstrated in the context of photocatalytic water splitting. A fully titanium-based oxychloride, La2.1Bi2.9Ti2O11Cl, with a double-layered Sillén-Aurivillius intergrowth structure is fabricated via a facile one-step solid-state synthesis. A detailed crystal structure analysis is performed via powder X-ray diffraction and correlated to density functional theory calculations, providing a detailed understanding of the site occupancies in the unit cell. The chemical composition and the morphology are studied using scanning and transmission electron microscopy together with energy-dispersive X-ray analysis. The capability of the compound to absorb visible light is demonstrated by UV-vis spectroscopy and analyzed by electronic structure calculations. The activity toward the hydrogen and the oxygen evolution reaction is evaluated by measuring anodic and cathodic photocurrent densities, oxygen evolution rates, and incident-current-to-photon efficiencies. Thanks to the incorporation of Ti4+, this Sillén-Aurivillius-type compound enables best-in-class photoelectrochemical water splitting performance at the oxygen evolution side under visible light irradiation. Thus, this work highlights the potential of Ti-containing Sillén-Aurivillius-type compounds as stable photocatalysts for visible light-driven solar water splitting.

A Relativity Enhanced, Medium-Strong Au(I)···H-N Hydrogen Bond in a Protonated Phenylpyridine-Gold(I) Thiolate.

Gold is an electron-rich metal with a high electronegativity comparable to that of sulfur. Hence, hydrogen bonds of the Au(I)···H-E (E = electronegative element) type should be possible, but their existence is still under debate. Experimental results are scarce and often contradictory. As guidance for possible preparative work, we have theoretically investigated (ppyH)Au(SPh) (ppy = 2-phenylpyridine) bearing two monoanionic ligands which are not strongly electronegative at the same time to further increase the charge density on the gold(I) atom. The protonated pyridine nitrogen atom in ppy is geometrically ideally suited to place a proton in close proximity to the gold atom in a favorable geometry for a classical hydrogen bond arrangement. Indeed, the results of the calculations indicate that the hydrogen bonded conformation of (ppyH)Au(SPh) represents a minimum geometry with bond metrics in the expected range for medium-strong hydrogen bonds [r(N-H) = 1.043 Å, r(H···Au) = 2.060 Å, a(N-H···Au) = 141.4°]. The energy difference between the conformer containing the H···Au bond and another conformer without a hydrogen bond amounts to 7.8 kcal mol-1, which might serve as an estimate of the hydrogen bond strength. Spectroscopic properties were calculated, yielding further characteristics of such hydrogen bonded gold species.

Resolving the structure of V3O7·H2O and Mo-substituted V3O7·H2O.

Vanadate compounds, such as V3O7·H2O, are of high interest due to their versatile applications as electrode material for metal-ion batteries. In particular, V3O7·H2O can insert different ions such as Li+, Na+, K+, Mg2+ and Zn2+. In that case, well resolved crystal structure data, such as crystal unit-cell parameters and atom positions, are needed in order to determine the structural information of the inserted ions in the V3O7·H2O structure. In this work, fundamental crystallographic parameters, i.e. atomic displacement parameters, are determined for the atoms in the V3O7·H2O structure. Furthermore, vanadium ions were substituted by molybdenum in the V3O7·H2O structure [(V2.85Mo0.15)O7·H2O] and the crystallographic positions of the molybdenum ions and their oxidation state are elucidated.

Modified H2 V3 O8 to Enhance the Electrochemical Performance for Li-ion Insertion: The Influence of Prelithiation and Mo-Substitution.

Nanostructured H2 V3 O8 is a promising high-capacity cathode material, suitable not only for Li+ but also for Na+, Mg2+ , and Zn2+ insertion. However, the full theoretical capacity for Li+ insertion has not been demonstrated experimentally so far. In addition, improvement of cycling stability is desirable. Modifications like substitution or prelithiation are possibilities to enhance the electrochemical performance of electrode materials. Here, for the first time, the substitution of vanadium sites in H2 V3 O8 with molybdenum was achieved while preserving the nanostructure by combining a soft chemical synthesis approach with a hydrothermal process. The obtained Mo-substituted vanadate nanofibers were further modified by prelithiation. While pristine H2 V3 O8 showed an initial capacity of 223 mAh g-1 and a retention of 79 % over 30 cycles, combining Mo substitution and prelithiation led to a superior initial capacity of 312 mAh g-1 and a capacity retention of 94 % after 30 cycles.

A Facile One-Pot Synthesis of Hierarchically Organized Carbon/TiO2 Monoliths with Ordered Mesopores.

Sol-gel processing combined with soft templating and gelation-induced phase separation is very sensitive to the precursor sol composition. In this work we present a straightforward synthesis towards hierarchically structured, macroporous carbon/titania monoliths with ordered mesopores derived from resorcinol/formaldehyde monoliths and a glycolated titanium precursor. We demonstrate the influence of various reaction solvents, where diol-based media and the proportion of the catalyst seem to be essential in controlling spinodal decomposition, obtaining similar monolithic structures under different synthesis conditions. Based on these observations, we further homogeneously incorporated TiO2 into the carbon structure by an in situ synthesis approach, obtaining structural features similar to pure carbon materials with surface areas of about 400 m2 g-1 , periodically arranged mesopores with a mean distance of 10-11 nm and cellular macroporosity.

Hybrid carbon spherogels: carbon encapsulation of nano-titania.

Extraordinarily homogeneous, freestanding titania-loaded carbon spherogels can be obtained using Ti(acac)2(OiPr)2 in the polystyrene sphere templated resorcinol-formaldehyde gelation. Thereby, a distinct, crystalline titania layer is achieved inside every hollow sphere building unit. These hybrid carbon spherogels allow capitalizing on carbon's electrical conductivity and the lithium-ion intercalation capacity of titania.