Visible Light Photo-oxidation of Model Pollutants Using CaCu₃Ti₄O₁₂: An Experimental and Theoretical Study of Optical Properties, Electronic Structure, and Selectivity

Charge transfer between metal ions occupying distinct crystallographic sublattices in an ordered material is a strategy to confer visible light absorption on complex oxides to generate potentially catalytically active electron and hole charge carriers. CaCu₃Ti₄O₁₂ has distinct octahedral Ti⁴⁺ and square planar Cu²⁺ sites and is thus a candidate material for this approach. The sol−gel synthesis of high surface area CaCu₃Ti₄O₁₂ and investigation of its optical absorption and photocatalytic reactivity with model pollutants are reported. Two gaps of 2.21 and 1.39 eV are observed in the visible region. These absorptions are explained by LSDA+U electronic structure calculations, including electron correlation on the Cu sites, as arising from transitions from a Cu-hybridized O 2p-derived valence band to localized empty states on Cu (attributed to the isolation of CuO₄ units within the structure of CaCu₃Ti₄O₁₂) and to a Ti-based conduction band. The resulting charge carriers produce selective visible light photodegradation of 4-chlorophenol (monitored by mass spectrometry) by Pt-loaded CaCu₃Ti₄O₁₂ which is attributed to the chemical nature of the photogenerated charge carriers and has a quantum yield comparable with commercial visible light photocatalysts

Ion Dynamics and CO₂ Absorption Properties of Nb‑, Ta‑, and Y‑Doped Li₂ZrO₃ Studied by Solid-State NMR, Thermogravimetry, and First-Principles Calculations

Among the many different processes proposed for large-scale carbon capture and storage (CCS), high-temperature CO₂ looping has emerged as a favorable candidate due to the low theoretical energy penalties that can be achieved. Many different materials have been proposed for use in such a process, the process requiring fast CO₂ absorption reaction kinetics as well as being able to cycle the material for multiple cycles without loss of capacity. Lithium ternary oxide materials, and in particular Li₂ZrO₃, have displayed promising performance, but further modifications are needed to improve their rate of reaction with CO₂. Previous studies have linked rates of lithium ionic conduction with CO₂ absorption in similar materials, and in this work we present work aimed at exploring the effect of aliovalent doping on the efficacy of Li₂ZrO₃ as a CO₂ sorbent. Using a combination of X-ray powder diffraction, theoretical calculations, and solid-state nuclear magnetic resonance, we studied the impact of Nb, Ta, and Y doping on the structure, Li ionic motion, and CO₂ absorption properties of Li₂ZrO₃. These methods allowed us to characterize the theoretical and experimental doping limit into the pure material, suggesting that vacancies formed upon doping are not fully disordered but instead are correlated to the dopant atom positions, limiting the solubility range. Characterization of the lithium motion using variable-temperature solid-state nuclear magnetic resonance confirms that interstitial doping with Y retards the movement of Li ions in the structure, whereas vacancy doping with Nb or Ta results in a similar activation energy as observed for nominally pure Li₂ZrO₃. However, a marked reduction in the CO₂ absorption of the Nb- and Ta-doped samples suggests that doping also leads to a change in the carbonation equilibrium of Li₂ZrO₃, disfavoring the CO₂ absorption at the reaction temperature. This study shows that a complex mixture of structural, kinetic, and dynamic factors can influence the performance of Li-based materials for CCS and underscores the importance of balancing these different factors in order to optimize the process

Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li_6+<i>x</i>P_1–<i>x</i>Si_<i>x</i>O₅Cl

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic (P213) structure at room temperature, undergoing a transition at 473 K to a Li+ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si4+ with P5+ in Li6+xP1–xSixO5Cl (0.3 x < 0.85) solid solution. The resulting delocalization of Li+ sites leads to a maximum ionic conductivity of 1.82(1) × 10–6 S cm–1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li6+xP1–xSixO5Cl is directly connected to structural changes occurring within the Li+ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites

Bi₄O₄Cu_1.7Se_2.7Cl_0.3: Intergrowth of BiOCuSe and Bi₂O₂Se Stabilized by the Addition of a Third Anion

Layered two-anion compounds are of interest for their diverse electronic properties. The modular nature of their layered structures offers opportunities for the construction of complex stackings used to introduce or tune functionality, but the accessible layer combinations are limited by the crystal chemistries of the available anions. We present a layered three-anion material, Bi₄O₄Cu_1.7Se_2.7Cl_0.3, which adopts a new structure type composed of alternately stacked BiOCuSe and Bi₂O₂Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi₂O₂Se_0.7Cl_0.3 blocks coupled to Cu-deficient Bi₂O₂Cu_1.7Se₂ blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi₄O₄Cu₂Se₃ is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi₄O₄Cu_1.7Se_2.7Cl_0.3: a room-temperature thermal conductivity (κ) of 0.4(1) W/mK was measured on a pellet with preferred crystallite orientation along the stacking axis, with perpendicular measurement indicating it is also highly anisotropic. This κ value lies in the ultralow regime and is smaller than those of both BiOCuSe and Bi₂O₂Se. Bi₄O₄Cu_1.7Se_2.7Cl_0.3 behaves like a charge-balanced semiconductor with a narrow band gap. The chemical diversity offered by the additional anion allows the integration of two common structural units in a single phase by the simultaneous and coupled creation of charge-balancing defects in each of the units