Defect engineering is a strategy that has been widely used to design active semiconductor photocatalysts. However, understanding the role of defects, such as oxygen vacancies, in controlling photocatalytic activity remains a challenge. Here we report the use of chemically-2 triggered fluorogenic probes to study the spatial distribution of active regions in individual tungsten oxide nanowires using super-resolution fluorescence microscopy. The nanowires show significant heterogeneity along their lengths for the photocatalytic generation of hydroxyl radicals. Through quantitative, coordinate-based colocalization of multiple probe molecules activated by the same nanowires, we demonstrate that the nanoscale regions most active fo the photocatalytic generation of hydroxyl radicals also possess a greater concentration of oxygen vacancies. Chemical modifications to remove or block access to surface oxygen vacancies, supported by calculations of binding energies of adsorbates to different surface sites on tungsten oxide, show how these defects control catalytic activity at both the ensemble and single-particle level. These findings reveal that clusters of oxygen vacancies activate surface-adsorbed water molecules towards photooxidation to produce hydroxyl radicals, a critical intermediate in several photocatalytic reactions.

Heteroanionic oxysulfide perovskite compounds represent an emerging class of new materials allowing for a wide range of tunability in the electronic structure that could lead to a diverse spectrum of novel and improved functionalities. Unlike cation ordered double perovskites—where the origins and design rules of various experimentally observed cation orderings are well known and understood—anion ordering in heteroanionic perovskites remains a largely uncharted territory. In this contribution, we present and discuss insights that have emerged from our first-principles-based electronic structure analysis of a prototypical anion-ordered SrHf(O0.5S0.5)3 oxysulfide chemistry, studied in all possible anion configurations allowed within a finite size supercell. We demonstrate that the preferred anion ordering is always an all-cis arrangement of anions around an HfO3S3 octahedron. As a general finding beyond the specific chemistry, the origins of this ordering tendency are traced back to a combined stabilization effect stemming from electronic, elastic, and electrostatic contributions. These qualitative notions are also quantified using state-of-the-art machine learning models. We further study the relative stability of the identified ordering as a function of A (Ca, Sr, Ba) and B (Ti, Zr, Hf) site chemistries and probe chemistry-dependent trends in the electronic structure and functionality of the material. Most remarkably, we find that the identified ground-state anion ordering breaks the inversion symmetry to create a family of oxysulfide ferroelectrics with a macroscopic polarization >30 μC/cm2, exhibiting a significant promise for electronic materials applications.

The properties of the high-temperature superconductor YBa 2 Cu 3 O 7−x (YBCO) depend on the concentration of oxygen vacancies (V O). It is generally agreed upon that V O form in the CuO chains, even at low concentrations where the critical temperature for superconductivity peaks (x ≈ 0.07), with only a handful of reports suggesting the presence of V O at the apical sites. In this paper, we show direct evidence of apical V O in optimally doped YBCO samples. Using density-functional-theory calculations, we predict that isolated V O are equally favorable to form in either the CuO chains or the apical sites, which we confirm using atomic-resolution scanning transmission electron microscope imaging and spectroscopy. We further show that apical V O lead to significant lattice distortions and changes in the electronic structure of YBCO, indicating they should be considered on an equal footing with chain V O to understand the superconducting properties of YBCO in the optimal doping region.

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The fluoride ion is well suited to be the active species of rechargeable batteries, due to its small size, light weight, and high electronegativity. While existing F-ion batteries based on...

The relationship between the formation of oxygen vacancies in the apical sites of the YBa2Cu3O7−x structure and the commonly observed Y2Ba4Cu8O16 intergrowth defect has been demonstrated by examination of thin-film and single crystal samples.

Lead halide perovskites have emerged as a promising class of semiconductors; however, they suffer from issues related to lead toxicity and instability. We report results of a first-principles-based design of heavy-metal-based oxynitrides as alternatives to lead halide perovskites. We have used density functional theory calculations to search a vast composition space of ABO2N and ABON2 compounds, where B is a p-block cation and A is an alkaline, alkali-earth, rare-earth, or transition metal cation, and identify 10 new ABO2N oxynitride semiconductors that we expect to be formable. Specifically, we discover a new family of ferroelectric semiconductors with A 3+SnO2N stoichiometry (A = Y, Eu, La, In, and Sc) in the LuMnO3-type structure, which combine the strong bonding of metal oxides with the low carrier effective mass and small, tunable band gaps of the lead halide perovskites. These tin oxynitrides have predicted direct band gaps ranging from 1.6 to 3.3 eV and a sizable electric polarization up to 17 μC/cm2, which is predicted to be switchable by an external electric field through a nonpolar phase. With their unique combination of polarization, low carrier effective mass, and band gaps spanning the entire visible spectrum, we expect ASnO2N ferroelectric semiconductors will find useful applications as photovoltaics and photocatalysts as well as for optoelectronics.

Multiferroic materials with simultaneous magnetic and ferroelectric ordering that persist above room temperature are rare. Using first-principles density functional theory calculations, we demonstrate fluorination of oxygen-deficient AA'FeO perovskites, where A and A' are cations with +3 and +2 oxidation states, respectively, and have a layered ordering, as an effective strategy to obtain room-temperature multiferroics. We show that by controlling the size of the A and A' cations, it is possible to stabilize a noncentrosymmetric phase arising due to the hybrid improper ferroelectricity mechanism, with polarization as high as 13 μC/cm. The fluorination also stabilizes Fe in +3 oxidation state, which results in superexchange interactions that are strong enough to sustain magnetic order well above room temperature. We also show the presence of a magnetoelectric coupling wherein the switching mode that reverses the direction of the spontaneous polarization also affects the strength of the magnetic interactions. The results show that low-temperature fluorination of anion-deficient perovskites with layered cation ordering can be an effective approach to design new multiferroics.