Enriching surface-ordered defects on WO3 for photocatalytic CO2-to-CH4 conversion by water.

Defect engineering has been widely applied in semiconductors to improve photocatalytic properties by altering the surface structures. This study is about the transformation of inactive WO3 nanosheets to a highly effective CO2-to-CH4 conversion photocatalyst by introducing surface-ordered defects in abundance. The nonstoichiometric WO3-x samples were examined by using aberration-corrected electron microscopy. Results unveil abundant surface-ordered terminations derived from the periodic {013} stacking faults with a defect density of 20.2%. The {002} surface-ordered line defects are the active sites for fixation CO2, transforming the inactive WO3 nanosheets into a highly active catalyst (CH4: O2 = 8.2: 16.7 µmol h-1). We believe that the formation of the W-O-C-W-O species is a critical step in the catalytic pathways. This work provides an atomic-level comprehension of the structural defects of catalysts for activating small molecules.

Superstructure-Assisted Single-Atom Catalysis on Tungsten Carbides for Bifunctional Oxygen Reactions.

Single-atom catalysis (SAC) attracts wide interest for zinc-air batteries that require high-performance bifunctional electrocatalysts for oxygen reactions. However, catalyst design is still highly challenging because of the insufficient driving force for promoting multiple-electron transfer kinetics. Herein, we report a superstructure-assisted SAC on tungsten carbides for oxygen evolution and reduction reactions. In addition to the usual single atomic sites, strikingly, we reveal the presence of highly ordered Co superstructures in the interfacial region with tungsten carbides that induce internal strain and promote bifunctional catalysis. Theoretical calculations show that the combined effects from superstructures and single atoms strongly reduce the adsorption energy of intermediates and overpotential of both oxygen reactions. The catalyst therefore presented impressive bifunctional activity with an ultralow potential gap of 0.623 V and delivered a high power density of 188.5 mW cm-2 for assembled zinc-air batteries. This work opens up new opportunities for atomic catalysis.

Establishing Multiple-Order Built-In Electric Fields Within Heterojunctions to Achieve Photocarrier Spatial Separation.

Hybridizing two heterocomponents to construct a built-in electric field (BIEF) at the interface represents a significant strategy for facilitating charge separation in carbon dioxide (CO2)-photoreduction. However, the unidirectional nature of BIEFs formed by various low-dimensional materials poses challenges in adequately segregating the photogenerated carriers produced in bulk. In this study, leveraging zinc oxide (ZnO) nanodisks, a sulfurization reaction is employed to fabricate Z-scheme ZnO/zinc sulfide (ZnS) heterojunctions featuring a multiple-order BIEF. These heterojunctions reveal distinctive interfacial structures characterized by two semicoherent phase boundaries. The cathodoluminescence 2D maps and density functional theory calculation results demonstrate that the direction of the multiple-order BIEF spans from ZnS to ZnO. This directional alignment significantly fosters the spatial separation of photogenerated electrons and holes within ZnS nanoparticles and enhances CO2-to-carbon monoxide photoreduction performance (3811.7 µmol h-1 g-1). The findings present a novel pathway for structurally designing BIEFs within heterojunctions, while providing fresh insights into the migratory behavior of photogenerated carriers across interfaces.

The Directional Crystallization Process of Poly (triazine imide) Single Crystals in Molten Salts.

Poly (triazine imide) photocatalysts prepared via molten salt methods emerge as promising polymer semiconductors with one-step excitation capacity of overall water splitting. Unveiling the molecular conjugation, nucleation, and crystallization processes of PTI crystals is crucial for their controllable structure design. Herein, microscopy characterization was conducted at the PTI crystallization front from meso to nano scales. The heptazine-based precursor was found to depolymerize to triazine monomers within molten salts and KCl cubes precipitate as the leading cores that guide the directional stacking of PTI molecular units to form aggregated crystals. Upon this discovery, PTI crystals with improved dispersibility and enhanced photocatalytic performance were obtained by tailoring the crystallization fronts. This study advances insights into the directional assembling of PTI monomers on salt templates, placing a theoretical foundation for the ordered condensation of polymer crystals.

Unveiling the charge transfer dynamics steered by built-in electric fields in BiOBr photocatalysts.

Construction of internal electric fields (IEFs) is crucial to realize efficient charge separation for charge-induced redox reactions, such as water splitting and CO2 reduction. However, a quantitative understanding of the charge transfer dynamics modulated by IEFs remains elusive. Here, electron microscopy study unveils that the non-equilibrium photo-excited electrons are collectively steered by two contiguous IEFs within binary (001)/(200) facet junctions of BiOBr platelets, and they exhibit characteristic Gaussian distribution profiles on reduction facets by using metal co-catalysts as probes. An analytical model justifies the Gaussian curve and allows us to measure the diffusion length and drift distance of electrons. The charge separation efficiency, as well as photocatalytic performances, are maximized when the platelet size is about twice the drift distance, either by tailoring particle dimensions or tuning IEF-dependent drift distances. The work offers great flexibility for precisely constructing high-performance particulate photocatalysts by understanding charge transfer dynamics.

Atomistic Observation of Temperature-Dependent Defect Evolution within Sub-stoichiometric WO3-x Catalysts.

Tunable crystalline defects endow WO3-x catalysts with extended functionalities for a broad range of photo- and electric-related applications. However, direct visualization of the defect structures and their evolution mechanism is lacking. Herein, aberration-corrected and in situ transmission electron microscopy was complemented by theoretical calculations to investigate the effect of temperature on the defect evolution behavior during hydrogenation treatment. Low processing temperature (100-300 °C) leads to the occurrence of randomly distributed oxygen vacancies within WO3-x nanosheets. At higher temperatures, oxygen vacancies become highly mobile and aggregate into stacking faults. Planar defects are prone to nucleate at the surface and develop in a zigzag form at 400 °C, while treating at 500 °C promotes the growth of {200}-type stacking faults. Our work clearly establishes that the atomic configuration of the defects in WO3-x samples could be manipulated by regulating the hydrogenation temperature. This study not only expands our understanding of the structure-function relationships of sub-stoichiometric tungsten oxides but also unlocks their full potential as advanced catalysts by tuning stoichiometry in a controlled manner.

Electrochemical Behaviors of Sulfurized-Polyacrylonitrile with Synthesized Polyacrylonitrile Precursors Based on the Radical Polymerization Through Monomer Acrylonitrile.

Polyacrylonitrile (PAN) precursors have been polymerized at different radical polymerization temperatures for preparing sulfurized-polyacrylonitrile (S-PAN) composite cathodes in rechargeable lithium sulfur battery. The physical properties of these composites have been investigated using X-ray diffraction, Fourier transform infrared spectrometry, Raman spectroscopy, Brunner-Emmet-Teller measurement and Gel permeation chromatography analysis. The electrochemical performance of the S-PAN composite cathodes made from the PAN precursor was investigated. The results showed that the molecular weight distribution of the PAN precursors affected the electrochemical performance of the S-PAN made from the PAN precursor. S-PAN composites derived from PAN with a narrower molecular weight distribution at 65 °C were exhibit the best electrochemical performance in lithium-sulfur battery.