Bimetallic Phosphide (Ni,Cu)₂P Nanoparticles by Inward Phosphorus Migration and Outward Copper Migration

Bimetallic phosphide nanoparticles are drawing a great interest as a family of nanomaterials. Controlling their fine features such as surface composition, core crystal structure, and overall composition is a key to further application. Copper and nickel are particularly interesting first-row metals for their abundance and relevance in several branches of catalysis. To synthesize crystalline bimetallic phosphide Ni–Cu–P nanoparticles, core–shell copper–nickel nanoparticles were reacted with white phosphorus (P4). Surprisingly, hollow monocrystalline (Ni,Cu)2P nanoparticles were formed alongside Cu nanoparticles and crystallized in a phase isostructural to Ni2P. Using a combination of local and ensemble analytic techniques, we showed that this unique structure is the result of several competing processes: phosphorus migration, interaction of stabilizing ligands with copper as well as metal phosphide phase crystallization. This study provides important mechanistic insights to rationalize bimetallic phosphide nanoparticles syntheses. Beyond metal phosphides, this well-characterized case study about competing diffusion and crystallization processes is of major relevance for the advancement of materials sciences at the nanoscale

Triazine-Based Covalent Organic Framework for Photocatalytic Water Oxidation: The Role of Bipyridine Ligand and Cobalt Coordination

Covalent organic frameworks (COFs) are crystalline porous conjugated polymers that have been widely used for photocatalytic hydrogen evolution and CO2 reduction. However, only few COFs showed photocatalytic oxygen evolution, which is a more challenging half-reaction of photocatalytic water splitting. Here, we presented visible-light-driven photocatalytic water oxidation of a triazine-based COF (TAPT-Bpy-COF) coordinated with cobalt as cocatalysts. The highest oxygen evolution rate (OER) was achieved at 483 μmol g–1 h–1 (≥420 nm) with an efficient apparent quantum efficiency (AQE) of 7.6% (420 ± 20 nm). The highly photocatalytic oxygen evolution activity of TAPT-Bpy-COF could be attributed to its highly ordered structures, high surface area, good wettability as well as enhanced charge separation. This work demonstrates the potential of COFs for photocatalytic oxygen evolution half-reaction and overall water splitting

Accelerated Synthesis and Discovery of Covalent Organic Framework Photocatalysts for Hydrogen Peroxide Production

A high-throughput sonochemical synthesis and testing strategy was developed to discover covalent organic frameworks (COFs) for photocatalysis. In total, 76 conjugated polymers were synthesized, including 60 crystalline COFs of which 18 were previously unreported. These COFs were then screened for photocatalytic hydrogen peroxide (H2O2) production using water and oxygen. One of these COFs, sonoCOF-F2, was found to be an excellent photocatalyst for photocatalytic H2O2 production even in the absence of sacrificial donors. However, after long-term photocatalytic tests (96 h), the imine sonoCOF-F2 transformed into an amide-linked COF with reduced crystallinity and loss of electronic conjugation, decreasing the photocatalytic activity. When benzyl alcohol was introduced to form a two-phase catalytic system, the photostability of sonoCOF-F2 was greatly enhanced, leading to stable H2O2 production for at least 1 week

MOF-Derived Multi-heterostructured Composites for Enhanced Photocatalytic Hydrogen Evolution: Deciphering the Roles of Different Components

Bimetal-organic-framework (Bi-MOF) NH2-MIL-125(Ti/Cu)-derived nanocomposites are systematically investigated to elucidate the role of individual species TiO2, CuxO and the porous carbon matrix in photocatalytic activity. Among the studied samples, the TiO2/CuxO/C nanocomposite derived from heat processing NH2-MIL-125(Ti/Cu) under Ar/H2O vapor demonstrates the highest photocatalytic H2 evolution performance due to the formation of a phasejunction between the well-crystallized anatase/rutile TiO2 polymorph, the optimized and codoped nitrogen/carbon in the composites, the formation of p–n heterojunctions between the TiO2 and CuxO nanoparticles, as well as their uniform distribution in a hydrophilic porous carbon matrix decorated with N and carboxylic functional groups. These parameters enable the in situ-formed multi-heterostructures in these nanocomposites to not only possess relatively narrower energy band gaps and improved spatial charge separation due to the formed type-II staggered p–n heterojunctions but also offer multiple pathways for charge diffusion, resulting in lower charge-transfer resistance, suppressed bulk charge recombination, and consequently, much improved visible-light absorption. Therefore, the Bi-MOF NH2-MIL-125(Ti/Cu)-derived TiO2/CuxO/C nanocomposite provides easily accessible active sites with an excellent photocatalytic H2 evolution activity of 3147 μmol gcat–1 h–1, 99 times higher than that of bare TiO2. This work provides a simple one-step approach to producing tunable novel nanocomposites for efficient photocatalytic H2 evolution without using expensive noble metals as cocatalysts

A Cubic 3D Covalent Organic Framework with nbo Topology

The synthesis of three-dimensional (3D) covalent organic frameworks (COFs) requires high-connectivity polyhedral building blocks or the controlled alignment of building blocks. Here, we use the latter strategy to assemble square-planar cobalt­(II) phthalocyanine (PcCo) units into the nbo topology by using tetrahedral spiroborate (SPB) linkages that were chosen to provide the necessary 90° dihedral angles between neighboring PcCo units. This yields a porous 3D COF, SPB-COF-DBA, with a noninterpenetrated nbo topology. SPB-COF-DBA shows high crystallinity and long-range order, with 11 resolved diffraction peaks in the experimental powder X-ray diffraction (PXRD) pattern. This well-ordered crystal lattice can also be imaged by using high-resolution transmission electron microscopy (HR-TEM). SPB-COF-DBA has cubic pores and exhibits permanent porosity with a Brunauer–Emmett–Teller (BET) surface area of 1726 m2 g–1

A Pyrene-4,5,9,10-Tetraone-Based Covalent Organic Framework Delivers High Specific Capacity as a Li-Ion Positive Electrode

Electrochemically active covalent organic frameworks (COFs) are promising electrode materials for Li-ion batteries. However, improving the specific capacities of COF-based electrodes requires materials with increased conductivity and a higher concentration of redox-active groups. Here, we designed a series of pyrene-4,5,9,10-tetraone COF (PT-COF) and carbon nanotube (CNT) composites (denoted as PT-COFX, where X = 10, 30, and 50 wt % of CNT) to address these challenges. Among the composites, PT-COF50 achieved a capacity of up to 280 mAh g–1 as normalized to the active COF material at a current density of 200 mA g–1, which is the highest capacity reported for a COF-based composite cathode electrode to date. Furthermore, PT-COF50 exhibited excellent rate performance, delivering a capacity of 229 mAh g–1 at 5000 mA g–1 (18.5C). Using operando Raman microscopy the reversible transformation of the redox-active carbonyl groups of PT-COF was determined, which rationalizes an overall 4 e–/4 Li+ redox process per pyrene-4,5,9,10-tetraone unit, accounting for its superior performance as a Li-ion battery electrode

Kinked Silicon Nanowires: Superstructures by Metal-Assisted Chemical Etching

We report on metal-assisted chemical etching of Si for the synthesis of mechanically stable, hybrid crystallographic orientation Si superstructures with high aspect ratio, above 200. This method sustains high etching rates and facilitates reproducible results. The protocol enables the control of the number, angle, and location of the kinks via successive etch-quench sequences. We analyzed relevant Au mask catalyst features to systematically assess their impact on a wide spectrum of etched morphologies that can be easily attained and customized by fine-tuning of the critical etching parameters. For instance, the designed kinked Si nanowires can be incorporated in biological cells without affecting their viability. An accessible numerical model is provided to explain the etch profiles and the physicochemical events at the Si/Au–electrolyte interface and offers guidelines for the development of finite-element modeling of metal-assisted Si chemical etching

MXene Aerogel Derived Ultra-Active Vanadia Catalyst for Selective Conversion of Sustainable Alcohols to Base Chemicals

Selective oxidation reactions are an important class of the current chemical industry and will be highly important for future sustainable chemical production. Especially, the selective oxidation of primary alcohols is expected to be of high future interest, as alcohols can be obtained on technical scales from biomass fermentation. The oxidation of primary alcohols produces aldehydes, which are important intermediates. While selective methanol oxidation is industrially established, the commercial catalyst suffers from deactivation. Ethanol selective oxidation is not commercialized but would give access to sustainable acetaldehyde production when using renewable ethanol. In this work, it is shown that employing 2D MXenes as building blocks allows one to design a nanostructured oxide catalyst composed of mixed valence vanadium oxides, which outperforms on both reactions known materials by nearly an order of magnitude in activity, while showing high selectivity and stability. The study shows that the synthesis route employing 2D materials is key to obtain these attractive catalysts. V4C3Tx MXene structured as an aerogel precursor needs to be employed and mildly oxidized in an alcohol and oxygen atmosphere to result in the aspired nanostructured catalyst composed of mixed valence VO2, V6O13, and V3O7. Very likely, the bulk stable reduced valence state of the material together coupled with the nanorod arrangement allows for unprecedented oxygen mobility as well as active sites and results in an ultra-active catalyst

Kinked Silicon Nanowires: Superstructures by Metal-Assisted Chemical Etching

We report on metal-assisted chemical etching of Si for the synthesis of mechanically stable, hybrid crystallographic orientation Si superstructures with high aspect ratio, above 200. This method sustains high etching rates and facilitates reproducible results. The protocol enables the control of the number, angle, and location of the kinks via successive etch-quench sequences. We analyzed relevant Au mask catalyst features to systematically assess their impact on a wide spectrum of etched morphologies that can be easily attained and customized by fine-tuning of the critical etching parameters. For instance, the designed kinked Si nanowires can be incorporated in biological cells without affecting their viability. An accessible numerical model is provided to explain the etch profiles and the physicochemical events at the Si/Au–electrolyte interface and offers guidelines for the development of finite-element modeling of metal-assisted Si chemical etching