Efficiency of layered photocatalysts such as graphitic carbon nitride (g‐CN) is still too low due to the poor utilization of photoexcited‐charge carriers. The major drawback is that the weak van der ...Waals force among g‐CN layers is unfavorable for the charge transfer between the adjacent layers and the intrinsically π‐conjugated planes with inefficient random in‐plane charge migration. Herein, an atomically dispersed Pd layered photocatalyst with both bridged sites of adjacent layers and surface‐sites of g‐CN is demonstrated, providing directional charge‐transfer channels and targeting active sites for photocatalytic water reduction. Both theoretical prediction and empirical characterizations are conducted to achieve the successful synthesis of single‐atom engineered Pd/g‐CN hybrid and the excellent separation of charge transfer as well as the efficient photocatalytic hydrogen evolution, much better than that of the optimized Pt/g‐CN benchmark. The finding in this work provides a rational way for tailoring the performance and engineering of single‐atomic noble metal.
A single‐atom engineered Pd/g‐CN hybrid with directional charge transfer channels and targeting active sites is constructed by interlayer intercalation and surface anchor of Pd atoms into the framework of graphitic carbon nitride, showing excellent ability of charge transfer/separation and efficient photocatalytic hydrogen evolution, much better than that of the optimized Pt/g‐CN benchmark.
In elderly people particularly in postmenopausal women, inadequate bone formation by osteoblasts originating from bone marrow mesenchymal stem cells (BMSCs) for compensation of bone resorption by ...osteoclasts is a major reason for osteoporosis. Enhancing osteoblastic differentiation of BMSCs is a feasible therapeutic strategy for osteoporosis. Here, bone marrow stromal cell (ST)-derived exosomes (STExos) are found to remarkably enhance osteoblastic differentiation of BMSCs
in vitro
. However, intravenous injection of STExos is inefficient in ameliorating osteoporotic phenotypes in an ovariectomy (OVX)-induced postmenopausal osteoporosis mouse model, which may be because STExos are predominantly accumulated in the liver and lungs, but not in bone. Hereby, the STExo surface is conjugated with a BMSC-specific aptamer, which delivers STExos into BMSCs within bone marrow. Intravenous injection of the STExo-Aptamer complex enhances bone mass in OVX mice and accelerates bone healing in a femur fracture mouse model. These results demonstrate the efficiency of BMSC-specific aptamer-functionalized STExos in targeting bone to promote bone regeneration, providing a novel promising approach for the treatment of osteoporosis and fracture.
A novel strategy to deliver therapeutic exosomes to bone is developed for the first time by conjugating a specific BMSC-targeting aptamer to the exosomal surface.
Biaryl atropisomers are of great importance in natural products, pharmaceuticals, and asymmteric synthesis. The efficient synthesis of these chiral scaffolds with full enantiocontrol and high ...diversity remains challenging. Reported herein is a Pd‐catalyzed atroposelective C−H allylation with tert‐leucine as an efficient catalytic chiral transient auxiliary. A wide range of enantioenriched biaryl aldehydes were prepared in synthetically useful yields with excellent enantioselectivity (up to >99 % ee) through β‐O elimination. The reaction could be carried out on a gram scale without erosion of the ee value. A variety of axially chiral carboxylic acids could be obtained with high enantiopurity. The resulting axially chiral biaryl aldehydes and carboxylic acids might be used in asymmetric synthesis as chiral ligands and/or organocatalysts.
A Pd‐catalyzed atroposelective C−H allylation with allylic surrogates is reported. tert‐Leucine was identified as an efficient catalytic transient chiral auxiliary. A range of enantioenriched biaryls were prepared in synthetically useful yields with enantioselectivities up to >99 % ee through β‐O elimination. The reaction could be scaled up and the products could be further converted into enantiomerically pure axially chiral carboxylic acids.
NiFe‐based layered double hydroxides (LDHs) are among the most efficient oxygen evolution reaction (OER) catalysts in alkaline medium, but their long‐term OER stabilities are questionable. In this ...work, it is demonstrated that the layered structure makes bulk NiFe LDH intrinsically not stable in OER and the deactivation mechanism of NiFe LDH in OER is further revealed. Both operando electrochemical and structural characterizations show that the interlayer basal plane in bulk NiFe LDH contributes to the OER activity, and the slow diffusion of proton acceptors (e.g., OH−) within the NiFe LDH interlayers during OER causes dissolution of NiFe LDH and therefore decrease in OER activity with time. To improve diffusion of proton acceptors, it is proposed to delaminate NiFe LDH into atomically thin nanosheets, which is able to effectively improve OER stability of NiFe LDH especially at industrial operating conditions such as elevated operating temperatures (e.g., at 80 °C) and large current densities (e.g., at 500 mA cm−2).
The interlayer basal plane in bulk NiFe layered double hydroxide (LDH) contributes to the oxygen evolution reaction (OER) activity. Restricted diffusion of proton acceptors within the interlayers of bulk NiFe LDH causes catalyst dissolution. Exfoliating multilayered NiFe LDH into single‐layered nanosheets greatly improves the catalytic stability of NiFe LDH in alkaline OER.
Hydroxide‐exchange membrane fuel cells can potentially utilize platinum‐group‐metal (PGM)‐free electrocatalysts, offering cost and scalability advantages over more developed proton‐exchange membrane ...fuel cells. However, there is a lack of non‐precious electrocatalysts that are active and stable for the hydrogen oxidation reaction (HOR) relevant to hydroxide‐exchange membrane fuel cells. Here we report the discovery and development of Ni3N as an active and robust HOR catalyst in alkaline medium. A supported version of the catalyst, Ni3N/C, exhibits by far the highest mass activity and break‐down potential for a PGM‐free catalyst. The catalyst also exhibits Pt‐like activity for hydrogen evolution reaction (HER) in alkaline medium. Spectroscopy data reveal a downshift of the Ni d band going from Ni to Ni3N and interfacial charge transfer from Ni3N to the carbon support. These properties weaken the binding energy of hydrogen and oxygen species, resulting in remarkable HOR activity and stability.
Dispersed Ni3N nanoparticles on a carbon support (Ni3N/C) were discovered as a new catalyst for the hydrogen oxidation reaction (HOR). A shift in the Ni d‐band and interfacial charge transfer from Ni3N to the carbon support may lead to the weak binding of hydrogen and oxygen species to the catalyst. In turn, this weak binding might be the origin of the remarkable HOR activity and stability.
Chiral aldehyde catalysis opens new avenues for the activation of simple amines. However, the lack of easy access to structurally diverse chiral aldehyde catalysts has hampered the development of ...this cutting‐edge field. Herein, we report a Pd‐catalyzed atroposelective C−H naphthylation with 7‐oxabenzonorbornadienes for the preparation of axially chiral biaryls with excellent enantioselectivities (up to >99 % ee). This reaction is scalable and robust, which serves as a key step to provide a rapid access to axially chiral aldehyde catalysts through a three‐step C−H functionalization sequence. These chiral aldehydes exhibit better activities and enantioselectivities than the previously reported organocatalysts in the asymmetric activation of glycine derived amides and dipeptides. Moreover, preliminary investigation also discloses that the aldehyde catalyst can effectively override the intrinsic facial selectivity of chiral dipeptide substrates, showcasing the strong chiral induction ability of this type of novel aldehyde catalysts.
A concise synthetic route to chiral aldehyde catalysts with Pd‐catalyzed atroposelective C−H naphthylation as a key reaction is reported. These chiral aldehyde catalysts were successfully employed as ligands in the asymmetric activation of glycine esters enabling a higher level of asymmetric induction and reaction activity. A wide range of enantioenriched biaryls were prepared in synthetically useful yields with excellent enantioselectivities (up to >99 % ee).
Abstract
Van der Waals heterobilayers of transition metal dichalcogenides with spin–valley coupling of carriers in different layers have emerged as a new platform for exploring spin/valleytronic ...applications. The interlayer coupling was predicted to exhibit subtle changes with the interlayer atomic registry. Manually stacked heterobilayers, however, are incommensurate with the inevitable interlayer twist and/or lattice mismatch, where the properties associated with atomic registry are difficult to access by optical means. Here, we unveil the distinct polarization properties of valley-specific interlayer excitons using epitaxially grown, commensurate WSe
2
/MoSe
2
heterobilayers with well-defined (AA and AB) atomic registry. We observe circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules for interlayer excitons. Using selective excitation schemes, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.
Three's a charm for iron and CO2Large-scale electrochemical reduction of CO2 to CO could be a promising first step in sustainable conversion of the greenhouse gas to commodity chemicals. Currently, ...gold and silver are the most active catalysts for this process, whereas more abundant, less expensive metals tend to require impractically high potentials. Jun Gu et al. now report an iron catalyst with activity equaling or exceeding that of the precious metals. The key proved to be stabilization of the dispersed single iron ions in the +3 oxidation state.Science, this issue p. 1091Currently, the most active electrocatalysts for the conversion of CO2 to CO are gold-based nanomaterials, whereas non–precious metal catalysts have shown low to modest activity. Here, we report a catalyst of dispersed single-atom iron sites that produces CO at an overpotential as low as 80 millivolts. Partial current density reaches 94 milliamperes per square centimeter at an overpotential of 340 millivolts. Operando x-ray absorption spectroscopy revealed the active sites to be discrete Fe3+ ions, coordinated to pyrrolic nitrogen (N) atoms of the N-doped carbon support, that maintain their +3 oxidation state during electrocatalysis, probably through electronic coupling to the conductive carbon support. Electrochemical data suggest that the Fe3+ sites derive their superior activity from faster CO2 adsorption and weaker CO absorption than that of conventional Fe2+ sites.
The oxygen evolution reaction (OER) is the performance-limiting half reaction of water splitting, which can be used to produce hydrogen fuel using renewable energies. Whereas a number of transition ...metal oxides and oxyhydroxides have been developed as promising OER catalysts in alkaline medium, the mechanisms of OER on these catalysts are not well understood. Here we combine electrochemical and in situ spectroscopic methods, particularly operando X-ray absorption and Raman spectroscopy, to study the mechanism of OER on cobalt oxyhydroxide (CoOOH), an archetypical unary OER catalyst. We find the dominating resting state of the catalyst as a Co(IV) species CoO2. Through oxygen isotope exchange experiments, we discover a cobalt superoxide species as an active intermediate in the OER. This intermediate is formed concurrently to the oxidation of CoOOH to CoO2. Combing spectroscopic and electrokinetic data, we identify the rate-determining step of the OER as the release of dioxygen from the superoxide intermediate. The work provides important experimental fingerprints and new mechanistic perspectives for OER catalysts.
Control of Polymer Properties by Entanglement: A Review Kong, De‐Chao; Yang, Ming‐Hao; Zhang, Xue‐Song ...
Macromolecular materials and engineering,
December 2021, 2021-12-00, 20211201, Letnik:
306, Številka:
12
Journal Article
Recenzirano
Chain entanglement, either cohesional or topological, distinguishes polymers from other engineering materials. It impedes the movement of molecular segments and influences the polymer rheology, ...morphology, and mechanical properties. Although a high level of entanglement can increase the polymer toughness, excessive entanglement should be avoided because it causes a high melt viscosity making the processing difficult. This review tended to elucidate the influence of entanglement on the polymer structure, determining the material properties and processability. A wide range of methods used to fine control the degrees of chain entanglement are summarized. The methods are applicable to polymers in solutions, melts, and condensed states with advantages and limitations discussed in detail. The authors also examined the effect of the entanglement on polymer crystallization—the mechanism remains a controversial issue. This review will provide general guidance to designing and processing polymer materials with desired properties via a rational route of controlling the chain entanglement.
In this work, a wide range of methods used to fine control the degrees of chain entanglement are summarized. The methods are applicable to polymers in solutions, melts, and condensed states with advantages and limitations discussed in details. This review will provide general guidance to designing and processing polymer materials with desired properties via a rational route of controlling the chain entanglement.
PDF
