Showing papers by "Cheng Wang published in 2020"

Heterostructured Inter-Doped Ruthenium-Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting.

Abstract: The development of transition-metal-oxides (TMOs)-based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter-doped ruthenium-cobalt oxide [(Ru-Co)Ox ] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru-Co)Ox nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm-2 and a small Tafel slope of 23.5 mV dec-1 , as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm-2 . As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm-2 for alkaline overall water splitting.

Photoactivation of Cu Centers in Metal–Organic Frameworks for Selective CO2 Conversion to Ethanol

Abstract: CO2 hydrogenation to ethanol is of practical importance but poses a significant challenge due to the need of forming one C–C bond while keeping one C–O bond intact. CuI centers could selectively ca...

Highly Dispersed Ni Catalyst on Metal–Organic Framework-Derived Porous Hydrous Zirconia for CO2 Methanation

Abstract: We report the preparation of porous hydrous zirconia by treatment of zirconium-based metal-organic framework (MOF) UiO-66 with a strong base. Microporosity of the original MOF was partially retained in the resultant porous hydrous zirconia. NiII centers were then adsorbed onto the OH-rich hydrous zirconia and in situ converted to highly dispersed Ni0 for CO2 hydrogenation to CH4. The activated catalyst after an induction period showed a turnover frequency of 345 h-1 or a space-time yield of 5851 mmol·gNi-1·h-1 with a CH4 selectivity of over 99%. The catalyst was tested for 100 h on stream, showing only a 4% decrease in activity, and was found to convert atmospheric CO2 to CH4 via CO2 collection through Na2CO3/NaHCO3 cycling. Thermal decomposition of NaHCO3 released CO2 for hydrogenation to CH4, and the resultant Na2CO3 absorbed CO2 from air to form NaHCO3. This work highlights the opportunity in using MOFs as precursors to prepare highly porous metal oxide/hydroxide supports for solid-gas phase catalysis.

Tunable Cobalt-Polypyridyl Catalysts Supported on Metal-Organic Layers for Electrochemical CO2 Reduction at Low Overpotentials.

Abstract: The Co center is active in electrochemical CO2 reduction (CO2RR), and its activity can be tuned by changing its coordination environment. However, the coordination number around the Co center cannot be readily changed in homogeneous systems owing to bimolecular decomposition of reduced low-coordinate Co species. Herein we report the systematic tuning of N atom numbers from 2 to 5 in the first coordination sphere around Co centers supported on two-dimensional metal-organic layers (MOLs) for the electrochemical CO2RR. The N atoms come from a combination of bipyridine, terpyridine, and phenylpyridine ligands. The Co centers are isolated and stabilized on the MOL to prevent bimolecular decomposition. All of the catalysts, denoted MOL-Co-Nx (x = 2-5), are active in reducing CO2 to CO electrochemically, but their activities are highly dependent on the number of coordinating N atoms. MOL-Co-N3 showed the highest current density of 2.3 A mg-1 with a CO Faradaic efficiency of 99% at an overpotential of only 380 mV. Density functional theory calculations attribute the high activity of the Co-N3 center to a balance of ligand field strength and open coordination site: the high ligand field strength promotes back-bonding, while the open coordination site allows HCO3- assistance, both of which accelerate C-O cleavage. MOLs thus provide a unique platform to systematically study the relationship between the coordination environment and the reactivity of open metal sites in electrocatalysis.

Metal-Organic Framework with Dual Active Sites in Engineered Mesopores for Bioinspired Synergistic Catalysis.

Abstract: Here we report the design of an enzyme-inspired metal-organic framework (MOF), 1-OTf-Ir, by installing strong Lewis acid and photoredox sites in engineered mesopores. Al-MOF (1), with mixed 2,2'-bipyridyl-5,5-dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands, was oxidized with ozone and then triflated to generate strongly Lewis acidic Al-OTf sites in the mesopores, followed by the installation of [Ir(ppy)2(dcbpy)]+ (ppy = 2-phenylpyridine) sites to afford 1-OTf-Ir with both Lewis acid and photoredox sites. 1-OTf-Ir effectively catalyzed reductive cross-coupling of N-hydroxyphthalimide esters or aryl bromomethyl ketones with vinyl- or alkynyl-azaarenes to afford new azaarene derivatives. 1-OTf-Ir enabled catalytic synthesis of anticholinergic drugs Pheniramine and Chlorpheniramine.

Machine-Learning-Guided Morphology Engineering of Nanoscale Metal-Organic Frameworks

Abstract: Summary Controlling morphology of nanocrystals is one of the central tasks of nanoscience. In this work, we studied nanoscale metal-organic frameworks (nMOFs) from Hf-oxo clusters and linear dicarboxylate ligands with the aid of machine-learning methods for data analysis. Ligand solubility and modulator concentration were found to quantitatively predict the growth of nMOFs with a specific morphology, such as ultrathin two-dimensional film, hexagonal nanoplate, octahedron, cuboctahedron, concave octahedron, or hollow octahedron morphology. With these insights, we use epitaxy growth sequences to design nMOFs of desirable nanostructures with enhanced substrate transport and, hence, increased activities for catalytic olefin hydrogenation. This work highlights new opportunities in using machine learning to guide morphology engineering of nMOFs and other nanomaterials.

Blue Energy Conversion from Holey-Graphene-like Membranes with a High Density of Subnanometer Pores.

Abstract: Blue energy converts the chemical potential difference from salinity gradients into electricity via reverse electrodialysis and provides a renewable source of clean energy. To achieve high energy c...

Nanoscale Metal-Organic Frameworks and Metal-Organic Layers with Two-Photon-Excited Fluorescence.

Abstract: Metal-organic frameworks (MOFs) based on 9,10-diphenylanthracene-derived ligands had been reported to exhibit upconverted fluorescence through triplet-triplet annihilation. We found that zirconium MOFs based on 9,10-diphenylanthracene can also give upconverted fluorescence via two-photon absorption without adding a triplet photosensitizer when a femtosecond pulsed laser is used as the excitation source. By tuning the synthetic condition, we obtained nanoscale MOFs of UiO structure in both octahedral and hexagonal nanoplate shapes, as well as a hexagonal nanoplate of MOFs of hcp-UiO structure and two-dimensional metal-organic layers. All of them, as well as a homogeneous solution of the 9,10-diphenylanthracene ligand, exhibit upconverted fluorescence upon excitation using a laser pulse of 60 fs with a pulse energy of ∼1.1 × 106 nJ/cm2 (unfocused). Moreover, we observed different emission spectra by two-photon excitation compared to those by one-photon excitation, which indicates access to a unique initial excited state via two-photon excitation. This phenomenon is not observed for a homogeneous solution of the ligand. These nanoscale MOFs may find application in two-photon fluorescence imaging.

Probing surface structure on two-dimensional metal-organic layers to understand suppressed interlayer packing

Abstract: Two-dimensional metal-organic layers (MOLs) from alternatively connected benzene-tribenzoate ligands and Zr6(μ3-O)4(μ3-OH)4 or Hf6(μ3-O)4(μ3-OH)4 secondary building units can be prepared in gram scale via solvothermal synthesis. However, the reason why the monolayers did not pack to form thick crystals is unknown. Here we investigated the surface structure of the MOLs by a combination of sum-frequency generation spectroscopy, nanoscale infrared microscopy, atomic force microscopy, aberration-corrected transmission electron microscopy, and compositional analysis. We found a partial coverage of the monolayer surface by dangling tricarboxylate ligands, which prevent packing of the monolayers. This finding illustrates low-density surface modification as a strategy to prepare new two-dimensional materials with a high percentage of exposed surface.

Photoresponsive 2D polymeric Langmuir–Blodgett films of 2,3,6,7,10,11-hexaiminotriphenylene

Abstract: 2,3,6,7,10,11-Hexaiminotriphenylene (HATP) was connected via aerobic oxidative coupling to form large-area polymeric 2D films using the Langmuir–Blodgett technique. These 2D polymers exhibited electronic transport properties as a p-type semiconductor with high conductivity (0.91 S cm−1), a high on/off current ratio (103), and a photo-responsivity (980 nm) up to 160 mA W−1.

Transport through a network of two-dimensional NbC superconducting crystals connected via weak links

Abstract: Recent progresses in the growth and fabrication techniques for preparing crystalline two-dimensional (2D) superconductors have stimulated intense interest in the studies of the electronic properties of these systems. Here we investigate the superconducting transport properties based on chemical vapor deposition-grown thin NbC crystals consisting of network structures. The 2D character of the superconductivity in individual NbC crystals is revealed by examining the angular dependence of magnetotransport measurements. At low temperatures, the samples show nonmonotonic double-step superconducting transitions as a function of temperature and magnetic field. We demonstrate that the observed transport characteristics can be understood in terms of coupled Josephson junctions forming between isolated NbC crystals, including the effects of Josephson and quasiparticle tunneling. In particular, detailed analysis of the magnetic field-driven transition suggests the existence of quantum flux-creep regime at low temperatures and small magnetic fields in such thin NbC superconducting crystals. Our work underlines the importance of the morphology on the transport properties of 2D superconducting crystals, providing a comprehensive understanding of crystalline 2D superconductors.