2D Free-Standing Nitrogen-Doped Ni-Ni3 S2 @Carbon Nanoplates Derived from Metal-Organic Frameworks for Enhanced Oxygen Evolution Reaction.

2D metal-organic frameworks (2D MOFs) are promising templates for the fabrication of carbon supported 2D metal/metal sulfide nanocomposites. Herein, controllable synthesis of a newly developed 2D Ni-based MOF nanoplates in well-defined rectangle morphology is first realized via a pyridine-assisted bottom-up solvothermal treatment of NiSO4 and 4,4'-bipyridine. The thickness of the MOF nanoplates can be controlled to below 20 nm, while the lateral size can be tuned in a wide range with different amounts of pyridine. Subsequent pyrolysis treatment converts the MOF nanoplates into 2D free-standing nitrogen-doped Ni-Ni3 S2 @carbon nanoplates. The obtained Ni-Ni3 S2 nanoparticles encapsulated in the N-doped carbon matrix exhibits high electrocatalytic activity in oxygen evolution reaction. A low overpotential of 284.7 mV at a current density of 10 mA cm-2 is achieved in alkaline solution, which is among the best reported performance of substrate-free nickel sulfides based nanomaterials.

Advanced electrocatalysts based on two-dimensional transition metal hydroxides and their composites for alkaline oxygen reduction reaction.

The electrocatalytic oxygen reduction reaction (ORR) is a crucial part in developing high-efficiency fuel cells and metal-air batteries, which have been cherished as clean and sustainable energy conversion devices/systems to meet the ever-increasing energy demand. ORR electrocatalysts currently employed in the cathodes of fuel cells and metal-air batteries are mainly based on high-cost and scarce noble metal elements. It is thus of great importance to develop cheap and earth-abundant ORR electrocatalysts. In this aspect, redox-active transition metal hydroxides, a class of multifunctional inorganic layered materials, have been proposed as prospective candidates on account of their abundance and high ORR activities. In this article, the preparation and structural evolution of transition metal hydroxides, in particular their exfoliation into two-dimensional (2D) nanosheets, as well as compositing/integrating with catalytic active and/or conductive components to overcome the insulating nature of hydroxides in alkaline ORR, are summarized. Recent advances have demonstrated that 2D transition metal hydroxides with carefully tuned compositions and elaborately designed nanoarchitectures can achieve both high activity and high pathway selectivity, as well as excellent stability comparable to those of commercial Pt/C electrocatalysts. To realize the dream of renewable electrochemical energy conversion, new strategies and insights into rational designing of 2D hydroxide-based nanostructures with further enhanced electrocatalytic performance are still to be vigorously pursued.

In situ growth of metallic Ag0 intercalated CoAl layered double hydroxides as efficient electrocatalysts for the oxygen reduction reaction in alkaline solutions.

Metallic Ag0 intercalated CoAl-layered double hydroxides (CoAl LDHs) have been successfully synthesized in situ through a simple redox process with ethylene glycol (EG) and triethanolamine (TEOA). The Ag(CN)2- anion-exchanged precursor was reduced by EG to form metallic Ag0. Furthermore, the effect of TEOA on confining the particle size of Ag0 was demonstrated. The oxygen reduction reaction (ORR) property of metallic Ag0 intercalated CoAl LDHs was examined in alkaline aqueous solution. A typical sample synthesized by the addition of TEOA for 180 min exhibited excellent ORR catalytic activity with a high current density of 5.5 mA cm-2 at 0.2 V (vs. a reversible hydrogen electrode (RHE)) and good stability. Koutecky-Levich (K-L) calculations and rotating ring-disk electrode (RRDE) measurements further revealed that the ORR of the as-prepared catalyst proceeded mainly via an almost ideal four-electron transfer process. The enhanced electrocatalytic activity was ascribed to the intercalated Ag0, confined nanoparticle size and the expanded interlayer space, which effectively facilitate the reactant transfer and electron migration.

Two-dimensional porous cuprous oxide nanoplatelets derived from metal-organic frameworks (MOFs) for efficient photocatalytic dye degradation under visible light.

Bottom-up synthesis is a promising method to design and control the morphology of metal-organic frameworks (MOFs). Here, square shaped two-dimensional (2D) MOF nanoplatelets with a thickness of ∼80 nm and a lateral dimension of 4-6 µm were successfully synthesized through a facile solvothermal treatment of Cu(NO3)2 and 4,4'-bipyridine in the presence of polyvinyl pyrrolidone (PVP). The growth of a cross-weaved structure assembled via 1D chains linked with 4,4'-bipyridine along the layer stacking direction was hindered by PVP, resulting in a high-aspect ratio of the nanoplatelets. Subsequent annealing treatment converted the Cu-based MOFs into porous N-doped Cu2O/carbon composites, retaining the 2D square morphology. This annealed product showed a higher performance in the degradation of methyl orange under visible light compared to previously reported Cu2O composites. By using a small amount of the catalyst, the degradation rate could reach up to 2.5 mg min-1 gcat-1 as a result of the efficient absorption of visible light and high surface area of the porous catalysts.

Controllable fabrication of urchin-like Co3O4 hollow spheres for high-performance supercapacitors and lithium-ion batteries.

Urchin-like cobalt oxide (Co3O4) hollow spheres can be successfully prepared by thermal decomposition of cobalt carbonate hydroxide hydrate (Co(CO3)0.5(OH)·0.11H2O) obtained by template-assisted hydrothermal synthesis. The morphology, crystal structure evolution and thermal decomposition behaviors of the as-prepared products have been carefully investigated. A plausible formation mechanism of the urchin-like Co3O4 hollow spheres in the presence of hexadecyl trimethyl ammonium bromide (CTAB) as the surfactant template is proposed. The urchin-like Co3O4 hollow spheres are further constructed as electrode materials for high-performance supercapacitors with a high specific capacitance of 460 F g-1 at a current density of 4 A g-1 and excellent cycling stability. Furthermore, as anode materials for lithium-ion batteries (LIBs), superior lithium storage performance of 1342.2 mA h g-1 (0.1 C) and 1122.7 mA h g-1 (0.2 C) can also be achieved. The excellent performances can be ascribed to the unique hierarchical urchin-like hollow structure of the electrode materials, which offers a large specific surface area, short electron and ion diffusion paths and high permeability while being directly in contact with the electrolyte. Moreover, the hollow structure with sufficient internal void spaces can self-accommodate volume change during electrochemical reactions, which improves the structural stability and integrity.

Controllable Fabrication and Optical Properties of Uniform Gadolinium Oxysulfate Hollow Spheres.

Uniform gadolinium oxysulfate (Gd2O2SO4) hollow spheres were successfully fabricated by calcination of corresponding Gd-organic precursor obtained via a facile hydrothermal process. The Gd2O2SO4 hollow spheres have a mean diameter of approximately 550 nm and shell thickness in the range of 30-70 nm. The sizes and morphologies of as-prepared Gd2O2SO4 hollow spheres could be deliberately controlled by adjusting the experimental parameters. Eu-doped Gd2O2SO4 hollow spheres have also been prepared for the property modification and practical applications. The structure, morphology, and properties of as-prepared products were characterized by XRD, TEM, HRTEM, SEM and fluorescence spectrophotometer. Excited with ultraviolet (UV) pump laser, successful downconversion (DC) could be achieved for Eu-doped Gd2O2SO4 hollow spheres.