Recent Advances in Ultrathin Two-Dimensional Nanomaterials.

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

Correction: Hybrid micro-/nano-structures derived from metal-organic frameworks: preparation and applications in energy storage and conversion.

Correction for 'Hybrid micro-/nano-structures derived from metal-organic frameworks: preparation and applications in energy storage and conversion' by Xiehong Cao et al., Chem. Soc. Rev., 2017, 46, 2660-2677.

Hybrid micro-/nano-structures derived from metal-organic frameworks: preparation and applications in energy storage and conversion.

Metal-organic frameworks (MOFs), an important class of inorganic-organic hybrid crystals with intrinsic porous structures, can be used as versatile precursors or sacrificial templates for preparation of numerous functional nanomaterials for various applications. Recent developments of MOF-derived hybrid micro-/nano-structures, constructed by more than two components with varied functionalities, have revealed their extensive capabilities to overcome the weaknesses of the individual counterparts and thus give enhanced performance for energy storage and conversion. In this tutorial review, we summarize the recent advances in MOF-derived hybrid micro-/nano-structures. The synthetic strategies for preparing MOF-derived hybrid micro-/nano-structures are first introduced. Focusing on energy storage and conversion, we then discuss their potential applications in lithium-ion batteries, lithium-sulfur batteries, supercapacitors, lithium-oxygen batteries and fuel cells. Finally, we give our personal insights into the challenges and opportunities for the future research of MOF-derived hybrid micro-/nano-structures.

Ionic Janus Liquid Droplets Assembled and Propelled by Electric Field.

Traditional Janus particle approaches to produce active motion are based on using solid particles, but it is interesting to consider liquid droplets instead, because for solid particles, the self-assembly of synthetic active matter requires moving objects to sit in a near-planar 2D geometry. Emulsions, cross-linked polymers, and porous materials have been proposed for 3D self-assembly but with limitations to propel them. It is now demonstrated that Janus liquid droplets can be used as building block in an active propulsion system. Using an ionic liquid motif, the droplet system can be tuned from core-shell to Janus and multipatches, using facile surfactant-based methods. The approach was stimulated by the success of electro-hydrodynamic flow produced by an alternating electric field to produce motion of colloidal particles; its usefulness to also propel ionic liquids is demonstrated.

Anodized Aluminum Oxide Templated Synthesis of Metal-Organic Frameworks Used as Membrane Reactors.

The incorporation of metal-organic frameworks (MOFs) into membrane-shaped architectures is of great importance for practical applications. The currently synthesized MOF-based membranes show many disadvantages, such as poor compatibility, low dispersity, and instability, which severely limit their utility. Herein, we present a general, facile, and robust approach for the synthesis of MOF-based composite membranes through the in situ growth of MOF plates in the channels of anodized aluminum oxide (AAO) membranes. After being used as catalysis reactors, they exhibit high catalytic performance and stability in the Knoevenagel condensation reaction. The high catalytic performance might be attributed to the intrinsic structure of MOF-based composite membranes, which can remove the products from the reaction zone quickly, and prevent the aggregation and loss of catalysts during reaction and recycling process.

Synthesis of WOn -WX2 (n=2.7, 2.9; X=S, Se) Heterostructures for Highly Efficient Green Quantum Dot Light-Emitting Diodes.

Preparation of two-dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WOn -WX2 (n=2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WOn nanomaterials. The WOn -WX2 heterostructures are composed of WO2.9 nanoparticles (NPs) or WO2.7 nanowires (NWs) grown together with single- or few-layer WX2 nanosheets (NSs). As a proof-of-concept application, the WOn -WX2 heterostructures are used as the anode interfacial buffer layer for green quantum dot light-emitting diodes (QLEDs). The QLED prepared with WO2.9 NP-WSe2 NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.

Preorganized Chromophores Facilitate Triplet Energy Migration, Annihilation and Upconverted Singlet Energy Collection.

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) has the potential to enhance significantly photovoltaic and photocatalytic efficiencies by harnessing sub-bandgap photons, but the progress of this field is held back by the chemistry problem of how to preorganize multiple chromophores for efficient UC under weak solar irradiance. Recently, the first maximization of UC quantum yield at solar irradiance was achieved using fast triplet energy migration (TEM) in metal-organic frameworks (MOFs) with ordered acceptor arrays, but at the same time, a trade-off between fast TEM and high fluorescence efficiency was also found. Here, we provide a solution for this trade-off issue by developing a new strategy, triplet energy migration, annihilation and upconverted singlet energy collection (TEM-UPCON). The porous structure of acceptor-based MOF crystals allows triplet donor molecules to be accommodated without aggregation. The surface of donor-doped MOF nanocrystals is modified with highly fluorescent energy collectors through coordination bond formation. Thanks to the higher fluorescence quantum yield of surface-bound collectors than parent MOFs, the implementation of the energy collector greatly improves the total UC quantum yield. The UC quantum yield maximization behavior at ultralow excitation intensity was retained because the TTA events take place only in the MOF acceptors. The TEM-UPCON concept may be generalized to collectors with various functions and would lead to quantitative harvesting of upconverted energy, which is difficult to achieve in common molecular diffusion-based systems.

Colloidal-sized metal-organic frameworks: synthesis and applications.

Colloidal metal-organic frameworks (CMOFs), nanoporous colloidal-sized crystals that are uniform in both size and polyhedral shape, are crystals composed of metal ions and organic bridging ligands, which can be used as building blocks for self-assembly in organic and aqueous liquids. They stand in contrast to conventional metal-organic frameworks (MOFs), which scientists normally study in the form of bulk crystalline powders. However, powder MOFs generally have random crystal size and shape and therefore do not possess either a definite mutual arrangement with adjacent particles or uniformity. CMOFs do have this quality, which can be important in vital uptake and release kinetics. In this Account, we present the diverse methods of synthesis, pore chemistry control, surface modification, and assembly techniques of CMOFs. In addition, we survey recent achievements and future applications in this emerging field. There is potential for a paradigm shift, away from using just bulk crystalline powders, towards using particles whose size and shape are regulated. The concept of colloidal MOFs takes into account that nanoporous MOFs, conventionally prepared in the form of bulk crystalline powders with random crystal size, shape, and orientation, may also form colloidal-sized objects with uniform size and morphology. Furthermore, the traditional MOF functions that depend on porosity present additional control over those MOF functions that depend on pore interactions. They also can enable controlled spatial arrangements between neighboring particles. To begin, we discuss progress regarding synthesis of MOF nano- and microcrystals whose crystal size and shape are well regulated. Next, we review the methods to modify the surfaces with dye molecules and polymers. Dyes are useful when seeking to observe nonluminescent CMOFs in situ by optical microscopy, while polymers are useful to tune their interparticle interactions. Third, we discuss criteria to assess the stability of CMOFs for various applications. In another section of this Account, we give examples of supracrystal assembly in liquid, on substrates, at interfaces, and under external electric fields. We end this Account with discussion of possible future developments, both conceptual and technological.

Voids and yolk-shells from crystals that coat particles.

We investigate curvature-driven core-shell morphology that emerges when polycrystalline shells of ZIF-8 (zeolitic imidazolate framework coordination polymer) grow on colloid-sized particles. In early growth stages, the shell is continuous, but it transforms to yolk-shell, with neither sacrificial template nor core etching, because of geometrical frustration. A design rule is developed regarding how local surface curvature matters. Comparing shells grown on cubic, rod-like, and peanut-shaped hematite core particles, we validate the argument.

Electric field-induced assembly of monodisperse polyhedral metal-organic framework crystals.

Monodisperse polyhedral metal-organic framework (MOF) particles up to 5 µm in size, large enough to enable in situ optical imaging of particle orientation, were synthesized by the strategy of simultaneous addition of two capping ligands with different binding strength during crystallization. Upon dispersing them in ethylene glycol and applying AC electric field, the particles facets link to form linear chains. We observe well-regulated crystal orientation not only for rhombic dodecahedra all of whose facets are equivalent, but also for truncated cubes with nondegenerate facets. After removing the electric field, chains disassemble if their facets contain even modest curvature, but remain intact if their facets are planar. This assembly strategy offers a general route to fabricate oriented polyhedral crystal arrays of potential interest for new applications and functions.

Triple-layer (au@perylene)@polyaniline nanocomposite: unconventional growth of faceted organic nanocrystals on polycrystalline Au.

Unconventional crystal growth: core/shell nanocrystals were obtained by growth of a dominant single-crystalline phase of perylene over polycrystalline Au nanoparticle seeds and isolated by coating with polyaniline (PANI) shells. Perylene is released in the presence of sodium dodecyl sulfate (SDS) micelles. The TEM images show (Au@perylene)@PANI nanocomposites before and after complete release of perylene leaving Au@PANI (inset).

Carbon-Based Functional Materials Derived from Waste for Water Remediation and Energy Storage.

Carbon-based functional materials hold the key for solving global challenges in the areas of water scarcity and the energy crisis. Although carbon nanotubes (CNTs) and graphene have shown promising results in various fields of application, their high preparation cost and low production yield still dramatically hinder their wide practical applications. Therefore, there is an urgent call for preparing carbon-based functional materials from low-cost, abundant, and sustainable sources. Recent innovative strategies have been developed to convert various waste materials into valuable carbon-based functional materials. These waste-derived carbon-based functional materials have shown great potential in many applications, especially as sorbents for water remediation and electrodes for energy storage. Here, the research progress in the preparation of waste-derived carbon-based functional materials is summarized, along with their applications in water remediation and energy storage; challenges and future research directions in this emerging research field are also discussed.

Synthesis of Ultrathin PdCu Alloy Nanosheets Used as a Highly Efficient Electrocatalyst for Formic Acid Oxidation.

Inspired by the unique properties of ultrathin 2D nanomaterials and excellent catalytic activities of noble metal nanostructures for renewable fuel cells, a facile method is reported for the high-yield synthesis of ultrathin 2D PdCu alloy nanosheets under mild conditions. Impressively, the obtained PdCu alloy nanosheet after being treated with ethylenediamine can be used as a highly efficient electrocatalyst for formic acid oxidation. The study implicates that the rational design and controlled synthesis of an ultrathin 2D noble metal alloy may open up new opportunities for enhancing catalytic activities of noble metal nanostructures.

Improved Reversibility of Fe3+ /Fe4+ Redox Couple in Sodium Super Ion Conductor Type Na3 Fe2 (PO4 )3 for Sodium-Ion Batteries.

The methodology employed here utilizes the sodium super ion conductor type sodium iron phosphate wrapped with conducting carbon network to generate a stable Fe3+ /Fe4+ redox couple, thereby exhibiting higher operating voltage and energy density of sodium-ion batteries. This new class of sodium iron phosphate wrapped by carbon also displays a cycling stability with >96% capacity retention after 200 cycles.

Submonolayered Ru Deposited on Ultrathin Pd Nanosheets used for Enhanced Catalytic Applications.

Ultrathin Pd nanosheets (NSs) coated with submonolayered Ru, referred to as Pd@Ru NSs, are synthesized via a seed-mediated growth method. The underpotential deposition can be the driving force for the formation of Pd@Ru NSs. The Pd@Ru NSs exhibit superior catalytic properties in the reduction of 4-nitrophenol and the semihydrogenation of 1-octyne, compared to the pure Pd NSs and Ru NSs.

Shape-selected colloidal MOF crystals for aqueous use.

Methods are described to synthesize shape-selectable, monodisperse, aqueous-stable metal-organic frameworks (MOFs) by the reaction of aluminium nitrate with benzene tricarboxylic acid in various aqueous solvent mixtures and acetic acid as the capping ligand. Environmental stability was confirmed by thermal analysis and immersion in aqueous acidic media.

Near-incompressible faceted polymer microcapsules from metal-organic framework templates.

Faceted polymer microcapsules are prepared from metal-organic framework (MOF) templates. The MOF templates are removable under mild aqueous conditions. The obtained microcapsules are stiffer than their spherical counterparts, reflecting the near-incompressibility of the facet edges, and indicating that the faceting might be a useful strategy for controlling the mechanical properties of polymer microcapsules.

3D dendritic gold nanostructures: seeded growth of a multi-generation fractal architecture.

In this report, we focus on the synthetic challenges for nanoscale 3D fractal architectures, namely the multi-generation growth with control in both size uniformity and colloidal stability; by directing the simultaneous growth of Au and polyaniline on Au seeds, fractal nanoparticles can be achieved with a topology distinctively different from those of spheres, cubes or rods.