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Layered lithium cobalt oxide (LiCoO 2 , LCO) is the most successful commercial cathode material in lithium-ion batteries. However, its notable structural instability at potentials higher than 4.35 V (versus Li/Li + ) constitutes the major barrier to accessing its theoretical capacity of 274 mAh g −1 . Although a few high-voltage LCO (H-LCO) materials have been discovered and commercialized, the structural origin of their stability has remained difficult to identify. Here, using a three-dimensional continuous rotation electron diffraction method assisted by auxiliary high-resolution transmission electron microscopy, we investigate the structural differences at the atomistic level between two commercial LCO materials: a normal LCO (N-LCO) and a H-LCO. These powerful tools reveal that the curvature of the cobalt oxide layers occurring near the surface dictates the structural stability of the material at high potentials and, in turn, the electrochemical performances. Backed up by theoretical calculations, this atomistic understanding of the structure–performance relationship for layered LCO materials provides useful guidelines for future design of new cathode materials with superior structural stability at high voltages. A three-dimensional continuous rotation electron diffraction method allows atomistic characterization of the chemistry of curved layered cathode materials.

Covalent organic framework (COF) materials have been difficult to characterize structurally and to exploit because they tend to form powders or amorphous materials. Ma et al. studied a variety of three-dimensional COFs based on imine linkages (see the Perspective by Navarro). They found that the addition of aniline inhibited nucleation and allowed the growth of crystals large enough for single-crystal x-ray diffraction studies. Evans et al. describe a two-step process in which nanoscale seeds of boronate ester–linked two-dimensional COFs can be grown into micrometer-scale single crystals by using a solvent that suppresses the nucleation of additional nanoparticles. Transient absorption spectroscopy revealed superior charge transport in these crystallites compared with that observed in conventional powders. Science , this issue p. 48 , p. 52 ; see also p. 35 The addition of aniline enables the growth of single crystals of imine-based covalent organic framework materials. The crystallization problem is an outstanding challenge in the chemistry of porous covalent organic frameworks (COFs). Their structural characterization has been limited to modeling and solutions based on powder x-ray or electron diffraction data. Single crystals of COFs amenable to x-ray diffraction characterization have not been reported. Here, we developed a general procedure to grow large single crystals of three-dimensional imine-based COFs (COF-300, hydrated form of COF-300, COF-303, LZU-79, and LZU-111). The high quality of the crystals allowed collection of single-crystal x-ray diffraction data of up to 0.83-angstrom resolution, leading to unambiguous solution and precise anisotropic refinement. Characteristics such as degree of interpenetration, arrangement of water guests, the reversed imine connectivity, linker disorder, and uncommon topology were deciphered with atomic precision—aspects impossible to determine without single crystals.

Electrically conducting 2D metal–organic frameworks (MOFs) have attracted considerable interest, as their hexagonal 2D lattices mimic graphite and other 2D van der Waals stacked materials. However, understanding their intrinsic properties remains a challenge because their crystals are too small or of too poor quality for crystal structure determination. Here, we report atomically precise structures of a family of 2D π -conjugated MOFs derived from large single crystals of sizes up to 200 μm, allowing atomic-resolution analysis by a battery of high-resolution diffraction techniques. A designed ligand core rebalances the in-plane and out-of-plane interactions that define anisotropic crystal growth. We report two crystal structure types exhibiting analogous 2D honeycomb-like sheets but distinct packing modes and pore contents. Single-crystal electrical transport measurements distinctively demonstrate anisotropic transport normal and parallel to the π -conjugated sheets, revealing a clear correlation between absolute conductivity and the nature of the metal cation and 2D sheet packing motif. Two-dimensional MOFs can possess porosity and electrical conductivity but are difficult to grow as single crystals. Here, by balancing in-plane and out-of-plane interactions, single crystals of sizes up to 200 µm are grown, allowing in-plane transport measurements and atomic-resolution analysis.

The tuning of molecular switches in solid state toward stimuli-responsive materials has attracted more and more attention in recent years. Herein, we report a switchable three-dimensional covalent organic framework (3D COF), which can undergo a reversible transformation through a hydroquinone/quinone redox reaction while retaining the crystallinity and porosity. Our results clearly show that the switching process gradually happened through the COF framework, with an almost quantitative conversion yield. In addition, the redox-triggered transformation will form different functional groups on the pore surface and modify the shape of pore channel, which can result in tunable gas separation property. This study strongly demonstrates 3D COFs can provide robust platforms for efficient tuning of molecular switches in solid state. More importantly, switching of these moieties in 3D COFs can remarkably modify the internal pore environment, which will thus enable the resulting materials with interesting stimuli-responsive properties. Tuning of molecular switches in solid state toward stimuli-responsive materials attracted attention in recent years but has not yet been realized in three-dimensional (3D) covalent organic frameworks (COFs). Herein, the authors demonstrate a stable and switchable 3D COF which undergoes reversible transformation through a hydroquinone/quinone redox reaction.

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.

Nonlinear optical (NLO) materials are the vital components of future photoelectric technologies as they can broaden the tunable wavelength range supplied by common laser sources. However, the necessary prerequisites for a practical NLO material are rather strict. Accordingly, considerable efforts have been focused on finding potential NLO materials. Here we report two asymmetric beryllium-free borates Pna 2 1 - and P 6 ̄ 2 m -Ba 3 Mg 3 (BO 3 ) 3 F 3 featuring NLO-favorable 2 ∞ [Mg 3 O 2 F 3 (BO 3 ) 2 ] layered structures. The reversible phase transition among two polymorphs was demonstrated by multiple experimental tests. The optical measurements reveal that Pna 2 1 -Ba 3 Mg 3 (BO 3 ) 3 F 3 possesses the optical properties required for ultraviolet NLO applications. Remarkably, Pna 2 1 -Ba 3 Mg 3 (BO 3 ) 3 F 3 has a large laser damage threshold, a deep-ultraviolet cutoff edge, a favorable anisotropic thermal expansion as well as the capacity of insolubility in water. These optical properties can be comparable or superior to that of commercial NLO material β -BaB 2 O 4 , which make Pna 2 1 -Ba 3 Mg 3 (BO 3 ) 3 F 3 a promising ultraviolet NLO material. Nonlinear optical crystals suitable for the UV spectral region could simplify short-wavelength generation and make it more efficient. Here, the authors design and demonstrate that one of two asymmetric borate polymorphs exhibits promising optical and mechanical properties for generating UV light.

Crystal size effect is of vital importance in materials science by exerting significant influence on various properties of materials and furthermore their functions. Crystal size effect of covalent organic frameworks (COFs) has never been reported because their controllable synthesis is difficult, despite their promising properties have been exhibited in many aspects. Here, we report the diverse crystal size effects of two representative COFs based on the successful realization of crystal-size-controlled synthesis. For LZU-111 with rigid spiral channels, size effect reflects in pore surface area by influencing the pore integrity, while for flexible COF-300 with straight channels, crystal size controls structural flexibility by altering the number of repeating units, which eventually changes sorption selectivity. With the understanding and insight of the structure-property correlation not only at microscale but also at mesoscale for COFs, this research will push the COF field step forward to a significant advancement in practical applications. Crystal size effects are of vital importance to understand various properties and functions of a material but have not been reported for Covalent Organic Frameworks (COFs). Here, the authors report a crystal-size-controlled synthesis of two COFs and look into different crystal size effects.

The design and synthesis of three-dimensional covalent organic frameworks (3D COFs) have still been considered as a big challenge. Here we report the design and synthesis of an AIEgen-based 3D COF (3D-TPE-COF), with a high surface area (1084 m 2  g −1 ). According to powder X-ray diffraction and continuous rotation electron diffraction analyses, 3D-TPE-COF is identified to adopt a seven-fold interpenetrated pts topology. Interestingly, 3D-TPE-COF emits yellow fluorescence upon excitation, with a photoluminescence quantum yield of 20%. Moreover, by simply coating 3D-TPE-COF onto a commercial blue light-emitting diode (LED), a prototype white LED (WLED) under continuously driving without degradation for 1200 h was demonstrated. The present work suggests the possibility of using COF materials for stable WLEDs, which will greatly inspire us to design and synthesize fluorescent 3D COFs and facilitate the development of COF-based WLEDs in future. 3D covalent organic frameworks (COF) show interesting hierarchical arrangements of nanopores and open sites but their synthesis remains challenging. Here the authors report a fluorescent AIEgen-based 3D COF and demonstrate application as a coating material for white LEDs and for sensing of picric acid

Although polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesised directly from solution. One-dimensional (1D) covalent polymers that are packed into a framework structure can be viewed as a 1D covalent organic framework (COF), but making a single crystal of this has been elusive. Herein, by combining labile metal coordination and dynamic covalent chemistry, we discover a strategy to synthesise single-crystal metallo-COFs under solvothermal conditions. The single-crystal structure is rigorously solved using single-crystal electron diffraction technique. The non-centrosymmetric metallo-COF allows second harmonic generation. Due to the presence of syntactic pendant amine groups along the polymer chains, the metallopolymer crystal can be further cross-linked into a crystalline woven network. Although polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesized directly from solution. Here, the authors demonstrate a strategy to synthesize single crystalline 1D metallo-covalent organic frameworks by combining dynamic covalent chemistry and metal-ligand coordination.

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C 2 H 6 ) from ethylene (C 2 H 4 ), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C 2 H 4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C 2 H 6 adsorption capacity (110.4 cm 3 g −1 at 293 K and 1 bar) and good C 2 H 6 /C 2 H 4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C 2 H 6 . To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C 2 H 4 directly from a C 2 H 6 /C 2 H 4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation. The construction of three-dimensional covalent organic frameworks for gas separation is challenging due to the similar physicochemical properties of the gas mixture. Here, the authors report functional three-dimensional covalent organic frameworks by fine-tunning the pore environment with pyridine units to achieve effective separation of ethane from ethylene.