Exfoliation of Metal-Organic Frameworks to Give 2D MOF Nanosheets for the Electrocatalytic Oxygen Evolution Reaction.

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond-breaking strategy is demonstrated for synthesizing ultrathin metal-organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs' structure due to the instant temperature drop and drastic expansion of liquid N2, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.

Mechanochemical Lithography.

Surfaces with patterned biomolecules have wide applications in biochips and biomedical diagnostics. However, most patterning methods are inapplicable to physiological conditions and incapable of creating complex structures. Here, we develop a mechanochemical lithography (MCL) method based on compressive force-triggered reactions. In this method, biomolecules containing a bioaffinity ligand and a mechanoactive group are used as mechanochemical inks (MCIs). The bioaffinity ligand facilitates concentrating MCIs from surrounding solutions to a molded surface, enabling direct and continuous printing in an aqueous environment. The mechanoactive group facilitates covalent immobilization of MCIs through force-triggered reactions, thus avoiding the broadening of printed features due to the diffusion of inks. We discovered that the ubiquitously presented amino groups in biomolecules can react with maleimide through a force-triggered Michael addition. The resulting covalent linkage is mechanically and chemically stable. As a proof-of-concept, we fabricate patterned surfaces of biotin and His-tagged proteins at nanoscale spatial resolution by MCL and verify the resulting patterns by fluorescence imaging. We further demonstrated the creation of multiplex protein patterns using this technique.

Dip-Pen Nanolithography(DPN): from Micro/Nano-patterns to Biosensing.

Dip-pen nanolithography is an emerging and attractive surface modification technique that has the capacity to directly and controllably write micro/nano-array patterns on diverse substrates. The superior throughput, resolution, and registration enable DPN an outstanding candidate for biological detection from the molecular level to the cellular level. Herein, we overview the technological evolution of DPN in terms of its advanced derivatives and DPN-enabled versatile sensing patterns featuring multiple compositions and structures for biosensing. Benefitting from uniform, reproducible, and large-area array patterns, DPN-based biosensors have shown high sensitivity, excellent selectivity, and fast response in target analyte detection and specific cellular recognition. We anticipate that DPN-based technologies could offer great potential opportunities to fabricate multiplexed, programmable, and commercial array-based sensing biochips.

Modifiers versus Channels: Creating Shape-Selective Catalysis of Metal Nanoparticles/Porous Nanomaterials.

Shape-selective catalysis plays a key role in chemical synthesis. Porous nanomaterials with uniform pore structures are ideal supports for metal nanoparticles (MNPs) to generate efficient shape-selective catalysis. However, many commercial irregular porous nanomaterials face the challenge to realize satisfactory shape selectivity due to the lack of molecular sieving structures. Herein, we report a concept of creating shape selectivity in MNPs/porous nanomaterials through intentionally poisoning certain MNPs using suitable modifiers. The remaining MNPs within the substrates can cooperate with the channels to generate selectivity. Such a strategy not only applies to regular porous nanomaterials (such as MOFs, zeolites) but also extended to irregular porous nanomaterials (such as active carbon, P25). Potentially, the matching among different MNPs, corresponding modifiers, and porous nanomaterials makes our strategy promising in selective catalytic systems.

Metal-Organic Frameworks as Metal Ion Precursors for the Synthesis of Nanocomposites for Lithium-Ion Batteries.

Metal-organic frameworks (MOFs) are promising materials with fascinating properties. Their widespread applications are sometimes hindered by the intrinsic instability of frameworks. However, this instability of MOFs can also be exploited for useful purposes. Herein, we report the use of MOFs as metal ion precursors for constructing functional nanocomposites by utilizing the instability of MOFs. The heterogeneous growth process of nanostructures on substrates involves the release of metal ions, nucleation on substrates, and formation of a covering structure. Specifically, the synthesized CoS with carbon nanotubes as substrates display enhanced performance in a lithium-ion battery. Such strategy not only presents a new way for exploiting the instability of MOFs but also supplies a prospect for designing versatile functional nanocomposites.

Crystal-Growth-Dominated Fabrication of Metal-Organic Frameworks with Orderly Distributed Hierarchical Porosity.

Constructing architectures with hierarchical porosity has been widely considered as the most efficient way to bypass the problems related to slow mass transfer and inaccessibility of internal space in MOFs. Now, a crystal-growth-dominated strategy is proposed to fabricate hierarchically porous MOFs (HP-MOFs). When the crystal growth is dominated by the monomer attachment, the aggregation of nonionic surfactant or polymer can be easily captured and released during the crystal growth process, resulting in the formation and ordering hierarchical pores along the radial direction. Owing to the accelerated mass diffusion and more exposed active sites of this design, HP-MOFs exhibited an enhanced catalytic efficiency in styrene oxidation.

Multiple Active Sites of Carbon for High-Rate Surface-Capacitive Sodium-Ion Storage.

Although sodium ion batteries (SIBs) possess many beneficial features, their rate performance, cycling stability, and safety need improvement for commercial applications. Based on the mechanisms of the sodium ions storage in carbon materials, herein we present a multiple active sites decorated amorphous carbon (MAC) with rich structural defects and heteroatom doping as an anode material for SIBs. The full utilization of fast bonding-debonding processes between the active sites and sodium ions could bring a capacitive strategy to achieve superior sodium storage properties. Consequently, after materials characterization and electrochemical evaluation, the as-prepared electrode could deliver high rate and long-life performance. This active-site-related design could be extended to other types of electrode materials, thereby contributing to future practical SIB applications.

High-Precision Size Recognition and Separation in Synthetic 1D Nanochannels.

Covalent organic frameworks (COFs) allow elaborate manufacture of ordered one-dimensional channels in the crystal. We defined a superlattice of COFs by engineering channels with a persistent triangular shape and discrete pore size. We observed a size-recognition regime that is different from the characteristic adsorption of COFs, whereby pore windows and walls were cooperative so that triangular apertures sorted molecules of one-atom difference and notch nanogrooves confined them into single-file molecular chains. The recognition and confinement were accurately described by sensitive spectroscopy and femtosecond dynamic simulations. The resulting COFs enabled instantaneous separation of mixtures at ambient temperature and pressure. This study offers an approach to merge precise recognition, selective transport, and instant separation in synthetic 1D channels.

Recent advances in understanding of the mechanism and control of Li2O2 formation in aprotic Li-O2 batteries.

Aprotic Li-O2 batteries represent promising alternative devices for electrical energy storage owing to their extremely high energy densities. Upon discharge, insulating solid Li2O2 forms on cathode surfaces, which is usually governed by two growth models, namely the solution model and the surface model. These Li2O2 growth models can largely determine the battery performances such as the discharge capacity, round-trip efficiency and cycling stability. Understanding the Li2O2 formation mechanism and controlling its growth are essential to fully realize the technological potential of Li-O2 batteries. In this review, we overview the recent advances in understanding the electrochemical and chemical processes that occur during the Li2O2 formation. In the beginning, the oxygen reduction mechanisms, the identification of O2-/LiO2 intermediates, and their influence on the Li2O2 morphology have been discussed. The effects of the discharge current density and potential on the Li2O2 growth model have been subsequently reviewed. Special focus is then given to the prominent strategies, including the electrolyte-mediated strategy and the cathode-catalyst-tailoring strategy, for controlling the Li2O2 growth pathways. Finally, we conclude by discussing the profound implications of controlling Li2O2 formation for further development in Li-O2 batteries.

Correction: Recent advances in understanding of the mechanism and control of Li2O2 formation in aprotic Li-O2 batteries.

Correction for 'Recent advances in understanding of the mechanism and control of Li2O2 formation in aprotic Li-O2 batteries' by Zhiyang Lyu et al., Chem. Soc. Rev., 2017, DOI: 10.1039/c7cs00255f.

Surface Functionalization of Black Phosphorus via Potassium toward High-Performance Complementary Devices.

Two-dimensional black phosphorus configured field-effect transistor devices generally show a hole-dominated ambipolar transport characteristic, thereby limiting its applications in complementary electronics. Herein, we demonstrate an effective surface functionalization scheme on few-layer black phosphorus, through in situ surface modification with potassium, with a view toward high performance complementary device applications. Potassium induces a giant electron doping effect on black phosphorus along with a clear bandgap reduction, which is further corroborated by in situ photoelectron spectroscopy characterizations. The electron mobility of black phosphorus is significantly enhanced to 262 (377) cm2 V-1 s-1 by over 1 order of magnitude after potassium modification for two-terminal (four-terminal) measurements. Using lithography technique, a spatially controlled potassium doping technique is developed to establish high-performance complementary devices on a single black phosphorus nanosheet, for example, the p-n homojunction-based diode achieves a near-unity ideality factor of 1.007 with an on/off ratio of ∼104. Our findings coupled with the tunable nature of in situ modification scheme enable black phosphorus as a promising candidate for further complementary electronics.

Metal-Organic Framework Wears a Protective Cover for Improved Stability.

Metal-organic frameworks (MOFs) with an ordered channel and porosity show great promise for a myriad of purposes. Unfortunately, the coordination bond of metal ions and organic ligands easily weakens in unfavorable environments, which poses a key problem in expanding the application of MOFs. Herein, we report a general and efficient strategy to enhance the stability and preserve the porosity of MOFs by coating them with reduced graphene oxide (rGO). The prepared hybrid material consisted of MOFs and rGO, as the core and the protective shell, respectively. It is worth noting that the obtained MOFs@rGO composite material maintained a well-defined crystal structure and showed good catalytic activity as well as enhanced stability. Notably, this novel and general method of coating MOFs with a thin protective rGO shell will broaden the application fields of MOFs and open up a new avenue for the research of MOFs.

Multi-shelled Hollow Metal-Organic Frameworks.

Hollow metal-organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.

Alcohol-Mediated Resistance-Switching Behavior in Metal-Organic Framework-Based Electronic Devices.

Metal-organic frameworks (MOFs) have drawn increasing attentions as promising candidates for functional devices. Herein, we present MOF films in constructing memory devices with alcohol mediated resistance switching property, where the resistance state is controlled by applying alcohol vapors to achieve multilevel information storage. The ordered packing mode and the hydrogen bonding system of the guest molecules adsorbed in MOF crystals are shown to be the reason for the alcohol mediated electrical switching. This chemically mediated memory device can be a candidate in achieving environment-responsive devices and exhibits potential applications in wearable information storage systems.

Well-Dispersed and Size-Controlled Supported Metal Oxide Nanoparticles Derived from MOF Composites and Further Application in Catalysis.

Supported metal oxide nanoparticles are important in heterogeneous catalysis; however, the ability to tailor their size, structure, and dispersion remains a challenge. A strategy to achieve well-dispersed and size-controlled supported metal oxides through the manageable growth of a metal organic framework (Cu-BTC) on TiO2 followed by pyrolysis is described.

Centimeter-scale subwavelength photolithography using metal-coated elastomeric photomasks with modulated light intensity at the oblique sidewalls.

By coating polydimethylsiloxane (PDMS) relief structures with a layer of opaque metal such as gold, the incident light is strictly allowed to pass through the nanoscopic apertures at the sidewalls of PDMS reliefs to expose underlying photoresist at nanoscale regions, thus producing subwavelength nanopatterns covering centimeter-scale areas. It was found that the sidewalls were a little oblique, which was the key to form the nanoscale apertures. Two-sided and one-sided subwavelength apertures can be constructed by employing vertical and oblique metal evaporation directions, respectively. Consequently, two-line and one-line subwavelength nanopatterns with programmable feature shapes, sizes, and periodicities could be produced using the obtained photomasks. The smallest aperture size and line width of 80 nm were achieved. In contrast to the generation of raised positive photoresist nanopatterns in phase shifting photolithography, the recessed positive photoresist nanopatterns produced in this study provide a convenient route to transfer the resist nanopatterns to metal nanopatterns. This nanolithography methodology possesses the distinctive advantages of simplicity, low cost, high throughput, and nanoscale feature size and shape controllability, making it a potent nanofabrication technique to enable functional nanostructures for various potential applications.

Parallel near-field photolithography with metal-coated elastomeric masks.

Developing a cost-effective nanolithography strategy that enables the production of subwavelength features with various shapes over large areas is a long-standing goal in the nanotechnology community. Herein, an inexpensive nanolithographic technique that combines the wafer-scale production capability of photolithography with the subwavelength feature size controllability of near-field photolithography was developed to fabricate centimeter-scale up to wafer-scale sub-100-nm variously shaped nanopatterns on surfaces. The wafer-scale elastomeric trench-based photomasks with subwavelength apertures created at the apexes were compatible with mask aligners, allowing for the production of wafer-scale subwavelength nanopatterns with adjustable feature sizes, shapes, and periodicities. The smallest feature sizes of 50 and 80 nm were achieved on positive tone and negative tone photoresist surfaces, respectively, which could be ascribed to a near-field optical effect. The fabricated centimeter-scale nanopatterns were functionalized to study cell-matrix adhesion and migration. Compared to currently developed nanolithographic methods that approach similar functionalities, this facile nanolithographic strategy combines the merits of low cost, subwavelength feature size, high throughput, and varied feature shapes, making it an affordable approach to be used in academic research for researchers at most institutions.

Hybrid crystals comprising metal-organic frameworks and functional particles: synthesis and applications.

Hybrid crystals containing encapsulated functional species exhibit promising novel physical and chemical properties. The realization of many properties critically depends on the selection of suitable functional species for incorporation, the rational control of the crystallinity of the host materials, and the manipulation of the distribution of the encapsulated species; only a few hybrid crystals achieve this. Here, a novel synthetic method enables the encapsulation of functional species within crystalline metal-organic frameworks (MOFs). Various kinds of single-crystalline MOFs with incorporated particles are presented. The encapsulated particles can be distributed in a controllable manner, and the hybrid crystals are applied to the heterogeneous catalysis of the reduction of nitroarenes. These findings suggest a general approach for the construction of MOF materials with potential applications; by combining species and MOFs with suitable functionalities, new properties--not possible by other means--may arise.

Self-assembled metal-organic frameworks crystals for chemical vapor sensing.

A 3D metal-organic frameworks (MOFs) crystals film is obtained via Langmuir-Blodgett technique and used as a photonic sensor for chemical vapor detection. The MOFs crystals film exhibits both acute responses towards various chemical vapors and high controllability in terms of peak intensity and position. The method represents a general, facile and flexible strategy for the fabrication of MOFs-based photonic sensors.

Microstructured graphene arrays for highly sensitive flexible tactile sensors.

A highly sensitive tactile sensor is devised by applying microstructured graphene arrays as sensitive layers. The combination of graphene and anisotropic microstructures endows this sensor with an ultra-high sensitivity of -5.53 kPa(-1) , an ultra-fast response time of only 0.2 ms, as well as good reliability, rendering it promising for the application of tactile sensing in artificial skin and human-machine interface.