Phase-dependent growth of Pt on MoS2 for highly efficient H2 evolution.

Crystal phase is a key factor determining the properties, and hence functions, of two-dimensional transition-metal dichalcogenides (TMDs)1,2. The TMD materials, explored for diverse applications3-8, commonly serve as templates for constructing nanomaterials3,9 and supported metal catalysts4,6-8. However, how the TMD crystal phase affects the growth of the secondary material is poorly understood, although relevant, particularly for catalyst development. In the case of Pt nanoparticles on two-dimensional MoS2 nanosheets used as electrocatalysts for the hydrogen evolution reaction7, only about two thirds of Pt nanoparticles were epitaxially grown on the MoS2 template composed of the metallic/semimetallic 1T/1T' phase but with thermodynamically stable and poorly conducting 2H phase mixed in. Here we report the production of MoS2 nanosheets with high phase purity and show that the 2H-phase templates facilitate the epitaxial growth of Pt nanoparticles, whereas the 1T' phase supports single-atomically dispersed Pt (s-Pt) atoms with Pt loading up to 10 wt%. We find that the Pt atoms in this s-Pt/1T'-MoS2 system occupy three distinct sites, with density functional theory calculations indicating for Pt atoms located atop of Mo atoms a hydrogen adsorption free energy of close to zero. This probably contributes to efficient electrocatalytic H2 evolution in acidic media, where we measure for s-Pt/1T'-MoS2 a mass activity of 85 ± 23 A [Formula: see text] at the overpotential of -50 mV and a mass-normalized exchange current density of 127 A [Formula: see text] and we see stable performance in an H-type cell and prototype proton exchange membrane electrolyser operated at room temperature. Although phase stability limitations prevent operation at high temperatures, we anticipate that 1T'-TMDs will also be effective supports for other catalysts targeting other important reactions.

Solution-Processable and Printable Two-Dimensional Transition Metal Dichalcogenide Inks.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with layered crystal structures have been attracting enormous research interest for their atomic thickness, mechanical flexibility, and excellent electronic/optoelectronic properties for applications in diverse technological areas. Solution-processable 2D TMD inks are promising for large-scale production of functional thin films at an affordable cost, using high-throughput solution-based processing techniques such as printing and roll-to-roll fabrications. This paper provides a comprehensive review of the chemical synthesis of solution-processable and printable 2D TMD ink materials and the subsequent assembly into thin films for diverse applications. We start with the chemical principles and protocols of various synthesis methods for 2D TMD nanosheet crystals in the solution phase. The solution-based techniques for depositing ink materials into solid-state thin films are discussed. Then, we review the applications of these solution-processable thin films in diverse technological areas including electronics, optoelectronics, and others. To conclude, a summary of the key scientific/technical challenges and future research opportunities of solution-processable TMD inks is provided.

Phase Engineering of Nanomaterials: Transition Metal Dichalcogenides.

Crystal phase, a critical structural characteristic beyond the morphology, size, dimension, facet, etc., determines the physicochemical properties of nanomaterials. As a group of layered nanomaterials with polymorphs, transition metal dichalcogenides (TMDs) have attracted intensive research attention due to their phase-dependent properties. Therefore, great efforts have been devoted to the phase engineering of TMDs to synthesize TMDs with controlled phases, especially unconventional/metastable phases, for various applications in electronics, optoelectronics, catalysis, biomedicine, energy storage and conversion, and ferroelectrics. Considering the significant progress in the synthesis and applications of TMDs, we believe that a comprehensive review on the phase engineering of TMDs is critical to promote their fundamental studies and practical applications. This Review aims to provide a comprehensive introduction and discussion on the crystal structures, synthetic strategies, and phase-dependent properties and applications of TMDs. Finally, our perspectives on the challenges and opportunities in phase engineering of TMDs will also be discussed.

Recent Progress on Phase Engineering of Nanomaterials.

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Alkoxy Diazomethylation of Alkenes by Photoredox-Catalyzed Oxidative Radical-Polar Crossover.

Herein, we present the discovery and development of the first photoredox-catalyzed alkoxy diazomethylation of alkenes with hypervalent iodine reagents and alcohols. This multicomponent process represents a new disconnection approach to diazo compounds and is featured by a broad scope, mild reaction conditions, and excellent selectivity. Key to the process was the generation of diazomethyl radicals, which engaged alkenes and alcohols in an inter- and intramolecular fashion by a photoredox-catalyzed oxidative radical-polar crossover leading to unexplored ß-alkoxydiazo compounds. The synthetic utility of such diazo compounds was demonstrated with a series of transformations involving C-H, N-H, and O-H insertions as well as in the construction of complex sp3-rich heterocycles.

Association of blood cadmium concentration with chronic obstructive pulmonary disease progression: a prospective cohort study.

BACKGROUND: Prior studies in patients with chronic obstructive pulmonary disease (COPD) had indicated a potential correlation between cadmium (Cd) exposure and reduction in lung function. Nevertheless, the influence of Cd exposure on the progression of COPD remained unknown. Exploring the relationship between Cd exposure and the progression of COPD was the aim of this investigation. METHODS: Stable COPD patients were enrolled. Blood samples were collected and lung function was evaluated. Regular professional follow-ups were conducted through telephone communications, outpatient services, and patients' hospitalization records. RESULTS: Each additional unit of blood Cd was associated with upward trend in acute exacerbation, hospitalization, longer hospital stay, and death within 2 years. Even after adjusting for potential confounding factors, each 1 unit rise in blood Cd still correlated with a rise in the frequencies of acute exacerbation, longer hospital stay, and death. Moreover, COPD patients with less smoking amount, lower lung function and without comorbidities were more vulnerable to Cd-induced disease deterioration. CONCLUSION: Patients with COPD who have higher blood Cd concentration are susceptible to worse disease progression.

Monolayer atomic crystal molecular superlattices.

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

Reversible Semimetal-Semiconductor Transition of Unconventional-Phase WS2 Nanosheets.

Phase transition with band gap modulation of materials has gained intensive research attention due to its various applications, including memories, neuromorphic computing, and transistors. As a powerful strategy to tune the crystal phase of transition-metal dichalcogenides (TMDs), the phase transition of TMDs provides opportunities to prepare new phases of TMDs for exploring their phase-dependent property, function, and application. However, the previously reported phase transition of TMDs is mainly irreversible. Here, we report a reversible phase transition in the semimetallic 1T'-WS2 driven by proton intercalation and deintercalation, resulting in a newly discovered semiconducting WS2 with a novel unconventional phase, denoted as the 1T'd phase. Impressively, an on/off ratio of >106 has been achieved during the phase transition of WS2 from the semimetallic 1T' phase to the semiconducting 1T'd phase. Our work not only provides a unique insight into the phase transition of TMDs via proton intercalation but also opens up possibilities to tune their physicochemical properties for various applications.

On-Chip Investigation of Electrocatalytic Oxygen Reduction Reaction of 2D Materials.

The on-chip electrocatalytic microdevice (OCEM) is an emerging platform specialized in the electrochemical investigation of single-entity nanomaterials, which is ideal for probing the intrinsic catalytic properties, optimizing performance, and exploring exotic mechanisms. However, the current catalytic applications of OCEMs are almost exclusively in electrocatalytic hydrogen/oxygen evolution reactions with minimized influence from the mass transfer. Here, an OCEM platform specially tailored to investigate the electrocatalytic oxygen reduction reaction (ORR) at a microscopic level by introducing electrolyte convection through a microfluidic flow cell is reported. The setup is established on gold microelectrodes and later successfully applied to investigate how Ar-plasma treatment affects the ORR activities of 2H MoS2 . This study finds that Ar-plasma treatment significantly enhances the ORR performance of MoS2 nanosheets owing to the introduction of surface defects. This study paves the way for highly efficient microscopic investigation of diffusion-controlled electrocatalytic reactions.

Metastable 1T'-phase group VIB transition metal dichalcogenide crystals.

Metastable 1T'-phase transition metal dichalcogenides (1T'-TMDs) with semi-metallic natures have attracted increasing interest owing to their uniquely distorted structures and fascinating phase-dependent physicochemical properties. However, the synthesis of high-quality metastable 1T'-TMD crystals, especially for the group VIB TMDs, remains a challenge. Here, we report a general synthetic method for the large-scale preparation of metastable 1T'-phase group VIB TMDs, including WS2, WSe2, MoS2, MoSe2, WS2xSe2(1-x) and MoS2xSe2(1-x). We solve the crystal structures of 1T'-WS2, -WSe2, -MoS2 and -MoSe2 with single-crystal X-ray diffraction. The as-prepared 1T'-WS2 exhibits thickness-dependent intrinsic superconductivity, showing critical transition temperatures of 8.6 K for the thickness of 90.1 nm and 5.7 K for the single layer, which we attribute to the high intrinsic carrier concentration and the semi-metallic nature of 1T'-WS2. This synthesis method will allow a more systematic investigation of the intrinsic properties of metastable TMDs.

High-Yield Exfoliation of Ultrathin 2D Ni3 Cr2 P2 S9 and Ni3 Cr2 P2 Se9 Nanosheets.

Multinary layered 2D nanomaterials can exhibit distinct physicochemical properties and innovative applications as compared to binary 2D nanomaterials due to their unique crystal structures. However, it still remains a challenge for the high-yield preparation of high-quality multinary 2D nanosheets. Here, the high-yield and large-scale production of two quaternary metal thiophosphate nanosheets are reported, i.e., Ni3 Cr2 P2 S9 and Ni3 Cr2 P2 Se9 , via the liquid exfoliation of their layered bulk crystals. The exfoliated single-crystalline Ni3 Cr2 P2 S9 nanosheets, with a lateral size ranging from a few hundred nanometers to a few micrometers and thickness of 1.4 ± 0.2 nm, can be easily used to prepare flexible thin films via a simple vacuum filtration process. As a proof-of-concept application, the fabricated thin film is used as a supercapacitor electrode with good specific capacitance. These high-yield, large-scale, solution-processable quaternary metal thiophosphate nanosheets could also be promising in other applications like biosensors, cancer therapies, and flexible electronics.

On-chip electrocatalytic microdevice: an emerging platform for expanding the insight into electrochemical processes.

Electrochemical conversion is an important process in renewable energy conversion, and electrocatalysts play a vital role since they can improve the rate and efficiency of chemical transformations. Thus, the continuing interest in electrocatalysis is fueled both in terms of mechanism exploration and performance optimization, and this field is continuously being updated. However, conventional electrochemical methods still have room to be explored, such as in situ dynamic monitoring, external field regulation, and single-entity electrocatalytic detection. Noteworthily, inspired by the recent success in nanoelectronic semiconductor devices, the emerging field of on-chip electrocatalytic microdevices, focusing on the electrochemical behaviors at individual nanowire/nanosheet as the working electrode, has emerged as a powerful alternative platform to the traditional techniques. This unique device configuration enables several advantages, such as in situ electronic/electrochemical measurements and adjustable microstructure of individual catalysts, which is constantly expanded to directly probe electrochemical processes to obtain previously inaccessible information. Hence, herein, we first introduce the device configuration and its advantages as an emerging platform. Subsequently, the attempts to expand the insight into electrochemical processes through this type of microdevice are explicitly analyzed and summarized including dynamic monitoring, external field regulation, identification of active sites, and single structural factor regulation. Finally, some personal perspectives on the challenges and future research directions in this promising area are also presented. We believe that this review will provide new insight into electrochemical processes, ranging from dynamic exploration to performance optimization.

Palladium-Catalyzed Site-Selective [3+2] Annulation via Benzylic and meta C-H Bond Activation.

The palladium-catalyzed [3+2] annulation of aromatic amides with maleimides via the activation of ortho benzylic C-H and meta C-H bonds is reported. Carboxamide and anilide type substrates that contain a 2-methylthiophenyl group both participate in this [3+2] annulation, indicating that the presence of a 2-methylthiophenyl directing group is a key for the success of the reaction. The first C-H bond activation at the benzylic C-H bond is followed by a second C-H bond activation at the meta C-H bond to give five-membered cyclic products. The cleavage of these C-H bonds is irreversible.

Self-gating in semiconductor electrocatalysis.

The semiconductor-electrolyte interface dominates the behaviours of semiconductor electrocatalysis, which has been modelled as a Schottky-analogue junction according to classical electron transfer theories. However, this model cannot be used to explain the extremely high carrier accumulations in ultrathin semiconductor catalysis observed in our work. Inspired by the recently developed ion-controlled electronics, we revisit the semiconductor-electrolyte interface and unravel a universal self-gating phenomenon through microcell-based in situ electronic/electrochemical measurements to clarify the electronic-conduction modulation of semiconductors during the electrocatalytic reaction. We then demonstrate that the type of semiconductor catalyst strongly correlates with their electrocatalysis; that is, n-type semiconductor catalysts favour cathodic reactions such as the hydrogen evolution reaction, p-type ones prefer anodic reactions such as the oxygen evolution reaction and bipolar ones tend to perform both anodic and cathodic reactions. Our study provides new insight into the electronic origin of the semiconductor-electrolyte interface during electrocatalysis, paving the way for designing high-performance semiconductor catalysts.

In Situ Probing Molecular Intercalation in Two-Dimensional Layered Semiconductors.

The electrochemical molecular intercalation of two-dimensional layered materials (2DLMs) produces stable and highly tunable superlattices between monolayer 2DLMs and self-assembled molecular layers. This process allows unprecedented flexibility in integrating highly distinct materials with atomic/molecular precision to produce a new generation of organic/inorganic superlattices with tunable chemical, electronic, and optical properties. To better understand the intercalation process, we developed an on-chip platform based on MoS2 model devices and used optical, electrochemical, and in situ electronic characterizations to resolve the intermediate stages during the intercalation process and monitor the evolution of the molecular superlattices. With sufficient charge injection, the organic cetyltrimethylammonium bromide (CTAB) intercalation induces the phase transition of MoS2 from semiconducting 2H phase to semimetallic 1T phase, resulting in a dramatic increase of electrical conductivity. Therefore, in situ monitoring the evolution of the device conductance reveals the electrochemical intercalation dynamics with an abrupt conductivity change, signifying the onset of the molecule intercalation. In contrast, the intercalation of tetraheptylammonium bromide (THAB), a branched molecule in a larger size, resulting in a much smaller number of charges injected to avoid the 2H to 1T phase transition. Our study demonstrates a powerful platform for in situ monitoring the molecular intercalation of many 2DLMs (MoS2, WSe2, ReS2, PdSe2, TiS2, and graphene) and systematically probing electronic, optical, and optoelectronic properties at the single-nanosheet level.

Strong Charge Transfer at 2H-1T Phase Boundary of MoS2 for Superb High-Performance Energy Storage.

Transition metal dichalcogenides exhibit several different phases (e.g., semiconducting 2H, metallic 1T, 1T') arising from the collective and sluggish atomic displacements rooted in the charge-lattice interaction. The coexistence of multiphase in a single sheet enables ubiquitous heterophase and inhomogeneous charge distribution. Herein, by combining the first-principles calculations and experimental investigations, a strong charge transfer ability at the heterophase boundary of molybdenum disulfide (MoS2 ) assembled together with graphene is reported. By modulating the phase composition in MoS2 , the performance of the nanohybrid for energy storage can be modulated, whereby remarkable gravimetric and volumetric capacitances of 272 F g-1 and 685 F cm-3 are demonstrated. As a proof of concept for energy application, a flexible solid-state asymmetric supercapacitor is constructed with the MoS2 -graphene heterolayers, which shows superb energy and power densities (46.3 mWh cm-3 and 3.013 W cm-3 , respectively). The present work demonstrates a new pathway for efficient charge flow and application in energy storage by engineering the phase boundary and interface in 2D materials of transition metal dichalcogenides.

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.

A Synthesis of 3,4-Dihydroisoquinolin-1(2H)-one via the Rhodium-Catalyzed Alkylation of Aromatic Amides with N-Vinylphthalimide.

The alkylation of C-H bonds with N-vinylphthalimide by a rhodium-catalyzed reaction of aromatic amides containing an 8-aminoquinoline moiety as the directing group is reported. N-Vinylphthalimide functions as a 2-aminoethylating reagent. The resulting alkylated products can be converted into 3,4-dihydroisoquinolin-1(2H)-one derivatives in a one-pot transformation. Deuterium-labeling experiments suggest that the reaction proceeds through a carbene mechanism.

Two-dimensional nanomaterial-based field-effect transistors for chemical and biological sensing.

Meeting the increasing demand for sensors with high sensitivity, high selectivity, and rapid detection presents many challenges. In the last decade, electronic sensors based on field-effect transistors (FETs) have been widely studied due to their high sensitivity, rapid detection, and simple test procedure. Among these sensors, two-dimensional (2D) nanomaterial-based FET sensors have been demonstrated with tremendous potential for the detection of a wide range of analytes which is attributed to the unique structural and electronic properties of 2D nanomaterials. This comprehensive review discusses the recent progress in graphene-, 2D transition metal dichalcogenide-, and 2D black phosphorus-based FET sensors, with an emphasis on rapid and low-concentration detection of gases, biomolecules, and water contaminants.

Vertical Charge Transport and Negative Transconductance in Multilayer Molybdenum Disulfides.

Negative transconductance (NTC) devices have been heavily investigated for their potential in low power logical circuit, memory, oscillating, and high-speed switching applications. Previous NTC devices are largely attributed to two working mechanisms: quantum mechanical tunneling, and mobility degradation at high electrical field. Herein we report a systematic investigation of charge transport in multilayer two-dimensional semiconductors (2DSCs) with optimized van der Waals contact and for the first time demonstrate NTC and antibipolar characteristics in multilayer 2DSCs (such as MoS2, WSe2). By varying the measurement temperature, bias voltage, and body thickness, we found the NTC behavior can be attributed to a vertical potential barrier in the multilayer 2DSCs and the competing mechanisms between intralayer lateral transport and interlayer vertical transport, thus representing a new working mechanism for NTC operation. Importantly, this vertical potential barrier arises from inhomogeneous carrier distribution in 2DSC from the near-substrate region to the bulk region, which is in contrast to conventional semiconductors with homogeneous doping defined by bulk dopants. We further show that the unique NTC behavior can be explored for creating frequency doublers and phase shift keying circuits with only one transistor, greatly simplifying the circuit design compared to conventional technology.