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.

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.

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.

Isoreticular Series of Two-Dimensional Covalent Organic Frameworks with the kgd Topology and Controllable Micropores.

Two-dimensional (2D) covalent organic frameworks (COFs) possess designable pore architectures but limited framework topologies. Until now, 2D COFs adopting the kgd topology with ordered and rhombic pore geometry have rarely been reported. Here, an isoreticular series of 2D COFs with the kgd topology and controllable pore size is synthesized by employing a C6-symmetric aldehyde, i.e., hexa(4-formylphenyl)benzene (HFPB), and C3-symmetric amines i.e., tris(4-aminophenyl)amine (TAPA), tris(4-aminophenyl)trazine (TAPT), and 1,3,5-tris[4-amino(1,1-biphenyl-4-yl)]benzene (TABPB), as building units, referred to as HFPB-TAPA, HFPB-TAPT, and HFPB-TABPB, respectively. The micropore dimension down to 6.7 Å is achieved in HFPB-TAPA, which is among the smallest pore size of reported 2D COFs. Impressively, both the in-plane network and stacking sequence of the 2D COFs can be clearly observed by low-dose electron microscopy. Integrating the unique kgd topology with small rhombic micropores, these 2D COFs are endowed with both short molecular diffusion length and favorable host-guest interaction, exhibiting potential for drug delivery with high loading and good release control of ibuprofen.

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.

Seeded Synthesis of Unconventional 2H-Phase Pd Alloy Nanomaterials for Highly Efficient Oxygen Reduction.

Crystal phase engineering of noble-metal-based alloy nanomaterials paves a new way to the rational synthesis of high-performance catalysts for various applications. However, the controlled preparation of noble-metal-based alloy nanomaterials with unconventional crystal phases still remains a great challenge due to their thermodynamically unstable nature. Herein, we develop a robust and general seeded method to synthesize PdCu alloy nanomaterials with unconventional hexagonal close-packed (hcp, 2H type) phase and also tunable Cu contents. Moreover, galvanic replacement of Cu by Pt can be further conducted to prepare unconventional trimetallic 2H-PdCuPt nanomaterials. Impressively, 2H-Pd67Cu33 nanoparticles possess a high mass activity of 0.87 A mg-1Pd at 0.9 V (vs reversible hydrogen electrode (RHE)) in electrochemical oxygen reduction reaction (ORR) under alkaline condition, which is 2.5 times that of the conventional face-centered cubic (fcc) Pd69Cu31 counterpart, revealing the important role of crystal phase on determining the ORR performance. After the incorporation of Pt, the obtained 2H-Pd71Cu22Pt7 catalyst shows a significantly enhanced mass activity of 1.92 A mg-1Pd+Pt at 0.9 V (vs RHE), which is 19.2 and 8.7 times those of commercial Pt/C and Pd/C, placing it among the best reported Pd-based ORR electrocatalysts under alkaline conditions.

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.

Ethylene Selectivity in Electrocatalytic CO2 Reduction on Cu Nanomaterials: A Crystal Phase-Dependent Study.

The crystal phase of metal nanocatalysts significantly affects their catalytic performance. Cu-based nanomaterials are unique electrocatalysts for CO2 reduction reaction (CO2RR) to produce high-value hydrocarbons. However, studies to date are limited to the conventional face-centered cubic (fcc) Cu. Here, we report a crystal phase-dependent catalytic behavior of Cu, after the successful synthesis of high-purity 4H Cu and heterophase 4H/fcc Cu using the 4H and 4H/fcc Au as templates, respectively. Remarkably, the obtained unconventional crystal structures of Cu exhibit enhanced overall activity and higher ethylene (C2H4) selectivity in CO2RR compared to the fcc Cu. Density functional theory calculations suggest that the 4H phase and 4H/fcc interface of Cu favor the C2H4 formation pathway compared to the fcc Cu, leading to the crystal phase-dependent C2H4 selectivity. This study demonstrates the importance of crystal phase engineering of metal nanocatalysts for electrocatalytic reactions, offering a new strategy to prepare novel catalysts with unconventional phases for various applications.

Intramolecular Hydrogen Bonding-Based Topology Regulation of Two-Dimensional Covalent Organic Frameworks.

Creating molecular networks with different topologies using identical molecular linkers is fundamentally important but requires precise chemistry control. Here, we propose an effective strategy to regulate the network topologies of two-dimensional (2D) covalent organic frameworks (COFs) through the conformational switching of molecular linkages. By simply altering the substituents of an identical molecular linker, the topology-selective synthesis of two highly crystalline 2D COFs can be readily achieved. Their distinct crystal structures are observed and determined by low-dose, high-resolution transmission electron microscopy imaging, indicating that the driving force for linkage conformation switching is intramolecular hydrogen bonding. Our strategy would greatly diversify the COF topologies and enable vast postsynthetic modifications such as boron complexation, endowing these structures with a unique optical property such as fluorescence turn on and aggregation-induced emission.

Elemental Segregation in Multimetallic Core-Shell Nanoplates.

In this work, we report an element segregation phenomenon in two-dimensional (2D) core-shell nanoplates, subsequently resulting in the formation of yolk-cage nanostructures after selective electrochemical etching. By using PtCu nanoplates as templates, PtCu@Pd core-shell nanoplates are formed. Interestingly, during the growth of Ru on the PtCu@Pd core-shell nanoplates, due to the selective element diffusion, PtCuPd@PdCu@Ru nanoplates are obtained. After selectively etching of PdCu in PtCuPd@PdCu@Ru using electrochemical method, the PtCuPd@Ru yolk-cage nanostructures are obtained. As a proof-of-concept application, this unique nanostructure shows superior electrocatalytic activity and stability toward the methanol oxidation reaction as compared to the PtCu nanoplates and commercial Pt/C catalyst.

Novel structured transition metal dichalcogenide nanosheets.

Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their unique properties and great potential in a wide range of applications. Great efforts have been devoted to the preparation of novel-structured TMD nanosheets by engineering their intrinsic structures at the atomic scale. Until now, a lot of new-structured TMD nanosheets, such as vacancy-containing TMDs, heteroatom-doped TMDs, TMD alloys, 1T'/1T phase and in-plane TMD crystal-phase heterostructures, TMD heterostructures and Janus TMD nanosheets, have been prepared. These materials exhibit unique properties and hold great promise in various applications, including electronics/optoelectronics, thermoelectrics, catalysis, energy storage and conversion and biomedicine. This review focuses on the most recent important discoveries in the preparation, characterization and application of these new-structured ultrathin 2D layered TMDs.

Preparation of 1T'-Phase ReS2 xSe2(1- x) ( x = 0-1) Nanodots for Highly Efficient Electrocatalytic Hydrogen Evolution Reaction.

As a source of clean energy, a reliable hydrogen evolution reaction (HER) requires robust and highly efficient catalysts. Here, by combining chemical vapor transport and Li-intercalation, we have prepared a series of 1T'-phase ReS2 xSe2(1- x) ( x = 0-1) nanodots to achieve high-performance HER in acid medium. Among them, the 1T'-phase ReSSe nanodot exhibits the highest hydrogen evolution activity, with a Tafel slope of 50.1 mV dec-1 and a low overpotential of 84 mV at current density of 10 mA cm-2. The excellent hydrogen evolution activity is attributed to the optimal hydrogen absorption energy of the active site induced by the asymmetric S vacancy in the highly asymmetric 1T' crystal structure.

Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets: Preparation and Application in Highly Sensitive and Selective DNA Detection.

The ability to prepare ultrathin two-dimensional (2D) covalent organic framework (COF) nanosheets (NSs) in high yield is of great importance for the further exploration of their unique properties and potential applications. Herein, by elaborately designing and choosing two flexible molecules with C3v molecular symmetry as building units, a novel imine-linked COF, namely, TPA-COF, with a hexagonal layered structure and sheet-like morphology, is synthesized. Since the flexible building units are integrated into the COF skeletons, the interlayer stacking becomes weak, resulting in the easy exfoliation of TPA-COF into ultrathin 2D NSs. Impressively, for the first time, the detailed structural information, i.e., the pore channels and individual building units in the NSs, is clearly visualized by using the recently developed low-dose imaging technique of transmission electron microscopy (TEM). As a proof-of-concept application, the obtained ultrathin COF NSs are used as a novel fluorescence sensing platform for the highly sensitive and selective detection of DNA.

Single-Layer Ternary Chalcogenide Nanosheet as a Fluorescence-Based "Capture-Release" Biomolecular Nanosensor.

The novel application of two-dimensional (2D) single-layer ternary chalcogenide nanosheets as "capture-release" fluorescence-based biomolecular nanosensors is demonstrated. Fluorescently labeled biomolecular probe is first captured by the ultrathin Ta2 NiS5 nanosheets and then released upon adding analyte containing a target biomolecule due to the higher probe-target affinity. Here, the authors use a nucleic acid probe for the model target biomolecule Plasmodium lactate dehydrogenase, which is an important malarial biomarker. The ultrathin Ta2 NiS5 nanosheet serves as a highly efficient fluorescence quencher and the nanosensor developed from the nanosheet is highly sensitive and specific toward the target biomolecule. Apart from the specificity toward the target biomolecule in homogeneous solutions, the developed nanosensor is capable of detecting and differentiating the target in heterogeneous solutions consisting of either a mixture of biomolecules or serum, with exceptional specificity. The simplicity of the "capture-release" method, by eliminating the need for preincubation of the probe with the test sample, may facilitate further development of portable and rapid biosensors. The authors anticipate that this ternary chalcogenide nanosheet-based biomolecular nanosensor will be useful for the rapid detection and differentiation of a wide range of chemical and biological species.

Ternary Chalcogenide Nanosheets with Ultrahigh Photothermal Conversion Efficiency for Photoacoustic Theranostics.

2D materials (TDMs) have been explored for photonic theranostics. To achieve deep-tissue penetration, near-infrared (NIR) light is essential for photoacoustic (PA) theranostics. However, because the absorption profiles of existing TDMs are generally featureless with no obvious NIR absorption peaks, their PA signals and therapeutic efficacies are limited. This paper herein reports the synthesis and application of ternary chalcogenide nanosheets (Ta2 NiS5 -P) for PA theranostics. In contrast to the current TDMs for such application, Ta2 NiS5 -P has a ternary instead of binary composition. This difference brings in the strong and featured NIR for Ta2 NiS5 -P. To the best of the knowledge, this is the first example using ternary chalcogenide nanosheets for such application; moreover, the photothermal conversion efficiency of Ta2 NiS5 -P is the highest (35%) among all the reported TDMs based on the same calculation method. These advantages allow Ta2 NiS5 -P to passively target, effectively delineate, and completely eradicate the tumor of living mice after systemic administration.

Preparation of Ultrathin Two-Dimensional Tix Ta1-x Sy Oz Nanosheets as Highly Efficient Photothermal Agents.

Although two-dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high-yield preparation of solution-processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, Tix Ta1-x Sy Oz (x=0.71, 0.49, and 0.30), from Tix Ta1-x S2 precursors. The nanosheet exhibits strong absorbance in the near-infrared region, giving a large extinction coefficient of 54.1 L g-1 cm-1 at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid-conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets.

Preparation of Single-Layer MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 Nanosheets with High-Concentration Metallic 1T Phase.

The high-yield and scalable production of single-layer ternary transition metal dichalcogenide nanosheets with ≈66% of metallic 1T phase, including MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 is achieved via electrochemical Li-intercalation and the exfoliation method. Thin film MoS(2x)Se2(1- x) nanosheets drop-cast on a fluorine-doped tin oxide substrate are used as an efficient electrocatalyst on the counter electrode for the tri-iodide reduction in a dye-sensitized solar cell.

Solution-Processed Two-Dimensional MoS2 Nanosheets: Preparation, Hybridization, and Applications.

As one member of the emerging class of ultrathin two-dimensional (2D) transition-metal dichalcogenide (TMD) nanomaterials, the ultra-thin MoS2 nanosheet has attracted increasing research interest as a result of its unique structure and fascinating properties. Solution-phase methods are promising for the scalable production, functionalization, hybridization of MoS2 nanosheets, thus enabling the widespread exploration of MoS2 -based nanomaterials for various promising applications. In this Review, an overview of the recent progress of solution-processed MoS2 nanosheets is presented, with the emphasis on their synthetic strategies, functionalization, hybridization, properties, and applications. Finally, the challenges and opportunities in this research area will be proposed.

A facile and universal top-down method for preparation of monodisperse transition-metal dichalcogenide nanodots.

Despite unique properties of layered transition-metal dichalcogenide (TMD) nanosheets, there is still lack of a facile and general strategy for the preparation of TMD nanodots (NDs). Reported herein is the preparation of a series of TMD NDs, including TMD quantum dots (e.g. MoS2 , WS2 , ReS2 , TaS2 , MoSe2 and WSe2 ) and NbSe2 NDs, from their bulk crystals by using a combination of grinding and sonication techniques. These NDs could be easily separated from the N-methyl-2-pyrrolidone when post-treated with n-hexane and then chloroform. All the TMD NDs with sizes of less than 10 nm show a narrow size distribution with high dispersity in solution. As a proof-of-concept application, memory devices using TMD NDs, for example, MoSe2 , WS2 , or NbSe2 , mixed with polyvinylpyrrolidone as active layers, have been fabricated, which exhibit a nonvolatile write-once-read-many behavior. These high-quality TMD NDs should have various applications in optoelectronics, solar cells, catalysis, and biomedicine.