Phase-selective synthesis of 1T' MoS2 monolayers and heterophase bilayers

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.

Quantum spin Hall state in monolayer 1T'-TMDCs

Although the 1T'phase is rare in the transition metal dichalcogenides (TMDCs) family, it has attracted rapid growing research interest due to the coexistence of superconductivity, unsaturated magneto-resistance, topological phases etc. Among them, the quantum spin Hall (QSH) state in monolayer 1T'-TMDCs is especially interesting because of its unique van der Waals crystal structure, bringing advantages in the fundamental research and application. For example, the van der Waals two-dimensional (2D) layer is vital in building novel functional vertical heterostructure. The monolayer 1T'-TMDCs has become one of the widely studied QSH insulator. In this review, we review the recent progress in fabrications of monolayer 1T'-TMDCs and evidence that establishes it as QSH insulator.

Stable mid-infrared polarization imaging based on quasi-2D tellurium at room temperature

Next-generation polarized mid-infrared imaging systems generally requires miniaturization, integration, flexibility, good workability at room temperature and in severe environments, etc. Emerging two-dimensional materials provide another route to meet these demands, due to the ease of integrating on complex structures, their native in-plane anisotropy crystal structure for high polarization photosensitivity, and strong quantum confinement for excellent photodetecting performances at room temperature. However, polarized infrared imaging under scattering based on 2D materials has yet to be realized. Here we report the systematic investigation of polarized infrared imaging for a designed target obscured by scattering media using an anisotropic tellurium photodetector. Broadband sensitive photoresponse is realized at room temperature, with excellent stability without degradation under ambient atmospheric conditions. Significantly, a large anisotropic ratio of tellurium ensures polarized imaging in a scattering environment, with the degree of linear polarization over 0.8, opening up possibilities for developing next-generation polarized mid-infrared imaging technology.

Photodetectors operating within scattering environment can be realized with anisotropic materials. Here, the authors report polarization sensitive photodetectors based on thin tellurium nanosheets with high photoresponsivity of 3.54 × 10² A/W, detectivity of ~3.01 × 10⁹ Jones in the mid-infrared range and an anisotropic ratio of ∼8 for 2.3 μm illumination to ensure polarized imaging.

Designed Growth of Large-Size 2D Single Crystals

In the "post-Moore's Law" era, new materials are highly expected to bring next revolutionary technologies in electronics and optoelectronics, wherein 2D materials are considered as very promising candidates beyond bulk materials due to their superiorities of atomic thickness, excellent properties, full components, and the compatibility with the processing technologies of traditional complementary metal-oxide semiconductors, enabling great potential in fabrication of logic, storage, optoelectronic, and photonic 2D devices with better performances than state-of-the-art ones. Toward the massive applications of highly integrated 2D devices, large-size 2D single crystals are a prerequisite for the ultimate quality of materials and extreme uniformity of properties. However, at present, it is still very challenging to grow all 2D single crystals into the wafer scale. Therefore, a systematic understanding for controlled growth of various 2D single crystals needs to be further established. Here, four key aspects are reviewed, i.e., nucleation control, growth promotion, surface engineering, and phase control, which are expected to be controllable at different periods during the growth. In addition, the perspectives on designed growth and potential applications are discussed for showing the bright future of these advanced material systems of 2D single crystals.

In situ monitoring of the electrochemically induced phase transition of thermodynamically metastable 1T-MoS2 at nanoscale

1T-MoS2 is widely used in the hydrogen evolution reaction (HER) due to its abundant active sites and good conductivity. However, 1T-MoS2 is thermodynamically metastable due to the distorted crystal structure. Recently, researchers have detected the J1 and A1g Raman peaks after the HER process and confirmed that the 2H-1T phase possesses good stability. Therefore, continuous HER is likely to transform 1T-MoS2 into a stable 2H-1T mixed phase. The in situ characterization of 1T-MoS2 individual nanosheets in the HER process is important to understand the intrinsic electrocatalytic behaviour at confined nanoscale, which has rarely been investigated. Herein, we built an individual 1T-MoS2 nanosheet micro-nano device by the intercalation of N-butyllithium into 2H-MoS2. Then, the device was kept at an overpotential (η) of 450 mV, which was much lower than the onset potential, for 20 minutes to ensure continuous HER. Through this electrochemical treatment, we successfully obtained a mixed phase of 2H-1T and monitored the electrochemical phase transition by in situ Raman mapping and atomic force microscopy (AFM). The HER performance of the 2H-1T phase was superior to that of 1T-MoS2 and 2H-MoS2. Additionally, computational simulations demonstrated that the 2H-1T phase exhibited optimal hydrogen adsorption energy. The presented work displays the excellent catalysis of the mixed phase obtained by the electrochemical phase transition, which provides new directions for improving the catalytic activity of TMDs.

Phase engineering of nanomaterials

Phase has emerged as an important structural parameter - in addition to composition, morphology, architecture, facet, size and dimensionality - that determines the properties and functionalities of nanomaterials. In particular, unconventional phases in nanomaterials that are unattainable in the bulk state can potentially endow nanomaterials with intriguing properties and innovative applications. Great progress has been made in the phase engineering of nanomaterials (PEN), including synthesis of nanomaterials with unconventional phases and phase transformation of nanomaterials. This Review provides an overview on the recent progress in PEN. We discuss various strategies used to synthesize nanomaterials with unconventional phases and induce phase transformation of nanomaterials, by taking noble metals and layered transition metal dichalcogenides as typical examples. Moreover, we also highlight recent advances in the preparation of amorphous nanomaterials, amorphous-crystalline and crystal phase-based hetero-nanostructures. We also provide personal perspectives on challenges and opportunities in this emerging field, including exploration of phase-dependent properties and applications, rational design of phase-based heterostructures and extension of the concept of phase engineering to a wider range of materials.

FIB-Patterned Nano-Supercapacitors: Minimized Size with Ultrahigh Performances

Advances in microelectronic system technology have necessitated the development and miniaturization of energy storage devices. Supercapacitors are an important complement to batteries in microelectronic systems; and further reduction of the size of micro-supercapacitors is challenging. Here, a novel strategy is demonstrated to break through the resolution limit of micro-supercapacitors by preparing nano-supercapacitors (NSCs) with interdigital nanosized electrodes using focused ion beam technology. The minimization of the size of the NSCs leads to a large increase in capacitance, with a high areal capacitance of 9.52 mF cm^-2 and a volumetric capacitance of 18 700 F cm^-3 , far superior to those of other reported works. Size reduction and the narrowing of the physical separation between nanoelectrodes are proved to be the most crucial factors in the enhancement of capacitive performances. New charge-storage mechanisms are discovered with a remarkable nonfaradaic double-layer capacitance that exists due to the considerable inner electric field force at the nanoscale. The developed strategy and the first set of data provided here shed light on the design and fabrication of flexible interdigitated NSCs that rival state-of-the-art supercapacitors in performance.

Ultralow Power Wearable Heterosynapse with Photoelectric Synergistic Modulation

Although the energy consumption of reported neuromorphic computing devices inspired by biological systems has become lower than traditional memory, it still remains greater than bio-synapses (≈10 fJ per spike). Herein, a flexible MoS2-based heterosynapse is designed with two modulation modes, an electronic mode and a photoexcited mode. A one-step mechanical exfoliation method on flexible substrate and low-temperature atomic layer deposition process compatible with flexible electronics are developed for fabricating wearable heterosynapses. With a pre-spike of 100 ns, the synaptic device exhibits ultralow energy consumption of 18.3 aJ per spike in long-term potentiation and 28.9 aJ per spike in long-term depression. The ultrafast speed and ultralow power consumption provide a path for a neuromorphic computing system owning more excellent processing ability than the human brain. By adding optical modulation, a modulatory synapse is constructed to dynamically control correlations between pre- and post-synapses and realize complex global neuromodulations. The novel wearable heterosynapse expands the accessible range of synaptic weights (ratio of facilitation ≈228%), providing an insight into the application of wearable 2D highly efficient neuromorphic computing architectures.

Effect of Structural Phases on Mechanical Properties of Molybdenum Disulfide

Molybdenum disulfide (MoS₂) is a promising layer-structured material for use in many applications due to its tunable structural and electronic properties in terms of its structural phases. Its performance including efficiency and durability is often dependent on its mechanical properties. To understand the effects of the structural phase on its mechanical properties, a comparative study on the mechanical properties of bulk 2H, 3R, 1T, and 1T' MoS₂ was conducted using the first-principles density functional theory. Since considerable applications of MoS₂ are developed through strain engineering, the impact of the external pressure on its mechanical properties was also considered. Our results suggest a strong relationship between the mechanical properties of MoS₂ and the structural symmetry of its crystal. Accordingly, the impacts of the external pressure on the mechanical properties of MoS₂ also greatly vary with respect to the structural phases. Among all of the considered phases, the 2H and 3R MoS₂ have a larger bulk modulus, Young's modulus, shear modulus, and microhardness due to their higher stability. Conversely, 1T and 1T' MoS₂ are less strong. As such, 1T and 1T' MoS₂ can be a better candidate for strain engineering.

Epitaxial synthesis of ultrathin β-In2Se3/MoS2 heterostructures with high visible/near-infrared photoresponse

van der Waals (vdWs) heterostructures, combining different two-dimensional (2D) layered materials with diverse properties, have been demonstrated to be a very promising platform to explore a new physical phenomenon and realize various potential applications in atomically thin electronic and optoelectronic devices. Here, we report the controlled growth of vertically stacked β-In2Se3/MoS2 vdWs heterostructures (despite the existence of large lattice mismatching ∼29%) through a typical two-step chemical vapor deposition (CVD) method. The crystal structure of the achieved heterostructures is characterized by transmission electron microscopy, where evident Moiré patterns are observed, indicating well-aligned lattice orientation. Strong photoluminescence quenching is obeserved in the heterostructure, revealing effective interlayer charge transfer at the interface. Electrical devices are further constructed based on the achieved heterostructures, which have a high on/off ratio and a typical rectifying behavior. Upon laser irradiation, the devices show excellent photosensing properties. A high responsivity of 4.47 A W-1 and a detectivity of 1.07 × 109 Jones are obtained under 450 nm laser illumination with a bias voltage of 1 V, which are much better than those of heterostructures grown via CVD. Most significantly, the detection range can be extended to near-infrared due to the relatively small bandgap nature of β-In2Se3. With 830 nm laser illumination, the devices also show distinct photoresponses with fast response speed even when operating at room temperature. The high-quality β-In2Se3/MoS2 heterostructures broaden the family of the 2D layered heterostructure system and should have significant potential applications in high-performance broadband photodetectors.

Super strong 2D titanium carbide MXene-based materials: a theoretical prediction

The discovery of strong materials is essential in materials science and engineering. It becomes more significant to the practical applications of two-dimensional (2D) materials. In this study, the mechanical properties of all known 2D titanium carbide-based MXene monolayers have been systematically investigated by means of the density functional theory computations. Both the impacts of the thickness of the MXenes and the surface functionalization have been considered. Our results reveal that the in-plane planar elastic constants, Young's moduli and Shear moduli increase over the thickness. Moreover, they are enhanced by the terminal groups of surface functionalization. And the oxygen terminal group has the largest influence. As a result, the 2D Ti₄C₃O₂ is the strongest one among all 2D titanium carbide-based MXene, which is even stronger than the graphene. Our prediction provides the theoretical foundation for the specific application of MXenes that demands superior mechanical properties.

Direct Growth of Continuous and Uniform MoS2 Film on SiO2/Si Substrate Catalyzed by Sodium Sulfate

Because of its unique electronic band structure, molybdenum disulfide (MoS₂) has been regarded as a star semiconducting material. However, direct growth of continuous and high-quality MoS₂ films on SiO₂/Si substrates is still very challenging. Here, we report a facile chemical vapor deposition (CVD) method based on synergistic modulation of precursor and Na₂SO₄ catalysis, realizing the centimeter scale growth of a continuous MoS₂ film on SiO₂/Si substrates. The as-grown MoS₂ film had an excellent spatial homogeneity and crystal quality, with an edge length of the composite domain as large as 632 μm. Both experimental and theoretical results proved that Na tended to bond with SiO₂ substrates rather than to interfere with as-grown MoS₂. Thus, they showed decent and uniform electrical performance, with electron mobilities as high as 5.9 cm² V^-1 s^-1_. We believe our method will pave a new way for MoS₂ toward real application in modern electronics.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

An in situ grown lanthanum sulfide/molybdenum sulfide hybrid catalyst for electrochemical hydrogen evolution

An La₂S₃–MoS₂ catalyst with expanded interlayer spacing and engineered nano-interfaces was facilely synthesized, demonstrating enhanced catalytic activity for electrochemical hydrogen evolution.

Nitrogen-Plasma-Treated Continuous Monolayer MoS2 for Improving Hydrogen Evolution Reaction

Theoretically, the edges of a MoS2 flake and S-vacancy within the lattice have nearly zero Gibbs free energy for hydrogen adsorption, which is essentially correlated to the exchange currents in hydrogen evolution reaction (HER). However, MoS2 possesses insufficient active sites (edges and S-vacancies) in pristine form. Interestingly, active sites can be effectively engineered within the continuous MoS2 sheets by treating it with plasma in a controlled manner. Here, we employed N2 plasma on a large-area continuous-monolayer MoS2 synthesized via metal-organic chemical vapor deposition to acquire maximum active sites that are indeed required for an efficient HER performance. The MoS2 samples with maximum active sites were acquired by optimizing the plasma exposure time. The newly induced edges and S-vacancies were directly verified by high-resolution transmission electron microscopy. The 20 min treated MoS2 sample showed maximum active sites and thereby maximum HER activity, onset overpotential of ∼-210 mV vs reversible hydrogen electrode (RHE), and Tafel slope of ∼89 mV/dec. Clearly, the above results show that this approach can be employed for improving the HER efficiency of large-scale MoS2-based electrocatalysts.

Local engineering of topological phase in monolayer MoS2

Monolayer transition metal dichalcogenides (TMDCs) with the 1T' structure are a new class of large-gap two-dimensional (2D) topological insulators, hosting topologically protected conduction channels on the edges. However, the 1T' phase is metastable compared to the 2H phase for most of 2D TMDCs, among which the 1T' phase is least favored in monolayer MoS₂. Here we report a clean and controllable technique to locally induce nanometer-sized 1T' phase in monolayer 2H-MoS₂ via a weak Argon-plasma treatment, resulting in topological phase boundaries of high density. We found that the stabilization of 1T' phase arises from the concerted effects of S vacancies and the tensile strain. Scanning tunneling spectroscopy (STS) clearly reveals a spin-orbit band gap (~60 meV) and topologically protected in-gap states residing at the 1T'-2H phase boundary, which are corroborated by density-functional theory (DFT) calculations. The strategy developed in this work can be generalized to a large variety of TMDCs materials, with potentials to realize scalable electronics and spintronics with low dissipation.

Anion Extraction-Induced Polymorph Control of Transition Metal Dichalcogenides

Controlled phase conversion in polymorphic transition metal dichalcogenides (TMDs) provides a new synthetic route for realizing tunable nanomaterials. Most conversion methods from the stable 2H to metastable 1T phase are limited to kinetically slow cation insertion into atomically thin layered TMDs for charge transfer from intercalated ions. Here, we report that anion extraction by the selective reaction between carbon monoxide (CO) and chalcogen atoms enables predictive and scalable TMD polymorph control. Sulfur vacancy, induced by anion extraction, is a key factor in molybdenum disulfide (MoS₂) polymorph conversion without cation insertion. Thermodynamic MoS₂-CO-CO₂ ternary phase diagram offers a processing window for efficient sulfur vacancy formation with precisely controlled MoS₂ structures from single layer to multilayer. To utilize our efficient phase conversion, we synthesize vertically stacked 1T-MoS₂ layers in carbon nanofibers, which exhibit highly efficient hydrogen evolution reaction catalytic activity. Anion extraction induces the polymorph conversion of tungsten disulfide (WS₂) from 2H to 1T. This reveals that our method can be utilized as a general polymorph control platform. The versatility of the gas-solid reaction-based polymorphic control will enable the engineering of metastable phases in 2D TMDs for further applications.

Electronic properties of polymorphic two-dimensional layered chromium disulphide

Two-dimensional (2D) Cr-based layered and non-layered materials such as CrI₃, Cr₂Ge₂Te₆, Cr₂S₃, CrSe, and CrOX (X = Cl and Br) have attracted considerable attention due to their potential application in spintronics. Despite few experimental studies, theoretical studies reported that 2D chromium dichalcogenide (CrS₂) materials show unique properties such as valley polarization, piezoelectric coupling, and phase dependent intrinsic magnetic properties. Here, we report for the first time the synthesis of 2D layered CrS₂ flakes down to the monolayer via the chemical vapor deposition (CVD) method, its phase structures and electronic properties. We observed the 2H, 1T, and 1T' phases coexisting in CVD grown monolayer CrS₂. The formation of 1T' phases from 1T phases is described by dimerization of metal atoms at room temperature according to our molecular dynamics studies. The coexistence of 1T and 1T' phases with 2H phases is referred to as the 1T and 1T' puddling phenomenon. We theoretically showed that the monolayer 2H-CrS₂ is a direct bandgap semiconductor with a gap of approximately 0.95 eV predicted by the PBE functional, while the 1T- and 1T'-CrS₂ are metallic and semi-metallic with approximately 10 meV gap, respectively. Furthermore, 2H CrS₂ exhibits nonmagnetic semiconducting properties while for ferromagnetic spin configuration, the 1T and 1T' CrS₂ show magnetic characteristics with 0.531μ_B and 2.206μ_B magnetic moment per Cr atom respectively, for ferromagnetic spin configuration as predicted from DFT+U calculation. Importantly, CrS₂-based field-effect transistors exhibit a p-type behavior. Our study would stimulate further exploration of 2D layered CrS₂ with astonishing properties and open up a whole new avenue for the urgent need for developing multifunctional 2D materials for nanoelectronics, valleytronics, and spintronics.

Simultaneously Engineering Electron Conductivity, Site Density and Intrinsic Activity of MoS2 via the Cation and Anion Codoping Strategy

The catalytic activity of 2H-MoS₂ is retarded by the deficiency in active sites, inferior intrinsic activity, and slow electron transfer kinetics. However, the strategies to concurrently resolve these issues have been challenging and rarely reported. Herein, we successfully endow MoS₂ with exceptional acidic HER performance by concurrently doping nitrogen and metal atoms into the basal plane of MoS₂. The experimental results reveal that the N dopant that induces the intervalence charge transfer between two ions (Mo⁴⁺/Mo³⁺) and the atoms rearrangement can enable the successful synthesis of 1T MoS₂ on reduced graphene oxides, which can concurrently increase the active-site density and facilitate the charge transfer from the substrate to the catalyst active sites. The spontaneous doping of metal cation atoms further improves the intrinsic activity of MoS₂ by creating more sulfur vacancy sites and tailoring the energy level matching. The optimized electrocatalyst exhibited unprecedented activity and stability for HER with a low overpotential of 143 mV at 150 mA cm^-2 and a high exchange current density of 1 mA cm^-2. Therefore, our work opens up possibility to manipulate the MoS₂ catalytic performance to rival Pt, which is of significant importance to both fundamental study and industry applications.

Two-dimensional inorganic molecular crystals

Two-dimensional molecular crystals, consisting of zero-dimensional molecules, are very appealing due to their novel physical properties. However, they are mostly limited to organic molecules. The synthesis of inorganic version of two-dimensional molecular crystals is still a challenge due to the difficulties in controlling the crystal phase and growth plane. Here, we design a passivator-assisted vapor deposition method for the growth of two-dimensional Sb₂O₃ inorganic molecular crystals as thin as monolayer. The passivator can prevent the heterophase nucleation and suppress the growth of low-energy planes, and enable the molecule-by-molecule lateral growth along high-energy planes. Using Raman spectroscopy and in situ transmission electron microscopy, we show that the insulating α-phase of Sb₂O₃ flakes can be transformed into semiconducting β-phase under heat and electron-beam irradiation. Our findings can be extended to the controlled growth of other two-dimensional inorganic molecular crystals and open up opportunities for potential molecular electronic devices.

Two-dimensional molecular crystals made of inorganic units would be desirable for their chemical and electronic properties, but have been difficult to obtain. Here the authors show the synthesis of monolayer Sb₂O₃ molecular crystals on mica substrates by passivator-assisted vapor deposition.

Precise Vapor-Phase Synthesis of Two-Dimensional Atomic Single Crystals

Two-dimensional atomic single crystals (2DASCs) have drawn immense attention because of their potential for fundamental research and new technologies. Novel properties of 2DASCs are closely related to their atomic structures, and effective modulation of the structures allows for exploring various practical applications. Precise vapor-phase synthesis of 2DASCs with tunable thickness, selectable phase, and controllable chemical composition can be realized to adjust their band structures and electronic properties. This review highlights the latest advances in the precise vapor-phase synthesis of 2DASCs. We thoroughly elaborate on strategies toward the accurate control of layer number, phase, chemical composition of layered 2DASCs, and thickness of non-layered 2DASCs. Finally, we suggest forward-looking solutions to the challenges and directions of future developments in this emerging field.

A facile alkali metal hydroxide-assisted controlled and targeted synthesis of 1T MoS2 single-crystal nanosheets for lithium ion battery anodes

High-quality metallic 1T phase MoS2 single-crystal nanosheets were synthesized by a facile eco-friendly alkali metal hydroxide-assisted controlled and targeted approach via the calcination of lithium hydroxide and ammonium tetrathiomolybdate under argon atmosphere at 1000 °C. The 1T MoS2 single-crystal nanosheets were used as lithium-ion battery anodes, exhibiting superior electrochemical performances, including long cycle life and high capacities. The first charge and discharge capacities were up to 889.1 mA h g-1 and 892.5 mA h g-1, respectively, giving a first cycle coulombic efficiency of 98.0%, which exceeded 99.1% in the second cycle. In the 400th cycle, the charge and discharge capacities were 737.2 mA h g-1 and 738.0 mA h g-1, respectively, with 82.9% capacity retention ratio. This study not only provides a novel strategy for fabricating metallic 1T phase MoS2, which can be used as lithium ion battery anodes but also introduces a facile eco-friendly lithium hydroxide-assisted controlled and targeted approach for different phase MoS2 (1T and 2H). Moreover, it can be extended to other alkali metal hydroxide-assisted (NaOH and KOH) approaches for the MoS2 preparation.

Shape-Engineered Synthesis of Atomically Thin 1T-SnS2 Catalyzed by Potassium Halides

Shape engineering plays a crucial role in the application of two-dimensional (2D) layered metal dichalcogenide (LMD) crystalline materials in terms of physical and chemical property modulation. However, controllable growth of 1T phase tin disulfide (SnS₂) with multifarious morphologies has rarely been reported and remains challenging. Herein, we report a direct synthesis of large-size, uniform, and atomically thin 1T-SnS₂ with multiple morphologies by adding potassium halides via a facile chemical vapor deposition process. A variety of morphologies, i.e., from hexagon, triangle, windmill, and dendritic to coralloid, corresponding to fractal dimensions from 1.01 to 1.81 are accurately controlled by growth conditions. Moreover, the Sn concentration controls the morphology change of SnS₂. The edge length of the SnS₂ dendritic flake can grow larger than 500 μm in 5 min. Potassium halides can significantly reduce the surface migration barrier of the SnS₂ cluster and enhance the SnS₂ adhesion force with substrate to facilitate efficient high in-plane growth of monolayer SnS₂ compared to sodium halides by density functional theory calculations. More branched SnS₂ with higher fractal dimension provides more active sites for enhancing hydrogen evolution reactions. Importantly, we prove that potassium halides are preferable for 1T-phase LMDs structures, while sodium halides are more suitable for 2H-phase materials. The growth mechanism proposed here provides a general approach for controllable-phase synthesis of 2D LMD crystals and related heterostructures. Shape engineering of 2D materials also provides a strategy to tune LMD properties for demanding applications.

Distorted 1T-ReS2 Nanosheets Anchored on Porous TiO2 Nanofibers for Highly Enhanced Photocatalytic Hydrogen Production

Recently, loading TiO₂ with transition-metal disulfides (TMDs) to construct dual functional heterostructures has been widely researched as an effective strategy to improve the photocatalytic performance of a TiO₂ photocatalyst. For the TMD cocatalysts, the 2H-MoS₂ and 1T-MoS₂ have been widely studied and researched. However, they suffer from poor catalytic activity sites/low charge transfer ability and an unstable structure. In this regard, distorted 1T-phase TMDs with a stable structure are greatly fit for the cocatalyst due to their high charge transfer ability and rich catalytic sites on both the edge and basal plane. Therefore, it is highly desirable to develop distorted 1T-phase TMD/TiO₂ heterostructures with well-identified interfaces for highly enhanced photocatalytic performance. Herein, we first introduce distorted 1T-ReS₂ anchored on porous TiO₂ nanofibers as a promising photocatalyst for achieving an excellent photocatalytic hydrogen production. The excellent performance is attributed to the strong chemical interaction of the Ti-O-Re bond between TiO₂ and ReS₂, the excellent electron mobility of distorted 1T-ReS₂, and the abundant catalytic activity sites on both the plane and edge of the ReS₂ cocatalyst.

High Phase Purity of Large-Sized 1T'-MoS2 Monolayers with 2D Superconductivity

The development of transition metal dichalcogenides has greatly accelerated research in the 2D realm, especially for layered MoS₂ . Crucially, the metallic MoS₂ monolayer is an ideal platform in which novel topological electronic states can emerge and also exhibits excellent energy conversion and storage properties. However, as its intrinsic metallic phase, little is known about the nature of 2D 1T'-MoS₂ , probably because of limited phase uniformity (<80%) and lateral size (usually <1 µm) in produced materials. Herein, solution processing to realize high phase-purity 1T'-MoS₂ monolayers with large lateral size is demonstrated. Direct chemical exfoliation of millimeter-sized 1T' crystal is introduced to successfully produce a high-yield of 1T'-MoS₂ monolayers with over 97% phase purity and unprecedentedly large size up to tens of micrometers. Furthermore, the large-sized and high-quality 1T'-MoS₂ nanosheets exhibit clear intrinsic superconductivity among all thicknesses down to monolayer, accompanied by a slow drop of transition temperature from 6.1 to 3.0 K. Prominently, unconventional superconducting behavior with upper critical field far beyond the Pauli limit is observed in the centrosymmetric 1T'-MoS₂ structure. The results open up an ideal approach to explore the properties of 2D metastable polymorphic materials.

Phase Control in Inorganic Nanocrystals through Finely Tuned Growth at an Ultrathin Scale

Crystalline polymorphs have been considered a prevailing phenomenon in inorganic nanocrystals and provide approaches to modulate fundamental properties and innovative advanced applications. As a basic demand for phase engineering, accessible and controllable synthetic methodologies are indispensable for acquisition of high-quality products in expected phases. Phase stability is also a non-negligible issue that determines continuous gains of functionality and long-term sustainability of characteristic features. Maintaining structural stability of metastable phases provides challenges and opportunities for investigations on fascinating properties and intriguing applications of inorganic nanocrystals. Phase engineering is of great significance to acquire metallic (1T) and semiconducting (2H) Mo- and W-based dichalcogenides for hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR), respectively. The catalysts in 1T phase have superior electron transfer kinetics and abundant active sites on both basal planes and edges for HER, while ones in 2H phase are preferentially deployed for CO₂RR to utilize edge sites for catalysis and restrain competitive HER activity. In addition, the photocatalytic performance for HER has been enhanced by combining anatase and rutile phases because electron transfer between the two phases during photocatalysis facilitates the separation of charge carriers and thus impedes the recombination of electron-hole pairs. Although ample effort has been devoted to developing phase engineering, principle understanding at an ultrathin scale remains obscure. In this Account, we provide comprehensive insight into work from our group regarding controllable synthesis of inorganic nanocrystals with phase engineering, critical effects on phase stability, and noteworthy studies on phase-related properties and applications. For bulk materials, phase control and transition have a large energy barrier, so they can only be achieved under rigorous conditions. However, at the initial stage of synthesis, especially for nucleation, there are a small quantity of chemical bonds that contribute to regulate phase and structure with ease. In our work, we mainly modulate nucleation and growth at an ultrathin scale to demonstrate facile approaches for phase engineering. This unique perspective makes for a distinct guidance of controllable synthesis and deliberate stabilization of inorganic nanomaterials with phase engineering. We have developed a series of synthetic strategies for phase engineering to fabricate inorganic nanocrystals in a specific phase with controlled size and composition and adjustable morphologies and surface features. Four sorts of models (MoS₂, ZrO₂, In₂O₃, and TiO₂) are used for demonstrating finely tuned growth at an ultrathin scale. However, phase engineering has been regarded as immature because only one phase in polymorphs is thermodynamically stable generally. Phase stability of metastable nanocrystals has attracted much interest. Our substantial investigations illustrate several crucial factors on phase stability, leading to inspiration for facilitating persistent emergence of characteristics and functionalities. By full use of the features of a specific phase, we spotlight ligand-induced surface interactions on coverage-dependent electronic structures and chemisorption effects at one-unit thickness of TiO₂(B) nanomaterials with phase engineering. Meanwhile, an energy conversion system for overall water splitting (OWS) drives forward steps in function-oriented synthesis of MoS₂-based nanomaterials with phase engineering. In the last section, we summarize this theme and highlight several promising directions for future development.

Progress in Electrocatalytic Hydrogen Evolution Based on Monolayer Molybdenum Disulfide

Energy and environmental issues raise higher demands on the development of a sustainable energy system, and the electrocatalytic hydrogen evolution is one of the most important ways to realize this goal. Two-dimensional (2D) materials represented by molybdenum disulfide (MoS2) have been widely investigated as an efficient electrocatalyst for the hydrogen evolution. However, there are still some shortcomings to restrict the efficiency of MoS2 electrocatalyst, such as the limited numbers of active sites, lower intrinsic catalytic activity and poor interlayer conductivity. In this review, the application of monolayer MoS2 and its composites with 0D, 1D, and 2D nanomaterials in the electrocatalytic hydrogen evolution were discussed. On the basis of optimizing the composition and structure, the numbers of active sites, intrinsic catalytic activity, and interlayer conductivity could be significantly enhanced. In the future, the study would focus on the structure, active site, and interface characteristics, as well as the structure-activity relationship and synergetic effect. Then, the enhanced electrocatalytic activity of monolayer MoS2 can be achieved at the macro, nano and atomic levels, respectively. This review provides a new idea for the structural design of two-dimensional electrocatalytic materials. Meanwhile, it is of great significance to promote the study of the structure-activity relationship and mechanism in catalytic reactions.

Direct solution-phase synthesis of 1T' WSe2 nanosheets

Crystal phase control in layered transition metal dichalcogenides is central for exploiting their different electronic properties. Access to metastable crystal phases is limited as their direct synthesis is challenging, restricting the spectrum of reachable materials. Here, we demonstrate the solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T’ phase) of WSe₂ nanosheets. We design a kinetically-controlled regime of colloidal synthesis to enable the formation of the metastable phase. 1T’ WSe₂ branched few-layered nanosheets are produced in high yield and in a reproducible and controlled manner. The 1T’ phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400 °C. The 1T’ WSe₂ nanosheets demonstrate a metallic nature exhibited by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe₂ nanosheets and comparable to other 1T’ phases. This synthesis design can potentially be extended to different materials providing direct access of metastable phases.

1T’ phases of transition metal dichalcogenides show promise for electrocatalysis, energy storage, and spintronic applications but are difficult to obtain. Here the authors synthesize 1T’ WSe2 few-layered nanosheets by kinetically-controlled colloidal synthesis, and test their electrocatalytic activity.

Using ligands to control reactivity, size and phase in the colloidal synthesis of WSe2 nanocrystals

Colloidal synthesis enables size- and phase-tuning of WSe₂ nanocrystals.

Synergistic additive-mediated CVD growth and chemical modification of 2D materials

This review summarizes significant advances in the use of typical synergistic additives in growth of 2D materials with chemical vapor deposition, and the corresponding performance improvement of field effect transistors and photodetectors.