High-Gain MoS2/Ta2NiSe5 Heterojunction Photodetectors with Charge Transfer and Suppressing Dark Current.

Emerging two-dimensional narrow band gap materials with tunable band gaps and unique electrical and optical properties have shown tremendous potential in broadband photodetection. Nevertheless, large dark currents severely hinder the performance of photodetectors. Here, a MoS2/Ta2NiSe5 van der Waals heterostructure device was successfully fabricated with a high rectification ratio of ∼104 and an ultralow reverse bias current of the pA level. Excitingly, the charge transfer and the generation of the built-in electric field of heterostructures have been proved by theory and experiment, which effectively suppress dark currents. The dark current of the heterostructure reduces by nearly 104 compared with the pure Ta2NiSe5 photodetector at Vds = 1 V. The MoS2/Ta2NiSe5 device exhibits excellent photoelectric performance with the maximum responsivity of 515.6 A W-1 and 0.7 A W-1 at the wavelengths of 532 and 1064 nm under forward bias, respectively. In addition, the specific detectivity is up to 3.1 × 1013 Jones (532 nm) and 2.4 × 109 Jones (1064 nm). Significantly, the device presents an ultra-high gain of 6 × 107 and an exceptional external quantum efficiency of 1.2 × 105% under 532 nm laser irradiation. The results reveal that the MoS2/Ta2NiSe5 heterostructure provides an essential platform for the development and application of high-performance broadband optoelectronic devices.

Nonlayered Tin Thiohypodiphosphate Nanosheets: Controllable Growth and Solar-Light-Driven Water Splitting.

As a promising candidate in various fields, including energy conversion and electronics, layered van der Waals metal phosphorus trichalcogenides (MPX3) have been widely explored. In addition to the layered structures, MPX3 comprising post-transition metals (i.e., Sn and Pb) are known to form a unique 3D framework with nonlayered structure. However, the nonlayered two-dimensional (2D) crystals of this family have remained unexplored until now. Herein, we successfully synthesized 2D nonlayered tin thiohypodiphosphate (Sn2P2S6) nanosheets, having an indirect bandgap of 2.25 eV and a thickness down to ∼10 nm. The as-obtained nanosheets demonstrate promising photocatalytic water splitting activity to generate H2 in pure water under simulated solar light (AM 1.5G). Moreover, the ultrathin Sn2P2S6 catalyst shows auspicious performance and stability with a continuous operation of 40 h. This work is not only an expansion of the MPX3 family, but it is also a major milestone in the search for new materials for future energy conversion.

Layered metal phosphorous trichalcogenides nanosheets: facile synthesis and photocatalytic hydrogen evolution.

Layered transition metal phosphorous trichalcogenide (MPX3) materials have attracted immense attention due to their excellent optical and electrical properties. However, the controllable synthesis of ultrathin MPX3 nanosheets is still challenging. Here, we present a facile phosphosulfurization scheme to prepare high-quality layered FePS3 nanosheets, with ∼20 nanometers in thickness. The optical properties of the as-obtained FePS3 show good light-harvesting ability, which endows it with excellent photocatalytic hydrogen evolution performance (402.4 µmol g-1 h-1) under the simulated solar illumination. We further show that other MPX3 family members such as In2/3PS3 and CdPS3 can be also synthesized by the same approach. Our finding can offer a feasible approach for rationally designing other MPX3 materials for various applications.

Ultrathin Magnetic 2D Single-Crystal CrSe.

2D magnetic materials have generated an enormous amount of attention due to their unique 2D-limited magnetism and their potential applications in spintronic devices. Recently, most of this research has focused on 2D van der Waals layered magnetic materials exfoliated from the bulk with random size and thicknesses. Controllable growth of these materials is still a great challenge. In contrast, 2D nonlayered magnetic materials have rarely been investigated, not especially regarding their preparation. Crn X (X = S, Se and Te; 0 < n < 1), a class of nonlayered transition metal dichalcogenides, has rapidly attracted extensive attention due to its abundance of structural compounds and unique magnetic properties. Herein, the controlled synthesis of ultrathin CrSe crystals, with grain size reaching the sub-millimeter scale, on mica substrates via an ambient pressure chemical vapor deposition (CVD) method is demonstrated. A continuous CrSe film can also be achieved via precise control of the key growth parameters. Importantly, the CVD-grown 2D CrSe crystals possess obvious ferromagnetic properties at temperatures below 280 K, which has not been observed experimentally before. This work broadens the scope of the CVD growth of 2D magnetic materials and highlights their significant application possibilities in spintronics.

High-Yield Production of Monolayer FePS3 Quantum Sheets via Chemical Exfoliation for Efficient Photocatalytic Hydrogen Evolution.

2D layered transition metal phosphorus trichalcogenides (MPX3 ) possess higher in-plane stiffness and lower cleavage energies than graphite. This allows them to be exfoliated down to the atomic thickness. However, a rational exfoliation route has to be sought to achieve surface-active and uniform individual layers. Herein, monolayered FePS3 quantum sheets (QSs) are systematically obtained, whose diameters range from 4-8 nm, through exfoliation of the bulk in hydrazine solution. These QSs exhibit a widened bandgap of 2.18 eV as compared to the bulk (1.60 eV) FePS3 . Benefitting from the monolayer feature, FePS3 QSs demonstrate a substantially accelerated photocatalytic H2 generation rate, which is up to three times higher than the bulk counterpart. This study presents a facile way, for the first time, of producing uniform monolayer FePS3 QSs and opens up new avenues for designing other low-dimensional materials based on MPX3 .

Efficient Photocatalytic Hydrogen Evolution via Band Alignment Tailoring: Controllable Transition from Type-I to Type-II.

Considering the sizable band gap and wide spectrum response of tin disulfide (SnS2 ), ultrathin SnS2 nanosheets are utilized as solar-driven photocatalyst for water splitting. Designing a heterostructure based on SnS2 is believed to boost their catalytic performance. Unfortunately, it has been quite challenging to explore a material with suitable band alignment using SnS2 nanomaterials for photocatalytic hydrogen generation. Herein, a new strategy is used to systematically tailor the band alignment in SnS2 based heterostructure to realize efficient H2 production under sunlight. A Type-I to Type-II band alignment transition is demonstrated via introducing an interlayer of Ce2 S3 , a potential photocatalyst for H2 evolution, between SnS2 and CeO2 . Subsequently, this heterostructure demonstrates tunability in light absorption, charge transfer kinetics, and material stability. The optimized heterostructure (SnS2 -Ce2 S3 -CeO2 ) exhibits an incredibly strong light absorption ranging from deep UV to infrared light. Significantly, it also shows superior hydrogen generation with the rate of 240 µmol g-1 h-1 under the illumination of simulated sunlight with a very good stability.

An efficient ternary CoP2xSe2(1-x) nanowire array for overall water splitting.

Developing earth-abundant and efficient bifunctional electrocatalysts for realizing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline conditions is an intriguing challenge. Here, ternary necklace-like CoP2xSe2(1-x) nanowire arrays are synthesized via simultaneously phosphorizing and selenizing Co(OH)2 nanowires. Owing to the substitution of the P atom in the ternary system, the optimal electronic structure of CoP2xSe2(1-x) can be obtained and the stability can also be enhanced for hydrogen evolution. Thus, the ternary CoP2xSe2(1-x) NWs are highly active for electrochemical hydrogen evolution in both acidic and alkaline media, achieving a current density of 10 mA cm -2 at overpotentials of 70 mV and 98 mV, respectively. To realize the overall water splitting, we further performed the experiment using the CoP2xSe2(1-x) NWs as a cathode and Co(OH)2 NWs as an anode, which requires a cell voltage of 1.65 V to afford a water splitting current density of 10 mA cm -2 in strong alkaline media (1.0 M KOH).

Efficient Catalysis of Hydrogen Evolution Reaction from WS2(1-x) P2x Nanoribbons.

The rational design of Earth abundant electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of carbon neutral energy carrier. Owing to their fascinating chemical and physical properties, transition metal dichalcogenides (TMDs) are widely studied for this purpose. Of particular note is that doping by foreign atom can bring the advent of electronic perturbation, which affects the intrinsic catalytic property. Hence, through doping, the catalytic activity of such materials could be boosted. A rational synthesis approach that enables phosphorous atom to be doped into WS2 without inducing phase impurity to form WS2(1-x) P2x nanoribbon (NRs) is herein reported. It is found that the WS2(1-x) P2x NRs exhibit considerably enhanced HER performance, requiring only -98 mV versus reversible hydrogen electrode to achieve a current density of -10 mA cm-2 . Such a high performance can be attributed to the ease of H-atom adsorption and desorption due to intrinsically tuned WS2 , and partial formation of NRs, a morphology wherein the exposure of active edges is more pronounced. This finding can provide a fertile ground for subsequent works aiming at tuning intrinsic catalytic activity of TMDs.

CoS(2x)Se(2(1-x)) nanowire array: an efficient ternary electrocatalyst for the hydrogen evolution reaction.

Binary transition metal dichalcogenides (TMDs) have emerged as efficient catalysts for the hydrogen evolution reaction (HER). Co-based TMDs, such as CoS2 and CoSe2, demonstrate promising HER performance due to their intrinsic metallic nature. Recently, the ternary electrocatalysts were widely acknowledged for their prominent efficiency as compared to their binary counterparts due to increased active sites caused by the incorporation of different atoms. Herein, we successfully grew the ternary CoS2xSe2(1-x) (x = 0.67) nanowires (NWs) on a flexible carbon fiber. As a superior electrocatalyst, ternary CoS2xSe2(1-x) NWs arrays demonstrated excellent catalytic activity for electrochemical hydrogen evolution in acidic media, achieving current densities of 10 mA cm(-2) and 100 mA cm(-2) at overpotentials of 129.5 mV and 174 mV, respectively. Notably, the high stability of CoS2xSe2(1-x) NWs suggested that the ternary CoS2xSe2(1-x) NWs are a scalable catalyst for electrochemical hydrogen evolution.

Recent advances in transition-metal dichalcogenide based nanomaterials for water splitting.

The desire for sustainable and clean energy future continues to be the concern of the scientific community. Researchers are incessantly targeting the development of scalable and abundant electro- or photo-catalysts for water splitting. Owing to their suitable band-gap and excellent stability, an enormous amount of transition-metal dichalcogenides (TMDs) with hierarchical nanostructures have been extensively explored. Herein, we present an overview of the recent research progresses in the design, characterization and applications of the TMD-based electro- or photo-catalysts for hydrogen and oxygen evolution. Emphasis is given to the layered and pyrite-phase structured TMDs encompassing semiconducting and metallic nanomaterials. Illustrative results and the future prospects are pointed out. This review will provide the readers with insight into the state-of-the-art research progresses in TMD based nanomaterials for water splitting.

A vertical-oriented WS2 nanosheet sensitized by graphene: an advanced electrocatalyst for hydrogen evolution reaction.

Electrocatalytic hydrogen production at low overpotential is a promising route towards a clean and sustainable energy. Layered transition metal dichalcogenides (LTMDs) have attracted copious attention for their outstanding activities in hydrogen evolution reaction (HER). However, the horizontally laid nanosheets suffer from a paucity of active edge sites. Herein, we report the successful synthesis of vertical-oriented WS2 nanosheets through a hydrothermal method followed by a facile sulfurization process. Furthermore, the surface of synthesized WS2 nanosheets was decorated by ultrathin reduced graphene oxide (rGO) nanoplates. This is achieved for the first time by bringing the rGO on the surface of vertical-oriented WS2 nanosheets, which is conducive to rapid electron transport during the HER process. Significantly, the as-synthesized rGO/WS2 nanosheets exhibit improved HER activity as compared to the undecorated ones. It needs a low overpotential of only 229 mV vs. RHE to afford a current density of 10 mA cm(-2). We believe that this hybrid structure demonstrated remarkable HER activity brought about by a compatible synergism between rGO and WS2.

van der Waals epitaxial ultrathin two-dimensional nonlayered semiconductor for highly efficient flexible optoelectronic devices.

Despite great progress in synthesis and application of graphene-like materials, it remains a considerable challenge to prepare two-dimensional (2D) nanostructures of nonlayered materials that may bring us surprising physical and chemical properties. Here, we propose a general strategy for the growth of 2D nonlayered materials by van der Waals epitaxy (vdWE) growth with two conditions: (1) the nonlayered materials satisfy 2D anisotropic growth and (2) the growth is implemented on the van der Waals substrates. Large-scale ultrathin 2D Pb(1-x)Sn(x)Se nanoplates (∼15-45 nm) have been produced on mica sheets by applying this strategy. Benefiting from the 2D geometry of Pb(1-x)Sn(x)Se nanoplates and the flexibility of mica sheet, flexible photodetectors that exhibit fast, reversible, and stable photoresponse and broad spectra detection ranging from UV to infrared light (375, 473, 632, 800, and 980 nm) are in situ fabricated based on Pb(1-x)Sn(x)Se nanoplates. We anticipate that more nonlayered materials will be developed into 2D nanostructures through vdWE, enabling the exploitation of novel electronic and optoelectronic devices.

Tungsten oxide@polypyrrole core-shell nanowire arrays as novel negative electrodes for asymmetric supercapacitors.

Among active pseudocapacitive materials, polypyrrole (PPy) is a promising electrode material in electrochemical capacitors. PPy-based materials research has thus far focused on its electrochemical performance as a positive electrode rather than as a negative electrode for asymmetric supercapacitors (ASCs). Here high-performance electrochemical supercapacitors are designed with tungsten oxide@PPy (WO3 @PPy) core-shell nanowire arrays and Co(OH)2 nanowires grown on carbon fibers. The WO3 @PPy core-shell nanowire electrode exhibits a high capacitance (253 mF/cm2) in negative potentials (-1.0-0.0 V). The ASCs packaged with CF-Co(OH)2 as a positive electrode and CF-WO3 @PPy as a negative electrode display a high volumetric capacitance up to 2.865 F/cm3 based on volume of the device, an energy density of 1.02 mWh/cm3 , and very good stability performance. These findings promote the application of PPy-based nanostructures as advanced negative electrodes for ASCs.

Component-controllable WS(2(1-x))Se(2x) nanotubes for efficient hydrogen evolution reaction.

Owing to the excellent potential for fundamental research and technical applications in optoelectronic devices and catalytic activity for hydrogen evolution reaction (HER), transition metal dichalcogenides have recently attracted much attention. Transition metal sulfide nanostructures have been reported and demonstrated promising application in transistors and photodetectors. However, the growth of transition metal selenide nanostructures and their applications has still been a challenge. In this work, we successfully synthesized high-quality WSe2 nanotubes on carbon fibers via selenization. More importantly, through optimizing the growth conditions, ternary WS2(1­x)Se2x nanotubes were synthesized and the composition of S and Se can be systematically controlled. The as-grown WS2(1­x)Se2x nanotubes on carbon fibers, assembled as a working electrode, revealing low overpotential, high exchange current density, and small series resistance, exhibit excellent electrocatalytic properties for hydrogen evolution reaction. Our study provides the experimental groundwork for the synthesis of low-dimensional transition metal dichalcogenides and may open up exciting opportunities for their application in electronics, photoelectronics, and catalytic electrochemical reactions.

Construction of 3D V2O5/hydrogenated-WO3 nanotrees on tungsten foil for high-performance pseudocapacitors.

3D semiconductor nanostructures have proved to be a rich system for the exploring of high-performance pseudocapacitors. Herein, a novel 3D WO3 nanotree on W foil is developed via a facile and green method. Both capacitance and conductivity of the WO3 nanotree electrode are greatly improved after hydrogenation treatment (denoted as H-WO3). First-principles calculation based on the experiments reveals the mechanism of the hydrogenation treatment effect on the 3D WO3 nanotrees. The surface O of 3D WO3 nanotrees gains electrons from the adsorbed H, and consequently certain electrons are back-donated to the neighboring W, thus providing the conducting channel on the surface. Ultrathin V2O5 films were coated on the H-WO3 nanotrees via a simple, low-cost, environmentally friendly electrochemical technique. This V2O5/H-WO3 electrode exhibited a remarkable specific capacitance of 1101 F g(-1) and an energy density of 98 W h kg(-1). The solid-state device based on the V2O5/H-WO3 electrodes shows excellent stability and practical application. Our work opens up the potential broad application of hydrogenation treatment of semiconductor nanostructures in pseudocapacitors and other energy storage devices.