Synthesis of Ultrathin Topological Insulator ß-Ag2 Te and Ag2 Te/WSe2 -Based High-Performance Photodetector.

ß-Ag2 Te has attracted considerable attention in the application of electronics and optoelectronics due to its narrow bandgap, high mobility, and topological insulator properties. However, it remains a significant challenge to synthesize 2D Ag2 Te because of the non-layered structure of Ag2 Te. Herein, the synthesis of large-size, ultrathin single crystal topological insulator 2D Ag2 Te via the van der Waals epitaxial method for the first time is reported. The 2D Ag2 Te crystal exhibits p-type conduction behavior with high carrier mobility of 3336 cm2 V-1 s-1 at room temperature. Taking advantage of the high mobility and perfect electron structure of Ag2 Te, the Ag2 Te/WSe2 heterojunctions are fabricated via mechanical stacking and show an ultrahigh rectification ratio of 2 × 105 . Ag2 Te/WSe2 photodetector also exhibits self-driven properties with a fast response speed (40 µs/60 µs) in the near-infrared region. High responsivity (219 mA W-1 ) and light ON/OFF ratio of 6 × 105 are obtained under the photovoltaic mode. The overall performance of the Ag2 Te/WSe2 photodetector is significantly competitive among all reported 2D photodetectors. These results indicate that 2D Ag2 Te is a promising candidate for future electronic and optoelectronic applications.

The Controllable Synthesis of High-Quality Two-Dimensional Iron Sulfide with Specific Phases.

2D Fe-chalcogenides have drawn significant attention due to their unique structural phases and distinct properties in exploring magnetism and superconductivity. However, it remains a significant challenge to synthesize 2D Fe-chalcogenides with specific phases in a controllable manner since Fe-chalcogenides have multiple phases. Herein, a molecular sieve-assisted strategy is reported for synthesizing ultrathin 2D iron sulfide on substrates via the chemical vapor deposition method. Using a molecular sieve and tuning growth temperatures to control the partial pressures of precursor concentrations, hexagonal FeS, tetragonal FeS, and non-stoichiometric Fe7 S8 nanoflakes can be precisely synthesized. The 2D h-FeS, t-FeS, and Fe7 S8 have high conductivities of 5.4 × 105 S m-1 , 5.8 × 105 S m-1 , and 1.9 × 106 S m-1 . 2D tetragonal FeS shows a superconducting transition at 4 K. The spin reorientation at ≈30 K on the non-stoichiometric Fe7 S8 nanoflakes with ferrimagnetism up to room temperature has also been observed. The controllable synthesis of various phases of 2D iron sulfide may provide a route for synthesizing other 2D compounds with various phases.

Edge-Riched MoSe2 /MoO2 Hybrid Electrocatalyst for Efficient Hydrogen Evolution Reaction.

Molybdenum diselenide (MoSe2 ) is widely considered as one of the most promising catalysts for the hydrogen evolution reaction (HER). However, the absence of active sites and poor conductivity of MoSe2 severely restrict its HER performance. By introducing a layer of MoO2 on Mo foil, MoSe2 /MoO2 hybrid nanosheets with an abundant edge and high electrical conductivity can be synthesized on the surface of Mo foil. Metallic MoO2 can improve the charge transport efficiency of MoSe2 /MoO2 , thereby enhancing the overall HER performance. MoSe2 /MoO2 exhibits fast hydrogen evolution kinetics with a small overpotential of 142 mV versus RHE at a current density of 10 mA cm-2 and Tafel slope of 48.9 mV dec-1 .

Infrared Interlayer Excitons in Twist-Free MoTe2/MoS2 Heterobilayers.

Excitonic devices based on interlayer excitons in van der Waals heterobilayers are a promising platform for advancing photoelectric interconnection telecommunications. However, the absence of exciton emission in the crucial telecom C-band has constrained their practical applications. Here, this limitation is addressed by reporting exciton emission at 0.8 eV (1550 nm) in a chemically vapor-deposited, strictly aligned MoTe2/MoS2 heterobilayer, resulting from the direct bandgap transitions of interlayer excitons as identified by momentum-space imaging of their electrons and holes. The decay mechanisms dominated by direct radiative recombination ensure constant emission quantum yields, a basic demand for efficient excitonic devices. The atomically sharp interface enables the resolution of two narrowly-splitter transitions induced by spin-orbit coupling, further distinguished through the distinct Landé g-factors as the fingerprint of spin configurations. By electrical control, the double transitions coupling into opposite circularly-polarized photon modes, preserve or reverse the helicities of the incident light with a degree of polarization up to 90%. The Stark effect tuning extends the emission energy range by over 150 meV (270 nm), covering the telecom C-band. The findings provide a material platform for studying the excitonic complexes and significantly boost the application prospects of excitonic devices in silicon photonics and all-optical telecommunications.

Directional Construction of a 1T0.63-MoSe2@MoP Multiphase-Interface Catalyst for Highly Efficient Alkaline Hydrogen Evolution.

Alkaline water electrolysis is the most widely used technology for industrial hydrogen production. However, transition-metal dichalcogenides as inert alkaline hydrogen evolution electrocatalysts suffer from sluggish water adsorption and dissociation dynamics originating from the inappropriate intrinsic electronic structure. To address this issue, we report the synthesis of a type of multiphase-interface catalyst (MPIC), 1T0.63-MoSe2@MoP (1T = octahedral phase; MoSe2 = molybdenum selenide; MoP = molybdenum phosphide), that tunes the intrinsic interfacial electronic structure by multiphase synergy, promoting the alkaline hydrogen evolution reaction (HER). Consequently, the self-standing 1T0.63-MoSe2@MoP MPIC requires a small overpotential of 358 mV to reach a large current density of 1000 mA cm-2 in an alkaline freshwater electrolyte, along with impressive HER activity and stability at large current densities in an artificial alkaline seawater electrolyte. This work unravels the potential of Mo-based electrocatalysts for hydrogen evolution at high current densities, owing to the simple and mature synthesis process, which offers a vision to enable large-scale commercial hydrogen generation by seawater electrolysis. Meanwhile, density functional theory studies consistently confirm that the combination of metallic phase and intrinsic HER-active MoP in MoSe2 could successfully tune its electronic structure to improve the HER catalytic activity.

Water assisted growth of two-dimensional MoS2/MoSe2 vertical heterostructures on molten glass.

Herein, we demonstrate a chemical vapor deposition route to the controlled growth of large scale MoS2/MoSe2 vertical van der Waals heterostructures on a molten glass substrate using water as the oxidizing chemical to guarantee a sufficient and uniform delivery of the metal precursor. This work offers an efficient way for developing other layered heterostructures for integrated electronic and optoelectronic devices.

Interfacial engineering of tungstic disulfide-carbide heterojunction for high-current-density hydrogen evolution.

Developing low-cost and high-efficiency electrocatalysts to electrolyze water is an effective method for large-scale hydrogen production. For large-scale commercial applications, it is crucial to call for more efficient electrocatalysts with high-current density (≥1000 mA cm-2). However, it is challenging to simultaneously promote the large-scale production and hydrogen evolution reaction (HER) activity of these hydrogen catalysts. Herein, we report the large area tungstic disulfide-carbide (W/WS2-WC) heterojunction electrode vertically grown on an industrial-grade tungsten substrate by the solid-state synthesis method. The W/WS2-WC heterojunction electrode achieves a low overpotential of 473 mV at 1000 mA cm-2 in alkaline electrolytes.

A three-dimensional macroporous framework molybdenum disulfide-carbide heterojunction for highly efficient electrocatalytic hydrogen evolution at high current densities.

Herein, we report a facile solid-state synthesis strategy for the synthesis of a three-dimensional (3D) macroporous framework molybdenum disulfide-carbide (MoS2-Mo2C) heterojunction. The MoS2-Mo2C/Mo electrode exhibits excellent HER activity with low overpotentials of 56 and 446 mV at current densities of 10 and 1000 mA cm-2, respectively, in alkaline electrolytes.

Nonlayered 2D ultrathin molybdenum nitride synthesized through the ammonolysis of 2D molybdenum dioxide.

We report a new scalable strategy for the synthesis of nonlayered ultrathin two-dimensional (2D) molybdenum nitride (MoN) on a SiO2/Si substrate by converting 2D molybdenum dioxide (MoO2) through an ammonolysis process. The edge of MoN shows higher performance than that of the basal plane in both acidic and alkaline solutions.

Boosting the quantum efficiency of the BiVO4 photoanode by increasing the oxygen vacancies for highly-efficient solar water oxidation.

BiVO4 (BVO) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, it is severely restricted by its short charge diffusion length and poor charge transport. Introducing oxygen vacancies into BVO is an effective method to solve these problems because they serve as surface electron capture sites and facilitate charge separation. In this work, a novel gas reaction method using chemical vapor deposition was used to produce abundant oxygen vacancies in single-crystal BVO. Oxygen vacancies in BVO acted as hole donors. This method effectively reduced the surface agglomeration and produced uniform BVO crystals. The optimized BVO photoanode achieved a photocurrent density of 2.44 mA cm-2 (1.23 V vs. RHE) and an incident photon-to-current efficiency of 90% (450 nm). This work provides an effective strategy to prepare high-performance BVO photoanodes by chemical vapor deposition, electrodeposition and thermal evaporation.

A high-performance silicon photoanode enabled by oxygen vacancy modulation on NiOOH electrocatalyst for water oxidation.

Silicon (Si) is an attractive photoanode material for photoelectrochemical (PEC) water splitting. However, Si photoanode towards the oxygen evolution reaction (OER) is highly challenged due to its poor stability and catalytic inactivity. The integration of highly active electrocatalysts with Si photoanodes has been considered to be an effective strategy to improve their OER performance by accelerating the reaction kinetics and inhibiting Si photocorrosion. In this work, ultra-small NiFe nanoparticles are deposited onto the n-Si/Ni/NiOOH surface to improve the activity and stability of Si photoanodes by engineering the electrocatalyst and Si interface. Ultra-small NiFe nanoparticles can introduce oxygen vacancies via modulating the local electronic structure of Ni hosts in NiOOH electrocatalysts for fast charge separation and transfer. Besides, NiFe nanoparticles can also serve as a co-catalyst exposing more active sites and as a protection layer preventing Si photocorrosion. The as-prepared n-Si/Ni/NiOOH/NiFe photoanode exhibits excellent OER activity with an onset potential of 1.0 V versus reversible hydrogen electrode (RHE) and a photocurrent density of ∼25.2 mA cm-2 at 1.23 V versus RHE. This work provides a promising approach to design high-performance Si photoanodes by surface electrocatalyst engineering.

High-Performance Silicon Photoanode Enhanced by Gold Nanoparticles for Efficient Water Oxidation.

Ni catalyst is a low-cost catalyst for oxygen evolution reaction (OER) on silicon metal-insulator-semiconductor photoanode. We found that Au nanoparticles incorporated with Ni nanoparticles can enhance the OER activity and stability of Ni nanoparticles due to the local surface plasmon resonance (LSPR) effect of the Au nanoparticles. The efficiency of NiAu/TiO2/n-Si photoanode can be boosted at least three times under the illumination (100 mW/cm2) by LSPR effect of the Au nanoparticles. A small onset potential of 1.03 V versus reversible hydrogen electrode (overpotential, η0 = -0.20 V) and a current density of 18.80 mA/cm2 at 1.23 V versus reversible hydrogen electrode can be obtained. The NiAu/TiO2/n-Si photoanode exhibits a high saturation current density of 35 mA/cm2, which is greater than that of most of the state-of-the-art silicon photoanodes.