Jianwei Chai

Preparation of MoS2–MoO3 Hybrid Nanomaterials for Light‐Emitting Diodes

A facile strategy to prepare MoS2 -MoO3 hybrid nanomaterials is developed, based on the heat-assisted partial oxidation of lithium-exfoliated MoS2  nanosheets in air followed by thermal-annealing-driven crystallization. The obtained MoS2 -MoO3 hybrid nanomaterial exhibits p-type conductivity. As a proof-of-concept application, an n-type SiC/p-type MoS2 -MoO3 heterojunction is used as the active layer for light-emitting diodes. The origins of the electroluminescence from the device are theoretically investigated. This facile synthesis and application of hybrid nanomaterials opens up avenues to develop new advanced materials for various functional applications, such as in electrics, optoelectronics, clean energy, and information storage.

Metal-containing amorphous carbon films for hydrophobic application

Abstract Tetrahedral amorphous carbon (ta-C) and metal-containing amorphous carbon (a-C/Me) films including a-C/Ni, a-C/Fe and a-C/Al films were deposited using metal–carbon composite targets, by a filtered cathodic vacuum arc (FCVA) technique. The wettability of the films was examined by the contact angle measurement. Three types of liquid with different polarity were used to study the changes in the surface energy including the dispersive and polar components. Micro-Raman spectroscopy and X-ray induced photoelectron spectroscopy were used to characterize the structural and surface properties of the films. Atomic force microscopy (AFM) was used to characterize the morphology and roughness of the films. The contact angle decreases after incorporating Ni into the films. However, the contact angle of films increases after incorporating Fe and Al into the films, respectively. The contact angle of a-C/Al film with water reaches as high as 104°. The roughness of the films has no obvious effect on the contact angle. The structure and surface analysis show that the surface energy is related to sp 3 /sp 2 content in the film. The surface adsorption plays an important role in the change of the surface energy.

X-ray photoelectron spectroscopy studies of nitridation on 4H-SiC (0001) surface by direct nitrogen atomic source

A Si3N4 passivation layer has been successfully grown on the 4H-SiC (0001) surface by direct atomic source nitridation at various substrate temperatures. In situ x-ray photoelectron spectroscopy measurements show that higher substrate temperature leads to higher nitridation rate and good crystallinity of the passivation layer. A thin oxynitride layer on the top of the Si3N4 was observed due to the residual O in the vacuum system, but was decomposed during annealing. In the meantime, excess C was found to be effectively removed by the reactive atomic N source.

Surface energy of amorphous carbon films containing iron

Iron containing diamond-like amorphous carbon (a-C:Fe) films were deposited by filtered cathodic vacuum arc technique. The influences of Fe content and substrate bias on the surface energy of the films were investigated. The surface energy of a-C:Fe films was determined by the contact angle measurement. Atomic force microscopy, Raman spectroscopy, and x-ray induced photoelectron spectroscopy were employed to analyze the origin of the variation of surface energy with various Fe content and substrate bias. It is found that the contact angle for water increases significantly after incorporating Fe into the films and the films become hydrophobic. The roughness of these films has no effect on the contact angle. The surface energy is reduced from 42.8 to 25 dyne/cm after incorporating Fe into the a-C film (10% Fe in the target), which is due to the reduction of both dispersive and polar component. The reduction in dispersive component is ascribed to the decrease of atomic density of the a-C:Fe films due to the ...

Large-scale two-dimensional MoS 2 photodetectors by magnetron sputtering

We report on the demonstration of photodetectors based on large scale two-dimensional molybdenum disulfide (MoS2) transition metal dichalcogenides. Excellent film uniformity and precise control of the MoS2 thickness down to a monolayer (~0.75nm) were achieved by magnetron sputtering synthesis approach. In particular, the photodetectors integrated with five MoS2 monolayers exhibit a high photoresponsivity of 1.8 A/W, an external quantum efficiency exceeding 260%, and a photodetectivity of ~5 x 10(8) Jones for a wavelength of 850 nm, surpassing the performance of mechanically exfoliated based photodetectors.

Effect of interfacial coupling on photocatalytic performance of large scale MoS2/TiO2 hetero-thin films

Interface electronic behavior of two-dimensional large scale MoS2/TiO2 hetero-thin films has been studied using photoemission spectroscopy. We show a clear experimental evidence for type II band alignment and upward band bending (∼0.55 eV) at the interface of this system. The valence band offset at monolayer MoS2/TiO2 interface was measured to be 2.15 eV, while the conduction band offset was 1.00 eV. The unique interface band positions introduce a strong build-in electric field for efficient electron-hole separation. In addition, thermal treatment results in better interfacial coupling and charge separation efficiency thus enhanced visible light photoactivity. Our results explicate the mechanism and emphasize its huge potential in visible light photocatalysis.

A Robust Hybrid Zn-Battery with Ultralong Cycle Life

Advanced batteries with long cycle life and capable of harnessing more energies from multiple electrochemical reactions are both fundamentally interesting and practically attractive. Herein, we report a robust hybrid zinc-battery that makes use of transition-metal-based redox reaction (M-O-OH → M-O, M = Ni and Co) and oxygen reduction reaction (ORR) to deliver more electrochemical energies of comparably higher voltage with much longer cycle life. The hybrid battery was constructed using an integrated electrode of NiCo2O4 nanowire arrays grown on carbon-coated nickel foam, coupled with a zinc plate anode in alkaline electrolyte. Benefitted from the M-O/M-O-OH redox reactions and rich ORR active sites in NiCo2O4, the battery has concurrently exhibited high working voltage (by M-O-OH → M-O) and high energy density (by ORR). The good oxygen evolution reaction (OER) activity of the electrode and the reversible M-O ↔ M-O-OH reactions also enabled smooth recharging of the batteries, leading to excellent cycling stabilities. Impressively, the hybrid batteries maintained highly stable charge-discharge voltage profile under various testing conditions, for example, almost no change was observed over 5000 cycles at a current density of 5 mA cm-2 after some initial stabilization. With merits of higher working voltage, high energy density, and ultralong cycle life, such hybrid batteries promise high potential for practical applications.

Synergistic effect of 2D Ti2C and g-C3N4 for efficient photocatalytic hydrogen production

Photocatalytic water splitting is an environmentally friendly technique for hydrogen production. In this work, we report a novel photocatalyst consisting of two-dimensional (2D) titanium carbide (Ti2C) and graphitic carbon nitride (g-C3N4). We observe substantially enhanced water splitting activities due to the efficient synergistic interaction between Ti2C and g-C3N4. Optimal properties are achieved in the g-C3N4 with a loading of 0.4 wt% Ti2C with a hydrogen production rate of 47.5 μmol h−1, which is 14.4 times as much as that in the case using pure g-C3N4, and it even outperforms Pt-loaded g-C3N4. We further show that the Ti2C/g-C3N4 has high stability and good reproducibility. We expect that the Ti2C/g-C3N4 can be a photocatalyst for large scale applications because both Ti2C and g-C3N4 are low-cost, abundant, and nontoxic.

Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction

Ni3S2 nanowire arrays doped with vanadium(V) are directly grown on nickel foam by a facile one-step hydrothermal method. It is found that the doping can promote the formation of Ni3S2 nanowires at a low temperature. The doped nanowires show excellent electrocatalytic performance toward hydrogen evolution reaction (HER), and outperform pure Ni3S2 and other Ni3S2-based compounds. The stability test shows that the performance of V-doped Ni3S2 nanowires is improved and stabilized after thousands of linear sweep voltammetry test. The onset potential of V-doped Ni3S2 nanowire can be as low as 39 mV, which is comparable to platinum. The nanowire has an overpotential of 68 mV at 10 mA cm-2, a relatively low Tafel slope of 112 mV dec-1, good stability and high Faradaic efficiency. First-principles calculations show that the V-doping in Ni3S2 extremely enhances the free carrier density near the Fermi level, resulting in much improved catalytic activities. We expect that the doping can be an effective way to enhance the catalytic performance of metal disulfides in hydrogen evolution reaction and V-doped Ni3S2 nanowire is one of the most promising electrocatalysts for hydrogen production.

Deposition of iron containing amorphous carbon films by filtered cathodic vacuum arc technique

Ž. Iron containing amorphous carbon a-C:Fe films have been deposited with an Fegraphite composite target with different Fe Ž. Ž . content by filtered cathodic vacuum arc FCVA technique. X-Ray induced photoelectron spectroscopy XPS was used to analyze the Fe content in the films. Micro-Raman spectroscopy was employed to characterize the structural changes of a-C:Fe films. The properties of the a-C:Fe films such as the intrinsic stress, morphology and roughness investigated by the profiler, atomic force Ž. microscope AFM . The XPS results show that there exists small amount of oxygen in the form of FeO in the films and the Fe fraction in the films is always larger than that in the target. Compared with pure amorphous carbon films the intrinsic stress was effectively reduced by incorporating Fe into the films, and decreases with increasing Fe content. As increasing the Fe content, the clusters in the films become finer and the roughness increases The studies of Raman spectra show that the positions of G peak and D peak shift to low and high wavenumbers, respectively, and the ratio of the intensity of D and G peaks increases with an increase in Fe content, that suggests that the sp 2 -bonded carbon and the size of the sp 2 -bonded cluster increases with an increase in the Fe content. 2001 Elsevier Science B.V. All rights reserved.