Chang-Lung Hsu

Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene‐Protected 3D Ni Foams

A three-dimensional Ni foam deposited with graphene layers on surfaces is used as a conducting solid support to load MoS(x) catalysts for electrocatalytic hydrogen evolution. The graphene sheets grown on Ni foams provide robust protection and efficiently increase the stability in acid. The superior performance of hydrogen evolution is attributed to the relatively high catalyst loading weight as well as its relatively low resistance.

Self-assembly of hierarchical MoSx/CNT nanocomposites (2<x<3): towards high performance anode materials for lithium ion batteries.

Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 ≤ x ≤ 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 200°C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., > 1000 mAh/g) was achieved at the current density of 50 mA g−1, which is much higher than theoretical numbers for either MWNTs or MoS2 along (~372 and ~670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.

Layer-by-Layer Graphene/ TCNQ Stacked Films as Conducting Anodes for Organic Solar Cells

Large-area graphene grown by chemical vapor deposition (CVD) is a promising candidate for transparent conducting electrode applications in flexible optoelectronic devices such as light-emitting diodes or organic solar cells. However, the power conversion efficiency (PCE) of the polymer photovoltaic devices using a pristine CVD graphene anode is still not appealing due to its much lower conductivity than that of conventional indium tin oxide. We report a layer-by-layer molecular doping process on graphene for forming sandwiched graphene/tetracyanoquinodimethane (TCNQ)/graphene stacked films for polymer solar cell anodes, where the TCNQ molecules (as p-dopants) were securely embedded between two graphene layers. Poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction polymer solar cells based on these multilayered graphene/TCNQ anodes are fabricated and characterized. The P3HT/PCBM device with an anode structure composed of two TCNQ layers sandwiched by three CVD graphene layers shows optimum PCE (∼2.58%), which makes the proposed anode film quite attractive for next-generation flexible devices demanding high conductivity and transparency.

Label-free detection of DNA hybridization using transistors based on CVD grown graphene.

The high transconductance and low noise of graphene-based field-effect transistors based on large-area monolayer graphene produced by chemical vapor deposition are used for label-free electrical detection of DNA hybridization. The gate materials, buffer concentration and surface condition of graphene have been optimized to achieve the DNA detection sensitivity as low as 1 pM (10(-12) M), which is more sensitive than the existing report based on few-layer graphene. The graphene films obtained using conventional PMMA-assisted transfer technique exhibits PMMA residues, which degrade the sensing performance of graphene. We have demonstrated that the sensing performance of the graphene samples prepared by gold-transfer is largely enhanced (by 125%).

Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy

band gap engineering of TMD has become an important topic. In early studies the TMD solid solutions both in the metal (e.g., Mo x W 1−x S 2 ) and chalcogen (e.g., MoS 2x Se 2(1−x) ) sublattice forms have been realized by the direct vapor transport growth, where the stoichiometric amounts of desired powder elements were introduced into a quartz ampoule for crystal growth. [ 17,18 ] Meanwhile, the growth of MoS 2 , WSe 2 and WS 2 monolayers has been reported recently by using sulfurization or selenization of transition metal oxides with chemical vapor deposition (CVD) techniques. [ 19–21 ] The density-functinoal theory (DFT) calculations show that the single layers of mixed TMDs, such as MoS 2x Se 2(1−x) are thermodynamically stable at room temperature, [ 22 ] so that such materials can be manufactured using chemical-vapor deposition technique. It is therefore useful to know whether it is possible to realize the synthesis of MoS 2x Se 2(1−x) monlayers which exhibit intriguing electronic properties and tunable optical band gaps. Very recently, the transition-metal dichalcogenide monolayer alloys (Mo 1–x W x S 2 ) have been obtained by mechanical cleaving from their bulk crystals, [ 23 ] where the band gap emission ranges from 1.82 eV to 1.99 eV. Note that the mechanical cleavage is valuable for fundamental research; however, a simple and scalable method to obtain TMD monolayers with controllable optical energy gaps is still urgently needed. In this contribution, we report that the MoS 2 monolayer fl akes prepared by CVD can be selenized in the presence of selenium vapors to form MoS x Se y monolayers. The optical band gap of the obtained MoS x Se y , ranging from 1.86 eV to 1.57 eV, is easily controllable by the selenization temperature. It is key demonstration for controlling electronic and optoelectronic structures of TMD monolayers using a simple method, where pproach is straightforward and applicable to the band gap engineering for other TMD monolayers. The CVD-grown MoS 2 monolayers were synthesized based on our previous reports. [ 19 ] In brief, the triangular MoS 2 fl akes are formed by the vapor phase reaction of MoO 3 with S powders, where the MoS 2 monolayers with a lateral size up to tens micron can be obtained and which growth method has been adopted by many other groups . [ 24,25 ] To modulate the electronic structures and optical band gaps of the MoS 2 monolayer, we perform the selenization in a hot-wall furnace at various temperatures. The scheme in Figure 1 a illustrates the experimental set-up for the selenization process, where the inlet gas (a mixture of Ar and H 2 ) carries the vaporized DOI: 10.1002/smll.201302893 2D Materials

Three‐Dimensional Molybdenum Sulfide Sponges for Electrocatalytic Water Splitting

Electroactive MoSx catalysts on porous 3D sponges synthezied by a simple and scalable thermolysis process are proposed. Although no conducting materials are used to host the MoSx catalysts, they still serve as efficient electrodes for hydrogen evolution. The high current density of the MoSx-coated sponges are attributed to the large electrochemical surface area and their S-rich chemical structure.

Growth selectivity of hexagonal-boron nitride layers on Ni with various crystal orientations

Layered hexagonal-boron nitride (h-BN) films were synthesized by chemical vapor deposition (CVD) on Ni foils using ammonia borane as a precursor. Confocal Raman spectroscopy and electron backscatter diffraction (EBSD) were used to probe the effect of underlying Ni crystals with various orientations on growth behaviors of h-BN layers. The growth of the h-BN layers strongly depends on the Ni crystal orientations, where the growth rate of h-BN is larger on Ni(100)-like crystal surfaces but the growth on Ni(111)-like surfaces is not detectable, suggesting that Ni (100)-like facets are likely to promote the growth of h-BN compared with Ni (111)-like surfaces. The observation is in clear contrast to the reported growth of h-BN on Ni(111) in an ultrahigh vacuum environment. The as-grown CVD h-BN films on Ni exhibit a layered structure as revealed by atomic force microscopy (AFM). Thin h-BN layers are found on the Ni domain with a low growth rate. The observation of h-BN growth on various Ni grains may provide insights for the control of thickness, size and morphology of CVD h-BN films.

Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment

Atomically thin two-dimensional transition-metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe2 monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap-state emission and promote the neutral exciton and trion emission in defective MoSe2 monolayers through the p-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe2 layers. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs.

Controllable Synthesis of Band-Gap-Tunable and Monolayer Transition-Metal Dichalcogenide Alloys

The electronic and optical properties of transition metal dichalcogenide (TMD) materials are directly governed by their energy gap; thus, the band gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayer TMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here we systematically compared the two approaches of forming MoS2xSe2(1-x) monolayer alloys: selenization of MoS2 and sulfurization of MoSe2. The optical energy gap of as-grown CVD MoS2 can be continuously modulated from 1.86 eV (667 nm) to 1.57 eV (790 nm) controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe2 and thus the MoSe2/MoS2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.

Enhanced Electrocatalytic Activity of MoSx on TCNQ-Treated Electrode for Hydrogen Evolution Reaction

Molybdenum sulfide has recently attracted much attention because of its low cost and excellent catalytical effects in the application of hydrogen evolution reaction (HER). To improve the HER efficiency, many researchers have extensively explored various avenues such as material modification, forming hybrid structures or modifying geometric morphology. In this work, we reported a significant enhancement in the electrocatalytic activity of the MoSx via growing on Tetracyanoquinodimethane (TCNQ) treated carbon cloth, where the MoSx was synthesized by thermolysis from the ammonium tetrathiomolybdate ((NH4)2MoS4) precursor at 170 °C. The pyridinic N- and graphitic N-like species on the surface of carbon cloth arising from the TCNQ treatment facilitate the formation of Mo(5+) and S2(2-) species in the MoSx, especially with S2(2-) serving as an active site for HER. In addition, the smaller particle size of the MoSx grown on TCNQ-treated carbon cloth reveals a high ratio of edge sites relative to basal plane sites, indicating the richer effective reaction sites and superior electrocatalytic characteristics. Hence, we reported a high hydrogen evolution rate for MoSx on TCNQ-treated carbon cloth of 6408 mL g(-1) cm(-2) h(-1) (286 mmol g(-1) cm(-2) h(-1)) at an overpotential of V = 0.2 V. This study provides the fundamental concepts useful in the design and preparation of transition metal dichalcogenide catalysts, beneficial in the development in clean energy.