Yung-Huang Chang

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.

Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures

Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.

High‐Gain Phototransistors Based on a CVD MoS2 Monolayer

A phototransistor based on a chemical vapor deposited (CVD) MoS2 monolayer exhibits a high photoresponsivity (2200 A W(-1) ) and an excellent photogain (5000). The presence of shallow traps contributes to the persistent photoconductivity. Ambient adsorbates act as p-dopants to MoS2 , decreasing the carrier mobility, photoresponsivity, and photogain.

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.

Giant photoluminescence enhancement in tungsten-diselenide–gold plasmonic hybrid structures

Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.

Low-temperature growth of ZnO nanorods in anodic aluminum oxide on Si substrate by atomic layer deposition

Low-temperature growth of self-organized ZnO nanorods on Si substrate is achieved using anodic aluminum oxide and atomic layer deposition at 250°C without catalyst or seed layer. Photoluminescence spectrum indicates that the ZnO nanorod arrays exhibit a blue∕green luminescence at 480nm. In addition, the nanorod arrays demonstrate excellent field-emission properties with a turn-on electric field of 6.5Vμm−1 and a threshold electric field of 9.8Vμm−1, which are attributed to the perfectly perpendicular alignment of ZnO nanorods to the Si substrate.

Amorphous Molybdenum Sulfide on Graphene–Carbon Nanotube Hybrids as Highly Active Hydrogen Evolution Reaction Catalysts

In this study, we report on the deposition of amorphous molybdenum sulfide (MoSx, with x ≈ 3) on a high specific surface area conductive support of Graphene-Carbon Nanotube hybrids (GCNT) as the Hydrogen Evolution Reaction (HER) catalysts. We found that the high surface area GCNT electrode could support the deposition of MoSx at much higher loadings compared with simple porous carbon paper or flat graphite paper. The morphological study showed that MoSx was successfully deposited on and was in good contact with the GCNT support. Other physical characterization techniques suggested the amorphous nature of the deposited MoSx. With a typical catalyst loading of 3 mg cm(-2), an overpotential of 141 mV was required to obtain a current density of 10 mA cm(-2). A Tafel slope of 41 mV decade(-1) was demonstrated. Both measures placed the MoSx-deposited GCNT electrode among the best performing molybdenum sulfide-based HER catalysts reported to date. The electrode showed a good stability with only a 25 mV increase in overpotential required for a current density of 10 mA cm(-2), after undergoing 500 potential sweeps with vigorous bubbling present. The current density obtained at -0.5 V vs SHE (Standard Hydrogen Electrode potential) decreased less than 10% after the stability test. The deposition of MoSx on high specific surface area conductive electrodes demonstrated to be an efficient method to maximize the catalytic performance toward HER.

Electronic structure of the carbon nanotube tips studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

Angle-dependent x-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been performed to differentiate local electronic structures of the tips and sidewalls of highly aligned carbon nanotubes. The intensities of both π*- and σ*-band C K-edge XANES features are found to be significantly enhanced at the tip. SPEM results also show that the tips have a larger density of states and a higher C 1s binding energy than those of sidewalls. The increase of the tip XANES and SPEM intensities are quite uniform over an energy range wider than 10 eV in contrast to earlier finding that the enhancement is only near the Fermi level.

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.