Size-Dependent Properties of Two-Dimensional MoS₂ and WS₂

The characteristic differences between MoS₂ and WS₂ nanosheets and nanodots are investigated. The nanosheets were formed by liquid-phase sonication, while the nanodots were formed by breaking the nanosheets through heating the solvent ethylene glycol. The nanosheets and nanodots were approximately 0.7–2 nm thick, with slight deviation. Most of the nanosheets were longer than 100 nm, and most of the nanodots were shorter than 5 nm. As the bulk materials were transformed into nanosheets and/or nanodots, the absorption peaks and Raman peaks shifted to shorter wavelengths. Photoluminescence peaks were observed at 500 and 445 nm in the MoS₂ and WS₂ samples smaller than 100 nm. In the X-ray diffraction spectra, only the (002) peak was present in the nanosheets, while no peak was detected for the nanodots due to their small size. No detectable differences between the nanosheets and nanodots were observed in the transmission electron micrographs, synchrotron radiation photoemission spectra, or work function measurements, suggesting that exfoliation did not affect the crystal structure or bonding configuration of MoS₂ and WS₂. These results could potentially be used for the application of MoS₂ and WS₂ nanosheets and nanodots in optical devices, hydrogen evolution reaction catalysts, bioapplicable devices, and so on

Embossed TiO₂ Thin Films with Tailored Links between Hollow Hemispheres: Synthesis and Gas-Sensing Properties

Embossed TiO2 thin films with high surface areas were achieved using soft templates composed of monolayer polystyrene beads. The structure of links between beads in the templates could be controlled by varying O2 plasma etching time, resulting in a variety of templates with close-linked, nanolinked, or isolated beads. Room-temperature deposition of TiO2 on the plasma-treated templates and calcination at 550 °C resulted in embossed films with tailored links between anatase TiO2 hollow hemispheres. Although all embossed TiO2 films displayed a similar increase in the surface-to-volume ratio compared with a plain TiO2 thin film, the response of embossed TiO2 films with nanolinked hollow hemispheres to CO or ethanol gases was much higher than the response of films with close-linked or isolated hollow hemispheres. The strong correlation between gas sensitivity and the structure of links between the TiO2 hollow hemispheres revealed the critical importance of tailoring links between individual oxide nanostructures for enhancing gas-sensing properties of the ensemble of the individual nanostructures. The facile and large-scale synthesis of embossed TiO2 films with nanolinked hollow hemispheres on Si substrates and the high sensitivity that is achieved without the aid of additives provide a sustainable competitive advantage over other methods for fabricating highly sensitive metal oxide gas sensors

Tailorable Topologies for Selectively Controlling Crystals of Expanded Prussian Blue Analogues

Chemical manipulations of Prussian blues and Prussian blue analogues (PBAs) beyond first-row transition-metal cations have remained quite preliminary to this day. The presented report demonstrates the feasibility of using different types of cations, including general transition-metal ions, p region elements in the periodic table, lanthanide elements, and overlooked cations such as Al3+ and Mo3+ to build unique PBAs. A systematic study of the different types of PBAs is provided in terms of physical and chemical features by means of transition electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption near-edge structure. Diverse PBAs can be synthesized with different morphologies. The [Ni­(CN)4]2–-based PBAs mainly exhibited layered products owing to their 4-fold-coordinated anions. The 6-fold-coordinated anion-based PBAs displayed cubic or distorted cubic crystal structures following the same method of ion arrangements with conventional [Fe­(CN)6]2‑/3–-based PBAs. Furthermore, bonding conditions are greatly affected by the introduced cations. In addition, the PBAs constructed using cations with more unpaired free electrons displayed intense paramagnetic performance. This study provides discoveries regarding innovative PBAs and gives new insights into materials exploration for different target applications

Understanding the Enhancement of the Catalytic Properties of Goethite by Transition Metal Doping: Critical Role of O* Formation Energy Relative to OH* and OOH*

Goethite (α-FeOOH), thermodynamically the most stable phase among various iron (oxy)­hydroxides, is getting attention as an oxygen evolution reaction (OER) catalyst due to its terrestrial abundance. But goethite suffers from an inferior catalytic activity like other iron-based oxides. To enhance its catalytic performance, doping has been applied universally. However, due to the lack of a systematic approach to doping, the choice of dopant element has been carried out without standards. Herein, we provide a comprehensive study on a critical factor to evaluate the activity of an introduced dopant at the goethite surface based on both theoretical investigation and experimental verification. For the pristine goethite, the most dominant surface for OER is determined. To enhance the catalytic property of pristine goethite, transition metals (TM = Cr, Mn, Co, and Ni) are substituted with the surface layer iron atom, and substituted dopants are all confirmed to be the active site of OER. The Co-doped goethite has the oxygen-adsorbed state (O*) formation energy near the optimal value. Lowered overpotential in doped goethite mainly originates from the O* formation energy, which is proportional to the occupied p-band center of adsorbed oxygen. To verify the calculation result, pristine and TM-doped goethite is synthesized. The measured overpotential value has the same tendency as the calculated overpotential value

Reduction of Structural Defects in the GaSb Buffer Layer on (001) GaP/Si for High Performance InGaSb/GaSb Quantum Well Light-Emitting Diodes

Monolithic integration of GaSb-based optoelectronic devices on Si is a promising approach for achieving a low-cost, compact, and scalable infrared photonics platform. While tremendous efforts have been put into reducing dislocation densities by using various defect filter layers, exploring other types of extended crystal defects that can exist on GaSb/Si buffers has largely been neglected. Here, we show that GaSb growth on Si generates a high density of micro-twin (MT) defects as well as threading dislocations (TDs) to accommodate the extremely large misfit between GaSb and Si. We found that a 250 nm AlSb single insertion layer is more effective than AlSb/GaSb strained superlattices in reducing both types of defects, resulting in a 4× and 13× reduction in TD density and MT density, respectively, compared with a reference sample with no defect filter layer. InGaSb quantum well light-emitting diodes were grown on the GaSb/Si templates, and the effect of TD density and MT density on their performance was studied. This work shows the importance of using appropriate defect filter layers for high performance GaSb-based optoelectronic devices on standard on-axis (001) Si via direct epitaxial growth

Rendering Redox Reactions of Cathodes in Li-Ion Capacitors Enabled by Lanthanides

Capacitors allow extremely fast charge and discharge operations, which is a challenge faced by recent metal-ion batteries despite having highly improved energy densities. Thus, combined type electric energy storage devices that can integrate high energy density and high power density with high potentials, can overcome the shortcomings of the current metal-ion batteries and capacitors. However, the limited capacities of cathode materials owing to the barren redox reactions are regarded as an obstacle for the development of future high-performance hybrid metal-ion capacitors. In this study, we demonstrate the redox-reaction-rendering effect of the much overlooked lanthanide elements when used as the cathode of lithium-ion capacitors using the mesoporous carbon (MC) as a matrix material. Consequently, these lanthanide elements can effectively enrich the redox reaction, thus improving the capacity of the matrix materials by more than two times. Typically, the Gd-elemental decoration of MC surprisingly enhances the capacity by almost two times as compared with the underacted MC. Furthermore, the La nanoparticles (NPs) decoration depicts the same behavior. Evident redox peaks were formed on the original rectangular cyclic voltammetry (CV) curves. This study provides the first example of embedding lanthanide elements on matrix materials to enrich the desired redox reactions for improving the electrochemical performances

Stacking-Order Dependence of Strain in Bilayer Graphene: Implications for High-Performance Electronics

The Cu step bunches formed during the synthesis of graphene by chemical vapor deposition (CVD) have been intensively studied to optimize the electrical and mechanical properties of graphene. For example, it has been reported that the compressive strain due to the mismatch between the thermal expansion coefficients of Cu and graphene tends to be released by forming periodic steps depending on the number of graphene layers. However, the stacking-order dependence of the step bunches in multilayer graphene has not yet been investigated. Here, we show that the twisted bilayer graphene (tBLG) with less compressive strain induces the formation of considerably smaller step bunches compared to the case of AB-stacked bilayer graphene (BLG), as evidenced by atomic force microscopy (AFM) and Raman spectroscopy. It is supposed that interlayer slipping between the weakly coupled tBLG layers weakens mechanical stiffness as well as compressive strain to deform the Cu surface. In addition, we also find that the direction of Cu step bunches depends on the lattice orientation of tBLG. Thus, our findings are expected to provide insights into understanding and improving the electrical and mechanical properties of multilayer CVD graphene for high-performance device applications

Transition Metal Disulfide Nanosheets Synthesized by Facile Sonication Method for the Hydrogen Evolution Reaction

Two-dimensional transition metal disulfide (TMD) nanosheets, including MoS₂, WS₂, TaS₂, and TiS₂, were used to catalyze the hydrogen evolution reaction (HER). The TMDs were exfoliated by sonication to generate nanosheet layers that were approximately a few hundred nanometers in size. X-ray diffraction and transmission electron microscope data indicated that the major plane of the exfoliated nanosheets was the (002) plane and that the hexagonal structure is maintained after exfoliation with lattice constants of 0.32 nm for MoS₂ and WS₂ and 0.34 nm for TaS₂ and TiS₂. Exfoliated MoS₂, WS₂, TaS₂, and TiS₂ loaded on Au electrodes exhibited good electrocatalytic activity with low onset potentials of ∼100, 150, 175, and 135 mV, respectively, at a current density of −1 mA/cm². MoS₂ and TiS₂ exhibited the best HER performance with Tafel slopes of 94.91 and 91 mV/decade. These results indicated that TMD nanosheets have potential applications as HER catalysts for the mass production of hydrogen