Half-Metallicity in Co-Doped WSe₂ Nanoribbons

The recent development of two-dimensional transition-metal dichalcogenides in electronics and optoelelectronics has triggered the exploration in spintronics, with high demand in search for half-metallicity in these systems. Here, through density functional theory (DFT) calculations, we predict robust half-metallic behaviors in Co-edge-doped WSe₂ nanoribbons (NRs). With electrons partially occupying the antibonding state consisting of Co 3d_yz and Se 4p_z orbitals, the system becomes spin-polarized due to the defect-state-induced Stoner effect and the strong exchange splitting eventually gives rise to the half-metallicity. The half-metal gap reaches 0.15 eV on the DFT generalized gradient approximation level and increases significantly to 0.67 eV using hybrid functional. Furthermore, we find that the half-metallicity sustains even under large external strain and relatively low edge doping concentration, which promises the potential of such Co-edge-doped WSe₂ NRs in spintronics applications

α-Sulfur Crystals as a Visible-Light-Active Photocatalyst

We show that in contrast to conventional compound photocatalysts, α-sulfur crystals of cyclooctasulfur (S₈) are a visible-light-active elemental photocatalyst. The α-S crystals were found to have the ability not only to generate ·OH radicals but also to split water in a photoelectrochemical process under both UV–vis and visible-light irradiation. Although the absolute activity obtained was low because of the large particle size and poor hydrophilicity of the α-S crystals studied, there is great potential for increasing the activity with the assistance of known strategies such as surface modification, nanoscaling, doping, and coupling with other photocatalysts

Enrichment of Semiconducting Single-Walled Carbon Nanotubes by Carbothermic Reaction for Use in All-Nanotube Field Effect Transistors

Selective removal of metallic single-walled carbon nanotubes (SWCNTs) and consequent enrichment of semiconducting SWCNTs were achieved through an efficient carbothermic reaction with a NiO thin film at a relatively low temperature of 350 °C. All-SWCNT field effect transistors (FETs) were fabricated with the aid of a patterned NiO mask, in which the as-grown SWCNTs behaving as source/drain electrodes and the remaining semiconducting SWCNTs that survive in the carbothermic reaction as a channel material. The all-SWCNT FETs demonstrate improved current ON/OFF ratios of ∼10³

NiPS₃ Nanosheet–Graphene Composites as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Developing new electrocatalysts is essentially important for efficient water splitting to produce hydrogen. Two-dimensional (2D) materials provide great potential for high-performance electrocatalysts because of their high specific surface area, abundant active edges, and tunable electronic structure. Here, we report few-layer NiPS₃ nanosheet–graphene composites for high-performance electrocatalysts for oxygen evolution reaction (OER). The pure NiPS₃ nanosheets show an overpotential of 343 mV for a current density of 10 mA cm^–2, which is comparable to that for IrO₂ and RuO₂ catalysts. More importantly, the NiPS₃ nanosheet–graphene composites show significantly improved OER activity due to the synergistic effect. The optimized composite shows a very low overpotential of 294 mV for a current density of 10 mA cm^–2, 351 mV for a current density of 100 mA cm^–2, a small Tafel slope of 42.6 mV dec^–1, and excellent stability. These overall performances are far better than those of the reported 2D materials and even better than those of many traditional materials even at a much lower mass loading of NiPS₃

Hollow Anatase TiO₂ Single Crystals and Mesocrystals with Dominant {101} Facets for Improved Photocatalysis Activity and Tuned Reaction Preference

Faceting photocatalysts has attracted increasing interest to improve photocatalytic activity by optimizing surface charge carrier separation/transfer. In principle, a high photocatalytic activity is co-contributed by both high surface separation/transfer and low bulk recombination of charge carriers. However, little effort focuses on lowering bulk recombination of charge carriers in faceted photocatalysts. In this work, we report the synthesis of hollow anatase TiO₂ single crystals and mesocrystals with dominant {101} facets by a new route with PO₄^3–/F^– as morphology controlling agent. It is found that with respect to solid crystals, being hollow crystals and mesocrystals can substantially improve photocatalytic activity (O₂/H₂ evolution from water splitting, CH₄ generation from photoreduction of CO₂) as a result of the synergistic effects of shortened bulk diffusion length of carriers for the decreased bulk recombination and increased surface area. Furthermore, the photocatalysis reaction preference toward O₂ and H₂ evolution from water splitting can be tuned

Switching Photocatalytic H₂ and O₂ Generation Preferences of Rutile TiO₂ Microspheres with Dominant Reactive Facets by Boron Doping

Revealing the key factors of controlling the reduction and oxidation half reactions of photocatalysis is necessary in order to obtain the implications for designing and developing efficient photocatalysts. In this work, boron-doped TiO₂ microspheres consisting of rutile nanorods with the top reactive {111} facets were synthesized by the acidic hydrolysis of TiB₂. The thermal diffusion of boron from the inner to surface part of the microspheres results in switching of the preference from photocatalytic H₂ evolution to O₂ evolution. This switching is caused by the downward shift of surface band edges with the incorporation of boron in surface

Two-Dimensional MoS₂ Confined Co(OH)₂ Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes

The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS₂ confined Co­(OH)₂ nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS₂ nanosheets followed by Co²⁺ exchange in alkaline media to form MoS₂ intercalated with Co­(OH)₂ nanoparticles (denoted Co-Ex-MoS₂), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER activity and excellent stability, with an onset overpotential and Tafel slope as low as 15 mV and 53 mV dec^–1, respectively, which are among the best values reported so far for the Pt-free HER in alkaline media. Furthermore, density functional theory calculations show that the cojoint roles of Co­(OH)₂ nanoparticles and MoS₂ nanosheets result in the excellent activity of the Co-Ex-MoS₂ electrocatalyst, and the good stability is attributed to the confinement of the Co­(OH)₂ nanoparticles. This work provides an imporant strategy for designing HER electrocatalysts in alkaline solutions, and can, in principle, be expanded to other materials besides the Co­(OH)₂ and MoS₂ used here

Scalable Fabrication of Photochemically Reduced Graphene-Based Monolithic Micro-Supercapacitors with Superior Energy and Power Densities

Micro-supercapacitors (MSCs) hold great promise as highly competitive miniaturized power sources satisfying the increased demand of smart integrated electronics. However, single-step scalable fabrication of MSCs with both high energy and power densities is still challenging. Here we demonstrate the scalable fabrication of graphene-based monolithic MSCs with diverse planar geometries and capable of superior integration by photochemical reduction of graphene oxide/TiO₂ nanoparticle hybrid films. The resulting MSCs exhibit high volumetric capacitance of 233.0 F cm^–3, exceptional flexibility, and remarkable capacity of modular serial and parallel integration in aqueous gel electrolyte. Furthermore, by precisely engineering the interface of electrode with electrolyte, these monolithic MSCs can operate well in a hydrophobic electrolyte of ionic liquid (3.0 V) at a high scan rate of 200 V s^–1, two orders of magnitude higher than those of conventional supercapacitors. More notably, the MSCs show landmark volumetric power density of 312 W cm^–3 and energy density of 7.7 mWh cm^–3, both of which are among the highest values attained for carbon-based MSCs. Therefore, such monolithic MSC devices based on photochemically reduced, compact graphene films possess enormous potential for numerous miniaturized, flexible electronic applications

Titanium Dioxide Crystals with Tailored Facets

Titanium Dioxide Crystals with Tailored Facet

Lithiation of Silicon Nanoparticles Confined in Carbon Nanotubes

Silicon has the highest theoretical lithium storage capacity of all materials at 4200 mAh/g; therefore, it is considered to be a promising candidate as the anode of high-energy-density lithium-ion batteries (LIBs). However, serious volume changes caused by lithium insertion/deinsertion lead to a rapid decay of the performance of the Si anode. Here, a Si nanoparticle (NP)-filled carbon nanotube (CNT) material was prepared by chemical vapor deposition, and a nanobattery was constructed inside a transmission electron microscope (TEM) using the Si NP-filled CNT as working electrode to directly investigate the structural change of the Si NPs and the confinement effect of the CNT during the lithiation and delithiation processes. It is found that the volume expansion (∼180%) of the lithiated Si NPs is restricted by the wall of the CNTs and that the CNT can accommodate this volume expansion without breaking its tubular structure. The Si NP-filled CNTs showed a high reversible lithium storage capacity and desirable high rate capability, because the pulverization and exfoliation of the Si NPs confined in CNTs were efficiently prevented. Our results demonstrate that filling CNTs with high-capacity active materials is a feasible way to make high-performance LIB electrode materials, taking advantage of the unique confinement effect and good electrical conductivity of the CNTs