In Situ XPS Investigation of Transformations at Crystallographically Oriented MoS₂ Interfaces

Nanoscale transition-metal dichalcogenide (TMDC) materials, such as MoS₂, exhibit promising behavior in next-generation electronics and energy-storage devices. TMDCs have a highly anisotropic crystal structure, with edge sites and basal planes exhibiting different structural, chemical, and electronic properties. In virtually all applications, two-dimensional or bulk TMDCs must be interfaced with other materials (such as electrical contacts in a transistor). The presence of edge sites vs basal planes (i.e., the crystallographic orientation of the TMDC) could influence the chemical and electronic properties of these solid-state interfaces, but such effects are not well understood. Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate how the crystallography and structure of MoS₂ influence chemical transformations at solid-state interfaces with various other materials. MoS₂ materials with controllably aligned crystal structures (horizontal vs vertical orientation of basal planes) were fabricated, and in situ XPS was carried out by sputter-depositing three different materials (Li, Ge, and Ag) onto MoS₂ within an XPS instrument while periodically collecting photoelectron spectra; these deposited materials are of interest due to their application in electronic devices or energy storage. The results showed that Li reacts readily with both crystallographic orientations of MoS₂ to form metallic Mo and Li₂S, while Ag showed very little chemical or electronic interaction with either type of MoS₂. In contrast, Ge showed significant chemical interactions with MoS₂ basal planes, but only minor chemical changes were observed when Ge contacted MoS₂ edge sites. These findings have implications for electronic transport and band alignment at these interfaces, which is of significant interest for a variety of applications

Seeded Nanowire and Microwire Growth from Lithium Alloys

Although vapor–liquid–solid (VLS) growth of nanowires from alloy seed particles is common in various semiconductor systems, related wire growth in all-metal systems is rare. Here, we report the spontaneous growth of nano- and microwires from metal seed particles during the cooling of Li-rich bulk alloys containing Au, Ag, or In. The as-grown wires feature Au-, Ag-, or In-rich metal tips and LiOH shafts; the results indicate that the wires grow as Li metal and are converted to polycrystalline LiOH during and/or after growth due to exposure to H₂O and O₂. This new process is a simple way to create nanostructures, and the findings suggest that metal nanowire growth from alloy seeds is possible in a variety of systems

Operando Synchrotron Measurement of Strain Evolution in Individual Alloying Anode Particles within Lithium Batteries

Alloying anode materials offer high capacity for next-generation batteries, but the performance of these materials often decays rapidly with cycling because of volume changes and associated mechanical degradation or fracture. The direct measurement of crystallographic strain evolution in individual particles has not been reported, however, and this level of insight is critical for designing mechanically resilient materials. Here, we use operando X-ray diffraction to investigate strain evolution in individual germanium microparticles during electrochemical reaction with lithium. The diffraction peak was observed to shift in position and diminish in intensity during reaction because of the disappearance of the crystalline Ge phase. The compressive strain along the [111] direction was found to increase monotonically to a value of −0.21%. This finding is in agreement with a mechanical model that considers expansion and plastic deformation during reaction. This new insight into the mechanics of large-volume-change transformations in alloying anodes is important for improving the durability of high-capacity batteries

Reversible Tuning of the Surface Plasmon Resonance of Indium Tin Oxide Nanocrystals by Gas-Phase Oxidation and Reduction

Heavily doped oxide nanocrystals exhibit a tunable localized surface plasmon resonance (LSPR) in the infrared, a property that is promising for applications in photonics, spectroscopy, and photochemistry. Nanocrystal carrier density and, thus, spectral response are adjustable via chemical reaction; however, the fundamental processes that govern this behavior are poorly understood. Here, we study the time dependence of the LSPR supported by indium tin oxide (ITO) nanocrystals during O₂ and N₂ annealing with <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that the LSPR red-shifts upon oxidation in O₂ and blue-shifts to its original position upon reduction in N₂. A reaction–diffusion model allows us to rationalize the underlying physicochemical processes and quantitatively connect nanocrystal redox chemistry, solid-state diffusion, carrier density, and the LSPR

A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes

Silicon is regarded as one of the most promising anode materials for next generation lithium-ion batteries. For use in practical applications, a Si electrode must have high capacity, long cycle life, high efficiency, and the fabrication must be industrially scalable. Here, we design and fabricate a yolk-shell structure to meet all these needs. The fabrication is carried out without special equipment and mostly at room temperature. Commercially available Si nanoparticles are completely sealed inside conformal, thin, self-supporting carbon shells, with rationally designed void space in between the particles and the shell. The well-defined void space allows the Si particles to expand freely without breaking the outer carbon shell, therefore stabilizing the solid-electrolyte interphase on the shell surface. High capacity (∼2800 mAh/g at C/10), long cycle life (1000 cycles with 74% capacity retention), and high Coulombic efficiency (99.84%) have been realized in this yolk-shell structured Si electrode

Common Capacity Fade Mechanisms of Metal Foil Alloy Anodes with Different Compositions for Lithium Batteries

Metal foils are attractive anode candidates for replacing graphite in lithium-ion batteries, since metal alloys feature high lithium storage capacity and their direct use as foils could avoid slurry coating during manufacturing. Aluminum foil is highly abundant and low-cost, but aluminum foil anodes have generally shown poor cyclability. Here, we fabricate aluminum alloy foils (aluminum–tin, aluminum–zinc, and aluminum–gallium) and examine their electrochemical behavior to understand how composition and microstructure influence cycling performance of metal foil anodes. We show that the addition of alloy components can increase the cycle life of aluminum foil anodes by up to a factor of 2, and both the composition and microstructure of foils influence the cycling capability. We find an approximate power-law dependence of cycle life on the extent of lithiation per cycle for most aluminum-based foils as well as other metal foil compositions, suggesting a common “electrochemical fatigue” degradation mechanism arising from internal porosity formation during alloying/dealloying that governs the behavior of a wide variety of metal foil-based anodes. This understanding, as well as the improved cyclability of the alloy foils, suggests possible pathways to enhance performance of foil anodes for lithium batteries

Common Capacity Fade Mechanisms of Metal Foil Alloy Anodes with Different Compositions for Lithium Batteries

Metal foils are attractive anode candidates for replacing graphite in lithium-ion batteries, since metal alloys feature high lithium storage capacity and their direct use as foils could avoid slurry coating during manufacturing. Aluminum foil is highly abundant and low-cost, but aluminum foil anodes have generally shown poor cyclability. Here, we fabricate aluminum alloy foils (aluminum–tin, aluminum–zinc, and aluminum–gallium) and examine their electrochemical behavior to understand how composition and microstructure influence cycling performance of metal foil anodes. We show that the addition of alloy components can increase the cycle life of aluminum foil anodes by up to a factor of 2, and both the composition and microstructure of foils influence the cycling capability. We find an approximate power-law dependence of cycle life on the extent of lithiation per cycle for most aluminum-based foils as well as other metal foil compositions, suggesting a common “electrochemical fatigue” degradation mechanism arising from internal porosity formation during alloying/dealloying that governs the behavior of a wide variety of metal foil-based anodes. This understanding, as well as the improved cyclability of the alloy foils, suggests possible pathways to enhance performance of foil anodes for lithium batteries

Stable Solar-Driven Water Oxidation to O₂(g) by Ni-Oxide-Coated Silicon Photoanodes

Semiconductors with small band gaps (<2 eV) must be stabilized against corrosion or passivation in aqueous electrolytes before such materials can be used as photoelectrodes to directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of photoelectrodes is required for efficient oxidation of H₂O to O₂(g) and reduction of H₂O or H₂O and CO₂ to fuels. We report herein the stabilization of np⁺-Si­(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O₂(g) in 1.0 M KOH­(aq) by an earth-abundant, optically transparent, electrocatalytic, stable, conducting nickel oxide layer. Under simulated solar illumination and with optimized index-matching for proper antireflection, NiO_<i>x</i>-coated np⁺-Si­(100) photoanodes produced photocurrent-onset potentials of −180 ± 20 mV referenced to the equilibrium potential for evolution of O₂(g), photocurrent densities of 29 ± 1.8 mA cm^–2 at the equilibrium potential for evolution of O₂(g), and a solar-to-O₂(g) conversion figure-of-merit of 2.1%

Passivation Coating on Electrospun Copper Nanofibers for Stable Transparent Electrodes

Copper nanofiber networks, which possess the advantages of low cost, moderate flexibility, small sheet resistance, and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the premier transparent electrode. However, the chemical activity of copper nanofibers causes a substantial increase in the sheet resistance after thermal oxidation or chemical corrosion of the nanofibers. In this work, we utilize atomic layer deposition to coat a passivation layer of aluminum-doped zinc oxide (AZO) and aluminum oxide onto electrospun copper nanofibers and remarkably enhance their durability. Our AZO–copper nanofibers show resistance increase of remarkably only 10% after thermal oxidation at 160 °C in dry air and 80 °C in humid air with 80% relative humidity, whereas bare copper nanofibers quickly become insulating. In addition, the coating and baking of the acidic PEDOT:PSS layer on our fibers increases the sheet resistance of bare copper nanofibers by 6 orders of magnitude, while the AZO–Cu nanofibers show an 18% increase

Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS₂ and MoSe₂

Single crystals of n-type MoS₂ and n-MoSe₂ showed higher electrocatalytic activity for the evolution of H₂(g) in alkaline solutions than in acidic solutions. The overpotentials required to drive hydrogen evolution at −10 mA cm^–2 of current density for MoS₂ samples were −0.76 ± 0.13 and −1.03 ± 0.21 V when in contact with 1.0 M NaOH­(aq) and 1.0 M H₂SO₄(aq), respectively. For MoSe₂ samples, the overpotentials at −10 mA cm^–2 were −0.652 ± 0.050 and −0.709 ± 0.073 V in contact with 1.0 M KOH­(aq) and 1.0 M H₂SO₄(aq), respectively. Single crystals from two additional sources were also tested, and the absolute values of the measured overpotentials were consistently less (by 460 ± 250 mV) in alkaline solutions than in acidic solutions. When electrochemical etching was used to create edge sites on the single crystals, the kinetics improved in acid but changed little in alkaline media. The overpotentials measured for polycrystalline thin films (PTFs) and amorphous forms of MoS₂ showed less sensitivity to pH and edge density than was observed for single crystals and showed enhanced kinetics in acid when compared to alkaline solutions. These results suggest that the active sites for hydrogen evolution on MoS₂ and MoSe₂ are different in alkaline and acidic media. Thus, while edges are known to serve as active sites in acidic media, in alkaline media it is more likely that terraces function in this role