Gold-copper oxide core-shell plasmonic nanoparticles: the effect of pH on shell stability and mechanistic insights into shell formation

Plasmonic nanoparticles play an important role in applications for chemical sensing, catalysis, medicine, and biosensing. The localized surface plasmon resonance (LSPR) of a nanoparticle is determined by factors such as size, shape, and the local dielectric environment. Here, we report a simple colloidal synthesis method to create core-shell plasmonic nanoparticles with a gold core and a copper oxide (Cu2O) shell. The gold cores are particles of various shapes and sizes, including nanorods, nanobipyramids, and nanoshells, and the Cu2O shell is on the order of 30-40 nm thick. The growth of the oxide shell red shifts the plasmonic absorption of the gold core particle by up to 250 nanometers, resulting in a particle that can absorb into the near-infrared (NIR). Additionally, we report the unique ability to immediately remove and regrow the oxide shell by simple changes to the solution pH. We demonstrate the repeated dissolution and nucleation of the oxide shell through the addition of an acid and a base, respectively. The process is confirmed by characterization using Ultraviolet-Visible-Near-IR (UV-Vis-NIR) spectroscopy and electron microscopy of the particles. After several iterations of this process, we report the formation of large Cu2O spheres, where Cu2O nucleation on other Cu2O particles is favored over the gold nanoparticles. In addition, we provide insight into the role of ligands in shell formation.

Cellulose Nanofiber-Alginate Biotemplated Cobalt Composite Multifunctional Aerogels for Energy Storage Electrodes

Tunable porous composite materials to control metal and metal oxide functionalization, conductivity, pore structure, electrolyte mass transport, mechanical strength, specific surface area, and magneto-responsiveness are critical for a broad range of energy storage, catalysis, and sensing applications. Biotemplated transition metal composite aerogels present a materials approach to address this need. To demonstrate a solution-based synthesis method to develop cobalt and cobalt oxide aerogels for high surface area multifunctional energy storage electrodes, carboxymethyl cellulose nanofibers (CNF) and alginate biopolymers were mixed to form hydrogels to serve as biotemplates for cobalt nanoparticle formation via the chemical reduction of cobalt salt solutions. The CNF-alginate mixture forms a physically entangled, interpenetrating hydrogel, combining the properties of both biopolymers for monolith shape and pore size control and abundant carboxyl groups that bind metal ions to facilitate biotemplating. The CNF-alginate hydrogels were equilibrated in CaCl2 and CoCl2 salt solutions for hydrogel ionic crosslinking and the prepositioning of transition metal ions, respectively. The salt equilibrated hydrogels were chemically reduced with NaBH4, rinsed, solvent exchanged in ethanol, and supercritically dried with CO2 to form aerogels with a specific surface area of 228 m2/g. The resulting aerogels were pyrolyzed in N2 gas and thermally annealed in air to form Co and Co3O4 porous composite electrodes, respectively. The multifunctional composite aerogel's mechanical, magnetic, and electrochemical functionality was characterized. The coercivity and specific magnetic saturation of the pyrolyzed aerogels were 312 Oe and 114 emu/gCo, respectively. The elastic moduli of the supercritically dried, pyrolyzed, and thermally oxidized aerogels were 0.58, 1.1, and 14.3 MPa, respectively. The electrochemical testing of the pyrolyzed and thermally oxidized aerogels in 1 M KOH resulted in specific capacitances of 650 F/g and 349 F/g, respectively. The rapidly synthesized, low-cost, hydrogel-based synthesis for tunable transition metal multifunctional composite aerogels is envisioned for a wide range of porous metal electrodes to address energy storage, catalysis, and sensing applications.

Elucidation of Molybdenum Trioxide Sulfurization: Mechanistic Insights into Two-Dimensional Molybdenum Disulfide Growth

Powder vaporization is a common method for the generation of large-area, single-crystal, two-dimensional molybdenum disulfide. While commonly employed as a growth method, the fundamental molecular mechanisms are not well understood. Recent ab initio analyses have shown that molybdenum oxysulfide rings play a key role in the sulfurization of molybdenum trioxide from elemental sulfur. In this study, we utilize molecular dynamics simulations with a reactive force field and ab initio calculations to elucidate the reaction pathway of sulfur with molybdenum trioxide. The molecular dynamics simulations demonstrated that for all sulfur allotropes the reaction pathway could be reduced to that of disulfur, trisulfur, or a combination of the two and that molybdenum trioxide can catalyze the decomposition of larger sulfur allotropes. Ab initio calculations were used to illuminate the intermediates and transition states in the reaction pathways for disulfur and trisulfur. Analysis of the temperature dependence of the transition state energies shows that the maximum reaction rates occur between 1000 and 1100 K, which corresponds with commonly reported experimental growth temperatures for molybdenum disulfide.

A Salt-Templated Synthesis Method for Porous Platinum-based Macrobeams and Macrotubes

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed by rinsing and supercritical drying steps, often resulting in mechanically fragile materials. Here, a method to synthesize nanostructured porous platinum-based macrotubes and macrobeams with a square cross section from insoluble salt needle templates is presented. The combination of oppositely charged platinum, palladium, and copper square planar ions results in the rapid formation of insoluble salt needles. Depending on the stoichiometric ratio of metal ions present in the salt-template and the choice of chemical reducing agent, either macrotubes or macrobeams form with a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Elemental composition of the macrotubes and macrobeams, determined with x-ray diffractometry and x-ray photoelectron spectroscopy, is controlled by the stoichiometric ratio of metal ions present in the salt-template. Macrotubes and macrobeams may be pressed into free standing films, and the electrochemically active surface area is determined with electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials.

Vanadium disulfide flakes with nanolayered titanium disulfide coating as cathode materials in lithium-ion batteries

Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g-1 @200 mA g-1 current density) and rate capability (~70 mAh g-1 @1000 mA g-1), while achieving capacity retention close to 100% after 400 charge-discharge steps.

The Role of Molybdenum Oxysulfide Rings in the Formation of Two-Dimensional Molybdenum Disulfide by Powder Vaporization

Sulfurization of molybdenum trioxide by elemental sulfur through powder vaporization is a common method used for growth of molybdenum disulfide. Optimization of complexes between sulfur allotropes and molybdenum species using Density Functional Theory has revealed the molecular mechanism of sulfurization. Complete sulfurization of molybdenum trioxide to molybdenum disulfide requires at least three sets of nucleophilic addition-elimination reactions that generate the experimentally observed molybdenum oxysulfide intermediates along the reaction pathway. Each nucleophilic addition reaction of a sulfur allotrope to a molybdenum species gives rise to a molybdenum oxysulfide ring, which can dissociate into a more sulfurized molybdenum intermediate. At the typical growth temperatures used in powder vaporization, the equilibrium constants for these reactions are essentially unity. Thus, sulfurization is driven by excess sulfur and gas flow through the growth furnace.

Utilizing van der Waals Slippery Interfaces to Enhance the Electrochemical Stability of Silicon Film Anodes in Lithium-Ion Batteries

High specific capacity anode materials such as silicon (Si) are increasingly being explored for next-generation, high performance lithium (Li)-ion batteries. In this context, Si films are advantageous compared to Si nanoparticle based anodes since in films the free volume between nanoparticles is eliminated, resulting in very high volumetric energy density. However, Si undergoes volume expansion (contraction) under lithiation (delithiation) of up to 300%. This large volume expansion leads to stress build-up at the interface between the Si film and the current collector, leading to delamination of Si from the surface of the current collector. To prevent this, adhesion promotors (such as chromium interlayers) are often used to strengthen the interface between the Si and the current collector. Here, we show that such approaches are in fact counter-productive and that far better electrochemical stability can be obtained by engineering a van der Waals "slippery" interface between the Si film and the current collector. This can be accomplished by simply coating the current collector surface with graphene sheets. For such an interface, the Si film slips with respect to the current collector under lithiation/delithiation, while retaining electrical contact with the current collector. Molecular dynamics simulations indicate (i) less stress build-up and (ii) less stress "cycling" on a van der Waals slippery substrate as opposed to a fixed interface. Electrochemical testing confirms more stable performance and much higher Coulombic efficiency for Si films deposited on graphene-coated nickel (i.e., slippery interface) as compared to conventional nickel current collectors.

Protecting Silicon Film Anodes in Lithium-Ion Batteries Using an Atomically Thin Graphene Drape

Silicon (Si) shows promise as an anode material in lithium-ion batteries due to its very high specific capacity. However, Si is highly brittle, and in an effort to prevent Si from fracturing, the research community has migrated from the use of Si films to Si nanoparticle based electrodes. However, such a strategy significantly reduces volumetric energy density due to the porosity of Si nanoparticle electrodes. Here we show that contrary to conventional wisdom, Si films can be stabilized by two strategies: (a) anchoring the Si films to a carbon nanotube macrofilm (CNM) current collector and (b) draping the films with a graphene monolayer. After electrochemical cycling, the graphene-coated Si films on CNM resembled a tough mud-cracked surface in which the graphene capping layer suppresses delamination and stabilizes the solid electrolyte interface. The graphene-draped Si films on CNM exhibit long cycle life (>1000 charge/discharge steps) with an average specific capacity of ∼806 mAh g^-1. The volumetric capacity averaged over 1000 cycles of charge/discharge is ∼2821 mAh cm^-3, which is 2 to 5 times higher than what is reported in the literature for Si nanoparticle based electrodes. The graphene-draped Si anode could also be successfully cycled against commercial cathodes in a full-cell configuration.

Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems.