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1.
J Vis Exp ; (159)2020 May 18.
Artigo em Inglês | MEDLINE | ID: mdl-32478752

RESUMO

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.

2.
Nat Commun ; 10(1): 1764, 2019 04 16.
Artigo em Inglês | MEDLINE | ID: mdl-30992432

RESUMO

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.

3.
J Phys Chem A ; 122(37): 7320-7327, 2018 Sep 20.
Artigo em Inglês | MEDLINE | ID: mdl-30092134

RESUMO

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.

4.
ACS Appl Mater Interfaces ; 10(16): 13442-13451, 2018 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-29620865

RESUMO

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.

5.
J Colloid Interface Sci ; 524: 204-208, 2018 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-29655138

RESUMO

Micro- and nanostructured surfaces are known to induce anti-wetting and self-cleaning properties. However, traditional formation of these structures is difficult and requires high-resolution micro- and nanofabrication methods. Here, we demonstrate a facile method for the formation of superhydrophobic self-cleaning surfaces by laser pulse heating of a carbon nanotube-polymer composite. Laser treatment exposes a carbon nanotube network which controls surface wetting properties. Advancing and receding contact angle measurements demonstrate that these surfaces are superhydrophobic. Additionally, surfaces show anti-wetting and self-cleaning properties.

6.
ACS Nano ; 11(5): 5051-5061, 2017 05 23.
Artigo em Inglês | MEDLINE | ID: mdl-28414906

RESUMO

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.

7.
ACS Nano ; 8(5): 5282-9, 2014 May 27.
Artigo em Inglês | MEDLINE | ID: mdl-24754702

RESUMO

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.

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