No abstract

The ability to intercalate guest species into the van der Waals gap of 2D layered materials affords the opportunity to engineer the electronic structures for a variety of applications. Here we demonstrate the continuous tuning of layer vertically aligned MoS 2 nanofilms through electrochemical intercalation of Li + ions. By scanning the Li intercalation potential from high to low, we have gained control of multiple important material properties in a continuous manner, including tuning the oxidation state of Mo, the transition of semiconducting 2H to metallic 1T phase, and expanding the van der Waals gap until exfoliation. Using such nanofilms after different degree of Li intercalation, we show the significant improvement of the hydrogen evolution reaction activity. A strong correlation between such tunable material properties and hydrogen evolution reaction activity is established. This work provides an intriguing and effective approach on tuning electronic structures for optimizing the catalytic activity.2D materials | layer vertically standing | electrochemical catalysis L ayer-structured 2D materials are an interesting family of materials with strong covalent bonding within molecular layers and weak van der Waals interaction between layers. Beyond intensively studied graphene-related materials (1-4), there has been recent strong interest in other layered materials whose vertical thickness can be thinned down to less than few nanometers and horizontal width can also be reduced to nanoscale (5-9). The strong interest is driven by their interesting physical and chemical properties (2, 10) and their potential applications in transistors, batteries, topological insulators, thermoelectrics, artificial photosynthesis, and catalysis (4,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25).One of the unique properties of 2D layered materials is their ability to intercalate guest species into their van der Waals gaps, opening up the opportunities to tune the properties of materials. For example, the spacing between the 2D layers could be increased by intercalation such as lithium (Li) intercalated graphite or molybdenum disulfide (MoS 2 ) and copper intercalated bismuth selenide (26-29). The electronic structures of the host lattice, such as the charge density, anisotropic transport, oxidation state, and phase transition, may also be changed by different species intercalation (26,27).As one of the most interesting layered materials, MoS 2 has been extensively studied in a variety of areas such as electrocatalysis (20)(21)(22)(30)(31)(32)(33)(34)(35)(36). It is known that there is a strong correlation between the electronic structure and catalytic activity of the catalysts (20,(37)(38)(39)(40)(41). It is intriguing to continuously tune the morphology and electronic structure of MoS 2 and explore the effects on MoS 2 hydrogen evolution reaction (HER) activity. Very recent studies demonstrated that the monolayered MoS 2 and WS 2 nanosheets with 1T metallic phase synthesized by chemical exfoliation exhibited superio...

We show H2O2 is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 μm to 20 µm in diameter). Production of H2O2, as assayed by H2O2-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H2O2. The generated H2O2 concentration was ∼30 µM (∼1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O2 or bubbling O2 decreased the yield of H2O2 in microdroplets, indicating that pure water microdroplets directly generate H2O2 without help from O2 either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H2O2 formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH− at or near the surface of the water microdroplet. This catalyst-free and voltage-free H2O2 production method provides innovative opportunities for green production of hydrogen peroxide.

We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.

Thin-film solid oxide fuel cell ͑SOFC͒ structures containing electrolyte membranes 50-150 nm thick were fabricated with the help of sputtering, lithography, and etching. The submicrometer SOFCs were made of yttria-stabilized zirconia ͑YSZ͒ or YSZ/ gadolinium-doped ceria composites electrolyte and 80 nm porous Pt as cathode and anode. The peak power densities were 200 and 400 mW/cm 2 at 350 and 400°C, respectively. The high power densities achieved are not only due to the reduction of electrolyte thickness but also to the high charge-transfer reaction rates at the interfaces between the nanoporous electrodes ͑cathode and/or anode͒ and the nanocrystalline thin electrolyte.

Yttria-stabilized zirconia (YSZ) films were synthesized by atomic layer deposition (ALD). Tetrakis(dimethylamido)zirconium and tris(methylcyclopentadienyl)yttrium were used as ALD precursors with distilled water as oxidant. From X-ray photoelectron spectroscopy (XPS) compositional analysis, the yttria content was identified to increase proportionally to the pulse ratio of Y/Zr. Accordingly, the target stoichiometry ZrO2/Y2O3 = 0.92:0.08 was achieved. Crystal and grain structures of ALD YSZ films grown on amorphous Si3N4 were analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM). The microstructure of the polycrystalline films consisted of grains of tens of nanometers in diameter. To evaluate ALD YSZ films as oxide ion conductor, freestanding 60 nm films were prepared with porous platinum electrodes on both sides of the electrolyte. This structure served as a solid oxide fuel cell designed to operate at low temperatures. Maximum power densities of 28 mW/cm2, 66 mW/cm2, and 270 mW/cm2 were observed at 265 °C, 300 °C, and 350 °C, respectively. The high performance of thin film ALD electrolyte fuel cells is related to low electrolyte resistance and fast electrode kinetics. The exchange current density at the electrode−electrolyte interface was approximately 4 orders of magnitude higher compared to reference Pt-YSZ values.

A low temperature micro solid oxide fuel cell with corrugated electrolyte membrane was developed and tested. To increase the electrochemically active surface area, yttria-stabilized zirconia membranes with thickness of 70 nm were deposited onto prepatterned silicon substrates. Fuel cell performance of the corrugated electrolyte membranes released from silicon substrate showed an increase of power density relative to membranes with planar electrolytes. Maximum power densities of the corrugated fuel cells of 677 mW/cm2 and 861 mW/cm2 were obtained at 400 and 450 degrees C, respectively.

The photocathodic hydrogen evolution reaction (HER) from p-type Si nanowire (NW) arrays was evaluated using platinum deposited by atomic layer deposition (ALD) as a HER cocatalyst. ALD of Pt on the NW surface led to a highly conformal coating of nanoparticles with sizes ranging from 0.5 to 3 nm, allowing for precise control of the Pt loading in deep submonolayer quantities. The catalytic performance was measured using as little as 1 cycle of Pt ALD, which corresponded to a surface mass loading of ∼10 ng/cm(2). The quantitative exploration of the lower limits of Pt cocatalyst loading reported here, and its application to high-surface-area NW photoelectrodes, establish a general approach for minimizing the cost of precious-metal cocatalysts for efficient and affordable solar-to-fuel applications.