Iodine Catalyzed Propane Oxidative Dehydrogenation Using Dibromomethane as an Oxidant

Propane oxidative dehydrogenation is a promising candidate for on-purpose propylene production. However, in oxidative dehydrogenation the propylene yield is limited by the simultaneous oxidization of propane to multiple oxygenated byproducts. We show that a small amount of I₂ is highly effective in catalyzing the dehydrogenation of propane into propylene, using dibromomethane (DBM), a byproduct of the activation of methane by bromine, as the oxidant. Single-pass “C₃H₆+C₃H₇X” (X = Br, I; C₃H₇X can be easily converted to C₃H₆ and HX) yields of up to 80% can be easily achieved, with the highly selective conversion of DBM to methyl bromide, which is readily converted into either high-market-value petrochemicals or liquid fuels. Bearing in mind that the formation of DBM is one of the major undesirable byproducts in the bromine-mediated gas-to-liquid technology, our findings create a win-win situation. On the one hand, this approach is promising for developing a low-cost, on-purpose propylene technology using natural gas as a feedstock. On the other hand, DBM is shown to be a useful reactant for the industrial application of the bromine-mediated gas-to-liquid technology

Plasmonic Photoanodes for Solar Water Splitting with Visible Light

We report a plasmonic water splitting cell in which 95% of the effective charge carriers derive from surface plasmon decay to hot electrons, as evidenced by fuel production efficiencies up to 20-fold higher at visible, as compared to UV, wavelengths. The cell functions by illuminating a dense array of aligned gold nanorods capped with TiO₂, forming a Schottky metal/semiconductor interface which collects and conducts the hot electrons to an unilluminated platinum counter-electrode where hydrogen gas evolves. The resultant positive charges in the Au nanorods function as holes and are extracted by an oxidation catalyst which electrocatalytically oxidizes water to oxygen gas

Large Format Surface-Enhanced Raman Spectroscopy Substrate Optimized for Enhancement and Uniformity

Gratings have been widely investigated both theoretically and experimentally as surface-enhanced Raman spectroscopy (SERS) substrates, exhibiting, under appropriate circumstances, increased far-field extinctions and near-field intensities over those of an appropriately equivalent number of isolated particles. When the grating order transitions from evanescent to radiative, narrow resonance peaks are observed in the extinction spectrum whose properties can be manipulated by controlling the grating’s geometric parameters. Here we report the application of the architectural principles of grating fabrication using a square two-dimensional array of gold-coated nanostructures that achieves SERS enhancements of 10⁷ uniformly over areas of square centimeters. The high-performance grating substrates were fabricated using commonly available foundry-based techniques that have been chosen for their applicability to large-scale wafer processing. Additionally, we restricted ourselves to a parametric regime that optimizes SERS performance in a repeatable and reproducible manner

On the Plasmonic Photovoltaic

The conversion of sunlight into electricity by photovoltaics is currently a mature science and the foundation of a lucrative industry. In conventional excitonic solar cells, electron–hole pairs are generated by light absorption in a semiconductor and separated by the “built in” potential resulting from charge transfer accompanying Fermi-level equalization either at a p–n or a Schottky junction, followed by carrier collection at appropriate electrodes. Here we report a stable, wholly plasmonic photovoltaic device in which photon absorption and carrier generation take place exclusively in the plasmonic metal. The field established at a metal–semiconductor Schottky junction separates charges. The negative carriers are high-energy (hot) electrons produced immediately following the plasmon’s dephasing. Some of the carriers are energetic enough to clear the Schottky barrier or quantum mechanically tunnel through it, thereby producing the output photocurrent. Short circuit photocurrent densities in the range 70–120 μA cm^–2 were obtained for simulated one-sun AM1.5 illumination with devices based on arrays of parallel gold nanorods, conformally coated with 10 nm TiO₂ films and fashioned with a Ti metal collector. For the device with short circuit currents of 120 μA cm^–2, the internal quantum efficiency is ∼2.75%, and its wavelength response tracks the absorption spectrum of the transverse plasmon of the gold nanorods indicating that the absorbed photon-to-electron conversion process resulted exclusively in the Au, with the TiO₂ playing a negligible role in charge carrier production. Devices fabricated with 50 nm TiO₂ layers had open-circuit voltages as high as 210 mV, short circuit current densities of 26 μA cm^–2, and a fill factor of 0.3. For these devices, the TiO₂ contributed a very small but measurable fraction of the charge carriers

Properly Structured, Any Metal Can Produce Intense Surface Enhanced Raman Spectra

While silver and gold have been the dominant plasmonic metals used for surface-enhanced Raman spectroscopy (SERS) since the field’s inception. We argue that virtually any metal, when appropriately nanostructured as a grating, has the potential to be an efficient SERS substrate. This conclusion provides the basis for making SERS a general tool for studying surface processes and catalysis and allows SERS substrates to be routinely based on earth-abundant, low-cost, and chemically interesting metals. We illustrate the above premise by producing highly performing SERS substrates using aluminum, nickel, and copper in addition to silver and gold as benchmarks. All five metals were found to yield high SERS intensities. The approximately three orders enhancement variation among the five substrates based on differing metals is ascribed mainly to local field effects associated with individual grating elements. This conclusion is supported by local field calculations. This suggests that the largest contribution to the enhancement is a (radiative) nonlocal grating-based (plasmonic) effect which is approximately equal for all of the gratings we studied regardless of metal from which they were fabricated, so long as the structural details of the gratings were kept constant

Correction to “Anisotropic Growth of TiO₂ onto Gold Nanorods for Plasmon-Enhanced Hydrogen Production from Water Reduction”

Correction to “Anisotropic Growth of TiO₂ onto Gold Nanorods for Plasmon-Enhanced Hydrogen Production from Water Reduction

Synthesis of Chemicals Using Solar Energy with Stable Photoelectrochemically Active Heterostructures

Efficient and cost-effective conversion of solar energy to useful chemicals and fuels could lead to a significant reduction in fossil hydrocarbon use. Artificial systems that use solar energy to produce chemicals have been reported for more than a century. However the most efficient devices demonstrated, based on traditionally fabricated compound semiconductors, have extremely short working lifetimes due to photocorrosion by the electrolyte. Here we report a stable, scalable design and molecular level fabrication strategy to create photoelectrochemically active heterostructure (PAH) units consisting of an efficient semiconductor light absorber in contact with oxidation and reduction electrocatalysts and otherwise protected by alumina. The functional heterostructures are fabricated by layer-by-layer, template-directed, electrochemical synthesis in porous anodic aluminum oxide membranes to produce high density arrays of electronically autonomous, nanostructured, corrosion resistant, photoactive units (∼10⁹–10¹⁰ PAHs per cm²). Each PAH unit is isolated from its neighbor by the transparent electrically insulating oxide cellular enclosure that makes the overall assembly fault tolerant. When illuminated with visible light, the free floating devices have been demonstrated to produce hydrogen at a stable rate for over 24 h in corrosive hydroiodic acid electrolyte with light as the only input. The quantum efficiency (averaged over the solar spectrum) for absorbed photons-to-hydrogen conversion was 7.4% and solar-to-hydrogen energy efficiency of incident light was 0.9%. The fabrication approach is scalable for commercial manufacturing and readily adaptable to a variety of earth abundant semiconductors which might otherwise be unstable as photoelectrocatalysts

Coupling between the 2D “Ligand” and 2D “Host” and Their Assembled Hierarchical Heterostructures for Electromagnetic Wave Absorption

Constructing the strong interaction between the matrix and the active centers dominates the design of high-performance electromagnetic wave (EMW) absorption materials. However, the interaction-relevant absorption mechanism is still unclear, and the design of ultrahigh reflection loss (RL < −80 dB) absorbers remains a great challenge. Herein, CoFe-based Prussian blue (PB) nanocubes are coprecipitated on the surface of ultrathin CoAl-LDH nanoplates with the assistance of unsaturated coordination sites. During the subsequent pyrolysis process, CoAl-LDH serves as a “ligand” providing a Co source and reacts with Fe or C in the CoFe-PB “host” to form stable CoFe alloys or CoCx species. As a result, strong reactions emerged between the CoAl-LDH matrix and the active CoFe-CoCx@NC centers. Based on the experimental results, the CoAl/CoFe-CoCx@NC hierarchical heterostructure delivers good dielectric losses (dipolar polarization, interface polarization, and conductive loss), magnetic losses (eddy current loss, natural resonance, and exchange resonance), and impedance matching, resulting in a remarkable EMW absorption performance with a reflection loss (RL) value of −82.1 dB at a matching thickness of 3.8 mm. Theoretical results (commercial CST) identify that the strong interaction between the 2D CoAl-LDH “ligand” and 2D CoFe-CoCx “host” promotes a robust heterointerface among the nanoparticles, nanosheets, and nanoplates, which extremely contribute to the dielectric loss. Meanwhile, the coupling effect of nanosheets and nanoplates greatly contributes to the matching performance. This work provides an aggressive strategy for the effect of ligands and hosts on high-performance EMW absorption

Sulfur-Functionalized Mesoporous Carbons as Sulfur Hosts in Li–S Batteries: Increasing the Affinity of Polysulfide Intermediates to Enhance Performance

The Li–S system offers a tantalizing battery for electric vehicles and renewable energy storage due to its high theoretical capacity of 1675 mAh g^–1 and its employment of abundant and available materials. One major challenge in this system stems from the formation of soluble polysulfides during the reduction of S₈, the active cathode material, during discharge. The ability to deploy this system hinges on the ability to control the behavior of these polysulfides by containing them in the cathode and allowing for further redox. Here, we exploit the high surface areas and good electrical conductivity of mesoporous carbons (MC) to achieve high sulfur utilization while functionalizing the MC with sulfur (S–MC) in order to modify the surface chemistry and attract polysulfides to the carbon material. S–MC materials show enhanced capacity and cyclability trending as a function of sulfur functionality, specifically a 50% enhancement in discharge capacity is observed at high cycles (60–100 cycles). Impedance spectroscopy suggests that the S-MC materials exhibit a lower charge-transfer resistance compared with MC materials which allows for more efficient electrochemistry with species in solution at the cathode. Isothermal titration calorimetry shows that the change in surface chemistry from unfunctionalized to S-functionalized carbons results in an increased affinity of the polysulfide intermediates for the S–MC materials, which is the likely cause for enhanced cyclability

Microwave Synthesis of Microstructured and Nanostructured Metal Chalcogenides from Elemental Precursors in Phosphonium Ionic Liquids

We describe a general approach for the synthesis of micro-/nanostructured metal chalcogenides from elemental precursors. The excellent solubility of sulfur, selenium, and tellurium in phosphonium ionic liquids promotes fast reactions between chalcogens and various metal powders upon microwave heating, giving crystalline products. This approach is green, universal, and scalable