Investigation of arrays of photosynthetically active heterostructures using conductive probe atomic force microscopy.

Control and optimization of optically excited charge and energy transport across solid-liquid interfaces are essential for many applications including artificial photosynthesis, photocatalysis, and photopolymerization. Nanostructures are especially suited for this purpose, because the exciton diffusion length is typically much larger than the dimension of the particle, enabling efficient charge transport from the bulk to the nanoparticle surface for use in chemical transformations. However, characterization of charge transfer processes at nanoscale interfaces involving either isolated or assembled optoelectronic components remains a major challenge. Here, we use conductive probe-atomic force microscopy (cp-AFM) to spatially characterize the photovoltaic and photoelectrochemical properties of individual nanostructured photosynthetically active heterostructure (PAH) units in large area scans and compare them to thin-film photoelectrode devices. For CuInSe2/Au Schottky barrier PAH devices electrochemically synthesized inside porous anodic aluminum oxide, we observed a significant increase in solid-state photovoltages (∼0.5 V) and applied bias photocurrents (∼5 pA at +2 V) with much less spatial variation compared to thin film devices (<0.1 V and ∼2 pA at +2 V). We identified that the key reasons for the low performance of CuInSe2/Au thin film devices were an increased number of short-circuit pathways formed as a result of the fabrication process, and a lower density of grain boundaries leading to reduced photoelectrochemically active surface area. When photoanodes were fabricated with these PAH units, the electrodes showed superior and stable photoelectrochemical performance due to their inherent fault tolerance. Our results demonstrate the potential of using cp-AFM as a tool to characterize spatially resolved photoelectrochemical performance over device structures designed for areal production of chemicals and to provide us with a means of investigating optimal structural configurations and to better understand charge transfer processes across solid-liquid interface.

Investigation of the active sites of rhodium sulfide for hydrogen evolution/oxidation using carbon monoxide as a probe.

Carbon monoxide (CO) was observed to decrease the activity for hydrogen evolution, hydrogen oxidation, and H2-D2 exchange on rhodium sulfide, platinum, and rhodium metal. The temperature at which the CO was desorbed from the catalyst surface (detected by recovery in the H2-D2 exchange activity of the catalyst) was used as a descriptor for the CO binding energy to the active site. The differences in the CO desorption temperature between the different catalysts showed that the rhodium sulfide active site is not metallic rhodium. Using density functional theory, the binding energy of CO to the Rh sites in rhodium sulfide is found comparable to the binding energy on Pt. Coupled with experiment this supports the proposition that rhodium rather than sulfur atoms in the rhodium sulfide are the active site for the hydrogen reaction. This would indicate the active sites for hydrogen evolution/oxidation as well as oxygen reduction (determined by other groups using X-ray absorption spectroscopy) may be the same.

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 (~10(9)-10(10) PAHs per cm(2)). 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.

Mesoporous delafossite CuCrO2 and spinel CuCr2O4: synthesis and catalysis.

Delafossite CuCrO2 and spinel CuCr2O4 with mesoporous structures have been successfully synthesized using nanocasting methods based on a KIT-6 template. The functional activity of the mesoporous materials was evaluated in applications as heterogeneous catalysts. The activity for photocatalytic hydrogen production of the delafossite structures with different morphologies was characterized and the oxidation state changes associated with photocorrosion of Cu(+) investigated using electron energy loss spectroscopy (EELS). Mg(2+) doping was found to facilitate the casting of ordered structures for CuCrO2 and improves the photocorrosion resistance of delafossite structures. The mesoporous spinel CuCr2O4 nanostructures were found to be active for low temperature CO oxidation.

Improved light olefin yield from methyl bromide coupling over modified SAPO-34 molecular sieves.

As an alternative to the partial oxidation of methane to synthesis gas followed by methanol synthesis and the subsequent generation of olefins, we have studied the production of light olefins (ethylene and propylene) from the reaction of methyl bromide over various modified microporous silico-aluminophosphate molecular-sieve catalysts with an emphasis on SAPO-34. Some comparisons of methyl halides and methanol as reaction intermediates in their conversion to olefins are presented. Increasing the ratio of Si/Al and incorporation of Co into the catalyst framework improved the methyl bromide yield of light olefins over that obtained using standard SAPO-34.

Synthesis and characterization of sintering-resistant silica-encapsulated Fe3O4 magnetic nanoparticles active for oxidation and chemical looping combustion.

A nanocomposite catalyst composed of ferromagnetic magnetite cores (15.5 +/- 2.0 nm) and silica shells with a thickness of 4.5 +/- 1.0 nm (Fe(3)O(4)@SiO(2)) was prepared by a two-step microemulsion-based synthesis. X-ray photoelectron spectroscopy and Raman spectroscopy after oxidation support the presence of a stable Fe(3)O(4) core and a surface phase of gamma-Fe(2)O(3). The nanocomposite structure exhibited 100% conversion of CO in oxygen at a residence time of 0.1 s at 310 degrees C. When pre-oxidized, the Fe(3)O(4)@SiO(2) catalyst is shown to be a suitable solid oxygen carrier for chemical looping combustion of methane at 700 degrees C. The nanocomposites retain their magnetism following the reaction which provides the potential for use of magnetic separation and capture in moving bed reactor applications. The core magnetite within the silica shell is resistant to sintering and a bulk phase transition to temperatures as high as 700 degrees C. These catalysts can be of use in applications of high temperature applications where catalyst recovery by magnetic separation may be required.

Improved photoelectrochemical performance of Ti-doped alpha-Fe2O3 thin films by surface modification with fluoride.

CoF(3) aqueous solution was used to modify the surface of Ti-doped iron oxide thin film photoanodes to negatively shift the flat-band potential and allow photogenerated electrons to directly reduce water to hydrogen without an external bias; the zero bias performance was further improved by the use of glucose (a biomass analog) to bypass the relatively slow oxygen evolution reaction to provide a source of electrons to rapidly consume photogenerated holes.

Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production.

Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H2 production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO2 passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ > 500 nm, Pin = 80 mW cm-2). The incident photon-to-current efficiency at 0 V versus RHE for H2 production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H2 production.

Catalytic molten metals for the direct conversion of methane to hydrogen and separable carbon.

Metals that are active catalysts for methane (Ni, Pt, Pd), when dissolved in inactive low-melting temperature metals (In, Ga, Sn, Pb), produce stable molten metal alloy catalysts for pyrolysis of methane into hydrogen and carbon. All solid catalysts previously used for this reaction have been deactivated by carbon deposition. In the molten alloy system, the insoluble carbon floats to the surface where it can be skimmed off. A 27% Ni-73% Bi alloy achieved 95% methane conversion at 1065°C in a 1.1-meter bubble column and produced pure hydrogen without CO2 or other by-products. Calculations show that the active metals in the molten alloys are atomically dispersed and negatively charged. There is a correlation between the amount of charge on the atoms and their catalytic activity.

Efficient electrocatalyst utilization: electrochemical deposition of Pt nanoparticles using nafion membrane as a template.

We deposit Pt particles electrochemically on an electrode covered with a Nafion membrane. Platinum ions travel through the hydrophilic channels of the membrane, and platinum deposits are formed at the place where the channels make contact with the planar electrode. This procedure deposits the catalyst only at the end of the hydrophilic channels that cross the membrane; no catalyst is placed under the hydrophobic domains, where it would not be in contact with the electrolyte. By performing a series of cyclic voltammograms with this system, we show that deposition of the platinum through the membrane achieves better platinum utilization than deposition of platinum on the naked electrode followed by the placement of the membrane on top.

Solid Suspension Flow Batteries Using Earth Abundant Materials.

The technical features of solid-electrode batteries (e.g., high energy density, relatively low capital cost ($/kWh)) and flow batteries (e.g., long cycle life, design flexibility) are highly complementary. It is therefore extremely desirable to integrate their advantages into a single storage device for large-scale energy storage applications where lifetime cost ($/kW-h/cycle) is an extremely important parameter. Here, we demonstrate a non-Li-based-flow battery concept that replaces the aqueous solution of redox-active molecules found in typical redox flow batteries with suspensions of hydrophilic carbon particles ("solid suspension electrodes") coated with earth-abundant redox-active metals. The solid suspension electrodes charge by depositing earth-abundant redox-active metals onto the carbon particle suspension, which are then stripped during discharge operation. The electrical contact to the solid suspension electrodes is fed through fixed redox-inert hydrophobic carbon current collectors through "contact charge transfer" mechanism. The hydrophobicity of the current collectors prevents direct plating of redox-active metals onto their surfaces. The above concept was successfully used to demonstrate several non-Li-based battery chemistries including zinc-copper, zinc-manganese oxide, zinc-bromine, and zinc-sulfur, providing a pathway for potential applications in medium and large-scale electrical energy storage.

Where next on e-waste in Australia?

For almost two decades waste electrical and electronic equipment, WEEE or e-waste, has been considered a growing problem that has global consequences. The value of recovered materials, primarily in precious and base metals, has prompted some parts of the world to informally and inappropriately process e-waste causing serious environmental and human health issues. Efforts in tackling this issue have been limited and in many ways unsuccessful. The global rates for formal e-waste treatment are estimated to be below the 20% mark, with the majority of end-of-life (EoL) electronic devices still ending up in the landfills or processed through rudimentary means. Industrial confidentiality regarding device composition combined with insufficient reporting requirements has made the task of simply characterizing the problem difficult at a global scale. To address some of these key issues, this paper presents a critical overview of existing statistics and estimations for e-waste in an Australia context, including potential value and environmental risks associated with metals recovery. From our findings, in 2014, on average per person, Australians purchased 35kg of electrical and electronic equipment (EEE) while disposed of 25kg of WEEE, and possessed approximately 320kg of EEE. The total amount of WEEE was estimated at 587kt worth about US$ 370million if all major metals are fully recovered. These results are presented over the period 2010-2014, detailed for major EEE product categories and metals, and followed by 2015-2024 forecast. Our future projection, with the base scenario fixing EEE sales at 35kg per capita, predicts stabilization of e-waste generation in Australia at 28-29kg per capita, with the total amount continuing to grow along with the population growth.

Electrolithographic investigations of the hydrophilic channels in Nafion membranes.

Nafion membranes are used as semisolid electrolytes in methanol and hydrogen fuel cells. The ion conduction takes place through those hydrophilic channels in the Nafion that can provide continuous pathways through the membrane. There is as yet limited information about the density, the size, and the shape of these channels. We have developed two electrochemical methods of visualizing the pore structure which involve the creation of metal lithographs using the membrane pores as templates. In the experiments, the membrane is supported on a flat solid surface on one side, and is in contact with an electrolyte on the other side. Using hydrogen-terminated n-doped Si(111), we deposited gold from an electrolyte containing a gold salt. The Au ions traverse the membrane through the pores, reach the silicon surface, and are spontaneously reduced. A metallic Au deposit is formed on the silicon surface, at the base of the hydrophilic channel. The Au deposits are imaged after the membrane is dissolved. Another method involves supporting the membrane on a Pt surface and depositing silver wires through the hydrophilic channels of the membrane. The scanning electron microscope pictures of these wires provide an image of the size and the shape of the hydrophilic channels.

Synthesis and characterization of Pt-WO3 as methanol oxidation catalysts for fuel cells.

Several compositions of Pt-WO3 catalysts were synthesized and characterized for the electro-oxidation of methanol and CO. The surface morphologies of the catalysts were found to be dependent on the composition. X-ray energy dispersive spectroscopy and X-ray photoelectron spectroscopy results suggest a surface enrichment of WO3 in the codeposited Pt-WO3 catalysts. Cyclic voltammetry and chronoamperometry in methanol show an improvement in catalytic activity for the Pt-WO3 catalysts. A significant improvement in the poison tolerance toward CO and other organic intermediates was observed in the mixed metal-metal oxide catalyst. The catalytic performance of the different compositions was directly compared by normalization of the current to active sites. CO-stripping voltammetry suggests the involvement of WO3 in the catalytic process as opposed to a mere physical effect as suggested by previous work. A possible mechanism for this improvement is proposed based on the electrochemical data.

Direct propylene epoxidation on chemically reduced Au nanoparticles supported on titania.

A highly selective titania supported propylene epoxidation catalyst was synthesized from chemically reduced, thiol capped, gold nanoparticles, and characterized by TEM and XPS.

Self-assembly of CdSe/CdS quantum dots by hydrogen bonding on Au surfaces for photoreception.

CdSe/CdS core-shell quantum dots have been self-assembled onto thiolcarboxylic acid functionalized gold surfaces by hydrogen bonding; control of the pH during deposition allows producing a high coverage photoactive surface for use in a surface sensitized Schottky barrier photovoltaic structure.

Synthesis and photocatalytic properties of highly crystalline and ordered mesoporous TiO2 thin films.

Highly crystalline and ordered mesoporous TiO2 thin films have been synthesized by stabilization of the mesostructure with confined carbon; the films exhibit 2.5% photoconversion efficiency for the water photolysis at zero-bias and Xe lamp illumination of 40 mW cm(-2).

Parallel synthesis and characterization of photoelectrochemically and electrochromically active tungsten-molybdenum oxides.

Single phase tungsten-molybdenum mixed oxide films (W(1-x)Mo(x)O(3)) were successfully synthesized by automated parallel electrodeposition, and distinct structural changes were observed as a function of composition. A monoclinic structure (beta-phase) was observed in mixed oxides with less than 90% Mo, and above 90% Mo, orthorhombic structure (alpha-phase) was identified.

A general integrated process for synthesizing olefin oxides.

Metal oxide solid reactants are shown to react with vicinal alkyl dibromides to selectively form olefin oxides and fully regenerable metal bromides.