Showing papers by "Nathan S. Lewis published in 2014"
TL;DR: The activity was essentially unchanged after 400 cyclic voltammetric sweeps, suggesting long-term viability under operating conditions, and CoP is amongst the most active, acid-stable, earth-abundant HER electrocatalysts reported to date.
Abstract: Nanoparticles of cobalt phosphide, CoP, have been prepared and evaluated as electrocatalysts for the hydrogen evolution reaction (HER) under strongly acidic conditions (0.50 M H_2SO_4, pH 0.3). Uniform, multi-faceted CoP nanoparticles were synthesized by reacting Co nanoparticles with trioctylphosphine. Electrodes comprised of CoP nanoparticles on a Ti support (2 mg cm^(−2) mass loading) produced a cathodic current density of 20 mA cm^(−2) at an overpotential of −85 mV. The CoP/Ti electrodes were stable over 24 h of sustained hydrogen production in 0.50 M H_2SO_4. The activity was essentially unchanged after 400 cyclic voltammetric sweeps, suggesting long-term viability under operating conditions. CoP is therefore amongst the most active, acid-stable, earth-abundant HER electrocatalysts reported to date.
1,118 citations
TL;DR: It is shown that TiO2 coatings grown by atomic layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors.
Abstract: Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel production, these materials are unstable in aqueous media. We show that TiO2 coatings (4 to 143 nanometers thick) grown by atomic layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors. In conjunction with a thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous oxidation of 1.0 molar aqueous KOH to O2 for more than 100 hours at photocurrent densities of >30 milliamperes per square centimeter and ~100% Faradaic efficiency. TiO2-coated GaAs and GaP photoelectrodes exhibited photovoltages of 0.81 and 0.59 V and light-limiting photocurrent densities of 14.3 and 3.4 milliamperes per square centimeter, respectively, for water oxidation.
1,117 citations
TL;DR: In this paper, the major design constraints that motivate continued research in the field of solar-driven water splitting are summarized, and key device components that are now available for use in demonstration systems and prototypes.
Abstract: Through decades of sustained effort, researchers have made substantial progress on developing technologies for solar-driven water splitting. Nevertheless, more basic research is needed before prototype devices with a chance for commercial success can be demonstrated. In this Perspective, we summarize the major design constraints that motivate continued research in the field of solar-driven water splitting. Additionally, we discuss key device components that are now available for use in demonstration systems and prototypes. Finally, we highlight research areas where breakthroughs will be critical for continued progress toward commercial viability for solar-driven water-splitting devices.
626 citations
TL;DR: Under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles deposited on TiO2 produced H2 at rates and amounts that begin to approach those of Pt/TiO2, therefore FeP is a highly Earth-abundant material for efficiently facilitating the HER both electrocatalytic and photocatalytically.
Abstract: Nanostructured transition-metal phosphides have recently emerged as Earth-abundant alternatives to platinum for catalyzing the hydrogen-evolution reaction (HER), which is central to several clean energy technologies because it produces molecular hydrogen through the electrochemical reduction of water. Iron-based catalysts are very attractive targets because iron is the most abundant and least expensive transition metal. We report herein that iron phosphide (FeP), synthesized as nanoparticles having a uniform, hollow morphology, exhibits among the highest HER activities reported to date in both acidic and neutral-pH aqueous solutions. As an electrocatalyst operating at a current density of −10 mA cm–2, FeP nanoparticles deposited at a mass loading of ∼1 mg cm–2 on Ti substrates exhibited overpotentials of −50 mV in 0.50 M H2SO4 and −102 mV in 1.0 M phosphate buffered saline. The FeP nanoparticles supported sustained hydrogen production with essentially quantitative faradaic yields for extended time periods...
389 citations
TL;DR: Amorphous molybdenum phosphide (MoP) nanoparticles have been synthesized and characterized as electrocatalysts for the hydrogen evolution reaction (HER) in 0.50 M H2SO4 (pH 0.3).
Abstract: Amorphous molybdenum phosphide (MoP) nanoparticles have been synthesized and characterized as electrocatalysts for the hydrogen-evolution reaction (HER) in 0.50 M H2SO4 (pH 0.3). Amorphous MoP nanoparticles (having diameters of 4.2 ± 0.5 nm) formed upon heating Mo(CO)6 and trioctylphosphine in squalane at 320 °C, and the nanoparticles remained amorphous after heating at 450 °C in H2(5%)/Ar(95%) to remove the surface ligands. At mass loadings of 1 mg cm–2, MoP/Ti electrodes exhibited overpotentials of −90 and −105 mV (−110 and −140 mV without iR correction) at current densities of −10 and −20 mA cm–2, respectively. These HER overpotentials remained nearly constant over 500 cyclic voltammetric sweeps and 18 h of galvanostatic testing, indicating stability in acidic media under operating conditions. Amorphous MoP nanoparticles are therefore among the most active known molybdenum-based HER systems and are part of a growing family of active, acid-stable, non-noble-metal HER catalysts.
353 citations
TL;DR: Amorphous tungsten phosphide (WP), which has been synthesized as colloidal nanoparticles with an average diameter of 3 nm, has been identified as a new electrocatalyst for the hydrogen-evolution reaction (HER) in acidic aqueous solutions.
249 citations
TL;DR: In this paper, a CoP film was synthesized by cathodic deposition from a boric acid solution of Co2+ and H2PO2 on copper substrates followed by operando remediation of exogenous contaminants.
Abstract: Films of CoP have been electrochemically synthesized, characterized, and evaluated for performance as a catalyst for the hydrogen-evolution reaction (HER) The film was synthesized by cathodic deposition from a boric acid solution of Co2+ and H2PO2– on copper substrates followed by operando remediation of exogenous contaminants The films were characterized structurally and compositionally by scanning-electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman spectrophotometry The catalytic activity was evaluated by cyclic voltammetry and chronopotentiometry Surface characterization prior to electrocatalysis indicated that the film consisted of micrometer-sized spherical clusters located randomly and loosely on a slightly roughened surface The composition of both the clusters and surface consisted of cobalt in the metallic, phosphide, and amorphous-oxide forms (CoO·Co2O3) and of phosphorus as phosphide and orthophosphate The orthophosphate species, produced
203 citations
TL;DR: In this paper, a triple-junction amorphous hydrogenated Si (a-Si:H) cell was used as the light absorber, Pt and cobalt phosphate (Co-Pi) as electrocatalysts for the hydrogen-evolution reaction (HER) and oxygen-evolutions reaction (OER), respectively, and a bulk aqueous solution buffered at pH = 9.2 by 1.0 M of boric acid/borate as an electrolyte.
Abstract: The efficiency limits, gas-crossover behavior, formation of local pH gradients near the electrode surfaces, and safety characteristics have been evaluated experimentally as well as by use of multi-physics modeling and simulation methods for an integrated solar-driven water-splitting system that operates with bulk electrolyte solutions buffered at near-neutral pH. The integrated membrane-free system utilized a triple-junction amorphous hydrogenated Si (a-Si:H) cell as the light absorber, Pt and cobalt phosphate (Co–Pi) as electrocatalysts for the hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER), respectively, and a bulk aqueous solution buffered at pH = 9.2 by 1.0 M of boric acid/borate as an electrolyte. Although the solar-to-electrical efficiency of the stand-alone triple-junction a-Si:H photovoltaic cell was 7.7%, the solar-to-hydrogen (STH) conversion efficiency for the integrated membrane-free water-splitting system was limited under steady-state operation to 3.2%, and the formation of pH gradients near the electrode surfaces accounted for the largest voltage loss. The membrane-free system exhibited negligible product-recombination loss while operating at current densities near 3.0 mA cm−2, but exhibited significant crossover of products (up to 40% H2 in the O2 chamber), indicating that the system was not intrinsically safe. A system that contained a membrane to minimize the gas crossover, but which was otherwise identical to the membrane-free system, yielded very low energy-conversion efficiencies at steady state, due to low transference numbers for protons across the membranes resulting in electrodialysis of the solution and the consequent formation of large concentration gradients of both protons and buffer counterions near the electrode surfaces. The modeling and simulation results showed that despite the addition of 1.0 M of buffering agent to the bulk of the solution, during operation significant pH gradients developed near the surfaces of the electrodes. Hence, although the bulk electrolyte was buffered to near-neutral pH, the electrode surfaces and electrocatalysts experienced local environments under steady-state operation that were either highly acidic or highly alkaline in nature, changing the chemical form of the electrocatalysts and exposing the electrodes to potentially corrosive local pH conditions. In addition to significant pH gradients, the STH conversion efficiency of both types of systems was limited by the mass transport of ionic species to the electrode surfaces. Even at operating current densities of <3 mA cm−2, the voltage drops due to these pH gradients exceeded the combined electrocatalyst overpotentials for the hydrogen- and oxygen-evolution reactions at current densities of 10 mA cm−2. Hence, such near-neutral pH solar-driven water-splitting systems were both fundamentally limited in efficiency and/or co-evolved explosive mixtures of H2(g) and O2(g) in the presence of active catalysts for the recombination of H2(g) and O2(g).
159 citations
TL;DR: In this paper, the tandem devices exhibited open-circuit potentials of E_(∝) = −1.21 V versus E^0′(O_2/H_2O) and resulted in unassisted H 2 production during twoelectrode measurements (anode: tandem device, cathode: Pt disc).
Abstract: Tandem junction (n–p^+-Si/ITO/WO_3/liquid) core–shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. The tandem devices exhibited open-circuit potentials of E_(∝) = −1.21 V versus E^0′(O_2/H_2O), demonstrating additive voltages across the individual junctions (n–p^+-Si E_(∝) = −0.5 V versus solution; WO_3/liquid E_(∝) = −0.73 V versus E^0′(O_2/H_2O)). Optical concentration (12×, AM1.5D) shifted the open-circuit potential to E_(∝) = −1.27 V versus E^0′(O_2/H_2O) and resulted in unassisted H_2 production during two-electrode measurements (anode: tandem device, cathode: Pt disc). The solar energy-conversion efficiencies were very low, 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H_2, respectively, due to the non-optimal photovoltage and band-gap of the WO_3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes.
147 citations
TL;DR: In this paper, the energy-conversion efficiency of systems that rely on internal photoemission processes at metal-semiconductor Schottky-barrier diodes was evaluated.
Abstract: Collection of hot electrons generated by the efficient absorption of light in metallic nanostructures, in contact with semiconductor substrates can provide a basis for the construction of solar energy-conversion devices. Herein, we evaluate theoretically the energy-conversion efficiency of systems that rely on internal photoemission processes at metal-semiconductor Schottky-barrier diodes. In this theory, the current-voltage characteristics are given by the internal photoemission yield as well as by the thermionic dark current over a varied-energy barrier height. The Fowler model, in all cases, predicts solar energy-conversion efficiencies of <1% for such systems. However, relaxation of the assumptions regarding constraints on the escape cone and momentum conservation at the interface yields solar energy-conversion efficiencies as high as 1%–10%, under some assumed (albeit optimistic) operating conditions. Under these conditions, the energy-conversion efficiency is mainly limited by the thermionic dark current, the distribution of hot electron energies, and hot-electron momentum considerations.
132 citations
TL;DR: In this paper, crystallographically amorphous films of cobalt selenide have been deposited from aqueous solution onto planar Ti supports and evaluated as electrocatalysts for the hydrogen evolution reaction.
Abstract: Using an electrochemical method under ambient conditions, crystallographically amorphous films of cobalt selenide have been deposited from aqueous solution onto planar Ti supports. These films have been evaluated as electrocatalysts for the hydrogen-evolution reaction. In 0.500 M H2SO4, the cobalt selenide films required an overpotential of ∼135 mV to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm−2. Galvanostatic measurements indicated stability of the electrocatalytic films for >16 h of continuous operation at −10 mA cm−2. The facile preparation method, and the activity of the cobalt selenide films, suggest that electrodeposited metal chalcogenides are potentially attractive earth-abundant electrocatalysts for the hydrogen-evolution reaction.
TL;DR: In this article, ultrathin dual layers of TiO_2 and Ni have been used to stabilize polycrystalline BiVO_4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte.
Abstract: Ultrathin dual layers of TiO_2 and Ni have been used to stabilize polycrystalline BiVO_4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO_2 layers were deposited on BiVO_4 thin films by atomic-layer deposition, with Ni deposited onto the TiO_2 by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4 photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO_4 film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems.
TL;DR: In this paper, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained MDEs, MoSe3, MoO3, and elemental selenium, and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate
Abstract: The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hy...
TL;DR: A commercially available bipolar membrane that can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water has been identified and its performance has been evaluated quantitatively.
Abstract: A bipolar membrane can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water. A commercially available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quantitatively, and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their physical properties such as optical transparency, electronic conductivity and kinetics of water dissociation.
TL;DR: In this article, an n-CdTe/TiO2/Ni oxide electrode with a thin overlayer of a Ni-oxide based oxygen-evolution electrocatalyst was shown to achieve stable solar-driven photoelectrochemical oxidation of H2O to O2(g).
Abstract: Although II–VI semiconductors such as CdS, CdTe, CdSe, ZnTe, and alloys thereof can have nearly ideal band gaps and band-edge positions for the production of solar fuels, II–VI photoanodes are well-known to be unstable towards photocorrosion or photopassivation when in contact with aqueous electrolytes. Atomic-layer deposition (ALD) of amorphous, “leaky” TiO2 films coated with thin films or islands of Ni oxide has been shown to robustly protect Si, GaAs, and other III–V materials from photocorrosion and therefore to facilitate the robust, solar-driven photoelectrochemical oxidation of H2O to O2(g). We demonstrate herein that ALD-deposited 140 nm thick amorphous TiO2 films also effectively protect single crystalline n-CdTe photoanodes from corrosion or passivation. An n-CdTe/TiO2 electrode with a thin overlayer of a Ni-oxide based oxygen-evolution electrocatalyst produced 435 ± 15 mV of photovoltage with a light-limited current density of 21 ± 1 mA cm−2 under 100 mW cm−2 of simulated Air Mass 1.5 illumination. The ALD-deposited TiO2 films are highly optically transparent and electrically conductive. We show that an n-CdTe/TiO2/Ni oxide electrode enables the stable solar-driven oxidation of H2O to O2(g) in strongly alkaline aqueous solutions, where passive, intrinsically safe, efficient systems for solar-driven water splitting can be operated.
TL;DR: In this paper, the authors describe the deposition of thin overlayers of TiO2 (4-143 nm) on Si, GaAs, and GaP electrodes by atomic layer deposition.
Abstract: Thick overlayers of TiO2 (4—143 nm) are deposited on Si, GaAs, and GaP electrodes by atomic layer deposition.
TL;DR: In this article, high-structured silicon microwire (Si MW) arrays have been synthesized and characterized as absorbers for solar energy-conversion systems, allowing for efficient collection of photogenerated carriers from impure materials.
Abstract: Highly structured silicon microwire (Si MW) arrays have been synthesized and characterized as absorbers for solar energy-conversion systems. These materials are of great interest for applications in solar energy conversion, including solar electricity and solar fuels production, due to their unique materials properties, form factors, ease of fabrication, and device-processing attributes. The Si MW array geometry allows for efficient collection of photogenerated carriers from impure materials that have short minority-carrier diffusion lengths while simultaneously allowing for high optical absorption and high external quantum yields for charge-carrier collection. In addition, Si MW arrays exhibit unique mesoscale optical behavior and can be removed from the growth substrate to provide flexible, processable arrays of Si microwires ordered in a variety of organic polymers and ionomers. The unique photon-management properties of Si MW arrays, combined with their high internal surface area and controlled morpho...
TL;DR: The solution-phase deposition of inorganic semiconductors is a promising, scalable method for the manufacture of thin film photovoltaics as discussed by the authors, which offers the possibility of expensive, rapid, high-throughput thin film fabrication through processes such as spray coating.
Abstract: The solution-phase deposition of inorganic semiconductors
is a promising, scalable method for the manufacture of
thin film photovoltaics. Deposition of photovoltaic materials
from molecular or colloidal inks offers the possibility of
inexpensive, rapid, high-throughput thin film fabrication
through processes such as spray coating. For example, CdTe,
Cu(In,Ga)(S,Se)_2 (CIGS), and CH_3NH_3Pb(Cl,I)_3 perovskite-based
thin film solar cells have been previously deposited using
solution-based processes. Inks have also recently been
developed for the solution deposition of Cu_2ZnSn(S,Se)_4
(CZTS) and FeS_2 (iron pyrite) absorber layers for thin film
solar applications, in order to provide sustainable alternatives to
materials that contain environmentally harmful heavy metals
(e.g., Cd, Pb) and/or scarce elements (e.g., Te, In).
TL;DR: Compared to conformal, unstructured WO3 shells on Si microwires, the hierarchically structured core-shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected.
Abstract: WO_3 thin films have been deposited in a hierarchically structured core–shell morphology, with the cores consisting of an array of Si microwires and the shells consisting of a controlled morphology WO_3 layer. Porosity was introduced into the WO_3 outer shell by using a self-assembled microsphere colloidal crystal as a mask during the deposition of the WO_3 shell. Compared to conformal, unstructured WO_3 shells on Si microwires, the hierarchically structured core–shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected. The use of structured substrates also improved the growth rate of microsphere-based colloidal crystals and suggests strategies for the use of colloidal materials in large-scale applications.
TL;DR: Studies of methyl-terminated Si(111), which have shown that the H-Si(111) and CH3-Si (111) surfaces are structurally nearly identical, yet impart significantly different chemical and electronic properties to the resulting Si surface, demonstrate the potential use of a wet-chemical surface modification scheme that combines bothchemical and electronic passivation.
Abstract: ConspectusThe chemical, electronic, and structural properties of surfaces are affected by the chemical termination of the surface. Two-step halogenation/alkylation of silicon provides a scalable, wet-chemical method for grafting molecules onto the silicon surface. Unlike other commonly studied wet-chemical methods of surface modification, such as self-assembly of monolayers on metals or hydrosilylation on silicon, the two-step method enables attachment of small alkyl chains, even methyl groups, to a silicon surface with high surface coverage and homogeneity. The methyl-terminated Si(111) surface, by comparison to hydrogen-terminated Si(111), offers a unique opportunity to study the effects of the first surface bond connecting the overlayer to the surface. This Account describes studies of methyl-terminated Si(111), which have shown that the H–Si(111) and CH3–Si(111) surfaces are structurally nearly identical, yet impart significantly different chemical and electronic properties to the resulting Si surface...
TL;DR: ZnSn1-x Gex N2 direct bandgap semiconductor alloys, with a crystal structure and electronic structure similar to InGaN, are earth-abundant alternatives for efficient, high-quality optoelectronic devices and solar-energy conversion.
Abstract: ZnSn_(1-x)Ge_xN_2 direct bandgap semiconductor alloys, with a crystal structure and electronic structure similar to InGaN, are earth-abundant alternatives for efficient, high-quality optoelectronic devices and solar energy conversion. The bandgap is tunable almost monotonically from 2 eV (ZnSnN_2) to 3.1 eV (ZnGeN_2) by control of the Sn/Ge ratio.
TL;DR: In this article, the photocurrent as a function of the thickness of a light absorber was measured by scanning an illuminated spot of photons with an energy well above the band gap of the material was scanned along the thickness gradient of a wedge-shaped, rear-illuminated semiconducting light absorbers.
Abstract: Measurement of the photocurrent as a function of the thickness of a light absorber has been shown herein both theoretically and experimentally to provide a method for determination of the minority-carrier diffusion length of a sample. To perform the measurement, an illuminated spot of photons with an energy well above the band gap of the material was scanned along the thickness gradient of a wedge-shaped, rear-illuminated semiconducting light absorber. Photogenerated majority carriers were collected through a back-side transparent ohmic contact, and a front-side liquid or Schottky junction collected the photogenerated minority carriers. Calculations showed that the diffusion length could be evaluated from the exponential variation in photocurrent as a function of the thickness of the sample. Good agreement was observed between experiment and theory for a solid-state silicon Schottky junction measured using this method. As an example for the application of the technique to semiconductor/liquid-junction photoelectrodes, the minority-carrier diffusion length was determined for graded thickness, sputtered tungsten trioxide and polished bismuth vanadate films under back-illumination in contact with an aqueous electrolyte. This wedge technique does not require knowledge of the spectral absorption coefficient, doping, or surface recombination velocity of the sample.
TL;DR: In this paper, the authors used linear sweep voltammetry to assess the ability of WSe_2 films to catalyze the hydrogen-evolution reaction and chronopotentiometry was used to gauge the temporal stability of the catalytic performance of the films under cathodic conditions.
TL;DR: Surface-initiated polymerization can provide a straightforward, reproducible method for large-scale functionalization of nanosensors.
Abstract: Surface-initiated polymerization has been used to grow thick, uniform poly(methyl methacrylate) films on nanocantilever sensors. Cantilevers with these coatings yielded significantly greater sensitivity relative to bare devices as well as relative to devices that had been coated with drop-cast polymer films. The devices with surface-initiated polymer films also demonstrated high selectivity toward polar analytes. Surface-initiated polymerization can therefore provide a straightforward, reproducible method for large-scale functionalization of nanosensors.
TL;DR: Electricchemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active, however the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry.
Abstract: Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2′-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2′-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry.
TL;DR: In this article, two designs for an integrated photo-electrolysis system that uses a 10× concentrating solar collector have been investigated in detail using a multi-physics model that accounts for the properties of the tandem photoabsorbers, mass transport, and the electrocatalytic performance of the oxygen evolution and hydrogen evolution reactions.
Abstract: Two designs for an integrated photoelectrolysis system that uses a 10× concentrating solar collector have been investigated in detail. The system performance was evaluated using a multi-physics model that accounted for the properties of the tandem photoabsorbers, mass transport, and the electrocatalytic performance of the oxygen-evolution and hydrogen-evolution reactions (OER and HER, respectively). The solar-to-hydrogen (STH) conversion efficiencies and the ohmic losses associated with proton transport in the solution electrolyte and through the membrane of the photoelectrolysis system were evaluated systematically as a function of the cell dimensions, the operating temperatures, the bandgap combinations of the tandem cell, and the performance of both the photoabsorbers and electrocatalysts. Relative to designs of optimized systems that would operate without a solar concentrator, the optimized 10× solar concentrator designs possessed larger ohmic losses and exhibited less uniformity in the distribution of the current density along the width of the photoelectrode. To minimize resistive losses while maximizing the solar-to-hydrogen conversion efficiency, η_(STH), both of the designs, a two-dimensional “trough” design and a three-dimensional “bubble wrap” design, required that the electrode width or diameter, respectively, was no larger than a few millimeters. As the size of the electrodes increased beyond this limiting dimension, the η_(STH) became more sensitive to the performance of the photoabsorbers and catalysts. At a fixed electrode dimension, increases in the operating temperature reduced the efficiency of cells with smaller electrodes, due to degradation in the performance of the photoabsorber with increasing temperature. In contrast, cells with larger electrode dimensions showed increases in efficiency as the temperature increased, due to increases in the rates of electrocatalysis and due to enhanced mass transport. The simulations indicted that cells that contained 10% photoabsorber area, and minimal amounts of Nafion or other permselective membranes (i.e. areal coverages and volumetric fractions of only a few percent of the cell), with the remaining area comprised of a suitable, low-cost inert, non porous material (flexible polymers, inert inorganic materials, etc.) should be able to produce high values of η_(STH), with η_(STH) = 29.8% for an optimized design with a bandgap combination of 1.6 eV/0.9 eV in a tandem photoabsorber system at 350 K.
TL;DR: Investigation of the biological and biomedical imaging roles and impacts of fluorescent metallocorrole-TiO2 nanoconjugates as potential near-infrared optical contrast agents in vitro in cancer and normal cell lines revealed that 1-Al- TiO2 was found inside the cancer cells even at low doses and localized in the cytosol.
TL;DR: In this paper, the electrocatalytic behavior of Ni and Pt nanoparticles for the hydrogen-evolution reaction (HER) on p-type Si photocathodes was measured experimentally and the current density vs. potential (J-E) characteristics of a general metal catalyst on pSi was modeled as a combination of a Si photodiode in series electrically with metal electro catalysts.
TL;DR: The International Symposia for advancing the Chemical Sciences (ISACS 12) as mentioned in this paper focused on challenges in Chemical Renewable Energy (CRE) with oral presentations organized along five themes: photovoltaics, solar fuels, molecular and bio-inspired catalysts, new material for batteries, and fuel cells.
Abstract: The International Symposia for
advancing the Chemical Sciences, a
partner of the journal Chemical Science,
held its 12th meeting (ISACS 12) at the
University of Cambridge on September
3–6 2013. ISACS 12 focused on “Challenges
in Chemical Renewable Energy”,
with oral presentations organized along
five themes: photovoltaics, solar fuels,
molecular and bio-inspired catalysts, new
materials for batteries, and fuel cells.
ISACS 12 also included a presentation on
the sugar cane-based energy industry in
Brazil, a recording by the BBC World
Service, and two poster sessions. This
conference was an exciting, busy place to
meet people, exchange ideas, and foster
collaboration.
TL;DR: The junction between n-type silicon microwires and p-type conducting polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) was investigated using a soft contact method as mentioned in this paper.
Abstract: The junction between n-type silicon microwires and p-type conducting polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) was investigated using a soft contact method. Dopant levels within the microwires were varied during growth to give a highly-doped region for improved contact and a low-doped region for light absorption. The low-doped region of the microwires had a dopant density of 5 X 10(17) cm(-3) while the highly-doped region had an increased dopant density of 5 X 10(18) cm(-3) over similar to 20 mu m. Uniform, highly-doped microwires, with a dopant density of 4 X 10(19) cm(3), were used as a comparison. Regions of highly-doped n-type Si microwires (N-D = 5 X 10(18) cm(-3) and 4 X 10(19) cm(-3)) contacted by PEDOT:PSS showed a significantly lower junction resistance compared to the low-doped (3 X 10(17) cm(-3)) regions of the microwire. Junctions incorporating the metal catalyst used during growth were also investigated. Microwires with copper at the interface had similar currentvoltage characteristics to those observed for the highly-doped microwire/conducting polymer junction; however, junctions that incorporated gold exhibited significantly lower resistances, decreasing the iR contribution of the junction by an order of magnitude with respect to the total voltage drop in the entire structure.