Nathan S. Lewis

Bio: Nathan S. Lewis is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Semiconductor & Silicon. The author has an hindex of 112, co-authored 720 publications receiving 64808 citations. Previous affiliations of Nathan S. Lewis include Lawrence Berkeley National Laboratory & Massachusetts Institute of Technology.

Design of a 13% efficient n-GaAs 1−x P x semiconductor–liquid junction solar cell

Abstract: We report here the design of the most efficient non-aqueous semiconductor–liquid junction solar cell studied to date. Our approach involves the use of ternary semiconductor electrodes made from solid solutions of a large hand gap material, GaP, and a small band gap material, GaAs. We demonstrate here that photoanodes consisting of such materials are capable of simultaneously yielding high open circuit voltages and favourable wavelength response to the solar spectrum. A few n-type semiconductor–liquid junction solar cells in aqueous solutions have been reported to yield high (>10%) solar-to-electrical conversion efficiencies1–3. However, for most materials, rapid photoanodic corrosion dominates the interfacial photochemistry4–8. Non-aqueous solvent systems can suppress electrode decay due to corrosion4,7,8; but modest (<6%) conversion efficiencies have been observed for all photoanodes studied in solar irradiation conditions9–13. The photoanodes used here yield over 13% solar-to-electrical conversion efficiencies, or more than double the efficiency of any other non-aqueous semiconductor–liquid junction solar cell previously reported.

Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO2

Abstract: The operational constraints for a 6-electron/6-proton CO2 reduction system that operates at the concentration of CO2 in the current atmosphere (pCO2 = 400 ppm) have been evaluated on a variety of scale lengths that span from laboratory scale to global scale. Due to the low concentration of CO2 in the atmosphere, limitations due to mass transport of CO2 from the tropopause have been evaluated through five different regions, each with different characteristic length scales: the troposphere; the atmospheric boundary layer (ABL); the canopy layer; a membrane layer; and an aqueous electrolyte layer. The resulting CO2 conductances, and associated physical transport limitations, will set the ultimate limit on the efficiency and areal requirements of a sustainable solar-driven CO2 reduction system regardless of the activity or selectivity of catalysts for reduction of CO2 at the molecular level. At the electrolyte/electrode interface, the steady-state limiting current density and the concomitant voltage loss associated with the CO2 concentration overpotential in a one-dimensional solar-driven CO2 reduction cell have been assessed quantitatively using a mathematical model that accounts for diffusion, migration and convective transport, as well as for bulk electrochemical reactions in the electrolyte. At pCO2 = 400 ppm, the low diffusion coefficient combined with the low solubility of CO2 in aqueous solutions constrains the steady-state limiting current density to 10% efficient solar-driven CO2 reduction system (based on the solar collection area). This flux limitation is consistent with estimates of oceanic CO2 uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in coupled atmosphere/ocean general circulation models. Two strategies to improve the feasibility of obtaining efficient and sustainable CO2 transport to a cathode surface at pCO2 = 400 ppm are described and modeled quantitatively. The first strategy employs yet unknown catalysts, analogous to carbonic anhydrases, that dramatically accelerate the chemically enhanced CO2 transport in the aqueous electrolyte layer by enhancing the acid–base reactions in a bicarbonate buffer system. The rapid interconversion from bicarbonate to CO2 in the presence of such catalysts near the cathode surface would in principle yield significant increases in the steady-state limiting current density and allow for >10% solar-fuel operation at the cell level. The second strategy employs a thin-layer cell architecture to improve the diffusive transport of CO2 by use of an ultrathin polymeric membrane electrolyte. Rapid equilibration of CO2 at the gas/electrolyte interface, and significantly enhanced diffusive fluxes of CO2 in electrolytes, are required to increase the steady-state limiting current density of such a system. This latter approach however only is feasible for gaseous products, because liquid products would coat the electrode and therefore thicken the hydrodynamic boundary layer and accordingly reduce the diffusive CO2 flux to the electrode surface. Regardless of whether the limitations due to mass transport to the electrode surface are overcome on the laboratory scale, at global scales the ultimate CO2 flux limitations will be dictated by mass transport considerations related to transport of atmospheric CO2 to the boundary plane of the solar-driven reactor system. The transport of CO2 across the troposphere/ABL interface, the ABL/canopy layer interface, and the canopy layer/electrolyte interface have therefore been assessed in this work, to provide upper bounds on the ultimate limits for the solar-to-fuel (STF) conversion efficiency for systems that are intended to effect the reduction of atmospheric CO2 in a sustainable fashion at global scale.

Photoelectrochemical oxidation of anions by WO3 in aqueous and nonaqueous electrolytes

Abstract: The behavior of WO3 photoanodes has been investigated in contact with a combination of four anions (Cl−, CH3SO3−, HSO4−, and ClO4−) and three solvents (water, acetonitrile, and propylene carbonate), to elucidate the role of the semiconductor surface, the electrolyte, and redox kinetics on the current density vs. potential properties of n-type WO3. In 1.0 M aqueous strong acids, although the flat-band potential (Efb) of WO3 was dominated by electrochemical intercalation of protons into WO3, the nature of the electrolyte influenced the onset potential (Eon) of the anodic photocurrent. In aprotic solvents, the electrolyte anion shifted both Efb and Eon, but did not significantly alter the overall profile of the voltammetric data. For 0.50 M tetra(n-butyl)ammonium perchlorate in propylene carbonate, the internal quantum yield exceeded unity at excitation wavelengths of 300–390 nm, indicative of current doubling. A regenerative photoelectrochemical cell based on the reversible redox couple B10Br10˙−/2− in acetonitrile, with a solution potential of ∼1.7 V vs. the normal hydrogen electrode, exhibited an open-circuit photovoltage of 1.32 V under 100 mW cm−2 of simulated Air Mass 1.5 global illumination.

Chemical modification of n-GaAs electrodes with Os3+ gives a 15% efficient solar cell

Abstract: The minimization of interfacial recombination losses is a key factor in the operation of any semiconductor-based solar-energy-conversion device, including solid-state junctions, semiconductor/liquid junctions and colloidal suspensions of semiconductors. A frequently cited advantage of semiconductor/liquid junctions is the ability to manipulate surface recombination rates by chemical reactions1–7. A notable example is the improvement in current–voltage properties of n-GaAs photoanodes which have been exposed to aqueous solutions of Ru3+ ions8–10. Here we report new surface-modification procedures for GaAs which have produced the most efficient photoelectrochemical cell reported to date. We also report experiments which indicate that the current–voltage improvements in this system are accompanied by increased interfacial hole transfer rates at the GaAs/liquid interface.

Theoretical and Experimental Upper Bounds on Interfacial Charge-Transfer Rate Constants between Semiconducting Solids and Outer-Sphere Redox Couples

Abstract: Theoretical expressions for the charge-transfer rate constant at a semiconductor/liquid junction have been modified to include the effects of adiabaticity and the existence of a Helmholtz layer at the solid/liquid interface. These expressions have yielded an estimate of the maximum interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorbing, outer-sphere redox species. An experimental upper bound on this interfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semiconductor electrodes in contact with outer-sphere redox species. For this purpose, n-Si/CH3OH−dimethylferrocenium−dimethylferrocene, n-GaAs/CH3CN−ferrocenium−ferrocene, and p-InP/CH3CN−cobaltocenium−cobaltocene contacts were investigated using a combination of current density-potential and differential capacitance-potential methods. The upper limits for the interfacial charge-transfer ra...

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films

Abstract: THE large-scale use of photovoltaic devices for electricity generation is prohibitively expensive at present: generation from existing commercial devices costs about ten times more than conventional methods1. Here we describe a photovoltaic cell, created from low-to medium-purity materials through low-cost processes, which exhibits a commercially realistic energy-conversion efficiency. The device is based on a 10-µm-thick, optically transparent film of titanium dioxide particles a few nanometres in size, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. Because of the high surface area of the semiconductor film and the ideal spectral characteristics of the dye, the device harvests a high proportion of the incident solar energy flux (46%) and shows exceptionally high efficiencies for the conversion of incident photons to electrical current (more than 80%). The overall light-to-electric energy conversion yield is 7.1-7.9% in simulated solar light and 12% in diffuse daylight. The large current densities (greater than 12 mA cm-2) and exceptional stability (sustaining at least five million turnovers without decomposition), as well as the low cost, make practical applications feasible.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Heterogeneous photocatalyst materials for water splitting

Abstract: This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)