earned a B.S. degree in chemistry from the University of Dayton in 2001 and as an undergraduate worked on conductive polymer syntheses at the Air Force Research Laboratory at Wright Patterson Air Force Base. He completed an M.S. degree in 2004 and Ph.D. degree in 2008 at Portland State University and joined the Lewis group at Caltech in 2008. He is currently an NSF-ACCF postdoctoral fellow (2009) and has been studying the electrical characteristics of inorganic semiconductors in contact with conductive polymers. His research interests include molecular semiconductors for solar energy conversion, porphyrin macrocycles for optoelectronic applications, and catalyst materials for photoelectrolysis. Emily L. Warren received a B.S. in chemical engineering at Cornell University in 2005. She received an M.Phil in Engineering for Sustainable Development from Cambridge University in 2006. She is currently a graduate student in Chemical Engineering at the California Institute of Technology. Her research interests include semiconductor photoelectrochemistry, solar energy conversion, and semiconductor nanowires. She is currently a graduate student in Chemical Engineering at the California Institute of Technology, working under Nathan S. Lewis. James R. McKone is in his third year of graduate studies in the Division of Chemistry and Chemical Engineering at the California Institute of Technology, working under Nathan S. Lewis and Harry B. Gray. In 2008 he graduated from Saint Olaf College with a Bachelor of Arts degree, double-majoring in music and chemistry. His current research focuses on semiconductor-coupled heterogeneous catalysis of the hydrogen evolution reaction using mixtures of earth-abundant transition metals. Shannon W. Boettcher earned his B.A. degree in chemistry from the University of Oregon, Eugene (2003), and, working with Galen Stucky, his Ph.D. in Inorganic Chemistry from the University of California, Santa Barbara (2008). Following postdoctoral work with Nate Lewis and Harry Atwater at the California Institute of Technology (2008-2010), he returned to the University of Oregon to join the faculty as an Assistant Professor. His research interests span synthesis and physical measurement with the goal of designing and understanding solid-state inorganic material architectures for use in solar-energy conversion and storage.

Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO 2 emissions in the atmosphere demands that holding atmospheric CO 2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

Nanoparticles of nickel phosphide (Ni2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solutions, under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni2P nanoparticles were hollow and faceted to expose a high density of the Ni2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H2(g) with nearly quantitative faradaic yield, while also affording stability in aqueous acidic media.

Keeping semiconductors safe from harm Solar cells harvest the energy of sunlight to create electricity, but electricity is hard to store. Solar cells could also be used to make hydrogen from water, which can be stored as a fuel. Separating water into hydrogen and oxygen, however, presents challenges, especially if this is done directly by illuminating the anode that oxides water. Under the acidic or alkaline conditions needed for practical devices, semiconducting anode materials corrode during operation. Hu et al. now show that amorphous titanium dioxide coatings can protect semiconductors from alkaline corrosion while still allowing light through. Science , this issue p. 1005

At present, solar energy conversion technologies face cost and scalability hurdles in the technologies required for a complete energy system. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in a cost-effective fashion. New developments in nanotechnology, biotechnology, and the materials and physical sciences may enable step-change approaches to cost-effective, globally scalable systems for solar energy use.

A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio ͑length/diameter͒ nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: ͑1͒ In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. ͑2͒ The rate of carrier recombination in the depletion region must not be too large ͑for silicon this means that the carrier lifetimes in the depletion region must be longer than ϳ10 ns͒. If only condition ͑1͒ is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail.

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