Ceramics in Energy

janfeb13

ceramics in energy ARPA-E award helps Berkeley Oxide Nanocrystals,” which appeared the films are designed as optical cavities Lab groups shine smart in 2011 in Nano Letters (doi:10.1021/ that trap light and efficiently collect nl203030f). n the charge carriers. The penetration windows tech depth of visible light in iron oxide is A research group led by Delia Ultrathin rust films trap sunlight about a micrometer, but the photogen- Milliron at the Lawrence Berkeley erated charge carriers are collected only National Lab has been hammering for splitting water in a 2–20-nanometer range. So, the away for several years to forge smart Water molecules are a great place key is to push, or trap, the light into a window technologies that can drive to store hydrogen. Now that the 20-nanometer range that is located near down costs and address the practicali- “hydrogen economy” is getting some the place where the charge is needed, ties involved with bringing such ener- traction, the question is how to get the i.e., a surface. gy-saving materials in reach of consum- hydrogen “out of storage.” One way Rothschild explains, “The light is ers. In December, Milliron’s efforts were to unlock the hydrogen is photoelec- trapped in quarter-wave or even deeper rewarded with a $3 million ARPA-E trolysis—using the sunlight to split the sub-wavelength films on mirror like grant to further efforts to improve the water. The photoelectrolysis process back reflector substrates. Interference performance and lower production costs involves capturing sunlight, convert- between forward- and backward- for materials that will yield commercial ing it to current, using the current to propagating waves enhances the light electrochromic windows. electrically split the water molecule, absorption close to the surface wherein But Milliron’s group, part of LBL’s and harvest out the hydrogen. Some the photogenerated charge carriers are Molecular Foundry, along with the lab’s semiconducting materials are able to collected before recombination takes Environmental Energy Technologies convert sunlight into charge carriers, place. The escaped (back reflected) Division, believes the current line of i.e., current. photons are retrapped by a second commercial smart windows is not agile Because we expect to need a lot of ultrathin-film photoanode in front of enough and still too far from affordable hydrogen, we are going to need a lot of the first photoanode, thereby leading for most applications. semiconducting material that is stable to efficient photon harvesting using ARPA-E would like to see smart in aqueous environments, nontoxic, 20–30-nanometer-thick α-Fe2O3 films.” windows that can separate the filter- abundant, inexpensive, and able to In this way, the light intensity is absorb visible light. Rust, in thin- amplified near the surface of the pho- ing of visible light from the filtering of film form, meets those requirements. toanode, where it oxidizes the water near-infrared radiation (NIR), along However, α-Fe O (hematite) has poor before recombining. with a technology that efficiently uses 2 3 transport properties, and the “photo- The second aspect to maximizing current glassmaking techniques. generated” charge carriers generally efficiency involves smart design of the According to a press release, the recombine before they can be used to photoanode geometry. The abstract researchers believe they have candidate do any work. reports that V-shaped cells, for exam- nanocrystal thin films that can indi- Researchers at the Technion-Israel ple, are especially efficient at harvesting vidually block the NIR and visible light Institute of Technology may have light in these ultrathin films. components, and additionally have an found a way around the recombination Rothschild says the new technology inexpensive approach for applying the problem in photoelectrolysis anodes, could lead to cost-effective, integrated film that is similar to spray-painting a or photoanodes, according to a new solar cells that combine the ultrathin car. Ultimately, they want to deliver a paper published in Nature Materials. In iron oxide photoelectrodes with stan- low-cost window that can be toggled a press release, lead researcher Avner dard silicon-based solar cells and there- among various settings, such as fully Rothschild, associate professor in the by produce electricity and hydrogen. opaque, transparent for visible light but at Technion’s Department of Materials He also says the light-trapping research not NIR, transparent for NIR but not Science and Engineering, says, “Our could reduce the need for rare elements visible light, or fully transparent. light-trapping scheme overcomes this in so-called second generation photo- Berkeley Lab has already spun off a trade off between light absorption and voltaic cells, such as tellurium in CdTe company, Heliotrope Technologies, to charge carrier recombination, enabling cells or indium in Cu-In-Ga-Se cells. work on commercial development of efficient absorption in ultrathin films The paper is “Resonant light trap- the electrochromic applications. wherein the photogenerated charge car- ping in ultrathin films for water split- For more about Milliron’s research riers are collected efficiently.” ting,” H. Dotan, O. Kfir, E. Sharlin, into the above-mentioned materials, The efficiency of the 20–30-nanome- O. Blank, M. Gross, I. Dumchin, G. see, for example, “Tunable Infrared ter-thick α-Fe2O3 films has two sources. Ankonina, and A. Rothschild, Nature Absorption and Visible Transparency First, according to the paper’s abstract, Materials (doi: 10.1038/nmat3477). n of Colloidal Aluminum-Doped Zinc American Ceramic Society Bulletin, Vol. 92, No. 1 | www.ceramics.org 15


janfeb13
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