{"id":16761,"date":"2013-06-16T00:00:00","date_gmt":"2013-06-15T22:00:00","guid":{"rendered":"http:\/\/www.bio-based.eu\/news\/index.php?startid=20130616-01n"},"modified":"2013-06-16T00:00:00","modified_gmt":"2013-06-15T22:00:00","slug":"artificial-forest-for-solar-water-splitting","status":"publish","type":"post","link":"https:\/\/renewable-carbon.eu\/news\/artificial-forest-for-solar-water-splitting\/","title":{"rendered":"Artificial Forest for Solar Water-Splitting"},"content":{"rendered":"

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\"Schematicof a Si nanowire (gray) and the two absorbing different regions of the
solar spectrum. Insets display photoexcited electron−hole pairs sepa-
rated at the semiconductor-electrolyte interface to carry out water
splitting with the help of co-catalysts (yellow and gray dots). “><\/td>\n<\/tr>\n
Schematic shows TiO2 nanowires (blue) grown on the upper half
of a Si nanowire (gray) and the two absorbing different regions of the
solar spectrum. Insets display photoexcited electron−hole pairs sepa-
rated at the semiconductor-electrolyte interface to carry out water
splitting with the help of co-catalysts (yellow and gray dots). <\/td>\n<\/tr>\n<\/table>\n<\/div>\n

In the wake of the sobering news that atmospheric carbon dioxide is now at its highest level in at least three million years, an important advance in the race to develop carbon-neutral renewable energy sources has been achieved. Scientists with the U.S. Department of Energy (DOE)\u2019s Lawrence Berkeley National Laboratory (Berkeley Lab) have reported the first fully integrated nanosystem for artificial photosynthesis. While “artificial leaf\u201d is the popular term for such a system, the key to this success was an “artificial forest.\u201d<\/b><\/p>\n

“Similar to the chloroplasts in green plants that carry out photosynthesis, our artificial photosynthetic system is composed of two semiconductor light absorbers, an interfacial layer for charge transport, and spatially separated co-catalysts,\u201d says Peidong Yang, a chemist with Berkeley Lab\u2019s Materials Sciences Division, who led this research. “To facilitate solar water- splitting in our system, we synthesized tree-like nanowire heterostructures, consisting of silicon trunks and titanium oxide branches. Visually, arrays of these nanostructures very much resemble an artificial forest.\u201d<\/p>\n

Yang, who also holds appointments with the University of California Berkeley\u2019s Chemistry Department and Department of Materials Science and Engineering, is the corresponding author of a paper describing this research in the journal NANO Letters. The paper is titled “A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting.\u201d Co-authors are Chong Liu, Jinyao Tang, Hao Ming Chen and Bin Liu.<\/p>\n

Solar technologies are the ideal solutions for carbon-neutral renewable energy \u2013 there\u2019s enough energy in one hour\u2019s worth of global sunlight to meet all human needs for a year. Artificial photosynthesis, in which solar energy is directly converted into chemical fuels, is regarded as one of the most promising of solar technologies. A major challenge for artificial photosynthesis is to produce hydrogen cheaply enough to compete with fossil fuels. Meeting this challenge requires an integrated system that can efficiently absorb sunlight and produce charge-carriers to drive separate water reduction and oxidation half-reactions.<\/p>\n

“In natural photosynthesis the energy of absorbed sunlight produces energized charge-carriers that execute chemical reactions in separate regions of the chloroplast,\u201d Yang says. “We\u2019ve integrated our nanowire nanoscale heterostructure into a functional system that mimics the integration in chloroplasts and provides a conceptual blueprint for better solar-to-fuel conversion efficiencies in the future.\u201d<\/p>\n

When sunlight is absorbed by pigment molecules in a chloroplast, an energized electron is generated that moves from molecule to molecule through a transport chain until ultimately it drives the conversion of carbon dioxide into carbohydrate sugars. This electron transport chain is called a “Z-scheme\u201d because the pattern of movement resembles the letter Z on its side. Yang and his colleagues also use a Z-scheme in their system only they deploy two Earth abundant and stable semiconductors \u2013 silicon and titanium oxide \u2013 loaded with co-catalysts and with an ohmic contact inserted between them. Silicon was used for the hydrogen-generating photocathode and titanium oxide for the oxygen-generating photoanode. The tree-like architecture was used to maximize the system\u2019s performance. Like trees in a real forest, the dense arrays of artificial nanowire trees suppress sunlight reflection and provide more surface area for fuel producing reactions.<\/p>\n

\n\n\n\n
\"Arraysconsisting of Si trunks and TiO2
branches facilitate solar water-
splitting in a fully integrated
artificial photosynthesis system.”><\/td>\n<\/tr>\n
Arrays of tree-like nano-wires
consisting of Si trunks and TiO2
branches facilitate solar water-
splitting in a fully integrated
artificial photosynthesis system.<\/td>\n<\/tr>\n<\/table>\n<\/div>\n

“Upon illumination photo-excited electron−hole pairs are generated in silicon and titanium oxide, which absorb different regions of the solar spectrum,\u201d Yang says. “The photo-generated electrons in the silicon nanowires migrate to the surface and reduce protons to generate hydrogen while the photo-generated holes in the titanium oxide nanowires oxidize water to evolve oxygen molecules. The majority charge carriers from both semiconductors recombine at the ohmic contact, completing the relay of the Z-scheme, similar to that of natural photosynthesis. \u201dArrays of tree-like nanowires consisting of Si trunks and TiO2 branches facilitate solar water-splitting in a fully integrated artificial photosynthesis system.<\/p>\n

Under simulated sunlight, this integrated nanowire-based artificial photosynthesis system achieved a 0.12-percent solar-to-fuel conversion efficiency. Although comparable to some natural photosynthetic conversion efficiencies, this rate will have to be substantially improved for commercial use. However, the modular design of this system allows for newly discovered individual components to be readily incorporated to improve its performance. For example, Yang notes that the photocurrent output from the system\u2019s silicon cathodes and titanium oxide anodes do not match, and that the lower photocurrent output from the anodes is limiting the system\u2019s overall performance.<\/p>\n

“We have some good ideas to develop stable photoanodes with better performance than titanium oxide,\u201d Yang says. “We\u2019re confident that we will be able to replace titanium oxide anodes in the near future and push the energy conversion efficiency up into single digit percentages.\u201d<\/p>\n

This research was supported by the DOE Office of Science.<\/p>\n

About the Lawrence Berkeley National Laboratory<\/i><\/b>
Lawrence Berkeley National Laboratory addresses the world\u2019s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab\u2019s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy\u2019s Office of Science.<\/p>\n

About the DOE’s Office of Science<\/b>
DOE\u2019s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.<\/p>\n

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