Versatility of a new material makes for more efficient solar cells

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(PhysOrg.com) -- A Colorado State University professor has successfully turned a mineral compound into a material that can pass current through a nanoparticle network– an important discovery into more efficient, inexpensive solar cell materials.

Amy Prieto, professor of chemistry and founder of Prieto Battery, discovered that dramatic reactions occur with copper selenide at the nanoscale, according to the cover story in the Feb. 9 issue of theJournal of the American Chemical Society. Reactions with air allow Prieto and her students to manipulate or“tune” the properties of the device– such as a solar cell– containing the copper compound.

That’s an important discovery for looking further into earth-abundant, non-toxic materials that could help makesolar cellsinexpensive and absorb sunlight more efficiently than silicon, Prieto said.

“Nanoparticlesare so small, therefore most of the surface reactions that you would never notice in bulk materials are pretty dramatic in a nanoparticle,” Prieto said.“There is getting to be a lot of interest in making devices like solar cells from nanoparticles.

"There is still much to be understood about how the material we're now using works– how it absorbs photons and converts them to current, that then has to traverse a tortuous path through the nanoparticle network” she said.

Prieto and her team tested the copper nanomaterial by attaching electrodes to thin films of copper selenide nanoparticles and watching how the thin films pass electric charges. They found that, with prolonged air exposure, the current changed dramatically.

Prieto’s research focuses on creating new inorganic materials– to replace silicon or cadmium telluride, for example– that could be incorporated into solar panels to produce electrical current.

She joined Colorado State University in 2005 as an assistant professor. She is part of the university’s Clean Energy Supercluster commercialization arm, Cenergy. In 2009, Prieto co-founded Cenergy’s first startup company, Prieto Battery, a company expected to produce batteries theoretically up to 1,000 times more powerful and 10 times longer lasting and cheaper than traditional batteries. The development of this technology could revolutionize the military, automobile and healthcare industries.

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Tuning graphene film so it sheds water

Windshields that shed water so effectively that they don't need wipers. Ship hulls so slippery that they glide through the water more efficiently than ordinary hulls.

These are some of the potential applications for graphene, one of the hottestnew materialsin the field ofnanotechnology, raised by the research of James Dickerson, assistant professor of physics at Vanderbilt.

Dickerson and his colleagues have figured out how to create a freestanding film of graphene oxide and alter itssurface roughnessso that it either causeswaterto bead up and run off or causes it to spread out in a thin layer.

"Graphene films are transparent and, because they are made of carbon, they are very inexpensive to make,"Dickerson said."The technique that we use can be rapidly scaled up to produce it in commercial quantities."

His approach is documented in an article published online by the journalACS Nanoon Nov. 26.

Graphene is made up of sheets ofcarbon atomsarranged in rings– something like molecular chicken wire. Not only is this one of the thinnest materials possible, but it is 10 times stronger than steel and conducts electricity better at room temperature than any other known material. Graphene's exotic properties have attracted widespread scientific interest, but Dickerson is one of the first to investigate how it interacts with water.

Many scientists studying graphene make it using a dry method, called"mechanical cleavage,"that involves rubbing or scraping graphite against a hard surface. The technique produces sheets that are both extremely thin and extremely fragile. Dickerson's method can produce sheets equally as thin but considerable stronger than those made by other techniques. It is already used commercially to produce a variety of different coatings and ceramics. Known as electrophoretic deposition, this"wet"technique combines an electric field within a liquid medium to create nanoparticle films that can be transferred to another surface.

Dickerson and his colleagues found that they could change the manner in which the graphene oxide particles assemble into a film by varying the pH of the liquid medium and the electric voltage used in the process. One pair of settings lay down the particles in a"rug"arrangement that creates a nearly atomically smooth surface. A different pair of settings causes the particles to clump into tiny"bricks"forming a bumpy and uneven surface. The researchers determined that the rug surface causes water to spread out in athin layer, while the brick surface causes water to bead up and run off.

Dickerson is pursuing an approach that could create film that enhances these water-associated properties, making them even more effective at either spreading out water or causing it to bead up and run off. There is considerable academic and commercial interest in the development of coatings with these enhanced properties, called super-hydrophobic and super-hydrophilic. Potential applications range from self-cleaning glasses and clothes to antifogging surfaces to corrosion protection and snow-load protection on buildings. However, effective, low-cost and durable coatings have yet to make it out of the laboratory.

Dickerson's idea is to apply his basic procedure to"fluorographene"– a fluorinated version of graphene that is a two-dimensional version of Teflon– recently produced by Kostya S. Novoselov and Andre K. Geim at the University of Manchester, who received the 2010 Nobel Prize for the discovery of graphene. Normal fluorographene under tension should be considerably more effective in repelling water thangrapheneoxide. So there is a good chance a"brick"version and a"rug"version would have extreme water-associated effects, Dickerson figures.

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Researcher investigates new material grown from sugar

(PhysOrg.com) -- Ordinary table sugar could be a key ingredient to developing much lighter, faster, cheaper, denser and more robust computer electronics for use on U.S. military aircraft.

Though admittedly far in the future, recent results from a program led by chemist and Rice University professor, Dr. James Tour demonstrate another example of the cutting-edge basic research.

Tour and his colleagues at Rice have developed a relatively easy and controllable method for making pristine sheets ofgraphene--- the one-atom-thick form of carbon --- from regular tablesugarand other solid carbon sources.

"Dr. Tour is exploring a chemical approach to producing high quality carbon based nanostructures such as nanotubes and graphenes with well defined properties,"said AFOSR program manager, Dr. Charles Lee.

In their method, a small amount of sugar is placed on a tiny sheet of copper foil. The sugar is then subjected to flowing hydrogen and argon gas under heat and low pressure. After 10 minutes, the sugar is reduced to a pure carbon film, or a single layer of graphene. Adjusting thegas flowallowed the researchers to control the thickness of the film.

The use of solid carbon sources like sugar has allowed Tour to stay away from the more cumbersomechemical vapor depositionmethod and the high temperatures associated with it. His one-step, low-temperature process makes graphene considerably easier to manufacture.

"In a traditional CVD point of view, it was straightforward to optimize the pristine graphene's quality through adjusting the growth conditions and themetal catalystswith continuous gas sources (CH₄or C₂H₂),"explained Tour."With this technique using different kinds of solidcarbonsources, more benefits such as graphene doping and thickness control could be realized."

According to Tour, doped graphene opens more possibilities for both Air Force and commercial electronics applications. Pristine graphene has no bandgap, but doped graphene allows for manipulation of electronic and optical properties, important factors for making switching and logic devices.

"These materials can be used in advanced electronics, photonics as well as structural applications for the Air Force,"explained Lee.

While the Air Force is focusing primarily on potential electronics applications, many other commercial and medical uses could be possible, including transparent touch screen devices, special biocompatible films for surgery of traumatic brain injuries, faster transistors in personal computers or thin materials for solar energy harvesting.

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Chemists turn gold to purple -- on purpose

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Professor Richard Watt and his chemistry students suspected that a common protein could potentially react with sunlight and harvest its energy– similar to what chlorophyll does during photosynthesis.

The story of how they proved it sounds as colorful as the legend of the leprechaun who hid his pot ofgoldat the end of the rainbow.

They started with citric acid from oranges and mixed it with the protein. Next they dissolved gold powder into the solution. Then they put vials of the yellow-colored mixture in direct sunlight and crossed their fingers in the hope that it would turn purple.

Here's the reason why: If it turned purple, that would signal that the gold atoms had received electrons and used the donated energy to bunch together as small, purple-colorednanoparticles. And that would mean that the protein used the sunlight to excite the citric acid and trigger a transfer of energy.

While direct sunlight did the trick in about 20 minutes, a high-powered tungsten mercury lamp worked much faster.

"We set the system up, turned on the light, and the solution turned purple,"Watt said."We knew that we'd proved the concept."

The beauty of this experiment lies not in its colors– unless, of course, you're thinking of it as a potential"green"energy source that keeps the environment clean.

The BYU researchers published their experiments in theJournal of Nanoparticle Research. The final step of this project will involve connecting theproteinto an electrode to channel the energy into a battery or fuel cell. The BYU chemists will partner with Jae-Woo Kim of the National Institute of Aerospace for this next stage of the work.

Professor Watt's pedigree includes a post-doc at Princeton, a father who developed a fuel cell that runs on sugar and weed-killer and a more distant ancestor credited with inventing the first practical steam engine. That ancestor is also the Scottish engineer for whom the unit of power"watt"is named.

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New approach to solar cells

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An interdisciplinary team of UC Davis and UC Santa Cruz researchers is taking a novel approach to solar power, one that promises to lead to a technological breakthrough. By using nanoparticles of germanium, silicon and other materials, the researchers hope to produce solar cells far more efficient than the current state of the art.

The project was recently awarded $1.5 million over three years from the National Science Foundation.

Conventional solar cells all operate on the same principle of"one photon in, one electron out,"said Gergely Zimanyi, professor of physics at UC Davis and principal investigator on the NSF grant. In other words, one particle of light, or photon, hits the solar cell and generates one electron to produce an electrical current.

The efficiency— energy out compared to energy in— of a solar cell operating according to this principle is capped by a theoretical maximum of 31 percent. But by constructing solar cells from extremely small nanoparticles, the UC researchers aim to generate severalelectronsfor each photon, raising the maximum efficiency to between 42 and 65 percent.

The one-photon-in/multiple-electrons-out paradigm has been demonstrated at the Los Alamos National Laboratory, Zimanyi said— but the Los Alamos group did not build a functioning solar cell based on this paradigm. The UC Davis/UC Santa Cruz team includes scientists with experience makingsolar cellsfrom nanoparticles, giving hope that the group will be able to construct a fully functioning and well-optimized solar cell fromgermaniumandsiliconnanoparticles, he said.

The team members are: Zimanyi; UC Davis chemistry professors Susan Kauzlarich and Delmar Larsen; Professor Giulia Galli, who holds a joint appointment in physics and chemistry; Professor Zhaojun Bai, Department of Mathematics and Computer Science; Debashis Paul, professor in the Department of Statistics; and Susan Carter, professor of physics at UC Santa Cruz.

The interdisciplinary nature of the team was crucial to getting the proposal funded, Zimanyi said."NSF asked for a collaborative effort between materials sciences, chemistry and mathematical sciences,"he said.

Zimanyi, Galli and Bai will conduct theoretical and computer-modeling studies, with Paul providing statistical expertise; Kauzlarich's lab will synthesize the newnanoparticles, Larsen's group will characterize them and Carter's lab at UCSC will develop a working device. A prototype cell has been already constructed prior to getting the grant and exhibited an efficiency of about 8 percent, which Zimanyi described as a very encouraging result given the limited resources going into its construction.

The team will collaborate with the California Solar Energy Collaborative, which is based at UC Davis and led by Pieter Stroeve, professor of chemical engineering and materials science. The team also plans an outreach effort, primarily via its public webpage:http://www.solarwi… cdavis.edu/.

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A mix of tiny gold and viral particles -- and the DNA ties that bind them

Scientists have created a diamond-like lattice composed of gold nanoparticles and viral particles, woven together and held in place by strands of DNA. The structure– a distinctive mix of hard, metallic nanoparticles and organic viral pieces known as capsids, linked by the very stuff of life, DNA– marks a remarkable step in scientists' ability to combine an assortment of materials to create infinitesimal devices.

The research, done by scientists at the University of Rochester Medical Center, Scripps Research Institute, and Massachusetts Institute of Technology, was published recently inNature Materials.

While people commonly think ofDNAas a blueprint for life, the team used DNA instead as a tool to guide the precise positioning of tiny particles just one-millionth of a centimeter across, using DNA to chaperone the particles.

Central to the work is the unique attraction of each of DNA's four chemical bases to just one other base. The scientists created specific pieces of DNA and then attached them to gold nanoparticles and viral particles, choosing the sequences and positioning them exactly to force the particles to arrange themselves into a crystal lattice.

When scientists mixed the particles, out of the brew emerged a sodium thallium crystal lattice. The device"self assembled"or literally built itself.

The research adds some welcome flexibility to the toolkit that scientists have available to create nano-sized devices.

"Organic materials interact in ways very different from metal nanoparticles. The fact that we were able to make such different materials work together and be compatible in a single structure demonstrates some new opportunities for building nano-sized devices,"said Sung Yong Park, Ph.D., a research assistant professor of Biostatistics and Computational Biology at Rochester.

Park and M.G Finn, Ph.D., of Scripps Research Institute are corresponding authors of the paper.

Such a crystal lattice is potentially a central ingredient to a device known as a photonic crystal, which can manipulate light very precisely, blocking certain colors or wavelengths of light while letting other colors pass. While 3-D photonic crystals exist that can bend light at longer wavelengths, such as the infrared, this lattice is capable of manipulating visible light. Scientists foresee many applications for such crystals, such as optical computing and telecommunications, but manufacturing and durability remain serious challenges.

It was three years ago that Park, as part of a larger team of colleagues at Northwestern University, first produced a crystal lattice with a similar method, using DNA to link gold nanospheres. The new work is the first to combine particles with such different properties– hard goldnanoparticlesand more flexible organic particles.

Within the new structure, there are actually two distinct forces at work, Park said. Thegoldparticles and theviral particlesrepel each other, but their deterrence is countered by the attraction between the strategically placed complementary strands of DNA. Both phenomena play a role in creating the rigidcrystal lattice. It's a little bit like how countering forces keep our curtains up: A spring in a curtain rod pushes the rod to lengthen, while brackets on the window frame counter that force, creating a taut, rigid device.

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A paperweight for platinum: Bracing catalyst in material makes fuel cell component work better, last longer

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A new combination of nanoparticles and graphene results in a more durable catalytic material for fuel cells, according to work published today online at the Journal of the American Chemical Society. The catalytic material is not only hardier but more chemically active as well. The researchers are confident the results will help improve fuel cell design.

"Fuel cells are an important area of energy technology, but cost and durability are big challenges,"said chemist Jun Liu."The unique structure of this material provides much needed stability, goodelectrical conductivityand other desired properties."

Liu and his colleagues at the Department of Energy's Pacific Northwest National Laboratory, Princeton University in Princeton, N.J., and Washington State University in Pullman, Wash., combined graphene, a one-atom-thick honeycomb of carbon with handy electrical and structural properties, withmetal oxidenanoparticles to stabilize afuel cellcatalystand make it better available to do its job.

"This material has great potential to make fuel cells cheaper and last longer,"said catalytic chemist Yong Wang, who has a joint appointment with PNNL and WSU."The work may also provide lessons for improving the performance of other carbon-based catalysts for a broad range of industrial applications."

Muscle Metal Oxide

Fuel cells work by chemically breaking down oxygen and hydrogen gases to create an electrical current, producing water and heat in the process. The centerpiece of the fuel cell is the chemical catalyst -- usually a metal such asplatinum-- sitting on a support that is often made of carbon. A good supporting material spreads the platinum evenly over its surface to maximize the surface area with which it can attackgas molecules. It is also electrically conductive.

Fuel cell developers most commonly useblack carbon-- think pencil lead -- but platinum atoms tend to clump on such carbon. In addition, water can degrade the carbon away. Another support option is metal oxides -- think rust -- but what metal oxides make up for in stability and catalyst dispersion, they lose in conductivity and ease of synthesis. Other researchers have begun to explore metal oxides in conjunction with carbon materials to get the best of both worlds.

As a carbon support, Liu and his colleagues thought graphene intriguing. The honeycomb lattice of graphene is porous, electrically conductive and affords a lot of room for platinum atoms to work. First, the team crystallized nanoparticles of the metal oxide known as indium tin oxide -- or ITO -- directly onto specially treated graphene. Then they added platinum nanoparticles to the graphene-ITO and tested the materials.

Platinumweight

The team viewed the materials under high-resolution microscopes at EMSL, DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. The images showed that without ITO, platinum atoms clumped up on the graphene surface. But with ITO, the platinum spread out nicely. Those images also showed catalytic platinum wedged between the nanoparticles and the graphene surface, with the nanoparticles partially sitting on the platinum like a paperweight.

To see how stable this arrangement was, the team performed theoretical calculations of molecular interactions between the graphene, platinum and ITO. This number-crunching on EMSL's Chinook supercomputer showed that the threesome was more stable than the metal oxide alone on graphene or the catalyst alone on graphene.

But stability makes no difference if the catalyst doesn't work. In tests for how well the materials break down oxygen as they would in a fuel cell, the triple-threat packed about 40% more of a wallop than the catalyst alone on graphene or the catalyst alone on other carbon-based supports such as activated carbon.

Last, the team tested how well the new material stands up to repeated usage by artificially aging it. After aging, the tripartite material proved to be three times as durable as the lone catalyst ongrapheneand twice as durable as on commonly used activated carbon. Corrosion tests revealed that the triple threat was more resistant than the other materials tested as well.

The team is now incorporating the platinum-ITO-graphene material into experimental fuel cells to determine how well it works under real world conditions and how long it lasts.

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