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graphene-nanonporous-mem-ornlledteamd Less than 1 percent of Earth’s water is drinkable. Removing salt and other minerals from our biggest available source of water—seawater—may help satisfy a growing global population thirsty for fresh water for drinking, farming, transportation, heating, cooling and industry. But desalination is an energy-intensive process, which concerns those wanting to expand its application.

Now, a team of experimentalists led by the Department of Energy’s Oak Ridge National Laboratory has demonstrated an energy-efficient desalination technology that uses a porous made of strong, slim graphene—a carbon honeycomb one atom thick. The results are published in the March 23 advance online issue of Nature Nanotechnology.

“Our work is a proof of principle that demonstrates how you can desalinate saltwater using free-standing, porous graphene,” said Shannon Mark Mahurin of ORNL’s Chemical Sciences Division, who co-led the study with Ivan Vlassiouk in ORNL’s Energy and Transportation Science Division.

“It’s a huge advance,” said Vlassiouk, pointing out a wealth of travels through the porous graphene membrane. “The flux through the current graphene membranes was at least an order of magnitude higher than [that through] state-of-the-art polymeric membranes.”

Current methods for purifying water include distillation and reverse osmosis. Distillation, or heating a mixture to extract volatile components that condense, requires a significant amount of energy. Reverse osmosis, a more energy-efficient process that nonetheless requires a fair amount of energy, is the basis for the ORNL technology.

Making pores in the graphene is key. Without these holes, water cannot travel from one side of the membrane to the other. The water molecules are simply too big to fit through graphene’s fine mesh. But poke holes in the mesh that are just the right size, and water molecules can penetrate. Salt ions, in contrast, are larger than and cannot cross the membrane. The allows osmosis, or passage of a fluid through a semipermeable membrane into a solution in which the solvent is more concentrated. “If you have saltwater on one side of a porous membrane and freshwater on the other, an osmotic pressure tends to bring the water back to the saltwater side. But if you overcome that, and you reverse that, and you push the water from the saltwater side to the freshwater side—that’s the reverse osmosis process,” Mahurin explained.

Graphene Nanonporous Mem ornlledteamd

Researchers created nanopores in graphene (red, and enlarged in the circle to highlight its honeycomb structure) that are stabilized with silicon atoms (yellow) and showed their porous membrane could desalinate seawater. Orange represents a …more

Today reverse-osmosis filters are typically polymers. A filter is thin and resides on a support. It takes significant pressure to push water from the saltwater side to the freshwater side. “If you can make the membrane more porous and thinner, you can increase the flux through the membrane and reduce the pressure requirements, within limits,” Mahurin said. “That all serves to reduce the amount of energy that it takes to drive the process.”

Graphene to the rescue Graphene is only one-atom thick, yet flexible and strong. Its mechanical and chemical stabilities make it promising in membranes for separations. A porous graphene membrane could be more permeable than a , so separated water would drive faster through the membrane under the same conditions, the scientists reasoned. “If we can use this single layer of graphene, we could then increase the flux and reduce the membrane area to accomplish that same purification process,” Mahurin said.

To make graphene for the membrane, the researchers flowed methane through a tube furnace at 1,000 degrees C over a copper foil that catalyzed its decomposition into carbon and hydrogen. The chemical vapor deposited carbon atoms that self-assembled into adjoining hexagons to form a sheet one atom thick.

The researchers transferred the graphene membrane to a silicon nitride support with a micrometer-sized hole. Then the team exposed the graphene to an oxygen plasma that knocked carbon atoms out of the graphene’s nanoscale chicken wire lattice to create pores. The longer the graphene membrane was exposed to the plasma, the bigger the pores that formed, and the more made.

The prepared membrane separated two water solutions—salty water on one side, fresh on the other. The silicon nitride chip held the graphene membrane in place while water flowed through it from one chamber to the other. The membrane allowed rapid transport of water through the membrane and rejected nearly 100 percent of the salt ions, e.g., positively charged sodium atoms and negatively charged chloride atoms.

To figure out the best pore size for desalination, the researchers relied on the Center for Nanophase Materials Sciences (CNMS), a DOE Office of Science User Facility at ORNL. There, aberration-corrected scanning transmission electron microscopy (STEM) imaging, led by Raymond Unocic, allowed for atom-resolution imaging of graphene, which the scientists used to correlate the porosity of the graphene membrane with transport properties. They determined the optimum pore size for effective desalination was 0.5 to 1 nanometers, Mahurin said.

They also found the optimal density of pores for desalination was one pore for every 100 square nanometers. “The more pores you get, the better, up to a point until you start to degrade any mechanical stability,” Mahurin said.

Vlassiouk said making the porous membranes used in the experiment is viable on an industrial scale, and other methods of production of the pores can be explored. “Various approaches have been tried, including irradiation with electrons and ions, but none of them worked. So far, the oxygen plasma approach worked the best,” he added. He worries more about gremlins that plague today’s reverse osmosis membranes—growths on membrane surfaces that clog them (called “biofouling”) and ensuring the mechanical stability of a membrane under pressure.

Explore further: Imperfect graphene opens

MIT-FuturePV-01x250 (From R & D Magazine)  Use of solar photovoltaics has been growing at a phenomenal rate: Worldwide installed capacity has seen sustained growth averaging 43% per year since 2000.

To evaluate the prospects for sustaining such growth, the MIT researchers look at possible constraints on materials availability, and propose a system for evaluating the many competing approaches to improved solar-cell performance.

In a broad new assessment of the status and prospects of solar photovoltaic technology, Massachusetts Institute of Technology (MIT) researchers say that it is “one of the few renewable, low-carbon resources with both the scalability and the technological maturity to meet ever-growing global demand for electricity.”

The analysis is presented in Energy & Environmental Science; a broader analysis of solar technology, economics and policy will be incorporated in a forthcoming assessment of the future of solar energy by the MIT Energy Initiative.

The team comprised MIT professors Vladimir Bulović, Tonio Buonassisi and Robert Jaffe, and graduate students Joel Jean and Patrick Brown. One useful factor in making meaningful comparisons among new photovoltaic technologies, they conclude, is the complexity of the light-absorbing material.

The report divides the many technologies under development into three broad classes: wafer-based cells, which include traditional crystalline silicon, as well as alternatives such as gallium arsenide; commercial thin-film cells, including cadmium telluride and amorphous silicon; and emerging thin-film technologies, which include perovskites, organic materials, dye-sensitized solar cells and quantum dots.

MIT-FuturePV-01x250

Illustration shows the MIT team’s proposed scheme for comparing different photovoltaic materials, based on the complexity of their basic molecular structure. The complexity increases from the simplest material, pure silicon (single atom, lower left), to the most complex material currently being studied for potential solar cells, quantum dots (molecular structure at top right). Materials shown in between include gallium aresenide, perovskite and dye-sensitized solar cells. Image courtesy of the researchers.

With the recent evolution of solar technology, says Jean, the paper’s lead author, it’s important to have a uniform framework for assessment. It may be time, he says, to re-examine the traditional classification of these technologies, generally into three areas: silicon wafer-based cells, thin-film cells and “exotic” technologies with high theoretical efficiencies.

“We’d like to build on the conventional framework,” says Jean, a doctoral student in MIT’s Dept. of Electrical Engineering and Computer Science. “We’re seeking a more consistent way to think about the wide range of current photovoltaic technologies and to evaluate them for potential applications. In this study, we chose to evaluate all relevant technologies based on their material complexity.”

Under this scheme, traditional silicon—a single-element crystalline material—is the simplest material. While crystalline silicon is a mature technology with advantages including high efficiency, proven reliability and no material scarcity constraints, it also has inherent limitations: Silicon is not especially efficient at absorbing light, and solar panels based on silicon cells tend to be rigid and heavy. At the other end of the spectrum are perovskites, organics and colloidal quantum dots, which are “highly complex materials, but can be much simpler to process,” Jean says.

The authors make clear that their definition of material complexity as a key parameter for comparison does not imply any equivalency with complexity of manufacturing. On the contrary, while silicon is the simplest solar-cell material, silicon wafer and cell production is complex and expensive, requiring extraordinary purity and high temperatures.

By contrast, while some complex nanomaterials involve intricate molecular structures, such materials can be deposited quickly and at low temperatures onto flexible substrates. Nanomaterial-based cells could even be transparent to visible light, which could open up new applications and enable seamless integration into windows and other surfaces. The authors caution, however, that the conversion efficiency and long-term stability of these complex emerging technologies is still relatively low. As they write in the paper: “The road to broad acceptance of these new technologies in conventional solar markets is inevitably long, although the unique qualities of these evolving solar technologies—lightweight, paper-thin, transparent—could open entirely new markets, accelerating their adoption.”

The study does caution that the large-scale deployment of some of today’s thin-film technologies, such as cadmium telluride and copper indium gallium diselenide, may be severely constrained by the amount of rare materials that they require. The study highlights the need for novel thin-film technologies that are based on Earth-abundant materials.

The study identifies three themes for future research and development. The first is increasing the power-conversion efficiency of emerging photovoltaic technologies and commercial modules.

A second research theme is reducing the amount of material needed per cell. Thinner, more flexible films and substrates could reduce cell weight and cost, potentially opening the door to new approaches to photovoltaic module design.

A third important research theme is reducing the complexity and cost of manufacturing. Here the researchers emphasize the importance of eliminating expensive, high-temperature processing, and encouraging the adoption of roll-to-roll coating processes for rapid, large-scale manufacturing of emerging thin-film technologies.

“We’ve looked at a number of key metrics for different applications,” Jean says. “We don’t want to rule out any of the technologies,” he says—but by providing a unified framework for comparison, he says, the researchers hope to make it easier for people to make decisions about the best technologies for a given application.

Source: Massachusetts Institute of

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Rice U. Researchers Fine-Tune Quantum Dots from Coal

rice QD finetuneGraphene quantum dots made from coal, introduced in 2013 by the Rice University lab of chemist James Tour, can be engineered for specific semiconducting properties in either of two single-step processes.

In a new study this week in the American Chemical Society journal Applied Materials & Interfaces, Tour and colleagues demonstrated fine control over the graphene oxide dots’ size-dependent band gap, the property that makes them semiconductors. Quantum dots are semiconducting materials that are small enough to exhibit quantum mechanical properties that only appear at the nanoscale.

Tour’s group found they could produce quantum dots with specific semiconducting properties by sorting them through ultrafiltration, a method commonly used in municipal and industrial water filtration and in food production.

The other single-step process involved direct control of the reaction temperature in the oxidation process that reduced coal to quantum dots. The researchers found hotter temperatures produced smaller dots, which had different semiconducting properties.

Tour said graphene quantum dots may prove highly efficient in applications ranging from medical imaging to additions to fabrics and upholstery for brighter and longer-lasting colors. “Quantum dots generally cost about $1 million per kilogram and we can now make them in an inexpensive reaction between coal and acid, followed by separation. And the coal is less than $100 per ton.”

rice QD finetune

The dots in these experiments all come from treatment of anthracite, a kind of coal. The processes produce batches in specific sizes between 4.5 and 70 nanometers in diameter.

Rice University scientists have produced graphene quantum dots produced from coal with tuned band gaps and photoluminescent properties. These quantum dots, seen with an electron microscope, average 70 nanometers in diameter. Credit: Tour Group/Rice University

Graphene quantum dots are photoluminescent, which means they emit light of a particular wavelength in response to incoming light of a different wavelength. The emitted light ranges from green (smaller dots) to orange-red (larger dots). Because the emitted color also depends on the dots’ size, this property can also be tuned, Tour said. The lab found quantum dots that emit blue light were easiest to produce from bituminous .

The researchers suggested their quantum dots may also enhance sensing, electronic and photovoltaic applications. For instance, catalytic reactions could be enhanced by manipulating the reactive edges of . Their fluorescence could make them suitable for metal or chemical detection applications by tuning to avoid interference with the target materials’ emissions.

Rice University scientists have produced graphene quantum dots produced from coal with tuned band gaps and photoluminescent properties. These quantum dots are about 4.5 nanometers in diameter. Credit: Tour Group/Rice University

Explore further: Making quantum dots glow brighter

Berkley Lab: A Better Way of Scrubbing CO2

Jeff-Long-CO2-New-284x300Berkeley Lab Researchers Find Way to Improve the Cost-Effectiveness Through the Use of MOFs

A means by which the removal of carbon dioxide (CO2) from coal-fired power plants might one day be done far more efficiently and at far lower costs than today has been discovered by a team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab). By appending a diamine molecule to the sponge-like solid materials known as metal-organic-frameworks (MOFs), the researchers were able to more than triple the CO2-scrubbing capacity of the MOFs, while significantly reducing parasitic energy.

“We’ve shown that diamine-appended MOFs can function as phase-change CO2 adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature and result in a much higher separation capacity,” says Jeffrey Long, a chemist with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Berkeley. “The step-shaped adsorption isotherms are the product of an unprecedented cooperative process in which CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate.”

Jeff-Long-CO2-New-284x300

Appending a diamine molecule to this manganese-based MOF greatly increased its capacity for adsorbing CO2. Green, gray, red, blue and white spheres represent Mn, C, O, N and H atoms respectively.

Details on this research are reported in a paper published in the journal Nature. The paper is titled “Cooperative insertion of CO2 in diamine-appended metal-organic frameworks.” Long is the corresponding author. The lead authors are Thomas McDonald and Jarad Mason. (See below for a complete list of co-authors.)

(From left) Tom McDonald, Jeffrey Long and Jarad Mason were part of a collaboration that discovered a way to improve the carbon-scrubbing capabilities of MOFs. (Photo by Roy Kaltschmidt)

Approximately 13 billion tons of carbon dioxide are released into the atmosphere each year as a result of burning coal for the production of electricity. These carbon emissions are major contributors to global climate change and the acidification of our planet’s oceans. However, given that the United States holds the world’s largest estimated recoverable reserves of coal, coal-burning power plants will continue to be a major source of our nation’s electricity generation for the foreseeable future. This makes the wide-spread implementation of carbon capture and storage technologies at coal-fired power plants an imperative.

Current carbon capture and storage technologies are based on aqueous amine scrubbers that impose a substantial energy penalty for their use. If widely implemented, these scrubbers would consume about one-third of the energy generated by a power plant and this would substantially drive up the price of electricity. MOFs have been proposed as a highly promising alternative to amine scrubbers. Consisting of a metal center surrounded by organic “linker” molecules, MOFs form a highly porous three-dimensional crystal framework with an extraordinarily large internal surface area – a MOF the size of a sugar cube if unfolded and flattened would blanket a football field. By altering their composition, MOFs can be tailored to serve as highly effective storage vessels for capturing and containing carbon dioxide.

Long and colleagues at the Center for Gas Separation Relevant to Clean Energy Technologies, a DOE-funded Energy Frontier Research Center (EFRC) hosted by UC Berkeley, have been exploring various ways to functionalize MOFs for the selective adsorption of CO2.

“An ideal MOF will selectively bind CO2 in the presence of nitrogen and release it under mild regeneration conditions,” Long says. “We were experimenting with the appending of diamines to the open coordination sites in the framework pores of a MOF as a way of increasing the adsorption of CO2. Once we saw the formation of the isotherm steps we knew we had something different and important going on.”

A portion of the ammonium carbamate chain formed along the pore surface of a MOF when CO2 molecules are inserted into metal-amine bonds. Green, gray, red, blue and white spheres represent Mn, C, O, N and H atoms respectively.

The appending of the diamine to the metal sites set off a chain reaction of events in which the carbon, metal and amines cooperatively reconfigured into the ammonium carbonates that enabled the CO2 isotherm adsorption steps. The researchers then found that the pressure at which these adsorption steps occur can be tuned in accordance with the strengths of the metal-amine bonds. By starting with magnesium ions, then strategically replacing them with ions of manganese, iron, cobalt and zinc, Long and his colleagues were able to create the first solid phase-change CO2 scrubbing materials.

“With our technique, large CO2 separation capacities can be achieved with small temperature swings and regeneration energies that are appreciably lower than what can be achieved with state-of-the-art aqueous amine solutions,” Long says. “We now understand how this CO2 cooperative process works and should be able to use the mechanism to design highly efficient adsorbents for removing CO2 from various gas mixtures.”

To adapt this technique to real-world applications, Long is now working with chemist Steven Kaye on the Mosaic Materials Project. Aimed at replacing today’s energy-intensive and expensive distillation and adsorption processes with high-efficiency MOFs, the Mosaic Materials Project is being funded by Cyclotron Road, a technology-incubation program established by Berkeley Lab.

In addition to Long, McDonald and Mason, other authors of the Nature paper that describes this study were Xueqian Kong, Eric  Bloch, David Gygi, Alessandro Dani, Valentina Crocella, Filippo Giordanino, Samuel O. Odoh, Walter Drisdell, Bess Vlaisavljevich, Allison Dzubak, Roberta Poloni, Sondre Schnell, Nora Planas, Kyuho Lee, Tod Pascal, Liwen F.Wan, David Prendergast, Jeffrey Neaton, Berend Smit, Jeffrey Kortright, Laura Gagliardi, Silvia Bordiga and Jeffrey Reimer.

Portions of this research were supported by DOE’s Advanced Research Projects Agency–Energy (ARPA-E).

Additional Information

For more about the research of Jeffrey Long go here

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