Platinum-graphene fuel cell catalysts show superior stability over bulk platinum – Georgia Institute of Tecnology


Seung Soon Jang, an associate professor, Faisal Alamgir, an associate professor, and Ji Il Choi, a postdoctoral researcher, all in Georgia Tech’s School of Materials Science and Engineering, examine a piece of platinum-graphene catalyst. Credit: Allison Carter

Films of platinum only two atoms thick supported by graphene could enable fuel cell catalysts with unprecedented catalytic activity and longevity, according to a study published recently by researchers at the Georgia Institute of Technology.

Platinum is one of the most commonly used catalysts for fuel cells because of how effectively it enables the oxidation reduction reaction at the center of the technology. But its high cost has spurred research efforts to find ways to use smaller amounts of it while maintaining the same .

“There’s always going to be an initial cost for producing a fuel cell with , and it’s important to keep that cost as low as possible,” said Faisal Alamgir, an associate professor in Georgia Tech’s School of Materials Science and Engineering. “But the real cost of a fuel cell system is calculated by how long that system lasts, and this is a question of durability.

“Recently there’s been a push to use catalytic systems without , but the problem is that there hasn’t been a system proposed so far that simultaneously matches the catalytic activity and the durability of platinum,” Alamgir said.

The Georgia Tech researchers tried a different strategy. In the study, which was published on September 18 in the journal Advanced Functional Materialsand supported by the National Science Foundation, they describe creating several systems that used atomically-thin  of platinum supported by a layer of graphene—effectively maximizing the total surface area of the platinum available for catalytic reactions and using a much smaller amount of the precious metal.

Most platinum-based catalytic systems use nanoparticles of the metal chemically bonded to a support surface, where surface atoms of the particles do most of the catalytic work, and the catalytic potential of the atoms beneath the surface is never utilized as fully as the surface atoms, if at all.

This graphic shows how the graphene layer in gray provides structure and stability to the two atomic layers of platinum above represented in blue. Credit: Ji Il Choi

Additionally, the researchers showed that the new platinum films that are at least two atoms thick outperformed nanoparticle platinum in the dissociation energy, which is a measure of the energy cost of dislodging a surface platinum atom. That measurement suggests those films could make potentially longer-lasting catalytic systems.

To prepare the atomically-thin films, the researchers used a process called electrochemical atomic layer deposition to grow platinum monolayers on a layer of graphene, creating samples that had one, two or three atomic layers of atoms. The researchers then tested the samples for dissociation energy and compared the results to the energy of a single atom of platinum on graphene as well as the energy from a common configurations of platinum nanoparticles used in catalysts.

“The fundamental question at the heart of this work was whether it was possible that a combination of metallic and  can render the platinum atoms in a platinum-graphene combination more stable than their counterparts in bulk platinum used commonly in catalysts that are supported by metallic bonding,” said Seung Soon Jang, an associate professor in the School of Materials Science and Engineering.

The researchers found that the bond between neighboring platinum atoms in the film essentially combines forces with the bond between the film and the graphene layer to provide reinforcement across the system. That was especially true in the platinum film that was two atoms thick.

“Typically metallic films below a certain thickness are not stable because the bonds between them are not directional, and they tend to roll over each other and conglomerate to form a particle,” Alamgir said. “But that’s not true with graphene, which is stable in a two-dimensional form, even one atom thick, because it has very strong covalent directional bonds between its neighboring . So this new catalytic system could leverage the directional bonding of the graphene to support an atomically-thin film of platinum.”

Future research will involve further testing of how the films behave in a catalytic environment. The researchers found in earlier research on graphene-platinum films that the material behaves similarly in catalytic reactions regardless of which side—graphene or platinum—is the exposed active surface.

“In this configuration, the graphene is not acting as a separate entity from the platinum,” Alamgir said. “They’re working together as one. So we believe that if you’re exposing the  side, you get the same catalytic activity and you could further protect the platinum, potentially further enhancing durability.”

More information: Ji Il Choi et al, Contiguous and Atomically Thin Pt Film with Supra‐Bulk Behavior Through Graphene‐Imposed Epitaxy, Advanced Functional Materials(2019).  DOI: 10.1002/adfm.201902274

Journal information: Advanced Functional Materials

Provided by Georgia Institute of Technology

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Stretchy Plastic Electrolytes could Enable NEW lithium-ion Battery Design – Replaces Expensive metals and traditional liquid Electrolyte with Lower Cost transition metal fluorides and a Solid Polymer Electrolyte


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A lithium-ion battery is shown using a promising new cathode and electrolyte system that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a solid polymer electrolyte. Credit: Allison Carter

The growing popularity of lithium-ion batteries in recent years has put a strain on the world’s supply of cobalt and nickel—two metals integral to current battery designs—and sent prices surging.

In a bid to develop alternative designs for lithium-based batteries with less reliance on those , researchers at the Georgia Institute of Technology have developed a promising new  and  system that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a .

“Electrodes made from transition metal fluorides have long shown stability problems and rapid failure, leading to significant skepticism about their ability to be used in next generation batteries,” said Gleb Yushin, a professor in Georgia Tech’s School of Materials Science and Engineering. “But we’ve shown that when used with a solid polymer electrolyte, the metal fluorides show remarkable stability—even at —which could eventually lead to safer, lighter and cheaper lithium-ion batteries.”

In a typical lithium-ion battery, energy is released during the transfer of lithium ions between two electrodes—an anode and a cathode, with a cathode typically comprising lithium and transition metals such as cobalt, nickel and manganese. The ions flow between the electrodes through a liquid electrolyte.

For the study, which was published Sept. 9 in the journal Nature Materials and sponsored by the Army Research Office, the research team fabricated a new type of cathode from iron fluoride active material and a solid polymer electrolyte nanocomposite. Iron fluorides have more than double the lithium capacity of traditional cobalt- or nickel-based cathodes. In addition, iron is 300 times cheaper than cobalt and 150 times cheaper than nickel.

To produce such a cathode, the researchers developed a process to infiltrate a solid polymer electrolyte into the prefabricated iron fluoride electrode. They then hot pressed the entire structure to increase density and reduce any voids.

Stretchy plastic electrolytes could enable new lithium-ion battery design

Gleb Yushin, a professor in Georgia Tech’s School of Materials Science and Engineering and Kostiantyn Turcheniuk, research scientist in Yushin’s lab, inspect a battery using a new cathode design that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a solid polymer electrolyte. Credit: Allison Carter

Two central features of the polymer-based electrolyte are its ability to flex and accommodate the swelling of the iron fluoride while cycling and its ability to form a very stable and flexible interphase with iron fluoride. Traditionally, that swelling and massive side reactions have been key problems with using iron fluoride in previous battery designs.

“Cathodes made from iron fluoride have enormous potential because of their high capacity, low material costs and very broad availability of ,” Yushin said. “But the volume changes during cycling as well as parasitic side reactions with liquid electrolytes and other degradation issues have limited their use previously. Using a solid electrolyte with elastic properties solves many of these problems.”

The researchers then tested several variations of the new solid-state batteries to analyze their performance over more than 300 cycles of charging and discharging at elevated temperature of 122 degrees Fahrenheit, noting that they outperformed previous designs using metal  even when these were kept cool at room temperatures.

The researchers found that the key to the enhanced battery performance was the solid polymer electrolyte. In previous attempts to use metal fluorides, it was believed that metallic ions migrated to the surface of the cathode and eventually dissolved into the liquid electrolyte, causing a capacity loss, particularly at elevated temperatures. In addition,  fluorides catalyzed massive decomposition of liquid electrolytes when cells were operating above 100 degrees Fahrenheit. However, at the connection between the solid electrolyte and the cathode, such dissolving doesn’t take place and the solid electrolyte remains remarkably stable, preventing such degradations, the researchers wrote.

“The polymer electrolyte we used was very common, but many other solid electrolytes and other battery or electrode architectures—such as core-shell particle morphologies—should be able to similarly dramatically mitigate or even fully prevent parasitic side reactions and attain stable performance characteristics,” said Kostiantyn Turcheniuk, research scientist in Yushin’s lab and a co-author of the manuscript.

In the future, the researchers aim to develop new and improved solid electrolytes to enable fast charging and also to combine solid and liquid electrolytes in new designs that are fully compatible with conventional cell manufacturing technologies employed in large battery factories.


Explore further

Study: New solvent-free, single lithium-ion conducting covalent organic framework

A Path to Cheaper Flexible Solar Cells -Researchers at Georgia IT and MIT are Developing the Potential Perovskite-Based Solar Cells


Perovskite GT 190207142218_1_540x360
A researcher at Georgia Tech holds a perovskite-based solar cell, which is flexible and lighter than silicon-based versions. Credit: Rob Felt, Georgia Tech

There’s a lot to like about perovskite-based solar cells. They are simple and cheap to produce, offer flexibility that could unlock a wide new range of installation methods and places, and in recent years have reached energy efficiencies approaching those of traditional silicon-based cells.

But figuring out how to produce perovskite-based energy devices that last longer than a couple of months has been a challenge.

Now researchers from Georgia Institute of Technology, University of California San Diego and Massachusetts Institute of Technology have reported new findings about perovskite solar cells that could lead the way to devices that perform better.

“Perovskite solar cells offer a lot of potential advantages because they are extremely lightweight and can be made with flexible plastic substrates,” said Juan-Pablo Correa-Baena, an assistant professor in the Georgia Tech School of Materials Science and Engineering. “To be able to compete in the marketplace with silicon-based solar cells, however, they need to be more efficient.”

In a study that was published February 8 in the journal Science and was sponsored by the U.S Department Energy and the National Science Foundation, the researchers described in greater detail the mechanisms of how adding alkali metal to the traditional perovskites leads to better performance. Perov SCs 091_main

“Perovskites could really change the game in solar,” said David Fenning, a professor of nanoengineering at the University of California San Diego. “They have the potential to reduce costs without giving up performance. But there’s still a lot to learn fundamentally about these materials.”

To understand perovskite crystals, it’s helpful to think of its crystalline structure as a triad. One part of the triad is typically formed from the element lead. The second is typically made up of an organic component such as methylammonium, and the third is often comprised of other halides such as bromine and iodine.

In recent years, researchers have focused on testing different recipes to achieve better efficiencies, such as adding iodine and bromine to the lead component of the structure. Later, they tried substituting cesium and rubidium to the part of the perovskite typically occupied by organic molecules.

“We knew from earlier work that adding cesium and rubidium to a mixed bromine and iodine lead perovskite leads to better stability and higher performance,” Correa-Baena said.

But little was known about why adding those alkali metals improved performance of the perovskites.

To understand exactly why that seemed to work, the researchers used high-intensity X-ray mapping to examine the perovskites at the nanoscale.

Structure-of-perovskite-solar-cells-a-Device-architecture-and-b-energy-band-diagram

“By looking at the composition within the perovskite material, we can see how each individual element plays a role in improving the performance of the device,” said Yanqi (Grace) Luo, a nanoengineering PhD student at UC San Diego.

They discovered that when the cesium and rubidium were added to the mixed bromine and iodine lead perovskite, it caused the bromine and iodine to mix together more homogeneously, resulting in up to 2 percent higher conversion efficiency than the materials without these additives.

“We found that uniformity in the chemistry and structure is what helps a perovskite solar cell operate at its fullest potential,” Fenning said. “Any heterogeneity in that backbone is like a weak link in the chain.”

Even so, the researchers also observed that while adding rubidium or cesium caused the bromine and iodine to become more homogenous, the halide metals themselves within their own cation remained fairly clustered, creating inactive “dead zones” in the solar cell that produce no current.

“This was surprising,” Fenning said. “Having these dead zones would typically kill a solar cell. In other materials, they act like black holes that suck in electrons from other regions and never let them go, so you lose current and voltage.

“But in these perovskites, we saw that the dead zones around rubidium and cesium weren’t too detrimental to solar cell performance, though there was some current loss,” Fenning said. “This shows how robust these materials are but also that there’s even more opportunity for improvement.”

The findings add to the understanding of how the perovskite-based devices work at the nanoscale and could lay the groundwork for future improvements.

“These materials promise to be very cost effective and high performing, which is pretty much what we need to make sure photovoltaic panels are deployed widely,” Correa-Baena said. “We want to try to offset issues of climate change, so the idea is to have photovoltaic cells that are as cheap as possible.”

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Materials provided by Georgia Institute of TechnologyNote: Content may be edited for style and length.

Remote-control shoots laser at nano-gold to turn on cancer-killing immune cells – could one day send immune cells on a rampage against a malignant tumor


Nano Thermo Cancer 55092A heat-sensitive gene switch implanted in a sample of T-cells works in an in vitro check. Gentle pulses from a near-infrared laser directed at gold nanoparticles, which are also in the sample with the T-cells, transform into gentle heat and flip the switch on, activating the T-cells. The resulting signal appears as orange dots on a monitor in the background. CREDIT Georgia Tech / Allison Carter

Abstract:
A remote command could one day send immune cells on a rampage against a malignant tumor. The ability to mobilize, from outside the body, targeted cancer immunotherapy inside the body has taken a step closer to becoming reality.

Remote-control shoots laser at nano-gold to turn on cancer-killing immune cells

Bioengineers at the Georgia Institute of Technology have installed a heat-sensitive switch into T-cells that can activate the T-cells when heat turns the switch on. The method, tested in mice and published in a new study, is locally targeted and could someday help turn immunotherapy into a precision instrument in the fight against cancer.

Immunotherapy has made headlines with startling high-profile successes like saving former U.S. President Jimmy Carter from brain cancer. But the treatment, which activates the body’s own immune system against cancer and other diseases, has also, unfortunately, proved to be hit-or-miss.

“In patients where radiation and traditional chemotherapies have failed, this is where T-cell therapies have shined, but the therapy is still new,” said principal investigator Gabe Kwong. “This study is a step toward making it even more effective.”

Laser, gold, and T-cells

In the study, Kwong’s team successfully put their remote-control method through initial tests in mice with implanted tumors (so-called tumor phantoms, specially designed for certain experiments). The remote works via three basic components.

First, the researchers modified T-cells, a type of white blood cell, to include a genetic switch that, when switched on, increased the cells’ expression of specific proteins by more than 200 times. That ability could be used to guide T-cells’ cancer-fighting activities.

The T-cells, with the switch off, were introduced into the tumor phantom which was placed into the mice. The tumor phantom also included gold nanorods, just dozens of atoms in size. The researchers shone pulses of a gentle laser in the near-infrared (NIR) range from outside the mouse’s body onto the spot where the tumor was located.

The nanorods receiving the light waves turned them into useful, localized mild heat, allowing the researchers to precisely warm the tumor. The elevated heat turned on the T-cells’ engineered switch.

Hyper-activated T-cells

This study honed the method and confirmed that its components worked in living animals. It was not the intention of the study to treat cancer yet, although undertaking that is the next step, which is already on its way.

“In upcoming experiments, we are implementing this approach to treat aggressive tumors and establish cancer-fighting effectiveness,” said Kwong, who is an assistant professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University.

The researchers published their results in the current edition of the journal ACS Synthetic Biology. The study’s first author was graduate research assistant Ian Miller. The research was funded by the National Institutes of Health, the National Science Foundation, the Burroughs Wellcome Fund, and the Shurl and Kay Curci Foundation.

Better immunotherapy

Bioengineers have been able to do a lot with T-cells already when they’re outside of the body.

“Right now, we’re adept at harvesting a patient’s own T-cells, modifying to target cancer, growing them outside the body until there are hundreds of millions of them,” Kwong said. “But as soon as we inject them back into a patient, we lose control over the T-cells’ activity inside the body.”

Cancer is notoriously wily, and when T-cells crawl into a tumor, the tumor tends to switch off the T-cells’ cancer-killing abilities. Researchers have been working to switch them back on.

Kwong’s remote control has done this in the lab, while also boosting T-cell activity.

T-cell toxicities

Having an off-switch is also important. If T-cells were engineered to be always-on and hyper-activated, as they moved through the body, they could damage healthy tissue.

“There would be off-target toxicities, so you really want to pinpoint their activation,” Kwong said. “Our long-term goal for them is to activate site-specifically, so T-cells can overcome immunosuppression by the tumor and become better killers there.”

When the heat remote is turned off, so are Kwong’s engineered T-cells, because customary body temperatures are not high enough to activate their switch.

Heat-shock switch

The switch is a natural safety mechanism in human cells that has evolved to protect against heat shock and turns on when tissue temperatures rise above the body’s normal operating range, which centers on 37 degrees Celsius (98.6 F). But the researchers re-fitted T-cells with the switch to make it turn on other functions, and it could be used to hyper-activate the cells.

The Georgia Tech bioengineers found that the switch worked in a range of 40 to 42 degrees Celsius (104 – 107.6 F), high enough to not react to the majority of high fevers and low enough to not damage healthy tissue nor the engineered T-cells.

“When the local temperature is raised to 45 degrees (113 F), some cells in our body don’t like it,” Kwong said. “But if heating is precisely controlled in a 40 to 42 degrees window with short pulses of the NIR light, then it turns on the T-cells’ switch, and body cells are still very comfortable.”

Immuno-goals and dreams

The researchers want to combine the switch with some additional cancer-fighting weapons they envision engineering into T-cells.

For example, secreted molecules called cytokines can boost immune cells’ ability to kill cancer, but cytokines, unfortunately, can also be toxic. “Our long-term goal is to engineer T-cells to make and release powerful immune system stimulants like cytokines on command locally and sparingly,” Kwong said.

In other studies, gently heated gold nanorods have been shown to kill tumors or hinder metastasis. But T-cell treatments could be even more thorough and, in addition, hopefully, one day give patients treated with them a long-lasting memory immune response to any recurrence of their cancer.

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Citation: This experimental method is in laboratory stages in mice and is not yet available as a treatment of any type for human patients. The study was co-authored by Marielena Castro, Joe Maenza and Jason Weis of Coulter BME at Georgia Tech. The research was funded by the National Institutes of Health Director’s New Innovator Award (grant #DP2HD091793), the NIH National Center for Advancing Translational Sciences (grant #UL1TR000454), the NIH GT BioMAT Training Grant (#5T32EB006343), the National Science Foundation (grant # DGE-1451512), the Shurl and Kay Curci Foundation, and the Burroughs Wellcome Fund. Any findings or opinions are those of the authors and not necessarily of the funding agencies.

Making Solar Cells Obsolete with GIT’s Optical ‘Rectenna’ Technology ~ 40% to 90% Conversion Effciency: YouTube Video


Optical Rectenna download

Georgia Tech Professor of Mechanical Engineering, Dr. Bara Cola, shares how his childhood dreams of playing professional football turned into an exciting research career and important nanoengineering innovations. Dr. Cola’s breakthrough optical rectenna technology can be viewed here https://smartech.gatech.edu/handle/18….”

Watch the YouTube Video:

 

e9cf3-nanorectannaA new kind of nanoscale rectenna (half antenna and half rectifier) can convert solar and infrared into electricity, plus be tuned to nearly any other frequency as a detector.

Right now efficiency is only one percent, but professor Baratunde Cola and colleagues at the Georgia Institute of Technology (Georgia Tech, Atlanta) convincingly argue that they can achieve 40 percent broad spectrum efficiency (double that of silicon and more even than multi-junction gallium arsenide) at a one-tenth of the cost of conventional solar cells (and with an upper limit of 90 percent efficiency for single wavelength conversion).

It is well suited for mass production, according to Cola. It works by growing fields of carbon nanotubes vertically, the length of which roughly matches the wavelength of the energy source (one micron for solar), capping the carbon nanotubes with an insulating dielectric (aluminum oxide on the tethered end of the nanotube bundles), then growing a low-work function metal (calcium/aluminum) on the dielectric and voila–a rectenna with a two electron-volt potential that collects sunlight and converts it to direct current (DC).

“Our process uses three simple steps: grow a large array of nanotube bundles vertically; coat one end with dielectric; then deposit another layer of metal,” Cola told EE Times. “In effect we are using one end of the nanotube as a part of a super-fast metal-insulator-metal tunnel diode, making mass production potentially very inexpensive up to 10-times cheaper than crystalline silicon cells.”

Read the full Article Here: Solar Cells Will be Made Obsolete by 3D rectennas aiming at 40-to-90% efficiency

 

Georgia Institute of Technology: New Low-Cost Technique Converts Bulk Alloys to Oxide Nanowires


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Researchers have developed a new low-cost technique for converting bulk powders directly to oxide nanowires. Shown is a crucible in which an alloy of lithium and aluminum is being formed. Credit: Rob Felt, Georgia Tech

 

A simple technique for producing oxide nanowires directly from bulk materials could dramatically lower the cost of producing the one-dimensional (1D) nanostructures. That could open the door for a broad range of uses in lightweight structural composites, advanced sensors, electronic devices – and thermally-stable and strong battery membranes able to withstand temperatures of more than 1,000 degrees Celsius.

The technique uses a solvent reaction with a bimetallic alloy – in which one of the metals is reactive – to form bundles of nanowires (nanofibers) upon reactive metal dissolution. The process is conducted at ambient temperature and pressure without the use of catalysts, toxic chemicals or costly processes such as chemical vapor deposition. The produced nanowires can be used to improve the electrical, thermal and mechanical properties of functional materials and composites.

The research, which was reported this week in the journal Science, was supported by the National Science Foundation and California-based Sila Nanotechnologies. The process is believed to be the first to convert bulk powders to nanowires at ambient conditions.

“This technique could open the door for a range of synthesis opportunities to produce low-cost 1D nanomaterials in large quantities,” said Gleb Yushin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “You can essentially put the bulk materials into a bucket, fill it with a suitable solvent and collect nanowires after a few hours, which is way simpler than how many of these structures are produced today.”

Yushin’s research team, which included former graduate students Danni Lei and James Benson, has produced oxide nanowires from lithium-magnesium and lithium-aluminum alloys using a variety of solvents, including simple alcohols. Production of nanowires from other materials is part of ongoing research that was not reported in the paper.

The dimensions of the nanowire structures can be controlled by varying the solvent and the processing conditions. The structures can be produced in diameters ranging from tens of nanometers up to microns.

“Minimization of the interfacial energy at the boundary of the chemical reaction front allows us to form small nuclei and then retain their diameter as the reaction proceeds, thus forming nanowires,” Yushin explained. “By controlling the volume changes, surface energy, reactivity and solubility of the reaction products, along with the temperature and pressure, we can tune conditions to produce nanowires of the dimensions we want.”

One of the attractive applications may be separator membranes for lithium-ion batteries, whose high power density has made them attractive for powering everything from consumer electronics to aircraft and motor vehicles. However, the polymer separation membranes used in these batteries cannot withstand the high temperatures generated by certain failure scenarios.

As result, commercial batteries may induce fires and explosions, if not designed very carefully and it’s extremely hard to avoid defects and errors consistently in tens of millions of devices.

Using low-cost paper-like membranes made of ceramic nanowires could help address those concerns because the structures are strong and thermally stable, while also being flexible – unlike many bulk ceramics. The material is also polar, meaning it would more thoroughly wetted by various battery electrolyte solutions.

“Overall, this is a better technology for batteries, but until now, ceramic nanowires have been too expensive to consider seriously,” Yushin said. “In the future, we can improve mechanical properties further and scale up synthesis, making the low-cost ceramic separator technology very attractive to battery designers.”

Fabrication of the nanowires begins with formation of alloys composed of one reactive and one non-reactive metal, such as lithium and aluminum (or magnesium and lithium). The alloy is then placed in a suitable solvent, which could include a range of alcohols, such as ethanol. The reactive metal (lithium) dissolves from the surface into the solvent, initially producing nuclei (nanoparticles) comprising aluminum.

Though bulk aluminum is not reactive with alcohol due to the formation of the passivation layer, the continuous dissolution of lithium prevents the passivation and allows gradual formation of aluminum alkoxide nanowires, which grow perpendicular to the surface of the particles starting from the nuclei until the particles are completely converted. The alkoxide nanowires can then be heated in open air to form aluminum oxide nanowires and may be formed into paper-like sheets.

The dissolved lithium can be recovered and reused. The dissolution process generates hydrogen gas, which could be captured and used to help fuel the heating step.

Though the process was studied first to make magnesium and aluminum oxide nanowires, Yushin believes it has a broad potential for making other materials. Future work will explore synthesis of new materials and their applications, and develop improved fundamental understanding of the process and predictive models to streamline experimental work.

The researchers have so far produced laboratory amounts of the nanowires, but Yushin believes that the process could be scaled up to produce industrial quantities. Though the ultimate cost will depend on many variables, he expects to see fabrication costs cut by several orders of magnitude over existing techniques.

“With this technique, you could potentially produce nanowires for a cost not much more than that of the raw materials,” he said. Beyond battery membranes, the nanowires could be useful in energy harvesting, catalyst supports, sensors, flexible electronic devices, lightweight structural composites, building materials, electrical and thermal insulation and cutting tools.

The new technique was discovered accidentally while Yushin’s students were attempting to create a new porous membrane material. Instead of the membrane they had hoped to fabricate, the process generated powders composed of elongated particles.

“Though the experiment didn’t produce what we were looking for, I wanted to see if we could learn something from it anyway,” said Yushin. Efforts to understand what had happened ultimately led to the new synthesis technique.

In addition to those already named, the research included Alexandre Magaskinski of Georgia Tech and Gene Berdichevsky of Sila Nanotechnologies.

Different aspects of this work were supported by NSF (grant 0954925) and Sila Nanotechnologies, Inc. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Gleb Yushin and Gene Berdichevsky are shareholders of Sila Nanotechnologies.

CITATION: Danni Lei, Jim Benson, Alexandre Magasinski, Gene Berdichevsky, Gleb Yushin, “Transformation of bulk alloys to oxide nanowires,” (Science, 2017).

Uniform ‘hairy’ nanorods have potential energy, biomedical applications


hairy-nanorods-123632_webIMAGE: IMAGE SHOWS MAGNETIC NANORODS IN THE VIAL ATTRACTED TO THE MAGNET. GEORGIA TECH RESEARCHERS HAVE DEVELOPED A NEW STRATEGY FOR CRAFTING ONE-DIMENSIONAL NANORODS BASED ON CELLULOSE USING A WIDE RANGE… view more

CREDIT: CREDIT: ROB FELT, GEORGIA TECH

 

GEORGIA INSTITUTE OF TECHNOLOGY

Materials scientists have developed a new strategy for crafting one-dimensional nanorods from a wide range of precursor materials. Based on a cellulose backbone, the system relies on the growth of block copolymer “arms” that help create a compartment to serve as a nanometer-scale chemical reactor. The outer blocks of the arms prevent aggregation of the nanorods.

The produced structures resemble tiny bottlebrushes with polymer “hairs” on the nanorod surface. The nanorods range in size from a few hundred nanometers to a few micrometers in length, and a few tens of nanometers in diameter. This new technique enables tight control over diameter, length and surface properties of the nanorods, whose optical, electrical, magnetic and catalytic properties depend on the precursor materials used and the dimensions of the nanorods.

The nanorods could have applications in such areas as electronics, sensory devices, energy conversion and storage, drug delivery, and cancer treatment. Using their technique, the researchers have so far fabricated uniform metallic, ferroelectric, upconversion, semiconducting and thermoelectric nanocrystals, as well as combinations thereof. The research, supported by Air Force Office of Scientific Research, will be reported on September 16 in the journal Science.

“We have developed a very general and robust strategy to craft a rich variety of nanorods with precisely-controlled dimensions, compositions, architectures and surface chemistries,” said Zhiqun Lin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “To create these structures, we used nonlinear bottlebrush-like block copolymers as tiny reactors to template the growth of an exciting variety of inorganic nanorods.”

Nanorod structures aren’t new, but the technique used by Lin’s lab produces nanorods of uniform sizes – such as barium titanate and iron oxide, which have not yet been demonstrated via wet-chemistry approaches in the literature – and highly-uniform core-shell nanorods made by combining two dissimilar materials. Lin and former postdoctoral research associate Xinchang Pang say the precursor materials applicable to the technique are virtually limitless.

“There are many precursors of different materials available that can be used with this robust system,” Lin said. “By choosing a different outer block in the bottlebrush-like block copolymers, our nanorods can be dissolved and uniformly dispersed in organic solvents such as toluene or chloroform, or in water.”

Fabrication of the nanorods begins with the functionalization of individual lengths of cellulose, an inexpensive long-chain biopolymer harvested from trees. Each unit of cellulose has three hydroxyl groups, which are chemically modified with a bromine atom. The brominated cellulose then serves as macroinitiator for the growth of the block copolymer arms with well-controlled lengths using the atom transfer radical polymerization (ATRP) process, with, for example, poly(acrylic acid)-block-polystyrene (PAA-b-PS) yielding cellulose densely grafted with PAA-b-PS (i.e., cellulose-g-[PAA-b-PS]) that give the bottlebrush appearance.

The next step involves the preferential partitioning of precursors in the inner PAA compartment that serves as a nanoreactor to initiate the nucleation and growth of nanorods. The densely grafted block copolymer arms, together with the rigid cellulose backbone, give researchers the ability to not only prevent aggregation of the resulting nanorods, but also to keep them from bending.

“The polymers are like long spaghetti and they want to coil up,” Lin explained. “But they cannot do this in the complex macromolecules we make because with so many block copolymer arms formed, there is no space. This leads to the stretching of the arms, forming a very rigid structure.”

By varying the chemistry and the number of blocks in the arms of the bottlebrush-like block copolymers, Lin and coworkers produced an array of oil-soluble and water-soluble plain nanorods, core-shell nanorods, and hollow nanorods – nanotubes – of different dimensions and compositions.

For example, by using bottlebrush-like triblock copolymers containing densely grafted amphiphilic triblock copolymer arms, the core-shell nanorods can be formed from two different materials. In most cases, a large lattice mismatch between core and shell materials would prevent the formation of high-quality core-shell structures, but the technique overcomes that limitation.

“By using this approach, we can grow the core and shell materials independently in their respective nanoreactors,” Lin said. “This allows us to bypass the requirement for matching the crystal lattices and permits fabrication of a large variety of core-shell structures with different combinations that would otherwise be very challenging to obtain.”

Lin sees many potential applications for the nanorods.

“With a broad range of physical properties – optical, electrical, optoelectronic, catalytic, magnetic, and sensing – that are dependent sensitively on their size and shape as well as their assemblies, the produced nanorods are of both fundamental and practical interest,” Lin said. “Potential applications include optics, electronics, photonics, magnetic technologies, sensory materials and devices, lightweight structural materials, catalysis, drug delivery, and bio-nanotechnology.”

For example, plain gold nanorods of different lengths may allow effective plasmonic absorption in the near-infrared range for use in solar energy conversion with improved harvesting of solar spectrum. The upconversion nanorods can preferentially harvest the IR solar photons, followed by the absorption of emitted high-energy photons to generate extra photocurrent in solar cells. They can also be used for biological labeling because of their low toxicity, chemical stability, and intense luminescence when excited by near-IR radiation, which can penetrate tissue much better than higher energy radiation such as ultraviolet, as is often required with quantum dot labels.

The gold-iron oxide core-shell nanorods may be useful in cancer therapy, with MRI imaging enabled by the iron oxide shell, and local heating created by the photothermal effect on the gold nanorod core killing cancer cells.

In addition to the researchers already mentioned, co-authors included graduate research assistant Yanjie He and postdoctoral researcher Jaehan Jung in Georgia Tech’s School of Materials Science and Engineering.

This research was supported by the Air Force Office of Scientific Research under grant FA9550-16-1-0187. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor.

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CITATION: Xinchang Pang, Yanjie He, Jaehan Jung, Zhiqun Lin, “1D nanocrystals with precisely controlled dimensions, compositions, and architectures,” (Science 2016).

Solar Cells Will be Made Obsolete by 3D rectennas aiming at 40-to-90% efficiency


Rectenna Naval Optical 150928122542_1_540x360A new kind of nanoscale rectenna (half antenna and half rectifier) can convert solar and infrared into electricity, plus be tuned to nearly any other frequency as a detector.

Right now efficiency is only one percent, but professor Baratunde Cola and colleagues at the Georgia Institute of Technology (Georgia Tech, Atlanta) convincingly argue that they can achieve 40 percent broad spectrum efficiency (double that of silicon and more even than multi-junction gallium arsenide) at a one-tenth of the cost of conventional solar cells (and with an upper limit of 90 percent efficiency for single wavelength conversion).

It is well suited for mass production, according to Cola. It works by growing fields of carbon nanotubes vertically, the length of which roughly matches the wavelength of the energy source (one micron for solar), capping the carbon nanotubes with an insulating dielectric (aluminum oxide on the tethered end of the nanotube bundles), then growing a low-work function metal (calcium/aluminum) on the dielectric and voila–a rectenna with a two electron-volt potential that collects sunlight and converts it to direct current (DC).

“Our process uses three simple steps: grow a large array of nanotube bundles vertically; coat one end with dielectric; then deposit another layer of metal,” Cola told EE Times. “In effect we are using one end of the nanotube as a part of a super-fast metal-insulator-metal tunnel diode, making mass production potentially very inexpensive up to 10-times cheaper than crystalline silicon cells.”

For commercialization, billions or even trillions of carbon-nanotube bundles could be grown side-by-side, ramping up the power output into the megaWatt range, after optimization for higher efficiency.

“We still have a lot of work to do to lower contact resistance which will improve the impedance match between the antenna and diode, thus raising efficiency,” Cola told us.”Our proof-of-concept was tuned to the near-infrared. We used infrared-, solar- and green laser-light and got efficiencies of less than one percent, but what was key to our demo was we showed our computer model matched our experimental results, giving us the confidence that we can improve the efficiency up to 40 percent in just a few years.”

For the future, Cola’s group has a three tiered goal–first develop sensor applications that don’t require high efficiencies, second to get the efficiency to 20 percent for harvesting waste heat in the infrared spectrum, then start replacing standard solar cells with 40 percent efficient panels in the visible spectrum. The team is also seeking suitable flexible substrates for applications that require bending.

 

Schematic of the components making up the optical rectenna–carbon nanotubes capped with a metal-oxide-metal tunneling diode. (Credit: Thomas Bougher)
(Source: Georgia Tech)

Nature Nanotechnology – A carbon nanotube optical rectenna

An optical rectenna—a device that directly converts free-propagating electromagnetic waves at optical frequencies to direct current—was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 pHz (switching speed on the order of 1 fs).

Diodes operating at these frequencies are feasible if their capacitance is on the order of a few attofarads but they remain extremely difficult to fabricate and to reliably couple to a nanoscale antenna. Here we demonstrate an optical rectenna by engineering metal–insulator–metal tunnel diodes, with a junction capacitance of ∼2 aF, at the tip of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna. Upon irradiation with visible and infrared light, we measure a d.c. open-circuit voltage and a short-circuit current that appear to be due to a rectification process (we account for a very small but quantifiable contribution from thermal effects). In contrast to recent reports of photodetection based on hot electron decay in a plasmonic nanoscale antenna a coherent optical antenna field appears to be rectified directly in our devices, consistent with rectenna theory. Finally, power rectification is observed under simulated solar illumination, and there is no detectable change in diode performance after numerous current–voltage scans between 5 and 77 °C, indicating a potential for robust operation.

 

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Georgia Institute of Technology: New Kind of ‘Smart-Glass Changes Color – Produces Electricity


Smart Glass 041015 5526909ae33ceA team of researchers working at the Georgia Institute of Technology has developed a type of smart-glass that not only changes color, but creates electricity. They have published a description of their work and the glass they have produced and some ideas on what the new kind of glass might be used for in their paper published in ACS Nano.

Many types of smart-glass have been created, some that display a tint when it gets sunny out, others that change to prevent heat from coming in, etc. In this new effort, the researchers sought to add something new—production of . Realizing that many types of glass are subjected to rain and wind, they sought to find a way to coat a window that would take advantage of triboelectrics—capturing the energy in that occurs when two materials meet.

They came up with a two layer solution, one layer to capture the energy in raindrops, the other to do the same for wind. In the first layer, the researchers developed nano-sized generators that would take advantage of the in raindrops that develops as it rubs against air on its way down from clouds and then as it crashes into a car’s windshield. The second layer consisted of a sandwich of two charged sheets of plastic with tiny springs between them. As wind pressure develops on an accelerating vehicle, the plastic sheets are pushed closer together, creating an .

Smart Glass 041015 5526909ae33ce

Together the two layers result in a glass that is initially clear, but then develops a blue tint—they also generated as much as 130 milliwatts of electricity per square meter of glass, which the researchers point out, is enough to charge a sleeping smartphone. Moving forward, the team suggests that such types of glass could be used with wireless networks because it is not based on a separate power source. But, before that can happen, the team is looking into ways to store the power that is generated. They think it might be possible to embed see-through super-capacitors in the glass as well. At this time, it is not clear how much with all that embedded technology would cost.

Explore further: Researchers invent smart window that tints and powers itself

More information: Motion-Driven Electrochromic Reactions for Self-Powered Smart Window System, ACS Nano, Article ASAP. DOI: 10.1021/acsnano.5b00706

Abstract
The self-powered system is a promising concept for wireless networks due to its independent and sustainable operations without an external power source. To realize this idea, the triboelectric nanogenerator (TENG) was recently invented, which can effectively convert ambient mechanical energy into electricity to power up portable electronics.

In this work, a self-powered smart window system was realized through integrating an electrochromic device (ECD) with a transparent TENG driven by blowing wind and raindrops. Driven by the sustainable output of the TENG, the optical properties, especially the transmittance of the ECD, display reversible variations due to electrochemical redox reactions.

The maximum transmittance change at 695 nm can be reached up to 32.4%, which is comparable to that operated by a conventional electrochemical potentiostat (32.6%). This research is a substantial advancement toward the practical application of nanogenerators and self-powered systems.

Polymer Structures Serve as ‘Nanoreactors’ for Nanocrystals


QDOTS imagesCAKXSY1K 8Using star-shaped block co-polymer structures as tiny reaction vessels, researchers have developed an improved technique for producing nanocrystals with consistent sizes, compositions and architectures – including metallic, ferroelectric, magnetic, semiconductor and luminescent nanocrystals. The technique relies on the length of polymer molecules and the ratio of two solvents to control the size and uniformity of colloidal nanocrystals.

 

The technique could facilitate the use of nanoparticles for optical, electrical, optoelectronic, magnetic, catalysis and other applications in which tight control over size and structure is essential to obtaining desirable properties. The technique produces plain, core-shell and hollow nanoparticles that can be made soluble either in water or in organic solvents.

“We have developed a general strategy for making a large variety of nanoparticles in different size ranges, compositions and architectures,” said Zhiqun Lin, an associate professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “This very robust technique allows us to craft a wide range of nanoparticles that cannot be easily produced with any other approaches.”

The technique was described in the June issue of the journal Nature Nanotechnology. The research was supported by the Air Force Office of Scientific Research.

Georgia Tech professor Zhiqun Lin examines a gold nanoparticle toluene solution. The work is part of research on using star-shaped block co-polymers to create nanocrystals of uniform size and shape.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

The star-shaped block co-polymer structures consist of a central beta-cyclodextrin core to which multiple “arms” – as many as 21 linear block co-polymers – are covalently bonded. The star-shaped block co-polymers form the unimolecular micelles that serve as a reaction vessel and template for the formation of the nanocrystals.

The inner blocks of unimolecular micelles are poly(acrylic) acid (PAA), which is hydrophilic, which allows metal ions to enter them. Once inside the tiny reaction vessels made of PAA, the ions react with the PAA to form nanocrystals, which range in size from a few nanometers up to a few tens of nanometers. The size of the nanoparticles is determined by the length of the PAA chain.

The block co-polymer structures can be made with hydrophilic inner blocks and hydrophobic outer blocks – amphiphilic block co-polymers, with which the resulting nanoparticles can be dissolved in organic solvents. However, if both inner and outer blocks are hydrophilic – all hydrophilic block co-polymers – the resulting nanoparticles will be water-soluble, making them suitable for biomedical applications.

Lin and collaborators Xinchang Pang, Lei Zhao, Wei Han and Xukai Xin found that they could control the uniformity of the nanoparticles by varying the volume ratio of two solvents – dimethlformamide and benzyl alcohol – in which the nanoparticles are formed. For ferroelectric lead titanate (PbTiO3) nanoparticles, for instance, a 9-to-1 solvent ratio produces the most uniform nanoparticles.

The researchers have also made iron oxide, zinc oxide, titanium oxide, cuprous oxide, cadmium selenide, barium titanate, gold, platinum and silver nanocrystals. The technique could be applicable to nearly all transition or main-group metal ions and organometallic ions, Lin said.

“The crystallinity of the nanoparticles we are able to create is the key to a lot of applications,” he added. “We need to make them with good crystalline structures so they will exhibit good physical properties.”

Earlier techniques for producing polymeric micelles with linear block co-polymers have been limited by the stability of the structures and by the consistency of the nanocrystals they produce, Lin said. Current fabrication techniques include organic solution-phase synthesis, thermolysis of organometallic precursors, sol-gel processes, hydrothermal reactions and biomimetic or dendrimer templating. These existing techniques often require stringent conditions, are difficult to generalize, include a complex series of steps, and can’t withstand changes in the environment around them.

Georgia Tech professor Zhiqun Lin (standing) watches research scientist Xinchang Pang tuning the experimental condition in the nanocrystal synthesis.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

By contrast, nanoparticle production technique developed by the Georgia Tech researchers is general and robust. The nanoparticles remain stable and homogeneous for long periods of time – as much as two years so far – with no precipitation. Such flexibility and stability could allow a range of practical applications, Lin said.

“Our star-like block co-polymers can overcome the thermodynamic instabilities of conventional linear block co-polymers,” he said. “The chain length of the inner PAA blocks dictates the size of the nanoparticles, and the uniformity of the nanoparticles is influenced by the solvents used in the system.”

The researchers have used a variety of star-like di-block and tri-block co-polymers as nanoreactors. Among them are poly(acrylic acid)-block-polystyrene (PAA-b-PS) and poly(acrylic acid)-blockpoly(ethylene oxide) (PAA-b-PEO) diblock co-polymers, and poly(4-vinylpyridine)-block-poly(tert-butyl acrylate)-block-polystyrene (P4VP-b-PtBA-b-PS), poly(4-vinylpyridine)-block-poly (tert-butyl acrylate)-block-poly(ethylene oxide) (P4VP-b-PtBA-b-PEO), polystyrene-block-poly(acrylic acid)-block-polystyrene (PS-b-PAA-b-PS) and polystyrene-block-poly(acrylic acid)-block-poly(ethylene oxide) (PS-b-PAA-b-PEO) tri-block co-polymers.

For the future, Lin envisions more complex nanocrystals with multifunctional shells and additional shapes, including nanorods and so-called “Janus” nanoparticles that are composed of biphasic geometry of two dissimilar materials.

Georgia Tech professor Zhiqun Lin (standing) and research scientist Xinchang Pang compare two cadmium selenide (CdSe) nanocrystals made by Pang. The researchers are examining the absorption spectra of the nanocrystals in front of the computer.

(Photo Credit:  Georgia Tech Photo: Gary Meek)