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Rice group: Silica + titania = cheap, green, improved water purification
Tanzania water source. Credit: Bob Metcalf
I don’t have enough historical perspective to know if this is truly a eureka moment or not, but a group from Rice University reports that adding a small amount of silica (or another inexpensive silicon source) can amp up the anti-viral, anti-bacterial power of titanium dioxide for producing disinfected drinking water.
The researchers Huma R. Jafry, Michael V. Liga, Qilin Li and Andrew R. Barron report in Environmental Science & Technology that, at least in non in situ testing with model bacteriophage MS2, adding a dose of silica to a commercial TiO2 product, P25, triples the material’s ability to kill viruses and increases the adsorption of viruses onto P25 nanoparticles.
Liga’s website mentions that researchers are looking at several TiO2 dopants, and have also been testing the improved photocatalyst against a model organic pollutant, congo red dye, and discusses the possibility of even stronger effects:
“Our composite catalysts have been found to inactivate viruses over five times faster than the base titanium dioxide material. Future research will be focused on inactivating pathogenic adenovirus, which is of concern to the drinking water treatment industry. We will also attempt the construction of a lab scale treatment reactor employing the photocatalyst.”"If you’re using titanium dioxide, just take it, treat it for a few minutes with silicone grease or silica or silicic acid, and you will increase its efficiency as a catalyst,” he said.
In a news release from Rice, Barron says they have supercharged the performance of the titania with little increase in cost. “Basically, we’re taking white paint pigment and functionalizing it with sand,” he says.
Barron and the others say they are setting the performance bar fairly high by looking at how the silica-titania mix would perform against Yangtze river contamination. “We chose the Yangtze River as our baseline for testing, because it’s considered the most polluted river in the world, with the highest viral content. Even at that level of viral contamination, we’re getting complete destruction of the viruses in water that matches the level of pollution in the Yangtze,” he says.
A couple of the researchers admit they accidentally stumbled into this revelation. Liga says he noticed a jump in the TiO2 performance when then-grad student Huma Jafry was heating a titania solution in a sealed flask. Jafry reported that she had done nothing knowingly different, but eventually she and Liga realized it might be the silicone grease used to lubricate the sealing stopper, a hypothesis they later confirmed.
Barron attributes the improved power of TiO2 to band bending that creates a path for electrons freed by the UV to react with water to create a surge of hydroxyl radicals. “Because the silicon-oxygen bond is very strong, you can think of it as a dielectric. If you put a dielectric next to a semiconductor, you bend the conduction and valence bands. And therefore, you shift the absorption of the ultraviolet (used to activate the catalyst) . . . If your conduction band bends to the degree that electrons find it easier to pop out and do something else, your process becomes more efficient,” he says.
“In places where they don’t have treatment plants or even electricity, the SODIS [solar disinfection - ed.] method is great, but it takes a very long time to make water safe to drink,” says coauthor Quilin Li. “Our goal is to incorporate this photocatalyst so that instead of taking six hours, it only takes 15 minutes.”
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Pore-enhanced silicon used to increase capacity of Li-ion batteries
Top and side view of etched silicon battery material. (Credit: Rice Univ.)
According to a press release, a team of Rice University and Lockheed Martin scientists has discovered a way to use simple silicon to increase the capacity of lithium-ion batteries by enhancing the inherent ability of silicon to absorb lithium ions
Rice University is famed for the buckyball discovery 25 years ago for nanotechnology development. The new battery work was introduced this week at Rice’s Buckyball Discovery Conference, part of a yearlong celebration of the 25th anniversary of the Nobel Prize-winning discovery.
“The anode, or negative, side of today’s batteries is made of graphite, which works. It’s everywhere,” says Michael Wong, a professor in chemical and biomolecular engineering and in chemistry. “But it’s maxed out. You can’t stuff any more lithium into graphite than we already have.”
Silicon has the highest theoretical capacity of any material for storing lithium, but there’s a serious drawback to its use. “It can sop up a lot of lithium, about 10 times more than carbon, which seems fantastic,” Wong said. “But after a couple of cycles of swelling and shrinking, it’s going to crack.”
However, the researchers say they discovered that putting micron-sized pores into the surface of a silicon wafer gives the material sufficient room to expand. While common Li-ion batteries hold about 300 milliamp hours-per-gram of carbon-based anode material, the researchers determined the treated silicon could theoretically store more than 10 times that amount.
The pores, a micron wide and 10-50 microns long, form when positive and negative charge is applied to the sides of a silicon wafer, which is then bathed in a hydrofluoric solvent. “The hydrogen and fluoride atoms separate. The fluorine attacks one side of the silicon, forming the pores. They form vertically because of the positive and negative bias,” says Sibani Lisa Biswal, an assistant professor in chemical and biomolecular engineering.
Putting pores in silicon requires a real balancing act, as the more space is dedicated to the holes, the less material is available to store lithium. And if the silicon expands to the point where the pore walls touch, the material could degrade.
The researchers are confident that cheap, plentiful silicon combined with ease of manufacture could help push their idea into the mainstream.
“We are very excited about the potential of this work. This material has the potential to significantly increase the performance of lithium-ion batteries, which are used in a wide range of commercial, military and aerospace applications,” says Steven Sinsabaugh, a Lockheed Martin fellow.
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Reversible silicon/silicon-oxide nanowire switch developed for 3D storage
A 5 nm silicon nanowire can be repeatedly broken and reconnected by applying a pulse of varying voltage through the silicon oxide, creating a two-terminal resistive switch. A chip with 1000 of these silicon-oxide/silicon nanowire memory elements has been assembled as a proof-of-concept.(Credit: Jun Yao/Rice.)
It’s not often that “plain vanilla” silicon oxide makes the front page of a paper like the New York Times, but it happened when Rice University scientists announced that they have created the first two-terminal memory chips based on silicon oxide in a way that they say should be easily adaptable to nanoelectronic manufacturing techniques, and promises to extend the limits of miniaturization subject to Moore’s Law.
Their technique creates nanocrystal wires that are as small as 5 nanometers wide, far smaller than circuitry in even the most advanced computers and electronic devices. The Rice group believes its nanocrystal conductors could lead to massive, robust 3-D storage.
“The beauty of it is its simplicity,” says James Tour, Rice’s T.T. and W.F. Chao chair in chemistry as well as a professor of mechanical engineering and materials science and of computer science. That, he said, will be key to the technology’s scalability. Silicon-oxide switches or memory locations require only two terminals, not three (as in flash memory), because the physical process doesn’t require the device to hold a charge.
According to the Rice press release:
“[Graduate student] Jun Yao sandwiched a layer of silicon oxide, an insulator, between semiconducting sheets of polycrystalline silicon that served as the top and bottom electrodes.
“Applying a charge to the electrodes created a conductive pathway by stripping oxygen atoms from the silicon oxide and forming a chain of nano-sized silicon crystals. Once formed, the chain can be repeatedly broken and reconnected by applying a pulse of varying voltage.”
Layers of silicon-oxide memory can be stacked in three-dimensional arrays. “I’ve been told by industry that if you’re not in the 3-D memory business in four years, you’re not going to be in the memory business. This is perfectly suited for that,” Tour says.
Silicon-oxide memories are compatible with conventional transistor manufacturing technology, says Tour, who recently attended a workshop by the National Science Foundation and IBM on breaking the barriers to Moore’s Law, which states the number of devices on a circuit doubles every 18 to 24 months.
Austin tech design company PrivaTran is already bench testing a silicon-oxide chip with 1,000 memory elements built in collaboration with the Tour lab.
The findings were published in Nano Letters.
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Researchers build first nanodragster
Live Science reported that researchers have built a new super-small “nanodragster” that could speed up efforts to craft molecular machines.
“We made a new version of a nanocar that looks like a dragster,” said James Tour, a chemist at Rice University who was involved in the research. “It has smaller front wheels on a shorter axle and bigger back wheels on a longer axle.”
The vehicle is about 50,000 times thinner than a human hair and is pushed along by heat or an electric field.
Spherical molecules called buckyballs made of 60 carbon atoms each serve as the big rear wheels. Due to chemical attractions, these wheels nicely grip the “dragstrip,” which is made of a superfine layer of gold rather than pavement. For the front wheels, the scientists opted for a less sticky compound, p-carbonane.
Tour’s group built nanocars before with buckyballs as all four wheels, but these autos hug the road too tightly and require temperatures around 400 F to get rolling. Nanocars with all p-carbonane wheels, on the other hand, slip and slide as if on ice, said Tours, making them difficult to image and study.
By incorporating both wheel types, the nanodragster can cruise at lower temperatures with greater agility and range of motion.
To make the new nanodragster, Tour’s team started with a previously built, off-the-shelf short axle and front wheel unit in their lab, which is sort of a nano-Monster Garage. They then chemically hooked this up to a pair of aligned hydrocarbon molecules called phenylene-ethynylene-the vehicle’s chassis. The rear axle came next and finally the buckyball wheels went on.
Once the new nanocar gets rolling, it can reach speeds of up to nine nanomiles, or 0.014 millimeters (.0005 inches), per hour, which is relatively fast for their size, said Tour.
The tiny hot rods can also do tricks. “Because the front wheels don’t stick to the surface as strongly, they’re more prone to lift up, so [the nanodragster] does seem to pop a wheelie at times,” Tour told Top Ten Reviews.
By learning how to drive nanovehicles, Tour hopes to pave the way for small but technologically useful structures, such as electronics, that could be built atom-by-atom.
The research appeared in a recent issue of the journal Organic Letters.
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Rice research use “kites” to pull nanotubes to new heights
Credit: Rice University
A group of Rice University researchers, led by Bob Hauge, think they have an approach that could ultimately lead to single-walled carbon nanotubes of unlimited length.
Credit: Rice University
Hauge’s group started playing with making nanotubes using the same machinery the U.S. Treasury uses to embed special anti-counterfeiting materials and markings in paper currency. The group used this specialized printing process to lay down a three-part surface on a Mylar roll. One layer was made up of iron particles. A layer of flakes of alumina is then spread over the iron. The iron and alumina act as a catalyst for nanotube growth. The bottom layer is a release layer that can be activated by acetylene.
Next, the film is put in a furnace with an acetylene-hydrogen atmosphere. As the temperature increases, the alumina flakes lift in the chemical vapor while arrays of nanotubes grew vertically in tight, forest-like formations atop the iron particles. They say that under a microscope, the bundles of tubes looked the pile of a carpet.
Hauge’s group says the flakes look like they are “flying” in the vapor and “pulling up” the nanotubes. They call the product made using this process “odako” SWNTs, named for the huge, multistringed traditional Japanese kites they resemble.
The team has also grown odako on pure carbon, Grafoil (a flexible graphite material) and carbon fiber that has been woven into a material.
So far they have been able to make SWNTs measured in centimeters, and, he said, the process could eventually yield tubes of unlimited length. They also believe that odako-type growth may be possible on such other materials as quartz fibers and some metals.
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