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Scanning transmission electron microscope (STEM) image of PbSe quantum dots. Inset shows Pb atomic columns of single quantum dot. (Credit: John Silcox, Cornell University)

In a recent post at ScientificAmerican.com, David Bielo details many of the inefficiencies of photovoltaic cells. Although layered cells composed of various elements can convert more than 40 percent of sunlight into electricity, more simple semiconducting materials such as silicon hover around 20 percent when mass-produced. And, at best, such cells could convert only a third of incoming sunlight due to physical limits.

However, researchers at the universities of Minnesota and Texas may have reduced on of the major inefficiencies: heat loss. Nanosize crystals of semiconducting material, in this case a mixture of lead and selenium, move electrons fast enough to channel some of them faster than they can be lost as heat.

According to Bielo:

Solar cells employ semiconducting material because when a photon of sunlight of the right wavelength strikes that material, it knocks loose an electron, which can then be harvested as electrical current. But many of those loosened electrons dissipate as heat rather than being funneled out of the photovoltaic cell. Previous work in 2008 had shown that nanocrystals of semiconducting material can, in effect, slow down such “hot” electrons. As a result, these nanocrystals, also known as quantum dots, might be able to boost the efficiency of a solar cell.

In the June 18 issue of Science, researchers claim that quantum dots capture some of the “hot” electrons but they can also channel them to a typical electron-accepting material-the same titanium dioxide used in conventional solar cells. Because that transfer is so fast, fewer of the excited electrons are lost as heat, thus boosting the theoretical efficiency to as high as 66 percent.

Unfortunately, according to Bielo, that’s not all that’s required to build such a highly efficient solar cell. The next step would be to show that the captured electrons and transferred current can be carried away on a wire, as in a conventional solar cell. The challenge will be making a wire small enough to connect to a solar cell incorporating a quantum dot no bigger than 6.7 nanometers in diameter-and one that won’t lose much of the current as heat. And it would be years if not decades before such quantum dot-based solar cells might be manufactured.

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In a recent interview with Nanowerk, Ayusman Sen, a professor at the Department of Chemistry at Penn State, explained how he uses titanium dioxide to convert optical energy to mechanical energy via photocatalysis. The team’s findings were published recently in Advanced Functional Materials.

“The whole system consists only of titania, water, sometimes organics, and light input. The system is very forgiving, requiring no careful control of substrate concentration or catalyst conditioning, and is easily controllable by external light,” says Sen.

Via Nanowerk:

“There are two categories of autonomous movement associated with titanium dioxide: the photo-induced motility of titanium dioxide particles (ranging from 0.2 to 2.5 µm) and the photo-induced reversible ‘microfireworks’ where silicon dioxide particles, which tend to gather around titanium dioxide particles, were shown to immediately move away from the titanium dioxide particles upon exposure to UV light, thereby creating an exclusion zone cleared of particles around each individual titanium dioxide particle. When the UV source is removed, the tracer particles pull back toward the titanium dioxide particle and form the aggregates again.”

Sen explains that the photoactivity of titanium dioxide comes from its hole–electron separation triggered by photons of energy equal to or higher than its bandgap.

“The reactions produce more product molecules than the reactants consumed, making it possible to propel a titanium dioxide particle by the mechanism of osmotic propulsion,” he says.

Sen claims that the titanium dioxide-based motor system is highly active, inexpensive, clean and simple.

Self-propelled motion of synthetic materials can be useful in applications such as bottom-up assembly of structures, pattern formation and drug delivery at specific locations.

The video above shows the photoactivity of a large titania particle in 1M methanol. The surrounding tracer particles are silica.

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I don’t know much about this other than what was posted at PNNL’s website last week, but I hope to see something published on this soon:

Researchers would like to develop lithium-ion batteries using titanium dioxide, an inexpensive material. But titanium dioxide on its own doesn’t perform well enough to replace the expensive, rare-earth metals or fire-prone carbon-based materials used in today’s lithium-ion batteries. To test whether graphene, a good conductor on its own, can help, PNNL’s Gary Yang and colleagues added graphene, sheets made up of single carbon atoms, to titanium dioxide. When they compared how well the new combination of electrode materials charged and discharged electric current, the electrodes containing graphene outperformed the standard titanium dioxide by up to three times. Graphene also performed better as an additive than carbon nanotubes. Yang discussed this work and provided an overview of the field of electrical storage materials.

Just for the record, it appears from a note on the website that ACerS member Jun Liu partnered with Yang on this research. Yang recently spoke at Oregon State University’s “Micro Nano Breakthrough Conference” held last week in Portland, Ore.

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E. coli (Credit: FDA)

Nanotechnology is key to the development of a new paint that reportedly has the ability to kill antibiotic-resistant superbugs, according to a report from Manchester Metropolitan University researcher Lucia Caballero at a September 2008 meeting of the Society for General Microbiology. Caballero reports that, when exposed to fluorescent light or the sun’s ultraviolet rays, paints containing nanoparticles of titanium dioxide can kill potentially fatal bacteria. The particles do this, the researcher says, by absorbing the light and producing active molecules that “clean up the painted surfaces.” (more…)

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