وبلاگ شیمی آلی سعید عظیمی

Dancing 'Adatoms' Help Chemists Understand How Water Molecules Split

ScienceDaily (Mar. 16, 2009) — Single oxygen atoms dancing on a metal oxide slab, glowing brighter here and dimmer there, have helped chemists better understand how water splits into oxygen and hydrogen. In the process, the scientists have visualized a chemical reaction that had previously only been talked about. The new work improves our understanding of the chemistry needed to generate hydrogen fuel from water or to clean contaminated water

While exploring titanium dioxide as a way to split water into its hydrogen and oxygen pieces, researchers can use a technique called scanning tunneling microscopy to watch the chemical reaction. The surface of a slab of titanium dioxide is like a corn field: rows of oxygen atoms rise from a patch of titanium atoms. The alternating oxygen and titanium rows look like stripes.

Scientists can also see some atoms and molecules that come to rest on the surface as bright spots. One such visible atom is a single oxygen atom that comes to rest on a titanium atom, called an "adatom". Chemists can only see water molecules if they drop the temperature dramatically -- at ambient temperature, water moves too fast for the method to pick them up.

In this work, PNNL scientists studied water's reactions with titanium dioxide at ambient temperature at EMSL, the DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. Starting with a surface plated with a few oxygen adatoms, they added water -- and the adatoms started to dance.

"Suddenly, almost every adatom started to move back and forth along the titanium row," said Lyubinetsky. "From theory and previous work, we expected to see this along the row."

Remarkably, the adatoms didn't just slide up and down the stripes. They also bounced out of them and landed in others, like pogoing dancers in a mosh pit.

"We saw quite unexpected things. We thought it was very strange -- we saw adatoms jump over the rows," Lyubinetsky said. "We just couldn't explain it."

Calculating how much energy it would take for the adatoms to move by themselves, much less hop over an oxygen row, the chemists suspected the adatoms were getting help -- most likely from the invisible water molecules.

The Unseen Enabler

To make sense of the dancing adatoms, the team calculated how much energy it would take to move adatoms with the help of water molecules. If a water molecule sits down next to an adatom, one of the water's hydrogen atoms can jump to the adatom, forming two oxygen-hydrogen pairs.

These pairs are known as hydroxyls and tend to steal atoms from other molecules, including each other. One of the thieving hydroxyls can then nab the other's hydrogen atom, turning back into a water molecule. The water molecule floats off, leaving behind an adatom. Half the time, that adatom is one spot over -- which makes the original appear to have moved.

The chemists determined that water can help the adatom jump a row as well: If a water molecule and an adatom are situated on either side of a raised oxygen row, a row oxygen can serve as the middleman, handing over a hydrogen from the water molecule to the adatom. Again, two hydroxyls form, one ultimately stealing both hydrogens (with the help of the middleman) and zipping away as water. If the incoming water molecule has been stripped, the adatom appears to have hopped over.

The calculated energy required for these different scenarios fit well with the team's experimental data. When a row oxygen serves as a middleman, the process is known as "pseudo-dissociation", a reaction suggested by chemists but until now, never verified experimentally.

"We realized that only if we involved the pseudo-dissociative state of the water can we explain it," said Lyubinetsky. "Otherwise, all the calculations show there's too high a barrier, the adatom just cannot jump by itself."

Lyubinetsky points out that this shows that water itself can work as a catalyst. A catalyst is a molecule that can help a chemical reaction along and remain unchanged by the experience.

"Water is required to move the adatoms around, but like a catalyst it is not consumed in the reaction," he said. "You start with water and you end with water."

In the future, the team plans on determining if water can make the adatoms move other species and more than one space at a time. In addition, they will investigate how light affects the reaction.

This work was supported by the Department of Energy's Office of Science.

The magnetic magic of liquid mirrors

July 17th, 2008 | by KFC |

Liquid mirror telescopes are amazing contraptions. They start life as a puddle of mercury in a bowl. Set the whole thing spinning and the mercury spreads out in a thin film up the sides of the bowl.

The result is a fabulously cheap mirror that can be used for a variety of astronomical surveys. If we ever put a telescope on the moon, many astronomers have suggested that it should be one of this type.

It won’t have escaped your attention that liquid mirrors have important limitations. First, they can only point straight up. One or two people have played with fluids that have a higher viscosity than mercury and so can be tilted a few degrees this way or that but with limited success. And second, they cannot be made adaptive to correct for blurring introduced by the Earth’s atmosphere.

But that may change thanks to some interesting work being done by Denis Brousseau at Université Laval in Quebec et amis. Their machine controls the shape of the surface of a liquid mirror using a magnetic field. Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.

Instead the team have used a suspension of ferromagnetic nanoparticles in oil. A thin highly reflectivity layer of silver particles can then be spread across the surface of the ferrofluid to create a mirror.

Brousseau and co use an array of tiny coils behind the liquid to create a field that deforms the fluid surface as required. Their tests show this can be done fast and furiously enough to cope with the usual array of optical aberrations that the atmosphere throws up.

However, it may also be possible to use this technique to tilt liquid mirrors further than ever before. Ferrofluids can easily be made much more viscous than mercury and so combat the deforming pull of gravity. But they can also be deformed in a way that opposes gravity during each rotation of the supporting bowl. That could make them much more tiltable than mercury mirrors.

Of course, such a mirror would be mechanically more complex than the spinning bowls we have today and correspondingly more expensive. And sending one to the moon seems an unnecessary extravagance given the absence of an atmosphere there.

But here on Earth they could be made much more useful. It’s a combination of new-found utility and value for money that many astronomy projects on a budget will find irresistible.

Ref: arxiv.org/abs/0807.2397: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments

	American Chemical Society
	January 16, 2007