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.
The scientists made the discovery while trying to determine the basics of how titanium dioxide -- a compound sometimes found in sunscreen -- breaks down water. The chemical reactions between water and oxygen are central to such varied processes as hydrogen production, breaking down pollutants, and in solar energy.
"Oxygen and water are involved in many, many reactions," said physicist Igor Lyubinetsky at the Department of Energy's Pacific Northwest National Laboratory, who reported the team's results in March 6 issue of the Physical Review Letters. "This mobility might interfere with some reactions and help others."
Bustling Bright Spots
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.
Reference: Y. Du, N. A. Deskins, Z. Zhang, Z. Dohnálek, M. Dupuis, and I. Lyubinetsky, Two Pathways for Water Interaction with Oxygen Adatoms on TiO2(110), Phys Rev Letters, March 6, 2009, DOI 10.1103/PhysRevLett.102.096102
This work was supported by the Department of Energy's Office of Science.
Mary Beckman | EurekAlert!
Further reports about: > Dancing adatoms > Hydrogen > Oxygen > PNNL > Science TV > Single oxygen atoms > adatom > chemical processes > chemical reaction > contaminated water > dancing > energy generation > hydrogen atom > hydrogen production > metal oxide slab > oxygen atom > pollution cleanup > solar energy > titanium dioxide > water molecules
APEX takes a glimpse into the heart of darkness
25.05.2018 | Max-Planck-Institut für Radioastronomie
First chip-scale broadband optical system that can sense molecules in the mid-IR
24.05.2018 | Columbia University School of Engineering and Applied Science
The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.
Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
25.05.2018 | Event News
02.05.2018 | Event News
13.04.2018 | Event News
25.05.2018 | Event News
25.05.2018 | Machine Engineering
25.05.2018 | Life Sciences