Argonne scientists discover new pathway for artificial photosynthesis

(Press-News.org) Contact information: Jared Sagoff
jsagoff@anl.gov
630-252-5549
DOE/Argonne National Laboratory
Argonne scientists discover new pathway for artificial photosynthesis ARGONNE, Ill. (Jan. 13, 2014) -- Humans have for ages taken cues from nature to build their own devices, but duplicating the steps in the complicated electronic dance of photosynthesis remains one of the biggest challenges and opportunities for chemists.

Currently, the most efficient methods that we have of making fuel – principally hydrogen – from sunlight and water involve rare and expensive metal catalysts, like platinum. In a new study, researchers at the U.S. Department of Energy's Argonne National Laboratory have found a new, more efficient way to link a less expensive synthetic cobalt-containing catalyst to an organic light-sensitive molecule, called a chromophore.

Even though cobalt is significantly less efficient than platinum when it comes to light-induced hydrogen generation, the drastic price difference between the two metals makes cobalt the obvious choice as the foundation for a synthetic catalyst, said Argonne chemist Karen Mulfort.

"Cobalt doesn't have to be as efficient as platinum because it is just so much cheaper," she said.

The Argonne study wasn't the first to look at cobalt as a potential catalytic material; however, the paper did identify a new mechanism by which to link the chromophore with the catalyst. Previous experiments with cobalt attempted to connect the chromophore directly with the cobalt atom within the larger compound, but this eventually caused the hydrogen generation process to break down.

Instead, the Argonne researchers connected the chromophore to part of a larger organic ring that surrounded the cobalt atom, which allowed the reaction to continue significantly longer.

"If we were to directly link the chromophore and the cobalt atom, many of the stimulated electrons quickly fall out of the excited state back into the ground state before the energy transfer can occur," Mulfort said. "By coupling the two materials in the way we've described, we can have much more confidence that the electrons are going to behave the way we want them to."

One additional advantage of working with a cobalt-based catalyst, in addition to its relatively low price and abundance, is the fact that scientists understand the atomic-level mechanisms at play.

"There's a lot of different ways in which we already know we can modify cobalt-based catalysts, which is important because we need to make our devices more robust," Mulfort said.

Future studies in this arena could involve nickel- and iron-based catalysts—metals which are even more naturally abundant than cobalt, although they are not quite as effective natural catalysts. "We want to extrapolate from what we've gained by looking at this kind of linkage in respect to other catalysts," Mulfort said.

### Mulfort and her Argonne colleagues used the high-intensity X-rays provided by the laboratory's Advanced Photon Source as well as precise spectroscopic techniques available at Argonne's Center for Nanoscale Materials.

A paper based on the study appeared in the journal Physical Chemistry Chemical Physics. The research was supported by DOE's Office of Science. Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy's Office of Science to carry out applied and basic research to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels, provide the foundations for new energy technologies, and support DOE missions in energy, environment, and national security. To learn more about the Office of Science X-ray user facilities, visit http://science.energy.gov/user-facilities/basic-energy-sciences/.

The Center for Nanoscale Materials at Argonne National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE's Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit http://nano.energy.gov.

DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov. For more information, contact Jared Sagoff at (630)-252-5549 or at jsagoff@anl.gov.