Researchers observed the molecular structure of liquid water at a gold surface under different charging conditions, the findings could help researchers understand a number of biological phenomena.
When a solid material is immersed in liquid, the liquid next to the object is molecularly different than the bulk, and when this surface is charged it can spur even further changes, DOE/Lawrence Berkeley National Laboratory reported. In the past researchers have had trouble observing the molecular structure of the described "solid-liquid interface."
"At an electrode surface, the build-up of electrical charge, driven by a potential difference (or voltage), produces a strong electric field that drives molecular rearrangements in the electrolyte next to the electrode," said Miquel Salmeron, a senior scientist in Berkeley Lab's Materials Sciences Division (MSD) and professor in UC Berkeley's Materials Science and Engineering Department.
The researchers developed a method to look at the molecules next to the electrode surface and determine their arrangement. The team used x-ray absorption spectroscopy (XAS) to probe the interface and reveal how the local molecules were arranged. The XAS technique was slightly tweaked because the method is usually conducted in a vacuum, which will cause liquids to evaporate. By using a very thin (100 nm, or a tenth of a micrometer) x-ray transparent window, with a thin coating of gold (20nm) the team was able to safely expose the water molecules to X-rays.
To make their findings it was critical for the researchers to determine which part of the electrical current was caused by the X-rays or battery. To accomplish this the researchers "pulsed the incoming x-rays from the synchrotron at a known frequency," which allowed them to separate the current.
The experiments allowed the team to achieve absorption versus x-ray energy curves (spectra) that reflected how water molecules within the nanometers of the gold surface absorbed the X-ray. They then performed a sophisticated theoretical analysis to determine how this information translated into molecular structures. Using a supercomputer the team conducted simulations of the gold-water interface and predicted the x-ray absorption spectra of the predicted structures.
"These are first-principles calculations," explained David Prendergast, a staff scientist in the Molecular Foundry and researcher in the Joint Center for Energy Storage Research (JCESR). "We don't dictate the chemistry: we just choose what atomic elements are present and how many atoms. That's it. The chemistry is a result of the calculation."
"The main thing we know about the gold electrode surface from the x-ray absorption spectra: how many water molecules are tilted one way or another, and if their hydrogen bonds are broken or not," Salmeron concluded. "Water next to the electrode has a different molecular structure than it would in the absence of the electrode."
The team found the structure of the water is limited to the first two layers above the surface spanning only about one nanometer; to make these observations in the experimental spectra with varying voltage, suggest the measurements are sensitive to a shorter length scale than ever thought possible. The study is the first to show such high sensitivity in an in-situ environment under working electrode conditions.
The findings were published in a recent edition of the journal Science.
