X-ray absorption reveals water structure mystery

A new twist on x-ray absorption spectroscopy has allowed American scientists to reveal the molecular structure of water at gold electrodes. When a solid is immersed in liquid, the liquid immediately next to its surface differs from the bulk of the liquid – when the solid surface is charged it can drive further changes, but elucidating the molecular structure at the interface has proved difficult. Now, scientists at the Lawrence Berkeley National Laboratory have observed this molecular structure at a gold surface under different conditions. The team used a 100nm x-ray transparent window with a 20nm coating of gold on a sealed liquid sample holder to expose water molecules in the liquid to x-rays. Upon absorption, the excited water molecule either emits electrons or photons. Measuring the fluorescence – amount of photons emitted – can tell scientists how many x-ray photons have been absorbed, but interference from molecules in the gold means this dominated the spectrum. So the team measured electron emission because electrons emitted from x-ray excited water molecules travel nanometres distances through matter. The electrons arriving at the gold electrode surface can be detected as an electrical current travelling through a wire attached to it. The experiment resulted in absorption versus x-ray energy spectra that reflected how water molecules with nanometres of the fold surface absorbed the rays. The results, published in Science, revealed that for a neutral gold surface, a significant number of water molecules next to the surface form hydrogen bond with the gold. When negatively charge, the number of water molecules bound to the gold increased, while a positively charges gold surface caused the water to bond between itself. “That’s 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,” said Miquel Salmeron, senior scientist in the Materials Science Division. “Water next to the electrode has a different molecular structure that it would in the absence of the electrode.” The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy