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NOTTINGHAM, UK – Scientists have, for the first time, probed the inner core of electrochemical interactions as they happen. The application of this new spectroelectrochemistry technique could lead to a more efficient and more intelligent design of new materials to be used as catalysts or sensors, said Peter Licence, associate chemistry professor at the University of Nottingham. “Applications could be as broadly ranging as microelectronics, medical, catalysis and performance materials.”
To accomplish this, researchers have devised a way to do spectroelectrochemistry in a high vacuum. Traditionally, such studies are done using infrared, visible or ultraviolet techniques, but these methods yield information only about the outermost electron structure of the electroanalytes.
Getting to the core requires the use of x-rays, which, in turn, means that the reactions must be studied under a high vacuum. Typically, the pressure needs to approach a billionth of a torr, or about a trillionth of normal atmospheric pressure. Unfortunately, most solvents evaporate in a high vacuum. Thus, researchers have used x-rays to analyze electrochemical products after the fact.
To get around this problem, Licence and his group turned to a new class of solvents called room temperature ionic liquids. These are salts that form a liquid at room temperature with very low vapor pressure. Consequently, the materials do not evaporate in a vacuum, and they are electrolytic by nature, allowing them to act as both solvent and electrolyte.
Use of an ionic liquid enabled researchers to perform spectroscopy of electrochemically generated species during a reaction in an ultrahigh vacuum. X-rays produce photoelectrons from the electronic core of analytes placed in the ionic liquid. Courtesy of Peter Licence, University of Nottingham.
In a proof of concept, Licence and his colleagues monitored the electrochemical reduction of iron, capturing Fe (III) being transformed into Fe (II). They used a cell coated with a millimeter layer of gold and a millimeter-diameter platinum electrode, bombarding the area near the electrode with x-rays. The result was the production of photoelectrons, which the investigators captured using a spectrometer from Kratos Analytical of Manchester, UK.
X-ray photoelectron spectroscopy is a surface-sensitive technique with a penetration depth of only a few nanometers. Even a thin layer on top of the electrode would be enough to keep the x-rays from reaching its surface, which is where the chemistry of interest would take place. The researchers couldn’t stir the electrode around in the ionic liquid as it sat in a vacuum. Thus, it was difficult to see new electrochemically generated species without waiting too long. The group was able to solve this problem and probe the near-electrode area through careful cell and electrode design, Licence said.
Their measurements showed expected results, with photoelectron peaks at the beginning indicating only Fe (III). They then captured the growth of other photoelectron peaks of a lower binding energy, indicating Fe (II). They reported these results in the October 2009 issue of Chemical Communications.
Applications of the technique could include construction of molecularly clean surfaces for sensors and other uses. The group already plans to put the tool to use, Licence said. “Our work will now move toward metal deposition and stripping in a controlled environment, hopefully leading to higher-quality film-metal layers for high-tech applications.”