Surfaces are very interesting to material scientists. The reactions that happen at the point where inside and outside meet, and elements interact with other chemicals or radiation, are important for developing new technology for batteries, fuel cells or photovoltaic panels, for catalysts for the chemical industry, and for understanding environmental chemistry and pollution. Now researchers at UC Davis and the Advanced Light Source at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory have combined two existing methods techniques to come up with a new method for studying surfaces with X-rays. This new technique is called SWAPPS, for Standing Wave Ambient Pressure Photoelectron Spectroscopy.
“SWAPPS enables us to study a host of surface chemical processes under realistic pressure conditions and for systems related to energy production, such as electrochemical cells, batteries, fuel cells and photovoltaic cells, as well as in catalysis and environmental science,” says Charles Fadley, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Davis, where he is a Distinguished Professor of Physics. “SWAPPS provides all the advantages of the widely used technique of X-ray photoelectron spectroscopy, including element and chemical-state sensitivity, and quantitative analysis of relative concentrations of all species present. However with SWAPPS we don’t require the usual ultrahigh vacuum, which means we can measure the interfaces between volatile liquids and solids.”
Fadley is one of three corresponding authors of a paper, with Hendrik Bluhm of the Berkeley Lab’s Chemical Sciences Division and Slavomír Nemšák, now with Germany’s Jülich Peter Grünberg Institute, describing SWAPPS in Nature Communications.
X-rays probe surfaces
In terms of energies and wavelengths, X-rays serve as excellent probes of chemical processes. Fadley’s group at ALS originally developed standing wave photoelectron spectroscopy, which uses X-rays to probe buried surfaces, while a team including Bluhm developed high ambient pressure photoelectron spectroscopy, which made it possible to use X-ray spectroscopy under pressures and humidities similar to those encountered in natural or practical environments. The new technique combines the best features of both. That means the researchers can probe the composition of surfaces and interfaces with unprecedented resolution under the conditions where batteries, fuel cells or other devices actually work.
Says Fadley, “We believe SWAPPS will deliver vital information about the structure and chemistry of liquid/vapor and liquid/solid interfaces, in particular the electrical double layer whose structure is critical to the operation of batteries, fuel cells and all of electrochemistry, but which is still not understood at a microscopic level.”
The researchers used the technique to probe an experimental system of sodium and cesium hydroxide, layered on iron oxide (hematite).
“We determined that the sodium ions are located close to the iron oxide/solution interface, while cesium ions are on average not in direct contact with the solid/liquid interface,” Bluhm says. “We also discovered that there are two different kinds of carbon species, one hydrophobic, which is located exclusively in a thin film at the liquid/vapor interface, and a hydrophilic carbonate or carboxyl that is evenly distributed throughout the liquid film.”
In their Nature Communications paper, the authors say that future time-resolved SWAPPS studies using free-electron laser or high-harmonic generation light sources would also permit, via pump-probe methods, looking at the timescales of processes at interfaces on the femtosecond time scale.
“The range of future applications and measurement scenarios for SWAPPS is enormous,” Fadley says.
In addition to Fadley, Bluhm and Nemšák, other authors of the paper describing SWAPPS were Andrey Shavorskiy, Osman Karslioglu, Ioannis Zegkinoglou, Peter Greene (UC Davis), Edward Burks (UC Davis), Arunothai Rattanachata (UC Davis), Catherine Conlon, (UC Davis) Armela Keqi (UC Davis), Farhad Salmassi, Eric Gullikson, See-Hun Yang and Kai Liu (UC Davis).
This research was primarily funded by the Department of Energy’s Office of Science. The Advanced Light Source is a DOE Office of Science User Facility.
Adapted from an original story by Lynn Yarris, LBL.