Computer simulations hold tremendous promise to accelerate the molecular engineering of green energy technologies, such as new systems for electrical energy storage and solar energy usage, as well as carbon dioxide capture from the environment. However, the predictive power of these simulations depends on having a means to confirm that they do indeed describe the real world.
Such confirmation is no simple task. Many assumptions enter the setup of these simulations. As a result, the simulations must be carefully checked by using an appropriate “validation protocol” involving experimental measurements.
To address this challenge, a team of scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, the Department of Chemistry and the Pritzker School of Molecular Engineering (PME) at the University of Chicago, and the University of California, Davis, developed a groundbreaking validation protocol for simulations of the atomic structure of the interface between a solid (a metal oxide) and liquid water. The team was led by Giulia Galli, Liew Family Professor of Molecular Engineering at Pritzker Molecular Engineering, and Paul Fenter, an Argonne experimentalist.
“We focused on a solid/liquid interface because interfaces are ubiquitous in materials, and those between oxides and water are key in many energy applications,” said Galli.
“To date, most validation protocols have been designed for bulk materials, ignoring interfaces,” added Fenter. “We felt that the atomic-scale structure of surfaces and interfaces in realistic environments would present a particularly sensitive, and therefore challenging, validation approach.”
The validation procedure they designed uses high-resolution X-ray reflectivity (XR) measurements as the experimental pillar of the protocol. The team compared XR measurements for an aluminum oxide/water interface, conducted at beamline 33-ID-D at Argonne’s Advanced Photon Source (APS), with results obtained by running high-performance computer simulations at the Argonne Leadership Computing Facility (ALCF). Both the APS and ALCF are DOE Office of Science User Facilities.
“These measurements detect the reflection of very high energy X-ray beams from an oxide/water interface,” said Zhan Zhang, a physicist in Argonne’s X-ray Science Division. At the beam energies generated at the APS, the X-ray wavelengths are similar to interatomic distances. This allows the researchers to directly probe the molecular-scale structure of the interface.
“This makes XR an ideal probe to obtain experimental results directly comparable to simulations,” added Katherine Harmon, a graduate student at Northwestern University and the first author of the paper. The team ran the simulations at the ALCF using the Qbox code, which is designed to study finite temperature properties of materials and molecules using simulations based on quantum mechanics.