An article published in Nature Communications on January 16, 2018 provides answers to a decades-long question: What happens in the short time period following the injection of an electron into water? The research is a result of a collaboration between scientists at the University of Chicago, University of California in San Diego, as well as Argonne (ANL) and Lawrence Livermore National Laboratories (LLNL), and is led by Giulia Galli, Liew Family Professor of Molecular Engineering at University of Chicago, who also has a joint appointment at ANL.
“When an electron is injected into water, it is captured by the liquid, and the energy gain due to this process is called the electron affinity of water," said Alex Gaiduk, NSERC postdoctoral fellow at the University of Chicago and the lead author of this study. “Knowing the electron affinity of liquid water is crucial to understanding and modeling processes involving electron transfer between solids and the liquid, such as those occurring in photoelectrochemical cells to split water and generate oxygen and hydrogen.”
“Unfortunately, the electron affinity of water has not yet been measured experimentally due to a number of technical difficulties,” Galli said. “Most of the results quoted in the literature as experimental numbers are actually values obtained by combining some measured quantities with crude theoretical estimates.” Paesani, co-author of the study who has spent years developing an accurate potential for the modelling of liquid water, added “On the other hand, accurate theoretical estimates have been out of reach for a long time, due to a lack of appropriate theoretical methods and the high cost of direct simulations.”
The interaction potential between water molecules developed by Paesani from UC San Diego was used to model the structure of liquid water and also that of the surface of water. Once the structure was obtained, highly accurate theoretical methods and software to study excited states of matter, developed in the Galli group, were utilized to understand what happens when an electron is injected in water; the researchers sought to understand whether the electron can reside in the liquid and eventually participate in chemical reactions. The central question was: Is the electron bound right away by the liquid?
The researchers found that the electron is bound; however its binding energy as soon as it is injected in water is tiny, much smaller than previously believed. This prompted them to revisit a number of well-accepted data and models for the electron affinity of water.
The methods for excited states used in this study were developed over the years by Galli and co-workers, within collaborations involving T. A. Pham, from LLNL, and Marco Govoni, from ANL, co-authors of the study. "Using the software developed to study excited state phenomena in realistic systems (WEST) and leadership computing resources at Argonne National Laboratory, we were finally able to generate data for samples both large enough and on sufficiently long timescales, to study the electron affinity of liquid water," Govoni said. "We found large differences between the affinity at the surface and in the bulk liquid. We also found values rather different from those accepted in the literature, which prompted us to revisit the full energy diagram of an electron in water," Pham said.
This finding has important consequences both for the fundamental understanding of the properties of water, as well as for the description of reduction/oxidation reactions in aqueous solutions, which are widespread in chemistry and biology. "This is an extremely important and potentially influential study, making a strong case for a fundamental redrawing (qualitatively as well as quantitatively) of the bulk energy-level diagram for liquid water," one of the peer reviewers stated in the referee report. "The older number is still referenced frequently and taken seriously by a certain community interested in computing redox potentials of aqueous solutes [...] so any adjustment of the electron affinity potentially readjusts a lot of other numbers."
Particularly, the information about the energy levels of water is often used during the computational screening of materials for photoelectrochemical cells. Having a reliable estimate of the water electron affinity (which the researchers of the study provided for both bulk water and its surface) will lead to more robust and reliable computational protocols, and improve computational screening of materials.
Funding for the work by Gaiduk and co-workers was provided by Department of Energy Office of Science, Basic Energy Sciences through the Midwest Integrated Center for Computational Materials, Natural Sciences and Engineering Research Council of Canada, the National Science Foundation, and the Lawrence Fellowship. Computer time was provided by the INCITE, XSEDE, and LLNL Grand Challenge programs.