James Stewart

Figure SEQ Figure \* ARABIC 1. After getting excited with the pump laser, a sound wave pulse is sent through the sample. This scatters some of the probe laser which scatters out of the sample and interferes with light reflecting off the surface.

In the pursuit of making renewable fuels from water, scientists at the Ćuk research group at CU Boulder have made a remarkable discovery: they can "hear" water molecules breaking apart during the initial stages of the process. By using a new technique, they have successfully identified the timing involved in the complex reaction. The Ćuk group is affiliated with the Center for Electrochemical Dynamics and Reactions on Surfaces (CEDARS), an EFRC that focuses on understanding and optimizing various catalysts for water splitting. As renewable energy sources continue to emerge, finding a clean way to store energy remains a challenge. Producing hydrogen gas through water splitting holds great promise in this regard.

The concept of splitting water has been known since the 1800s; by connecting power cables to two metal pieces, submerging them in salt water, and applying a strong enough voltage to the system causes the water to break down into its elemental gases. The metal acts as an electrode catalyst, providing a surface for the water molecules to attach to and interact with one another more effectively. This process can be accomplished using many conductor or semiconductor materials, so a wide range of potential catalyst options are accessible.

While water splitting is conceptually straightforward, its current implementation is highly inefficient and not commercially viable for solving our energy storage challenges. The complexity arises from the multiple steps and intermediates involved in water splitting. Moreover, certain steps in the chemical reaction occur within a fraction of a second (10-15 seconds!), making it difficult to observe and study each step individually. To address this challenge, a specialized technique known as pump–probe spectroscopy is employed.

Similar to the sports mode on a camera, pump–probe spectroscopy captures rapid snapshots in time. Instead of applying a high voltage to the electrodes to initiate the water splitting reaction, researchers can initiate it by using a powerful laser called the pump. The pump laser provides the necessary energy to start the reaction. Subsequently, a second laser, called the probe, captures an image of the reaction. By adjusting the timing between the pump laser's initiation and the probe laser's snapshot, a frame-by-frame “movie” of the reaction process can be created.

During the analysis of these reaction movies, researchers observed an intriguing phenomenon—a consistent wave in the data. When water begins to split on the catalyst surface following the pump laser pulse, it causes a slight displacement of the surface atoms. This displacement creates a sound wave pulse that propagates through the material by causing neighboring atoms to vibrate. The observed wave behavior in the data results from the interaction between this sound wave and the probe laser used to capture the reaction.

When light interacts with a new material, it can either reflect off the surface or refract and travel through the material. Even subtle differences, such as slight variations in bond length during a sound wave pulse, act as a new material to the light.

When the probe laser reaches the catalyst surface, some of the light penetrates the material and interacts with the sound wave. Most of the light continues its path through the material, but a small portion scatters out of the material and into the detector.

The scattered light interferes with the desired light that was intentionally captured. By comparing the interference at different points in time, it is possible to identify changes caused by the sound wave as it propagates away from the reaction site. This process is shown in Figure 1. The change in interference can be predicted, modeled, and studied to extract valuable information.

By modeling the intricate details of wave interference, researchers gained insights into the initial step of water splitting. They discovered that it takes a specific amount of time for the first intermediate to appear and accumulate on the surface after the pump laser excites the system. This formation time affects the phase and intensity of the sound wave. Through comparison with other techniques and previous measurements, they established a correlation between the formation time of the first intermediate and the generated sound wave. Consequently, by analyzing the sound wave produced during the initial stage of the reaction, researchers can effectively "listen" to the first intermediate form.

Within the CEDARS EFRC, researchers have unearthed a novel technique for studying the water splitting reaction on catalyst surfaces. By analyzing the sound wave propagated through the catalyst during the reaction initiation, they extracted crucial information about the early timeline of the reaction. This pioneering technique contributes to the toolset utilized in the CEDARS project, further advancing the investigation of catalysts for water splitting.

More Information

1. Singh, S.; Lyle, H.; D’Amario, L.; Magnano, E.; Vinogradov, I.; Ćuk, T. Coherent Acoustic Interferometry during the Photodriven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate (TIOH*). Journal of the American Chemical Society 2021, 143 (39). DOI:10.1021/jacs.1c04976.s001.

The experimental work was supported by the Director, Office of Science, Office of Basic Energy Sciences, and by the Division of Chemical Sciences, Geosciences and Biosciences of the U.S. Department of Energy at RASEI (Boulder, CO) under Contract No. DE-SC0018939. The experimental work was also supported by the Director, Office of Science, Office of Basic Energy Sciences, and by the Division of Material Sciences of the U.S. Department of Energy at SLAC under Contract No. DE-SC18050129 for RASEI (Boulder, CO).

About the author(s):

James Stewart is a graduate student working in the Ćuk research group at the University of Colorado, Boulder. His work as part of the CEDARS EFRC focuses on investigating the water evolution reaction on different semiconductor catalysts with the goal of creating better solar fuels. This is done using ultrafast IR spectroscopy to look at vibrations at the solid/liquid interface and how they change within the initial stages of the reaction. The highly interdisciplinary field involves physical and inorganic chemistry, condensed matter and device physics, and surface science. ORCID ID #0009-0000-8988-0398.

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