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Femtosecond mid-infrared study of water at electrified interfaces

Final Report Summary - WATERDYNAMICS (Femtosecond mid-infrared study of water at electrified interfaces)

The interaction of water with metal surfaces plays a central role in many disciplines such as catalysis, electrochemistry, and biochemistry. However, little is known about the properties of surface water on metals, especially at electrochemical interfaces, and no information at all is available about the dynamics of water on metals.

One possible approach to perform such a study is the application of femtosecond mid-infrared pump-probe spectroscopy to measure the properties of water molecules at electrified, metal interfaces. With this method, the dynamics of water molecules in the vicinity of a metal surface would be studied for the first time, as a function of the applied electrostatic potential. In these experiments, an intense pump pulse is used to excite a fraction of the water molecules in the sample, and the dynamics of the excited molecules are probed by a second pulse of which the time delay with respect to the pump is controlled by a delay stage.

In the first stage of this project, Dr Garcia-Araez was trained in the use of the pump-probe mid-infrared femtosecond spectroscopy through a collaboration project with Dr K. J. Tielrooij and Prof. M. Bonn. In this study, the dynamics of water molecules around ions were studied in detail, and it was found that, in certain cases, the interaction between water and ions is affected by strong cooperative effects (Tielrooij, K. J. et al., Science 2010, 328, 1006 - 1009). These results are key to understand the dynamics of water on electrochemical interfaces, since charging an electrode produces a layer of highly concentrated ions in the solution near the electrode surface.

An important challenge of the study of water at electrified interfaces is the distinction of the response of interfacial water from that of bulk water. In most techniques, the signal of surface water is hard to distinguish because of the overwhelming response of bulk water. However, this problem can be solved by taking advantage of the enhancement of the electric field at the surface associated with surface plasmon excitation in a technique called surface-enhanced IR absorption spectroscopy (SEIRAS) in an attenuated total reflection (ATR) configuration. In these experiments, the working electrode is a thin gold layer (circa 20 nm), deposited on a silicon hemisphere by vacuum deposition. When the IR beams enter through the silicon hemisphere, they will be totally reflected at the gold-water interface. Under these conditions, the response from the interfacial water molecules dominates. In these studies, we characterised the molecular properties of water on gold electrodes with different surface structures. (Garcia-Araez, N. et al., J. Phys. Chem. C 2011, 115, 21249 - 21257; Garcia-Araez, N. et al., J. Phys. Chem. C. 2012, 116, 4786 - 4792).

The application of the pump-probe mid-infrared femtosecond spectroscopy to water-metal interfaces involved other complications. From parallel measurements with only the silicon support, in the absence of water and the gold layer, we concluded that the measured pump-probe signal is largely dominated by the response of the silicon support. It might be possible to modify this technique to study the water-metal interface by, for example, selectively acquiring the electrochemical potential dependent pump-probe signal. However, from our experiments, it seems more feasible to employ another surface-sensitive non-linear spectroscopy like surface sum-frequency generation (SFG).

SFG is a second-order nonlinear optical process in which two light beams at frequencies omega1 and omega2 generate light at their sum frequency (omega3 = omega1 + omega2). The process of SFG can be used for highly surface-specific vibrational spectroscopy, as the SFG process is forbidden in centrosymmetric media (within the electric dipole approximation). Hence, for centrosymmetric media like liquid water, the SFG is only generated in the top molecular water layer, thus providing extremely high interfacial specificity. Therefore, SFG is an ideal technique to study the properties of water at the metal-water interface. We performed SFG measurements of water on several electrode surfaces (gold, ITO, TiO2 and platinum), and we discovered that Fresnel factors produce a major influence on the SFG spectra [Garcia-Araez, N. et al., in preparation).

In conclusion, the Marie-Curie fellowship has been extremely fruitful. Dr Garcia-Araez became an expert in many optical and nonlinear optical techniques including like surface-enhanced IR absorption spectroscopy, femtosecond mid-infrared pump-probe spectroscopy and surface sum-frequency generation. With this technique the properties of water near ions and electrified metal surfaces were studied, and new insights in the effects of electric fields on the structure and dynamics of nearby water molecules were obtained. The results of these studies have been published in three articles (one in Science, two in the Journal of Physical Chemistry C) and will be published in a forthcoming paper (to be submitted to the Journal of Chemical Physics). Thanks to the Marie-Curie fellowship Dr Garcia-Araez is in an excellent position to pursue her academic career. In fact, she has started a new position as a project leader at the Paul Scherer Institute in Switzerland. In this position, she will start her own research group.

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