Time-Resolved Near-IR Spectroscopy Probes Potential Jump at Electrochemical Interface

Anne L. Fischer

The electrochemical interface where a solid meets a solution is of interest in the development of biosensors and fuel cells because it is where electron transfer occurs between electrodes and molecules. The techniques used to study reaction dynamics at the interface, such as voltametry and amperometry, have been based on the measurement of electrochemical quantities, including the electrode potential, the current that flows across the interface and the impedance of the interface. However, they do not offer direct structural information about the interface or about the molecules involved in a reaction.

The graph shows the redshift observed in the potential-dependent C-O stretching vibration of CO (circles) and the changes in the intensity of the reflected near-IR probe pulses (triangles). The temporal profile of the 532-nm pump pulse is indicated by the dashed line. Reprinted with permission. ©2006 American Chemical Society.

Scientists from Hokkaido University in Sapporo and from Tokyo Institute of Technology in Yokohama, both in Japan, hope to characterize electrochemical interfaces at atomic and molecular scales, with the goal of improving understanding of reactions and enabling better interface designs and control of reactions. They have developed a surface-enhanced IR absorption spectroscopy method to observe the potential jump at an electrochemical interface induced by thermal effects resulting from its irradiation with a pulse of laser light.

According to researcher Masatoshi Osawa of Hokkaido University, they demonstrated that picosecond time-resolved IR measurements are possible using this technique. They also determined how much and how quickly the electrode potential — the potential difference between the working (or test) and the reference electrodes — can be changed by irradiation with a visible laser pulse, which may have an impact on controlling reactions.

In the technique, the surface of the electrode (a platinum film) is irradiated by a visible laser pulse from the front side through the solution. Near-IR probe pulses irradiate the sample from the back side through the prism and are directed to a deuterated triglycine sulfate detector.

In the experiments, they used 35-ps pulses of 1064-nm radiation produced by a Continuum Inc. Nd:YAG laser operating at 10 Hz to probe the change in the potential of a platinum electrode in contact with perchloric acid. The 532-nm pump pulses, also 35 ps long, were generated by a homebuilt system. Specifically, they monitored the potential-dependent C-O stretching vibration of CO adsorbed on the electrode, observing that the peak redshift was ~200 ps delayed from the pump pulse. They used the technique in an attenuated total reflection mode to record the time-resolved spectra, which enhanced the signal-to-noise ratio and prevented the water from absorbing the green pump pulses.

They chose to use near-IR pulses to observe molecular vibrations, Osawa said, because these wavelengths provide rich information on the structure and chemical nature of molecules. The pulses penetrate only to a depth on the order of the wavelength of the radiation, but that is sufficient because the scientists are interested only in molecules near the electrode surface. Until they developed the approach, they found it difficult to detect very weak signals from a single molecular layer. Now they know how to make the measurements and are doing so in microseconds.

The next step is to study why and how the electrode potential changes with the jump in temperature. They plan to compare their results with the predictions of the Marcus theory, which they hope will enable a better understanding of the electron transfer between the electrode surface and molecules.

Journal of Physical Chemistry B, April 6, 2006, pp. 6423-6427.