Electrochemical tip-enhanced Raman spectroscopy : development and applications
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Abstract
Understanding the complex interplay of electrical and chemical reactions at the electrochemical interface is essential to optimize electrochemical devices in a large variety of applications ranging from energy-conversion to catalysis and synthesis. However, there is a lack of analytical techniques providing a full picture of the electrochemical interface. Tip-enhanced Raman spectroscopy (TERS) is a surface-specific vibrational spectroscopy providing chemical information with (sub)monolayer sensitivity and nanometric spatial resolution. TERS has been applied mostly to interrogate interfaces in the air and ultra-high vacuum studies, but the characteristics of the technique make it an ideal candidate to study electrochemical interfaces. The objective of this work is to develop a TERS system able to work in an electrochemical environment.
As a first step, a TERS setup operating in a simplified liquid/solid interface is developed. Despite a focus distortion at the air/liquid interface, which leads to a reduction of the scattering intensity by a factor of 3, we are able to efficiently collect TER spectra from few hundreds of small resonant and non-resonant molecules adsorbed at flat gold electrodes. Low TERS intensity in liquid can be compensated by phase modulation of the beam, however, enhancement factors of 10^5 can be reached with the setup without distortion correction. In air, TERS intensity has been found be proportional to d^{-10}, being d the tip-sample distance. In STM-based TERS, the gap distance is determined by the STM bias voltage and tunneling current, however, no systematic analysis on the dependence of the spectral features into these parameters was available up to now. By a series of experiments in argon and water, we find that the distance-dependent model is valid also in liquid experiments. Working at tunneling currents higher than 1 nA and bias voltages lower than 0.1 V results in an efficient rise of the TERS intensity in both air and liquid experiments. In this work, we demonstrate systematic and reproducible TERS characterization of solid/liquid interfaces.
Finally, we demonstrate the extension of the system to electrochemical environment. As a test experiment, we measure a monolayer of adenine on a Au(111) electrode in acidic media. EC-TERS data combined with theoretical simulations offer a full picture of this electrochemical interface. We find that protonated adenine is physisorbed onto the gold substrate in a tilted configuration at potentials lower than the potential of zero charge, and it adopts a vertical orientation at higher potentials. After deprotonation at 0.6 V (versus Ag/AgCl), neutral adenine is chemically adsorbed at the surface with a planar orientation.
The results obtained in this thesis demonstrate the promising capabilities of the instrument to characterize liquid and electrochemical interfaces, providing a general overview of chemical conversion, electron transfer reactions and orientation at the interface. The extension of TERS to electrochemical and liquid environments paves the way for powerful in-situ chemical nano-characterization of a wide range of electrified interfaces.