Spectroscopy and Electrochemistry of Nanometre-Scale Plasmonic Gaps

The work in this thesis presents a modern nano-electrode for spectro-electrochemistry: the Nanoparticle-on-Mirror (NPoM). I utilise the extreme confinement of light by coupled plasmons to understand different electrochemical processes at the electrode/molecule interface and learn about ion diffusion and catalysis in a confined environment. Both electrochemistry and vibrational spectroscopy are well utilised techniques to learn about chemical reactions but inhomogeneity at traditional rough-electrode interfaces makes systematic study difficult. By contrast, NPoM electrodes are highly homogeneous and I show that they enable quantitative detection of as few as eight molecules undergoing chemical reactions. In the first part of this thesis, I focus on quantifying electrochemical fields within the first nanometre of the working electrode and mapping out the electrical double layer as it decays through molecular layers. I spectroscopically track the transient charging of nanoparticles by dissolved electroactive species and challenge the pervasive belief that electrochemical potentials cannot be formed in NPoM gaps. I extend this work by quantifying photovoltage forming across NPoM junctions and connect this to single atom movement and single molecule spectroscopy. Moving towards chemical reactions, I study the protonation and deprotonation of acidic molecules inside NPoM and find that local pH is subtly modulated by the proximity of the nanoparticle. Electrochemical protonation is an important process for catalysis but is relatively poorly understood. I unveil some complications with the interpretation of electrochemically protonated molecular layers and devise a method to approach the problem. Having proven that NPoM is a viable electrode for surface-bound electrochemistry, I investigate the catalysis of a CO₂ reduction catalyst and propose a previously unresolved mechanism. Importantly, the first step in the cycle directly involves the molecular anchoring group which highlights the importance of these groups