Toward nanoscale reactivity mapping under electro-catalytic reaction conditions : Plasmon-enhanced vibrational spectroscopy of the electrochemical gold oxidation and gold oxide reduction
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Abstract
Detailed molecular understanding of surface chemistry under realistic non-equilibrium working conditions is pivotal for the rational design of efficient electrocatalytic energy conversion devices or effective measures against surface corrosion. Operando monitoring of reacting species with high surface-molecular sensitivity and chemical specificity under reaction conditions, ideally with nanometer spatial resolution, can provide the necessary molecular insights to understand the underlying surface electrochemistry. However, detection of reaction intermediates is challenging; in particular assessing simultaneously nano-structure surface topography and local chemical information on the nanoscale.
In this thesis, we investigate one of the most important model electrodes in fundamental electrochemistry: polycrystalline and single crystal Au surfaces in sulfuric acid electrolytes. We show how new molecular and unprecedented nanoscale insight into the electrochemical Au oxidation and Au oxide (AuOx) reduction can be obtained using electrochemical surface-enhanced Raman and infrared (EC-SERS/EC-SEIRAS) as well as tip-enhanced Raman spectroscopy (EC-TERS).
We use potential-jumps to the AuOx electro-reduction onset combined with EC-SERS to monitor the evolution of short-lived reaction intermediates. Our results confirm that Au-OH intermediates are formed also in acidic media and give spectral evidence of Au atoms in a 4-fold coordination with oxygen resembling the bulk Au2O3 coordination. The EC-SER spectra further attest the adsorption of (bi)sulfate ions during surface reduction. For sulfate adsorption on Au(111), we show how the potential-dependent vibrational Stark effect and coverage-dependent contributions to the observed EC-SEIRA signal can be disentangled. Using EC-SEIRAS potential-jump experiments combined with a model equation that can describe the potential-induced peak shift of the sulfate stretch vibrational mode, we quantify the coverage-dependent contribution to be 15.6 ± 1.2 cm−1/θSO and the Stark effect to be 75.6 ± 2.7 cm−1/V.
Importantly, we demonstrate how near-field optical nanoscopy, EC-TERS, offers a unique approach for corrosion and electrocatalysis to map nanoscale defect reactivity under electrocatalytic reaction conditions with a chemical spatial sensitivity of ∼10 nm. We find that the electro-oxidation reactivity of Au nanodefects is directly correlated to their surface topography and is limited to an oxide depth of ∼3 nm. The local Raman fingerprint indicates the presence of at least two spatially separated AuOx species, namely Au2O3 and Au2O, on the nanodefects. Finally, we discuss the implications of all presented results for future EC-TERS studies to identify potential-dependent reaction pathways and
their active sites at the sub-10 nm level, which will aid to push our understanding of defect reactivity to the molecular or atomistic level.