Abstract
Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto- and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions.
Highlights
Nanomaterials, like small nanoparticles (NP), show a high surface area to volume ratio and an altered electronic structure, as compared to their bulk counterparts (Kreibig, 1974; Zhu et al, 2013)
The second peak can be assigned to the indium tin oxide (ITO) substrate, as it is seen in the first cycle of blank experiments in the absence of nanoparticles, represented in Supplementary Figure S1
It was shown that individual immobilized silver nanoparticles can dissolve during electrochemical oxidation in a nitrate containing electrolyte
Summary
Nanomaterials, like small nanoparticles (NP), show a high surface area to volume ratio and an altered electronic structure, as compared to their bulk counterparts (Kreibig, 1974; Zhu et al, 2013). There is a huge demand to get deeper insights into reaction mechanisms, especially for electrochemically (and potential) driven reactions, at and of nanostructures on the single entity nanomaterial level The latter one is of special interest, because even small changes in the size and shape of the materials (which likely alters by the synthesis) strongly influence and vary their electrochemical and physical behavior (Roduner, 2006; Viñes et al, 2014; Khan et al, 2017). The excitation energy and intensity of the localized surface plasmon resonance of nanomaterials can be probed with plasmon-based electrochemical current microcopy (PECM) and with dark-field microscopy (DFM) The latter is able to precisely study the spectral positions and intensities of the extinction spectra (LSPR) of individual nanoparticles by hyperspectral imaging (HSI) 30–60 μL of electrolyte solution was added and the cell was closed by a cover slip
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