Operando Studies of the Electrochemical Dissolution of Silver Nanoparticles in Nitrate Solutions Observed With Hyperspectral Dark-Field Microscopy

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.

Optical monitoring of the electrochemical nucleation and growth of silver nanoparticles on electrode: From single to ensemble nanoparticles inspection

“…Dark-field microscopy is also frequently coupled to spectroscopy to obtain chemical information through changes in the localized surface plasmon resonance. [43][44][45] Interferometric configuration allows increasing the OPT signal to background ratio, as in the interferometric scattering microscope (iSCAT), but it cannot apply to electrochemistry since it relies on reflections at an insulating interface. 46,47 However, recent works have shown that ultrathin layer of absorbing and conducting materials deposited onto a glass substrate could be used in an interferometric observation mode named BALM for backside absorbing layer microscopy.…”

Electrochemistry and Optical Microscopy

“…Color camera or hyperspectral dark field microscopes, e.g. via commercial platform (Cytoviva®, [60][61][62][63]) or hyperspectral camera, collect images with both full spectral identification and 2D localization of single NPs.…”

Substrate mediated dissolution of redox active nanoparticles; electron transfer over long distances

“…The surface plasmon resonance peak position depends on a number of factors including the particle size, shape and agglomeration state [14,52]. Under dark field illumination individual silver nanoparticles as small as 10-15 nm in diameter can be observed under optimal conditions [22,23]. If the dimensions of the nanoparticle are below the wavelength of the light, on a microscope image a single nanoparticle appears as an individual diffraction-limited spot.…”

“…Metallic nanoparticles are visible using a dark-field optical microscope within a wide size limit between ca. 10 and 100 nm in diameter [22,23] and under suitable conditions individual particles or their clusters can be separately monitored [24]. This is in contrast to many other experimental techniques which are often limited to measuring the average response across a nanoparticle population.…”