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Soft X-ray coherent diffraction imaging for tracking electrodeposition dynamics

In situ physico-chemical analysis of nanostructured functional materials is crucial for advances in their design and production. X-ray microscopy (XRM) provides a powerful tool for investigating materials at sub-micron resolution. Among the different XRM techniques, X-ray Coherent Diffraction Imaging (CDI) methods have recently demonstrated impressive potential for characterizing such materials with elemental sensitivity and high spatial resolution, not limited by the probe size.  However, moving from the current ex-situ static regime to the in situ dynamic one remains a challenge, especially when operating in the soft X-ray regime that requires in-vacuum conditions. Ptychography is a CDI technique that requires scanning of the sample at a step size smaller than the probe size, insuring adequate overlapping of subsequent illuminated areas. The redundant information due to the oversampling allows computer algorithms to reconstruct reliably both the sample transmission function and the illuminating probe, recovering amplitude and phase of both these signals. As inclassical scanning X-ray transmission microscopy (SXTM), ptychography also provides elemental and chemical information by recovering the phase and amplitude functions from two images, obtained using photon energies above and below atomic resonances, or recovering the near edge absorption spectrum from sets of images taken at photon energies across the atomic absorption edges. On the other hand keyhole CDI is a single-shot technique, therefore in general preferable for in situ dynamic studies. By using a known illumination, keyhole CDI allows a single shot image of an extended specimen to be taken with directly quantitative phase and amplitude.
The reported investigation is an ongoing joint project of the scientific computing team and the TwinMic beamline at the Elettra laboratory, the SXRI beamline at the Australian Synchrotron and the Università del Salento aimed at development of methodology for spatio-temporal monitoring morphological and chemical changes of complex functional materials during fabrication and/or operation processes by combining X-ray ptychography and Keyhole CDI.
Beyond the experiment design and the sample preparation, this study of co-electrodeposition of Mn-Co/polypyrrole (PPy) nanocomposites, has a dominant mathematical and computational aspect. This has been imposed by the need to use and develop advanced computational methods for handling ptychographic scans at Big Data scale.
Combining soft X-ray ptychography and single-shot keyhole CDI we performed the first in situ spatiotemporal study of an electrodeposition process in a sealed wet environment, employed for the fabrication of oxygen reduction catalysts, a key component for alkaline fuel cells and metal-air batteries. The obtained results provide the first experimental demonstration of theoretically predicted Turing-Hopf electrochemical pattern formation resulting from morpho-chemical coupling, adding a new dimension for in-depth characterization of electrodeposition processes in space and time in situ.
Up to date neither in situ ptychography nor dynamic Keyhole CDI has been successfully undertaken individually, let alone combined in a single study. The present results demonstrate that the combination of these methods will find applications far beyond the case highlighted in the manuscript, attracting the interest of broader scientific community.
Figure 1 shows a picture of the electrochemical cell (a) keyhole CDI images (b and c) acquired after the application of electrodeposition cycles, within the typical range of electrocatalyst amount employed in air cathodes. The images are centered at the Working-Electrode/electrolyte interface, the region (see red box in Figure 1a) where the electrodeposition rate is higher due to the current density distribution of the implemented electrode configuration. The distinct darker features exhibit the evolution of the two most typical electrodeposition features: globular nuclei (γ) and dendrites (β). One of the larger dendrites (β in panel b) is attached to the WE edge by a relatively thin stalk (α); the formation of this type of features is due to growth instabilities resulting from opposed voltage and concentration gradients.

Figure 1. Optical micrograph (a) of the electrochemical microcell with working electrode (WE), counter electrode (CE), and reference electrode (RE). The red frame in (a) indicates a typical area selected for investigation. A region of the WE/electrolyte interface measured after 30 (b) and 40 (c) electrodeposition cycles. Comparison of (b) and (c) shows the movement of the feature marked β (attached to the WE via the ‘stalk’ marked α) with successive electrodeposition pulses. (e) and (f) outline a small region of interest, indicated by black boxes in (b) and (c), respectively. It highlights the dynamic evolution of the WE/electrolyte interface (dotted greenline), where small features change in size and shape (highlighted by the gray and blackarrows). (d) illustrates the first and last stages of in operando monitoring during biasing (region indicated in the gray frame in (e))

 

In order to explore the spatial details of the co-deposited composite material, getting insight into the elemental distribution at the nanoscale, we performed in situ ptychography of the same area after individual growth steps scanning the photon energies across the Mn L edge. This allowed us to extract Mn L3 absorption spectra and to spatially evaluate the distribution of different Mn oxidation states. Figure 2 shows the distribution of Mn2+ and Mn4+ in a sub-region of the WE, together with the average absorption spectrum and typical spectra of region showing high Mn2+ and Mn4+content respectively.
 

Figure 2. (a) Absorption image acquired at 636 eV (below the Mn L absorption edge) of a sub-region of the WE electrode. (b) The same area where the distribution of Mn2+ and Mn4+ states are indicated with red and green, overlapping the absorption contrast dominated by Mn species. The results are based on scans at 18 different energies from 636 to 647 eV. Panel (d) depicts the average absorption spectrum collected over the whole area shown in panel (a), while panel (c) shows the average absorption spectra of the region where Mn2+ state dominates (green plot) compared to where Mn4+ state dominates (red plot).


 

This research was conducted by the following research team:


Georgios Kourousias1Benedetto Bozzini2Alessandra Gianoncelli1Michael M.W. Jones3Mark Junker4, Grant Van Riessen4 and Maya Kiskinova1 

 

Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
Dipartimento di Ingegneria dell'Innovazione, Università del Salento, via Monteroni, Lecce, Italy.
Australian Synchrotron, Clayton, Australia and ARC Centre of Excellence for Advanced Molecular Imaging, La Trobe Institute of Molecular Science, La Trobe University, Bundoora, Australia
Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Bundoora, Australia.



Local contact person:

Georgios Kourousias, email: george.kourousias@elettra.eu

 

Reference

Georgios Kourousias, Benedetto Bozzini, Alessandra Gianoncelli, Michael MW Jones, Mark Junker, Grant van Riessen, Maya Kiskinova "Shedding light on electrodeposition dynamics tracked in situ via soft X-ray coherent diffraction imaging"  Nano Research 9, 2046 (2016) doi: 10.1007/s12274-016-1095-9
Last Updated on Tuesday, 19 July 2016 13:28