Depth-Dependent Scanning Photoelectron Microspectroscopy Unravels the Mechanism of Dynamic Pattern Formation in Alloy Electrodeposition

Abstract

Fascinating spatiotemporal patterns forming during the electrodeposition of some alloys have attracted the interest of the scientific communities dealing with electrochemical materials science and dynamic processes. Notwithstanding extensive experimental work and recently achieved theoretical insights, several aspects of the physical chemistry of these dynamic structures are still elusive. In particular, the analytical methods employed so far to characterize these structures invariably failed to pinpoint any chemical or structural patterns correlated to those perceived by the naked eye or with a light microscope. In this work, we have made systematic use of the extreme surface sensitivity provided by synchrotron-based scanning photoelectron microspectroscopy, combined with progressive erosion by precisely controlled Ar + sputtering, to achieve quantitative 3D understanding of the compositional and chemical-state distribution of an Ag−In electrodeposited layer, following the key elements Ag, In, and O. The results revealed that the pattern is present only in the topmost region (ca. 100 nm) of the layer and exhibits a regular distribution of the alloying elements in certain chemical states. Specifically, pattern formation in Ag−In electrodeposits is crucially controlled by the space distribution of surface In 3+ oxi-/ hydroxides, deriving from reaction-diffusion processes taking place during alloy growth, and this pattern disappears in depth because of the delayed reduction of In 3+ present in this film to elemental In, followed by intermetallic formation.

Spatially Resolved XPS Characterization of Electrochemical Surfaces

“…The sample exhibits a 3-phase crystallographic structure, featuring α−Ag-rich terminal solid solution (Figure 6; marked with red dots) and the Ag 3 In and AgIn 2 intermetallics, as shown by the Powder Diffraction Files (PDF) Cards No: 29-0677 and No: 65-1552, respectively, in keeping with the literature [27,28]. The same phase structure was found in the single-spiral sample studied in [26]: the quantitative differences in phase composition and preferential orientations are compatible with differences in electrodeposition current density.…”

“…More experimental and theoretical details, as well as comprehensive reference to relevant literature, can be found in ref. [26]. The sample analyzed in this research (Figure 5) shows a doublespiral pattern with a spiral-core distance of ca.…”

“…Sequential sputtering periods were applied, corresponding to depths of 2, 4, 10, 20, 50, 65 and 90 nm, estimated according to standard models for sputtering depth ( [29] and references therein): for full methodological and quantitative details, refer to ref. [26]. These sputtering steps effectively allow for monitoring of progressive depth variations in the alloy pattern.…”

“…Details on the image processing procedure are reported in the Supporting Information of ref. [26]. Ar + bombardment (1.5 × 10 −5 mbar Ar, 1-1.5 kV, sample current of 0.8 µA) erosion was employed for depth-profiling purposes; the ion beam was delivered at 45 • with respect to the sample.…”

“…As proved in ref. [26,36], SPEM, owing to its high chemical-state sensitivity combined with surface sensitivity and lateral resolution, is the method of choice for unravelling the morphochemical details of electrodeposited alloy patterns. In view of completing and further validating the scenario set up in the two publications mentioned just above, in this study we have explored the 3D space elemental and chemical-state distribution of an Ag-In sample, fabricated under different conditions with respect to the one investigated in ref.…”