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

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 In3+ oxi-/ hydroxides, deriving from reaction-diffusion processes taking place during alloy growth, and this pattern disappears in depth because of the delayed reduction of In3+ present in this film to elemental In, followed by intermetallic formation.