Detection of Ge-Containing Adlayers at the Liquid Hg/Water Interface by In Situ X-ray Reflectivity in Aqueous Borate Electrolytes Containing Dissolved GeO₂

Abstract

The local structure of the liquid metal/electrolyte interface for Hg electrodes immersed in aqueous solutions suitable for crystalline Ge electrodeposition was probed by in situ X-ray reflectivity. A series of X-ray reflectivity measurements were collected in both pure 0.1 M Na 2 B 4 O 7 solution and 0.1 M Na 2 B 4 O 7 with 0.05 M GeO 2 at potentials where there are no faradaic redox reactions. Quantitative analysis of these data indicates that the interface structure in pure Na 2 B 4 O 7 solution is indistinguishable from that of Hg immersed in NaF solution, suggesting nonspecific adsorption of the borate ions. In GeO 2 -containing electrolytes, the presence of Ge species could be detected over a wide potential regime. Between −0.9 and −0.5 V vs saturated calomel electrode (SCE), the reflectivity data revealed the adsorption of dissolved H 2 GeO 3 and/or HGeO 3 − at a coverage that appears to be independent of potential. At potentials more positive than −0.5 V, an incomplete ultrathin film of a condensed phase forms via slow nucleation and growth at the Hg surface. This solid phase is consistent with a form of GeO 2 and tentatively assigned to polygermanates. This film likely forms through a condensation reaction driven by the concentration of HGeO 3 − at the liquid Hg/ electrolyte interface at positive potentials.

In Situ and Operando X-ray Scattering Methods in Electrochemistry and Electrocatalysis

“…Surface layering has been shown to be a general property of liquid Hg and other liquid metals , and was also observed in other electrolytes, including electrolytes that contain chemisorbing species. − In the latter studies, also insight into the adlayer structure could be obtained. Specifically, measurements in solutions containing Pb and halide ions revealed the formation of salt-like layers at the interface, , similar to those observed on solid electrodes. ,, As an example, the electron density profile and interface structure of Hg in PbBr 2 -containing NaF solution is shown in Figure d, where the formation of a defined layer of crystalline c -axis oriented PbFBr with a thickness of one unit cell was observed …”

“…96 In the context of studies of the electrochemical liquid−liquid-solid deposition of Ge on Hg and HgIn alloys, in situ XRR was used to determine the influence of the interface structure on the growth process. 368,597 In situ X-ray scattering techniques have also been applied to study growth at interfaces between other immiscible liquids, e.g., between aqueous and nonaqueos solutions, 598−602 but corresponding studies of electrodeposition in such systems have not been reported yet.…”

“…Detailed studies of PbBrF growth indicated with increasing overpotential a crossover from the slow growth of a small number of large crystals to the formation of a compact film with a saturation thickness of 25 nm . In the context of studies of the electrochemical liquid–liquid-solid deposition of Ge on Hg and HgIn alloys, in situ XRR was used to determine the influence of the interface structure on the growth process. , In situ X-ray scattering techniques have also been applied to study growth at interfaces between other immiscible liquids, e.g., between aqueous and nonaqueos solutions, − but corresponding studies of electrodeposition in such systems have not been reported yet.…”

Understanding and Expanding the Prospects for Electrosynthesis with Liquid Metal Electrodes

“…Extensive XRR measurements were collected to compare Hg immersed in this electrolyte and NaF(aq). 45 The latter system is the most well-studied liquid metal/water interface, 41 with Hg exhibiting atomic structure nearly identical to that of the Hg/vacuum interface. 17 Since the XRR data for Hg/Na 2 B 4 O 7 (aq) contacts were statistically indistinguishable from data for Hg/NaF(aq) junctions, the former interface was concluded to have essentially the same pristine character and structure as the latter.…”

“…The experimental measurements were well fit with a model consisting of Hg atoms arranged with order that dissipated when moving perpendicular from the surface plane toward the bulk (Figure a inset). Extensive XRR measurements were collected to compare Hg immersed in this electrolyte and NaF­(aq) . The latter system is the most well-studied liquid metal/water interface, with Hg exhibiting atomic structure nearly identical to that of the Hg/vacuum interface .…”