Mechanism of the potential-triggered surface transformation of germanium in acidic medium studied by ATR-IR spectroscopy

Abstract

In acidic solution, germanium surfaces undergo a transformation to hydrogen-terminated surfaces at sufficiently negative electrode potentials. Herein, we used in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy coupled to electrochemical experiments to study the details of this surface transformation on Ge(111) and Ge(100) in 0.1 M HClO. The ATR-IR data gathered during the surface transformation are consistent with an interpretation according to which an intermediate state exists of a surface with mixed termination. In the mixed termination, both H and OH are bound to the surface, which showed a Ge-H stretching mode at ∼2025-2030 cm. At sufficiently negative potentials, the surfaces became fully hydrogen terminated. ATR-IR spectra can be understood by assigning the peak at ∼1977-1990 cm to the stretching mode of GeH species on Ge(111), and the peak at ∼2000-2015 cm to a stretching mode of GeH species on Ge(100). Measurements of the linear dichroism showed the GeH species to be oriented predominantly upright. The transition dipole moment of the GeH species was oriented parallel to the surface, as expected for an antisymmetric stretching mode.

Vibrational spectroscopic study of pH dependent solvation at a Ge(100)-water interface during an electrode potential triggered surface termination transition

“…1. The CV at the most acidic pH in Ar saturated electrolyte is consistent with previously published CVs, 18,21 with an anodic peak assigned to the formation of an oxide, 18,19,49 and a cathodic current…”

“…2) show a qualitatively similar trend as the spectra at pH 1.4. 42 The appearance of the mode at 1970 cm −1 , assigned to a Ge-H stretching mode, 19,21,22 occurred at the same potential as the negative difference absorbance in the region of the OH stretching modes was observed. The appearance of this peak also shifts to negative potentials with increasing pH, confirming the expectations on the basis of the CVs.…”

“…17 Germanium also possesses an interesting feature that allows the control the hydration shells with electrode potential: the germanium surface transforms from hydrophilic, OH-terminated to hydrophobic, H-terminated, when the applied electrode potential is sufficiently negative. [18][19][20][21] This surface transformation was so far investigated in acidic solution. 18,21,22 During the surface transformation, an intermediate surface state with both H and OH surface groups was inferred from a detailed analysis of the Ge-H stretching modes in between the initial OH-terminated state and the final H-terminated state.…”

“…[18][19][20][21] This surface transformation was so far investigated in acidic solution. 18,21,22 During the surface transformation, an intermediate surface state with both H and OH surface groups was inferred from a detailed analysis of the Ge-H stretching modes in between the initial OH-terminated state and the final H-terminated state. 21 Some authors treat liquid water at a solid-liquid and gas-liquid interface as a separate thermodynamic system from bulk water.…”

“…18,21,22 During the surface transformation, an intermediate surface state with both H and OH surface groups was inferred from a detailed analysis of the Ge-H stretching modes in between the initial OH-terminated state and the final H-terminated state. 21 Some authors treat liquid water at a solid-liquid and gas-liquid interface as a separate thermodynamic system from bulk water. 23 This interfacial water phase was investigated using x-ray reflectivity, [24][25][26] vibrational sum frequency generation (SFG) spectroscopy, 27,28 and atomic force microscopy.…”