Templating the near-surface liquid electrolyte: In situ surface x-ray diffraction study on anion/cation interactions at electrified interfaces

We present an in situ surface x-ray diffraction study on the structuring impact of an anion-modified electrode surface on the near-surface liquid electrolyte. This templating effect of the so-called inner Helmholtz layer of specifically adsorbed anions affects not only the interfacial structure parallel to the surface normal by layering the liquid in the near-surface regime but induces moreover a lateral ordering of water dipoles and solvated counter ions in the so-called outer Helmholtz layer. In this respect, we observe a symmetry transfer from the inner Helmholtz layer into the liquid electrolyte next to the electrode surface. Our prototypical model system is a Cu(100) surface on which chloride adsorbs under the formation of a simple $p\left(1\times 1\right)$ adlayer phase (this notation refers to the fcc unit cell of Cu) that serves as structural template for the coadsorption of monovalent potassium and hydronium cations from the acidified supporting electrolyte. A layer of interfacial water is interpreted as a part of the remaining solvation shell of potassium cations in the outer Helmholtz layer.

Figure 1

(Color) (a) Schematical drawing of the electrochemical SXRD cell. $k_{\text{i}}$ and $k_{\text{f}}$ are the wave vectors of the incoming and diffracted x-rays, respectively; (b) picture of the copper sample in the hanging droplet configuration; (c) cyclic voltammogram (CV) of Cu(100) in the 5 mM ${\text{H}}_2{\text{SO}}_4/10\text{mM}\text{KCI}$ electrolyte indicating the stability range of the $p\left(1\times 1\right)\text{-Cl}$ chemisorption layer (inner Helmholtz layer); CV obtained in the in situ SXRD cell.

Figure 2

(a) Scanning tunnel microscope image of the Cu(100) surface morphology in the presence of the $p\left(1\times 1\right)\text{-Cl}$ adlayer (inner Helmholtz layer), T1–T3 indicate different terraces separated by monatomically high substrate steps, $13\text{nm}\times 13\text{nm}$, $I_{\text{t}}=5\text{nA}$, $U_{\text{bias}}=-312\text{mV}$; (b) atomic structure of the $p\left(1\times 1\right)\text{-Cl}$ adlayer on Cu(100), $4.2\text{nm}\times 4.2\text{nm}$, $I_{\text{t}}=0.67\text{nA}$, $U_{\text{bias}}=81\text{mV}$.

Figure 3

(a) Experimental data and best fit of the $\left(00L\right)$ specular rod and the $\left(20L\right)$ crystal truncation rod, $E_{\text{sample}}=+150\text{mV}$ (CTR: crystal truncation rod; ADR: adlayer rod); (b) experimental data and best fit of the $\left(10L\right)$ and $\left(12L\right)$ adlayer rods in comparison with model (dashed line) proposed by Gründer et al.,[11] $E_{\text{sample}}=+150\text{mV}$. The unit $2\pi /c$ is the length of the momentum transfer vector perpendicular to the surface, $q_z$, with $c=a_{\text{Cu}}=3.61\text{Å}$ as the lattice constant of Cu. The inset indexes the reciprocal lattice points in the $\left(HK0\right)$ plane with reference to the conventional fcc unit cell of copper.

Figure 4

(Color) Full 3D structure model of the metal/electrolyte interface of Cu(100) in; (a) out-of plane structure (side view); (b) in-plane structure (top view). The dashed lines indicate the size and position of the Cu bulk unit cell. HML: Helmholtz layer.