In Situ STM and Impedance Spectroscopy Study of 4,4'-Bipyridine Adsorption on Cd(0001) and Sb(111) Single Crystal Electrode Surfaces

Molecular self-assembly at solid surfaces, resulting in the formation of the nanostructures with well-controlled properties and functionality, reveals fascinating perspectives in science and technology at nanoscale [1,2]. For instance, the smart tailoring of the structural properties of the functionalised electrodes attracts a lot of attention due to an ability to trigger the specific electrochemical processes.

In situ STM and impedance spectroscopy methods have been used for the study of 4,4’-bipyridine (4,4’-BP) adsorption at the electrochemically polished Cd(0001) and Sb(111) single crystal electrodes from weakly acidified Na₂SO₄ (pH ≈5.5÷ 6.0) supporting electrolyte solution.
The Cd(0001) and  Sb(111) electrodes were electrochemically polished in a heated solution of concentrated phosphoric acid and Milli Q⁺ water and in a KI + HCl aqueous solution, respectively. After polishing the electrodes were submerged under cathodic polarisation (E= -1.0 V vs. Ag/AgCl in sat. KCl for Cd(0001) and E= -0.8 V for Sb(111)) into electrolyte solution, saturated with Ar (95%) and H₂ (5%) gas mixture. In the case of Cd(0001) the studied electrolyte was 1.5 mM 4,4’-BP + 0.1 M Na₂SO₄ + 1×10^-5 M H₂SO₄ aqueous solution. For studying the adsorption process at Sb(111) 3 mM 4,4’-BP + 0.5 M Na₂SO₄ + 3×10^-4 M H₂SO₄ aqueous solution was used.
The atomic resolution for electrochemically polished Cd(0001) and Sb(111) electrodes has been obtained and the in situ STM data show that there is no quick surface reconstruction process. The surface structure is stable within the potential region investigated [3,4].

Within the region of maximal adsorption, the capacitance pits in the differential capacitance vs. electrode potential curves have been observed. In the case of Sb(111) the adsorption of 4,4’-BP is a very slow process, forming almost irreversible capacitance maximum at more anodic electrode potential. For the Cd(0001) electrode very pronounced capacitance peak appears at the cathodic potential, E = -1.2 V, where the compact adsorption layer formation takes place.

From the in situ STM data it has been found that 4,4’-BP  adsorbs in a form of a stripe pattern on cadmium and antimony single crystal electrodes, similarly to previously studied Bi(111) electrode surface [5]. In the case of electrochemically polished Cd(0001) single crystal electrode the pattern appears at E = -1.25 V (Fig. 1) and completely disappears at E = -1.15 V. In the case of electrochemically polished Sb(111) single crystal electrode the pattern appears at E = -0.7 V and disappears at E= -0.6 V.

Acknowledgements:The present study was supported by the Estonian Center of Excellence: High technology materials for sustainable development, by the Estonian Ministry of Education and Research project SF0180002s08, Estonian Science Foundation project ETF8786 and by graduate school „Functional materials and processes” (European Social Fund, project 1.2.0401.09-0079).

References:
1. L.-J. Wan, Acc. Chem. Res., 39 (2006), pp. 334–342
2. T. Wandlowski, Phase Transitions in Two-dimensional Adlayers at Electrode Surfaces: Thermodynamics, Kinetics, and Structural Aspects, A.J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry, Wiley VCH, Weinheim (2002)
3. P. Pikma, V. Grozovski, H. Kasuk, E. Lust, In situ STM studies of Cd(0001) electrode in aqueous electrolyte solutions, Surf. Sci (submitted)
4. V. Grozovski, S. Kallip. E. Lust, Surface Science, 613(2013) 108-113.
5. V. Grozovski, V. Ivaništšev, H. Kasuk. T. Romann, E. Lust, Electrochimica Acta, 120 (2014) 86-95.