The brightest Au(111) surface
- Weronica Linpé (Lund University, Sweden)
- Monday 30 March 2020
- Hotel NH Noordwijk Conference Centre Leeuwenhorst
W. Linpe1, J. Evertsson1, G. Abbondanza1, A. Larsson1, G. Harlow1, J. Zetterberg2, L. Rämisch2, S. Pfaff2, L. Jacobse3, A. Stierle3 and E. Lundgren1
1Division of Synchrotron Radiation Research, Lund University, SE-221 00, Sweden
2Division of Combustion Physics, Lund University, SE-221 00 Lund, Sweden
3Photon Science, DESY, Notkestrasse 85, Hamburg 22607, Germany
In situ studies of surfaces during a reaction are essential for understanding the interaction between the surface and the surrounding environment. Cyclic Voltammetry (CV) has been extensively and successfully used to understand reactions occurring at the surface during electrochemical reactions. The measurements provide information on the overall reaction between the sample surface and the liquid electrolyte but contain little information regarding structural changes occurring on the sample surface during the CV. To characterize the surface changes during the reaction, in situ tools which can penetrate the liquid environment and probe the electrode surface are required [1, 2]. Recently, we have used 2D Surface Optical Reflectance (2D-SOR) to follow the surface changes of a model catalyst during a catalytic reaction [3, 4]. We have also demonstrated that 2D-SOR can track the surface changes of the electrode during a CV measurement , however no atomistic changes can be obtained from 2D-SOR. To this end, we have combined 2D-SOR and High Energy Surface X-Ray Diffraction (HESXRD) at the Swedish beamline P21.2 at PetraIII, Desy, Hamburg, to study a Au(111) surface during CV in 0.1 M HClO4 and 0.05 M H2SO4 electrolytes.
The Au(111) crystal is known to have a surface (Herringbone, HB) reconstruction [6, 7]. In electrochemistry, this reconstruction is observable with SXRD as a superlattice rod at low applied potentials , while only a (1x1) bulk terminated surface is observed by the presence of the Crystal Truncation Rods (CTRs) at higher potentials. In addition, the applied potential will induce different reactions on the surface, such as oxidation and reduction, as well as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) at higher and lower potentials respectively [9-11]. A better understanding of these reactions may promote more efficient electrolyzers and fuel cells, which are essential devices for a future sustainable society.
In the first experiments, we measured 2D-SOR and HESXRD during 10 cycles of the CV in 0.1 M HClO4. The measurements show that the 2D-SOR reflectance decreases as the potential is increased and oxide formation starts. As the potential reaches the reduction peak, the reflectance once again increases since the oxide is reduced. The HESXRD measurements showed that the HB signal appeared at low potentials when the oxide was reduced and was present when the 2D-SOR intensity was highest. The appearance of the HB could also be correlated to a decrease in the CTR intensity, which is attributed to the mixed FCC and HCP sites in the HB reconstruction, inducing destructive interference. The CTR signal could be shown to follow the 2D-SOR and CV quite well, except for the period when the HB reconstruction is present. This indicates that HESXRD and 2D-SOR can both detect the surface changes of the sample as a CV is scanned, while the 2D-SOR images the overall surface changes, the HESXRD investigates the change at an atomistic scale.
In a second set of experiments, using a 0.1 M HClO4 electrolyte, we increased the potential in steps, and at each step we performed a full rotational scan. The detector images in such a scan can be combined into a single image depicting a large part of reciprocal space. In this way, new structures such as surface reconstructions or oxide formation can be detected. It was shown that the HB is lifted at potentials above 0.5 V, and that the CTRs becomes increasingly weaker at higher potentials, indicating roughening of the surface. At the highest potential (2.2V) above the onset of the OER, clear Au powder rings were observed, suggesting either an extremely rough powder-like surface or re-deposited Au from dissolved Au in the electrolyte. The detector images showed no new scattering features, thus no Au oxide formation or any chloride induced structure was detected. Finally, we will also report on the usefulness of 2D-SOR to detect X-ray induced beam damage, which may be significant in a water environment .
We have shown that the Au(111) surface is brightest when the HB is present on the surface. Further, the HESXRD data reveals no oxygen or chloride induced structures indicating that additional experiments with surface chemical sensitivity should be performed.
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