SPring-8, the large synchrotron radiation facility

Chemical Science

New structures of water adsorbed on Au(111) electrode surface

The two-dimensional structure of water molecules on a surface gives an insight into important mechanisms concerning electron and proton transfer or weak interactions of water molecules with surfaces. In order to understand the role of hydration water molecules on electrochemical double layers, the structures of hydration water molecules adsorbed on electrode surfaces under an electrode potential control were determined by in situ surface X-ray diffraction. A cyclic voltammogram (CV) of Cu UPD on Au(111) electrode in H₂SO₄ solution showed two kinds of redox peaks, which divide the whole potential range into three parts : A (more negative than Ia), B (between Ia and IIa) and C (more positive than IIa) as shown in Fig. 1.
In the potential range B (0.35 V) where Cu UPD honeycomb structure forms on Au(111) electrode in 0.5 M H₂SO₄ + 1mM CuSO₄ solution, we succeeded in obtaining the location of hydration water molecules on top of each UPD copper atom, so that a 1x1 closest pack oxygen adlayer is formed on the UPD copper surface, which is a new phase of water with a high density as shown in Fig. 2 and 3. Intensities in a non-integer reflection, (1/5,1/5, 0.2) and its equivalent reflections, developed in the region C, which measure the growth of 3^1/2x7^1/2 structure domains of bisulfate anion on the surface. The ball model in Fig.1 shows the best fit of the observation. In the region A, the strong intensity at (1/3,1/3,1.5) reflection continues to evolve even at the potential more negative than 100 mV, where multilayers of copper deposit epitaxially on Au(111). This indicates that copper and sulfate anion still form the similar 3^1/2x3^1/2 structure shown in the ball model in Fig.1.

provided by Masashi Nakajima and Masatoki Ito, Keio University

		Fig. 2. ORTEP drawing of the structure based on the final positional parameters and temperature factors for Cu UPD on Au(111) electrode in 0.5 M H2SO4 + 1mM CuSO4 solution. Electrode potential was 0.32 V(RHE).
Fig. 1. CV of Au(111) electrode in 0.5 M H2SO4 + 1mM CuSO4 at a scan rate of 50 mV/s. In situ STM images of adsorbed molecules on Au(111) electrode in 0.5 M H2SO4 + 1mM CuSO4 solution are shown at A and C ranges in the inset. STM (A range) : In situ STM of sulfate + UPD copper adsorbed on Au(111) electrode surface. The electrode potential was 0.35 V(RHE). It = 2.8 nA, Vbias = -65.6 mV. STM (C range) : In situ STM of HSO4- + H5O2+ adsorbed on Au(111) electrode surface. The electrode potential was 1.20 V(RHE). It = 4.2 nA, Vbias = -967.2 mV
		Fig. 3. Schematic model (top and side view) of UPD copper, hydration water molecule and sulfate anion on Au(111) electrode at the potential of 0.32 V(RHE).