SURFACE-CHEMISTRY OF CU(100) IN ACIDIC SULFATE-SOLUTIONS

The surface chemistry of Cu(100), in H2SO4 and acidic K2SO4 solutions, has been studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). Experiments were conducted in an electrochemical cell coupled directly to a UHV surface analysis chamber. Structures formed on the Cu(100) surface emersed (removed) from sulfate solutions were the principal focus of this study. Structures and coverages were primarily dependent on sulfate concentration and the presence of coadsorbates such as Cl and K+. Emersion of Cu(100) from 1 mM H2SO4 resulted in a Cu(100)(2 × 2)-SO2-4 adlattice with a 1 4 coverage of sulfate. A 1 5 coverage Cu(100)( 5 × 5)R26.6°-SO2-4 structure formed upon emersion mM H2SO4 containing trace Cl- contaminants, and a 1 3 coverage Cu(100)(2 × 100)-SO2-4 structure formed following e from 10 mM H2SO4. Emersion of Cu(100) from 1 mM K2SO4 (pH = 3.6) resulted in a Cu(100)(4 × 2 5)-SO2-4, K+ surface structure, with K+ and SO2-4 coadsorbed in a 1:1 stoichiometry, each at 1 4 coverage. No significant potential-dependent variatio in surface structure or coverage was observed when the electrode was emersed from H2SO4 at potentials in the double-layer charging region. For emersion from 1 mM K2SO4 (pH = 3.6), a reduction in K+ coverage along with a change in the surface structure to a (2 × 2) occurred at positive potentials. Comparisons were made of sulfate adsorption on the low-index planes of Cu using a Cu single crystal polished on three different faces to the (111), (110) and (100) planes. This electrode was emersed from 1 mM K2SO4 (pH = 3.6) at several potentials. A c(8 × 2) and a diffuse (1 × 1) LEED pattern were observed on Cu(110) and Cu(111), respectively. The K+ and SO2-4 coverages differed appreciably between the three surfaces. Significant differences were observed in the thermal desorption spectra of Cu(100) emersed from H2SO4 and K2SO4. Water desorption occurred following emersion from H2SO4, but not after emersion from K2SO2. For Cu(100) emersed from 1 mM H2SO4 and l mM K2SO4 (pH = 3.6), sulfate species decomposed into SO2(g) and adsorbed oxygen at 670 and 780 K, respectively for the two solutions. This behavior indicates sulfate stabilization by coadsorbed K+ following emersion from K2SO4. Lastly, TDS of an Au(100) surface following emersion from 1 mM H2SO4 indicated that sulfate is considerably more strongly bound to Cu(100). © 1990.