Abstract

Silicotungstic acid, α-H₄SiW₁₂O₄₀, modified Ag and Au surfaces were characterized. STM images showed that α-H ₄SiW₁₂O₄₀ spontaneously formed adherent, ordered monolayers on both Ag(100) and Ag(111). Under potential control, different ordered structures were observed and analyzed. Although STM images provide 2-D structural information on how these molecules arrange on Ag surfaces, the interaction between α-SiW₁₂O₄₀⁴⁻ and Ag surfaces can only be elucidated by means of X-ray reflectivity measurements. The oscillatory behavior is associated mainly with constructive and destructive interference of the X-rays by the planes of tungstens from the α-H₄SiW₁₂O₄₀ molecule. The Si-Ag layer spacing from the best fit to the S₄ model is 4.90 ± 0.02 Å. Only in the configuration where α-H₄SiW ₁₂O₄₀ is centered in the hollow site with the terminal W-O bonds aligned with the (10) and (01) substrate lattice axes are all the O atoms far enough from the first layer of Ag substrate, so that the shortest Ag-O bond length is not smaller than the sum of their covalent radii (2.00 Å). The bond lengths (with Ag-O = 2.06 Å) calculated from the interface structures determined in this study uniquely establish that α-H ₄SiW₁₂O₄₀ chemisorbs on Ag(100). The α-H ₄SiW₁₂O₄₀/Ag system was also characterized by in situ IR, ex situ Raman, XPS.

Passivation behavior of α-H₄SiW₁₂O₄₀ on Ag surfaces was studied electrochemically and compared with the electrochemical responses on Au surfaces. The results showed that on Au electrodes, α-SiW ₁₂O₄₀⁴⁻ behaved as a large anion which did not substantially inhibit solution redox response. In contrast, on Ag surfaces with the appropriate potential treatment, α-SiW₁₂O ₄₀⁴⁻ passivated the electrode toward subsequent solution redox chemistry. The appearance of surface-confined peaks of α-SiW ₁₂O₄₀⁴⁻ on Ag(111) also suggested the stronger interaction between α-SiW₁₂O₄₀ ⁴⁻ and Ag surfaces.

Multilayer-like features were observed on Ag surfaces when the potential was cycled negative of the onset of the first redox couple of α-H ₄SiW₁₂O₄₀. QCM measurements suggested that negative potentials attracted α-SiW₁₂O₄₀⁴⁻ near Ag surfaces. XPS data qualitatively indicated that more α-SiW ₁₂O₄₀⁴⁻ was on Ag surfaces after potential cycle treatment. The multilayer-like features of α-H₄SiW ₁₂O₄₀ on Ag surfaces were also suggested by X-ray specular reflectivity and STM measurements.