Self-Assembled Monolayer of Polyoxometalate on Gold Surfaces: Quartz Crystal Microbalance, Electrochemistry, and in-Situ Scanning Tunneling Microscopy Study

Abstract

A new kind of inorganic self-assembled monolayer (SAM) was prepared by spontaneous adsorption of polyoxometalate anion, AsMo11VO404-, onto a gold surface from acidic aqueous solution. The adsorption process, structure, and electrochemical properties of the AsMo11VO404- SAM were investigated by quartz crystal microbalance (QCM), electrochemistry, and scanning tunneling microscopy (STM). The QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at −20 KJ mol-1. The maximum surface coverage of the AsMo11VO404- SAM on gold surface was determined from the QCM data to be 1.7 × 10-10 mol cm-2, corresponding to a close-packed monolayer of AsMo11VO404- anion. The analysis of the voltammograms of the AsMo11VO404- SAM on gold electrode showed three pairs of reversible peaks with an equal surface coverage of 1.78 ×10-10 mol cm-2 for each of the peaks, and the value was agreed well with the QCM data. In-situ STM image demonstrated that the AsMo11VO404- SAM was very uniform and no aggregates or multilayer could be observed. Furthermore, the high-resolution STM images revealed that the AsMo11VO404- SAM on Au(111) surface was composed of square unit cells with a lattice space of 10−11 Å at +0.7 V (vs Ag|AgCl). The value was quite close to the diameter of AsMo11VO404- anion obtained from X-ray crystallographic study. The surface coverage of the AsMo11VO404- SAM on gold electrode estimated from the STM image was around 1.8 × 10-10 mol cm-2, which was consistent with the QCM and electrochemical results.