Structure and dynamics of tetramethylthiourea adsorption on Au(111) studied by in situ scanning tunneling microscopy

We have examined tetramethylthiourea (TMTU) adsorption on Au(111) with cyclic voltammetry, capacitance measurements, and in situ scanning tunneling microscopy (STM), Capacitance measurements indicate that TMTU is adsorbed over a wide potential range. A capacitance peak at higher potentials rca. 650 mV vs SCE) is interpreted as arising from the displacement of TMTU by bisulfate/sulfate ions. High-resolution STM images show an ordered structure of TMTU on the gold surface. Three distinctive spots for each molecule are seen in the STM images. From these molecular resolution images a (3 x 3) structure for the TMTU adsorbate is deduced. Large changes are observed in the step and terrace morphology of the Au(111) substrate at potentials (+400 mV) slightly before the capacitance maximum assigned to TMTU desorption. The STM images indicate a slow etching of the gold surface at potentials slightly before the onset of TMTU desorption. This deduction has been further supported by inductively coupled plasma optical emission spectroscopy (ICP-OES) and atomic absorption spectroscopy (AAS), with gold being detected in electrolyte solution which had been in contact with the gold electrode polarized at +400 mV, The etching is seen in our images as a slow retreat of step edges, which have the same height as monoatomic gold steps. Upon subsequently lowering the potential to below 0.3 V, a slow advance of step edges is then seen and this is assigned to the deposition of complexed gold from solution. At more positive potentials, where TMTU desorption occurs, the formation of pits is seen, which expand and coalesce. These pits have a height of 0.12 +/- 0.02 nm, which is significantly less than that of a monoatomic step on a Au(lll) surface. This observation has been assigned to the dissolution of the TMTU film.