Layer-by-layer growth of zinc during electrodeposition on Au(111) from a room temperature molten salt

The initial stages of electrochemical phase formation of Zn on Au(111) in a solution of AlCl3-MBIC (58:42) containing 1 mmol l(-1) of Zn(II) have been investigated by in situ electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS). In the underpotential range (UPD) three successive Zn monolayers are clearly resolved by STM which are correlated with reduction peaks in the cyclic voltammogram at 300, 100 and 10 mV vs. Zn(II)/Zn, respectively. This layer-by-layer growth mechanism seems to continue into the bulk deposition regime. The thickness of the Zn layers in the UPD and OPD range as determined from STM measurements has a value of 2.4 +/- 0.2 Angstrom, with the exception of the first UPD Zn monolayer which is 2.2 +/- 0.2 Angstrom thick. Measurements of the effective tunneling barrier phi by STS indicate no significant variation of the interfacial electronic structure at anodic potentials, with the exception of a clearly reduced value of phi similar to0.7 eV for the first monolayer. These observations are indicative of Zn-Au surface alloying. This is strongly supported by STM images of the Au(111) substrate taken after dissolution of bulk Zn deposits. Surface alloying is manifested by holes with a depth of 1 or 2 monolayers in the Au(111) terraces.