Electrochemical in situ STM study of Al and Ti-Al alloy electrodeposition on Au(111) from a room temperature molten salt electrolyte

The electrodeposition of Al and Ti-Al alloys on Au(111) from an acidic aluminium chloride-1-methyl-3-butylimidazolium chloride (AlCl3-MBImC) room-temperature molten salt electrolyte containing TiCl4 was studied by cyclic voltammetry and chronoamperometry and in particular, nucleation and growth processes were monitored in situ by means of electrochemical scanning tunneling microscopy (EC-STM) for the first time. Two underpotential deposition (UPD) states for Al were observed in the cyclic voltammogram at 0.51 and +0.22 V vs. Al/Al3+. At corresponding UPD conditions STM images show different nucleation behavior for Al: first a growth of clusters all over the Au surface, and later preferentially along the step edges leading to a step edge decoration. Potential step experiments imply progressive nucleation and growth and are evidenced by STM images obtained at different time intervals showing the growth of Al clusters of up to 8 nm diameter in size. After anodic dissolution of the deposited Al film, the appearance of circular holes of a size comparable with the Al clusters notably along Au step edges indicates Al-Au surface alloy formation. Tunneling current vs. bias voltage (I-U) curves of the Al clusters exhibit a reduced tunneling barrier height (Phi) of 0.23 eV. A distinct two-dimensional nucleation of Ti-Al clusters begins to occur at 0.5 V vs. Al/Al3+ exclusively on the Au terraces. They grow in the form of chains reaching a size of about 2-3 nm diameter in size, smaller in comparison with Al clusters. X-Ray photoelectron spectra of the potentiostatically deposited film under overpotential deposition (OPD) conditions confirm the formation of a Ti-Al alloy phase.