In situ STM study of the growth and dissolution processes on bimetallic Pd/Au(111) substrates and of ultrathin alloy layers on Au(111)
Résumé
In the present work, we use in situ Scanning Tunneling Microscopy (STM) to study different electrochemical processes related to bimetallic surfaces in the ultrathin film limit (i.e. between submonolayer coverage and few atomic planes in thickness). First, a PdAu bimetallic surface, which consists in Au(111) single crystal partially covered by monoatomic Pd islands, is used to directly compare the deposition and dissolution of one Ni monolayer on these two surfaces and get better insight into the influence of the substrate chemical nature on the heteroepitaxial processes. Our in situ STM investigations at the nanometer scale reveal the existence of a strongly selective Ni growth on Au(111) terraces in a wide potential range. Conversely, in a specific potential range, a Ni monolayer covering the entire bimetallic surface can be selectively dissolved from Pd islands. We show that the Ni-substrate interactions play a key role in this selectivity, and we estimate the binding energy of Ni to Pd to be 80 meV smaller than that of Ni to Au. Such experimental approach opens up perspectives to determine small differences in binding energies for other adsorbate/substrate couples. Second, we investigate the growth/dissolution of 2D monolayer thick NixPd1-x and NixAu1-x alloys on Au(111). We find that NixAu1-x alloy films are phase separated, in agreement with bulk thermodynamic data, but with a very small segregation characteristic scale (~ 1-3 nm), reflecting the interplay between strain relaxation in the Ni film and growth kinetics. As expected for bulk alloys, we find that NixPd1-x alloy films form solid solutions and we correlate the average inter-atomic distance of the alloy with the alloy Ni content. In addition, according to time-resolved STM measurements, we succeed in establishing a close correlation between the dissolution potential of a Ni atom and the number of its Pd neighbors. We also studied the dissolution process by Monte Carlo simulations, allowing us to support the conclusions derived from the experimental results.