Density Functional Calculations of Self-Diffusion and Au Island Nucleation on Au(001) in the Presence of a Cl Adlayer
Homoepitaxy on metal surfaces at electrochemical environment has been adopted as model system to investigate the dependence of growth modes on sample potential and metal deposition rate as well as the influence of anions e.g. halides. In operando surface X-ray scattering (SXS) experiments at the interface between an Au(001) electrode and HCl + HAuCl_4 containing electrolyte Krug et al. have observed that the growth mode depends on the electrode potential and Au ion concentration. Transitions from 3D to layer-by-layer and to step-flow growth mode were observed at sample potentials, where the unreconstructed substrate surface is stable. To understand the morphology and differences of growing structures during electrodeposition model calculations for atomistic processes accounting for coadsorbed atoms are required. In this work density functional theory (DFT) calculations have been employed to investigate self-diffusion of a Au adatom and nucleation of Au islands on Cl covered unreconstructed Au(001) terraces. As step-flow growth mode involves rapid mass transport on flat terraces, diffusion mechanisms of a Au adatom on c(sqrt2 × 2sqrt2)R45°-Cl and c(2 × 2)-Cl Au(001) surfaces have been calculated with Cl coverage of 1/2 and in the presence of a Cl vacancy in the adlayer structure. A prerequisite for calculating diffusion mechanisms is the search of global energy minimum configurations, our DFT calculations have shown that the Au_(ad)Cl_2 complex stays stable at different Cl coverages. It has been found that in case of the fully Cl covered Au(001) surfaces the diffusion energy barriers are ≥ 0.44 eV. In the presence of a neighboring Cl vacancy the diffusion barriers are drastically reduced to about half of the value for the fully Cl covered surface. Thus, Au self-diffusion on covered surfaces is promoted by Cl vacancies. DFT calculations addressing the initial steps toward Au island nucleation on c(2 × 2)-Cl covered Au(001) surfaces have been carried out. The atomic geometry of Au_n adatom structures for n ≤ 4, as well as their stability as a function of Cl chemical potential has been determined. A weakly bonded (AuCl_2)-chain as the most stable structure has been identified in case of Cl chemical potentials such that Cl vacancies are suppressed. In the range of Cl chemical potential, where Cl vacancies occur in equilibrium with the adlayer and bind to a Au adatom structure, significant changes in regard to the Au_(ad) -structure are induced. Moreover, the formation of a dimer from two monomers is accompanied by an energy gain between 0.06 – 0.23 eV. For the formation of Au trimer and tetramer structures the calculations suggest a similar tendency. Results indicate that on c(2 × 2)-Cl covered Au(001) surfaces the Au_(ad) -island nucleation is supported by the presence of vacancies in the Cl adlayer. The dependence of the diffusion energy barriers on sample potential is estimated from the variation of the dipole moment along the diffusion path. However, for more reliable estimations the double layer capacity and the potential of zero charge for the Au(001)/electrolyte interface are required. This work reveals the complex influence of the coadsorption of Cl atoms on the self-diffusion of Au adatom and the nucleation of Au islands. Our results refer to intralayer processes and nucleation on Cl covered terraces. The Cl adlayer strongly modifies the adsorption properties and energies of small Au clusters, which results in different diffusion mechanisms and Au cluster equilibrium structures compared to the clean surface. The concentration of halide vacancies in the Cl adlayer may play a decisive role for the self-diffusion and for the energetics of island nucleation.