Adsorption of small Au, Cu and Ag clusters on hematite. A comparative density functional study

CAROLINA ZUBIETA; RICARDO M. FERULLO; FUENTE SILVIA A; PATRICIA G. BELELLI

Montevideo

Congreso; XLII Congreso Latinoamericano de Químicos Teóricos de Expresión latina (QUITEL 2016); 2016

Universidad de la República Uruguay

In this work we have investigated the adsorption of very small Mn clusters (M = Cu, Ag and Au, with n = 1-5) on the Fe-terminated (0001) surface of -Fe2O3 (hematite). The main objectives were to identify preferred adsorption sites and geometries of deposited metal clusters, to explore the nature of the metal-support bonding, and to quantify nucleation energies. The calculations have been performed within density functional theory including an on-site Coulomb term (DFT+U; U=6 eV) as implemented in the VASP (Vienna Ab-Initio Simulation Package) code. To model the cluster growth on the surface, an ideal atom-by-atom nucleation process was simulated. The corresponding nucleation energy was defined as: Enucl = E(Mn/surf) ? E(Mn-1/surf) ? E(M), with n=2-5. It therefore corresponds to a sequential growth obtained by adding M atoms from the gas phase. In Fig. 1 the Enucl values are presented. Cu1 and Ag1 atoms prefer to interact with the surface at the O-hollow sites, while for Au1 the most stable site is on the top of surface Fe ions. The results show that Cu1, Ag1 and Au1 bind with adsorption energies of -2.31, -1.38 and -0.97 eV, respectively. From these adsorbed M1 states, the sequential growth was modeled by exploring the different possible isomers for each case and selecting the most stable one. Calculations evidenced that for the three metals, dimers and trimers (triangle-shaped) adsorbed tilted with respect to the hematite surface. Whereas Au4 adopts a planar structure, Cu4 and Ag4 are tetrahedron-shaped. For pentamers, the three metals take flattened geometries with all the M atoms directly interacting with surface ions. Concerning the total charge of adsorbed Mn clusters, Aun aggregates result in null or slightly negative values; for their part, Cun and Agn acquire positive values, with higher values for n=3-5 (0.8-1.3e). With respect to the total spin of adsorbed Mn, Cun and Agn present low values (< 0.05 B), except for the case of Cu2. Aun show an even-odd zigzag behavior as it was observed previously on goethite [1]. According to these results, the Enucl variations (Fig. 1) seem to be correlated mainly with the spin of each particular aggregate. Indeed, when an adsorbed Mn particle with an odd number of atoms interacts with an M atom coming from the gas phase a strong interaction is expected. This indeed is the case for Au. For its part, Ag shows almost constant values of Enucl in line with the corresponding states of spin. Finally, Cu presents a maximum absolute value of Enucl at Cu3, when adsorbed Cu2 (with 0.3 B) interacts with an incoming Cu atom.