Cu, Ag and Au adsorption on hematite: A comparative density functional study

ZUBIETA C.; FUENTE SILVIA A; AQUINO LINARES LUIS G; PATRICIA G. BELELLI; FERRULO RICARDO M

Congreso; VI San Luis Conference; 2018

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=4 eV) as implemented in the VASP (Vienna Ab-Initio Simulation Package) code. To model the atom-by-atom nucleation until obtaining Cu5, Ag5 and Au5 clusters on the hematite surface, we have made a detailed study of the atomic preferential adsorption site for each metal. On the Fe-terminated (0001) surface, whose layers are ordered according to Fe-O3-Fe-Fe-O3....., there are five adsorption sites: on-top of Fe, on-top of O, and three types of O-hollow sites according to the ion located symmetrically bellow the center of the O-hollow (with an Fe ion at the third layer, with an Fe ion at the fourth layer, or with an O ion at the fifth layer). 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. Cu and Ag atoms prefer to link on an O-hollow site (that one with an Fe ion lying at the fourth layer); the metal atom interactions occur through three O ions at 2.00 Å and 2.34 Å for Cu and Ag, respectively. For Au, the most stable site was on-top of Fe, with an Au-Fe distance of 2.51 Å. These results are as expected because Cu and Ag are more oxidizable metals than Au. The adsorption energies indicate that Cu atom binds strongly to the hematite surface with a value of -2.32 eV, Ag with a lower value of -1.38 eV, and Au atom with the weakest interaction (-0.97 eV). 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 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.