Structural and Electronic Properties of Au and Au₂on an MgO(100) Surface: A DFT Cluster Embedding Approach

Abstract

The structural and electronic properties of Au and Au2 adsorbed on five coordinated oxygen and magnesium regular terrace sites, on the neutral Fs color center (oxygen vacancy containing two electrons), and on the charged Fs + color center (oxygen vacancy containing one electron) of the MgO(100) surface have been investigated using a density functional theory cluster embedding approach. Two types of calculations have been performed. In one, we optimized and calculated the energetics of the systems using a generalized gradient approximation, and in the second, we used a combined approach, optimizing the geometries using a local density approximation and then performing a single point energy calculation using a generalized gradient approximation. Our studies also examine the effect of relaxation of the substrate. This was accomplished through two studies: first, both the MgO surface and the Au atoms were optimized, and second, the MgO surface atoms were kept fixed while the Au atoms were allowed to relax. We report the geometry, adsorption, and dimerization energies of the lowest energy and low energy isomers at each adsorption site, charge transfers from the MgO surface to the Au atoms in the lowest energy isomers, and compare our findings with previous theoretical studies. This comparison allowed us to establish in most of the cases limits for the adsorption energies, binding energies, and geometrical parameters at each adsorption site. For the case of Au2, for each adsorption site, our studies confirm the lowest energy isomer and most of the low energy isomers, previously reported, when both the surface and the Au2 are optimized. For most cases, the studies with frozen MgO surface atoms produced similar results as the studies when the MgO surface atoms were optimized. However, for the Mg site we found that the restrictions introduced by keeping the surface frozen favored the coexistence of an additional low energy isomer, and for the Fs + site, the restrictions favored the dissociation of the adsorbed Au2 dimer into two surface adsorbed Au atoms. In general, both fully generalized gradient and combined calculations produced similar results, though, in the case of the Fs site the combined calculations found another low lying isomer, and in the case of the Fs + site it predicted the dissociation of the adsorbed Au2 dimer, and inverted the energetic order of the lowest energy and low energy isomers. An increase in the charge transfer from the MgO surface to the Au atoms at the different adsorption sites according with the number of localized electrons at the adsorption sites was found. Finally, we found that the site with the largest adsorption and dimerization energies, and where an Au atom is most stable and can grow, is the neutral Fs center.

Catalytic Activity of Au and Au2on the h-BN Surface: Adsorption and Activation of O2

“…In addition, the presence of the vacancy and impurity point defects on the h-BN surface results in a drastic change in the charge state of the trapped Au and Au 2 and hence also influences the catalytic activity of the supported gold particles. Although it is well established that F-center defects in metaloxides can promote catalytic activity of gold clusters via the charge transfer, 19,35,74 to the best of our knowledge the similar effect for gold clusters supported on the h-BN surface has not been studied yet. In the case of O 2 adsorption on free Au, the Bader charge localized on O 2 is −0.19e.…”

AuNclusters (N=1--6)supported on MgO(100) surfaces: Effect of exact exchange and dispersion interactions on adhesion energies

“…One Au atom is bound to an oxygen surface atom at a distance of 2.166Å, while the dimer bond length remains practically unchanged (2.507Å) compared to the gas-phase structure (2.511Å), in good agreement with previous theoretical results. 11,49,52,[96][97][98] The ground state of both gas-phase and supported Au 2 is a singlet. Furthermore, due to the strong attractive interaction at the F s center, the most stable Au dimer configuration is tilted toward an Mg surface atom (67 • to the surface normal), with an elongated dimer bond length of 2.70 A, compared to the dimer supported on the pristine surface (see Fig.…”

Formation of Ag2, Au2 and AgAu particles on MgO(100): DFT study on the role of support-induced charge transfer in metal–metal interactions

“…Concerning the catalytic activity, it was observed that when the particles are supported, typically on an oxide surface, their electronic structure can be strongly affected by the support which in turns modifies the surface reactivity [8,9]. For example, it is well known that the interaction with an oxide can cause charging of Au aggregates [10][11][12][13][14][15][16][17][18][19]. When the supported particles are composed by two different metals, the composition and segregation properties are also important in determining their catalytic activity.…”

“…It presents different type of defects which are relatively simple to model. The deposition of coinage metal atoms and clusters on regular and defective MgO(1 0 0) surface was recently studied from DFT calculations [11][12][13][14][15][16][17][18][19]21]. Among the different type of defects present on terrace, the neutral O-vacancy (F s centers) has been widely analyzed.…”

“…This site is generated by the removal of a neutral O atom; as a consequence, two electrons are trapped inside the cavity. When Au particles are anchored on F s sites, a charge transfer occurs from the vacancy to the metal enhancing the metal-oxide interaction [10][11][12][13][14][15][16][17][18][19]. This process is important in this particular case because Au presents a high electron affinity.…”