Au/TiO2(110) Interfacial Reconstruction Stability from ab Initio

Bulk metallic Au is chemically inert and catalytically inactive as a consequence of combination of valence d orbitals and diffused valence s and p orbitals. Recently, Au nanoparticles have been found to be catalytically active when supported on metal oxides such as TiO2, SiO2, Fe2O3, Co3O4, NiO, Al2O3, MgO, etc. 1 6 For example, Au nanoparticles supported on a TiO2(110) surface demonstrate catalytic activity to promote the reaction between CO and O2 to form CO2 at T < 40 K with 3.5 nm Au nanoparticles maximizing activity. The catalytic activity is remarkably sensitive to the support material, Au particle size, and Au support interaction; in addition, the reaction mechanism of CO oxidation over Au/TiO2 system remains under debate. 9 High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) have characterized the atomic structure of nanocrystal interface. However, the atomic structure of Au/TiO2 interface is difficult to determine in HRTEM image simulations due to several issues, such as the thickness of nanoparticles and metal oxide substrates are not determined, the positions of atoms in the direction parallel to the electron beam are not determined, and the very low contrast for oxygen atoms. New HRTEM experiments observed Au nanoparticles on TiO2(110) surfaces with both the Au(111) and the Au(100) epitaxies, with the Au(111) epitaxy more frequently observed than Au(100). Their analysis with HAADF-STEM analyzed the reconstructed interface of epitaxial Au(111) sitting on a TiO2(110) 1 2 surface and extracted important geometric information such as interlayer separations, the presence of Au in the interface of a 1 2 reconstruction, and estimates of the work of adhesion. Density functional theory (DFT) calculations have studied the optimum size and stable adsorption of Au nanoparticles on rutile TiO2(110). A single Au atom is energetically favorable on the site above 5-fold coordinated (5c) Ti atom on a stoichiometric TiO2 surface 14 and is most stable on the 2-fold coordinated (2c) bridging-O vacancy site on a reduced surface. 17 Oxygen vacancies cause a stronger binding of Au atoms, nanoclusters, 21 and nanorows to the reduced TiO2 surface than to the stoichiometric surface. Apart from the stoichiometric and reduced TiO2 surfaces, Shi et al. found the O-rich interface is the most stable at low temperature of catalytic reaction after examining the Au-rod/TiO2(110) in the orientation Au(111)// TiO2(110) with different interface stoichiometry and various rigid-body translations. Recently, Shibata et al. examined two and nine Au(110) atomic layers supported on reduced TiO2(110) and demonstrated that both the atomic and the electronic structure of two-layer Au are reconstructed, while the lattice coherency decays rapidly across the interface for nine-layer Au. We compare different Au/TiO2 interfaces: Au(111)//TiO2(110) and Au(100)//TiO2(110), with and without bridging oxygen, Au(111) on 1 2 added-row TiO2(110) reconstruction, and Au(111) on a new proposed 1 2 TiO reconstruction.We use the newly reformulated density functional theory energy density method to evaluate energy for each atom in the interfacial reconstruction. This provides insight into interfacial stability from the changes in atomic energy from the formed interface and corrects for spurious errors in the work of adhesion from the remaining free surfaces in the computational cell. The new information of atomic energies extracted from density functional theory shows the response to bonding environment changes in interfaces. The comparison with experimental geometry and work of adhesion allows us to validate our predicted structures.