Abstract
Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO2 films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO2 films as well as the (1̅11) surface of monoclinic ZrO2. STM shows decreasing cluster size, indicative of increasing metal–oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO2/Pt3Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal–oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO2, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO2 film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between −1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO2, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO2{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces.
Highlights
ZMirectoanl iacliunsteterrfascessuaprpeoirntteedresotningzfirocrohneiater(oZgerOne2o)usancdatalmyseitsa1l−−4 and important for solid oxide fuel cells (SOFCs)[5] and oxygen gas sensors.[6]
All experiments were performed in two interconnected ultrahigh vacuum (UHV) chambers: one mainly for sample preparation and the other for scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED)
We prepared ultrathin ZrO2 films different from those described in ref 9, i.e., by annealing at slightly higher temperature (≈950 °C instead of 850−900 °C)
Summary
ZMirectoanl iacliunsteterrfascessuaprpeoirntteedresotningzfirocrohneiater(oZgerOne2o)usancdatalmyseitsa1l−−4 and important for solid oxide fuel cells (SOFCs)[5] and oxygen gas sensors.[6] In SOFCs and zirconia-based gas sensors, zirconia (with dopants such as yttria, named yttria-stabilized zirconia, YSZ) is used as an electrolyte. Zirconia is sandwiched between the cathode and anode materials, which are often porous metals. These applications rely on the favorable properties of zirconia: high mechanical strength, high melting point (2983 K), electronic insulation with a large band gap (≈5 eV) even in doped form, and dopant-induced oxygen ion conductivity at high temperature (≳600 °C). Despite the importance for applications, atomic-scale studies of zirconia surfaces and metal−ZrO2 interfaces are very rare, mainly because most conventional surface sensitive probing techniques require electronic conductivity. In order to overcome this shortfall, ultrathin zirconia films were grown by reactive evaporation of Zr onto Pt surfaces.[7,8]
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