Ordered Arrays of Size-Selected Oxide Nanoparticles

A bottom-up approach to produce a long-range ordered superlattice of monodisperse and isomorphic metal-oxide nanoparticles (NP) supported onto an oxide substrate is demonstrated. The synthetic strategy consists of self-assembling metallic NP on an ultrathin nanopatterned aluminum oxide template followed by a morphology-conserving oxidation process, and is exemplified in the case of Ni, but is generally applicable to a wide range of metallic systems. Both fully oxidized and core-shell metal-metal-oxide particles are synthesized, up to 3-4 nm in diameter, and characterized via spectroscopic and theoretical tools. This opens up a new avenue for probing unit and ensemble effects on the properties of oxide materials in the nanoscale regime

Growth of Ceria Nano-Islands on a Stepped Au(788) Surface

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm2) has been generated on the Au(788) surface by the postoxidation method. This reflects the periodic anisotropy of the template surface and has been ascribed to the pinning of ceria clusters and thus nucleation on the fcc domains of the herringbone reconstruction on the Au terraces. In contrast, the reactive evaporation method yields ceria islands elongated in [01-1] direction, i.e., parallel to the step edges, with high aspect ratios (~6). Diffusion along the Au step edges of ceria clusters and their limited step crossing in conjunction with a growth front perpendicular to the step edges is tentatively proposed to control the ceria growth under reactive evaporation conditions. Both deposition recipes generate two-dimensional islands of CeO2(111)-type O–Ce–O single and double trilayer structures for submonolayer coverages

Cerium Oxide Nanoclusters on Graphene/Ru(0001): Intercalation of Oxygen <i>via</i> Spillover

Cerium oxide is an important catalytic material known for its ability to store and release oxygen, and as such, it has been used in a range of applications, both as an active catalyst and as a catalyst support. Using scanning tunneling microscopy and Auger electron spectroscopy, we investigated oxygen interactions with CeO_<i>x</i> nanoclusters on a complete graphene monolayer-covered Ru(0001) surface at elevated temperatures (600–725 K). Under oxidizing conditions (<i>P</i>_O₂ = 1 × 10^–7 Torr), oxygen intercalation under the graphene layer is observed. Time dependent studies demonstrate that the intercalation proceeds <i>via</i> spillover of oxygen from CeO_<i>x</i> nanoclusters through the graphene (Gr) layer onto the Ru(0001) substrate and extends until the Gr layer is completely intercalated. Atomically resolved images further show that oxygen forms a <i>p</i>(2 × 1) structure underneath the Gr monolayer. Temperature dependent studies yield an apparent kinetic barrier for the intercalation of 1.21 eV. This value correlates well with the theoretically determined value for the reduction of small CeO₂ clusters reported previously. At higher temperatures, the intercalation is followed by a slower etching of the intercalated graphene (apparent barrier of 1.60 eV). Vacuum annealing of the intercalated Gr leads to the formation of carbon monoxide, causing etching of the graphene film, demonstrating that the spillover of oxygen is not reversible. In agreement with previous studies, no intercalation is observed on a complete graphene monolayer without CeO_<i>x</i> clusters, even in the presence of a large number of point defects. These studies demonstrate that the easily reducible CeO_<i>x</i> clusters act as intercalation gateways capable of efficiently delivering oxygen underneath the graphene layer