Interaction of Au with CeO2(111): A photoemission study

In Situ Spectroscopy and Microscopy Insights into the CO Oxidation Mechanism on Au/CeO2(111)

In this work, we prepared and investigated in ultra-high vacuum (UHV) two stoichiometric CeO₂(111) surfaces containing low and high amounts of step edges decorated with 0.05 ML of gold using synchrotron-radiation photoelectron spectroscopy (SRPES) and scanning tunneling microscopy (STM). The UHV study helped to solve the still unresolved puzzle on how the one-monolayer-high ceria step edges affect the metal-substrate interaction between Au and the CeO₂(111) surface. It was found that the concentration of ionic Au⁺ species on the ceria surface increases with increasing number of ceria step edges and is not correlated with the concentration of Ce³⁺ ions that are supposed to form on the surface after its interaction with gold nanoparticles. We associated this with an additional channel of Au⁺ formation on the surface of CeO₂(111) related to the interaction of Au atoms with various peroxo oxygen species formed at the ceria step edges during the film growth. The study of CO oxidation on the highly stepped Au/CeO₂(111) model sample was performed by combining near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), UHV-STM, and near-ambient-pressure STM (NAP-STM). This powerful combination provided comprehensive information on the processes occurring on the Au/CeO₂(111) surface during the interaction with CO, O₂, and CO + O₂ (1:1) mixture at conditions close to the real working conditions of CO oxidation. It was found that the system demonstrates high stability in CO. However, the surface undergoes substantial chemical and morphological changes as the O₂ is added to the reaction cell. Already at 300 K, gold nanoparticles begin to grow using a mechanism that involves the disintegration of small gold nanoparticles in favor of the large ones. With increasing temperature, the model catalyst quickly transforms into a system of primarily large Au particles that contains no ionic gold species.

Magnetism and plasmonic performance of mesoscopic hollow ceria spheres decorated with silver nanoparticles

We investigate the role of interfaces and surfaces in the magnetic and surface enhanced Raman spectroscopy (SERS) properties of CeO2 hollow spheres decorated with Ag nanoparticles (H-CeO2@Ag). The composites, H-CeO2@Ag, were synthesized using a newly developed two-step process. The CeO2 hollow sphere diameter ranges from 100 nm to 2 μm and the grafted Ag nanoparticle (NP) size varies from 5 to 50 nm with a controllable coverage ratio. Spectroscopic and microscopic characterization confirms the formation of an interface between the Ag and ceria and shows different charge rearrangements occurring at both the interface and the surface. Room temperature ferro-magnetism was observed in all composites, and is associated mostly with ceria surface defects. A strong SERS effect was reported with a detection limit down to 10-14 M for the rhodamine 6G analyte. Scanning transmission electron microscopy and electron energy loss spectroscopy investigation reveals that hot-spots are associated with the silver NP surfaces and also with the Ag/CeO2 interface. This interfacial hot spot occurs for metallic particles above 30 nm and is strongly red shifted with respect to the Ag surface plasmon. The strong SERS activity is then attributed to the presence of several types of hot-spots and the geometrical features (buoyant hollow sphere and size dispersion) of the composite.

Interface interactions and enhanced room temperature ferromagnetism of Ag@CeO2 nanostructures

Enhancement of room temperature ferromagnetism (RTFM) has been achieved with core-shell metal-oxide nanoparticles (Ag@CeO₂). To enhance the magnetic properties, interfacial charge transfer is achieved via the formation of a core-shell interface. Furthermore, by varying the shell thicknesses, additional control of the RTFM can be obtained. The Ag@CeO₂ core-shell nanoparticles are synthesized successfully via a two-step method. Ag nanoparticles (NPs) are first synthesized on a TiO₂ substrate by a thermally assisted photoreduction method, and then CeO₂ NPs are deposited on the surface of Ag NPs by chemical reduction. No surfactants or organic compounds are used during the synthesis. At the interface between the core and the shell, electron transfers from the Ag-p orbital to the Ag-d and Ce-f orbitals are evidenced by X-ray absorption spectroscopy and electron energy loss spectroscopy. Such interfacial charge transfer results in enhanced room temperature ferromagnetism in the Ag@CeO₂ core-shell NPs compared to the magnetism arising for bare Ag or CeO₂ NPs. This study suggests that tailoring the interface, the surface and their coupling in nanostructured metal-oxide core shell nanoparticles is an effective way to enhance their magnetic properties.

Controlling the physics and chemistry of binary and ternary praseodymium and cerium oxide systems

Binary and ternary PrO_xand CeO_xfilms grown on Si(111) are most versatile systems available in a variety of stoichiometries and surface structures.

Titration of Ce3+ ions in the CeO2(111) surface by Au adatoms

The role of surface and subsurface O vacancies for gold adsorption on crystalline CeO2(111) films has been investigated by scanning tunneling microscopy and density functional theory. Whereas surface vacancies serve as deep traps for the Au atoms, subsurface defects promote the formation of characteristic Au pairs with a mean atom distance of two ceria lattice constants (7.6 Å). Hybrid density functional theory calculations reveal that the pair formation arises from a titration of the two Ce3+ ions generated by a single O vacancy. The Au-Ce3+ bond forms due to a strain effect, as the associated charge transfer from the spacious Ce3+ into the adgold enables a substantial relaxation of the ceria lattice. Also the experimentally determined Au-pair length is reproduced in the calculations, as we find a Ce3+-Ce3+ spacing of two ceria lattice parameters to be energetically preferred. Single Au atoms can thus be taken as position markers for Ce3+ ion pairs in the surface, providing unique information on electron-localization phenomena in reduced ceria.

Epitaxial Cubic Ce2O3 Films via Ce-CeO2 Interfacial Reaction

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO2 films. The method yields bulk-truncated layers of cubic c-Ce2O3. Compared to CeO2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce2O3(111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO2(111). In addition, c-Ce2O3 layers represent an experimental realization of a normally unstable polymorph of Ce2O3. During interfacial reaction, c-Ce2O3 nucleates on the interface between CeO2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal-oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.

Interaction of tungsten with CeO2(111) layers as a function of temperature: a photoelectron spectroscopy study

The interaction of tungsten with CeO(2)(111) layers grown on Cu(111) was studied in the temperature range between 300 and 870 K by photoelectron spectroscopy of the core levels and resonant valence band spectroscopy. The interaction was found to be very strong even at 300 K, leading to the formation of cerium tungstate Ce(6)WO(12) in which the metal atoms were in Ce(3+) and W(6+) chemical states. The growth was limited by the diffusion of W atoms into the ceria layer, so subsequent tungsten deposition led to formation of W suboxides with consecutively lower chemical oxidation states, i.e. W(4+), W(2+) and metallic W(0) with an almost negligible contribution of W(5+). Step-wise annealing of the layer showed that due to stimulated diffusion of tungsten into ceria at higher temperature, Ce(6)WO(12) was formed more easily. Larger W overlayer thicknesses needed higher annealing temperature to promote diffusion. The thickest sample studied, 1.4 nm W/CeO(2), was transformed by annealing to 870 K to the Ce(6)WO(12)/W system with a tungsten monoxide (WO) interface, whereas the rest of the tungsten was converted to the W(6 + ) oxidation state.

Atomic resolution non-contact atomic force microscopy of clean metal oxide surfaces

In the last two decades the atomic force microscope (AFM) has become the premier tool for topographical analysis of surface structures at the nanometre scale. In its ultimately sensitive implementation, namely dynamic scanning force microscopy (SFM) operated in the so-called non-contact mode (NC-AFM), this technique yields genuine atomic resolution and offers a unique tool for real space atomic-scale studies of surfaces, nanoparticles as well as thin films, single atoms and molecules on surfaces irrespective of the substrate being electrically conducting or non-conducting. Recent advances in NC-AFM have paved the way for groundbreaking atomic level insight into insulator surfaces, specifically in the most important field of metal oxides. NC-AFM imaging now strongly contributes to our understanding of the surface structure, chemical composition, defects, polarity and reactivity of metal oxide surfaces and related physical and chemical surface processes. Here we review the latest advancements in the field of NC-AFM applied to the fundamental atomic resolution studies of clean single crystal metal oxide surfaces with special focus on the representative materials Al(2)O(3)(0001), TiO(2)(110), ZnO(1000) and CeO(2)(111).