The morphology and interfacial energetics of vapor-deposited Cu on slightly reduced CeO 2 (111) surfaces at 300 K have been studied using single crystal adsorption calorimetry (SCAC), He + low-energy ion scattering spectroscopy (ISS), Xray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Copper grows as three-dimensional nanoparticles with a density of ∼10 13 particles/cm 2 on CeO 2−x (111) (x = 0.05, 0.1, and 0.2). The initial heat of adsorption of Cu decreased with the extent of reduction, showing that stoichiometric ceria adsorbs Cu more strongly than oxygen vacancies. On CeO 1.95 (111), the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Cu clusters on stoichiometric steps, followed by the Cu particles spreading onto less favorable sites (step vacancies and terraces). Above ∼0.1 ML (>0.8 nm in diameter), the Cu adsorption energies showed no variation with extent of ceria reduction: the heat of adsorption increased slowly with coverage (particle size) due to the formation of more Cu−Cu bonds per adatom as the size grows, finally approaching the heat of sublimation of bulk Cu by 3.5 ML (2.5 nm). The adhesion energy of Cu(solid) to CeO 1.95 (111) was found to be 3.52 J/m 2 for 2.2 nm diameter particles, decreasing slightly with the extent of reduction. The Ce 3d XPS line shape showed an increase in the Ce 3+ /Ce 4+ ratio with Cu coverage, corresponding to donation of at most ∼0.17 and 0.06 electrons per Cu atom to CeO 1.95 (111) and CeO 1.8 (111), respectively. INTRODUCTIONHeterogeneous catalysts are generally composed of late transition metal nanoparticles dispersed over high surface area oxide supports. The interaction of the supported metal and underlying oxide can greatly influence catalytic properties such as long-term sinter resistance, activity, and selectivity. To improve our understanding of how the choice of metal and support can influence catalytic properties, detailed studies of model systems, where metal atoms are vapor deposited onto single crystal oxide supports, are often employed. With these model systems, the structure of the support surface, the size of the metal particles, and surface cleanliness can be better controlled. 1−5 Studies of this type provide the basic understanding necessary for the intelligent design of new, more efficient, and greener catalysts. Here we apply that approach to study model Cu/CeO 2 catalysts consisting of Cu nanoparticles grown by vapor deposition on CeO 2 (111) surfaces with controlled extents of reduction.We study the energies of the Cu atoms in this system using single crystal adsorption calorimetry (SCAC). This method directly measures the adsorption energy of the incoming metal atoms as they bind to the oxide surface, and to metal nanoparticles on that surface as they grow in size. 6−11 The Cu nanoparticle morphology is characterized using ion scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS). Adsorption energies measured using SCAC along with detailed adsorbate structura...

Many catalysts consist of late transition metal nanoparticles dispersed across oxide supports. The chemical potential of the metal atoms in these particles correlate with their catalytic activity and long-term thermal stability. This chemical potential versus particle size, across the full size range between the single isolated atom and bulk-like limits, is reported here for the first time for any metal on any oxide. The chemical potential of Cu atoms on CeO 2 (111) surfaces, determined by single crystal adsorption calorimetry (SCAC) of gaseous Cu atoms onto slightly-reduced CeO 2 (111) at 100 and 300 K, is shown to decrease dramatically with increasing Cu cluster size. The Cu chemical potential is ~110 kJ/mol higher for isolated Cu adatoms on stoichometric terrace sites than for Cu in nanoparticles exceeding 2.5 nm diameter, where it reaches the bulk Cu(solid) limit. In Cu dimers, Cu's chemical potential is ~57 kJ/mol lower at step edges than on stoichiometric terrace sites. Since Cu avoids oxygen vacancies, these monomer and dimer results are not strongly influenced by the 2.5% oxygen vacancies present on this CeO 2 surface, and are thus considered representative of stoichiometric CeO 2 (111) surfaces.

Nanoparticles of one element or compound dispersed across the surface of another substance form the basis for many materials of great technological importance, like catalysts, fuel cells, sensors and biomaterials. Nanoparticles also often grow during thin film deposition. The size and number density of such nanoparticles are important, often estimated with electron or scanning tunneling microscopies. However, these are slow and often unavailable with sufficient resolution for particles near 1 nm. Because the probe depth of low-energy ion scattering spectroscopy (LEIS) with He + and Ne + is so shallow (less than one atom), it provides quantitative information on the fraction of the surface that is covered by such nanoparticles. Combined with the total amount per unit area, this fraction provides the average particle thickness. When the ions are incident or detected at some angle away from the surface normal, macroscopic screening effects cause interpretation of LEIS signals in terms of area fraction covered to be complicated. In this paper, we report a geometric analysis of particles with the shape of hemispherical caps so that LEIS signals obtained in any measurement geometry can also be used to quantitatively determine the area fraction, average particle thickness and diameter, or number density of particles.

The adsorption of vapor-deposited Au onto CeO2‑x(111) thin films (x = 0.05 and 0.2) at 300 and 100 K was studied using single crystal adsorption calorimetry (SCAC). The morphology of Au on these films was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) by monitoring the changes in substrate and adsorbate signals with Au coverage. Both techniques indicate that Au grows on CeO1.95(111) as three-dimensional particles in the approximate shape of hemispherical caps with a density of 2.8 × 1012 particles/cm2 at 300 K and 7.8 × 1012 particles/cm2 at 100 K. At 300 K, Au initially grows on CeO1.80(111) with a shape similar to hemispherical caps with a density of 5.4 × 1012 particles/cm2 until ∼1.6 ML Au coverage, above which the Au particles become thicker than hemispherical caps. At 300 K, the initial heat of adsorption of Au onto CeO1.95(111) is 259 kJ/mol, which is 37 kJ/mol lower than that on CeO1.80(111). This indicates stronger binding of Au to oxygen vacancies. On both surfaces, the Au heats of adsorption increase slowly with coverage, approaching the bulk heat of sublimation of Au­(solid) (368 kJ/mol) by ∼2 ML (3.2 nm in diameter on CeO1.95(111) and 2.4 nm on CeO1.80(111)). The heat of adsorption remains higher on the reduced surface than on the oxidized surface at all particle sizes. At 100 K, the initial heat of Au adsorption onto CeO1.95(111) is 209 kJ/mol (50 kJ/mol lower than at 300 K), which is due to a higher fraction of Au atoms adsorbing to terraces rather than at step sites. The adhesion energy of Au­(solid) to CeO1.95(111) at 300 K was found to be 2.53 J/m2 for 3.6 nm diameter particles and 2.83 J/m2 onto CeO1.80(111) for 2.5 nm diameter particles. This further indicates that Au particles bind more strongly to surfaces with a larger fraction of oxygen vacancies.

Thin films of metals and other materials are often grown by physical vapor deposition. To understand such processes, it is desirable to measure the adsorption energy of the deposited species as the film grows, especially when grown on single crystal substrates where the structure of the adsorbed species, evolving interface, and thin film are more homogeneous and well-defined in structure. Our group previously described in this journal an adsorption calorimeter capable of such measurements on single-crystal surfaces under the clean conditions of ultrahigh vacuum [J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instrum. 69, 2427 (1998)]. Here we describe several improvements to that original design that allow for heat measurements with ~18-fold smaller standard deviation, greater absolute accuracy in energy calibration, and, most importantly, measurements of the adsorption of lower vapor-pressure materials which would have previously been impossible. These improvements are accomplished by: (1) using an electron beam evaporator instead of a Knudsen cell to generate the metal vapor at the source of the pulsed atomic beam, (2) changing the atomic beam design to decrease the relative amount of optical radiation that accompanies evaporation, (3) adding an off-axis quartz crystal microbalance for real-time measurement of the flux of the atomic beam during calorimetry experiments, and (4) adding capabilities for in situ relative diffuse optical reflectivity determinations (necessary for heat signal calibration). These improvements are not limited to adsorption calorimetry during metal deposition, but also could be applied to better study film growth of other elements and even molecular adsorbates.