Ion scattering spectroscopy intensities for supported nanoparticles: The hemispherical cap model

Abstract

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.

Energetics of Ag Adsorption on and Adhesion to Rutile TiO2(100) Studied by Microcalorimetry

“…The normalized Ag and Ti LEIS signals directly measure the fraction of the surface area that is covered by the Ag particles including a shadowing effect. 73 The evolution of the normalized Ag and Ti LEIS signals is plotted versus Ag coverage in Figure 3 and compared with two typical growth models. The Ag coverages reported in this paper are given in monolayer (ML), where 1 ML is defined as 7.36 × 10 18 atoms/m 2 , which is the areal number density of coordinatively unsaturated O atoms on the ideal bulk-terminated TiO 2 (100) surface.…”

“…The solid curved lines correspond to the best fit of these data to a 3D growth model called the hemispherical cap model. 73 It assumes that Ag grows as 3D particles with the shape of hemispherical caps and that the Ag particle number density n is saturated by the first dose after which the Ag particles only grow in size and that the Ag particles all have the same shape and same size at any given coverage. Although no study of Ag nanoparticles on TiO 2 (100) has been reported before, STM studies of Ag nanoparticles grown on rutile TiO 2 (110) show that the majority of the islands have nucleated by 0.05 ML (i.e., the saturation density of islands is nearly reached already by ∼0.05 ML) 68 as is generally the case for such systems consisting of late transition metal nanoparticles supported on oxide surfaces.…”

“…78 This model is applied up to the coverage where ∼35% of the surface area is covered by particles since the particles may begin to overlap with each other beyond this coverage. This model predicts that the normalized Ag LEIS intensity [I(θ)/I(∞)] increases with the Ag coverage (θ) and Ag particle number density (n) as follows 73 θ π…”

“…The normalized LEIS signals of Ag and Ti give the fraction of surface area that is covered by Ag nanoparticles at a given coverage, as described previously. 73…”

Energetics of Cu Adsorption and Adhesion onto Reduced CeO2(111) Surfaces by Calorimetry

“…The data are well fit using the hemispherical cap model described by Diebold et al, 25 but modified to better describe macroscopic shadowing of the incident ion beam by hemispherical particles. 26 It assumes that the film grows as 3D particles with the shape of 111), respectively) as a function of Cu coverage after deposition onto CeO 1.95 (111) (diamonds) and CeO 1.8 (111) (triangles) at 300 K. The dashed line corresponds to the normalized ISS signal that would be observed if Cu grew in a layer-by-layer fashion, while the solid line corresponds to Cu growing as hemispherical caps with a fixed radius and a fixed particle density of 7.8 × 10 12 particles/ cm 2 . This model is only reasonable up to ∼35% of the surface being covered by particles, since they may start to overlap with each other at higher coverage, so the dotted lines after that are only a guide to the eye.…”