Catalyst particle sizes from Rutherford scattered intensities

Abstract

SUMMARY We show that the number of atoms in a small supported catalyst cluster can be estimated from the strength of electron scattering into a high angle annular detector in the STEM. The technique is related to the Z contrast methods developed by Crewe, Wall, Langmore and Isaacson. It works best for high atomic number catalyst particles when supported on low atomic number supports, such as Pt on γ‐aluminium oxide. The method is particularly useful for detecting and measuring particles in the sub‐nanometre size range where bright field images are unreliable. Unlike the Z contrast methods, a high angle annular detector is used, which avoids intensity modulations arising from Bragg reflections. The signal is mostly high angle diffuse scattering, which is predominantly Rutherford scattering, and is proportional to the number of atoms probed by the beam, weighted by their individual scattering cross‐sections. Scattering strengths of individual clusters are computed from digitized high angle annular detector images. Data for Pt on γ‐aluminium oxide, when plotted as imaged area1/2 against intensity1/3, define a straight line. Such plots provide calibration of the intensity increment per atom without the need of external calibration, although assumptions about particle morphology must be made. Reliable results require high signal‐to‐noise and optimum sampling of the specimen. For an STEM probe size of 0.35 nm, Pt clusters containing as few as three atoms can be detected when supported on typical, 20 nm thick, γ‐aluminium oxide supports.

Weighing Supported Nanoparticles: Size-Selected Clusters as Mass Standards in Nanometrology

“…Cantilever-based techniques have recently been demonstrated for mass detection at the zeptogram level (10 À21 g) [8,9]. Electronmicroscopy-based techniques have a long history [6,7,[10][11][12][13][14][15][16][17][18]. In Zeitler and Bahr's early work of 1962, the mass of nanoparticles has been determined down to 10 À18 g using STEM with an accuracy of 10% [17].…”

Direct Imaging of Zirconia Pillars in Montmorillonite by Analytical Electron Microscopy

“…The ADF image is formed by collecting electrons scattered through large angles as the electron probe is rastered over the sample. For sufficiently high angles, the electron scattering has a strong dependence on the average atomic number of the sample (Treacy et al, 1980;Treacy and Rice, 1989). In an ADF image, bright contrast corresponds to regions of high atomic number.…”

Development of Quantitative STEM for a Conventional S/TEM and Real-Time Current Calibration for Performing QSTEM with a Cold Field Emission Gun

“…Through careful calibration and measurement of the image and microscope, the contrast (scattered electron intensity) can be explicitly related back to the number of atoms involved in the scattering. This presentation discusses our developments on two different QSTEM approaches, one based on a conventional TEM/STEM and another on an aberration-corrected dedicated STEM.The earliest QSTEM work performed demonstrated the feasibility of counting the number of atoms in ultra-small NP using sufficiently high collection angles (≥100 mrad) [1,2]. It was further shown that this method could also indirectly recover details of NP shape (e.g., spherical, hemispherical, or plate-like) [3].…”