Quantitative High-Angle Annular Dark-Field Scanning Transmission Electron Microscope (HAADF-STEM) Tomography and High-Resolution Electron Microscopy of Unsupported Intermetallic GaPd₂ Catalysts

Apart from describing the occurrence and detailed crystallographic nature of novel five-fold twinned nanoparticles (<5 nm) of the selective hydrogenation catalyst GaPd₂, the main thrust of this work is to demonstrate a method of characterizing, by electron tomography, the structural morphologies of large agglomerates, consisting of ca. 1800 nanoparticles, the individual sizes of which fall in the range of 1–30 nm in equivalent diameter. The so-called segmentation of electron tomograms, usually evaluated manually and vulnerable to subjective bias (as well as being laborious) is carried out by utilizing sophisticated, yet readily implementable, image processing techniques that facilitate versatile 3D nanometrological analysis. Such procedures will play an important role in the move toward quantitative 3D characterization at the nanoscale and are applicable to numerous other systems of technological and catalytic interest (such as fuel-cell electrodes) where there are agglomerates of nanoparticles and nanoclusters of various compositions, including those that are supported on high-surface-area solids.