Nanotomography is a technique of growing importance in the investigation of the shape, size, distribution and elemental composition of a wide variety of materials that are of central interest to investigators in the physical and biological sciences. Nanospatial factors often hold the key to a deeper understanding of the properties of matter at the nanoscale level. With recent advances in tomography, it is possible to achieve experimental resolution in the nanometre range, and to determine with elemental specificity the three-dimensional distribution of materials. This critical review deals principally with electron tomography, but it also outlines the power and future potential of transmission X-ray tomography, and alludes to other related techniques.

[1] Particle size distributions for soluble and insoluble species in Mt. Etna's summit plumes were measured across an extended size range (10 nm < d < 100 mm) using a combination of techniques. Automated scanning electron microscopy (QEMSCAN) was used to chemically analyze many thousands of insoluble particles (collected on pumped filters) allowing the relationships between particle size, shape, and composition to be investigated. The size distribution of fine silicate particles (d < 10 mm) was found to be lognormal, consistent with formation by bursting of gas bubbles at the surface of the magma. The compositions of fine silicate particles were found to vary between magmatic and nearly pure silica; this is consistent with depletion of metal ions by reactions in the acidic environment of the gas plume and vent. Measurements of the size, shape and composition of fine silicate particles may potentially offer insights into preemission, synemission, and postemission processes. The mass flux of fine silicate particles from Mt. Etna released during noneruptive volcanic degassing in 2004 and 2005 was estimated to be $7000 kg d À1 . Analysis of particles in the range 0.1 < d/mm < 100 by ion chromatography shows that there are persistent differences in the size distributions of sulfate aerosols between the two main summit plumes. Analysis of particles in the range 0.01 mm < d < 0.1 mm by scanning transmission electron microscopy (STEM) shows that there are significant levels of nanoparticles in the Mt. Etna plumes although their compositions remain uncertain.

The size and shape of pores is a key factor in the successful deployment of mesoporous silicas as supports for high-activity catalytic nanoparticles. Critical concerns are the accessibility of catalyst particles to reactant species, the effect of the particle-support interaction on catalytic activity, and the stability of the system with respect to degradation or sintering. In the present work, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography has provided quantitative three-dimensional information about the location of bimetallic nanoparticles supported on and within disordered mesoporous silica (Grace Davison 634-type). The surface of the pore network was found to be fractal in nature (fractal dimension D s ∼ 2.4) with implications for the selectivity of the catalyst-support system. By measuring the location of catalyst particles as a function of the local curvature of the support, particle adsorption sites were classified. The distribution of nanoparticles within the interior of the support showed preferential adsorption on anticlastic ("saddle-shaped") surfaces, whereas nanoparticles adsorbed on the exterior surfaces of the support structure also demonstrated a strong preference for concave ("cup-like") regions. These results highlight the critical importance of three-dimensional characterization for quantitative evaluation of porous media and catalytic supports.