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Highlights in Chemical Technology

Chemical technology news from across RSC Publishing.

Instant insight: The shape of things to come

20 August 2007

Paul Midgley, Edmund Ward, Ana Hungria and John Meurig Thomas discuss using nanotomography to take a 3D glimpse at the nanoworld.

Several 3-D nanotomography images

A montage of projections of a scanning electron tomogram. It shows the distribution of catalyst nanoparticles within a mesoporous silica support.

There is little doubt that medicine has benefited greatly from the ability to visualise the internal organs of the human body using a variety of radiation including X-rays, positrons, ultrasound and nuclear magnetic resonance. The invention of the 'cat-scan', or CT-scan, in the 1960s enabled such views to be further improved by allowing a full three-dimensional reconstruction of the internal architecture of the body. The basis behind the reconstruction is the technique of tomography, from the Greek word 'tomos' meaning 'slice' or 'section', in which a series of images, or projections, is used to create a three-dimensional view by back-projecting these images into a 3D space in a computer.

In the chemical sciences, relatively little advantage has been taken of tomographic techniques even though it has long been clear that the spatial resolution ultimately attainable by the use of X-rays and electron beams far exceed those associated with CT scans and NMR imaging of the human body.

"In the chemical sciences, relatively little advantage has been taken of tomographic techniques"
Just as the morphology and size of organs is of key importance in the human anatomy, in nanoscience and nanotechnology the size and shape of an object may play a key role in determining its electronic and chemical behaviour. There are many examples where the physical and chemical properties of nanocrystals and clusters deviate significantly from their bulk crystalline phase. Gold in its bulk state displays no catalytic activity and yet in nanoparticle form it is an extremely good catalyst for selective oxidation of hydrocarbons and the complete combustion of carbon monoxide in air. The shape or crystal morphology can be equally important, such as in ceria nanoparticles for automotive catalysis.

The shape, size and distribution of nanoparticles and nano-structures are all key to their function and the need for tomographic methods applicable to chemical systems (ranging from the physical to the biological) is therefore pressing, just as it is in the engineering and earth sciences.

In our Critical Review we investigate nanotomographic methods that are open to the materials-oriented chemist and present a range of illustrative examples taken from nanoscale chemistry, along with contiguous sub-disciplines encompassing parts of biology and medicine.

"Efforts are afoot worldwide to reach atomic resolution in three dimensions using electron tomography"
We focus mainly on electron tomography (of which there are several variants), and its life sciences application, such as the study of cellular organelles, magnetotactic bacteria and the nuclear pore complex, and in the physical sciences, including supported catalysts, nanoalloys and binary II-VI compounds and polymers. Three-dimensional spatial resolution of 1 nm3 is now possible and efforts are afoot worldwide to reach atomic resolution in three dimensions using electron tomography. In anticipation of future developments, we also outline the rudiments of tomography via transmission X-ray microscopy, a technique that will undoubtedly be of tremendous importance, especially with greater access to next-generation synchrotron X-ray sources, such as the new Diamond Light Source in the UK. Other nanotomographic techniques highlighted in the review include atom probe field-ion microscopy (APFIM), a destructive technique applicable to conducting and semi-conducting samples. It uses time-of-flight mass spectrometry to identify single ions combined with position sensitive detection to produce a sensor capable of determining three-dimensional information with excellent resolution in both location and chemical identity. Serial sectioning, in which a three-dimensional model may be constructed from a series of slices, is possible with techniques such as atomic force microscopy and scanning electron microscopy (SEM), coupled with a focussed ion beam (FIB) workstation.

The need for three-dimensional visualisation and analysis at high spatial resolution is likely to increase as nanoscience and nanotechnology become increasingly important - nanotomography will play a key role in understanding structure, composition and physico-chemical properties at the nanoscale.

Read the full Critical Review 'Nanotomography in the chemical, biological and materials sciences' in issue 9 of Chemical Society Reviews.

Link to journal article

Nanotomography in the chemical, biological and materials sciences
Paul A. Midgley, Edmund P. W. Ward, Ana B. Hungría and John Meurig Thomas, Chem. Soc. Rev., 2007, 36, 1477
DOI: 10.1039/b701569k

Related Links

Link icon Paul Midgley's website at the University of Cambridge
Read more about electron microscopy

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Also of interest

Direct visualisation, by aberration-corrected electron microscopy, of the crystallisation of bimetallic nanoparticle catalysts
Edmund P. W. Ward, Ilke Arslan, Paul A. Midgley, Andrew Bleloch and John Meurig Thomas, Chem. Commun., 2005, 5805
DOI: 10.1039/b511004a

High-resolution transmission electron microscopy: the ultimate nanoanalytical technique
John Meurig Thomas and Paul A. Midgley, Chem. Commun., 2004, 1253
DOI: 10.1039/b315513g

Z-Contrast tomography: a technique in three-dimensional nanostructural analysis based on Rutherford scattering
Paul A. Midgley, Matthew Weyland, John Meurig Thomas and Brian F. G. Johnson, Chem. Commun., 2001, 907
DOI: 10.1039/b101819c