Materials science

Pore characterization

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Porous materials are extremely useful in catalysis and selective molecular filtration, and can even serve as miniature laboratories for conducting chemistry with just a few molecules at a time.

Most natural porous minerals, such as zeolites, are laced with pores whose width is no greater than the size of small molecules (less than 2 nm). But materials scientists and chemical engineers want to make synthetic 'mesoporous' solids with larger pores (2–50 nm) that are selective for larger molecules.

Although much effort has gone into finding new ways of making such mesoporous solids, the atomic-scale structure of the pores is not well known and so refining their structure is a matter of trial-and-error. This is due partly to the difficulty in obtaining large single crystals for conventional diffraction studies using X-rays or neutrons. Elsewhere in this issue ( Nature 408, 449–453; 2000), Galen Stucky and colleagues report a method that uses high-resolution electron microscopy to obtain images that can be mathematically treated to give a full three-dimensional (3D) structure of mesoporous materials.

Electron microscopy uses electrostatic lenses to form a magnified image of the sample (by recombining the diffracted electron beams). This has the advantage that it can be carried out on very small areas, because the focused electron beam is only a few micrometres in diameter. The technique does not depend on the actual resolution of the images, but provides more complete information about the structure. So, compared with conventional methods for structural analysis of crystalline and amorphous materials, this approach is generally applicable to soft materials that have disorder at the atomic length scale, but order at the mesoscopic scale.

The picture here shows the 3D structure of a mesoporous silica material obtained by Stucky and colleagues. This image provides information about the sizes and shape of the pores at the nanoscale level, as well as their connectivity. Further analysis reveals an unusual bimodal structure, in which there are different sized micro- and mesopores.

What makes this imaging method so attractive is that, compared with X-ray diffraction, it provides full structural information without using pre-assumed models or parameters. Moreover, it can reveal high-resolution details of pore structure at different scales in porous composites. The technique could be used to characterize the detailed structure of a wide range of composite materials.

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