The structures of many solids are only partially known. This arises because diffraction and spectroscopic methods produce only an average picture of the atomic arrangement, and require specimens composed of identical, repeating units if they are to yield the actual detail. For many materials this condition is not fulfilled: particularly important cases occur in defect oxides and heterogeneous catalysts, and indeed, any material where reactivity depends on inhomogeneity.
My interest is in determining the real structure of such solids, via direct imaging of the atomic arrangement. For atoms other than those on exposed surfaces this requires high resolution electron microscopy (HREM). Metal atoms can be resolved using HREM, irrespective of whether the structure is periodic or not. The structural information from HREM at this level has opened up for study a whole range of solids which were hitherto intractable. Foremost among these are defect oxides with applications in high-Tc superconductivity and fast-ion conductors: with improved data from HREM, we can now start to relate physical properties to actual structure and associated variations in stoichiometry.
Similar advances have been made in microcrystalline catalysts, where changes in co-ordination influencing reactivity, particularly in surface and sub-surface regions, can be monitored directly. Two examples of this, involving platinum metal on titanium dioxide and nanocrystalline gold, are shown below. It has been found that these 'nanoparticles' possess such a high ratio of surface to bulk atoms that their chemical properties can be drastically affected, and their behaviour is nothing like bulk materials. For example, nanocrystalline alumina, normally exceptionally inert, has an extremely reactive surface, and this is believed to play an important part in the catalytic activity of metals supported on this oxide.
Photocatalytic properties of TiO2 modified with gold nanoparticles in the degradation of 4-chlorophenol in aqueous solution, Catal. Letters, 92, 41-47, (2004)
High Resolution Transmission Electron Microscopy Studies of Sol-Gel Derived Cobalt-Substituted Barium Ferrite, J. Solid State Chem. , 167, 254-257, (2002)
Quaternary lead-Niobium-Tungsten Oxides based on the Tetragonal Tungsten Bronze Structure, J. Solid State Chem. , 161, 135-151, (2001)
The surface activity of ultrafine particles, Phil. Trans. R. Soc. Lond. , A358, 2683-2692, (2000)
The structural and physical chemistry of nanoparticles, Particulate Matter - Properties and Effects on Health, R.L.Maynard and C.V.Howard eds., BIOS Scientific publishers, Oxford 63-84, (1999)
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