At the heart of photocatalysis lies the recognition that chemical reactions are catalyzed on specific surfaces of catalyst particles. Profound knowledge about the relationship between processing conditions and the abundance of specific crystal facets would allow for tailoring improved catalysts that show predominantly those surfaces that are catalytically most active for specific reactions. Today, titanium dioxide, in the crystalline modifications anatase and rutile, is by far the most important photocatalyst material. There is some evidence that small crystals prefer to adopt the anatase modification, and large titania crystals are thermodynamically favoured if they have the rutile structure. Therefore, rutile surfaces have been studied extensively on macroscopic crystals, while the knowledge about specific anatase surfaces is less of experimental than of theoretical nature. Here, we present a methodical approach that combines geometric estimations by transmission electron microscopy (TEM) and the elucidation of detailed three-dimensional shapes of anatase nanoparticles by a Wulff-type construction [1]. Relative and absolute surface areas of different crystal facets are estimated, surface-to-volume ratios are deduced and compared to integral measurements of BET surfaces. Facets of the type (101) and (001), which are widely assumed to make up the majority of exposed facets on anatase particle surfaces, were found to be in the minority. Surfaces of commercial powder nanoparticles under investigation are rather dominated by facets of the type (100) or (111), and they show also (110), (112), (102), (103), (104), (106), and (108) facets, depending on the particle shape. The investigations clearly show that the shape of naturally occurring macroscopic anatase is not the key to a better understanding of nanocrystalline materials (i.e. “little is different”).

Reference

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1. Photocatalytic One-pot N-alkylation of Nitroaromatic Compounds Using Pt/TiO₂
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