Atomic structure of ultrafine catalyst particles resolved with a 200-keV transmission electron microscope

Abstract

Small atomic assemblies (diameter 10–100 A) possess several unusual physical and catalytic properties1–9. Speculations regarding the crystalline nature of individual particles of supported metals dispersed to this degree, as well as colloidal metallic sols, date back to Faraday's researches on divided metals. Interest has been renewed following the demonstration10,11 that metals could readily be prepared in colloidal form. Similarly, binary semiconductors such as CdS exhibit novel properties which are of potential value in photoelectrochemistry. Thus, the electronic band-gap of bulk CdS is 2.4 eV, whereas a CdS cluster of ∼18° diameter—corresponding to one of the ‘magic agglomeration numbers’—has a band-gap closer to 3.4 eV. Moreover, the conduction-band edges of such ultrafine semiconductors functioning as microelectrodes are sufficiently shifted to favour the photoreduction of H+ (as gaseous H2) from water4. Reliable methods are needed to probe the ultrastructure of very fine particles, especially as such methods can elucidate the nature of many other ultrafine materials. Examples of such materials are: (1) bimetallic systems8, such as Pt/Ir on oxide supports for the catalytic re-forming of hydrocarbons to improve octane ratings; (2) semiconductors such as TiO2 overlaid with comminuted Pt for the photocleavage of water12,14; (3) dispersed Rh or Pt on oxide supports for controlling automobile exhausts15; and (4) large clusters of transition metal and noble metals of the type Au55 [P(C6H5)3]12Cl6, which bridge the gap between molecular and extended metallic structures6. High-resolution electron microscopy is a possibility as it probes structures directly in real-space16–18. Very-high-voltage transmission electron microscopes (TEM), operating between 400 and 1,500 keV, would provide the best interpretable resolution; however, they cannot usually be fitted with in situ micro-analytical facilities based on X-ray emission—essential for sample characterization of mixed phases—and voltage stability is harder to achieve as accelerating voltage increases. Here we report results obtained with a modified commerical 200-keV TEM. A new type of side-entry specimen stage permits the investigation of atomic arrays in minute assemblies of Au and in colloidal catalysts of Pt, such as those used in photoredox processes4,19,20. These are shown to be crystalline but relatively rich in disordered regions, both within their bulk and on their surface.