Thermal and chemical stability and adhesion strength of Pt nanoparticle arrays supported on silica studied by transmission electron microscopy and atomic force microscopy

The thermal, chemical, and mechanical stability of Pt nanoparticles support ed on silica has been measured with transmission electron microscopy (TEM) and atomic force microscopy (AFM). The nanoparticle arrays were fabricated using electron beam lithography, which produced uniform particle sizes (20 +/- 1 nm) and uniform interparticle distances (150 +/- 1 nm). TEM studies p rovided information about the array periodicity, particle dimensions, and c rystallinity of individual particles. Before heat treatments, individual Pt nanoparticles were found to be polycrystalline with crystalline domain siz es of 4-8 nm. After heating to 1000 K in high vacuum (10(-7) Torr) and 1 at m H-2, the crystalline domain sizes within individual particles grew larger , without noticeable deformation of the array. A similar enlargement of cry stalline domains was seen in 1 arm O-2 at a lower temperature of 700 K. Usi ng contact mode AFM, the height, periodicity, and adhesion of the particles were determined. On a newly prepared sample, Pt particles were displaced f rom the silica support by the AFM tip with approximately 10 nN lateral forc e. The interfacial adhesion energy between the Pt and SiO2 was on the order of 1 mJ/m(2), which is relatively weak bonding. After heating, the Pt part icles could not be displaced by the AFM tip, suggesting that heat treatment s had increased the bonding between the Pt and SiO2. The stability and unif ormity of the nanoparticle arrays make them ideal model catalysts for react ions in either oxidizing or reducing conditions.