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 supported 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 provided information about the array periodicity, particle dimensions, and crystallinity of individual particles. Before heat treatments, individual Pt nanoparticles were found to be polycrystalline with crystalline domain sizes of 4?8 nm. After heating to 1000 K in high vacuum (10-7 Torr) and 1 atm H2, the crystalline domain sizes within individual particles grew larger, without noticeable deformation of the array. A similar enlargement of crystalline domains was seen in 1 atm O2 at a lower temperature of 700 K. Using contact mode AFM, the height, periodicity, and adhesion of the particles were determined. On a newly prepared sample, Pt particles were displaced from the silica support by the AFM tip with approximately 10 nN lateral force. The interfacial adhesion energy between the Pt and SiO2 was on the order of 1 mJ/m2, which is relatively weak bonding. After heating, the Pt particles could not be displaced by the AFM tip, suggesting that heat treatments had increased the bonding between the Pt and SiO2. The stability and uniformity of the nanoparticle arrays make them ideal model catalysts for reactions in either oxidizing or reducing conditions.