Metal nanoparticles several tens of nanometers in size have different properties from bulk, and are being applied to various field. At 20 nm or less, however, aggregation is hastened because surface energy increases as particle size decreases. Thus, new methods suppressing and moderately promoting aggregation are being developed.
Platinum nanoparticles have highly catalytic activity and are expected to have wide applications. However, it is not easy to maintain their high catalytic activity without causing aggregation or surface deterioration. Using simultaneous measurement of X-ray diffraction and differential scanning calorimetry (DSC), it is easy to estimate the effects of temperature and atmospheric condition on aggregation.
A platinum catalyst used in a polymer electrolyte fuel cell (supported at a 40% concentration on carbon, with a particle size of 2 nm) was examined using Rigaku's Ultima IV multipurpose diffraction system. Simultaneous XRD-DSC measurements were performed with increasing temperature in different atmospheres.
The figure below shows the results obtained in dry nitrogen. Between 250°C and 290°C, an exothermic peak was observed in the DSC chart and the platinum (111) diffraction peak becomes sharp. This indicates that aggregation is promoted and the crystallite size grows up to 5 nm.
The figure below shows the results obtained in 3% H2/He. Exothermic change and aggregation of nanoparticles occurs in the vicinity of 0°C.