Precious-metal nanoparticles are well known as good candidates for electrocatalysis, for example in fuel cell applications.One of the most important objectives for using precious metal electrocatalysts is how to reduce loading whilst also increasing electrochemical activity. Generally,this is achieved by minimizing the cluster diameter to increase electrochemical surface area (ECSA), whilst at the same time decreasing the required amount of catalyst. On the other hand, small particle diameters (e.g. clusters containing less than 300 atoms) are thought to have very different behavior from larger, bulk-like particles. Substrate and relaxation effects have been explored by experimental approach and computer simulation. [1,2] However, to the best of our knowledge, microscopy and quantitative analysis of such effects are scarce.
Pt clusters on carbon black were prepared. Cs-corrected STEM analysis and quantitative measurement was applied to predict changes in catalytic activity. We used a JEOL ARM200F equipped with a Cs-corrector and Cold FEG. From atomic resolution STEM images of the Pt clusters, we can see direct evidence of strain. We attempt to quantify this strain by compensating from the crystal model and the measured lattice spacing. At first, attempted strain mapping. We selected a constant area of the Pt cluster from phase mapping, generated from the FFT peak with an applied Gaussian filter. Our atom by atom inspection using geometrical phase analysis revealed the strain and relaxation of the surface area quantitatively. If strain effects catalytic activity, this can explain why we have a decrease in activity below a 3 nm Pt cluster size. In addition, this work may be able to predict what type and cluster size of catalyst could be good candidates for practical fuel cells.
[1] Stamenkovic, Vojislav, et al. "Changing the activity of electrocatalysts for oxygen reduction by tuning the surface electronic structure." Angewandte Chemie 118.18 (2006): 2963-2967.
[2] Power, Timothy D., and David S. Sholl. "Effects of surface relaxation on enantiospecific adsorption on naturally chiral Pt surfaces." Topics in catalysis 18.3-4 (2002): 201-208.
The authors gratefully acknowledge the Center of Innovation.The International Institute for Carbon-Neutral Energy Research was established by World Premier International Research Center Initiative (WPI), MEXT, Japan.References