The level of catalytic activity mediated by one type of crystal facet can vary across the surface of that facet and also among several facets of a single type. This finding by Cornell University chemists suggests that knowing the surface structure of crystalline catalyst particles alone is not sufficient for predicting activity (Nat. Nanotechnol., DOI: 10.1038/nnano.2012.18). Decades ago, researchers found that some crystal facets, or faces, of solid catalysts are more catalytically active than others. Catalyst manufacturers have used that information to fine-tune preparation methods to enhance exposure of the most active facets. Cornell’s Peng Chen and coworkers used fluorescence microscopy with single-molecule resolution to monitor light-producing catalytic reactions on gold nanorods and discovered that facet-dependent activity turns out to be unexpectedly complex. By examining hundreds of highly faceted nanorods they determined that catalytic activity within a single facet exhibits a reactivity gradient that tends to increase from the center toward the facet edges. In addition, the relative reactivities of the ends and sides of nanorods vary widely among samples even though they exhibit the same types of facets, the team reports.