A Photochemical, Room-Temperature, and Aqueous Route to the Synthesis of Pd Nanocubes Enriched with Atomic Steps and Terraces on the Side Faces

Palladium nanocubes enriched with atomic steps and terraces on the side faces were synthesized at room temperature in an aqueous solution containing a salt precursor, ascorbic acid, KBr, and poly(vinylpyrrolidone) under UV-vis irradiation. The key to the success of this synthesis was the use of PdBr₄^2- as a precursor with strong absorption in the region of 300-600 nm. Single-crystal seeds were formed in the reaction system upon exposure to a medium pressure Hg arc lamp, and the seeds then grew into cubic nanocrystals under extended irradiation. Because the adatoms inherently diffuse slowly across the surface at room temperature, the nanocrystals favored an asymmetric growth mode, generating side faces covered by a series of atomic steps and terraces rather than smooth {100} facets. The steps and terraces would disappear to generate smooth side faces if the sample was subjected to postsynthesis annealing at 130 °C. When applied as catalysts toward the electrochemical oxidation of formic acid, the low-coordination surface atoms such as those presented on the atomic steps resulted in significant reduction of activity relative to the smooth {100} facets. This trend could be rationalized through the overbinding of reactive species to the low-coordination sites.