Exploring the Limitations of the Use of Competing Reducers to Control the Morphology and Composition of Pt and PtCo Nanocrystals

We have explored size- and shape-controlled synthesis of platinum nanocrystals (Pt NCs) by systematically comparing the differential reducing performance of two competing reducing agents in a one-pot synthesis: hexadecanediol, a weak reducer, and metallic cobalt or superhydride, stronger reducers of Pt. In addition to its role as a metal reducer, Co also functions as a shape-directing agent and is incorporated into the Pt NCs, forming a PtCo alloy structure. By maintaining a constant HDD concentration and systematically increasing the Co content, the shape of the resulting NCs was found to alter from polypods, when no Co was present, to cuboctahedrons and cubes when trace amounts of Co were added, and back to polypods when Co dominated the reduction process. On the other hand, when the concentration of HDD was systematically increased (with Co kept constant), evolution from polypod morphology to prismatic/spherical/cubic NCs, followed by irregular shapes was observed. Both experimental results indicate the importance of the competitive role between the reducing agents, their concentration limits for achieving a controlled morphology, and the presence of Co as a shape-directing agent to alter the NC shape. This allows the exploration of a wide range of NC morphologies without significant modification of the synthesis recipe.