5-fold Twinned Nanowires and Single Twinned Right Bipyramids of Pd: Utilizing Small Organic Molecules To Tune the Etching Degree of O2/Halides

We report that 5-fold twinned nanowires and single twinned right bipyramids of Pd with high yields can be selectively synthesized in a hydrophilic system with the assistance of acetonitrile and ethanol, respectively. The controlled synthesis is based on an idea that small organic molecules (SOMs) that can attract halide ions via electrostatic interactions of different strengths could well adjust their activity to tune the etching degree of O2/halides for protecting the twinned Pd crystal nucleus. We consider that relatively stronger interaction between acetonitrile and halide ions for the formation of nanowires is due to the existence of three C–Hδ+ bonds induced by the electron-withdrawing CN group of CH3CN, which is confirmed by an as-called iodine starch test, 1H nuclear magnetic resonance spectra, and theoretical calculations. On the basis of this finding, we then have successfully expanded SOMs to other molecules, including acetone, 1,4-dioxane, and 1,3,5-trioxane, which have a function similar to that of acetonitrile for the production of Pd nanowires, and 2-propanol, which has a function similar to that of ethanol for the fabrication of right bipyramids. Although nanowires and bipyramids are both mainly bound by {001} planes, nanowires show better catalytic performance toward the reduction of 4-nitrophenol, indicating that more twin boundaries could offer more active catalytic sites. This work not only provides new information to decrease the degree of etching of O2/halides for controlling twin structure of noble metals but also supports the idea that creating a twin structure is good for enhancing catalytic activity.