We report that five-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) which can attract halide ions via different strength of electrostatic interactions could well adjust their activity to tune the etching degree of O2/halides for protecting 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 electron withdrawing –CN group in CH3CN, which is confirmed by an as-called iodine-starch test, 1H-NMR spectra, and theoretical calculations. Based on this finding, we then have successfully expanded SOMs to other molecules including acetone, 1, 4-dioxane, and 1, 3, 5-trioxane which have similar function to acetonitrile for production of Pd nanowires, and isopropanol that is similar to ethanol for 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 a new thinking clue to lessen the etching of O2/halides for controlling twin structure of noble metals, but also supports an idea that creating twin structure is good for enhancing catalytic activity.

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