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Nam Heon Cho1 Hye-Eun Lee1 Hyo-Yong Ahn1 Ki Tae Nam1

1, Seoul National University, Seoul, , Korea (the Republic of)

Morphology dependent properties of nanoparticles have been utilized in various applications and especially nanoparticles composed of high-index facets have attracted significant attention in catalysis with its high density of atomic steps serving as active sites for reactions. However, despite vigorous efforts for delicate control of nanoparticle morphology, controlled fabrication of specific surface structures within a few nanometer scale still remains as a big challenge due to instability of high-index facets. Previously, we demonstrated aqueous based seed-mediated two step growth method to synthesize high-indexed gold nanoparticles with morphological variations by control of reaction parameter and introduction of organo-thiol shape controlling additives. Strong interactions between the organo-thiol based additives and the metal surface induces directional growth depending on the shape-modifier structures.
Herein, based on this established system, we demonstrate precise surface structure control with organo-thiol additive during palladium nanoparticle synthesis. Resulting nanoparticles show multi-stepped surface structures which varies from cubic based planar step patterns to spiral step patterns composed of multiple high-index facets. We demonstrate superior catalytic activity of “multi-stepped” nanoparticles. Morphology variation is originated from collaborative interactions between kinetic modifiers and shape modifying additives during the growth step. Reduction kinetic was controlled by growth solution pH, where various types of acids were introduced to the growth solution. Higher proton concentration in the synthesis environment induced slower electron withdrawing of reducing agent to affect geometrical shape and uniformity. Additionally, dramatic change in nanoparticle morphology was observed respect to the acid type when shape modifying organo-thiol group was introduced to the reaction solution. These anion dependent morphology change is related to specific interactions between organo-thiol molecule and anions to modify growth directionality. Our study provides crucial design constraints for fine morphology tuning during nanoparticle synthesis which allows systematical control of nanoparticle surface structures for potential catalysis application.

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