Electrolyte effects on the shape-controlled synthesis of Pt nanocrystals by electrochemical square-wave potential method

Abstract

Surface structures or shapes of metal nanocrystals (NCs) greatly affect their catalytic performance. Electrochemical square-wave potential (SWP) method shows great advantages in the shape-controlled synthesis of Pt-group metal nanocrystals with high reactive high-index facets, due to the ease in controlling the growth and oxidation (or etching) by potentials. The potential conditions have been investigated substantially for the SWP method previously. However, the effects of electrolytes on the shapes of NCs are rarely explored. Herein, we investigated the effect of electrolyte (H2SO4 and Na2SO4) on the shape evolution of Pt NCs prepared by electrochemical SWP method. We found that high-index faceted Pt NCs, including {hk0}-faceted tetrahexahedral (THH) and {hkk}-faceted trapezohedral (TPH) Pt NCs can be obtained in 0.1 M H2SO4 in broad potential window, while mainly low-index {111}-faceted octahedral Pt NCs are formed in 0.1 M Na2SO4 electrolyte. Through evaluating the effects of cations, anions, and solution pH value, we observed that the pH value plays a key role in shape evolution. Low acidity in Na2SO4 electrolyte enhances the oxygen adsorption on Pt on the one hand, and promotes the hydrolysis of PtCl62− that decreases the growth rate of Pt NCs on the other hand. These two factors enhance the oxidative etching effect at upper potential limit of SWP, which can remove low-coordinated step atoms, and form flat low-index facets. This study shows insight into the electrolyte effects on electrochemical shape-controlled synthesis of Pt NCs, and helps to understand the growth mechanism.