Toward a Quantitative Understanding of the Reduction Pathways of a Salt Precursor in the Synthesis of Metal Nanocrystals

Abstract

Despite the pivotal role played by the reduction of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the solution phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by reduction through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the reduction kinetics involved. Our quantitative analyses of the reduction kinetics of PdCl and PdBr by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl was reduced in the solution phase while PdBr was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr by ascorbic acid could be switched from surface to solution by raising the reaction temperature.

Quantitative Analysis of the Multiple Roles Played by Halide Ions in Controlling the Growth Patterns of Palladium Nanocrystals

“…For the Na 2 PdCl 4 plus KBr system, the absorption peak appeared at 332 nm, which is consistent with what was reported for PdBr 4 2À . [20,33,38,41] This result suggests that the presence of Br À ions at a high concentration was able to convert most of the PdCl 4 2À to PdBr 4 2À through a ligand exchange process. To avoid confusion, we still use the chemical formulas of the original compounds to denote the Pd(II) precursor involved in each synthesis.…”

“…This result indicates that the proportion of PdBr 4 2À increased with the concentration of Br À ions in the solution. [20,33,38,41] In addition, the almost identical spectra observed at R = 400 and 800 suggest that the Pd(II) precursor was dominated by PdBr 4 2À when the molar ratio of KBr to Na 2 PdCl 4 was over 400. Due to the complexity of ligand exchange, it is very difficult to resolve the actual species in an aqueous solution of the Pd(II) precursor.…”

“…[24][25][26][27][28] All these changes can impact the reduction kinetics and even reduction pathway of a precursor, enabling the formation of nanocrystals with diverse shapes. [20,[32][33][34][35][36] However, the understandings derived from early studies were primarily based on qualitative observations and empirical laws, in which the reduction was typically described using qualitative terms such as "slow" and "fast" and the synthesis usually involved a trial-and-error approach in reaching the optimal conditions. These qualitative insights tend to fall short in elucidating the mechanistic details involved in both the nucleation and growth processes.…”

“…These qualitative insights tend to fall short in elucidating the mechanistic details involved in both the nucleation and growth processes. [33,37,38] To this end, it is of critical importance to achieve a quantitative understanding of how halide ions affect the reduction kinetics and reduction pathway of a salt precursor during seed-mediated growth.…”

The Mixed-Electrode Concept for Understanding Growth and Aggregation Behavior of Metal Nanoparticles in Colloidal Solution

“…At a metal surface and even after the formation of nuclei, the redox process is enforced by the separation of metal ion reduction and oxidation of reducing agent due to an electron transfer via the preformed metal. The result is a strong autocatalytic activity of the metal taking place as it was observed in the chemical formation of Pd nanocubes [29].…”