A Simple Method for Producing Colloidal Palladium Nanocrystals: Alternating Voltage-Induced Electrochemical Synthesis

摘要

Colloidal palladium nanocrystals (Pd-NCs) are important materials for many applications, such as heterogeneous catalysis, ~([1–11]) hydrogen purification-and-storage, ~([12–14]) chemical sensors, ~([15, 16]) and electronics. ~([17, 18]) Their performances are strongly dependent on their sizes, size distributions, shapes, and capping ligands. ~([19–25]) The structure-function relationship has stimulated tremendous effort to develop effective and efficient methods for synthesizing well-dispersed Pd-NCs. Currently, there are four main methods (Figure S1 in the Supporting Information): 1) the chemical reduction of Pd(II) or Pd(IV) anions PdX _n ~(m ?); ~([5, 8, 19, 26]) 2) the thermal decomposition of Pd compounds such as palladium acetylacetonate Pd(acac) _2; ~([27–29]) 3) the electrochemical reduction of Pd(II) or Pd(IV) anions PdX _n ~(m ?), and; ~([6, 11, 30–32]) 4) the anodic dissolution of Pd electrode to PdX _n ~(m ?) anions followed by a cathodic deposition. ~([33, 34]) The first three methods rely on expensive Pd(II) or Pd(IV) compounds, and the last two methods require specialized potentiostats. Therefore, it is highly desirable to produce high-quality Pd-NCs via simpler methods, particularly through environmentally friendly processes. ~([35]) Herein we report an alternating voltage induced electrochemical synthesis (AVIES) method, which allows for synthesizing welldispersed, size-controlled, and single-crystalline colloidal Pd-NCs. This method involves only two Pd wires, a conventional transformer, and an electrolyte solution containing capping ligands. No special reducing agents are needed. It enables us to synthesize not only hydrophilic Pd-NCs, but also hydrophobic Pd-NCs. The reaction mechanism is unveiled to be a direct Pd-NC ejection off the Pd electrodes through cathodically reducing the PdO intermediates, which have been produced in the prior anodic half-cycles.