Formation Of Au Nanoparticles Through Reaction Of Na₃Au(SO₃)₂ By Electrochemically Evolved Hydrogen

Abstract

Hydrogen evolution during electrochemical deposition has long been regarded as undesired and deliberately suppressed since small hydrogen bubbles could cause a rough surface of deposits. Here, we demonstrate a new role for electrochemically evolved hydrogen, serving as both templates and reducing agents for synthesizing Au nanoparticles via electroless depositions from Na₃Au(SO₃)₂ electrolytes. A series of experiments have been designed to investigate this new role. A working electrode of patterned Ag strips on the non-conductive substrates has been used. It has been observed that when a potential more negative than hydrogen evolution potential was applied to metal areas, a large amount of Au nanoparticles formed on the non-conductive regions. This strongly suggests that hydrogen evolution and electroless deposition play critical roles for the formation of Au nanoparticles. Au nanoparticles have also been generated on TEM grid, which provided a convenient way for analyzing the structure of Au nanoparticles. The hollow feature of Au nanoparticles was revealed by high-resolution transmission electron microscopy (HRTEM) suggesting electrochemically evolved hydrogen bubbles act as templates for the formation of Au nanoparticles. It has been found that the size and size distribution of Au nanoparticles were affected by applied potential and addition of Ni²⁺ ions in the electrolyte. The higher applied potential usually results in larger hydrogen bubbles thus a bigger size of Au nanoparticles. Ni²⁺ ions can enhance hydrogen evolution efficiency which gives the better size distribution of Au nanoparticles. Moreover, the effects of hydrophobicities of substrates have been studied. It shows that Au nanoparticles can form on both hydrophobic and hydrophilic surfaces, and the hydrophobicity does not have effects on the morphology of Au nanoparticles. This result is not fully understood, and more studies are needed to investigate the formation mechanisms of nanobubbles on different substrates. Au metal formation around electrochemically evolved hydrogen bubbles has been directly observed. However, no reaction takes place around hydrogen gas bubbles. This may be due to that hydrogen gas bubbles are less reactive than electrochemically hydrogen bubbles.