The Mechanism Of The Electrochemical Synthesis Of Gold Nanoparticles And Noble Metal Nanoparticles Technology Transfer

In recent years, electrochemical synthesis of metal nanoparticles have received increasing interest because of simple and convenient operation, high purity of products, easy control in the size and morphology of nanoparticles by adjusting applied current or voltage. Here we used a rotating cathode to electrochemically synthesize metal nanoparticles. The advantages of a rotating cathode over stationary working electrode is that the nanoparticles synthesized at the cathode/solution interface can be forced to leave the cathodic region and enter into the bulk solution. Meanwhile, we used poly(N-vinylpyrrolidone) (PVP) as a stabilizing agent for metal nanoparticles, which not only protected nanoparticles from aggregation but also control the size and morphology of nanoparticles to some extent during the electrochemical synthesis.PVP-protected metal nanoparticles have quite high stability and can be reserved for several months, without any precipitate, due to the strong interaction between PVP macromolecules and the metal nanoparticles. But on the other hand, PVP's strong adsorption on the surface of metal nanoparticles greatly lowers the catalytic activity of particles. It is necessary to remove a portion of PVP polymers that wrap the metal nanoparticles for enhancing the catalytic activity of metal nanoparticles. In this paper, we developed a novel phase-transfer method of nanoparticles based on the decrease of the PVP's solubility in water with the temperature increase: the PVP-protected metal nanoparticles can spontaneously leave the aqueous phase and enter into an organic phase when heating the oil-water mixture containing metal nanoparticles to an appropriate temperature. The metal nanoparticles transferred to the organic phase exhibit a high catalytic activity towards methanol oxidation since most PVP polymers were removed in the course of phase-transfer.We also studied electrochemical formation mechanism of nanoparticles with special shapes. We noticed such a fact that the electrolyte in anodic and cathodic regions became acidic and alkaline respectively after the electrochemical synthesis. It is possible that a small portion of PVP monomers hydrolyzed to form N-vinyl-γ-amino-butanoic acid or the corresponding carboxylate anions in the process of electrolysis. The random polymer may serve as both reducing agent for Au(III) ions and shape-controller for selective growth of gold cluster, directing the reduced gold clusters to grow into nanocrystals with different shapes depending on different pH values. This implies that pH value should have a great influence on size and morphology of gold nanocrystals in a mixed solution of PVP and HAuCl₄, This inference was verified by the following experimental results: nanocrystals of different morphologies can be obtained only by adjusting pH of PVP + HAuCl₄ mixed solutions, which provides a simple and effective synthetic method for preparation of shape- or size-controlled metal nanoparticles.Gold nanofilms were formed on ITO conductive glass through heterogeneous nucleation and subsequent growth in PVP + HAuCl₄ mixed solutions of appropriate pH values. Using the ITO glass covered by gold nanofilms as the working electrode, we characterized the electrochemical properties of the gold nanofilms and found that the films had an obvious catalytic activity towards methanol oxidation. This kind of research work provides a great probability for application of gold nanofilms as electrocatalysts.