Paper PS+AS+SE-MoM8
Efficiency of Electrolytic Reduction of Aqueous Metal Salts to Metal Nanoparticles at a Plasma-Liquid Interface

Monday, October 30, 2017, 10:40 am, Room 21

Electrolytic cells with a plasma serving as one or both of the electrodes eliminate the solid metal and allow electrochemical reactions to be carried out at a gas-liquid interface. This is particularly beneficial for the synthesis of metal nanoparticles from metal salts since the deposition of a thin film onto the electrode is avoided. However, because of the complexity of the plasma and the resulting interfacial reactions, the mechanism for metal nanoparticle formation remains unknown.

Here, we designed experiments to understand the mechanism of the reduction of silver nitrate (AgNO₃) to silver (Ag) nanoparticles by a previously reported atmospheric-pressure, direct current microplasma operated as the cathode. We applied a well-known methodology in electrodeposition to assess the faradaic efficiency whereby the mass of the synthesized material is compared with the theoretical amount of mass estimated from the charge injected into solution. A faradaic efficiency of 100% would indicate that all the charge is going towards the desired reduction of Ag cations to solid Ag, Ag⁺+e^- --> Ag⁰, whereas an efficiency less than 100% would suggest that there are side reactions, most probable of which is the second order recombination of (solvated) electrons to form hydrogen gas and hydroxide ions, e^-_(aq)+e^-_(aq)+2H₂O_(l) --> H_2(g)+2OH^-_(aq).

We find that at a relatively high AgNO₃ concentration in the bath, the faradaic efficiency depends weakly on the current, reaching values of 100% at 2 mA and decreasing to slightly less than 100% at 6 mA. To corroborate these measurements, the mass change of a Ag foil anode which oxidizes in solution by the reverse of the cathode reaction, Ag⁰ --> Ag⁺+e^-, was compared and found to yield slightly lower efficiencies, but with the same overall trend. At constant current and varying AgNO₃ concentration in the bath, the faradaic efficiency was found to drastically decrease to less than 100%. We interpret these results as follows. The kinetics of the primary reactions, Ag⁺ reduction and second order recombination, depend on the respective rate constants which are similar (3.7 x 10¹⁰ M/s and 5.5 x 10⁹ M/s) and the reactant concentrations. At low current or high AgNO₃ concentration, the rate of Ag⁺ reduction is higher than second order recombination and the faradaic efficiency approaches 100%. Conversely, the rate of second order recombination is higher than Ag⁺ reduction at high current or low AgNO₃ concentration, lowering the faradaic efficiency. A reaction model was developed to support these interpretations.