Quantitative Study of Electrochemical Reduction of Ag⁺to Ag Nanoparticles in Aqueous Solutions by a Plasma Cathode

Abstract

Replacing the solid metal cathode in an electrolytic cell with a plasma (gas discharge) allows the reduction of metal salt solutions to metal nanoparticles. Here, we apply gravimetric analysis to quantify the charge transfer (faradaic) efficiency of this reduction process and understand reaction pathways at the plasma-liquid interface. Silver powder synthesized from aqueous solutions of silver nitrate is collected and the weight is compared to the theoretical amount of silver obtained by applying Faraday's law of electrolysis. We find that the faradaic efficiencies are near 100% when the initial silver nitrate concentration is large (>100 mM), but are much less than 100% at low salt concentrations (<15 mM) or at high applied currents (>6 mA). These results are explained in terms of reaction kinetics involving solvated electrons and two important reactions: the reduction of silver ions (Ag + ) and the second order recombination of solvated electrons which releases hydrogen gas. Plasmas formed at the surface of liquids including water have the potential to carry out electrochemical reactions through the interaction between gas-phase and solution species.1-3 In a direct-current (DC) configuration where the plasma is one electrode and a solid metal or plasma is another electrode with an aqueous electrolyte in between, charge is transferred from gas-phase electrons or ions in the plasma to aqueous ions in solution through charge transfer reactions at the plasma-liquid interface. 4,5 In contrast to typical electrochemical reactions at the interface of a solid metal electrode and ionic solution, the reduction or oxidation reactions in plasma-assisted electrochemistry occur substrate-free. For example, solutions containing metal cations can be reduced by a cathodic plasma to produce colloidallydispersed metal nanoparticles rather than thin films of metal deposited on a substrate. [6][7][8] This approach to synthesizing metal nanoparticles is high-purity and environmentally-friendly, avoiding chemical reducing agents such as sodium borohydride, 9 can eliminate the need for organic capping agents by electrostatically stabilizing the particles, 10,11 and has the potential to create novel materials such as metal oxides via non-equilibrium, solution-phase radical chemistry. 12,13 While the ability of plasmas to function as an electrochemical electrode and reduce metal cations to metal nanoparticles has been clearly demonstrated, the fundamental reaction mechanisms remain elusive, limiting the ability to optimize or tailor the process. A critical reason is that plasmas contain a variety of reactive species that originate from the inert carrier gas and, in many applications, ambient air, including electrons, ions, and metastable neutrals, and these reactive species have a distribution of energies.14 Many of these species can react directly with and dissociate other gas or vapor phase molecules such as water, 15 or dissolve into the solution and then react with solution species to create a number of products.16 For example, in ...

Facile Preparation of Ni-Co Bimetallic Oxide/Activated Carbon Composites Using the Plasma in Liquid Process for Supercapacitor Electrode Applications

“…Nickel and cobalt precipitated uniformly on AC. Ni 2+ and Co 2+ were produced in the aqueous reactant solution by the dissociation of nickel and cobalt precursors were reduced to Ni 0 and Co 0 due to chemical activated species in the plasma field, particularly hydrogen radicals, and precipitated on the AC surface [28,29]. On the other hand, the mapping result shown in the upper right corner of Figure 2 shows the presence of many oxygen elements on the AC surface.…”

Lightning under water: Diverse reactive environments and evidence of synergistic effects for material treatment and activation

“…Since most of the plasma-forming excited species are consumed for the decomposition and activation of neutral precursors in the solution, the plasma should be intense enough to produce the supercritical density of active reactants capable to sustain continuous nucleation in the solution. 176 Plasma and plasma-generated electric current densities, i.e., power density in liquid precursor, are the triggers that overcome thresholds of nucleation How does the growth of complex structures proceeds in liquids under the action of electric current or other energy fluxes that ensure ionization, dissociation, and other types of reactant activation? The key mechanisms that manifest in all of these processes can be illustrated using an example of a discharge-driven electrolytic oxidation with alternating pulsed unipolar and bipolar regimes.…”

Experimental and computational investigation of polylactic acid/silver‐NP nanocomposite with antimicrobial activity prepared by plasma in liquid

“…At the same time, the formation of nanoparticles is also possible from dissolved precursor substances in the processes of reduction by active particles formed by a similar method of organizing a plasma−solution system. [ 4 ]…”