Session QR1: Plasma Interaction with Liquids II

Understanding Charge Transfer Reactions at the Interface of Plasmas in Contact with Liquids

Plasmas in and in contact with liquids offer a very rich physical and chemical environment where a multitude of species (electrons, ions, neutrals) and physical phenomena (light, electric fields) intersect. With emerging applications in medicine, environmental remediation, and materials synthesis, it has become paramount to understand the many processes occurring at the interface in order to design and optimize new technologies. Perhaps the most important plasma species is the electron, and it thus reasonable to assume it can play a critical role when plasmas are brought in contact with liquids as well. Over the past several years, our group has focused on deciphering the nature of electron transfer from a plasma to liquid and the subsequent chemistry the electrons induce. Our experimental configuration is the plasma equivalent of an electrochemical or electrolytic cell, where the cathode and anode are submerged in an electrolyte solution and current is carried by reduction reactions at the cathode and oxidation reactions at the anode. When the cathode is replaced by a plasma, the circuit is explicitly completed by the injection of plasma electrons into the solution where they stably solvate before inducing reduction reactions. Recently, we have demonstrated the first direct detection of these stably solvated electrons using a novel total internal reflection absorption spectroscopy experiment, resulting in the first measurement of the optical absorption spectrum for plasma-solvated electrons. Further, we have shown that the lifetime of these electrons can be significantly reduced if suitable solution- and plasma-phase scavengers are used to react quickly with these electrons. These results highlight the complexity of the plasma-liquid interface and how charge-transfer processes often compete with other chemistry that occurs at the plasma-liquid interface, such as the dissolution of plasma species into the liquid.   

Hydrated Electrons at the Plasma-Water Interface

When atmospheric pressure plasma interacts with liquid water surfaces, complex processes involving both charged and neutral species generally occur but the details of the processes are not well understood. One plasma-generated specie of considerable interest that can enter an adjacent liquid water phase is the electron. Hydrated electrons are well known to be important in radiation chemistry as initiating precursors for a variety of other reactive compounds. Recent experimental evidence for hydrated electrons near the atmospheric pressure plasma-water interface was reported by Rumbach et al. [1]. We present results from a model of a dc argon plasma coupled to an anodic adjacent water layer that aims to simulate this experiment. The coupled plasma-electrolyte model illustrates the nature of the plasma-water interface and reveals important information regarding the self-consistent electric fields on each side of the interface as well as time- and space-resolved rates of reaction of key reactive species. We suggest that the reducing chemistry that results from electron hydration may be useful therapeutically in countering local excess oxidative stress. \\[4pt] [1] Rumbach et al., The solvation of electrons by an atmospheric-pressure plasma, Nature Communications, in press, 2015.   

Plasma Functionalized Nanocarbon Materials and Their Applications

The plasma treatment method is important for modifying carbon nanomaterials since it has the advantage of being nonpolluting. It has the possibility of scaling up to produce large quantities necessary for commercial use. The liquid-related plasma is especially advantageous in avoiding use of toxic stabilizers and reducing agents during the nanoparticle formation process. In this work, both gas phase and liquid phase plasmas are used to modify nanocarbon materials including graphene and carbon nanotubes. The synthesis of metal nanoparticles functionalized nanocarbon materials including carbon nanotubes and graphene has been realized by an environmentally-friendly gas-liquid interfacial method. Furthermore, the new catalysts based on hybrid of nanocarbon materials and metal nanoparticles have been proved to be stable and high catalytic performance in organic molecule transformation reactions. In addition, the modification of few-layer graphene grown by chemical vapour deposition via the nitrogen plasma ion irradiation has been performed, and the modified graphene sheets as counter electrodes in bifacial dye-sensitized solar cells exhibit high performance.   

Synthesis of magnetic nanoparticles by atmospheric-pressure plasma electrolysis and observation of liquid flow induced by plasma

For the synthesis of magnetic metal NPs (nanoparticles), we used the electrolysis combined with atmospheric-pressure plasma. Plasma irradiated positive ions or electron to the solution surface; it worked as electrode of electrolysis. In the case of using aqueous solutions of FeCl2, magnetic NPs were synthesized at plasma-liquid interface when electron was irradiated to liquid surface. The plasma was generated in a miniature helium gas flow surrounded by a shielding gas flow controlling the gas condition around the plasma. The condition of magnetic NPs synthesis depended on the shielding gas species of plasma. In the case of using Ar or N2 shielding gas, magnetic NPs were synthesized. On the other hand, in the case of using O2 shielding gas or without shielding gas, magnetic NPs were not synthesized. To synthesize NPs without chemicals such as FeCl2 solutions, we use plasma electrolysis with iron electrode which is immersed in liquid. When plasma electrolysis was operated, iron electrode eluted to Fe cation and it becomes magnetic NPs at plasma-liquid interface. By using this method much of Fe3O4 is synthesized. In addition, we investigated liquid flow of plasma electrolysis by using Schlieren visualization. Liquid flow was observed when plasma electrolysis was operated.   

Gas phase microreaction: nanomaterials synthesis via plasma exposure of liquid droplets

Plasma-liquid interactions are complex but offer considerable scope for use in nanomaterials synthesis. The introduction of individual picolitre micro-droplets into a steady-state low temperature plasma at atmospheric pressure, offers opportunities for enhanced scope and control of plasma-liquid chemistry and material properties. The gas-phase micro-reactor is similar in concept to liquid bubble microfluidics currently under intense research but with enhanced opportunities for scale-up. For nanomaterials and quantum dot synthesis, the addition of a liquid phase within the plasma expands considerably the scope for core-shell and alloy formation. The synthesis and encapsulation within a liquid droplet allows continuous delivery of nanoparticles to remote sites for plasma medicine, device fabrication or surface coating. We have synthesized Au nanoparticles in flight using AuHCl4 droplets with plasma flight times \textless 0.1 ms. Also, Ag nanoparticles have been synthesized downstream via the delivery of plasma exposed water droplets onto AgNO3 laden substrates.   

Facile synthesis of cuprous oxide nanoparticles by plasma electrochemistry

Cuprous oxide nanoparticles were synthesized by plasma electrochemistry. In the plasma electrochemistry system, plasma was contacted with the solution as one electrode and a Cu plate immersed in the solutions as the counter electrode. NaCl solution was used as the basic electrolyte and glucose was added as a reaction mediator and/or a reducing agent. The plasma created many reducing and oxidizing species which can react with the Cu ions released from the Cu plate in the solution. Cu2O nanoparticles with an average diameter of about 30 nm were formed under the competition of reducing and oxidizing reactions. The results show that the concentration of added glucose strongly affects the properties of the products. Corresponding to high, medium and low concentrations of glucose, the products were, respectively, nanoparticles of amorphous Cu2O, polycrystalline Cu2O, and a mixture of polycrystalline Cu2O and Cu2Cl(OH)3.