Experimental and Theoretical Studies of the Solvated Electron at Plasma-Liquid Interfaces

Plasma-liquid interactions is an extremely wide field, encompassing a wide variety of applications: the reduction of metal cations in solution to form colloidal nanoparticles, the generation of reducing and oxidizing species to destroy contaminants in wastewater, the application of plasma to tissue in order to promote wound healing or kill malignant cells, and as an analytical tool to determine the constituent species in a solution. There are also a number of ways to apply plasmas to a liquid, including forming a discharge inside a gas bubble in solution, via advection in a plasma jet, and with a direct electrical connection as in plasma electrolysis, which is the focus of this work. Many of the reactive species formed at the plasma-water interface are well known, most notably the solvated electron and hydroxyl radical, and their reactions have been well characterized in the field of radiation chemistry. However, there are many open questions about the physics of their formation, and their subsequent dynamics and lifetime in a plasma-liquid system. It is only recently that solvated electrons were directly observed at the plasma-liquid interface, using total internal reflection absorption spectroscopy (TIRAS). This work builds on these results, and uses the TIRAS diagnostic to assess theoretical models of the behavior of primary products at the plasma-liquid interface. First, a study is conducted of the dynamics of the TIRAS discharge, observing its changing size as a function of current to determine the corresponding effect on the plasma-laser overlap. The size is observed to change throughout the duration of a TIRAS cycle at 20 kHz, but slowly enough for the discharge to reach a steady state at its maximum and minimum current. Quantities extracted from TIRAS, interfacial concentration and penetration depth, are also observed to be extremely sensitive to even small errors in measurement of current density. This motivates modifications to the TIRAS system, setting the low current state to 0 A, which simplifies measurement of current density and removes a large source of potential error. Experimental measurements are then used to confirm an outstanding theoretical model of interfacial solvated electron concentration and penetration depth as a function of current density in plasma electrolysis with a liquid anode, by observing the scaling behavior of current density. These results are used to develop an improved theoretical model, which accounts for both current density and modulation frequency, and predict upper bounds of penetration depth and concentration in plasma electrolysis, 120 nm and 0.6 mM respectively. Next, the formation of solvated electrons in a discharge with a liquid cathode is studied. TIRAS is used to confirm the presence of solvated electrons in this configuration. Previous measurements of yield from the dissociative attachment of chloroacetate are used to measure their yield, 1.04±0.59 per incident ion. Based on this high yield, a mechanism is proposed for the ionization of water in a liquid-cathode discharge, wherein excess energy from ion neutralization drives a proton-coupled electron transfer to an existing solvation trap. To test this, measurements of secondary emission coefficient are were carried out in both krypton and xenon, which have a lower ionization energy and should drive fewer ionization events. However, secondary emission was instead found to increase in these gases, as a function of ion mass, indicating secondary emission is a separate process from solvated electron formation driven by physical emission of water molecules. Solvated electron yield is then measured by comparing the faradaic efficiency of chloride production from chloroacetate in krypton and argon discharges, which is found to be the same within error. Lastly, these results, as well as TIRAS’ current limitations, serve as motivation for a proposal of future experiments using this measurement, and of a proposed future system, which might enable TIRAS to perform more consistent, quantitative measurements.等离子体-液体相互作用是一个覆盖范围极广的研究领域，包含诸多应用方向：将溶液中的金属阳离子还原以制备胶体纳米颗粒；生成氧化还原活性物种以降解废水中的污染物；将等离子体作用于生物组织以促进伤口愈合或杀灭恶性细胞；还可作为分析工具测定溶液中的组成组分。等离子体作用于液体的方式也多种多样，包括在溶液内的气体气泡中形成放电、通过等离子体射流的对流实现作用，以及如等离子体电解（plasma electrolysis）般采用直接电连接的方式——而本研究的聚焦对象正是等离子体电解。 在等离子体-水界面生成的诸多活性物种已得到广泛研究，其中最具代表性的是溶剂化电子（solvated electron）与羟基自由基（hydroxyl radical），它们的反应过程在辐射化学领域已得到充分表征。然而，关于这些活性物种的形成物理机制，以及它们在等离子体-液体体系中的后续动力学过程与存活寿命，仍存在诸多悬而未决的问题。直至近期，研究人员才借助全内反射吸收光谱法（total internal reflection absorption spectroscopy, TIRAS）直接在等离子体-液体界面观测到溶剂化电子。本研究正是基于上述成果，利用TIRAS诊断技术对等离子体-液体界面初级产物行为的理论模型进行评估。 首先，本研究对TIRAS放电的动力学过程展开研究，通过观测其尺寸随电流的变化情况，以明确其对等离子体与激光重叠区域的对应影响。在20 kHz的TIRAS周期内，放电尺寸始终处于变化之中，但变化速率足够缓慢，使得放电能够在最大与最小电流处达到稳态。从TIRAS数据中提取的界面浓度与穿透深度等参数，即便在电流密度存在微小测量误差的情况下，也会表现出极强的敏感性。这一发现推动了TIRAS系统的改进方案：将低电流状态设定为0 A，此举简化了电流密度的测量流程，同时消除了一大潜在误差来源。 随后，本研究通过观测电流密度的标度行为，开展实验测量以验证一项针对液阳极等离子体电解中界面溶剂化电子浓度与穿透深度随电流密度变化的经典理论模型。基于上述结果，我们开发了一款改进后的理论模型，该模型同时考虑了电流密度与调制频率的影响，并预测了等离子体电解中穿透深度与浓度的上限值，分别为120 nm与0.6 mM。 其次，本研究针对液阴极放电体系中的溶剂化电子生成过程展开研究。借助TIRAS技术，我们证实了该配置下溶剂化电子的存在。以往基于氯乙酸盐解离附着过程的产率测量数据，被用于测定溶剂化电子的产率：每个入射离子对应1.04±0.59个溶剂化电子。基于这一较高的产率，我们提出了液阴极放电中水的电离机制：离子中和过程释放的多余能量，驱动质子耦合电子转移至已存在的溶剂化陷阱中。 为验证该机制，我们分别在氪气与氙气环境中开展了二次发射系数的测量——这两种气体的电离能较低，理论上应会引发更少的电离事件。然而实验结果却显示，二次发射系数反而随离子质量的增加而升高，这表明二次发射是一个与溶剂化电子生成相互独立的过程，由水分子的物理发射所驱动。随后，我们通过比较氪气与氩气放电中氯乙酸盐生成氯化物的法拉第效率，对溶剂化电子的产率进行了测定，结果显示二者在误差范围内并无显著差异。 最后，上述研究成果以及TIRAS当前存在的局限性，为后续基于该测量技术的实验方案以及新型系统的开发提供了研究方向，有望实现TIRAS更具一致性与定量性的测量能力。