Understanding and Expanding the Prospects for Electrosynthesis with Liquid Metal Electrodes.

ConspectusThis Account describes and summarizes the latest work from our laboratory on developing and maturing strategies based on low-temperature liquid metals as reaction environments for materials synthesis. The electrochemical liquid-liquid-solid (ec-LLS) crystal growth concept is a hybrid method that combines electrodeposition and melt crystal growth. Using liquid metals as both electrodes and solvents for the purpose of producing inorganic crystals and materials, a simple and environmentally friendly process is possible. The impetus is to address the key deficiency in the inorganic crystalline materials that are the basis of modern optoelectronics and renewable energy capture/conversion systems. Specifically, existing methods for synthesizing crystalline inorganic materials for these purposes are largely energy- and resource-intensive, with a substantial impact on the environment when scaled. A long-term goal of our work with ec-LLS is to realize a materials synthetic process that is matured without requiring intensive resources or negatively impacting the environment. To this end, the factors that both limit and govern ec-LLS processes must be identified and understood. To date, questions regarding the factors that affect crystal nucleation and growth, form factors, and overall composition remain.Previous work established concretely ec-LLS as a versatile method for synthesizing and producing crystalline semiconductors at low temperatures as either particles, nanowires, or microwires. Subsequent experiments have focused on two tiers. First, the microscopic details of the liquid metal and its interfaces that dictate materials synthesis and crystal growth must be identified. Second, strategies that widen the attainable material form factors to facilitate device architectures must be realized. Hence, this Account describes results aimed at answering three questions: (1) What are the consequences of reaching supersaturation by an electrochemical rather than a thermal driving force for crystal growth in ec-LLS? (2) Can the location of nucleation and subsequent crystal growth be controlled? (3) Does the atomic structure of the liquid metal affect product formation in ec-LLS? The science described herein illustrates the value of in situ methods spanning transmission electron microscopy, X-ray diffraction, and X-ray reflectance for revealing the role that liquid metal composition and structure can play in ec-LLS. Additionally, we summarize work that shows for the first time that it is possible to produce both single-crystalline epitaxial films and complex intermetallic compounds through ec-LLS by tuning the cell design, electrochemical excitation waveform, and composition of the liquid metal electrodes.The cumulative findings described here substantially enrich our understanding of the ec-LLS concept while simultaneously motivating further questions moving forward. Is it possible to attain complete control over the crystalline quality and composition of ec-LLS products? Can the materials produced by ec-LLS provide tailored functional properties for targeted applications? Can the ec-LLS strategy be further refined to allow material synthesis and deposition at precise locations with deterministically chosen form factors? What synthetic pathways are accessible when even more sophisticated electrochemical waveforms and cell designs are used? Our hope is that this Account will spur additional researchers to help answer such questions.