Quintuple P-N Junction Nanowires for Wireless Water Splitting in Particle Suspension Reactors

利用太阳光从水中廉价地产生氢（“光催化水分解”）将提供丰富的可再生燃料来源。 高效的太阳能水分解反应器将需要新型的光催化材料和先进的反应器设计。 硅是地壳中第二丰富的元素，无毒，是几乎所有现代电子产品的基础。该研究项目将探索一种方法，通过悬浮在水中的细长颗粒（“纳米线”）将硅用于光催化水分解。 这些纳米线将使用一种化学过程生长，这种化学过程能够同时产生数亿根纳米线，其尺寸比人类头发小100-1000倍。 虽然这些颗粒很小，但它们具有复杂的内部结构。当用灯光照射时，内部结构允许电线产生类似于AA或AAA电池的电压。该研究项目将探索硅纳米线生长的基本方面，并将研究结构的设计如何影响吸收的光和产生的电压。这些颗粒悬浮在水中时利用光诱导的电压进行化学反应的能力将在一种新型反应堆设计中进行测试。该项目还将为高中到研究生院水平的学生提供研究经验和培训，并将为公众提供了解光催化水分解的多种机会。这些结果将为硅的新技术应用打开大门，这些应用可以通过在微观尺度上控制粒度和成分来实现。颗粒悬浮反应器，其中光活性纳米颗粒悬浮在水中，是一种潜在的低成本设计，用于太阳能驱动的光电化学水分解以产生负担得起的氢气。为了实现这种器件架构，需要开发一种既能产生足够电压又能吸收宽光谱可见光到近红外光的光活性纳米颗粒。该研究项目将解决硅纳米线的合成和开发，可以用任意数量的p-i-n结进行编码，以产生超过水分解所需的1.23 V热力学势的大光电压。采用金属催化生长的方法，利用气-液-固（VLS）生长机制，合成了多结硅纳米线，并在纳米线生长过程中通过掺杂剂的原位调节形成了p-n或p-i-n结。该研究项目的目标是对单个纳米线的生长，加工和特性进行基础研究，以设计可用于水分解的高性能五重p-i-n结纳米线。在这种结构中，五个结中的每一个都必须单独作为高效太阳能电池工作，此外，它们必须通过高效隧道结串联连接。除了合成轴向p-（p-i-n）x-n纳米线（x范围从1到15个结）外，该项目还将采用单纳米线光伏性能的实验和计算评估。电性能和光吸收特性都将被评估，结果将反馈到结构的合成和设计中。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估的支持。

The inexpensive generation of hydrogen from water using sunlight ("photocatalytic water splitting") would provide an abundant source of renewable fuel. High efficiency solar water splitting reactors will require new types of photocatalytic materials and advanced reactor designs. Silicon is the second most abundant element in the Earth's crust, is non-toxic, and is the basis for nearly all modern electronics. This research project will explore a way to use silicon in photocatalytic water splitting by way of elongated particles ("nanowires") suspended in water. These nanowires will be grown using a chemical process that enables hundreds of millions of nanowires to be created at once with a size that is 100-1000 times smaller than a human hair. Although these particles are small, they have a complex internal structure. When illuminated with light, the internal structure allows the wires to generate a voltage similar to the that of AA or AAA batteries. This research project will explore the fundamental aspects of the growth of silicon nanowires and will examine how the design of the structures affects the light absorbed and the voltage generated. The ability of these particles to perform chemical reactions when suspended in water, using the voltage induced by light, will be tested in a novel reactor design. This project will also provide research experiences and training for students from the high school through graduate school level and will provide multiple opportunities for the public to learn about photocatalytic water splitting. The results should open the door to new technological applications of silicon that are made possible by the control of particle size and composition at a microscopic scale. Particle suspension reactors, in which photoactive nanoparticles are suspended in water, are a potentially low-cost design for solar-driven photoelectrochemical water splitting to produce affordable hydrogen. To realize this device architecture, the development of a photoactive nanoparticle that both produces enough voltage and absorbs a broad spectrum of visible to near-infrared light is needed. This research project will address the synthesis and development of silicon nanowires that can be encoded with an arbitrary number of p-i-n junctions to produce large photovoltages in excess of the 1.23 V thermodynamic potential needed for water splitting. The multijunction silicon nanowires are synthesized by metal-catalyzed growth using the vapor-liquid-solid (VLS) growth mechanism, and p-n or p-i-n junctions are formed by in situ modulation of dopants as the nanowire grows. The goal of this research project is to perform fundamental studies on the growth, processing, and properties of single nanowires to enable the design of high-performance quintuple p-i-n junction nanowires that can be used for water splitting. In this structure, each of the five junctions must individually operate as an efficient solar cell, and in addition, they must each be connected in series by an efficient tunnel junction. In addition to the synthesis of axial p-(p-i-n)x-n nanowires, where x ranges from 1 to 15 junctions, the project will employ a combined experimental and computational evaluation of single-nanowire photovoltaic performance. Both the electrical performance and light absorption characteristics will be evaluated, and the results will feed back into the synthesis and design of structures. Proof-of-concept single-nanowire water splitting devices and particle suspension reactors will be developed for initial evaluation of the proof of concept.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.