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气(“光催化分解水”)将提供丰富的可再生燃料来源。高效太阳能裂水反应堆将需要新型光催化材料和先进的反应堆设计。硅是地壳中第二丰富的元素，无毒，几乎是所有现代电子产品的基础。这项研究项目将探索一种方法，通过悬浮在水中的细长粒子(纳米线)，将硅用于光催化分解水。这些纳米线将使用一种化学过程生长，这种过程使得一次可以创造出数亿条纳米线，其尺寸比人类头发的尺寸小100-1000倍。虽然这些粒子很小，但它们的内部结构很复杂。当用光照明时，内部结构允许导线产生类似于AA或AAA电池的电压。这项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所吸收的光和产生的电压。这些颗粒悬浮在水中时，利用光感应电压进行化学反应的能力将在一种新型的反应堆设计中进行测试。该项目还将为从高中到研究生院的学生提供研究经验和培训，并将为公众提供多种机会了解光催化分解水。这一结果应该会为硅的新技术应用打开大门，这些技术应用是通过在微观尺度上控制颗粒大小和组成来实现的。粒子悬浮反应器将光活性纳米颗粒悬浮在水中，是一种潜在的低成本设计，可以利用太阳能驱动的光电化学分解水来生产负担得起的氢气。为了实现这种装置架构，需要开发一种光活性纳米颗粒，既能产生足够的电压，又能吸收从可见光到近红外光的广泛光谱。这项研究项目将致力于硅纳米线的合成和开发，这种纳米线可以用任意数量的p-i-n结进行编码，以产生超过分解水所需的1.23V热力学电势的巨大光伏。采用气-液-固(VLS)生长机制，采用金属催化法生长多结硅纳米线，并在纳米线生长过程中原位调制掺杂形成p-n或p-i-n结。本研究项目的目标是对单根纳米线的生长、加工和性能进行基础研究，以便设计出可用于水分离的高性能五元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运行，此外，它们必须通过一个高效的隧道结串联连接。除了合成轴向p-(p-i-n)x-n纳米线，其中x的范围从1到15个结，该项目将采用单纳米线光伏性能的综合实验和计算评估。将对其电性能和光吸收特性进行评估，并将结果反馈给结构的综合和设计。概念验证单纳米线水分离装置和粒子悬浮反应堆将被开发用于概念验证的初步评估。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.