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

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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.