Novel Strategies for the Mass Production of Corrosion-Resistant Nanowires and Their Bottom-up Assembly into Isotropic/Anisotropic Energy Conversion Devices
Authors: Vaddiraju Sreeram 
Published: February 2020

Over the last two decades, nanomaterials have been demonstrated to be useful in renewable energy generation. In particular, nanowires, a subset of the broad class of nanomaterials, have been employed not only for efficiently converting solar energy into electricity (using dye-sensitized solar cells), but also efficiently converting heat energy into electricity (using thermoelectrics). In addition, they have been utilized as components in the design and fabrication of optoelectronics, sensors, and photocatalysts for producing hydrogen fuel from water. Enhanced charge transfer resulting from the single-crystalline nature of nanowires, coupled with the control of their electrical, thermal and electronic properties through size variations, allowed for accomplishing this task. Most of these demonstrations were made using devices either based on single nanowires or small-scale nanowire arrays and mats. From these studies, however, it is not yet clear whether novel properties exhibited by individual nanowires and small-scale nanowire mats and arrays are extendable to devices utilizing large-scale nanowire assemblies. More specifically, these studies do not indicate whether it is possible to assemble these single-crystalline nanowires in an interface-engineered manner into assemblies with controlled porosity so as to ensure that the resulting assemblies also exhibit the superior properties observed in individual nanowires. These studies also fail to account for the environmental impact of nanowires. They do not account for the air-, moisture- and acid-assisted degradation of the nanowires. This data is essential for two reasons: the enhanced surface areas of nanowires make them more vulnerable to air- and moisture-assisted degradation, compared to bulk materials; and the operation of devices employing these nanowires in ambient air necessitates that the nanowire components of these devices be resistant to acid-assisted degradation, as rain water is acidic in nature. Therefore, at this juncture, it is imperative to ask the following questions. What quantities of nanowires need to be produced for mass producing the renewable energy devices needed for mankind? What should be the set of properties nanowires need to exhibit for their use in renewable energy device fabrication for mankind? Finally, what strategies should be employed for industrial scale production and assembly of nanowires into energy conversion devices? Answering the first two questions is possible using a photocatalysts useful for spilling water into hydrogen fuel with the aid of sunlight as an illustrative example.

Considering a hydrogen production rate of 10 mM/hour per gram of nanowire photocatalyst, 1.65 x 108 Kg of nanowires are required to generate energy from hydrogen at a rate equal to 1% of the current mankind’s energy consumption rate (estimated to be 13 terawatts as of 2007). To put this in perspective, the scale of production of nanowires should be on a level similar to that accomplished by the steel industry (≈2x1012 kg of steel was produced in the world in 2005). Therefore, photoelectrochemical production of hydrogen from water requires the production of the photocatalysts on million metric ton scales.

Secondly, the nanowires produced should exhibit the following characteristics: They should be (a) stable in water for extended periods of time (corrosion resistant), (b) not degrade and release their constituents into water, (c) resistant to fouling for extended periods of time, (d) made from inexpensive raw materials, (e) bandgap engineerable, (f) easily removable from water following disinfection, without the need for specialized membranes and filters (i.e., they should also be amenable to removal by filtration from water through conventional water treatment facilities used by municipalities, counties and states across US).

Mass producing such corrosion-resistant nanowires is ideally possible by developing strategies that rely only on direct reactions of the raw materials at low/moderate temperatures, without reliance on expensive catalysts/templates. Assembly strategies developed for consolidating nanowires should ensure control over the properties of the interfaces between them after assembly. In an ideal case, interfaces (or bridges) between the nanowires should have the same chemical composition and phase as the nanowires themselves. In addition, assembly strategies should offer the possibility of consolidating nanowires either in a randomly oriented fashion or aligned fashion. Assemblies of aligned nanowires aid in utilizing any direction dependent properties of nanostructured materials. This cannot be accomplished using hot pressing or spark plasma sintering.

In this talk, strategies developed for the mass production of nanowires by direction reaction of component elements, their assembly via welding and imparting stabilities to them using non-conformal/conformal decoration of their surfaces with organic/inorganic molecules will be discussed. In addition, shear assisted consolidation of nanowires into aligned and welded nanowire assemblies will be discussed. Finally, the electrical and thermal transport behavior through these nanowire assemblies will be discussed.