Nanopurification of silicon from 84% to 99.999% purity with a simple and scalable process.

Silicon, with its great abundance and mature infrastructure, is a foundational material for a range of applications, such as electronics, sensors, solar cells, batteries, and thermoelectrics. These applications rely on the purification of Si to different levels. Recently, it has been shown that nanosized silicon can offer additional advantages, such as enhanced mechanical properties, significant absorption enhancement, and reduced thermal conductivity. However, current processes to produce and purify Si are complex, expensive, and energy-intensive. Here, we show a nanopurification process, which involves only simple and scalable ball milling and acid etching, to increase Si purity drastically [up to 99.999% (wt %)] directly from low-grade and low-cost ferrosilicon [84% (wt %) Si; ∼$1/kg]. It is found that the impurity-rich regions are mechanically weak as breaking points during ball milling and thus, exposed on the surface, and they can be conveniently and effectively removed by chemical etching. We discovered that the purity goes up with the size of Si particles going down, resulting in high purity at the sub-100-nm scale. The produced Si nanoparticles with high purity and small size exhibit high performance as Li ion battery anodes, with high reversible capacity (1,755 mAh g(-1)) and long cycle life (73% capacity retention over 500 cycles). This nanopurification process provides a complimentary route to produce Si, with finely controlled size and purity, in a diverse set of applications.

Direct Conversion of Perovskite Thin Films into Nanowires with Kinetic Control for Flexible Optoelectronic Devices.

With significant progress in the past decade, semiconductor nanowires have demonstrated unique features compared to their thin film counterparts, such as enhanced light absorption, mechanical integrity and reduced therma conductivity, etc. However, technologies of semiconductor thin film still serve as foundations of several major industries, such as electronics, displays, energy, etc. A direct path to convert thin film to nanowires can build a bridge between these two and therefore facilitate the large-scale applications of nanowires. Here, we demonstrate that methylammonium lead iodide (CH3NH3PbI3) nanowires can be synthesized directly from perovskite film by a scalable conversion process. In addition, with fine kinetic control, morphologies, and diameters of these nanowires can be well-controlled. Based on these perovskite nanowires with excellent optical trapping and mechanical properties, flexible photodetectors with good sensitivity are demonstrated.

Simultaneous Purification and Perforation of Low-Grade Si Sources for Lithium-Ion Battery Anode.

Silicon is regarded as one of the most promising candidates for lithium-ion battery anodes because of its abundance and high theoretical capacity. Various silicon nanostructures have been heavily investigated to improve electrochemical performance by addressing issues related to structure fracture and unstable solid-electrolyte interphase (SEI). However, to further enable widespread applications, scalable and cost-effective processes need to be developed to produce these nanostructures at large quantity with finely controlled structures and morphologies. In this study, we develop a scalable and low cost process to produce porous silicon directly from low grade silicon through ball-milling and modified metal-assisted chemical etching. The morphology of porous silicon can be drastically changed from porous-network to nanowire-array by adjusting the component in reaction solutions. Meanwhile, this perforation process can also effectively remove the impurities and, therefore, increase Si purity (up to 99.4%) significantly from low-grade and low-cost ferrosilicon (purity of 83.4%) sources. The electrochemical examinations indicate that these porous silicon structures with carbon treatment can deliver a stable capacity of 1287 mAh g(-1) over 100 cycles at a current density of 2 A g(-1). This type of purified porous silicon with finely controlled morphology, produced by a scalable and cost-effective fabrication process, can also serve as promising candidates for many other energy applications, such as thermoelectrics and solar energy conversion devices.

Scalable Production of Si Nanoparticles Directly from Low Grade Sources for Lithium-Ion Battery Anode.

Silicon, one of the most promising candidates as lithium-ion battery anode, has attracted much attention due to its high theoretical capacity, abundant existence, and mature infrastructure. Recently, Si nanostructures-based lithium-ion battery anode, with sophisticated structure designs and process development, has made significant progress. However, low cost and scalable processes to produce these Si nanostructures remained as a challenge, which limits the widespread applications. Herein, we demonstrate that Si nanoparticles with controlled size can be massively produced directly from low grade Si sources through a scalable high energy mechanical milling process. In addition, we systematically studied Si nanoparticles produced from two major low grade Si sources, metallurgical silicon (∼99 wt % Si, $1/kg) and ferrosilicon (∼83 wt % Si, $0.6/kg). It is found that nanoparticles produced from ferrosilicon sources contain FeSi2, which can serve as a buffer layer to alleviate the mechanical fractures of volume expansion, whereas nanoparticles from metallurgical Si sources have higher capacity and better kinetic properties because of higher purity and better electronic transport properties. Ferrosilicon nanoparticles and metallurgical Si nanoparticles demonstrate over 100 stable deep cycling after carbon coating with the reversible capacities of 1360 mAh g(-1) and 1205 mAh g(-1), respectively. Therefore, our approach provides a new strategy for cost-effective, energy-efficient, large scale synthesis of functional Si electrode materials.

Tailoring Graphene Oxide-Based Aerogels for Efficient Solar Steam Generation under One Sun.

Graphene oxide-based aerogels with carefully tailored properties are developed to enable efficient solar steam generation. Aerogels, with inherent porous structures, are excellent thermal insulators and provide channels for water supply and vapor escape. With enhanced absorption and hydrophilicity by incorporation of carbon nanotubes and sodium alginate, the resulting aerogels can enable efficient (≈83%) solar steam generation under one-sun illumination.

Scalable Production of the Silicon-Tin Yin-Yang Hybrid Structure with Graphene Coating for High Performance Lithium-Ion Battery Anodes.

Alloy anodes possessed of high theoretical capacity show great potential for next-generation advanced lithium-ion battery. Even though huge volume change during lithium insertion and extraction leads to severe problems, such as pulverization and an unstable solid-electrolyte interphase (SEI), various nanostructures including nanoparticles, nanowires, and porous networks can address related challenges to improve electrochemical performance. However, the complex and expensive fabrication process hinders the widespread application of nanostructured alloy anodes, which generate an urgent demand of low-cost and scalable processes to fabricate building blocks with fine controls of size, morphology, and porosity. Here, we demonstrate a scalable and low-cost process to produce a porous yin-yang hybrid composite anode with graphene coating through high energy ball-milling and selective chemical etching. With void space to buffer the expansion, the produced functional electrodes demonstrate stable cycling performance of 910 mAh g-1 over 600 cycles at a rate of 0.5C for Si-graphene "yin" particles and 750 mAh g-1 over 300 cycles at 0.2C for Sn-graphene "yang" particles. Therefore, we open up a new approach to fabricate alloy anode materials at low-cost, low-energy consumption, and large scale. This type of porous silicon or tin composite with graphene coating can also potentially play a significant role in thermoelectrics and optoelectronics applications.

Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation.

The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as critical for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an average measured absorbance of ~99% across the wavelengths from 400 nm to 10 µm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber-based solar steam generation has over 90% efficiency under solar irradiation of only 4-sun intensity (4 kW m(-2)). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufacturing of other nanophotonic structures and devices.