Nanosynthesis of Iron Based Material for Green Energy

In this work, nanosynthesis of multiple iron-based materials are explored to further their use in green renewable-energy applications. First, the nanosynthesis of the abundant, non-toxic semi-conductor Iron Disulfide (Iron Pyrite, Fool's Gold, FeS2) is investigated. Within these studies, it became possible to tune the shape of the FeS2 nanoparticles easily by modifying injection temperatures and iron precursors. From here, the growth mechanisms of the different shapes were elucidated by examining different time points within the synthesis. It was discovered that the FeS2 did not grow by Ostwald Ripening, but instead by Oriented Attachment. Knowing this, it was possible to not only further the shapes of FeS2 nanoparticles, but also manipulate the size and crystallinity. Focus was then shifted to creating larger micron sized FeS2 crystals. Larger crystals where achieved by a unique FeS nanowire precursor followed by sulfurization. The dominant crystal surface of these crystals could be regulated simply by the time and temperature of the sulfurization. Second, synthetic control of magnetic nanoparticles was examined. A novel synthesis of Iron Palladium (FePd) made possible by interdiffusion of iron into palladium nanocores was identified. Furthermore, a shell of Iron oxide (Fe2O3) could facilely be grown on the FePd nanoparticles, generating a FePd/Fe2O3 core/shell nanoparticle. These FePd/Fe2O3 core/shell particles provided an excellent foundation to create an L10- FePd/α-Fe exchange-coupled nanocomposite that exhibited improved magnetic properties compared to its single phase FePd counterpart. However, the stabilizing ligand used within this FePd synthesis doped into the final nanoparticles, degraded the magnetic properties. iii To overcome the dopant ligand problem, a novel nanoalloy synthetic strategy of Metal Redox was developed. The Metal Redox strategy utilized the inherent reducing power of zero-valent metal sources to create a vast sampling of metal nanoalloys without the need of ligands or excess reducing agents. Stoichiometry of these nanoalloys could be readily adjusted by temperature and explained by simple chemical equilibrium concepts. The Metal Redox methodology was then expanded to shape control and tri-metallic alloys. Finally, the unique MnBi nanoalloy system was created using Metal Redox, making it the first ever reported solution processed formation of this material

Symmetry-Defying Iron Pyrite (FeS2) Nanocrystals through Oriented Attachment

A grant from the One-University Open Access Fund at the University of Kansas was used to defray the author’s publication fees in this Open Access journal. The Open Access Fund, administered by librarians from the KU, KU Law, and KUMC libraries, is made possible by contributions from the offices of KU Provost, KU Vice Chancellor for Research & Graduate Studies, and KUMC Vice Chancellor for Research. For more information about the Open Access Fund, please see http://library.kumc.edu/authors-fund.xml.Iron pyrite (fool's gold, FeS2) is a promising earth abundant and environmentally benign semiconductor material that shows promise as a strong and broad absorber for photovoltaics and high energy density cathode material for batteries. However, controlling FeS2 nanocrystal formation (composition, size, shape, stoichiometry, etc.) and defect mitigation still remains a challenge. These problems represent significant limitations in the ability to control electrical, optical and electrochemical properties to exploit pyrite's full potential for sustainable energy applications. Here, we report a symmetry-defying oriented attachment FeS2 nanocrystal growth by examining the nanostructure evolution and recrystallization to uncover how the shape, size and defects of FeS2 nanocrystals changes during growth. It is demonstrated that a well-controlled reaction temperature and annealing time results in polycrystal-to-monocrystal formation and defect annihilation, which correlates with the performance of photoresponse devices. This knowledge opens up a new tactic to address pyrite's known defect problems

Ionic-passivated FeS2 photocapacitors for energy conversion and storage

This is the publisher's version, also available electronically from http://pubs.rsc.org/en/Content/ArticleLanding/2013/CC/c3cc45088k#!divAbstrac

Surface-passivated plasmonic nano-pyramids for bulk heterojunction solar cell photocurrent enhancement

This is the published version. ©Copyright 2012 Royal Society of ChemistryWe report that self-assembled gold (Au) nanopyramid arrays can greatly enhance the photocurrent of narrow bandgap organic solar cells using their plasmonic near-field effect. The plasmonic enhanced power conversion efficiency exhibited up to 200% increase under the AM 1.5 solar illumination

Nanocarbon-Based photovoltaics

Carbon materials are excellent candidates for photovoltaic solar cells: they are Earth-abundant, possess high optical absorption, and superior thermal and photostability. Here we report on solar cells with active layers made solely of carbon nanomaterials that present the same advantages of conjugated polymer-based solar cells - namely solution processable, potentially flexible, and chemically tunable - but with significantly increased photostability and the possibility to revert photodegradation. The device active layer composition is optimized using ab-initio density functional theory calculations to predict type-II band alignment and Schottky barrier formation. The best device fabricated is composed of PC70BM fullerene, semiconducting single-walled carbon nanotubes and reduced graphene oxide. It achieves a power conversion efficiency of 1.3% - a record for solar cells based on carbon as the active material - and shows significantly improved lifetime than a polymer-based device. We calculate efficiency limits of up to 13% for the devices fabricated in this work, comparable to those predicted for polymer solar cells. There is great promise for improving carbon-based solar cells considering the novelty of this type of device, the superior photostability, and the availability of a large number of carbon materials with yet untapped potential for photovoltaics. Our results indicate a new strategy for efficient carbon-based, solution-processable, thin film, photostable solar cells

Interdiffusion Induced Exchange Coupling of L1₀‑FePd/α-Fe Magnetic Nanocomposites

One-pot synthesis of FePd and FePd/Fe₂O₃ (core/shell) nanoparticles via interdiffusion is reported for the first time. It was found that the size of FePd particles and Fe₂O₃ shell thickness could be controlled by the ligand and iron precursor amounts, respectively. These FePd/Fe₂O₃ particles can be reductively annealed at 500 °C to produce exchanged coupled L1₀-FePd/α-Fe magnetic nanocomposites. The effect of the phosphine ligand on magnetic characteristics of synthesized particles and final annealed nanocomposite is discussed. Finally, it was found that the magnetic properties of the final L1₀-FePd/α-Fe nanocomposites could be tuned by Fe₂O₃ shell thickness and can reach a coercivity (<i>H</i>_c) of up to 2.4 kOe and a saturation magnetization (<i>M</i>_s) of 141 emu/g

Phase Transformation-Induced Tetragonal FeCo Nanostructures

Tetragonal FeCo nanostructures are becoming particularly attractive because of their high magnetocrystalline anisotropy and magnetization achievable without rare-earth elements, . Yet, controlling their metastable structure, size and stoichiometry is a challenging task. In this study, we demonstrate AuCu templated FeCo shell growth followed by thermally induced phase transformation of AuCu core from face-centered cubic to L1₀ structure, which triggers the FeCo shell to transform from the body-centered cubic structure to a body-centered tetragonal phase. High coercivity, 846 Oe, and saturation magnetization, 221 emu/g, are achieved in this tetragonal FeCo structure. Beyond a critical FeCo shell thickness, confirmed experimentally and by lattice mismatch calculations, the FeCo shell relaxes. The shell thickness and stoichiometry dictate the magnetic characteristics of the tetragonal FeCo shell. This study provides a general route to utilize phase transformation to fabricate high performance metastable nanomagnets, which could open up their green energy applications

Metal-Redox Synthesis of MnBi Hard Magnetic Nanoparticles

High coercivity MnBi alloy is a promising candidate as earth abundant permanent magnet for energy-critical technologies. We report here a new metal-redox method to synthesize colloidal MnBi nanoparticles, exhibiting a saturation magnetization of 49 emu/g and coercivity of 15 kOe. It is shown that the magnetic properties of the MnBi nanoalloys can be readily modified by precursor stoichiometry, temperature ramp rate, and reaction temperature, making it a versatile scalable strategy for generation of MnBi

Extraordinary Photocurrent Harvesting at Type-II Heterojunction Interfaces: Toward High Detectivity Carbon Nanotube Infrared Detectors

Despite the potentials and the efforts put in the development of uncooled carbon nanotube infrared detectors during the past two decades, their figure-of-merit detectivity remains orders of magnitude lower than that of conventional semiconductor counterparts due to the lack of efficient exciton dissociation schemes. In this paper, we report an extraordinary photocurrent harvesting configuration at a semiconducting single-walled carbon nanotube (s-SWCNT)/polymer type-II heterojunction interface, which provides highly efficient exciton dissociation through the intrinsic energy offset by designing the s-SWCNT/polymer interface band alignment. This results in significantly enhanced near-infrared detectivity of 2.3 × 10⁸ cm·Hz^1/2/W, comparable to that of the many conventional uncooled infrared detectors. With further optimization, the s-SWCNT/polymer nanohybrid uncooled infrared detectors could be highly competitive for practical applications

Synthesis and Optoelectronic Properties of Two-Dimensional FeS₂ Nanoplates

There is a growing interest in the earth abundant and nontoxic iron disulfide (FeS₂) photovoltaic materials. Here, we report the synthesis of FeS₂ nanoplates with different spectral features which we have associated with thicknesses and crystallization. The structure and crystalline order of ultrathin FeS₂ nanoplates have a strong influence on the carrier lifetime, electronic and optical properties. We demonstrate that two-dimensional FeS₂ nanoplates show great promise for fabrication of hybrid bulk heterojunction solar cells. This opens up a host of applications of these materials as inexpensive solar cells and photocatalysts