Investigations in Nanotechnology: Tailoring of Magnetic Nanomaterials for Electromagnetic Wave Absorption

In recent years, there has been a growing interest in magnetic nanomaterials used as microwave-absorbing materials due to military and civil applications such as stealth technology and electromagnetic interference. They are used in portable electronic devices such as smart phones and mobile PCs to protect humans from exposure to electromagnetic pollution, which could increase the risk of cancer or other neural illnesses. In defense applications, the surface of ships, submarines, and aircrafts are coated with electromagnetic absorbing materials to reduce the radar cross section (RCS) and increase stealth capabilities. Magnetic materials have long been used as radar absorbers on aircrafts, e.g., in the form of iron ball paint. The absorber is usually applied by painting the metal surface with mixtures of carbonyl iron and polymer which generates magnetic iron or ferrite particles in situ by decomposition and/or oxidation of iron carbonyl. While this approach has been demonstrated to be successful in reduction of the RCS, it can be expected that the poorly defined synthetic approach produces inhomogeneous layers of magnetic materials with a wide distribution of particle sizes and may also generate magnetic particles containing impurities. This lack of control over material structure makes it challenging to systematically study and improve magnetic materials for electromagnetic wave absorption. This research explores the application of magnetic nanomaterials for electromagnetic wave absorption. We present organic solution synthesis routes towards preparation of a wide range of magnetic nanoparticles, including soft and hard magnetic materials. We have been able to manipulate the synthetic conditions to tune the particle size, shape, and composition to tailor the magnetic properties in terms of coercivity (Hc) and magnetization (M0). We have evaluated the dependence of the permittivity (ε) and permeability (μ) of the nanoparticle assemblies on these parameters, based on which we have further derived the correlation between the electromagnetic wave absorption characteristics and the nanoscale architectures of the materials. Our research will substantially advance the fundamental understanding of the electromagnetic wave absorption behavior of magnetic nanomaterials. The materials and technology developed in this research will also have great applications in both civilian and defense industries

Advances in Middle Infrared Laser Crystals and Its Applications

In the last twenty years, there has been a growing interest in middle infrared (mid-IR) laser crystals and their application to achieve mid-IR laser radiations, which has benefited from the development of novel mid-infrared crystals and the improving quality of traditional mid-IR crystals. Moreover, these works have promoted the development of related technical applications. This Special Issue of the journal Crystals focuses on the most recent advances in mid-IR laser crystals, from materials to laser sources and applications. It aims to bring together the latest developments in novel mid-IR crystals, improvements in the quality of mid-IR crystals, mid-IR non-linear crystals and mid-IR lasers, as well as the application of mid-IR technology in spectroscopy, trace gas detection and remote sensing, optical microscopy and biomedicine. Aspiring authors are encouraged to submit their latest original research, as well as forward-looking review papers, to this Special Issue

Electrospun Polyaniline-Reduced Graphene Oxide Composite Nanofibers Based High Sensitive Ammonia Gas Sensor

Ammonia is an important gas in many power plants and industrial processes so its detection is of extreme importance in environmental monitoring and process control due to its high toxicity. Ammonia’s threshold limit is 25 ppm and the exposure time limit is 8 h, however exposure to 35 ppm is only secure for 10 min. In this work a brief introduction to ammonia aspects are presented, like its physical and chemical properties, the dangers in its manipulation, its ways of production and its sources. The application areas in which ammonia gas detection is important and needed are also referred: environmental gas analysis (e.g. intense farming), automotive-, chemical- and medical industries. In order to monitor ammonia gas in these different areas there are some requirements that must be attended. These requirements determine the choice of sensor and, therefore, several types of sensors with different characteristics were developed, like metal oxides, surface acoustic wave-, catalytic-, and optical sensors, indirect gas analyzers, and conducting polymers. All the sensors types are described, but more attention will be given to polyaniline (PANI), particularly to its characteristics, syntheses, chemical doping processes, deposition methods, transduction modes, and its adhesion to inorganic materials. Besides this, short descriptions of PANI nanostructures, the use of electrospinning in the formation of nanofibers/microfibers, and graphene and its characteristics are included. The created sensor is an instrument that tries to achieve a goal of the medical community in the control of the breath’s ammonia levels being an easy and non-invasive method for diagnostic of kidney malfunction and/or gastric ulcers. For that the device should be capable to detect different levels of ammonia gas concentrations. So, in the present work an ammonia gas sensor was developed using a conductive polymer composite which was immobilized on a carbon transducer surface. The experiments were targeted to ammonia measurements at ppb level. Ammonia gas measurements were carried out in the concentration range from 1 ppb to 500 ppb. A commercial substrate was used; screen-printed carbon electrodes. After adequate surface pre-treatment of the substrate, its electrodes were covered by a nanofibrous polymeric composite. The conducting polyaniline doped with sulfuric acid (H2SO4) was blended with reduced graphene oxide (RGO) obtained by wet chemical synthesis. This composite formed the basis for the formation of nanofibers by electrospinning. Nanofibers will increase the sensitivity of the sensing material. The electrospun PANI-RGO fibers were placed on the substrate and then dried at ambient temperature. Amperometric measurements were performed at different ammonia gas concentrations (1 to 500 ppb). The I-V characteristics were registered and some interfering gases were studied (NO2, ethanol, and acetone). The gas samples were prepared in a custom setup and were diluted with dry nitrogen gas. Electrospun nanofibers of PANI-RGO composite demonstrated an enhancement in NH3 gas detection when comparing with only electrospun PANI nanofibers. Was visible higher range of resistance at concentrations from 1 to 500 ppb. It was also observed that the sensor had stable, reproducible and recoverable properties. Moreover, it had better response and recovery times. The new sensing material of the developed sensor demonstrated to be a good candidate for ammonia gas determination.O amoníaco é um elemento importante em muitas centrais elétricas e processos industriais, tornando-se extremamente importante a sua deteção na monitorização ambiental e para o controlo dos processos devido à sua alta toxicidade. O limite máximo de exposição é de 25 ppm para um limite de tempo de 8 h sendo que para 35 ppm o limite de exposição é drasticamente reduzido para apenas 10 min. Neste trabalho é apresentada uma breve introdução às características do amoníaco tais como suas propriedades físicas e químicas, os perigos na sua manipulação, as suas formas de produção e as suas fontes. Também serão indicadas as áreas de aplicação onde é importante e necessário a deteção do gás amoníaco sendo elas a monitorização dos gases ambientais (por exemplo, agricultura intensiva), as indústrias automóveis, as indústrias químicas e as indústrias médicas. Com a finalidade de monitorar as diversas áreas tem de se cumprir alguns requisitos, os quais irão condicionar a escolha do sensor a utilizar. Devido a esse fator vários tipos de sensores foram desenvolvidos com diferentes características, tais como, os óxidos metálicos, os de onda acústica de superfície, os catalíticos, os óticos, os detetores de gás que o fazem de forma indireta e os polímeros condutores. Todos os tipos de sensores serão descritos mas será dada maior atenção aos sensores modificados que utilizam a polianilina (PANI). Assim, serão descritas as suas características, formas de síntese, processos de doping, formas de a depositar, modos de transdução e a formas de adesão aos materiais inorgânicos. Será ainda incluída uma descrição das suas nanoestruturas, da técnica electrospinning usada na criação de nanofibras e microfibras e ainda também do grafeno tal como as suas características. O sensor criado procura ser um instrumento que vá de encontro com um objetivo da comunidade médica no controlo dos níveis de amoníaco presentes na respiração, sendo um método fácil e não-invasivo para o diagnóstico do mau funcionamento dos rins e/ou úlceras gástricas. Para isso, o dispositivo teria de ser capaz de detetar diferentes níveis de concentrações de gás amoníaco. Portanto, para este trabalho foi desenvolvido um sensor para a deteção de gás amoníaco utilizando um polímero condutor composto o qual foi imobilizado num transdutor de superfície em carbono (substrato comercial). O trabalho experimental realizado foi direcionado para a deteção de várias concentrações de gás amoníaco na escala ppb. As medições de gás amoníaco foram realizadas num intervalo de concentrações que vai de 1 ppb até 500 ppb. Após um pré-tratamento adequado do substrato, a área de trabalho dos elétrodos foi coberta por nanofibras de um polímero composto. O polímero composto, obtido através de síntese química, foi PANI dopada (H2SO4) com óxido de grafeno reduzido (RGO). Este composto foi a base para a formação de nanofibras através da técnica de electrospinning. As nanofibras vão potenciar a sensibilidade do material sensitivo. As fibras de PANI-RGO foram posteriormente depositadas no substrato comercial e, em seguida, procedeu-se à sua secagem à temperatura ambiente. Foram efetuadas medições amperométricas para diferentes concentrações de gás amoníaco (1 a 500 ppb). Foram também obtidas as características I-V dos sensores e foi realizado um estudo de interferência de alguns gases (NO2, etanol e acetona) na análise. As amostras de gás foram preparadas num sistema de configuração personalizada e as diluições realizaram-se com azoto gasoso seco. As nanofibras criadas a partir do composto PANI-RGO por meio da técnica de electrospinning, demonstraram uma melhoria na deteção de gás amoníaco quando comparado com as nanofibras preparadas só de PANI. Foi visível um maior intervalo de valores de resistência para concentrações de 1 a 500 ppb. Também foi observado que o sensor possui boas propriedades tais como a estabilidade, reprodutibilidade e capacidade de recuperação. Além disso, apresentou melhores tempos de resposta e de recuperação. O novo material sensitivo do sensor desenvolvido demonstrou ser um bom candidato para a determinação de gás amoníaco

Production and processing of graphene and related materials

© 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown

41st Rocky Mountain Conference on Analytical Chemistry

Final program, abstracts, and information about the 41st annual meeting of the Rocky Mountain Conference on Analytical Chemistry, co-sponsored by the Colorado Section of the American Chemical Society and the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Denver, Colorado, August 1-5, 1999