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Plasma engineering of advanced functional materials for photocatalytic wastewater treatment
Semiconductor metal oxide photocatalyst with favourable light absorption and charge transport characteristics have been widely used as a photocatalyst in various applications, to name a few, energy harvesting and storage, environmental remediation and air pollution. Energy harvesting which comprises the full utilisation of the wide solar light (wavelength) spectrum has become a central point of research in the field of materials science and engineering. Hence, the development of sustainable materials from environmentally sustainable techniques which can absorb majority of the solar light spectrum has become a huge challenge. For efficient utilisation of solar energy in catalytic applications, it is important to create photocatalyst that can absorb the full solar spectrum involving ultraviolet (UV), visible (VIS) and near infrared (NIR) wavelengths. More than three decades, TiO2 and its composites have been widely researched academically and used industrially as a low-cost material for photocatalytic applications. However, the large bandgap of TiO2 limits its photocatalytic activity to the UV region which is just 3-5% of sunlight on Earthâs atmosphere. TiO2 also suffers from rapid recombination of photogenerated carriers (i.e., holes and electrons) thereby affecting its photocatalytic efficiency. Over the years, there has been active research in altering the chemistries of TiO2 to overcome these aforementioned shortcomings. The most recent advantage is the use of two dimensional (2D) materials because of its layered structure One of the unexplored and interesting layered structure is MXene. The aim of this thesis is to modify the chemical structure of Ti2C MXene to produce TiO2 as an efficient photocatalyst for absorbing solar energy especially in the UV and visible regions. As a compound of titanium and carbon, Ti2C MXene could facilitate the creation of TiO2 and carbonaceous materials hereby improving the photocatalytic performance. The abundance of surface terminal groups on Ti2C MXene allow for ease of surface modification and functionalisation. In this thesis, for the first time, the functionalisation of TiO2 from Ti2C MXene using a dry and low powered system, atmospheric pressure plasma jet (APPJ) is reported. This process involved using Ti2C nano colloidal ink with highly reactive oxygen plasma source which can tune the electronic properties (engineering bandgap) of Ti2C MXene in-situ while simultaneously printing on to a substrate. X-ray/Ultraviolet Photoelectron spectroscopy showed an additional density of states (DOS) close to valence band edge and changes to the Ti, O core level spectra due to the oxygen plasma functionalisation. Density functional Theory calculation suggests that the changes in the electron structure might be due to the influence of oxygen vacancies and hence the increase in efficiency of catalytic process. Also, time dependent oxygen plasma functionalisation studies reveal the morphology and size of the in-situ generated TiO2 nanoparticles varied from 5-8 nm with exceptionally high photocatalytic performance.
The second aim of the thesis is to create a heterostructure of Ti2C MXene with low cost and earth abundant graphitic carbon nitride, g-C3N4 (GCN) with visible light properties. For the first time, a lower power APPJ method was reported to produce a ternary in-situ TiO2/Ti2C/GCN heterostructure. In this thesis, GCN nanosheets were used as a semiconducting photocatalyst that could efficiently harvest the energy from visible light. Ti2C MXene nanosheets acted as an excellent electron sink while providing enhanced surface area which could facilitate the interfacial charge carriers. Structural studies show the formation of heterostructure formation between Ti2C MXene and GCN. Influence of morphology and hence changes to the optical properties were discussed. The synthesized ternary in-situ TiO2/Ti2C/GCN nanosheets showed enhancement in photocatalytic performance.
The third aim of my research was to integrate TiO2 onto earth abundant natural cellulose fibres. Utilising the power of low power atmospheric pressure plasma (APPJ) to in-situ anchor TiO2 onto cellulose fibres to prevent the thermal degradation and chemical instability leading to leaching of the oxides from the cellulose fibres. APPJ in the presence of highly oxidised species caused an increase in COO- bonds which provided a strong linkage between TiO2 and cellulose materials. Also, structural studies revealed polymorphic changes in the structure of cellulose materials that improved its crystallinity and surface area for photocatalytic applications. APPJ is also able to create oxygen vacancies in the TiO2 which further reduced the bandgap of as synthesized TiO2/cellulose nanocomposites that enhanced photocatalytic applications. Toxicity studies showed that TiO2 was not cytotoxic.
This plasma modified surfaces (of all the samples) show exceptional degradation of wastewater with ternary in-situ TiO2/Ti2C/GCN showing two times more improvement in methylene blue degradation (84% degradation) as compared to in-situ TiO2/Ti2C MXene (42% degradation). Also, TiO2/cellulose bionanocomposite showed excellent adsorptive-photocatalytic performance in degrading industrial waste dye providing a clear route as nanocomposites from research into industrialisation
Zur Struktur von Kohlenstoffnanopartikeln
Kohlenstoffnanopartikeln sind ambivalenter Natur. Mit einem Produktionsvolumen von sind sie das weltweit am hĂ€ufigsten hergestellte Nanopartikelsystem (Singh und Vander Wal 2019). Die nanoskaligen Teilchen werden als Funktions- oder Elektromaterial industriell eingesetzt, finden sich aber ebenfalls in Aerosolen aus Verbrennungsprozessen, als Schadstoffe in Schadensfeuern, Energieumwandlungsprozessen und verbrennungsbasierten Antrieben. Dabei sind die globalen Gesamtemissionen aus genannten Quellen, verglichen mit der intendierten Herstellung, von gleicher GröĂenordnung (Bond et al. 2013). In diesem Zusammenhang sind sie gesundheitsgefĂ€hrdend (Kennedy 2007) und tragen erheblich zur globalen ErwĂ€rmung bei (Ramanathan und Carmichael 2008). Da sich sowohl die positiven als auch die negativen Aspekte auf den strukturellen Bauplan der Teilchen zurĂŒckfĂŒhren lassen, bespricht die vorliegende Arbeit die Struktur von Kohlenstoffnanopartikeln und deren Auswirkung auf ihre strukturassoziierten Eigenschaften. WĂ€hrend die Meso- und Mikrostruktur die GröĂenverteilungen der Aggregate sowie der PrimĂ€rteilchen beschreibt, definiert die Nanostruktur den molekularen Aufbau der PrimĂ€rpartikeln, die aus aromatischen Ringstrukturen, den Basisstruktureinheiten, mit statistisch verteilter LĂ€ngenausdehnung aufgebaut sind.
Bereits Minutolo et al. (1996), JĂ€ger et al. (1999) und Williams et al. (2007) vermuteten einen Einfluss der Ordnung, Ausdehnung und Orientierung der Basisstruktureinheiten auf das wellenlĂ€ngenabhĂ€ngige Absorptionsvermögen. Diese Hypothese wird sorgfĂ€ltig geprĂŒft, wobei ein quantitativer Zusammenhang zwischen der StrukturlĂ€nge und dem VerhĂ€ltnis der Brechungsindex-Absorptionsfunktion bei zwei monochromatischen WellenlĂ€ngen abgeleitet werden kann. Der gefundene Zusammenhang ist linearer Natur und lĂ€sst sich auf die mit wachsender StrukturlĂ€nge abnehmende optische BandlĂŒckenenergie, die wiederum das Absorptionsvermögen der Partikeln im nahinfraroten Spektralbereich determiniert, zurĂŒckfĂŒhren. Werden nun quantitative VerknĂŒpfungen zwischen einer gesuchten, fĂŒr eine spezielle Fragestellung relevanten strukturassoziierten Eigenschaft und der nanostrukturellen Teilchenkonfiguration abgeleitet, wird deren berĂŒhrungslose in situ Quantifizierung durch die Erfassung leicht zugĂ€nglicher Messinformationen möglich. Der Grundsatzbeweis einer schnellen, berĂŒhrungslosen in situ Diagnostik einer exemplarischen strukturassoziierten Teilcheneigenschaft - der OxidationsreaktivitĂ€t - wird mit der im Rahmen dieser Arbeit entwickelten Doppelpuls zeitaufgelösten laserinduzierten Inkandeszenz, DP-TiRe-LII, im Abgastrakt eines Serienmotors, der sowohl unter stationĂ€ren als auch transienten Bedingungen betrieben wird, erbracht. Damit wird gleichzeitig der erste berĂŒhrungslose Sensor fĂŒr Nanostruktur und OxidationsreaktivitĂ€t vorgestellt.
Zu verstehen, wie sich Meso-, Mikro- und Nanostruktur wĂ€hrend der Partikelbildung in AbhĂ€ngigkeit von den Bildungsrandbedingungen entwickeln, eröffnet die Möglichkeit einer gezielten Synthese von Teilchen maĂgeschneiderter Topologie. Unter Zuhilfenahme neuentwickelter invasiver und laseroptischer Methoden wird deshalb die Dynamik der StrukturverĂ€nderung der in Gegenstromflammen synthetisierten Teilchen wĂ€hrend ihrer Bildungssequenz untersucht. Sowohl der PrimĂ€rteilchendurchmesser als auch die LĂ€ngenausdehnung der Basisstruktureinheiten wachsen mit zunehmendem Volumenbruch. Folgerichtig sind Mikro- und Nanostruktur miteinander korreliert, wobei lange Struktureinheiten in vergleichsweise groĂe Partikeln eingebettet sind und vice versa.
Ein abschlieĂender Aspekt widmet sich der AufklĂ€rung der StrukturverĂ€nderung wĂ€hrend der Partikeloxidation mit molekularem Sauerstoff, wobei der Fokus auf die AbhĂ€ngigkeit von der initialen Teilchenstruktur gelegt wird. Die Mesostruktur verĂ€ndert sich mit wachsendem Oxidationsfortschritt stetig, da die Partikeldichte der Aggregate durch vollstĂ€ndige Oxidation einzelner Teilchen abnimmt und sich folglich auch der Aggregatdurchmesser reduziert. Die VerĂ€nderung der Mikrostruktur bewegt sich in AbhĂ€ngigkeit der nanostrukturellen Konfiguration zwischen zwei gekoppelten GrenzfĂ€llen, der internen und der OberflĂ€chenoxidation. Reaktionsfreudige Teilchen, die aus kurzen, gekrĂŒmmten Basisstruktureinheiten aufgebaut sind, reagieren bevorzugt in einer OberflĂ€chenoxidation. Hingegen neigen Teilchen mit langen Basisstruktureinheiten zur internen Oxidation und verlieren vergleichsweise langsam an Masse. Einer Fragmentierung langer Struktureinheiten folgt die Konversion der einzelnen Segmente
Synthesis of Quasi-Freestanding Graphene Films Using Radical Species Formed in Cold Plasmas
For over a decade, the Stinespring laboratory has investigated scalable, plasma assisted synthesis (PAS) methods for the growth of graphene films on silicon carbide (SiC). These typically utilized CF4-based inductively coupled plasma (ICP) with reactive ion etching (RIE) to selectively etch silicon from the SiC lattice. This yielded a halogenated carbon-rich surface layer which was then annealed to produce the graphene layers. The thickness of the films was controlled by the plasma parameters, and overall, the process was readily scalable to the diameter of the SiC wafer.
The PAS process reproducibly yielded two- to three-layer thick graphene films that were highly tethered to the underlying SiC substrate via an intermediate buffer layer. The buffer layer was compositionally similar to graphene. However, a significant number of graphene carbons were covalently bound to silicon atoms in the underlying substrate. This tethering lead to mixing of the film and substrate energy bands which degraded many of grapheneâs most desirable electrical properties.
The research described in this dissertation was aimed at improving graphene quality by reducing the extent of tethering using a fundamentally different plasma etching mechanism while maintaining scalability. In the ICP-RIE process, the etchant species include F and CFx (x = 1-3) radicals and their corresponding positive ions. These radicals are classified as âcold plasma speciesâ in the sense that they are nominally in thermal equilibrium with the substrate and walls of the system. In contrast, the electrons exist at extremely high temperature (energy), and the ionic species are accelerated to energies on the order of several hundred electron volts by the plasma bias voltage that exists between the plasma and substrate. As a result, the ionic species create a directional, high rate etch that is dominated by physical etching characterized by energy and momentum transfer. In contrast, the neutral radicals chemically etch the surface at a much lower rate.
In this work, the effects of physical etching due to high energy ions were eliminated by shielding the SiC substrate using a mask (e.g., quartz) supported by silicon posts. In this way, a microplasma consisting of chemically reactive cold plasma species was created in the small space between the substrate surface and the backside of the quartz mask. This process, referred to here as microplasma assisted synthesis (MPAS), was used to produce graphene films.
A parametric investigation was conducted to determine the influence of MPAS operating parameters on graphene quality. The key parameters investigated included ICP power, RIE power, etch time, various mask materials, microreactor height, substrate cooling, initial surface morphology and SiC polytype. The resulting graphene films were characterized by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM).
Following optimization of the MPAS process, some tethering of the graphene films remained. However, films produced by MPAS consistently exhibited significantly less tethering than those produced using the PAS process. Moreover, both XPS and Raman spectroscopy indicated that these films were quasi-free standing, and, in some cases, they approached free standing graphene. From a wide view, the results of these studies demonstrate the potential of MPAS as a technique for realizing the controlled synthesis of high-quality, lightly tethered mono-, and few-layer graphene films directly on an insulating substrate. On a more fundamental level, the results of these studies provide insight into the surface chemistry of radical species
Design and Fabrication of Photo-Thermal and Thermo-Electric Materials and Systems
Thermal energy is all around us. How to control and utilize it is an important topic in advanced manufacturing. This dissertation is focused on the design and fabrication of photo-thermal and thermo-electric materials and systems and is divided into three topics: pH-responsive Au@silica semi-shell nanoparticles (NPs), thermo-osmotic ionogel (TOI), and HClOâ-enhanced Fe(III/II) thermocells (TECs).
pH-responsive photothermal therapy holds promise for non-invasive antitumor treatment, but the preparation of smart photothermal agents (PTAs) remains challenging. In the first topic (Chapter 3), a simple two-step approach was developed for the precise synthesis of anisotropic Au@silica semi-shell NPs, which were then used as pH-responsive PTAs for non-invasive antitumor therapy. In the synthesis of Au@silica semi-shell NPs, the isotropic solution-synthesized Au@silica core-shell NPs were firstly self-assembled on silicon wafers to form monolayer films by drop-casting technique. Then, Au@silica semi-shell NPs were obtained after selective and directional removal of part of the silica shell by reactive ion etching. After functionalization with pH-sensitive 4-mercaptobenzoic acid (4-MBA) molecules, the semi-shell NPs achieved pH-responsive rod-shaped assembly/disassembly in physiological saline solution, thereby exhibiting pH-responsive photothermal effects. In addition, the 4-MBA-semi-shell NPs have been successfully applied to in vitro photothermal therapy of tumor cells, showing great application potential in non-invasive antitumor therapy.
Low efficiency in recovering low-grade heat remains unresolved despite decadesâ attempts. In the second topic (Chapter 4), a novel thermo-osmotic ionogel (TOI) composite was designed and fabricated to recover low-grade heat to generate electric power through thermo-induced ion gradient and selective ion diffusion. The TOI composite was assembled with crystalline ionogel (polymer-confined LiNOâ-3HâO) film, cation exchange membrane, and hydrogel film. With a 90 °C heat supply, the single TOI composite produced a high open-circuit voltage of 0.52 V, a differential thermal voltage of ~26 mV/K, a peak power density of 0.4 W/mÂČ, and a ground-breaking peak energy conversion efficiency of 11.17%. Eight pieces of such TOI composite were connected in series, demonstrating an open-circuit voltage of 3.25 volts. Such a TOI system was also demonstrated to harvest body temperature for powering a LED, opening numerous opportunities for powering wearable devices. This work opens a new door for efficient harvesting of low-grade heat by embedding thermo-osmotic conversion as an intermedia stage of thermo-electric conversion.
In addition to thermo-ionic capacitors (Chapter 4), emerging thermocells (TECs) can convert a temperature gradient into electricity continuously and thus are promising for low-grade heat harvesting. However, itâs challenging to simultaneously improve the thermopower (Se, a thermodynamics parameter) and ionic conductivity (Ïᔹ, a kinetics parameter) of TECs due to the well-known inherent interdependence between thermodynamics and kinetics. In the third topic (Chapter 5), a simple perchloric acid (HClOâ) incorporation method has been developed to enhance the charge density of the oxidant Fe(III) ions in the state-of-the-art n-type Fe(III/II)-ClOâ redox pair, thereby improving the Se and Ïᔹ simultaneously. In Fe(III/II)-ClOâ electrolyte, the addition of HClOâ composed of protons and weakly coordinating anions suppresses the deprotonation of [Fe(HâO)â]Âłâș without inducing Fe(III)-anion coordination. The n-type TEC using HClOâ-acidified Fe(III/II)-ClOâ as electrolyte and hydrophilic carbon fiber cloth as the electrode was charactered and demonstrated a Se of 1.5 mV/K (comparable to -1.4 mV/K of benchmark p-type Fe(CN)âÂłâ»/Fe(CN)ââŽâ» TECs) and an excellent temperature normalized power density of 1.19 mW/mÂČ/KÂČ (2.64 times higher than that of the state-of-the-art n-type TECs using carbon electrodes), overcoming barriers for practical p-n integrated TEC applications
Natural Toxins: Environmental Fate and Safe Water Supply
Plants, bacteria, cyanobacteria, algae and other organisms produce a vast diversity of bioactive and toxic natural compounds. We know that many of these toxins are mobile and can be produced in high amounts close to or within drinking water reservoirs. Natural toxins represent emerging classes of environmental contaminants for which we have very limited insight on occurrence, fate and effects. The konference âNatural toxins: Environmental Fate and Safe Water Supplyâ addresses knowledge gaps within the field of natural toxins, target, non-target, suspect and effect-directed analysis, distribution, fate, toxicity and management of natural toxins in aquatic environments and drinking water reservoirs. These proceedings are a collection of the abstracts to contributions presented at the conference
Aiding the conservation of two wooden Buddhist sculptures with 3D imaging and spectroscopic techniques
The conservation of Buddhist sculptures that were transferred to Europe at some point during their lifetime raises numerous questions: while these objects historically served a religious, devotional purpose, many of them currently belong to museums or private collections, where they are detached from their original context and often adapted to western taste.
A scientific study was carried out to address questions from Museo d'Arte Orientale of Turin curators in terms of whether these artifacts might be forgeries or replicas, and how they may have transformed over time. Several analytical techniques were used for materials identification and to study the production technique, ultimately aiming to discriminate the original materials from those added within later interventions
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