Bismuth-Based Nanoparticles as Photocatalytic Materials

Bismuth-based nanoparticles are a unique category of materials that possess interesting properties such as excellent chemical, electrical, optical and catalytic activities among others. The application of bismuth-based nanoparticles as photocatalytic materials has caught the interest of the scientific community in recent times due to these unique properties. Consequently, a number of data have been generated in relation to the photocatalytic application of these nanoparticles. This chapter intends to organise and provide the recently generated information on the use of bismuth-based nanoparticles in photocatalytic degradation processes. A detailed discussion is provided on bismuth-based nanoparticles including bismuth chalcogenides, bismuth vanadate, bismuth oxyhalides and other bismuth-related nanoparticles. Attention was also paid to the modification of these nanoparticles to improve their photocatalytic activities. The application of the modified nanoparticles in various photocatalytic processes with emphasis on water treatment, waste gas treatment, hydrogen production and air purification has also been thoroughly discussed

Chemically Engineered Metal Sulfides and Oxides as Electrode Materials for Li-Ion and Photochargeable Batteries

Lithium-ion batteries have dominated battery technologies for portable electronic devices for decades, commanded by intercalation chemistry for electrochemical energy storage while pushing the limits of storage capacity worldwide to the terawatt-hour level. State-of-the-art intercalation cathodes for Li-ion batteries operate within the limits of transition metal oxide electrochemistry. However, conversion-redox processes have rich opportunities for substantially increasing energy densities. Due to the limitations of both the Li content and the extraction of one electron per transition metal, the target energy density of 500 Wh kg-1 of classical layered oxides at the cell level remains elusive. The diversity of compositions that exhibit high reversible capacities following a conversion redox reaction in the solid state has inspired the exploration of new materials for next-generation cathodes for lithium batteries and beyond. While thinking beyond, the electrification of the aviation sector is a game-changer for future transport. Therefore, the requirements on battery technologies must focus even more on high power density, high energy density with fast-charging capability and having low weight and compacted cell design. In addition, battery safety aspects and sustainability of energy materials are key challenges. To address some of the challenges, synthesis and surface engineering of high energy density and fast-charging materials, as well as the development and comprehensive characterization of metal-sulfur batteries for Li-S, Mg-S, and hybrid Li/Mg-S systems, were studied in this thesis. In terms of fast-charging electrode materials, TiNb2O7 was modified by a carbon-coating to improve the charge conduction and specific capacity at high current rates. Further, novel concepts towards photoresponsive cathode materials, such as vanadium pentoxide were investigated in a lithium-ion photo battery. This study reports on the optimization of dual functionality by chemical surface engineering of electrospun vanadium pentoxide fibers as photoresponsive cathodes in lithium-ion batteries. To meet the demand for high energy density in metal-sulfur batteries, lithium-sulfur and mag- nesium-sulfur batteries were explored. For Li-S system, a synthetic approach based on a new molecular precursor [(LiSC2H4)2NCH3] to form lithium sulfide/carbon nano- fibers as cathode was pursued. Suitable cathode materials based on metal sulfides, mainly copper, iron, and copper-iron-sulfides, were investigated in terms of their suitability for rechargeable magnesium batteries due to their high theoretical capacities (Mg: 3,833 mAh/cm3; 2,205 mAh/g) and high abundance in the earth’s crust (23,300 ppm). The influence of their crystal structures, particle morphologies, and nano-sized effects were tested to elucidate and further understand the electrochemical behavior with Mg2+ as the active ion. Introducing a small amount of Li-containing salts to the magnesium electrolyte, resulted in a hybrid electrolyte that showed great potential to improve the electrochemical behavior in combination with metal sulfides following a conversion reaction mechanism

LAYER-BY-LAYER ASSEMBLY OF {NANOCLAY-(SOL-GEL OXIDE)}N AND {NANOCLAY-(OXIDE NANOPARTICLE)}N MULTILAYERS: SYNTHESIS AND GROWTH MECHANISMS

Two new classes of all-inorganic nanostructured multilayers, {MMTx-(sol-gel oxide)}n and {MMTx-(oxide nanoparticle)}n, have been successfully synthesized for the first time. They were made by adapting a layer-by-layer (LbL) assembly method initially developed to synthesize polyelectrolyte-based multilayers. In most previous studies, this electrostatic-assisted LbL assembly method used polyelectrolytes/polymers as the \u27structural glue\u27 to prepare multilayers. The synthesis of {MMTx-(sol-gel ZrO2)}n multilayers demonstrates for the first time the feasibility of making sol-gel oxide \u27glued\u27 all-inorganic multilayers, thereby introducing an innovative nanoscale fabrication concept. The synthesis of {MMTx-(oxide nanoparticle)}n multilayers further illustrates the versatility of LbL assembly technique by achieving a second new type of all-inorganic multilayers with a novel \u27plate-ball\u27 architecture. The feasibility of synthesizing other types of multilayer structures, including {MMTx-(ionic liquid)}n, {(carbon nanotube)-(sol-gel ZrO2)}n, and {polymer-(sol-gel ZrO2)}n, were also explored. Systematical investigations of the growth kinetics of {MMTx-(sol-gel ZrO2)}n multilayers reveal unique underlying mechanisms for electrostatic-assisted growth of sol-gel films and LbL assembly. The growth of the MMT and sol-gel ZrO2 layers is strongly coupled. For fresh aqueous ZrO2 precursors, the growth rates of sol-gel ZrO2 layers on MMT surfaces as functions of time and precursor concentration do not follow the standard mass transfer or interfacial reaction controlled kinetic models. Furthermore, the growth of the sol-gel oxide layers on MMT surfaces is self-limited to a maximum thickness of ~50-60 nm. These observations suggest a surface-mediated growth of sol-gel oxide layers on MMT surfaces, one that is likely controlled by electrostatic interactions. These new findings significantly advance the general understanding of the LbL electrostatic assembly process. For the aged precursors, the growth mechanism differs; the growth of sol-gel oxide layers is controlled by hydrodynamics and follows the Landau-Levich model. For as-deposited multilayers, isothermal annealing at ~400 °C dehydrates them and removes the residue acetate groups without damaging the MMT nanoplatelets and the ordered layer structures. Nanomechanical measurements show that the elastic modulus of the multilayers can be intentionally tuned by changing the multilayer design and that significant porosity is present in the multilayers even after annealing. In addition, free-standing multilayers are successfully made via using sacrificial substrates, and the newly developed methodology for {MMTx-(sol-gel oxide)}n can be extended, using other metal oxides, e.g., SnO2, as the inorganic \u27glue.\u27 Potential applications of these new nanostructured multilayers are discussed

Accessing Metastable Solid-Solution Nanoparticles from Solution-Phase Condensation Reactions: Applications in High-K Dielectrics, Geopolymerization, and X-Ray Phosphors

This dissertation focuses on the design, synthesis, and functional applications of ceramic materials prepared with precise compositional, dimensional, and structural control from molecular precursors using a versatile sol—gel condensation process. Three primary thrusts have stemmed from this central idea: (i) mapping the size-dependent phase diagram of HfOv2 and stabilizing the metastable tetragonal phase of HfOv2 at room temperature as a result of dimensional confinement, thereby obtaining a technologically important high-dielectric-constant polymorph that is only accessible above a temperature of 1720°C in the bulk; (ii) developing a method to cross-link plant fibers through creation of siloxane frameworks, resulting in the stabilization of a mechanically resilient load-bearing composite for roadworks in the Alberta Oil Sands; and (iii) stabilizing solid-solution rare earth oxychloride (REOCl) nanocrystals across a broad compositional range to obtain a full palette of X-ray phosphors, allowing for elucidation of activation channels, sensitization mechanisms, and recombination pathways underpinning X-ray-activated optical luminescence. The dissertation develops a versatile synthetic toolbox for defining oxide and oxyhalide frameworks. The choice of molecular precursors and ligands added during synthesis strongly influence kinetics of particle growth and allow for compositional control as well as tunability of particle dimensions. The metastable materials synthesized in this work have allowed for exploration of the size-dependent phase diagram of HfO2 and have enabled the development of quaternary and quintary solid-solution phosphors based on the PbFCl-type LaOCl and GdOCl frameworks

The recent advances in the mechanical properties of self-standing two-dimensional MXene-based nanostructures: Deep insights into the supercapacitor

MXenes have emerged as promising materials for various mechanical applications due to their outstanding physicochemical merits, multilayered structures, excellent strength, flexibility, and electrical conductivity. Despite the substantial progress achieved in the rational design of MXenes nanostructures, the tutorial reviews on the mechanical properties of self-standing MXenes were not yet reported to our knowledge. Thus, it is essential to provide timely updates of the mechanical properties of MXenes, due to the explosion of publications in this filed. In pursuit of this aim, this review is dedicated to highlighting the recent advances in the rational design of self-standing MXene with unique mechanical properties for various applications. This includes elastic properties, ideal strengths, bending rigidity, adhesion, and sliding resistance theoretically as well as experimentally supported with various representative paradigms. Meanwhile, the mechanical properties of self-standing MXenes were compared with hybrid MXenes and various 2D materials. Then, the utilization of MXenes as supercapacitors for energy storage is also discussed. This review can provide a roadmap for the scientists to tailor the mechanical properties of MXene-based materials for the new generations of energy and sensor devices. 2020 by the authors. Licensee MDPI, Basel, Switzerland.Scopu

Novel liquid phase routes for the synthesis of metal sulphide nanomaterials and their thin films

The unique properties exhibited by many sulphide materials have driven research interest in recent years. As new dimensionalities and morphologies continue to be isolated, a vast array of useful materials is discovered and their emerging applications are realised. Amongst these morphologies, two-dimensional transition metal dichalcogenides have gained significant attention. This PhD research focuses on some of the most important aspects of two-dimensional metal sulphides: their exfoliation, conversion of their dispersions into thin films and eventually enhancing their optical properties. Due to the general dimension-dependent properties of layered metal sulphide crystals, it is important to have control over the thickness and lateral size during preparation of such two-dimensional nanosheets. Despite many past advances, there is still ample opportunity to develop improved exfoliation techniques, which becomes one of the main focuses of this research. Traditionally, liquid-phase exfoliation of layered sulphide materials is performed using organic solvents, due to their surface energies providing superior nanoflake dispersibility. More recently, the use of surfactants has been established, to improve the nanoflake yield in alternative solvents. During this PhD research, the author established a new sonication-assisted biocompatible molybdenum disulphide (MoS2) exfoliation technique using the bile salt, chenodeoxycholic acid, in a water-ethanol solution. The method was shown to produce high quality nanoflakes in a reasonable yield. A mechanically gentle, reductive exfoliation method was also explored for the synthesis of laterally large ultrathin nanosheets of MoS2. Following successful exfoliation of layered MoS2, the method was then further explored to exfoliate quasi-stratified Bi2S3 crystals. The crystal structure of Bi2S3 comprises rows of stacked ribbons which are held together through van der Waals forces in two directions. The anisotropic structure favours the formation of one-dimensional nanomaterials and exfoliation of two-dimensional layers has not previously been demonstrated. As such, the work presented in this thesis introduces the first report of exfoliation of Bi2S3 into micron-scale ultrathin nanosheets as a novel step in creating planar structures from crystals that are not completely stratified. The lateral dimensions of the sheets were in the order of 10s of μm wide, with thickness reduced to one or two fundamental layers. The p-type nanosheets, which contained sulphur vacancies, were shown to be selectively sensitive to NO2 gas, with a fast response attributed to strong physisorption. Another area that requires research attention is the translation of suspended exfoliated metal sulphide nanoflakes into thin films; especially for the development of future functional systems. In most cases, techniques such as spin-coating and drop-casting are used to deposit suspended two-dimensional materials, resulting in films that are non-uniform and have poor coverage. A recent report outlines the treatment of exfoliated transition metal dichalcogenide nanoflakes with chemical modifiers, before injection into a pre-defined liquid-liquid interface, resulting in an assembled film. In this PhD research, the author explored a more efficient assembly process for exfoliated nanoflakes, where a liquid-liquid interface was established directly from the suspended particles without the addition of any inducing agents. Advantageously, avoiding chemical processing reduced the influence on the properties of the nanoflakes in the resulting film. Controlled deposition of thin films from assembled nanoflakes was also achieved through the use of hydrophobic patterned substrates. An efficient film assembly and dip coating of the substrates results in large-scale uniform patterned thin films of tungsten disulphide (WS2) and MoS2 nanoflakes. Composite films are also established through simply mixing two different nanoflake suspensions prior to the formation of the liquid-liquid interface. Film characterisation showed that the MoS2 and WS2 were evenly dispersed throughout the composite thin film, with no isolated regions of the individual materials. One of the most promising properties of monolayer MoS2 is that it displays photoluminescence, although the low emission is not ideal for practical optical applications. Extensive studies on quantum dot emission optimisation, through surface passivation, drew our attention to the possibility of incorporating MoS2 into a hybrid structure to enhance its photoluminescence. Recent studies report composites of MoS2 nanosheets with nanoparticle decoration, or stacked in layered heterostructures, but no hybrid quantum dots have been demonstrated. In this thesis, the author presents the hydrothermal conversion of MoS2 nanoflakes into quantum dots with simultaneous ZnS growth. The hybrid particles were found to have a narrow size distribution and enhanced photoluminescence quantum yield compared to the exfoliated nanoflakes. The emission was found to be excitation-wavelength-independent, which would make it easier to monitor their response in optical sensing applications. The developed water-stable hybrid quantum dots provide a biocompatible alternative to the traditional synthesis of toxic core-shell quantum dots in harsh organic solvents. Overall, the author of this thesis believes that this research contributed to the advancement of nanotechnology through the development of several new morphologies of metal sulphides and added to the ever-growing body of knowledge in the field of two-dimensional materials

Towards Green, Enhanced Photocatalysts for Hydrogen Evolution

This book gathers selected research on the preparation, characterization and application of new organic/inorganic composites endowed with photo(electro)catalytic properties for the photocatalytic production of H2. In these pilot studies, the photoactive materials were tested under either UV-visible or, even more conveniently, under visible light for H2 evolution in “sacrificial water splitting” or “photoreforming” systems. In addition, a review article on the use of 2D materials and composites as potential photocatalysts for water splitting is included

Morphological influences on the photocatalytic activity of a CsTaWO6 model system

In the present work, the influence of different morphologies on the photocatalytic activity of a semiconducting transition metal oxide was investigated. The quaternary photocatalyst caesium tantalum tungstate (CsTaWO6) was therefore used as a model system. This compound exhibits a number of advantages compared to other photocatalysts such as titanium dioxide (TiO2) due to its crystallization in just one cubic crystal structure and its beneficial band positions for the test reaction of photocatalytic hydrogen generation. CsTaWO6 was nanostructured via hydrothermal and sol-gel processes to investigate the influence of surface area, crystallinity and pore sizes on the photocatalytic activity. It was possible to synthesize single crystal CsTaWO6 nanoparticles with different crystallite sizes and mesoporous materials with a number of pore sizes, pore ordering and resulting surface areas. When looking at the photocatalytic hydrogen evolution of all the investigated samples, it could be shown that an increase in surface area does not correlate with an increase in activity. In fact, the optimum crystallite size was found to be the most important aspect in nanostructured photocatalyst, whereas for CsTaWO6 this optimum lies at 12 to 13 nm. Furthermore, it could be shown that larger pores lead to an enhanced hydrogen production in mesoporous photocatalysts. In general, a decrease of the synthesis temperature resulted in an increase in the defect concentration (estimated from strain parameters) and therefore a lower activity. The optimum morphology for the CsTaWO6 system was found to be a mesoporous material with large pores (up to 40 nm), thin pore walls and an optimum crystallite size of approximately 12 nm.In der vorliegenden Arbeit wurde der Einfluss unterschiedlicher Morphologie auf die photokatalytische Aktivität eines halbleitenden Übergangsmetalloxids untersucht. Dazu wurde der quaternäre Photokatalysator Cäsiumtantalwolframat (CsTaWO6) als Modellsystem verwendet. Diese Verbindung besitzt gegenüber anderen Photokatalysatoren wie Titandioxid (TiO2) den Vorteil, dass es nur in einer einzigen, kubischen Kristallstruktur kristallisiert und zudem über Bandpositionen verfügt, die für die Testreaktion der photokatalytischen Wasserstoffproduktion optimal liegen. CsTaWO6 wurde über Hydrothermal- und Sol-Gel-Synthesen nanostrukturiert, um den Einfluss von Oberfläche sowie Kristallit- und Porengröße auf die photokatalytische Aktivität zu überprüfen. Dabei konnten einkristalline CsTaWO6-Nanopartikel in unterschiedlichen Kristallitgrößen hergestellt werden, sowie mesoporöse Materialien mit einer Reihe von Porengrößen, Porenordnung und daraus resultierenden Oberflächen. Bei Betrachtung der photokatalytischen Wasserstoffentwicklung aller präparierten Proben konnte gezeigt werden, dass eine Oberflächenvergrößerung nicht mit einer Vergrößerung der Aktivität korreliert. Vielmehr kann die optimale Kristallitgröße als wichtigster Aspekt in nanostrukturierten Photokatalysatoren angesehen werden, wobei dieser Wert im Falle des CsTaWO6 bei 12 bis 13 nm liegt. Zudem konnte herausgefunden werden, dass große Poren in mesoporösen Photokatalysatoren zu einer verbesserten Wasserstoff¬entwicklung beitragen. Die Abhängigkeit der Synthesetemperatur auf die Aktivität wurde auf die steigende Defektkonzentration mit sinkender Temperatur zurückgeführt, wobei mit dieser auch die photokatalytische Wasserstoffproduktion abnahm. Als optimale Morphologie konnte für CsTaWO6 ein mesoporöses Material mit großen Poren (bis 40 nm), dünnen Porenwänden und Kristallitgrößen um das für CsTaWO6 gefundene Optimum von 12 nm bestimmt werden

Interfacial Chemistry at the Surfaces of Engineered and Natural Nanomaterials

The central theme of my research seeks to understand the interfacial chemistry of engineered and natural nanomaterials. Manipulation of the surface chemistry of nanostructures is an important tool in tuning their properties for various applications, given that these properties are greatly influenced by the high abundance of defects and dangling bonds on the surface. When an ad-atom, molecule, or periodic solid interacts with the surface of a material, the interaction can be classified as either physisorptive or chemisorptive. Herein, I present three disparate areas of research, which explore interfacial interactions at nanostructured surfaces. Emphasis on the chemisorption and physisorption on engineered nanomaterials is provided in the first and third projects. The first project illustrates that graphene oxide is partially reduced when used as a substrate for the atomic layer deposition (ALD) of amorphous HfOv2. Understanding the interfacial chemistry between graphene oxide and HfOv2 provides new knowledge on designing graphene-based field effect transistors and semiconductor heterostructures for catalysis. In the third project, a novel ex situ doping technique for modulating the metal—insulator transition (TvMIT) of VOv2 has been developed. Initial deposition of the molecular boron precursor 2-allyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane on the surface of VOv2 nanowires allows for the subsequent incorporation of B atoms in the tetrahedral interstitial sites of VOv2 upon rapid thermal annealing, which results in the stabilization of the rutile phase in greater proximity to room temperature. The diffusive annealing process can be tuned to program the TvMIT of VOv2 for applications such as thermochromic fenestration and the design of memory devices. The second project is focused on natural nanomaterials, wherein the interactions of Ag-Au bimetallic alloy nanoparticles in aquatic media have been investigated. The growth of these alloy nanoparticles is mediated by dissolved organic matter (DOMs) such as fulvic and humic acids, with or without photoillumination from natural sunlight. In the absence of natural sunlight, Ag- and Au-ions are first complexed with Lewis basic groups (carbonyls, carboxyls, thiols) on the DOM; subsequently, alloy formation is facilitated by galvanic replacement. Under visible light irradiation, Ag, Au, and Ag-Au bimetallic alloy nanocrystals are grown via a plasmon-induced mechanism. The study of the interfacial chemistry at the surfaces of these nanomaterials paves way for the rational design of various architectures which can be used for various applications such as catalysis, environmental remediation, and thermochromic fenestration

Faceted nanomaterial synthesis, characterizations and applications in reactive electrochemical membrane filtration

Facet engineering of nanomaterials, especially metals and metal oxides has become an important strategy for tuning catalytic properties and functions from heterogeneous catalysis to electrochemical catalysis, photocatalysis, biomedicine, fuel cells, and gas sensors. The catalytic properties are highly related to the surface electronic structures, surface electron transport characteristics, and active center structures of catalysts, which can be tailored by surface facet control. The aim of this doctoral dissertation research is to study the facet-dependent properties of metal or metal oxide nanoparticles using multiple advanced characterization techniques. Specifically, the novel atomic force microscope-scanning electrochemical microscope (AFM-SECM) and density functional theory (DFT) calculations were both applied to both experimentally and theoretically investigate facet dependent electrochemical properties, molecular adsorption, and dissolution properties of cuprous oxide and silver nanoparticles. To promote the facet engineered nanomaterials for environmental engineering apparitions, our research has evaluated the performances of electrochemically reactive membranes that were prepared with novel 2D nanomaterials with surface functioal modifications to enable electrochemical advanced oxidation processes (EAOPs) in membrane filtration process. Our results demonstrated many advantages such as tunable reactivity, tailored surface reactions, antifouling features, and feasibility of large-scale continuous operations. Specifically, this dissertation will introduce our electrochemical membrane synthesis, reactivity, aging, byproducts formation and electrochemical adsorption and desorption, oxidation of pollutants such as two typical per-and poly-fluoroalkyl substances (PFAS), perfluorooctanoic Acid (PFOA) and perfluorobutanoic acid (PFBA)