De nos jours, les émulsions stabilisées par adsorption de particules colloïdales à l'interface liquide-liquide (émulsions de Pickering) présentent un intérêt pour une grande variété d'applications allant des produits pharmaceutiques ou alimentaires aux modèles pour la préparation de nouveaux matériaux. Dans cette thèse, des émulsions huile-dans-eau (H/E) et eau-dans-huile (E/H) ont été efficacement stabilisées grâce à des particules inorganiques colloïdales (oxyde de graphène (GO) et nanofeuillets de nitrure de bore (h-BNNS)). L'adsorption de particules à l'interface huile-eau est induite par l'ajustement de la mouillabilité des particules dans les milieux liquides. Deux types d'émulsions, H/E et E/H, sont formées en utilisant des matériaux bidimensionnels qui possèdent des comportements hydrophiles différents. Les conditions requises pour atteindre l'émulsion la plus stable sont étudiées en variant la formulation de l’émulsion pour chaque type de particules. Les microstructures finales des émulsions peuvent être modifiées en ajustant leur composition initiale. L'utilisation d'une concentration élevée de particules améliore la stabilité des émulsions. Des émulsions à base de h-BNNS ont été rapportées dans ce travail pour la première fois et leur comportement a été profondément étudié. De plus, une nouvelle approche verte pour obtenir des membranes poreuses à base d'alcool polyvinylique (PVA) a été rapportée. Dans ce cas, l'ajout de PVA à l'émulsion augmente sa stabilité à long terme et permet sa mise en forme à l'aide de technologies conventionnelles telles que l’étalement. Les composites polymères obtenus à partir d'émulsions Pickering présentent une microporosité de 0,19 ± 0,03 µm ou 1,1 ± 0,3 µm en fonction du temps de séchage. Les membranes poreuses obtenues présentent de bonnes performances en matière de perméabilité à l'eau et de rejet des particules. Pour des membranes ayant une taille de pores d'environ 1,1 µm et une perméabilité à l'eau d'environ 2000 L / h / m2, un taux de rejection de 86% a été mesuré avec des particules de la même taille que les pores. / Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have been of interest in a wide variety of applications, ranging from pharmaceutical or food products to templates for the preparation of new materials. In this thesis, oil-in-water (O/W) and water-in-oil (W/O) emulsions are efficiently stabilized using colloidal inorganic particles (graphene oxide (GO) and hexagonal boron nitride nanosheets (h-BNNS)). The adsorption of particles to the oil-water interface is induced by adjusting the particle wetting behavior in the liquid media. Two types of emulsions, O/W and W/O are formed by using two-dimensional materials possessing different hydrophilic behaviors. The conditions required to reach the most stable emulsion using two different types of particles at different formulations are investigated. The final microstructures of the mixtures are tailored by adjusting the initial composition of emulsion. The use of high concentration of particles leads to enhanced stability of particles-stabilized emulsions. h-BNNS based emulsions were reported in this work for the first time and their behavior was deeply investigated. Furthermore, a novel green approach to obtain polyvinyl alcohol (PVA)-based porous membranes was reported. In this case, the addition of PVA to the emulsion increases its long term stability and allows its shaping using conventional technologies such as casting. The polymer composites obtained from emulsions stabilized with inorganic particles exhibit microporosity, showing typical pore dimensions of 0.19 ± 0.03 µm or 1.1 ± 0.3 µm depending on the curing time. These obtained porous membranes display good performance in water permeability and particle rejection. Membranes displaying a pore size about 1.1 µm showed water permeability about 2000 L/h m2 bar, and a rejection rate of 86 % with particles of the same size than the pores.

Following the discovery of graphene, 2D nanostructures have been noted for their potential in a range of high-impact applications, such as sensing, catalysis, and composite reinforcement. Liquid-phase exfoliation and chemical vapour deposition have been demonstrated and indicate the feasibility of mass-scale production. With the advent of mass-produced 2D nanostructures a key focus of research is to characterise these materials. This thesis is concerned with imaging and structural properties of the 2D nanomaterials, hexagonal boron nitride (h-BN), molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), titanium disulfide (TiS₂) and hexabenzocoronene (HBC), produced via liquid phase exfoliation. HBC strictly speaking is not 2D nanomaterial, however, it can be viewed as transition molecule from benzene to graphene. The data used for characterisation is based primarily on electron diffraction and, in particular, aberration corrected annular dark field (ADF) scanning transmission electron microscopy (STEM). The incoherent nature of ADF STEM provides direct atomic imaging without the contrast reversals upon focus changes seen in conventional high-resolution transmission electron microscopy (HRTEM). The main structural feature investigated in this thesis was the stacking sequences in few-layers h-BN, MoS₂, WS₂ and TiS₂. Simple stacking (AAA) can be distinguished from Bernal (ABA) and rhombohedral (ABC) on the basis of intensity ratio, I_{10̅10}/I_{11̅20} , in diffraction patterns and indirectly in HRTEM images. Nonetheless acquisition of the diffraction patterns suitable for analysis can be challenging due to the sample issues. Non-bulk stacking sequences were reliably confirmed for all above 2D nanomaterials on the basis of atomically resolved ADF STEM. 20 h-BN, 28 MoS₂, 5 WS₂ and 6 TiS₂ nanoflakes were imaged and analysed. Amongst them 2 h-BN, 9 MoS2, 4 WS2 and 1 TiS2 nanoflakes displayed non-bulk stacking. Hence, it appears that 2D WS2 has the greatest affinity for non-bulk stacking. Finally, an interesting structural transformation was observed in HBC molecules. Under the influence of electron beam HBC agglomerates were transformed into crystalline phase with 90^o symmetry.

abstract: Liquid-phase exfoliation (LPE) is a straightforward and scalable method of producing two-dimensional nanomaterials. The LPE process has typical been applied to layered van der Waals (vdW) solids, such as graphite and transition metal dichalcogenides, which have layers held together by weak van der Waals interactions. However, recent research has shown that solids with stronger bonds and non-layered structures can be converted to solution-stabilized nanosheets via LPE, some of which have shown to have interesting optical, magnetic, and photocatalytic properties. In this work, two classes of non-vdW solids – hexagonal metal diborides and boron carbide – are investigated for their morphological features, their chemical and crystallographic compositions, and their solvent preference for exfoliation. Spectroscopic and microscopic techniques are used to verify the composition and crystal structure of metal diboride nanosheets. Their application as mechanical fillers is demonstrated by incorporation into polymer nanocomposite films of polyvinyl alcohol and by successful integration into liquid photocurable 3D printing resins. Application of Hansen solubility theory to two metal diboride compositions enables extrapolation of their affinities for certain solvents and is also used to find solvent blends suitable for the nanosheets. Boron carbide nanosheets are examined for their size and thickness and their exfoliation planes are computationally analyzed and experimentally investigated using high-resolution transmission electron microscopy. The resulting analyses indicate that the exfoliation of boron carbide leads to multiple observed exfoliation planes upon LPE processing. Overall, these studies provide insight into the production and applications of LPE-produced nanosheets derived from non-vdW solids and suggest their potential application as mechanical fillers in polymer nanocomposites. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

Indiana University-Purdue University Indianapolis (IUPUI) / The field of two-dimensional (2D) nanomaterials first began in earnest with the discovery of graphene in 2004 due to their unique shape-dependent optical, electronic, and mechanical properties. These properties arise due to their one-dimensional confinement and are further influenced by the elemental composition of the inorganic crystal lattice. There has been an intense focus on developing new compositions of 2D nanomaterials to take advantage of their intrinsic beneficial properties in a variety of applications including catalysis, energy storage and harvesting, sensing, and polymer nanocomposites. However, compared to the field of bulk materials, the influence of surface chemistry on 2D nanomaterials is still underdeveloped. 2D nanomaterials are considered an “all-surface” atomic structure with heights of a single to few layers of atoms. The synthetic methods used to produce 2D materials include bottom-up colloidal methods and top-down exfoliation related techniques. Both cases result in poorly controlled surface chemistry with many undercoordinated surface atoms and/or undesirable molecules bound to the surface. Considering the importance surfaces play in most applications (i.e., catalysis and polymer processing) it is imperative to better understand how to manipulate the surface of 2D nanomaterials to unlock their full technological potential. Through a focus of the ligand-surface atom bonding in addition to the overall ligand structure we highlight the ability to direct morphological outcomes in lead free halide perovskites, maximize optoelectronic responses in substoichiometric tungsten oxide, and alter physicochemical properties titanium carbide MXenes. The careful control of precursor materials including poly(ethylene glycol) (PEG) surface ligands during the synthesis of bismuth halide perovskites resulted in the formation of 2D quasi-Ruddlesden-Popper phase nanomaterials. Through small angle X-ray scattering (SAXS) and in conjunction with X-ray photoelectron spectroscopy (XPS) we were able to conclude that an in-situ formation of an amino functional group on our PEG-amine ligand was inserted into the perovskite crystal lattice enabling 2D morphology formation. Additionally, through UV-vis absorption and ultraviolet photoelectron spectroscopies we were able to develop a complete electronic band structure of materials containing varying halides (i.e., Cl, Br, and I). Furthermore, through the increased solubility profile of the PEG ligands we observed solvent controlled assemblies of varying mesostructures. We developed an ex-situ ligand treatment to manipulate the localized surface plasmon resonance (LSPR) response of anion vacancy doped tungsten oxide (WO3-x) nanoplatelets (NPLs). Upon ligand treatment to alter the surface passivating ligand from carboxylic acid containing myristic acid (MA) to tetradecylphosphonic acid (TDPA) we observed a >100 nm blue shift in the LSPR response. Using Fourier transform infrared (FTIR) and Raman spectroscopies in conjunction with DFT calculated Raman spectra we were able to conclude this shift was due to the formation of tridentate phosphonate bonds on the NPLs surface. Phosphonate bonding allows for an increase in surface passivation per ligand decreasing surface trapped electrons. These previously trapped electrons were then able to participate as free electrons in the LSPR response. Electron paramagnetic spectroscopy (EPR) further supported this decrease in surface traps through a decrease and shift of the EPR signal related to metal oxide surface trapped electrons. Lastly, using our knowledge of PEG ligands we were able to modify esterification chemistry to covalently attach PEG ligands to a MXene surface. The successful formation of an ester bond between a carboxylic acid containing PEG ligand and hydroxyl terminating group on the MXene surface was supported by FTIR spectroscopy and thermogravimetric analysis. The attachment of PEG resulted in a drastic change in the hydrophilicity of the MXene surface. Where MXenes were previously only processed in extremely polar solvents the PEG attachment allowed for high dispersibility in a wide range of polar and non-polar organic solvents, effectively increasing their processability. Further, this chemistry was modified to include an additional functional group on the PEG ligand to increase the valency of the post-modification MXene nanoflakes. Overall, work presented in this dissertation represents the development and application of surface chemistry to relatively new 2D nanomaterials. We believe our work significantly increases the knowledge of 2D halide perovskite formation, manipulation of LSPR active metal oxide materials, and the future processing of MXene materials.

The popularity of graphene owing to its unique properties has triggered huge interest in other two-dimensional (2D) nanomaterials. Among them, silicene shows considerable promise for electronic devices due to the expected compatibility with silicon electronics. However, the high-end potential application of silicene in electronic devices is limited owing to the lack of an energy band gap. Hence, the principal objective of this research is to tune the electronic and magnetic properties of silicene related nanomaterials through first-principles models. I first explored the impact of edge functionalization and doping on the stabilities, electronic, and magnetic properties of silicene nanoribbons (SiNRs) and revealed that the modified structures indicate remarkable spin gapless semiconductor and half-metal behaviors. In order to open and tune a band gap in silicene, SiNRs were perforated with periodic nanoholes. It was found that the band gap varies based on the nanoribbon’s width, nanohole’s repeat periodicity, and nanohole’s position due to the quantum confinement effect. To continue to take advantage of quantum confinement, I also studied the electronic and magnetic properties of hydrogenated silicene nanoflakes (SiNFs). It was discovered that half-hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flake’s size. Next, I studied the adsorption behavior of various gas molecules on SiNRs. Based on my results, the SiNR could serve as a highly sensitive gas sensor for CO and NH3 detection and a disposable gas sensor for NO, NO2, and SO2. I also considered adsorption behavior of toxic gas molecules on boron carbide (BC3) and found that unlike graphene, BC3 has good sensitivity to the gas molecules due to the presence of active B atoms. My findings divulged the promising potential of BC3 as a highly sensitive molecular sensor for NO and NH3 detection and a catalyst for NO2 dissociation. Finally, I scrutinized the interactions of CO2 with lithium-functionalized germanene. It was discovered that although a single CO2 molecule was weakly physisorbed on pristine germanene, a significant improvement on its adsorption energy was found by utilizing Li-functionalized germanene as the adsorbent. My results suggest that Li-functionalized germanene shows promise for CO2 capture.

<p>This dissertation describes how the surface chemistries of 2D nanomaterials can be modified to alter overall material properties. Specifically, through a focus of the ligand-surface atom bonding in addition to the overall ligand structure we highlight the ability to direct morphological outcomes in lead free halide perovskites, maximize optoelectronic responses in substoichiometric tungsten oxide, and alter physicochemical properties titanium carbide MXenes.   </p>