Novel liquid phase routes for the synthesis of metal sulphide nanomaterials and their thin films
Creators: name: Clark, Rhiannon
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401807
subject: Nanomaterials

description: 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.<br><br>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.<br><br>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.<br><br>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.<br><br>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.<br><br>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.<br><br>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.<br><br>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.<br><br>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.