Synthesis and application of two dimensional non-layered nanomaterials derived from liquid gallium

Syed, N 2019, Synthesis and application of two dimensional non-layered nanomaterials derived from liquid gallium, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Synthesis and application of two dimensional non-layered nanomaterials derived from liquid gallium
Author(s) Syed, N
Year 2019
Abstract The field of two dimensional (2D) materials has received considerable attention and experienced substantial development over the past two decades. Various families of 2D materials have been explored to date leading to ample success in wide range of applications. However, despite many significant advances the synthesis of large area, ultra-thin 2D materials remains a great technological challenge, limiting their commercial application. Additionally, the field of 2D materials is mostly focused on stratified materials with layered crystal structures due to the prevailing synthesis techniques. This leaves many non-layered materials unexplored, which could be suitable candidates in their 2D morphologies for future applications. Recent progress in the field of liquid metal chemistry can offers new avenues of investigation and a range of unexploited possibilities in the domain of 2D materials. Liquid gallium and gallium based alloys feature low melting points that are close to room temperature and undergo Cabrera-Mott oxidation process in air, creating atomically thin 2D metal oxides on the surface. In addition, the developed surface oxides demonstrate minimal adhesion to the parent metal which allows to delaminate these naturally occurring 2D materials with relative ease. Alloying can be used to expand the number of accessible 2D materials, unfolding new opportunities for the synthesis of many previously inaccessible 2D materials. Therefore, this PhD thesis aims to develop 2D metal oxide and further metal compounds that are derived from liquid metal gallium through the harvesting of its interfacial oxide skin. The isolated metal oxides will be then subjected to further chemical conversion to create desired metal compounds in 2D morphology. The synthesized 2D materials will be explored for potential applications in catalysis and different functional devices.

The first objective of this thesis was to exfoliate 2D gallium oxide nanoflakes from gallium droplets immersed in aqueous solution. Gallium is a near room temperature liquid metal with extraordinary properties that partly originate from the self-limiting oxide layer formed on its surface. Taking advantage of the surface gallium oxide, this work has introduced a novel technique to synthesize porous gallium oxide (Ga2O3) nanoflakes at high yield by harvesting the native oxide skin of gallium. The synthesis process followed a facile two-step method comprising liquid gallium metal sonication in DI water and subsequent annealing. In order to explore the functionalities of the product, the obtained α-Ga2O3 nanoflakes were used as a photocatalytic material to decompose organic model dyes. Excellent photocatalytic activity was observed under solar light irradiation. To elucidate the origin of these enhanced catalytic properties, the electronic band structure of the synthesized α-Ga2O3 was carefully assessed. It was found that the excellent photocatalytic performance is associated with the presence of trap states which are located at ~1.65 eV below the conduction band minimum.

To broaden the versatility of 2D materials for different future "wafer-scale" applications, scalable and low cost synthetic routes must be developed to deposit large-area 2D gallium compounds utilizing liquid gallium as a precursor. In the second part of this thesis, large-area 2D nanosheets piezoelectric material gallium phosphate (GaPO4) was synthesized providing new opportunities for piezosensors and energy harvesting. GaPO4 is an archetypal piezoelectric material which does not naturally crystallize in a stratified structure and hence cannot be exfoliated using conventional methods. Until now, no 2D piezoelectric material has been manufactured in large sheets, making it impossible to integrate these materials into silicon chips or use them in large-scale surface manufacturing. This work reports a low temperature liquid metal based two-dimensional printing and synthesis strategy to achieve this goal. The synthesis process was consist of surface-printing of the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide band-gap two-dimensional GaPO4 nanosheets of unit-cell thickness, while featuring lateral dimensions reaching centimetres. The unit-cell thick nanosheets presented a large effective out-of-plane piezoelectric coefficient of 7.5±0.8 pm V-1. The printing process developed here was also suitable for the synthesis of free-standing GaPO4 nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future two-dimensional piezoelectric materials.
Following the successful exfoliation of non-layered GaPO4, liquid gallium was further used as a platform to exfoliate centimetre-scale 2D gallium nitride (GaN) nanosheets. Gallium nitride is a semiconductor of great technological importance with excellent electronic and optical properties. The synthesis relied on the ammonolysis of liquid metal derived 2D oxide sheets that were squeeze transferred onto desired substrates. Wurtzite GaN nanosheets featured typical thicknesses of 1.3 nm, an optical band-gap of 3.5 eV and a carrier mobility of 21.5 cm2 V-1s-1. In order to assess the versatility of the synthesis method, an adapted synthesis process utilizing liquid indium instead of liquid gallium was explored to synthesize 2D indium nitride (InN) featuring a thickness of 2.0 nm. The method provides a scalable approach for the integration of 2D morphologies of industrially important semiconductors into emerging electronics and optical devices.

Overall, the author of this PhD thesis believes that the outcomes presented herein will contribute to the advancement of nanotechnology through the development of scalable and low cost synthesis paths and will work as a platform for many future investigations into the synthesis and applications of 2D materials.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Nanomaterials
Nanofabrication, Growth and Self Assembly
Surfaces and Structural Properties of Condensed Matter
Functional Materials
Keyword(s) Two dimensional
Liquid metal
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Created: Fri, 01 Nov 2019, 09:52:26 EST by Adam Rivett
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