Synthesis of Oxide-based Nanoplates and Nanosheets for Photocatalysis and Optoelectronics

Rafiezadeh, Somayeh

Synthesis of Oxide-based Nanoplates and Nanosheets for Photocatalysis and Optoelectronics

Rafiezadeh, Somayeh

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Publication Type:: Thesis
Issue Date:: 2025

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Field	Value	Language
dc.contributor.author	Rafiezadeh, Somayeh
dc.date.accessioned	2026-03-16T05:55:31Z
dc.date.available	2026-03-16T05:55:31Z
dc.date.issued	2025
dc.identifier.uri	http://hdl.handle.net/10453/194347
dc.description	University of Technology Sydney. Faculty of Science.	en_US.UTF-8
dc.description.abstract	Gallium oxide (Ga2O3) is an emerging ultrawide bandgap semiconductor with strong potential for optoelectronic and power electronic applications. However, its intrinsic n-type conductivity and the challenge of achieving stable p-type doping hinder its suitability for bipolar devices. This motivates the exploration of alternative materials such as ZnGa2O4, which exhibits both n-type and p-type conductivity, making it strong candidate for next-generation electronics. Additionally, bandgap engineering in Ga2O3 alloys is constrained by high- temperature deposition techniques and limited dopant incorporation. To address these challenges, this thesis synthesises ZnGa2O4 nanoplates via hydrothermal method and investigates the role of native defects, particularly cation site conversion, in tuning luminescence and photocatalytic properties. The second part of this work develops low-temperature synthesis approach for fabricating Al-enriched β-(AlxGa1-x)2O3 nanosheets, enabling compositional tuning for bandgap engineering without complex fabrication techniques. The effect of Al incorporation on electronic structure, defect formation, and optical properties is examined, providing insights into defect engineering strategies for optimising material performance. ZnGa2O4 nanoplates with a pure spinel phase, lateral dimensions up to 10 μm, and thicknesses around 40 nm were synthesized. Photoemission and Raman spectroscopy revealed significant cation inversion, with Ga3+ occupying tetrahedral sites (GaZn) and Zn2+ occupying octahedral sites (ZnGa), forming anti-site defects. The inversion parameters were 0.36 ± 0.04 (GaZn) and 0.25 ± 0.02 (ZnGa). These nanoplates exhibited stable, bright broadband luminescence, including UV emission at 3.2 eV (self-trapped holes) and three visible defect bands. Furthermore, ZnGa2O4 nanoplates demonstrated superior photocatalytic efficiency in degrading Rhodamine B (RhB) under ultraviolet A (UVA) irradiation compared to Ga2O3. Band structure analysis revealed strong tail states extending the valence and conduction band edges, reducing the bandgap to 3.9 eV and enhancing hydroxyl radical production. To address the challenge of compositional tuning at the nanoscale, this thesis investigates low-temperature liquid metal-based synthesis method for selective Al enrichment in β-Ga2O3 nanosheets. This approach yielded β-(AlxGa1- x)2O3 nanosheets with monoclinic crystal structure and dominant (- 201) orientation. The synthesised nanosheets exhibited large lateral dimensions (> 100 m) and average thickness of 3.2 ± 0.5 nm, making them suitable for nanoscale device applications. By varying the Al content in liquid metal from 0 to 10 at%, bandgap modulation was achieved from 4.5 eV (pure β-Ga2O3) to 6.4 eV (β-(Al0.88Ga0.12)2O3). Cathodoluminescence spectroscopy revealed that the β-(AlxGa1- x)2O3 nanosheets exhibit broadband luminescence, retaining the characteristic self-trapped hole emission of β-Ga2O3 around 3.2 eV, while Al incorporation introduces an additional distinct deep-UV emission.	en_US.UTF-8
dc.format	Thesis (PhD)
dc.language.iso	en	en_US.UTF-8
dc.relation	https://opus.lib.uts.edu.au/bitstream/10453/194347/1/thesis.pdf
dc.rights	info:eu-repo/semantics/openAccess
dc.rights	The author owns the copyright in this thesis including all reproduction and reuse rights for the work. The work may not be altered without the permission of the copyright owner. Attribution is essential when quoting or paraphrasing from this thesis.
dc.rights	©2025 Somayeh Rafiezadeh
dc.rights	au.edu.uts.lib/nph
dc.subject	spinel ZnGa2O4, cation inversion, photoemission, broadband phosphor, photocatalysis, 2D materials; β-AlGaO, monoclinic oxide semiconductors, liquid metal synthesis, bandgap engineering	en_US.UTF-8
dc.title	Synthesis of Oxide-based Nanoplates and Nanosheets for Photocatalysis and Optoelectronics	en_US.UTF-8
dc.type	Thesis
utslib.copyright.status	open_access	*

Abstract:

Gallium oxide (Ga2O3) is an emerging ultrawide bandgap semiconductor with strong potential for optoelectronic and power electronic applications. However, its intrinsic n-type conductivity and the challenge of achieving stable p-type doping hinder its suitability for bipolar devices. This motivates the exploration of alternative materials such as ZnGa2O4, which exhibits both n-type and p-type conductivity, making it strong candidate for next-generation electronics. Additionally, bandgap engineering in Ga2O3 alloys is constrained by high- temperature deposition techniques and limited dopant incorporation. To address these challenges, this thesis synthesises ZnGa2O4 nanoplates via hydrothermal method and investigates the role of native defects, particularly cation site conversion, in tuning luminescence and photocatalytic properties. The second part of this work develops low-temperature synthesis approach for fabricating Al-enriched β-(AlxGa1-x)2O3 nanosheets, enabling compositional tuning for bandgap engineering without complex fabrication techniques. The effect of Al incorporation on electronic structure, defect formation, and optical properties is examined, providing insights into defect engineering strategies for optimising material performance. ZnGa2O4 nanoplates with a pure spinel phase, lateral dimensions up to 10 μm, and thicknesses around 40 nm were synthesized. Photoemission and Raman spectroscopy revealed significant cation inversion, with Ga3+ occupying tetrahedral sites (GaZn) and Zn2+ occupying octahedral sites (ZnGa), forming anti-site defects. The inversion parameters were 0.36 ± 0.04 (GaZn) and 0.25 ± 0.02 (ZnGa). These nanoplates exhibited stable, bright broadband luminescence, including UV emission at 3.2 eV (self-trapped holes) and three visible defect bands. Furthermore, ZnGa2O4 nanoplates demonstrated superior photocatalytic efficiency in degrading Rhodamine B (RhB) under ultraviolet A (UVA) irradiation compared to Ga2O3. Band structure analysis revealed strong tail states extending the valence and conduction band edges, reducing the bandgap to 3.9 eV and enhancing hydroxyl radical production. To address the challenge of compositional tuning at the nanoscale, this thesis investigates low-temperature liquid metal-based synthesis method for selective Al enrichment in β-Ga2O3 nanosheets. This approach yielded β-(AlxGa1- x)2O3 nanosheets with monoclinic crystal structure and dominant (- 201) orientation. The synthesised nanosheets exhibited large lateral dimensions (> 100 m) and average thickness of 3.2 ± 0.5 nm, making them suitable for nanoscale device applications. By varying the Al content in liquid metal from 0 to 10 at%, bandgap modulation was achieved from 4.5 eV (pure β-Ga2O3) to 6.4 eV (β-(Al0.88Ga0.12)2O3). Cathodoluminescence spectroscopy revealed that the β-(AlxGa1- x)2O3 nanosheets exhibit broadband luminescence, retaining the characteristic self-trapped hole emission of β-Ga2O3 around 3.2 eV, while Al incorporation introduces an additional distinct deep-UV emission.

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