Abstract
This project aims to explore green and sustainable methods for synthesising high-quality graphene oxide (GO) materials using renewable biomass resources and to study the effects of different process parameters on the characteristics and application potential of GO materials. By systematically investigating the influence of factors such as the fundamental components (organic components and minerals) and type of biomass, pyrolysis temperature, concentration of liquid medium, and exfoliation time, this project reveals how optimising these parameters aids in enhancing the structural and functional properties of GO, thereby advancing its applications in energy storage, environmental remediation, and biomedicine.The project first conducted the analysis of the thermal behaviours of 135 biomass samples (including dicotyledonous, monocotyledonous, and gymnosperm), systematically analysing how the fundamental components of biomass affect thermal conversion performance and product characteristics. This research demonstrates how the contents of cellulose, hemicellulose, lignin (CHL, where C refers to cellulose, H refers to hemicellulose, and L refers to lignin), and minerals [such as calcium (Ca), potassium (K), sodium (Na), and silicon (Si)] affect product properties during combustion under industrial conditions. To improve the understanding and control of these complex components, the study developed a support vector machine (SVM) model for highly accurate predictions of mineral content in ash after combustion, particularly excelling in predicting sulphur (S) content. The development of this model enables the precise selection of biomass resources with as-desired properties for various applications. These findings not only support the broader utilisation of biomass but also indicate the significant potential of SVM models in large-scale biomass characterisation and process optimisation, especially for exploring energy potential. Further pyrolysis experiments explored the migration behaviour of minerals in biomass at different pyrolysis temperatures in detail. The results presented that different types of minerals exhibit distinctive migration mechanisms during pyrolysis. Mobile elements such as Na, K, chlorine (Cl), and magnesium (Mg) primarily migrate at lower temperatures (<600 °C) via direct volatilisation and at higher temperatures (800-1000 °C) through reactions with free radicals that are generated from biochar. In contrast, immobile elements are not easily volatilised during pyrolysis but may redistribute at specific temperatures, such as Si stabilising in the epidermis while iron (Fe) migrates from parenchymatous cells to the more mechanically robust vascular bundles. Understanding this migration behaviour helps optimise the distribution of minerals in biochar by regulating pyrolysis temperature, thereby improving the quality and functional properties of the products and providing scientific insights for developing high-performance biomass-based advanced carbon materials.
Based on these distinct features of thermal conversion, the research investigated the potential of using a liquid exfoliation method that combines ultrasonic technology in an ethanol-water solvent system to synthesise GO materials from rice stem-derived biochar. The experiments demonstrated that ethanol plays an essential role in the solvent system, mainly by reducing surface tension, creasing viscosity and enhancing the acoustic impedance of the solution, thereby directly affecting the exfoliation efficiency and the properties of the GO product. It was found that the exfoliation of GO was most effectively achieved at an ethanol concentration of 74%, with the resulting product predominantly consisting of monolayer GO and over 92% of the product having fewer than three layers. The adjustment of ethanol concentration involves balancing multiple physicochemical parameters. Within the optimal range of ethanol concentration, the number of cavitation bubbles during the exfoliation process is moderate, effectively exfoliating biochar without causing reaggregation, thereby ensuring a high-quality GO product. Furthermore, the high content of oxygenated functional groups (such as epoxy and hydroxyl groups) and the presence of nanoporous structures retained from biomass significantly improve some of the properties of biomass-derived GO, greatly enhancing the potential applications in catalysis, electrochemistry, environmental remediation, and biomedical field.
Moreover, this project explored the further optimisation of biomass derived GO properties for biomedical applications by adjusting pyrolysis temperature and ultrasonic exfoliation time. The results indicated that GO produced at higher pyrolysis temperatures contains a moderate level of oxygen containing functional groups, including hydroxyl, carboxyl, and epoxy groups. These groups enhance the functionality and reduce the toxicity of GO materials in biomedical applications by increasing their hydrophilicity, improving their dispersibility, and providing sites for chemical modification, respectively. The adjustment of ultrasonic time directly affected the number of GO layers with longer ultrasonic duration, reducing the number of layers but potentially leading to smaller sheet sizes. Biocompatibility tests demonstrated that GO samples with fewer layers and moderate levels of oxygenated functional groups exhibited the lowest cytotoxicity, resulting in an approximate 500% higher survival rate during cellular tests when compared to the commercialised GO product. This enhanced biocompatibility is attributed to its unique physicochemical properties, which result in a reduced generation of reactive oxygen species (ROS) in the cell culture environment with GO, thereby preventing severe cell death in the short term. Additionally, these properties promote an effective dispersion of GO materials in the culture medium, significantly reducing the aggregation of GO on the cell membrane and the damage caused by the penetration into the cell membrane, thus protecting the integrity and function of the cell structure and internal organelles. These findings suggest that biomass-derived GO presents more significant potential for biomedical applications compared to commercialised products. Additionally, the precise control of process parameters allows for the production of GO materials with specific physiochemical characteristics and biocompatibility.
This project plays a crucial role in not only demonstrating the efficient synthesis of as-desired GO materials from renewable resources but also promoting green chemistry principles by providing environmentally friendly approaches to waste management and resource processing. By exploring environmentally friendly GO synthesis methods, this research establishes a solid foundation for promoting the usage of renewable resources, minimising environmental impact, and fostering the development of a circular economy. Additionally, the outcomes of this project provide new perspectives on customising high-quality advanced carbon materials, particularly in the control of material structure and functional properties tailored to specific applications. By systematically exploring the effects of various process parameters on GO materials, this work offers valuable insights for the future development of materials science and engineering, with significant implications for achieving sustainable development goals.
| Date of Award | 15 Jul 2025 |
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| Original language | English |
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| Supervisor | Cheng Heng Pang (Supervisor), Edward Lester (Supervisor) & Siew Shee Lim (Supervisor) |