Two-dimensional Graphene Derivatives and Metal Oxide for Room-temperature Gas Sensors

Gas sensing plays a crucial role in a wide range of industries, from environmental monitoring to healthcare, detecting toxic gases, and enabling early warning systems for hazardous conditions. As urbanization and industrial activities intensify, the demand for efficient, sensitive, and highly selective gas sensors has never been more critical. Advanced gas sensing technologies have become essential to addressing global challenges such as pollution control and workplace safety. This growing need has driven extensive research into novel materials capable of meeting these requirements, particularly in applications requiring high precision and stability. Two-dimensional (2D) semiconductor materials have attracted substantial attention in scientific research and technological applications compared to other dimensionalities such as zero-dimensional (0D), one-dimensional (1D), and three-dimensional (3D) nanostructures. Due to their high surface-to-volume ratio, 2D materials are characterized by significantly enhanced surface reactivity, leading to substantial improvements in adsorption/desorption processes and other distinctive physiochemical properties. These characteristics make 2D materials particularly well-suited for gas sensing applications, positioning them as a natural choice for various gas sensors. Conventional metal oxides, such as SnO₂, ZnO, TiO₂, and WO₃, have been widely utilized in chemiresistive sensors due to their outstanding long-term stability in ambient environments. However, the need for a microheater to maintain an elevated operating temperature, stemming from their chemisorption mechanisms, imposes severe limitations on their industrial potential. Over the last decade, considerable research on 2D materials for room temperature gas sensors has been conducted. For instance, certain 2D metal chalcogenides have demonstrated room temperature gas sensing responses with the assistance of external stimuli (ES) such as ultraviolet (UV) lamps. Nevertheless, ES introduce practical challenges in terms of power consumption and implementation complexity. Among these materials, distinctive room temperature physisorption-based gas sensing capabilities are exhibited by graphene derivatives such as graphene oxide, hexagonal metal oxide, and their heterostructures, developed by our research group, owing to their inherent electrical and optical properties. Low-power visible light excitation is unnecessary, yet the electrical conductivity of these 2D materials can still dynamically adjust to changes in adsorption/desorption interactions between the material surface and gas molecules. Consequently, physisorption-based gas sensing responses are produced in the ambient gas environment at room temperature. The synthesis of 2D materials is often costly due to the need for precise control over their properties. Techniques like atomic vapor deposition (AVD), chemical vapor deposition (CVD), molecular beam epitaxy (MBE) require sophisticated equipment, high-purity precursors, and stringent conditions to achieve desired thickness, lateral dimensions, and defect-free structures. Additionally, these methods frequently involve low production yields, post-synthesis treatments to remove impurities, and significant material waste, further driving up costs. It has been reported that expensive materials like graphene can be developed from plastic and organic carbon-based waste. This upcycling process involves various techniques such as solvolysis, pyrolysis, catalytic depolymerization, and flash joule heating, which break the carbon-based material lifecycle by converting plastic scrap into graphene with less production waste and higher yield. This method effectively lowers the cost of this prominent 2D material, thereby creating opportunities for its use in various industries. Recognizing the strengths of these novel materials and synthesis approaches, an intermediary class of materials between graphene derivatives and metal oxides emerges as a promising candidate for a new generation of room temperature sensing materials. After a thorough review of the literature, the author found that new plastic upcycling methods like solvolysis and flash joule heating offer distinctive features compared to the previously mentioned oxide materials. These materials show significant promise for use in practical room temperature gas sensors based on physisorption mechanisms. However, research on upcycling methods for plastic waste, hexagonal titanium dioxide, and their heterostructures is still in its early stages. A comprehensive understanding of the physicochemical and electro-optical properties of these materials is still needed. Therefore, advancing our knowledge of oxide materials and their gas sensing capabilities presents both challenges and exciting research opportunities in this emerging field.In this dissertation, various types of plastic waste, including HIPS, PP, PVC and PET, were initially selected to demonstrate the potential of upcycling techniques. These methods effectively transform low-cost landfill waste into high-value carbon-based materials such as graphene oxide (GO), which is the first generation of 2D material. This newly synthesized material exhibits excellent gas sensing capabilities without the need for external stimuli, potentially serving as a substitute for traditional high-temperature and energy-intensive UV lamps in gas sensing. Most existing research focuses on achieving room-temperature gas sensing with GO using Hummer’s method from pristine graphene nano powder rather than upcycling from plastic waste. The novel synthesis approach presented here offers a more environmentally friendly alternative. This was accomplished through a combination of chemical and thermal treatment of mixed plastic wastes, followed by calcination at 600°C, rendering it into a metastable state conducive for gas sensing. Notably, under dark conditions without any visible light excitation, the graphene oxide sensor demonstrated a consistent and repeatable response to 1260 ppb NO2 gas, achieving a response of ~11.84% at room temperature in dark environment. Linearity assessments highlighted its versatility across various NO2 concentrations, demonstrating a leading-edge limit of detection of 1.43 ppb among chemiresistive sensors. Although graphene oxide sensors have an impressive LOD, the prolonged recovery which can take up to ~115 minutes, could hinders the sensor’s real-time usability in dynamic environments where fast response and recovery are crucial. To address these challenges, the focus shifted to exploring new materials with improved recovery rates, leading to the investigation of an emerging 2D material called hexagonal titanium dioxide (h-TiO₂). It was especially intriguing to uncover ultra-fast gas sensing properties of titanium dioxide in hexagonal crystal structure, as pure TiO2 typically does not exhibit such capabilities without external stimuli. The material was synthesized in a controlled environment inside the glovebox at 2000C, then mechanically transfer up on the silicon wafer for field-effect transistor gas sensor fabrication. Material characterization revealed a distinctive honeycomb like crystal structure, validated the titanium’s metastable state and confirmed its elemental composition after calcination. The synthesized titanium dioxide showed a response factor of ~4.2% to 0.000126% to NO2 at room temperature, while it remained unresponsive to prevalent industrial gases, paving the path for other hexagonal oxide to be studied and applied for ultra-fast response room temperature gas sensors.Heterostructures are well-regarded for combining the strengths of two distinct materials to overcome the limitations inherent in each. By integrating different properties, heterostructures offer a unique synergy that overcome the weaknesses of individual materials, such as low sensitivity or slow recovery times, while enhancing the gas sensing performance. With this potential in mind, PhD candidate started the exploration towards the creation of a graphene and titanium dioxide heterostructure to investigate its gas sensing capabilities. The Graphene/TiO2 heterostructure was synthesized via Flash Joule Heating method, using carbon black nano powder as a conductive agent. It was discovered that applying a high-power electric shock for a brief duration could induce a transformation of mix plastic wastes and carbon black powder into graphene, simultaneously doped with titanium dioxide powder, producing an artificial two-dimensional material called Graphene/TiO2 heterostructure. Material characterization highlighted that this novel material exhibited a unique crystal morphology characterized by a micro-cage structure which doped from hexagonal sheets. Significantly, this newly achieved metastable state introduced novel room-temperature gas sensing capabilities not previously observed with pure graphene or conventional TiO2. The Graphene/TiO2 demonstrated superior performance in detecting NO2 gas in room-temperature in a dark environment, with the response magnitude of ~11.2% and remarkable selectivity against common industry gases such as CH4, H2 and CO2. These findings validate the feasibility of synthesizing novel heterostructures from plastic waste and metal oxide, underscore their potential as cutting-edge materials for high-performance gas sensing applications.In summary, this research has pioneered the synthesis and application of 2D graphene oxide derived from upcycling plastic wastes, 2D titanium dioxide in hexagonal structure, and their heterostructure. The study has demonstrated their potential as exceptional materials for room-temperature gas sensing without reliance on external stimuli. These findings beside enhance the understanding of oxide materials but also lay the foundation for developing a new class of cutting-edge, physisorption-based room-temperature gas sensing materials suitable for various applications.