Implementation of novel carbon-based nanomaterials for high-performance gas sensors

Abstract

Environmental pollution has emerged as a critical dilemma due to the rapid escalation of industrial activities on a global scale. Toxic gas emissions stand out as a primary contributor to numerous climate issues, such as global warming and acid rain, posing a threat to both the environment and public health in the short and long term. The precise detection of such pollutants holds immense significance across various sectors, including environmental management, defense, healthcare, and industry. Substantial research efforts have been dedicated to advancing high-performance gas sensors that can accurately detect low gas concentrations while exhibiting robust sensing characteristics and durability Carbon nanomaterials have gained significant attention for numerous applications. Their outstanding physical and chemical properties. Extensive research has been conducted to assess the potential of various carbon-based nanomaterials, such as fullerenes, carbon onions, carbon quantum dots, nanodiamonds, carbon nanotubes, and graphene, as gas sensing materials. This thesis aims to explore the potential of novel carbon materials and their implementation in gas sensing applications. The thesis consists of five chapters and is organized as follows: The first chapter comprises a published comprehensive review of the literature discussing recent progress in the utilization of carbon nanomaterials and their composites in gas sensing devices. The chapter introduces the sensing mechanism, design, and preparation techniques of such sensors. It also discusses the modification of carbon-based nanostructures with other nanomaterials and their effects on sensing performance. The second, third, and fourth chapters consist of published and in-press articles presenting the research findings obtained in the thesis research. The research reported in these three chapters and the related findings are summarized in the following paragraphs. The final chapter provides a complementary conclusion, addresses existing challenges, and offers inspiring recommendations for future research. In the second chapter, the synthesis of a novel composite involving quantum dots enhanced carbon nanotubes (CNTs) and graphene nanoplates (GNPs) is reported, along with its application as a sensing material for detecting various concentrations of ethanol at room temperature. Carbon quantum dots (CQDs) employed in this study were synthesized via a solvothermal process and integrated with CNTs and GNPs to investigate their synergistic effects on the structure of the resulting composite and its sensing properties. Transmission electron microscopy (TEM) images provided evidence of the successful integration between CNTs and GNPs. CNTs were observed to interconnect with GNPs, forming a web-like three-dimensional hybrid structure that significantly enhanced the specific surface area (SSA) of the composite. The introduction of CQDs influenced the final hybrid structure by introducing zero-dimensional roughness, achieved through the attachment of CQDs to the surfaces of both CNTs and GNPs. The hybrid composite served as a sensitive film deposited onto the surface of a 5 MHz quartz crystal microbalance (QCM) sensor through drop-casting. The hybrid nanocomposite-based sensor exhibited significantly enhanced sensing sensitivity. At a concentration of 500 ppm, the CQD-enhanced CNT-GNP composite showed approximately 10- and 15-fold higher responses compared to CNT- and GNP-coated sensors, respectively. The response and recovery times of the CQD-enhanced CNT-GNP composite sensor were found to be approximately 2 minutes and 0.5 minutes, respectively. The sensor demonstrated reasonable repeatability and good recovery. The sensing mechanism was attributed to the adsorption and desorption processes via interactions between ethanol molecules and the composite surface functional groups. In the pursuit of cost-efficient alternative sensing materials, asphaltenes, a byproduct of the petroleum industry, have garnered attention as a potentially valuable waste material. The third chapter of the dissertation presents the initial utilization of asphaltenes as an affordable carbon-based material for gas sensing. Asphaltenes, derived from various oil sources, underwent facile cross-linking reactions to produce nanoporous carbon materials, where asphaltene molecules from different layers are interconnected via covalent bonds. Characterization results of these cross-linked asphaltenes revealed a substantial enhancement in their SSA and surface functionality. QCM sensors with sensing films derived from various asphaltene samples were prepared to detect different ethanol concentrations at room temperature. All cross-linked asphaltene samples exhibited a significant enhancement in the sensing response (up to 430%) compared to their respective raw parent samples. This response of the cross-linked asphaltene samples was comparable to that obtained from graphene oxide. The sensor based on cross-linked asphaltenes demonstrated good linearity, with a response time of approximately 2.4 minutes, a recovery time of around 8 minutes, and excellent response repeatability. After 30 days, the sensor based on cross-linked asphaltenes showed an approximate 40% reduction in its response, suggesting long-term aging. This decline is partially attributed to the observed swelling. This study opens the door to a deeper exploration of asphaltenes and highlights their potential as a promising carbon-based material for sensing applications. In the fourth chapter, the thesis research went far to an interesting unexplored form of carbon materials. Despite all carbon nanomaterials being composed of sp2 and sp3 hybridized carbons, the one-dimensional (1D) sp carbon, known as Carbyne, remains elusive, and the properties of this novel carbon form have not been fully discovered yet. However, the unique structure of carbyne suggests its potential possession of significant chemical, optical, and magnetic properties. In this chapter of the study, the synthesis and characterization of these carbyne nanostructures were investigated to gain a better understanding of their unique properties and potentials. Carbyne synthesis was achieved through two different processes: ion-assisted pulse-plasma deposition (IA-PPD) and laser ablation in liquid (LAL). Raman, XPS, and FTIR observations for the LA-PPD sample indicated the successful synthesis of sp carbon chains of carbyne. However, these chains existed at low concentrations in the obtained nanofilms, alongside a high concentration of sp2 and sp3 carbon. On the other hand, characterization results of the LAL sample showed higher carbyne content, as confirmed by Raman spectra measurements, along with high crystallinity observed from XRD results. The practical application of the synthesized carbyne as sensing materials was investigated on QCM sensors to detect various pollutants at room temperature. The LAL carbyne also exhibited higher sensitivity in gas experiments compared to IA-PPD carbyne. In detecting various analytes, LAL carbyne showed greater selectivity for ammonia gas. The sensor exhibited a moderate response time of 4.7 minutes with full recovery in approximately 9.3 minutes. However, compared to other available carbon materials, the sensitivity of carbyne was found to be relatively low, revealing the need for further research to optimize carbyne synthesis and the fabrication of its sensors.