Innovative heterojunction devices for advanced gas sensing technologies: Exploring hybrids from metal phthalocyanine nanomaterials and two-dimensional metal organic frameworks

Abstract

As we move forward, the need for toxic gas sensors will increase in environmental monitoring, industrial safety, aerospace operations, and biomedical diagnostics, where timely detection of hazardous gases can help prevent equipment failures, ecological damage, and serious health consequences. Among these toxic gases are ammonia (NH₃) and nitrogen dioxide (NO₂); both have proven useful in everyday life. NH₃ is an extremely useful gas, as it is used in various industries, including agriculture, refrigeration, and disease diagnosis. NO₂, unlike NH₃, is used in the manufacture of other chemicals, metal refining, and, in aerospace operations, as rocket fuel. While these gases are beneficial in various applications, this does not negate their harmful short- and long-term effects. In the short term (acute), NH₃ and NO₂ can cause lung burns, skin irritation, eye damage, and, in cases of high concentrations, death. Long-term (chronic) exposure to low concentrations of the toxic gases includes respiratory problems, skin conditions, cancer, asthma, and other diseases. Considering that these toxic gases can lead to various health consequences, the need for gas sensors that can accurately, quickly, and selectively detect toxic gases is a key priority. Despite these demanding needs, current gas sensors rely on the use of metal oxides. While metal oxides have proven to be effective sensors, they tend to suffer from high operating temperatures, cross-sensitivity, and slow recovery times. In contrast, organic semiconductors, such as metal phthalocyanine (MPc), have proven reliable for use in gas sensors.<br /> <br /> However, critical limitations still affect MPc-based gas sensors. MPc thin films indeed often exhibit poor conductivity, slow response, and incomplete recovery after exposure to strongly interacting gases; in addition, the relationships between substitution pattern, film morphology, device architecture, and gas-sensing performance remain poorly understood. Physical vapor deposition strategies for growing MPc nanowires are not yet fully optimized for reproducibility. At the same time, MPc-based metal-organic frameworks (MOFs), which combine porosity with electronic functionality, remain a novel platform for chemiresistive detection of NH₃ and NO₂. To address these gaps, this dissertation explored different platforms of MPc-based materials assembled into distinct nanostructured architectures, including FePcF_16-rGO hybrid films, CuPc-S-TFMP/FePcF₁₆ nanowire-on-thin-film (NWs-on-thin-film) heterojunctions, and a CuPc-O₈-Pt framework. This work will address molecular design, dimensionality, and interfacial engineering that govern charge transport and chemiresistive response by systematically comparing the gas-sensing behavior of thin films, nanowires, heterojunctions, and a framework architecture under controlled exposure to NH₃ and NO₂, and by suggesting design principles for next-generation room-temperature gas sensors.<br /> <br /> In this dissertation, we evaluated three distinct state-of-the-art gas sensor architectures that incorporate different MPc compositions and device configurations. In Chapter 3:we present the development of a hybrid material composed of hexadecafluorinated iron phthalocyanine (FePcF₁₆) and reduced graphene oxide (rGO) as a conductive support. The effect of the FePcF_16-rGO hybrid was evaluated by measuring NO₂ at room temperature and compared with the individual components. The FePcF_16-rGO hybrid gas sensor exhibits a significant increase in baseline current, enhanced NO₂ response, and improved recovery time upon exposure to UV light. This demonstrated that combining carbon-based materials can enhance the performance of MPcs-based gas sensors.<br /> <br /> Chapter 4:presents an organic heterojunction gas sensor in which two different MPc nanostructures are combined for the detection of NH₃. This is the first gas sensor to utilize two different types of MPcs nanostructures, tetra[3,5-bis(trifluoromethyl)-4-thio-copper phthalocyanine (CuPc-S-TFMP) and hexadecafluorinated iron phthalocyanine (FePcF₁₆), in a NWs-on-thin-film architecture. This gas sensor demonstrated a bias-dependent ambipolar behavior when exposed to NH₃. This chapter highlights the critical role that an interface can have in the behavior of gas sensors.<br /> <br /> Furthermore, building on the structure&ndash;property insights from the previous chapters, Chapter 5:presents the development of a CuPc-O₈-Pt metal-organic framework (MOF)- like architecture as a gas-sensing platform. In this system, an octahydroxy-substituted copper phthalocyanine is coordinated to platinum (Pt) centers, forming an extended, porous network that modifies both the adsorption environment and charge-transport pathways. The incorporation of this CuPc-O₈-Pt framework into chemiresistive devices results in enhanced NO₂ sensing performance at room temperature, with ultra-low sensitivity. These results demonstrate that MPc-based MOF architectures can effectively couple porosity and electronic functionality, offering a complementary design strategy to the hybrid and heterojunction sensors.<br /> <br /> In recent years, MPc-based materials have attracted increasing attention as low-power alternatives and as more flexible materials than conventional metal oxide gas sensors. In this dissertation, we demonstrated that the different uses of MPc composition, nanostructure, and device architecture can overcome key limitations of traditional MPc thin films. The FePcF_16-rGO hybrid sensor improved conductivity and NO₂ response and enhanced recovery with UV light; the CuPc-S-TFMP/FePcF₁₆ NWs-on-thin-film heterojunction enabled bias-dependent NH₃ sensing and a new device architecture; and the CuPc-O₈-Pt framework coupled porosity with electronic functionality to enhance NO₂ detection. Together, these MPc-based gas sensors serve as practical platforms for room-temperature gas sensing and provide nanostructured architectures that can be extended to future sensors targeting other toxic or emerging analytes.