Highly sensitive and selective gas sensors based on metal phthalocyanines nanostructures and multi walled carbon nanotubes

Abstract

Gas sensors (GS) are considered among the most important invented devices due to their ability in detecting and preventing human exposure to potentially dangerous conditions. GS are devices that can produce measurable changes when interacting with a target gas. There are different types of GS, <em>electrochemical, optical, infrared and metal semiconductors</em>, depending on the measurable change during the testing. GS materials can detect a diversity of gases (NH₃, NO₂, etc.). However, various of the detectable gases present in the environment are of a toxic nature.<br /> <br /> The production of these harmful gases is a problem that involves the automotive combustion processes, agricultural production, and the overproduction of other manufacturing goods. Among such toxic gases, nitrogen dioxide (NO₂) is a commonly produced toxic gas formed, and constant exposure (> 0.12 parts-per-million/hour) can result in pulmonary diseases. Another example of a toxic gas is ammonia (NH₃), which is produced during the use of fertilizers in the farm animal husbandry. Exposure to doses of NH₃ above > 50 ppm/8 hours can cause serious lung problems. The detection of gases is not only limited to environmental monitoring but also encompasses applications in the health field. Human breath can be a source of volatile organic compounds (VOCs). High concentrations of VOCs in the human breath can be the result of different diseases. For instance, the presence of NO^- in the human breath in concentrations higher than 100 ppb could suggest pulmonary diseases such asthma. Consequently, the development of highly effective and fast responsive gas sensors is of extreme importance to environmental and health monitoring.<br /> <br /> Organic semiconducting materials are an excellent choice for gas sensor development due to their low cost, practical and facile preparation. Many semiconductors' materials have been XX used for GS that include metal oxides, carbon-based materials and organic semiconductors. There are existing limitations of the currently used materials as GS that include the necessity of high working temperatures, poor sensitivity at low gas concentrations (ppb) and the lack of selectivity.<br /> <br /> Metal phthalocyanines (MPc) is an effective material with semiconducting capabilities that are ideal for gas sensing purposes. Its structure allows incorporating diverse metal centers (Co, Cu, Pd, Ni) to enhance the properties of the material. In addition, substituents can be added to the macrocyclic structure and can result in changes in the semiconduction nature of the MPc. For instance, non-substituted MPcs show a “p” type semiconductor behavior whereas adding electron withdrawing substituents (such as F), changes the semiconduction to an “n” type. This allows major control on the selectivity of the material when interacting with an oxidant or reducing gas. For instance, non-substituted MPcs show higher selectivity towards oxidant gases (NO₂, NO^-, etc.). On the other hand, fluorinated MPcs show higher selectivity towards reducing gases (NH₃, etc.).<br /> <br /> In terms of the gas sensing device development, MPcs allows the formation of different nanostructures such as thin films (TFs) and nanowires (NWs) that can be deposited on a chip and/or interdigitated gold electrode (IDE). Furthermore, MPc nanostructures can be developed by utilizing methods such as drop casting (DC) and physical vapor deposition (PVD). MPcs have been widely explored for gas sensing applications in TF nanostructures. Recent studies of MPcs nanostructures such as NWs have shown superior performance in terms of sensitivity, saturation and low detection limits (parts-per-billion) when compared to TF of the same material. Another MPc limitation is their low conductivity that can result in a much slower response when being exposed to the target gas. A solution to the low conductivity for a faster XXI response is the use of a material that offers a large surface area and has optimal conductivity such as Multi Walled Carbon Nanotubes (MWCNTs).<br /> <br /> The presented doctoral thesis work focuses on the synthesis and characterization of “d” metals MPcs based on cobalt (Co), copper (Cu), palladium (Pd), nickel (Ni) and platinum (Pt) unsubstituted (MPcs). Additionally, the hexadecafluorinated (F₁₆MPcs) structures using Pd and Pt metal centers were also developed. For the synthesis, <strong>Chapter 2</strong> covers the cyclotetramerization solid state reaction followed by the development technique of the NWs using as the precursor each MPc/F₁₆MPcs powder for the PVD crystalline growth. The NWs were deposited on IDE and aluminum substrates to perform the sensing experiments and structural characterizations. <strong>Chapter 3</strong> includes spectroscopic characterizations (UV-vis, Raman spectroscopy) to confirm metal bonding and macrocycle formation. Moreover, for the structural studies, SEM and EDS were performed. In addition, the evaluation of possible changes in crystalline phases after high deposition temperatures (> 400 ^◦C) using Raman spectroscopy and X-ray Diffraction are shown. Finally, the normalized response (S %) of the gas sensing experiments towards NO₂, NH₃ and CO^- from 0.500 - 4 ppm are presented and evaluation of the performance of the GS based on the metal center (Pd, Pt) is also discussed.<br /> <br /> In the following <strong>Chapter 4</strong>, the incorporation of MPcs (M= Co, Cu, Pd, Ni) to acid modified multi walled carbon nanotubes (AMWCNTs) to create highly sensitive and selective TFs for gas sensing at low ppb concentrations are described. The MWCNTs were modified, using acidic conditions. For the MPc-AMWCNTs TF hybrids composites DC was performed on a gas sensing chip. Spectroscopic (UV-vis, FT-IR, Raman spectroscopy) and structural (SEM, EDS, elemental mapping) characterizations were performed to the MPc/AMWCNTs materials. Furthermore, gas sensing experiments results are shown towards N₂O, NO^-, NH₃, toluene and XXII acetone in range concentrations between 40 ppb-6000 ppb. Additionally, due to the higher observed responses towards the oxidant gases (NO-/N₂O), the fast response values were calculated to evaluate sensitivity. Moreover, to understand the effect of the metal center in the interactions between the material and the studied gases, DFT calculations were also conducted. Results showed a trend based on each developed metal center hybrid (Co, Cu, Pd, Ni). Finally, the conclusions of the project, contributions to the field and future work are discussed in <strong>Chapter 5</strong>. This doctoral thesis led to the development of eight gas sensors based on MPc NWs and MPc- AMWCNT TFs, demonstrating effective responses toward NO⁻, N_₂O, acetone, and toluene.