TAILORING MXENE AND BOROPHENE BASED CHEMIRESISTIVE SENSORS FOR SELECTIVE GAS SENSING

Abstract

The necessity for selective and sensitive detection of toxic gases and volatile organic compounds (VOCs) in food quality control, environmental monitoring, industrial process management, medical diagnostics, and occupational safety has increased in recent years. NOx concentration in the atmosphere has increased due to rising traffic and building construction, as they produce nitrogen oxides through combustion and car exhaust emissions, leading to damage to the ozone layer and causing acid rains. Moreover, when the concentration of NO2 gas exceeds 410 parts per billion (ppb) in an hour and 82 ppb in a year in the air, it leads to significant health issues, as stated by the world health organization (WHO). The chemical compound NH3 (ammonia), is widely used in manufacturing chemical compounds, refrigerants, agrichemicals, etc. Concurrently, the health of human beings will be negatively impacted by ammonia gas once its aggregation exceeds 25 ppm for 8 h as a measure of the standard of indoor pollutants. So, we need devices called gas sensors to help us keep track of these harmful substances in very small amounts. Currently, chemical sensors operating at ambient temperature have become an entirely novel sensing device with reduced device power requirements, improved safety, and wearables projected to replace the traditional sensors in the coming years. This can be accomplished with the help of two-dimensional (2D) nanostructures. Several benefits of 2D nanostructures include their large surface area, abundance of active sites, ease of surface functionalization, good compatibility with device integration, and potential for assembly into three-dimensional (3D) architectures. In this present research work, MXene and borophene, a new family of 2D nanomaterials are explored for gas sensing application. Large surface area and abundant surface functional groups favor the MXene (Nb2CTx and Ti3C2Tx) for potential gas sensor applications. However, atmospheric instability is the major limitation of the MXenes. In the case of Nb2CTx, (3-aminopropyl) triethoxysilane (APTES), a popular silane coupling agent, with predetermined concentrations is used for surface modification of MXene. This forms homogeneous, thick protective layer on the Nb2CTx MXene structures and minimizes the oxidation by covalent contact through silylation processes. This allows simultaneous incorporation of additional reactive groups such as (-NH2) thus facilitating sensing of acidic gases like NO2. In further research of this thesis, by employing Ti3C2Tx MXene as a metal precursor and 2-aminoterapthalic acid as an organic linker, MXene-derived MOFs with high surface reactivity were synthesized. The Ti3C2Tx MXene sensor, initially selective towards NH3 gas, was tailored to detect NO2 gas by integrating it with MXene-derived MOFs, which offer abundant active sites on the surface. Sensing materials can react similarly to different gases, limiting selectivity, and are sensitive to humidity. Higher gas selectivity has been achieved by combining polymer membranes with metal-organic frameworks (MOFs) to prepare mixed matrix membranes (MMMs). In the realm of NH3 gas sensing applications, the electrically conductive nature of Ti3C2Tx MXene, adorned with surface terminations such as -O and -OH groups, renders it a compelling material. However, the inherent challenges of atmospheric instability and selectivity in the presence of gas mixtures have prompted the exploration of innovative solutions. This work introduces a strategic solution through the deposition of a Mixed Matrix Membrane (MMM) composed of polyvinylidene fluoride (PVDF) as the matrix and zeolitic imidazolate framework-67 (ZIF-67) as the filler. This composite membrane acts as a selective filter, permitting the passage of a specific gas, namely NH3. Leveraging the hydrophobic and chemically inert nature of PVDF, the MMM enhances the atmospheric stability of Ti3C2Tx by impeding water molecules from interacting with the MXene. Furthermore, ZIF-67 is selective to NH3 gas via acid-base interactions within zeolite group and selective pore size. In further research of selective gas sensing work, a recent 2D material, borophene, is used as the NO2 gas-sensing material. Polyetherimide (PEI) incorporated with the Zeolitic imidazole framework (ZIF-8) with different loading (mixed matrix membrane, MMM) is used as a selective gas separation membrane for its solubility (imide, isopropylidene and ether groups on PEI) and diffusion (porosity) properties. The MMM was selective to NO2 gas as ZIF-8 is selective towards NO2 gas, and PEI is resistant to substances like hydrocarbons, alcohols, and halogenated solvents. A wireless handheld sensor device was developed for real time monitoring of the toxic gases using Wi-Fi enabled technology. The entire NO2 and NH3 gas sensing measurements was carried out at room temperature under different relative humidity (35 - 80% RH). The simple and cost-effective developed sensors can be used for environmental monitoring by detecting NH3 and NO2 gas.