PhD Thesis Defence Presentations - Stefanos Matsalis

One of the major challenges of the 21st century is achieving a cleaner and healthier environment. Industrial evolution has led to the release of various gases and pollutants as byproducts of the primary manufacturing processes, contributing to environmental degradation. The excess concentration of these gases is posing daily health risks and are responsible for millions of deaths each year. This has created a growing demand for humidity and gas sensors capable of detecting different vapors with high accuracy, excellent sensitivity, and fast response and recovery times.      

The primary objective of this thesis is to develop and chemically modify various two-dimensional materials to enhance their humidity and gas sensing capabilities. Following the isolation of graphene in 2004, a new class of materials emerged, the two-dimensional materials, whose unique nature allows them to have a thickness of only a few nanometers. This structure offers a wide range of exceptional electrochemical and mechanical properties, making them perfect candidates for sensing applications, such as high surface area to volume ratio and the ability to be modified. The latter can be achieved by introducing new molecules, removing the already existing or even creating structural defects to their lattice. 

Initially, novel 3D printed electrodes were produced utilizing the Fused Deposition Modeling (FDM) technique. For this reason, electrically conductive nanocomposites filaments were produced based on the polymer Polylactic Acid (PLA) mixed with the nanofiller multiwall carbon nanotubes (MWCNTs). A melt premixing step was introduced followed by extrusion through a single screw extruder to produce the electrically conductive filament. The nanocomposite filament was then inserted into the FDM 3D printer and interdigitated electrodes were printed over neat PLA substrate. The electrodes were extensively characterized in order to identify their physical and electrical properties along with other critical parameters including flexibility and their mechanical strength.

Subsequently, various materials were synthesized and chemically modified in order to investigate their humidity and gas sensing capabilities. Initially, graphene oxide (GO) was produced following a top-down technique, a modification of Hummer’s method. According to this, flakes of graphite were implemented into a two-step reaction which exfoliates the graphite and introduces oxygen functional groups to its lattice. Then, the GO was extensively physicochemically characterized to understand its structure and properties. After coating the 3D printed electrodes, it exhibited excellent humidity sensitivity, and especially pairing with the polymer Polyvinyl Alcohol (PVA), which acted as an intermediate layer between the electrodes and the sensing layer, its stability was significantly enhanced. Furthermore, the effect of temperature on the humidity sensing measurements of the GO/ PVA sensor was investigated, and a mathematical model was derived, capable of providing real-time measurements by converting the capacitance and temperature to relative humidity. Subsequently, the GO/ PVA sensor was integrated into a helicopter’s wing to evaluate its ability to detect humidity under extreme weather conditions simulating flight environments, including elevated humidity levels, freezing temperatures and the presence of ice. The GO/ PVA sensor demonstrated excellent stability, accurately measuring the humidity without the performance being hindered by the harsh environmental conditions. Following this, the effect of mechanical strain on the flexible sensor was examined, revealing that it is capable of detecting the strain, without affecting the humidity measurements, thus providing multifunctional abilities. Finally, the GO was employed as gas sensor detecting various gases including ammonia and formaldehyde.

Furthermore, Layered Double Hydroxides (LDHs) were synthesized and tested for their gas and humidity sensing properties. Specifically, different LDHs based on the metals Mg and Al were synthesized using a coprecipitation method followed by an ageing step with intercalated CO₃^2- ions and GO. The final materials were characterized and found that they possess an increased concentration of oxygen functional groups which yielded high sensitivity towards humidity. At the same time, the effect of coating thickness on humidity sensing performance was evaluated, and impedance spectroscopy was used to examine the mechanism of physisorption and absorption of water molecules. Regarding the gas sensitivity, the LDH with intercalated CO₃^2- is able to detect both formaldehyde and ammonia with great accuracy.

Finally, another nanomaterial, the hexagonal Boron Nitride (hBN) was employed and underwent chemical modification. After using a single step strong oxidation procedure, the hBN became highly oxidized and were introduced various oxygen functional groups in its lattice, and obtained excellent humidity sensing capabilities and sensitivity towards ethanol, hydrochloric acid and ammonia.