Carbon dioxide (CO2) detection is of vital importance in numerous fields, such as the health sector, food industry, agriculture and environmental monitoring. Metal oxides (MOx) rely on surface interactions with gas molecules and have drawn significant attention as sensing materials due to their high sensitivity, small size and micro-fabrication compatibility. Nanostructured MOx offer a high surface-to-volume ratio, providing abundant active sites for gas interaction. Pristine MOx possess limited sensitivity towards CO2 due to its chemically stable nature. A common sensitization method is introducing noble metals to create a composite layer. In this study, spark ablation coupled with a dry printing system is utilized to produce and deposit high-purity Ru-loaded SnO2 nanoporous layers (NPLs) on interdigitated electrodes (IDEs), in order to test their reproducibility as chemiresistive gas sensing layers. Three geometrical patterns were printed, namely a dot, a line and a stitched rectangle. The printing reproducibility of the NPLs is evaluated through electrical measurements on a large number of devices, followed by a statistical analysis. The same layers are also printed on IDEs with an integrated coplanar Pt microheater to realize gas sensing devices for CO2 detection. Due to the inability of the coplanar microheater to reach the desired temperature (300 °C), an industrial MEMS-type microheater platform was adopted. Both the reproducibility test devices and gas sensors were annealed at 500 °C for 10 minutes to stabilize their operation, increase conductivity and introduce more oxygen vacancies. The line and dot patterns represented the highest degrees of printing reproducibility, following a log-normal distribution. Sixty-eight percent of the line resistances are 2.12 times larger or smaller than their median value, and this factor rises to 4.5 for 95% of the devices. For the dots, these factors are 2.23 and 4.98 for 68% and 95% of the devices, respectively. The rectangle pattern demonstrated multi-modal behavior, with factors of 3.15 and 9.9 for 68% and 95% of the devices, respectively. The gas sensing results revealed an optimal operating temperature of 350 °C for all three sensors, which were highly sensitive even for low CO2 concentrations (300 ppm). However, all sensors exhibited oscillations in their baseline resistance, masking their actual response. Tests in a humid environment exhibited a linear increase in the response with increasing relative humidity levels. Nevertheless, the response was lower compared to the dry-air environment. ... Read more

Graphene as an allotrope of carbon is promising for the detection of gaseous molecules due to its extremely high surface-to-volume ratio. However, its low selectivity poses a major problem for practical applications. The work presented in this thesis addresses the selectivity problem by depositing metal/metal oxide nanoparticles (NPs) on the surface of graphene. For this purpose, gas sensors with multiple multi-layer graphene (MLG) strips were fabricated. Four types of metal/metal oxide nanoparticles were investigated: Gold (Au), Platinum (Pt), Copper (Cu), and Iron (Fe). Various techniques were developed to study the properties of graphene and NPs. In addition, an automatic measurement setup with multi-strips switching is developed for data acquisition. Finally, the sensors decorated with different metal types and coverages are investigated for their response to H2O and NO2. The study shows that both the pristine devices and the sensors decorated with NPs hardly response to water molecules, while the responses to NO2 are larger and vary depending on the NPs. The results are promising for the development of a gas sensor array based on this methodology with improved selectivity for gas detection. ... Read more