Gas sensing with modified carbon nanotubes, graphene and diamondoids

Resumen

Gas sensors constitute an essential option for the early detection of harmful gases in the atmosphere. Thus, an extensive sensor network is required for continuous monitoring in order to preserve the human health and the environment from dangerous levels of pollutants. Nowadays, most of inexpensive commercial gas sensors are made-up with metal oxides. However, these oxides usually require high operating temperatures, which results in high power consumption and reduced sensor shell-life because of their chemical degradation. Additionally, metal oxide-based gas sensors show low selectivity, being difficult their integration in devices able to detect a wide variety of gas mixtures. Conversely, carbon nanomaterials such as carbon nanotubes, graphene or diamondoids, present outstanding electronic, physicochemical and mechanical properties. However, their implementation in commercial gas sensors is still an issue due to their inherent problems. For instance, carbon nanomaterials usually show low reactivity with gas species, poor selectivity and high response/recovery times due to the slow adsorption/desorption rates of gas molecules. Nevertheless, if these problems are solved, carbon nanomaterials show interesting properties such as low-cost, possibility to work at room temperature (therefore, low power consumption) and straightforward options to be implemented in miniaturised devices. In consequence, carbon nanomaterials show high potential to be employed in the next generation of gas sensors, provided further modifications to these nanomaterials are implemented for overcoming their main drawbacks. Thus, the aim of this thesis is to explore different modifications of carbon nanomaterials in order to improve essential gas sensing parameters such as selectivity, sensitivity or response times. To do that, several approaches were developed, by functionalizing the carbon nanomaterials with specific molecules or atoms, by decorating them with metal oxide nanoparticles or by favouring the formation of self-assembled monolayers, among others. The results obtained with these modified carbon nanomaterials demonstrated the achievement of enhanced sensing parameters. Several methodologies and techniques were used in this thesis. First of all, the sensing nanomaterials were deposited onto silicon dioxide or alumina substrates to create chemical-sensitive layers that can be implemented in electronic devices. Afterwards, the resistance changes upon the exposure to gases were monitored and analysed. Extensive material characterization was performed by using spectroscopic and microscopic techniques such as X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopies (SEM and TEM, respectively), Raman spectroscopy, x-ray diffraction (XRD), and others. Additionally, the physicochemical interactions between gas molecules and sensitive carbon layers were studied, resulting in the discussion of several gas sensing mechanisms. In summary, several gas sensing characteristics were improved, demonstrating that modified carbon nanomaterials are a suitable option to be employed in the new generation of inexpensive gas sensors. Furthermore, we achieved the detection of pollutants at trace concentration levels, even below the threshold limit values established by law for human exposure.