Synthesis and gas sensing properties of inorganic semiconducting, p-n heterojunction nanomaterials

Resumen

Due to the large increase in environmental pollution burden and especially the threats to both the environment and human health in the short and long term resulting of breathing atmospheres in which pollutant gases are present, there is a strong need to develop gas sensors able to detect such gases at trace concentrations in the air. The gas sensors can have a myriad of applications but the most direct is the control of air quality for example in closed areas such as laboratories, industrial facilities or in domestic environments as well as outdoor areas such as parks industrial or large cities. In industrial areas, gas sensors can be used to prevent gas leaks during processes, to monitor emissions into the atmosphere and to be aware of what workers may be potentially exposed to. In domestic environments for example stoves, burners, etc, are potentially dangerous due to the emission of carbon monoxide which can cause suffocation and therefore the sensors can act as an alarm. Thus, a real and increasingly need to develop state-of-the-art sensors that are able to detect ever-lower concentrations of potentially harmful gases has risen. In addition, these sensors must be easily manufacturable in order to scale their production as well as consume as little energy as possible. For these reasons, gas sensors based on metal oxide nanostructures are ideal candidates for performing the task of gas sensing as chemoresistive gas sensors. A large number of sensors based on metal oxide nanostructures, especially tin oxide, are currently available on the market. However, the growing need to analyze a wider range of different gases while at different concentrations makes it necessary to research new materials based on metal oxides, as well as new nanostructures while understanding the fundamental mechanisms taking place once the metal oxide interacts with the target gas. Therefore, in this thesis, using mainly Aerosol Assited Chemical Vapor Deposition, AACVD, as a synthesis methodology for nanostructured tungsten oxide, different gas sensors have been manufactured. To study the improvement in the selectivity and sensitivity of gas sensors based on tungsten oxide, they have been decorated, via AACVD, with nanoparticles of other metal oxides to create heterojunctions to obtain an increase in electronic sensitivity, in the chemical properties of the material or at the same time in both. Particularly, we have worked on different tungsten oxide nanowire sensors decorated with nanoparticles of nickel oxide, cobalt oxide and iridium oxide resulting in sensors with a large increase in response and selectivity towards hydrogen sulfide, for ammonia. and for nitrogen oxide respectively at trace concentrations. In addition, the reaction mechanisms that take place between oxygen species adsorbed on the sensor surface when it interacts with a gas have been also studied. Furthermore, efforts have been put on trying to control the surface potential of the nanostructured layers to control the drift in the signal over time, when operating the sensors, through temperature control.