Tungsten oxide nanocrystalline powders for gas sensing applications

Abstract

[eng] This doctoral dissertation deals with the study of nanocrystalline tungsten oxide-based powders for gas sensing applications. Gas monitoring is receiving increasing attention due to environmental and safety reasons. Gas sensors based on heated semiconducting metal oxides are appreciated as they can detect low concentrations of certain gases by a variation of conductance of the metal oxide layer. Among the different metal oxides proposed, tungsten oxide (WO3) is considered one of the most promising materials for the detection of ammonia (NH3), hydrogen sulphide (H2S) and nitrogen dioxide (NO2). The first chapter of this dissertation presents the general framework where this investigation is placed. It includes a brief overview of chemical sensors, metal oxide-based gas sensors and the reported properties of WO3 for gas sensing applications. Finally, motivation and main targets of this investigation, as well as the organisation of this dissertation, are presented and argued. Chapter 2 presents the experimental details of this study. It discusses the preparation of WO3 nanocrystalline powder, the experimental techniques used to analyse the structural properties of the WO3 powders and the implementation and test of gas sensor devices. For this work, XRD, Raman spectroscopy, TEM-EELS, XPS, EPR, TPD and DRIFTS have been used. Thick-film screen-printed gas sensors based on WO3-powders were tested. The target of Chapter 3 is to present an investigation into the structural and spectroscopic properties of pure and catalysed nanocrystalline WO3 powder. Firstly, the characterisation of pure nanocrystalline WO3 powder is considered. The main parameter analysed is the influence of the annealing treatment on the structural properties. Structural and spectroscopic properties of catalysed WO3 are also reported here. In this case, the emphasis lays on the characterisation of catalytic centres, rather than on bulk WO3. The catalytic additives introduced were copper (Cu), vanadium (V) and chromium (Cr). Results concerning gas sensors based on WO3 nanocrystalline powders for the detection of the previously mentioned gases are reported in Chapter 4. The purpose of this investigation was to evaluate the sensing properties of thick-film gas sensors based on WO3 obtained from tungstic acid and to determine the effect of different additives on sensor response to NH3, H2S and NO2. Interference of humidity on the detection of these gases was also evaluated. A tentative interpretation of the reported results based only on the test data presented is also provided. The aim of Chapter 5 is to explore the implementation of real condition characterisation techniques to WO3-based nanopowders in order to study surface species and reactions involved in gas sensing. By real condition, we refer to a characterisation under controlled conditions of temperature and gas concentration on the sample, as similar to test conditions as possible. These studies, which are standard in the field of catalysis, are not so common for gas sensors. This is still a key point in the field of gas sensors at the moment: to obtain a deeper understanding of what occurs on the surface of the sensing material. The chapter has two main parts, corresponding to the results of DRIFTS and TPD techniques. By means of DRIFTS it is possible to identify surface species that present infrared vibrations. By TPD, the desorption of adsorbed target gases is analysed. Finally, Chapter 6 aims at discussing the results previously presented as a whole, as well as presenting the main conclusions that can be drawn from this investigation and some proposals for a future research grounded on the present work.