Noble Metal / Metal Oxide nanocomposite thin films for optical gas sensors

Content Provider	Semantic Scholar
Author	Gaspera, Enrico Della
Copyright Year	2011
Abstract	In the last decades, the research field known as nanotechnology has been deeply investigated since it helps to understand the properties of the materials, and provides a useful tool to design materials with tailored properties, that can be exploited for many applications across the whole field of science. Nanomaterials exhibit distinctive size-dependent properties, and a high surface to volume ratio, extremely useful in applications like sensing and catalysis. In this doctoral project, different combinations of noble metals and transition metal oxides have been used to prepare inorganic thin films to be used as reducing gases sensors through an optical interface: while the semiconductive metal oxide is usually responsible for the detection mechanism, metal nanoparticles play the role of optical probes, enhancing the optical response, and/or catalysts, improving the sensor performances. The main work presented here was focused on the synthesis of these nanocomposite materials through different strategies, according to the desired quality of the final material, the easiness of the procedure, the control on key aspects like size and shape of the particles, their size distribution, the crystallinity of the different components, the porosity. In the first part, noble metal (Au, Ag, Pt) ions have been embedded inside oxide matrixes by means of sol-gel or impregnation processes, and reduced to metal nanoparticles through high temperature annealing, which is necessary also to promote the oxides crystallization: remarkable gas sensing properties have been observed for NiTiO3-TiO2-Au films for hydrogen sulfide detection, with extremely good sensitivity and selectivity towards interfering gases like CO and H2. The experimental results suggest a catalytic oxidation of H2S to sulfur oxides promoted by NiTiO3 crystals, while Au nanoparticles are not involved directly in the reaction mechanism, but act as probes providing an easily detectable optical signal. Quite good sensing properties for CO and hydrogen detection have been presented for other nanocrystalline thin films like SiO2-NiO-Ag prepared combining sol-gel and impregnation processes, sol-gel ZnO-NiO-Au nanocomposites, and microstructured WO3-Au-Pt films synthesized with the sputtering technique and a subsequent impregnation process. The second part is based on the colloidal synthesis of metal (Au, Pt, Au@Pt core@shell) and oxide (TiO2, ZnO pure and doped with transition metal ions) nanoparticles with desired size and distribution: purification and concentration protocols have been developed and the final colloidal solutions have been directly used for films deposition, obtaining nanocrystalline coatings at low temperatures. TiO2-based films show good sensitivity for CO and H2, with a detection threshold of about 2 ppm, quite remarkable considering that films are only 40-60 nm thick. These materials were also able to detect ethanol vapors at room temperature. Moreover samples containing both Au and Pt NPs are able to reversibly detect hydrogen at room temperature, thanks to the synergetic effect occurring between the optical properties of Au and the catalytic properties of Pt. ZnO-based samples have been tested as CO sensors with a detection limit down to 1-2 ppm, and a relationship between type of dopant (Ni, Co, Mn) and response intensity has been presented. The third part is focused on the deposition of Au nanoparticles layers on properly functionalized substrates, and their subsequent coating with sol-gel films: when Au nanoparticles are in close contact with each other, a coupling of the plasmon frequencies is found to occur, and this effect can be used to enhance sensing, SERS and catalytic performances. Au nanoparticles layers covered with NiO or TiO2 films showed promising gas sensing properties for CO and hydrogen detection at high temperatures, and for ethanol sensing at low temperatures. More complex structures composed of an Au nanoparticles layer sandwiched between two different oxide layers (NiO, TiO2, ZnO) are also prepared, trying to enhance the selectivity towards interfering gases by providing two different noble metal / metal oxide interfaces.
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Language	English
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Content Type	Text
Resource Type	Article