Noble metal nanoparticles as active materials for solar energy conversion and chemical sensing

The aim of this thesis is to assess the possible applications of noble metal nanoparticles (NPs) in the fields of solar energy production and chemical sensing. The research project arises directly from the well-known extraordinary properties of noble metal NPs, namely the optical and electrical properties, high surface area, high catalytic activity and surface enhanced Raman scattering effect. More specifically, concerning the energy production field, the first part of the thesis will deal with the application of Au NPs as counter electrode material in dye sensitized solar cells (DSSCs). This study was conducted in the S.O.L.A.R.E. laboratory at the Istituto per i Processi Chimico-Fisici of the Italian National Research Council (IPCF-CNR) in Messina, under the supervision of Dr. Giuseppe Calogero. To the best of our knowledge, the role of noble metal NPs as exclusive counter electrode material in DSSCs has not been explored yet, despite their catalytic activity (i.e. applications in water purification) and electron conductivity have been massively studied. On the other hand, gold mirror thin films were used in combination with graphene nanoplatelets for the fabrication of the counter electrode in the most efficient DSSC reported in literature. For this reason, the work conducted towards the realisation of this thesis has been focused on the development of an efficient and reproducible method for the fabrication of Au based counter electrodes for DSSCs. Two different methods have been proposed, namely the thermal decomposition of HAuCl4 as bottom-up method and the pulsed laser ablation of a gold target as top-down method. They were compared on the basis of the counter-electrodes optical properties, surface morphology, catalytic activity and performance both in dummy cells and fully assembled DSSCs. In the latter case, the adequate photoanode configuration has been studied, in order to reduce the electron recombination and maximize the solar-to-electric power conversion efficiency. In addition, the fabricated gold counter-electrodes were compared to standard platinum ones. In the second part of the thesis, the use of Ag NPs for the fabrication of multiresponsive plasmonic sensing platforms for surface enhanced Raman scattering (SERS) and chemiresistive sensing will be presented. This work has been conducted in the Nanochimie laboratory at the Institut de Science et d'Ingénierie Supramoléculaires (ISIS) in Strasbourg, under the supervision of Prof. Paolo Samorì. The development of SERS based sensors has experienced an enormous growth in the last decades, as a consequence of their high versatility, high sensitivity, ease of fabrication and low cost. However, despite the inherently higher SERS activity of Ag compared to Au, sensing platforms based on Au NPs are already commercially available, while Ag NPs are mainly employed as colloidal dispersions, due to their lower chemical stability arising from oxidation. On the other hand, the extraordinary electrical properties of noble metal NPs made them suitable as active conductive materials in chemiresistors. Within this context, different synthetic procedures have been used to obtain citrate stabilized Ag NPs, tannic acid stabilized Ag NPs and Au@Ag core@shell NPs. Besides the study of their optical properties in colloidal solutions, they were compared by means of their ability to give rise to uniform thin films on chemiresistive devices, using both the cross-linking with dithiols and the electrostatic layer-by-layer deposition. The best results were obtained with the electrostatic layer-by-layer deposition of tannic acid stabilized Ag NPs, so that the optical properties, surface morphology, SERS and chemiresistive activity of these devices was largely studied. Furthermore, the possibility to use the fabricated sensing platforms for the sensing of mercury ions in water by both indirect SERS and resistance variation was explored.