Functional nanoparticles: tin monoxide and molybdenum disulfide quantum dots on graphene nanosheets
Abstract
Light harvesting can be referred to the use of an ensemble of different nanoparticles, or quantum dots, or other absorbers to optimize the ability to capture a given spectrum of electromagnetic radiation (for example the solar spectrum under specific atmospheric conditions) in a light-absorbing system. To this end, different nanoparticles play complementary functions within the system and absorb or scatter light at different wavelength intervals. Light harvesting finds applications in fields as diverse as solar cells, photosynthesis and photocatalysis. Graphene supporting a set of different semiconducting nanoparticles has often been proposed as light harvesters. To further this concept, my thesis has explored the effect of few-layer graphene nanosheets on the optical properties of colloidal molybdenum disulfide quantum dots (MoS2-QDs) and investigated the synthesis of tin/tin oxide nanoparticles (SnOx-NPs) with a process compatible with their integration on graphene. Using synchrotron X-ray absorption near-edge spectroscopy, we find that the density of unoccupied electronic-states in MoS2-QDs is profoundly affected by their Van der Waals interaction with few-layer graphene nanosheets (principally affecting the S K-edge) as well as quantum confinement (mainly shifting the Mo L3 edge). As far as tin-based nanoparticles are concerned, our revisitation, via X-ray photoelectron spectroscopy (XPS), of a popular synthesis approach based on stannous acetate reduction, shows that this method is normally not yielding to metallic Sn-NPs, but tin monoxide, even in oxygen-free atmospheres. Only the subsequent disproportionation of SnO may perhaps lead to SnO2 and metallic tin because of heat released from the reduction process. As SnO-NPs are an excellent optical absorber in the visible-near infrared photon energy range, they will be an excellent complement to MoS2-QDs on few-layer graphene nanosheets, in a novel graphene-based light-harvesting system.