Nanomaterials synthesis and application in the photoconversion of carbon dioxide and the photo(electro)chemical degradation of organic pollutants

Abstract

The rising level of CO2 are one of the modern major concerns related to climate change and the need of a concrete action addressed the attention on the possibility to convert CO2 in re-usable products. The use of sunlight as energy source has also been broadly employed to ensure the sustainability of the process. Many metals and semiconductors were proven as able to provide either electro- or photo-driven CO2 conversion, with the main overall limitations consisting in a) costs for highly efficient – and less abundant – metals and b) efficiency and corrosion for highly abundant semiconductors. Factors as size, shape, presence of co-catalysts and reaction conditions, were al reported as able to affect the catalytic performance. To address the current technological needs using a cost-effective and environmentally sustainable approach, in this study nanostructured materials consisting in earth-abundant semiconductors such as ZnO, TiO2 have been synthesized and their photo-oxidizing properties were compared and tested in the CO2 conversion to CH4 in gas phase. Photo/degradation of methylene blue in liquid phase was also tested. ZnO and TiO2 were additionally tested as photoanodes in a photoelectrochemical cell for both the water oxidation and the photodegradation of methylene blue. Eventually, the employment of co-catalysts such as Cu, Ag – and TiO2 for ZnO based materials – aimed to improve the photocatalytic properties, to provide protection from photo-corrosion as well as investigating the presence of a charge transfer mechanism as responsible for the enhanced properties. The use of a scaffold – such as SiO2 – aimed to enhance light scattering and promote gas adsorption, while Ce was introduced to increase light absorption in the visible region. Characterization techniques such as physisorption analysis, photoluminescence, UV-Vis absorption spectroscopy, XPS, SEM-EDX, XRD, TEM, TPO and TPR were performed to assess the morphological and optical features of the materials and used as tool of comparison and evaluation of their photo-catalytic properties. Techniques such as electrochemical impendence spectroscopy, linear sweep voltammetry, quantum efficiency and cyclic voltammetry were also used to evaluate the electrochemical and photo(electro)chemical properties of the samples. In conclusion, the CO2 photoconversion in valuable products mediated by cheap and abundant materials has been tested in mild temperature, pressure and light irradiance conditions. The photoactivity of differently shaped and synthesized ZnO-based and TiO2-based materials was compared, and the underlying mechanism studied. The catalytic properties enhancement was attempted by introduction of co-catalysts or a scaffold, and a charge transfer mechanism has been proposed. The photocurrent produced by semiconductor-based photo-anodes was also measured and the effect of a bias on stability and catalytic properties has also been investigated. Eventually, the ability of both ZnO and TiO2 synthesized by different methods to photo-oxidize dye pollutants was also compared.