Sustainable solar energy conversion with defined ferrite nanostructures

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Photocatalysis and photoelectrochemistry with semiconductor materials is an important method for the sustainable utilization of sunlight for chemical reactions. Both methods are especially used in the degradation of recent environmental impacts like greenhouse gases and pollutants, and also for securing the future energy needs. Especially research regarding the photocatalytic and photoelectrochemical splitting of water has been intensified in recent years. However, it can be observed that research has mainly focused on binary oxides like TiO2, WO3 und a-Fe2O3, which however have major drawbacks (too large band gap, low charge carrier lifetimes) to find industrial application for water splitting. The task for materials scientists is now to increase the materials library for water splitting, and to find alternatives in ternary or quaternary oxide materials. Moreover, no expensive but earth-abundant element should comprise those materials, to keep the costs for sustainable hydrogen low.The given project focuses on the investigation of the properties of ternary spinel-type ferrites (MFe2O4) for photocatalytic and photoelectrochemical water splitting. The aimed materials consist only of earth-abundant elements (Ca, Mg, Fe, Zn, O), absorb to a large extend visible light (maximum band gap 2.2 eV), and are therefore theoretically able to reach more than 10% solar-to-hydrogen efficiency. The diffusion properties and lifetimes of charge carriers in these ferrites will be investigated, as well as the influence of nanostructuring on the properties of ferrite photoelectrodes. Both dense and mesoporous ferrite photoelectrodes will be prepared, by using both molecular and nanoparticular building blocks. Block-copolymers will be used as templates for the preparation of ordered mesoporous photoelectrodes via dip-coating.Finally, a self-sustaining tandem cell is targeted, by using both n- and p-type ferrites for the respective half reaction, to achieve spatially separated evolution of hydrogen and oxygen. To increase light absorption, deposition of ferrite thin films onto doped silicon wafers is targeted, to prepare monolithic p-n-heterojunction tandem photoelectrodes.