Sustainable solar energy conversion with defined ferrite nanostructures

利用半导体材料进行光催化和光电化学是可持续利用太阳光进行化学反应的重要方法。这两种方法都特别用于减少温室气体和污染物等最近的环境影响，也用于确保未来的能源需求。特别是对水的光催化和光电化学分解的研究近年来得到了加强。然而，可以观察到，研究主要集中在TiO2， WO3和a-Fe2O3等二元氧化物上，但它们存在较大的缺点（带隙太大，载流子寿命低），无法在工业上应用于水裂解。材料科学家现在的任务是增加用于水分解的材料库，并在三元或季氧材料中寻找替代品。此外，这些材料中不应该含有昂贵但地球上丰富的元素，以保持可持续氢的低成本。本课题主要研究了三元尖晶石型铁氧体（MFe2O4）的光催化和光电化学水分解性能。目标材料仅由地球上丰富的元素（Ca, Mg, Fe, Zn， O）组成，可以吸收大量可见光（最大带隙2.2 eV），因此理论上能够达到10%以上的太阳能制氢效率。研究了这些铁氧体中载流子的扩散特性和寿命，以及纳米结构对铁氧体光电极性能的影响。致密和介孔铁氧体光电极将通过使用分子和纳米颗粒构建块来制备。嵌段共聚物将作为模板，通过浸涂法制备有序介孔光电极。最后，通过使用n型和p型铁氧体分别进行半反应，实现氢和氧在空间上的分离演化，建立了一个自我维持的串联电池。为了增加光吸收，将铁氧体薄膜沉积在掺杂硅片上，以制备单片p-n异质结串联光电极。

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.