Photochemistry of metal cluster - GaN semiconductor hybrid materials

使用光作为驱动化学反应的能源早已被设想用于各种应用。由于利用太阳能获得绿色能源燃料的可能性，光催化过程目前在基础和应用研究中受到特别关注。在多相催化剂中最有前途的系统是包含半导体与助催化剂的混合系统。通常认为，利用高于半导体带隙的光子能量的照射产生电子和空穴，电子和空穴由于半导体表面处的电子带的弯曲而分离。结果，电子或空穴行进到助催化剂，在助催化剂处它们能够进行催化反应。绝大多数的研究都致力于寻找具有改进的光催化活性的新材料。虽然更有效的催化剂的合理设计非常可取的，更基本的方面的研究是稀缺的。他们的调查是本项目的主题。混合III-氮化物半导体装饰与大小选择的Pt-簇的大小范围内高达约100个原子将被用作模型系统，用于调查光催化过程。与其他光催化材料（如TiO 2或ZnO）相比，GaN允许通过合金化以及n型和p型掺杂的带隙工程来调整其电子特性。对于光催化反应，我们重点关注加氢反应。混合体系的光催化效率受以下因素的影响：（i）团簇与反应物的相互作用，（ii）团簇-基底相互作用，以及（iii）光生载流子的动力学。虽然前两个已经在黑暗中的反应速率是必不可少的，电荷载流子动力学控制照明下的量子效率。我们对这个项目的策略是理清所有三种效应的影响。我们将分别研究热反应对团簇尺寸的依赖性、载体对反应性的影响以及尺寸选择团簇的光化学反应性。在这里，半导体制备和表征将应用于设计具有定制特性的新结构，例如定制的n-p（p-n）二极管结构，以确定性地控制电荷分离。此外，探索性研究将在用大小选择的Pt簇修饰的纳米结构支持物上进行。控制反应机制的关键参数的识别，如簇的大小，半导体的电子性质，和电荷载流子动力学，将使更深入地了解金属簇/半导体系统的催化性能。这将最终允许基于此类材料的新型催化剂的定制设计。

The use of light as energy source for driving chemical reactions has long been envisioned for various applications. Fostered by the possibility of using sun power for obtaining green-energy fuels, photocatalytic processes currently receive particular attention in both, fundamental and applied research. The most promising systems in heterogeneous photocatalysis are hybrid systems comprising a semiconductor with a co-catalyst. It is generally believed that illumination with photon energies above the semiconductor band gap generates electrons and holes which are separated due to a bending of the electronic bands at the semiconductor surface. As a result, electrons or holes travel to the co-catalyst where they enable catalytic reactions. The vast majority of studies has been dedicated to the search of new materials with improved photocatalytic activity. Although highly desirable for the rational design of more efficient catalysts, studies on more fundamental aspects are scarce. Their investigation is subject of the present project. Hybrid III-nitride semiconductors decorated with size-selected Pt-clusters in the size range of up to about 100 atoms will be used as a model system for investigating photocatalytic processes. In contrast to other photocatalytic materials such as TiO2 or ZnO, GaN allows for the tuning of its electronic properties by band gap engineering via alloying as well as both n- and p-type doping. For the photocatalytic reactions we focus on hydrogenation reactions. The photocatalytic efficiency of hybrid systems is influenced by (i) the interaction of the clusters with the reactants, (ii) the cluster-substrate interaction, and (iii) the dynamics of photogenerated charge carriers. While the first two are already essential for the reaction rate in the dark, the charge carrier dynamics govern the quantum efficiency under illumination. Our strategy for the project is to disentangle the influence of all three effects. We will study the dependence of the thermal reaction on the cluster size, the influence of the support on reactivity, and photochemical reactivity of size-selected clusters, respectively. Here, semiconductor preparation and characterization will be applied to design new structures with tailored properties, e.g. customized n-p (p-n) diode structures to deterministically control charge separation. Moreover, exploratory studies will be performed on nanostructured supports decorated with size-selected Pt-clusters. The identification of key parameters controlling the reaction mechanisms, such as the size of the clusters, the electronic properties of the semiconductor, and the charge carrier dynamics, will enable a deeper understanding of the catalytic properties of metal cluster/semiconductor systems. This will ultimately allow the tailored design of novel catalysts based on such materials.