Photochemistry of metal cluster - GaN semiconductor hybrid materials

长期以来，人们一直在设想将光作为驱动化学反应的能源，用于各种应用。在利用太阳能获取绿色能源燃料的可能性的推动下，光催化过程目前在基础和应用研究中都受到了特别的关注。在多相光催化中最有前途的体系是由半导体和助催化剂组成的混合体系。通常认为，在半导体带隙以上的光子能量照射会产生电子和空穴，这是由于半导体表面的电子带弯曲而分离的。结果，电子或空穴传播到辅助催化剂，在那里它们能够进行催化反应。绝大多数研究一直致力于寻找具有更高光催化活性的新材料。尽管对于合理设计更有效的催化剂来说是非常可取的，但对更基本方面的研究却很少。他们的调查是本项目的主题。用尺寸选择的铂原子团簇装饰的混合III-氮化物半导体将被用作研究光催化过程的模型系统。与二氧化钛或氧化锌等其他光催化材料不同，GaN可以通过合金化以及n型和p型掺杂的带隙工程来调节其电子性质。对于光催化反应，我们主要关注加氢反应。杂化体系的光催化效率受(I)团簇与反应物的相互作用、(Ii)团簇-衬底相互作用和(Iii)光生载流子动力学的影响。虽然前两者对黑暗中的反应速度是必不可少的，但电荷载流子动力学决定了照明下的量子效率。我们对该项目的策略是理清这三种效应的影响。我们将分别研究热反应对团簇尺寸的依赖，载体对反应活性的影响，以及尺寸选择的团簇的光化学反应活性。这里，半导体制备和表征将被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，以确定地控制电荷分离。此外，还将对用尺寸选定的铂团簇装饰的纳米结构载体进行探索性研究。识别控制反应机理的关键参数，如团簇的大小、半导体的电子性质和电荷载流子动力学，将有助于更深入地了解金属团簇/半导体体系的催化性质。这最终将使基于这种材料的新型催化剂的定制设计成为可能。

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.