Photochemistry of metal cluster - GaN semiconductor hybrid materials

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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.