Charge Density in Solar-Driven Noble Metal / TiO2 Catalysts

The proposed Lise Meitner project aims at bringing together Dr. Saji Thomas Kochuveedu, a highly motivated, talented and, with respect to her academic age, extremely successful and productive young scientist with Prof. Dr. Thomas A. Klar, who runs one of the world-leading groups in spectroscopy of single metallic nanoparticles (NPs). With this project, the profound knowledge of Dr. Kochuveedu in the fields of nanoparticle synthesis and photocatalysis will be matched with the comprehensive arsenal of single nanoparticle and ultrafast (ensemble) spectroscopic tools in the group of Prof. Klar. As a specific topic to be tackled within this unique set of joint knowledge, the applicant wants to investigate noble metal nanoparticles which release or accept electrons in solar-driven processes such as photocatalysis or optically driven hydrogen production. Obviously, these topics are of extremely high relevance (specifically in her home country, India) as they address the use of abundantly available solar energy as energetic source for chemical process engineering, for solar fuels and for solar electricity. However, fundamental research on optically driven noble metal / oxide hybrid photocatalysts is required on the way towards applications. One unsolved question is how many electrons are really transiently stored on (or how many electrons are displaced from) noble metal nanoparticles in metal / semiconductor nanoparticle hybrid systems during and shortly after irradiation with (sun-) light. In addition, the processes of electron transfer from noble metal nanoparticles to the semiconductor particles are not sufficiently understood. For example, it is largely unclear whether a plasmonic excitation decays directly into an exciton on the semiconductor (adsorbate induced quenching of the plasmon), whether it decays into an inter- or an intraband excitation of an electron in the metal (Landau damping) with subsequent transfer of the excited electron (likewise: the hot hole) over a Schottky-type barrier, or if the single hot electron thermalizes and the hot electron sea in the metal emits electrons into the semiconductor. Dr. Kochuveedu will address these questions using femtosecond pump probe spectroscopy and, most important, optical spectroscopy of the charge state of the metallic nanoparticles, a method recently developed in the group of Prof. Klar.

Combustion of fossil fuel and consequent carbon dioxide emission is one of the main sources of climate change and other detrimental environmental effects. Finding alternative fuel without compromising the quality of the environment has been a constant challenge to researchers. Considering the future energy demand, development of clean and renewable energy has attracted much interest, in particular hydrogen generation by splitting of water. Developing suitable catalysts including semiconductor-plasmonic metal composites to achieve targeted fuel generation and to investigate the mechanism of charge transfer between the catalysts` components were the aim of this project. Since the planned materials did not show sufficient efficiency, a new set of catalysts with visible light absorption characteristics was designed to demonstrate splitting of water. Iron oxide (hematite) is well-known for its oxygen generation activity and cadmium sulfide (CdS) is considered as very good hydrogen generation catalyst. During irradiation of light, these two catalysts produce excited electrons in the conduction band and holes in the valence band. In the case of hematite, the holes in the valence band have sufficient energy to produce oxygen and the electrons in the conduction band of CdS are capable to generate hydrogen. The performance of a catalyst is often measured by its surface properties, such as surface area, porosity, etc.. Therefore, a catalytic structure is designed which compiles the mentioned criteria for the catalyst. Highly porous hematite is prepared using a silica template-assisted method, and CdS nanoparticles (NPs) are trapped inside the pores. The scheme on the left hand side explains the structure and the generation of hydrogen and oxygen by the respective catalyst during the process. The blue colored outer walls are made of a silica template, the brick color cylindrical pores are constituted of hematite, and the yellow spheres represent CdS NPs inside the pores. When the catalyst is dispersed in water, the water molecules come in contact with the catalyst, and when light is passed through the system, the catalysts generate electrons and holes within them. The electrons in CdS produce hydrogen from water, whereas holes in hematite produce oxygen. The charge transfer processes is explained in the scheme on the right side.