Electron transfer in organic and inorganic light-converting systems
- Principal Investigators:
- Prof. Dr. Stephan Kümmel
- Project Manager:
- Dr. Ingo Schelter
- additional Affiliation:
- Sonderforschungsbereich 1585 “Structured functional materials for multiple transport in nanoscale confinements”
- HPC Platform used:
- NHR@FAU: Fritz
- Project ID:
- b163cb
- Date published:
- Researchers:
- MSc Matthias Knodt, MSc Emmi Gareis
- Introduction:
- This project aims at designing new materials for photocatalytic water splitting. The computational project is closely interlinked with an experimental partner project within the Sonderforschungsbereich 1585. By splitting water into its components hydrogen and oxygen with the help of sunlight, the energy carrier hydrogen can be generated in a sustainable way. Attaching molecular complexes to a semiconductor surface is a key step in the material design. In quantum mechanical simulations that are conducted at NHR Erlangen, we explore material combinations and calculate, e.g., whether certain complexes bind properly to the surfaces. Successful combinations can then be realized in experiment.
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Photocatalytic water splitting is an attractive option for generating the green energy carrier hydrogen. However, designing materials that can achieve this is a challenge. The main bottleneck in the water splitting reaction is the water oxidation. For the overall process to run efficiently, a material is required that can absorb light and at the same time is able to accept electrons from oxygen. In this project, light is absorbed in a WO3 crystal and the aim is to use the thus generated excitation “hole” to oxidize water. This requires to transport charges from the electrolyte into the crystal. The oxidation- and transport-step can greatly be facilitated by attaching specific molecular complexes to the crystal surface. Supported by the German Science Foundation (DFG) and as part of the Sonderforschungsbereich 1585, a joint theoretical and experimental effort is made to design such a surface-functionalized water-splitting system.
An important part of the project is the computational analysis of the electronic structure of the systems and processes. The WO3 crystals and the molecular complexes can be described with Density Functional Theory (DFT). DFT is in principle excellently suited for the first-principles calculation of electronic properties. However, the water splitting problem poses a considerable challenge because many properties have to be represented faithfully: The bond-lengths and bond-angles of the WO3 crystal, its band gap, and the geometry and ionization potential of the molecular complex. At the same time, the combined system comprises hundreds of electrons. The calculations are therefore computationally expensive, and methods and procedures have to be carefully selected to achieve a balance between the required accuracy and the affordable computational cost.
In the initial phase of the project we have explored the computational methods. Standard plane-wave representations allow for an efficient description of the WO3 slabs that model the cystal surface. For the transport calculations, calculations that represent the physical quantities on a grid in real space have been found to be very efficient. They allow to study the flow of charge through the system with atomistic resolution in real time. Furthermore, we have established that a new class of Meta-Generalized Gradient Approximations, which have recently been developed in a different DFG project [1,2], can be used here when they are combined with new, consistent effective core potentials [3].
In a second phase we have used these methods to test which molecular complexes bind to the WO3 surface [4]. The binding strength is important, because if the complexes bind too weakly, they will be washed off by the water. Furthermore, a sufficient electronic coupling between the complex and the WO3 crystal is required for allowing charges to move between complex and crystal. Fig. 1 shows one possible scenario for the binding to a WO3 (001) surface: Here, Catechol is used as an anchoring group and a monodentate binding mechanism attaches the molecule to the crystal surface. The binding is further supported by a hydrogen bridge-like interaction.
In addition to single-crystal surfaces, we also study how the molecular complexes interact with nanocrystallites that have structural defects. The defects tend to increase the binding strength. Furthermore, we probed the strength of the electronic coupling by calculating the electronic excitations. Fig. 2 shows the transition density that is associated with the optical excitation of catechol bound to a WO3 crystallite via dispersive interactions (left) and via a covalent bond (right). In the first case the excitation almost only affects the WO3 crystallite, whereas in the second case, it spreads over both the molecule and the crystallite. This shows that in the latter case, the light can transfer charge between the molecule and the solid, and this nourishes hope that a water splitting system can be based on on this concept of surface modification. The computing resources provided by NHR Erlangen are an invaluable tool in this research effort.
[1] T. Aschebrock and S. Kümmel, Phys. Rev. Research 1, 033982 (2019)
[2] T.Lebeda, T. Aschebrock, S. Kümmel, Phys. Rev. Lett. 133, 136402 (2024)
[3] E.Gareis, I. Schelter, S. Kümmel, Phys. Rev. B (in preparation)
[4] M. Knodt, L. Mayer, E. Gareis, G. Hörner, B. Weber, L. Leppert, R. Marschall, S. Kümmel, PCCP (in preparation)
- Institute / Institutes:
- Physik
- Affiliation:
- University of Bayreuth
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