Exploring photocatalytic processes at the atomic scale
Final Report Abstract
Overarching goal of this DFG project, carried out as a collaboration between the University of Oldenburg and the Fritz-Haber-Institute in Berlin, was to move towards a mechanistic understanding of photocatalytic processes at surfaces, in particular of photo-induced water splitting. For this purpose, atomically flat and crystalline oxide films with potential applications in photocatalysis have been prepared and explored down to atomic length scales with regard to their stoichiometry, surface termination and defect landscape. In particular, the Cu2O(111), MnO(100), Mn3O4(001) and Fe3O4(111) surfaces have been in the focus of research, given their medium-sized band gaps that enable photo-stimulation with solar radiation. To establish realistic starting conditions for a photo-induced process, the different model systems were exposed to water and the water-oxide interactions were analyzed with a combination of experimental techniques, e.g. IRAS, TDS, XPS and STM, and theoretical approaches. The studies provided insight into the nature of water adsorption (dissociative versus associative), the atomic binding sites (regular versus defective) and typical adsorption energies at the different oxide surfaces. Moreover, several means have been tested to tailor the binding properties of molecules on the oxide films, for instance via doping with transition and rare-earth metal ions. In addition, the optical properties of the materials were optimized with respect to potential photo-applications. For example, optically active impurities and plasmonic nanoparticles have been embedded into MgO(001) and ZnO(0001) thin films, respectively, in order to increase the absorption cross section for light and to provide photo-generated carries in the hybrid systems. The DFG project was less successful when it came to the initialization and direct visualization of photoactivity on the model surfaces. Our attempts to monitor either desorption, dissociation or diffusion processes of molecules following a controlled photo-stimulation failed, because the applied light sources (conventional arc lamps and continuous lasers) were too weak and the thermal load to the sample was too high to exclude non-optical effects. A main outcome of this project was therefore an increased awareness that femtosecond lasers are inevitable for generating the precise optical stimuli required for photocatalytic studies. To circumvent this problem at least partly, a novel nanoscale light source has been developed in the project that is based on a conventional STM, in which the metallic tip is replaced with an optical fiber pumped by a laser diode. First measurements demonstrated the capabilities of the instrument, for instance to probe the absorption cross section of individual Ag-nanoparticles on a TiO2 support. The technique will be extended towards controlled desorption and dissociation experiments carried out on small molecular ensembles or even single molecules in the future. In summary, the combined Oldenburg-Berlin DFG project has provided a good foundation to prepare and explore model systems for photocatalytic studies at a mechanistic level. To realize the next step that is initiating and monitoring photo-induced processes at the atomic scale, further instrumental and scientific hurdles need to be taken, most of them being connected to better photon sources. The DFG project has nonetheless generated a countable scientific success.
Publications
- J. Phys. Chem. C 120 (2016) 13604-13609. Dopant-induced diffusion processes at metal-oxide interfaces studied for Fe-and Cr-doped MgO/ Mo(001) model systems
S. Benedetti, N. Nilius, S. Valeri, S. Tosoni, E. Albanese, G. Pacchioni
(See online at https://doi.org/10.1021/acs.jpcc.6b04182) - J. Phys. Chem. C 120 (2016) 21962–21966. Gold-Isophorone Interaction - Driven by Keto-Enol-Tautomerization
C. Stiehler, N.Nilius, J. Nevalaita, K. Honkala, H. Häkkinen H.-J. Freund, H. Häkkinen
(See online at https://doi.org/10.1021/acs.jpcc.6b06254) - Phys. Chem. Chem. Phys. 18 (2016) 6729 – 6733. Incorrect DFT-GGA predictions of the stability of non-stoichiometric/polar dielectric surfaces: The case of Cu2O(111)
N. Nilius, H. Fedderwitz, B. Groß, J. Goniakowski, C. Noguera
(See online at https://doi.org/10.1039/c5cp06933e) - Phys. Rev. Lett. 116 (2016) 236101. Diffusion barriers block defect occupation on reduced CeO2(111)
P. G. Lustemberg, Y. Pan, B.-J. Shaw, D. Grinter, Chi Pang, G. Thornton, R. Pérez, M. V. Ganduglia- Pirovano, N. Nilius
(See online at https://doi.org/10.1103/PhysRevLett.116.236101) - J. Phys. Chem. C 121 (2017) 20877–20881. Water Adsorption on Cu2O(111) Surfaces – An STM Study
Ch. Möller and N. Nilius
(See online at https://doi.org/10.1021/acs.jpcc.7b06996) - J. Phys. Chem. C 121 (2017) 4318–4323. Formation of magic isophorone islands on Au(111)
Y. Pan, Y. Cui, T. Meyer, N. Nilius
(See online at https://doi.org/10.1021/acs.jpcc.6b11781) - J. Phys. Chem. C 122 (2018) 2195–2199. Water Adsorption on Crystalline Cu2O Thin Films – Structural and Vibrational Properties
C. Möller, J. Barreto, F. Stavale, H. Tissot, S. Shaikhutdinov, H. J. Freund, and N. Nilius
(See online at https://doi.org/10.1021/acs.jpcc.7b10835) - Phys. Chem. Chem. Phys. 20 (2018) 5636 – 5643. Temperature-Dependent Phase Evolution of Copper-Oxide Thin-Films on Au(111)
C. Möller, H. Fedderwitz, C. Noguera, J. Goniakowski, N. Nilius
(See online at https://doi.org/10.1039/c7cp08387d) - J. Phys. Chem. C: Manganese Oxide Thin Films on Au(111): Growth Competition between MnO and Mn3O4
C. Möller, F. Stavale, N. Nilius
(See online at https://doi.org/10.1021/acs.jpcc.8b04176)
