Fundamentals of photocatalytic water splitting at spinel oxide interfaces
Subproject P11
Obtaining mechanistic insights into photocatalytic processes in their natural aqueous environment is challenging since it is difficult to probe just the interfacial molecules in the presence of the bulk. Moreover, typical reaction timescales are very short, making high time resolution necessary.
The long-term objective of P11 is to unravel the reaction mechanism of photocatalytic processes in real-time directly at the water-catalyst interface, where the reaction is taking place. In the first project period, we focus on photocatalytic water splitting at iron spinel oxide interfaces. The project will benefit from high-quality and well-characterized spinel samples prepared by P04 Parkinson. We will use vibrational spectroscopy to probe only the interfacial molecules. In this way, we study the strength of hydrogen bonds and the orientation of the water molecules, i.e., how water binds to various spinel oxides. Subsequently, photodissociation will be initiated by a laser pulse mimicking sunlight. We will follow the reaction on sub-picosecond timescales by probing the water molecules and potential reaction intermediates and products. The collaboration with theoretical projects helps with data interpretation and fundamental insights into the reaction (P12 Dellago). The water structure and reaction dynamics will be linked to reported efficiencies of the photocatalysts to connect molecular-level details to high catalytic efficiency (P10 Föttinger).
Expertise
Our expertise is ultrafast and nonlinear optical spectroscopy, predominantly in the infrared spectral region. We have two femtosecond amplifiers and several optical parametric amplifiers to convert the amplifier’s 800 nm output into other frequencies.
To study the solid-liquid interface, our focus in the TACO project, we will have two time-resolved sum-frequency generation setups, of which one is phase-resolved as well.
Team

Associates
Publications
2020

Backus, Ellen H. G.; Schaefer, Jan; Bonn, Mischa
Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy
Journal ArticleOpen AccessIn: Angewandte Chemie - International Edition, vol. 60, no. 19, pp. 10482–10501, 2020.
Abstract | Links | BibTeX | Tags: P11, pre-TACO
@article{Backus2020,
title = {Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy},
author = {Ellen H. G. Backus and Jan Schaefer and Mischa Bonn},
doi = {10.1002/anie.202003085},
year = {2020},
date = {2020-06-19},
urldate = {2020-06-19},
journal = {Angewandte Chemie - International Edition},
volume = {60},
number = {19},
pages = {10482--10501},
publisher = {Wiley},
abstract = {The interaction between minerals and water is manifold and complex: the mineral surface can be (de)protonated by water, thereby changing its charge; mineral ions dissolved into the aqueous phase screen the surface charges. Both factors affect the interaction with water. Intrinsically molecular-level processes and interactions govern macroscopic phenomena, such as flow-induced dissolution, wetting, and charging. This realization is increasingly prompting molecular-level studies of mineral–water interfaces. Here, we provide an overview of recent developments in surface-specific nonlinear spectroscopy techniques such as sum frequency and second harmonic generation (SFG/SHG), which can provide information about the molecular arrangement of the first few layers of water molecules at the mineral surface. The results illustrate the subtleties of both chemical and physical interactions between water and the mineral as well as the critical role of mineral dissolution and other ions in solution for determining those interactions.},
keywords = {P11, pre-TACO},
pubstate = {published},
tppubtype = {article}
}

Lesnicki, Dominika; Zhang, Zhen; Bonn, Mischa; Sulpizi, Marialore; Backus, Ellen H. G.
Journal ArticleOpen AccessIn: Angewandte Chemie - International Edition, vol. 59, no. 31, pp. 13116–13121, 2020.
Abstract | Links | BibTeX | Tags: P11, pre-TACO
@article{Lesnicki2020,
title = {Surface Charges at the CaF_{2}/Water Interface Allow Very Fast Intermolecular Vibrational-Energy Transfer},
author = {Dominika Lesnicki and Zhen Zhang and Mischa Bonn and Marialore Sulpizi and Ellen H. G. Backus},
doi = {10.1002/anie.202004686},
year = {2020},
date = {2020-04-02},
urldate = {2020-04-02},
journal = {Angewandte Chemie - International Edition},
volume = {59},
number = {31},
pages = {13116--13121},
publisher = {Wiley},
abstract = {We investigate the dynamics of water in contact with solid calcium fluoride, where at low pH, localized charges can develop upon fluorite dissolution. We use 2D surface-specific vibrational spectroscopy to quantify the heterogeneity of the interfacial water (D_{2}O) molecules and provide information about the sub-picosecond vibrational-energy-relaxation dynamics at the buried solid/liquid interface. We find that strongly H-bonded OD groups, with a vibrational frequency below 2500 cm^{−1}, display very rapid spectral diffusion and vibrational relaxation; for weakly H-bonded OD groups, above 2500 cm^{−1}, the dynamics slows down substantially. Atomistic simulations based on electronic-structure theory reveal the molecular origin of energy transport through the local H-bond network. We conclude that strongly oriented H-bonded water molecules in the adsorbed layer, whose orientation is pinned by the localized charge defects, can exchange vibrational energy very rapidly due to the strong collective dipole, compensating for a partially missing solvation shell.},
keywords = {P11, pre-TACO},
pubstate = {published},
tppubtype = {article}
}
2019

Schlegel, Simon J; Hosseinpour, Saman; Gebhard, Maximilian; Devi, Anjana; Bonn, Mischa; Backus, Ellen H. G.
How water flips at charged titanium dioxide: an SFG-study on the water–TiO2 interface
Journal ArticleOpen AccessIn: Physical Chemistry Chemical Physics, vol. 21, no. 17, pp. 8956–8964, 2019.
Abstract | Links | BibTeX | Tags: P11, pre-TACO
@article{Schlegel2019,
title = {How water flips at charged titanium dioxide: an SFG-study on the water–TiO_{2} interface},
author = {Simon J Schlegel and Saman Hosseinpour and Maximilian Gebhard and Anjana Devi and Mischa Bonn and Ellen H. G. Backus},
doi = {10.1039/c9cp01131e},
year = {2019},
date = {2019-04-05},
urldate = {2019-04-05},
journal = {Physical Chemistry Chemical Physics},
volume = {21},
number = {17},
pages = {8956--8964},
publisher = {Royal Society of Chemistry (RSC)},
abstract = {Photocatalytic splitting of water into hydrogen and oxygen by utilizing sunlight and a photocatalyst is a promising way of generating clean energy. Here, we report a molecular-level study on heavy water (D_{2}O) interacting with TiO_{2} as a model photocatalyst. We employed the surface specific technique Sum-Frequency-Generation (SFG) spectroscopy to determine the nature of the hydrogen bonding environment and the orientation of interfacial water molecules using their OD-stretch vibrations as reporters. By examining solutions with various pD-values, we observe an intensity-minimum at around pD 5, corresponding to the balance of protonation and deprotonation of TiO_{2} (point of zero charge). The majority of water molecules’ deuterium atoms point away from the interface when the pD is below 5, and point towards the surface when the pD is higher than 5, with strong hydrogen bonds towards the surface.},
keywords = {P11, pre-TACO},
pubstate = {published},
tppubtype = {article}
}