Fundamentals of the structure of
liquid/oxide interfaces

@article{Zelenka_2025a,

title = {MgO–water interface: structure and surface dissolution depend on flow and pH},

author = {Moritz Zelenka and Ellen H. G. Backus},

doi = {10.1039/D5CP03295D},

year  = {2025},

date = {2025-10-10},

journal = {Physical Chemistry Chemical Physics},

abstract = {Magnesium oxide (MgO) is frequently in contact with water throughout numerous research and industrial applications and in nature. Remarkably, we found that there is a substantial influence on the interfacial structure and dissolution process whether water is flowing or static at the MgO(100) surface. Sum frequency generation spectroscopy revealed that flowing acidic solutions enhance the charging of the MgO surface, which leads to an increased net orientation of water close to the surface. Contrary, the MgO surface resembles a near neutrally charged state when in contact with static liquid for all tested solutions between pH 3 and pH 11. We explain this surprising observation with the dissolution of MgO in aqueous solutions, which effectively removes charge from the interfacial region. The continuous solution exchange due to flowing liquid shifts the equilibrium towards a more charged state in comparison to static liquid. Additionally, by investigating the transition from flowing to static liquid we found a reaction order of around 0.5 for the dissolution reaction with respect to the H^{+} concentration. Furthermore, the significant effect of the MgO surface dissolution on the interfacial structure points out that other solid–liquid interfaces with similar or higher solubility might exhibit similar properties.},

keywords = {P11},

pubstate = {published},

tppubtype = {article}

}

@article{Romano_2024b,

title = {Structure of the water/magnetite interface from sum frequency generation experiments and neural network based molecular dynamics simulations},

author = {Salvatore Romano and Harsharan Kaur and Moritz Zelenka and Pablo Montero De Hijes and Moritz Eder and Gareth S. Parkinson and Ellen H. G. Backus and Christoph Dellago},

url = {https://arxiv.org/abs/2410.12717},

year  = {2024},

date = {2024-10-16},

urldate = {2024-10-16},

journal = {arXiv},

abstract = {Magnetite, a naturally abundant mineral, frequently interacts with water in both natural settings and various technical applications, making the study of its surface chemistry highly relevant. In this work, we investigate the hydrogen bonding dynamics and the presence of hydroxyl species at the magnetite-water interface using a combination of neural network potential-based molecular dynamics simulations and sum frequency generation vibrational spectroscopy. Our simulations, which involved large water systems, allowed us to identify distinct interfacial species, such as dissociated hydrogen and hydroxide ions formed by water dissociation. Notably, water molecules near the interface exhibited a preference for dipole orientation towards the surface, with bulk-like water behavior only re-emerging beyond 60 Å from the surface. The vibrational spectroscopy results aligned well with the simulations, confirming the presence of a hydrogen bond network in the surface ad-layers. The analysis revealed that surface-adsorbed hydroxyl groups orient their hydrogen atoms towards the water bulk. In contrast, hydrogen-bonded water molecules align with their hydrogen atoms pointing towards the magnetite surface.},

keywords = {P04, P11, P12},

pubstate = {published},

tppubtype = {article}

}

@article{Backus2024,

title = {Ultrafast Surface‐Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water‐Splitting Photocatalyst},

author = {Ellen H. G. Backus and Saman Hosseinpour and Charusheela Ramanan and Shumei Sun and Simon J. Schlegel and Moritz Zelenka and Xiaoyu Jia and Maximilian Gebhard and Anjana Devi and Hai I. Wang and Mischa Bonn},

doi = {10.1002/anie.202312123},

issn = {1521-3773},

year  = {2023},

date = {2023-11-27},

urldate = {2023-11-27},

journal = {Angewandte Chemie - International Edition},

number = {e202312123},

publisher = {Wiley},

abstract = {A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO_{2} in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H_{2}O into Ti−OH groups and the deprotonation of pre-existing Ti−OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts.},

keywords = {P11},

pubstate = {published},

tppubtype = {article}

}

@article{Buessler2023,

title = {Unravelling the interfacial water structure at the photocatalyst strontium titanate by sum frequency generation spectroscopy},

author = {Martin Buessler and Shingo Maruyama and Moritz Zelenka and Hiroshi Onishi and Ellen H. G. Backus},

doi = {https://doi.org/10.1039/D3CP03829G},

year  = {2023},

date = {2023-10-31},

urldate = {2023-10-31},

journal = {Physical Chemistry Chemical Physics},

volume = {25},

pages = {31471--31480},

abstract = {The direct conversion of solar energy to hydrogen is considered as a possible method to produce carbon neutral hydrogen fuel. The mechanism of photocatalytic water splitting involves the chemical breakdown of water and re-assembly into hydrogen and oxygen at the interface of a photocatalyst. The selection rules of a suitable material are well established, but the fundamental understanding of the mechanisms, occurring at the interface between the catalyst and the water, remains missing. Using surface specific sum frequency generation spectroscopy, we present here characterisation of the interface between water and the photocatalyst strontium titanate (SrTiO_{3}). We monitor the OH-stretching vibrations present at the interface. Their variations of intensities and frequencies as functions of isotopic dilution, pH and salt concentration provide information about the nature of the hydrogen bonding environment. We observe the presence of water molecules that flip their orientation at pH 5 indicating the point of zero charge of the SrTiO_{3} layer. These water molecules are oriented with their hydrogen away from the surface when the pH of the solutions is below 5 and pointing towards the surface when the pH is higher than 5. Besides, water molecules donating a H-bond to probably surface TiOH groups are observed at all pH.},

keywords = {P11},

pubstate = {published},

tppubtype = {article}

}