Publications
2023
Backus, Ellen H. G.; Hosseinpour, Saman; Ramanan, Charusheela; Sun, Shumei; Schlegel, Simon J.; Zelenka, Moritz; Jia, Xiaoyu; Gebhard, Maximilian; Devi, Anjana; Wang, Hai I.; Bonn, Mischa
Journal ArticleOpen AccessIn: Angewandte Chemie - International Edition, no. e202312123, 2023, ISSN: 1521-3773.
Abstract | Links | BibTeX | Tags: P11
@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}
}
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 TiO2 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 H2O 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.
Buessler, Martin; Maruyama, Shingo; Zelenka, Moritz; Onishi, Hiroshi; Backus, Ellen H. G.
Journal ArticleOpen AccessIn: Physical Chemistry Chemical Physics, vol. 25, pp. 31471–31480, 2023.
Abstract | Links | BibTeX | Tags: P11
@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}
}
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 (SrTiO3). 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 SrTiO3 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.
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}
}
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.
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}
}
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 (D2O) 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.
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}
}
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 (D2O) interacting with TiO2 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 TiO2 (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.