TY - JOUR A1 - Yue, Yanhua A1 - Melani, Giacomo A1 - Kirsch, Harald A1 - Paarmann, Alexander A1 - Saalfrank, Peter A1 - Campen, Richard Kramer A1 - Tong, Yujin T1 - Structure and Reactivity of a-Al2O3(0001) Surfaces: How Do Al-I and Gibbsite-like Terminations Interconvert? JF - The journal of physical chemistry / publ. weekly by the American Chemical Society. C, Energy, materials, and catalysis N2 - The alpha-Al2O3(0001) surface has been extensively studied because of its significance in both fundamental research and application. Prior work suggests that in ultra-high-vacuum (UHV), in the absence of water, the so-called Al-I termination is thermodynamically favored, while in ambient, in contact with liquid water, a Gibbsite-like layer is created. While the view of the alpha- Al2O3(0001)/H2O(l) interface appears relatively clear in theory, experimental characterization of this system has resulted in estimates of surface acidity, i.e., isoelectric points, that differ by 4 pH units and surface structure that in some reports has non-hydrogen-bonded surface aluminol (Al-OH) groups and in others does not. In this study, we employed vibrational sum frequency spectroscopy (VSFS) and density functional theory (DFT) simulation to study the surface phonon modes of the differently terminated alpha-Al2O3(0001) surfaces in both UHV and ambient. We find that, on either water dosing of the Al-I in UHV or heat-induced dehydroxylation of the Gibbsite-like in ambient, the surfaces do not interconvert. This observation offers a new explanation for disagreements in prior work on the alpha-Al2O3(0001)/liquid water interface -different preparation methods may create surfaces that do not interconvert-and shows that the surface phonon spectral response offers a novel probe of interfacial hydrogen bonding structure. Y1 - 2022 U6 - https://doi.org/10.1021/acs.jpcc.2c03743 SN - 1932-7447 SN - 1932-7455 VL - 126 IS - 31 SP - 13467 EP - 13476 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Wirth, Jonas A1 - Saalfrank, Peter T1 - Vibrational spectroscopy of hydroxylated alpha-Al2O3(0001) surfaces with and without water BT - an ab initio molecular dynamics study JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Using gradient- and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated alpha-Al2O3(0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm(-1) as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5023347 SN - 0021-9606 SN - 1089-7690 VL - 149 IS - 1 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Saalfrank, Peter T1 - Vibrational energy relaxation of interfacial OH on a water-covered alpha-Al2O3(0001) surface BT - a non-equilibrium ab initio molecular dynamics study JF - Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies N2 - Vibrational relaxation of adsorbates is a sensitive tool to probe energy transfer at gas/solid and liquid/solid interfaces. The most direct way to study relaxation dynamics uses time-resolved spectroscopy. Here we report on a non-equilibrium ab initio molecular dynamics (NE-AIMD) methodology to model vibrational relaxation of OH vibrations on a hydroxylated, water-covered alpha-Al2O3(0001) surface. In our NE-AIMD approach, after exciting selected O-H bonds their coupling to surface phonons and to the water adlayer is analyzed in detail, by following both the energy flow in time, as well as the time-evolution of Vibrational Density of States (VDOS) curves. The latter are obtained from Time-dependent Correlation Functions (TCFs) and serve as prototypical, generic representatives of time-resolved vibrational spectra. As most important results, (i) we find a few-picosecond lifetime of the excited modes and (ii) identify both hydrogen-bonded aluminols and water molecules in the adsorbed water layer as main dissipative channels, while the direct coupling to Al2O3 surface phonons is of minor importance on the timescales of interest. Our NE-AIMD/TCF methodology is powerful for complex adsorbate systems, in principle even reacting ones, and opens a way towards time-resolved vibrational spectroscopy. Y1 - 2021 U6 - https://doi.org/10.1039/d0cp03777j SN - 1463-9076 SN - 1463-9084 VL - 23 IS - 13 SP - 7714 EP - 7723 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Melani, Giacomo A1 - Nagata, Yuki A1 - Campen, Richard Kramer A1 - Saalfrank, Peter T1 - Vibrational spectra of dissociatively adsorbed D2O on Al-terminated alpha-Al2O3(0001) surfaces from ab initio molecular dynamics JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - Water can adsorb molecularly or dissociatively onto different sites of metal oxide surfaces. These adsorption sites can be disentangled using surface-sensitive vibrational spectroscopy. Here, we model Vibrational Sum Frequency (VSF) spectra for various forms of dissociated, deuterated water on a reconstructed, Al-terminated α-Al2O3(0001) surface at submonolayer coverages (the so-called 1-2, 1-4, and 1-4′ modes). Using an efficient scheme based on velocity-velocity autocorrelation functions, we go beyond previous normal mode analyses by including anharmonicity, mode coupling, and thermal surface motion in the framework of ab initio molecular dynamics. In this way, we calculate vibrational density of states curves, infrared, and VSF spectra. Comparing computed VSF spectra with measured ones, we find that relative frequencies of resonances are in quite good agreement and linewidths are reasonably well represented, while VSF intensities coincide not well. We argue that intensities are sensitively affected by local interactions and thermal fluctuations, even at such low coverage, while absolute peak positions strongly depend on the choice of the electronic structure method and on the appropriate inclusion of anharmonicity. Y1 - 2019 U6 - https://doi.org/10.1063/1.5099895 SN - 0021-9606 SN - 1089-7690 VL - 150 IS - 24 PB - American Institute of Physics CY - Melville ER - TY - THES A1 - Melani, Giacomo T1 - From structural fluctuations to vibrational spectroscopy of adsorbates on surfaces T1 - Von Strukturfluktuationen bis zur Schwingungsspektroskopie von Adsorbaten auf Oberflächen BT - a theoretical study of H2O on α-Al2O3(0001) BT - eine theoretische Untersuchung von H2O auf α-Al2O3 (0001) N2 - Aluminum oxide is an Earth-abundant geological material, and its interaction with water is of crucial importance for geochemical and environmental processes. Some aluminum oxide surfaces are also known to be useful in heterogeneous catalysis, while the surface chemistry of aqueous oxide interfaces determines the corrosion, growth and dissolution of such materials. In this doctoral work, we looked mainly at the (0001) surface of α-Al 2 O 3 and its reactivity towards water. In particular, a great focus of this work is dedicated to simulate and address the vibrational spectra of water adsorbed on the α-alumina(0001) surface in various conditions and at different coverages. In fact, the main source of comparison and inspiration for this work comes from the collaboration with the “Interfacial Molecular Spectroscopy” group led by Dr. R. Kramer Campen at the Fritz-Haber Institute of the MPG in Berlin. The expertise of our project partners in surface-sensitive Vibrational Sum Frequency (VSF) generation spectroscopy was crucial to develop and adapt specific simulation schemes used in this work. Methodologically, the main approach employed in this thesis is Ab Initio Molecular Dynamics (AIMD) based on periodic Density Functional Theory (DFT) using the PBE functional with D2 dispersion correction. The analysis of vibrational frequencies from both a static and a dynamic, finite-temperature perspective offers the ability to investigate the water / aluminum oxide interface in close connection to experiment. The first project presented in this work considers the characterization of dissociatively adsorbed deuterated water on the Al-terminated (0001) surface. This particular structure is known from both experiment and theory to be the thermodynamically most stable surface termination of α-alumina in Ultra-High Vacuum (UHV) conditions. Based on experiments performed by our colleagues at FHI, different adsorption sites and products have been proposed and identified for D 2 O. While previous theoretical investigations only looked at vibrational frequencies of dissociated OD groups by staticNormal Modes Analysis (NMA), we rather employed a more sophisticated approach to directly assess vibrational spectra (like IR and VSF) at finite temperature from AIMD. In this work, we have employed a recent implementation which makes use of velocity-velocity autocorrelation functions to simulate such spectral responses of O-H(D) bonds. This approach allows for an efficient and qualitatively accurate estimation of Vibrational Densities of States (VDOS) as well as IR and VSF spectra, which are then tested against experimental spectra from our collaborators. In order to extend previous work on unimolecularly dissociated water on α-Al 2 O 3 , we then considered a different system, namely, a fully hydroxylated (0001) surface, which results from the reconstruction of the UHV-stable Al-terminated surface at high water contents. This model is then further extended by considering a hydroxylated surface with additional water molecules, forming a two-dimensional layer which serves as a potential template to simulate an aqueous interface in environmental conditions. Again, employing finite-temperature AIMD trajectories at the PBE+D2 level, we investigated the behaviour of both hydroxylated surface (HS) and the water-covered structure derived from it (known as HS+2ML). A full range of spectra, from VDOS to IR and VSF, is then calculated using the same methodology, as described above. This is the main focus of the second project, reported in Chapter 5. In this case, comparison between theoretical spectra and experimental data is definitely good. In particular, we underline the nature of high-frequency resonances observed above 3700 cm −1 in VSF experiments to be associated with surface OH-groups, known as “aluminols” which are a key fingerprint of the fully hydroxylated surface. In the third and last project, which is presented in Chapter 6, the extension of VSF spectroscopy experiments to the time-resolved regime offered us the opportunity to investigate vibrational energy relaxation at the α-alumina / water interface. Specifically, using again DFT-based AIMD simulations, we simulated vibrational lifetimes for surface aluminols as experimentally detected via pump-probe VSF. We considered the water-covered HS model as a potential candidate to address this problem. The vibrational (IR) excitation and subsequent relaxation is performed by means of a non-equilibrium molecular dynamics scheme. In such a scheme, we specifically looked at the O-H stretching mode of surface aluminols. Afterwards, the analysis of non-equilibrium trajectories allows for an estimation of relaxation times in the order of 2-4 ps which are in overall agreement with measured ones. The aim of this work has been to provide, within a consistent theoretical framework, a better understanding of vibrational spectroscopy and dynamics for water on the α-alumina(0001) surface,ranging from very low water coverage (similar to the UHV case) up to medium-high coverages, resembling the hydroxylated oxide in environmental moist conditions. N2 - Das Hauptziel dieser Doktorarbeit war die Untersuchung der Wechselwirkung zwischen Aluminiumoxid (α-Al2O3) und Wasser, wobei die (0001)-Oberfläche von α-Al2O3 im Fokus der Betrachtungen stand. Infolge der Entwicklung moderner oberflächensensitiver spektroskopischer Methoden, insbesondere in der Schwingungsspektroskopie, ist es inzwischen experimentell möglich, die Oberflächenchemie dieser Oxidmaterialien zu untersuchen und atomistische Informationen über ihre wässrigen Grenzflächen zu erhalten. Unsere Gruppe hat mit der Interfacial Molecular Spectroscopy Gruppe des Fritz-Haber-Instituts der MPG in Berlin zusammengearbeitet, deren Expertise in der oberflächensensitiven Vibrational Sum Frequency (VSF) Generation liegt. Diese Technik gestattet den spezifischen Nachweis von Adsorbatvibrationen, beispielsweise des Produkts der Wasserdissoziation und -adsorption. Zur theoretischen Untersuchung dieses Systems wurden verschiedene Berechnungsmethoden genutzt, wobei bevorzugt die Ab-initio-Molekulardynamik (AIMD) zum Einsatz kam. Darüber hinaus konzentrierten wir uns auf die Simulation von Schwingungsfrequenzen und -spektren für verschiedene Systeme unter Verwendung eines neuen, effizienten Algorithmus, der auf Grundlage klassischer Zeitkorrelationsfunktionen entwickelt wurde. Das erste Projekt untersuchte die einfachsten Adsorptionsprodukte von Wasserreaktionen auf der Al-terminierten α-Al2O3(0001)-Oberfläche, die unter Ultrahochvakuumbedingungen (UHV) als stabilste Struktur bekannt ist. Mit AIMD-basierten Methoden haben wir verschiedene Spezies simuliert, die aus unimolekular adsorbiertem Wasser stammen, und ihre Schwingungsfrequenzen im Vergleich zum Experiment bestimmt. In einem zweiten Projekt widmeten wir uns einer Situation, in der eine vollständig hydroxylierte α-Aluminiumoxid-Oberfläche mit einer ebenfalls vollständig hydroxylierten, aber um eine zusätzliche Wasserschicht erweiterten α-Aluminiumoxid-Oberfläche wechselwirkt. Diese beiden Strukturen dienen als Grundlage zur weitergehenden Untersuchung von Aluminiumoxid-Oberflächen mit größeren Wasser-Bedeckungsgraden unter Umgebungsbedingungen. Wiederum simulierten wir vollständige Schwingunsspektren (einschließlich IR und VSF). Hierzu wurden etablierte Methoden genutzt, die auf der Verwendung von Zeitkorrelationsfunktionen basieren, jedoch von uns im Rahmen dieser Doktorarbeit weiterentwickelt wurden. Schließlich haben wir in einem dritten Projekt in Zusammenarbeit mit Experimentatoren die Schwingungsrelaxation an der Wasser / α-Al2O3-Grenzfläche beschrieben. Motiviert durch Pump-Probe-VSF-Experimente zur Bestimmung der Schwingungslebensdauer adsorbierter OH-Spezies entwickelten und verwendeten wir einen AIMD-Ansatz zur Simulation der Anregungs- und Relaxationsprozesse. Auf Grundlage von Nichtgleichgewichtstrajektorien konnten wir auch eine Relaxationszeitskala für OH-Gruppen ermitteln, die zwischen 2 und 4 Pikosekunden beträgt und gut mit experimentellen Werten übereinstimmt. KW - theoretical chemistry KW - molecular dynamics KW - DFT KW - vibrational spectroscopy KW - theoretische Chemie KW - Molekulardynamik KW - DFT KW - Schwingungsspektroskopie Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-441826 ER - TY - JOUR A1 - Bouakline, Foudhil A1 - Lorenz, Ulrich J. A1 - Melani, Giacomo A1 - Paramonov, Guennaddi K. A1 - Saalfrank, Peter T1 - Isotopic effects in vibrational relaxation dynamics of H on a Si(100) surface JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - In a recent paper [U. Lorenz and P. Saalfrank, Chem. Phys. 482, 69 (2017)], we proposed a robust scheme to set up a system-bath model Hamiltonian, describing the coupling of adsorbate vibrations (system) to surface phonons (bath), from first principles. The method is based on an embedded cluster approach, using orthogonal coordinates for system and bath modes, and an anharmonic phononic expansion of the system-bath interaction up to second order. In this contribution, we use this model Hamiltonian to calculate vibrational relaxation rates of H–Si and D–Si bending modes, coupled to a fully H(D)-covered Si(100)-(2×1) surface, at zero temperature. The D–Si bending mode has an anharmonic frequency lying inside the bath frequency spectrum, whereas the H–Si bending mode frequency is outside the bath Debye band. Therefore, in the present calculations, we only take into account one-phonon system-bath couplings for the D–Si system and both one- and two-phonon interaction terms in the case of H–Si. The computation of vibrational lifetimes is performed with two different approaches, namely, Fermi’s golden rule, and a generalized Bixon-Jortner model built in a restricted vibrational space of the adsorbate-surface zeroth-order Hamiltonian. For D–Si, the Bixon-Jortner Hamiltonian can be solved by exact diagonalization, serving as a benchmark, whereas for H–Si, an iterative scheme based on the recursive residue generation method is applied, with excellent convergence properties. We found that the lifetimes obtained with perturbation theory, albeit having almost the same order of magnitude—a few hundred fs for D–Si and a couple of ps for H–Si—, are strongly dependent on the discretized numerical representation of the bath spectral density. On the other hand, the Bixon-Jortner model is free of such numerical deficiencies, therefore providing better estimates of vibrational relaxation rates, at a very low computational cost. The results obtained with this model clearly show a net exponential decay of the time-dependent survival probability for the H–Si initial vibrational state, allowing an easy extraction of the bending mode “lifetime.” This is in contrast with the D–Si system, whose survival probability exhibits a non-monotonic decay, making it difficult to define such a lifetime. This different behavior of the vibrational decay is rationalized in terms of the power spectrum of the adsorbate-surface system. In the case of D–Si, it consists of several, non-uniformly distributed peaks around the bending mode frequency, whereas the H–Si spectrum exhibits a single Lorentzian lineshape, whose width corresponds to the calculated lifetime. The present work gives some insight into mechanisms of vibration-phonon coupling at surfaces. It also serves as a benchmark for multidimensional system-bath quantum dynamics, for comparison with approximate schemes such as reduced, open-system density matrix theory (where the bath is traced out and a Liouville-von Neumann equation is solved) or approximate wavefunction methods to solve the combined system-bath Schrödinger equation. Y1 - 2017 U6 - https://doi.org/10.1063/1.4994635 SN - 0021-9606 SN - 1089-7690 VL - 147 IS - 14 PB - American Institute of Physics CY - Melville ER -