Session S25: Focus Session Chemical Physics Frontiers at Interfaces II

Frontiers at Interfaces

Interfacial acid-base equilibrium of carboxyl groups tethered to gold nanoparticles is a key factor that stabilizes gold nanoparticles in aqueous solution. In this study we used Second Harmonic (SH) generation to measure interfacial potential and interfacial pH, and thereby obtained a surface pKa value of 3.3±0.1 for the carboxyl group at a gold nanoparticle/aqueous interface. This pKa value is smaller than its bulk counterpart in aqueous solution, which suggests that the charged carboxyl group is favored at the gold nanoparticle surface. The SH findings that the pKa of the carboxyl group at the gold nanoparticle/water interface is more acidic than carboxyls free in bulk solution, is consistent with the effects of the noble metal (gold) surface on a charge in solution, as predicted by the method of images.   

Spectro-microscopic investigations of ion reconfiguration in ionic liquids

Ionic liquids are being used in range of applications, such as in batteries, catalysts, and transistors. This creates a need for better understanding of their dynamics and the nature of the interactions with solid interfaces, particularly under operating conditions. In this work, we present results of an in situ spectro-microscopic investigations of a few monolayers of 1 ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (EMMIM TFSI) deposited on an a surface with electrodes patterned on it. We show that long-range and correlated ionic reconfigurations occur near the electrodes when the bias is applied to them. These processes are temperature- and thickness-dependent, which in turn is related to ionic mobility and different configurations for out-of-plane ion ordering near the electrodes.   

Molecular-Scale Investigation of Heavy Metal Ions at a Charged Langmuir Monolayer

Solvent extraction -- the surfactant-aided preferential transfer of a species from an aqueous to an organic phase -- is an important technique used in heavy and precious metal refining and reprocessing [1]. Solvent extraction requires transfer through an oil/water interface, and interfacial interactions are expected to control transfer kinetics and phase stability, yet these key interactions are poorly understood [2]. Langmuir monolayers with charged headgroups atop concentrated salt solutions containing heavy metal ions act as a model of solvent extraction interfaces; studies of ions at a charged surface are also fundamentally important to many other phenomena including protein solvation, mineral surface chemistry, and electrochemistry. We probe these charged interfaces using a variety of surface-sensitive techniques -- vibrational sum frequency generation (VSFG) spectroscopy, x-ray reflectivity (XRR), x-ray fluorescence near total reflection (XFNTR), and grazing incidence diffraction (GID). We integrate experiments with Molecular Dynamics (MD) simulations to uncover the molecular-level interfacial structure. [1] P. Tasker et al., Comp. Coord. Chem. II, 2003, 759. [2] W. Rock et al., Langmuir, 2016, 32 (44), 11532.   

Interaction of ions with rough solid-liquid interfaces with dielectric contrast.

Ions dissolved in liquids have electrostatic interactions with the solid boundaries of the medium. Typically, these solid boundaries have dielectric properties different from those of the liquid. For a flat boundary, the electrostatic energy of interaction between a single ion and the solid-liquid interface is obtained using the classical method of images. It is of interest to extend this result to cases where roughness is present at the interface. This roughness can be modeled as a sinusoidal wave. This presentation will address the calculation of the ion-interface interaction energy using perturbation theory on the roughness amplitude. The main result is that the energy can be written in closed analytical form as a function of the distance between ion and surface, the roughness wavelength and the dielectric properties of the medium.   

Validating First Principles Molecular Dynamics Calculations of Solid/Liquid Interfaces with X-ray Reflectivity

The interface between a metal oxide electrode and liquid water plays a crucial role in energy conversion processes such as photo-electrochemical water splitting and CO$_{\mathrm{2}}$~reduction to create fuel from sunlight.~ First principles electronic structure calculations are useful tools to obtain electronic properties relevant to catalysis, such as electron energy alignment between the electrode surface and a reacting molecule, but~their ability to predict realistic structural models requires careful validation. We combine calculations of interfacial X-ray structure factors [1] from~ab-initio molecular dynamics trajectories with experimental X-ray reflectivity measurements [2] to obtain a validation protocol~for structural models of aqueous interfaces. Simultaneously, we provide a means to interpret X-ray reflectivity measurements without the need for~model-dependent empirical fitting, offering unique insight into the influence of atomic-scale structure upon reaction mechanisms at surfaces.~We will discuss in particular the case of the water/Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ (001) interface [3]. [1] M. Plaza et al, \textit{JACS }\textbf{138}, 4 (2016) [2] J. G. Catalano, \textit{Geochimica et Cosmochimica Acta} \textbf{75}, 10 (2011) [3] P. Huang et al, \textit{J Phys. Chem. C} \textbf{118}, 8 (2014).   

Charge Transfer across Quantum Dot-Oxide Interfaces for High-Efficiency Photovoltaics

Metal oxides constitute robust and relatively cheap semiconductor materials that are finding increasing applications in opto-electronics, but their band gaps are typically prohibitively wide for the generation of free charges through the absorption of visible light. Several approaches have been developed to circumvent this drawback. Specifically, the sensitization of mesoporous oxides by semiconductor quantum dot (QD) nanocrystals represents a promising route for the development of low-cost photovoltaics in QD sensitized solar cells. In addition to their tuneable band gap, QDs have the ability to generate multiple charge carriers from single photons by a process called carrier multiplication (CM), which potentially provides a means towards high-efficiency photovoltaics. Although CM has been widely interrogated in colloidal QDs in solution, the collection of those multiple charge carriers at oxide electrodes has not been clearly elucidated. The contribution of CM towards the overall device performance is ultimately determined by a competition between transfer to the electrode material and charge recombination within the QDs. We report interfacial electron transfer dynamics from quantum dots grown directly onto mesoporous oxide films. Such systems are well-suited for achieving efficient multiple charge transfer by CM, as electron transfer from QD-to-oxide is substantially faster than charge recombination. However, despite CM occurring in the QD, only one electron is transferred to the oxide. This seemingly counterintuitive result can be understood by noting that efficient hot electron transfer at the QD-oxide interface can compete with CM within the QDs. Hot electron transfer is observed to occur on sub-100 fs timescales, nulling the CM efficiency. Implications of these results for solar energy conversion are discussed.   

Novel applications of X-ray photoelectron spectroscopy on unsupported nanoparticles

X-ray photoelectron spectroscopy (XPS) is a powerful technique for chemical analysis of surfaces. We will present novel results of XPS on unsupported, gas-phase nanoparticles using a velocity-map imaging (VMI) spectrometer. This technique allows for probes of both the surfaces of nanoparticles via XPS as well as their interiors via near edge X-ray absorption fine structure (NEXAFS) spectroscopy. A recent application of this technique has confirmed that arginine's guanidinium group exists in a protonated state even in strongly basic solution. Moreover, the core-level photoelectron spectroscopy can provide information on the effective attenuation length (EAL) of low kinetic energy electrons. This contradictory value is important for determining the probing depth of XPS and in photolithography. A new method for determining EALs will be presented.   

Water Adsorption on the Gypsum (010) Surface: a theoretical and experimental study

In this work we present a combined experimental/theoretical study which provides a molecular-level understanding of the ubiquitous and important gypsum/water interface. We investigate the structural and dynamic properties of adsorbed water on (010) gypsum single crystal surface, at room temperature, combining Sum Frequency Generation (SFG) Spectroscopy experiments and first principles simulations. Gypsum is a layered crystal with molecular planes linked through weak hydrogen bonding, allowing the perfect cleavage of (010) faces. The SFG spectra of gypsum in low relative humidity (RH) 0.1\% under N$_{2}$ atmosphere showed anisotropic arrangement of H$_2$O structural molecules and the presence of dangling OH groups. When studying higher RH the adsorbed water also revealed an anisotropic arrangement although different than the one present in the structural water and with the free OH signal significantly suppressed. The theoretical results corroborates the experimental ones and allows to identify the structure of the water molecules at the interface.   

Water adsorption on Sr$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$Ru$_{2}$O$_{7}$

Perovskite oxides are promising materials for a wide range of applications as diverse as sensors, fuel cells and catalysts. Surprisingly an atomic scale knowledge of their surface chemistry is still rather poor. Here we present a combined DFT, low-temperature STM and XPS study of the first monolayer H$_{2}$O formation on the (001) surfaces of two Ruddelsden-Popper type compounds, Sr$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$Ru$_{2}$O$_{7}$. Both cleave nicely, yielding flat surfaces of rocksalt-like SrO and CaO type. Adsorbed H$_{2}$O monomers dissociate on both surfaces by transferring a proton to an apical O while the remaining OH resides at a nearby Sr-Sr or Ca-Ca bridge, respectively. The different tilting and rotations of the RuO$_{6}$ octahedra in the Sr and Ca perovskites cause different behaviour of the dissociated fragments: for Sr$_{3}$Ru$_{2}$O$_{7}$ a lone Sr-Sr bridge OH circles the apical OH with an activation energy of $\approx$187meV. At higher coverages dimers, followed by chains and cages are formed[1]. In contrast, the dissociated monomer fragments are immobile on Ca$_{3}$Ru$_{2}$O$_{7}(001)$ and only chains along the [010] direction occur. [1]D. Halwidl et. al., Nature Materials, 15(4), 450-455.   

Water dissociate on faceted NiO(111)

The interaction of water with metal oxide surface is important in heterogeneous catalysis, geochemistry, electrochemistry and corrosion science. It has been reported that water molecules physically absorb on NiO(100) surface while they dissociate as OH on NiO(111) surface. As a typical polar surface, NiO(111) is instable, and the polar compensation may lead to surface reconstruction, segregation of oxygen vacancies, or formation of facets. Faceted NiO(111) surface, consisting of Ni(100) or NiO(110) facets, introduces new surface states and provides complex chemical environment for water adsorption and dissociation. We prepare faceted NiO(111) films with different thickness and observe water dissociation on the surface. The dissociated OH can be detected at high temperature up to 700K on the thin film surface, while on the surface of relative thick film, the OH amount is significantly reduced and the thermo stability lowered. The morphology characterization of the film shows that the size and number density of the facets are thickness dependent -- the thinner the films is, the smaller the facets are, and the higher the density of facets is. We conclude that the boundary sites on NiO(111) facilitate the dissociative adsorption of water. This work is supported by Chinese NSFC (11474334 {\&} 11274237).   

First Principles Study of Energetics, Local Electronic States and Adsorption of H2O and H2O2 on Reduced CeO2 Surfaces

The importance of ceria (CeO2) in catalysis originates from its remarkable redox and oxygen storage capability. It undergo repeatable Ce4$+$/Ce3$+$ redox cycles depending on the operation conditions. The great effort has been made to improve ceria reducibility. The reduction of ceria can be controlled by the oxygen vacancies. We study the energetics, local electronic states, and oxygen vacancy formation energies for the (111), (110) and (100) surfaces of stoichiometric and reduced ceria by DFT$+$U calculations. We find that ceria (111) surface is most stable, while (100) and (110) surfaces have higher formation energies. Both subsurface and surface oxygen vacancies induce the electron localization of reduced CeO2 on all these three terminations of the surface, leading to the appearance of Ce3$+$ sites. In the case of ceria (111) surface, oxygen vacancy at the surface and subsurface forms Ce3$+$ at next nearest neighbor to the vacancy. This is consistent with the experimental finding that the ratios of these two types of oxygen vacancy are almost same. In the case of ceria (100) and (110) surfaces, Ce3$+$ is formed at the sites nearest to the oxygen vacancy sites. The H2O and H2O2 adsorption and dissociation on three reduced surfaces are also investigated. Ce3$+$ generated by oxygen vacancy can promote the adsorption of H2O and H2O2.