Surface Potential and Interfacial Water Order at the Amorphous TiO2 Nanoparticle/Aqueous Interface

Marie Bischoff, Arianna Marchioro, Sylvie Roke
AMER CHEMICAL SOC, 2020
Journal paper

Abstract

Colloidal nanoparticles exhibit unique size-dependent properties differing from their bulk counterpart, which can be particularly relevant for catalytic applications. To optimize surface-mediated chemical reactions, the understanding of the microscopic structure of the nanoparticle-liquid interface is of paramount importance. Here we use polarimetric angle-resolved second harmonic scattering (AR-SHS) to determine surface potential values as well as interfacial water orientation of similar to 100 nm diameter amorphous TiO2 nanoparticles dispersed in aqueous solutions, without any initial assumption on the distribution of interfacial charges. We find three regions of different behavior with increasing NaCl concentration. At very low ionic strengths (0-10 mu M), the Na+ ions are preferentially adsorbed at the TiO2 surface as innersphere complexes. At low ionic strengths (10-100 mu M), a distribution of counterions equivalent to a diffuse layer is observed, while at higher ionic strengths (>100 mu M), an additional layer of hydrated condensed ions is formed. We find a similar behavior for TiO2 nanoparticles in solutions of different basic pH. Compared to identically sized SiO2 nanoparticles, the TiO2 interface has a higher affinity for Na+ ions, which we further confirm with molecular dynamics simulations. With its ability to monitor ion adsorption at the surface with micromolar sensitivity and changes in the surface potential, AR-SHS is a powerful tool to investigate interfacial properties in a variety of catalytic and photocatalytic applications.

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Marie Bischoff, Arianna Marchioro, Sylvie Roke
AMER CHEMICAL SOC, 2020
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/278091?ln=en

About this result

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Photocatalytic applications play an essential role in the search for alternative energy sources and environmental decontamination techniques. It is of fundamental interest to understand on a molecular level the aqueous solid/liquid interface, where photocatalytic reactions occur, to improve the reaction efficiency.Surface charge, ion diffusion and adsorption are all elements that can have a significant impact on reaction rates and, in the case of nanoparticles, colloidal stability. However, not many experimental methods are able to probe the electrical double layer (EDL) and its peculiar properties, particularly when it comes to colloidal nanoparticles directly dispersed in solution. In this thesis we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to study the EDL of colloidal oxide particles in aqueous environments. The insulating oxide silica (SiO2) and the semiconducting metal oxide titanium dioxide (TiO2) are investigated and further compared. We show that from AR-SHS measurements, we can extract the surface potential of the particles with respect to bulk liquid and the surface susceptibility, as a measure of interfacial molecular orientation. This can be achieved without assuming a specific charge distribution in the EDL, and hence without employing a model such as the Gouy-Chapman (GC) or the Gouy-Chapman-Stern (GCS). Through electrophoretic mobility measurements, we additionally obtain the zeta potential. Knowing the surface potential, the surface susceptibility and the zeta potential enables us to establish a molecular-level picture of the oxide particle/liquid interface as a function of ionic strength and pH. First, we provide evidence that a compact layer of hydrated ions is formed at the 300 nm amorphous SiO2/aqueous interface at high pH and high NaCl concentrations (> 1mM). Computation of surface charge density values based on our measured surface potential values and using the GC and GCS models are in good agreement with literature values.Next, we study the EDL of 100 nm amorphous TiO2 particles as a function of NaCl concentration and of basic pH. Three regions, reflecting three different phenomena occurring as a function of ionic strength, can be identified. First, inner-sphere adsorption at the lowest concentrations, then formation of a diffuse layer of counterions and finally, accumulation of hydrated counterions near the interface. Similar regions are observed for 100 nm SiO2 particles as a function of NaCl concentration. The TiO2 surface is found to have a stronger affinity for Na+ ions than SiO2.We further evaluate ion-specific effects at the SiO2 and amorphous TiO2 colloidal aqueous interface. The addition of NaCl, RbCl or CaCl2 to the solution leads to relativedifferences in surface potential and in the evolution of the interfacial H-bonding network as a function of ionic strength, which reveal surface and cation- specific preferences for inner- and outer-sphere adsorption.Finally, we demonstrate that AR-SHS measurements as a function of pH can be used to determine the pKas of 100 nm anatase TiO2 particles. Our data reveals a correlation between the change in orientation of interfacial water molecules and the pKas of surface TiO2 groups. This work shows that AR-SHS is a powerful tool to uncover the EDL structure of colloidal oxide particles, thus contributing to a better understanding of interfacial processes important to a variety of (photo-)catalytic applications.

EPFL2022

Electrochemical promotion of Pt catalysts for gas phase reactions

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The subject of this work is the study of the electrochemical promotion of catalysis (EPOC), also called non-Faradaic electrochemical modification of catalytic activity (NEMCA). In this phenomenon, application of small currents or potentials on catalysts in contact with solid electrolytes leads to very pronounced strongly non-Faradaic and reversible changes in catalytic activity and selectivity. This work concerns the system composed of platinum film electrodes, deposited on the YSZ electrolyte (ZrO2 doped with 8% Y2O3), in contact with an oxygen containing atmosphere (the O2(g),Pt/YSZ system). The aim of this work was to focus on two points in the phenomenon of EPOC: 1) The nature of the species present on the surface of the catalyst and their impact on the catalytic activity. 2) The phenomenon of permanent promotion of catalysis (P-EPOC), for which the catalytic rate enhancement is not totally reversible, leading to a long lasting activated state. The Pt electrodes were prepared via three different methods (sputtering, thermal decomposition and screen-printing) and, as expected, the resulting microstructure, observed by scanning electron microscopy (SEM), was found to depend greatly on the preparation technique. The electrochemical response of each electrode was characterized by the technique of cyclic voltammetry. Due to the occurrence of different site-related electrochemical reactions, the voltammogram shape was highly influenced by the microstructure of the Pt/YSZ contact. Based on the preliminary studies, the cermet electrode was chosen for additional electrochemical experiments, using chronoamperometry, chronopotentiometry, cyclic voltammetry, steady state polarization and impedance spectroscopy. The results of all these techniques suggest that under anodic polarization, two parallel reactions take place. The first reaction is the oxygen evolution, giving rise to a steady state current under a constant applied potential. The other reaction is the oxygen storage (oxygen injection) in the form of Pt-O species. This second process gives rise to a time dependant current under a constant applied potential, and is therefore responsible for the pseudocapacitive behavior of the electrode. Moreover, linear sweep voltammetry measurements indicated that, by application of an anodic potential, at least three types of Pt-O species were stored, following distinct kinetics. Based on the amount of stored Pt-O and on the corresponding storage kinetics, they were attributed to three different locations on the electrode: 1) at the Pt/YSZ interface, 2) diffusing from the tpb toward the Pt/gas interface, 3) diffusing from the Pt/YSZ interface toward the bulk of the platinum electrode. It has been shown that the Pt film exhibited various activity states toward the catalytic oxidation of ethylene, depending on the oxygen species present at its surface. As evidenced by XPS measurements and in agreement with the thermodynamic data for the given system, the inactive (deactivated) state has been attributed to the formation of surface platinum oxide at T > 550°C. Decreasing the reaction temperature to 525°C caused the slow spontaneous reactivation of the catalyst and several days were necessary to reach the active state, where most of the catalyst was present in its metallic state. The catalyst activity recovery could be accelerated by the application of anodic potential pulses. In agreement with the mechanism of EPOC, this was explained by the oxide destabilizing effect of the anodically produced Oδ- species. Anodic polarization also caused reversible electrochemical promotion of catalysis at unusual high temperature (600°C). In addition, very intriguing catalytic relaxation transients were observed after prolonged anodic polarization times. Indeed, it was found that enhanced catalytic activity could be maintained for up to 10 hours after current interruption. Combination of these latter results, the related literature, the state of the art model of EPOC, and the results obtained by electrochemical techniques led to the proposition of an original model based on the processes of Pt-O storage/release at various locations of the O2(g),Pt/YSZ system. According to this model, anodic polarization produces Oδ- back-spillover species, promoting the catalytic activity, in agreement with the mechanism of EPOC. In parallel, hidden oxygen species are stored at the Pt/YSZ interface and diffuse toward the Pt phase. When the polarization is switched off, these hidden oxygen species diffuse to the gas-exposed surface and cause non-Faradaic promotion, as back-spillover Oδ- do. The large amount of stored charge and its slow diffusion controlled emergence causes the rate enhancement to last for hours.

EPFL2007

Membranes, composed of a variety of lipids and other biomolecules, mediate signaling processes between cells and their aqueous environment. To fulfill this function, membranes can vary their composition leaflet-specific and thus alter their surface properties. To fully understand the impact of these processes on the molecular level, it is necessary to develop tools that can access the molecular properties of free-floating model membranes label-free. These tools are ideally surface-specific. In this thesis, we apply the nonlinear optical techniques second harmonic scattering (SHS) and vibrational sum-frequency scattering (SFS) together with electrokinetic measurements to label-free characterize the interfacial properties, hydration structure and surface potentials of liposomes in aqueous solutions. First, we generalize the nonlinear optical theory to describe the second-order surface response from interfaces with aqueous solutions independent of the ionic strength for reflection, transmission and scattering geometries. We demonstrate that interference effects from oriented water molecules in the bulk aqueous solution alter the probing depth and the expected second-order response at low ionic strengths. Then, we apply this theory to demonstrate that SHS patterns of liposomes and oil droplets contain all necessary information to extract the absolute surface potential of the respective particles without assuming a model for the interfacial structure. By analyzing scattering patterns that capture the orientational distribution of water around the particles, we find surface potentials of -38 mV for bare oil-droplets and -11 mV for zwitterionic liposomes in water. For anionic liposomes the surface potential varies between -150 mV and -23 mV in solutions containing different amounts of NaCl ranging from

\sim

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EPFL2017