Browsing by Subject "X-ray Photoelectron Spectroscopy"

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Open Access
Investigation of electronic properties of ionic liquid electrochemical devices by X-ray photoelectron spectroscopy
(2016-12) Camcı, Merve
Attention towards electrochemical energy storage devices assembled with innovative solvent-free electrolytes ‘ionic liquids’ (ILs) has been progressively rising over the last two decades. In order to design a particular electrochemical device it becomes crucial to understand the structure of interfacial region and the electrical response of ILs. Accordingly, this thesis focuses on Xray Photoelectron Spectroscopic (XPS) investigations of electrochemical devices containing ILs, that is compatible with ultra high vacuum condition needed for XPS. Towards better understanding the fundamental aspects of certain electrochemical issues, electrochemical devices consisting of two metalelectrodes, which contains N,N – Diethyl -N- methyl -N- (2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide, (DEME-TFSI) IL-electrolyte between them, have been investigated by XPS under external electrical stimuli control, as a novel analytical tool for elucidating; (i) charging/ discharging phenomena, (ii) electrical double layer (EDL) formation and (iii) electrochemical reaction products. In the first part, a co-planar electrochemical device, with two gold electrodes on porous polyethylene membrane (PEM) plus DEME-TFSI impregnated between the electrodes, has been studied using external DC bias, for recording the position dependent electrical potential variations. In addition, AC bias is used to harvest temporal behavior. For the AC bias a square wave excitation is used, for which two frequencies are adopted corresponding to slow (10 mHz) and fast (1 kHz) time scales, for probing the response of the system at infinite- and zero-time onset, respectively. In all cases XP spectra have been recorded at different lateral positions. As a result of these DC and AC applications a new understanding has surfaced. Accordingly, although at the metal-electrolyte interface the EDL formation is limited to lateral dimensions at the nanometer scale, its visualization through the analysis of the XPS-probed voltage transients can be extended to very large distances from the interface, in the millimeters scale. These responses have also been modeled using a simple equivalent circuit with two oppositely polarized electrodes and an ionic conducting medium in between. In the second part, re-arrangement of the DEME-TFSI’s ionic constituents at the Au electrode/IL-electrolyte interface has been monitored by the dynamic-XPS approach under application of electrical pulses in the form of a slow (1 mHz) triangular wave with an amplitude of 5V, while recording the intensity fluctuations of the two N1s peaks corresponding to the anionic and the cationic fragments. In the last part, the externally bias XPS analysis has been used for insitu and in-vacuo monitoring of anodically triggered electrochemical preparation and characterization of Au NPs in both a co-planar and also in a wire-plane electrode electrochemical geometries. The small sized Au NPs’ formation within the DEME-TFSI medium has been confirmed by the characteristic peak around 470 nm in the Visible spectrum and with the spherical and well-dispersed (~4 nm) particles in TEM images.
Open Access
Sorption studies of cesium and barium on magnesite using radiotracer and x-ray photoelectron spectroscopy
(1997-06) Shahwan, Talal
As the consumption of the radioactive materials is continuously increasing, the problem of disposing the resulting v^^astes safely is becoming more challenging. One way through which these radioactive wastes could be isolated from the biological environment is by disposing them in deep geological formations. Clay minerals are proposed as backfill buffering materials that can delay the migration of the radionuclides and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. Up to now, several studies were carried out to examine the effect of such parameters on the sorption behavior of different radionuclides on various kinds of minerals. This study was conducted to investigate the effects of time, concentration and temperature on the sorption behavior of cesium and barium ions on magnesite. Cesium 137 140 and barium have the radioactive isotopes Cs (t^^^ = 30.1 y) and Ba (t^^^ = 12.8 d) 1/2 produced in high yields during the fission process which are important in radioactive waste considerations. Magnesite is a mineral composed mainly of magnesium carbonate 2+ together with minor amounts of quartz and has a single exchangeable cation. Mg . The radiotracer method and x-ray photoelectron spectroscopy, which is a powerful surface sensitive tool, were used in this study. The results obtained from both methods complemented each others and were in good agreement. Kinetic studies of the sorption process show that equilibrium was approached within one day of contact for both of cesium and barium ions. The data of the sorption of both cations using different concentrations at various temperatures were most adequately described by the Freundlich type isotherms which correspond to multilayer adsorption on heterogeneous surfaces. The values of the Freundlich constants k and n imply that barium ions have slightly larger adsorption affinity and adsorption intensity than cesium ions. The adsorption data at low concentrations were also observed to obey the Dubinin-Radushkevich type isotherms which describe monolayer adsorption on heterogeneous or homogeneous surfaces. The adsorption data were very poorly described by the Langmuir type isotherms. Thermodynamic parameters such as enthalpy change, AH°, entropy change, AS° and free energy change of adsorption, AG°, were calculated from the sorption data of cesium and barium ions at different temperatures. The values obtained for AH° and AS° were -37 kJ/moL, -0.09 kJ/moL.K and -13 kJ/moL, -0.009 kJ/moL.K for cesium and barium ions respectively. The negative AH° values indicate the exothermic nature of adsorption which means that low temperatures are favored. The decrease in entropy upon adsorption implied by the negative AS° values is indicative of the stability of adsorption for both cations. The values of AG° at different temperatures were all negative indicating the spontaneity of the adsorption process for both cesium and barium ions. The magnitudes of AG° were seen to be within the 8-16 kJ/moL range which is the energy range of ion- exchange type processes.
Open Access
X-Ray photoelectron spectroscopy for chemical and electrical characterization of devices extended to liquid/solid interfaces
(2018-12) Göktürk, Pınar Aydoğan
Understanding of electrical and electrochemical devices in operating conditions is vital for development of new technologies. Many important characteristics that determine the performance of such devices lie on their surfaces and interfaces which significantly deviate from the bulk properties. However, particularly for the liquid based devices, carrying out surface analysis is challenging and requires highly sophisticated instrumentation. In this PhD. thesis, we aim to unravel the potential development on liquids, dielectrics as well as the liquid/solid interfaces during AC and DC excitation in a chemically resolved fashion using the UHV compatible non-aqueous liquids in a basic electrowetting on dielectrics configuration within X-Ray Photoelectron Spectroscopy (XPS) chamber. Low molecular weight Polyethylene glycol (PEG) and a particular ionic liquid Diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI] are used to represent two extreme cases as being non-ionic and fully-ionic liquids. Application of external electrical bias to these devices either from the top or the bottom electrode during data acquisition enabled us to investigate the electrowetting phenomenon, in a chemically addressed fashion In the first part of the thesis, geometrical changes that the drop undergoes during electrowetting have been monitored both by steady state areal maps and by dynamic XPS point analysis where the potential was altered periodically. In the second part, we have focused only on the DC electrowetting of liquids. We probed the potential developments in the dielectric layer and on the liquid by monitoring the changes in the binding energy of the representative XPS peaks with respect to the applied potential. We showed that the conductivity of the liquid has no influence on the potential and the entire potential drop occurs at the liquid/dielectric interface. Dielectric breakdown and its effect on the potential developments were also investigated in this part. In the third part, we have tried to understand the frequency dependent potential developments of the ionic liquid and the polyethylene glycol based EWOD devices by AC electrowetting. Our time dependent XPS measurements under AC excitation with sweeping frequency have demonstrated that EWOD devices exhibit two different behaviors separated by a critical frequency, which is dependent on the AC resistance (impedance) or ionic content of the liquid and also the electrical characteristics of the dielectric layer. Below the critical frequency, XPS spectra are mainly affected by the capacitive component of the dielectric, hence the liquid completely screens the applied electrical field. However, for frequencies above the critical, the resistive component of the liquid dominates and the drop behaves like a resistive strip, resulting in the formation of equipotential surface contours which are shown experimentally for the first time in this study. In the last part of the thesis, an equivalent circuit model was developed to electrically describe the electrowetting behavior of PEG on dielectric and also to generate a solid-state mimicking device to produce the same XPS spectral observations.
Open Access
An XPS study of the interaction of model Ba/TiO2 AND Ba/ZrO2 NSR catalysts with NO2
(Pleiades Publishing, 2014) Smirnov, M. Y.; Kalinkin, A. V.; Nazimov, D. A.; Bukhtiyarov, V. I.; Vovk, E. I.; Ozensoy, E.
X-ray photoelectron spectroscopy is used to study the interaction of model NO2 storage-reduction catalysts (NSR catalysts) Ba/TiO2 and Ba/ZrO2 with NO2. The catalysts are prepared on the surface of ultrathin Al2O3 film substrates obtained by the FeCrAl alloy oxidation. It is shown that at room temperature the model catalysts react with NO2 with the successive formation of surface barium nitrite and nitrate. The NO2 reduction with the formation of barium nitrite at the initial step of the interaction is assumed to be accompanied by the oxidation of residual metallic barium and amorphous carbon impurity. It is found that the formation of barium nitrate proceeds more efficiently on Ba/ZrO2 rather than on Ba/TiO2.