Cation Segregation Research Articles

Surface Segregation and Inter-Diffusion of Cations and Impurities in Microelectrodes for Solid Oxide Fuel Cells and Electrolyzers

The surface segregation and inter-diffusion of cations and impurities in La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) micro-patterned electrode structures on a single crystal Zr0.84Y0.16O1.92(YSZ) electrolyte was investigated by high-resolution ion beam analysis (Time-of Flight Secondary Ion Mass Spectrometry and Low-Energy Ion Scattering). The outermost and near surface of these microelectrodes showed significant changes in the distribution of the cations and impurities after micro-patterning, annealing and electrochemical testing of the cells. As occurring for bulk LSCF ceramics, strong segregation of Sr towards the electrode surface is observed during annealing of the microelectrode, as well as due to the thermal treatments required for electrochemical measurements. This segregation might have a drastic effect on the surface oxygen exchange reaction as the electrochemically active transition metals (Fe, Co) are not directly exposed to the gas phase. Impurities, such as Si and Ca, are also observed at the annealed electrolyte surface. Furthermore, the microstructured electrode showed that impurities can also migrate between the different components (i.e. Na and Si impurities migrate from the electrolyte to the LSCF electrode, showing a “cleaner” surface after the thermal treatments). This study has provided a further understanding of the migration and segregation of cations and impurities taking place in real devices, building upon previous studies performed on ceramics materials. Figure 1

Surface Segregation in Solid Oxide Electrode Materials Occurring at Intermediate Temperatures

The exchange of oxygen between a solid oxide electrode and the gas phase is key to the operation of electrochemical energy conversion devices such as Solid Oxide Fuel Cells(SOFCs) and Electrolysers (SOECs), as well as for related applications such as oxygen separation membranes. Whilst the mechanisms of bulk transport in these materials are, for the most part, well understood, a precise description of the surface exchange process in atomistic terms is still unavailable. To understand the mechanisms of oxygen surface exchange in these materials, and its influence on device performance and degradation in the long term, we must know the composition of the oxide surface after heat treatments representative of the processing and operation conditions relevant to these devices. To this end, our group has recently been applying Low Energy Ion Scattering (LEIS), which is capable of analyzing the very outer atomic layer of a surface, to the study of these materials [1,2,3]. This contribution summarizes some of our recent studies of the evolution of surface and near surface composition in typical solid oxide electrode materials, namely La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) and GdBaCo2O5.5+d(GBCO). After sintering, the surfaces of both materials are covered by the preferentially segregated (divalent) alkaline earth cation (Sr2+ or Ba2+respectively), with the transition metal, often assumed to catalytically active site for the oxygen incorporation reaction, apparently absent from the surface [3]. Sputter depth profiling analyses of the sintered samples reveal a sub-surface region enriched in the transition metal cation, apparently associated with the segregation of the larger rare earth / lanthanide cations to the surface. Furthermore, we show that these surface composition changes do not only occur at the high temperatures used for device processing, or time scales comparable to device lifetimes. In fact, significant changes in surface composition are seen at even modest temperatures and relatively short times. For example, an enrichment in Sr and a decrease in the concentration of transition metal cations exposed at the surface has been in seen in LSCF annealed at 400 °C for 8 hours [1] (fig. (a)). In a later set of experiments (fig. (b)) on GBCO focusing on shorter timescales, segregation of the Ba2+cation was evident after only 15 minutes at 400 °C [2]! These results underline the very strong tendencies for cation segregation in these materials. As measurable changes in surface composition occur even at these low temperatures, it is clear that for any electrode material that has been exposed to elevated temperatures, whether during processing or during operation, the surface composition will not be a simple termination of the “ideal” bulk, and that cation segregation processes will play a role in determining the surface kinetics.

(Invited) High Temperature CO2 Electrolysis on La(Sr)Fe(Mn)O3 Oxide Cathode by Using LaGaO3 Based Electrolyte

Solid oxide electrolysis cells (SOECs) for converting CO2 to more useful chemical species are attracting considerable attention because of their high electrolytic efficiency, offering the possibility of direct conversion of CO2 to CO (CO2 → CO + 1/2O2). Heat energy supplied to SOECs can maximize electrolysis efficiency, offering both thermodynamic and kinetic advantages. The generated CO can be used as a fuel gas, converted into syngas via reaction with H2, or to reduce iron oxide to pure iron in the iron-making process. The application of SOECs and related technologies can contribute to a more sustainable economy. For the electrolyte in SOECs, ZrO2 doped with 8 mol% of Y2O3 (YSZ) has always been used because of its chemical stability and mechanical strength. However, an electrolyte for CO2 electrolysis with lower internal resistance and increased efficiency is highly desirable. LaGaO3-based electrolytes have higher oxide-ion conductivities and transport numbers, which can lead to higher energy conversion efficiency; hence, a doped-LaGaO3 electrolyte (La0.9Sr0.1Ga0.8Mg0.2O3, LSGM) has been tested in this study. For CO2 electrolysis, another drawbacks is formation of C since electrolysis potential is close to that of CO formation. Although metals, particularly Ni, are widely used as cathodes in CO2 electrolysis cells, we found that a LaFeO3−δ-based perovskite cathode (La0.6Sr0.4Fe0.8Mn0.2O3−δ, LSFM6482) showed the highest activity for CO2 electrolysis among the examined oxides with a current density for CO2 electrolysis of 0.52 A/cm2 at 1.6 V and 1173 K. Among the cells using a La0.6Sr0.4Fe0.9M0.1O3−δ-based oxide cathode (M = Mn, Co, Ni, or Cu) for CO2 electrolysis at 1073K. Mn was the most effective Fe replacement on the B site, and the La0.6Sr0.4Fe0.9Mn0.1O3−δ cathode showed the highest activity and current density for CO2 electrolysis of this set of cathodes. Among the tested dopants, Mn and Co have ionic sizes most similar to Fe and a smaller size mismatch between the host and dopant cations in the perovskite structure, which should suppress dopant segregation and minimize detrimental effects of cation segregation. Overall, among the examined cations, Mn was the best dopant for the B site of the LaFeO3 cathode. A cell consisting of BLC64/LSGM/LSFM6482 exhibited the highest CO2 electrolysis activity (a current density of 0.52 A/cm2 at 1.6 V and 1173 K) of all cathodes investigated in this study and reduced CO2 at a rate of 153 μmol/cm2•min at 1173 K and 1.6 V with negligible carbon formation. The observed electrolysis current density was close to that of a Ni cathode but smaller than that of a Ni–Fe cathode, which is one of the most active cathodes for CO2 electrolysis. An LSFM6482 oxide cathode seems to be effective for improving not only initial cathodic performance but also long-term stability. LSFM6482 could be a potential candidate for high-temperature CO2 electrolysis. Further details of electrode reaction will be discussed based on impedance analysis and CO2 partial pressure.

Surface Segregation and Inter-Diffusion of Cations and Impurities in Microelectrodes for Solid Oxide Fuel Cells and Electrolyzers

The surface segregation and inter-diffusion of cations and impurities in La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) electrode structures micro-patterned on a Zr0.84Y0.16O1.92 (YSZ) single crystal was investigated by high-resolution Time-of-Flight Secondary Ion Mass Spectrometry and Low-Energy Ion Scattering. The outermost and near surface of the microelectrodes showed significant changes after patterning and electrochemical testing of the cell. As occurring in bulk LSCF ceramics, Sr segregation towards the surface is observed after annealing at 500°C. Furthermore, the electrode showed that impurities (i.e. Na, Si and Ca) originating from the YSZ electrolyte bulk migrate actively to the LSCF electrode during electrochemical testing. The impurity migration has implications for oxygen surface exchange at the cathode-electrolyte and air interface for solid oxide fuel cells: the electrolyte YSZ surface becomes cleaner whereas the segregation of Sr to the LSCF outer surface seems to be suppressed with respect to the operation time of the device.

Examination of the mechanism for the reversible aging behavior at open circuit when changing the operating temperature of (La0.8Sr0.2)0.95MnO3 electrodes

The aging behavior of symmetrical cells, consisting of either (La0.8Sr0.2)0.95MnO3 (LSM) or La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) electrodes screen printed on either 8mol% yttria-stabilized zirconia (YSZ) or Ce0.8Gd0.2O2 (GDC) electrolyte substrates, is reported as the symmetrical cell is thermally cycled between 700°C and 800°C. For LSM, between 700°C and 850°C, the polarization resistance exhibits slow increases or decreases with time (on the order of days) after a quick change in temperature. When increasing the temperature, the polarization resistance decreases with time, and when decreasing the temperature, the polarization resistance slowly increases with time. In a previous work, the authors had explained these results with LSM by connecting the testing conditions to literature reports of surface analysis of LSM thin films which demonstrated a change in the amount of surface cation segregation as a function of temperature. In this work, TEM/EDS/XPS analysis of dense LSM pellets thermally cycled under the same conditions as the symmetrical cells does not indicate any significant reversible change in the surface composition of the LSM pellet between 700°C and 800°C. An alternative hypothesis is proposed to explain the relationship between polarization resistance and the LSM cation/anion vacancy concentrations controlled by the Schottky reaction. The timescale of aging behavior is related to the time necessary for the cations to move to or from the LSM surface to adjust to the new equilibrium at each temperature. The relevance in understanding the mechanism behind the aging behavior is emphasized with respect to fuel cell sample/stack modeling as well as to proper testing procedures for reaching reliable conclusions when comparing different electrode samples.

Cationic surface segregation in doped LaMnO3

The surface cation chemistry in (La, A)MnO3 (A = Ca, Sr and Ba) is investigated using first-principles thermodynamics. We find that, all three dopants tend to segregate to the surface over a wide range of T–\( p_{{{\text{O}}_{2} }} \) conditions and the tendency for segregation increases with the increase in the dopant cationic size. Moving toward the low oxygen pressure, dopants prefer to remain in the surface regions accompanied by the appropriate number of charge compensating oxygen vacancies. The situation when dopants remain in the bulk regions tends to occur close to the thermodynamic conditions that also favor the decomposition of LaMnO3. The present work serves as an important step toward understanding of factors governing the cationic surface segregation in doped LaMnO3 and opens a pathway to study other important chemical environments (such as water- and CO2-containing air) which are crucially given the fact that the ‘real-world’ air enhances cationic segregation.

Relating surface chemistry and oxygen surface exchange in LnBaCo2O(5+δ) air electrodes.

The surface and near-surface chemical composition of electroceramic materials often shows significant deviations from that of the bulk. In particular, layered materials, such as cation-ordered LnBaCo2O(5+δ) perovskites (Ln = lanthanide), undergo surface and sub-surface restructuring due to the segregation of the divalent alkaline-earth cation. These processes can take place during synthesis and processing steps (e.g. deposition, sintering or annealing), as well as at temperatures relevant for the operation of these materials as air electrodes in solid oxide fuel cells and electrolysers. Furthermore, the surface segregation in these double perovskites shows fast kinetics, starting at temperatures as low as 400 °C over short periods of time and leading to a decrease in the transition metal surface coverage exposed to the gas phase. In this work, we use a combination of stable isotope tracer labeling and surface-sensitive ion beam techniques to study the oxygen transport properties and their relationship with the surface chemistry in ordered LnBaCo2O(5+δ) perovskites. Time-of-Flight Secondary-Ion Mass Spectrometry (ToF-SIMS) combined with (18)O isotope exchange was used to determine the oxygen tracer diffusion (D*) and surface exchange (k*) coefficients. Furthermore, Low Energy Ion Scattering (LEIS) was used for the analysis of the surface and near surface chemistry as it provides information from the first mono-atomic layer of the materials. In this way, we could relate the compositional modifications (e.g. cation segregation) taking place at the electrochemically-active surface during the exchange at high temperatures and the oxygen transport properties in double perovskite electrode materials to further our understanding of the mechanism of the surface exchange process.

Improved electrochemical stability at the surface of La(0.8)Sr(0.2)CoO3 achieved by surface chemical modification.

The degradation of the surface chemistry on perovskite (ABO3) oxides is a critical issue for their performance in energy conversion systems such as solid oxide fuel/electrolysis cells and in splitting of H2O and CO2 to produce fuels. This degradation is typically in the form of segregation and phase separation of dopant cations from the A-site, driven by elastic and electrostatic energy minimization and kinetic demixing. In this study, deposition of Ti at the surface was found to hinder the dopant segregation and the corresponding electrochemical degradation on a promising SOFC cathode material, La(0.8)Sr(0.2)CoO3 (LSC). The surface of the LSC films was modified by Ti (denoted as LSC-T) deposited from a TiCl4 solution. The LSC and LSC-T thin films were investigated by electrochemical impedance spectroscopy, nano-probe Auger electron spectroscopy, and X-ray photoelectron spectroscopy (XPS), upon annealing at 420-530 °C in air up to about 90 hours. The oxygen exchange coefficient, k(q), on LSC-T cathodes was found to be up to 8 times higher than that on LSC cathodes at 530 °C and retained its stability. Sr-rich insulating particles formed at the surface of the annealed LSC and LSC-T films, but with significantly less coverage of such particles on the LSC-T. From this result, it appears that modification of the LSC surface with Ti reduces the segregation of the blocking Sr-rich particles at the surface, and a larger area on LSC surface (with a higher Sr doping level in the lattice) is available for the oxygen reduction reaction. The stabilization of the LSC surface through Ti-deposition can open a new route for designing surface modifications on perovskite oxide electrodes for high temperature electro- and thermo-chemical applications.

A variational approach to surface cation segregation in mixed conducting perovskites

A variational, phase-field methodology describing the segregation of dopant cations to the free surfaces of mixed conducting perovskite oxides is introduced. Based on the Cahn–Hilliard formalism for solute segregation in alloys and the Cahn theory of wetting of a solution at a solid surface, the model qualitatively predicts the experimentally observed behavior of solute segregation in oxides, which shows segregation layers on the order of tens of nanometers. Critical in this description are the interactions among defects and the gradient energy – two related concepts that can be expected to become influential at relatively high dopant concentrations. The analysis predicts that a first-order phase transition – a surface-mediated spinodal – may occur depending on the strength of dopant interactions and the affinity of dopant cations and oxygen vacancies for the free surface.

Surface segregation and poisoning in materials for low-temperature SOFCs

Abstract

Periodic cation segregation in Cs0.44[Nb2.54W2.46O14] quantified by high-resolution scanning transmission electron microscopy.

The atomic structure of Cs0.44[Nb2.54W2.46O14] closely resembles the structure of the most active catalyst for the synthesis of acrylic acid, the M1 phase of Mo10V2(4+)Nb2TeO42-x. Consistently with observations made for the latter compound, the high-angle electron scattering signal recorded by scanning transmission electron microscopy shows a significant intensity variation, which repeats periodically with the projected crystallographic unit cell. The occupation factors for the individual mixed Nb/W atomic columns are extracted from the observed intensity variations. For this purpose, experimental images and simulated images are compared on an identical intensity scale, which enables a quantification of the cation distribution. According to our analysis specific sites possess low tungsten concentrations of 25%, whereas other sites have tungsten concentrations above 70%. These findings allow us to refine the existing structure model of the target compound, which has until now described a uniform distribution of the niobium and tungsten atoms in the unit cell, showing that the similarity between Cs0.44[Nb2.54W2.46O14] and the related catalytic compounds also extends to the level of the cation segregation.

Invited Presentation: Effect of Elastic Strain and Dislocations on Oxide Ion Diffusion and Oxygen Exchange Kinetics on Oxides

How can the oxygen non-stoichiometry, oxygen diffusion and oxygen reduction kinetics be altered in functional oxides by lattice strain? This question has recently attracted wide interest for enabling a new route for attaining fast oxygen exchange and transport kinetics, and obtaining fundamental answers to this question is important for a range of technologies including high performance solid oxide fuel cells, oxide based sensors, separation membranes and memristors. By applying mechanical stress, the inherent energy landscape of the reactions involved on the material can be altered. For example, in theory one can turn an endothermic reaction into an exothermic one, and reduce the energy barriers by applying lattice strain. This is the so-called mechano-chemical coupling, and when applied on electrochemical systems we can introduce the term mechano-electro-chemistry.In this talk, we will discuss recent work that probes quantitatively the mechanisms by which elastic strain impacts surface reactivity and diffusion kinetics in fluorite and perovskite oxides. Computationally, using ab initio and atomistic simulations, we discovered that lattice strain can accelerate the ionic transport in 8%Y2O3-ZrO2 by reducing the ion migration energy barriers. Combining surface chemical characterization on model thin films with ab initio modeling, we demonstrated for the first time that tensile lattice strain favors oxygen exchange kinetics on both the oxygen-deficient (La,Sr)CoO3-d and (La,Sr)MnO3-d and the oxygen-excess Nd2NiO4+d by impacting the energies of oxygen adsorption and dissociation, oxygen vacancy formation, oxygen diffusion, cation segregation and the electron transfer process. The favorable impact of tensile lattice strain on these elementary reactions was validated at the collective level by quantifying the oxygen surface exchange and diffusion kinetics with isotope exchange measurements on (La,Sr)CoO3-d. Lastly, we will discuss the impact of dislocations in ionic crystals, as doped CeO2 and ZrO2 on the distribution of charged defects and mobility of oxygen vacancies. These results together put forth both elastic strain and the dislocations to be important factors that impressively changes the kinetics of charge transport and reactivity in functional oxides.

Swelling due to the partition of soluble fission products between the grey phase and uranium dioxide

The change in volume associated with the partition of soluble cations from uranium dioxide into the (Ba,Sr)ZrO3 grey phase has been investigated using atomic scale simulations. Here past work on the thermodynamic drive for the segregation of trivalent and tetravalent cations from uranium dioxide is built upon in the context of fuel swelling. Only small tetravalent cations segregate into the grey phase and this is predicted to result in an overall reduction in fuel volume. Individual trivalent cations that segregate, can cause either a contraction or an expansion of the overall fuel volume. Cr2O3 doped UO2 promotes co-partition forming mixed cation clusters in the grey phase and causing an overall reduction in fuel volume for all trivalent cations. This may have implications for fuel performance and may alter other fuel swelling mechanisms.

Surface Chemistry and Non-Stoichiometry of Nd2NiO4+ δ Epitaxial Thin Films with Different Orientation and Strain

The influence of lattice strain on non-stoichiometry and surface chemical composition was investigated for epitaxial Nd2NiO4+ ä (NNO) films during annealing in ultra high vacuum (below 10-8 mbar) and temperatures of up to 700oC. (100)- and (001)-oriented films with tensile and compressive lattice strain along c-axis were fabricated using pulsed laser deposition method. A significant decrease in the c-lattice parameter during annealing was found by x-ray diffraction (XRD) for the tensile strained films. X-ray photoelectron spectroscopy (XPS) showed that Ni reduction during annealing takes place only in compressively strained films, indicating the lower content of oxygen interstitials. A lower interstitial content in the compressively strained NNO films is consistent with the smaller c-lattice parameter measured by XRD and the easier reducibility of Ni measured by XPS. Cation segregation and morphological changes were found only for the compressively strained film surfaces. These results show that lattice strain along the c-axis is an important parameter that can alter the surface chemistry, and thus the oxygen exchange kinetics, on Nd2NiO4+ ä at elevated temperatures.

Factors that Influence Cation Segregation at the Surfaces of Perovskite Oxides

As the oxygen reduction reaction (ORR) becomes more critical for development of solid oxide fuel cells (SOFCs) that operate at 500-700 °C, the correlation between the surface chemistry and electrochemical performance is important to understand and enable design of cathode materials with optimal surface chemistry. Recently we demonstrated that elastic and electrostatic interactions of the dopant with the host lattice drive dopant segregation, a detrimental process on the surface of perovskite cathodes [1]. Motivated by those results, here we investigated the effects of A-site stoichiometry in La0.8Sr0.2MnO3 (LSM) thin films on the surface chemistry and electrochemical activity. Angle-resolved X-ray photoelectron spectroscopy was employed to identify the surface cation content and chemical bonding states. A-site deficient LSM films showed higher chemical stability against Sr segregation and secondary phase formation upon annealing. This was correlated with a higher electrochemical activity measured by AC impedance spectroscopy. Given the insulating nature of secondary phases created on the surface upon annealing, observed higher electrochemical stability in A-site deficient LSM films can be ascribed to the suppressed surface segregation and phase separation.

Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites

Cation segregation on perovskite oxide surfaces affects vastly the oxygen reduction activity and stability of solid oxide fuel cell (SOFC) cathodes. A unified theory that explains the physical origins of this phenomenon is therefore needed for designing cathode materials with optimal surface chemistry. We quantitatively assessed the elastic and electrostatic interactions of the dopant with the surrounding lattice as the key driving forces for segregation on model perovskite compounds, LnMnO3 (host cation Ln = La, Sm). Our approach combines surface chemical analysis with X-ray photoelectron and Auger electron spectroscopy on model dense thin films and computational analysis with density functional theory (DFT) calculations and analytical models. Elastic energy differences were systematically induced in the system by varying the radius of the selected dopants (Ca, Sr, Ba) with respect to the host cations (La, Sm) while retaining the same charge state. Electrostatic energy differences were introduced by varying the distribution of charged oxygen and cation vacancies in our models. Varying the oxygen chemical potential in our experiments induced changes in both the elastic energy and electrostatic interactions. Our results quantitatively demonstrate that the mechanism of dopant segregation on perovskite oxides includes both the elastic and electrostatic energy contributions. A smaller size mismatch between the host and dopant cations and a chemically expanded lattice were found to reduce the segregation level of the dopant and to enable more stable cathode surfaces. Ca-doped LaMnO3 was found to have the most stable surface composition with the least cation segregation among the compositions surveyed. The diffusion kinetics of the larger dopants, Ba and Sr, was found to be slower and can kinetically trap the segregation at reduced temperatures despite the larger elastic energy driving force. Lastly, scanning probe image contrast showed that the surface chemical heterogeneities made of dopant oxides upon segregation were electronically insulating. The consistency between the results obtained from experiments, DFT calculations, and analytical theory in this work provides a predictive capability to tailor the cathode surface compositions for high-performance SOFCs.

Atomic resolution mapping of interfacial intermixing and segregation in InAs/GaSb superlattices: A correlative study

We combine quantitative analyses of Z-contrast images with composition analyses employing atom probe tomography (APT) correlatively to provide a quantitative measurement of atomic scale interfacial intermixing in an InAs/GaSb superlattice (SL). Contributions from GaSb and InAs in the Z-contrast images are separated using an improved image processing technique. Correlation with high resolution APT composition analyses permits an examination of interfacial segregation of both cations and anions and their incorporation in the short period InAs/GaSb SL. Results revealed short, intermediate, and long-range intermixing of In, Ga, and Sb during molecular beam epitaxial growth and their distribution in the SL.

Cation size effects in mixed-ion metaphosphate glasses: structural characterization by multinuclear solid state NMR spectroscopy

Metaphosphate glasses with two monovalent species A(1-x)B(x)PO3 (0 ≤x≤ 1) show mixed-ion effects (MIE) in the dc conductivities and glass transition temperatures, which are strongly dependent on the cation size mismatch between the two mobile species. In the present contribution, mixed-ion metaphosphate glasses based on the cation combinations Cs-Li, Rb-Li, and Cs-Ag, exhibiting particularly large size mismatches, are analyzed by (31)P, (87)Rb, (109)Ag and (133)Cs NMR to determine possible correlations between this mismatch and some of the structural properties critical to the development of the MIE: the local environments around the mobile species and their spatial distribution relative to each other. The results are compared with those obtained in the Na-Ag metaphosphate series, which serves as a reference system, with minimized cation mismatch MIE. The local coordination environments of the Ag(+), Rb(+) and Cs(+) ions follow analogous compositional trends as previously observed in Na-based mixed-ion metaphosphate glasses: for a given cation species A, the average A-O distance shows an expansion/compression when this cation is replaced by a second species B with smaller/bigger ionic radius, respectively. This compositional differentiation of the structural sites for the mobile species may contribute to the MIE. Concerning the relative spatial distribution of the mobile ions, results from (7)Li-(133)Cs (SEDOR) experiments indicate a random mixture of Cs and Li in Cs-Li metaphosphate glasses. While this result is in agreement with one of the fundamental hypotheses of the models proposed to describe the MIE, it is at variance with the observation of various partial cation segregation phenomena observed in Na-based mixed alkali glasses. This result suggests that cation size mismatch is not the decisive parameter in determining segregation or non-statistical mixing of cations in the glass. In the Cs-Ag and Na-Ag glasses, (109)Ag spin-echo NMR reveals a progressive slowing down of the Ag(+) diffusion dynamics as this species is replaced by Cs(+) or Na(+). The substitution by the bigger Cs(+) ion causes a strong reduction in Ag(+) mobility suggesting the existence of separated diffusion pathways for these cations. In contrast, substitution by the similarly-sized Na(+) causes a much weaker mobility reduction consistent with the existence of Ag-Na cooperative hopping.

Characterisation, phase stability and surface chemical properties of photocatalytic active Zr and Y co-doped anatase TiO2 nanoparticles

We report on the characterization, phase stability, surface chemical and photocatalytic properties of Zr and Y co-doped anatase TiO2 nanoparticles prepared by homogenous hydrolysis methods using urea as precipitating agent. The materials were analyzed by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, BET isotherm and BJH pore size distribution measurements. It is shown that Y and Zr ions replace Ti ions in the anatase TiO2 structures up to a critical total dopant concentration of approximately 13wt%. The co-doped particles show increased phase stability compared to pure anatase TiO2 nanoparticles. The anatase to rutile phase transformation is shown to be preceded by cation segregation and dissolution with concomitant precipitation of Y2Ti2−xZrxO7 and ZrTiO4. Co-doping modifies the optical absorption edge with a resulting attenuation of the Urbach tail. The band gap is slightly blue-shifted at high doping concentrations, and red shifted at lower doping concentrations. Formic acid adsorption was used as a probe molecule to investigate surface chemical properties and adsorbate structures. It was found that the relative abundance of monodentate formate compared to bidentate coordinated formate decreases with increasing doping concentration. This is attributed to an increased surface acidity with increasing dopant concentration. Photodegradation of formic acid occurred on all samples. With mode-resolved in situ FTIR spectroscopy it is shown that the rate of photodegradation of monodentate formate species are higher than for bidentate formate species. Thus our results show that the trend of decreasing photo-degradation rate with increasing dopant concentration can be explained by the adsorbate structure, which is controlled by the acidity of the surface.

Improved Dehydrogenation Properties of Ti-Doped LiAlH4: Role of Ti Precursors

The dehydrogenation properties of LiAlH4 doped with different Ti precursors (Ti, TiO2, and TiCl3) via ball milling are investigated. The results not only show significant decreases in the decomposition temperatures (Tdec) and activation energies (EA) of the first two dehydrogenation reaction steps of LiAlH4 by doping with TiO2 or TiCl3, but also reveal how each Ti precursor affects the dehydrogenation process. Although doping LiAlH4 with TiCl3 induced the largest decrease in Tdec and EA, TiO2-doped LiAlH4 produced a decrease in Tdec and EA that is quite close to the TiCl3-doped sample as well as superior short-term stability, suggesting that doping with TiO2 has certain advantages over doping with TiCl3. Further, the underlying mechanisms associated with the Ti precursors during the dehydrogenation reaction of LiAlH4 have been studied using quasi in situ X-ray photoelectron spectroscopy. The results reveal that the Ti4+ and Ti3+ reduction processes and the segregation of Li cations to the surface of LiAlH4 during ball milling play critical roles in the improved dehydrogenation properties observed.