In situ high-resolution X-ray photoelectron spectroscopy – Fundamental insights in surface reactions

Since the advent of third generation synchrotron light sources optimized for providing soft X-rays up to 2 keV, X-ray photoelectron spectroscopy (XPS) has been developed to be an outstanding tool to study surface properties and surface reactions at an unprecedented level. The high resolution allows identifying various surface species, and for small molecules even the vibrational fine structure can be resolved in the XP spectra. The high photon flux reduces the required measuring time per spectrum to the domain of a few seconds or even less, which enables to follow surface processes in situ. Moreover, it also provides access to very small coverages down to below 0.1% of a monolayer, enabling the investigation of minority species or processes at defect sites. The photon energy can be adjusted according to the requirement of a particular experiment, i.e., to maximize or minimize the surface sensitivity or the photoionization cross-section of the substrate or the adsorbate. For a few instruments worldwide, a next step forward was taken by combining in situ high-resolution spectrometers with supersonic molecular beams. These beams allow to control and vary the kinetic and internal energies of the incident molecules and provide a local pressure of up to ~10^?5 mbar, which can be switched on and off in a controllable way, thus offering a well-defined time structure to study adsorption or reaction processes.Herein, we will review some specific scientific aspects which can be addressed by in situ XPS in order to demonstrate the power and potential of the method: In particular, the following topics will be addressed: (1) The sensitivity of the binding energy to adsorption sites will be analyzed, using CO on metals as example. From measurements at different temperatures, the binding energy difference between different sites can be derived, and exchange processes between different adsorbate species at step edges can be followed. (2) The vibrational fine structure of adsorbed small hydrocarbon species on metal surfaces will be analyzed in detail. We will first introduce the linear coupling model, then discuss the properties of adsorbed methyl and of a number of other small hydrocarbons, and show that the vibrational signature can be used as fingerprint for identifying surface species. (3) It is demonstrated that the binding energy of equivalent atoms in a molecule can be differentially changed by adsorption to a substrate; this sensitivity to the local environment will be discussed for adsorbed ethylene, benzene and graphene. (4) By temperature programmed XPS, the thermal evolution of adsorbed species can be followed in great detail, allowing for the identification of reaction intermediates and the determination of their stabilities. (5) The investigation of reaction kinetics by isothermal XPS measurements will be discussed; here results for the oxidation of sulfur and of CO will be presented and the corresponding activation energies of the rate limiting steps will be determined.     

Non-destructive and in situ analysis of Egyptian wall paintings by X-ray diffraction and X-ray fluorescence portable systems

The concurrence and complementarities of obtained images under various wavelengths and the elemental and structural analyses provided by XRF and XRD, using portable non-invasive systems, have allowed for obtaining accurate data about the employed pictorial technique in two Egyptian wall paintings dating from the New Kingdom. Thus, compounds such as Egyptian blue, Egyptian green, goethite, jarosite, hematite, calcite, anhydrite or huntite have been detected in the paintings. The performance of the measurements by the different techniques and its contribution to the knowledge of the materials are discussed. They notably give a clue on the origin of arsenic compounds, unexpectedly detected in some decors.     

Dynamics of iron in Fe-porphyrin aggregates studied by X-ray absorption and Mössbauer spectroscopy

The iron-porphyrin aggregates were studied by optical absorption and fluorescence method, infrared spectroscopy, X-ray absorption and Mössbauer spectroscopy. The aggregation of porphyrin molecules strengthens the Fe-ligands bonds and accelerates the spin-spin relaxations. A significant speeding-up of relaxation was observed with lowering the temperature down to 25 K. The comparison of the EXAFS (Extended X-ray Absorption Fine Structure) and Mössbauer spectroscopy results enabled some separation of the individual Fe vibration from its collective movement with ligands.     

Mapping of the metal intake in plants by large-field X-ray microradiography and preliminary feasibility studies in microtomography

This paper reports on dual energy micro-radiography and tomography techniques applied both to thin plant leaves treated with copper or lead solutions and on Cu-treated small roots and stem sections, performed at the SYRMEP X-ray beamline of ELETTRA synchrotron facility in Trieste (Italy). The features of the source allowed us to apply different imaging techniques with an extremely vast field of view, up to 160 ×6 mm2 and 28×6 mm2 for micro-radiography and tomography experiments, respectively. The feasibility of getting positive indications on metal accumulation in leaves, sections of roots and stems, stem and root whole cylindrical pieces has been checked.     

In situ observation of cell-detachment process initiated by femtosecond laser-induced stress wave

When a stress wave generated by focusing a femtosecond laser is loaded on an animal cell adhered on a substrate, the cell is detached from the substrate. There are two possible mechanisms for the cell detachment: (a) The cell is detached from a scaffold coated on a glass plate, and (b) the cell is detached from the glass plate with the scaffold. In this work, we have studied the cell-detachment mechanism by visualizing the scaffold with a fluorescence probe of quantum dots. When the cell was detached from the substrate, fluorescence from the scaffold simultaneously disappeared from the glass plate, although the scaffold was not irradiated by the laser. This indicates that detachment due to the stress wave is attributed to mechanism (a). On the other hand, when the cell was detached from the substrate by a trypsin treatment, the fluorescence from the scaffold remained, suggesting mechanism (b). By comparing both results, it is considered that physiological damage of the cell membrane during the detachment process by femtosecond laser-induced stress wave is less than that due to the trypsin treatment.     

A portable X-ray diffraction apparatus for in situ analyses of masters’ paintings

It is rare that the analyses of materials in paintings can be carried out by taking micro-samples. Valuable works of art are best studied in situ by non-invasive techniques. For that purpose, a portable X-ray diffraction and fluorescence apparatus has been designed and constructed at the C2RMF. This apparatus has been used for paintings of Rembrandt, Leonardo da Vinci, Van Gogh, Mantegna, etc. Results are given to illustrate the performance of X-ray diffraction, especially when X-ray fluorescence does not bring sufficient information to conclude.     

Mechanical properties of in situ consolidated nanocrystalline multi-phase Al–Pb–W alloy studied by nanoindentation

Formation of chunks of various sizes ranging between 2 and 6 mm was achieved using high-energy ball milling in Al–1at.%Pb–1at.%W alloy system at room temperature during milling itself, aiding in in situ consolidation. X-ray diffraction and transmission electron microscopy (TEM) studies indicate the formation of multi-phase structure with nanocrystalline structural features. From TEM data, an average grain size of 23 nm was obtained for Al matrix and the second-phase particles were around 5 nm. A high strain rate sensitivity (SRS) of 0.071 ± 0.004 and an activation volume of 4.71b³ were measured using nanoindentation. Modulus mapping studies were carried out using Berkovich tip in dynamic mechanical analysis mode coupled with in situ scanning probe microscopy imaging. The salient feature of this investigation is highlighting the role of different phases, their crystal structures and the resultant interfaces on the overall SRS and activation volume of a multi-phase nc material.     

Structural investigation of the zincblende-β tin transition for gasb by in situ X-ray absorption spectroscopy

Abstract

The transition to the β tin structure has been observed by X-ray absorption spectroscopy for GaSb at 7.9 GPa. In the low pressure phase, the bulk modulus has been determined. In the high pressure phase, the results for the Ga-Sb and Ga-Ga distances are in good agreement with previous x-ray diffraction data. The pseudo Debye-Waller factor associated to the Ga-Sb distance is abnormally high, indicating that disorder exists for this bond.     

In situ high-pressure study of LiNbO3-type FeTiO3: X-ray diffraction and Mössbauer spectroscopy

The structure of LiNbO₃-type FeTiO₃ and the oxidation state of Fe have been investigated using X-ray diffraction and Mössbauer spectroscopy in the diamond anvil cell up to 18 GPa at room temperature. A structural phase transition is observed at 15.7 GPa from LiNbO₃-type to perovskite-type, accompanied by a volume collapse of 1.5%. LiNbO₃-type FeTiO₃, which is shown to contain only ferrous iron up to this pressure, and no charge transfer is observed. In addition to the c/a axial ratio that has been used to distinguish between ilmenite and LiNbO₃-type FeTiO₃, the hyperfine parameters (isomer shift and quadrupole splitting) provide an efficient way to discriminate between these two phases.     

In situ synchrotron X-ray imaging on morphological evolution of dendrites in Sn–Bi hypoeutectic alloy under electric currents

The growth behavior and morphological evolution of dendrites in solidifying Sn–Bi alloy under electric currents [e.g., direct current (DC) and electric current pulse (ECP)] are in situ studied using synchrotron radiation X-ray imaging technique. The suppression of dendrite growth, floating and rotation of dendrites, refinement and remelting of dendrites are investigated by analyzing a series of animated images captured during the experiments. The modification mechanisms of dendrite morphology by electric fields are discussed based on the in situ and real-time observations. When DC is imposed on the samples, the growth of dendrites is significantly suppressed due to the effect of Joule heat, and a small dendrite freely flows up and rotates due to the common effect of natural convection. When ECP is imposed in the whole solidification process, the outset of solidification is delayed by Joule heat. And due to the accumulation of undercooling, dendrites suddenly nucleate, grow and finally become fine primary dendrite arm spacing. When ECP is imposed during the crystal growth stage only, the dendrites are remelted at first and then reappear along the original growing trajectories, showing the hereditary feature.     

In situ synchrotron X-ray microdiffraction analysis of thermomechanically induced phase transformations in Cu–Al–Ni shape-memory alloy

Phase transformations in a single-crystal Cu–Al–Ni shape-memory alloy induced by thermomechanical effects were investigated in situ by high-resolution synchrotron X-ray microdiffraction. Contrary to the common belief, austenite texture maps revealed that austenite-to-martensite transformation occurred during heating of the partially transformed material under fixed specimen elongation. Twinned and detwinned types of martensite coexisted during this austenite-to-martensite phase transformation. Twinning and detwinning structures evolved to accommodate changes in stress and strain generated in the temperature-varying environment. Small amounts of austenite exhibiting distorted crystallographic orientation were detected in regions of stress-induced martensite during heating of the partially transformed material. The results of this investigation provide insight into intriguing stress rate-dependent phenomena intrinsic of shape-memory alloys and elucidate complex phase transformations due to thermal and mechanical stress effects.     

Effect of titanium ion substitution in the barium hexaferrite studied by M?ssbauer spectroscopy and X-ray diffraction

A series of M-type barium hexaferrite has been synthesized in a glass melt by partially substituting the Fe₂O₃ with TiO₂ for investigation of their structure. The glass melt has the basic composition (mol%): 40 BaO + 33 B₂O₃ + (27-x) Fe₂O₃ + x TiO₂ with x =?0, 3.6, 5.4 and 7.2 mol% TiO₂. The substituted ferrites were studied by means of X-ray diffraction, Mössbauer spectroscopy and vibration sample magnetometer. X-ray diffraction studies revealed that not all samples have a single ferritic phase, a small second phase corresponding to BaTi₆O₁₃ was also observed to form. The Mössbauer spectra changed from magnetically ordered (x =?0) to magnetically ordered with strong line broadening. Moreover, the broadening increases with TiO₂ content. The Mössbauer parameters suggested that Ti^4?+? occupies the 2a and 12k crystal sites, and the Ti^4?+? substitution on the 2b and 4f₂ site also occurs at high melt dopings. Therefore, coercivity and saturation magnetization decreased.     

Nucleation of the titanium α and β phases. in situ study of the dislocation role by synchrotron X-ray topography

This “in situ” and real time study is an approach to the role of matrix dislocations in the nucleation of the α or β phases in titanium single crystals. When the dislocations interact, forming tangles, subboundaries…, the residual stresses, present at the transformation temperature, trigger the new phase. If the dislocations are isolated at the transformation temperature they are destabilized by the large crystalline anisotropy resulting from the vibrational entropy dependence with the temperature. These dislocations disappear and do no act as preferential nucleation sites.     

Superexcited states of carbon monoxide studied by fast-electron impact

Absolute optical oscillator strength density and double differential cross section spectra of CO below 120 eV are determined by fast electron impact. Some peaks above the first ionization threshold stand out as the momentum transfer square K2 increases. The doubly excited Rydberg states converging to C 2∑^＋, D ^2∏, and F ^2∏ states of CO^＋, respectively, are confirmed in our spectra. Another peak at around 32eV is assigned to the transition of （3σ）^-1（2π）^1^1∏←X^1∑^＋.     

Structure of Na2S–GeS2 glasses studied by using neutron and X-ray diffraction and reverse Monte Carlo modeling

Neutron and X-ray diffraction measurements were performed to investigate the structure of Na₂S–GeS₂ glasses synthesized by mechanical alloying. The Ge–S coordination numbers calculated from the total correlation functions show that GeS₄ tetrahedra form the basic framework structure of Na₂S–GeS₂ glasses. In addition, a three-dimensional structural model of the (Na₂S)₅₀(GeS₂)₅₀ glass was obtained by using reverse Monte Carlo (RMC) simulation on neutron and X-ray diffraction data, and it was compared with the RMC model previously obtained for a (Li₂S)₅₀(GeS₂)₅₀ glass. The results show that the Ge–S framework structures in the two glasses are almost the same in the short and intermediate ranges; that is, the Ge–S framework structures are formed mainly by the connection of corner-sharing GeS₄ tetrahedra. Many of the Na ions are coordinated by four S atoms, which are non-bridging S atoms and/or bridging S atoms.     

Enhancement of convective drying by application of airborne ultrasound – A response surface approach

Drying is one of the oldest and most commonly used processes in the food manufacturing industry. The conventional way of drying is by forced convection at elevated temperatures. However, this process step often requires a very long treatment time, is highly energy consuming and detrimental to the product quality. Therefore, an investigation of whether the drying time and temperature can be reduced with the assistance of an airborne ultrasound intervention is of interest.Previous studies have shown that contact ultrasound can accelerate the drying process. It is assumed that mechanical vibrations, creating micro channels in the food matrix or keeping these channels from collapsing upon drying, are responsible for the faster water removal. In food samples, due to their natural origin, drying is also influenced by fluctuations in tissue structure, varying between different trials. For this reason, a model food system with thermo-physical properties and composition (water, cellulose, starch, fructose) similar to those of plant-based foods has been used in this study.The main objective was, therefore, to investigate the influence of airborne ultrasound conditions on the drying behaviour of the model food. The impact of airborne ultrasound at various power levels, drying temperature, relative humidity of the drying air, and the air speed was analysed. To examine possible interactions between these parameters, the experiments were designed with a Response Surface Method using Minitab 16 Statistical Software (Minitab Inc., State College, PA, USA). In addition, a first attempt at improving the process conditions and performance for better suitability and applicability in industrial scale processing was undertaken by non-continuous/intermittent sonication.     

In situ X-ray diffraction studies of phase transition in Pb1− x La x Zr0.40Ti0.60O3 ferroelectric ceramics

The temperature dependence of the crystalline structure and the lattice parameters of Pb_{1? x} La _x Zr_0.40Ti_0.60O₃ ferroelectric ceramic system with 0.00 ≤ x ≤ 0.21 was determined. The samples with x ≤ 0.11 show a cubic-to-tetragonal phase transition at the maximum dielectric permittivity, T _max. Above this amount and especially for the x = 0.12 sample, a spontaneous phase transition from a relaxor ferroelectric state (cubic phase) to a ferroelectric state (tetragonal phase) is observed upon cooling below the T _max. Unlike what has been reported in other studies, the x = 0.13, 0.14, and 0.15 samples, which present a more pronounced relaxor behavior, also presents a spontaneous normal-to-relaxor transition, indicated by a cubic to tetragonal symmetry below the T _max. The origin of this anomaly has been associated with an increase in the degree of tetragonality, confirmed by the measurements of the X-ray diffraction patterns. The differential thermal analysis (DSC) measurements also confirm the existence of these phase transitions.