Time-resolved Energy Dispersive X-ray Research Articles

Tracking Minority Species in Redox Reactions: A Quantitative Combined XAS and Modulation Excitation Study.

Understanding the nature of intermediates/active species in reactions is a major challenge in chemistry. This is because spectator species typically dominate the experimentally derived data and consequently active phase contributions are masked. Transient methods offer a means to bypass this difficulty. In particular, modulation excitation with phase-sensitive detection (ME-PSD) provides a mechanism to distinguish between spectator and reacting species. Herein, modulation excitation (ME) time-resolved (energy dispersive) X-ray absorption spectroscopy, assisted by phase sensitive detection (PSD) analysis, has been applied to the study of a liquid phase process; in this case the classic ferrocyanide/ferricyanide redox couple. Periodic switches of the electrical potential (anodic/cathodic) enabled the use of the ME approach. Structural changes at fractions as low as 2 % of the total number of electroactive species were detected within the X-ray beam probe volume containing ~30 pmol of Fe(II)/Fe(III).

An in situ study of thermal crystallization of amorphous calcium phosphates.

The heat-induced crystallization of amorphous calcium phosphate (ACP) is an intriguing process not yet well comprehended. This is because most of the works on this topic are based on ex situ studies where the materials are characterized after the heat and cooldown cycles, thus missing transient structural changes. Here, we used time-resolved energy dispersive X-ray diffraction and infrared spectroscopy to study, for the first time, the thermal crystallization of ACP in situ. The thermal crystallization of two kinds of citrate-stabilized carbonated ACP was studied, as they are promising materials for the preparation of advanced bioceramics. The behavior of these samples was compared to that of two citrate-free ACPs, either doped or non-doped with carbonate ions. Our results evinced that several phenomena occur during ACP thermal annealing. Before crystallization, all ACP samples undergo a decrease in the short-range order process, followed by several internal reorganizations. We have assessed that differently from carbonate-free ACP, carbonated ACPs with and without citrate directly crystallize into a biomimetic poorly crystalline carbonated hydroxyapatite. Citrate-stabilized ACPs in comparison to citrate-free ACPs have a faster hydroxyapatite formation kinetics, which is due to their higher specific surface area. This work reveals the necessity and the potentialities of using in situ techniques to effectively probe complex processes such as the heat-induced crystallization of ACPs.

Following a Chemical Reaction on the Millisecond Time Scale by Simultaneous X-ray and UV/Vis Spectroscopy.

An innovative approach aimed at disclosing the mechanism of chemical reactions occurring in solution on the millisecond time scale is presented. Time-resolved energy dispersive X-ray absorption and UV/vis spectroscopies with millisecond resolution are used simultaneously to directly follow the evolution of both the oxidation state and the local structure of the metal center in an iron complex. Two redox reactions are studied, the former involving the transformation of FeII into two subsequent FeIII species and the latter involving the more complex FeII-FeIII-FeIV-FeIII sequence. The structural modifications occurring around the iron center are correlated to the reaction mechanisms. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase and represents a new powerful tool to characterize short-lived intermediates that are silent to common spectroscopic techniques.

Electric field effect on chemical and phase equilibria in nano-TiB2–TiO2–TiBO3system at <650 °C: an in situ time-resolved energy dispersive x-ray diffraction study with an ultrahigh energy synchrotron probe

Abstract

The dynamics of pseudocapacitive phenomena studied by Energy Dispersive X-Ray Absorption Spectroscopy on hydrous iridium oxide electrodes in alkaline media

The behavior of highly hydrated IrOx porous electrodes under potentiostatic conditions is studied by operando time-resolved Energy Dispersive X-Ray Absorption Spectroscopy (EDXAS) at the Ir LIII edge. The potential steps are selected in order to drive the electron and charge transfers into the Ir(III) ⇄ Ir(IV) and Ir(IV) ⇄ Ir(V) transition domains. The adoption of sodium hydroxide aqueous solutions provides the best conditions for the potentiostatic control of the iridium oxidation states, as evidenced by the two well-separated, symmetric peaks observed in cyclic voltammetry. The intrinsic time resolution of ca. 10ms implies the acquisition of 10000–20000 spectra per run, prompting the introduction of an effective procedure for treating this large number of data for analysis and comparison with the chronoamperometric response. The combined treatment of electrochemical and XAS signals mutually supports the working hypothesis of full complementarity between the two techniques, that allow observing the pseudocapacitive reactions and its time evolution even in the presence of possible parasitic side-processes, and highlights the XAS features to be used as effective kinetic parameters.

Impact of the Nature of the Organic Spacer on the Crystallization Kinetics of UiO-66(Zr)-Type MOFs.

The influence of the constitutive dicarboxylate linkers (size, functional group) over the crystallization kinetics of a series of porous Zr metal-organic frameworks with the UiO-66 topology has been investigated by in situ time-resolved energy dispersive X-ray diffraction (EDXRD). Both large aromatic spacers (2,6-naphthalene-, 4,4'-biphenyl- and 3,3'-dichloro-4,4'-azobenzene-dicarboxylates) and a series of X-functionalized terephthalates (X=NH2 , NO2 , Br, CH3 ) were investigated in dimethylformamide (DMF) at different temperatures and compared with the parent UiO-66. Using different crystallization models, rate constants and further kinetic parameters (such as activation energy) have been extracted. Finally, the impact of the replacement of the toxic DMF by water on the crystallization kinetics was studied through the synthesis of the functionalized UiO-66-NO2 solid.

Microwave-assisted synthesis and characterization of silver nanowires by polyol process

Silver nanowires have been synthesized by the polyol process with ethylene glycol as a reducing agent and polyvinylpyrrolidone as a stabilizer, using microwave technique. Crystallographic, topographic, and morphological characterizations of the synthesized nanostructures have been studied via powder X-ray diffraction and electron microscopy (Field Emission Scanning Electron Microscope) and Transmission Electron Microscope, respectively. Fourier Transform-Infrared Spectroscopy, UV–Vis absorption spectroscopy, Raman spectroscopy, Thermogravometric analysis, and X-ray photoelectron spectroscopy have been carried out for the detailed quantitative and qualitative analyses. Optical characterizations of the synthesized silver nanowires have been concluded via energy-resolved and time-resolved photoluminescence and energy dispersive X-ray spectroscopic studies, respectively. Spectroscopic studies confirm the formation of good-quality silver nanowires. The X-ray photoelectron spectroscopy investigation and Raman spectra further show that the PVP molecules are adsorbed on the surface of Ag nanowires through Ag: O coordination. A possible growth mechanism of the Ag nanowires has been proposed. It is implied that the PVP molecules are used as both a protecting agent and a structure-directing agent for the growth of Ag nanowires. These studies confirm the formation of high-quality silver nanowires. Topographic and morphological studies confirm that average grain size of silver nanowires is in the nanometer range.

Effect of a crystalline phase of TiO2 photocatalysts on the photodeposition of Rh metal nanoparticles

Effect of a crystalline phase of TiO2 is investigated in the photodeposition processes of Rh metal nanoparticles on TiO2 photocatalysts having anatase and rutile phase by means of in situ time-resolved energy dispersive X-ray absorption fine structure spectroscopy (DXAFS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The important factor was neither its crystallite size nor its specific surface area, and was the crystalline phase. In situ time-resolved DXAFS analysis clarified that the Rh metal nanoparticles with a uniform size appear on both anatase and rutile phases of TiO2, whereas the appearance rate of Rh metal nanoparticles decreases with the photoirradiation time only on the rutile phase of TiO2. The TEM observation and XPS depth analysis revealed that the bare surface of photodeposited Rh metal nanoparticles on the rutile is exposed, although that on the anatase is partially covered with TiO2 due to strong metal-support interaction (SMSI) between Rh metal nanoparticle and TiO2−δ.

Enhancement of photo/thermal stability of organic bulk heterojunction photovoltaic devices via gold nanoparticles doping of the active layer

This study focuses on the crucial problem of the stability of organic photovoltaic (OPV) devices, aiming to shed light on the photo and thermal degradation mechanisms during prolonged irradiation under ambient conditions. For this purpose, the stability enhancement of bulk heterojunction OPV devices upon embedding surfactant free Au nanoparticles (NPs) into the photoactive layer is investigated by in situ time-resolved energy dispersive X-ray reflectometry (EDXR), photoluminescence (PL) and Raman spectroscopy as well as device degradation electrical measurements. It is shown that besides the improved cell efficiency attributed to plasmon absorption and scattering effects, the embedded NPs act as performance stabilizers, giving rise to enhanced structural stability and, in turn, to reduced photodegradation rate of the respective OPV devices. It is particularly clarified that, in addition to further stabilization of the polymer-fullerene blend, the observed improvement can be ascribed to a NP-mediated mitigation of the photooxidation effect at the cathode-active layer interface. Our work suggests the exploitation of surfactant free NPs to be a successful approach to address aging effects in OPV devices.

A time-resolved diffraction study of a window of stability in the synthesis of a copper carboxylate metal–organic framework

An in situ, time-resolved energy dispersive powder X-ray diffraction study of the solvothermal crystallisation of the copper(II) 4,4′,4″-benzene-1,3,5-triyl-tris(benzoate) metal–organic framework MOF-14 shows how reaction conditions must be carefully chosen to allow successful preparation of the material, since on prolonged heating at ≥120 °C the material irreversibly collapses into Cu2O under solvothermal conditions in less than 2 hours. This situation is in contrast to the related Cu(II)-containing metal–organic framework HKUST-1, which shows solvothermal stability over similar temperatures and reaction times. The kinetics of crystallisation of both MOFs are examined using a mathematical model proposed by Gualtieri for zeolite crystallisation: this allows separation of the nucleation and growth regimes to yield two rate constants. Arrhenius analysis gives activation energies that reveal in both cases the crystallisations are nucleation controlled. For MOF-14 we can additionally simulate its decomposition as dissolution of the first-formed interpenetrating structure: this produces a complete picture of the solvothermal stability of MOF-14 as nucleation-growth crystallisation, with some evidence of secondary nucleation, followed by dissolution.

Critical investigation of high temperature gas nitriding of a PM tool steel

AbstractThe nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogenbased case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metallurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230°C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified.

The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, viavaterite.

The kinetics and mechanisms of nanoparticulate amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite, were studied at a range of environmentally relevant temperatures (7.5-25 °C) using synchrotron-based in situ time-resolved Energy Dispersive X-ray Diffraction (ED-XRD) in conjunction with high-resolution electron microscopy, ex situ X-ray diffraction and infrared spectroscopy. The crystallization process occurs in two stages; firstly, the particles of ACC rapidly dehydrate and crystallize to form individual particles of vaterite; secondly, the vaterite transforms to calcite via a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite. The second stage of the reaction is approximately 10 times slower than the first. Activation energies of calcite nucleation and crystallization are 73±10 and 66±2 kJ mol(-1), respectively. A model to calculate the degree of calcite crystallization from ACC at environmentally relevant temperatures (7.5-40 °C) is also presented.

Crystallization of Hematite (α-Fe2O3) under Alkaline Condition: The Effects of Pb

The transformation of ferrihydrite (5Fe2O3·9H2O) to hematite (α-Fe2O3) under alkaline condition in the presence and absence of lead was for the first time investigated using in situ, time-resolved synchrotron-based energy dispersive X-ray diffraction combined with off-line chemical characterization and imaging. The results showed that the crystallization of hematite occurred via a two-stage process with goethite (α-FeOOH) as an intermediate phase. The presence of lead enhanced the formation of hematite and reduced the induction times (∼20−30%) but had little effect on the mechanism of the transformation reactions. The reaction rates for the two systems (with and without lead) ranged from 12 to 259 × 10−4 s−1 and 19 to 461 × 10−5 s−1 for the first and second stage, respectively. The activation energies of nucleation of the two systems were 16(±3) and 9(±2) kJ/mol, while the activation energies for crystallization ranged from 41(±7) to 77(±14) kJ/mol. During the hematite crystallization, the majority of the lead in the system was rapidly and irreversibly incorporated into the final hematite, while only minor amounts of lead were released back into solution.

In Situ and Real-Time Monitoring of Oxide Growth in a Few Monolayers at Surfaces of Platinum Nanoparticles in Aqueous Media

The electrochemical oxidation behaviors of the surfaces of platinum nanoparticles, one of the key phenomena in fuel cell developments, were investigated in situ and in real time, via time-resolved hard X-ray diffraction and energy dispersive X-ray absorption spectroscopy. Combining two complementary structural analyses, dynamical and inhomogenous structural changes occurring at the surfaces of nanoparticles were monitored on an atomic level with a time resolution of less than 1 s. After oxidation at 1.4 V vs RHE (reversible hydrogen electrode) in a 0.5 M H(2)SO(4) solution, longer Pt-O bonds (2.2-2.3 A that can be assigned to OHH and/or OH species) were first formed on the surface through the partial oxidation of water molecules. Next, these species turned to shorter Pt-O bonds (2.0 A, adsorbed atomic oxygen), and atomic oxygen was incorporated into the inner part of the nanoparticles, forming an initial monolayer oxide that had alpha-PtO(2)-like local structures with expanded Pt-Pt bonds (3.1 A). Finally, quasi-three-dimensional oxides with longer Pt-(O)-Pt bonds (3.5 A, precursor for beta-PtO(2)) grew on the surface, at almost 100 s after oxidation. Despite the very complex oxidation mechanism on the atomic level, XANES analysis indicated that the charge transfer from platinum to the adsorbed oxygen species was almost constant and rather small, that is, about 0.5 electrons per oxygen, up to two monolayers of oxygen. This means that ionic polarization hardly develops at this stage of the surface platinum's "oxide" growth.

Time and position resolved in situ X-ray diffraction study of the hydrothermal conversion of gypsum monoliths to hydroxyapatite

Time-resolved energy dispersive X-ray diffraction (EDXRD) data have been measured in situ from cast blocks of gypsum, CaSO(4) x 2 H(2)O, in the presence of reactive phosphate solutions under hydrothermal conditions (100 < or = T < or = 180 degrees C) in order to understand the formation of hydroxyapatite monoliths, for applications such as in artificial bone or dental materials. Measurement of data in short (60 s) intervals has thus permitted information about the kinetics and mechanism of transformation of gypsum to hydroxyapatite to be obtained in a non-invasive way, avoiding the irreversible conversion of hydrous intermediate phases that would occur on quenching. At the lower temperatures used gypsum first converts to an amorphous intermediate phase during reaction, but as the temperature is raised to 130 degrees C and above, hydrothermal dehydration to the subhydrate CaSO(4) x 1/2 H(2)O always occurs before hydroxyapatite crystallises. The final crystal size of the hydroxyapatite is estimated from the peak broadening of the EDXRD data and this reveals an increase in crystal dimension with increasing reaction temperature. Comparing measurements from the surface and from the core of the blocks shows that an additional phosphate phase, CaHPO(4), is observed in the core at two temperatures and also that the crystal growth of hydroxyapatite does not penetrate the core of the block on the time scales we have investigated (up to 6 hours). The observations have important implications in the fabrication of hydroxyapatite monoliths from cast gypsum for applications, since the conversion via several intermediate phases may compromise the integrity and mechanical properties of the monoliths.

Dynamic “operando” observation of 1 wt% Pd-based TWCs: Simultaneous XAS/DRIFTS/mass spectrometry analysis of the effects of Ce 0.5Zr 0.5O 2 loading on structure, reactivity and performance

The behavior of 1 wt% Pd-TWCs (three-way catalysts), containing up to 33 wt% Ce 0.5Zr 0.5O 2 is followed under reducing (CO) and oxidizing (NO) cycling conditions. The dynamic behavior of these systems is analyzed using a synchronous, time-resolved energy dispersive X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) set-up with subsecond time resolution. Two main physico-chemical phenomena corresponding to noble metal morphological (size/shape) changes and the redox behavior of the noble metal–promoter interface are shown to control the TWC response to NO/CO cycling conditions. Metal-only aspects strongly influence N–O dissociation and N–N coupling steps while the metal–promoter interface has a global influence on both N 2 and CO 2 formation via oxygen handling (storage/release) properties. The relative importance of these two phenomena is studied as a function of the Ce 0.5Zr 0.5O 2 promoter content of the catalysts.

Time-resolved morphological study of organic thin film solar cells based on calcium/aluminium cathode material

The stability and degradation of calcium/aluminium cathode organic solar cells are investigated in situ by time-resolved energy dispersive X-ray reflectometry. They combine the good charge carrier separation and transport properties of the poly(3-hexylthiophene-2,5-diyl):C 61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction blend and the capability of the calcium/aluminium cathode to improve the fill factor and the open circuit voltage, with respect to aluminium cathodes cells. The study focuses on the crucial problem of the device structural/morphological stability in working condition. It aims to detect and control possible morphological variations at the various interfaces and to correlate these changes to the device aging.

Time-resolved Morphological Study of Bulk Heterojunction Films for Efficient Organic Solar Devices

The crucial requirement of device stability in the development of organic electronics was addressed. In particular, the nanoscale morphology of bulk heterojunction organic films for photovoltaic applications was studied by time-resolved energy dispersive X-ray reflectometry in synergy with atomic force microscopy analysis. A reorganization of the organic molecules in the film upon illumination was detected. The occurrence of two distinct processes (characterized by a reorganization of the blend bulk and an increase of its surface roughness, respectively) was revealed. Furthermore, the effect of the morphological instability on the device efficiency over time was quantified. Finally, the effect of thermal annealing treatments and of the choice of different cathodes was verified.

Green rust as a precursor for magnetite: an in situ synchrotron based study

AbstractIn this study, direct evidence for the formation of magnetite via a green rust intermediate is reported. The Fe(II) induced transformation of ferrihydrite, was quantified in situ and under O2-free conditions using synchrotron-based time-resolved energy dispersive X-ray diffraction. At pH 9 and Fe(II)/Fe(III) ratios of 0.5 and 1, rapid growth (6 min) of sulphate green rust and its subsequent transformation to magnetite was observed. Electron microscopy confirmed these results, showing the initial rapid formation of hexagonal sulphate green rust particles, followed by the corrosion of the green rust as magnetite growth occurred, indicating that the reaction proceeds via a dissolution-reprecipitation mechanism. At pH 7 and Fe(II)/Fe(III) ratio of 0.5, sulphate green rust was the stable phase, with no transformation to magnetite.

Controlling photoinduced degradation in plastic photovoltaic cells: A time-resolved energy dispersive x-ray reflectometry study

The electrode-active layer interface of organic photovoltaic cells, a critical point in the development of organic devices, was studied by the energy dispersive x-ray reflectivity (EDXR) technique applied in situ. An EDXR-based protocol allowing discrimination between the possible mechanisms that produce the aging process at the interface was established. The study detects photoinduced oxidation of the electrode at the buried interface, to which fading of the device performances could be attributed. This conclusion was further confirmed by results obtained on a new cell, of selectively modified architecture, whose performances turned out to be stable in time.