Mesoscale Characterization of Nanoparticles Distribution Using X‐ray Scattering

The properties of many functional materials depend critically on the spatial distribution of an active phase within a support. In the case of solid catalysts, controlling the spatial distribution of metal (oxide) nanoparticles at the mesoscopic scale offers new strategies to tune their performance and enhance their lifetimes. However, such advanced control requires suitable characterization methods, which are currently scarce. Here, we show how the background in small‐angle X‐ray scattering patterns can be analyzed to quantitatively access the mesoscale distribution of nanoparticles within supports displaying hierarchical porosity. This is illustrated for copper catalysts supported on meso‐ and microporous silica displaying distinctly different metal distributions. Results derived from X‐ray scattering are in excellent agreement with electron tomography. Our strategy opens unprecedented prospects for understanding the properties and to guide the synthesis of a wide array of functional nanomaterials.     

Elucidation of the Structure of Organic Solutions in Solvent Extraction by Combining Molecular Dynamics and X‐ray Scattering

Knowledge of the supramolecular structure of the organic phase containing amphiphilic ligand molecules is mandatory for full comprehension of ionic separation during solvent extraction. Existing structural models are based on simple geometric aggregates, but no consensus exists on the interaction potentials. Herein, we show that molecular dynamics crossed with scattering techniques offers key insight into the complex fluid involving weak interactions without any long‐range ordering. Two systems containing mono‐ or diamide extractants in heptane and contacted with an aqueous phase were selected as examples to demonstrate the advantages of coupling the two approaches for furthering fundamental studies on solvent extraction.     

Time‐Resolved In Situ Synchrotron X‐ray Study and Large‐Scale Production of Magnetite Nanoparticles in Supercritical Water

A general solution : In situ synchrotron X‐ray scattering in a high‐pressure pulsed injection reactor (see picture) shows that magnetite nucleation and growth are temporally separated. Gram‐scale crystalline, pure phase, superparamagnetic magnetite nanoparticles were synthesized without surfactants in supercritical water in less than one hour using a laboratory‐scale continuous‐flow reactor.

     

In Situ Synchrotron Radiation X‐Ray Microspectroscopy of Polymer Microcontainers

Direct, real‐time analytical techniques that provide high‐resolution information on the chemical composition and submicrometer structure of various polymer micro‐ and nanoparticles are in high demand in a range of life science disciplines. Synchrotron‐based scanning transmission X‐ray microspectroscopy (STXM) combines both local‐spot chemical information (assessed via near‐edge X‐ray absorption fine structure spectroscopy) and imaging with resolution of several tens of nanometers, and thus can yield new insights into the nanoscale properties of these materials. Furthermore, this method allows in situ examination of soft‐matter samples in aqueous/gaseous environments and under external stimuli, such as temperature, pressure, ultrasound, and light irradiation. This Minireview highlights some recent progress in the application of the STXM technique to study the temperature‐dependent behavior of polymer core–shell microcapsules and to characterize the physicochemical properties of the supporting shells of gas‐filled microbubbles in their natural hydrated state.     

Online wide‐angle X‐ray diffraction/small‐angle X‐ray scattering measurements for the CO2‐laser‐heated drawing of poly(ethylene terephthalate) fiber

Fiber‐structure‐development in the poly(ethylene terephthalate) fiber drawing process was investigated with online measurements of wide‐angle and small‐angle X‐ray scattering with both a high‐luminance X‐ray source and a CO₂‐laser‐heated drawing system. The intensity profile of the transmitted X‐ray confirmed the location of the neck‐drawing point. The diffraction images had a time resolution of several milliseconds, and this still left much room for improvement. Crystal diffraction appeared in the wide‐angle X‐ray images almost instantaneously about 20 ms after necking, whereas a four‐point small‐angle X‐ray scattering pattern appeared immediately after necking. With the elapse of time after necking, the four‐point scattering pattern changed into a meridional two‐point shape. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1090–1099, 2005     

Probing the Self‐Assembled Nanostructures of Functional Polymers with Synchrotron Grazing Incidence X‐Ray Scattering

For advanced functional polymers such as biopolymers, biomimic polymers, brush polymers, star polymers, dendritic polymers, and block copolymers, information about their surface structures, morphologies, and atomic structures is essential for understanding their properties and investigating their potential applications. Grazing incidence X‐ray scattering (GIXS) is established for the last 15 years as the most powerful, versatile, and nondestructive tool for determining these structural details when performed with the aid of an advanced third‐generation synchrotron radiation source with high flux, high energy resolution, energy tunability, and small beam size. One particular merit of this technique is that GIXS data can be obtained facilely for material specimens of any size, type, or shape. However, GIXS data analysis requires an understanding of GIXS theory and of refraction and reflection effects, and for any given material specimen, the best methods for extracting the form factor and the structure factor from the data need to be established. GIXS theory is reviewed here from the perspective of practical GIXS measurements and quantitative data analysis. In addition, schemes are discussed for the detailed analysis of GIXS data for the various self‐assembled nanostructures of functional homopolymers, brush, star, and dendritic polymers, and block copolymers. Moreover, enhancements to the GIXS technique are discussed that can significantly improve its structure analysis by using the new synchrotron radiation sources such as third‐generation X‐ray sources with picosecond pulses and partial coherence and fourth‐generation X‐ray laser sources with femtosecond pulses and full coherence.

     

Investigation of the deformation of homogeneous poly(ethylene‐co‐1‐octene) by wide‐ and small‐angle X‐ray scattering using synchrotron radiation

The deformation behavior of homogeneous ethylene‐1‐octene copolymers was investigated as a function of the crystallinity and the crystal size and perfection, respectively, by wide‐ and small‐angle X‐ray scattering using synchrotron radiation. The crystallinity and the crystal size and perfection, respectively, are controlled by the copolymer composition and the condition of melt crystallization. The deformation includes rotation of crystals, followed by plastic deformation and complete melting of the initial crystal population, and final formation of microfibrils. The process of rotation, plastic deformation, and melting of crystals of the initial structure is completed at lower strain if the size and perfection of the crystals, respectively, decrease, that is, if crystals thermally melt at lower temperature. The kinetics of the fibrillation of the initial structure seems independent of the crystal symmetry, that is, rotation and melting of pseudohexagonal and orthorhombic polyethylene crystals (as evident in low‐crystalline specimens) are similar. The structure of the microfibrils, before and after stress release, is almost independent of the condition of prior melt crystallization, which supports the notion of complete melting of the initial crystal population. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1919–1930, 2002     

Structural investigations of carrageenan–surfactant systems by synchrotron X‐ray scattering

Small‐angle X‐ray scattering by means of synchrotron radiation was used to study the interaction of κ‐ and ι‐carrageenan of different molar mass in the presence of the gel‐inducing ions, K⁺, with the ionic surfactants cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DPC). This interaction resulted in a more or less complete shrinking of the gel and in the formation of ordered periodic structures of the surfactant in conjunction with the carrageenan molecules. The influence of the polymer concentration for a given surfactant concentration, the content of surfactant for the same concentration of the polysaccharide, the molar mass, and the linear charge density of the polymer were all investigated. Decreasing the length of the alkyl chain of the surfactant, increasing the charge density of the polymer chain, and increasing the polymer concentration for the samples explored improved the ordering in the carrageenan–surfactant complexes. The structures of the κ‐carrageenan–CPC complexes were investigated as a function of temperature during reversible heating–cooling cycles, and it was shown that the addition of the surfactant lead to a more pronounced temperature stability of polymer network. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2851–2859, 2000     

Structural Tracking of a Bimolecular Reaction in Solution by Time‐Resolved X‐Ray Scattering

Molecular movies : Time‐resolved X‐ray scattering provides direct structural information on an electronically excited complex while it is formed in the bimolecular reaction between excited octahydrogen[tetrakis‐μ‐diphosphito‐1κP:2κP′‐diplatinate](4‐) (PtPOP*) and thallium ions. In the exciplex one thallium(I) and two platinum(II) ions are found to be collinear.

     

Crystal structure of poly(octamethylene terephthalate) determined by X‐ray fiber diffraction and molecular modeling

Poly(octamethylene terephthalate) (POT), a semicrystalline aromatic polyester, is synthesized by melt‐condensation reaction, and its thermal property and crystal structure are investigated by using differential scanning calorimetry, X‐ray diffraction, and molecular modeling methods, respectively. It is revealed that the synthesized POT sample has comparably low melting temperature of 131 °C and forms one crystalline phase. Based on two‐dimensional X‐ray fiber diagram and molecular modeling analyses, the crystal structure of POT is identified to be triclinic with dimensions of a = 4.560 Å, b = 5.597 Å, c = 18.703 Å, α = 104.87°, β = 119.45°, and γ = 100.32°, in which one chemical repeating unit of POT with all‐trans conformation of octamethylene group is packed according to the space group of . © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 276–283, 2009     

Bismuth Tri‐ and Tetraarylcarboxylates: Crystal Structures,In Situ X‐ray Diffraction,Intermediates and Luminescence

A systematic investigation of the systems Bi³⁺/carboxylic acid/HNO₃ for the tri‐ and tetracarboxylic acids pyromellitic acid (H₄Pyr), trimellitic acid (H₃Tri) and trimesic acid (H₃BTC) acid led to the discovery of five new bismuth carboxylates. Structural characterisation allowed the influence of the linker geometry and the Bi³⁺:linker molar ratio in the starting solution on the crystal structure to be determined. The crystallisation of three selected compounds was investigated by in situ energy‐dispersive X‐ray diffraction. Three new crystalline intermediates were observed within minutes, and two of them could be isolated by quenching of the reaction mixture. Their crystal structures were determined from laboratory and synchrotron X‐ray powder diffraction data and allowed a possible reaction pathway to be established. In depth characterisation of the luminescence properties of the three bismuth pyromellate compounds was carried out. Fluorescence and phosphorescence could be assigned to (mainly) ligand‐ and metal‐based transitions. The polymorphs of Bi(HPyr) exhibit different luminescence properties, although their structures are very similar. Surprisingly, doping of the three host structures with Eu³⁺ and Tb³⁺ ions was only successful for one of the polymorphs.     

Microsecond X‐ray Absorption Spectroscopy Identification of CoI Intermediates in Cobaloxime‐Catalyzed Hydrogen Evolution

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time‐resolved X‐ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co^I intermediate of cobaloxime, which is a non‐noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X‐ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co^I state under similar conditions. Possible deactivation mechanisms are discussed.     

In Situ Observation of the Thermally Induced Growth of Platinum‐Nanoparticle Catalysts Using High‐Temperature X‐ray Diffraction

Fundamental understanding about the thermal stability of nanoparticles and deliberate control of structural and morphological changes under reactive conditions is of general importance for a wide range of reaction processes in heterogeneous and electrochemical catalysis. Herein, we present a parametric study of the thermal stability of carbon‐supported Pt nanoparticles at 80 °C and 160 °C, with an initial particle size below 3 nm, using in situ high‐temperature X‐ray diffraction (HT‐XRD). The effects on the thermal stability of carbon‐supported Pt nanoparticles are investigated with control parameters such as Brunauer–Emmet–Teller (BET) surface area, metal loading, temperature, and gas environment. We demonstrate that the growth rate exhibits a complex, nonlinear behavior and is largely controlled by the temperature, the initial particle size, and the interparticle distance. In addition, an ex situ transmission electron microscopy study was performed to verify our results obtained from the in situ HT‐XRD study.     

Watching Nanoparticles Form: An In Situ (Small‐/Wide‐Angle X‐ray Scattering/Total Scattering) Study of the Growth of Yttria‐Stabilised Zirconia in Supercritical Fluids

Understanding nanoparticle‐formation reactions requires multi‐technique in situ characterisation, since no single characterisation technique provides adequate information. Here, the first combined small‐angle X‐ray scattering (SAXS)/wide‐angle X‐ray scattering (WAXS)/total‐scattering study of nanoparticle formation is presented. We report on the formation and growth of yttria‐stabilised zirconia (YSZ) under the extreme conditions of supercritical methanol for particles with Y₂O₃ equivalent molar fractions of 0, 4, 8, 12 and 25 %. Simultaneous in situ SAXS and WAXS reveals a quick formation (seconds) of sub‐nanometre amorphous material forming larger agglomerates with subsequent slow crystallisation (minutes) into nanocrystallites. The amount of yttria dopant is shown to strongly affect the crystallite size and unit‐cell dimensions. At yttria‐doping levels larger than 8 %, which is known to be the stoichiometry with maximum ionic conductivity, the strain on the crystal lattice is significantly increased. Time‐resolved nanoparticle size distributions are calculated based on whole‐powder‐pattern modelling of the WAXS data, which reveals that concurrent with increasing average particle sizes, a broadening of the particle‐size distributions occur. In situ total scattering provides structural insight into the sub‐nanometre amorphous phase prior to crystallite growth, and the data reveal an atomic rearrangement from six‐coordinated zirconium atoms in the initial amorphous clusters to eight‐coordinated zirconia atoms in stable crystallites. Representative samples prepared ex situ and investigated by transmission electron microscopy confirm a transformation from an amorphous material to crystalline nanoparticles upon increased synthesis duration.     

Correlations and Fluctuations of Charged Colloids as Determined by Anomalous Small‐Angle X‐Ray Scattering

Summary: We performed molecular dynamics simulation of a charged colloidal particle with explicit counterions. Our work provides a direct comparison between simulations and ASAXS‐experiments, offering insight into the counterion distribution of charged colloidal suspensions. We give a detailed constitution of the appearing scattering terms with their physical meaning. It is shown that the cross‐correlation between a macroion and its counterions gives the meanfield approximation of the counterion density even if the counterion system is highly fluctuating. Furthermore, it is shown that cross‐correlations can be negative due to oscillations of the density amplitudes of the macroion and counterions and, therefore, must be distinguished from other scattering contributions. These oscillations become more pronounced if the counterions exhibit a fixed shape and if the size of the macroion and that of the counterion system are different.

Simulation sanpshot of a charged colloid (big central sphere) with counterions (small spheres).     

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

Pair distribution function analysis of in situ total scattering data recorded during formation of WO₃ nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO₆ polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO₃.     

The Crystal Structure of Trimethyl Borate by Neutron and X‐ray Powder Diffraction

The crystal structure of one of the simplest organoboron compounds, trimethyl borate does not appear to have been determined hitherto. The compound is of interest for the study of π‐donor ligands and their interaction with the π‐acceptor behavior of trigonal boron and the consequences of such interactions on molecular structure. We used powder neutron (with isotopically labeled material) and X‐ray diffraction to determine the crystal structure of trimethyl borate at 15 K and 200 K (neutron) and 200 K (X‐ray). The material is hexagonal (Z = 2) with a = b = 6.950(8) Å and c = 6.501(3) Å at 15 K. The unit cell volume is 272.00(1) ų. The space group is P6₃/m (SG 176) at 15 K and 200 K. This is the first crystal structure solved on the Neutron Powder Diffractometer (NPDF) at the Lujan Center.     

Time‐resolved crystallization study of absorbable polymers by synchrotron small‐angle X‐ray scattering

Changes in the lamellar morphology that occurred during the quiescent isothermal crystallization of absorbable poly(p‐dioxanone) (PDS) and PDS/poly(glycolide) block copolymer were studied by synchrotron small‐angle X‐ray scattering. Important morphological parameters such as the lamellar long period, the thicknesses of the crystal and amorphous phases, and the scattering invariant were estimated as a function of time, and trends observed over a wide range of experimental conditions are discussed. Thicker but more perfect lamellae were detected at higher crystallization temperatures. The breadth of the normalized semilog Lorentz‐corrected intensity peak systematically decreased with increasing temperature. In addition, the values of the crystallization half‐time and the Avrami exponent (n = 2.5), determined from the real‐time changes in the lamellar development, showed superb agreement with the bulk crystallinity data generated from other experimental techniques, such as calorimetry and dielectric relaxation spectroscopy. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 153–167, 2001     

In Situ High‐Energy Synchrotron Radiation Study of Boehmite Formation,Growth, and Phase Transformation to Alumina in Sub‐ and Supercritical Water

Boehmite (AlOOH) nanoparticles have been synthesized in subcritical (300 bar, 350 °C) and supercritical (300 bar, 400 °C) water. The formation and growth of AlOOH nanoparticles were studied in situ by small‐ and wide‐angle X‐ray scattering (SAXS and WAXS) using 80 keV synchrotron radiation. The SAXS/WAXS data were measured simultaneously with a time resolution greater than 10 s and revealed the initial nucleation of amorphous particles takes place within 10 s with subsequent crystallization after 30 s. No diffraction signals were observed from Al(OH)₃ within the time resolution of the experiment, which shows that the dehydration step of the reaction is fast and the hydrolysis step rate‐determining. The sizes of the crystalline particles were determined as a function of time. The overall size evolution patterns are similar in sub‐ and supercritical water, but the growth is faster and the final particle size larger under supercritical conditions. After approximately 5 min, the rate of particle growth decreases in both sub‐ and supercritical water. Heating of the boehmite nanoparticle suspension allowed an in situ X‐ray investigation of the phase transformation of boehmite to aluminium oxide. Under the wet conditions used in this work, the transition starts at 530 °C and gives a two‐phase product of hydrated and non‐hydrated aluminium oxide.