Martin Bremholm

Experimental setup for in situ X-ray SAXS/WAXS/ PDF studies of the formation and growth of nanoparticles in near- and supercritical fluids

The growing interest in inorganic nanoparticles for a wide range of applications is spurring a need for synthesis methods that allow a highly specific tailoring of material properties. Synthesis in supercritical fluids holds great promise for solving this problem, but so far the fundamental chemical processes taking place under these conditions are to a large extent unknown. Here the design, construction and application of a versatile experimental setup are reported; this setup enables in situ synchrotron small-angle X-ray scattering/wide-angle X-ray scattering/pair distribution function (SAXS/WAXS/PDF) studies of the formation and growth of nanoparticles under supercritical fluid conditions.

In Situ Studies of Solvothermal Synthesis of Energy Materials

Solvothermal and hydrothermal synthesis, that is, synthesis taking place in a solvent at elevated temperature and pressure, is a powerful technique for the production of advanced energy materials as it is versatile, cheap, and environmentally friendly. However, the fundamental reaction mechanisms dictating particle formation and growth under solvothermal conditions are not well understood. In order to produce tailor-made materials with specific properties for advanced energy technologies, it is essential to obtain an improved understanding of these processes and, in this context, in situ studies are an important tool as they provide real time information on the reactions taking place. Here, we present a review of the use of powder diffraction and total scattering methods for in situ studies of synthesis taking place under solvothermal and hydrothermal conditions. The experimental setups used for in situ X-ray and neutron studies are presented, and methods of data analysis are described. Special attention is given to the methods used to extract structural information from the data, for example, Rietveld refinement, whole powder pattern modelling and pair distribution function analysis. Examples of in situ studies are presented to illustrate the types of chemical insight that can be obtained.

Critical Size of Crystalline ZrO2 Nanoparticles Synthesized in Near- and Supercritical Water and Supercritical Isopropyl Alcohol

Nanocrystalline ZrO(2) samples with narrow size distributions and mean particle sizes below 10 nm have been synthesized in a continuous flow reactor in near and supercritical water as well as supercritical isopropyl alcohol using a wide range of temperatures, pressures, concentrations and precursors. The samples were comprehensively characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS), and the influence of the synthesis parameters on the particle size, particle size distribution, shape, aggregation and crystallinity was studied. On the basis of the choice of synthesis parameters either monoclinic or tetragonal zirconia phases can be obtained. The results suggest a critical particle size of 5-6 nm for nanocrystalline monoclinic ZrO(2) under the present conditions, which is smaller than estimates reported in the literature. Thus, very small monoclinic ZrO(2) particles can be obtained using a continuous flow reactor. This is an important result with respect to improvement of the catalytic properties of nanocrystalline ZrO(2).

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.

Location of Cu2+ in CHA zeolite investigated by X-ray diffraction using the Rietveld/maximum entropy method

Rietveld/MEM analysis applied to synchrotron powder X-ray diffraction data of dehydrated CHA zeolites with catalytically active Cu2+ reveals Cu2+ in both the six- and eight-membered rings in the CHA framework, providing the first complete structural model that accounts for all Cu2+. Density functional theory calculations are used to corroborate the experimental structure and to discuss the Cu2+ coordination in terms of the Al distribution in the framework.

Phase Separation and Bulk p‐n Transition in Single Crystals of Bi2Te2Se Topological Insulator

Bismuth chalcogenide (Bi 2 Ch 3 ) alloys are among the most extensively studied and commonly used thermoelectric materials. [ 1 ] They are also currently of great interest in condensedmatter physics as prototypical three-dimensional topological insulators (TIs), due to the existence of stable and topologically protected Dirac like states on the surface. [ 2 , 3 ] Shubnikov– de Haas and weak-fi eld Hall anomalies show a substantially enhanced surface current and mobility of the surface states over bulk Bi 2 Te 3 values. [ 4 ] However, it is challenging to investigate the charge-transport characteristics of the surface states directly, as the charge transport is dominated by the bulk properties. [ 3–5 ] Most studies of TIs in the Bi 2 Ch 3 family have focused on binary Bi 2 Se 3 and Bi 2 Te 3 systems. Both compounds are semiconductors, but they display high bulk conductivities due to intrinsic defects. [ 4–6 ] A conversion from nto p-type conduction in Bi 2 Te 3 thin fi lms is observed when the growth condition changes. [ 5b ] Recently, the ternary compound Bi 2 Te 2 Se has been suggested to be the best material for studies of the surface transport due to its large bulk resistivity. [ 3 , 6–8 ] However, controlling the bulk conductivity is diffi cult, even for Bi 2 Te 2 Se, due to unintentional doping by crystal defects. [ 6 , 8 ] Here we show that the diffi culty of making high-quality Bi 2 Te 2 Se single crystals originates from the internal features of the specifi c solid-state composition and phase separation in Bi 2 Te 2 Se. Bi 2 Ch 3 has the tetradymite-type rhombohedral structure, which can be described by the R-3m space group. The lattice can be regarded as a hexagonal layered structure in which the layers stack in the sequence of Ch 1 -Bi-Ch 2 -Bi-Ch 1 , where Ch is Te or Se in the present study. Each unit cell consists of three of these fi velayer groups, which are connected by bonds with a high degree of van der Waals character between the Ch 1 -Ch 1 layers. Early phase-diagram studies showed that Bi 2 Te x Se y ( x + y = 3) compounds form continuous solid solutions at temperatures above 500 ° C; however, the crystal structures tend to be ordered at

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 degrees C) and supercritical (300 bar, 400 degrees 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 degrees C and gives a two-phase product of hydrated and non-hydrated aluminium oxide.

In situ total X-ray scattering study of the formation mechanism and structural defects in anatase TiO2 nanoparticles under hydrothermal conditions

Polymorphism, morphology, particle size, and defects play key roles in the physical and chemical properties of nanoparticles. In hydrothermal synthesis of metal oxide nanoparticles, it is important to understand the influence of the specific precursor on these characteristics. Here, the formation mechanism of anatase TiO2 nanoparticles by hydrolysis of titanium isopropoxide under hydrothermal conditions is studied by in situ total X-ray scattering and pair distribution function (PDF) analysis. It is shown that the amorphous precursor structure has short-range order up to ~6.5 A consisting of titanium hydroxide clusters made from TiO6/TiO5 units in an arrangement related to anatase. Insight into the structural disorder of the anatase TiO2 nanocrystals is obtained from both PDF and powder X-ray diffraction (PXRD) analyses. Defects of OH species are present on the surface of the nanocrystals, and their concentration correlates strongly with the particle size. Even though the formation of anatase TiO2 under hydrothermal conditions resembles a solid-state phase transition from amorphous titania, the crystallization and grain growth kinetics of the nanocrystals are different due to the effects of the solvent.

Redox‐Driven Migration of Copper Ions in the Cu‐CHA Zeolite as Shown by the In Situ PXRD/XANES Technique

Using quasi-simultaneous in situ PXRD and XANES, the direct correlation between the oxidation state of Cu ions in the commercially relevant deNOx NH3 -SCR zeolite catalyst Cu-CHA and the Cu ion migration in the zeolitic pores was revealed during catalytic activation experiments. A comparison with recent reports further reveals the high sensitivity of the redox-active centers concerning heating rates, temperature, and gas environment during catalytic activation. Previously, Cu+ was confirmed present only in the 6R. Results verify a novel 8R monovalent Cu site, an eventually large Cu+ presence upon heating to high temperatures in oxidative conditions, and demonstrate the unique potential in combining in situ PXRD and XANES techniques, with which both oxidation state and structural location of the redox-active centers in the zeolite framework could be tracked.

Interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator.

The discovery of high-temperature superconductivity in Fe-based compounds triggered numerous investigations on the interplay between superconductivity and magnetism, and on the enhancement of transition temperatures through interface effects. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture remains elusive because of the lack of atomically resolved data. Here we present spin-polarized scanning tunnelling spectroscopy of ultrathin FeTe1−xSex (x=0, 0.5) films on bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level, indicating superconducting correlations up to Tc∼6 K for one unit cell FeTe grown on Bi2Te3, in contrast to the non-superconducting bulk FeTe. The gap spatially coexists with bi-collinear AFM order. This finding opens perspectives for theoretical studies of competing orders in Fe-based superconductors and for experimental investigations of exotic phases in superconducting layers on topological insulators.