V. Honkimäki

Molecular layering of fluorinated ionic liquids at a charged sapphire (0001) surface

Room-temperature ionic liquids (RTILs) are promising candidates for a broad range of “green” applications, for which their interaction with solid surfaces plays a crucial role. In this high-energy x-ray reflectivity study, the temperature-dependent structures of three ionic liquids with the tris(pentafluoroethyl)trifluorophosphate anion in contact with a charged sapphire substrate were investigated with submolecular resolution. All three RTILs show strong interfacial layering, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period point to an interfacial ordering mechanism, akin to the charge inversion effect, which is suggested to originate from strong correlations between the unscreened ions. The observed layering is expected to be a generic feature of RTILs at charged interfaces.

Real-time and in situ monitoring of mechanochemical milling reactions

Chemical and structural transformations have long been carried out by milling. Such mechanochemical steps are now ubiquitous in a number of industries (such as the pharmaceutical, chemical and metallurgical industries), and are emerging as excellent environmentally friendly alternatives to solution-based syntheses. However, mechanochemical transformations are typically difficult to monitor in real time, which leaves a large gap in the mechanistic understanding required for their development. We now report the real-time study of mechanochemical transformations in a ball mill by means of in situ diffraction of high-energy synchrotron X-rays. Focusing on the mechanosynthesis of metal-organic frameworks, we have directly monitored reaction profiles, the formation of intermediates, and interconversions of framework topologies. Our results reveal that mechanochemistry is highly dynamic, with reaction rates comparable to or greater than those in solution. The technique also enabled us to probe directly how catalytic additives recently introduced in the mechanosynthesis of metal-organic frameworks, such as organic liquids or ionic species, change the reactivity pathways and kinetics.

High-resolution in situ x-ray study of the hydrophobic gap at the water–octadecyl-trichlorosilane interface

The knowledge of the microscopic structure of water at interfaces is essential for the understanding of interfacial phenomena in numerous natural and technological environments. To study deeply buried liquid water–solid interfaces, high-energy x-ray reflectivity measurements have been performed. Silicon wafers, functionalized by a self-assembled monolayer of octadecyl-trichlorosilane, provide strongly hydrophobic substrates. We show interfacial density profiles with angstrom resolution near the solid–liquid interface of water in contact with an octadecyl-trichlorosilane layer. The experimental data provide clear evidence for the existence of a hydrophobic gap on the molecular scale with an integrated density deficit ρd = 1.1 Å g cm−3 at the solid–water interface. In addition, measurements on the influence of gases (Ar, Xe, Kr, N2, O2, CO, and CO2) and HCl, dissolved in the water, have been performed. No effect on the hydrophobic water gap was found.

Fast microtomography using high energy synchrotron radiation

At the High Energy Beamline ID15A at the European Synchrotron Radiation Facility we have developed a fast three-dimensional x-ray microtomography system, which acquires a complete dataset in typically less than 10s. This unprecedented speed is achieved by combining a high efficiency phosphor screen, a reflecting microscope objective and a fast charge coupled device detector with the very intense high-energy white beam radiation provided by a wiggler source. The achieved spatial resolution is 2μm. The available x-ray energy spectrum spans from 20to250keV and can therefore be used for low and high Z materials. The spectrum can be modified by inserting different filters into the x-ray beam in order to optimize the signal-to-noise ratio and to avoid beam-hardening artifacts. Different phosphors with different energy sensitivity can be used. The very high speed allows in situ studies of systems evolving on the time scale of a few seconds or minutes. Three examples are given on sintering of metallic powders, so...

Layering of [BMIM]+-based ionic liquids at a charged sapphire interface.

The structure of two model room temperature ionic liquids, [BMIM](+)[PF(6)](-) and [BMIM](+)[BF(4)](-), near the solid/liquid interface with charged Al(2)O(3)(0001) (sapphire) was determined with subnanometer resolution by high energy (72.5 keV) x-ray reflectivity. [BMIM](+)[PF(6)](-) exhibits alternately charged, exponentially decaying, near-surface layering. By contrast, the smaller-anion compound, [BMIM](+)[BF(4)](-), shows only a single layer of enhanced electron density at the interface. The different layering behaviors, and their characteristic length scales, correspond well to the different bulk diffraction patterns, also measured in this study. Complementary measurements of the surface and interface energies showed no significant different between the two RTILs. The combined bulk-interface results support the conclusion that the interfacial ordering is dominated by the same electrostatic ion-ion interactions dominating the bulk correlations, with hydrogen bonding and dispersion interactions playing only a minor role.

Formation of CuInSe2 by the annealing of stacked elemental layers—analysis by in situ high-energy powder diffraction

Phases and phase transitions in three binary systems, Cu–Se, In–Se, Cu–In and in the ternary and quaternary systems Cu–In–Se (Ga) were investigated by in situ high energy powder diffraction in a temperature range from 25 to 550 °C. Results for the binary systems are compared to the known equilibrium phase diagrams of Cu–In, Cu–Se and In–Se. Above 225 °C Cu–In and Cu–Se follow the equilibrium phase diagrams. For In–Se significant deviations from the equilibrium diagram are observed. The results on binary systems yielded the basis for the qualitative phase analysis of the phase sequences observed in the CuInSe2 precursors during thermal anneal. On ternary and quaternary systems Cu–In–Se–(Ga) the reaction path to the formation of CuInSe2 could be determined in real time. Deviations of phase sequences from the equilibrium phase diagrams are attributed to the digression from the chemical rather than the thermal equilibrium. CuInSe2 finally crystallises from the direct precursors Cu2Se (Cu2−xSe, respectively) and InSe within a melt rich in selenium. The influence of gallium and sodium on the phase sequences and the resulting formation of CuInSe2 are discussed. Both, Na and Ga promote the crystallisation of Cu2Se, the direct precursor phase for CuInSe2. A comparison of the crystallographic structures of Cu2Se and InSe shows that epitaxial growth of InSe (0001) on Cu2Se (111) lattice planes is feasible. Based upon the experimental and crystallographic analysis a qualitative model for CuInSe2 crystallisation from the precursors in the melt is developed.

Real-time in situ powder X-ray diffraction monitoring of mechanochemical synthesis of pharmaceutical cocrystals.

Looking in: The penetrating power of high-energy X-rays provides a means to monitor in situ and in real time the course of ball-milling reactions of organic pharmaceutical solids by detecting crystalline phases and assessing the evolution of their particle sizes. Upon switching from neat grinding to liquid-assisted grinding, cocrystal formation is enabled or tremendously accelerated, while the reaction mechanism alters its course.

Water and ice in contact with octadecyl-trichlorosilane functionalized surfaces: a high resolution x-ray reflectivity study

We present a high energy x-ray reflectivity study of the density profiles of water and ice at hydrophobic and hydrophilic substrates. At the hydrophobic water/octadecyl-trichlorosilane (water-OTS) interface, we find clear evidence for a thin density depletion layer with an integrated density deficit corresponding to approximately 40% of a monolayer of water molecules. We discuss the experimental results in terms of a simple model of hydrophobic/hydrophilic solid-liquid interfaces. Our results also exclude the presence of nanobubbles. A detailed study of possible radiation damage induced by the intense x-ray beam at the dry OTS surface and at the ice-OTS, as well as at water-OTS interfaces, discloses that noticeable damage is only induced at the water-OTS interface, and thus points to the dominant role of highly mobile radicals formed in bulk water close to the interface.

In situ and real-time monitoring of mechanochemical milling reactions using synchrotron X-ray diffraction

We describe the only currently available protocol for in situ, real-time monitoring of mechanochemical reactions and intermediates by X-ray powder diffraction. Although mechanochemical reactions (inducing transformations by mechanical forces such as grinding and milling) are normally performed in commercially available milling assemblies, such equipment does not permit direct reaction monitoring. We now describe the design and in-house modification of milling equipment that allows the reaction jars of the operating mill to be placed in the path of a high-energy (∼90 keV) synchrotron X-ray beam while the reaction is taking place. Resulting data are analyzed using conventional software, such as TOPAS. Reaction intermediates and products are identified using the Cambridge Structural Database or Inorganic Crystal Structure Database. Reactions are analyzed by fitting the time-resolved diffractograms using structureless Pawley refinement for crystalline phases that are not fully structurally characterized (such as porous frameworks with disordered guests), or the Rietveld method for solids with fully determined crystal structures (metal oxides, coordination polymers).

Focusing Optics for High-Energy X-ray Diffraction

Novel focusing optical devices have been developed for synchrotron radiation in the energy range 40-100 keV. Firstly, a narrow-band-pass focusing energy-tuneable fixed-exit monochromator was constructed by combining meridionally bent Laue and Bragg crystals. Dispersion compensation was applied to retain the high momentum resolution despite the beam divergence caused by the focusing. Next, microfocusing was achieved by a bent multilayer arranged behind the crystal monochromator and alternatively by a bent Laue crystal. A 1.2 micro m-high line focus was obtained at 90 keV. The properties of the different set-ups are described and potential applications are discussed. First experiments were performed, investigating with high spatial resolution the residual strain gradients in layered polycrystalline materials. The results underline that focused high-energy synchrotron radiation can provide unique information on the mesoscopic scale to the materials scientist, complementary to existing techniques based on conventional X-ray sources, neutron scattering or electron microscopy.