Lithium insertion into silicon electrodes studied by cyclic voltammetry and operando neutron reflectometry.

Operando neutron reflectometry measurements were carried out to study the insertion of lithium into amorphous silicon film electrodes during cyclic voltammetry (CV) experiments at a scan rate of 0.01 mV s-1. The experiments allow mapping of regions where significant amounts of Li are incorporated/released from the electrode and correlation of the results to modifications of characteristic peaks in the CV curve. High volume changes up to 390% accompanied by corresponding modifications of the neutron scattering length density (which is a measure of the average Li fraction present in the electrode) are observed during electrochemical cycling for potentials below 0.3 V (lithiation) and above 0.2 V (delithiation), leading to a hysteretic behaviour. This is attributed to result from mechanical stress as suggested in the literature. Formation and modification of a surface layer associated with the solid electrolyte interphase (SEI) were observed during cycling. Within the first lithiation cycle the SEI grows to 120 Å for potentials below 0.5 V. Afterwards a reversible and stable modification of the SEI between 70 Å (delithiated state) and 120 Å (lithiated state) takes place.

BioRef II-Neutron reflectometry with relaxed resolution for fast, kinetic measurements at HZB.

We present an upgrade to the time-of-flight neutron reflectometer BioRef at the research reactor BER II of the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). Through the integration of an additional chopper into the existing setup, the available wavelength resolution is significantly extended. Now two distinct operation modes can be used: a high resolution mode with Δλ/λ ranging from 1% to 5%, which allows for the investigation of thick films up to 4000 Å, and a high flux mode with Δλ/λ = 7%-11%. In the high flux mode, reflectivity curves from 0.007 Å-1 to 0.2 Å-1 with three angular settings can be recorded in 7 min. For a single angular setting and its respective window in Q-space, a time resolution of even less than 4 min is reached. The different configurations are documented by respective measurements (a) on a Ni-Ti multilayer and (b) the swelling kinetics of a solid-supported phospholipid coating upon incubation in a polyelectrolyte solution.

Comparative Adsorption of Saturated and Unsaturated Fatty Acids at the Iron Oxide/Oil Interface.

A detailed comparison of the adsorption behavior of long straight chain saturated and unsaturated fatty acids at the iron oxide/oil interface has been considered using a combination of surface study techniques. Both depletion isotherms and polarized neutron reflectometry (PNR) show that the extent of adsorption decreases as the number of double bonds in the alkyl chains increases. Sum frequency generation spectroscopic measurements demonstrate that there is also an increase in chain disorder within the adsorbed layer as the unsaturation increases. However, for the unsaturated analogues, a decrease in peak intensity is seen for the double bond peak upon heating, which is thought to arise from isomerization in the surface-bound layer. The PNR study of oleic acid adsorption indicates chemisorbed monolayer adsorption, with a further diffuse reversible adsorbed layer formed at higher concentrations.

Neutron reflectometry studies on the lithiation of amorphous silicon electrodes in lithium-ion batteries.

Neutron reflectometry is used to study in situ the intercalation of lithium into amorphous silicon electrodes. The experiments are done using a closed three-electrode electrochemical cell setup. As a working electrode, an about 40 nm thick amorphous silicon layer is used that is deposited on a 1 cm thick quartz substrate coated with palladium as a current collector. The counter electrode and the reference electrode are made of lithium metal. Propylene carbonate with 1 M LiClO4 is used as an electrolyte. The utility of the cell is demonstrated during neutron reflectometry measurements where Li is intercalated at a constant current of 100 µA (7.8 µA cm(-2)) for different time steps. The results show (a) that the change in Li content in amorphous silicon and the corresponding volume expansion can be monitored, (b) that the formation of the solid electrolyte interphase becomes visible and (c) that an irreversible capacity loss is present.

Impact of a model synovial fluid on supported lipid membranes.

The interaction of a model synovial fluid, here a solution of 3mg/mL hyaluronic acid (HA) in heavy water (D(2)O), with an oligolamellar stack of lipid (DMPC) membranes on silicon support has been studied by neutron reflectometry and infrared spectroscopy on the molecular scale at non-physiological and physiological conditions. The system under investigation represents a simple model for lipid-coated mammalian joints and other artificial implant surfaces. When exposed to pure D(2)O at 21°C, i.e. below the main phase transition of the system, the lipid membranes show a lamellar spacing of 65Å. Heating to 26°C results in detachment of all lipid bilayers except for the innermost lipid lamella directly adsorbed to the surface of the silicon support. On the contrary, when incubated in the solution of HA in D(2)O the oligolamellar lipid system starts swelling. In addition, heating to 39°C does not result in loss of the lipid membranes into the liquid phase. The interfacial lipid coating adopts a new stable lamellar state with an increase in d-spacing by 380% to 247Å measured after 43 days of incubation with the model synovial fluid. Potential consequences for joint lubrication and protective wear functionality are considered.

Selective self assembly of glutamate molecules on polyelectrolyte multilayers.

The adsorption behavior of neurotransmitter biomolecule, glutamate, on terminal poly-(allylamine)hydrochloride (PAH) polyelectrolyte multilayer is compared with its adsorption on a terminal poly(styrenesulfonate) (PSS) polyelectrolyte multilayer. Using X-ray and neutron reflectivity experiments, the internal structure of such a supramolecular film has been revealed with high resolution and the volume fraction of the adsorbed glutamate is determined. It has been shown that the glutamate binds only to the terminal PAH multilayer. Multiple attenuated total reflection infrared spectroscopy indicates that glutamate is electrostatically physisorbed on PAH surface in the zwitterionic form. Index matching neutron experiments have been done where the scattering length density of the solvent is varied, by changing the ratio of heavy water and light water, until it completely matches with that of the polyelectrolyte layer. The resulting absorption of the glutamic acid leads to changes in scattering profile which are analyzed and it is seen that the adsorption is restricted only to the surface layers. On the other hand, terminal poly(styrenesulfonate) multilayers show resistance toward glutamate. Such repulsion and adsorption between the neurotransmitter and polyelectrolytes could be potentially used in a variety of medicinal applications.

BioRef: a versatile time-of-flight reflectometer for soft matter applications at Helmholtz-Zentrum Berlin.

BioRef is a versatile novel time-of-flight reflectometer featuring a sample environment for in situ infrared spectroscopy at the reactor neutron source BER II of the Helmholtz Zentrum Berlin für Materialien und Energie (HZB). After two years of design and construction phase the instrument has recently undergone commissioning and is now available for specular and off-specular neutron reflectivity measurements. BioRef is especially dedicated to the investigation of soft matter systems and studies at the solid-liquid interface. Due to flexible resolution modes and variable addressable wavelength bands that allow for focusing onto a selected scattering vector range, BioRef enables a broad range of surface and interface investigations and even kinetic studies with subsecond time resolution. The instrumental settings can be tailored to the specific requirements of a wide range of applications. The performance is demonstrated by several reference measurements, and the unique option of in situ on-board infrared spectroscopy is illustrated by the example of a phase transition study in a lipid multilayer film.

Simultaneous polarized neutron reflectometry and anisotropic magnetoresistance measurements.

A novel experimental facility to carry out simultaneous polarized neutron reflectometry (PNR) and anisotropic magnetoresistance (AMR) measurements is presented. Performing both techniques at the same time increases their strength considerably. The proof of concept of this method is demonstrated on a CoO/Co bilayer exchange bias system. Although information on the same phenomena, such as the coercivity or the reversal mechanism, can be separately obtained from either of these techniques, the simultaneous application optimizes the consistency between both. In this way, possible differences in experimental conditions, such as applied magnetic field amplitude and orientation, sample temperature, magnetic history, etc., can be ruled out. Consequently, only differences in the fundamental sensitivities of the techniques can cause discrepancies in the interpretation between the two. The almost instantaneous information obtained from AMR can be used to reveal time-dependent effects during the PNR acquisition. Moreover, the information inferred from the AMR measurements can be used for optimizing the experimental conditions for the PNR measurements in a more efficient way than with the PNR measurements alone.

Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers.

This study addresses the effect of ionic strength and type of ions on the structure and water content of polyelectrolyte multilayers. Polyelectrolyte multilayers of poly(sodium-4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared at different NaF, NaCl and NaBr concentrations have been investigated by neutron reflectometry against vacuum, H(2)O and D(2)O. Both thickness and water content of the multilayers increase with increasing ionic strength and increasing ion size. Two types of water were identified, "void water" which fills the voids of the multilayers and does not contribute to swelling but to a change in scattering length density and "swelling water" which directly contributes to swelling of the multilayers. The amount of void water decreases with increasing salt concentration and anion radius while the amount of swelling water increases with salt concentration and anion radius. This is interpreted as a denser structure in the dry state and larger ability to swell in water (sponge) for multilayers prepared from high ionic strengths and/or salt solution of large anions. No exchange of hydration water or replacement of H by D was detected even after eight hours incubation time in water of opposing isotopic composition.

Laminar order within Langmuir-Blodgett multilayers from phospholipid and myelin basic protein: a neutron reflectivity study.

Multilayers consisting of negatively charged phospholipid DMPA and myelin basic protein (MBP) were assembled by Langmuir-Blodgett deposition of floating Langmuir monolayers from the air/water interface to solid substrates. Protein/lipid samples were obtained by binding MBP from the aqueous subphase to the phospholipid monolayers before deposition. The vertical organization of these model membranes (i.e., with organization perpendicular to the substrate surface) was investigated in detail by neutron reflectivity measurements, and the internal distribution of water molecules was determined from the change of contrast after in-situ H2O/D2O exchange. The multilayers were well ordered, with repeating lipid bilayers as fundamental structural unit. MBP was inserted in between adjacent lipid headgroups, such as in the natural myelin membrane. Water molecules in the multilayers were present mainly in the lipid headgroup and protein slab. On exposition of the pure lipid multilayers to a dry atmosphere, a reduction of the bilayer spacing was determined, whereas the global lamellar order was not affected. In contrast, drying of the protein/lipid multilayers induced degradation of the laminar order. The data demonstrate that ordered Langmuir-Blodgett multilayers are versatile model systems for studying how competing interactions between lipid, protein, water, and ions affect the global organization of such multilamellar lipid/protein assemblies. Here, the water molecules were found to be a necessary mediator to maintain the laminar order in a multilayer from DMPA and myelin basic protein.

Density depletion at solid-liquid interfaces: a neutron reflectivity study.

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Swelling behavior of self-assembled monolayers of alkanethiol-terminated poly(ethylene glycol): a neutron reflectometry study.

The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., approximately 45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from approximately 25 A in the dry state to approximately 70 A in contact with D2O, which corresponds to a total water uptake of approximately 38 mass %. In contrast, quasi-crystalline monolayers exhibit only a small amount of water penetrating into the film (approximately 8 mass %) with a corresponding change of the layer thickness from approximately 110 to approximately 125 A. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.

Neutron reflectivity studies of critical adsorption: the correspondence between a critical adsorption profile and specular neutron reflection.

For interfacial order parameter profiles which decay as Az(-mu), such as the composition profile of the noncritical interface of a binary liquid mixture at a critical end point, there is a reported one-to-one correspondence between the profile and the reflectivity which can be described by an analytical theory [S. Dietrich and R. Schack, Phys. Rev. Lett. 58, 140 (1987)]. Evidence for mu=0.53+/-0.02 for adsorption at the hydrophilic silicon/liquid surface of the mixture (2-butoxyethanol+deuterium oxide) near its lower critical end point, determined from neutron reflectivity measurements, is presented. This value is in good agreement with the theoretical prediction of mu=0.516+/-0.004. Further examination of the data permits the determination of the asymptotic surface enrichment scaling factor amplitude P0 approximately 0.11, which is not in agreement with the theoretical value P(0) = 0.94+/-0.05 and values determined by other experimental methods.

The Evaporation Resistance of Mixed Monolayers of Octadecanol and Cholesterol.

The high resistance to water evaporation exhibited by monolayers of octadecanol is markedly reduced if small quantities of cholesterol are present, an effect that mirrors the behavior of contaminants in octadecanol monolayers. Surface pressure-area isotherms show that octadecanol and cholesterol are almost (but not entirely) immiscible. Grazing-incidence synchrotron X-ray diffraction (GIXD) of the floating monolayers shows that cholesterol produces no diffraction pattern and thus is amorphous, octadecanol gives a clear diffraction peak which can be deconvoluted into a major and a minor peak corresponding to a slightly distorted hexagonal packing of the alkyl chains, and the alcohol-rich mixtures give diffraction patterns like that of pure octadecanol. The latter result shows that cholesterol does not enter the octadecanol domains in the mixed monolayers. The evaporation data are explained by permeation through the irregularly packed domain boundary regions in a pure octadecanol monolayer and by the rapid increase in the size of these regions when cholesterol is added. Copyright 1998 Academic Press.