David G. Wiesler

Distribution of water molecules at Ag(111)/electrolyte interface as studied with surface X-ray scattering

The spatial distribution of water molecules at solid-electrolyte interfaces has received extensive theoretical study, due to the importance of this interface in electrochemistry and other sciences. Such studies suggest that adjacent to the interface water is arranged in several layers, that the molecular arrangements in the inner layer is similar to bulk water, and that the inner-layer molecules have an oxygen-up (oxygen-down) average orientation for negative (positive) electrode charge (or, equivalently, potential). However, little of this has been verified by experimental measurements. In this paper we report surface X-ray scattering measurements of the water distribution perpendicular to a Ag(111)-electrolyte interface in 0.1M NaF at two potentials: +0.52 and −0.23 V from the potential of zero charge (PZC) on the electrode. We find that, first, the water is ordered in layers extending about three molecular diameters from the electrode. Second, the extent of ordering and the distance between the electrode and first water layer depend on potential, the latter being consistent with an oxygen-up (oxygen-down) average molecular orientation for negative (positive) electrode potential. Third, the inner water layer contains 1.55 × 1015 (at −0.23 V) and 2.6 × 1015 (at +0.52 V) water molecules per cm−2, remarkably more than expected from the bulk water density (i.e., ∼ 1.15 × 1015cm−2). Such a large compression shows that the molecular arrangements in the inner layer are significantly different from bulk, which has not been anticipated in current models of charged, aqueous interfaces. We give a qualitative explanation of this large density as resulting from the strong electric field at the charged Ag(111) electrode and present a tentative model of the molecular arrangements.

Structure and epitaxy of anodic TiO2/Ti(1120)

We report the results of a study by synchrotron X-ray scattering of the anodic oxide formed on Ti(1120). The oxide was grown slowly by ramping the potential to 9–10 V at 0.1 mV/s in 0.1N H2SO4. We find that the film consists of crystalline TiO2, present in nearly equal amounts as rutile and anatase. There is no evidence for amorphous oxides or oxides with stoichiometry different from TiO2. The rutile is orietationally ordered to within 8° of the Ti substrate; its tetragonal a- and b-axes are in the Ti[1100] and [0001] directions, and the c-axis is along the surface normal. The rutile diffraction peak widths correspond to an average in-plane domain size of 40 A. The anatase has domains twice this size, but it is not as orientationally ordered as the rutile. The dominant anatase orientation has its (013) planes in the surface, rotated azimuthally so that the near-neighbor Ti-Ti bonds are matched as closely as possible between the Ti and the oxide. Because both rutile and anatase have well-defined orientations with respect to the Ti substrate, but not to each other, we propose that both oxides are formed at the metal/oxide interface.

X-ray diffraction from anodic TiO2 films : in situ and ex situ comparison of the Ti(0001) face

X-ray diffraction with synchrotron radiation is used to study the structure and epitaxy of a thin (~ 250 A) anodic film grown slowly on the basal plane of single-crystal Ti. Anatase crystallites, roughly 130 A in diameter, are observed, with no indication of other forms of TiO2 or films with lower oxidation state. The oxide is more orientationally disordered than films grown on the (1120) and (1010) faces of Ti, but it exhibits weak six- and twelve-fold texturing by the metal sublayer. In situ and ex situ measurements are qualitatively similar, suggesting that the oxide does not change appreciably upon emersion.

Synchrotron x‐ray diffraction studies of the lattice and magnetic structure of epitaxial Dy films in LaF3/Dy/LaF3 sandwiches (abstract)

Lattice and magnetic x‐ray diffraction from a 2000 A thick film of Dy, sandwiched by LaF3 films on a GaAs(111) substrate, are reported. The structure was grown by molecular beam epitaxy with the c axis of the Dy parallel to the LaF3 c axis and GaAs [111] axis. We find that the c‐axis lattice constant of the Dy has a similar temperature dependence to bulk Dy from room temperature to about 110 K, but below this, the film is different from bulk. The transition to ferromagnetic ordering at ∼86 K exhibits temperature hysteresis which is also evident in the magnetic x‐ray scattering and SQUID magnetometry data. This hysteresis may arise from a combination of the strain‐energy barrier accompanying the transition and magnetic inhomogeneities in the film. The temperature dependence of the magnetic wave vector is qualitatively similar to bulk, although a weaker dependence is observed below ∼110 K, which is possibly caused by magnetoelastic effects. The magnetic coherence length (310 A) is significantly shorter than...

An in Situ Small Angle Neutron Scattering Investigation of Ag 0.7 Au 0.3 Dealloying Under Potential Control

In situ small angle neutron scattering (SANS) was used to characterize the formation of the random 3-dimensional porous structure created during the dealloying of a Ag 0.7 Au 0.3 alloy under potential control. Using a model developed by N.F. Berk (Phys. Rev. Lett., 58 (1987) 2718) for the scattering properties of random porous media, we were able to characterize the average ligament width as a function of time during the dealloying process. We find that the coarsening of the average ligament width is strongly dependent upon the value of the applied potential. Our results represent the first in situ nanoscale characterization of the 3-dimensional formation of porosity in a dealloying system under potential control.

Structural and Magnetic X-RAY Scattering Measurements of Epitaxial Dy thin Films

We report structural and magnetic X-ray scattering measurements of a 2000A thick Dy film, sandwiched by LaF 3 films on a GaAs( 111 ) substrate. The structure was grow by molecular beam epitaxy with the c-axis of the Dy parallel to the LaF 3 c-axis and GaAs[ 111 ] axis. For the magnetic X-ray scattering, we used a polarization analyzer and an X-ray energy near the Dy L 111 absorption edge. ∆t this energy there is a resonant enhancement in the magnetic scattering intensity, and we obtained ≃-50 counts per second in the magnetic diffraction peaks. Between 85 and 179K, bulk Dy forms a helical antiferromagnetic structure, and below 85K, it transforms into a ferromagnet. We find that the c-axis lattice constant of the Dy film has a similar temperature dependence to bulk Dy from room temperature to about 110K, but below this, the film behaves differently from bulk and is strained. The temperature dependence of the turn-angle in the helical antiferromagnetic state is similar to bulk, although a weaker dependence is observed below ∼1 10K due to magnetoelastic effects. Surprisingly, the magnetic coherence length (≃310A) is significantly shorter than the structural coherence length (≃730A). The transition to ferromagnetic ordering at 86K exhibits temperature hysteresis as is evident in the structural and magnetic X-ray data and in SQUID magnetometry data. We believe this hysteresis arises from a combination of the strain-energy barrier accompanying the transition and magnetic inhomogeneities in the film.