Ronald P. Chiarello

Lead adsorption at the calcite-water interface: Synchrotron x-ray standing wave and x-ray reflectivity studies

Abstract By combining synchrotron X-ray standing wave (XSW) measurements with synchrotron X-ray reflectivity measurements, we have determined: (1) the precise three-dimensional location within the calcite unit cell of submonolayer Pb ions adsorbed at the calcite (104) surface from dilute aqueous solutions, and (2) the precise one-dimensional location of these unit cells relative to the calcite surface. Our XSW measurements, using three separate calcite Bragg reflections for triangulation, show that most adsorbed Pb ions occupy Ca sites in the calcite lattice with an ordered coverage of 0.05 equivalent monolayers, while the remaining Pb ions are disordered with a coverage of 0.03 equivalent monolayers. Our X-ray reflectivity measurements show that the ordered Pb ions occur primarily (>70%) in the surface atomic layer of calcite. Atomic force microscopy (AFM) was used to characterize the topography of the calcite (104) surface under conditions similar to the X-ray experiments. The quantitative morphological information obtained by AFM was used to develop realistic models of the calcite surface. The calculated X-ray reflectivities for these model surfaces were compared with the measured X-ray reflectivities. The new combined X-ray method that we have developed can be used to determine the atomic-scale structure of other metals adsorbed at mineral-water interfaces. Such high-resolution structural determinations are essential before detailed conceptual and theoretical models can be further developed to understand and predict the behavior of dissolved metals in mineral-water systems.

In-situ synchrotron X-ray reflectivity measurements at the calcite-water interface

Abstract In-situ synchrotron X-ray reflectivity (SXR) measurements of the calcite-water interface were made. These data were used to model the angstrom-scale interfacial width and electron density profile of the interface. Measurements were made on the (10T4) cleavage surface of Iceland spar calcite in contact with pure N 2 , humid N 2 , deionized water, and dilute nitric acid solutions at 25°C. The measurements were performed in an X-ray transparent reaction cell in which fluid pH was monitored during SXR data acquisition. Analysis of the data indicated that the pristine calcite surface in contact with pure N 2 has an interfacial width (statistical roughness) of 2.60 ± 0.15 A. Exposure to water vapor (humid N 2 ) resulted in the deposition of a water film 19.9 ± 0.4 A thick. There was no evidence from the SXR data that the adsorbed water film caused roughening of the calcite surface. However, when reacted with deionized water the calcite was roughened, yielding an interfacial width of 23.1 A. During successive reactions with fluids having decreasing initial pH the calcite/fluid interfacial width increased from 23.1 to 45.3 A. The SXR results indicate that intraterrace areas participate in the dissolution of calcite. A linear correlation was observed between interfacial width of the intraterrace areas and the quantity of calcite dissolved.

Otavite-calcite solid-solution formation at the calcite-water interface studied in situ by synchrotron X-ray scattering

Abstract Synchrotron X-ray scattering measurements were performed in situ during the formation of thin (50–600 A) overgrowths of otavite-calcite solid-solutions at the (10 1 4) cleavage surface of single-crystal calcite. These solid-solutions were precipitated from EDTA-bearing aqueous solutions having varied initial saturation states of otavite and calcite. From repetitive X-ray diffraction scans, the Cd/(Ca + Cd) ratios and the effective thicknesses (average domain size perpendicular to the calcite cleavage surface) of the solid-solutions were determined as a function of time. Additional in-plane X-ray diffraction scans were done to further characterize the relationship between the solid-solutions and the calcite cleavage surface. The solid-solution phase grew epitaxially with a (1014) growth plane oriented parallel to the calcite (10 1 4) cleavage surface. The compositions of the solid-solutions evolved with time, while their growth rates (increases in effective thickness) remained fairly constant (10–54 A/hr). In each experiment, the coverage of the initial surface by the. solid-solution (calculated from the difference between the initial and final Cd concentrations in the aqueous solution) was about 20%. Glancing-incidence X-ray reflectivity scans were also monitored as a function of time. From these scans, we determined that the solid-water interface did not become significantly rougher during the nucleation and growth of the solid-solution phase. These observations indicate that the solid-solution grew by layer spreading and that most growth may have occurred preferentially at macrostep faces produced during cleavage.

Epitaxial growth of otavite on calcite observed in situ by synchrotron X-ray scattering

Synchrotron X-ray reflectivity and X-ray diffraction techniques were used to characterizean otavite (CdCO3) overgrowth during its precipitation from an aqueous solution onto a calcite (1014) cleavage surface. X-ray reflectivity was used to measure the otavite thickness and the roughness of the calcite/otavite and otavite/fluid interfaces. Specular and off-specular X-ray diffraction were used to measure the crystallographic orientation and long-range atomic order of the otavite overgrowth. The otavite grew coherently with a (1014) growth plane oriented parallal the calcite (1014) cleavage surface. The average growth rate of the otavite for the first 9 hours was 15 A · h−. During the early growth stage (≤50 A), the otavite (1014) lattice spacing (d-value) was compressed by as much 2.2% in the direction perpendicular to the calcite cleavage surface. As the otavite thickness increased, this d-value approached that of bulk otavite. At a thickness of 443 A, the otavite was determined to be of single-crystal quality (0.4° mosaic) and epitaxial with calcite. This study demonstrates a new and accurate approach for measuring in situ precipitation rates and growth mechanisms in mineral-fluid systems.

The calcite (10l̄4) cleavage surface in water: Early results of a crystal truncation rod study

Abstract This paper introduces the application of the crystal truncation rod (CTR) technique to the study of mineral-fluid interfaces. The CTR of the calcite (10l4) cleavage surface in contact with a calcite-saturated aqueous solution was measured as a function of time by using synchrotron X-ray radiation. These results are consistent with the presence of steps of monomolecular height (3.04 A high) separated by terrace areas, as observed in previous atomic force microscope studies of the calcite surface. We found that the step density decreased with time of exposure to solution, implying that the surface healed by migration and annihilation of monomolecular steps. An atomic model of the CTR measurements of the healed surface indicates that the atomic structure of the cleavage surface is not measurably different than that of (10l4) crystallographic planes within the crystal and the root mean square roughness of the terrace areas between steps is 0.35 ± 0.1 A. Higher resolution CTR measurements could reveal exact atomic positions of the calcite surface and the structure of aqueous fluids in the interfacial region.

Electromagnetic modes and prism-film coupling in anisotropic planar waveguides of epitaxial (101) rutile thin films

We report optical waveguiding in single‐crystal, epitaxial (101) oriented rutile (TiO2) thin films grown on (1120) sapphire (α‐Al2O3) substrates using the metal‐organic chemical vapor deposition technique. The electromagnetic field distributions and propagation constants for asymmetric planar waveguides composed of an anisotropic dielectric media applicable to these films are derived. Modifications to the prism‐film coupling theory for this anisotropic case are also discussed. By application of this model to (101) oriented rutile thin films, we directly obtain values of the ordinary and extraordinary refractive indexes, n0 and ne, of the rutile thin films as well as film thicknesses. We obtain typical values of the refractive indexes (n0=2.5701±0.0005; ne=2.934±0.001) near to those for bulk rutile single crystals indicating the exceptional quality of these films.

Raman scattering and x‐ray diffractometry studies of epitaxial TiO2 and VO2 thin films and multilayers on α‐Al2O3(112̄0)

Epitaxial thin films of TiO2 and VO2 single layers and TiO2/VO2 multilayers were grown on (1120) sapphire (α‐Al2O3) substrates using the metalorganic chemical vapor deposition technique and were characterized using Raman scattering and four‐circle x‐ray diffractometry. X‐ray diffraction results indicate that the films are high quality single crystal material with well defined growth plane and small in‐plane and out‐of‐plane mosaic. Single‐layer films are shown to obey the Raman selection rules of TiO2 and VO2 single crystals. The close adherence to the Raman selection rules indicates the high degree of orientation of the films, both parallel and perpendicular to the growth plane. Selection rule spectra of two and three layer TiO2/VO2 multilayers are dominated by the VO2 layers with only minimal signature of the TiO2 layers. Due to the low band gap of semiconducting vanadium dioxide, we attribute the strong signature of the VO2 layers to resonant enhancement of the VO2 Raman component accompanied with abs...

Epitaxial growth of Cu sub 2 O films on MgO by sputtering

The epitaxial growth of Cu{sub 2}O films is of significant interest for the unique potential they offer in the development of multilayer devices and superlattices. While fundamental studies may be carried out on epitaxial films prepared by any technique, the growth of artificially layered superlattices requires that films grow epitaxially during deposition. The present study examined the growth of Cu{sub 2}O on MgO substrates directly during deposition by sputtering. Although epitaxial thin films of Cu{sub 2}O could be produced, a mosaic structure was observed. The structure of the film may be related to the growth mechanism in which islands coalesce to form a continuous film.

X-Ray Diffraction Study of Heteroepitaxy of MOCVD Grown TiO 2 and VO 2 Films on Sapphire Single Crystals

A four-circle diffractometry technique is used to determine the heteroepitaxial relations of VO 2 and TiO 2 thin films grown by an MOCVD technique on sapphire (0001) and (1120) surfaces. The use of a reflective geometry eliminates special sample preparation of the sample for the x-ray diffraction measurements. The distribution of epitaxial domains is found to depend strongly on the symmetry of the underlying substrate.

In-situ x-ray reflectivity study on growth dynamics of sputter deposited gold on silicon

We performed in-situ x-ray reflectivity measurements to study the growth dynamics of gold sputter deposited onto silicon, using an x- ray scattering chamber equipped with a faced magnetron source where the substrate is held at a right angle relative to the sputtering guns. By operating the guns at low power (1 watt) and under 20 mTorr Ar, we could control the gold deposition rate to less than 1{Angstrom}/sec. The observed x-ray reflectivity for gold deposited onto a silicon substrate held at room temperature may be consistent with nucleated island growth for average gold particle sizes less than 50 {Angstrom}. Above 50 {Angstrom}, the reflectivity data indicates that the gold film uniformly covers the silicon surface, and that as the film thickness is increased the gold-vacuum interface gets progressively rougher. Detailed analysis of room temperature data is in progress, as is the temperature dependence on the roughness of the gold vacuum interface.