Low-Energy Ion Scattering (LEIS) Laboratory

The surface condition of a hydrogen storage material affects the kinetics of hydrogen uptake and release. Contaminants can impede H₂ uptake/release kinetics while some types of foreign atoms can function as catalysts and improve the kinetics. As part of an effort to understand the influence of contaminants and catalysts, we examined several model systems using the surface-specific technique of low-energy ion scattering spectroscopy (LEIS), which can detect and identify top-layer atoms (including hydrogen and its isotopes) as well as provide information about local atom arrangement on ordered surfaces.

Considering the high current interest in nanoconfinement of MgH₂ , the use of MgH₂ in destabilized borohydride systems and Mg(BH₄)₂ itself, we are focusing on Mg and MgB₂ surfaces. On Mg, we studied the binding site location of hydrogen on the pure crystalline surface to provide baseline data for subsequent work concerning the effect of contaminants and catalysts. On MgB₂, we measured the surface composition of MgB₂ powders prior to hydrogenating to determine their cleanliness and stoichiometry. We also examined the rate of adsorption of atomic and molecular hydrogen on the prepared surface.

Figure 1. Hydrogen atom map of the Mg(0001) surface obtained from LEIS

The surface of Mg(0001) was probed with a 2 keV Ne⁺ beam and the energy distribution of scattered and recoiled ions was measured as a function of sample orientation. Over six thousand spectra were recorded at selected polar and azimuthal angles, from which it was possible to construct a real-space map of both Mg and H surface atoms, as shown in Figure 1. This is the first time adsorbed hydrogen has been directly imaged on Mg. Adsorbed hydrogen was found to occupy 3-fold hollow sites, in agreement with DFT calculations. In the future, we plan to obtain similar data on surfaces with adsorbed impurities or transition metal atoms to see how their presence affects hydrogen binding.

We have also analyzed pressed pellets of MgB₂ powder, which surprisingly was found to consist of oxidized Mg with almost no B present. Even after sputter cleaning the surface composition was Mg (0.7) O (0.2) B (0.1). We monitored the cleaned material's reactivity to both molecular and atomic hydrogen. There was no uptake of molecular D₂, but when the sample was exposed to a flux of atomic D, noticeable uptake of D occurred. An accompanying drop in scattering intensity from Mg suggested that D chemisorbs on surface Mg atoms. Our initial LEIS measurements demonstrate how it is possible to monitor in great detail the surface condition of model hydrogen storage materials and their reactivity with hydrogen. We plan to apply this technique to Ca(BH₄)₂ as samples become available. These LEIS studies were performed as part of the MHCoE technical program.

Mass Spectrometry Laboratories

In order to fully understand how metal hydride surfaces are behaving, how catalysts affect their performance, and how contamination may effect their properties, we must have quantitative element-specific information about these surfaces. In addition to the LEIS apparatus, we have two state-of-the-art experimental facilities for exploring these materials via mass spectrometry: The Simultaneous Thermo-gravimetric Modulated-Beam Mass Spectrometry (STMBMS) Laboratory and the Surface Chemical Imaging with Precision Mass Analyzer (ChIPMA) Laboratory.

Simultaneous Thermo-gravimetric Modulated-Beam Mass Spectrometry (STMBMS) Laboratory

Experiments investigating the chemical processes and kinetics of light-metal hydride oxidation are being conducted in the simultaneous thermo-gravimetric modulated-beam mass spectrometry apparatus depicted in Figure 1.

Figure 1: Schematic of the STMBMS apparatus

The metal hydride of interest is held in the reaction cell and heated up to 1000°C. The gas species leaving the sample are identified with a mass spectrometer, while at the same time the mass loss from the sample is recorded. The combination of the mass loss data and mass spectra provide the identities and partial pressures of each gaseous species within the reaction cell as a function of time, thereby providing molecular analysis and rates of reactions during the hydrogen desorption process. The STMBMS apparatus can be configured to examine the rate of oxidation of hydride samples over a range of different conditions.

Surface Chemical Imaging with Precision Mass Analyzer (ChIPMA) Laboratory

The ChIPMA instrument, pictured below in Figure 2, is used identify the surface species that result when metal hydrides are contaminated with foreign gases (O₂, H₂O, N₂, etc.) with a high spatial resolution of order 200 nm.

The ChIPMA instrument is divided into three sections, as shown in Figure 3. One section consists of the surface analysis system, shown on the left-hand side of Figure 3, which provides for sample exposure, heating and surface ablation (for species analysis) using ion and laser beams. The second section consists of a reflection time-of-flight mass spectrometer (shown in the middle of the diagram) for imaging mass spectrometer measurements. The third system consists of the ion trap interface that allows ions to be transported from the surface analysis system, stored in the ion trap, and then passed on for analysis to the Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer for identification. The FTICR mass spectrometer enables accurate identification of complex molecules due to its high mass accuracy (< 1 ppm) and high resolution (m/Δm ~ 200,000) features.

A two-part experimental methodology is used to examine samples in the ChIPMA apparatus. First, fast acquisition Time-of-Flight (ToF) mass spectra of sample surfaces and material interfaces are imaged with high spatial resolution and analyzed for localized regions of chemical interest. This information may be used to probe for unusual mass spectral signatures, indications of known chemical degradation/decomposition of chemical constituents, or indicators of chemical migration/diffusion across interfaces. Second, the localized regions of chemical ambiguity are further analyzed with the high mass resolution and accuracy of the FTICR instrument to obtain exact mass (empirical formula) and chemical structure information. These two techniques, coupled together, provide unambiguous chemical mappings of sample interfaces at high spatial resolution.

Figure 2: Photograph of the ChIPMA Laboratory at Sandia/CA

Figure 3: Schematic diagram of ChIPMA instrument. Inset shows example FTMS/SIMS spectrum of polydimethylsiloxane polymer.

LEIS Contact:

Dr. Rob Kolasinski, Sandia National Laboratories
Phone: (925) 294-2872
Email: rkolasi@sandia.gov

Mass Spectrometry Contact:

Dr. Rich Behrens, Sandia National Laboratories
Phone: 925-294-2170
Email: rbehren@sandia.gov