Watching in situ the hydrogen diffusion dynamics in magnesium on the nanoscale

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Science Advances  08 May 2020:
Vol. 6, no. 19, eaaz0566
DOI: 10.1126/sciadv.aaz0566
  • Fig. 1 In situ s-SNOM principle.

    (A) Schematic drawing of the principle for in situ s-SNOM. We use free-standing thin films realized by thermally evaporating 10 nm Pd, 5 nm Ti, and 50 nm Mg on a Pd-Au membrane. This allows hydrogenation from below. The metalized AFM tip of the s-SNOM setup is scanning the top surface to investigate the local optical properties, while the Mg thin film is absorbing hydrogen. In addition, a characteristic IR phonon of MgH2 enables chemically specific imaging. The Mg layer is in contact with air, causing oxidization. However, the very thin MgO layer is transparent for imaging at the frequency of the MgH2 phonon and is barely influencing our s-SNOM measurements. (B and C) Optical images (taken in reflection) showing the s-SNOM cantilever and the free-standing Mg film in its pristine state and after 60 min of hydrogen gas exposure (2% at 1 bar), respectively. Photo credits: J. Karst (University of Stuttgart).

  • Fig. 2 Near-field appearance of the Mg-MgH2 phase transition.

    (A to D) s-SNOM measurements depicting the same area of a 50 nm Mg film in its pristine state and after 10 min of hydrogenation at room temperature. (A) The topography depicts the expansion of the individual nanocrystallites of the polycrystalline Mg film during hydrogenation. (B) The mechanical phase φmech indicates clear grain boundaries between the individual nanocrystallites of the polycrystalline Mg film. By applying an edge detection filter, we extract a mask of these grain boundaries. (C) The scattering amplitude s4 (fourth demodulation order) drops when metallic Mg changes to dielectric MgH2. However, the scattering amplitude is also highly influenced by the surface roughness, as grain boundaries are visible in the two-dimensional (2D) scans plotted in (C). This leads to an inaccuracy in the determination where Mg has switched to MgH2, as both, a change in the optical properties and a change in the surface morphology/roughness, change the scattering amplitude. (D) The scattering phase φ4 displays a very high material contrast between metallic Mg (blue appearance) and dielectric MgH2 (red appearance). This is achieved by performing s-SNOM measurements at a characteristic IR phonon resonance of MgH2 and allows a chemically specific nanoscale imaging of the hydrogen diffusion without the influence of the surface topography. The 2D images are overlaid with the grain boundary mask from (B). (E) Nano-FTIR spectra of the near-field scattering phase taken on Mg (blue) and MgH2 (red). The plot shows the average and SD of four positions each. The distinct phonon resonance of MgH2 peaks at v¯=1320 cm1 and causes a maximum scattering phase difference of Δφ ≈ 130° between MgH2 and Mg.

  • Fig. 3 Chemically specific in situ nanoscale imaging of the diffusion dynamics of hydrogen into a 50nm Mg thin film.

    We plot 2D s-SNOM images of the scattering phase φ4 at several time steps of the hydrogen loading process. See the Supplementary Materials for a video of the nanoscale hydrogen diffusion process, showing also the corresponding scattering amplitude images, and Materials and Methods for details about the exact measurement and hydrogenation procedure. All scans are performed with an illumination frequency of v¯=1280 cm1. Hydrogenated areas (dielectric MgH2) lead to a large shift of the optical phase compared to metallic Mg, as visualized by a blue-to-red transition. An overlay with grain boundary masks allows an excellent tracking of the MgH2 formation and a detailed study of the diffusion mechanism of hydrogen in Mg thin films. We find that hydride formation is nucleated at grain boundaries and is followed by a growth process of these nucleation centers. The hydrogenation front progresses from grain to grain until channels of MgH2 have formed all over the film surface. The phase formation stops, although the surface is not completely switched from Mg to MgH2.

  • Fig. 4 Dynamics of the hydride phase propagation in individual grains versus the entire film.

    We plot the relative hydrogenated area AMgH2(t)/AGrain/Film in dependence of the hydrogen diffusion time. The relative hydrogenated area is obtained by determining the total area of MgH2 (from the optical phase data shown in Fig. 3) in four individual grains and in the entire film, respectively. Then, we normalize it to the size of the respective grain and entire film, respectively. In addition, we plot the relative hydrogenated area in the whole film when normalized to the maximum hydrogenated area of the entire film. The analyzed grains are marked in the near-field scattering phase image of the pristine Mg film on the right with the respective color. The inset shows a zoom-in of the time dependency of the hydride formation between 1 and 10 min, allowing a better comparison of the loading times. The loading in individual Mg grains is faster than the diffusion of hydrogen in the entire film. We observe, e.g., for grain 2, a loading of 75% of its area in less than 1 min.

  • Fig. 5 Vertical expansion during hydrogenation.

    (A) Topography of the Mg thin film after 2, 10, 20, and 60 min of hydrogen exposure. First, there are small peaks appearing. The longer the hydrogenation takes, the rougher/more uneven the surface becomes. (B and C) 2D images of the local vertical expansion and their histograms for the same time steps as in (A) showing a local vertical expansion of more than 60%. The average vertical expansion is calculated by integrating each histogram. (D) Average vertical expansion versus time. For a fully hydrogenated Mg film, one would expect the expansion to be 30%. As the hydrogen absorption in our 50-nm Mg film has saturated while still areas of metallic Mg were left, we reach a maximum average vertical expansion of approximately 25%. This can be explained with the hydrogenation front propagation in the vertical direction through the Mg film as indicated in fig. S6.

Supplementary Materials

  • Supplementary Materials

    Watching in situ the hydrogen diffusion dynamics in magnesium on the nanoscale

    Julian Karst, Florian Sterl, Heiko Linnenbank, Thomas Weiss, Mario Hentschel, Harald Giessen

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    • Figs. S1 to S11

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