In situ detection of ​hydrogen-induced phase transitions in individual ​palladium nanocrystals

Journal name:
Nature Materials
Volume:
13,
Pages:
1143–1148
Year published:
DOI:
doi:10.1038/nmat4086
Received
Accepted
Published online

Abstract

Many energy- and information-storage processes rely on phase changes of nanomaterials in reactive environments. Compared to their bulk counterparts, nanostructured materials seem to exhibit faster charging and discharging kinetics, extended life cycles, and size-tunable thermodynamics. However, in ensemble studies of these materials, it is often difficult to discriminate between intrinsic size-dependent properties and effects due to sample size and shape dispersity. Here, we detect the phase transitions of individual ​palladium nanocrystals during ​hydrogen absorption and desorption, using in situ electron energy-loss spectroscopy in an environmental transmission electron microscope. In contrast to ensemble measurements, we find that ​palladium nanocrystals undergo sharp transitions between the α and β phases, and that surface effects dictate the size dependence of the ​hydrogen absorption pressures. Our results provide a general framework for monitoring phase transitions in individual nanocrystals in a reactive environment and highlight the importance of single-particle approaches for the characterization of nanostructured materials.

At a glance

Figures

  1. Pd nanocube TEM and in situ EELS set-up.
    Figure 1: Pd nanocube TEM and in situ EELS set-up.

    a, Schematic of the in situ measurement set-up. EEL spectra are collected by focusing a monochromated 80 kV electron beam in the centre of a ​Pd nanocube exposed to a controlled ​H2 pressure. b, Aberration-corrected TEM image of a representative single-crystalline ​Pd nanocube on an ultrathin (~3 nm) ​carbon substrate. The adjacent panels show a zoomed-in view of the cube and its corresponding Fourier transform. c, Low-magnification annular dark-field STEM image showing the 12 ​Pd nanocubes characterized in this study (yellow circles). The nanocubes are dispersed on a 20-nm-thick ​SiO2 TEM membrane. All the measured particles are separated from each other by at least 2× their edge length to minimize any interparticle near-field interaction.

  2. EEL spectra of a single Pd nanocrystal at varying H2 pressures.
    Figure 2: EEL spectra of a single ​Pd nanocrystal at varying ​H2 pressures.

    a, Filled areas: raw EEL spectra recorded on the 23 nm ​Pd nanocube (shown in the inset) at the beginning of the experiment (top, p(H2) = 4 Pa), after ​hydrogen absorption (centre, p(H2) = 98 Pa), and after ​hydrogen desorption (bottom, p(H2) = 4 Pa). The peaks at energy losses of 10.6 eV and 12.6 eV originate from the ​SiO2 substrate and the ​H2 gas, respectively. White lines: background EEL spectra recorded on a ​SiO2 area close to the ​Pd nanocube (the spectra have been rescaled for clarity). b, Complete EEL spectrum evolution on ​H2 pressure increase (loading) and decrease (unloading). The corresponding ​H2 pressures in Pa are indicated next to each spectrum. The spectra are smoothed with a locally weighted second-order polynomial (LOESS) regression and normalized by the height of the bulk plasmon resonance peak. The peak maxima are indicated with a red (loading) or yellow (unloading) circle. The increased signal at E > 9 eV at high ​H2 pressures is due to increased electron energy losses in the ​H2 gas.

  3. Single-particle isotherms.
    Figure 3: Single-particle isotherms.

    Loading (red) and unloading (yellow) isotherms and corresponding STEM images (shown to scale) of the ​palladium nanocubes characterized in this study. All isotherms are measured at 246 K. The dashed white lines at 338 Pa correspond to the bulk loading pressure of ​palladium at 246 K, obtained using a previously published expression for the ​hydrogen chemical potential42 and the standard partial molar enthalpy and entropy of ​hydrogen in ​palladium48 (see also Supplementary Information 4). The error bars for both pressure and energy are within the size of the data points.

  4. Surface stress effect on the loading equilibrium pressures.
    Figure 4: Surface stress effect on the loading equilibrium pressures.

    a, Concentration dependence of the chemical potential of ​hydrogen in bulk ​Pd, μH,bulk (blue solid line), and in a ​Pd nanoparticle with a 15 nm diameter, μH,nano (magenta solid line). The difference between the two is the stress-induced term ΔμH,stress (red solid line), which accounts for the tensile hydrostatic pressure exerted on the core by the ​hydrogen-saturated 1-nm-thick shell. The concentration in the shell is fixed at xshell = 0.51, which corresponds to the value fitted to our experimental loading pressures. The plots for μH,bulk and μH,nano have been shifted vertically by +0.1 eV for easier comparison with ΔμH,stress. b, Equilibrium loading pressures measured on 12 ​Pd nanocubes as a function of the nanocube average edge length, h, calculated as the square root of the particle area in the STEM images. The vertical uncertainties span the difference in pressure between the highest pressure in the α phase and the lowest pressure in the β phase. To determine the size of the particles, the STEM images were fitted five separate times and the horizontal error bars correspond to the standard errors. The dashed lines indicate the calculated loading pressures using equation (3), with xshell = 0.41, xshell = 0.51 (the fitted value) and xshell = 0.61. In the fit we used t = 1 nm (refs 38, 40), Å3/atom = 1.57 × 10−6 m3 mol−1 (ref. 49), m3 mol−1, K = 187 GPa (ref. 38), ν = 0.39 (ref. 38) and T = 246 K. The inset shows a schematic of the core–shell spherical model we used to approximate our ​Pd nanocubes.

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Author information

  1. These authors contributed equally to this work.

    • Andrea Baldi &
    • Tarun C. Narayan

Affiliations

  1. Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Stanford, California 94305, USA

    • Andrea Baldi,
    • Tarun C. Narayan &
    • Jennifer A. Dionne
  2. FOM Institute DIFFER, Dutch Institute for Fundamental Energy Research, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands

    • Andrea Baldi
  3. Stanford Nanocharacterization Laboratory, Stanford University, Stanford, California 94305, USA

    • Ai Leen Koh
  4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Jennifer A. Dionne

Contributions

A.B., T.C.N. and J.A.D. designed the experiments and A.B., T.C.N. and A.L.K. performed the experiments. A.B. and T.C.N. analysed the data and wrote the initial draft of the manuscript. J.A.D. supervised the project, and all authors discussed the results and contributed to final manuscript preparation.

Competing financial interests

The authors declare no competing financial interests.

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