Revealing the Bonding Nature and Electronic Structure of Early-Transition-Metal Dihydrides

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Revealing the Bonding Nature and Electronic Structure of Early-Transition-Metal Dihydrides

Curran Kalha, Laura E. Ratcliff, Giorgio Colombi, Christoph Schlueter, Bernard Dam, Andrei Gloskovskii, Tien-Lin Lee, Pardeep K. Thakur, Prajna Bhatt, Yujiang Zhu, Jürg Osterwalder, Francesco Offi, Giancarlo Panaccione, and Anna Regoutz

PRX Energy 3, 013003 – Published 16 January 2024

Abstract

Metal hydrides are potential candidates for applications in hydrogen-related technologies, such as energy storage, hydrogen compression, and hydrogen sensing, to name just a few. However, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using hard x-ray photoelectron spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: $YH$ $_{2 - δ}$ and $Ti H$ $_{2 - δ}$ . By taking advantage of the tunability of synchrotron radiation, a nondestructive depth profile of the chemical states is obtained using core-level spectra. Combining experimental valence-band (VB) spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of $d$ versus $s p$ contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal $s$ -band features close to the Fermi energy in the HAXPES VB spectra.

Received 30 May 2023
Revised 28 September 2023
Accepted 2 November 2023

DOI:https://doi.org/10.1103/PRXEnergy.3.013003

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Hydrogen storage

Hydrides Synchrotron radiation facilities

Density functional theory Hard x-ray photoelectron spectroscopy X-ray photoelectron spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Curran Kalha ¹, Laura E. Ratcliff ², Giorgio Colombi ³, Christoph Schlueter⁴, Bernard Dam³, Andrei Gloskovskii⁴, Tien-Lin Lee⁵, Pardeep K. Thakur⁵, Prajna Bhatt ¹, Yujiang Zhu¹, Jürg Osterwalder ⁶, Francesco Offi ⁷, Giancarlo Panaccione ^8,*, and Anna Regoutz ^1,†

¹Department of Chemistry, University College London (UCL), 20 Gordon Street, London WC1H 0AJ, United Kingdom
²Centre for Computational Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
³Materials for Energy Conversion and Storage, Department of Chemical Engineering, Delft University of Technology, Delft NL-2629HZ, The Netherlands
⁴Deutsches Elektronen-Synchrotron (DESY), Notkestraße 85, Hamburg 22607, Germany
⁵Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom
⁶Physik-Institut, Universität Zürich, Zürich CH-8057, Switzerland
⁷Dipartimento di Scienze, Università di Roma Tre, Rome 00146, Italy
⁸Istituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, in Area Science Park, S.S.14, Km 163.5, Trieste I-34149, Italy

^*panaccione@iom.cnr.it
^†a.regoutz@ucl.ac.uk

Popular Summary

Metal hydrides hold significant promise in various hydrogen-related technologies, such as energy storage, hydrogen compression, and hydrogen sensing. Yet, unlocking their full capabilities requires a profound understanding of their electronic structure and chemical characteristics. This study introduces an innovative approach to comprehensively explore these critical aspects within technologically relevant systems, employing hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory. By examining thin films of two prototypical metal dihydrides, yttrium and titanium dihydride, this research offers valuable insights into the nature of chemical bonds present and the occupied states close to the Fermi energy that largely control their behavior. By combining experiment and theory, a direct connection is found between specific electronic states within the materials and their enthalpy of formation. This work bolsters our understanding of metal hydrides, which is crucial for driving advancements in hydrogen-related technologies.

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Vol. 3, Iss. 1 — January - March 2024

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Figure 1
The chemical states and the probing depth. (a),(b) Annotated core-level spectra of (a) $YH$ $_{2 - δ}$ ( $Y$ $3 d$ ) and (b) $Ti H$ $_{2 - δ}$ ( $Ti$ $2 p$ ), respectively, as a function of the photon energy. The spectra are normalized to their respective areas (after removing a Shirley-type background) and plotted on a calibrated BE scale. An indication of the percentage split between the hydride (denoted as $Ti H$ $_{2 - δ}$ or $YH$ $_{2 - δ}$ ) and nonhydride (denoted as $Ti$ — $O$ or $Y$ — $O$ for simplicity) contributions as determined from peak-fit analysis is included adjacent to the legends. The shaded regions in (b) containing an asterisk correspond to areas where additional lower-valence-state metal-oxide environments are present. (c) Estimation of the probing depth and the oxide-layer thickness. The evolution of the hydride intensity as a function of the information depth (black axes) of the respective core-level kinetic energies [integrating over 95% of the depth-distribution function (DDF)] for $Ti H$ $_{2 - δ}$ and $YH$ $_{2 - δ}$ . The solid lines show the DDF curves plotted as a function of the depth (red axes) and the data points represent the percentage contribution of the hydride components to the total spectral area determined from the $Y$ $3 d$ , $Ti$ $2 p$ , and $Ti$ $1 s$ core-level spectra. Both depth axes—depth and information depth—are plotted on the same scale for ease of analysis. The horizontal shaded regions separated by dashed gray guidelines represent the estimated maximum oxide overlayer thickness, $d_{oxide}$ . This value was determined using the DDF derived from the core-level spectra collected at $h ν = 7.2$ keV. The oxide thicknesses were estimated to be $8.2 \pm 2$ and $18.4 \pm 2$ nm for $Ti H$ $_{2 - δ}$ and $YH$ $_{2 - δ}$ , respectively.
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Figure 2
The electronic structure of $YH$ $_{2 - δ}$ and $Ti H$ $_{2 - δ}$ . VB spectra collected at 3.3 and 7.2 keV are presented for (a) $YH$ $_{2 - δ}$ and (b) $Ti H$ $_{2 - δ}$ . These spectra are normalized to the total area of features I and II adjacent to the $E_{F}$ . (c), (d) Magnified views of the density of states (DOS) adjacent to the $E_{F}$ , along with a direct comparison with the total photoionization cross-section-weighted PDOS for (c) $Ti H$ $_{2}$ and (d) $YH$ $_{2}$ , calculated with PBE (i.e., the sum of all PDOS). Underneath the main panels of (c) and (d) are the PDOS differences between the $d$ and $s p$ states when cross-section weighted at photon energies of 3.3 and 7.2 keV. Parts (a)–(d) are all plotted on the same $y$ -axis scale but note the different $x$ -axis scales. The PDOS differences at the bottom of panels (c) and (d) are plotted on a $\times 2$ magnified $y$ -axis scale compared to the main-panel $y$ -axis scale.
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Figure 3
The bond nature of metal hydrides. Using the relaxed $Ti H$ $_{2}$ and $YH$ $_{2}$ structures (calculated with PBE), electron-density maps were obtained, with (a) and (b) showing the 3D density maps for $Ti H$ $_{2}$ and $YH$ $_{2}$ , respectively. Two-dimensional (2D) electron-density maps for $Ti H$ $_{2}$ and $YH$ $_{2}$ are displayed in (c) and (d), respectively. The 2D electron-density maps are plotted along the (011) body-diagonal lattice plane (1 $d$ from the origin) and on the same intensity scale to allow for a direct comparison. Note that the 3D maps are for reference only, to show where the 2D maps have been extracted from (the color scale does not match the 2D maps). Contour lines have been added to the 2D electron-density maps. These contour lines have been plotted using the function $F (N) = A \times B^{N / step}$ , where $A = 1$ , $B = 10$ , $N_{\min} = - 1$ , $N_{\max} = 3$ , and step = 5. The maps are temperature scaled, with the blue regions indicating a low electron density and the red regions indicating a high electron density.
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Figure 4
The enthalpy of formation. A comparison between the $Δ E$ values obtained by various methods (left $y$ axis) and the resultant $Δ H_{f}$ values calculated by inputting said $Δ E$ values into Eq. (2) (right $y$ axis). (a) On the left-hand side, the $Δ E$ values extracted from Ref. [35, Fig. 4] are displayed. These values, termed “Original,” were determined by the authors of Ref. [35] by calculating the energy where the integrated total DOS of the host metal was equal to one. On the right-hand side of (a), the optimized $Δ E$ values for $Ti$ and $Y$ taken from Ref. [35, Table II] are displayed. (b) $Δ E$ values extracted from the PDOS of the metals and dihydrides calculated in this work and termed “Theory.” Abiding by the definitions of $Δ E$ for the dihydrides, the position of the $s$ band closest to $E_{F}$ was used. For the metal, the main intensity $s$ -state peak position was used to determine $Δ E$ . (c) $Δ E$ values determined using the position of feature II in the VB spectra collected with HAXPES and termed “Experiment.” For (b) and (c), the left-hand side displays the metal value and right-hand side the dihydride value. Horizontal dashed reference lines are plotted to indicate the enthalpy of formation determined using DFT (darker line) and reported in the literature (“Literature”) using experimental methods (lighter line).
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