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Water- and acid-stable self-passivated dihafnium sulfide electride and its persistent electrocatalytic reaction

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Science Advances  05 Jun 2020:
Vol. 6, no. 23, eaba7416
DOI: 10.1126/sciadv.aba7416
  • Fig. 1 Two-dimensional layered structure and electronic structure of Hf2S.

    (A) Calculated crystal structure with a hexagonal unit cell (a = b = 3.38 Å, c = 11.85 Å, α = β = 90°, γ = 120°) represented by solid black lines. Dumbbell-shaped excess anionic electrons (e) are localized between [Hf2S]2+ layers. Maxima of excess anionic electrons are located at the Wyckoff position (0.333, 0.667, 0.57), which indicates strong localization at the position of e. (B) Total and projected band structures. Identical band dispersions imply the strong hybridization state of Hf and excess anionic electron. (C) Total and projected DOS per unit energy per formula unit (f.u.) on spheres located at Hf, S, and e positions. The DOS of e is filled green. (D) ELF, (E) CDM, and (F) PCDM on the (21¯0) plane. Positions of Hf, S, and e are denoted by white dashed circles. (G) Fermi surface in the first Brillouin zone.

  • Fig. 2 Characterizations of crystal structure and chemical states for [Hf2S]2+∙2e electride.

    (A) XRD patterns for [Hf2S]2+∙2e single crystal. Out-of-plane 2θ scan (main) and in-plane ϕ scan (inset) for cleaved surface showing well-constructed hexagonal structure with exclusive (00l) diffraction and sixfold symmetry, respectively. (B) Rietveld refinement of powder XRD pattern. Inset shows a photograph of a cleaved shiny surface. Detailed crystal information from Rietveld refinement listed in table S1. (C) Atomic-scale high-angle annular dark-field STEM image and EDX elemental mapping result for the (21¯0) plane. The calculated structure is superimposed on the observed image and shows good agreement. (D) XPS spectra for Hf 4f, S 2p, and O 1s with UHV-cleaved, air-exposed, and water-exposed single crystals. All spectra are measured at the (00l) plane and calibrated from the spectrum of reference Au metal (fig. S3). Typical Hf4+, S2−, and O2− spectra are filled green, yellow, and red, respectively. The blue spectrum is expected to be Hf2+ because the peak of Hf 4f7/2 (15.5 eV) is located between the Hf metal (14.0 eV, 0 valence state) and HfO2 (17.9 eV, 4+ valence state). a.u., arbitrary units.

  • Fig. 3 Water-durable [Hf2S]2+∙2e electride passivated by double amorphous layers.

    Comparison of (A) powder XRD patterns, (B) electrical transport properties, and (C) UPS spectra for UHV-cleaved (black), air-exposed (red), and water-exposed (blue) samples (Materials and Methods). XRD and UPS results indicate the increased resistivity of air- and water-exposed samples attributed to increased contact resistance between the electrode and insulating a-HfO2 at the [Hf2S]2+∙2e electride surface. Carrier concentration (NH) of 3.04 × 1022 cm−3 agrees well with the calculated value of 3.44 × 1022 cm−3 based on two electrons per formula unit with a chemical formula of [Hf2S]2+∙2e. Inset of (C) shows the metallic Fermi edge of the UHV-cleaved sample. (D) HRTEM images of surface structures for UHV-cleaved, air-exposed, and water-exposed [Hf2S]2+∙2e electrides. (E) STEM-EDX elemental mapping profile in the region of the white dashed box in (D). S- and O-dominant amorphous layers highlighted with different colors are distinguishable by a contrast difference (fig. S6). Oxygen signal underneath a-HfO2 layer ascribed to oxidation of top and bottom surface [perpendicular direction to the images in (D)] of the TEM sample. (F) STEM-EDX elemental-mapping images for the water-exposed [Hf2S]2+∙2e electride.

  • Fig. 4 Persistent electrocatalytic hydrogen evolution reaction of self-passivated [Hf2S]2+∙2e electride in acid solution.

    (A) Polarization curves and (B) Tafel plots for self-passivated [Hf2S]2+∙2e electride, HfO2 powder, and thin films. The thick HfO2 powder (~45 μm) and film (~100 nm) are nonactive, whereas thin enough HfO2 film (~10 nm) shows HER activity with poor performance due to the tunneling current. HER performances of the self-passivated [Hf2S]2+∙2e electride maintained over 1 week (5000 cycles). The 5000th cycle of polarization curves selected to plot the Tafel slope (fig. S7). Plausible band diagrams of (C) self-passivated [Hf2S]2+∙2e electride and (D) HfO2 thin film on Au for electron transfer to evolve the hydrogen gas. The WF values are acquired from DFT calculations (for the [Hf2S]2+∙2e electride), UPS experiments (for the a-Hf2−xS and a-HfO2), and literature (37) for Au.

  • Fig. 5 Schematic model for self-passivation and electrocatalytic reaction of [Hf2S]2+∙2e electride.

    Structural model and possible pathway for HER over the self-passivated [Hf2S]2+∙2e electride as a working electrode. Volmer (H+ + e → Hads, ion discharge with the formation of adsorbed hydrogen, Hads), Heyrovsky (H+ + e + Hads → H2, subsequent desorption of Hads), and Tafel (2Hads → H2, recombination of Hads) reactions occur at the surface of a-HfO2 with transferring excess electrons from electride.

Supplementary Materials

  • Supplementary Materials

    Water- and acid-stable self-passivated dihafnium sulfide electride and its persistent electrocatalytic reaction

    Se Hwang Kang, Joonho Bang, Kyungwha Chung, Chandani N. Nandadasa, Gyeongtak Han, Subin Lee, Kyu Hyoung Lee, Kimoon Lee, Yanming Ma, Sang Ho Oh, Seong-Gon Kim, Young-Min Kim, Sung Wng Kim

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