Ferromagnetism in hexagonal-close-packed elements

Stabilizing and Activating Metastable Nickel Nanocrystals for Highly Efficient Hydrogen Evolution Electrocatalysis

Exploring high-performance and cost-efficient electrocatalysts with unusual metastable phase offers opportunities for improving the electrochemical hydrogen generation, while it remains a great challenge to achieve them with desirable activity and stability. Herein, we report that the doping engineering in a metastable, hexagonal-close-packed nickel (hcp Ni) electrocatalyst is a largely unrevealed yet important factor in achieving an extremely active and stable electrocatalyst toward alkaline hydrogen evolution reaction (HER). Theoretical predications and experimental results suggest that, while the stability of metastable hcp Ni electrocatalyst can be largely improved via the manganese (Mn) doping due to the lower formation energy and lattice stabilization, the MnO/hcp Ni surface promotes the HER via intrinsic favorable H₂O adsorption and fast water dissociation kinetics. Consequently, the Mn-doped hcp Ni electrocatalyst shows a small overpotential of 80 mV at 10 mA/cm² and a low Tafel slope of 68 mV/dec. The result is even approaching that of the commercial Pt/C, being one of the best reported non-noble metal HER electrocatalysts in alkaline media. Under long-term chronopotentiometry measurement, such electrocatalyst can endure at least 10 h with negligible activity decay and structure change. The present work demonstrates the dimension in boosting the electrocatalysis by doping engineering of metastable electrocatalysts.

61Ni synchrotron radiation-based Mössbauer spectroscopy of nickel-based nanoparticles with hexagonal structure

We measured the synchrotron-radiation (SR)-based Mössbauer spectra of Ni-based nanoparticles with a hexagonal structure that were synthesised by chemical reduction. To obtain Mössbauer spectra of the nanoparticles without ⁶¹Ni enrichment, we developed a measurement system for ⁶¹Ni SR-based Mössbauer absorption spectroscopy without X-ray windows between the ⁶¹Ni₈₄V₁₆ standard energy alloy and detector. The counting rate of the ⁶¹Ni nuclear resonant scattering in the system was enhanced by the detection of internal conversion electrons and the close proximity between the energy standard and the detector. The spectrum measured at 4 K revealed the internal magnetic field of the nanoparticles was 3.4 ± 0.9 T, corresponding to a Ni atomic magnetic moment of 0.3 Bohr magneton. This differs from the value of Ni₃C and the theoretically predicted value of hexagonal-close-packed (hcp)-Ni and suggested the nanoparticle possessed intermediate carbon content between hcp-Ni and Ni₃C of approximately 10 atomic % of Ni. The improved ⁶¹Ni Mössbauer absorption measurement system is also applicable to various Ni materials without ⁶¹Ni enrichment, such as Ni hydride nanoparticles.

Anomalous self-reduction of layered double hydroxide (LDH): from α-Ni(OH)2 to hexagonal close packing (HCP) Ni/NiO by annealing without a reductant

An anomalous self-reduction phenomenon can occur for Ni LDH synthesized from an ionic liquid system.

Ferromagnetic hcp chromium in Cr/Ru(0001) superlattices

We report the first observation of a weak ferromagnetic state of Cr in Cr/Ru(0001) superlattices, based on magnetic hysteresis and corroborated by x-ray magnetic circular dichroism at the CrL(2,3) edges. In situ reflection high-energy electron diffraction, x-ray diffraction, and Cr K-edge polarized x-ray absorption investigations have shown that the Cr layers thinner than 8 angstroms adopt a slightly distorted hcp structure, accompanied by a large atomic volume expansion of up to 14% compared to the bcc packing volume. The expanded hcp structure clearly induces the observed ferromagnetism, in agreement with theory.