Nano-scale new Heusler compounds NiRh 2 Sb and CuRh 2 Sb: synthesis, characterization, and application as electrocatalysts

Two Heusler compounds NiRh 2 Sb and CuRh 2 Sb were synthesized for the first time and their crystal structures were determined by combined X-ray diffraction and high-resolution electron microscopy. Two synthesis routes were applied: a one-pot polyol reduction of metal cations at 280–290 °C under microwave radiation yielded nanocrystalline particles of 3–6 nm, whereas intergrown microcrystalline samples were obtained by conventional high-temperature synthesis at 700 °C starting from the elements. NiRh 2 Sb is an orthorhombic Heusler compound that crystallizes in space group Cmcm with lattice parameters a = 6.9188(1) Å, b = 5.7917(1) Å and c = 5.7374(1) Å. CuRh 2 Sb is dimorphic; depending on the synthesis conditions, it either adopts the cubic space group Fm 3̄ m with a lattice parameter of a = 6.1235(1) Å, or the tetragonal space group I 4/ mmm with lattice parameters a = 4.1859(1) Å and c = 6.6375(1) Å. The transition between the cubic and tetragonal structures of CuRh 2 Sb is attributed to a reversible band Jahn–Teller distortion as suggested by density functional theory-based calculations. Electrical resistivities evidence metallic behavior for both compounds in accordance with the computed band structures and the density-of-states. Both compounds are Pauli paramagnetic and show no magnetic ordering between room temperature and 2 K. The electrocatalytic performances of the nano-scale intermetallics were tested with the electrolysis of water, and both were found to greatly reduce the overpotential of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with a large electrochemically active surface area.