Facet-Controlled Synthesis\nof Unconventional-Phase\nMetal Alloys for Highly Efficient Hydrogen Oxidation

Facet control and phase engineering of metal nanomaterials\nare\nboth important strategies to regulate their physicochemical properties\nand improve their applications. However, it is still a challenge to\ntune the exposed facets of metal nanomaterials with unconventional\ncrystal phases, hindering the exploration of the facet effects on\ntheir properties and functions. In this work, by using Pd nanoparticles\nwith unconventional hexagonal close-packed (<i>hcp</i>,\n2H type) phase, referred to as 2H-Pd, as seeds, a selective epitaxial\ngrowth method is developed to tune the predominant growth directions\nof secondary materials on 2H-Pd, forming Pd@NiRh nanoplates (NPLs)\nand nanorods (NRs) with 2H phase, referred to as 2H-Pd@2H-NiRh NPLs\nand NRs, respectively. The 2H-Pd@2H-NiRh NRs expose more (100)<sub>h</sub> and (101)<sub>h</sub> facets on the 2H-NiRh shells compared\nto the 2H-Pd@2H-NiRh NPLs. Impressively, when used as electrocatalysts\ntoward hydrogen oxidation reaction (HOR), the 2H-Pd@2H-NiRh NRs show\nsuperior activity compared to the NiRh alloy with conventional face-centered\ncubic (<i>fcc</i>) phase (<i>fcc</i>-NiRh) and\nthe 2H-Pd@2H-NiRh NPLs, revealing the crucial role of facet control\nin enhancing the catalytic performance of unconventional-phase metal\nnanomaterials. Density functional theory (DFT) calculations further\nunravel that the excellent HOR activity of 2H-Pd@2H-NiRh NRs can be\nattributed to the more exposed (100)<sub>h</sub> and (101)<sub>h</sub> facets on the 2H-NiRh shells, which possess high electron transfer\nefficiency, optimized H* binding energy, enhanced OH* binding energy,\nand a low energy barrier for the rate-determining step during the\nHOR process.