Crystal Structure Dependent Dissolution of Non‐Cubic Au Crystallites in Aqua Regia

Abstract

Properties of metal crystallites are governed by their morphologies and inherent crystal structures. In this work, bipyramidal Au microcrystallites hosting non-cubic lattices, body-centered orthorhombic and tetragonal (together termed as bc(o,t)), are investigated for their stability in aqua regia. Specifically, microcrystallites comprising 92 % of bc(o,t) have been subjected to aqua regia of different concentrations and the changes in morphology and lattice phases have been monitored using scanning electron microscopy and X-ray diffraction techniques. The dissolution process was found to be crystal structure dependent and begin at the bipyramidal tips enriched with fcc lattice while retaining the bc(o,t) rich body. Interestingly, with increasing the reaction times, the remaining core was found to be highly reluctant to dissolution and instead, transformed to tetragonal lattices which with increasing treatment, exhibited lattice parameters closer to that of fcc. The study reveals the presence of a bc(o,t)-fcc core-shell structure with the tips enriched with fcc.

Tuning Noncubic Phases of Au at Microscale using Low-Energy Ar+ Ion Irradiation

“…In the quest for novel optical and catalytic properties, extensive studies on the phase engineering of noble fcc Au have led to the stabilization of noncubic phases: the hcp 4H-phase in Au nanowires or nanoribbons, , 2H- hcp Au square sheets with a thickness of ∼2–3 nm, and fcc / hcp heterophase nanostructures . While most studies investigate phase transitions in Au at the nanoscale, synthesis and metastability of unconventional non- fcc phases of Au at the microscale has fascinated us for quite some time. − These phases emerge from a constrained penta-twinned bipyramidal morphology (∼3 μm in length and ∼400 nm in width) and encompass locked strains in the elastic regime with respect to fcc Au. With small strains (<2%) in the elastic regime, the metastable non- fcc phases are expected to revert readily to the native fcc phase when subjected to external perturbations such as temperature, pressure, or exposure to a charged particle beam.…”

“…The penta-twinned structure of these microscaled corrugated bipyramids hosts body-centered orthorhombic ( bco ) and body-centered tetragonal ( bct ) lattices (together called as bc ( o , t ) phases) in its core, , while being enveloped by high-index corrugated facets {0 kl } ( k or l ≥ 2) along the length, and (111) closed-packed fcc structure at the tips . The non- fcc metastable phases, as characterized by varying degrees of elastic strains with respect to fcc Au, are formed due to the kinetic arrest of residual stresses in the Au microcrystallites during their growth in the intermediate temperature range (∼200–250 °C), essentially due to the sluggish rate of thermal diffusion.…”

Adsorbate-Induced Phase Transformation of Ambient Stable Noncubic Lattices in Au Microcrystallites

“…35 A recent report showcases extraordinary stability of the non-fcc Au microcrystallites in Hg and aqua-regia environments unlike to the conventional Au. 36,37 This surprising behavior is mainly governed by the lattice anisotropy associated with the various lattices present within the crystallites. The crystallites undergo lattice transformation to fcc while treatment with tetrabutylammonium bromide at moderate temperatures by oxidative etching.…”