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

Metals possess crystal structure dependent properties, be it in the bulk, micro- or in nanostructures. The stability of Au microcrystallites stabilized in unconventional body-centered orthorhombic and tetragonal [termed as bc(o,t)] lattices, under common chemical adsorbates, hexadecanethiol (HDT) and Na₂S, is explored in this study. Treatment with HDT selectively enhances the (101)_bc(o,t) diffraction intensity while with Na₂S results in an irreversible bc(o,t) to fcc lattice transformation, which is remarkable, given the extraordinary stability of these lattices, even under high pressures and temperatures. These observations were further supported by selected area electron diffraction measurements. Importantly, the overall crystallite morphology remained similar as examined using high-resolution scanning electron microscopy. The calculated adsorption energies using density functional theory (DFT) for S adsorption on various crystallite facets reveal higher stability for fcc over the metastable bc(o,t) lattices, and the trend is opposite for the adsorption of thiol. With the latter, the (101)_bc(o,t) facets are favored over (002)_bc(o,t), which reflects in the selective enhancement of the diffraction intensity and indeed in overall crystallinity itself. The aspect related to facet reorientation induced by adsorbates relates to similar changes observed in polycrystalline fcc Au itself under similar conditions.