Noncubic to Cubic Structural Transformation in Au Microcrystallites by Oxidative Etching

Modulating crystal structures of noble metals can provide a library of new properties, optical, electrical, catalytic and so on. Surprisingly, most lattice conversions occur at high temperatures and pressures. Recently, Au microcrystallites have been stabilized in body-centered orthorhombic and body-centered tetragonal lattices, together termed as bc(o,t), via chemical route which exhibit exuberant catalytic performance. The non-cubic lattices are found to be robust at pressures up to ~ 40 GPa and temperature, 700 C. Herein, we report the irreversible phase transformation to thermodynamically stable face-centered cubic (fcc) lattice induced by surfactants (such as tetrabutylammonium bromide) at high but near ambient temperatures mediated by (002)bc(o,t)  (002)fcc orientational changes involving an intermediate phase, termed bct-I. The phase transformation is essentially due to oxidative etching at the nanoscale and followed by redeposition of the metal. Interestingly, the conversion is governed by the binding strength of the adsorbent and thereby dependent on the alkyl chain length of the participating quaternary ammonium cation, the most effective one being the butyl chain. The study also unravels a core-shell structure of the microcrystallite, i.e., fcc capped bc(o,t).