Noncubic to Cubic Structural Transformation in Au Microcrystallites by Oxidative Etching

Abstract

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 routes that exhibit exuberant catalytic performance. The noncubic lattices are found to be robust at pressures up to ∼40 GPa and temperature of 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 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).

Optical Properties of Noncubic Au Microcrystallites

“…The formation of bc­(o,t) phases is hindered at lower temperatures (≤220 °C) by the slower growth kinetics and at higher temperatures (≥260 °C) by the annealing effects . Interestingly, as the bc­(o,t) fraction increased, a drastic decrease in the intensity of the XRD pattern was observed (Figure i), and the observed reduction in the intensity of XRD patterns may be due to a reduction in size of the crystallite (as detailed below) and overall crystallinity . In addition, the formation of three types of Au bipyramids was observed in the temperature window of 200–300 °C.…”

“…33 Interestingly, as the bc(o,t) fraction increased, a drastic decrease in the intensity of the XRD pattern was observed (Figure 2i), and the observed reduction in the intensity of XRD patterns may be due to a reduction in size of the crystallite (as detailed below) and overall crystallinity. 32 In addition, the formation of three types of Au bipyramids was observed in the temperature window of 200− 300 °C. Au bipyramids with an average size ∼4 μm, possessing a highly corrugated surface and sharp tips (Figure 2b,c), were formed for lower thermolysis temperatures as previously reported from this laboratory (Figure S3).…”

“…30 distribution of these lattices revealed the bc(o,t)-rich body and fcc-enriched tips 31 while hosting a bc(o,t)-fcc core−shell structure. 32 In this work, we have investigated the optical properties of bc(o,t) microcrystallites by monitoring the extinction spectra. While doing so, we have enhanced the bc(o,t) mole fraction in the crystallites from 85 to 92%, a 7% increase from the earlier reported value.…”

“…Further, the microcrystallites showed high catalytic activity (unlike fcc Au) and stability in Hg and aqua regia . Spatial distribution of these lattices revealed the bc­(o,t)-rich body and fcc-enriched tips while hosting a bc­(o,t)-fcc core–shell structure . In this work, we have investigated the optical properties of bc­(o,t) microcrystallites by monitoring the extinction spectra.…”

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

“…The crystallites undergo lattice transformation to fcc while treatment with tetrabutylammonium bromide at moderate temperatures by oxidative etching. 38 We considered it worthwhile to investigate the interaction of sulphydryls on the non-fcc Au microcrystallites. Accordingly, hexadecanethiol (HDT) and Na 2 S were made to interact with bc(o,t) lattices of Au microcrystallites at a moderate temperature (25−200 °C).…”