Nobler than the Noblest: Noncubic Gold Microcrystallites

Abstract

Conventional gold comprising the cubic lattice is universally known for its stability. However, well known to chemists and metallurgists, this nobility is challenged by reagents such as aqua regia, which dissolve gold to form a salt solution. Among metals, mercury blends with gold to form amalgam, otherwise transition metals such as copper tend to interact with gold surfaces in electrochemical media. Herein, we report a combined experimental and theoretical investigation of the stability of Au microcrystallites bearing unconventional crystal lattices that exhibit enhanced stability towards Hg and aqua regia and practically no interaction with Cu during electroless plating. The unconventional gold is undoubtedly nobler.

Unraveling the Spatial Distribution of Catalytic Non-Cubic Au Phases in a Bipyramidal Microcrystallite by X-ray Diffraction Microscopy

“…The central part of the crystallite appears to be dominated by bco lattices as the spreads, particularly the circumferential spreads, are higher (see Figure ). We may therefore consider the crystallite volume to consist of three regions, the strained body and less strained tips (shown in SI, Figure S17), which also explains the anisotropic deposition of Cu on the crystallite, preferring tips over the central portion (see Figure a–d) as also observed in an earlier study . Additional support comes from the selective dissolution of the tips in aqua regia (see Figure e).…”

“…The crystallites are stable at ambient conditions as examined over the last four years and are also robust under moderate temperatures (<400 °C) and pressures (<15 GPa). 28 In addition, the phases show high chemical stability in harsh chemical environments, 16 but surprisingly are found to be catalytically active in certain reactions unlike the bulk Au which is always inactive. 15 Thus, the bc(o,t) Au behaves quite differently from the conventional fcc phase.…”

“…We may therefore consider the crystallite volume to consist of three regions, the strained body and less strained tips (shown in SI, Figure S17), which also explains the anisotropic deposition of Cu on the crystallite, preferring tips over the central portion (see Figure 6a−d) as also observed in an earlier study. 16 Additional support comes from the selective dissolution of the tips in aqua regia (see Figure 6e). From multiple crystallites as shown in SI, Figure S30, one may generalize the exclusively distinct chemical behavior of the tips and, in turn, understand that the occurrence of distribution of lattices is quite common among such crystallites.…”

“…For example, fcc Ru nanoparticles (instead of conventional hcp) act as a superior catalyst in CO oxidation reaction . While bulk Au is inactive, Au crystallites hosting noncubic lattices are shown to serve as a catalyst in a reduction reaction while possessing oxidant resistant property in Hg and aqua regia . Ag film in 4H lattice appears golden-yellow in reflected light (instead of shiny-white) and is found to be less metallic than the fcc counterpart .…”

“…11 While bulk Au is inactive, Au crystallites hosting noncubic lattices are shown to serve as a catalyst in a reduction reaction 15 while possessing oxidant resistant property in Hg and aqua regia. 16 Ag film in 4H lattice appears golden-yellow in reflected light (instead of shiny-white) and is found to be less metallic than the fcc counterpart. 7 4H Au nanoribbons possess lower bulk modulus than the fcc, indicating softer behavior to compression.…”