Toward a Quantitative Understanding of Crystal-Phase Engineering of Ru Nanocrystals

Abstract

The crystal phase with a specific stacking sequence of atoms largely affects the catalytic performance of metal nanocrystals. Since the control of the phase at the same composition is extremely difficult, the phase-dependent performance of metal nanocrystals is studied rarely. Here, we show the synthesis of Ru nanocrystals with different percentages of face-centered cubic (FCC) and hexagonal close-packed (HCP) phases via kinetic control, further revealing a quantitative correlation between the phase percentage of Ru nanocrystals and the initial reduction rate of Ru(III) precursors. Specifically, we manipulate the single parameter-initial reduction rate by controlling the Ru(III) injection rate into the dropwise synthesis at a fixed reaction temperature and correlate the kinetic data with the Ru phase percentage analyzed by atomic-resolution electron microscopy and synchrotron X-ray scattering. Based on the quantitative analysis, the ranges of initial reduction rates of Ru precursors can be determined for synthesizing Ru nanocrystals with the percentages of unusual FCC phase from 9.0 to 55.1%. We demonstrate that a low initial reduction rate corresponds to the crystallization of the Ru HCP phase, while a high initial reduction rate favors the crystallization of the FCC lattice. Furthermore, we also systematically examine the catalytic performance of Ru nanocrystals with different phases.

Elucidating the Role of Reduction Kinetics in the Phase-Controlled Growth on Preformed Nanocrystal Seeds: A Case Study of Ru

“…The distinct atomic arrangements in the hcp phase (ABABAB) and fcc phase (ABCABC) could be clearly resolved along the [21̅10] and [011] zone axes, respectively, perpendicular to the basal plane of the nanoplate. The interface between the hcp and fcc phases, indicated by a stacking fault, could be formed due to the symmetry alignment ( C 3 ) of the fcc -{111} and hcp -{0001} facets. ,, The FFT pattern taken from the middle area of the core (blue boxed) matches the diffraction pattern of the hcp phase along the [21̅10] zone axis, while the FFT pattern acquired from the shell region (red boxed) is consistent with the diffraction pattern of the fcc phase along the [011] zone axis (Figure G, H). The deposition of fcc -Ru overlayers predominantly occurred on the top half of the Ru hcp seed, and this could be attributed to an asymmetrical growth mode initiated following the formation of the fcc phase in the first few layers.…”

“…This finding highlights the influence of the chemical species on phase control; yet, it is debatable whether this effect was actually exerted through the reduction kinetics. A recent study shed light on this issue by elucidating a quantitative correlation between the crystal phase of seeds formed during nucleation and the initial reduction rate of the precursor . While the authors were able to obtain Ru nanocrystals with varying percentages of the metastable fcc phase, decoupling the effect of reaction kinetics from the nucleation process would lead to a more precise control over the crystal phase and thus enhance the purity of the products.…”

Recent Progress on Phase Engineering of Nanomaterials

“…42 MoO x species were simultaneously formed during the reduction of Ru, which originated from the Mo(CO) 6 compound acting as the reductant. Reduction kinetics, e.g., the reduction rate of metal precursors, also plays a crucial role in determining the structures, 43,117,118 including the crystal phase, of obtained metal nanomaterials. In a typical example, Lin et al synthesized a mixture of hcp-and fcc-Ru nanocrystals with different hcp/fcc phase ratios via the modulation of the reduction rate of the Ru precursor by adjusting its injection rate in synthesis.…”

“…Reduction kinetics, e.g. , the reduction rate of metal precursors, also plays a crucial role in determining the structures, ,, including the crystal phase, of obtained metal nanomaterials. In a typical example, Lin et al .…”

“…In a typical example, Lin et al . synthesized a mixture of hcp - and fcc -Ru nanocrystals with different hcp / fcc phase ratios via the modulation of the reduction rate of the Ru precursor by adjusting its injection rate in synthesis . Experimental and characterization data indicated that a higher injection rate of Ru precursor contributed to the synthesis of unconventional fcc phase, as the resultant higher reduction rate of Ru favored the formation of fcc -phase seeds.…”