![]() ![]() The analytical model (Wulffmaker-based) is purely thermodynamic, whereas kinetically enhanced growth at re-entrant corners was included in the numerical Crystal Creator approach. Diffraction spots areas scale with intensity. (c–h) Projected thickness maps as a proxy for HAADF-STEM images, thickness profile along the yellow line, and simulated diffraction patterns, with parent in black and twin in red. Simulated images and diffraction patterns for BCC single crystal (left) and twinned NPs (right) from Fig. Taking a similar shape approach, one could use STEM-HAADF 3D tomography, yet again rounding and small sizes will cause issues unless atomic resolution is achieved. † The cuboctahedral single crystal and related twinned NPs look virtually identical along the, and zone axes, for instance ( Fig. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) offers improved resolution over SEM and more straightforwardly interpretable images than TEM it produces thickness projections such as those predicted in Fig. Simple shape characterization approaches such as scanning electron microscopy (SEM) or atomic force microscopy (AFM) reveal out-of-plane topography however, given the typically small size (<50 nm) of catalytically relevant NPs and experimental edge/corner rounding, low resolution and shape similarities limit their use. This similarity between single crystal and twinned shapes undermines shape-based experimental identification of twinned BCC NPs. The twinned shapes produced are remarkably similar to single crystal shapes ( Fig. Wulffmaker and Crystal Creator were then modified to model (112)-twinned BCC crystals, using surface energies/growth velocities mapped on those of single crystals. 12 In Mo, first-principles calculations and bond-cutting calculations yield differently proportioned Bijinski dodecahedra. In Fe NPs, truncated cuboctahedra have been observed and modelled 24 truncated nanocubes have also been seen. The relative surface energies/growth velocities differ widely under varying experimental conditions, and in turn so do the shapes obtained. Complementarily, relative surface energies were extracted from matching published NP shapes from ref 12 ( Fig. Wulffmaker 26 and Crystal Creator 19, 27 were used to predict single crystalline NP shapes based on the relative surface energies or growth velocities reported for Fe and Mo 24, 25 ( Fig. To establish realistic shapes and surface energies, we modelled shapes inspired by numerical and experimental results. (b, d, f and h) Analytical thermodynamic and (c, e, g and i) numerical kinetic shapes from the surface energies/growth velocities listed twin planes are shown as a black line, viewing directions are 0] or these and the x, y, z directions refer to the parent crystal, on, for 0], the bottom left of and, for, behind the crystal. (a) Dense crystallographic planes in BCC and their color-coding. 6– 8 For instance, twinned icosahedral Pd and Pt–Ni NPs have higher activities than single crystal octahedra for formic acid oxidation and oxygen reduction, respectively, despite both being bound exclusively by facets with their relative values depending on reaction conditions. the presence of one or more planar crystallographic defects, leads to not only novel shapes but also strain that influences catalytic properties. 5 Twinning and grain boundaries are known to affect catalytic activity, 6– 10 yet there is little recorded evidence for or against the presence of twinning in BCC NPs.Ĭatalytic properties are controlled by NP shape, composition, and crystallinity. Nanoparticles (NPs) of these metals are finding applications in strain sensing, 1 catalysis, 2– 5 and, for Fe, medical diagnosis, treatment, and electronic media. $$\sin\theta=\frac are present for the primitave cubic, $h k l=$ odd are absent in bcc and for face centred lattices for a reflection to be present h, k, l are all odd or all even.About one third of metallic elements crystallise in a body-centred cubic (BCC) structure, including the transition metals Cr, Mo, W and Fe. ![]()
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