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 or these and the x, y, z directions refer to the parent crystal, on, for, 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. About one third of metallic elements crystallise in a body-centred cubic (BCC) structure, including the transition metals Cr, Mo, W and Fe. We conclude by outlining how nanoparticles can be characterized to conclusively prove the presence or absence of twinning. BCC single crystal and twinned shapes often appear similar and diffraction patterns along common, low-index zone axes are often indistinguishable, casting doubt on many claims of single crystallinity. Here, we explore the potential shapes of twinned BCC nanoparticles, and predict their electron microscopy and diffraction signatures. Twinning has a marked effect on catalytic activity, yet there is little evidence for or against the presence of twinning in BCC nanoparticles. Many metals and alloys, including Fe and W, adopt body-centred cubic (BCC) crystal structures and nanoparticles of these metals are gaining significant scientific and industrial relevance.

Find a plane in crystalmaker