Nanoparticle shape, thermodynamics and kinetics

Abstract

Nanoparticles can be beautiful, as in stained glass windows, or they can be ugly as in wear and corrosion debris from implants. We estimate that there will be about 70,000 papers in 2015 with nanoparticles as a keyword, but only one in thirteen uses the nanoparticle shape as an additional keyword and research focus, and only one in two hundred has thermodynamics. Methods for synthesizing nanoparticles have exploded over the last decade, but our understanding of how and why they take their forms has not progressed as fast. This topical review attempts to take a critical snapshot of the current understanding, focusing more on methods to predict than a purely synthetic or descriptive approach. We look at models and themes which are largely independent of the exact synthetic method whether it is deposition, gas-phase condensation, solution based or hydrothermal synthesis. Elements are old dating back to the beginning of the 20th century-some of the pioneering models developed then are still relevant today. Others are newer, a merging of older concepts such as kinetic-Wulff constructions with methods to understand minimum energy shapes for particles with twins. Overall we find that while there are still many unknowns, the broad framework of understanding and predicting the structure of nanoparticles via diverse Wulff constructions, either thermodynamic, local minima or kinetic has been exceedingly successful. However, the field is still developing and there remain many unknowns and new avenues for research, a few of these being suggested towards the end of the review.

Single-particle mapping of nonequilibrium nanocrystal transformations

“…These data are consistent with the expectation that, because the rod tips are curved along two orthogonal dimensions while the sides are curved along one, the tips possess a lower areal ligand density and are therefore more chemically accessible and etch faster (fig. S14) (22)(23)(24); this interpretation is supported by Monte Carlo simulations of rod etching (fig. S15 and movie S6).…”

“…Whereas in the near-equilibrium case, atoms at the tips have an opportunity to relax (e.g., via surface diffusion) (fig. S16 and movie S7) (20,23), the persistent oxidative environment reacts with these species selectively, accelerating the reaction and driving the transient stabilization of the ellipsoidal intermediate. Etching reactions conducted on rods with synthetically blunted tips follow the same reaction trajectory (ellipsoidal intermediate), albeit with slower initial kinetics corresponding to selective removal of the blunt features (Fig.…”

“…4E). This mechanism, known as step-recession (29,30), is the reverse of the well-known step-flow growth process (31, 32) which occurs in many examples of crystal growth and serves as a foundation for continuum models of nanoparticle shape development (e.g., kinetic Wulff construction) and modern thin-film fabrication techniques (e.g., molecular beam epitaxy) (23,32).…”

On the disclination nature of pentagonal pyramids with high growth steps of electrolytic origin

“…Такие пентагональные объекты выращены на основе не только чистых металлов, но и различных соединений [1]. В частности, в работах [2 -5] получены пентагональные кристаллы меди [2], серебра [3], ни-келя [4] и других металлов [5]. В ряде работ [1,4,5] та-кие микрообъекты скорее являются исключением, чем правилом.…”

“…В частности, в работах [2 -5] получены пентагональные кристаллы меди [2], серебра [3], ни-келя [4] и других металлов [5]. В ряде работ [1,4,5] та-кие микрообъекты скорее являются исключением, чем правилом. В них авторы наблюдают одиночные кри-сталлы, которые образуются в узком интервале техно-логических параметров.…”