On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals

Abstract

Controlling the shape or morphology of metal nanocrystals is central to the realization of their many applications in catalysis, plasmonics, and electronics. In one of the approaches, the metal nanocrystals are grown from seeds of certain crystallinity through the addition of atomic species. In this case, manipulating the rates at which the atomic species are added onto different crystallographic planes of a seed has been actively explored to control the growth pattern of a seed and thereby the shape or morphology taken by the final product. Upon deposition, however, the adsorbed atoms (adatoms) may not stay at the same sites where the depositions occur. Instead, they can migrate to other sites on the seed owing to the involvement of surface diffusion, and this could lead to unexpected deviations from a desired growth pathway. Herein, we demonstrated that the growth pathway of a seed is indeed determined by the ratio between the rates for atom deposition and surface diffusion. Our result suggests that surface diffusion needs to be taken into account when controlling the shape or morphology of metal nanocrystals. seeded growth | shape control | noble metals S urface diffusion is a general process that involves the motion of adsorbed atoms (adatoms), molecules, or atomic clusters on the surface of a solid material (1, 2). Over the past decades, it has emerged as an important concept in many areas of surface science, including catalysis, epitaxial growth, and electromigration of voids (3-7). Here, we demonstrated that surface diffusion also plays a pivotal role in determining the growth pathway of a seed and thus the shape or morphology taken by the final product in a solutionphase synthesis of metal nanocrystals. Fig. 1 schematically illustrates four possible pathways for the growth of a cubic seed. As a model system, we focused on Pd nanocubes with slight truncation at corners and edges, together with six side faces passivated by chemisorbed Br -ions. In the following discussion, we refer to them as "Pd cubic seeds" for simplicity. We chose them as seeds for two major reasons: (i) they had a well-defined shape, together with a set of low-index facets on the surface (8, 9); and (ii) their side faces are blocked by Br -ions to ensure selective deposition of atoms onto the corner sites during seed-mediated growth (10-12). These two distinctive features allowed us to easily track the deposition of atoms and their surface diffusion during a growth process by analyzing the shape or morphology of the final product.The newly formed Pd atoms resulting from the reduction of a Pd precursor are expected to deposit at the corners of a cubic seed because the side faces are blocked by the chemisorbed Br -ions (Fig. 1A, 1). Upon deposition, there will be two different options for these adatoms: staying at the corner sites or migrating to other sites, including edges and side faces, through surface diffusion (Fig. 1A, 2 and 3). It should be pointed out that only surface diffusion was allowed here to move atoms from corners to edg...

25th Anniversary Article: Galvanic Replacement: A Simple and Versatile Route to Hollow Nanostructures with Tunable and Well‐Controlled Properties

“…where D is the interdiffusion coeffi cient; C is the atomic concentration of metal A as a function of time ( t ) and distance ( x ) from the A -B interface; and C s is initial concentration of metal A at the A-B interface ( x = 0), which is in direct proportion to the number of A atoms deposited on metal B ; D is strongly dependent on temperature and the energy barrier to diffusion, [ 76,78 ] and can be described by the Arrhenius equation:…”

“…here D 0 is the pre-exponential factor; E diff is the energy barrier to diffusion, which is determined by a set of parameters such as bonding energy and lattice mismatch between the two metals; [ 76 ] T is the absolute temperature; and R is the ideal gas constant. Clearly, D has a strong dependence on the temperature at which the galvanic replacement reaction is conducted.…”

“…The impacts of other capping agents on different metals and their facets, including citrate for Ag{111} and hexadecylamine for Cu{100}, [ 66,71 ] are worthy of a thorough investigation in the future. In addition to the capping agents, other factors such as reaction kinetics, [ 72 ] the presence of twin defects, [73][74][75] and involvement of surface diffusion [ 76 ] can all affect the facet selectivity of galvanic replacement and thus deserve careful study. in the galvanic replacement or by utilizing a conventional wet etchant.…”

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“…2A), rather than after the formation of perfect Pt overlayers. The deposition was conducted at a relatively high temperature so as to give Pt atoms an opportunity to spread across the Pd surface and thus grow in a layer-by-layer fashion (18). Using DFT, we obtained an energy barrier of 0.99 eV for the diffusion of Pt adatoms across a Pd(100) surface (a "hopping" mechanism).…”

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