Coalescence of Au Nanoparticles without Ligand Detachment

Pan Guo and Yi Gao

Phys. Rev. Lett. 124, 066101 – Published 11 February 2020

Abstract

Repulsion of ligands is known as the key factor for hindering nanoparticle (NP) coalescence. Thus, during the past decade, it has generally accepted that the full removal of capping ligands of the contact surface is the first step for NP coalescence. Herein, using molecular dynamics simulations, we have identified a new mechanism for the coalescence of $S {({CH}_{2})}_{n} COOH$ -coated Au NPs in water without ligand detachment. In contrast to the traditional mechanism, the aggregation of the NPs is induced by the twined hydrophobic chains of the ligands rather than the hydrophilic carboxyl tails as believed previously. Next, the exposed surface atoms attach to form the neck, and extend with the atomic rearrangement of the contact interface to merge the NPs, which do not need the removal of ligands as expected from traditional supposition. This finding refreshes the understanding of the atomic mechanism of the coalescence of NPs, which paves the way for the rational design and synthesis of NPs.

Received 4 April 2019
Revised 7 November 2019
Accepted 15 January 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.066101

Physics Subject Headings (PhySH)

Growth Nanocrystals Structural properties

Condensed Matter, Materials & Applied PhysicsParticles & FieldsAtomic, Molecular & OpticalGeneral Physics

Authors & Affiliations

Pan Guo¹ and Yi Gao ^1,2,*

¹Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
²Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China

^*gaoyi@zjlab.org.cn

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 124, Iss. 6 — 14 February 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Snapshots of the coalescence trajectory of two ${Au}_{147} {({SCH}_{2} COOH)}_{27}$ ( $Σ = 4.1 {nm}^{- 2}$ ) in water. The original NPs are shown in yellow with Au atoms in the final contact interface as orange spheres; among them, the ligand-coated Au atom being expulsed from the contact area as colored spheres and the resulting new surfaces formed by Au atoms from the same NP in pink. The contact area of two NPs is shown as blue lines and green surfaces. (S atom, yellow; Au-S bond, pink; ligands, black; water formed hydrogen bonds (H-bonded) with ligands, red and white; water not H-bonded with ligands are omitted for clear views). (b) The interaction energy between NPs ( $E_{tot}$ , black), including the interaction from ligands ( $E_{l l}$ , blue), Au atoms ( $E_{a a}$ , brown), and between ligands and Au atoms ( $E_{l a}$ , magenta), together with the separation distance between the center of mass of the two ${Au}_{147}$ ( $d$ , green, right green axis) during the coalescence process. Blue vertical regions indicate the time of coalescence, including neck formation and growth. Black, red, and green shaded areas indicate the states before and after coalescence. (c) Free-energy profile of the coalescence process (PMF, red) in comparison with the interaction energy between NPs. The free-energy profile is defined to zero at $d = 3.5 nm$ . $E_{l l}$ is further divided into the van der Waals portion $E_{l l -vdw}$ (green) and the electrostatic portion $E_{l l -coul}$ (purple) as shown in the inset. (d) Number of ligand H-bonded with water molecules (blue), with ligands from the same NP (black), and from the other NP (red) as a function of $d$ .
Reuse & Permissions
Figure 2
(a) Scheme for ligands twining the surface of not fully coated NPs. (b) The effective radius of $S {({CH}_{2})}_{n} COOH$ coated ${Au}_{147}$ in water ( $R_{NP}$ , black) as a function of $n$ , the radii of ${Au}_{147}$ ( $R_{Au 147}$ , red) are also shown as a reference. (c) The coverage of ligands ( $S_{ligands} / S_{NP}$ , red) and the exposure of surface atoms ( $S_{Au_exposed} / S_{Au 147}$ , blue) on NP in water as a function of $n$ . $S_{ligands} / S_{NP}$ is further divided in to the COOH portion ( $S_{COOH} / S_{NP}$ ) and the alkyl chains portion ( $S_{alkyl_chian} / S_{NP}$ ).
Reuse & Permissions
Figure 3
(a) Snapshots of the rearrangement of the ligand-coated Au atoms during the coalescence process. The expulsed ligands are shown in the same color as the coated Au atoms, other colors used are the same as Fig. 1. (b) Enlarged coalescence region in Fig. 1. (c) Angles between orientations of different NPs. The value of angle changes as the relative rotation of NPs occurs. The orientations of NPs are defined as two vectors pointing from the center of mass of ${Au}_{147}$ to two adjacent vertices of ${Au}_{147}$ .
Reuse & Permissions
Figure 4
Scheme for the coalescence mechanism of Au NPs in water mediated by atomic rearrangement of surface atoms without ligand detachment.
Reuse & Permissions

Physical Review Letters