Article pubs.acs.org/JPCC
Equilibrium Morphology of Plasmonic Au/Polystyrene Dimeric Nanoparticle Myung-Seok Yang,†,§ Sunil Jeong,‡,§ Taewook Kang,*,‡ and Dongchoul Kim*,† †
Department of Mechanical Engineering and ‡Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, 121-742, Korea S Supporting Information *
ABSTRACT: Growth of a metal on nanoparticles has been considered to be a useful synthetic tool in a wide variety of applications ranging from catalysis to nanomedicine. This technique can combine more than two functionalities into a single nanoparticle. Most of the methods for such growth rely on a trial-and-error approach to produce grown nanoparticles with the desired sizes and shapes, which is rather time consuming and difficult to reproduce. Here we systematically studied the equilibrium morphology of metal/dielectric dimeric nanoparticle. A computational model was developed by considering the diffusion and surface energy of a metal and the interface energy between the metal and a dielectric. As a proof-of-concept, the growth of Au on a dimeric nanoparticle consisting of Au and polystyrene (PS) was considered. The effects of the surface and interface energy, the concentration of Au ion over the course of the growth, and the size of PS on the shape (i.e., morphology) of the grown nanoparticle were examined. Interestingly, the effects of the surface and interface energy of Au on its coverage of PS are found to be relatively negligible compared to the other two factors. A diagram for the equilibrium morphology with respect to the concentration of Au ion and the size of PS is proposed, which is qualitatively consistent with the experiment.
1. INTRODUCTION Growth of a metal on a wide variety of nanoparticles enables the production of novel nanoparticles with superior physical and chemical properties. This is a result of the properties of the metal often being complementary to the functionality of the nanoparticle prior to growth. Hence, the growth reaction has been widely used in applications ranging from catalysis to nanomedicine.1−3 For example, the surface microstructure of catalysts can be altered, which significantly improves their activity, selectivity, and long-term stability. In addition, more than two functionalities, such as optical and magnetic or optical and chemical properties, can be integrated into a single particle, which has wide-reaching implications for biomedical applications ranging from diagnostics to the treatment of diseases. Until recently, such growth reaction has been performed either on two-dimensional substrates by using nanosphere lithography4,5 or in a solution.6,7 Many factors such as nanoparticle’s geometry and surface property and the experimental conditions were roughly estimated to have an influence on the final morphology which dictates the optical properties of the resulting nanoparticle. However, owing to an absence of design criteria that aid in the rational synthesis of the grown nanoparticles with desired sizes and shapes, most methods for this purpose rely on a trial-and-error approach, which is rather time consuming and difficult to reproduce. Here we report a quantitative analysis of the equilibrium morphology of plasmonic Au/polystyrene (Au/PS) dimeric nanoparticle after growth of Au. To predict the equilibrium morphology, a © XXXX American Chemical Society
computational model was developed by considering the diffusion and surface energy of Au and the interface energy between Au and PS.
2. MODEL Figure 1 is a schematic representation of the possible equilibrium morphology when Au was grown on Au/dielectric dimeric nanoparticle. Three density field variables, c1(x, y, z, t), c2(x, y, z, t), and c3(x, y, z, t), are defined as cubic shapes consisting of a 2 nm axis and are time dependent and spatially continuous. c1, c2, and c3 are defined as the volume fractions of Au, the dielectric, and the medium solution, respectively. To predict the morphological evolution under growth conditions, we considered the reduction of Au ions on the Au surface of the particle and the diffusive flux of reduced Au atoms along the interface. Note that surface charge and crystallinity of Au are not considered in our simulation. First, the diffusion of the Au atoms is denoted by ∇·J1. The flux of Au atoms induced by the chemical potential is represented by J1 = − M1∇μ1. M1 is the mobility of Au, which is defined as a function of c1 and a material constant, M0, that is proportional to the diffusivity of Au. Thus, the mobility is represented by M1(c1) = M0c13(6c12 − 15c1 + 10). The chemical potential, μ1, is μ1 = ∂F/∂c1. F, which Received: September 19, 2014 Revised: December 29, 2014
A
DOI: 10.1021/jp509508s J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
consisting of Au with a 50 nm diameter and a dielectric with a 60 nm diameter was prepared. The increase in m by unity corresponds to a 10-fold volumetric increase of Au (VF/VI) judged by TEM images.
3. RESULTS AND DISCUSSION As shown in Figure 2a, changes in Au coverage of a dielectric are investigated by varying α and m. The equilibrium
Figure 2. Effect of the ratio of the Au/polystyrene (Au/PS) interface energy to Au surface energy on the Au coverage after growth of Au. Au coverage refers to the ratio of the Au-covered area to the total surface area of the PS. (a) Changes in the percent coverage of Au (y axis). m is changed from 1 to 2 and 4, as depicted by squares, circles, and triangles, respectively. (b) Snapshots of time-dependent structural evolution of Au/PS nanoparticle. (c) Time-dependent structural evolution and changes in the Au coverage, when m is 2 and α is 0.95.
Figure 1. (a) Conceptual illustration of representative morphologies of a dimeric nanoparticle consisting of Au and a dielectric under the growth of Au. (b) Proposed model to predict the equilibrium morphology after growth of Au.
morphology is transformed from nanocap (
