Article pubs.acs.org/JPCB
Ag Nanocrystals: 1. Effect of Ligands on Plasmonic Properties Jingjing Wei,†,‡ Nicolas Schaeffer,†,‡ and Marie-Paule Pileni*,†,‡,§ †
Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, F-75005, Paris, France CNRS, UMR 8233, MONARIS, F-75005, Paris, France § CEA/IRAMIS, CEA Saclay, 91191, Gif-sur-Yvette, France ‡
S Supporting Information *
ABSTRACT: Silver nanocrystals (NCs) stabilized using amine-terminated coating agents (oleylamine or dodecylamine), their size ranging between 2 and 12 nm in diameter, are synthesized by hot injection methods. Their dispersion in size is relatively low (typically below 10%) without the need for a postsynthesis size segregation process. The amineterminated coating agents are replaced by thiol-terminated molecules (dodecanethiol or hexadecanethiol) by ligand exchange, allowing the formation of alkanethiol coated Ag colloids. All NCs with various surface coatings are dispersed in toluene. Regardless of the nature of the coating agent, the surface plasmon resonance (SPR) is red-shifted with decreasing the NC size. For a given size, the SPR peak of thiol-stabilized NCs is shifted to lower energies compared to that of amine-stabilized NCs. Furthermore, with thiol-stabilized Ag NCs, the position of the SPR peak was found to be sensitive to the length of the alkyl chains of the coating agent, whereas minor differences are detected for Ag NCs coated with amines terminated with differing alkyl chain lengths.
1. INTRODUCTION Metallic nanocrystals (NCs) stabilized by a monolayer of organic coating agent have attracted growing interest during the last few decades due to their size- and shape-dependent physical properties, making them potential candidates for the design of new functional materials.1−3 The main role of the organic layer anchored to the metallic core is to avoid coalescence between the NCs in solution and to counterbalance the van der Waals attractive forces. This layer also governs the physical properties of the NCs surface.1 The coating agent is usually a relatively large organic molecule or polymer bearing one or more anchoring groups for bonding onto the surface of the NCs.4,5 Functional groups such as thiols, amines, or silanes are routinely used to prepare stable metallic NCs, and a handful of efficient synthetic procedures have been developed for the production of NCs. In some cases, the NC size and shape can be controlled through changes in the nature or amount of coating agent. Thus, optimization of the NCs properties in view of specific applications requires careful considerations when choosing the nature of the coating agent.6 The optical properties of noble-metal NCs have fascinated scientists because they exhibit a strong surface plasmon resonance (SPR) effect that is characteristic of the size regime at the nanoscale, hence making them potentially interesting units for designing optical devices,7 or for optical energy transport applications,8,9 and surface-enhanced Raman scattering (SERS) spectroscopy.10−14 This SPR results from the collective resonance oscillation of conduction electrons upon light irradiation.15−17 As reported previously, the oscillation © 2014 American Chemical Society
frequency is dependent on several factors, such as the density of electrons and the shape and size of the charge distribution.16,18,19 Thus, in order to fully understand the optical properties of noble-metal NCs, different parameters must be considered: the type of material, the size and shape of the NCs, the dielectric properties of the solvent, the nature of the stabilizing agent, and the possible interparticle coupling interactions.20−23 For example, extinction spectra of gold nanorods exhibit two plasmon bands that correspond to longitudinal and transverse SPR, respectively.21,24 In the case of spherical NCs, size-dependent surface plasmon resonances have been studied; Peng et al.25 reported a gradual blue-shift of the SPR absorption band when decreasing Ag NCs size down to 12 nm, followed by a strong red-shift when decreasing the size further. In the case of Ag NCs, various reports describe their size- or shape-dependent optical properties. However, only a handful of investigations have included an in-depth study of the influence of the coating agent on these optical properties.26−29 Here, we report the synthesis of Ag NCs of various sizes using different coating agents, with all the specimens being characterized by a low size distribution (70%), 1,2-dodecanediol (>99%), 1-dodecanethiol (>98%) and 1-hexadecanethiol (>95%) from Sigma-Aldrich. All reagents were used as received without further purification. In a typical synthesis of 6 nm Ag NCs coated with oleylamine, 0.1 g of AgNO3 was dissolved in 5 mL of DCB 14071
dx.doi.org/10.1021/jp5050699 | J. Phys. Chem. B 2014, 118, 14070−14075
The Journal of Physical Chemistry B
Article
3. RESULTS AND DISCUSSION The synthetic procedure for the preparation of Ag NCs coated with oleylamine (C18-NH2) and dodecylamine (C12-NH2) is based on a hot-injection method. Briefly, an AgNO3 odichlorobenzene (DCB) solution mixed with oleylamine or dodecylamine was rapidly injected into a hot dodecanediol DCB solution (180 °C) and the mixture was allowed to react for 3 min; the solution was then cooled down to room temperature. With oleylamine (C18-NH2), the size of the Ag NCs produced by this procedure can be controlled from 2.2 to 7 nm by controlling the relative ratio of AgNO3 and oleylamine (Table S1). For larger NC size (8.7 and 12.9 nm), oleylamine coated NCs were prepared following a modified one-pot method as described elsewhere.30 Hence, this procedure permits control of the NC size from 2.2 to 12.9 nm. The TEM images in Figure 1 clearly show that all the NCs, regardless of their size, are well arranged in a two-dimensional hexagonal closed packed array; this is typical of NCs with low size distribution (typically below 10%).31 Note that these NCs were obtained without any postsynthesis size selection process or size-focusing ripening process. This is, to the best of our knowledge, a new platform of size-controllable synthesis of Ag NCs without a size-focusing process.32 High-resolution TEM images of the NCs reveal that all NCs are multiply twined particles (MTPs), probably holding icosahedral morphologies, and the same icosahedral crystals were reported by Peng et al.25 When the oleylamine (C18-NH2) is replaced by dodecylamine (C12-NH2), which is a similar -NH2-terminated coating agent with a shorter alkyl chain, stable monodisperse Ag NCs coated by dodecylamine are produced, as shown in Figures 2a and S2. In this case, the size of the Ag NCs can be controlled from 3.0 to 6.3 nm. Again, the size distribution of these dodecylamine coated NCs is kept below 10% without the need for size segregation procedures. This hot injection process cannot be used for the preparation of alkanethiol coated Ag NCs, since AgNO3 cannot be dissolved in DCB in the presence of -SH-terminated coating agent. When a mixture of AgNO3 and dodecanethiol is injected into hot solutions containing polyols, only polydisperse small NCs are produced. Hence, in order to obtain Ag NCs of similar sizes and coated with alkanethiol, a ligand exchange procedure is carried out. Thiol coated Ag NCs are prepared from the preformed oleylamine coated NCs described above and a ligand exchange procedure. A small amount of dodecanethiol (20 μL of dodecanethiol per 10 mg of Ag NCs) is added to the suspension of Ag NCs, and the mixture is kept under vigorous stirring for 1 h. Then, ethanol is added to the solution, inducing flocculation. The excess dodecanethiol is removed from the supernatant, and the NCs are redispersed in toluene. The sizes and size distributions of the various NCs prepared throughout this study are listed in Table S1 (Supporting Information). The efficiency of the ligand exchange procedure was assessed by tracking by ICP-AES the relative amounts of Ag, N, and S in the preparation before and after exchanging the ligands. The results shown in Table 1 show that only traces of N could be detected after this procedure, inferring than at least 80% of the amine-terminated ligand was exchanged for the thiol containing compound. Figures 2b and S3 show TEM images of the Ag NCs coated with dodecanethiol, and their size can be controlled from 2.2 to 6.0 nm. Figure 2c and Figure S4 (Supporting Information) show TEM images of Ag NCs coated with hexadecanethiol prepared in a similar manner. It is worth
Table 1. 6 nm Ag Nanocrystals Coated with Oleylamine Were Analyzed by ICP-AES (Inductively Coupled Plasma− Atomic Emission Spectroscopy) before and after Ligand Exchange
a
elements
N (w/w)
S (w/w)
Ag (w/w)
Before ligand exchange After ligand exchange
0.44%
