Chapter 1: Introduction
1.7 Surface Functionalization of Metal Nanoparticles
The surface functionalization of metal nanoparticles is often required to provide stability as well as compatibility of the resulting particles towards different media. In the case of the use of metal nanoparticles as conductive fillers for high permittivity materials, surface functionalization is essential due to the increasing dielectric loss and electrical conductivity at fc where the insulator-conductor transition occurs thus diminishing the filler’s prospect of being used for the preparation of high performance materials in spite of their high permittivities.80,81,104,193,194 At filler contents approaching fc, the possibility of agglomerated particles forming a percolation pathways increases thus leading to electric shortcuts (Fig.
20).
Fig. 20: Agglomeration of conductive particles within the polymeric matrix leading to percolation pathways (left) and insulated particles without agglomeration (right).96
1. Introduction
In order to overcome these limitations and prevent electric shortcuts from occurring, the insulation of the particles prior to their dispersion into the matrix has become a common method to avoid electric breakdown due to agglomerated particles. For instance, Ni, Cu, and Al can undergo self-passivation in air which leads to the growth of an oxide shell on the surface.195–197 For noble metals, this process does not occur under ambient conditions due to their high reduction potential.198 Therefore, additional materials need to serve as surface coating. In the case of AuNPs and AgNPs, it is possible to coat them with silica (SiO2) with controllable shell thickness by using a modified Stöber method.170,199–203 Back in 1968, Stöber and coworkers reported the preparation of monodisperse SiO2 colloids with varying sizes through the hydrolysis of tetraethoxysilane (TEOS) which was catalyzed by ammonia.204 Silica colloids prepared with this method are depicted in Fig. 21
.
Fig. 21: Silica nanoparticles prepared with the Stöber method.204
Further research conducted over the years enabled this method to be extended to the coating of colloids and particles. For example, the main motivation to coat noble metal nanoparticles with SiO2, in particular Ag and Au, is the control over their plasmonic properties by stabilizing the dielectric environment of the metal core and by controlling the interparticle distance through the silica shell thickness.205–207 The SiO2 shell also allows the deposition of other molecular structures such as quantum dots, dyes and
1. Introduction
biomolecules.208–211 The silica-coating of the metal nanoparticles has initially been conducted in the presence of surface coupling agents such as 3-(aminopropyl)trimethoxysilane (APTMS) or 16-mercaptohexadecanoic acid (MHA).212–214 However, the coating process could be performed directly on particles prepared via the polyol synthesis which was investigated by Graf et al.215 Due to the alkaline conditions of the Stöber method, the enol tautomer of the PVP is predominant, and therefore the hydroxyl group serves as an anchoring group for the deposition of the hydrolyzed TEOS monomers which form the SiO2 shell (Fig. 22).199,216 This coating-procedure has been demonstrated for various particle morphologies, including nanowires and nanoprisms.214,217 Metal colloids prepared by the Turkevich method could also be directly coated since the citrate ligand also serves as an interfacial layer essential for the growth of the SiO2
shell.201,206,218 Various improvements in conducting the silica-coating have been reported throughout the years. The use of microwaves and the replacement of ammonia by other amine bases such as dimethylamine reduced the reaction time from hours to minutes and hindered the dissolution of Ag caused by the complexation with ammonia.218 Furthermore, the one-pot synthesis of Ag@SiO2 core-shell particles has been reported using micelle templates.213,219 The advantage of this method is the protection of the AgNPs from aggregation with the help of the micelles which provide stability, especially since particles below 50 nm are unstable and tend to agglomerate during the conventional Stöber method.204,220
Fig. 22: Transformation of the PVP molecule to the enol conformation under basic conditions.215
1. Introduction
Aside from silica, various other inorganic materials have also been used to coat metal colloids. The preparation of core-shell particles with titania (TiO2) has also been a hot topic in the research community due to the possibility of combining the properties of the two moieties. TiO2 is very attractive due to its chemical and thermal stability as well as its excellent electronic, optical and photocatalytic activity.221–223 Although numerous efforts on the preparation of Ag@TiO2 core-shell particles exist,151,202,224–227 the obtained TiO2
shells do not feature a well-defined uniformity and some of the reported synthesis routes are difficult to reproduce. While the Stöber method is used for the silica-coating, there are few existing reports on the preparation of TiO2 shells using this method.228 In one of the few reports on the titania-coating with the Stöber method, the TiO2 shells are produced through the hydrolysis and condensation of tetrabutyltitanate which was realized on the surface of Fe2O3 particles. In comparison to the silica-coating, the preparation of TiO2
shells is quite difficult as the reaction kinetics of the hydrolysis/condensation of the titania precursor has to be controlled carefully in order to obtain core-shell structures with uniform TiO2 shells.228
Other inorganic insulating shells which have also been demonstrated on metal cores include Al2O3, Fe3O4, Ni(OH)2, MnO, Eu2O3, CdS, ZnS, ZnO and ZrO2.112,226,229–231
While the insulation of metal nanoparticles has usually been achieved with inorganic materials, organic-based shell materials have also been successful in providing metal nanoparticles with an insulating layer. Shen and coworkers successfully insulated silver nanoparticles (AgNPs) with an organic shell and the produced Ag@C core-shell particles also featured a further advantage through the enhanced dispersibility in organic polymers.
As a result, the organic shell facilitated the dispersion of the particles in order to form a homogenous composite. In direct comparison, surface treatment is generally required in order to enhance the compatibility of the core-shell particles with the matrix which is usually hydrophobic (e.g. PDMS). The shells could be tuned from 4-6 nm up to 8-10 nm.
Qi and coworkers have used mercaptosuccinic acid (MSA) and dodecanoic acid (DDA) as a surfactant solution to prepare AgNPs which could be readily dispersed in epoxy in and
1. Introduction
form composites. Other routes to equip metal nanoparticles with a surrounding organic shell include the preparation of polymer-coated metal nanoparticles either through the grafting of the polymers onto the surface or by conducting a surface-initiated polymerization. The organic shell could be tuned from 2-50 nm although some of the reported polymer shells are hydrophilic in nature and therefore do not facilitate the dispersion in hydrophobic media.232–238 Further treatment such as the cross-linking of the polymer chains dangling around the particles can provide additional stability to the particles towards agglomeration, especially at high temperatures where polymer desorption from the surface can occur.239
1.8 Current State of Functionalized Metals/Polymer Nanocomposites