Steric stabilization, metal colloid synthesis

A promising strategy towards stable and catalyticaUy active metal colloids is their preparation inside the core of micelles formed by amphiphilic block copolymers. This strategy offers a number of advantages (i) micelles represent a nano-structured environment which can be exactly tailored by block copolymer synthesis (ii) polymers act as effective steric stabilizer ]36] (iii) metal leaching might be avoided (iv) micelles allow control over particle size, size distribution and particle solubility [37] and (v) micelles are also supposed to effect catalytic activity and selectivity [38]. 

Controlled hydrolysis is one of the most popular methods for processing silica spheres in the range of 10-1,000 nm. The method was developed by Stober, Fink, and Bohn (SFB) [226-229] and is based on the hydrolysis of TEOS in a basic solution of water and alcohol. Particle size depends on the reactant concentration, i.e., the TEOS/alcohol ratio, water concentration, and pH (>7). This method has been extended to other metal oxide systems with similar success, particularly for Ti02 synthesis [85,230]. The hydrous oxide particles precipitated by the hydrolysis of an alkoxide compound have the same tendency to agglomerate as that described for metal colloid systems. Different stabilizers can be used to stabilize these particles and prevent coagulation (step 2). These stabilizers control coagulation by electrostatic repulsion or by steric effects [44], similarly to the metal colloid systems. 

Only a few studies concerning the synthesis of ruthenium particles have been published, most of them reporting the use of RuCh as metal precursor. For example, stable colloidal solutions of monodisperse Ru particles can be obtained by reduction of RuCh in polyols. The stabilization is then achieved by addition of poly (vinyl) pyrrolidone (PVP) (steric stabilization) or of sodium acetate (electrostatic stabilization) [165]. 

Electrostatic and steric stabilization are, in a sense, combined in the use of long-chain alkylammonium cations and surfactants, either in single-phase sols or in the reverse micelle synthesis of colloidal metals. 

The main role of stabilizers (surfactants or polymers) is to provide a steric or an electrostatic barrier between particles, thereby preventing inhibition of aggregation. Furthermore, stabilizers play an essential role in the control of both size and shape of nanoparticles. Generally, polymers are recommended as stabilizers for metal colloids due to their transparent, permeable, and nonconductive properties and also because they do not influence the optical, electrical, and catalytic properties of the nanoparticles. In addition, investigation of polymer-stabilized MNPs appears as a suitable way for solving the stability of MNPs. For this reason, great attention has been focused on the incorporation of MNPs into a polymer matrix, a procedure based on the synthesis of nanometer-sized metallic filler particles (Giannazzo et al. 2011). 

Recently, DHBCs have been used as a good stabilizer for the in-situ formation of various metal nanocolloids and semiconductor nanocrystals such as Pd, Pt [328-330], Au [280,328-330], Ag [331], CdS [332], and lanthanum hydroxide [333]. PAA-fe-PAM and PAA-fc-PHEA were used as stabihzer for the formation of hairy needle-Uke colloidal lanthanum hydroxide through the complexation of lanthanum ions in water and subsequent micelhzation and reaction [333]. The polyacrylate blocks induced the formation of starshaped micelles stabilized by the PAM or PHEA blocks. The size of the sterically stabilized colloids was controlled by simply adjusting the polymer-to-metal ratio, a very easy and versatile synthesis strategy for stable colloids in aqueous environment [333]. The concept of induced micelhzation of anionic DHBCs by cations was also apphed in a systematic study of the direct synthesis of highly stable metal hydrous oxide colloids of AP+, La +, Ni +, Zn ", Ca ", or Cu " via hydrolysis and inorganic polycondensation in the micelle core [334,335]. The AP+ colloids were characterized in detail by TEM [336], and the intermediate species in the hydrolysis process by SANS, DLS, and cryo-TEM [337]. 

---