Characterization of bimetallic
P
d
A
g nanoparticle arrays by the diblock copolymer micelle approach
Authors: Ehret Eric , Beyou Emmanuel , Mamontov Grigory V , Bugrova Tatiana A. , Domenichini Bruno , Prakash Swamy , Aouine Mimoun , José Cadete Santos Aires Francisco 
Published: December 2016

Bimetallic nanoparticles (NPs) display unique properties drastically different from those of the corresponding single‐component particles. These properties are assumed to result from both the electronic and structural effects of the bimetallic NP. As these properties depend also on the preparation conditions, the synthesis of bimetallic NPs with accurately controlled structures and compositions is essential to obtaining advanced materials for electronic, magnetic, optic and catalytic properties. In the present study, 2D ordered arrays of bimetallic PdAg NPs were successfully synthesized via the copolymer micelle approach and characterized by various spectroscopic and microscopic characterization methods. A special focus was laid on the influence of the type of reduction treatments on the chemical nature and the stability of the PdAg NPs. A comparison with the synthesis of single metal (Pd and Ag) NPs obtained by the same method was made.

A series of polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymers of various compositions and molecular weights was synthesized by nitroxide mediated radical polymerization using an alkoxyamine unimolecular initiator (Styryl‐SG1). Intermolecular interactions between the PdCl2 and/or AgNO3 salts and P4VP micellar core and formation of nanoparticles from the micellar complex were investigated by various spectroscopic (UV‐vis, XPS) and microscopic (AFM, HRTEM/EDX) characterization methods.

Once the base solutions were made, three ways to produce the final arrays of bimetallic NPs were investigated (Figure 1). One consisted in using an oxygen‐plasma treatment after dip‐coating of the SiO2 surface (Figure 1a); this method uses the large amount of electrons generated by the plasma to reduce the metallic cations and lead to the formation of the NPs that eventually oxidize under the oxygen‐plasma. A second method consisted in introducing a reducing agent (hydrazine) to the base solution (Figure 1b‐path1). The third method consisted on flowing a vapour of the reducing agent (hydrazine) over the film after dip‐coating of the SiO2 surface (Figure 1b‐path2). Spectroscopic and microscopic characterization of the resulting films before and after reduction showed that the two first methods consistently lead to the formation of rather regular arrays of NPs. In the case of the films before reduction we observe that, for the three methods, the metallic loaded micelles preferentially arrange to form a quasi‐hexagonal pattern on the carbon coated copper grid; a close observation of the P4VP cores revealed a fine grain substructure inside every micelle corresponding to ultrasmall (bi)metallic NPs (Figure 2e). After reduction either by oxygen‐plasma or by hydrazine in the solution (Figure 2) we obtain rather organized arrays of bimetallic NPs. The NPs have uniform sizes (Figure 2d‐insert) and compositions. Unlike the NPs obtained by oxygen plasma, those obtained by hydrazine in the solution are not oxidized. A full characterization of the physicochemical properties of the NPs was enabled by the use of different methods (ICP, AFM, HRTEM/EDX, XPS, UV‐Vis).

The copolymer micelle approach is an excellent method to obtain ordered arrays of bimetallic nanoparticles supported on flat surfaces with controlled sizes, spacing and compositions [1]. These collections of NPs can thus be used as model catalysts (for COV abatement in the case of PdAg) where important parameters that influence their catalytic behaviour can be finely monitored and modulated.[2]