Effect Of Preparation Of Au/Ag Multilayer Via Layer-by-Layer Self-assembly Based On Spherical Polyelectrolyte Brushes And Its Catalytic Activity

Due to high surface-to-volume ratio, metal nanoparticles (NPs) with high catalytic activity were applied in water pollution treatment, photo-catalysis and photoelectric systems. However, with the decrease diameter of metal NPs, its stability became worse which restricted NPs’ application. Polyelectrolytes were deposited onto spherical polyelectrolyte brushes (SPBs) via layer-by-layer self-assembly. The size distribution, dispersant and stability of Au/Ag multilayer were tuned by pH value, temperature and others reaction conditions. We studied the catalytic activity of the reduction reaction of 4-nitrophenol as the catalyst of Au/Ag bilayer NPs.In this paper, SPBs consisting of polystyrene (PS) core and poly(2-aminoethyl methacrylate hydrochloride) (PAEMH) shell were prepared by photo-emulsion polymerization. Au NPs with controlled size and size distribution were synthesized in-situ using SPBs as nanoreactors. Via layer-by-layer deposition technique on the surface of SPBs, nano-composite particles with Au/Ag-NPs bilayer and Au/Ag/Au-NPs trilayer were prepared. Microstructure of layer-by-layer structure and the content of Au and Ag NPs in every step of preparation of Au/Ag/Au trilayer can be identified by Zeta potential, UV-vis, TEM, ICP-AES and DLS.There can be different distribution order of metal NPs. We also described another synthesis of Ag/Au/Ag trilayer NPs. Firstly, Ag NPs were prepared onto anionic poly(acrylic acid) (PAA) chains. PAH-Au and PAsp-Ag were alternately deposited onto anionic spherical polyelectrolyte brushes (SPB-Ag) via layer-by-layer self-assembly.At the end, the catalytic activity of Au/Ag NPs, versus Au or Ag NPs, in the reduction reaction of 4-nitrophenol was monitored by UV-vis.The main conclusion in this paper is summarized as follows:(1) Polyelectrolytes PAH-Au and PAsp-Ag were alternately deposited onto countra-ion spherical substrate of SPBs by layer-by-layer self-assembly to prepare Au/Ag multilayer.(2) Au NPs with diameter of 1.4±0.5 nm and Ag NPs with diameter of 3.0±0.9 nm were synthesized with controlled size and size distribution. The content of Au and Ag NPs in Au/Ag/Au trilayer NPs were respectively 50 mg/g and 33 mg/g. Au/Ag/Au trilayer NPs were kept stably for few months.(3) The catalytic activity of the reduction reaction in presence of Au/Ag bilayer was compared with Au NPs and Ag NPs. It could be concluded that the apparent activation energy of the reduction reaction as the catalyst of Au/Ag bilayer nanoparticles is 41.2 KJ/mol, which was between Au (Ea=23.34 KJ/mol) and Ag (Ea=59.5 KJ/mol). The rate constant K1 (normalized to the surface of particles) of catalytic reaction in presence of Au/Ag bilayer is 1.7×10-2 s-1 m-2 L, which was also between Au NPs (K,=2.1×10-2 s-1 m-2 L) and Ag NPs (K,=0.9×10-2 s-1 m-2 L). In conclusion, the Au/Ag-NPs bilayer nano-composite particles showed relatively high catalytic efficiency.