Why the size of copper nanoparticles depends on the nature of the reducing agent in the preparation of sols in a cationic polyelectrolyte solution

The influence of different borane reducing agents (hydrazine borane and sodium borohydride) on the size of copper nanoparticles in sols prepared at 20°C through the reduction of Cu(II) ions in diluted aqueous solutions of high-molecular-mass poly(1,2-dimethyl-5-vinylpyridinium methyl sulfate) is studied. When hydrazine borane is used as a reducing agent, the nanoparticle size (3–14 nm in diameter) is independent of the solution pH. In the case of sodium borohydride, the transition from a highly acidic (pH 2.0) solution to solution with pH 4.5–8.5 during sol synthesis leads to a shift in the size distribution of nanoparticles toward larger diameters (from 3–14 to 10–20 nm). During the reduction of Cu(II) ions with sodium borohydride, the polymer-analogous reaction, which includes the reduction of pyridine rings of the polymer, proceeds simultaneously with the main process. The as-modified polymer is a weak polybase and, therefore, is protonated to a high extent via nitrogen atoms only in a strongly acidic solution. As pH is increased in the range 4.5–8.5, the positive charge of chains is decreased. The relationship between the nanoparticle size and variation in the positive charge of macromolecules is discussed in terms of the concept of the pseudomatrix synthesis of polymer-metal nanocomposite sols.