Metal nanoclusters exhibit unique properties which differ from their bulk materials, owing to the quantum size effects. However, nanoclusters tend to be fairly unstable in solution and thus special precautions have to be taken to avoid their aggregation or precipitation during the preparation of such nanoclusters in solution . In order to obtain stable nanoclusters dispersed in solution, a stabilizing agent is usually added into the reaction system. In the literature of colloidal stability and in Derjaguin-Landau-Verway-Overbeek (DLVO) theory, colloidal stabilization is well established to involve both: (i) electrostatic stabilization by the surface adsorbed anions such as chloride or citrate ions and (ii) steric stabilization by the presence of polymers such as the often used poly(vinylpyrrolidone). The use of polymeric matrix as stabilizer improves some properties of the nanoclusters such as the solubility, thermal stability and catalytic activity.

A variety of preparative methods is available for obtaining polymer-stabilized metal nanoclusters. The most widely used synthetic method involves reduction of the metal ion in solution to the colloidal metal in zerovalent state within the polymer medium, followed by coalescence of the polymer onto the nanoclusters formed.

In our group, we have developed many water soluble polymer stabilized transition metal(0) nanoclusters by using (i) the reduction of metal precursors in the presence of water soluble polymers by sodium borohydride in methonol after 1 h reflux (Figure 1a, 1b, Figure 1c and Figure 1d (ii) in-situ  reduction of metal precursors in the presence of water soluble polymers in the hydrolysis of ammonia borane (Figure 2a, 2b, 2c, 2d). Then, we used water soluble transition metal(0) nanoclusters as catalyst in hydrogen generation from the hydrolysis of sodium borohydrides and ammonia borane.

TEM images of poly(N-vinyl-2-pyrrolidone) stabilized

(a)   Ni(0) Nanoclusters (b) Co(0) Nanoclusters (c) Ru(0) Nanoclusters (d) Pd(0) Nanoclusters