Nanoclusters size stabilization

The average size and size distribution of the microgel-stabilized metal nanoclusters were investigated by TEM. Some representative TEM micrographs are reported in Figure 2. The numerical results are displayed in Figures 3-5 (the vertical bars show the standard deviations in the average metal nanocluster size). 

To the best of our knowledge, such a control of nanocluster size through the nanomorphology of the stabilizer cannot be achieved with any other system, not even with those in which the metal nanocluster precursors are physically confined within nanometer-sized cavities [23]. 

In contrast, Zuo et al. prepared highly efficient Pt nanocluster catalysts stabilized with poljwinylpyrrohdone pol5mier (PVP) modified with Cnd and noted that achievement of high ee s in the formation of MeLaPd requires the existence of very small erystallite sizes. On finely dispersed catalysts MePy and EtPy were hydrogenated into (/ )-laetates with ee s of 97.6% and 92.2%, respectively. Zuo et al.demonstrated that the reaction is structure insensitive on clusters of about 1.4 nm, whieh is quite different from conventional supported chiral catalysts. 

Figure 1. TEM images and size distributions of Pt, Rh and Ru nanoclusters stabilized by EG and simple ions [11] (a) Pt nanoclusters (0.37g/1) (b) Pt nanoclusters (3.7g/1) (c) Rh nanoclusters (0.31 g/1) (d) Ru nanoclusters (0.32g/1). (Reprinted from Ref [11], 2000, with permission from American Chemical Society.)...

Figure 3. TEM image and size distribution of Pt/Ru bimetallic nanoclusters (1.85g/l) stabilized by EG and simple ions [13].

The emphases of future investigation on these unprotected metal nanoclusters should be mainly placed on (1) further controlling the size, composition and shape of the unprotected metal or alloy nanoclusters (2) better understanding the stabilizing mechanism of the unprotected metal nanoclusters in colloidal solutions prepared by the alkaline EG synthesis method (3) developing novel catalytic and other functional systems for real applications. 

Figure 3. Dependence of the size of microgel-stabilized Pd nanoclusters from the crosslinking degree of the microgel and from the nature of the nonfunctional comonomer.

In addition to Au and noble metals, Ni-Zn nanoclusters with an amorphous structure were successfully deposited on Ti02 nanoclusters. The state of Ni was metallic. The catalytic activity of Ni-Zn/Ti02 in olefin hydrogenation was ca. 10 times higher than unsupported Ni nanoclusters. Selective deposition onto Ti02 and the addition of Zn seemed to play an important role to stabilize Ni nanoclusters and to decrease the size of Ni nanoclusters, respectively. Also, clearly Zn promoted the hydrogenation activity of Ni and inhibit the growth of the size, but did not substantially affect Ni nature itself... 

---