Structure and Magnetization of Small Monodisperse Platinum Clusters

Magnetization measurements of well-characterized monodisperse Pt clusters consisting of $13\pm 2$ atoms in a zeolite confirm the predicted extraordinary magnetic polarization with up to 8 unpaired electrons on a cluster, corresponding to a magnetic moment of $0.65(5){\mu }_B$ per atom. The effect is partly quenched by hydrogen chemisorption. The study provides insight into the electronic structure of the cluster and is fundamental for an understanding of how magnetism develops in small clusters.

Figure 1

(a) Comparison of the $k^2$-weighted $\chi (k)$ functions of the ${\mathrm{H}}_2$-desorbed (solid line) samples in comparison with bulk Pt (dashed line). Background correction was carried out using the AUTOBK program package [10]. As a result of the very small particle size, the oscillatory fine structure of the nanoparticles is completely different from that of bulk Pt; (b) spectra of ${\mathrm{H}}_2$-adsorbed (dotted line) and ${\mathrm{H}}_2$-desorbed (solid line) samples in detailed comparison. From the identity of the two spectra it can be concluded that no structural changes are induced by hydrogen adsorption.

Figure 2

(a) Fourier transforms of the ${\mathrm{H}}_2$-adsorbed (dotted line) and ${\mathrm{H}}_2$-desorbed (solid line) samples in comparison with bulk Pt (dashed line). The intensity loss and distance contraction of the Pt shell at 2.6 Å due to extremely small cluster size is obvious. At 2.0 Å the oxygen shell is observed; (b) shows the enlarged spectra of the samples containing nanoclusters.

Figure 3

(a) Inverse magnetic susceptibility (magnetization divided by field) per mol of of Pt atoms on NaY in the range of 1.8–120 K in an external field of 0.2 T before (I) and after hydrogen desorption (II); (b) Molar magnetization at 1.8 K of $\mathrm{Pt}/\mathrm{NaY}$ before and after hydrogen desorption, compared with literature data for analogous measurements of Pt nanoparticles embedded in a poly-$N$-vinyl-2-pyrrolidone (PVP) polymer [5]. The particles have diameters of 2.3 nm (ca. 420 atoms, III), 3.0 nm (940 atoms, IV) and 3.8 nm (V, 1900 atoms).