Palladium clusters in nanoporous carbon samples: Structural properties

The structure of nanoporous carbon samples (prepared from silicon and boron carbides) with incorporated palladium clusters was studied. X-ray and electron diffraction studies show that most Pd clusters have an fcc lattice. Using small-angle X-ray scattering measurements, the metal cluster sizes are determined under certain assumptions. The sizes are not very close to the cluster sizes (4–14 nm) that are found from photomicrographs obtained using a transmission electron microscope (TEM). The difference is likely due to the local character of the TEM measurements. Apart from relatively coarse clusters of the sizes indicated above, the samples contain very small clusters, which are smaller in size than a micropore. Such small clusters are particularly large in number in C(SiC)B: Pd, where they are 0.5–0.7 nm in size. The content of small clusters in C(B4C)B: Pd is substantially lower, and they are somewhat larger in size (1.2–1.6 nm). The possible reasons of the ferromagnetism observed in these samples are discussed. It is assumed that the magnetism may be due to the small clusters, which do not have cubic symmetry.