Negatively charged subnanometer-sized silicon clusters and their reversible migration into AFI zeolite pores studied with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy

Subnanometer sized silicon clusters were deposited on AFI zeolite AlPO4-5: one-dimensional channel diameter -0.73 nm. by pulsed laser ablation of silicon wafer. Their electronic structures were elucidated in situ by X-ray photoelectron spectroscopy XPS.and ultraviolet photoelectron spectroscopy UPS.. Core level Si 2p spectra were analyzed into five components, Si I. to Si V.. Si I. and Si II. species selectively increased with a constant ratio during pulsed laser silicon ablation. Their binding energies BEs.were below 99.5 eV implying negatively charged states. Charge transfer occurred between silicon clusters and framework oxygen and phosphor ions. It was interpreted that the stability of negative charge is due to large electron affinity of silicon clusters. The intensity of XPS signals decreased as a function of time and at the same time the channels were blocked. These results were interpreted due to migration of silicon clusters into zeolite pores. The estimated activation energy 57 kJrmol.suggests that rate-determining step of the migration is reflected by a weak adsorbed state of silicon clusters similar to physisorbed state. The silicon clusters were partially oxidized at 573 K, which was interpreted as a driving force of backward migration from zeolite pores to the external surface. The composition of silicon cluster was discussed based on homogeneous dispersion of single species. q1999 Elsevier Science B.V. All rights reserved.