pubs.acs.org/Langmuir © 2009 American Chemical Society
Subnanometer CdS Clusters Self-Confined in MFI-Type Zeolite Nanoparticles and Thin Films Ka-Lun Wong,† Abdelhafid Souici,‡ Vincent De Waele,‡ Mehran Mostafavi,‡ Till H. Metzger,*,§ and Svetlana Mintova*,† †
Laboratoire Catalyse & Spectrochimie, ENSICAEN - Universit e de Caen - CNRS, 6, boulevard du Mar echal Juin, 14050 Caen, France, ‡Laboratoire de Chimie Physique, UMR-8000, CNRS - UPS, Bat. 349, Orsay 91405, France, and §European Synchrotron Radiation Facility, ESRF, BP 220, Grenoble 38043, France Received September 17, 2009. Revised Manuscript Received October 13, 2009
One-step colloidal synthesis of subnanometer CdS clusters in hydrophobic MFI-type zeolite crystals in the presence of 3-mercaptopropyl-trimethoxysilane (MPS), cadmium precursor, and tetrapropylammonium hydroxide (TPAOH) is performed. MPS is used as the bifunctional agent, as it hydrolyzes fast, cross-links with the silica framework, and provides thiol groups to anchor Cd2þ, and subsequently forms CdS clusters. The MFI crystals with the thiol groups not only function as a nanochamber for the formation of CdS but also prevent further moisture-induced agglomeration of the clusters. Direct evidence for the presence of asymmetric shaped subnanometer CdS clusters aligned in the channels of MFI crystals stabilized in suspensions and films is provided by high resolution transmission electron microscopy (HRTEM), grazing incidence X-ray diffraction (GI-XRD), and photoluminescence spectra (emission < 350 nm). The CdS clusters are stable for months in colloidal suspensions and films without any particular precaution against water. The hydrophobicity of the MFI zeolite and the presence of the organic template in the channels favor the stabilization of small CdS clusters, which are available for further applications.
1. Introduction Semiconductor (SC) clusters are potential materials for a variety of optoelectronic, solar photovoltaic, and sensor devices.1,2 The tunable optical and electronic properties of SC clusters, which differ significantly from those of the bulk material, are mainly dependent on their particle sizes. Due to their very reactive surfaces, the SC clusters prefer to grow or agglomerate. Therefore, the formation of stable clusters with good monodispersity and welldefined shape has been the aim of many researchers. Numerous synthetic methods have been reported involving colloidal synthesis, controlled precipitation, and molecular precursor approach consisting of decomposition of metal-organic compounds.2-4 In order to prepare SC clusters using these methods, stabilizers or capping agents have been used to protect their active surface and hence to prevent them from aggregation. Another technique involves confinement or structured medium synthesis, which aims to form SC clusters inside an ordered porous matrix. The pores or cages of the matrix function as nanochambers that limit the size to which the SC clusters can grow. Zeolites, in particular, with their uniformed micropores (
