Preparation and Characterization of Ultra-Small Sized Metal and Semiconductor Particles in Sol-Gel Materials
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ABSTRACT Poly (1,4-phenylene)-bridged and poly (l,6-hexylene)-bridged silsesquioxanes (PPS and HPS) were prepared by the sol-gel process. The surface areas and pore diameters of these porous xerogels were obtained by BET and BJH methods, respectively. These porous materials were used as a confinement matrix for the growth of small-sized semiconductor and transition metal clusters. Quantum-sized CdS particles in PPS (approximately 58±12 A) and HPS (91±16 A) matrices were prepared by first soaking the xerogel in a CdCl2 solution. Following a washing with water, a Na2S solution was then added. EDAX and electron diffraction techniques were used to identify the CdS particles. The particle sizes of CdS in PPS and HPS were determined by both UV measurements and from TEM images. Small-sized Cr clusters were prepared in dried xerogels by an internal doping method. Mixed Cr/CdS phases were also prepared by internal loading of a chromium metal precursor. Following deposition of CdS the xerogel was heated at 120 *C under high vacuum, resulting in formation of intimately mixed phases of Cr metal and CdS. Changes in morphology, in particular the surface area and pore size distribution were noted. A decrease in surface area and an increase in pore size were observed as a result of Cr metal deposition.
INTRODUCTION Hybrid organic-inorganic glasses are an interesting class of versatile materials. Their syntheses by sol-gel chemistry allows for control of microstructure. The variable organic component provides a range of properties for use in optical devices, chemical sensors and as catalysts. Previous reports described synthesis of a new class of modified silicates that incorporate aryl and alkyl spacers. 1-3 The silicate framework of alternating silicon and oxygen atoms was modified by inserting an alkyl or aryl group in the lattice to obtain highly porous hybrid glassy materials (Figure 1). These bridged poly silsesquioxane matrices were prepared by hydrolysis/condensation of the appropriate hexafunctional monomeric building blocks by acid or base catalyzed sol-gel chemistry. 6 Small-sized particles have recently attracted considerable attention for use as catalystS.4Enhanced catalytic activity can be expected since small-sized particles have high surface area-tovolume ratios when compared to bulk phases. Small-sized particles may also be useful for photocatalysts. Small-sized particles show an increased effective band gap energy with decreasing particle size. This property allows for variation of the redox potential, resulting in a highly tunable catalyst.7 In addition, since the particle diameter is often much less than the excitation wavelength (< 380 nm), light scattering may be negligible. For this reason, the exact determination of the photochemical quantum yield is possible. Particularly interesting opportunities are possible from dispersions of small particles in a microporous or mesoporous matrix. For example, when small-sized particles are prepared in a porous xerogel, changes in catalytic behavior resulting from inter
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