Characterization of di-phasic nanoscale composites derived from xerogels

Di-phasic xerogel-derived composites, such as SiO₂AgCl, SiO₂-AIPO₄, SiO₂-CePO₄, SiO₂ -Nd₂O₃, SiO₂-CdS, SiO₂CrPO₄, SiO₂-BaSO₄ and SiO₂-PbCrO₄ have been characterized in detail by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) techniques. The SiO₂-AgCl photochromic composites with small amounts of AgCl did not show any crystallinity either by XRD or by SAED. Thin edges of these SiO₂-AgCl composites did not reveal discrete AgCl particles because these are too small to be resolved even by TEM and are expected to be in the range 1.5 to 2.5 nm in size based on the pore size of silica gel. A few large AgCI-Ag particles precipitated on the outside of silica gel were, however, detected by TEM-SAED in silica gels with higher concentrations of AgCl. The SiO₂-AIPO₄ and SiO₂-Nd₂O₃ composites are noncrystalline and did not show any periodic structure by TEM and SAED. Heat treatments to 400 or 600° C did not crystallize the AIPO₄ or Nd₂O₃ phases. On the other hand, SiO₂-CePO₄ and SiO₂-CdS composites showed lath-like particles of CePO₄ and irregular particles of presumably CdS on the surfaces of silica gels. The SiO₂-BaSO₄ and SiO₂-PbCrO₄ composites showed crystals of BaSO₄ and PbCrO₄ which are too large to be incorporated in the silica gel pores. These results show that the size and crystallinity of a second phase within silica gels can be controlled by the appropriate manipulation of the different parameters, and to do so is an important advantage for this new class of diphasic nanoscale composite xerogel materials.