Comparative studies of p6m siliceous mesostructures by powder X-ray diffraction and nitrogen adsorption

摘要

Two-dimensional mesostructures with p6m symmetry such as MCM-41 and SBA-15 are the subject of intensive studies by using nitrogen and argon adsorption, transmission electron microscopy, and powder X-ray diffraction/scattering techniques. The latter may involve the X-ray diffraction (XRD) or small angle X-ray scattering (SAXS) measurements. The XRD/SAXS patterns for the aforementioned ordered mesostructures often exhibit four or more reflections, the most intensive one, 100 peak, and three less intensive peaks, 110, 200, 210. So far, analysis of these patterns was usually limited for the evaluation of the unit cell parameter and the identification of the observed peaks. In this work we present an attempt to analyze the XRD/SAXS patterns by including not only the position of observed peaks but also their intensity. It is shown that the intensity of these peaks, especially 110 peak, depends on the ratio of the pore diameter to the unit cell parameter. In particular, this dependence was studied for the intensity ratio of 110 and 200 peaks assuming 200 peak as a reference reflection because of its nearness to 110 peak (which minimizes the influence of other factors than the structural ones on the 110/200 intensity ratio analysis) and similar intensity to that of 110 peak. The values of the 110/200 intensity ratio were determined for many MCM-41 and SBA-15 samples and analyzed in relation to the pore width/ unit cell ratio, where the pore width was estimated on the basis of nitrogen adsorption data or by the XRD/SAXS structure modeling. Comparative analysis of this intensity ratio for numerous MCM-41 and SBA-15 samples allows for a quick estimation of the pore width and provides some information about hexagonality of mesopores in these materials.