Photoluminescence Study of the Introduction of V in Si-MCM-41: Role of Surface Defects and Their Associated SiO- and SiOH Groups

Abstract

The photoluminescent properties of highly ordered siliceous and vanadium-substituted mesoporous molecular sieves (MCM-41) are examined. Both Si-MCM-41 and V-loaded MCM-41 show green/blue/violet photoluminescence at 490−400 nm, likely due to surface defects associated with SiO- and/or SiOH groups. The intensity of the green/blue/violet emission strongly decreases when MCM-41 materials contain well-dispersed tetrahedral VV species. This intensity decrease is related to the consumption of surface defects and their associated SiO- and/or SiOH groups. Photoluminescence spectroscopy distinguishes only one kind of tetrahedral VV species, characterized by a vibrational fine structure and energy corresponding to a short VO bond. The intensity of photoluminescence from vanadyl groups in V-loaded MCM-41 increases as a function of V content. The addition of an anti-foaming agent into the synthesis solution enhances reproducibility of V-MCM-41 materials and causes a highly ordered hexagonal MCM-41 structure. The local symmetry of vanadium ions dispersed in highly ordered MCM-41 structure is closely related to the pore size of the mesoporous matrix.