Intrazeolite photochemistry. 26. Photophysical properties of nanosized TiO2 clusters included in zeolites Y, beta, and mordenite

The photophysical properties of TiO2 included in zeolites Y, beta, and mord enite are determined using time-resolved techniques. The shift in the groun d-state absorption spectra for the TiO2-zeolite samples, compared to bulk p hase anatase, is attributed to a direct transition in the semiconductor, ba sed on calculated electronic transition energies. Excitation of the samples at 355 nm results in the formation of a trapped electron. Time-resolved fl uorescence spectra are dependent upon the sample treatment, that is, whethe r the sample is heated prior to monitoring of the emission. Significant cha nges were found in the emission lambda (max) and decay kinetics. These chan ges that occur are due to the presence of two different Ti species in the n anoclusters, differing in the presence of hydroxyl groups bonded to Ti atom s, that in turn depends on the thermal treatment and the relocation/redistr ibution of water around the Ti atoms. Changes in relative intensities betwe en the two sets of samples are attributed to defect sites that are produced upon heating of the sample at high temperatures.