UV–Vis spectroscopy and desorption/ionization mass spectrometry as the tools for investigation of adsorbed dye photodegradation

The photodegradation of the cationic dyes methylene blue, acridine yellow and acridine orange adsorbed on TiO 2 , TiO 2 /SiO 2 , SiO 2 and Ag/SiO 2 mesoporous films has been studied by electronic spectroscopy and laser desorption/ionization mass-spectrometry methods. Qualitative difference of peak intensity distribution for cation M + and its reduced forms [M + H] +· and [M + 2H] + has been detected in the laser desorption/ionization mass spectra of the dye adsorbed on the three film types. The results point out the sensitivity of the laser desorption/ionization mass spectra parameters to specific features of the dye cation adsorption on films of different compositions and photocatalytic activity. Characteristic dependencies of mass spectra intensities upon the duration of UV irradiation have been obtained for dyes adsorbed on the catalytically active films. Some products of the dye photo-destruction have been identified, and it has been shown that photo-catalytic activity decreased in the order TiO 2 > SiO 2 /TiO 2 > SiO 2 . In the presence of titania, photobleaching of adsorbed dyes occurs faster in comparison with this process in the presence of SiO 2 and leads to the full discoloration and degradation of adsorbed dye molecules. Efficient photodegradation of acridine dyes occurs through the following steps: 1-N-demethylation (deamination for AY); 2-photodimerization; 3-photodegradation. It is shown that both the rise of Ag nanoparticle concentration on the surface of mesoporous silica films and the enlargement of the nanoparticles’ size from 3 to 6–12 nm intensified methylene blue desorption/ionization. The efficiency of mesoporous nanosized TiO 2 , SiO 2 /TiO 2 , SiO 2 and Ag/SiO 2 films as substrates for laser desorption/ionization mass-spectrometry method has been demonstrated.